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Rapid plate motion, alongside pronounced variations in age and thickness of the Australian continental lithosphere, makes it an excellent location to assess the relationship between seismic anisotropy and lithosphere‐asthenosphere dynamics. In this study, SKS and PKS shear‐wave splitting is conducted for 176 stations covering the transition from the South Australian Craton to eastern Phanerozoic Australia. Comparisons are made with models of lithospheric thickness as well as numerical simulations of mantle flow. Splitting results show uniform ENE‐WSW aligned fast directions over the Gawler Craton and broader South Australian Craton, similar to the orientation of crustal structures generated during an episode of NW‐SE directed compression and volcanism ∼1.6 billion years ago. We propose that heat from volcanism weakened the lithosphere, aiding widespread lithospheric deformation, which has since been preserved in the form of frozen‐in anisotropy. Conversely, over eastern Phanerozoic Australia, fast directions show strong alignment with the NNE absolute plate motion. Overall, our results suggest that when the lithosphere is thin (<125 km), lithospheric contributions are minimal and contributions from asthenospheric anisotropy dominate, reflecting shear of the underlying mantle by Australia's rapid plate motion above. Further insights from geodynamical simulations of the regional mantle flow field, which incorporate Australian and adjacent upper mantle structure, predict that asthenospheric material would be drawn in from the south and east toward the fast‐moving continental keel. Such a mechanism, alongside interactions between the flow field and lithospheric structure, provides a plausible explanation for smaller‐scale anomalous splitting patterns beneath eastern Australia that do not align with plate motion. Plain Language Summary: The Australian continent is moving rapidly northward at around 7–8 cm per year. As the continent moves, it is expected to shear or deform the warmer and weaker layer of the Earth below, called the mantle. The actual pattern of deformation within the mantle can be investigated by studying how seismic waves are polarized as they pass through this material. Results show that for one of the geologically oldest regions in Australia, an area in South Australia, the deeper part of the continent here was substantially deformed 1.6 billion years ago. This deformation was likely aided by volcanism that occurred at the same time that would have warmed and weakened the material, making it easier to deform. This material has since cooled and strengthened over time, freezing in the ancient pattern of deformation. Meanwhile in eastern Australia, the continental material here has a much younger geological age (<550 million years old). The results from this region instead show agreement with the present‐day direction of shear due to the fast northward motion of the Australian continent, as initially expected. Key Points: Fossilized anisotropy dominates where lithosphere is old and thick, with deformation of the Gawler Craton from ∼1.6 Ga preservedAsthenospheric shear relating to Australia's fast plate motion dominates where lithosphere is young and thinLithospheric thickness variations likely induce further deviations in mantle flow [ABSTRACT FROM AUTHOR]

Highlights • Upper mantle anisotropy structure revealed beneath Bass Strait, SE Australia. • Large delay times (>2.5 s) in vicinity of recent intraplate volcanism. • Postulated Precambrian microcontinent appears to be defined by fast polarisation directions oriented approximately NE. • Overall complex anisotropy pattern that seldom correlates with magnetic anomalies and absolute plate motion. Abstract We investigate the structure of the upper mantle using teleseismic shear wave splitting measurements obtained at 32 broadband seismic stations located in Bass Strait and the surrounding region of southeast Australia. Our dataset includes ∼366 individual splitting measurements from SKS and SKKS phases. The pattern of seismic anisotropy from shear wave splitting analysis beneath the study area is complex and does not always correlate with magnetic lineaments or current N-S absolute plate motion. In the eastern Lachlan Fold Belt, fast shear waves are polarized parallel to the structural trend (∼N25E). Further south, fast shear wave polarization directions trend on average N25–75E from the Western Tasmania Terrane through Bass Strait to southern Victoria, which is consistent with the presence of an exotic Precambrian microcontinent in this region as previously postulated. Stations located on and around the Neogene-Quaternary Newer Volcanics Province in southern Victoria display sizeable delay times (∼2.7 s). These values are among the largest in the world and hence require either an unusually large intrinsic anisotropy frozen within the lithosphere, or a contribution from both the lithospheric and asthenospheric mantle. In the Eastern Tasmania Terrane, nearly all observed fast directions are approximately NW-SE. Although part of our data set strongly favours anisotropy originating from "fabric" frozen in the lithospheric mantle, a contribution from the asthenospheric flow related to the present day plate motion is also required to explain the observed splitting parameters. We suggest that deviation of asthenospheric mantle flow around lithospheric roots could be occurring, and so variations in anisotropy related to mantle flow may be expected. Alternatively, the pattern of fast polarisation orientations observed around Bass Strait may be consistent with radial mantle flow associated with a plume linked to the recently discovered Cosgrove volcanic track. However, it is difficult to characterise the relative contributions to the observed splitting from the lithospheric vs. asthenospheric upper mantle due to poor backazimuthal coverage of the data. [ABSTRACT FROM AUTHOR]

