An Upper-Crustal Fold Province in the Hinterland of the Sevier Orogenic Belt, Eastern Nevada, U.S.A.: a Cordilleran Valley and Ridge in the Basin and Range
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Long An upper-crustal fold province in the hinterland of the Sevier orogenic belt, eastern Nevada, U.S.A.: A Cordilleran Valley and Ridge in the Basin and Range Sean P. Long1,* 1Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada 89557, USA ABSTRACT basin ~220 km eastward during the Sevier system (Taylor et al., 2000; Long, 2012; Greene, orogeny. Low deformation magnitudes in the 2014; Long et al., 2014). The region of eastern Overprinting Cenozoic extension hinders hinterland are attributed to the rheological Nevada between the Central Nevada thrust belt analysis of Cordilleran contractional defor- competence of this thick basin. and Western Utah thrust belt is interpreted to mation in the hinterland of the Sevier thrust have experienced relatively little upper-crustal belt in Nevada. In this study, a 1:250,000 INTRODUCTION Cordilleran contractional deformation (e.g., scale paleogeologic map of eastern Nevada, Armstrong, 1972; Speed et al., 1988). This is showing spatial distributions of Paleozoic– The Jurassic–Paleogene Cordilleran oro- primarily based on compilation maps of strati- Mesozoic rocks beneath a Paleogene uncon- genic belt is the result of over 100 m.y. of con- graphic levels exposed beneath a regional formity, is combined with dip magnitude tractional deformation of the North American unconformity that places Paleogene rocks over maps for Paleozoic–Mesozoic and Tertiary plate above an Andean-style subduction zone Paleozoic–Mesozoic rocks, which show that the rocks, published sedimentary thickness (Allmendinger, 1992; Burchfi el et al., 1992; total postorogenic structural relief of this region records, and a published reconstruction of DeCelles, 2004; Dickinson, 2004). In the west- was low (2–4 km; Armstrong, 1972; Gans and extension, in order to defi ne and regionally ern interior United States, the majority of Cor- Miller, 1983; Long, 2012). In addition, early correlate thrust faults and folds, and estimate dilleran crustal shortening was accommodated mapping studies in eastern Nevada documented the pre-extensional amplitude, wavelength, in the Sevier thrust belt in western Utah (Fig. 1; that angularity across the unconformity is typi- and limb dips of folds. A new structural prov- e.g., Armstrong, 1968; Royse et al., 1975; Vil- cally ≤10°–15° (Young, 1960; Kellogg, 1964; ince, the Eastern Nevada fold belt, is defi ned, lien and Kligfi eld, 1986; DeCelles and Coogan, Moores et al., 1968), which has led to a prevail- consisting of a 100-km-wide region contain- 2006). Decades of study in the Sevier thrust belt ing view that any pre-Tertiary deformation was ing fi ve fi rst-order folds that can be traced leave the geometry, kinematics, magnitude, and gentle and minor. for map distances between 100 and 250 km timing of deformation well documented (e.g., However, despite the low structural relief of and have amplitudes of 2–4 km, wavelengths Jordan, 1981; Allmendinger et al., 1983, 1987; this region, the map patterns of regionally trace- of 20–40 km, pre-extensional limb dips typi- Royse, 1993a; DeCelles et al., 1995; Mitra, 1997; able (≥100 km) folds are defi ned on subcrop cally between 10° and 30°, and deform rocks DeCelles and Coogan, 2006). However, in the maps (Gans and Miller, 1983; Long, 2012), and as young as Jurassic and Early Cretaceous. hinterland of the thrust belt in eastern Nevada, detailed geologic maps (Humphrey, 1960; Bro- No regional-scale thrust faults or décolle- fundamental questions remain regarding the kaw and Barosh, 1968; Nolan et al., 1971; Nutt, ment horizons breach modern exposure style, geometry, and magnitude of Cor dilleran 2000) show that many more folds are present, levels in the Eastern Nevada fold belt. First- deformation, and how it relates tem porally to although to date they have not been regionally order folds of the Eastern Nevada fold belt deformation in the frontal thrust belt (e.g., Arm- correlated or described in detail. Recent work are interpreted to have formed above a deep strong, 1972; Miller et al., 1988; Miller and near Eureka that documents 60°–70° of angu- (≥10 km below the Paleogene unconformity), Hoisch, 1995; Camilleri and Chamberlain, 1997; larity across the Paleogene unconformity (Long blind décollement or shear zone, perhaps the Taylor et al., 2000; Long, 2012; Greene, 2014; et al., 2014), and geologic maps showing fold westward projection of the master décolle- Long et al., 2014). This can be attributed in part limb dips ranging from 30° to overturned (Hum- ment of the Sevier thrust belt. to sparse preservation of synorogenic sedimen- phrey, 1960; Larson and Riva, 1963; Brokaw Three hinterland structural provinces, the tary rocks, but it is primarily due to the com- and Barosh, 1968; Nolan et al., 1971) both chal- Central Nevada thrust belt, Western Utah plex dismemberment of the region by Cenozoic lenge the interpretation of minimal pre-Tertiary thrust belt, and the intervening Eastern extension (e.g., Gans and