Multichannel seismic survey along the southern Antarctic Peninsula
orso-ila DF8O-177 DF80144
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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
r:z/i
intersections of these fracture zones with the continental shelf are now better constrained (figure 1). The depositional processes governing the stratigraphic development of the continental margin along the Antarctic Peninsula may be regulated by the complex interaction between ancient subduction, the advance and retreat of ice sheets across the continental shelf, and the presence of bottom currents along the slope and rise. Bathymetric profiles across the middle to outer continental shelf indicate that the shelf shoals in a seaward direction. Water depths, however, typically remain deeper than 400 meters. Wide-angle seismic records from sonobuoys deployed across the shelf indicate that sediment velocities are greater than 2 kilometers per second. These relatively high velocities suggest that the sediments are overcompacted, a condition that may have been caused by the previous grounding of ice. The shelf sediments are characterized by high-amplitude, oblique progradational seismic reflectors, similar to those documented by Larter and Barker (1989) farther to the northeast. Large U-shaped canyons extend across the shelf, and many canyon-cutting events are recorded at the base of the slope and across the upper rise. Numerous unconformities are recognized in the sedimentary column on seismic-reflection profiles across the rise. Many of these unconformities appear to have formed from preferential erosion and may indicate the presence of strong bottom currents along the rise. An exciting discovery includes the identification of large sediment drifts across the lower slope and upper rise (figure 3) documenting the influence of bottom currents along the rise. These drift deposits appear to
J:.
-78
DSDP 325
.66.
1073
Adel eldeo(
1079 %
-70*
8-7674 -72
Figure 1. Seismic tracklines of the southern portion of the Ewing survey EW91 -01 along the Pacific side of the Antarctic Peninsula. The fracture zones, identified from the seismic-reflection profiles, are depicted as heavy dashed lines. Selected isobaths in meters.
along the upper rise. A Moho reflection is observed on most of the deep-water profiles and, where imaged, is characterized by discontinuous reflectors ranging between 1.5 to 2 seconds two-way travel time beneath oceanic basement (figures 2 and 3). A series of planar reflectors dipping landward and extending through middle to lower oceanic crust have been tentatively identified on the dip sections (figure 2). These reflectors have higher amplitudes and appear to dip less steeply than the diffractions generated off acoustic basement. These intracrustal reflectors are similar to those previously recognized in seismic reflection profiles of slow-spreading oceanic crust from the western north Atlantic (see, for example, White et al. 1990; Mutter and Karson 1992). Depth to oceanic basement diminishes as it approaches the shelf edge, and there is no evidence from the seismic data of an ancient trench beneath the outer margin. The trench topography generated during subduction, therefore, has either been destroyed or exists farther landward beneath the continental shelf. A number of fracture zones, including the Tula and Adelaide, have been Figure 2. Portion of seismic record, 1073, extending across the upper rise. The label, "DR?", denotes identified on our strike lines apparent intracrustal dipping reflectors tentatively interpreted on dip lines in this region. Location of this across the upper rise, and the section is illustrated in figure 1. (TWT denotes two-way travel time.)
ANTARCTIC JOURNAL - REVIEW 1993 94
Figure 3. Portion of seismic record, 1079, acquired along the upper rise. The southern part of one of the contourites is indicated. Location of this section is illustrated in figure 1. (TWT denotes two-way travel time.) straddle the major fracture zones; this straddling suggests that the drift morphology may be controlled in some fashion by the pre-existing basement topography. Sediment thicknesses along the rise range from a minimum of approximately 1.5 to approximately 3 seconds two-way travel time beneath the drift deposits. These sediments are divided into two major seismic facies units. The internal reflection configuration of the lower unit (unit 1, figures 2 and 3) is characterized by parallel to subparallel, low-amplitude reflectors, which can be traced laterally along the rise for many kilometers. An automatic gain control was applied during processing to amplify the seismic reflectors within this package. The upper seismic unit (unit 2, figures 2 and 3) is separated from the lower unit by a prominent, regional erosional unconformity. This deepsea unconformity may document the onset of intensified circulation along the Antarctic Peninsula. The reflector configuration of the upper unit varies dramatically ranging from highamplitude, laminated reflectors to very chaotic, discontinuous reflectors representing high-energy gravity flows or slumps sourced from the continental shelf and slope. The majority of unconformities and canyon-cutting events occur within the
upper unit. Faults associated with minor sediment slumps are observed on seismic profiles across the rise (figure 2). We would like to thank Neal Driscoll for critical review of this manuscript. This research was supported by National Science Foundation grant OPP 89-17332.
References Barker, P.F. 1982. The Cenozoic subduction history of the Pacific margin of the Antarctic Peninsula: Ridge crest-trench interactions. Geological Society of London Journal, 139(6), 787-801. Hawkes, D.D. 1981. Tectonic segmentation of the northern Antarctic Peninsula. Geology, 9(5), 220-224. Larter, R.D., and P.F. Barker. 1989. Seismic stratigraphy of the Antarctic Peninsula Pacific margin: A record of Pliocene-Pleistocene ice volume and paleoclimate. Geology, 17(8), 731-734. Mutter, J.C., and J.A. Karson. 1992. Structural processes at slowspreading ridges. Science, 257(5070), 627-634. Pelayo, AM., and D.A. Wiens. 1989. Seismotectonics and relative plate motions in the Scotia Sea region. Journal of Geophysical Research, 94(B6), 7293-7320. White, R.S., R.S. Detrick, J.C. Mutter, P. Buhl, T.A. Minshull, and E. Morris. 1990. New seismic images of oceanic crustal structure. Geology, 18(5), 462-465.
