Crustal Model of the Indo-Burma Region based on Seismically Constrained 3-D Gravity Inversion

Indo-Burma ranges mark the boundary between the Indian and Eurasian plates. Whether the subduction in this region is still active is highly debated due to the oblique convergence of the Indian with the Eurasian plates. In the present study, non-linear 3-D gravity inversion of residual gravity anomalies was performed to model crustal structure and Moho geometry of the subduction zone. To compute the residual gravity anomalies, the free-air gravity anomalies from the GOCE satellite mission are corrected for the effect of both upper mantle density heterogeneities compiled from the seismic tomography model and sediments considering the exponential density-depth relation based on the seismic refraction. The residual gravity anomalies show highly positive values (150-320 mGal) over the Bay of Bengal and moderately positive values (50-150 mGal) over the central Myanmar basin (CMB), whereas relatively low gravity values (-150 to -20 mGal) over the Indo-Burma Range (IBR) and Shan plateau. A tesseroid-based gravity inversion was performed in spherical coordinates to obtain the 3-D Moho geometry of the Indo-Burma region. The inversion result depicted a shallow Moho (~20 km) below the Bengal Basin, whereas a deeper Moho (~52 km) beneath the northern Indo-Burma range. The central Myanmar basin shows a moderate Moho at a depth of ~33 km, which deepens in the east after the Sagaing fault and reaches ~42 km beneath the Shan plateau. The estimated Moho depth correlates well with the receiver function data (Std ~5.7 km) and the Crust1.0 model (Std 6.9 km). It is observed that the dip of the Moho decreases towards the south, and it becomes flat after 20°N latitude, which suggests the absence of subducting slabs. These observations are also consistent with the 3-D seismic tomography model available in this region.