1Dipartimento di Scienze della Terra, Università di Siena, via delle Cerchia 3, 53100 Siena, Italy
²AGIP, DESI/ESNI, via Emilia 1, S. Donato Milanese (Mi), Italy
³Department of Land Resource Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1

Seismic and stratigraphic data of the inland Volterra Basin and of the offshore Tuscan shelf have been analyzed to determine the tectono- sedimentary evolution of this part of the northern Apennines from the early Miocene (about 20 millions years ago) to the Present. 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 units are separated by major unconformities.
   The geometries of the deposits of these basins, as detectable through seismic, change in a quasi-regular manner through time and space. Some care should be taken in their interpretation because the geometries are also affected by the strong erosional unconformities that bound them.
   Early stages (late Burdigalian to early Tortonian) of evolution of the basins are marked by either flat lying deposits, quasi-uniform in thickness, probably remnants of originally wider and shallow settings, or, in places, by relatively small, scoop-like basins (small bowl-shaped). The samll bowl-shaped basins may have been strongly affected by the substrate topography and tectonics, as they developed at or near the leading edges of basement thrusts. These early deposits may represent sedimentation during a transitional period at the end of the compressional tectonic and the start of the extensional regime that has affected this region. In any case all these early deposits represent a pre-narrow rift stage of evolution of the region. The subsequent stage of tectonic evolution (late Tortonian to early Messinian), where preserved, is recorded in triangular geometries representing half-graben basins bounded by active syn-sedimentary normal faults. 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 type of basins, showing wide and deep bowl shaped geometries in the offshore, and new triangular shaped deposits, indicating the formation of new narrow rifts inshore in the Volterra Basin. This period, thus, represents a transitional stage where part of the system is still affected by syn-rift sedimentation and part is developing into post- rift conditions. These transitional stage was followed in early to middle Pliocene by one characterized by wide bowl-shaped deposits everywhere, indicating regional post narrow-rifting conditions. The pre- syn- and post narrow rift stages have grossly followed each other through time and space, starting first in the westernmost offshore area and migrating in a semi-regular fashion toward the east, inshore.