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| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
1 GeoLogic, 9501 Nettleton Drive, Anchorage, Alaska 99507, USA
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONDOMAIN DESCRIPTIONSDISCUSSION AND REGIONAL...SUMMARYREFERENCES CITED |
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Ten domains are described. They include lower Paleozoic domains based on paleogeo-graphic facies, the Carbonate Shelf, Slope and Basin domains; the Nolan Belt domain, a structurally complex domain that includes Precambrian and lower Paleozoic slope and basin facies rocks; the Dutch Flat domain, an Upper Devonian feldspathic sandstone of exotic origin; an Upper Devonian to Lower Pennsylvanian siliciclastic Foreland basin domain resting conformably over the Shelf domain; the Pennsylvanian and Permian siliciclastic and carbonate Antler Overlap domain, which sits unconformably over all of the older domains; the Golconda domain of deformed upper Paleozoic oceanic, carbonate and siliciclastic rocks, which is faulted over the Antler Overlap domain; the upper Paleozoic and Mesozoic volcaniclastic Black Rock-Jackson domain; and numerous carbonate, siliciclastic, and volcaniclastic Mesozoic terranes and assemblages that were either accreted to the margin or deposited unconformably over previously accreted Paleozoic terranes.
Interpretations of these domains define multiple, distinct, lower Paleozoic tectonic environments. They suggest that the "Antler Orogeny" can be reinterpreted as a sequence of tectonic events involving deformation of the margin and the accretion of multiple terranes to the margin over an extended period from the Late Devonian to the Early Pennsylvanian in a complex transpressive tectonic regime. Some of the accreted terranes contain rocks unlike those from the adjacent margin or other terranes and suggest they are far traveled. A change in the plate boundary configuration in the Middle Pennsylvanian led to the development of a new margin that reflected the effects of a new plate boundary farther to the west. Accretion to the margin of upper Paleozoic oceanic terranes at the close of the Paleozoic redefined the margin once again as it changed from a transpressive accretion regime to a true backarc plate tectonic setting in the Mesozoic. East-vergent and west-vergent, thick-skinned thrusting and exhumation coupled with significant translation of components of Mesozoic and older terranes rearranged the Paleozoic rocks of the shelf and earlier accreted terranes during Jurassic and Cretaceous time. Viewing the geologic history of the region in the context of terrane accretion provides new insight into the complex processes that shaped the continental margin of western North America.
Keywords: Nevada • Tectonic • Paleozoic • Antler • terrane
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONDOMAIN DESCRIPTIONSDISCUSSION AND REGIONAL...SUMMARYREFERENCES CITED |
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Traditional interpretations of Paleozoic tectonic events in Nevada have primarily relied on pre-plate tectonic or early plate tectonic ideas of displacement of the Earth's crust that do not necessarily address the complexity of structural and stratigraphic evidence that has been observed since they were first proposed (Brueckner and Snyder, 1985; Burchfiel and Davis, 1972, 1975; Burchfiel and Royden, 1991; Miller et al., 1984; Roberts et al., 1958; Speed, 1979; Speed and Sleep, 1982). Specifically, ideas of terrane accretion and displacement have only been applied either very generally to the Paleozoic and Mesozoic rocks within Nevada (Dickinson and Gehrels, 2000; Geissman et al., 1984; Silberling et al., 1987; Silberling et al., 1992), or to a few specific terranes (Blome and Reed, 1995; Darby et al., 2000; Gehrels and Dickinson, 2000; Gehrels et al., 2000a; Gehrels et al., 2000b; Gehrels et al., 1995; Harwood and Murchey, 1990; Jones, 1990; Ketner et al., 2005; Madden-McGuire and Marsh, 1991a; Smith and Gehrels, 1994). The two primary tectonic events of the Paleozoic, the Antler and Sonoma orogenies, are discussed in detail in this paper together with evidence for other less well known Paleozoic tectonic events. The "Antler Orogeny" (Roberts, 1951) refers to the folding and faulting of pre-Pennsylvanian rocks that is observed throughout northern Nevada and is generally considered to be Late Devonian and Mississippian in age. The "Sonoma Orogeny" (Silberling and Roberts, 1962) has been defined as a Late Permian to Early Triassic tectonic event that deformed Upper Paleozoic oceanic facies rocks and emplaced them over the Upper Paleozoic margin of northern Nevada.
The observations derived from viewing the Paleozoic and Mesozoic geology of Nevada as regional tectonic domains pose more questions than they answer, but they also demonstrate that early models of tectonic events affecting Paleozoic rocks in the region can benefit from being analyzed in the context of terrane accretion, and that important components of these events can be enhanced and updated. Defining domains helps to provide a spatial context for the various rock units. How the natures of the boundaries of the domains are interpreted provides important constraints on the timing and orientation of tectonic events affecting the different domains and the margin. Understanding tectonic relations between the various terranes and the margin is necessary for the geology to be an effective predictive tool for resource exploration and regional tectonic synthesis.
The terminology "tectonic domain" is explicitly intended to encompass the variety of geologic entities including stratigraphic sequences, assemblages, and terranes, deliberately utilized and described in detail on the new geologic map (Crafford, 2007); but it is also meant to distinguish "domains" as interpretive groupings derived from the more explicit geologic groupings used on the map. On the map, traditional stratigraphic units are grouped into sequences; "terrane" is used in the classic sense for fault-bounded geologic entities of regional extent, each characterized by a geologic history that is different from the histories of contiguous terranes (Jones et al., 1983); and "assemblage" is used for a group of related rock units within a terrane, or for a unit (or units) that has a known basement but is geographically isolated and lithologically and (or) structurally distinct from other coeval rocks. "Assemblage" is also used for grouping rock units that have been historically interpreted as geologically related to each other, but the relationship is unclear based on existing geologic data. In many cases, a specific tectonic domain is synonymous with a particular terrane or assemblage, and in other circumstances, the domain represents a grouping of these entities.
