GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Volume 11 2024 This is an open-access article in which the Utah Geological Association permits unrestricted use, distribution, and reproduction of text and figures that are not noted as copyrighted, provided the original author and source are credited. Email inquiries to GIW@utahgeology.org. THE WASATCH MONOCLINE, CENTRAL UTAH—A PRE-BASIN AND RANGE EXTENSIONAL STRUCTURE Shelley A. Judge, David H. Elliot, Terry J. Wilson, and Kenneth A. Foland GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Production Cover Design and Desktop Publishing Douglas A. Sprinkel Cover View to the northeast of the Wasatch monocline as ex- pressed along Manti Canyon near Manti, Utah. Signif- icant east-west drainage transects are perpendicular to the monocline axial trace, exposing strata folded into the limb of the monocline. In Manti Canyon, the variegated beds of the Cretaceous-Paleocene North Horn Forma- tion are capped by cliff-forming strata of the Paleocene Flagstaff Formation. Monocline flexure is punctuated by east-dipping antithetic normal faulting that displaces strata by tens to hundreds of meters. i Become a member of the UGA to help support the work of the Association and receive notices for monthly meetings, annual field conferences, and new publi- cations. Annual membership is $30 and annual student membership is only $5. Visit the UGA website at www.utahgeology.org for information and membership application. The UGA board is elected annually by a voting process through UGA members. However, the UGA is a volunteer-driven organization, and we welcome your voluntary service. If you would like to participate please contact the current president or committee member corresponding with the area in which you would like to volunteer. Utah Geological Association formed in 1970 from a merger of the Utah Geological Society, founded in 1946, and the Intermountain Association of Geologists, founded in 1949. Affiliated with the American Association of Petroleum Geologists. Volume 11 2024 Geology of the Intermountain West (GIW) is an open-access journal in which the Utah Geological As- sociation permits unrestricted use, distribution, and reproduction of text and figures that are not noted as copyrighted, provided the original author and source are credited. 2023–2024 UGA Board President Eugene Syzmanski eugenes@utah.gov 801.537.3364 President-Elect Keilee Higgs keileeann@utah.gov 801.537.3304 Program Chair Chris Stallard cstallard@utah.gov 801.386.0976 Treasurer Aubry DeReuil aubry@zanskar.us 850.572.2543 Secretary Trae Boman tboman@teanues.com 801.648.5206 Past President Rick Ford rford@weber.edu 801.915.3188 UGA Committees Environmental Affairs Craig Eaton eaton@ihi-env.com 801.633.9396 Geologic Road Sign Greg Gavin greg@loughlinwater.com 801.541.6258 Historian Paul Anderson paul@pbageo.com 801.364.6613 Outreach Greg Nielsen gnielsen@weber.edu 801.626.6394 Public Education Zach Anderson zanderson@utah.gov 801.537.3300 Matt Affolter gfl247@yahoo.com Publications Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 Publicity Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 Social/Recreation Roger Bon rogerbon@xmission.com 801.942.0533 AAPG House of Delegates 2023–2026 Term David A. Wavrek dwavrek@petroleumsystems.com 801.322.2915 State Mapping Advisory Committee UGA Representative Bill Loughlin bill@loughlinwater.com 435.649.4005 UGA Newsletter Newsletter Editor William Lund uga.newsletter@gmail.com 435.590.1338 UGA Website — www.utahgeology.org Webmaster Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 Scholarship Golf Tournament Co-Chair Rick Ford rford@weber.edu 801.915.3188 Co-Chair John South jsouth@utah.gov 385.266.2113 Earthquake Safety Committee Chair Grant Willis gwillisgeol@gmail.com 801.537.3355 Douglas A. Sprinkel Azteca Geosolutions 801.391.1977 GIW@utahgeology.org dsprinkel@gmail.com Thomas C. Chidsey, Jr. Utah Geological Survey, Emeritus 801.824.0738 tomchidsey@gmail.com Bart J. Kowallis Brigham Young University 801.380.2736 bkowallis@gmail.com Steven Schamel GeoX Consulting, Inc. 801.583.1146 geox-slc@comcast.net John R. Foster Utah Field House of Natural History State Park Museum 435.789.3799 johnfoster@utah.gov William R. Lund Utah Geological Survey, Emeritus 435.590.1338 williamlundugs@gmail.com Editors GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Volume 11 2024 1 ABSTRACT The Wasatch monocline is a major structure in the transition zone between the Basin and Range and Colorado Plateau