Higher(Greater) Himalayas The Higher Himalayas are also known as the Central Crystalline zone, comprised of ductily deformed metamorphic rocks and mark the axis of orogenic uplift. Mica schist, quartzite, paragneiss, migmatite, and leucogranite bodies characterize this uppermost Himalayan zone. They represent a multiphase deformation event, the first being Barrovian type, or normal geothermal gradient conditions. There was then a shift to Buchan-type metamorphism, low pressure and high temperature conditions, with temperatures greatly exceeding normal gradient temperatures(Sorkhabi 1999). Local retrograde events have also been noted. Analyses show that peak orogenic temperatures and pressures were 475-825 degrees Celsius and 500-800 megapascals. Corresponding minerals assemblages are dominated by biotite to sillmanite, representing greenschist to amphibolite facies deformation. Deformation seems to have occured in a north to south direction and is associated with the Main Central Thrust Fault (MCT), which brings the higher Himalayas on top of the lower Himalayas (Sorkhabi 1999). Initially, it was thought that approximately 350km of shortening had occured in the Greater Himalayan sequence of rocks. However, through studies by DeCelles etal. (1998), a major thrust fault within the zone was discovered. As a result, it is now estimated that between 600 and 650km of shortening occured here. There was also a question of provenance for Great Himalayan rocks. Previous work suggested that lower Indian crust comprised this area. New interpretations of rocks there indicate that the higher Himalayas are actually made of supercrustal rock. This idea states that upper crustal material of India accreted northward onto the Asian continent and that crustal material was origanlly an appendage of India that was, itself, accreted to India during Paleozoic time. This study implies that India probably had significantly more continental crust than previously thought, much more crust to be shortened in the formation of the Greater Himalayas.
Posted on: Thu, 28 Nov 2013 10:45:13 +0000
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