Surface roughness of ancient seismic faults exhumed from seismogenic depths (Gole Larghe Fault, Italian Alps)

Fault surface roughness is a principal factor controlling earthquake rupture nucleation, propagation and arrest, and, possibly, dynamic friction during seismic slip. However, the characterization of fault roughness is limited to a few examples of fault zones exhumed from < 5 km depth and hosted in sedimentary and volcanic lithologies. Here we investigate the roughness of slip surfaces from the seismogenic strike-slip Gole Larghe Fault Zone, exhumed from ~10 km depth and hosted in granitoid rocks of the Adamello batholith (Italian Alps). This fault zone is composed of numerous sub-parallel and interconnected slip surfaces that are exposed in continuous glacier-polished outcrops with undulating topography, providing 1D fault traces oriented at a variety of angles relative to the net slip vector. Ancient seismicity is corroborated by the occurrence of continuous layers of pseudotachylyte which very often line the slip surfaces. We determined the geometry of the fault traces over seven orders of magnitude using the following methods: (1) terrestrial laser-scanning (LIDAR: 100 - 102 m scale), (2) 3D mosaics of high-resolution rectified digital photographs (10-3 - 101 m scale), and (3) scans of thin sections from cores of the slipping zones (10-5 - 10-2 m scale). LIDAR scans and photomosaics were georeferenced in 3D using a Differential Global Positioning System allowing detailed reconstruction of fault traces in gOcad®. 1D fault trace roughness data collected along different orientations can be then used to reconstruct the 2D fault surface roughness by means of 2D Fourier synthesis. From a methodological point of view, compared to direct measurements of exposed fault surfaces, the technique used here has the advantage that (1) both hangingwall and footwall are preserved and separations and displacements can be easily measured (using markers separated by the slip surfaces), (2) fault rocks (pseudotachylyte and cataclasite) are well-preserved and can be related to roughness and offset, and (3) the measured roughness is not affected by weathering processes. Results from the Gole Larghe Fault shed light on the role that pre-existing anisotropy, in the form of preexisting joints, play on the roughness of the seismogenic fault surfaces.