New Mexico Geological Society Annual Spring Meeting — Abstracts

The Defiance Uplift is an asymmetrical basement cored, fault bounded uplift of Ancestral Rockies and Laramide age located entirely on the Navajo Nation in NE Arizona and NW New Mexico. Small outcrops of Precambrian rocks occur in four canyons (Blue Canyon, Meadow Wash, Fork Canyon, and Hawk Canyon) that drain the east side of the uplift (total outcrop area of < 1 km² ). Blue Canyon and Meadow Wash contain quartzites, with no contact relationships exposed. These are medium-grained quartz arenites with well preserved ripple marks and cross beds, lithologically very similar to the Uncompahgre Quartzite of SW Colorado exposed about 200 km to the NE. Fork and Hawk canyons contain thinly laminated low grade argillite and grayish green highly altered greenstone (metabasalt). These are intruded by several different granitoids: fine grained granite (probably 1.7-1.65 Ga), altered granite porphyry, megacrystic granite (likely 1.4 Ga), and aplite and pegmatite dikes. Based on correlation of aeromagnetic highs with granites encountered in drill holes, the basement of the uplift is appears dominated by granitoids. U-Pb zircon ages on basement xenoliths from the nearby Tertiary diatremes (Condie et al., 1999) include both Paleoproterozoic granites (1725 to 1603 Ma) and Mesoproterozoic granites (1453 to 1381 Ma). The assemblage and ages of Proterozoic rocks are thus similar to the Yavapai- Mazatzal transitional boundary zone elsewhere in New Mexico. Structure contour and isopach maps show a gentle westward dip of the Great Unconformity accompanied by westward thickening of Paleozoic rocks (Werme, 1981), indicating an Ancestral Rockies history. The faulted east side, coincident with East Defiance monocline, is likely Laramide in age, with possible right strike slip movement (Kelley, 1955). Evidence for basement controlled localization of Phanerozoic structures includes parallelism of a prominent NE-trending aeromagnetic high and the Nazlini NE-trending fault and deflection of the East Defiance monocline at the same basement anomaly.