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Seismicity and Shape of the Subducted Nazca Plate

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Seismicity and Shape of the Subducted Nazca Plate JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. B12, PAGES 17,503-17,529, NOVEMBER 10, 1992 Seismicity and Shapeof the SubductedNazca Plate THOMASCAHILL 1 AND BRYANL. ISACKS Institutefor the Studyof Continentsand Departmentof GeologicalSciences, Cornell University,Ithaca New York An updatedcompilation of earthquakelocations and focal mechanismsolutions from the International SeismologicalCentre and Preliminary Determination of Earthquakes is the basis of a comprehensivestudy of the geometry of the Wadati-Benioff zone beneath western South America. The new data support previous mapping of a sharp flexure rather than a tear in the subductedNazca plate beneath southern Peru and provide evidencefor a similar flexure in the southwardtransition from nearly horizontal subductionto a slab with -30 ø dip at latitude 33øS. In contrast, the transition from 30 ø slab dip beneath Bolivia to a nearly horizontal dip in the region between 28øS - 32øS is more gradual, occurring over several hundred kilometers of along-strike distance between 20øS and 32øS. This southward flattening corresponds to a broadeningof a horizontal, benchlike part of the subductedplate formed between 100 and 125 km depth. The transition in continental tectonic style near 27øS-28øS, from a wide, volcanically active plateau to a narrow, nonvolcaniccordillera, appearsnot to be associatedwith the main slab flattening, which begins to the north of these latitudes, but with a more abrupt changein curvature of the subductedslab, from convex upward to concaveupward, immediately below the plate boundary interface. The concept of Gaussian curvature is applied to slab bending to explain how subductiongeometry is affected by the shape of the South American plate. We hypothesize that the polarity of vertical curvature in the subducting slab is governed by the orientation of lateral curvature of the plate margin. Focal mechanism solutions for intermediate and deep earthquakesare grouped by geographicregion and inverted for the orientation and relative magnitudesof the principal stresses. Results of the inversion indicate that downdip extension dominatesin the slab above 350 km while downdip compressiondominales at greaterdepths. INTRODUCTION Barazangi and lsacks [1976] first interpretedthe teleseismic Details of subductiongeometry are crucial to refine models locationsas indicating the individual segmentsare boundedby relating subduction geometry to upper-plate tectonics; this is tears in the subductingslab. They visualized the two shallow- especially true in the case of the South American Wadati- dipping segmentsof slab having edges sharply offset froIn the Benioff zone (WBZ), the east-dipping zone of mantle adjoining segments by a throw that increases toward the earthquakes that delineates the position and shape of the downdip direction. Hasegawa and Sacks [1981] later inferred, subducted Nazca plate. Hamilton [1969] and Jordan et al. from local seismic data gathered in the vicinity of the segment [1983] and othershave recognizedthe importanceof using this boundary beneath southern Peru, a continuous flexure of the active example of an ocean-continentconvergence system for slab rather than atear. Beyis [1986] and Beyis anttlsacks developing tectonic models of the late Mesozoic in the western [1984] analyzed the distribution of teleseismic hypocenters United States. We presentan updated,three-dimensional view using trend surface analysis and showed that it was possible to of the WBZ beneath western South America, produced by interpret every segment boundary in the region in terms of a adding 9 yearsof teleseismicdata to previousco•npilations of smooth contortion rather than a tear. mantle seismicity. Tears may exist in the vicinity of one or several of the large The feature that lnost strongly characterizes the subduction aseismic areas within the central Andean WBZ. However, the geometry beneath the Andean cordillera is the along-strike teleseisrnic hypocenters indicate such tears would not be variation in dip of the subductedNazca plate [Sykesand Hayes, segment-bounding faults with large vertical offset but rather 1971; Barazangi and lsacks, 1976; Beyis and Isacks, 1984]. would separate gaps lying within the space of the curved Barazangi and lsacks [1976] described the subductedslab as regional trend. Nothing short of one or more large tear-faulting having four 'segments' between 4øS and 45øS, with each focal mechanisrnscould establish the existenceof lithosphere- segment distinguished from its neighbors by a significant scale ripping of the subductedslab; to date, no such event has changein the angle of dip. Beneathsouthern Peru and northern been recognized within the set of central