Magnetic and seismic-reflection observations from the western Ross Sea
Magnetic and seismic-reflection observations from the western Ross Sea: More evidence of rift-associated tectonic activity in the Victoria Land Basin, Antarctica Louis R. BARTEK Department of Geology and Geophysics Rice University Houston, Texas 77251 CAROL RAYMOND
Earth and Space Sciences Division Jet Propulsion Laboratory California institute of Technology Pasadena, California 91109
The Ross Sea, the focus of this study, is a large embayment of the antarctic coastline that lies at the boundary of the two major subcontinents of Antarctica (East and West Antarctica). The Ross embayment is best characterized as a broad rift (1,000 kilometers wide) Davey (1987). Cooper, Davey, Behrendt (1987) attribute the formation of the Ross embayment to the breakup of Gondwanaland. Three major basins have been identified within the Ross embayment: the Eastern Basin, the Central Trough, and the Victoria Land Basin (Davey, Bennett, and Houtz 1982). The Victoria Land Basin and the Central Trough are thought to have formed by continued rifting, while the Eastern Basin is thought to be a product of crustal downwarping (Davey 1987). The presence of numerous near-surface normal faults and submarine volcanoes documents Cenozoic rifting in the Victoria Land Basin (Cooper et al. 1987). In this article, we report more evidence of rift-associated tectonic activity within the Victoria Land Basin. During Leg 2 of the R'V Polar Duke expedition to the Ross Sea seismic, gravity, and magnetic data were collected in the western portion of the Ross. The scientific objective of the cruise was to acquire a high-resolution, seismic-reflection database that will facilitate the development of a high-resolution seismic stratigraphy for the late Paleogene and Neogene strata of the continental shelf (see figures 1 and 2 of Anderson and Bartek, Antarctic Journal, this issue, for the location of cruise tracklines). Existing drill core from the shelf (DSDP leg 28 sites 270, 271, 272, and 273 (Deep Sea Drilling Project) and McMurdo Sound (MSSTS-1 (McMurdo Sound Sediment and Tectonic Studies) and CIROS 1 and 2 (Cenozoic Investigation of the Ross Sea)) provide the chronostratigraphy for the high resolution seismic database. The drill sites in the Ross Sea are not numerous, however, and they are widely dispersed, so an attempt to make regional correlations in the Ross Sea demands that stratigraphic continuity along the seismic profiles be maintained. Therefore, when planning our cruise tracks we used existing data sets (Hinz and Block 1983 and Cooper et al. 1987) to avoid the structural features that disrupt stratigraphic con68
tinuity. Our efforts to avoid structural complexity were temporarily foiled, however, in the western Ross Sea when we crossed over an uncharted bathymetric feature that we interpret as a submarine volcano. Seismic and magnetic data support the interpretation of this feature, which is located on the eastern flank of the Discovery Graben, on the Lee Arch, in the Victoria Land Basin (76°15.3'S 166°23.7'E) (see figures 24 and 25 of Cooper et al. 1987 for locations), as a submarine volcano (figure 1 illustrates the location of the profile over the volcano). It appears to a part of the chain of small volcanoes that lie on the axis of the Lee Arch, south of 76°S (Total Magnetic Anomaly Map, Victoria Land/Ross Sea, Antarctica 1987). The volcano was overflown during the BGR-USGS GANO VEX IV aeromagnetic survey of the Ross Sea in 1984-1985 (Duerbaum and Tessensohn 1986; and Behrendt, Cooper, and Yuan 1987), and it lies in an area that Davey and Cooper (1987) indicate has a free-air gravity anomaly of zero. During the aeromagnetic survey, which was flown at a height of 2,000 feet (610 meters), a 65-nanotesla magnetic anomaly was recorded in this area (Duerbaum and Tessensohn 1986; and Behrendt et al. 1987). This anomaly is actually a narrow cone that is approximately 10 kilometers in diameter. The crest of it lies at a depth of 158 meters and rises approximately 500 meters above the surrounding seafloor and the slopes on the flanks of this knoll are approximately 3 degrees. The massive acoustic character of the knoll (figure 2) is similar to the acoustic character of the submarine volcanoes and near-surface intrusions described by Behrendt et al. (1987), Cooper et al. (1987), and Karl, Reimnitz, and Edwards (1987). A 600-nanotesla anomaly, with a wavelength of approximately 10 kilometers and a sharp peak over the northeastern flank of the knoll, was observed during our survey over the area (figure 2). The peak over the northeastern flank may indicate that it was the most recently active portion of the volcano. Southwest of the volcano, the presence of several subseafloor intrusions is indicated by smaller, symmetrical short-wavelength anomalies of 100 and 200 nanoteslas (figure 2). The small intrusions