31003 e Structural

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LATE CENOZOIC NORMAL AND STRIKE-SUP FAULTlNG IN NORTHPERUWAN WESTERN CORDILLERA : AN EXAMPLE OF ALTERNATE EXTENSIONAL

AND COMPRESSIONAL TECTONIC REGIMES IN HIGH ANDES

0fM.r BELLlER*, Mid.1 SEBBiER*

URACNBS (73g) G6ophysiquo Olodynamiqur htemo. BLt.BIg. Unlversh6 PARS-SLID, 014w ORSAY

RESUME

Au coun du MiocAno, de8 deml-grabenr s’organisent arall6lemmt aux awidmtr majours do la Gordill6rrOwidontslr du Nord POrou. l r’y awumulmt de8 d6p6ts fluvio-laoustror u wun d’unr phase d’extmsion ENE-WSW. Au N6og6nr sup. et au Guatemaire inf., la r6gion rst alon soumir 1 drux r6gimrr d6crochants dont Iordireotlons o oompresslon ent WNWESE puir N-S. Depuit Ie Pl6istoo6ne moyen, Ie r6gime ett caract6ris6 par uneextension de dimwon N-S. Ges r6gimer teotoniquer sent interpr6t6r wmme la wndquenw der forces de volumeli6er 6 la haute topographic et de8 foroer de bordure i6er 6 la wnvergenoa de8 plaques NaxwAm6rique du Sud.

Key Words : Halfqraben, Extension, Compression, ectonic egimes, Western Gordillera, Northern Peru.

I - INTRODUGTION_

The Peruvian Andes is a wrdilleran margin where the subduction s Uve since he Jurassic. The,period romEarly Miocene to Present-day s characterbed by a high convergence ata between the oceanic Naxu Md thewntinentai South Ameriun plates: convergence direction being nearly orthogonal o the Peruvian rench. S6brierand Soler (1989) have reported 4 compressional ectonic pulses which affect the Gentral Andes after the Eoceneand Oligocene main deformation phases 3.e; 42 Ma Md 23-23 Ma dated phases). These pulses are dated: lowerMiocene (1517 Ma), middle Miocene (10 Ma), upper Miocene (7 Ma) and early Quaternary 2.5 Ma). The lower andmiddle Miocene pulses correspond o locally deformational crisis. After the maln upper Eooene shortening Incaicphase) hat affects he High Andes, several basins have been initiated along major faults in the Western Cordilleraof northern Peru as for examples the Cajabamba, San Marwr, and Namora baslnr (fig.1). These ones were half-gralwnr infilled by Miwene fiuvidacustrine deposits Md bordered by normal faults that reactivated oldwmpresslonal structures. he Gajabamba basin s situated n the western edge of the NNWSSE trending Ma&onThrust Md Paid Belt (see M6gard, 1984), the Namora basin Is wlthln the E-W Gajamarca deflection and the SanMamor baskt, situated between he Gajamarca defieotion Md the Marafion Thrust and Fold Belt, is bordered byNW-SE trending rtructurer. The moan altitude of this studied region Is 3100 m (calculated on an area of 10 Oookm2 [Belller, 19891). The study of the 8edimematY Md tectonic evolution of these basins, wnstraint by radiometricand paleontologlc aget, permit to show idenw for longer perioda of moderated Md lwailzed extensionaldeformation separated by short periods of compressional eformation.

II - MIOCENE SEDIMENTARY EVOLUTION

The Gajabamba Md San Marws bashr were filled by a thick fluvio-facu8trlne edimentary Group (fig.2). Thisgroup is characterized by a mean 350 m thick Iacustrine ormaUon ermed the Csjabamba fm and wnatituted byalternated shales, marls, lignited shales, gypsum, diatomits. silts and sandstones eposits which are overlying ifflewnglomeratio fluvial series. ln this lacustrine ormation we observed volcanidoam facier Md aynudimentarynormal faults indicating that voicanism and tectonios were active during the basin evolution. This Iacustrlnefomatlon Is overlaped by a meen 500 m thkk furglomerate formation ermed the Gondebamba m. This one, Iswnstituted by mud-flow and fiuvfal sequencer at the bottom and warser torrential sequences oward the top. Theage of this group was Umated rfy to middle Miocene hanks o diatom assemblages exposed n U-IO acustrinedeposits Bellier et al., le Fourtarrier t al, in prepuation). The tectonic ramework permit to Umate he age ofthis Group probably ranglng between 13-17 h6a Md 10 f& I.e., between the iower Md middle Miowno tOCtCfIi

pulses. The Gondebamba m Indicate that a climatic change had owured between the Gajabamba fm and the

