CORDANI, LJ.G./ MILANI, E.J. I THOMAZ flLHO. A.ICAMPOS. D.A.TECTON IeEVOLUTION OF SOUTH AMERICA. P. 481·559 j RIO DE JANEIRO, 2000

TECTONIC EVOLUTION OF THE

ANDES OF ECUADOR, PERU, BOLIVIA

E. Jaillard, G. Herail, T. Monfret, E. Dfaz-Martfnez, P. Baby, A, Lavenu, and J.F. Dumont

This chapterwasprepared underthe co-ordinationofE.[aillard.Together withG.Herail andT. Monfret,hewrotethe Introduction. Enrique Dfaz-Martinez prepared thesection on the Pre-Andean evolution of the Central Andes.Again Iaillard, onthe Pre-orogenic evolution of the North­Central Andes. E.[aillard, P. Baby, G. Herail.A, Lavenu, andJ.E Dumont wrote the texton the orogenic evolution of theNorth-Central Andes, And, finally, [aillard dosed themanuscript with the conclusions.

INTRODUCTION:

THE PRESENT-DAY NORTH-CENTRAL

ANDES (jON - 23°5) _

TheAndean Chain isthemajormorphological feature oftheSouth American Continent. ThisB(){)()kmlongmountainbeltextends alongthewesternborderoftheSouth AmericanPlate, and can be divided into three segments of distinctorientations, separated bytwomajorbends. TheNNE-SSWtrendingColombian-Ecuadorian segment (12°N - 5°S) is2000 km long and includes part of northernmost Peru andeasternmost Venezuela. It is separated from the Peruviansegment by the Huancabamba Bend (Megard, 1987). Itsnorthern end(eastern Venezuela) exhibits achange to anE­Worientation. dueto its connection to theSouth Caribbeandextral transform system. ThePeruvian segment (S·S ~ 18°5)is 2000 km long and its orientation is closeto NW-SE. Itincludes northern Bolivia, and isseparated from theChileansegments by the Arica Bend. The Chilean segment (IBCS ­56°5) is 4000 km long and trends N-S. Its southern tipexhibitsa change to an E-W direction along the Scotiasinistral transform system.

Thewidthofthe Andean Chain variesfrom250km innorthern Peru (5·5) or southernmost Chile (52°5 - 55°5).to as muchas 750 km in Bolivia (18°S).

The area described in this chapter includes Ecuador.Peru, Bolivia and,therefore, northernmost Chile (Nof23°S).It includes parts ofthethreesegments described above, theorientation of which played a significant role in the pre­orogenic period.

The lowest pass of the studied area is situated innorthernPeru(Abra dePorculla, 2300 m),a zonewhere the

chainisvery narrow. Thehighest average altitudeis reachedbetween 15°5 and 23°S, where the Altiplano ofBolivia andsouthernPerureaches anearly 4000 mofaverage elevation,andcorresponds to thewidest partofthechain. TheAndeanChain is usually highly asymmetric, with a steep westernslope. and a large and complex eastern side. In Peru,thedistance between the trench and the hydrographic dividevariesfrom 240 to }OO km.whereas. the distancebetweenthehydrographic divide and the200m contourlinerangesbetween 280 km(5°N) and about1000 kIn (Lima Transect,8·S - 12°5). In northern Chile and Argentina (23·5), thesedistances become 300 krn and 500 km, respectively. Tnsouthern Peru,as littleas 240 km separates the CoropunaVolcano (6425 m) from the Chile-Peru Trench (- 6865 m).This, together with the western location of the Andesrelative to theSouth American Continent,explains whytheriversflowing toward the Pacific Ocean do not exceed 300kmlong, whereas thoseflowing to theAtlantic Ocean reach4000 kmlong.We haveto notetwo exceptions to this rule.

In Ecuador thisasymmetry disappears, duetopeculiartectonic history and deformational processes. Thetrench­hydrographic divide distance roughly equals the distancebetween the waterdivide and the 200 m contourline,andranges between 280and 350km. Between U·S and 24°$(southern Peru-Bolivia). there are two hydrographicdivides, delimiting a wide, flat, endorheic basin known asthe Altiplano, which coincides with the zone of highestaverage elevation and largest widthofthe chain(fig. 1).

The highest summits are usually recent or activevolcanoes situated on the deformed chain or onmetamorphic or graniticslices uplifted by reverse faults.Among the former are the Cotopaxi (5897 m) andChimborazo volcanoes (6310 m) in Ecuador, the Coropuna(6425 m) andAmpatovolcanoes (6310 m) in Peru,andtheSajama Volcano (6520 m) in Bolivia, near the Bolivia-Chileborder. Among the latter, we may note the mainly graniticCordillera Blanca in Peru,which culminates at 6768 m(Nevada Huascaran), and the metamorphic EasternCordillera ofsouthernPeru and northernBolivia (NevadoIllimani, 6682 m; Nevado Illampu, 6485 m). However.deformed and uplifted sediments may also form highsummits in Peru, suchas theNevado Yerupaja (6632 m) inthe CordiJIera Huayhuash and the Nevado Ausangate (63&4m) in southern Peru or the Cordillera de Apolobamba(Nevado Cololo, 5975 m) in Bolivia. 481

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Manazo FTB) involving mainly Mesozoic sedimentary rocks.Insouthern Peru, the EasternCordillera is thrust over theAltiplano bymeans ofSW-verging reverse faults. Longitudinalvalleys, the trend ofwhich iscontrolled bythe mainAndeanstructures, separate N of 13°5 the Eastern and WesternCordilleras. 5of 13°S, theEastern andWestern Cordilleras aremore distantand areseparated bylarge valleys (13° - 14"S)evolvingsoutheastwards intoanendorheic basin (Altiplano).which considerably enlarges inBolivia.

Thedeformed subandean zone is muchlargerthan inEcuador (120to 250km) and enlarges southeastwards (Fig.2). Deformation involve the Mesozoic and Tertiarysedimentary rocks, together with their Paleozoic,sedimentary or metamorphic basement. The eastwardmigration ofdeformation influenced thecourse oftherivers,which trend mainly parallel to theAndeanstructures.Theeastern lowlands are made of either wide alluvial plainsreceiving Quaternary sediments (northernPeru),orslightlyuplifted, fault-controlled zones underlainby the BrazilianShield (southern Peru,Bohvia).

In northern Peru (7"S), where the chain is relativelynarrow, crustal thickness is 40 - 45 km below the WesternCordillera and decreases rapidly eastwards,whereas insouthernPeru(13°5),wherethechain ismuch wider, crustalthickness reaches 65 km below the Western and EasternCordilleras and drastically decreases below theSubandeanZone(Fukao etal., 1989).

The northern part of the Chilean-Bolivian segment ismorecomplex. To the W, the 50km wideCoastal Cordillerais madeof chiefly Jurassic-Early Cretaceous magmatic arcrocks, cut bythe majorN-StrendingAtacama wrench faultsystem. The Longitudinal Valley, underlain by MiddleCretaceous arcrocks separates it from thePrecordillera. The3000 mhighPrecordillera ismadeoflatestCretaceous-earlyPaleogene volcanic and sedimentary rocks overlyingJurassic-Early Cretaceous back-arc sedimentary rocks, andoverlain by Neogene-Quaternary tuff and piedmontdeposits. To the N of 21"S, it merges with the WesternCordillera, whereas to the Sof21"S,it isseparatedfromthelatter by the Preandean Depression occupied by the Salarde Atacama. As a whole, these four desert units are 70 to250kmwide.

TheWestern Cordillera is made ofNeogene-Quaternaryarc rocks, and comprises active volcanoes culminating atmore than 6000 m.Isolated outcropsshow thatTertiary arcrocksdirectlyover!iePrecambrian basement,atleastlocally.TheWestern Cordillera forms the Chilean-Bolivian borderandlimitstotheWthe200 krnwide, flat Bolivian Altiplano,in-filled bythousandsof metres of Teitiaryto Quaternaryrocks. The latterexhibits an average altitudeof morethan3900m and contains endorheic lakesevolving locally intowide salars. The Eastern Cordillera is made of Paleozoicfolded metasediments which arethrust tothe SW orWontotheAltiplano border, andtothe NE or EontotheSubandeanZone. At 19°5, crustalthickness exceeds 60km and reaches70to75kmbelow theWestern and Eastern Cordilleras (Becketal; 1996; Fig. 2).TheSubandean Zone is madeoffaultedand folded sedimentaryrocksof Paleozoic to Miocene age,which are thrust to the NE or Eonto the easternlowlands.The latter is a large alluvial plain where products of theerosion of the Andes are presentlybeingdeposited.

Plate tectonic setting

As first proposed byWegener early in this century, theAndean Orogeny is related to the convergence between thePacific Ocean Plate and the SouthAmerican Continent, thelatter being pushedwestwards bytheopeningoftheSouthAtlantic Ocean. We know nowadays that not only SouthAmerica, but also the Pacific oceanic Plate moves in anabsolute reference frame. SouthAmerica migratesroughlywestwards at a rate of 3 cm/y. Between 2"5 and 23"5, theNazca Oceanic Plate migrates to the E to ENE, and theconvergence rate between the Nazca and South Americanplates is around8 cmly, although it seems to be lower at 0°(Fig. 3).Thewhole Andeanmarginis, therefore, submittedto a roughly E-W convergence between the subductingNazca Plateand the SouthAmerican continental margin.This phenomenon is long regarded as the majorcause ofthe Andean Orogeny (Rutland, 1971; James, 1971).

TheNEtrendingGrijalva FractureLone (GFI) locatedbetween 3°S and 50S (Fig. 3) separatesthe Nazca OceanicPlate into two portions.To the N of the GFZ, the currentlysubducting oceanic plateis 23to 10Maold (Miocene) andthe depth of the oceanic bottom is 2800 to 3500 m belowsealevel. Most of the Ecuadorian -Colombian trenchislessthan 4000 m deep. The NazcaPlate is overlain bythe E-Wtrending Carnegie aseismic ridge, which is presentlysubducting offshore Ecuador between 0" and 2°S. TheCarnegie Ridge is regarded as formed by the Galapagoshotspot,situated about}000km Wofthe Ecuadorian coast.

Southof the GFZ, the subducting oceanic plateis 50to30 Maold (Eocene - earlyOligocene) and 4000 to 5600 mdeep. ThePeruvian-Chilean trench reaches depth of 6600m (11-s, 17"S), 7500 m(I 9"S) and even8055 m (23°S). TheNazca Oceanic Platecomprises two submarinehighs. TheNE-trending Nazca aseismic ridge, entered into the trench3 to 10Maago and ispresently subducted at 15"S - 16"5. Itpresentssummitsculminating at -330m,-841 mand -2059m, from 5Wto NE, thus showing a NE plunge ofits crestalline.To the Sof L8°S, a N-S trending submarineridgerunsalongl'oide the trench of northern Chile,and culminateslocally at -1800 m,

Recent improvement in hypocenter relocation at thescale of South America has been done, giving a gooddefinition of the Benioff-Wadati Zone geometry (Engdahlet al., 1998). It shows along-strike segmentation of theAndes but without tears even around the latitude 15°Swheretheslabhasa severecontortionduetochanges in Itsdip.And onlyaverysmall portionoftheobserved seismicityoriginates within the Andean crust.

From NtoS,theNazca Platesubductsatan angle of19"-30· toadepth which variesfrom 250 km to 650km.NandSof theBolivian Orocline, at latitudes2°5- 15°S and 26°S ­33°S, seismic activity indicates that theNazca Plate isbeingsubducted nearlyhorizontal at a depth ofabout 100 km fora distance of over 300km before descending steeply intothe mantle.Thereare novolcanic arcsbecause ofthelackofasthenosphere wedge between the subductingflatslabandthe upper continental plate (Barazangi and [sacks, 1976).These horizontal segments partially correlate with theoceanic ridges (Nazca Ridge.Juan Fernandez Ridge) or totally

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I~I(IncaPlateau?) subducted (Gutsheretal.,1999). Inaddition,it isaccompanied by importantshallow earthquakes (< 20depth) related to crustal shortening.

The contact zone between the Nazca and SouthAmerican plales concentrates only a partoftheconvergencemotion as shown byGPS measurements (Kendrick et al.,1999). In the case of the Antofagasta 1995 earthquake, thelastgreatChilean event (M"= 8.1),a locked portionof theplate interface was ruptured. The observed slip release(between 2and7m)was lessthantheexpected plate motionaccumulation (over 8 m) and suggests that the resultantmotion causes permanent deformation through uplift oftheAltiplano and crustal shortening in eastern Andes(Norabuena etal.,1998).

Low magnitude earthquakes recorded bylocal seismicarraysare revealing small-scale differences inseismicity ofthe Benioff-Wadati Zone and thecontinental crust (Comteet al., 1999), and should be related with some specifictectonic processes. Body-wave velocity models at the scaleoftheSouth American Plate(Bijwaard et al., 1998) and atsmaller scale (Comte etaL, 1994; Dorbath et aL, 1996; Myerset aI., 1998) show a medium lateral heterogeneity wherethe descending oceanic Nazca Platecan be observed over600km depth as zonesof faster seismic velocities due tothe colder, denser material than the underlyingasthenosphere. The crustal thickness underneath theAltiplano Plateau isof60- 75krn,twice as thickasnormal,but its exact origin remains enigmatic.

Volcanic and seismic activity

Like all activemargins, the Andesare submitted tointense volcanic activity, which presents, however, severalpeculiarities. Distribution of the active volcanoes indicatesthat the volcanic arc activity is not continuous along theAndean margin, defining a Northern, a Central and aSouthern Volcanic Zone (NVZ, CVZ and SVZ; Fig. 3). thathave beeninterpreted asrelated tothedipofthesubductionplane.As a matteroffact,theNorthern andCentral VolcanicZones overlie segments with relatively steep Wadatti­BenioffZones. whereas the volcanic gap of southernEcuador and northern-central Perucorresponds to a zoneof flat subduction. In this latter case, the lack of anasthenosphere wedge between thesubducting slaband theupper continental plate would prevent arc magmageneration (Barazangi and (sacks, 1976). Moreover, theNVZ is characterized by basaltic andesite, whereasandesite, dacite lavaandignimbrite dominate theCVZ. Thelatter feature is interpreted as due to fractionalcrystallization during the ascent of the andesite magmathrough a thickened continental crust

Inaddition, thispartoftheworldhasalready generatedgreat earthquakes ofmagnitude over 8.2followed ingeneralby destructive tsunamis. They occurredrepeatedly with acertainperiodicity of more than 100 years as the greatestinstrumentally ever-recorded earthquake located insouthern Chile (37"5 - 46.5°5). This earthquake occurredon May, 22 1960 (Mw .... 9.4, depth .... 25 km) and wasgenerated bya 20m slipuponan areaof about 1000 x 200

kmalong the contact zone between the two plates. Overmagnitude 8.7, thesouthernPeru onAugust 14,1868 (M=9.0), the northern Chile on May 9,1877(M

w=9.0)and the

Ecuador-Colombia on January 31, 1906 (Mw

= 8.8) greatearthquakes and their associated tsunamiscaused a lot ofdamage. Ontheotherhand,onJune 9,1994 thelargest deepearthquake (M

w=8.2, depth=630 km) in recorded history

occurred beneaththeAndes providing moreconstraints onthedynamics andgeometry oftheNazca Plate atthat depth.

PRE-ANDEAN EVOLUTION OF THE

NORTH-CENTRAL ANDES DOMAIN

This sectionattempts to provide a coherent overviewof the paleogeographic and geodynamic evolution of theCentralAndes Domain during the Paleozoic, along withsomecomments onitsProterozoic basementThesynthesishasbeencompiled from different sources, andalsoincludessome new concepts and interpretations. The area hereconsidered as Central Andes is roughly 5"-27'S, coveringpart of Peru, Bolivia, northern Chile and northwesternArgentina (Fig. 4),and therefore excludes the Arnotapes ofnorthwestern Peru and southern Ecuador, and thePrecordillera ofwestern Argentina.

In contrast with the rather complex Phanerozoicaccretionary historyof the SouthAmerican margin in theNorthern and Southern Andes (Colombia-Ecuador andChile-Argentina, respectively), the evolution of the CentralAndes appears to be somewhat simpler. as there is noevidence fortheaccretion ofallochthonous terranesduringthe Phanerozoic. Recent studies suggest that the crustalbasementin mostof theCentral Andeanarea formed partofthe Grenville Orogen, as a resultofthe collision betweenLaurentia andAmazonia intheMesoproterozoic (Wasteneysetal., 1995; SadowskyandBettencourt, 1996; Tosdal, 1996).Paleozoic rocksoftheCentral Andes record the break-up ofthe Neoproterozoic Proto-Pangea (Rodinia) in the latestNeoproterozoic-Cambrian to form a passive margin alongwestern Gondwana (Bond et al., 1984; PoweJl et al., 1993),and its later evolution as an active margin during mostofthe Paleozoic and untilpresent times(Sempere, 1995). Thecontinuous superposition of magmatic, tectonic andsedimentary events has led to complex lateralvariations,bothin cross section and along strike,and hasoriginated awide range of settings for the development of mineraldeposits (Fontbote etat. ,1990; Schneider, 1990; Fornari andHerail, 1991) and hydrocarbon generation andaccumulation (Gohrbandt, 1992; Moretti et al., 1995;Tankard etal. ,1995) related with Paleozoic rocks.

Proterozoic basement

Due to the protracted superposition oforogenic eventsin the Central Andes, the interpretation of the Proterozoicevolution of the region is inevitably very fragmentary, andshould be considered as a mereworking hypothesis. ThemodembasementoftheCentral Andesconsistsoftwocrustalblocks with different origins: theArequipa-Antofalla Craton

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(Ramos et al; 1986) and theAmazonian Craton (Teixeira etal., 1989) orCentral Brazil Shield (figs. 1and2).TheArequipa­Antofalla Craton isa Proterozoic terraneinterpreted to haveoriginated asthetipofapre-Grenville Laurentianpromontory(comprising Labrador, Greenland and Scotland) that wasincorporated into the Grenville Orogen (Dalziel, 1994;Wasteneys et al., 1995). Pb isotope composition seems tocontradict thismodel, indicating instead closer tieswith theAmazonian Craton (Tosdal, 1996). Thereconstruction oftheremnants of the Grenville Orogen in South America(Sadowski andBettencourt, 1996) indicates that theCentralAndes corresponds to an area intermediate between themagmatic arc(represented by theSunsas igneous province,in eastern Bolivia and western Brazil) and the thrust belt(southeastern Canada) oftheGrenville Orogen (Fig. 5b),andexplains the similar trends identified between theProterozoic outcrops along the Andes, and those of theBrazilian Shield (Litherland et al., 1985, 1989).

Paleoproterozoic ages indicated by Rb/Sr whole-rockisochrons and bulk U/Pb zircon geochronology representthe pre-Grenville Laurentian-Amazonian protolith,andMesoproterozoic ages of granulite-facies metamorphismindicated by U/Pb Single-grain zircon geochronologyrepresent themaincollisional events oftheGrenville Orogen(Wasteneys et al., 1995; Sadowski and Bettencourt, 1996;Tosdal, 1996). Rifting during break-up of Rodinia in theNeoproterozoic-Cambrian led to separation of Laurentiafrom Amazonia (Fig. 5-c), leaving behind theparautochthonous Arequipa-Antofalla Craton attached toAmazonia (Central Brazil Shield). The boundary zonebetween the two crustal blocks, which is located beneaththe Eastern Altiplano and Eastern Cordillera, thusconstitutes a paleosuture and crustal weakness zoneinherited from the Mesnproterozoic evolution of theGrenville Orogen (Figs. 1and2).Thiszoneremained activeduringthePaleozoic, andever since,withvariable behaviourdepending on the regional stateof stresses (Ramos, 1988;Dorbath et al., 1993; Forsythe et al., 1993).

Tectonomagmatic episodes

Tectonic and magmatic events tookplace in the CentralAndes in a rather continuous manner during the wholePaleozoic, shifting their foci and areal extent with timeas aresult of plate interactions, variable geometry of the platesinvolved, and the resulting stress regimes. Thesevariableconditions led to apparently different styles of evolutiondepending onthelocal areaunderstudy. However, an overalltendency for the whole region may be discerned andsimplified as follows. Apart from the aforementionedMesoproterozoic (1.2Ga- 900 Ma) ages resulting from theGrenville granulite-facies metamorphism, and thePaleoproterozoic (2.0 -1.9 Ga) ages obtained from itsmetagranitoid protolith, thecrystalline basement underlyingthe Central Andes also presents Neoproterowic to MiddleCambrian (600- 520 Ma) ages ofigneous and metamorphicevents.These events arecommonlyassigned tothelastphasesofthe BrasilianoOrogeny, which arereferred to as PampeanOrogeny inthesouthernCentral Andes (Rapela etaL,1998).

Break-up of Rodinia and rifting of eastern Laurentia fromGondwana in the Neoproterozoic and Early Cambrian ledtothe development of passive margins on both sidesof theSouthern IapetusOcean (Fig. 5-c).

Along the pre-Andean margin ofGondwana Sof27·S,a westward shift of the spreading centre is interpreted tohave left oceanic crustbetween adetached continentalblock(Pampean Terrane) and the Gondwana margin (Rapela etal., 1998), Later eastward subduction and closure of thisremnantseaduringtheEarly Cambrian led to thecollisionof the Pampean Terrane in the Middle Cambrian. To the Nof27"S,a similar history isalsoprobable,with theArequipa­Antofalla Craton alsopartially rifting andthenlatercollidingalong its southeastern boundary with the AmazonianCraton. Meanwhile, thewestern margin ofboththePampeanTerrane and the Arequipa-Antofalla Craton remainedpassive untiltheLateCambrian (Fig. Soc).

Beginning in the Late Cambrian or earliest Ordovician,this western passive margin became an active continentalmargin (Fig. S-d).TheSan Nicolas Batholith insouthwesternPeruis interpreted as the rootofthemagmatic arcresultingfrom eastward subduction ofoceanic crust along theactivemargin of Gondwana during the Paleozoic (Mukasa andHenry, 1990).Ordovician-Devonian ages obtained forlowerintercepts in U/Pb geochronology ofbasement rocks alongthe western Arequipa-Antofalla Craton reflect thermaloverprinting and Pb-Ioss coinciding with peaks of thisPaleozoic magmatic activity (Shackleton etai, ,1979; Dammetai.,1990; 1994; Mukasa and Henry, 1990; Tosdal, 1996).Different rates of plate activity and sense of migration ofthemagmatic arcdeveloped depending onregional stressesand inhomogeneities, andbasindevelopment alsochangedin accordance withtheseplateinteractions.

The Paleozoic development of this active margin ischaracterized bybackarcextensional conditions duringtheearly phase (latest Cambrian-Early Ordovician) and latephase(LateCarboniferous-Pennian),resulting in astronglysubsiding thinned crust, with partial rifting and syn­sedimentary basic volcanism (Fig. Sd). In contrast, acompressional regime (retro-arc foreland) characterized theintermediate phase (Middle Ordovician-EarlyCarboniferous; Fig. 5e). Anapparent lackof evidence forSilurian and Devonian tectonomagmatic activity in thesouthwestern Central Andes has been interpreted asevidence fora passive margin resulting from riftingoffofpartoftheArequipa-Antofalla Craton (Bahlburg andHerve,1997). However, this interpretation is difficult to reconcilewith the evidence for a coeval active plate margin insouthern Peru (down to 17"5) and northern Chile andArgentina (up to 27·S),as well as withthe Silurian ageofigneous andmetamorphic events inthesameregion (Dammetal., L99O, 1991, 1994).

With regard to Late Paleozoic tectonomagmatic eventsintheCentral Andes, thesehavebeentraditionally assignedtoa"Eohercynian"Orogeny (Megard et al., 1971; Bard et al.,

1974; Dalmayrac et aL,1980), including Late Devonian­Carboniferous K/Ar and Rb/Sr ages, and regionalstratigraphic andpetrographic evidence.However,onlyoneU/Pb zircon age is reported (330 ± 10 Ma; Carlier et al.,

1982). OtherUlPbzircon datesongranitoid along the NWtrending segment of the EasternCordillera of Peru and

TECTONIC EVOLUTION OF SOUTH AMERICA

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Bolivia establish theirtime of emplacement as Permian oryounger (McBride etal., 1983; Heinrich et al.,1988; Farraretal.,1990; Kontak et al., 1990).

This more recent evidence questions the relation ofthegranitoid plutons with widespread Late Devonian-EarlyCarboniferous "Eohercynian" magmatism and orogenesis,aspreviously interpreted. Nevertheless, thereisevidence forlocaltranspressional uplift and deformation ofthe EasternCordillera in the latest Devonian and Early Carboniferous,which separated an Altiplano basin from a Subandean­Chaco basin (Megard et al., 1971; Dalmayrac et al., 1980;Kley and Reinhardt, 1994; Sempcre, 1995; Dfaz-Martlnez,1996). Maximum burialdepths (locally exceeding 10 km)were attained in different areas of the Central AndeanPaleozoic basinat different timesduringthe Late Paleozoic(Late Devonian-Permian interval). Asa result of this deepburial, and probably in conjunction with transcurrentstresses along the aforementioned suture zone (Sempere,1995), the stratigraphically lower units (Ordovician­Silurian) reached very low-grade to low-grademetamorphism, as indicated by vitrinite reflectance andillite crystallinity (Kley and Reinhardt, 1994).Thisthermaloverprint resulted in the reset of K/Ar system, thusexplaining theCarboniferous-Early Permian ages obtainedbysome authors (Dalmayrac etal.,1980; Paton, 1990).

At the same time, erosion related with the mid­Carboniferous global regression resulted ina disconformityor low-angle unconformity throughout the Central Andes,with Upper Carboniferous and Permian units directlyoverlying Devonian, Silurian or Ordovician units (Kley andReinhardt, I994;Jsaacson and Dfaz-Martfnez, 1995; Diaz­Martinez, 1996, 1998b).Asimilar unconformity isobservedin the southern Central Andes (northern Chile andnorthwesternArgentina), where the development of anactive margin withrelated fore-arc, intra-arc and back-arcbasins took place during the Late Carboniferous andPermian (Breitkreuz et al., 1988,1989; Bahlburg andBreitkreuz, 1991; Breitkreuz and Zeil, 1994). Thus, thereinterpretation of the evidence indicates that the alleged"Eohercynian" Orogeny may be no more than theconjunction of different processes and events takingplaceduring the Late Paleozoic throughout the Central Andeanregion, but not a single tectonomagmatic event or beltlocalized in space and lime.

