Dalziel, I. W. D. 1969. Structural studies of the Scotia Series of the Antarctic Peninsula, and late Mesozoic Arc: Livingston Island. Antarctic Journal of the U.S., conglomerates. The metamorphic (and late Mesozoic) IV (4) : 137. 1970. Structural studies in the Scotia Arc: the rocks are cut by a few undeformed diabase dikes. Patagonian and Fuegian Andes. Antarctic Journal of Earlier work in the group has been carried out by the U.S., V (4): 99-100. members of the Scottish National Antarctic Expedi- 1971. Structural studies in the Scotia Arc: the tion (Pine, 1905 and unpublished), and by the South Orkney Islands, R/V Hero Cruise 7 1-1. Antarctic British Antarctic Survey (Adie, 1964). Journal of the U.S., VI (4): 124. In press. Large-Scale Folding in the Scotia Arc. Owing to the rugged nature of their terrain it In: Antarctic Geology and Geophysics. Oslo, Universitets- proved most efficient to study Laurie, Powell, forlaget. Fredriksen, and Coronation Islands (see map) using D. P. Price, and G. L. Stirewalt. 1970. Structural the ship as a base and making landings using an studies in the Scotia Arc: Elephant Island, Gibbs Island, Hope Bay, and Livingston Island. Antarctic Journal of inflatable rubber boat. Over 150 landings were made the U.S., V (4): 100-101. on Laurie Island, 10 on Powell Island, eight on and D. H. Elliot. In press. The Scotia Arc and Fredriksen Island, and two on Coronation Island. antarctic margin. In: The Ocean Basins and Margins; I. Signy Island was studied for 3 1/2 weeks from a base The South Atlantic. New York, Plenum Publishing Corpo- camp. ration. Katz, H. R. In press. Some New Concepts in Geosynclinal Developments and Mountain Building at the Southern Preliminary scientific results End of South America. XXII Session of the International Geological Congress, New Delhi. Section IV. p. 241-255. Laurie Island. The whole highly indented coastline Kranck, E. H. 1932. Geological investigations in the Cor- was mapped in detail. The suggestion by Matthews dillera of Tierra del Fuego. Acta Geogràphica, 4 (2): and Maling (1967, p. 2) of the presence of late 1-231. Mesozoic conglomerates is incorrect unless these rocks are confined to nunataks. Only rocks of the Gray- wacke-Shale Formation (formerly Graywacke-Shale Series) were seen. Structural studies in the Scotia Arc: In lithology and structural style the Graywacke- the South Orkney Islands. Shale Formation is virtually indistinguishable from R/V Hero Cruise 71-1 the Trinity Peninsula Series, the Miers Bluff Forma- tion in the South Shetland islands, and some rocks of the Madre de Dios basin in Chile previously IAN W. D. DALZIEL studied by the author (Dalziel, 1969, 1970, and in press; Dalziel et al., 1970). Lamont-Doherty Geological Observatory The rocks are highly deformed but essentially Columbia University unmetamorphosed. The reported north-northwest to south-southwest trending strike and fold axis orienta- tion (Pine, 1905 and unpublished; Adie, 1964) is The South Orkney Islands phase of Hero Cruise in fact confined to a narrow zone near the Argenti ae 71-1 took place during February and March 1971. base Orcadas, built on the site of the Scottish Expedi- It started from Palmer Station and terminated at tions base. The dominant strike is east-west in tae Punta Arenas, Chile. The purpose of the cruise was central part of the island and north-south at trie to study the geology of the South Orkneys as part east end. Large-scale recumbent and reclined folds of a continuing program supported by the National involving over 1,500 m of strata are present, arid Science Foundation to elucidate the structure and their limbs are deformed by later structures. tectonic history of the Scotia Arc (Dalziel, 1969, Fredriksen Island. Although similar to the Gr y- 1970, 1971, and in press; Dalziel et al., 1970). The wacke-Shale Formation of Laurie Island, the sequence author was assisted by Stephan and Alice Brocoum on Fredriksen Island contains more shale. The rocks and Mark Barsdell, all of Columbia University. are again highly deformed but unmetamorphos d. The South Orkney Islands are located on the They generally dip at moderate angles to the souh- South Scotia Ridge some 650 km east of the Ant- southwest. British Antarctic Survey geologists rep rt arctic Peninsula. Geologically they are composed the presence of a "greenstone" sill at one locality. (see map) of a metamorphic complex of unknown Powell Island. The southern part of the island age, comparable to that of Elephant, Gibbs, and consists of flat-lying late Mesozoic conglomerates. Clarence Islands in the South Shetlands group, a The northern part is composed of medium grade thick succession of graywackes and shales comparable metamorphic rocks like those of Signy and Corona- to the probably late Paleozoic Trinity Peninsula tion Islands, and not Graywacke-Shale Formation as previously mapped (Adie, 1964, p. 129; Matthews Senior scientist, Cruise 71-1, South Orkneys phase. and Maling, 1967, fig. 1). The metamorphics have

