JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. B8, PAGES 11,945-11,982, JULY 30, 1992

Changesin the TectonicRegime Above a SubductionZone of AndeanType' The Andesof Peru and Bolivia During the Pliocene-Pleistocene

JACQUESLouIs MERCIER,• MICHEL SEBRIER, • ALAIN LAVENU, 2 JUSTO CABRERA, • OLIVIERBELLIER, • JEAN-FRANCOIS DUMONT, 2 AND JOSE MACHARE •'•

This paper adressesthe changesin the tectonicregime in the Peruvianand Bolivian Andes that have occurredsince the upper Miocene when the present-dayelevation of the Cordillera above sea level has beenalmost reached. The stresspatterns are deducedessentially from a field studyof fault kinematicsand a numericalinversion of the slip vectordata measuredon the fault planes. The Cuzco fault system in southern Peru is chosenas an exampleto illustratethe methodologyused. In this region, striationson both active and Holocenefaults are in agreement with a N-S extension.But faults affecting early Pleistocenedeposits exhibit two families of striations. •l•e younger resultsfrom the previous N-S extension: the older, involving reversemotions, resultsfrom either an E-W or a N-S compression.Faults affectingPliocene formations often show an oldestfamily of striationsresulting from a NE-SW or an E- W trendingextension. Thus threetectonic regimes are demonstratedwhich are also supportedby regional unconformitiesand sedimentologicaldata: (1) a Plioceneextensional regime, (2) a lower Pleistocene compressionalregime, and (3) a mid-Pleistocene-present-dayextensional regime. Similar analyses conductedin the Pacific and sub-Andean lowlands allow sketchingof the successive Pliocene- Pleistocenestress patterns in the Central Andes. The Quaternaryand present-daystress pattern is characterizedby a N-S extensionin the High Andes and in the Pacific lowlands and by an E-W compressionin the sub-Andeanlowlands and at the contactbetween the Nazca and South American plates.Thisstress pattern is interpretedat a large wavelength(>100 km) as an effect of compensated topography. This model supposesthat the vertical lithospheric stress, Ozz, increaseswith the topography,the crustalthickness, and the low-densitymantle beneathand that the lithosphericmaximum (compressional)horizontal stress Oilmax,trending E-W roughlyparallel to the convergence,is fairly constant.On both edgesof the Andes,the tectonicsbeing compressional, Ozz is o 3 andOilmax is ol. In the High Andes,Ozz becomes 01, thenthe E-W trendingOilmax is o 2 andOHmin trendingN-S is 03, allowingextension to occurin this direction. The Pliocenestress pattern was characterizedby a NE-SW or an E-W trendingextension in the High Andes,in the Pacific lowlands,and possiblyin the sub-Andean lowlands.This stresspattern was clearly differentfrom the present-dayone becausethe E-W trendingstress was OHmin. This requireda weak pushor, eventually,tractional boundary forces acting on the South Americanlithosphere. It is suggestedthat this might result from a strongslab pull due to a long, steeply dippingslab which decreasedthe value of the Oxx stresstransmitted to the overridingplate. The early Pleistocenestate of stresswas compressional.Since the elevationof the Andeshad not markedly decreased duringthis period,this requiredan increaseof the E-W trendingstress value. This resultedfrom a strong couplingbetween the two lithosphericplates, possiblydue to a rupture of a long slab under its own weight.Other spatialchanges in the stresspattern are related to the particularsituation of the forearc,to the subductionof the buoyantNazca ridge, and to the different dips of the slab. Extensionin the High Andesis of small magnitude,ofthe order of 1% during the last 1-2 m.y.; in a few basins,it may have attained40% duringthe Pliocene(--5-3 m.y.).

1. INTRODUCTION is scarceand is located only in the High Andes. But, for a long time, normal faulting has been reported in the central Andes Our field study has been conductedin Peru and Bolivia. [Heirn, 1949; Silgado, 1951; Audebaudet a/.,1973; Aubouin et There, the Andean subduction zone shows two different slab al., 1973], and detailed studies have clearly demonstrated geometries[Stauder, 1973, 1975; Barazangiand lsacks, 1976, Quaternary and Recent normal faulting [see Dalmayrac, 1974; Lavenu, 1978; Soulas, 1978;Yonekura et al., 1979; S•brier et 1979]: a flat segment beneath central Peru and a 30ø east al., 1985, 1988a; Mercier, 1981; Cabrera et al., 1987; Lavenu dipping segment beneath southern Peru-Bolivia-northern and Mercier, 1991]. Chile. The shallow seismicityof the Andes is clearly related to However, field work has also demonstrated the occurrence compressionaltectonics. It is concentratedon'both sides of of folding, of reverse faulting, and of several regional the Cordillera, i.e., along the contact between the Nazca and angularunconformities which show that the High Andes have South American plates and in the sub-Andes, along the also sufferedcompressional tectonics during the Cenozoic. Six Amazonianplain. Seismicity related to extensionaltectonics discrete compressional pulses, which took place during the last 4045 m.y., have been distinguished [seeSgbrier et al., •URA CentreNational de la RechercheScienfifique, Giophysique et 1988b]. During the same period, the Cordillera has been G•odynamiqueInterne, Universit• de ParisSud, Orsay,France. uplifted. Analysis of the Late Cenozoic morphological •Officede In Recherche5cienfifique etTechnique Outre-Mer, Paris, surfaces(the "Punas") on the Pacific side of the Andes has France. ßIow at InstitutoGeofisico del PeA, Lima. shown that the high Cordillera topographywas produced essentiallybetween 26 and 6 Ma [Sgbrier et al., 1988b]. The presentwork concernsmainly the period subsequent to the Copyright1992 by the AmericanGeophysical Union. compressionalevent of uppermostMiocene age (--7 Ma), when Paper number 90/B02473. the present-dayelevation of the Cordillera above sea level has 0145-0227/92/90JB-02473 $05.00 been almost reached.

11,945 11,946 MERCIERET AL.: CHANGESIN TH• ANDEANTEC'rONIC REGIMES

The stresspatterns have been establishedprimarily from the fault plane, a mean deviatoricstress tensor may be field analysisof faultkinematics and also using published focal computed,within a factork, froma setof striatedfaults [Carey mechanismsof earthquakes.Field work hasbeen focusedon and Brunier, 1974]. Several quantitative computer-aided the late Cenozoic basins where the successivekinematics of methods have been proposed to solve this problem by the faults may be distinguishedand dated. Thus it is minimizingthe deviationsbetween the measuredand the necessarilydiscontinuous in space but hasbeen complemented computed slip vectors [seeZoback, this issue]. Here, we use by analysisof aerialphotographs and Landsat images. More the algorithmproposed by Carey [1976, 1979].However, the than 200 faulted siteshave been analyzedin Peru and Bolivia; use of suchmethods without caution may lead to a misleading theyhave yielded about 5000 fault slip vectordata, about computationof severaldifferent stress tensors instead of a 500 of which comefrom major faultsseveral to 10 km long singleactual state of stress[see Mercier and Carey-Gailhardis, borderingthe sedimentarybasins. These have yielded325 1989]. Therefore, a separationof familiesof striafionsmust independentdirections of stressand of deformationof different be basednecessarily on geologicaldata demonstratingtheir qualifies(for definitionof qualityranking, see Zoback et at. chronologyand their relationswith regionaltectonic events. [1989]).Moreover, this structuralwork has been supported by Similar inversionmethods may be usedfor focal mechanism detailed geologicalanalyses [Blanc, 1984; Bonnot, 1984; populations(see Carey-Gailhardisand Mercier, 1987,and Huaman, 1985; Lavenu, 1986; Macharg, 1987; Sdbrier, 1987; referencestherein); they permitone to choosethe preferred Cabrera, 1988; Betlier, 1989]. seismic fault plane and to compute the stress tensor which explains their kinematics. Synsedimentary faults have been particularly studied 2. METHODOLOGY becausetheir kinematics are easily dated. To compute the related statesof stress we have used data from faults sealedby deposits belonging to the same lithological formation that 2.1. Fault Kinematics Analysis and Computation they offset (see sections 2.2.2 and 2.2.3). The of a Stress Deviator From Slip. Vector Data synsedimentaryfaults have been analyzed in details in our previousworks to which the readermay refer [Cabrera et al., Kinematics of a fault population is defined using the 1991, Bellier et al., 1989a, b; Bonnot et al., 1988]. In section striations measured on the fault planes. Supposingthat 2.2, the Cuzco fault system is chosen as an example to slidingoccurs in the directionof the shearstress resolved on illustrate the usedmethodology.

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Fig. 1. Structuralsketch of the Ayacucho,Cuzco et Vilcanota fault systemsdrawn from Landsatimages and field observations.1, Ice caps;2, Quaternarybasins; 3, Mesozoicand Cenozoicformations; 4, Paleozoicrocks; 5, mid- Pleistocene-present-daynormal faults; 6, pre-Quaternaryfaults; 7, anticlines;8, synclines;9, flexures;10, strike-slip hults; 11, villagesdestroyed by historicalearthquakes; 12, Locationof recentearthquake epicenters (from U.S. Geological Survey catalog). MERCIEREl' AL.: CHANGES IN THEANDEAN TEC'rONIC RF.•IMES 11,947

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Fig. 2. Structuralmap of the Cuzcofault system. 1, morainesof thelast major glaciadon; 2, mid-Pleistocene-Holocene alluvial terraces;3, late Pleistocene-Holocenefans; 4, mid-Pleistocenefans and terraces;5a, early Pleistocene fiuviolacustrineSan SebastianFormation; 5b, Pliocenefiuviolacustrine Huaypo-P'iuray units; 6, Plio-Quaternaryvolcanic centers;7, mid-lateMiocene Chinceros Formation; 8, Mesozoicand Cenozoicformations; 9, Paleozoicrocks; 10, Quaternaryand active normal faults; 11, fault segmentsreactivated during the April 5, 1986,earthquake; 12, faults reactivatedduring the earlyPleistocene compression; 13, reversefaults; 14, flexures;15, analyzedfaulted sites. Cross sectionBB' is locatedon the map. Solid circles,recent earthquake epicenters.

2.2. The Pliocene-Pleistocene Tectonic Regimes strike-slipmovements which predateQuaternary normal Along the Cuzco Fault System displacements[Sgbrier et al., 1985]. A south facingscarp, approximately300-400 m high,exhibiting triangularfacets (Figure4) and2- to 4-m-highscarplets located at thefoot of TheCuzco fault system (Figures 1 and2) is a partof a more this scarpclearly demonstrate these Quaternary normal fault than 200-km-long, E-W strikingfault zone which is displacements.The N-S striking,west facing Tamboray fault superimposedto the "Abancaydeflection" [Marocco, 1977], a (Figure2) is expressedin thefield by a scarplet3.5 km long majorAndean transverse structure. This deflectionis located and2-4 m high.Displacements of streamscut by thisscarplet justnorth of thetransitional zone between the flat subductingshow that the motion on the fault is also essentially slabof centralPeru andthe 30ø eastdipping slab of southern normal. In the Pachatusanfault sector [Cabrera et al., 1987], Peru.It comprisessix discontinuousfault sectors(Figure 2) scarplets offset major fresh morainesand minor arcuate withlengths ranging between 3 and18 km [seeSgbrier et al., moraines(Figure 5) whichare attributedto the 14,000 years 1985; Cabrera et al., 1987]. B.P. lastmajor glaciation and to the=11,000 years B.P. minor 2.2.1. Quaternarynormal fault motionson the Cuzco fault readvancesdated [Mercer and Palacios, 1977; Mercer, 1983] in system.The Tambomachayfault sectoris composedof four the CordilleraVilcanota-Ausangate, respectively, east of Cuzco fault segments(Figures 2 and 3), 5-6 km long, dipping at an (Figure1). Thatthis faultsystem is still activeis shownby angleof 60ø-70øS.The major fault planesshow reverse and historicaldestructive earthquakes (dots on Figure2) [Silgado 11,948 MERCIER ET AL.: C•IANG• IN THJEANDEAN TECTONIC REGIM•

Fig. 3. Aerial photographof the Tambomachay(TM) fault (underlinedby arrowheads) and of the Cuzcobasin (location on Figure2). PU, PucaPucara; SS, San Sebastianvillage. Numbers give locationof the analyzedsites. et al., 1950; Silgado, 1978; Ericksen et al., 1954] and formedat the foot of the Holocenescarplets (Figure 6a) during instrumentalseismicity [Instituto Geofisico del Per•, (IGP), the main shock [Cabrera ,1988]. Along the Chincherosfault 1980].The lastmajor crustal earthquakes occurred on May 21, (Figure2), they attainedmaximum 3-cm verticaloffsets and 1950 (M = 6), andApril 5, 1986 (rob= 5.4). 3-cm-wideopenings, and alongthe Qoricochafault (Figure2) Kinematicsof the seismicfaults of the April 5, 1986, they attained maximum 10-cm vertical offsets. Trenches Chincheros-Qorichochaearthquake: Surface seismic cracks (Figure 6b) have been excavatedacross the reactivated .MERCIERET AL.: CHANG• IN THE ANDEAN TECTONIC REGIMES I 1,949

Fig. 4. Field view of the Tambomachayfault nearPuca Pucara (PU, Figure3). The Pleistocenescarp forms clear triangular facets(some 80 m high). It dominatesa 2- to 4-m-highscarplet (vertical arrows) resulting from Holocenemovements. scarpletsto observethe main fault plane affectingthe bedrock trenches result from tensional (•3) directions trending or Quaternarydeposits. Striations (Figure 6c) observedon the betweenN8øand N12øE (stereonet TQS, Figure7, andsites Chincherosmajor fault planesshow a normalmotion with a 103-105,Table 1). At site104 (Figure2) thedisplacement small dextral strike-slip component (numbers 2 and 5 on measuredon the surface crack had the same azimuth that the stereonetGQK, Figure 7), while on the major plane of the striationmeasured on the major fault plane observedin the Qoricochafault, they demonstratea purely normal motion trenchat the sameplace (number 16 on stereonetGQK, Figure (numbers16 and50 on stereonetGQK, Figure7). Kinematics 7). This suggeststhat the striations measured on the of thesemajor faults are in agreementwith a roughlyNNE-SSW reactivatedfault segmentscould be mostlycoseismic. lengthening.Kinematics of the minor fault measuredin the Holocenekinematics of the Cuzcofault system:The major

Fig. 5. Aerialphotograph showing the last glaciation deposits (a) andthe minor Holocene glacial readvances (b, c) offset by thePachatusan scarplets underlined by arrowheads (locations on Figure2). 11,950 MERCIERET AL.:CHANGES IN THEANDEAN TECTONIC REGIMF_S

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Fig. 6. (a) Field view of the HoloceneChincheros scarplet (underlined by arrowheads) reactivated during the April 5, 1986, earthquake(site 103, Figure2). (b) Reactivatedraftjot fault plane (arrowheads) of the Qoricochasegment observed in an excavatedtrench (site 104, Figure 2). (c) Normal striation(arrow) on the reactivatedmajor fault plane observedin the trench. U, upthrow; D, downthrowblocks. MERCIER Er AL.: CHANGESIN THE ANDEAN TECTONIC REGIMES 11,951

TABLE 1. Parametersof the StressDeviators Computed From Fault Kinematicsof Mid-Pleitocene-Present-DayAge

Site ND Latitude Longitude ol o2 o3 R MMA, S.D., D References

øS •V Azim.dip Azim.dip Azirn.dip deg deg

Northwestern Coast 1 nc 4.500 81.083 ..... normal ...... Q 1(C) 4 03 5.817 81.017 .... reverse ...... Q 5(21) Narnora 7(L) 02 7.172 78.326 ..... N-S ...... Mz 2(N9-F3) 10 06 7.186 78.350 069ø 74ø 291ø 12ø 199ø 11ø 0.77 11.6 5.5 uM 2(N8) 11 23 7.186 78.354 327ø 73ø 092ø 100 184ø 13ø 0.35 12.6 3.0 uM 2(N2-F2) 12 14 7.188 78.354 191ø 84ø 086ø 01ø 356ø 05ø 0.37 12.5 3.2 Q 2(N21) 14(L) 04 7.189 78.356 ...... N-S -- Mz 2(NIl-F3) 17 08 7.190 78.358 232ø 79ø 117ø 05ø 027ø 100 0.83 6.9 2.7 uM 2(N15) 18 06 7.191 78.333 107ø 70ø 288ø 20ø 198ø 01ø 0.14 5.8 2.5 Mz 2(N17) 20(L) 04 7.200 78.354 ...... N-S ...... uM 2(N6-F3) 21 15 7.204 78.335 263ø 67ø 114ø 20ø 020ø 11ø 0.62 9.0 2.5 uM 2(N23) 22 09 7.207 78.239 229ø 78ø 115ø 050 024ø 100 0.57 14.8 6.9 Mz 2(N13) 23 13 7.216 78.306 098ø 64ø 287ø 26ø 195ø 04ø 0.54 12.7 3.9 Mz 2(N10) NA(S) 11 150ø 88ø(16ø) 286ø 02ø(16ø) 015ø 01ø(11ø)...... 2(RST) San Marcos 29 24 7.336 78.103 088ø 77ø 288ø 13ø 197ø 040 0.41 13.5 3.3 mM 3(S9-F4) 33 11 7.341 78.139 240ø 72ø 106ø 13ø 014ø 13ø 0.66 9.1 2.9 mM 3(S2) 35(L) 01 7.353 78.106 ..... N-S .... mM 3(S13-F2) Cajabam• 41 09 7.621 78.075 099ø 720 273ø 18ø 004ø 02ø 0.40 14 5.4 mM 3(C2-3) 43 11 7.683 77.983 264ø 83ø 097ø 070 007ø 02ø 0.64 24.1 8.1 H* 3(CH) NSC(S)16 147ø 87o(11o) 284ø 02o(12o) 014ø 020(90) m m _ 3(RE3) Quiches 44 15 8.400 77.516 343ø 83ø 116ø 050 207ø 05ø 0.61 11 3.5 H* 4(9C) Cordillera Blanca 47(L) 15 9.083 77.700 100ø 90ø 280ø 000 0100 00ø 0.25 13.3 3.3 H 5(33)-6(D5) 51(L) 54 9.450 77.500 033ø 80ø 274ø 050 183ø 09ø 0.55 14.0 2.3 H 5(34)-6(D7) 53 15 9.633 77.466 294ø 89ø 1000 01ø 190ø 01ø 0.70 12.7 3.8 P 5(35)-6(D8) 55(L) 15 10.000 77.250 131ø 84ø 259ø 040 350ø 050 0.69 12.6 3.8 H 5(36)-6(D9) Sub-Andes:Satipo-San Ramon 57 25 11.166 75.366 289ø 040 021ø 260 190ø 640 0.65 14.2 3.5 Q 5(45)-13(D.F4) Huancayo 66 84 11.933 75.050 075ø 030 168ø 32ø 340ø 57ø 0.84 38.1 5.2 H* 5(41)

