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Home , Aguilera (volcano), Altiplano Basin, Andean Geology, Andean orogeny, Antofalla, Carnegie Ridge

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

TECTONIC OF THE OF , ,

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

This chapterwasprepared underthe co-ordination chainisvery narrow. Thehighest average altitudeisreached ofE.[aillard. Together withG.Herail andT. Monfret,hewrote between 15°5 and 23°S, where the ofBolivia and the Introduction. Enrique Dfaz-Martinez prepared the southernPerureaches anearly 4000 mofaverage elevation, section on the Pre-Andean evolution ofthe Central Andes. andcorresponds tothewidest partofthechain. TheAndean Again Iaillard, onthe Pre-orogenic evolution ofthe North­ Chain is usually highly asymmetric, witha steep western Central Andes. E.[aillard, P. Baby, G. Herail.A, Lavenu, and slope. and a large and complex eastern side. In Peru,the J.E Dumont wrote the texton theorogenic evolution of the distance between the trench and the hydrographic divide North-Central Andes, And, finally, [aillard dosed the variesfrom 240 to }OO km.whereas. the distancebetween manuscript with theconclusions. thehydrographic divide and the200m contourlineranges between 280 km(5°N) and about1000 kIn ( Transect, 8·S - 12°5). In northern and (23·5),these distances become 300 krn and 500 km, respectively. Tn INTRODUCTION: southern Peru,as littleas 240 km separates the THE PRESENT-DAY NORTH-CENTRAL (6425 m) from the Chile-Peru Trench (- 6865 m). This, together with the western location of the Andes ANDES (jON - 23°5) _ relative to theSouth American Con tinent,explains whythe riversflowing toward the do not exceed 300 TheAndean Chain isthemajormorphological feature of kmlong, whereas thoseflowing to theAtlantic Ocean reach theSouth American Continent. ThisB(){)() kmlongmountain 4000 kmlong.We haveto notetwo exceptions to this rule. beltextends alongthewestern borderoftheSouth American In Ecuador thisasymmetry disappears, duetopeculiar Plate, and can be divided into three segments of distinct tectonic history and deformational processes. Thetrench­ orientations, separated bytwomajorbends. TheNNE-SSW hydrographic divide distance roughly equalsthe distance trendingColombian-Ecuadorian segment (12°N - 5°S) is between the waterdivide and the 200 m contourline,and 2000 km long and includes part of northernmost Peru and ranges between 280and 350km. Between U·S and 24°$ easternmost . It is separated from the Peruvian (southern Peru-Bolivia). there are two hydrographic segment by the Huancabamba Bend (Megard, 1987). Its divides, delimiting a wide, flat, endorheic basin known as northern end(eastern Venezuela) exhibits achange to anE­ the Altiplano, which coincides with the zone of highest Worientation. dueto itsconnection to theSouth Caribbean average elevation and largest widthofthe chain(fig. 1). dextral transform system. ThePeruvian segment (S·S ~ 18°5) The highest summits are usually recent or active is 2000 km long and its orientation is closeto NW-SE. It volcanoes situated on the deformed chain or on includes northern Bolivia, andisseparated from theChilean metamorphic or graniticslices uplifted by reverse faults. segments by the Arica Bend. The Chilean segment (IBCS ­ Among the former are the (5897 m) and 56°5) is 4000 km long and trends N-S. Its southern tip volcanoes (6310 m) in Ecuador, the Coropuna exhibitsa change to an E-W direction along the Scotia (6425 m) andAmpatovolcanoes (6310 m) in Peru,andthe sinistral transform system. Sajama Volcano (6520 m) in Bolivia, nearthe Bolivia-Chile Thewidthofthe Andean Chain variesfrom250km in border. Among the latter, we may notethe mainly granitic northern Peru (5·5) or southernmost Chile (52°5 - 55°5). Blanca in Peru,which culminates at 6768 m to as muchas 750 km in Bolivia (18°S). (Nevada Huascaran), and the metamorphic Eastern Thearea described in this chapter includes Ecuador. Cordillera ofsouthernPeru and northernBolivia (Nevado Peru, Bolivia and,therefore, northernmost Chile (Nof23°S). Illimani, 6682 m; Nevado Illampu, 6485 m). However. It includes parts ofthethreesegments described above, the deformed and uplifted sediments may also form high orientation of which played a significant role in the pre­ summits in Peru, suchas theNevado Yerupaja (6632 m) in orogenic period. the CordiJIera Huayhuash andthe Nevado Ausangate (63&4 The lowest pass of the studied is situated in m) in southern Peru or the Cordillera de Apolobamba northernPeru(Abra dePorculla, 2300 m),a zonewhere the (Nevado Cololo, 5975 m) in Bolivia. 481 "''''''''' '-"'- ""', ~ .~ ...... ,! ! ,• 0 -­ .(21 .-..._- ,...... ! ~ Eo>lom e-. , E3 - ~ r.:-::.J ~ ­ I ',:,{:JB s- ; •

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I (IncaPlateau?) subducted (Gutsher etal.,1999). Inaddition, kmalong the contact zone between the two plates. Over W l­ v> it isaccompanied byimportantshallow (< 20 magnitude 8.7, thesouthernPeru onAugust 14,1868 (M= o :::l: depth) related to crustalshortening. 9.0), the northern Chile on May 9,1877(M =9.0)and the z w tx: l.I.I The contact zone between the Nazca and South Ecuador- on January 31, 1906 (Mw = 8.8) great I ~ American plales concentrates only a partoftheconvergence earthquakes and theirassociated tsunamiscaused a lot of o z motion as shown byGPS measurements (Kendrick et al., damage. Ontheotherhand,onJune 9,1994 thelargest deep c z «1: 1999). In the case ofthe Antofagasta 1995 , the earthquake (Mw =8.2, depth=630 km)in recorded history «1: lastgreatChilean event (M"= 8.1),a locked portionofthe occurred beneaththeAndes providing moreconstraints on s::::; o plate interface was ruptured. The observed slip release thedynamics andgeometry oftheNazca Plate atthat depth. co (between 2and7m)was lessthantheexpected plate motion a:=i l.I.I accumulation (over 8 m) and suggests that the resultant e; ct! motion causes permanent deformation through uplift ofthe o PRE-ANDEAN EVOLUTION OF THE o Altiplano and crustal shortening in eastern Andes « NORTH-CENTRAL ANDES DOMAIN ~ (Norabuena etal.,1998)...... v ..... o Low magnitude earthquakes recorded bylocal seismic ..... This sectionattempts to provide a coherent overview LU arraysare revealing small-scale differences inseismicity of C of the paleogeographic and geodynamic evolution of the z the Benioff-Wadati Zone and thecontinental crust(Comte ..0:: I.U CentralAndes Domain during the Paleozoic, along with I et al., 1999), and should be related with some specific t- tectonic processes. Body-wave velocity models at thescale somecomments onitsProterozoic basementThesynthesis o z oftheSouth American Plate(Bijwaard et al., 1998) and at hasbeencompiled from different sources, andalsoincludes o ;:::: smaller scale (Comte etaL, 1994; Dorbath et aL, 1996; Myers some new concepts and interpretations. The area here :::> considered as Central Andes is roughly 5"-27'S, covering o et aI., 1998) show a medium lateral heterogeneity where .....> the descending oceanic Nazca Platecanbe observed over part of Peru, Bolivia, northern Chile and northwestern w z Argentina (Fig. 4),and therefore excludes the Arnotapes of o 600km depth as zonesof fasterseismic velocities due to >­ w northwestern Peru and southern Ecuador, and the ~ the colder, denser material than the underlying >- asthenosphere. The crustal thickness underneath the Precordillera ofwestern Argentina. Altiplano isof60- 75krn,twice asthickasnormal, In contrast with the rather complex Phanerozoic but its exact origin remains enigmatic. accretionary historyof the SouthAmerican margin in the Northern and Southern Andes (Colombia-Ecuador and Chile-Argentina, respectively), the evolution of theCentral Andes appears to be somewhat simpler.as there is no Volcanic and seismic activity evidence fortheaccretion ofallochthonous terranesduring the Phanerozoic. Recent studies suggest that the crustal Like all activemargins, the Andesare submitted to basementin mostof theCentral Andeanareaformed part intense volcanic activity, which presents, however, several ofthe Grenville Orogen, as a resultofthe collision between peculiarities. Distribution of theactive volcanoes indicates Laurentia andAmazonia intheMesoproterozoic (Wasteneys that the activity is not continuous along the etal., 1995; Sadowsky andBettencourt, 1996; Tosdal, 1996). Andean margin, defining a Northern, a Central and a Paleozoic rocksoftheCentral Andes record the break-up of Southern Volcanic Zone (NVZ, CVZ and SVZ; Fig. 3). that the Proto-Pangea () in the latest have beeninterpreted asrelated tothedipofthesubduction Neoproterozoic- to form a along plane.As amatteroffact,theNorthern andCentral Volcanic western (Bond et al., 1984; PoweJl et al., 1993), Zones overlie segments with relatively steep Wadatti­ and itslater evolution as an active margin duringmostof BenioffZones. whereas the volcanic gap of southern the Paleozoic and untilpresent times(Sempere, 1995). The Ecuador and northern-central Perucorresponds to a zone continuous superposition of magmatic, tectonic and of flat . In this latter case, the lack of an sedimentary events has led to complex lateralvariations, asthenosphere wedge between thesubducting slaband the bothin cross section and along strike,and hasoriginated a upper continental plate would prevent arc wide range of settings for the development of generation (Barazangi and (sacks, 1976). Moreover, the deposits (Fontbote etat. ,1990; Schneider, 1990; Fornari and NVZ is characterized by basaltic , whereas Herail, 1991) and hydrocarbon generation and andesite, lavaandignimbrite dominate theCVZ. The accumulation (Gohrbandt, 1992; Moretti et al., 1995; latter feature is interpreted as due to fractional Tankard etal. ,1995) related with Paleozoic rocks. crystallization during the ascent of the andesite magma through a thickened Inaddition, thispartoftheworldhasalready generated Proterozoic basement great earthquakes ofmagnitude over 8.2followed ingeneral by destructive tsunamis. They occurredrepeatedly with a Due to the protracted superposition oforogenic events certainperiodicity of morethan 100 years as the greatest in the Central Andes, the interpretation ofthe Proterozoic instrumentally ever-recorded earthquake located in evolution of the region is inevitably very fragmentary, and southern Chile (37"5 - 46.5°5). This earthquake occurred should be considered as a mereworking hypothesis. The .... on May, 22 1960 (Mw 9.4, depth .... 25 km) and was modembasement oftheCentral Andes consists oftwocrustal generated bya 20m slipuponan areaof about 1000 x 200 blocks with different origins: theArequipa- 487 TECTONIC EVOLUTION OF SOUTH AMERICA

1~?~1~viJI ..... z o ::; (Ramos et al; 1986) and theAmazonian Craton (Teixeira et Break-up of Rodinia and rifting of eastern Laurentia from ::;) Cl al., 1989) orCentral Shield (figs. 1and2).TheArequipa­ Gondwana in the Neoproterozoic and Early Cambrian ledto Antofalla Craton isaProterozoic terraneinterpreted to have the development of passive margins on both sidesof the originated asthetipofapre-Grenville Laurentian promontory Southern IapetusOcea n (Fig. 5-c). (comprising Labrador, Greenland and Scotland) that was Along the pre-Andean margin ofGondwana Sof27·S, incorporated into the Grenville Orogen (Dalziel, 1994; a westward shift of the spreading centre is interpreted to Wasteneys et al., 1995). Pb isotope composition seems to have left oceanic crustbetween adetached continental block contradict thismodel, indicating instead closer tieswith the (Pampean ) and the Gondwana margin (Rapela et (Tosdal, 1996). Thereconstruction ofthe al., 1998), Later eastward subduction and closure of this remnants of the Grenville Orogen in South America remnantseaduringtheEarly Cambrian led to thecollision (Sadowski andBettencourt, 1996) indicates that theCentral of the Pampean Terrane inthe Middle Cambrian. To the N Andes corresponds to an area intermediate between the of27"S,a similar history isalsoprobable, with theArequipa­ magmatic arc(represented bytheSunsas igneous province, Antofalla Craton alsopartially rifting andthenlatercolliding in eastern Bolivia and western Brazil) and the thrust belt along its southeastern boundary with the Amazonian (southeastern ) oftheGrenville Orogen (Fig. 5b),and Craton. Meanwhile, thewestern margin ofboththePampean explains the similar trends identified between the Terrane and the Arequipa-Antofalla Craton remained Proterozoic outcrops along the Andes, and those of the passive untiltheLateCambrian (Fig. Soc). Brazilian Shield (Litherland et al., 1985, 1989). Beginning in the Late Cambrian or earliest , Paleoproterozoic ages indicated by Rb/Sr whole-rock this western passive margin became an active continental isochrons and bulk U/Pb zircon geochronology represent margin (Fig. S-d).TheSan Nicolas insouthwestern the pre-Grenville Laurentian-Amazonian protolith,and Peruisinterpreted asthe rootofthemagmatic arcresulting Mesoproterozoic ages of granulite-facies from eastward subduction ofoceanic crust along theactive indicated by U/Pb Single-grain zircon geochronology margin of Gondwana during the Paleozoic (Mukasa and represent themaincollisional events oftheGrenville Orogen Henry, 1990).Ordovician- ages obtained forlower (Wasteneys et al., 1995; Sadowski and Bettencourt, 1996; intercepts in U/Pb geochronology ofbasement rocks along Tosdal, 1996). Rifting during break-up of Rodinia in the the western Arequipa-Antofalla Craton reflect thermal Neoproterozoic-Cambrian led to separation of Laurentia overprinting and Pb-Ioss coincidingwith peaks of this from Amazonia (Fig. 5-c), leaving behind the Paleozoic magmatic activity (Shackleton etai, ,1979; Damm parautochthonous Arequipa-Antofalla Craton attached to etai.,1990; 1994; Mukasa and Henry, 1990; Tosdal, 1996). Amazonia (Central Brazil Shield). The boundary zone Different rates of plate activity and sense of migration of between the twocrustal blocks, which is located beneath themagmatic arcdeveloped depending onregional stresses the Eastern Altiplano and Eastern Cordillera, thus and inhomogeneities, andbasindevelopment alsochanged constitutes a paleosuture and crustal weakness zone in accordance withtheseplateinteractions. inherited from the Mesnproterozoic evolution of the The Paleozoic development of this active margin is Grenville Orogen (Figs. 1and2).Thiszoneremained active characterized bybackarcextensional conditions duringthe duringthePaleozoic, andever since,withvariable behaviour early phase (latest Cambrian-Early Ordovician) and late depending on the regional stateof stresses (Ramos, 1988; phase(LateCarboniferous-Pennian),resulting in astrongly Dorbath et al., 1993; Forsythe et al., 1993). subsiding thinned crust, with partial rifting and syn­ sedimentary basic (Fig. Sd). In contrast, a compressional regime (retro-arc foreland) characterized the intermediate phase (Middle Ordovician-Early Tectonomagmatic episodes ; Fig. 5e). Anapparent lackof evidence for and Devonian tectonomagmatic activity in the Tectonic and magmatic events tookplace inthe Central southwestern Central Andes has been interpreted as Andes in a rather continuous manner during the whole evidence fora passive margin resulting from riftingoffof Paleozoic, shifting their foci and areal extent with timeas a partoftheArequipa-Antofalla Craton (Bahlburg andHerve, result of plate interactions, variable geometry of the plates 1997). However, this interpretation is difficult to reconcile involved, and the resulting stress regimes. Thesevariable with the evidence for a coeval active plate margin in conditions led to apparently different styles of evolution southern Peru (down to 17"5) and northern Chile and depending onthelocal areaunderstudy. However, an overall Argentina (up to 27·S),as well as withthe Silurian ageof tendency for the whole region may be discerned and igneous andmetamorphic events inthesameregion (Damm simplified as follows. Apart from the aforementioned etal., L99O, 1991, 1994). Mesoproterozoic (1.2Ga- 900 Ma) ages resulting from the With regard to Late Paleozoic tectonomagmatic events Grenville granulite-facies metamorphism, and the intheCentral Andes, thesehavebeentraditionally assigned Paleoproterozoic (2.0 -1.9 Ga) ages obtained from its toa"Eohercynian"Orogeny (Megard et al., 1971; Bard etal., metagranitoid protolith, thecrystalline basement underlying 1974; Dalmayrac et aL,1980), including Late Devonian­ the Central Andes also presents Neoproterowic to Middle Carboniferous K/Ar and Rb/Sr ages, and regional Cambrian (600- 520 Ma) ages ofigneous and metamorphic stratigraphic andpetrographic evidence. However, onlyone events.These events arecommonly assigned tothelastphases U/Pb zircon age is reported (330 ± 10 Ma; Carlier et al., ofthe BrasilianoOrogeny, which arereferred to as Pampean 1982). OtherUlPbzircon datesongranitoid along the NW 488 Orogeny inthesouthernCentral Andes (Rapela etaL,1998). trending segment of the EasternCordillera of Peru and TECTONIC EVOLUTION OF SOUTH AMERICA I~t.'~2~~·'{:~;c;'~'>" I ~

:::c: Bolivia establish theirtime of emplacement as or towards the S, developing in Bolivia during the Middle u ...... ,., younger (McBride etal., 1983; Heinrich et al.,1988; Farrar to Early Jurassic (McBride et al., 1983; Heinrich er a ~ etal.,1990; Kontak et al., 1990). al., 1988). The activity was not related to subduction Z a: I.U This more recent evidence questions the relation ofthe processes, but instead, consists of alkaline volcanism and :::c: ....0: granitoid plutons with widespread Late Devonian-Early plutonism which are interpreted asa result ofpartialrifting o Z Carboniferous "Eohercynian" magmatism and orogenesis, and transtension along the zone between the o z aspreviously interpreted. Nevertheless, thereisevidence for Arequipa- Antofalla Craton andtheAmazonian Craton (Fig.

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'""'m· F .._ _ _ ...... ~ ""' - ... -- ,~-~ ,.., PRE..OROGENIC (LATE PERMIAN" volcanic rocks are known.In the coast of southern Peru (Arequipa Massif), metamorphic and intrusive rocks MIDDLE ) EVOLUTION yielded KlAr ages rangingfrom 213to 187 Ma (Stewart et OF THE NORTH..CENTRAL ANPES aI., 1974; Romeuf etal., 1993). In southern Peru,a Triassic­ Liassic age is assumed by Boily et al. (1984) for the arc Thisperiod maybedivided into two mainsub-periods, volcanism, sinceit is post-datedby, and locally intercalated of Late Permian-Late Jurassic and latest[urassic-Paleogene with,Sinemurian marinedeposits(, 1970; Jaillardet age, corresponding todifferent geodynamic, paleogeographic al., 1990).However, one of the plutons intruding these and tectonic settings. volcanic rocks yielded a 211 Maage,thus indicatinga pre­ Liassic minimumage(Romeuf et al.,1993). In northern Chile (26 - 31°S), basic to granitic,I-type Late Permian to Late intrusionsyielded ages between 231 and 201 Ma(Berg and Baumann, 1985; Irwin et al., 198B; Cana, 1991),and are Kimmeridgian (255 • 140 Ma) roughly coeval withsyn-metamorphicdeformationdated at220- 201 Ma (Irwinetal., 1987). Fartherto the E, inPeru, Priorto the Sinernurian, the magmaticarc is not well­ thick sequences of fine- to coarse-grained, defined. During Jurassic, the paleogeographic framework and conglomerate beds (MituGroup) weredeposited in a exhibits theclassical three-fold division ofan activemargin subaerial environment, and filled fault controlled grabens (Fig. 6). The fore-arc realm,is nearly unknown,becauseof (Megard, 1978; Kontak et al., 1985; Carlotto, 1998). The subsequent tectonic erosion and/or deformation. It was associated volcanic rocks are alkaline with located in the present-day Eastern Cordillera of Ecuador subordinate tholeiitic basalt,and acidic, dacitictorhyolitic (Cordillera Real), whereas it probably liesoffshore Peruand pyroclastites (Fig.7). Shoshonitic and peralkaline suites Chile. The magmaticarc controlledthe paleogeographic (Kontaketal., 1985) aswell ascomendite(Noble era!', 1978) pattern.It trends NN Ein the northern segment;runs along are present, indicalingan intraplate extensional tectonic the present-day Subandean Zoneof Ecuadorand crosscut regime. Because theyoverlie fusulinid-bearing limestone obliquely the northern Peruvian margin. Farther S, it ofEarly Permianage, the volcanic andsedimentaryrocks of appearslocally on the SEtrending coastof southern Peru, the Mitu Group have been ascribed to the LatePermian­ and extends into northern Chile,where its present-day Triassic (Newell et ai, 1953; Laubacher, 1978). Farther to orientation isroughly N-S. Theback-arc domaincovers what the N (Ecuador), this period corresponds to non-marine arenow theOriente Basin ofEcuador, theWesternCordillera clastics and subordinate volcanics (Sacha Formation, ofPeruandmostof thecoastof northernmostChile. Distal Rivadeneira and Sanchez. 1991). Deposits similarto those back-arc areas covering eastern Peruand Bolivia (Fig. 6) ofthe MituGrouphavebeen recentlyidentifiedin Bolivia, bordered it tothe E. wheretheyart:foundinmajorpaleogeographic structures, which governed theAndean (Sempere er al., 1999; LatePermian - MiddleSinemurian Fig. 7). fn northern Chile, deep transtensional basins (255 -195 Ma) were filled by thick sequences of subaerial basic, intermediate and acid volcanic rocks and subaerial This period ischaracterized by Late Permian-Triassic siliciclastic and volcaniclastic rocks of Late Permian­ alongNW-SE trending grabens,and Triassic age (Chong, 1976; Suarez and Bell, 1991; Flint er byLate Triassic-Early Liassic marinetransgressions. Coeval al., 1993). alkaline volcanism and intrusionsdueto partial melting of TheLate Permian-Early Triassic agehasbeenconfirmed the lower crust were locally associatedwith deep-seated in southern Peru by 270 - 200 Ma ages obtained from metamorphism. intrusionsand shoshonite, which, however, extentintothe Early Jurassic (200-180Ma,KontaketaL, 1985; ClarketaL, Late Permian- EarlyNorian (255 - 215 Ma) 1990). In southern Peru,coeval biotitegranodiorite and Very littleisknownaboutthe Westernmost areasofthe monzogranite derived from partial melting of the lower margin.Coastal areasare,however, affected bysignificant crust (Granitoid province) are crosscut by dykes although poorly understood deformation and showing chemistryand mineralogy similar to that of the metamorphism. In the EasternCordillera ofEcuador, type alkalibasaltoftheMita Group (Kontak etal.,1985). Farther I granitoid(Ires Lagunas) yielded ages ranging from 257 the SE, mostofthe graniticto granodioritic, peraluminous (SmlNd) to 200 ~ 189Ma(Rb/Sr, Aspden and Litherland, intrusionsofthe Bolivian CordiJIera Real yielded agesfrom 1992; Litherland etaL,I994),and in southwestern Ecuador 225 to 19$Ma (Avila.Salinas, 1990). These sedimentary, orthogneiss yielded metamorphic agesof234to 198 Ma(KI tectonic and magmatic phenomena were interpreted as Ar,Feininger and Silberman, 1982; Rb/Sr, Aspden et al., resulting froma NW-SE trendingrifting of LatePermian­ 1995).Intrusion andmetamorphismareinterpretedas due Triassic age,relatedto the break-up of Gondwana and/or to a strongthermal event of LateTriassic age,resulting in to theTethyan rifting(Vivier etal., 1976; Kontaketal., 1985; partial crustal melting and high temperature Jail/ard etat,199Q; Hint et ai., 1993; Sempere etal.,1998). metamorphism. Theywould be associated with significant Nodear evidences of subduction-related volcanism have dextral movements relatedto the Tethyan oblique rifting been foundas yet in theseareas. (Litherland et al."1994). In northern and centralPeru,no pre-LateJurassic arc 493 ' H'O"", ...... ,. ...." • • ~ " " . 10-X> I- -,- - ---~- j ~ - -­"" . i ...... ­ ,l ------i -- ~:<-- --, - .. • I • •;

