Lithosphere tearing and foundering during continental subduction: Insights from Oligocene-Miocene magmatism in southern Tibet

The distribution of Oligo-Miocene magmatic rocks from southern Tibet in space and time yields critical information on the geometry and deformation of the subducted Indian lithosphere which impacts on plateau growth following the India and Eurasia collision. A growing body of geophysical evidence has shown that the subducted Indian lithosphere beneath the Tibetan Plateau has been torn apart. However, the spatiotemporal distribution and cause of the tearing remain enigmatic. Timing of the post-collisional magmatic rocks in southern Tibet exhibits four patterns of decreasing ages; magmatism began earlier in the west and east Himalayan syntaxis and evolved to two age undulations in the central southern Tibet. Seismic images show that regions of slab window (both 90 degrees E and 84 degrees E) and flattened subducted lithosphere (both 86 degrees E and 81 degrees E) are present at depth of 135 km. Correspondingly, increasing mineral crystallization temperatures (absolute value of 50 degrees C) were recorded in the Oligo-Miocene ultrapotassic-potassic rocks at 90 degrees E and 84 degrees E, while opposing trends were shown by coeval ultrapotassic-potassic rocks at 86 degrees E and 81 degrees E. Besides, the melting depth of the Oligo-Miocene ultrapotassic-potassic primitive melts decreases from nearly 100 km to 70 km between 81 degrees E and 90 degrees E, probably indicating progressive rising of the lithosphere-asthenosphere boundary. Such variations were possibly the results of the focused flow and upwelling of asthenosphere, which advanced rapidly but diachronously through weakened and torn sectors within the overlying Indian slab. The upwellings probably induced diachronously upward bending of the residual Indian slab and its flattening, which accelerated the tearing of the Indian lithosphere during continental subduction.