physiographic provinces. The timing of formation of the monocline and its tectonic significance has been the subject of debate because it is an anomalous structural style for central Utah. Constraining the age for flexure and outlining the tectonic regime responsible for Wasatch monocline formation are principal objectives of this research. Field mapping near the southern end of the mono- cline revealed an unconformity between the older, middle Eocene Crazy Hollow Formation, included in monocline folding, and the younger, middle Eocene (Bartonian) formation of Aurora deposited against the monocline. We report an incremental step-heating and direct single-grain laser fusion age of 38.0 ± 0.2 Ma for biotite from an ash-flow tuff within the formation of Aurora, which constrains monocline formation to no younger than the mid-Eocene. Structural data from the Wasatch monocline, in the context of regional structural and tectonic analysis, indicate the monocline formed in an extensional regime as a forced fold or a rollover fold of the east-facing Sanpete half-graben, formed during pre-Basin and Range extension, also recorded in the Paleogene-Neogene basins of Utah Valley and Salt Lake Valley regions. The Wasatch Monocline, Central Utah—A Pre-Basin and Range Extensional Structure Shelley A. Judge1, David H. Elliot2, 3,Terry J. Wilson2, 4, and Kenneth A. Foland2, 5 1Department of Earth Sciences, The College of Wooster, Wooster, OH 44691 USA; sjudge@wooster.edu 2 School of Earth Sciences, The Ohio State University, Columbus, OH 43210 USA; 3elliot.1@osu.edu; 4wilson.43@osu.edu; 5kfoland@ geology.ohio-state.edu Citation for this article. Judge, S.A., Elliot, D.H., Wilson, T.J., and Foland, K.A., 2024, The Wasatch monocline, central Utah—a pre-Basin and Range extensional structure: Geology of the Intermountain West, v. 11, p. 1–19, https://doi.org/10.31711/giw.v11.pp1-19. INTRODUCTION Importance of Study Most monoclines within the Colorado Plateau are prominent Laramide structures, formed as drag folds above reactivated, high-angle, reverse faults due to contractional tectonics (e.g., Kelley and Clinton, 1960; Davis, 1978, 1999; Anderson and Barnhard, 1986; Er- slev and Rogers, 1993; Bump, 2003). Central Utah is the physiographic transition zone along the boundary between the Colorado Plateau province to the east and the Basin and Range province to the west. The Wasatch monocline is a broad and open fold with the form of typical regional monoclines; it extends for about 110 km (68 mi) through central Utah and is the structural back- bone of the transition zone (Figure 1). The west-facing monocline also forms the western front of the Wasatch Plateau, a notable geomorphic feature of the region (Figure 2). The Wasatch monocline is in an area that has been subject to both contractional and extensional tectonics since the Early Cretaceous. The development of the monocline by either tectonic regime is of specif- ic interest, because it is an anomalous structural style for central Utah. No indisputable Laramide-style defor- mation has been documented in the Wasatch Plateau region of central Utah, and this research confirms that the Wasatch monocline is not a Laramide-style feature, characterized by monocline flexure due to high-angle, 2 The Wasatch Monocline, Central Utah—A Pre-Basin and Range Extensional Structure Judge, S.A., Elliot, D.H., Wilson, T.J., and Foland, K.A. Geology of the Intermountain West 2024 Volume 11 reverse dip-slip motion of basement blocks (Davis, 1978). New information on the Wasatch monocline is invaluable for improving knowledge on the timing and extent of contraction or extension in the region, includ- ing correlating the timing of tectonic regimes to known events to the north and west. Although workers have reported on Wasatch mono- cline development for over 140 years (e.g., Dutton, 1880; Spieker, 1946; Erb, 1971; Willis, 1986; Anderson et al., 2001; Schelling et al., 2007), the timing and formation of the Wasatch monocline has remained uncertain and controversial. Previous workers have suggested a wide range of possible ages for monocline formation from the Middle Eocene through the Early Miocene (ca. 40 to 20 Ma; e.g., Spieker, 1949; Erb, 1971; Willis, 1986; Wit- kind, 1994; Anderson et al., 2001). Spieker (1949) and his students generally accepted the monocline as late Eocene to Miocene in age using only stratigraphic and structural relationships. More recent age constraints have been based on radiometric age dates from sever- al different formations (Willis, 1986), but these radio- metric dates have not extended the inferred age range significantly. The exceptions are Anderson et al. (2001) and Frank Royse, Jr. (Chevron, written communication, 2020) who proposed that the monocline is Miocene or younger, essentially making it a Basin and Range struc- ture. Purpose of Research Here we report new data to constrain the timing of monocline flexure and its relationship to contrac- tional and/or extensional tectonics. New field-based information obtained to determine the timing of flex- ure includes studies on stress regimes, paleocurrents, stratigraphic field relationships, and 40Ar/39Ar dating. Focused mapping near the southern end of the struc- ture reveals an angular unconformity between older strata included in monocline folding and younger strata deposited unconformably against the monocline. These stratigraphic data bracket the timing of flexure by pro- viding maximum and minimum ages of monocline formation. Wasatch monocline age constraints can be combined with established tectonic information to pro- vide a more comprehensive picture of changing stress regimes in a regional context. GEOLOGIC SETTING Regional Tectonic Setting The Wasatch monocline resides in a key location in the complex Mesozoic-Cenozoic tectonic framework of central Utah. The monocline lies in the transition zone, which separates the western margin of the Col- orado Plateau Province and the eastern margin of the Basin and Range Province. It also lies along the late Pre- cambrian to Paleozoic Cordilleran sedimentary hinge- line of central Utah and with the eastern limit of the Figure 1. Regional DEM map of central Utah displaying the topographic highs and adjacent valleys. Important geograph- ic or geologic localities from the text are mapped for refer- ence, including: the Wasatch Plateau, San Pitch Mountains, Cedar Hills, and Valley Mountains (black), Sanpete, Sevier, and Juab Valleys (black), the axial trace of the Wasatch mono- cline (blue), and the Wasatch and Gunnison faults (red). The locations of Rock and Dry Canyons and gulley south of Black Cap Mountain (yellow) are also mapped. 3 The Wasatch Monocline, Central Utah—A Pre-Basin and Range Extensional Structure Judge, S.A., Elliot, D.H., Wilson, T.J., and Foland, K.A. Geology of the Intermountain West 2024 Volume 11 Figure 2. (A) Photograph of the Wasatch monocline in profile, as seen in Manti Canyon on the Wasatch Plateau. Manti Canyon is within the central part of the fold, signifying its highest structural relief. A series of east-dipping antithetic normal faults displace strata of the Flagstaff Formation and the North Horn Formation along the monocline limb, which dips west toward Sanpete Valley. View to the northeast. (B) Schematic cross section across Sanpete Valley. Modified from Schelling et al. (2007). The red box indicates the placement of the photograph along the cross-section profile. The locations of the Gunnison fault and Ephraim faults bound Sanpete Valley to the west and east, respectively. Ja = Jurassic Arapien Formation; Jt+Kcm = Jurassic Twist Gulch Formation and Cretaceous Cedar Mountain Formation; KiL = Cretaceous Indianola Group, lower; KiU = Cretaceous Indianola Group, upper; PC = Precambrian strata; PgKn+Pgf = Cretaceous-Paleogene North Horn Formation and Paleogene Flagstaff Formation; Pz = Paleozoic strata; TR+J = Triassic and Jurassic strata. 4 The Wasatch Monocline, Central Utah—A Pre-Basin and Range Extensional Structure Judge, S.A., Elliot, D.H., Wilson, T.J., and Foland, K.A. Geology of the Intermountain West 2024 Volume 11 Cretaceous Sevier fold-thrust belt. The Utah hingeline, which geographically is oriented approximately north- south along the center of the state, has long been recog- nized as a zone of crustal weakness and tectonic activity during many periods of Utah’s geologic history (Baer, 1976; Ritzma, 1981; Stokes, 1988; Hintze and Kowallis, 2021). Central Utah has undergone changes from one stress regime to another through time. Structural overprint- ing shows both contractional features associated with Sevier orogenesis and extensional features associated with Oligocene-Miocene and then Basin and Range ex- tension. Several studies have outlined the regional tec- tonic evolution. DeCelles and Coogan (2006) proposed a regional thrusting chronology associated with the Sevier fold-and-thrust belt of central Utah (c.a. 145 to 65 Ma; latest Jurassic-Early Cretaceous to Early Paleo- cene). Their chronology rewrote the previous tectonic