Investigations of seismic anisotropy beneath the Macquarie Ridge Complex

Abstract

The Macquarie Ridge Complex (MRC) is located along the southeastern Australian-Pacific plate boundary, south of New Zealand. Given its remote location in the Southern Ocean, seismic station deployments are lacking, and therefore very few direct observations of seismic anisotropy have been made. The dynamics of this plate boundary are however of great interest due to changes in the plate boundary style both along strike and over time. Particular segments of the MRC are associated with trans-pressional motion (e.g. McDougall and Macquarie segments), while subduction initiation has been inferred for others (e.g. Puysegur and Hjort segments). Here we conduct source-side splitting analysis to investigate upper mantle dynamics using earthquakes that have occurred along this plate boundary. We combine this with a detailed investigation of SKS/PKS receiver-side splitting at the long-running permanent station on Macquarie Island (station: MCQ). We find the results from shear-wave splitting are locally consistent but vary along the plate boundary, potentially suggesting changes in seismic anisotropy and the pattern of the underlying mantle deformation. Analysis of the First Fresnel Zone are applied to explore how the different ray-paths may sample the sub-surface anisotropy at various depths. A comparison is then made with expected patterns of anisotropy in the lithosphere versus from the asthenosphere, by comparing with predictions from seafloor spreading, plate motions, and global azimuthal anisotropy models. Following this, the primary source of the anisotropy detected is considered more likely from the asthenospheric upper mantle, rather than the oceanic lithosphere which is relatively young (~10-35 million years) and thin (less than ~77 km) in this region.

Altogether, our splitting observations demonstrate the pattern of mantle deformation beneath the MRC changes both with proximity to the plate boundary, as well as along the plate boundary itself, potentially reflecting the ongoing evolution of this plate boundary both in space and time.