Dr Desmond Lascelles1
1University Of Western Australia, Perth, Australia
An alternative model of Archean tectonics suggests that early (before 2.5 Ga) crustal plate collisions may have resulted in folding of the thin ocean crust into upright folds with a total amplitude of up to 50 km instead of subduction beneath ocean plates or continental masses. The folded crust would be deeply depressed into the mantle by isostatic compensation with consequent partial melting at depth forming a felsic melt and a dense ultramafic restite. Anatectic replacement of the down-folded ocean crust by granite and extended erosion of the upper parts of the folds resulted in the formation of an Archean granite and greenstone style craton with an ultimate thickness of around 30-35 km.
The collision of two cratons folded the intervening mafic crust, including sediment deposited on the ocean floor, to form a new craton that combined the cratons into a continent. Uplift and erosion of continents during the late Archean deposited sediments on the thinner margins of the continents forming extensive continental margin basins. The subduction of ocean crust did not occur until the formation of deep sedimentary accumulations on the sea floor beyond the margins of the continents depressed the ocean crust and was able to subduct the oceanic plate beneath the continental margin. The subducting ocean crust caused compression of the sedimentary basin and the formation of modern style orogenic fold belts and volcanic arc systems that were not present during the Archean era.
I obtained B.Sc.(hons) (London) in 1964 and M.Sc (Macquarie) in 1973. I worked mainly as a self-employed consultant, for exploration and mining companies with approximately 30 years spent in the iron ore industry. I received my PhD in 2007 on the origin of BIF and iron ore.