Lima Coast 80(L) 05 12.865 76.500 ...... norm-fault --- Q 7(E4) 81 17 13.083 76.333 179ø 80ø 288ø 03ø 019ø 09ø 0.65 13.2 4.0 Q 5(27)-7(E5) Ayacucho 83 15 13.050 74.133 124ø 74ø 293ø 15ø 024ø 030 0.35 15.4 5.6 PQ 5(42a-b-c) 85 44 13.082 74.166 128ø 70ø 294ø 19ø 026ø 040 0.54 12.5 2.4 PQ 5(42) 87 29 13.166 74.200 175ø 71ø 280ø 05ø 012ø 18ø 0.59 11.7 2.7 uM 5(42d) CPA(S) 113ø 87ø 281ø 020 011ø 050 0.58 ..... 5(CPA) Pisco Coast 97(L) 03 13.683 76.150 ..... N-S • -- uM 7(E7) Cuzco 103 12 13.404 71.971 254ø 81ø 103ø 080 012ø 050 0.60 7.5 2.4 H* 8(17-19-20) 104 24 13.422 71.933 242ø 82ø 1000 06ø 0100 050 0.55 13.1 3.3 H* 8(21) 105 24 13.433 71.925 241ø 790 099ø 09ø 008ø 070 0.56 11.5 2.8 H* 8(23) GQK(L)05 13.416 71.950 244 90 109 00 019 00 0.60 6.0 2.8 H* 8(GQK) TQS 60 13.416 71.950 254ø 81ø 103ø 080 012ø 050 0.60 12.4 1.9 H* 8(TQS) 107 28 13.434 72.324 288ø 51ø 076ø 34ø 177ø 16ø 0.17 9.8 2.1 H 0(13-14) 108 26 13.430 72.303 165ø 83ø 277ø 030 007ø 06ø 0.61 10.0 2.3 H 0(15) 109 26 13.433 72.258 246ø 720 078ø 18ø 347ø 030 0.62 13.1 2.9 H 0(16) 115 43 13.450 71.016 237ø 78ø 353ø 050 084ø 11ø 0.63 15.9 1.9 H 9(1) 116 50 13.458 72.017 266ø 85ø 001o 00o 091ø 050 0.53 10.1 1.6 H 9(2) 117 40 13.463 72.015 141ø 85ø 285ø 04ø 015ø 02ø 0.66 10.9 2.0 H 9(3) 118 64 13.461 72.008 266ø 81ø 087ø 070 356ø 06ø 0.58 10.9 1.6 H 9(4) 119 50 13.458 71.988 210ø 83ø 092ø 030 001ø 06ø 0.59 10.5 1.7 H 9(5) 120 45 13.459 71.977 195ø 85ø 093ø 01ø 002ø 04ø 0.70 11.9 2.1 H 9(6) 121 20 13.469 71.939 161ø 81ø 281ø 05ø 012ø 08ø 0.68 9.0 2.3 H 9(7-8-9) 122 28 13.522 71.883 142ø 83ø 282ø 06ø 012ø 050 0.52 12.0 2.7 H 9(10) 123 04 13.511 71.883 111ø 86ø 2740 04ø 0040 01ø 0.83 13.8 7.8 H 9(11) 124 28 13.494 71.848 170ø 89ø 266ø 080 356ø 01ø 0.72 11.4 2.5 H 0(26-28-34) 125 23 13.488 71.836 331ø 89ø 101ø 01ø 191ø 01ø 0.68 10.9 2.8 H 0(29) 126 24 13.510 71.811 145ø 84ø 288ø 050 019ø 040 0.59 9.8 2.3 H 0(31) 11,952 MERCIERET AL.:CHANGES IN TI-IEANDEAN TECTONIC REGIMES

TABLE 1. (continued)

Site ND Latitude Longitude 61 (•2 (•3 R MMA, S.D., D References

AV Azim.dip Azim.dip Azim.dip deg deg

127 26 13.529 71.796 292 ø 80ø 098 ø 09ø 188ø 02ø 0.86 11.3 2.7 H 0(32) 130 11 13.522 71.929 308 ø 77 ø 081ø 09 ø 173ø 09 ø 0.65 11.8 4.7 LQ 9(12a.b,c,F2) 136 14 13.684 71.625 279 ø 770 111ø 13ø 020 ø 03ø 0.10 10.7 3.3 LQ 0(36-37,F2) MP(L) 11 13.450 72.000 111ø 86 ø 274 ø 04 ø 004 ø 01ø 0.83 5.9 1.9 --- 9(MP) RTCO(L)37 13.500 72.000 345 ø 85ø 095 ø 020 185ø 040 0.69 10.8 2.1 --- 8(RTCO) RECO(L)31 230ø 87ø 095ø 02ø(7ø) 005ø 020(6ø) ___ -- -- (P._ECO)

138 21 13.600 71.553 089 ø 84ø 276 ø 06ø 181ø 01ø 0.75 13.6 3.5 PQ 0(38-39) 139 14 13.639 71.558 010 ø 88 ø 279 ø 03 ø 189 ø 02o 0.49 11.1 3.2 PQ 0(40) Ocongate 140 20 13.693 71.303 238 ø 84ø 092 ø 05ø 001 o 030 0.65 9.5 2.3 H 17(1-3) 141 21 13.693 71.283 349 ø 87ø 100ø 01ø 190ø 030 0.52 9.8 2.4 H 17(5-6) 142 14 13.688 71.261 058 ø 87 ø 264 ø 030 174 ø 020 0.50 7.5 2.1 H 17(7) 143 22 13.700 71.271 226 ø 84ø 085 ø 040 354 ø 030 0.64 10.7 2.8 H 17(8-9) 144 28 13.711 71.291 001ø 79ø 260 ø 02ø 170ø 11ø 0.61 10.4 2.3 H 17(10-11) 145 20 13.646 71.241 296 ø 790 083 ø 09 ø 174ø 06ø 0.56 9.7 2.6 H 17(12) TGO(L)10 13.683 71.266 334 ø 80ø 087 ø 04o 177 ø 10ø 0.84 7.0 2.5 H 17(TGO) Vilcanota 146(L) 03 14.950 71.600 .... q 10(10) Ananea 147 18 14.683 69.500 121ø 81ø 265 ø 07 ø 356 ø 050 0.79 7.2 1.6 Q 10(13) 148 06 14.683 69.500 170ø 80ø 267 ø 01ø 356 ø 10o 0.77 5.8 3.4 Q 10(13b) 149 09 14.683 69.500 099 ø 80ø 267ø 10o 357 ø 020 0.76 6.7 2.3 Q 10(13c) Ica-Nazca Coast 150 24 14.700 75.283 288ø 85ø 083ø 05o 173ø 02ø 0.92 7.0 2.5 P 5(30)-7(E9) 152 10 15.366 75.150 191ø 85o 092ø 01o 002ø 05o 0.86 11.8 2.7 Q 10(1)-7(El0) Altiplano 153(L)02 15.101 70.203 .... N-S Q 10(12) Southern Peru Volcanic Arc 156 12 15.633 72.016 161o 82o 279o 04o 009ø 07ø 0.77 8.3 2.8 H 10(8) Arequ•oaCoast 157 03 15.833 74.300 ...... N-S Q 10(2) 173 16 !6.183 72.033 121ø 71ø 282ø 18ø 014ø 06ø 0.71 11.3 4.3 OM 10(5)-7(E17) 175(L)03 16.383 73.233 ...... N-S -- P 10(3-4) 181 08 17.466 70.466 115ø 75ø 266ø 13ø 357ø 07ø 0.83 6.2 2.3 H 10(6-7)-7(E19) Northern Chile 182(L)02 18.383 69.633 .... NNW-SSE -- -- p Bolivia Sub-Andes:Tipuani 183 08 15.450 68.000 ...... N30øE -- --- Q 12(4,F2) Za Paz 185 11 16.450 68.116 019ø 73ø 111ø 01ø 201ø 17ø 0.46 13.7 4.8 P 11(5E) 187 24 16.516 68.083 089ø 84ø 277ø 06ø 186ø 01ø 0.71 11.7 2.7 P 11(5B,F2) 188 24 16.533 68.133 104ø 72ø 284ø 18ø 014ø Off' 0.57 14.8 3.7 P 11(SA) 189 24 16.600 68.133 288ø 83ø 085ø 06ø 175ø 030 0.94 11.2 2.5 P 11(5C) Ayo-Ayo 191 11 17.166 68.000 100o 86ø 258ø 040 348ø 01ø 0.50 15.5 5.5 Q 11(6) Cochabamba 193 11 17.383 66.016 212ø 79o 310ø 01ø 041ø 10ø 0.83 13.6 4.7 P 11(8B) San IsMro 204 nc 18.050 64.416 N60øE ...... PQ 11(9) Tarija 206 104 21.533 64.700 000o 90o 291o 03o 201o 04o 0.94 5.0 2.0 mQ 11(18) 206 7 21.533 64.700 085ø 04o 329ø 82ø 176ø 07ø 0.55 5.0 1.0 mQ 11(18)

Sitenumbers refer to Figures13, 19a and 19b, 23a and 23b; letters in parenthesesindicate L, populationsof major faults several kilometers to 10km long;S, statisticsof thestress directions deduced from populations of minorfaults several tens to hundredsof meterslong (numbers in parentheses followingstress axis parameters give the 95% confidence angle); without letters, minor fault populations. ND, numberof dataused for computation; (•1, (•2,(•3 givethe principal stress directions and R thestress ratios ((•2 -(•3/(•2 - (•1)of theoptimum models; MMA, meanmisfit angle; S.D.: standard deviation;D is the ageof the depositsaffected by thefaults: H, Holocene;Q, Quaternary;LQ, earlyPleistocene; Q-P, Plio- Quaternary; uP, upper Pliocene;P, Pliocene;uM, upperMiocene; M, midlate Miocene; N, Neogene;OM, Oligocene-Miocene;Mz, Mesozoicbedrock. References:0, unpublisheddata; 1, Blanc [1984]; 2, Bellieret al. [1989a];3, Bellieret al.[1989b], 4, Bellier[1989]; 5, Sdbrieret al. [1988b];6, Bonnotet al. [1988];7, Machardet al. [1986];8, Cabrera[1988]; 9, Cabreraet al. [1987];10, Sdbrier et al. [1985];11, Lavenu and Mercier [1991]; 12, Fornariet al.[ 1987]; 13, Dumont [1988]; 14, Cabreraet al. [1991]; 15, Huaman [1985]; 16, Carey-Gailhardis and Mercier [ 1987]; 17, Cabrera[1989]; 18,Lavenu [1986]. Letters and numbers in parenthesesfollowing the referencegive the original number of thesite in thatreference eventually followed by F1, F2, F3 if successivestriation families have been separated at a givensite. *Active fault if associatedwith H andsynsedimentary faults if associatedto anotherletter. MERCIERET AL.: CHANGESIN THE ANDEANTECTONIC REGIMES 11,953

N TQS • N12'-5' 3 101•C•;.

35

38

22

36

4

•6

0 •0 20 30 ,•0 I(•' •'•1 122 '-' RTCO

5 22' 1 • 33

2' 10 ',lS133135 't14i32i34

N 130 N•-2 N• RE C O 9 5 •/' 0-3

•' • '" "o: ß ß ß • N173'-9

Fig. 7. Slip vectordata of the Quaternaryand active normalfaults of the Cuzco region. Numbersoutside the stereonetsrefer to data inside the histograms.Arrows attachedto fault tracescorrespond to the measuredslip vectors(Wulff stereonet,lower hemisphere).Thick segmentson the fault tracesand histogramsshow deviations between measured (S) and preticted(t) slip vectorson each fault plane. The tensor GQK cannotbe computeddue to few data; the best fitting tensorGQK has been obtainedfrom a test program(A), (B) is a test with the RTCO tensor.Solution RECO is a statisticalanalysis of 24 (53 directionscomputed using a momentof inertiaanalysis [Watson, 1960], then the FisherJan statistics gives the 0•95 cone of confidence(stippled area). 11,954 MERCIERETAL.: CHANGES INTHE ANDEAN TECTONIC REGIMES fault planes(Figures 2 and 3) have been observed[Cabrera et compressional directionstrend N260øE+ 10ø and N8øE + 7ø al., 1987] either where these are crossedby streams(13 sites) (stereonetsRCa and RCb, Figure 9). These are in agreement or in trenches (24 sites) excavated across the scarplets. with the compressionaldirections obtainedfrom the reverse Inversionof the slip vector data providedby thesemajor fault fault slip vectorswhich are observedon the major fault planes planes yields a tensional principal stressaxis ((•3) trending (stereonetsMRCa and MRCb, Figure 9) and predatethe normal motionsresulting from the QuaternaryN-S extension. N185øE (site RTCO, Figure 7). The ratio R = 0.70 indicams 2.2.3. Pliocene normal fault motions on the Cuzco fault that the intermediatedeviatoric stressvalue (•J2) is tensional. system.In the Piuray lake basin (Figure 2), extension is This explains why the N-S striking Tamboraymajor fault coeval with lacustrine sedimentationand volcanic activity. (numbers9 and 10 on stereonetRTCO, Figure 7), oriented These sedimentsprobably correspondto the lower part of the nearly parallel to the (•3 direction, has beenreactivamd with a formation which contains vertebrate fossils of early slip vector trendingE-W, roughly in the directionof the Pleistoceneage in its upperpart [Cabrera et al., 1991]. On axis. Kinematicsof the minor faults (stereonet122, Figure 7) one hand, they were depositedin depressionscarved in a measuredat the 37 faulted sites give tensionalprincipal stress Neogeneformation (Figure 10) which was previously folded, directions(•J3) trendingbetween N13øW and N20øE (sims107- probablyduring the uppermostMiocene (=7 Ma). On the other 136, Table 1). Yet the minor faults observed in two trenches hand, these lacustrinesediments are themselvesaffected by alongthe Tamborayfault indicatean E-W trendingextension compressionaldeformations attributed to the early Pleistocene. This suggeststhat the synsedimentaryextension is of Pliocene (63) (sites115 and 116, Table 1), in agreementwith the major age. fault motion (numbers9 and 10, stereonetRTCO, Figure 7). Synsedimentary faults with metric offsets show This shows that motions on minor faults located closed to a kinematics in agreementwith an extensiontrending between major fault may be controlledby the motion of this latter. N60 ø and N90øE (stereonets100 and 101, Figure 11; sites98 Therefore,the regionalstress pattern deduced from minor fault and 100-102, Table 3). On the major fault planes of the kinematics has been obtained using a statistical approach Tambomachayfault system, the oldest family of normal (stereonetRECO, Figure 7). striationsis often observedto predate the family of reverse Kinematicsof the faults affecting the Quaternary of the striations attributed to the early Pleistocenecompression and Cuzco basin: Normal faults offset an early Pleistocene that of normal striationsresulting from the mid-Pleistocene- lacustrine formation (Figures 2 and 3) and crosscutearlier present-day extension. Inversion of these oldest normal compressionalstructures. They have offsetsranging between striationdata gives an extensionalprincipal stress(53 direction several centimeters and 3 m. Kinematics of these faults trending N85øE (sites ZTa and ZTb, Figure 11) in agreement measuredat three different places (sites 130, Figures 2 and 3) with thoseobtained from the minor synsedimentaryfaults. resultfrom a roughlyN-S extension(smreonet 130, Figure7). In conclusion, the superimposedfamilies of striationson In conclusion, along the Cuzco fault system, minor faults the major and minor fault planes of the Cuzco fault system affecting early Pleistocene or Holocene deposits, or those permit one to establisha successionof regional statesof stress produced during historical seismicity, show kinematics which arguesfor three regional tectonic regimes in the resultingfrom a mean extensionaldirection trending N5øE + 6ø Cuzco region: (1) a Plioceneexmnsional regime having an E- (stereonetRECO, Figure 7). This is in agreementwith the W (=N85 ø) trending extension, (2) an early Pleistocene N185øE trending •J3 direction (stereonetRTCO, Figure 7) compressionalregime showing two families of fault deducedfrom the youngestnormal motions of the major faults. motionsresulting from either an E-W (N85øE + 7ø) or a N-S Such a result is not due to the particular E-W strike of this (N5øE + 10ø) compression,and (3) a mid-Pleistocene-present- fault system because the same extensional directions are day extensionalregime having a N-S (N5øE + 6ø) trending deducedfrom the NW-SE strikingTambomachay, Qoricocha, extension.These changesin the tectonic regimes have been and Pachatusanfault segments. observedin the whole High Andes. 2.2.2. Lower Pleistocenereverse fault motionson the Cuzco Now, we examine the stress pattern of mid-Pleistocene- fault system.Compressional deformations have affected the present-day age in the three major regions which are fluviolacustrine formation of the Cuzco basin whose middle distinguishedin the centralAndes: (1) the High Andes,(2) the andupper partscontain vertebrate fossils of early Pleistocene sub-Andeanlowlands, and (3) the Pacific lowlands(Figure 12). age [Gregory, 1916; Cabrera et al., 1991]. These compressionaldeformations predate the normalfaults analyzed above. The earlier formed before the depositionof the upper 3. STRESSPATTERN OF MID-PLEISTOCENE part of this early Pleistocene formation (Figure 8a). The TOPRESENT-DAY AGE ( FIGURE13) latter postdate this formationand predamthe depositionof lacustrineterraces of middle Pleistoceneage (T4, sectionBB', 3.1. ExtensionalTectonics in the High Andes Figure 2). Synsedimentarystructures are severalmeter-sized folds, reverse faults (Figure 8a; sites 129 and 135, Figure 2) 3.1.1. Normal surfacefaulting in the High Andes. In and flexures (site 131, Figure 2). Postsedimentary structures northernPeru, the more recent fault motionswhich affect the are metric to kilometric sized flexures (sectionBB', Figure late Cenozoic sediments of the Namora (sites 7-23), San 2), folds, and reverse faults (Figure 8b; site 128, Figure 2). Marcos (sites29-35), and Cajabamba(sites 41-43, Figure They result from the reactivationof E-W to NW-SE, possibly 14) basinsresult from a NNE-SSW extension.Some faults N-S, striking major faults. affect Pleistocenefluvial terraces(site 12, Figure 14) and the Kinematicsof the synsedimentaryreverse faults [Cabrera et 1.5-kin-longChaquilbamba fault (site43, Figure14) hasbeen al., 1991] are complexand result from eitheran E-W (site 129, reactivatedduring the April 1937 earthquake. Its motion Figure9) or a N-S (stereonet131, Figure 9; sims131 and 135, observedin excavatedtrenches is in agreement with a nearly Table 2) trending compression.Postsedimentary reverse faults N-S extension. More to the south, the Quiches fault (site 44) also showtwo families of striationsresulting from an E-W and reactivatedduring the 1946 Ancash earthquake[Silgado, a N-S trending compression, respectively(Table 2). Thus 1951], strikesNNW-SSE to WNW-ESE and showsa 1- to 1.5- thereare no argumentsto distinguishtwo successiveE-W and m-high scarpletaffecting glacial deposits[Sdbrier et al., N-S trending compressions. Corresponding statistical 1988a]. Its motion observedon the major fault plane at the MERCIERET AL.: CHANGES IN THEANDEAN TECTONIC I•,GIM• 11,955

