Deciphering the crustal structure of the Tasmanides in southeastern Australia with anisotropic short-period Rayleigh wave tomography

The Tasmanides of Australia were formed by a series of tectonic supercycles that determined the crustal - and probably the lithospheric - structure of the eastern third of the Australian continent during the Phanerozoic. In southeast Australia, the Tasmanides essentially consist of the Delamerian, Lachlan and New England orogens whose mutual boundaries are obscured by the presence of large Mesozoic to Cenozoic sedimentary basins: the Sydney Basin at the boundary separating the Lachlan and the New England orogens, and the Murray Basin at the boundary between the Delamerian and the Lachlan orogens. In this work, we exploit the ambient noise wavefield recorded by the largest transportable seismic array experiment in the southern hemisphere, which has operated in eastern Australia from 1998 to present and involves the deployment of over 700 temporary stations with an average interstation distance of about 50 km. We analyze Rayleigh wave phase dispersion curves obtained in a previous study on more than 8,200 cross-correlograms using data from 450 sites, and we perform an anisotropic tomography inversion for periods ranging from 1 to 20 s in order to account for the apparent dependence of Rayleigh wavespeeds on azimuthal propagation direction. The resulting maps allow us to identify several crustal elements in terms of lateral extent, wavespeed and anisotropy. The Archean Gawler Craton and Proterozoic Curnamona Province, as well as the Eastern and Central Subprovinces of the Lachlan Orogen, are characterized by high velocities, while the Western Subprovince of the Lachlan Orogen and the Adelaide Fold Belt exhibit lower velocities. The transition from Delamerian to Lachlan orogens is marked by a clear change from high to low velocity and a change in anisotropy pattern, especially in the northern sector of the model. In the western Lachlan subprovince, the fast direction changes from SE-NW in the South to SW-NE in the North, and follows the orientation of its boundary with the Delamerian Orogen. This suggests that the proto-Pacific margin of east Gondwana was significantly curved, in contrast with the consistently NS fast axis orientation of the eastern Lachlan Orogen that suggests a more linear margin. It also appears that crustal anisotropy in the Lachlan Orogen was not significantly affected by events that have followed its formation such as Cenozoic volcanism or the separation between Australia and Antarctica. Largely on the basis of high resolution aeromagnetic maps, several recent studies have identified the possible presence of a remnant fragment of Precambrian lithosphere - possibly originating from the break-up of Rodinia - embedded within the Lachlan Orogen. Intriguingly, our anisotropic tomography results show the fast axis of anisotropy almost mimicking the magnetic lineations which appear to wrap around a region that is now referred to as the Hay-Booligal Zone. The close correlation between the patterns of azimuthal anisotropy and lineations identified in potential field data is one of the most remarkable outcomes of this high resolution study.