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-'IO«'lf---' __... -""J ....- _,,- .....- . ~ . -.M_-_""'....-.. " ... - --...... - -.....­ - «--,_,__- _ ~...4. ­,. •'.•. -~=- -~ - .- - - -_--. -- --- •Eil - :::I: Late(?) Norian - Middle Sinemurian interrupted by a Middle Jurassic tectonic event; (3) in u l­ (215 - 195 Ma) southern Peru and northern Chile, the western magmatic V">o ~ arc was flanked to the E by a marine,subsiding back-arc z Two maininarinetransgressions anda relative tectonic cr: LU basin,bordered by emergenteasternareas (Fig. 8). :r quiescence characterize this time-span. The Late Norian I- In Colombia (Mojica and Dorado, 1987), southern o reached the western part of the z"" Ecuador (Romeufetal.,1997) and northwestern Peru( Andean marginfrom southernColombia to northernChile C> and Sanz, 1979), Middle to Late Sinernurian marine «z (Fig. 8). Norian deposits usually overlie Permian-Triassic « deposits are intercalated with alkaline basaltic flows, subaerial volcanic and sedimentary rocks, locally in angular :> whereas subduction-related volcanic arc rocks appear in ~ unconformity, or may overlie the Paleozoic basement. In the immediately overlying deposits, which. evolve toward '"::i a: thecentral andnorthernparts ofthebasin(Ecuador.Peru], LU subaerial environments. The Late Sinemurian age of the "'- Norian deposits consist of shallow-marine shelflimestone ~ beginning ofthemagmatic arcactivity isconfirmed bydates o and dolomite (Megard, 197B; Pardo and Sanz, 1979; o obtainedfromassociated arc-related intrusions (195 - L90 « Loughman and Hallam, 1982; Prinz,1985; Rivadenei ra and ~ Ma.Aspden etal.,1987; Litherland etal.,L994). Theactivity u..... Sanchez, L991; Rosas el ai., 1997), whereas in the eastern u.. ofthisNNE trendingcontinental magmatic arc(Misahualli o (easternPeru,Megard, L978)and southernareas(southern .....V"> and Colan formations) continued on during the Middle o Bolivia, northern Chile, Groschke el al., 1988; Suarez and z Jurassic, and eventually ceasedbylatestJurassic times(L50 ....« Dalenz, 1993; Ardill et al., L998), they begin with a :c - 140Ma, Aspden etal., 1987; Litherland etal.,1994, Romeuf I­ transgressive clastic, locally evaporitic sequence, evolVing .... et al., L995). To the W of the magmatic are, marine o toward marinedeposits. z volcaniclastic sediments, datedaslate Middle to earlyLate o A stratigraphic hiatus of latest Norian-Rhetian age i= Jurassic in southernEcuador and northern Peru(Mourier, => occurred locally in Peru (Megard, 1978; Loughman and o-' 1988), are interpreted as fore-arc deposits. > Hallam, L982; Prinz, 1985), which may explain local ..... In southwestern Peru, the disconforrnable marine u Z disconformities (Pardo and Sanz, 1979). Thesecond major o transgression is datedas Early Sinernurian, Early Toarcian, l­ marine transgression is of Late Hettangian to Early .....V Late Toarcian (commonly), or even late Bajocian­ I- Sinemurian age (Fig. 8).InPeru,thesetransgressive deposits Bathonian, according to the regions, suggesting either a onlap onto the Permian-Triassic volcanogenic rocks contrasted and complex paleogeography, or the (Meganl, 1978; Loughman andHallam, 1982).lnthecentral juxtaposition of distinct units due to subsequent partofthebasin,they consist ofdarklimestone unitsrichin displacements (Kurth etal.,1996). Most ofthe successions organic material tobituminous shale, theupperpartofwhich display a sedimentary hiatusofLateBajocian-Bathonian age isrichinphosphate (Megard, 1978; Loughman and Hallam, (Vicente, 1981, 1989). Volcanic arcrocks formerly ascribed 1982; Prinz, 1985; Rosas etal., 1997). Inmanysouthern areas, to the Triassic-Liassic (Chocolate Formation) yielded the Sinernurian transgressive beds are the first marine radiometric datesof 177- 157 Maand Aalenian-Bathonian deposits to be recorded. They may locally onlap onto the paleontological ages (Roperch and earlier, 1992; Romeuf el Paleozoic basement (Antofagasta, Munoz et ai., 1988; Baeza al., 1995),thus suggesting that they are coeval with the andQuinzio, L991),or overlie undated (Triassic?) arcvolcanic magmatic arcofEcuador, northern PeruandnorthernChile. rocks (southern Peru,Vargas, 1970; Vicente et aI., 1982; These volcanicrocks are overlain by volcaniclastic and lquique and Arica, Muiioz andCharrier, L993). siliciclastic marineshelfdeposits ofAalenian-Callovian age Intheback-arc areas ofcentral Peru, scattered lavaflows (Upper Rio Grande andGuaneros formations, RUegg, 1956; interbedded in Early Liassic marine sediments display Aguirre and Oftler, 1985; Romeuf et al., 1995), thus alkaline chemistry, indicating thatextensionaltectonic regime indicating a progressive and significant decrease of thearc stillprevailed (Rosas andPontbote, 1995; Romeuf etal., 1997; activityduring the Middle Jurassic. No volcanic rocks of Figs. 9and 12). In summary, theLate Triassic-Early Jurassic LateJurassic ageknown. period is marked bya mild extensional regime, interpreted In the coastof northern Chile, outpouringof a thick as related to the break-up ofGondwana and tothe Tethyan accumulation ofsubduction-related dacite, andesite, basalt rifting. Large-scale marinetransgressions occurred intheLate and tuff (LaNegraFormation, Fig. 10)began alsoduring Norian and Early Sinemurian, respectively. Subduction ofa the LateSinernurian-Pliensbachian (Munoz et al., 1988; paleo- Pacific Plate beneath this part of theSouth American Baeza and Quinzio, L991; Munoz and Charrier, 1993). They Plate hasnotbeenproved as yet. follow basic intrusions related to probably transtensional sinistral movements (Pichowiak et al., 1990). In Late Sinemurian - Kimmeridgian northernmost Chile, this volcanic series is overlain by (195·150 Ma) marinesediments ofearlyMiddle Jurassic to Late Jurassic age,according to the areas (Munoz and Charrier, 1993; This interval is marked by active subduction-related Kessler, 1997), butvolcanic activity isassumed to have lasted magmatism, sinistralwrenching movements along NW-SE untilthe Late Jurassic (Charrierand Munoz, L994) oreven faults, and tectonically controlled sedimentation. Three theEarly Cretaceous (Mpodozis andRamos, 1989; Scheuber main domains are distinguished: (1) the NNE trending etaL, 1994), fartherS.Extensional ortranstensional regime Ecuadorian-northern Peruvian segmentischaracterized by prevailed duringthe Jurassic, favouring the formation and a NNE-trending magmatic arcborderedto the Ebya back­ play oftheAtacama wrench fault system (Brown etal., 1993). arc subaerial basin; (2) in northern and central Peru,the East of the magmatic are,the Jurassic back-arc basin carbonate shelf sedimentation continued and was received poorly dated, mainly argillaceous and volcaniclastic 495 TECTONIC EVOLUTION OF SOUTH AMERICA ~;;£¥:T::h IAa:.7~~ I

subaerial sedimentation (Chapiza and Sarayaquillo Early to early Middle Jurassic.Soler and Sempere,1993; formations; Tschopp, 1953; Seminario and Guizado, 1976; Sernpere et al.•1998),and volcanic flows interbedded in Rivadeneira and Sanchez, 1991; Fig. 6).Thesesedimentary the Bajocian to Callovian deposits of the Domeyko Basin rocksreston the Liassic carbonatebeds to the W, or on the are regarded as related to transtensional movements Paleozoic basement to the E. In the latter case, they may (Ardillet al., 1998; Fig. 14).On the other hand. the mid­ include unfossiliferous Late Triassic-Liassic beds. They Jurassic Arequipa Troughhas been interpreted as a pull­ comprisealowersequence ofshale,fine-grained sandstone, apart basin due to sinistral movements (Vicente et al., dolomite and of sabkha environment, and an 1982).Therefore, this period seems to be dominated by a uppersequence consistingofcoarse-grained sandstoneand mild extension,probablydue to a sinistral transtensional conglomerate of fluvial environment (Tschopp, 1953; regime (see also Scheuberetal.,1994). Megard, I978).However, Jurassic marineplatform limestone However, the southern part of the area (10"5 - 25°5) is units are locally known in easternmost Ecuador characterized by a sedimentary hiatus of Late Bajocian­ (Petroproduccion, unpublisheddata). Bathonian age (Megard,1978; Vicente, 1989; Ardill et al., In the back-arc areas of Peru, platform carbonate 1998).This "Bathonian phase" is classically regarded as deposition,started in the LateTriassic, went on until the responsible for the disconforrnity that separates the lower earlyMiddleJurassic (Figs. I I and 12).Thiscarbonateshelf and upperredbeds ofthe PeruvianEasternBasin (Megard, was bordered to the E by continental back-arc deposits 1975}.ltis coeval with a rapid exhumation(3000m) ofthe consisting influvial redbeds tothe N,and locally significant Eastern Cordillera of central Peru (Laubacher and Naeser, accumulations of eolian sandstone to the S (Sernpere, 1994),withmetamorphismofthearczoneofnorthern Chile 1995). No information is available on the western part of (170 - 150Ma, Lucassen and Thirlwall, 1998) and with central Peru. On the Peruvian Platform,the bituminous intensefolding and reverse faulting ofthe arczoneof north facies andphosphate-richcarbonatebedsoftheSinernurian (163- 140 Ma,Irwinetal.•1997). Therefore, a transgression (Aramachay Formation) are overlain by poorlydocumented compressional deformation seems to stratified oolitic and skeletal limestone presenting local have occurredin the Late Bajocian-Bathcnian. cross-stratified calcarenitebeds of Late Sinemurlan to Late Insummary, between 195 and 140 Ma,theNNE trending Toarcian, maybe Aalenian age (Condorsinga formation; Colombian-Ecuadorian and the N-S trending Chilean Megard, 1978; Westermann et al., 198D; Loughman and segmentsare characterized by abundant arc magmatism, Hallam, 1982; Prinz, 1985). These Liassic deposits are unstable tectonic regime, subaerial back-arc sedimentation overlain bysandy-argillaceous limestone of Late Aalenian­ and locally marine fore-arc deposits. At the same time, Late Bajocian age (Chunumayo Formation, Megard, 1978; transtensional, probably sinistraltectonic regime prevailed Westermann et al.,1980). Thecarbonateseriesends locally alongtheNW-SE trendingPeruvian segment, which resulted (Huancayo) with unconformable fluvial sandstone inlocalized pull-apartbasinsandinthealternation ofmarine {Cen.:apuqui

'-'_'M""'''o-_ • j I ,• ,I • I "',... .. "'-.,...."' - .. • ft...... _--.,.-..' _, ,-- i ,...... " "4 ...... _ r .....__"..-.._.. --...... -.,.,. • ,.,,,..-__ n<'U" , _ i <-.4_-'.... -._..- -_,_.., ! <_...... " • --• - ._- -• ~:;Str : ~:] :{

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, ~.... . - ....., I~\·t-"-;;.,.~ }.~. . . :.....,.,;'. ) signatures ofthe Cretaceous oceanic terranessubsequently bya thickseriesofcoarse-grainedvolcaniclastic turbidite accreted to the Ecuadorian margin. beds of late Tithonian age,which reworked the Jurassic ln Peru,theback-arcareacompriseda western, mobile volcanic arc.Thisevolution pointstothe creation ofa deep, and subsidingbasin (West-Peruvian Trough ofBenavides, N-S trending sedimentarybasininterpretedasa pull-apart 1956, present-dayWestern Cordillera), an axialpositive basin(Jaillard and [acay, 1989), which extends southwards threshold (MaranonGeanticline, Cuzco-Puno Swell, and (Fig. 6). This succession follows (Jaillard and Iacav, 1989) present-dayEasternCordillera) and an eastern,stableand with Late Tithonian black shale of shelf environment, moderately to little subsiding basin (present-day Tithonian-Berriasian massive sandstone of nearshore Subandean Zone and eastern basin, Fig. 16). The latter environment (Tinajones Formation;Wilson, 1984), and extends northwards into Ecuador (present-day disconformable massive dean sandstone of presumably Basin),and southwards into Bolivia (Potosi Basin).The Berriasian age(Chimu Formation, Benavides, 1956; laillard Kimmeridgian-late Paleocene lime-span(150 - 55Ma) can and Jacay, 1989). be divided into three periods, characterized by distinct In the back-arcareasof Ecuadorand northern Peru, paleogeographic and tectonicsettings, controlled by the the LateJurassic back-arcredbeds seemtograde upwards platekinematicsevolution. into coarser-grainedred beds interbedded with basaltic to rhyolitic flows (Yaupi Mb.[aillard,1997), locally dated as earliestCretaceous (Hall and Calle, 1982; Iaillard, 1997). Kimmeridgian? - Aptian This magmaticactivitywould be coeval with small-sized (I50 -110 Ma) stocks, which intrude the Jurassic red beds and are disconformably capped by Early Cretaceous sandstone Latest Jurassic toearliestCretaceous timesare marked (Tschopp, 1953; Tafur, 1991). bya complete reorganization ofthepaleogeographic pattern IncentralandsouthernPeru,Kimmeridgian times were and tectonic evolution of the Andean margin,interpreted markedbythe arrivalofclastic deposits. Siliciclastic shelf astheresult ofadrasticchange intheconvergence direction, sedimentsabruptlyoverlie the Callovian blackshaleof the which triggered a magmatic arc re-organization and Arequipa Trough (Vicente et al., 1982; Fig. 13). These significant tectonic deformation (Aspdenetal.,1987;Iaillard sediments are correlated with undated unconformable et al., 1990, 1995). These events were followed in the Early conglomerate of the Peruvian (Chupa Formation, Klink et Cretaceous by an important diachronous marine al., 1986; Iaillard, 1995) and Bolivian Altiplano (Condo transgression in thewhole area. Formation), which, however, might be younger. The Early Tithonian marinetransgression is recorded in the western Kimmeridgian? - Berriasian (150 • 135Ma) part of central and southern Peru by shallow marine In Ecuador and northern Peru (and Colombia), the limestone (Jaguay Formation, Ruegg, 1961; Gramadal activity ofthecontinental volcanic arc(Misahualli andColan Formation, Chavez, 1982; Batty and Iaillard, 1989), which formations) ceased bytheendof Kimmeridgian times (ISO appeartobeoverlain byTithonian-Berriasian (?)black shale to 140 Ma ago,Aspden etaL, 1987; Mourier, 1988; Litherland and sandstone (Oy6n Formation, Megard, 1978; Tiabaya et ai; 1994; Fig. 17). The deformed and eroded magmatic outcrops, Geyer, 1982). arc is then overlain by unconformable fluvio-rnarine Volcanic arc activity is known in the Lima area sandstone, the diachronous base of which is dated as (Atherton etal.,1985;Aleman, 1996);Itseemstohave begun Valanginian (?)toAlbian, from SW toNE (Villagomez etal., byLate Tithonian times.since the upperpartofthearcseries 1996; Robert et al., 1998). This major hiatus and yielded ammonites of LateTithonian age(Bulot, personal unconformity suggests theoccurrence ofasignificant latest communication, 1998; formerly ascribedto the Berriasian; lurassic-Early Cretaceous tectonic event(Litherland et aL, Wiedmann, 1981), and have continued during part of 1994). No Kimmeridgian depositsare known in the fore­ Berriasian times (Aleman, 1996). Disoonformable dean arcareasof Ecuador and northern Peru. massive sandstone is ascribed to the Berriasian in the In southwesternEcuador. the Raspas high pressure western parts of central and southern Peru [Chimu, metamorphic complexyielded a 132MaK/Arage(Feininger Goyllarisquizga andHualhuani formations, Benavides, 1956, and Silberman, 1982) and 130- llS MaAr/Ar and Sm/Nd Batty and Iaillard, 1989). However, in spite of limited ages (Malfere, 1999). Theseare interpreted ascooling ages, paleontological evidence, thebase ofthesedeposits seems subsequent to the and HP metamorphism of au to be diachronous, being m'.lch younger toward the £ and accretionary prisminthelatest[urassic­ (Wilson, 1963; Iaillard, 1995; Robert et al., 1998). In the earliest Cretaceous (Gabriele etal.,1999;Malfere etal;1999). easternbasins of Peru and Bolivia, no earliest Cretaceous Thissuture extends northwards alongthe western edgeof deposits have been recognized so far, below the the Eastern Cordillera of Ecuador (Peltetec suture, unconformable Early Cretaceous sandstoneunits. Litherland etal., 1994; Aspden et al.; 1995) and to theWof In northernmost Chile, the Kimmeridgian phase is the Central Cordillera of Colombia (Amaime Terrane, marked bya marineregression which a culminatedwitha Aspden andMcCourt, 1986;Toussaint and Restrepo, 1994). local hiatus and an angular unconformity, by sinistral According toLitherland etaL (1994),thiseventcorresponds wrench movements along the N-S trendingAtacama Fault to the accretion of a continental microplate lChaucha System, and bysubsidence ofthe back-arcareas(Bogdanic Terrane) to theEcuadorian margin. andEspinosa, 1994). Intheback-arc basinofnorthernmost In northwestern Peru, Early (?) Tithonian lagoonal Chile, Oxfordian limestone and shalegrade upwards into deposits areabruptlyoverlain bydeepshelfshale, and then limestone and shalewithinterbeds ofevaporite inthe[ower 499 TECTONIC EVOLUTION OF SOUTH AMERICA l~l·<~~\1 ~~ ..... :z o :::;;: part,andsandstoneintercalations in theupperpart (Munoz margins, bytheongoing, although mild. volcanic arcactivity. => n et aL, 1988). Farther S (Domeyko Basin), the Callovian In Ecuador and northern Peru, verylittleis knownabout marine black shale beds are overlain by evaporite beds thisperiod.In theback-arc areasofEcuador, one KI Ardate deposited in basin to sabkha environments. and then by and one palynological age suggest that subaerial red bed innershelflimestone ofLate Kimmeridgian- Early Tithonian depositioncontinued during part of this period (Bristow age, which laterally gradesouthwards intomarinesiltstone and Hoffstetter, 1977; Baldock, 1982). The overlying (Chong. 1976; Ardill et ai., 1998; Fig. 14).Farther5 (31°5), transgressive disconformable sandstonebedsareofAlbian the Coastal Cordillera wasdeformed byW-verging opento age (laillard, 1997). In the Cordillera Real of Ecuador, tightfolds between140 and 126 Ma(lrwinetai; 1987). The Litherland et al. (I 994)mentionednumerous KJ Arresets deformation is coeval with significant sinistral lateral in the Jurassic granite, interpreted as due to significant displacements offore-arc and arcslivers alongtheAtacama dextral movements related to the collision of displaced Fault System (26°S),dated between 145 and 125 Ma(Kurth . In the neighbouring Maranon Basin of etal,1996). northeasternPeru,a sedimentaryhiatus seems to separate During thelatestJurassic-earliest Cretaceous, magmatic the Jurassic redbedsand the disconformable, diachronous arc activity seems to have continued without changes of transgressive sandstone beds of Early Cretaceous age location. However, a magmatic gapseemsto have occurred (Laurent,19&5). between 150and 140 Ma(Hammerschmidt etal.; 1992; Fig. Meanwhile, somewhere in the southeastern paleo­ 15) and the magmatic activity appears to decrease Pacific domain,a mantle plume was responsible for the significantly near the Jurassic-Cretaceous boundary formation of a large and thick Oceanic Plateau that was (Mpodozis and Ramos, 1989: Scheuber etal.,1994; Charrier accreted to the Andean margin in times and Munoz, 1994). (Cosma er al., 199&; Reynaud et al., 1999), and crops out TheendoftheNNE trending Ecuadorian- north-Peruvian presently in theWestern Cordillera ofEcuador whereit has magmatic arcbyTithoniantimes, and thebeginning ofthe beendated as Barremian or Hau terivian (123Ma,Lapierre activityoftheNW trendingPeruvian magmatic arc inthelate et al..1999). Tithonian, expresses a drastic change in the convergence In Central and Southern Peru, scarce outcrops of direction. which passed from nearly southwards to nearly volcanic rocks in thecoastal and western areassuggest that northeastwards (Aspden et al., 19&7; [aillard et ai" 1990, volcanic arc activity continued at leastlocally untilAptian 1995). This plate kinematics reorganization correlates with times (Bellido, J956;Vidal et al., 1990; Aleman, 1996). geodynamic events inthesoutheastern Pacific andtheTethys However, the occurrence of thick intercalations of quartz­ Ocean. During the LateJurassic, the southeastern Pacific rich sandstone and unusual volcanic-free shelflimestone accretion ridge would have been oriented roughly NE~SW intheLima area(Rivera ernl ,1975;Aleman, 1996) suggests and connected with the Tethyan accretionary system that,eithervolcanic activity waslocal and sporadic, or part (Caribbean and Central Atlantic oceans, Duncanand of the coastal terranes havebeen subsequently displaced Hargraves, 1984). Theoutpouring ofa majorplumealon.gthe (May and Butler, 1985). Pacific accretionary ridgeinTithonian times(Chalzkrridge) Farther E, in the back-arcareas, well-sorted, clean, would have disturbedtheaccretion direction; broughtabout quartz-sandstone bedswere deposited from Late Berriasian thebreak-upoftheEastern paleo-Pacific Plate, andcreated a timesonwards (Benavides, 1956; Wilson, 1963) .In northern (Nakanishi et al., 1989). Meanwhile, the PeruandEcuador, thesewere datedasLateBerriasian-Early Tethyan realm wasmarked bya Significant slowdown in the Valanginian in the easternregion, (Benavides, 1956; Rivera spreading rates,allowing the newly created NW-SE trending et aI., 1975; Bulot, personal communication, 199&), Southeastern Pacific Ridge to impose a northeastward drift Berriasian-Barremian in the western part of the Eastern direction for the Eastern paleo-Pacific Plate. These events Basin of Peru (Tarazona, 19&5), Aptian in the centreof the would haveprovoked, intheTithonian (l40 Ma), achange of basin, and earlyLate in the eastern parts of the convergence direction, according to the process proposed Eastern Basin (ViIlag6mez etal.,1996; Robert et al., 1998, byDuncan andHargraves (l984) forEarly Cretaceous times. Fig. 18). In centralPeru,sucha diachronism wassuggested This major geodynamicchange mayaccount for the by Wilson (1963), and. although paleontological evidence accretion of the oceanic terrane of Ecuador and is poor, a comparable diachronism mayoccurin southern Colombia, the creation ofthe ChicamaBasinof northern Peru (Jaillard, 1995). Peru, the Widespread emergenceand subsequent gap of Paleocurrents indicate that the Guiana and Brazilian Late Tithonian-Berriasian deposits, the unconformity shields were the sources of the clastic supply. of the Early Cretaceous deposits and the possible Paleoenvironments evolve from subaerlal/fluvial to compressional deformation recorded in northern Peru. nearshore/shallow marine from E to W, and deposition is These events may be correlated with the Araucanphase mainly controlled byeustaticvariations (Moulin, 1989). The of northern Chile (Stipanicic and Rodrigo. 1969; isopach mapclearly indicates aneasterndepocenter situated Scheuber et al., 1994). in northern Peru (present Maraiion River), and western depocenters NofLima andaroundArequipa (Iaillard, 1994; Fig. J9}. Scarce syn-sedimentarytectonic features suggest a Berriasian-Aptian (140 - 110Ma) mildextensional regime (Moulin, 1989). Thisperiod is markedby the widespread deposition of Intheback-arcareaofnorthernmost Chile, LateJurassic disconformable, diachronous quartz-sandstone units,by a strataareoverlain byfine-grained sandstone.siltstone and 500 relative tectonic quiescence; andalong the Peruvian andChilean shale with occasional tuff and andesitic lava of ---_., ! I ,, ! • j , • '0. • . . -- e • .-_­_ j ","'", " '.._ - _---*' .-...... , ,_ _- ~ "", I