interpretation for synorogenic deposits (see Villien and Kligfield, 1986). Constenius (1996) and Constenius et al. (2003) were instrumental in outlining the timing of pre-Basin and Range extension (ca. 49 to 20 Ma; Middle Eocene to Early Miocene) throughout the western U.S. Cordillera. They noted that the change from compres- sional to tensional stress regime was not synchronous everywhere, with northern segments of the Sevier fold- and-thrust belt showing extensional overprinting earli- er in time. Other studies conducted near the southern and northern extent of the monocline have focused on is- sues of structural overprinting. Toward the southern end, Cline and Bartley (2007) tested five competing hy- potheses on observed relationships of the Cenozoic-Ju- rassic geology of Sevier Valley. They determined that the principal process responsible for a unique contact in Sevier Valley was regional extension and called the fault the Salina detachment. Anderson et al. (2001) noted many Neogene extensional features in the region near the town of Salina, and therefore suggested a Miocene or younger age for the Wasatch monocline. They based their results on structural concordance and the absence of deposition against paleotopography of the mono- cline. North of the monocline, in the Charleston-Ne- bo segment of the Sevier thrust belt, Constenius et al. (2003) provided evidence for crustal extension (ca. late Eocene to middle Miocene) evidenced by reactivation of thrust structures to form half grabens. Monocline Expression The Wasatch monocline is a north-northeast-trend- ing, west-verging monocline in Sanpete-Sevier Valleys of central Utah exposed between Millburn and Salina Canyon for approximately 110 km (68 mi) (Figure 1; Willis, 1986). Its northern segment trends 020° to 030°; however, there is a noticeable change in the anticlinal axial trace to 000° to 005° near its southern end. The Wasatch monocline, a major geomorphic feature, is a doubly plunging fold, with its highest topographic ex- pression and structural relief (1710 m; 5600 ft) in the central part of the fold (Figure 2). Topographic relief in the northern and southern ends is approximately 850 m (2790 ft) and 700 m (2297 ft), respectively. McGookey (1958) attributed this reduced relief at the southern end to less displacement, to less resistant strata exposed at the surface, and to the overlap of folded units by young- er strata. Access to monocline exposures is excellent not only along the trend of the monocline, but also across the monocline flexure due to several cross-strike struc- tural transects formed by large drainage basins. In profile, nearly flat-lying Paleogene sedimentary strata that dip less than 10° west form the plateau summit; these strata are displaced by several northeast-south- west-striking en echelon grabens. The plateau grabens are large-scale features that dominate the landscape of the summit. Smaller shoulder grabens and east-dipping antithetic normal faults break the continuity of the fold- ed strata within the anticlinal hinge zone of the Wasatch monocline. Along the limb of the monocline, strata dip 15° to 40° west—steepest within the central part of the monocline limb—and are cut by a series of antithetic normal faults that divide the limb into distinct structural domains (Figure 2). In each structural domain, small- er-scale mesoscopic normal faults, joints, and veins are prominent. Eventually, limb strata plunge beneath the adjacent Sanpete Valley; a broad, synclinal hinge has been interpreted at depth in several balanced structural cross sections of the region (Schelling et al., 2007). 5 The Wasatch Monocline, Central Utah—A Pre-Basin and Range Extensional Structure Judge, S.A., Elliot, D.H., Wilson, T.J., and Foland, K.A. Geology of the Intermountain West 2024 Volume 11 Regional Stratigraphy Geologic units are well exposed along the length and width of the Wasatch monocline due to the sever- al east-west-drainage transects. Units exposed in the central part of the monocline include the Cretaceous to Paleogene North Horn Formation and the Paleo- gene Flagstaff, Colton, Green River, and Crazy Hollow Formations (Figure 3). The North Horn Formation represents shallowing of the foredeep during Gunni- son thrust emplacement during the Sevier orogeny (DeCelles and Coogan, 2006). Flagstaff through Crazy Hollow strata were deposited during times of Laramide uplift, which produced areas of high topographic relief separated by numerous basins (Dickinson et al., 1988). In central