Assessing temporal relationships between foreland and hinterland deformation in fold-thrust belts is critical to understanding the dynamics of orogenic systems. In the western U.S. Cordillera, the central Nevada thrust belt (CNTB) has been interpreted as a hinterland component of the Sevier fold-thrust belt in Utah. However, imprecise timing constraints on CNTB deformation have hindered evaluation of space-time patterns of strain partitioning between these two thrust systems. To address this problem, new 1:24,000-scale geologic mapping and balanced cross sections are presented through the CNTB near Eureka, Nevada, in conjunction with industry drill-hole data, conodont age determinations, and 40Ar/39Ar and U-Pb ages from volcanic, intrusive, and sedimentary rocks.
Our mapping redefines the first-order structures and deformation geometry of the CNTB at the latitude of Eureka. Contractional structures include two north-striking, east-vergent thrust faults, the Prospect Mountain thrust and Moritz-Nager thrust, which are connected as the same fault in cross section, several north-striking map-scale folds, and a Cambrian over Silurian relationship observed in multiple drill holes, corresponding to repetition of ∼2–2.5 km of stratigraphy, that defines the blind Ratto Canyon thrust. Two distinct sets of normal faults cut the contractional structures, and are overlapped by a regional late Eocene (ca. 37 Ma) subvolcanic unconformity. Retrodeformation of both sets of normal faults reveals the existence of the Eureka culmination, a 20-km-wide, 4.5-km-tall anticline with limb dips of 25°–35°, that can be traced for ∼100 km north-south on the basis of Paleogene erosion levels. The culmination is interpreted as a fault-bend fold that formed from ∼9 km of eastward displacement of the Ratto Canyon thrust sheet over a buried footwall ramp.
The type exposure of the Early Cretaceous (Aptian) Newark Canyon Formation (NCF) is preserved on top of Mississippian, Pennsylvanian, and Permian rocks on the eastern limb of the Eureka culmination. We propose that the NCF was deposited in a piggyback basin on the eastern limb of the culmination as it grew, which is consistent with published east-directed paleocurrents and provenance data suggesting derivation from proximal late Paleozoic subcrop units. Syncontractional deposition of the NCF is used to define the probable Aptian construction of the Eureka culmination and associated slip on the Ratto Canyon thrust at depth. After deposition, the NCF continued to be folded during late-stage growth of the culmination.
Aptian deformation in the CNTB at Eureka postdated migration of the Sevier thrust front into Utah by at least ∼10 m.y. and possibly as much as ∼30 m.y., and therefore represents out-of-sequence hinterland deformation. CNTB deformation was coeval with emplacement of the Canyon Range thrust sheet in the type-Sevier thrust belt in western Utah, and may represent internal shortening of this orogen-scale thrust sheet that acted to promote further eastward translation.
The Jurassic–Paleogene North American Cordillera is the type example of an ancient orogen that formed between converging continental and oceanic plates (e.g., Armstrong, 1968; Burchfiel and Davis, 1975; Oldow et al., 1989; Allmendinger, 1992; Burchfiel et al., 1992; DeCelles, 2004; Dickinson, 2004). In the western interior United States, major components of the Cordillera include the Sierra Nevada magmatic arc in California, and a broad retroarc region across Nevada and western Utah in which most crustal shortening was accommodated (Fig. 1). The retroarc region has been divided into distinct tectonic zones, including the Jurassic Luning-Fencemaker thrust belt in western Nevada (e.g., Oldow, 1983, 1984; Wyld, 2002) and the Cretaceous frontal Sevier thrust belt in Utah (e.g., Armstrong, 1968; Burchfiel and Davis, 1975; Royse et al., 1975; Villien and Kligfield, 1986; DeCelles and Coogan, 2006). Between these two loci of Cordilleran crustal shortening is a broad hinterland region that underwent distinct Jurassic and Cretaceous magmatic, metamorphic, and shortening events, although the timing and relative magnitude of Jurassic versus Cretaceous deformation within this region are debated (e.g., Armstrong, 1972; Allmendinger and Jordan, 1981; Gans and Miller, 1983; Miller et al., 1988; Speed et al., 1988; Thorman et al., 1991, 1992; Bjerrum and Dorsey, 1995; Miller and Hoisch, 1995; Camilleri and Chamberlain, 1997; Taylor et al., 2000; Wyld et al., 2001; Martin et al., 2010; Greene, 2014).
One debate centers on the timing of deformation in the central Nevada thrust belt (CNTB), defined by Taylor et al. (1993) as a series of dominantly east-vergent thrust faults and folds exposed between the towns of Alamo and Eureka, Nevada (Fig. 1). Thrust systems in the southern part of the CNTB connect southward with thrust faults of the Sevier belt in southern Nevada (Bartley and Gleason, 1990; Armstrong and Bartley, 1993; Taylor et al., 1993, 2000; Long, 2012), implying an overlap in deformation timing between these two thrust systems. However, on the basis of crosscutting relationships, southern CNTB thrust faults can only be broadly bracketed between the Pennsylvanian and the Late Cretaceous (Taylor et al., 2000), hindering evaluation of space-time patterns of strain partitioning between the CNTB and the Sevier thrust belt.
This problem is compounded in the northern part of the CNTB, near the town of Eureka (Fig. 1), an area that has been interpreted as occupying a zone of thrust faults and folds in several generations of studies (e.g., Nolan et al., 1971, 1974; Taylor et al., 1993; Carpenter et al., 1993). Here dating of contractional deformation has been hindered by varying interpretations of the style and geometry of large-offset faults (e.g., Nolan, 1962; Nolan et al., 1971, 1974; Roeder, 1989; Carpenter et al., 1993; Ransom and Hansen, 1993; Taylor et al., 1993; Lisenbee et al., 1995; Lisenbee, 2001a) and their relationships to the Early Cretaceous Newark Canyon Formation (NCF), a sedimentary unit preserved in several isolated exposures in the region (Nolan et al., 1971, 1974; Hose, 1983; Vandervoort and Schmitt, 1990) (Fig. 2). Contractional deformation in the Eureka region has been proposed to be bracketed between the Permian and Early Cretaceous (Nolan, 1962; Taylor et al., 1993; Lisenbee et al., 1995), to be dominantly Early Cretaceous, coeval with deposition of the NCF (Vandervoort and Schmitt, 1990; Carpenter et al., 1993; Ransom and Hansen, 1993), and possibly to be as young as Late Cretaceous (Vandervoort and Schmitt, 1990; Taylor et al., 1993). These varying interpretations are further compounded by complex, overprinting extensional deformation, which has hindered reconstruction of the preextensional structural geometry.
