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Victoria University Antarctic Research Expedition Science and Logistics Reports 1979-80: VUWAE 24

Sea Ice Gravity Survey (BAS)

Sea Ice Gravity Survey (BAS)

Gravity was observed with a Worden gravity meter at 2 1/2 km intervals along all profiles except that at Cape Armitage (Fig. 10). All observations on the sea ice were repeated and the standard error of observation found to be 0.4 mgals. The positions of stations were determined by starting profiles at known locations and setting a course directly toward another known feature. Distances were measured by metre wheel. Precise surveys were subsequently made of the entire Strand Profile and of three beacons in the Ferrar Profile including one at the snout of the Ferrar Glacier. In all cases station positions are known to better than ± 200 m. Elevations on the sea ice are known to within 0.1 m. The total error in computing Bouguer Anomalies is less than 0.7 mgals.

Bouguer Anomalies computed for a crustal density of 2.67 gm/cc and water density of 1.00gm/cc were used to construct Figure 10, a Bouguer Anomaly map of McMurdo Sound Dry Valley region. The map includes additional data from land based surveys discussed later (Event 13). Bouguer Anomaly profiles along the lines of observation are given in Figure 11. The main feature is steep positive eastward gradient across the Victoria Land Coast previously described by Bull (1962) and Smithson (1972). Smithson attributed his results to crustal thinning under McMurdo Sound together with a +0.2 gm/cc intrusion at a depth of 4 km in basement under the Sound.

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Our survey also shows a 15 mgal anomaly in the reverse sense to the main gradient and not found in the previous surveys. The anomaly has a wavelength of 5 km and is centred 10 km east of the Victoria Land Coast. Preliminary models show that it is consistent with a vertical 0.4 gm/cc density discontinuity having a throw of about 1 km and mid depth of about 1 km, the feature could be a basement fault down thrown to the east.

Figure 10 Preliminary Bouguer Anomaly Map of western McMurdo Sound with gravity and bathymetry profile lines shown.

Figure 10 Preliminary Bouguer Anomaly Map of western McMurdo Sound with gravity and bathymetry profile lines shown.

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Figure 11 Bouguer anomalies for profile lines shown in Figure 10.

Figure 11 Bouguer anomalies for profile lines shown in Figure 10.