Pilo-Pleistocene uplift of the McMurdo Dry Valley sector of the ...
Lambrecht, L.L., W.S. Lacey, and C.S. Smith. 1973. Observations on the Permian flora of the Law Glacier area, central Transantarctic Mountains. Bulletin of the Society of Belgian Geologists, Palaeontologists, and Hydrologists, 81, 161-167. Rigby, J.F. 1963. On a collection of plants of Permian age from Baralaba, Queensland. Proceedings of the Linnean Society of New South Wales, 87, 341-351. Rigby, J.F. 1978. Permian glossopterid and other cycadopsid fructifi-
Pilo-Pleistocene uplift of the McMurdo Dry Valley sector of the Transantarctic Mountains T.I. WILCH
Department of Geological Sciences
and
Institute for Quaternary Studies University of Maine Orono, Maine 04469-0110
D.R. Lux Department of Geological Sciences, University of Maine Orono, Maine 04469-0110
W.C. MCINTOSH New Mexico Institute for Mining and Technology Socorro, New Mexico 87801
G.H. DENTON Department of Geological Sciences
and
Institute for Quaternary Studies University of Maine Orono, Maine 04469-0110
Most evidence for past fluctuations of the antarctic ice sheet comes from glacial erosional and depositional features in the Transantarctic Mountains. Many such features occur at high elevations and date to Pliocene time. Because of the antiquity and elevation of these features, uplift history of the Transantarctic Mountains is critical for accurate ice-sheet reconstructions. Plio-Pleistocene uplift of the McMurdo Dry Valley region of the Transantarctic Mountains can be constrained by determining the elevation of subaerial basaltic cinder-cone deposits 30
cations from Queensland. (Geological Survey Queensland.) Palaeontology, Paper 41, Publication 367, 1-21. Schopf, J.M. 1976. Morphologic interpretation of fertile structures in glossopterid gymnosperms. Review of Palaeobotany and Palynology, 21, 25-64. Taylor, E.L. 1987. Glossopteris reproductive organs: An analysis of structure and morphology. 14th International Botany Congress (Abstracts), West Berlin.
perched on the walls of middle Taylor Valley between Nussbaum Riegel and Borns Glacier (figure). Taylor Valley is glacially carved and opens to McMurdo Sound at its eastern end. 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. Fieldwork in Taylor Valley was carried out during the 19871988 and 1988-1989 austral summers as part of an extensive surficial geology mapping program. Two primary objectives were to establish whether the volcanic rocks were erupted subaerially and to determine whether they were in situ. Eighteen geographically and/or mineralogically distinct alkalic basalt localites all contained in situ outcrops and all were erupted subaerially. Over 80 samples were collected for argon-40/argon-39 isotopic age determinations. Elevations of sample localities and lowest in situ outcrops were surveyed using a T-2 Theodolite and Electronic Distance Measurer. Despite careful selection, petrographic observations in many of the dating samples revealed the presence of small xenocrysts. These xenocrysts likely originated in the Paleozoic country rocks and therefore probably contain an older radiogenic argon-40 component than is contained in the basaltic groundmass. For this reason, magnetic and heavy-liquid separation techniques were used during sample preparation to remove the xenocrystic contaminants from the groundmass. McDougall and Harrison (1988) provide a review of the argon-40/argon-39 dating method. Specific analytical techniques used at the University of Maine follow those described by Lux (1986). Samples were irradiated in the H5 facility of the Ford Nuclear Reactor at the University of Michigan. Neutron flux 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 isotopic compositions were determined by Nuclide 6-60-SGA 1.25 mass spectrometer for between 4 to 12 heating increments for each sample. Ages based on these ratios were calculated using the decay constants recommended by Steiger and Jaeger (1977). One advantage of the argon-40/argon-39 method over the conventional potassium/argon method is that the gas is released in steps, thus allowing isolation and recognition of inherited argon-40. An age is determined for the gas released in each heating increment. A weighted average age of the sepANTARCTIC JOURNAL
-
Lower Marr Site V
-
AW
V
-
V
Upper Marr $0
fT:;S;iUs !
