Boundary Layers in the Mantle; a Tomographer’s View
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Transcript of Boundary Layers in the Mantle; a Tomographer’s View
Boundary Layers in the Mantle;
a Tomographer’s View
Adam M. Dziewoński
VLAB Workshop, August 10, 2007
In cooperation with:Gőran EkstrőmYu Jeffrey GuBogdan Kustowski
Data – the critical element
Three types of data are needed: 1. Fundamental mode surface waves to resolve the near surface structure;
2. Overtone data to resolve structure in the transition zone and
3. Travel time data to resolve the lower mantle structure.
Only three research groups (Berkeley, Caltech/Oxford and Harvard) use this, or equivalent, combination of data
Velocity
lowattenuation
highattenuation
Attenuation
Recent velocity modelsare very well correlatedin the top 200 km, or so, of the mantle
Shapiro & Ritzwoller (2002)
Very sharp bottom of the cratonic lithosphere
From Kustowski et al. (2006)
Questions:• What processes cause radial anisotropy in a 100 Ma old oceanic lithosphere at a depth of 150 km?
• Why are the isotropic velocity anomalies correlated with the age of the oceanic plate down to a depth of 200 km, even though plate cooling models prefer 100 km thickness?
• What is the thickness of the continental lithosphere
Degree-2 Upper Mantle SignalShifts of the Spectral Peaks Plotted at the Pole Positions
Masters et al., 1982
Patterns of velocity anomalies above and below the 670 km discontinuity are not similar
From Gu et al. (2001)
Model S362ANIDepth 750 km
Strong, but spatially limited fast anomalies in the lower mantle may represent regions of limited penetration of subducted material accumulated in the transition zones
l
Pacific “superplume”
Romanowicz and Gung, 2002
QRLf12
(Q-1)
SAW24B16
(Vs)
The slow velocities in a superplume andhigh attenuation in the transition zonecould be explained if transition zone isalso a thermal boundary layer.
EPR
Conclusions• Overtone/waveform data are critical for resolution of the transition zone structure.
• The change in the spectrum across the discontinuity is as sharp as can be resolved at the present time.
• We conclude that the transition zone is a boundary layer that could be penetrated by episodic events, but does not permit steady state circulation across the 650 km discontinuity.
Megaplumes span whole lower
mantle
3-D view of +0.5% and -0.5% isosurfaceof S-velocity modelof Masters et al. (2000). The uppersurface is truncated at 800 km depthand lower – at CMB
Usually, models of the shear and compressional velocity are obtained independently. However, P-velocity depends both on shear modulus and bulk modulus. To isolate this interdependence, Su and Dziewonski (1997) formulated the inverse problem for a joint data set and derived 3-D perturbations of bulk sound velocity and shear velocity
Model of shear and bulk sound velocities
K-μ Inversion550 km Depth
At this depth, the shear and bulk sound velocities show high correlation; both show fast values in the Western Pacific and South America, likely associated with the ponding of subducted slabs.
K-μ Inversion2800 km Depth
Near the CMB, there is a distinct negative correlation between K/ρ and μ/ρ under the Pacific and African Superplumes.
Bulk Sound and Shear Velocity Anomalies
Correlation between the bulk sound and shear velocity anomalies changes from +0.7 in the transition zone to –0.8 in the lowermost mantle. From Su and Dziewonski, 1997.
K-μ Inversion2800 km Depth
In the vicinity of the “China High”, the correlation remains positive. This is likely an expression of compositional heterogeneity, perhaps, combined with a thermal one.