SW NE

m

o-o. o •,o.•,o.•.oO.•)Oo. o. o'o 'o o o. •. 'o

W

m Huatacay River

20

10

o 1'o 2•) m

Fig.8. (a) Synsedimentaryreverse fault (site 129) affecting the early Pleistocene San Sebastian Formation (1) andcovered bya Recentsoil (2) cover,drawn from field view. (b) Crosssection (site 128) showing the bedrock (4) overthrustingthe early PleistoceneSan SebastienFormation (3); 1, late Pleistocenetorrential fan' 2,middlePleistocene torrential fan.

surfaceor in trenchesalso agrees with a NNE-SSWtrending has a continuationtoward the NW into the normalfaults of the extension[Bellier et al., 1991]. Ayacuchobasin (sites 83-87) whose motions result from an In central Peru, the major normal fault zone of the extensiontrending N19øE + 7ø. To theSE, it is relayedby the CordilleraBlanca (Figure 15) is more than 200 km long Vilcanotafault system(Figure 1), which showsevidence of [Wilsonet al., 1967;Dalrnayrac, 1974; Yonekuraet al., 1979; recentfaulting (site 146) and of historicalseismicity [Silgado, Bonnot,1984; Schwartz, 1988]. The fault scarp. whichcuts 1978].Through the Mataro fault (site 153), this fault system throughthe batholithof the CordilleraBlanca, dips 45øW and extendsinto the BolivianAltiplano. There, the majorfaults is about1000 m high (Figure16). At the foot of the scarp, arelocated at thefoot of theEastern Cordillera (Figure 17). scarpletsoffset old (--50,000years B.P.) and recent (--20,000 Theyaffect glacial and fluvioglacial deposits [Sdbrier et aL, yearsB.P.) moraines andlate glacial deposits (=10,000 years 1982;Lavenu et al., 1984]and showQuaternary cumulative B.P.) with vertical offsetsof 40-30 m, --15 m and 10-8 m, verticaloffsets up to 300m. Theirkinematics are in agreement respectively.The more recentnormal motions (Figure 15) witha N-S to NNE-SSWextension (sites 185-189). showfight- and left-lateralcomponents on the major fault In the Eastern Cordillera of southernPeru, the Ccatca planesstriking N15øE and N150øE,respectively, indicating a (sites138 and 139) and Ocongate (sites 140-145) major fault roughlyN-S lengthening. These major faults (sites47, 51, zonesstrike NW-SE and E-W, respectively(Figure 1). In the and55) andthe minor faults affectingthe Plio-QuaternaryOcongate basin, faultsaffect Quaternarymoraines and sedimentsof the Callejonde Huaylasgraben (site 53) give fluvioglacialdeposits and show Pleistocene scarps up to 15 m extensional directionstrending N00 ø 2 10ø and N10øE, high. Trenchesexcavated across the Holocene scarplets respectively. [Cabrera,1989] show striations on themajor fault planes in The Cuzco fault system(sites 103-136, section 2.2.1) agreementwith a N-S extension(TGO, Table1). In theAnanea 11,956 MERCIERETAL.: CHANGES INTItE ANDEAN TECTONIC REGIMES

129 131

n46 N3••••I16_

37 48 • 5N83' 12 3•

•3 38 34

OS IG •0

08 13 18 OS,O• I1I0 04Ifi 2•IS •140 l I(•,S )1 O I O Po

N2o • 134b N12'- 6• 1/TM ,.. 3 i-i ?2

n

2;7 28•8

1• 25 26 112 20 24 IO• 1721 lOG16 18 lOS 11] 1116 4 [04

I(g, S )l I(g,S)l 0 I 0 PO 0 10 20

N RCb

N260-

N MRCb

N268-6

' " =' " '"I(•.s•

Fig. 9. Slip vectordata from earlyPleistocene reverse faults of the Cuzcoregion. Minor faultsaffecting the early PleistoceneSan SebastianFormation: sites 129 and 131, synsedimentaryfaults; sites 134a and 134b, postsedimentary faults;Rcb and Rca,statistical analyses of the compressionaldirections deduced from the nilnorfaults; Mcb andMca: slip vectordata from the majorfault planesrelated to the lower Pleistocenecompression. MRCa is a testwith the Rcatensor, MRCbis notstrongly constrained due to similarorientations of thefault. Same symbols as on Figure7. MERCIERET AL.: CHANG• IN THEANDEAN TECTONIC REGIMES 11,957

TABLE 2. Parametersof theStress Deviators Comput• FromReverse Fault Kinematics of UpperPliocene-Early Pleistocene Age

Site ND Latitude Longitude O1 0'2 O3 MMA, S.D. D 'lB References

% øw Azin•dip Azin•dip Azin•dip d• d•g

Northwestern Coast 3 02 4ø56'0 81ø03' EW ...... LQ (-/I.,Q) 0(Colan) 6 03 6ø00'0 80o57' NS ...... N (-/N) 0(!lescas) 6 04 6ø00'0 80ø57' EW ...... N (-/N) O(mescas)

7(L) 04 7ø10'40 78ø18'50 NS ...... Mz (-/-) 2(N9-F2) 13 05 7ø11'30 78ø21'30 013ø 32ø 146ø 47ø 266ø 25ø 0.55 9.0 4.6 tim (mQD) 2(N•) 14(L• 7ø11'30 78O21'30 EW ...... Mz (-/-) 2(NIl-F2) 15 13 7ø11'30 78O2135 026ø 09ø 292ø 21ø 139ø 67ø 0.80 15.4 6.2 uM (mQt'7) 2(N25C) 15 04 7ø11'30 78O21'35 291ø 15ø 201ø 02ø 110o 74ø 0.71 5.0 2.5 uM (mQt'7) 2(N25C) 20 04 7ø12'0 78ø21'20 NS ...... uM (mQ/7) 2(N6-F2) San Marcos 26(L)047ø19•)8 78ø05'07 NS ...... Mz (-/raM) 3(S5-F2) 28 02 7O20'15 78ø05'15 EW ...... mM (-/raM) 3(S6-F1) 28 04 7O20'15 78ø05'15 NS ...... ram (-/mlVl) 3(S6-F2) 29 10 7O20'15 78ø06'15 112ø 04ø 022ø 09ø 229ø 80ø 0.83 13.5 5.6 mM (-/raM) 3(S9) 29 11 7O20'15 78ø06'15 193ø 09ø 286ø 19ø 077ø 69ø 0.41 15.5 5.3 mM (-/raM) 3(S9) 30 15 7O20'15 78o06'00 017ø 07ø 114ø 45ø 280ø 44ø 0.90 11.7 3.9 mM (-/raM) 3(S4) 31 18 7020'23 78006'37 303ø 09ø 036ø 15ø 182ø 73ø 0.89 15.0 4.2 mM (-/raM) 3(S3) 32 09 7020,32 78o05,45 281ø 23o 191ø 03o 101ø 67ø 0.78 12.2 4.8 mM (-/raM) 3(Sll) 32 31 7020'32 78005'45 351ø 11ø 247ø 52ø 089ø 35ø 0.80 15.3 3.3 mM (-/raM) 3(Sll) 33 11 7020,45 78008,30 271ø 02o 180ø 39ø 003ø 51ø 0.88 18.6 6.3 mM (-/raM) 3(S2) 33 15 7o20,45 78o08,30 178ø 01o 268ø 070 077ø 82ø 0.67 14.7 4.3 mM (-/raM) 3(S2) 34 17 7O20'52 78ø05'15 358ø 170 191ø 72o 089ø 04o 0.95 14.1 4.0 mM -/raM) 3(S12) 35 10 7ø21'15 78o06'30 356ø 18ø 219ø 66ø 091ø 16ø 0.16 12.5 4.9 mM (-/raM) 3(S13) 36 10 7ø21'15 78006'00 254ø 25ø 126ø 52ø 358ø 26ø 0.85 20.0 7.2 mM (-/raM) 3(S17)

38a(L)08 7o33'30 78ø01'45 230ø 01ø 138ø 59ø 320ø 31ø 0.87 11.3 4.6 Mz (-/-) 0(C12) 38b 24 7o34'45 78o03'52 233ø 01ø 324ø 50ø 142ø 40ø 0.53 11.0 2.8 mM (-/raM) 3(C10) 38c 07 7o34'55 78o03'50 220ø 29o 021ø 59ø 125ø 08o 0.63 8.6 3.9 Mz (-/-) 0(Cll) 40(L) 08 7o37'00 78ø01'53 301ø 08ø 033ø 15ø 183ø 73ø 0.48 14.4 6.0 Mz (-/-) 3((28) NSCI(S) 7O20'0 78ø10' 285ø 10o 021ø (X)ø 047ø 86ø 0.78 .... 3(RC1) NSC2(S) 7O20' 78010' 010ø 10ø .... 0.64 .... 3(RC2)

44(L)14 8O24' 77o31' 068o 030 338ø 05o 191ø 84ø 0.43 5.5 2.3 Mz (H/-) 0(QU-F3) Cordillera Bianca 45 57 8ø59' 77o47' 069ø 05ø 159ø O1ø 266ø 85ø 0.18 21.2 3.4 P (mQ/5) 6(SCauz) 45 16 8059' 77047' 0020 050 093ø 04o 226ø 84ø 0.18 26.6 8.9 P (mQ/5) 6(SCmz) 46 53 9002' 77046' 068ø 09ø 337ø 06ø 213ø 78ø 0.27 16.3 3.1 P (mQ/5) 6(Paron) 46 32 9o02' 77o46' 343ø 12ø 249ø 16ø 108ø 70ø 0.55 20.2 4.3 P (mQ/5) 6(Paron) 48 33 9ø08' 77o43' 089ø 01ø 179ø 02ø 333ø 88ø 0.22 22.0 4.8 P (mQ/5) 6(Llang) 48 18 9008' 77o43' 143ø O1o 233ø 05o 035ø 85ø 0.27 15.0 4.4 P (rnQ/5) 6(Llang) 49 24 9ø14' 77o38' 252ø 01ø 162ø 10O 349ø 80ø 0.44 14.7 4.3 P (mQ/5) 6(Shilla) 49 10 9ø14' 77038' 162ø 040 254ø 25ø 064ø 65ø 0.49 18.0 6.6 P (mQ/5) 6(Shilla) 53 20 9 38.0 77 28 265 04 175 08 023 81 0.73 24.0 6.9 P (mQ/5) 6(Llocl) 53 43 9O38' 77028' 182ø 07ø 092ø 01ø 352ø 82ø 0.34 28.9 5.6 P (mQ/5) 6(Llocl) 54 14 9043' 77O20' 174ø 12ø 076ø 36ø 280ø 52ø 0.97 12.9 4.2 P (mQ/5) 6(QuerA) 54 15 9o43' 77o20' 310o 01ø 040ø 02ø 210o 88ø 0.85 15.0 4.6 P (rnQ/5) 6(•) Sub-Andes:San;go-SanRamo 57 13 11ø10' 75o22' NS ...... N (Q/N) 13(C-F3) 57 25 11 10' 75ø22' 289ø 040 021o 260 190ø 64ø 0.65 17.4 4.9 Q (-/Q) 5(45)-13(D-F4) 58 19 1lø20 ' 74o31' 282ø 02o 192ø 08o 023ø 81ø 0.76 21.3 6.6 NQ (-/-) 5(47al) 58 15 1lo20 74o31' 357ø 12ø 089ø 10o 220ø 74ø 0.71 17.7 6.4 NQ (-/-) 5(47a2) 59 17 1lø21 ' 74o31' 052ø 09ø 320ø 12ø 176ø 74ø 0.77 12.4 3.4 NQ (-/-) 5(47bl) 59 20 1lø21' 74o31' 002o 04o 272ø 020 159ø 85ø 0.88 13.5 3.3 NQ (-/-) 5(4762) Huancayo 60 06 11o49' 75o30' 095ø 01ø 186ø 10o 002ø 80ø 0.27 11.7 5.5 PQ (mQ/5.6) I(A) 60 10 11o49' 75o30' 016ø 01ø 106ø 41ø 285ø 49ø 0.99 12.0 4.9 PQ (mQ/5.6) I(A) 61 13 11050' 75o30' 253ø 04ø 344ø 20ø 154ø 69ø 0.40 17.7 5.9 uMP (mQ/-) I(B) 61 16 11o50' 75o30' 034ø 01ø 304ø 03ø 127ø 87ø 0.53 24.4 8.2 uMP (mQ/-) I(B) 62 09 1lø51.6' 75O24.6' 058ø 10o 326ø 11ø 1870 75ø 0.47 12.8 4.7 uMP (mQ/-) l(C) 62 08 11o51.6' 75o24.6' 015ø 16ø 111ø 20ø 250ø 64ø 0.76 11.9 4.7 uMP (mQ/-) l(C) 63 25 1lø53.7' 75o24' 259ø 09ø 359ø 48ø 162ø 41ø 0.98 16.6 4.0 PQ (mQ/5.6) I(D,F2) 63 17 11o53.?' 75O24' 178ø 14ø 083ø 20ø 300ø 65ø 0.76 16.8 4.9 PQ (mQ/5.6) I(D, F3) 64 09 11ø54' 75024' 076ø 09ø 342ø 24ø 185ø 64ø 0.96 7.8 2.9 PQ (mQ/5.6) I(E) 64 14 1lø54 ' 75O24' 184ø 10o 276ø 13ø 0570 14ø 0.49 16.1 5.7 PQ (mQ/5.6) I(E) 65 20 1lø55.7' 75o22.5' 297ø 02ø 203ø 62ø 029ø 27ø 0.88 10.5 3.0 PQ (mQ/5.6) I(F) 67 08 12ø 75ø11.4' 206ø 15ø 098ø 50ø 308ø 37ø 0.98 20.0 8.0 uMP (mQ/-) I(G) 68 18 12o01.4' 75O22.8' 064o 040 344ø 01ø 239ø 86ø 0.90 22.5 6.2 PQ (mQ/5.6) I(H) 11,958 MERCIER ET AL.: CHANGES IN THE ANDEAN TECTONIC REGIMES

TABLE 2. (continued)

