A Re-look at the Himalayan metamorphism

Abstract

Regional metamorphic rocks preserve records of geodynamic evolution of orogenic belts. The Himalaya represents an evolving mountain belt with a complex geological history and is considered here as a composite of Trans-Himalaya and the Himalaya per se and the intervening Indus-Tsangpo Suture Zone. Each of these three tectonic domains is evaluated in the context of their metamorphic evolution. An episodic evolution is inferred and described. The beginning of the Himalayan orogeny is ascribed to the Cretaceous initiation of the suturing of the 'island arc' rocks with the Asian block, followed by underthrusting of the Indian block ~57Ma ago. Differences in metamorphism of underthrust and overthrust blocks are reflected in their P-T-t evolution. The exhumation along the strike length was unequal with significantly higher exhumation rates along the syntaxes. The anatectic melt produced at the time of the Tertiary age Himalayan metamorphism got emplaced within the Himalaya as well as within the Trans-Himalaya, possibly through crustal-scale faults such as the Karakoram fault.

The Indian Subcontinent: Its Tectonics

“…Most of the terrain has elevation from 600 to 4,500 m and is highly dissected by southflowing rivers. Two sedimentary belts characterize the Lesser Himalaya between the Main Boundary Thrust (MBT) and the Munsiari Thrust (MT)/Main Central Thrust (MCT) in Uttarakhand-Nepal: (i) southernmost Neoproterozoic-Early Paleozoic Outer Lesser Himalayan (oLH) sedimentary belt, and (ii) northernmost Paleoproterozoic-Mesoproterozoic Inner Lesser Himalayan (iLH) sedimentary belt (Valdiya, 1980;Jain et al, 2020). These belts are separated by the Lesser Himalayan Crystalline (LHC) nappe.…”

“…The HHC belt is exposed along numerous sections from Zanskar to Arunachal Himalaya where it reveals classic Himalayan Inverted Metamorphism (HIM) (Jain and Manickavasagam, 1993;Kohn, 2014;Pant et al, 2020). This belt is comprised of two litho-tectonic units: (i) a lower Munsiari Group/Lesser Himalayan Crystalline Sequence (LHCS)/Main Central Thrust Zone (MCTZ) of amphibolite, schist and augen mylonite, delimited by the Munsiari Thrust at the base (sensu stricto Main Central Thrust-Heim and Gansser, 1939) and the Vaikrita Thrust at the top, and (ii) the upper main group of High Himalaya Crystalline Sequence (HHCS)/Great Himalayan Sequence (GHS)(also known as the Vaikrita Group in Uttarakhand) is bounded by the MCT (sensu lato)/Vaikrita Thrust at the base and the South Tibetan Detachment System (STDS) near the top (Jain et al, 2014).…”

“…Since then, the MCT has been investigated by various workers along its entire length. Definition of the MCT has undergone numerous changes with time due to difficulties in its recognition (see Searle et al, 2008;Martin, 2017;Carosi et al, 2018;Jain et al, 2020).…”

“…Two distinct MCT are now recognized throughout the Himalaya: the Ramgarh/Munsiari and Vaikrita Thrusts in the NW Himalaya (Valdiya, 1980;Sheshrtha et al, 2015;Mukherjee et al, 2019;Jain et al, 2020), the MCT-1 and the MCT-2 in Nepal (Arita, 1983) Step-ages of biotite between 8.6 and 16 Ma and muscovite between 3.6 and 7.8 Ma reflect samplespecific recrystallization markers, hence these ages were linked to the activity along the Vaikrita Thrust at least from 9 to 6 Ma at c. 600°C and termination by 6 Ma.…”

“…Kohn et al (2010) visualized a Paleoprotezoic arc system and argued that linear belts of magmatic and clastic metasediments developed in a very short duration of about 100 Mya only. Mukherjee et al (2019) andJain et al (2020) opined that granitoids of the Munsiari, Chiplakot, Askot, Ramgarh and basal Almora klippen were generated by subduction-related hydrous partial melting of the Paleoproterozoic mafic source and sediments and is similar to an Andean-type magmatic arc-back-arc system on the Indian northern margin in the Columbia…”