Mapping upper mantle discontinuities beneath the Australian continent using multimode surface waves and receiver functions

Kotaro Tarumi , Kazunori Yoshizawa 

Spatial distributions of upper mantle discontinuities beneath the Australian continent are mapped using a large number of station-based 1-D S-velocity profiles with radial anisotropy, derived from joint Bayesian inversions of multimode surface waves and P-wave receiver functions. Numerous permanent and temporary stations across Australia enable us to constrain localized seismic velocity structures, including the lithospheric-asthenosphere boundary (LAB), mid-lithospheric discontinuities (MLDs), and X-discontinuities (X-Ds) below the LAB, providing quantitative constraints on the variations of seismic velocity and radial anisotropy across these interfaces. Subsequent statistical cluster analysis of locally detected discontinuities identifies multiple MLDs and X-Ds. The LAB is shallow (60–100 km) in Phanerozoic eastern Australia and deepens toward western cratonic regions (120–200 km). A thick lithosphere (170–200 km) with significant radial anisotropy but minimal S-velocity reductions is observed beneath central Australia. MLDs in cratonic regions are detected at three distinct depths (from 60 to 130 km), with either negative or positive S-velocity changes depending on the tectonic setting. X-Ds, characterized by velocity increases beneath the LAB, are detected at multiple depths (~170 km, 220–270 km, and ~310 km), commonly associated with the weakening of radial anisotropy underneath. The shallowest X-D (~170 km) is found mainly beneath eastern Australia and is likely influenced by thermal effects in the asthenosphere, while deeper X-Ds may correspond to the Lehmann discontinuity or phase transitions of pyroxene. These findings provide a comprehensive view of the upper mantle discontinuities beneath Australia, offering new insights into the tectonic evolution of the Australian continent.