564

Early Cretaceous construction of a structural culmination, Eureka, Nevada, U.S.A.: Implications for out-of-

sequence deformation in the Sevier hinterland

Sean P. Long1,*, Christopher D. Henry1, John L. Muntean1, Gary P. Edmondo2, and Elizabeth J. Cassel3

1Nevada Bureau of Mines and Geology, Mail Stop 178, University of Nevada, Reno, Nevada 89557, USA2Timberline Resources Corporation, 101 East Lakeside, Coeur D’Alene, Idaho 83814, USA3Department of Geological Sciences, University of Idaho, P.O. Box 443022, Moscow, Idaho 83844, USA

ABSTRACT

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 evalu-ation of space-time patterns of strain par-titioning between these two thrust systems. To address this problem, new 1:24,000-scale geologic mapping and balanced cross sec-tions 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 redefi nes the fi rst-order structures and deformation geometry of the CNTB at the latitude of Eureka. Contrac-tional structures include two north-striking, east-vergent thrust faults, the Prospect Moun-tain thrust and Moritz-Nager thrust, which are connected as the same fault in cross sec-tion, several north-striking map-scale folds, and a Cambrian over Silurian relationship observed in multiple drill holes, correspond-ing to repetition of ~2–2.5 km of stratigraphy, that defi nes the blind Ratto Canyon thrust. Two distinct sets of normal faults cut the con-tractional 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 culmi-nation is interpreted as a fault-bend fold that formed from ~9 km of eastward displace-ment 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, Penn-sylvanian, and Permian rocks on the eastern limb of the Eureka culmination. We propose that the NCF was deposited in a piggy back basin on the eastern limb of the culmina-tion as it grew, which is consistent with published east-directed paleocurrents and provenance data suggesting derivation from proximal late Paleozoic subcrop units. Syn-contractional deposition of the NCF is used to defi ne 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 defor-mation. 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.

INTRODUCTION

The Jurassic–Paleogene North American Cordillera is the type example of an ancient orogen that formed between converging con-tinental and oceanic plates (e.g., Armstrong,

1968; Burchfi el and Davis, 1975; Oldow et al., 1989; Allmendinger, 1992; Burchfi el et al., 1992; DeCelles, 2004; Dickinson, 2004). In the western interior United States, major com-ponents 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 accom-modated (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; Burchfi el and Davis, 1975; Royse et al., 1975; Villien and Kligfi eld, 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 deforma-tion in the central Nevada thrust belt (CNTB), defi ned 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 deforma-

For permission to copy, contact editing@geosociety.org© 2014 Geological Society of America

*Corresponding author: splong@ unr .edu

Geosphere; June 2014; v. 10; no. 3; p. 564–584; doi:10.1130/GES00997.1; 13 fi gures; 1 table; 3 plates; 1 supplemental fi le.Received 24 October 2013 ♦ Revision received 25 February 2014 ♦ Accepted 8 April 2014 ♦ Published online 25 April 2014

Construction of a structural culmination, Eureka, Nevada

Geosphere, June 2014 565

tion timing between these two thrust systems. However, on the basis of crosscutting relation-ships, 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 north-ern 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 relation-ships 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; Vander-voort and Schmitt, 1990) (Fig. 2). Contractional deformation in the Eureka region has been pro-posed 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; Carpen-ter et al., 1993; Ransom and Hansen, 1993), and possibly to be as young as Late Creta-ceous (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 defi nition of the CNTB at the latitude of Eureka. To achieve this goal, a new 1:24,000-scale geologic map of the north-ern 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 sedi-mentary rocks. These data facilitate redefi nition of the fi rst-order structures that compose the CNTB, and reconstruction of the regional pre-extensional deformation geometry. In addition, a structural model is presented that relates deposi-tion in the type-NCF basin to CNTB deforma-tion, including growth of a regional-scale anti-cline 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 pre-served in a rock record that spans the Paleozoic, Mesozoic, and Tertiary (Plate 1; Fig. 3).

Paleozoic

From the late Neoproterozoic to the Devo-nian, the Cordilleran passive margin sequence, a westward-thickening section of clastic and car-bonate rocks, was deposited on the rifted North American continental shelf in what is now east-ern Nevada and western Utah (e.g., Stewart and Poole, 1974). Eureka was situated near the dis-tal, western margin of the shelf (Cook and Cor-boy, 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 domi-nated 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 Moun-tains allochthon, composed of distal slope and basinal sediments, was thrust to the east over the western edge of the continental shelf dur-ing the Antler orogeny (e.g., Burchfi el 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

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Long et al.

566 Geosphere, June 2014

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Construction of a structural culmination, Eureka, Nevada

Geosphere, June 2014 567

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Long et al.

568 Geosphere, June 2014

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 conglom-erate, including the Ely Limestone and Carbon Ridge Formation (Fig. 3).

Mesozoic

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 Mis-sissippian 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, mud-stone, and limestone (Vandervoort, 1987). Rocks in the type section exposure are interpreted to record fl uvial and lacustrine deposition (Vander-voort and Schmitt, 1990). The NCF type section has yielded Aptian–Albian fossil fi sh, ostra-cods, 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 sec-tion defi nes an Aptian deposition age, and a ca. 122 Ma youngest U-Pb detrital zircon age popu-lation obtained from sandstone in the type sec-tion defi nes 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 uncon-formably overlies Ordovician, Mississippian, and Permian rocks on Hoosac Mountain (Fig. 2;

0

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Secret Canyon Shale (305 m)

Hamburg dolomite (350 m) Dunderberg Shale (45 m)Catlin mbr. (55 m)Bullwhacker mbr. (60 m)

Goodwin Fm. (365 m)

Ninemile Fm. (120 m)

Antelope Valley Fm. (305 m)

Eureka Quartzite (107 m)Hanson Creek Fm. (45 m)

Silurian Lone Mountain Dolomite (395 m)

Beacon Peak Dolomite mbr. (190 m)Oxyoke Canyon Sandstone mbr. (130 m)Sentinel Mtn. Dolomite mbr. (170 m)Woodpecker Limestone mbr. (75 m)Bay State Dolomite mbr. (235 m)Meister mbr. (85 m)

Hayes Canyon mbr. (260 m)Pilot Shale (85 m)

Dale Canyon Fm. (305 m)

Chainman Shale (715 m)

Diamond Peak Fm. (365 m)

Ely Limestone (≤150 m)

Carbon Ridge Fm. (580 m)

Carbon Ridge Fm., upperconglomerate (>460 m)

Joana Limestone (40 m)

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Devonian section in Fish Creek/Mountain Boy ranges:

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Figure 3. (A) Stratigraphic col-umn of Paleozoic rock units in the study area. Column on right shows map unit divisions used in Plates 1 and 2, and column on left shows simplifi ed unit divisions used on balanced cross sections in Plate 3 (Cl—lower Cambrian, Cu—upper Cam-brian, O—Ordovician, S—Silu-rian, D—Devonian, M—Missis-sippian, IPP—Pennsylvanian and Permian). (B) Devonian stratigraphy of Fish Creek and Mountain Boy Ranges (note: stratigraphic column in A shows Devonian stratigraphy of Diamond Mountains).

