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The Muddy and North Muddy Mountains were highly deformed during the Cretaceous and early Tertiary(?) Sevier orogeny. Two major thrust plates, the Muddy Mountain and Summit-Willow Tank plates, and a well-developed belt of recumbent folds are present there. A foreland basin developed east of the thrust plates as well. Many of the Paleozoic rock units exposed in these ranges vary in thickness and character between the thrust plates and the authochthon; the thicker and more shelflike units occur in the thrust plates, and the thinner, cratonic ones occur in the autochthon. This is especially true of the lower Paleozoic formations, whereas Mississippian through Permian formations vary much less in thickness. Pre-Sevier age Mesozoic formations exposed locally in the upper plate of the Summit-Willow Tank thrust and in the autochthon are consistent in thickness, and they are stratigraphically similar to equivalent rocks on the nearby Colorado Plateau. The oldest syntectonic Albian and Cenomanian(?) rocks of the foreland basin are cut by the Summit-Willow Tank thrust, but the youngest of them overlap it. The Muddy Mountain thrust overrode all of the foreland deposits.

New interpretations are presented herein regarding many of the Mesozoic structures of the Muddy and North Muddy Mountains. The Muddy Mountain, Glendale, and Arrowhead faults are interpreted to be parts of the same thrust and are collectively called the Muddy Mountain thrust. Three major faults, the Summit, the Willow Tank, and the North Buffington, form the older and structurally lower Summit-Willow Tank thrust. The North Muddy Mountain fold belt is interpreted to have developed synchronously with thrusting as it occurs between the two major thrusts. The monocline or drape fold described by early workers is reinterpreted as a truncation of one thrust plate by another. The North Muddy Mountains are now interpreted to underlie the Muddy Mountain plate and are thus autochthonous with respect to it.

After thrusting and foreland sedimentation, the next major tectonic and sedimentologic events occurred during Miocene time. Several complex, nonmarine, closed basins of the Horse Spring Formation formed in taphrogenic environments in conjunction with left slip on the Lake Mead fault system and simultaneous growth of the andesitic Hamblin-Cleopatra volcano. Basin and ranges of late Miocene age are superposed over all of these earlier-formed structures.

South of the Muddy Mountains, upper plate Paleozoic rocks dip southward at varying degrees and are overlapped by Tertiary rocks. The upper plate rocks are in apparent tectonic contact with autochthonous Mesozoic rocks in the Gale Hills, Bowl of Fire, northern Bitter Spring Valley, and east of the Longwell Ridges. Unfortunately, this contact is almost completely covered and it is poorly understood. Available data suggest that the contact is a fault that formed prior to the deposition of the Thumb Member of the Horse Spring Formation (pre-15 m.y.), and it might have been (1) a north-dipping normal fault with a large amount of displacement; (2) the trace of the Las Vegas Valley shear zone with a large amount of right slip; or (3) the trace of the Muddy Mountain thrust (Mississippian over Cretaceous and Jurassic rocks at that point). The interpretation that it is the trace of the Las Vegas Valley shear zone is favored herein because neither of the other interpretations is as consistent with the known local and regional geology.

Post-15-m.y.-old deformation south of the Muddy Mountains includes a complex group of tectonic elements. A relatively minor amount of extensional tectonism occurred in the Gale Hills and was bounded to the north by the northwest-striking Gale Hills fault and to the south by the east-trending West Bowl of Fire fault. Uplift and relative southward displacement of the Bowl of Fire horst occurred on major oblique slip faults probably at the same time as the extension in the Gale Hills. North-south directed compression in the Gale Hills also apparently occurred at this time, as east-trending folds and reverse faults are present there. Normal faulting and graben formation occurred in White Basin in conjunction with oblique slip on the Rogers Spring fault. All of these local tectonic elements were probably active synchronously with left slip on the Lake Mead fault system and right slip on the Las Vegas Valley shear zone, but the details of interaction are poorly understood.

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