Seismic stratigraphy of the Miocene-Pleistocene sedimentary basins of the Northern Tyrrhenian Sea and western Tuscany (Italy)

Seismic and stratigraphic data of the inland Volterra Basin and of the Tusc an Shelf (Northern Tyrrhenian Sea) have been analysed to determine the tect ono-sedimentary evolution of this part of the Northern Apennines from the e arly Miocene (about 20 Ma) to the present. The area is a good example for b etter understanding the evolution of postcollisional related basins. The study area is characterized by a series of sedimentary basins separated by tectonic ridges. Similar environmental conditions existed both onshore and offshore as indicated by the occurrence of similar seismic units. The u nits are separated by major unconformities. The cross-sectional geometries of the deposits of these basins, as defined through seismic reflection prof iles, change in a quasi-regular manner through time and space. Early stages (late Burdigalian to early Tortonian) of evolution of the basi ns are marked by either flat-lying deposits, quasi-uniform in thickness, pr obably remnants of originally wider and shallow settings, or, in places, by relatively small bowl-shaped basins. The latter may have been strongly aff ected by the pre-existing topography and tectonics, as they developed at or near the leading edges of pre-Neogene substrate thrusts. These early depos its represent sedimentation during a transitional period from the end of co mpressional tectonics to the start of an extensional phase and represent a pre-narrow rift stage of evolution of the region. The subsequent stage of t ectonic evolution (late Tortonian to early Messinian), where preserved, is recorded by fault-bounded triangular-shaped basins interpreted as half-grab ens. This is one of the periods of major development of narrow rifts in the area. The following stage (late Messinian to Early Pliocene) is marked by variable types of basins, showing wide and deep bowl-shaped geometries pers istent in the offshore, whereas inshore (Volterra Basin) they alternate wit h half-graben, synrift deposits. This period thus represents a transitional stage where part of the system is still affected by synrift sedimentation and part is developing into incipient post-rift conditions. This stage was followed in early to middle Pliocene times by wide bowl-shaped to blanket-t ype deposits both in offshore and in inshore areas, indicating regional pos t-rifting conditions. The pre-, syn- and post-rift stages have followed eac h other through time and space, starting first in the westernmost offshore area and shifting later toward the east, inshore.