An updated compilation of earthquake locations and focal mechanism solutions from the International Seismological Centre and Preliminary Determination of Earthquakes is the basis of a comprehensive study 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 subducted Nazca plate beneath southern Peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with ~30¿ dip at latitude 33 ¿S. In contrast, the transition from 30¿ slab dip beneath Bolivia to a nearly horizontal dip in the region between 28 ¿S--32 ¿S is more gradual, occurring over several hundred kilometers of along-strike distance between 20 ¿S and 32 ¿S. This southward flattening corresponds to a broadening of a horizontal, benchlike part of the subducted plate formed between 100 and 125 km depth. The transition in continental tectonic style near 27 ¿S--28 ¿S, from a wide, volcanically active plateau to a narrow, nonvolcanic cordillera, appears not to be associated with the main slab flattening, which begins to the north of these latitudes, but with a more abrupt change in curvature of the subducted slab, from convex upward to concave upward, immediately below the plate boundary interface. The concept of Gaussian curvature is applied to slab bending to explain how subduction geometry 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 earthquakes are grouped by geographic region and inverted for the orientation and relative magnitudes of the principal stresses. Results of the inversion indicate that downdip extension dominates in the slab above 350 km while downdip compression dominates at greater depths. ¿ American Geophysical Union 1992