Construction of a structural culmination, Eureka, Nevada

Geosphere, June 2014 569

Plate 1). Nolan et al. (1974) mapped this con-glomerate as the NCF. However, the conglom-erate contains detrital zircon that have a ca. 72 Ma youngest U-Pb age population, defi ned by 3 concordant grains that overlap within error (sample NV11DZ-017EU; Fig. 4; see Supple-mental File1 for methods, concordia plots, and data from individual analyses), that defi nes a Late Cretaceous maximum depositional age (e.g., Dickinson and Gehrels, 2009). The con-glomerate is overlain by late Eocene dacite, defi ning a minimum deposition age.

Tertiary

Nevada was the site of the Great Basin ignim-brite fl areup, 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 fl areup 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 north-east parts of the map area (Fig. 2; Plate 1), and include the Pinto Peak and Target Hill rhyolite dome systems, Ratto Springs dacite, and Rich-mond 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., Dickin-son, 2006), and by ca. 10 Ma either transitioned into, or was followed by a separate episode of, lower magnitude extension that formed the mod-ern Basin and Range topography (e.g., Dickin-son, 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 com-plexes (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 deforma-tion. 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; Ran-som and Hansen, 1993; Taylor et al., 1993; Lisenbee, 2001b), resulting in signifi cant dis-agreement over deformation geometry and tim-ing. In the following sections, map-scale faults and folds are described, along with observations that support interpretations of their style and geometry, and fi eld relationships that bracket their motion timing. These observations are used to support construction of three 1:24,000-scale deformed cross sections (Plates 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 simplifi ed 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 geom-etry, with motions on extensional faults retro-deformed. 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 topogra-phy 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 sup-porting data). It was assumed that the fault dip angle estimated at the modern erosion surface stays the same both above and below the mod-ern erosion surface. Apparent dips of supporting attitude measurements were projected onto the cross sections (Plates 1 and 2), and areas of simi-lar 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 inter-limb angle. Division of areas of the cross sec-tions into dip domains, combined with quanti-tative constraints on fault dip angles, allowed estimation of the cutoff angles that strata make with faults at the modern erosion surface. All off-set estimates listed in the following for specifi c faults were measured from the 1:24,000-scale cross sections in Plates 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 justifi ca-tions 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 Moun-tain (Fig. 2; Plate 1), drilling by Timberline Resources Corporation has revealed an older-

(Ma)

70–73 Ma (n=3)

100–115Ma (n=31)

NV11DZ-017EU (n=99)Late Cretaceous to late Eocene conglomerate

Figure 4. U-Pb detrital zircon age spectrum of Late Cretaceous to late Eocene conglomerate sample NV11DZ-017EU collected on Hoosac Mountain (location: 39.48535°N, 115.94587°W; Fig. 2; Plate 1). Graph is a relative probability plot, which represents the sum of probability distributions from ages and corresponding errors (input errors are 2σ) for all analyses. Cre-taceous age populations are labeled. Refer to Supplemental File (see footnote 1) for discus-sion of methods, concordia plots, and data from individual analyses.

1Supplemental File. Zipped fi le containing 6 sup-plemental fi gures, 4 supplemental tables, supplemen-tal ArcGIS fi les, and a supplemental text document. If you are viewing the PDF of this paper or reading it off-line, please visit http:// dx .doi .org /10 .1130 /GES00997 .S1 or the full-text article on www .gsapubs .org to view the Supplemental File.

Long et al.

570 Geosphere, June 2014

over-younger relationship at depth, defi ned by Cambrian Geddes Limestone, and locally Cam-brian Secret Canyon Shale, over Silurian Lone Mountain Dolomite (Fig. 5), corresponding to structural repetition of ~1800–2300 m of stratig-raphy. 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 Can-yon thrust is corroborated by Silurian and Ordo-vician conodont age determinations from sam-pled 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 Pros-pect 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 Mississip-pian rocks (Plate 1; Fig. 2), and has an estimated top-to-the-east displacement between 1.0 and 1.8 km (Plates 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 Quater-nary 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 defi ned by outcrops of Devo-nian, Mississippian, and Pennsylvanian strata on the eastern fl ank of the Diamond Moun-tains, 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 defi ned 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 Moun-tain 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 vol canism (see following discussion), and the Moritz-Nager thrust and Sentinel Mountain syncline are overlapped by late Eocene volcanic rocks.

Normal Faults

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; Plates 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 sec-tions as a series of subvertical structures (Plates 1–3). The easternmost, master fault of the sys-tem places Mississippian and Permian rocks over Ordovician rocks (Fig. 7A; Plates 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 dip-ping 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

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Long et al.

572 Geosphere, June 2014

(Table SM4 in the Supplemental File [see foot-note 1]), demonstrate their steep eastward dip, which refutes the thrust fault interpretation. The easternmost two faults of the system are over-lapped 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 foot-note 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 fl at that can be traced for ~5 km across strike. Hanging-wall stratigraphic levels are typi cally in Silurian dolomite, indicating a maxi mum 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 detach-ment 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 strati-graphic omission of 1200–1500 m. Cumulative offset estimates for the Reese and Berry detach-ment system are 1.5–2.6 km in the Fish Creek Range and 4.7 km in the Mountain Boy Range (Plates 1 and 2). Similar low-cutoff-angle nor-mal 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, includ-ing the Dugout Tunnel, Lookout Mountain, and Pinto Summit faults (Fig. 2; Plates 1–3). The Dugout Tunnel fault places shallowly east dip-ping Ordovician rocks over steeply east dip-ping Cambrian rocks, and has between 2500 and 2900 m of offset (Plates 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 Moun-

