Direct Inversion of S-P Differential Arrival Times for VpVs Ratio in SE Asia

Abstract

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 Vp, Vs, and (Formula presented.) structure using arrival time information from the ISC-EHB catalog. The oceanic lithosphere subducting beneath Java is imaged as a positive dVp and negative d(Vp/Vs) anomaly. At 200 km depth, the forearc mantle beneath Sumatra and Java is revealed by positive dVp and d(Vp/Vs) anomalies which cease at Sumba island, where negative d(Vp/Vs) anomalies mark the presence of cold Australian lithosphere (down to 200–250 km depth) which is colliding with Sundaland. These negative d(Vp/Vs) anomalies depict a ∼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(Vp/Vs) 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(Vp/Vs) 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.