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Mesozoic-Cenozoic Metamorphic Core Complex Formation

The term metamorphic core complex has been applied to isolated complexes of regionally metamorphosed rocks in the Cordillera of western North America (Coney, 1980, Snoke, 1980; Armstrong, 1982). Core complexes contain an infrastructure composed of an older metamorphic-plutonic basement within which metamorphism dies out progressively upsection. The infrastructure is overlain by a transition or detachment zone which shows an abrupt change in lithology, and is characterized by a prominent foliation and elongation lineation induced by intense strain. The younger unmetamorphosed cover or suprastructure is characterized by brittle deformation along high-angle listric and planar normal faults, and low-angle normal faults which attenuate or eliminate thick stratigraphic sections (Coney, 1980; Stewart, 1980).

The uplift, doming, and unroofing of metamorphic core complexes within the Basin and Range occurred along detachment faults and related low-angle and high-angle normal fault systems that appear to be primarily Middle Miocene in age (Stewart, 1983). The age of regional metamorphism however is Mesozoic according to many workers, although it is far from unambiguous as a result of argon loss associated with deformational stress along low-angle detachment faults, or during cooling after uplift (Kistler and Willden, 1969; Kistler and O'Neil, 1975). This argon loss results in Mesozoic metamorphic and plutonic rocks showing locally reset Tertiary ages. Hose and Blake (1976) suggest that regional metamorphism within the Snake, Schell Creek, Kern and Deep Creek Ranges may have occurred as early as Jurassic and continued into the Tertiary. A Jurassic Rb-Sr age in the Harrison Pass pluton in the Ruby Range is progressively reset with K-Ar ages from 36 to 21 Ma, and reset metamorphic rocks in the Ruby, Deep Creek, Schell Creek and Snake Ranges range from 40 to 21 Ma (Kistler and Willden, 1969, Armstrong, 1963).

Two major metamorphic core complexes have been identified in the Ruby Mountains-East Humboldt Range, and in the Snake Range of northeastern Nevada. The Grant Range may represent a third core complex, although less spectacularly displayed than those in the Ruby and Snake Ranges (Kleinhampl and Ziony, 1985). Along with well developed core complexes, discontinuous patches of greenschist to amphibolite facies metamorphic rocks are exposed within the lower plates of Tertiary low-angle normal faults within the Schell Creek, northern Egan-southern Cherry Creek, Kern-Deep Creek, Toana-Goshute, northern Pequop and Pilot Ranges, and the Wood Hills (Misch, 1960; Misch and Hazzard, 1962; Dechert, 1967; Woodward, 1963; Thorman, 1970; Miller and Lush, 1981). This regional metamorphism may represent deep structural levels of the Sevier fold and thrust belt which have been exhumed by Tertiary detachment and high-angle faulting.

In the Ruby Mountains, the Precambrian(?) through Devonian metasediments of the infrastructure are metamorphosed to the amphibolite or sillimanite zone of the amphibolite facies. These rocks are extensively migmatized with over one third of the terrane composed of pegmatitic granite sills, dikes, lenses and layers which grade into the metasedimentary rocks (Howard, 1966). These rocks have been recumbently folded on a large-scale with variable orientations of folds that are commonly overturned to the west (Howard, 1966; Hose and Blake, 1976; Snoke, 1980). Lineations within these metamorphosed rocks indicate west-northwest directed extension (Snoke and Howard, 1984). The upper plate or suprastructure of the Ruby Mountains is deformed by low-angle normal faults which place younger-over-older rocks as well as older-over-younger rocks, and attenuate or eliminate several thousand feet of stratigraphic section. A few high-angle faults with small to moderate displacements also offset the Devonian through Miocene rocks in the upper plate.

The timing of metamorphism and deformation within the Ruby Mountains-East Humboldt Range are still poorly understood despite intensive study (Snoke and Lush, 1984). The best estimated age for metamorphism is probably Cretaceous, about 72 Ma (Snoke and Howard, 1984). The metamorphic rocks were unroofed and exposed during the Neogene. Rb-Sr whole rock isochrons indicate a Miocene-Oligocene age for mylonitization and stratigraphic attenuation along low-angle faults (Snoke and Lush, 1984). A progressive thermal resetting to a minimum age of 21 Ma can be documented in cataclastic rocks near the detachment surface (Kistler and Willden, 1969). The low-angle fault system cuts 17 Ma volcanic rocks along the flank of the Ruby Mountains (Snoke, 1980).

