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Young Tyrrhenian Sea evolved very quickly
Young Tyrrhenian Sea evolved very quickly* In t h e M e d i t e r r a n e a n
thin crust. Because earlier evidence indicated that the southeastern (Vavilov) basin was younger than the northeastern (Marsili) basin, the Tyrrhenian Sea was considered a good field area tk)r testing the hypothesis of expansion of a backare basin through seaward migration of the are and subduction zone. And because the Tyrrhenian is bounded to the northeast and southeast by orogenic belts, interactions between extension and collision could be explored. Like other passive margins, the western Tyrrhenian Sea has continental crust that has been stretched and thinned by listric faulting. A main goal of drilling on the Tyrrhenian margin was to determine the timing and rate of extension and subsidence during the stretching phase and during oceanic-crust emplacement. Because the Tyrrhenian passive
The Tyrrhenian Sea is a small triangular part of the Mediterranean Sea, surrounded by mainland Italy and the
islands of Sicily, Sardinia and Corsica. Drilling objectives for Leg 1()7 of the Ocean Drilling Program considered the Tyrrhenian Sea from these perspectives: as a backarc basin: as a young, passive margin: and as a stratigraphic type locality. Like other backarc basins, the Tyrrhenian Sea has a Benioff zone, a calcalkaline volcanic belt, thinned continental crust on the margins, tholeiitic volcanism, high heat flow and high-amplitude magnetic anomalies. The southeastern half of the Tyrrhenian Sea contains 2 small deep basins floored with * This and subsequent reports have been reproduced from Geotimes, with kind permission of the Ocean Drilling Program
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Figure 1 The Tyrrhenian Sea is surrounded by mainland Italy and the islands of Sardinia, Corsica, and Sicily. Leg 107 of the Ocean Drilling Program drilled a northwest/southeast transect of 7 sites across the Tyrrhenian from the Sardinian passive margin across 2 small deep ocean basins. (From the Ocean Drilling Program; bathymetry from Internationalbathymetricchart of the Mediterranean, Intergovernmental Oceanographic Commission [Unesco), 1981) 258
M a r i n e a n d P e t r o l e u m G e o l o g y , 1987, V o l 4, A u g u s t
Ocean Drifting Program
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Figure 2 Simplified lithologic columns for each site are projected onto a bathymetric and crustal thickness cross-section of the Tgrrhenian Sea. The position of the transect is shown in the map, upper right. Graphic symbols for lithologic columns of Sites 650 to 656: (1) nannofossil ooze; (2) nanno-foram or foram-nanno ooze; 13) calcareous ooze; (4) nannofossil chalk; (5) nanno-foram or foram-nanno chalk; (6)calcareous chalk; (7)clag/cla`/stone; (8)mud/mudstone; (9)sand,/ mud/sand,/ mudstone; (10)silt/siltstone; (11) sand/sandstone; (12} gravel; (13) conglomerate; (14) basic igneous; (15) pebbles and red carbonate mud matrix; (16) dolomite; (17) volcanic ash; (18) volcanic lapilli; (19) gypsum; (20) complex transition zone; (21) peridotite. (From the Ocean Drilling Program; cross section from 'A 560 km long Moho traverse in the l-`/rrhenian Sea from O.B.S. recorded Pn waves' by k. Steinmetz otaL Geophys. Res. Let&, 1982,, 111, [61) margin borders on a young ocean basin and has a low sedimentation rate, the pre-rift, syn-rift and post-rift sediment sections are more accessible for drilling than on other margins, Another goal of Leg 107 was to obtain a near-continuous Plio-Pleistocene sequence of pelagic sediments to serve as a deep-sea type section. Plio-Pleistocene stages were defined originally in land sections around the Tyrrhenian Sea but until now stratigraphic correlations between the enclosed Mediterranean and open-ocean records have been ambiguous. Therefore, a correlation of various chronologies (biostratigraphy, magnetostratigraphy, tephrochronology and stable-isotope stratigraphy) was considered essential. Leg 107 drilled a transect of 7 sites from a young passive continental margin to an even younger oceanic-type basin. Results from each site are described from northwest to southeast, Site 654 is a fault-bounded tilted block on the slightly stretched upper contintental margin. Seismic reflection profiles show westward-dipping subparallel reflectors at depth (interpreted as the pre-rift sediment sequence). Those are overlain by a westward-thickening wedge of sediment interpreted as the syn-rift sequence. Flat-lying subparallel reflectors interpreted as the post-rift sequence overlie the syn-rift wedge, At the base of the hole, we recovered a transgressive sequence caused by the tectonic subsidence of the rifting margin. Subaerial conglomerates are overlain by shallow-marine oyster-bearing glauconitic sand. The sand is Marine
