Heterogeneous shear zone evolution: The role of shear strain hardening/softening

Mathematical models are proposed to describe heterogeneous shear zones characterized by variation thickness with time. These models take into account several combinations of deformation type regime (simultaneous simple shear, pure shear, and volume change) and degree of strain localization. Increasing or decreasing shear zone thickness results, respectively, from strain hardening where deformation progressively diffuses into the host rock, and strain softening where deformation gradually localizes within a narrow zone. Shear zone models are made of a series of layers, each characterized by homogenous strain. A quantitative evaluation of displacement–thickness relationship and finite strain development may be obtained from each model. Temporal evolution depends on the following parameters: (i) rate of shear strain hardening/softening, (ii) initial shear strain increment imposed to the system, and (iii) thickness of homogenously deformed units within a layered shear zone. Based on data coming from natural shear zones, displacement rates may be obtained independently of shear zone thickness.