Trans-Himadri Fault: Tectonics of a Detachment System in Central Sector of Himalaya, India

First recognized as the Malari Thrust Fault in northern Kumaun Himalaya in early seventies, as the South Ebetan Detachnzent System in southern Tibet adjoining northeastern Nepal in early eighties and as the Zanskar Shear Zone in northwestern Himachal Pradesh in Iate eighties, the fault between the high-grade metamorphic and granitic rocks of the Himadri (E Great Himalaya) and the Tethyan sedimentary pile, ranks in importance among the terranedefining boundary thrusts like the Main Central Thrust (MCT) and the Main Boundary Thrust (MBT). Not only is there an abrupt change in the metamorphic grade across the tectonic plane, but also an attenuation and wholesafe elimination of some lithostratigraphic formations of the hanging wall. Moreover, the strongly Iineated metamorphic rocks of the basement complex in the footwall, characterized by isoclinal reclined folds, give way in the hanging wall to the Tethyan sedimentary rocks lacking in similar deformation and metamorphism. Later movements have caused shearing and brecciation of the basement rocks.

Exhibiting predominant dip-slip movement in the central sector of the Himalayan arc, the Trans-Himadri Fizult (T-HF) was formed as a consequence of the Tethyan sedimentary cover detaching from its rigid foundation of the basement complex, following blocking or slowing down on India-Asia collision zone of tectonic movements related to India-Asia convergence. As the basement complex was squeezed up to great height during a phase of compression, the sedimentary cover on the over-steepened northern slope of the Himadri slid down, toppling over northward and giving rise to back-folds and back-thrusts. In the compressive regime, there was strike-slip movement and attendant extension parallel to the orogen in eastern and western sectors of the Himalaya. In Kashmir, western Himachal Pradesh, central Nepal and northwestern Bhutan the compression was so strong that the Tethyan sedimentary rocks along with a slice of the basement in the hanging wall advanccd southwards across the Himadri and were emplaccd as nappes and klippen even south of the Main Central Thrust.

The T-HF movement occurred in the period 23.5 and 19.5 Ma, say approximately around 20.9 Ma. Quaternary reactivation resulted in river ponding and development of huge lakes such as the Garbyang palaeolake in the Kali valley and the - 40,000 year old Goting palaeolake in the Western Qhauli valley.