ANTARCTIC JOURNAL - REVIEW 1993 95
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
160
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
r:z/i
intersections of these fracture zones with the continental shelf are now better constrained (figure 1). The depositional processes governing the stratigraphic development of the continental margin along the Antarctic Peninsula may be regulated by the complex interaction between ancient subduction, the advance and retreat of ice sheets across the continental shelf, and the presence of bottom currents along the slope and rise. Bathymetric profiles across the middle to outer continental shelf indicate that the shelf shoals in a seaward direction. Water depths, however, typically remain deeper than 400 meters. Wide-angle seismic records from sonobuoys deployed across the shelf indicate that sediment velocities are greater than 2 kilometers per second. These relatively high velocities suggest that the sediments are overcompacted, a condition that may have been caused by the previous grounding of ice. The shelf sediments are characterized by high-amplitude, oblique progradational seismic reflectors, similar to those documented by Larter and Barker (1989) farther to the northeast. Large U-shaped canyons extend across the shelf, and many canyon-cutting events are recorded at the base of the slope and across the upper rise. Numerous unconformities are recognized in the sedimentary column on seismic-reflection profiles across the rise. Many of these unconformities appear to have formed from preferential erosion and may indicate the presence of strong bottom currents along the rise. An exciting discovery includes the identification of large sediment drifts across the lower slope and upper rise (figure 3) documenting the influence of bottom currents along the rise. These drift deposits appear to
J:.
-78
DSDP 325
.66.
1073
Adel eldeo(
1079 %
-70*
8-7674 -72
Figure 1. Seismic tracklines of the southern portion of the Ewing survey EW91 -01 along the Pacific side of the Antarctic Peninsula. The fracture zones, identified from the seismic-reflection profiles, are depicted as heavy dashed lines. Selected isobaths in meters.
along the upper rise. A Moho reflection is observed on most of the deep-water profiles and, where imaged, is characterized by discontinuous reflectors ranging between 1.5 to 2 seconds two-way travel time beneath oceanic basement (figures 2 and 3). A series of planar reflectors dipping landward and extending through middle to lower oceanic crust have been tentatively identified on the dip sections (figure 2). These reflectors have higher amplitudes and appear to dip less steeply than the diffractions generated off acoustic basement. These intracrustal reflectors are similar to those previously recognized in seismic reflection profiles of slow-spreading oceanic crust from the western north Atlantic (see, for example, White et al. 1990; Mutter and Karson 1992). Depth to oceanic basement diminishes as it approaches the shelf edge, and there is no evidence from the seismic data of an ancient trench beneath the outer margin. The trench topography generated during subduction, therefore, has either been destroyed or exists farther landward beneath the continental shelf. A number of fracture zones, including the Tula and Adelaide, have been Figure 2. Portion of seismic record, 1073, extending across the upper rise. The label, "DR?", denotes identified on our strike lines apparent intracrustal dipping reflectors tentatively interpreted on dip lines in this region. Location of this across the upper rise, and the section is illustrated in figure 1. (TWT denotes two-way travel time.)
ANTARCTIC JOURNAL - REVIEW 1993 94
Figure 3. Portion of seismic record, 1079, acquired along the upper rise. The southern part of one of the contourites is indicated. Location of this section is illustrated in figure 1. (TWT denotes two-way travel time.) straddle the major fracture zones; this straddling suggests that the drift morphology may be controlled in some fashion by the pre-existing basement topography. Sediment thicknesses along the rise range from a minimum of approximately 1.5 to approximately 3 seconds two-way travel time beneath the drift deposits. These sediments are divided into two major seismic facies units. The internal reflection configuration of the lower unit (unit 1, figures 2 and 3) is characterized by parallel to subparallel, low-amplitude reflectors, which can be traced laterally along the rise for many kilometers. An automatic gain control was applied during processing to amplify the seismic reflectors within this package. The upper seismic unit (unit 2, figures 2 and 3) is separated from the lower unit by a prominent, regional erosional unconformity. This deepsea unconformity may document the onset of intensified circulation along the Antarctic Peninsula. The reflector configuration of the upper unit varies dramatically ranging from highamplitude, laminated reflectors to very chaotic, discontinuous reflectors representing high-energy gravity flows or slumps sourced from the continental shelf and slope. The majority of unconformities and canyon-cutting events occur within the
upper unit. Faults associated with minor sediment slumps are observed on seismic profiles across the rise (figure 2). We would like to thank Neal Driscoll for critical review of this manuscript. This research was supported by National Science Foundation grant OPP 89-17332.
References Barker, P.F. 1982. The Cenozoic subduction history of the Pacific margin of the Antarctic Peninsula: Ridge crest-trench interactions. Geological Society of London Journal, 139(6), 787-801. Hawkes, D.D. 1981. Tectonic segmentation of the northern Antarctic Peninsula. Geology, 9(5), 220-224. Larter, R.D., and P.F. Barker. 1989. Seismic stratigraphy of the Antarctic Peninsula Pacific margin: A record of Pliocene-Pleistocene ice volume and paleoclimate. Geology, 17(8), 731-734. Mutter, J.C., and J.A. Karson. 1992. Structural processes at slowspreading ridges. Science, 257(5070), 627-634. Pelayo, AM., and D.A. Wiens. 1989. Seismotectonics and relative plate motions in the Scotia Sea region. Journal of Geophysical Research, 94(B6), 7293-7320. White, R.S., R.S. Detrick, J.C. Mutter, P. Buhl, T.A. Minshull, and E. Morris. 1990. New seismic images of oceanic crustal structure. Geology, 18(5), 462-465.
ANTARCTIC JOURNAL - REVIEW 1993 95
