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| Titel |
D" anisotropy and slip systems in post-perovskite |
| VerfasserIn |
Andy Nowacki, James Wookey, J.-Michael Kendall |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250033068
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| Zusammenfassung |
| The lowermost few hundred kilometres of the Earth’s mantle—known as D″—form the
boundary between it and the core below, control the Earth’s convective system, and are the
site of probable large thermochemical heterogeneity. Seismic observations of D″ show a large
(~2%) increase in S-wave velocity and significant seismic anisotropy (the variation of wave
speed with direction) are present in many parts of the region. On the basis of continuous
regions of fast shear velocity (V S) anomalies in global models, it is also proposed as the
resting place of subducted slabs, notably the Farallon beneath North America. The
MgSiO3-post-perovskite mineral phase is the most compelling explanation for
observations of anisotropy, though an outstanding question is how post-perovskite
and other mineral phases may deform to produce this: different mechanisms are
possible. With knowledge either of mantle flow or which slip system is responsible for
causing deformation, we can determine the other with the seismic anisotropy which is
created.
We investigate the dynamics at the CMB beneath North America using differential shear
wave splitting in S and ScS phases from earthquakes of magnitude MW > 5.5 in South and
Central America, Hawaii the Mid-Atlantic Ridge and East Pacific Rise. They are detected on
~500 stations in North America, giving ~700 measurements of anisotropy in D″. We achieve
this by correcting for anisotropy in the upper mantle (UM) beneath both the source
and receiver. The measurements cover three regions beneath western USA, the
Yucatan peninsula and Florida. In each case, two different, crossing ray paths are
used, so that the style of anisotropy can be constrained—only one azimuth cannot
distinguish differing cases. Our results showing ~1% anisotropy dependent on azimuth
are not consistent with transverse isotropy with a vertical symmetry axis (VTI)
anywhere. The same but with a tilted axis is possible (TTI) and would be consistent with
inclusions of seismically-distinct material such as melt. TTI planes of isotropy dip south
beneath Florida, southwest beneath western USA and southeast beneath Yucatan.
However we test other slip systems in MgO, pv and ppv to determine if deformation in
these phases can account for the observed anisotropy. The systems [100](010) and
[¯110](110) in ppv are consistent everywhere; pv is not beneath Yucatan. If we assume a
general downwelling and displacement of mantle material in the seismically fast D″,
corresponding to the impingement of slab material, slip along [100](010) seems more
likely. With a new breed of detailed mantle deformation models, or experimental
evidence of which slip system dominates, seismic anisotropy may be used to map
deformation in D″ and provide greater insight into Earth’s convecting interior. |
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