Insights Into Passive Margin and Back-Arc Basin Evolution

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

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M. Barone o

1000

2000

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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

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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 lithostratigraphic unit numbers (right); correlation between cored depths and seismic profile is based on acoustic velocities measured in cored samples and calculated from MCS data. The geometry of the seismic units suggests prerift, synrift, and postrift deposits. Our preferred interpretation of the seismic stratigraphy places the prerift/synrift contact at 580 msec and the synrift/postrift transition at 300-320 msec below sea floor at the drillsite.

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1977) suggested that there were two regions of rotational normal faulting prior to the onset of ticularly intriguing because this margin bounds a basaltic crust formation within the Tyrrhenian formation of basaltic oceanic crust (Moussat and back-arc basin. Subsidence curves for basaltic Sea: a western center in the Magnaghi-Vavilov others, 1985; Malinverno, 1981; Malinverno crust in back-arc basins differ systematically basin (Deep Sea Drilling Project Site 373; Hsu, and others, 1981). As is the case off the coast of from open-ocean basins (Kobayashi, 1984); dif- Montadert, and others, 1978) and an eastern Spain (Montadert and others, 1979) or Long ferences may be expected in the rifting-phase center in the Marsili Basin (Fig. 1). Several in- Island (Hutchison and others, 1986), seismic subsidence history as well. Because of the young vestigators postulated that the Vavilov center reflection profiles reveal asymmetrical, appar- age of the Tyrrhenian Sea and the relatively formed earlier than the Marsili center and that ently fault-bounded basement blocks. The best- slow rate of shedding of terrigenous sediment the site of basaltic crust formation moved to- imaged blocks show a deep-lying sequence of from neighboring Sardinia, the synrift/postrift ward the subduction zone (southeastward) over uniformly tilted strata, overlain by a landward- and even the prerift/synrift contacts are accessi- time (Alvarez and others, 1974; Moussat and thickening and landward-tilted wedge of dip- ble to drilling. Sites 654, 652, and 656 form a others, 1985; Hutchison and others, 1985; Re- ping reflectors, capped with subparallel, subhor- transect across the Sardinia continental margin. hault, 1984; Malinverno and Ryan, 1986). izontal reflectors conformable with the sea floor. By drilling several synrift sequences along an Thus, the Tyrrhenian was seen as a suitable field This geometry suggests sediments which have onshore-offshore transect, we were able to ex- area in which to test the hypothesis of expansion been deposited on subhorizontal depositional plore the possibility that extension did not occur of a back-arc basin through seaward migration surfaces before, during, and after slip on bound- uniformly in space and/or time. of the arc and subduction zone (Dewey, 1980; ing rotational normal faults (Wernicke and Ritsema, 1979). Sites 650 and 651 were de- Burchfiel, 1982; Rehault and others, 1987). OBSERVATIONS signed to address this objective by documenting These units are termed "prerift," "synrift," and the age and nature of basement in Marsili Basin "postrift" sediments, with the implicit assump- Observations will be presented site by site and Vavilov Basin, respectively. tion that the phase of stretching and thinning of from west to east. Within each site, observations continental crust (that is, the rifting phase) is will be described in stratigraphie order from The Western Tyrrhenian as a Young coincident with the period of motion on the oldest to youngest; however, note that in ac- Passive Margin normal faults of the passive margin. cordance with shipboard procedures lithostrati- Much of the work on passive continental graphic unit numbers increase down-section. The western Tyrrhenian is an example of the margins focuses on timing and rates of extension Additional detail and documentation may be class of margins in which continental crust is and subsidence. The early subsidence and found in Kastens, Mascle, Auroux, and others thought to have been stretched and thinned by stretching history of the Sardinia margin is par- (1987).

(a) 654A- 28R-1 (b) 654A- 38R-I (c) 654A- 46R-CC (d) 654-49R-I

85-1

Figure 3. Core photographs from Site 654. In this and subsequent core photographs, numbers across the top indicate hole-core-section, and numbers along the side indicate centimeters downcore within the section, (a) Lithostratigraphic Unit 2, 252 mbsf: finely laminated gypsum intervals, such as that shown here, alternate with clay-rich intervals containing dwarfed marine fossils, (b) Lithostratigraphic Unit 3,330 mbsf: the lower Messinian claystones and siltstones are finely laminated, organic carbon rich, and contain siliceous fossils. Micro-faults, such as the one shown here, are common in this unit, (c) Lithostratigraphic Unit 5, 407 mbsf: glauconitic sand containing oysters was found below Tortonian-age marine marl, (d) Lithostratigraphic Unit 6, 439 mbsf: Site 654 bottomed in a coarse conglomerate with continental clasts.

