Exhumation of Ultrahigh-Pressure Metamorphic Terranes

Abstract

Most ultrahigh-pressure (UHP) metamorphic rocks are not simply isolated tectonic fragments but belong to nappe units composed of coherent crustal material (basement and cover sequences) that were parts of passive continental margins. Their formation results from subduction of continental crust to mantle depths. Although subduction along convergent plate boundaries is the rule, UHP metamorphic terranes are only found in some collisional orogens. This has bearing on exhumation of UHP slices to the surface. Because they show thin-aspect ratio and occur as ductile deformed nappes, it is common to assume their formation through subduction and exhumation along continental subduction channels. Nevertheless, there are two types of mechanism for exhumation of UHP slices from deep to shallow. One is the thrust of UHP slices to yield asymmetric structures, and the other is the extrusion of UHP domes to produce more symmetric structures. In either case, the extension a necessary condition for exhumation. In addition to the internal force of buoyancy for exhumation of low-density crustal rocks, external forces such as contraction, shear traction or erosion also play a role in facilitating the exhumation of UHP slices. Although the two types of exhumation mechanism often occur in different collisional orogens, many UHP terranes contain more than one UHP slices showing thrusting before extrusion. Temporally, the slice thrust would proceed in the late stage of continental collision for exhumation of some UHP terranes from mantle depths to the shallowest crust along subduction channels, whereas the domical extrusion would operate in the post-collisional stage for exhumation of many UHP terranes from the deepest to shallowest crustal depths.