Analysis of antarctic geophysical data
Brown, G. M. 1957. Pyroxenes from the early and middle stages of fractionation of the Skaergaard intrusion, East Greenland. Mineralogical Magazine, 31: 511-543. Brown, G. M., and E. A. Vincent. 1963. Pyroxenes from the late stages of fractionation of the Skaergaard intrusion, East Greenland. Journal of Petrology, 4: 175-197. Ford, A. B. 1970. Development of the layered series and capping granophyre of the Dufek intrusion of Antarctica. In: Symposium on the Bushveld Igneous Complex and Other Layered Intrusions (D. J . L. Visser and G. von Gruenewaldt, eds.). Geological Society of South Africa. Special Publication, 1: 494-510. Ford, A. B., and W. W. Boyd, Jr. 1968. The Dufek intrusion, a major stratiform gabbroic body in the Pensacola Mountains, Antarctica. Proceedings of the 23rd International Geological Congress, 2: 213-228. Hess, H. H. 1941. Pyroxenes of common mafic magmas. American Mineralogist, 26: 515-535, 573594. Hess, H. H. 1960. Stillwater igneous complex, Montana: a quantitative mineralogical study. Geological Society of America. Memoir, 80. p. 121. Jackson, E. D. 1970. The cyclic unit in layered intrusions-a comparison of repetitive stratigraphy in the ultraniafic parts of the Stillwater, Great Dyke, and Bushveld Complexes. In: Symposium on the Bushveld Igneous Complex and Other Layered Intrusions (D. J . L. Visser and G. von Gruenewaldt, eds.). Geological Society of South Africa. Special Publication, 1: 391-424. Page, N. J . , R. Shimek, and R. Huffman, Jr. 1972. Grain-size variations within an olivine cumulate, Stillwater Complex, Montana. U.S. Geological Survey. Professional Paper, 800-C, C29-C37. Poldervaart, A., and H. H. Hess. 1951. Pyroxenes in the crystallization of basaltic magma. Journal of Geology, 59, 472-489. Walker, K. R., N. G. Ware, and J . R. Lovering. 1973. Com positional variations in the pyroxenes of the differentiated Palisades Sill, New Jersey. Geological Society of America. Bulletin, 84, 89-110.
Analysis of antarctic geophysical data C. R. BENTLEY, H. K. ACHARYA, J . L. CLAPP, W. CLOUGH, H. KOHNEN, and J . D. ROBERTSON J.
Geophysical and Polar Research Center Department of Geology and Geophysics University of Wisconsin, Madison Continuing analysis of antarctic geophysical data follows several lines, including studies of ice properties (as revealed by seismic and electromagnetic wave propagation experiments near Byrd Station), west antarctic gravity maps, Roosevelt Island strain data, and theoretical studies of seismic wave propagation. Appended is a bibliography of papers on these subjects (since Contribution number 299, Geophysical and Polar Research Center, Department of Geology and Geophysics, University of Wisconsin, Madison. September-October 1973
Bentley et al., 1969). What follows is a summary of recent results not yet published. 1. Seismic velocities obtained from short refraction profiles can be used to predict density at depths between o and 10 meters, with a standard error of about 0.01 gm/cm3. 2. A newly recognized and extensive horizon at depths of 25 to 30 meters, marking an apparent change in the densification rate, has been found in West Antarctica. The horizon's existence suggests that two distinct mechanisms successively dominate the metamorphic process between the depth of closest packing of snow grains and the urn-ice boundary. 3. Measurement of P-wave attenuation in ice near Byrd Station led to the determination of a very low value for the internal friction. From comparison with laboratory measurements (Kuroiwa, 1964), it appears that a slight but significant contamination of the antarctic ice by ionic impurities (Gow, 1968) and the ambient ice temperature (-28 0 C.) result in the falling of seismic frequencies at a dissipation minimum between spectral regions dominated respectively by grain-boundary phenomena, and by the fundamental relaxation spectrum. 4. Analysis of electromagnetic wide-angle reflection measurements shows that correction for refraction in the upper portion of an ice sheet need not be made when calculating mean velocities. Even for a reflector as shallow in depth as 100 meters, the error introduced by assuming straight line geometry is only about 10 nanosec, well below the time resolution of the measurements. For accurate measurements, however, the length of wideangle profiles must be limited to distances corresponding to the reflection path for a ray at grazing incidence on the surface. The amplitude of the reflected wave, which changes markedly along the length of a profile, can play an important role in the measurements. 5. Previously existing maps of gravity anomalies in West Antarctica have been supplemented by new data and have been contoured by a computer. Three-dimension al modeling has been used to prepare an Airy isostatic gravity anomaly map. This map reveals several imbalances, which may be caused by: up-warping of the Mdiscontinuity in Ellsworth Land and beneath the HollickKenyon Plateau; dense, lower-crustal material unusually near the surface southwestward from the Whitmore Mountains; the extension beneath the Rockefeller Plateau of a pre-Cretaceous geosyncline known to exist in the Edsel Ford Ranges; the southern boundary of the Cenozoic volcanic province in Marie Byrd Land. A deep negative anomaly of unknown cause exists along the Bakutis Coast. On the hypothesis of a recent retreat of the ice sheet, no more than 40 percent (and probably much less) of the anomaly can be attributed to incomplete isostatic rebound. 6. A Rayleigh wave group-veloiity curve, applicable to the known velocity-depth and density-depth curves in 263
West Antarctica, has been computed with the use of a finite difference technique. Results agree well with observed data. Comparison with calculations based on approximations commonly made in surface wave analyses (Poisson's ratio = 1/4 ; density = constant) surprisingly shows that the group velocities are relatively more sensitive to incorrect densities than to incorrect shear wave velocities. 7. Final strain-rate calculations for a grid network across Roosevelt Island show a strongly asymmetrical profile, with the longitudinal extensional strains twice as great on the northeast as on the southwest flank of the island. Since accumulation rates on the two flanks are about the same the difference in strain rates is probably attributable to the effect of the Ross Ice Shelf.
