Block‐in‐Matrix Deformation and Veining in Alpine Subducted Oceanic Metasediments at Blueschist Facies Conditions

Abstract In subduction zones, the locked seismogenic zone is bordered by an upper and a lower transition zone. In these transition zones, deformation is primarily accommodated by an interplay of diffuse viscous deformation within a weak matrix and brittle deformation in stiffer bands and blocks. This style of deformation is supported by numerical modeling, which emphasizes the block and matrix ratio and the contrast in viscosity between the different shear zone components as primary contributors. However, natural case studies are still limited, especially for the lower transition zone, and more constraints are needed to disentangle this intricate deformation pattern. This study examines a subduction‐related blueschist‐facies block‐in‐matrix shear zone in oceanic metasediments (NW Italian Alps). Integrating field observations, multiscale structural analysis, image analysis and thermodynamic modeling, this study proposes that the development of broadly coeval carpholite‐bearing veins and associated mylonitic foliation are primarily governed by the rheological contrast and the relative abundance of competent blocks and weak matrix, complementing and expanding numerical modeling findings. Thermodynamic modeling indicates that alternating brittle and ductile deformation occurred at pressure >∼1.0 GPa and temperature of 300–350°C, consistent with blueschist facies conditions supported by the presence of carpholite. This documentation of a fossil block‐in‐matrix shear zone contributes to a broader understanding of fluid pathways and deformation mechanisms occurring at >30 km of depth in subduction zones and offers insights into a possible geological record of slow earthquakes.