Date of Award

Summer 8-2016

Level of Access Assigned by Author

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor

Christopher Gerbi

Second Committee Member

Nicholas Culshaw

Third Committee Member

Edward Grew

Additional Committee Members

Scott Johnson

Peter Koons

Martin Yates

Abstract

The rheology of the deep crust influences the transmission of mantle stresses to the surface as well as the topographic relief of orogenic terrains. Since deformation in the deep crust is often localized in shear zones, we investigate the mechanical processes associated with strain localization to understand the rheology of the deep crust. Strain localizes due to a strength heterogeneity, which can be created by the activation of one or more weakening factors. Studying the mechanical processes and weakening factors that sustain strain localization in the deep crust is becoming increasingly important as geodynamic models become more computationally robust, and therefore more intricate. To research the processes associated with strain localization this study explores ancient shear zones in the denuded core of the Grenville Orogen in Ontario, Canada. This study focuses on a meta-granitoid unit, the Bad River Granite (BRG), of the Grenville Front Tectonic Zone to explore several aspects of strain localization at the m- and km-scale in granitic orogenic crust. The km-scale strain gradients are associated with shear zones along the lithologic boundaries at the BRG’s eastern and western edges. Microstructural and chemical evidence from these gradients show a spatial evolution of the active deformation mechanisms indicating that the shear zone narrowed over time and highlighting the temporal and spatial dynamism associated with shear zone formation. Similar studies of a mylonitic, m-scale shear zone in the BRG show the importance of mass transfer and grain boundary sliding as deformation mechanisms in deep crustal deformation and mylonitization. Lastly, using observations from the km- and m-scale gradients this study also explores six weakening factors to determine whether shear zone formation can be parameterized in a way that allows for prediction of shear zone formation in numerical models. Also, the development of stress heterogeneities at lithologic boundaries is explored through numerical modeling. Results indicate that due to the number of weakening factors and their associated feedbacks, initial variables, and uncertainty in natural conditions shear zone formation is not predictable within reasonable limits. The results presented in this thesis can inform numerical and/or theoretical models as well as other rheologic studies.

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