Low Angle Normal Fault (LANF)-zone architecture and permeability features in bedded carbonate from inner Northern Apennines (Rapolano Terme, Central Italy)

Abstract

Fault zones have the capacity to be hydraulic conduits within upper crustal levels, allowing migration of large volume of fluids through shallow and deeper geological environments. Low-angle normal faults (LANFs) crosscutting carbonate rocks produce damaged volumes that may have a relevant role in channelling or hosting geothermal fluids, therefore deserving of investigation to better predict mining targets. Deformation along LANFs zones, dissecting carbonate successions, produces permeable volumes presently exploited in the Larderello and Monte Amiata geothermal areas (Italy). In this paper, the architectural and permeability features of an exhumed LANF-zone exposed in the Northern Apennines, (Rapolano Terme, central Italy), affecting Cretaceous bedded limestone, are presented. Such a fault was not affected by circulation of geothermal fluids, but its features could reveal much on the potential impact on fluids migration in the active geothermal areas, therefore resulting an intriguing analogue. The study LANF-zone consists of faults, which enucleated at depth >4km. During its earlier stage of evolution, dissolution seams, often arranged in s–c fabric, characterised the whole damage zone. Dissolution seams developed under very low-grade metamorphism (T=100–150°C) as indicated by illite crystallinty analyses. Fault zone architecture and permeability features changed during the fault growth and exhumation. Permeability heterogeneity and anisotropy characterised the LANF zone during its development. If geofluids circulated within the fault zone, it could be an effective barrier during its earlier evolution, being accompanied by dissolution seams. On the contrary, it could play as combined barriers–conduits during its later evolution (progressively at shallower levels) being characterised by intersecting fault planes, which define pipe-like conduits parallel to the direction of the tectonic transport. Such a configuration could have the capacity to impact on fluids migration for long paths across the upper crustal levels. This adds significant information for predicting deeper geothermal circuits and the location of potential exploitable reservoirs in active tectonic environments.