FLOW OF DEEPEST OROGENIC CRUST: PRINCIPLES UNDERLYING THE EXHUMATION OF HIGH-PRESSURE AND ULTRAHIGH-PRESSURE ROCKS

The deep orogenic crust is characteristically weak (high T, low viscosity) and is therefore able to flow in response to gradients in gravitational potential energy. 2D and 3D numerical experiments show that the deep crust can flow laterally over large distances (100s of km) and move upward into accommodation space that is created by mid- to shallow crust extension. The combination of lateral and vertical flow allows the orogenic crust to thin while maintaining a flat Moho, and to exhume the deep crust in metamorphic core complexes, a process that over time stabilizes orogens. The main physical principle uncovered by experiments is that the deepest/weakest orogenic crust, located just above the Moho, flows most efficiently and is able to traverse the entire crustal column to ultimately occupy the shallowest levels of exhumed crust. In essence, this process results in partial overturning of the crust, with deepest rocks being exhumed to shallowest levels. Predicted P-T-t paths for the deep crust include isothermal decompression (from ~1.5 to <0.5 GPa) and then rapid cooling from >700-800°C to <200°C over timescales of <10 million years. In nature, felsic migmatites crystallize at relatively low P (~0.5 GPa) and are therefore poor recorders of original, pre-exhumation depths. However, inclusions (pods, layers) of high-P granulite or eclogite occur in migmatite terranes as prime recorders of a deep crustal, near-Moho origin. For example, eclogites have been documented in the Variscan Montagne Noire migmatite dome, French Massif Central. The same principle applies to the metamorphic history of the Caledonian orogen, where continental subduction drove continental rocks to mantle depths (>100 km). In the exhumed ultrahigh-P (UHP) terrane in Norway, pods of eclogite (>3.0 GPa) are dispersed in felsic migmatites that record much lower P (<1.0 GPa). Extension of the Caledonian orogen triggered exhumation of the lowest viscosity rocks, containing inclusions of UHP eclogite, within the subducted continental crust. The same partial overturning of crust described above for metamorphic core complexes operated, placing UHP eclogite at shallow crustal levels directly underneath Devonian sedimentary basins that were deposited in this extensional-transtensional system.