Michael R Riedel (miker@GFZ-Potsdam)1, Shun-Ichiro Karato (firstname.lastname@example.org)2 & David A. Yuen (email@example.com)2
1GFZ Potsdam, Telegrafenberg C7, Potsdam, D-14473, FR Germany
2University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, MN 55455, U.S.A.
Rheology of a subducting slab has an important influence on its behaviour in the deep mantle. Slab rheology is likely to change both across its thickness and with depth due to large variation in temperature. Variation in grain-size caused by phase transformations could further complicate the rheological structure. Therefore estimation of slab rheology needs to incorporate large spatial variation in strength caused by these effects. We have developed a new "self-consistent" formulation to calculate slab strength in which high resolution numerical modeling of grain-size evolution is incorporated.
Here, we investigate the effects arising from the two non-linear feed-back mechanisms (i) latent heat release and (ii) kinetic grain-size reduction accompanying the olivine-spinel transformation on the dominating slab deformation mechanism. Since both mechanisms exert effects which are self-accelerating to slab rheology, they demand an extremely high spatial resolution in order to resolve the multiscale features present, typically with a spatial resolution of 100 meters or finer.
For this purpose, we assume a simple but efficient momentum balance equation for calculating the slab stress response to an external bending force. For each mineral, 3 independent creep mechanisms are considered: dislocation creep, diffusion creep and Peierls stress limited creep (Karato and Wu, 1993). It appears that the slab has a complicated viscosity distribution after the phase transformation: a very weak zone at the center due to small spinel grain size surrounded by relatively strong shoulders and finally warm and weak outer zones. Localized stress spikes appear in the neighbourhood of the phase transition region with large viscosity contrast.
For fast subduction (v~10 cm/yr), the cold and mechanically strong slab core bifurcates below 450km depth into 2 sideways branches, which can sustain stresses up to 5 kbar down to 600 km depth. Surprisingly, among the 6 different mechanisms, dislocation creep of olivine does not play any significant role for deformation here.
For slow slabs (v~4 cm/yr), there is no weak zone developing in the slab core and therefore, the resulting viscosity and stress profiles reveal a much simpler structure. Olivine dislocation creep appears to be one of the dominating deformation mechanisms.
The flexural viscosity of the slab vs. depth has been calculated in a 2-D parameter space consisting of the subduction velocity and the exerted bending moment. It is shown that, mainly due to the strong temperature dependence of spinel grain-size, the slab resistance against deformation increases anomalously with temperature in most of the temperature range.
Karato S & Wu P, Science 260, 771-778, (1993).
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