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 et Technique 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. 0 20 40 kin. Ayacucho'•1•.' x_.x,, ß ß Colombia -•-•] 1 [-'--]2 ß •--]3 5 •6 to. Langui Layo L. •5' • 9 •o \ /e * 11 (•) 12 \xx • 15' i i i 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 7•i 45' "'-;•.'•..ß ...... 4,øø" ..... • "'L ;.;.... !....;11. i i :4' i .,..,:: ;.-:-:.:•.:.""": •-;-.;......:-..