Constraints on the LAB from Seismology, Petrology and GeodynamicsMineral Physics

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Constraints on the LAB from Seismology, Petrology
and Geodynamics/Mineral Physics
A. Bengston, M. Blondes, M. Collier, J. Gaherty,
T. Höink, M. Jiang, E. Kite, C.-T. Lee, A.
Levander, J. Li, Q. Li, P. Luffi, M. Manga, M.
Miller, J. Naliboff, T.-L. Tseng, D. Weeraratne,
Y. Xu, T. Yano, Z. Yang, Y. Zhang
www.physicalgeography.net/ fundamentals/10h.html
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Understanding the nature of the
lithosphere-asthenoshpere boundary (LAB)
Hypotheses
Solid state anelastic effects
Wet/damp asthenosphere
Partial melting in the asthenosphere
Stixrude and Lithgow-Bertelloni 2005
Hirth and Kohlstedt 1996
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H0 The Asthenosphere results from solid-state
anelasticity. H1 The Asthenosphere is partially
molten.
Establish reference model for solid
state (anharmonicity and anelasticity )
?
Refine estimates of Q beneath ocean basins

Petrologic constraints on the origin depths of
magmas
Seismic constraints on depth of LVZ
?

Geodynamics of a low viscosity channel (solid
state creep reference) Dynamic Topography
Modeling
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Melting depths vs seismic lid (also dynamic
topography and surface heat flow)
Observed Q vs theoretical Q
Geodynamics
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Testing LVZ Hypotheses with Thermodynamically
Calculated Seismic Velocities and Estimates of Q
Input (P,T,C)
Calculate equilibrium phase assemblages elastic
constants
Test null hypothesis by comparing calculated
seismic velocities with Q corrections to seismic
observations.
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Null Hypothesis for LVZ

• For a given composition and temperature,
  solid-state anhydrous processes can explain the
  low-velocity zone observed in some regions
  beneath the lithosphere.
• Solid-state processes
• Attenuation related to anelasticity
• Seismic anisotropy related to solid-state
  dislocation creep.
• Estimates of attenuation in the upper mantle
• Romanowicz (1995), Faul and Jackson (2005), this
  group.

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Solid-State LVZ?
Stixrude and Lithgow-Bertelloni, JGR 2005
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Estimate Q models for LVZ under West Pacific
TanHelmberger(2007)
Data Source 30 events with intermediate depth
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Example of synthetic/observed seismograms with
pa5_Q50 model
6 different Q models with PA5 as velocity
model Q30 Q50g Q50 (original PA5) Q70g Q70 Q90g
PA5 velocity model
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More sensitive to Q
Test of data sensitivity to Q in LVZ Synthetic
SS/S ratios, relative to Q50
Observed SS/S ratios relative to Q models
Preliminary Result High Q in West Pacific?
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Japan
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Non-Plume Intraplate Magmas near Japan Motivations
Partial melting in asthenosphere or plume?
Hirano et al., 2006
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Inferred Pressure and Temperature
Pressure MORB Temperature MORB Consistent
with plate model -- Not plume
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How to get the melt Up?
Current stress pattern (fps) consistent with the
model prediction Extension predicted by slab pull
model The extension may facilitate the melt
rising up
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Western USA
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59 events
556 stations
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SRF vs PRF
Moho
LAB
Pds
Moho
LAB
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Basalt whole rock data from NAVDAT
database Black all data Red most likely to be
unaffected by petrologic complexity 1)
likely not highly modified 2) likely
saturated only in olivine
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Viscous Radial Forces Acting on the Base of the
Lithosphere Dynamic Topography
E
Ref
Lith
?c
Moucha et al. (2008)
?c
Residual Topography
Observed topography - Isostatic Elevation (E)
?m
?m
?m
???m ?constant, ?(P,TC)
Pref
Plith? Pref
PrefPlith
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Variations in Isostatic Elevation
Isostatic Elevation - Mean Isostatic Elevation
(meters)
63 km
45 km
30 km
Depleted Mantle Density (kg/m3)
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Compositional and Thermal Constraints
Residual Topography (meters)
Average Mantle Density (kg/m3)
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L?Z from dynamic rheology?
use rheologic flow lax simple flow consistent
computing strategy
flow law
simple flow
effective viscosity
plume
slab
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generic dry oceanic system (dislocation creep)
solidus

• prediction
• developed L?Z
• without melt or water
• strain rate localization
• anisotropy maximized
• descends with age

60 Ma 1450 K
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generic dry continental system (dislocation creep)
solidus
adiabat

• prediction
• strong continental lithosphere
• pronounced L?Z from solid state effects
  without melt or water

surface heat flow 41 mW/m2
crustal heat production 0.6 ?W/m2
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The LAB is hot, weak, produces melt (at least in
some places) and might be wet.
A. Bengston, M. Blondes, M. Collier, J. Gaherty,
T. Höink, M. Jiang, E. Kite, C.-T. Lee, A.
Levander, J. Li, Q. Li, P. Luffi, M. Manga, M.
Miller, J. Naliboff, T.-L. Tseng, D. Weeraratne,
Y. Xu, T. Yano, Z. Yang, Y. Zhang