Intragranular stress heterogeneity records transient creep dy dislocation interactions in the Oman-UAE ophiolite

Abstract:

Large earthquakes transfer stress from the upper crust to the underlying viscoelastic lower crust and upper mantle, inducing postseismic creep. Recent experiments on olivine have provided a new rheological model for this transient creep based on accumulation and release of back stresses among dislocations. Here, we test whether natural rocks preserve dislocation-induced stress heterogeneity consistent with the back-stress hypothesis by mapping olivine from the palaeosubduction interface of the Oman-UAE ophiolite with high-angular resolution electron backscatter diffraction. The olivine preserves heterogeneous residual stresses that vary in magnitude by several hundred megapascals over length scales of a few micrometres. Large stresses are commonly collocated with elevated densities of geometrically necessary dislocations within subgrain interiors. These spatial relationships, along with characteristic probability distributions of the stresses, confirm that the stress heterogeneity results from the dislocation content. Images of dislocations revealed by oxidation decoration display bands of high and low dislocation density, often bounded by subgrain boundaries, suggesting that dislocation interactions contribute to organisation of the substructure. These results support the applicability of the back-stress model of transient creep to deformation in the mantle portion of plate-boundary fault zones.