Extension rates, crustal melting, and core complex dynamics

Two-dimensional thermomechanical experiments reveal that the crystallization versus exhumation histories of migmatite cores in metamorphic core complexes give insights into the driving far-field extensional strain rates. At high strain rates, migmatite cores crystallize and cool along a hot geothermal gradient (35–65 °C km−1) after the bulk of their exhumation. At low strain rates, migmatite cores crystallize at higher pressure before the bulk of their exhumation, which is accommodated by solid-state deformation along a cooler geothermal gradient (20–35 °C km−1). In the cases of boundary-driven extension, space is provided for the domes, and therefore the buoyancy of migmatite cores contributes little to the dynamics of metamorphic core complexes. The presence of melt favors heterogeneous bulk pure shear of the dome, as opposed to bulk simple shear, which dominates in melt-absent experiments. The position of migmatite cores in their domes reveals the initial dip direction of detachment faults. The migmatitic Shuswap core complex (British Columbia, Canada) and the Ruby–East Humboldt Range (Nevada, United States) possibly exemplify metamorphic core complexes driven by faster and slower extension, respectively.