Dome Formation During Crustal Extension in the Himalaya: Kinematic and Pressure-Temperature-Time-Deformation Constraints on Extensional Exhumation Along the Southern Margin of the Tibetan Plateau

The Himalaya and Tibetan Plateau were built by a combination of south-directed thrusting, north-directed extension, and generally east-west-directed extension within the Himalaya and Tibetan Plateau all to accommodate convergence between the Indian and Eurasian plates that began in the Eocene. Normal-sense shear zones that accommodate roughly east-west-directed extension across the southern margin of the Himalaya have exhumed young metamorphic domes across the Himalayan front. These metamorphic domes contain high-grade metamorphic rocks bound by normal-sense shear zones. The purpose of this study is to evaluate how these normal-sense shear zones develop and how they exhume metamorphic domes, which remains poorly understood and is critical for understanding the processes that accommodate extensional exhumation in this convergent setting. Two examples of metamorphic domes that were exhumed by east-west-directed normal-sense shear zones in the Himalaya are the Ama Drime Massif, southern Tibet, and the Leo Pargil dome, northwest India-Tibet. The Ama Drime Massif is a 30 km-wide north-south-striking structure that thins toward the north. It is located ~50 km northeast of Mount Everest and is bound by the Nyönno Ri detachment on the eastern flank and the Ama Drime detachment on the western flank. The Leo Pargil dome, ~950 km west of Ama Drime, is a 20 km-wide, northeast-southwest-striking structure composed of high-grade metamorphic rocks and leucogranite. It is bound on the east by the Qusum detachment and on the west by the Leo Pargil shear zone. Field mapping and sample collection were combined with kinematic, microstructural, thermobarometric, and geochronologic methods to constrain the metamorphic conditions, the kinematics of deformation during shearing, the amount of exhumation, and the timing of metamorphism and shear zone initiation. These data demonstrate the exhumation on these normal-sense shear zones in the Himalaya are controlled by an interplay between various processes including a regional kinematic setting that favored extension which led to strain partitioning, fault reactivation, decompression-driven melting, and the development of these deeply-rooted extensional systems.