A thermotectonic evolution for the main central thrust and higher Himalaya, western Garhwal, India.

Subsequent to Lower Eocene (ca. 50Ma) collision of the Indian and Asian plates, continental subduction occurred along the N-dipping Main Central Thrust (MCT) of the Himalaya. In western Garhwal, NW India, upper amphibolite facies Vaikrita Group gneisses of the High Himalayan Slab (HHS) were thrust southwards over unmetamorphosed to greenschist facies Garhwal Group quartzites, carbonates and metabasics of the Lesser Himalaya. In the Bhagirathi valley, the MCT forms a ca. 10km thick shear zone composed of mylonitic Munsiari Group augen gneiss, amphibolite and metasediments. Metamorphic grade increases both northwards and with structural height. The MCT zone is bounded to the N by the Vaikrita roof thrust (VT) and by the Munsiari floor thrust (MT) to the S. The VT is a diffuse high-temperature shear zone recognised through a difference in lithology, metamorphic history, and tectonic style between the Vaikrita and Munsiari Groups. The MT is a relatively discrete fault formed at conditions approaching the brittle-ductile transition. N of the MCT zone, the Jhala Normal Fault (JNF) is a ductile to brittle N-dipping extensional shear zone that was responsible for the downthrow of HHS gneisses and Tethyan sediments in response to gravitational instability of the uplifting orogen. Garnet compositional zoning was produced during growth in both the MCT zone and the lower HHS. In the central and upper HHS it resulted from high-temperature homogenization followed by retrogressive re-equilibration. Diffusion studies suggest rapid cooling of the upper HHS garnets may have been caused by crustal thinning across the JNF. The inverted metamorphic sequence is the cumulative result of polyphase metamoiphism. M1 was a post-collisional Barrovian event of garnet to sillimanite grade restricted to the HHS. M2 was contemporaneous with D2 MCT kinematics and was prograde only in the MCT zone and lower HHS possibly as a result of conductive footwall heating. M3 resulted from nearly isothermal decompression of the upper HHS as a consequence of JNF activation. Thermobarometic transects reveal a significant increase in both P and T across the VT with subsequent decreases accompanying structural height in the HHS. Reliable K-Ar (muscovite) cooling ages from a transect through the MCT zone and HHS are progressively younger towards the S. Ages of ca. 22Ma to ca. 8Ma reflect the piggy-back style deformation sequence; disruptions to the younging sequence are interpreted as localised resetting of ages due to out-of-sequence shearing events. Biotite ages commonly suffered from excess argon and were unreliable. An40Ar/39Ar (hornblende) cooling age suggests rocks of the lower MCT zone were not heated above ca. 500°C since the Precambrian. A ca. 20Ma age dates the last high-temperature motion in the upper MCT zone. The decrease in cooling rate obtained from cooling ages for specific mineral blocking temperatures for the upper MCT zone may be hnked to a return to erosion-controlled denudation after JNF extension