overlain by marine nannofossil ooze of upper Tortonian and lowermost Messinian (latest Miocene)age. Like the rest of the Mediterranean Sea, Site 654 experienced drastic environmental changes and dessication during the upper Messinian. Finely laminated mudstones containing siliceous biogenic remains are overlain by cycles of laminated gypsum alternating with mud and marl. Rifting slowed or stopped during the upper Messinian. Pliocene and Pleistocene open-marine ooze then draped conformably over the syn-rift strata. Site 653 is a bathymetric terrace midway down the margin of Sardinia, less than a kilometer from Site 132 of the Deep Sea Drilling Project. In this region, seismic-reflection data show a thick sequence of Messinian evaporites filling a north-south trending structural basin. The 2 holes drilled at Site 653 recovered a nearly complete Pliocene/Pleistocene pelagic sequence, which should become a type section for comparative stratigraphic studies. Also, Site 653 documented significant lateral lithofacies variability m the uppermost Messinian sediments. Site 652 is in a structural setting comparable to Site 654, but on thinner continental crust. The lower part of the section comprises a wedge of barren well-bedded subaqueous elastic sediments. Thin layers and nodules of gypsum or anhydrite are common. We think that the barren elastic sequence wits deposited in a lacustrine environment during the Messinian episode of lowered sea level. A comparison between seismic-reflection data and recovered lithologies suggests that the transition from pre-rift to syn-rift sediments occurs in the lowest part of the recovered sequence. and Petroleum
Geology,
1 9 8 7 , V o l 4, A u g u s t
259
Ocean Drilling Program The barren inferred-Messinian sediments are overlain by open-marine oozes of Pliocene and Pleistocene age. An upsection decrease in the debris flows suggests that the transition from syn-rift to post-rift sedimentation occurred in the lower Pliocene. Site 656 is on the western flank of the de Marchi Seamount, which is the last tilted continental block, just west of the transition to inferred oceanic crust. Beneath a Pleistocene turbidite cover, we recovered a condensed Pliocene sequence with several major hiatuses. The Pliocene sequence overlies a conglomerate with a dolomitic iron-oxide rich matrix, interpreted as a Messinian subaerial deposit. Loose fragents of amphibolite metadolerite, metaquartzite, marble, greenstone and chert, derived apparently from an underlying Alpine-type basement, were recovered at the base of the cored section. Site 655 is on a narrow north-south trending ridge near the transition between thinned continental and inferred oceanic crust. 115 m of porphyritic basalt were recovered below a thin upper Pliocene and Pleistocene pelagic cover. Curved, glassy, chilled margins suggest that the ridge at the drill site is constructed of numerous pillow-basalt flows. Two lines of evidence constrain the age of eruption of the pillows. FirsL the entire basalt pile is reverse magnetized. Second, carbonate within cracks in the basalt contains planktonic foraminifera of zone MPL-4 and nannofossils of zone NN-15. Together. those observations indicate an age of 3.4 to 3.6 million years. Site 6.51 is on the eastern flank of a north-south trending basement swell. The swell bisects the Vavilov Basin, the northwestern of the 2 deep Tyrrhenian Sea basins. The Plio-Pleistocene sedimentary section consists of pyroclastic debris in hemipelagic ooze. The basement at Site 651 is unexpectedly complex: 3 0 m of mantle-derived serpentinized periodotite are overlain by 30 m of basalt and basaltic breccia, then by a 28 m thick assemblage of dolerite, metasediments and metadolerite. The shallowest basement unit is another basalt unit 78 m thick.
