DECOMPRESSION OF SUBDUCTED CONTINENTAL CRUST AND PARTIAL MELTING OF OROGENS

The middle crust of active orogens is characterized by a thick layer of partially molten crust (20-40% melt), consistent with the presence of large regions of migmatite in exhumed orogens, but the origin of large-scale crustal melting is unresolved. A deluge of recent studies has proposed that partial melting is driven by delamination of mantle lithosphere beneath thickened crust. The replacement of lithosphere by convecting mantle would melt the deep crust, but this hypothesis is contradicted by seismic-tomography studies of the Tibetan plateau. These studies indicate that, between depths of 100-300 km, the mantle is faster/colder under Tibet than beneath adjacent regions (Tapponnier et al. 2001, Science 294). We propose that melting of continental crust is a direct consequence of continental subduction. Most orogens contain high to ultrahigh-pressure (UHP) rocks, indicating that continental crust is indeed subducted to depths of 50-150 km. Only a small proportion of the large volume of subducted continental crust is exhumed to the surface, forming UHP terrains. Some of the subducted crust melts during ascent and ponds at the base of the orogenic crust to form the observed partially molten layer. In Tibet, a series of south-directed subduction zones allowed the successive growth of the plateau northward during Tertiary time. Continental crust was entrained in these subduction zones to a depth of ~100 km, as is the case of the Tarim crust today. This subducted crust underwent partial melting because, at these depths, continental crust is gravitationally unstable and rises buoyantly, leading to dehydration melting during decompression. Buoyant ascent and decompression melting develop a positive feedback, suggesting that decompression of subducted continental crust, and not mantle delamination, is the dominant melting process in orogens. Material from the partially molten layer may ascend as diapirs if the balance between vertical flow (buoyancy) and lateral flow (thinning/extrusion) allows a diapir to rise to shallow levels without significant cooling. Gneiss domes are migmatite diapirs that survived ascent and were emplaced high enough in the crust that they froze and retained their domal shape.