The purpose of this paper is to present a new structural and geometric definition of the CNTB at the latitude of Eureka. To achieve this goal, a new 1:24,000-scale geologic map of the northern Fish Creek Range and southern Diamond Mountains is presented (Plate 1; Fig. 2), along with deformed and restored cross sections (Plates 1–3), industry drill-hole data, conodont age determinations, and U-Pb and 40Ar/39Ar geochronology of volcanic, intrusive, and sedimentary rocks. These data facilitate redefinition of the first-order structures that compose the CNTB, and reconstruction of the regional preextensional deformation geometry. In addition, a structural model is presented that relates deposition in the type-NCF basin to CNTB deformation, including growth of a regional-scale anticline and associated motion on a thrust fault at depth, thereby placing the CNTB and NCF into the broader spatiotemporal framework of Sevier deformation and synorogenic deposition. This work is aided by recent U-Pb geochronology in the NCF type section (Druschke et al., 2011) and a recent synthesis of the timing and magnitude of crustal shortening accommodated in the type-Sevier thrust belt at the latitude of our study area (DeCelles and Coogan, 2006), both of which facilitate assessment of temporal relationships between foreland and hinterland shortening. Implications of this work for the dynamics of the Cordilleran orogenic wedge are then explored.
GEOLOGIC SETTING AND STRATIGRAPHY
Eastern Nevada has occupied a diverse range of tectonic settings that have evolved through the Phanerozoic (e.g., Dickinson, 2006). In the study area this tectonic development is preserved in a rock record that spans the Paleozoic, Mesozoic, and Tertiary (Plate 1; Fig. 3).
From the late Neoproterozoic to the Devonian, the Cordilleran passive margin sequence, a westward-thickening section of clastic and carbonate rocks, was deposited on the rifted North American continental shelf in what is now eastern Nevada and western Utah (e.g., Stewart and Poole, 1974). Eureka was situated near the distal, western margin of the shelf (Cook and Corboy, 2004). In the study area the exposed part of the passive margin sequence is 4.7 km thick, and consists of lower Cambrian clastic rocks, including the Prospect Mountain Quartzite, and a middle Cambrian to Devonian section dominated by limestone and dolomite, with lesser quartzite and shale (Fig. 3) (e.g., Palmer, 1960; Nolan, 1962; Ross, 1970; Nolan et al., 1974).
During the Mississippian, the Roberts Mountains allochthon, composed of distal slope and basinal sediments, was thrust to the east over the western edge of the continental shelf during the Antler orogeny (e.g., Burchfiel and Davis, 1975; Dickinson, 1977; Speed and Sleep, 1982). Eureka is immediately to the east of the Antler thrust front (Fig. 1), but was the site of synorogenic deposition of Mississippian clastic sediments that were sourced from the Roberts Mountains allochthon (e.g., Poole, 1974; Smith and Ketner, 1977; Poole and Sandberg, 1977). In the study area the Mississippian section consists of 1.5 km of conglomerate, sandstone, and shale, including the Chainman Shale and Diamond Peak Formation (Fig. 3) (e.g., Brew, 1961a, 1961b, 1971; Stewart, 1962; Nolan et al., 1974).
During the Pennsylvanian and early Permian, eastern Nevada underwent a protracted series of deformation and erosion events recorded by unconformity-bound packages of carbonate and clastic rocks (e.g., Trexler et al., 2004). In the study area, the Pennsylvanian–Permian section consists of 1.3 km of limestone and conglomerate, including the Ely Limestone and Carbon Ridge Formation (Fig. 3).
Across eastern Nevada and western Utah, a regional unconformity places Tertiary and in some places Cretaceous rocks over Paleozoic to Triassic rocks (e.g., Armstrong, 1972; Gans and Miller, 1983; Long, 2012). This unconformity records the net erosion that occurred during Jurassic and Cretaceous deformation within the Sevier hinterland and Cretaceous uplift of the hinterland that accompanied crustal thickening in the frontal Sevier thrust belt (e.g., Coney and Harms, 1984; DeCelles, 2004; Long, 2012).
In the Eureka region, the Mesozoic section is represented by the Early Cretaceous NCF, which is preserved in a series of isolated exposures in the Diamond Mountains and Fish Creek Range (Fig. 2) (Nolan et al., 1971, 1974; Hose, 1983). The NCF is only exposed in the northeast corner of the map area (Plate 1), where it overlies Mississippian and Pennsylvanian rocks. However, the NCF type section is exposed 1–5 km north of the map area in the Diamond Mountains (Fig. 2), and consists of ∼500 m of conglomerate, mudstone, and limestone (Vandervoort, 1987). Rocks in the type section exposure are interpreted to record fluvial and lacustrine deposition (Vandervoort and Schmitt, 1990). The NCF type section has yielded Aptian–Albian fossil fish, ostracods, and nonmarine mollusks (MacNeil, 1939; Fouch et al., 1979). A ca. 116 Ma U-Pb zircon age obtained from an air-fall tuff in the type section defines an Aptian deposition age, and a ca. 122 Ma youngest U-Pb detrital zircon age population obtained from sandstone in the type section defines an Aptian maximum deposition age (Druschke et al., 2011).
One other rock unit of possible Mesozoic age in the study area is a conglomerate that unconformably overlies Ordovician, Mississippian, and Permian rocks on Hoosac Mountain (Fig. 2; Plate 1). Nolan et al. (1974) mapped this conglomerate as the NCF. However, the conglomerate contains detrital zircon that have a ca. 72 Ma youngest U-Pb age population, defined by 3 concordant grains that overlap within error (sample NV11DZ-017EU; Fig. 4; see Supplemental File1 for methods, concordia plots, and data from individual analyses), that defines a Late Cretaceous maximum depositional age (e.g., Dickinson and Gehrels, 2009). The conglomerate is overlain by late Eocene dacite, defining a minimum deposition age.
