Abstract
This study sheds light on the deformation mecha- nisms of subducted mafic rocks metamorphosed at amphi- bolite and granulite facies conditions and on their impor- tance for strain accommodation and localization at the top of the slab during subduction infancy. These rocks, namely metamorphic soles, are oceanic slivers stripped from the downgoing slab and accreted below the upper plate man- tle wedge during the first million years of intraoceanic sub- duction, when the subduction interface is still warm. Their formation and intense deformation (i.e., shear strain ≥ 5) attest to a systematic and transient coupling between the plates over a restricted time span of ∼ 1 Myr and specific rheological conditions. Combining microstructural analyses with mineral chemistry constrains grain-scale deformation mechanisms and the rheology of amphibole and amphibolites along the plate interface during early subduction dynamics, as well as the interplay between brittle and ductile deforma- tion, water activity, mineral change, grain size reduction and phase mixing.Results indicate that increasing pressure and temperature conditions and slab dehydration (from amphibolite to gran- ulite facies) lead to the nucleation of mechanically strong phases (garnet, clinopyroxene and amphibole) and rock hard- ening. Peak conditions (850 ◦C and 1 GPa) coincide with a pervasive stage of brittle deformation which enables strain localization in the top of the mafic slab, and therefore pos- sibly the unit detachment from the slab. In contrast, dur- ing early exhumation and cooling (from ∼ 850 down to ∼ 700 ◦C and 0.7 GPa), the garnet–clinopyroxene-bearing am- phibolite experiences extensive retrogression (and fluid in- gression) and significant strain weakening essentially accommodated in the dissolution–precipitation creep regime including heterogeneous nucleation of fine-grained materi- als and the activation of grain boundary sliding processes. This deformation mechanism is closely assisted with con- tinuous fluid-driven fracturing throughout the exhumed am- phibolite, which contributes to fluid channelization within the amphibolites. These mechanical transitions, coeval with detachment and early exhumation of the high-temperature (HT) metamorphic soles, therefore controlled the viscosity contrast and mechanical coupling across the plate interface during subduction infancy, between the top of the slab and the overlying peridotites. Our findings may thus apply to other geodynamic environments where similar temperatures, lithologies, fluid circulation and mechanical coupling be- tween mafic rocks and peridotites prevail, such as in mature warm subduction zones (e.g., Nankai, Cascadia), in lower continental crust shear zones and oceanic detachments.