Late Glacial and Holocene history of the western Ross Sea: An initial ...
In summary, this study provides us with a detailed image of the lower crust beneath an extensional basin located within a large-scale continental rift zone. Assuming that the original crustal thickness beneath the Ross Sea was 30-40 km, the crust beneath the flanks of the central basin was uniformly stretched by a factor of 1.5-2.0. The absence of a thick, highvelocity, lower crustal layer beneath the flanks of the central basin suggests that significant extension occurred with only a small amount of melt generation. Beneath the central basin, the pre-rift crust was thinned by a factor of 5-8, and about 8 km of melt was added to the crust (including up to 1 km of extrusive material; Tréhu et al. 1993). This volume of melt is consistent with the mantle decompression melting model of McKenzie and Bickle (1988) for an upper mantle potential temperture anomaly of only about 50-75°C relative to normal oceanic upper mantle. Although the data do not place any firm constraints on the relative timing of extensional episodes, we assume that extension was broadly distributed at first and later localized beneath one or more rift basins, the locations of which may have been controlled by preexisting crustal structure. The balance between crustal thinning, volume of melt added, and postrift subsidence and sedimentation was such that the final crustal thickness is approximately constant. We thank Martin Uyesugi for having operated the OBSs, Tim Holt for preliminary data processing, and Stefan Orbach and Jannis Makris for having provided copies of the data collected by the University of Hamburg. Fieldwork was supported by the Bundesanstalt für Geowissenschaften und Rohstoffe and the U.S. Geological Survey. Data analysis was supported by National Science Foundation grant OPP 88-17040 to Oregon State University.
beneath the western Ross Sea Continental Shelf, Antarctica— Interpretation of an aeromagnetic survey. In M.R.A. Thomson, J.A. Crame, and J.W. Thomson (Eds.), Antarctic geoscience. Cambridge: Cambridge University Press. Cooper, A.K., F.J. Davey, and K. Hinz. 1991. Crustal extension and origin of sedimentary basins beneath the Ross Sea and Ross Ice Shelf, Antarctica. In M.R.A. Thomson, J.A. Crame, and J.W. Thomson (Eds.), Antarctic geoscience. Cambridge: Cambridge University Press. Davey, F.J., and A.K. Cooper. 1987. Gravity studies of the Victoria Land Basin and Iselin Bank. In A.K. Cooper and F.J. Davey (Eds),
The antarctic continental margin: Geology and geophysics of the western Ross Sea (Earth Sciences Series, Vol. 5B). Houston: Cir-
cum-Pacific Council for Energy and Mineral Resources. Hinz, K., and M. Block. 1984. Results of geophysical investigations in the Weddell Sea and in the Ross Sea. Proceedings of the 11th World Petrologic Congress, London 1983 (Vol. 2, Geology Exploration Reserves). New York: John Wiley and Sons. Hinz, K., M. Hemmerith. U. Salge, and 0. Eiken. 1990. Structures in rift-basin sediments on the conjugate margins of western Tasmania, South Tasman Rise and Ross Sea, Antarctica. In U. Bleil and J.T. Theide (Eds.), Geological history of the polar oceans: Arctic versus Antarctic. Norwell, Massachusetts: Kluwer Academic Publishers. LeMasurier, W. 1990. Late Cenozoic volcanism on the Antarctic plate—An overview. In W.E. LeMasurier and J.W. Thomson (Eds.), Volcanoes of the antarctic plate and southern oceans (Antarctic Research Series, Vol. 48). Washington, D.C.: American Geophysical Union. McKenzie, D.P., and M.J. Bickle. 1988. The volume and composition of melt generated by extension of the lithosphere. Journal of Petrology, 29, 625-679. Mooney , W.D., M.C. Andrews, A. Ginzburg, D.A. Peters, and R.M. Hamilton. 1983. Crustal structure of the northern Mississippi Embayment and a comparison with other continental rift zones. Tectonophysics, 94, 327-348. Tréhu, A., J. Behrendt, and I. Fritsch. 1993. Generalized crustal structure of the Central Basin, Ross Sea, Antarctica. In D. Damaske and J. Fritsch (Eds.), GANO VEX V, GeologischesJahrbuch, E47,291-313. Tréhu, A., P. Morel-à-Huissier, R. Meyer, Z. Hajnal, J. Karl, R. Mereu, J. Sexton, J. Shay, W.-K. Chan, D. Epili, T. Jefferson, X.-R. Shih, S. Wendling, B. Milkereit, A. Green, and D. Hutchinson. 1991. Imaging the midcontinent rift beneath Lake Superior using large aperture seismic data. Geophysics Research Letters, 18(4), 625-628.