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DOMAIN DESCRIPTIONS |
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TOPABSTRACTINTRODUCTIONDOMAIN DESCRIPTIONSDISCUSSION AND REGIONAL...SUMMARYREFERENCES CITED |
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The geologic units described on the new map (Crafford, 2007) utilize stratigraphic nomenclature as well as informal assemblages and terranes. In many cases, there is a one-to-one correlation between an assemblage or terrane described on the new geologic map, and a tectonic "domain" described in this paper. In other cases, a tectonic domain represents a group of assemblages or stratigraphic units on the map. The text that accompanies the map describes all of the geologic units and their groupings as sequences, assemblages, or terranes in detail (Crafford, 2007), and is not covered in this paper. Table 1 lists each tectonic domain, the correlative units from Crafford (2007), and a brief description of the nature of the boundaries and extents of the tectonic domains. The distribution of the tectonic domains displayed in the figures with this paper represents a simplified grouping of rock units from Crafford (2007). The actual exposure of the rock units from the map is shown within the extent of the domain. Since such a large area of Nevada is covered by younger volcanic rocks and alluvium, the simplified distribution of the domain is intended as a reasonable extrapolation of the extent of the occurrence of the actual units included in the domain underneath Tertiary and Quaternary cover.
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Lower Paleozoic Shelf Domain
This domain is defined by the sequence of passive margin carbonate shelf rocks exposed in the eastern half of Nevada (Cook and Corboy, 2004), sitting depositionally on Proterozoic North American basement (Fig. 1). It is distinguished from other tectonic domains by its relative lack of Paleozoic deformation east of its western boundary, its well-defined carbonate platform paleogeographic setting, and its unequivocal stratigraphic link to the autochthonous coeval rocks of the Colorado Plateau.
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Extent and Boundaries
The extent of the Lower Paleozoic Shelf domain is shown in Figure 1. The northern boundary of the domain trends east-west across central Elko County. The western boundary trends northeasterly in north-central Nevada and then bends toward the southeast near the Lander-Nye County boundary. To the south, it bends abruptly westward in southernmost Nye County and continues south and west from there into California. This domain extends eastward into Utah along most of Nevada's eastern border and southward until Proterozoic basement crops out in southernmost Nevada.
Tectonic Events
Paleozoic, Mesozoic, and Cenozoic age thrusting and exhumation from both contractional and extensional tectonics have blurred the original autochthonous stratigraphic western boundary of this domain (Coats and Riva, 1983; Johnson and Pendergast, 1981; Johnson and Visconti, 1992; Ketner and Smith, 1982; McFarlane, 1997; McKee, 1976b; Riva, 1970). In the southern parts of the state, the faulting of lower Paleozoic rocks generally involves Mesozoic rocks, demonstrating a Mesozoic or younger age for most faulting (Burchfiel and Davis, 1972; Burchfiel et al., 1974; Burchfiel et al., 1970; Carr, 1983) in that region. In central and east central Nevada, a paucity of Mesozoic sedimentary rocks makes it difficult to distinguish the effects of Paleozoic tectonism versus younger folding events on these rocks, but evidence suggests that they have been primarily affected by multiple episodes of Mesozoic deformation (Armstrong, 1968; Camilleri and Chamberlain, 1997; Hudec, 1992; McKee, 1976b).
Discussion
Much of the present distribution and exposure of the rocks of the Lower Paleozoic Shelf domain (apart from Tertiary and Quaternary cover) is the result of exhumation of the Paleozoic shelf by Mesozoic and Tertiary age thrusting and extension. Imbrication of the rocks of this Shelf domain with rocks of both the Slope and Basin domains, discussed below, has been documented (Fig. 2) in the Snake Mountains (McFarlane, 1997), the Independence Mountains (Kerr, 1962), the Tuscarora Mountains (Peters, 1997a, 1997c), the Shoshone Range (Gilluly and Gates, 1965), the Roberts Mountains (Murphy et al., 1978), the Toiyabe Range (Means, 1962; Stewart and McKee, 1968; Stewart and Palmer, 1967), and the Toquima Range (McKee, 1976b). Some of this faulting is interpreted as relating to Paleozoic (Antler) events between Upper Devonian and Middle Pennsylvanian time (McFarlane, 1997; Silberling et al., 1997), and elsewhere it has been inferred to be younger (Coats and Riva, 1983; Ketner, 1984; Ketner et al., 1993; Murphy et al., 1978), although actual age constraints are notably rare. The Middle Pennsylvanian unconformity that defines the deformation associated with the Antler Orogeny lies above Foreland Basin domain rocks that depositionally overlie the Shelf domain (Crafford, 2007; Trexler et al., 2004). It is therefore reasonable to infer involvement of the western edge of the Shelf domain in Upper Devonian to pre-Middle Pennsylvanian folding and thrusting (Silberling et al., 1997), but it is also clear that Mesozoic thrusting has subsequently imbricated rocks of these domains together as well (Coats and Riva, 1983; Ketner and Smith, 1982; Oversby, 1972; Riva, 1970).
