Title: Constraints on the LAB from Seismology, Petrology and GeodynamicsMineral Physics
1Constraints 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
2Understanding 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
3H0 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
4Melting depths vs seismic lid (also dynamic
topography and surface heat flow)
Observed Q vs theoretical Q
Geodynamics
5Testing 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.
6Null 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.
7Solid-State LVZ?
Stixrude and Lithgow-Bertelloni, JGR 2005
8Estimate Q models for LVZ under West Pacific
TanHelmberger(2007)
Data Source 30 events with intermediate depth
9Example 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
10More 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?
11Japan
12Non-Plume Intraplate Magmas near Japan Motivations
Partial melting in asthenosphere or plume?
Hirano et al., 2006
13Inferred Pressure and Temperature
Pressure MORB Temperature MORB Consistent
with plate model -- Not plume
14How 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
15Western USA
1659 events
556 stations
17SRF vs PRF
Moho
LAB
Pds
Moho
LAB
18(No Transcript)
19Basalt 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
20(No Transcript)
21Viscous 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
22Variations in Isostatic Elevation
Isostatic Elevation - Mean Isostatic Elevation
(meters)
63 km
45 km
30 km
Depleted Mantle Density (kg/m3)
23Compositional and Thermal Constraints
Residual Topography (meters)
Average Mantle Density (kg/m3)
24L?Z from dynamic rheology?
use rheologic flow lax simple flow consistent
computing strategy
flow law
simple flow
effective viscosity
plume
slab
25generic 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
26generic 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
27The 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