Utah, intermontane ponded basins dom- inated, and the Flagstaff and Green River Formations represent lacustrine intervals that interfingered with siliciclastic-rich Colton and Crazy Hollow fluvial units (Stanley and Collinson, 1979; Weiss and Warner, 2001; Davis et al., 2009; Dickinson et al., 2012). Additional units with significant volcanic/volcani- clastic components are exposed along the monocline at its extreme northern and southern ends. At the north- ern end, the Moroni Formation laps onto the Green River Formation in several places, and in all probability, also laps onto the Crazy Hollow Formation along the eastern flanks of the Cedar Hills (Fograscher, 1956; Al- brecht, 2001; Weiss and Warner, 2001). Moroni K/Ar dating from Cedar Hills samples, conducted by Albrecht (2001), provided a 34.3 ± 0.3 Ma age for the formation. At the southern end, beds assigned to the formation of Aurora by Willis (1986) lap onto the monocline; we show here that Aurora strata figure significantly in the geologic history of monocline flexure. The forma- tion of Aurora was proposed by Willis (1986; replaced Bald Knoll Formation) but not formally defined. Willis (1986) described it as a sequence of “interbedded mud- stone, bentonitic shale, limestone and sandstone.” He further mentions that the formation becomes increas- ingly volcanic toward the south (away from Salina) and includes rhyolitic ash-flow tuff. For purposes of this re- search, we follow the stratigraphic definition of the for- mation of Aurora set by Willis (1986) but acknowledge that Aurora Formation was used in a stratigraphic col- umn by Hintze and Kowallis (2021). COMPETING HYPOTHESES FOR MONOCLINE DEVELOPMENT There are several competing hypotheses to explain the origin and timing of Wasatch monocline develop- ment: (1) contractional folding associated with the final stages of the Sevier orogeny (e.g., Spieker, 1946; Standlee 1982; Lawton, 1985; Villien and Kligfield, 1986); (2) dif- ferential subsidence due to salt tectonics (e.g., Witkind, 1992, 1994); (3) Laramide-style contractional deforma- tion above a reactivated, high-angle reverse fault, mod- eling many other monoclines in the Colorado Plateau (for research on verified Laramide monocline exam- ples, see Davis, 1978; Reches and Johnson, 1978; Yin, 1994; Davis and Bump, 2009; Keating et al., 2012); and Figure 3. Generalized stratigraphic column of principal units affected by flexure of the Wasatch monocline in the south- ern portion of the study area. Thickness data taken from the mouth of Salina Canyon. Modified from Hintze and Kowal- lis, 2021). K = Cretaceous; M = Maastrichtian. 6 The Wasatch Monocline, Central Utah—A Pre-Basin and Range Extensional Structure Judge, S.A., Elliot, D.H., Wilson, T.J., and Foland, K.A. Geology of the Intermountain West 2024 Volume 11 (4) extensional deformation associated with a younger tensional regime (Anderson et al., 2001; Schelling et al., 2007; Frank Royse, Jr., Chevron, written communica- tion, 2020). These several competing hypotheses coin- cide with three different orogenic events affecting the region (i.e., Sevier, Laramide, and Basin and Range tec- tonism) and with the wide range of proposed ages for development of the monocline (Middle Eocene through the Early Miocene or even younger; e.g., Spieker, 1949; Erb, 1971; Willis, 1986; Witkind, 1994; Anderson et al., 2001; Frank Royse, Jr., Chevron, written communica- tion, 2020). METHODS This research tests the hypotheses for monocline or- igin through an integrated, primarily field-based study of the development and timing of flexure. We obtained a series of data sets (i.e., field mapping, stereonet anal- ysis on stratal bedding, and ash-flow tuff geochronolo- gy) and combined data about the geologic structures of the region and the age, lateral extent, and sedimentary characteristics of regional formations. Field Mapping The most important locality with respect to mono- cline timing is in a gulley southwest of the summit of Black Cap Mountain and west of Twist Canyon (UTM 12 S. 0423792 E., 4310373 N.; Figures 4 and 5). Here, Aurora beds are in contact with the Crazy Hollow Formation, and the nature of this contact has been uncertain for decades in the literature. Willis (1986) indicated the importance of finding a depositional rela- tionship between the two units in this region; however, it was Erb (1971) who first provided clues as to where this depositional relationship might be located, even though he did not definitively identify the Crazy Hol- low Formation. In his thesis, Erb (1971) stated, “Unless some future worker can determine, without doubt, the identity of the Crazy Hollow(?) wedge, this particular angular unconformity will be of little use in determin- ing the validity of…flexing of the Wasatch Monocline.” The suggestion of an angular unconformity in this area provided incentive to include Black Cap Mountain as a Figure 4. Geologic map of the area south of Black Cap Mountain. The red star in the center of the map marks the viewpoint for Figure 5 (photograph of the gulley transect). 