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*Present address: Antarctica New Zealand, Christchurch, New Zealand.
Since the earliest geological work in the Transantarctic Mountains (e.g., Gould 1935), it has been suggested that the Transantarctic Mountains are divided into a number of fault blocks separated by transverse structural features. Major outlet glaciers of the east antarctic ice sheet often occupy these structural features, which may represent transfer faults or accommodation zones.
The objective of this project is to study the thermal history of the Transantarctic Mountains in the Shackleton-Liv Glacier region using the fission track and argon-40/argon-39 (40Ar/39Ar) techniques. The thermal history is used to constrain the timing, amount, and rate of rock uplift and denudation. Results will be used to map the variation of these parameters across the range, between the Shackleton and Liv Glaciers, across these major outlet glaciers, and then in conjunction with other studies, map the variation along the length of the Transantarctic Mountains.
The resulting data, in conjunction with field observations, will also be used to determine the structure of the Transantarctic Mountains Front and to test the hypothesis that the mountains are segmented and that different segments have different tectonomorphological histories. It is important for constraining rift flank uplift models that not only are different denudation events in the Transantarctic Mountains identified but also that patterns of denudation across the mountains are delineated. Although the dominant uplift and denudation event that has shaped the present form of the Transantarctic Mountains began in the early Cenozoic (e.g., Gleadow and Fitzgerald 1987; Fitzgerald and Gleadow 1988; Fitzgerald 1992), the record of Cretaceous denudation events is more subtle and variable. Episodes of denudation have been recorded in the early, mid, and late Cretaceous (e.g., Stump and Fitzgerald 1992; Fitzgerald 1994, 1996, p. 35). A better understanding of Cretaceous events within the Transantarctic Mountains is critical given that most of the extension within the Ross embayment is Cretaceous in age (e.g., Lawver and Gahagan 1994) and most denudation in the Ellsworth-Whitmore Mountains crustal block is Cretaceous (Fitzgerald and Stump 1992, pp. 331-340). The little evidence we have on mid-Cretaceous denudation patterns within the Transantarctic Mountains (Fitzgerald 1994, 1995, p. 133) indicates greater denudation along the inland flank of the Transantarctic Mountains, suggesting that the subglacial basins inland of the Transantarctic Mountains are Cretaceous in age (Fitzgerald 1996, p. 35). Our sampling strategy was designed to address these questions.
Six weeks were spent in the area between the Shackleton and Liv Glaciers (figure 1), operating first out of tent camps near Cape Surprise and Mount Daniel, and then using helicopter close support from the Shackleton Glacier deep field camp (85°05.62'S 175°22.93'W). Cape Surprise is an important locality because of the unique presence of Beacon Supergroup sedimentary strata downfaulted to coastal levels (Barrett 1965). Our work at Cape Surprise (see Miller et al., Antarctic Journal, in this issue) suggests that downfaulting along the Transantarctic Mountains Front is accommodated via multiple faults rather than a single fault and that a significant component of dextral strike-slip movement as well as dip-slip movement occurred along the faults. Except for Cape Surprise, where Beacon strata and Ferrar dolerite have been downfaulted, fault location and offset, as well as the overall structure of the range, are difficult to document within the predominantly granitic basement. A series of "vertical" sampling profiles across the range (figure 2) were collected to reveal information on the denudation history, as well as the structure. Vertical profiles were collected from Mount Munson [84°48'S 174°23.3'W; 1,500 meters (m) vertical profile], Mount Olds (84°40.2'S 174°40.9'W, 900 m profile), Pyramid Peak (informal name, 84°34.3'S 174°58.6'W, 500 m profile), spot-height 700 (84°31'S 174°55'W, 500 m profile), and spot-height 950 (84°33'S 4°15'W, 550 m profile), and individual samples were collected in between these profiles at strategic locations as well as from granite near Cape Surprise (see figure in Miller et al., Antarctic Journal, in this issue). Samples were also collected in a transect from the Sage Nunataks inland along the Olliver Peak ridge system.
On the west side of the Shackleton Glacier, we collected vertical profiles at Mount Speed (84°29.8'S 176°37'W, 825 m vertical profile), Mount Wasko (84°34.1'S 176°56.3'W, 800 m vertical profile), Mount Franke (84°37'S 177°W, 1,250 m profile), and Mount Butters (84°53.5'S 177°W, 550 m profile). We anticipate this series of profiles along the Shackleton Glacier will reveal the most information related to variations in Cenozoic versus Cretaceous patterns of denudation. The Kukri Peneplain at Mount Butters, on the west side of the Shackleton Glacier, dips inland (i.e., south) at approximately 17° compared to the dip of the peneplain at Mount Munson and Mount Wade where it is approximately 2-3° to the south. These contrasting dips indicate that a transfer fault (or accommodation zone) transects the range along the Shackleton Glacier. The overall structure of the range appears to be a series of large blocks tilted south (dipping variably) both west and east of the Shackleton Glacier (figure 1).
On the west side of the Liv Glacier, we collected limited vertical profiles from Mount Daniel (84°54'S 170°W, 1,150 m vertical profile) and Mount Koob (84°48'S 174°20'W, 300 m profile) and also samples on a north-south transect along the Mount Dryfoose spur. In this area, the Kukri Peneplain can be traced from the dramatic frontal escarpment of the Prince Olav Mountains to the summit of Mount Daniel, almost halfway across the Transantarctic Mountains Front. This location for the peneplain suggests that the frontal escarpment of the mountains in this area has formed by scarp retreat rather than faulting, implying that the mountains and the frontal scarp are relatively old features, rather than having formed by relatively young tectonic uplift and faulting. Furthermore, it indicates that the Transantarctic Mountains Front occupies only the outer half of the area between the frontal escarpment and the coast.
On the east side of the Liv Glacier, we collected an approximately 800-m vertical profile by helicopter between Mount Schevill (85°5'S 167°18'W) and Mount Blood (85°1'S 167°28'W) and also a north-south transect between Duncan Mountains and the Kukri erosion surface under spot-height 2070 (85°12'S 167°6.5'W). The uniform attitude of the Kukri Peneplain on either side of the Liv Glacier does not indicate the presence of a structural feature there, although the presence of such a large glacier outlet does suggest a transverse structural feature may exist.
We thank the National Science Foundation, Antarctic Support Associates, the Antarctic Development Squadron 6 (VXE-6), the Air National Guard, Ken Borek Air, Helicopters New Zealand, and the staff at the Shackleton Glacier camp for support during the season. This research was supported by National Science Foundation grant OPP 93-16720.
References
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