Archean continental evolution is still poorly understood due to the poor preservation and overprinted nature of many Archean-age terranes. The southwest Yilgarn Craton is a well-preserved and relatively well-exposed portion of Archean crust, which forms an ideal natural laboratory to test tectonic models for Archean continental evolution. In this area the boundary between the South West and Youanmi Terranes was recently shifted to be congruent with extensive geological and geophysical data that indicate that a large portion of the old South West Terrane is in fact a high-grade equivalent to the rest of the Youanmi Terrane; and that the two terranes were juxtaposed along a major crustal discontinuity possibly during the 2665–2635 Ma granulite-facies tectono-magmatic event that formed most of the structures of the sinistral transpressive Corrigin Tectonic Zone ​(Quentin de Gromard et al., 2021)​. In this study, we combine field based structural and lithostratigraphic investigations with metamorphic petrology and geochronology in order to develop a tectono-magmatic framework for the region and the influence of these processes in the wider Yilgarn Craton. Preliminary field observations of the South West Terrane directly west of the terrane boundary reveal well-preserved meta-sedimentary outcrops that retain their original depositional features such as ripples, crossbedding and, in one case, an angular unconformity, giving important way-up information. Structural investigation shows a complex polyphase deformation history with at least three phases of folding. This interpretation allows the stratigraphy to be simplified and the characteristic quartzite in the area to be considered as one unit rather than multiple units juxtaposed with each other, as was previously thought ​(Nieuwland, 1979)​. Towards the Youanmi–South West terrane boundary, strain is observed to increase and the adjacent stratigraphy is rotated 90° to become parallel with it. The presence of alumina-rich metamorphic minerals such as sillimanite and andalusite indicate that this folding is associated with at least one episode of metamorphism and a high thermal gradient. When integrated with geochronological analysis the new structural data from this previously understudied area provides new insights into the assembly of the southwestern portion of the Yilgarn Craton. In addition to testing the validity of the new terrane boundary, our findings have broad implications for our understanding the timeline of tectonic events across the wider Yilgarn Craton and regional exploration strategies based upon reconstructing the large-scale processes behind mineral enrichment.

Reference(s)

Nieuwland, D. A. (1979). Structural geology and geochronology of the Toodyay district, Western Australia: Australian National University, PhD thesis (unpublished). 

Quentin de Gromard, R., Ivanic, T.J. and Zibra, I. (2021). Pre-Mesozoic interpreted bedrock geology of the southwest Yilgarn, in Accelerated Geoscience Program extended abstracts: Geological Survey of Western Australia, Record 2021/4, p. 122–144.