Integrating Gravity, Seismic, AEM and MT Data to Investigate Crustal Architecture and Cover Thickness: Modelling New Geophysical Data from the Southern Thomson Region

Abstract

New geophysical data, including gravity, airborne electromagnetic (AEM), and both audio frequency and broadband magnetotelluric (AMT, BBMT) were collected along a series of traverses in the southern Thomson Orogen region of north-western New South Wales and southwestern Queensland in 2014 as part of the Southern Thomson Project. Comparing and integrating these data over the same spatial extents aimed to provide a better understanding of the crustal architecture of this region, and help estimate cover thicknesses above basement rocks. When comparing all available datasets, AEM cannot be reliably used when cover thickness is > ~150 m because of limitations in Depth of Investigation (DOI), and BBMT tends to overestimate cover thickness where it is less than 50 m. AMT likely provides the best resolution for estimating cover thicknesses of 0–1000 m on this regional scale. Forward modelling of the gravity data along selected traverses tested the interpreted crustal architecture and cover thicknesses inferred from available seismic images and the new AEM and MT conductivity models. The variable cover thicknesses interpreted from this combined approach produces a closer match with the observed gravity response when compared to a uniform, average cover thickness. The most accurate crustal-scale forward model is a thickened crust north of the Olepoloko Fault (the proposed southern boundary of the southern Thomson), split into simplified lower, middle and upper layers with basement lithologies immediately beneath cover based on the most recent basement interpretation map. Resistive bodies shown in the MT models were included in the gravity modelling, producing a good match between the observed and calculated gravity responses. These results demonstrate the utility in using a combination of different geophysical techniques to understand crustal architecture and estimations of basement depths in regions of Australia with little surface outcrop and thick cover sequences.