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Introduction

It has been hypothesized that deep earthquakes in subducting slabs might result from transformational faulting in cold peridodite wedges containing metastable olivine to depths of more than 600 km. The slab instability arises then from sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser phase [1,2]. One pre-supposition of this hypothesis is that the untransformed cold interior of fast subducting slabs is acting as a stress-guide for the slab down to 600 km depth. The viscosity of a cold (olivine) wedge extrapolated to low temperatures of about $\sim$ 850 K is indeed extremely high and no deformation seem to be possible on the geological timescale. On the other hand, seismic tomography reveals that slab bending seems to be possible in some of the western Pacific subduction zones [3,4]. Hence, mechanisms for the rheological weakening of subducted slabs are called for.

One mechanism that has been suggested is the weakening due to the grain-size reduction as a result of the olivine to spinel transformation [5,6,7]. The distinct possibility of a grain-size reduction accompanying this transformation was indeed indicated in several kinetic high-pressure experiments [5,8,9,10].

More recently, an attempt was made to quantify the extent of grain-size reduction under subduction conditions, based on a scaling model of nucleation and growth [11]. The estimate of the degree of grain-size reduction was done using available experimental data on the olivine-spinel transformation kinetics as reviewed, e.g., by Rubie and Ross [12], Kirby et al. [2] and Karato [13].

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Michael Riedel
1999-01-27