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The Passive Margin East of Sardinia of predominantly finely laminated ("balatino- thick hemipelagic Plio-Pleistocene sequence is type," Ogniben, 1957; Cita, Wright, and others, similar to that recovered at DSDP Site 132 Site 654 (Upper Sardinia Margin). Site 654 1978) gypsum were encountered, ranging from (Ryan, Hsu, and others, 1973). is located on a fault-bounded, tilted block on the 0.15 to 7 m in thickness (Fig. 3a). Gypsum-poor Site 652 (Lower Sardinia Continental upper continental margin of Sardinia. Seismic interbeds of calcareous clay contain both plank- Margin). Site 652 lies near the inferred profiles across the site (Fig. 2; see also line MS-1 tonic foraminifera and nannofossils of Messinian continent-ocean transition of the lower Sardin- in Finetti and others, 1970) exhibit a geometry age, generally of smaller than normal size. ian margin, where continental crust has appar- suggestive of prerift, synrift, and postrift se- The 243-m-thick Plio-Pleistocene unit (unit 1; ently been extensively stretched and thinned. quences. Hole 654A penetrated into the wedging Fig. 2) consists of nannofossil ooze and calcare- The site is on the easternmost of the tilted blocks seismic unit interpreted as synrift sediments but ous mud, with occasional terrigenous clastics, on which prerift, synrift, and postrift sediments failed to reach the subparallel dipping reflectors volcanic ash, and organic-carbon-rich layers could be clearly discerned in reflection profiles that are interpreted as the prerift sequence. ("sapropels" and "sapropelic layers"). Indica- (Fig. 4). The lowermost 140 m of Hole 654A are a tions of sedimentary instability, such as micro- The bottom 533 m of Hole 652A comprises textbook example of a transgressive sequence. faults and slumps, are rare. A 2-m-thick interval monotonous, barren, gypsiferous, calcareous, At the base of the hole, we drilled through 60 m of alkaline or transitional basalt was encoun- sandy and silty mud and mudstone (units 4 and of conglomerate, gravel, and gravelly mudstone tered near the Plio-Pleistocene contact. 5; Fig. 4). Not a single autochthonous normal- (Fig. 3d; unit 6 in Fig. 2); the hole was aborted Comparison between the seismic stratigraphy marine foraminifera or nannofossil was found in with the drill bit stuck in this stubborn lithology. and the recovered lithologies (Fig. 2) shows that this entire interval, although reworked Creta- The matrix, where present, is red colored and the silica-bearing Messinian sediments (unit 3), ceous and Paleogene nannofossils were present. rich in iron oxides. Clasts are subrounded peb- the Tortonian/Messinian oozes (unit 4), and the Organic remains were limited to scattered plant bles apparently derived from a metamorphosed undated basal sand and conglomerate (units 5 debris, one 2-cm-thick layer rich in euryhaline carbonate sequence and associated quartzitic and 6) are all unambiguously within the synrift algae, three specimens of the brackish-water basement. We infer a subaerial depositional en- wedge (deeper than 405-m-sec sub-bottom). The benthic foraminifera Ammonia becarii tepida, vironment such as an alluvial fan. The subaerial Plio-Pleistocene sediments (unit 1), which corre- and a few possible ostracod fragments. Repeated sediments are overlain by oyster-bearing glau- late with the surficial, relatively transparent, centimetric layers are typically normally graded, conitic sands (Fig. 3c), presumably emplaced in seismic unit (shallower than 290-m-sec sub- less commonly reversely graded, and occasion- a nearshore environment (unit 5; Fig. 2). Neither bottom), are unambiguously in the postrift ally micro-cross laminated. Gypsum occurs as unit 6 nor unit 5 has been dated directly. They drape. The intervening gypsiferous upper Mes- dispersed detrital grains; anhydrite is found as are overlain by nannofossil ooze of uppermost sinian unit (unit 2) correlates with a sequence of nodules and in discrete layers. Indications of sed- Tortonian and lowermost Messinian (both very fine, closely spaced, laterally continuous re- imentary instability, including micro-faults, mi- upper Miocene) age (unit 4; Fig. 2). Benthic flectors. Although the seismic unit thickens crobreccias, and water-escape structures are foram assemblages indicate that water depth in- westward, the individual reflectors do not di- abundant. The lowermost 38 m of the drilled creased throughout the deposition of the nanno- verge appreciably. Instead, they onlap eastward section (unit 5b; Fig. 4) is distinguished by its fossil ooze unit; calcareous nannofossils suggest as if onto a pre-existing west-dipping surface. extreme degree of induration. A layer of smooth, a change from restricted-marine to open-marine The lack of westward divergence of reflectors rounded, sedimentary and metamorphic pebbles environments over the same interval. We attrib- suggests that tilting of the fault block stopped (unit 4c; Fig. 4) occurs in the upper half of the ute the transgressive sequence of units 6,5, and during or before deposition of the gypsiferous barren unit. The collection of varied lithologies 4 to subsidence of the continental crust during Messinian; however, the very high sedimenta- represented by these pebbles occurs in the the rifting stage of the Tyrrhenian Basin opening. tion rate that might be expected in this evap- Apenninic nappes of peninsular Italy and Sicily but is unknown in Sardinia or Calabria. During the middle and late Messinian (latest orative facies could mask a slow tilting. The Miocene), water circulation between the Medi- observed frequency of micro-faults and slumps The age and environment of deposition of the terranean and the world ocean was restricted supports the inference that the synrift-postrift barren sequence are somewhat uncertain. The and eventually cut off altogether (Hsu and oth- transition occurred near the end of the Mes- entirei sequence is reversely magnetized (Fig. 4), ers, 1973). Drastic changes in water chemistry sinian; such structures are common in the Mes- which, in the context of the magnetostratigraphy were followed by the desiccation of much or all sinian (Fig. 3) and nearly absent in the Plio- of the overlying sediments, suggests deposition of the Mediterranean, with the consequent dep- Pleistocene sediments (unit 1). during the lowermost reversed polarity event of osition of widespread evaporites. At Site 654, Site 653 (Cornaglia Terrace, Middle Sar- the Gilbert epoch (that is, between 4.79 and the onset of restricted conditions is recorded in a dinia Margin). Site 653 is located one-half mile 5.41 Ma; Harland and others, 1982). We as- 36-m-thick unit of organic-carbon-rich (1.0% to northeast of DSDP Site 132, in an area where sume a Messinian age for the barren sediments 2.4%) claystone and dolomitic/calcareous silt- relatively flat-lying sea floor is underlain by a based on their sub-Pliocene stratigraphic posi- stone (unit 3; Fig. 2). This unit is remarkable for structurally deep, evaporite-bearing trough (see tion, their unusual lithology, and their reversed the occurrence of radiolarians, diatoms, and Ryan, Hsu, and others, 1973, their Fig. 2). Holes magnetic polarity. The environment of deposi- sponge spicules; siliceous fossils are generally 653A, 653B, and 132 each penetrated several tion of the pre-Pliocene sediments seems to have uncommon in the circum-Mediterranean Neo- tens of meters into restricted marine and evapo- been variable through time. The repeated, thin, gene. Small-scale debris-flow deposits, convo- ritic Messinian sediments. Messinian lithologies well-graded layers, interpreted as suspensites, lute laminations, and micro-faults are abundant include balatino-type laminated gypsum; nanno- suggest a dominantly subaqueous setting. The in these nonbioturbated, finely laminated sedi- fossil mud; calcareous mud; nannofossil and lack of marine fossils, the presence of minor ments (Fig. 3b). During the pan-Mediterranean foraminifera-bearing marly calcareous mud; and evaporites, the rare brackish-water fauna, and desiccation event, 70 m of gypsum interbedded brilliant yellow and red muds containing hema- the euryhaline algae are compatible with an with calcareous clay, mudstone, minor sand- tite, limonite, sulfur, and sulfates. As at Site 654, origin in a closed lake in which salinity fluc- stone, and dolostone (unit 2; Fig. 2) was depos- the clay-rich beds often contain nannofossils, tuated widely. The pebble horizon suggests a ited at Site 654. At least five discrete intervals usually of smaller than normal size. The 220-m- temporary fluviatile or beach environment.

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Figure 4. (Upper) Composite core log (see Fig. 2 caption) indicating lithostratigraphic units recovered at Site 652 on the lower Sardinia margin. Lithostratigraphic Unit 3, a 40-cm-thick transitional interval of bright red/brown/gray clays and muds, is too thin to indicate at this scale. Magnetostratigraphy is indicated to the right; the entire barren interval (lithostratigraphic units 4 and 5) appears to be in the lowermost reversed polarity event of the Gilbert epoch, and thus Messinian in age. (Lower) Seismic reflection profile across Site 652 on the lower Sardinia margin shows a fault-bounded, tilted block not unlike the tectonic setting of Site 654. Our preferred interpretation of the seismic stratigraphy places the prerift/synrift and the synrift/postrift contacts, respectively, at 720-750 msec and 200-230 msec below the sea floor at the drillsite.

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Calcareous Biogenic T~] Terrigenous Sand/silt/clay Conglomerate

WEST

Figure 5. (Upper) Composite core logs showing the lithostratigraphic units recovered at Hole 656A. The upper Pleistocene sediments were drilled without coring ("washed") in Holes 6S6A and 656B. The contact between subaerial conglomerate (Unit 4) and barren dolomitic mud (Unit 3) is gradational and could result from tectonic subsidence of this site during the Messinian. (Lower) Seismic reflection profile showing the location of Site 656 on the easternmost tilted fault block of the Sardinian margin. Hole 656A, sited slightly downdip from the pinch-out of a thin wedge of inferred synrift sediment, terminated with the bit stuck in gravel, and so hole 656B was drilled farther updip.

