Tectonic control of the subducting Juan Fernández Ridge on the Andean margin near Valparaiso, Chile

Near the latitude of Valparaiso, Chile, a fundamental change in configuration of the Benioff Zone, volcanic arc activity, and the structure of the continental margin occurs opposite the subducting Juan Fernández 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 across the eastern end of Juan Fernández Ridge. Near the O'Higgins group of seamounts, the Juan Fernández Ridge strikes northeast rather than continuing its east‐west trend across the Pacific Basin. The ridge uplifts the upper plate, and sediments of the Valparaiso Basin are deformed against its southern flank. This deformation is consistent with the southward migration required by an oblique trending ridge and the nearly 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 margin not yet affected by ridge subduction is fronted by an accretionary prism about 25 km wide. Nazca Plate relief clearly influences tectonism of the margin where it subducts beneath thin continental crust; its relation to deeper processes segmenting the Andean Orogen appears to involve prior tectonic events.