Structural study of the Leap Year Fault, northern Victoria Land

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OWEN CONGLOMERATE CAMP RIDGE QUARTZITE vIvv LATE CAMBRIAN TYNDALL CARRYER GP CONGL t44 DUNDAS MIDDLE GP MARINER Mt RE CAMBRIAN Mt READ GP VOLC. AD SLEDGERS VOLC. GP EQUIVALENT REMOVED CRIMSON. CK. FM. CONTACT NOT EXPOSED - SUCCESS CK. FM.

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PROTEROZOIC BASEMENT Figure 2. Tentative correlation between Bowers and Dundas Troughs.

Zealand, Australia, and Antarctica. In C. Craddock (Ed.), Antarctic geoscience. Madison: University of Wisconsin Press. Jago, J . B. 1979. Tasmanian Cambrian biostratigraphy—A preliminary report. Journal of the Geological Society of Australia, 26,223-230. Jago, J . B., Reid, K. D., Quilty, P. C., Green, G. R., and Dailey, B. 1972. Fossiliferous Cambrian limestone from within the Mt. Read Volcanics, Mt. Lyell mine area, Tasmania. Journal of the Geological Society of Australia, 19,379-382. Laird, M. G., Bradshaw, J . D., and Wodzicki, A. 1982. Stratigraphy of the upper Precambrian and lower Paleozoic Bowers Supergroup, northern Victoria Land, Antarctica. In C. Craddock (Ed.), Antarctic geoscience. Madison: University of Wisconsin Press. Laird, M. G., Cooper, R. A., and Jago, J . B. 1977. New data on the lower Paleozoic sequence of northern Victoria Land, Antarctica, and its

significance for Australian-Antarctic relations in the Paleozoic. Nature, 265,107-110. Solomon, M. 1979. Discussion: Delamerian unconformities in Tasmania. Journal of the Geological Society of Australia, 26,435-438. Solomon, M. 1981. An introduction to the geology and metallic ore deposits of Tasmania. Economic Geology, 76,194-208. Tessensohn, F., Duphorn, K., Jordan, H., Kleinschmidt, G., Skinner, D. N. B., Vetter, U., Wright, T. 0., and Wyborn, D. 1981. Geological comparison of basement units in north Victoria Land, Antarctica. Geologisches Jahrbuch, B41,31-88. Wodzicki, A., Bradshaw, J . D., and Laird, M. G. 1982. Petrology of the Wilson and Robertson Bay Groups and Bower Supergroup, northern Victoria Land, Antarctica. In C. Craddock (Ed.), Antarctic geoscience. Madison: University of Wisconsin Press.

Structural study of the Leap Year Fault, northern Victoria Land

Bowers Group on the west and also divides areas containing 360-million-year-old Admiralty intrusions from older (470 million years) Granite Harbor granites (Gair et al. 1969; Kreuzer et al. 1981; Tessensohn et al. 1981). The history of movement on this fault, including sense of motion, displacement, and timing, is critical to reconstructing Paleozoic geographies and plate tectonic settings for this part of the ancient Gondwana continental margin (Craddock 1972; Elliot 1975). During the last part of December 1981 and the first half of January 1982, I visited 15 localities near the trace of the fault, from McKenzie Nunatak in the north to the Mount McCarthyMount Burton area just east of the head of the Mariner Glacier. Operating out of the northern Victoria Land base camp on the Evans Névé, I used field camps and close-support helicopter traverses to examine the sites. Assisting in the field was Ellen K. Wright.

