Marine geology and geophysics
Marine geology and geophysics Marine magnetic and aeromagnetic evidence for late Cenozoic submarine and subglacial volcanism in the west antarctic rift system JOHN C. BEHRENDT, U.S. Geological Survey, Regional Geophysics Branch, Denver Federal Center, Denver, Colorado 80225 GERMAN ANTARCTIC NORTH VICTORIA LAND EXPEDITION (GANOVEX) GROUP CORRIDOR AEROGEOPHYSICS OF THE SOUTHEAST Ross TRANSECT ZONE (CASERTZ) GROUP
xtensive geophysical surveys over and adjacent to the E Ross Sea Continental Shelf have revealed much information about the structure of the extended crust in this area of the volcanically active west antarctic rift system MR). The areas of the rift underlying the Byrd Subglacial Basin are much less well known. The late Cenozoic rift activity, characterized by exposures of alkaline bimodal volcanic rocks along the flanks of the WR (LeMasurier 1990), has been dated from approximately 30 million years ago to the present. Geophysical data (Behrendt et al. 1991, in press) and ocean island basalt chemistry (Hole and LeMasurier 1990; LeMasurier, Behrendt, and Hole 1992) of the volcanic rocks suggest that a large mantle plume head origin may have been responsible for the late Cenozoic rift activity in the WR. The west antarctic ice sheet (WAIS) flows through the Byrd Subglacial Basin into the Ross Ice Shelf, and its regime is probably partially controlled by rift tectonism, including active volcanism (Blankenship et al. 1993) and rift shoulder mountain uplift. The magnetic gradiometer survey by the research vessel S.P. Lee in 1984 (Behrendt, Cooper, and Yuan 1987) and the German antarctic north Victoria Land expedition-U.S. Geological Survey (GANOVEX-USGS) aeromagnetic survey during the 1984-1985 field season, over the western Ross Sea Continental Shelf and adjacent rift shoulder (Bosum et al. 1989; Behrendt et al. 1991), and a 1990-1991 (Damaske et al. 1992) GANOVEX-USGS aeromagnetic survey over the northwest Ross Ice Shelf, supplemented by aeromagnetic profiles from Pederson et al. (1981), indicate numerous magnetic anomalies in the WR interpreted to be the result of late Cenozoic magmatism by comparison to anomalies over the exposed late Cenozoic volcanic rocks (LeMasurier and Thomson 1990). About 100 circular short wavelength [1-10-kilometers (km)] 20- to more than 1,000-nanotesla (nT) positive anomalies are interpreted as evidence of submarine volcanoes concentrated along north-northwest trending zones associated with magnetic lineations interpreted as "rift fabric" (Behrendt et al. 1991). Marine seismic reflection profiles support the volcanic interpretation and provide additional evidence for a Holocene age of one of the features.
During the 1988-1989 field season, GANOVEX-(JSGS collected large offset seismic profiles (Tréhu, Behrendt, and Fritsch in press) over the Ross Sea Continental Shelf; these and other data (Cooper, Davey, and Cochrane 1987; Beaudoin, ten Brink, and Stern 1992) have shown extended continental crust about 18 to 20 km thick. The Central Basin (Tréhu et al. in press) is underplated by a "rift cushion" with a seismic velocity of more than 7 km per second (km/sec) which is interpreted as the source of associated positive magnetic and gravitational anomalies. A regional positive Bouguer gravity anomaly [0 to +50 milliGals (mGal) contrasted to -150 mGal across the rift shoulder] stretches from the extended continental crust of the Ross Sea Continental Shelf (Behrendt et al. 1991) throughout the Ross embayment and Byrd Subglacial Basin area. Behrendt et al. (1991) interpret this anomaly as indicative that the Moho beneath the extended crust of the Ross embayment-Byrd Subglacial Basin is probably closer to a 20-km depth (probably coincident with the top of the upwarped asthenosphere) than to the 30 km reported in earlier interpretations made from gravity data alone. If