Structure and tectonics of the central Chilean margin (31°–33°S): implications for subduction erosion and shallow crustal seismicity

Abstract

The pre- and current collision of the Juan Fernandez Ridge with the central Chilean margin at 31°–33°S is characterized by large-scale crustal thinning and long-term subsidence of the submarine forearc caused by subduction erosion processes. Here, we study the structure of the central Chilean margin in the ridge–trench collision zone by using wide-angle and multichannel seismic data. The transition from the upper to middle continental slope is defined by a trenchward dipping normal scarp with variable offsets of 500–2000 m height. Beneath the scarp, the 2-D velocity–depth models show a prominent lateral velocity contrast of >1 s−1 that propagates deep into the continental crust defining a major lateral seismic discontinuity. The discontinuity is interpreted as the lithological contact between the subsided/collapsed outermost forearc (composed of eroded and highly fractured volcanic rocks) and the seaward part of the uplifted Coastal Cordillera (made of less fractured metamorphic/igneous rocks). Extensional faults are abundant in the collapsed outermost forearc, however, landward of the continental slope scarp, both extensional and compressional structures are observed along the uplifted continental shelf that forms part of the Coastal Cordillera. Particularly, at the landward flank of the Valparaiso Forearc Basin (32°–33.5°S), shallow crustal seismicity has been recorded in 2008–2009 forming a dense cluster of thrust events of Mw 4–5. The estimated hypocentres spatially correlate with the location of the fault scarp, and they highlight the upper part of the seismic crustal discontinuity.