Geochronologic and thermobarometric constraints on the evolution of the Main Central Thrust, central Nepal Himalaya

Abstract

The Main Central Thrust (MCT) juxtaposes the high‐grade Greater Himalayan Crystallines over the lower‐grade Lesser Himalaya Formation; an apparent inverted metamorphic sequence characterizes the shear zone that underlies the thrust. Garnet‐bearing assemblages sampled along the Marysandi River and Darondi Khola in the Annapurna region of central Nepal show striking differences in garnet zoning of Mn, Ca, Mg, and Fe above and below the MCT. Thermobarometry of MCT footwall rocks yields apparent inverted temperature and pressure gradients of ∼18°C km−1 and ∼0.06 km MPa−1, respectively. Pressure‐temperature (P‐T) paths calculated for upper Lesser Himalaya samples that preserve prograde compositions show evidence of decompression during heating, whereas garnets from the structurally lower sequences grew during an increase in both pressure and temperature. In situ (i.e., analyzed in thin section) ion microprobe ages of monazites from rocks immediately beneath the Greater Himalayan Crystallines yield ages from 18 to 22 Ma, whereas late Miocene and Pliocene monazite ages characterize rocks within the apparent inverted metamorphic sequence. A Lesser Himalayan sample collected near the garnet isograd along the Marysandi River transect contains 3.3±0.1 Ma monazite ages (P ≈ 0.72 GPa, T ≈ 535°C). This remarkably young age suggests that this portion of the MCT shear zone accommodated a minimum of ∼30 km of slip over the last 3 Ma (i.e., a slip rate of >10 mm yr−1) and thus could account for nearly half of the convergence across the Himalaya in this period. The distribution of ages and P‐T histories reported here are consistent with a thermokinematic model in which the inverted metamorphic sequences underlying the MCT formed by the transposition of right‐way‐up metamorphic sequences during late Miocene‐Pliocene shearing.