Fluid‐Driven Shear Instabilities in the Subducted Oceanic Mantle at Intermediate Depths: Insights From Western Alps Meta‐Ophiolites

Abstract Serpentinites are major carriers of volatiles in deep subduction zones, releasing most fluids in the 500–650°C range. Despite fundamental implications for mass transfer and intermediate‐depth seismicity, the mechanical role of these fluids is unclear. To characterize the mechanical role of fluids at (ultra)high‐pressure conditions, we perform a petro‐structural analysis on olivine‐rich veins from the Western Alps meta‐ophiolite. Some veins formed through dilational and mixed dilational‐shear fracturing without significant shear‐related deformation. However, field and microstructural observations indicate transient shearing and dilational fracturing at high pore fluid pressures. These include: (a) foliated sheared veins; (b) newly formed olivine and Ti‐clinohumite within mineral lineations coating sheared veins and shear bands; (c) Olivine and Ti‐clinohumite mineral fibers sealing porphyroclasts; (d) mutual crosscutting relationships among dilational and shear features. Dilational veins prevail in low‐strain areas, while sheared veins and shear bands dominate within high‐strain zones toward the ultramafic sliver boundaries. These strain variations underscore the role of local stress regimes during serpentinite dehydration. Consequently, areas experiencing stronger shear stresses around large‐scale blocks or mechanical weakening during fluid circulation are prone to draining overpressurized fluids. These interface‐parallel and fracture‐controlled pathways thus facilitate fluid escape from the dehydrating downgoing slab. Transient events of dilational fracturing and brittle‐ductile shearing, along with strain localization in highly comminuted olivine‐bearing sheared veins, may have resulted from strain rate bursts potentially related to (sub)seismic deformation. These observations are in line with geophysical data indicating high pore fluid pressures within the intermediate‐depth seismicity region.