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

Abstract

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.