Deep crustal structure in the southwestern Yilgarn Craton, Western Australia

Abstract

The Archaean rocks of the Yilgarn Craton comprise the largest geological entity in Western Australia and one of the largest areas of Archaean crust anywhere in the world. However, poor outcrop has prevented a thorough understanding of the structure of the crust in the region, a subject of particular interest being the relationship between high-grade metamorphic rocks in the west, and lower-grade granitoid-greenstone terrains further east. Interpretation of previously unpublished seismic refraction data, combined with the re-interpretation of published data, allows crustal velocity structure in the southwest of the Craton to be resolved. Overall, the structure is similar to shield areas elsewhere in the world, the data defining a two-layered crust with an average thickness of about 35 km. However, significant lateral variations in velocity structure are also observed, and these coincide with a terrane boundary previously postulated on the basis of geological observations and seismic reflection data. The terrane boundary dips to the east, extends through the entire crust, and is notable for an area of anomalously high seismic velocity in the lower crust. Two possible interpretations of the high-velocity zone are proposed. Firstly, it is due to the presence of mafic to ultramafic intrusions. Secondly, and the preferred interpretation, the high-velocity zone is a fault-bounded mega-sliver, which may be a suspect terrane in its own right, perhaps with oceanic affinities. The results of the seismic experiment strongly support tectonic models for Yilgarn Craton based on terrane tectonics. The possibility that the terrane boundary is being reactivated by contemporary stresses also provides a plausible explanation for the local intra-plate seismicity.