7 The Wasatch Monocline, Central Utah—A Pre-Basin and Range Extensional Structure Judge, S.A., Elliot, D.H., Wilson, T.J., and Foland, K.A. Geology of the Intermountain West 2024 Volume 11 field mapping site in this study. This research clarifies the nature of the spatial re- lationships and the contact between the Crazy Hollow Formation and the formation of Aurora. The goal of field mapping near Black Cap Mountain was (1) to iden- tify definitively each unit in the field, and (2) to locate and to interpret either a conformable or unconformable contact between the two units (Figure 4). Although we had scoured the trend of the monocline for exposures that provided concrete spatial relationships, only the southern end of the monocline was suitable for detailed mapping. Using unit descriptions for Crazy Hollow and for Aurora strata (Willis, 1986; Weiss and Warner, 2001), we identified both units. A northeast-southwest transect through one gulley was used to measure and to document the numerous bedding attitudes of both units near the contact (Crazy Hollow, n = 22; Aurora, n = 38) (Figure 5). We constructed a detailed stratigraph- ic column (Figure 6) that starts above the characteristic red beds of the Crazy Hollow Formation but includes its typical ‘salt-and-pepper’ sandstone and terminates at the unconformably overlying ash-flow tuff within Aurora strata. Careful field mapping in the area docu- mented bedding patterns, small-scale faulting, and con- tacts between exposed units (Figure 4). Finally, we gath- ered stratigraphic thickness data between two selected beds—one bed in the Crazy Hollow and the other in the Aurora—to test for stratigraphic consistency along three separate transects, each progressively closer to the monocline fold axial trace. This was done to discount any other structural explanation responsible for bed- ding attitude disparities. Stereonet Analysis We undertook data analysis of bedding attitudes in both the Crazy Hollow Formation and the formation of Aurora (Figure 7). Because of the impact of multiple deformation events in the area and small-scale faulting east of the gulley, only measurements from the north- east-southwest-gulley transect were used for final anal- ysis to prevent any chance of comparing attitudes from Figure 5. Photograph of the gulley south of Black Cap Mountain that shows the spatial relationships between the Crazy Hollow Formation (Pgch) and the overlying and onlapping formation of Aurora (Pgau). The ash-flow tuff is apparent in the bottom right of the photograph, and an arrow designates where samples were collected for 40Ar/39Ar age dating. The thickness associated with each resistant bed correlate to the stratigraphic placement within the graphic column in Figure 6. 8 The Wasatch Monocline, Central Utah—A Pre-Basin and Range Extensional Structure Judge, S.A., Elliot, D.H., Wilson, T.J., and Foland, K.A. Geology of the Intermountain West 2024 Volume 11 different fault blocks. Bedding measurements from a single formation within a single fault block were plot- ted, and the average bedding orientation was deter- mined from the contoured maxima of poles to bedding planes. Both the Crazy Hollow and Aurora average bed- ding planes were rotated to restore bedding in Aurora strata back to horizontal to simulate its attitude at the time of its deposition. The resultant stereonet shows the attitude of the Crazy Hollow Formation at the time of deposition of Aurora strata (Figure 7), documenting their angular discordance. Ash-Flow Tuff Exposures Field observations at Black Cap Mountain focused on white to gray ash-flow tuff beds in the lower part of Aurora strata. For this study, samples for analysis were taken from the least weathered interval near the base of the ash-flow tuff and again at about 7.6 m (25 ft) above the base of the unit (Figure 7). Biotites within the ash-flow tuff were dated using two different 40Ar/39Ar techniques, incremental step-heating and direct, sin- gle-grain