MOVING MOUNTAINS: REACTIVATION OF THE HIMALAYAN MAIN CENTRAL THRUST AT 4 MA, BHAGIRATHI RIVER, NW INDIA

The Himalayan Main Central Thrust (MCT) zone is 2-10 km thick, juxtaposing kyanite to sillimanite-grade gneisses (Greater Himalayan Crystallines) over kyanite to zeolite-grade rocks (Lesser Himalayan Sequence). Although the structure accommodated several hundred km of Indo-Asia convergence, it was long thought to have be inactive subsequent to the Early Miocene. Several outstanding questions remain, including the origin of footwall inverted metamorphism and the presence of a prominent topographic break along the MCT zone. In central Nepal, Lesser Himalayan monazites dated by the Th-Pb ion-microprobe method in thin section yield ages as young as ~3 Ma, suggesting the MCT zone continued activity into the Pliocene. To explore the existence of this event along strike of the Himalaya, samples were collected along the Bhagirathi River, NW India. Sample BR14 is from the Greater Himalaya-Lesser Himalaya contact, whereas BR43A is from the Munsiari Thrust, the lowermost fault of the MCT zone. BR14 matrix monazites yield an age of 4.5±1.1 Ma (n=8, MSWD=0.8), whereas ages of two matrix monazites from BR43A are 1.0±0.5 Ma and 0.8±0.2 Ma. Rocks between BR14 and BR43A yield ages form 23.5±0.7 Ma to 2.6±0.7 Ma, indicating a complicated sequence of deformation and/or fluid-flow events. BR14 contains euhedral garnets with inclusion patterns that extend into the matrix, suggesting coeval growth with monazite, whereas BR43A monazites are associated with chalcopyrite in a vein, indicating a hydrothermal origin. The ~4 Ma age is further supported by the presence of 4.1±0.1 Ma to 4.8±0.2 Ma monazites south of BR14, and a single 3.4±0.7 Ma monazite grain we dated from the Sutlej River further west. These results indicate that modeling the Himalayas as a region where plate convergence shifts solely progressively and temporally towards the foreland is an oversimplification. Contraction may progress at the regional scale towards the foreland, but the hinterland continues to internally thicken.