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Abstract

Microstructural and petrological data from >60 samples, collected by L.R. Wager in 1933, have been used alongside existing data to investigate temperature gradients and deformational style in four profiles across the South Tibetan Detachment shear zone, over a north–south distance of 35 km in the Mt Everest area, east-central Himalaya. The ductile shear zone, defined on petrographic criteria, extends for c. 900 m beneath the brittle Qomolangma Detachment (QD). New thermobarometry from the north flank of Mt Everest reveals a gradient from 440°C at the QD down to samples recording peak conditions around 650°C, 5.5 kbar. The upper limit of leucogranite sheets forms an approximately isothermal surface at 600–650°C within the developing shear zone. The recrystallized grain size of quartz shows a systematic increase down-section in four transects. Profiles of deformation temperature reveal gradients of up to 200°C km−1 whose formation and preservation required a combination of processes: a shear zone active for a short period (≤18–15.5 Ma) at high strain rates, with a component of vertical shortening, and a contribution of latent heat from emplacement of sheeted granites. The likely horizontal displacement was >40 km, with up to 10 km of vertical exhumation.

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