AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA
Lateral Faults and the Northern Nevada Rift
The importance of abundant, poorly exposed, lateral fault zones has been generally overlooked, although the structural alignment and control of igneous rocks and ore deposits within Nevada has been documented for 30 years (Roberts, 1960, 1964a). The major mining districts in north-central Nevada can be plotted along northwest trending linears such as the Battle Mountain-Eureka belt in the south and the Lynn-Railroad belt to the north (Roberts 1964; 1966). The Battle Mountain-Eureka belt contains the Battle Mountain, Cortez, Roberts, Lone Mountain, and Eureka Mining districts. The Lynn-Railroad belt includes the Lynn and Maggie Creek districts. Many of these deposits are hosted by lower plate carbonates exposed as windows within the upper plate of the Roberts Mountains thrust zone.
Strike-slip faults are probably produced as conjugate shears with small cumulative displacements, concentrated in areas of large-magnitude extension and as structural boundaries between terranes of differential extension. The lack of recognition of lateral faults is in part a result of the subtle and ambiguous ways that these faults affect surface geology (Kleinhampl and Ziony, 1985). Strain related to these lateral faults is commonly expressed at the surface as dip-slip northeast trending normal faults at oblique angles to the primary transform zone below, and as small lateral offsets of less than 1 mile to as much as 10 miles. Many major lateral fault zones are probably unrecognized in northeastern Nevada.
The largest of these lateral fault zones in northeastern Nevada is the Northern Nevada Rift, a 250 km long Mid-Miocene Rift or lateral fault zone which appears to mark the inception of Basin and Range extension within northern Nevada. The rift is marked by a 5-7 km wide North-northwest-trending zone of basalt, basaltic andesite and rhyolite flows and dikes which give radiometric ages of 13.8-16.3 Ma, are up to 850 m in thickness, and extend a total of 700 km into Oregon (Gilluly and Masursky, 1965; Stewart and others, 1975; Christiansen and McKee, 1977).
The Nevada portion of the rift was first suggested by Roberts (1960, 1966) who observed a northwest lineation of mineral belts, such as the Eureka-Battle Mountain Belt which followed linear aeromagnetic highs. This strongly positive magnetic signature is present from Mount Jefferson in north-central Oregon through north-central Nevada as far as Eureka, Nevada and is known as the Oregon-Nevada Lineament (Mabey, 1966; Robinson, 1970; Walker, 1969; Stewart and others, 1975). Zoback (1978) suggested this induced aeromagnetic anomaly can be best modeled as a deep-seated diabase dike swarm, and that the Northern Nevada Rift is in fact a leaky transform system. Left-lateral offsets of the aeromagnetic anomaly of up to 1.4 miles occur along oblique slip north-northeast-trending normal faults such as the fault bounding the Roberts Mountains (Mabey, 1966; Robinson, 1970; Zoback, 1978). The best dike exposures are in the Cortez and Roberts Mountains. Hydrothermally altered dikes in the Cortez Range are up to 3 miles in length, 10-800 feet in width, and exposed across a 6 km wide zone. Fine-grained and columnar jointed basaltic dikes in the Roberts Mountains are present over a 20 km length and 6 km width with individual dikes 10-150 ft wide.
The lineament is also defined by a north-northwest-trending belt of closely spaced and partly en echelon faults such as the Brothers Fault Zone in Oregon (Stewart and others, 1975). In Nevada the rift is magnificently expressed by a northwest-trending fault zone intersected at high angles (60 degrees) by relatively unique northeast-trending Basin and Range normal faults in Humboldt, Elko and Eureka Counties. Zoback (1978) considers these northeast oblique-slip faults to be post-rift, and to represent a conjugate fault set required to accommodate a 45 degree rotation in extension direction in northern Nevada at about 6 Ma. This clockwise rotation in the extension direction has been explained by inboard right-lateral shear as a result of oblique tensional fragmentation of western North America.
Stratigraphic and paleomagnetic data suggest a rapid southward rift propagation spanning one reverse to one normal polarity event, and a much higher volume of Mid-Miocene volcanics extruded in the northern portion of the rift as opposed to the younger southern portion. Rhyolites and rhyodacites are interfingered with andesite, basalt and dacite flows. However, basaltic andesite flows as old as 16.3 Ma characterize the southern portion of the rift in the northern Shoshone, Cortez and Roberts Mountains with rhyolite and minor basalt and andesite in the northern portion of the rift (Stewart and others, 1975; Zoback, 1978).
Zoback (1978) has suggested that dike intrusion created a 20-30 percent extension across the rift. She estimated 10.7 km (9.0-12.0 km) of extension across the 52 km wide southern portion of the Northern Nevada Rift, or essentially 20 percent total extension based upon fault offsets. Surrounding areas in Nevada have been assigned recent estimates of extension between 30-50 percent in the western and central portion of the state (Anderson, 1971, Proffett, 1977) to 100-200 % in the eastern low-angle normal fault terranes (Wernicke, 1981; Miller and Gans, 1983).
Consistent fault offsets of dikes suggest a North 76 West extension direction within this rift which is intermediate between the west-southwest-east-northeast (South 68 West +/- 5) Mid-Miocene extension which opened the rift, and a modern west-northwest-east-southeast (North 65 West +/-20) extension suggested by historical faults and earthquake foci (Zoback, 1978). This represents a 45 degree clockwise rotation in the extension direction between 16 Ma and 6 Ma. Dikes can be a much more reliable extension indicator than faults since fault trends are somewhat variable even within regionally homogenous stress fields.
