An integrative approach to investigating crustal architecture and cover thickness in the Southern Thomson region: modelling new geophysical data

The under-cover geology of the southern Thomson Orogen in north-western New South Wales and south-western Queensland is largely unknown due to the extensive, up to 600 m thick Cenozoic and Mesozoic cover. This cover (mainly consisting of Eromanga Basin and Lake Eyre Basin rocks) results in very restricted basement outcrop, with a subsequent lack of understanding of subsurface lithologies, structures and the potential for the location of economic resources. As a result, this area was selected for a regional, multi-disciplinary project (the Southern Thomson Project) by Geoscience Australia and its State partners the Geological Survey of New South Wales and the Geological Survey of Queensland. The Project reflects the focus of the UNCOVER Initiative (Australian Academy of Science 2012) that aims to form the basis for Australia's potential future discovery and development of new economic mineral resources.
The Southern Thomson Project involves many geoscientific disciplines including geophysics, geochronology, geochemistry and stratigraphy to better understand the region and promote mineral exploration by reducing exploration risk. This report focuses on some of the initial reconnaissance and pre-drilling geophysical data collected in 2014 - in particular gravity data, AEM (Airborne Electromagnetics) and MT (Magnetotellurics) along two regional north-south traverses, and a shorter east-west traverse in the northern part of the region. The major aim of this study is to compare AEM and MT electrical conductivity data acquired along these traverses, and integrate them with interpretation of available deep seismic reflection data to generate a series of 2D geological models, which can be tested via forward gravity modelling and subsequent density inversions. This integrative approach allows for a more robust understanding of the crustal architecture and cover thickness (or depth to basement) variations in the Southern Thomson region.
The main findings of this report are:
1) Cover thicknesses of 0 to &gt;500 m were initially estimated along various traverses through a combination of AEM and MT data interpretation as well as existing data from drill holes and water bores. Most datasets yield broadly similar results in terms of relative cover thickness variations, although AEM cannot be reliably used when cover thickness is greater than ~150 m due to limitations in the Depth Of Investigation (DOI), and Broadband MT (BBMT) tends to overestimate cover thickness where it is known to be less than 50 m. Cover thickness estimates using MT methods (especially AMT - Audio-frequency Magnetotellurics) agree with other datasets such as existing drill holes/water bores, GABWRA (Great Artesian Basin Water Resource Assessment; Ransley and Smerdon 2012) depth to basement results, and targeted high-resolution ground geophysical surveys (Goodwin et al. in prep). On this regional scale, AMT likely provides the most suitable resolution for estimating cover thicknesses of 0 - 1000 m. 
2) Cover thicknesses estimated by AEM and MT conductivity sections have been tested by forward gravity modelling and produce better matches with the observed gravity responses compared to an averaged, uniform cover thickness. This observation shows cover thickness variations can produce discernible variations in gravity responses and need to be taken into account in gravity modelling. Further, this supports the use of a combined approach in using AEM, MT and gravity models to asses cover thickness variations over a broad region.
3) Several alternative interpretations of deep seismic reflection data along the southern part of one of the regional MT traverses (Line 3) were performed to assess crustal architecture. These were tested by forward gravity modelling with subsequent inversions (allowing modelled bodies' density to vary) producing a close match between the observed and modelled gravity responses with reasonable geological densities of crustal units given the limited known and/or inferred rock properties in this region.
4) Two-dimensional (2D) cross-section models along each line were generated by integrating the recent interpretation of basement geology (Purdy et al. 2014) with AEM and MT conductivity sections. These models were tested via forward gravity modelling and subsequent inversions (allowing modelled bodies' density to vary). This approach showed that the most accurate model was a thickened crust north of the Olepoloko Fault (the Southern Thomson region). 
5) Many of the 2D forward models produced reasonable matches between the observed and calculated (modelled) gravity responses with respect to the large scale crustal architecture and location of prominent resistive bodies (inferred as felsic igneous intrusions) observed in MT conductivity sections. However, gravity inversions sometimes produced unrealistic densities of crustal units given the (limited) known rock properties in this region. Despite these limitations, the simplistic 2D forward models provide a good starting point for future refinement as more geological, geophysical, geochemical and petrophysical data become available.

Tags: AEM, Earth Sciences, Gravity, MT, Published_External, Thomson, UNCOVER
Organization: Geoscience Australia Data