Middle and Late Cenozoic tectonics within northeastern Nevada were dominated by local and regional extension which has been related to a broad belt of right-lateral shear induced by differential motion between the North American and Pacific plates (Atwater, 1970).

Extensional faults in northeastern Nevada include planar and listric high-angle normal faults, and low-angle normal faults which in some cases form regional detachment surfaces. A planar normal fault is a fault with a planar surface which shows no major or systematic change in dip with depth, and no apparent rotation of beds. A listric fault is a fault with an upwardly convex surface which decreases in dip and flattens with depth. Because of this geometry, beds in the hanging wall of listric faults show steeper stratal tilts than those in the footwall. In cross-section listric faults often actually consist of several short en echelon segments. A series of imbricate listric fault blocks will show successively steeper tilts in the downthrown direction, while a row of planar normal faults will all be tilted domino style by the same amount (Wernicke and Burchfiel, 1982).

These planar and listric normal faults typically root or sole in a basal low-angle normal fault or detachment surface or surfaces. Where exposed, the low-angle normal fault zones commonly dip from 0-20 degrees cutting downsection in the direction of tectonic transport, and show younger-over-older stratigraphic relationships with elimination rather than repetition of strata. Thick zones of intense brecciation and monlothilogic breccia sheets are common in the upper plate of low-angle normal faults. In many cases several low-angle normal faults are present with older low-angle faults being cut by successively younger faults.

Estimates of the total amount of extension across northern Nevada vary from 10 to 35 percent if a simple horst and graben structure related to planar normal faults is assumed (Stewart, 1971). Estimates of 50 to greater than 100 percent are obtained if downward flattening listric fault geometries are assumed (Proffett, 1977; Gans and Miller, 1983; Wernicke and Burchfiel, 1982). The amount of regional extension due to Early Tertiary low-angle normal faults is difficult to assess but has certainly been significant over large areas of northeastern Nevada. A modest 40-60 percent figure is probably an appropriate estimate for regional extension across northeastern Nevada.

Unfortunately, high and low-angle fault surfaces in northeastern Nevada are generally poorly exposed. Well developed and exposed high-angle fault surfaces are occasionally exposed along range fronts as along the Cortez, Sulphur Spring, Adobe and Grant Ranges. Low-angle normal faults, dipping 10-30 degrees are also well displayed in several ranges, particularly in the Schell Creek, northern Egan, Grant, and Snake Ranges. These exposures rarely reveal the cross-sectional geometry of faulting.

In the Yerington district of western Nevada and in the Egan and Snake Ranges of northeastern Nevada, well-exposed cross-sections of faults and drill-core data suggest listric geometries for most high-angle faults, and imply or display basal low-angle detachment zones (Proffet, 1977; Miller and others, 1983). Faulting in the volcanic sequence at Yerington suggests that nearly horizontal Middle Miocene faults were originally high angle faults 17-14 Ma, and have been rotated into their present position about 8 Ma (Proffett, 1977). Similar mechanisms appear to have been important in the Snake, Schell Creek, Grant, White Pine, Egan, and Deep Creek Ranges (Miller and others, 1983; Young, 1960; Kleinhampl and Ziony, 1985; Rogers, 1985).

Subsurface seismic reflection data (Anderson and others, 1983; Effimoff and Pinezich, 1981) indicate that faulting within the Tertiary basins is characterized by major steep planar rotational normal faults and shallow and deep listric faults which merge with detachment surfaces. Tertiary basins can be classified with three relatively consistent geometries. These are simple sags associated with steep planar normal faults, prisms above tilted bedrock ramps associated with listric faults, or complexly deformed subbasins associated with both listric and planar normal faults which sole into a basal detachment. As basins mature, early formed sub-basins appear to coalesce, and paleo-topographic ridges which separate the sub-basins are buried. As basins enlarge, their structure appears simpler since fewer faults control their development (Anderson and others, 1983).

Pre-Tertiary reconstructions suggest that Tertiary rocks generally lie on Upper Paleozoic rocks that have been deformed by broad open folds with little associated high or low-angle faulting (Moores and others, 1968; Armstrong, 1972). The Eocene Sheep Pass provides the earliest record of Tertiary deformation in northcentral Nevada where it forms an eastward thickening wedge of fluvial and lacustrine sediments against the Shingle Pass fault in the southern Egan Range (Kellogg, 1964). Nine hundred meters of conglomerate, shale, and lacustrine limestone record the presence of Late Cretaceous to Eocene fault-bounded basins (Fouch, 1979; Newman, 1979). The presence of relatively large Paleozoic slide blocks within the Sheep Pass Formation indicate an adjacent high area. Pre-Eocene Tectonic relief appears to have been minor however since clasts older than Mississippian are rare in the Sheep Pass (Kellogg, 1964).