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IPP D

S

M

IPP

DMIP

P

D

S

M D

M

D

ClCu

O

O

O

OD

IPP

O

Cu Cu

Cu

S

S O

MIP

PCl

CuO

DS

S

Cl

IPP

IPPIP

P

S OS O

Cu

SO

MM

Cl

Cu

O

IPP M

S

D

ClCu

O

IPP

M

SD

S O

ClS O

Cu

Cu

Cu

Cu

CuS

ClCuS O

Cu

Wes

t

U.S

.H

wy

50

East

A. 3

9°27

’30”

N

Rees

e an

d Be

rry

deta

chm

ent s

yste

m

Hoo

sac

faul

tsy

stem

Dug

out

Tunn

elfa

ult

Rees

e an

d Be

rry

deta

chm

ent s

yste

m

Pint

oSu

mm

itfa

ult

Hoo

sac

faul

tsy

stem

Mor

itz-

Nag

erth

rust

Pros

pect

Mou

ntai

nth

rust

end of map

end of map

Hoo

sac

faul

tsy

stem

Dug

out

Tunn

elfa

ult

A

A’

A

A’

Rees

e an

d Be

rry

deta

chm

ent s

yste

m

Hoo

sac

faul

t sys

tem D

ugou

tTu

nnel

faul

t

Pint

oSu

mm

itfa

ult

Mor

itz-

Nag

erth

rust

Pros

pect

Mou

ntai

nth

rust

AA’

AA’

Wes

t

U.S

.H

wy

50

East

B. 3

9°26

’15”

N

Upp

er R

eese

and

Berr

y de

tach

men

t

Hoo

sac

faul

tsy

stem

Dug

out

Tunn

elfa

ult

Low

er R

eese

and

Berr

y de

tach

men

t

Pint

oSu

mm

itfa

ult

Hoo

sac

faul

tsy

stem

Mor

itz-

Nag

erth

rust

Pros

pect

Mou

ntai

nth

rust

end of map

Hoo

sac

faul

tsy

stem

Low

er R

eese

and

Berr

y de

tach

men

t

Dug

out

Tunn

elfa

ult

end of map

Upp

er R

eese

and

Berr

y de

tach

men

t

Hoo

sac

faul

tsy

stem

Dug

out

Tunn

elfa

ult

Pint

oSu

mm

itfa

ult

Mor

itz-

Nag

erth

rust

Pros

pect

Mtn

. thr

ust

Low

er R

eese

and

Berr

y de

tach

men

t

B

B’

BB’

BB’

BB’

Wes

tEa

st

Wes

t

U.S

.H

wy

50

East

C. 3

9°24

’20”

N

Upp

er R

eese

and

Berr

y de

tach

men

t

Hoo

sac

faul

tsy

stem

Dug

out

Tunn

elfa

ult

Look

out

Mou

ntai

nfa

ult

Pint

oSu

mm

itfa

ult

Hoo

sac

faul

tsy

stem

Mor

itz-

Nag

erth

rust

end of map

Hoo

sac

faul

tsy

stem

Low

er R

eese

and

Berr

y de

tach

men

t

end of map

Hoo

sac

faul

tsy

stem

Wes

tEa

st

Upp

er R

eese

and

Berr

y de

tach

men

tD

ugou

tTu

nnel

faul

t

Look

out

Mou

ntai

nfa

ult

Pint

oSu

mm

itfa

ult

Mor

itz-

Nag

erth

rust

Pros

pect

Mou

ntai

nth

rust

Low

er R

eese

and

Berr

y de

tach

men

tH

oosa

c fa

ult

syst

em

CC’

C

C’

CC’

CC’

Sent

inel

Mou

ntai

nsy

nclin

eSe

ntin

elM

ount

ain

sync

line

Sent

inel

Mou

ntai

nsy

nclin

e

Sent

inel

Mou

ntai

nsy

nclin

e

Plat

e 3

Ratt

oCa

nyon

thru

st

Not

es c

omm

on to

all

cros

s-se

ctio

ns:

I. H

oosa

c fa

ult s

yste

m s

impl

ified

as

one

mas

ter,

east

ern

norm

al fa

ult,

with

a s

mal

ler-

offse

t wes

tern

nor

mal

faul

t tha

t mer

ges

into

mas

ter f

ault

abov

e an

d be

low

mod

ern

eros

ion

surf

ace.

II. H

oosa

c fa

ult s

yste

m s

how

n re

tain

ing

a st

eep

dip

at d

epth

bec

ause

it d

oes

not b

reac

h m

oder

n er

osio

n su

rfac

e in

foot

wal

l of P

into

Sum

mit

faul

t.III

. Hoo

sac

faul

t sys

tem

abo

ve m

oder

n er

osio

n su

rfac

e is

dra

fted

with

a fl

atte

r dip

as

it cr

osse

s fo

ld a

xes,

in o

rder

to re

tain

~20

-30°

han

ging

wal

l and

foot

wal

l cut

off-a

ngle

rela

tive

to b

eddi

ng. T

his

is b

ased

on

low

ha

ngin

g w

all c

utoff

-ang

le o

f Per

mia

n ro

cks

obse

rved

on

cros

s-se

ctio

n A

. A

lso,

Hoo

sac

faul

t sys

tem

is s

how

n cu

ttin

g Re

ese

and

Berr

y de

tach

men

t sys

tem

abo

ve m

oder

n er

osio

n su

rfac

e. T

his

rela

tions

hip

is n

ot

obse

rved

, and

was

dra

fted

this

way

sol

ely

base

d on

Hoo

sac

faul

t sys

tem

hav

ing

a hi

gher

cum

ulat

ive

offse

t.IV

. Lo

wer

and

upp

er R

eese

and

Ber

ry d

etac

hmen

ts a

re s

how

n w

ith ~

20-3

0° c

utoff

-ang

le a

bove

mod

ern

eros

ion

surf

ace,

and

cut

dow

n-se

ctio

n w

estw

ard

to d

ecol

lem

ont a

t top

of E

urek

a qu

artz

ite, a

fter

obs

erva

tions

in

Mou

ntai

n Bo

y Ra

nge

and

Rees

e an

d Be

rry

Cany

on.

V. ~

40° c

hang

e in

dip

acr

oss

Dug

out T

unne

l fau

lt m

odel

ed a

s ju

xtap

ositi

on o

f opp

osite

sid

es o

f a k

ink

surf

ace

with

ste

eper

dip

dom

ain

on e

ast.

