Sn-wave velocity structure of the uppermost mantle beneath the Australian continent

We have extracted a data set of more than 5000 Sn traveltimes for source-station pairs within continental Australia, with 3-D source relocation using Pn arrivals to improve data consistency. We conduct tomographic inversion for S-wave-speed structure down to 100 km using the Fast Marching Tomography (FMTOMO) method for the whole Australian continent. We obtain a 3D-model with potential resolution of 3.0 degrees x3.0 degrees. The newS-wave-speed model provides strong constraints on structure in a zone that was previously poorly characterized. The S velocities in the uppermost mantle are rather fast, with patterns of variation generally corresponding to those for Pn. We find strong heterogeneities of S wave speed in the uppermost mantle across the entire continent of Australia with a close relation to crustal geological features. For instance, the cratons in the western Australia usually have high S velocities (> 4.70 km s-(1)), while the volcanic regions on the eastern margin of Australia are characterized by low S velocities (< 4.40 km s(-1)). Exploiting an equivalent Pn inversion, we also determine the V-p/V-s ratios across the whole continent. We find that most of the uppermost mantle has V-p/V-s between 1.65 and 1.85, but with patches in central Australia and in the east with much higher V-p/V-s ratios. Distinctive local anomalies on the eastern margin may indicate the positions of remnants of mantle plumes.