Exhumation of the Main Central Thrust from Lower Crustal Depths, Eastern Bhutan Himalaya

Geothermometry and mineral assemblages show an increase of temperature structurally upwards across the Main Central Thrust (MCT); however, peak metamorphic pressures are similar across the boundary, and correspond to depths of 35-45 km. Garnet-bearing samples from the uppermost Lesser Himalayan sequence (LHS) yield metamorphic conditions of 650-675 ! C and 9-13 kbar. Staurolite-kyanite schists, about 30 m above the MCT, yield P-T conditions near 650 ! C, 8-10 kbar. Kyanite-bearing migmatites from the Greater Himalayan sequence (GHS) yield pressures of 10-14 kbar at 750-800 ! C. Top-to-the- south shearing is synchronous with, and postdates peak metamorphic mineral growth. Metamorphic monazite from a deformed and metamorphosed Proterozoic gneiss within the upper LHS yield U⁄Pb ages of 20-18 Ma. Staurolite-kyanite schists within the GHS, a few metres above the MCT, yield monazite ages of c. 22 ± 1 Ma. We interpret these ages to reflect that prograde meta- morphism and deformation within the Main Central Thrust Zone (MCTZ) was underway by c. 23 Ma. U⁄Pb crystallization ages of monazite and xenotime in a deformed kyanite-bearing leucogranite and kyanite-garnet migmatites about 2 km above the MCT suggest crystallization of partial melts at 18- 16 Ma. Higher in the hanging wall, south-verging shear bands filled with leucogranite and pegmatite yield U⁄Pb crystallization ages for monazite and xenotime of 14-15 Ma, and a 1-2 km thick leuco- granite sill is 13.4 ± 0.2 Ma. Thus, metamorphism, plutonism and deformation within the GHS con- tinued until at least 13 Ma. P-T conditions at this time are estimated to be 500-600 ! C and near 5 kbar. From these data we infer that the exhumation of the MCT zone from 35 to 45 km to around 18 km, occurred from 18 to 16 to c. 13 Ma, yielding an average exhumation rate of 3-9 mm year )1 . This process of exhumation may reflect the ductile extrusion (by channel flow) of the MCTZ from between the overlying Tibetan Plateau and the underthrusting Indian plate, coupled with rapid erosion.