Extension and compression in the Northern Apennines (Italy) hinterland: Evidence from the late Miocene‐Pliocene Siena‐Radicofani Basin and relations with basement structures
Authors: Bonini Marco , Sani Federico 
Published: October 2002

The Northern Apennines (NA) hinterland area is characterized by a complex Neogene‐Quaternary tectonics where both crustal extension, associated with the Tyrrhenian Basin opening, and crustal shortening in the onshore area took part in the deformation. Analysis of synorogenic deposits preserved in the NNW trending Siena‐Radicofani Basin (SRB), extending along a large part of the NA hinterland, documents the evolution of deformation of this sector during the last 9 Ma. Information from subsurface geology (deep seismic lines, commercial seismic lines, and deep wells), surface geology (mapping and structural analysis), and from analogue modeling was integrated and used to infer the tectono‐sedimentary history of the SRB and its relation with structures in the substratum, as well as the possible implications for the evolution of the NA hinterland. The results we obtain indicate that the SRB and the adjoining hinterland basins were bounded by thrust anticlines controlling basin development and deformation. Seismic lines across the SRB display various examples of compressional structures affecting the basin fill and the substratum, such as thrust anticlines and reverse faults. Notably, these basin‐scale structures exhibit a good correlation with those observed in the field and are consistent with the kinematics of the outcrop‐scale compressional structures. The thrust anticlines bounding the basins are often cored by Triassic evaporites (Burano Formation), suggesting that this weak layer decoupling the sedimentary cover from the underlying crystalline basement controlled their evolution. In this circumstance, three series of scaled brittle‐ductile physical models have been used to investigate the development of a basement cover system, with a décollement ductile layer at the base of the sedimentary cover. These models simulate the evolution of the Northern Apennines hinterland, where the basement is involved in the thrusting, and the sedimentary cover is shortened above a basal ductile layer given by the Burano Formation. Longitudinal models cross sections display similar deformation patterns to those observed in the NA hinterland, such as characteristic wavelength of both basement and cover structures, as well as detachment and fault propagation folding in the cover. Extensional tectonics is instead found to control sedimentation in the southern part of the SRB (at ∼8 Ma) or representing recent deformation. The older extensional event is here related to the forelandward propagation of Tyrrhenian‐related extension that could reactivate suitably oriented basement thrusts. Transfer zones produced a differential propagation of extension, such that along‐strike sectors of the hinterland were eventually deformed by different stress fields. Analysis of the SRB and of other adjoining basins reveals that the NA hinterland experienced alternated periods of forward and backward migration of the compression‐extension transition. In our interpretation, the competition between extension and compression in the hinterland is related to the NNW directed Africa (Pelagian) indentation responsible for the lateral extrusion of the NA and the contemporaneous opening of the Tyrrhenian Basin in the inner zone. Increased marginal loading at the foreland promoting reactivation of hinterland thrusts may have influenced this process as well.