Transcript Constraints on the LAB from Seismology, Petrology and Geodynamics/Mineral Physics A. Bengston, M.

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
Understanding the nature of the lithosphereasthenoshpere boundary (LAB)
Hypotheses
Partial melting in the
asthenosphere
Wet/damp asthenosphere
Solid state anelastic effects
T
ZLAB
Z
Zbase
Stixrude and Lithgow-Bertelloni 2005
Hirth and Kohlstedt 1996
H0: The Asthenosphere results from solid-state anelasticity.
H1: The Asthenosphere is partially molten.
Establish reference
model for solid state
(anharmonicity and
anelasticity )
Petrologic constraints
on the origin depths of
magmas
?
=
?
=
Refine estimates of Q
beneath ocean basins
Seismic constraints on
depth of LVZ
Geodynamics of a low viscosity channel
(solid state creep reference)
Dynamic Topography Modeling
Melting depths vs seismic lid
(also dynamic topography
and surface heat flow)
Observed Q vs theoretical Q
Geodynamics
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.
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.
Solid-State LVZ?
Stixrude and Lithgow-Bertelloni, JGR 2005
Estimate Q models for LVZ under West Pacific
Tan&Helmberger(2007)
Data Source: 30 events with
intermediate depth
Example of synthetic/observed
seismograms with pa5_Q50 model
6 different Q models with PA5 as velocity model:
Q30
Q50g
PA5 velocity
model
Q50 (original PA5)
Q70g
Q70
Q90g
More sensitive to Q
Test of data sensitivity to Q in LVZ
Observed SS/S ratios relative to Q models
Synthetic SS/S ratios, relative to Q50
Residual
Sum
Q30
Q50g
Q50
Q70g
Q70
Q90g
13.2
12.6
9.2
9.7
7.4
7.8
Preliminary Result: High Q in West Pacific?
Japan
Non-Plume Intraplate Magmas near Japan
Motivations
Partial melting in asthenosphere or
plume?
Hirano et al., 2006
Inferred Pressure and Temperature
Pressure ~ MORB
Temperature ~ MORB
Consistent with plate model -- Not plume
How to get the melt Up?
Modified from
Garcia-Castellanos 2000
Current stress pattern (fps)
consistent with the model
prediction
Extension predicted by slab
pull model
The extension may facilitate
the melt rising up
Western USA
Teleseismic
S wave
59 events
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
556 stations
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TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
SRF vs PRF
Sdp
Moho
LAB
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Pds
LAB
Moho
Latitude 37 deg
Moho
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TIFF (Uncompressed) decompressor
are needed to see this picture.
LAB
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Longitude -119 deg
Moho
LAB
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Sierra drip
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TIFF (Uncompressed) decompressor
are needed to see this picture.
Zandt Nature 2004
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
Viscous Radial Forces Acting on the Base of the
Lithosphere ~ Dynamic Topography
Ref
Lith
c
m
m
Pref
Plith≠ Pref
E
c
m
Pref=Plith
Moucha et al. (2008)
Residual Topography =
Observed topography - Isostatic Elevation (E)
m => constant, (P,TC)
Variations in Isostatic Elevation
Isostatic
Elevation
-
63 km
45 km
Mean
Isostatic
30 km
Elevation
(meters)
Depleted Mantle Density (kg/m3)
Compositional and Thermal Constraints
Residual
Topography
(meters)
Average Mantle Density (kg/m^3)
LZ from dynamic rheology?
use rheologic flow lax + simple flow = consistent computing strategy
flow law
simple flow:
effective viscosity
plume
slab
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
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
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