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The goal of this chapter is to integrate old and new data from the Tyrrhenian Sea into an internally consistent, chronologically ordered, interdisciplinary story that can serve as an overview of the highlights of Leg 107, or as an introduction to more detailed studies in this volume. Among the more intriguing aspects of this Tyrrhenian research are the following: 1. The extensional basin developed within thickened continental crust on the site of former orogenic belts. Extension in the Tyrrhenian proceeded simultaneously with compression in the circum-Tyrrhenian. 2. The onset of major rifting in the Tyrrhenian (~9 Ma) coincided with a major CCW change in the direction of Africa/Europe motion such that the direction of major plate motion was more nearly parallel to the dip of the downgoing slab under Calabria. This coincidence suggests that the decrease in obliquity of convergence may have triggered or favored backarc extension. 3. Rifting was rapid and diachronous across the Sardinia margin: on the upper margin, subsidence and tilting of fault blocks were most active in the Tortonian to Messinian, whereas on the lower margin these symptoms of rifting are concentrated in the Messinian to Pliocene sequences. Such diachronous development is compatible with an evolution controlled by east-dipping, low-angle detachment faults, accompanying "rollback" of the hinge zone of the downgoing African plate. 4. Of the two small basalt-floored sub-basins, the one closer to the subduction zone (Marsili Basin) is younger. This observation is compatible with development controlled by "rollback" of the downgoing slab. 5. Rapid subsidence has been documented in the younger of the two basalt-floored basins. Rapid subsidence is predicted for a spreading segment that is short in the along-strike direction because of along-strike heat loss. 6. A body of mantle-derived peridotite subcrops in the axis of the Vavilov Basin. Its origin at such a shallow crustal depth remains mysterious, but unroofing by shallow-dipping, deep-cutting detachment faults may have been important. 7. The Messinian salinity crisis produced a wide variety of lithologies, including subaerial conglomerates, dolomitic mudstones, clastic lacustrine sediments, and laminated gypsum. Magnetostratigraphy of the unusually complete lower Pliocene section at Site 652 places the Miocene/Pliocene boundary at 4.8 Ma, during a eustatic rise in sea level that may have increased the rate of inflow of Atlantic-derived water into the Mediterranean and thus ended the evaporative phase. Establishment of stable oceanographic conditions occurred within approximately 240,000 yr after the initial marine incursion. 8. In the modern Mediterranean, evaporation exceeds runoff plus rainfall, and the circulation pattern is anti-estuarine, i.e., Atlantic-derived water flows in (eastward) at the surface and deep/intermediate water flows out near the seafloor. Early Pliocene circulation, on the contrary, was estuarine. Leg 107 stable isotope data suggest that the change to the modern hydrographic regime occurred about 2.5 Ma, in response to increased aridity associated with the beginning of Northern Hemisphere glaciation. 9. Tephra layers are present in the Messinian sediments and abundant in Pleistocene sediment, but totally lacking in Pliocene sediments. Thus, either the Pliocene-age Vavilov Basin opened in a backarc setting without accompanying explosive arc-type vulcanism, or the volcanic arc was underwater.

The Geological Evolution of the Tyrrhenian Sea: An Introduction to the Scientific Results of ODP Leg 107