THOMAS 0. WRIGHT

Division of Earth Sciences National Science Foundation Washington, D.C. 20550

The Leap Year Fault is a 250-kilometer-long linear feature in the Transantarctic Mountains of northern Victoria Land (figure 1). It separates the Robertson Bay Group on the east from the 1982 REVIEW

11



from the Robertson Bay shales and graywacke (figure 2). This feature was observed in a strip up to 15 kilometers wide paralleling the fault trace, although there are a few areas much nearer the fault that do not show this feature. The 15-kilometer-wide strip probably indicates the schistosity and mineral lamination discussed by Bradshaw, Laird, and Wodzicki (1977) and possibly that of Crowder (1968). This banding appears to be an accentuation of preexisting cleavage because the orientation parallels cleavage regionally. Gradations from cleavage to this banding were noted, but the banding probably occurred during an additional transposition or overprinting cleavage-forming event. These features are best developed in the finer grained units. Considerable strain analysis on collected oriented-band specimens remains to be done; however, very preliminary indications (early, steep fold axes in quartz bands) are that the

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Figure 1. Index map showing location of the Leap Year Fault in northern Victoria Land. Dots indicate localities studied.

The fault itself was mostly under ice (this was expected, since fault zones normally are more easily eroded than are unfractured rocks at a distance from fault zones). Consequently, the strategy was to observe such secondary features as minor folds, faults, striations on fault planes, and kink bands on exposures as near the fault as possible and to use this information as an indication of the major motions on the main fault. The main fault was seen only in the Mount McCarthy-Mount Burton area on a relatively limited exposure. Results indicate several periods of motion on this important fault. The first, and probably the major event in terms of displacement, involved a relatively plastic response, primarily on the Robertson Bay side of the fault, that produced a strongly deformed rock with spectacular mineral banding or segregation Figure 2. Line drawing of transposed bedding and strongly developed mineral banding or schistosity. Hammer point approximately 20 centimeters.

Figure 3. Equal-angle stereoplot of poles to late stage quartz veins (crosses) and striae orientations on slickensided surfaces (dots).

motion was largely westward directed, implying thrusting or reverse faulting rather than strike-slip displacement. This plastic behavior was not recognized in the quartzites west of the fault except in the Mount McCarthy-Mount Burton area. This may be due to different lithologic response or may reflect original deformation asymmetry. If the motion were primarily thrusting, such asymmetry—with the overriding plate being deformed more than the overridden one would be expected. Also, the extraordinarily wide fault-related deformation zone may be a result of a low-angle fault plane dipping eastward under the Robertson Bay Group rocks. Superimposed on this early deformation are at least three generations of brittle faulting. Sequences of faulting were worked out for individual areas by crosscutting relationships; however, correlations between local sequences have not been resolved. Both compressional features (thrusts and kink bands) and extensional features (quartz veins with growth fibers) were 12

ANTARCTIC JOURNAL

present. Orientations of abundant striations on fault surfaces (slickensides) and other features clearly indicate normal or reverse faulting with little or no strike-slip movement (figure 3). The last of these appears related to intrusion of diabase. Several small dikes and sills of diabase were seen on McKenzie Nunatak and on the ridge 8 kilometers south of the summit of Mount Verhage and are known from other places along the fault (Laird personal communication). Should these date as Farrar equivalents, as seems likely on the basis of lithologic similarity and lack of deformation, the age of these brittle deformations would be restricted to the pre-Jurassic. To place further constraints on the faulting history, oriented samples will be slabbed and thin sections made for fabric analysis, especially to resolve the early plastic deformation style. Fluid inclusions in the brittle deformations will be studied in an attempt to improve correlation of brittle events, and argon-40/ argon-39 ( 40Ar/39 Ar) incremental release spectra will be obtained in an attempt to date both the early faulting and possibly the late kinking events. These preliminary conclusions about the Leap Year Fault kinematics and other regional relations imply that the Robertson Bay Group may have originated well to the east of its present position and arrived by thrusting during the end of the Ross Orogeny, or possibly as late as the late Devonian Admiralty intrusive event. Therefore, in addition to the better known late high-angle faults that produced the large structural blocks in northern Victoria Land, large-scale thrust faulting may play an

Beacon fossils from northern Victoria Land WILLIAM

R.

HAMMER

Department of Geology Augustana College Rock island, Illinois 61201 JOHN

M.

