3D mantle upwelling beneath the South China Sea and Southeast Asia: Insights from geodynamic modeling

We conduct geodynamic simulations to investigate 3D mantle upwelling beneath the South China Sea (SCS) and Southeast Asia, focusing on plate motion-driven flow in the upper mantle. We simulate the 3D flows of a 2,900-km-thick mantle layer in a spherical Earth using the spherical harmonic simulation platform of ASPECT. Time-dependent flow in the upper mantle is driven primarily by the surface plate motion as constrained by the latest global plate reconstruction models, as well as subduction-induced mantle return flow. The model assumes relatively high-viscosity at the top 100 km and the lower mantle and phase changes at the lower boundary of the mantle transition zone. Modeling results reveal multiple modes of mantle upwelling of distinctive characteristics: (1) During seafloor spreading in the SCS from 33 Ma to 15 Ma, localized linear zone of mantle upwelling as driven by seafloor spreading is calculated to be limited to the top 100-200 km of the mantle; the center of this linear upwelling zone migrated southward as a result of ridge jump at about 25 Ma. (2) The westward subduction of the Philippine Sea Plate beneath the Eurasian Plate is calculated to induce westward mantle return flow at depth of ~200-670 km, resulting in a broad zone of upwelling centered on the western SCS. (3) A separated zone of mantle upwelling is calculated to exist beneath the northern SCS, which is in general consistent with seismic tomographic observations.