Publication Name: The North West Shelf Australia
Authors: V. Anfiloff
Date Published: July 1988
Number of Pages: 25
Reference Type: Book Section
Abstract:Marine gravity surveys of the North West Shelf by the BMR between 1965 and 1968 provide a good indication of the tectonic framework. In most areas, gravity trends generally agree with seismic trends. In the Browse Basin, prominent linear gravity highs may be caused by extensive volcanism; in the Bonaparte Basin strong gravity anomalies reflect major deep structures. The marine magnetic coverage is less revealing; it shows some igneous activity, but little of the primary basement related structures, and is too sparse to show textural information. A systematic detailed aeromagnetic coverage would provide a better insight into deep igneous intrusives and shallow volcanic activity.
A polycyclic rift model developed in the Eromanga and Murray Basins and consistent with African rifting, can explain much of the primary framework. This framework, defined by gravity anomalies greater than 100 ums-2, is attributed to basement topography associated with vertical faults of the "Cardinal System", and igneous activity controlled by those faults. Magnetics show little of this framework. In many areas, basement ridges form a continuous bifurcating network, in which ridges isolate troughs from each other. Ridges also cut across troughs, producing compartments which can be intruded by acid or basic igneous rocks. It appears that basement ridges can also include igneous rocks from a previous cycle of rifting. Ridges may represent the propagation of horizontal
compression; and troughs, the zones of tension between ridges.
Structures under the Bonaparte Basin are interpreted in terms of polycyclic intracratonic rifting producing the characteristic gravity "dipole". The first cycle produces a basement ridge and a flanking trough which fills with dense volcanics. Later, the volcanics are incorporated in a new basement ridge, and two flanking rifts develop. The Dampier Sub-Basin may also be a polycyclic rift zone, underlain by granites of an older rift, and flanked by dense volcanics in the Rankin Platform, also from an older rift.