Structural Geology, Geochronology and Tectonic Evolution of the Eastern Himalaya: Implications for the Geologic Correlation of the Himalayan Orogen and Indian Craton

The lack of geologic information from the eastern Himalaya has contributed to the great uncertainty in determining the growth mechanism(s) of the Himalayas. To decipher the three-dimensional evolution of the Himalayan orogen, detailed field mapping, U-Pb zircon dating, P-T study, and 40Ar/39Ar thermochronology were performed along two geologic transects across the eastern Himalaya. Dating results indicate that the region has experienced four main episodes of magmatism at 1745-1760 Ma, 825-878 Ma, 480-520 Ma and 28-20 Ma. The first three events also occurred in northeastern India and the youngest event was produced by the Cenozoic Indo-Asian collision. Correlation of magmatic events in conjunction with the similar age ranges of major lithologic units suggests that the eastern Himalaya was developed in situ by basement-involved thrusting in the northern Indian continent. This discovery is inconsistent with the interpretations that high-grade metamorphic rocks in the core of the Himalaya were derived from Tibetan lower crust via channel flow or accretion of an exotic terrane in the early Paleozoic. The Main Central Thrust (MCT) in the eastern Himalaya forms the roof thrust of two thrust duplexes. The northern duplex was initiated at 13-14 Ma, while the southern duplex formed at 10-11 Ma. Duplex development in the MCT footwall caused exhumation of the MCT from ~30 km to <16 km from 10 Ma to 6 Ma. Coeval with duplex development in the MCT footwall, crustal thickening developed in the high-grade metamorphic rocks of the MCT hanging wall and was expressed by ductile thrusting between 12 Ma and 7 Ma. Our interpretations suggest that the timing of initiation for the major contractional structures in the eastern Himalaya was ~10 Ma younger than the equivalent structures in the central Himalaya.