On the structure of Archean greenstone belts

Abstract

Abstract Archean cratons comprise two principal elements: low-grade volcano-sedimentary greenstone belts enveloped in high-grade tonalitic gneisses/plutons and amphibolites. Younger linear mobile belts generally deform and frame present cratonic margins. The high-grade gneisses are believed to consist of polycyclic sialic crust which was finally stabilized and subjected to weathering prior to 3,800 Ma as extensive, relatively thick crustal segments. The greenstone belts were deposited on sialic crust in the interval between 3,500 and 2,500 Ma as sheet-like shield volcanoes. Subsequent deformation was dominantly gravitational resulting from the density inversion of mafic volcanic rocks overlying lighter sial. A pro posed model of deformation (based on Ramberg's model experiments) can be described in two steps: 1. (a) Marginal subsidence of the greenstones with concomitant parautochtonous basement uplift (as nappes?) compressing the volcanic rocks and resulting in thrusting which repeats the greenstone succession and inverts metamorphic isograds. Repetition of the successions can lead to discrepancies between the stratigraphic thickness and gravitational reponse of the greenstones. 2. (b) Central subsidence of the greenstones assumes the shape of an inverted “mushroom” with basement remobilization as “gregarious batholiths”. Deformation of the greenstones is compressive: tight isoclinal folding, high-angle thrust faulting and longitudinal strike-slip faulting. Partial melting of the subsiding amphibolites produces the so-called calc-alkaline Archean volcanism. Examples of the structures expected within the greenstone belts from the model are reported and suspected in the greenstone belts of Rhodesia, South Africa, Australia, and Canada. It is suggested that discrepancies between the measured stratigraphic thicknesses of greenstone successions, their gravitational response, and their grades of metamorphism (depths of burial) may indicate that there has been tectonic thickening of the greenstones by thrusting.