References Behrendt, J.C., H.J. Duerbaum, D. Damaske, R. Saltus, W. Bosum, and A.K. Cooper. 1991. Extensive volcanism and related tectonism
Late Glacial and Holocene history of the western Ross Sea: An initial survey of available cores ANNE E. JENNINGS, KERSTIN M. WILLIAMS, KATHY J. LIGHT, and JOHN T. ANDREWS, Institute ofArctic and Alpine Research and Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309
part of the west antarctic ice sheet initiative (BindN chadler 1991), investigators from Rice University, Hamilton College, and the University of Colorado at Boulder teamed up to develop an integrated approach to describing and dating the retreat of the west antarctic ice sheet across the Ross Sea during the last glacial/deglacial cycle. A knowledge of the glacial history and the paleoceanography of Antarctica is vital to resolving the large uncertainties about
the global water budget during deglaciation (Tushingham and Peltier 1991; Andrews 1992) and is also important for assessing the potential global risk of rapid glacial ice recession and collapse. Productivity changes, as recorded by phytoplankton in the sediment, may provide information on global changes in Holocene deep-water circulation. The research has two main phases: the first phase (1992-1994) involves a study of all existing cores, which are
ANTARCTIC JOURNAL - REVIEW 1993 91
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18o
ments to Holocene diatomaceous muds. There is a distinct difference in the susceptibility signature: "tills" in transect 2 have a median value of 90 (x10 5) System International (SI) compared to 20 and 40 (x10-5) SI in transects 1 and 3, respectively. In contrast, the diatomaceous sediments were normally less than 5 (x10 5) SI (figure 2). Seven cores have been analyzed as part of the preliminary study in the western Ross Sea (figure 3). We have obtained an accelerator mass spectrometry carbon-14 date of 5,390±70 years (uncorrected for reservoir effect) at 6 centimeters (cm) on organics from core DF80- 112. We intend to pursue this method where we do not find enough calcareous material for dating. The diatom analyses (diatoms per gram) often reveal considerable changes within core sections that often have been described as "unchanging" in the core logs. We feel that with a concerted dating effort, we can determine variability in surface productivity over the timespan of the diatomaceous sediment by calculating accumulation rates. Diatom production depends on temporal and spatial sea-ice extent and is normally not nutrient limited in the southern oceans (that is, Harrison and Cota 1991). Core DF80-102 is closest to the coast of all the cores. It shows a great deal of variability in diatoms per gram over the top 100 cm and probably indicates first-order changes in the neighboring glaciers and in sea-ice activity. Except for core E51-8, which was barren of diatoms, the rest of the cores (figure 1) show varying numbers of diatoms per gram. The tops of the cores all have a productivity spike (figure 2), which may be correlative. We intend to date the spikes and any other part of the cores which will yield enough organic material. Core 108 has an occurrence of Actinocyclus ingens (1 percent) at approximately 15 cm. This Lower Pleistocene species could indicate reworking of the sediment, although such small percentages are probably not meaningful. A. ingens also occurs in low percentages (less than 2 percent) at 30 cm in core 111, at 40 cm and below in core 144, at 120 cm in 177, and at 90 cm in 102. This research was supported by National Science Foundation grant OPP 91-17958.
170'W
72S
74S
76'S
i60'E 170'E 180' 170'W 160W •DVIS FJtaIti
Figure 1. Locations of some of the cores being studied at present. (See figure 2.)
stored in the Antarctic Research Facility at Florida State University, applicable to our project. The second phase will involve cruises in 1994 and 1995 to the Ross Sea to obtain additional high-resolution seismic lines and new cores, the locations of which will depend on the results of our analyses during the first phase. We used a Bartington magnetic susceptibility meter and a 70-millimeter (mm) loop to record variations in the volume magnetic susceptibility of sediments in over 40 cores. Magnetic susceptibility measurements are nondestructive and provide a rapid way of logging 0(80-111 0180-11 a 0180-177 0(80-144 changes in sediment prove- 0180-102 Df80-10 I 1 o nance and depositional history a 'j 0_' (Thompson and Oldfield 1986; 50I Andrews and Jennings 1987). 50 50_i 100.1 The cores studied at the -1 50 100 Institute of Arctic and Alpine j 75Studies (INSTAAR) were locat100 1 1501 75 ed along three transects run-' too 150 200 125 fling from the ice shelf out 175 100 125___ 150 toward the shelf break (figure 1). Transect 1 is closest to the __________ 175 i ___ 25 ] 300 I I I 225-! 1501 iU] 150 i Victoria Land coast, and 3 is 0 50 100 150 0 0 150 0 25 50 75 100 0 25 50 75 0 75 150 225 300 0 50 100 150 further east. Our studies indiMS (10A 5) SI cate that magnetic susceptibility easily detects the transition Figure 2. Plots of downcore variations in volume mass susceptibility for a selection of cores. (See figure from till/glacial marine sedi1 for locations.)
