YUANCHENGGUNG, MARKPANNING

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Global anisotropy and the thickness of
continents

• YUANCHENG GUNG, MARK PANNING BARBARA ROMANOWICZ
  
• Nature 422 , 707 - 711 (2003)

Or What is the Tectosphere, and can we see it?
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The Tectosphere Continental Roots
Postulated deep continental root depleted
material due to ?ancient? partial melting
(supported by velocities and by xenoliths),
hard to find - obscured by Lehman seismic
discontinuity (L) Originally thought to be
300-400 km deep, new data 200-250 km deep
(lithosphere is 100 km deep)
Importance Existence would resolve questions
about plate tectonic driving forces Explain
motions and rates of plates w/o connected
subduction zones Plates with large continents
-- slower, often no subducting slab,
?asthenospheric drag?
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S-Wave anisotropyGlobal vs continents only
Reduced correlation in the depth range
250400 km between SH/hybrid models and SV models
is strongly accentuated over continents.
Anisotropy is stronger, and more specific to
particular wave-forms under continents
Correlation as a function of depth Different
tomographic slowness models SV Vertical
component data SH Horizontal component
data RTS and L18 3 component data
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Depth range of tomographic anomalies
Different models reveal different depths and
extents of slowness anomalies beneath
continents Continental roots SV models -
lt200-250 km SH models - gt300-350 km
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Anisotropy maps at different depths
Relative lateral variations in the anisotropic
parameter  ln  gt 0 in regions where VSH gtVSV
 ln  lt 0 in regions where VSV ltVSH .
Significant   anisotropy at 175 km depth beneath
Pacific. 175 - 300 km depths shift in
postitive anisotropy regions from C, Pacific to
continents lt200 km continental shields
generally negative anisotropy 300 km depth,
continental shields little or no vertical
component gt350 km, subduction zones are more
prominent, EPR mantle is prominent Depth
resolution 50 km
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Cross Sections across cratons
SH and SV data Different results SH - deep
horizontal anisotropy tectosphere 300-350 km
thick SV shallower vertical anisotropy tectosp
here 220 km thick Implications for mantle
driving force cratons w/o subduction zones?