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.