Evol.geodinamica.andesCentralesyNorte.mesozoico.temprano.intermedio1

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, Lt 'CL A, " I 3ii"-Journal of the Geological Society, London, Vol. 141, 1990, pp. 1009-1022, 9 figs. Printed in Northern Ireland

Geodynamic evolution of the northern and central Andes during early to

middle Mesozoic times: a Tethyan model

ETIENNE JAILLARD1,3, PIE RRE S OLER', GXB RIË GCA RLI ËR~__& THoMAs MOU RI ER~-----

OR S T OM, UR l H , 213 rue La Fayette, 75480Paris Cedex 10, France

' 2 rue Richer, 75009Paris, France

3Formerly at: Institut Français d'Etudes Andi nes, Lim a, Peru and Laboratoire de géologie alpine, UA CNRS 69,

Grenoble, France

Abstract: The early to middle Mesozoic sedimentary and magmatic history of the northern andcentral Andes (i.e. the "E-trending Colombian-Ecuadorian segment, and the NW-trending Peru-vian segment) exhibits a spatially contrasted evolution involving several successive tectonic andgeodynamic settings.

The late Triassic to late Liassic period began with a widespread marine transgression. From the

latest Triassic, the Colombian marine shelf was progressively destroyed by southward propagating

extensional tectonic activity but marine sedimentation continued in Peru. During this time, nosignificant magmatic activity is recorded except in the emerging Colombian area. This period is

interpreted as the result of rifting of a Tethyan oceanic arm which separated the Colombian andpalaeo-Mexican margins.

From latest Liassic, an important calc-alkaline magmatic arc developed along the emergent

Colombian segment. Further south, the north-Peruvian shelf probably emerged, but marine sedimen-tation continued in southern Peru. During middle and early Late Jurassic times, the Colombiansegment was characterized by important magmatic activity and by coarse clastic continental sedimen-tation. Along the Peruvian segment, a turbiditic trough, emergent areas, and continental basins were

created, and the scarcity of calc-alkaline magmatism suggests that only very local subduction tookplace. This period is regarded as one of the southeastward subduction beneath the Colombiansegment, and of Tethyan oceanic crust originating in the newly formed 'Colombian' oceanic arm. This

pattern would have induced a chiefly left-lateral transform motion along the Peruvian segment.By Kimmeridgian-Tithonian times, the palaeogeographic framework had drastically changed.

Along the Colombian segment, magmatic activity ceased, and continental accretions occurred along

dextral strike-slip sutures. In Peru, tectonic activity was recorded by the creation of a new turbiditictrough and by the resumption of detrital sedimentation. In the coastal area, arc-related volcanismindicated that subduction took place beneath this segment. This geodynamic change is interpreted asthe result OE a sharp decrease in the spreading activity of the Tethyan ridges and replacement byPacific spreading centres inducing a roughly northeastward convergence direction.

The central Andes are considered a typical example of anorogenic belt related to the subduction of an oceanic platebeneath a continental margin. Many authors consider thatsubduction beneath the South American continent has takenplace continuously since either late Palaeozoic or Liassictimes (James 1971; Audebaud et al . 1973; Mégard 1978,1987; Dalziel 1985). Nevertheless, others (Dalmayrac et al.

1980; Martinez 1980; McCourt et al . 1984; Aspden et al .1987) have suggested that the Andean margin may haveacted as a transform zone during part of Mesozoic times.This latter option is supported by new data relating to thesedimentary, tectonic, and magmatic evolution of thenorthern and central Andes during Jurassic times (seeAspden et al. 1987; Mojica & Dorado 1987; Mourier et al .1988a; Jaillard & Jacay 1989). The aim of this paper is tosynthetize the stratigraphical, sedimentological, tectonic andmagmatic data, in order to propose an integrated, althoughprobably oversimplified, model of the geodynamic evolutionof the central and northern Andes during Triassic andJurassic times.

The Huancabamba deflexion, situated at c . 5"s(Fig. 1)

roughly separates the Andes into: (1) a northern, NE-to"E-trending, 'Colombian' segment whose western part ismade up of allochtonous terranes accreted during Mesozoicand Tertiary times (Feininger & Bristow 1980; McCourt etal . 1984; Aspden & McCourt 1986; Aspden et al . 1987;Bourgois et al . 1987; Feininger 1987; Mourier et al . 1988b);and ( 2 ) a southern, NW- to "-trending, 'Peruvian'

segment, where no exotic blocks have been recognized(Mégard 1987; Beck 1988).This paper is concerned only with the evolution of the

'integral', non-allochtonous South American continentalplate which can be conveniently divided into severalpalaeogeographic domains (Fig. 1): (1) the easternsedimentary basins, now exposed in the subandean zone andOriente, which were bounded eastward by the Brazilian andGuianese Precambrian shields; (2) the axial Cordilleras(Central Cordillera of Colombia and Eastern Cordilleras ofPeru and Ecuador) which acted as swells during part ofMesozoic times; (3) the western Andean basin (orwest-Peruvian Trough) which is now exposed along thePeruvian western Cordillera; and (4) the Altiplano which

1009 ORSTOM Fdnds Documg@rg



E. J A I L L A R D E T A L .

a o w

VENEZUELA

AND

SOUTHERN COLOM

NORTHERNMOST

NORTHERN

CENTRAL PERU

SOUTHERN PERU

- ' AXIAL CORDILLERA ALLOCHTONOUS TERRANES

WEST PERUVIAN TROUQH PRECAMBRIAN SHIELDS

Fig. 1. Major tectono-stratigraphicunits of the northern and central

Andes (mainly after Cretaceous outcrops).The present paper only deals with the evolution of the integralSouth American margin. i.e. the non-allochtonous units.

constitutes a distinct. southern structural zone. and developssouthward into Bolivia.

In this paper. the time-scale of Haq cf al . (1987) is used.

Evolution of the northern and central Andes duringEarly to middle Mesozoic times

The middle Triassic (240 o 220 M u )

The Hercynian cycle ended with the deposition ofcontinental red beds of late Permian to middle Triassic age(Pre-Payandé, EI Sudan and Luisa Formations of Colombia:Geyer 1973: Macia er ul. 1985; Mitu Group of Peru: Mégard1978; Dalmayrac et al. 1980) (Fig. 2) .

In Colombia. a few synkinematic plutons of this age arerecorded and they have been related to suhductionprocesses, to extensional tectonics (Macia & Mojica 1981 .

or to sinistral wrenching (McCourt et al. 1984, Aspden ct al.

