Structure of the Isan Orogeny under cover to the east of the Mount Isa Inlier revealed by multiscale edge analysis and forward and inverse modelling of aeromagnetic data

Aeromagnetic data reveals the presence of three north–south orientated, tens of kilometre scale, elliptical structures under cover to the east of the Mount Isa Inlier in Queensland, Australia. The Holy Joe Creek, Boot Creek and Nora Creek structures are typically defined by three narrow, continuous magnetic ridges. Highly magnetically susceptible amphibolite bodies, similar to those exposed in the Inlier, are probably the source of most of these magnetic anomalies. Multiscale edge analysis, 2D forward modelling and 3D inverse modelling methods have been used to resolve the geometry and near surface dip of the structures. These techniques indicate that the structures dip moderately to steeply around their edges. The Holy Joe Creek Structure exhibits sub-vertical to steeply west-dipping sides and a moderate plunge around the northern closure. The Boot Creek and Nora Creek structures are more complex magnetically, but their eastern edges also have sub-vertical to steep westerly dips. Diapirism and domes in an egg-box style (type 1) fold interference pattern are considered as possible origins for the elliptical structures. However, disparity between the shape of the structures and possible diapiric bodies and the lack of complementary basin structures to the domes, makes these two explanations unlikely. A more plausible alternative is that the structures are the result of deformation in a single phase, producing folds with a variable hinge-line plunge. These results indicate that east–west shortening during the D2 event of the Isan Orogeny was the major deformation in the buried rocks of the study area, suggesting a shared structural history with the Mount Isa Inlier to the west.