Date of Award


Document Type

Honors Thesis (Open Access)


Colby College. Geology Dept.


Walter A. Sullivan

Second Advisor

Rachel Beane

Third Advisor

Robert Gastaldo


S-C mylonites form as a result of non-coaxial deformation in ductile shear zones. Planes of weakness develop and delineate two domains, the S and the C Each of these domains accommodates variable amounts of strain. Sample WMSZ-188 exhibits subgrain rotation (SGR) recrystallization in the C domains, and grain boundary migration (GBM) recrystallization in the S domains. This partitioning of recrystallization mechanisms indicates either: (1) strain-rate partitioning during a single deformation event; or (2) a second, lower-temperature deformation event that over-printed the C domains. This study tests these two hypotheses. Optical and EBSD analyses were used to look for evidence of a second phase of lower-temperature deformation in the S domains. The quartz grains in the S domains exhibit some areas of internal misorientation, however these zones are not interpreted as being a result of a SGR recrystallization overprint. High-temperature microstructures in feldspar and biotite also are preserved throughout the sample, indicating that no late-stage, low-temperature deformation over-printing occurred. These observations, in conjunction with optical analyses of samples from the region, indicate that these two recrystallization mechanisms are coeval. Because the sample does not record a syndeformational decrease in temperature and probably records nearly isobaric deformation due to its setting in a strike-slip fault system, the simultaneous generation of the recrystallization mechanisms is attributed to domainal strainrate partitioning during a single deformation event. One to three orders of magnitude of difference in strain-rate is required to transition from one recrystallization mechanism to another between the S and C domains. The implication of this domainal strain-rate partitioning is that caution must be exercised when using recrystallization mechanisms to determine deformation temperature.


quartz, recrystallization

Multimedia URL