Constraints on the LAB from Seismology, Petrology and

Geodynamics/Mineral Physics

www.physicalgeography.net/ fundamentals/10h.html

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

Understanding the nature of the lithosphere-asthenoshpere boundary (LAB)

Partial melting in the asthenosphere

Wet/damp asthenosphere Solid state anelastic effects

Stixrude and Lithgow-Bertelloni 2005Hirth and Kohlstedt 1996

ZLAB

ZbaseZ

T

Hypotheses

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

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.

Stixrude and Lithgow-Bertelloni, JGR 2005

Solid-State LVZ?

Tan&Helmberger(2007)

Estimate Q models for LVZ under West Pacific

Data Source: 30 events with intermediate depth

6 different Q models with PA5 as velocity model:

Q30

Q50g

Q50 (original PA5)

Q70g

Q70

Q90g

Example of synthetic/observed seismograms with pa5_Q50 model

PA5 velocity model

Preliminary Result: High Q in West Pacific?Preliminary Result: High Q in West Pacific?

Test of data sensitivity to Q in LVZ

Synthetic SS/S ratios, relative to Q50

More sensitive to Q

Q30 Q50g Q50 Q70g Q70 Q90g

Residual Sum

13.2 12.6 9.2 9.7 7.4 7.8

Observed SS/S ratios relative to Q models

Japan

Non-Plume Intraplate Magmas near Japan

MotivationsPartial melting in asthenosphere or plume?

Hirano et al., 2006

Pressure ~ MORB

Temperature ~ MORB

Consistent with plate model -- Not plume

Inferred Pressure and Temperature

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

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Teleseismic S wave

59 events

556 stations

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Sdp SRF vs PRF

Pds

Moho

Moho

LAB

LAB

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Latitude 37 deg

Moho

LAB

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Longitude -119 deg

Moho

LAB

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Sierra drip

Zandt Nature 2004

Basalt whole rock data from NAVDAT database

Black: all dataRed: 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

Moucha et al. (2008)

Pref Plith≠ Pref Pref=Plith

E

c

c

mmm

Residual Topography = Observed topography - Isostatic Elevation (E)

m => constant, (P,TC)

Ref Lith

Variations in Isostatic Elevation

Depleted Mantle Density (kg/m3)

Isostatic

Elevation

-

Mean

Isostatic

Elevation

(meters)

63 km

45 km

30 km

Compositional and Thermal Constraints

Average Mantle Density (kg/m^3)

Residual Topography

(meters)

LZ from dynamic rheology?

flow law

simple flow:

slabplume

use rheologic flow lax + simple flow = consistent computing strategy

effective viscosity

generic dry oceanic system (dislocation creep)

60 Ma1450 K

solidus

prediction:

• developed LZ

• without melt or water

• strain rate localization

• anisotropy maximized

• descends with age

generic dry continental system (dislocation creep)

prediction:

• strong continental lithosphere

• pronounced LZ from solid state effects without melt or water

solidus adiabat

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