Thermo-Mechanical Modeling of Continental Extension: the Transition From Core Complexes to Rigid Block Faulting

Extension of overthickened continental crust is commonly characterized by an early "core complex" stage of extension followed by a later stage of crustal-scale rigid block faulting. These two stages are clearly recognized during the extensional destruction of the Alpine orogen in northeast Corsica, where rigid block faulting overprinting core complex formation eventually led to crustal separation and the formation of a new oceanic back-arc basin (the Ligurian Sea). Here we investigate the geodynamic evolution of continental extension by using a novel, fully coupled thermomechanical numerical model of the continental crust. We consider that the dynamic evolution is governed by fault weakening, which is generated by the evolution of the natural-state variables (i.e., pressure, deviatoric stress, temperature, and strain rate) and their associated energy fluxes. Our results show the appearance of a detachment layer that controls the initial separation of the brittle crust on characteristic listric faults, and a core complex formation that is exhuming strongly deformed rocks of the detachment zone and relatively undeformed crustal cores. This process is followed by a transitional period, characterized by an apparent tectonic quiescence, in which deformation is not localized and energy stored in the upper crust is transferred downward and causes self-organized mobilization of the lower crust. Eventually, the entire crust ruptures on major crosscutting faults, shifting the tectonic regime from core complex formation to wholesale rigid block faulting. Our model shows that extension develops from an early core complex mode of extension to a rigid block mode of extension. The earlier extensional phase involves high-strain ductile deformation along a detachment contemporaneously with the exhumation of core complexes. This deformational stage is followed by an apparent lull prior to the initiation of crustal-scale rigid block faulting. It should be noted that at shallow crustal levels, tilting of rigid blocks also occurs during the early (core complex) stage of extension. Our model suggests, however, that the extending continental crust reaches a critical point at which core complexes cease to be active, and all the extensional strain is accommodated by high-angle normal faults.