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The northeast-trending western edge of exposure of the Shelf domain (Fig. 1) in northern Nevada may be close to the actual depositional boundary of this domain based on observed facies changes, although a truncation of the margin subparallel to the facies changes would be difficult to detect and thus cannot be ruled out. The regional change in orientation of the trend of the western edge of exposure of the Shelf domain, however, is significant. The northern and northwestern boundaries are subparallel to facies changes within the shelf rocks (Stevens, 1991, and other papers of that volume) which is consistent with the idea that the trace of this boundary today is a reflection, more or less, of the orientation of the trend of the carbonate shelf boundary during the lower Paleozoic, unless the entire block has been rotated as one. In contrast, in southern Lander County, the western boundary of the domain turns and trends southeast. It is oblique to the facies pattern in the rocks, suggesting that this boundary postdates the original Paleozoic shelf boundary and formed as a truncation of the margin sometime after the Middle Devonian. The second abrupt change in trend of the western boundary at its southern end could suggest either a return to the original shelf margin orientation or rotation of the whole region due to younger tectonic events.
Slope Domain
The Slope domain is defined by the specific slope facies characteristics of its rocks, the fact that some of them are part of a continuous stratigraphic sequence that includes rocks of the Shelf domain (Crafford, 2007), and its generally east-vergent, large-scale Paleozoic folding (Fig. 3).
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Extent and Boundaries
Slope domain rocks (Fig. 3) are exposed in a north-northeast–trending belt across the center of the state that turns toward the east at its northern edge, and in a few localized outliers to the northwest and south. Nearly all of the extent of the Slope domain overlaps exposures of Ordovician and younger rocks of the Shelf domain. The southwestern edge of the primary exposures of the Slope domain coincides with the southwestern edge of the Shelf domain, trending southeast across the regional facies changes in the rocks. Four outliers of rocks that are generally lithologically consistent with Slope domain rocks are shown as isolated areas west and south of the main area of exposure. Only the southernmost is depositional with Shelf domain rocks. In a number of cases, rocks are not well distinguished between the Slope and Basin domains, and there is significant uncertainty as to which group to assign them.
Tectonic Events
Late Devonian to pre-Middle Pennsylvanian generally east-vergent, large-scale folding and thrusting is present throughout this domain (Finney and Perry, 1991; Finney et al., 1993; Madden-McGuire and Marsh, 1991a; Noble and Finney, 1999; Thoreson et al., 2000). In the Tuscarora Mountains, thrusting is interpreted to be late Paleozoic (Theodore et al., 1998). East-vergent, post-Early Triassic folding, thrusting, and exhumation have affected these rocks in the central part of the state (Bartley, 1990; Bartley and Gleason, 1987; Cameron and Chamberlain, 1988; Carpenter et al., 1993; Coats and Riva, 1983).
Discussion
Most, but not all of the contacts between rocks of the Slope domain and the rocks of the Shelf domain are structural (Crafford, 2007), as discussed above. The few depositional contacts help to define an important link between these two domains. In a number of places, the Slope and Basin domain rocks have not been adequately distinguished from each other due primarily to the structural complexity of the rocks. Another difficult distinction is between the Upper Devonian and Lower Mississippian Slope domain rocks and the coeval rocks of the Foreland Basin domain discussed below (Silberling et al., 1997). Regional mapping has not distinguished them in a consistent way. The overlap of the western third of the Shelf domain and the Slope domain (Fig. 3) is indicative of both the tectonic interlayering of these domains and the migration over time during the lower Paleozoic of the shelf-slope break (Cook and Corboy, 2004), as discussed above. The southeast-trending, southwestern edge of the domain is suggestive of a truncation or structural break similar to that observed in the Shelf domain rocks. Rocks of the Slope domain are imbricated with rocks of the Shelf, Basin, and the Foreland Basin domains (Fig. 4) (Silberling et al., 1997, see references under Shelf domain discussion as well).
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The abundant gold resources of Nevada are strongly concentrated in rocks of the Slope domain (Cook and Corboy, 2004; Crafford, 2005, 2007). A small number of these auriferous rocks are in depositional contact with Carbonate Shelf sequence rocks (the Shelf domain), but most are in imbricate fault slices between the rocks of Basin and Shelf domains (Peters, 1997a, 1997b, 1997c).
The current distribution of Slope domain rocks is the result of faulting and folding caused by more than one Paleozoic tectonic episode (Silberling et al., 1997; Theodore et al., 1998; Theodore et al., 2003) and at least one and more likely two distinct Mesozoic tectonic events (see references above) addressed further in the discussion and regional synthesis section of this paper. These tectonic imbrications are superimposed on an original stratigraphic distribution of slope facies rocks.
Basin Domain
The Basin domain (Fig. 5) is defined by its predominance of lithologic components derived from a basin facies environment—the presence of significant lithology not derived from the Nevada region of the continental margin; and a moderate to strongly deformed, generally east-vergent structural style. It is a composite domain consisting of several poorly defined tectonic elements.
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Extent and Boundaries
The Basin domain (Fig. 5) overlaps extensively with and is poorly differentiated from the Slope domain in some areas. The primary area of exposure forms a north- to northeast-trending belt in the center of the state that trends sharply eastward in northern Elko County and ends abruptly at its southern edge at the Lander-Nye County boundary. Outliers of the Basin domain are exposed to the north, west, and southwest and are separated from other exposures of similar rocks to the east by rocks of the Nolan Belt (see next section) and younger Paleozoic rocks. All of the boundaries of the Basin domain are structural.
Tectonic Events
At the type locality of the Antler Orogeny at Battle Mountain, Devonian through Ordovician rocks of the Basin domain and the Dutch Flat terrane (the Harmony Formation) are both unconformably overlain by a Middle Pennsylvanian conglomerate (Roberts, 1951). The Basin domain is also structurally overlain by the Upper Devonian Dutch Flat terrane (the Harmony Formation) (Roberts, 1964). The folding in the Basin domain rocks is complex but generally suggests an east-directed Paleozoic transport direction (Evans and Theodore, 1978; Peters, 1997a, 1997b, 1997c; Theodore et al., 2003). In the Sonoma and East Ranges, the Basin domain is additionally faulted over and folded with Middle and Upper Triassic carbonates (Gilluly, 1967; Silberling, 1975; Stahl, 1989) in generally west-vergent structures. Detailed structural relations studied in the Roberts Mountains (Murphy et al., 1978; Murphy et al., 1984; Noble and Finney, 1999) and elsewhere demonstrate that multiple phases of Paleozoic and Mesozoic deformation have affected rocks of the Basin domain, and that the oldest permissible age for the imbrication of the rocks within the domain is Late Devonian (Silberling et al., 1997) but is not well constrained.
