The basal mantle of the Semail ophiolite directly overlying the metamorphic sole has been affected by a late and relatively low temperature ductile deformation event, ascribed to the deformation of the mantle just above the plate interface during subduction infancy (which ultimately led to ophiolite obduction). We show that this deformation results in the formation of proto-mylonitic (∼850–750°C) to ultra-mylonitic (∼750–650°C) shear zones wrapping lenses of porphyroclastic tectonites deformed at higher temperature (∼1200°C). In proto- to ultra-mylonites, syn-deformation hydrous fluid/peridotite interaction triggered the dissolution of coarser grains (olivine1, orthopyroxene1 and clinopyroxene1) and the crystallization of polymineralic mixtures of smaller grains (olivine2, orthopyroxene2, clinopyroxene2, spinel2 ± sulfide, together with magnesio-hornblende to pargasitic amphibole). This modal metasomatism is associated with cryptic metasomatism marked by striking mineral enrichment in some fluid-mobile-elements (especially B and Li). From the major and trace elements composition of the newly precipitated grains and estimated conditions of fluid/peridotite interaction (∼800°C), we interpret the metasomatic agent as a fluid derived from the dehydration of the amphibolitic to granulitic metamorphic sole (i.e. the metamorphosed crust from the downgoing plate) underlying the mantle during incipient subduction. We propose that this high temperature sole was accreted to the proto-mylonitic banded unit while dehydrating and that both units were exhumed together during peridotite ultra-mylonitic deformation, thereby preserving a fossilized warm subduction interface and evidence of an incipiently (de)forming mantle wedge.