VI. ~

40° d

ip c

hang

e ac

ross

Pin

to S

umm

it fa

ult m

odel

ed a

s co

mbi

natio

n of

~20

° of l

istr

icity

and

juxt

apos

ition

of o

ppos

ite s

ides

of k

ink

surf

ace

with

~20

° hig

her d

ips

on w

est.

VII.

Pros

pect

Mou

ntai

n an

d M

oritz

-Nag

er th

rust

s ar

e co

nnec

ted

at d

epth

, bas

ed o

n si

mila

r ver

genc

e an

d off

set m

agni

tude

, rel

ativ

e st

ratig

raph

ic le

vels

they

def

orm

, and

lack

of s

urfa

ce-b

reac

hing

thru

st fa

ults

ob

serv

ed b

etw

een

thei

r tra

ces.

Mod

eled

with

~30

° han

ging

wal

l and

foot

wal

l cut

off-a

ngle

s ea

st o

f Pro

spec

t Mou

ntai

n.VI

II. S

entin

el M

ount

ain

sync

line

mod

eled

as

faul

t-be

nd fo

rmed

fold

from

mot

ion

of M

oritz

-Nag

er th

rust

she

et o

ver f

ootw

all r

amp.

Ram

p m

odel

ed a

s in

crea

se in

foot

wal

l cut

off a

ngle

from

30°

to ~

70-8

0°.

Not

es fo

r C-C

’ cro

ss-s

ecti

on:

4. L

ower

Ree

se a

nd B

erry

det

achm

ent p

roje

cts

abov

e m

oder

n er

osio

n su

rfac

e, a

nd b

ased

on

strik

e lik

ely

has

a co

mpo

nent

of m

otio

n in

and

out

of p

age;

Silu

rian

sect

ion

is s

chem

atic

ally

thin

ned

on d

efor

med

cr

oss-

sect

ion

to s

how

this

. Si

luria

n se

ctio

n is

rest

ored

to fu

ll th

ickn

ess

on re

stor

ed c

ross

-sec

tion.

5. U

pper

Cam

bria

n se

ctio

n is

thin

ned

~300

m h

ere

base

d on

col

laps

e of

Ham

burg

dol

omite

to ~

50-1

00 m

of b

recc

ia in

alte

ratio

n zo

ne in

foot

wal

l of L

ooko

ut M

ount

ain

faul

t. T

his

is in

par

t res

pons

ible

for s

teep

eas

twar

d di

p of

Rat

to C

anyo

n th

rust

sho

wn

here

. U

pper

Cam

bria

n se

ctio

n is

rest

ored

to fu

ll th

ickn

ess

on lo

wer

, par

tially

-res

tore

d cr

oss-

sect

ion.

6. S

truc

tura

l ele

vatio

n an

d ea

stw

ard

dip

of R

atto

Can

yon

thru

st c

ould

indi

cate

em

plac

emen

t of a

sm

all,

youn

ger t

hrus

t she

et c

ompr

ised

of u

nits

O a

nd S

car

ved

off o

f top

of f

ootw

all r

amp

that

cut

s th

e Ra

tto

Cany

on

thru

st a

nd fe

eds

slip

into

Mor

itz-N

ager

thru

st.

Not

es fo

r B-B

’ cro

ss-s

ecti

on:

2. S

ectio

n of

Dug

out T

unne

l fau

lt is

mod

eled

as

flat b

ecau

se c

ross

-sec

tion

line

cros

ses

an e

ast-

strik

ing

port

ion

of fa

ult,

whi

ch li

kely

has

dom

inan

t obl

ique

or s

trik

e-sl

ip c

ompo

nent

of m

otio

n. S

pace

in h

angi

ng w

all o

f thi

s fla

t por

tion

is le

ft e

mpt

y fo

r pur

pose

of r

esto

ratio

n, b

ecau

se d

omin

ant c

ompo

nent

of m

otio

n is

like

ly in

and

out

of p

age.

3. ~

20-3

0° e

astw

ard

tiltin

g of

Lat

e Cr

etac

eous

to la

te E

ocen

e co

nglo

mer

ate

on H

oosa

c M

ount

ain

is a

ttrib

uted

to d

own-

to-w

est m

otio

n on

list

ric P

into

Sum

mit

faul

t.

Not

es fo

r A-A

’ cro

ss-s

ecti

on:

1. R

eese

and

Ber

ry d

etac

hmen

t sys

tem

is s

impl

ified

as

one

deta

chm

ent f

ault,

bec

ause

upp

er a

nd lo

wer

det

achm

ents

mer

ge im

med

iate

ly s

outh

of c

ross

-sec

tion

line.

II

I

II

IIII

III

IIIIII

III

IIIIII

IV1

IVIV

IV

IV

IV

4

4

2

V

V

V

V

VV

VIVI

VI

VIVI

VI

Pint

o Cr

eek

sync

line

Pint

o Cr

eek

sync

line

VII

VII

VII

VII

VII

VII

VIII

VIII

VIII

VIII

VIII

VIII

5

5

?

25.1

km

17.7

km

25.1

km

17.5

km

24.7

km

16.6

km

3, IX

Not

es c

omm

on to

all

cros

s-se

ctio

ns, c

onti

nued

:IX

. ~20

-30°

eas

twar

d til

ting

of L

ate

Cret

aceo

us to

late

Eoc

ene

cong

lom

erat

e on

Hoo

sac

Mou

ntai

n is

att

ribut

ed to

dow

n-to

-wes

t mot

ion

on li

stric

Pin

to S

umm

it fa

ult (

cros

s-se

ctio

n B-

B’).

Ret

ro-d

efor

mat

ion

of a

sim

ilar

amou

nt o

f tilt

ing

is a

pplie

d to

cro

ss-s

ectio

ns A

-A’ a

nd C

-C’.

X. ~

10-2

0°E

dips

in D

evon

ian

rock

s to

wes

t of m

ap a

rea

in M

ahog

any

Hill

s ar

e re

port

ed o

n m

ap o

f Sch

alla

(197

8); t

his

supp

orts

loca

tion

of w

este

rnm

ost k

ink

surf

ace.

XI. L

ine

conn

ectin

g ba

sal c

onta

ct o

f low

er C

ambr

ian

sect

ion

at it

s in

ters

ectio

n w

ith w

este

rnm

ost k

ink

surf

ace

(com

men

t X) a

nd P

into

Cre

ek s

yncl

ine

axis

is in

terp

rete

d as

pal

eo-h

oriz

onta

l. T

hese

fold

axe

s de

fine

wes

tern

and

eas

tern

lim

its o

f Eur

eka

culm

inat

ion.