Site 650 is near the western rim of Marsili Basin, the southeastern of the 2 deep Tyrrhenian basins. Seismicreflection data show that the sediment cover at Site 650 is among the thickest in the Marsili Basin. The sedimentary sequence of Site 650 is similar to that of Site 651. The age of the base of the sediment column, constrained by both biostratigraphy and magnetostratigraphy, is surprisingly young (1.7 to 1.9 million years). Basement consists of vesicular basalt; vesicles are large (up to 3 or 4 millimetres diameter) and abundant (about 2(l% of the rock volume). Textural evidence suggests that the basalt was emplaced as a flow rather than a sill. The high degree of vesicularity implies emplacement at a depth significantly shallower than the present depth (4,118 m below sea level).
Leg 107 touched on a wide range of geologic questions, including passive-margin and backarc-basin cwHution, crustal heterogeneity and protrusion of upper-mantle material to the sea floor, chronology of circum-Tyrrhenian eruptive volcanism, cyclic evaporite deposition, origin of sapropels, origin of metalliferous "basal" sediments, and definition of the Miocene/Pliocene boundary. The rest of this report focuses on the outstanding tectonic results: evidence that the Tyrrhenian Sea is very young and has evolved quickly, and that the locus of intense tectonic activity and of ocean-crust formation has migrated through time toward the subduction zone. On the upper Sardinian margin (Site 654), rifting began in the upper Tortonian and slowed significantly or stopped in the upper Messinian. However, on the lower Sardinian margin (Sites 652 and 656), rifting apparently began during the Messinian and probably ended in the lower Pliocene. Those observations indicate that rifting and subsidence were diachronous across the continental margin. By the Messinian, rifting and subsidence of what is no~, the upper and middle Sardinian margin were advanced enough to
Figure 3 This photograph from Site 650 shows the contact between vesicular basalt and upper Pliocene dolomitic nannofossil ooze. The extreme vesicularity of the basalt implies that it was emplaced in water much shallower than its present depth, 4,118 m below sea level. (From Ocean Drilling Program) 260
M a r i n e a n d P e t r o l e u m G e o l o g y , 1987, V o l 4, A u g u s t
Ocean Drilling Program allow deposition of basinal evaporites at Sites 654 and 653. Rifting and subsidence on what is now the lower Sardinian margin was less advanced resulting in lacustrine and subaerial Messinian facies at Sites 652 and 656. Since the water depth at Site 654 is now 1.3 km shallower than at Site 652, a drastic reorganization of the morphology of the ancestral Tyrrhenian basin must have occurred during and since the Messinian. We attribute the reorganization to progressive rifting. Neither the Vavilov Basin nor the Marsili Basin seems to have existed as a deep ocean basin during the Messinian. Just as the locus of intense rifting migrated from northwest to southeast, a comparable shift in the location of crustal accretion occurred from the northwestern deep basin (Vavilov) to the southeastern deep basin (Marsili). Basalts cored in the Vavilov Basin (Sites 655 and 651) are estimated to be about 3.5 million years old, based on biostratigraphitally controlled dating of sediment overlying and filling cracks in the basalt. Seismic-reflection data imply that we did not
sample the oldest basement of Vavilov Basin. Since Site 650 was in the area of thickest sediment cover in the Marsili Basin, the basalt sampled was probably among the oldest in the basin. The age of the basalt at the Site 650 sediment/basalt contact is biostratigraphically and magnetostratigraphically constrained at 1.6 to 1.9 million years. Therefore. injection of basaltic crust began at least 1.5 million years later in Marsili Basin than in Vavilov Basin. Leg 107 has shown that rifting and basin formation in the Tyrrhenian Sea migrated southeastward toward the subducting plate. Furthermore. the process is very recent: the present phase of rifting began as recently as upper Tortonian, and the younger of the deep basins appears to be less than 2 million years old.
