Dislocation creep of olivine: Backstress evolution

13 Transient creep occurs during geodynamic processes that impose stress changes on rocks at high 14 temperatures. The transient is manifested as evolution in the viscosity of the rocks until steady-state flow 15 is achieved. Although several phenomenological models of transient creep in rocks have been proposed, 16 the dominant microphysical processes that control such behavior remain poorly constrained. To identify 17 the intragranular processes that contribute to transient creep of olivine, we performed stress-reduction 18 Preprint submitted to ESSOAr and Journal of Geophysical Research Solid Earth 2 tests on single crystals of olivine at temperatures of 1250–1300°C. In these experiments, samples undergo 19 time-dependent reverse strain after the stress reduction. The magnitude of reverse strain is ~10 and 20 increases with increasing magnitude of the stress reduction. High-angular resolution electron backscatter 21 diffraction analyses of deformed material reveal lattice curvature and heterogeneous stresses associated 22 with the dominant slip system. The mechanical and microstructural data are consistent with transient 23 creep of the single crystals arising from accumulation and release of backstresses among dislocations. 24 These results allow the dislocation-glide component of creep at high temperatures to be isolated, and we 25 use these data to calibrate a flow law for olivine to describe the glide component of creep over a wide 26 temperature range. We argue that this flow law can be used to estimate both transient creep and steady27 state viscosities of olivine, with the transient evolution controlled by the evolution of the backstress. This 28 model is able to predict variability in the style of transient (normal versus inverse) and the load-relaxation 29 response observed in previous work. 30