Abstract

Despite being the largest active collisional orogen on Earth, the growth mechanism of the Himalaya remains uncertain. Current debate has focused on the role of dynamic interaction between tectonics and climate and mass exchanges between the Himalayan and Tibetan crust during Cenozoic India-Asia collision. A major uncertainty in the debate comes from the lack of geologic information on the eastern segment of the Himalayas from 91°E to 97°E, which makes up about one-quarter of the mountain belt. To address this issue, we conducted detailed field mapping, U-Pb zircon age dating, and 40Ar/39Ar thermochronology along two geologic traverses at longitudes of 92°E and 94°E across the eastern Himalaya. Our dating indicates the region experienced magmatic events at 1745–1760 Ma, 825–878 Ma, 480–520 Ma, and 28–20 Ma. The first three events also occurred in the northeastern Indian craton, while the last is unique to the Himalaya. Correlation of magmatic events and age-equivalent lithologic units suggests that the eastern segment of the Himalaya was constructed in situ by basement-involved thrusting, which is inconsistent with the hypothesis of high-grade Himalaya rocks derived from Tibet via channel flow. The Main Central thrust in the eastern Himalaya forms the roof of a major thrust duplex; its northern part was initiated at ca. 13 Ma, while the southern part was initiated at ca. 10 Ma, as indicated by 40Ar/39Ar thermochronometry. Crustal thickening of the Main Central thrust hanging wall was expressed by discrete ductile thrusting between 12 Ma and 7 Ma, overlapping in time with motion on the Main Central thrust below. Restoration of two possible geologic cross sections from one of our geologic traverses, where one assumes the existence of pre-Cenozoic deformation below the Himalaya and the other assumes flat-lying strata prior to the India-Asia collision, leads to estimated shortening of 775 km (∼76% strain) and 515 km (∼70% strain), respectively. We favor the presence of significant basement topography below the eastern Himalaya based on projections of early Paleozoic structures from the Shillong Plateau (i.e., the Central Shillong thrust) located ∼50 km south of our study area. Since northeastern India and possibly the eastern Himalaya both experienced early Paleozoic contraction, the estimated shortening from this study may have resulted from a combined effect of early Paleozoic and Cenozoic deformation.

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