Inversion in the Eastern Succession of the Mt Isa inlier: A key to crustal architecture

Abstract

New insights into the crustal structure and evolution of the Mt Isa Eastern Succession have come from constructing a crustal scale 3D model. A revised chronostratigraphy, detailed aeromagnetic data, multiscale wavelet edges of potential field data, and reprocessing and reinterpretation of seismic data suggest hitherto unsuspected links between upper and lower crustal structures. Three major approximately NS structures are interpreted to underlie the Eastern Succession, respectively along the edge of the Wonga belt, under the Mitakoodi anticline and at the western margin of the contiguous Soldiers Cap outcrop. Major contrasts in potential field data occur along these structures; they also correspond to major changes in stratigraphy. They are interpreted to have originated as extensional faults during basin formation, which were positively inverted during the Isan orogeny from 1.6 to 1.5 Ga. Inversion apparently involved both the cover sequences and basement rocks, although net extension is preserved on some parts of the major structures. Reactivation of these structures in the Isan orogeny may explain some of the complexity of structure within the Eastern Succession, which has hitherto been interpreted as evidence for multiple overprinting deformation. Furthermore, the localisation of contractional features over extensional structures suggests that the Eastern Succession is parautochthonous. The two largest mineral deposits in the Eastern Succession (Cannington and Ernest Henry) are proximal to the easternmost major crustal structure, suggesting the importance of these structures to mineralization.