Crustal dynamics and active fault mechanics during subduction erosion. Application of frictional wedge analysis on to the North Chilean Forearc

Abstract

The forearc region of the non-accreting South American Plate margin in northern Chile is characterised by subduction erosion and regional uplift. Neotectonic deformation structures reflect simultaneous extensional and contractional fault kinematics. In the outer forearc, where the brittle crust directly overlies the subducting Nazca Plate, the stress regime changes from extension in the upper part to compression at the base of the forearc wedge as seen in neotectonic surface structures and seismic data. In the inner forearc, surface structures indicate a compressional stress regime also affecting the western rim of the magmatic arc. This stress regime is limited to a brittle crustal wedge segment which overlies the ductile part of the inner forearc lithosphere. The shape of the two brittle forearc wedges at the leading edge of the South American Plate is controlled by exogenetic surface processes, internal deformation processes, contemporaneous basal tectonic erosion and underplating. Mechanical parameter sets controlling and reflecting the recent tectonic processes and geometrical wedge segmentation within the forearc system are evaluated and applied to general frictional wedge models. The states of stress within the crustal wedges are controlled by spatial variations of the basal mechanical parameters in the down-dip direction of the forearc wedge base. The new models illustrate the fundamental kinematics and dynamic processes of ‘Coulomb-type’ basal tectonic erosion and mass transfer modes along active non-accretive convergent margins. The frictional wedge models explain the dynamics of the simultaneous and contrary deformation processes affecting the forearc crust at the North Chilean convergent margin shaped by active basal tectonic erosion.