Exhumation of migmatites in two collapsed orogens: Canadian Cordillera and French Variscides

This paper is part of the special publication No.154, Exhumation processes: normal faulting, ductile flow and erosion (eds: U.Ring, M.T. Brandon, G.S. Lister and S.D. Willett). The Shuswap Metamorphic Core Complex in the Canadian Cordillera and the Velay Dome in the French Massif Central are examples of metamorphic core complexes formed in the hinterland of collapsed orogens. They display structural sections which allow the assessment of the efficiency of various exhumation mechanisms at different crustal levels. Erosion affected the relief generated during crustal thickening and redistributed the upper level of the mountain belt into nearby extensional basins or larger foreland basins. Faulting accommodated horizontal extension and lateral sliding of the upper crust. A detachment zone separates the upper brittle crust from ductilely deformed lower crust and achieved mechanical decoupling between these two major crustal layers. Below the detachment zone, the major fabric of high-grade rocks developed in the presence of melt and indicates vertical thinning along with horizontal extension. The lowest structural level exposed consists of migmatites which rose during crustal-scale boudinage of the overlying units and appear in the cores of dome-shaped culminations. The combination of these various exhumation mechanisms within a period of a few million years brought into close proximity migmatites which formed at depth of about 20-25 km and sediments deposited at the surface.