Interplay Between Migmatites and Deep Crustal Shear Zones

The presence of melt and associated fluids profoundly weakens the continental crust, promoting strain localization and establishing a close link between migmatites and ductile shear zones. Here we compare four migmatite case studies developed within major crustal-scale shear zones formed in contrasting tectonic settings, from collisional to extensional regimes: the Kinawa migmatite (Brazil), Opatica migmatite (Canada), Saint-Malo migmatite (France), and the Øksfjord Shear Zone (Norway). Our goal is to evaluate the connection between migmatites and shear zones, their impact on shear zone evolution, and the main macro- and microstructural features of migmatites in shear zones. We also examine the extent to which shear zones may serve as conduits for magma transport within the crust.

All migmatites formed at mid- to lower-crustal conditions (4–9 kbar; 650–820 °C) under both fluid-present and fluid-absent regimes. Macro- and microstructural observations reveal that the evolution of melt connectivity and permeability was strongly controlled by shear zone kinematics. In the Kinawa and Opatica examples, preservation of magmatic microstructures indicates that deformation ceased shortly after melt crystallization, suggesting limited post-melting deformation. In contrast, the Saint-Malo and Øksfjord shear zones record pervasive solid-state deformation overprinting magmatic fabrics, implying sustained deformation and continued microstructural reorganization after partial melting.

Across all examples, the spatial association between migmatites and shear zones highlights the role of deformation in enhancing melt segregation, extraction, and transient permeability. However, only some shear zones evolved into efficient pathways for melt migration. These and other case studies from the literature illustrate how ductile shear zones function as dynamic crustal domains in which deformation, partial melting, and fluid transport are tightly coupled, and where porosity and permeability evolve through time in response to changing rheology and strain.