Plio/Quaternary sediments at Site 652 com- ers, 1986) on the steeper eastern flank of the 656-9R-CC prise hemipelagic marine sediments with minor seamount have documented Paleozoic and volcanic glass (Fig. 4; units 1 and 2). Eight sap- Mesozoic sedimentary and low-grade meta- ropels or sapropelic layers occur in the Pleisto- morphic rocks. cene. Near the Mio/Pliocene boundary, the Both holes bottomed in a matrix-supported sediments take on a strong reddish color, which conglomerate with an oxide-rich, barren matrix we attribute to iron oxide in sediments that were (Fig. 6). The clasts range in size from coarse subaerially weathered during the Messinian sands to cobbles. They represent a wide range of draw-down and then reworked during the ter- rock types, indicating a provenance from an minal Messinian transgression (unit 3; Fig. 4). ophiolitic nappe and its metamorphosed sedi- Comparison between the lithostratigraphic mentary cover. Of the many clasts examined, and seismic reflection data (Fig. 4) indicates that none suggests a Miocene marine environment. A the prerift/synrift contact falls within or at the 10-m-thick layer of dark gray, barren, calcare- top of the highly indurated basal 40 m of the barren Messinian(?) interval. Our preferred interpretation of the synrift/postrift contact on the Figure 6. Photograph of the basal con- seismic profile falls within the lower Pliocene of glomerate (lithostratigraphic Unit 4) of Hole the drilled sequence. The distribution of slumps, 656A, 188 mbsf. The iron oxide-rich matrix debris flows, and micro-faults confirms that tec- suggests a subaerial environment of deposi- tonic activity was vigorous through the end of tion. Oasts in this unit include greenish silt- the Messinian. stone, line-grained calcarenite, altered green- Site 656 (Lowermost Sardinia Margin; de stone, chert, silicified micritic limestone, Marchi Seamount). Holes 656A and 656B are limonitic clay, mudstones, tremolite-rich al- located on the western flank of de Marchi Sea- tered metagabbro, amphibolites, metadoler- mount, the easternmost tilted block on the Sar- ite, metaquartzite, and serpentinized perido- dinia margin (Figs. 1 and 5). Dredging (Heezen tite; the lithologies of the clasts suggest and others, 1971; Colantoni and others, 1981) erosion of an ophiolitic nappe and its meta- and submersible diving (Gennesseaux and oth- morphosed sedimentary cover.

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ous dolomitic mudstone overlies the conglomer- tinental and basaltic crust at the western edge of swell on which the site is located lies along strike ate in Hole 656A (unit 3, Fig. 5). Because of its Vavilov Basin (Figs. I and 7). The bottom of the from a major elongate volcano (Vavilov Sea- stratigraphie position and resemblance to up- drilled section comprises 120 m of tholeiitic ba- mount). Seismic reflection profiles show that permost Messinian sediments at Sites 652 and salt. The recovery of basalt at Site 655 narrows Vavilov Basin is floored with up to 1 sec 653, the dolomitic mudstone is assigned a Mes- down the location of the contact between basal- (~1 km) of ponded, presumably turbiditic, sinian age. The contact between conglomerate tic and continental crust to the 25-km-wide gap sediments. and dolomitic mudstone is gradational and between this site and de Marchi Seamount to the The basement stratigraphy at Site 651 is un- seems to represent a transgression from a sub- west. Little vertical variability in structure or expectedly complex (Fig. 8). The hole bottomed aerial to a subaqueous environment. Such a composition was observed in the basalt. Curved, in 30 m of strongly serpentinized peridotite (Fig. transgression could have resulted from tec- glassy margins, spaced about 2 m apart, suggest 9). Relict mineralogy suggests that the original tonic subsidence during the Messinian and/or successive flows of pillow basalts. A consistent rocks were primarily lherzolite. Both high- and from flooding near the end of the Messinian magnetic inclination of 50°-60° rules out signif- low-temperature phases of deformation can be desiccation. icant tectonic tilting of the basalt pile. The age of recognized in the peridotite, but no cumulate Both Hole 656A and Hole 656B recovered emplacement is constrained by the age of nanno- textures were identified. The peridotite is over- lower Pliocene to middle Pleistocene nannofos- fossils (zone NN15; 3.5-3.6 Ma) and planktonic lain by 30 m of basalt and basalt breccia, fol- sil ooze overlain by upper Pleistocene detrital foraminifera (zone MP1-4; 3.1-3.6 Ma) within lowed up-section by a 28-m-thick assemblage of and volcanogenic sediments (units 2 and 1; carbonate-cemented fractures in the basalt, and dolerite, metasediments, metadolerite, and Fig. 5). In the down-dip hole (656A), the Plio- the observation that the entire basalt column is highly altered peridotite fragments, followed by Pleistocene nannofossil succession is in correct reversely magnetized. Together, these observa- a second 78-m-thick basalt and basalt-breccia stratigraphie order, but it contains several un- tions suggest that the basalt erupted during the unit. The basalts are aphanitic, with sparse vesi- conformities. In the up-dip hole (656B), the later half of the reversed-polarity event at the top cles. Chilled glass margins indicate that both ba- Plio-Pleistocene sequence is disrupted, appar- of the Gilbert magnetic epoch, that is, 3.4-3.6 salt units were emplaced as flows. ently by extensive slumping. Ma (Harland and others, 1982). The basalt is The oldest sedimentary unit at Site 651 is a overlain by 80 m of upper Pliocene to Pleisto- 40-m-thick brightly colored dolostone (unit 2b; The Basalt-Floored Basins cene marly nannofossil ooze with occasional Fig. 8). The dolostone lacks fossils and is not volcaniclastic layers and sapropels (Fig. 7). suitable for paleomagnetic dating. Immediately Site 655 (Western Rim of Vavilov Basin, Site 651 (Axis of Vavilov Basin). Site 651 is above the dolostone, calcareous ooze and cal- Gortani Ridge). Site 655 was located on the on the eastern flank of a north-south-trending careous claystone have been biostratigraphically crest of a north-south-trending ridge (Gortani basement swell within the western of the two and paleomagnetically dated as late Pliocene Ridge), near the contact between inferred con- deep Tyrrhenian basins (Figs. 1 and 8). The (Réunion magnetic sub-chron, -2.0-2.1 Ma;

Calcareous Marly nannofossil ooze, Biogenic Sapropels, dolomite ^ Terrigenous near basement sand/silt/clay (m) 100 Basalt with limestone Dolomite in fractures Basalt

655

• -immimsse&ssgsmi^SSi -5

L6 WEST

Figure 7. (Upper) Core log showing the lithostratigraphic units recovered at Site 655. The recovery of 120 m of basalt flows narrows down the gap between continental and basaltic crust to the 25-km-wide gap between this site and de Marchi Seamount. (Lower) Seismic reflection profile showing the location of Site 655 on the crest of a north-south-trending ridge near the transition from basaltic to continental crust.

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 [' I j I Calcareous Volcanogenic sediments, 11 i I I Biogenic 65IA- 57R-I

marly riannofossil-rich mud [»'„»/•fi Volcanogenic t-I-1 Terrigenous sand/silt/clay P~pi Marly nannofossil chalk, U J Dolomite volcanic ash, Mafic Igneous calcareous siltstone and mudstone Peridotite

60- Dolostone Basalt with carbonate-filled veins Dolerite and Metadolerite Basalt and basalt breccia Serpentinlzed peridotite

651A a 65- c 1 o o 10 1) 2a 0.

•3- ?b •9 B1

70-

f igure y. core pnotograpn snows serpentinized peridotite at 532 mbsf in Hole 651A. WEST Motion along a shallow-dipping, deeply penetrating detachment fault during rifting could have facilitated the emergence of this mantle- Figure 8. (Upper) Core log indicating the lithostratigraphic units identified at Site 651. Note the complex nature of the basement with a unit of derived material. dolerites, metasediments, metadolerite, and altered peridotite fragments sandwiched between two layers of basalt and basalt breccia, all underlain by serpentinized peridotite. (Lower) Seismic reflection profile showing the location of Site 651 on the flank of a basement swell in the Vavilov Basin.

