Midmantle deformation between the Australian continent and the Fiji‐Tonga subduction zone?

Abstract

Shear wave splitting measurements are performed at temporary stations deployed across the Australian continent for direct S waves from events occurring in the neighboring subduction zones, especially Fiji‐Tonga. Unlike core refracted shear waves, direct S waves are not polarized at the core‐mantle boundary and are therefore subject to influences on the source side, on the receiver side, and along the ray path in between. Source side contamination due to the development of lattice preferred orientation of olivine can be minimized by considering deep events with an epicentral location below 410 km depth, the olivine‐spinel phase transition depth. Anisotropic contributions from the transition zone or lower mantle can only be reliably inferred in the case of high delays that cannot be generated solely by the upper mantle structure underneath a station. The analysis of a comprehensive data set for direct S waves from deep events leads to an average delay time of 0.9 s at stations deployed across the continent. Despite the limitations of such an arithmetic average, the result highlights the lack of evidence for midmantle deformation between the Fiji‐Tonga subduction zone and the Australian continent, unlike previously suggested in studies dealing with the permanent seismological stations only. A striking consistency between measurements performed across several stations belonging to an array in western Australia suggests a very coherent lithospheric structure underneath the Yilgarn craton.