Und

er th

is in

terp

reta

tion,

~20

° of e

astw

ard

tilt w

as re

tro-

defo

rmed

in h

angi

ng w

all o

f Pin

to S

umm

it fa

ult (

with

loca

lly h

ighe

r tilt

s [~

30°]

in it

s im

med

iate

han

ging

w

all;

this

is c

orro

bota

ted

by e

astw

ard

tilt o

f the

Lat

e Cr

etac

eous

-Ear

ly Te

rtia

ry c

ongl

omer

ate

[com

men

t IX]

), an

d ~1

0° o

f eas

twar

d til

t was

retr

o-de

form

ed in

foot

wal

l of P

into

Sum

mit

faul

t.

IXIX

IXIX

IX

X

X

X

XX

X

XIXI

XI

Ratt

oCa

nyon

thru

st

undi

ffere

ntia

ted

Cam

bria

n to

Silu

rian

rock

s

Ratt

oCa

nyon

thru

stun

diffe

rent

iate

dCa

mbr

ian

to S

iluria

n ro

cksRa

tto

Cany

onth

rust

?

? undi

ffere

ntia

ted

Cam

bria

n to

Silu

rian

rock

s

?

No

vert

ical

exa

gger

atio

n

No

vert

ical

exa

gger

atio

n

No

vert

ical

exa

gger

atio

n

No

vert

ical

exa

gger

atio

nN

o ve

rtic

al e

xagg

erat

ion

No

vert

ical

exa

gger

atio

n

Mod

ern

eros

ion

surf

ace

(res

tore

d cr

oss-

sect

ions

onl

y)La

te E

ocen

e, s

ub-v

olca

nic

eros

ion

surf

ace

(def

orm

ed c

ross

-sec

tions

onl

y)Lo

wes

t pos

sibl

e er

osio

n le

vel p

rior t

o m

otio

n on

Ree

se a

nd B

erry

det

achm

ent s

yste

m (d

efor

med

cro

ss-s

ectio

ns o

nly)

Eros

ion

surf

ace

belo

w L

ate

Cret

aceo

us to

late

Eoc

ene

cong

lom

erat

e on

Hoo

sac

Mou

ntai

n (d

efor

med

cro

ss-s

ectio

n B-

B’ o

nly)

App

roxi

mat

e st

ratig

raph

ic le

vel o

f Ear

ly C

reta

ceou

s er

osio

n su

rfac

e at

bas

e of

New

ark

Cany

on F

orm

atio

n (p

roje

cted

from

~1-

5 km

N o

f map

are

a, fr

om m

appi

ng o

f Nol

an e

t al.,

197

1; 1

974)

Eros

ion

leve

ls:

6Pros

pect

Mou

ntai

nth

rust

-3,000

8,000 4,000Thickness (feet)

020,000 16,000 12,00024,000

Thickness (meters)6,000 4,000 2,000 09,000 1,00010,000 8,000 3,0007,000 5,000

28,00032,000

12,00013,000 11,000

36,00040,000

Elevation (meters)6,000 4,000 2,000 0 -2,000 -4,0001,000 -1,0003,0007,000 5,000

8,000 4,000 0Elevation (feet)

-4,000 -8,000 -12,00020,000 16,000 12,000

Elevation (meters)6,000 4,000 2,000 0 -2,0001,000 -1,000 -3,0003,0007,000 5,000

8,000 4,000 0Elevation (feet)

-4,000 -8,00020,000 16,000 12,000

8,000

24,000

8,000 4,000Thickness (feet)

020,000 16,000 12,00024,000

Thickness (meters)6,000 4,000 2,000 09,000 1,00010,000 8,000 3,0007,000 5,000

28,00032,000

12,00013,000 11,000

36,00040,000

Elevation (meters)6,000 4,000 2,000 0 -2,0001,000 -1,000 -3,0003,0007,000 5,000

8,000 4,000 0Elevation (feet)

-4,000 -8,00020,000 16,000 12,00024,000

8,000 4,000Thickness (feet)

020,000 16,000 12,00024,000

Thickness (meters)6,000 4,000 2,000 09,000 1,00010,000 8,000 3,0007,000 5,000

28,00032,000

12,00013,000 11,000

36,00040,000

Pla

te 3

. 1:1

00,0

00-s

cale

bal

ance

d cr

oss

sect

ions

, wit

h si

mpl

ifi ed

Pal

eozo

ic s

trat

igra

phic

div

isio

ns (

Cl—

low

er C

ambr

ian,

Cu—

uppe

r C

am-

bria

n, O

—O

rdov

icia

n, S

—Si

luri

an,

D—

Dev

onia

n, M

—M

issi

ssip

pian

, IP

P—

Pen

nsyl

vani

an a

nd P

erm

ian;

see

Fig

. 3)

, an

d al

l fa

ults

wit

h <3

00 m

of o

ffse

t om

itte

d. Q

uate

rnar

y an

d Te

rtia

ry r

ocks

om

itte

d fo

r si

mpl

icit

y. T

op r

ow s

how

s pr

esen

t-da

y de

form

ed g

eom

etry

, and

bot

tom

ro

w s

how

s pa

rtia

lly r

esto

red

geom

etry

, wit

h m

otio

n on

ext

ensi

onal

fau

lts

retr

odef

orm

ed. T

rans

luce

nt a

reas

abo

ve m

oder

n er

osio

n su

rfac

e re

pres

ent e

rode

d ro

ck. J

usti

fi cat

ions

for

indi

vidu

al d

ecis

ions

mad

e in

dra

ftin

g an

d re

trod

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Construction of a structural culmination, Eureka, Nevada

Geosphere, June 2014 573

tain fault places Devonian rocks over Cambrian rocks, and has ~2000 m of down-to-the-west offset at Lookout Mountain (Plates 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 pre-serve Devonian rocks. In addition, 3–6 km south of the map area, late Eocene volcanic rocks are mapped on both sides of the Look-out Mountain fault (Nolan et al., 1974; Cowell, 1986), with Cambrian erosion levels in the foot-wall and Devonian erosion levels in the hang-ing 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 Dug-out Tunnel fault places unmetamorphosed Ordovician limestone over Cambrian Secret Canyon Shale that is metamorphosed to horn-fels, which is interpreted as contact metamor-phism associated with intrusion of Late Creta-ceous 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 associ-ated 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

