Geochronology and thermal history of the Mount Everest massif

New in situ apatite U–Pb and mica Rb–Sr and 40 Ar/ 39 Ar geochronology outline a break in geochronology within the Mount Everest massif at an elevation of c . 8000 m. Above the break, most chronometers record dates from the Eocene epoch, whereas below, most dates are from the Middle Miocene epoch. Raman spectroscopy of carbonaceous material and Ti-in-biotite-based thermometry results outline a break in temperatures at the same elevation. From the summit downwards, temperatures increase to a maximum of c . 500°C at c . 8500 m before decreasing to c . 400°C at c . 7900 m. The coinciding breaks in the independent datasets are interpreted to reflect movement along a previously unrecognized structure, here termed the Adrishya thrust that was active c. 18 Ma. The Adrishya thrust may reflect re-equilibration of the evolving orogenic wedge, perhaps further influenced by cooling and strain localization structurally away from the exhuming orogenic core.