Dynamics and structural development of metamorphic core complexes

The development of metamorphic core complexes (MCCs) in a thickened continental lithosphere is studied using fully coupled thermomechanical numerical code, accounting for elastic-brittle-ductile properties of constituent rocks. For MCCs to develop, the middle lower crust and the sub-Moho mantle are required to be weak enough to flow laterally, so as to simultaneously feed the exhuming dome and enable the Moho to keep a flat geometry. The conditions are satisfied with initial Moho temperatures of 800°C or higher, with crustal thicknesses of 45 km or greater, and with initial effective viscosities lower than 10²⁰ Pa s and 10²² Pa s in the lower crust and the underlying mantle, respectively. A compositional (mainly density) anomaly with the properties of granite is placed centrally in the crust to localize strain at the onset of deformation. During a first stage of "upper crust necking," the deformation pattern is relatively symmetrical and dominated by graben formation in the upper crust. When the first ductile layers reach the surface, a second stage of "dome amplification and widening" occurs. Dome amplification is accommodated by horizontal flow in the ductile crust, giving an early symmetrical pattern of conjugate shear zones with no obvious detachment zone. The system then rapidly becomes asymmetric, with the localization of a detachment zone along one dome limb, further accommodating dome widening. Thus the exhumation process of a metamorphic dome results in the progressive development of a detachment zone. Depending on initial Moho temperature, the detachment zone can migrate in space or die out and be replaced by a new one with an opposite dip.