Structure and Damage Patterns of a Seismic Fault Zone

Fault zone structure over a wide range of scales strongly influences earthquake mechanics. We present results from an ongoing project that aims to quantify the structure and fracture network characteristics of the seismic Gole Larghe Fault Zone (GLFZ) in the Italian Alps, exhumed from 8-10km depth. The GLFZ is c.500m thick and accommodates a total displacement of c.1000m, distributed amongst hundreds of first- and second-order oblique-slip cataclasite- and pseudotachylyte-bearing faults. The faults nucleated on pre-existing joints in granitoids of the Adamello batholith. Due to the continuous, glacier-polished nature of the exposures, we are using a range of digital mapping techniques, together with sample collection, to survey the fault at scales of centimetres to kilometres. The main results to date are: 1) Joints outside the GLFZ formed predominantly at temperatures >500C, whilst cataclasite- and pseudotachylyte-bearing fault strands within the GLFZ were active at 200-300C. The transition from jointed 'wall rock' to 'fault zone' is marked by an abrupt increase in macroscopic fracture density; 2) Second-order faults inside the GLFZ are strongly clustered around first-order faults. However, in around 70% of cases, second-order faults are asymmetrically distributed on the northern side of first-order faults. This damage asymmetry is not explained by lithological variation, but may reflect the fact that propagating earthquake ruptures preferentially follow one of the boundaries between a pre-existing joint cluster and relatively intact host rock; 3) P-wave velocities measured on 37 samples collected along a c.300m profile across the wall rock and fault zone increase from 3500-4500m/s in the wall rock to 5000-6000m/s inside the fault zone. We suggest that the increase in P-wave velocity may reflect sealing of pervasive microcracks by minerals such as epidote and K-feldspar; future work will quantify microcrack densities and fills. The above field observations suggest that the GLFZ is markedly different from other seismic fault zones, where fracture density increases exponentially towards the fault zone and P-wave velocities decrease from the wall rocks towards the fault zone.