A thermomechanical model of exhumation of high pressure (HP) and ultra-high pressure (UHP) metamorphic rocks in Alpine-type collision belts

Using a fully coupled numerical thermomechanical model handling strain localization, surface processes and ultra-high viscosity contrasts (11 orders of magnitude) we test a number of possible mechanisms of High Pressure (HP)-Low Temperature (LT)/High Temperature (HT) exhumation in continental collision zones. The model considers two end-member cases, low or high buoyancy of the downgoing crust. The first model case predicts three levels of exhumation in the same collisional context: the ''classical'' corner flow LP-LT (Low Pressure-Low Temperature) exhumation in the accretionary prism; deeper (70 km) HP-KT exhumation for the thickened subducting crustal-sedimentary wedge, and ultra HP-HT exhumation from the ''lower'' crustal chamber, forming at the depth of 100-120 km and separated from the upper one by a narrow crustal channel. The width of this channel can oscillate in the process of shortening, thus controlling the quantity of the crustal material exchanged between the crustal wedge and the lower crustal chamber. Although both zones of crustal accumulation and the narrow channel between them resemble a vortex-shaped nozzle, this nozzle appears to be too soft to produce any significant overpressures. From the upper crustal wedge, the material is exhumed following the ascending shear flow created by the overriding plate assisted by positive buoyancy of the heated crustal material. From the lower crustal chamber, the material is transported upward to the upper crustal wedge by a flow induced by the asthenospheric traction and a small-scale convective instability forming in the lower crustal chamber due to its heating by the overriding asthenosphere. In the second modelled case of high buoyancy, the latter mechanisms become dominant resulting in hyper fast exhumation of the crust to the surface, accelerated or slowed subduction in case of full or partial crustal decoupling, respectively, and upper plate extension. (C) 2001 Elsevier Science B.V. All rights reserved.