Date of Award

Summer 8-23-2019

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)


Earth Sciences


Scott Johnson

Second Committee Member

Christopher Gerbi

Third Committee Member

Alicia Cruz-Uribe

Additional Committee Members

Martin Yates


Localized shear zones are important rheological features that influence deformation behavior throughout the Earth’s middle-to-lower crust. Therefore, the processes through which shear zones initiate and localize remains an important geologic question. The study of strain localization and shear zone initiation is made difficult due to continued deformation overprinting the microstructures which lead to initiation and obfuscating the context in which localization occurred. The Marcy anorthosite in the Adirondack Highlands, New York, is a nominally granulite-facies, plagioclase-rich massif cut by centimeter-to-meter scale shear zones which provides a natural example of shear zone localization within the middle-to-lower crust. My work focuses on the microstructural examination of shear zones at Bennies Brook within the Marcy massif to construct the sequence of geologic events which lead to shear zone initiation. I used field observations combined with optical and electron microscope observations and electron probe geochemistry to investigate how microstructural conditions changed over time as the sequence progressed, as well as explore the tectonic implications of shear zone development within the massif. My results suggest that the initiation of viscous shearing was facilitated by a combination of physical and chemical weakening during exhumation. The country rock anorthosite was pervasively fractured and/or crushed which increased permeability sufficiently to allow for the infiltration of chlorine-rich hydrothermal fluids, primarily along centimeter-wide brittle fault zones where permeability was greatest. These fluids triggered the retrograde replacement of plagioclase feldspar to scapolite and pyroxene to amphibole and quartz. This weakened the rock through the introduction of the relatively weak minerals as well as an associated reduction in grain size. In the planar zones of greatest metasomatism, this weakening was sufficient for viscous shearing to initiate. The relative timing and orientation of the shear zones supports previous work suggesting that the Adirondack Highlands underwent exhumation associated with orogenic collapse during the Ottawan orogeny (ca. 1080–1000 Ma).