Continental rifting and the Archean-Proterozoic transition

Abstract

In the Western Australian shield, the Norseman-Wiluna belt, dominated by lowermost sequences of deformed and metamorphosed basalts and komatiites, and the Fortescue Group, with successions of gently dipping, little-metamorphosed basalts and rare komatiites, are typical examples of Archean-style greenstones and Proterozoic-style platform sequences, respectively. Isotopic evidence, however, indicates that they are broadly coeval and both Archean (ca. 2.7 ±0.05 Ga), and geologic evidence suggests that both relate to continental rifting. Their contrasting volcanism and subsequent deformation are inferred to result from their positions relative to rift axes and the state of the basement, the Norseman-Wiluna belt representing a failed rift in stretched, relatively hot crust, and the Fortescue Group representing a continental basalt succession on thick, relatively cold crust. The Fortescue Group immediately follows a major period of crustal reworking and stabilization, whereas eruption of the greenstones of the Norseman-Wiluna belt was closely followed by such an event. Thus, the diachronous change in tectonic style from greenstones to platforms, the Archean-Proterozoic transition, in this case appears to relate to a finite period of stabilization of continental crust rather than to a fundamental change in tectonic style or process with time.