a ' ' a
______________
1
so ic
rl
ANTARCTIC JOURNAL - REVIEW 1993
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1
orso-ila DF8O-177 DF80144
DF80102 DF80-108 0F80-111 0 0 20
20 - - . 20 - - -
40 40 60 V
80 100
60
40
80
60
_
100
120
240 260
80
120 100
I>
60
90
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20 -V __•40 J1 60 80 100 120 140 150 180 200 220
100
140 120
0
.120 ____________ __
0 500 1000 0 200 400 0 100 200
_
-
ro 40 60 80
100 12 140 160
r.
0 000 400 0 20 40 00 0 10 20
Figure 3. Plots of downcore variations in the numbers of diatoms (x106) per gram dry weight for a selection of cores. (See figure 1 for locations.) Note the structure in the data. The dashed line represents a possible isochronous horizon above which diatom numbers rise; however, note that the diatom flux (numbers per square centimeter per unit of time) may not mimic these variations. The carbon-14 date on DF80-112 is shown uncorrected for reservoir effect.
dlVtoms/g(1 02.6)
References Harrison, W.G., and G.F. Cota. 1991. Primary production in polar waters: Relation to nutrient availability. Polar Research, 10, 87-104. Thompson, R., and F. Oldfield. 1986. Environmental magnetism. Winchester, Massachusetts: Allen & Unwin. Tushingham, A.M. and W.R. Peltier. 1991. Ice 3G: A new global model of late Pleistocene deglaciation based upon geophysical predictions of post-glacial relative sea level change. Journal Geophysical Research, 96(3), 4497-4523.
Andrews, J.T. 1992. A case of missing water. Nature, 358(6384), 281. Andrews, J.T., and A.E. Jennings. 1987. Influence of sediment source and type on the magnetic susceptibility of fiord and shelf deposits. Baffin Island and Baffin Bay, N.W.T. Canadian Journal Earth Sciences, 24(7), 1386-1401. Bindschadler, R.A. (Ed.) 1991. West antarctic ice sheet initiative (Vol. 1, Science and Implementation Plan, Conference Publication 3115). Washington, D.C.: National Aeronautics and Space Administration.
Fracture zone control on continental margin development: Multichannel seismic survey along the southern Antarctic Peninsula JOHN P. McGINNIS, DENNIS E. HAYES, JOHN C. MUTTER, PETER BUHL, and JOHN B. DIEBOLD, Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964
with the continental margin led to the cessation of subduetion (Barker 1982). Subduction initially terminated in the southern portion of the peninsula during the Eocene (approximately 50 million years ago), and due to several large left-lateral offsets of the ridge axis, subsequently ceased in a timetransgressive fashion to the northeast (Barker 1982). Active subduction may still be continuing beneath the Shetland Islands (Pelayo and Wiens 1989). This plate kinematic scenario provides the opportunity to examine the impact of ridge-trench collision on the tectonic and stratigraphic evolution of this margin. Previous investigators have argued that the Antarctic Peninsula is segmented into a series of discrete petrologic provinces whose boundaries may coincide with major fracture zones subducted beneath the margin (see, for example, Hawkes 1981). These fracture zones may also serve as preferential sites for ophiolite emplacements. The southern portion of the survey consists of 11 seismicreflection profiles totaling greater than 2,500 km (figure 1). Six dip lines extend from the continental shelf to the upper rise. Two long strike lines were acquired along the shelf and three
