Fluid flow during accretion in sediment-dominated margins: evidences 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 de´collement-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 de´collement zone of modern margins. The more competent layers are truncated by high angle fractures representing the transient connectivity that existed between horizontal conduits.