Continental extension in orogenic belts : modes of extension, origin of core complexes, and two-phase postorogenic extension
Continental extension principally occurs in orogenic belts, however, most of numerical simulations use uniform crust that cannot represent an orogenic belt. We simulate lithospheric extension in an orogenic hinterland approximated by a crustal wedge. We first show that the presence of a preexisting weak mid-crustal shear zone dipping at low angle exerts a critical control on whether crustal and mantle deformation are decoupled or coupled. When the lower crust and the mid-crustal shear zone are weak, decoupling occurs and crustal deformation is compensated by lower crustal flow. When the lower crust is strong or a weak shear zone is absent, coupling occurs and crustal deformation is compensated by flow in the mantle. By varying the strength of the lower crust and the weak shear zone in numerical lithospheric extension experiments, we examine structures developed and compare them with structures observed in extended and collapsed orogenic belts. In models with a weak mid-crustal shear zone, we find that decoupling is particularly effective. In these models, we distinguish three modes of extension: 1) localized, asymmetric crustal exhumation in a single metamorphic massif with a weak lower crust, 2) the formation of rolling-hinge normal faults and the exhumation of lower crust in multiple metamorphic core complexes with an intermediate strength lower crust, and 3) distributed domino faulting over the weak mid-crustal shear zone with a strong lower crust. In models without a mid-crustal shear zone, extension is coupled and structures similar to those observed in continental margins form. We further analyze my model to better explain and understand the core complexes and low-angle normal faults which develop when a preexisting weak mid-crustal shear zone is present. We define three types of detachment systems and present four models which produce core complexes that bear striking resemblance to natural examples: 1) bivergent core complexes, 2) metamorphic core complexes, 3) boudinage structures, and 4) flexural core complexes. We also discuss intracrustal isostasy and the thermal history of material particles sampled in modeled detachment. Finally, based on a geological and geophysical synthesis and using numerical simulations, we propose a two-phase postorogenic extensional scenario that approximates the evolution and the structures observed in the South China Sea margins.