disrupt seismic reflectors near the top of the stratigraphic succession (figure 2). Stratigraphic and structural relationships in the seismic data (figure 2) indicate that the rift-associated tectonic activity in this area is late Cenozoic in age. Post-late Oligocene rocks lie at a depth of approximately 0.5 seconds two-way travel time below the seafloor in this area (Cooper et al. 1987, figure 9-B). Downiap of reflectors, within a massive and hummocky wedge of strata on the north flank of the volcano, onto a surface that lies at a depth of approximately 0.53 seconds two-way travel time below the seafloor (figure 2) suggests that volcanic activity in this area was initiated in early Neogene time. The presence of another massive and hummocky wedge of reflectors on top of a near-surface unconformity that is late Miocene(?) to late Pleistocene(?) in age (Karl et al. 1987) suggests that volcano may still be active. Other evidence of recent rift-associated tectonic activity includes the normal faults (figure 1) that dissect the Neogene (Cooper et al. 1987) strata in this area. We wish to thank John Anderson, the Chief Scientist of the leg 2 RIV Polar Duke 1990 cruise, for informative discussions and for releasing data presented in this paper. We also thank Alan Cooper for assistance in planning the cruise. Adrien Pascouet and Jim Hedger of Seismic Systems, Inc., loaned a generator injector gun to us and trained us to use and maintain it. Rick Shiftman of Teledyne, Inc., provided assistance in streamer repair. We thank the highly professional crew of the RIV Polar Duke for their assistance during the cruise and SteANTARCTIC JOURNAL
160 E
165' E
1700 E
175' E
180'
175' W
170' W
1650 W
160 W 71 S
Cape Adare
72S
es 73 S ations
.740 S
¼
TO 75 S
Nc
• Drygals Ice ToTe 76' S
:.
Granite Harbor
77 S
78 S 474
Ross Ice Shelf
RooseveIr77) Island
79%S
Figure 1. Map showing the location of seismic profiles collected in the Ross Sea during Leg 2 of the R/V Polar Duke expedition in the Ross Sea. The area highlighted along PD-90-45 is the location of the profile that is displayed in figure 2. (DSDP denotes Deep Sea Drilling Project.) phani Staples for her assistance with drafting. This project was funded by National Science Foundation grant number DPP 8818523. References Anderson, J.B., and L.R. Bartek. 1990. Preliminary results of a highresolution seismic reflection survey of the Ross Sea continental shelf. Antarctic Journal of the U.S., 25(5). Behrendt, J.C., A.K. Cooper, and A. Yuan. 1987. Interpretation of marine magnetic gradiometer and multichannel seismic-reflection observations over the western Ross Sea shelf, Antarctica. In A.K.
Cooper and F.J. Davey (Eds.), The Antarctic Continental Margin: Geology and Geophysics of the Western Ross Sea. (Circum-Pacific Council for Energy and Mineral Resources Earth Science Series, Vol. 5B.) Houston, Texas: Circum-Pacific Council for Energy and Mineral Resources. Cooper, A.K., F.J. Davey, and J.C. Behrendt. 1987. Seismic stratigraphy and structure of the Victoria Land Basin, western Ross Sea, Antarctica. In A.K. Cooper and F.J. Davey (Eds.), The Antarctic Continental Margin: Geology and Geophysics of the Western Ross Sea. (Circum-Pacific Council for Energy and Mineral Resources Earth Science Series, Vol. 5B.) Houston, Texas: Circum-Pacific Council for Energy and Mineral Resources. 1990 REVIEW
Davey, F.J. 1987. Geology and structure of the Ross Sea region. In
A.K. Cooper and F.J. Davey (Eds.), The Antarctic Continental Margin: Geology and Geophysics of the Western Ross Sea. (Circum-Pacific Council for Energy and Mineral Resources Earth Science Series, Vol. 5B.) Houston, Texas: Circum-Pacific Council for Energy and Mineral Resources. Davey, F.J., D.J. Bennett, and R.E. Houtz. 1982. Sedimentary basins
of the Ross Sea, Antarctica. New Zealand Journal of Geology and Geophysics, 25, 245-255. 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. (Circum-Pacific Council for Energy and Mineral Resources Earth Science Series, Vol. SB.) Houston, Texas: Circum-Pacific Council for Energy and Mineral Resources. Duerbaum, H.J., and F. Tessensohn. 1986. Aeromagnetic survey of the Transantarctic Mountains and the Ross Sea, Antarctica. Bundesanstalt für Geowissenschaften und Rohstofe (Circular 3.) Hinz, K., and M. Block. 1983. Results of geophysical investigations in the Weddell Sea and in the Ross Sea, Antarctica. Proceedings of the 11th World Petroleum Congress (London), PD 2, Westchester. New York: Wiley and Sons. Karl, H.A., E. Reimnitz, and B.D. Edwards. 1987. Extent and nature of Ross Sea Unconformity in the western Ross Sea, Antarctica. In
A.K. Cooper and F.J. Davey (Eds.), The Antarctic Continental Margin: 69
Geology and Geophysics of the Western Ross Sea. (Circum-Pacific Council for Energy and Mineral Resources Earth Science Series, Vol. 513.) Houston, Texas: Circum-Pacific Council for Energy and Mineral Resources.
