Moho beneath Tibet based on a joint analysis of gravity and seismic data

We use the improved Parker’s and Parker−Oldenburg’s formulas that include a reference depth into the exponential term and employ the Gauss-FFT method to investigate Moho depth beneath the Tibetan Plateau. The synthetic models demonstrate that the improved Parker’s formula has high accuracy with the maximum absolute error less than 2 mGal compared to an analytical solution. Two inversion parameters, such as the reference depth and the density contrast are essential for the Moho estimation based on the gravity field, and they need to be determined in advance to obtain correct results. Therefore, Moho estimates derived from existing seismic studies are used to reduce the non-uniqueness of gravity inversion and to determine these parameters by searching for the maximum correlation between the gravity-inverted and seismic-derived Moho depths. We apply the improved Parker−Oldenburg’s formula to obtain the Moho interface in Tibet and surroundings using the recent Earth gravity model EIGEN-6C4. Another critical issue is to remove beforehand the effects of other factors, which can influence the observed gravity field. In addition to the topography, the gravity effects of the mass anomalies in the sedimentary layer and crystalline crust are removed based on existing crustal models, while the upper mantle impact is determined based on seismic tomography. The inversion results show that the Moho structure under the Tibetan plateau is very complex with the depths varying from about 30 ~ 40 km in the surrounding basins (e.g., Ganges basin, Sichuan basin, and Tarim basin) to 60 ~ 80 km in the plateau. This considerable difference up to 40 km on the Moho depth reveals the substantial uplift and thickening of the crust in the Tibetan Plateau. Furthermore, two visible “Moho depression belts” are observed within the plateau and between them a basin-shaped relatively shallow Moho feature is also obvious. The southern “belt” may be formed in a compressional environment, where the Indian plate underthrusts northwards beneath the Tibetan Plateau, while the northern one may be formed by the southward underthrust of the Asian lithosphere beneath Tibet.