Structural Studies in the Scotia Arc: Livingston Island
Structural Studies in the Scotia Arc: Livingston Island IAN W. D. DALZIEL Lamont-Doherty Geological Observatory Columbia University
The South Shetland Islands lie on the southern limb of the Scotia Arc, separated from the Antarctic Peninsula by the Bransfield Strait (Fig. 1). During the 1968-1969 antarctic summer, the author and his assistant, S. J. Brocoum, spent approximately seven weeks in the southeastern part of Livingston Island, the largest of the group, carrying out geological and geophysical studies pertinent to the tectonic history of the Arc. This particular location was selected in order to study the so-called False Bay schists, which were tentatively assigned a Precambrian age by Hobbs (1968), and a deformed but unmetamorphosed sedimentary sequence, the Miers Bluff Formation (Hobbs, 1968). The False Ba y schists occur oil eastern side of False Bay. They are mainly hornblendic, look like part of a reuiona1lv metamorphosed assemblage, but occur only as xenoliths within a coarse-grained tonalite. As pointed out by Hobbs (1968), there is no conclusive evidence oil Island concerning their age. In an attempt to settle this question, material was collected for radiometric dating, which is now being undertaken by Martin Halpern of the Southwest Center for Advanced Studies. Specimens of the tonalite are also being dated; it is presumabl y a member of the Andean intrusive suite. Most of the available time in the field was spent sttl(lving the Miers Bluff Formation, t1iic1i forms the Hurd Peninsula between South Bay arid False Bay. It is a flvsch-type sequence of alternating shales and thin graywackes which also contains numerous beds of
South ha'. is
The Scotia Arc showing location of Livingston Island and (inset) the geology of the southwestern part of the island.
\
relatively mature sandstone. Only plant fossils were found and it is not yet known whether any of these are diagnostic. Although tentatively correlated with the Trinity Peninsula Series by Hobbs (1968) on lithic grounds, the rocks are remarkably similar to the Cretaceous sediments at Cape Legoupil, directly across the Bransfield Strait on the Antarctic Peninsula, as described by Halpern (1965; see also Adie, 1964). The rocks of the Miers Bluff Formation, dip northwest at a low or moderate angle. They were regarded by Hobbs (1968) as a normal upward-facing succession. However, the present study has shown that they lie on the inverted limb of a recumbent fold of nappe proportions which is refo!ded by at least two later sets of structures. The hinge line of the recumbent fold can he deduced from cleavage/bedding relations to strike northeast-southwest parallel to the trend of the Scotia Arc at this point, and the fold faces southeast. The structural history of the Miers Bluff Formation is similar to that of the Cape Legoupil Formation as out1in€d by Miller (1966). Two other projects were undertaken during the field season. Approximately 1 50 cores were collected for paleomagnetic study from the Miers Bluff Formation, the tonalite body, and numerous crosscutting dikes. Also, a portable hi g h- g ain seismnogra)h was operated for 10 days before it was damaged in transit. References Adie, R. J . 1964. Geological history. In: Antarctic Research. Butter-worths, London, P. 118-162. Halpern, M. 1965. The geology of the General Bernardo O'Higgins area, northwest Antarctic Peninsula. Antarctic Research Series, 6: 177-209. Hobbs, C. J . 1968. The geology of the South Shetland Islands: TV. The geology of Livingston Island. British Antarctic Surz'ey. Scientific Report, no. 47, 34 p. Miller, H. 1966. Kleintektonische Untersuchungen in der Umngebung der Station "General Bernardo O'Higgins," Nordwestspitze der Antarktisclien 1-lalbinsel Geological Research, 55: 809-819.
