Thermal Evolution of the Pamir Deep Crust Constrained using Lu-Hf and U-Pb Geochronology, and Garnet Thermometry

English Abstract:

The large hot orogen exposed in the Pamir-Karakorum-Himalaya is a focal point of research into the links between active plate dynamics, magmatism, and tectonics. Extensive research accomplishment during the past three decades has allowed detailed reconstruction of Miocene exhumation, uplift and magmatism. Earlier orogenic processes, however, are still largely obscured, owing to an apparent scarcity of well-preserved assemblages and the general difficulty in resolving prograde histories from these. Recent research efforts demonstrated the widespread occurrence of Cenozoic deep-crustal rocks in the Pamir plateau. In this study, we investigated the timing and cause of prograde metamorphism of these rocks using Lu-Hf geochronology, U-Pb rutile thermochronology, and garnet thermometry. Regional prograde metamorphism and heating to 750-830 °C, as constrained by Zr-in-rutile and diffusion thermometry, occurred between 37-27 Ma. Prograde growth of garnet first occurred in the South Pamir at c. 37 Ma and spread to the Central Pamir during the following 10 Myr. By c. 27 Ma, the deep crust had heated beyond c. 750 °C and was partially molten on a regional scale. We attribute this thermal evolution to enhanced mantle heat flow following the c. 45-Ma slab break-off of Indian lithosphere underneath the Pamir. The history uncovered here confirms a long-lived thermal history of the Pamir deep crust and advocates a causal link between break-off, enhanced mantle heat flow, and heating of the deep crust along margins of large hot orogens.