Site-o'00 (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 adjacent increments with concordant ages is a plateau age. Samples yielding discordant age spectra are difficult to interpret, although Lo Bello (1987) reported that samples with excess argon components are characterized by interpretable saddle-shaped spectra. In this situation, the plateau age is derived from steps at the base of the saddle. Such a plateau age should be considered a maximum value for the entire sample. The table lists elevation and preliminary argon-40/argon-39 plateau age data from 11 of the 18 volcanic units in middle Taylor Valley. Most of these ages agree with previously published potassium/argon ages (Armstrong 1978). Several release spectra were saddle-shaped, and the base of the saddle was interpreted as an approximate age of the volcanic event. Consistency of ages within each volcanic unit adds credence to the 1989 REVIEW
ages. The maximum uplift rate for each unit, also listed in the table, is determined by dividing the lowest elevation of in situ volcanics by the age. The lowest maximum uplift rate is 137 meters per million years since 3 million years ago at the Lower Marr site (west side). Two other models of Transantarctic Mountain uplift have recently been proposed. One model (Gleadow and Fitzgerald 1987) used fission-track dating of basement apatites in Wright Valley to approximate the timing and amount of uplift. They concluded that asymmetric uplift to a maximum elevation of 4.8-5.3 kilometers commenced 50 million years ago. An average uplift rate since 50 million years ago was calculated to be 100 + 5 meters per million years. After analyzing the thermobarometry of two-phase granulite inclusions in Cenozoic volcanics in the McMurdo Sound area, Berg and Herz (1986) 31
Elevation and preliminary argon-40largon-39 whole rock ages and uplift rates from volcanic outcrops in Taylor Valley, Antarctica
Locality
Plateau age Elevation (in meters) Maximum uplift rate Sample number (in millions of years) (lowest in situ outcrop) (in meters per million years)
East Rhone Site
86K-20 86A-26 86A-31 TWV87-20
2.90 ± .10 2.79 ± .24 1.81 ± .12 1.69 ± .27
671.0 ± 5.0 671.0 ± 5.0 671.0 ± 5.0 671.0 ± 5.0
231.4 ± 9.7 240.5 ± 30.2 370.7 ± 27.3 397.0 ± 66.4
West Matterhorn Sites
TWV87-37 TWV87-44 TWV87-42 TWV87-48 86K-5 86K-6 86K-2 86K-3 86K-21 TWV87-1 7
3.03 ± .21 3.04 ± .30 3.53 ± .18 3.12 ± .15 3.72 ± .17 3.71 ± .13 3.72 ± .17 2.96 ± .16 3.71 ± .15 3.42 ± .31
1,054.0 ± 0.5 1,054.0 ± 0.5 602.4 ± 0.5 602.4 ± 0.5 821.8 ± 0.5 821.8 ± 0.5 821.8 ± 0.5 821.8 ± 0.5 821.8 ± 0.5 821.8 ± 0.5
347.9 ± 24.3 346.7 ± 34.4 170.7 ± 8.8 193.1 ± 9.4 220.9 ± 10.2 221.5 ± 10.3 220.9 ± 10.2 277.6 ± 15.2 221.6 ± 9.1 240.3 ± 21.9
Calkin Site
84A-1 86A-35
1.58 ± .17 1.29 ± .35
425.0 ± 5.0 425.0 ± 5.0
269.0 ± 32.1 329.5 ± 93.3
Sollas Sites (lower)
84A-9 84A-1 1 TWV87-1 2N TWV87-1 4 TWV87-65 TWV87-1 0
2.01 ± .03 2.19 ± .12 2.20 ± .08 2.40 ± .30 1.74 ± .20 3.63 ± .30
323.5 ± 0.5 323.5 ± 0.5 323.5 ± 0.5 323.5 ± 0.5 416.3 ± 0.5 644.5 ± 5.0
160.9 ± 2.7 147.7 ± 8.3 147.0 ± 5.6 134.8 ± 17.06 239.3 ± 68.9 177.5 ± 27.8
Upper Marr Site
TWV87-04
2.79 ± .08
640.5 ± 5.0
229.6 ± 6.8
Lower Mark Sites (west) (east)
TWV87-62H TWV87-1 8 TWV87-1 02
3.09 ± .14 2.69 ± .24 2.52 ± .23
424.1 ± 0.5 432.3 ± 0.5 432.3 ± 0.5
137.3 ± 6.4 160.7 ± 14.52 171.5 ± 15.86
(west) (east)
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, = Elevation error ] ± [Age ± elevation (Age error ) ± Age 2].
