Fault zone architecture and permeability features in siliceous sedimentary rocks: Insights from the Rapolano geothermal area (Northern Apennines, Italy)

Abstract

Studies of fracture networks and hydrothermal mineralisation of several faults affecting Jurassic siliceous sedimentary rocks, exposed in the Rapolano geothermal area (hinterland of the Northern Apennines, Italy), allow us to characterise the fault zone architectures and their permeability features. The study structures are normal faults with displacements of anything up to 30–40 m, belonging to a fossil hydrothermal system, Pleistocene in age. The fault zones are characterised by asymmetrical damage zones which are thickest in the hangingwall blocks. Fracture networks mainly consist of a subvertical fracture set at about 45° with respect to the fault plane. Widespread hydrothermal alteration (illite/smectite and kaolinite) and mineralisation consisting of quartz, calcite, dolomite, malachite, azurite and iron oxides characterise the fractures of the damaged rocks. This mineralisation suggests the occurrence of extraformational fluid circulation during the latest stage of faulting. Fault cores are characterised by cemented fault rock up to 25 cm thick, consisting of protocataclasite and ultracataclasite layers, grading to crush and fine crush breccia strongly affected by minor C 1-type shear planes. Fault cores represented barriers to fluid flow during the latest stage of faulting, whereas they acted as conduits during the initial stages. Fault zones with similar features are presently affected by hydrothermal circulation (thermal water up to 39 °C, and CO 2). The hydrothermal fluids give rise to several thermal springs and are exploited at depth for thermal resorts and CO 2 extraction, both in the Rapolano area and surroundings. A similar scenario characterises the Larderello-Travale and Mt. Amiata geothermal areas, where hydrothermal fluids and steam are industrially exploited for electricity production. The main results of this study are that the damage zone is asymmetrical and widest in the hangingwall. This could represent a useful contribution to the prediction of hydrothermal fluid pathways and geothermal targets in all the geothermal areas with similar features to those described here.