Insights into shallow-level processes of mountain building from the Northern Apennines, Italy

Recent observations from submarine convergent margins reveal that sediments at shallow levels of burial undergo intricate and irregular fluctuations i n physical conditions as they progressively deform and lithify-the early st ages of mountain building. Similar on-land evidence is comparatively sparse , as a result of difficulties of exposure and of early features having been obscured. Here we describe aspects of three structures from the Northern A pennines of Italy that corroborate these notions of complex shallow-level o rogenic processes. The Upper Cretaceous Castiglioncello melange formed during the subduction p hase of Apennine orogenesis; the major Cervarola thrust and the smaller, in tra-formational, Balduini thrust, both of Tortonian age, represent the ensu ing collisional phase. All three structures formed at burial depths less th an 2 or 3 km and are syn-diagenetic. Although differing in detail, all thre e show features such as clay fabrics and mineralized veins that record comp lex histories of fluid flow, overpressure, and deformation. The clays show varying types of scaliness, some of which had to involve pore dilation and collapse operating in cycles. The veins typically evolved from networks of bedding-parallel and bedding-perpendicular veinlets, through dilation brecc ias and mosaic textures, to shear and extensional fibre-growth during hydro fracture. Such observations imply fluctuating physical conditions and dynam ic tectonic processes operating on shallow-level sediments in a way that is highly heterogeneous-with depth, areally and through time.