Structure and tectonic style of the Western Australian Shield

Abstract

The Western Australian Shield consists of two large Archaean cratons that are partly covered by remnant Proterozoic sedimentary basins and partly surrounded by Proterozoic mobile belts. Archaean terrains are either granitoid-greenstone, or high-grade gneiss, the regional distribution of which influences the style of Proterozoic tectonism. Granitoid-greenstone terrains consist of thick volcanogenic sequences, now occurring as dismembered synclinal keels within voluminous granitoidand display features that are uniquely Archaean. The gneiss terrains, although severely modified and dismembered by metamorphism and plutonism, seem to display a more uniformitarian tectonic style than the granitoid-greenstones. Mounting evidence in the Yilgarn Block suggests that the gneiss terrains represent a pre-greenstone basement, which was probably very extensive, both outside and within the greenstone areas. The most extensive area of gneiss lies in a huge arc around the western part of the Yilgarn Block, creating a novel situation where older rocks seemingly “wrap around” younger rocks. It is postulated that the precursors of the two major granitoidgreenstone terrains were huge, discrete, somewhat rounded volcanic basins that developed within extensive and perhaps continuous crust. At least in the Pilbara, there is a phenomenally continuous volcanic stratigraphy. Despite the basic similarities there are sufficient differences between the two volcanic basins to suggest independent evolution, whereby similar processes operated in different places in different times. These granite—greenstone areas had largely stabilised by about 2500 m.y. and, during the Proterozoic, behaved as cratonic blocks that tended not to participate in the mobile belts. Thus, the Capricorn Orogen developed as an ensialic geosyncline, on gneiss basement, between the two cratons. Where Proterozoic sedimentary basins transgress on to the cratons, they are preserved as gently folded and virtually unmetamorphosed covers. Within the orogenic zone itself, trough sedimentation, prograde metamorphism, basement reworking, multiple deformation and granitoid emplacement were active over the period 2000-1600 m.y. Superimposed on the Capricorn Orogen is the intracratonic Bangemall Basin (about 1100-1000 m.y.) which displays patterns of cratonic deformation that relate closely to the underlying structures. Along the southeastern margin of the Yilgarn Block is the Albany-Fraser Province which developed over an interval from 1900 m.y., or older, to 1100 m.y. Tectonic zonation is expressed by a linear striping of contrasting rocks that become younger away from the Yilgarn Block. Rather than an accretionary origin, voluminous granitoid, basemeni reworkingand absence of geosynclinal sedimentation suggest a discrete zone of high crustal strain and high thermal activity, and the belt is likened to an arrested rift in a continental setting.