Deep crustal structure of North Queensland

The interpretation of a reconnaissance gravity survey of North Queensland has shown that the area is composed of normal continental crust approximately 40 km thick, and is consistent with relief at the Moho of approximately 7 km. The gravity anomalies were separated into regional and residual components using four successive running means, a process equivalent to filtering with a sinc4 band pass filter. Spectral analysis of the filtered data showed that an effective regional-residual separation was performed by this process. The parameters required for three-dimensional crustal gravity modelling, crustal thickness and density contrast across the Moho, were derived from available crustal seismic refraction experiments together with analyses of correlations of crustal parameters on a world-wide basis. There are no large departures from isostatic equilibrium in the area. The southeastern part of the area is isostatically compensated and the crustal thickness reaches 43 km. The Mt. Isa Block is not isostatically compensated and it coincides with an area of thin (38 km) crust. This area is stable and, due to its size, is unlikely to achieve local isostatic equilibrium. The Cape York area has not reached isostatic equilibrium. The gravity anomaly pattern suggests that the area may have approached equilibrium progressively with those parts of the area farthest from the centre of the Tasman Geosyncline having the smallest departure from isostatic equilibrium. This agrees with the history of development of the northern part of the Tasman Geosyncline which is younger to the north and east. The Palmerville Fault is a major surface structure but has no gravity effect originating at Moho depths, and hence may be only an upper crustal feature. The Coen Inlier also has little or no influence on the regional Bouguer anomalies and is therefore probably a shallow crustal feature, of less significance than its surface outcrop suggests. The Cape York-Oriomo Ridge, on the other hand, has little surface expression, but is shown by the gravity data to be a major feature of the crust. Further refinement of the gravity interpretation would require a knowledge of the structure within the crust. Compilation of geophysical surveys, carried out principally for oil exploration, provides sufficient information to structure-contour the base of the Mesozoic. The Mesozoic sediments are relatively thin and do not exceed 1000 metres in the onshore portion of the Carpentaria Basin or 3000 metres in the Laura Basin. None of this exploration has provided significant information below the sedimentary basement. Deep reflection seismic work is suggested as the most effective technique to obtain velocities (and hence densities) and depths within the crustal section down to the Moho