Major shear zone within the Greater Himalayan Sequence and sequential evolution of the metamorphic core in Sikkim, India

Abstract

The Greater Himalayan Sequence (GHS), bounded by the South Tibetan detachment system, and the Main Central thrust (MCT), constitutes majority of the Himalayan Metamorphic Core. Recent studies identified several shear zones within the GHS, which played significant role in its evolution. Here, we report the presence of a major shear zone, the Chungthang-Thambi thrust (CTT), which is situated within the GHS in Sikkim, eastern India. The CTT exhibits “zone-type” geometry, with mylonite core flanked by protomylonite zones, and its thickness varies from ∼1km to ∼5.6km along the transport direction. New 40Ar/39Ar mica ages, and apatite fission track dates from this study, when integrated with published UThPb monazite, and LuHf garnet ages indicate that the CTT, the MCT, and the Munsiari thrust (MT), at base of the crystalline Lesser Himalayan Sequence (LHCS), developed in-sequence between ∼27Ma and ∼12Ma, and controlled the sequential evolution of the metamorphic core (GHS+LHCS) in Sikkim. Our imbricate model suggests that the upper-middle GHS was initially underthrusted below the Tethyan unit, and underwent prograde metamorphism from ∼37Ma to ∼28Ma. Thrusting on the CTT exhumed the middle GHS, and underthrusted the lower GHS between ∼27Ma and ∼22.5Ma. The MCT exhumed the lower GHS between ∼22.5Ma and ∼12.1Ma, and underthrusted the LHCS, which was later exhumed by the MT between ∼12Ma and ∼10Ma. The thrust sheets exhumed to near-surface conditions between ∼2.6 and ∼0.5Ma, during growth of the underlying Lesser Himalayan Duplex. The MCT showed protracted activity till ∼7.2Ma, and leading edge of the CTT was later reactivated at ∼13.5Ma, and at ∼1.8Ma. The CTT is equivalent to the High Himalayan Discontinuity, and forms part of a regional structure that, spans ∼800km arc-length of the Himalaya from Sikkim to west-central Nepal.