Formation and structure of lithospheric shear zones with damage

Shear localization is an integral part of tectonics on Earth. We examine the role of two different forms of microstructural weakening (voids and grain size reduction) in the formation of shear zones as a function of depth. The evolution of grain size and voids employed in this study is determined within the framework of two-phase damage theory. The shear zone model is characterized by two-dimensional simple shear, which allows for lithostatic pressure to influence the evolution of void-generating damage. We consider cases with pure void-generating damage, pure grain size reducing damage, and combined void-generating and grain size reducing damage. The introduction of lithostatic pressure alters void evolution, and specifically leads to a suppression of void-generation at depth. Grain size reducing damage produces the most significant localization. Differences in the time scale and efficacy of the two different damage mechanisms result from the mechanical constraints of viscous compaction and dilation imposed on void-generating damage. Cases with combined void-generating and grain size reducing damage lead to a more complicated interaction, with grain size reduction driving porosity to the flanks of the central shear zone. This result is proposed to be relevant for interpreting the void and grain size microstructures observed in faults on Earth.