Structural geology along the southeastern margin of the Tucson basin, Pima County, Arizona

Abstract

The Cienega Gap area, located about 27 miles southeast of Tucson, Arizona, has long been noted for its structural complexity. Discordances between sedimentary rocks of Paleozoic, Mesozoic, and Cenozoic age and between these rocks and their granitic basement have led previous workers to propose large -scale northward thrusting in an effort to explain the complex structures observed. An alternative hypothesis invoking southward gliding has been considered by several authors but has been assigned a subordinate role in explaining the deformation. The present study was undertaken in an effort to evaluate the relative suitability of dominantly northward versus dominantly southward movement. In the course of this study nine separate localities were examined and mapped in detail in an effort to evaluate movement direction. Evidence of displacement on low -angle faults or glide surfaces was found in each of the areas examined, and definite evidence of movement direction was recognized in five of these. Large-scale recumbent folding, previously unrecognized in the Colossal Cave and Agua Verde Wash areas, was found to be closely related to local uplift. The asymmetry of the folds, plus a very few observed offsets marginal to the uplifts, were the only criteria found for determining the direction of movement. Evidence of northward, southward, and nearly westward movement was found in the course of the study. Areas in the northern Empire Mountains south of Cienega Gap gave evidence of west – northwestward and northward movement. Areas in the southern Rincon Mountains north of Cienega Gap were found to have undergone dominantly southward movement. Involvement of the Pantano Formation in several of the localities suggests that deformation occurred at least as late as early Oligocene time and probably after middle Miocene time. Cienega Gap was therefore the focus of movement for material which was moving laterally away from areas undergoing uplift in Tertiary time.