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Structural Studies in the Scotia Arc: "Basement" Rocks of the South

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Structural Studies in the Scotia Arc: An electronic salinity-temperature-depth (STD) re- Structural studies in the corder was used at each station to measure the verti- cal temperature and salinity structure. With the Scotia Arc: "basement" rocks of aid of an acoustic pinger, the STD was positioned the South Shetland Islands to within about 20 meters of the bottom at most (R/V Hero cruise 76-1) stations. The STD was standardized against revers- ing thermometers and salinity determinations of water samples taken with Nansen bottles placed on the STD wire. Complete Nansen casts were made at 16 stations, and water samples were analyzed IAN W. D. DALZIEL for salinity, oxygen, phosphate, silicate, nitrate, Lamont-Doherty Geological Observatory and nitrite. Further, 129 water samples were col- Columbia University lected and shipped back to the United States for Palisades, New York 10964 tritium analysis. A high-resolution electronic con- ductivity-temperature-depth (cTD) recorder (figure 2) was used to investigate the fine structure of tem- During December 1975 and January 1976, Rich- perature and salinity profiles at selected stations. ard Schweickert, Richardson Allen, Randall For- Altogether, 86 lowerings of the CTD were made. sythe, and I, all of Lamont-Doherty, spent 4 weeks Four net tows were made in the upper ocean to studying all occurrences of probable pre-Jurassic obtain krill, but large numbers of krill were only "basement" rocks in the South Shetland Islands as caught at one station. These were frozen and part of the Scotia Arc tectonics project (Daiziel, shipped back to the United States for heavy metal 1975). Supported by RIV Hero (cruise 76-1), our analysis. party mapped the coastal exposures of Elephant, Clarence, Aspland, OBrien, Eadie, and Smith A highpoint this season was the recovery of two islands (see figure, next page). A brief landing was current meter moorings that had been set in 1975 made on Gibbs Island to supplement our early- 1975 (figure 3). Although the records have not been work (Dalziel et al., 1975). Further studies also were analyzed, it appears that one current meter re- made of the Miers Bluff Formation on Livingston corded current speed and direction, temperature, Island (Hobbs, 1968; Dalziel, 1969; Dalziel, 1972). and conductivity in the bottom water for over a Much laboratory work remains before a defini- year. This record may show whether formation of tive account of the rocks studied and an interpre- Antarctic Bottom Water is an intermittent process. tation of their origin and significance can be pre- sented. We plan to prepare such an account for the Symposium on Antarctic Geology and Geophy- Figure 4 shows a preliminary analysis of the ver- sics to be held in Madison, Wisconsin, in August tical section of potential temperature from the 1977. Meantime, we state that the "basement" of central Weddell Sea to the continental shelf east the South Shetlands consists of a variety of meta- of the tip of the Antarctic Peninsula. A thick layer sedimentary and metavolcanic rocks with obvious of very cold, newly formed bottom water may be affinities to those comprising the pre-Jurassic "base- traced continuously up the continental slope, indi- ment" of the South Orkney Islands and the south- cating that bottom water may form even during the ernmost Andes (Matthews and Maling, 1967; austral summer. Thomson, 1968; Dalziel, 1971; Dalziel and Elliot, 1973). The ultramafic rocks of Gibbs Island do not outcrop on the coast of Aspland, OBrien, or Eadie islands. Blueschists comparable to those recently discovered by British and Chilean parties at Cape Jason Middleton collaborated with me in the Smith, Smith Island (Smellie and Clarkson, 1975; physical oceanographic work. Robert Michel was in Rivano and Cortés, 1976), were studied around charge of the chemistry program. Technical sup- that islands entire coast and were also found on port was provided by Richard Mead, Walter Rich- northern Elephant Island. The asymmetric folds ter, Alan Rowe, James Schmitt, and Robert Yates. reported by Smellie and Clarkson (1975) as the All of the aforementioned persons are from earlier of two sets of structures on Smith Island Scripps. Terence Long, Monash University, Aus- are the dominant folds there, but they postdate tralia, participated in the data collection program. earlier isoclinal structures. The assistance of Glaciers marine science division is gratefully acknowledged. This work was sup- ported by National Science Foundation grant This research was supported by National Science 75-14936. Foundation grant opp 74-21415. June 1976 75 Index map of South Shet- land Islands and northern Antarctic Peninsula. 