ALBERT

Description: Subduction erosion has dominated the evolution of the north Chile convergent continental margin since at least the Mesozoic. We investigate the structure of the Antofagasta (23°S) sector of this margin along a transect using coincident wide-angle and near-vertical seismic profiling and gravity data. A 2-D velocity field of the overriding and subducting plates was obtained using joint refraction and reflection travel time tomography. A velocity-derived density distribution was used to model marine gravity data and substantiate the velocity model. The gravity and velocity models imply that the overriding plate is mainly made of arc-type igneous basement. The upper plate is constructed of two main rock bodies separated by a subhorizontal layer defined by a velocity inversion, the top coincident with a reflection in near-vertical seismic images. The seismic boundary is interpreted as a detachment separating an upper extended domain with large-scale normal faulting from a lower domain apparently undergoing a different type of deformation. Velocity-derived porosity indicates that the front of the margin is probably fluid-saturated and disaggregated by fracturation as a consequence of frontal subduction erosion. Fluids carried into the subduction channel within slope debris filling underthrusting grabens reduce basal friction and probably induce hydrofracturing and basal erosion along the underside of the overriding plate. At depths greater than ∼20 km, porosity and density values imply that most fluids have been exhausted and the lower part of the upper plate is structurally coherent and little fractured. The change in physical properties leads to increased mechanical coupling along the plate boundary and occurs at the updip limit of the distribution of aftershocks of the 1995 Antofagasta earthquake (M w = 8.0) defining the seismogenic zone.