Marine seismic refraction studies on the continental margin to the south of Australia

Abstract A normal crustal transition to a true ocean basin is indicated across the narrow continental margin to the southwest of Kangaroo Island. A depth of between 11 and 12 km is obtained for the Mohorovicic discontinuity in the ocean basin and a P n velocity of 8·27 km/sec. This velocity apparently decreases across the margin of the continent to a value of 8·05 km/sec which was obtained by Doyle and Everingham (1964) from atomic explosion studies. Howevever, it is possible that the latter value may have been affected by the slope of the discontinuity. The results across the continental margin to the south of Pt. D'Entrecasteau are not as conclusive. The base of the oceanic crustal layer is indicated as shallowing towards the continent to a depth of about 10·5 km, roughly 120 km south of Pt. D'Entrecasteau. The underlying velocity is calculated as 8·07 km/sec but this figure could include travel times from an intermediate layer in the zone of crustal transition. Although insufficient data are available to resolve this point, the likelihood of an intermediate layer is enhanced by the recording of velocities of 7·3 km/sec at Mundaring Observatory near Perth from explosions off the coast and from quarry blasts ( Doyle and Everingham 1964).