References
Bentley, C. R., H. K. Acharya, J . E. Beitzel, and J . W. Clough. 1969. Analysis of antarctic geophysical data, 1968-1969. Antarctic Journal of the United States, IV(5) 219. Gow, A. J . 1968. Electrolytic conductivity of snow and glacier ice from Antarctica and Greenland. Journal of Geophysical Research, 73(12): 3643-3649. Kuroiwa, D. 1964. Internal friction of ice. Contributions from the Institute of Low Temperature Science, Hokkaido University. Series A. 18.
Bibliography Acharya, H. K. 1970. Reflection from the free surface of an inhomogeneous media. Bulletin of the Seismological Society of America, 60(4): 1101-1104. Acharya, H. K. 1972. Surface-wave dispersion in Byrd Land, Antarctica. Bulletin of the Seismological Society of America, 62(4): 955-959. Acharya, H. K. In press. Investigation of surface wave dispersion in inhomogeneous media by the finite difference method. Proceedings of the Ninth Annual Symposium on Geophysical Theory and Computer Applications. Bentley, C. R. 1972. Seismic-wave velocities in anisotropic ice: a comparison of measured and calculated values in and around the deep drill hole at Byrd Station, Antarctica. journal of Geophysical Research, 77(23): 4406-4420. Bentley, C. R., 1972. Suglacial rock surface topography of Antarctica. Antarctic Map Folio Series, 16. Bentley, C. R. In press. Crustal structure of Antarctica. Proceedings of IUMC Symposium: crustal structure based on seismic data. Tectono physics. Bentley, C. R., and J . W. Clough. 1971. Electromagnetic sounding of ice thickness In: Propagation Limitations in Remote Sensing (J. B. Lomax, ed.). AGARD Conference Proceedings, North Atlantic Treaty Organization. 90: 18-1-18-7. 264
Bentley, C. R., and J . W. Clough. 1972. Seismic refraction shooting in Ellsworth and Dronning Maud Lands. In: Antarctic Geology and Geophysics (R. J . Adie, ed.). Oslo, Universitetsforlaget. 169-172. Clough, J . W. 1973. Radio-echo sounding: brine percolation layer. Journal of Glaciology, 12(64): 141-143. Kohnen, H. 1971. The relation between seismic urn structure, temperature, and accumulation. Zeitschrift für Gletscherkunde und Glazialgeologie, VII( I-2): 141-151. Kohnen, H. 1972. Uber die beziehung zwischen seismischen geschwindigkeiten und der dichte in firn and eis. Zeitschrift für Geophysik, 38: 925-935. Kohnen, H., and C. R. Bentley. 1973. Seismic refraction and reflection measurements at Byrd Station, Antarctica. Journal of Glaciology, 12(64): 101-111. Kososki, B. A. 1972. A gravity study of West Antarctica. M. S. Thesis, University of Wisconsin. Robertson, J . D. 1972. A seismic study of the structure and metamorphism of 6rn in West Antarctica. M. S. Thesis, University of Wisconsin.
