Deep crustal structure in the southwestern Yilgarn Craton, Western Australia

The Archaean rocks of the Yilgarn Craton comprise the largest geological en tity in Western Australia and one of the largest areas of Archaean crust an ywhere in the world. However, poor outcrop has prevented a thorough underst anding of the structure of the crust in the region, a subject of particular interest being the relationship between high-grade metamorphic rocks in th e west, and lower-grade granitoid-greenstone terrains further east. Interpr etation of previously unpublished seismic refraction data, combined with th e re-interpretation of published data, allows crustal velocity structure in the southwest of the Craton to be resolved. Overall, the structure is simi lar to shield areas elsewhere in the world, the data defining a two-layered crust with an average thickness of about 35 km. However, significant later al variations in velocity structure are also observed, and these coincide w ith a terrane boundary previously postulated on the basis of geological obs ervations and seismic reflection data. The terrane boundary dips to the eas t, extends through the entire crust, and is notable for an area of anomalou sly 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 interpretati on, the high-velocity zone is a fault-bounded mega-sliver, which may be a s uspect terrane in its own right, perhaps with oceanic affinities. The resul ts of the seismic experiment strongly support tectonic models for Yilgarn C raton based on terrane tectonics. The possibility that the terrane boundary is being reactivated by contemporary stresses also provides a plausible ex planation for the local intra-plate seismicity. (C) 2000 Elsevier Science B .V. All rights reserved.