In northern Nevada, initiation of Basin and Range extension is recorded in upper Eocene (42-35 Ma) fluvial and lacustrine sediments exposed in the Bull Run Basin (Clark and others, 1985). Conglomerates of the Mori Road Formation contain clasts of the upper Paleozoic Schoonover Sequence exposed in the Bull Run and Independence Mountains. These Late Eocene sediments thicken and coarsen towards the buttressing Bull Run Fault, a normal fault which dips 40 to 60 degrees southeast towards the Bull Run Basin and may flatten at depth to extend under both the Bull Run Basin and Independence Mountains (Clark and others, 1985).

Paleocene and Eocene normal faults can also be documented in many other areas in northeastern Nevada including the Fish Creek Range (Nolan, 1962), Schell Creek Range (Drewes, 1967), Grant, White Pine, and Horse Ranges (Moores and others, 1968), Egan Range (Kellogg, 1964), Carlin-Pinon Range (Smith and Ketner, 1975), and the Jarbidge Quadrangle (Coats, 1964).

The major displacement on most high-angle faults within northeastern Nevada appears to be post-Upper Oligocene to Lower Miocene in age, with a particularly intense regional deformation from the Middle Miocene through Early Pliocene. Several workers have suggested that major high-angle displacements occurred between 20 Ma and 10 Ma (Moores and others, 1968; Kellogg, 1964) while others suggest most of the high-angle displacement occurred during the last 7 Ma (McKee and Noble, 1974). No obvious spatial distribution in the age of initiation of high-angle faults appear to be present in central Nevada or the surrounding region (Loring, 1972).

At least two major extensional episodes after initial Paleocene-Eocene faulting are indicated in northeastern Nevada. The earlier extension appears to be a high strain event at about 40-30 Ma, and is related to east-directed low-angle normal faults. The later extension was a lower strain-rate "Basin and Range" extension at around 14-17 Ma. These events are suggested to some degree by the variance in dips in the Tertiary section. Eocene and Oligocene sediments commonly show relatively high dips of 40 to 60 degrees, while perhaps 80 percent of the Miocene through Holocene sediments dip less than 25 degrees. This writer has seen no evidence for the large-scale Tertiary age compressive fold systems suggested by Smith and Ketner (1977). The local folding and overturning of the Oligocene Elko Formation described by Smith and Ketner (1977) in the Carlin-Pinon area is related to local rotation along high-angle listric-type faults rather than regional compression.

"Basin and Range" faulting over the past 15 Ma (Stewart and McKee, 1977; Zoback and others, 1981) produced the present day physiography which characterizes northeastern Nevada. These faults have produced elongate ranges 10-20 km across separated by comparably wide alluvial valleys. Major faults with several thousands of feet of displacement bound the ranges on one, or in some cases, both sides. Most of the range bounding faults are relatively straight to curvilinear north-northeast striking faults which are commonly segmented in either colinear or en echelon fault segments (Kleinhampl and Ziony, 1985). High-angle normal and reverse faults with displacements generally less than 3000 feet are distributed throughout the ranges and valleys as well. This faulting creates a complex and variable mosaic of unique structural blocks within ranges and beneath basin fill within the valleys.

The present extension direction in northeastern Nevada is about North 65 West-South 65 East (+/- 20) (Zoback and Thompson, 1978). North-northwest oriented basaltic dike swarms, from 13.8 to 16.3 Ma indicate a North 68 East-South 68 West (+/- 5) extension vector during the middle Miocene (Zoback and Thompson, 1978). These directions are relatively consistent for large areas of the Basin and Range and suggest that extension vectors have rotated clockwise from the Middle Miocene through the present. In general, the early formed high-angle faults were oriented northwest-southeast, with later formed high-angle faults generally trending north-northeast-south-southwest. Several of these late formed Basin and Range faults show fresh scarps suggesting Pleistocene and Holocene movement (Kleinhampl and Ziony, 1985).

Several local areas show the effects of gravity sliding during the Tertiary, with the development of local landslide and meggabreccia masses. Many of the gravity slides have accompanied uplift of ranges such as along the Diamond Mountains and Grant Ranges, while others are within the margins of caldera complexes as in the Shoshone, Toquima and Hot Creek Ranges. Several gravity slides are related to low-angle normal faults. These include blocks in the Egan and Schell Creek Ranges where brecciated portions of the Pennsylvanian Ely Limestone have slid on the Chainman Formation (Kellogg, 1960), the Harrison Pass area of the Ruby Range with brecciated Paleozoic rocks emplaced over the Miocene Humboldt Formation (Moores, 1966), and megabreccias in the Sacramento Pass area of the Snake Range (Misch, 1960).

A characteristic and perhaps the most important structural element of northeastern Nevada, is widespread low-angle younger-over-older faults which result in an attenuation or thinning (rather than repetition) of the stratigraphic section. Nearly all of the low-angle normal faults, where described at all, were originally mapped as Mesozoic age thrust faults. The omission of section and younger on older structural relationships indicate an extensional rather than compressional origin. These faults are designated as such on the geologic map compiled during this evaluation. This map is the only place at present that all of these faults have been so designated and mapped. Undoubtedly some of the faults retained as thrusts, particularly in northern Elko County, will prove on further examination to be low-angle normal faults.

