Holocene shortening across the Main Frontal Thrust zone in the eastern Himalaya

Abstract

How plate-boundary processes control intra-continental deformation is a fundamental question in Earth sciences. Although it is long known that the active India–Asia convergence rate increases eastward, how this boundary condition impacts on active growth of the Himalaya is unclear. To address this issue, we conducted a geologic investigation of the Main Frontal Thrust (MFT), the largest and the most dominant active structure in the Himalayan orogen. Using the age and geometry of uplifted river terraces, we establish a minimum Holocene slip rate of 23±6.2mm/yr along the decollement of the 10km wide MFT zone in the far eastern Himalaya. This slip rate is partitioned on three structures: at ∼8.4mm/yr on the Bhalukpong thrust in the north, at ∼10mm/yr across the growing Balipara anticline in the middle, and at ∼5mm/yr on the Nameri thrust in the south. Our estimated minimum total shortening rate is significantly higher than the Holocene slip rate of 9±3mm/yr for the MFT in the western Himalaya. However, this rate is similar to or potentially greater than the slip rate of 21±1.5mm/yr for the MFT in the central Himalaya. Our results support an early interpretation that active shortening across the Himalayan orogen is mostly accommodated by slip along the narrow MFT zone and its linked decollement beneath the Himalaya. The eastward increase in the Holocene MFT slip rate requires that the plate-boundary force rather than gravitational spreading is the fundamental control on active growth of the Himalayan orogen.