The gravity field of offshore Australia

Maintenance and Update Frequency: unknown | Statement: Unknown | The free-air anomaly field of offshore Australia has been divided into about fifty regional gravity provinces, each of which is characterized by uniformity of trend, free-air anomaly level, or degree of disturbance. These are discussed in relation to structural and/or bathymetric features in each region. The Bouguer anomalies are used as a rough guide to variations in crustal thickness. On the continental shelf the free-air anomaly provinces generally correlate with the main structural elements. The Precambrian shields are associated mainly with regional gravity lows, and the peripheral mobile belts mainly with gravity ridges. On the northwest and southern margins these mobile belts cut across the continental shelf and appear to be truncated at the shelf-break. On the marginal plateaus and terraces the free-air anomaly pattern largely reflects the relative elevation of basement and the thickness of sediment. The well-defined gravity highs on the Lord Howe Island and Tasmantid seamount chains are caused by the combined effects of sea-floor topography and high-density igneous bodies. Regional positive free-air anomaly values over the broad continental shelves of the Northwest and Southern Margins, and negative values on the adjacent abyssal plains, indicate that slight readjustment of the crust/mantle interface must occur in these regions if isostatic equilibrium is to be attained. However, the well-defined free-air anomaly ridges and troughs which correspond to the top and foot of the continental slope respectively, are largely a gravity edge effect caused by abrupt changes in water depth and crustal thickness. This can reach ±70 mGal over steep slopes, such as that bordering the Tasman Basin. Regional positive free-air anomaly values over the Queensland and Marion Plateaus indicate that slight subsidence of these features is necessary if they are to attain isostatic equilibrium. The Bouguer anomaly values indicate that the crust thins oceanward except in the Timor Sea area where crustal thickening occurs, probably due to interaction of lithospheric plates along the Inner Banda Arc. Crust of typically oceanic thickness (10-15 km) is confined to the lower part of the continental slopes and the abyssal plains, generally oceanward of the 4000 m isobath.