cm per year, or about 10 cm of water. The mean an- nual temperature is estimated to be —22°C. The party was returned to McMurdo Station by a C-130 flight on February 9, 1970; total time in the field was 91 days.
References , Adie, R. J. 1955. The petrology of Graham Land; II, The Andean granite-gabbro intrusive suite. Falkland Islands - Dependencies Survey. Scientific Report, 12. 39 p. Halpern, Martin. 1967. Rubidium-strontium isotopic age - measurements of plutonic igneous rocks in eastern Ells- worth Land and northern Antarctic Peninsula, Antarctica. Journal of Geophysical Research, 72(20) : 5133-5142. Intrusive contact exposed at Mount Wood, Latady Mountains. Laudon, T. S., L. L. Lackey, P. G. Quilty, and P. M. Otway. Folded Jurassic marine siltstone and shale (left) intruded by 1969. Geology of eastern Ellsworth Land. Antarctic Map granodiorite of probable Cretaceous age. Folio Series, 12, pl. III. flora that are tentatively identified as Jurassic. In- tensely folded felsic to intermediate flows and ash- Structural Studies in the Scotia Arc: flow tuffs occur at Mount Poster. The plutonic rocks are mostly medium- to coarse- The Patagonian and Fuegian Andes grained granodiorites and adamellites, but granites and diorites also occur. The plutons are 10-15 km IAN W. D. DALZIEL in diameter, and contacts with the sedimentary rocks Lamont-Doherty Geological Observatory are well exposed at many localities (see figure) ; the Columbia University plutons are surrounded by metamorphic aureoles several hundred meters wide. The plutonic rocks are In the course of Cruise 69-6 of R/V Hero, the probably a part of the Cretaceous and Lower Terti- author was able to examine a complete structural ary Andean intrusive suite found throughout the cross-section of the Patagonian Andes at 50°-52°S., Antarctic Peninsula (Adie, 1955). Felsic to basaltic and to make limited observations in the Fuegian dikes and sills, in part associated with the plutons, Andes at 54°-55°S. (see map). All the basic tectono- occur throughout the area. stratigraphic units of the cordillera were studied. The sedimentary rocks were tightly, and in places It was not possible to undertake regional struc- isoclinally, folded about northeast-trending, gently tural mapping in the time available. Instead, em- plunging axes. Many folds are asymmetrical with phasis was placed on the establishment of basic field axial planes steeply inclined to the northwest, which relations, tectonic history, and local structural ge- implies yielding toward the southeast. Near the ometry in each of the units. These are sufficiently plutons, fold axes are commonly deflected and local- continuous along the strike of the cordillera (see ly are steeply plunging. Penetrative deformation ac- map) to allow some extrapotation to be made with companied thermal metamorphism of the country a reasonable degree of confidence. rock, suggesting that folding and plutonism over- Irrefutable structural evidence was obtained of at lapped in time. least two major orogenic events in the Andes—at Geologically, the area is similar to the mountains 50°-52°S. and also at 54°S. The deformed Mesozoic of eastern Ellsworth Land (Laudon et al., 1969), (late Jurassic-Lower Cretaceous) sediments and where fossiliferous Jurassic sedimentary rocks and volcanics rest unconformably on the metamorphic dacitic volcanics have been intruded by plutons of complex, not only east of the High Cordillera as Cretaceous age (Halpern, 1967). Fold axes on the reported by Cespedes (unpublished) and Katz (in Orville Coast are east-west and thus form a con- press), but also to the west of it (Cortés and Daiziel, necting part of a great orogenic arc extending from 1970). The metamorphic complex underwent poly- the Lassiter Coast to eastern Ellsworth Land, where phase ductile deformation and broadly synchronous fold axes trend west-northwest. low-grade regional metamorphism in pre-late Juras- A line of stakes to measure ice movement was sic times. Late Mesozoic-early Tertiary deformation extended across the 5-mile-wide Wetmore Glacier. of the cover rocks resulted in the formation of tight A maximum movement of nearly 6 meters was eastward-facing folds with well developed, steeply recorded during the month of January. Accumula- dipping axial surface foliation. However, no evi- tion studies indicate a snowfall of approximately 30 dence was observed of high-grade regional metamor-
July–August 1970 99 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