The P-T-t-D evolution of the Mahabharat, east-central Nepal: The out-of-sequence development of the Himalaya

A garnet-bearing schist from the southernmost such exposure along the Himalaya in east central Nepal records prograde metamorphism at 32.4 ​± ​0.3 ​Ma. Phase equilibria modelling, combined with Ti-in-biotite and quartz c-axis thermometry, outline a tight-to-hairpin pressure-temperature (P-T) path extending from ~515 ​°C and 5.5 ​kbar to peak conditions at ~575 ​°C and 7 ​kbar followed by deformation during the retrograde phase at 480–515 ​°C and 6–7 ​kbar. The new geochronology data place an upper bound on the evolution of metamorphism and deformation in the frontal-most part of the Himalaya, which lasted until 17.5 ​Ma, as indicated by previously published ⁴⁰Ar/³⁹Ar data. The P-T-time data from this part of the Himalaya, as well as that from more hinterland-ward portions of the orogen, outline a progressive, stepwise, commonly out-of-sequence evolution. Further data from along the orogen indicates that this evolution is not a local phenomenon, but instead characterizes the tectonics of this system as a whole.