Discussion
The Basin domain represents a subset of the "western facies" rocks described by Roberts et al. (1958) or the "siliceous" grouping of Stewart and Carlson (1978). The Basin domain consists of a wide variety of lithologies including Silurian feldspathic rocks that were not derived from the now adjacent North American margin (Gehrels et al., 2000b; Girty et al., 1985), and basaltic rocks formed as seamounts and mid-ocean crust (Leslie et al., 1991; Watkins and Browne, 1989). This suggests that parts of the Basin domain may consist of a number of distinct, possibly far-traveled accreted terranes (Wright and Wyld, 2006).
The Basin domain is clearly "allochthonous" in a traditional sense, but to describe it solely as the "upper plate" of a regional thrust fault, the Roberts Mountains thrust (Roberts et al., 1958), is no longer a geologically defensible interpretation of its tectonic history or the process by which the rocks were emplaced in their present position. The rocks of the Basin domain are repeatedly imbricated with rocks of the Slope, Shelf, and Foreland Basin domains (Noble and Finney, 1999; Silberling et al., 1997; Theodore et al., 1998). Significant lateral movement of rocks was indeed a far-reaching geologic concept of the 1950's geosynclinal world. It was the foresight of the originators of the idea (Merriam and Anderson, 1942; Nolan et al., 1956) that helped lead to our understanding of plate tectonics and the idea that pieces of the Earth's crust have indeed moved enormous distances (not just tens of kilometers) around the planet in the processes of formation and accretion (Coney et al., 1980).
The geologic evidence for the unconformity between the rocks of the Basin domain and the Antler Overlap domain that defines the Antler Orogeny is robust (Doebrich, 1994, 1995; Roberts, 1951, 1964; Theodore, 1991, 1994; Theodore et al., 1994)—the youngest deformed rocks are Upper Devonian or possibly even Lower Mississippian (Boundy-Sanders et al., 1999; Coles and Snyder, 1985). The oldest uncontroversial overlap is Middle Pennsylvanian (Roberts, 1964), and this relationship can be observed regionally in many places across Nevada (Dott, 1955; Hotz and Willden, 1964; Larson and Riva, 1963; McFarlane, 1997; McKee, 1972; Riva, 1970; Theodore et al., 2003; Trexler et al., 2004).
Large regions of the Basin domain consist of basalt, tuffaceous rocks, and deep-water cherts and argillites. These rocks likely formed in an ocean basin of unknown size and were subsequently accreted to the western margin of North America through a series of tectonic events. Whether this was the result of arc-related tectonism as suggested by early workers (Burchfiel and Davis, 1972, 1975; Speed and Sleep, 1982) or transpressional strike-slip plate movements as suggested by others (Eisbacher, 1983), or a combination of these events, remains to be determined. The scattered outliers of rocks included in the Basin domain that are outboard (west) of younger accreted terranes like Dutch Flat and Golconda demonstrate that the rocks of the Basin domain have been further disrupted and dislocated by younger Upper Paleozoic and Mesozoic tectonic events.
Nolan Belt Domain
Lower Paleozoic rocks that share affinity to a continental margin but demonstrate unusual structural characteristics form a discrete belt west and northwest of displaced rocks of the Slope and Basin domains (Fig. 6). Earlier maps and interpretations included these rocks in either "transitional" or "siliceous" groupings (Roberts et al., 1958; Stewart, 1980; Stewart and Carlson, 1978). They are different from the other lower Paleozoic rocks in a number of important ways that warrant distinction as a separate group (Crafford, 2007; Crafford and Grauch, 2002). These rocks have structural characteristics of an accreted terrane; that is, they exhibit complex polyphase deformation and metamorphism distinct from adjacent, coeval rocks, but also appear to have stratigraphic ties to a craton that suggest they have not traveled great distances laterally from a continental margin. This does not preclude the idea that rocks of this domain may have traveled great distances longitudinally along or around the continental margin.
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Rocks
Strongly deformed Precambrian to Cambrian quartzite and Cambrian and Ordovician (and younger?) schist, phyllite, shale, chert, quartzite, and thin bedded limestone are the principal rock types found within the Nolan Belt domain. The Cambrian section lies conformably on Pre-cambrian-Cambrian quartzite. In a few cases, Cambrian phyllite and shale have been separately recognized (Decker, 1962; Ehman, 1985; Ferguson and Cathcart, 1954; Means, 1962), but in most places they are intimately deformed together with Ordovician phyllite, schist, shale, and chert and are not distinguished separately on regional maps. The age relations between the Ordovician rocks deformed within the Nolan belt and the Ordovician rocks of the Basin domain are not well constrained, but they are inferred to be partly coeval based on limited data (Churkin and Kay, 1967; Finney et al., 1993; Madden-McGuire, 1991; Madden-McGuire and Palmer, 1990; Ross and Berry, 1963). The rock units included in this domain are referred to as the Nolan Belt (Table 1) on the geologic map (Crafford, 2007).
