Origin of Azimuthal Seismic Anisotropy in Oceanic Plates and Mantle

Abstract:

Seismic anisotropy is strongest in Earth's thermo-mechanical boundary
layers where azimuthal anisotropy should be straightforward to relate
to mantle flow. However, both frozen-in and active mantle convection
scenarios have been invoked, and no simple, global relationships
exist. We show that lattice preferred orientation (LPO) inferred from
mantle flow computations, in fact, produces a plausible global
background model for asthenospheric anisotropy underneath oceanic
lithosphere. The same is not true for absolute plate motion (APM)
models. A ~200 km thick layer where the flow model LPO matches
observations from tomography lies just below the ~1200C isotherm of a
half-space cooling model, indicating strong temperature-dependence of
the processes that control the development of azimuthal anisotropy. We
infer that the depth extent of shear, and hence the thickness of a
relatively strong oceanic lithosphere, can be mapped this way. These
findings for the background model, and ocean-basin specific deviations
from the half-space cooling pattern, are found in all of the three
recent surface wave models we considered. Further exploration of
deviations from the background model may be useful for general studies
of oceanic plate formation and dynamics as well as regional-scale
tectonic analyses.