TEC'rO••, VOL. 12, NO. 1, PAGES 63-76, FEBRUARY 1993 BASEMENT SEISMICITY BENEATH THE by subductionof the oceanic Nazca plate beneath the ANDEAN PRECORDILLERA THIN- continentalSouth American plate. The modernAndes are due SKINNED THRUST BELT AND to Neogene tectonics characterized by along-strike IMPLICATIONS FOR CRUSTAL AND segmentationof majorfeatures of thesubducted and overriding LITHOSPHERIC BEHAVIOR plates [Barazangiand Isacks, 1976; Jordan et al., 1983] (Figures1 and2). In addition,the along-strikefeatures of the Robert Smalley,Jr., 1 Jos6Pujol, 1 MarcRegnier, 2 two platesare correlated,both spatiallyand in their temporal Jer-MingChiu, 1 Jean-LucChatelain, 2,3Bryan L. Isacks,4 development.Between 15øS and 24øS the Altiplano-Puna MarioAraujo, 5 and N. Puebla5 plateau, the EasternCordillera and SubandeanZone thrust belts,and an activevolcanic arc are located above a moderately Abstract.Data froma digitallyrecording seismic network dippingWadati-Benioff zone (WBZ). In contrast,a narrow in SanJuan, Argentina, provide the first imagesof crustal thin-skinned thrust belt (lyecordillera), a wide zone of scalebasement faults beneath the lyecordillera. This seismicity basementuplifts (SierrasPampeanas), and an extinctare are is nearthe boundary between the lyecordillera (a thin-skinned locatedabove an intermediate-depthsubhorizontally subducting thrustbelt) and the SierrasPampeanas (a regionof thick- WBZ between 28øS and 33øS. Shutdownof volcanism, onset skinnedbasement deformation), two seismically active tectonic of Pampean and Precordilleran crustal deformation, and provincesof theAndean foreland. The seismicity data support developmentof flat subductionare thoughtto coincide at modelsfor thisregion in whichcrustal thickening, rather than 10-15Ma [Jordanet al., 1983]. The SierrasPampeanas and magmaticaddition or thermaluplift, plays the dominant the lYecordilleraare thoughtto have a tectonicrelationship mountainbuilding role. The lYecordillera seismicity occurs in similarto thatof theMesozoic Sevier and Laramide provinces three segmentsdistributed north to south.The southern of North America,which are alsothought to havedeveloped segmentis an areaof diffuseactivity extending across the overa regionof flat subduction[Dickenson and Snyder, 1978; Precordilleraand eastwardinto the SierrasPampeanas that Fieldingand Jordan, 1988]. showsno patternsin mapor crosssection. The northernand The Andeanorogen is seismicallyactive, and variations of centralsegments have well-defined dipping planes that define crustalintraplate seismicity in the overridingplate correlate crustalscale faults extending from 5 to 35 km depth.It is with the along-strikevariations of tectonicstyle (Figure2). clear from the relativefault geometriesthat the overlying Crustalseismicity is concentratedin a narrowband along the Precordillerais not simplyrelated to the basementactivity. easternmargin of theforeland in areasover normal subduction, Theseismicity here may result from reactivation of anancient while above flat subductiona wide area of the foreland, with suturebetween the lYecordillera and Pampeanas terranes or be thin- and thick-skinnedtectonic provinces, is highly active occurringin basementof unknownaffinity west of thesuture. seismically.The easternmostlimit of crustal seismicity Theseismicity provides the first constraints on basement fault coincideswith the easternmostWBZ activityin bothflat and geometries,and we present models integrating this information steepsubduction regions. The depthof crustalearthquakes also with the surfacegeology. These basement faults may have correlateswith tectonicstyle. In regionsof normalsubduction, been responsiblefor the 1944 Ms 7.4 earthquakethat seismicityoccurs in the upper and mid crust to depthsof destroyedthe city of SanJuan. The imagingof thesefaults 25 km [Cahill et al., 1992], while in areasof flat subduction suggeststhat seismic risk estimates for SanJuan made on the crustalevents occur at mid and lower crustaldepths to 40 km basisof surfacegeologic studies may be too low. ISmalleyand Isacks,1990; Su•ez et al., 1983].This variation may be due to regionaldifferences in lithosphericor crustal INTRODUCTION stress,strain rate, crustalgeotherm, or rheology,both across andalong the orogen. The Andesprovide a uniquelaboratory to studymountain belt formation in a subduction-driven but noncollisional GEOLOOY environment.The presentorogen began in theJurassic, driven Precordillera. The lYecordillera is a thin-skinned thrust belt of Paleozoicelastics divided into three structural subprovinces: 1 Centerfor Earthquake Research and Information, Memphis Western,Central, and Eastern(Figure 3) [Baldis,1975; Ortiz StateUniversity, Memphis, TN. and Zambrano, 1981; Baldis et al., 1982]. The Central and 2 InstitutFranqais de RechercheScientifique pour le Westernprovinces form the forelandthrust belt foundon the D6veloppementen CoopdrationNoumea, New Caledonia, cratonicside of compressionalorogens. These mountain belts South West Pacific. are interpretedas the resultof a crustalshortening across an 3 Nowat Institut Franqais deRecherche Scientifique pour le orogenin which crustalthickening associated with A-type D6veloppementen Coop6ration,Domaine Universitaire, subductionoccurs along a majorintracontinental thrust [Bally, Grenoble,France. 