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In the Western Alps, oceanic lithosphere fragments recovered from subduction are exposed continuously across the Liguro‐Piemont domain. In this nappe‐stack, the Schistes Lustrés metasediments are volumetrically dominant and contain large amounts of high‐pressure lawsonite‐ and Fe‐Mg carpholite‐bearing veins. These veins formed close to peak burial conditions at 30–60 km depth where deep slow slips and tremors occur. In the 12 studied outcrops, vein thickness distribution fit power laws while vein spacings and clustering show significant deviations from power laws, interpreted as the result of truncation artifacts and, possibly, at least in part, of later ductile deformation. Vein distribution at the outcrop scale suggests that fluids mostly circulated pervasively through the rock rather than along major localized conduits, in agreement with geochemical studies. Through the study of vein textures at macroscopic and microscopic scales, we showed that these high‐pressure veins formed by an incremental crack‐seal mechanism under tensile and shear failure and possibly between extremely weak planes. The spacings between crack‐seal inclusion trails and bands, which is in the same order as slip increments for low frequency‐earthquakes, fit a power law for a small fractal range with a fractal exponent similar to those estimated for slow slip events and ordinary earthquakes. In addition, the shear stress drop estimated for these veins is consistent with those inferred for slow slips and tremors. Data suggest that these veins, formed at the downdip end of the seismogenic zone, may correspond to the record of successive low‐frequency earthquakes during subduction of the Liguro‐Piemont ocean. Modern subduction zones—places where a tectonic plate sinks under another—host a zone at ∼25–55 km depth, where anomalous seismic velocities and atypical earthquakes suggest the presence of fluids. In this manuscript, we study a fossil subduction zone preserved in the Western Alps where rocks were buried at the same depth range. The occurrence of metamorphic veins (mineralized fractures) witnesses the fossil fluid circulation and fluid‐rock interactions. Their statistical distributions suggest that fluid flow was pervasive in the rocks, rather than localized in major conduits. The textures of these veins show their formation by successive fracturing events. The distribution of cracking increments fits features of the seismic events classically described at these depths (the slow slips and tremors). These veins are therefore considered to be a likely record of these slow earthquakes during subduction. Lawsonite‐ and Fe‐Mg carpholite‐bearing crack‐seal veins formed during subduction, at the depth of the slow slip and tremor regionVein thicknesses, spacings and clustering suggest a pervasive fluid flow during subduction, in agreement with geochemical studiesThe distribution of crack‐seal inclusions in the veins fits slow slip and tremor features suggesting a likely record of these events Lawsonite‐ and Fe‐Mg carpholite‐bearing crack‐seal veins formed during subduction, at the depth of the slow slip and tremor region Vein thicknesses, spacings and clustering suggest a pervasive fluid flow during subduction, in agreement with geochemical studies The distribution of crack‐seal inclusions in the veins fits slow slip and tremor features suggesting a likely record of these events

Abstract Optically homogeneous pseudomorphs occur in high-pressure metapelites from the Alpine Rif chain, Morocco, along with talc and kyanite. Further characterization using optical microscopy, electron microprobe, single- crystal X-ray diffraction, and HRTEM reveals a complex metamorphic replacement sequence of a previous highpressure metamorphic mineral, possibly magnesiochloritoid. Within the pseudomorphs, three zones are recognized based on the occurrence of distinct layer silicates, which crystallized under different P-T conditions along the retrograde metamorphic P-T path. These layer silicates are, from core to rim: (1) a relic mixture of margarite and chlorite, (2) oriented chlorite-smectite intergrowths ± pyrophyllite, and (3) chlorite and abundant smectite (at the periphery of the pseudomorph). These pseudomorphs represent an uncommon record of the retrograde metamorphic P-T path because the earliest smectite formed while the rocks still experienced P-T conditions in the stability field of pyrophyllite. This observation suggests that clay minerals may crystallize as regional metamorphic products, and that their stability field might not be restricted to late shallow-depth alteration.