The effects of lower crustal strength and preexisting midcrustal shear zones on the formation of continental core complexes and low-angle normal faults

Abstract

To investigate the formation of core complexes and low-angle normal faults, we devise thermo-mechanical simulations on a simplified wedge-like orogenic hinterland that has initial topography, Moho relief, and a preexisting mid-crustal shear zone that can accommodate shear at very low angles (<20°). We mainly vary the strength of the lower crust and the frictional strength of the preexisting mid-crustal shear zone. We find that the strength of the lower crust, and the existence and strength of a preexisting shear zone significantly affect the formation and evolution of core complexes. With increasing lower crustal strength, we recognize varying extensional features with decreasing exhumation rate: these are characterized by bivergent metamorphic massifs, classic Cordilleran metamorphic core complexes, multiple consecutive core complexes (or boudinage structures), and a flexural core complex underlined by a large subsurface low-angle detachment fault with a small convex curvature. Topographic loading and mantle buoyancy forces, together with divergent boundaries, drive a regional lower crustal flow that leads to the exhumation of the lower crust where intensive upper crustal faulting induces strong unloading. The detachment fault is a decoupling zone that accommodates large displacement and accumulates sustained shear strain at very low angle between upper and lower crust. Though the regional stress is largely Andersonian, we find non-Andersonian stress in regions adjacent to the preexisting shear zone and those with high topographic gradient. Our new models provide a view that is generally consistent with geological and geophysical observations on how core complexes form and evolve.