A geophysical study of the southern continental margin of Australia: Great Australian Bight and western sections

摘要

Marine gravity, magnetic, bathymetric, and seismic reflection and refraction measurements, most of them made aboard U.S.N.S. Eltanin, were used to study the western and central sections of the southern continental margin of Australia. The data are presented as profiles across the margin. A magnetic quiet zone extends from west of Tasmania to the western border of the Great Australian Bight. In the Great Australian Bight area, the seaward quiet-zone boundary parallels the oldest marine magnetic lineation south of Australia, anomaly 22. An isostatic gravity high very nearly coincides with this isochron. There are no corresponding anomalous topographic features, but seismic profiler records generally show basement peaks occurring in the vicinity and seaward of anomaly 22. A conspicuous magnetic low is located near the landward boundary of the quiet zone; it separates the subdued signature of the magnetic quiet zone proper from high-amplitude, short-wavelength magnetic anomalies of shallow origin on its continentward side. It is associated with an isostatic gravity high and a significant topographic break of the continental slope. Most seismic profiler records show probable shallow basement corresponding to the rough magnetic signature landward of the prominent magnetic low. Seismic profiler and limited refraction data indicate greater depths to basement underlying the magnetic quiet zone than on either side. To the west of the Great Australian Bight, the magnetic lineations older than anomaly 19 are not well defined. The prominent magnetic low is continuous along the western and central margin, as are the isostatic gravity high and the topographic break in the slope associated with it. The area immediately seaward of the magnetic low is occupied by irregular, noncorrelatable magnetic anomalies. Available seismic data indicate a greater thickness of sediments for this zone than for the adjacent seaward area. We have examined several magnetics and gravity models to explain the geophysical anomalies of the Great Australian Bight area. One possible model assumes that non-oceanic subsided basement underlies the magnetic quiet zone, that the seaward quiet-zone boundary marks the juxtaposition of shallow oceanic and deeper non-oceanic basement, and the landward boundary represents a structural discontinuity, probably a normal fault, within continental-type basement. In this model, the basement subsidence of the magnetic quiet zone is isostatically compensated by a lateral change in density. It is not possible to eliminate all other models for the origin of the area occupied by the magnetic quiet zone.