124 ANTARCTIC JOURNAL undergone at least two episodes of deformation. The mens from the diabase dikes for paleomagnetic structure is dominated by gently south-plunging analysis. The metamorphic country rocks on Corona- asymmetric folds with an easterly vergence (sense of tion immediately north of Signy Island have a struc- overturning). These refold small isoclinal folds. The tural history similar to the rocks there. gentle east to west folding of the conglomerates Material was collected for petrologic study from (Adie, 1964) may be the effect of initial dip rather all the islands visited, and from Laurie Island for than diastrophic deformation. geochronologic analysis and examination for micro- Signy Island. The metamorphic rocks of Signy fossils. Island have been affected by at least three deforma- This work could not have been accomplished with- tion episodes. The north to south trending, gently out the cooperation and seamanship of Captain plunging asymmetric folds described earlier (Adie, Richard J . Hochban and the crew of R/V Hero. 1964; Matthews and Maling, 1967; Thomson, 1968) are in fact the second set of structures. They refold isoclinal folds recognizable only on a mesoscopic scale, and their axial surfaces are locally warped by References open flexures. The asymmetric folds consistently over- Adie, R. J . 1964. Geological history. In: Antarctic Research. tlrn to the west rather than the east as stated else- London, Butterworths, p. 118-162. where (Adie, 1964, p. 128). One macroscopic fold Daiziel, I. W. D. 1969. Structural studies in the Scotia of this type runs the length of the island. The struc- Arc: Livingston Island. Antarctic Journal of the U.S., IV(4) : 137. ture indicates that the stratigraphic succession is more 1970. Structural studies in the Scotia Arc: the complex than that advanced by Matthews and Maling Patagonian and Fuegian Andes. Antarctic Journal of the (1967), and it does not support their contention U.S., V(4): 99-100. that an unconformity is present within the sequence. 1971. Structural studies in the Scotia Arc: Canal The structural and metamorphic history of the Beagle, Tierra del Fuego. Antarctic Journal of the U.S., VI (4) : 122. rocks on Signy Island is very similar to that of the In press. Large-scale folding in the Scotia Arc. rocks examined during the 1969-1970 season on In: Antarctic Geology and Geophysics, Universitets- Elephant Island in the South Shetland group (Daiziel forlaget, Oslo. et al., 1970). D. J . Price, and G. L. Stirewalt. 1970. Structural studies in the Scotia Arc: Elephant Island, Gibbs Island, Coronation Island. Two landings were made on Hope Bay, and Livingston Island. Antarctic Journal of Coronation Island, mainly to collect oriented speci- the U.S., V(4): 100-101.

CONGLOMERATE GRAYWACKE - SHALE FORMATION

METAMORPHIC COMPLEX

DIABASE DIKES D

SCALE

0 20 Hi/en,e/ers

POWELL I. 60040

Orcadas

FREDRIKSEN I.

