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| Titel |
Viscous Dissipation and Criticality of Subducting Slabs |
| VerfasserIn |
Mike Riedel, Shun Karato, Dave Yuen |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250135286
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| Publikation (Nr.) |
 EGU/EGU2016-16133.pdf |
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| Zusammenfassung |
| Rheology of subducting lithosphere appears to be complicated. In the shallow part,
deformation is largely accomodated by brittle failure, whereas at greater depth, at higher
confining pressures, ductile creep is expected to control slab strength.
The amount of viscous dissipation ΔQ during subduction at greater depth, as constrained
by experimental rock mechanics, can be estimated on the basis of a simple bending moment
equation [1,2]
2ε˙0(z) ∫ +h∕2 2
M (z) = h ⋅ −h∕2 4μ(y,z)y dy ,
(1)
for a complex multi-phase rheology in the mantle transition zone, including the effects of a
metastable phase transition as well as the pressure, temperature, grain-size and stress
dependency of the relevant creep mechanisms; μ is here the effective viscosity and ε˙0(z) is a
(reference) strain rate.
Numerical analysis shows that the maximum bending moment, Mcrit, that can be
sustained by a slab is of the order of 1019 Nm per m according to Mcrit∼=σp ∗h2∕4, where
σp is the Peierl’s stress limit of slab materials and h is the slab thickness. Near Mcrit, the
amount of viscous dissipation grows strongly as a consequence of a lattice instability of
mantle minerals (dislocation glide in olivine), suggesting that thermo-mechanical instabilities
become prone to occur at places where a critical shear-heating rate is exceeded, see figure.
This implies that the lithosphere behaves in such cases like a perfectly plastic solid
[3].
Recently available detailed data related to deep seismicity [4,5] seems to provide support
to our conclusion. It shows, e.g., that thermal shear instabilities, and not transformational
faulting, is likely the dominating mechanism for deep-focus earthquakes at the bottom of
the transition zone, in accordance with this suggested “deep criticality” model.
These new findings are therefore briefly outlined and possible implications are
discussed.
References
[1] Riedel, M. R., Karato, S., Yuen, D. A. Criticality of Subducting Slabs. University of Minnesota
Supercomputing Institute Research Report, UMSI 99/129: 21 pages, 1999.
[2] Karato, S., Riedel, M. R., Yuen, D. A. Rheological structure and deformation of subducted slabs
in the mantle transition zone: implications for mantle circulation and deep earthquakes. Physics of the
Earth and Planetary Interiors, 127, doi:10.1016/S0031-9201(01)00223-0, 2001.
[3] Buffett, B. A., Becker, T. W., Bending stress and dissipation in subducted lithosphere. Journal of
Geophysical Research, 117, doi:10.1029/2012JB009205, 2012.
[4] Zhan, Z., Kanamori, H., Tsai, V. C., Helmberger, D. V., Wei, S., Rupture complexity of the
1994 Bolivia and 2013 Sea of Okhotsk deep earthquakes. Earth and Planetary Science Letters, 385,
doi:10.1016/j.epsl.2013.10.028, 2014.
[5] Meng, L., Ampuero, J.-P., Bürgmann, R., The 2013 Okhotsk deep-focus earthquake: Rupture beyond
the metastable olivine wedge and thermally controlled rise time near the edge of a slab. Geophys. Res.
Lett., 41, doi:10.1002/2014GL059968, 2014. |
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