Exhumation of deep orogenic crust

Abstract

Exhumation of deeply buried material at plate convergence settings brings up fragments of oceanic crust and continental margin, and even larger segments of continental crust that equilibrated at high or ultrahigh pressure (mantle depths). Subduction channels are capable of cycling sizable crustal blocks from downward to upward trajectories, depending on the viscosity and buoyancy of material at the subduction interface and the geometric evolution of the channel. In collision zones, the exhumation of deeply buried continental crust also relies on the nature of the coupling between the subducted and overlying plates (if there is partial melting of subducted crust, hydration of mantle wedge, etc.), as well as the evolution of boundary conditions (e.g., switch to extension or transtension, break-off of mantle slab). Exhumation in mature collisional orogens is dominated by erosion of the thrust wedge at the front of the overthickened continental plateau. For the case of the Himalaya, debate exists on the nature and path of material that has fed the orogenic front and constructed the Greater Himalayan Sequence, particularly in the Miocene. The work by Grujic et al. (2011) adds significantly to this debate and documents mid-Miocene exhumation of rocks that were at the base of the south Tibetan crust in mid-Miocene time. Within only a few million years, these young eclogites were heated to granulite facies and rapidly exhumed along with their partially molten crustal host. This discovery offers new insight on the relative role of thrusting versus crustal fl ow in the construction of orogenic wedges; links mantle heat input, crustal melting, and crustal fl ow with the dynamics of the orogenic front; and provides renewed knowledge of lithospheric evolution in collisional orogens. © 2011 Geological Society of America.