BUILDING THE PAMIRS: THE VIEW FROM THE UNDERSIDE

Abstract

The Pamir mountains are an outstanding example of extreme crustal shortening during continental collision that may have been accommodated by formation of a thick crust-much thicker than is currently thought-and/or by continental subduction. We present new petrologic data and radiometric ages from xenoliths in Miocene volcanic rocks in the southeastern Pamir mountains that suggest that Gondwanan igneous and sedimentary assemblages were underthrust northward, buried to >50-80 km during the early stage of the India-Asia collision, and then heated and partly melted during subsequent thermal relaxation before finally being blasted to the surface. These xenoliths, the deepest crustal samples recovered from under any active collisional belt, provide direct evidence for early Cenozoic thickening of the Pamirs and lower-crustal melting during collision; the xenoliths also suggest that the present mountain range was a steady-state elevated plateau for most of the Cenozoic.