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• - - TECTONIC EVOLUTION OF SOUTH AMERICA l~~~~1 ~ ~ :r cross-cutbybasicto intermediateintrusionsdated at 104 1979; Iaillard et al., 1995). Note that, there too, volcanic v I­ - 101 Ma(Wilson, 1975; Cobbing r!tal., 1981; Bussel, 1983), activityseemstohaveceased byCenomanian times. o'" ~ thus indicating that compressional deformations beganby In the back-arc areasofEcuadorand Peru,theAptian­ z a: u.I Middle Albiantimes(Cobbing etal.,19B1). Albian boundary is marked by scattered volcanic :I: t­ o::: Deformation was associatedwith significant dextral manifestations, varying from basaltic flows to rhyolithic o z movements (Myers, L975; Bussel and Pitcher, 1985). In the tuff,intercalated withinthe first transgressive units. This Clz arc zones of Peru and Ecuador, the Late Albian tectonic bimodalvolcanism as beenlocally determined as alkaline, « ::::; end of marine sedimentation, by a general decrease of Earliest Albian timesare markedby thebeginningofa o magmatic activity (Soler andBonhomme, 1990), and bythe major large-scale marine transgressionwhichreached its ::i"" 0::: W replacement of volcanic effusions by plutonic intrusions, maximum extentby Turonian times (Figs. 13and 21).The 0.. 0:::' suggesting that the arc zones were significantly uplifted Lima area alreadyreceived a marine sedimentationfrom o Cl (Cobbing et 01., 198J; Soler, 199]). These calc-alkaline Early Cretaceous times (Rivera etal; 1975;Aleman, 1996). :3u plutons, which intrudethevolcanic are,definetheso-called TheEarly Albian transgression firstreached thewestern part w Coastal Batholith ofPeru(Pitcher. 1978; Cobbinget al.•1981; oftheback-arc areas(Benavides, 1956; laillard, 1995; Robert o '"..... Soler, 1991). Effusive magmatism ceased by earliest et al., 1998), where it deposited red to coloured silt Cl Z Cenomanian limes, and incipient plutonic activity was andsandstone, withglauconitic and locally oolitic limestone -c I.l.I :I: ratherlow(Solerand Bonhomme, 1990), except in central­ in the upper part (Inca and Pariahuanca formations, I­ southern Peru where intrusions are dated at 101 - 94Ma Benavides, 1956; Wilson, 1963; Moulin, 1989). "-o z (Beckinsale es al., 1985).Thestability ofthe magmatic front TheAlbian transgression isthenmarkedbythreepulses o ;:::: incentralPerusuggests that the LateAlbian tectonicevent ofmidEarly, earlyMiddle andearlyLate Albian age(Robert et o::::> did notchangesignificantly theshapeof theactivemargin al., 1998). The firstpulseonly reached the western part of ~ (Soler and Bonhomme, 1990; [aillardand Soler, 1996; Fig. the Eastern basins (Chulec Formation, Benavides 1956; v Z o 20).Mostofthearczoneof Ecuadorand Peruseems tohave Wilson, 1963; Robert, in progress). whereas the third one l­ V..... remained emergentduring Cenomanian-Turonian times, reached locally the easternborder of the Eastern Basin of t- sincethe Albian volcanic piles are usually unconformably Ecuador (Basal Napo Shales) andnorthernPeru,where itmay capped bySantonian toCampanian transgressive sediments rest on Paleozoic rocks (Jaillard, 1997). This latter (Iaillard, 1995; laillard eral; 1996). Plutonic activity was high transgression probably reached also the axial swell of during the Cenomanian (Beckinsale et al., 19B5; Mukasa, southernPeruandtriggered thedeposition of transgressive, 1986,94 - 90 Mapulseof Soler and Bonhomme, 1990), but partly marinesandstone (Huancane Formation, Carlotto et no intrusions -of Turonian age are known (90 ~ 85 Ma al, 1995; [aillard, 1995), which gradeto the E into thicker plutonic gap,Soler, 1991). deposits (lower part ofPatinaGroup, Audebaud et al; 1976; In northern Chile, the locus of the magmatic arc [aillard, 1995).ltcould havereached alsothe Bolivian Potosi significantly shiftedeastwardduringAptian-Albian times Basin where a few lens ofmetresofcoarse-grained, locally (Hammerschmidt et al., 1992), and the Middle Cretaceous conglomeratic sandstone areknown(LaPuertaFormation, magmatic arc (lIS - 90 Ma) is partlysuperimposed on the Sempere, 1994). The early Middle and early Late Albian Jurassic arc (Charrier and Munoz, 1994; Scheuberet al., transgressive pulses areassociated withwidespread anoxic 1994; Fig. 15). In northernmostChile, arc-related andesite. deposits in central and northern Peru and in Ecuador , agglomerate, tuff,sandstoneand conglomerate of (Pariatambo, Basal Napa, Chonta formations, Villagomez et Albian ageoverlie theEarly Cretaceous depositsoftheback­ al., 1996; Robertet aI., 1998). LateAlbian times are then arcbasin(Scheuber etaI., 1994). Theyyielded us -104Ma marked by the development of carbonate shelves in the datesandarecrosscutbylIS - 80Maintrusions(Bogdanic western part (Yumagual Formation, part of Mujarnin, and Espinosa, 1994).An extensional or transtensional lumashaandArcurquina formations, Benavides, 1956,1962; regime isassumedtohaveprevailed (Scheuber etaI., 1994). Wilson, 1963; Jaillard,I987},and bythedeposition ofdeltaic­ However, although no deformationof Albianagehas been fluvial sandstonein theeastern parts of the back-arc areas recognized. the significant eastward shift of the Middle (Agua Caliente Formation, Tsandstone, PutinaGroup), the Cretaceous arc (50km) mayresultfromashorteningevent. progradation and retreatofwhich are mainlycontrolled by Volcanic arc activity continued untilearlyLateCretaceous eustaticvariations witha subordinate influence of tectonic times(Scheuber et al., 1994). events(faillard, 1994, 1997). No arc-related magmatism isknownonthe continental Theeffects oftheLate Albian tectonic eventaremildinthe margin N of 3°S (Ecuador). Therefore, the Peruvian back-arc areas. TheLate Albian shelfcarbonate sedimentation subduction zoneextended probably northwestwards intothe exhibits slumps, syn-sedimentary faults and breccia, clastic oceanic domainby meansof an intra-oceanicsubduction dykes and differential subsidence, which together express an lone, which gaveway to the formation of island arcs of extensional regime (Audebaud, 1971; laillard, 1987, 1994). Albian to early LateCretaceous age.This interpretation is Farther to the E, progradation of deltaic systems maybe supported by the occurrence, on the accreted OCeanic regarded astheresult ofaslight uplift related totheLate Albian terranes of Ecuador, of pre-Cenomanian rocks tectonic event (JallJard. 1987. 1997). In northernArgentina. (LasOrquideas andIoachf units,Jaillard etaI., 1995; , alkaline volcanic rocks (L10- 100 Ma)areinterpretedasrelated 1995; Cosma etal; 1998),overlainbyvolcaniclastic arcseries toa rift episode (Viramonte etat, 1999). of Cenomanian to Santonian age (Cayo and Pilaton In the westernpart of the back-arc areas of Ecuador formations, Faucher etaL, 1971; Kehrer andVan derKaaden, and Peru, the carbonate shelf sedimentation recorded 505 TECTONIC EVOLUTION OF SOUTH AMERICA

. '.~-,: .. , . -' ,.. --; I~·;<·~'";.;;. '<''>"';;,~ \

significant eustatic transgression near the Albian­ Turonian-Coniacian boundary event and Cenomanian boundary, in Middle Cenomanian, earlyLate Coniacian - Early Santonian evolution Cenomanian (widely characterized by Neolobites (88· 85Ma) vibrayeanus (= N.kummelii,and Early Turonian times.Each transgression ismarkedbyconspicuous marlylevels which In the fore-arc Celica-Lancones Basin(northern Peru­ grade upwards into massive platform carbonate exhibiting southern Ecuador), the youngest fossil recovered from the frequently desiccation features, thus indicatinga shallow turbidite series is of EarlyConician age (Petersen, 1949; marineenvironment (Iaillard,1987,1995,1997; Fig. 22).In Iaillard etal; 1999).A sedimentary gapoccurred thenduring some southern parts of the Eastern Basin of Peru, the Late Coniacian and Santoniantimes,In the Talara fore-arc Albian-Cenomanian f1uvio- marinesandstone beds(Oriente basinof northwestern Peru,nodepositsareknownbetween and Putinagroups)are overlain 'oy Earlyluronian marine theAlbian shelfcarbonateandtheCampanian transgressive shale, illustrating the large extent of the Turonian marine deposits (Gonzalez, 1976; Morales, 1993). No transgression. Insomeareas,the upperpartoftheTuronian information isavailable on theother fore-arc zones.In the shelf limestone exhibits mild syn-sedimentary tectonic oceanic fragments accreted to Ecuador, the Turonian­ features announcing the tectonic event of the Turonian­ Coniacian bQundary toughlycoincides withthe beginning Coniacian boundary (Jaillard, 1987, 1995, 1997). of the Cayo island-arc activity, as expressed by thickseries Subsidence wasintense inthewestern areasofnorthern of coarse-grained volcaniclastic turbidite beds (Cayo and Peru,and decreased drastically towardthe NEand SE. The Pilaton formations, [aillard et al., 1995; Benitez, 1995; Albian-Turonian seriesreaches nearly 2000 minthewestern Kehrer and Van der Kaaden, 1979). partofnorthernPeru{Benavides, 1956; Wilson, 1963},about TheAlbian volcanic arcseriesofsouthern Ecuadorare 300m in the OrienteBasin ofEcuador (Iaillard, 1997),600 unconformably cappedby Early Campanian transgressive m in southwestern Peru (Benavides, 1962; jaillard, 1995), marine deposits (Naranjo Formation. 'aillard, 1997), In and 30 m in the PotOSI Basin of Bolivia (Sempere, 1994). southern Peru,Albianvolcanic rocksare unconformably Theaxialswell continuously behaved as a positive area. capped byundatedshelflimestone ofSenonianage(Ornoye The Middle to Late Albian deformation was the first Formation, Vicente, 1981; [aillard, 1994), In the arc zones, significant compressional deformation recorded in the the effects of the Late Albian and Turonian-Coniacian Cretaceous evolution of theAndean margin, which affected deformation are,therefore, indistinguishable. However, the mainly the fore-an: and arczones.hcoincided with aperiod deformational eventseemstonaveoccurredbeforetheEarly (If highconvergence rateand withthe opening ofthe South Campanian transgression of southern Ecuador. In the Atlantic Ocean at equatorial latitudes, which triggered the Coastal Batholith of Peru, significant wrench movements westward driftoftheSouthAmerican Plate andtherefore, the associated witha variable compressional regimehas been trenchward motion of the overridingplate (Prutos, 1981; recognized duringTuronian-Coniadan times (Russel and Jarrard, 1986; Soler and Bonhomrne 1990; Iaillard and Soler, Pitcher, 1985). Plutonic intrusions are very scarce, and 1996). The strong dextral wrench component of this volcanic activity is unknown (90 - 85 Ma magmaticgap, deformation (Bussel and Pitcher, 1985; Soler, 1991; jaillard, Soler, 1991; Fig. 20).However, although the magmatic front 1994) resulted from thenortheastward motionoftheFarall6n appearsto be nearly stable,a slighteastward shift of some Plate, indicated alsobythelackofanyarcmagmatism along kilometres can be detected (Soler and Bonhomme, 1990j theColombian-Ecuadorian margin(Aspden eral;1987). Due [aillard and Soler, 1996). In northern Chile, the magmatic totheoblique direction oftheoceanic plate, convergence was arc is markedbya well-expressed magmatic gap between accommodated by lateral displacement and wrenching 90and80Ma(Hammerschmidt etaL, 1992; Scheuber etal., deformation alongtheedgeoftheactive margin, ratherthan 1994), whichfollowed (Hammerschmidt etal.•1992) or was by shortening of the whole margin. The resumption of associated with the eastward shift of the magmatic front volcanic activity along the Peruvian marginmaybe related (Scheuber etal.•1994). to the beginning of the Middle Cretaceous period of high In the westernpart of the back-arcareas of Ecuador, convergence rate(Soter, 1991). Peru and Bolivia,the end of the carbonate platform sedimentation markstheTuronian-Coniacian boundary. In Coniacian ~ late Paleocene the N,it is replaced by ammonite-rich marine shalewith (88 - 57Ma) limestoneinterbeds (Celendfn, Upper Napo, Upper Chonta formations, Tschopp, 1953: Benavides, 1956; Wilson, 1963; This period is marked by a major change in the JailIard.1987, 1997; Figs. 21and 22),whereas in the S,the paleogeographic pattern, by the occurrence of Turonian limestoneunits areoverlain by red shaleand silt compressional tectonic events, the intensity of which with abundant evaporite layers (Chilcane, Aroifilla increased through time, and by the incipient eastward formations,Vicente, 198); Sempere et al; 1997; Figs. 13 migrationofthe arczonein Peru.Aprogressive but general and 21}. In Ecuadorand northern Peru,twomain marine , the arrival of fine-grained detrital transgressions are recognized, of Early Coniacian and tate deposits and theeastward shiftofdepocenters intheeastern Coniacian-EarlySantonian age,respectively. They determine basins markedsedimentation. Tectonic events are of Late two thickening-upward progradational sequences, of Turonian-E.arly Coniacian (88Ma),Santonian(35Ma),Late Coniacian and 'Early Santonian age,respectively (Jaillard, Campanian? (80- 75Ma)and LateMaastrichtian age(70­ 1997). The appearance of detrital quartz in the Coniacian 65Ma).Someoftheseevents coincide withtheaccretion of sequence indicates the creation ofnewsource areas.No Late 506 oceanic terranes in Ecuadoror northern Peru, Santonianfaunahasbeen foundso farin thesesequences. "~

--- ""co --- -... --- CJ - -- ... --- CJ -_---._- ... B _._- ~----",....".

_ UH 0 44' ·_ ""_ <-~_...... _ -.,...... ­ ,--", "--'-- '~- ~ &' ...... ".,' -- ..------__-_ _- -.,..... -__,--'...._---.~e-"4.,- ..., ­ TECTONtC EVOLUTION OF SOUTH AMERICA

N :~', :t:'i "' - ' :> I~"'~'"'.~:~. -"'~G··l ':-;/

Since the Coniacian to Early Santonian marine shale is Campanian transgressive shelfsedimentsunconformably generally overlain bytransgressive sandstoneofCampanian overlie the deformed turbidite seriesof pre-Santonian age age,asedimentary gapofLate Santonian-Early Campanian deposits (Iaillard et al., 1997, 1999; Fig. 24). The age is inferred. In southern Peru,and probably in Bolivia, compressional closureof thebasin seemsto be associated the Late Coniacian-Early Santonian transgression is well withtheintrusion ofsyn-tectonic gabbro (Reyes and Caldas, marked and forms a thin marine layer(MiddleQuerque; 1987),locally datedat 82Ma(Mourier, 1988).In theTalara Vicente, 1981;Iaillard, 1994),whichisused as a correlation fore-arc basin of northwestern Peru, the Albian shelf layer(Middle Vilq uechico, Chaunaca formations, [aillard et carbonate units are also covered disconformably by al., 1993; Sempere etal., 1997;Carlotto, 1998;Fig. 23). Campaniantransgressive marinedeposits(Gonzalez, 1976; In the back-arc areas, subsidence significantly Machan: et al., 1986; Seranne, 1987; Morales, 1993).In the increased during the Coniacian-Early Santonian time­ fore-arc zoneof (northern Peru),Middle Campanian span. In Bolivia, thespectacular increasein subsidenceis transgressive deposits restunconformably on the Paleozoic regarded as the result of a foreland-type, flexural basement(Bengtson and Jail lard,1997;[aillard etal., 1999). subsidence (Sempere, 1994;Sempere et al., 1997;Fig. 23), Therefore, the main deform ation of these fore-arc zones dueto significanttectonicshorteningin the westernareas occurred during the latest Coniacian-EarlyCampanian (northern Chile).Extension prevailed, however, in these time-span. back-arcareas(Solerand Sernpere, L993). Theoccurrence In the Celica-Lancones Basin, the middle Campanian of disoxigenated deposits (northern Peru, Ecuador) and transgressive beds are overlain by basinal dark shale of (southern Peru, Bolivia) suggests that the interbedded with fine-grained turbidite beds of Late o :z back-arc basin was separated from the open sea by an Campanian-Early age (Iaillard et al.. 1999). o i= incipient morphological barrier, which was more South ofPaita,theMiddle Campanian transgressive slXJuence ~ -'o :;,­ pronounced to the S. In the whole area, Coniacian­ consists oftransgressive marlstone and sandstone, rudist­ u.J Santonian marine deposits onlap eastwards onto the bearing massivelimestone, and transgressive marl and ':::: z o Guianaand Brazilian shields (Sempere, 1994). In eastern limestone grading upwards into sandstone and l­ V u.J Ecuador, northeastern and southern Peru and in eastern conglomerate, suggesting the occurrence of a Late I- Bolivia, the Coniacian-Santonian beds are the first Campanian tectonic event(LaMesa, Bengtson andJaillard, Cretaceous marine shalesto be deposited,and overlie the 1997).Fartherto theW (La'Iortuga), the succession follows Albian U)-Turonian fluvio-rnarine massive sandstone with a 3000 104000 m-thick series of alluvial to marine units (laillard, 1995,1997; Sempere et al., L997).This breccia, overlain by transgressive nearshore sandstone significant eastward migration of the early Senonian containing ammonites of Maastrichtian (probablyMiddle depocenter is associated with a reorganization of the Maastrichtian) age (Bengtson and Iaillard, L997; Fig. 24). isopach maps, which become narrower and elongated Theseareunconformably overlain bylatestPaleocene-early parallelto the present-daychain,suggesting that the back­ Eocene coarse-grained conglomerate, suggesting that a new arcbasins beganto behaveas distal forelandbasins. tectonic eventdeformed this area in the Late Maastrichtian This LateTuronian-Early Coniacian paleogeographic orPaleocene. Noinformation isavailable about theotherfore­ reorganization is associated with local tectonic arc zones, manifestations. In Bolivia, continental red beds In Santonian-Early Campanian times, the Ecuadorian unconformably overlie Middle Cretaceous marine strata margin underwent the accretion of an oceanic terrane (Vilcapujio event, Sempere, 1994)and in Ecuador (Jaillard, constituted by an oceanic plateau dated at 123 ± 12 Ma 1997), Coniacian silt or shale disconformably overlie the (Lapierre etal., 1999;Reynaud etal., L999)overlain byintra­ eroded Turonian limestone. In the Oriente Basin ofEcuador, oceanicislandarc series(Fig. 25).Thiseventismarked by the LateTuronian-Coniacian deposits exhibit significant a regional hiatus of Campanian age on the continental thickness variations related to syn-sedimentary faulting of margin,by a significant thermalevent which affected the Late Turonian-Coniacian age(Christophoul et al., 1999).In EasternCordillera ofEcuador around85- 80Ma(Cordillera this area and in the EasternBasin of northern Peru,mild Real, Litherlandet al., 1994) and by the abrupt arrival of compressional deformation hasbeenrecognized (Dashwood disconformable quartz-richturbidites of Late Campanian andAbbots, 1990; GiletaI., 1996;Rivadeneira andBaby, 1999). (?)-Maastrichtian age on the accreted oceanic series This, together with the change in sedimentation and (Yunguilla Formation; Faucheretal., 1971; Kehrer and Van paleogeography; the slightretreatof the magmatic arc and derKaaden, 1979; Cosma etal;1998).Intheoceanic domain, theincrease insubsidence, indicate thattectonic deformation the collision led to the endof the Middle Cretaceous island andmildshortening affected the Western areas. arc activity (Cayo Pormanon.Denttez, 1995), and to the onset, farther W, of a new island arc of LateCampanian­ Maastrichtian age (San Lorenzo Formation, Lebrat er al., Santonian-Early Campanian tectonic event 1987;Ordonez, 1996).Since theaccretedislandarcseriesis and Campanian-Middle Maastrichtian locallydatedas Coniacian inthe Western Cordillera (Faucher evolution (85 - 68 Ma) etal.,1971),theaccretions occurredbetweenthe Coniacian The Santonian (Early Campanian?) event is a major and the Late Campanian. This arc jump expresses a turning point in the Andeanevolution, recognized a long significant reorganization of the intra-oceanic subduction timeago as the Peruvianphase (Steinmann, 1929).In the zonegeometry(Cosma et al., 1998).Theaccretedoceanic fore-arc Celica-Lancones-Basin (northern Peru-southern terrane( unit,McCou rt etal; 1998),characterized Ecuador), diachronous, latest Santonian to Middle by its association with the Yunguilla Formation, crops out 509 TECTONIC EVOLUTION OF SOUTH AMERICA I~?~I