Nevada was the site of the Great Basin ignimbrite flareup, an episode of silicic volcanism that swept from northeast to southwest across the state from the late Eocene to the early Miocene (e.g., Coney, 1978; Armstrong and Ward, 1991; Best and Christiansen, 1991; Henry, 2008; Best et al., 2009). Ignimbrite flareup rocks in the Eureka region include late Eocene silicic lavas, tuffs, shallow intrusive rocks, and associated volcaniclastic and sedimentary rocks. Volcanic rocks are concentrated in the central and northeast parts of the map area (Fig. 2; Plate 1), and include the Pinto Peak and Target Hill rhyolite dome systems, Ratto Springs dacite, and Richmond Mountain andesite. K-Ar ages of these volcanic rocks range between ca. 37 and 33 Ma (Nolan et al., 1974), and new 40Ar/39Ar dates (Table 1; see Supplemental File [see footnote 1] for age spectra, inverse isochron plots, and data from individual analyses) show that the oldest volcanic rocks are 37.4 Ma (late Eocene).
From the Neogene to the present, much of Nevada and western Utah has been the site of regional extensional tectonism. Widespread, large-magnitude, upper crustal extension began in the early Miocene (ca. 17.5 Ma) (e.g., Dickinson, 2006), and by ca. 10 Ma either transitioned into, or was followed by a separate episode of, lower magnitude extension that formed the modern Basin and Range topography (e.g., Dickinson, 2002; Colgan and Henry, 2009). However, although more local in scale, evidence for pre–middle Miocene extension in areas of Nevada and Utah has also been presented, including tectonic denudation of metamorphic core complexes (e.g., Snoke and Miller, 1988; Hodges and Walker, 1992; Camilleri and Chamberlain, 1997; McGrew et al., 2000), and development of latest Cretaceous to Eocene extensional basins (e.g., Vandervoort and Schmitt, 1990; Potter et al., 1995; Druschke et al., 2009a, 2009b).
STRUCTURES AND DEFORMATION TIMING CONSTRAINTS
The Eureka area has experienced a multiphase tectonic history of contractional deformation complexly overprinted by extensional deformation. Interpretations of the style, geometry, and origin of many large-offset faults in the Fish Creek Range and Diamond Mountains have varied widely in previous studies (e.g., Nolan et al., 1971, 1974; Carpenter et al., 1993; Ransom and Hansen, 1993; Taylor et al., 1993; Lisenbee, 2001b), resulting in significant disagreement over deformation geometry and timing. In the following sections, map-scale faults and folds are described, along with observations that support interpretations of their style and geometry, and field relationships that bracket their motion timing. These observations are used to support construction of three 1:24,000-scale deformed cross sections (Plate 1 and 2), and three 1:100,000-scale deformed and restored balanced cross sections (Plate 3). The balanced cross sections are based on the 1:24,000-scale cross sections, but with simplified Paleozoic stratigraphy (Fig. 3), and with faults with <300 m of offset omitted. The top row of Plate 3 shows the modern, deformed geometry, and the bottom row shows a partially restored geometry, with motions on extensional faults retrodeformed. Angles, line lengths, and offsets on faults were matched between the deformed and restored cross sections (e.g., Dahlstrom, 1969).
The dip directions of faults were determined by the interactions of fault traces with topography in Plate 1, and quantitative estimates of the dip angles of faults were determined using three-point problems on fault traces (see Table SM4 in the Supplemental File [see footnote 1] for supporting data). It was assumed that the fault dip angle estimated at the modern erosion surface stays the same both above and below the modern erosion surface. Apparent dips of supporting attitude measurements were projected onto the cross sections (Plate 1 and 2), and areas of similar apparent dip were divided into dip domains. Boundaries between adjacent dip domains were treated as kink surfaces (e.g., Suppe, 1983), their orientations determined by bisecting the interlimb angle. Division of areas of the cross sections into dip domains, combined with quantitative constraints on fault dip angles, allowed estimation of the cutoff angles that strata make with faults at the modern erosion surface. All offset estimates listed in the following for specific faults were measured from the 1:24,000-scale cross sections in Plate 1 and 2, by measuring the displacement of matching hanging-wall and footwall cutoffs across the structure. Additional details on fault and fold geometry, and justifications for individual decisions made in drafting and retrodeforming the balanced cross sections are annotated in footnotes in Plate 3. All cross sections were drafted by hand.
Contractional Structures: The CNTB
On the eastern flank of Lookout Mountain (Fig. 2; Plate 1), drilling by Timberline Resources Corporation has revealed an older-over-younger relationship at depth, defined by Cambrian Geddes Limestone, and locally Cambrian Secret Canyon Shale, over Silurian Lone Mountain Dolomite (Fig. 5), corresponding to structural repetition of ∼1800–2300 m of stratigraphy. This is interpreted as evidence for a large-throw thrust fault at depth, here named the Ratto Canyon thrust. The thrust has been intercepted in 5 drill holes over a north-south distance of 3 km, at depths between 350 and 450 m, and does not breach the modern erosion surface within the map area. In addition to lithologic logs of drill core, the existence of the Ratto Canyon thrust is corroborated by Silurian and Ordovician conodont age determinations from sampled footwall rocks (Fig. 5) (see Supplemental File [see footnote 1] for drill-hole lithologic logs and conodont age determination report).
On Prospect Mountain (Figs. 2 and 6), a ∼10°E dipping fault with ∼60° hanging-wall and footwall cutoff angles exhibits a top-to-the-east motion sense, and places older rocks over younger rocks. This fault was mapped as an unnamed thrust fault by Nolan (1962) and Nolan et al. (1974), and is here named the Prospect Mountain thrust. The thrust places Prospect Mountain Quartzite, Pioche Shale, and Eldorado Dolomite over Secret Canyon Shale (Fig. 6), and has ∼1.0 km of top-to-the-east displacement (Plate 1). The Prospect Mountain thrust is cut by several normal faults (Fig. 6; Plate 1), including the Dugout Tunnel fault.
In the southern Diamond Mountains, the ∼60°W dipping Moritz-Nager thrust (French, 1993) places Devonian rocks over Mississippian rocks (Plate 1; Fig. 2), and has an estimated top-to-the-east displacement between 1.0 and 1.8 km (Plate 1 and 2). In its hanging wall, the Sentinel Mountain syncline (Nolan et al., 1974) strikes parallel to the thrust trace. This upright, open syncline plunges 15°N and has a western limb that dips 20°E and an eastern limb that dips 20°–40°W. Both the Moritz-Nager thrust and the axis of the Sentinel Mountain syncline are overlapped by late Eocene tuff (Plate 1).
