3-D seismic tomography of the Adelaide fold belt

A lithospheric tomography experiment was conducted in the Adelaide Fold Belt of South Australia, to elucidate the 3-D velocity structure down to depths of several hundred kilometres in a region of major geological and tectonic significance. The data comprised arrival times for compressional waves from 148 teleseismic events recorded on a temporary array of 40 digital seismographs spanning an area of 200 x 300 km. A new irregular shortest path ray tracer and non-linear inversion scheme were used to map the crust- and upper-mantle structure. Synthetic 3-D imaging tests were first performed to validate the procedure and establish resolution limits for the particular earthquake distribution and recording geometry. Wave speed anomalies of up to 4 per cent relative to the IASP91 layered earth model were found. The boundary between a prominent velocity high and a prominent velocity low in the shallow (crustal) depth slices seems to coincide with the synclinal axis of Adelaidean and Early Palaeozoic sediments. Other velocity features at shallow depths correlate with gravity highs and lows, and compare favourably with structure revealed from explosion seismic profiles recorded in the area. At depths greater than 60 km, the tomographic images reveal isolated velocity highs enclosed by regions of lower velocity within the fold belt. These features, of dimensions tens of kilometres, could indicate the existence of fragments of old mantle broken off from the thick subcontinental lithosphere beneath the Gawler Craton and scattered in the Rodinian break-up rift zone.