1987). In Ecuador. no intrusions of this age are recorded(Kennerley 19M; Baldoch 1982).

In the Eastern Cordillera of Peru. cnlc-alkalinegranodiorites and granites (Carlier er t i l . I%'?), and alkalineto peralhaline comendites. basalts and syenites (C d ie r ct t i l .

1982: Mégard 1987) are present. These rocks are interpretedas not directly related to subduction processes; rather theywould be associated to intracontinental, rifting-typeextension (Kontak et al . 1985). associated with the offset ofNW-trending normal faults (Dalmayrac er al. 1980).

In southwestern Peru. a thick sequence of undatedandesitic flows (Chocolate and Junerata Fms, Fig. 2) overlieearly Permian rocks and is capped by early Sinemurian

limestones (Benavides 1962; Vicente 1981: Vicente et u¿.1982). It exhibits the characteristics of a continental volcanicarc (James er al. 1975; Boily et al . 1984). We regard theseformations as pre-Norian.

Norian to Hettungiun (220 o 200M a )

In Co!omhia, Norian massive platform limestones (PayandéFm. Fig. 2) are restricted to the southern areas (Fig. 3;Kummel & Fuchs 1953; Geyer 1973: Mojica & Dorado1987), and these locally grade upward into black-colouredbreccias and olistolith-hearing limestones and shales ofRhaetian age (Mojica & L h a s 1984). Then most of the areaemerged and continental red beds intercalated with andesitic

to acidic volcanic rocks were deposited along "E-SSWtrending grabens, indicating an extensional tectonic activity(Macia er al. 1985; Mojica Xr Dorado 1987). Plutons of thisage are known only in the central part of Colombia (Aspdenet al . 1987).

In Ecuador. though the Norian stage has not heenrecognized, its presence is assumed (Geyer 1982)at the haseof the thick Santiago Formation which is partly ofSinemurian age (Tschopp 1953: Geyer 1974: Bristow &Hoffstetter 1977). No coeval intrusions are known within thecontinental margin (Baldock 1982, Hall & Calle 1982).

In northern and central Peru (Fig. 3), a firsttransgression took place during the Norian (Mégard 1968:Geyer 1982; Pardo & Sanz 1979: Prinz 1985). and isrecorded by massive. cherty, dark-coloured platformlimestones and dolostones of late Triassic age, which formthe basal part of the Pucara Group (Chambara Fm : Mégard1968: Loughmann & Hallam 1982. Fig. 3 ) . The second,early Liassic transgression led to the deposition of

bituminous. sandy and cherty. black-coloured limestones ofHettangian to Sinemurian age (Aramachay Fm: Megard1968; Palacios 1980: Prinr 1985). deposited in a deep shelfenvironment (Pardo 61 Sanz 1979; Loughman &k Hallam1982). Differential subsidence was controlled by NW-trending normal faults (Szekely L! Grose 1972: Megard1978).

In southwestern Peru (Fig. 3) . no late Triassic to earlyLiassic marine sediments are known.

Sinernurian to uppermost Liassic ( 3 0 o 180M u )

In Colombia (Fig. 4). the limestones of the Sinemurianmarine transgression are restricted to NW-trending (fault-controlled'?) troughs (Bata and Morrocoyal Fms: Geyer1973; Maresch 1983: Mojica 61 Dorado 1987; Fig. 2) . Theother parts of Colomhia remained emergent and receivedred. continental sedimentation (Saldaña and La QuintaFms: Mojica & Dorado 1987). Throughout Colomhia,post-Sinemurian deposits are essentially of volcanic ordetrital continental origin: however. poorly dated. paralicsediments arc reported locally (Fig. 2. Bata, El Indio. andMontchel Fnis: Geyer 1973: hlojica & Dorado 1987).



STRATIGRAPHY-M. A-

130

140

150

160

170

180

190

200

21 0

220

-

STAGES~

VAL.Wpc EARLY-

BERR

TITH.

CALL-_

o BAT.

Q AAL.er

TOAR7

RIEN

EARLY-INE.

HETT.

v j RHAE

LATE NOR.a

CARN

COLOMBIA IECUADOR~ PERU

COARSE DETRITAL DEPOSITS SHAL ES VOLCANIC ROCKS

......................SANDSTONES EdLIMESTONES MAR INE D EPOS TS

Fig. 2. Stratigraphic framework of the early to middle Mesozoic deposits of the northern and central Andes.

1

xON

CI

mcO

e

z

El

2ö

+zU

z

Or

h)O

m

mm

o



1012 E. J A I L L A R D E T A L .

-r

B O W

1-

70W

N ik 1 0 N

t10 s

. .

. .. ... . ..

: + ,:-losi. . ... .

NORIAN TO HETTANGIAN

2 2 0 to 2 0 0 Ma

BATHOLITHS

RADIOMETRIC AGES ) :A <....................

MARINE CARBONATE SHELF:....................... COASTAL FACIES

_----------_ _ _ _ _* ' VOLCANO-SEDIMENTARY, XZssx

SHALLOW MARINE_ _ _ - -Kj.y CONTINENTAL BEDS E___-p j

DEEPER MARINE- -- - .

Fig. 3. Palaeogeogrriphic reconstructionof the northern lind centralAndean margin during Norian to Hettangiiin times.

quiet shelf sedimentatinn continued in the south.

The northern areas (Colomhia) progressively emerged whereas

Some 198 to 191Ma monzonites. granodiorites andgranites of poorly known composition occur in the northernpart of Colomhia. principally in the Santander massif (Fig.4). and have been related either to ;ipeculiar subductionprocess. or to ensialic rifting (Aspden er ul. lYS7). Theseintrusions seem to cease after Sinemurian times. Neverthe-less. these ages could represent older agcs reset bysubsequent intrusions (Aspden er t i l . 3987).

In Ecuador. the Sinemurian horizons in the SantiagoFormation exhibit flysch-like facies (Geyer 1974). Towardthe west. intercalations of volcanic breccias and tuffs arepresent (Tschopp 1953: Bristow ¿k Hoffstettcr 1977). hut

(Mégard

FIO N

t o

t i o s

Liassic intrusions have so far not been reported in Ecuador(Hall & Calle 1983: Baldock 1982).

