Fracture network heterogeneity in a carbonate-hosted brittle shear zone (Gavorrano, Tuscany, Italy)

Fracture networks strongly influence permeability in low-permeability carbonate rocks, with important implications for geothermal, hydrocarbon and mineral resources. In low-permeability carbonates, where bedding and other stratigraphic anisotropies are weak or absent, variations in fracture distribution, topology and connectivity can determine whether a fault zone acts as a barrier, a conduit or a hybrid system, with fault architecture and strain partitioning exerting major control on fault-related damage. We investigate a carbonate-hosted brittle shear zone exposed in abandoned quarries near Gavorrano, southern Tuscany, Italy. The shear zone cuts Early Jurassic massive limestone and records the polyphase evolution of a high-angle fault system, initially developed under strike-slip to oblique-slip, locally transtensional kinematics and subsequently modified by normal movement during extension in the inner Northern Apennines. The study integrates structural analysis, 26 scan line surveys, fracture mapping, topological analysis of nine scan areas, and digital outcrop modelling. Results show that the shear zone consists of closely-spaced, hierarchically-organized fault segments and subsidiary fractures. Their close spacing and interaction generate a laterally-continuous fractured rock volume in which individual domains display moderate to relatively high topological connectivity. Although fracture density, intensity and topology vary locally, fracturing remains broadly distributed across the shear zone, reflecting the combined effects of fault-segment interaction and the relative mechanical homogeneity of the host rock. The Gavorrano shear zone is an outstanding example of the structurally-complex distributed fracture networks, locally variable connectivity and persistent fracture-controlled anisotropy, that can result in massive carbonates from brittle deformation in an evolving strain field.