Ice thickness determination at Wilkes

BY

G. A. ALLEN and R. WHITWORTH Bureau of Mineral Resources, Geology and Geophysics, Melbourne

Introduction Aerial photography by the U.S. Navy Antarctic Expedition of 1947-48 was the beginning of detailed mapping in the area of Antarc- tica. Summer surveys of ice-free areas by Australian, Russian, and U.S. parties continued until Wilkes station was set up in in 1956 by the U.S.A. During the IGY, U.S. scientists established a sub- sidiary station called S-2 about 50 miles south-east of Wilkes for glacio- logical purposes. Heights along the trail to S-2 were determined by barometric altimeters. Some gravity observations were made around Wilkes by Sparkman during a gravity tie from McMurdo Sound. A spot ice thickness by radio echo sounder was reported by Waite in 1958 but its locality is uncertain. In 1959, control of the station was transferred to the Australian National Antarctic Research Expedition (ANARE). In 1961 the first seismic ice thickness measurements were made by geophysicists of the Bureau of Mineral Resources (BMR) in co-ordination with ANARE. This survey was along a north-south traverse, and reached 300 miles south of S-2 (Jewell, 1962). It revealed a valley in the ice 40 to 80 miles south of S-2, beneath which the rock plunged to 7000 ft. below sea-level. Jewell suggested that the trough diverted ice flow into the Totten and Quincy Adams-Vanderford Glaciers. A traverse to the south-east of S-2 in 1962 showed that the ice surface to the east of Wilkes was dome-shaped (Walker, 1966). Gravity work on this traverse and on the summer traverse to Vostok suggested that there was a rock ridge along the southern edge of this dome. Bouguer anomaly gradients caused by rock structure were large enough to indicate a major tectonic feature coincident with the trough. A reconnaissance traverse east of S-2 by Kirton in 1963 further outlined the shape of the dome and rock surface in the eastern part of the dome (Kirton, 1965). To summarize, these surveys outlined an ice dome about 100 miles in diameter on the edge of the Antarctic continent. The dome was separated from the continent proper by a trough in the rock that could be a major tectonic feature sub-parallel to the coast, as had been reported elsewhere in (Zhivago, 1962). The ice dome formed a small-scale ice-cap conveniently situated for intensive glaciological and geophysical studies. As a result, ANARE glaciologists commenced a detailed investigation 405 406 ISAGE of ice movement and snow accumulation over the northern sector of the dome in 1964 (Morgan, 1966; McLaren, 1967; Budd, 1968), whilst the BMR began a systematic geophysical survey over the dome and trough.

The systematic survey As the previous surveys had shown the Bouguer anomalies to be greatly influenced by the underlying geology, a grid of seismic stations with spacing of 10 miles was adopted. Gravity readings were made at one-mile intervals between shot points; readings at this spacing were expected to define the rock surface adequately under moderate ice thickness when tied into the seismic control. Barometric readings at one-fifth mile intervals with simultaneous observations every mile were used for height control. The survey formed a rectangular grid of traverses between shot-points. In this way uncertainties in the Worden gravity meter drift and barometric heighting were considerably reduced after adjustment of the network of observations by least squares. The grid method proved very effective for the gravity observations. For example, on the spring traverse of 1964, the standard deviation of the adjusted intervals between shot points was 0-08 milligals, despite a very erratic meter drift. The accuracy of the barometric levelling was an order of magnitude lower by comparison. During the same traverse, the barometer pair difference was fairly consistent with a standard deviation of variation equivalent to 6 ft. Yet the standard deviation of the adjusted 10-mile traverses was 27 ft, an error several times larger than expected. The major cause of error appears to be local atmospheric fluctuations such as temperature inversions. Partly as a result of these errors, it has proved difficult to combine the previous surveys and glaciological traverse observations with the grid survey. Differences of up to 160 ft have occurred at traverse inter- sections. Ad hoc adjustments to the heights on other surveys were made to make the results compatible with the grid survey. After adjustment of the heights, differences in Bouguer anomaly values at the intersections were found. These are thought to be caused by navigational errors in an area of rapid horizontal changes in gravity.