I­ Z o :2 presently along the eastern edge of the Western Cordillera zone, thetectonic ofthe Domeyko Cordillera and ::> C> of central and northern Ecuador (San luan-Pujilf Suture, creationof the retro-arc Purilactis Basin (Mpodozis and [uteau etal., 1977; McCourt etal., 1998). Ramos 1989; Scheuber et al., 1994). TheEarly Maastrichtian Yunguilla Formation (Bristow Intheback-arc areasofEcuador andnorthern Peru, Late and Hoffstetter, 1977) consistsof alternations of basinal Santonian-Early Campanian timesaremarked bya regional shale and medium-grained turbidite beds reworking sedimentary gap (Tschopp, 1953; Benavides 1956; volcaniclastic and siliciclastic material. These locally overlie Serninario andGuizado 1976; [aillard L987, 1997; Mathalone units of transgressive limestone of Late Campanian­ andMontoya, 1995), which coincides withtheaccretion and Maastrichtian age (Kehrer and Kehrer, 1969). This related deformations recorded in the westerly zones, In succession, comparable to thatof the Celica-Lancones and northern Peru and eastern Ecuador, theSantonian marine Paita areas, indicates the creation of a widefore-arc basin deposits exhibit a thickening-upward evolution expressing ofMiddle Campanian-Middle Maastrichtian age (Fig. 24), thearrival ofsandydetritalmaterial regarded as related to which extended at least from the Paitaarea (5'S) to N of the incoming Late Santonian tectonic movements. In (0'). southern Peru andBolivia.stratigraphic dataareinsufficient TheAlbian volcanic arc series of southernEcuador are to demonstrate the occurrence and durationof this hiatus unconformably capped byLate Santonian-Early Campanian in the mostly continental deposits (Middle Vilquechico, transgressive marinedeposits (Naranjo Formation, Iaillard Middle Yuncaypata, Chaunaca formations; Sempere et al., etal, 1997), which allow refining theage ofthemaintectonic 1997; Jaillard et al., 1993; Carlotto, 1998). The Early event as pre-Campanian. In southern Peru, the Albian Santonian ageofthelastmarine deposits in northern Peru volcanics areunconformably cappedbytheundated Omoye andEcuador,however, supports a Late Santonian agefor the Formation (Vicente, 1981), which hasbeenascribed to the maindeformational event. Santonian (Iaillard, L994),although it mightbe younger Campanian times are then markedby a short-lived, (Campanian?) .Inthearczones, theeffects ofthe Late Albian, regional marine transgression, locallydated as Middle Turonian-Coniacian and Late Santonian deformation are, Campanian (northern Peru, Mourier et al., 1988), and therefore, indistinguishable. Inbothareas, thetransgressive therefore, probably coeval with the main transgression in sequence grades intocoarser-grained, locally conglomeratic, thefore-arc zone (Fig. 24).ln Ecuador andnortheastern and nearshore to continental deposits, dated in southern central Peru, this transgression is associated with Ecuador as Maastrichtian (Cosanga Formation, Baudino, conspicuous disconforrnable transgressive sandstone (M-l 1995; [aillard, 1997), which indicate new tectonic .LowerVivian Formation) overlain byathinlayer movements in theMaastrichtian. ofmarine shale (Augustoet al; 1990; Salas, 1991; Mathalone TheSantonian-Early Campanian eventcoincided with and Montoya, 1995; [aillard, 1997). In southernPeru and thebeginning ofasignificant retreatoftheCoastal Batholith Bolivia, the Middle Campanian transgression is correlated ofPeru(Soler andBonbornme, 1990; Fig.20). This,together with a thin layer of charophyte-bearing shaleoverlain by withthe subsidence of the LateCampanian-Maastrichtian fine-grained red beds of presumed Late Campanian age fore-arc basinofnorthernPeru-southern Ecuador, suggests (Middle Vilquechico, Middle ¥uncaypata, Upper Chaunaca that tectonic erosion began to act as a significant mass formations, [aillardet al., 1993; Sempere et aI., 1997; Figs. transferprocess in the fore-arc and arc zones at that time 23 and 27). The hiatus between Campanian and (Iaillard andSoler, 1996; [aillard, 1997). TheLate Santonian­ Maastrichtian deposits suggests theoccurrence ofatectonic Early Campanian event isfollowed bya major plutonic pulse event in theLate Campanian, but an eustatic originforthis intheCoastal Batholith ofcentral Peru, duringwhich mainly sedimentary gap cannot be ruled out. Mid-Campanian granodiorite bodieswere emplaced (85- 77Maepisode of alkaline volcanic rocks (SO - 75Ma) pointto anextensional Soler, 1991). Aprobable magmatic gap(77- 74Malmight strain in northernArgentina (Viramonte etal; 1999). correspond totheLateCampanian event, andisfollowed by A newregional marine transgression occurred in the a newmagmatic episode (74 - 69 Ma), which beganwith Early Maastrichtian, which deposited transgressive dyke swarm emplacement (Soler, 1991 ).In southernPeru,a sandstone unitsgradingupwards intomarineshale,which plutonic gap (84- 70Ma)maycoincide withtheSantonian restdisconformably ontheCampanian beds.In Ecuador and and Late Campanian events. Thelatterare responsible for northern andcentral Peru, theseEarlyMaastrichtian marine the majorNE-vergent Uuta Thrust,near Arequipa, which layers are dated by marine microfossils and very scarce resulted inthethrustof rocks ontoCretaceous ammonites (Lower Tena, Upper Vivian, Areniscas deAzucar sediments (Vicente eral., 1982). Since it involves Coniacian­ formations, Koch and Blissenbach, 1962; Rodriguez and Early Santonian bedsand isconcealed bylatestCretaceous­ Chalco, 1975; Vargas, 1988; Mourier et al., 1988; laillard, early Paleogene unconformable conglomerate beds,it isof 1997), They aregenerally overlain by charophyte-bearing Late Cretaceous age(Vicente, 1989). fine-grained continental red beds of Maastrichtian age. A In northern Chile, magmatic activity resumed around disconformity separates thesedeposits from the overlying 80Ma ago (Early Campanian, Hammerschmidt etaL, 1992), fine-grained, continental Paleocene redbeds(UpperIena, and thelocation ofthe magmatic arc significantly shifted Yahuarango, Sol formations), suggesting theoccurrence of eastward, thus indicating that the Middle and Late tectonic movements near the Maastrichtia n-Paleocene Cretaceous tectonic events resulted in significant uustal boundary (Mathalone and Montoya 1995; Iaillard, 1997; shortening and/or crustal erosion (Scheuber et ai., 1994). Christophoul, in progress). This significant contractional event dated as 90 - 78Ma, In southernPeruand Bolivia, the Early Maastrichtian 510 resulted in the folding, emergence and erosion of the arc maximum flooding is marked by ephemeral marine ,"'...,..,.-,..."" ""',...... ,~X> i I• ------_ . i - ~ ) I ""~,: , )i---' ' > 0 - - Ie , - . -~ - ---_. ,I . -- • I I '.."

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'" TECTONIC EVOLUTION OF SOUTH AMERICA

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~

bJr-E__ """""'.. =... e-.>... ' -r-oc.a 515 'N'_ ...... _ ...... OM"KO , ,, - , ::::::::::- I - ...... :. ------~ - - • ,r - - - - I - - - ._ ' -- j - , ..-- WJ_ --- _._- - - - ., .. " ------'-. ------, ,-- • -- • --- "'"-'.... <' . ~ _ ...._---~ .. - .._ _ C__, _ , . ' • , ~ ....-.

'" TECTONIC EVOLUTION OF SOUTH AMERICA I~~~I ~ ~ :I: Sandstone} seems to be associated with Eocene pelagic 1999; Formation, Steinmann, 1997; Playas v l­ chert,and wasthereforenot yetin contactwiththe Andean Formation, Hungerbuhler, 1997; Fig. 33).Therenewal ofarc V>o ::2' margin(Hughes and Pilatasig, 1999). activity along the Ecuadorian margin is due to the more :z 0:: In the fore-arc zone of Peru, although early Eocene easterly convergence direction, subsequent to the late :r:UJ I­ depositshavebeen locally mentioned (Kulm et al., 1982, Paleocene plate tectonic re-organization (Pilger, 1984; 0::o Z Suess etal., 1988), depositsof that agearewellknownonly Pardo-Casas and Molnar, 1987). Q :z in the Talara Basin (Fig. 29).There.the very thick Eocene In northern Peru,thickcalc-alkaline subaerial volcanic .q; « series comprises threemainsedimentary sequences 1irn ited seriesaredatedbetween S5and40Ma(Laughlin etal., 1968; s Cobbing etal., 1981;Noble etal, 1990; Soler, 1991),and post­ ~ by disconformities of earliest Eocene, latest earlyto early a> middle Eocene, and late middle Eocene age,respectively date compressional deformation (Cobbing er al., 1981; :i 0:: UJ (Gonzalez, 1976; Seranne, 1987; Morales. 1993). The Bussel, 1983; Bussel and Pitcher, 1985). Although the Q., a= diachronous transgression begins with disconformable chemical signature did not change with respect to the o Q marine sandstoneand conglomerate of latestPaleocene to Paleocene magmatic arc(Soler, 1991 ),theEocene magmatic :3 earlyEocene age, from W to E (Morales, 1993), the base of activityis markedbya slighteastward shiftof thewestern .....v which contains boulders ofoceanic origin,which post-date magmatic front, a decrease of plutonic intrusions in the o V>..... the accretion of oceanic terranes (Pecoraet al., 1999). The Coastal Batholith and the beginningof the enlargement of Q Z Eocene sequences generally consistofmarineshaleand fine the magmatic arc,which reached the present-day Western « UJ::r: to coarse-grained sandstone, whichgrade laterally Eor NE Cordillera (Noble etal., 1990iSoler, 1991; Fig. 20).Incontrast, I­ u.. into coarse-grained continental sandstone and a magmatic gapoccurredin southernPeruduringtheearly o z conglomerate (Gonzalez, 1976; Seranne,1987). Pre-rn iddle Eocene (Mukasa, 1986; Boily etal., 1990; Clarket al., 1990; o ;:::: Eocene removed part oftheearlyEocene sequence, Soler. 1991). However, subvolcanic stocks and associated o~ and the second sequence locally rests on the basal porphyrycopperdepositswereemplaced in the Toquepala ~ prospect between57and 52Ma(Sebrieretal.• 1988; Clark '--.' conglomerate (Paredes, 1958). The upper part of the late :z Q middle Eocene sequence exhibits compressional syn­ et al., 1990). Fartherto the E (Inner Arc), the enlargement l­ V UJ sedimentary deformations related to a transpressional ofthemagmatic arcismarkedbytheemplacement ofseveral t- regime (Seranne, 1987; Becerra et al., 1990).The third calc-alkaline plutons, among which the large is the sequence (late middle to early late Eocene) is made of Andahuaylas-Yauri Batholith (48-34Ma,Carlier etal., 1996). unconformable massive coarse-grained sandstone of In northern Chile, the magmatic arc drastically shifted nearshore environment (Gonzalez, 1976; Morales, 1993), eastwardbetween55 and 48 Ma (Hammerschmidt et al., announcing the lateEocenetectonicevent. Overlying open 1992; Scheuber etal., 1994), thus suggesting that significant marineshalebedsareascribedtothelateEocene (Gonzalez, crustal shortening and/or tectonic erosionoccurredin the 1976) or the earlyOligocene (Morales, 1993). earlyEocene (Fig. 15).This event has been ascribedto the Neartheearly-middle Eocene boundary,subsidence of lateEocene tectonic phase(Scheuber etai., 1994).butisbetter thefore-arc zonetriggeredthe creationofa newgeneration correlated withthelatePaleocene event. Thelatteris followed of fore-arc basins,wherethick middleto earlylate Eocene by a significant resumption of the magmatic activityalong depositsunconformably restuponPaleozoic to earlyEocene the whole margin.In northern Chile, the resumption of arc units.Intheoffshore fore-arc basinsofcentraland northern magmatism is expressed by numerousvolcanic outcrops Peru, drillholes crosscut as much as 2000 m of middle datedasmiddleEorene(48-38Ma,Hammerschmidt etal; Eocene shale,siltstone and sandstone of shallow marine, 1992). Products of this arc weredeposited in a proximal, high-energy environment (Ballesteros et al., 1988; Suess it extensional back-arc basin, which received a thick, al., 1988), which usually unconformably overlies Cretaceous coarsening-upward pileofmainly volcaniclastic rocks,which to Paleozoic rocks(Machare etal., 1986). It ends uplocally may rest conformable on Late Cretaceous sediments or withbreccia, suggesting a tectonicinstability oflatemiddle unconformably on olderrocks(Hartley etal., 1992; Hintit Eocene to lateEocene age.Subsidence and basin tectonics al., 1993; Purilactis Formation ofCharrierand Reutter, 1994; is controlled by NNE trending normal faults (Machare et Fig. 30).Tectonic regimein the back-arc zone is no longer al., 1986; AzaIgara etal., 1991).ThemiddleEocene seriesis extensional(ScheuberetaI., 1994). commonly directly overlainbyMiocene marinesediments, In the westernbad-arc areas of Ecuador(Cordillera thus providing evidence for a widespread sedimentary Real), KJArageresets near65-50Maindicate theOCl;UITenCf hiatusencompassing lateEocene, Oligocene, andoftenearly ofa noticeable thermalevent(AspdenandLitherland, 1992; Miocene times (Suess etal; 1988; Von Huene etaL, 1988). Litherland etal; 1!194), related tothelatePaleocene tectonic In the arc zone of southern Ecuador, subaerial arc event. In theAndes of northern Peru,earlyEocene volcanic volcanism and associated continental volcaniclastic rocks datedat 55to50Maunconformably reston deformed sedimentation occurredduringthelatePaleocene and early Cretaceous sediments (Cobbing et aI., 1981; Bussel, 1983; Eocene (Sacapalca Formation, laillard, 1997; Hungerblih1er, Noble etai; 1990; Soler, 1991). In theAndes of central Peru, 1997). In the rest of Ecuador, resumption of arcvolcanism continental red bedsbearingPaleocene-Eocene charophytes isdatedasearlyor middleEocene (53- 45Ma, Eguez 1986; unconformably restonCretaceous sediments (Ml!gard, 1978; Wallrabe·Adams, 1990; Van Thournout et aI., 1990; Megard itel; 1996). Steinmann, 1997; Dunkley andGaibor, 1998). Itiscommonly In the present-day Andes of central Peru, associated withsubaerialvolcaniclastic red beds ofmiddle unconformable continental red beds are 10l;a1Iy dated by Eocene agedeposited in prorimalback-arcbasins (Silante latest Maastrichtian charophytes, whereasin other parts, Formation, Wallrabe-Adams, 1990; Hughes and Pilatasig, apparentlysimilar, but conformable, red beds yielded late 517 TECTONIC EVOLUTION OF SOUTH AMERICA

l~;:,,,p~l., ..~.. ! ....,.,. ~ ~, ..... z o ~ Eocene-early Oligocene charophyte oogonsand 40 to 37 culminated in the late Eocene (37 ~ 35 Ma) with the ::J o Ma KJArdates (Megard et al., 1996). Therefore, somered deformation and emergence of many external fore-arc bedsmaybeo(late Paleocene-middle Eocene age,but their basins (Machan! et al.,1986; Seranne,1987; Ballesteros et characteristics and extensionare unknown so far. Undated al., 1988; Iaillardet al., 1995; figs. 25and 29).Thisevent, fluviatile red beds (Casapalca Formation) overlyingthe together withthe lateOligocene crisis,is. responsible fora latestCretaceous strata havebeenascribedtothe Paleocene widespread Oligocene hiatus inthe fore-arc zone. However, (Jacay, 1994), although they may be younger. In southern sedimentation occured in a few basins (Talara,locally), Peru,Paleocene fluviatile redbeds(Quilque Formation) are and someinternal (eastern) fore-arcbasins wereaffected disconformably overlain by lacustrine deposits (Chilca by significantsubsidence, which allowed the deposition Formation, Jaillard et al., 1993; Carlotta, 1998), possibly of of late Eocene to early Oligocene marine (Pisco) or Eocene age. continental sequences(Moquegua)(Machan! etal., 1988; The eastern back-arcareas are marked by a regional DeVries, 1998). This subsidence pulse announced the unconformity below massive coarse-grained sandstone and accelerated subsidence, related to tectonic erosion conglomerate of earlyEocene age(Tiyuyacu Formation of processes, whichaffectedtheAndeanfore-arc zonesfrom Ecuador, Dashwood and Abbots, 1990; Benitez etal., 1993; the Eocene(Suess et al., 1988; Bourgois et al., 1990; Von laillard, 1997; Rivadeneira and Baby, 1999; Rentema and Hueneand Scholl, 1991), Basal Pozo formations ofPeru,Naeser etal; 1991; Robertson In Ecuador, undatedcoarse-grained conglomerate beds Research, 1990; Cayara Formation ofBolivia, Sempere, 1994; offan-delta environment, which unconformably overlie the Sempereetal., 1997; Figs. 13,23,and Jl). Theyoftenpost­ Eocene sequence maybeascribed eithertothelatestEocene­ date asedimentarygap, which encompasses a largepart of earlyOligocene (Jaillard et al., 1995; Fig. 25), or to the late thePaleocene. Moreover, noearlyEocene deposits have been Oligocene (Benitez, 1995). Marineshale,siltstoneand fine­ accurately datedsofarintheback-arcareasofEcuadorand grainedsandstonearedatedas middleOligocene (Playa Rica Peru. Early to middle Eocene times are then markedby a Formation, Benitez, 1995). Theyrest disconformably onthe regional marine transgression. Eocene sequence and are separated from the Miocene In Ecuador, the unconformable Lower Tiyuyacu deposits byasedimentary hiatus(Benitez, 1995). Innorthem Formation isoverlain bymarinetobrackishbeds ofEocene Peru(Talara Basin), the latemiddleEocene sandstone beds age(Benitez etal.; 1993). Ineastern Peru,the earlyEocene are overlain by pelagic shale of debated, possibly early basal transgressive lag is overlainbya marine to brackish Oligocene, age(Chira Formation, Morales, 1993). Insouthern fine-grained layerofEocene ageand bycoarsening-upward Central Peru (PiscoBasin), 600 m of transgressive shale, lacustrineto fluviatile redbedsof middletolateEocene age siltstone and subordinate sandstone of intertidal to (Pozo Formation, Kummel, 1948; MUller and Aliaga, 1981; nearshore environments artregarded asoflateEocene, maybe Robertson Research, 1990; upper part of Sol3 Formation, earlyOligocene (?),age(ParacasFormation, Newell, 1956; Koch and Blissenbach, 1962; Gutierrez, 1982). The Ucayali Marocco and De Muizon, 1988; Machare et al., 1988).A Basin seemstobe markedbya sedimentaryhiatusofearly middle Oligocene marine sequence has been recently Eocene age (Koch and Blissenbach, 1962). Onthe western described (DeVries, 1998), which probably correlates with borderof the basin (Rentema), conglomerate bedsdated at the Oligocene beds of Ecuador (and northern Perui). In 54Maare conformably overlain byearlyto middleEocene Southern Peru(Moquegua), transgressive fanconglomerate, lacustrine deposits, equivalent to the Pow Formation fluvial sequences and evaporite-bearing lacustrinesilt and (Naeser et al.; 1991; Iaillard, 19~4). Comparable lacustrine shale are ascribedto the Eocene, and infill an extensional, deposits of Eocene age are known in the Altiplano of fault-controlled basin,probably createdafterthelate Eocene southernPeru(Chilca Formation,Carlotto, 1998) and locally event(Marocco et al., 1985). M for manyfore-arc basinsof in Bolivia (Cayara Formation,Sempere etal; 1997). Probably Peru, these beds unconformably overlie Precambrian to due to subsequent erosions, the overlying late Eocene Mesozoic rocks deformed by the LateCretaceous to late succession isfrequently lackingonthebordersofthebasin Eocene tectonic phases. (Ecuador -Subandean Zone; Peru- Rentema, Ucayali Basin). In the arc zone of central Ecuador(Cuenca),volcanic rocks ofearlymiddleEocene age(43Ma,Steinmann,1997) Middle-late Eocene event(40-35Ma) are overlain by a 1000 m thick series of fluvial and Oligocene evolution (35- 28Ma) conglomerateand sandstone beds of late middle to late Eoceneage(Quingeo Formation,42 - 35 Ma,Steinmann, Thelate Eocene eventhas longbeen recognized in the 1997: Fig.33).In central and northern Peru,intrusions in Andes ofPeru(lncaicphase,Steinmann, 1929),and hasbeen the CoastalBatholith ceased by latest Eocenetimes (35 further documentedon the basisof radiometricdata. It is Ma.Beckinsale etal., 1985; Mukasa and Tilton,1985; Soler, followed bythe depositionof unconformable bedsoflatest 1991). With respect to the Cretaceous-Paleocene Eocene-middle Oligocene age. Sedimentation is chiefly intrusions,the lateEocene-recent arc magmatism exhibits tectonically driven.in theeasternintermtmtaneandforeland significant geochemical changes, regarded as resulting continental basins. This period ended with the late from the late Eocenetectonicevent(Soler, 1991; Fig. 20). Oligocene Aymara tectonicevent(28- 26Ma,Sebrieretal., Inthe present-dayAndesof northern and centralPeru,the 1988;SempereetaL, 1990;Fig.32). late Eocene event is materialised by a widespread In the fore-arcbasins, the late Eocenetectonicevent unconformity, the age of which is bracketed between44 was announced by the deposition of late middle Eocene and 40 Ma (Noble etal., 1974, 1979, 1990; Megard et al.,

518 disconformable coarse-grained deposits (42 + 40 Ma).It 1996). In southern Peru, the middle to late Eocene TECTONIC EVOLUTION OF SOUTH AMERICA

<:)1. ,:<~. I~'~"~Ii(,,:~. ~ ......

