Oblique Convergence along the Chilean Margin: Partitioning, Margin-Parallel Faulting and Force Interaction at the Plate Interface

Abstract

The Chilean fore-arc exhibits margin-parallel strike-slip faulting and associated fore-arc sliver formation. Comparison of the long-term (geologic timescale) and short-term (human timescale) record of margin-parallel faulting along the oblique Chilean subduction margin between 15° S and 46° S reveals significant spatio-temporal heterogeneity. We have reviewed newly compiled data on the geometric, kinematic and mechanical properties and their variation along-strike of the Chilean margin and evaluated their competing influence on fore-arc deformation. Among the parameters considered are the plate kinematics (e.g., convergence obliquity and rate), overriding plate heterogeneities that affect its capability for localizing horizontal shear (e.g., thermal and structural weaknesses) or resistance to block motion (e.g., plate margin curvature) as well as properties governing or indicating force interaction at the plate interface (e.g., trench sediment-fill, geodetic and seismic coupling depth). Most remarkably, the short-term GPS-derived fore-arc velocity field, dominated by elastic loading processes, shows little variation along-strike of the margin, despite the significantly changing conditions (e.g., trench sediment-fill, mass transfer mode at the tip of the overriding plate, plateau or no plateau, and slab-dip variations). Variations in recent and past strike-slip motion do not appear to depend on the rate or obliquity of convergence, nor on the mode of mass transfer at the subduction front. The frictionally coupled area on the plate contact increases southwards and a decreasing taper along the Chilean margin can be reconciled in the framework of taper theory by a southward decrease of the effective coefficient of friction on the plate interface. The development of fore-arc slivers seems to be primarily controlled by mechanisms that cause effective rheological weakening of parts of the upper plate and/or by geometries that hamper margin-parallel sliver motion. While the seaward concave-shaped margin in North Chile hinders the margin-parallel motion of a fore-arc sliver, the present strike-slip activity of the Liquiñe-Ofqui Fault Zone in southern Chile is likely facilitated by the superposition of two conditions: a shallowly dipping slab and an exceptionally small arc to trench distance.