Pyroxene compositional trends in the Dufek Intrusion, Pensacola ...
kilometers to the east, must have risen at least 2000 and 2100 meters since their formation, if they were formed entirely beneath ice level (as evidenced by the palagonite breccias). Mount Siple, 400 kilometers northwest of Toney Mountain, may have risen as much as 3000 meters. These displacements appear to have been differential, and related to movements along late Cenozoic block faults (LeMasurier, 1972a, 1972b), rather than to glacio-isostatic rebound. Hollin (1962, 1968) considered the maximum increases in Quaternary ice level that may be used as a second limiting case. In his 1962 paper, Hollin presented a profile suggesting that the ice level may have been about 1000 meters higher than today at Mount Murphy (located very near the present grounding line), and roughly 250 meters higher than today at Mount Takahe (about 175 kilometers inland), during the Würm maximum glaciation. In Hollin's 1968 paper, however, it was noted that these increases in ice level probably were overestimated because the effects of isostatic loading (which would depress the bed of the glacier) were underestimated. Thus, if an ice level increase of 250 meters is considered an overestimate at Mount Takahe, it seems reasonable to estimate that approximately 2000 meters of uplift occurred since the formation of Mount Takahe. It therefore appears that Mount Takahe has been rising at an average rate of about 1 centimeter per year (or 36 feet per 1000 years), if one assumes essentially a maximum age for the volcano (200,000 years). This rate favorably compares with uplift rates in active orogenic belts (Schumm, 1963). Similar rates may have characterized the uplift of Mount Murphy and Mount Siple, because glacial erosion in Marie Byrd Land appears to depend on the exposure of rock above ice level, and the glacial dissection of these two mountains is much more advanced than that of Mount Takahe (Andrews and LeMasurier, 1973). Marie Byrd Land appears to be part of a large tectonic province characterized by late Cenozoic block faulting and basalt—trachyte volcanism that extends from the Ross Sea region, through Marie Byrd Land to the Antarctic Peninsula. Webb (1972) reviews evidence for block faulting in the McMurdo Sound region and presents evidence for a minimum of 262 meters of uplift in Wright Valley within the last 3.4 million years. Evidence from Mount Takahe suggests that tectonic activity in this province may be more recent than formerly thought and, in some places, may be continuing today. If this is true, then the relative aseismicity of Antarctica is more puzzling than ever. The explanation that Antarctica is aseismic and tectonically dormant because it lies entirely within a lithospheric plate is not very satisfying from a geologic point of view. This work is supported by National Science Foundation grant 2 5 328. 260
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References Andrew's, J . T., and W. E. LeMasurier. 1973. Rates of Quarternary erosion and corrie formation, Marie Byrd Land, Antarctica. Geology, 1: 75-80. Hollin, J . T. 1962. On the glacial history of Antarctica. Journal of Glaciology, 4: 173-195. Hollin, J . T. 1968. The antarctic ice sheet and the Quaternary history of Antarctica. In: Pilaeoecology of Africa and Antarctica (E. M. van Zinderen Bakker, ed.). Capetown, A. A. Balkema, 5: 109-138. LeMasurier, W. E. 1972a. Volcanic record of Cenozoic glacial history of Marie Byrd Land. In: Antarctic Geology and Geophysics (R. J . Adie, ed.). Oslo, Universitetsforlaget. 251-260. LeMasurier, W. E. 1972b. Volcanic record of antarctic glacial history: implications with regard to Cenozoic sea levels. In: Polar Geomorphology (R. J . Price and D. E. Sugden, compilers). Institute of British Geographers. Special Publication, 4:59-74 LeMasurier, W. E. 1972c. Marie Byrd Land Quaternary volcanism: Byrd ice core correlations and possible climatic influences. Antarctic Journal of the U.S., Vu(S) : 139-141. Schurnrn, S. A. 1963. The disparity between present rates of denudation and orogeny. U.S. Geological Suriey. Professional Paper, 454-H.
