The South Tibetan Detachment (STD) in the Himalayan orogen juxtaposes low-grade Tethyan Himalayan sequence sedimentary rocks over high-grade metamorphic rocks of the Himalayan crystalline core. We document infiltration of meteoric fluids into the STD footwall at ~17–15 Ma, when recrystallized hydrous minerals equilibrated with low-δD (meteoric) water. Synkinematic biotite collected over 200m of structural section in the STD mylonitic footwall (Rongbuk Valley, near Mount Everest) record high-temperature isotopic exchange with D-depleted water (δDwater = –150 ± 5‰) that infiltrated the ductile segment of the detachment most likely during mylonitic deformation, although later isotopic exchange cannot be definitively excluded. These minerals also reveal a uniform pattern of middle Miocene (15 Ma)40Ar/39Ar plateau ages. The presence of low-δD meteoric water in the STD mylonitic footwall is further supported by hornblende and chlorite with very low δD values of –183‰ and –162‰, respectively. The δD values in the STD footwall suggest that surface-derived fluids were channeled down to the brittle-ductile transition. Migration of fluids from the Earth’s surface to the active mylonitic detachment footwall may have been achieved by fluid flow along steep normal faults that developed during synconvergent extension of the upper Tethyan Himalayan plate. High heat flow helped sustain buoyancy-driven fluid convection over the timescale of detachment tectonics. Low δD values in synkinematic fluids are indicative of precipitation-derived fluids sourced at high elevation and document that the ground surface above this section of the STD had already attained similar-to-modern topographic elevations in the middle Miocene.
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