Results Heights (Fig. 1). The ice dome forms a fairly symmetrical feature separated from the main Antarctic plateau by a valley that lies between the Totten and Vanderford Glaciers. Several minor valleys run into the main valley. The symmetry of the dome may be due in part to the sparsity of data in the east and north-west. Small waves of a few tens of feet occur in the ice surface along the southern flank of the dome. In the south- west, the waves are sufficiently intense to cause tensional crevassing. The waves probably reflect underlying rock ridges rather than pressure waves in the ice. MASS BUDGETS : REGIONAL STUDIES 407

CAK POINSETT

FIG. 1. Ice surface topography.

Free-air anomalies (Fig. 2). If isostatic compensation occurs, the free- air anomalies would be expected to average zero. Compensation of a feature appears to occur regionally over an area of 250 km across. It is therefore marginal whether a body of the size of the ice dome would be compensated, or whether the strength of the crust would bear the load. The rugged rock surface and intense geological gravity anomalies make the free-air anomalies complex. This complexity, plus the present lack of data in some areas makes it impossible to determine the degree of compensation reliably. Ice Bouguer anomaly results (Fig. 3). Over most of the dome, the Bouguer anomaly features are broad and shallow, with an easterly trending "high" in the north and a "low" with indistinct trend in the S-2 area. Localized features occur over rock ridges under the fairly thin ice cover. An arcuate gravity ridge marks the southern edge of the dome area. The northern boundary is not very marked, but the southern edge exhibits extremely high gradients, particularly in the west where gradients exceed 20 milligals/mile. The areas of maximum gradient form a linear feature aligned with the northern edge of the valley. 408 ISAGE

LEGEND

u~Ll • 'Hlfh' anomaly 'Low' anomaly Contour Intervall 10 mllllpls Free air anomalies In mllllgali

STATUTE MILES

FIG. 2. Free air anomalies.

South of the dome there is an intense, complex Bouguer anomaly "low"; in the west its trend is north-westerly, but it swings round to an east-west trend towards the . Seismic shooting. It is useful to digress at this point to mention certain aspects of the seismic work. Shallow 3- to 6-ft holes were used almost exclusively. The corrections for the uppermost low-velocity layers were obtained using a power-law function, of the type V(z) = azb for the velocity depth relationship (Faust, 1951). The applicability of this func- tion to the seismic time-distance plots is of some interest as it implies that considerable anisotropy in velocity may therefore occur (Gassman, 1951 ; White and Sengbush, 1953). A maximum velocity of 12,500 ft/sec was determined by several refraction profiles in the survey area. MASS BUDGETS : REGIONAL STUDIES 409

_ _| Hljh anomaly r —i "Low' anomaly lur Inccrvil 10 mllllgals Bou|uer anomaly In mllllgalt

FIG. 3. Ice Bouguer anomalies.

Difficulty was experienced in identifying the ice/bedrock reflection on some seismic records. Instead of a single discrete reflection event, often a zone or zones of events were recorded. Possibly these are reflections from several areas of a rough bedrock surface, or multiple reflections from a thick basal moraine layer. As we had no definite evidence to the contrary, the beginning of the first event was taken as the bedrock reflec- tion. Ice thickness measurements uncorrected for dip were used to control the gravity reductions. Rock Bouguer anomalies (Fig. 4). The Bouguer anomalies caused by geological structure were derived from the ice Bouguer anomaly at each seismic shot—point by correcting for the thickness of ice. The formula 410 ISAGE

