Date of Award


Document Type

Honors Thesis (Open Access)


Colby College. Geology Dept.


Walter A. Sullivan

Second Advisor

Robert A. Gastaldo

Third Advisor

Robert E. Nelson


Mylonite formation and development is dependent on a number of factors including temperature, strain rate, and fluid fugacity. All three factors affect flow stress and viscosity; thus, a variance in each will affect the development of mylonites. This study focuses on factors leading to the formation of mylonite surrounded by ultramylonite of the same protolith, along-strike to one another at the center of a fault zone. The rocks in question are from the Deblois granite cut by the Kellyland fault zone, a ductile strike-slip fault zone within the larger Norumbega fault system in eastern Maine. The Deblois granite as a whole experienced three stages of deformation within the Kellyland fault zone: a primary high-temperature phase, a second brief brittle phase, and a third long-lived ductile phase. The brittle phase prompted the widespread development of ultramylonite in the center of the shear zone; the zone of mylonite records no evidence for this brittle phase. The lack of brittle deformation in the mylonites was probably due to prolonged locally elevated temperatures and to a high fluid content during later stages of deformation. The implications for this study are that even in a setting of prolonged high-strain shearing, ultramylonites may not form without a primary brittle phase to weaken the overall rheology, and may not form in the presence of a fluid.


mylonite, deformation mechanisms, kellyland fault zone, norumbega fault system, dynamic recrystallization, microstructure

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