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

Abstract. This study sheds light on the deformation mechanisms of subducted mafic rocks metamorphosed at amphibolite and granulite facies conditions, and on their importance 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 plastered below the upper plate mantle wedge during the first million years of intra-oceanic subduction, 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 My and specific rheological conditions. Combining micro-structural 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 deformation, water activity, mineral change, grain size reduction and phase mixing. Results indicate, in particular, that increasing pressure-temperature conditions and slab dehydration (from amphibolite to granulite facies) lead to the crystallization of mechanically strong phases (garnet, clinopyroxene and high-grade amphibole) and rock hardening. In contrast, during early exhumation and cooling (from ~850 down to ~700 °C – 0.7 GPa), the garnet-clinopyroxene-bearing amphibolite experiences pervasive retrogression (and fluid ingression) and significant strain weakening essentially accommodated by dissolution-precipitation and grain boundary sliding processes. Observations also indicate cyclic brittle deformation near peak conditions and throughout the early exhumation, which contributed to fluid channelization within the amphibolites, and possibly strain localization accompanying detachment from the slab. These mechanical transitions, coeval with detachment and early exhumation of the HT metamorphic soles, controlled mechanical coupling across the plate interface during subduction infancy, between the top of the slab and the peridotites above. Our findings may thus apply to other geodynamic environments where similar temperatures, lithologies, fluid circulation and mechanical coupling between mafic rocks and peridotites prevail, such as in mature warm subduction zones (e.g., Nankai, Cascapedia), in lower continental crust shear zones and oceanic detachments.