Pilo-Pleistocene Uplift of the Mcmurdo Dry Valley Sector of The

Home , McMurdo Dry Valleys

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 isotopic 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 argon-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)

East Rhone

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. uplift could have occurred in pulses rather than continuously G. Falloon and P. Sole of the New Zealand Department of during the past 50 million years. Because the averages fall Surveying and Land Information provided generous surveying beneath the constraints imposed by the lowest maximum uplift support. R. Garster assisted with surveying. We thank N. Dun- rates in Taylor Valley, however, we conclude that the averages bar for assistance with mapping. D.P. West, Jr., assisted with cannot be discounted for this latest time interval. argon-40/argon-39 geochronologic analyses and interpreta- A second model holds that the glacigene Sirius Formation tions. and Dominion Erosion Surface located in the Beardmore Gla- cier area of the Transantarctic Mountains originated near sea level and have since undergone uplift of 1,000-3,000 meters (Webb et al. 1986). Biostratigraphic ages of less than 3 million References years for the Sirius Formation are inferred from marine diatom assemblages (Harwood 1983, 1986; Webb et al. 1983, 1984, Alexander, E.C., Jr., G.M. Michelson, and M.A. Lanphere. 1978. MMhb- 1986). These data yield average uplift rates from 333 to 1,000 1: A new 40Ar/39Ar dating standard. In R.E. Zartman (Ed.), Fourth meters per million years since 3 million years in the Beardmore International Conference on Geochronology, Cosinochronology, and Osotope Glacier region. Geology. U.S. Geological Survey, Open-File Report 78-701, 6-8. The discrepancy between the high diatom-based uplift rates Armstrong, R.L. 1978. K-Ar Dating: Late Cenozoic McMurdo Volcanic Group and dry valley glacial history, Victoria Land, Antarctica. New of the Transantarctic Mountains in the Beardmore Glacier re- Zealand Journal of Geology and Geophysics, 21, 685-689. gion and the lower uplift rates of the Transantarctic Mountains Berg, J.H., and D.L. Herz. 1986. Thermobarometry of two-pyroxenc- in the McMurdo Dry Valleys determined from two indepen - granulite inclusions in Cenozoic volcanic rocks of the McMurdo dent methods is at least an order of magnitude. This difference Sound region. Antarctic Journal of the U.S., 21(5), l) 20.

32 AN I \I Gleadow, A.J.W., and P.C. Fitzgerald. 1987. Uplift history and struc- mochronology by the 40Ar/39Ar Method. New York: Oxford Uni- ture of the Transantarctic Mountains: New evidence from fission versity Press. track dating of basement apatites in the Dry Valleys area, southern Steiger, RH., and E. Jaeger. 1977. Subcommission on Geochronology: Victoria Land. Earth and Planetary Science Letters, 82, 1-14. Convention of the use of decay constants in geo- and cosmochron- Harwood, D.M. 1983. Diatoms from the Sirius Formation. Antarctic ology. Earth and Planetary Science Letters, 36, 359-362. Journal of the U.S., 18(5), 98-100. Webb, P.-N., D.M. Harwood, B.C. McKelvey, and L.D. Stott. 1983. Harwood D.M. 1986. Recycled siliceous microfossils from the Sirius Late Neogene and older Cenozoic microfossils in high elevation Formation. Antarctic Journal of the U.S., 21(5), 101-103. deposits of the Transantarctic Mountains: Evidence for marine sedi- Lo Bello, Ph., C. Feraud, C.M. Hall, D. York, P. Lavina, and M. Bernat. mentation and ice volume variation on the East Antarctic Craton. 1987. 41 Ar/39Ar step-heating and laser fusion of a Quaternary pumice Antarctic Journal of the U.S., 18(5), 96-97. from Neschers Massif, Central France: The defeat of exnocrystic Webb, P.-N., D.M. Harwood, B.C. McKelvey, and L.D. Stott. 1984. contamination. Chemical Geology, (isotope geoscience section), 66, Cenozoic marine sedimentation and ice-volume variation on the East 61-71. Antarctic craton. Geology, 12, 287-291. Lux, D.L. 1986. 40Ar/39Ar ages for minerals from the amphibolite dy- Webb, P.-N., D.M. Harwood, B.C. McKelvey, M.C.G. Mabin, and namothermal aureole, Mont Albert, Gaspe, Quebec. Canadian Journal J.H. Mercer. 1986. Late Cenozoic tectonic and glacial history of the of Earth Science, 23, 21-26. Transantarctic Mountains. Antarctic Journal of the U.S., 21(5), 99-100. McDougall, I., and T.M. Harrison. 1988. Geochronology and ther-