Pyroxene compositional trends in the Dufek Intrusion, Pensacola Mountains G. R. HIMMELBERG and A. B. FORD Department of Geology, University of Missouri, Columbia U.S. Geological Survey, Menlo Pat-k, California The Dufek intrusion in the northern Pensacola Mountains is a stratiform mafic complex with many similarities in primary structure and stratigraphy to the Stillwater and Bushveld complexes of Montana and South Africa, as well as to the much smaller Skaergaard intrusion of east Greenland. The layered gabbros are exposed in two partial, non-overlapping stratigraphic sections, one in the Dufek Massif and the other in the Forrestal Range. Each section is nearly 2 kilometers thick. The intrusion, described in Ford and Boyd (1968) and Ford (1970), consists dominantly of gabbro interlayed with minor anorthosite and pyroxenite and capped concordantly by granophyre. Ford (1970) demonstrated that the most pronounced whole-rock chemical trend, stratigraphically ascending in the layered series of gabbros, is progressive ironenrichment. We recently initiated mineralogical and chemical studies of coexisting Ca-rich and Ca-poor pyroxenes, the principal mafic minerals in the complex, ANTARCTIC JOURNAL
I Figure 1 Trend of crystallization of Ca-rich and Capoor pyroxenes from the lower part of the Dufek intrusion. Open circles are intercumulus pyroxenes, closed circles are cumulus pyroxenes. Because of crowding, tie lines have not been shown for all pairs. The Skaergaard (solid lines) and Palisade (broken lines) trends are shown for comparison.
Ca
P__-
Mg
in order to further elucidate the crystallization history and differentiation trends of the intrusion. This progress report summarizes compositional trends of the pyroxenes thus far analyzed. Fifteen of the samples are from the lower partial section, Dufek Massif, and one is from the lowermost exposed gabbro in the upper section in the Forrestal Range. Analyses were made with an AR L E MX-SM electron microprobe. Both the Ca-rich and Ca-poor pyroxenes have abundant exsolved Ca-poor and Ca-rich pyroxene, respectively. In most of the Ca-poor pyroxene hosts the exsolved phase occurs as lamellae and irregularly distributed blebs. In the Ca-rich pyr?xene hosts the exsolved phase occurs as lamellae. In order to determine as close as possible the bulk composition for the pyroxenes, two grains each of Ca-poor and Ca-rich pyroxene in each sample were traversed with adjacent analytical areas by using a 20 to 25 micron diameter beam. The data presented in this report are based on averages of the two grains. Differences in major oxide content between the two analyzed grains rarely exceed ± 3 percent of the amount present. The standards were minerals and synthetic glasses of known composition. Corrections were made for the background, mass absorption, secondary fluorescence, and atomic number. Iron values are total iron computed as Fe2+. The variations, with fractionation, of some of the major cations (iron, magnesium, and calcium) are shown in the conventional pyroxene quadrilateral (fig. 1). Those pyroxenes plotted with open circles are intercumulus and occur in anorthosite from the lowest exposed rocks of the Dufek Massif. All the other plotted pyroxenes are cumulus in origin (they accumulated on the chamber floor by settling from the melt). The most Mgrich and Ca-poor pyroxene analyzed probably crystallized as a primary hypersthene. The twinning, character of exsolution textures and compositions of all the other Ca-poor pyroxenes, lead us to interpret them as hyperSeptember-October 1973
atomic %
Fe
sthene inverted from pigeonite (Hess, 1941; Poldervaart and Hess, 1951; Brown, 1957). The Ca-rich pyroxenes are augite. The coexisting pyroxenes show a general iron-enrichment, with fractionation. Details of this trend are illustrated by the plot of the cation ratio Fe/(Fe + Mg) versus stratigraphic height (fig. 2). Through a cumulative thickness of approximately 1000 meters, between about the 500-meter and 1500-meter levels, the iron enrichment is small and accompanied by minor reversals in the cation ratio of Fe/(Fe + Mg). Ford (1970) demonstrated a similar trend in the gabbros for the variation in whole-rock mafic index, (FeO + Fe203/ FeO + Fe 2O + MgO) X 100, which he interpreted as being related to chemical changes accompanying convective activity. The mafic-index trend line is approximately parallel to the pyroxenes' trend line below the top 500 meters of the Dufek Massif section—that is, below the level at which cumulus magnetite first appears. Minor reversals in pyroxene composition in the Stillwater Complex also were interpreted as being related to convective overturn (Hess, 1960). Jackson (1970) and Page et al. (1972) have shown that even within individual cyclic units of the Stillwater Complex cumulus minerals can vary widely in composition. Page et al (1972) ascribe the variations to oscillatory or cyclic processes within a single. magma batch. Comparison with pyroxene trends of other large stratiform intrusions suggests that the pyroxenes from the Dufek Massif are more like those from the Skaergaard intrusion (Brown and Vincent, 1963) than like those from the Bushveld complex (Atkins, 1969), with respect to the miscibility gap's extent. Striking similarities also exist when compared to the trend established for the Palisades Sill pyroxenes (Walker et al., 1973). More Mg-rich pyroxenes are expected to occur in the unexposed basal layers of the Dufek intrusion. Similarily, the slight compositional gap between the
261
6-
granophyre /
'C
o'c
90
80
Forrestol Range sect/on
layered gabbro
5
.E
A
(concealed)
-c G) -C
"t. ,71•
C.)
