Mimicking postseismic creep in the laboratory: testing dislocation-based models for transient creep in the upper mantle

Studies of postseismic creep often infer a strong reduction in upper-mantle viscosity after large earthquakes, yet the microphysical mechanism responsible for this behavior remains subject of debate. Here, we present new gas-medium apparatus experiments at 1100–1200°C on pre-dried, annealed Åheim dunite (grain size of ~400 µm) that mimic the postseismic creep after rapid 'coseismic' loading. We demonstrate that the transient rheological behavior of these rocks matches remarkably well with predictions of the backstress model, which involves the evolution of dislocation density and long-range interactions among dislocations. Microstructural analyses link the evolution of viscosity up to 1.3 orders of magnitude in our experiments to an increase in dislocation density. Extrapolation to upper mantle conditions illustrates that the backstress model can explain the 1–2 orders of magnitude reduction in upper-mantle viscosity that is inferred from studies of postseismic creep, whereas alternative models involving the plastic anisotropy of olivine cannot.