How Does the Mid-crust Accommodate Deformation in Large, Hot Collisional Orogens? Insight from the Himalaya-Tibet System

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Hot, weak crust is a central component of new hypotheses about the evolution of continent-continent collisions, and in particular may play an important role in accommodating the greater than 3000 kilometers of convergence within the Himalaya-Tibetan collision over the last 50 million years. Models that implicate flow of semi-viscous midcrustal rocks south towards the front of the Himalayan orogen, 'channel flow', are able to account for many geologic observations in the Himalaya. However, alternative models of collision, particularly 'thrust-wedge taper', demonstrate that the observed geology could have formed in the absence of a low-viscosity midcrustal layer. Whether channel flow and thrust-wedge taper models are mutually exclusive or whether they represent a continuum of deformation in time and/or space within a single collisional system is of crucial importance for explaining the evolution of the Himalayan orogen and, by extension, for understanding the evolution of Earth?s many continent-continent collision zones. A key difference in these contrasting models is the location, nature and relationships of thrust faults within the Himalayan foreland; specifically, the spatial and kinematic association of the Main Central Thrust and the Ramargh Thrust. In the channel flow model of the Himalaya, exhumed midcrustal rocks are predicted to act as a pervasively-deformed tectonic package bounded at its base by a single major fault; the Ramgarh and Main Central thrust are synthetic and coeval components of that diffuse structure. In contrast, thrust-wedge taper models interpret the Main Central thrust and Ramgarh thrust to be kinematically distinct, and formed in sequence within a foreland propagating fold and thrust belt. These predictions are being tested by analyzing samples at three key sites across the central to eastern Himalaya via a combination of structural analysis, uranium-thorium-lead accessory phase petrochronology coupled to thermobarometry, detrital thermochronology and bulk rock neodymium isotope geochemistry. These data provide essential information about the timing of the Main Central Thrust and the Ramgarh Thrust, the timing of deformation and metamorphism in the midcrust, and source characteristics of the rocks that give clues as to their pre-Himalaya origins. These data also yield insight into the evolution in time and space of the transition between deep crustal hinterland-style deformation recorded within the core and the shallow foreland-style deformation recorded near the front of the Himalaya. This project supports both graduate and undergraduate students, providing them with hands-on research experience in both field investigation methods and a variety of analytical facilities, particularly taking advantage of state-of-the-art high-resolution and high-precision electron- and ion-beam instrumentation at the University of California, Santa Barbara. The project is also supporting the research efforts of two early career researchers, and is contributing to the broadening of participation of underrepresented groups in the earth sciences. This project is part of an international scientific collaboration, bringing together researchers and students from the University of California, Santa Barbara, Canada, Nepal and India.