Interpretation of the crustal structure between the Hamersley and Ashburton Basins from gravity and magnetic data in the Wyloo area, Western Australia

Abstract

The crustal structure beneath the eastern part of the boundary between the Hamersley and Ashburton Basins near Paraburdoo in the northwest of Western Australia has been investigated previously using seismic refraction and gravity data obtained in the late 1970s. The availability of recently acquired gravity data over the area of the WYLOO 1:250,000 map sheet and a newly established density database of the Hamersley Basin have prompted this study of the western part of the boundary north of the Wyloo Dome. On WYLOO the Hamersley Basin is characterised by short wavelength and high-amplitude magnetic anomalies produced by banded iron formations. These are in striking contrast to the short to medium wavelength and moderate-amplitude magnetic anomalies that occur over the Ashburton Basin. A well-defined positive gravity anomaly along the contact zone between the two basins can be explained with a model of a southwesterly dipping contact between the higher density rocks of the Hamersley Basin and the lower density rocks in the Ashburton Basin. Whereas the interpreted supracrustal geology and basin structure largely explain the aeromagnetic and shorter-wavelength gravity responses on WYLOO, they do not account for the regional decrease in the Bouguer gravity values, from +100 gu in the Ashburton Basin to about -600 gu in the Hamersley Basin. Using constraints imposed by the earlier seismic investigations, the observed gravity response has been simulated with a model of a two-layer crust that thins southwards from 30-34 km beneath the Hamersley Basin to 27-30 km below the western part of the Ashburton Basin. This is in contrast to the southwards-thickening model that was derived from the earlier seismic refraction and gravity investigations to the east. It suggests that there may be a significantly different crustal structure beneath the western and eastern parts of the boundary between the Hamersley and Ashburton Basins.