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. ... Read more

Since the Cenozoic, a series of extensional south-north normal faults and gneiss-granite domes evolved in the southern Tibet Plateau, the formation mechanism of which is of scientific interest and which has implications for the tectonic dynamics of the plateau. Typical of such features are the Xainza-Dinggye rift and the Mabja gneiss dome, which are located in the Xainza-Xietongmen-Dinggye region in central Tibet. In this study, Magnetotelluric measurements across this region are used to generate a high-resolution 3-D electrical resistivity model of the subsurface and to analyze the cause of the conductive zones. The large-scale conductive zones identified in the middle-lower crust may result from aqueous melt partial melting, whereas the smaller-scale conductive zones in the upper-middle crust may result from saline fluids, possibly with varying minor volumes of melts. Subsequently, based on the electrical resistivity model, and combined with the spatiotemporal coupling of the geological, geochemical and geophysical data, the state and migration features of crustal materials are discussed. The results show that the upwelling of mantle materials along subduction channels and slab-windows related to the tearing of the Indian lithospheric plate contributed to the partial melting of the middle-lower crust in the Lhasa terrane. Furthermore, partial melting of the upper-middle crust in the Tethys-Himalaya terrane resulted from southern extrusion of crustal materials in the Lhasa terrane. These two mechanisms can significantly reduce the effective viscosity. We speculate that the deformation of the brittle upper crust that is controlled by large-scale ductile layers characterized by weak rheology is the main dynamic mechanism of rift evolution. Meanwhile, the metamorphism and anatexis in the upper-middle crust of the Tethys-Himalaya terrane related to the southern extrusion of materials contributed to the evolution of the Mabja gneiss dome. During the middle Miocene, the southern extrusion of crustal materials may have been influenced by the cooling events beneath the Mabja gneiss dome, which can explain why the deep areas beneath the Mabja gneiss dome have middle-high resistivity. In addition, our study region is located in the Mediterranean-Himalayan seismic belt, and mainly includes shallow-focus earthquakes and intermediate-depth earthquakes. In the north, shallow-focus earthquakes are mainly controlled by the accumulation of stress in the brittle layer of the overlying crust related to the ductile layer of the middle and lower crust. In the south, shallow-focus earthquakes (e.g., Dingri Ms 6.8 earthquake) mainly occur in the rigid, resistive block, which is surrounded by conductive zones, possibly because fluid migration may be hindered by these resistive blocks. The intermediate-depth earthquakes are mainly related to the detachment of the Indian lithospheric mantle from the Indian crust. ... Read more