Crustal structure under the Lachlan Fold Belt, southeastern Australia

Abstract

Seismic investigations of the Earth's crust in the Lachlan Fold Belt of southeastern Australia, under the continent's highest mountain ranges, indicate a depth to the Moho of between 40 and 52 km, with P-wave velocities in the upper crust of 5.6–6.3 km s −1, lower crustal velocities of 6.7–7.4 km s −1, and upper mantle velocities of 8.0–8.1 km s −1. Velocity changes within the crust and at the Moho boundary are transitional rather than discontinuous. A well-defined low-velocity layer in the crust, at a depth of about 15–20 km, is evident throughout the region, and localized low-velocity layers may be present in the upper crust at shallower depths. The thickness of the upper crust increases considerably (from 26 to 35 km) along a 300 km NE-SW traverse, with the greatest thickness occurring under the highest mountains. S-wave velocities are close to 3.65 km s −1 in the upper crust and 3.9 km s −1 in the lower crust. Many previous descriptions of the orogenic evolution of southeastern Australia invoke models analogous to the tectonic processes occurring in western Pacific island arcs. The very thick crustal section interpreted in the Lachlan Fold Belt indicates that such orogenic evolution was unlikely, and that an Andean-type model is probably more appropriate. Major granitic batholiths in the Lachlan Fold Belt have a P-wave velocity of about 5.9–6.0 km s −1, compared with interspersed metamorphosed Palaeozoic sediments which have a velocity of about 6.25–6.5 km s −1. There is evidence that the thickness of the batholiths ranges from 15 to 22 km.