laser fusion, following the methods described in McDougall and Harrison (1988, 1999) and Foland et al. (1993). Our goal was to compare these 40Ar/39Ar dates to previous age dating of the Aurora. Willis (1986) described three K/Ar radiometric dates from nearby lo- calities: 40.5 ± 1.7 Ma from the lower part of the unit in the southernmost Salina 7.5-minute quadrangle, about 5.5 km (3.4 mi) to the south-southeast, and 39.6 ± 1.5 Ma and 38.4 ± 1.5 Ma from near the top of the unit in the adjacent Aurora 7.5-minute quadrangle, about 9.3 km (5.8 mi) to the west. FIELD RESULTS AND ANALYSIS Angular Discordance Near Black Cap Mountain Constraining the timing of Wasatch monocline flex- ure is a primary aim of this research. Black Cap Moun- tain is the only known place where a depositional con- tact between the beds involved in Wasatch monocline flexure and overlapping beds are exposed. Due to sub- sequent erosion and deformation along the trend of the monocline, no other localities provide this critical field relationship. At this locality, red beds assignable to the Crazy Hollow Formation are overlain by 43.3 m (142 Figure 6. Stratigraphic column of part of the Crazy Hollow Formation and the formation of Aurora. The column was measured in a prominent gulley south of Black Cap Moun- tain and shown in Figure 5. 9 The Wasatch Monocline, Central Utah—A Pre-Basin and Range Extensional Structure Judge, S.A., Elliot, D.H., Wilson, T.J., and Foland, K.A. Geology of the Intermountain West 2024 Volume 11 ft) of strata comprising mudstone, siltstone, sandstone, and thin micrite beds (Figures 5 and 6). This sequence includes thin sandstones with black chert grains char- acteristic of the Crazy Hollow Formation, but otherwise it is atypical of that formation and somewhat similar to Aurora strata as described by Willis (1986, 1988). This 43.3 m-thick (142 ft) interval is overlain by a white ash- flow tuff, which itself is overlain by a sequence of vol- caniclastic sandstones and tuffaceous beds that share similarities with both Aurora strata and the “unnamed sandstone, mudstone and conglomerate beds” of Willis (1986). Field mapping supports a difference in the attitudes between the Crazy Hollow Formation and Aurora strata (Figure 4). Bedding analysis along our northeast-south- west transect demonstrates an angular discordance of about 15° between the Aurora ash-flow tuff and the un- derlying beds (Figure 7). The average bedding attitude for the Crazy Hollow beds beneath the unconformity is 316°, dip 29° NE. (N. 44° W., dip 29° NE.), whereas that Figure 7. (A) Stereonet plots showing the average bedding attitude from measurements taken from the Crazy Hollow For- mation and the formation of Aurora in the vicinity of the mapped area shown in Figures 4 and 5. Field data from the Crazy Hollow show an average orientation of 316°, dip 29° NE. (N. 44° W., dip 29° NE.), whereas field data from the Aurora show an average orientation of 319°, dip 45° NE. (N. 41° W., dip 45° NE.). (B) Raw field data of Aurora strata were rotated back to horizontal (a fold test) to determine the orientation of the Crazy Hollow Formation at the time the Aurora was being de- posited. When the Aurora was horizontal (0° dip), the Crazy Hollow was oriented 144°, dip 16° SW. (S. 36 E., dip 16° SW.). An idealized cross section (without antithetic normal faults) shows the discordant relationship between Crazy Hollow and Aurora strata. 10 The Wasatch Monocline, Central Utah—A Pre-Basin and Range Extensional Structure Judge, S.A., Elliot, D.H., Wilson, T.J., and Foland, K.A. Geology of the Intermountain West 2024 Volume 11 for the Aurora ash-flow tuff and overlying strata is 319°, dip 45° NE. (N 41° W., dip 45° NE.). When the ash-flow tuff is rotated back to horizontal during the fold test, the underlying fine-grained strata and the definitive Crazy Hollow beds assume an average attitude of 144°, dip 16° SW. (S 36° E., dip 16° SW.) (Figure 7). Thus, at the time of deposition of the ash-flow tuff, the underlying eroded and truncated beds were dipping about 15° southwest and therefore were part of the west-dipping limb of the Wasatch monocline. These transect bedding results rep- licated in all bedding data produced from field mapping of both units in the area. Our stratigraphic column (Figure 6) starts above the characteristic red beds of the Crazy Hollow For- mation, includes its typical salt-and-pepper sandstone, and terminates at the unconformably overlying the ash- flow tuff. Thin micrite beds in that interval are a