suggested that the fission-track uplift rate should be halved, because only 2,000 meters of uplift has occurred in the McMurdo Dry Valleys. A problem with using these data for ice sheet reconstructions is that the uplift rates are only averages and may have varied since 50 million years ago. In other words, uplift could have occurred in pulses rather than continuously during the past 50 million years. Because the averages fall beneath the constraints imposed by the lowest maximum uplift rates in Taylor Valley, however, we conclude that the averages cannot be discounted for this latest time interval. A second model holds that the glacigene Sirius Formation and Dominion Erosion Surface located in the Beardmore Glacier 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 years for the Sirius Formation are inferred from marine diatom assemblages (Harwood 1983, 1986; Webb et al. 1983, 1984, 1986). These data yield average uplift rates from 333 to 1,000 meters per million years since 3 million years in the Beardmore Glacier region. The discrepancy between the high diatom-based uplift rates of the Transantarctic Mountains in the Beardmore Glacier region and the lower uplift rates of the Transantarctic Mountains in the McMurdo Dry Valleys determined from two indepen dent methods is at least an order of magnitude. This difference 32
needs to be reconciled because of its importance to Pliocene ice-sheet reconstructions. Either Plio-Pleistocene uplift rates are vastly different in the McMurdo Dry Valleys and near Beardmore Glacier, or one of the methods for calculating uplift rates is flawed. G. Falloon and P. Sole of the New Zealand Department of Surveying and Land Information provided generous surveying support. R. Garster assisted with surveying. We thank N. Dunbar for assistance with mapping. D.P. West, Jr., assisted with argon-40/argon-39 geochronologic analyses and interpretations.
References Alexander, E.C., Jr., G.M. Michelson, and M.A. Lanphere. 1978. MMhb1: A new 40Ar/39Ar dating standard. In R.E. Zartman (Ed.), Fourth International Conference on Geochronology, Cosinochronology, and Osotope Geology. U.S. Geological Survey, Open-File Report 78-701, 6-8. Armstrong, R.L. 1978. K-Ar Dating: Late Cenozoic McMurdo Volcanic Group and dry valley glacial history, Victoria Land, Antarctica. New Zealand Journal of Geology and Geophysics, 21, 685-689. Berg, J.H., and D.L. Herz. 1986. Thermobarometry of two-pyroxencgranulite inclusions in Cenozoic volcanic rocks of the McMurdo Sound region. Antarctic Journal of the U.S., 21(5), l) 20. AN I \I
Gleadow, A.J.W., and P.C. Fitzgerald. 1987. Uplift history and structure of the Transantarctic Mountains: New evidence from fission track dating of basement apatites in the Dry Valleys area, southern Victoria Land. Earth and Planetary Science Letters, 82, 1-14. Harwood, D.M. 1983. Diatoms from the Sirius Formation. Antarctic Journal of the U.S., 18(5), 98-100. Harwood D.M. 1986. Recycled siliceous microfossils from the Sirius Formation. Antarctic Journal of the U.S., 21(5), 101-103. Lo Bello, Ph., C. Feraud, C.M. Hall, D. York, P. Lavina, and M. Bernat. 1987. 41 Ar/39Ar step-heating and laser fusion of a Quaternary pumice from Neschers Massif, Central France: The defeat of exnocrystic contamination. Chemical Geology, (isotope geoscience section), 66, 61-71. Lux, D.L. 1986. 40Ar/39Ar ages for minerals from the amphibolite dynamothermal aureole, Mont Albert, Gaspe, Quebec. Canadian Journal of Earth Science, 23, 21-26. McDougall, I., and T.M. Harrison. 1988. Geochronology and ther-
Ship-to-shore seismic refraction investigation of the lithospheric structure of the Transantarctic Mountain front
mochronology by the 40Ar/39Ar Method. New York: Oxford University Press. Steiger, RH., and E. Jaeger. 1977. Subcommission on Geochronology: Convention of the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters, 36, 359-362. Webb, P.-N., D.M. Harwood, B.C. McKelvey, and L.D. Stott. 1983. Late Neogene and older Cenozoic microfossils in high elevation deposits of the Transantarctic Mountains: Evidence for marine sedimentation and ice volume variation on the East Antarctic Craton. Antarctic Journal of the U.S., 18(5), 96-97. Webb, P.-N., D.M. Harwood, B.C. McKelvey, and L.D. Stott. 1984. Cenozoic marine sedimentation and ice-volume variation on the East Antarctic craton. Geology, 12, 287-291. Webb, P.-N., D.M. Harwood, B.C. McKelvey, M.C.G. Mabin, and J.H. Mercer. 1986. Late Cenozoic tectonic and glacial history of the Transantarctic Mountains. Antarctic Journal of the U.S., 21(5), 99-100.