Extent and Boundaries
The Nolan Belt domain crops out in four distinct areas that together form a sinuous, north-south belt through the center of the state (Fig. 6). The northern end of the belt located in northern Elko County trends northeastward through the Bull Run Mountains and the Copper Mountains (the Mountain City block) (Bushnell, 1967; Coash and Hoare, 1967; Coats, 1964; Decker, 1962; Ehman, 1985). There is a significant gap produced by cover rocks from there southwest to north-central Nevada, where it crops out in the distinctive Osgood block in the Osgood Mountains (Crafford, 2000a; Hotz and Willden, 1964), Edna Mountain (Erickson and Marsh, 1974a, 1974b), the Sonoma Range (Gilluly, 1967), and the East Range (Whitebread, 1994). Fragments of Cambrian phyllite are exposed in the Shoshone Range (Gilluly and Gates, 1965), and more extensive exposures crop out to the south in the Toiyabe Range (Ferguson and Cathcart, 1954; McKee, 1976a; Means, 1962; Stewart and McKee, 1968; Washburn, 1970) and at the southern end of the Toquima Range near Manhattan (Shawe, 1995). Metamorphosed and deformed lower Paleozoic rocks depositionally overlying Precambrian rocks are well exposed in Esmeralda County trending westward into California (Crafford, 2007). Whether they should be assigned to the Nolan Belt or another domain is uncertain. All of the boundaries of the Nolan Belt domain are structural (Table 1).
Tectonic Events
The rocks of this domain have been affected by pre-Middle Pennsylvanian metamorphism, deformation, and exhumation quite distinct from the deformation measured in rocks of the Basin and Slope domains (Crafford and Grauch, 2002; Erickson and Marsh, 1974c; Madden-McGuire and Marsh, 1991a). While the Paleozoic deformation in the Basin and Slope domains is primarily east-vergent, the pervasive deformation in the Nolan Belt domain is distinctly west-vergent. Limited evidence suggests it has also been affected by east-vergent deformation that may be similar to that in the Basin domain (Means, 1962; Oldow, 1984b). Additionally, in a few places these rocks have also been involved in west-vergent Mesozoic folding and thrusting (Gilluly, 1967; Hotz and Willden, 1964; Silberling, 1975; Stahl, 1989; Whitebread, 1994). Importantly, rocks of the Antler Overlap domain unconformably overlie both the rocks of the Nolan Belt domain and those of the Basin and Slope domains (Ehman, 1985; Erickson and Marsh, 1974b, 1974c; Hotz and Willden, 1964), indicating that deformation in the Nolan Belt and that in the combined Slope and Basin domains predates the Middle Pennsylvanian and that these rocks were juxtaposed in their positions relative to each other by that time.
Discussion
The nature of the unusual complex polyphase deformation in the Nolan belt has been locally observed for some time (Boskie and Schweickert, 2001; Crafford and Grauch, 2002, and references therein); Ehman, 1985; Erickson and Marsh, 1974b, 1974c; Madden-McGuire and Marsh, 1991a; Means, 1962) but not incorporated into a regional tectonic framework. Whether the youngest age of rocks deformed in this domain is Ordovician, Silurian, or Devonian also is not well constrained. These are critical factors in understanding and more clearly defining this domain. Northern portions of the Nolan belt in the Osgood Mountains (the Osgood block) and the Bull Run Mountains (the Mountain City block) are outboard (west) of large areas of exposure of the significantly younger Golconda terrane (Fig. 7). Additional exposures of the Basin domain and Golconda terrane crop out still farther west. These observations require relative tectonic displacements between these rocks that are likely related to Jurassic or younger west-vergent folding and thrusting (Stahl, 1989, 1992) or exhumation.
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Dutch Flat Domain
The Dutch Flat domain is distinguished by its unusual coarse-grained feldspathic sandstone lithology that is unknown elsewhere in the Great Basin (Fig. 8). Its Late Devonian age is constrained by conodont fragments recovered from turbiditic, quartzose, limestone horizons inter-bedded with the feldspathic sandstone found in the Hot Springs Range in Humboldt County (Jones, 1997a).
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Extent and Boundaries
The Dutch Flat terrane (Fig. 8), more commonly known as the Harmony Formation, is only exposed in north-central Nevada at Battle Mountain (Roberts, 1964), in the Hot Springs Range north of Winnemucca (Jones, 1997a, 1997b), the Sonoma Range (Gilluly, 1967; Silberling, 1975), and in a small area in the East Range in Pershing County (Ferguson et al., 1951; Whitebread, 1994). However, fragments of feldspathic debris for which the Harmony Formation is the only plausible source turn up in places across a large area of northern Nevada from the Foreland Basin domain to the Golconda terrane (Ketner et al., 2005). All known boundaries of the Dutch Flat terrane are structural.
Tectonic Events
The Harmony Formation is unconformably overlain by the Middle Pennsylvanian Battle Formation of the Antler Overlap domain at the type locality of the Antler Orogeny at Battle Mountain (Roberts, 1951). The folding in the Harmony Formation at Battle Mountain has been reported to be similar to the folding in the rocks of the Basin domain (Evans and Theodore, 1978). However, in the Hot Springs Range, the Harmony Formation has been involved in a folding event that has overturned many folds westward (Jones, 1993, 1997a). In the Sonoma Range, the Harmony Formation is also thrust westward over Triassic rocks (Gilluly, 1967; Silberling, 1975; Stahl, 1987, 1989) suggesting that the west-vergent folding in the Harmony in the Hot Springs Range is likely related to the Jurassic Winnemucca fold and thrust belt (Speed et al., 1982; Stahl, 1992) or an even younger deformation event.