Thesedimentary record in Peru and western Boliviapresents evidence ofmarginal arcvolcanismbeginninginthelateEarlyCarboniferous (Ambo Group, 60S - 17"S; DalmayracetaI., 1980; Dfaz-Martfnez, 1995, 1998b). Thissubduction­related magmatic activity propagated to theS (16°5 - 24°S)along the active margin duringthe Late Carboniferous andintothe Permian (Carlier et al., 1982; Bell, 1987; Breitkreuzetal.; 1988, 1989; Bahlburg andBreitkreuz, 1991; Breitkreuzand Zeil, 1994; Sempere, 1995; Isaacson and Dfaz-Martinez,1995; Bahlburg and Herve, 1997). Fartherto the S (20°5 ­42°5), thick volcano-sedimentary successions developed inthe Permian and Triassic (Choiyoi Group), withassociatedhigh-level intrusions (Kay et ai; 1989).

Furtherevidence forLatePaleozoic magmatic activityis also found in the Eastern Cordillera of Peruand Bolivia.Thisactivity beganearlierin the Peruvian sector, withLatePermian-Triassic ages (Kontak et aL, 1990), and migrated

towards the S, developing in Bolivia during the MiddleTriassic to Early Jurassic (McBride et al., 1983; Heinrich eral., 1988). The activity was not related to subductionprocesses, but instead, consists of alkaline volcanism andplutonism which are interpreted asa result ofpartialriftingand transtension along the suture zone between theArequipa-Antofalla Craton and theAmazonian Craton (Fig.Sf), due to regional stresses during Pangea break-up(Kontak et al., 1990; Atherton and Petford, 1991).

Tectono-sedimentary cycles

Most ofthe Late Cambrian toDevonian sedimentationin theCentral Andes tookplace alonga wide and elongatedepicontinental marinebasinbroadly parallel to themarginofGondwana (Fig. 4).The mainsourcearea for this basinwas situated totheWand SW, although thissourceareawasnota rifted-off formerPacificcontinent" (asconsidered byBahlburg andHerve, 1997).1 nstead, it isconsidered thatthesource area was the active margin of Gondwana itself(Isaacson and Diaz-Marttnez, 1995; Sempere, 1995),influenced byits discontinuous activity, specially the upliftand forward migration ofthe fold -thrust belt.Deepening ofthe basinand increased subsidence ratesbroadly coincidein time with the aforementioned Ordovician-Devonianpeaks of magmatic activity, and demonstrate the syn­tectonic character of deposition, in dose relation withtectonic piling and uplift along this fold-thrust belt(Sempere, 1995; Diaz-Martinez etal.,1996).

ThePaleozoic marginal orogen which fed the basin istodaycompletely dismantled, with its rootscropping out inthe Arequipa Massif and other smalleroutcrops scatteredthroughout theArequipa-Antofalla Craton. Extreme burialdepthslocally exceeding 10km were achieved during theLatePaleozoic, leading to the reset of isotope ages duringtheir maxima, and previous to the widespread erosion ofdepocenter areas. Paleozoic basin fills were later variablyeroded, andtheresulting stratigraphic gapsarewiderat themargins ofthebasin,especially towards thewestern upliftedareas.The originalwidth of the Paleozoic orogen seemsnarrower todaydue to tectonic erosion along the westernmarginof the Arequipa-Antofalla Craton (Stern, 1991), aswell as compressional shortening due to Cenozoic foldingand thrust-piling. Both processes (tectonic erosion anddismantling of the orogen and basinfill) continued duringMesozoicandCenozoic subduction, uplift anderosion intheCentral Andes, and resulted in the scarcity of evidence forthe moremarginal (fore arcandintraarc)andolderbasins.

Paleozoic sedimentation in the CentralAndes maybesubdivided into three main tectono-sedimentarycycles(limited by major unconformities): latest Proterozoic­Middle Cambrian,Late Cambrian-EarlyCarboniferous,andLate Carboniferous-Early Triassic. The first tectono­sedimentary cycle equates to the Pampean Cycle ofAcenolaza et aL (1990). Thesecond tectono-sedimentarycycle equates to the Tacsarian and Cordilleran cycles ofSuarez-Soruco (1989), and to the Tacsara, Chuquisaca andVillamontes supersequences of Sempere (1995). Thethirdtectono-sedimentary cycle equates to the Cueva

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superse<juence of Sernpere (1995). Overall, Paleozoic

sedimentation wasrathercontinuous, although remarkablechanges in subsidence ratestook place duringcertaintimeintervals. Sedimentary evidence for increased tectonicinstability anduplift occurneartheOrdovician-Silurian andDevonian-Carboniferous boundaries(Dfaz-Martinez etal.,1996), which have been traditionally used for furthersubdivision of the sedimentary pile(Suarez-Soruco, 1989;

Gohrbandt, 1992; Sempere, 1995). However, as mentionedabove, tectonomagmatic processes along the marginhavebeen rather continuous and synchronous with thedevelopment ofbasins.Hence.it doesnotseemappropriateto maintain the traditional subdivisions of the Paleozoicsedimentaryrecordbasedon discrete orogenic events.

Paleogeography andpaleoclimates

Deposition withina large marine basin (Peru-BoliviaBasin) located adjacent to a marginal orogen prevailedduringmostofthePaleozoic (Figs. 1and2).Theoverall andprogressive increase of rigidity and thickness of the crustbeneaththe Central Andesin thePaleozoic, ledtoa gradualincrease of terrestrial (subaerial and lacustrine)sedimentation intheLatePaleozoic. Fluvial deposits areveryrare until the Devonian, and become frequent in theCarboniferous. Eolian deposits begin to be present in theLateCarboniferous and are frequent until the Jurassic. Theareal extentof marine deposition in the LatePermian andTriassic wasvery limited.

The CentralAndes underwent important latitudinalmovements duringits drift as part of Gondwana's westernmargin in the Paleozoic. Despite the scarcity of confidentpaleomagnetic data,several models havebeen proposed forthe Paleozoic paleogeographic evolution of the region. Theoverall trend consists of a subtropical (30") latitudinalposition during the EarlyCambrian, and a shift towardshigher latitudes in the Middle and Late Cambrian(Courjault-Rade etal., 1992).During theOrdovician,Silurianand Devonian, the CentralAndes remained at mid to highlatitudes.withvariable shifts.Agradualshifttowards lowerlatitudestookplace duringtheEarly Carboniferous.and thearea has remainedat tropicallatitudeseversincethe LateCarboniferous (Dfaz-Martinez er al., 1993; IsaacsonandDfaz-Martfnez, 1995; Sempere,1995; Lopez-Garnundi andBreitkreuz, 1997). Paleozoic tropical carbonates andevaporites in the Central Andes are present only withinEarly-Middle Cambriandepositsand LateCarboniferous­Permian deposits.Thincarbonate-bearing unitsarepresentin theOrdovician.Silurian andDevonian systems containingcool-water faunal associations. Plantremains are frequentin thesedimentary record beginning intheMiddle Silurian.and coal development is observed in the late EarlyCarboniferous (Ambo Group) and in the LateCarboniferous-Early Permian (Copacabana Formation).Glacially influenced deposits are preserved within latestOrdovician-Early Silurian units,andwithinLateDevonian­Early Carboniferous units.However, truetillitesandglacial!ystriated pavements are only very rarely found (Starck,

1995), andmostoftheevidence consists ofglacially facetedand striated clasts, both as dropstones and withinresedimented units (Diaz-Martinez et al., 1999). Theseglacially-related deposits are interpreted as a result ofglaciation of tectonically uplifted highlands, and thereforeunrelated withthe largecontinental icecaps inGondwana.

Inherited pre-Andeanstructures

Structures inherited fromthepre-Andean geodynamicevolution of the Central Andeshave exerted an importantinfluence on its later development. The crust beneaththeCentral Andes originally formed as part of Proterozoicorogens. Thetrend of the structures developed during theformation of these orogens greatly conditioned laterPaleozoic basinformation andcrustalweakness zones.andtherefore influenced the geometry and distribution oftectonism and deposition. Theboundaryzonebetween theArequipa-Antofalla Craton and the Amazonian Craton istheprincipalof these features(Fig. 4), probablyinheritedasa suture zonefromthe collision betweenLaurentiaandtheAmazonian Cratonas part of theGrenville Orogen dig.S).Thiscrustal weakness zonewasthelocation of(a) riftingin the Neoproterozoic and Cambrian, (b) back arc andforeland successor basin formation fromtheOrdovician to

:arboniferous, (c) syn-sedimentary magmatism in theOrdovician and Silurian, (d) transpressive stressesoriginating local uplifts in the Devonian and EarlyCarboniferous, and (e) syn-sedimentary magmatism,andtranstensional stresses originating semigrabens andgrabens in the Late Carboniferous to Jurassic. In turn, theresulting Paleozoic features alsoinfluenced laterevents. Forinstance, (a) Paleozoic basin geometry and faciesdistribution greatly conditioned thelocation ofdecollementlevels and lateral variations in the propagation of thrustsduringCenozoic tectonism (Baby etal., 1989, 1992;Sempereet al., 1991), and (b) listric faults originated during LatePaleozoic- Early Mesozoic extensional conditions werereversed duringtheCenozoic. Thesameboundaryzone wasactive with magmatism and strong tectonism during theCenozoic, resulting intheformationoftheEasternCordilleraand Subandean fold-thrust belts.

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MIDDLE OLIGOCENE) EVOLUTION

OF THE NORTH..CENTRAL ANPES

Thisperiod maybedivided into two mainsub-periods,of Late Permian-Late Jurassic and latest[urassic-Paleogeneage, corresponding todifferentgeodynamic, paleogeographicand tectonic settings.

Late Permian to LateKimmeridgian (255 • 140 Ma)

Priorto the Sinernurian, the magmaticarc is not well­defined. During Jurassic, the paleogeographic frameworkexhibits theclassical three-fold division ofan activemargin(Fig. 6). The fore-arc realm,is nearly unknown,becauseofsubsequent tectonic erosion and/or deformation. It waslocated in the present-day Eastern Cordillera of Ecuador(Cordillera Real),whereas it probably liesoffshore PeruandChile. The magmaticarc controlledthe paleogeographicpattern.It trends NNEin the northern segment;runs alongthe present-day Subandean Zoneof Ecuadorand crosscutobliquely the northern Peruvian margin. Farther S, itappearslocally on the SEtrending coastof southern Peru,and extends into northern Chile,where its present-dayorientation isroughly N-S. Theback-arc domaincovers whatarenow theOriente Basin ofEcuador, theWesternCordilleraofPeruand mostof thecoastof northernmostChile. Distalback-arc areas covering eastern Peruand Bolivia (Fig. 6)bordered it to the E.

LatePermian - MiddleSinemurian(255 -195 Ma)

This period ischaracterized by Late Permian-Triassicextensional tectonics alongNW-SE trending grabens,andbyLate Triassic-Early Liassic marinetransgressions. Coevalalkaline volcanism and intrusionsdue to partial melting ofthe lower crust were locally associatedwith deep-seatedmetamorphism.

Late Permian- EarlyNorian (255 - 215 Ma)

Very littleisknownaboutthe Westernmost areasofthemargin.Coastal areasare,however, affected by significantalthough poorly understood deformation andmetamorphism. In the EasternCordillera ofEcuador, typeI granitoid (Ires Lagunas) yielded ages ranging from 257(SmlNd) to 200 ~ 189Ma(Rb/Sr, Aspden and Litherland,1992; Litherland etaL,I994),and in southwestern Ecuadororthogneiss yielded metamorphic agesof234to 198 Ma(KI

Ar,Feininger and Silberman, 1982; Rb/Sr, Aspden et al.,1995).Intrusion and metamorphismare interpretedas dueto a strong thermal event of LateTriassic age,resulting inpartial crustal melting and high temperaturemetamorphism. Theywould be associated with significantdextral movements related to the Tethyan oblique rifting(Litherland et al."1994).

In northern and central Peru,no pre-LateJurassic arc

volcanic rocks are known. In the coast of southern Peru(Arequipa Massif), metamorphic and intrusive rocksyielded KlAr ages rangingfrom 213 to 187 Ma (Stewart etaI., 1974; Romeuf etal., 1993). In southern Peru,a Triassic­Liassic age is assumed by Boily et al. (1984) for the arcvolcanism, sinceit is post-datedby, and locally intercalatedwith,Sinemurian marinedeposits(Vargas, 1970; Jaillardet

al., 1990).However, one of the plutons intruding thesevolcanic rocks yielded a 211 Maage,thus indicatinga pre­Liassic minimumage(Romeuf et al.,1993).

In northern Chile (26 - 31°S), basic to granitic,I-typeintrusionsyielded ages between 231 and 201 Ma(Berg andBaumann, 1985; Irwin et al., 198B; Cana, 1991),and areroughly coeval withsyn-metamorphicdeformationdatedat220- 201 Ma (Irwinetal., 1987). Fartherto the E, inPeru,thick sequences of fine- to coarse-grained, red sandstoneand conglomerate beds (Mitu Group) weredeposited in asubaerial environment, and filled fault controlled grabens(Megard, 1978; Kontak et al., 1985; Carlotto, 1998). Theassociated volcanic rocks are alkaline basalt withsubordinate tholeiitic basalt,and acidic, dacitic torhyoliticpyroclastites (Fig.7). Shoshonitic and peralkaline suites(Kontaketal., 1985) aswell ascomendite(Noble era!', 1978)are present, indicalingan intraplate extensional tectonicregime. Because theyoverlie fusulinid-bearing limestoneofEarly Permianage, the volcanic andsedimentaryrocks ofthe Mitu Group have been ascribed to the Late Permian­Triassic (Newell et ai, 1953; Laubacher, 1978). Farther tothe N (Ecuador), this period corresponds to non-marineclastics and subordinate volcanics (Sacha Formation,Rivadeneira and Sanchez. 1991). Deposits similarto thoseofthe MituGrouphavebeen recentlyidentifiedin Bolivia,wheretheyart:foundinmajorpaleogeographic structures,which governed theAndean tectonics (Sempere er al., 1999;Fig. 7). fn northern Chile, deep transtensional rift basinswere filled by thick sequences of subaerial basic,intermediate and acid volcanic rocks and subaerialsiliciclastic and volcaniclastic rocks of Late Permian­Triassic age (Chong, 1976; Suarez and Bell, 1991; Flint eral., 1993).

TheLate Permian-Early Triassic agehasbeenconfirmedin southern Peru by 270 - 200 Ma ages obtained fromintrusionsand shoshonite, which, however, extent into theEarly Jurassic (200-180Ma,KontaketaL, 1985; ClarketaL,1990). In southern Peru,coeval biotitegranodiorite andmonzogranite derived from partial melting of the lowercrust (Granitoid province) are crosscut by mafic dykesshowing chemistryand mineralogy similar to that of thealkalibasaltof theMita Group (Kontak etal.,1985). Fartherthe SE, mostofthe graniticto granodioritic, peraluminousintrusionsofthe Bolivian CordiJIera Real yielded agesfrom225 to 19$Ma (Avila.Salinas, 1990). These sedimentary,tectonic and magmatic phenomena were interpreted asresulting froma NW-SE trendingrifting of LatePermian­Triassic age,relatedto the break-up of Gondwana and/orto theTethyan rifting(Vivier etal., 1976; Kontaketal., 1985;

Jail/ard etat,199Q; Hint et ai., 1993; Sempere etal.,1998).

Nodear evidences of subduction-related volcanism havebeen foundas yet in theseareas.

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Late(?) Norian - Middle Sinemurian(215 - 195 Ma)

Two maininarinetransgressions anda relative tectonicquiescence characterize this time-span. The Late Norianmarine transgression reached the western part of theAndean marginfrom southernColombia to northernChile(Fig. 8). Norian deposits usually overlie Permian-Triassicsubaerial volcanic and sedimentary rocks, locally in angularunconformity, or may overlie the Paleozoic basement. Inthecentral andnorthernparts ofthebasin(Ecuador.Peru],Norian deposits consist of shallow-marine shelflimestoneand dolomite (Megard, 197B; Pardo and Sanz, 1979;Loughman and Hallam, 1982; Prinz,1985; Rivadeneira andSanchez, L991; Rosas el ai., 1997), whereas in the eastern(easternPeru,Megard, L978)and southernareas(southernBolivia, northern Chile, Groschke el al., 1988; Suarez andDalenz, 1993; Ardill et al., L998), they begin with atransgressive clastic, locally evaporitic sequence, evolVingtoward marinedeposits.

A stratigraphic hiatus of latest Norian-Rhetian ageoccurred locally in Peru (Megard, 1978; Loughman andHallam, L982; Prinz, 1985), which may explain localdisconformities (Pardo and Sanz, 1979). Thesecond majormarine transgression is of Late Hettangian to EarlySinemurian age (Fig. 8).InPeru,thesetransgressive depositsonlap onto the Permian-Triassic volcanogenic rocks(Meganl, 1978; Loughman andHallam, 1982).lnthecentralpartof thebasin,they consist ofdarklimestone unitsrichinorganic material tobituminous shale, theupperpartofwhichisrichinphosphate (Megard, 1978; Loughman and Hallam,1982; Prinz, 1985; Rosas etal., 1997). Inmanysouthern areas,the Sinernurian transgressive beds are the first marinedeposits to be recorded. They may locally onlap onto thePaleozoic basement (Antofagasta, Munoz et ai., 1988; BaezaandQuinzio, L991),or overlie undated (Triassic?) arcvolcanicrocks (southern Peru,Vargas, 1970; Vicente et aI., 1982;lquique and Arica, Muiioz andCharrier, L993).

Intheback-arc areas ofcentral Peru, scattered lavaflowsinterbedded in Early Liassic marine sediments displayalkaline chemistry, indicating thatextensionaltectonic regimestillprevailed (Rosas andPontbote, 1995; Romeufetal., 1997;Figs. 9and 12). In summary, theLate Triassic-Early Jurassicperiod is marked bya mild extensional regime, interpretedas related to the break-up of Gondwana and to the Tethyanrifting. Large-scalemarinetransgressions occurred intheLateNorian and Early Sinemurian, respectively. Subduction ofapaleo-Pacific Plate beneath this part of theSouth AmericanPlate hasnotbeenproved as yet.

Late Sinemurian - Kimmeridgian(195·150 Ma)

This interval is marked by active subduction-relatedmagmatism, sinistralwrenching movements along NW-SEfaults, and tectonically controlled sedimentation. Threemain domains are distinguished: (1) the NNE trendingEcuadorian-northern Peruvian segmentischaracterized bya NNE-trending magmatic arcborderedto the Ebya back­arc subaerial basin; (2) in northern and central Peru, thecarbonate shelf sedimentation continued and was

interrupted by a Middle Jurassic tectonic event; (3) insouthern Peru and northern Chile, the western magmaticarc was flanked to the E by a marine,subsiding back-arcbasin,bordered by emergenteasternareas (Fig. 8).

In Colombia (Mojica and Dorado, 1987), southernEcuador (Romeufetal.,1997) and northwestern Peru(Pardoand Sanz, 1979), Middle to Late Sinernurian marinedeposits are intercalated with alkaline basaltic flows,whereas subduction-related volcanic arc rocks appear inthe immediately overlying deposits, which. evolve towardsubaerial environments. The Late Sinemurian age of thebeginningofthemagmatic arcactivity isconfirmed bydatesobtainedfromassociated arc-related intrusions (195 - L90Ma.Aspden etal.,1987; Litherland etal.,L994). TheactivityofthisNNE trendingcontinental magmatic arc(Misahualliand Colan formations) continued on during the MiddleJurassic, and eventually ceasedbylatestJurassic times(L50- 140Ma, Aspden etal., 1987; Litherland etal.,1994, Romeufet al., L995). To the W of the magmatic are, marinevolcaniclastic sediments, datedaslate Middle to earlyLateJurassic in southern Ecuador and northern Peru (Mourier,1988), are interpreted as fore-arc deposits.

In southwestern Peru, the disconforrnable marinetransgression is datedas Early Sinernurian, Early Toarcian,Late Toarcian (commonly), or even late Bajocian­Bathonian, according to the regions, suggesting either acontrasted and complex paleogeography, or thejuxtaposition of distinct units due to subsequentdisplacements (Kurth etal.,1996). Most of the successionsdisplaya sedimentary hiatusofLateBajocian-Bathonian age(Vicente, 1981, 1989). Volcanic arcrocks formerly ascribedto the Triassic-Liassic (Chocolate Formation) yieldedradiometric datesof 177- 157 Maand Aalenian-Bathonianpaleontological ages (Roperch and earlier, 1992; Romeuf el

al., 1995),thus suggesting that they are coeval with themagmatic arcofEcuador, northern PeruandnorthernChile.These volcanic rocks are overlain by volcaniclastic andsiliciclastic marineshelfdeposits ofAalenian-Callovian age(Upper Rio Grande andGuaneros formations, RUegg, 1956;Aguirre and Oftler, 1985; Romeuf et al., 1995), thusindicating a progressive and significant decrease of thearcactivityduring the Middle Jurassic. No volcanic rocks ofLateJurassic ageknown.

In the coastof northern Chile, outpouring of a thickaccumulation ofsubduction-related dacite, andesite, basaltand tuff (LaNegraFormation, Fig. 10)began alsoduringthe LateSinernurian-Pliensbachian (Munoz et al., 1988;Baeza and Quinzio, L991; Munoz and Charrier, 1993). Theyfollow basic intrusions related to probably transtensionalsinistral movements (Pichowiak et al., 1990). Innorthernmost Chile, this volcanic series is overlain bymarine sediments ofearlyMiddle Jurassic to Late Jurassicage,according to the areas (Munoz and Charrier, 1993;Kessler, 1997), butvolcanic activity isassumed to have lasteduntil the Late Jurassic (Charrierand Munoz, L994) oreventheEarly Cretaceous (Mpodozis andRamos, 1989; ScheuberetaL, 1994), fartherS.Extensional ortranstensional regimeprevailed duringthe Jurassic, favouring the formation andplay oftheAtacama wrench fault system (Brown etal., 1993).

East of the magmatic are,the Jurassic back-arc basinreceived poorly dated, mainly argillaceous and volcaniclastic

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TECTONIC EVOLUTION OF SOUTH AMERICA

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subaerial sedimentation (Chapiza and Sarayaquilloformations; Tschopp, 1953; Seminario and Guizado, 1976;Rivadeneira and Sanchez, 1991; Fig. 6).Thesesedimentaryrocks reston the Liassic carbonatebeds to the W, or on thePaleozoic basement to the E. In the latter case, they mayinclude unfossiliferous Late Triassic-Liassic beds. Theycomprisea lowersequence ofshale,fine-grained sandstone,dolomite and evaporite of sabkha environment, and anuppersequence consistingofcoarse-grained sandstoneandconglomerate of fluvial environment (Tschopp, 1953;Megard, I978).However, Jurassic marineplatform limestoneunits are locally known in easternmost Ecuador(Petroproduccion, unpublisheddata).

In the back-arc areas of Peru, platform carbonatedeposition, started in the LateTriassic, went on until theearlyMiddleJurassic (Figs. I I and 12).Thiscarbonateshelfwas bordered to the E by continental back-arc depositsconsisting in fluvial redbeds to the N,and locally significantaccumulations of eolian sandstone to the S (Sernpere,1995). No information is available on the western part ofcentral Peru. On the Peruvian Platform, the bituminousfacies andphosphate-richcarbonatebedsoftheSinernuriantransgression (Aramachay Formation) are overlain bystratified oolitic and skeletal limestone presenting localcross-stratified calcarenitebeds of Late Sinemurlan to LateToarcian, maybe Aalenian age (Condorsinga formation;Megard, 1978; Westermann et al., 198D; Loughman andHallam, 1982; Prinz, 1985). These Liassic deposits areoverlain bysandy-argillaceous limestone of Late Aalenian­Late Bajocian age (Chunumayo Formation, Megard, 1978;Westermann et al.,1980). Thecarbonateseriesends locally(Huancayo) with unconformable fluvial sandstone{Cen.:apuqui<l Fmmation)overlainbylaminatelkarbonate,shaleand evaporitebeds oftidal flatertvi ronment, ascribedto theBajocian (Chaucha Formation; Megard, 1978; Moulin,1989: Fig. 11).However, inmanyareas, the upperpartof theplatformcarbonate sequence is eroded (Prinz, 1985). dueto the Bathoniantectonic event.

In the back-arcareasofnorthern Chile, theSinemuriantransgressive beds are overlain by open marine deposits(Chong, 1976; Vicente et al., 1982; Munoz etal., 1988; Ardillet al., 1998). In southern Peru (Arequipa Trough), thissuccession comprises aPliensbachian toBajocian diachronictransgression, Bathonian turbidite beds, Callovian blackshaleandOxfordian-Kimmeridgian shelfsandstone (Vicente,1981, 1989; Vicente et al., 1982; Fig. 13).In northernmostChile, the turbidite beds progressively disappear andcarbonate interbeds appear in the succession (Munoz etal.,1988}. Farther S (Dorneyko Cordillera), the succession isdominated byoffshore shaledepositslocally rich inorganicmatteru.hong, 1976; Ardilletal.,1998). These marineback­arc basins are bordered to the E (southernPeru, Bolivia) byemergentareaswhich received locally thick, coarse-grained,quartz-rich eoliansandstoneascribed to theEarly toMiddleJurassic (Ravelo and Ichoa formations; Oller and Sempere,1990; Carlotto, 1998; Sempereet al., 1998).

Scattered alkaline lava flows are known during thisinterval in the back-arc areas. Alkaline basalt flows areinterbedded in the Sinernurian limestoneof central Peru(Rosas et al., 1997), scarce sills and intracontinentaltholeiitic flows dated at 185- 170Ma0 ccur in Bol ivia(late

Early to early Middle Jurassic.Soler and Sempere,1993;Sernpere et al.•1998),and volcanic flows interbedded inthe Bajocian to Callovian deposits of the Domeyko Basinare regarded as related to transtensional movements(Ardill et al., 1998; Fig. 14).On the other hand. the mid­Jurassic Arequipa Troughhas been interpreted as a pull­apart basin due to sinistral movements (Vicente et al.,

1982).Therefore, this period seems to be dominated by amild extension,probably due to a sinistral transtensionalregime (see also Scheuberetal.,1994).