Though its axis is concealed under Quaternary sediment within much of the map area, the Pinto Creek syncline, originally described by Nolan et al. (1974) immediately north of our map area, is defined by outcrops of Devonian, Mississippian, and Pennsylvanian strata on the eastern flank of the Diamond Mountains, in the footwall of the Moritz-Nager thrust (Fig. 2; Plate 1). It is an upright, open syncline that plunges ∼17°N, and has a western limb that dips 30°–60°NE and an eastern limb that dips 10°–20°NW.
A north-striking anticline axis can be traced along the topographic crest of the Fish Creek and Mountain Boy Ranges (Fig. 2; Plate 1). It is an upright, open fold, with a western limb that dips 10°–20°W and an eastern limb that dips 20°–30°E, here named the White Mountain anticline. The fold axis is cut by several normal faults (Plate 1), including the lower Reese and Berry detachment.
In summary, contractional structures in the map area include (1) two east-vergent thrust traces, the Prospect Mountain thrust and Moritz-Nager thrust, which are connected as the same structure in the cross sections (Plate 3) on the basis of the relative stratigraphic levels that they deform, similar offset magnitude (∼1 km), and a lack of any surface-breaching thrust faults observed between their traces; (2) the Ratto Canyon thrust, which is defined by a Cambrian over Silurian relationship observed in drill holes (Fig. 5), corresponding to repetition of ∼2–2.5 km of stratigraphy; and (3) three north-striking, upright, open folds, the White Mountain anticline, Sentinel Mountain syncline, and Pinto Creek syncline. The Prospect Mountain thrust and White Mountain anticline are cut by normal faults that predate late Eocene volcanism (see following discussion), and the Moritz-Nager thrust and Sentinel Mountain syncline are overlapped by late Eocene volcanic rocks.
Normal faults in the map area can be divided into an older set, consisting of two oppositely verging fault systems with low (≤20°) cutoff angles to bedding, including the Hoosac fault system and Reese and Berry detachment system (Fig. 2; Plate 1), and a younger set of high dip-angle, down-to-the-west normal faults.
The down-to-the-east Hoosac fault system (Fig. 2; Plate 1–3) is an anastomosing series of ∼70°–90°E dipping normal faults (Table SM4 in the Supplemental File [see footnote 1]) with low (typically ∼20°) hanging-wall and footwall cutoff angles, and a cumulative offset of ∼5 km. This fault system is modeled on the cross sections as a series of subvertical structures (Plate 1–3). The easternmost, master fault of the system places Mississippian and Permian rocks over Ordovician rocks (Fig. 7A; Plate 1–3), corresponding to an omission of ∼2–3 km of stratigraphy, and a series of subsidiary faults west of the master fault deform Ordovician and late Cambrian rocks (Plate 1). The master fault has been interpreted as a steeply east dipping normal fault (Hague, 1892; Nolan et al., 1974) and a shallowly west dipping thrust fault (Nolan, 1962; Taylor et al., 1993; Lisenbee, 2001b) in previous studies. However, the map patterns of both the master fault and subsidiary faults (Plate 1), combined with multiple three-point problems calculated along their length (Table SM4 in the Supplemental File [see footnote 1]), demonstrate their steep eastward dip, which refutes the thrust fault interpretation. The easternmost two faults of the system are overlapped by late Eocene (ca. 36–37 Ma) dacite and rhyolite (Plate 1; Fig. 7A), and the westernmost two faults are cut by a late Eocene rhyolite dike (Plate 1). In addition, an angular unconformity at the base of the Late Cretaceous to late Eocene conglomerate on Hoosac Mountain overlaps the master Hoosac fault (Figs. 2 and 8; Plate 1).
The down-to-the-west Reese and Berry detachment system (Figs. 2 and 7B; Plate 1) consists of two ∼10°–20°W dipping faults (Table SM4 in the Supplemental File [see footnote 1]) with hanging-wall and footwall cutoff angles typically between 0° and 20°. The lower detachment is developed along the top of the Ordovician Eureka Quartzite, which forms a footwall flat that can be traced for ∼5 km across strike. Hanging-wall stratigraphic levels are typically in Silurian dolomite, indicating a maximum stratigraphic omission of 400 m. In addition, a bedding-parallel detachment fault that bounds the base of several klippen of Eureka Quartzite on White Mountain (Fig. 2; Plate 1) merges to the west with the lower detachment. The upper Reese and Berry detachment places Devonian dolomite over Silurian dolomite, omitting between 600 and 900 m of stratigraphy. In the Mountain Boy Range, the upper and lower detachments merge, and place Devonian rocks over Ordovician rocks, a stratigraphic omission of 1200–1500 m. Cumulative offset estimates for the Reese and Berry detachment system are 1.5–2.6 km in the Fish Creek Range and 4.7 km in the Mountain Boy Range (Plate 1 and 2). Similar low-cutoff-angle normal faults have been documented in other areas of the Fish Creek and Mountain Boy Ranges (Cowell 1986; Lisenbee et al., 1995; Simonds, 1997; Lisenbee, 2000). Approximately 1.5 km south of the map area, the lower Reese and Berry detachment is cut by dikes of porphyritic granite, which are correlated with late Eocene (34.1 ± 1.5 Ma; Marvin and Cole, 1978) granite in the Mahogany Hills (Cowell, 1986).
