Pressure, temperature and time constraints on metamorphism across the Main Central Thrust zone and High Himalayan Slab in the Garhwal Himalaya

Abstract

Following the early Eocene collision of India and Asia, continental subduction occurred on the northward-dipping Main Central Thrust (MCT). In western Garhwal, N. India, upper amphibolite-facies gneisses on the High Himalayan Slab are thrust southwards over unmetamorphosed to greenschist facies quartzites, carbonates and metabasics of the Lesser Himalaya. In the Bhagirathi valley, the MCT forms a c. 10 km thick shear zone composed of mylonitic augen gneiss, amphibolite and metasediments. Metamorphic grade increases both northwards and with structural height. The MCT zone is bounded to the north by the Vaikrita (roof) Thrust and to the south by the Munsiari (floor) Thrust. The Vaikrita Thrust is a diffuse high-temperature shear zone, whereas the Munsiari Thrust is a relatively discrete fault formed under brittle-ductile conditions. North of the MCT zone, at the top of the High Himalayan Slab a northward-dipping extensional shear zone, the Jhala normal fault, was responsible for the downthrow of the Tethyan sediments to the north with respect to the uplifting High Himalayan Slab gneisses to the south. Thermobarometic transects reveal a sudden increase in both pressure and temperature across the Vaikrita Thrust from south to north but with subsequent decreases accompanying structural height in the High Himalayan Slab. Temperatures increase going up-structural section from about 500° C to 770° C across the MCT zone, but then decrease again to the north varying between about 550 and 640° C. Similarly, pressures increase sharply up-structural section across the MCT zone from 6 to 12 kbar, then decrease towards the top of the slab to between 7 and 8.9 kbar. The inverted P-T gradient across the MCT zone changes to approximately isothermal and isobaric conditions in the top 9 km (horizontal distance) of the High Himalayan slab. Cooling rates for the upper MCT zone determined from 40 Ar/ 39 Ar (hornblende) and K-Ar (muscovite and biotite) cooling ages suggest a return to erosion-controlled denudation following extension at the top of the High Himalayan Slab. Additional K-Ar (muscovite) cooling ages from a transect through the MCT zone and High Himalayan Slab are progressively younger towards the south, reflecting the southward propagation of the deformation sequence with time. Hornblende 40 Ar/ 39 Ar cooling ages from the MCT zone suggest that structurally lower rocks have not been heated above c . 500° C since the Precambrian, whilst a 19.8 ± 2.6 Ma hornblende age from the MCT zone dates the latest high-temperature shearing at higher structural levels in the MCT zone and places a minimum age constraint on Himalayan metamorphism in the Garhwal sector of the Himalaya.

A stream sediment geochemical survey of the Ganga River headwaters in the Garhwal Himalaya

“…Recent workers propose that the MCT is a wide zone of high strain ductile shear corresponding to the entire thickness of Munsiari/Baijnath/Almora crystalline rocks, associated with inverted metamorphism from south to north with increasing structural height rather than a single thrust plane as perceived earlier (Valdiya et al, 1999, Metcalfe, 1993. We also prefer to refer to this as the Main Central Thrust Zone (MCTZ) for simplicity.…”

“…The sedimentary rocks (PMS) are metamorphosed to chlorite grade, whereas the PH demonstrate biotite grade of metamorphism evidenced by augen-gneisses and some biotite-bearing metapelite rocks. In contrast, the MCTZ rocks show low to medium garnet-staurolite grade of metamorphism and goes locally in pockets up to kyanite grade to the northern most contact near Vaikrita Thrust (Metcalfe, 1993). There is a southward younging of thrust sheets (Metcalfe, 1993;Oliver et al, 1995) starting from Vaikrita (HHC) through Munsiari (MCTZ) to Ramgarh (PH).…”

“…Each is bounded generally by northwest-southeast trending regional tectonic contacts with a gentle northerly dip that is locally variable. These are, from north to south: 1) the Main Central Thrust Zone (MCTZ, Metcalfe, 1993) that includes Valdiya's Munsiari, Baijnath, and Almora groups; Fig. 2) the Porphyroids (PH) that include Valdiya's Debguru formation of Ramgarh Group; and 3) a Proterozoic metasedimentary sequence (PMS) that includes Valdiya's Gangolihat, Rautgara, and Berinag formations (also referred to as Garhwal Group).…”

The thermodynamics of Himalayan orogenesis

“…Detailed structural studies show that the earliest shearing fabrics related to the MCT were synmetamorphic with respect to Neohimalayan metamorphism in the Everest (Hubbard 1988;Hubbard & Harrison 1989), Langtang , Annapurna Hodges et al 1996;Coleman 1996), and Garhwal (Hodges & Silverberg 1988;Metcalfe 1993) regions of Nepal and India (Fig. 1).…”

“…Evidence for such P-T-t paths is very common in the Himalaya (e.g. Hodges et al 1988Hodges et al , 1993Metcalfe 1993;Pognante & Benna 1993;Vannay & Hodges 1996;Winslow et al 1995;Zeitler et al 1993), indicating fast denudation during the late stages of Neohimalayan metamorphism. While very rapid physical erosion may have occurred in some regions, it seems likely that STD movement played an important role in unroofing the Greater Himalayan sequence in most areas, because normal faulting is a more efficient mode of denudation than physical erosion.…”