| Summary: | Based on a compilation of published and new seismic refraction and multichannel seismic reflection data along the southern central Chile margin (33°-46°S), we study the processes of sediment accretion and subduction. In terms of the frontal accretionary prism (FAP) size, the marine southern central Chile forearc can be divided in two main segments: (1) the Maule segment (south of the Juan Fernández Ridge and north of the Mocha Block) characterized by a relative large FAP (20-40 km wide), and (2) the Chiloé segment (south of the Mocha Block and north of the Nazca-Antarctic-South America plates junction) characterized by a small FAP (~10 km wide). In addition, the Maule and Chiloé segments correlate with a thin (< 1 km thick) and thick (1.0-1.5 km thick) subduction channel, respectively. The Mocha Block lies between ~37.5° and 40°S, and is configured by the Chile trench, Mocha and Valdivia Fracture Zones. This region separates young (0-25 Ma) oceanic lithosphere in the south from old (30-35 Ma) oceanic lithosphere in the north, and it represents a fundamental tectonic boundary separating two different styles of sediment accretion and subduction, respectively. A process responsible for this segmentation could be related to differences in initial angles of subduction which in turn depend on the amplitude of the downdeflected oceanic lithosphere under trench sediment loading.
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