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MP1-6 foraminiferal zone, 1.67-2.0 Ma). Con- the seamount show a thick sequence of flat- comprise occasional volcaniclastic layers inter- tinuing up-section through the Plio-Quaternary, lying, ponded turbidites overlying an irregular, bedded with calcareous muds and oozes (unit 2, volcanogenic sediment gradually becomes vol- diffractive acoustic basement (Fig. 10). Fig. 10). As at Site 651, the upper Pleistocene of umetrically more important than the calcareous Site 650 basement is basaltic. Textural evi- Site 650 is dominated by gravitationally rede- biogenic component. Much of the Pleistocene dence suggests that the basalt was emplaced as posited volcanogenic sediments (unit 1; Fig. 10) sequence is reworked volcanogenic debris that flows rather than as a sill. Vesicles in the basalt emplaced at very high sedimentation rates. has been deposited at extremely high accumula- are large (up to 3 or 4 mm) and abundant (from tion rates (>27-68 cm/1,000 yr). -10% to >30% of rock volume). The sediment/ IMPLICATIONS FOR THE TECTONIC Site 650 (Western Rim of Marsili Basin). basement contact was recovered intact (Fig. 11). HISTORY OF THE TYRRHENIAN Site 650 is located near the western rim of the Biostratigraphic and magnetostratigraphic con- BASIN southeastern of the two deep Tyrrhenian basins straints date the sediment immediately above (Figs. 1, 10). A large (>3 km relief) seamount basement at 1.67 to 1.87 Ma (Olduvai magnetic Evolution of the Basalt-Floored Basins rises in the center of Marsili Basin. Seismic re- event; biostratigraphic zones NN18/MP16). flection profiles across the bathyal plain around The Pliocene and lower Pleistocene sections Vavilov Basin Is Older Than Marsili Basin. Leg 107 has confirmed that the two bathymetri- cally deep, thin-crusted basins of the Tyrrhenian Volcanoclastic Sea are floored with basaltic crust. In addition, turbidites, the drilling results support earlier suggestions calcareous mud that Vavilov Basin is older than Marsili Basin; Pumice that is, that the site of basaltic crust formation migrated or jumped toward the subduction zone Volcanoclastic over time (Moussât, 1983; Moussât and others, turbidites, 1985; Malinverno and Ryan, 1986). calcareous mud

Calcareous mud, minor turbidites Calcareous Biogenic Calcareous n Volcanogenic mud Terrigenous ---: j sand/silt/clay

Dolomite Nannofossil ooze, dolomitized near basement ','*/< Basalt Vesicular basalt 1 II M I 0 100 650A

-5.0

o a> tn

-5.5

SOUTH

Figure 10. (Upper) Core log indicating the lithostratigraphic units recovered at Site 650. The percentage of volcanogenic sediments increased upsection throughout the sediment column. A thick pumice layer (Unit 2b) correlates with a seismic reflector which can be traced throughout the Marsili Basin. (Lower) Seismic reflection profile showing the location of Site 650 in the turbidite-filled Marsili Basin.

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We reached this conclusion through the fol- 650A- 66X-2 ence a thin, acoustically unresolvable, Mes- lowing line of reasoning. Site 650 was located sinian interval overlying the irregular acoustic near the western rim of the Marsili Basin. It is basement, however. Furthermore, we have not Upper situated as far as possible from Marsili Sea- conclusively ruled out the possibility that the mount within the confines of the basin as de- Pliocene dolostone at Site 651 is Messinian, although the fined by deep-lying acoustic basement and thin Nanno- preferred interpretation for the dolomite-rich crust. The sediments of Marsili Basin are ponded Fo ssili layers immediately above basement at Sites 650 turbidites, which form strong subhorizontal Ooze (Fig. 11), 655, and 651 is that they are altered seismic reflectors. Correlation of these unambig- hemipelagic sediments. uous seismic reflectors throughout a dense net- In summary, then, it seems safe to say that the work of reflection profiles shows that the oldest basaltic basement in the Marsili Basin is sediment overlying acoustic basement at Site probably uppermost Pliocene, whereas the old- 650 is as old as or older than the sediment over- est basaltic basement in Vavilov Basin is not lying acoustic basement anywhere else within Dolomite younger than early Pliocene and may be as old the Marsili bathyal plain. Thus we believe that Rich as late Miocene. In other words, basaltic crust the basement under Site 650 is among the oldest Mud was forming in the basin that is farther from the to be found within Marsili Basin. present volcanic arc and subduction zone (the Biostratigraphic and paleomagnetic dating Vavilov Basin) more than a million years before place the age of the sediments immediately it began to form in the basin that is nearer to the above basement between 1.67 and 1.87 m.y. Al- arc (the Marsili Basin). A somewhat analogous though it is subject to reinterpretation by radio- evolutionary sequence is observed in the western metric dating, we tentatively take this age for the Pacific where the locus of back-arc extension on recovered basalt. As hole 650A penetrated only the Philippine plate has migrated through time 32 m into basalt, there remains the possibility toward the down-going Pacific plate (Karig, that the cored basalt flows are underlain by ad- 1975). Such observations are compatible with ditional sediment and then by older basalt. The the hypothesis of growth of a back-arc basin velocity computed from site survey multichan- Vesicular through seaward migration of the "hinge zone" nel seismic data for the uppermost acoustic Basalt of a high-density down-going slab (Fig. 12A; see basement near Site 650 (2.7 km/sec), however, also Dewey, 1980; and Carlson and Melia, is similar to that measured in laboratory speci- 1984). Concepts similar to Dewey's (1980) mens of cored basalt (2.8-3.2 km/sec). Given "rollback" model have been applied to the that laboratory basalt velocities nearly always Tyrrhenian by Ritsema (1979; "passive subduc- exceed in situ velocities due to large-scale voids tion"), Moussat (1983,1985), Rehault and oth- and fractures (Broglia and Moos, 1987), it seems ers (1984, 1987), and Malinverno and Ryan unlikely that there is a substantial thickness of (1986). low-density sediment below the base of hole Significance of the Seamounts and Bathy- 650A. Given the high sedimentation rate at this Figure 11. Core photograph from Site 650. metric Saddle. We cannot, as yet, constrain the site, it seems unlikely that basalt flows spanning The sediment/basalt contact was recovered timing of the end of basaltic crust formation in a long interval of time would accumulate with- intact at 602 mbsf and dated as upper Plio- Vavilov Basin. Pleistocene basalts have been out significant intercalations of sediment. In cene (1.67-1.87 m.y.). Vesicles are large and recovered from Vavilov volcano (Gennesseaux summary, then, we believe that the initiation of abundant in the basalt, suggesting a relatively and others, 1986). It is important, however, to basaltic crust formation in Marsili Basin oc- shallow depth of eruption. distinguish between (1) eruptions on an isolated curred not earlier than about 2 Ma. volcano and (2) formation of new sea floor. For Several lines of evidence imply an older age example, in the Shikoku back-arc basin, mag- for the basement of Vavilov Basin. Potassium/ these oldest datable sediments fall within the netic lineations indicate that back-arc sea-floor argon measurements on basalts from DSDP subhorizontal, subparallel reflectors typical of spreading ceased—15 Ma; a linear chain of sea- Site 373 yield ages of 3.0 to 7.3 m.y., with a turbidites. If one traces the reflector correlative mounts then developed along the former axis of cluster around 4 m.y. (early Pliocene) (Barberi with the oldest datable sediment laterally into spreading (Kobayashi, 1984). We visualize a and others, 1978; Kreuzer and others, 1978; Sa- the structurally deepest part of Vavilov Basin, it similar sequence of events for Vavilov Basin: velli and Lipparini, 1978). At ODP Site 655, on falls approximately midway between the sea during a period of extensional stress in Vavilov a ridge near the western rim of the basin, the age floor and the basement. If, as is generally the Basin, the low-lying crust of the basin floor of the 120-m-thick section of recovered basalt is case in basinal settings in the Mediterranean formed in a process not unlike a somewhat dis- 3.4-3.6 m.y. (early Pliocene). This date is well (Cita and others, 1978a), Pliocene sedimenta- organized version of sea-floor spreading. (Note constrained by the observation of reversed mag- tion rates are equivalent to, or slower than, Pleis- the implicit assumption that the low-lying crust netic polarity throughout the basalt column, and tocene rates, then the oldest sediment in this of Vavilov Basin is basaltic; all basalt recovered by the occurrence of carbonates from biostrati- deepest part of the Vavilov Basin would be early by dredging or drilling has come from basement graphic zones NN15 (~3.5-3.6 Ma) and MP1-4 Pliocene at the youngest. The lack of an unam- highs.) Then extension slowed or stopped, but (3.1-3.6 Ma) in cracks in the basalt. At ODP biguous M-reflector under the Vavilov bathyal magma continued to be delivered along the Site 651 in the axis of Vavilov Basin, the old- plain (Fabbri and Curzi, 1979; Malinverno and same plumbing system. Without the sea-floor est datable sediments are late Pliocene in age others, 1981; Moussat, 1983) may be inter- spreading conveyor belt to carry basalt away (-2.0 m.y., biozone MP1-6/NN18); these were re- preted as an indication that much of this crust from its exit vent, a large volcano, Vavilov Sea- covered at 348 mbsf or 40 m above the sediment- had not yet been created during the Messinian mount, built up in the axis of the basin, elongate basalt contact. On seismic reflection records, desiccation event. We cannot rule out the pres- parallel to the feeder dikes from the former

