A review of crust/upper mantle structure in the Precambrian areas of Australia and implications for Precambrian crustal evolution

Abstract

Listen

Translate article

The western two thirds of the Australian continent are underlain by Precambrian rocks. Seismically, this region is characterised by negative teleseismic travel-time residuals indicating a thick, cold tectosphere. Despite extensive platform cover and Phanerozoic crustal reworking in a few intracratonic sedimentary basins, the Precambrian crust can be classified broadly into Archaean and Proterozoic cratons and Proterozoic mobile belts. The Archaean cratons have a two-layered crust 25–35 km thick, with observed velocities in the lowermost crust in the range 6.8–7.2 km s −1. The Moho is a distinct feature with a velocity contrast of > 1.0 km s −1. Vertical reflection probes in the southern Yilgarn Block yielded record sections that have numerous, laterally discontinuous but distinct reflections, especially in the lower crustal layer, that are especially concentrated around the boundary between the two crustal layers and the Moho. The seismic reflections and velocities are consistent with a predominantly felsic crust, with mafic lenses constituting 20–35% of the lowermost crust, and with the metamorphic grade ranging from granulite in the upper to middle crust to garnet granulite and even eclogite at the base of the crust. The Proterozoic mobile belts and cratons a have multi-layered crust 45–50 km thick. Velocities below 25 km depth are almost always > 7.0 km s −1, and reach 7.4–7.6 km s −1 in the lowermost crust; velocities such as these are characteristic of gabbroic (basaltic) underplate. The velocity contrast is < 1.0 km s −1 at the Moho, which is less distinct than in Archaean terranes. Record sections from vertical reflection probes show a highly reflective crust with no correlation between seismic layers in the refraction models and the distribution of vertical reflections. The contrasting character of the crust in Archaean and Proterozoic provinces favours a tectonic model for the evolution of Precambrian crust in which predominantly felsic Archaean protocrust was tectonically reworked and underplated to produce Proterozoic crust. The simple two-layered protocrust became more complicated in the process. The introduction of vast amounts of underplate, especially near the crust/mantle boundary, thickened the crust and destroyed the character of the Moho. It overprinted the reflection character of the crust so that the bands of reflections now do not correlate with the boundaries between the crustal layers. The velocities in the lower crust are high because of the underplate.