Leg 107 Scientific Drilling Party Ocean Drilling Program, Texas A & M University, College Station, 77843-3669, USA
Paleoclimate studied in the east Atlantic On Leg 108 Leg 108 of the Ocean Drilling Program drilled 27 holes at 12 sites from February 12 to April 17, in the low-latitude eastern Atlantic Ocean. The purpose of the cruise was to study the evolution of Neogene paleoclimate and paleoceanography. Favorable coring conditions led to recovery of about 3,850 m of cores, whictl represented 90% of the drilled section. That is the greatest amount of cores ever collected on a single ocean-drilling cruise. It will take years to investigate in detail the samples obtained from those sediments in order to extract high-resolution signals at the periods of Earth-orbital changes (20,000 to 10(I,000 years). Evidence from late Pleistocene piston cores suggests that the orbital rhythms carry much of the key paleoclimatic response of those regions. However, important contributions arc already apparent from the coarse-resolution shipboard analvses obtained during the cruise. The tropical-subtropical eastern Atlantic Ocean was chosen for drilling during Leg 108 because it contains a critical boundary zone of surface-water oceanography that includes the lntertropical Convergence Zone (ITCZ) and the thermal equator. In the deep water, the Sierra Leone Rise forms an almost continuous, tectonically inactive barrier between the basins of the south and north Atlantic. Long-term changes in and along those major equatorial boundaries are linked closely to the history of Neogene global climate change. The main objectives of Leg 108 were to study whether and to what degree the changes are controlled by polar components of the climate system such as the build-up of ice sheets and the formation of sea ice, and to what degree the low-latitude ocean-atmosphere components evolve independentb, such as those changes that result from variations in low-latitude solar isolation. It was also important to evaluate the relative importance of the 2 polar regions in influencing climate change near the equator. To investigate such complex questions, we studied microfossil surface-ocean indicators in the sediment sequences. We paid particular attention to changes in the tropical ocean at times of prominent changes in Northern Hemisphere ice volumes at about 3.0 to 2.5 and (1.9 million years ago, and in Southern Hemisphere ice-sheet changes at 5 to 6 million years ago. Leg 108 also aimed to study the history of upwelling intensity and biologic productivity in the eastern equatorial and subtropical Atlantic. The shipboard part of that work was based on monitoring changes in the abundance of the opaline plankton and of organic carbon in the sediment. By those means, we intended to define the latitudinal persistence of the upwelling cells during varying climates and to assess their significance in the broader climate context of variations in
global CO:, the evaporation balance of the equatorial ocean. and the deposition of sediments rich in organic carbon. Another objective was to measure variations of wind circulation along the ITCZ as deduced from the abundance of wind-blown particles in the sediment. The changes may be vital for understanding the evolution of the tropical Hadley Cell and the fluctuating (orbital) cvcles in African aridity and monsoon rains in the past. Leg 108 also tried to determine the Neogene history of deep-water exchange through fracture zones between the western and eastern Atlantic basins and between the southern and northern east Atlantic, as well as to investigate the cause of incursions of Antarctic-source bottom water into the northeastern Atlantic. Based on evidence from upper Quaternary sediments recovered by gravity cores, differences in the bathymetric distribution of calcium carbonate and stable carbon isotopes were expected to provide insights into dramatic short-term changes in the exchange of deep water and oxygen between the diflerent basins. Several seismic reflectors also suggest long-term changes of deep-water circulation, events that have been undated and unexplained in a paleoclimatic context. In pursuing those objectives, Leg 108 cored a latitudinal transect of 12 sections of middle to late Cenozoic, and in one case upper Cretaceous, sediments. The transect spans 24° of latitude from 22°N to almost 2%. DSDP Leg 94 cored earlier 6 sites in the transitional to subpolar latitudes of North Atlantic (37°-55°N). Those sites include every major surface-water mass between the south equatorial divergence and the northern part of the North Atlantic Drift, and the northeastern extension of the Gulf Stream. The advanced hydraulic piston corer and extended core barrel systems were both used to obtain cores. Two or three holes