[SIO GP Seminars] Thorsten Becker, USC, Today at 3:00

[Matt Wei]

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Today, Mar. 16, 3:00 PM
(refreshments served at 2:45 PM)
Munk Conference Room

Thorsten Becker
USC

"Isotropic, azimuthal, and radially anisotropic mantle structure: The  
new, true, blue."

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Abstract:

Seismic shear waves travel faster in the upper mantle when they are
polarized horizontally (SH) rather than vertically (SV), and
deformation of rocks under dislocation creep in mantle convection has
been invoked as a general explanation for such radial anisotropy.  Flow
in the upper mantle may thus potentially be constrained by modeling the
textures that form by lattice preferred orientation of intrinsically
anisotropic grains.  While there have been detailed models of the
azimuthal type of anisotropy as seen by surface waves, the stronger
radial version has previously only been considered in semi-quantitative
models.
Here, I will review basic constraints on mantle structure
(tectospheria, slabland, and pile-o-tania) and new work on improving
synthetic anisotropy models, including the first quantitative models of
radial anisotropy.  We show that both the depth-dependence of average
amplitudes as well as lateral deviations from the mean at
sub-lithospheric depths are predicted by improved mantle flow
computations.  These new models include texture formation as well as
global power-law rheology and were recently shown to explain texture
heterogeneity in laboratory samples and mantle xenoliths.  We find that
radial anisotropy amplitudes provide a strong constraint for mantle
rheology and are able to associate the "unique" fast and slow SV
regions within the Pacific basin at ~150 km depth with upwellings
underneath the East Pacific Rise and shearing in the asthenosphere,
respectively.
By adding radial anisotropy as a tool for applied geodynamics, a more
comprehensive understanding of the origin of upper mantle anisotropy is
emerging. Improved models of asthenospheric flow predict the strongest
global anisotropy features underneath oceanic plates; continental
anisotropy at shallow depths is, however, under-predicted, which
implies a signature of frozen-in deformation in cratons. At the same
time, geodynamics over-predicts azimuthal anisotropy from surfaces
waves but matches regional SKS splitting. This complication may be
telling us something about the averaging properties of surface waves
and the complex nature of texture formation and archiving over billions
of years.

Have a good day.

Matt

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Meng Wei ( Matt )
Institute of Geophysics and Planetary Physics
Scripps Institution of Oceanography
University of California, San Diego
La Jolla, CA 92093-0225
mwei at ucsd.edu
(858) 822-4347
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