Site ND Latitude Longitude ol o2 o3 R MMA, S.D. D •B Re&a'ences

69 11 12ø02.3' 75ø22.6' 089ø 03ø 355ø 46ø 182ø 44ø 0.93 12.3 4.5 uMP (mQ/-) l(I) 69 22 12002.3' 75022.6' 258ø 07ø 357ø 52ø 163ø 37ø 0.90 14.1 3.8 uMP (mQ/-) l(J) 69 06 12002.3' 75022.6' 208ø 25ø 101ø 32ø 323ø 47ø 0.21 9.2 4.0 uMP (mQ/-) l(J) 70 18 12002.3' 75022.8' 245ø 08ø 153ø 15ø 003ø 72ø 0.91 16.4 4.9 uMP (mQ/-) 1CK) 70 11 12002.3' 75022.8' 211ø 02ø 301ø 020 079ø 88ø 0.99 11.4 4.5 uMP (mQ/-) 1CK) 71 18 12003' 75022.5' 009ø 14ø 101o 070 218ø 75ø 0.23 18.9 5.2 PQ (mQ/5.6) I(M) 72 26 12ø03.1' 75021' 245ø 05ø 155ø 07ø 011ø 81ø 0.75 22.7 6.3 PQ (mQ/5.6) I(P) 73 13 12003.2' 75022' 057ø 12ø 321ø 26ø 170ø 60ø 0.77 13.5 4.3 PQ (mQ/5.6) 1(o) 74 13 12005, 75019, 036ø 020 126ø 050 283ø 85ø 0.25 12.7 4.2 PQ (mQ/5.6) I(N) 75 08 12007' 75028' 194ø 16ø 296ø 37ø 084ø 49ø 0.80 13.1 5.2 uMP (mQ/-) I(Q) 76 10 12010' 75008.5' 021ø 28ø 289ø 030 194ø 62ø 0.41 11.0 4.5 uMP (mQ/-) l(U) 77 18 12o10.9, 75014, 061ø 080 152ø 020 256ø 82ø 0.90 14.7 4.1 uMP (mQ/-) I(R) 78 06 12ø12.1' 75ø10.8' 180ø 01ø 270ø 19ø 087ø 71ø 0.45 14.2 4.2 PQ (mQ/5.6) l(S) 79 14 12ø12.9' 75ø10.3' 035ø 11ø 129ø 17ø 273ø 69ø 0.55 12.9 4.3 P (mQ/5.6) 1(7) Ayacucho 82 21 13002, 74008, 079ø 030 348ø 14ø 180ø 76ø 0.96 11.4 3.1 uM (-/uM) 0(42a-b) 82 05 13002' 74008' NS ...... uM (-hdVl) 0(42a-b) 86 35 13o05, 74009, 060o 020 315ø 81ø 150ø 09ø 0.89 14.4 3.1 Q (-/Q) 0(42c) 87 63 13007' 74ø12' 263ø 04ø 355ø 21ø 163ø 69ø 0.97 16.3 2.9 uM (-/6) 0(42xi) 87 17 13ø07' 74ø12' 281ø 030 130ø 86ø 011ø 020 0.93 12.4 3.5 uM (-/6) 0(42dR) Sub-Andes:Pillcopata-Salvacion-Quince Mil 88 22 12047.8, 71022, 088ø 040 357ø 11ø 196ø 78ø 0.30 20.5 6.4 NQ (Q/NQ) 0(RYungu) 89 17 12ø48' 71021.5' 250ø 09ø 160ø 020 057ø 81ø 0.37 10.3 2.7 NQ (Q/NQ) O(QYungu-F1) 89 08 12048' 71021.5' 019ø 040 110O 11ø 269ø 79ø 0.89 15.6 6.2 NQ (Q/NQ) O(QYungu-F2) 90 21 12ø50' 71020' 191ø 01ø 282ø 050 095ø 85ø 0.63 11.4 3.0 NQ (Q/NQ) 10(15-Calv) 91 21 12055, 71o23, 081ø 050 348ø 26ø 182ø 64ø 0.64 18.5 5.5 NQ (Q/NQ) 10(16-Ubald) 91 10 12ø55' 71023' 187ø (12)o 2770 040 095ø 86ø 0.81 18.8 8.8 NQ (Q/NQ) 10(16-Ubald) 92 26 13o05' 70o23' 016ø 16ø 283ø 11ø 160ø 70ø 0.54 21.2 5.9 NQ (Q/NQ) 1IX18a) 93 13 13o08, 70o23, 003ø 08o 094ø 05o 215ø 80ø 0.57 11.2 3.5 NQ (Q/NQ) 10(18b) 94 13 13Oll, 70o42, 016ø 080 284ø 10O 145ø 77ø 0.83 21.9 8.5 NQ (Q/NQ) 10(17b) 95 16 13012' 70042' 195ø 01ø 105ø 32ø 287ø 58ø 0.63 19.1 6.9 NQ (Q/NQ) 10(17a) Pisco Coast 96 22 13020' 76015' 016ø 020 107ø 06ø 267ø 83ø 0.16 19.7 7.3 Q (-/Q) 7(E6b) 96 11 13o20, 76015, 259ø 04ø 168ø 17ø 001o 720 0.66 13.5 5.1 Q (-/Q) 7(F_,6a) 97(L)10 13o41, 76o09, 258ø 02ø 167ø 25o 353ø 65ø 0.65 11.0 4.2 PQ (uQ/PQ) 7(E7) Cuzco 99 10 13022.9, 72007.2, 007ø 050 277ø 06ø 137ø 82ø 0.34 8.5 3.1 LQ (mQ/LQ) 14(41-F1) 99 13 13022.9' 72007.2' 251ø 01ø 161ø 01ø 042ø 89ø 0.85 13.1 4.2 LQ (mQ/LQ) 14(41-F2) 106 13 13ø26.2' 72021.4' 181ø 080 272ø 06ø 036ø 80ø 0.86 11.5 3.5 LQ (mQ/LQ) 14(46) 110 13 13o27.6, 72008.8, 171ø (12)o 262ø 080 081ø 82ø 0.90 11.2 3.7 LQ (mQ/LQ) 14(47-F1) 110 38 13ø27.6' 72008.8' 269ø 040 3600 080 155ø 81ø 0.90 9.9 1.8 LQ (mQ/LQ) 14(47-F2) 111 20 13o28.4, 72005.4, 018ø 050 108ø 040 237ø 84ø 0.64 11.7 2.9 LQ (mQ/LQ) 14(48-F1) 111 46 13o28.4, 72o05.4, 091o 030 001o 080 200ø 81ø 0.97 11.4 2.1 LQ (mQ/LQ) 14(48-49-F2) 112 32 13o28.7, 72004.4, (Xy9o 020 099ø 050 258ø 84ø 0.98 10.8 2.3 LQ (mQ/LQ) 14(49) 113 35 13ø29.0' 72ø03.1' 193ø 00ø 283ø 09ø 101ø 81ø 0.96 12.9 2.5 LQ (mQ/LQ) 14(50-51-F1) 113 48 13o29.0, 72o03.1, 245ø 020 155ø 050 356ø 85ø 0.94 11.1 1.9 LQ (mQ/LQ) 14(50-F2) 114 44 13o29.1, 72o03.1, 262ø 030 172ø O1ø 056ø 87ø 0.92 11.3 2.0 LQ (mQ/LQ) 14(51-F2) 128 17 13o31.6' 71ø58.8' 205ø 01ø 115ø 09ø 303ø 81ø 0.88 10.0 2.8 LQ (mQ/LQ) 14(52-53) 128 24 13o31.6' 71o58.8' 279ø 11ø 187ø 11ø 053ø 74ø 0.65 9.4 2.1 LQ (mQ/LQ) 14(52-F1) 129 12 13o32.4, 71059.0, 083ø 13ø 353ø 01o 258ø 770 0.59 10.8 3.7 LQ* (*/*) 14(54) 130 25 13031.4' 71ø55.6' 191ø 09ø 101ø (Dø 009ø 81ø 0.75 10.0 2.2 LQ (mQ/LQ) 14(12-55-F1) 131 54 13o31.6' 71ø55.4' 016ø 030 106ø 020 234ø 87ø 0.88 10.5 1.7 LQ* (*/*) 14(56-F1) 131 26 13"31.6' 71o55.4' 230O 02o 139ø 16ø 327ø 74ø 0.85 13.1 3.0 LQ (mQ/LQ) 14(56-F2) 132 33 13o31.5' 71ø55.2' 012ø 01o 282ø 02o 139ø 88ø 0.76 10.9 2.2 LQ (mQ/LQ) 14(57) 133 31 13ø31.5' 71ø54.5' 321ø 09ø 230o 02o 125ø 81ø 0.66 13.5 2.8 LQ (mQ/LQ) 14(58-F1) 133 16 13ø31.5' 71o54.5' 262ø (12)ø 352ø 11ø 172ø 79ø 0.80 10.0 2.8 LQ (mQ/LQ) 14(58-F2) 134 16 13o31.4, 71o54.4, 012ø 06ø 103ø 09ø 249ø 79o 0.42 7.2 2.2 LQ (mQ/LQ) 14(59-F1) 134 31 13o31.4' 71o54.4' 095ø 06ø 186ø 08o 329ø 80ø 0.95 12.9 2.7 LQ (mQ/LQ) 14(59-F2) 135 25 13ø31.4' 71o55.6' NS ...... LQ (mQ/LQ) 14(55) 136 13ø41.1' 71o37.6' EW ...... LQ (mQ/LQ) 14(36-37-F1) 137 14 13o36.0' 71o34.1' 069ø (12)ø 159ø 16ø 339ø 74ø 0.65 14.6 4.4 LQ (mQ/LQ) 14(60) RCI(S) 260O 05...... LQ (mQ/LQ) 14(RCb) RC2(S) - 008ø 01ø -- ...... LQ (mQ/LQ) 14(RCa) Vilcanota 146(L)03 14ø57' 71o36' NS ...... Mz (-/-) 10(11) Ica-Nagca Coast 150 24 14o42' 75o17' 274ø 15ø 005 03o 106ø 75ø 0.19 13.2 3.6 P (Q/N) 7(E9a-F1) 150 18 14o42, 75o17, 341ø 07o 251ø 02o 144ø 83ø 0.53 8.5 2.3 P (Q/N) 7(F_.9b-F1) 152 27 15022' 74058' 075ø • • 165ø • • uM (-/uM) 7(Ell• MERCIER ET AL.: CHANGESIN THE ANDEAN TECTONIC REGIMF3 11,959

TABLE 2. (continued)

Site ND Latitude Longitude c•l 02 c•3 R MMA, S.D. D 215 References

•s øw Azim.dip Azim.dip Azim.dip deg deg

Altiplano 154 13 15041.38' 70008.52' 087ø 13ø 356ø 04ø 248ø 77ø 0.42 16.7 6.5 P (uQ/5.7) 0(ATUN-F1) 154 12 15041.38' 70008.52' 040ø 03ø 150ø 81ø 310ø 08ø 0.52 20.4 7.8 P (uQ/5.7) 0(ATUN-F2) A requipaCoast 174 12 16021' 73016' 091ø 040 181ø 030 307ø 85ø 0.53 12.5 4.3 P (-/P 15(Cal)-7(E14) 175 03 16023' 73014' 015ø 01ø 105ø 06ø 275ø 84ø 0.23 -- P (-/P) 15(LaPlanchada) Northern Chile 182(L)02 18ø23' 69038' NS ...... P (-/P) 0(Copaqui-F1) Bolivia Sub-Andes:Tipuani 183 23 15027' 68ø 050ø - ...... uM (PQ/9) 12(4-F1) Titicaca: Isla De La Luna 184 01 16002' 69004' EW ...... P (-/P) 18(13)fold LaPaz 186 01 16030.7' 68004' EW .... P (Q/2.8) 11(5D)fold 188 10 16032' 68008' 094ø 15ø 189ø 19ø 329ø 65ø 0.73 13.1 4.7 P (Q/2.8) 11(5A1) 188 08 16032' 68008' NS .... P (Q/2.8) 11(5A2) 189 10 16036' 68008' 078ø 07ø 169ø 030 280 ø 82ø 0.43 11.5 4.2 P (Q/2.8) 11(5C) Nazao• 190 01 16059' 68043.5' EW P (-/P) 18(14)fold Ayo Ayo 191 10 17ø07' 680 265 ø 01ø 172ø 70ø 356 ø 20ø 0.83 11.5 4.6 P (Q/2.6) 11(6) Topohoco 192 01 17ø11' 68023' P (-/P) 18(15)fold Cochat•rnba 193 08 17ø23' 66001' P (Q/P) 1l(8B1) 193 01 17023' 66001' 065 ø ...... P (Q/P) 1l(8B)fold 194 23 17026' 65058' 081ø 01ø 351ø 09ø 176ø 81ø 0.87 11.5 3.0 P (Q/P) 11(8A1) 194 06 17026' 65058' NS ...... P (Q/P) 11(8A1) Mattamasa 197 08 17034' 69019' PQ (Q/2.2-3) 11(17) Curahuara 199 13 17049' 68022' 217ø 06ø 309ø 06ø 086ø 81ø 0.17 8.4 2.6 P (Q/5.5) 11(1N) 200 06 17051' 68019' 052ø ...... P (Q/5.5) 1l(1H-M)fold

201 05 17051.5' 68035' EW P (43.3) 11(7A) Caracollo 202 01 17040' 67015' EW PQ (-/PQ) 18(16)fold Culluri 203 02 18ø18'30 67o35 ' EW P (Q/P) 11(11) San Isidro 204 01 18003 ' 64025 ' 060 ø PQ (-/PQ) 18(9)fold Sucre 205 09 19003, 65014, 270ø 02ø 000o 020 138ø 87ø 0.80 15.0 6.8 PQ (-/PQ) 11(10B) 205 08 19o03' 65014' NNE-SSW ...... PQ (-/PQ) 11(10C) 205 01 19003' 65014' EW ...... PQ (-/PQ) 11(10A)fold rar0• 206 104 21032' 64ø42' 000o 90ø 291ø 030 201ø 040 0.94 5.0 2.0 Q (uQ/mQ) 11(18) 206 07 21032' 64ø42' 085ø 040 329ø 82ø 176ø 070 0.55 5.0 1.0 Q (uQ/mQ) 11(18)

Samesymbols as on Table 1 andin addition,TB givesthe time brackets of thecorresponding deformation at eachsite. On theleft- handside of the slashare the paleontological (same symbols as D) or radiometric(in Ma) dataof theformations not affectedby the analyzeddeformation which lie unconformablyonthe top of thedeformed unit. On the right-hand side of theslash are the chronological dataof the deformedformations. Asterisks indicate synsedimentary deformation and hyphens indicate sites lacking more precise chronologicalconstraints. When only one direction is given,it is onlyindicative, data being insufficient to computea solution. basin (sites 147-149), normal faults offset Plio-Pleistocene showingnormal fault motions.Faults in the High Andesdo not depositsand moraines of the penultimate glacial epoch result from simple, superficiallandsliding. The Cordillera (older than 100,000 years B.P.); they indicate a roughly N-S Bianca and the Cuzco-Vilcanota fault systems, each being extension. In the volcanic region of the Western Cordillera more than 200 km long, exhibit Pleistocenescarps 1000 and (site 156), two E-W striking major faults, some 10 -15 km 400 m high, respectively. These faults are deep seatedas long, show scarpsup to 30 m high. They offset slope shownby associatedshallow seismicitygenerally no more depositsof the last glacial epoch (roughly 10,000-50,000 than 10-15 km deep. Few focal mechanismsshowing normal yearsB.P.)and present-day soils. Their motionsresult from a fault motionsare known; they are locatedin the Western N9øEtrending extension. Cordillera. One is a compositesolution (CHO, Figure 13) 3.1.2. High Andesfocal mechanismsof earthquakescorresponding to a microseismicactivity recorded froIn a local 11,960 MERCIERET AL.' CHANGES IN TttEANDEAN TECTONIC REGIMES

Normal faults FluviolacustrineHuaypo Unit

w E

......

m •o:•...... o ..... • ...... oøøøøøøøøø oø...... ,,,.

.! ...... CNncherosFm (Neogene)...... o o o lm

Fig. 10. Crosssection drawn from a field view (site98) showingsynsedimentary normal faults in the fiuviolacustrine Huaypounit (2) attributedto thePliocene. 1, Recentsoil; 3, Pliocenelahar flows; 4, Pliocenescoria flows; 5, theMiocene Chincheros formation.

N 89'-2'

09 11 11 10 08 10 06 08 09 o, 03 05 07 I _1• 02 04 O1 12 I(•, s )1

O' 10'

N:,a • ZTa 14 11 ZTb 5 26 14 N83'- 6' N84'- 5' 24 23 24 19 7

2O

3 1 n

17

., 16 15 22 1

20

• 19 10 20

08 12 27

07 11 17 22 27 12 8 2 7 06 09 IE; 16 25 26 10 OS 02 03 01 21 04

Fig. 11. Slip vectordata of the Pliocenenormal faults of the Cuzcoarea. Sites 100 and 101, synsedimentaryfaults; ZTa and ZTb, oldestnormal striations on the majorfault planes.Same symbols as on Figure7. MERCIERE'r AL.: CHANGESIN THE ANDEANTECTONIC REGIMES 11,961

TABLE 3. Parametersof theStress Deviators Computed From Normal Faults of Mioceneand Pliocene Age

Site ND Latitude Longitude ol 02 o3 MMA, S.D. D •IB References

% øw Azim.dip Azim.dip Azim.dip deg deg

Sa•q•gga 8 18 7.186 78.356 051ø 74ø 169ø 080 261ø 14ø 0.87 14.7 4.1 uM* (*/*) 2(N1) 9 16 7.187 78.354 251ø 86ø 354ø 01ø 084ø 04ø 0.82 14.1 4.3 uM (mQ/7) 3(N7) 11 11 7.188 78.356 255ø 80ø 151ø 02ø 061ø 10O 0.58 10.0 3.5 uM (mQ/7) 3(N2) 13 10 7.192 78.358 127ø 82ø 359ø 05ø 209ø 06ø 0.83 06.0 2.0 uM (mQ/7) 3(N24) 16 14 7.193 78.348 357ø 82ø 164ø 08ø 255ø 02ø 0.87 08.9 2.8 uM* (*/*) 3(N4) 19 09 7.196 78.347 062ø 72ø 158ø 02ø 249ø 17ø 0.60 09.4 3.7 uM (mQ/7) 3(N22) NA(S)06 049ø 85ø(10ø) 166ø 02ø(9ø)257 ø 04ø(12ø)0.70 ...... (mQn) 2(1) San Marcos 24 10 7.302 78.111 126ø 87ø 353ø 02ø 263ø 02ø 0.50 12.0 4.8 mM (Q/raM) 3(S16) 25(L)09 7.308 78.092 094ø 83ø 307ø 06ø 217ø 040 0.65 05.6 1.9 Mz (-/-) 3(S15) 26(L)09 7.319 78.085 039ø 67ø 158ø 11ø 252ø 19ø 0.65 08.3 3.1 Mz (-/-) 3(S5-8,F1) 26(L)10 7.319 78.085 219ø 720 312ø 0.7ø 041ø 18ø 0.84 .... Mz (-/-) 3(S5-8,F2) 27 09 7.329 78.146 315ø 85ø 168ø 040 077ø 03ø 0.90 12.2 2.8 mM (Q/raM) 3(S1) 28 07 7.338 78.088 068ø 73ø 337ø 0.2ø 247ø 17ø 0.42 10.5 4.3 mM (Q/mM) 3(S6-7,F1) 28 07 7.338 78.088 007ø 84ø 125ø 02ø 215ø 05ø 0.52 07.1 2.8 mM (Q:mM) 3(S6-7,F2) 29 07 7.338 78.104 045ø 88ø 187ø 01ø 277ø 01ø 0.41 13.3 6.2 mM (Q/mM) 3(S9) 31 09 7.340 78.110 152ø 49ø 020ø 31ø 274ø 25ø 0.93 06.7 2.4 mM (Q/raM) 3(S3) 37 11 7.356 78.111 085ø 87ø 184ø 0.5ø 274ø 03ø 0.80 09.1 3.4 raM* (*/*) 3(S14)

