Seismic Structure and Tectonic Evolution of Borneo and Sulawesi

Abstract:

Southeast Asia (SEA) is one of the most active tectonic regions on Earth due to its convergent setting, which accommodates rapid northward motion of the Indo-Australian plate and westward motion of the Philippines Sea plate. Borneo is located at the centre of SEA, on the leading edge of the Sundaland block of the Eurasian plate, and exhibits lower rates of seismicity when compared to the surrounding regions due to its intraplate setting. Sulawesi, an island which lies adjacent to Borneo in the east, is characterised by intense seismicity due to multiple subduction zones in its vicinity. The tectonic relationship between the two islands is poorly understood, including the provenance of their respective lithospheres, which may have Eurasian and/or East Gondwana origin.

Here, we showcase receiver function (RF) results from temporary and permanent broadband seismic stations in the region, which can be used to help improve our understanding of the crust and mantle lithosphere beneath Borneo and Sulawesi. H-K stacking, receiver function migration and inversion are all applied in an effort to determine robust crustal thickness estimates and variations in shear wave speed with depth. Our preliminary results indicate that the crust beneath Sabah (northern Borneo), which is a post-subduction setting, appears to be much more complex and is overall thicker (more than 35 km) than the rest of the island. Furthermore, we find that crustal thickness varies between different tectonic blocks defined from surface mapping, with the thinnest crust (23 to 25 km) occurring beneath Sarawak in the west-northwest as well as in the east.

Results from Virtual Deep Seismic Sounding (VDSS) in Sabah, where the tectonic setting is known to be complicated, are also presented. Here, the RF analysis could not reliably determine the Moho depth for stations located in the southeast, whereas VDSS is able to yield well constrained results.