Wollondilly-Blue Mountains Gravity Gradient and its bearing on the origin of the Sydney Basin

Some 526 gravity measurements were made to an accuracy of 1 mgal in order to further delineate the main gravity features of the Sydney Basin. Corrections for an Airy-type isostatic model and for the effect of the basin sediments are applied to a profile between Bathurst and Mona Vale. An asymmetric gravity high of 44 mgal amplitude, centred about 10 km E of the Lapstone Monocline, emerges as the major feature. This is interpreted to indicate the presence of a mafic body, of density 2.9 t m-3 and a maximum thickness of 12 km, underlying the basin within the upper crust. The base of the body has an easterly dip of 53° on the western side, representing a major basement fault beneath the Lapstone Monocline-Kurrajong Fault System. The westerly dips on the eastern flank range from 21° to 33° and these may indicate a series of step faults. The large size of the source implies a significant crustal extension preceding the development of the Sydney Basin. Although the basement faults did not propagate upwards into the Permo-Triassic strata, the western fault controlled sedimentation and is expressed as a warp — the Lapstone Monocline.