The region of northern Borneo in South East Asia sits within a post-subduction setting formed by the recent termination of two sequential but opposed subduction systems. In this study we use seismic data from a recent temporary array deployment to image the crustal velocity structure beneath northern Borneo using a two-stage Bayesian trans-dimensional tomography scheme, in which period dependent phase velocity maps are first generated, and then used to build a 3-D shear wave model through a series of 1-D inversions. In the second stage, we also apply an Artificial Neural Network to solve the 1D inverse problem, which results in a smoother 3-D model compared to the TransD approach without sacrificing data fit. Our shear wave velocity model reveals a complex crustal structure. Under the Crocker Range, a heterogeneous velocity structure likely represents remnants of early Miocene subduction, including underthrust continental crust from subsequent continent-continent collision. In the east we observe high velocities that are interpreted to be igneous rocks in the crust generated by melting due to mid Miocene Celebes Sea subduction and later decompression melting as well as a low velocity zone that could represent underthrust sediment or duplexes from Celebes Sea subduction. A low velocity zone in the lower crust is present in a region of apparent crustal thinning. Our preferred explanation for this anomaly is remnant thermal upwelling within a failed rift that represents the on-shore continuation of the extension of the Sulu Sea, most likely caused by rollback of the Celebes Sea slab.

Studies attempting to gain new insights into the last stage of the subduction cycle are typically challenged by limited direct observations owing to a lack of recent post-subduction settings around the world. Central to unravelling how the subduction cycle ends is an understanding of crust and mantle processes that take place after subduction termination. Northern Borneo (Malaysia) represents a unique natural laboratory because it has been the site of two sequential subduction episodes of opposite polarity since the mid-Paleogene. The region exhibits several enigmatic post-subduction (after ~10 Ma) features, including: subsidence followed by rapid uplift, localised intraplate volcanism, possible orogen collapse, and a pluton that emerged to become the third highest peak in southeast Asia, Mt Kinabalu (4095 m). Arrival-time residuals from distant earthquake data recorded by the nBOSS seismic network have been used to investigate P- and S-wavespeed variations in the crust and underlying upper mantle beneath northern Borneo. Our 3-D tomographic images consistently show a high-velocity perturbation in western Sabah that we associate with an upper-mantle remnant of the Proto South-China Sea slab, thus providing important constraints for tectonic reconstructions of SE Asia. The tomographic models, combined with other seismological and geological information, reveal evidence for lithospheric removal in eastern Sabah via a drip instability. Our results suggest that lithospheric drips can be smaller than previously thought, yet their effects on the post-subduction evolution of continental lithosphere can be significant.

The post-subduction tectonic evolution of northern Borneo, which experienced two sequential subduction episodes of opposite polarity in the Neogene, is still widely debated with first-order questions such as whether the region has been in a state of compression or extension remaining unresolved. We use waveform data recorded from a dense seismic network in northern Borneo to investigate crustal thickness variations through the application of Virtual Deep Seismic Sounding (VDSS). The new results reveal an extensive area of thin crust in central and southeastern Sabah that appears to extend northeast into the Sulu Sea, where rifting initiated. We also compute local earthquake focal mechanisms, which suggest that extension is ongoing, though now dominated by orogen collapse in the NW. Together these results point to the pervasiveness of regional extension tectonics over the last 15-20 Myr and its role in the post-subduction cycle of plate tectonics in SE Asia.