Metamorphic core complexes; windows into the mechanics and rheology of the crust

Metamorphic core complexes are products of normal-fault displacements sufficient to exhume rocks from below the brittle-ductile transition. These faults (detachments) may initiate within the brittle crust at steep angles, but they sole into the ductile middle crust, and during displacement rotate to gentler dips due to hanging-wall extension. The exhumed footwall commonly adopts an arched or domed geometry owing to flexural isostatic readjustment, and may be overlain by strongly extended upper crustal rocks that slipped on gently dipping, low-friction shallow segments of the detachment. Metamorphic rocks exhumed beneath the detachment record progressively increasing flow stress, strain localization and strain-rate with decreasing temperature, providing a window into physical conditions and deformational processes in the mid-crust. The metamorphic and deformational history of the footwall rocks may reflect tectonic processes that predate formation of the detachment fault, in addition to those accompanying exhumation. These processes may include diapiric emplacement of gneiss domes, or exhumation in a subduction channel, and may not be directly related to formation of the core complex. Factors favouring core complex formation are high gravitational potential energy of the extending crust, weak rheology and a change in the tectonic boundary conditions such as a cessation or slowing of plate convergence.