Backstress governs transient postseismic creep in the continental lithospheric mantle

Sheng Fan, Thomas Breithaupt, Claudia A. Trepmann, David Wallis

Understanding the physical processes that control how the continental lithospheric mantle responds to sudden stress changes during the earthquake cycle is essential for interpreting postseismic deformation and constraining the rheological behaviour of the mantle. Laboratory studies have revealed that long-range elastic interactions among dislocations generate backstresses that strongly influence the mechanical response of olivine during transient creep, and the relevance of this mechanism to natural contexts can be confirmed by analysing the stress fields of dislocations in various tectonic settings. Here, we examine peridotites from the Finero Complex in the Ivrea-Verbano Zone, European Alps, which preserve microstructures formed during transient, earthquake-related deformation in a slice of exhumed continental lithospheric mantle. High-angular resolution electron backscatter diffraction reveals pronounced intragranular stress heterogeneity in olivine, with stresses varying by hundreds of megapascals over length scales on the order of 1–10 µm and correlated with elevated densities of geometrically necessary dislocations. These observations demonstrate that long-range dislocation interactions occur in naturally deformed olivine of the continental lithospheric mantle. Superimposing the grain size-stress data on deformation-mechanism maps indicates that a burst of dislocation glide dominates immediately after a stress increase, whereas both dislocation-mediated deformation and diffusion creep contribute at steady state, with diffusion creep relatively more important in finer-grained domains. These inferences are consistent with variations in intragranular substructure and crystallographic preferred orientation. Having demonstrated that long-range elastic interactions contribute to transient creep of the continental lithospheric mantle, we use a dislocation-based microphysical to explore the characteristics of such transients in contexts similar to those represented by the Finero peridotite. This analysis indicates that an earthquake-induced stress increase produces a short-lived reduction by 2–3 orders of magnitude in viscosity followed by progressive restrengthening as backstress builds up over plastic strains on the order of 10-3. These results demonstrate that time-dependent microphysical models, rather than flow laws constructed for steady state alone, are required to describe the postseismic evolution of mantle viscosity.