Effects of temperature-dependent thermal diffusivity on shear instability in a viscoelastic zone: Implications for faster ductile faulting and earthquakes in the spinel stability field

The introduction of a new model of thermal diffusivity has motivated us to reinvestigate a one-dimensional viscoelastic shear zone model with realistic rheology, temperature-dependent thermal diffusivity (κ(T)) and viscous dissipation. Although thermal diffusivity in the shear zone is spatially heterogeneous with κ(T) and viscous heating, the spatial distribution of κ(T) does not affect shear zone evolution for the mesh resolution used in the model. As temperatures increase above room temperature, thermal diffusivity decreases. The lower thermal diffusivity causes a slight spatial thinning of the shear zone and an acceleration of the onset of instability relative to cases using a room temperature value of thermal diffusivity. Increasing the nonlinearity of κ(T) enhances shear zone thinning and speed-up of instability; the amount of enhancement depends on temperature, mineralogy and the rate of shear heating. The rheology of spinel creates a more unstable situation for the shear zone than that of olivine, but the boundary separating instability and stability is sensitive to changes in material properties. A decrease in the grain size does not influence the timescale of instability, unless grain size reduction causes diffusion creep to be the dominant deformation mechanism. Viscoelastic thermal-mechanical instabilities occur on timescales ranging from a few hundred to several thousand years. In most slabs, no instability is found to occur in spinel regions at temperatures above 1200 K. Likewise, shear instability in olivine at upper mantle depths will not occur at temperatures greater than 1100 K. (C) 2000 Elsevier Science B.V. All rights reserved.