A new radially anisotropic shear wave speed model for the Australasian region is constructed from multi-mode phase dispersion of Love and Rayleigh waves. An automated waveform fitting technique based on a global optimization with the Neighbourhood Algorithm allows the exploitation of large numbers of three-component broad-band seismograms to extract path-specific dispersion curves covering the entire continent. A 3-D shear wave model is constructed including radial anisotropy from a set of multi-mode phase speed maps for both Love and Rayleigh waves. These maps are derived from an iterative inversion scheme incorporating the effects of ray-path bending due to lateral heterogeneity, as well as the finite frequency of the surface waves for each mode. The new S wave speed model exhibits major tectonic features of this region that are in good agreement with earlier shear wave models derived primarily from Rayleigh waves. The lateral variations of depth and thickness of the lithosphere–asthenosphere transition (LAT) are estimated from the isotropic (Voigt average) S wave speed model and its vertical gradient, which reveals correlations between the lateral variations of the LAT and radial anisotropy. The thickness of the LAT is very large beneath the Archean cratons in western Australia, whereas that in south Australia is thinner. The radial anisotropy model shows faster SH wave speed than SV beneath eastern Australia and the Coral Sea at the lithospheric depth. The faster SH anomaly in the lithosphere is also seen in the suture zone between the three cratonic blocks of Australia. One of the most conspicuous features of fast SH anisotropy is found in the asthenosphere beneath the central Australia, suggesting anisotropy induced by shear flow in the asthenosphere beneath the fast drifting Australian continent. [ABSTRACT FROM AUTHOR]

Felsic episodes in young SE Australian volcanism were studied using new combined zircon U–Pb, feldspar40Ar–39Ar and fission-track dating. Trachytes, xenocrysts in basalts and derived detrital crystals yielded an 8 Ma range for felsic sequences in the Macedon–Trentham (ca8–5 Ma) and Western District (< 5–0.0 Ma) provinces of Victoria. At Newham, zircon and feldspar ages of 6.3–6.1 ± 0.1 Ma agree with the local basalt stratigraphy, while near Trentham zircon dating suggests felsic activity at ca 8.3 Ma and 6–5 Ma. Zircons crystallised in high-temperature crustal trachytes that evolved from alkali basalts, following amphibole crystallisation in the mantle (6.3 Ma Brimbank complex). The 8–5 Ma felsic episodes are attributed to lithospheric passage over an asthenospheric plume-like upwelling, now centred under Bass Strait. The Western District Province includes quartz-normative trachyte near Creswick (40Ar–39Ar ageca2.4 ± 0.4 Ma), zircon xenocrysts in basalt near Daylesford (U–Pb age 1.8 ± 0.3 Ma) and zircon megacrysts in tuff at Bullenmerri maar (U–Pb zircon age 0.28 ± 0.04 Ma). The Creswick and Daylesford felsic phases may represent fractionation of basaltic icelandites during peak Western District volcanic activity. Bentonitic beds of trachyandesite affinities in NW Victoria–SE New South Wales lie in strata dated atca2 Ma and may mark a separate distal phase of peak Western District felsic volcanism. The E–W trend of post-5 Ma Western District basaltic activity has been attributed to lithospheric edge-driven or Tasman Fracture Zone fault-driven magmatic up-wells. However, new tomographic modelling of sublithospheric upper mantle suggests that Bassian asthenospheric inputs may explain young felsic components in adjacent basalts. Multiple felsic inputs allow greater appraisal of the young volcanic genesis and eruptive risks for the area. [ABSTRACT FROM AUTHOR]

