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The main objective of this paper is to review and synthesize all the paleoenvironmental information gained by the shipboard sedimentological party during Leg 107 of the Ocean Drilling Project in the Tyrrhenian Sea. The drilling on this leg was so successful that it is possible to construct an almost unique synthesis of an entire small ocean basin. ODP Leg 107 drilled a transect of ten holes at seven sites across the Tyrrhenian Sea from southeast to northwest, document­ ing the sedimentary and paleoceanographic evolution of a small ocean basin created by crustal extension in an area of broadly continental collision. Following early Miocene juxtaposition of the Sardinia-Corsica block with Adria to the east, crustal extension was initiated and rifting of the northeast Tyrrhenian Sea took place in late Miocene and early Pli­ ocene to form the Sardinian passive margin. A classic transgressive succession comprises coarse subaerial clastic sedi­ ments (Site 654) derived from metamorphic basement and sedimentary cover rocks, similar to Sardinia, overlain by coastal to shallow marine (glauconitic), hemipelagic, and pelagic carbonates (Site 654). During early Messinian, or­ ganic-rich siliceous sediments, comparable with the Sicilian Tripoli, accumulated in a productive basin that was anoxic at depth (Site 654). Deposition during the Messinian salinity crisis comprised the following facies: finely-laminated gyp­ sum precipitated in a desiccating basin affected by periodic marine incursions (Site 653); siliciclastic, organic-rich muds and nodular anhydrite apparently accumulated in a closed inward-draining fresh water, brackish and/or hypersaline lake, or desiccating basin (Site 652) subject to possible marine incursion; sulfur-rich sediments, possibly deposited near the shores of a saline lake (Holes 653A, -B); and, lastly, coarse subaerial elastics shed from an active fault-block com­ posed of metamorphosed ophiolitic rocks of Ligurian or Piedmontese type (Holes 656A, -B). Along the eastern basin margin (Site 652) provenance was from low-grade metamorphic and Mesozoic-Tertiary cover rocks, as exposed in Penin­ sular Italy and Sicily. An end Messinian marine transgression reworked terrestrial sands (Holes 65 3A, -B), reddish silts (Holes 656A, -B, Hole 652), and multicolored clays (Site 654) into basal Pliocene hemipelagic clays and muds. During latest Messinian-Pliocene new oceanic-type crust was created in the Vavilov Basin to the southeast (Site 651), associated with mantle protrusion, basaltic lava eruption, sill intrusion, and breccia formation. Overlying pelagic carbonates were dolomitized by low-temperature hydrothermal fluids (ODP Sites 651, 650; DSDP Site 373). Several generations of car­ bonate sediment and hydrothermal cement fill cracks in pillow basalt on the Cortani Ridge, a tholeiitic basalt Seamount (Site 655). Late Pliocene and Pleistocene intervals in both the igneous-floored basins, Marsili and Vavilov, largely comprise tur­ biditic volcaniclastic sediments derived from the Southern Apennine potassic Roman province to the east (Site 651) and the calc-alkaline and shoshonitic Eolian islands to the southeast (Site 650). In situ tephra is rare. Volcanic glass is com­ monly altered to zeolite and smectite. Quaternary sea-level changes influenced deposition; sediment was trapped in nearshore basins during sea-level highstands and reworked to depocenters as sea level fell. Sapropels and sapropelic lay­ ers are now well documented from the Tyrrhenian Sea in Leg 107 late Pliocene and Pleistocene successions. Volcanic, tectonic, and climatic effects caused sediment instability in the Vavilov and Marsili Basins (Sites 650, 651), evidenced by turbidities, mass-flow, slumping, dewatering structures, and syn-sedimentary faulting. The igneous basement of the Tyrrhenian Sea formed above a subduction zone and has a high potential for ultimate preservation on land during conti­ nental collision, comparable with some emplaced Tethyan ophiolite complexes. I N T R O D U C T I O N Drilling the Tyrrhenian Sea in the Western Mediterranean (Fig. 1) during Leg 107 exemplifies the tectonic and sedimentary evolution of a small oceanic basin created by crustal extension 1 Kastens, K. A., Mascle, J., et al., 1990. Proc. ODP, Sci. Results, 107: Col­ lege Station, TX (Ocean Drilling Program). 2 Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, United Kingdom. 3 Lehrstuhl fiir Algemeine, Angewandte und Ingenieur-Geologie, Technische Universitat Miinchen, Federal Republic of Germany. 4 Institut Dolomieu, Universite Scientifique et Medicale de Grenoble, Greno­ ble, France. 5 Lamont-Doherty Geological Observatory of Columbia University, Pali­ sades, NY 10964. 6 Department of Geology, University of Florida, Gainesville, FL 32611. 7 Laboratoire de Geodynamique Sous-Marine, Universite Pierre et Marie Cu­ rie, 06230 Villefranche-sur-Mer, France. 8 Istituto di Geologia Marina, 40127 Bologna, Italy. in a setting of broadly continental collision. The purpose of this paper is to summarize and discuss the sedimentological and pa­ leoceanographic data and interpretations from Leg 107, based mainly on shipboard data . The paper can be considered as a background to specialized sedimentological studies in this vol­ ume. Comparative aspects of the on-land geology (Sartori et al . , 1987a, b) and the geophysics of the Tyrrhenian Sea (Rehault et al . , 1987) are summarized in Kastens, Mascle, et al. (1987). Tectonic and magmatic aspects of the evolution of the Tyrrhe­ nian Sea, based on Leg 107, are treated elsewhere in this volume (see also Stanley and Wezel, 1985). During Leg 107, a transect of ten holes was drilled at seven sites across the Tyrrhenian Sea (Figs. 2, 5, 7, 9, 10, 12, 13, 15). The results support earlier hypotheses of the Tyrrhenian Sea as a back-arc basin formed above a northwestward-dipping sub­ duction zone (Boccaletti and Guazzone, 1972; Malinverno and Ryan, 1986). As summarized by Rehault et al . (1987), a subduc­ tion zone dips northwestward under Calabria and the northeast-

proceedings of the ocean drilling program

Summary and Synthesis of Late Miocene to Recent Sedimentary and Paleoceanographic Evolution of the Tyrrhenian Sea, Western Mediterranean: Leg 107 of the Ocean Drilling Program