In northern and central Peru (Fig. 4) , Sinemuriandeposits consist of bituminous. cherty limestones repre-senting a relatively deep marine environment (AramachayFm: Mégard 1968; Fig. 1).They locally disconformablyoverlie the Hettangian sediments (Wilson Cy r Reyes 1964)

and often contain significant amounts of sandy material

1968; Palacios 1980). Calciturbidites have been

T

80 wT

70W

F i

...

SINEMURIAN TO

UPPERMOST LIAS

i 10 si

L P

20 S-I19 2 \

200 to 180 Ma__ - -_ _ _- -II,ATHOLITHS (WITH

E]INEMURIAN-LATEST

_----

RADIOMETRIC AGES) LIA SSIC LIMESTONES171OLCANO-SEDIMENTARY - -- SINEMURIAN MARINE

v - ~ ONTINENTAL BEDS LIMESTONES

-.-.- -SINEMURIAN BRACKISH- +ZIA+SINEMURIAN............

............................ WATER DEP OSITS CALCITURBIDITES..........

Fig. 4. Palaeogengraphic recon!druction of the northern and ccntr:il

Andean margin during Sinemurian to latest Liassic times.Extensional tectonic activity occurred in the northern areds

(emergent in Colnnihia), and progressed \outh\vv;ard (Sinemuriantorhidites, m d post-Sinemurian prohahle emergence in Ecuadorand northern Peru) . In southern Peru. the Liassic trun\grehninncovered ;i wide area.



ME SOZ OIC E VOL UT ION OF ANDE S N OF 2 0 " s 1013

reported in northern Peru (Pardo & Sanz 1979). The upperpart of the Pucara Group comprises massive dolostones oflate Liassic age (Condorsinga Fm: Mégard 1968; Fig. 2),which were deposited on a shallow platform (Loughman &Hallam 1982). The top of the sequence, which is of Toarcianage, is locally sandy (Szekely & Grose 1972; Mégard 1978;Palacios 1980). During Liassic times, sedimentation isthought to have been controlled by NW-trending normal

faults (Szekely & Grose 1972; Mégard 1978).Either because of subsequent erosion, or owing tonon-deposition, the uppermost preserved sediments youngtoward the southeast: Sinemurian in northwestern Peru(Wilson & Reyes 1964; Pardo & Sanz 1979; Prinz 1985),Pliensbachian to Toarcian in northeastern Peru (Prinz 1985),and probably early Aalenian in central Peru (Mégard 1968,1978; Westermann et al. 1980) (Fig. 2).

In southern Peru (Fig. 4), the andesitic flows of theChocolate Formation are capped by shallow marinevolcaniclastic rocks and limestones of early Sinemurian age(top of the Chocolate Fm: Benavides 1962; Vicente 1981;Vicente et al. 1982), which locally exhibit a thick basalconglomerate (Pelado Fm: Vicente 1981; Fig. 2). Farther

east, adjacent to the Altiplano, the Sinemurian sedimentsare represented by dark-coloured limestones deposited in adeep marine platform environment; the base of thelimestones is unknown (Lower Lagunillas Gp: Portugal1974; Vicente 1981). In the whole area, late Liassic shelflimestones (Socosani and San Francisco Fms, part of theLagunillas Gp, Fig. 2) locally overlying the ChocolateFormation through a basal conglomerate, indicate a newtransgressive stage of Toarcian age (Vicente et al. 1982).They are commonly associated with extensional synsedi-mentary tectonic processes. (Vicente et al. 1982).

In southern Peru, radiometric ages of 196 to 182Mahave been obtained for tonalitic plutons in the coastal areaand in the Arequipa region (Beckinsale et al. 1985; Mukasa& Tilton 1985; Mukasa 1986) (Fig. 4). In the south PeruvianAxial Cordillera, 175 to 185 Ma radiometric ages have beenobtained from syenitic and granodioritic intrusions (Stewartet al. 1974; Kontak et al. 1985). However, these ages can beinterpreted as having been reset (Laubacher 1978; Kontak1984).

Aalenian and Bajocian (180 to 1 6 5 M a )

This period is generally poorly understood because ofthe widespread occurrence of undated continentalsedimentation.

In Colombia, the post-Sinemurian deposits consist firstlyof subaerial volcanic rocks (Motema, La Mojana andGuatapuri Fms, Fig. 2) and then of detrital continentaldeposits (GirBn Gp, Bocas and Jordan Fms), deposited infault-controlled grabens (Mojica & Dorado 1987). However,post-Sinemurian deposits are often absent, owing tosubsequent erosion or to non-deposition (see Geyer 1973;Maresch 1983; Mojica & Dorado 1987 for reviews). Duringthis period, and especially between 176 and 166Ma,numerous tonalites and granodiorites were emplaced alongthe eastern edge of the Axial Cordillera (McCourt et al.1984; Aspden et al. 1987) (Fig. 5). These plutons hostcopper mineralization of mid-Jurassic age (Sillitoe 1988).This magmatic belt extends southward into Ecuador andnorthernmost Peru (Hall & Calle 1982; Baldock 1982;Mourier et al. 1988~).

k 1 0 N

t o

t10 s

T8 0 W

T

7 0 W ,

'

AALENIAN AND BAJOCIAN

180 to 165 Ma

"os.,.p".o*~.OARSE-GRAINED

CONTINENTAL RED BEDS

FINE-GRAINED EVAPORITIC

CALC-ALKALINE

PLUTONS (WITH AGES) "o " - . " '

SUBDUCTION-RELATED %* LAGOONAL DEPOSITS

SUBAERIAL VOLCANISM - - -.- - - MARINE HEMIPELAGIC

DARK LIMESTONES

Fig. 5. Palaeogeographic econsrruction of the northern and centralAndean margin during Aalenian and Bajocian times. (Subaerialvolcanism and continental deposits are poorly dated) .

A "E-trending magmatic arc developed along the Colombiansegment, together with local continental detrital sedimentation.Along the Peruvian segment, palaeogeographic zonation occurred

with northwestern, subcontinental detrital sedimentation, andsoutheastern, hemipelagic calcareous sedimentation.

In the Oriente of Ecuador and Peru (Fig. 5), a thick,undated, terrigenousJlagoona1 sequence of sandstones,shales, dolostones and gypsum (Lower Chapiza, LowerSarayaquillo and Boqueron Fms, Fig. 2) disconformablyoverlies the Liassic limestones (Kummel 1948; Tschopp1953; Faucher & Savoyat 1973; Pardo & Zuniga 1976;Laurent 1985). These formations are generally thought to beMiddle Jurassic (Bristow & Hoffstetter 1977; Mégard 1978),



1014 E. J A I L L A R D E T A L .

but their lower parts could be locally equivalent to the topof the Pucara Group (Mégard 1978).