39(L)04 7.594 78.067 ...... E-W ..... mM (Q/raM) 3(C4,F1) 39(L)04 7.594 78.067 ...... NE-SW ...... mM (Q/raM) 3(C4,F2) 40(L)10 7.629 78.029 021ø 58ø 164ø 26ø 262ø 16ø 0.28 10.0 3.7 Mz (-/-) 3(C7-8,F2) 41 07 7.623 78.079 315ø 32ø 164ø 54ø 054ø 14ø 0.38 08.6 2.9 mM (Q/raM) 3(C2-3) 42 08 7.667 78.071 094ø 79ø 348ø 03ø 258ø 10O 0.80 11.2 4.5 mM (Q/mM) 3(05) REI(S) 7.334 78.167 063ø 83ø(10o) 170ø 02ø(10ø)260 ø 08ø(9.5ø)0.68 .... mM (Q/raM) 3(RE1) RE2(S) 7.334 78.167 300ø 81ø 308ø 02ø 042ø 06ø 0.60 .... mM (Q:mM) 3(RE2) Qu/ches 44(L)07 8.400 77.517 ...... WSW-ENE .... Mz (-/-) 3(QU,F2) Corch'llera Bianca 47(L)19 9.083 77.700 199ø 50ø 018ø 40ø 108ø 01ø 0.34 14.5 3.9 uM (-/10) 5(33)-6(D4,F2) 47(L)39 9.083 77.700 205ø 77ø 320ø 06ø 051ø 12ø 0.51 09.0 2.1 uM (-/10) 5(33)-6(D1,F1) 50(L)25 9.450 77.500 188ø 720 326ø 14ø 059ø 12ø 0.56 08.3 2.3 uM (410) 6(D2,F1) 51(L)22 9.467 77.500 134ø 61ø 022ø 11ø 286ø 26ø 0.18 09.8 2.6 uM (-/10) 5(34)-6(D5,F2) 52(L)42 9.483 77.467 304ø 730 155ø 15ø 063ø 08ø 0.81 13.2 2.4 uM (-/10) 6(D3,F1) 53 17 9.633 77.467 093ø 86ø 195ø 01o 286ø 040 0.69 12.3 3.7 P* (*/*) 6(D6) Sub-Andes:Oxapampa-San Ramon 56 22 10.550 75.417 130ø 86ø 340ø 040 250ø 10O 0.75 -- NQ (Q/NQ) 0(Oxapampa) 57 - 11.100 75.367 ..... WNW-ESE ...... NQ (Q•Q) 13(B,F2) Huancayo 60 06 11.817 75.500 097ø 81ø 251ø 080 341ø 040 0.16 -• -- P (mQ/5.6) I(A) 61 12 11.833 75.500 071ø 76ø 271ø 13ø 180ø 05ø 0.50 .... P (mQ/5.6) I(B) 63 10 11.895 75.400 109ø 82ø 254ø 06ø 344ø 06ø 0.66 .... P (mQ/5.6) (D) 70 14 12.038 75.375 273ø 65ø 041ø 16ø 136ø 19ø 0.27 .... P (mQ/5.6) I(L) 74 07 12.083 75.317 037ø 69ø 195ø 19ø 287ø 070 0.17 .... P (mQ/5.6) I(N) Ayacucho 84 08 13.083 74.167 150ø 80ø 317ø 10ø 047ø 02ø 0.38 .... uMP (duMP) 0(Quina) Cuzco 98 20 13.380 72.110 3080 85ø 151ø 05ø 061ø 020 0.21 13.2 3.4 N?*. (*/*) 14(42) 100 13 13.387 72.082 342ø 87ø 171ø 03ø 081ø 01ø 0.25 12.3 4.0 N* (*/*) 14(43) 101 12 13.408 72.053 248ø 88ø 359ø 01ø 089ø 02ø 0.80 10.4 3.2 N* (*/*) 14(44) 102 14 13.438 72.058 165ø 80ø 349ø 10O 259ø 01ø 0.77 10.3 3.0 N* 5*/*) 14(45) ZTa 33 13.417 72.300 262ø 84ø 353ø (Dø 083ø 06ø 0.11 17.7 3.6 Mz (-/-) 14(ZFa) ZTb 27 13.467 72.933 200ø 78ø 353ø 11ø 084ø 05ø 0.20 15.2 3.5 Mz (-/-) 14(ZTb) Ica-Nazca Coast 152 14 15.367 75.150 226ø 89ø 317ø (Do 047ø 01ø 0.85 16.4 5.2 N* (*/*) 7(El0) ArequipaCoast 176 05 16.400 73.217 000o 90ø 136ø (Do 046ø (Do 0.72 10.0 5.1 uP* (*/*) 7(E15) 179 09 16.533 72.867 121ø 66ø 330ø 22ø 236ø 11ø 0.72 05.5 1.9 OM (dOM) 15(Km806) 180 08 17.117 71.750 000o 90ø 045ø (Do 135ø (Do 0.90 13.1 6.1 P?* (*/*) 7(E18) Bolivia Subandes: Tipuani 183 08 15.450 68.000 ...... N30OE ...... PQ (-/PQ) 12(4) /at Paz 187 13 16.517 68.083 294ø 88ø 186ø 05ø 095ø 01ø 0.78 18.8 6.0 P (2.8/5.5) 11(5B,F2) 188 • 16.533 68.133 ..... E-W .... 13.6 4.7 P . (2.8/5.5) 11(5A) 189 -- 16.600 68.133 ...... E-W ..... P (2.8/5.5) 1(5C) Ayo Ayo 191 -- 17.167 68.000 ..... E-W ...... -•- P (Q/2.6) 11(6) 11,962 MERCIERE'r AL.: CHANGES IN TIlEANDEAN TECTONIC REGIMES

TABLE 3, (continued)

Site ND Latitude Longitude el 0'2 o3 R MMA, S.D. D TB References

% øw Azim.dip Azim.dip Azim.dip deg deg

Coc• 193 m 17.383 66.017 m -- E-W ..... P (Q/P) 11(8B2) 193 -11 17.383 66.017 212 ø 79ø 310 ø 01ø 041 ø 10 ø 0.83 -- P (Q/P) 11(8B1) Curahuara 200 nc 17.850 68.317 E-W P (Q/P) 11(1) lirata 201(L)nc 17.858 68.583 E-W P (3.3/5.5) 11(7A)

Same footnotes as Tables 1 and 2. network[Grange et al., 1984a]. It is in agreementwith a N-S faulting. In the appendix, we examine the particular trending extension. Another concernsthe 1946 Ancash microseismicactivity of the Cordillera Blanca. earthquakeresulting from the reactivationof the Quichesfault analyzed above. Two recent focal solutionshave been 3.2. CompressionalTectonics in the Sub-AndeanLowlands computed:one indicates a purenormal motion on theNNW-SSE strikingseismic fault [Suarezet al. , 1983], the otherindicates The sub-Andean hills form a typical fold-and-thrust belt a normal motion involving a left-lateral component[Doser, indicatinga compressionaltectonic regime. 1987]. This latter is in agreement[Bellier et al., 1991] with 3.2.1. Sub-Andean focal mecanisms oJ earthquakes. In the NNE-SSW extensiondeduced from the slip vectormeasured central Peru, the sub-Andeanseismic activity is high. There, on the Quichesfault plane (site 44). Thus althoughthe High 14 focal mechanismsare available (Figure 13): 12 come from Andes focal mechanisms are scarce and often poorly World-Wide Standard SeismographNetwork (WWSSN) data constrained,they do not disagreewith the mid-Pleistocene- corresponding to six distinct events and two have been present-dayextensional tectonic regimeevidenced by surface obtained from a local network. Inversion of these focal

0'2 o'1

0'2 3 2 SURFACE FAULTING

IPACIFICLOWLANDS SUBANDEAN LOWLANDS HIGH A '.ONIAN FORELAND P. Pmdmont Western Cordillera Costat Cord•ller. a Eastern Cord•l lera Trench

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0'1 0'2 LLU 02 101 FOCAL MECHANISMS

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J

Antofagasta ß . o 240 24 c - 0 500km o 82 ø 78 ø 72 ø 66 ø

Fig. 13. Principal stressdirections deduced from the analysisof mid-Pleistoceneand active faults of Peru and Bolivia. Divergent arrows, tensionalhorizontal stressdirections (o3); convergentarrows, compressionalhorizontal stress directions(Ol); solid circlesattached to the arrows,computed directions; open circles, graphically defined directions (sites are listed in Table 1); balloons,focal mechanismsof earthquakes;letters refer to Ab, Abe [1972]; St, Stauder [1975]; Sp, Dewey and Spence [1979]; Pe, Pennington [1981]; Su, Suarez [1982] and Suarezet al. [1983]; C, Chinnand Isacks[1983]; CHO, Grange et al. [1984a]. Compressionalstress directions FA and SA are obtainedfrom the inversionof the teleseismic focal mechanismsand LLU from the inversionof microseismicfocal mechanisms[Grange et al., 1984b](see Table 4); large arrow, directionof the Nazca-SouthAmerican plate convergence[Minster and Jordan, 1978].

mechanismsgives a compressional(•1 axis trending N86øE 3.2.2. Field data in the sub-Andes. In the sub-Andes, (Table 4; stereonet SA, Figure 18). Thus the present-day conglomeraticdeposits are clearly folded,but theseare poorly deformationsclearly result from a roughlyE-W compressionas dated.Two series are distinguished.The older, generallytilted, previously suggested[Stauder, 1975; Suarez et al., 1983]. folded and strongly weathered series is of Neogene age. The This is also supportedby a recent study of the microseismic younger series, made of poorly weathered fan terraces, lay activity of this region [Dorbath et al., 1986]. unconformablyupon the older series; it is attributed to the 11,964 MERCIERET AL: CHANGESIN THEANDEAN TECTONIC REGIM•

both in central [Snoke et al., 1979; Suarez, 1982] and southern [Isacks and Barazangi, 1977] Peru. The deeper earthquakes located within the oceanic slab result from normal faulting [Stauder, 1973, 1975; Isacks and Barazangi, 1977; Malgrange et al., 1981; Chinn and Isacks, 1983]. The shallower earthquakes,located in the vicinity or at the plate interface, result from compressionalfaulting. Inversion of 12 available thrust-typefocal mechanism data (see Figure 13) givesa N68øE trendingcompression (stereonet FA, Figure 18, and Table 4). The NW-SE striking, gently east dipping preferred seismic planes are probably located at the contact between the two plates. But the others, markedly different in dips and orientations, are probably located in the deeperpart of the

0 10km forearccontinental wedge, suggestingthat this is affectedby i 78*30 compression. The compressional direction slightly differs I from the N86øE compressionin the sub-Andes(stereonet SA, Figure 18). But it is not sure that this deviationis significant because the focal mechanisms of the forearc are not well constrained. sc 3.3.2. Surfacefaulting along the Pacific coast. In contrast, numerousnormal faults are observed at the surface along the Pacific coast.In central Peru, the subsidingPacific coastlineis coveredby continentalPleistocene deposits cut by numerous small normal faults. Their orientations often are oblique or orthogonal to the coastline,suggesting a roughly N-S extension [Soulas, 1978; Machard, 1981; Machard et al., 1986]. Some slickensides(sites 81 and 97, Figure 13) cut upper

9 200k• •.

- 8'30' • ' • Huaytapallana• Fig. 14. Structural sketch of the northernPeru basinsof Late Cenozoic age. 1, Pliocene-Quaternary;2, upperMiocene (Namora Formation); 3, 4, lower(?) to late MioceneCondebamba Formation; 4, Cajabamba Formation; 5, Tertiary volcanism; 6, pre-Cenozoicformations; 7, faults; 8, normalfaults activated during the Neogene;9, thrustsof the o \• Maranon fold-and-thrust belt. Arrows, tensional directions of \ Quaternaryage (listedon Table 1). ; Queruran•

Quaternary. In central Peru, compressionaldeformations affectingthe youngerseries have been observedonly in the Ulta • San Ramonpiedmont basin (site 57). Thesedeformations result from two N-S and WNW-ESE trending compressions. The WNW-ESE compressionpostdating the N-S one is in agreementwith the E-W compressionobtained from the focal Honda mechanisms.However, it is possiblethat theseN-S and E-W compressionsare both of lower Pleistoceneage (see section 4.2). In the San Isidro basin in Bolivia (site 204), sedimentary formations attributed to the Pliocene-Quaternary [Lavenu, -9'30' 1984a] are folded as a result of a N60øE trendingshortening. But theseformations are not preciselydated, and consequently, the compressionaldeformations might be possiblyolder than •'Querococha •\ mid-upperPleistocene. In conclusion, focal mechanismsand possiblysome field data demonstrate a present-day, roughly E-W trending compressionin the sub-Andes of centralPeru. That it may be the tectonicregime in all the sub-Andeanzone is suggestedby the focal mechanismsof earthquakes(Figure 13) of southern San Cristobal Bolivia and northwesternArgentina which also indicate an E- nord W compression[Chinn and Isacks,1983; Jordan et al., 1983]. • San Cristobal Unfortunately, there is neither seismic nor field evidence •,..centre concerningthe tectonic regime of the sub-Andesin southern 10kmI •- •,----...••'/ •$ud San Cristol•al Llaca",\'t• Peru. 78ø 7?30' 717ø 3.3. Compressionaland ExtensionalTectonics Fig. 15. Structuralsketch of the CordilleraBianca fault zone activated in the PacificLowlands duringthe Pliocene-Quaternary.Stereonets give the strikesof the major fault segmentswith the slip vectors(arrows). Balloons, single focal 3.3.1. Forearc focal mechanismsof earthquakes.Beneath mechanismsobtained from a local seismicnetwork [Deverch•re, 1988] the forearc,seismic foci define a 30ø east dippingBenloft zone with the slip vectorsarrows on the preferredseismic fault planes. MERCIERET AL.:CHANGES IN THEANDEAN TECTONIC REGIMES 11,965

Fig. 16. Obliqueaerial photograph of theCordillera Blanca fault scarp, west of Huaraz(location on Figure15). The •-1000- m-highscarp cuts through the batholithand forms triangular facets (some 300 m high).At the foot of the scarp,scarplets (arrows)affect glacial depositsyounger than 50,000 yearsB.P.

Miocene-Pliocene marine beds and early Pleistocene zone. The seismic activity of this fault zone has been studied conglomerates.They postdate compressionalstructures of usinga local seismicnetwork [Grange et al., 1984a]. Events early Pleistoceneage and indicate a N19øE trendingextension. whosedepths range between12 and 40 km are probably Opposite to the Nazca ridge and in southernPeru, the locatedin the vicinity of a fault dippingroughly 45 ø $W. upliftingPacific coastline exhibitsstepped Quaternary marine Elevenfocal mechanisms have been computed. In contrastwith terraces; the oldest onesreach altitudesranging between 700 the normalsurface faulting, theyshow reverse, strike-slip, and 250 m. Normal faults affect these marine terraces(sites and normalfault motions[Grange et al., 1984b]. Inversionof 151 and 157) and showdownthrows ranging between some tens thesedata showsthat these fault slips are explainedby of centimeters to several meters. Normal faults have been also horizontalN290øE trending o 1 andN31øE trending 03 axes; observed in Pliocene marine beds (sites 150 and 175), the(•2 axishas a 60ø dip (stereonetLLU, Figure18 andTable postdating synsedimentarynormal faults and subsequent reversefaults. Their kinematicsalso result from a roughly N-S 4). The tensional{x3 axis deviatesby about20 ø from the N14ø extension. Thus the most conspicuousdeformations on the trendingtensional axis computedfrom field data (site 173). coast are extensional. However, small reverse faults have been But it is not sure that this is significantbecause the seismic also reported in deposits attributed to mid-late Quaternary. networkwas large, and thusthe computed focal solutionsare They are scarceand isolated [Machar• et al., 1986; S•brier et not well constrained.Thus alongthis fault zone the stateof al., 1988a], but they indicate that the superficial deformation stresscorresponds to a strike-slipregime at depth (nearly may have been occasionallycompressional. This adding to vertical{x2 axis) and to an extensionalregime at the surface the small magnitude of either the extensional or the (vertical•1 axis). compressionaldeformations argues for a nearly neutral stateof Thus,in general,during the mid-Pleistocene-present-day stress on the Pacific coast with a tendancy to N-S trending extension. period, the average stresspattern of the Central Andes is characterizedby a roughlyE-W compressionin the sub-Andean 3.3.3. Surface faulting in the fault zone separating the lowlandsand at thecontact between the two plates and by a N-$ Pacific lowlandsfrom the High Andes.In southernPeru, a NW- extensionin the High Andesand the Pacific lowlands. SE striking west facing belt of faults is located between the Pacific Piedmont and the High Andes in a region where the elevationrapidly increasesfrom 2000 to 4000 m. This fault 4. STRESSPATTERN OF LOWER PleISTOCENE Ac• belt is constitutedby the Incapuquio, Lluclla, and Pampacolca (FIGURE 19) fault segmentswhich generally show high-angle dips and superimposedreverse, strike-slipand normalmotions [S•brier For the legibility of the data, the roughly E-W and N-$ et al., 1985]. On the Lluclla fault segment(site 173), the last trending compressionaldirections have been reported on motions are normal. The Pampacolca fault affects early Figures 19a and 19b, respectively.But, as discussedbelow, Pleistocenedeposits with a 20-m normal throw. The Chulibaya this doesnot mean that these representtwo different tectonic fault belongingto the Incapuquiofault zone (site 181) showsa regimes. 2.5-m-high scarplet resulting from an historical seismic motion. The final kinematics and the active motions on this 4.1. CompressionalTectonics in the High Andes fault belt result from a roughlyN-S trendingextension, •1 In central Peru, north of the Cuzco basin (sites 99-136, being vertical. section2.2.2), the Ayacuchoand Huancayobasins show clear 3.3.4. Focalmechanisms of earthquakesin theLluclla fault evidencesof compression.The more spectacularrecent folds in fl,966 MERCI•ET AL: CHANOES iNTIlE ANDEAN TECIONIC REGIM•

187

Villa

o 5km

Fig. 17. Structuralsketch of the La Paz (Bolivia) area: 1, Quaternary;2, Pliocene;3, Pre-Plioceneformations; 4, Quaternary flat surfaceof the Altiplano borderingthe La Paz depression;5, tensionaldirections of Quaternaryage; 6, folds of Pliocene age (a, synclinal;b, anticline;c, fold coveredby Quaternarydeposits) (recognized by seismicprospection); 7, faults (a, observed;b, inferred normal faults; c, strike-slipfaults).

