Early Cretaceous vs. Early Jurassic tectonics: role of inherited structures in the post-rift evolution

While the subject of Early Jurassic rifting in the Western Tethys, with its accompanying set of variable facies and stratigraphic thickness, has been thoroughly addressed over the last decades, post-rift Mesozoic extension is a far lesser known theme. Stratigraphic and structural evidence document a late Early Cretaceous extensional phase, which can be identified across Italy (outcrop and subsurface), in the form of mega-clastic bodies, neptunian dykes, syn-sedimentary faults and angular unconformities. The pre-rift substrate (Calcare Massiccio Fm., a shallow-water limestone) was exposed at the footwall of Early Jurassic basin-margin rift faults/paleoescarpments as the submarine tectonic topography was draped and eventually levelled by pelagic sediments. Where present, Cretaceous faults reactivated Early Jurassic structures, although not necessarily reutilizing the original fault planes in a strict sense, causing downstepping-rejuvenation of the inherited margins. The “Mt. Cosce Breccia”, outcropping on the eastern slopes of the homonymous hill (Narni-Amelia Range, Central Apennines), is a polygenic breccia with lithoclasts of the Calcare Massiccio Fm. and of Jurassic basinal and condensed units, and rests unconformably on the Jurassic horst-block of Mt. Cosce. This deposit indicates reactivation of an Early Jurassic fault, and exhumation of a paleoescarpment tract that had been buried by the lower part of the Maiolica Fm. Stratigraphic evidence indicates an ?Hauterivian-Barremian age for the breccia. Neptunian dykes made of Maiolica-type facies, referable to the same tectonic event, penetrate a structural-high top condensed succession about 4 km east of Mt. Cosce, in the Sabini Mts. Calcirudites and calcarenites embedded in the upper Maiolica Fm. occur in the Sibillini Mts. and the Umbria-Marche and Marche Ranges (Northern Apennines), also with clasts of the unroofed pre-rift substrate. A comparable situation to Mt. Cosce, albeit at a larger scale, is represented in the Southern Alps by the “Ballino Breccia”, having an Early Cretaceous age and found at the Jurassic western margin of the Trento Plateau. In the subsurface of the Po plain the “Cavone Breccia”, a Lower Cretaceous mega-clastic deposit sealed by the Marne del Cerro Fm. (Aptian-Albian) is related to tectonic rejuvenation of the buried Bagnolo carbonate platform margin. Early Cretaceous paleoescarpments have been described in the Ligurian Briançonnais, as well as in the exposed margins of the Apulia carbonate platform (Maiella Range - a still debated subject - and Gargano peninsula). Syn-sedimentary faults and breccias sealed by the Marne a Fucoidi Fm. (Aptian-Albian), identified in seismic lines, affect the buried eastern and northern margins of the Ombrina-Rospo plateau (Apulia Platform, Adriatic offshore). These data suggest that evidence for Early Cretaceous extension are more widespread than expected. As plates re-arrangement took place in the Mediterranean region in the “Middle Cretaceous” due to opening of the Southern Atlantic, causing the inception of Alpine compression, an extensional phase is a rather unexpected event that will deserve future investigation.