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Zusammenfassung:
Rheological properties of Earth can be inferred from several types of time-dependent deformation including the post-glacial isostatic adjustment (GIA) and post-seismic deformation (PSD). The time scale of deformation is ~103-104 y for GIA, and 1-10 y for PSD, as compared to 107-108 y for geological deformation. The stress magnitude associated with GIA and PSD is not much different from that with geological deformation, but the strain is vastly different: ~ elastic strain (or less) for GIA and PSD, while it is much larger than elastic strain for geological deformation.In order to link rheological properties inferred from small strain phenomena to rheological behavior associated with large strain, we need to understand the physical mechanisms of transient creep. Currently two models are proposed: (i) intra-granular and (ii) inter-granular transient creep. In the former, transient creep behavior is attributed to dislocation dynamics within a grain (with a given slip system), while in the latter, it is attributed to grain-grain interaction involving multiple slip systems. We have conducted deformation experiments of olivine aggregates and single crystals together using a synchrotron facility to test these models. Strain is measured by X-ray imaging while stress is measured by radial X-ray diffraction form a polycrystalline olivine as well as a pyrope stress sensor. The results show that in a polycrystal, the average stress is near the lower end of stress levels inferred from various diffraction planes (hkl) at small strains, but it becomes close to the upper end at large strains (with a transition strain close to the elastic strain). This suggests a transition in the rate controlling slip systems with strain. In comparison, the transition strain for a single crystal (for the [001] slip) is substantially smaller than elastic strain. We conclude that in a deforming polycrystal olivine, single crystals attain steady-state quickly and the transient behavior is largely controlled by the evolution of controlling slip systems caused by grain-grain interactions. Because sensitivity of deformation on some parameters such as water content differs among different slip systems, applications of rheological properties inferred from short-term geophysical with long-term deformation needs to be made with a great care.