The attenuation of seismic waves is strongly influenced by temperature and rheological changes, and so provides an important supplement to information from seismic wavespeeds. Differential attenuation measurements are made for seismic waves refracted through the mantle beneath the Australasian region and are then used to construct 3D images of the attenuation of shear and compressional waves. The differential attenuation between different seismic phases is estimated using a spectral ratio method in the frequency band 0.25-1.0 Hz using a multi-taper method. Over this band the attenuation is nearly independent of frequency so that the logarithmic spectral ratio is a linear function of frequency. Differential measurements are made either between P and S phases on the same record or between stations for P and S waves separately. Images of seismic attenuation for the Australasian region are produced using a tomographic inversion, with the fast marching method employed to trace ray paths in an initial 3-D model derived from surface wave tomography. There is a deep seated horizontal contrast between central Australia and the eastern seaboard. The crustal and lithospheric mantle beneath the Archean and Proterozoic rocks in the west and in the middle of the continent have low seismic attenuation, whereas the Phanerozoic material in the east is more attenuative. Regions with recent volcanism, most likely associated with hot spots such as near Bass Strait and the Coral Sea, display high seismic attenuation anomalies. There is a strong contrast in attenuation between the relatively low loss lithosphere and the high loss asthenosphere beneath with a change by a factor of 6-10. Anisotropy in attenuation is slight even though some would be expected from the radial anisotropy seen in shear wavespeed in the lithosphere. [ABSTRACT FROM AUTHOR]

Abstract: Sp and Ps scattered wave receiver functions were calculated for nineteen stations across Australia and the island of Tasmania in order to image the lithosphere–asthenosphere boundary and layering within the lithosphere. Within Phanerozoic eastern Australia and the eastern margin of the South Australia Craton, prominent Sp phases from a negative velocity contrast were found at depths of 61±11km to 131±9km, consistent with the lithosphere–asthenosphere boundary depth range from surface wave tomography. These phases imply significant velocity drops over depth ranges of 30–40km or less, and thus cannot be explained by a lithosphere–asthenosphere boundary that is controlled by temperature alone. Rather, they imply that the asthenosphere is hydrated with respect to a drier, depleted lithosphere or contains a small amount of partial melt. The shallowest Sp phases have the largest amplitudes and occur in regions with the most recent, voluminous volcanism, strengthening the link to partial melt at the base of the lithosphere. In contrast, no significant negative Sp phases were found at the base of the thick cratonic lithosphere at the stations in central and western Australia, implying that the cratonic lithosphere–asthenosphere velocity gradient is distributed over more than 50–70km in depth. This gradient may be purely thermal in origin, although gradational changes in composition or melt content cannot be ruled out. A negative Sp phase was observed at depths of 69±8km to 85±14km at stations in central and western Australia, indicating the presence of a drop in velocity internal to the lithosphere. This interface within the lithosphere may be a relic of cratonic mantle formation, or the result of alteration by melt and metasomatism. [ABSTRACT FROM AUTHOR]

A three-dimensional lithospheric electrical resistivity model of the Archaean-Proterozoic Gawler Craton in southern Australia has been developed, to define the tectonic framework of the craton and identify craton margins under regolith cover. Knowledge of cratonic margins is important as upwards of 60% of known mineral wealth is located in palaeoconvergent zones between ancient cratons. The research was conducted in three phases. First, all previous magnetotelluric (MT) and geomagnetic depth sounding (GDS) data were compiled to establish a regional scale induction database. Almost 400 observation sites were found in the literature and from personal communication, collected over the last 30 years by various research groups. Of these, most measurements were GDS only, but recent 2D MT sites along major profiles provide further depth constraints. Second, eight long period MT sites were collected from central areas of the craton to provide a window into the deeper lithosphere and asthenosphere. Data were collected over a number of weeks to establish high quality MT responses in the bandwidth 10–104 s. Onedimensional modelling indicated that the lithosphere was resistive (> 100 Ω m), with a more conductive midmantle at 70 km and a decrease in resistivity at the transition zone. These MT responses agree with the long period (104-107 s) continental lithosphere induction responses of Olsen (N. Olsen: 'Long-period (30 days–1 year) electromagnetic sounding and the electrical conductivity of the lower mantle beneath Europe', Geophys. J. Int., 1999, 138, 179–187). Finally, a 3D resistivity model of the lithosphere was compiled from 2D inversions of MT profiles, with constraints from lithosphere asthenosphere soundings, and regional scale GDS data. The Gawler Craton is shown to have a resistive core of dimension 500 km, surrounded by more conductive terrains. The core is largely, but not exclusively, Archaean crust, surrounded by conductive Proterozoic crust. The edge of the core on the eastern side of the Gawler Craton correlates with the location of several iron oxide–copper–gold deposits, including the world class Olympic Dam deposit, potentially making mapping of large scale resistivity structures a useful tool for regional scale exploration. [ABSTRACT FROM AUTHOR]