Secular change in Archaean crust formation recorded in Western Australia

The formation mechanisms for early Archaean continental crust are controversial. Continental crust may have accumulated via horizontal accretion in modern-style subduction zones or via vertical accretion above upper mantle upwelling zones. However, the characteristics of the continental crust changes at the transition between the Archaean and Proterozoic eons, suggesting that continental crust did not form in subduction zones until at least the late Archaean. Here I use seismic receiver function data to analyse the bulk properties of continental crust in Western Australia, which formed and stabilized over a billion years in the Archaean. I find that the bulk seismic properties of the crust cluster spatially, with similar clusters confined within the boundaries of tectonic terranes. I use local Archaean crustal growth models to show that both plume and subduction processes may have had a role in creating crust throughout the Archaean. A correlation between crustal age and the bulk seismic properties of the crust reveals a trend: from about 3.5 Gyr ago (Ga) to the end of the Archaean, the crust gradually thickened and simultaneously became more evolved in composition. I propose that this trend reflects the transition between crust dominantly formed above mantle plumes, to crust formed in subduction zones-a transition that may reflect secular cooling of Earth's mantle.