Geology and evolution of fissure systems in fractured basement rocks, Calabria, southern Italy: implications for sub-unconformity reservoirs and aquifers

Basement-hosted fissure-fill networks in sub-unconformity settings are increasingly recognized globally and have the potential to act as important subsurface reservoirs and/or migration pathways for hydrocarbons, geothermal fluids and groundwater. We examine well-exposed fissures from exhumed crystalline upper Carboniferous basement rocks in southern Italy (Calabria) and describe their nature, origin and evolution. The basement rocks record the emplacement and exhumation of their plutonic protoliths. The evolution of these rocks includes their initial intrusion in the late Carboniferous, followed by veining, folding and rifting events, to eventual exhumation at the surface when fissuring occurred in the mid-Miocene. The fissure network hosts fossiliferous marine sediments, wall rock collapse breccias and limited mineralization with vuggy cavities. In the basement below the main erosional unconformity, fissure-fills form up to 50% by volume of the exposed rock. The fills are notably more porous (up to 15-25% matrix porosity) than the ultra-low-porosity (<1%) crystalline host rocks. We present field observations, palaeostress analyses of fault slickenlines and fracture topology analyses demonstrating that these exceptionally well-connected fissure networks are related to rifting and penetrated to depths of at least 150 m below the main Miocene erosion surface.