Modelling source depth and possible origin of magnetic anomalies associated with the Yallalie impact structure, Perth Basin, Western Australia

Abstract

The Yallalie impact structure is located within the Mesozoic sedimentary rocks of the Perth Basin, Western Australia. To map the geometry of the structure beneath its cover of Cainozoic sediments, an aeromagnetic survey was flown at 200 m line spacing and 60 m flying height. Several sub-circular magnetic anomalies, which are concentric about a central magnetic peak, correlate closely with faults interpreted from seismic data that displace reflectors within the sequence comprising the impact structure. Semi-automated interpretation tools, including power spectrum analysis, pseudo-depth separation, wavenumber analysis, Euler deconvolution, and inversion modelling, were used to constrain the depths of the tops and bases of the magnetic sources. The tops of the sources responsible for the concentric magnetic anomalies are estimated to be between 100 and 300 m below the surface. The thickness of the sources is poorly constrained, although maximum source depths between a few hundred and 1500 m are predicted by separation filtering. The causes of magnetic anomalies associated with meteorite impacts in sedimentary basins are poorly understood in comparison with those in crystalline basement rocks. Possible explanations for the magnetic sources associated with the Yallalie structure are: the production of new magnetic mineral species and resetting of remanence within impact melt and suevite, or by post-impact hydrothermal systems, and post-impact magnetic sedimentary fill. Comparison of source depth and geometry with seismic reflection data suggests the most likely causes of the magnetic anomalies at Yallalie are remanently magnetised impact breccia, or creation of magnetic minerals within faults by impact-related hydrothermal alteration of iron-bearing minerals within the target rocks.