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| Titel |
Viscosity in transition zone and shallow lower mantle - Insight from numerical models of subduction |
| VerfasserIn |
Javier Quinteros, Stephan V. Sobolev |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250056704
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| Zusammenfassung |
| A viscosity profile throughout the upper 1000 km of mantle is far from being well known.
Different models were proposed to find the best-fit viscosity distribution based on
the imposition of different sets of constraints (e.g. geoid, Haskell, tomography
model). There are some common factors in all the possible profiles: 1) there seems
to be a sharp viscosity increment between transition zone (TZ) and lower mantle
(LM) and 2) a slow increase in viscosity with depth in the first hundreds of km of
LM.
The positive Clapeyron slope associated with the olivine-spinel phase transition at ~410
km favors that the cold slab can go through it, accelerating the subduction process. The
opposite situation occurs on the spinel-perovskite phase transition at ~660 km, where the
Clapeyron slope is negative and inhibits or slows the subduction. Whether the slab is
able to sink beyond this boundary depends, not only on the value of the Clapeyron
slope, but also on other factors like absolute velocities of the plates, viscosity of
the underlying mantle (TZ and LM) and discontinuities in its distribution, among
others.
In this set of experiments, we represent the viscosity profile of TZ and LM in a simplified
way by means of two parameters: viscosity in TZ and viscosity step at the spinel-perovskite
phase transition.
We used a coupled elasto-visco-plastic thermomechanical numerical model based on code
SLIM-3D to run all experiments. The model has true free surface and elastic deformation is
included. Realistic rheology was applied to the subducting and overriding plates as well
as the upper mantle. In this part of the model, viscosity is stress and temperature
dependent. Three different types of creep (diffusion, dislocation and Peierls) are
included. In particular, Peierls creep plays a key role to reduce stress in the deep cold
slab.
From our results we can conclude that the most reasonable range of viscosity values
for the TZ would be (3 * 1020 - 1021) Pa s. A value of 3 * 1021 is too high to
allow a subducting slab to penetrate into the LM with a reasonable velocity if a
viscosity step is present. The same situation arises if the viscosity step is greater
than 10, in which case most of the times a piling up of material next to the LM or
flat slabs can be seen, but no direct penetration. It should also be noted that, for
higher jumps, viscosity in LM would be too large under certain combinations. A
substantial reduction of the Clapeyron slope at the 660 km boundary did not help to
enhance subduction and showed only a small influence over the evolution of the
slab.
On the other hand, when viscosity in TZ is significantly lower than 3 * 1020 Pa s slab
accelerates to more than 30 cm/yr while (or before) penetrating the TZ, a phenomenon that is
rarely seen in nature. If viscosity step is not present, exaggerated or very high velocity values
are also obtained for the most reasonable viscosity. The best results were obtained with a step
around 5. Even if overriding velocity can change the style of subduction toward more
flattened slab, it cannot much reduce the exaggerated velocities obtained with the low values
of both variables.
With a viscosity of 3 * 1020 Pa s in TZ, a step between 5 and 10 gives the best velocity
patterns, considering that this is a case of fast subduction. If viscosity is 1021 Pa s, the step
between TZ and LM should be smaller than 5 in order to get reasonable subduction
velocities. |
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