46 Geologic map of the South Orkney Islands.

J ly—August 1971 125

Matthews, D. H., and D. H. Maling. 1967. The geology of out to 700 m were chosen for a detailed study of the the South Orkney Islands, I. Signy Island. British Ant- velocity distribution in the upper few hundred meters arctic Survey. Scientific Reports, 25. 32 p. Pine, J. H. H. 1905. On the graptolite-bearing rocks of of the finn layer. Distances were measured with a the South Orkneys. Royal Society of Edinburgh. Proceed- 50-rn tape with an estimated error of less than 0.1 ings, 25(6): 463-470. percent. An HTL 7000B seismograph system was used Unpublished. Geology of the South Orkneys. together with 7- and 20-Hz vertical geophones and Scottish National Antarctic Expedition Reports, p. 1-10. Thomson, J . W. 1968. The geology of the South Orkney 7-Hz horizontal geophones. Islands. II: The petrology of Signy Island. British The maximum velocities from the refraction pro- Antarctic Survey. Scientific Reports, 62. 30 p. files, corresponding t6 propagation in ice of density 0.91 g/cm3, are— Profile I: V, = 3.863 ± 0.003 km/sec V 1.949 ± 0.017 km/sec Seismic refraction measurements Profile II: V = 3.857 ± 0.004 km/sec at Byrd Station V5 = 1.949 ± 0.016 km/sec

HEINZ KOHNEN and CHARLES R. BENTLEY Profile III: V, = 3.859 ± 0.004 km/sec = 1.950 ± 0.013 km/sec Department of Geology and Geophysics and from these results we can give the overall mean University of Wisconsin velocities for horizontally traveling P- and S-waves as V, = 3.860 ± 0.003 km/sec and V = 1.949 ± 0.010 km/sec. The maximum depth of penetration for Seismic investigations in Antarctica and Greenland both wave types is approximately 200 m. Fig. 1 shows have shown that wave velocities in ice are affected by the reduced P-wave travel time curve for profile II. anisotropic crystal orientation and by different modes A mean attenuation constant for P-wave ampli- of densification. To study these effects, seismic refrac- tudes of a = 0.18 X 10 3m at 100 Hz has been tion measurements were made near Byrd Station, calculated. This value is smaller by a factor of about where densities and crystal orientations through the 2 than the attenuation constants derived by Robin ice sheet are known from deep and shallow drill holes. (1958) for the antarctic ice sheet and Kohnen (1969) The refraction measurements comprised a common- for the Greenland ice sheet. In the investigations at reflection-point profile 10 km long (profile I) and two Byrd Station the amplitudes were not affected by single-ended profiles 10.5 km long (profile II) and automatic gain control, and the recording system was 7.7 km long (profile III), angled 60° to one another. carefully calibrated. The common central point of the profiles was 10 km A plot of attenuation constant versus frequency southeast of the station. The ice thickness at the cen- (fig. 2) gives a linear relationship of the form a = tral point was found to be 2,030 m, approximately (0.15 f + 3) >< 10-s with a correlation coefficient of 100 m less than at Byrd Station. 0.95. A linear law is consistent with the theory of The geophone spacing was generally 30 m. Closer White (1966), in which attenuation is related to spacings of 2 m on the first 92 m and then 5 to 15 m sliding and static friction at the grain boundaries in

I Il I I I t sec] / / REDUCED TRAVEL-TIME CURVE ATTENUATION CONSTANT a 401— PROFILE. IT I / C1

I +/ [md] I / 0.0005 tO.O3?6 x 35H ,/ + 3.85±O.003 [sec] + /

+ 30 oH a = ( O.15f+3) x105

25 1 I I I I 4 8 X[km] 0 2 6 50 100 150 200 f [cp$]

Figure 1. P-wave travel time curve, profile II. Figure 2. Attenuation constant versus frequency.

126 ANTARCTIC JOURNA