/ 70 4/s
-c ci 0
CP 0
)
layered gabbro
/ AAS5 AAss As
5.4-
(I)
As AS AA
Dufek
Massif sect/on
I
50 -' anorthosi te 0A 02
augite o hypersthene
0.6 04 Cation ratio Fe/(Fe+Mg)
0.8
Figure 2. Variation of pyroxene cation ratio Fe! (Fe + Mg) with stratigraphic height. Open symbols are intercumulus pyroxenes, closed symbols are cumulus pyroxenes. The broken line shows the approximate trend of mafic index in gabbroic rocks and granophyre (from Ford, 970). Numbers indicate numerical value of maflc index.
pyroxenes from the uppermost part of the Dufek Massif study, with assistance from National Science Foundaand those from the lowermost exposed gabbros of the tion grant GA-18445. Forrestal Range probably is because of concealment of an intermediate section of the complex. We expect that the iron-enrichment trend of the pyroxenes will be extended when analyses of higher level pyroxenes are References completed. This work is supported by National Science Foundation grant AG-238. The University of Missouri pur- Atkins, F. B. 1969. Pyroxenes of the Bushveld Intrusion chased the electron microprobe that was used for this South Africa. Journal of Petrology, 10: 222-249. 262
ANTARCTIC JOURNAL
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
GV-
References Andrew's, J . T., and W. E. LeMasurier. 1973. Rates of Quarternary erosion and corrie formation, Marie Byrd Land, Antarctica. Geology, 1: 75-80. Hollin, J . T. 1962. On the glacial history of Antarctica. Journal of Glaciology, 4: 173-195. Hollin, J . T. 1968. The antarctic ice sheet and the Quaternary history of Antarctica. In: Pilaeoecology of Africa and Antarctica (E. M. van Zinderen Bakker, ed.). Capetown, A. A. Balkema, 5: 109-138. LeMasurier, W. E. 1972a. Volcanic record of Cenozoic glacial history of Marie Byrd Land. In: Antarctic Geology and Geophysics (R. J . Adie, ed.). Oslo, Universitetsforlaget. 251-260. LeMasurier, W. E. 1972b. Volcanic record of antarctic glacial history: implications with regard to Cenozoic sea levels. In: Polar Geomorphology (R. J . Price and D. E. Sugden, compilers). Institute of British Geographers. Special Publication, 4:59-74 LeMasurier, W. E. 1972c. Marie Byrd Land Quaternary volcanism: Byrd ice core correlations and possible climatic influences. Antarctic Journal of the U.S., Vu(S) : 139-141. Schurnrn, S. A. 1963. The disparity between present rates of denudation and orogeny. U.S. Geological Suriey. Professional Paper, 454-H.
Pyroxene compositional trends in the Dufek Intrusion, Pensacola Mountains G. R. HIMMELBERG and A. B. FORD Department of Geology, University of Missouri, Columbia U.S. Geological Survey, Menlo Pat-k, California The Dufek intrusion in the northern Pensacola Mountains is a stratiform mafic complex with many similarities in primary structure and stratigraphy to the Stillwater and Bushveld complexes of Montana and South Africa, as well as to the much smaller Skaergaard intrusion of east Greenland. The layered gabbros are exposed in two partial, non-overlapping stratigraphic sections, one in the Dufek Massif and the other in the Forrestal Range. Each section is nearly 2 kilometers thick. The intrusion, described in Ford and Boyd (1968) and Ford (1970), consists dominantly of gabbro interlayed with minor anorthosite and pyroxenite and capped concordantly by granophyre. Ford (1970) demonstrated that the most pronounced whole-rock chemical trend, stratigraphically ascending in the layered series of gabbros, is progressive ironenrichment. We recently initiated mineralogical and chemical studies of coexisting Ca-rich and Ca-poor pyroxenes, the principal mafic minerals in the complex, ANTARCTIC JOURNAL
I Figure 1 Trend of crystallization of Ca-rich and Capoor pyroxenes from the lower part of the Dufek intrusion. Open circles are intercumulus pyroxenes, closed circles are cumulus pyroxenes. Because of crowding, tie lines have not been shown for all pairs. The Skaergaard (solid lines) and Palisade (broken lines) trends are shown for comparison.