LU ...J I of the inner arc are intruded by acid, calc­ 1978; Angeles, 1987; Mourier, 1988).]n southernPeru, late v t­ v'> alkaline subvoJcanic stocksof earliestOligocene age(34­ Eocene times(42- 38 Ma) arealso marked bythrustingto o ::;; 32 Ma), thus indicating a strong uplift event during the the NE along the southern border of the Altiplano z lateEocene (Carlier et al., 1996; Carlotto, 1998). (Laubacher, 1978; Farrar et al., 1988; Carlotto, 1998), and :c"'"LU 1­ e<: In the arczoneof northernChile, a significant angular also by SW-verging thrust faults NE of the Altiplano o z unconformity is dated at 39 - 38 Ma (Hammerschmidt et (Huancane Fault Zone, Laubacher, 1978). Farthertothe NE, o z al., 1992). Late Eocene uprightanticlines, which account for the middle-late Eocene event isresponsible forwidespread « « 25% shortening in the arc zone, were associated with arc­ unconformities in the arc zone and the Altiplano, and for s parallel dextral strike-slip movements and with E-vergent disconformities in the eastern areas (Sebrier et ai., 1988; i5 u groups (Flintetal., 1993; Fig. 30). ina50km-large, NE-verging fold andthrustbelt(Maranon LU In the arc zone of central-southernEcuador, the late HB, Megard 1984, 1987), which occurs on the western o u> Eocene event is followed by an important pulse of arc borderoftheMesozoic positive zone (Maranon Geanticline), '"n z « volcanism (andesite, dacite and subordinate ) and interpreted as the result of the tectonicinversion of ..... :I: dated as latest Eocene-Oligocene (39 - 23 Ma, Saraguro normal paleo-faults (Mourier, 1988). Thisbeltexpresses a t- Group; Eguez et al., 1992; Dunkley and Gaibor, 1998). significant shortening of the continental crust and its o Within this pile, Dunkley and Gaibor (1998) identified overlying cover. Theyare associated with coarse-grained oZ ;::: ::> erosional periodsoflatestEocene- earliestOligocene (36­ deposits exhibiting internal unconformities (Pacobamba ...J ~ 34Ma)andmiddleOligocene age(30- 27Mar.ln northern Formation, Angeles, 1999). In southem Peru,the Incaic ..... Ecuador, volcanic activityseemsto have decreasedin the Deformation resulted inacomparable NE-verging fold and V Z o Oligocene, butchronological dataare scarce (Eguez, 1986; thrust beltto theSoftheCusco-Puno Swell (Manazo FTB, t­ V UJ Wallrahe-Adams, 1990). [aillard andSantander, 1992; Carlotto, 1998), andin theSW­ l- In the arczoneand the paleo- Andes of centralPeru,a verging Huancane Fault Zone (Audebaud et al., 1976; plutonic pulseof latemiddle and late Eocene age (42 - 36 Laubacher, ]978). The subsequent erosion period is Ma) is followed by a minorpulseof middleOligocene age concealed bythedeposition of widespread, unconformable (31 - 30Mal.thelatterbeingrestricted to the Paleo-Andes coarse-grained conglomerate (Chanove et al., 1969), and (Soler, 1991).Volcanic activity displays a correlative documented locally by Oligocene terrestrial evolution, sincethevolcanic Calipuy Formation yielded ages preserved in karstexcavations (Hartenberger etaI., 1984). of41 -35Ma.and31- 29Ma(McKee andNoble, 1982; Noble Farther to the NE, the middle-late Eocene event is etal., 1979; Soler, 1991).ThemiddletoearlylateOligocene responsible forwidespread unconformities ontheAltiplano, magmatic quiescence is correlated witha lowconvergence and by disconformities in the eastern areas (Laubacher, period(31- 26Ma.Sebrier and Soler, 1991), and is marked 1978; Sebrier etal., 1988; FarraretaI., 1988). by a subtle change in the geochemical composition of the In the area (southern Peru), 5000 to 6000 m of arcmagmatism (Soler, 1991). In the Altiplano and Eastern alluvial red beds(SanJeronimo Group), formerly ascribed Cordillera ofsouthernPeru,a significant episodeofhigh- K to the Late Cretaceous (Gregory, 1916; [aillard etai; 1993; alkaline magmatism occurred between 30 and 27 Ma Noblet et al., 1995), are presently dated as late Eocene(?)­ (Bonhomme et al., 1985; Bonhomme and Carlier, 1990), middle Oligocene age (Carlotto, 1998; Fig. 34). They which express alocal extensional regime (Carlier etal., 1996) comprise twothickcoarsening-upward sequences affected andis coeval withtheemplacement of monzogabbro at the bylarge-scale progressive unconformities showing evidence southernedge of the Altiplano (30Ma, Clark et al, 1990). for syn-sedimentary compressional or transpressional These lateOligocene-earliest Miocene alkaline, shoshonitic deformation (C6rdova, 1986; Noblet et al; 1987; Carlotto, and high-K calc-alkaline effusions and intrusions are ]998).Fartherto the E,as muchas 2000 m ofverycoarse­ interpreted as the resultof partial melting of an enriched grained fanglomerate and sandstone of late Eocene to mantle wedge (Sebrier and Soler, 1991). middle Oligocene age (Anta Formation) unconformably In thearc zoneof northern Chile, Oligocene timesare overly Cretaceous to middle Eocene rocks, thus providing markedby the deposition of mainlysedimentary, fluvial evidence fur strong magmatic andtectonic activity (Carlotto, beds,which indicateaperiodof magmatic quiescence (40 1998). Coeval deposits are known farther to the SEfrom • 28Ma,Azapa Formation and Paciencia Group, Coiraet isolated basins exhibiting changing sedimentary and al., 1982; Hint et al., 1993; Garda, 1997; Fig. 30). In the paleogeographicevolutions, and yielding scarce early Paleo-Andes, the middle-late Eocene event is well­ Oligocene ages(30- 27Ma,Carlotto, 1998). expressed. In the Western Cordillera of Ecuador, late IntheAltiplano Basin ofBolivia, thePaleozoic basement Eocene times are markedby the deposition of subaerial is unconformably overlain bya 3000 m thickseriesof red conglomerate onthe Eocene marinesequence.interpreted shaleand sandstone beds,withevaporite unitsin thelower by some authors as the result of the accretion of the part,dated at 30- 29Ma(Tiwanacu Formation, Rochat et Western Cordillera terrane (Bourgois et al., 1990; al., 1998; Fig. 35). This succession exhibits eastward Litherland etal; 1994; McCourt et al., 1998). paleocurrents and is interpreted as the foreland sequence Deformation is maximum in the Western Cordillera of theWestern Cordillera deformed duringthelate Eocene where Evergingfold and thrust belts developed (Megard, event(Sernpere et ai; 1990; Sempere, 1995, Rochat et al, 519 TECTONIC EVOLUTION OF 50UTH AMERICA I~I I­ Z a :2 1998). However, Lamb et al. (1997) determined westward This tectonic event was marked by regional o=­ paleocurrents andproposed thattheEastern Cordillera was unconformities inthe Andes (Sebrier etal: 1988; Sempere also uplifted alongan E-verging during the et al., 1990), by the deposition of disconforrnable coarse­ middle-late Eocene deformation, and thus, separated the grainedconglomerate inthe Eastern Basin,bytheinception Altiplano Basin from the incipienteasternforeland basin ofeastward thrustingin theSub-Andean Zone (Sempere et (Lamb and Hoke, 1997). al., 1990),and bya sharpincrease ofthesubsidence ratesin Intheback-arc basins ofEcuador lateEocene-Oligocene the Eastern Basin (Thomas et ai., 1995; Berrones and times are represented by disconformable quartz-rich Corrina, 1996),Italsotriggered oraccelerated thesubsidence conglomerate (Upper Tiyuyacu Formation), overlain byfine­ related to subduction-related tectonic erosion in the fore­ grained red beds (Orteguaza Formation) exhibiting a arc zones, sincein most areas, pelagic Miocene deposits conspicuous transgressive layer ofpartlymarineglauconitic disconformably overlie Eocene shelfdeposits (Machan! et sandstone (Benftez etal; 1993; Rivadeneira andBaby, 1999). al., 1986; Suess et al., 1988). Thisevent is also marked by In eastern Peru, the [ate Eocene event accounts for a pre-23 Madisconformities in the Andes of northern Peru widespread sedimentary hiatus encompassing the late (Mourier, 1988),by some resetsof KlArages in theEastern Eocene-middle Oligocene time-span in the western and Cordillera of Ecuador (35- 25Ma.Litherland et at., 1994), southernzones(Koch and Blissenbach, 1962; Naeser etaL, byunconformities atthe baseofthelateOligocene volcanics 1991; Figs. 31and36)andforaslightunconformity farther of Ecuador (base of the Saraguro Formation. 29 - 26 Ma, totheEandNE. There, Robertson Research (1990) identified Dunkley and Gaibor, 1998; Steinmann etal., 1999), and by a thin lacustrine unit of probable Oligocene age. Although unconformities and syn-tectonic sediments in Bolivia available stratigraphic data are scarce and sometimes (Sempere etaL, 1990; Rochatet al., 1998). conflicting, theysuggest a noticeable decrease ofthetectonic subsidence during the late middle to late Eocene interval Latest Oligocene-early Miocene (40-35Ma.Thomas et aL, 1995; Berrones andCotrina, 1996; evolution (26- 17Ma) Contreras etal: 1996). The frequent lack of late Eocene-middle Oligocene The major plate dynamics reorganization of [ate deposits in the Oriente Basin contrasts with the thick Oligocene age provoked a renewal oftectonic activity, which accumulations ofcoeval deposits in thePaleo-Andes,which accelerated the shorteningand upliftof the Andes, and seemtohave beenmarked, however, byan EtoNE drainage induced thick continental sedimentation in the system. Thissuggests that, eitherthesedeposits have been intermontane and retro-arc foreland basins. The late eroded due to a significant late Oligocene uplift of the Oligocene tectonic event is post-dated by the creation of a Eastern Basin, or the entire detrital sediments have been nearly continuous beltoffore-arc (Machareer aL, 1986) and trappedwithintheAndeanbasins,which actedtherefore as bya sharp increase of the subsidence rates in the Eastern the proximal foreland basinsoftheWestern Cordillera FTB Basin (Thomas et al., 1995). Offshore northern Peru, (Sernpere, 1995; Carlone, 1998), the Eastern Basin is markedby the unconformable rest constituting a by-passzoneforlow discharge rivers. of Middle Miocene pelagic deposits upon Eocene shelf deposits (Machare etal; 1986; Suess etal., 1988; Von Huene etal.; 1988; Bourgois etal., 1990; Fig. 37). OROGENIC EVOLUTION In coastal Ecuador, theearlyMiocene sequence begins locally withtransgressive conglomerate overlain bymarine OF THE NORTH-CENTRAL ANDES shale and siltstone rich in planktic foraminifera and 1LATE OLIGOCENE - PRESENy)'---__ radiolaria (Dos Bocas andVillingota formations, Evans and Whittaker, 1982; Benitez, 1995). The upper part, of early Late Oligocene - middle middleMiocene age, locally gradeslaterally into coarser­ grained subaerial deposits (Benitez, 1995). Rapid subsidence Miocene evolution (28 .. 10 Ma) of the northern Talara and Tumbes basins is expressed by thedeposition ofa250to 1000 m thicktransgressive series ThelateOligocene "Aymara" tectonic of locally conglomeratic sandstonebeds, with marlyand event(28-26 Ma) carbonate-rich intercalations of paralic environment (Mancora Formation), which unconformably rest on Amajortectonic andgeodynamic event occurred inthe Paleozoic rocks (Leon, 1983). Furthersubsidence allowed lateOligocene (28·26 Ma). Ithasbeendescribed bySebrier the deposition of as much as 1000 m of shale, madstone et aL (1988) and Sempere et aL (1990; Fig. 32). The[ate andsporadic turbiditeunitsrichinplanktonic foraminifera, Oligocene event is related to a major plate dynamics which indicate a significant deepening of the basinduring reorganization that occurredat 26 Ma. This consisted of theearly tomiddle Miocene (Heath Formation, Leon, 1983). thebreakupofthe Farallon Plateintothe Cocos and Nazca Offshore northernPeru.earlyMiocene marinedeposits are plates, accompanied by a change in the direction of mentioned onlylocally. They overlie directly middJe Eocene convergence (Pilger, 1984; Pardo-Casas andMolnar, 1987). strata(Ballesteros etal; 1988). Convergence became approximately E-W, which triggered In southern centraJ Peru (), the latest a progressive rotationofthestrain,from NNE-SSW during Oligocene-early Miocene deposits consistof a 60 to 300m thelateOligocene, toE-Wat theendoftheMiocene (Noblet thickseries oftransgressive shale,siltstone andfine-grained 520 et aL, 1988). sandstone. which unconformably restson Paleozoic toearly ~» """'''' _.. " ," ~- .. 1 _..,' ...-~" , ' '._ ,,- ....·.k' '." ,",.. ~ ~ M ' ~" '.. " ~,...... J,,_ , _ ", , ~ ,, __ ! ..._.>-_ J_ -..« ,r ol., ,_, I '" __ '" _ ,_ _ " C B "" ~ "' _ ~ , --- ~ 1 >'> " " "' l~"'''''~_.. ""-''''''~--,I'''. "-""~ ... 100>''''...w1o.0 ~ _ ... I ,,-...... ,,"-...-...... --.,....."' _._+ • •_ ..... 1 ~ . '--.,, -_ ! -- • ...... "'(_.. , - ~ ... "'--_._ _~ _ ...... _ _ol- • -am. "'" ~,,"." _ .-.. i ...... ~ I _ 1. ~ " K .." . , ,...... ' " ,' -~ _ _"' _ 'r ~ _-, ."J • .... _ __ ".. 'n ...... _ -- ...... I ..... _"'_ _ _ "' ' J] . " ..."'_ __ ,.." « i .., I..'. I ~ I· .." ,..,•...... ,. , ' - --__ ~ " "'... " .".....". " _ '''1 - ...... ,.."~ ... -,,..._._'.", ~ " ' "-, , ,_...... ""' ...... ~ _.. _JJ "'_• ••.....(_"'.--...~ "'- I -JJ ""' ".,. "" ...... ~.... _- ""-.. _...... "- ...... ,..."" . ,...... -...'_. ",_".."_.'..,... ~"- ...._...... -_-- -- ..... = "'- ".., [1]-- .[Q)'­_ ...... "."...... _ol, -.."' __ G:il!l_ . .. _ ...... , ... ,._•._._-"-...... _..... -, _ -"·ll 'm -«"'-'..., ._- "'-. _1_ ..--..._.."'-1_,""'...... -..-. "'-,""'.. ~ , " .. _._m_-_, , ~"-,..",...-_,'--. -.. . --""_ -...,.r- .....,.,.~"'-' '"" ...... ""w __ .. ... l """" d ~ .__ ..,.,, _-"' I..'-, ' . • ...... '""""--.""'-... "'."_«- ...... ,'"'- "'- ' ,'...' ..... ,.,...... -- _ -- "".... ,-.."...... ,...... :~: ,! ~. • I ,• ,I • , ..'''--.. ­-- ,• _- • ----- , ~, ;. ;, ;. ,,, ,,,~ --_ • ; i I .. mTI , ·E-- ••• ,-- I ::-.:: ~ ;; f I ~ . ­_.. I .. -_-.. -- .... " ...... "="'~ 0 "'''·''' -- .,.,,'. ..- ,- - .- - :;:"'- ,'--­­ .- ~ - ­ '-" - JlIIl = , _ --- --'-- - "'"""''' '' 1 ;. "'_ ...... ,_ .- ~...... " -~ ... ".,,

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...,...,..... ,...... "",... " "'."n.. --,_~" ~ - """ "' '''- ....,- .._",_. ,_. _~ ",- ,-- " , -...._ ---"', _ ., .....'.._' 1'"""...... ,...... _'...'-'-. "'- '... _n __ _.... _ , ...." no '-'"''r'-- 4~...-.. ""' ''... _._---,~ _,..,,_ ,_-_, ...... ~ .. ,. ,,_, ...... " O _ _ ..- _ __ '''''.4.._ _ ... I. ...~ -...... - ,_ oJ ..o4ot_ " '~ , ..,"• • " .. .. ,.. .. _ ~" " _'M ....,' , ...... ;,_ _"... " , ... _ r ...... ot, ",. , "'''_ ...... ,-..-.,-.. "'...... ~..,.' , ,..~ "" ... - " ..,...... ,..., " ..~" _ _ Cool " ... -_ .._, _, , , m -_...- -m_- ~­ ,- ' ~-- i • •

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. - Iill­ - D ­ - m _ r• ClI ---.­ -- rn''''''-'' 11I - .._ - - TECTONIC EVOLUTION OF SOUTH AMERICA I~·~~·{';·l ~