In northern Peru. outcrops of mid-Jurassic rocks areunknown.

In the central Peruvian Andes (Fig. 5). Aalenian toBajocian shallow marine limestones and sandstones(Chunumayo Fm: Megard 1968. 1978; Westermann et al .

1980) conformahly overlie the Pucara Group. but Bathonian

deposits are lacking (Fig. 2). Further north. fluvialplant-bearing red shales and sandstones (Cercapuquio Fm)are overlain by lagoonal limestones and silts (Chaucha Fm)and have been tentatively ascribed to the Aalenian andBajocian respectively (Mégard 1968; Moulin 1989).

However. a late Jurassic age is also possible (Mégard 1978).

Similar Aalenian to Bajocian deposits are known alongthe coast (Rio Granae Fm: Riiegg 1956: Ivíégard 1975;

Westermann et ul . 1980).

In southern Peru (Fig. 5). the calcareous hemipelagicsedimentation continued until Bajocian times (upper part ofthe Socosani Fm. San Francis0 Fm, part of the LagunillasGp, Fig. 2) indicating a marked increase of water depthbetween Toarcian and Bajocian times related to extensional

tectonic suhidence (Vicenteet

al . 1982).No midJurassic deposits are known in the AxialCordillera and the Altiplano (Fig. 5). probably hecause ofsubsequent erosion, rather than to non-deposition. since thefacies of the eastern areas are very similar to those of thewestern and coastal areas.

Bathonian to Kimmeridgiun (165 to 140 145M a )

In Colnmbia, the lack of fauna precludes precisereconstructions, nevertheless, both erosion and fault-controlled red bed sedimentation continue (Jordan andGiron Fms: Geyer 1973; Mojica & Dorado 1987). Volcanicactivity seems to decrease during late mid-Jurassic times.Locally. a poorly dated transgression hegan during the lateJurassic (Valle Alto and Cheterl6 Fms: Geyer 1973; Mojica& Dorado 1987) (Fig. 2).

Following a period of apparent magmatic quiescence(160-151 Ma), a new calc-alkaline plutonic pulse occurredalong the Colombian segment (151-142 Ma. Baldock 1982;Hall & Calle 1982: McCourt et N Z. 1954: Aspden et al. 1987)

(Fig. 6) accompanied in Colomhia hy porphyry-coppermineralization (Sillitoe 1988).

In Ecuador and northeastern Peru (Fig. 6) , a thick,eastward tapering wedge of shaly, sandy and conglomeratic.continental red beds (middle Chapiza. upper SarayaquilloFms, Fig. 2 ) disconformably overlies the lagoonal deposits(Kummel 1948; Tschopp 1953; Pardo 8r Zuñiga 1976;

Bristow & Hoffstetter 1977). Plant remains indicate a

Jurassic age (Seminario & Guizado 1976). Sedimentationthickness indicates a westerly source for detritus supply(Portugal X r Gordon 1976; Laurent 1985), suggesting thatthe Axial Cordillera was undergoing erosion (Tschopp 1953;

Audebaud et al . 1973; Pardo & Zuñiga 1976).

In northernmost Peru, thick andesitic flows andvolcanoclastic rocks were deposited (Coliin Fm: Pardo &Sanz 1979; Fig. 2 ) , partly during Callovian-Oxfordian times(Mourier et u[. 19XXu), and they are considered to representthe southern extension of the Colombian-Ecuadorianmagmatic belt (Fig. 6). In the northern and central PeruvianAndes. south ot 7”s. no deposits of late Middle Jurassic toearly Late Jurassic age are known.

-!-

sowT

7 0 W

t o

I-10

BATH

o i

lOSi

K lM M ER l f f i lAN

165 o 140 Ma

CALC-ALKALINE

PLUTONS (WITH AGES)

SUBDUCTION RELATED SUBMARINE FAN

” ” SUBAERIAL VOLCANISM

MARINE SEDIMENTATION

Fig. 6. Palaeogeographic reconstruction of th e northern and centralAndean margin during Bathonian to Kimmeridgiantimes.

dated. coarse. detrital continental sedimentation extended

southward. In Peru, NW-trending emergent areas and troughs werecreated.

In Colombia, magmatic activity continued. and th e poorly

In the southern coastal area, a thick series ofvolcanogenic sandstones and hlack shales of Callovian andprobahly early Late Jurassic age were deposited (GuanerosFm: Olchauski 1980: Vicente 1981; Fig. 2). Our fieldobservations show that the volcanic rocks formerlyconsidered as part of this formation. actually correspond toyounger sills and dykes.

In southern Peru (Fig. h ) , the Bajocian hemipelagiclimestones are overlain by thick. southeastward progradingturbiditic fans (Puente and Ataspaca Fms: Vicente 1981;

-



MES OZOIC EVOLUTION OF A N D E S N OF 20"s 1015

Vicente et al. 1982) of Bathonian to Callovian age(Benavides 1962; Westermann et al. 1980; Vicente 1985).They pass northeastwards into hemipelagic slope deposits(part of the Lagunillas Gp, Fig. 2). Callovian-Oxfordiantimes are characterized by widespread pelagic black shales(Cachios Fm: Vicente 1985; part of Lagunillas Gp: Portugal1974; Piste Fm: Pecho 1981), part of which exhibitsynsedimentary normal faulting, slumping (Vicente et al.1982), olistoliths and clastic dykes (Santander, pers. comm.1988). The inferred extensional tectonic phase took place atthe Middle to Late Jurassic boundary. In the Altiplano, nodeposits of this age have been recognized.

During this period, magmatic activity along the Peruviansegment is very restricted (Fig. 6). Along the coast, acalc-alkaline, subduction-related andesitic flow of the upperRio Grande Fm (Caldas 1978; Fig. 2) yielded a mid-Jurassicage (164Ma, Aguirre & Offler 1985). On the other hand,some ages in the range 155-152Ma were obtained byMcBride (1977) and Mukasa (1986) from a few granitoids insouthern Peru.

