New multichannel seismic results from Bransfield Strait, Antarctica
EW91-01: New multichannel seismic results from Bransfield Strait, Antarctica DANIEL H.N. BARKER, Department of Geological Sciences and Institute for Geophysics, University of Texas, Austin, Texas 78759 JAMES A. AUSTIN, JR., Institute for Geophysics, University of Texas, Austin, Texas 78759
versity of Texas Institute for Geophysics through to 30-fold stacked sections, with poststack migration applied. Preliminary results of processing of these data have already been presented (Barker, Hoar, and Austin 1992), following up on initial postcruise presentation of the data (Austin etal. 1991). Listric normal faulting is observed on the peninsula margin of Bransfield Strait (figure 2). In general, the peninsula margin faulting undergoes a complex evolution along strike, with numerous changes in polarity and direction of apparent block rotation. The South Shetland Islands basin margin also shows seismic evidence for large-offset normal faults. A neovolcanic ridge, which runs along the deep basin axis connecting the active volcanoes of Deception Island to the southwest with Bridgeman Island to the northeast (figure 1), is welldefined but shows apparent complex sill intrusions into adjacent and overlying sediment. There are also preliminary indications of flexural loading of preexisting crust by these basin volcanics. In addition to this main volcanic ridge, which has been previously presumed to be the focus of active modern basin extension, the new multichannel seismic results indicate that there are other loci for extension, perhaps associated with intracrustal diapirism (figure 3; Barker and Austin in preparation). This evidence suggests that this whole region may be undergoing more diffuse extension than previously suggested. Work is continuing on processing multichannel seismic data across the inner trench wall and the South Shetland
ransfield Strait is a young basin still apparently undergoB ing active extension. It is located between the northern tip of the Antarctic Peninsula and the South Shetland Islands, a small chain of islands to the northwest trending roughly parallel with the peninsula (figure 1). Although the South Shetland Islands themselves show no recent arc volcanism, they lie above a subduction zone that appears still to be active, albeit with subduction occurring at a very slow rate. This setting, coupled with interpretation of other tectonic events affecting plate motions in the vicinity, have led to the currently held view that Bransfield Strait is a back-arc basin. Relatively little is known, however, about its deeper crustal structure, and there is no real consensus as to details of tectonics active in the region. A joint cruise by the University of Texas Institute for Geophysics and the Lamont-Doherty Earth Observatory in February and March 1991 acquired deep-penetration multichannel seismic reflection data in Bransfield Strait and offshore the South Shetland "arc" as part of cruise EMI-01 of the Lamont-Doherty research vessel Maurice Ewing (see also McGinnis et al., Antarctic Journal, in this issue). These data give the best existing opportunity for imaging the deep structure of the basin. The data comprise more than 2,000 kilometers of reflection profiles (figure 1). Also acquired were Hydrosweep multibeam bathymetry, gravity, and magnetic data. Most of the northern EMI-01 lines have been processed at the Uni-
Figure 1. Tectonic setting of Bransfield Strait and the South Shetland Trench [after British Antarctic Survey (1985)]. HFZ denotes Hero Fracture Zone; NSA denotes North Scotia Ridge; SFZ denotes Shackleton Fracture Zone; SSR denotes South Scotia Ridge. Inset shows the track for cruise EW91 -01 Highlighted section indicates portion of line AP-3 shown in figures 2 and 3.
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Figure 2. Listric faulting of the Antarctic Peninsula margin of Bransfield Strait, as shown on line AP-3. This type of normal faulting is common along the margin, but polarity of faulting switches a number of times along strike, suggesting possible cross-basin transfer zones. If present, these transfer zones should be visible in the two along-axis seismic lines (see figure 1). back-arc basin. EOS, Transactions of the American Geophysical Union, 72(44), 246. Barker, D.H.N., and J.A. Austin, Jr. In preparation. Crustal diapirism in Bransfield Strait, West Antarctica—Evidence for distributed extension in marginal basin formation. Barker, D.H.N, T.J. Hoar, and J.A. Austin, Jr. 1992. Extensional crustal structure in Bransfield Strait, West Antarctic Peninsula. EOS, Transactions of the American Geophysical Union, 73(43), 563. British Antarctic Survey. 1985. Tectonic map of the Scotia Arc, 1:3,000,000 (BAS miscellaneous 3). Cambridge: British Antarctic Survey. McGinnis, J.P., D.E. Hayes, J.C. Mutter, P. Buhl, and J.B. Diebold. 1993. Fracture zone control on continental margin development: Multichannel seismic survey along the southern Antarctic Peninsula. Antarctic Journal of the U.S., 28(5).
Trench. At the same time, we are engaged in a detailed structural analysis of Bransfield Strait to elucidate the interaction of active fore-arc and back-arc magmatic and extension processes there. This work was supported by National Science Foundation grant OPP 89-16436 awarded to I.W.D. Dalziel of the Uni versity of Texas Institute for Geophysics. (UTIG contribution number 1013.)
References Austin, l.A., Jr., T.H. Shipley, L.A. Law yer, D.E. Hayes, J.C. Mutter, and J. McGinnis. 1991. Initial multichannel seismic results from the northern West Antarctic Peninsula: Bransfield Strait, a nascent
Figure 3. On the left, data from the Antarctic Peninsula margin of Bransfield Strait are shown. Features to note are fanning normal faults and the high-amplitude reflection event between 2.9 and 3.2 seconds. The fan faulting associated with an apparent structural high is reminiscent of sediment deformation above diapirs. On the right are data from the Gulf of Mexico (courtesy of Carl Fiduk, University of Texas Institute for Geophysics) showing typical sediment structures above a salt diapir (convex-upward, high-amplitude reflection event peaking at approximately 1.9 seconds is the top of salt). Note that both figures are at the same scale.
