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Results of a seismic and gravity study at the SouthPole during the 1961-62 austral summer are presented. Seismic compressional waves in the ice at the South Pole are shown to reach a maximum velocity of 3,925 m/xc at a depth of 186 m. This depth is attributed to the density profile of the ice. An ice thicknessof 2,900 m lsindicated from seismic reflection shooting. Gravity data reveal flat topography underlying the ice in the vicinity of the South Pole.

An unusual assemblage of geological and geophysical anomalies in Wilkes Land, Antarctica, suggests that these features may owe their origin to hypervelocity impact by an extraterrestrial body. The evidence which supports this conclusion includes a 158.3-mGal negative free air gravity anomaly, apparent structural deformation, basin-shaped profiles in the ice sheet and in the subglacial topography, australite distribution, the probable geologic recency of the features, the presence of rim structures similar to terrestrial, lunar, Martian, and Mercurian impact crater rims, lack of isostatic equilibrium, and the excellent agreement between the morphologic dimensions of observed features and the morphologic dimensions of terrestrial, lunar, Martian, and Mercurian impact craters, namely, crater rim widths, crater rim heights, crater depths, and crater diameters. This evidence reveals a subglacial topography with a depression which is a minimum of 848 m deep and 243 km across. In addition, the 5600 km postulated by other investigators as-the distance between the australites and their source impact crater on earth is in excellent agreement with the 4800-to 5800-km distance of the subglacial depression in Wilkes Land from the tektite strewn fields of Australia. Absolute determination of the origin of the feature is dependent upon obtaining more direct evidence, such as shock metamorphic features or fragments of the impacting body, or defining the complete morphology of the subglacial depression. INTRODUCTIONThere exists in Wilkes Land, Antarctica, a region of at least 27,000 km •' that exhibits an unusual assemblage of geological and geophysical anomalies which have been a source of puzzlement since they were discovered in 1959 [Weihaupt, 1961]. The discovery region, centered at approximately 71ø30'S to 140ø00'E, is marked by a broad, shallow surface depression in the Antarctic ice • sheet and is underlain by a greater than normal ice sheet thickness and a depression in the subglacial rock surface topography. An unusually large negative gravity anomaly is found over the region [Weihaupt, 1961;Schmidt, 1962].The existence of these geological circumstances, in apparent contrast to the surrounding regions ,of Victoria Land and Wilkes Land, suggests that geological processes may have operated which are not normally considered in Antarctic studies. It is the purpose of this paper to describe this assemblage of features, to attempt to interpret them in an integrated way, and to suggest a cause for their occurrence. DESCRIPTION OF THE ANOMALOUS REGION dence for a change in the character of the ice surface of the plateau was noted at station 525D (72ø05.8'S-140ø39'E) (Figure 1), where the ice surface assumes an undulating appearance. Station 525D is located on the WSW rim of a broad, shallow basin in the ice surface which was estimated in the field to be 5.Q-6.5 km in diameter and roughly 30 m deep. Approximately 1.5-3.0 km to the ENE of station 525D a very large depression or trough was identified, with an orientation of about 3...

Anomalous free-air gravity signals in and around the Antarctic continent have been reported for some decades. Recent defi nition of the Antarctic gravity fi eld from fi eld-based oversnow traverses and supporting data from Earth-orbiting satellites reveal discrete regions of both negative and positive free-air gravity anomalies. The data from these observations have enabled us to construct a free-air gravity anomaly map of Antarctica. Negative free-air gravity anomalies are found to occur mainly on the Antarctic continent, in particular, in the Wilkes Land, Ross Sea, central continental, and Weddell Sea sectors. Positive free-air gravity anomalies are found to occur mainly in the offshore circumcontinental sectors. While each of these regions of anomalies provides excellent opportunities for further investigation, including identifi cation of the causes of the negative and positive free-air gravity anomalies, special attention is given to the negative free-air gravity anomaly sites of the continent proper. Three potential sources of the negative free-air gravity anomalies are identifi ed: the mantle, lithosphere, and crust. Examination of thermally induced density variations in the mantle based upon seismic tomography, and analysis of mantle-related gravity anomaly wavelengths favor a gravity anomaly source other than the mantle. Examination of the subcrustal lithosphere based on upper-mantle thermal structure, the origin of the lithosphere, and crustal infl uences on the underlying lithosphere, including radiogenic heat, implies that the source of the negative free-air gravity anomalies is less likely to be the subcrustal lithosphere and more likely to be located in the Antarctic crust. Examination of possible crustal features that might account for these anomalies leads to a consideration of subglacial topography and specifi c locales of anomalously low rock density.

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