Insights into the structure of the upper mantle beneath the Murray Basin from 3D teleseismic tomography

Distant earthquake records from a passive array of 20 short-period seismometers are used to image the 3D P-wave velocity structure of the upper mantle beneath the Murray Basin in southern New South Wales and northern Victoria. During the five-month deployment period of the array, 158 teleseisms with good signal-to-noise ratios were recorded, allowing 3085 relative arrival time residuals to be picked with high accuracy. These arrival time residuals are mapped as 3D perturbations in P-wave velocity with respect to the ak135 global reference model using teleseismic tomography. The resulting images exhibit lateral variations in wave speed in the upper mantle to depths of nearly 300 km, and help to reveal the deep structure beneath a region almost devoid of Palaeozoic outcrop. Of particular significance is an east-west high-low-high relative wave speed variation that is present throughout the upper mantle between 70 and 250 km depth. The transition from faster to slower wave speeds in the west is indicative of a change from Proterozoic to Phanerozoic lithosphere that has also been observed further south in a previous teleseismic study. The transition from slower to faster wave speeds in the east has a less obvious association, but may well signal an underlying change in lithospheric structure related to the convergence of the western and central Lachlan subprovinces. At shallower depths (< 70 km), the structural pattern is dominated by a fast perturbation (> 3.5%) which overlies the slower region across the northern margin of the Bendigo Zone, and may be a consequence of localised thrusting and emplacement of denser lithosphere during the Early-Middle Devonian Tabberabberan Orogeny.