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

Northern Borneo (Sabah) has a complex geological history, having experienced multiple episodes of subduction, magmatism, uplift, subsidence, and extension since the Mesozoic. This includes the subduction of the proto-South China Sea beneath what is now the north-western margin of Sabah, which terminated ~21 Ma; a postulated later phase of northward subduction of the Celebes Sea plate, which terminated ~9 Ma; extension in central Sabah ~9-10Ma; rapid emplacement and exhumation of a granite intrusion ~7Ma, which forms Mt Kinabalu today, and the development of a fold and thrust belt offshore during the last 5 Myr. While these events have all left an imprint in the rock record at the surface, it has not been possible, until recently, to investigate deeper lithospheric processes that have shaped Sabah. However, the installation of 46 broadband seismometers with a ~40 km station spacing as part of the northern Borneo Orogeny Seismic Survey (nBOSS) between 2018 and 2020 means that for the first time it is now possible to constrain the architecture of the crust and uppermost mantle beneath Sabah. Here we present the results of receiver function analysis using two years of passive seismic data recorded by the nBOSS network, and an additional 24 Malaysian Meteorological Service broadband seismometers also located in Sabah. We calculate P-wave receiver functions and use these in a joint inversion with surface wave data to obtain shear velocity models of crustal structure. We find that the crustal thickness in northern Borneo varies between 24 and 60 km. The thickest crust occurs beneath the Crocker Range, while the thinnest crust 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.