We investigate the two-dimensional (2-D) nature of the coherence between
Bouguer gravity anomalies and topography on the Australian
continent. The coherence function or isostatic response is commonly
assumed to be isotropic. However, the fossilized strain field recorded
by gravity anomalies and their relation to topography is manifest in a
degree of isostatic compensation or coherence which does depend on
direction. We have developed a method that enables a robust and
unbiased estimation of spatially, directionally, and
wavelength-dependent coherence functions between two 2-D fields in a
computationally efficient way. Our new multispectrogram method uses
orthonormalized Hermite functions as data tapers, which are optimal
for spectral localization of nonstationary, spatially dependent
processes, and do not require solving an eigenvalue problem. We
discuss the properties and advantages of this method with respect to
other techniques. We identify regions on the continent marked by
preferential directions of isostatic compensation in two wavelength
regimes. With few exceptions, the short-wavelength coherence
anisotropy is nearly perpendicular to the major trends of the suture
zones between stable continental domains, supporting the geological
observation that such zones are mechanically weak. Mechanical
anisotropy reflects lithospheric strain accumulation, and its presence
must be related to the deformational processes affecting the
lithosphere integrated over time. Three-dimensional models of seismic
anisotropy obtained from surface wave inversions provide an
independent estimate of the lithospheric fossil strain field, and
simple models have been proposed to relate seismic anisotropy to
continental deformation. We compare our measurements of mechanical
anisotropy with our own model of the azimuthally anisotropic seismic
wave speed structure of the Australian lithosphere. The correlation of
isostatic anisotropy with directions of fast wave propagation gleaned
from the azimuthal anisotropy of surface waves decays with depth. This
may support claims that above 150-200 km, internally coherent
deformation of the entire lithosphere is responsible for the
anisotropy present in surface wave speeds or split shear
waves.
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