However, the southern part of the area (10"5 - 25°5) ischaracterized by a sedimentary hiatus of Late Bajocian­Bathonian age (Megard,1978; Vicente, 1989; Ardill et al.,

1998).This "Bathonian phase" is classically regarded asresponsible for the disconforrnity that separates the lowerand upperredbeds ofthe PeruvianEasternBasin (Megard,1975}.ltis coeval with a rapid exhumation(3000m) oftheEastern Cordillera of central Peru (Laubacher and Naeser,1994),withmetamorphismofthearczoneofnorthern Chile(170 - 150Ma, Lucassen and Thirlwall, 1998) and withintense folding and reverse faulting ofthe arczoneof northcentral Chile (163- 140 Ma,Irwin etal.•1997). Therefore, apoorlydocumented compressional deformation seems tohave occurred in the Late Bajocian-Bathcnian.

Insummary, between 195 and 140 Ma,theNNE trendingColombian-Ecuadorian and the N-S trending Chileansegmentsare characterized by abundant arc magmatism,unstable tectonic regime, subaerial back-arc sedimentationand locally marine fore-arc deposits. At the same time,transtensional, probably sinistral tectonic regime prevailedalongtheNW-SE trendingPeruvian segment, which resultedinlocalized pull-apartbasinsandin thealternation ofmarinesedimentation andemergent periods.Thissituation hasbeeninterpreted as the resultofa southeastward convergence ofthe paleo-Pacific Platebeneath South America (Aspden etal.,1987; [aillardet ai., 1990,1995).

Kimmeridgian to latePaleocene (140 - 57 Ma)

During the Kimmeridgian-Paleocene interval, thepaleogeographic framework wasstillcontrolledbythe fore­arc,arc and back-arc zones.However, the trend of the arczonebecameNE in Peruand N-S in Chile. and the back-arcarea presented a three-foldsubdivision inherited from theJurassic structures.Nomagmaticarc is known at this timein Ecuador, whichconstituted a back-arcdomain.

The Kimmeridgian-early LateCretaceous fore-arc zoneis virtually unknown. Duringthe latestCretaceous, fore-arczones appear in northernmost Peru-southwesternmostEcuador and southern Peru-northern Chile, due to theeastward shiftofthemagmatic arcfrom 80Ma.Themagmaticarc crops out Widely in coastal Peruand northern Chile. Thenorthernextension oftheNW' trendingPeruvian arccrosscutsobliquely the southernmost part of Ecuador, since thesubduction zoneprobably extended northwestwards intotheoceanic realm. However, insights into the geodynamicevolution of the East-Pacific oceanic realm are provided bythe analysis of the stratigraphiccontent and geochemical

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signatures of the Cretaceous oceanic terranessubsequentlyaccreted to the Ecuadorian margin.

ln Peru,theback-arcareacompriseda western, mobileand subsidingbasin (West-Peruvian Trough ofBenavides,1956, present-dayWestern Cordillera), an axialpositivethreshold (MaranonGeanticline, Cuzco-Puno Swell, andpresent-dayEasternCordillera) and an eastern,stableandmoderately to little subsiding basin (present-daySubandean Zone and eastern basin, Fig. 16). The latterextends northwards into Ecuador (present-day OrienteBasin),and southwards into Bolivia (Potosi Basin).TheKimmeridgian-late Paleocene lime-span(150 - 55Ma) canbe divided into three periods, characterized by distinctpaleogeographic and tectonic settings, controlled by theplatekinematicsevolution.

Kimmeridgian? - Aptian(I50 -110 Ma)

Latest Jurassic toearliestCretaceous timesare markedbya complete reorganization ofthepaleogeographic patternand tectonic evolution of the Andean margin, interpretedastheresult ofadrasticchange intheconvergence direction,which triggered a magmatic arc re-organization andsignificant tectonic deformation (Aspdenetal.,1987;Iaillardet al., 1990, 1995). These events were followed in the EarlyCretaceous by an important diachronous marinetransgression in thewhole area.

Kimmeridgian? - Berriasian (150 • 135Ma)

In Ecuador and northern Peru (and Colombia), theactivityofthecontinental volcanic arc(Misahualli andColanformations) ceased bytheendof Kimmeridgian times (ISOto 140 Ma ago,Aspden etaL, 1987; Mourier, 1988; Litherlandet ai; 1994; Fig. 17). The deformed and eroded magmaticarc is then overlain by unconformable fluvio-rnarinesandstone, the diachronous base of which is dated asValanginian (?)toAlbian, from SW toNE (Villagomez etal.,1996; Robert et al., 1998). This major hiatus andunconformitysuggests theoccurrence ofasignificant latestlurassic-Early Cretaceous tectonic event(Litherland et aL,1994). No Kimmeridgian deposits are known in the fore­arcareasof Ecuador and northern Peru.

In southwesternEcuador. the Raspas high pressuremetamorphic complexyielded a 132MaK/Arage(Feiningerand Silberman, 1982) and 130- llS MaAr/Ar and Sm/Ndages (Malfere, 1999). Theseare interpreted ascooling ages,subsequent to the accretion and HP metamorphism of auoceanic plateau and accretionary prisminthelatest[urassic­

earliestCretaceous (Gabriele etal.,1999;Malfere etal;1999).Thissuture extends northwards alongthe western edge ofthe Eastern Cordillera of Ecuador (Peltetec suture,Litherland etal., 1994; Aspden et al.; 1995) and to theWofthe Central Cordillera of Colombia (Amaime Terrane,Aspden andMcCourt, 1986;Toussaint and Restrepo, 1994).

According toLitherland etaL (1994), this eventcorrespondsto the accretion of a continental microplate lChauchaTerrane) to theEcuadorian margin.

In northwestern Peru, Early (?) Tithonian lagoonaldeposits areabruptlyoverlain bydeepshelfshale, and then

bya thickseries ofcoarse-grainedvolcaniclastic turbiditebeds of late Tithonian age,which reworked the Jurassicvolcanic arc.Thisevolution pointstothe creation ofa deep,N-S trending sedimentarybasininterpretedasa pull-apartbasin (Jaillard and [acay, 1989), which extends southwards(Fig. 6). This succession follows (Jaillard and Iacav, 1989)

with Late Tithonian black shale of shelf environment,Tithonian-Berriasian massive sandstone of nearshoreenvironment (Tinajones Formation;Wilson, 1984), anddisconformable massive dean sandstone of presumablyBerriasian age(Chimu Formation, Benavides, 1956; laillardand Jacay, 1989).

In the back-arcareasof Ecuadorand northern Peru,the LateJurassic back-arcredbeds seemtograde upwardsinto coarser-grainedred beds interbedded with basalticto rhyolitic flows (Yaupi Mb.[aillard,1997), locally datedas earliestCretaceous (Hall and Calle, 1982; Iaillard, 1997).

This magmaticactivitywould be coeval with small-sizedstocks, which intrude the Jurassic red beds and aredisconformably capped by Early Cretaceous sandstone(Tschopp, 1953; Tafur, 1991).

IncentralandsouthernPeru,Kimmeridgian times weremarkedbythe arrival ofclastic deposits. Siliciclastic shelfsedimentsabruptlyoverlie the Callovian blackshaleof theArequipa Trough (Vicente et al., 1982; Fig. 13). Thesesediments are correlated with undated unconformableconglomerate of the Peruvian (Chupa Formation, Klink etal., 1986; Iaillard, 1995) and Bolivian Altiplano (CondoFormation), which, however, might be younger. The EarlyTithonian marinetransgression is recorded in the westernpart of central and southern Peru by shallow marinelimestone (Jaguay Formation, Ruegg, 1961; GramadalFormation, Chavez, 1982; Batty and Iaillard, 1989), whichappeartobeoverlain byTithonian-Berriasian (?)black shaleand sandstone (Oy6n Formation, Megard, 1978; Tiabayaoutcrops, Geyer, 1982).

Volcanic arc activity is known in the Lima area(Atherton etal.,1985;Aleman, 1996);Itseemstohave begunbyLate Tithonian times.since the upperpartofthearcseriesyielded ammonites of LateTithonian age(Bulot, personalcommunication, 1998; formerly ascribedto the Berriasian;Wiedmann, 1981), and have continued during part ofBerriasian times (Aleman, 1996). Disoonformable deanmassive sandstone is ascribed to the Berriasian in thewestern parts of central and southern Peru [Chimu,Goyllarisquizga andHualhuani formations, Benavides, 1956,

Batty and Iaillard, 1989). However, in spite of limitedpaleontological evidence, thebase of thesedeposits seemsto be diachronous, being m'.lch younger toward the £(Wilson, 1963; Iaillard, 1995; Robert et al., 1998). In theeasternbasins of Peru and Bolivia, no earliest Cretaceousdeposits have been recognized so far, below theunconformable Early Cretaceous sandstoneunits.

In northernmost Chile, the Kimmeridgian phase ismarked bya marineregression which a culminatedwithalocal hiatus and an angular unconformity, by sinistralwrench movements along the N-S trendingAtacama FaultSystem, and bysubsidence ofthe back-arcareas(BogdanicandEspinosa, 1994). Intheback-arc basinofnorthernmostChile, Oxfordian limestone and shalegrade upwards intolimestone and shalewithinterbeds ofevaporite in the[ower 499

TECTONIC EVOLUTION OF SOUTH AMERICA

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part,andsandstoneintercalations in theupperpart (Munozet aL, 1988). Farther S (Domeyko Basin), the Callovianmarine black shale beds are overlain by evaporite bedsdeposited in basin to sabkha environments. and then byinnershelflimestone ofLate Kimmeridgian-Early Tithonianage, which laterally gradesouthwards into marinesiltstone(Chong. 1976; Ardill et ai., 1998; Fig. 14).Farther5 (31°5),the Coastal Cordillera wasdeformed byW-verging opentotightfolds between140 and 126 Ma(lrwinetai; 1987). Thedeformation is coeval with significant sinistral lateraldisplacements offore-arc and arcslivers alongtheAtacamaFault System (26°S),dated between 145 and 125 Ma(Kurthetal,1996).

During thelatestJurassic-earliest Cretaceous, magmaticarc activity seems to have continued without changes oflocation. However, a magmatic gapseemsto have occurredbetween 150and 140 Ma(Hammerschmidt etal.; 1992; Fig.15) and the magmatic activity appears to decreasesignificantly near the Jurassic-Cretaceous boundary(Mpodozis and Ramos, 1989: Scheuber etal.,1994; Charrierand Munoz, 1994).

TheendoftheNNE trending Ecuadorian- north-Peruvianmagmatic arcbyTithoniantimes, and thebeginning of theactivityoftheNW trendingPeruvian magmatic arc inthelateTithonian, expresses a drastic change in the convergencedirection. which passed from nearly southwards to nearlynortheastwards (Aspden et al., 19&7; [aillard et ai" 1990,1995). This plate kinematics reorganization correlates withgeodynamic events inthesoutheastern Pacific andtheTethysOcean. During the LateJurassic, the southeastern Pacificaccretion ridge would have been oriented roughly NE~SW

and connected with the Tethyan accretionary system(Caribbean and Central Atlantic oceans, DuncanandHargraves, 1984). Theoutpouring ofa majorplumealon.gthePacific accretionary ridgeinTithonian times(Chalzkrridge)would have disturbedtheaccretion direction; broughtaboutthebreak-up oftheEastern paleo-Pacific Plate, andcreated atriple junction (Nakanishi et al., 1989). Meanwhile, theTethyan realm wasmarked bya Significant slowdown in thespreading rates,allowing the newly created NW-SE trendingSoutheastern Pacific Ridge to impose a northeastward driftdirection for the Eastern paleo-Pacific Plate. These eventswould haveprovoked, intheTithonian (l40 Ma), achange ofconvergence direction, according to the process proposedbyDuncan andHargraves (l984) forEarly Cretaceous times.

This major geodynamicchange may account for theaccretion of the oceanic terrane of Ecuador andColombia, the creation ofthe ChicamaBasinof northernPeru, the Widespread emergenceand subsequent gap ofLate Tithonian-Berriasian deposits, the unconformityof the Early Cretaceous deposits and the possiblecompressional deformation recorded in northern Peru.These events may be correlated with the Araucan phaseof northern Chile (Stipanicic and Rodrigo. 1969;Scheuber et al., 1994).

Berriasian-Aptian (140 - 110Ma)

Thisperiod is markedby the widespread deposition ofdisconformable, diachronous quartz-sandstone units,by arelative tectonic quiescence; andalongthe Peruvian andChilean

margins, bytheongoing, although mild. volcanic arcactivity.In Ecuador and northern Peru, verylittle is knownaboutthisperiod.In theback-arc areasofEcuador, one KI Ardateand one palynological age suggest that subaerial red beddepositioncontinued during part of this period (Bristowand Hoffstetter, 1977; Baldock, 1982). The overlyingtransgressive disconformable sandstonebedsareofAlbianage (laillard, 1997). In the Cordillera Real of Ecuador,Litherland et al. (I 994)mentionednumerous KJArresetsin the Jurassic granite, interpreted as due to significantdextral movements related to the collision of displacedterranes. In the neighbouring Maranon Basin ofnortheasternPeru,a sedimentaryhiatus seems to separatethe Jurassic redbedsand the disconformable, diachronoustransgressive sandstone beds of Early Cretaceous age(Laurent,19&5).

Meanwhile, somewhere in the southeastern paleo­Pacific domain,a mantle plume was responsible for theformation of a large and thick Oceanic Plateau that wasaccreted to the Andean margin in Late Cretaceous times(Cosma er al., 199&; Reynaud et al., 1999), and crops outpresently in theWestern Cordillera of Ecuador whereit hasbeendated as Barremian or Hauterivian (123Ma,Lapierreet al..1999).

In Central and Southern Peru, scarce outcrops ofvolcanic rocks in thecoastal and western areassuggest thatvolcanic arc activity continued at leastlocally untilAptiantimes (Bellido, J956;Vidal et al., 1990; Aleman, 1996).However, the occurrence of thick intercalations of quartz­rich sandstone and unusual volcanic-free shelf limestoneintheLima area(Rivera ernl , 1975;Aleman, 1996) suggeststhat,eithervolcanic activity waslocal and sporadic, or partof the coastal terranes have been subsequently displaced(May and Butler, 1985).

Farther E, in the back-arcareas, well-sorted, clean,quartz-sandstonebedswere deposited from Late Berriasiantimesonwards (Benavides, 1956; Wilson, 1963) .In northernPeruandEcuador, thesewere datedasLateBerriasian-EarlyValanginian in the easternregion, (Benavides, 1956; Riveraet aI., 1975; Bulot, personal communication, 199&),Berriasian-Barremian in the western part of the EasternBasin of Peru (Tarazona, 19&5), Aptian in the centreof thebasin, and earlyLate Albian in the eastern parts of theEastern Basin (ViIlag6mez etal.,1996; Robert et al., 1998,Fig. 18). In centralPeru,sucha diachronism wassuggestedby Wilson (1963), and. although paleontological evidenceis poor, a comparable diachronism mayoccur in southernPeru (Jaillard, 1995).

Paleocurrents indicate that the Guiana and Brazilianshields were the sources of the clastic supply.Paleoenvironments evolve from subaerlal/fluvial tonearshore/shallow marine from E to W, and deposition ismainly controlled byeustaticvariations (Moulin, 1989). Theisopach mapclearly indicates aneasterndepocenter situatedin northern Peru (present Maraiion River), and westerndepocenters NofLima andaroundArequipa (Iaillard, 1994;Fig. J9}. Scarce syn-sedimentarytectonic features suggest amildextensional regime (Moulin, 1989).

Intheback-arcareaofnorthernmost Chile, LateJurassicstrataareoverlain byfine-grained sandstone.siltstone andshale with occasional tuff and andesitic lava of

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cross-cutbybasic to intermediate intrusionsdated at 104- 101 Ma(Wilson, 1975; Cobbing r!tal., 1981; Bussel, 1983),thus indicating that compressional deformations beganbyMiddle Albiantimes(Cobbing etal.,19B1).

Deformation was associatedwith significant dextralmovements (Myers, L975; Bussel and Pitcher, 1985). In thearc zones of Peru and Ecuador, the Late Albian tectoniccompression is marked bylocalfolding and faulting, bytheend of marine sedimentation, by a general decrease ofmagmatic activity (Soler and Bonhomme, 1990), and bythereplacement of volcanic effusions by plutonic intrusions,suggesting that the arc zones were significantly uplifted(Cobbing et 01., 198J; Soler, 199]). These calc-alkalineplutons, which intrudethevolcanic are,definetheso-calledCoastal Batholith ofPeru(Pitcher. 1978; Cobbinget al.•1981;

Soler, 1991). Effusive magmatism ceased by earliestCenomanian limes, and incipient plutonic activity wasratherlow(Solerand Bonhomme, 1990), except in central­southern Peru where intrusions are dated at 101 - 94 Ma(Beckinsale es al., 1985).Thestability ofthe magmatic frontincentralPerusuggests that the LateAlbian tectoniceventdid notchangesignificantly theshapeof the activemargin(Soler and Bonhomme, 1990; [aillardand Soler, 1996; Fig.20).Mostof thearczoneof Ecuadorand Peruseems tohaveremained emergentduring Cenomanian-Turonian times,sincethe Albian volcanic piles are usually unconformablycapped bySantonian toCampanian transgressive sediments(Iaillard, 1995; laillarderal; 1996).Plutonic activity was highduring the Cenomanian (Beckinsale et al., 19B5; Mukasa,1986,94 - 90 Mapulseof Soler and Bonhomme, 1990), butno intrusions -of Turonian age are known (90 ~ 85 Maplutonic gap,Soler,1991).

In northern Chile, the locus of the magmatic arcsignificantly shiftedeastwardduringAptian-Albian times(Hammerschmidt et al., 1992),and the Middle Cretaceousmagmatic arc (lIS - 90 Ma) is partlysuperimposed on theJurassic arc (Charrier and Munoz, 1994; Scheuberet al.,1994; Fig. 15). In northernmostChile, arc-related andesite.breccia, agglomerate, tuff,sandstone and conglomerate ofAlbian ageoverlie theEarly Cretaceous depositsoftheback­arcbasin(Scheuber etaI., 1994).Theyyielded us -104MadatesandarecrosscutbylIS - 80Maintrusions(Bogdanicand Espinosa, 1994).An extensional or transtensionalregime isassumedtohaveprevailed (Scheuber etaI., 1994).

However, although no deformationof Albianagehas beenrecognized. the significant eastward shift of the MiddleCretaceous arc (50km) mayresultfroma shorteningevent.Volcanic arc activity continued untilearlyLateCretaceoustimes (Scheuber et al., 1994).

No arc-related magmatism isknownon the continentalmargin N of 3°S (Ecuador). Therefore, the Peruviansubduction zoneextended probably northwestwards intotheoceanic domainby meansof an intra-oceanicsubductionlone, which gaveway to the formation of island arcs ofAlbian to early LateCretaceous age.This interpretation issupported by the occurrence, on the accreted OCeanicterranes of Ecuador, of pre-Cenomanian island arc rocks(LasOrquideas andIoachfunits,Jaillard etaI., 1995;Benitez,1995; Cosma etal; 1998),overlainbyvolcaniclastic arcseriesof Cenomanian to Santonian age (Cayo and Pilatonformations, Faucher etaL, 1971; Kehrer andVan derKaaden,

1979; Iaillard et al., 1995). Note that, there too, volcanicactivityseemstohaveceased byCenomanian times.

In the back-arc areasofEcuadorand Peru,theAptian­Albian boundary is marked by scattered volcanicmanifestations, varying from basaltic flows to rhyolithictuff, intercalated withinthe first transgressive units. Thisbimodalvolcanism as beenlocally determined as alkaline,indicating an intracontinental extension (Soler, 1989).

Earliest Albian timesare markedby thebeginningofamajor large-scale marine transgressionwhichreached itsmaximum extentby Turonian times (Figs. 13and 21).TheLima area alreadyreceived a marine sedimentationfromEarly Cretaceous times (Rivera etal; 1975;Aleman, 1996).TheEarly Albian transgression firstreached thewestern partof theback-arc areas(Benavides, 1956; laillard, 1995;Robertet al., 1998), where it deposited red to yellow coloured siltandsandstone,withglauconitic and locally oolitic limestonein the upper part (Inca and Pariahuanca formations,Benavides, 1956; Wilson, 1963; Moulin, 1989).

TheAlbian transgression isthenmarkedbythreepulsesofmidEarly,earlyMiddle andearlyLateAlbianage(Robertetal., 1998). The firstpulseonly reached the western part ofthe Eastern basins (Chulec Formation, Benavides 1956;Wilson, 1963; Robert, in progress). whereas the third onereached locally the eastern border of the Eastern Basin ofEcuador (Basal Napo Shales) andnorthernPeru,where itmayrest on Paleozoic rocks (Jaillard, 1997). This lattertransgression probably reached also the axial swell ofsouthernPeruand triggered thedeposition of transgressive,partly marinesandstone (Huancane Formation, Carlotto etal, 1995; [aillard, 1995), which gradeto the E into thickerdeposits (lower part ofPatinaGroup, Audebaud et al; 1976;[aillard, 1995).ltcould havereached alsothe Bolivian PotosiBasin where a few lens ofmetresofcoarse-grained, locallyconglomeratic sandstone areknown(LaPuertaFormation,Sempere, 1994). The early Middle and early Late Albiantransgressive pulses areassociated withwidespread anoxicdeposits in central and northern Peru and in Ecuador(Pariatambo, Basal Napa, Chonta formations, Villagomez etal., 1996; Robertet aI., 1998). LateAlbian times are thenmarked by the development of carbonate shelves in thewestern part (Yumagual Formation, part of Mujarnin,lumashaandArcurquina formations, Benavides, 1956,1962;Wilson, 1963; Jaillard,I987},and bythedeposition ofdeltaic­fluvial sandstonein the eastern parts of the back-arc areas(Agua Caliente Formation, Tsandstone, PutinaGroup), theprogradation and retreatofwhich are mainlycontrolled byeustaticvariations witha subordinate influence of tectonicevents(faillard, 1994, 1997).

TheeffectsoftheLate Albian tectonic eventaremildintheback-arc areas. TheLate Albianshelfcarbonate sedimentationexhibits slumps, syn-sedimentary faults and breccia, clasticdykes and differential subsidence, which together express anextensional regime (Audebaud, 1971; laillard, 1987, 1994).Farther to the E, progradation of deltaic systems mayberegarded astheresult ofaslightuplift related to theLate Albiantectonic event (JallJard. 1987. 1997). In northernArgentina.alkaline volcanic rocks (L10-100Ma)areinterpretedasrelatedtoa rift episode (Viramonte etat, 1999).

In the western part of the back-arc areas of Ecuadorand Peru, the carbonate shelf sedimentation recorded

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significant eustatic transgression near the Albian­Cenomanian boundary, in Middle Cenomanian, earlyLateCenomanian (widely characterized by Neolobitesvibrayeanus (= N.kummelii,and Early Turonian times.Eachtransgression ismarkedbyconspicuous marlylevels whichgrade upwards into massive platform carbonate exhibitingfrequently desiccation features, thus indicatinga shallowmarineenvironment (Iaillard,1987,1995,1997; Fig. 22).Insome southern parts of the Eastern Basin of Peru, theAlbian-Cenomanian f1uvio- marinesandstone beds(Orienteand Putina groups) are overlain 'oy Earlyluronian marineshale, illustrating the large extent of the Turoniantransgression. Insomeareas,the upperpart oftheTuronianshelf limestone exhibits mild syn-sedimentary tectonicfeatures announcing the tectonic event of the Turonian­Coniacian boundary (Jaillard, 1987, 1995, 1997).

Subsidence wasintense in thewestern areasofnorthernPeru,and decreased drastically towardthe NEand SE. TheAlbian-Turonian seriesreaches nearly 2000 min thewesternpartofnorthernPeru{Benavides, 1956; Wilson, 1963},about300m in the OrienteBasin ofEcuador (Iaillard, 1997),600m in southwestern Peru (Benavides, 1962; jaillard, 1995),and 30 m in the PotOSI Basin of Bolivia (Sempere, 1994).Theaxialswell continuously behaved as a positive area.

The Middle to Late Albian deformation was the firstsignificant compressional deformation recorded in theCretaceous evolution of theAndean margin, which affectedmainly the fore-an: and arczones.hcoincided with aperiod(If highconvergence rateand withthe opening of the SouthAtlantic Ocean at equatorial latitudes, which triggered thewestward driftoftheSouthAmerican Plate andtherefore, thetrenchward motion of the overridingplate (Prutos, 1981;Jarrard, 1986; Soler and Bonhomrne 1990; Iaillard and Soler,1996). The strong dextral wrench component of thisdeformation (Bussel and Pitcher, 1985; Soler, 1991; jaillard,1994) resulted from thenortheastward motionoftheFarall6nPlate, indicated alsoby thelackofanyarcmagmatism alongtheColombian-Ecuadorian margin(Aspdeneral;1987). Dueto theoblique direction oftheoceanic plate, convergence wasaccommodated by lateral displacement and wrenchingdeformation alongtheedge oftheactive margin, ratherthanby shortening of the whole margin. The resumption ofvolcanic activity along the Peruvian marginmaybe relatedto the beginning of the Middle Cretaceous period of highconvergence rate (Soter, 1991).

Coniacian ~ late Paleocene(88 - 57 Ma)

This period is marked by a major change in thepaleogeographic pattern, by the occurrence ofcompressional tectonic events, the intensity of whichincreased through time, and by the incipient eastwardmigrationofthe arczonein Peru.Aprogressive but generalmarine regression, the arrival of fine-grained detritaldeposits and theeastward shiftofdepocenters in theeasternbasins markedsedimentation. Tectonic events are of LateTuronian-E.arly Coniacian (88Ma),Santonian(35Ma),LateCampanian? (80- 75Ma)and LateMaastrichtian age(70­65Ma).Someof theseevents coincide withthe accretion ofoceanic terranes in Ecuadoror northern Peru,

Turonian-Coniacian boundary event andConiacian - Early Santonian evolution(88· 85Ma)

In the fore-arc Celica-Lancones Basin(northern Peru­southern Ecuador), the youngest fossil recovered from theturbidite series is of Early Conician age (Petersen, 1949;Iaillard etal; 1999).A sedimentary gapoccurred thenduringLate Coniacian and Santoniantimes,In the Talara fore-arcbasinof northwestern Peru,nodepositsareknownbetweentheAlbian shelfcarbonateand theCampanian transgressivemarine deposits (Gonzalez, 1976; Morales, 1993). Noinformation isavailable on the other fore-arc zones.In theoceanic fragments accreted to Ecuador, the Turonian­Coniacian bQundary toughlycoincides with the beginningof the Cayo island-arc activity, as expressed by thickseriesof coarse-grained volcaniclastic turbidite beds (Cayo andPilaton formations, [aillard et al., 1995; Benitez, 1995;Kehrer and Van der Kaaden, 1979).

