The Gneiss Canyon shear zone in the Lower Granite Gorge of the Grand Canyon represents an important Paleoproterozoic tectonic boundary between the Mojave and Yavapai provinces, and records west-side-up shear and NW-SE contraction at elevated temperature (~600 degrees Celsius). Movement took place on subvertical NE-striking foliation that forms the main tectonic fabric (S2). Travertine Canyon, east of the shear zone, preserves lower amphibolite grade Paleoproterozoic rocks with subvertical NE-trending fabric (S2). Prophyroblasts contain inclusion trails parallel to, and similar in morphology to, S2 indicating that porphyroblasts grew late or after S2 fabric development. Post-porphyroblast movements on S2 fabric involved rotation of porphyroblasts relative to matrix, shearing, and bending of S2 around porphyroblasts. Post-porphyroblast matrix coarsening indicates that metamorphism accompanied late adjustments on S2. Collectively, these data suggest that peak metamorphism occurred during the latest stages of NW-SE shortening on the previously formed S2 crenulation cleavage. In contrast, Spencer Canyon, west of the shear zone, preserves upper amphibolite grade rocks and porphyroblast textures that indicate peak metamorphism was synchronous with progressive deformation on northwest-trending fabrics. These NW fabrics are overprinted by the Gneiss Canyon shear zone at map scale and hence are at least in part older than S2. These fabrics and porphyroblasts are interpreted to record thrusting and decompression of high-grade upper plate rocks on the west side against colder lower plate rocks on the east side of the reverse sense Gneiss Canyon shear zone.
Earth and Planetary Sciences
Level of Degree
Department of Earth and Planetary Sciences
First Committee Member (Chair)
Kudo, Albert M.
Grand Canyon, Porphyroblast, Porphyroblast growth, Gneiss Canyon, Lower Granite Gorge
Hutchison, Jimmie. "Relative Timing of Porphyroblast Growth and Peak Metamorphism in the Lower Granite Gorge, Grand Canyon, Arizona.." (1996). http://digitalrepository.unm.edu/eps_etds/39