Sismicity 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 Subducted Nazca Plate
THOMASCAHILL1 AND BRYANL. ISACKS
Institutefor the Studyof Continents and Departmentof GeologicalSciences, Cornell University,Ithaca New York An updatedcompilationof earthquake locationsand focal mechanismsolutionsfrom the InternationalSeismological Centreand PreliminaryDeterminationof Earthquakesis 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 southwardtransitionfrom nearly horizontal subductionto a slabwith -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 severalhundred 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 kmdepth. 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 curvatureof the subductedslab, from convex upward to concaveupward, immediately below the plate boundaryinterface. Theconcept 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 andinverted 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 compression dominalesat greaterdepths.

INTRODUCTION

Details of subductiongeometry are crucial to refine models relating subduction geometry to upper-plate tectonics; this is especially true in the case of the SouthAmerican WadatiBenioff zone (WBZ), the east-dipping zone of mantle earthquakes that delineates the position and shape of the subductedNazca plate. Hamilton [1969] and Jordan et al. [1983] and othershave recognizedthe importanceof usingthis active example of an ocean-continentconvergencesystem for developing tectonic models of the late Mesozoic in the western United States. We presentanupdated,three-dimensional view of the WBZ beneath western South America, produced by adding9 yearsof teleseismicdata to previousco•npilationsof mantle seismicity. The feature that lnost strongly characterizes the subduction geometry beneath the Andean cordillera is the along-strike variation in dip of the subducted Nazca plate [Sykesand Hayes, 1971; Barazangi and lsacks, 1976; Beyis and Isacks, 1984].Barazangi and lsacks [1976] described the subductedslab as having four 'segments' between 4øS and 45øS, with each segment distinguished from its neighbors by a significant changein the angle of dip. BeneathsouthernPeru and northern Chile (15øS to 27øS), and again south of 33øS, the WBZ dips moderately to the east at an angle of about 30 ø. Beneath northern and central Peru (8øS to 14øS)andwesternArgentina (28øS to 32øS) the subductedlithosphereextendseastwardfor hundreds kilometersat a depth near 100 km before resuming of
its downward descent.

Barazangi and lsacks [1976] first interpreted teleseismic the locationsas indicating the individual segments boundedby are tears in the subductingslab. They visualized the two shallowdipping segmentsof slab having edges sharply offset froIn the...