Andean WBZ Chile (15øS to 27øS), and again south of 33øS, the WBZ dips earthquakes. moderately to the east at an angle of about 30 ø. Beneath Associatedwith the changesin angle of subductionbeneath northern and central Peru (8øS to 14øS) and western Argentina western South America are changes in the regional-scale (28øS to 32øS) the subductedlithosphere extends eastwardfor tectonic character of the overriding plate [Barazangi and hundredsof kilometers at a depth near 100 km before resuming Isacks,1976; Jordan et al. 1983]. A distinctive feature of central its downward descent. Andean geology is the spatial correlation between late Cenozoic arc volcanism, plateau formation, and steeper subduction angle. In this paper, digitized topographic and tNow at EngineeringTectonics, P.A. Winston-Salem,North bathyrnetric data and an updated catalog of young volcanic Carolina. centers from lsacks [1988] are used to detail the spatial relationships between subduction geometry, active volcanism, Copyright 1992 by the American GeophysicalUnion. and the physiographyof the Andean cordillera. Paper Number 92JB00493. Along with its effect on the upper plate, the geometry of 0148-0227/92/92JB-00493505.00 subducted Nazca plate also strongly influences deformation 17,503 17,504 CAHILL AND ISACKS:SEISMICITY OF THE S UBDUCTEDNAZCA PLATE within the slab itself [ Isacks and Barazangi, 1977' Schneider longer be discerned. This observation could be predicted from and Sacks, 1987]. The orientation of the stress field in noting that events with n<15 often have a large standard descending slabs is manifested in the focal mechanism deviation of travel time residuals (s), one indicator of location solutions for the intraplate events [Isacks and Molttar, 1971' quality (Figure la). Creager andBoyd, 1991]' within the set of South American The hypocentral data presentedin the cross sections and mantle earthquakes are many that are large enough for regional seismicity maps include all events reported by 15 or determinationof reliable fault-planesolutions. We use several more stations. In determiningthe best fitting depthcontours, a previously unpublishedsolutions along with Harvard Centroid more restrictive subsetof the data was used; all hypocenters Moment Tensor (HCMT) solutions and those of Stauder [ 1973, based on 40 or more (n_>40) P arrival time reports were 1975] to invert for the principal axes and relative magnitudesof included in the selection of events used; in areas where the stress in the subducted slab. seismicity represented byn>40 was sparse or nonexistent, DATA eventswith fewer reports (first 25 _<n _<39 , then 15 _<n _<24) Wadati-BenioffZone Itypocenters were considered; these smaller events were rejected if arrival The catalogs of the International SeismologicalCentre time residualsfrom nearby stations(A<5 ø) were very large. (ISC) for the period of 1964-1985 and the United States In the course of selecting data to use for this study we Geological Survey Preliminary Determination of Epicenters consideredthe complete set of available teleseismiclocations. (PDE) for the periodsof 1963 and 1986-1988 were the primary In addition to the data sources mentioned above, this included sourcesof data used in this study. Intermediate-depthevents the International Seismological Summary catalog between (h>60 kin) were selected from thesetwo catalogson the basis 1950 and 1963, the PDE catalog from 1950-1982, and an of the number of arrival time reports (n) used to locate the unpublished 1964 compilation of Sykes and Hayes [1971]. earthquake. Comparingmaps made from thesedata sets,which spanahnost In a studyof WBZ morphology,some grading and culling 40 years, we observed that consistency in the spatial of the data are necessary to assure use of only reliable distribution of intermediate-depth seismicity is a fundamental hypocentrallocations. We chose to employ n as our sole feature of the South American WBZ. The clusteringof events selection criterion. Standard hypocenter grading schemes, into tight bands of seismicity noted by Santo [1969] for the based on the geographic distribution of teleseismic reporting period of 1960-1968 and by Stauder[1973, 1975] for 1962-1971 stations,local station geometry, and surface-reflectedphases to is reproduced faithfully by events from other time intervals of verify focal depths, are difficult to apply to South American similar length from any of the other compilations. WBZ earthquakes. The azimuthal and distancedistribution of Furthermore,no event in the period between1950 and 1985 seismographstations for events in this region is peculiar; there has been reliably located in any of the substantialspatial gaps are few stationslying between A=10 ø and A=85 ø to the west in seismicity that also characterize the South American WBZ. (the eastern Pacific Ocean) and few stations between A=10 ø Data
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