Plateau icecap (figure 1). The array consisted of a 1.2-kilometer Ship-to-shore seismic refraction linear array of eight vertical seismographs, deployed at 150- investigation of the lithospheric meter intervals away from the array center, and an equilateral structure of the Transantarctic (160-meter sides) triangular deployment of three-component Mountain front

DANIEL R.H. OCONNELL and RALPH R.B. VON FRESE 162.0 163.0 164.0 165.0 166.0 -74. -74 Byrd Polar Research Center and Department of Geology and Mineralogy Ohio State University Columbus, Ohio 43210

JOHN PASKIEVITCH -74. 74.5 U.S. Geological Survei Branch of Alaskan Geology Anchorage, Alaska 99508

As part of the 1988-1989 German Antarctic Northern Victoria Land Expedition V (GANOVEX V) offshore-onshore seismic re- traction experiment, we established a seismic recording array in -75 75 the Transantarctic Mountains to record a tuned airgun array operated in the Ross Sea. The objective was to determine the crustal structure of the transition zone between the Transantarctic Mountains and the Ross Sea by recording three onshore-offshore seismic refraction profiles across the Transantarctic Mountains near Terra Nova Bay. Unfavorable ice conditions in Terra Nova Bay reduced two onshore-offshore profile lengths and necessi- tated reorientation of the third profile. Other GANOVEX V groups -7E 5 deployed onshore and offshore seismographs to provide con- 162.0 153.0 154.0 165.0 156.0 straints on the crustal structure of the Ross Sea and inland Tran- santarctic Mountain portions of the profiles. Figure 1. Map of the study area showing the position of the seismic An 11-element digital seismograph array was established 65 recording array (triangle) in relation to seismic lines 1, 4, and 5 kilometers inland from the Terra Nova Bay on the Tourmaline (circles).

1989 REVIEW 33

III LD FH H—

TAN Ross Sea

DISTANCE (KM) I 20 40 60 80 100 120 140 160 180 0 I I I I

10 Moho

0 w 1olo Cr)

Figure 3. A model of the Transantarctic Mountains/Ross Sea transition structure is shown at the bottom with P-wave ray paths for line 1. The C denotes the position of the coastline. Synthetic seismograms are plotted at the top using a reduction velocity of 8 kilometers per second. The synthetic seismograms approximately reproduce the P-wave first arrival patterns of the observed data. (km denotes kilometer. s denotes seconds.)

References The Antarctic Continental Margin: Geology and geophysics of the western Ross Sea. Houston: American Association of Petroleum Geologists. Cooper, A.K., F.J. Davey, and J.C. Behrendt. 1987. Seismic stratig- Earth Science Series, 5B, 119-137. raphy and structure of the Victoria Land Basin, western Ross Sea, Fitzgerald, PG., and A.J.W. Gleadow. 1988. Fission-track geochro- Antarctica. In A.K. Cooper and F. J. Davey (Eds.), The Antarctic nology, tectonics and structure of the Transantarctic Mountains in Continental Margin: Geology and geophysics of the western Ross Sea. Northern Victoria Land, Antarctica. Chemical Geology (isotope geo- Houston: American Association of Petroleum Geologists. Earth Sci- science section), 73, 169-198. ence Series, 5B, 27-76. McGinnis, L.D., R.H. Bowen, J.M. Erickson, B.J. Allred, and J.L. Cooper, A.K., F.J. Davey, and G.R. Cochrane. 1987. Structure of ex- Kreamer. 1985. East-west Antarctic boundary in McMurdo Sound. tensionally rifted crust beneath the western Ross Sea and Iselin Tectonophysics, 114: 341-356. Bank, Antarctica, from sonobuoy data. In A.K. Cooper and F.J. Smithson, S.B. 1972. Gravity interpretation in the Transantarctic Davey (Eds.), The Antarctic Continental Margin: Geology and geophysics Mountains near McMurdo Sound, Antarctica. Geological Society of of the western Ross Sea. Houston: American Association of Petroleum America Bulletin. 83, 3437-3442. Geologists. Earth Science Series, 5B, 93-118. ten Brink, U., T. Stern, I. Paintin, B. Beaudoin, T. Hefford, and J. Davey, F.J., and A.K. Cooper. 1987. Gravity studies of the Victoria McGinnis. 1989. Seismic investigation of lithospheric flexure within Land Basin and Iselin Bank. In A.K. Cooper and F.J. Davey (Eds.), the Ross Embayment, Antarctica. EOS, 70(15), 468.

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