Miocene infiltration of rainfall in the South Tibetan Detachment (Everest)

What is it about?

The South Tibetan Detachment (STD) is a fundamental structure within the Himalayan orogenic belt that juxtaposes low-grade Tethyan Himalayan Sequence sedimentary rocks over high-grade metamorphic rocks of the Himalayan crystalline core. We document infiltration of D-depleted meteoric fluids ) in the STD mylonitic footwall over a minimum duration of ca. 2 Myr between 17 and 15 Ma where recrystallized hydrous minerals equilibrated with evolved meteoric water and therefore recorded the hydrogen isotope composition of water present during deformation. We determined the origin of fluids and the timing of fluid flow by using a combination of hydrogen isotope (D) and 40Ar/39Ar geochronology measurements performed on hydrous minerals from mylonites collected over 200 m of structural section in the STD footwall at Rongbuk Valley, near Mount Everest. Migration of fluids from the Earth’s surface to the active mylonitic detachment footwall may have been achieved via steep normal faults that developed during syn-convergent extension of the upper Tethyan Himalayan plate, while at the same time high heat flow sustained buoyancy-driven fluid convection. Low D 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 at 15 Ma.

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Why is it important?

The results presented here shed new light on Miocene crustal hydrology in the Himalayan region, and offer new ideas regarding the relationship among crustal deformation, Himalayan topography, and fluid circulation. This research employs a multi-disciplinary approach and is of direct relevance to a wide swathe of the Earth sciences community. It is of considerable interest for geologists interested in the Himalaya-Tibet tectonics. It also informs those who are interested in surface dynamics, geochemistry and hydrogeologic studies.