A preliminary interpretation of deep seismic reflection and other geophysical data from the Darling Fault Zone, Western Australia

Abstract

Deep seismic reflection data were recorded along a 75 km long traverse that covered some 50 km within the Yilgarn Craton and 25 km within the Perth Basin, down to 12 s two-way time (TWT). Gravity data were recorded at 500 m intervals and magnetic data at 100 m intervals along the same traverse. The data were processed to stack stage using a conventional land seismic processing sequence. Migration proved difficult because of the poor knowledge of the subsurface velocity distribution in the Yilgarn. Good reflections were obtained for the full 12 s TWT. The preferred interpretation of the seismic reflection data suggests that the crust beneath the western Yilgarn Craton may be divided vertically into three structural zones: 1 (0 km to 7 km) a zone of thin-skinned compressional tectonism which probably occurred between 2650 Ma and 2500 Ma; 2 (7 km to 25 km) a zone that appears to contain several minor detachments (or shear zones) and seismic events characteristic of intruded lenses of magma; and 3 (25 km to 40 km) easterly dipping continuous seismic events. Reflection events between 11 s and 12 s TWT in the east of the profile may represent the top of the Moho "reflection package", but previous work suggests that it may be slightly deeper than this. The seismic reflection data also image well the fault plane of the Phanerozoic Darling Fault. Modelling of the gravity data suggests that the Darling Fault Zone may separate the crust (43 km thick) beneath the Yilgarn Craton from the crust (27 km thick) beneath the Darling Mobile Zone and Perth Basin. This change of crustal thickness is probably coincident with the Proto-Darling Fault, and is not related to the Phanerozoic Darling Fault that is evident on the seismic reflection data. Modelling of the magnetic data indicates numerous intrusive bodies between the surface and approximately 20 km depth. Some of these bodies exhibit strong magnetic remanence.