Map units:Qc - Quaternary colluviumTcbr - Tertiary collapse brecciaTj - Tertiary jasperoidTi - Late Eocene(?) intrusiveTrsv - Late Eocene Ratto Springs volcanicsDevonian Nevada Formation: Ds - Simonson dolomite member Doc - Oxyoke Canyon sandstone mbr. Dsr - Sadler Ranch member Db - Bartine member Dbp - Beacon Peak memberSlm - Silurian Lone Mtn. dolomite

DugoutTunnel fault Hoosac

faultsystem

PintoSummit

fault

Reese and Berrydetachment system

LookoutMtn. fault

Moritz-Nager thrust

ProspectMtn.

thrust

Locations:

Tcbr and Tj(Ds protolith)

Tcbr(Ch protolith)

CsCd

Ch

Cw

Slm

Qc

Ch

Trsv

Cg

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Ds

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Cambrian: Cw - Windfall Formation Cd - Dunderberg shale Ch - Hamburg dolomite Cs - Secret Canyon shale

BHSE

-007

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605

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Figure 5. Cross sections through Lookout Mountain and Ratto Canyon, showing sub-surface relationships revealed by industry drill holes (green lines). Locations of cross-section lines are shown in lower right inset (D–D′ is also shown in Plate 1 and Fig. 2). The Ratto Canyon thrust places Cambrian Geddes Limestone over Silurian Lone Mountain Dolomite. Silurian and Ordovi-cian conodonts obtained from footwall rocks corroborate drill-hole lithologic interpreta-tions (see Supplemental File [see footnote 1] for supporting lithologic logs and conodont age determinations for drill holes BH-0605 and BHSE-007C).

Long et al.

574 Geosphere, June 2014

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 asso-ciated 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; Plates 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]), defi ne a steep (~65°–75°) westward dip, which refutes the thrust fault interpretation. Offset esti-mates for the Pinto Summit fault range from 3.7 to 4.5 km (Plates 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

ProspectMountain

thrust

lowerthrustfault

Prospect Mountain thrust:

footwallhanging wallN

500 m

N500 m

A B

Map units:Qc - Quaternary colluviumTtrd - Tertiary rhyolite dikeCh - Cambrian Hamburg dolomiteCs - Cambrian Secret Canyon shaleCe - Cambrian Eldorado dolomiteCp - Cambrian Pioche shaleCpm - Cambrian Prospect Mountain quartzite

Ce

DugoutTunnel fault Hoosac

faultsystem

PintoSummit

fault

Reese and Berrydetachment system

LookoutMtn. fault

Moritz-Nager thrust

ProspectMtn.

thrust

Location:

Figure 6. (A) Geologic map of the Prospect Mountain thrust (location shown in Fig. 2 and bottom right inset). Crosscutting normal faults drop trace of thrust below modern erosion surface on east and west. (B) Map of the same area as A, with hanging wall and footwall of Prospect Mountain thrust labeled.

Figure 7 (on following page). Annotated photographs (locations and view directions shown in Fig. 2, unit abbreviations shown in Plate 1). Dashed lines with dip numbers are apparent dip symbols. Red lines represent late Eocene unconformity. (A) Hoosac fault system viewed from Hoosac Mountain. Eastern, master fault places Permian rocks over Ordovician rocks, and westernmost fault omits units On and Oav. Late Eocene unconformity at base of unit Trsv overlaps master fault. Qu—undifferentiated Quaternary deposits. (B) Reese and Berry detachment system in Reese and Berry Canyon. Lower detachment occupies upper contact of Eureka Quartzite (Oe) for several kilometers across strike. Upper detachment places top of Devonian section (units Ds and Ddg) over lower Devonian rocks and Silurian rocks. Unit Du represents map units Dbp, Db, Dsr, and Doc undifferentiated. (C) Pinto Summit fault placing Permian rocks over Silurian rocks; note steeper dips in hanging wall. Late Eocene unconformity at base of unit Ttt overlaps the fault. (D) Late Eocene unconformity at base of Pinto Peak rhyolite (Tpr) and associated breccia (Tprx) and sedimentary rocks (Ts) is negligibly tilted, and overlies 60°–70°E dipping Mississippian and Permian rocks.

Construction of a structural culmination, Eureka, Nevada

Geosphere, June 2014 575

Oe

Oe

Oe

PcrcPcrQu

Oav

Trsv

Mc Mdp Pcr

Qu

Qu

Qu

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Cd

ChCs

Ch

CdQu

Hoosacfault

system

Windfall Canyon

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Pcrc

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81E65E

68E72E

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Reese and Berry Canyon

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Upper Reese andBerry detachment

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masterfault

approximate scaleon skyline (m)

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0 100 300200

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U.S. Highway 50

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Pcrc

Pcr

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Pcr

Pinto Summitfault

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64E74E

64E61E

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Mdp

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Tprx

TprPintoPeak

S ~200mD

78EQc

Qc

Mc

Mc

SE

52E

Figure 7.

Long et al.

576 Geosphere, June 2014

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 pre-dates 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, includ-ing 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 vol-canic 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 refl ect deposition on paleo-topography, but are capped by conglomerate and gravel that exhibit subhorizontal basal con-tacts 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 expo-

sures 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 expo-sure 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 unconform-ably 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 Missis-sippian 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 conglom-erate 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 vari-ety of different rock units, including Permian conglom erate, Late Cretaceous to late Eocene conglomerate, sedimentary rocks associated with late Eocene volcanism, and Quaternary col-luvium. 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 fi eld photographs of outcrops, for areas of key importance where we have dis-agreed with their mapping of the NCF. These areas include: (1) the axis of the Sentinel Moun-tain syncline (Fig. SM6A in the Supplemental File [see footnote 1]), where we map unconsoli-dated 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, tuffa-ceous matrix that overlies Silurian rocks as con-glomerate associated spatially and temporally with late Eocene volcanism, similar to other syn-volcanic conglomerate mapped near Pinto Sum-mit; 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

N N

Late Cretaceous to late Eocene conglomerate (n=29)Ordovician rocks in footwall of master Hoosac fault (n=21)Mississippian-Permian rocks in hanging wall of master Hoosac fault (n=24)

A B

Late K-late Eo cgl:020, 32SE

Hoosac FW:345, 72E

Hoosac HW:355, 67E

Hoosac FW:332, 48NE

Hoosac HW:342, 39NE

Figure 8. Equal-area stereonet plots (generated using Stereonet 8; Allmendinger et al., 2011). (A) Poles to planes of bedding of Ordovician rocks in footwall (FW) of master Hoosac fault (purple), Mississippian–Permian rocks in its hanging wall (HW; red), and overlapping Late Cretaceous to late Eocene conglomerate (Late K–late Eo cgl; green). Great circles show median bedding orientation for all measurements. Plot defi nes angular unconformity with ~40° of differential tilt between Paleozoic rocks and conglomerate. (B) Same data as in A, but rotated so average bedding of Late Cretaceous to late Eocene conglomerate (020°, 32°SE) is restored to horizontal. Great circles for median bedding of rocks in footwall and hanging wall of Hoosac fault are an approximation for original eastern limb dip of Eureka culmination at this locality.

