oa Potential field “worms” and models as the basis of a 3D tectonic model of the Koonenberry Belt, north-western NSW

Completion of a 1:100 000 geological mapping program over the Koonenberry region of north-western New South Wales has allowed the Geological Survey of New South Wales to develop techniques for potential field modelling of serial, sheet-by-sheet cross-sections, with the ultimate aim of constructing a 3D interpretative tectonic model for this Delamerian margin. Modelling has been aided by comparison of magnetic and gravity structural trends, revealed by edge analysis of upwardly continued fields (“worming”), made practical by improved data density and quality resulting from the Exploration NSW initiative. Worms aid modelling in two senses: indirectly, through qualitative assessment of tectonic styles; and directly, by fixing the position and trend of otherwise poorly imaged deep sources to constrain inversion. We exploit the different rate of decrease during upward continuation of anomalies due to dipole (magnetic) and monopole (gravity) sources, to distinguish structural differences between relatively shallow (<2–5 km) and deeper rocks. This has been particularly useful for examining the fold-and-thrust tectonics of the region. Comparison of synthetic worms derived from the model anomalies with worms derived from the gridded data allows a powerful test of the validity of the model.

Resulting profiles have been combined in GOCAD to produce a 3D model of the belt. Definition of surfaces linking the profiles is guided by an inverted image of the potential field worms. The final 3D model reveals a series of features related to repeated late Neoproterozoic to Cambrian rifting, amalgamation of the Delamerian and Thomson orogens, and subsequent transpression.