TheAlbian volcanic arcseriesofsouthern Ecuadorareunconformably capped by Early Campanian transgressivemarine deposits (Naranjo Formation. 'aillard, 1997), Insouthern Peru,Albianvolcanic rocksare unconformablycapped byundatedshelflimestone ofSenonianage(OrnoyeFormation, Vicente, 1981; [aillard, 1994), In the arc zones,the effects of the Late Albian and Turonian-Coniaciandeformation are,therefore, indistinguishable. However, thedeformational eventseemstonaveoccurredbeforetheEarlyCampanian transgression of southern Ecuador. In theCoastal Batholith of Peru, significant wrench movementsassociated witha variable compressional regimehas beenrecognized during Turonian-Coniadan times (Russel andPitcher, 1985). Plutonic intrusions are very scarce, andvolcanic activity is unknown (90 - 85 Ma magmaticgap,Soler, 1991; Fig. 20).However, although the magmatic frontappearsto be nearly stable,a slighteastward shift of somekilometres can be detected (Soler and Bonhomme, 1990j[aillard and Soler, 1996). In northern Chile, the magmaticarc is markedbya well-expressed magmatic gap between90and80Ma(Hammerschmidt etaL, 1992; Scheuber etal.,1994),whichfollowed (Hammerschmidtetal.•1992) or wasassociated with the eastward shift of the magmatic front(Scheuber etal.•1994).

In the westernpart of the back-arcareas of Ecuador,Peru and Bolivia,the end of the carbonate platformsedimentation markstheTuronian-Coniacian boundary. Inthe N,it is replaced by ammonite-rich marine shalewithlimestoneinterbeds (Celendfn, Upper Napo, Upper Chontaformations, Tschopp, 1953: Benavides, 1956;Wilson, 1963;JailIard.1987, 1997; Figs. 21and 22),whereas in the S,theTuronian limestoneunits areoverlain by red shaleand siltwith abundant evaporite layers (Chilcane, Aroifillaformations,Vicente, 198); Sempere et al; 1997; Figs. 13and 21}. In Ecuadorand northern Peru,twomain marinetransgressions are recognized, of Early Coniacian and tateConiacian-EarlySantonian age,respectively. Theydeterminetwo thickening-upward progradational sequences, ofConiacian and 'Early Santonian age, respectively (Jaillard,1997). The appearance of detrital quartz in the Coniaciansequenceindicates the creation ofnewsource areas.No LateSantonianfaunahasbeen foundso far in thesesequences.

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TECTONtC EVOLUTION OF SOUTH AMERICA

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Since the Coniacian to Early Santonian marine shale isgenerally overlain bytransgressive sandstoneofCampanianage,asedimentary gapofLate Santonian-Early Campanianage is inferred. In southern Peru,and probably in Bolivia,the Late Coniacian-Early Santonian transgression is wellmarked and forms a thin marine layer (MiddleQuerque;Vicente, 1981;Iaillard, 1994),whichisused as a correlationlayer(Middle Vilquechico, Chaunaca formations, [aillard etal., 1993; Sempere etal., 1997;Carlotto, 1998;Fig. 23).

In the back-arc areas, subsidence significantlyincreased during the Coniacian-Early Santonian time­span. In Bolivia, thespectacular increasein subsidence isregarded as the result of a foreland-type, flexuralsubsidence (Sempere, 1994;Sempere et al., 1997;Fig. 23),due to significanttectonicshortening in the westernareas(northern Chile).Extension prevailed, however, in theseback-arcareas(Solerand Sernpere, L993). Theoccurrenceof disoxigenated deposits (northern Peru, Ecuador) andof evaporites (southern Peru, Bolivia) suggests that theback-arc basin was separated from the open sea by anincipient morphological barrier, which was morepronounced to the S. In the whole area, Coniacian­Santonian marine deposits onlap eastwards onto theGuianaand Brazilian shields (Sempere, 1994). In easternEcuador, northeastern and southern Peru and in easternBolivia, the Coniacian-Santonian beds are the firstCretaceous marine shales to be deposited,and overlie theAlbian U)-Turonian fluvio-rnarine massive sandstoneunits (laillard, 1995,1997; Sempere et al., L997).Thissignificant eastward migration of the early Senoniandepocenter is associated with a reorganization of theisopach maps, which become narrower and elongatedparallelto the present-daychain,suggesting that the back­arcbasins beganto behaveas distal forelandbasins.

This LateTuronian-Early Coniacian paleogeographicreorganization is associated with local tectonicmanifestations. In Bolivia, continental red bedsunconformably overlie Middle Cretaceous marine strata(Vilcapujio event, Sempere, 1994)and in Ecuador (Jaillard,1997), Coniacian silt or shale disconformably overlie theeroded Turonian limestone. In the Oriente Basin ofEcuador,the LateTuronian-Coniacian deposits exhibit significantthickness variations related to syn-sedimentary faulting ofLate Turonian-Coniacian age(Christophoul et al., 1999).Inthis area and in the EasternBasin of northern Peru,mildcompressional deformation hasbeenrecognized (DashwoodandAbbots, 1990;GiletaI., 1996;Rivadeneira andBaby, 1999).

This, together with the change in sedimentation andpaleogeography; the slightretreatof the magmatic arc andtheincrease insubsidence, indicate thattectonic deformationand mildshortening affected the Western areas.

Santonian-Early Campanian tectonic eventand Campanian-Middle Maastrichtianevolution (85 - 68 Ma)

The Santonian (Early Campanian?) event is a majorturning point in the Andeanevolution, recognized a longtimeago as the Peruvianphase (Steinmann, 1929).In thefore-arc Celica-Lancones-Basin (northern Peru-southernEcuador), diachronous, latest Santonian to Middle

Campanian transgressive shelfsedimentsunconformablyoverlie the deformed turbidite seriesof pre-Santonian agedeposits (Iaillard et al., 1997, 1999; Fig. 24). Thecompressional closureof thebasin seems to be associatedwiththeintrusion ofsyn-tectonic gabbro (Reyes and Caldas,1987),locally datedat 82Ma(Mourier, 1988).In theTalarafore-arc basin of northwestern Peru, the Albian shelfcarbonate units are also covered disconformably byCampaniantransgressive marinedeposits(Gonzalez, 1976;Machan: et al., 1986; Seranne, 1987; Morales, 1993).In thefore-arc zoneof Paita(northern Peru),Middle Campaniantransgressivedeposits restunconformably on the Paleozoicbasement(Bengtson and Jail lard,1997;[aillard etal., 1999).

Therefore, the main deform ation of these fore-arc zonesoccurred during the latest Coniacian-EarlyCampaniantime-span.

In the Celica-Lancones Basin, the middle Campaniantransgressive beds are overlain by basinal dark shaleinterbedded with fine-grained turbidite beds of LateCampanian-Early Maastrichtian age (Iaillard et al.. 1999).South ofPaita,theMiddle Campanian transgressive slXJuenceconsists of transgressive marlstone and sandstone, rudist­bearing massive limestone, and transgressive marl andlimestone grading upwards into sandstone andconglomerate, suggesting the occurrence of a LateCampanian tectonic event(LaMesa, Bengtson andJaillard,1997).Fartherto theW (La'Iortuga), the succession followswith a 3000 104000 m-thick series of alluvial to marinebreccia, overlain by transgressive nearshore sandstonecontaining ammonites of Maastrichtian (probablyMiddleMaastrichtian) age (Bengtson and Iaillard, L997; Fig. 24).

Theseareunconformably overlain bylatestPaleocene-earlyEocene coarse-grained conglomerate, suggesting that a newtectonic eventdeformed this area in the Late MaastrichtianorPaleocene. Noinformation isavailable about theotherfore­arc zones,

In Santonian-Early Campanian times, the Ecuadorianmargin underwent the accretion of an oceanic terraneconstituted by an oceanic plateau dated at 123 ± 12 Ma(Lapierre etal., 1999;Reynaud etal., L999)overlainbyintra­oceanicislandarc series(Fig. 25).Thisevent ismarked bya regional hiatus of Campanian age on the continentalmargin, by a significant thermalevent which affected theEasternCordillera ofEcuador around85- 80Ma(CordilleraReal, Litherlandet al., 1994) and by the abrupt arrival ofdisconformable quartz-richturbidites of Late Campanian(?)-Maastrichtian age on the accreted oceanic series(Yunguilla Formation; Faucheretal., 1971; Kehrer and VanderKaaden, 1979; Cosma etal;1998).Intheoceanic domain,the collision led to the endof the Middle Cretaceous islandarc activity (Cayo Pormanon.Denttez, 1995), and to theonset, farther W, of a new island arc of LateCampanian­Maastrichtian age (San Lorenzo Formation, Lebrat er al.,1987;Ordonez, 1996).Since theaccretedislandarcseries islocallydatedas Coniacian inthe Western Cordillera (Faucheretal.,1971),theaccretions occurredbetweenthe Coniacianand the Late Campanian. This arc jump expresses asignificant reorganization of the intra-oceanic subductionzonegeometry(Cosma et al., 1998).Theaccretedoceanicterrane(Pallatanga unit,McCourt etal; 1998),characterizedby its association with the Yunguilla Formation, crops out

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presently along the eastern edge of the Western Cordilleraof central and northern Ecuador (San luan-Pujilf Suture,[uteau etal., 1977; McCourt etal., 1998).

TheEarly Maastrichtian Yunguilla Formation (Bristowand Hoffstetter, 1977) consistsof alternations of basinalshale and medium-grained turbidite beds reworkingvolcaniclastic and siliciclastic material.These locallyoverlieunits of transgressive limestone of Late Campanian­Maastrichtian age (Kehrer and Kehrer, 1969). Thissuccession, comparable to that of the Celica-Lancones andPaita areas, indicates the creation of a widefore-arc basinofMiddle Campanian-Middle Maastrichtian age (Fig. 24),which extended at least from the Paitaarea (5'S) to N ofQuito (0').

TheAlbian volcanic arc series of southernEcuador areunconformably capped byLateSantonian-Early Campaniantransgressive marinedeposits (Naranjo Formation, Iaillardetal, 1997), which allow refining theage ofthemaintectonicevent as pre-Campanian. In southern Peru, the Albianvolcanics areunconformably cappedbytheundated OmoyeFormation (Vicente, 1981), which hasbeenascribed to theSantonian (Iaillard, L994),although it mightbe younger(Campanian?) .Inthearczones, theeffects ofthe Late Albian,Turonian-Coniacian and Late Santonian deformation are,therefore, indistinguishable. Inbothareas, thetransgressivesequence grades intocoarser-grained, locally conglomeratic,nearshore to continental deposits, dated in southernEcuador as Maastrichtian (Cosanga Formation, Baudino,1995; [aillard, 1997), which indicate new tectonicmovements in the Maastrichtian.

TheSantonian-Early Campanian eventcoincided withthebeginning ofasignificant retreatoftheCoastal BatholithofPeru(Soler andBonbornme, 1990; Fig.20). This,togetherwiththe subsidence of the LateCampanian-Maastrichtianfore-arc basinofnorthernPeru-southern Ecuador, suggeststhat tectonic erosion began to act as a significant masstransferprocess in the fore-arc and arc zones at that time(Iaillard andSoler, 1996;[aillard, 1997).TheLate Santonian­Early Campanian event isfollowed bya major plutonic pulseintheCoastal Batholithofcentral Peru, duringwhich mainlygranodiorite bodieswere emplaced (85- 77Maepisode ofSoler, 1991). Aprobable magmatic gap(77- 74Mal mightcorrespond tothe LateCampanian event,and isfollowed bya newmagmatic episode (74 - 69 Ma), which beganwithdyke swarm emplacement (Soler, 1991 ).In southernPeru,aplutonic gap (84- 70Ma)maycoincide withtheSantonianand Late Campanian events. Thelatterare responsible forthe major NE-vergent Uuta Thrust,near Arequipa, whichresulted inthethrustof Precambrian rocks ontoCretaceoussediments (Vicente eral., 1982). Since it involves Coniacian­Early Santonian bedsand isconcealed bylatestCretaceous­early Paleogene unconformable conglomeratebeds,it isofLate Cretaceous age(Vicente, 1989).

In northern Chile, magmatic activity resumed around80Ma ago (Early Campanian, Hammerschmidt etaL, 1992),and thelocation of the magmatic arc significantly shiftedeastward, thus indicating that the Middle and LateCretaceous tectonic events resulted in significant uustalshortening and/or crustal erosion (Scheuber et ai., 1994).This significant contractional event dated as 90 - 78Ma,resulted in the folding, emergence and erosion of the arc

zone, thetectonic inversion ofthe Domeyko Cordillera andcreationof the retro-arc Purilactis Basin (Mpodozis andRamos 1989; Scheuber et al., 1994).

Intheback-arc areasofEcuador andnorthern Peru, LateSantonian-Early Campanian timesaremarked bya regionalsedimentary gap (Tschopp, 1953; Benavides 1956;Serninario andGuizado 1976;[aillard L987, 1997; MathaloneandMontoya, 1995), which coincides withtheaccretion andrelated deformations recorded in the westerly zones, Innorthern Peru and eastern Ecuador, theSantonian marinedeposits exhibit a thickening-upward evolution expressingthearrival ofsandydetritalmaterial regarded as related tothe incoming Late Santonian tectonic movements. Insouthern Peru andBolivia.stratigraphic dataareinsufficientto demonstrate the occurrence and durationof this hiatusin the mostly continental deposits (Middle Vilquechico,Middle Yuncaypata, Chaunaca formations; Sempere et al.,1997; Jaillard et al., 1993; Carlotto, 1998). The EarlySantonian ageof the lastmarine deposits in northern PeruandEcuador,however, supports a Late Santonian agefor themaindeformational event.

Campanian times are then markedby a short-lived,regional marine transgression, locallydated as MiddleCampanian (northern Peru, Mourier et al., 1988), andtherefore, probably coeval with the main transgression inthefore-arc zone (Fig. 24).ln Ecuador andnortheastern andcentral Peru, this transgression is associated withconspicuous disconforrnable transgressive sandstone (M-lSandstones.LowerVivian Formation) overlain byathinlayerofmarine shale (Augustoet al; 1990; Salas, 1991; Mathaloneand Montoya, 1995; [aillard, 1997). In southernPeru andBolivia, the Middle Campanian transgression is correlatedwith a thin layer of charophyte-bearing shaleoverlain byfine-grained red beds of presumed Late Campanian age(Middle Vilquechico, Middle ¥uncaypata, Upper Chaunacaformations, [aillardet al., 1993; Sempere et aI., 1997; Figs.23 and 27). The hiatus between Campanian andMaastrichtian deposits suggests theoccurrence ofatectonicevent in theLate Campanian,but an eustatic originforthissedimentary gap cannot be ruled out. Mid-Campanianalkaline volcanic rocks (SO - 75Ma) pointto anextensionalstrain in northernArgentina (Viramonte etal; 1999).

A newregional marine transgression occurred in theEarly Maastrichtian, which deposited transgressivesandstone unitsgradingupwards intomarineshale,whichrestdisconformably ontheCampanian beds.In Ecuadorandnorthern andcentral Peru, theseEarlyMaastrichtian marinelayers are dated by marine microfossils and very scarceammonites (LowerTena,UpperVivian, Areniscas deAzucarformations, Koch and Blissenbach, 1962; Rodriguez andChalco, 1975; Vargas, 1988; Mourier et al., 1988; laillard,1997), They aregenerally overlain by charophyte-bearingfine-grained continental red beds of Maastrichtian age. Adisconformity separates thesedeposits from the overlyingfine-grained, continental Paleocene redbeds (UpperIena,Yahuarango, Sol formations), suggesting the occurrence oftectonic movements near the Maastrichtia n-Paleoceneboundary (Mathalone and Montoya 1995; Iaillard, 1997;Christophoul, in progress).

In southernPeruand Bolivia, the Early Maastrichtianmaximum flooding is marked by ephemeral marine

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TECTONIC EVOLUTION OF SOUTH AMERICA

I~·<I~conditions Gaillard etal., 1993; Gayet et al., 1993) and theMaastrichtian deposits significantly onlapontotheeasternborderof the Cretaceous basin (Sempere et aL, 1997), thusexpressing the gradual eastward shift of the depocenterduringtheLate Cretaceous (Fig. 26).Lacustrine conditionsprevailed inpartoftheBolivian-Argentine partofthebasin(Rouchy et al., 1993; Salfity and Marquillas, 1994). Insouthern Peru,an erosional disconformity separates theMaastrichtianandTertiary beds(Jaillardetal; 1993;Carlotta,1998), but sedimentation is assumed to be continuous inBolivia (Sempere et al., 1997). The tectonic regime in theback-arc areasof northern Chile and Bolivia is assumed tohave been extensional during the Maastrichtian, thusallowing sporadic shallow marine incursion and theoutpouring of alkaline basicvolcanics (Reyes et al., 1976;Scheuber etal.,1994;Viramonteet al, 1999).

Late Maastrichtian - early late Paleocene(68- 57 Ma)

The early to early late Paleocene sequencedisconformably overlies latestCretaceous strata.Itconsistsof volcanic rocks (arczone), and of fine-grained deposits,eithermarine (fore-arc zone) orcontinental (back-arezone).In the arcand fore-arc zones, besidethe regional hiatusofLateMaatrichtian-early Paleocene age and the frequentdisconformities between Cretaceous and Tertiary beds, aLateMaastrichtian tectonic event issuggested bynumerousintrusions (Mukasa, 1986), the emplacement of a centredcomplex (Mukasa and Tilton, 1985) and high strike-slipratesbetween 68and 64Mainthe Coastal Batholith ofPeru(Busse! and Pitcher, 1985), and by numerous KiAr resetsindicating a thermal event at 70 - 60 Ma in the CordilleraReal ofEcuador (Litherland etal; 1994).

Paleocene marinedeposits areonlyknown in the fore­arc Talara Basin of northwestemmost Peru (Iddings andOlsson, 1928; Gonzalez, 1976; Zuniga and Cruzado, 1979;Seranne,1987; Morales, 1993; Fig. 29)and maybe inthe off­shorefore-arc zoneofnorthern Peru,where thepresence ofpre-late Eocene sediments has been assumedlocally (9°S,Kulm et al., 1982). In theTalara Basin. the LateCretaceousdeposits are disconformably overlain by early Paleocenetransgressive sandstone andconglomerate intercalated withmarine greyshale, which grade upward into marine darkshale of middle Paleocene age (Weiss, 1955; Paredes, 1958;Gonzalez, 1976). Theupperunitoverlaps to the E(Seranne,1987), and is strongly eroded toward the S by pre-Eoceneerosion (Paredes, 1958). To the E, thin continental tonearshore clastic facies grade westward into thick fine­grained deposits of relatively deep marine environment(Seranne, 1987). Farther to the S (Paita),the Cretaceoussedimentation endsupwiththe 3500 m thickbreccia oftheMaastrichtian La Tortuga Formation (Olsson, 1944; Fig. 24).

In the presently accreted oceanic terranesof Ecuador,sincetheSanLorenzo island-arc did notyieldagesyoungerthanMaastrichtian (Lebratetal., 1987; Ordonez, 1996), itsactivity mayhave ceased by Paleocene times. Moreover,McCourt etaL (1998) recently identified an early to middlePaleocene quartz-rich turbidite series resting on accretedoceanic island arc series in the Western Cordillera ofEcuador. Since these early Paleocene turbidite bedsarefound

totheWoftheoutcropping beltoftheYunguilla Formation,they might indicate that an other fragment of oceanicterrane has been accreted during the Late Maastrichtianand/or the earliest Paleocene. However, in someparts, theMaastrichtian-Paleocene sedimentation continuedwithout any noticeable changes (Guayaquil Formation,Benitez, 1995; Keller et al., 1997; Fig. 25).

Inthemagmatic arcofsouthwestern Ecuador, subaerialandesiticlava.breccia.egglomerate andacid tuff(SacapalcaFormation) are of latest Maastrichtian (67 Ma) to earlyEocene age (Jaillard etal., 1996; Hungerbuhler, 1997; Prattet al., 1998). indicating that volcanic art activity resumedafter a gap that lasted from Late Albian times. In centraland northern Ecuador, volcanic arc activity did not startbefore theearly Eocene, Incentraland northernPeru,ring­complexes (68 - 64 Ma,Cobbing et al., 1981; Soler, 1991)and calc-alkaline intrusions were emplaced (64 - 59 Ma,Cobbing et al., 1981; Beckinsale et al., 1985) and werepossibly associated with coeval volcanism (PararinFormation, Bussel, 1983). These intrusions arevolumetrically important,indicating the local resumptionof magmatic art activity, but no significant compositionalchanges is noted with respect to the Late Cretaceousmagmatism (Soler, 1991; Fig. 20).The68- 64Ma periodisalso marked by important dextral wrench movements(Bussel, 1983; Bussel and Pitcher, 1985), which may be theexpression of a Late Maastrichtian tectonic event. Amagmatic gap thenoccurred duringthelatePaleocene (59- 54Ma, Soler, 1991). In southernPeru,plutonic intrusionsbeganduringlatestCretaceous times(78Ma)and exhibit amajorpulseduringthe earlyto middle Paleocene (62 - 57Ma.Beckinsale etal; 1985, Mukasa, 1986; OarketaL, 1990).Associated volcanism (Toquepala Formation) consists of3000 m of dacitic to rhyolitic tuff with minor andesiticintercalations, the composition of which suggests that theAndeancrust was not thickened (Boily et al., 1990). It iscrosscut by gabbroic andgraniticintrusionsdated mainlyat 66• 63 Ma(Laughlin etal., 1968; Vatin-Perignon et al.,1982; Mukasa and Tilton, 1985; Clarket al. 1990). Theresumption of art magmatism recognised in Ecuador andPeru is expressed in northern Chile by abundant LateMaastrichtian-early Paleocene ages in the magmatic art

rocks (Hammerschmidt et al., 1992; Charrier and Reutter,1994), and by coeval volcanic intercalations in red bedsdeposited in proximal back-arc basins (Purilactis Group.64Ma,Flint etaL, 1993).

In the proximal back-arc zonesof Peru (present-dayAndes), Paleocene deposits unconformably rest on LateCretaceous rocks (Noble et at 1990; laillardet al., 1993;

Megard et aL, 1996), whereas the contact is only locallydisconformable intheEastern Basin (Vargas, 1988; Augustoet al., 1990; Salas, 1991; Mathalone and Montoya, 1995; Gilet aL, 1996; Figs. 27 and 31). In addition, compressionaldeformationduetotectonic inversionsneartheCretaceous­Tertiaryboundary arecommon andwidespread intheback­arcareasofeasternEcuador (Dashwood andAbbots, 1990;Rivadeneira and Baby, 1999; Christophoul, in progress),northeastern Peru (Contreras et al; 1996; Gil et al; 1996)and Colombia (Cheilletz et ai, 1997).

In the distalback-arc areasof Ecuador and Peru,theLateCampanian-Maastrichtian sequence is overlain by a

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thickseriesofPaleocene fine-grained redbeds(UpperIena,Yahuarango, 5011, Quilque, Chilca, SantaLucia formations,Kummel, 1948; Koch and Blissenbach, 1962; Mathalone andMontoya, 1995; [aillard, 1997; Sempere et al., 1997;Christophoul, in progress)which wedges out toward theW,mainlybecauseofpre-Eocene erosion(Naeser et at, 1991;[aillard et al.• 1993; Carlotto, 1998). In the western zones.Paleocene deposits are usually lacking. However. in theAndes ofCentral Peru,a seriesoffluviatile redbedshasbeenassigned to the Paleocene (Casapalca Formation, Iacay,1994), although it maybe younger. ThesePaleocene fine­grained red beds were deposited in wide, distal alluvialplainsor in coastalsetting.Clastic materialproceeded fromthe smooth relief of the Paleo-Andes. Microfossils aredominated by charophyte associations (Gutierrez, 1982;[aillard, 1994),but scarce foraminifera indicate local andsporadic marine influences (Koch and Blissenbach, 1962).In northeastern Peru. the Paleocene beds aredisconformably overlain bytransgressive conglomerate andmarine to brackish beds of Eocene age(Pow Formation).In Bolivia and northern Argentina, extensional conditionsare marked by early Paleocene K-rich lava flows (65 - 60Ma,Viramonteetal., 1999).

The widespread hiatus, unconformities and detritalsedimentation.aswell asthedeformation and thermaleventsuggest that a significant, althoughpoorlyknown,tectoniceventoccurred neartheMaastrichtian-Paleocene boundary.Thiseventmightcorrespondto the accretion of an oceanicterrane, since part of western Ecuador received earlyPaleocene quartz-richsedimentation, and recorded theendofthe activityof an islandarc(arc jump).

Late Paleocene - lateOligocene

Thisperiodcorresponds toa transitionbetween thepre­orogenic and the syn-orogenic periods. Compressionaldeformations becamesignificant and involved the westernparts oftheback-arcareas,wheremarinesedimentation nolongeroccurred.except locally in Ecuador. Thesubsidenceof fore-arc zones, whichfollows the compressional events,createdsedimentarybasins. Detritalsedimentationin theeastern area shows evidence for tectonically-induceddisconformities.Finally. activity ofthevolcanic arcresumed,including along the Ecuadorian margin where arcmagmatism wasunknownsinceLateJurassic times.

Because of the ongoingcrustal shortening, eastwardmigrationof the magmaticfront,and upliftof theAndeandomain.thepaleogeographic patternprogressively changedduring this period (Fig. 28). The fore-arc zones roughlycorrespond to the present-daycoastaland offshore parts ofthe margin. Due to tectonic erosion, shortening, and/orflattening of the slab,the arc zonemigratedand enlargedeastwards throughtime.Itcorresponded to thewestern partof the present-day Western Cordillera. The back-arcareascan be divided into a western, deformed and usuallyemergent area,alsoreferredto as the"Paleo-Andes", andaneasternarea,whichstillreceived sedimentation,and evolvedthroughtime towarda foreland retro-arcbasin.

