Presentation by Dr. Roberts
Not so simple: How coaxial narrowing drove century-scale transient behavior in the 96 Mile Mylonite Zone, Grand Canyon, USA
Large scale shear zone systems evolve dynamically through time, often with punctuated, transiently high strain rates and differential stresses. Typically, this transient behavior is explained by a material transformation (e.g., phase, fluids, grain size) that weakens part of the shear zone and localizes deformation into a narrower zone. In models of shear zone development, the assumption of simple shear is ubiquitous and leads to the problem that, without any further material transformation or feedback mechanism (e.g., shear heating), the deformation is steady state and does not lead to a run-away effect. This talk questions the assumption of simple shear and suggests that shear zone narrowing due to even small percentages of coaxial (e.g., pure shear) deformation is a viable, material independent mechanism for transiently increasing strain rate and differential stress in shear zone systems on the century timescale. I will compare a non-steady state strain, strain rate, and stress trajectory model of monoclinic transpression with strain, stress, and strain rate analysis from transpressional, quartz-vein-hosted micro shear zones within the 1.4 Ga 96 Mile Mylonite Zone, Grand Canyon. The quartz veins preserve evidence for microstructural evolution from low stress to high stress, preserve a perfect flattening strain, and record strain rates of up to ~10-9 s-1. A comparison of the measured finite strain and strain rate to the numerical model suggests that deformation was localized within the quartz veins for only 2-3 centuries before the quartz veins were boudinaged and deformation migrated elsewhere. I propose a conceptual model for the role that coaxial narrowing may play — in tandem with material transformation — in shear zones throughout the lithosphere.