FIG. 4. Rock Bouguer anomalies. used was: RBA = IBA — hr (pr — pi), where RBA = rock Bouguer anomaly, IBA = ice Bouguer anomaly, hr = thickness of rock above or below sea level, pr = density of rock and pi = density of ice. The total range in rock Bouguer anomalies exceeded 120 milligals, and gradients as high as 7 milligals/mile were detected. The major features are the broad "low" over most of the dome area, which may extend as far as the coast in the east, and the complicated pattern of "highs" and "lows" in the south. The Bouguer anomaly "highs" generally correspond with considerable depressions in the rock surface suggesting that these areas may be underlain by corresponding rises in the base of the crust. MASS BUDGETS : REGIONAL STUDIES 411

Rock elevation In feet above teilevel Value» bated on Selimlc lutlom only

FIG. 5. Rock topography.

Rock topography (Fig. 5). The application of corrections for geology greatly alters the picture of the rock topography given by the ice Bouguer anomaly results. For example, within a distance of ten miles the ice thickness estimated using gravity in 1962 differs by up to 2000 ft from the thickness measured by seismic shooting in 1965. Therefore only rock height estimates tied directly into seismic shot-points have been computed. The trends of the smaller features have been defined using field observa- tions of ice ridges and crevassing where this information has been available. Parallelism between the rock surface and rock Bouguer anomaly contours is evident. Several subsidiary peaks rise to 1500 ft above sea level. The southern ridge may possibly continue to the north-east; if so, the ridge would become an almost complete ring structure. 412 ISAGE

FIG. 6. Preliminary geological interpretation. The most prominent feature in the south is the deep trench plunging to 8000 ft below sea level. It trends north-west towards the mouth of the Vanderford Glacier, where a sea bottom depression of similar shape and depth has been observed. The northern side of the trench is steep along its entire length, and a dip of 60° has been determined by seismic means in this zone (Walker, 1966). In the east of the survey area the trough continues with an easterly trend, but at a shallower level. Another depression flanks the trough in the west and a minor valley runs into the main trench from the north.

Conclusions The major factor in the development of the ice dome appears to have been the deep trough, [t is probably deep enough to cause ice flowing off the main plateau to be channelled around the dome. Thus the dome MASS BUDGETS : REGIONAL STUDIES 413 may be an isolated feature that has built up as a result of local snow accumulation, its shape controlled by ice flow dynamics. If this is so, it should prove possible to set up a mathematical model simulating the genesis and growth of the ice dome to its present form (vide Campbell and Rasmussen, 1968). Variation in rate of ice flow on the dome would presumably be con- trolled by irregularities in the rock surface. Along the coast between Cape Folger and Cape Poinsett there are several topographic "lows" which would accentuate flow northwards. The ice cape at Cape Poinsett does not appear to be associated with a valley in the rock surface, though data are sparse in the area. In the S-2 area, an easterly-trending valley would tend to divert ice flow towards the east where a glacier does exist, but there is no definite evidence to connect the two at the moment. The valley on the northern side of the trench may channel ice towards the Vanderford valley. As mentioned previously, the rock surface parallels the rock Bouguer anomaly features, so that the rock surface appears to be structurally controlled. Metamorphic and igneous provinces have been distinguished in the northern and southern parts of the by Robertson (1959). The dissimilarity in rock Bouguer anomaly pattern in the dome and trough areas suggests that this division could continue inland. The rock Bouguer anomaly "low" and associated peripheral rock ridges in the north suggest a structural "low". The "low" has an east-west trend and possibly plunges to the east. The linearity and steep slope of the south-western flank of the dome together with the high Bouguer anomaly gradients indicate a fault downthrown to the south-west. The downthrown block would form the trough of the Vanderford Glacier, suggesting that the total throw could be as high as 6000 ft. The Bouguer anomaly "lows" in the trough area may correspond to intruded granites. The fact that the positive Bouguer anomaly associated with the depressed areas is greater than would be expected if no isostatic compensation had occurred, suggests that partial compensation may exist in the region of the trough. The trough could be the local equivalent of an extensive offshore faulted trench mapped further west by Russian scientists (Stroev and Frolov, 1967). Hence the dome area could form an anomalous structural feature "offshore" from the Antarctic continent. The present programme of continuous profiling with the radio echo sounder should provide improved ice thickness measurements which in turn should allow a better definition of the rock Bouguer anomaly features to be made. If this work were supplemented by continuous magnetic profiling it would allow a more sophisticated geological inter- pretation to be made. Completion of the systematic survey will not only provide the glaciologists with detailed maps of the ice surface and sub-ice features but will enable computation of the load on the crust, assessment of the degree of isostatic compensation, and possibly a determination of the strength of the crust. 414 ISAGE A cknowledgements The authors wish to thank the Director of the Bureau of Mineral Resources, Geology and Geophysics, for permission to present this paper, and the Acting Director of the Antarctic Division, Department of Supply, for the opportunity to carry out the work.