1300
1400
1500
Total Magnetic Anomaly Map, Victoria Land/Ross Sea Antarctic. 1987. Sheet 6 (Franklin Island), GANOVEX IV 1984-1985, BGR (Bundesanstalt für Geowissenschaften und Rohstofe)-USGS (U.S. Geological Survey). Hannover, Federal Republic of Germany.
1600
1700
1800
0.0i..
U
C) .!.
0.5
C)
E
IC) >
I>' (a
1.0
1.5
1300 1400 1500 1600 0.0 , i I I I I I ,-,, Direct Arrival rrivaV' PD-90-45 Submarine Volcano 0.5
1700
C)
E I-
C) >
I-
Massive, Hummocky Wedge of Strata 1.0
\
1800
lntruslves Sea floor
/ Onlp -'
>, Ca
0
_-----
Multiple
_ 1.5
Faults
Faults ipIe 1300
Post-Latel:ene Surface 1400
1500
1600
10
kilometers
Vertical Exaggeration = 16x 1700 1800
I I I I I I I t
I
600500U, 4)
0
400-
Ca
3
300-
Figure 2. These profiles are, from top to bottom, high resolution air gun data, a line drawing interpretation, and magnetic data acquired over a submarine volcano in the western Ross Sea.
70
ANTARCTIC JOURNAL
Earth and Space Sciences Division Jet Propulsion Laboratory California institute of Technology Pasadena, California 91109
The Ross Sea, the focus of this study, is a large embayment of the antarctic coastline that lies at the boundary of the two major subcontinents of Antarctica (East and West Antarctica). The Ross embayment is best characterized as a broad rift (1,000 kilometers wide) Davey (1987). Cooper, Davey, Behrendt (1987) attribute the formation of the Ross embayment to the breakup of Gondwanaland. Three major basins have been identified within the Ross embayment: the Eastern Basin, the Central Trough, and the Victoria Land Basin (Davey, Bennett, and Houtz 1982). The Victoria Land Basin and the Central Trough are thought to have formed by continued rifting, while the Eastern Basin is thought to be a product of crustal downwarping (Davey 1987). The presence of numerous near-surface normal faults and submarine volcanoes documents Cenozoic rifting in the Victoria Land Basin (Cooper et al. 1987). In this article, we report more evidence of rift-associated tectonic activity within the Victoria Land Basin. During Leg 2 of the R'V Polar Duke expedition to the Ross Sea seismic, gravity, and magnetic data were collected in the western portion of the Ross. The scientific objective of the cruise was to acquire a high-resolution, seismic-reflection database that will facilitate the development of a high-resolution seismic stratigraphy for the late Paleogene and Neogene strata of the continental shelf (see figures 1 and 2 of Anderson and Bartek, Antarctic Journal, this issue, for the location of cruise tracklines). Existing drill core from the shelf (DSDP leg 28 sites 270, 271, 272, and 273 (Deep Sea Drilling Project) and McMurdo Sound (MSSTS-1 (McMurdo Sound Sediment and Tectonic Studies) and CIROS 1 and 2 (Cenozoic Investigation of the Ross Sea)) provide the chronostratigraphy for the high resolution seismic database. The drill sites in the Ross Sea are not numerous, however, and they are widely dispersed, so an attempt to make regional correlations in the Ross Sea demands that stratigraphic continuity along the seismic profiles be maintained. Therefore, when planning our cruise tracks we used existing data sets (Hinz and Block 1983 and Cooper et al. 1987) to avoid the structural features that disrupt stratigraphic con68