Thorman and Ketner (1979) have proposed several major northwest-trending strike-slip faults within northeastern Nevada, including the Wells, Dry Creek, and Ely-Black Rock faults, to explain apparent abrupt offset in Paleozoic facies trends. The validity of these faults has been questioned by many geologists since there appears to be little demonstrable offset on some portions of these faults, and are essentially no piercement points. Still, portions of these faults do appear to be valid structural elements.
As defined, the northeast-southwest-trending left-lateral Dry Creek Fault in Eureka and Lander Counties may have a length of about 150 miles and an offset of about 25 miles. Evidence for the fault includes the abrupt northward termination of Diamond Valley and offset features in the Sulphur Spring and Pinon Ranges (Thorman and Ketner, 1979). The Dry Creek fault probably does not exist as defined by Thorman and Ketner (1979). Instead segments of the fault represent part of lateral fault zones within the Northern Nevada Rift as shown on Overlay III.
As defined by Howard (1976), the northeast-southwest-trending Ely-Black Rock fault extends from the northern White Pine Range through Sacramento Pass in the Snake Range. The Ely-Black Rock fault is invoked to explain deflections in several ranges and the axis of the Butte-Deep Creek Basin from the Butte Mountains to the northern Egan Range (Howard, 1976). The Ely-Black Rock fault does not appear to be a valid fault. There is no hard geological or geophysical evidence for this fault.
The east-southeast right-lateral-trending Wells fault does appear to be a valid structure for part of the length proposed by Thorman and Ketner (1979). It is included here within a major structural zone which we have termed the Humboldt-Wells lineament (see Overlay III). This lineament is a northeast-southwest-trending zone which extends form Palisade at the northern end of the Cortez Mountains, along the southeast margin of the Adobe Range to Wells where it trends east to cut the northern Pequop Mountains and pass through Silver Zone Pass in the northern Toana Range. Right-lateral offset of a few miles is apparent along several portions of the Humboldt-Wells lineament. It terminates or bends several basins and ranges and is offset by at least two major northwest-southeast trending faults near Carlin and north of Elko Mountain. The lineament forms a structural break between low-angle faulted and regionally metamorphosed rocks to the south and unmetamorphosed and essentially unattenuated rocks to the north in the Pequop Mountains (Thorman, 1970). The age of the Humboldt-Wells lineament is cryptic. It may include several faults of different ages which have had a complicated history of re-activation.
Immediately north of the Humboldt-Wells lineament another major arcuate and generally east-west-trending structural lineament is present in northeastern Nevada. We have termed this the Independence-Pequop lineament and the block between the two lineaments can be referred to as the Adobe block. The Independence-Pequop lineament can be mapped from Taylor Canyon in the Independence Mountains, to the east where it bends the northern Adobe Range, cuts the Windermere Hills, and terminates the northern Pequop Mountains and southern Leach Mountains. The lineament is apparent in gravity data and Cenozoic isopach thicknesses as well. The Independence-Pequop lineament shows post-Triassic right-lateral movement. Displacements are poorly constrained because of a lack of piercement points.
The eastward curve of Paleozoic assemblage boundaries along the leading edge of the Roberts Mountains thrust can be explained by lateral offset along this lineament (Poole and others, 1977; Thorman, 1970; Thorman and Ketner, 1979). Other explanations are also possible however, including an original irregularity along the Paleozoic margin of western North America, and southeast directed faulting (Poole and others, 1977; Coats and Riva, 1983).
Several long northwest to west striking lineaments and left-lateral strike-slip faults are present in the central portion of northern Nye County and include the Warm Springs, Pritchards Station, and Pancake Range lineaments (Ekren and others, 1974; Ekren and others, 1976). These lineaments also terminate ranges and valleys, demarcate breaks in structural style, and coincide with aeromagnetic anomalies.
The east-west-trending Warm Springs lineament forms a structural break between the southern Hot Creek and Kawich Ranges, and the Pancake and northern Reveille Ranges to the east along U.S. Highway 6. It continues to the west of the evaluation area. At the northern end of the Reveille Range, a chaotic brecciated mass of tuff and lava formed during intense and recurrent strike-slip movement along this fault between 18.5 and 26 Ma (Ekren, Rogers and others, 1973).
The east-northeast-trending Pancake Range lineament is present to the west of the evaluation area. It passes through the Manhattan area at the southern tip of the Toquima Range, through the southern Monitor Range, south of Morey Peak in the Hot Creek Range, and through the Pancake Range along Wood Canyon (Quinlivan and others, 1974; Ekren and others, 1976). This left-lateral fault may continue to the east through Railroad Valley and into the northern Grant Range along the Ragged Ridge fault of Moores and others (1968).
The Pritchards Station lineament passes through the southern Toquima Range north of Mount Jefferson, east through the central Monitor Range near Tulle Creek, across the northern Hot Creek Range and between the Park Range and Squaw Hills, and through the Pancake Range north of Portuguese Mountain. This left-lateral fault continues to the east across Railroad Valley and merges with the Miocene-age left-lateral Currant Summit fault which terminates the southern White Pine and northern Horse Ranges (Lumsden, 1964; Ekren and others, 1976; Moores and others, 1968; Dixon and others, 1972). Left-lateral motion along the Pritchards Station lineament is post-23 Ma according to Ekren and others (1974).
Several structural blocks in the White Pine, Grant, Horse Egan and Schell Creek Ranges also appear to have been displaced along local lateral faults (Lumsden, 1964; Moores and others, 1968). West-northwest-trending lateral faults offset the southern Egan Range near Shingle Pass and terminated the Egan Range and southern portion of the Schell Creek Range (Kellogg, 1960; Tschanz and Pampeyan, 1970). Minor lateral faults are present in many other ranges within the evaluation area. Many of these are discussed in the following sections on range and basin structure.