Seismic investigation of the continental margin off- and onshore Valparaiso, Chile

At the latitude of Valparaiso, Chile, a fundamental change in the configuration of the Benioff zone, volcanic activity, and the structure of the continental margin occurs opposite the subducting Juan Fernandez Ridge. Three legs of the German R/V Sonne (cruises SO101, SO103 and SO104) surveyed the continental margin and oceanic plate offshore Valparaiso, aiming at studying the crustal structure and investigating possible causes for the change in slab configuration. Senile cruise SO101 investigated the tectonic setting with swath-mapping bathymetry, magnetics and high-resolution seismics. Following these investigations cruise SO103 collected land-sea wide-angle seismic data, and coincident deep seismic reflection data were acquired during cruise SO104. Coincident near-vertical and wide-angle seismic measurements were made along two profiles. Profile 1, located at the south of the study area, away from the influence of the subducting ridge, crosses the margin where thick trench sediment and an accretionary wedge near the trench is observed. Profile 2, located in the north, runs from the Juan Fernandez Ridge to the Chilean coast. The crustal velocity models obtained for the two profiles show that the continental crust extends to the middle-lower slope boundary, which is also reflected in morphology, In addition, they show that the crustal structure of the oceanic plate is rather similar, but the plate seems to be slightly more inclined along the northern profile (13 degrees versus 10 degrees in the south). The two profiles are only about 70 km apart but their structures differ significantly. No straightforward correlation exists between the two profiles that can be attributed to ridge collision. The data support that the 1985 central Chile earthquake ruptured the plate boundary in the area that includes the segment boundary and mainly where continental crust forms the upper plate. (C) 1998 Elsevier Science B.V. All rights reserved.