Oceanic plate subduction and continental extrusion in Sumatra: Insight from S-wave anisotropic tomography

Abstract

We determine the first high-resolution 3-D model of S-wave azimuthal anisotropy down to ∼200 km depth of the Sumatra subduction zone using high-quality amplitude and phase data of teleseismic fundamental mode Rayleigh waves at periods of 20-70 s. The subducting Indo-Australian slab is imaged as a high-velocity (high-V) zone beneath the Sumatra arc, whereas obvious low-V anomalies exist in the mantle wedge beneath the arc and back-arc areas. Our results also reveal significant depth-varying azimuthal anisotropy beneath Sumatra. The subducting slab mainly exhibits a trench-parallel (NW-SE) fast velocity direction (FVD) normal to the fossil seafloor isochrons on the slab, except for an area near the Toba volcano where trench-normal (NE-SW) FVDs exist in the slab. The mantle wedge is dominated by trench-normal FVDs parallel or sub-parallel to the subduction direction of the oceanic slab, which may indicate trench-normal corner flow in the mantle wedge. The overriding plate beneath the Sumatra Island exhibits FVDs sub-parallel or oblique to the Sumatra fault, which may be related to the continental extrusion in Southeast Asia due to the Cenozoic collision between the Indo-Australian plate and the Eurasian plate. These results suggest that structural fabrics related to the oceanic plate subduction and continental extrusion are decoupled from each other beneath Sumatra.