"Quechua 1» event (17- 15 Ma) and (ZapahuiraFormation, 13- 11 Ma)unconformably overlie middle tolateMiocene evolution earlyMiocene tuffandconglomerate (Garcia, 1997; Fig. 38). Fartherto the5, avolcanic unit (SanBartolo Group,17Ma) (I5-9Ma) overlies Oligocene fluvial sediments with an angular In thetare-arc basins of Ecuador, nogoodrecord of the unconformity (Flintet al: 1993; Fig. 30). event, which occurred near the early-middle Miocene In the paleo-Andes of Ecuador and Peru,the opening boundary (I7 - 15 Ma), is known. This event may be of the "Miocene" intermontane basins had been dated as recorded by the arrival of moderate amounts of detrital mainlyearlyMiocene (KJAr,26- 22Ma.Lavenu etal., 1992) sedimentsand by localtransgression(ProgresoBasin). In and wereregarded as pull-apartbasins,openedbythe play the Andes of Ecuador, F-T dates on sedimentsindicatean of an oblique NE trending strain exerted on pre-existing uplift stagearound 18 Ma(Steimann et al., 1999). In the NNE and ENE trendingfaults (Noblet et al., 1988; Baudino Eastern Cordillera ofEcuador. resetsof KJAr agesat20 - 15 et ul., 1994, Marocco et al; 1995; Barragan et al.; 1996). Mamaybeduetocompressional deformations(Litherland However, recent F-T concordant dates support a lateearly etal., 1994). Thisintervalis followed by the openingofthe toearlymiddleMiocene age(I6 -14 Ma)forthe creationof Guayaquil Gulf(Deniaud etal., 1999). most intermontane basins of Ecuador (Figs. 33 and 40), IncentralPeru,themiddle-lateEocene structureswere which havebeen alternatively regarded as the resultof an re-activated in the Western Cordillera and in the Marafion E-W extensional regime(Steinmann, 1997, Hungerbuhler, Foldand Thrust Belt (Megard, 1984; Megard et al., 1984), 1997; Steinmannetal.,1999). Anextensional regimeisalso which fold lateCretaceous red beds. A possiblemagmatic thought to have governed the creation of many Miocene gap occurred around 19- 18Ma in the arc of centralPeru intermontane basinsofPeru (Noble etal., 1999). (Soler, 1991). In southern Peru, it is responsible for In Ecuador, the rapidly opened intermontane basins (16­ monoclinal folds and reverse faults, large scale incisions 15Ma} werefilled bymiddleMiocene fine-grained lacustrine due to the resumptionof erosionstriggeredby a pulse of deposits (14-10 Ma) representing aperiod ofrelative tectonic uplift (Sebrier et al., 1988). The latter is thought to be quiescence (Noblet etaL, 198B;Marocco etai, 1995). Thefine­ responsible for a 400m uplift(Sebrieret al., 1988). grained deposits areoverlain byacoarsening-upward sequence Inthearcandfore-arc zones ofnorthernChile (Fig. 42), ofsandstone andconglomerate, coeval with thecompressional W-verging fault thrusts are assumed to havebeganaround dosureofthesebasins (Megard etaJ., 1984; Noblet eral, 1988; 16 - 15Ma(Garcia et al; 1996; Munoz and Charrier. 1996; Baudino etol; 1994; Marocco etaL, 1995; Hungerbtihler etol; Charrier et al., 1999). This event is associated in 1995; Hungerbtihler, 1997; Fig. 40).TheCuenca andLoja basins northernmost Chile, with unconformable late middle of southern Ecuador contain middle Miocene marine Miocene lava flows (Zapahuira Formation, 13- 11 Ma) on intercalations, indicatingthatthey werelocated at or very dose the early MiO£enf tuffandconglomerate (Garcia, 1997; Fig. tosealevel andareregarded asembayments offore-arc basins 38).Farthertothe S(Antofagasta), thevolcanic rocks ofthe (IS - II Ma, Hungerbiihler 1997; Steinmann etai, 1999; Fig. San Bartolo Group (17 Ma) overlie Oligocene fluvial 40).InPeru, thenonmarinevolcanidastic andsedimentary in­ sediments withanangularunconformity (Flint et al., 1993). fill of the middleMiocene basins is usually unconformably In the fore-arc basins of Ecuador, early Miocene overlain by mainlyvolcanic deposits dated at 10 to 7 Ma marlstone beds are disconformably overlain by middle (Megard etaL, 1984; MaroccoetaL, 1995; Nobleetai, 1999). Miocene sandstone and marlstone (Subibaja-Angostura The Altiplano Basin of Bolivia is a peculiar case of Formation). They areinturn overlainbyearlylateMiocene intermontane basinconsisting ofN-S-trending half-grabens conformable mudstone(Onzole Formation, Benitez, 1995). (Fig.35). In this basin,shale,sandstoneand subordinate In the Progreso Basin, the middleMiocene disconformity conglomerate of middleMiocene agerest conformably on (Subibaja and San Antonio transgressive limestone the earlyMiocene deposits(Lamb etal; 1997; Rochatetal., members) is followed by deposition ot nearshore 1998). The middleMiocene sequence was deposited with sandstone of late Miocene age (Progreso Formation), veryhighsedimentation rates,especially intheCorque Basin relatedto the beginningof the first opening stagesof the ofsouthernBolivia (Roperch etal., 1999a).Clastic sediments Guayaquil Gulf. After anopeningstage(lateOligocene-early mainly derived from the EasternCordillera, the erosionof Miocene) the Tumbes Basinof northernmostPerureceived which allowed the development of regional-scale flat deep-marineturbiditebeds (Le6n,1983). In the forearcof morphological surfaces. These are the Chayanta (13 - 14 centraland northern Peru,the earlyMiocene depositsare Ma) and San [uan de Oro(10 Ma) surfaces,which can be conformablyoverlain by thick middle Miocene marine observedand tracedfromnorthern Argentina up to the La mudstone(Machare et al., 1986; Ballesteros et al., 1988; pazregion (Servant etaI., 1989, Herail etal., 1993; Gubbels Fig. 37). etal., 1993). This periodcoincides witha lowupliftrate of Except in Ecuador, the arc zones are marked by the the EasternCordillera. resumption ofsignificant amountsofvolcanic products.In In the back-arc basins, a conspicuous shallow marine central Peru, a magmatic pulse occurred in the Eastern transgression is recorded during the late middleMiocene Western Cordillera and the Altiplano between 18and 13 (15Ma, of Peru,Hoorn, 1993), which was Ma, which comprises abundant volcanism (Soler, 199 I; Fig. connected totheopenmarinerealmthroughtheMaracaibo 20). In southern Peru, the shoshonitic and High-K area(Hoornetal; 1995) and possibly theGuayaquil seaway magmatism carne10anend,while calc-alkalinernagmatism in southern Ecuador (Steinmann et al., 1999). A similar wenton (Carlieretal.,1996). In thearcand fore-arc zoneof shallow marineinvasion ofearlylateMiocene ageisknown northernmost Chile, late middle Miocene lava flows in the EasternBasin ofBolivia (Yecua Formation, Marshall 525 TECTONIC EVOLUTION OF SOUTH AMERICA I~l ..... z o ::; et al., 1993). which was connected southward to the ThePaleo-Andes andsurrounding areasaremarked by :::I Cl Atlantic Ocean. In Ecuador andnorthernPeru,thisperiodis ageneral andrapiduplift (Sebrier etal.,1988; Steinmann et marked by slight decrease of the tectonic subsidence al., 1999), thelocal ratesofwhich remain 10bespecified. In (Thomas et al., 1995; Contreras et al., 1996; Fig.39). Ecuador, the estimates of mean rock uplift rate since the However, in eastern Bolivia, a strongincrease of tectonic lateMiocene (9 Ma) is of 0.7 mm/y and the meansurface subsidence has been related to the deformation of the upliftisof0.3mmiy (Hungerbuhler, 1997; Steinmann etal., easternCordillera (Marshall et al., 1993; Sempere, 1995). 1999), which is consistent withestimates by Delfaud et a1. (1999) inthesamearea,bySebrier erci (1988) insouthern Peru, andbyParraguez era1. (1997) in northernChiIe.In the uQuechua 2" event (9 - 8 Ma) latterareaand in Bolivia, uplift involved both the Western and late Miocene evolution (9 (Sebrier et al; 1988) and Eastern(Benjamin et al., 1987) - 6 Ma) Cordilleras. In Ecuador, thisperiodismarked bythecompressional closure ofthe Miocene intramontane basins (Noblet et al., ...u Thisperiodbeginswiththe Quechua 2tectonic phase 1988; Marocco etal., 1995; Hungerbuhler, 1997; Steinmann .....' UJ (9 - 8Ma,Megard, 1984; Sebrieret al., 1988). Ratherthan etaI., 1999), which are filled bycoarsening and thickening­ ~ z a a major deformational event, the Quechua 2 event is a upward clastic deposits. Thisis interpreted astheresultofa ::; ...., turning point in the evolution of the northern Central change from an extensional (15- 10Ma) toacompressional Andes, which corresponds to the change from a regime (9- 8 Ma), which resulted in the uplift of southern depositional periodcharacterized by thick and relatively Ecuador, the establishment of terrestrialconditions in the widespread fining-upward sequences, to a compressional intermontane basinsof southernEcuador and risingrelief and uplift period marked by erosions and depositional in the Eastern Cordillera (Hungerbuhler, 1997). Latest areasrestricted to the fore arc and retroarcdomains. This Miocene times are then marked by the development of isinterpretedas the resultofthebeginning ofthenearly en smaller-scale intermontane basins filled with mainly bloc eastward thrusting of the Paleo-Andes onto the volcanic and volcanogenic rocks (Lavenu et al. 1996; Brazilian and Guiana shields, which resulted in crustal Hungerbuhler; 1997; Fig. 41), thickeningand rapid uplift of the arc zones and paleo­ In northern Peru, although timing constraints are Andes, and the transfer of active deformation into the poorer, and the change fromextensional tocompressional Subandean thrust and fold belts, regime isassumed tobeoflateMiocene age. In central Peru, Thefore-arc zonearemainly marked byuplift (Sebrier N-S shortening induced mainly dextralmovements along etal., 1988), unconformities and reverse faulting. Incoastal NW trendingfaults (Megard, 1984). In theAyacucho Basin, Ecuador, the middleMiocene disconformity is overlain by compressional deformation occurred between 9.5and 8.5 transgressive marine sandstone (Angostura Formation) and Ma(Megard et al., 1984). In southernPeru,latestMiocene thenbyathinning- upwards succession ofmarine nearshore transpressional stress is thought to be responsible for the sandstone oflateMiocene age(Progreso and LowerOnzole closure ofintermontane basins (Paruro Basin) opened about formations), related to the beginning of theopening of the 12Maago (Carlotto, 1998). Fartherto the S,late Miocene Guayaquil Gulf (Deniaud et al., 1999). In the fore-arc zone timesaremarked bythecontraction oftheAltiplano, related ofPeru, theunconformity between middle andlateMiocene to the tectonic inversion of the pre-existing normal faults marinedeposits in thefore-arc basinsofcentral Peru(Lima defining thehemi-grabens locatedWoftheAltiplano Basin Basin, 11 °S)has not been recognized farther N (Yaquina (Kennan et aI., 1995; Lamb et ai" 1997; Rochat etal., 1998, Basin, 9°S, Ballesteros et al., 1988). In the Lima Basin.low 1999; Fig. 35).In the Altiplano Basin, tuff beds datedat 9 energyshelfdeposits of late Miocene age evolved toward Ma are disconformably overlain by a thin sequence of shelftoslopedeposits (Ballesteros etal., 1988),indicating a conglomerate reworked Paleozoic basement rocks (Lamb et noticeable deepening of thedepositional environment, due al., 1997jRochatetaL, 1998). to thesubsidence related to tectonic erosion ofthefore-arc In northernChile, compressional tectonic activity went zones (Von Huene etaI., 1988).1n theYaquina Basin, middle oninthePre-Cordillera,withthedevelopment ofthewestern Miocene low energy turbidite beds grade upwards into thrusts of the W-vergent thrust system (Fig. 42),whereas higher energyturbidites of late Miocene- age an extensional regime prevailed farther to the W (Ballesteros etal., 1988). (Longitudinal Valley and Coastal Cordillera; Munoz and In thefore-arc zoneofnorthernChile, where W-verging Charrier, 1996; Garcia et al., 1999). thrustingwenton(Fig. 42),a samekindofunconformity is Regarding the EasternBasin, the eastward migration recognized between late middle Miocene lava and late of the deformation front during the late Miocene is the Miocene alluvial deposits (Huaylas Formation, 9 - 8 Ma, prevailing feature (Sheffels, 1990; Baby etal; 1997). From Garcia, 1997; Fig. 38). In the magmatic arc of Peru, no thistimeonwards, mostofthedeformation and shortening location changeis observed in the magmatic activity, with oftheAndean margin isaccommodated bytheeastern areas, respect to the former periods. A major magmatic pulse which received W-proceeding coarse-grained clastic, occurred between 12and7Ma(peak. activity around10Ma). terrestrial sediments.In theOriente Basin of Ecuador, late Itcorresponds to numerous intrusions, and veryabundant Miocene deposits consistofthicksequences ofpoorlydated effusive products (Soler, 1991). A possible gap,or at least coarse-grained conglomeratic sequences separated from magmatic quiescence, is mentioned at9 - 8 Ma, which may each otherbydisconformities (Christophoul, 1999). In the 526 coincide with a compressional event (Soler, 1991; Fig. 20). western part oftheEasternBasin ofnorthernPeru(Bagua tECTONIC (VOLUt IO OF SOUTH AMER ICA ~~

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527 TECTONIC EVOLUTION OF SOUTH AMERICA

I~~I~ ..... z o ::2 area), a marked unconformity (10 Ma) separates fluvial centred around 5 - 4 Ma.In Peru,it corresponds mainlyto ::> o sandstoneand conglomerate of middleMiocene age,from ignimbritic tuff associated with rhyolitic dykes in the late Miocene ccarse-grained fanglomerate {SanAntonio Western ComilleTa (Soler, 1991). In southern Peru, the Formation, Mourieret al., 1988; Naeser eral., 1991; Fig.3l). emplacement of alkaline,peraluminous and shoshonitic In the Ucayali Basin, fluvial siltstoneand sandstoneoflate suitesalongmajorfault systems suggests thatan extensional Miocene ageabruptlyoverlie marinetobrackish beds (Koch regime prevailed around 6 - 5 Ma (Carlier et aI.. 1996). and Blissenbach, 1962; Fig. 36). Effusions of shoshonite, minette, lamproite and peraluminous rhyolite anddacitewentonduringthe past3 Quechua 3event(7-5Ma) and latest Ma, and took place along the fault systems limiting the Miocene-Present evolution(6 - 0 Ma) Altiplano, andinterpretedassinistralwrench-faults (Carlier etal.,1996; Carlotto, 1998). Atthebeginningofthis period,thesignificant 7- 5Ma In theeasternbasins.latestMiocene- timesare contractional eventis markedbya chiefly E-W shortening marked by a strong flexural subsidence allowing thick (Megard, 19M;Sebrieretal., 1988).Itis marked by folding accumulations of foreland clasticdeposits (Thomaset al., and reverse and strike-slipfaulting in southwestern Peru 1995; Contreras et al., 1996; Baby et al., 1995; Fig. 39), (Sebrier et al.. 1988), and by theonset of the sub-Andean whereas Recent times are marked bya strongdecrease of thrust and fold belts, which accommodate most of the the sedimentation rate and local uplifts. However, Recent shortening in the Andean Chain during the Pliocene sedimentationcontinues in restrictedand/or moreeasterly (Roeder, 1988; Sheffels, 1990; Baby etal., 1992). areas (Ucamaradepression of easternmostPeru;Pastaza­ In the fore-arc zonethe deformation and uplift were Maranonalluvial fan), influenced by the incipient subduction of the Nazca and In the Oriente Basin of Ecuador, localized coarse­ Carnegie aseismicridges (7 to 3 Maago, Suess etal., 1988; grained fanglomerate are incised by present-day rivers Von Huene andScholl, 1991, Benitez, 1995), andtheongoing (Christophoul, 1999).ln northeasternPeru,apatite fission tectonicerosionof the fore-arc zones.In the fore-arc zones tracks data indicatethat the Basin underwent a of Ecuador, a regional disconformity datedat the Miocene­ rapid uplift (0.4 mmfy) during the last 10 Ma,probably Pliocene boundary (5.5 Ma) precedes the depositionof a related to the onset of the Santiago coarseningand shallowing-upward sequence, related tothe during the latestMiocene (Pardo, 1982; Megard, 1984). In increased uplift and erosion of the AndeanChain from 9 theMarafion Basin.asedimentaryhiatusseparatesmiddle­ Ma(Benitez, 1995; Deniaudet al., 1999). In the Guayaquil late Miocene fine-grained deposits from disconformable Gulf, however, a strong subsidence due to transtensional coarse-grained fanglomerate of latestMiocene- Recent age movements allowedthe deposition of huge volumes of (Mathalone and Montoya, 1995) and Pliocene times are clasticsediments, especially during the early Pleistocene markedby the upliftof the area (Contreras et al., 1996). In (Deniaudet al.; 1999). Asimilar disconformity and hiatus the Ucayali Basin,no post-Miocene deposits are known seemto be recorded at 5 Main the offshore basins of Peru (Koch and Blissenbach, 1962). In the Madre de Dios Basin, (Von Hoene etal., 198B). late Miocene deposits fill incised valleys, and the recent In central Peru W - 14°S), thesubsidence of the fore­ alluvial terrace morphology shows a Pleistocene uplift of arczonesrelatedto the tectonicerosionis recorded in the the area. In the sub-Andean of northern LimaBasin by Pliocene lowenergy turbidite beds, which Bolivia, asignificant increasein thesubsidence allowed the indicatedeepeningof the environment (Ballesteros et al. accumulation of about 5000 m of late Miocene-Pliocene 1988), by the localtransition fromupliftto subsidence at6 clastic sediments.This is interpreted as the result of the Ma (Von Hueneet al. 1988),and by the lack of uplifted rapideastwardmigrationofthe Andeandeformation 10to terraces in the coastal zone (Machare and Ortlieb, 1993). 6 Maago(Gubbels et al., 1993; Baby etal; 1995). However, On the contrary, significant upliftmovements affected the noforedeep sedimentation occursat present (Roeder, 1988). coastofnorthern (4°- 60S, <0.2 mm/y)andsouthern Peru (14° - 18°S, < 0.7 mm/y) sincethe late Pliocene (Machare and Ortlieb, 1992;Von Huene et aI" 1988).In thislauer case, TECTONIC AND KINEMATIC upliftisdueto the subductionof the Nazca aseismic ridge, EVOLUTION OF THE which induced a regional extensional stress regime (Machare and Ortlieb, 1992) . NORTH-CENTRAL ANDES All segments of the Andes are marked by the continuation ofthe majorand rapiduplift.I nEcuador, uplift From Bolivia to Ecuador, structural styleof the Andes rateestimatedby P- Tevidencea slow downbetween 6 and changes dramatically. Geometry of the present-day 4 Ma,and an increasefrom3 Ma (Steinmannet al., 1999). deformation of the Bolivian and North-Chilean Andes Othermethods estimate1000 - 1200 m of net upliftsince5 results from Neogene thin-skinned tectonics, whereas the Ma (0.2 mmfy, Delfaud et aI., 1999). Pliocene-Quaternary Ecuadorian Andes havebeen structured by thick-skinned timesarealsomarkedbytheupliftofthe sub-AndeanZo ne and wrench tectonics since Cretaceous times. Figures 2A, ofEcuador(Baby et al; 1999). In theAndes ofsouthernPeru, 28 and 2C illustrateglobal changesin structural geometry upliftisestimatedat 1300 msincethelateMiocene.ofwhich and chain width.These two parts of the Andes form two 200- 300m would be ofQuaternaryage. extremes. Neogene tectonic events seem to occur Thearczonesof Ecuador(Steinmannet al., 1999), and contemporaneously, hutexpress twoshapesoforogenic belt. 528 Northernand CentralPeruare markedbyaneffusive pulse Three Neogene orogenic stages, late Oligocene-early ,, n E; , o \m I -cg, ii

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~ < o ...c~ o !mmI Er.y_ It o !lHBiI S"'~ -...-.. o ~ ~ LJrII _ o 1, ,11IC:1:Jn1t ~ "...z •J: \II .. N '" ID i ~ MloCOM to Prutnl

IJ lo U fJ . T....-..c rPt., "" CI.,,...J A.In.. /Jyoo ::i< They have been recorded in fore-arc, intermontane and Ordovician anchimetamcrphic sedimentary rocks. z back-arc basins.Sedimentary successions involved in the Shortening isconcentrated in theW-verging thrust system UJ"" ...... :I: deformations consist ofCambrian toOligocene pre-orogenic of the westernpart of the Cordillera Oriental and of the ""o z strata, and Oligocene-Miocene to Recent continentalsyn­ Interandean Zone. TheCordillera Oriental ischaracterized Cl z orogenic in-fill. by small Neogene piggyback basins (Fornari et al., 1987; « « Herail etal; 1996).Good surface dataallowed ustoconstruct :::;s some balanced cross sections, according to which total o Crustal structures and Neogene -'o shortening by the reactivation of high angle faults and the lack of >..... Paleozoic cover. Ontheeastern partofthe Precordillera.back u Z TheCentral Andes are divided fromE to W intoseveral thrusts limit a blind pop-up structure below the Tertiary S2 u u.J morpho-tectonic units (Fig. 43).TheChaco and Beni plains deposits (Riquelme and Herail, 1997). Shortening is dose I- correspond to a slightly deformed Neogene foreland basin to18krn.In theCentral Valley,withinwhich Late Cretaceous­ underlain by the Brazilian Shield. It is overthrust by the Paleocene magmatic arc and associated deposits are Subandean Zone, a complex thin-skinned fold andthrustbelt deformedby a mild Plio-Pleistocene extensional tectonics characterized initscentral part (Santa Cruz elbow) bylarge­ (Parraguez et al., 1997). TheCoastal Cordillera shows low scale transferzones (Baby etai; 1996). Thenorthernbranch reliefconstitutedby jurassic: - EarlyCretaceous magmatic oftheSubandean Zone ischaracterized by large scale thrust arc rocks. TheChilean marginexhibits a and sheets (to - 20km of offset) and broad (Roeder, topography, and in its central part, a well expressed 1988) filled by up to 6000 m of syn-tectonic Neogene extensional, asymmetric basin (Munozand Fuenzalida, sedimentary rocks (Baby et al; 1995a). Surface mapping, 1997), similarto theNeogene basinslocatedfartherN(Von seismic reflection data, and drilling information showthat Huene and Scholl, 1991). the main detachment planes are located in the Ordovician, Crustal balancingacross the CentralAndes between Silurian, Devonian and Permian shaly levels (Baby et al., IS"Sand I80S (Fig. 2)onthebasisofa normalpre-orogenic 1995b). Theslope ofthebaseofthe foredeep is4°toward the crustalthickness (according tothelocationofthePalaeozoic SW. Theamount ofshortening is74 km,i.e, 50%. basinand thelackofsignificant Meso-Cenozoic extension) In the southern branch of the Sabandean Zone, a allows us to calculate 210 km of shortening during the regional E-verging thrust (Mandiyuti Thrust) divides the Neogene (Baby e ai, 1997).At thelatitude oftheArica elbow, southernBolivian Zoneintotwo fold andthrust belts, which shortening is associated with the clockwise rotation of differaccording to their thrust system geometry. Mainly crustal blockscontrolled by inherited faults (Fig. 43), due fault-propagation fold sand fault-bend fold scharacterize the to thecompression exertedbythefore-arc zone that behaves western belt,whereas fault-propagation folds and passive­ as a rigid buttress. These rotations are coeval with the roof duplexes characterize the eastern belt. Main compressional deformatiun,bu\ the elhowshape of the detachments are located in Silurian dark shale, Lower Bolivian orocline has been acquired prior to this Devonian shale,and at the base and top of the Middleto deformation, and isprobably ofLate Cretaceous or Eocene Upper Devonian dark shale. The Silurian-Devonlan age(Roperchetal., 1999b). succession is covered by more than 2000 m of upper TheMoho shapeand the Nazca Plategeometryat this Paleozoic and Mesozoic sandstone with no potential latitudearewell constrained bygeophysical studies(James, detachments; in some placesit is also covered by several 1971; Cahill and Isacks, 1992; Dorbath et aL, 1993; Beck et thousand metres of syn-orogenic Neogene sedimentary al; 1996; Zandt et al, 1996). Deep crustal structures are rocks(Moretti et al; 1996). Thebaseof the foredeep slopes imaged by lower crust reflectors located at different at 2°W. Total shorteningdecreases southward from 140km structural levels (Wigger et al., 1994; Allmendinger and (50%)at 20"5, to 86km (35%) at22·S. Zapata,1996). Thecrustal duplexes below theEasternand The Interandean Zone and Cordillera Oriental are Western Cordillera are insufficient to producethe crustal deformed by E-vergent thrusts which involve basement thickening evidenced by geophysical data below the rocks (K1ey, 1996),and associated thin-skinnedthrusts and Altiplano and the fore-arc zone. Duplex structures in the backthrusts, Mainly Silurian, Devonian, and Carboniferous lower crust (Lamb and Hoke. 1997) cannotexplain theover strataareexposed intheInterandeanZone. IntheCordillera balanced volume(7216 km'in cross-section; Fig. 2),since 531 TECTONIC EVOLUTION OF SOUTH AMERICA I~I

the lowercrust structures havebeentakenintoaccountin sedimentation-erosionanddeeperosionunderplating was the crusta] balancing. Asthenosphere wedge as well as broken. In an ongoingconvergence tectoniccontext, deep significant volumes of magmatic addition cannotaccount up-drive willinvolve destructionoftheAltiplano byerosion for the observed thickness (Rochat et al.. 1999). The and associatedcollapse. significant tectonicerosionofthe Chilean margin(Rutland, 1971; Cloos and Shreve. 1996; Von Huene and Scholl, 1991) and associated extensional deformations suggestthat deep crust materialremoved from the continental edgehas been Andes of Ecuador underplated below the fore-arc zone andAltiplano (Schmitz, 1994; Baby er al., 1997). The Ecu adorian Andes (ION - 4OS), are one of the narrowestand most active part of the Andean Belt. It is deformed by NNE-SSW right-lateral transpressive shear Timing of Neogene deformations zones(Tibaldi and Ferrari.1992)and uooemenl anintense In the Central Andes, theback-arcthrustingstarted in tectonic and volcanic activity. The Dolores­ thelateOligocene (Sempere etal., 1990; Baby et ai., 1997). Guayaquil Megashear constitutes an important dextral The first W-vergent thrust motions in the fore-arc zone transcurrent boundarywhich marksroughly thesuture zone occurred in the late Oligocene-lower Miocene along the between the SouthAmerican continental marginand the median thrust plane (Garcia et al., 1996). Meanwhile, the Coastal Block withoceanic basement,accretedduringLate Altiplano corresponded to an endorhelc basin (Rochat et Cretaceous-Paleogene times (Tuteau et al., 1977; Lebrat et al., 1998, 1999) situated at the back of the more internal al., 1987; Cosma et al., 1998; Reynaud er aI,. 1999). Deep crustal thrust of the EasternCordillera. During the upper geophysical data arenot numerous enoughtoconstrainthe Miocene, the median thrust planeof the W-vergent thrust Moho geometry. Below the chain,the average depthof the system wasreactivated (Garciaeral., 1996) andcrustalback Moho isabout50km(Prevot etai; 1996). thrusts produced the partial expulsion of the Altiplano, which represented, therefore, a broad piggy-back basin Crustal structures and Neogene carried over the crustal duplex of the Eastern Cordillera deformations (Baby etal; 1997). Activity oftheSubandean fold andthrust system startedat the sameperiod (Gubbels et al., 1993); its TheEcuadorian Andes aredivided fromEtoWintosix eastward propagation accelerated in the Pliocene and morphotectonic units (Fig. 2). The Amazonas foreland continues presently. basin is deformed by two major NNE-SSW trending, transpressional right-lateral fault zones, which correspond Kinematic anddynamic analysis toinverted Mesozoic riftsystems (Baby et al., 1999). Positive flower structures were developed along these trends and Tectono-sedimentary studiesof the Altiplano (Rochat formed the main oil fields of Ecuador. No Quaternary etal; 1998) indicate a localtype isostatic behaviour (deep sedimentary sequences arecropping outinthisbasin,which basincontrolled byvertical motionalongpre-existing high seems to undergo upliftpresently. anglefaults). Predicted topography from10kmofdeposits, TheSubandean Zone isformed bytwoenechelon NNE­ assuminga normal crust isostatically compensated, is 1.5 SSW trendingpositiveflower structures(NapaandCutuco km (Rochat et ai., 1999). However, no significant absolute uplifts, Baby et al., 1999). which are still seismically and subsidence and upliftoccurredin the Altiplano duringthe volcanically active. They result also from transpressional Neogene. Thecontinuityofthe sedimentation inthe centre dextral movements, andare separatedbyaQuaternary pull­ oftheAltiplano shows that thetopography wasarchived by apart basin(Pastaza Depression). filling up of the thick syn-orogenic deposits and TheCordillera Real isa metamorphic belt,intrudedby progradationoverthe upliftingborders. Jurassic batholiths and strongly deformed by wrench The Neogene fillingof the fore-arc extensional basin tectonics. A W-dipping, high angle reverse fault zone (Von Huene and Scholl, 1991) is coeval with the separatesit fromthe Subandean Zone. In the Interandean sedimentary overloading of the Altiplano, which Valley, thick alluvial, lacustrine and volcaniclastic corresponds to 30%of the volumeeroded fromthe back­ continental sediments were deposited in several Neogene arc and the Cordillera Occidental (Rschat, 1999). Timing intermontane basins,controlled byregional strike-slip faults of both processes indicates that deeptectonicerosionand limiting the Interandean Valley (Marocco et al., 1995; underplating were able to maintain isostaticequilibrium Barragan etal., 1996; Hungerbuhler, 1997). and consequently the verticalaggradationoftheAltiplano TheWestern Cordillera and Coastal areaarepartofthe level. Along the fore-arc zone, structural traps (like the allochthonous oceanic terranes accreted to the Andean Altiplano crustal piggyback basin) do not exist. The marginduringLate Cretaceous-early Tertiary times (Lebrat intensityof Neogene erosion shows that these areas were etal.,1987; Cosma etal.• 1998; Hughes and Pilatasig, 1999; overcompensated bythedeepunderplating. TheupperPlio­ Reynaud et al.• 1999). The Western Cordillera is made of Pleistocene decrease ofsedimentation areasintheAltiplano oceanic plateau and island arc magmatic rocksand their (Rochatetai., 1998) wasassociated withexorheic drainage. Cretaceous-Eocene flysch cover, overlain and crosscut by Consecutive minor uplift,as is shownbylacustrine over­ magmatic rocks. The Coastal area is deepening and extensional deformations(Lavenu, 1995), characterized byfourmainNeogene fore arcbasins(Borbon, 532 show that the equilibrium between superficial Manabl,Progreso,Guayaquil; Fig. 1)related todextralstrike- !lClONIC I ~ O I U " O N or SOUI H AMI "ltA

