Geochronologic constraints across the Main Central Thrust shear zone, Bhagirathi River (NW India): Implications for Himalayan tectonics

Abstract

The Main Central Thrust shear zone is the dominant crustal thickening feature in the Himalayas, largely responsible for the extreme relief and mass wasting of the range. Along the Bhagirathi River in NW India, the Main Central Thrust is several kilometers thick and separates high-grade gneisses of the Greater Himalayan Crystallines from Lesser Himalayan metasedimentary rocks. Th-Pb ion microprobe ages of monazite dated in rock thin section from the Greater Himalayan Crystallines are Eocene (38.0 ± 0.8 Ma) to Miocene (19.5 ± 0.3 Ma), consistent with the burial of the unit during imbrication of the northern Indian margin and subsequent exhumation due to Main Central Thrust activity, respectively. However, two samples directly beneath the Main Central Thrust yield 4.5 ± 1.1 Ma (T = 540 ± 25 °C and P = 700 ± 180 MPa from coexisting assemblage) and 4.3 ± 0.1 Ma (five grains) matrix monazite ages, suggesting Pliocene reactivation of the structure. Hydrothermal monazites at the base of the Main Central Thrust shear zone record Th-Pb ages of 1.0 ± 0.5 Ma and 0.8 ± 0.2 Ma, the youngest ever reported for the Himalayas. These ages postdate or overlap activity along structures closer to the Indian foreland and show that the zone of Indo-Asia plate convergence did not shift systematically southwestward from the Main Central Thrust toward the foreland during the mountain-building process. Instead, age data support out-of-sequence thrusting and reactivation consistent with critical-taper wedge models of the Himalayas.