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CondObunk m hQ8 h rdditbn, ths 08t8strOphlc 008s~ 0on@0msr8tlo f8d.s of this Qroup probrbly m8rksdths obsinQ of ths b8sins 8ssooirtsd to the miHl0 htbcens tsotonb pubs. Ws can Um8ts 8 moM drposit rsts8nd oonsquontly 8 m88n subsldona nts in ths b8sins during this p d: t8kin~ into 8ccount hs docomp8ctionp8ruMtrrsws osn Um8ts 8 m88n loo0 m thbk, dsoomp8otsd ssdimsnWy cdumn whbh hu bson doposItsddu~w~Th~~ta~~~~mr~wbJdrnor~ ~ 0.17tO03 mm/vr.

~NunonbutnbRllrdby81~~thidc2~~~~trnnrd~~8fmloullyovorfylng hs Condob8mb8 m wlth 8 silQht unoonfwtty. Thir fomutbn w8s ch8motsrizsd by mont8ry fluvklsqurnorr whbh puud pro~rouhnfy tow8rd ths top to boustkw soqusrws m8ds of rurds, siits. sh8bs,dbtomlts, IiQnibd shsbs, oubon8tsd oonwstbns 8nd Umsstonoa h the middb put of ths boustdnr ssqusnw,soms d8dUo tuffs wsro lntorbsddsd within di8tom bsds. This ktdbsts 8 wb8nb soth4ty contsmpor8nsous o thsb8sln filling. 0-1 ths othsr hsnd, synssdknsntsry nomul f8ults Indk~@ 8 Nomul furlUng 8othAty. ThspsbontobQbsl~8lysls of dbtom ssmpb Qw 8 upper Mbosno s~s (Ssllbr ot 8l.. loBob; Fourt8nbr st 8l., Inprspu8tbn). Rsoantfy, wdlomotrb 8n8lysss of ths N8mon d8oltb tuffs (Klk 8nslysss by M.G. BonhOmms) givs

M rgs Of 7.2tO6 Ms for ths middb-uppor put cf ths Nsmorsfm. Thb b ln rqrssmont with rgss of ths N8mor8 fmrsnQlnQ m ths middb 8nd uppsr Mbo8nr tsotonb ph8sss; I..., bstmn 10 8nd 7a h Ws h8vs ti~tsd 8msxlmum 300 m thbk, dooomp8otod sdinwnuly column of ths N8mor8 fm whbh hu bssn dspositsd during 2-3

Ma This psrmlt to o8bul8ts 8 subsidsnos rrts of 0.12t0.03 mm/r.Ul MlCCENE TO QUATERNARY TECTONIC REGIMES

Ths Mice8110 o Cu8tsm8ry tsotonb svolution of northern Psru intr8uwdillsrM b8sins shows sxtsnsbnrlpsrbds sspsr8tsd by comprsssiorul dsformrtbn crbia This wok n wss invssti~8tsd uslnQ tsotonb M8lysb 8ndfsult kkwn8tlcr 8rulysls. Ansfysb of thsss fsult kinsm8tbs In tsmu of strsss wu dsvsbpsd usinQ ths oomputsr-sided msthod pmpossd by Cusy (1979). This 8nslysis show svidsncs for 5 succsssivs Qsodynsmb st8t.s (Ssllisr,1Qw):

1 : ?ho San Mucos ad Cajabamba 8lf+rsbsns wars dsvsb+sd during ths mkldk k cans

contsmpor8nsously with ths Wsstsm @rdilbrs uplift snd. Thb Mbcws b8sin8l svolutbn wss oe8v8l with snsxtsnsbn8l t8otonb roglms (l-110.3) ch8r8otsrixsd by 8 domirmnt ENE-WSW trsndlng tsnrlon sxpossd bysynssdfmonwy nomul f8ult 8ffsctkw th8 cJ8bsmb8 fm. mi8 wwsw trsnding xtmrbn pdsts 8 minor NE-SWtrsndin~ sxtsnrlon (2-fio.3) whbh 8ffsots 8lI ths reeks prbr to ths Nsmorr fm. Ths Furglomor8ts Condsb8rnb8fm s8sms to indluts 8 tsctonb oh8nQS whbh m8y corrsspond o ths 10 Ms oomprsssion8l pub.. mi8 pubs m8yh8VOpfOdUordthrOlOsingOfthObuiM.

