Bachelor of Science
Date of Expected Graduation
Ductile deformation of the middle to lower crust during interseismic periods influences loading of brittle faults in the upper crust. Quartz is a common mineral in the middle to lower continental crust and is thought to significantly affect the strength of this zone. Quartz deformed by dislocation creep produces a crystallographic alignment of quartz grains, or lattice preferred orientation (LPO), because deformation is easier along specific crystallographic planes and directions. However, formation of a LPO may cause the strength of the quartz aggregates to be anisotropic, approaching the properties of a single crystal.
In order to determine the viscous anisotropy caused by a pre-existing LPO, we deformed a natural quartzite with a cross-girdle LPO from the Moine Thrust in Scotland. This quartzite has aligned, but distributed fine-grained muscovite which defines a foliation and lineation. Samples were cored perpendicular, parallel, and at 45 degrees to the foliation and lineation and deformed at the same temperature (T) of 800°C, confining pressure (Pc) of 1500 MPa and strain rate () of 1.6 * /s. The peak stresses range from 1100 MPa to 500 MPa at a strain of 10-20%, depending on the angle of the foliation or lineation to the compression direction. All samples strain weaken and evolve to a similar stress of 460 MPa (+/-50 MPa). Optical microstructures include undulatory extinction, deformation lamellae, and at high strain (58%), the quartzite is more than 50% recrystallized. Scanning electron microscope (SEM) - electron backscatter diffraction (EBSD) analyses indicate that the pre-existing LPO is obliterated by recrystallization in high strain zones. Therefore, these results suggest that a pre-existing LPO may cause strength anisotropy in rocks in the middle to lower continental crust, but this anisotropy may be transient.
Dr. Caleb W. Holyoke, III
Dr. John Peck
Dr. James Thomka
Braccia, Casey, "Effect of a pre-existing lattice preferred orientation on the strength of quartzite" (2018). Williams Honors College, Honors Research Projects. 795.