Slip-partitioning and fore-arc deformation at the Sunda Trench, Indonesia

Oblique subduction at the Sunda Trench has produced transpressive deformation of the plate leading edge. A major feature is the right-lateral Great Sumatran Fault (GSF) which probably absorbs a significant fraction of the trench-parallel shear. The kinematics of Sunda relative to Australia are discussed on the basis of available GPS data, and geologically determined slip rates on the GSF. In spite of the uncertainty on the plate motion, several robust conclusions can be drawn. The predicted obliquity of the convergence increases northward along the Sumatra Trench, up to about 30°. Slip partitioning is nearly complete along the northern segment of the Sumatra Trench, where the GSF probably accommodates most of the trench parallel shear. Along the southern segment, where obliquity is less than about 20°, slip-partitioning is not complete as indicated by oblique thrusting at the subduction. There, only a fraction of the trench parallel motion of Australia relative to SE Asia is accommodated along the GSF. These observations suggest that the leading edge behaves like a plastic wedge, except that slip-partitioning, although incomplete, is observed even at low obliquities.