A Discussion on global tectonics in Proterozoic times - General Discussion

There is a general concensus amongst geologists today that the geotectonic evolution of Archaean ‘greenstone’ belts was causatively different to that of Phanerozoic orogens. That such a concensus seems not to have been reached for Proterozoic orogens is at first surprising. However, during the course of the discussion meeting it became clear that there is a fundamental difference in philosophy between those who work in the African, Laurentian or Baltic Shields and those who work on the Canadian or Australian shields. Intercratonic mobile belts in Africa consist typically of highly deformed and metamorphosed sequences lying between cratonic basement. These belts are described as ensialic both because they contain remnants of the cratons and because sedimentary sequences can be traced from the craton into the mobile belt. The fact that they are eroded deeply while the surrounding cratons are not bespeaks of strong differential vertical movements, a fact that has been used to discriminate against a plate-tectonic origin, as have the absences from these belts of calcalkaline igneous rocks and molasse. Finally the branching, arcuate pattern of Pan-African mobile belts has been contrasted with the pattern of interplate orogens.