ODP Leg 107 in the Tyrrhenian Sea: Insights into passive margin and back-arc basin evolution KIM KASTENS Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964 JEAN MASCLE Laboratoire de Géodynamique Sous-Marine, Université Pierre et Marie Curie, BP 48, 06230 Ville-franche-sur-Mer, France CHRISTIAN AUROUX Ocean Drilling Program, Texas A&M University, College Station, Texas 77843 FNRTCO BONATTT 1 r^cTixuDDArr ta f Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964 LHK1M1MA dKUULIA ) JAMES CHANNELL Department of Geology, University of Florida, Gainesville, Florida 32611 PIETRO CURZI Instituto di Geologia Marina, 40127 Bologna, Italy KAY-CHRISTIAN EMEIS Ocean Drilling Program, Texas AàM University, College Station, Texas 77843 GEORGETTE GLAÇON Laboratoire de Stratigraphie et de Paleoécologie, Centre Saint-Charles, Université de Provence, France SHIRO HASEGAWA Institute of Geology, Tohoku University, Aobayama, Sendai 980, Japan WERNER HIEKE Lehrstuhl für Allgemeine, Angewandte und Ingenieur-Geologie, Technische Universität München, Federal Republic of Germany GEORGES MASCLE Institut Dolomieu, Université Scientifique et Médicale de Grenoble, Grenoble, France FLOYD McCOY Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964 JUDITH MCKENZIE Department of Geology, University of Florida, Gainesville, Florida 32611 JAMES MENDELSON Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02142 CARLA MÜLLER Universität Frankfurt/Main, D-6000 Frankfurt/Main 1, Federal Republic of Germany JEAN-PIERRE RÉHAULT Laboratoire de Géodynamique Sous-Marine, Université Pierre et Marie Curie, 06230 Ville-franche-sur-Mer, France ALAST AIR ROBERTSON Department of Geology, University of Edinburgh, Edinburgh EH9 3JW, United Kingdom RENZO SARTORI Instituto di Geologia Marina, 40127 Bologna, Italy RODOLFO SPROVIERI Instituto di Geologia, Palermo, Italy MASAYUKI TORII Department of Geology and Mineralogy, Kyoto University, Kyoto, 606, Japan ABSTRACT their Messinian facies are subaerial and lacus- depth of >4,100 m below sea level nearly trine, respectively. We infer from these lines three times as fast as normal subsidence of Leg 107 of the Ocean Drilling Program of evidence that tilting and subsidence oc- crust formed at a mid-ocean ridge. drilled a west-northwest-east-southeast tran- curred more than a million years earlier on INTRODUCTION sect of seven sites across the Tyrrhenian Sea, the upper margin than on the lower margin. the youngest of the sub-basins of the Medi- Such diachroneity can be interpreted in terms The Mediterranean region records a long and terranean Sea. Sites 654, 653, 652, and 656 of migration of the zone of maximum exten- complex history of tectonic interactions, featur- document the rifting and subsidence of the sion above a "rolling-back" subduction zone, ing numerous episodes of creation and destruc- Sardinia passive continental margin. On the or in terms of extension of continental crust, tion of ocean floor. Included within this small upper margin (Site 654), we cored a classic by shear along a deep "detachment fault." sea are fragments of what may be the oldest transgressive sequence: subaerial conglomer- Sites 655, 651, and 650 were drilled into unsubducted and unobducted oceanic crust re- ates, overlain by oyster-bearing sands, over- two small basalt-floored basins of the central maining in the world's oceans: fragments of the lain by marine marl. Comparison between the and eastern Tyrrhenian. Emplacement of ba- Mesozoic Tethys under the eastern Mediterra- recovered lithologies and seismic reflection saltic crust in the central Tyrrhenian (Vavilov nean (Argand, 1924; Biju-Duval and others, profiles suggests that the synrift sediments on Basin) apparently began more than a million 1977; Robertson and Dixon, 1984). Within a the upper margin are Tortonian (late Mio- years before, emplacement of basaltic crust in few hundred kilometers of this ancient crust can cene) to Messinian (latest Miocene) in age, the eastern Tyrrhenian (Marsili Basin). This be found one of the youngest basalt-floored ba- whereas synrift sediments on the lower mar- observation is compatible with previous sug- sins in the world's oceans: the Marsili Basin of gin are Messinian to Pliocene in age. During gestions that the Tyrrhenian has grown the Tyrrhenian Sea. Unraveling the interrela- the Messinian desiccation of the Mediterra- southeastward in response to "rollback" of tionships between microplates, zones of diffuse nean, Sites 654 and 653, now on the upper the down-going slab that currently dips deformation, back-arc basins, orogenic belts, Sardinian margin, apparently occupied a ba- northwestward under the toe of Italy. At the