Age of the Falla Formation (Triassic), Queen Alexandra Range G. FAURE and
R. L. HILL
Department of Geology and Mineralogy Institute of Polar Studies The Ohio State University A whole-rock rubidium-strontium age determination of tuff from the Triassic Falla Formation, containing Dichroidiurn odontopieroides, indicates a date of 190±9 million years. Five whole-rock specimens collected from the type section located 293 to 414 meters above the base of the Falla Formation on the northwest face of Mt. Falla, Queen Alexandra Range, were analyzed for an age determination by using the rubidium -strontium method. P. J . Barrett collected the samples from his section F-2 (Barrett, 1968). He described these rocks as fine-grained tuffs composed of fresh to slightly devitrified or zeolitized glass shards, and grains of quartz and plagioclase in a matrix with low birefringence which is not optically resolvable. Barrett (1968) reported finding Dicroidiunu odontoptet-oides in a shale bed 135 meters above the base of the Falla Formation, at the type locality. According to Townrow (1967), this fossil occurs elsewhere in rocks of Middle to Upper Triassic age. The samples used in this report were originally analyzed by Hill (1969), who calculated a whole-rock rubidium-strontium isochron date of 203±12 million years, based on four of the five analyzed spçcimens. The only other age determination of the Falla Formation is a whole-rock potassium-argon date of 197.7±2.7 million years for a trachyte pebble collected 280 meters above ANTARCTIC JOURNAL
Analysis of antarctic geophysical data C. R. BENTLEY, H. K. ACHARYA, J . L. CLAPP, W. CLOUGH, H. KOHNEN, and J . D. ROBERTSON J.
Geophysical and Polar Research Center Department of Geology and Geophysics University of Wisconsin, Madison Continuing analysis of antarctic geophysical data follows several lines, including studies of ice properties (as revealed by seismic and electromagnetic wave propagation experiments near Byrd Station), west antarctic gravity maps, Roosevelt Island strain data, and theoretical studies of seismic wave propagation. Appended is a bibliography of papers on these subjects (since Contribution number 299, Geophysical and Polar Research Center, Department of Geology and Geophysics, University of Wisconsin, Madison. September-October 1973
Bentley et al., 1969). What follows is a summary of recent results not yet published. 1. Seismic velocities obtained from short refraction profiles can be used to predict density at depths between o and 10 meters, with a standard error of about 0.01 gm/cm3. 2. A newly recognized and extensive horizon at depths of 25 to 30 meters, marking an apparent change in the densification rate, has been found in West Antarctica. The horizon's existence suggests that two distinct mechanisms successively dominate the metamorphic process between the depth of closest packing of snow grains and the urn-ice boundary. 3. Measurement of P-wave attenuation in ice near Byrd Station led to the determination of a very low value for the internal friction. From comparison with laboratory measurements (Kuroiwa, 1964), it appears that a slight but significant contamination of the antarctic ice by ionic impurities (Gow, 1968) and the ambient ice temperature (-28 0 C.) result in the falling of seismic frequencies at a dissipation minimum between spectral regions dominated respectively by grain-boundary phenomena, and by the fundamental relaxation spectrum. 4. Analysis of electromagnetic wide-angle reflection measurements shows that correction for refraction in the upper portion of an ice sheet need not be made when calculating mean velocities. Even for a reflector as shallow in depth as 100 meters, the error introduced by assuming straight line geometry is only about 10 nanosec, well below the time resolution of the measurements. For accurate measurements, however, the length of wideangle profiles must be limited to distances corresponding to the reflection path for a ray at grazing incidence on the surface. The amplitude of the reflected wave, which changes markedly along the length of a profile, can play an important role in the measurements. 5. Previously existing maps of gravity anomalies in West Antarctica have been supplemented by new data and have been contoured by a computer. Three-dimension al modeling has been used to prepare an Airy isostatic gravity anomaly map. This map reveals several imbalances, which may be caused by: up-warping of the Mdiscontinuity in Ellsworth Land and beneath the HollickKenyon Plateau; dense, lower-crustal material unusually near the surface southwestward from the Whitmore Mountains; the extension beneath the Rockefeller Plateau of a pre-Cretaceous geosyncline known to exist in the Edsel Ford Ranges; the southern boundary of the Cenozoic volcanic province in Marie Byrd Land. A deep negative anomaly of unknown cause exists along the Bakutis Coast. On the hypothesis of a recent retreat of the ice sheet, no more than 40 percent (and probably much less) of the anomaly can be attributed to incomplete isostatic rebound. 6. A Rayleigh wave group-veloiity curve, applicable to the known velocity-depth and density-depth curves in 263
West Antarctica, has been computed with the use of a finite difference technique. Results agree well with observed data. Comparison with calculations based on approximations commonly made in surface wave analyses (Poisson's ratio = 1/4 ; density = constant) surprisingly shows that the group velocities are relatively more sensitive to incorrect densities than to incorrect shear wave velocities. 7. Final strain-rate calculations for a grid network across Roosevelt Island show a strongly asymmetrical profile, with the longitudinal extensional strains twice as great on the northeast as on the southwest flank of the island. Since accumulation rates on the two flanks are about the same the difference in strain rates is probably attributable to the effect of the Ross Ice Shelf.