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"spreading center." A similar sequence of events considerably shallower depths. First, the assem- ing sands/marine marls), suggesting tectonic may have occurred in Marsili Basin, with Mar- blage of benthic foraminifera found in the subsidence; and (c) the observation of numerous sili Seamount marking the location of the former lowermost 25 m of the sedimentary section sug- micro-faults and slumps in the Messinian sedi- axis of spreading. gests a water depth of 1,000-2,000 m. Second, ments, contrasted with the relative scarcity of The inference that the locus of basaltic crust the Site 650 basalts are extremely vesicular (10% such structures higher in the section. Drilling at formation jumped from Vavilov to Marsili Basin to >30% by volume). Because gas solubility in a Site 654 was halted for technical reasons with- raises the question of the nature of the interven- magma is pressure dependent, basalt with out reaching the seismic reflector that had been ing structural and bathymetric saddle across greater than 10% vesicle volume is usually found interpreted as the prerift/synrift contact. The which the alleged jump occurred. Dredging on only at water depths shallower than 1,000 m sands and conglomerates that comprise the low- the two southward-protruding prongs on the (Moore, 1965, 1970; Jones, 1969; Moore and ermost 74 m of the drilled section have not north side of the saddle (Mount Issel and Mount Schilling, 1973). This line of reasoning needs to been directly dated. Thus no lower (older) limit Poseidon; Fig. 1) yielded a variety of continental be viewed with caution because the intra-plate can be placed on the timing of onset of extension lithologies appropriate for an extension of the and mid-ocean ridge basalts on which system- on the upper margin. We note, however, that Italian margin: phyllites, schists, quartzite, and atic-depth versus vesicle-abundance studies have coarse conglomerates such as those at the base of shallow-water carbonates (Colantoni and oth- been conducted may have had parent magmas the drilled section at Site 654 are typically de- ers, 1981). Dredge hauls from the south side of with significantly lower volatile content than a posited extremely rapidly, so that the undated the saddle (M. Glauco), however, contained back-arc basin magma (Saunders and Tarney, section (~ 150 m thick) between the prerift/syn- middle to upper Pliocene calc-alkaline rocks 1979; Dick, 1980; Jenner and others, in press). rift contact and the lowermost dated sediments (Colantoni and others, 1981; Beccaluva and Balancing the foram data, the basalt vesicularity, (upper Tortonian) need not be very long in geo- others, 1981). The physiography and thicker and the potentially high volatile content of back- logic time. crust (Steinmetz and others, 1983) of the saddle, arc basin basalts, it seems probable that the Site 652, on the lower continental margin, is as well as the M. Glauco dredging results, can be depth of emplacement was shallower than 2,500 in a tectonic setting very similar to that of Site explained if the saddle between Marsili and m. If the 2.0-m.y. age and the 2,500-m maxi- 654. Here, however, the relative abundance of Vavilov Basins is in part a remnant volcanic arc, mum depth of emplacement are approximately micro-faults and slumps, plus comparison of analogous to the Palau-Kyushu and West Mari- correct, then Site 650 has subsided at a rate on litho- and seismic-stratigraphies, place the synrift/ anas Ridges in the Philippine Sea (Scott and the order of 700 m/m.y. (1,400 m/2 m.y.). This postrift transition in the early Pliocene. Thus the others, 1980). If the middle to upper Pliocene rate is nearly three times faster than is observed half-graben in which Site 652 is located became age of the dredged calc-alkaline rocks (Colan- on oceanic crust, which subsides -500 m in its inactive after, possibly more than a million years toni and others, 1981) can be trusted, the activ- first 2 m.y. due to lithospheric cooling (Parsons after, fault motion slowed or stopped in Site ity of this postulated volcanic arc would have and Sclater, 1977). Our data thus support the 654's half-graben. At Site 652, we were able to coincided with the opening of the Vavilov Basin, suggestion of Kobayashi (1984) that back-arc penetrate through the wedging seismic unit in- as is to be expected from their relative positions. basins follow a subsidence curve which is interpreted as the synrift sequence and to reach the The presence of a Pliocene calc-alkaline arc in itially steeper than that of sea floor formed at uppermost prerift sediments. Unfortunately, the the central Tyrrhenian would fill the previously mature open-ocean spreading centers. Such lower 534 m of the drilled section lacks age- puzzling temporal and spatial gap between the rapid, recent subsidence is also compatible with diagnostic fossils. A plausible interpretation of Oligocene-Miocene (29-13 Ma; Macciotta and the structural evidence cited by Selli and Fabbri the magnetostratigraphy (Fig. 4) places the en- others, 1978) calc-alkaline volcanism on Sardin- (1971) in support of their model of post- tire barren sequence, including the prerift/synrift ia and the Quaternary (1.5-0 Ma; Beccaluva Miocene "foundering" of the southeastern contact, within the lowermost reversed polarity and others, 1985) arc volcanism of the Eolian Tyrrhenian. event of the Gilbert epoch. This interpretation Islands. implies that prerift/synrift transition occurred Rate of Growth and Subsidence of Marsili Evolution of the Sardinia Passive Margin more recently than 5.4 Ma, that is, within the Basin. If our estimate of 2 m.y. for the age of Messinian. Since the prerift/synrift transition at Marsili Basin is correct, then the -70-km-wide Extension and Subsidence Were Diachro- Site 654 cannot have been younger than upper basin has opened at a full spreading rate of 3.5 nous across the Margin. We infer that exten- Tortonian, we again find that a tectonic event on cm/yr, comparable to the rate estimated by sion and subsidence began and ended earlier on the upper margin has preceded an analogous Moussat and others (1985) for the entire Tyr- what is now the upper continental margin of event on the lower margin, probably by a mil- rhenian Basin. This is a relatively slow spreading Sardinia than on the present-day lower conti- lion years or more. rate for a back-arc basin. By comparison, the nental margin. Our evidence comes from two We recovered no early Tortonian or pre- Mariana Trough and Havre Basin have opened lines of reasoning: first, a comparison of the age Tortonian sediments. Our data therefore can at full spreading rates of 4-5 cm/yr (Hussong of the inferred synrift sediments on the upper neither support nor refute the suggestion by Re- and Uyeda, 1981; Malahoff and others, 1982), and lower margins; second, a consideration of hault and others (1984, 1987) that parts of the the East Scotia Basin has opened between 5 and the facies and inferred paleodepths of the upper western Tyrrhenian experienced an earlier phase 7 cm/yr (Barker and Hill, 1981), and the Manus and lower margins during the Messinian of rifting and subsidence prior to the late Torto- Basin has opened at 13 cm/yr (Taylor, 1979). desiccation. nian event. The basalt/sediment contact at Site 650 was Evidence from Age of Synrift Sediments. Evidence from Messinian Facies. In today's encountered at 4,100 m below sea level. If the At Site 654, on the upper margin, we drilled into Tyrrhenian Sea, water depth increases from west sediment load were removed, the basement a landward-dipping wedge of upper Tortonian to east along the transect formed by Sites 654 depth would be -3,930 m below sea level (cal- to middle Messinian sediments. Our interpreta- (2,218 m), 653 (2,832 m), 652 (3,471 m), and culated using Airy isostasy and average sediment tion of this wedge of sediments as synrift depos- 656 (3,606 m). Examination of the distribution density of 1.75 g/cm3 derived from measure- its is supported by (a) the geometry of the of facies deposited during the Messinian desicca- ments on cores; Parsons and Sclater, 1977). seismic unit, with its landward-fanning and tion event suggests that this relief relationship Two lines of reasoning suggest that the basalt -dipping reflectors; (b) the occurrence of a tras- did not exist 5 m.y. ago. As noted by Montadert and oldest sediment at this site were emplaced at gressive sequence (conglomerates/oyster-bear- and others (1978, their Fig. 1), Malinverno and