the hypothesis for a mantle plume is correct, we would expect extensive, late Cenozoic, rift-related volcanism in the Byrd Subglacial Basin beneath the WAIS. The widely spaced aeromagnetic profiles collected in the 1960s (Behrendt et al. 1991) and the combined aero magnetic/ radar ice-sounding profiles with data collected during the 1978-1979 field season (Jankowski, Drewry, and Behrendt 1983) over the Ross Ice Shelf and Byrd Subglacial Basin and the ice streams draining the WAIS indicate many short-wavelength, high-amplitude (more than 100 nT) magnetic anomalies that were interpreted as having as their sources volcanic rocks (late Cenozoic?) at the base of the ice. During the 1991-1992 and 1992-1993 field seasons, the corridor aerogeophysics of the southeast Ross transect zone (CASERTZ) program collected 50,000 km of radar ice sounding, laser altimeter, aerogravity, and aeromagnetic profiles on a 5-km orthogonal grid over an approximately square area (roughly 330 km per side) centered approximately 82030'S 1170 W. On the basis of these data, Blankenship et al. (1993)
ANTARCTIC JOURNAL - REVIEW 1993 87
Behrendt, J.C., W.E. LeMasurier, and A.K. Cooper. In press. The West Antarctic rift system—A propagating rift "captured" by a mantle plume. In K. Kaminuma and Y. Yoshida (Eds.), Proceedings of the 6th International Symposium on Antarctic Earth Sciences, Tokyo, Japan, 1992. Tokyo: Terra Scientific. Behrendt, J.C., W.E. LeMasurier, A.K. Cooper, F. Tessensohn, A.M. Tréhu, and D. Damaske. 1991. Geophysical studies of the west antarctic rift system. Tectonics, 10(6), 1257-1273. Bosum, W., D. Damaske, N.W. Roland, J.C. Behrendt, and R. Saltus. 1989. The GANOVEX N Victoria Land/ Ross Sea aeromagnetic survey: Interpretation of anomalies. Geologisches Jahrbuch, E38, 153-230. Blankenship, D.D., R.E. Bell, S.M. Hodge, J.M. Brozena, J.C. Behrendt, and C. Finn. 1993. Active volcanism beneath the west antarctic ice sheet. Nature, 361, 526-529. Cooper, A.K., F.J. Davey, and G.R. Cochrane. 1987. Structure of extensionally rifted crust beneath the western Ross Sea and Iselin Bank, Antarctica, from sonobuoy seismic data. In A.K. Cooper and F.J.
reported the presence of a subglacial active volcano, confirming that in at least one location in the WR the magnetic anomalies over the WAIS, and interpreted as volcanic in origin, overlie a volcanically active source area. The CASERTZ corridor, when completed, will cross the WR, the sub-sea-level Byrd Subglacial Basin and the WAIS, which flows through the WR. The aeromagnetic data collected by CASERTZ during the 1991-1992 field season have been compiled into aeromagnetic maps with a 5-nT-contour interval. The 1991-1992 CASERTZ aeromagnetic data, generally observed approximately 1 km above the ice and approximately 3 km above bedrock, show 30-40 shallow-source anomalies with amplitudes from a few hundred to more than 1,000 nT, similar to those interpreted from the older aeromagnetic surveys over the WR to be produced by subglacial volcanic rocks. These anomalies are concentrated along north-south trends, which we interpret as rift fabric similar in appearance to those over the western Ross Sea Continental Shelf. The north-south trend in the CASERTZ data is slightly different from that expected on the basis of published maps of bedrock topography, a finding that is indicative of the complexity of the WR. Models fit to several typical anomalies in the CASERTZ survey indicate high magnetizations in the present field direction, which would be expected for late Cenozoic volcanic rocks. The results of the aeromagnetic surveys, which indicate a substantial volume of volcanic rocks beneath the WAIS in the Byrd Subglacial Basin, support a plume origin for the late Cenozoic rifting in the WR. This work was partially supported by National Science Foundation grants OPP 89-19100, OPP 89-19661, and OPP 9203170.
