Abstract

Recent observations from submarine convergent margins reveal that sediments at shallow levels of burial undergo intricate and irregular fluctuations in physical conditions as they progressively deform and lithify—the early stages 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 Apennines of Italy that corroborate these notions of complex shallow‐level orogenic processes.

The Upper Cretaceous Castiglioncello mélange formed during the subduction phase of Apennine orogenesis; the major Cervarola thrust and the smaller, intra‐formational, Balduini thrust, both of Tortonian age, represent the ensuing collisional phase. All three structures formed at burial depths less than 2 or 3 km and are syn‐diagenetic. Although differing in detail, all three show features such as clay fabrics and mineralized veins that record complex 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 breccias and mosaic textures, to shear and extensional fibre‐growth during hydrofracture. Such observations imply fluctuating physical conditions and dynamic tectonic processes operating on shallow‐level sediments in a way that is highly heterogeneous—with depth, areally and through time.