--> --> Distribution of mid-Mesozoic to Cenozoic deformations in East-Central Nevada

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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Distribution of mid-Mesozoic to Cenozoic deformations in East-Central Nevada

Abstract

East-central to southeastern Nevada (~36-40áµ′N, ~114-116áµ′W) experienced 3-5 distinct deformations from mid-Mesozoic to today. Most areas underwent Mesozoic shortening, Miocene extension and Pliocene-Quaternary extension, but in some areas additional extensions occurred. However, none of the deformations are evenly distributed throughout this region. Shortening occurred in the Central Nevada Thrust and Eastern Nevada Fold belts. This hinterland deformation is associated with the Jurassic to Paleogene Sevier orogeny. The deformation caused low-, to locally moderate-, magnitude shortening and crustal thickening. The Central Nevada Thrust Belt runs ~N-S through the western part of the area. These thrusts have long, steep ramps that contribute to vertical uplift and creation of a high plateau, the Nevadaplano. The southern edge of the Nevadaplano lies near 37áµ′-37áµ′ 30′N latitude. This edge had streams that ran SE as shown by area-restricted conglomerates that fine SE to sandstones deposited in buttress unconformity against an irregular paleo-surface underlain by various thrust sheets. In the north, syn-thrusting extension occurred. Cretaceous-Eocene extensional structures and sedimentary deposits, including the Sheep Pass Formation in the Egan Range, suggest this deformation occurred, but was restricted in extent. Post-shortening, Cenozoic extension occurred first in the east on the Seaman breakaway, Stampede detachment and Southern Snake Range detachment. These faults are overlain by Oligocene tuffs. Some slip may have occurred on the Northern Snake Range detachment at this time. In addition, local sedimentary basins with conglomerates containing ~34 Ma clasts and Oligocene freshwater limestones in the North Pahroc and southern Egan ranges record this early extension. Western and southern areas were unextended at this time. The ~35-18 Ma ignimbrite flare-up is represented by the Central Nevada, Indian Peak and Caliente caldera complexes. Limited extension occurred during this magmatism. The addition of magma to the crust during this flare-up necessarily changed the crustal thermal structure. The ignimbrite flare-up was followed by widespread ~E-W extension that locally began as early as 24 Ma, but dominantly occurred after the ~18 Ma eruption of the last regional ash-flow tuff, the Hiko Tuff. Numerous post-volcanic, Miocene normal faults cut the tuffs. However, some areas such as the Timpahute and Mt. Irish ranges and the northern White Pine Range were blocks of low-magnitude extension during the Miocene. Areas of geometrically similar Miocene normal faults are separated by E-W to NE-striking left-lateral faults such as the Currant Summit fault, northern faults of the Timpahute lineament and Pahranagat shear zone. Recent mapping shows that those faults are transfer faults that accommodated differences in the amount or spatial distribution of extension. The transfer faults segment this part of the Basin and Range rift. Late Pliocene to Quaternary extension occurs on widely spaced normal faults that distribute extension more evenly than in the Miocene. Faults of this deformation such as the Railroad Valley, Dry Lake and the White River Valley faults either reactivate parts of older structures or cut across them. The deeper basins in the region preserve parts of the Oligocene and/or Miocene basins at depth below the Pliocene-Quaternary basins, consistent with seismic reflection and drill hole data.