Ultramylonitic carpholite-bearing veins as a proxy for deformation mechanisms from deeply subducted oceanic units (Liguro-Piemontese Zone, Western Alps)

Dilational hydroshear veins are hybrid veins that involve slip along weak planes and simultaneous extension under local hydrofracturing conditions (sensu Fagereng et al. 2010). These structures are considered as a possible record of episodic tremors and slow slip events (ETS). Carpholite-bearing dilational hydroshear veins and cyclic brittle-ductile deformation have been suggested to represent possible markers of these phenomena occurring at depth >30 km in subduction zones (Giuntoli & Viola 2022). In the Western Alps, similar structures in the form of lawsonite/carpholite-bearing veins have recently been reported (Herviou et al. 2023).

In this study, we analyzed the Lago Nero Unit (Western Alps), representing a fragment of the Liguro-Piemontese oceanic lithosphere and the related metasedimentary cover, deformed at 300-350 °C and 0.8-1.3 GPa during the Alpine Orogeny (Agard, 2021). We performed a detailed meso and microstructural characterization of mylonitic marble lenses wrapped by weak metapelite, both deformed by sheath folds. Hybrid veins in mylonitic marbles occur with crack-seal textures, oriented both parallel and at high angles to the main metamorphic foliation. The regional stretching mineral lineation oriented NE-SW is both parallel to the carpholite fiber composing veins and to the sheath fold axes. A few carpholite veins are folded within mylonitic marble, attesting to cyclic switches between brittle and ductile deformation in the stability field of carpholite, i.e. under blueschist facies conditions.

We focus on veins parallel to the foliation mainly composed of Ca-carbonate (now calcite, formerly aragonite), quartz and Fe-Mg carpholite (0.32<XMg<0.43). Frequently, large quartz and carpholite fibers form shear boudins in a plastically deformed Ca-carbonate mylonitic and ultramylonitic matrix, with a top-to-SW shear sense. Therefore, elevated strain partitioning is visible between host mylonitic marbles and veins and within single veins. Optical cathodoluminescence analysis shows different carbonate generations: larger and more luminescent fibers surrounded by small equant less luminescent grains. Electron Backscattered Diffraction analyses highlight that large Ca-carbonate fibers (50-500µm) deformed preferentially by subgrain rotation recrystallization, with the most deformed domains composed of smaller equant grains (<10µm) deforming by diffusion creep and grain boundary sliding. Summarizing, Ca-carbonate grew as fibers in veins and subsequently was affected by local strong grain size reduction due to strain partitioning at the microscale that activated grain size sensitive creep mechanisms along bands of accelerated creep.  Strain partitioning was likely favored by differences in the initial carbonate grain size and/or crystallographic orientation and by the presence of stiffer quartz and carpholite. Paleopiezometry is underway to constrain differential stresses and strain rates responsible for the formation of the observed microstructures.

In conclusion, oceanic metasedimentary covers record evidence of transient and cyclic pore fluid pressure fluctuation, reaching sub-lithostatic values and elevated strain partitioning under transiently high strain rates. These structures likely reflect cyclic seismic and aseismic creep occurring at >30 km depth in the Alpine subduction zone. Our results may be compatible with the geophysical and geological data ascribed to deep ETS in subduction zone contexts.