Fabric Evolution and the Development of Ductile Shear Zones

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It is well known that deformation is localized to some extent at every depth in the lithosphere in shear zones. The origin and overall mechanical behavior of these shear zones remain poorly understood. To address their role in tectonics, it is important to understand their origin, whether a scar of earlier tectonics events or a spontaneously generated weak zone. A key is to constrain their rheology, and for this, it is critical to understand deformation processes active in ductile shear zones as well as their architecture. This project aims at better constraining, from a theoretical and mechanical point of view, the structure of the lithosphere beneath seismically active faults. Specifically, the project will address the generation of ductile shear zones and their link with the seismic cycle. Using a software developed specifically for this project, the development of a ductile shear zone beneath a strike-slip fault will be simulated, revealing in particular how the shear zone structure, width, and strength are likely to change with depth. This project would address one of the significant weaknesses in current computer models, which is that they do not adequately incorporate deformation localization processes. The project has potential to advance desired societal outcomes through: 1) development of a globally competitive STEM workforce through training of a post-doctoral fellow; 2) increased partnerships through international collaboration; and 3) development of research infrastructure through the development of new open access software that would be available for reuse by others.The objective of this project is to develop numerical models of the lithosphere deforming in a strike-slip environment. The model will include fabric evolution in the crust and grain size evolution in the mantle to determine the conditions under which ductile flow may localize and form a ductile shear zone. Three kinds of models will be developed in this project: 1) a 1-D column model that couples a local shear zone evolution model; 2) a 2-D antishear model, using the finite element library deal.II, that will follow development a shear zone below a brittle fault; and 3) the same model subjected to episodic loading by earthquakes on the brittle fault. The project will evaluate three different scenarios for localization through the lithosphere. First, spontaneous localization is the only player throughout the brittle as well as ductile levels of the lithosphere to determine how grain size evolution and the development of a layered structure in the ductile upper mantle and mid to lower crust interacts with strain-weakening in the brittle regime to form a lithospheric-scale shear zone. Second, the study will impose a preexisting brittle fault and determine how ductile shear may localize beneath it, a scenario that includes both imposed and spontaneous localization. Finally, the influence of the earthquake cycle, which periodically imposes episodes of enhanced stress on the evolving shear zones, will be considered. These models will provide basic knowledge on the structure, strength, and width of ductile shear zones that may be expected at depth in actively deforming regions.