Tasman orogenic system; a model for its subdivision and growth history based on gravity and magnetic anomalies

Abstract

The basement of the Tasman orogenic system can be subdivided using gravity and magnetic anomalies into 11 (geophysical) domains. The unity of each domain is given by subparallel trends, which show the direction of the dominant tectonic structure in the nonsedimentary part of the upper crust (basement). Crosscutting trends at domain boundaries indicate the relative ages of the domain cratonization. These relative ages are consistent with the dated ages of first deformation, and they show that cratonization occurred in numerous episodes and resulted in an apparent growth of the Tasman orogenic system from west to east between the Cambrian and the Carboniferous. Each domain can be subdivided into bands of different character. In the main area of basement outcrop, the southern part of the Lachlan orogen, the position of the boundaries of these domains and bands agree with boundaries based on the composition of granites, structure, and strontium isotope ratios. Over the larger part of the Tasman orogenic system, where basement is under cover, these geophysically defined domains and bands are our best estimate of basement subdivision and history. Near domain boundaries there are characteristic associations of gravity and magnetic anomalies due to reworking of the margin of the older crust by deformation, intrusion, and metamorphism; differences between domains in mean upper crustal density and magnetization; and the common occurrence of volcanic bands, ophiolite-serpentinite, and deep elongate sedimentary troughs caused by crustal extension. The domains can be divided into a sedimentary basement type formed by crustal extension and sedimentation or by the addition of an accretionary wedge, and a volcanic-arc basement type added by the accretion.