Direct inversion of S-P differential arrival-times for Vp/Vs ratio in SE Asia

Southeast Asia lies within one of the most complex tectonic settings on Earth and exhibits a range of features, including strongly curved subduction zones, arc-continent collision, and slab break-off, which are not well understood. To help gain insight into mantle structure and processes beneath this region, we perform an inversion for variations in V-p, V-s, and V-p/V-s structure using arrival time information from the ISC-EHB catalog. The oceanic lithosphere subducting beneath Java is imaged as a positive dV(p) and negative d(V-p/V-s) anomaly. At 200 km depth, the forearc mantle beneath Sumatra and Java is revealed by positive dV(p) and d(V-p/V-s) anomalies which cease at Sumba island, where negative d(V-p/V-s) anomalies mark the presence of cold Australian lithosphere (down to 200-250 km depth) which is colliding with Sundaland. These negative d(V-p/V-s) anomalies depict a similar to WE trending structure that appears to correspond with the underthrusting of Australian continental crust. One notable salient has a location and shape which appears to coincide with those of ancient terranes or a Gondwana-related microcontinent reconstructed by paleogeographic studies and may have been entrained in the subduction process. The velocity and d(V-p/V-s) patterns beneath the Banda Arc support the existence of a single curved subducting slab associated with rollback. The extreme extensional strike-slip setting in Seram produces the highest positive d(V-p/V-s) anomalies in the model which may be due to one or more of widespread serpentinization, high concentrations of intraslab fluid-filled faulting, and mantle upwelling.