Deep dynamic mechanism of N-S-directed rifts on the Southern Tibetan Plateau and implications for the subducted Indian plate

Abstract

The tectonic-dynamic mechanisms that formed N-S-directed rifts in Southern Tibet remain unknown. Their formation is believed to be related to the subduction pattern of the Indian Plate; however, this pattern remains highly debated. For example, different studies have proposed various ideas about where a slab tear might be located. We created lithospheric-scale 3-D electrical resistivity model along a line that crosses the Xainza-Dinggye rift and Yadong-Gulu rift. The results reveal crustal low-resistivity zones beneath the rifts and high-resistivity zones adjacent to them. The low-resistivity zones are interpreted to indicate a weak middle-lower crust caused by the upwelling of mantle-derived materials, and leading to deformation of the crust, thereby contributing to the development of the rifts. High-resistivity zones between the rifts may be ancient crust in the central Lhasa terrane. In the southern region of the high-resistivity zones it is speculated that, due to break-off of the frontier of the eclogitized Indian lower crust, the Indian lithospheric mantle delaminated from the Indian crust and rolled back, causing steep-angled subduction. The results are interpreted to indicate that, because of the roll-back of the Indian Plate at depth in the regions between the two adjacent rifts, the subduction angle in these regions is steeper than that on both sides. The findings of our study offer an alternative to the traditional view of the subduction pattern of the Indian Plate in southern Tibet, and are in contrast to previous models that suggested smooth and monotonic variations from west to east.