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I~·~"t. ~R" -.r~.';.~~ ~~,~ ~;.. \I ..... =! :I: slip displacements (Deniaud et al., 1999). The Gulf of type of neotectonic studies is dedicated to the foreland U l­ Guayaquil is the deepest Neogene fore-arc basin. It basins,where river locations and shifts are controlled by V'o .::!i corresponds to a pull-apart basin developed between the neotectonic deformation ofthebasin surface (Fig.45). :z oc The convergence vector is oblique with respect to the u.I Dolores-Guayaquil Megashear Zone totheE,andtheoblique i= D:: convergent Nazca- South America Plate boundarytothe W plate boundary zone. The mode of the oblique o Z (Deniaudetal.,1999). accommodation is problematic, specially the relationship o z between the overriding plate deformations and the -« « subduction. Thegeometry of the coast and the subduction :;: Timing of Neogene deformations ::::; system suggests that itstrongly controls thebuilding of the o Neogene deformations havebeenrecorded inthefore­ range. The coastalareas of South America are generally '"~j D:: arc,intermontane and foreland basins.Thecreation ofthe '->.J submitted to extensional tectonics, mostly because it lies o, intermontane basinsstartedat about28 - 26Ma (Marocco D::- over the subducted plate without lateral constraints. o Cl etal., 1995), like the Bolivian Altiplano Basin. Inthecoastal area, the evolution of the Manabi and Progreso Neogene u oblique convergence and relatively wide coastal zone ...... basinsbegan intheearlyMiocene. In theAmazonas Basin, (comprised between trench and Western Cordillera) it is o V'..... stratigraphic data are insufficient to specify the ageofthe orthogonal to thedirection ofplateconvergence (Ecuador, o onset of the Neogene foreland basin. It is marked by the :z« southern Peru). In Chile, where there is a narrow belt ..... eastward wedge of the subaerial Arajuno Formation :I: between the trenchandthe Main Cordillera, the extension ...... (Petroproduccion seismic information), of probable early is orthogonal to the margin, and interpreted as related to o z Miocene age. gravitational post-seismic effects. o ;:: TheupperMiocene is characterized bytheclosure and In the coastal region of Ecuador, the stresspattern is =:> 5 the piggyback evolution of the intermontane basins > dominated by a N-S extension (Dumont es al., 1997), due u.I (Marocco et al., L995), and the closure ofthe Manabf and U eithertothegeneral northward escape oftheAndean Block, Z Progreso fore-arc basins(Deniaud etal., 1999).At thesame o or,more locally, tothenorthward increasing obliquity ofthe l­ V time, the Amazonas foreland was mildlydeformed and '->.J convergence, from the to Colombia. The I- invaded bymarineincursions (Hoorn el ai., 1995). triangle-shaped Andean Block accommodates the ThePliocene showed anacceleration ofthedeformation deformation at the triple junction between the South and marked theonsetofthe strongest orogenic stage ofthe American, Caribbean, and Nazca plates. Atthesouthern tip Andes, which is stillactive. The opening of the pull-apart ofthe Ecuadorian Coastal Block, which forms thesouthern basinof theGulfof Cuayaquil startedduringthe Pliocene, cornerof theAndean Block, the Gulf of Guayaquil opened and sedimentation rate reached a maximum (8600 mlMa asa result oftherightlateral movement oftheAndeanBlock inthedepocenter) in the lower Pleistocene (Deniaud ef aI., withrespect to the . Thisrightlateral 1999b). TheupliftoftheSubandean Zone (Napa andCutucu movement is accommodated along the Pallatanga Fault, uplifts) occurred duringthisperiodandcontinues presently. which extends northeastwards towards the Interandean Depression (located between the Eastern and Western Cordilleras) and farther N to other fault segments (e.g., Chingual-Sofia Fault). Southwestwards, the Pallatanga Fault OVERVIEW OF THE NEOTECTONICS extends intothe GulfofGuayaquil, bythe means ofa system OF THE NORTH-CENTRAL ANDES of transcurrentand normalfaults. The calculated average Quaternary extensional rateof the Gulf of Guayaquil is of (ECUADOR, PERU, BOLIVIA, AND 2.5± 1.1 mrn/y, NORTHERN CHllE) _ Thesubduction oftheCarnegie Ridge duringearly and middle Pleistocene isanimportant parameterofthe coastal The main features of the Andean Cordillera were uplift. Several Quaternary abrasion surfaces at elevations acquired during theMiocene, andfew changes occured since ranging from 7m to asmuchas330m (e.g., the surfaces of then.However, significant modifications of the topography the'Iablazos Formation) areobserved between the Gulfof areproduced byneotectonic deformations, resulting inthe Guayaquil and Esmeraldas, suggesting a maximum uplift present-day topography. During thisperiod, altiplano basins rate of about0.2 mmJy duringthe Quaternary. Along the areformed or maintained in Ecuador andBolivia. theNazca Pacific coast ofPeru.the Quaternary faults evidence a N-S and Carnegie aseismic ridges are introduced in the' trending extension. InthePacific lowlands ofsouthernPeru, subductionzone,leading the coast to rise, and the two this stateof stress is about neutral,due to a topographic depressions of the Marafion and Beni basins are effect related totheproximity ofthedeep Peru-Chile Trench. individualized, giving birthto the presentAmazonas River. Onthe northernPeruvian Coast, the present-day elevation This evolution ofthelandscape isbetterapproached and ofthe abrasionsurfaces suggests an upliftrateof0.2mmJ understood considering three different aspects of y duringthe Quaternary. In southernPeru,in frontof the neotectonic studiesandmethods.Thefirst determines the Nazca Ridge, uplifted marineterraces located at 300to700 stateofstress(Fig. 44),asdeducedfromfaultanalysis. As m high,suggest an average uplift rateof O. L8 mmJy and a far as Quaternary terranes are considered, a comparison maximum uplift rate of 0.7 mm/y for the same period with the present state of stress can be carried out. The (Machan: and Ortlieb, 1993). second dealswiththevertical movements alongthecoast, InChile, inlocalised coastal areas, which aretheclosest asdetermined from thestudyofmarineterraces. Thethird to the trench (80 to lOll km); the observed slate of stress 535 TECTONIC EVOLUTION OF SOUTH AMERICA I~l

duringthe upper Pleistocene isan E-W extension. ThisE­ compressional stress sHmax is considered constant and Wstretchingis related to uplifted terraces, located overa trends E-W, i.e., roughly parallel to the convergence crustal bulgedue to the subduction. Thusthe extension is direction. In the Western Cordillera ofthe HighAndes, szz interpreted as resulting from an accommodation of the becomes l.Hmax is s2 andHminiss3,trendlngN-S.ln the risingtopography, related to bodyforces. The£.Wtrending EasternCordillera, szzbecomes s2,sHmax iss1and trends SIT is s3 (sHmin,Horizontal minimumprincipalstress,or E-W,andsHmin iss3trending N-S.ln theEastern Cordillera, tensional deviatoric stress), syy N-S trending is s2 thecompressional strike-slip faulting maybeexplained by (intermediate deviatoric stress) and szz,vertical, is s1 aneffect oftopography, between thehighWestern Cordillera (Hmax, maximum principal stress or compressional andtheSubandeanLowlands. TheEasternCordillera being deviatoric stress).Thestateof stress is szz > syy> SIT. In undercompensated, its elevation should be lower in an northernChile (230 to27"S) theQuaternary marineabrasion isostatic equilibrium. The change between the surfaces, located at an elevation of 200 m, suggest a compressional regime intheSubandean Zoneandthestrike­ maximumupliftrateofaboutO.2 mmty{ortheQuaternary slip regime in the Eastern CordilJera should take place (Ortlieb et aL, 1996). between 1000 and 2000 rnin elevation. u.i In theMainRange ofCentral Andes, present-day stress In southern Peru,the state ofstress is different in the >-'" u.J field and crustaldeformations are nothomogeneous along High Andes; there,szzis sl , sHmaxis s2 and sHmin is s3 ...... ""z strike.In the whole Andeanrangeof Ecuador, the present­ and trends N-S, i.e., roughly perpendicular to the o ::iii day stress field appears to be homogeneous and the convergence vector (Sebriereral., 1985,1988; Mercier etal., 0-: Quaternary dominanttectonicregimeis an E-W trending 1992). In the Bolivian HighAndes, during thePliocene (6­ compression. Near the trench, the state of stress is sl = 3Ma),thetectonic regime was extensional; s3is sHmax and N8Iv E; inthe HighCordillera between 0° and l"S,the state trendsE-W(Lavenu and Mercier ]991). Duringuppermost of stress is N77°E < sl < N1200E; and in the Subandean Pliocene-lower Pleistocene (3 - 2 Ma) a compressional Zone. sl is N99°E (Ego er al., 1996). In the northern part of tectonics affected this region, which is characterized bys1 Ecuador, alongthe right lateraJ ChingaI-La Sofia Fault, the (rending E-W, parallel to theconvergence. This tectonic e~ent lateral slip rate displacementis of 7 ± 3 mm/yforthe last is characterized by a weak deformation, and by the 37kaBP(Ego, 1995). ln the Interandean Depression, inthe reactivation of old faults as reverse and strike-slip faults. restrainingbendofthe Zone, the shortening rate This stress regime is followed by a nearly coeval N-S is of 1.4 ± 0.3 mm/y since 1.4Ma (Lavenu et al; 1995). trending compressional tectonics. Since lower Pleistocene Along the right lateral strike-slip Pallatanga Fault, the to Present, the whole range is affected by an extensional horizontal rate motion isof4 ± 1mm/yforthesameperiod tectonics with s3 trending N-S (kilometric normal faults (Winter and Lavenu, 1989; Winter, 1990; Winter et al., with hectornetric throw). In the Altiplano and the High 1993). Andes, Quaternary tectonic regime is extensional with The present-daystate of stress in the Peruvian Andes sHmin= s3 and trends N-S, sHmax= s2 and trends E-W, hasbeendeducedfrom thestructuralanalysis ofQuaternary and s1 is vertical. As in Peru,this stress field resultsfrom andactive faults and seismic data(Sebrieretai; 1985, 1988, bodyforces duetoa compensed hightopography. The E-W Mercier et al., 1992). The crustaldeformation of the High trendinghorizontal stresssHmin=s2 isroughly parallel to Andes ischaracterized bynormalfaulting, excepted within the convergence direction; szz (s1) increases with the the EasternCordillera ofcentralPeru,whereas the western topography dueto the rangeload. and eastern boundaries of the HighAndes(fore-arc and The intermediate zones ie.g., Tarija, 1900 m in foreland) are characterized by thrust mechanisms that elevation) are characterized by two superposed stress indicatecompressional deformations. regimes. Oneisa relatively weakstrike-slipcompressional In the HighAndes, twotectonicregimes occur. In the stress,withs2vertical,s1=sHmax,E-Wtrending,and s3 Western Cordillera, recent and activedeformations result =sllrnin, N-S trending.The other one,more intensive, is froma N-S trending extensional tectonics. In the Eastern an extensional, axial stress, with s1 vertical;s2 trends E­ Cordillera, seismicity andactivestrike-slip faults result from Wand is equivalent to s3,which trends N-S. [fwe admit both a N-S trending extension and an E-W trending that the verticalstress szzis the resultof the weightof an compression. In the SubandeanZone, reverse faulting isin isostatically compensatedtopography. the strike-slipslate agreementwithan E-Wtrendingcompression. Close to the of stress is consistent with the intermediate location of trench, at the contact between the Nazca and South the basin, between the Subandean Zone and the High American plates, focal mechanisms ofearthquakes evidence Andes(Lavenu and Mercier,1991). an E-W trending compression roughly parallel to the Along the Chilean coast. the Quaternary regime is convergence betweenthe twoplates. In southern Peru,the extensional and of an E-W strikes. This deformation state of stress in the HighAndes as well as in the Pacific characterizes the westernmost portionsof the continental lowlands resultsfroma N-S trendingextension. fore-arc, dose to the trenchaxis(80km).Thisdeformation Thus, the state of stress in the Andes of central Peru doesnotappeartobedirecdylinkedtoboundaryforces due and thoseoftheAndesofsouthernPerumaybeinterpreted to the convergence. but could be the consequence of co­ as an effectof compensated high topography. However, seismic crustal bending with subduction-related compressional tectonicsaffects the HighAndes of central earthquakes. Itcouldbetopographic accommodation tothe Perubut not those ofsouthernPeru. uplift ofthispartofthecoast(bodyforce duetotopography), IncentralPeru,thestressmodel issuchthatthevertical sxxstrikingE-Wbecomes s3,syystrikingN-S is s2,and szz 536 stress szz increases with the topography and the is s1.The stateof stress is such that szz> syy> SIX. This TECTONIC EVOLUTION OF SOUTH AMERJCA I~CW;:~t:[~t~~ I ~ phenomenon could be related to the zones of maximum northward migrationof the deflection point made bythe coupling between theoceanicand continentalplatesin the Beni River enteringthebasin.AN-Strendingfaultcrossing Central Andes, which couldactas a buttress zone. the foothill margin controlsthis downstream increment. The partition'of the deformation across the plate Thepresentregional stateofstressinthe Subandean region boundary zone shows that the tectonic regime of the is roughly E-W, exceptin the southernpart oftheMarafion Quaternary ismorecomplex than previously recognized. In Basinwhere it is NE-SW (Assumpcao, 1992). Quaternary thesouthernAndes (Chile), aswell asin thenorthernAndes normal faults displaying a NNW-SSE extension in the (Ecuador), the Cord ilIeran segments,linkedtolargestrike­ distal part oftheMaranonBasin, as well as risingofbulges slip faults and high angle convergence obliquity,slides on theeastern margins ofboth basins are consistentwith toward the North. A part of the energy, transmitted from the present-day state of stress. The interpretation the subducting plateto theoverridingplate,is absorbed by emphasizesthat in the distal areas of the Maranon Basin thefreeescapeoffore arcslivers, parallel tothe margin.The the river traces are guided by topographic lows along lackof important crustal thickening and widening of the tensional faults, or basement blocks, uplifted or range characterizes these parts of the Andes. On the downwarped by tensional faulting. Near the piedmont, contrary, in the Cordilleran segments linked to low angle river shifts are controlled by the increment of fault convergence obliquity, the progressive stop of the lateral movements towardthe basin. However, theeffectof faults movements is due to buttress zones and the energy is in the near piedmont is moredifficult to explainthan in absorbed by the crustal thickening and widening of the the distal areas. Topographic effect between the High range(Bolivia). Andes and the foreland basin (Assumpcao and Araujo, The Subandean Zones of the Central Andes are 1993) may explainthat the geometry of active faults on dominated by a compressional stress regime. In the the foothills piedmont depend also on the localtrend of Subandean Zoneof Ecuador, the Quaternarystress field is the Cordillera. The interpretation of the successive shifts compressional and trends E-W. In the SubandeanZoneof involves the knowledge of paleoclimate oscillations. It centralPeru,reverse faulting is in agreementwith an E-W appears that a river moves toward a new formed trendingcompression, whereasdeformations resultfrom a depocenterat theonset of awetterperiod,and can stayin N-S trendingcompression (sl is sHmaK) in theSubandean place, even if tectonic deformation progress, during Zone of southern Peru. In the Subandean lowlands of relatively dryer periods (Schummetal., 1998). Bolivia, deformations arecompressional, withsHmax assxx, In summary, the study of the Recent state of stress in is sI,E-W trending. the Andes shows several types of behaviour of the TheMaranon and Beni basins arerespectively situated continental platealongtheactivemargin.Thisbehaviouris at the northern and southern ends of the Peru-Bolivia linked to the dip of the subdueted plate, the obliquityof Andeansegment(Fig. 45A). Thissegment corresponds to convergence between theoceanic andcontinental plates, the the flat slab subducrion of the beneath the bodyforces and boundary forces, the presenceor absence Andes. Spedfie structuresofthe foothills ofthe Subandean ofbuttress zones in the upper plate,and the possibility for Zone controltwo basins,eachone having onlyone outlet, the coastal blocks of freeescaping(Fig. 46). the Amazonas and Madeira rivers, respectively. The flat InEcuador, where theconvergence obliquity isveryhigh surface ofthesebasinsshowsa complexnetwork offlowing (g""O 31°to 45°),the Coastal Block is pushed northwards, and fossil fivertraces.Theseactiveand abandoned fluvial and is affected by a N-5 trending extensian. Since the tracesare used, together withneotectonic, seismotectonic elevation of the Andean range is relatively low, the and subsurface structural data (Dumont and Fournier, topographic effect isweak. Therangeis separatedfromthe 1994), to determine the neotectonic evolution of the Coastal Block by a large strike-slip fault, and an E-W Peruvian and Bolivian foreland basins(Dumont, 1996).The trendingcompressional stressdeveloped, dueto boundary phenomena exemplified below refer to short term forces. neotectonics, occurringduring the Holocene (0.1 - 0 Ma). In Peruand Bolivia, convergence obliquity is relatively In both basins, recent directional shifts of the main low (g = 20" - 24°). The HighAndes, which present the rivers arecontrolled bytheoffsetoffaults,TheUcayali River highest average altitude. are affected by a N-S extension, flows northwards along a N-S intra-subandeanbasin,then mainlydueto the bodyforces. Acompressional regimeis enterstheMaranon Basin whereithasbeen deflected tothe observed only alongthe boundarybetweenthe rangeand NE (Fig.45B).The successive deflection points shifted the Brazilian Shield (boundary forces). upstream and along the foothills. The line joining the In centraland southernChile, convergence obliquity is deflection points(Fig. 45B, pointsA, BandC)liesjustbehind intermediate (g =22° to 30°). The fore-arc and intra-arc the Andean Frontal Thrust.representedherebythe'Iapiche zonesof the Cordillera, the topography of which is lower Fault (Fig. 458).Contemporaneously,theMarafion Riverwas than in Peru and Bolivia, are affected by a N-S to NE-5W deftected tothe NE, linedup withthe straight,NEtrending compression. lower reaches ofthe Huallaga River, which is controlled bya Regarding theuplifted marineterraces, thetypeandrate fault observed onsatellite images. In the easternpart of the of vertical movements are thought to be relatively Marafion Depression,the riverstrend NE-SW,parallelto the independenton the rate and directionof convergence, the strikeofthemainbasementfaults oftheMarafion Structural convergence obliquitj.and theageand dipofthe subducted Zone (Laurent, 1985).Elongated lakes are situated over plate(Machare and Ortlieb, 1993).Conversely, thesevertical structurallydownwarped blocks (Dumont,1993). movements aretightly dependentonthe morphology ofthe Successive shiftsof the BeniRiver (Fig.45C) showthe subductingplate(aseismic ridges),andonthestructureand 537 ,tu_....."00 .. ..." .....".