76 ANTARCTIC JOURNAL References antarctic margin. In: The Ocean Basins and Margins: 1. The South Atlantic (Nairn, A. E. M., and Stehli, F. G., editors). New York, Plenum Press. 171-245. Hobbs, G. J . 1968. The geology of the South Shetland Islands: Daiziel, I. W. D. 1969. Structural studies in the Scotia Arc: IV. Livingston Island. British Antarctic Survey Scientflc Report, Livingston Island. Antarctic Journal of the U.S., IV(4): 137. 47: 34. Daiziel, I. W. D. 1971. Structural studies in the Scotia Arc: the Matthews, D. H., and D. H. Maling. 1967. The geology of the South Orkney Islands. Antarctic Journal of the U.S., VI(4): 124- South Orkney Islands: I. Signy Island. British Antarctic Sur- 126. vey Scientfic Report, 25: 32. Dalziel, I. W. D. 1972. Large-scale folding in the Scotia Arc. In: Rivano, S., and R. Cortés. 1976. Note on the presence of the Antarctic Geology and Geophysics (Adie, R. J . , editor). Oslo, Uni- Lawsonite-sodic amphibole association on Smith Island, South versitetsforlaget. 47-55. Shetland Islands, Antarctica. Earth and Planetary Science Daiziel, I. W. D. 1975. Scotia Arc tectonics project, 1969-1975. Letters, 29: 34-36. Antarctic Journal of the U.S., X(3): 70-8 1. Smellie,J. L., and P. D. Clarkson. 1975. Evidence for pre-Juras- Dalziel, I. W. D., M. J. de Wit, and C. R. Stern. 1975. Structural sic subduction in western Antarctica. Nature, 258: 701-702. and petrologic studies in the Scotia Arc. Antarctic Journal of Thomson, J. W. 1968. The geology of the South Orkney Islands: the U.S., X(4): 180-182. II. The petrology of Signy Island. British Antarctic Survey Dalziel, I. W. D., and D. H. Elliot. 1973. The Scotia Arc and Scientific Report, 62: 30. Piston coring in the Ross Sea seawater). Samples were taken from the tops and bottoms of most of the cores while aboard ship; these are being analyzed. The quality of the individ- ual cores and their suitability for paleoclimatic analysis has not yet been determined because the THOMAS B. KELLOGG and ROBERT TRUESDALE cores were taken in plastic liners. Analyses of the Institute for Quaternary Studies remainder of each core will be made after they have University of Maine, Orono been shipped back to the United States and their Orono, Maine 04473 liners have been opened. Earlier in the season, Dr. Kellogg accompanied another University of Maine field party, under the In January 1976 we boarded USCGC Glacier at direction of George Denton, on a number of trips McMurdo Station to take a series of piston cores in to the dry valleys. Closely spaced samples of marine the southern Ross Sea. Unfortunately, the coring silts and clays were obtained from exposed sections winch was severely damaged by heavy seas while at New Harbor and at South River (near Marble Glacier was en route south from New Zealand. The Point). Samples of marine (?) or lacustrine (?) clays damage, restricted to electric and hydraulic sys- and silts also were taken from an entrenched stream tems, took 10 days to repair and resulted in a 2- valley east of Commonwealth Glacier in Taylor Val- day delay in starting our coring program. The ley. These samples and the Dry Valley Drilling Proj- resulting time limitations made us decide to concen- ect core from New Harbor should be helpful in trate our efforts in the restricted area along the correlating the glacial stratigraphy of the dry val- ice barrier between 173°F. and 173°W. and within leys with that to be worked out from the piston 50 nautical miles of the Ross Ice Shelf, an area from cores in the Ross Sea. which there are no Eltanin cores suitable for our study. In 5 days we occupied seventeen oceanographic We thank the officers and crew of Glacier for their stations. Piston cores were obtained at fifteen of able and untiring assistance during the coring pro- these. The cores range in length from about 10 gram, and Dr. Denton and his field party for their centimeters to 10 meters with an average length assistance. We were in the field from 5 December of about 3.5 meters. Sea surface temperature and 1975 to 20 January 1976. This research was sup- salinity data also were obtained at each station, ported by National Science Foundation grant along with modern diatom samples (filtered from 75-15524. June 1976 77.
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