Late Paleocene event(58-55Ma)and Eocene Sequence (55- 40Ma)

ThelatePaleocene event,first suspectedbyCobbing etal. (l98l) and Bussel and PitcherI1985), isone ofthe majorevents in theAndeanhistory(Marocco etal; 1987; Noble aal., 1990; Sernpere et al., 1997; Iaillard, 1997). It is coevalwith an important plate kinematic reorganization in thePacific realm, datedat 58- 56Ma,which resulted inachangein theconvergence directionof theFarallonPlate.Thelatterchanged direction from N or NNE to NE (Pilger, 1984;Gordon and Iurdy, 1986; Pardo-Casas and Molnar, 1987;Atwater, 1989). Thelate Paleocene eventis followed by thedeposition ofdisconformable,well-identifiable sedimentaryor volcanic sequences of earlyto earlylate Eocene age.

In the fore-arc zone of Ecuador, the collision of anoceanic terrane resulted in locally intense deformation ofearly late Paleocene chert (Santa Elena Formation)belonging to the accretedterrane (Benitez, 1995; Iaillard etal., 1995).Since a thickseriesofquartz-richcoarse-grainedturbiditebedsoflatestPaleocene ageconceals the accretion,the latteroccurred in thelate Paleocene (Iaillard, 1997; Fig.25).Afurther tectonic eventofprobableearliestEoceneagedeformed the unconformable quartz-rich turbidite beds(Jaillard, 1997).In theTalara fore arcbasinofnorthernPeru:sandstone andconglomerate (Basal Salinas) disconfonnablyoverlie Paleocene marine shale(Seranne,1987) and gradesouthwards into diachronous. latest Paleocene to earlyEocene, coarse-grainedalluvial conglomerate (MogollonFormation, Morales, 1993; Fig.29).InthePaitaarea,thelattercontain boulders of intra-oceanic origin, thusdemonstrating that accretion occurred before the earlyEocene in northern Peru. Since oceanic terranes arepresently in Ecuador, they were subsequently displacednorthwardsalongdextralfaults, witha minimumestimaterateof 5 mm/y (Pecoraet al., 1999).

Due to the late Paleocene event, the fore-arc zones ofEcuadorare marked by a widespread sedimentary hiatusencompassing most of the early Eocene (Benftez, 1995;[aillardet al, 1995).Sedimentation resumed diachronicallysince the end of the early Eocene. The Eocene sequencetypically beginswithbreccia.slumpedshaleor transgressiveperi-reefal limestone concealing fault-controlled relief.Diachronism and tectonic figures indicatean extensionaltectonic regime, related to the subsidence that led to thedeposition oftheoverlying thinningandshallowing-upwardssequence of marls interbedded with turbiditic sandstone(Jaillard et al., 1995). The Eocenesequence ends up withdisconforrnable, locally conglomeratic sandstone ofnearshore to continental environment, dated as latemiddleto earlylateEocene (Iaillard etal., 1995). Theseindicate thebeginningof the lateEocene tectonicmovements (Iaillard,1997).A comparable sequence is known in most of thecoastal area (Benitez, 1995; [aillard et al; 1995) and theWestern Cordillera (Santos et al. 1986; Egilez, 1986;Bourgois et al; 1990), This suggests that most of westernEcuador underwenta similarsedimentaryevolution duringthe middleEocene. and that mostofoceanic terraneswereprobably already accreted to the continental margin bymiddleEocene times (Iaillard, 1997; Cosma etal; 1998).lntheWestern Cordillera.however, an islandarcunit (Macuchi

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Sandstone} seems to be associated with Eocene pelagicchert,and wasthereforenot yet in contactwiththe Andeanmargin(Hughes and Pilatasig, 1999).

In the fore-arc zone of Peru, although early Eocenedepositshavebeen locally mentioned (Kulm et al., 1982,Suess etal., 1988), depositsof that agearewellknownonlyin the Talara Basin (Fig. 29).There.the very thick Eoceneseries comprises threemainsedimentary sequences1irnitedby disconformities of earliest Eocene, latest early to earlymiddle Eocene, and late middle Eocene age,respectively(Gonzalez, 1976; Seranne, 1987; Morales. 1993). Thediachronous transgression begins with disconformablemarine sandstoneand conglomerate of latestPaleocene toearlyEocene age, from W to E (Morales, 1993), the base ofwhich contains boulders ofoceanic origin,which post-datethe accretion of oceanic terranes (Pecoraet al., 1999). TheEocene sequences generally consistofmarineshaleand fineto coarse-grained sandstone, whichgrade laterally Eor NEinto coarse-grained continental sandstone andconglomerate (Gonzalez, 1976; Seranne,1987). Pre-rn iddleEocene erosions removed part oftheearlyEocene sequence,and the second sequence locally rests on the basalconglomerate (Paredes, 1958). The upper part of the latemiddle Eocene sequence exhibits compressional syn­sedimentary deformations related to a transpressionalregime (Seranne, 1987; Becerra et al., 1990).The thirdsequence (late middle to early late Eocene) is made ofunconformable massive coarse-grained sandstone ofnearshore environment (Gonzalez, 1976; Morales, 1993),announcing the lateEocenetectonicevent. Overlying openmarineshalebedsareascribedto thelateEocene (Gonzalez,1976) or the earlyOligocene (Morales, 1993).

Neartheearly-middle Eocene boundary,subsidence ofthefore-arc zonetriggeredthe creationofa newgenerationof fore-arc basins,wherethick middle to earlylate Eocenedepositsunconformably restuponPaleozoic to earlyEoceneunits.In theoffshore fore-arc basinsofcentraland northernPeru, drillholes crosscut as much as 2000 m of middleEocene shale,siltstone and sandstone of shallow marine,high-energy environment (Ballesteros et al., 1988; Suess it

al., 1988),which usually unconformably overlies Cretaceousto Paleozoic rocks(Machare etal., 1986). It ends up locallywithbreccia, suggesting a tectonicinstability of latemiddleEocene to late Eocene age.Subsidence and basin tectonicsis controlled by NNE trending normal faults (Machare etal., 1986; AzaIgara etal., 1991).ThemiddleEocene series iscommonly directly overlainbyMiocene marinesediments,thus providing evidence for a widespread sedimentaryhiatusencompassing lateEocene, Oligocene, andoftenearlyMiocene times (Suess etal; 1988; Von Huene etaL, 1988).

In the arc zone of southern Ecuador, subaerial arcvolcanism and associated continental volcaniclasticsedimentation occurredduringthelatePaleoceneand earlyEocene (Sacapalca Formation, laillard, 1997; Hungerblih1er,1997). In the rest of Ecuador, resumption of arcvolcanismisdatedasearlyor middleEocene (53- 45Ma, Eguez 1986;Wallrabe·Adams, 1990; Van Thournout et aI., 1990;

Steinmann, 1997; Dunkley andGaibor, 1998). Itiscommonlyassociated withsubaerialvolcaniclastic red beds ofmiddleEocene agedeposited in prorimal back-arcbasins (SilanteFormation, Wallrabe-Adams, 1990; Hughes and Pilatasig,

1999; Quingeo Formation, Steinmann, 1997; PlayasFormation,Hungerbuhler, 1997; Fig. 33).The renewal ofarcactivity along the Ecuadorian margin is due to the moreeasterly convergence direction, subsequent to the latePaleocene plate tectonic re-organization (Pilger, 1984;Pardo-Casas and Molnar, 1987).

In northern Peru,thickcalc-alkaline subaerial volcanicseriesaredatedbetween S5and40Ma(Laughlin etal., 1968;

Cobbing etal., 1981;Noble etal, 1990; Soler, 1991),and post­date compressional deformation (Cobbing er al., 1981;

Bussel, 1983; Bussel and Pitcher, 1985). Although thechemical signature did not change with respect to thePaleocene magmatic arc(Soler, 1991 ),theEocene magmaticactivity is markedbya slighteastward shiftof thewesternmagmatic front, a decrease of plutonic intrusions in theCoastal Batholith and the beginningof the enlargement ofthe magmatic arc,which reached the present-day WesternCordillera (Noble etal., 1990iSoler, 1991; Fig. 20).Incontrast,a magmatic gapoccurredin southernPeruduringtheearlyEocene (Mukasa, 1986; Boily etal., 1990; Clarket al., 1990;

Soler. 1991). However, subvolcanic stocks and associatedporphyrycopperdepositswereemplaced in the Toquepalaprospect between57and 52Ma(Sebrieretal.• 1988; Clarket al., 1990). Farther to the E (Inner Arc), the enlargementofthemagmatic arc ismarkedbytheemplacement ofseveralcalc-alkaline plutons, among which the large is theAndahuaylas-Yauri Batholith (48-34Ma,Carlier etal., 1996).

In northern Chile, the magmatic arc drastically shiftedeastwardbetween55 and 48 Ma (Hammerschmidt et al.,1992; Scheuber etal., 1994), thus suggesting that significantcrustal shortening and/or tectonic erosionoccurredin theearlyEocene (Fig. 15).This event has been ascribed to thelateEocene tectonic phase(Scheuber etai., 1994).butisbettercorrelated withthelatePaleocene event. Thelatteris followedby a significant resumption of the magmatic activityalongthe whole margin.In northern Chile, the resumption of arcmagmatism is expressed by numerous volcanic outcropsdatedasmiddleEorene(48-38Ma,Hammerschmidt etal;

1992). Products of this arc weredeposited in a proximal,extensional back-arc basin, which received a thick,coarsening-upward pileofmainly volcaniclastic rocks,whichmay rest conformable on Late Cretaceous sediments orunconformably on olderrocks(Hartley etal., 1992; Hint ital., 1993; Purilactis Formation ofCharrierand Reutter, 1994;Fig. 30).Tectonic regimein the back-arc zone is no longerextensional(ScheuberetaI., 1994).

In the western bad-arc areas of Ecuador(CordilleraReal), KJArageresets near65-50Maindicate theOCl;UITenCfofa noticeable thermalevent(Aspdenand Litherland, 1992;Litherland etal; 1!194), related to thelatePaleocene tectonicevent. In theAndes of northern Peru,earlyEocene volcanicrocks datedat 55to50Maunconformably reston deformedCretaceous sediments (Cobbing et aI., 1981; Bussel, 1983;Noble etai; 1990; Soler, 1991). In theAndes of central Peru,continental red bedsbearingPaleocene-Eocene charophytesunconformably restonCretaceous sediments (Ml!gard, 1978;Megard it el; 1996).

In the present-day Andes of central Peru,unconformable continental red beds are 10l;a1Iy dated bylatest Maastrichtian charophytes, whereas in other parts,apparentlysimilar, but conformable, red beds yielded late

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Eocene-early Oligocene charophyte oogonsand 40 to 37Ma KJArdates (Megard et al., 1996). Therefore, someredbedsmaybeo(late Paleocene-middle Eocene age,but theircharacteristics and extensionare unknown so far. Undatedfluviatile red beds (Casapalca Formation) overlyingthelatestCretaceous strata havebeenascribedto the Paleocene(Jacay, 1994), although they may be younger. In southernPeru,Paleocene fluviatile redbeds(Quilque Formation) aredisconformably overlain by lacustrine deposits (ChilcaFormation, Jaillard et al., 1993; Carlotta, 1998), possibly ofEocene age.

The eastern back-arcareas are marked by a regionalunconformity below massive coarse-grained sandstone andconglomerate of earlyEocene age (Tiyuyacu Formation ofEcuador, Dashwood and Abbots, 1990; Benitez etal., 1993;

laillard, 1997; Rivadeneira and Baby, 1999; Rentema andBasal Pozo formations ofPeru,Naeser etal; 1991; RobertsonResearch, 1990; Cayara Formation ofBolivia,Sempere, 1994;Sempereetal., 1997; Figs. 13,23,and Jl ). Theyoftenpost­date a sedimentarygap, which encompasses a largepart ofthePaleocene. Moreover, noearlyEocene deposits have beenaccurately datedsofar in theback-arcareasofEcuadorandPeru. Early to middle Eocene times are then markedby aregional marine transgression.

In Ecuador, the unconformable Lower TiyuyacuFormation isoverlain bymarinetobrackishbeds ofEoceneage(Benitez etal.; 1993). In eastern Peru,the earlyEocenebasal transgressive lag is overlainbya marine to brackishfine-grained layerofEocene ageand bycoarsening-upwardlacustrineto fluviatile redbedsof middletolateEocene age(Pozo Formation, Kummel, 1948; MUller and Aliaga, 1981;Robertson Research, 1990; upper part of Sol3 Formation,Koch and Blissenbach, 1962; Gutierrez, 1982). The UcayaliBasin seemstobe markedbya sedimentaryhiatusofearlyEocene age (Koch and Blissenbach, 1962). Onthe westernborderof the basin (Rentema), conglomerate bedsdated at54Maare conformably overlain byearlyto middleEocenelacustrine deposits, equivalent to the Pow Formation(Naeser etal.; 1991; Iaillard, 19~4). Comparable lacustrinedeposits of Eocene age are known in the Altiplano ofsouthernPeru(Chilca Formation,Carlotto, 1998) and locallyin Bolivia (Cayara Formation,Sempere etal; 1997). Probablydue to subsequent erosions, the overlying late Eocenesuccession is frequently lackingon thebordersof thebasin(Ecuador -Subandean Zone; Peru- Rentema,UcayaliBasin).

Middle-late Eoceneevent (40-35Ma)and Oligocene evolution (35- 28Ma)

Thelate Eocene eventhas longbeen recognized in theAndes ofPeru(lncaicphase,Steinmann, 1929),and hasbeenfurther documentedon the basisof radiometricdata. It isfollowed by the depositionof unconformable bedsof latestEocene-middle Oligocene age. Sedimentation is chieflytectonically driven. in the easternintermtmtaneandforelandcontinental basins. This period ended with the lateOligocene Aymara tectonicevent(28- 26Ma,Sebrieretal.,1988;SempereetaL, 1990;Fig.32).

In the fore-arcbasins, the late Eocenetectoniceventwas announced by the deposition of late middle Eocenedisconformable coarse-grained deposits (42 + 40 Ma).It

culminated in the late Eocene (37 ~ 35 Ma) with thedeformation and emergence of many external fore-arcbasins (Machan! et al.,1986; Seranne,1987; Ballesteros etal., 1988; Iaillardet al., 1995; figs. 25and 29).Thisevent,together with the lateOligocene crisis,is. responsible forawidespread Oligocene hiatus in the fore-arc zone. However,sedimentation occured in a few basins (Talara, locally),and someinternal (eastern) fore-arcbasins wereaffectedby significantsubsidence, which allowed the depositionof late Eocene to early Oligocene marine (Pisco) orcontinental sequences(Moquegua)(Machan! etal., 1988;DeVries, 1998). This subsidence pulse announced theaccelerated subsidence, related to tectonic erosionprocesses, whichaffectedtheAndeanfore-arc zonesfromthe Eocene(Suess et al., 1988; Bourgois et al., 1990; VonHueneand Scholl, 1991),

In Ecuador, undatedcoarse-grained conglomerate bedsoffan-delta environment, which unconformably overlie theEocene sequence maybeascribed eithertothelatestEocene­earlyOligocene (Jaillard et al., 1995; Fig. 25), or to the lateOligocene (Benitez, 1995). Marineshale,siltstoneand fine­grainedsandstonearedatedas middleOligocene (Playa RicaFormation, Benitez, 1995). Theyrest disconformably on theEocene sequence and are separated from the Miocenedeposits byasedimentaryhiatus(Benitez, 1995). InnorthemPeru(Talara Basin), the late middleEocene sandstone bedsare overlain by pelagic shale of debated, possibly earlyOligocene, age(Chira Formation, Morales, 1993). InsouthernCentral Peru (PiscoBasin), 600 m of transgressive shale,siltstone and subordinate sandstone of intertidal tonearshore environments art regarded asoflateEocene, maybeearlyOligocene (?),age (ParacasFormation, Newell, 1956;Marocco and De Muizon, 1988; Machare et al., 1988).Amiddle Oligocene marine sequence has been recentlydescribed (DeVries, 1998), which probably correlates withthe Oligocene beds of Ecuador (and northern Perui). InSouthern Peru(Moquegua), transgressive fanconglomerate,fluvial sequences and evaporite-bearing lacustrinesilt andshaleare ascribedto the Eocene, and infill an extensional,fault-controlled basin,probably createdafterthelate Eoceneevent(Marocco et al., 1985). M for manyfore-arc basinsofPeru, these beds unconformably overlie Precambrian toMesozoic rocks deformed by the LateCretaceous to lateEocene tectonic phases.

In the arc zone of central Ecuador(Cuenca),volcanicrocks ofearlymiddleEocene age(43Ma,Steinmann,1997)are overlain by a 1000 m thick series of fluvialconglomerateand sandstone beds of late middle to lateEoceneage (Quingeo Formation,42 - 35 Ma,Steinmann,1997: Fig.33).In central and northern Peru,intrusions inthe CoastalBatholith ceased by latest Eocene times (35Ma.Beckinsale etal., 1985; Mukasa and Tilton,1985; Soler,1991). With respect to the Cretaceous-Paleoceneintrusions,the lateEocene-recent arc magmatism exhibitssignificant geochemical changes, regarded as resultingfrom the late Eocenetectonicevent(Soler, 1991; Fig. 20).In the present-dayAndesof northern and centralPeru,thelate Eocene event is materialised by a widespreadunconformity, the age of which is bracketed between44and 40 Ma (Noble etal., 1974, 1979, 1990; Megard et al.,1996). In southern Peru, the middle to late Eocene

TECTONIC EVOLUTION OF SOUTH AMERICA

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batholiths of the inner arc are intruded by acid, calc­alkaline subvoJcanic stocksof earliestOligocene age(34 ­32 Ma), thus indicating a strong uplift event during thelateEocene (Carlier et al., 1996; Carlotto, 1998).

In the arczoneof northernChile, a significant angularunconformity is dated at 39 - 38 Ma (Hammerschmidt etal., 1992). Late Eocene uprightanticlines,which account for25% shortening in the arc zone, were associated with arc­parallel dextral strike-slip movements and with E-vergentfolds and reverse faults in the back-arc zone (Scheuber etal., 1994}.ln the proximal back-arc area, the middle-lateEocene tectonic event is recorded by the post 42 Maunconformity which separates the Purilactis and Pacienciagroups (Flintetal., 1993; Fig. 30).

In the arc zone of central-southernEcuador, the lateEocene event is followed by an important pulse of arcvolcanism (andesite, dacite and subordinate rhyolite)dated as latest Eocene-Oligocene (39 - 23 Ma, SaraguroGroup; Eguez et al., 1992; Dunkley and Gaibor, 1998).Within this pile, Dunkley and Gaibor (1998) identifiederosional periodsoflatest Eocene- earliestOligocene (36­34Ma)andmiddleOligocene age(30- 27Mar.ln northernEcuador, volcanic activityseemsto have decreased in theOligocene, butchronological dataare scarce (Eguez, 1986;Wallrahe-Adams, 1990).

In the arczoneand the paleo-Andes of centralPeru,aplutonic pulseof latemiddle and late Eocene age (42 - 36Ma) is followed by a minorpulseof middleOligocene age(31 - 30Mal.the latterbeingrestricted to the Paleo-Andes(Soler, 1991).Volcanic activity displays a correlativeevolution, sincethevolcanic Calipuy Formation yielded agesof41 -35Ma.and31- 29Ma(McKee andNoble, 1982; Nobleetal., 1979; Soler, 1991).ThemiddletoearlylateOligocenemagmatic quiescence is correlated witha lowconvergenceperiod(31- 26Ma.Sebrier and Soler, 1991), and is markedby a subtle change in the geochemical composition of thearcmagmatism (Soler, 1991). In the Altiplano and EasternCordillera ofsouthernPeru,a significant episodeofhigh- Kalkaline magmatism occurred between 30 and 27 Ma(Bonhomme et al., 1985; Bonhomme and Carlier, 1990),which express alocal extensional regime (Carlier etal., 1996)

andis coeval withtheemplacement of monzogabbro at thesouthernedge of the Altiplano (30Ma, Clark et al, 1990).These lateOligocene-earliest Miocene alkaline, shoshoniticand high-K calc-alkaline effusions and intrusions areinterpreted as the resultof partial melting of an enrichedmantle wedge (Sebrier and Soler, 1991).

In thearc zoneof northern Chile, Oligocene timesaremarkedby the deposition of mainlysedimentary, fluvialbeds,which indicatea periodof magmatic quiescence (40• 28Ma,Azapa Formation and Paciencia Group, Coiraetal., 1982; Hint et al., 1993; Garda, 1997; Fig. 30). In thePaleo-Andes, the middle-late Eocene event is well­expressed. In the Western Cordillera of Ecuador, lateEocene times are markedby the deposition of subaerialconglomerate onthe Eocene marinesequence.interpretedby some authors as the result of the accretion of theWestern Cordillera terrane (Bourgois et al., 1990;

Litherland etal; 1994; McCourt et al., 1998).

Deformation is maximum in the Western Cordillerawhere Evergingfold and thrust belts developed (Megard,

1978; Angeles, 1987; Mourier, 1988).]n southernPeru, lateEocene times(42- 38 Ma) arealso marked bythrustingtothe NE along the southern border of the Altiplano(Laubacher, 1978; Farrar et al., 1988; Carlotto, 1998), andalso by SW-verging thrust faults NE of the Altiplano(Huancane Fault Zone, Laubacher, 1978). Farthertothe NE,the middle-late Eocene event isresponsible forwidespreadunconformities in the arc zone and the Altiplano, and fordisconformities in the eastern areas (Sebrier et ai., 1988;

Farraretal., 1988; Noble etal., 1999).

In the present-day Western Cordillera of northern andcentral Peru, LateCretaceous strata arefolded and faulted,and intheeasternpart,compressional deformations resultina50km-large, NE-verging fold andthrustbelt(MaranonHB, Megard 1984, 1987), which occurs on the westernborderoftheMesozoic positive zone (Maranon Geanticline),and interpreted as the result of the tectonic inversion ofnormal paleo-faults (Mourier, 1988). Thisbelt expresses asignificant shortening of the continental crust and itsoverlying cover. Theyare associated with coarse-graineddeposits exhibiting internal unconformities (PacobambaFormation, Angeles, 1999). In southem Peru,the IncaicDeformation resulted inacomparable NE-verging fold andthrust beltto theSofthe Cusco-Puno Swell (Manazo FTB,[aillard andSantander, 1992; Carlotto, 1998), andin theSW­verging Huancane Fault Zone (Audebaud et al., 1976;

Laubacher, ]978). The subsequent erosion period isconcealed bythedeposition of widespread, unconformablecoarse-grained conglomerate (Chanove et al., 1969), anddocumented locally by Oligocene terrestrial faunaspreserved in karst excavations (Hartenberger etaI., 1984).

Farther to the NE, the middle-late Eocene event isresponsible forwidespread unconformities ontheAltiplano,and by disconformities in the eastern areas (Laubacher,1978; Sebrier etal., 1988; FarraretaI., 1988).

In the Cusco area (southern Peru), 5000 to 6000 m ofalluvial red beds(SanJeronimo Group), formerly ascribedto the Late Cretaceous (Gregory, 1916; [aillard etai; 1993;

Noblet et al., 1995), are presently dated as late Eocene(?)­middle Oligocene age (Carlotto, 1998; Fig. 34). Theycomprise twothickcoarsening-upward sequences affectedbylarge-scale progressive unconformities showingevidencefor syn-sedimentary compressional or transpressionaldeformation (C6rdova, 1986; Noblet et al; 1987; Carlotto,]998).Fartherto the E,as muchas 2000 m ofverycoarse­grained fanglomerate and sandstone of late Eocene tomiddle Oligocene age (Anta Formation) unconformablyoverly Cretaceous to middle Eocene rocks, thus providingevidence fur strong magmatic and tectonic activity (Carlotto,1998). Coeval deposits are known farther to the SEfromisolated basins exhibiting changing sedimentary andpaleogeographicevolutions, and yielding scarce earlyOligocene ages(30 - 27Ma,Carlotto, 1998).

IntheAltiplano Basin ofBolivia, thePaleozoic basementis unconformably overlain bya 3000 m thickseriesof redshaleand sandstonebeds,withevaporite unitsin thelowerpart, dated at 30- 29Ma(Tiwanacu Formation, Rochat etal., 1998; Fig. 35). This succession exhibits eastwardpaleocurrents and is interpreted as the foreland sequenceof theWestern Cordillera deformed duringthe late Eoceneevent (Sernpere et ai; 1990; Sempere, 1995, Rochat et al,

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1998). However, Lamb et al. (1997) determined westwardpaleocurrents and proposed thatthe Eastern Cordillerawasalso uplifted along an E-verging thrust fault during themiddle-late Eocene deformation, and thus, separated theAltiplano Basin from the incipienteasternforeland basin(Lamb and Hoke, 1997).

Intheback-arc basins ofEcuador lateEocene-Oligocenetimes are represented by disconformable quartz-richconglomerate (UpperTiyuyacu Formation),overlain byfine­grained red beds (Orteguaza Formation) exhibiting aconspicuous transgressive layerofpartlymarineglauconiticsandstone (Benftez etal; 1993; Rivadeneira andBaby, 1999).In eastern Peru, the [ate Eocene event accounts for awidespread sedimentary hiatus encompassing the lateEocene-middle Oligocene time-span in the western andsouthernzones(Koch and Blissenbach, 1962; Naeser etaL,1991; Figs. 31and36)andforaslightunconformity farthertotheEandNE. There,Robertson Research (1990) identifieda thin lacustrine unit of probable Oligocene age. Althoughavailable stratigraphic data are scarce and sometimesconflicting, theysuggest a noticeable decrease ofthetectonicsubsidence during the late middle to late Eocene interval(40-35Ma.Thomas et aL, 1995; BerronesandCotrina, 1996;Contreras etal: 1996).

