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Report on the Victoria University of Wellington Antarctic Expedition 1960-61: VUWAE 4



Bouguer Anomaly Map

In Figure 1 the values of Bouguer anomaly at a representative number of gravity stations are plotted, together with the isogals obtained by considering all of the station values. The Bouguer anomaly decreases from a value of 0 mgals in the northeast to −90 mgals at the extreme western point of the survey. A full consideration of the results has not yet been made but the following points may be significant.

1.In the northeast part of the area the isogals are approximately north-south, as they are in the Wright Valley further north. This probably indicates that if the coastline is fault controlled, that the fault continues southwards from Cape Chocolate and does not swing southwesterly to follow the northern side of the Koettlitz Glacier.
2.The gravity gradient in the north-east part of the area is smaller than the 10 mgals/mile which occurs at the coastal end of the Wright Valley, and which has been interpreted in accord with the existence of a major fault along the coastline.page 11
3.A gravity graidnet of about 10 mgals/mile does occur a few miles to the west, between the mouth of the Marshall Valley and the head of Miers Valley. This large gravity gradient is not associated with any major fault.
4.Heald Island appears to be gravitationally associated with the area on the mainland immediately west of it. It is unlikely that the northern side of the Koettlitz Valley is fault-controlled.
5.Values of Bouguer anomaly are significantly higher near the Pyramid, which was one of the main centres of recent volcanic activity in the area.
6.The overall east-west gradient is approximately 2 mgals/mile. This is a value similar to that found in traverses from the lowlands of Austria or Italy into the regionally isostatically compensated Alps.
7.In the western end of the Wright Valley the gravity gradient changes sign, and higher values of Bouguer anomaly are found to the west. This probably indicates that the western side of the Victoria Land mountain chain is also fault controlled and that the formation is a horst. No similar change in the gravity gradient is found in the present survey; values of the Bouguer anomaly continue to decrease westwards. It is regretted that time was not available to continue the survey into the Royal Society Range.

Glacier Thicknesses

The route taken in this gravity traverse included the crossing of three glaciers. From the measurements of gravity on the glacier approximate estimates can be made of the thickness of ice under the gravity staton. The method has been fully described elsewhere (C. Bull and J. R. Hardy, “Gravity Measurements on a Glacier,” Journal of Glaciology, March 1957). Values of the Bouguer anomaly obtained on rock at the sides of the glacier are interpolated across rhe glacier, to determine the value of the Bouguer anomaly which would occur at the gravity stations on the glacier, if the ice of the glacier (density about 0.9 gm.cm−3) were replaced by “standard” rock of density 2.67 gm.cm−3. In fact the values of gravity on the glacier ore less than those required to give the values of Bouguer anomaly obtained by interpolation in this way; the deficit is attributed directly to the replacement of “standard” rock by the much less dense ice.

In the three glaciers measured by this method the Bouguer anomaly expected at the glacier stations (assuming that all of the glacier ice were replaced by rock) can be interpolated with some confidence to an accuracy of 2 milligals. The maximum gravity deficiencies on the three Glaciers are 25 mgals (Koettlitz Glacier), 12 mgals (Radian Glacier) and 16 mgals (Hobbs Glacier) so that the error in the estimate of thickness should not exceed 20 per cent.

The three profiles are given in Figure 2. At the position of the profile the Hobbs is a noticeably underfit glacier flowing from a deep valley into a lowland region. In the profile of the Radian Glacier there is some suggestion that the valley is a pair of parallel troughs. Since the glacier is fed by two approximately equal size tributaries this is not completely unexpected, but it is unusual for the two tributaries to maintain their identity so far down the glacier. A similar feature has been observed on a Norwegian glacier by the writer, but only within a short distance of the confluence of the two tributaries (C. Bull and J. R. Hardy, loc. cit.). The southern arm of the Koettlitz Glacier, between Heald Island and the Pyramid area, is a slow-moving or dead relic of a much larger glacier fed from the inland ice of eastern Antarctica. However, at the position of the traverse its thickness exceeds 1,100 feet and the sub-glacial surface is below sea level. The glacier volley is appreciably deeper on the northern side, the outer side of the curve of the arm of the glacier as it swings northward.

These profiles of the glaciers and their significance will be discussed further in the Report on the Glaciological work of the Expedition.

Note: The calculations of the values of gravity and all the attendant corrections have not yet been checked. These preliminary results and considerations may not remain unaltered in the light of recalculations and more mature judgement.

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