New structural model to explain geophysical features in northwestern New South Wales: implications for the tectonic framework of the Tasmanides

Paleozoic and older basement rocks in northwestern New South Wales and southern and central Queensland are largely obscured by sedimentary rocks of the Mesozoic Eromanga Basin and younger Cenozoic sediments. Interpretations of the basement geology for that area have been heavily reliant upon interpretations of the regional aeromagnetic and gravity data. Geophysically, northwestern New South Wales is characterised by an east-west-trending belt of magnetic features which trend northwest in the western part and northeast to east-northeast in the eastern part; and by an east-west-trending gravity ridge. These east-west-trending features have previously been interpreted to represent the southern part of the Thomson Orogen, a separate tectonic domain to the Lachlan Orogen to the south. However, apart from these east-west geophysical trends there is sufficient geophysical and lithological evidence to suggest continuity of the Lachlan Orogen into southern and central Queensland. A new model is herein proposed whereby the geophysical features are attributed to structures resulting from crustal thickening of a Cambro-Ordovician backarc package, west of a volcanic arc, which occurred during the Late Ordovician to Early Silurian Benambran Orogeny. While the principal stress direction was oriented east-northeast-west-southwest, the structures formed via a process of tectonic escape or extrusion due to differential strain caused by the shape of the Precambrian margin in western New South Wales. The model is consistent with field observations, geophysical imagery, borehole data and seismic data. Analogue models using sand and gelatine were constructed to illustrate the concept and demonstrate that it is feasible. The model permits the extension of the Delamerian and Lachlan Orogens into southern and central Queensland to join with similar rock packages in northern Queensland, consistent with some previous tectonic models for the Tasmanides. This brings into question the need to delineate a separate Thomson Orogen.