Significance of strain localization in the lower crust for structural evolution and thermal history of metamorphic core complexes

Numerical simulations demonstrate that the dynamic behavior of the lower crust can exert a crucial control on the tectonic denudation of metamorphic core complexes. Our model suggests that the structural architecture and thermal history of terrains exhumed by extensional shear zones can be controlled by whether or not localization of deformation occurs in the lower crust. Predicted architectures range from midcrustal gneiss and granite domes to upper crustal core complexes with brittle deformation in both the upper and lower plates. We simulate continental crust in extension, with a strong, viscous/frictional-plastic upper crust underlain by a weak, viscoplastic lower crust. Strain is accommodated by plastic shear zones in the upper crust, while viscous flow and plasticity compete in the lower crust. We explore a number of scenarios by varying the viscosity and yield limit of the lower crust. Deformation patterns and thermal architecture change significantly, depending on whether or not localization occurs in the lower crust. For a relatively high viscosity lower crust with a low yield stress limit, deformation is dominated by localization along discrete plastic shear zones. Along these shear zones, increasingly deeper material is exhumed, creating an asymmetrically denuded lower plate with metamorphic grade increasing in the direction of upper plate displacement. In relatively weak, low-viscosity lower crust, however, the yield limit may never be reached, and strain in the lower crust in this case is accommodated by distributed flow. The denuded lower plate in the distributed flow case resembles a dome-shaped structure, bounded by opposite facing shear zones.