Mesozoic and Cenozoic tectonics of the Sevier thrust belt in the Virgin River Valley area, southern Nevada

Abstract

The frontal portion of the Cordilleran thrust belt at latitude 37°N is characterized by a major, east-directed décollement-style thrust system developed within Paleozoic and Mesozoic strata intermediate in thickness between the craton to the east and the Cordilleran miogeocline to the west. Large-magnitude Neogene extension dismembered the thrust system, resulting in unusually complete exposures of it, both along and across strike, and providing an ideal setting in which to study the influence of older thrust structure on extensional faults. A shallow thrust flat was reactivated as a low-angle normal fault over a large area, but the thrust ramp was not. Due to structural duplication by the thrust system, Tertiary normal faults commonly place older rocks on younger, having excised or reactivated thrusts. The hanging walls of large normal faults are commonly brecciated for 10 to 100 m above the fault, whereas thrusts caused little disruption via brecciation, even within centimeters of the fault plane. This criterion was found to be more useful than stratigraphic juxtaposition for distinguishing thrust faults from normal faults. Detailed mapping has allowed identification and correlation both along and across strike of the following Mesozoic structural sequence: (1) autochthonous crystalline basement and depositionally overlying Phanerozoic cover (bottom), (2) subregionally developed duplexes torn from footwall ramps, (3) a regional décollement thrust that generally carries Middle Cambrian dolostone in its hanging wall, (4) an internally imbricated duplex composed of Middle and Upper Cambrian strata, and (5) an overlying roof thrust that detached at the same horizon as level (4) and carried a comparatively undisrupted sequence as young as Mississippian-Permian. The previously identified Glendale, Mormon, and Tule Springs thrusts are herein correlated and correspond to the thrust at the base of (3). The 50-km-long Weiser syncline (Longwell, 1949) formed as an inclined footwall syncline and was rotated into its recumbent attitude during Tertiary extension. Extension in the central Mormon Mountains-East Mormon Mountains-Tule Springs Hills transect was controlled by two major west-dipping detachment faults: the older and structurally higher Mormon Peak detachment and the younger, structurally lower Tule Springs detachment. The Mormon Peak detachment cut gradually (5 to 25°) down to the west in its initial trajectory from structurally high levels of the Tule Springs thrust plate in the east to autochthonous crystalline basement in the west. By contrast, the Tule Springs detachment followed the Jurassic footwall flat of the Tule Springs thrust in the Tule Springs Hills, then ramped downward through the Mesozoic autochthon into crystalline basement, probably flattening again at depth beneath the Mormon Mountains. Much of the uplift and eastward tilting of the East Mormon Mountains was probably caused by isostatic response to differential tectonic unloading in this extensional ramp zone. The entire Mormon Mountains-Tule Springs Hills area was translated westward, with only minor internal disruption, on the younger Castle Cliff detachment exposed along the edge of the Colorado Plateau to the east. The Mormon Peak detachment postdates ca. 14-Ma ignimbrites, the Tule Springs detachment probably predates the deposition of much or all of the Miocene-Pliocene Muddy Creek Formation, and the Castle Cliff detachment was active during Muddy Creek and more recent time. A broad east- to east-northeast-trending zone of Tertiary dextral normal oblique-slip faulting and oroflexure in the southern Mormon and East Mormon Mountains (the Moapa Peak shear zone) was active synchronously with detachment faulting. East-west-trending faults that separate differentially extended blocks to the north and south make up the north margin of the shear zone. Gravity, magnetic, and seismic reflection data suggest that the southern boundary is a south-southeast-facing bedrock scarp that forms the north edge of the Mormon Mesa basin. We interpret this scarp as the north edge of a scoop-shaped fault. The shear zone is apparently a transfer structure between areas of differential extension in the east-central and southern Mormon Mountains and southern East Mormon Mountains. The youngest faults in the area are steep, east- and west-dipping faults in and adjacent to the East Mormon Mountains and Tule Springs Hills. They may be related to movement on the Castle Cliff detachment. Locally, these faults cut Plio-Quaternary(?) pediment gravels, and they are associated with opening the Tule Desert basin and the basin east of the East Mormon Mountains.

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