DAMAGE, PERMEABILITY AND SEALING PROCESSES OF AN EXHUMED SEISMIC FAULT ZONE; THE GOLE LARGHE FAULT ZONE, ITALIAN ALPS

The Gole Larghe Fault Zone (GLFZ) in the Italian Alps shows evidence of ancient seismicity attested by the widespread occurrence of cataclasites associated with pseudotachylytes formed at 9 to 11 km depth in tonalite host rock at temperatures of 250 to 300°C. We combine systematic meso- and micro-structural data with permeability and ultrasonic velocity laboratory measurements from samples collected along fault transects to define the damage structure, fluid flow properties and sealing history of a seismogenic source in crystalline basement. The GLFZ (~600 m wide) can be divided into five distinct zones, defined by large variations in fracture density, pseudotachylyte distribution, volume of fault rock materials, and fracture sealing characteristics. The ~80 m wide central zone exhibits pervasive damage in the form of dense cataclastic fault-fracture networks, bordered by two thick (2 m) and continuous cataclastic horizons. This central zone is flanked by outer damage zones ~250 m wide with reduced fracture densities. Within and immediately surrounding the central zone, the syn-tectonic sealing of both micro- and macro-fractures evidenced by formation of epidote, K-feldspar, and chlorite minerals was pervasive, resulting in low permeabilities (~10^-21 m²). Fault-fracture networks hosted pervasive fluid-rock interaction, leading to a ~200 m wide alteration zone bounded by fluid infiltration fronts with irregular geometry. Fracture density is lower in the damage zones, but incomplete healing results in higher sample permeabilities (~10^-18 m²). Laboratory P-wave velocities correlate with both the damage and sealing characteristics of the fault zone. P-wave velocities are uniformly high (~ 6 km/s) both within and immediately surrounding the central zone, consistent with pervasive sealing of fractures and low sample permeability. In the damage zones P-wave velocities are much lower due to the presence of open fractures. These measurements highlight the close interplay between fracturing, fluid flow and mineralization of large fault zones. Importantly, they demonstrate that seismic wave velocities and permeability depend on both fracture density and orientation, and the degree of fracture sealing, which has implications for the interpretation of active fault zone structures based on geophysical data.