The younger set of normal faults in the study area consists of three high-dip-angle (typically ∼60°–70°), multiple-kilometer-throw (∼2–4.5 km), down-to-the-west faults, including the Dugout Tunnel, Lookout Mountain, and Pinto Summit faults (Fig. 2; Plate 1–3). The Dugout Tunnel fault places shallowly east dipping Ordovician rocks over steeply east dipping Cambrian rocks, and has between 2500 and 2900 m of offset (Plate 1 and 2). The ∼40° change in dip angle observed across the fault is modeled as juxtaposition of opposite sides of a kink surface (Plate 3). The Lookout Mountain fault places Devonian rocks over Cambrian rocks, and has ∼2000 m of down-to-the-west offset at Lookout Mountain (Plate 2 and 3). The Dugout Tunnel and Lookout Mountain faults are both overlapped by the late Eocene subvolcanic unconformity. In the footwall of the Lookout Mountain fault, the unconformity is on Cambrian rocks (Plate 1; Fig. 5), while modern erosion levels in its hanging wall preserve Devonian rocks. In addition, 3–6 km south of the map area, late Eocene volcanic rocks are mapped on both sides of the Lookout Mountain fault (Nolan et al., 1974; Cowell, 1986), with Cambrian erosion levels in the footwall and Devonian erosion levels in the hanging wall. In the footwall of the Dugout Tunnel fault, ∼200 m south of the map area, Nolan et al. (1974) mapped late Eocene rhyolite in depositional contact over Cambrian rocks, while modern erosion levels in its hanging wall preserve strata as young as Ordovician (Fig. 9), indicating that the majority of motion had to be pre–late Eocene.
In Rocky Canyon (Fig. 2; Plate 1), the Dugout Tunnel fault places unmetamorphosed Ordovician limestone over Cambrian Secret Canyon Shale that is metamorphosed to hornfels, which is interpreted as contact metamorphism associated with intrusion of Late Cretaceous granite dikes at depth. This is supported by drill hole NMC609C, located 600 m to the southeast (Fig. 2; Plate 1), which drilled through Silurian and Ordovician dolomite, quartzite, and limestone before abruptly intercepting Secret Canyon Shale metamorphosed to hornfels and skarn. The metamorphism is spatially associated with numerous two-mica granite dikes intercepted in the drill hole. We present a new U-Pb zircon crystallization age of 86.1 ± 0.8 Ma (2σ) from a granite dike sampled from drill core (sample NMC609; see Supplemental File [see footnote 1] for methods, concordia plot, and data from individual analyses), which overlaps within error with the 84 ± 2 Ma age obtained by Barton (1987) from a granite dike sampled from the same drill hole. This indicates that the Dugout Tunnel fault cuts a contact aureole associated with a ca. 86 Ma intrusive system, which provides a maximum motion age. The Dugout Tunnel fault also cuts a detachment fault that bounds a klippe of Eureka Quartzite (Plate 1), which is correlated with the lower Reese and Berry detachment.
The Pinto Summit fault (Lisenbee, 2001b) places steeply east dipping Permian rocks on the west against shallowly east dipping Silurian rocks (Figs. 2 and 7C; Plate 1 and 2). The Pinto Summit fault has previously been interpreted as a down-to-the-east normal fault (Nolan et al., 1974; Lisenbee, 2001b) and a top-to-the-west reverse fault (Taylor et al., 1993). However, the map patterns of exposures on the north and south ends of the map area (Plate 1), combined with the results of three-point problems (Table SM4 in the Supplemental File [see footnote 1]), define a steep (∼65°–75°) westward dip, which refutes the thrust fault interpretation. Offset estimates for the Pinto Summit fault range from 3.7 to 4.5 km (Plate 1–3). The fault is overlapped along most of its length by late Eocene volcanic rocks (Plate 1). Eastward tilting accompanying motion on the Pinto Summit fault is interpreted as the most likely mechanism for tilting the Late Cretaceous to late Eocene conglomerate on Hoosac Mountain 20°–30° to the east (Plate 1; Fig. 8). This brackets the maximum motion age on the Pinto Summit fault as ca. 72 Ma, the maximum deposition age of the conglomerate. In addition, because the conglomerate overlaps the Hoosac fault system, but was likely tilted eastward by motion on the Pinto Summit fault, this indicates that the Hoosac fault system predates the Pinto Summit fault (Plates 1–3).
In summary, normal faults in the map area can be divided into two distinct sets, both of which predate late Eocene (ca. 37 Ma) volcanism: (1) an older set consisting of the down-to-the-east Hoosac fault system and down-to-the-west Reese and Berry detachment system, which have low (≤20°) cutoff angles to bedding, and offset magnitudes between ∼2 and 5 km; and (2) a younger set consisting of high-dip-angle (60°–70°), down-to-the-west normal faults with offset magnitudes between 2 and 4.5 km, including the Dugout Tunnel, Lookout Mountain, and Pinto Summit faults. Faults of set 1 are cut and tilted by faults of set 2, and faults of set 2 cut a Late Cretaceous (ca. 86 Ma) contact aureole and tilt conglomerate that has a Late Cretaceous (ca.72 Ma) maximum deposition age. Motion on faults of set 2 was accompanied by as much as to ∼20°–30° of eastward tilting.
Geometry of the Late Eocene Subvolcanic Unconformity
The map patterns of late Eocene volcanic rocks, as well as the map-scale geometry of the subvolcanic unconformity, indicate that only minor (≤10°) tilting or vertical throw on normal faults has taken place in most of the map area since the late Eocene. For example, the ∼9 km2 Pinto Peak rhyolite dome is rimmed with volcanic breccia and pyroclastic deposits that exhibit subhorizontal map patterns (Plate 1; Fig. 7D). Volcaniclastic deposits associated with the Ratto Springs dacite have a general ∼10°SE dip, which may partly reflect deposition on paleotopography, but are capped by conglomerate and gravel that exhibit subhorizontal basal contacts and yield variable strike directions and dip angles typically ≤7° (Plate 1).
The late Eocene unconformity is exposed in three or more localities on each cross section, distributed across the eastern two-thirds of the map area (Plates 1–3). The cross sections illustrate that the late Eocene erosion surface mimics the modern erosion surface, and is not tilted at a regional scale (Plate 3). Notably, the topographic lows of several prominent canyons incised into Paleozoic bedrock contain exposures of late Eocene volcanic rocks (Plate 1), indicating that these canyons must have been incised by the late Eocene.
Geometry and Stratigraphic Level of the Early Cretaceous Unconformity
The Early Cretaceous NCF is only preserved in the northeast corner of the map area, in a series of exposures that unconformably overlie the Mississippian Diamond Peak Formation and Pennsylvanian Ely Limestone (Plate 1). However, the 5-km-long, semicontinuous exposure that contains the type section of the NCF is located 1–5 km north of the map area (Figs. 2 and 10), where the formation unconformably overlies the Mississippian Diamond Peak Formation, Pennsylvanian Ely Limestone, and Permian Carbon Ridge Formation (Nolan et al., 1971, 1974). Nolan et al. (1971) also mapped the NCF unconformably overlying the Mississippian Diamond Peak Formation and Permian Carbon Ridge Formation 1–4 km north of the map area, at and south of the town of Eureka (Fig. 2).