Kimmeridgian-Tithonian to Berriasian (1401145 o

1 2 5 M a )In parts of eastern Colombia, the sedimentation resumedwith disconformable, undated conglomerates (Buena Vistabreccias, Rio Negro Fm, Fig. 2), followed by undated redbeds (Yavi Fm?) or plant-bearing dark shales andvolcanogenic sandstones (Shame Fm), locally and partlydated by a Kimmeridgian to Tithonian fauna (Cuiza Fm:Geyer 1973; Wiedmann 1981; Maresch 1983; Mojica &

Dorado 1987). In other places, red bed sedimentationcontinues (Gir6n Fm, Boinet et al. 1985).

In Colombia, a significant gap in plutonism occurredduring latest Jurassic to earliest Cretaceous times (142-124 Ma, Aspden & McCourt 1986; Aspden et al. 1987).

In Ecuador (Fig. 7), the upper part of the ChapizaFormation (Misahualli Mbr, Fig. 2) is composed of redshales, sandstones and conglomerates associated withbasaltic and trachytic lavas, and then with rhyolitic tuffs(Baldock 1982; Hall & Calle 1982). It disconformablyoverlies the middle part of the Chapiza Formation (Tschopp1953), and is considered as latest Jurassic to earliestCretaceous according to both palynological (Bristow &

Hoffstetter 1977; Canfield et al. 1982) and radiometric data(132Ma, Canfield, in Feininger & Bristow 1980; Espin, inHall & Calle 1982). It could be coeval with the intrusion ofnumerous basic, porphyritic bodies which cross-cut the latestJurassic rocks and are capped by the Neocomian sandstones(Kummel 1948; Tschopp 1953; Bristow & Hoffstetter 1977).In the westernmost areas, volcanogenic quartzites verysimilar to the Neocomian quartzites are reported (Tschopp1953), and may be correlated with the Bemasian TinajonesFormation (see below).

In northernmost and eastern Peru, there is no definitivereport of deposits of this age. However they may exist aspart of some late Jurassic deposits (e.g. the Sávila Fm:Reyes & Caldas 1987; Mourier et al. 1988a) (Fig. 2).

In northern Peru (Fig. 7), a lagoonal sequence oflimestones and sandstones (Simba1 Fm: Jaillard & Jacay1989; Fig. 2) is sharply overlain by a thick sequence ofproximal, southward-flowing turbidites in early LateTithonian, followed by slumped, block-bearing,hemipelagicslope deposits, indicating notable tectonic activity (Punta

b 1 0 N

t o

t l0 s

T80 W

T

7 0W

TlTHONlAN TO

o i

..: lOSi

L P

BER RIASIAN \ .': 20 s i0 .

140 t0 125 Ma TITHONIAN-BERRIASIAN

SHALES AND SANDS

DISSEMINATE Da ORPHYRITIC BODIES E]- - - EARLY TlTHONlAN

LAGOONAL DEPOSITS- . - -

TITHONIAN-BERRIASIAN

SUBMARINE FAN

VOLCANISM

CONTINENTAL

NORMAL FAULTS

s RED BEDS

Fig. 7. Palaeogeographic econstructionof the northern and centralAndean margin during Tithonian to Berriasian times.

segment, and oblique collisionstook place, which probablytriggered the opening of the north-Peruvian turbiditic trough. Alongthe Peruvian segment, an extensional regime is recorded, whilevolcanic activity resumed along the coast (Lima).

Except in Ecuador, magmatism ceased in the Colombian

Moreno Fm). These are overlain by black shales of lateTithonian to Bemasian age (Stappenbeck 1929; Geyer 1983;Zapotal Fm: Jaillard & Jacay 1989), and then by afining-upward sequence of plant-bearing volcanogenicsandstones and shales with subordinate conglomerates(Tinajones Fm: Cobbing et al. 1981; Reyes & Caldas 1987;Mourier et al. 1988a; Fig 2). The latter detrital sequencedisconformably overlies Liassic to Berriasian rocks, and



1l) lh E . J A I L L A R D E T A L .

exhibits numerous extensional synsedinientary tectonicfeatures (Jaillard Xr Jacay 1980).

In central Peru. south of 11"s.no equivalent of the lateTithonian turbiditic sequence of northern Peru is known.The late Tithonian to Berriasian coal-bearing dark shalesand sandstones (Oy6n Fm: Wilson 1963; MCgard 1978:Cohhing ct al. 1981) would be the southern equivalent of theTinajones Fm (Fig. 3). Further east. red shales and

sandstones were deposited during Tithonian-Berriasiantimes in fluvial to estuarine environments (lower Goyllar-isquizga Fni: Moulin 1989). and disconformably overlieLiassic rocks or the Cercapuquio and Chaucha Formations.A s mentioned above, these latter formations could be ofKimmeridgian-Tithonian age. At this time. sedimentationwas controlled by NE-SW and NW-SE-trending normalfmits (Arthaud cr i l l . 1977: Moulin 1989).

In the coastal area of central Peru (Fig. 7) . undatedporphyritic andesites intercalated with grepvackes andshales are overlain by basaltic flows and volcaniclasticdeposits (Puente Piedra Fm: Rivera et d. 1975: Mégard1978) of latest Berriasian age (Wiedmann 1981: Fig. 2).

They have heen interpreted as suhduction-related, volcanic

arc flows (Atherton et al. 1983. 1985). Further south, theJaguay Formation of early Tithonian age (Riiegg 1961;Caldas 1978) comprises brecciated and siliceous limestonesdeposited in a shallow marine environment. Porphyriticdykes. sills and small stocks cross-cut the pre-Valanginianrocks along the southern Peruvian coast.

In southern Peru (Fig. 7 ) . siliciclastic shelf depositspartly of Kimmeridgian age (Labra Fm: Benavides 1962:Vicente 1985; Vicente er d. 1981: Chuquibambilla Fm:Pecho 1981: Chachacumane Fm: Vicente 1981. 1985; Fig. 2 )overlie the late Jurassic black shales. with a localunconformity (Yauca Fm: Olchauski 1980). They areoverlain by limestones and sandstones representing alagoon-harrier environment (Gramadal Fm: Benavides

1962; Vicente cr ill. 1982). The limestones have been locallydated as early Tithonian (Vicente 1985: Batty & Jaillard1989). In some areas, late Tithonian fine-grained quartzites(Geyer lW3) or Berriasian hlack shales (Bellido 19%) areknown. but emergence serms to have occurred elsewhereduring late Tithonian-Berriasian times (Batty SL Jaillard1989: Fig 2 ) .