n a collaborative effort, Lamont-Doherty Earth Observatory I and the University of Texas Institute for Geophysics acquired approximately 6,100 kilometers (km) of multichannel seismic reflection, gravity, magnetics, and swath bathymetry data along the Antarctic Peninsula during February and March 1991. The survey, conducted aboard the Maurice Ewing, was divided into two parts. The northern part focused on the Bransfield Strait and Shetland Trench. The southern part, discussed here, concentrated along the Pacific side of the Antarctic Peninsula (figure 1). One of the scientific objectives for the southern portion of the survey was to use geophysical observations of the deep crustal structure to determine how convergent tectonics may have contributed to the apparent segmentation of the Antarctic Peninsula. Marine magnetic anomalies show that the crust increases in age away from the margin, a finding that indicates that portions of the original Phoenix plate were subducted beneath the peninsula. Relative motion between the Phoenix and antarctic plates was such that the fracture zones were subducted nearly perpendicular to the peninsula. Subsequently, collision of the ridge
ANTARCTIC JOURNAL - REVIEW 1993 93
beneath the western Ross Sea Continental Shelf, Antarctica— Interpretation of an aeromagnetic survey. In M.R.A. Thomson, J.A. Crame, and J.W. Thomson (Eds.), Antarctic geoscience. Cambridge: Cambridge University Press. Cooper, A.K., F.J. Davey, and K. Hinz. 1991. Crustal extension and origin of sedimentary basins beneath the Ross Sea and Ross Ice Shelf, Antarctica. In M.R.A. Thomson, J.A. Crame, and J.W. Thomson (Eds.), Antarctic geoscience. Cambridge: Cambridge University Press. Davey, F.J., and A.K. Cooper. 1987. Gravity studies of the Victoria Land Basin and Iselin Bank. In A.K. Cooper and F.J. Davey (Eds),
The antarctic continental margin: Geology and geophysics of the western Ross Sea (Earth Sciences Series, Vol. 5B). Houston: Cir-
cum-Pacific Council for Energy and Mineral Resources. Hinz, K., and M. Block. 1984. Results of geophysical investigations in the Weddell Sea and in the Ross Sea. Proceedings of the 11th World Petrologic Congress, London 1983 (Vol. 2, Geology Exploration Reserves). New York: John Wiley and Sons. Hinz, K., M. Hemmerith. U. Salge, and 0. Eiken. 1990. Structures in rift-basin sediments on the conjugate margins of western Tasmania, South Tasman Rise and Ross Sea, Antarctica. In U. Bleil and J.T. Theide (Eds.), Geological history of the polar oceans: Arctic versus Antarctic. Norwell, Massachusetts: Kluwer Academic Publishers. LeMasurier, W. 1990. Late Cenozoic volcanism on the Antarctic plate—An overview. In W.E. LeMasurier and J.W. Thomson (Eds.), Volcanoes of the antarctic plate and southern oceans (Antarctic Research Series, Vol. 48). Washington, D.C.: American Geophysical Union. McKenzie, D.P., and M.J. Bickle. 1988. The volume and composition of melt generated by extension of the lithosphere. Journal of Petrology, 29, 625-679. Mooney , W.D., M.C. Andrews, A. Ginzburg, D.A. Peters, and R.M. Hamilton. 1983. Crustal structure of the northern Mississippi Embayment and a comparison with other continental rift zones. Tectonophysics, 94, 327-348. Tréhu, A., J. Behrendt, and I. Fritsch. 1993. Generalized crustal structure of the Central Basin, Ross Sea, Antarctica. In D. Damaske and J. Fritsch (Eds.), GANO VEX V, GeologischesJahrbuch, E47,291-313. Tréhu, A., P. Morel-à-Huissier, R. Meyer, Z. Hajnal, J. Karl, R. Mereu, J. Sexton, J. Shay, W.-K. Chan, D. Epili, T. Jefferson, X.-R. Shih, S. Wendling, B. Milkereit, A. Green, and D. Hutchinson. 1991. Imaging the midcontinent rift beneath Lake Superior using large aperture seismic data. Geophysics Research Letters, 18(4), 625-628.