In the Snake Range core complex to the east, the lower plate Cambrian and Precambrian sediments are metamorphosed to the amphibolite facies, are foliated and microfolded, and are thrown into major recumbent folds which are overturned to both the north and south (Misch, 1960). The intensity of deformation and degree of metamorphism are directly related and are affected by the proximity to Mesozoic intrusives within the lower plate (Hose and Blake, 1976). In some places swarms of granitic dikes comprise 80 percent of the lower plate (Miller and others, 1983).

Armstrong (1963) determined K-Ar ages of about 28.5 +/- 0.7 Ma from two samples of schist and mylonitic gneiss in the northern Snake Range, while K-Ar dates on metamorphic hornblende from the Pioche Shale near Mount Moriah are 56 Ma in age. This suggests a Paleocene age for metamorphism since overlapping Oligocene volcanics are unmetamorphosed. It is probable that these ages are reset and younger than the original metamorphism. A 72 +/- 7.0 Ma Rb-Sr isochron on metamorphosed plutonic rocks in the Kern Mountains just north of the Snake Range suggests Paleocene to latest-Cretaceous metamorphism (Armstrong, 1963).

The Snake Range shows a sharp tectonic contact between the infrastructure and suprastructure, long referred to as the Snake Range decollement (Misch, 1960; Miller and others, 1983). This gently dipping detachment fault separates the ductily deformed and metamorphosed lower plate from highly faulted Upper Paleozoic rocks within the upper plate. A northwest-southeast extension direction has been established for the upper plate of this detachment (Miller and others, 1983).

The base of the suprastructure or upper plate in the Snake Range is characterized by a series of low-angle normal faults which eliminate several thousand feet of stratigraphic section. Two generations of high-angle listric faults merge with or offset the basal detachment surface. In the southern portion of the range this faulting truncates 35 Ma volcanics (Miller and others, 1983). Potassium-argon ages decrease towards the detachment surface with a minimum age of about 17 Ma (Lee and others, 1980).

Kleinhampl and Ziony (1985) have proposed that the Grant Range along the eastern margin of Railroad Valley is a metamorphic core complex. A regionally metamorphosed lower structural level is separated from a brittlely deformed upper structural level (Moores and others, 1968; Cebull, 1967; Hyde, 1963; Kirkpatrick, 1960) The lower structural level in the Grant Range is mainly composed of schistose Cambrian and Ordovician rocks metamorphosed to the chlorite zone of the greenschist facies and intruded by Tertiary monzonitic intrusives (Cebull, 1970). Large-scale recumbent folds, in part overturned to the east are exposed in the lower plate rocks (Cebull, 1967).

Several low-angle younger-over-older normal faults cut away several thousand feet of Lower and Middle Paleozoic rocks at the base of the upper plate in the Grant Range (Huttrer, 1963; Moores and others, 1968; Kirkpatrick, 1960). High-angle normal faults also cut the upper plate rocks, commonly with less than 2000 feet of throw. There appears to be two generations of high-angle faults in the Grant Range. One set of faults cuts the low-angle normal faults while an apparently earlier set of high-angle faults are cut by the low-angle normal faults (Hyde, 1963). These high-angle and low-angle faults appear to be both Oligocene and Miocene in age.

Regional metamorphism of folded Ordovician through Devonian rocks in the Wood Hills and northern Pequop Mountains occurred in two phases with prograde metamorphism reaching the kyanite-staurolite zone of the amphibolite facies (Thorman, 1962). These rocks are deformed by large-scale northeast-trending recumbent and northward overturned folds. The Devonian metasediments in the Wood Hills are the youngest regionally metamorphosed rocks recognized to date in northeastern Nevada.

To the east, local metamorphism of Cambrian and Ordovician rocks in the northern Toana Range reached the lower portion of the greenschist facies (Pilger, 1972). In the Pilot Range, Precambrian and Cambrian sediments are metamorphosed to greenschist and locally to the amphibolite facies. The timing of this metamorphism is poorly constrained but appears to be Jurassic and/or Early Cretaceous based upon crosscutting plutonic bodies (Miller and Lush, 1981).

Farther south in the Kern Mountains-Deep Creek Range, Precambrian and Lower Cambrian sediments have been regionally metamorphosed to the staurolite zone of the amphibolite facies (Nelson, 1966). In the nearby Schell Creek Range, lower plate amphibolite facies Precambrian and Cambrian sediments show cleavage and are poorly foliated (Misch, 1960). In the northern Egan Range, lower plate Precambrian rocks have been metamorphosed to the lower greenschist facies (Woodward, 1962).


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