Stratification of Anisotropy in the Pacific Upper Mantle

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Stratification of Anisotropy in the Pacific Upper
Mantle

• Daniel B. Smith, Michael H. Ritzwoller, and
  Nikolai M. Shapiro
• University of Colorado at Boulder

Motivating Question Is azimuthal anisotropy
related simply to plate motions now (conforming
anisotropy) or at the time of formation of the
plate (fossil anisotropy)?
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Data and Method
Data
Shorter period group speeds (lt50 sec)
provide sensitivity to shallow lithosphere.
Contrast anisotropy at 25 - 50 sec -- 25
- 50 km peak sensitivity. 100 - 150 sec -- 90
- 150 km peak sensitivity.
Rayleigh wave group speeds
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Data and Method
Method Isotropic part Finite frequency
tomography (spatially extended sensitivity
kernels). Anisotropic part Ray theory with
ad-hoc smoothing. Absolute amplitudes
smaller scale patterns (lt1000 km) are
indeterminate. Large-scale spatial patterns and
relative amplitudes (e.g., function of age)
are robust.
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Amplitude Problem
underdamped
overdamped
50 s Rayleigh wave
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Results 2-Psi Fast Axis Directions
Amplitudes larger in the young than the old
Pacific. Large amplitudes trend NW from the
EPR. Fast axes align perpendicular to the EPR
at all periods. 100 s 150 s maps similar
across the Pacific. 25 s 50 s maps mostly
similar. Contrast 50 s 150 s maps in the
old Pacific.
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Azimuthal Anisotropy Fast Axis Directions vs
Present Plate Motions
Present-Day Plate Motion
HS3-NUVEL-1A (Gripp Gordon, 2002)
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Azimuthal Anisotropy Fast Axis Directions vs
Present Plate Motions
Present-Day Plate Motion
HS3-NUVEL-1A (Gripp Gordon, 2002)
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Azimuth Difference Fast Axes vs Present Plate
Motions
50 sec Avg difference 34 deg lt30 Ma -- 20
deg gt70 Ma -- trend from 25 - 60 deg
150 sec Avg difference 17 deg lt30 Ma -- 10
deg gt70 Ma -- 20 deg
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Azimuth Difference Fast Axes vs Present Plate
Motions
Near the EPR fast axes align nearly perpendicular
to PPM at all periods. At 70 My, agreement
begins to break down at short and
intermediate periods. Beyond 100 My,
disagreement is severe at periods below 80
s. Thus, in the shallow lithosphere (lt75 km) in
the old Pacific fast axes are not aligned with
present plate motions, but they are in the
underlying asthenosphere (gt125 km).
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Azimuthal Anisotropy Fast Axis Directions vs
Paleo-Spreading Directions
Paleo-Spreading Directions
Determined from the gradient of lithospheric age.
Mueller et al., 1997
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Azimuthal Anisotropy Fast Axis Directions vs
Paleo-Spreading Directions
Paleo-Spreading Directions
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Azimuth Difference Fast Axes vs Paleo-Spreading
Directions
50 sec Avg difference 17 deg lt30 Ma -- 15
deg gt70 Ma -- no trend
150 sec Avg difference 25 deg lt30 Ma --
20 deg gt70 Ma -- trend from 30 - 45 deg
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Stratification of Anisotropy Beneath the Pacific

• At large scales the story that emerges is a
  pretty simple one of
• anisotropy stratification
• In the deep lithosphere
• asthenosphere, anisotropic
• fast axes conform with current
• plate motions.
• In the shallow lithosphere
• (lt75 km), fast axes appear to
• be fossilized, aligning closer
• to the paleo-spreading directions.
• There are regional variations and the story at
  very short periods (20 s)
• Is more complicated -- both issues deserve
  further study.

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Mechanisms to Produce Stratification of
Anisotropy Beneath the Pacific
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Amplitude of Azimuthal Anisotropy as a Function
of Age
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Azimuthal Anisotropy Fast Axis Directions vs
Paleo-Spreading Directions
Paleo-Spreading Directions
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