Quantifying the structure of an exhumed seismogenic fault zone using Differential GPS and Terrestrial Laser-Scanning

Fault zone structure over a wide range of scales strongly influences earthquake mechanics, including the sites of earthquake nucleation and arrest, co-seismic strength and slip distribution, and the amount of energy expended during frictional heating and creation of wall-rock damage. Here, we present preliminary results from an ongoing project that aims to use digital methodologies (e.g. LIDAR, GPS, photogrammetry) to quantify the hierarchical structure of the seismogenic Gole Larghe Fault Zone (GLFZ) in the Italian Alps, exhumed from ~10km depth. We focus on oblique-slip cataclasite- and pseudotachylyte-bearing fault networks that nucleated on pre-existing joint sets in granitoids of the Adamello batholith. The fault networks are 100% exposed in continuous, glacier-polished outcrops for distances of tens to hundreds of metres.

Fractures outside the GLFZ are represented by joints and sheared joints formed predominantly at temperatures >500C, whilst fractures inside the GLFZ are represented by cataclasite- and pseudotachylyte-bearing fault strands active at 200-300C. The transition from 'wall rock' to 'fault zone' is marked by an abrupt increase in macroscopic fracture density, in contrast to the more gradual increases in fracture density typically reported in the damage zones of other large faults in crystalline basement. 'First-order' faults that accommodated metres to tens-of-metres displacement have spacing distributions and mean spacing values that are similar to joints outside the fault zone. Overall, faults inside the GLFZ are strongly clustered, reflecting clustering of minor faults around first-order faults. However, in 70% of cases, minor faults are asymmetrically distributed around first-order faults (preferentially on the northern side), suggesting an asymmetric damage distribution within the GLFZ that cannot be explained by lithological variations. One explanation for the asymmetric damage distribution may be that propagating earthquake ruptures preferentially followed one of the boundaries between a pre-existing joint cluster and relatively intact host rock. We use our observations to comment on the development and behaviour of seismogenic fault systems in areas containing strong pre-existing anisotropies.