ZAWISKIE

Department of Geology and Mineralogy Ohio State University Columbus, Ohio 43210

An extensive search for fossils in the Beacon Supergroup of northern Victoria Land during the 1981-82 field season yielded no vertebrates; however, numerous plant and trace fossil localities were discovered. Our season ran from 30 November 1981 to 15 January 1982, and all our field studies were in conjunction with J. W. Collinson's group, which included Collinson and Barry Roberts of Ohio State University and N. R. Kemp of the Tasmanian Museum and Art Gallery (Hobart, Australia). The Beacon explored in the Freyberg Mountains included exposures at Smiths Bench (72°10'S 163°8'E), Mount Baldwin 1982 REVIEW

important part in the tectonic history, especially during the Paleozoic. References Bradshaw, J. D., Laird, M. G., and Wodzicki, A., 1982. Structural style and tectonic history in northern Victoria Land. In C. Craddock (Ed.), Antarctic geoscience. Madison: University of Wisconsin Press. Craddock, C. 1972. Antarctic tectonics. In R. J . Adie (Ed.), Antarctic geology and geophysics. Oslo: Universitetesforlaget. Crowder, D. F. 1968. Geology of a part of north Victoria Land, Antarctica. In U.S. Geological Survey Professional Paper 600-D. Washington, D.C.: U.S. Government Printing Office. Elliot, D. H. 1975. Tectonics of Antarctica: A review. American Journal of Science, 275A, 45-106. Gair, H. S., Sturm, A., Carryer, S. J . , and Grindley, G. W. 1969. The geology of northern Victoria Land (Folio 12, Plate 12). In V. C. Bushnell (Ed.), Geologic maps of Antarctica 1:100,000, Antarctic maps folio series. New York: American Geographical Society. Kreuzer, H., Hohndorf, A., Lenz, H., Vetter, U., Tessensohn, F., Muller, P., Jordan, H., Harre, W., and Besang, C. 1981. K/Ar and Rb/Sr dating of igneous rocks from north Victoria Land, Antarctica. Geologisches Jahrbuch, B41, 267-273. Laird, M. G. Personal communication, 1982. Tessensohn, F., Duphorn, K., Jordan, H., Kleinschmidt, G., Skinner, D., Vetter, U., Wright, T. 0., and Wyborn, D. 1981. Geological comparison of basement units in north Victoria Land, Antarctica. Geologisches Jahrbuch, B41, 31-88.

(72°15'S 163°15'E), Monte Cassino (72°21'S 163°40'E), a series of unnamed ridges along the western side of Moawhango Névé (72°17'S 163°30'E), the eastern edge of Mount Massell (72°30'S 163°28'E), and an unnamed peak at the northernmost end of the Alamein Range (72°2'S 163°16'E). Localities south of Neall Massif in the Salamander Range (72°9'S 164°30'E), at Boggs Valley (71°56'S 161°30'E) in the Helliwell Hills and at DeGoes Cliff (71°45'S 162°00'E), and at the head of the Jupiter Amphitheatre (71°35'S 161°53'E) in the Morozumi Range also were searched. Farther south, Roberts Butte (72°39'S 160°07'E), the Lichen Hills (73°20'S 162°07'E), the Vantage Hills (73°32'S 162°28'E), and the western flank of Gair Mesa (73'31'S 162°45'E) received attention. These areas contain nearly all of the mapped Beacon north of 73°37'S in the Transantarctic Mountains (Gair et al. 1969). Although none of-the northern Victoria Land Beacon sections mentioned contained vertebrates, nearly all of them contained fossils of some type, usually unidentifiable, poorly preserved wood impressions in sandstone. Boggs Valley in the Helliwell Hills and DeGoes Cliff in the Morozumi Range yielded wellpreserved, carbonized leaf and stem impressions. So far identified are both Glossopteris sp. and Gangamopteris sp. (figure 1). As yet unidentified reproductive structures also were discovered, including a seed from ?Glossopteris sp. and a fertiliger capitula of ?Glossopteris stricta. Specimens of Vertebraria, which apparently represent glossopterid-gangamopterid roots (Schopf 1976), occur in association with the leaves. Exposures at Monte Cassino produced scrappy remains of Glossopteris sp. At all three localities, the fossils were obtained from black shales. Inter13