Construction of a structural culmination, Eureka, Nevada

Geosphere, June 2014 577

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 Mis-sissippian Diamond Peak Formation were not breached by the Early Cretaceous. This indi-cates 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 Creta-ceous unconformity also has important implica-tions 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 uncon-formity. Subcrop relationships defi ned by our new mapping show that the NCF overlies simi-lar stratigraphic levels on either side of the Pinto Summit fault, indicating that it cuts the Early Cretaceous unconformity (Plate 3). Our map-ping and geochronology data show that the NCF is not preserved in the footwall of the Hoosac fault system, which suggests that this fault sys-tem also cuts the Early Cretaceous unconformity.

EUREKA CULMINATION

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 displace-ments of matching hanging-wall and footwall cutoffs across all normal faults; and (3) retro-deformation 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 simi-lar preextensional deformation geometry on all three cross sections, defi ned by an upright anti-cline 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 exis-tence of this regional structural high is indepen-dently corroborated by Paleogene subcrop and erosion patterns. In Long (2012), a paleogeo-logic map of the Sevier hinterland was presented that showed the distribution of Neoproterozoic to Triassic rocks exposed beneath the regional Paleogene, subvolcanic unconformity, and illus-trated the approximate map patterns of regional-scale, pre-unconformity structures. Figure 11 shows a Paleogene subcrop map of the Eureka region (modifi ed from Long, 2012) that reveals the approximate map patterns of several north-trending, erosionally beveled, pre-Paleogene folds, with erosion levels typically in Mississip-pian to Triassic rocks. However, the Eureka cul-mination 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 Cam-brian, 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 ero-sional exhumation of the Eureka culmination during and after its construction, and tectonic exhumation by pre–late Eocene normal faulting.

Structural Model

In regions that have undergone superposed contractional and extensional tectonism, one of the largest uncertainties in restoring and quantifying deformation is how to differenti-ate 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 specifi c structures at depth, because folding

OgCwQu

Cs

Cs

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faultour mappingNolan et al. (1974)

western-mostfault ofHoosacsystem

500 m

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Map units:Qu - Quaternary deposits, undiff.Tu - late Eocene volcanics, undiff.Oe - Ordovician Eureka quartziteOg - Ordovician Goodwin Fm.

Cw - Cambrian Windfall Fm.Cs - Camrian Secret Canyon shaleCg - Cambrian Geddes limestoneCe - Cambrian Eldorado dolomite

DugoutTunnel fault Hoosac

faultsystem

PintoSummit

fault

Reese and Berrydetachment system

LookoutMtn. fault

Moritz-Nager thrust

ProspectMtn.

thrust

Location:

Figure 9. Geologic map of southern part of Secret Canyon (loca-tion shown in lower inset and in Fig. 2), showing fi eld relationships of late Eocene unconformity on either side of Dugout Tunnel fault. Unconformity is on Cambrian Secret Canyon Shale in footwall (green arrow) and on Ordovician Goodwin Formation in hanging wall (blue arrow). Units as high in the section as Eureka Quartzite are locally preserved in hanging wall (red arrow).

Long et al.

578 Geosphere, June 2014

Newark Canyon road

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approximate trace ofPinto Creek syncline

northerncontinuation

of PintoCreek syncline

mbr

Pcrc

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IPMe

Mdp

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QTu undifferentiated Tertiary volcanics and Quaternary sedimentlandslide masses (megabreccias) composed of map unit IPMe

Newark Canyon Formation

Carbon Ridge Formation upper conglomerate

Carbon Ridge Formation

Ely limestone

Diamond Peak Formation

Chainman shale

N

Knc attitude(n=31)

Paleozoicattitude (n=41)

W limb, medianorientation:

331, 37NE(n=54)

E limb, medianorientation356, 20W(n=18)

Figure 10. Geologic map of part of southern Diamond Mountains, adjoining north edge of Plate 1 (location shown in Fig. 2), simpli-fi ed from Nolan et al. (1971, 1974), illustrating map pattern of exposure that contains type section of Newark Canyon Formation (type section is along Newark Canyon road). Early Cretaceous unconformity is on map units Mdp, IPMe, Pcr, and Pcrc. Stereo-net plot (equal-area, generated using Stereonet 8; Allmendinger et al., 2011) shows poles to planes of bedding for Paleozoic and Cretaceous rocks, and median limb orientations of Pinto Creek syncline, based on all Paleozoic–Cretaceous attitudes in each limb.

Construction of a structural culmination, Eureka, Nevada

Geosphere, June 2014 579

Y

Y

Y

Y

Y

Y

Y

Y

Y

Y

Y

Y Y

Y

Y

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Duckwaterthrust

MoodyPeak

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Pancakethrust

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fault

Buttesynclin-orium

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fault

Eurekaculmination

RobertsMts.

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RobertsMts. thrust

116°

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NScale (km)

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Subcrop map explanation:

Mississippian

Devonian

Silurian

Ordovician

Cambrian

Pennsylvanian

Lower Permian

Upper Permian

Lower Triassic

Early Cretaceous

Y

Subcrop data explanation:Subcrop beneath exposure of Paleogene unconformity

Youngest possible subcrop beneath Paleogene unconformity

Youngest possible subcrop beneath Paleogene unconformity in drill hole

Subcrop beneath Paleogene unconformity in drill hole

EEasternmost exposure of rocks in hanging wall of Roberts Mountains thrust

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Ranges and valleys:

Eureka

Area ofPlate 1

Area ofPlate 1

U.S. 50

Figure 11. Paleogene subcrop map of Eureka region (modifi ed from Long, 2012), showing subcrop patterns of regional-scale, erosionally beveled, pre-Paleogene folds (including Eureka culmination), Newark Canyon Formation exposures, and pre–late Eocene normal faults in study area.

