We interpret the three-dimensional seismic wavespeed
structure of the Australian upper mantle by comparing
its azimuthal anisotropy to estimates of past and
present lithospheric deformation. We infer the fossil
strain field from the orientation of gravity anomalies
relative to topography, bypassing the need to
extrapolate crustal measures, and derive the current
direction of mantle deformation from present-day plate
motion. Our observations provide the depth resolution
necessary to distinguish fossil from contemporaneous
deformation. The distribution of azimuthal seismic
anisotropy is determined from multi-mode surface-wave
propagation. Mechanical anisotropy, or the directional
variation of isostatic compensation, is a proxy for the
fossil strain field and is derived from a spectral
coherence analysis of digital gravity and topography
data in two wavelength bands. The joint interpretation
of seismic and tectonic data resolves a rheological
transition in the Australian upper mantle. At depths
shallower than 150-200 km strong seismic anisotropy
forms complex patterns. In this regime the seismic fast
axes are at large angles to the directions of principal
shortening, defining a mechanically coupled crust-mantle
lid deformed by orogenic processes dominated by
transpression. Here, seismic anisotropy may be
considered ``frozen'', which suggests that past
deformation has left a coherent imprint on much of the
lithospheric depth profile. The azimuthal seismic
anisotropy below 200 km is weaker and preferentially
aligned with the direction of the rapid motion of the
Indo-Australian plate. The alignment of the fast axes
with the direction of present-day absolute plate motion
(APM) is indicative of deformation by simple shear of a
dry olivine mantle. Motion expressed in the hot spot
reference frame matches the seismic observations better
than the no-net-rotation reference frame. Thus, seismic
anisotropy supports the notion that the hot-spot
reference frame is the most physically
reasonable. Independently from plate motion models,
seismic anisotropy can be used to derive a best-fitting
direction of overall mantle shear.
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