CONSTRAINTS ON ARCHEAN CRUSTAL COMPOSITION AND STRUCTURE PROVIDED BY DEEP SEISMIC-SOUNDING IN THE YILGARN BLOCK

New seismic reflection data from the Eastern Goldfields and Southern Cross Provinces, coupled with results from earlier seismic refraction profiles recorded in the western Southern Cross Province indicate that, with the exception of the greenstone belts that occupy much of the surface of the Eastern Goldfields Province, the crust in the Yilgarn Block is composed predominantly of felsic material with only minor mafic material within the lowest part of the crust. The greenstone belts themselves, are shallow features and not deep-rooted features as often proposed. The transition from the crust to the mantle is sharp, occurs over only a few kilometres, and marks a significant change in composition and density. The middle and lower crust along the section have no marked lateral change in reflection character between the greenstones terranes and the adjacent granite-dominated terranes, except that both the mid-crustal and crust-mantle boundaries have been bowed downwards by approximately 6 km under the greenstones. The greenstone belts of the Eastern Goldfields Province are 6-9 km thick. They are superimposed on an otherwise uniform crust. Their base is planar and subhorizontal, and appears to be block faulted in places. The Ida Fault, which defines the western boundary of the greenstone belts, is planar, dips to the east at 25-degrees-30-degrees and can be traced to depths of approximately 25 km (8 s).