Davey (Eds.), Antarctic continental margin geology and geophysics of the western Ross Sea (Earth Science Series, Vol. 5B). Houston,
Texas: Circum-Pacific Council for Energy and Natural Resources. Damaske, D., U. Meyer, A. McCafferty, J.C. Behrendt, and H. Hoppe. 1992. An aeromagnetic survey over the northwestern Ross Ice Shelf and the McMurdo Sound area. Polarforschung, 2(60), 152-156. Hole, M.J., and W.E. LeMasurier. 1990. Geochemical variations and tectonic affinities of Cenozoic alkali basaltic rocks from the Pacific margin of West Antarctica. EOS, Transactions of the American Geophysical Union, 71(43), 1699. [Abstract]
Jankowski, E.J., D.J. Drewry, and J.C. Behrendt. 1983. Magnetic studies of upper crustal structure. In R.L. Oliver, P.R. James, and J.B. Jago (Eds.), Antarctic earth science. Canberra: Australian Academy of Science. LeMasurier, W.E. 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. LeMasurier, W.E., J.C. Behrendt, and M.J. Hole. 1992. Volcanism in the west antarctic rift: What kind of a plume? EOS, Transactions of the American Geophysical Union, 73(43), 541. [Abstract] LeMasurier, W.E., and J.W. Thomson (Eds). 1990. Volcanoes of the antarctic plate and southern oceans. (Antarctic Research Series, Vol. 48.) Washington, D.C.: American Geophysical Union. Pederson, D.R., G.E. Montgomery, L.D. McGinnis, and C.P. Ervin. 1981. Aeromagnetic survey of Ross Island, McMurdo Sound, and the Dry Valleys. In L.D. McGinnis (Ed.), Dry Valleys Drilling Project (Antarctic Research Series, Vol. 33.) Washington, D.C.: American Geophysical Union. Tréhu, A.M., J.C. Behrendt, andJ.C. Fritsch. In press. Crustal structure of the Central Basin, Ross Sea, Antarctica. In D. Damaske and J.C.
References Beaudoin, B.C., U.S. ten Brink, and T.A. Stern. 1992. Characteristics and processing of seismic data collected on thick, floating ice: Results from the Ross Ice Shelf, Antarctica. Geophysics, 57(10), 1359-1372. 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 (Earth Science Series, Vol 5B). Houston, Texas: Circum-Pacific Council for Energy and Natural Resources.
Fritsch (Eds.), GANOVEK V, GeologischesJahrbuch.
ANTARCTIC JOURNAL - REVIEW 1993 88
xtensive geophysical surveys over and adjacent to the E Ross Sea Continental Shelf have revealed much information about the structure of the extended crust in this area of the volcanically active west antarctic rift system MR). The areas of the rift underlying the Byrd Subglacial Basin are much less well known. The late Cenozoic rift activity, characterized by exposures of alkaline bimodal volcanic rocks along the flanks of the WR (LeMasurier 1990), has been dated from approximately 30 million years ago to the present. Geophysical data (Behrendt et al. 1991, in press) and ocean island basalt chemistry (Hole and LeMasurier 1990; LeMasurier, Behrendt, and Hole 1992) of the volcanic rocks suggest that a large mantle plume head origin may have been responsible for the late Cenozoic rift activity in the WR. The west antarctic ice sheet (WAIS) flows through the Byrd Subglacial Basin into the Ross Ice Shelf, and its regime is probably partially controlled by rift tectonism, including active volcanism (Blankenship et al. 1993) and rift shoulder mountain uplift. The magnetic gradiometer survey by the research vessel S.P. Lee in 1984 (Behrendt, Cooper, and Yuan 1987) and the German antarctic north Victoria Land expedition-U.S. Geological Survey (GANOVEX-USGS) aeromagnetic survey during the 1984-1985 field season, over the western Ross Sea Continental Shelf and adjacent rift shoulder (Bosum et al. 1989; Behrendt et al. 1991), and a 1990-1991 (Damaske et al. 1992) GANOVEX-USGS aeromagnetic survey over the northwest Ross Ice Shelf, supplemented by aeromagnetic profiles from Pederson et al. (1981), indicate numerous magnetic anomalies in the WR interpreted to be the result of late Cenozoic magmatism by comparison to anomalies over the exposed late Cenozoic volcanic rocks (LeMasurier and Thomson 1990). About 100 circular short wavelength [1-10-kilometers (km)] 20- to more than 1,000-nanotesla (nT) positive anomalies are interpreted as evidence of submarine volcanoes concentrated along north-northwest trending zones associated with magnetic lineations interpreted as "rift fabric" (Behrendt et al. 1991). Marine seismic reflection profiles support the volcanic interpretation and provide additional evidence for a Holocene age of one of the features.
