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Structural Studies in the Scotia Arc: Elephant Island, Gibbs Island, Hope

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Structural Studies in the Scotia Arc: Elephant Island, Gibbs Island, Hope MADRE QUATENAP Y AND DE DIOS Eli ANDEAN IGNEOUS COUPLED CRETACEOUS SEDIMENTS JURASSIC AND CRETACEOUS VOLCANIC ROCKS Simplified geologic map of southern South U. PALEOZOIC SEDIMENTS America. The black arrows show the general lo- cation of the traverses made during Hero Cruise MFIAMORPHIC 69-6. ca I 7UI phism or basement reactivation (apart from clean- Structural Studies in the Scotia Arc: cut thrusting) at this time. At least two fold episodes affected the Pennsyl- Elephant Island, Gibbs Island, Hope vanian-Permian marine succession of the Madre de Bay, and Livingston Island Dios basin on the west coast of the continent. How- ever, there is no direct evidence of the age relation- IAN W. D. DALZIEL, DAVID P. PRICE, ship between the polyphase deformation recognizable and GERRY L. STIREWALT in the pre-late Jurassic metamorphic complex to the east and that observed in these sediments. Pebbles of Lamont-Doherty Geological Observatory granitic rocks were found in the sediments. Columbia University The large granitic (and earlier basic) intrusives seen to cut the Pennsylvanian-Permian sediments, During January and February 1970, the authors the metamorphic basement, and the Mesozoic cover carried out geologic studies at a number of localities rocks are mainly post-tectonic. There is no structural in the South Shetland Islands and the Antarctic evidence that any of these igneous rocks are older Peninsula (see map). Ten days were spent on the than latest Mesozoic-Tertiary. Their foliation is west coast of Elephant Island, ten days at the north- primary. eastern end of the Antarctic Peninsula (Hope Bay), A large number of specimens were collected for and a brief landing was made on Gibbs Island. In microscopic studies. It is hoped that these will shed addition, one of us (D.P.P.) worked with geologists further light on the nature of the various deforma- from the Instituto Antártico Chileno (Francisco tional events for which field evidence was observed. Hervé and Eduardo Valenzuela) on western Livings- The specimens of the metamorphic rocks will make ton Island for two weeks in early February. it possible to establish the relationship in time be- The rocks seen on western Elephant Island con- tween deformation and metamorphism. sisted of metasedimentary schists similar to those Over 100 oriented specimens were collected for reported from elsewhere on the island (Wordie, 1921; paleomagnetic analysis. Tilley, 1930; Tyrrell, 1945; Araya and Hervé, 1966). However, the authors were able to establish a his- References tory of polyphase deformation, which had not previ- Cortés, R. and I. W. D. Dalziel. 1970. The tectonic history ously been recognized in the metamorphic rocks of of the Patagonian Andes (abstract). American Geophys- the islands in the Scotia Arc. ical Union. Transactions, 51:421. The visit to Hope Bay was improvised when Katz, H. R. In press. Some new concepts on geosynclinal development and mountain building at the southern end logistic problems prevented the establishment of an- of South America. International Geological Congress, other camp on Elephant Island. Hope Bay is the New Delhi, 1964. "type" locality for studying the relationship between 100 ANTARCTIC JOURNAL principal investigator in the Miers Bluff Formation of eastern Livingston Island during the 1968-1969 season (Dalziel, 1969). The Jurassic volcanics and ELIPHANY ISIAND sediments which crop out at a higher topographic level (and have a basal conglomerate containing clasts from the Trinity Peninsula Series), on the other hand, show no signs of having been affected by more than a mild regional tilting to the south- 50 W west. Thus, although it is not possible to prove un- equivocally that an unconformable relationship exists between the two sequences, as the actual contact is covered by a scree slope (Adie, 1957; Bibby, 1966; OINVILLE Schopf, in press), there is little room for doubt that such an unconformity exists, as Adie (1957) has long KING =GE maintained. On western Livingston Island, Price examined the sedimentary and volcanic sequence of the Byers Peninsula previously assigned to the Miocene (Hobbs, 1968), but now recognized as Mesozoic (Araya and Hervé, 1966; 0. Gonzalez, personal communication). The rocks are much less intensely LIVINGSTON deformed than the sediments of the Miers Bluff ISLAND Formation (? Trinity Peninsula Series) of eastern Livingston Island (Daiziel, 1969). On Gibbs Island, time was available only for col- lecting. A helicopter reconnaissance indicated, how- ever, that ultramafic rocks are not confined to the eastern end of the island (Narrow Isle) as previously reported (Tyrrell, 1945). Material was collected for petrologic, paleon- tologic, paleomagnetic, and geochronologic study. References Adie, R. J. 1957. The petrology of Graham Land, ITT: Metamorphic rocks of the Trinity Peninsula Series. Falk- land Islands Dependencies Survey. Scientific Report No. 20. 26 p. Araya, Roberto and Francisco Hervé. 1966. Estudio geo- BEEN morfológico y geológico en ]as Islas Shetland del Sur, Antártica. Instituto Antdrtico Chileno. Publicación No. 8, QUATERNARY AND TERTIARY .;5 TRINITY PENINSULA SERIES ANDEAN IGNEOUS COMPLEX AND EQUIVALENTS p. 1-15. CRETACEOUS AND JURASSIC METAMORPHIC BASEMENT(?) Bibby, J S. 1966. The stratigraphy of part of north-east SEDIMENTS COMPLEX . JURASSIC VOICANIC ROCKS Graham Land and the James Ross Island Group. British Antarctic Survey. Scientific Report No. 53. 37 p. Geological map of the South Shetland Islands and the northern Daiziel, I. W. D. 1969. Structural studies in the Scotia Arc: Antarctic Peninsula (modified, with permission, from a British Ant- Livingston Island. Antarctic Journal of the U.S., IV (4) arctic Survey map compiled by R. J. Adie). Localities visited are 137. indicated by black arrows. Hobbs, G. J . 1968. The geology of the South Shetland Is- lands: IV, The geology of Livingston Island. British Ant- arctic Survey. Scientific Report, No. 47. 34 p. two of the most important lithostratigraphic units Schopf, J . M. In press. Plant material from the Miers Bluff of the Antarctic Peninsula area, namely the Trinity Formation of the South Shetland Islands. Peninsula Series (? late Paleozoic) and the volcanic Tilley, C. E. 1930. Petrographical notes on rocks from Ele- pliant Island, South Shetlands. In: Report on the Geo- and sedimentary Jurassic sequence (Adie, 1957). logical Collections made during the Voyae of the Quest. While the details of their structural history could British Museum of Natural History, p. 55-62. not be established within the limited area studied, Tyrrell, G. W. 1945. Report on rocks from West Antarctica the northwesterly dipping sediments of the Trinity and the Scotia Arc. Discovery Reports, 23: 76-88. Peninsula Series have clearly been affected by a Wordie, J . M. 1921. Shackleton Antarctic Expedition, 1914- 1917. Geological observations in the Weddell Sea area. number of episodes of folding and faulting. Their Royal Society of Edinburgh. Transactions, 53 (pt. I, no. structural style is similar to that observed by the 2): 17-27. July—August 1970 101.
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