Transient Creep in Olivine at Shallow Mantle Pressures: Implications for Time‐Dependent Rheology in Post‐Seismic Deformation.

Transient creep in olivine aggregates has been studied by stress‐relaxation experiments at pressures of 1.7–3.6 GPa and at temperatures of ≤1020 K in a DIA apparatus. Time‐dependent deformation of olivine at small strains (<0.07) was monitored with an ∼1 s of time resolution using a combination of a high‐flux synchrotron X‐ray and a cadmium telluride imaging detector. The observed deformation was found to follow the Burgers creep function with the transient relaxation time ranging from 50 (±20) to 1,880 (±750) s. We show that the Burgers creep for olivine cannot account for the low viscosities in early post‐seismic deformation reported by geodetic observations (<7 × 1017 Pa·s). In contrast, the time‐dependent increase in viscosity observed in late post‐seismic deformation (1018−1020 Pa·s) is explained by the Burgers rheology, suggesting that the combination of the Burgers model and another model is needed for the interpretation of post‐seismic deformation. Plain Language Summary: Geodetic observations reveal that the deformation of the crust and upper mantle after a great earthquake continues for decades. Viscosities of the upper mantle estimated from early post‐seismic deformation are often significantly low (1017−1018 Pa·s) and continuously increase to a typical value (∼1020 Pa·s). This characteristic of post‐seismic mantle flow cannot be explained by partial melting (nor water weakening) of upper mantle rocks. Here we performed small‐strain deformation experiments on natural olivine, which is the major mineral in the upper mantle, via a state‐of‐the‐art technology large‐volume deformation apparatus combined with high‐flux synchrotron X‐ray observations. We have successfully shown that the reported time‐dependent crustal deformation, which continues for decades after a great earthquake, is explained by the transient creep of olivine. Key Points: In situ deformation experiments on olivine were performed using a high‐flux synchrotron X‐ray and a cadmium telluride imaging detectorTransient creep of olivine aggregates follows the Burgers creep function at upper mantle pressures and temperaturesTime‐dependent rheology of the shallow mantle observed in the late post‐seismic deformation is explained by the Burgers model [ABSTRACT FROM AUTHOR]