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| Titel |
The viscosity of Earth's lower mantle inferred from sinking speed of subducted lithosphere |
| VerfasserIn |
H. Cizkova, A. P. van den Berg, W. Spakman, C. Matyska |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250063003
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| Zusammenfassung |
| The viscosity of the mantle is indispensable for predicting Earth´ s mechanical
behavior at scales ranging from deep mantle material flow to local stress accumulation
in earthquakes zones. Mantle viscosity is, however, not well determined. For the
lower mantle, particularly, only few constraints result from elaborate high-pressure
experiments (Karato, 2008) and a variety of viscosity depth profiles result from joint
inversion of the dynamic geoid and postglacial rebound data (Forte and Mitrovica,
1996; Kaufmann and Lambeck, 2000; Mitrovica and Forte, 2004). Here we use
lower-mantle sinking speed of lithosphere subduction remnants as a unique internal
constraint on modeling the viscosity profile. We perform a series of dynamic subduction
calculations in the models with complex composite rheology spanning a range of
viscosity profiles in the lower mantle. We focus on the models with detached remnants
resulting from the slab break-off, that sink to the lower mante. Using these models
we select profiles that predict the inferred sinking speed of 12 ± 3 mm/yr (van
der Meer et al., 2010). Our modeling shows that sinking speed is very sensitive
to lower mantle viscosity. The best-fitting viscosity profiles are associated with
subduction models that show accumulation or thickening of the slab, but minor
temporal stagnation associated with the phase change at 660 km and a mild increase of
viscosity in the top of the lower mantle by a factor of about three. The sinking
speed constrains almost uniform viscosity models of the lower mantle to a viscosity
value of 1 - 2 Ã 1022 Pas. Higher amplitudes of the lower mantle viscosity (and an
associated step-wise increase at the 660 km phase boundary) are responsible for the
detached slab being stagnant for several 10s of millions of years at the top of the
lower mantle. This yields a corresponding delay in age-depth curves and leads to
average deviating from the inferences of van der Meer et al. (2010). A weaker lower
mantle, on the other hand, produces slabs that are too fast and reach the base of
the mantle in much less than 200 Myr. Viscosity profiles incorporating a viscosity
maximum in the deep lower mantle, as proposed in numerous studies, only lead to
a good prediction if a significant viscosity reduction is prescribed just above the
core-mantle boundary. The low viscosity anomalies at the bottom boundary layer could be
explained by the presence of the weak post-perovskite (Ammann et al., 2010) or by the
steep temperature gradient in the D” layer. Our prefered model with a viscosity
maximum at 2500 km depth has an average lower mantle viscosity of 3 Ã 1022 Pas. |
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