Fluid flow during accretion in sediment-dominated margins: Evidence of a high-permeability fossil fault zone from the Internal Ligurian accretionary units of the Northern Apennines, Italy

We report here a detailed structural study carried out in the Internal Ligurian Units of the Northern Apennines, Italy, formed during the building of the Alpine accretionary complex through subduction of the sediment-filled Ligure–Piemontese oceanic basin. The deformation mechanisms associated with fluid migration across an accretion-related fault zone have been studied through a detailed analysis of different generations of syn-tectonic veins. Hydrofracturing occurred mainly sub-parallel to bedding in unlithified to semi-lithified sediments. Transient, upward-directed fluid injection locally connected the décollement-parallel veins through bedding-normal hydrofractures of lithified sandstone layers. A third vein system comprises fibrous hydrofractures developed on the limbs of accretion-related folds. Crosscutting vein sets and the peculiar features of each identified vein set suggest that deformation was intricately associated with lithification and diagenetic processes. Dehydration-produced fluids transiently injected the lithifying sediments leading to local stress permutations. The proposed model provides a “ramp-flat” migration of fluids in which fluid flow is enhanced along high permeability, less cohesive layers, leading to the development of regional dilated hydrofracture channels like those recognized along the décollement zone of modern margins. The more competent layers are truncated by high angle fractures representing the transient connectivity that existed between horizontal conduits.