During the 1988-1989 field season, GANOVEX-(JSGS collected large offset seismic profiles (Tréhu, Behrendt, and Fritsch in press) over the Ross Sea Continental Shelf; these and other data (Cooper, Davey, and Cochrane 1987; Beaudoin, ten Brink, and Stern 1992) have shown extended continental crust about 18 to 20 km thick. The Central Basin (Tréhu et al. in press) is underplated by a "rift cushion" with a seismic velocity of more than 7 km per second (km/sec) which is interpreted as the source of associated positive magnetic and gravitational anomalies. A regional positive Bouguer gravity anomaly [0 to +50 milliGals (mGal) contrasted to -150 mGal across the rift shoulder] stretches from the extended continental crust of the Ross Sea Continental Shelf (Behrendt et al. 1991) throughout the Ross embayment and Byrd Subglacial Basin area. Behrendt et al. (1991) interpret this anomaly as indicative that the Moho beneath the extended crust of the Ross embayment-Byrd Subglacial Basin is probably closer to a 20-km depth (probably coincident with the top of the upwarped asthenosphere) than to the 30 km reported in earlier interpretations made from gravity data alone. If the hypothesis for a mantle plume is correct, we would expect extensive, late Cenozoic, rift-related volcanism in the Byrd Subglacial Basin beneath the WAIS. The widely spaced aeromagnetic profiles collected in the 1960s (Behrendt et al. 1991) and the combined aero magnetic/ radar ice-sounding profiles with data collected during the 1978-1979 field season (Jankowski, Drewry, and Behrendt 1983) over the Ross Ice Shelf and Byrd Subglacial Basin and the ice streams draining the WAIS indicate many short-wavelength, high-amplitude (more than 100 nT) magnetic anomalies that were interpreted as having as their sources volcanic rocks (late Cenozoic?) at the base of the ice. During the 1991-1992 and 1992-1993 field seasons, the corridor aerogeophysics of the southeast Ross transect zone (CASERTZ) program collected 50,000 km of radar ice sounding, laser altimeter, aerogravity, and aeromagnetic profiles on a 5-km orthogonal grid over an approximately square area (roughly 330 km per side) centered approximately 82030'S 1170 W. On the basis of these data, Blankenship et al. (1993)
ANTARCTIC JOURNAL - REVIEW 1993 87
Behrendt, J.C., W.E. LeMasurier, and A.K. Cooper. In press. The West Antarctic rift system—A propagating rift "captured" by a mantle plume. In K. Kaminuma and Y. Yoshida (Eds.), Proceedings of the 6th International Symposium on Antarctic Earth Sciences, Tokyo, Japan, 1992. Tokyo: Terra Scientific. Behrendt, J.C., W.E. LeMasurier, A.K. Cooper, F. Tessensohn, A.M. Tréhu, and D. Damaske. 1991. Geophysical studies of the west antarctic rift system. Tectonics, 10(6), 1257-1273. Bosum, W., D. Damaske, N.W. Roland, J.C. Behrendt, and R. Saltus. 1989. The GANOVEX N Victoria Land/ Ross Sea aeromagnetic survey: Interpretation of anomalies. Geologisches Jahrbuch, E38, 153-230. Blankenship, D.D., R.E. Bell, S.M. Hodge, J.M. Brozena, J.C. Behrendt, and C. Finn. 1993. Active volcanism beneath the west antarctic ice sheet. Nature, 361, 526-529. Cooper, A.K., F.J. Davey, and G.R. Cochrane. 1987. Structure of extensionally rifted crust beneath the western Ross Sea and Iselin Bank, Antarctica, from sonobuoy seismic data. In A.K. Cooper and F.J.
