New combined nonparametric/parametric statistical analysis of fracture networks to unravel the evolution of fault zones: a study of the exhumed seismogenic Gole Larghe Fault Zone (Italian Southern Alps)

The spatial distribution of “fractures” (faults, shear fractures, joints and veins) in fault zones evolves from an early stage where fractures show a random Poisson distribution, towards more mature stages where the distribution becomes increasingly regular, tending to complete saturation (log-normal to normal distribution of spacing with a well-defined mean). This evolution can be characterised with scanline surveys, where fracture intersections along 1D transects perpendicular to the fault zone are recorded. Here we present a study of the paleoseismic Gole Larghe Fault Zone (GLFZ), composed of hundreds of sub-parallel faults hosted in tonalites of the Adamello Massif (Italian Southern Alps), where we collected a complete transect across the fault zone, over a distance of >1km. First, we studied the correlation between fracture spacing and position with a robust non-parametric approach. This analysis, new for fracture distribution studies, allowed detecting volumes of the fault zone with clustering or a trend in spacing, versus volumes where the spatial distribution is stationary. We then subdivided the fault zone in “stationary volumes” that we further investigated. The GLFZ shows a characteristic zonation. Considering larger faults and fractures sub-parallel to the fault zone, including hundreds of seismogenic pseudotachylyte-bearing faults, we observe (i) outer zones showing a random Poisson spatial distribution (exponential spacing distribution), and (ii) an inner zone with a more regular log-normal spacing distribution. The outer zones do not show pervasive evidence of fluid flow, while the inner zone is strongly altered. Minor fractures (Riedel and tensional veins) always show a regular log-normal distribution. Our novel analysis reveals a clear evolutionary pattern. From the outer towards the inner zones, larger sub-parallel fractures and faults tend towards a regular spacing, possibly due to stress-shadowing effects. The distribution of minor high-angle fractures - always regular - can be explained since they are confined by larger structures. The near-saturated fracture network of the inner zone is highly interconnected and permeable, and this explains the evidence of fluid flow. This new statistical approach leads to robust predictions of fracture network parameters in fault zones.