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Pilo-Pleistocene Uplift of the Mcmurdo Dry Valley Sector of The Lambrecht, L.L., W.S. Lacey, and C.S. Smith. 1973. Observations on cations from Queensland. (Geological Survey Queensland.) Palaeon- the Permian flora of the Law Glacier area, central Transantarctic tology, Paper 41, Publication 367, 1-21. Mountains. Bulletin of the Society of Belgian Geologists, Palaeontologists, Schopf, J.M. 1976. Morphologic interpretation of fertile structures in and Hydrologists, 81, 161-167. glossopterid gymnosperms. Review of Palaeobotany and Palynology, Rigby, J.F. 1963. On a collection of plants of Permian age from Baralaba, 21, 25-64. Queensland. Proceedings of the Linnean Society of New South Wales, 87, Taylor, E.L. 1987. Glossopteris reproductive organs: An analysis of 341-351. structure and morphology. 14th International Botany Congress (Ab- Rigby, J.F. 1978. Permian glossopterid and other cycadopsid fructifi- stracts), West Berlin. Pilo-Pleistocene uplift perched on the walls of middle Taylor Valley between Nuss- baum Riegel and Borns Glacier (figure). Taylor Valley is gla- of the McMurdo Dry Valley sector cially carved and opens to McMurdo Sound at its eastern end. of the Transantarctic Mountains Therefore, the lowest elevation of each in situ volcanic outcrop erupted subaerially represents the maximum amount of uplift at that location since the time of the eruption. T.I. WILCH Fieldwork in Taylor Valley was carried out during the 1987- 1988 and 1988-1989 austral summers as part of an extensive Department of Geological Sciences surficial geology mapping program. Two primary objectives and were to establish whether the volcanic rocks were erupted Institute for Quaternary Studies subaerially and to determine whether they were in situ. Eigh- University of Maine teen geographically and/or mineralogically distinct alkalic bas- Orono, Maine 04469-0110 alt localites all contained in situ outcrops and all were erupted subaerially. D.R. Lux Over 80 samples were collected for argon-40/argon-39 iso- topic age determinations. Elevations of sample localities and Department of Geological Sciences, lowest in situ outcrops were surveyed using a T-2 Theodolite University of Maine and Electronic Distance Measurer. Despite careful selection, Orono, Maine 04469-0110 petrographic observations in many of the dating samples re- vealed the presence of small xenocrysts. These xenocrysts likely W.C. MCINTOSH originated in the Paleozoic country rocks and therefore prob- ably contain an older radiogenic argon-40 component than is New Mexico Institute for Mining and Technology contained in the basaltic groundmass. For this reason, mag- Socorro, New Mexico 87801 netic and heavy-liquid separation techniques were used during sample preparation to remove the xenocrystic contaminants G.H. DENTON from the groundmass. McDougall and Harrison (1988) provide a review of the ar- gon-40/argon-39 dating method. Specific analytical techniques Department of Geological Sciences and used at the University of Maine follow those described by Lux Institute for Quaternary Studies (1986). Samples were irradiated in the H5 facility of the Ford University of Maine Nuclear Reactor at the University of Michigan. Neutron flux Orono, Maine 04469-0110 gradients within the reactor were monitored by Fish Canyon Tuff (FCT-3, 27.68 million years old) and University of Maine standard IEH, (180.9 million years old relative to standard MMhb-1; Alexander, Michelson, and Lanphere 1978). Argon Most evidence for past fluctuations of the antarctic ice sheet isotopic compositions were determined by Nuclide 6-60-SGA comes from glacial erosional and depositional features in the 1.25 mass spectrometer for between 4 to 12 heating increments Transantarctic Mountains. Many such features occur at high for each sample. Ages based on these ratios were calculated elevations and date to Pliocene time. Because of the antiquity using the decay constants recommended by Steiger and Jaeger and elevation of these features, uplift history of the Transant- (1977). arctic Mountains is critical for accurate ice-sheet reconstruc- One advantage of the argon-40/argon-39 method over the tions. conventional potassium/argon method is that the gas is re- Plio-Pleistocene uplift of the McMurdo Dry Valley region of leased in steps, thus allowing isolation and recognition of in- the Transantarctic Mountains can be constrained by determin- herited argon-40. An age is determined for the gas released in ing the elevation of subaerial basaltic cinder-cone deposits each heating