Publications

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

Chakraborty, S. and Mukul, M. and Mathew, G. and Pande, K.

TECTONOPHYSICS
Volume: 770 Pages:
DOI: 10.1016/j.tecto.2019.228183
Published: 2019

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 ∼1 km to ∼5.6 km along the transport direction. New 40Ar/39Ar mica ages, and apatite fission track dates from this study, when integrated with published U[sbnd]Th[sbnd]Pb monazite, and Lu[sbnd]Hf 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 ∼27 Ma and ∼12 Ma, 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 ∼37 Ma to ∼28 Ma. Thrusting on the CTT exhumed the middle GHS, and underthrusted the lower GHS between ∼27 Ma and ∼22.5 Ma. The MCT exhumed the lower GHS between ∼22.5 Ma and ∼12.1 Ma, and underthrusted the LHCS, which was later exhumed by the MT between ∼12 Ma and ∼10 Ma. The thrust sheets exhumed to near-surface conditions between ∼2.6 and ∼0.5 Ma, during growth of the underlying Lesser Himalayan Duplex. The MCT showed protracted activity till ∼7.2 Ma, and leading edge of the CTT was later reactivated at ∼13.5 Ma, and at ∼1.8 Ma. The CTT is equivalent to the High Himalayan Discontinuity, and forms part of a regional structure that, spans ∼800 km arc-length of the Himalaya from Sikkim to west-central Nepal. © 2019 Elsevier B.V.

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