the High Andes are observedon the westernborder of the and rhyolitic tuffs datedat •,4 Ma [Mdgard et aL, 1984] and Huancayobasin (Figure 20). They affect a 100-m-thick 2.5 Ma [Kaneoka and Guevara, 1984], respectively.Minor conglomeratic series overlying a formation whose base reverse faults affect these fans. Their kinematics agree contains andesitic lavas dated at 5.6 Ma [Blanc, 1984]. The essentiallywith an E-W trendingcompression (sites 82, 86 majorfold axesstrike N140ø-150øE, although some of themare and 87), althoughsome fault motions(site 82) resultfrom a N- arcuatestriking between N150 ø and N225øE.They arguefor a S compression. Farther north, the dated Pliocene deposits roughlyE-W trendingshortening because they are associated which infdl the Callejon de Huaylasgraben (Figure 17) are with major reversefaults, strikingN150 ø, probablyalready affectedby small folds, scarcereverse faults, and pressure- active during the upper Miocene, which overthrustthe solutionmarks on the pebblesof the conglomerates.These Pliocene-early Pleistocene formations. The minor faults give results(sites 45-53, Figure 19a and sites45-54, Figure affecting these formationsshow two families of striations; 19b) similarto that obtainedin the Huaneayobasin. they are kinematicallyincompatible [Blanc, 1984] andresult In northem.Peru,the last compressionaldeformations are from either roughly E-W (sites 60-77) or N-S (sites 60-79) subsequentto depositsof Mioceneage in the San Mareosand compression.Scarce superimposed striations are in favorof the Cajabambabasins and of upperMiocene age in the Namora latterpostdating the former. The Ayacuchobasin contains thin basin(Figure 14). Theypredate the N-S trendingextension of alluvialfans that are partly time equivalentwith latitic lavas Quaternaryage. Compressional deformations are expressed by MERCIERET AL.: CHANGESIN THE ANDEANTECTONIC REGIMES 11,967

several meter-sized folds and pressure-solution marks on pebblesof the conglomeraticformations. They also yield two mean directions of compressiontrending N285øE (sites 14, 28-36, and 40) and N10øE (sites 7-20 and 28-38). It is admitted that the latter postdatethe former and that both are of lower Pleistoceneage [Bellier et al., 1989b], although an uppermost Miocene age (--7 Ma) cannotbe excluded. In the Bolivian Altiplano, near La Paz, dated Pliocene formations are folded [Martinez, 1980] and unconformably coveredby glacial and fluvioglacial depositsattributed to the upper Pliocene(?)to Pleistocene[Servant, 1977; Lavenu et al., 1984]. Several kilometer-sized folds striking N-S (sites 186, 188, and 189, Figure 17) indicate a roughly E-W shortening. Kinematics of minor reverse faults (sites 188, 189, and 191) which affect these Pliocene formations also result essentially from an E-W trending compression.However, somerare faults (site 188) show other kinematics in agreement with a N-S shortening. On the western border of the Altiplano (sites 199 and 200), a Pliocene tuff whose base is dated at 5.6 Ma [Evernden et al., 1977] is also folded. The several kilometer- sized folds result from a N52ø+20ø trendingshortening. Some of them (site 199) have been subsequentlycut by reversefaults resultingfrom a N-S trending compression. Farther west (site 197), reverselow-angle faults resulting also from a roughlyN- S trending shortening cut ignimbrites dated at 2.2-3 Ma [Evernden et al., 1966] but do not affect the overlying lacustrinedeposits of early Pleistoceneage (R.Hoffstetter and L.Branisa, as cited by Blanco [1980]). In the intramountain basins of the Eastern Cordillera, few data have been obtained. In the Cochabamba basin, dated Pliocene formations [Mancilla, 1979] are folded and unconformablycovered by fluviolacustrinedeposits of recent Quaternaryage [Lavenu and Ballivian, 1979]. Fold axes strike N155ø indicatingan ENE-WSW shortening. Kinematicsof the reverse faults affecting these formationsagree with a N81øE trending compression(site 194), although some rare faults result from a roughly N-S compression(sites 193 and 194). In the Sucre basin, sediments attributed to the Pliocene, but not preciselydated, are folded and faulted.Folds result from a N70 ø trendingshortening. Reverse faults show complex kinematics resultingfrom roughly E-W (site 205) and NNE-SSW (site 205) shortenings.

4.2. CompressionalTectonic in the Sub-AndeanLowlands

In central Peru, compressional deformations have been analyzed in the San Ramon (site 57) and Satipo (sites58 and 59) basins. The Paleozoic formations of the Eastern Cordillera overthrust the older Neogeneconglomeratic series of the San Ramon basin [Dumont, 1988]. Theseare affected by reverse faults resultingfrom a NE-SW trending compression.The younger conglomeraticseries comprisefluvial terraceslying unconformablyupon the older Neogeneseries. They are not affected by the previous deformations.However, pressure- solutionmarks and striationson pebblesof the oldest fluvial terrace (San Ramon conglomeraticfan) indicate that this has been affected by two roughly N-S and E-W trending compressions which are also observedin the older Neogene series [Sdbrier et al., 1988a; Dumont, 1988]. The E-W trending compressionmight be related to the active E-W compressionshown by the seismicactivity. However, if the San Ramon conglomeraticfan is of early Pleistoceneage as supposedby Dumont [1988], then both the E-W and N-S compressionsin the Satipo and San Ramon basins (sites 57, 58, and 59) must be more likely related to the lower Pleistocenecompressional event. This should imply that the older Neogeneseries and the older NE-SW compressionwhich affectsthem predatethe lower Pleistocene. 11,968 MERCIER ET AL.: CHANGESIN THE ANDEAN TECTONIC REGIMES

TABLE 4. Parametersof the StressDeviators Computed From FocalMechanisms of Earthquakesof Peru

Site NS ND c•l c•2 c•3 R MMA, S.D, References

Azim.Dip Azim.Dip Azim.Dip deg deg

S.A. 7 13 086 ø 01ø 356 ø 11ø 177 ø 79 ø 0.71 10.0 3.4 10(SA) F.A. 10 12 068 ø 02ø 158 ø 02 ø 298 ø 87ø 0.82 8.3 2.9 10(FA) LLU 9 9 298 ø 05ø 200" 59 ø 031 ø 31ø 0.46 5.6 1.9 16(LLU2)

SeeFigure 13. NS, numberof events;ND, numberof availablesolutions. Same abreviations as on Table 1.

78 ø 72 ø 66 ø I I

3

,/ 6

icla,

- 8 ø

12 - 12ø

- t6 o 16 c 184

200-20! 202 Santa Cruz ! / // -5000•

2O o - 20-' ß ß

Antofagasta 24o

I 82 ø 78 ø 72 ø 66 ø

Fig. 19a.Principal compressional stress directions deduced from kinematics of reversefaults of earlyPleistocene age. E-W trendingcompressional directions. Same symbols as on Figure 13. (Sitesare listed in Table2.) MERCIERETAL.: CHANGES INTHE ANDEAN TECTONIC REGIMES 11,969

78ø 72ø 66ø

1 •'.• S - 160 16 •

197• 193_1941 .•99 SantaCruzi

A 20 ø 2•- A A

A I

, ,% I , , , , J • Antofagastaß 240 24• - 500kin • ß 82• 78= 72ø 66ø Fig.19b. N-• trendingcompressional directions.

In southernPeru, compressional deformations have been 110øE.Slip motions in variousstructural settings: reverse and also observedin basinswhich are in a structuralsituation strike-slipfaulting, thrusting associated with foldingand similarto that of the Satipoand San Ramon basins. In the discontinuousdeformations of fold limbsare in agreementwith QuinceMil basin(sims 94 and95), reversefaults affecting the a N-S trendingcompression. But this is not well dated; olderseries show kinematics in agreementwith a N15ø trending accordingtoLaubacher etal. [1984],it affectsearly Quaternary compression.In the Pillcopata basin (site 91), striationson deposits.Quaternary alluvial terraces unconformably covering reversefault planes affecting the older series result from a N10ø the foldedseries do not show any evidenceof deformation. trendingcompression, but therethey predate striations Thus, in the sub-Andeanhills the N-S compressionmay be of resultingfrom an E-W compression.In these two basins, no lowerPleistocene age as in theSan Ramon basin. deformationshave been observed in the youngerseries. In the sub-Andeanhills, deformationshave been analyzedin the 4.3 CompressionalTectonics Along the PacificCoast Salvacion(site 90) and Mazuko(sims 92 and 93) areas.The Neogene(?)to early Quaternary conglomerafic series, at least Some scarce folds and reverse faults demonstrate that this 3000 m thick, are folded. Asymmetric,NE verging folds domainhas been also submittedto compressionduring the associatedwith steeplydipping reverse faults strike N100- lower Pleistocene. In central Peru, nearby Pisco (site 97), 11,970 MERCIERET AL.:CHANGES IN THEANDEAN TECTONIC REGIMES

...... :.::.' ':.:.'.'.:.:::::...... :•:•-•::•:::•.,:•s:•:::• -.-.-:.======...... :.:...:.:.:.:...... :..;.,:.:;•::.:.::....;;;...... :;:,:...:::::

......

...... ,:•,,:,:•,--..:,::::.•:,.,:-!...... - ...... :.. :.:::...... •,::::.....:.-::::...... •!•i•!•!•!:•/:!•/•!!•:i!i•/•ii!•:•!•:;•:!•!<•:•::.•::•:-...... ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

:::::::::::::::::::::.:..,:::::::.::... :,:,•;: ...... ======:..:::•.,....-,;::.;•:;::;;::::;•:::•;::• ...... " :::::::::::::::::::::::•,::: :,,:::::•..•:,:•:,,.%...•e.a:;::,•,• ;•:•::•:•;•.•::•::•`:•::•?======

...... ,:::•.:.:.:.:.:.:,,.:.::,...... !t:!•:::':Z:Z•Z:.: :Z:Z:..'.:.:.:.'.:.'.'.':.::.;.•!::.:.:.:'.•;:.;::.:.•:'•;:.'•:'.:•.•.--.':':

...... :%.::.,:.....======

......

...... :,:,,...... :;:::;'?'%ii:•i:½ii•i.-•...... ?:•::!½•,.;-::::::' ......

::

Fig.20. Fieldview, seen southward, of the Huarisca faulted anticline (site 72), Huancayobasin. The antidine (underlined by dashedline) affects a conglomeraticformation of upperPliocene-early Pleistocene age. Arrows show the reverse fault plane.

several kilometer-sized folds affect the Pliocene marine beds

o and the overlyingearly Quaternaryconglomerates (Figure 21). o o o o o o o A fold, easily seen at Puente Huamani (Figure 22) strikes o o o roughlyN-S and indicatesan E-W trendingshortening [Soulas, o o o 1977]. Kinematics of reverse faults associatedwith the western limb of this fold agreewith a N258øEtrending compression. At site 96, minor reverse faults affect alluvial fans attributed to the Pleistocene; they predate the mid-upper Pleistocene N-S PAMPA trendingextension. Reverse striations belong to two families, one resulting from a N259 ø, the other from a N16 ø trending AGUA compression. The latter compression is considered as postdating the former, but chronology establishedfor few superimposedstriations cannot be consideredas representative of all the fault motions. On the southern Peru coast, at the confluence of the Nazca and Rio Grande rivers (site 150) a several kilometer-sized flexure affects continental sedimentsof Pliocene age. Reverse faults affecting this flexure predate the N-S trending tensionof mid-upperQuaternary age. They show striationsbelonging to two families: one resultsfrom a N274øE trendingcompression and seems to be related to the formation of the flexure, the other results from a N341øE trending compression. No argumentspermit one to establisha chronologyof these two families. The Calaveritasfault (site 174) moved as a strike-slip reverse fault during the lower Pleistocene reactivating a synsedimentarynormal fault which separated two coastal blocks during the Pliocene [Huarnan, 1985; Machard et al, 1986]. In conclusion, a compressionaltectonic regime affectedthe whole Central Andes during the early Pleistoceneand possibly 0 3kin I ' • I the latest Pliocene. Most of the major structures, folds, faulted 76?0' 76o8' folds, and major faults, created during this tectonic regime Fig. 21. Structuralsketch of the monoclinalfolds of early Pleistocene result from a roughly E-W compressionexcept in the sub- age near Pisco.1, Pliocenemarine Pisco Formation; 2, early Quaternary Andean hills of southern Peru. There, the fold-and-thrust belt conglomeraticCanete Formation;3, monoclinalfolds; 4, Panamerican striking N100-110øE (sites 88-90 and 92-93) results from a highway. roughly NNE-SSW trending shortening. Two directions of MERCIER ET AL.: CHANGESIN THE ANDEAN TECTONIC REGIMF3 11,971

Fig. 22. Field view, seen northward,of the monoclinalfold of early Pleistoceneage near Puente Huamani (site 97); PI, Pliocenemarine PiscoFormation; Q, early Quaternary. compression have been evidenced from fault kinematics that in the older seriesthe ENE-WSW trendingextension both analysis.They trend roughly E-W and N-S. In the Cuzco basin predatesand postdatesthe NE-SW one, whereasin the younger (section 2) synsedimentary and postsedimentary faults seriesonly the ENE-WSW extensionis evidenced. Therefore affecting the early Pleistocene formations result from both the earlierENE-WSW and the NE-SW extensionsare probably the E-W and N-S compressions[Cabrera et al., 1989]. Thus of mid(?) to upper Miocene age and the later ENE-WSW there is no convincing reason to distinguish two successive extensionof upper Miocene,possiblyPliocene age. compressional tectonic regimes each of which being In centralPeru, the Callejonde Huaylashalf graben(Figure characterizedby one directionof compressioneither E-W or N- 15) is infilled with a fluviolacustrine formation whose lower S. member containsa tuff bed dated at 5.4-6.65 m.y. [Bonnot et al., 1988]. Synsedimentaryfaults are observedin this Pliocene 5. STRESSPATTERN DURING THE LATE MIOCENE AND formation; their kinematics are in agreement with a N105 ø THEPLIOCENE (FIOURE 23) trendingtension (site 53). The major faults borderingthe half graben to the east (Figure 15) are associatedwith a 30-m- Severalcompressional events occurred during the Neogene thick, plastic and cataclasticdeformed zone affecting the [Sdbrier et al., 1988b]; one of the most important being of Cordillera Blanca batholith. The rocks are foliated, and the C uppermostMiocene age (=7 Ma). Sincewe havenot collecteda planes indicate a normal motion. Stretching lineations are large set of data concerningthis last event, it is not analyzed seen on the fault planes; they are parallel to subsequent in this paper. However, many synsedimentarynormal faults striationsoften associatedwith aligned chlorite. The foliation have been also observed in the Miocene and Pliocene correspondsto the older stage of extension in a high- formations of Peru and Bolivia. They resulted from two temperature gradient since deformation took place at a extensionaldirections trending roughly E-W and NE-SW which maximum 4500 m depth which is the maximum downthrowof are reported on Figures 23a and 23b respectively. In the the graben.These striations are in agreementwith a N50ø-60øE following, we focus on the most relevant data concerningthe trendingtension (sites 47-52). A brittledeformation postdates chronologyof these two directionsof extension. theseductile to ductile-brittledeformations; the corresponding striations(sites 47 and 51) are in agreementwith a N105ø 5.1. Extensional Tectonicsin the High Andes trendingtension similar to that demonstratedby the Pliocene synsedimentaryfaulting (site 53). Thusthe NE-SWtrending In northernPeru, the Cajabamba, San Marcos, and Namora tensionis subsequentto the crystallization of the batholith half grabens(Figure 14) subsidedduring the depositionof the datedat 12-9 Ma [Giletti and Day, 1968;Stewart et al., 1974; 1000- to 1300-m-thick, fluvial and fluviolacustrine formations Cobbinget al., 1981;Beckinsale et al., 1985]and predates the of mid-upperMiocene age [Bellier et al., 1989a, b]. In the San Pliocene ENE-WSW tension. Marcos and Cajabamba basins, synsedimentary(site 37) and In the Bolivian Altiplano, near La Paz, faults affect postsedimentaryfaults affect the older seriesof probablymid Pliocene depositsand predatethe oldest erosionalsurface of Miocene age. They result from two directions of tension Quaternaryage. Sedimentologicalstudies [Lavenu, 1984b] trendingENE-WSW (sims24-42) and NE-SW (sims25-28 and demonstratethat the Altiplano basinenlarged to the east 41). In the Namorabasin, faultswere activeduring (sims 8 and during the Plioceneas a consequenceof a synsedimentary 16) and subsequentto the depositionof the younger series extension. Normal faults affectingthe Pliocene deposits datedof upper Mioceneage. They resultfrom a meanN257øE often show two superimposedfamilies of striations. The trending tension(sims 8-19). Superimposedstriations show younger one results from the N-S trending extension also 11,972 MER• ETAL.: CHANGES IN THEANDEAN TECTONIC RF•IMES

/,

- 8o

i

.• •..,•,'"'? 4

•1•- 12ø ) ,, '2'

0

•4 • $

ß I•1 I Sohio Cruz

-

(

82 ø 780 72 ø ß Fig. 23a. Principaltensional stress directions deduced from kinematicsof normalfaults of Miocene-Plioceneage. E-W trendingtensional directions. Same symbols as on Figure13. (Sitesare listedin Table3.) deduced from faults affecting the overlying Quaternary [Fo;•ari et al., 1987]. Therefore it seems possible that the deposits. The older one resulting from a roughly E-W overthrustingof the Eastern Cordillera onto the older seriesof extension is of Pliocene age (sites 187-191). On the western the SanRamon and Tipuani basinsis of upperMiocene (--7 Ma) border of the Altiplano (site 201), a several kilometer-sized age. In this case, the subsequentnormal faulting in the San graben (Figure 24) striking NNE-SSW affects Pliocene tuffs Ramon basin (sites56 and 57) resultingfrom a roughlyE-W dated at 5.6 m.y. [Everden et al., 1977]; most of the trending tension might be of Pliocene age. In the Tipuani downthrow predates ignimbritic beds probably of upper basin, normal faults also affect the old terraces,indicating a Pliocene-Quaternaryage. NE-SW tension [Fornari et al., 1987]. These terracesare not preciselydated; they are consideredas early Quaternaryin age, 5.2. A PossibleTensional Tectonic Regime in the Sub-Andes but an upper Pliocene age is also possible.In this latter case, the NE-SW tensionmight be alsoof upperPliocene age. A tensionaltectonic regime in the sub-Andesduring the late Neogene is suggestedby some scarcedata in the San Ramon 5.3. ExtensionalTectonics in the Pacific Lowlands basin. There the Paleozoic formations of the Eastern Cordillera overthrust an older series (Canon Formation) of Previous works have already mentioned normal faults Neogene age (see section4.2). This is affected by reverse affecting the late Tertiary depositsof the forearc basins faults which are cut by subsequentnormal faults. Reverseand [Travis, 1953; Ochoa, 1980; Thornburg and Kulm, 1981; normal faults predatethe depositionof the San Ramon fan of Leon, 1983; Machard et al , 1986], but the related tensional probablyQuaternary age (see section4.2). The older series is directions have not been defined. generally consideredas Pliocene to early Quaternary in age Along the central Peru coast, synsedimentaryfaults have [Mdgard, 1978; Sdbrier et al.,1988a]. But Dumont [1988] been also observedin early Miocene marine beds (site 152); doesnot exclude that it may be the equivalentof red formations their kinematicsresults from a NE-SW trendingtension. Along of Mio-Pliocene age which crops out farther north. Moreover, the southernPeru coast, scarcenormal synsedimentaryfaults the Canon Formation is situatedin a geologicalsetting similar have been observedoffsetting upper Pliocenemarine deposits. to that of the Cangall Formation in the Tipuani basin (site At La Planchada(site 176), theyresult from a NE-SW trending 183). This latter containscinerite beds recently dated at 9 Ma tension. MERCIER ET AL.: CHANG• I•T THE ANDEAN TECTONIC REGIMES 11,973

•18ø

8 ø

j•, 12ø

!