In the Altiplano area. coarse conglomerates (Lauhacher1978: Chupa Fm: Klinck cf u1. 1986) disconformably overliePalaeozoic rocks. They are followed by lagoonal limestones.shales and sandstones (Sipin Fm), which grade upward intotidal red shales and sandstones (hfuni Fm: Newell 1949:Audehaud et id. 147h: Fig. 1).These formations exhibit astrong synsedimentary ENE-WSW-trending extension

(Batty & Jaillard 1989).Near the Berriasian-Valanginian boundary, a major

disconformity occurred and led to the deposition. in most ofthe central and northern Andes. of a westward propagatingsequence of clean and mature quartzites, which disconfor-mably overlies Palaeozoic to Berriasian rocks (Kummel1948; Tschopp 1053: Benavides 1956: Pardo Csr Zufiiga 197h:Laurent 1985: Jaillard X r Jacay l%Y: Batty S Jaillard 1989)(Fig. 3 ) .

Synthesis of tectonic, magmatic, and sedimentaryfealires

The e\olutinn of the northern and central Andes during the

early and middle Mesozoic c m he dkided intn three mitin

periods, each one showing distinctive sedimentary, tectonicand magmatic features (Fig. 8) .

Middle Triassic I'o late Liussic (240to 180 M a )

Continental red bed sedimentation of Permo-Triassic age isfollowed by platform sedimentation which began with thewidespread Norian transgression. During early Sinemurianand early Toarcian times. new transgressive stages arerecorded. The marine carbonate shelf is diachronouslydestroyed by a southward migrating extensional tectonicactivity which induced the immersion of the northern areasbetween the Rhaetian (Colombia), and the late Liassic(Ecuador and northern Peru). This tectonic activity isrecorded in central Peru by local unconformities and bypdrtly detrital sedirneniaîion, and in scmthern Peru hyextension-related suhsidence (Fig. 8).

During this period. magmatic activity is mainly restrictedto Colombia. and to southern Peru (Fig. 8) . In the formerarea. most of the intrusions seem to he related toextensional or strike-slip processes, whereas in southernPeru. plutons intrude suhduction-related volcanic rocks

(Chocolate Fm). This poorly dated volcanic arc is the onlypreserved evidence that subduction nccurred along the studyarea during an undetermined time-span between earlyPermian and middle Liassic times.

As a whole. this period is dominated by a southwardpropagating extensional tectonic regime. No clear evidenceof subduction processes is preserved. except perhaps insouthern Peru.

~hfert i f l rzo Kinznzeridgicirz (180to 14()/.145M u 1This time-span begins with a palaeogeographic changemarked by widespread disconformable detrital deposits andlocal tectonic suhsidence. A Bathonian tectonic phase

resulted in the formation of the south Peruvian turhiditictrough, the probable emergence of the Axial Cordillera andthe deposition of coarse continental beds in the easternbasins (Fig. 8 . By Callovian-Osfordian times. tectonicinstahility occurred and was contemporaneous with localblack shale deposits.

The NNE-trending calc-alkaline. locally Cu-mineralizedmagmatic belt which extends all along Colombia andEcuador (Fig. 8). unamhiguously indicates that subductiontook place beneath this segment (Aspden et d. 987). Thebeginning of the suhduction. as expressed hy the 18OMaintrusions roughly coincides with detrital sedimentation inEcuador and in northern and central Peru. and with thetectonic subsidence of southern Peru.

In contrast. the scarcity of the intrusions along thePeruvian segment (Fig. 8 ) suggests that only local andlorepisodic subduction occurred at this time. On the otherhand. the creation of the south Peruvian turhiditic troughindicates an extensional tectonic regime.

In agreement with Aspden er ul. (1987). we interpret thispattern as the result of a very oblique, southeastwardconvergence direction which would have induced a strong.sinistral strike-slip component along the Peruvian segment(Fig. 8) . The southern turbiditic trough may be interpretedas ;i pull-apart basin. Nevertheless, the Bathonian,subduction-related andesites of the upper Ri o GrandeFormation (Aguirre Xr Offier 1985) suggest that ohliquesubduction may have taken place locally and sporadically

along the Peruvian margin.



ME SOZ OIC E VOL UT ION OF ANDE S N OF 20"s

STRATIGRAPHY

a f te r Haq et al. 1987

1017

M A G MAT I S MSEGMENTS

COLOMBIAN I PERUVIANU.0.

140-

150.

160-

170.

180-

190.

200-

220.

230-

c:wœ

1 3 0 . 2

u

M

M

œœ

-

7

210.-

o

v,

M

c-œI-

-

STAGES

-ALATE

INTRUSIONS VOLCAN. INTR. VOLCAN

MIDDLE

EARLY

-EARLY3

I--UENTE

PIEDRA

?-

MOTEMA

SALDAÑA

UPPER-.GRANDE

LATE9 I - I Ï _ , -CAR

L A D

...5.. lgIDDLEI

M A I N E V E NT S

SEGMENTSCOLOMBIAN I PERUVIAN

MAJO R DlSCONTlNUlTLOBLIQUE1

TUABlDlTHCONTINENTAL TROUGH

DEPOSITS

CONTROLLED

BY NORMAL EXTENSION

RELATED

SUBSlDENa

TRANSGRESSIONIXTE NSI ON

NTERPRETATION

SEGMENTS

OCEANIC

KIMMERIDGIAN-TlTHONlAN EVENTS

SE-WARDSINISTRAL

OCEANIC

MOVEMENTSUBDUCTION

SOUTH WARD

PROPAGATING

EXTENSIONAL

TECTONIC ACTIVITY

?

Fig. 8. A synthesis of the evolution of the Colombian and Peruvian segments. Intrusions are represented according to the number of knownradiometric ages (Ecuadorian plutons in white). Volcanic formations illustrate the volcanic activity.

progressively destroyed the Norian-Liassic carbonate shelf.

subduction beneath this segment, whereas ridges and detritus-filled trough were created in the Peruvian segment, as the result of sinistralwrenching tectonics.

From Kimmeridgian-Tithonian to Berriasian times, magmatism ceased abruptly along the Colombian segment which underwent dextral,

oblique collisions. Along the Peruvian segment, new troughs and ridges were created, and the creation of the Lima marginal basin indicatesthat a grossly northeastward subduction ook place beneath this segment.

