Reconnaissance helicopter gravity surveys by the Bureau of Mineral Resources between 1969 and 1972 covered the southern two-thirds of Western Australia and the western part of South Australia at a station density of at least 1 per 120 km2 . Gravity provinces defined from the surveys are mainly correlated with divisions of the Precambrian shield. The Archaean Pilbara and Yilgarn Cratons are represented by gravity provinces of complex contour pattern, in which local highs correlate with greenstone belts, and lows with granitic intrusions. The Pilbara Craton is much more extensive than its exposed part, as it includes the basement of the Proterozoic Hamersley Basin as well as the Pilbara Block. The central Yilgarn C r a t o n is divided into two gravity provinces along a sinuous line extending between Norseman and Wiluna. In the western province, intense local gravity highs are short and disjointed and trend mainly north, whereas in the eastern province, local highs are longer, trend mainly north-northwest, and are of smaller amplitude and intensity. The Yilgarn Craton is flanked by elongate gravity lows. One of them correlates with the Perth Basin, but the others extend over Precambrian crystalline rocks and must correspond to mass deficiencies within the shield or at depth within the crust or upper mantle. The deep gravity depression over the Perth Basin is continuous into the Byro and Coolcalalaya Basins, indicating that they are sub-basins of the Perth rather than the Carnarvon Basin. Gravity ridges bordering the Pilbara Craton to the south and northeast and the Yilgarn Craton to the southeast are attributed to Proterozoic mobile belts containing dense metamorphic rocks. Dense granulites crop out at Fraser Range, on the southeast margin of the Yilgarn Craton, where the amplitude of the gravity ridge is particularly large. A gravity ridge over the Musgrave Block in South Australia is believed to reflect a zone of relatively thin crust; intense local highs in the west are caused by basic intrusions of the Giles Complex. Gravity relief over the Eucla Basin reflects mainly lateral density variations in the basement, except for a low in the southwestern corner of South Australia, which can be correlated with the Denman Basin. Farther east, discrete highs over the Gawler Block are attributed to local concentrations of gneiss and amphibolite in a mainly granitic basement. A broad gravity depression across the central Gawler Block may be caused by a mass deficiency in the deep crust or upper mantle. The Officer Basin in South Australia coincides roughly with a deep gravity depression. However, the known thickness of sediments cannot entirely account for the depression, which must be partly due to a regional mass deficiency beneath the basin. Gravity evidence suggests that a basement ridge separates the eastern and western parts of the Officer Basin.
