Real and virtual outcrops to characterize the internal structure of a carbonate-hosted fault zone: the Tre Monti fault case study in Central Italy

A fault zone is composed of one or multiple fault cores, which are located within a complex network of fractures and secondary slip surfaces (i.e., the damage zone) that determine the mechanical behaviour. For example, fractures within the damage zone control fluid circulation and have a strong impact on the elastic properties of the host rock. Furthermore, multiple fault cores within the same structure can host both foreshocks and aftershocks during seismic sequences. All these observations suggest that fault zone structure exerts a primary control on the hydromechanical properties of faults. To perform a detailed reconstruction of fault zone internal structure, we have integrated standard structural geology investigations with structural analyses obtained from a terrestrial laser-scanner survey. We have investigated the internal structure of the Tre Monti fault, a SSE dipping carbonate-hosted right-transtensional fault in the Central Apennines. The fault is exposed for a length of ~ 8 km and is ~ 1 km wide, with a throw of ~ 1500 m accommodated by at least three sub-parallel main slip surfaces. Structural data have been gathered from exceptional exposures preserved within a quarry. In particular, the intensity and the orientation of joint sets within the damage zone were obtained both using a classical approach (i.e., scanline surveys on the quarry wall) and a semi-automatic extraction from the laser-scanner point cloud. Fracturing data were integrated with a detailed mapping of the secondary faults. Fracture orientations extracted semi-automatically from the point cloud are consistent with data derived from scanlines, suggesting that once the appropriate calibration procedure is adopted, the laser scanner analysis is a valuable complementary tool in structural geology, capable of reproducing a huge amount of data in a short time. The integrated analysis shows that secondary faults exhibit various orientations, from low angle antithetic normal faults to nearly-vertical oblique-slip faults with direction orthogonal to the main fault surface. They control local fracture density and geometry so that fractures are heterogeneously distributed within the quarry, with a density ranging between 10 and 50 m-1 and do not show exponential increase approaching the main fault. In addition, a nearly-vertical joint-set with WNW-ESE to WSW-ENE direction is widespread. Such an orientation is consistent with the horizontal, approximately N-S trending, 3 inferred from the right-lateral transtensional slip observed on the main fault surface. Our preliminary data highlight a wide fault zone structure, formed by parallel fault cores that are surrounded by a complex network of fractures and minor faults with multiple orientations. Further studies on similar faults will be performed to develop a reference framework of fault-zone structure that can be useful for all the studies aimed at the characterization of the hydromechanical properties of carbonate-bearing faults.