Late Miocene movement within the Himalayan Main Central Thrust shear zone, Sikkim, north‐east India

Abstract

In the Sikkim region of north-east India, the Main Central Thrust (MCT) juxtaposes high-grade gneisses of the Greater Himalayan Crystallines over lower-grade slates, phyllites and schists of the Lesser Himalaya Formation. Inverted metamorphism characterizes rocks that immediately underlie the thrust, and the large-scale South Tibet Detachment System (STDS) bounds the northern side of the Greater Himalayan Crystallines. In situ Th-Pb monazite ages indicate that the MCT shear zone in the Sikkim region was active at c. 22, 14-15 and 12-10 Ma, whereas zircon and monazite ages from a slightly deformed horizon of a High Himalayan leucogranite within the STDS suggest normal slip activity at c. 17 and 14-15 Ma. Although average monazite ages decrease towards structurally lower levels of the MCT shear zone, individual results do not follow a progressive younging pattern. Lesser Himalaya sample KBP1062A records monazite crystallization from 11.5 ± 0.2 to 12.2 ± 0.1 Ma and peak conditions of 610 ± 25°C and 7.5 ± 0.5 kbar, whereas, in the MCT shear zone rock CHG14103, monazite crystallized from 13.8 ± 0.5 to 11.9 ± 0.3 Ma at lower grade conditions of 525 ± 25°C and 6 ± 1 kbar. The P-T-t results indicate that the shear zone experienced a complicated slip history, and have implications for the understanding of mid-crustal extrusion and the role of out-of-sequence thrusts in convergent plate tectonic settings.

Tectonic interleaving along the Main Central Thrust, Sikkim Himalaya

“…This results in the abandonment of the old thrust surface and the development of new thrusts in the footwall that eventually leads to the formation of an imbricate stack (Butler 1982). This suggests that as movement on the Main Central Thrust occurred in the Sikkim Himalaya at c. 22-10 Ma (Catlos et al 2004), deformation migrated down-section from the original isotopic break, interpreted as the location of the original décollement zone of the Main Central Thrust, into the underlying Lesser Himalayan rocks.…”

“…Studies have variously bounded the Main Central Thrust zone with two named thrusts (Catlos et al 2004;Dubey et al 2005;Bhattacharyya & Mitra 2009, placed the Main Central Thrust at the top of the Main Central Thrust zone (Ghosh 1956;Acharyya 1975;Banerjee et al 1983) or placed the Main Central Thrust at the base of the Main Central Thrust zone (Searle & Szulc 2005). Furthermore, the distinctive Palaeoproterozoic Lingtse gneiss, strongly sheared along the Main Central Thrust zone throughout the Sikkim Himalaya, has been used in other studies as a defining lithology for determining the location of the Main Central Thrust (Neogi et al 1998;Chakraborty et al 2003;Dasgupta et al 2004).…”

Channel flow, ductile extrusion and exhumation in continental collision zones: an introduction

“…Other models propose a specific style of thrusting along the base of the GHS as an alternative model to explain both the distribution of metamorphic zones and the timing of metamorphism (e.g. Harrison et al 1998;Catlos et al 2004).…”

“…At the top of the GHS and at the base of the Tethyan sedimentary sequence, a strongly attenuated, rightway-up decrease in metamorphic grade is present. Models for inverted metamorphism include: (1) overthrusting of hot material ('hot iron effect'; Le Fort 1975); (2) imbricate thrusting (Brunel & Kienast 1986;Harrison et al 1997bHarrison et al , 1998Harrison et al , 1999a; (3) folding of isograds (Searle & Rex 1989); (4) transposition of a normally zoned metamorphic sequence due to either localized simple shear along the base of the GHS (Jain & Manickavasagam 1993;Hubbard 1996), heterogeneous simple shear distributed across the Lesser Himalayan sequence and GHS (Grujic et al 1996;Jamieson et al 1996;Searle et al 1999) or general shear of previously foreland-dipping isograds (Vannay & Grasemann 2001); and (5) shear heating (England et al 1992;Harrison et al 1998;Catlos et al 2004). The metamorphic isograds can be deformed passively according to various kinematic models that are compatible with either extrusion or channel flow, or both (e.g.…”

The September 2011 Sikkim Himalaya earthquake Mw 6.9: is it a plane of detachment earthquake?

“…There are a few works on surface geological and geo-chemical studies carried out in this area (Acharya & Sastry 1979;Mohan et al 1989;Catlos et al 2004). However, the geophysical investigation in this region is limited to seismic studies only (Engdhal et al 1998;De & Kayal 2003;Nath et al 2005;Monsalve et al 2006;Tiwari et al 2006;Joshi et al 2010).…”