increment. A weighted average age of the sep- 30 ANTARCTIC JOURNAL Lower Marr Site - - V - V AW - V Upper Marr $0 fT:;S;iUs Site-o00 ! (west) V East Rhone p- balkin Site A The subaerial basaltic cinder-cone deposits perched on the walls of middle Taylor Valley. arate increments is a total gas age; an average of selected ad- ages. The maximum uplift rate for each unit, also listed in the jacent increments with concordant ages is a plateau age. Samples table, is determined by dividing the lowest elevation of in situ yielding discordant age spectra are difficult to interpret, al- volcanics by the age. The lowest maximum uplift rate is 137 though Lo Bello (1987) reported that samples with excess argon meters per million years since 3 million years ago at the Lower components are characterized by interpretable saddle-shaped Marr site (west side). spectra. In this situation, the plateau age is derived from steps Two other models of Transantarctic Mountain uplift have at the base of the saddle. Such a plateau age should be con- recently been proposed. One model (Gleadow and Fitzgerald sidered a maximum value for the entire sample. 1987) used fission-track dating of basement apatites in Wright The table lists elevation and preliminary argon-40/argon-39 Valley to approximate the timing and amount of uplift. They plateau age data from 11 of the 18 volcanic units in middle concluded that asymmetric uplift to a maximum elevation of Taylor Valley. Most of these ages agree with previously pub- 4.8-5.3 kilometers commenced 50 million years ago. An av- lished potassium/argon ages (Armstrong 1978). Several release erage uplift rate since 50 million years ago was calculated to spectra were saddle-shaped, and the base of the saddle was be 100 + 5 meters per million years. After analyzing the ther- interpreted as an approximate age of the volcanic event. Con- mobarometry of two-phase granulite inclusions in Cenozoic sistency of ages within each volcanic unit adds credence to the volcanics in the McMurdo Sound area, Berg and Herz (1986) 31 1989 REVIEW Elevation and preliminary argon-40largon-39 whole rock ages and uplift rates from volcanic outcrops in Taylor Valley, Antarctica Plateau age Elevation (in meters) Maximum uplift rate Locality Sample number (in millions of years) (lowest in situ outcrop) (in meters per million years) East Rhone Site 86K-20 2.90 ± .10 671.0 ± 5.0 231.4 ± 9.7 86A-26 2.79 ± .24 671.0 ± 5.0 240.5 ± 30.2 86A-31 1.81 ± .12 671.0 ± 5.0 370.7 ± 27.3 TWV87-20 1.69 ± .27 671.0 ± 5.0 397.0 ± 66.4 West Matterhorn Sites TWV87-37 3.03 ± .21 1,054.0 ± 0.5 347.9 ± 24.3 TWV87-44 3.04 ± .30 1,054.0 ± 0.5 346.7 ± 34.4 TWV87-42 3.53 ± .18 602.4 ± 0.5 170.7 ± 8.8 TWV87-48 3.12 ± .15 602.4 ± 0.5 193.1 ± 9.4 86K-5 3.72 ± .17 821.8 ± 0.5 220.9 ± 10.2 86K-6 3.71 ± .13 821.8 ± 0.5 221.5 ± 10.3 86K-2 3.72 ± .17 821.8 ± 0.5 220.9 ± 10.2 86K-3 2.96 ± .16 821.8 ± 0.5 277.6 ± 15.2 86K-21 3.71 ± .15 821.8 ± 0.5 221.6 ± 9.1 TWV87-1 7 3.42 ± .31 821.8 ± 0.5 240.3 ± 21.9 Calkin Site 84A-1 1.58 ± .17 425.0 ± 5.0 269.0 ± 32.1 86A-35 1.29 ± .35 425.0 ± 5.0 329.5 ± 93.3 Sollas Sites (lower) 84A-9 2.01 ± .03 323.5 ± 0.5 160.9 ± 2.7 84A-1 1 2.19 ± .12 323.5 ± 0.5 147.7 ± 8.3 TWV87-1 2N 2.20 ± .08 323.5 ± 0.5 147.0 ± 5.6 TWV87-1 4 2.40 ± .30 323.5 ± 0.5 134.8 ± 17.06 (west) TWV87-65 1.74 ± .20 416.3 ± 0.5 239.3 ± 68.9 (east) TWV87-1 0 3.63 ± .30 644.5 ± 5.0 177.5 ± 27.8 Upper Marr Site TWV87-04 2.79 ± .08 640.5 ± 5.0 229.6 ± 6.8 Lower Mark Sites (west) TWV87-62H 3.09 ± .14 424.1 ± 0.5 137.3 ± 6.4 (east) TWV87-1 8 2.69 ± .24 432.3 ± 0.5 160.7 ± 14.52 TWV87-1 02 2.52 ± .23 432.3 ± 0.5 171.5 ± 15.86 The ages are expressed in millions of years. The errors associated with the plateau ages are two standard deviation units. The errors associated with the elevation data are estimates given by surveyors. The errors for the maximum uplift rates are propagated using the following formula: [Rateerro, = Elevationerror] ± [Age ± elevation (Age error) ± Age 2]. suggested that the fission-track uplift rate should be halved, needs to be reconciled because of its importance to Pliocene because only 2,000 meters of uplift has occurred in the McMurdo ice-sheet reconstructions. Either Plio-Pleistocene uplift rates Dry Valleys. A problem with using these data for ice sheet are vastly different in the McMurdo Dry Valleys and near reconstructions is that the uplift rates are only averages and Beardmore Glacier, or one of the methods for calculating uplift may have varied since 50 million years ago. In other words, rates is flawed.
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