Seismic and gravity investigations along the geotraverse, Western Australia, 1969 A BMR contribution to the Upper Mantle Project

On the recommendation of the Australian Upper Mantle Committee, the Bureau of Mineral Resources carried out a seismic survey across the Precambrian shield in southwestern Australia between June and December 1969. Deep crustal reflection and refraction information was obtained from explosions set off in five areas along the Geotraverse, a line extending from Perth through Coolgardie in the east to Point Culver in the southeast. The results have been combined with those from gravity surveys and from other seismic refraction surveys in southwestern Australia to obtain an integrated interpretation of the structure and composition of the crust and upper mantle. The reflection surveys included continuous profiling, offset recording, expanded spread velocity profiling and common-depth-point profiling along short traverses in the five areas. The quality of the analogue-processed sections was generally poor, and the probable reflection events, of very low relative amplitude, could be picked only from repetition of events over a few traces. Digital processing of the data effectively increased the relative amplitudes of the events and generally improved their continuity, but it also raised the level of some noise events and produced a smearing of all events over a greater number of traces than before processing. The lack of any distinctive reflection character and definitive vertical velocity information in the poor-quality data makes it impossible to correlate probable reflection events from one area to another along the Geotraverse. The reflection information was, however, useful in providing support to the interpretation of the refraction data between Perth and the Jubilee Mine about 60 km northeast of Kalgoorlie. Refractions from the large reflection shots were also recorded at temporary and permanent seismograph stations between Perth and the Jubilee Mine, and between Perth and Albany. The quality of the refraction data recorded during the Geotraverse project, and of those recorded earlier but used in the analysis here, suffers from irregular spacing of the recording stations, insufficient lengths of some traverses, and irregular clusters of data points near the critical recording distances for the various layers. The reliability of the refraction analysis depends largely on the correlation of second and later arrivals. However, the generalized structure interpreted from the refraction data is considered to be the most probable one as it is consistent with the interpretation of reflection and gravity results. The analysis of the seismic data indicates that the crust is of normal continental type in the east but changes towards the Perth Basin in the west. Near Kalgoorlie it consists of two layers with velocities of 6.12 and 6.66 km/s and is 34 km thick, whereas near Perth, close to the continental margin, it is 44 km thick and includes an extra basal layer of velocity 7.42 km/s, which thins out towards the east and southeast. The upper two crustal layers near Perth, on the other hand, thicken to the east and southeast. In the Perth Basin, about 7.5 km of sediments overlies a block of the crust which has been thrown down to the west along the Darling Fault. Southeast of Coolgardie, the high-velocity basal layer is shown to be thin and the southeastern part of the crustal block has been upthrust to the northwest along the Fraser Fault. The measured velocity of the upper mantle underneath the abnormal crust is 8.25 km/s. The seismically determined structure is consistent with a crust in or close to isostatic equilibrium, and with the observed gravity anomaly field in southwestern Australia for two possible density models of the crust and upper mantle. One of the density models is also consistent with the hypothesis that the highvelocity basal layer in the crust is garnet-granulite overlying eclogitic mantle.

Tags: AU-WA, Bulletin, Earth Sciences, GA Publication, Published_External, gravity, seismics
Organization: Geoscience Australia Data