Lithospheric Structure Near the Northern Xainza‐Dinggye Rift, Tibetan Plateau–Implications for Rheology and Tectonic Dynamics.

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. Plain Language Summary: The Xainza‐Dinggye rift is a north‐south trending fault in the central‐southern Tibetan Plateau. The 3‐D electrical structure is imaged using the magnetotelluric method, which uses natural electromagnetic signals to measure the subsurface electrical resistivity structure and is particularly sensitive to the presence of fluids and partial melts. The resulting model shows resistive regions beneath the Tethys‐Himalaya terrane that represent the subducted Indian plate. Several conductors in the mid‐lower crust may be related to partial melting. With the constraints of the temperature, pressure and water content, estimates of pure melt conductivity were obtained. This analysis is used to evaluate the strength characteristic of the mid‐lower crust. The results show that the formation and scale of the rift‐zone is likely caused by subduction features of the Indian plate (e.g., tearing of the plate) and the deformation characteristic of the mid‐lower crust (e.g., partial melting). Additionally, the rise of hot materials and fluids may have heated the circulating system of underground water and led to the formation of the Xainza‐Dinggye thermal belt and the formation of widespread hot springs. This work provides critical constraints for southward extrusion along the Main Himalaya Thrust and for possible conditions of east‐west fluid migration. Key Points: 3‐D electrical resistivity model used to constrain crustal melt fraction using experimentally derived estimates of pure melt resistivityHeterogeneous distribution of melt zones has implications for the possible east‐west migration of fluid along the Indian Plate below TibetTearing of the plate (East part steeper) and melting of the mid‐lower crust caused congruent rifting, thermal springs, and ore districts [ABSTRACT FROM AUTHOR]