The frequent lack of late Eocene-middle Oligocenedeposits in the Oriente Basin contrasts with the thickaccumulations ofcoeval deposits in thePaleo-Andes,whichseemtohave been marked, however,byan EtoNE drainagesystem. Thissuggests that, eitherthesedeposits have beeneroded due to a significant late Oligocene uplift of theEastern Basin, or the entire detrital sediments have beentrappedwithintheAndeanbasins,which actedtherefore asthe proximal foreland basinsoftheWestern Cordillera FTB(Sernpere, 1995; Carlone, 1998), the Eastern Basinconstituting a by-passzoneforlow discharge rivers.

OROGENIC EVOLUTION

OF THE NORTH-CENTRAL ANDES

1LATE OLIGOCENE - PRESENy)'---__

Late Oligocene - middleMiocene evolution (28 .. 10 Ma)

ThelateOligocene"Aymara" tectonicevent(28-26 Ma)

Amajortectonic andgeodynamic event occurred inthelateOligocene (28·26 Ma). Ithasbeendescribed bySebrieret aL (1988) and Sempere et aL (1990; Fig. 32). The[ateOligocene event is related to a major plate dynamicsreorganization that occurred at 26 Ma. This consisted ofthebreakupofthe Farallon Plateintothe Cocos and Nazcaplates, accompanied by a change in the direction ofconvergence (Pilger, 1984; Pardo-Casas andMolnar, 1987).Convergence became approximately E-W, which triggereda progressive rotationofthestrain,from NNE-SSW duringthelateOligocene, toE-Wat theendoftheMiocene (Nobletet aL, 1988).

This tectonic event was marked by regionalunconformities inthe Andes (Sebrier etal: 1988; Sempereet al., 1990), by the deposition of disconforrnable coarse­grainedconglomerate inthe Eastern Basin,bytheinceptionofeastward thrustingin theSub-Andean Zone (Sempere etal., 1990),and bya sharpincrease ofthesubsidence ratesinthe Eastern Basin (Thomas et ai., 1995; Berrones andCorrina, 1996),Italsotriggered oraccelerated thesubsidencerelated to subduction-related tectonic erosion in the fore­arc zones, since in most areas, pelagic Miocene depositsdisconformably overlie Eocene shelfdeposits (Machan! etal., 1986; Suess et al., 1988). Thisevent is also marked bypre-23 Madisconformities in the Andes of northern Peru(Mourier, 1988),by some resetsof KlArages in the EasternCordillera of Ecuador (35 - 25Ma.Litherland et at., 1994),

byunconformities atthe baseofthelateOligocene volcanicsof Ecuador (base of the Saraguro Formation. 29 - 26 Ma,Dunkley and Gaibor, 1998; Steinmann etal., 1999), and byunconformities and syn-tectonic sediments in Bolivia(Sempere etaL, 1990; Rochatet al., 1998).

Latest Oligocene-early Mioceneevolution (26- 17Ma)

The major plate dynamics reorganization of [ateOligocene age provoked a renewal oftectonic activity, whichaccelerated the shorteningand uplift of the Andes, andinduced thick continental sedimentation in theintermontane and retro-arc foreland basins. The lateOligocene tectonic event is post-dated by the creation of anearly continuous beltoffore-arc (MachareeraL, 1986) andbya sharp increase of the subsidence rates in the EasternBasin (Thomas et al., 1995). Offshore northern Peru,tectonic subsidence is markedby the unconformable restof Middle Miocene pelagic deposits upon Eocene shelfdeposits (Machare etal; 1986; Suess etal., 1988;Von Hueneetal.; 1988; Bourgois etal., 1990; Fig. 37).

In coastal Ecuador, theearlyMiocene sequence beginslocally withtransgressive conglomerate overlain bymarineshale and siltstone rich in planktic foraminifera andradiolaria (Dos Bocas andVillingota formations, Evans andWhittaker, 1982; Benitez, 1995). The upper part, of earlymiddleMiocene age, locally gradeslaterally into coarser­grained subaerial deposits (Benitez,1995).Rapid subsidenceof the northern Talara and Tumbes basins is expressed bythedeposition ofa250to 1000 m thicktransgressive seriesof locally conglomeratic sandstonebeds, with marlyandcarbonate-rich intercalations of paralic environment(Mancora Formation), which unconformably rest onPaleozoic rocks (Leon, 1983). Furthersubsidence allowedthe deposition of as much as 1000 m of shale, madstoneandsporadic turbiditeunitsrichinplanktonic foraminifera,which indicate a significant deepening of the basinduringtheearly tomiddleMiocene (Heath Formation, Leon, 1983).Offshore northernPeru.earlyMiocene marinedeposits arementioned onlylocally.They overlie directly middJe Eocenestrata (Ballesteros etal; 1988).

In southern centraJ Peru (Pisco Basin), the latestOligocene-early Miocene deposits consistof a 60 to 300mthickseries oftransgressive shale,siltstoneandfine-grainedsandstone. which unconformably restson Paleozoic toearly

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"Quechua 1» event (17- 15 Ma) andmiddle to lateMiocene evolution(I5-9Ma)

In thetare-arc basins of Ecuador, nogoodrecord of theevent, which occurred near the early-middle Mioceneboundary (I7 - 15 Ma), is known. This event may berecorded by the arrival of moderate amounts of detritalsedimentsand by localtransgression(ProgresoBasin). Inthe Andes of Ecuador, F-T dates on sedimentsindicateanuplift stagearound 18 Ma(Steimann et al., 1999). In theEastern Cordillera ofEcuador. resetsof KJAr agesat20 - 15Mamaybeduetocompressional deformations(Litherlandetal., 1994). This intervalis followed by the openingof theGuayaquil Gulf(Deniaud etal., 1999).

IncentralPeru,themiddle-lateEocene structureswerere-activated in the Western Cordillera and in the MarafionFoldand Thrust Belt (Megard, 1984; Megard et al., 1984),

which fold lateCretaceous red beds. A possiblemagmaticgap occurred around 19- 18Ma in the arc of centralPeru(Soler, 1991). In southern Peru, it is responsible formonoclinal folds and reverse faults, large scale incisionsdue to the resumptionof erosions triggeredby a pulse ofuplift (Sebrier et al., 1988). The latter is thought to beresponsible for a 400m uplift(Sebrieret al., 1988).

In thearcandfore-arc zones ofnorthernChile (Fig. 42),

W-verging fault thrusts are assumed to havebeganaround16 - 15Ma(Garcia et al; 1996; Munoz and Charrier. 1996;

Charrier et al., 1999). This event is associated innorthernmost Chile, with unconformable late middleMiocene lava flows (Zapahuira Formation, 13- 11 Ma) onthe early MiO£enf tuffandconglomerate (Garcia, 1997; Fig.38).Fartherto the S(Antofagasta), thevolcanic rocks oftheSan Bartolo Group (17 Ma) overlie Oligocene fluvialsediments withanangularunconformity (Flint et al., 1993).

In the fore-arc basins of Ecuador, early Miocenemarlstone beds are disconformably overlain by middleMiocene sandstone and marlstone (Subibaja-AngosturaFormation). They are in turn overlainbyearlylateMioceneconformable mudstone(Onzole Formation, Benitez, 1995).In the Progreso Basin, the middleMiocene disconformity(Subibaja and San Antonio transgressive limestonemembers) is followed by deposition ot nearshoresandstone of late Miocene age (Progreso Formation),related to the beginningof the first opening stagesof theGuayaquil Gulf. After anopeningstage(lateOligocene-earlyMiocene) the Tumbes Basinof northernmostPerureceiveddeep-marineturbiditebeds (Le6n,1983). In the forearcofcentral and northern Peru,the earlyMiocene depositsareconformablyoverlain by thick middle Miocene marinemudstone (Machare et al., 1986; Ballesteros et al., 1988;

Fig. 37).

Except in Ecuador, the arc zones are marked by theresumption ofsignificant amountsofvolcanic products.Incentral Peru, a magmatic pulse occurred in the EasternWestern Cordillera and the Altiplano between 18and 13Ma, which comprises abundant volcanism (Soler, 199 I; Fig.20). In southern Peru, the shoshonitic and High-Kmagmatism carne10anend,while calc-alkalinernagmatismwenton (Carlieretal.,1996). In the arcand fore-arc zoneofnorthernmost Chile, late middle Miocene lava flows

(ZapahuiraFormation, 13- 11 Ma)unconformably overlieearlyMiocene tuffandconglomerate (Garcia, 1997; Fig. 38).Fartherto the5, avolcanic unit (SanBartolo Group,17Ma)overlies Oligocene fluvial sediments with an angularunconformity (Flintet al: 1993; Fig. 30).

In the paleo-Andes of Ecuador and Peru,the openingof the "Miocene" intermontane basins had been dated asmainlyearlyMiocene (KJAr,26- 22Ma.Lavenu etal., 1992)

and wereregarded as pull-apartbasins,openedbythe playof an oblique NE trending strain exerted on pre-existingNNE and ENE trendingfaults (Noblet et al., 1988; Baudinoet ul., 1994, Marocco et al; 1995; Barragan et al.; 1996).However, recent F-T concordant dates support a lateearlytoearlymiddleMiocene age(I6 -14 Ma)forthe creationofmost intermontane basins of Ecuador (Figs. 33 and 40),which havebeen alternatively regarded as the resultof anE-W extensional regime(Steinmann, 1997, Hungerbuhler,1997; Steinmannetal.,1999).Anextensional regimeisalsothought to have governed the creation of many Mioceneintermontane basinsofPeru (Noble etal., 1999).

In Ecuador, the rapidlyopened intermontane basins (16­15Ma} werefilled bymiddleMiocene fine-grained lacustrinedeposits (14-10 Ma) representingaperiod ofrelative tectonicquiescence (Noblet etaL, 198B;Marocco etai, 1995).Thefine­graineddepositsareoverlain byacoarsening-upward sequenceofsandstone andconglomerate, coevalwith thecompressionaldosureofthesebasins (Megard etaJ., 1984;Nobleteral, 1988;Baudino etol; 1994; Marocco etaL, 1995; Hungerbtihler etol;1995; Hungerbtihler, 1997; Fig. 40).TheCuenca andLojabasinsof southern Ecuador contain middle Miocene marineintercalations, indicatingthatthey werelocatedator very dosetosealevel andareregardedasembaymentsoffore-arcbasins(IS - II Ma, Hungerbiihler 1997; Steinmann etai, 1999; Fig.40).InPeru, thenonmarinevolcanidastic andsedimentary in­fill of the middleMiocene basins is usually unconformablyoverlain by mainlyvolcanic deposits dated at 10 to 7 Ma(Megard etaL, 1984; MaroccoetaL, 1995; Nobleetai, 1999).

The Altiplano Basin of Bolivia is a peculiar case ofintermontane basinconsisting ofN-S-trending half-grabens(Fig.35). In this basin, shale, sandstone and subordinateconglomerate of middleMiocene agerest conformably onthe earlyMiocene deposits(Lamb etal; 1997; Rochatetal.,1998). The middleMiocene sequence was deposited withveryhighsedimentation rates,especially in theCorqueBasinofsouthernBolivia (Roperch etal., 1999a).Clastic sedimentsmainly derived from the EasternCordillera, the erosionofwhich allowed the development of regional-scale flatmorphological surfaces. These are the Chayanta (13 - 14Ma) and San [uan de Oro(10 Ma) surfaces,which can beobservedand tracedfromnorthern Argentina up to the Lapazregion (Servant etaI., 1989, Herail etal., 1993; Gubbelsetal., 1993).This periodcoincides witha lowuplift rate ofthe EasternCordillera.

In the back-arc basins, a conspicuous shallow marinetransgression is recorded during the late middleMiocene(15Ma, Pebas Formation of Peru,Hoorn, 1993), which wasconnected to theopenmarinerealmthroughtheMaracaiboarea(Hoornetal; 1995) and possibly theGuayaquil seawayin southern Ecuador (Steinmann et al., 1999). A similarshallow marine invasion ofearlylateMiocene ageisknownin the EasternBasin ofBolivia (Yecua Formation, Marshall 525

TECTONIC EVOLUTION OF SOUTH AMERICA

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et al., 1993). which was connected southward to theAtlantic Ocean. In Ecuador andnorthernPeru,thisperiodismarked by slight decrease of the tectonic subsidence(Thomas et al., 1995; Contreras et al., 1996; Fig.39).However, in eastern Bolivia, a strongincrease of tectonicsubsidence has been related to the deformation of theeasternCordillera (Marshall et al., 1993; Sempere, 1995).

uQuechua 2" event (9 - 8 Ma)and late Miocene evolution (9- 6 Ma)

Thisperiodbeginswiththe Quechua 2 tectonic phase(9 - 8 Ma,Megard, 1984; Sebrieret al., 1988). Ratherthana major deformational event, the Quechua 2 event is aturning point in the evolution of the northern CentralAndes, which corresponds to the change from adepositional periodcharacterized by thick and relativelywidespread fining-upward sequences, to a compressionaland uplift period marked by erosions and depositionalareas restricted to the fore arc and retroarcdomains. Thisisinterpretedas the resultofthebeginning ofthenearly enbloc eastward thrusting of the Paleo-Andes onto theBrazilian and Guiana shields, which resulted in crustalthickeningand rapid uplift of the arc zones and paleo­Andes, and the transfer of active deformation into theSubandean thrust and fold belts,

Thefore-arc zonearemainly marked byuplift (Sebrieretal., 1988),unconformities and reverse faulting. IncoastalEcuador, the middleMiocene disconformity is overlain bytransgressive marine sandstone (Angostura Formation) andthenbya thinning-upwards succession ofmarine nearshoresandstone oflateMiocene age(Progreso and LowerOnzoleformations), related to the beginning of theopening of theGuayaquil Gulf (Deniaud et al., 1999). In the fore-arc zoneofPeru, theunconformity between middle andlateMiocenemarinedeposits in thefore-arc basinsofcentral Peru(LimaBasin, 11 °S) has not been recognized farther N (YaquinaBasin, 9°S, Ballesteros et al., 1988). In the Lima Basin.lowenergyshelf deposits of late Miocene age evolved towardshelftoslopedeposits (Ballesteros etal., 1988),indicating anoticeable deepening of thedepositional environment, dueto thesubsidence related to tectonic erosion of the fore-arczones (Von HueneetaI., 1988).1n theYaquina Basin, middleMiocene low energy turbidite beds grade upwards intohigher energy turbidites of late Miocene-Pleistocene age(Ballesteros etal., 1988).

In thefore-arc zoneofnorthernChile, where W-vergingthrustingwenton(Fig. 42),a samekind ofunconformity isrecognized between late middle Miocene lava and lateMiocene alluvial deposits (Huaylas Formation, 9 - 8 Ma,Garcia, 1997; Fig. 38). In the magmatic arc of Peru, nolocation changeis observed in the magmatic activity, withrespect to the former periods. A major magmatic pulseoccurred between 12and7Ma(peak. activityaround10Ma).Itcorresponds to numerous intrusions, and veryabundanteffusive products (Soler, 1991). A possible gap,or at leastmagmatic quiescence, is mentioned at9 - 8 Ma, which maycoincide with a compressional event (Soler, 1991; Fig. 20).

ThePaleo-Andes andsurrounding areasaremarked byageneral and rapiduplift (Sebrier etal.,1988; Steinmann etal., 1999), thelocal ratesofwhich remain 10bespecified. InEcuador, the estimates of mean rock uplift rate since thelateMiocene (9 Ma) is of 0.7 mm/y and the meansurfaceupliftisof0.3mmiy (Hungerbuhler, 1997; Steinmann etal.,1999), which is consistent withestimates by Delfaud et a1.(1999) inthesamearea,bySebrier erci (1988) insouthernPeru, andbyParraguez era1. (1997) in northernChiIe.In thelatterareaand in Bolivia, uplift involved both the Western(Sebrier et al; 1988) and Eastern(Benjamin et al., 1987)Cordilleras.

In Ecuador, thisperiodismarked bythecompressionalclosure of the Miocene intramontane basins (Noblet et al.,1988; Marocco etal., 1995; Hungerbuhler, 1997; SteinmannetaI., 1999), which are filled bycoarsening and thickening­upward clastic deposits. Thisis interpreted as theresultofachange from an extensional (15- 10Ma) toacompressionalregime (9- 8 Ma), which resulted in the uplift of southernEcuador, the establishment of terrestrial conditions in theintermontane basinsof southernEcuador and risingreliefin the Eastern Cordillera (Hungerbuhler, 1997). LatestMiocene times are then marked by the development ofsmaller-scale intermontane basins filled with mainlyvolcanic and volcanogenic rocks (Lavenu et al. 1996;Hungerbuhler; 1997; Fig. 41),

In northern Peru, although timing constraints arepoorer, and the change from extensional tocompressionalregime isassumed tobeoflateMiocene age. In central Peru,N-S shortening induced mainly dextralmovements alongNW trendingfaults (Megard, 1984). In theAyacucho Basin,compressional deformation occurred between 9.5and 8.5Ma(Megard et al., 1984). In southernPeru,latestMiocenetranspressional stress is thought to be responsible for theclosure ofintermontane basins (Paruro Basin) opened about12Maago (Carlotto, 1998). Farther to the S,late Miocenetimesaremarked bythecontraction oftheAltiplano, relatedto the tectonic inversion of the pre-existing normal faultsdefining thehemi-grabens locatedWoftheAltiplano Basin(Kennan et aI., 1995; Lamb et ai" 1997; Rochat etal., 1998,1999; Fig. 35). In the Altiplano Basin, tuff beds datedat 9Ma are disconformably overlain by a thin sequence ofconglomerate reworked Paleozoic basement rocks (Lamb etal., 1997jRochatetaL, 1998).

In northernChile, compressional tectonic activity wentoninthePre-Cordillera,withthedevelopmentofthewesternthrusts of the W-vergent thrust system (Fig. 42),whereasan extensional regime prevailed farther to the W(Longitudinal Valley and Coastal Cordillera; Munoz andCharrier, 1996; Garcia et al., 1999).

Regarding the EasternBasin, the eastward migrationof the deformation front during the late Miocene is theprevailing feature (Sheffels, 1990; Baby etal; 1997). Fromthistimeonwards, mostofthedeformation and shorteningoftheAndean margin isaccommodatedbytheeastern areas,which received W-proceeding coarse-grained clastic,terrestrial sediments.In theOriente Basin of Ecuador, lateMiocene deposits consistofthicksequences ofpoorlydatedcoarse-grained conglomeratic sequences separated fromeach otherbydisconformities (Christophoul, 1999). In thewestern part of the EasternBasin ofnorthernPeru(Bagua

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area), a marked unconformity (10 Ma) separates fluvialsandstoneand conglomerate of middleMiocene age,fromlate Miocene ccarse-grained fanglomerate {SanAntonioFormation, Mourieretal., 1988; Naeser eral., 1991; Fig.3l).In the Ucayali Basin, fluvial siltstoneand sandstoneof lateMiocene ageabruptlyoverlie marinetobrackishbeds (Kochand Blissenbach, 1962; Fig. 36).

Quechua 3event (7-5Ma) and latestMiocene-Present evolution(6 - 0 Ma)

Atthebeginningof this period,thesignificant 7- 5Macontractional event is markedbya chiefly E-W shortening(Megard, 19M;Sebrieretal., 1988).Itis marked by foldingand reverse and strike-slipfaulting in southwestern Peru(Sebrier et al.. 1988), and by the onset of the sub-Andeanthrust and fold belts, which accommodate most of theshortening in the Andean Chain during the Pliocene(Roeder, 1988; Sheffels, 1990; Baby etal., 1992).

In the fore-arc zone the deformation and uplift wereinfluenced by the incipient subduction of the Nazca andCarnegie aseismicridges (7 to 3Maago, Suess etal., 1988;

Von Huene andScholl, 1991, Benitez, 1995), andtheongoingtectonicerosionof the fore-arc zones.In the fore-arc zonesof Ecuador, a regional disconformity datedat the Miocene­Pliocene boundary (5.5 Ma) precedes the depositionof acoarseningand shallowing-upward sequence, related to theincreased uplift and erosion of the AndeanChain from 9Ma(Benitez, 1995; Deniaudet al., 1999). In the GuayaquilGulf, however, a strong subsidence due to transtensionalmovements allowed the deposition of huge volumes ofclasticsediments, especially during the early Pleistocene(Deniaudet al.; 1999). Asimilar disconformity and hiatusseemto be recorded at 5 Main the offshore basins of Peru(Von Hoene etal., 198B).

In central Peru W - 14°S), the subsidence of the fore­arczonesrelated to the tectonicerosionis recorded in theLimaBasin by Pliocene lowenergy turbidite beds, whichindicatedeepeningof the environment (Ballesteros et al.1988), by the localtransition from uplift to subsidence at6Ma (Von Hueneet al. 1988),and by the lack of upliftedterraces in the coastal zone (Machare and Ortlieb, 1993).On the contrary, significant upliftmovements affected thecoastofnorthern (4°- 60S, <0.2 mm/y)andsouthern Peru(14° - 18°S, <0.7 mm/y) since the late Pliocene (Machareand Ortlieb, 1992;Von Huene et aI" 1988).In thislauer case,uplift isdue to the subductionof the Nazca aseismic ridge,which induced a regional extensional stress regime(Machare and Ortlieb, 1992) .

All segments of the Andes are marked by thecontinuation ofthe majorand rapiduplift.I nEcuador, upliftrateestimatedby P-Tevidencea slow downbetween 6 and4 Ma,and an increasefrom3 Ma (Steinmannet al., 1999).

Othermethods estimate1000 - 1200 m of net upliftsince5Ma (0.2 mmfy, Delfaud et aI., 1999). Pliocene-Quaternarytimesarealsomarkedby theupliftofthe sub-AndeanZo neofEcuador(Babyet al; 1999). In theAndes ofsouthernPeru,upliftisestimatedat 1300 msincethelateMiocene.ofwhich200- 300m would be ofQuaternaryage.

Thearczonesof Ecuador(Steinmannet al., 1999), andNorthernand CentralPeruare markedbyan effusive pulse

centred around 5 - 4 Ma.In Peru, it corresponds mainlytoignimbritic tuff associated with rhyolitic dykes in theWestern ComilleTa (Soler, 1991). In southern Peru, theemplacement of alkaline,peraluminous and shoshoniticsuitesalongmajorfault systems suggests thatan extensionalregime prevailed around 6 - 5 Ma (Carlier et aI.. 1996).Effusions of shoshonite, minette, lamproite andperaluminous rhyolite anddacitewentonduringthe past3Ma, and took place along the fault systems limiting theAltiplano, and interpretedassinistralwrench-faults (Carlieretal.,1996; Carlotto, 1998).

In theeasternbasins.latestMiocene-Pliocene timesaremarked by a strong flexural subsidence allowing thickaccumulations of foreland clasticdeposits (Thomaset al.,1995; Contreras et al., 1996; Baby et al., 1995; Fig. 39),whereas Recent times are marked bya strongdecrease ofthe sedimentation rate and local uplifts. However, Recentsedimentationcontinues in restrictedand/or moreeasterlyareas (Ucamaradepression of easternmostPeru;Pastaza­Maranonalluvial fan),

In the Oriente Basin of Ecuador, localized coarse­grained fanglomerate are incised by present-day rivers(Christophoul, 1999).ln northeasternPeru,apatite fissiontracks data indicatethat the Santiago Basin underwent arapid uplift (0.4 mmfy) during the last 10 Ma,probablyrelated to the onset of the Santiago Fold and Thrust Beltduring the latestMiocene (Pardo, 1982; Megard, 1984). IntheMarafion Basin.asedimentaryhiatusseparatesmiddle­late Miocene fine-grained deposits from disconformablecoarse-grained fanglomerate of latestMiocene-Recent age(Mathalone and Montoya, 1995) and Pliocene times aremarkedby the upliftof the area (Contreras et al., 1996). Inthe Ucayali Basin,no post-Miocene deposits are known(Koch and Blissenbach, 1962). In the Madre de Dios Basin,late Miocene deposits fill incised valleys, and the recentalluvial terrace morphology shows a Pleistocene uplift ofthe area. In the sub-Andean foreland basin of northernBolivia,asignificant increasein thesubsidence allowed theaccumulation of about 5000 m of late Miocene-Plioceneclastic sediments.This is interpreted as the result of therapideastwardmigrationof the Andeandeformation 10to6 Maago(Gubbels et al., 1993; Baby etal; 1995). However,noforedeep sedimentation occursat present (Roeder, 1988).

TECTONIC AND KINEMATIC

EVOLUTION OF THE

NORTH-CENTRAL ANDES

From Bolivia to Ecuador, structural styleof the Andeschanges dramatically. Geometry of the present-daydeformation of the Bolivian and North-Chilean Andesresults from Neogene thin-skinned tectonics, whereas theEcuadorian Andes havebeen structured by thick-skinnedand wrench tectonics since Cretaceous times. Figures 2A,28 and 2C illustrateglobal changesin structural geometryand chain width. These two parts of the Andes form two

extremes. Neogene tectonic events seem to occurcontemporaneously, hutexpress twoshapesoforogenicbelt.Three Neogene orogenic stages, late Oligocene-early

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TECTONIC EVOLUTION Of SOUTH AMERICA

I~'~Miocene, lateMiocene,and Pliocene-Quaternary, havebeendistinguished accordingto the deformation styleand rate.They have been recorded in fore-arc, intermontane andback-arc basins. Sedimentary successions involved in thedeformations consist ofCambrian toOligocene pre-orogenicstrata, and Oligocene-Miocene to Recent continentalsyn­orogenic in-fill.

Central Andes of Bolivia

Recent work has shown the importance of crustalshortening forthe development of thestructural patternofthis part of the Andes (Allmendinger et al., 1983; Isacks,1988; Roeder, 1988; Sheffels, 1990; Sempere etal., 1990; Babyetal., 1992, 1997; Gubbels et al., 1993; Schmitz, 1994; KIeyand Reinhardt, 1994; Dunnet al., 1995; Rochat etat., 1999;Giese etal., 1999).

Crustal structures and Neogeneshortening

TheCentral Andes are divided fromE to W intoseveralmorpho-tectonic units (Fig. 43).TheChaco and Beni plainscorrespond to a slightly deformed Neogene foreland basinunderlain by the Brazilian Shield. It is overthrust by theSubandean Zone, a complex thin-skinned fold andthrustbeltcharacterized in itscentral part (Santa Cruz elbow) bylarge­scale transferzones (Baby etai; 1996). ThenorthernbranchoftheSubandean Zone ischaracterizedby large scale thrustsheets (to - 20km of offset) and broad synclines (Roeder,1988) filled by up to 6000 m of syn-tectonic Neogenesedimentary rocks (Baby et al; 1995a). Surface mapping,seismic reflection data, and drilling information showthatthe main detachment planes are located in the Ordovician,Silurian, Devonian and Permian shaly levels (Baby et al.,1995b).Theslope ofthebaseofthe foredeep is4°toward theSW. Theamount ofshortening is74 km,i.e, 50%.

