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Victoria University Antarctic Research Expedition Science and Logistics Reports 1985-86: VUWAE 30

Introduction and Background

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.

Previous work in Victoria Land (Gleadow and Fitzgerald, 1984; Fitzgerald and Gleadow, 1984) have shown a two-stage uplift history for the Transantarctic Mountains. Prior to about 50 Ma uplift could be interpreted as a steady 15 m/Ma but at 50 Ma this changed dramatically to about 100 m/Ma. This period of "slow uplift" prior to 50 Ma has now been reinterpreted as an uplifted 'partial annealing zone' (Fitzgerald and Gleadow, 1985), the "slope" of 15 m/Ma indicating not an "uplift and erosion" rate but representing an artifact of the thermal profile prior to uplift. Preliminary results for samples collected off the eastern end of the Kukri Hills and basement cored in the CIROS 2 drillhole (Barrett, 1985) in New Harbour are presented here (Fig. 3). The 'break in slope' which is clearly recognizable at about 50 Ma represents the base of the uplifted 'partial annealing zone' (Fitzgerald and Gleadow 1985, Gleadow and Fitzgerald in prep.). This marks the start of uplift of the Transantarctic Mountains. Apatite fission track ages from wells in the Otway Basin in southeastern Australia show the base of the 'partial annealing zone' lies at a temperature of 125°c (Gleadow and Duddy, 1981). Prior to uplift this probably lay close to a depth of 4 km below sea-level, calculated from an estimated paleo-landsurface elevation of 150 m above sea-level, a paleo-geothermal gradient of 30°C/km and a mean annual surface temperature of about 0°c at that time. The geothermal gradient of 30°C/km is taken from the DVDP-6 drillhole at Lake Vida (Decker and Bucher, 1982), which although some 40 km inland from the Transantarctic Mountain Front is thought to be representative of the situation at the start of uplift when a normal continental gradient probably existed, at least for the shallow depths being discussed here. The mean annual temperature of 0°C is an estimate and takes into account that the East Antarctic Ice Cap did not exist at that time (Hayes et.al., 1975). The 'break in slope' of the graph now lies at an elevation page 14
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.

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.

of about 900 m which means that in the 50 Ma, 4.9 km of uplift has occurred at an average rate of approximately 100 m/Ma.

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).