INVESTIGATING HIGH- AND LOW-TEMPERATURE SHEAR ZONES IN THE WESTERN GRAND CANYON

The Gneiss Canyon shear zone (GCsz) is a ~15 km-wide set of ductile and brittle-ductile shear zones that coincides with a major Paleoproterozoic tectonic boundary between the Mojave and Yavapai crustal provinces of southwestern North America. The GCsz is defined by several high strain zones that are 100% exposed in the Lower Granite Gorge of the Western Grand Canyon and in other parts of northwest Arizona. Shearing involves the dominantly NE-striking, subvertical tectonic fabric (regionally defined as S₂) with a wide range of intensities, structural styles, and temperature regimes including migmatite, ultramylonite, and pseudotachylyte. The various shear zone styles and intensities were previously interpreted to have formed during a single progressive west-up/ dextral event, the 1.7 Ga Yavapai orogeny. Alternatively, this study aims to test the hypothesis for lower temperature reactivation (perhaps ca. 1.65 or 1.45 Ga) of earlier shear zones. The study integrates new digital geologic mapping, microstructural analysis, geochronology and thermochronology. Preliminary work shows at least five mylonite – ultramylonite – pseudotachylyte zones (m to 10-m width) embedded in the GCsz that record varying stretching lineation attitudes from dip-slip to strike-slip. Structural analysis of shear sense and estimations of deformation temperatures using quartz and feldspar microstructures suggests that west-side up oblique slip dominates the higher temperature shearing and dextral strike-slip is observed in the low-temperature mylonites. In-situ monazite geochronology on a sample from river mile 236, located in the center of the high-temperature GCsz, yielded core dates of 1.70 Ga and rim dates of 1.67 and 1.65 Ga. ⁴⁰Ar/³⁹Ar thermochronology on hornblende, muscovite, biotite, and K-feldspar indicates cooling through 500 °C by ~1.63 Ga, 350-300 °C by ~1.53-1.40 Ga, and 250-175 °C by 1.40 to 0.50 Ga. Refined analyses are underway to apply in-situ monazite and xenotime dating and pseudo in-situ ⁴⁰Ar/³⁹Ar dating of fine grained minerals in both high- and low-temperature shear zones to better test whether the GCsz records a progressive 1.71-1.65 Ga event followed by slow cooling or intraplate reactivation of older fabrics ca. 1.65 or 1.45 Ga.