Late Miocene movement within the Himalayan Main Central Thrust shear zone, Sikkim, north-east India

In the Sikkim region of north-east India, the Main Central Thrust (MCT) juxtaposes high-grade gneisses of the Greater Himalayan Crystallines over lower-grade slates, phyllites and schists of the Lesser Himalaya Formation. Inverted metamorphism characterizes rocks that immediately underlie the thrust, and the large-scale South Tibet Detachment System (STDS) bounds the northern side of the Greater Himalayan Crystallines. In situ Th-Pb monazite ages indicate that the MCT shear zone in the Sikkim region was active at c. 22, 14-15 and 12-10 Ma, whereas zircon and monazite ages from a slightly deformed horizon of a High Himalayan leucogranite within the STDS suggest normal slip activity at c. 17 and 14-15 Ma. Although average monazite ages decrease towards structurally lower levels of the MCT shear zone, individual results do not follow a progressive younging pattern. Lesser Himalaya sample KBP1062A records monazite crystallization from 11.5 +/- 0.2 to 12.2 +/- 0.1 Ma and peak conditions of 610 +/- 25 degreesC and 7.5 +/- 0.5 kbar, whereas, in the MCT shear zone rock CHG14103, monazite crystallized from 13.8 +/- 0.5 to 11.9 +/- 0.3 Ma at lower grade conditions of 525 +/- 25 degreesC and 6 +/- 1 kbar. The P-T-t results indicate that the shear zone experienced a complicated slip history, and have implications for the understanding of mid-crustal extrusion and the role of out-of-sequence thrusts in convergent plate tectonic settings.