Date of Award

5-2012

Level of Access Assigned by Author

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor

Scott E. Johnson

Second Committee Member

Christopher C. Gerbi

Third Committee Member

Peter O. Koons

Abstract

Determining the structure and rheology of a seismogenic fault at frictional-to-viscous transition (FVT) depths is vital for understanding its strength and behavior. Few studies describe a fault from within this depth level, so the architecture of a shear zone at these depths as well as the effect of transient coseismic and postseismic deformation on the rheology of the shear zone is poorly-understood. The Sandhill Corner strand of the Paleozoic Norumbega fault system of Maine is the one of the few known examples of a subvertical, strike-slip fault exhumed from FVT depths. Using a suite of samples collected from the Sandhill Corner shear zone, this study (a) identifies coseismic and postseismic structures; (b) investigates the history of deformation from quartz data; (c) characterizes the across-strike structure; and (d) considers the strength and rheology of a shear zone within the FVT. The shear zone initially localized along the contact between two rheologically-contrasting units. Quartz microstructural data from monomineralic quartz ribbons suggest a history of initial localization at relatively higher temperatures influenced by a strong pre-existing crystallographic preferred orientation (CPO) followed by a lower temperature overprint. Based on maps showing the spatial distribution of rock type, pseudotachylyte, quartz microstructure, and quartz grain sizes, this study proposes across-strike divisions of an outer shear zone, an inner shear zone, and a shear zone core. In the outer shear zone, the lower temperature quartz overprint is variable with grain sizes of 10-80μm. In the inner shear zone, the quartz grains are completely overprinted with grain sizes of 10-20μm, indicating flow stresses of 60-100MPa. Contrasting quartz misorientation and CPO data also distinguish the inner shear zone from the outer shear zone. The shear zone core is a zone of ultramylonite/phyllonite contained within the inner shear zone that is coincident with the lithologic contact. The ultra fine-grained, micaceous ultramylonite/phyllonite of the shear zone core derived largely from deformed pseudotachylyte would have promoted grain size sensitive, diffusion-mediated creep in the matrix surrounding quartz ribbons, leading to the formation of the weakened fault core that flowed at stresses lower than those estimated from the quartz grain sizes.

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