Veining style and size-scaling behavior throughout the generation and exhumation of subduction mélange: Evidence from Japan and Alaska

We document patterns of cemented opening-mode fractures (veins) across the thickness of subduction mélanges, focusing on the Shimanto belt of SW Japan with ancillary observations from the Kodiak accretionary complex, Alaska, USA. We interpret at least two distinct phases of vein formation at seismogenic depths in Cretaceous mélanges of the Shimanto belt. Early veins (Set 1) can be ptygmatically folded, indicating formation prior to the completion of sedimentary compaction. Set 1 is crosscut by pervasive mélange foliation. Cement deposits within Set 1 comprise wall-lining, blocky quartz cement having blue cathodoluminescence (CL) cores and orange rims; calcite generally overgrows quartz, filling fracture pore space left between quartz crystals. Both minerals contain abundant fluid inclusions. Set 1 aperture sizes are power-law distributed, with most strain associated within the largest veins. Late veins (Set 2) crosscut Set 1 and mélange foliations. Set 2 veins have relatively planar walls, crack-seal texture, blue CL, a low density of fluid inclusions, and only sparse overgrowing calcite. Set 2 is numerically dominated by microfractures, likely a consequence of rapid healing at high temperature during fracture opening. Vein intensity is heterogeneous at outcrop and thin-section scales, with veins of both generations preferentially forming within brittle, sand-silt dominated facies, as opposed to intervening shales. Vein and host rock geometries indicate that the mélange deforms by early pure shear that evolves to simple shear as subduction proceeds. Vein formation accommodates plate convergence throughout numerous earthquake cycles on the subduction interface and comprises opening-mode fractures whose formation and cementation reflect long-term, interseismic fluid conditions. • Two generations of veins are present in subduction mélanges in Japan and Alaska. • Vein generations show contrasts in morphology, CL color, and size-scaling properties. • Early veins are a component of interseismic plate convergence. • Late veins formed during exhumation amid declining fluid overpressures.