2 : Than, the N8nmf8 b8sins wu f~msd, bstwsan 10 snd (I M8, sbn~ WNW-ESE trsnding Md S dippingmJoIfurltr.~~8~mmr~by~~ntuynomulf~WIwhiohind~~8ENE_WSW~nJonJtsotonbs (l-fio.3). This synssdlmontuy f8ulting dsfomwd ths d8dtb tuff lswls lndbstino that vokmim,8xtonsbnal tsotonbs, snd ths N8mor8 fillip mm oosv8f.

Thsn, ths Nsmor8, SM Msroos 8nd Csjsb8mbr buins mro 8ffscted by WNW-ESE snd N-S shortwings (38nd 4-flQ.3). Struotuml obswationr show th8t ths N-S trsndinQ oomprossb+l postd8tss ths WNW-ESE trsndinQon..

2 : lho WNW-ESE trsnding aw~prsssbn (3-fig.3) produw fokfs, rsvsrss 8nd striks-slip f8ults sffsctlng sll thsMbosns dsposita This dsformsUon ph8ss msy bs centisr first, 8s ths lsts Mioosn8 pubs drtsd st 7 Ms In

othr Csntr8l Andss put: thsn, ss M other comprss8bn8l pubs, prob8bty bts Noogsns too; Lo., y-oungsr h8n thsNun0r8 fm.

5 : Tha N-S rondlnQ oomprsssion (4-fig.3) 8pf~us m8jor with nspscf to thr WfVW-ESE ons Md msinly in thrcJ8msrca dsfbcUon. 2 produced sgbrul fbxuntbn, folds, futltsd ovsrfokfs, rsvsrss 8nd striks-slip fruits 8s wsll8s bobtod 8nd ‘sn s n* twktnsl~8shss. lhsss dsfomwUons wsrs ths sffsot of ths Esrly Curtsmuy pubs.

Ths kinsmatbs uulysb drmonstr8tss th8t ths Nin (WNWESE snd N-S) oompresaionrl dsfomwUon8 wsrsproduosd with strlks-sllp roglmoa

S : lho Early Ploi8tocmo omprsssion8l pubs b follovnd by 8 ns8rly KS trsndinQ sxtsnsiona tectoncs (5

fig.3). Thls tsctonbs b m8rksd by minor nomA f8ults whbh 8ffec ts ths Mssozoio bsd-rock, hr Nsogsns b8sinfilling 8nd ths Cu8tsm8ry fluvbl tsrmoss 8s wall 8s by 8 nomwl8 s f8ultz ths Chsquilb8mb8 nomul fsult. Thison8 wu rssothmtsd In 1937, 8nd shows 10 m high vsrUc8l dlspbwmsnt of ~18cJsl mor8kw crssts of ths lastGboirtbn (Ssllisr 8l., lQ3Qc) ndicrtinQ hat ths N-S sxtsnsion is ths prsuntdsy ststs of strssa

ln oonduskw, this study shows 8 doss rsbtbnshlp bstwssn tsotonb, ssdimsntuy Md volunb 8otivlUssdurlnQ hs Mboww to Prsuntdry svolution of ths Wsstsrn Cordillsrs ln Northsm Psru. This ngbn is dw8ctsrizsdby 4 m8jor tsctonlo rsglmos: 8 Miocuns sxtsnsbnal rrgkns hsvlng s nuinly ENEWSW trsndlng tsnsbn oosvsl withIntr8oordilbrM bulns svolution snd volunism; L8ts Nsogons snd E8rfy Pbistocsns comprssslonal tsctoniorsgimss having WNWESE Md N-S trsndlng shortsning: 8nd, s Mid-Pbistocsns to Presentday sxtsnsionsltsctOnicr h8vinQ 8 N-S trsndlng t8nriOn.

