Fluid Flow and Shear Instabilities in the Subducted Mantle at Intermediate-depths: insights from the Western Alps meta-ophiolites

Intermediate-depth earthquakes (IDEQs), which occur at depths of 50 to 300 km, are relatively poorly understood compared to shallow seismicity, and their source mechanisms and physical environment remain ignored. This scientific gap exists because obtaining data from these depths—whether through geophysical imaging or geological sampling—is exceptionally challenging. The dehydration of serpentinites, which can release up to 13 wt% of H2O at these depths, is thought to play a key role in driving deformation associated with IDEQs. However, the mechanical role of the fluids released during these metamorphic reactions remains unclear. To provide new insights into the physical habitat of IDEQs, we investigate olivine- and Ti-clinohumite-rich veins in the Zermatt-Saas meta-ophiolite, a natural laboratory that records dehydration and fluid flow processes under (ultra)high-pressure (UHP) conditions typical of IDEQ depths.

We conducted petro-structural analyses and identified three main vein types: dilational, hybrid dilational-shear, and highly strained sheared veins. Key observations include (i) foliated sheared veins; (ii) newly formed olivine and Ti-clinohumite aligned in mineral lineations within sheared veins and shear bands; (iii) olivine and Ti-clinohumite fibers sealing porphyroclasts; and (iv) mutual crosscutting relationships between dilational and shear features. These features indicate cyclic brittle fracturing and ductile shearing at 2.3–2.7 GPa and 520–650°C, reflecting transient shearing and dilational fracturing under conditions of elevated pore fluid pressures, potentially approaching or exceeding lithostatic levels. The observed structures suggest that fluid escape occurs through interface-parallel, fracture-controlled pathways localized in high-strain zones, particularly near ultramafic sliver boundaries.

Strain gradients reveal distinct deformation styles, with dilational veins prevalent in low-strain regions and sheared veins and shear bands dominating within high-strain zones. These findings highlight the role of local stress regimes during serpentinite dehydration. Cyclic brittle-ductile deformation and fracturing, potentially linked to seismic or sub-seismic strain rate bursts, may have facilitated fluid migration and strain localization along olivine-bearing vein networks. These results align with geophysical observations suggesting high pore fluid pressures within the intermediate-depth seismicity region, providing insights into the mechanisms linking dehydration, fluid flow, and seismicity at depth.