Explosion seismic determination of crustal structure beneath the Adelaide Geosyncline, South Australia

Abstract

The structure of the Earth's crust beneath the Adelaide Geosyncline, South Australia, has been investigated over the last few decades using both earthquake and explosion seismic techniques. These studies have led to the simple average model of a single, homogeneous layer having a P-wave velocity of 6.32 km s −1 and thickness of 38 km overlying a homogeneous mantle with a velocity of 8.05 km s −1. The only compelling evidence for crustal layering comes from two recent long-range refraction profiles, recorded along (N-S) and perpendicular (E-W) to the axis of the geosyncline, using quarry blast sources. Head waves and wide-angle reflections have been identified as originating from a mid-crustal boundary separating an upper crust of velocity 5.94 km s −1, from a lower crust of velocity 6.42 km s −1, at a depth of 10 to 18 km. The depth variation to this boundary, and the corresponding Moho topography, were interpreted using an iterative ray tracing technique. Both discontinuities shallow towards the Murray Basin which flanks the Geosyncline to the south-east. The crustal model obtained was further investigated by means of a synthetic aperture seismic array experiment, carried out on the N-S refraction profile, at a source offset of 120 km. Recordings were made on a 21 element unreversed linear profile, using digital seismographs spaced at 0.5 km intervals. An accurate P 1 velocity of 5.88 km s −1 was obtained. Secondary P arrivals were studied with the aid of beam formers and tau-p processors. Tentative reflections from within the crust and upper mantle were obtained. However, the variation in seismic waveform generated by different blasts was a definite drawback of the experiment. Prominent dipping reflectors and faults at depths of several kilometres have been observed in near-normal incidence reflection surveys shot in Porterozoic rocks just west of the Adelaide Geosyncline. The reflectors may represent layering within the crystalline basement, or they may relate to the rifting stage of basin development.