Santa Cruz -•ooo.,•

ß ß

ß

' I ' [ I' 500kin •A

Fig.23b. NE-SW trending tensional directions.

Thus data from the basins of northern and central Peru and SouthAmerican plates, tectonicsare compressional;O l demonstratethe following statesof stress: an E-W extension trends roughly E-W, nearly parallel to the N80 ø trending of mid-upperMiocene age (sites24-42), a NE-SW extensionof convergence [Minster and Jordan, 19.78], and {J3 is vertical. upper Miocene age (sites 25-28, 41, and 47-52), an E-W Similar roughly N-S tensionand E-W compressionhave been extensionof Pliocene age (sites 8-19 and 53). For the other also observedin the southernPuna and SierrasPampeanas of sites, it has not been possibleto establishthe sequencesof the northwestern Argentina, respectively [Allmendinger et al., statesof stress;in the Bolivian Altiplano (sites 187-191) an 1989]. E-W extensionacted during the Pliocene, and along the Pacific This Andean stateof stress(Figure 12) has been interpreted coast (site 176) a NE-SW extension acted during the upper in terms of a balance between boundary forces due to the Pliocene. We hypothesizethat during the mid-upper Miocene plate convergence and gravitational body forces due to and the Pliocene, the extensionalperiods separatedby the elevatedmasses isostatically compensated [e.g.,Dalmayrac and compressional events were characterized by a NE-SW Molnar, 1981; Suarez et al., 1983; Froidevaux and lsacks, extensional state of stress following an E-W extensional 1984; Froidevaux and Ricard, 1987; Sdbrier et al., 1985, one. This seemsus to be the best interpretationof our data, but 1988a]. At a large wavelength (>100 kin), the average other scenari are possible.The reader will find the obtained chronologicalconstrains for each site in Table 3. lithosphericvertical stress{Jzz in excessto a reference(sea level) lithostaticstress increaseswith the topography, the 6. LATE CENOZOIC REGIONAL STRESS PA'ITF_•NS crustal thickness, and the anomalouslight density mantle IN THE ANDES AT A LARGE WAVEI .ENGTH: beneath [see Fleitout and Froidevaux, 1982]. The average A BALANC•E BETWEEN BOUNDARY FORCES AND lithospherichorizontal stress value {Jxx due to the slabpush GRAVITATIONAL BODY FORC•ES on the overridingplate is consideredas roughly constant during the last 2 m.y., the shear stressat the base of the 6.1. Quaternary and Active Stress Pattern: Extension in the lithospherebeing neglicted.There is a generalagreement on High AndesOrthogonal to Compressionin the Sub-Andes the compressionalnature of Oxx for low angledipping slabs [seeUyeda and Kanamori, 1979;Dalmayrac and Molnar, 1981; In the High Andes of Peru and Bolivia, tectonics are Froidevaux and Isacks, 1984; Jarrard, 1986; Bott et al., 1989]. tensional(Figure 13); c•3 trendsN-S to NNE-SSW and C•l is Thus, if in the lowlandsOxx>Ozz, tectonicsare compressional, vertical. In the sub-Andes and at the contact between the Nazca and if in the highlands,Ozz becomes>Oxx, tectonicsare 11,974 MERCIERETAL.: CHANGES INTIdE ANDEAN TECTONIC REGIMES

Fig. 24. Field view, seennorthward, of the Tirata graben(site 201) on the westernborder of the Altiplano.The grey welded tuff of Lower Plioceneage (a) is offset by a normalfault (doublearrows). The grabenis infilled by a volcano-sedimentary formation(b) thickerin the grabenthat outsideindicating a normalfault motion prior to the ignimbriticflow (c) of upper Pliocene-early Pleistoceneage. tensional.In a triaxial stateof stress(Figure 12), (•xx which 6.2. Early PleistoceneStress Pattern: trendsE-W is the maximumhorizontal stress ((•Hmax) and CompressionalTectonics in the WholeAndes OHmin trendsN-S. Thus, in the lowlands where tectonicsare compressional:gzz is •3, Oilmax is •1, andgHmin is •2. In During the late Pliocene-early Pleistocene (=2.5 Ma), major fault motionsand major folds result from a roughlyE-W the highlands where tectonics are extensional, gzz is •1; trendingshortening. However, kinematicsof somefaults may consequently,gHmax is •2 andOHmin is •3, allowing also result from a N-S compression. extension to occur in a N-S direction which is observed in the The above model easily explains the E-W compression field (Figure 13). Indeedin Peru, the •3 statisticaldirection trending N5 ø to N15øE is not strickly orthogonalto the N80 ø (Figure 19a). If during this period the (•Hmax value became convergencedirection. It is not known if this is significant . higher than the (•zz value in the high Andeanplateau, the and possiblydue to the obliquity of the Andean topography compressionalprincipal deviatoricstress direction •1 trended with respectto the convergence. roughlyE-W in the wholeAndes (Figure 25c). Increasingof the 6zz value predictsanother possibility: The N-S trending compression(Figure 19b) is not easily between the lowlands and the highlands (Figure 12), the explained. It has been shown that there is no convincing vertical stressis the intermediate•2 one. In the denserainy reason to separate two successivecompressional tectonic forest between the sub-Andes and the Eastern Cordillera, this regimes,each of which being characterizedby either the E-W has not been observed. However, in the Eastern Cordillera of or the N-S compressionaldirection. Sdbrieret al. [1988a] centralPeru, the Huaytapallana active fault (site 66, Figure have suggestedthat the two differentfault kinematics might 13) shows a compressionalstrike-slip motion, suggestinga be due to a single compressionalevent during which the •2 vertical stressaxis. A similar situationis observedin NW compressionalaxis alternatedE-W to N-S. Taking into Argentina [Marrett et al., 1989]. At the transition between the account the results obtained from theoritical modelling of Pacific Piedmont and the western Cordillera, data deduced from angularplate boundaries[Shimazaki et a/.,1978; Kato et al., microseismicactivity also agreewith a nearly vertical •2 axis 1980], this alternationhas been consideredto be possiblydue (stereonetLLU, Figure 18). to the Santa Cruz (18øS) and Huancabamba (5øS) orocline In summary(Figure 25d), at a large wavelength, the trend bends. An alternative view is that kinematics of the faults of the horizontalmaximum stressis essentiallycontrolled by resulting from the N-S compressionare kinematic instabilities the convergence and in the high plateau, extensionoccurs as due to interactionsof blocksmoving in a highly fracturedbody a result of isostatically elevated masses; the extensional of rocks submittedto a triaxial state of stressinvolving an E- directiontrends nearly orthogonalto the convergence(Figure W regional compression. Fault kinematics instabilities have 12). This has been alsoobserved in the Himalayasand the high beenanalyzed from aftershocksequences [Mercier and Carey- Tibetan plateau [Tapponnier and Molnar, 1976; Armijo et al., Gailhardis, 1989]. It has been shown that a regional 1986; Mercier et al., 1987a]. compressional direction trending, for example, E-W may MERCIERET AL.: CHANGESIN TI-IEANDEAN TECTONIC REGIMES 11,975

o'1

-0

oxx

A LATE MIOCENE PLIOCENE1

0.1 •2••.(73NE SW r •

km

extensional -.•weak-- medium weak7,

o Trench••/ ' ' ' •---•-' -0

lOO ...... / / • i "•xx ; ;;".:'i '. ': ".', ..'-;"."-;;'; :''.' ;.".' :_ :.'i.: :'. ".'.; i :':'. '..' .' :'. !. •'•'•;•;•?.• • ..•

200

B i ,=-- Ioxx(o¾¾(ozzI . LATE--MiOCENE PLIOCENE 2

O3

km compressional ,• weak -- -weak----strong

0 0

oxx

200. : . . •:••:•`•:•.•;••••.•...•.;....:.•.:...... :. ß'.. '.'.'. -.....x,.....-... ß;:-...... -....'.....'.....;...... '.•'. i. ßß .. ß . . '.

i• 1 ,.. ioxx>ovv>ozz I__ ::EARLYPLEISTOCENE

Fig.25. Qualitative scenario suggested to interpret the changes in the Andean state of stressduring the Pliocene- Pleistocene.Seeexplanation intext. VNA and VSOM, velocities ofthe Nazca and South America plates, respectively; VT, velocityofthe westward slab retreat. This scenario ishypothesized forthe evolution ofthe Andes of southern Peru during thePliocene-Quaternary period.For the Andes of centralPeru, due to the southward displacement of the Nazca ridge with respecttothe coast [Pilger, 1981; Machard etal., 1986] and to its subduction, thesituation was more complicated. Theslab flattenedprogressively fromnorth central Peru around 4 Ma to its present-day location. As a result,the situation shown on Figure25b probably didnot occur in the whole central Peru during the Pliocene, butit didpossibly during the late Miocene. 11,976 MERCIERET AL.:CHANGES IN TI-IEANDEAN TECTONIC REGIMF_S

•zz_ N 03 02 O'1 0'2 0'.30'2 L/•xx E WS• /'f 0'1•'1 0'1 2 1 1

•compres_sio• - -•.•-n• e.eSlil• . [strikeslip_• km •' •:•'• extensional weak•compressional• VNA Vs• I ; • VSOM

-o

oxx

lOO ...... ' ß ß ß ' ß'. ß ß '""::::::•...... ß .' .--:"•".'•T"'•' ß . .. .y.?`•:•:J.•::::::::•::::::::•::::•?:::•:::::::.•::::•?•?:?•::?•:•:•:•:•:•:z:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•::• ß'.-'.....' ...... •..-.....: :'.•.• ( •.'•;':•.•.•.-.-.••.•-••••:•••.••••:••••••••••::••••.•.;"••' :...':....'..: ...',,: ...:.. "'ß' ' '-'"-"' "'-''.'.' '- "'"'"" ''' ß." ' '-" ß "•'••'•"••••-••'•:'•••••u:.h.•' .... '• • •"'••:••••••'••"; C".- :"'." • ' '•"'""" "•' .'' "':' ::"':.' ß :"'I.''': '- "'-" ß '. ß . ß . ß ß . ß ß ...• .... , ß i . ß ß ß ....::::•:•:: ...... ß• ' ß.i ß ß ß ß ß ß ' .' i ' ß ß ß ' ß ß ß' ' 200 •': ß' ' .' ß.' .'; '.'..' .'.' ' • "•-"••:••••••••:••:•••:•••••I:'.I'.'.-' ß'.' ' ;' ' ßß •' ' ' ß ' : ß' ß' ....ß"'.'.'.'.'.'.'.'..".'..:I..'.'-',...,.' ...... , ...... '.' ß .'...... i.'..... I ...... ,. ',...... :... . ß . . . ß . . . ß. .... , . ._ • - . -•••••••••q•:• .•' . ß . . . . ß ....' . ß ,.. ß . ß . - ...... ß ...... ß . ß ...... f...' . .•' . '•. • ß . ß ..... ß . . ß . -...... •...,• ß . . .-...... ß; • ß . ß ...... ß ...... "' '•' I' •'•1 •: ..... i ..... '•:•': ..... I .... • ...... ' '