\ \ \ \ \
False
South Ceongia \,t
SCOTIA SEA
5'. Sout h Orkney Islands
1/-
LEGEND
Bluff :17 N (2' Antarctic
South Sandwich Islands +507
ri Livingston Deception Island C Cape Legoupil
17
Peninsula
July—August 1969
Miers Bluff Formation N False Bay schists ** Tonalite
137
The South Shetland Islands lie on the southern limb of the Scotia Arc, separated from the Antarctic Peninsula by the Bransfield Strait (Fig. 1). During the 1968-1969 antarctic summer, the author and his assistant, S. J. Brocoum, spent approximately seven weeks in the southeastern part of Livingston Island, the largest of the group, carrying out geological and geophysical studies pertinent to the tectonic history of the Arc. This particular location was selected in order to study the so-called False Bay schists, which were tentatively assigned a Precambrian age by Hobbs (1968), and a deformed but unmetamorphosed sedimentary sequence, the Miers Bluff Formation (Hobbs, 1968). The False Ba y schists occur oil eastern side of False Bay. They are mainly hornblendic, look like part of a reuiona1lv metamorphosed assemblage, but occur only as xenoliths within a coarse-grained tonalite. As pointed out by Hobbs (1968), there is no conclusive evidence oil Island concerning their age. In an attempt to settle this question, material was collected for radiometric dating, which is now being undertaken by Martin Halpern of the Southwest Center for Advanced Studies. Specimens of the tonalite are also being dated; it is presumabl y a member of the Andean intrusive suite. Most of the available time in the field was spent sttl(lving the Miers Bluff Formation, t1iic1i forms the Hurd Peninsula between South Bay arid False Bay. It is a flvsch-type sequence of alternating shales and thin graywackes which also contains numerous beds of
South ha'. is
The Scotia Arc showing location of Livingston Island and (inset) the geology of the southwestern part of the island.
\
relatively mature sandstone. Only plant fossils were found and it is not yet known whether any of these are diagnostic. Although tentatively correlated with the Trinity Peninsula Series by Hobbs (1968) on lithic grounds, the rocks are remarkably similar to the Cretaceous sediments at Cape Legoupil, directly across the Bransfield Strait on the Antarctic Peninsula, as described by Halpern (1965; see also Adie, 1964). The rocks of the Miers Bluff Formation, dip northwest at a low or moderate angle. They were regarded by Hobbs (1968) as a normal upward-facing succession. However, the present study has shown that they lie on the inverted limb of a recumbent fold of nappe proportions which is refo!ded by at least two later sets of structures. The hinge line of the recumbent fold can he deduced from cleavage/bedding relations to strike northeast-southwest parallel to the trend of the Scotia Arc at this point, and the fold faces southeast. The structural history of the Miers Bluff Formation is similar to that of the Cape Legoupil Formation as out1in€d by Miller (1966). Two other projects were undertaken during the field season. Approximately 1 50 cores were collected for paleomagnetic study from the Miers Bluff Formation, the tonalite body, and numerous crosscutting dikes. Also, a portable hi g h- g ain seismnogra)h was operated for 10 days before it was damaged in transit. References Adie, R. J . 1964. Geological history. In: Antarctic Research. Butter-worths, London, P. 118-162. Halpern, M. 1965. The geology of the General Bernardo O'Higgins area, northwest Antarctic Peninsula. Antarctic Research Series, 6: 177-209. Hobbs, C. J . 1968. The geology of the South Shetland Islands: TV. The geology of Livingston Island. British Antarctic Surz'ey. Scientific Report, no. 47, 34 p. Miller, H. 1966. Kleintektonische Untersuchungen in der Umngebung der Station "General Bernardo O'Higgins," Nordwestspitze der Antarktisclien 1-lalbinsel Geological Research, 55: 809-819.
\ \ \ \ \
False
South Ceongia \,t
SCOTIA SEA
5'. Sout h Orkney Islands
1/-
LEGEND
Bluff :17 N (2' Antarctic
South Sandwich Islands +507
ri Livingston Deception Island C Cape Legoupil
17
Peninsula
July—August 1969
Miers Bluff Formation N False Bay schists ** Tonalite
137
