Dislocations and their stress field in natural peridotites from the palaeosubduction interface of the Oman-UAE ophiolite.

Abstract

Major seismic events induce period of transient creep, during which the effective viscosity of the rocks evolves with time, before steady-state creep is reached. Recent experimental work on olivine deformation resulted in a new rheological model for transient and steady-state creep based on back stresses between dislocations. However, observational research on structures in natural rocks that support this new model have not yet been conducted. Here, I analyse natural peridotites from the Oman-UAE ophiolite, where microstructural evidence for the processes involved in transient creep is potentially preserved, to compare their microstructures to those formed by experimental olivine deformation. This research uses oxidation decoration and high-angular resolution electron backscatter diffraction to characterise the microstructure and to map ‘free’ dislocation densities and stress heterogeneities. The results reveal banded structures of high dislocation densities, often bounded by subgrain boundaries and colocated with high stress heterogeneities of hundreds of megapascals over length scales of only a few micrometres. These results combined with the characteristic probability distribution of the stresses indicate that the stress heterogeneities originate from long-range dislocation interactions and that these interactions contribute to the organisation of the substructure. These results are similar to those from the experimental work on transient creep in olivine and support the applicability of the new rheological model to large-scale modelling of plate-boundary fault zones.