Seismic Anisotropy Analysis Across Southwestern Australia Reveals ENE-Trending Lithospheric Architecture Linked to Archean Yilgarn Craton Formation

Abstract

The southwest region of Western Australia is one of the oldest continental regions on Earth, hosting the Archean Yilgarn Craton, bounded by the Proterozoic Albany-Fraser and Pinjarra orogens. Here we calculate shear wave splitting of the PKS and SKS teleseismic phases using new broadband arrays with unprecedented station spacing across the region. We find evidence for coherent seismic anisotropy, with the regional average delay time ((Formula presented.) s) comparable to the global average, (Formula presented.) t = 1 s. Although fast polarization orientations show variation, they are not aligned with current plate motion and the expected mantle flow direction. In the South West Terrane and Albany-Fraser Orogen, fast polarization orientations match the trend of ancient structural faults. In contrast, structural faults in the Youanmi Terrane are oriented at an angle compared to the E–W and NE–SW fast polarizations. Instead, seismic anisotropy patterns show an intriguing similarity to E–W trending Precambrian (2.42 Ga) dykes that extend uninterrupted across the Yilgarn Craton. We propose that lithospheric fabrics frozen-in at the time of craton formation (2.76–2.65 Ga) generated a mechanical weakness which subsequently influenced the orientation and emplacement of the dykes. Further evidence for a similar, ancient (2.73 Ga) architectural fabric comes from recent isotope geochemistry analysis of primary ENE-trends within the Yilgarn Craton. Overall, these results point toward large-scale, fossilized lithospheric fabric within the Yilgarn Craton, preserved for over two billion years, offering a unique window into the formation and early evolution of the continent.