REFERENCES BUDD, W. T. 1970. The Wilkes Ice Cap Project. In: Gow et al., Eds. Inter- national Symposium on Antarctic Exploration (IS AGE), Hanover, New Hampshire, USA, 3-7 September, 1968. Cambridge (Pub. No. 8607 IASH), p. 414-29. CAMPBELL, W. J. and RASMUSSEN, L. A. 1970. An heuristic humerical model for three dimensional time-dependent glacier flow. In: Gow et al., Eds. Inter- national Symposium on Antarctic Exploration {ISAGE), Hanover, New Hampshire, USA, 3-7 September, 1968. Cambridge (Pub. No. 8607 IASH) p. 177-90. FAUST, L. Y. 1951. Seismic velocity as a function of depth and geological time. Geophysics, Vol. 16, p. 192-206. GASSMAN, F. 1951. Elastic waves through a packing of spheres. Geophysics, Vol. 16, p. 673-85. JEWELL, F. 1962. Wilkes Ice Thickness Measurements, Antarctica 1961. Bur. Min. Resour. Atist. Record No. 1962/162 (unpubl.). KIRTON, M. 1965. Wilkes Geophysical Surveys, Antarctica 1963. Bur. Min. Resour. Aust. Record No. 1965/24 (unpubl.). MCLAREN, A. 1967. Wilkes 1965, Glaciological Programme, M.Sc. Thesis, Melbourne University (unpubl.). MORGAN, P. J. 1966. Wilkes 1964, Glaciological Programme, M.Sc. Thesis Melbourne University (unpubl.). STROEV, P. A. and FROLOV, A. L. 1967. On the sublatitudinal fracture in the Earth's crust at the edge of the Eastern Antarctic. Physics of the Solid Earth, No. 5, 1967, p. 319-22. WALKER, D. J. 1966. Wilkes Geophysical Surveys, Antarctica, 1962. Bur. Min. Resour. Aust. Record No. 1966/129 (unpubl.). WHITE, J. E. and SENGBUSH, R. L. 1953. Velocity measurements in near surface formations. Geophysics, Vol. 18, p. 54-69. ZHIVAGO, A. V. 1962. Outlines of Southern Ocean geomorphology. In: Antarctic Research. Wexler, H. et al. eds. Washington, D.C., American Geophysical Union, p. 74-80.

The Wilkes Ice Cap Project BY W. F. BUDD Antarctic Division, Department of Supply, Melbourne

ABSTRACT The Wilkes ice cap is about 200 km in diameter and rises to 1390 m at its centre. The first phase of a long-term project to determine its dynamics has now been completed and the major results are presented. Surface velocities and longitudinal strain rates have been measured by repeated tellurometer traverses around a large northern sector and a small southern sector. Associated measurements include a series of strain rosettes along the triangle sides yielding lateral strain rates. Seismic and gravity surveys over the region, supplemented in 1967 and 1968 by radio echo soundings, have been carried out to determine the ice thickness which