Within our map area, Nolan et al. (1974) mapped several isolated exposures of conglomerate and other lithologies that unconformably overlie rocks between Ordovician and Permian in age as the NCF. We have reexamined all of these exposures, and we map them as a variety of different rock units, including Permian conglomerate, Late Cretaceous to late Eocene conglomerate, sedimentary rocks associated with late Eocene volcanism, and Quaternary colluvium. Figure SM6 in the Supplemental File (see footnote 1) shows a side by side comparison of our mapping and the mapping of Nolan et al. (1974), as well as field photographs of outcrops, for areas of key importance where we have disagreed with their mapping of the NCF. These areas include: (1) the axis of the Sentinel Mountain syncline (Fig. SM6A in the Supplemental File [see footnote 1]), where we map unconsolidated and locally cemented angular limestone cobbles and boulders that overlie Devonian rocks as Quaternary colluvium; (2) areas in the footwall of the Pinto Summit fault (Fig. SM6B in the Supplemental File [see footnote 1]) where we map pebble conglomerate with a white, tuffaceous matrix that overlies Silurian rocks as conglomerate associated spatially and temporally with late Eocene volcanism, similar to other synvolcanic conglomerate mapped near Pinto Summit; and (3) on Hoosac Mountain (Fig. SM6C in the Supplemental File [see footnote 1]), where red matrix, limestone-clast pebble conglomerate that overlies Ordovician rocks in the footwall of the Hoosac fault system, and Mississippian and Permian rocks in its hanging wall, yields a Late Cretaceous (ca. 72 Ma) youngest detrital zircon population, and is therefore a distinctly younger rock unit than the NCF.
Our new mapping and geochronology data refute the original interpretation of Nolan et al. (1974) that the NCF was deposited on strata as old as Ordovician, Silurian, and Devonian, and therefore have fundamental implications for the map-scale geometry and basal structural relief of the Early Cretaceous unconformity. The ages of subcrop units observed beneath NCF exposures in the northeast part of our map area (Plate 1), in the type exposure (Fig. 10) (Nolan et al., 1971, 1974), and at and south of the town of Eureka (Fig. 2) (Nolan et al., 1971, 1974), show that stratigraphic levels below the Mississippian Diamond Peak Formation were not breached by the Early Cretaceous. This indicates that the NCF was deposited on a substrate of Mississippian to Permian rocks that had a maximum regional structural relief of ∼1.5 km, estimated using stratigraphic thicknesses in Figure 3.
The low structural relief of the Early Cretaceous unconformity also has important implications for the maximum motion age of the Pinto Summit fault and Hoosac fault system, which Nolan et al. (1974) originally mapped as being overlapped by the Early Cretaceous unconformity. Subcrop relationships defined by our new mapping show that the NCF overlies similar stratigraphic levels on either side of the Pinto Summit fault, indicating that it cuts the Early Cretaceous unconformity (Plate 3). Our mapping and geochronology data show that the NCF is not preserved in the footwall of the Hoosac fault system, which suggests that this fault system also cuts the Early Cretaceous unconformity.
The cross sections on the bottom row of Plate 3 were retrodeformed for extensional deformation, which was accomplished by: (1) matching line lengths and angles between the deformed and restored cross sections (e.g., Dahlstrom, 1969); (2) restoring the displacements of matching hanging-wall and footwall cutoffs across all normal faults; and (3) retrodeformation of ∼25° of eastward tilting in the hanging wall of the Pinto Summit fault, based on the dip of the Late Cretaceous to late Eocene conglomerate on Hoosac Mountain. Details on additional constraints used in retrodeforming extension are annotated in footnotes in Plate 3. Comparison of deformed and restored lengths reveals similar extension magnitudes for all three cross sections, with a total range of 7.4–8.1 km (∼45%–50% extension).
Retrodeformation of extension reveals a similar preextensional deformation geometry on all three cross sections, defined by an upright anticline with a wavelength between 19 and 21 km and an amplitude between 4.3 and 5.0 km, which is here named the Eureka culmination. The existence of this regional structural high is independently corroborated by Paleogene subcrop and erosion patterns. In Long (2012), a paleogeologic map of the Sevier hinterland was presented that showed the distribution of Neoproterozoic to Triassic rocks exposed beneath the regional Paleogene, subvolcanic unconformity, and illustrated the approximate map patterns of regional-scale, pre-unconformity structures. Figure 11 shows a Paleogene subcrop map of the Eureka region (modified from Long, 2012) that reveals the approximate map patterns of several north-trending, erosionally beveled, pre-Paleogene folds, with erosion levels typically in Mississippian to Triassic rocks. However, the Eureka culmination exhibits erosion levels as much as 4 km deeper than the folds in the surrounding ranges, and can be traced along strike for ∼100 km, from the northern Pancake Range, where it exhibits Ordovician subcrop levels, through the study area, where subcrop levels are as deep as Cambrian, and into Diamond Valley, on the basis of Devonian and Mississippian subcrop levels observed in drill holes. In the study area deep subcrop levels represent a combination of erosional exhumation of the Eureka culmination during and after its construction, and tectonic exhumation by pre–late Eocene normal faulting.
In regions that have undergone superposed contractional and extensional tectonism, one of the largest uncertainties in restoring and quantifying deformation is how to differentiate between the effects of tilting and folding associated with motion on normal faults versus motion on thrust faults. In our cross sections, this equates to uncertainty in how to model the relationships of kink surfaces to the geometries of specific structures at depth, because folding can be attributed to changes in the geometry of nonplanar normal faults (e.g., Hamblin, 1965; Groshong, 1989; Dula, 1991; Xiao and Suppe, 1992) or thrust faults (e.g., Boyer and Elliott, 1982; Suppe, 1983; Mitra, 1990). Therefore, determining how to fill the space in the cross sections beneath the Moritz-Nager thrust and, more important, beneath the structurally lower, larger offset Ratto Canyon thrust, is the largest uncertainty in the cross sections. This space must be filled with structurally repeated Paleozoic rocks (Plate 3), and many geometric solutions are possible. Because the uncertainty in projection of kink surfaces and corresponding dip domains propagates with depth, as more structures are crossed, a simplified structural model for contractional deformation in the study area is presented here (Fig. 12), rather than attempting to balance fault offsets and cutoff angles for contractional structures on individual cross sections.
