Abstract

Based on a compilation of published 2-D velocity-depth models along the Chilean margin (22°–48°S), I review the structure and tectonic processes that govern this convergent margin in terms of sediment accretion/subduction and subduction erosion. North of the collision between the Juan Fernández Ridge with the overriding continental South America (Chile at ~32.5°S), subduction erosion has been active since Jurassic resulting in large-scale crustal thinning and long-term subsidence of the outermost forearc. Published 2-D velocity–depth models show a prominent lateral velocity contrast that propagates deep into the continental crust defining a major lateral seismic discontinuity (interpreted as the volcanic-continental basement contact of the submerged Coastal Cordillera characterized by a gravitational collapse of the outermost fore arc). Between the Juan Fernández Ridge and the Chile Triple Junction (CTJ) of the Nazca-Antarctic-South American plates (Chile at ~46.5°S), an accretionary prism 5-50 km wide has been formed due to an increase of trench sedimentation triggered by denudation processes of the Andes after the last Pleistocene Glaciation. However, the relatively small size of the accretionary prism is not compatible with an efficient history of sediment accretion, and sediment subduction is a dominant process specially south of the oceanic Mocha Fracture Zone (Chile at ~38°S) and north of the CTJ. In the overriding plate, seismic studies reveal two prominent velocity transition zones characterized by steep lateral velocity gradients, resulting in a seismic segmentation of the marine fore arc. The southern central Chilean margin is composed of three main domains: (1) a frontal prism below the continental slope, (2) a paleoaccretionary complex, and (3) the seaward edge of the Paleozoic continental framework that forms part of the Coastal Cordillera. Near the CTJ, where the Nazca-Antarctic spreading center (Chile Rise) collides with the margin, subduction erosion is active, and rapid uplift followed by subsidence of the forearc area and normal faulting and intensive sedimentary mass wasting are documented. South of the CTJ, the convergence between the oceanic Antarctic and continental South American plate is slow allowing more time sediment accumulation at the trench enhancing the formation of relatively large accretionary prisms (70-90 km wide).