Viscous Granular Flow: DEM Models of Diffusion Creep

Many near-surface geologic processes involving porous, unconsolidated granular material have been successfully modeled by the Distinct Element Method, which treats the elastic and frictional interactions of a packing of grains. At deeper levels, crustal and mantle rocks are typically modeled as a continuum material, i.e., with intrinsic properties varying smoothly throughout a particular rock formation or layer. However, all rocks are complex aggregates of individual mineral grains, typically polyphase, with varying size, shape and spatial distributions and crystallographic orientations. Changes in these properties during deformation, in turn, may alter the rheology of the aggregate. We use the unique advantages of DEM to develop a better fundamental understanding of how grain scale interaction evolves with the rheology of rocks in shear zones.

A process that may be significant in shear zones throughout much of the crust and upper mantle, and amenable to modeling with DEM, is diffusion-accommodated grain boundary sliding. Although there is little to no porosity in these rocks, grains rearrange without significant change in shape, similar to frictional granular flow. Because this “viscous granular flow” involves grain boundary sliding aided by diffusion and/or dislocation creep, the strength of grain contacts will depend on local stresses and grain properties in very different ways than in typical frictional sliding. We use a modified DEM code that explores a range of contact force laws designed to mimic these high temperature deformation mechanisms. We discuss the utility of the DEM method in describing evolving rheology during diffusion creep in mono- and polyphase material.