Dynamic models for metamorphic core complex formation and scaling: The role of unchannelized collapse of thickened continental crust

Metamorphic core complexes at collisions between cratons and softer terranes, such as in the northern North American Cordillera, share a set of characteristic features including spatial and temporal association of ductile mid-crustal deformation with brittle normal faulting, spatial coincidence with prior crustal thickening, characteristic spatial scaling and limited duration and extent of deformation. These properties are reproduced in numerical solutions for gravity-driven collapse of a viscous crustal region under conditions where vertical stress is continuous through thickened lithosphere (rigid, deformable conditions). Such solutions allow inversion for effective mechanical properties and crustal geometry from direct observations of aspect ratio and exhumation velocity; in the northern Rockies, core complex geometry is consistent with a twofold decrease in viscosity of the thickened Cordilleran crustal welt.