Discussion
While the unusual feldspathic characteristics of this unit have been recognized for many years (Roberts, 1951), the source of the feldspar has remained elusive and subject to varying interpretations (Jones, 1997a; Ketner et al., 2005; Roberts et al., 1958; Rowell et al., 1979; Smith and Gehrels, 1994; Stewart and Suczek, 1977; Wallin, 1990). Early workers included the Harmony Formation in the "transitional" assemblage (Roberts et al., 1958), a group of rocks that did not fit well into either the "eastern carbonate" or "western siliceous" assemblages. Zircon data indicate that some of the Harmony Formation was derived from an exotic source not near its present location (Dickinson and Gehrels, 2000; Gehrels et al., 2000b; Smith and Gehrels, 1994; Wallin, 1990). Its age and lithology require that its impact (literally and figuratively) on the continental margin did not begin until the end of the Devonian at the earliest. It is interpreted to be faulted over rocks of the Basin domain (the Valmy Formation) at Battle Mountain, and both it and the Valmy Formation at Battle Mountain are unconformably overlain by the Middle Pennsylvanian Battle Formation of the Antler Overlap domain (Roberts, 1964). This relationship at Battle Mountain is the "type locality" of the Antler Orogeny (Roberts, 1951), thus defining the accretion of the Dutch Flat terrane as an integral component of this tectonic event.
Because of its unusual lithologic characteristics, the Harmony Formation is considered an exotic accreted terrane that was emplaced against lower Paleozoic Basin domain (and/or Nolan Belt) rocks between Late Devonian and Middle Pennsylvanian time. The Harmony Formation originally mapped in the Osgood Mountains associated with Cambrian rocks (Hotz and Willden, 1964) consists of house-size blocks of feldspathic sandstone in a large mélange-like shear zone. Blocks of Cambrian limestone are also present in the mélange (Jones, 1991b). The matrix of the mélange contains Pennsylvanian radiolarians (McCollum and McCollum, 1991), suggesting additional upper Paleozoic disruption of the Dutch Flat terrane, possibly related to accretion of the Golconda terrane. The distinct structural characteristics of the Harmony Formation in different locales also indicate significant upper Paleozoic and Mesozoic disruption of various parts of the terrane since its original emplacement.
Foreland Basin Domain
The Foreland Basin domain is distinguished by the thick sequence of Lower Pennsylvanian through Upper Devonian hemipelagic, carbonate, and clastic rocks deposited over rocks of the Lower Paleozoic Shelf domain (Fig. 9). It is interpreted as a series of eastward-migrating, flexural-loading, foredeep and back bulge deposits (Goebel, 1991). At its western edge, these rocks are involved in mid-Paleozoic folding and thrusting related to the Antler orogeny (Silberling et al., 1997).
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Extent and Boundaries
The western boundary of the Foreland Basin domain is very abrupt (Fig. 9) and closely follows the eastern edge of the Slope domain along a small area of overlap. The distribution of Foreland Basin domain rocks extends far to the east all the way to the Nevada-Utah border (Poole and Sandberg, 1977, 1991) and beyond. On its northern edge, the boundary veers sharply eastward in northern Elko County, mimicking the other Paleozoic domain boundaries in that area (Crafford, 2007). To the south, the Foreland Basin domain narrows to a northeast-trending belt across western Lincoln County. Similar to Lower Paleozoic Shelf domain rocks, its trend takes a sharp turn to the west in southern Nye County. Foreland Basin domain rocks are only present in the far northwestern corner of Clark County in southernmost Nevada. An outlier of these rocks is also mapped in the Cactus Range of southern Nye County (Cornwall, 1972; Ekren et al., 1971).
Tectonic Events
Foreland Basin domain rocks have been involved in Paleozoic and Mesozoic folding and thrusting. The dramatic change in facies from the lower Paleozoic Shelf domain to the deposits of the Foreland Basin domain beginning in Late Devonian time is strong stratigraphic evidence for initiation of a Late Devonian tectonic event affecting the margin at this time (Goebel, 1991; Poole and Sandberg, 1977; Silberling et al., 1995; Silberling et al., 1997; Speed and Sleep, 1982). Studies of the structural characteristics of these Upper Devonian and Mississippian rocks have demonstrated that they are imbricated with older shelf, slope, and basin facies rocks near the western boundary of exposure of the Foreland Basin domain (Johnson and Visconti, 1992; McFarlane, 1997; Murphy et al., 1978; Murphy et al., 1984; Silberling et al., 1997; Smith and Ketner, 1977; Trexler and Cashman, 1991). The age of this faulting and the interpretation of which rocks belong in which domain have changed significantly over time (Ketner and Smith, 1982; Silberling et al., 1997; Smith and Ketner, 1968, 1978) and remain confusing. The impact of Mesozoic and younger structures on these rocks has not commonly been recognized but may be significant (Cameron and Chamberlain, 1988; Gilbert and Taylor, 2001; Nutt, 1997; Smith, 1984). Unconformities are interpreted within the Upper Devonian slope and basin facies rocks (Murphy et al., 1984), but the domain association of those rocks is unclear, and they are structurally bounded (Silberling et al., 1997). Interpretations of most boundaries as faulted or not can only be constrained by bio-stratigraphic evidence.
