Cenozoic structural history of selected areas in the eastern Great Basin, Nevada-Utah

Abstract

The Confusion Range structural trough (CRST) of west-central Utah predates the Oligocene rocks that are exposed along it. The northern part of the axial region of the CRST is complicated by structures that include reverse faults and associated folds, a large-amplitude mushroom fold, and belts of sharply flexed to overturned strata some of which are fault bounded. These structures, which also predate the Oligocene rocks, formed in a compressional regime that has been interpreted as resulting from thin-skinned gravitational gliding toward the axis of the CRST. Study of the sparse Tertiary rocks that are scattered along the axial region of the CRST reveals abundant evidence of Oligocene and younger deformation. The chief evidence includes (1) widespread Oligocene and Miocene coarse clastic rocks, many of which are conglomerates, that attest to local and distant tectonism, (2) faults that range from high-angle structures generally with less than 100 m of normal displacement to low-angle attenuation faults some of which may have large displacements, and (3) open asymmetric folds. Together with the distribution of sheet-form bodies of ash-flow tuffs, the Oligocene stratigraphic record allows for paleogeographic reconstruction of a lacustrine basin across what is now the northern Confusion Range and one or more basins in the southern part of the CRST. The basins are inferred to have been fault controlled by reactivation of previously formed faults or steep fold flanks. They may have been localized by differential vertical movements similar to those that produced the older systems of folds and faults. Parts of early formed basins were cannibalized as local syndepositional deformation took place in the axial region of the CRST. Both limbs of the CRST have been modified by folds that involve Oligocene rocks. Some of these folds appear to be genetically related to displacements on faults that bound them. They may record thin-skinned Neogene tectonic displacements toward the axis of the CRST. The most intensely faulted and tilted rocks along the axis of the CRST are located in the Tunnel Spring Mountains where Miocene(?) extension on closely spaced listric faults produced as much as 70 percent extension locally. Three episodes of Oligocene-Miocene deformation, all interpreted to have formed in an extensional environment, are recognized in the Tunnel Spring Mountains. The nearby Burbank Hills area may have been involved in the same deformational episodes, though there the relationships are not as clear-cut nor does evidence occur of extreme extension. Tight asymmetric folds in the Burbank Hills are interpreted as drape structures formed over buried normal faults. Other structures along the southern CRST have fold-like forms, but they result from cross-strike alternations in fault-related tilt directions, and they formed in an extensional stress regime. Least-principal stress directions inferred from orientations of extensional structures vary from ENE-WSW in the southern Tunnel Spring Mountains to approximately E-W in the Disappointment Hills and NW-SE in selected areas east of the axis of the CRST. The size, geographic distribution, and new data on the age of areas of major extensional faulting preclude previously published interpretations that the extension is related to major east-directed overthrusting of the Sevier orogeny in areas east of the hinterland of west-central Utah.