Plateau icecap (figure 1). The array consisted of a 1.2-kilometer linear array of eight vertical seismographs, deployed at 150meter intervals away from the array center, and an equilateral (160-meter sides) triangular deployment of three-component
162.0 163.0 164.0 165.0 166.0
DANIEL R.H. O'CONNELL and RALPH R.B. VON FRESE
-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 retraction experiment, we established a seismic recording array in 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 necessitated reorientation of the third profile. Other GANOVEX V groups deployed onshore and offshore seismographs to provide constraints on the crustal structure of the Ross Sea and inland Transantarctic Mountain portions of the profiles. An 11-element digital seismograph array was established 65 kilometers inland from the Terra Nova Bay on the Tourmaline 1989 REVIEW
-75
75
-7E
5 162.0 153.0 154.0 165.0 156.0
Figure 1. Map of the study area showing the position of the seismic recording array (triangle) in relation to seismic lines 1, 4, and 5 (circles). 33
0
GD U) III
LD
FH
H—
Cu
TAN
I
Ross Sea
DISTANCE (KM)
20 40 60 80 100 120 140 160 180
0
I I
I I
0
10
0 w
Moho
1olo Cr)
0
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 Cooper, A.K., F.J. Davey, and J.C. Behrendt. 1987. Seismic stratigraphy and structure of the Victoria Land Basin, western Ross Sea, Antarctica. In A.K. Cooper and F. J. Davey (Eds.), The Antarctic Continental Margin: Geology and geophysics of the western Ross Sea. Houston: American Association of Petroleum Geologists. Earth Science Series, 5B, 27-76.
Cooper, A.K., F.J. Davey, and G.R. Cochrane. 1987. Structure of extensionally rifted crust beneath the western Ross Sea and Iselin Bank, Antarctica, from sonobuoy data. In A.K. Cooper and F.J. Davey (Eds.), The Antarctic Continental Margin: Geology and geophysics
of the western Ross Sea. Houston: American Association of Petroleum Geologists. Earth Science Series, 5B, 93-118. Davey, F.J., and A.K. Cooper. 1987. Gravity studies of the Victoria Land Basin and Iselin Bank. In A.K. Cooper and F.J. Davey (Eds.),
1989 REVIEW
The Antarctic Continental Margin: Geology and geophysics of the western Ross Sea. Houston: American Association of Petroleum Geologists. Earth Science Series, 5B, 119-137.
Fitzgerald, PG., and A.J.W. Gleadow. 1988. Fission-track geochronology, tectonics and structure of the Transantarctic Mountains in Northern Victoria Land, Antarctica. Chemical Geology (isotope geoscience section), 73, 169-198. McGinnis, L.D., R.H. Bowen, J.M. Erickson, B.J. Allred, and J.L. Kreamer. 1985. East-west Antarctic boundary in McMurdo Sound. Tectonophysics, 114: 341-356. Smithson, S.B. 1972. Gravity interpretation in the Transantarctic Mountains near McMurdo Sound, Antarctica. Geological Society of America Bulletin. 83, 3437-3442. ten Brink, U., T. Stern, I. Paintin, B. Beaudoin, T. Hefford, and J. McGinnis. 1989. Seismic investigation of lithospheric flexure within the Ross Embayment, Antarctica. EOS, 70(15), 468.
