Steep subduction of the Indian continental mantle lithosphere beneath the eastern Himalaya revealed by gravity anomalies

The geometry and deformation of the Indian continental mantle lithosphere (ICML) beneath the India-Eurasia collision zone are critical to understanding the accommodation of continent-continent convergence. In this paper, the distribution of residual gravity anomalies in the upper mantle of southern Tibet is estimated using the gravity data and seismic velocity models, and the heterogeneous density distribution of the upper-mantle is then recovered through three-dimensional gravity inversion. The results reveal a low-density anomaly (similar to 300 km W-E and similar to 100 km N-S) in the upper mantle under the eastern Himalaya, while there is no obvious density anomaly under the western Himalaya. The western boundary of the low-density anomaly coincides with the Yadong-Gulu Rift (YGR) on the surface (89 degrees-90 degrees E), and its southern boundary is located at similar to 28 degrees N, approximately 130 km southward from the Indus-Yarlung suture, probably representing the mantle suture at depth. This observation indicates that, in contrast to the western ICML which is probably underthrusting at a shallow angle, the eastern ICML be likely subducting steeply, accompanying asthenosphere upwelling. Such a laterally varying geometry suggests that a major tearing of the ICML may have taken place from the intersection of the mantle suture and the YGR in the upper mantle. The tearing and the steep subduction of the ICML might be associated with the magmatic and mineralization events in the eastern Himalaya-Gangdese and the formation of the YGR.