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


This study focuses on a high-strain, strike-slip shear zone that juxtaposed metasedimentary rocks and a homogenous granite near the brittle-ductile transition, at a depth of 15-20 km. The metasedimentary rocks of the Flume Ridge Formation (FRF) exhibit a progressive deformation gradient of phase mixing and increasing fabric intensity across strike, northwest-to-southeast, approaching the center of the shear zone. Deformation mechanisms were controlled by metasomatic reactions in a fluid-rich fault zone. During deformation, biotite and potassium feldspar in the metasedimentary rocks reacted to form phengite—a much weaker phase. The nucleation and growth of weak phases, combined with phase mixing, decreased the shear strength of the rock. Phengite also branches at sharp angles to the foliation, and occurs preferentially along grain boundaries and dilatational sites indicating that these very fine-grained rocks deformed by granular flow. Hence, the cyclic process of solution precipitation and chemical alteration with granular flow governed the rheology of the metasedimentary rocks. Specific case studies such as this one can help to understand deformation mechanisms occurring in active fault zones at depth.


microstructure, reaction softening, granular flow, dissolution precipitation, Norumbega Fault System, Flume Ridge Formation

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