were cored at most sites to recover sediment sections that were as complete as possible. Previous DSDP and ODP paleoceanographic legs had difficulty obtaining continuous sections, despite coring 2 offset holes with the hydraulic piston corer at some sites: disturbed cores and under-recovery of some sections create gaps in the sediment sequence and thus in the climate record. Only DSDP Leg 94 succeeded in verifying continuity of section in detail (tens of centimetres) at sea by tracking CaCO3 layering in core photographs and then spot coring any gaps. New correlation techniques pioneered a fine-scale high-resolution analysis of cored sediment sections on Leg 108. Two signals (P-wave volocity and magnetic susceptibility) were measured continuously at intervals of a few centimetres on unsplit cores and formed the basis of this
Marine and Petroleum Geology, 1987, Vol 4, August
261
thin crust. Because earlier evidence indicated that the southeastern (Vavilov) basin was younger than the northeastern (Marsili) basin, the Tyrrhenian Sea was considered a good field area tk)r testing the hypothesis of expansion of a backare basin through seaward migration of the are and subduction zone. And because the Tyrrhenian is bounded to the northeast and southeast by orogenic belts, interactions between extension and collision could be explored. Like other passive margins, the western Tyrrhenian Sea has continental crust that has been stretched and thinned by listric faulting. A main goal of drilling on the Tyrrhenian margin was to determine the timing and rate of extension and subsidence during the stretching phase and during oceanic-crust emplacement. Because the Tyrrhenian passive
The Tyrrhenian Sea is a small triangular part of the Mediterranean Sea, surrounded by mainland Italy and the
islands of Sicily, Sardinia and Corsica. Drilling objectives for Leg 1()7 of the Ocean Drilling Program considered the Tyrrhenian Sea from these perspectives: as a backarc basin: as a young, passive margin: and as a stratigraphic type locality. Like other backarc basins, the Tyrrhenian Sea has a Benioff zone, a calcalkaline volcanic belt, thinned continental crust on the margins, tholeiitic volcanism, high heat flow and high-amplitude magnetic anomalies. The southeastern half of the Tyrrhenian Sea contains 2 small deep basins floored with * This and subsequent reports have been reproduced from Geotimes, with kind permission of the Ocean Drilling Program
t
o°
42 ° _
E
I
°
,.-
.
,
,
:"
ITALY
41 °
D#
4
653 e
•
.•',.
40 °
b ~/
D
~ ~f
Vavilov Basin
650 •
Basin
39 °
38° I
I 10 °
1 1 1°
...." - . 12 °
v.
13 °
..'-.-
..:
14 °
-.' 15°
". 16°
Figure 1 The Tyrrhenian Sea is surrounded by mainland Italy and the islands of Sardinia, Corsica, and Sicily. Leg 107 of the Ocean Drilling Program drilled a northwest/southeast transect of 7 sites across the Tyrrhenian from the Sardinian passive margin across 2 small deep ocean basins. (From the Ocean Drilling Program; bathymetry from Internationalbathymetricchart of the Mediterranean, Intergovernmental Oceanographic Commission [Unesco), 1981) 258
M a r i n e a n d P e t r o l e u m G e o l o g y , 1987, V o l 4, A u g u s t
Ocean Drifting Program
:-
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653
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Figure 2 Simplified lithologic columns for each site are projected onto a bathymetric and crustal thickness cross-section of the Tgrrhenian Sea. The position of the transect is shown in the map, upper right. Graphic symbols for lithologic columns of Sites 650 to 656: (1) nannofossil ooze; (2) nanno-foram or foram-nanno ooze; 13) calcareous ooze; (4) nannofossil chalk; (5) nanno-foram or foram-nanno chalk; (6)calcareous chalk; (7)clag/cla`/stone; (8)mud/mudstone; (9)sand,/ mud/sand,/ mudstone; (10)silt/siltstone; (11) sand/sandstone; (12} gravel; (13) conglomerate; (14) basic igneous; (15) pebbles and red carbonate mud matrix; (16) dolomite; (17) volcanic ash; (18) volcanic lapilli; (19) gypsum; (20) complex transition zone; (21) peridotite. (From the Ocean Drilling Program; cross section from 'A 560 km long Moho traverse in the l-`/rrhenian Sea from O.B.S. recorded Pn waves' by k. Steinmetz otaL Geophys. Res. Let&, 1982,, 111, [61) margin borders on a young ocean basin and has a low sedimentation rate, the pre-rift, syn-rift and post-rift sediment sections are more accessible for drilling than on other margins, Another goal of Leg 107 was to obtain a near-continuous Plio-Pleistocene sequence of pelagic sediments to serve as a deep-sea type section. Plio-Pleistocene stages were defined originally in land sections around the Tyrrhenian Sea but until now stratigraphic correlations between the enclosed Mediterranean and open-ocean records have been ambiguous. Therefore, a