Imbrication and Erosional Tectonics Recorded by Garnets in the Sikkim Himalayas

Abstract

The Sikkim region of the Himalayas (NE India) may form an important microplate between Nepal and Bhutan. Here we report high-resolution pressure-temperature (P-T) paths taken from garnet-bearing rocks across the northern and eastern portion of the region’s Main Central Thrust (MCT) shear zone. The MCT separates units affiliated with the Greater Himalayan Crystallines (GHC) in its hanging wall from the Lesser Himalayan Formation (LHF). Late Miocene monazite ages are reported from the LHF (10–14 Ma), whereas those from the GHC are Miocene (18–20 Ma). Some paths from the LHF and GHC show a P decrease before burial, consistent with erosion before compression. MCT shear zone and GHC rocks show a P increase and then decrease over a short T interval. This hairpin P-T path is consistent with an imbrication model for the Himalayas. LHF P-T path conditions and those obtained using conventional thermobarometry are best in agreement. These paths also are consistent with observed mineral assemblages and garnet zoning. Although we have the most confidence in LHF results, MCT shear zone and GHC P-T path shapes suggest processes to establish imbrication tectonics may have occurred here as early as the Miocene.