Low-angle extensional faults are dominantly exposed within a 125-185 mile (200-300 km) wide, north-northeast-trending zone within eastern Nevada and western Utah (Armstrong, 1972; McDonald, 1976; Almendinger and others, 1983; Zoback, 1983). These low-angle normal faults have been best imaged in the subsurface along the west dipping, 7.5-9 mile (12-15 km) deep Sevier Desert detachment in western Utah. Most workers would agree that the movement along these flat faults is in part, if not entirely, Middle or Late Tertiary (Late Eocene-Miocene) in age. It is highly probable that some of these faults are at least partially reactivated Mesozoic thrust faults; plutons from 109 Ma to 165 Ma crosscut imbricate faults in several ranges including the Egan, Snake, Ruby, HD and Dolly Varden Mountains (Armstrong and Hansen, 1966; Armstrong, 1972).

The first documented (Hazzard and others, 1953; Misch and Easton, 1954), and perhaps best understood of these low-angle fault complexes includes the Snake Range decollement which winds its way throughout the 50 mile long and 15 mile wide Snake Range. The Middle Cambrian upper plate rocks contain both low-angle and high-angle faults which have thinned and attenuated the Paleozoic section by as much as 20,000 feet by placing younger on older rocks.

Low-angle normal fault complexes with significant attenuation are also beautifully developed in the northern Egan and southern Cherry Creek Ranges (Woodward, 1962; Fritz, 1968), the Schell Creek Range (Young, 1960; Drewes, 1967; Dechert, 1967), Kern Mountains-Deep Creek Range (Nelson, 1959), the Grant and White Pine Ranges (Moores and others, 1968; Humphrey, 1960), as well as the Goshute Range (Pilger, 1972), Pequop Mountains and Wood Hills (Thorman, 1970), and the Bristol and West Ranges (Tschanz and Pampeyan, 1970). These faults are described in detail within the regional structure section of this volume.

A few low-angle faults with small displacement and attenuation are also present in the northern Shoshone Range (Hilltop area), Owyhee-Mountain City-Rowland Quadrangles, Toiyabe Range (Mount Callaghan area), northeastern Sulphur Springs Range, Adobe Range (Sillitonga, 1975), Independence Mountains (Rodeo Creek Quadrangle), Diamond Mountains, northern Fish Creek Range (Nolan and others, 1974), Lone Mountain, Pancake and Quinn Canyon Ranges (Kleinhampl and Ziony, 1985), northern Monitor and northern Reveille Ranges (J. Oldow, personal communication, 1983). Many of these areas contain portions of the Roberts Mountains thrust which appears to be locally reactivated as a low-angle normal fault surface.

Several of the ranges containing low-angle normal faults including the Snake, Schell Creek, Grant, Horse, and White Pine Ranges appear to have been domed during and after low-angle normal faulting in the Middle to Late Miocene (Moores and others, 1968; Hose and Blake, 1976). This doming is probably a reflection of isostatic adjustment as a result of stratigraphic attenuation along these faults (Spencer, 1984).

Commonly several major and minor low-angle normal faults are present in a given area and may locally merge with one another. These low-angle faults typically ride for considerable distances along contacts between structurally competent and incompetent lithologies, and locally cut across stratigraphic contacts at moderate angles. Structurally high faults truncate lower low-angle faults indicating progressive movement in an upsection direction. The major low-angle normal faults in northeastern Nevada cut consistently downsection to the east and show small-scale structures which indicate relative transport of the upper plate to the east.

The total amount of displacement as well as the total thickness of stratigraphic section eliminated along these flat faults requires a consideration of section removed by earlier truncated faults (Dechert, 1967). Horizontal displacements along low angle faults in the Deep Creek and Cherry Creek Ranges have been estimated at 12.4 miles (20 km) and 10.2 miles (16.5 km) respectively (Nelson, 1969; Wernicke, 1981). Stratigraphic attenuation varies from a few hundred to several tens of thousands of feet along various faults in various ranges. The details of geometry for many of these faults are given in the discussion of ranges in this volume.

The upper plate of these low-angle normal fault complexes is a mosaic of blocks bounded by either planar rotational or listric normal faults which intersect a deep detachment fault, commonly at a depth of 3-6 km, at high angles (Wernicke, 1981, Wernicke and Burchfiel, 1982, Gross and Hillemeyer, 1982). Some detachment models indicate listric or planar faults bottoming into an intact, but presumably plastically extending substratum (Proffett, 1977). Certainly some presently low-angle extensional faults have been rotated into their low angle positions by repetitive cutting of a high-angle fault surface by successive high angle faults.