The influence of lithospheric thickness variations beneath Australia on
seismic anisotropy and mantle flow
Abstract
Rapid plate motion, alongside pronounced variations in age and thickness
of the Australian continental lithosphere, make it an excellent location
to assess the relationship between seismic anisotropy and
lithosphere-asthenosphere dynamics. In this study, SKS and PKS
shear-wave splitting is conducted for 176 stations covering the
transition from the South Australian Craton to eastern Phanerozoic
Australia. Comparisons are made with models of lithospheric thickness as
well as numerical simulations of mantle flow. Splitting results show
uniform ENE-WSW aligned fast directions over the Gawler Craton and
broader South Australian Craton, similar to the orientation of crustal
structures generated during an episode of NW-SE directed compression and
volcanism ~1.6 billion years ago. We propose that heat
from volcanism weakened the lithosphere, aiding widespread lithospheric
deformation, which has since been preserved in the form of frozen-in
anisotropy. Conversely, over eastern Phanerozoic Australia, fast
directions show strong alignment with the NNE absolute plate motion.
Overall, our results suggest that when the lithosphere is thin
(<125 km), lithospheric contributions are minimal and
contributions from asthenospheric anisotropy dominate, reflecting shear
of the underlying mantle by Australia’s rapid plate motion above.
Further insights from geodynamical simulations of the regional mantle
flow-field, which incorporate Australian and adjacent upper mantle
structure, predict that asthenospheric material would be drawn in from
the south and east towards the fast-moving continental keel. Such a
mechanism, alongside interactions between the flow field and
lithospheric structure, provides a plausible explanation for
smaller-scale anomalous splitting patterns beneath eastern Australia
that do not align with plate motion.