tinuity. Our efforts to avoid structural complexity were temporarily foiled, however, in the western Ross Sea when we crossed over an uncharted bathymetric feature that we interpret as a submarine volcano. Seismic and magnetic data support the interpretation of this feature, which is located on the eastern flank of the Discovery Graben, on the Lee Arch, in the Victoria Land Basin (76°15.3'S 166°23.7'E) (see figures 24 and 25 of Cooper et al. 1987 for locations), as a submarine volcano (figure 1 illustrates the location of the profile over the volcano). It appears to a part of the chain of small volcanoes that lie on the axis of the Lee Arch, south of 76°S (Total Magnetic Anomaly Map, Victoria Land/Ross Sea, Antarctica 1987). The volcano was overflown during the BGR-USGS GANO VEX IV aeromagnetic survey of the Ross Sea in 1984-1985 (Duerbaum and Tessensohn 1986; and Behrendt, Cooper, and Yuan 1987), and it lies in an area that Davey and Cooper (1987) indicate has a free-air gravity anomaly of zero. During the aeromagnetic survey, which was flown at a height of 2,000 feet (610 meters), a 65-nanotesla magnetic anomaly was recorded in this area (Duerbaum and Tessensohn 1986; and Behrendt et al. 1987). This anomaly is actually a narrow cone that is approximately 10 kilometers in diameter. The crest of it lies at a depth of 158 meters and rises approximately 500 meters above the surrounding seafloor and the slopes on the flanks of this knoll are approximately 3 degrees. The massive acoustic character of the knoll (figure 2) is similar to the acoustic character of the submarine volcanoes and near-surface intrusions described by Behrendt et al. (1987), Cooper et al. (1987), and Karl, Reimnitz, and Edwards (1987). A 600-nanotesla anomaly, with a wavelength of approximately 10 kilometers and a sharp peak over the northeastern flank of the knoll, was observed during our survey over the area (figure 2). The peak over the northeastern flank may indicate that it was the most recently active portion of the volcano. Southwest of the volcano, the presence of several subseafloor intrusions is indicated by smaller, symmetrical short-wavelength anomalies of 100 and 200 nanoteslas (figure 2). The small intrusions disrupt seismic reflectors near the top of the stratigraphic succession (figure 2). Stratigraphic and structural relationships in the seismic data (figure 2) indicate that the rift-associated tectonic activity in this area is late Cenozoic in age. Post-late Oligocene rocks lie at a depth of approximately 0.5 seconds two-way travel time below the seafloor in this area (Cooper et al. 1987, figure 9-B). Downiap of reflectors, within a massive and hummocky wedge of strata on the north flank of the volcano, onto a surface that lies at a depth of approximately 0.53 seconds two-way travel time below the seafloor (figure 2) suggests that volcanic activity in this area was initiated in early Neogene time. The presence of another massive and hummocky wedge of reflectors on top of a near-surface unconformity that is late Miocene(?) to late Pleistocene(?) in age (Karl et al. 1987) suggests that volcano may still be active. Other evidence of recent rift-associated tectonic activity includes the normal faults (figure 1) that dissect the Neogene (Cooper et al. 1987) strata in this area. We wish to thank John Anderson, the Chief Scientist of the leg 2 RIV Polar Duke 1990 cruise, for informative discussions and for releasing data presented in this paper. We also thank Alan Cooper for assistance in planning the cruise. Adrien Pascouet and Jim Hedger of Seismic Systems, Inc., loaned a generator injector gun to us and trained us to use and maintain it. Rick Shiftman of Teledyne, Inc., provided assistance in streamer repair. We thank the highly professional crew of the RIV Polar Duke for their assistance during the cruise and SteANTARCTIC JOURNAL
160 E
165' E
1700 E
175' E
180'
175' W
170' W
1650 W
160 W 71 S
Cape Adare
72S
es 73 S ations
.740 S
¼
TO 75 S
Nc
• Drygals Ice ToTe 76' S
:.
