FISSION TRACK STUDIES (K044)
Sampling of granitoid rocks to determine the Late Mesozoic and Cenozoic uplift history of the Transantarctic Mountains was extended to the Beardmore Glacier region this season. Apatite fission track dating at the University of Melbourne will be used to determine uplift rates across the mountain range and position of faults at the mountain front. Samples were collected from both the north and south side of the lower Beardmore Glacier. Topographic features attributed to faulting here are an eastwards decrease in summit heights and the presence of escarpments, especially visible on the northern side of the Beardmore Glacier. Other areas sampled were the Queen Elizabeth and Miller Ranges on the inland side of the Transantarctic Mountains.
Introduction and Background
This was the last year of a Ph.D. study to determine the uplift history of the Transantarctic Mountains using fission track dating. The study of mountain uplift using this technique requires sampling at regular intervals over significant elevation ranges in order to gain information representing the greatest possible time period. Previous seasons have concentrated in southern Victoria Land but all field work this season was conducted out of the U.S. deep-field camp in the Beardmore Glacier area (84° 00′ 13.897″S, 164° 24′ 42.226″E). Two areas were visited from here; the coastal region around the mouth of the Beardmore Glacier and further inland In the Miller and Queen Elizabeth Ranges. The main field objective was the collection of samples to determine uplift rates and measure vertical displacement across faults. Sampling for fission track studies is limited to those rocks which contain uranium-enriched minerals. This study is looking mainly at apatite which is common in granitic rocks which in the Beardmore Glacier area outcrop mainly along the coast but make up a large proportion of the Miller Range and also occur as isolated plutons elsewhere.
Figure 3. Model for the uplift history of the Transantarctic Mountains based on a vertical sampling profile from the eastern end of the Kukri Hills, southern Victoria Land, and including a sample of basement gneiss from the CIROS 2 drillhole. The 'break in slope' in the graph at about 50 Ma marks the start of uplift of the mountains, giving an average uplift rate since that time of approximately 100 m/Ma. Errors plotted for the apatite ages are two standard deviations, for the elevations ±10 m.
DVDP 8 and 10, two holes drilled in New Harbour near the Transantarctic Mountain Front have geothermal gradients of 60°C/km (Decker and Bucher, 1982). This higher gradient is considered to be a result of the higher heat flow present in the Dry Valleys. - Ross Island area due to the present day extensional regime that is manifested by the presence of the McMurdo Volcanics.
It is also worth noting that the position of the vertical sample profile used to determine an uplift rate on the mountain front in relation to the point of maximum uplift is important. Profiles lying to the east of the point of maximum uplift in southern Victoria Land have been down faulted relative to it. Hence any uplift rate calculated from these will be a slight underestimate but still well within any error limits, given the assumptions made to calculate the uplift rate. Errors for ages less than 50 Ma are large compared to the change in elevation (Fig. 3) between successive page 15 samples. Errors in ages elder than 50 Ma are small compared to a change in elevation, a difference of a few hundred metres producing a significant age difference. This is an important point when generating artificial reference planes using apatite fission track ages as it is necessary to take samples from higher elevations to guarantee an age of over 50 Ma. In southern Victoria Land, near horizontal sills of dolerite within the basement can be used as reference surfaces to determine the amount of displacement across faults at the Transantarctic Mountain Front. The dolerite sills in the Beardmore Glacier area do not outcrop at the coast, hence the need for horizontal sampling traverses to confirm the position of faults suspected from analogy with other parts of the Transantarctic Mountains and from topographic evidence. To the south of the Beardmore Glacier at Cape Surprise a fault with 5 km of displacement downthrown to the east has been recorded (Barrett, 1965).
The geology of the field area consists of PreCambrian and Lower Palaeozoic folded metasedimentary rocks. The oldest unit, the Nimrod Group, is at least 1000 Ma old and is made up of schist, gneiss, marble and dolomite. This is overlain unconformably by the mainly terrigenous sediments of the PreCambrian Beardmore Group. These in turn are overlain unconformably by Cambrian carbonates of the Byrd Group. Numerous granitic plutons of the Granite Harbour Intrusives have intruded these three group (Gunner, in press). These basement rocks are unconformably overlain by the Devonian-Triassic Beacon Supergroup which is composed of glacial, alluvial and Shallow marine strata. These are intruded by numerous sills of the Jurassic Ferrar Dolerite which in places passes upwards into the Kirkpatrick Basalt. Sample localities are shown in Fig. 4.
Fieldwork in the Lower Beardmore Glacier Area
A vertical sampling profile of 1150 m was taken in the Mt Ida-Granite Pillars area lying just to the north of Beardmore Glacier. A horizontal sampling traverse collected by helicopter was taken from Mt Hope westwards to The Cloudmaker. In this area the only indication of faulting across the Transantarctic Mountain Front is given by topography: the decrease in height of the mountains in the form of a number of generalised benches and the presence of scarps, e.g. the large scarp formed by the east face of Mt Elizabeth and possibly the west wall of The Gateway. Samples were taken on each suspected fault block in order to determine relative displacements across the mountain front.
