Evidence of Subduction, Collision, and Extension in Northern Borneo: Constraints From Receiver Functions

What happens when subduction stops is a key, but poorly understood, part of the tectonic cycle. Northern Borneo (Sabah) with a complex geological history of multiple episodes of subduction, magmatism, uplift, subsidence, and extension since the Mesozoic, is an ideal location for studying post-subduction processes. Major events in this region include subduction of the proto-South China Sea beneath Sabah, terminating ∼21 Ma; postulated subduction of the Celebes Sea plate, terminating ∼9 Ma; extension in central Sabah ∼9–10 Ma; rapid emplacement and exhumation of a granite intrusion ∼7 Ma; 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. The installation of 46 broadband seismometers–the northern Borneo Orogeny Seismic Survey (nBOSS)–between 2018 and 2020 means we can now 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 24 Malaysian Meteorological Service broadband seismometers to calculate P-wave receiver functions. We then jointly invert these with surface wave data to obtain shear velocity models of crustal structure. The thickest crust (60 km) occurs beneath the Crocker Range, while the thinnest crust (24 km) is found in central Sabah, potentially recording Miocene extension. The crust beneath Mt Kinabalu (4,095 m) is also comparatively thin. Distinct, low-velocity, dipping anomalies identified in models provide clear evidence for under thrusting of Dangerous Grounds continental crust following subduction and collision.