correlation of various chronologies (biostratigraphy, magnetostratigraphy, tephrochronology and stable-isotope stratigraphy) was considered essential. Leg 107 drilled a transect of 7 sites from a young passive continental margin to an even younger oceanic-type basin. Results from each site are described from northwest to southeast, Site 654 is a fault-bounded tilted block on the slightly stretched upper contintental margin. Seismic reflection profiles show westward-dipping subparallel reflectors at depth (interpreted as the pre-rift sediment sequence). Those are overlain by a westward-thickening wedge of sediment interpreted as the syn-rift sequence. Flat-lying subparallel reflectors interpreted as the post-rift sequence overlie the syn-rift wedge, At the base of the hole, we recovered a transgressive sequence caused by the tectonic subsidence of the rifting margin. Subaerial conglomerates are overlain by shallow-marine oyster-bearing glauconitic sand. The sand is Marine
overlain by marine nannofossil ooze of upper Tortonian and lowermost Messinian (latest Miocene)age. Like the rest of the Mediterranean Sea, Site 654 experienced drastic environmental changes and dessication during the upper Messinian. Finely laminated mudstones containing siliceous biogenic remains are overlain by cycles of laminated gypsum alternating with mud and marl. Rifting slowed or stopped during the upper Messinian. Pliocene and Pleistocene open-marine ooze then draped conformably over the syn-rift strata. Site 653 is a bathymetric terrace midway down the margin of Sardinia, less than a kilometer from Site 132 of the Deep Sea Drilling Project. In this region, seismic-reflection data show a thick sequence of Messinian evaporites filling a north-south trending structural basin. The 2 holes drilled at Site 653 recovered a nearly complete Pliocene/Pleistocene pelagic sequence, which should become a type section for comparative stratigraphic studies. Also, Site 653 documented significant lateral lithofacies variability m the uppermost Messinian sediments. Site 652 is in a structural setting comparable to Site 654, but on thinner continental crust. The lower part of the section comprises a wedge of barren well-bedded subaqueous elastic sediments. Thin layers and nodules of gypsum or anhydrite are common. We think that the barren elastic sequence wits deposited in a lacustrine environment during the Messinian episode of lowered sea level. A comparison between seismic-reflection data and recovered lithologies suggests that the transition from pre-rift to syn-rift sediments occurs in the lowest part of the recovered sequence. and Petroleum
Geology,
1 9 8 7 , V o l 4, A u g u s t
259
Ocean Drilling Program The barren inferred-Messinian sediments are overlain by open-marine oozes of Pliocene and Pleistocene age. An upsection decrease in the debris flows suggests that the transition from syn-rift to post-rift sedimentation occurred in the lower Pliocene. Site 656 is on the western flank of the de Marchi Seamount, which is the last tilted continental block, just west of the transition to inferred oceanic crust. Beneath a Pleistocene turbidite cover, we recovered a condensed Pliocene sequence with several major hiatuses. The Pliocene sequence overlies a conglomerate with a dolomitic iron-oxide rich matrix, interpreted as a Messinian subaerial deposit. Loose fragents of amphibolite metadolerite, metaquartzite, marble, greenstone and chert, derived apparently from an underlying Alpine-type basement, were recovered at the base of the cored section. Site 655 is on a narrow north-south trending ridge near the transition between thinned continental and inferred oceanic crust. 115 m of porphyritic basalt were recovered below a thin upper Pliocene and Pleistocene pelagic cover. Curved, glassy, chilled margins suggest that the ridge at the drill site is constructed of numerous pillow-basalt flows. Two lines of evidence constrain the age of eruption of the pillows. FirsL the entire basalt pile is reverse magnetized. Second, carbonate within cracks in the basalt contains planktonic foraminifera of zone MPL-4 and nannofossils of zone NN-15. Together. those observations indicate an age of 3.4 to 3.6 million years. Site 6.51 is on the eastern flank of a north-south trending basement swell. The swell bisects the Vavilov Basin, the northwestern of the 2 deep Tyrrhenian Sea basins. The Plio-Pleistocene sedimentary section consists of pyroclastic debris in hemipelagic ooze. The basement at Site 651 is unexpectedly complex: 3 0 m of mantle-derived serpentinized periodotite are overlain by 30 m of basalt and basaltic breccia, then by a 28 m thick assemblage of dolerite, metasediments and metadolerite. The shallowest basement unit is another basalt unit 78 m thick.