On the southern side of the Beardmore Glacier, a partially completed vertical profile of 900 m was taken off Cleft Peak. Positions of suspected NE striking faults axe not as obvious here as on the northern side of the glacier, the relief is generally more rugged and granitic rocks are not exposed as far to the east. Nevertheless, a horizontal sampling traverse from Mt Robert Scott west to Mt Patrick was taken.
Fieldwork in the Miller and Queen Elizabeth Ranges
The Miller Range lies on the inland side of the Transantarctic Mountains and is an uplifted block of granitic and metamorphic rocks. Grindley (1967) worked on the geomorphology of the Miller Range and observed three high level glacial erosion surfaces. He calculated that these represented an uplift of about 1600 m since the onset of Antarctic glaciation, assumed at that time to be the early Quaternary. However the Cenozoic glaciation of the Antarctic continent is now thought to have begun 25 Ma ago (Hayes et al., 1975) which gives an average uplift rate of approximately 64 m/Ma, This allows us the opportunity of testing the fission track method against direct geological observations and so a vertical sampling profile of 800 m was taken off MacDonald Bluff, from spot height .2440 to the level of the Marsh Glacier. Moody page 16 Nunatak, lying across the Harsh Glacier was sampled as were various localities in the Queen Elizabeth Range to determine displacement across faults lying between here and the Miller Range (Grindley and Laird, 1969).
Publication and Future Research
Samples collected in the Beardmore Glacier region this season will be processed and dated at the fission track laboratory at Melbourne university. This completes for the time being the fission track study of the Transantarctic Mountains that was begun in 1981/82 by Gleadow working in Victoria Land (Gleadow, in Barrett, 1982). It is hoped that results of this study will be presented at the Fifth International Symposium on Antarctic Earth Sciences to be held in Cambridge, 1987.
We would like to thank John Splettstoesser, the NSF representative at Beardmore Camp as well as pilots and crews of VXE-6 based there, for helping to make this a successful and enjoyable season. Our thanks also to Peter Cresswell, OIC of Scott Baser end his staff.
Barrett, P.J. 1965. Geology of the area between the Axel Heiberg and Shackleton Glaciers, Queen Maud Range, Antarctica. Part 2 - Beacon Group. NZ Journal of Geology and Geophysics, 8(2), 344-370.
Barrett, P.J. 1981. Immediate report of Victoria university of Wellington Antarctic Expedition 26.
Barrett, P.J. 1985. Plio-Pleistocene glacial sequence cored at CIROS 2, Ferrar Fjord, Western McMurdo Sound. NZ Antarctic Record, 6(2), 8-19.
Decker, E.R. and Bucher, G.J. 1982. Geothermal studies in the Ross Island-Dry Valley region. In: Craddock, C.C. (ed.). Antarctic Geoscience, 887-901; Univ. Wisconsin Press, Madison.
Fitzgerald, P.G. and Gleadow, A.J.W. 1984. Uplift history of the Transantarctic Mountains, Victoria Land, Antarctic (Abstract). Workshop on fission-track analysis: principles and applications 4-6 September, James Cook University, Townsville, Australia.
Fitzgerald, P.G. and Gleadow, A.J.W. 1985. Uplift history of the Transantarctic Mountains, Victoria Land, Antarctica (Abstract). Sixth International Gondwana Symposium held at Ohio State University, U.S.A., 19-23 August, 1985.
Gleadow, A.J.W. and Duddy, I.R. 1981. A natural long-term track annealing experiment for apatite. Nucl. Tracks, 5(1-2), 169-174.
Gleadow, A.J.W., McKelvey, B.C. and Ferguson, M.U. 1984. Uplift history of the Transantarctic Mountains in the Dry Valleys area, southern Victoria Land, from apatite fission track ages. NZ Journal of Geology and Geophysics, 28(3), 457-464.
Gleadow, A.J.W. and Fitzgerald, P.G. 1984. Uplift history of the Transantarctic Mountains, Victoria Land, Antarctic (Abstract). Fourth International Fission Track Dating Workshop, July 31-August 3, Troy, New York.
Gleadow, A.J.W. and Fitzgerald, P.G. (in prep.). Uplift history and structure of the Transantarctic Mountains: Dew evidence from fission track dating of basement apatites in the Dry Valleys area, southern Victoria Land.
Grindley, G.W. 1967. The geomorphology of the Miller Range, Transantarctic Mountains with notes on the glacial history and neotectonics of East Antarctica. NZ Journal of Geology and Geophysics, 10(2), 557-598.
Grindley, G.W. and Laird, M.G. 1969. Geology of the Shackleton Coast. Antarctica Map Folio Series, Sheet 15, Folio 12, American Geophysical Society.
Gunner, J.D. (in press). Basement geology of the Beardmore Glacier region. In: Turner, M.D. and Splettstoesser, J.F. (eds). Geology of the Central Transantarctic Mountains. Antarctic Research Series, Vol.36. American Geophysical Onion, 1-9.