Long et al.

580 Geosphere, June 2014

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 fi ll the space in the cross sec-tions beneath the Moritz-Nager thrust and, more important, beneath the structurally lower, larger offset Ratto Canyon thrust, is the largest uncer-tainty in the cross sections. This space must be fi lled with structurally repeated Paleozoic rocks (Plate 3), and many geometric solutions are pos-sible. Because the uncertainty in projection of kink surfaces and corresponding dip domains propagates with depth, as more structures are crossed, a simplifi ed structural model for con-tractional deformation in the study area is pre-sented here (Fig. 12), rather than attempting to balance fault offsets and cutoff angles for con-tractional structures on individual cross sections.

First-order observations that must be satisfi ed in the structural model include the following: (1) the Eureka culmination has a wavelength of ~20 km, an amplitude of ~4.5 km, and pre-extensional 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 Devo-nian 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 culmina-tion 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 Moun-tain Quartzite to the Silurian Lone Mountain

Dolomite, and then forms a footwall fl at 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 Senti-nel Mountain syncline is interpreted as the trail-ing 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 west-ern and eastern limbs, 9 km of displacement on the Ratto Canyon thrust, and an additional 1 km of displacement on the Prospect Moun-tain–Moritz-Nager thrust, for a total of 10 km

A

B

C

9 km

7.5 km

1 km

27°

Ratto Canyon thrust

Eureka culmination

Prospect Mountain/Moritz-Nager thrustSentinel Mountain syncline axis

future Ratto Canyon thrust

4.5 km

future ProspectMountain/Moritz-

Nager thrust

ClCu

OS

D

M

IPP

Cl

CuOSD

M

IPP

Cl

CuOS

Cl

Cu

OSD

M

IPP

Cl Cu O S

ClCuOS

DM IPP

20 kmEureka culmination

NCF type-sectionpiggyback basin

erosion

approximate pre-NCF erosion level

folding>500 m

thick

??

top of IPP

top of IPP

folding

erosion

White Mtn.anticline axis

Pinto Creeksyncline axis

West East

West East

West East

Figure 12. Simplifi ed structural model for central Nevada thrust belt (CNTB) deformation in map area, showing construc-tion of Eureka culmination as a fault-bend fold (abbre-viations: Cl—lower Cam-brian, Cu—upper Cambrian, O—Ordovician, S—Silurian, D—Devonian, M—Mississip-pian, IPP—Pennsylvanian and Permian; see Fig. 3). (A) Unde-formed geometry. NCF—Newark Canyon Formation. (B) 9 km of eastward motion of Ratto Canyon thrust sheet over footwall ramp that cuts from base of unit Cl to top of unit S, and coeval deposition and fold-ing of Newark Canyon Forma-tion (NCF) in piggyback basin on eastern limb of culmination. (C) 1 km eastward displace-ment on Prospect Mountain/Moritz-Nager thrust, shown reactivating Ratto Canyon thrust along and west of foot-wall ramp, and branching from top of footwall ramp.

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Geosphere, June 2014 581

of horizontal shortening (Fig. 12). Because this is a simplifi ed geometric model, this shortening estimate should be considered approximate.

DISCUSSION

Relationship of Eureka Culmination to NCF Deposition

Fundamental questions remain regarding the timing and geometric relationship of NCF deposition to regional deformation, as demon-strated by the diverse range of scenarios pro-posed in previous studies (e.g., Nolan et al., 1974; Vandervoort and Schmitt, 1990; Carpen-ter 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 sub-crop units, Vandervoort and Schmitt (1990) and Druschke et al. (2011) suggested that deposition in the type-NCF basin may have been contem-porary with proximal CNTB deformation. How-ever, these studies lack the necessary structural context to relate NCF deposition to motion on specifi c 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 pro-pose that the NCF type exposure was deposited in a piggyback basin that developed on the east-ern 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 cul-mination 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 com-patible with provenance data suggesting deriva-tion 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 defi ned 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 con-text 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., defi ning 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 accom-modated 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 emplace-ment 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 Can-yon 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 propaga-tion is governed by the relative magnitudes of

Canyon Range thrustPavant thrustand duplex

Paxton thrustand duplex

Gunnisonthrust

frontal triangle zone

Total shortening in Sevier thrust belt:

Active structures in Sevier thrust belt:

Newark Canyon Formation type-section: Druschke et al. (2010): ~122 Ma youngest DZ peak and ~116 Ma tuff (Aptian)??

117 km

191 km 211 km 221 km

Eureka culmination(hinterland) Sevier culmination

708090100110120130140Time (Ma)

010

0km

200

Figure 13. Graph showing timing of active structures and cumulative shortening mag-nitudes reported for type-Sevier thrust belt (DeCelles and Coogan, 2006). Approximate Aptian deposition age range (125–112 Ma) is shown for Newark Canyon Formation. DZ—detrital zircon.

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582 Geosphere, June 2014

the frictional strength of the basal dècolle mont 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 appre-ciably 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 mecha-nism 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 east-ward 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 appre-ciably increased the overall surface slope angle of the orogenic wedge. However, instead of building taper, perhaps this hinterland deforma-tion acted as a mechanism to decrease the over-all frictional coupling of the basal dècollemont to promote forward propagation.

Studies focused on the kinematic develop-ment 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 west-ern 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.

CONCLUSIONS

New mapping and balanced cross sections redefi ne the fi rst-order structures and deforma-tion geometry of the CNTB at the latitude of Eureka. Retrodeformation of extension defi nes

the Eureka culmination, a north-striking anti-cline with a 20 km wavelength, a 4.5 km ampli-tude, and limb dips of 25°–35°, which is cor-roborated by deep Paleogene erosion levels that can be traced for ~100 km along strike. A Cam-brian over Silurian relationship observed in drill holes under the culmination crest defi nes 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 Can-yon Range thrust sheet, which acted as a mecha-nism to promote further eastward translation.

ACKNOWLEDGMENTS

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 Patter-son of the Timberline Resources Corporation Eureka exploration team. Comments from three anonymous reviewers and associate editor Terry Pavlis signifi -cantly improved this manuscript.

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