The Xainza-Dinggye rift, an approximately north-south trending Cenozoic fault zone across the Lhasa Terrane, is an ideal location to investigate extensional mechanisms in the upper crust and lithospheric deformation caused by the subducting Indian Plate beneath the central-southern Tibetan Plateau. The 3-D electrical resistivity structure was obtained by modeling magnetotelluric data from an array across the rift zone. Using the temperature distribution throughout the crust combined with the pressure and water content, we compute the pure melt conductivity. This enables estimation of the partial melt fraction of large-area conductive zones imaged throughout the crust, and thus allows their rheology and strength to be evaluated. The heterogeneous distribution of high melt fraction areas in the crust has implications for the local continuous migration of fluids in the east-west direction. The electrical structure also reveals a dipping resistive zone beneath the Tethys-Himalaya terrane that represents the subducted Indian Plate. Significantly, its depth is observed to vary substantially from east (deeper) to west (shallower), which may indicate tearing of the plate. We suggest that the cause of extension and the formation of the crustal rift zone is related to tearing of the plate directly below this location, and the subsequent partial melting of the mid-lower crust. Furthermore, the ascent of deep hydrothermal fluids, and heating of the shallow subsurface, likely led to the formation of the congruent Xainza-Dinggye Thermal Belt. Additionally, the emplacement of numerous adjacent magmatic-hydrothermal ore deposits is also likely related to partial melting and hydrothermal fluid migration in the crust.