EXRUSION PROCESS OF THE GREATER HIMALAYAN CRYSTALLINE COMPLEX EVIDENCED BY STRUCTURAL GEOLOGY AND METAMORPHISM

The India-Asia collision formed the huge Himalayan orogen. The greater Himalayan crystalline complex (GHC) is the core of this orogen and its extrusion process is the key for understanding the orogeny of the Himalayas. A modified model for this extrusion is proposed based on the structural, metamorphic and geochronological studies mainly along the Nyalam geo-transect from Nepal to Tibet. In this model, the Himalayan orogen had been thickened since the collision to Oligocene. This thickening caused the regional progressive metamorphism represented by the metamorphic zones of biotite – garnet – Kyanite – sillmanite – Cordieritesillmanite. Another important result of the thickening is the anatexis of the thickened crust. At this time, the south Tibetan detachment system (STDS) was a thrust. Since early Oligocene, the Himalayan orogen entered a period of thermal relaxation, with partially melten mid-lower crust of the Himalayas extruded laterally on the topographic front. The extrusion resulted in the exhumation of the high grade GHC and produced the deformed leucogranites of ~35Ma-18Ma in the GHC. One of most outstanding results of the GHC extrusion is the formation of an overturned antiform of metamorphic isotherm in the GHC. Also during the relaxation, the STDS became a top-to-north sliding zone and the extensional shearing thinned the upper limb of the thermal antiform. The extrusion and exhumation made the GHC became to be situated in a higher level and the younger anatexis beneath it caused the intrusion of un-deformed leucogranites of ~14Ma in the GHC. There is no MCT related to the extrusion of the GHC, because there is neither tectonic discontinuity nor abrupt change of P-T at the base of the GHC. The Himalayas seem to have experienced a complete orogenic cycle until the exhumation of the GHC. However, this orogen experienced thickening again after ~13Ma, and we can attribute this thickening as an intra-continental process. This process caused the piggyback thrusting of MBT, MFT and another more south thrust. This southward thickening resulted in the remelting of the newly thickened crust south to GHC, which produced the leucogranites intruding the LHS.