Frontal part of the northern Apennines 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 surv ey, including structural and biostratigraphic analysis, have been integrate d with subsurface (seismic lines and well logs) data in order to reconstruc t the tectonic evolution of the external zones of the northern Italian Apen nines in the Romagna-Marche foothills and Adriatic Sea areas. This integrat ed 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 (less than or equal to 20 %), southward increasing, amounts of shortening (obtained by the constructi on of line-length balanced and restored geological cross sections); (3) a r egional deformation style characterized by the presence of backthrusts asso ciated 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 pre sent 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 com bination of thin- and thick-skinned tectonic styles; and (5) contrasting st ructural styles characterizing deep features, as imaged by seismic reflecti on profiles, and shallow ones. Deep structures consist of growth anticlines bounded by major thrust ramps and back limb back thrusts, separated by bro ader, open synclines, both involving a Mesozoic-Paleogene, mainly carbonate , passive margin succession. In the crestal zones of major anticlines, shal low structures, affecting Neogene terrigenous foredeep sediments, show a co mplex pattern of upright to recumbent folds (of tens to hundreds of meters wavelength) related to minor thrusts and backthrusts. Deformation of the Me sozoic-Paleogene multilayer appears to be dominated by thrust propagation i n 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 struc tures in the crests of major anticlines, where the thrusts ramp to the surf ace cutting up section. Complex shallow structures, interpreted to accommod ate at shallow structural levels the deep deformation, would therefore repr esent a geometrical requirement for maintaining strain compatibility across the shallow detachment level located at the base of the foredeep successio n.