35
Pilo-Pleistocene uplift of the McMurdo Dry Valley sector of the Transantarctic Mountains T.I. WILCH
Department of Geological Sciences
and
Institute for Quaternary Studies University of Maine Orono, Maine 04469-0110
D.R. Lux Department of Geological Sciences, University of Maine Orono, Maine 04469-0110
W.C. MCINTOSH New Mexico Institute for Mining and Technology Socorro, New Mexico 87801
G.H. DENTON Department of Geological Sciences
and
Institute for Quaternary Studies University of Maine Orono, Maine 04469-0110
Most evidence for past fluctuations of the antarctic ice sheet comes from glacial erosional and depositional features in the Transantarctic Mountains. Many such features occur at high elevations and date to Pliocene time. Because of the antiquity and elevation of these features, uplift history of the Transantarctic Mountains is critical for accurate ice-sheet reconstructions. Plio-Pleistocene uplift of the McMurdo Dry Valley region of the Transantarctic Mountains can be constrained by determining the elevation of subaerial basaltic cinder-cone deposits 30
cations from Queensland. (Geological Survey Queensland.) Palaeontology, Paper 41, Publication 367, 1-21. Schopf, J.M. 1976. Morphologic interpretation of fertile structures in glossopterid gymnosperms. Review of Palaeobotany and Palynology, 21, 25-64. Taylor, E.L. 1987. Glossopteris reproductive organs: An analysis of structure and morphology. 14th International Botany Congress (Abstracts), West Berlin.
perched on the walls of middle Taylor Valley between Nussbaum Riegel and Borns Glacier (figure). Taylor Valley is glacially carved and opens to McMurdo Sound at its eastern end. 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. Fieldwork in Taylor Valley was carried out during the 19871988 and 1988-1989 austral summers as part of an extensive surficial geology mapping program. Two primary objectives were to establish whether the volcanic rocks were erupted subaerially and to determine whether they were in situ. Eighteen geographically and/or mineralogically distinct alkalic basalt localites all contained in situ outcrops and all were erupted subaerially. Over 80 samples were collected for argon-40/argon-39 isotopic age determinations. Elevations of sample localities and lowest in situ outcrops were surveyed using a T-2 Theodolite and Electronic Distance Measurer. Despite careful selection, petrographic observations in many of the dating samples revealed the presence of small xenocrysts. These xenocrysts likely originated in the Paleozoic country rocks and therefore probably contain an older radiogenic argon-40 component than is contained in the basaltic groundmass. For this reason, magnetic and heavy-liquid separation techniques were used during sample preparation to remove the xenocrystic contaminants from the groundmass. McDougall and Harrison (1988) provide a review of the argon-40/argon-39 dating method. Specific analytical techniques used at the University of Maine follow those described by Lux (1986). Samples were irradiated in the H5 facility of the Ford Nuclear Reactor at the University of Michigan. Neutron flux 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 isotopic compositions were determined by Nuclide 6-60-SGA 1.25 mass spectrometer for between 4 to 12 heating increments for each sample. Ages based on these ratios were calculated using the decay constants recommended by Steiger and Jaeger (1977). One advantage of the argon-40/argon-39 method over the conventional potassium/argon method is that the gas is released in steps, thus allowing isolation and recognition of inherited argon-40. An age is determined for the gas released in each heating increment. A weighted average age of the sepANTARCTIC JOURNAL
-
Lower Marr Site V
-
AW
V
-
V
Upper Marr $0
fT:;S;iUs !