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versity of Texas Institute for Geophysics through to 30-fold stacked sections, with poststack migration applied. Preliminary results of processing of these data have already been presented (Barker, Hoar, and Austin 1992), following up on initial postcruise presentation of the data (Austin etal. 1991). Listric normal faulting is observed on the peninsula margin of Bransfield Strait (figure 2). In general, the peninsula margin faulting undergoes a complex evolution along strike, with numerous changes in polarity and direction of apparent block rotation. The South Shetland Islands basin margin also shows seismic evidence for large-offset normal faults. A neovolcanic ridge, which runs along the deep basin axis connecting the active volcanoes of Deception Island to the southwest with Bridgeman Island to the northeast (figure 1), is welldefined but shows apparent complex sill intrusions into adjacent and overlying sediment. There are also preliminary indications of flexural loading of preexisting crust by these basin volcanics. In addition to this main volcanic ridge, which has been previously presumed to be the focus of active modern basin extension, the new multichannel seismic results indicate that there are other loci for extension, perhaps associated with intracrustal diapirism (figure 3; Barker and Austin in preparation). This evidence suggests that this whole region may be undergoing more diffuse extension than previously suggested. Work is continuing on processing multichannel seismic data across the inner trench wall and the South Shetland
ransfield Strait is a young basin still apparently undergoB ing active extension. It is located between the northern tip of the Antarctic Peninsula and the South Shetland Islands, a small chain of islands to the northwest trending roughly parallel with the peninsula (figure 1). Although the South Shetland Islands themselves show no recent arc volcanism, they lie above a subduction zone that appears still to be active, albeit with subduction occurring at a very slow rate. This setting, coupled with interpretation of other tectonic events affecting plate motions in the vicinity, have led to the currently held view that Bransfield Strait is a back-arc basin. Relatively little is known, however, about its deeper crustal structure, and there is no real consensus as to details of tectonics active in the region. A joint cruise by the University of Texas Institute for Geophysics and the Lamont-Doherty Earth Observatory in February and March 1991 acquired deep-penetration multichannel seismic reflection data in Bransfield Strait and offshore the South Shetland "arc" as part of cruise EMI-01 of the Lamont-Doherty research vessel Maurice Ewing (see also McGinnis et al., Antarctic Journal, in this issue). These data give the best existing opportunity for imaging the deep structure of the basin. The data comprise more than 2,000 kilometers of reflection profiles (figure 1). Also acquired were Hydrosweep multibeam bathymetry, gravity, and magnetic data. Most of the northern EMI-01 lines have been processed at the Uni-
Figure 1. Tectonic setting of Bransfield Strait and the South Shetland Trench [after British Antarctic Survey (1985)]. HFZ denotes Hero Fracture Zone; NSA denotes North Scotia Ridge; SFZ denotes Shackleton Fracture Zone; SSR denotes South Scotia Ridge. Inset shows the track for cruise EW91 -01 Highlighted section indicates portion of line AP-3 shown in figures 2 and 3.
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Figure 2. Listric faulting of the Antarctic Peninsula margin of Bransfield Strait, as shown on line AP-3. This type of normal faulting is common along the margin, but polarity of faulting switches a number of times along strike, suggesting possible cross-basin transfer zones. If present, these transfer zones should be visible in the two along-axis seismic lines (see figure 1). back-arc basin. EOS, Transactions of the American Geophysical Union, 72(44), 246. Barker, D.H.N., and J.A. Austin, Jr. In preparation. Crustal diapirism in Bransfield Strait, West Antarctica—Evidence for distributed extension in marginal basin formation. Barker, D.H.N, T.J. Hoar, and J.A. Austin, Jr. 1992. Extensional crustal structure in Bransfield Strait, West Antarctic Peninsula. EOS, Transactions of the American Geophysical Union, 73(43), 563. British Antarctic Survey. 1985. Tectonic map of the Scotia Arc, 1:3,000,000 (BAS miscellaneous 3). Cambridge: British Antarctic Survey. McGinnis, J.P., D.E. Hayes, J.C. Mutter, P. Buhl, and J.B. Diebold. 1993. Fracture zone control on continental margin development: Multichannel seismic survey along the southern Antarctic Peninsula. Antarctic Journal of the U.S., 28(5).
Trench. At the same time, we are engaged in a detailed structural analysis of Bransfield Strait to elucidate the interaction of active fore-arc and back-arc magmatic and extension processes there. This work was supported by National Science Foundation grant OPP 89-16436 awarded to I.W.D. Dalziel of the Uni versity of Texas Institute for Geophysics. (UTIG contribution number 1013.)
References Austin, l.A., Jr., T.H. Shipley, L.A. Law yer, D.E. Hayes, J.C. Mutter, and J. McGinnis. 1991. Initial multichannel seismic results from the northern West Antarctic Peninsula: Bransfield Strait, a nascent
Figure 3. On the left, data from the Antarctic Peninsula margin of Bransfield Strait are shown. Features to note are fanning normal faults and the high-amplitude reflection event between 2.9 and 3.2 seconds. The fan faulting associated with an apparent structural high is reminiscent of sediment deformation above diapirs. On the right are data from the Gulf of Mexico (courtesy of Carl Fiduk, University of Texas Institute for Geophysics) showing typical sediment structures above a salt diapir (convex-upward, high-amplitude reflection event peaking at approximately 1.9 seconds is the top of salt). Note that both figures are at the same scale.
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