This 8n8lysls shows that ths L8ts Csnoroio stress dirsctions us homogeneous oth In ths NNW-SSE strikingCerdilbr8n ssgmsnt md in ths E-W striking C8jsm8ro8 deflection. hb suggests thrt rincr the ssrly Mbcsns thsrs

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wu no eignlRoyrt dHtoronUal routIon botwoon the E-W CJvnuu dofkdon and the NNW-SSE striking &rnPoruvlanAndrrnBolt.

k lt hr boon propoaod In wuthrm and oontral Pwuvlan k&a (S&rlor ot al., was, roes), the vuiauonr ti

~~~ot~ouIn~m~n,~pur(okr~~~lottH~~~~bounduyfoc~e

du.to~~o~~~~~body~d~~Ohbhtopogr~.~v~tionrot~~~otmouurdwlomodiflcrtknrof~bwndrry~rkcru~~topoOnphyhunotdrutiully~g~du~gthoootuidorod time period. The ohangw of boundary forow may be the offool of modHio8tionr thor in theoonvugona mto (s(brior ot al., 1985: 1oeS) ti, In fho 8lab-pull for0 (Sorol ot al., 1Oee) or in thr &a-mtwt(Skrlor and Sokr; 1989). The MwPloi8tooww and wtlva KS trwIding toMbMl tootonicrofthogonalto~~dinction,irupWmdbynl8thn~ktrmn~bounduy~rmdbodytor#aThoktrMiooono and udy Ploietooono oompmaeional twtonloa aoomr to be nlatod to an inorour of the boundary foroorduo thor to igh oommrgonoa r& or to a’8l8b ruptum Lllkr, 1989; S&riar and Solr, 1gSg). The kte MiocwoENE-WSW Wondlng tanrknrl took&a, k awolatod with prodominwt gravitatIonal body foroor duo to thr hightopogmphy and dlmlnutlon ol tJw boundary fomor b&g duo thor to diminution ol oonvorgonoo rata, or to 8trong8lab-puuor- lnduerd by ong alab. Momover, iho mod roglon llluetr8tor that thr Inoroaso of thrvwikal8tmu la udfloionl to produoo tonsional drfonnrtlofu won H the moan topography ir nol wry high

(nruiy 31Co ml.

REFERENCES :

Bollhr 0. (1989). - Thbw Univ. ParCSud, Orsay, 2gSp.Sollior 0.. S6brirr M.. Gasu F., Fowtanirr E. and Roblrr I. (19esp). submitted to Gkdynamiqur - ORSTOM.bllkr O., S(brlrr M., Fowtanir E.. Ourr F. and Roblrr I. (1oBgb). Annalrr Tootonkao, Vol.lll, nY, in preu.blllor O., Machub J. and S(brirr M. (1oesC). Blotin do la Sooiodad Goologiu drl Peru, in prru.Cuoy, E. (1979). Rev, G6ol. Dyn. Ologr. Phyr. 21, i7-6&ward, F. (1984). Journal Goologioal SocJoty London. 141. p.893-ooo.8)brhr, M.. J. L M&or, F. ward, 0. Laubachrr and E. Cuoy-Gailhudir (1985). Teotoniu, 4, p.739760.s(brior M., Morok J.L, Maohub J., Sonnot D., Cabmm J. and Slant J.L (1oSe). Teotonios, 4. p.895928.S&ior M. and Solr P. (1980). G.S.A Special Papor : ‘Andoen magmatlem end its toot&o uttlng’, aocsptrd.Soml D.. Moroior J.L. Koraudron Rand Cuehing M. (1seS). C.R Aud. Scl. Park, 307, r(rio II, p.lgSl-1086.

Fig. 3 - Neogene to Oustemery gcodynrmic evolution inthe Weeccrn Gxdillcn of Nonhem Peru.

1 : lntnsordillcnn Miocene ENEWSW 1) and NESW (2) extcnsionel tectonics

2 : lntnsordillcran Lte Neogenc-Early PleistoceneWNW-@SE (3) and N-S (4) compressional ectonics

3 : Intn-cordilleran Quetemary NS extensionellecconicl 5)

Boxes en : 1) Miccene normrl faults; (2) regional faults;(3) Men6on Thrust md Fold Belt: (4) Subondeen Thrust endFold Belt; (5) Subduction trench; (6) studied basins (N)Nsmon SM) San Marcoc end (CA) Cajabrmbr bash% (7)compreeeionel direction: (8) Miocene enensionel direct ion;

(9) Neza plate eonvrrgencc direction

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31003 e Structural