Site-o'00 (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 adjacent increments with concordant ages is a plateau age. Samples yielding discordant age spectra are difficult to interpret, although Lo Bello (1987) reported that samples with excess argon components are characterized by interpretable saddle-shaped spectra. In this situation, the plateau age is derived from steps at the base of the saddle. Such a plateau age should be considered a maximum value for the entire sample. The table lists elevation and preliminary argon-40/argon-39 plateau age data from 11 of the 18 volcanic units in middle Taylor Valley. Most of these ages agree with previously published potassium/argon ages (Armstrong 1978). Several release spectra were saddle-shaped, and the base of the saddle was interpreted as an approximate age of the volcanic event. Consistency of ages within each volcanic unit adds credence to the 1989 REVIEW
ages. The maximum uplift rate for each unit, also listed in the table, is determined by dividing the lowest elevation of in situ volcanics by the age. The lowest maximum uplift rate is 137 meters per million years since 3 million years ago at the Lower Marr site (west side). Two other models of Transantarctic Mountain uplift have recently been proposed. One model (Gleadow and Fitzgerald 1987) used fission-track dating of basement apatites in Wright Valley to approximate the timing and amount of uplift. They concluded that asymmetric uplift to a maximum elevation of 4.8-5.3 kilometers commenced 50 million years ago. An average uplift rate since 50 million years ago was calculated to be 100 + 5 meters per million years. After analyzing the thermobarometry of two-phase granulite inclusions in Cenozoic volcanics in the McMurdo Sound area, Berg and Herz (1986) 31
Elevation and preliminary argon-40largon-39 whole rock ages and uplift rates from volcanic outcrops in Taylor Valley, Antarctica
Locality
Plateau age Elevation (in meters) Maximum uplift rate Sample number (in millions of years) (lowest in situ outcrop) (in meters per million years)
East Rhone Site
86K-20 86A-26 86A-31 TWV87-20
2.90 ± .10 2.79 ± .24 1.81 ± .12 1.69 ± .27
671.0 ± 5.0 671.0 ± 5.0 671.0 ± 5.0 671.0 ± 5.0
231.4 ± 9.7 240.5 ± 30.2 370.7 ± 27.3 397.0 ± 66.4
West Matterhorn Sites
TWV87-37 TWV87-44 TWV87-42 TWV87-48 86K-5 86K-6 86K-2 86K-3 86K-21 TWV87-1 7
3.03 ± .21 3.04 ± .30 3.53 ± .18 3.12 ± .15 3.72 ± .17 3.71 ± .13 3.72 ± .17 2.96 ± .16 3.71 ± .15 3.42 ± .31
1,054.0 ± 0.5 1,054.0 ± 0.5 602.4 ± 0.5 602.4 ± 0.5 821.8 ± 0.5 821.8 ± 0.5 821.8 ± 0.5 821.8 ± 0.5 821.8 ± 0.5 821.8 ± 0.5
347.9 ± 24.3 346.7 ± 34.4 170.7 ± 8.8 193.1 ± 9.4 220.9 ± 10.2 221.5 ± 10.3 220.9 ± 10.2 277.6 ± 15.2 221.6 ± 9.1 240.3 ± 21.9
Calkin Site
84A-1 86A-35
1.58 ± .17 1.29 ± .35
425.0 ± 5.0 425.0 ± 5.0
269.0 ± 32.1 329.5 ± 93.3
Sollas Sites (lower)
84A-9 84A-1 1 TWV87-1 2N TWV87-1 4 TWV87-65 TWV87-1 0
2.01 ± .03 2.19 ± .12 2.20 ± .08 2.40 ± .30 1.74 ± .20 3.63 ± .30
323.5 ± 0.5 323.5 ± 0.5 323.5 ± 0.5 323.5 ± 0.5 416.3 ± 0.5 644.5 ± 5.0
160.9 ± 2.7 147.7 ± 8.3 147.0 ± 5.6 134.8 ± 17.06 239.3 ± 68.9 177.5 ± 27.8
Upper Marr Site
TWV87-04
2.79 ± .08
640.5 ± 5.0
229.6 ± 6.8
Lower Mark Sites (west) (east)
TWV87-62H TWV87-1 8 TWV87-1 02
3.09 ± .14 2.69 ± .24 2.52 ± .23
424.1 ± 0.5 432.3 ± 0.5 432.3 ± 0.5
137.3 ± 6.4 160.7 ± 14.52 171.5 ± 15.86
(west) (east)
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, = Elevation error ] ± [Age ± elevation (Age error ) ± Age 2].
