Rheological properties of Earth’s mantle inferred from post-glacial rebound or post-seismic deformation likely correspond to transient creep because strain involved in these time-dependent deformation is small (e.g., [1-3]). However, most olivine deformation studies available, focused on “steady state” creep, and studies on transient creep are limited. Transient and “steady-state” creep involve different microscopic mechanisms particularly for dislocation creep (different slip systems) [3, 4].

Here I will be presenting results on transient creep obtained during in-situ high pressure deformation of olivine aggregate deformed in D-DIA for pressure between 3-7GPa and temperature ~ 1200C. strain was estimated from X-ray radiography of the samples during deformation. In all experiments, stress was estimated from X-ray diffraction of an isotropic stress sensor, Pyrope. The pyrope stress was compared with the estimated stress from the olivine aggregate deforming in dislocation creep regime.

Results show that, during deformation, olivine aggregate present a large (hkl) stress dependence due to large plastic anisotropy throughout the deformation, while pyrope shows much smaller (hkl) stress dependance (≤ 10%). Therefore, pyrope stresses represents the average stress of the olivine aggregate [5]. The comparison between the stress estimated from pyrope and the olivine aggregate seems to indicate a change in deformation mechanisms between transient creep and steady state creep. At low strain, pyrope stresses match the stresses in Olivine soft (hkl) (soft slip system). However, at large strain (steady state) stresses estimated from pyrope match the stresses in Olivine hard (hkl) (hard slip system).

These results have significant implications for small-strain geophysical processes, such as glacio-isostatic adjustment (GIA) and post-seismic creep. The depth and regional variations in rheological properties derived from GIA and post-seismic creep reflect largely the properties of the soft slip system(s) and are likely distinct from those relevant to mantle convection which is controlled largely by the hard slip system(s).

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