Between Norian and latest Liassic times, the magmatic activity remained low and the southward shift of the rifting-related ectonic activity

During middle and early-late Jurassic, an important magmatic arc developed in the Colombian segment, indicating roughly southeastward

Kimmeridgian to Berriasian (14011 45 to 1 2 5 M a )

This period begins with a widespread basal conglomerate,and with extensive shallow marine siliciclastic deposits whichpass upwards into. shallow marine limestones. In northernPeru, a turbiditic trough was created by late Tithonian times(Fig. 8). This period usually ends with the deposition offluvial to coastal sands and shales which are frequently

eroded and occur beneath the disconformably overlyingNeocomian quartzites.

Along the Colombian segment, accretions of alloch-tonous terranes occurred during latest Jurassic to earlyNeocomian times (McCourt et al . 1984; Roperch et al . 1987;Mourier et al . 19883; Restrepo & Toussaint 1988) (Fig. 8).These accretions are associated with a blueschist metamor-phic belt with ages of 132 to 125Ma (Feininger 1982;Bourgois et al . 1987), and are related to the 142 to 124Magap in plutonism in Colombia (McCourt et al . 1984; Aspden& McCourt 1986; Aspden et al . 1987). These events couldhave triggered the opening of the north Peruvian trough(Jaillard & Jacay 1989).

The Peruvian segment recorded both an extensionaltectonic regime, and a renewal of the volcanic activity in thecoastal area (Puente Piedra Fm, Fig. 8). The andesitic flowsof this formation show both subduction-related arc, andback-arc chemical affinities (Atherton et al. 1983, 1985).

Hence, the geodynamic pattern drastically changedduring Kimmeridgian-Tithonian times. The NW-trendingPeruvian segment records subduction processes while the

"E- trending Colombian segment was subjected to a strongdextral transform component (McCourt et al. 1984; Aspdenet al. 1987), which can be related to the accretion of terranesoriginating to the south (Mourier et al . 1988b). Theseobservations would therefore suggest a roughly north-eastward convergence direction of the oceanic Phoenix platerelative to continental South America (Aspden et al . 1987)(Fig. 8).

The major pre-Valanginian erosional disconformitycould represent either a sedimentary response to theColombian and Ecuadorian accretions (Mourier et al.1988a) or the effects of the incipient South Atlantic rifting(Jaillard & Jacay 1989).



1018 E . J A I L L A R D E T A L .

The early to middle Mesozoic history of thenorthern and central Andes: a Tethys-controlledevolution

Recently published Mesozoic geodynamic reconstructions ofthe Caribbean region (Pindell Cy ; Dewey 1982: Klitgord Cy ;

Schouten 198h; Ross & Scotese 1988) assume a continuouswestern margin for the Americas which united the westernmargin of northern Mexico with the western margin of

northern Peru by the means of various continentalmicroplates, remnants of which constitute present-dayCentral America (Fig. 9a). If accepted. importantdeductions may be inferred from these reconstructions.

During Early to Middle Jurassic times. the Tethyanrifting affected the Caribbean region. which acted as acomplex sinistral transform zone. The central Atlanticopening induced the separation between the North andSouth American plates (Pindell & Dewey 1982; Anderson Cy ;

Schmidt 1983). Therefore. the northwestward shift of NorthAmerica can be correlated with the creation of a Tethyanoceanic arm along the Colombian segment. This Tethyanridge extended southwestward through the Palaeopacificrealm (Mooney 1980). and separated the northern Farallon

oceanic plate from the southern Phoenix oceanic plate. As aconsequence, subduction beneath Colombia cannot havestarted before the beginning of the spreading in theTethyan-Colombian rift.

The rifting phase

In western Gondwana. the Tethyan rifting began during lateTriassic times and continued throughout Liassic times(Bernoulli & Lemoine 1980: Emery Cy: Uchupi 1984: Oliveter al. 1981; Savostin et al . 1986; Lemoine et (II. 1986). Itinduced an extensional regime in the Caribbean region(Pindell & Dewey 1982: Anderson Cy: Schmidt 1983:Klitgord & Schouten 1984; Ross Cy ; Scotese 1988). During

middle to late Liassic times, the Caribbean-Colombian riftwas deep enough to allow exchanges of pelagic faunasbetween the Tethyan and Caribbean realms (Hillebrandt1981; Thierry 1982).

The rifting-related extensional and possibly transcurrentstress is clearly recorded by the geological evolution of theColombian 5egment (Mojica & Dorado 1987). It is proposedthat the destruction of the Norian carbonate platform duringLiassic times in Ecuador and Peru is also related to theevolving rifting process, the effects of which progre5seddiachronously southward (Fig. 9a).

The Tethyun sribdrictioii plinse

The beginning of the spreading in the Weitern Tethys is of

late Middle Jurassic age in the Western Alps (Lemoine c’t al .

1986): of latest Liassic (180Ma, Olivet et ill. 1984: Savostinet al. 1986) to early Bajocian age in central Atlantic(170-175 Ma, Emery & Uchupi 1981: Khtgord & Schouten1984: Ziegler 1988): and of Aalenian to Bajocian age in theCaribbean region (175-165 Ma. Stephan et al. 1980; PindellCy; Dewey 1982: Anderson & Schmidt 1983: Ross&icotese1988). Initial spreading rate in Central Atlantic seems tohave been fint very low (Olivet et al. 1981).

In Colombia, the calc-alkaline arc magmas wereemplaced partly during the Toarcian ( 183Ma) and mainlyduring the Aaleninn and Bajocian ( 176-lhh M a. Aspden ct

í i l . 1987) (Fig. 8). This magmatic pulse may be regarded as

Fig. 9. Geodpnamic model for the evolution of the northern and

central Andes related to the early to middle Mesozoic Tethyanevolution. White areas. continental plates: AF. Africa: NOAM.North America: SOAM. South America. Shaded :ireas. oceanic

plates: FA . Farallon; PH, Phoenix; PaleoPAC. Palaeopacific.(a ) During Iate Triassic-Liassic times. the Tethyan rifting passed

hetween the Colomhirin and the palaeo-Mexican margins and

produced an extensional regime throughout the northern andcentral Andean margin. probably itssociated with some st ri bs l ip

movements.