suggested that the fission-track uplift rate should be halved, because only 2,000 meters of uplift has occurred in the McMurdo Dry Valleys. A problem with using these data for ice sheet reconstructions is that the uplift rates are only averages and may have varied since 50 million years ago. In other words, uplift could have occurred in pulses rather than continuously during the past 50 million years. Because the averages fall beneath the constraints imposed by the lowest maximum uplift rates in Taylor Valley, however, we conclude that the averages cannot be discounted for this latest time interval. A second model holds that the glacigene Sirius Formation and Dominion Erosion Surface located in the Beardmore Glacier 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 years for the Sirius Formation are inferred from marine diatom assemblages (Harwood 1983, 1986; Webb et al. 1983, 1984, 1986). These data yield average uplift rates from 333 to 1,000 meters per million years since 3 million years in the Beardmore Glacier region. The discrepancy between the high diatom-based uplift rates of the Transantarctic Mountains in the Beardmore Glacier region and the lower uplift rates of the Transantarctic Mountains in the McMurdo Dry Valleys determined from two indepen dent methods is at least an order of magnitude. This difference 32
needs to be reconciled because of its importance to Pliocene ice-sheet reconstructions. Either Plio-Pleistocene uplift rates are vastly different in the McMurdo Dry Valleys and near Beardmore Glacier, or one of the methods for calculating uplift rates is flawed. G. Falloon and P. Sole of the New Zealand Department of Surveying and Land Information provided generous surveying support. R. Garster assisted with surveying. We thank N. Dunbar for assistance with mapping. D.P. West, Jr., assisted with argon-40/argon-39 geochronologic analyses and interpretations.
References Alexander, E.C., Jr., G.M. Michelson, and M.A. Lanphere. 1978. MMhb1: A new 40Ar/39Ar dating standard. In R.E. Zartman (Ed.), Fourth International Conference on Geochronology, Cosinochronology, and Osotope Geology. U.S. Geological Survey, Open-File Report 78-701, 6-8. Armstrong, R.L. 1978. K-Ar Dating: Late Cenozoic McMurdo Volcanic Group and dry valley glacial history, Victoria Land, Antarctica. New Zealand Journal of Geology and Geophysics, 21, 685-689. Berg, J.H., and D.L. Herz. 1986. Thermobarometry of two-pyroxencgranulite inclusions in Cenozoic volcanic rocks of the McMurdo Sound region. Antarctic Journal of the U.S., 21(5), l) 20. AN I \I
Gleadow, A.J.W., and P.C. Fitzgerald. 1987. Uplift history and structure of the Transantarctic Mountains: New evidence from fission track dating of basement apatites in the Dry Valleys area, southern Victoria Land. Earth and Planetary Science Letters, 82, 1-14. Harwood, D.M. 1983. Diatoms from the Sirius Formation. Antarctic Journal of the U.S., 18(5), 98-100. Harwood D.M. 1986. Recycled siliceous microfossils from the Sirius Formation. Antarctic Journal of the U.S., 21(5), 101-103. Lo Bello, Ph., C. Feraud, C.M. Hall, D. York, P. Lavina, and M. Bernat. 1987. 41 Ar/39Ar step-heating and laser fusion of a Quaternary pumice from Neschers Massif, Central France: The defeat of exnocrystic contamination. Chemical Geology, (isotope geoscience section), 66, 61-71. Lux, D.L. 1986. 40Ar/39Ar ages for minerals from the amphibolite dynamothermal aureole, Mont Albert, Gaspe, Quebec. Canadian Journal of Earth Science, 23, 21-26. McDougall, I., and T.M. Harrison. 1988. Geochronology and ther-
Ship-to-shore seismic refraction investigation of the lithospheric structure of the Transantarctic Mountain front
mochronology by the 40Ar/39Ar Method. New York: Oxford University Press. Steiger, RH., and E. Jaeger. 1977. Subcommission on Geochronology: Convention of the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters, 36, 359-362. Webb, P.-N., D.M. Harwood, B.C. McKelvey, and L.D. Stott. 1983. Late Neogene and older Cenozoic microfossils in high elevation deposits of the Transantarctic Mountains: Evidence for marine sedimentation and ice volume variation on the East Antarctic Craton. Antarctic Journal of the U.S., 18(5), 96-97. Webb, P.-N., D.M. Harwood, B.C. McKelvey, and L.D. Stott. 1984. Cenozoic marine sedimentation and ice-volume variation on the East Antarctic craton. Geology, 12, 287-291. Webb, P.-N., D.M. Harwood, B.C. McKelvey, M.C.G. Mabin, and J.H. Mercer. 1986. Late Cenozoic tectonic and glacial history of the Transantarctic Mountains. Antarctic Journal of the U.S., 21(5), 99-100.
