Transient behavior in the lithosphere driven by coaxial narrowing in shear zones

Shear zones dynamically evolve through time due to a range of geologic processes, including changes in temperature, pressure, and deformation mechanism as well as chemical transformations and fluid-mediated processes. We consider a process that is capable of stress change throughout the lithosphere: shear zone narrowing due to a coaxial component of deformation. In shear zones bounded by high rheological contrasts, any coaxial (e.g., pure shear) component of deformation causes the shear zone boundaries to move toward one another. This narrowing leads to faster strain rates and thus higher differential stress. A 2-D cross-sectional analytical model of a lithospheric-scale shear zone system with 15° convergence angle (sub-simple shear) demonstrates that highly localized zones in the middle crust, deep crust, and upper mantle can experience significant stress increases on the decade to millennium scale due only to coaxial narrowing. This stress increase can lead to brittle failure and, coupled with other localizing or delocalizing mechanisms, drive cyclic, transient behavior throughout the lithosphere.