References Behrendt, J.C., H.J. Duerbaum, D. Damaske, R. Saltus, W. Bosum, and A.K. Cooper. 1991. Extensive volcanism and related tectonism
Late Glacial and Holocene history of the western Ross Sea: An initial survey of available cores ANNE E. JENNINGS, KERSTIN M. WILLIAMS, KATHY J. LIGHT, and JOHN T. ANDREWS, Institute ofArctic and Alpine Research and Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309
part of the west antarctic ice sheet initiative (BindN chadler 1991), investigators from Rice University, Hamilton College, and the University of Colorado at Boulder teamed up to develop an integrated approach to describing and dating the retreat of the west antarctic ice sheet across the Ross Sea during the last glacial/deglacial cycle. A knowledge of the glacial history and the paleoceanography of Antarctica is vital to resolving the large uncertainties about
the global water budget during deglaciation (Tushingham and Peltier 1991; Andrews 1992) and is also important for assessing the potential global risk of rapid glacial ice recession and collapse. Productivity changes, as recorded by phytoplankton in the sediment, may provide information on global changes in Holocene deep-water circulation. The research has two main phases: the first phase (1992-1994) involves a study of all existing cores, which are
ANTARCTIC JOURNAL - REVIEW 1993 91
170E
18o
ments to Holocene diatomaceous muds. There is a distinct difference in the susceptibility signature: "tills" in transect 2 have a median value of 90 (x10 5) System International (SI) compared to 20 and 40 (x10-5) SI in transects 1 and 3, respectively. In contrast, the diatomaceous sediments were normally less than 5 (x10 5) SI (figure 2). Seven cores have been analyzed as part of the preliminary study in the western Ross Sea (figure 3). We have obtained an accelerator mass spectrometry carbon-14 date of 5,390±70 years (uncorrected for reservoir effect) at 6 centimeters (cm) on organics from core DF80- 112. We intend to pursue this method where we do not find enough calcareous material for dating. The diatom analyses (diatoms per gram) often reveal considerable changes within core sections that often have been described as "unchanging" in the core logs. We feel that with a concerted dating effort, we can determine variability in surface productivity over the timespan of the diatomaceous sediment by calculating accumulation rates. Diatom production depends on temporal and spatial sea-ice extent and is normally not nutrient limited in the southern oceans (that is, Harrison and Cota 1991). Core DF80-102 is closest to the coast of all the cores. It shows a great deal of variability in diatoms per gram over the top 100 cm and probably indicates first-order changes in the neighboring glaciers and in sea-ice activity. Except for core E51-8, which was barren of diatoms, the rest of the cores (figure 1) show varying numbers of diatoms per gram. The tops of the cores all have a productivity spike (figure 2), which may be correlative. We intend to date the spikes and any other part of the cores which will yield enough organic material. Core 108 has an occurrence of Actinocyclus ingens (1 percent) at approximately 15 cm. This Lower Pleistocene species could indicate reworking of the sediment, although such small percentages are probably not meaningful. A. ingens also occurs in low percentages (less than 2 percent) at 30 cm in core 111, at 40 cm and below in core 144, at 120 cm in 177, and at 90 cm in 102. This research was supported by National Science Foundation grant OPP 91-17958.
170'W
72S
74S
76'S
i60'E 170'E 180' 170'W 160W •DVIS FJtaIti
Figure 1. Locations of some of the cores being studied at present. (See figure 2.)
stored in the Antarctic Research Facility at Florida State University, applicable to our project. The second phase will involve cruises in 1994 and 1995 to the Ross Sea to obtain additional high-resolution seismic lines and new cores, the locations of which will depend on the results of our analyses during the first phase. We used a Bartington magnetic susceptibility meter and a 70-millimeter (mm) loop to record variations in the volume magnetic susceptibility of sediments in over 40 cores. Magnetic susceptibility measurements are nondestructive and provide a rapid way of logging 0(80-111 0180-11 a 0180-177 0(80-144 changes in sediment prove- 0180-102 Df80-10 I 1 o nance and depositional history a 'j 0_' (Thompson and Oldfield 1986; 50I Andrews and Jennings 1987). 50 50_i 100.1 The cores studied at the -1 50 100 Institute of Arctic and Alpine j 75Studies (INSTAAR) were locat100 1 1501 75 ed along three transects run-' too 150 200 125 fling from the ice shelf out 175 100 125___ 150 toward the shelf break (figure 1). Transect 1 is closest to the __________ 175 i ___ 25 ] 300 I I I 225-! 1501 iU] 150 i Victoria Land coast, and 3 is 0 50 100 150 0 0 150 0 25 50 75 100 0 25 50 75 0 75 150 225 300 0 50 100 150 further east. Our studies indiMS (10A 5) SI cate that magnetic susceptibility easily detects the transition Figure 2. Plots of downcore variations in volume mass susceptibility for a selection of cores. (See figure from till/glacial marine sedi1 for locations.)
a ' ' a
______________
1
so ic
rl
ANTARCTIC JOURNAL - REVIEW 1993
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1
1
orso-ila DF8O-177 DF80144
DF80102 DF80-108 0F80-111 0 0 20
20 - - . 20 - - -
40 40 60 V
80 100
60
40
80
60
_
100
120
240 260
80
120 100
I>
60
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20 -V __•40 J1 60 80 100 120 140 150 180 200 220
100
140 120
0
.120 ____________ __
0 500 1000 0 200 400 0 100 200
_
-
ro 40 60 80
100 12 140 160
r.
