Deep crustal reflections in Australia 1957-1973--II. Crustal models

Abstract

In a companion paper, we described experiments conducted by the Australian Bureau of Mineral Resources (BMR) between 1957 and 1973 to record deep crustal seismic reflections. At 24 of the 46 sites investigated, events were recorded at times up to 10-13 s. At some sites, only one or two shots were recorded; where more comprehensive investigations were made, the events were consistent with deep reflections, though they lacked continuity in many cases. The results were sufficiently encouraging that we have now analysed the events on the hypothesis that they represented deep crustal reflectors, including the Moho. In 1976, BMR introduced digital recording, and successfully carried out several long deep crustal profiles since then. This paper analyses only the recordings made with analogue equipment prior to 1973. In a few places, the reflection structure could be compared with nearby refraction traverse results; some events correlated well with first or second-order discontinuities in the refraction velocity-depth curves, while for others there was no correspondence. It is difficult to generalize as to which types of refractors give rise to reflections. The refraction surveys show that the Moho is not always a boundary with a sharp change in velocity across it. In some areas in Queensland the deepest reflector correlates well with the refracting Moho. For reflection shots not near refraction surveys, we converted travel times to depths using simplified velocity models based on refraction surveys in similar geological environments, and on isostatic balance considerations using gravity and elevation data. In central Australia, if the deepest reflector is the Moho, a relatively thin and light crust is implied. However comparison with refraction models makes it seem probable that a heavier, thicker crust could exist, with the Moho being deeper than the deepest reflector. Also in the Tasman Geosyncline, it is probable that the Moho is deeper than the deepest reflector.