Anisotropic structure of the Australian continent

Abstract

The Australian continent preserves some of the oldest lithosphere on Earth in the Yilgarn, Pilbara, and Gawler Cratons. In this study we present shear wave splitting and Ps receiver function results at long running stations across the continent. We use these results to constrain the seismic anisotropic structure of Australia’s cratons and younger Phanerozoic Orogens. For shear wave splitting analysis, we utilize SKS and SKKS phases at 35 broadband stations. For Ps receiver function analysis, which we use to image horizontal boundaries in anisotropy, we utilize 14 stations. Shear wave splitting results at most stations show strong variations in both orientation of the fast direction and delay time as a function of backazimuth, an indication that multiple layers of anisotropy are present. In general, observed fast directions do not appear to be the result of plate motion alone, nor do they typically follow the strike of major tectonic/geologic features at the surface, although we do point out several possible exceptions. Our Ps receiver function results show significant variations in the amplitude and polarity of receiver functions with backazimuth at most stations across Australia. In general, our results do not show evidence for distinctive boundaries in seismic anisotropy, but instead suggest heterogenous anisotropic structure potentially related to previously imaged mid-lithospheric discontinuities. Comparison of Ps receiver function and shear wave splitting results indicates the presence of laterally variable and vertically layered anisotropy within both the thicker cratonic lithosphere to the west, as well as the Phanerozoic east. Such complex seismic anisotropy and seismic layering within the lithosphere suggests that anisotropic fabrics may be preserved for billions of years and record ancient events linked to the formation, stabilization, and evolution of cratonic lithosphere in deep time.

The Influence of Lithospheric Thickness Variations Beneath Australia on Seismic Anisotropy and Mantle Flow

“…This is consistent (within typical errors) with our previous study of permanent station MULG (Φ: 79°, δt: 1.1 s) located within the Gawler Craton (Eakin et al, 2021). Other recent studies have reported a similar NE-SW to ENE-WSW trend over the South Australian Craton (Ba et al, 2023;Birkey & Ford, 2022). The spatial extent of this shear-wave splitting pattern, throughout the Gawler Craton and extending eastwards into the Curnamona Province, can now be seen more clearly.…”

“…This orientation is very similar to the APM of the Australian plate, ∼6 cm per year at 21° (clockwise from North) in this location, as indicated by the black arrow in Figure 3. Hints of such a correspondence between the fast direction and the APM for eastern Phanerozoic Australia have been previously noted based on more limited results (Ba et al, 2023;Bello et al, 2019;Birkey & Ford, 2022). The results of our analysis confirm a similarity between the splitting fast direction and the APM for southeastern Australia that is spatially distinct from the pattern over cratonic Australia.…”

“…Within the South Australia Craton, upon first impressions, the ENE-WSW orientation of the anisotropy does not appear to match the outline of the major geological provinces or crustal boundaries (Figure S8 in Supporting Information S1), as noted by Birkey and Ford (2022). However, upon closer inspection, the ENE-WSW orientated fast directions agree very well with the structural trends found internally within the Gawler Craton, which are preserved as a series of E-W to NE-SW trending faults (Figure 6).…”

“…The number of previous shear‐wave splitting studies across Australia (∼10) is relatively limited compared to other continental landmasses. Previous authors have typically reported either (a) weak splitting (i.e., many null measurements or small delay times) (Ba et al., 2023; Chen et al., 2021; Eakin et al., 2021; Heintz & Kennett, 2005; Özalaybey & Chen, 1999; Vinnik et al., 1992); (b) complex patterns such as frequency dependence of the splitting parameters (Clitheroe & van der Hilst, 1998; Özalaybey & Chen, 1999); or (c) variability of the retrieved splitting parameters at a given station (Bello et al., 2019; Birkey & Ford, 2022; Chen et al., 2021; Heintz & Kennett, 2005, 2006). Various interpretations have been proposed to explain such results, including (a) a lack of azimuthal anisotropy present in the upper mantle (Chen et al., 2021; Özalaybey & Chen, 1999); (b) apparent isotropy due to two anisotropic layers with orthogonal fast directions (Heintz & Kennett, 2006); (c) contributions from frozen‐in lithospheric anisotropy but without clear correspondence to structural trends at the surface (Bello et al., 2019; Birkey & Ford, 2022; Clitheroe & van der Hilst, 1998; Heintz & Kennett, 2005); or (d) the possibility of asthenospheric flow that is not aligned with the APM, such as around a continental root (Ba et al., 2023; Clitheroe & van der Hilst, 1998; Heintz & Kennett, 2005).…”