Electrical structure and crustal architecture of the southern Mount Isa Province

Interpretation of the deep crustal architecture, including the nature and extent of the fundamental basement blocks, is hampered in many regions by limited exposure and because geoscience data tend to reflect the signatures of near-surface units, highlighting the need for tools that image deeper. The northern Mount Isa Province as imaged by long-period magnetotelluric AusLAMP data, has a resistive central core flanked to the east and west by more conductive crust. These structures are elongated in the north–south direction to the extent of current AusLAMP coverage. In the south of the province, sparse broadband magnetotelluric (BBMT) data are available to extend the understanding of the crustal-scale electrical structure of the Mount Isa Province. Two new profiles of BBMT data are used to generate two 3D inversions of impedance and tipper data over a bandwidth of 0.001 to 1.000 s. Data from the 2021 BBMT survey were supplemented with historical MT data in the area, for a total of 285 stations at approximately 10 km station spacing. Two overlapping inversion models, one east–west oriented and one north–south oriented, centred on the township of Boulia were run. A suite of inversion parameters was tested for the two models to establish the reliability of final model features. Consistent with the known electrical structures further north, the inversion models show a major longitudinal resistive block which forms the core of the province, flanked on either side by more conductive crust. Results from the preferred inversion for each model were then compared to deep crustal seismic data, gravity forward modelling and seismic interpretation. The eastern conductive feature is associated with the Numil basement terrane and Gidyea Structure which defines the eastern margin of the Mount Isa Province. The central resistive anomaly extends into the upper mantle and is associated with the Pitta Pitta and Altjawarra East basement terranes, representing the Archean-Paleoproterozoic cratonic core of the Mount Isa Province. The westernmost conductor is associated with the Altjawarra basement terrane and is an extension of the structure imaged in the AusLAMP model.