Cover Image Credits

The article by Davis and others, titled “Structure and thermochronology of basement/cover relations along the Defiance uplift (AZ and NM), and implications regarding Laramide tectonic evolution of the Colorado Plateau” (p. 1047–1087), leverages structural analysis of fold/fault relations and multi-method geochronology and thermochronology, including zircon U-Pb, zircon (U-Th)/He, apatite fissiontrack, apatite (U-Th-Sm)/He, and hematite (U-Th)/He. Laramide monoclinal folding is a trishear expression of oblique-slip along a blind master fault that partitioned ~8 km of strike-slip and ~1.5 km of reverse-slip displacement. Thermochronology was applied at two key localities: first, at rare basement exposures near the Hunters Point segment of the East Defiance monocline and second, to broadly exposed basement in the Zuni uplift. Data reveal a strong Laramide signature whereby basement rocks experienced initial cooling at <70 Ma, with the main phase of exhumation of the upper crust at ca. 60–40 Ma. These new data are consistent with support for tectonic models that bring the flat slab of North America along an east-northeast trajectory, rather than along a more northward-directed trajectory.

Explanation of cover figure: Structure-geological map of the East Defiance monocline and fault system portrayed within a digital structural elevation model (DSEM) at a base Cretaceous datum. The 3D surface was constructed from structure contour map by Hackman and Olson (1977). Color-coding shows stratal dip in degrees. WRfz = Wide Ruin fault zone. BCm = Black Canyon monocline. Inset photograph captures the Hunters Point monocline. Locations of quarry exposures of Precambrian basement (red dots) lie just south of Hunters Point monocline (HPm). Hunters Point is known to the Navajo People as “Tsé Nááchíí,” which means “Downturning Red Rock.” The downturned strata are Permian. Any persons wishing to conduct geologic investigations on the Navajo Nation must first apply for, and receive, a permit from the Navajo Nations Minerals Department, Window Rock, Arizona.