Ca
P__-
Mg
in order to further elucidate the crystallization history and differentiation trends of the intrusion. This progress report summarizes compositional trends of the pyroxenes thus far analyzed. Fifteen of the samples are from the lower partial section, Dufek Massif, and one is from the lowermost exposed gabbro in the upper section in the Forrestal Range. Analyses were made with an AR L E MX-SM electron microprobe. Both the Ca-rich and Ca-poor pyroxenes have abundant exsolved Ca-poor and Ca-rich pyroxene, respectively. In most of the Ca-poor pyroxene hosts the exsolved phase occurs as lamellae and irregularly distributed blebs. In the Ca-rich pyr?xene hosts the exsolved phase occurs as lamellae. In order to determine as close as possible the bulk composition for the pyroxenes, two grains each of Ca-poor and Ca-rich pyroxene in each sample were traversed with adjacent analytical areas by using a 20 to 25 micron diameter beam. The data presented in this report are based on averages of the two grains. Differences in major oxide content between the two analyzed grains rarely exceed ± 3 percent of the amount present. The standards were minerals and synthetic glasses of known composition. Corrections were made for the background, mass absorption, secondary fluorescence, and atomic number. Iron values are total iron computed as Fe2+. The variations, with fractionation, of some of the major cations (iron, magnesium, and calcium) are shown in the conventional pyroxene quadrilateral (fig. 1). Those pyroxenes plotted with open circles are intercumulus and occur in anorthosite from the lowest exposed rocks of the Dufek Massif. All the other plotted pyroxenes are cumulus in origin (they accumulated on the chamber floor by settling from the melt). The most Mgrich and Ca-poor pyroxene analyzed probably crystallized as a primary hypersthene. The twinning, character of exsolution textures and compositions of all the other Ca-poor pyroxenes, lead us to interpret them as hyperSeptember-October 1973
atomic %
Fe
sthene inverted from pigeonite (Hess, 1941; Poldervaart and Hess, 1951; Brown, 1957). The Ca-rich pyroxenes are augite. The coexisting pyroxenes show a general iron-enrichment, with fractionation. Details of this trend are illustrated by the plot of the cation ratio Fe/(Fe + Mg) versus stratigraphic height (fig. 2). Through a cumulative thickness of approximately 1000 meters, between about the 500-meter and 1500-meter levels, the iron enrichment is small and accompanied by minor reversals in the cation ratio of Fe/(Fe + Mg). Ford (1970) demonstrated a similar trend in the gabbros for the variation in whole-rock mafic index, (FeO + Fe203/ FeO + Fe 2O + MgO) X 100, which he interpreted as being related to chemical changes accompanying convective activity. The mafic-index trend line is approximately parallel to the pyroxenes' trend line below the top 500 meters of the Dufek Massif section—that is, below the level at which cumulus magnetite first appears. Minor reversals in pyroxene composition in the Stillwater Complex also were interpreted as being related to convective overturn (Hess, 1960). Jackson (1970) and Page et al. (1972) have shown that even within individual cyclic units of the Stillwater Complex cumulus minerals can vary widely in composition. Page et al (1972) ascribe the variations to oscillatory or cyclic processes within a single. magma batch. Comparison with pyroxene trends of other large stratiform intrusions suggests that the pyroxenes from the Dufek Massif are more like those from the Skaergaard intrusion (Brown and Vincent, 1963) than like those from the Bushveld complex (Atkins, 1969), with respect to the miscibility gap's extent. Striking similarities also exist when compared to the trend established for the Palisades Sill pyroxenes (Walker et al., 1973). More Mg-rich pyroxenes are expected to occur in the unexposed basal layers of the Dufek intrusion. Similarily, the slight compositional gap between the
261
6-
granophyre /
'C
o'c
90
80
Forrestol Range sect/on
layered gabbro
5
.E
A
(concealed)
-c G) -C
"t. ,71•
C.)
/ 70 4/s
-c ci 0
CP 0
)
layered gabbro
/ AAS5 AAss As
5.4-
(I)
As AS AA
Dufek
Massif sect/on
I
50 -' anorthosi te 0A 02
augite o hypersthene
0.6 04 Cation ratio Fe/(Fe+Mg)
0.8
Figure 2. Variation of pyroxene cation ratio Fe! (Fe + Mg) with stratigraphic height. Open symbols are intercumulus pyroxenes, closed symbols are cumulus pyroxenes. The broken line shows the approximate trend of mafic index in gabbroic rocks and granophyre (from Ford, 970). Numbers indicate numerical value of maflc index.
pyroxenes from the uppermost part of the Dufek Massif study, with assistance from National Science Foundaand those from the lowermost exposed gabbros of the tion grant GA-18445. Forrestal Range probably is because of concealment of an intermediate section of the complex. We expect that the iron-enrichment trend of the pyroxenes will be extended when analyses of higher level pyroxenes are References completed. This work is supported by National Science Foundation grant AG-238. The University of Missouri pur- Atkins, F. B. 1969. Pyroxenes of the Bushveld Intrusion chased the electron microprobe that was used for this South Africa. Journal of Petrology, 10: 222-249. 262
ANTARCTIC JOURNAL
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
