Frontal part of the northern Apennine foreland fold and thrust belt in the Romagna-Marche area (Italy): shallow and deep structural styles.

In this study, surface geological data resulting from a detailed field survey, including structural and biostratigraphic analysis, have been integrated with subsurface (seismic lines and well logs) data in order to reconstruct the tectonic evolution of the external zones of the northern Italian Apennines in the Romagna-Marche foothills and Adriatic Sea areas. This integrated analysis shows: (1) a late Messinian to lower Pleistocene progression of structural development from the hinterland to the foreland of the studied sector of the thrust belt; (2) relatively limited (≤ 20%), southward increasing, amounts of shortening (obtained by the construction of line-length balanced and restored geological cross sections); (3) a regional deformation style characterized by the presence of backthrusts associated with most foreland-vergent thrust ramps, leading to quasi-symmetric uplift and a low critical taper for the wedge, typical of foreland fold and thrust belts with a weak basal decollement (Triassic anhydrites in the present case); (4) an important influence of basement faulting which, despite a general basement-cover decoupling, appears to control stress localization in the latter, producing linkage of basement and cover structures in a combination of thin- and thick-skinned tectonic styles; and (5) contrasting structural styles characterizing deep features, as imaged by seismic reflection profiles, and shallow ones. Deep structures consist of growth anticlines bounded by major thrust ramps and back limb back thrusts, separated by broader, open synclines, both involving a Mesozoic-Paleogene, mainly carbonate, passive margin succession. In the crestal zones of major anticlines, shallow structures, affecting Neogene terrigenous foredeep sediments, show a complex pattern of upright to recumbent folds (of tens to hundreds of meters wavelength) related to minor thrusts and backthrusts. Deformation of the Mesozoic-Paleogene multilayer appears to be dominated by thrust propagation in the cores of early formed anticlines developed by buckling instabilities. The overlying Neogene deposits are detached from the carbonate substratum along the base of the foredeep succession. Bedding-parallel slip occurring along this detachment level appears to be accommodated by the complex structures in the crests of major anticlines, where the thrusts ramp to the surface cutting up section. Complex shallow structures, interpreted to accommodate at shallow structural levels the deep deformation, would therefore represent a geometrical requirement for maintaining strain compatibility across the shallow detachment level located at the base of the foredeep succession.