Fractal Distribution of Subduction‐Related Crack‐Seal Veins (Schistes Lustrés, W. Alps): Implications for Fluid Flow and Rupture Processes at the Downdip End of the Seismogenic Zone

Abstract

AbstractIn 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.