Structure and evolution of the intracratonic basins of central Australia

Abstract

The central Australian geology is dominated by a series of parallel sedimentary intracratonic basins whose history spans time from the Late Proterozoic to the Carboniferous. The basins are separated by regions of exposed Proterozoic crust that have a history of significant uplift. The formation of the basins cannot be explained by conventional thermal or stretching mechanisms. Nor does the foreland basin model provide an adequate explanation for their occurrence. The geophysical observations, particularly gravity, point to a structure whereby the crust is considerably thicker under the basins than it is under the regions where basement rock is exposed. This suggests that the crust as a whole is involved in the process that led to the basin formation. A mechanical model is developed which is based on the supposition that the lithosphere has been in a state of horizontal compression for long time intervals, The lithosphere is considered as a viscoelastic plate that has been subjected to a small normal load, distributed in an irregular manner. Under compression, initial deflections due to this load are magnified according to the plate's physical properties, the magnitude of the compressive force, and the wavelength spectrum of the initial perturbations. Because of the viscosity, the deflections grow with time at a rate that is determined by the balance of forces acting on the plate and by the lithosphere's rheology. Both the elastic and viscous deformations are much enhanced if erosion of the uplifted areas occurs, with the sediments deposited into the downwarps. These deformations take place at stresses that are much less than the buckling stresses. As bending stresses increase with time, failure by thrust faulting is predicted to occur within the basins, with the exposed basement being thrust over the basins. The model has been applied to the central Australian region starting at about 900 Myr ago. The first basins to develop are the Officer and Amadeus Basins with uplift occurring in the intervening Musgrave Block. Initial rates of uplift and basin subsidence are small but they can increase rapidly with time if the compressive force is maintained. A point may be reached where the uplift becomes sufficient for gravitational instabilities to occur and for the crust as a whole to fail. This occurred about 300Myr later and corresponds to the Petermann Ranges Orogeny. Further deformation is now shifted to areas where there has not been significant stress release at the time of failure. This corresponds to the southern Arunta Block, and the northern part of the Amadeus Basin is further developed. The main sequence of events leading to the formation of the Ngalia Basin also takes place now. Early basin growth is again slow but increases rapidly with time to culminate in the Alice Springs Orogeny in Early Carboniferous time. The proposed model reproduces many of the geological and geophysical aspects of the region and provides a framework for discussing the geological history deduced from the basin sediments and basement metamorphics. It also predicts a crustal structure that is in broad agreement with the available data and which can be further tested by geophysical observations. Show more