0 000 400 0 20 40 00 0 10 20
Figure 3. Plots of downcore variations in the numbers of diatoms (x106) per gram dry weight for a selection of cores. (See figure 1 for locations.) Note the structure in the data. The dashed line represents a possible isochronous horizon above which diatom numbers rise; however, note that the diatom flux (numbers per square centimeter per unit of time) may not mimic these variations. The carbon-14 date on DF80-112 is shown uncorrected for reservoir effect.
dlVtoms/g(1 02.6)
References Harrison, W.G., and G.F. Cota. 1991. Primary production in polar waters: Relation to nutrient availability. Polar Research, 10, 87-104. Thompson, R., and F. Oldfield. 1986. Environmental magnetism. Winchester, Massachusetts: Allen & Unwin. Tushingham, A.M. and W.R. Peltier. 1991. Ice 3G: A new global model of late Pleistocene deglaciation based upon geophysical predictions of post-glacial relative sea level change. Journal Geophysical Research, 96(3), 4497-4523.
Andrews, J.T. 1992. A case of missing water. Nature, 358(6384), 281. Andrews, J.T., and A.E. Jennings. 1987. Influence of sediment source and type on the magnetic susceptibility of fiord and shelf deposits. Baffin Island and Baffin Bay, N.W.T. Canadian Journal Earth Sciences, 24(7), 1386-1401. Bindschadler, R.A. (Ed.) 1991. West antarctic ice sheet initiative (Vol. 1, Science and Implementation Plan, Conference Publication 3115). Washington, D.C.: National Aeronautics and Space Administration.
Fracture zone control on continental margin development: Multichannel seismic survey along the southern Antarctic Peninsula JOHN P. McGINNIS, DENNIS E. HAYES, JOHN C. MUTTER, PETER BUHL, and JOHN B. DIEBOLD, Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964
with the continental margin led to the cessation of subduetion (Barker 1982). Subduction initially terminated in the southern portion of the peninsula during the Eocene (approximately 50 million years ago), and due to several large left-lateral offsets of the ridge axis, subsequently ceased in a timetransgressive fashion to the northeast (Barker 1982). Active subduction may still be continuing beneath the Shetland Islands (Pelayo and Wiens 1989). This plate kinematic scenario provides the opportunity to examine the impact of ridge-trench collision on the tectonic and stratigraphic evolution of this margin. Previous investigators have argued that the Antarctic Peninsula is segmented into a series of discrete petrologic provinces whose boundaries may coincide with major fracture zones subducted beneath the margin (see, for example, Hawkes 1981). These fracture zones may also serve as preferential sites for ophiolite emplacements. The southern portion of the survey consists of 11 seismicreflection profiles totaling greater than 2,500 km (figure 1). Six dip lines extend from the continental shelf to the upper rise. Two long strike lines were acquired along the shelf and three
n a collaborative effort, Lamont-Doherty Earth Observatory I and the University of Texas Institute for Geophysics acquired approximately 6,100 kilometers (km) of multichannel seismic reflection, gravity, magnetics, and swath bathymetry data along the Antarctic Peninsula during February and March 1991. The survey, conducted aboard the Maurice Ewing, was divided into two parts. The northern part focused on the Bransfield Strait and Shetland Trench. The southern part, discussed here, concentrated along the Pacific side of the Antarctic Peninsula (figure 1). One of the scientific objectives for the southern portion of the survey was to use geophysical observations of the deep crustal structure to determine how convergent tectonics may have contributed to the apparent segmentation of the Antarctic Peninsula. Marine magnetic anomalies show that the crust increases in age away from the margin, a finding that indicates that portions of the original Phoenix plate were subducted beneath the peninsula. Relative motion between the Phoenix and antarctic plates was such that the fracture zones were subducted nearly perpendicular to the peninsula. Subsequently, collision of the ridge
ANTARCTIC JOURNAL - REVIEW 1993 93
