The transition from the Thomson Orogen to the North Australian Craton from seismic data

Abstract

The transition between the North Australian Craton and the Thomson Orogen in the area south of the Mount Isa terrane lies under cover and is a critical element in interpretation of the nature of the Tasmanides. The location of the boundary between these domains is controlled by potential field results; the gravity and magnetic signatures are most sensitive to shallow structure, and so there is little information on structure at depth. Full-crustal reflection profiling crosses the boundary in a few locations but does not provide areal coverage. A deployment of 79 passive seismic stations spanning from southern Queensland into the Mount Isa terrane (AQT experiment) is exploited to examine the variations in crustal thickness and the nature of crustal structure across an area with no prior sampling. The analysis exploits the autocorrelation of the seismic signals that extracts the reflection response from the transmitted signals recorded at the surface, which can be migrated to provide an image of structure at depth. The combination of the active and passive seismic results and other geophysical data indicates that the structure of the northern and central Thomson Orogen is relatively homogeneous with a highly reflective, and magnetic, lower crust suggesting a common substrate across the region. The crust thickens and changes character in the North Australian Craton with variations under cover that can be linked to the areas of exposure. There is a zone approximately 100 km wide that separates the characteristic seismic signatures of the two domains indicating reworking at the craton margin. KEY POINTS A combination of passive and active seismic imaging sheds light on the transition from the Thomson Orogen to the North Australian Craton. The north central Thomson Orogen shows a consistent reflection character with a strong band of lower crustal reflectivity. The North Australian Craton shows seismic domains with varying character of reflectivity and crustal thickness. A transition zone about 100 km wide separates the distinctive styles of crustal architecture.