AN  INTEGRATED PETROLEUM  EVALUATION OF NORTHEASTERN  NEVADA


Introduction Evaluation Prospects


 

 

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Structural Events and Elements
Antler Orogeny / Roberts Mountains Thrust
Sonoma Orogeny / Golconda Thrust
Sevier Orogeny
Mesozoic(?) Folding
Mesozoic-Cenozoic Metamorphic Core Complex
Tertiary Extension
Lateral Faults / Northern Nevada Rift
 

 

 

 

 

 

 

STRUCTURE

STRUCTURAL SETTING

Northeastern Nevada lies in the western hinterland of the Jurassic through Paleocene Laramide-Sevier foreland fold and thrust belt of primarily unmetamorphosed sedimentary rocks, which traverses the eastern flank of the Cordillera from British Columbia south through southern Nevada and into the Mojave Desert of California.

This hinterland is characterized by structurally separated high-angle normal fault bounded ranges and valleys which expose discontinuous patches of locally intense metamorphism, ductile deformation, and associated plutonism within core complexes, and by low-angle detachment faults which attenuate or eliminate thousands of feet of stratigraphic section.

The western limit of Precambrian sialic crust in the Cordillera coincides with the boundary between plutons with initial 87Sr/86Sr greater than or less than 0.7060 (Kistler, 1983). This boundary transects central Nevada and represents the passive continental margin of western North America from the Proterozoic through the Middle Devonian.

Oceanic terranes were obducted above the slope and outer shelf during the Late Devonian through Mississippian and again in the Late Permian through Triassic, as the Roberts Mountains and Golconda allochthons. The eastward thrusting of these complexly and variably faulted and folded accretionary sheets onto the Paleozoic margin of western North America was not accompanied by significant magmatism or regional metamorphism (Speed, 1983). The colliding Sonoma Arc responsible for the Golconda allochthon is exposed in western Nevada, however an Antler Arc is not present; it may have thermally contracted and been subducted beneath the Golconda allochthon to the west (Speed and Sleep, 1982).

Tectonic loading and downflexing of the continental lithosphere by the Roberts Mountains allochthon created a broad asymmetric Antler foreland basin to the west of the orogenic highland. A small foreland basin is also preserved along the southern third of the smaller and thinner Golconda allochthon (Speed, 1983).

Mesozoic-Cenozoic regional uplift accompanying the thinning of supracrustal rocks has exposed deep structural levels that were variably deformed, metamorphosed, and intruded by granitic plutons during the Mesozoic. Metamorphic mineral assemblages indicate that Mesozoic metamorphism and deformation of Precambrian through Lower Paleozoic sediments occurred at depths as great as 15 km (Snoke, 1980; Howard, 1980).

Magmatism peaked within northeastern Nevada at about 155-165 Ma and again about 70-80 Ma (Almendinger and Jordan, 1981). The only Mesozoic volcanics present in northeastern Nevada are the Jurassic Pony Trail Group exposed in the Cortez Range (Muffler, 1964). East-west volcano-tectonic belts such as the Tobin-Toiyabe and Stillwater-Shoshone troughs were the loci of intense Oligocene-Early Miocene volcanic activity in central Nevada.

Oligocene and Miocene ash-flow tuffs were erupted from scattered, large and partially collapsed vents which formed calderas and caldron complexes. Calderas, volcanic vents, and caldron complexes are often marked with circular and elliptical faults that delineate collapse zones and commonly show thousands of feet of subsidence. Many of the caldera complexes are nested and exceedingly poorly exposed. Several of the ash-flow tuff sheets in the area have not been traced to a discrete vent or caldera. Large landslide and breccia masses of surrounding Tertiary and Paleozoic units are present in the interiors of these calderas which are locally resurged (Ekren and others, 1974; Stewart and Carlson, 1976; Stewart and McKee, 1977; Kleinhampl and Ziony, 1985). These calderas include the Lunar Lake, Williams Ridge, Hot Creek, Pancake Range, Northumberland, and Big Ten Peak Calderas in the Monitor, Toquima, Toiyabe and Shoshone Ranges, where volcanic sequences approach 15,000 feet in thickness (Kleinhampl and Ziony, 1985).

An early regional episode of extension, closely following the end of the Laramide orogeny, occurred prior to the formation of the present Basin and Range physiography of east-central Nevada. This extension is expressed by imbricate east-directed low-angle normal faulting of Oligocene and Miocene age, and was concurrent with calc-alkaine magmatism and the extrusion of 38 to 33 Ma andesitic to rhyolitic lava flows and tuffs (Hose and Blake, 1976).

Late Eocene through Holocene extension has created the series of high-angle fault bounded and internally segmented fault blocks which characterize the present physiography of northeastern Nevada as well as the remainder of the Basin and Range province. Extension direction and intensity have varied both geographically and chronologically across northeastern Nevada creating domains of differential extension. Many of these domains appear to be separated by lateral faults which form boundaries between differentially extended domains.


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Last modified: 09/12/06