References
Bentley, C. R., H. K. Acharya, J . E. Beitzel, and J . W. Clough. 1969. Analysis of antarctic geophysical data, 1968-1969. Antarctic Journal of the United States, IV(5) 219. Gow, A. J . 1968. Electrolytic conductivity of snow and glacier ice from Antarctica and Greenland. Journal of Geophysical Research, 73(12): 3643-3649. Kuroiwa, D. 1964. Internal friction of ice. Contributions from the Institute of Low Temperature Science, Hokkaido University. Series A. 18.
Bibliography Acharya, H. K. 1970. Reflection from the free surface of an inhomogeneous media. Bulletin of the Seismological Society of America, 60(4): 1101-1104. Acharya, H. K. 1972. Surface-wave dispersion in Byrd Land, Antarctica. Bulletin of the Seismological Society of America, 62(4): 955-959. Acharya, H. K. In press. Investigation of surface wave dispersion in inhomogeneous media by the finite difference method. Proceedings of the Ninth Annual Symposium on Geophysical Theory and Computer Applications. Bentley, C. R. 1972. Seismic-wave velocities in anisotropic ice: a comparison of measured and calculated values in and around the deep drill hole at Byrd Station, Antarctica. journal of Geophysical Research, 77(23): 4406-4420. Bentley, C. R., 1972. Suglacial rock surface topography of Antarctica. Antarctic Map Folio Series, 16. Bentley, C. R. In press. Crustal structure of Antarctica. Proceedings of IUMC Symposium: crustal structure based on seismic data. Tectono physics. Bentley, C. R., and J . W. Clough. 1971. Electromagnetic sounding of ice thickness In: Propagation Limitations in Remote Sensing (J. B. Lomax, ed.). AGARD Conference Proceedings, North Atlantic Treaty Organization. 90: 18-1-18-7. 264
Bentley, C. R., and J . W. Clough. 1972. Seismic refraction shooting in Ellsworth and Dronning Maud Lands. In: Antarctic Geology and Geophysics (R. J . Adie, ed.). Oslo, Universitetsforlaget. 169-172. Clough, J . W. 1973. Radio-echo sounding: brine percolation layer. Journal of Glaciology, 12(64): 141-143. Kohnen, H. 1971. The relation between seismic urn structure, temperature, and accumulation. Zeitschrift für Gletscherkunde und Glazialgeologie, VII( I-2): 141-151. Kohnen, H. 1972. Uber die beziehung zwischen seismischen geschwindigkeiten und der dichte in firn and eis. Zeitschrift für Geophysik, 38: 925-935. Kohnen, H., and C. R. Bentley. 1973. Seismic refraction and reflection measurements at Byrd Station, Antarctica. Journal of Glaciology, 12(64): 101-111. Kososki, B. A. 1972. A gravity study of West Antarctica. M. S. Thesis, University of Wisconsin. Robertson, J . D. 1972. A seismic study of the structure and metamorphism of 6rn in West Antarctica. M. S. Thesis, University of Wisconsin.
Age of the Falla Formation (Triassic), Queen Alexandra Range G. FAURE and
R. L. HILL
Department of Geology and Mineralogy Institute of Polar Studies The Ohio State University A whole-rock rubidium-strontium age determination of tuff from the Triassic Falla Formation, containing Dichroidiurn odontopieroides, indicates a date of 190±9 million years. Five whole-rock specimens collected from the type section located 293 to 414 meters above the base of the Falla Formation on the northwest face of Mt. Falla, Queen Alexandra Range, were analyzed for an age determination by using the rubidium -strontium method. P. J . Barrett collected the samples from his section F-2 (Barrett, 1968). He described these rocks as fine-grained tuffs composed of fresh to slightly devitrified or zeolitized glass shards, and grains of quartz and plagioclase in a matrix with low birefringence which is not optically resolvable. Barrett (1968) reported finding Dicroidiunu odontoptet-oides in a shale bed 135 meters above the base of the Falla Formation, at the type locality. According to Townrow (1967), this fossil occurs elsewhere in rocks of Middle to Upper Triassic age. The samples used in this report were originally analyzed by Hill (1969), who calculated a whole-rock rubidium-strontium isochron date of 203±12 million years, based on four of the five analyzed spçcimens. The only other age determination of the Falla Formation is a whole-rock potassium-argon date of 197.7±2.7 million years for a trachyte pebble collected 280 meters above ANTARCTIC JOURNAL
