Upper Mantle Seismic Anisotropy as a Constraint for Mantle Flow and
Continental Dynamics of the North American Plate
Abstract
The alignment of intrinsically anisotropic olivine crystals under
convection is typically invoked as the cause of the bulk of seismic
anisotropy inferred from shear-wave splitting (SWS). This provides a
means of constraining the interplay between continental dynamics and the
deep mantle, in particular for densely instrumented regions such as
North America after USArray. There, a comparison of “fast
orientations” from SWS with absolute plate motions (APM) suggests that
anisotropy is mainly controlled by plate motions. However, large
regional misfits and the limited realism of the APM model motivate us to
further explore SWS based anisotropy. If SWS is estimated from olivine
alignment in mantle circulation instead, plate-driven flow alone
produces anisotropy that has large misfits with SWS. The addition of
large-scale mantle density anomalies and lateral viscosity variations
significantly improves models. Although a strong continental craton is
essential, varying its geometry does, however, not improve the
plate-scale misfit. Moreover, models based on higher resolution
tomography degrade the fit, indicating issues with the flow model
assumptions and/or a missing contributions to anisotropy. We thus
compute a “lithospheric complement” to achieve a best-fit, joint
representation of asthenospheric and frozen-in lithospheric anisotropy.
The complement shows coherent structure and regional correlation with
independently imaged crustal and upper mantle anisotropy. Dense SWS
measurements therefore provide information on depth-dependent anisotropy
with implications for tectonics, but much remains to be understood about
continental anisotropy and its origin.