Effects of transverse structural lineaments on the Neogene–quaternary basins of Tuscany (inner Northern Apennines, Italy)

Several mountain arcs formed in the Mediterranean area during the Alpine orogeny, among them the Northern Apennines. They show diachronous development with the outer thrust front prograding eastward, and being progressively replaced to the west by relatively thin, post-orogenic extensional or transtensional basins. The outer and inner parts of the orogen are linked together through a series of transverse structural lineaments. Segments of such lineaments have through time acted as transcurrent faults, lateral ramps of thrusts, strike- and oblique-slip faults, and normal faults. The main lines of evidence about transverse lineaments of the Northern Apennines are reviewed, and their effects on some Neogene–Quaternary basins of the inner part of the orogen in Tuscany and the northern Tyrrhenian Sea shelf are assessed. New information from recently released commercial seismic profiles and from surface sedimentological studies has made it possible to confirm that the post-orogenic basins formed in tectonic depressions delimited by major, quiescent substrate thrusts. The depressions were longitudinally separated into basins by the transverse lineaments. The stratigraphy of the basins in each tectonic depression is similar; in most cases, initial narrow syn-rift sedimentation was followed by extensive post-rift successions due to thermal subsidence. However, the thickness and distribution of their sedimentary sequences vary according to different subsidence (or uplift) and extension that have occurred along each side of the same transverse lineament, or in blocks delimited by different lineaments. Furthermore, portions of the lineaments, such as those of the Livorno–Sillaro, may have temporarily acted as strike-slip faults (late Miocene–Pliocene, in this case), and equivalent substrate highs and parts of the same basin may have been shifted left-laterally for about 15–20 km. A further effect of the transverse lineaments on basin sedimentation has been the development of major alluvial fans at relay ramps developed near the intersection of lineaments and quasi-orthogonal, listric boundary faults