seismogenic zones, volcanism, and ophiolites of sinal setting, where they received nanno- easternmost site, high vesicularity of the ba- the Mediterranean region is a daunting geologic plankton-bearing clays interbedded with lami- salt and benthic foram assemblages in the puzzle (Dercourt and others, 1986). nated gypsum. Sites 656 and 652, now on the oldest sediments imply that the basalt erupted This paper summarizes results from a transect lower Sardinia margin, were apparently in water shallower than 2,500 m. It has ap- of seven drill sites across the youngest of the higher standing during the desiccation event; parently subsequently subsided to its present Mediterranean basins, the Tyrrhenian Sea (Fig. Geological Society of America Bulletin, v. 100, p. 1140-1156, 12 figs., July 1988 1140 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/100/7/1140/3380509/i0016-7606-100-7-1140.pdf by guest on 28 September 2021 ODP LEG 107 IN THE TYRRHENIAN SEA 1141 M. Barone o 1000 2000 <m> 3000 4000 Figure 1. (Upper) Location map. The Tyrrhenian Sea is the small triangular sea surrounded by mainland Italy, Sicily, Sardinia, and Corsica. Leg 107 drilled a west-northwest-east-southeast transect of seven sites across the basin. The shaded area is deeper than 3,400 m and is approximately coincident with the bathyal plain. (Lower) Profiles of bathymetry and crustal thickness along a north-northwest-south-southeast transect across the Tyrrhenian Sea; modified from Steinmetz and others (1983), assuming an average crustal seismic velocity of 5.S km/s. 1). Ocean Drilling Project Leg 107 considered young passive margin. This paper deals primar- The Tyrrhenian Sea as a Back-Arc Basin the Tyrrhenian from three different perspectives: ily with objectives two and three; for additional first, as a Neogene stratigraphic type locality; insight into the stratigraphic objective, see The Tyrrhenian Sea is an example of the class second, as a back-arc basin; and finally, as a Kastens, Mascle, and others (1987,1989). of small basins, floored with mafic crust, which Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/100/7/1140/3380509/i0016-7606-100-7-1140.pdf by guest on 28 September 2021 1142 KASTENS, MASCLE, AND OTHERS have opened behind the overriding edge of a ward edge of the basin (the Eolian Islands; Bar- Erickson and others, 1976; Delia Vedova and subduction plate boundary. In common with beri and others, 1974; Beccaluva and others, others, 1984; Hutchison and others, 1985); other back-arc basins, the Tyrrhenian exhibits 1985; Keller and others, 1974); (3) shoaling of and (6) high-amplitude magnetic anomalies (1) a Benioff zone (dipping to the northwest the Moho beneath the central part of the basin (Morelli, 1970; Vogt and others, 1971; Bolis beneath the toe of Italy; Peterschmitt, 1956; Ca- (Recq and others, 1984; Steinmetz and others, and others, 1981). puto and others, 1970; Ritsema, 1979; Gasparini 1983; Duschenes and others, 1985); (4) tho- Prior to Leg 107, heat-flow studies (Delia and others, 1982) corresponding to a dipping leiitic volcanism in the center of the basin (Bar- Vedova and others, 1984), refraction experi- zone of fast seismic-wave velocity (Spakman, beri and others, 1978; Dietrich and others, ments (Steinmetz and others, 1983), and distri- 1986); (2) an active volcanic belt on the arc- 1978); (5) high heat flow (up to 200 mW/m2; bution of dredged basalts (Selli and others, Calcareous Nannofossil ooze Biogenic with minor Volcanogenic 100 calcareous mud, Terrigenous volcanic ash, sand/silt/clay WSM sapropel t?0£ Conglomerate 200 •Vi'iO Dolomite (m) Gypsum interbedded with calcareous clay and mud Evaporite 300 Organic C-rich claystone, dolomitic calcareous siltstone with diatoms Basalt I and radiolarians Nannofossil ooze 400 Glauconitic sandstone with V molluscs and echinoderms Conglomerate, gravelly mudstone 654 A Figure 2. (Upper) Core log indicating lithostratigraphic units identified at Site 654. The sediment classifications are as defined by Ocean Drilling Project (1984). The width of each vertical division within a given lithostratigraphic unit is proportional to the percentage of that lithology recovered in the unit. Bar graph at left indicates percent recovery in each core. Units 6,5, and 4 record a transgressive sequence linked to tectonic subsidence; units 2 and 3 record distinctly different aspects of the Messinian salinity crisis. (Lower) Multi-channel seismic (MCS) reflection profile across Site 654. Simplified core log inserted into a break in the profile (no data missing) indicates ages (left) and lithostrati- graphic unit numbers (right); correlation between cored depths and seismic profile is based