LARGE-SCALE UNDERTHRUSTING AT THE WESTERN END OF THE INDO-ASIAN COLLISION ZONE: EVIDENCE FROM THE EASTERN MARGIN OF THE PAMIR

Within the Pamir salient at the western end of the Indo-Asian collision zone, Cenozoic metamorphic rocks are exposed in the fault bounded cores of two antiformal domes in the Central Pamir (the Muskol and Sares antiforms) and the footwall of the Kongur Shan normal fault (Robinson et al., 2004; Schwab et al., 2004). Where dated, these basement rocks record late Oligocene to late Miocene amphibolite facies metamorphism, synchronous with Miocene igneous activity in the Central Pamir. Field observations and previous studies show a lack of Cenozoic shortening in the upper crust of the Northern Pamir, suggesting that crustal thickening associated with Cenozoic metamorphism was largely driven by underthrusting of crustal material. Additionally, footwall rotation along the Kongur Shan normal fault indicates the eastern boundary of the Cenozoic metamorphic rocks, the steeply east-dipping right-lateral Ghez shear zone, has been rotated from an originally sub-horizontal orientation. The shear sense along the sub-horizontal Ghez shear zone would therefore record northward translation of the footwall relative to the hanging wall. We propose a model in which mid- to late-Cenozoic crustal thickening in the Pamir was primarily driven by northward underthrusting and internal thickening of the mid- to lower-crust along a sub-horizontal decollement. Thickening and heating in the hinterland of the underthrust sheet (beneath the Central and Southern Pamir) lead to partial melting in the Miocene. The increase in buoyancy and decrease in strength of the middle and lower crust resulted in extrusion of the Central Pamir gneiss domes, synchronous with continued underthrusting beneath the Northern Pamir and pervasive Miocene igneous activity in the Central Pamir.