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volcanic Figure 12. Working hypotheses to explain the asymmetrical opening of the Tyrrhenian Basin. A. "Roll-back" of the hinge-zone of the down-going African plate: VR is the horizontal velocity with which the hinge-zone

moves seaward, and V0 is the horizontal velocity of the stable part of the overriding plate, both positive to the right. Dewey

(1980) suggested that when VR =

gion, whereas VR > VQ should favor extension in the back-arc region. The figure has arc spreading been drawn with VQ = 0, and with slab dip alternating between steep and gentle, as suggested by Furlong and others (1982), but these details of the sketched scenario are not essential; the key point is that successive episodes of rollback have led to extension at successively more seaward locations. The saddle between the two basins could be either a remnant block of continental crust, as shown, and/or a previous volcanic arc, de- pending on whether extension in the overriding plate began behind or within the old relict bacIt- volcanic arc. arc basin

others (1981), and Moussat (1983), the only areas in the Tyrrhenian where seismic reflection profiles clearly indicate the presence of a thick evaporite sequence including the diapir-forming "lower evaporites" are in the western third of the Tyrrhenian: the deep structural basin under Cornaglia Terrace (near Site 653) and several small basins near Sardinia. From this distribu- relict back- active back- tion of acoustic facies, it was inferred that the western Tyrrhenian was the deepest part of the basin during the Messinian. Leg 107 results support this inference. At Sites 654 and 653, the shallowest of the Leg 107 sites, the Messinian sediments include alterna- tions of laminated gypsum intervals and clay- rich intervals. Forams and nannofossils are found in the clay-rich interbeds, suggesting periodic reflooding of a basin followed by evapora- tive brine formation and gypsum precipitation, in response to swinging of the "floodgate" of Hsu and others (1973). At Sites 652 and 656, presently the deepest of the passive margin sites, we recovered lacus- trine(?) and subaerial sediments underlying Pli- ocene marine sediments. Acknowledging the ambiguities in age and environment of deposition of these barren sediments, our preferred interpretation is that the present lower Sardinian margin was relatively high-standing during the Messinian. Consequently, Sites 652 and 656 received little or no input of Atlantic-derived sea water from the periodic refloodings of the Medi- (Kastens, Moscie et al., 1988 ; after Dewey, 1980-, Furlong et al., 1982; and MalinvernoSi Ryan, 1986) terranean during the desiccation event. (Note,

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' ••. Future fault

Figure 12. (Continued) B. Asymmetrical opening of a continental rift above detachment faults. The upper plate is stretched by slip on several generations of normal faults that sole in low-angle detachment faults (ti to t3>. What happens at the downdip end of the gently dipping faults is controversial: the illustrated ductile shear zone is after Lister and others (1988) but is not critical to the following discussion. As the overburden of the upper plate is removed, the lower plate bows upward isostatically O3, upper panel). This warps the early-formed detachment faults, which are then geometrically locked and unable to slip further. A new master detachment fault then slices into the intact lower plate, breaking the surface at a location farther to the right (east) than the original master detachment fault (t3, lower panel). Thus early stretching and subsidence, which occur above the older detachment faults, are observed farther west than late stretching and subsidence which occur above the new master detachment. This sketch is schematic only and is not intended to show individual specific Tyrrhenian fault blocks. The region marked "a", which stretched and subsided early, is broadly analogous to the region around Sites 654 and 653; whereas the region marked "b", which stretched and subsided later and more extremely, is analogous to Sites 652 and 656. Note that Figure 12B represents a smaller space and shorter time than does Figure 12A. The entire scenario of Figure B happens during the time interval between to and tj of Figure A. Consequently the two models are not mutually exclusive.

