Date of Award

Summer 8-19-2022

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

Peter O. Koons

Third Committee Member

Christopher C. Gerbi

Additional Committee Members

Martin G. Yates

Senthil S. Vel

Phaedra Upton

Abstract

Although the mechanics of continental, seismogenic strike-slip faults have been primarily studied around active faults near Earth’s surface, large earthquakes on these faults commonly extend to depths between 10 and 20 km. At the base of seismogenic strike-slip faults, interaction and feedback between coseismic brittle fracturing and post- and interseismic viscous flow affect transient and long-term changes in stress cycling, fluid and heat transport, fault strength, and associated strain localization and deformation mechanisms. A primary goal of my dissertation is to explore the deeper structures of damage zones near the base of the seismogenic zone and to better understand the influence of the damaged rocks on rupture dynamics, by examining microstructures of exhumed fault rocks. My study area, the Sandhill Corner shear zone that is the longest strand of the Paleozoic Norumbega fault system in Maine, USA, represents large-displacement, seismogenic strike-slip faults at frictional-to-viscous transition depths (corresponding to temperatures of ~400–500 °C). The shear zone contains mutually overprinting pseudotachylyte and mylonite, and juxtaposes quartzofeldspathic mylonites and mica-rich schists. I analyzed fractured and fragmented garnet grains using particle size distributions, microfracture patterns, and electron backscatter diffraction fabrics. Microstructural studies of fragmented garnets reveal asymmetric distribution of dynamic pulverization with a width of ~70 m in the Sandhill Corner shear zone, and these results imply that the same damage processes observed around active seismogenic strike-slip faults operate at the base of the seismogenic zone. Garnet microstructures formed during earthquake cycles at the frictional-viscous transition can also provide evidence for dynamic pulverization even though the particle size distribution is modified by quasi-static fragmentation during post- and interseismic shearing. Elastic and seismic properties of the quartzofeldspathic rock and the mica-rich schist are quantified using the Thermo-Elastic and Seismic Analysis (TESA) numerical toolbox. The results illustrate how elastic contrast across bimaterial faults separating two different anisotropic materials affects preferred rupture propagation and asymmetric damage distribution. Strong anisotropy occurs in fault zones where preferentially aligned phyllosilicate minerals are a major component of the modal mineralogy. My findings suggest that the orientation and proportion of preferentially aligned phyllosilicates, or other highly anisotropic minerals, should be considered when investigating fault ruptures in anisotropic rocks.

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