Graduate Thesis Or Dissertation

 

Structure of the southern Mormon Mountains, Clark County, Nevada and regional structural synthesis : fold-thrust and basin-range structure in southern Nevada, southwest Utah, and northwest Arizona Public Deposited

https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/2j62s922x

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  • Detailed geologic mapping in the Mormon Mountains and new geophysical data provide significant insight into contractional and extensional tectonics in southern Nevada, southwest Utah, and northwest Arizona. The rocks in the region were complexly deformed during two distinct tectonic episodes. Numerous interrelated events occurred within each episode. The first tectonic episode, related to the Sevier orogeny, was characterized by east-west crustal shortening which culminated in thin-skinned decollement style folding and thrusting during the Cretaceous. The Virgin-Beaver Dam Mountains anticline, a Laramide-type basement-involved uplift, represents the only thick-skinned contractional structure in the region. The second tectonic episode, related to basin-range rifting, was characterized by east-west crustal extension which was accommodated by high-angle normal faults, with dips averaging 60 degrees, in the brittle upper crust. In this area, basin-range rifting initiated in the Oligocene and continued to Recent time. Relations in the North Muddy Mountains in southern Nevada suggest that the Muddy Mountain thrust sheet advanced and overrode the Weiser syncline during the Cenomanian and may have continued to advance in Turonian time. In the southern Mormon Mountains, the Cambrian Bonanza King Formation lies in the hanging wall flat position in thrust contact with the overturned Petrified Forest Member of the Triassic Chinle Formation at the footwall ramp. The thrust sheet advanced eastward more than 30 km from the place of origin. Thrust imbrication, and probably the formation of hanging wall horses, likely occurred as the Muddy Mountain thrust sheet encountered and ascended up the footwall ramp zone (composed largely of competent carbonate rocks) where slices of the thrust sheet (hanging wall horses) splayed of f and accreted to the footwall ramp zone. A detailed retrodeformable (balanced) regional structure section suggests that fold-thrust shortening at the latitude of the Mormon Mountains is a minimum of about 26%. Extension-related structures overprint older fold-thrust structures in the Mormon Mountains. The west-plunging east-trending Candy Peak syncline is one of a family of fold structures related to basin-range rifting. The syncline formed in pre-Miocene time in association with the northeast-striking Reber Mountain normal fault directly north and the northeast-striking Dry Canyon right-lateral strike-slip fault directly south. The Tortoise Flat synform, which lies southeast of the Dry Canyon fault, developed in Miocene and possibly Pliocene time by right-lateral flexure of early Miocene Horse Spring beds as a result of drag associated with the Dry Canyon fault. The Dry Canyon fault and the Tortoise Flat synform are interpreted to be part of the right-lateral Moapa Peak-Reber Mountain shear zone system in the southern Mormon Mountains. Therefore, the time of formation of the Moapa Peak-Reber Mountain shear zone system is pre-Miocene to possibly Pliocene. The shear zone system formed in response to different amounts of west-directed extension-related movement of the hanging wall block of the high-angle Virgin Beaver Dam Mountains fault, which initiated in the Oligocene. From this, the timing of the Moapa Peak-Reber Mountain shear zone, system is interpreted as Oligocene to Miocene, and possibly Pliocene. The interpretation of 261 km of seismic reflection sections suggests that large-displacement high-angle normal faults, typically with 60 degrees of dip, control horst and graben structure and accommodate extension by simple shear in the upper brittle crust. Such faults likely extend to depths of 15 to 18 km. Below this depth extension is thought to be accommodated by penetrative ductile deformation. A detailed retrodeformable (balanced) regional structure section suggests that basin-range extension at the latitude of the Mormon Mountains is about 17%. The Virgin-Beaver Dam Mountains high-angle normal fault is a large-displacement master fault in the area, having more than 8,000 in of normal vertical separation at the latitude of the Virgin Valley basin depocenter. Miocene doming and uplift of the Mormon Mountains occurred in response to displacement on the Virgin-Beaver Dam Mountains fault. The Virgin Valley basin formed as the hanging wall block downdropped, and the Mormon Mountains dome formed by relative uplift at the opposite end of the hanging wall block. Half-grabens, and tilted, folded, and faulted range blocks characterize basin-range crustal structure. Depositional growth relations are interpreted in basins from fanning-upward reflector geometry, and the wedge-shape of Oligocene to Recent syntectonic basin-fill sediments. Non-overlapping opposing east- and west-tilted half-grabens compose the Meadow Valley-California Wash basin. Seismic sections, gravity data, well data, and geologic mapping demonstrate that the Mormon Peak, Tule Springs Hills, and Beaver Dam/Castle Cliff "detachments," which were thought to be rooted low-angle normal faults, do not exist. The Mormon Peak and Beaver Dam/Castle Cliff low-angle normal faults are denudational fault planes below gravity slid masses. The widely distributed translocated Paleozoic blocks, which were thought to be remnant pieces of large hanging wall sheets ("extensional allochthons"), are disjunct rootless gravity slide blocks of minor tectonic significance. A large number of these rootless slide blocks lie on Pliocene and Quaternary basin-fill deposits. The Muddy Mountain-Tule Springs thrust, of Sevier age, was not reactivated as a crustal penetrating Tule Springs Hills low-angle normal fault, but is affected by small-scale gravity slide features. Rootless gravity slide blocks, secondary features to high-angle normal faults, commonly occur from instability as a result of the loss of lateral support induced by block faulting and the associated erosion of range blocks.
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