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however, that we did not reach the base of the asymmetry in the underpinnings of a back-arc sion of mantle lithosphere which should exist barren, inferred Messinian interval at either Site basin and would not be predicted at passive downdip from the detachment faults (off the 652 or 656, and so more typical evaporites margins bounding a major ocean. right-hand edge of Fig. 12B)? We suggest that in could conceivably lie beneath the bottom of A second explanation may be found in recent a back-arc setting such as the Tyrrhenian, the either or both holes.) The recovery of cobbles of models which view continental rifting and zone of mantle extension may coincide with the inferred eastern provenance in the basal con- passive-margin evolution as inherently asym- zone of partial melt under the active or incipient glomerate of Site 656 is compatible with a metrical processes (Bosworth, 1985; Gibbs, volcanic arc. generally westward-dipping topographic gra- 1984; Lister and others, 1986, 1988). Such Rift to Drift Transition. An interesting point dient from the emerging Apennines to the axis of asymmetry might result from underlying deeply concerns the relative timing between stretching Cornaglia Basin. penetrating, gently dipping, normal "detach- of continental crust on the Sardinia margin and We cannot rule out the possibility that Sites ment" fault(s) (Wernicke, 1981, 1985; Wer- initiation of basaltic crust formation. Barberi and 652 and 656 were, in fact, deep-lying during the nicke and Burchfiel, 1982) that split the evolving others (1978) reported K/Ar ages of 6.8 and 7.5 Messinian, but that topographic barriers cut off basin into an upper plate and a lower plate (Lis- m.y. for samples from 450 mbsf at DSDP Site sea water as from the present Dead Sea. The ter and others, 1986; note that these "plates" are 373, and they inferred (p. 511) that basalt had location of Sites 652 and 656 in different half- within the crust as opposed to the lithospheric erupted at a slow rate throughout the interval grabens reduces the probability of this alterna- "plates" of plate tectonics). As the load of the from 7.5 to 3 Ma. If the earlier dates are re- tive. Furthermore, there is evidence that Site 656 upper plate is removed, the lower plate warps liable, then fault-bounded blocks on the Sar- was not in a marine environment immediately upward into a broad arch (Fig. 12B, time t3). dinian margin were being tilted landward, and prior to the desiccation event: pre-Messinian The early-formed detachment faults, also thus presumably continental crust was being Miocene marine sediments are totally lacking warped upward, are now in an unfavorable stretched, at the same time as the oldest dated among the many clasts examined in the basal geometry for further motion, and a new master basaltic crust was being erupted. This overlap in conglomerates of Site 656. A plausible interpre- detachment fault breaks through in a more sea- time between stretching on the margin of the tation is that Site 656 subsided through modern ward position. Eventually, the upper plate basin and basalt injection in the center of the sea level while the Mediterranean was desic- evolves into a narrow, steep continental margin basin suggests that the concept of a discrete "rift- cated. The high clastic sedimentation rate im- with normal faults that are only weakly rota- ing" phase followed by a discrete "drifting" plied by the great thickness of the Messinian tional. The lower plate evolves into a wide, phase may be inapplicable to the Tyrrhenian in sequence at Site 652 is compatible with rapid complex continental slope with numerous rota- specific and possibly to back-arc basins in subsidence of the lower margin at this time. tional faults, tilted blocks, and half-grabens (see general. The assumption has been that after sea- Lister and others, 1986, their Fig. 2). floor spreading begins, stresses can no longer be Why Did Rifting and Subsidence Occur transmitted across the basin, and stretching Earlier on the Upper Margin Than on the The Tyrrhenian margins can be interpreted in stops; but in a small basin, perhaps stress is Lower Margin? The inference that extension terms of such an asymmetrically rifted basin: the transmitted around the ends of localized and subsidence occurred earlier on the upper broad Sardinian margin with its half-grabens "spreading centers." continental margin than on the lower margin and tilted fault blocks would represent a lower- seems, at first glance, to be counterintuitive. One plate-type passive margin, whereas the steep, might expect that successively younger seaward- narrow margin of peninsular Italy would be the CONCLUSIONS dipping normal faults would develop in more upper-plate-type passive margin. The seaward continentward positions as extensional deforma- (eastward) migration of the locus of rifting and The observations of Leg 107, together with tion gradually spread outward and engulfed un- subsidence would then be a natural consequence the careful work of numerous previous investi- deformed portions of the continent. One might of the upbowing and locking of early detach- gators, prompt the following generalizations and predict that the more extensively stretched and ment faults, and the subsequent development of inferences. thinned lower margin (where crustal thickness is a more seaward detachment fault. In this scenar- 1. Site 654, on the upper Sardinian margin, 7-8 km; Recq and others, 1984; Steinmetz and io, the structurally deep basin under Cornaglia recovered the first deep-sea record of the onset others, 1983) would have experienced a longer Terrace, and possibly the Sardinia Basin, would of the Messinian salinity crisis. The lowermost period of stretching than the less extensively represent early developing rift basins within the Messinian sediments are open-marine nannofos- stretched upper margin (crustal thickness: 20 evolving rifted margin. Asymmetrical rifting sil ooze, lithologically indistinguishable from the km). Neither prediction is supported by our may also explain the near-surface recovery underlying Tortonian sediments. These are over- observations. of mantle peridotite at Site 651 (Lavecchia, lain by 36 m of finely laminated, organic- We suggest two potential resolutions for this in press) and the asymmetrical heatflow dis- carbon-rich claystone and siltstone, in which paradox. First, the position of most active rifting tribution of the Tyrrhenian Sea (Wang and siliceous fossils are preserved. The upper 70 m of and subsidence may have migrated eastward for Shi, in press). This interpretation of the Tyrrhe- the Messinian sequence comprises intervals of the same reason that the site of basaltic crust nian margins as a result of slip along a low-angle clays and of finely laminated gypsum. The clay- formation would migrate eastward several mil- detachment fault is not incompatible with the rich layers contain planktonic foraminifera and lion years later. We have tentatively associated interpretation of the Tyrrhenian as a back-arc nannofossils, suggesting occasional reflooding of the latter observation with "rollback" (sensu basin. In fact, the back-arc setting of the Tyr- the area by marine waters. Dewey, 1980) of the hinge zone of the down- rhenian may help to resolve a question common 2. Our data from the present-day, lower Sar- going plate. In this case, the observed seaward to all models involving nonuniform lithosphere dinian margin suggest that the physiographic po- shift in rifting would be an inherent result of the extension: where is the requisite zone of exten- sition of this area during the Messinian was such