Discussion
The abrupt change in the stratigraphy of the Upper Devonian continental margin has long been interpreted as the initiation of a tectonic event that has been attributed to the Antler Orogeny (Goebel, 1991; Poole, 1974; Speed and Sleep, 1982). While tectonism is the most plausible explanation for the important changes in lithology at this time, the association with the Antler Orogeny as it was originally defined at Battle Mountain is not as straightforward as it is often assumed to be. Early workers attributed creation of the "Antler Foreland Basin" to the shedding of debris off the "Antler Highland" (Poole, 1974) into an "exogeosynclinal trough." The general principle of a foreland basin forming as a result of tectonism to the west (Goebel, 1991; Speed and Sleep, 1982) has withstood geologic cross-examination (Silberling and Nichols, 1991; Silberling et al., 1995; Silberling et al., 1997), but the details of the nature and timing of the tectonic event(s) and the components that were accreted are still not well constrained. The regional tectonic relations that support the idea that the Foreland Basin domain formed as a response to the multiple tectonic events defining the Antler Orogeny are: (1) The narrow overlap and subparallel association of the western boundary of the Foreland Basin domain and the eastern boundary of the Slope domain; (2) the greatest thickness of Foreland Basin domain rocks is near its western edge (Poole and Sandberg, 1977); (3) the source rocks of the Foreland Basin domain primarily include rocks of the Basin and Slope (and other?) domains to the west (Harbaugh and Dickinson, 1981; Poole, 1974); (4) rocks of the Basin, Slope, Shelf, and Foreland Basin domains are all imbricated together along the Slope/Foreland Basin boundary (Jansma and Speed, 1993; Silberling et al., 1997); and (5) the rocks of the Foreland Basin domain are unconformably overlain by the Antler Overlap domain rocks (Dott, 1955; Trexler et al., 2004). The time represented from the Late Devonian to the Middle Pennsyl-vanian is more than 50 million years, suggesting the formation of a long-lived and complex basin in response to an extended period of episodic tectonism (Silberling et al., 1997; Trexler and Cashman, 1991; Trexler et al., 2004; Trexler and Nitchman, 1990).
Antler Overlap Domain
The Permian and Pennsylvanian Antler Overlap domain is characterized by the irregular distribution of siliciclastic and carbonate rocks that reflect local source areas across a large region (Fig. 10). Five distinct lower Paleozoic domains are overlain unconformably or disconformably by rocks of this domain. Important unconformities also exist within the rocks of the domain.
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Extent and Boundaries
Exposures of the Antler Overlap domain are widely scattered across central Nevada (Fig. 10) and are discontinuous, but its stratigraphic relations across the area are surprisingly consistent. It crops out as far northeast as the Snake Mountains and the HD Range, as far northwest as the Osgood Mountains, and south into northern Nye County (Crafford, 2007). Only the southwestern-most exposure in the Candelaria Hills includes Lower Triassic rocks in Mineral and Esmeralda Counties.
Tectonic Events
The extent of deformation within the Antler Overlap domain appears to be quite variable. It has been involved in important localized Mesozoic folding and thrusting in a number of places (Hotz and Willden, 1964; Ketner and Alpha, 1992; Ketner and Ross, 1990; Riva, 1970). Deformation within the Antler Overlap domain has placed Permian rocks directly on underlying folded Basin domain and Nolan belt rocks or unconformably on older Pennsylvanian and Lower Permian Antler Overlap domain rocks (Erickson and Marsh, 1974a, 1974b, 1974c; Trexler et al., 2004; Trexler et al., 1991; Villa et al., 2007), indicating that these rocks record changes within an active upper Paleozoic tectonic regime.
Discussion
The Antler Overlap domain constrains many components of Paleozoic tectonic events in north-central Nevada. The basal unconformity at Battle Mountain of the Battle Formation of the Antler Overlap domain over the Basin domain and the Dutch Flat terrane is the type locality of the Antler Orogeny (Roberts, 1951). The unconformity between the Antler Overlap domain and the Nolan Belt in the Edna Mountains and Osgood Mountains may be as old as Pennsylvanian but clearly predates the Upper Permian (Erickson and Marsh, 1974a, 1974b; Hotz and Willden, 1964; Villa et al., 2007). The rocks of the Antler Overlap domain unconformably overlie the Foreland Basin domain rocks near Carlin (Dott, 1955; Trexler et al., 2004). They also unconformably overlie the Lower Pennsylvanian Ely Limestone of the upper Paleozoic Shelf domain in the Diamond Mountains along the Eureka-White Pine County boundary (Dott, 1955; Nolan et al., 1956). The basal unconformity of this domain provides the only true upper age limit for the "end" of the Antler Orogeny, demonstrating that the complex sequence of events that involved the emplacement of the Nolan Belt, the accretion of the Dutch Flat terrane, folding and thrusting in the Slope, Basin, Shelf, and Foreland Basin domains, and the development of the foreland basin itself had ceased by the Middle Pennsylvanian.
Was the initiation of deposition of the rocks of the Antler Overlap domain indicative of a return to a passive margin or the initiation of a different kind of tectonic regime? It is consistent with existing geologic data (Theodore et al., 1998; Trexler et al., 2004) to argue that the rocks of the Antler Overlap domain represent the tectonic response of the new continental margin to the plate boundary events occurring offshore to the west in the upper Paleozoic oceanic basin that is now represented by rocks of the Golconda terrane. The places where Antler Overlap domain rocks themselves have been folded and faulted (Erickson and Marsh, 1974a, 1974b, 1974c; Hotz and Willden, 1964; Ketner and Alpha, 1992; Riva, 1970; Villa et al., 2007) are important constraints on the nature of the subsequent late Paleozoic and Mesozoic tectonic events of the region.
Golconda Domain
The Golconda domain is characterized by thick, deformed sequences of upper Paleozoic basin facies rocks (Fig. 11). These rocks are commonly bounded below by a regional thrust fault that emplaces them over coeval carbonate and clastic rocks of the Antler Overlap domain. Lower Triassic volcanic and carbonate rocks unconformably overlie the rocks of the Golconda domain.
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Extent and Boundaries
North and west of Ordovician through Lower Mississippian rocks of the Basin and Slope domains, rocks of the Golconda terrane and Home Ranch subterrane are distributed in three distinct areas (Crafford, 2007) (Figs. 7 and 11): (1) in northern Elko County at the northern end of the Independence Mountains and eastward; (2) in a large area in north-central Nevada near Golconda in the Sonoma, Tobin, and East Ranges, at Battle Mountain and Edna Mountain, and in the Shoshone Range, with outliers to the north in the Hot Springs Range and the Osgood Mountains; and (3) in west-central Nevada in the Toiyabe Range, the Pilot and Excelsior Mountains, and areas in between. The lithology and structural characteristics of each of these areas have similarities that warrant grouping them overall in the same terrane, although at a more localized level there may be significant distinctions. The Home Ranch subterrane is found in the northernmost Independence Mountains, the Hot Springs Range, the Osgood Mountains, and the northernmost East Range, where its outcrops are shown in a distinct color.
