Gravitational deformation and inherited structural control on slope morphology, north-central Chile (~29-33°S)

Abstract:

We studied the structure of the subduction zone system offshore the Pampean flat-slab segment (~29-33°S) using seismic and bathymetric constraints. Bathymetric data reveals three structural domains: (1) NW, NNW and NS oriented ridges and contractional geometries in the frontal prism and below the lower slope, (2) an extensional system controlled by NNW, NS and NE oriented westward dipping faults at the lower and middle slope, and (3) NNW-NS and NE oriented eastward dipping extensional faults at the upper continental slope. Overlying the acoustic top of the continental basement, at least two syn-extensional seismic sequences have been recognized that were correlated with onshore geological units and the Valparaíso Forearc Basin seismic sequences: Pliocene and Pre-Pliocene syn-extensional sequences. These sequences are separated by an erosional unconformity. Seismic reflection data reveal that the eastward dipping extensional system recognized at the upper slope can be extended to the middle slope and controlled the older seismic package. The westward dipping extensional system is essentially restricted to the middle slope. The tectonic boundary between the middle and upper continental slope is a prominent system of trenchward-dipping normal fault scarps (~1 km offset). This abrupt structural change can be readily detected along the Chilean erosive margin as well as the two extensional sets. Otherwise, evidences of positive inversion tectonics and thrusting have been recognized in the slope domain locally restricted to some eastern dipping faults. These contractional features could be related to gravitational deformation, which is favored by the regional inclination of the pre-Pliocene sequences. We propose that the structural configuration of the study area is dominantly controlled by tectonic erosion as well as the uplift of the Coastal Cordillera, and partially controlled by inherited architecture.