At the Core of Orogens

Nowhere is the continental crust evolving more rapidly than in the core of orogenic belts where the thickened crust typically undergoes partial melting and orogenic collapse, leading ultimately to crustal stabilization. While the processes of accretion and collision may take 10-100 Myr, the collapse instability, involving flow of deep crust and formation and emplacement of metamorphic core complexes and migmatite domes, is short lived (1-10 Myr). Material transfer during orogenic collapse is achieved by (1) lateral flow near the base of the evolving continental crust, where highly sheared partially molten rocks remain buried, and (2) upward flow within metamorphic core complexes (MCCs) below bounding extensional fault systems. These MCCs are commonly cored by migmatite domes that provide exceptional windows into the dynamics of the deep orogenic crust. Therefore, extensional detachment systems and migmatite domes are prime recorders of how the mechanical and thermal instability introduced by crustal thickening and melting, transitions to a gravitationally equilibrated crust. Some notable research results from these systems include the record of fluid-rock interaction across detachments and the provenance of partially molten crust as observed in migmatite domes.

MCCs are bounded by detachment shear zones that record large strain and metamorphic gradients as well as effective fluid-rock interaction enhanced by intense deformation and recrystallization processes at the grain scale. Stable isotope analyses across detachment systems have delineated the limit between a zone dominated by surface-derived (meteoric) fluids above and a zone of prevailing metamorphic fluids below. In some cases, the meteoric fluid is consistent with a surface fluid that precipitated at high elevation and was involved in convective flow from the surface down to the detachment shear zone, likely during the initial stages of orogenic collapse. The rocks within migmatite domes are varied and include refractory lithologies, typically mafic enclaves or pods, that inform the provenance of partially molten crust. In some domes, mafic pods are made of eclogite, and the age of eclogite metamorphism (pressures of 1.5-2.0 GPa) is close to the age of migmatite crystallization. In other cases, eclogite that was formed in subduction zones was incorporated as blocks or pods and transported laterally within the partially molten crust over long distances across the orogen before being exhumed in domes. The presence of eclogite within migmatite suggests that the partially molten crust is sourced at near-Moho depths and is extremely mobile, with the ability to travel laterally (order of >100 km) as well as vertically. Phanerozoic orogenic cores provide a template for understanding the extent of reworking of continental material and the flow of this material during the stabilization of continental crust over geologic time.