Amy Gilligan, David Cornwell , Nicholas Rawlinson, Felix Tongkul, Simone Pilia, Tim Greenfield, Conor Bacon

Northern Borneo (Sabah) has a complex geological history, experiencing multiple episodes of subduction, magmatism, uplift, subsidence, and extension since the Mesozoic. This includes the subduction of the proto-South China Sea beneath Sabah, terminating ~21 Ma; a postulated later phase of subduction of the Celebes Sea plate, terminating ~9 Ma; extension in central Sabah ~9-10Ma; rapid emplacement and exhumation of a granite intrusion ~7Ma, and the development of a fold-thrust belt offshore during the last 5 Myr. While these events have left imprints in the surface rock record, it has not been possible, until recently, to investigate deeper lithospheric processes that have shaped Sabah. The installation of 46 broadband seismometers - the northern Borneo Orogeny Seismic Survey (nBOSS) - between 2018 and 2020 means that it is now possible to constrain the architecture of the crust and uppermost mantle beneath Sabah. We use two years of passive seismic data recorded by the nBOSS network, and an additional 24 Malaysian Meteorological Service broadband seismometers in Sabah to calculate  P-wave receiver functions. We then use these in a joint inversion with surface wave data to obtain shear velocity models of crustal structure. The thickest crust (60km) occurs beneath the Crocker Range, while the thinnest crust (24km) is found in central Sabah, potentially recording Miocene extension. The crust beneath the 4095m high Mt Kinabalu is also comparatively thin. Distinct, low-velocity, dipping anomalies identified in our shear wave velocity models provide clear evidence for underthrusting of Dangerous Grounds continental crust following subduction and collision.