A growing set of data indicates a stark contrast between the evolution of two types of ultrahigh-pressure (UHP) terranes: large terranes that evolved slowly (over 10–30 Myr), and small terranes that formed and were exhumed on timescales of \u3c 10 Myr. Here we compare the characteristics – area, thickness, formation rate, exhumation rate, age, and tectonic setting – of these two endmember types of UHP terrane worldwide. We suggest that the two UHP terrane types may form during different orogenic stages because of variations in the buoyancy and traction forces due to different proportions of subducting crust and mantle lithosphere or to different rates of subduction. The initial stages of continent collision involve the subduction of thin continental crust or microcontinents, and thus tectonic forces are dominated by the density of the oceanic slab; subduction rates are rapid and subduction angles are initially steep. However, as collision matures, thicker and larger pieces of continental material are subducted, and the positive buoyancy of the down-going slab becomes more prominent; subduction angles become gentle and convergence slows. Assessing the validity of this hypothesis is critical to understanding the physical and chemical evolution of Earth\u27s crust and mantle.
Included here is the post-print copy of this article. The final publication is available via ScienceDirect at http://www.sciencedirect.com/science/article/pii/S0012821X1100756