In the southern branch of the Sabandean Zone, aregional E-verging thrust (Mandiyuti Thrust) divides thesouthernBolivian Zoneintotwo fold andthrust belts, whichdiffer according to their thrust system geometry. Mainlyfault-propagation fold sand fault-bend fold scharacterize thewestern belt,whereas fault-propagation folds and passive­roof duplexes characterize the eastern belt. Maindetachments are located in Silurian dark shale, LowerDevonian shale,and at the base and top of the MiddletoUpper Devonian dark shale. The Silurian-Devonlansuccession is covered by more than 2000 m of upperPaleozoic and Mesozoic sandstone with no potentialdetachments; in some placesit is also covered by severalthousand metres of syn-orogenic Neogene sedimentaryrocks(Moretti et al; 1996). Thebaseof the foredeep slopesat 2°W. Total shorteningdecreases southward from 140km(50%)at 20"5, to 86km (35%) at22·S.

The Interandean Zone and Cordillera Oriental aredeformed by E-vergent thrusts which involve basementrocks (K1ey, 1996),and associated thin-skinnedthrusts andbackthrusts, Mainly Silurian, Devonian, and Carboniferousstrataareexposed in theInterandeanZone. IntheCordillera

Oriental, the Neogene thrust systemis superimposed on adeeply eroded pre-Cretaceous fold belt that deformedOrdovician anchimetamcrphic sedimentary rocks.

Shortening isconcentrated in theW-verging thrust systemof the western part of the Cordillera Oriental and of theInterandean Zone. TheCordillera Oriental ischaracterizedby small Neogene piggyback basins (Fornari et al., 1987;Herail etal; 1996).Good surfacedataallowed ustoconstructsome balanced cross sections, according to which totalshorteningmaybe estimatedbetween 80 and 100krn,TheAltiplano is a complex Neogene intermontane basindeformed byboth extensional andcompressional tectonics.Surface mapping, seismic reflection data, and drillinginformation made possible the construction of balancedcross-sections. Thetotalshortening calculated is20km and13km in the southern and northern parts of the Altiplano,respectively.

The westernareasincludeseveral morphological units.The Cordillera Occidental includes Plio-Quaternaryvolcanoes. Itswesternpart is formed by aW-verging thrustsystem (Munoz and Charrier, 1996; Fig. 42), characterizedby the reactivation of high angle faults and the lack ofPaleozoic cover. Ontheeastern part ofthe Precordillera.backthrusts limit a blind pop-up structure below the Tertiarydeposits (Riquelme and Herail, 1997). Shortening is doseto18krn.In theCentral Valley,withinwhich Late Cretaceous­Paleocene magmatic arc and associated deposits aredeformedby a mild Plio-Pleistocene extensional tectonics(Parraguez et al., 1997). TheCoastal Cordillera shows lowreliefconstitutedby jurassic: - EarlyCretaceous magmaticarc rocks. TheChilean marginexhibits a horst and grabentopography, and in its central part, a well expressedextensional, asymmetric basin (Munozand Fuenzalida,1997), similarto theNeogene basinslocatedfartherN(VonHuene and Scholl, 1991).

Crustal balancingacross the CentralAndes betweenIS"Sand I80S (Fig. 2)on thebasisofa normalpre-orogeniccrustalthickness (according tothelocationof thePalaeozoicbasinand thelackofsignificant Meso-Cenozoic extension)allows us to calculate 210 km of shortening during theNeogene (Baby e ai, 1997).At thelatitude oftheArica elbow,shortening is associated with the clockwise rotation ofcrustal blockscontrolled by inherited faults (Fig. 43), dueto thecompression exertedbythefore-arc zone that behavesas a rigid buttress. These rotations are coeval with thecompressional deformatiun,bu\ the elhowshape of the

Bolivian orocline has been acquired prior to thisdeformation, and is probably ofLate Cretaceous or Eoceneage(Roperchetal., 1999b).

TheMoho shape and the Nazca Plategeometryat thislatitudearewell constrained bygeophysical studies(James,1971; Cahill and Isacks, 1992; Dorbath et aL, 1993; Beck et

al; 1996; Zandt et al, 1996). Deep crustal structures areimaged by lower crust reflectors located at differentstructural levels (Wigger et al., 1994; Allmendinger andZapata,1996). Thecrustal duplexes below theEasternandWestern Cordillera are insufficient to produce the crustalthickening evidenced by geophysical data below theAltiplano and the fore-arc zone. Duplex structures in thelower crust (Lamb and Hoke. 1997) cannotexplain theoverbalanced volume(7216 km'in cross-section; Fig. 2),since

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TECTONIC EVOLUTION OF SOUTH AMERICA

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the lowercrust structures havebeen taken intoaccount inthe crusta] balancing. Asthenosphere wedge as well assignificant volumes of magmatic addition cannot accountfor the observed thickness (Rochat et al.. 1999). Thesignificant tectonicerosionofthe Chilean margin(Rutland,1971; Cloos and Shreve. 1996; Von Huene and Scholl, 1991)and associated extensional deformations suggestthat deepcrust materialremoved from the continental edgehas beenunderplated below the fore-arc zone andAltiplano (Schmitz,1994; Baby er al., 1997).

Timing of Neogene deformationsIn the Central Andes, theback-arcthrustingstarted in

thelateOligocene (Sempere etal., 1990; Baby et ai., 1997).The first W-vergent thrust motions in the fore-arc zoneoccurred in the late Oligocene-lower Miocene along themedian thrust plane (Garcia et al., 1996). Meanwhile, theAltiplano corresponded to an endorhelc basin (Rochat et

al., 1998, 1999) situated at the back of the more internalcrustal thrust of the Eastern Cordillera. During the upperMiocene, the median thrust planeof the W-vergent thrustsystem wasreactivated (Garciaeral., 1996) andcrustalbackthrusts produced the partial expulsion of the Altiplano,which represented, therefore, a broad piggy-back basincarried over the crustal duplex of the Eastern Cordillera(Baby etal; 1997). Activity oftheSubandean fold and thrustsystem started at the sameperiod (Gubbels et al., 1993); itseastward propagation accelerated in the Pliocene andcontinues presently.

Kinematic anddynamic analysisTectono-sedimentary studiesof the Altiplano (Rochat

etal; 1998) indicate a localtype isostatic behaviour (deepbasincontrolled byvertical motionalongpre-existing highanglefaults). Predicted topography from10kmofdeposits,assuminga normal crust isostatically compensated, is 1.5km (Rochat et ai., 1999). However, no significant absolutesubsidence and upliftoccurredin the Altiplano duringtheNeogene. Thecontinuityofthe sedimentation in the centreof theAltiplano shows that the topography wasarchived byfilling up of the thick syn-orogenic deposits andprogradationoverthe upliftingborders.

The Neogene fillingof the fore-arc extensional basin(Von Huene and Scholl, 1991) is coeval with thesedimentary overloading of the Altiplano, whichcorresponds to 30%of the volumeeroded fromthe back­arc and the Cordillera Occidental (Rschat, 1999). Timingof both processes indicates that deeptectonicerosionandunderplating were able to maintain isostaticequilibriumand consequently the verticalaggradationof theAltiplanolevel. Along the fore-arc zone, structural traps (like theAltiplano crustal piggyback basin) do not exist. Theintensity of Neogene erosion shows that these areas wereovercompensated bythedeepunderplating. TheupperPlio­Pleistocene decrease ofsedimentation areasin theAltiplano(Rochatetai., 1998) wasassociated withexorheic drainage.Consecutive minor uplift, as is shownbylacustrine over­deepening and extensional deformations (Lavenu, 1995),show that the equilibrium between superficial

sedimentation-erosionanddeeperosionunderplating wasbroken. In an ongoingconvergence tectoniccontext, deepup-drive willinvolve destructionof theAltiplano byerosionand associatedcollapse.

Andes of Ecuador

The Ecu adorian Andes (ION - 4OS), are one of thenarrowestand most active part of the Andean Belt. It isdeformed by NNE-SSW right-lateral transpressive shearzones(Tibaldi and Ferrari.1992)and uooemenl an intenseHolocene tectonic and volcanic activity. The Dolores­Guayaquil Megashear constitutes an important dextraltranscurrent boundarywhich marksroughly thesuture zonebetween the SouthAmerican continental marginand theCoastal Blockwithoceanic basement,accretedduringLateCretaceous-Paleogene times (Tuteau et al., 1977; Lebrat etal., 1987; Cosma et al., 1998; Reynaud er aI,. 1999). Deepgeophysical data arenot numerous enoughtoconstraintheMoho geometry. Below the chain,the average depth of theMoho isabout50km (Prevot etai; 1996).

Crustal structures and Neogenedeformations

TheEcuadorian Andes aredivided fromEtoWintosixmorphotectonic units (Fig. 2). The Amazonas forelandbasin is deformed by two major NNE-SSW trending,transpressional right-lateral fault zones, which correspondtoinverted Mesozoic riftsystems (Baby et al., 1999).Positiveflower structures were developed along these trends andformed the main oil fields of Ecuador. No Quaternarysedimentary sequences arecropping out inthisbasin,whichseems to undergo upliftpresently.

TheSubandean Zone is formed bytwoenechelon NNE­SSW trendingpositiveflower structures(NapaandCutucouplifts, Baby et al., 1999). which are still seismically andvolcanically active. They result also from transpressionaldextral movements, andare separatedbyaQuaternary pull­apart basin(Pastaza Depression).

TheCordillera Real isa metamorphic belt,intrudedbyJurassic batholiths and strongly deformed by wrenchtectonics. A W-dipping, high angle reverse fault zoneseparatesit fromthe Subandean Zone. In the InterandeanValley, thick alluvial, lacustrine and volcaniclasticcontinental sediments were deposited in several Neogeneintermontane basins,controlled byregional strike-slip faultslimiting the Interandean Valley (Marocco et al., 1995;Barragan etal., 1996; Hungerbuhler, 1997).

TheWestern Cordillera and Coastal areaarepart of theallochthonous oceanic terranes accreted to the AndeanmarginduringLate Cretaceous-early Tertiary times (Lebratetal.,1987; Cosma etal.• 1998; Hughes and Pilatasig, 1999;Reynaud et al.• 1999). The Western Cordillera is made ofoceanic plateau and island arc magmatic rocksand theirCretaceous-Eocene flysch cover, overlain and crosscut bycontinental arc magmatic rocks. The Coastal area ischaracterized byfourmainNeogene fore arcbasins(Borbon,Manabl,Progreso,Guayaquil; Fig. 1)related todextralstrike-

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TECTONIC EVOLUTION OF SOUTH AMERICA

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slip displacements (Deniaud et al., 1999). The Gulf ofGuayaquil is the deepest Neogene fore-arc basin. Itcorresponds to a pull-apart basin developed between theDolores-Guayaquil Megashear Zone totheE,andtheobliqueconvergent Nazca- South America Plate boundarytothe W(Deniaudetal.,1999).

Timing of Neogene deformationsNeogene deformations havebeenrecorded in thefore­

arc,intermontane and foreland basins.Thecreation of theintermontane basinsstartedat about28 - 26Ma (Maroccoetal., 1995), like the Bolivian Altiplano Basin. In thecoastalarea, the evolution of the Manabi and Progreso Neogenebasinsbegan in theearlyMiocene. In theAmazonas Basin,stratigraphic data are insufficient to specify the ageof theonset of the Neogene foreland basin. It is marked by theeastward wedge of the subaerial Arajuno Formation(Petroproduccion seismic information), of probable earlyMiocene age.

TheupperMiocene is characterized bythe closure andthe piggyback evolution of the intermontane basins(Marocco et al., L995), and the closure of the Manabf andProgreso fore-arc basins(Deniaud etal., 1999).At thesametime, the Amazonas foreland was mildlydeformed andinvaded bymarineincursions (Hoorn elai., 1995).

ThePliocene showed anacceleration ofthedeformationand marked theonsetofthe strongest orogenic stage oftheAndes, which is still active. The opening of the pull-apartbasinof theGulfof Cuayaquil startedduringthe Pliocene,and sedimentation rate reached a maximum (8600 mlMain the depocenter) in the lower Pleistocene (Deniaud ef aI.,1999b).TheupliftoftheSubandean Zone (Napa andCutucuuplifts) occurred duringthisperiodandcontinues presently.

OVERVIEW OF THE NEOTECTONICS

OF THE NORTH-CENTRAL ANDES

(ECUADOR, PERU, BOLIVIA, AND

NORTHERN CHllE) _

The main features of the Andean Cordillera wereacquired during theMiocene, andfew changes occured sincethen.However, significant modifications of the topographyareproduced byneotectonic deformations, resulting in thepresent-day topography. During thisperiod,altiplano basinsareformed or maintained in Ecuador andBolivia. theNazcaand Carnegie aseismic ridges are introduced in the'subductionzone, leading the coast to rise, and the twodepressions of the Marafion and Beni basins areindividualized, giving birth to the presentAmazonas River.

This evolution ofthelandscape isbetterapproached andunderstood considering three different aspects ofneotectonic studiesandmethods.Thefirst determines thestateofstress(Fig. 44),asdeducedfromfaultanalysis. Asfar as Quaternary terranes are considered, a comparisonwith the present state of stress can be carried out. Thesecond dealswiththevertical movements alongthecoast,asdetermined from thestudyofmarineterraces. Thethird

type of neotectonic studies is dedicated to the forelandbasins,where river locations and shifts are controlled byneotectonic deformation ofthebasin surface (Fig.45).

The convergence vector is oblique with respect to theplate boundary zone. The mode of the obliqueaccommodation is problematic, specially the relationshipbetween the overriding plate deformations and thesubduction. Thegeometry of the coast and the subductionsystem suggests that it strongly controls thebuilding of therange. The coastalareas of South America are generallysubmitted to extensional tectonics, mostly because it liesover the subducted plate without lateral constraints.However, the direction of extension is variable. In areasofoblique convergence and relatively wide coastal zone(comprised between trench and Western Cordillera) it isorthogonal to thedirection ofplateconvergence (Ecuador,southern Peru). In Chile, where there is a narrow beltbetween the trenchandthe Main Cordillera, the extensionis orthogonal to the margin, and interpreted as related togravitational post-seismic effects.

In the coastal region of Ecuador, the stresspattern isdominated by a N-S extension (Dumont es al., 1997), dueeithertothegeneral northward escape oftheAndean Block,or,more locally, tothenorthward increasingobliquity oftheconvergence, from the Gulf of Guayaquil to Colombia. Thetriangle-shaped Andean Block accommodates thedeformation at the triple junction between the SouthAmerican, Caribbean, and Nazca plates. Atthesouthern tipofthe Ecuadorian Coastal Block, which forms thesoutherncornerof theAndean Block, the Gulf of Guayaquil openedasa result oftherightlateral movement oftheAndeanBlockwithrespect to the South American Plate. Thisrightlateralmovement is accommodated along the Pallatanga Fault,which extends northeastwards towards the InterandeanDepression (located between the Eastern and WesternCordilleras) and farther N to other fault segments (e.g.,Chingual-Sofia Fault). Southwestwards, the Pallatanga Faultextends intothe GulfofGuayaquil,bythe means ofa systemof transcurrentand normal faults. The calculated averageQuaternary extensional rateof the Gulf of Guayaquil is of2.5± 1.1 mrn/y,

Thesubduction oftheCarnegie Ridge duringearly andmiddle Pleistocene isan important parameterofthe coastaluplift. Several Quaternary abrasion surfaces at elevationsranging from 7m to asmuchas330m (e.g., the surfaces ofthe 'Iablazos Formation) areobserved between the GulfofGuayaquil and Esmeraldas, suggesting a maximum upliftrate of about0.2 mmJy during the Quaternary. Along thePacific coast ofPeru.the Quaternary faults evidence a N-Strending extension. InthePacific lowlands ofsouthernPeru,this stateof stress is about neutral,due to a topographiceffect related totheproximity ofthedeep Peru-Chile Trench.Onthe northernPeruvian Coast, the present-day elevationofthe abrasionsurfaces suggests an upliftrate of0.2mmJy during the Quaternary. In southernPeru,in frontof theNazca Ridge, uplifted marineterraces located at 300to700m high,suggest an average uplift rate of O. L8 mmJy and amaximum uplift rate of 0.7 mm/y for the same period(Machan: and Ortlieb, 1993).

InChile, inlocalised coastal areas, which aretheclosestto the trench (80 to lOll km); the observed slate of stress

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TECTONIC EVOLUTION OF SOUTH AMERICA

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duringthe upper Pleistocene isan E-W extension. ThisE­Wstretchingis related to uplifted terraces, located overa

crustal bulgedue to the subduction. Thus the extension isinterpreted as resulting from an accommodation of therisingtopography, related to bodyforces. The£.WtrendingSIT is s3 (sHmin,Horizontal minimumprincipalstress,or

tensional deviatoric stress), syy N-S trending is s2(intermediate deviatoric stress) and szz,vertical, is s1(Hmax, maximum principal stress or compressionaldeviatoric stress).Thestate of stress is szz > syy> SIT. InnorthernChile (230 to27"S) theQuaternary marineabrasionsurfaces, located at an elevation of 200 m, suggest amaximumupliftrateofaboutO.2 mmty{ortheQuaternary(Ortlieb et aL, 1996).

In theMainRange ofCentral Andes, present-day stressfield and crustaldeformations are nothomogeneous alongstrike.In the whole Andean rangeof Ecuador, the present­day stress field appears to be homogeneous and theQuaternary dominant tectonic regimeis an E-W trendingcompression. Near the trench, the state of stress is sl =N8IvE; inthe HighCordillera between 0° and l"S,the stateof stress is N77°E < sl < N1200E; and in the SubandeanZone. sl is N99°E (Ego er al., 1996). In the northern part ofEcuador, alongthe right lateraJ ChingaI-La Sofia Fault, thelateral slip rate displacementis of 7 ± 3 mm/yfor the last37kaBP(Ego, 1995). ln the Interandean Depression, in therestrainingbendofthe Latacunga Zone, the shortening rateis of 1.4± 0.3 mm/y since 1.4Ma (Lavenu et al; 1995).Along the right lateral strike-slip Pallatanga Fault, thehorizontal rate motion isof4 ± 1mm/yforthesameperiod(Winter and Lavenu, 1989; Winter, 1990; Winter et al.,1993).

The present-daystate of stress in the Peruvian Andeshasbeendeducedfrom thestructuralanalysis ofQuaternaryandactive faults and seismic data(Sebrieretai; 1985, 1988,Mercier et al., 1992). The crustal deformation of the HighAndes ischaracterized bynormalfaulting, excepted withinthe EasternCordillera ofcentralPeru,whereas the westernand eastern boundaries of the HighAndes (fore-arc andforeland) are characterized by thrust mechanisms thatindicatecompressional deformations.

In the HighAndes, twotectonic regimes occur. In theWestern Cordillera, recent and activedeformations resultfroma N-S trending extensional tectonics. In the EasternCordillera,seismicity andactivestrike-slip faults result fromboth a N-S trending extension and an E-W trendingcompression. In the SubandeanZone, reverse faulting is inagreementwithan E-W trendingcompression. Close to thetrench, at the contact between the Nazca and SouthAmerican plates, focal mechanisms ofearthquakes evidencean E-W trending compression roughly parallel to theconvergence betweenthe twoplates. In southern Peru,thestate of stress in the HighAndes as well as in the Pacificlowlands resultsfroma N-S trendingextension.

Thus, the state of stress in the Andes of central Peruand thoseoftheAndesofsouthernPerumaybe interpretedas an effectof compensated high topography. However,compressional tectonicsaffects the HighAndes of centralPerubut not those of southernPeru.

IncentralPeru,thestressmodel issuchthat theverticalstress szz increases with the topography and the

compressional stress sHmax is considered constant andtrends E-W, i.e., roughly parallel to the convergence

direction. In the Western Cordillera of the HighAndes, szzbecomes l.Hmax is s2 and Hmin iss3,trendlngN-S.ln theEasternCordillera, szzbecomes s2,sHmax iss1and trendsE-W,andsHmin iss3 trending N-S.ln theEastern Cordillera,the compressional strike-slip faulting maybeexplained by

aneffect oftopography, between thehighWestern Cordilleraand theSubandeanLowlands.TheEasternCordillera beingundercompensated, its elevation should be lower in anisostatic equilibrium. The change between thecompressional regime intheSubandean Zoneandthestrike­slip regime in the Eastern CordilJera should take placebetween 1000 and 2000 rnin elevation.

In southern Peru,the state of stress is different in theHigh Andes; there,szz is sl , sHmaxis s2 and sHmin is s3and trends N-S, i.e., roughly perpendicular to theconvergence vector (Sebriereral., 1985,1988; Mercier etal.,1992). In the Bolivian HighAndes, during thePliocene (6­3Ma),the tectonic regime was extensional; s3is sHmax andtrendsE-W(Lavenu and Mercier ]991). DuringuppermostPliocene-lower Pleistocene (3 - 2 Ma) a compressionaltectonics affected this region, which is characterized bys1

(rending E-W, parallel to theconvergence. This tectonic e~ent

is characterized by a weak deformation, and by thereactivation of old faults as reverse and strike-slip faults.This stress regime is followed by a nearly coeval N-Strending compressional tectonics. Since lower Pleistoceneto Present, the whole range is affected by an extensionaltectonics with s3 trending N-S (kilometric normal faultswith hectornetric throw). In the Altiplano and the HighAndes, Quaternary tectonic regime is extensional withsHmin= s3 and trends N-S, sHmax= s2 and trends E-W,and s1 is vertical. As in Peru,this stress field resultsfrombodyforces dueto a compensed high topography.The E-Wtrendinghorizontal stresssHmin=s2 is roughly parallel tothe convergence direction; szz (s1) increases with thetopography dueto the rangeload.

The intermediate zones ie.g., Tarija, 1900 m inelevation) are characterized by two superposed stressregimes. Oneisa relatively weakstrike-slipcompressionalstress,withs2vertical,s1=sHmax,E-Wtrending,and s3=sllrnin, N-S trending.The other one,more intensive, isan extensional, axial stress, with s1 vertical;s2 trends E­Wand is equivalent to s3,which trends N-S. [fwe admitthat the vertical stress szz is the resultof the weightof anisostatically compensatedtopography. the strike-slipslateof stress is consistent with the intermediate location ofthe basin, between the Subandean Zone and the HighAndes(Lavenu and Mercier,1991).

Along the Chilean coast. the Quaternary regime isextensional and of an E-W strikes. This deformationcharacterizes the westernmost portionsof the continentalfore-arc, dose to the trenchaxis(80km).Thisdeformationdoesnotappeartobedirecdylinkedtoboundaryforces dueto the convergence. but could be the consequence of co­seismic crustal bending with subduction-relatedearthquakes. Itcouldbe topographic accommodation to theuplift ofthispartofthecoast(bodyforce duetotopography),sxxstrikingE-Wbecomes s3,syystrikingN-S is s2,and szzis s1.The state of stress is such that szz> syy> SIX. This

TECTONIC EVOLUTION OF SOUTH AMERJCA

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phenomenon could be related to the zones of maximumcoupling between the oceanicand continentalplatesin theCentral Andes, which couldactas a buttress zone.

The partition'of the deformation across the plateboundary zone shows that the tectonic regime of theQuaternary ismorecomplex than previously recognized. InthesouthernAndes (Chile), aswell asin thenorthernAndes(Ecuador), the Cord ilIeran segments,linkedtolargestrike­slip faults and high angle convergence obliquity, slidestoward the North. A part of the energy, transmitted fromthe subducting plateto theoverridingplate,is absorbed bythefreeescapeoffore arcslivers, parallel tothe margin.Thelackof important crustal thickening and widening of therange characterizes these parts of the Andes. On thecontrary, in the Cordilleran segments linked to low angleconvergence obliquity, the progressive stop of the lateralmovements is due to buttress zones and the energy isabsorbed by the crustal thickening and widening of therange(Bolivia).

The Subandean Zones of the Central Andes aredominated by a compressional stress regime. In theSubandean Zoneof Ecuador, the Quaternarystress field iscompressional and trends E-W. In the SubandeanZoneofcentral Peru,reverse faulting is in agreementwith an E-Wtrendingcompression, whereasdeformations resultfrom aN-S trendingcompression (sl is sHmaK) in theSubandeanZone of southern Peru. In the Subandean lowlands ofBolivia,deformations arecompressional,withsHmax assxx,is sI,E-W trending.

TheMaranon and Beni basins are respectively situatedat the northern and southern ends of the Peru-BoliviaAndeansegment(Fig. 45A). This segment corresponds tothe flat slab subducrion of the Nazca Plate beneath theAndes. Spedfie structuresofthe foothills of the SubandeanZone controltwo basins,eachone having onlyone outlet,the Amazonas and Madeira rivers, respectively. The flatsurface ofthesebasinsshowsa complexnetwork offlowingand fossil fiver traces.Theseactiveand abandoned fluvialtracesare used, together with neotectonic, seismotectonicand subsurface structural data (Dumont and Fournier,1994), to determine the neotectonic evolution of thePeruvian and Bolivian foreland basins(Dumont, 1996).Thephenomena exemplified below refer to short termneotectonics, occurringduring the Holocene (0.1 - 0 Ma).

In both basins, recent directional shifts of the mainrivers arecontrolled bytheoffsetoffaults,TheUcayali Riverflows northwards along a N-S intra-subandeanbasin, thenenterstheMaranon Basin whereithasbeen deflected to theNE (Fig.45B).The successive deflection points shiftedupstream and along the foothills. The line joining thedeflection points(Fig. 45B, pointsA, BandC)liesjustbehindthe Andean Frontal Thrust .representedherebythe'IapicheFault (Fig. 458).Contemporaneously,theMarafion Riverwasdeftected to the NE, linedup with the straight,NEtrendinglower reaches ofthe Huallaga River, which is controlled byafault observed onsatellite images. In the eastern part of theMarafion Depression,the riverstrendNE-SW,parallelto thestrikeofthemainbasementfaults of theMarafion StructuralZone (Laurent, 1985).Elongated lakes are situated overstructurallydownwarped blocks (Dumont,1993).

