Reconstructing the proto-South China Sea and proto-Philippine Sea: insights from mantle structure and geodynamic models

Abstract:

Oceanic crust of early Cenozoic to mid-Cretaceous aged is largely missing within SE Asia and presumed to have been lost by subduction, driven in part by the opening of SE Asian marginal seas during the Cenozoic. As a result, strongly contrasted SE Asian plate tectonic reconstructions have been proposed. Here we examine the tectonic history of the regions currently occupied by the South China Sea and Philippine Sea, which are two of the largest marginal seas on Earth. Published paleomagnetism suggests the Philippine Sea is far-travelled, and was located near the equator during the Eocene. Therefore, it is possible that some other plate(s) occupied its present position, and we call these the ‘proto-Phliippine Sea’. Likewise, prior to South China Sea spreading, a proto-South China Sea plate or plates may have existed in the present South China Sea location. Although the proto-Philippine Sea and proto-South China Sea are now disappeared from the Earth’s surface, we search for their subducted lithospheric remnants from mantle tomography.

Here we document the SE Asian mantle structure from published tomography and compare to the results of recent TERRA global geodynamic models that implemented diverse SE Asian plate reconstructions. Within the Philippine Sea region, we find the closest match between predicted and imaged mantle structure in geodynamic models that implemented a smaller Philippine Sea plate with a short ~1000 km restored northern slab (i.e. Ryukyu slab). The small Philippine Sea plate consumed a now-vanished marginal sea south of the Eurasian margin during its northward motion towards Eurasia. In contrast, models that implemented very large Philippine Sea plates (>3000 km Ryukyu slab) were less favorable. Within the South China Sea region, outward, double-sided subduction of a proto-South China Sea produced the most favorable mantle structure, and in particular, reproduced some of the flat-lying slabs within the mantle transition under the South China Sea. However, pure southward proto-South China Sea subduction as shown in traditional plate models was also viable.