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New Stratigraphic Constraints on the South West Terrane of the Yilgarn Craton, Western Australia: Insights into the Tectonic Framework of Archean Craton Assembly

Session: Evolution of Orogenic Belts Through Time: Insights from Sedimentation, Deformation, Magmatism, and Metamorphism, Part I

Presenting Author:

Cat Gill

Authors:

Gill, Cat¹, Thébaud, Nico², Kemp, Anthony I.S.³, Tucker, Naomi⁴, Quentin de Gromard, Raphael⁵, Ivanic, Tim⁶

(1) School of Earth and Oceans, University of Western Australia, Perth, WA, Australia, (2) School of Earth and Oceans, University of Western Australia, Perth, WA, Australia, (3) School of Earth and Oceans, University of Western Australia, Perth, WA, Australia, (4) School of Earth and Oceans, University of Western Australia, Perth, WA, Australia, (5) Geological Survey of Western Australia, Perth, WA, Australia, (6) Geological Survey of Western Australia, Perth, WA, Australia,

Abstract:

Terrane nomenclature has long been applied to Archean cratons globally, typically reflecting an accretionary model in which exotic crustal fragments are assembled and stabilized to form cratons. Such models have frequently been used to explain the formation of the Yilgarn, Pilbara, Kaapvaal, Dharwar, North Atlantic, and Wyoming Cratons for example. However, limited consensus on Archean tectonic processes casts doubt on whether these subdivisions can be considered accurate and meaningful in terms of craton evolution (Bédard, 2018; Kusky et al., 2018; Van Kranendonk, 2010). When determining the origin of terranes, assessing the terrane boundaries is an obvious choice. Boundaries between these terranes should record the structural history of craton formation, however, protracted deformation and magmatic overprinting often hinder the geological record associated with their formation. One way to work around this issue is to document the stratigraphic and magmatic evolution of the terranes on either side of these boundaries to determine if they juxtapose exotic blocks (in agreement with the accretionary model) or if they divide blocks with similar histories (suggesting an alternative crust evolution).

In the Yilgarn Craton (Western Australia), competing models have been proposed for its formation, ranging from accretion to in-situ, mantle-driven growth (Cassidy et al., 2006; Schreefel et al., 2024). The South West Terrane, in particular, remains poorly constrained due to limited stratigraphic study, with recent interpretations still favoring an accretionary origin despite minimal evidence. This study assesses the northern South West Terrane to test its proposed exotic nature. Using U-Pb SIMS and LA-ICPMS data from detrital and igneous zircons, integrated with mapping and field observations, we document a previously unrecognized magmatic and sedimentary history predating 3000 Ma. This is overlain unconformably by ca. 2700 Ma volcaniclastic sequence—a contact relationship not observed elsewhere in the Yilgarn Craton. The unique stratigraphy and age relationships suggest that the South West Terrane may have experienced a significant level of of isolation from the rest of the Yilgarn, potentially supporting its exotic origin. When considered with evidence from the rest of the Yilgarn, the possibility of an exotic origin to the South West Terrane may be indicative of a geodynamic system cohabited by both autochthonous and allochthonous processes.