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_--­.o :::;; on the distance from the coast to the trench, and this crust of the Tethyan arm created between the Colombian z a::..... deformation characterizes the westernmost portionsofthe segmentand these blocks (Jaillard et al., 1990; Litherland :I: l­e::: continental fore are, 80 km close to the trench axis.The etal., 1994). o Z relationships between Quaternaryvertical movements and 2 - ]fthe Colombian segmentdirectly faced theoceanic o z seismic activity are stillpoorlyunderstood. paleo-Pacific Plate, subductionwasprobablyactivebefore -c -a: the Early Jurassic, and the creation of a magmatic arc may s :::J o have resulted from more rapid subduction, due to an co accelerated accretion ratein thepaleo-Pacific system. =>' .....or: CONCLUSIONS : GEODYNAMIC Whatever the case, the roughly southeastward c- e:::­ subduction beneath the Ecuadorian segment must have o PROCESSES OF THE NORTHERN­ C> induced oblique subduction along the Peruvian margin, CENmAL ANDEAN OROGENY :3 associatedwith a strong sinistral strike-slipcomponent, .....v manifested bythe creation ofthelargeNW trendingsouth­ '0 V> Plate Kinematics Framework Peruvian turbiditicpull-apart basin (Vicente et al.• 1982) oz and by transtensional features in the back-arc zones « LU :I: The evolution of the central and northern Andean (Sempere et al., 1998; Fig. 47). The Kimmeridgian­ t- '-'-- system canbedividedintorourmainperiodswithdifferent Berriasian time-span is transition period. Along the o z. sedimentary, tectonic and magmatic characteristics, Colombian-Ecuadorian segment,this period was marked o ;::: indicating distinctive geodynamic situations and by accretions of displaced terranes, compressional o~ convergence directions. deformation, and theend ofmagmatic activity, whilealong >..... thePeruvian segment varied tectonic eventswereassociated v Z o withtheresumption ofsubduction-relatedvolcanic activity l­ LatePermian - LateJurassic: V L.U Tethyan Period (Asp denetal., 1987; Jaillard etal.; 1990, 1995).A11 thisclearly t- resulted from an impcrtallt, global-scale geodynamic TheEarly Mesozoic evolutionofthe Andeanmarginis change. In the west-Tethyan realm(centralAtlantic.Alpine influenced bythe 'Tethyan riftingand evolution (Iaillard et oceanic ridges), spreading rates significantly decreased a/., 1990, 1995),and is characterized by the onset of a (Olivet eta/., 1984; Klitgord andSchouten, 1986; Savostin et southeastwards subduction along the Colombian, aL, 1986}.lfa Tethyan-Colombian oceanic arm didexist,the Ecuadorian and Peruvian margins (Fig. 47),Afterthe Late motionvector ofthe Phoenixoceanic platewas thesum of Paleozoic coalescence of Pangea, the Triassic evolution of the expansionvectors of the Tethyan and Pacific ridges the northern and central Andes was dominated by an (Duncan and Hargraves, 1984). As a result, slowdown of extensional regime responsible forthe creationof grabens Tethyan expansion would have induced a northeastward andtheextension o(alkaline volcanics. Thistectonic context convergence between the Phoenix and South American isclearly related tothe westward propagation oftheTethyan plates(Duncanand Hargraves, 19&4; laillardet aI., 1990). break-upbetweenLaurasia and Gondwana. Moreover, the outpouringof a largeoceanicplateaualong The Early Jurassic evolution of the northern Central thePacific Ridge in Tithoniantimes mayhavemodifiedthe Andes was dominated by the destruction of the Late accretiondirection oftheEast-pacific paleo-plate (Nakinishi Triassic-Liassic carbonate platform,caused by a general etaI.,1989). extensional tectonic activity that progressed diachronously southwards. This is thought to have been Early Cretaceous-Paleocene: induced bythe riftingof the E-WtrendingTethyan system. SouthAtlantic Period Meanwhile, no significant absolute motion of the South American Plate occurred relative to the surrounding During this period. the development of the South continentalplates (Africa, NorthAmerica). Atlantic Ocean controlled the westward drift of the South Between late Early and early LateJurassic times, the American Plateand the variations in the convergence rate Tethyan breakline resulted in the openingof NE-trending alongthe subduction zone. Thesearethoughtto determine oceanic-floored rhombochasms (Alpine andcentralAtlantic' the sedimentary, tectonicand magmatic evolution of the oceans, BernouUi and Lemoine, 1980) linkedby Eto ENE Andeanmargin.Duringthe Early Cretaceous, the sudden trending sinistral transform zones (e.g., the Caribbean arrival of a great amount of east-derived sands can be Transform Zone). Theopeningofthe centralAtlantic Ocean interpreted asthe resultofthe westward domingof eastern beganbefore thelateMiddleJurassic (I '57 Ma,Klitgord and South America due to the incipient rifting of the South Schouten, 1986) and possibly as earlyas latestLiassic (l80 Atlantic Ocean. Although no reliable geodynamic Ma, Scotese et al., 1988). In the Andes.this period (190­ reconstruction is available, the lackof significanttectonic 140 Ma)wasmarkedbythe emplacement ofl-type plutons or magmatic activityalong the Pacific marginofthe South and calc-alkaline volcanics along the NNE trending American Plate N of 18°S would indicate a slow, steep­ Ecuadorian-northern Peruvianmargin,whichshouldhave dippingsubduction ofthe paleo-Pacific slab. beencoeval withan activesubductionbeneath this part of The definitive openingof the SouthAtlantic Oceanat the Andean margin. According to the pre-break-up equatorial latitudesduringAlbian times(Emery andUchupi, reconstruction, thissituationcanbeinterpreted intwoways: 1984; Scotese et al., 1988)induced the beginning of the 539 TECTONIC EVOLUTION OF SOUTH AMER.ICA

I~I~

absolute westward motion of the South American Plate. Casas and Molnar, 1987), provoking the change from a Therefore, as noted by various authors (Prutos, 1981; dominantly dextral transform zone to a nearly normal Megard, 1987; Soler and Bonhomrne, 1990), the beginning convergent regime in theColombian-Ecuadorian segment of compressional deformation along the Peruvian and (Figs. 47 and 48). Such dramatic changes explain how Colombian segments duringthe Late Albian (100 - 95Ma) terranes that were previously situated W of the Andean coincides with the onset of the trenchward motion of the margin, were drifted eastwards andaccreted to thenorthern upperplate(Uyeda and Kanamori, 1979; Cross and Pilger, Andean margin at that time (Jaillard et al., 1995). On the 1982; Jarrard,1986). otherhand,themore easterly convergence direction allowed TheAlbian-Turonian periodcoincides with a periodof subduction to take place beneath the NNE trending high convergence rate and with the mid-Cretaceous Ecuadorian margin and triggered the resumption of arc magnetic quietzone(Larson, 1991). IntheCentral Andean magmatism in this areabyearlyEocene times. margin, it ischaracterized byimportantmagmatic activity, A second major reorganization occurred by late ahighaverage subsidence rate(Jaillard andSoler, 1996) and Oligocene times, astheFarallon Plate splitted intotheCocos probably significant dextralstrike-slip movement (Bussel andNazca plates (WortdandCloething, 1981 ).Convergence and Pitcher, 1985). The latter are probably related to the direction evolved from ENE to nearly W- Eandconvergence north-northeasterly motionassumed forthe paleo-Pacific ratesubsequently increased (Pilger, 1984; Pardo-Casas and slab during Late Cretaceous times (Pilger, 1984; Gordon Molnar, 1987; Tebbens and Cande, 1997; Somoza, 1998; Fig. and lurdy, 1986; Pardo-Casas and Molnar, 1987; Fig. 48). 48).As aconsequence,thisperiodismarked byasignificant This convergence direction accounts for the lack of increase of the orthogonal component of the convergence Cretaceous arc magmatism along the NNE trending velocity between the paleo-Pacific oceanic plateand the Ecuadorian margin. The NW trending subduction zone Andean margin. Thesubsequent increased coupling along along the Peruvian margin extended probably the subduction zone was responsible for a significant northwestwards intotheoceanic domain asanintra-oceanic eastward migration of the deformed zone, which involved subduction zoneallowing the development of Cretaceous the former arc zone and proximal back-arc areas,i.e, the islandarcs,suchasthoseknown inwestern Ecuador. present-day Eastern Cordillera of Ecuador, Western The Coniacian-late Paleocene interval wasmarked bya Cordillera of Peru andBolivia, and eventually theEastern significant slowdown in the convergence rate (85 - 75Ma), Cordillera ofBolivia. Thissuggests a significant decrease of followed bya periodoflowmeanconvergence ratebetween theplayoflateraldisplacement along theAndean marginin the Phoenix and SouthAmerican plates(80 - 58 Ma, Soler the accommodation of convergence, and a correlative and Bonhomme, 1990). This period was characterized, increase oftheshortening andthickening oftheoverriding however, by the beginningof the Late Cretaceous Andean continental plate. Note thatthisperioddoesnotcorrespond compressional events. In the northern part of the studied to thesubduction ofa younger plate(Fig. 49). area,significant deformation wasrestricted to thefore-arc and arczones. However, theLateCretaceous andPaleogene Late Oligocene to Present: tectonic eventsare coeval witha noticeable decrease ofthe Pacific Period subsidence rate in the back-arc areas, which favoured detritital deposits, sedimentary hiatusandunconformities, From thelateOligocene onwards, the northerncentral Thissuggests that duringthisperiod ofoblique subduction, Andean margin wascompletely controlled bytheWtoWNW most of the convergence was accommodated by lateral motion ofSouthAmerica and the E to ENE motion of the displacements of fore-arc slivers along the edge of the paleo-Pacific Plate, that determined a roughly E-Wcouple margin, rather than by shorteningand thickening of the and a nearly normal subduction system (Fig. 48).During upper plate. In the southern Peru and northern Chile, this period, the subducting slab is rejuvenating, the however, deformations seemto have beenmoreimportant, convergence rate is relatively high (Fig. 49) and aseismic and a significant increase of the subsidence ratein Bolivia ridges arrived in the subduction zone (Fig. 3). This is interpreted as the result of a foreland-type subsidence geodynamic pattern,which remains relatively stableand related to the deformation and tectonic loading of the differs significantly fromthe preceding ones,corresponds margin (Sempere, 1994). to theclassical Chilean-type convergent margin. From 30 Ma onwards,the age of the oceanic plate Late Paleocene-late Oligocene: rejuvenated slightly,becoming probably morebuoyant, and Transition Period favouring, therefore, a low-dipping angle of subduction. Although theconvergence ratedid notchange significantly, The late Paleocene to late Oligocene interval (55 - 25 late Oligocene-early Miocene times are marked by an Ma) isakeyperiodin thewhole Andean evolution. Displaced acceleration, while the Pliocene is marked by a slight terranes wereaccreted or obducted along the Colombian deceleration (Fig. 49).Correlation oftheseratevariations is segment, importantcompressional deformation occurred difficult to link with specific tectonic events. During this in the Andean realm and sedimentary gaps and period, thedeformed zone significantly migrated eastwards unconformities occurred in theeasterndomains. and enlarged, eventually involving crustal-Kale thrusting These events coincidedwith global plate kinematic in thePeruvian and Bolivian partsofthechain (Figs. 51 and reorganization (Scotese et al., 1988). In late Paleocene­ 52).Due tothislarge-scale thrust movement, thechainwas Eocene times, the convergence of the paleo-Pacific plate oonsiderably uplifted, most ofthepresent-dayaltitude being 540 changed from Nor NNE to NEor ENE (Pilger, 1984; Pardo- acquired during the last 8 to 9 Ma.On the other hand, nCTONIC EVOLUTION OF ~OU, " AMU ICA I , ~

n ifill 17• !Jufd..( dw,...... J of""AaJ<- -r-JilluLa",M ' 1i_ 1. ,,/_ "' 1 Iml 541 lE CTO""IC EYOUlTltI'" O' StlUTil """ule .. [~

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fiGURE" . E.w.oo. ofIJI. "'''"''I''.cr Toctcelc diorffricm .ftINptdft'·l'anfu OCN.1lU: ,w, rriuiw h' tM AllJ¥lI m.."u. Jjw~ tilt ""- r.....em...... r.jl<,1'Ul'f I",", r ON.­ e-..o. JMoI_., "1~

FlGt 'RE4' - ! -...I..rlon of lN --r<'." ' 1I 1Y . rut P I'~ IJ/ d ..,Ja_ .tll• . ..&.:t.d.-d,.ll..u /rfU .. en-riIC_ u. ,•.." loJut SMt, . ~ __.._ I ,," ~

- --.. - - 542 '::'::l r: subsidence in the eastern basins of Bolivia and southern phases roughly coincide with the rejuvenation of the v ..... Peru increased, due (0 ilexuralloading. In r;oaCrast, oceamcp/::::; an average subsidence rateofabout 1000 m/Ma. o CD In spiteof a nearly orthogonal convergence direction, Absolute trenchward motion => ...... a:: the rate of dextral displacements of fore-arc slivers or Co.. of theoverriding plate ",,­ terranes of western Ecuador are high (0.5 cmly). This o o much As rIMed illJity Jutnors, the opeiticrgo! the Soudt in the Late Cretaceous, when convergence rate was much Atlantic Ocean at the equatorial latitudes during Albian ....u u.. higher(Fig.49) andmuchmoreoblique thanin theNeogene times,which provoked the beginningof the westward shift o (Fig. 48). In the same way, local rotations have been of the South American Plate, roughly coincides with the u.J""o z significant during the Neogene, demonstrating that initiation of the contractional deformation along the ....« :I: contractional shortening was a leading process in the Peruvian-Ecuadorian margin.Thus,this parameterseems I- thickening and bending of the Altiplano Orocline,and to control the long-termed contractional regime of the o Z suggesting thatthe cumulated amountofrotation may have continental active margin. o ;::: beensignificant since the Cretaceous. Asemphasized bySebrier and Soler (1991) forthe late -'=­ o Tertiary Andean contr'lctional phases, only a slight ....> shorteningoccursin theAndean retro-arcforeland during u Z (he periods of teemII ir: qll.iescence, and chen most af the .....c» v Role of kinematic parameters ...... westward drift of the SOuth American Plate should be I- in the Andean Orogeny accommodated byan absillute westward overriding ofthe continental plateovera retreating oceanic slab. Ontheother Most classical geodynamic models forthe originofthe hand, the amount of tectC\nic shorteningobserved during tectonic phases in continental active marginsarebased on the contractional phases implies that virtually all the the observation and comparison of various present-day westward driftoftheSouth American Plate isaccommodated active margins (Uyeda and Kanamori, 1979; SchoU et al., by the shortening. Ther~fore, during the contractional 1980; Uyeda, 1982; Cross and Pilger, 1982; Jarrard, 1986), or phases, the western COntinental margin of the South throughphysical modelling (Bottet al., 1989; Whitta~r et American Plate is virtually motionless in an absolute al., 1992; Cloos, 1993; Shemenda, 1994). Only a few have reference frame(i.e., then:is a stopping ofthe slabretreat). beenelaborated throughthestudyofa single activemargin This recurrent stopping of tileslab retreat, the mechanical evolution through a longperiod of time.The study ofthe origins of which are unclear, mightbe one of the driving Andean margin since earliest Mesozoic times, however, phenomenon oftheshort-lived contractional tectonic crisis. provides some geological constraints on the origin and natureofthetectonic phasesofcontinental active margins. Collision of continental oroceanic Plate tectonic reconstructions arepoorlyconstrained for obstacles theLate Cretaceous period(Pardo-Casas and Molnar, 1987), especially as regards subduction of ridges. dip of the It hasbeenproposed thatthe arrival in the subduction subductingslab and direction of convergence. How~er, trenchofoceanic or continental obstacles (aseismic ridges, quantitative approximation ofsomeparameters,sucha. the sea-mounts,continental Jnicroplates) will to blocking convergence velocity (Soler and Bonhomrne, 1990) deduced ofsubduction, contractional deformation ofthecontinental from theglobal spreading rates(Larson, 199 I),the absolute marginand piate reorganisation (Schoff et at., Hl«

i ! - - l - - I - - ! - ,- m_ 0 - - rn -_ "' --~ 11- - u- fi..... --J - ...... _ Of .---- ~ ... __ ---_..._ - ,~ -- - ,_ . TECTONIC EVOLUTION OF SOUTH AMERICA l~! UJ ::::! ::t: southernPeru and northern Peru where no collisions are convergence direction not only control the normal U ..... known to haveoccurred. Thus, in this case,it seems that convergence rate, but also playa part in the regional '"a accretions or collisions of terranescannotbe the causeof subduction patternthatcouldinturn influence thetectonic ::Ez eo:: regional contractional phases. In the studiedarea, the fact regime. Such changes in the convergence direction during .... ::t:..... eo:: that contraction took place in non-accretionary settings the Paleogene can explain the contemporaneity of the a z at the sametime as accretions occurredsuggests that the contractional events in non-accretionary settings of the Clz accretion events and the coeval contractional phases are Central Andes and thecollisions ofisland arcs. « « consequences ofa sameglobal geodynamic mechanism. s ::::i a Relation convergence rates ­ co :::::l­ eo:: Convergence rate subsidence UJ a. eo::- According to Uyeda and Kanarnori (1979), Cross and In northern Peru,periods of slow plate convergence o o Pilger (1982); Pardo-Casas and Molnar (1987), a rapid correlate withlow subsidence rates(L30to 110 Ma, 75to45 :5u convergence between the oceanic and continental plates Ma, 35 to 25 Ma). Conversely, periodsof highconvergence LU provokes a compressional stress in thelatter. According to velocity arecoeval withperiods ofincreased subsidence rate a Soler andBonhomme (1990), periodsofhigh convergence (110 to85Ma, 50to 40Ma; figs. 49 and SO). Thiscannotbe '"UJ Clz rates along the Peruvian margin occurred in Albian­ explained by increased of the deep « LU Campanian and lateEocene-early Oligocene times,which continental margin (Von Huene andScholl.Issl), because ::t: I­..... coincide roughly with mainly contractional tectonic periods thislattermodel isonlyproved toaccount forthesubsidence o z (Fig.49). However, ratherthanwiththeconvergence velocity of the fore-arc or arc zones, whereas increased subsidence o i= itself, theshort-Iived tectonic events seemtocorrelatebetter is observed as far as the easterndomainbetween 110 and ....l=' o et al., 1996; 39 withchanges in the convergence velocity, whichever their 85Ma(Contreras figs. and SO). In contrast, >LU sign,eitherpositive (acceleration) ornegative (deceleration). theseobservations are consistent with the thermal model !::! z If the reconstruction of Solerand Bonhomme (1990) is of Mitrovica et al. (1989), that assumes that a fast .....o U correct, acceleration occurred inLate Aptian (L10 Ma), Late convergence provokes an increase ofthe subsidence rates .....LU Campanian (75Ma),early to middle Eocene (50Mal.latest along the whole continental margin, through mantle Eocene (38Ma), andlateOligocene-early Miocene times(25 convection (Gurnis, 1992; Sternand Holt, 1994). Thelack - 20Ma). whereas deceleration occurred in theLateAlbian of suchcorrelation in southernPeruis most probably due (100 - 95 Ma), Santonian (85Ma), middle-late Eocene (42 tothefact thatcontractional tectonic events occurred earlier Ma), and Pliocene (4 Ma). All these periods coincide with andwere stronger than innorthern Peru.There.tectonic uplift apparently extensional (LateAptian, early-middle Eocene of the marginby crustal shorteningand thickening, and boundary) or important contractional tectonic phases. overload tectonic subsidence of the foreland would have Therefore, short-lived contractional phases as well as prevailed since Senonian times (Sempere, 1994). extensional tectonic events seemtobe mainly controlled by changes in the convergence velocity. Dipof subduction and subduction erosion Direction of convergence The continentward shift of the volcanic front is Thegeometry ofthegeodynamic reconstructions aretoo interpreted classically as a result of the shorteningof the poorly constrained toallow avaluable discussion fortheLate continental margin, either by compressive tectonic Cretaceous. TheIncaic contractional tectonic phases oflate shortening, or by subduction erosion (Scholl et aL, 1980). Paleocene (58 - 55Mal.latemiddle Eocene (43- 42 Ma)and DuringAlbian and ea.rly Late Cretaceous times,thelocation late Oligocene age (26 Ma) coincide with successive of the magmatic arc of Peru was stable,indicating that clockwise rotation in the direction of convergence (Pilger, neither significant shortening nor subduction erosion L984; Pardo-Casas and Molnar, L987; Mayes et ai; 1990; occurred at this time (figs. 20 and 51). The eastward shift Tebbens andCande, 1997). These changes intheconvergence ofthemagmatic arcintheLate Campanian canbeexplained direction. which caused successive significant increases of mainly by the tectonic shortening related to the major thenormal convergence ratesseemto havethesame effects Peruvian phase. As a consequence. it seems that no as thoseassumed fora convergence acceleration. significant subduction erosion took placein Peru before Moreover, the important changes in the convergence latestCretaceous, and possibly before Paleocene times,as direction from NNE to ENE by late Paleocene must have indicated by the relative stability of the magmatic arc induced drasticchanges in subduction geometry. TheNNE location before this period. trending Ecuadorian margin changed from a mainly In Eocene times, the ongoing eastward shift of the transform to a chieOy convergent regime. This must have magmatic belt is associated with its abrupt widening induced theeastward drift and accretion of oceanic island interpreted as the result of a widening of the melting zone arcs along the Ecuadorian margin and the birth of new in the asthenosphere wedge linkedto a decrease of thedip subduction zones to the W of them (Jaillard et al., L995). of the BenioffZone, in turn controlled by the normal The changein the convergence direction of late middle convergence velocity (Soler, 1991). The coeval rapid Eocene agealsoresulted in anewevent ofcollision ofisland extensional subsidence observed in most of the fore-arc arcs along the Ecuadorian margin (Bourgois et aL, 1990; regions byearly middle Eocene times is toowidespread to Hughes and Pilatasig, 1999). Thus, changes in the resultfrom local tectonic events orpaleogeographic effects. 545 I ! - • ;• - • • -• ! j . l -=,,. ; -,. , I -. I '- _.."._­ • -~-_...... ; -~ --- ,--_-.. I -- ...... 01 _...... -.- ... ,", -.01_... _ ...... - _ . ~ ... ~,...... ,_ , ,.,..... -_...... __ ""'" ... .. '--""'"'__-.. __ .", , ...... -'_, I.W, '.."_ ... , ,-' , ...... -- ....""-J.....___.. .. "" ..... -...h_...- _' , ...... __ --_ --_ " ~,...,...... _ -_ _ e->...- , .. _ .... REfE RENCES .""...... ___ _ ,""- _.__ _- -.-,..,....."._-"',-",'""­ ..._....-- .... ". _....-... ,,"-...... ,...- .... _...... L"'.._._""'''__' _ . .. CornprnsioDal

L.U --' I u Angeles, C (1987). Les cheva uchementsdela Cordilln-e Occidentale Avila -Salinas, W.A. (1990). Tin-bearinggranites from the Cordillera ...... o par12°J5'5(Andes duPerou central). Thesis3rd cycle, Univ Real, Bolivia: a petrologi cal andgeochemical review. In:Plutonism ::2 z Montpellier II,1 map.h.r., 184p.(unpublished). fromAntarctica toAlaska, eds,Kay, S. andRapela,C.Geological .....a: :J: Angeles.C. (1999). Los sedimentoscenozoicos de Cerrode Pasco: Society ofAmerica, Special Paper, 241, 145-159. I­ a:: Estratigrafta, sedimentadonytectonica, Soc. Geol. Peru, Lirna, Azalgara, C., limenez, W., Morales, W. and Vergara,I. (1991). o Z Yol.jubilar 5,103-118. Posibilidades petroliferas de las cuencasdelAntearco, Peru. C> Z ArdiU, I., Flint, 5., Chong, G. and Wilke, H. (1998). Sequence Petroperu, report,37 p.(unpublished). .....u Aspden, I.A., Bonilla, W. and Duque, P. (I995). The EI Oro Baby, P..Colletta,B. and Zubieta, D. (I995a). Etude geometrique et ...... metamorphic complex, Ecuador: geology and economic experirnentale d'un bassin transporte: Exernple du bassin o ...... mineraldeposits. Overseas Geology andMi71erlll Reso urces, 67, subandin de l'Alto Beni (Andes centrales), Bulletin de la C> Z British Geological Surveypubl., Nottingham, 63 p. Societe Geologique deFrance, 166,797-811......

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Orstom ed., Paris,167·170. l'Equateur(0030'S): Implications tectoniques, Bulletinde la LoU ""c- Bussel, M.A. and Pitcher, W.S. (1985).The structural control of Societe geologique deFrance. Paris 169,739-751. a::­ o batholith emplacement. In: Magmatism at a Plate Edge: the Couch, R. andWhitsett, R.M. (1981). Structuresofthe Nazca Ridge n Peruvian Andes, eds. Pitcher, M.P., Cobbing, E.]. and and continental shelfand slopeof southern Peru.Geological ~ u...... Beckinsale, R.D..1'1'.167·176. Blackie Halsted Press,London. SodetyofAmerica Memoir, 154,569-586. '0 Bussel, M.A. (1983). Timingof lectonic and magmaticevents m Courjault-Rade.P.Debrenne.R andGandin,A.( 1992). Palaeogeographic V'> LoU the Central Andes of Peru.Journal of the Geological Society, andgeodynamic evolution oftheGondwana continental margins o Z London, 140, 279-286. during theCambrian. Ierra Nova, 4,657-667.

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