Plateau icecap (figure 1). The array consisted of a 1.2-kilometer linear array of eight vertical seismographs, deployed at 150meter intervals away from the array center, and an equilateral (160-meter sides) triangular deployment of three-component
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DANIEL R.H. O'CONNELL and RALPH R.B. VON FRESE
-74.
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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 retraction experiment, we established a seismic recording array in 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 necessitated reorientation of the third profile. Other GANOVEX V groups deployed onshore and offshore seismographs to provide constraints on the crustal structure of the Ross Sea and inland Transantarctic Mountain portions of the profiles. An 11-element digital seismograph array was established 65 kilometers inland from the Terra Nova Bay on the Tourmaline 1989 REVIEW
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75
-7E
5 162.0 153.0 154.0 165.0 156.0
Figure 1. Map of the study area showing the position of the seismic recording array (triangle) in relation to seismic lines 1, 4, and 5 (circles). 33
0
GD U) III
LD
FH
H—
Cu
TAN
I
Ross Sea
DISTANCE (KM)
20 40 60 80 100 120 140 160 180
0
I I
I I
0
10
0 w
Moho
1olo Cr)
0
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 Cooper, A.K., F.J. Davey, and J.C. Behrendt. 1987. Seismic stratigraphy and structure of the Victoria Land Basin, western Ross Sea, Antarctica. In A.K. Cooper and F. J. Davey (Eds.), The Antarctic Continental Margin: Geology and geophysics of the western Ross Sea. Houston: American Association of Petroleum Geologists. Earth Science Series, 5B, 27-76.
Cooper, A.K., F.J. Davey, and G.R. Cochrane. 1987. Structure of extensionally rifted crust beneath the western Ross Sea and Iselin Bank, Antarctica, from sonobuoy data. In A.K. Cooper and F.J. Davey (Eds.), The Antarctic Continental Margin: Geology and geophysics
of the western Ross Sea. Houston: American Association of Petroleum Geologists. Earth Science Series, 5B, 93-118. Davey, F.J., and A.K. Cooper. 1987. Gravity studies of the Victoria Land Basin and Iselin Bank. In A.K. Cooper and F.J. Davey (Eds.),
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The Antarctic Continental Margin: Geology and geophysics of the western Ross Sea. Houston: American Association of Petroleum Geologists. Earth Science Series, 5B, 119-137.
Fitzgerald, PG., and A.J.W. Gleadow. 1988. Fission-track geochronology, tectonics and structure of the Transantarctic Mountains in Northern Victoria Land, Antarctica. Chemical Geology (isotope geoscience section), 73, 169-198. McGinnis, L.D., R.H. Bowen, J.M. Erickson, B.J. Allred, and J.L. Kreamer. 1985. East-west Antarctic boundary in McMurdo Sound. Tectonophysics, 114: 341-356. Smithson, S.B. 1972. Gravity interpretation in the Transantarctic Mountains near McMurdo Sound, Antarctica. Geological Society of America Bulletin. 83, 3437-3442. ten Brink, U., T. Stern, I. Paintin, B. Beaudoin, T. Hefford, and J. McGinnis. 1989. Seismic investigation of lithospheric flexure within the Ross Embayment, Antarctica. EOS, 70(15), 468.
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