(b j During middle and early Late Jurassic. the southeastwardsubduction of the newly created Tethy:in-Colombian oceanic crustheneath Colomhia gave w a g to the hinistral transform offset of thePeruvian margin.( c ) During latest Jurassic and eurlie5t Cretaceou5 times. spreading

along the Tethyan ridges sharply decrcased (or even ceased) . andwas replaced hg the Pacific spreading centres. As :i result, :i

northeastward subduction commenced heneath the northern part of

the South American plate. and oblique, dextral collisinns nf

segment.

continental blocks :tnd i)ceanic ridges occurred along the Colnmhian



MESOZOIC EVOLUTION O F ANDES N OF 20"s 1019

being related to the subduction of the newly createdTethyan crust, formed in the Colombian oceanic arm (Fig.9b). It probably coincides with the beginning of the detritalsedimentation throughout Ecuador and Peru.

According to the classical model of Uyeda & Kanamori(1979; see also Cross & Pilger 1982; Soler & Bonhomme,1989), the strong tectonic stress (uplift and erosion), thevoluminous intrusiom and the Cu-mineralization recorded

by the Colombian segment suggest fast, low-anglesubduction of a young oceanic crust, which is in goodagreement with the Tethyan interpretation (Fig. 9b).

The Bathonian tectonic phase (165-160 Ma) does notshow clear correlations with global geodynamic events.Nevertheless, this diastrophic phase is widely knownthroughout South America (Zambrano 1978; Riccardi1983), and has been recently reported in the western marginof North America (169-161Ma, Dilles & Wright 1988;Wright & Fahan 1988).

In the Tethyan system, the Callovo-Oxfordian (155-150 Ma) is marked by a sharp increase in the spreading rate(Olivet et al. 1984; Savostin et al. 1986) and by tectonicevents in the Western Alps (Lemoine et al. 1986) and

Caribbean region (Anderson& Schmidt 1983). They can beregarded as coeval with part of the Nevadan orogeny(154 Ma: Schweickert et al. 1984; Ingersoll & Schweickert1986). The tectonic instability recorded in the Central andNorthern Andes could be related to these events.

The Bathonian and Callovo-Oxfordian Andean phasesroughly coincide with a decrease of the magmatic activityalong the Colombian segment, but the regional subductionpattern did not change significantly, since a new plutonicmaximum occurred between 151 and 142Ma in theColombian segment (Aspden et al. 1987).

Th e PaciJicsubductionphase

Kimmeridgian-Tithonian times (140-145 Ma) are markedby drastic geodynamic changes. In the central Atlantic, thespreading rate sharply decreased (Olivet et al. 1984;Klitgord & Schouten 1986), as well as in the Western Alps(Savostin et al. 1986). Moreover, in the Caribbean region,southward ophiolite obductions are concealed by Neoco-mian deposits in Venezuela (Stephan et al. 1980). On theother hand, the beginning of the south Atlantic rifting is oflate Oxfordian to early Tithonian age (147-136Ma, Herz1977; Ojeda 1982; Sibuet et al. 1985). The geodynamicchange along the South American margins can be regardedas a result of this kinematic reorganization.

Duncan & Hargraves (1984) showed the convergencevector of the Phoenix plate to be the sum of the expansionvectors of the NE-trending Tethyan ridge and the Pacificspreading system located farther southwest (Hilde et al.1977; Handschumacher et al. 1988). As a result, a fastspreading rate along the Tethyan ridge associated with a lowspreading rate in the Pacific ridges would have induced asoutheastward motion of the Phoenix oceanic plate (Duncan& Hargraves 1984). These conditions seem to have occurredbetween 180 and 145Ma (Fig. 9b). In contrast, a slowspreading rate in the Tethyan Colombian ridge would haveinduced a northeastward motion of the Phoenix plate(Duncan & Hargraves 1984). We propose that the abovementioned sharp decrease of the spreading rate in theTethyan ridges led to this situation after Kimmeridgiantimes (Fig. 9c).

Following Uyeda & Kanamori (1979), Cross & Pilger(1982) and Soler & Bonhomme (1989), the dominantextensional regime, together with the lack of any upliftindicated by the widespread Kimmeridgian-Tithoniantransgression along the Peruvian margin, suggest asteep-dipping subduction of a slow-converging, possibly oldoceanic slab.

Concluding remarks

The analysis of the evolution of the Andes north of theArica elbow shows that the subduction was not uniformduring early Mesozoic times. However, the proposed modelis speculative and several uncertainties remain.

The early volcanic arc in southern Peru (Chocolate Fm)suggests that active subduction took place sometimebetween early Permian and Sinemurian times. In this paper,we assume that subduction ceased before late Norian times.Actually it is quite difficult at the moment to state whethersubduction took place during the late Norian or not, andthis problem needs further investigation.

In this paper, we use present-day maps to draw thepalaeogeographic features. However, various tectonic andpalaeomagnetic studies suggest that at least parts of theAndean margin have undergone significant tectonic rotationand shortening from early Mesozoic times. Further studiesare needed in order to determine the palaeogeometry of thenorthern and central Andean margin.

We have considered the Kimmeridgian-Tithonian eventsof the central Andes to represent an unique andwell-defined boundary for geodynamic regimes. Actually,field data from Peru suggest that the whole Kimmeridgian-Berriasian time-span represents a transitional period.However, the model proposed is in good agreement with theavailable geological data which clearly indicate that the

various segments of the Andes underwent differentevolutions.From the late Triassic to the late Jurassic, Tethyan

rifting dominated the evolution of northern South America.It is expressed by a palaeogeographic zonation perpendicu-lar to the proposed convergence direction, with coarsecontinental sedimentation toward the northwest, shallowmarine, fine detrital sedimentation in northern and centralPeru, and deep marine carbonate sedimentation in southernPeru.

The late Jurassic events are related to a geodynamicrevolution which is marked by the end of the roughlyE-W-trending Tethyan break-up, and by the beginning ofthe roughly N-S-trending Atlantic rifting. In the study area,

the palaeogeographic framework drastically changed into aNE-SW to E-W orientation, with eastern red beds, centralemergent areas, and western shallow marine sedimentation.This displays the reaction of the South American margin tothe new, northeastward convergence direction.

The Tethyan realm between Colombia and palaeo-Mexico may explain the Tethyan affinities of some Pacificprovinces during early Mesozoic times (e.g. Marcoux et al.1982). The disruption of the Tethyan realm during theLiassic rifting, followed by the shift of some of thesecontinental microplates owing to the Tethyan and thenPacific spreading ridges during middle and late Jurassictimes, could explain the subsequent accretion of Tethyanterranes of southern origin along the western North





MESOZOIC EVOLUTION O F ANDES N OF 20"s 1021

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