Microstructural Evolution of High- and Low-Strain Serpentinites from the Zermatt-Saas Meta-Ophiolite: Insights into Antigorite and Olivine Deformation at Intermediate-Depth Seismicity Depths

Serpentinites are key players in subduction zones due to their unique mechanical behavior, which affects tectonic and seismic activity along the subduction interface. A persistent question is the mismatch between experimentally deformed serpentinites—typically showing brittle or brittle-ductile features—and naturally deformed ones, which display dominantly ductile microstructures. Debate also continues over whether deformation in antigorite-bearing rocks is driven by crystal plasticity, dissolution-precipitation, or both. Additionally, few studies have addressed deformation in hydrated or partially dehydrated serpentinites (with metamorphic olivine and clinopyroxene) at (ultra)high-pressure conditions.

To explore these issues, we analyzed a strain gradient zone within the Zermatt-Saas meta-ophiolite, focusing on antigorite and olivine deformation under conditions relevant to intermediate-depth earthquakes and mantle wedge exhumation. Low-strain samples show olivine-diopside veins from brucite-antigorite dehydration, with weak antigorite CPOs and evidence of twinning. Deformation localizes around olivine veins, where olivine shows a B-type CPO but no internal strain. With higher strain, antigorite forms a strong foliation, its CPO intensifies, and olivine experiences folding, boudinage, and CPO strengthening. High-strain zones exhibit mylonitic fabrics, intense antigorite foliation, and transposed olivine veins forming isoclinal folds. S-C’ foliations and pressure shadows develop with fine-grained olivine fibers and olivine-diopside mixtures.

These observations reveal a transition from brittle-ductile to fully ductile deformation in fluid-rich serpentinites, governed by both dissolution-precipitation and dislocation creep. This study offers rare insights into the deformation behavior of serpentinites at depth.