Seismic-constrained gravity inversion of Moho depth beneath Sabah, Malaysia: regional and tectonic implications

The geology of Sabah, the eastern Malaysian state in northern Borneo, is complex due to its location in a region of tectonic convergence between Asia, Australia and Pacific plates during the Cenozoic. Current tectonic models suggest that Sabah underwent crustal thickening in a double-subduction system - Oligocene–Early Miocene south-eastward subduction of the Proto-South China Sea beneath NW Sabah and Middle–Late Miocene north-westward subduction of Celebes Sea beneath the Sulu Arc in SE Sabah. In addition, Middle–Late Miocene crustal extension due to back-arc rifting in the Sulu Sea partly shaped the relatively subdued topography of the central-northeast coast of Sabah and the Kinabatangan River basin. Critical to understanding the complex geology of Sabah is its crustal structure. Previous gravity studies indicate a thickened crust beneath the Crocker Range and adjacent mountainous regions of southern Sabah. More recently, based on receiver function analysis and virtual deep seismic soundings (VDSS) the Moho depth beneath Sabah was determined to be between 21 and 55 km. In this study, Moho depths are estimated by gravity inversion of newly compiled gravity data and calibrated against seismic data derived from VDSS. For input to the gravity inversion, Mantle Bouguer Anomaly (MBA) was derived from free-air anomalies by applying Bouguer and mantle corrections based on topography from SRTM+ digital elevation model. A modified Parker-Oldenberg algorithm was used to invert the Moho depth from the MBA. The gravity inversion requires a mean Moho depth (zo) and a density contrast at the Moho (ρ_c) to be set at run-time. Two models of crustal density were investigated. In Model 1, using a uniform crustal density as input, different combinations of zo and ρ_c were tested. The highest correlation between the gravity-derived Moho and the seismic Moho (as calibration points) was attained when the ρ_c is unrealistically high (>3000 kg m⁻³) and, therefore, considered unlikely. In Model 2, we constrained the inversion by applying a laterally-varying crustal density model derived from the VDSS data. With this model, the Moho depth was estimated between 32.8 and 52.5 km, with a mean Moho depth of 40.8 km. Our analysis indicates that Sabah is not in complete isostatic equilibrium, and parts of the region appear to be highly overcompensated. In particular, across central Sabah a highly thickened crust indicated from gravity contradicts with seismic evidence for a thinner crust. This apparent disparity in Moho depths is the consequence of ignoring two density-related factors in the gravity inversion: (1) the presence of a deep sedimentary basin beneath the upper Kinabatangan River basin, and (2) the thermal effect of a cooling lithosphere beneath the basin, representing the onshore continuation of the SE Sulu Sea rift system. By correcting the input gravity for these effects, the difference between the gravity and seismic Moho is significantly reduced. The results suggest that the Kinabatangan River basin region may be dynamically supported by a cooling lithosphere associated with the rifting of the SE Sulu Sea and is still subsiding at the present-day.