TECTONIC CONTROL OF THE SUBDUCTING JUAN-FERNANDEZ RIDGE ON THE ANDEANMARGIN NEAR VALPARAISO, CHILE

Near the latitude of Valparaiso, Chile, a fundamental change in config uration of the Benioff Zone, volcanic are activity, and the structure of the continental margin occurs opposite the subducting Juan Fernande z Ridge, Upper plate tectonics related to subduction of the ridge were studied by an international group of geoscientists in the two-degree segment offshore Valparaiso, extending from the shelf edge seaward acr oss the eastern end of Juan Fernandez Ridge, Near the O'Higgins group of seamounts, the Juan Fernandez Ridge strikes northeast rather than c ontinuing its east-west trend across the Pacific Basin. The ridge upli fts the upper plate, and sediments of the Valparaiso Basin are deforme d against its southern flank, This deformation is consistent with the southward migration required by an oblique trending ridge and the near ly trench-normal vector of plate convergence. In the trench axis, the ridge forms a basement barrier behind which sediments 2.5 km deep have ponded. The lower slope over the ridge appears eroded, whereas the ma rgin not yet affected by ridge subduction is fronted by an accretionar y prism about 25 km wide. Nazca Plate relief clearly influences tecton ism of the margin where it subducts beneath thin continental crust; it s relation to deeper processes segmenting the Andean Orogen appears to involve prior tectonic events.