Miller, 1983; Coney deformation and justify a structural synthesis of Nevada fold belt, collectively record low-mag- and Harms, 1984; Dickinson, 2002). this region. nitude (a few tens of kilometers), upper-crustal Two zones of thrust faults, the Central In this study, the primary goal is to describe shortening that accompanied Cretaceous Nevada thrust belt and Western Utah thrust folding in the region between the Central translation of the Cordilleran passive-margin belt (Fig. 1), have been identifi ed in the Sevier Nevada thrust belt and Western Utah thrust belt; hinter land, and they are interpreted as contem- this exercise leads to defi nition of a new struc- *E-mail: [email protected]. porary, interior components of the Sevier thrust tural province, the Eastern Nevada fold belt. Geosphere; April 2015; v. 11; no. 2; p. 404–424; doi:10.1130/GES01102.1; 7 fi gures; 2 tables; 1 plate; 2 supplemental fi les. Received 11 July 2014 ♦ Revision received 2 December 2014 ♦ Accepted 16 January 2015 ♦ Published online 17 February 2015 404 For permissionGeosphere, to copy, contact April [email protected] 2015 © 2015 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/2/404/3335863/404.pdf by guest on 29 September 2021 A fold province in the Sevier hinterland in eastern Nevada CA OR ID A 120°WNV 117°W 114°W 111°W N RMT GC-RR-A Salt GT Lake WY 41°N Figure 1. (A) Map of the Cordi- Elko R-EH City lleran retroarc region in Nevada, area of Utah, and eastern California, Plate 1 superimposed over a base map from McQuarrie and Wernicke SR Reno Eureka ? (2005, their Fig. 7A), which shows Austin LFTB Ely present-day dimensions. Defor- 39°N old- mation fronts of Cordilleran Sierra Nevada magmatic arc thrust systems are shown, and WUTB ust belt Sevier f hr CNTB t spatial extents are shaded gray; ENFB Great Valley forearc basin locations are modifi ed from Long Tonopah et al. (2014), with additions from Cedar area of Greene (2014) and this study. City Figure 5 Area of Eastern Nevada fold belt UT 37°N (ENFB) is shaded blue. Paleozoic– AZ early Mesozoic thrust systems are shown in brown. Location of Sierra Franciscan Nevada magmatic arc is from Van accretionary Las Buer et al. (2009) and DeCelles complex Vegas Present day (2004). (B) Map of same area as 0200100 A, but superimposed over a base ESTB kilometers map from McQuarrie and Wer- B OR N nicke (2005, their Fig. 9E) which CA NV ID RMT shows a 36 Ma tectonic recon- GT struction. The map shows approx- imate pre-extensional dimensions WY of the Cordilleran retroarc region. Abbreviations: GT—Golconda thrust, RMT—Roberts Moun- tains thrust, LFTB—Luning- thrust belt Fencemaker thrust belt; ESTB— LFTB Sevier fold- Eastern Sierra thrust belt; CNTB—Central Nevada thrust belt; ENFB—Eastern Nevada ENFB fold belt; WUTB—Western Utah Sierra Nevada magmatic arc WUTB Franciscan accretionary complex thrust belt; R-EH—Ruby–East CNTB Humboldt core complex; SR— Great Valley forearc basin Snake Range core complex; GC-RR-A—Grouse Creek–Raft area of River–Albion core complex. State Figure 6 abbreviations: CA—California, UT OR—Oregon, NV—Nevada, ID— AZ Idaho, UT—Utah, WY—Wyo- ming, AZ—Arizona. 36 Ma 0 100 200 ESTB kilometers A 1:250,000 scale paleogeologic map of east- and these are combined with published sedi- gene limb dips. Fold axes are superimposed central Nevada, constructed by compiling for- mentary thicknesses (Stewart, 1980) to estimate over a pre-extensional tectonic reconstruction mation-scale divisions of Paleozoic–Mesozoic fold amplitude. The map is combined with dip (McQuarrie and Wernicke, 2005), which illus- rocks exposed beneath the Paleogene unconfor- magnitude maps for Paleozoic–Mesozoic and trates the synorogenic map dimensions of indi- mity, is presented. Subcrop patterns defi ned on Tertiary rocks, which corroborate the correlation vidual structures and structural provinces and the map allow regional correlation of fold axes, of fold axes and allow estimation of pre-Paleo- allows estimation of fold wavelength. Geosphere, April 2015 405 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/2/404/3335863/404.pdf by guest on 29 September 2021 Long In addition, the subcrop map provides a tral Nevada thrust belt and Western Utah thrust METHODS detailed view of the map patterns of thrust faults belt are interpreted to represent contemporary, and folds in the northern Central Nevada thrust interior components of the Sevier thrust system Paleogeologic Map belt, including the recently defi ned Eureka cul- (Taylor et al., 2000; Greene, 2014; Long et al., mination (Long et al., 2014). These observa- 2014). The region of eastern Nevada between Plate 1 presents a paleogeologic map of tions are combined with the structural synthesis these two thrust belts, which is the subject of east-central Nevada that shows the distribution of the Eastern Nevada fold belt and published this paper, exhibits upper-crustal folding but of Paleozoic and Mesozoic sedimentary rocks cross sections of the Western Utah thrust belt lacks regional-scale thrust faults (e.g., Arm- exposed under a regional unconformity beneath (Greene, 2014) and Sevier thrust belt (DeCelles strong, 1972; Gans and Miller, 1983).