The Wangkathaa Orogeny: an example of episodic regional ‘D2’ in the late Archaean Eastern Goldfields Province, Western Australia

Abstract

Abstract The Eastern Goldfields Province (EGP) is a late Archaean orogenic belt located on the eastern margin of the Yilgarn Craton. The EGP is characterised by a prominent NNW-trending tectonic grain developed largely during regional ∼E-W oriented shortening. A simplistic sequence of largely progressive compressional deformation events (D 1 to D 4+ ) has been used by previous workers to explain the development of the EGP. Regional ‘D 2 ’ was thought to be responsible for most of the finite strain, and therefore a reliable marker event for correlating deformation histories across the region. A re-examination of the deformation history has shown that ‘D 2 ’ (Kalgoorlie Orogen of Weinberg et al. [Precambrian Res. 120 (2003) 219–239]) was not a single progressive event. Rather, it was episodic and involved several stages of switching of the tectonic mode (compression–extension switching) between ∼2665 and ∼2655 Ma. The sequence of events during ‘D 2 ’ was an early stage of ∼E-W shortening (D 2a ), followed by extension with basin formation and normal fault movement (D 2E ), and finally a second stage of ∼E-W compression (D 2b ). We introduce this sequence of events as the Wangkathaa Orogeny, more complex in tectonics and broader in geographical scope than the Kalgoorlie Orogen. Features of the Wangkathaa Orogeny are well illustrated by the geometry and geological relationships that occur in the Kurnalpi Terrane (Welcome Well area) and the Kalgoorlie Terrane (Ora Banda, Kambalda, and the Boorara Domains) of the central and southern EGP. Available geochronology indicates that this sequence of events was likely to have been diachronous across the entire EGP and the eastern Southern Cross Province (of the Yilgarn Craton). Recognition of these separate (episodic) stages of ∼E-W oriented compression and extension is important in our understanding the tectonic evolution of the EGP. Switching tectonic mode between compression and extension will change the state of mean-stress in a rock mass (e.g. from net dilation to net constriction and vice versa), and is an effective method in transporting and trapping mineral-rich fluids. The paradox of the different timings established for ‘D 2 ’ across the province are explained by the Wangkathaa Orogeny, especially if it were diachronous, as different workers dated different phases of an episodic orogeny.