Successive shiftsof the BeniRiver (Fig.45C) showthe

northward migrationof the deflection point made by theBeni River enteringthebasin.AN-Strendingfaultcrossingthe foothill margin controls this downstream increment.Thepresentregional stateofstressin the Subandean regionis roughly E-W, exceptin the southernpart of theMarafionBasinwhere it is NE-SW (Assumpcao, 1992). Quaternarynormal faults displaying a NNW-SSE extension in thedistal part of theMaranonBasin, as well as risingofbulgeson the eastern margins ofboth basins are consistentwiththe present-day state of stress. The interpretationemphasizesthat in the distal areas of the Maranon Basinthe river traces are guided by topographic lows alongtensional faults, or basement blocks, uplifted ordownwarped by tensional faulting. Near the piedmont,river shifts are controlled by the increment of faultmovements towardthe basin. However, the effectof faultsin the near piedmont is more difficult to explain than inthe distal areas. Topographic effect between the HighAndes and the foreland basin (Assumpcao and Araujo,1993) may explain that the geometry of active faults onthe foothills piedmont depend also on the local trend ofthe Cordillera. The interpretation of the successive shiftsinvolves the knowledge of paleoclimate oscillations. Itappears that a river moves toward a new formeddepocenterat the onset of awetterperiod,and can stay inplace, even if tectonic deformation progress, duringrelatively dryer periods (Schummetal., 1998).

In summary, the study of the Recent state of stress inthe Andes shows several types of behaviour of thecontinental platealongtheactivemargin.Thisbehaviourislinked to the dip of the subdueted plate, the obliquityofconvergence between theoceanic andcontinental plates, thebodyforces and boundary forces, the presenceor absenceofbuttress zones in the upper plate,and the possibility forthe coastal blocks of freeescaping(Fig. 46).

InEcuador,where theconvergence obliquity isveryhigh(g""O 31°to 45°),the Coastal Block is pushed northwards,and is affected by a N-5 trending extensian. Since theelevation of the Andean range is relatively low, thetopographic effect is weak. The rangeis separatedfromtheCoastal Block by a large strike-slip fault, and an E-Wtrendingcompressional stressdeveloped, due to boundaryforces.

In Peruand Bolivia, convergence obliquity is relativelylow (g = 20" - 24°). The HighAndes, which present thehighest average altitude. are affected by a N-S extension,mainlydue to the bodyforces. Acompressional regimeisobserved only alongthe boundarybetweenthe rangeandthe Brazilian Shield (boundary forces).

In centraland southernChile, convergence obliquity isintermediate (g =22° to 30°). The fore-arc and intra-arczonesof the Cordillera, the topography of which is lowerthan in Peru and Bolivia, are affected by a N-S to NE-5Wcompression.

Regarding theuplifted marineterraces, thetypeandrateof vertical movements are thought to be relativelyindependenton the rate and directionof convergence, theconvergence obliquitj.and theageand dipofthe subductedplate(Machare and Ortlieb, 1993).Conversely, theseverticalmovements aretightly dependenton the morphology ofthesubductingplate(aseismic ridges),andonthestructureand 537

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i~1densitydistributionin theoverridingplate.InChile, aswellasin Peru, thevertical motionofcoastalareas isdependenton the distance from the coast to the trench, and thisdeformation characterizes the westernmost portionsofthecontinental fore are, 80 km close to the trench axis.Therelationships between Quaternaryvertical movements andseismic activity are stillpoorlyunderstood.

CONCLUSIONS : GEODYNAMIC

PROCESSES OF THE NORTHERN­

CENmAL ANDEAN OROGENY

Plate Kinematics Framework

The evolution of the central and northern Andeansystem canbedividedinto rourmainperiodswithdifferentsedimentary, tectonic and magmatic characteristics,indicating distinctive geodynamic situations andconvergence directions.

Late Permian - LateJurassic:Tethyan Period

TheEarly Mesozoic evolutionof the Andeanmarginisinfluenced bythe 'Tethyan riftingand evolution (Iaillard et

a/., 1990, 1995),and is characterized by the onset of asoutheastwards subduction along the Colombian,Ecuadorian and Peruvian margins (Fig. 47),Afterthe LatePaleozoic coalescence of Pangea, the Triassic evolution ofthe northern and central Andes was dominated by anextensional regime responsible for the creationof grabensandtheextension o(alkalinevolcanics. Thistectonic contextisclearly related tothe westward propagation oftheTethyanbreak-upbetweenLaurasia and Gondwana.

The Early Jurassic evolution of the northern CentralAndes was dominated by the destruction of the LateTriassic-Liassic carbonate platform, caused by a generalextensional tectonic activity that progresseddiachronously southwards. This is thought to have beeninduced bythe riftingof the E-WtrendingTethyan system.Meanwhile, no significant absolute motion of the SouthAmerican Plate occurred relative to the surroundingcontinentalplates (Africa, NorthAmerica).

Between late Early and early Late Jurassic times, theTethyan breakline resulted in the openingof NE-trendingoceanic-floored rhombochasms (Alpine andcentralAtlantic'oceans, BernouUi and Lemoine, 1980) linkedby Eto ENEtrending sinistral transform zones (e.g., the CaribbeanTransform Zone).Theopeningofthe centralAtlantic Oceanbeganbefore thelateMiddleJurassic (I '57 Ma,Klitgord andSchouten, 1986) and possibly as earlyas latestLiassic (l80Ma, Scotese et al., 1988). In the Andes.this period (190­

140 Ma)wasmarkedbythe emplacement ofl-type plutonsand calc-alkaline volcanics along the NNE trendingEcuadorian-northern Peruvianmargin,whichshouldhavebeencoeval withan activesubductionbeneath this part ofthe Andean margin. According to the pre-break-upreconstruction, thissituationcanbeinterpreted intwoways:

1 - ]f the Colombian segmentwas facing continentalblocks, thesubductionmusthaveinvolved the newoceaniccrust of the Tethyan arm created between the Colombiansegmentand these blocks (Jail lard et al., 1990; Litherlandetal., 1994).

2 - ]f the Colombian segmentdirectly faced theoceanicpaleo-Pacific Plate, subductionwasprobablyactivebeforethe Early Jurassic, and the creation of a magmatic arc mayhave resulted from more rapid subduction, due to anaccelerated accretion rate in thepaleo-Pacific system.

Whatever the case, the roughly southeastwardsubduction beneath the Ecuadorian segment must haveinduced oblique subduction along the Peruvian margin,associatedwith a strong sinistral strike-slipcomponent,manifested bythe creation of thelargeNW trendingsouth­Peruvian turbiditicpull-apart basin (Vicente et al.• 1982)and by transtensional features in the back-arc zones(Sempere et al., 1998; Fig. 47). The Kimmeridgian­Berriasian time-span is transition period. Along theColombian-Ecuadorian segment,this period was markedby accretions of displaced terranes, compressionaldeformation,and the end ofmagmatic activity, whilealongthePeruvian segment varied tectoniceventswereassociatedwiththeresumption of subduction-relatedvolcanic activity(Aspdenetal., 1987; Jaillard etal.; 1990, 1995).A11 thisclearlyresulted from an impcrtallt, global-scale geodynamicchange. In the west-Tethyan realm(centralAtlantic.Alpineoceanic ridges), spreading rates significantly decreased(Olivet eta/., 1984; Klitgord andSchouten, 1986; Savostin etaL, 1986}.lfa Tethyan-Colombian oceanic arm didexist,themotionvector of the Phoenixoceanic platewas thesum ofthe expansion vectors of the Tethyan and Pacific ridges(Duncan and Hargraves, 1984). As a result, slowdown ofTethyan expansion would have induced a northeastwardconvergence between the Phoenix and South Americanplates(Duncanand Hargraves, 19&4; laillardet aI., 1990).

Moreover, the outpouringof a largeoceanicplateaualongthePacific Ridge in Tithoniantimes mayhavemodifiedtheaccretiondirection oftheEast-pacific paleo-plate (NakinishietaI.,1989).

Early Cretaceous-Paleocene:SouthAtlantic Period

During this period. the development of the SouthAtlantic Ocean controlled the westward drift of the SouthAmerican Plateand the variations in the convergence ratealongthe subduction zone. Theseare thoughtto determinethe sedimentary, tectonicand magmatic evolution of theAndeanmargin.During the Early Cretaceous, the suddenarrival of a great amount of east-derived sands can beinterpreted as the resultof the westward domingof easternSouth America due to the incipient rifting of the SouthAtlantic Ocean. Although no reliable geodynamicreconstruction is available, the lackof significanttectonicor magmatic activityalong the Pacific marginof the SouthAmerican Plate N of 18°S would indicate a slow, steep­dippingsubduction ofthe paleo-Pacific slab.

The definitive openingof the SouthAtlantic Oceanatequatorial latitudesduringAlbian times(Emery andUchupi,1984; Scotese et al., 1988) induced the beginning of the

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TECTONIC EVOLUTION OF SOUTH AMER.ICA

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absolute westward motion of the South American Plate.Therefore, as noted by various authors (Prutos, 1981;Megard, 1987; Soler and Bonhomrne, 1990), the beginningof compressional deformation along the Peruvian andColombian segments duringthe Late Albian (100 - 95Ma)coincides with the onset of the trenchward motion of theupperplate(Uyeda and Kanamori, 1979; Cross and Pilger,1982; Jarrard,1986).

TheAlbian-Turonian periodcoincides with a periodofhigh convergence rate and with the mid-Cretaceousmagnetic quietzone(Larson, 1991). In theCentral Andeanmargin, it ischaracterized byimportantmagmatic activity,ahighaverage subsidence rate(Jaillard andSoler, 1996) andprobably significant dextralstrike-slip movement (Busseland Pitcher, 1985). The latter are probably related to thenorth-northeasterly motionassumed forthe paleo-Pacificslab during Late Cretaceous times (Pilger, 1984; Gordonand lurdy, 1986; Pardo-Casas and Molnar, 1987; Fig. 48).This convergence direction accounts for the lack ofCretaceous arc magmatism along the NNE trendingEcuadorian margin. The NW trending subduction zonealong the Peruvian margin extended probablynorthwestwards intotheoceanic domain asanintra-oceanicsubduction zone allowing the development of Cretaceousislandarcs,suchas thoseknown inwestern Ecuador.

The Coniacian-late Paleocene interval wasmarked byasignificant slowdown in the convergence rate (85 - 75Ma),followed bya periodoflowmeanconvergence ratebetweenthe Phoenix and SouthAmerican plates(80 - 58 Ma, Solerand Bonhomme, 1990). This period was characterized,however, by the beginningof the Late Cretaceous Andeancompressional events. In the northern part of the studiedarea,significant deformation wasrestricted to thefore-arcand arczones. However, the LateCretaceous andPaleogenetectonic eventsare coeval witha noticeable decrease ofthesubsidence rate in the back-arc areas, which favoureddetritital deposits, sedimentary hiatusandunconformities,Thissuggests that duringthisperiod ofoblique subduction,most of the convergence was accommodated by lateraldisplacements of fore-arc slivers along the edge of themargin, rather than by shorteningand thickening of theupper plate. In the southern Peru and northern Chile,however, deformations seemto have beenmoreimportant,and a significant increase of the subsidence rate in Boliviais interpreted as the result of a foreland-type subsidencerelated to the deformation and tectonic loading of themargin (Sempere, 1994).

Late Paleocene-late Oligocene:Transition Period

The late Paleocene to late Oligocene interval (55 - 25Ma) isa keyperiodin thewholeAndean evolution. Displacedterranes wereaccreted or obducted along the Colombiansegment, importantcompressional deformation occurredin the Andean realm and sedimentary gaps andunconformities occurred in theeasterndomains.

These events coincidedwith global plate kinematicreorganization (Scotese et al., 1988). In late Paleocene­Eocene times, the convergence of the paleo-Pacific platechanged from Nor NNE to NEor ENE (Pilger, 1984; Pardo-

Casas and Molnar, 1987), provoking the change from adominantly dextral transform zone to a nearly normalconvergent regime in theColombian-Ecuadorian segment(Figs. 47 and 48). Such dramatic changes explain howterranes that were previously situated W of the Andeanmargin, were drifted eastwards andaccreted to thenorthernAndean margin at that time (Jaillard et al., 1995). On theotherhand,themore easterly convergence direction allowedsubduction to take place beneath the NNE trendingEcuadorian margin and triggered the resumption of arcmagmatism in this areabyearlyEocene times.

A second major reorganization occurred by lateOligocene times, astheFarallon Plate splitted intotheCocosandNazca plates (WortdandCloething, 1981 ).Convergencedirection evolved from ENE to nearly W-Eandconvergenceratesubsequently increased (Pilger, 1984; Pardo-Casas andMolnar, 1987; Tebbens and Cande, 1997; Somoza, 1998; Fig.48).As aconsequence,thisperiodismarked byasignificantincrease of the orthogonal component of the convergencevelocity between the paleo-Pacific oceanic plateand theAndean margin. The subsequent increased coupling alongthe subduction zone was responsible for a significanteastward migration of the deformed zone, which involvedthe former arc zone and proximal back-arc areas, i.e, thepresent-day Eastern Cordillera of Ecuador, WesternCordillera of Peru andBolivia, and eventually theEasternCordillera ofBolivia. Thissuggests a significant decrease oftheplayoflateraldisplacement along theAndean margininthe accommodation of convergence, and a correlativeincrease oftheshortening andthickening oftheoverridingcontinental plate.Note thatthisperioddoesnotcorrespondto thesubduction ofa younger plate(Fig. 49).

Late Oligocene to Present:Pacific Period

From thelateOligocene onwards, the northerncentralAndean margin wascompletely controlled bytheWtoWNWmotion of SouthAmerica and the E to ENE motion of thepaleo-Pacific Plate, that determined a roughly E-Wcoupleand a nearly normal subduction system (Fig. 48).Duringthis period, the subducting slab is rejuvenating, theconvergence rate is relatively high (Fig. 49) and aseismicridges arrived in the subduction zone (Fig. 3). Thisgeodynamic pattern,which remains relatively stableanddiffers significantly fromthe preceding ones,correspondsto the classical Chilean-type convergent margin.

From 30 Ma onwards, the age of the oceanic platerejuvenated slightly,becoming probably morebuoyant,andfavouring, therefore, a low-dipping angle of subduction.Although theconvergence ratedid notchange significantly,late Oligocene-early Miocene times are marked by anacceleration, while the Pliocene is marked by a slightdeceleration (Fig. 49).Correlation oftheseratevariations isdifficult to link with specific tectonic events. During thisperiod, thedeformed zone significantly migrated eastwardsand enlarged, eventually involving crustal-Kale thrustingin thePeruvian and Bolivian partsofthechain (Figs. 51 and52).Due to thislarge-scale thrust movement, thechainwasoonsiderably uplifted, most ofthepresent-dayaltitude beingacquired during the last 8 to 9 Ma.On the other hand,

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subsidence in the eastern basins of Bolivia and southernPeru increased, due (0 ilexuralloading. In r;oaCrast,subsidence didnotincrease in Ecuador and even decreasedin northeastern Peru (Fig,39), thus indicating that thetectonic processes differ significant!y in both regions.Meanwhile, thefore-arc zones subsided considerably, insucha manner that Pliocene outer shelf deposits are foundpresently at more than 4000 m below sea-level, indicatingan average subsidence rateofabout 1000 m/Ma.

In spiteof a nearly orthogonal convergence direction,the rate of dextral displacements of fore-arc slivers orterranes of western Ecuador are high (0.5 cmly). Thissuggests that suchmovements maynave beenmuch higherin the Late Cretaceous, when convergence rate was muchhigher(Fig.49) andmuchmoreoblique thanin theNeogene(Fig. 48). In the same way, local rotations have beensignificant during the Neogene, demonstrating thatcontractional shortening was a leading process in thethickening and bending of the Altiplano Orocline,andsuggesting thatthe cumulated amountofrotation may havebeensignificant since the Cretaceous.

Role of kinematic parametersin the Andean Orogeny

Most classical geodynamic models forthe originof thetectonic phases in continental active marginsarebased onthe observation and comparison of various present-dayactive margins (Uyeda and Kanamori, 1979; SchoU et al.,1980; Uyeda, 1982; Cross and Pilger, 1982; Jarrard, 1986), orthroughphysical modelling (Bottet al., 1989; Whitta~r etal., 1992; Cloos, 1993; Shemenda, 1994). Only a few havebeenelaborated throughthestudyofa single activemarginevolution through a longperiod of time.The study of theAndean margin since earliest Mesozoic times, however,provides some geological constraints on the origin andnatureofthetectonic phasesofcontinental active margins.

Plate tectonic reconstructions arepoorlyconstrained fortheLate Cretaceous period(Pardo-Casas and Molnar, 1987),especially as regards subduction of ridges. dip of thesubductingslab and direction of convergence. How~er,quantitative approximation ofsomeparameters,sucha. theconvergence velocity (Soler and Bonhomrne, 1990) deducedfrom theglobal spreading rates(Larson, 199I), the absolutemotionof the SouthAmerican Platedrivenby theopeningand ridge activity of the SouthAtlantic Ocean (Niirnbergand Mtiller, 1991) and the age of the oceanic slab whiiesubducted,calculatedby Soler et al. (1989),allow us toanalyze them in relation to the earlytectonicevolution ofthenorthernCentral Andean margin.

Age of thesubducted slabClassical models assume thatthesubduction ofayoung,

buoyant oceanic lithosphere induces a contractionalstrainin the overriding continental plate (Molnar and Atwater,1978; Cross andPilger, 1982; Sacks, 1983).Acrording toSolerer al. (1989),the beginning of the contractional period(Albian) and tbe late Oligocene to Recent contractional

phases roughly coincide with the rejuvenation of theoceamcp/<Ite rubdua,:ag a:( t1t<It time. However, the Lste

Cretaceous and major Paleogene shortening phasesoccurred duringacontinuousincreaseintherelative ageofthe subducted slab(Fig. 49).Therefore. thelithosphereageof thesubductedslabmaycontributeto the appearance ofalong-termed contractional regime, butcannotaccount forshort-termed shortening phases.

Absolute trenchward motionof theoverriding plate

As rIMed or illJity Jutnors, the opeiticrgo! the Soudt

Atlantic Ocean at the equatorial latitudes during Albiantimes,which provoked the beginningof the westward shiftof the South American Plate, roughly coincides with theinitiation of the contractional deformation along thePeruvian-Ecuadorian margin.Thus,this parameterseemsto control the long-termed contractional regime of thecontinental active margin.

Asemphasized bySebrier and Soler (1991) for the lateTertiary Andean contr'lctional phases, only a slightshorteningoccursin theAndean retro-arcforeland during(he periods of teemII ir: qll.iescence, and chen most af the

westward drift of the SOuth American Plate should beaccommodated byan absillute westward overriding of thecontinental plateovera retreating oceanic slab. Ontheotherhand, the amount of tectC\nic shorteningobserved duringthe contractional phases implies that virtually all thewestward driftoftheSouth American Plate isaccommodatedby the shortening. Ther~fore, during the contractionalphases, the western COntinental margin of the SouthAmerican Plate is virtually motionless in an absolutereference frame(i.e., then:is a stopping of the slabretreat).This recurrent stopping of tileslab retreat, the mechanicalorigins of which are unclear, mightbe one of the drivingphenomenon oftheshort-lived contractional tectonic crisis.

Collision of continental oroceanicobstacles

It hasbeen proposed that the arrival in the subductiontrenchofoceanic or continental obstacles (aseismic ridges,sea-mounts,continental Jnicroplates) will lead to blockingofsubduction,contractional deformation ofthecontinentalmarginand piate reorganisation (Schoff etat., Hl«<J; Crossand Pilger, 1982; Ben-Avraham and NUT, 1987). Accordingto Cloos (1993), onlycontinental blocks andoceanic islandarcs with a crust more than 15to 20km thick or basalticplateaux of more than ~O km of crustal thickness willprovoke a jam in the subduction zone. The currentsubduction of the 15km-thickinactive Nazca Ridge resultsin theextensive subduction erosionofthefore-arc, andlocalupliftofc.900m associated withonlymoderatehorizontalcompressional stress(Couch and Whitsett, 1981; Machan:and Ortlieb, 1992). Thus, the arrival of moderately highobstacles in the trench seems to have moderatedeformational effects on the upper active margin.On theother hand,the accretions of oceanic islandarc terranesofCoastal Ecuador (Santonian,Iate Paleocene,late Eocene) arecoeval with contractional phases observed in Bolivia,

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TECTONIC EVOLUTION OF SOUTH AMERICA

l~!southernPeru and northern Peru where no collisions areknown to haveoccurred. Thus, in this case, it seems thataccretions or collisions of terranescannot be the causeofregional contractional phases. In the studiedarea, the factthat contraction took place in non-accretionary settingsat the sametime as accretions occurredsuggests that theaccretion events and the coeval contractional phases areconsequences ofa sameglobal geodynamic mechanism.

Convergence rateAccording to Uyeda and Kanarnori (1979), Cross and

Pilger (1982); Pardo-Casas and Molnar (1987), a rapidconvergence between the oceanic and continental platesprovokes a compressional stress in the latter. According toSoler andBonhomme (1990), periodsofhigh convergencerates along the Peruvian margin occurred in Albian­Campanian and lateEocene-early Oligocene times,whichcoincide roughly with mainly contractional tectonic periods(Fig.49). However, ratherthanwiththeconvergence velocityitself, theshort-Iived tectonic events seemtocorrelatebetterwithchanges in the convergence velocity, whichever theirsign,eitherpositive (acceleration) ornegative (deceleration).If the reconstruction of Solerand Bonhomme (1990) iscorrect, acceleration occurred inLate Aptian (L10 Ma), LateCampanian (75Ma),early to middle Eocene (50Mal.latestEocene (38Ma), andlateOligocene-early Miocene times(25- 20Ma). whereas deceleration occurred in the LateAlbian(100 - 95 Ma), Santonian (85Ma), middle-late Eocene (42Ma), and Pliocene (4 Ma). All these periods coincide withapparently extensional (LateAptian, early-middle Eoceneboundary) or important contractional tectonic phases.Therefore, short-lived contractional phases as well asextensional tectonic events seemtobe mainly controlled bychanges in the convergence velocity.

Direction of convergence

Thegeometryofthegeodynamic reconstructions aretoopoorly constrained toallow avaluable discussion fortheLateCretaceous. TheIncaic contractional tectonic phases oflatePaleocene (58 - 55Mal.latemiddle Eocene (43 - 42 Ma)andlate Oligocene age (26 Ma) coincide with successiveclockwise rotation in the direction of convergence (Pilger,L984; Pardo-Casas and Molnar, L987; Mayes et ai; 1990;

Tebbens andCande, 1997). These changes intheconvergencedirection. which caused successive significant increases ofthenormal convergence ratesseemto havethesame effectsas thoseassumed fora convergence acceleration.

Moreover, the important changes in the convergencedirection from NNE to ENE by late Paleocene must haveinduced drasticchanges in subduction geometry.TheNNEtrending Ecuadorian margin changed from a mainlytransform to a chieOy convergent regime. This must haveinduced theeastward drift and accretion of oceanic islandarcs along the Ecuadorian margin and the birth of newsubduction zones to the W of them (Jaillard et al., L995).

The changein the convergence direction of late middleEoceneagealsoresulted in a newevent ofcollision ofislandarcs along the Ecuadorian margin (Bourgois et aL, 1990;

Hughes and Pilatasig, 1999). Thus, changes in the

convergence direction not only control the normalconvergence rate, but also playa part in the regionalsubduction patternthatcouldinturn influence thetectonicregime. Such changes in the convergence direction duringthe Paleogene can explain the contemporaneity of thecontractional events in non-accretionary settings of theCentral Andes and thecollisions ofisland arcs.

Relation convergence rates ­subsidence

In northern Peru,periods of slow plate convergencecorrelate withlow subsidence rates(L30to 110 Ma, 75to45Ma, 35 to 25 Ma). Conversely, periodsof highconvergencevelocity arecoevalwithperiods ofincreased subsidence rate(110 to 85Ma, 50to 40Ma; figs. 49 and SO). Thiscannotbeexplained by increased subduction erosion of the deepcontinental margin (Von Huene andScholl.Issl), becausethislattermodel isonlyproved toaccount forthesubsidenceof the fore-arc or arc zones, whereas increased subsidenceis observed as far as the easterndomainbetween 110 and85Ma(Contreras et al., 1996;figs. 39 and SO). In contrast,theseobservations are consistent with the thermal modelof Mitrovica et al. (1989), that assumes that a fastconvergence provokes an increase of the subsidence ratesalong the whole continental margin, through mantleconvection (Gurnis, 1992; Sternand Holt, 1994). Thelackof suchcorrelation in southernPeruis most probably duetothefact thatcontractional tectonic events occurred earlierandwere stronger than innorthern Peru.There.tectonic upliftof the marginby crustal shorteningand thickening, andoverload tectonic subsidence of the foreland would haveprevailed since Senonian times (Sempere, 1994).

Dipof subduction and subductionerosion

The continentward shift of the volcanic front isinterpreted classically as a result of the shorteningof thecontinental margin, either by compressive tectonicshortening, or by subduction erosion (Scholl et aL, 1980).

DuringAlbian andea.rly Late Cretaceous times,thelocationof the magmatic arc of Peru was stable, indicating thatneither significant shortening nor subduction erosionoccurred at this time (figs. 20 and 51). The eastward shiftofthemagmatic arcintheLate Campanian canbeexplainedmainly by the tectonic shortening related to the majorPeruvian phase. As a consequence. it seems that nosignificant subduction erosion took placein Peru beforelatestCretaceous, and possibly before Paleocene times,asindicated by the relative stability of the magmatic arclocation before this period.

In Eocene times, the ongoing eastward shift of themagmatic belt is associated with its abrupt wideninginterpreted as the result of a widening of the melting zonein the asthenosphere wedge linkedto a decrease of thedipof the BenioffZone, in turn controlled by the normalconvergence velocity (Soler, 1991). The coeval rapidextensional subsidence observed in most of the fore-arcregions byearly middle Eocene times is toowidespread toresultfrom local tectonic events orpaleogeographic effects.

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TECTONIC EVOLUTION OF SOUTH AMERICA

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