Site 650 is near the western rim of Marsili Basin, the southeastern of the 2 deep Tyrrhenian basins. Seismicreflection data show that the sediment cover at Site 650 is among the thickest in the Marsili Basin. The sedimentary sequence of Site 650 is similar to that of Site 651. The age of the base of the sediment column, constrained by both biostratigraphy and magnetostratigraphy, is surprisingly young (1.7 to 1.9 million years). Basement consists of vesicular basalt; vesicles are large (up to 3 or 4 millimetres diameter) and abundant (about 2(l% of the rock volume). Textural evidence suggests that the basalt was emplaced as a flow rather than a sill. The high degree of vesicularity implies emplacement at a depth significantly shallower than the present depth (4,118 m below sea level).
Leg 107 touched on a wide range of geologic questions, including passive-margin and backarc-basin cwHution, crustal heterogeneity and protrusion of upper-mantle material to the sea floor, chronology of circum-Tyrrhenian eruptive volcanism, cyclic evaporite deposition, origin of sapropels, origin of metalliferous "basal" sediments, and definition of the Miocene/Pliocene boundary. The rest of this report focuses on the outstanding tectonic results: evidence that the Tyrrhenian Sea is very young and has evolved quickly, and that the locus of intense tectonic activity and of ocean-crust formation has migrated through time toward the subduction zone. On the upper Sardinian margin (Site 654), rifting began in the upper Tortonian and slowed significantly or stopped in the upper Messinian. However, on the lower Sardinian margin (Sites 652 and 656), rifting apparently began during the Messinian and probably ended in the lower Pliocene. Those observations indicate that rifting and subsidence were diachronous across the continental margin. By the Messinian, rifting and subsidence of what is no~, the upper and middle Sardinian margin were advanced enough to
Figure 3 This photograph from Site 650 shows the contact between vesicular basalt and upper Pliocene dolomitic nannofossil ooze. The extreme vesicularity of the basalt implies that it was emplaced in water much shallower than its present depth, 4,118 m below sea level. (From Ocean Drilling Program) 260
M a r i n e a n d P e t r o l e u m G e o l o g y , 1987, V o l 4, A u g u s t
Ocean Drilling Program allow deposition of basinal evaporites at Sites 654 and 653. Rifting and subsidence on what is now the lower Sardinian margin was less advanced resulting in lacustrine and subaerial Messinian facies at Sites 652 and 656. Since the water depth at Site 654 is now 1.3 km shallower than at Site 652, a drastic reorganization of the morphology of the ancestral Tyrrhenian basin must have occurred during and since the Messinian. We attribute the reorganization to progressive rifting. Neither the Vavilov Basin nor the Marsili Basin seems to have existed as a deep ocean basin during the Messinian. Just as the locus of intense rifting migrated from northwest to southeast, a comparable shift in the location of crustal accretion occurred from the northwestern deep basin (Vavilov) to the southeastern deep basin (Marsili). Basalts cored in the Vavilov Basin (Sites 655 and 651) are estimated to be about 3.5 million years old, based on biostratigraphitally controlled dating of sediment overlying and filling cracks in the basalt. Seismic-reflection data imply that we did not
sample the oldest basement of Vavilov Basin. Since Site 650 was in the area of thickest sediment cover in the Marsili Basin, the basalt sampled was probably among the oldest in the basin. The age of the basalt at the Site 650 sediment/basalt contact is biostratigraphically and magnetostratigraphically constrained at 1.6 to 1.9 million years. Therefore. injection of basaltic crust began at least 1.5 million years later in Marsili Basin than in Vavilov Basin. Leg 107 has shown that rifting and basin formation in the Tyrrhenian Sea migrated southeastward toward the subducting plate. Furthermore. the process is very recent: the present phase of rifting began as recently as upper Tortonian, and the younger of the deep basins appears to be less than 2 million years old.
