Abstract
AbstractSoutheast 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 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.
Highlights
The current configuration of landmasses which make up Southeast (SE) Asia is an assemblage of Tethyan sutures and Gondwana fragments (Sundaland) combined with island arcs
Despite our broad understanding of SE Asian assemblage, several outstanding questions regarding its tectonic evolution still remain, including where and how collision and subduction interact across the Sunda and Banda arcs; whether tears or holes are present in subducting slabs (Hall & Spakman, 2015), for example, due to the high stress exerted by the rapidly moving Australian plate (Debayle et al, 2005); how and where the Australian plate interacts and sutures with SE Asian lithosphere (Bird, 2003); and if preexisting forearc terranes consisting of various metamorphic/volcanic rocks and Gondwana-related microcontinents influence the formation and evolution of subduction zones along the northern margin of Australia (Hall, 2017; Metcalfe, 2011; Porritt et al, 2016)
We see evidence of short scale length structures that appear to be largely inherited from the dVs model (e.g., Figure 6). This is likely caused by the P wave and S wave models having anomalies of similar magnitude, but since the average P wave velocity is much higher than the average S wave velocity, the latter tends to dominate d(Vp/Vs)
Summary
The current configuration of landmasses which make up Southeast (SE) Asia is an assemblage of Tethyan sutures and Gondwana fragments (Sundaland) combined with island arcs. These structures were intertwined by the thousands of kilometers of oceanic lithosphere subduction that was precipitated by the closure of the Tethys, northward migration of the Australian-Indian plate, and westward convergence of the Philippine Sea plate from the Mesozoic to the Cenozoic (Hall, 2011, 2017; Hall & Spakman, 2015; Metcalfe, 2011). It has been demonstrated that well-constrained estimates of Vp ratio provide additional insight, into
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