First-order observations that must be satisfied in the structural model include the following: (1) the Eureka culmination has a wavelength of ∼20 km, an amplitude of ∼4.5 km, and preextensional average limb dips between 25° and 35°; (2) rocks as deep as the Cambrian Prospect Mountain Quartzite are exposed in the hanging wall of the Ratto Canyon thrust, indicating the highest permissible stratigraphic position for the regional basal dècollemont; (3) the Silurian Lone Mountain Dolomite is in the footwall of the Ratto Canyon thrust under the culmination crest; and (4) older-over-younger relationships are not observed in the footwall of the Moritz-Nager thrust, where strata as deep as the Devonian Oxyoke Canyon Sandstone are exposed (Plate 1); therefore, all rocks exposed in the Diamond Mountains are interpreted to be in the hanging wall of the Ratto Canyon thrust, which cannot have ramped upsection through more than 300 m of footwall stratigraphy between here and its interception recorded in drill holes ∼10 km to the west.
The simplest structural model that explains these observations is that the Eureka culmination represents a fault-bend fold (e.g., Suppe, 1983) constructed by eastward motion of the Ratto Canyon thrust sheet over a footwall ramp that cuts from the Cambrian Prospect Mountain Quartzite to the Silurian Lone Mountain Dolomite, and then forms a footwall flat within the Silurian section toward the east (Figs. 12A, 12B). The Prospect Mountain thrust is shown using the Ratto Canyon thrust surface along and west of its footwall ramp, and branching upward from the top of the footwall ramp at a ∼35° cutoff angle to strata (Figs. 12B–12C). Based on their similar offset sense and magnitude, the relative stratigraphic levels that they deform, and a lack of any surface-breaching thrust faults observed between their traces, the Prospect Mountain and Moritz-Nager thrusts are connected as the same structure at depth (Fig. 12C; Plate 3). The Sentinel Mountain syncline is interpreted as the trailing axis of a fault-bend fold that formed above a footwall ramp on the Moritz-Nager thrust (Fig. 12C).
The 20 km wavelength and 4.5 km amplitude of the Eureka culmination can be reproduced with a 27° footwall ramp angle, which is an approximate average of the dips of the western and eastern limbs, 9 km of displacement on the Ratto Canyon thrust, and an additional 1 km of displacement on the Prospect Mountain–Moritz-Nager thrust, for a total of 10 km of horizontal shortening (Fig. 12). Because this is a simplified geometric model, this shortening estimate should be considered approximate.
Relationship of Eureka Culmination to NCF Deposition
Fundamental questions remain regarding the timing and geometric relationship of NCF deposition to regional deformation, as demonstrated by the diverse range of scenarios proposed in previous studies (e.g., Nolan et al., 1974; Vandervoort and Schmitt, 1990; Carpenter et al., 1993; Ransom and Hansen, 1993; Taylor et al., 1993). Based on clast composition and detrital zircon provenance data indicating derivation from proximal late Paleozoic subcrop units, Vandervoort and Schmitt (1990) and Druschke et al. (2011) suggested that deposition in the type-NCF basin may have been contemporary with proximal CNTB deformation. However, these studies lack the necessary structural context to relate NCF deposition to motion on specific faults or growth of folds. The revised CNTB structural model presented here provides this context and facilitates interpretation of NCF deposition from the perspective of the regional preextensional deformation geometry. We propose that the NCF type exposure was deposited in a piggyback basin that developed on the eastern limb of the Eureka culmination as it grew (Figs. 12A, 12B). This implies that culmination growth, as well as related motion on the Ratto Canyon thrust at depth, can be indirectly dated as Aptian (ca. 116 Ma; Druschke et al., 2011).
The total preserved across-strike structural relief of the basal NCF unconformity is ∼1.5 km; structural relief of the eastern limb of the culmination approached 4.5 km by the end of its construction. This indicates that the NCF was deposited during the early stages of fold growth (Fig. 12B). The most likely sediment source was erosion of the culmination crest, which is compatible with provenance data suggesting derivation from proximal late Paleozoic subcrop units (Vandervoort and Schmitt, 1990; Druschke et al., 2011) and east-directed paleocurrent directions (Vandervoort, 1987). After deposition, the NCF continued to be folded during late-stage growth of the culmination, represented by the northward continuation of the Pinto Creek syncline into the NCF type exposure (Nolan et al., 1971) (Fig. 10), which in our structural model corresponds to the leading axis of the hanging-wall cutoff of the Ratto Canyon thrust sheet (Fig. 12C).
Lateral lithologic variability coupled with a lack of precise deposition age control make regional correlations between isolated NCF exposures tentative (e.g., Vandervoort, 1987; Carpenter et al., 1993). For example, on the basis of biostratigraphy, rocks mapped as NCF in the Fish Creek Range can only be narrowed between Barremian and Albian (Fouch et al., 1979; Hose, 1983), and rocks mapped as NCF in the Pinon Range and Cortez Mountains are broadly bracketed as Early to Late Cretaceous (Smith and Ketner, 1976; Fouch et al., 1979). In addition, in the Pancake Range and northern Diamond Mountains, rocks mapped as NCF (Stewart and Carlson, 1978; Kleinhampl and Ziony, 1985) have been alternatively interpreted as Permian (Larson and Riva, 1963; Perry and Dixon, 1993), with no published biostratigraphic age control. The Aptian deposition age defined in the type exposure (Druschke et al., 2011) provides the only precise age control of any NCF exposure to date. Therefore, although it is possible that other exposures mapped as NCF along strike were deposited in a regional belt of Early Cretaceous contractional deformation, the precise age control and detailed structural context necessary to convincingly demonstrate this are currently lacking, and indicate the need for additional mapping and geochronology.
