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. This study sheds light on the deformation mechanisms of subductedmafic rocks metamorphosed at amphibolite and granulite facies conditionsand on their importance for strain accommodation and localization at the topof the slab during subduction infancy. These rocks, namely metamorphicsoles, are oceanic slivers stripped from the downgoing slab and accretedbelow the upper plate mantle wedge during the first million years ofintraoceanic subduction, when the subduction interface is still warm. Theirformation and intense deformation (i.e., shear strain ≥5) attest to asystematic and transient coupling between the plates over a restricted timespan of ∼1 Myr and specific rheological conditions. Combiningmicrostructural analyses with mineral chemistry constrains grain-scaledeformation mechanisms and the rheology of amphibole and amphibolites alongthe plate interface during early subduction dynamics, as well as theinterplay between brittle and ductile deformation, water activity, mineralchange, grain size reduction and phase mixing. Results indicate that increasing pressure and temperature conditions and slabdehydration (from amphibolite to granulite facies) lead to the nucleation ofmechanically strong phases (garnet, clinopyroxene and amphibole) and rockhardening. Peak conditions (850 ∘C and 1 GPa) coincide with apervasive stage of brittle deformation which enables strain localization inthe top of the mafic slab, and therefore possibly the unit detachment fromthe slab. In contrast, during early exhumation and cooling (from∼850 down to ∼700 ∘C and 0.7 GPa),the garnet–clinopyroxene-bearing amphibolite experiences extensiveretrogression (and fluid ingression) and significant strain weakeningessentially accommodated in the dissolution–precipitation creep regimeincluding heterogeneous nucleation of fine-grained materials and theactivation of grain boundary sliding processes. This deformation mechanismis closely assisted with continuous fluid-driven fracturing throughout theexhumed amphibolite, which contributes to fluid channelization within theamphibolites. These mechanical transitions, coeval with detachment and earlyexhumation of the high-temperature (HT) metamorphic soles, therefore controlled the viscositycontrast and mechanical coupling across the plate interface duringsubduction infancy, between the top of the slab and the overlyingperidotites. Our findings may thus apply to other geodynamic environmentswhere similar temperatures, lithologies, fluid circulation and mechanicalcoupling between mafic rocks and peridotites prevail, such as in mature warmsubduction zones (e.g., Nankai, Cascadia), in lower continental crust shearzones and oceanic detachments.

Deformation mechanisms in mafic amphibolites and granulites: record from the Semail metamorphic sole during subduction infancy