Leg 107 Scientific Drilling Party Ocean Drilling Program, Texas A & M University, College Station, 77843-3669, USA
Paleoclimate studied in the east Atlantic On Leg 108 Leg 108 of the Ocean Drilling Program drilled 27 holes at 12 sites from February 12 to April 17, in the low-latitude eastern Atlantic Ocean. The purpose of the cruise was to study the evolution of Neogene paleoclimate and paleoceanography. Favorable coring conditions led to recovery of about 3,850 m of cores, whictl represented 90% of the drilled section. That is the greatest amount of cores ever collected on a single ocean-drilling cruise. It will take years to investigate in detail the samples obtained from those sediments in order to extract high-resolution signals at the periods of Earth-orbital changes (20,000 to 10(I,000 years). Evidence from late Pleistocene piston cores suggests that the orbital rhythms carry much of the key paleoclimatic response of those regions. However, important contributions arc already apparent from the coarse-resolution shipboard analvses obtained during the cruise. The tropical-subtropical eastern Atlantic Ocean was chosen for drilling during Leg 108 because it contains a critical boundary zone of surface-water oceanography that includes the lntertropical Convergence Zone (ITCZ) and the thermal equator. In the deep water, the Sierra Leone Rise forms an almost continuous, tectonically inactive barrier between the basins of the south and north Atlantic. Long-term changes in and along those major equatorial boundaries are linked closely to the history of Neogene global climate change. The main objectives of Leg 108 were to study whether and to what degree the changes are controlled by polar components of the climate system such as the build-up of ice sheets and the formation of sea ice, and to what degree the low-latitude ocean-atmosphere components evolve independentb, such as those changes that result from variations in low-latitude solar isolation. It was also important to evaluate the relative importance of the 2 polar regions in influencing climate change near the equator. To investigate such complex questions, we studied microfossil surface-ocean indicators in the sediment sequences. We paid particular attention to changes in the tropical ocean at times of prominent changes in Northern Hemisphere ice volumes at about 3.0 to 2.5 and (1.9 million years ago, and in Southern Hemisphere ice-sheet changes at 5 to 6 million years ago. Leg 108 also aimed to study the history of upwelling intensity and biologic productivity in the eastern equatorial and subtropical Atlantic. The shipboard part of that work was based on monitoring changes in the abundance of the opaline plankton and of organic carbon in the sediment. By those means, we intended to define the latitudinal persistence of the upwelling cells during varying climates and to assess their significance in the broader climate context of variations in
global CO:, the evaporation balance of the equatorial ocean. and the deposition of sediments rich in organic carbon. Another objective was to measure variations of wind circulation along the ITCZ as deduced from the abundance of wind-blown particles in the sediment. The changes may be vital for understanding the evolution of the tropical Hadley Cell and the fluctuating (orbital) cvcles in African aridity and monsoon rains in the past. Leg 108 also tried to determine the Neogene history of deep-water exchange through fracture zones between the western and eastern Atlantic basins and between the southern and northern east Atlantic, as well as to investigate the cause of incursions of Antarctic-source bottom water into the northeastern Atlantic. Based on evidence from upper Quaternary sediments recovered by gravity cores, differences in the bathymetric distribution of calcium carbonate and stable carbon isotopes were expected to provide insights into dramatic short-term changes in the exchange of deep water and oxygen between the diflerent basins. Several seismic reflectors also suggest long-term changes of deep-water circulation, events that have been undated and unexplained in a paleoclimatic context. In pursuing those objectives, Leg 108 cored a latitudinal transect of 12 sections of middle to late Cenozoic, and in one case upper Cretaceous, sediments. The transect spans 24° of latitude from 22°N to almost 2%. DSDP Leg 94 cored earlier 6 sites in the transitional to subpolar latitudes of North Atlantic (37°-55°N). Those sites include every major surface-water mass between the south equatorial divergence and the northern part of the North Atlantic Drift, and the northeastern extension of the Gulf Stream. The advanced hydraulic piston corer and extended core barrel systems were both used to obtain cores. Two or three holes were cored at most sites to recover sediment sections that were as complete as possible. Previous DSDP and ODP paleoceanographic legs had difficulty obtaining continuous sections, despite coring 2 offset holes with the hydraulic piston corer at some sites: disturbed cores and under-recovery of some sections create gaps in the sediment sequence and thus in the climate record. Only DSDP Leg 94 succeeded in verifying continuity of section in detail (tens of centimetres) at sea by tracking CaCO3 layering in core photographs and then spot coring any gaps. New correlation techniques pioneered a fine-scale high-resolution analysis of cored sediment sections on Leg 108. Two signals (P-wave volocity and magnetic susceptibility) were measured continuously at intervals of a few centimetres on unsplit cores and formed the basis of this
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