Implications for Out-Of-Sequence Deformation in the Sevier Hinterland
The timing of initiation of deformation in the Sevier thrust belt in Utah is a subject of long-standing debate (e.g., Armstrong, 1968; Jordan, 1981; Wiltschko and Dorr, 1983; Heller et al., 1986; DeCelles, 2004). The onset of subsidence in the Sevier foreland basin, which is attributed to loading from crustal thickening in the Sevier thrust belt, occurred by at least ca. 125 Ma (Aptian-Barremian boundary) (Jordan, 1981), and has been argued to be as old as ca. 145 Ma (latest Jurassic) (DeCelles, 2004; DeCelles and Coogan, 2006). Therefore, Aptian (ca. 116 Ma) growth of the Eureka culmination postdated migration of the Sevier thrust front into Utah by at least ∼10 m.y., and possibly as much as ∼30 m.y., defining out-of-sequence hinterland deformation.
Using a regional balanced cross section, DeCelles and Coogan (2006) documented 220 km of total shortening in the type-Sevier thrust belt in western Utah, at the approximate latitude of Eureka (Fig. 1). More than half (117 km) of this total shortening was accommodated by emplacement of the Canyon Range thrust sheet, which may have initiated as early as the latest Jurassic (ca. 145 Ma), and lasted until ca. 110 Ma, when the deformation front migrated forward to the Pavant thrust sheet (Fig. 13) (DeCelles and Coogan, 2006). Therefore, construction of the Eureka culmination in the hinterland was coeval with late-stage emplacement of the Canyon Range thrust sheet. This thick, areally extensive thrust sheet is exposed across all of westernmost Utah and eastern Nevada at this latitude, and was translated a total of 140 km eastward during Sevier orogenesis; this is attributed to high rheological competence as a result of carrying a ∼7-km-thick section of Neoproterozoic–early Cambrian quartzite (DeCelles, 2004). The dimensions of the Canyon Range thrust sheet, which approach 250 km across strike and 700 km along strike (estimate includes the correlative Paris-Willard thrust to the north; e.g., Armstrong, 1968), rank among the largest recorded thrust sheets in foreland thrust belts (Hatcher and Hooper, 1992; Hatcher, 2004), and may approach a critical mechanical upper bound for this tectonic setting.
Numerical and analog models predict that the critical taper angle of an orogenic wedge that must be achieved to allow forward propagation is governed by the relative magnitudes of the frictional strength of the basal dècollemont and the compressive strength of the rocks in the orogenic wedge (e.g., Chapple, 1978; Davis et al., 1983; Dahlen et al., 1984; Dahlen, 1990). Assuming that the strength of rocks within the Canyon Range thrust sheet did not change appreciably after accretion into the orogenic wedge, a decrease in basal frictional strength would be necessary for continued eastward translation. This could be accommodated through several different mechanisms, including progressive strain weakening of fault zone rocks, and/or an increase in pore pressure. An additional mechanism would be to decrease frictional strength by shortening the length of the basal dècollemont. This was undoubtedly accomplished through erosional beveling at the thrust front during translation (e.g., DeCelles, 1994), but could also have been assisted through internal shortening of the thrust sheet. The ∼10 km of hinterland shortening at Eureka could represent internal shortening of the Canyon Range thrust sheet during emplacement to promote further eastward translation. Because it was located ∼180–200 km west of the Canyon Range thrust front (preextensional map distance estimated from McQuarrie and Wernicke, 2005), construction of the Eureka culmination may not have appreciably increased the overall surface slope angle of the orogenic wedge. However, instead of building taper, perhaps this hinterland deformation acted as a mechanism to decrease the overall frictional coupling of the basal dècollemont to promote forward propagation.
Studies focused on the kinematic development of the frontal Sevier thrust belt in Utah have revealed the importance of out-of-sequence growth of structural culminations in the internal parts of the thrust belt as a mechanism to drive propagation of frontal thrust sheets (DeCelles, 1994, 2004; DeCelles and Coogan, 2006). At the latitude of our study area, this included growth of the Sevier culmination, a duplex system in western Utah composed of basement thrust sheets that folds and structurally elevates the Canyon Range thrust sheet (Allmendinger et al., 1986; DeCelles et al., 1995), and growth of duplexes further to the foreland that structurally elevate the Canyon Range and Pavant thrust sheets (DeCelles and Coogan, 2006). Our study demonstrates that, although more local in scale, similar processes were occurring earlier in the Sevier shortening history in the distal portions of the hinterland.
New mapping and balanced cross sections redefine the first-order structures and deformation geometry of the CNTB at the latitude of Eureka. Retrodeformation of extension defines the Eureka culmination, a north-striking anticline with a 20 km wavelength, a 4.5 km amplitude, and limb dips of 25°–35°, which is corroborated by deep Paleogene erosion levels that can be traced for ∼100 km along strike. A Cambrian over Silurian relationship observed in drill holes under the culmination crest defines the blind Ratto Canyon thrust. The culmination is interpreted as a fault-bend fold that formed from translation of the Ratto Canyon thrust sheet ∼9 km eastward over a footwall ramp at depth.
The Early Cretaceous (Aptian) type-NCF section was deposited in a piggyback basin on the eastern limb of the Eureka culmination as it grew. Aptian hinterland deformation postdated migration of the Sevier thrust front into Utah by ∼10–30 m.y., and was coeval with late-stage emplacement of the orogen-scale Canyon Range thrust sheet. Growth of the Eureka culmination could represent internal shortening of the Canyon Range thrust sheet, which acted as a mechanism to promote further eastward translation.
This work was funded by grants from the U.S. Geological Survey STATEMAP program and from Timberline Resources Corporation (Paul Dircksen, CEO). The 40Ar/39Ar ages were determined at the New Mexico Geochronology Research Laboratory (New Mexico Institute of Mining and Technology) under the guidance of Bill McIntosh, Matt Heizler, and Lisa Peters. U-Pb zircon data were acquired at the (U-Th)/He Geo- and Thermochronometry Lab at the University of Texas at Austin with guidance from Daniel Stockli. We thank Jennifer Mauldin and Irene Seelye of the Nevada Bureau of Mines and Geology cartography team, and Mitch Casteel, Bob Thomas, Peter Rugowski, Russell DiFiori, and Lucia Patterson of the Timberline Resources Corporation Eureka exploration team. Comments from three anonymous reviewers and associate editor Terry Pavlis significantly improved this manuscript.