Tectonic Events
Pervasive deformation of rocks unconformably overlain by relatively undeformed Lower Triassic rocks is a defining characteristic of the Golconda terrane, and was described long ago as the Sonoma Orogeny (Silberling and Roberts, 1962). The deformation has regional variability, but it is generally characterized by steeply west-dipping foliations that range in strike from north-south in the north central region of the state (Stewart et al., 1977; Stewart et al., 1986) to more northeasterly in the northeastern part of the state (Miller et al., 1984). In the north-central region of the State, the age of this deformation is constrained to Late Permian to Early Triassic (Silberling and Roberts, 1962) by unconformably overlying Lower Triassic rocks. In the Candelaria Hills area in Esmeralda and Mineral Counties in west-central Nevada, Lower Triassic rocks of the Candelaria Formation lay beneath the major structure bounding the Golconda terrane, suggesting that either faulting may have continued into the Early Triassic, or the rocks in this area were moved on a younger structure after initial accretion at the end of the Permian. In north-central Nevada, the structure bounding the Golconda terrane is the Golconda thrust (Ferguson et al., 1952), especially well exposed at its type locality at Edna Mountain, where motorists cross it on Interstate 80. It was originally interpreted as a Jurassic structure (Ferguson et al., 1952), but later work suggested that it could be as old as Late Permian to Early Triassic (Lupe and Silberling, 1985; Silberling, 1975; Silberling and Roberts, 1962). Uncertainty remains as to the age of the thrust and the relationship between the fault and the deformation within the terrane (Ketner, 1984; Northrup and Snyder, 1999). Regionally, however, there is a very consistent pattern of emplacement of the terrane on a low-angle structure over rocks of the Antler Overlap domain. Rocks of the Golconda terrane have also been involved in additional younger Mesozoic east-vergent and west-vergent thrusting (Jones, 1991b; Ketner et al., 1993).
Discussion
The variety of lithologies and the large age span of these rocks intimately imbricated together suggest that the Golconda terrane is made up of rocks formed in many depositional settings in and around an upper Paleozoic paleo-Pacific ocean of unknown size. With the exception of the Home Ranch subterrane, lithologic and biostratigraphic data from the Golconda terrane have not been regionally analyzed to distinguish other age-specific lithologic groupings that have been locally identified (Murchey, 1982; Murchey, 1990). Parts of the Golconda terrane are likely far traveled, while others clearly formed in proximity to a continental margin evidenced by deposition of siliciclastic material (Moore et al., 2000; Speed, 1979).
A number of structural and biostratigraphic studies have demonstrated the structural complexity of this terrane (Babaie, 1987; Brueckner and Snyder, 1985; Fagan, 1962; Jones, 1991a; Jones and Jones, 1991; Little, 1987; Miller et al., 1981; Miller et al., 1984; Murchey, 1990; Riley et al., 2000; Schweickert and Lahren, 1987; Silberling and Roberts, 1962; Speed, 1979; Stewart et al., 1977; Stewart et al., 1986). The pervasive deformation characterized by steeply dipping structures and large belts of mélange (Jones, 1991a, 1997b) within the terrane suggests proximity to a long-lived tectonic boundary that involved significant relative displacement of its components. Explanations for the origins of the deformation in the Golconda terrane have included microplate collisions, subsiding arcs, and translational plate boundaries (Brueckner and Snyder, 1985; Burchfiel and Davis, 1975; Jones, 1991a, 1991b; Miller et al., 1982; Speed, 1977, 1979). The bounding structure of the terrane as it is observed today, however, has minimal deformation in the lower plate and is a regionally characteristic thrust fault, suggesting it may relate to only the final emplacement of the terrane. This reinforces the idea that significant displacement within the terrane occurred when parts of it may have been far from the margin, and that the margin was not directly involved until a final emplacement event that may have occurred significantly later than the deformation within the terrane.
Black Rock-Jackson Domain
The Black Rock-Jackson domain (Silberling et al., 1992) is distinguished from coeval upper Paleozoic and lower Mesozoic rocks in Nevada by its lithologic correlation to terranes to the west in California, and its distinct Mesozoic deformation history (Fig. 11).
Rocks
This composite terrane consists of two sequences. Mid-Triassic to upper Paleozoic chert, siltstone, shale, sandstone, and tuffaceous and volcaniclastic rocks that formed in an oceanic-basin and island-arc setting were originally assigned to the Black Rock terrane. A mid-Jurassic to Upper Triassic sequence of volcanogenic and volcanic rocks was originally assigned to the Jackson terrane (Blome and Reed, 1995; Jones, 1990; Quinn et al., 1997; Russell, 1984; Silberling et al., 1992; Wyld, 1990). Rocks of this domain have affinities to correlative rocks in the Eastern Klamath and northern Sierra terranes, and not to the North American margin or the Golconda terrane (Jones, 1990; Skinner and Wilde, 1966). On the geologic map (Crafford, 2007), the rocks of this domain are referred to as the Black Rock-Jackson terrane (Table 1).
Extent and Boundaries
These rocks are exposed in far northwestern Nevada in southern Washoe, Humboldt, and Pershing Counties (Fig. 11) primarily in the Pine Forest Range, the Jackson Mountains, and the Granite Range.
Tectonic Events
Parts of the Black Rock terrane can be interpreted as