I Joxx)oz z) oyyJ Joxx)ozz > oyvl

Fig. 25. (continued) producecomplicated slips of small blocks resultingfrom In the second case (Figure 23b), the tensional direction axestrending either E-W or N-S. trendingNE-SW and the convergencedirection having not changeddrastically during the Miocene-Pliocene[Pilger, 6.3. Stateof StressDuring the Late Mioceneand the Pliocene: 1984; Pardo-Casas and Molnar, 1987] the above suggested Extension Parallel to the Convergenceor Orthogonalto the model cannot account for the data. It is remarkable that the Trench extensionaldirections roughly trend orthogonal to the trench. One might hypothesizethat at that time the leadingedge of the Two major directionsof extensiontrending roughly E-W overriding plate "flowed" towardthe retreatingslab (Figure (Figure 23a), and NE-SW (Figure 23b) have been observed. 25b). Such a situation would imply that the stateof stress As explained in section5.3, we suggestthat they characterize was also tensional in the lowlands. Synsedimentarynormal faulting along the Pacific coastis not really demonstrative two different types of states of stress. Paleomagnetic of this situationbecause it might result from the bending of measurementsin Peru and northern Chile [May and Butler, the forearc and/or from the gravitationaleffect due to the 1985; Kono et al., 1985; Beck, 1986; Laj et al., 1989] show that rotationsof material are of the order of 20ø-25ø during the nearbytrench. In contrast,normal faultingobserved in the San Ramon (site 84) and Tipuani (site 183) basins of the sub- Cenozoic and of =15 ø since the upper Miocene [Heki et al., 1983]. Therefore, they cannot account for the =45 ø angle Andes, if regionally significant,would be in agreementwith between the observed NE-SW and E-W mean tensional sucha stateof stress. Normal faultinghas been alsoobserved directions. The following discussion is conducted on the at moderate (2500 m) elevations at the foot of the Puna of present-day attitude of these mean extensional directions, northwesternArgentina [Allmendingeret al., 1989] and farther whose uncertaintiesare +10 ø, knowing that these may have south [ R.M. Allmendinger, personal communication,1989] been subjectedto a less than 15ø conterclockwiserotation. which are situatedin a structuralsetting similar to tlaatof the These Neogene extensionalstates of stressare markedly sub-Andes of Peru-Bolivia. different from the mid-Pleistocene-present-daystate of stress becausethe value of the E-W or NE-SW trendingstress axes are 6.4. Changesin the TectonicRegimes, Subsidence, and Uplift not Oilmax but OHmin. This implies that Oxx strongly in the Forearc Basinsas a Resultof the Slab Pull Changes? decreasedin the back arc region (Figure 23a) and possiblyin the whole belt (Figure 23b). It is assumed in the following that the horizontal In the first case (Figure 23a), the Oxx directiontrending E- lithosphericstresses are transmittedto the overriding plate W appearsto be still controledby the convergence(Figure mainly through the interplateboundary [e.g., Froidevaux and 25a). Finite elementanalyses have been used to investigate Isacks, 1984; Isacks, 1988] and that the elevation of the the 2-D stressdistribution in two convergingplates produced Andes abovesea level has not changedsignificantly during the by subductionprocesses [see Bischke, 1974; Bott et al., last 5 m.y. Supposing that the rheology of the Andean 1989]. They suggestthat an horizontaldeviatoric tension may lithospherehas not changedsignificantly during this period, be producedin the back arc regionof the overridingplate as an variationsin magnitudeof the boundaryforces are necessaryto effect of "trench suction" while local compressionoccurs explain the changesin the Andeantectonic regimes. These simultaneouslyin the vicinity of the forearc.This recalls the dependon the relative convergenceof the two plates at the blob model proposed for the Aegean Arc extension by interplate boundary and thus on the westwardmotion of the McKenzie [1978]. Although 3-D effects have not been southAmerican plate with respectto the seawardmotion of the slab flexure. investigated, this suggeststhat in an E-W convergencea processof "trench suction" might have produced an E-W Let us startfrom a situationsimilar to the present-dayone, trending extension. But coeval E-W compressionin the the horizontal stressOxx in the overridingplate being forearc has not been observed;it is possible that the forearc compressional(Figure 25d). The following hypothesized bendingand/or the gravitationaleffect due to the nearbytrench scenariois suggested.The dip of the slab increaseswith time may have cancelled the forearc compressionat the surface as its length increases.A high-angledipping slab favours a along the Pacific coast(see the appendix). seawardmigration of the slab flexure and thus a decreasein MERCIER ET AL.: CHANGESIN THE ANDEAN TECTONIC REGIMES 11,977 magnitude of the horizontal compressional stress C•xx At a first look, this scenario seems to disagree with the transmittedto the overridingplate which dependson the slab Isack's [1988] assumptionthat the Andean crust was thinned pull [Jarrard, 1986; Bott et al., 1989]. As said above, finite and weakened and then thickened by shortening of the elementanalyses [Bischke, 1974; Bott et al., 1989] suggest weakened crust. Indeed, we merely hypothesize that this that this situation allow tensional deviatoric stresses to occur scenariohas happenedseveral times duringthe last 30-25 m.y. in the back arc as an effect of "trench suction" while Weakening of the crust occurred for successive probably compression acts in the forearc (Figure 25a). If the slab extensional periods during which the slab progressively becomeslonger, the seawardmotion of the flexure may become lengthened and increased in dip. Thickening of the crust higher than the westwardmotion of the South Americanplate. occurred during the compressionalevents. Particular settings Then, the Andean lithosphere has to "flow" toward the may modify the stress pattern at a large wavelength. Some retreating slab and extensional tectonics may occur in the characteristicexamples are analyzedin the appendix. whole Andes (Figure 25b). Suchtectonic regimes must inducea subsidenceof the forearc[Bott et al., 1989] as alreadyshown 7. CONCLUSIONS in the AegeanArc [Mercier et al., 1987b; Sorel et al., 1988]. Indeed marine transgressionsonto the Peru coast are observed The changesin the tectonicregime of the Peruvian and during some extensional tectonic periods. The NE-SW Bolivian Andes during the Pliocene-Quaternaryhave been trendingtension was active during the deposition of the La analyzed from a field study of fault kinematicsand a Planchadaformation of upperPliocene age (site 176, Figure numerical inversion of slip vector data which yields the 23b). This formation lies on a marine abrasion surface and deviator responsible for the fault motions. The used clearly demonstratesa marine transgressioncorresponding to methodologyis illustratedby the analysisof the Cuzco fault a roughly 150-m elevationof the sealevel with respectto the systemsituated in the High Andes of centralPeru. Published continent. As during this period, the global sea level was focal mechanismsand trenchesdug across active faults have probably lower than nowadays;this demonstratesat least 150 been alsoused to precisethe present-daystate of stress.This m subsidenceprobably due to a downwarpingof the forearc. study has shown the following: (1) The mid-Pleistocene- The trangressiveCamana Formation of upper Oligocene age present-daystress pattern is characterizedby a roughlyE-W [Sdbrier et al., 1988b] also showsa strong subsidenceof the compressionin the sub-Andesand at the contactbetween the forearc of the order of 600 m. But synsedimentarynormal Nazca and southAmerican plates and by a roughlyN-S tension faulting during depositionof the Camana Formation. has not in the High Andes and the Pacific lowlands.(2) The upper yet been demonstrated. Pliocene(?) to lower Pleistocene tectonic regime is A low-angle dipping slab producescompression in the compressionalin the whole Andes. Most of the major overriding plate. Flattening of the slab may result from deformations result from an E-W compression. However, severalfactors includingoverriding of the forearc onto the certain faults result from a N-S trendingcompression. (3) The oceanicplate [Isacks, 1988], the effectsof the subductedplate Pliocenestress pattern is probablycharacterized by two, E-W age [Molnar and Atwater, 1978], of subductedtopographies and NE-SW trending, extensionsin the High Andes, the [Pilger, 1981], mantle flow, and relative and absoluteplate Pacific, and possiblythe sub-Andeanlowlands. Similar E-W velocity. Flatteningmay also result from a rupture of a long, and NE-SW extensionaldirections acted during the Miocenein high-angledipping slab under its own weight. Compressionin northernPeru. Other spatialchanges in the stresspattern at a the overridingplate due to a slab flattening is expectedto be large wavelengthare relatedto the particularsituation of the associatedwith an isostaticuplift of the forearc as it has been forearc, to the subductionof the buoyant Nazca ridge, and to shown in the Aegean Arc [Sorel et al., 1988]. Indeed differentdips of the slab(see the appendix). subsequentto the upper Pliocene extension and subsidenceof The stress patterns have been interpreted at a large the Andean forearc, uplift occurred which allowed the La wavelengthin termsof a balancebetween boundary forces due PlanchadaFormation to be elevatedas high as 150 m above to the slabpush on the overridingplate and body forcesdue the present-day sea level. Uplifts occurred during and to elevatedmasses isostatically compensated. (1) During the subsequentto the lower Pleistocene(=2.5 Ma) and upper mid-Pleistocene-present-dayperiod and with the slab dipping Miocene (=7 Ma) compressional events [Sdbrier et al., at a low angle, the Andeanhorizontal lithospheric stress gxx 1988b]. As a result of the slab flattening, a compressional due to the slab pushis compressionaland trendsE-W nearly event affects the whole belt (Figure 25c). Then, the length of parallel to the convergence. Therefore,in the sub-Andean the slab increasing,the overriding plate is still submittedto lowlands,tectonics result from an E-W compression(g 1), and an horizontal compressionbut whose value is less than the •3 is vertical.In the High Andes,due to the elevatedmasses, verticalC•zz stress value in the High Andesas it happensin the present-daysituation (Figure 25d). the valueof the verticallithospheric stress C•zz becomes higher Thus the followingscenario is surmized:in the overriding thanthe C•xxvalue. Thus •J1 becomesvertical, and c•3 trends plate the E-W extensionduring the late Miocene and Pliocene in the direction of the minimum horizontal stress, i.e., in a N- is transformedinto a NE-SW extension during the upper S directionorthogonal to the compression(•J1) in the adjacent Miocene and upper Pliocene as a result of a high-angle lowlands. Above the flat slab segment of central Peru, dipping slab due to its lengthening. Rupture of a long slab extensional tectonics occur at a mean elevation lower than leadsto its flatteningand to a compressionaltectonic regime 3000 m. (2) The E-W trending compressionduring the lower in thewhole Andes during thelower Pleistocene (=2.5 Ma), Pleistocenemay be interpretedby a similarmodel. The C•xx theuppermost Miocene (=7 Ma), andpossibly the older events of the Quechuaphase. The highvelocity of the Nazcaplate valuebeing higher than the C•zzone in theHigh Andes, an E- (=10cm yr -1) duringthe last 25-30 m.y. may have produced a W compression(C•l) affectedthe wholeAndes. It is suggested longslab and thus its rupturein lessthan 10 m.y. Thismay that the observedN-S compressionis a secondaryeffect due evenoccur in a shortertime spanif the SouthAmerica plate either to the orocline bends of the belt or to kinematics velocity increased.For the earlyPleistocene compressional instabilities of block motions in an E-W regional event, the lengthof the present-dayflat slab in centralPeru compression.(3) The Pliocenestate of stresswas markedly suggeststhat such a rupturemay have occured there =2.5 m.y. differentfrom the Quaternaryones because C•xx wasno longer ago in an arealocated beneath the WesternCordillera, i.e.,= the maximum but was the minimum horizontal stress. It is 250 km westof the present-daytips of the flat slab. proposedthat the E-W extensionmight result from a "trench 11,978 MERCIERET AL.: CHANGESIN TI-IEANDEAN TECTONIC REGIMF_S suction" and the NE-SW tension from a "flow" of the Andean occurs probably around 1500 m. These elevations at which crust toward a retreatingslab. changesoccur are similar to that observedby Allmendinger et It is suggestedthat thesechanges in the stresspatterns are al. [1989] in NW Argentina.Thus C•zz>{Xxx(Figure 12) occurs relatedto changesin the slabdip, andthe followingscenario is below 3000 m, that is, lower than the 4000 m value retainedby hypothesized. Starting from a low-angle dipping slab Froidevauxand Isacks[1984] to estimatethe C•xxvalue of the situation,the lengthof the slaband its dip increasewith time. slab push and than the 4500-5000 m elevation at which A high-angledip allows the "trenchsuction" to createE-W extension occurs in Tibet [Mercier et al., 1987a]. It is extensionin the backarcand E-W compressionin the forearc (Figure 25a), stress directions being controlled by the generally thoughtthat the {Xxx value is smaller above the 30ø convergence. The slab becominglonger, the slab flexure dipping slab segment. The data have not permitted us to retreats seaward at a higher rate allowing the Andean define the elevations at which the transitions between the lithosphereto "flow" in a roughlyNE-SW directiontoward the different tectonicregimes occur in southernPeru. But clearly, retreating slab (Figure 25b). Subsidenceof the forearc the present-dayelevation of the High Andesis higherthan that probablydue to its downwarpingoccurs during these periods. expectedfor a topographyin equilibrumwith the slabpush. The long slabmay rupture underits own weight andthen its upperpart flattens. This flat slab producesa compressional A.2. Stress Pattern in the Forearc and Forearc Slivers event in the overriding plate (Figure 25c). Then, the slab lengthensagain and compressionoccurs only in the lowlands; On the Pacific coast and Piedmont (Figure 12), the {XHmaxbeing smaller than the verticalstress in the highlands tensionalstress pattern deduced from surfacefaulting disagrees these are submitted to extensional tectonics (Figure 25d). with the compressionaltectonics predicted by the model. In Uplift of the forearcoccurred during the compressionalperiods. contrast, focal mechanismsof earthquakesagree with the We supposethat this scenariohave happenedseveral times model showing a compressionalregime in the forearc and a during the last 25-30 m.y. During the extensionalperiods, strike-slip faulting regime between the Piedmont and the thinning and thermal weakening of the crust probably western Cordillera. This means that the C•xx value decreases occurred. Shortening and thickening of the crust occurred from the depth toward the surface. This may result from a during the Incaic(--40-45 Ma.) andQuechua(=25, =17,--10, bending of the forearc [Sackset al., 1978; Froidevaux and --7,and •-2.5 Ma) compressionalevents. Since the mid- Isacks, 1984; Bott et al., 1989], producing extension above Pleistocene,lengthening along the 72 arc min meridian, the neutral surfaceof the plate. This is also possiblydue to a estimatedfrom downthrownsin the grabens, is of the orderof gravitationaleffect inducedby the nearby 7000-m-deeptrench 0.3% on a 400-kin-long section [Sdbrier et al., 1985]. within the part of the forearcsituated above the trench. Extensionoccurred at a higherrate duringthe Pliocene.Along The model predictsstrike-slip faulting between the forearc an E-W, about 150-km-longsection in centralPeru [Bonnotet and the back arc of the Andes. In southernPeru, the Incapuquio, at., 1988], extensionof the belt is probablyhigher than 4% Lluclla, and Pamcopalcafault zones(sites 173, and 181, Figure during3-4 Ma But in bulk, lengtheningof the High Andesis 13) are in such a situation. Moreover, there the obliquity much smaller than shortening [Isacks, 1988]. In the sub- between the strike of the Andean belt and the convergence Andes,compression is the dominatingprocess throughout the direction favors the formation of a strike-slip fault zone Cenozoic times. limiting a forearc sliver. At the surface, these fault zones show sinistral strike-slip motion with a normal or compressionalcomponent depending on the N-S tension (Figure 13) or the E-W compression(Figure19a) acting in A. 1. Stress Pattern in Central and Southern Peru theseregions. However, their motionsmight have been also VersusSlab Dip essentiallynormal (Figure 23b) with possiblya small dextral component (Figure 23a) dependingon the NE-SW or E-W While in the whole High Andes of southernPeru, the recent tensions acting in the forearc during the late Miocene and tectonicsare extensional,compressional faulting is known in Pliocene period. the EasternCordillera of centralPeru. There, the Huaytapallana fault (site 66) was reactivatedduring the July 24 and October1, A.3. Uplift and StressPattern of the OverridingPlate 1969, earthquakes.Focal mechanisms[Stauder, 1975; Suarez et in Front of the Nazca Ridge al., 1983], superficial seismic deformations [Philip and Mdgard, 1977], and slip vector measurementsperformed in Uplift is clearly observedon the Pacific coast in front of trenches[Blanc, 1984] show that they resultedfrom reverse the Nazca ridge. There, marine formations of the subsident strike-slip motions. Therefore it has been suggestedthat Piscoforearc basin emerged at theend of thePliocene [Machard compressionaltectonics might be the typical state of stressof et at., 1986]. The progressiveuplift is demonstratedby a set the central Andes above the subduetingflat slab segment of 22-27 steppedmarine terraces [Machard and Huaman, 1982; [Mdgardand Philip, 1976]. The Huaytapallanafault is observed Hsii and Bloom, 1985]. These are abrasion marine surfaces at an elevationof 4700 m, but if the topographyof this region sometimes covered by marine deposits of Pleistocene age is smoothedat a 100-km wavelength[Sdbrier et al., 1988a], [Teves, 1975]. The highest terraces of possibly upper the mean elevation appearsto be only 3700 m. Moreover, the Pliocene age attain 780 m in elevation.Their height decreases Eastern Cordillera is consideredas undercompensated,its rapidly northward,and then they disappear,giving place to the elevation would be about 3000 m according to the Airy subsidentcoast of centralPeru. Southward, theixheight also isostasy[Kono et al., 1985]. On the other hand, extensional decreases but down to 250 m which is the mean elevation of tectonicsoccur at an averageelevation of 3700 m [Cabrera et the Quaternary marine terraceson the uplifting southernPeru al., 1991] in the Cuzco region of central Peru and 3100 m coast. It is remarkablethat in spite of the low density of the [Belllet, 1989] in northern Peru. Therefore, above the flat Nazca ridge, its subductiondoes not produce compressional slab segment,transition between compressionalstrike-slip deformations in the on-shore continental crust above. On the tectonics((52 vertical) and extensionaltectonics ((51 vertical) coastaluplift (site 150) and at its southernborder (site 152), occurs around 3000 m in elevation. Transition between strike- Quaternaryfaulting result from a roughly N-S tensionas it is slip (c•2 vertical) and compressional(C•l vertical) tectonics generally along the coast (Figure 13). However, the ratios of MERCIERET AL.: CHANGESIN THEANDEAN TECTONIC REGIMES 11,979 the computeddeviators (Table 1) indicatea nearlyradial a Plate Edge, The Peruvian Andes, pp.177-202, John Wiley, New tension.This is probablydue to the oval-shapedupdoming of York,1985.. the edgeof the overridingplate which is alsopredicted by Belllet, O. , Tectonique en extension et changement d'ttats de numericalmodelling [Moretti, 1982]. contraintectnozoi'que en domaineintra-continental: Exemples des bassins intra-cordilltrains des hautes Andes (Nord Ptrou) et du grabende la Wei He (Chine du Nord), th•se, 286 pp., Univ. de A.4. Microseismicityof the Cordillera Blanca: Regional Paris-Sud,Orsay, 1989. StressPattern and a PossibleLocal Effect Due to the Batholith. Bellier, O.,M.Stbrier ,E.Fourtanier,F.Gasse, and I.Robles, Neogene and Quaternaryevolution of the intra-CordilleranNamora basin The microseismicactivity of the CordilleraB lanca has been (Northern Peruvian Andes), Ann. Tectonicae,3(2), 77-98, 1989a. analyzedin October-November1985 using a temporary Bellier, O., M.Stbrier, F.Gasse ,E.Fourtanier, and I.Robles, Evolution regionalnetwork [Deverch•re et al., 1989].Two maingroups g6odynamique mio-plioc•ne et quaternaire des bassins de la of seismic events have been recorded. One situated at a 5- to Cordill,re occidentaledu Nord Ptrou: Les bassinsde Cajabamba, 10-kindepth is relatedto the main fault and to its hanging SanMarcos et Namora(dtpartement de Cajamarca),Gdodynamique, wall. The otherone, just belowthe highest peaks, is located 4(2),93-118, 1989b. in a nearlyvertical zone at 5 + 2-kmdepth. Inversion of theP Bellier,O., J-F. Dumont, M.Stbrier, and J-L. Mercier,Geological contraints on the kinematics and fault-plane solution of the wavepolarities of a populationof theseearthquakes (0.8 < ML Quiches Fault zone: reactivated during the 10 November 1946 < 3.1) usingthe Rivera and Cisternas (1987) methodhas Ancash earthquake, Northern Peru, Bull. Seismol. Soc. Am., givena N60øEtrending tension (•3), thedeviator being nearly 81 (2),468-490, 1991. axisymmetricaround the horizontaltension. Bischke, R. E., A model of convergentplate margins based on the This tensional direction is markedly different from the recent tectonicsof Shikoku, Japan, J. Geophys.Res., 79, 4845- 4857,1974. present-daystress pattern of theAndes (Figure 13). However, Blanc, J. L., N6oteconiqueet Sismotectoniquedes Andes du Ptrou asshown by Deverch•re[1988], thefocal mechanisms related central dans la rtgion de Huancayo,th•se,161 pp., Univ.de Paris- to themajor fault planeshow kinematics which are in good Sud,Orsay, 1984. agreementwith thosemeasured on the fault planesat the Blanco, M, Evolucion plio-cuaternaria de la cuenca de surface(Figure 15), i.e., with a N10ø-20øEextension. In Chararia(Cordillera Occidental, Bolivia), tesis de grado, contrast,the focal mechanismsof the eventslocated beneath Univ.Mayor de San Andres, La Paz,Bolivia, 1980.. the highpeaks result from a N40ø-50øEextension. Although Bonnot,D,N6otectonique et Tectoniqueactive de la Cordill,re Blanche thereis no obviousexplanation for this discripancy,it seems et du Callejon de Huaylas:Andes Nord-ptruviennes,th•se, 96 pp., to usthat the kinematics at depthand at thesurface of themajor Univ.deParis-Sud,Orsay,1984. faultbordering the Cordillera Blanca (Figure 15) are consistantBonnot, D., M.Stbrier, and J. L. Mercier, Evolution g6odynamique plio-quaternairedu bassinintra-cordilltrain du Callejonde Huaylas with theregional state of stress.Thus kinematics of theminor et de la Cordill,re Blanche,Gdodynamique, 3(1-2),57-83, 1988. faults in or beneath the batholith would be due to the local Bott, M. H.,G.D. Waghorn,and A.Whittaker, Plate boundaryforces at variation of the tensional direction due to the batholith itself. subductionzones and trench-arc compression,Tectonophysics, 170,1-15, 1989. Cabrera,J., N6otectoniqueet sismotectoniquedans la cordill,reandinc Acknowledgments.Field work has beensupported by Instituto au niveaudu changementde g6omttriede la subduction:La rtgion Geofisicodel Peril, ORSTOM Lima (Institut Fran•ais de Recherche de Cuzco(Ptrou),th•se,275pp,Univ. de Paris-Sud,Orsay, 1988. Scientifiquepour le Dtveloppementen Cooptration),and ATP Cabrera,J., Fallas normalesactivas en la CordilleraOriental:La rtgion G6odynamiqueII, and ASP Blocset CollisionsPrograms (Institut de Ocongate(Ptru), Bol. Soc. Geol. Peru,80, 13-34, 1989. Nationaldes Sciences de l'Univers).We are gratefulto L. Fleitoutand Cabrera,J.,M.Stbrier, and J. L. Mercier, Active normal faulting in the R. Allmendingerfor theircritical reviews of thispaper. high plateausof Central Andes: The Cuzco region (Peru), Ann. Tectonicae, 1(2), 116-138, 1987. 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O. Bellier, I. Cabrera, I.L. Mercier, and M. Sgbrier,URA (CNRS) (ReceivedDecember 19, 1989; GgophysiqueetGgodynamique Interne, Universitg de Paris-Sud,Bitiment revisedNovember 9, 1990; 509, 91405 Orsay, France. acceptedNovember 9, 1990.)