Basement-hosted fissure fill networks in sub-unconformity settings are increasingly recognised globally and have the potential to act as important sub-surface reservoirs and/or migration pathways for hydrocarbons, geothermal fluids and groundwater. In the present paper 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, and an evolution which includes initial intrusion in the upper 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 porous (up to 15-25% matrix porosity) compared to the ultra-low porosity (< 1%) of the crystalline host rocks. We present field observations, palaeostress analyses of fault slickenlines, and fracture topology analyses which demonstrate 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.

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