Résumé
The knowledge and the understanding of the 3D lithospheric structure of the Himalayas and the Tibetan Plateau are still challenging although numerous geophysical studies have been performed in the region. Indeed, in many cases, the results we obtain differ from one another. This phenomenon can be explained by measurement errors, rough estimates or varying sensibility to physical parameters, among others. Joint inversions have been developed to better constrain geophysical models. Gravity - teleseismic P-wave tomography joint inversion is based on the existence of empiric laws linking velocity and density. The GOCE mission has the ambitious goal of mapping Earth's gravity field with unprecedented precision (i.e. an accuracy of 1-2 mGal for a spatial resolution of 100 km) to observe the lithosphere and upper mantle structure and consequently, to give new insights in the lithospheric structure beneath the Himalayas and the Tibetan Plateau. The GOCE gravity data now allow us to develop a new strategy for gravity-seismic inversion. Combined with teleseismic data over a large region in a joint inversion scheme, they will lead to lithospheric velocity-density models constrained in two complementary ways. We apply this joint inversion scheme to the Hi-CLIMB (Himalayan - Tibetan Continental Lithosphere during Mountain Building) seismological network, which was deployed in South Tibet and the Himalayas during almost three years. The large size of the network, the high quality of the seismic data and the new GOCE gravity dataset allow us to image the entire lithosphere of this active area in an innovative way. We image three-dimensional low velocity and density structures in the lower crust that fit the location of punctual low S-velocity zones revealed by receiver functions in previous geophysical studies. In the deeper parts of our velocity model we image a positive anomaly interpreted to be the heterogeneous Indian lithosphere vertically descending beneath the centre of the Tibetan Plateau.