Abstract
The Australian continental crust preserves a rich geological history, but it is unclear to what extent this history is expressed deeper within the mantle. Here an investigation of Quasi-Love waves is performed to detect scattering of seismic surface waves at mantle depths (between 100–200 km) by lateral gradients in seismic anisotropy. Across Australasia 275 new observations of Quasi-Love waves are presented. The inferred scattering source and lateral anisotropic gradients are preferentially located either near the passive continental margins, or near the boundaries of major geological provinces within Australia. Pervasive fossilized lithospheric anisotropy within the continental interior is implied, on a scale that mirrors the crustal geology at the surface, and a strong lithosphere that has preserved this signal over billions of years. Along the continental margins, lateral anisotropic gradients may indicate either the edge of the thick continental lithosphere, or small-scale dynamic processes in the asthenosphere below.
Highlights
The Australian continental crust preserves a rich geological history, but it is unclear to what extent this history is expressed deeper within the mantle
Since the break-up of eastern Gondwana, and the formation of its rifted passive margins, the Australian continent has moved northwards colliding with SE Asia[4,5]
The QL waves detected are of varying amplitude which is a function of the strength of the anisotropic gradient and the geometry of the anisotropy relative to the angle of the raypath
Summary
The Australian continental crust preserves a rich geological history, but it is unclear to what extent this history is expressed deeper within the mantle. An investigation of Quasi-Love waves is performed to detect scattering of seismic surface waves at mantle depths (between 100–200 km) by lateral gradients in seismic anisotropy. The inferred scattering source and lateral anisotropic gradients are preferentially located either near the passive continental margins, or near the boundaries of major geological provinces within Australia. Australia is the fastest moving continent on Earth[6], exerting considerable shear on the underlying mantle asthenosphere Such deformation is thought to cause a lattice preferred orientation (LPO) in olivine generating strong azimuthal anisotropy aligned with plate motion[7,8]. Observations of Love-to-Rayleigh surface wave scattering, termed Quasi-Love (QL) waves[14], can be used to pinpoint the location of lateral gradients in seismic anisotropy in the upper mantle (Fig. 1), such as those that may exist along present (and/or past) plate boundaries. These polarization-distortions are sometimes referred to in the literature as quasi-Love waves [18], their characteristics are distinguishable from QL wave scattering as described here (see ‘Methods’ for further details)
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