Defining the Himalayan Main Central Thrust in Nepal

Abstract

An inverted metamorphic field gradient associated with a crustal-scale south-vergent thrust fault, the Main Central Thrust, has been recognized along the Himalaya for over 100 years. A major problem in Himalayan structural geology is that recent workers have mapped the Main Central Thrust within the Greater Himalayan Sequence high-grade metamorphic sequence along several different structural levels. Some workers map the Main Central Thrust as coinciding with a lithological contact, others as coincident with the kyanite isograd, up to 1-3 km structurally up-section into the Tertiary metamorphic sequence, without supporting structural data. Some workers recognize a Main Central Thrust zone of high ductile strain up to 2-3 km thick, bounded by an upper thrust, MCT-2 (¼ Vaikrita thrust), and a lower thrust, MCT-1 (¼ Munsiari thrust). Some workers define an 'upper Lesser Himalaya' thrust sheet that shows similar P-T conditions to the Greater Himalayan Sequence. Others define the Main Central Thrust either on isotopic (Nd, Sr) differences, differences in detrital zircon ages, or as being coincident with a zone of young (,5 Ma) Th-Pb monazite ages. Very few papers incorporate any structural data in justifying the position of the Main Central Thrust. These studies, combined with recent quantitative strain analyses from the Everest and Annapurna Greater Himalayan Sequence, show that a wide region of high strain characterizes most of the Greater Himalayan Sequence with a concentration along the bounding margins of the South Tibetan Detachment along the top, and the Main Central Thrust along the base. We suggest that the Main Central Thrust has to be defined and mapped on strain criteria, not on stratigraphic, lithological, isotopic or geochronological criteria. The most logical place to map the Main Central Thrust is along the high-strain zone that commonly occurs along the base of the ductile shear zone and inverted metamorphic sequence. Above that horizon, all rocks show some degree of Tertiary Himalayan metamorphism, and most of the Greater Himalayan Sequence metamorphic or migmatitic rocks show some degree of pure shear and simple shear ductile strain that occurs throughout the mid-crustal Greater Himalayan Sequence channel. The Main Central Thrust evolved both in time (earlymiddle Miocene) and space from a deep-level ductile shear zone to a shallow brittle thrust fault.

Two comparable earthquakes produced greatly different coseismic landslides: The 2015 Gorkha, Nepal and 2008 Wenchuan, China events

“…1b), listed from south to north in order of increasing age of thrust initiation. The three thrusts divide this region into four tectonic zones: the autochthonous Indian Plate, the Sub-Himalayan zone (SHZ), the Lesser Himalayan sequence (LHS), and the Greater Himalayan sequence (GHS) (Mukherjee, 2015;Bhakuni et al, 2013;Searle et al, 2008;Upreti, 1999;Wesnousky et al, 1999;Le Fort, 1975). All the thrusts are believed to converge at depth into the Main Himalayan thrust (MHT).…”

Crustal–lithospheric structure and continental extrusion of Tibet

“…7. The Himalayan channel flow model after Searle et al (2006Searle et al ( , 2008Searle et al ( , 2010b showing the southward extrusion of a ductile partially molten layer of middle crust. Inset diagrams show the condensed right-way isograds beneath the South Tibetan Detachment and the condensed inverted isograds above the Main Central Thrust.…”

Tectonic interleaving along the Main Central Thrust, Sikkim Himalaya

“…Since that time there has been little agreement on the classification or location of the thrust in many Himalayan sections. A variety of factors have caused controversy over the Main Central Thrust: the divergent criteria used to define the thrust, differences in methods and approach, and variations in appearance of the thrust in the field (Searle et al 2008, and references therein). Different criteria used to define the thrust include the following: (1) lithological changes (Heim & Gansser 1939;Valdiya 1980;Gansser 1983;Pêcher 1989;Davidson et al 1997;Daniel et al 2003;Tobgay et al 2012); (2) high strain in a distinct zone (Stephenson et al 2001;Gupta et al 2010); (3) metamorphic discontinuities (Bordet 1961;Le Fort 1975;Hubbard & Harrison 1989;Stäubli 1989;Harrison et al 1997;Catlos et al 2001;Kohn et al 2001;Daniel et al 2003;Groppo et al 2009;Martin et al 2010); (4) structural criteria (Pêcher 1989;Martin et al 2005;Searle et al 2008); (5) isotopic breaks (Inger & Harris 1993;Parrish & Hodges 1996;Whittington et al 1999;Ahmad et al 2000;Robinson et al 2001;Martin et al 2005Martin et al , 2011Richards et al 2005Richards et al , 2006Ameen et al 2007;Imayama & Arita 2008;Gehrels et al 2011;Long et al 2011b;Tobgay et al 2010;...…”

“…It has been asserted that 'the essential criteria to define a shear zone are the identification of a strain gradient and the clear localisation of strain' (Passchier & Trouw 2005, p. 532;Searle et al, 2008). Although this approach is useful to define the Main Central Thrust in areas where structural criteria are clear-cut, it does not take into account the diffuse nature of the deformation that is associated with the Main Central Thrust in many other transects.…”