Field Collection for Apatite Fission Track Analysis, Ellsworth Mountains

Hammer, W.R. 1990. Triassic terrestrial vertebrate faunas of Antarc- New vertebrates from the Fremouw formation (Triassic) Beard- tica. In T.N. Taylor and E.L. Taylor (Eds.), Antarctic paleobiology. more Glacier region, Antarctica. Antarctic Journal of the U.S., 21(5), New York: Springer-Verlag. 24-26. Hammer, W.R., J.W. Collinson, and W.J. Ryan, III. 1990. A new Trias- Seeley, H.G. 1894. Researches on the structure, organization and clas- sic vertebrate fauna from Antarctica and its depositional setting. sification of the fossil Reptilia. Part IX, section 1: On the Therosuchia. Antarctic Science, 2(2), 163-167. Philosophical Transactions of the Royal Society of London, series B, 185, Hammer, W.R., W.J. Ryan, J.W. Tamplin, and S.L. DeFauw. 1986. 987-1,018. Field collection for apatite fission track 85°W analysis, Ellsworth Mountains EDMUND STUMP and PAUL C. FITZGERALD Department of Geology Arizona State University Tempe, Arizona 85287-1404 We collected rock samples for fission track analysis in the Ellsworth Mountains during the 1989-1990 austral summer to provide an understanding of the tectonic history of this crustal block. In broader terms, the study will attempt to relate the Figure 2 uplift history of the Ellsworth Mountains to the tectonic evo- lution of West Antarctica. Because the put-in date was delayed, we had to abbreviate the original field plan, which included collections in the Her- ,-Nunatak .690± itage Range (figure 1). We were put in on 21 November 1989 Bastien Hills 5 kilometers west of Mount Gardner in the central Sentinel 79°S Minisa (aCU " Range (figure 2). Bowers Corner On this expedition, all samples were collected from quartz- ites of the Crashsite Group, mainly from the Howard Nunataks Formation (Craddock et al. 1986). Sample size was typically about 5 kilograms. Over the span of 4 days, we collected a "vertical" profile of 26 samples from the Vinson Massif covering 2,500 meters of elp relief from the summit (4,897 meters) to the head of Nimitz c3 Glacier (1,400 meters). On the first day, we covered the lower ridge line from the terrace to the west of Vinson Massif down to Nimitz Glacier, with access by snowmobile from the west side of the glacier (figure 2). On the second day, we covered the western ridge line of the Vinson Massif from about the 2,900-meter level down to the terrace at the foot of the massif. Access was by snowmobile down a drift on the north of the 100 km glacier draining the face of Vinson Massif. On the third day, 1 we drove by snowmobile to the head of this glacier and climbed to the 3,800-meter level of the summit plateau, placing a tent and collecting down from 4,000 meters to the snowmobiles. On the fourth day (30 November), we drove to the head of Figure 1. Location map, Ellsworth Mountains. (km denotes kilo- the glacier, climbed to the tent, where we rested and cooked meter.) a meal, and then continued to the summit. From there, we sampled down to the previously collected level. The summit A 400-meter profile (5 samples) was collected at Bowers Corner day took 18 hours, basecamp to basecamp. extending the Vinson profile to the lowest elevation in the Subsequently, the party traversed down Nimitz Glacier to Sentinel Range (690 meters). From there, collections were made Bowers Corner (figure 1) collecting on both sides of the glacier. on the northern side of Minnesota Glacier from Nunatak 690 38 ANTARCTIC JOURNAL on the eastern side of the range, to the southern end of the Bastien Hills on the western side (figure 1). On the return traverse to basecamp, we made collections from six localities in the Bastien Hills. Collections were also made from several localities on nunataks and spurs in the vicinity of the basecamp. Our collections from the 1989-1990 austral summer provide a composite vertical profile of the Sentinel Range totalling ap- proximately 3,000 meters, a transverse profile of the range, and a suite of samples surrounding Nimitz Glacier. These samples offer the potential of determining the uplift history of the range, including time of initiation of uplift, average uplift rate, and whether multiple episodes of uplift occurred. Relative dis- placement across the range, and whether a major fault occurs down Nimitz Glacier also may be determined. Returned samples are being prepared for counting and track length mea- surements in the Fission Track Dating Laboratory at Arizona State University. This work was supported by National Science Foundation grant DPP 88-0981. Mountaineers Rob Hall and Mugs Stump assisted throughout the field season. Reference Figure 2. Location map, central Sentinel Range. Snowmobile routes Craddock, C., G. F. Webers, R. Rutford, B. Sporli and J .J . Anderson. are indicated by dashed lines. Collecting routes are indicated by 1986. Geologic map of the Ellsworth Mountains, Antarctica. (Map and dotted lines. (km denotes kilometer.) Chart Series MC-57). Boulder: Geological Society of America. 1990 REVIEW 39.

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