Granite Harbor
77 S
78 S 474
Ross Ice Shelf
RooseveIr77) Island
79%S
Figure 1. Map showing the location of seismic profiles collected in the Ross Sea during Leg 2 of the R/V Polar Duke expedition in the Ross Sea. The area highlighted along PD-90-45 is the location of the profile that is displayed in figure 2. (DSDP denotes Deep Sea Drilling Project.) phani Staples for her assistance with drafting. This project was funded by National Science Foundation grant number DPP 8818523. References Anderson, J.B., and L.R. Bartek. 1990. Preliminary results of a highresolution seismic reflection survey of the Ross Sea continental shelf. Antarctic Journal of the U.S., 25(5). Behrendt, J.C., A.K. Cooper, and A. Yuan. 1987. Interpretation of marine magnetic gradiometer and multichannel seismic-reflection observations over the western Ross Sea shelf, Antarctica. In A.K.
Cooper and F.J. Davey (Eds.), The Antarctic Continental Margin: Geology and Geophysics of the Western Ross Sea. (Circum-Pacific Council for Energy and Mineral Resources Earth Science Series, Vol. 5B.) Houston, Texas: Circum-Pacific Council for Energy and Mineral Resources. Cooper, A.K., F.J. Davey, and J.C. Behrendt. 1987. Seismic stratigraphy and structure of the Victoria Land Basin, western Ross Sea, Antarctica. In A.K. Cooper and F.J. Davey (Eds.), The Antarctic Continental Margin: Geology and Geophysics of the Western Ross Sea. (Circum-Pacific Council for Energy and Mineral Resources Earth Science Series, Vol. 5B.) Houston, Texas: Circum-Pacific Council for Energy and Mineral Resources. 1990 REVIEW
Davey, F.J. 1987. Geology and structure of the Ross Sea region. In
A.K. Cooper and F.J. Davey (Eds.), The Antarctic Continental Margin: Geology and Geophysics of the Western Ross Sea. (Circum-Pacific Council for Energy and Mineral Resources Earth Science Series, Vol. 5B.) Houston, Texas: Circum-Pacific Council for Energy and Mineral Resources. Davey, F.J., D.J. Bennett, and R.E. Houtz. 1982. Sedimentary basins
of the Ross Sea, Antarctica. New Zealand Journal of Geology and Geophysics, 25, 245-255. 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. (Circum-Pacific Council for Energy and Mineral Resources Earth Science Series, Vol. SB.) Houston, Texas: Circum-Pacific Council for Energy and Mineral Resources. Duerbaum, H.J., and F. Tessensohn. 1986. Aeromagnetic survey of the Transantarctic Mountains and the Ross Sea, Antarctica. Bundesanstalt für Geowissenschaften und Rohstofe (Circular 3.) Hinz, K., and M. Block. 1983. Results of geophysical investigations in the Weddell Sea and in the Ross Sea, Antarctica. Proceedings of the 11th World Petroleum Congress (London), PD 2, Westchester. New York: Wiley and Sons. Karl, H.A., E. Reimnitz, and B.D. Edwards. 1987. Extent and nature of Ross Sea Unconformity in the western Ross Sea, Antarctica. In
A.K. Cooper and F.J. Davey (Eds.), The Antarctic Continental Margin: 69
Geology and Geophysics of the Western Ross Sea. (Circum-Pacific Council for Energy and Mineral Resources Earth Science Series, Vol. 513.) Houston, Texas: Circum-Pacific Council for Energy and Mineral Resources.
1300
1400
1500
Total Magnetic Anomaly Map, Victoria Land/Ross Sea Antarctic. 1987. Sheet 6 (Franklin Island), GANOVEX IV 1984-1985, BGR (Bundesanstalt für Geowissenschaften und Rohstofe)-USGS (U.S. Geological Survey). Hannover, Federal Republic of Germany.
1600
1700
1800
0.0i..
U
C) .!.
0.5
C)
E
IC) >
I>' (a
1.0
1.5
1300 1400 1500 1600 0.0 , i I I I I I ,-,, Direct Arrival rrivaV' PD-90-45 Submarine Volcano 0.5
1700
C)
E I-
C) >
I-
Massive, Hummocky Wedge of Strata 1.0
\
1800
lntruslves Sea floor
/ Onlp -'
>, Ca
0
_-----
Multiple
_ 1.5
Faults
Faults ipIe 1300
Post-Latel:ene Surface 1400
1500
1600
10
kilometers
Vertical Exaggeration = 16x 1700 1800
I I I I I I I t
I
600500U, 4)
0
400-
Ca
3
300-
Figure 2. These profiles are, from top to bottom, high resolution air gun data, a line drawing interpretation, and magnetic data acquired over a submarine volcano in the western Ross Sea.
70
ANTARCTIC JOURNAL
