The Gneiss Canyon shear zone (GCSZ) in north-western Arizona sits between high grade migmatites on the west and lower amphibolite grade schists on the east. This 13 km-wide zone, contains domains of intense D2 strain up to 100m wide that separate blocks of preserved northwest-striking D1 fabric. Previous investigations of D2 domains show west-side-up and dextral displacement forming at amphibolite grade. D2 field and microstructures suggest deformation temperatures of >500 oC. Zoned monazite displays 1696 Ma cores and 1649 Ma outermost rims, suggesting protracted deformation associated with the Yavapai orogeny. Newly described retrograde D3 mylonites consists of discrete (cm to m-scale) zones, which occur within, and represent reactivation of the wider GCSZ. D3 microstructures indicate lower temperature dynamic recrystallization of quartz, as well as brittle deformation suggesting temperatures of <300-450 oC. These D3 domains within GCSZ also indicate west-side-up and/or dextral displacement, and could record retrograde shearing near the end of the Yavapai orogeny, or a younger shearing event (favored here). 40Ar/39Ar data from Lower Granite Gorge show disparate T-t paths for blocks on either side of the GCSZ. Both eastern and western blocks show initial ~1.7-~1.5 Ga cooling rate of ~2 oC/Ma, but with a 30-40 Ma time lag in cooling for the western block through muscovite closure (~350 oC). The disparity in biotite ages is greater. The eastern "cold" block cooled through biotite closure (~300 oC) at 1527 Ma; the western "hot" block apparently remained warmer, and cooled through ~300 oC by ~1400 Ma. K-feldspar multiple diffusion domain modeling shows an inversion of relative cooling rates from ~300 oC to ~150 oC. Results from Spencer Canyon and Gold Butte indicate cooling from 300-200 oC between ~1.4 and ~1.2 Ga, then slow cooling from 200-150 oC between 1.2-0.8 Ga. Travertine Grotto K-feldspar modeling suggests cooling from ~300 oC at ~1300 Ma to ~200 oC by ~1100 Ma. This slower late cooling suggests burial of the Travertine Grotto block as a result of ~1.4 Ga west-side-up thrusting across the GCSZ. The combined data suggests that D3 shear zones record reactivation of the GCSZ during regional ~1.4 Ga magmatism and associated deformation.
Earth and Planetary Sciences
Level of Degree
Department of Earth and Planetary Sciences
First Committee Member (Chair)
Second Committee Member
Gneiss Canyon shear zone, Grand Canyon, microstructures, thermochronology, geochronology, monazite, Spencer Canyon, Travertine Canyon, argon
McDermott, Jacob. "Microstructural, geochronologic and thermochronologic evidence for both Paleo- and Mesoproterozoic displacement across the Gneiss Canyon shear zone : lower Granite Gorge, Grand Canyon, Arizona." (2011). https://digitalrepository.unm.edu/eps_etds/52