1981]. Crustalmaterial from the orogenproper is thrust 4 Institutefor the Studyof the Continents,Cornell cratonwardover relatively undeformedforeland basement, University,Ithaca, New York. pushingthe forelandbasin sediments into a thin-skinnedthrust 5 InstitutoNacional de Prevenci6nSfsmica, San Juan, belt. This thrustbelt is typicallythe cratonwardmostlimit of Argentina. deformation associatedwith an orogenic system. The developmentof thesethrust belts generally follows a set of Copyfight1993 by the AmeficanGeophysical Union. rules, derived from geologic studies [Chappie, 1978; Dahlstrom,1970; Davis et al., 1983]and physical models of Paper number92TC01108. deformation[Panian and Pilant, 1990] describing the spatial 0278-7407/93/92TC-01108510.00 andtemporal evolution of their structures. 64 Smalleyet al.: BasementSeismicity in AndeanPrecordillera 20 S 25 30 75 W 7O 65 Fig. 1. Map of westernSouth America illustrating upper plate tectonic features, volcanic arc, and contours of theWadati-Benioff zone (WBZ). Along-strikesegmentation of the upperplate tectonic provinces correlateswith the along-strike segmentation of the WBZ [Jordanet al., 1983].The distribution of Neogene volcanoes(open circles) [Isacks, 1988] shows an active magmatic arc over the steep WBZ segmentand the absenceof anactive arc over the flat segment. The drainage divide, which defines the internally drained Puna/Altiplanoplateau and then follows the crest of theAndes southward between the Principal and Frontal cordilleras,is also shown. The inset shows the major topographic features of theoceanic Nazca plate (Juan Fernandezand Nazca ridges). 20 S 25 30 o o 75 W 70 65 Fig. 2. Shallowintraplate seismicity (solid circles) and damaging historic earthquakes (open circles) [Zamarbideand Castano,1978] of the Andeanback-arc. Epicenters are selectedfrom Preliminary Determinationof Epicenters(1963, 1983-1990)and International Seismological Center (1964-1982) catalogs.Selected events were located with 15or moreP arrivals,are shallower than 50 km,and are east of the 100km WBZ contourensuring they are in SouthAmerican lithosphere and are notinterplate or WBZ events.A selectionof focal mechanisms[Chinn and Isacks,1983; Dziewonski et al., 1987; Kadinsky-Cade, 1985;Stauder, 1973] is alsoshown. The level of seismicityin thePie dePalo area (31.5øS, 67.5øW) is so intensethat only focalmechanisms with accuratesource depths from syntheticmodeling are shown. Tectonicprovinces and inset are as in Figure1. 66 Smalleyet al.: BasementSeismicity in AndeanPrecordillera 894 .2 31 [] 1977 6.7 7A 32 Okm 50km Salinas i i 69 W 68 67 Fig.3. Geology,mapped and inferred faults (solid and dotted bold lines), seismic station sites (small triangles:solid for stations, open for arrays of 8 to 11stations with apertures of 1 m to 1 km),damaging historicalseismicity (dated open squares, 1944 location from Kadinsky-Cade [1985]), the 1944earthquake intensityVIII isoseismal(large triangle outline) [Castellanos, 1945], and two LANDSAT lineaments in SanJuan (heavy gray lines). The LANDSAT lineament near La Lajaincludes the fault suffering offset in 1944.Mountain range patterns are light gray for sedimentaryrocks (Precordillera) and dark gray for basementblocks (Sierras Pampeanas); intermontane valleys are in white.The Eastern Precordillera is the chainof mountainseast of the MatagusanosValley (SierrasChica de Zonda,Villicum, Morado, and extensionsnorth and south). West of thePrecordillera are the Calingasta and Iglesia valleys. The CentralPrecordillera is a typicalforeland thin-skinned Pampeanas,a province of Precambrianmetamorphic basement thrust belt. The Eastern Precordillera was fu'st described as the block uplifts [Caminoset al., 1982; Cingolaniand Varela, easternmostpart of this thrust belt [Orfiz and Zambrano, 1975]. Such deformationcratonward of the thin-skinnedthrust 1981], but it appearsto break two rules of thin-skinned belt is unusualand found only in regions overlying thrusting.First, the EasternPrecordillera is thrustwestward, subhorizontalsubducfion, a correlation also thought to holdin away from the craton.Second, the basalunit of the Eastern extinctorogens [Dickenson and Snyder, 1978]. The Pampeanas Precordillerais over 2 km down-sectionwith respectto the