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that the occasional refloodings from the Atlantic toward the subduction zone, as if by "passive was funded by the governments of Canada, the did not get here. At Site 656, the inferred Mes- subduction" (Ritsema, 1979) or "rollback" Federal Republic of Germany, France, Great sinian sediments comprise a few meters of barren (Dewey, 1980) of the high-density down-going Britain, Japan, and the United States. Prepara- dolomitic marl overlying subaerial conglomer- slab. tion of this manuscript was partially funded ates with an iron-oxide-rich matrix. At Site 652, 7. The axis of Vavilov Basin at Site 651 is by the U.S. National Science Foundation via the inferred Messinian sediments are primarily underlain by a body of peridotite, which could the United States Science Advisory Com- barren, rapidly deposited, clastic muds and silts, represent either a fragment of Mesozoic Ap- mittee (USSAC). This paper was written while with occasional lenses and nodules of chemical ennine ophiolite or a body of uplifted sub- K. Kastens was a Visiting Lecturer at Yale sediments. The environment of deposition is in- Tyrrhenian upper mantle. In the latter case, the University. She thanks Yale's Department of terpreted as a closed lake with extreme fluctua- mantle-derived body may have been emplaced Geology and Geophysics for their hospitality, tions of salinity. The lack of access to marine at the rift-to-drift transition, when extensive and the National Science Foundation's Program incursions can be attributed to either a high- stretching had thinned the crustal carapace but of Visiting Professorships for Women in Science standing position or to a position behind a topo- before mantle-melting and basalt-generating (Grant RII-8600385) for financial support. graphic barrier. processes were well established. Tectonic un- 3. The upper Sardinian margin (Site 654) roofing by extension along a "detachment fault" subsided through sea level during the Tortonian. might have facilitated the emergence of mantle- The lower Sardinian margin (Site 656) probably derived material. REFERENCES CITED subsided through modern sea level during the 8. Benthic foraminiferal assemblages and ves- Alvarez, W., Cocozza, T., and Wezel, F. C., 1974, Fragmentation of the Alpine Messinian. The fault block on which Site 654 is orogenic belt by microplate dispersal: Nature, v. 248, p. 309-314. icle abundance suggest that the basalt at Site 650 Argand, E., 1924, Tectonique de l'Asie, International Geological Congress, located (upper Sardinian margin) was actively erupted significantly shallower than its present 13th, Bruxelles, p. 171-372. Barberi, F., Innocenti, F., Ferrara, G., Keller, J., and Villari, L., 1974, Evolution tilting between Tortonian and Messinian times. depth. If our depth and age estimates are accu- of the Eolian arc volcanism (southern Tyrrhenian Sea): Earth and The fault block on which Site 652 is located Planetary Science Letters, v. 21, p. 269-276. rate, basement at Site 650 subsided to its present Barberi, F., Bizouard, H., Capaldi, G., Ferrara, G., Gasparini, P., Innocenti, F., (lower Sardinian margin) was actively tilting be- depth at a rate on the order of 700 m/m.y., Jordou, J. L., Lambert, B„ Treuil, M., and AUegre, G, 1978, Age and nature of basalts from the Tyrrhenian Abyssal Plain, in Hsu, K., Monta- tween Messinian and Pliocene times. supporting Kobayashi's (1984) suggestion that dert, L., and others, Initial reports of the Deep Sea Drilling Project, Volume 42, part 1: Washington, D.C., U S. Government Printing Of- 4. From observations 2 and 3, we conclude back-arc basin lithosphere follows a steeper sub- fice, p. 509-514. sidence curve than does open-ocean sea floor. Barker, P. F„ and Hill, I. A., 1981, Back-arc extension in the Scotia Sea: Royal that stretching of continental crust, as evidenced Society of London Philosophical Transactions, v. 300A, p. 249-262. by subsidence and tilting of near-surface sedi- 9. Volcaniclastic sediments dominate the Beccaluva, L., Gabbianelli, G., Lucchini, F., Rossi, P. L., Savelli, C., and Zeda, O., 1981, Magmatic character and K/Ar ages of volcanics dredged ments, occurred earlier on the upper margin Plio-Pleistocene turbidite fill of the Vavilov and from the Eolian seamounts (Tyrrhenian Sea), in Wezel, F. C., ed., Sedimentary basins of Mediterranean margins: C.N.R. Italian Project of than on the lower margin of Sardinia. Working Marsili Basin floors (Sites 650 and 651), and the Oceanography, Tecnoprint, p. 361-368. hypotheses to explain this sequence of events rate of volcaniclastic influx at both sites Beccaluva, L., Gabbianelli, G., Lucchini, F., Rossi, P. L„ and Savelli, C., 1985, Petrology and K/Ar ages of volcanics dredged from the Eolian sea- invoke (a) migration of the locus of maximum increases up-section. This could be a tectonically mounts: Implications for geodynamic evolution of the southern Tyr- rhenian basin: Earth and Planetary Science Letters, v. 74, p. 187-208. extension over a "rolling-back" subduction zone driven phenomenon, indicative of increased Biju-Duval, B., Dercourt, J., and Le Picbon, X., 1977, From the Tethys Ocean or (b) asymmetrical rifting over a deeply pene- volcanism in the Eolian Islands and Roman to the Mediterranean Seas: A plate tectonic model of the western Alpine system, in Biju-Duval, B., and Montadert, L., eds., Structural history of trating low-angle "detachment fault." volcanic provinces. Plausible alternative expla- the Mediterranean Basins: Paris, Technip, p. 143-164. Boccaletti, M., Horvath, F., Loddo, M., Mongelli, F., and Stegena, L., 1976, 5. In the Vavilov and Marsili Basins (Sites nations include increased turbidity current trans- The Tyrrhenian and Pannonian basins: A comparison of two Mediter- 650,651, and 655), where previous researchers port in response to eustatic sea-level fluctuations, ranean interarc basins: Tectonophysics, v. 35, p. 45-69. Bolis, G., Capelli, V., and Marinelli, M., 1981, Aeromagnetic data of the Italian had tentatively mapped subaerial clastics, pyro- and/or increased bypassing of nearshore sedi- area: Instrumental to a better comprehension of the basement main characteristic in Italy, Carta Magnetica d'ltalia: AGIP-43rd E.A.E.G. clastics, and thin ("marginal") evaporites (Mon- ment traps (Cita and others, 1978a). meeting in Venezia (Italy). tadert and others, 1978; Fabbri and Curzi, 1979; Bosworth, W., 1985, Geometry of propagating continental rifts:Nature , v. 316, p. 625. Malinverno and others, 1981; Moussat, 1983), ACKNOWLEDGMENTS Broglia, C., and Moss, D., 1988, In situ structure and properties of 110 Ma crust from geophysical logs in Deep Sea Drilling Project Hole 418A: Ocean we cored no Messinian facies. We infer that Drilling Program Proceedings, v. 102, part B (in press). much of the crust of these basins had not yet Caputo, M., Panza, G. F., and Postpischel, D., 1970, Deep structure id We gratefully acknowledge the competence the Mediterranean basins: Journal of Geophysical Research, v. 75, been formed at the time of the Messinian and cooperation of the Captain, officers, and rig p. 4919-4923. Carlson, R. L., and Melia, P. J., 1984, Subduction hinge migration: Tectono- desiccation. floor crew of the Sedco BP 471 (Joides Resolu- physics, v. 102, p. 399-411. Cita, M. B., Ryan, W.B.F., and Kidd, R. B., 1978a, Sedimentation rates in 6. Local formation of basaltic crust in the tion) during Ocean Drilling Program Leg 107. Neogene deep sea sediments from the Mediterranean and geodynamics Vavilov Basin may have occurred during the ODP Operations Superintendent David Huey, implications of their changes, in Hsu, K., Montadert, L., and others, Initial reports of the Deep Sea Drilling Project, Volume 42, part 1: upper Miocene, and the process was certainly Lab Officer "Gus" Gustaferson, and the ODP Washington, D.G, U.S. Government Printing Office, p. 991-1002. Cita, M. B, Wright, R. C„ Ryan, W.B.F., and Longinelli, A, 1978b, Messinian widespread by the lower Pliocene. There was odd-leg technicians were invaluable in the paleoenvironments, in Hsii, K., Montadert, L., and others, Initial reports collection of these data. Discussions with of Deep Sea Drilling Project, Volume 42, part 1: Washington, D.C., probably an overlap in time between apparent U.S. Government Printing Office, p. 1003-1035. stretching of the Sardinia margin and the forma- H. Chambley, G. Karner, G. Lavecchia, Colantoni, P, Fabbri, A., Gallignani, P, Sartori, R, and Rehault, J.-P., 1981, Lithologic and stratigraphic maps of the Italian Seas: Consiglio Nazio- tion of basaltic crust in Vavilov Basin. Forma- G. Lister, A. Malinverno, W.B.F. Ryan, C. nale delle Ricerche Publication, no. 4. tion of basaltic crust in the Marsili Basin Schreiber, and numerous other scientists have Delia Vedova, B„ Pellis, G, Foucher, J. P., and Rehault, J. P., 1984, Geo- thermal structure of the Tyrrhenian Sea: Marine Geology, v. 55, probably began about 2 Ma. The apparent later helped to clarify the thoughts expressed herein. p. 271-289. Dercourt, J., Zonenshain, L. P., Ricou, L. E., Kazmin, V. G., Le Pichon, X., date of initiation of basaltic crust formation in The manuscript was reviewed by A. Malin- Knipper, A. L., Grandjacquet, C., Sbortshikov, I. M., Geyssant, J., Lepvrier, C., Pechereky, D. H., Boulin, J., Sibuet, J. C., Savostin, L. A., Marsili Basin relative to Vavilov Basin is con- verno, B. C. Schreiber, B. Tucholke, and R. Von Sorokhtin, O., Westphat, M., Bazhenov, M. L., Laver, J. P., and Biju- sistent with previous suggestions that the Tyr- Herzen, whom we thank for their insights and Duval, B., 1986, Geologic»! evolution of the Tethys belt from the Atlantic to the Pamirs since the Lias: Tectonophysics, v. 123, rhenian Sea has grown southeastward, that is, suggestions. Ocean Drilling Program Leg 107 p. 241-315.

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MANUSCRIPT RECEIVED BY THE SOCIETY MAY 28,1987 Isotopic, trace and major element compositions of back arc basin vol- Ogniben, L, 1957, Petrografia délia Série Solfifera Siciliana e considerazioni REVISED MANUSCRIPT RECEIVED DECEMBER 22,1987 canics, Valu Fa Ridge, Lau Basin: Evidence for a subduction zone geologische relative: Memorie Descrittive délia Carta Geologica d'Italia, MANUSCRIPT ACCEPTED JANUARY 13,1988 influence: Journal of Geophysical Research. v. 33. LAMONT-DOHERTY GEOLOGICAL OBSERVATORY CONTRIBUTION NO. 4306

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