reported the presence of a subglacial active volcano, confirming that in at least one location in the WR the magnetic anomalies over the WAIS, and interpreted as volcanic in origin, overlie a volcanically active source area. The CASERTZ corridor, when completed, will cross the WR, the sub-sea-level Byrd Subglacial Basin and the WAIS, which flows through the WR. The aeromagnetic data collected by CASERTZ during the 1991-1992 field season have been compiled into aeromagnetic maps with a 5-nT-contour interval. The 1991-1992 CASERTZ aeromagnetic data, generally observed approximately 1 km above the ice and approximately 3 km above bedrock, show 30-40 shallow-source anomalies with amplitudes from a few hundred to more than 1,000 nT, similar to those interpreted from the older aeromagnetic surveys over the WR to be produced by subglacial volcanic rocks. These anomalies are concentrated along north-south trends, which we interpret as rift fabric similar in appearance to those over the western Ross Sea Continental Shelf. The north-south trend in the CASERTZ data is slightly different from that expected on the basis of published maps of bedrock topography, a finding that is indicative of the complexity of the WR. Models fit to several typical anomalies in the CASERTZ survey indicate high magnetizations in the present field direction, which would be expected for late Cenozoic volcanic rocks. The results of the aeromagnetic surveys, which indicate a substantial volume of volcanic rocks beneath the WAIS in the Byrd Subglacial Basin, support a plume origin for the late Cenozoic rifting in the WR. This work was partially supported by National Science Foundation grants OPP 89-19100, OPP 89-19661, and OPP 9203170.
Davey (Eds.), Antarctic continental margin geology and geophysics of the western Ross Sea (Earth Science Series, Vol. 5B). Houston,
Texas: Circum-Pacific Council for Energy and Natural Resources. Damaske, D., U. Meyer, A. McCafferty, J.C. Behrendt, and H. Hoppe. 1992. An aeromagnetic survey over the northwestern Ross Ice Shelf and the McMurdo Sound area. Polarforschung, 2(60), 152-156. Hole, M.J., and W.E. LeMasurier. 1990. Geochemical variations and tectonic affinities of Cenozoic alkali basaltic rocks from the Pacific margin of West Antarctica. EOS, Transactions of the American Geophysical Union, 71(43), 1699. [Abstract]
Jankowski, E.J., D.J. Drewry, and J.C. Behrendt. 1983. Magnetic studies of upper crustal structure. In R.L. Oliver, P.R. James, and J.B. Jago (Eds.), Antarctic earth science. Canberra: Australian Academy of Science. LeMasurier, W.E. 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. LeMasurier, W.E., J.C. Behrendt, and M.J. Hole. 1992. Volcanism in the west antarctic rift: What kind of a plume? EOS, Transactions of the American Geophysical Union, 73(43), 541. [Abstract] LeMasurier, W.E., and J.W. Thomson (Eds). 1990. Volcanoes of the antarctic plate and southern oceans. (Antarctic Research Series, Vol. 48.) Washington, D.C.: American Geophysical Union. Pederson, D.R., G.E. Montgomery, L.D. McGinnis, and C.P. Ervin. 1981. Aeromagnetic survey of Ross Island, McMurdo Sound, and the Dry Valleys. In L.D. McGinnis (Ed.), Dry Valleys Drilling Project (Antarctic Research Series, Vol. 33.) Washington, D.C.: American Geophysical Union. Tréhu, A.M., J.C. Behrendt, andJ.C. Fritsch. In press. Crustal structure of the Central Basin, Ross Sea, Antarctica. In D. Damaske and J.C.
References Beaudoin, B.C., U.S. ten Brink, and T.A. Stern. 1992. Characteristics and processing of seismic data collected on thick, floating ice: Results from the Ross Ice Shelf, Antarctica. Geophysics, 57(10), 1359-1372. 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 (Earth Science Series, Vol 5B). Houston, Texas: Circum-Pacific Council for Energy and Natural Resources.
Fritsch (Eds.), GANOVEK V, GeologischesJahrbuch.
ANTARCTIC JOURNAL - REVIEW 1993 88