key to demonstrating the angular discordance. Thickness transects of stratigraphic intervals between selected Crazy Hollow micrite marker beds and the ash-flow tuff demonstrate increasing thickness between the markers from east to west up the gulley wall. These differences in stratigraphic thickness show that the ash-flow tuff unit is progressively thinning to the east toward the mapped axial trace of the Wasatch monocline. These data demonstrate the presence of a westward-thicken- ing wedge that onlaps the monocline limb and can be dated to give an upper age limit for monocline flexure. The angular relationship just described raises sev- eral stratigraphic issues. If the 43.3-m (142 ft) section of beds above the typical red Crazy Hollow strata and below the unconformity are assigned to the Crazy Hol- low Formation, then it represents a depositional en- vironment unlike any other in that formation. If they should be assigned to the formation of Aurora, then the ash-flow tuff above the unconformity could be assigned to the “unnamed sandstone, mudstone and conglomer- ate beds” of Willis (1986). Alternatively, if the 43.3-m (142 ft) section and the overlying ash-flow tuff belong to Aurora strata, the angular relations could be evidence for a progressive unconformity developed by temporal coincidence of monocline deformation and deposition of Aurora strata. For the purposes of this discussion, the ash-flow tuff is assigned to the Aurora. Geochronology Dating the ash-flow tuff is critical to this study be- cause of its relationship to older Crazy Hollow beds near Black Cap Mountain. The ash-flow tuff is easily identified in the field. The white to pale-gray tuff is rhy- olitic in composition and contains abundant pumice fragments and hexagonal to pseudo-hexagonal biotite. The two different 40Ar/39Ar techniques of incremental step-heating and direct, single-grain laser fusion yield- ed consistent radiometric dates for the biotites (Figure 8). Two step-heating runs gave well-defined plateaus with remarkably similar plateau ages (tp) of 37.9 ± 0.2 Ma and 38.0 ± 0.2 Ma. For each run, the isotope correla- tion ages (tic) are identical to the plateau ages. One run by the single-grain laser fusion technique on 24 grains was conducted. Several grains had large uncertainties, and these were rejected, leaving 17 grains in the final estimated date of 38.2 ± 0.2 Ma (weighted by age un- certainties), which does not differ from the date derived from all 24 grains. The date obtained from weighting each analysis by the amount of 39Ar released is 38.5 ± 0.3 Ma. The isotope correlation age for the single grain analyses (n = 17) is 38.1 ± 0.2 Ma, although it is not a particularly well-constrained correlation. Dates obtained in this study range from 37.9 ± 0.2 Ma to 38.5 ± 0.3 Ma. Most dates cluster around 38.0 Ma. Therefore, a best estimate age of 38.0 ± 0.2 Ma is accepted for the Aurora ash-flow tuff. Because initiation of flexure is older than the base of the ash-flow tuff that onlaps the limb of the Wasatch monocline, the accepted 40Ar/39Ar date provides a minimum age for flexure of the monocline: namely, pre-late Eocene (pre-Priabonian). DISCUSSION Critical Age Relationships The Wasatch monocline did not have significant topographic relief during Eocene Crazy Hollow deposi- tion. Paleocurrent directions, collected from the Crazy Hollow in the Sanpete-Sevier Valley, indicate a general flow pattern from south-southeast to north-northwest through the area where the Wasatch monocline pres- ently stands (Judge, 2007). These patterns suggest that the Wasatch monocline did not affect paleoflow during 11 The Wasatch Monocline, Central Utah—A Pre-Basin and Range Extensional Structure Judge, S.A., Elliot, D.H., Wilson, T.J., and Foland, K.A. Geology of the Intermountain West 2024 Volume 11 Fi gu re 8 . A ge d at in g re su lts fr om th e as h- flo w tu ff in th e fo rm at io n of A ur or a. (A a nd B ) 40 A r/ 39 A r i nc re m en ta l-h ea tin g re le as e pa tte rn fo r b io tit e; t in t = in te gr at ed a ge ; t p = p la te au a ge . Th e a cc ep te d ag e f or th e a irf al l t uff n ea r t he b as e o f t he fo rm at io n of A ur or a is 38 .0 ± 0 .2 M a. (C ) 40 A r/ 39 A r s in - gl e gr ai n fu sio n pa tte rn fo r b io tit e; t av g = a ve ra ge a ge ; t ic = is ot op e- co rr el at io n ag e. Si ng le g ra in fu sio n ag es c om pa re fa vo ra bl y w ith th e in cr em en ta l st ep -h ea tin g re su lts . 12 The Wasatch Monocline, Central Utah—A Pre-Basin and Range Extensional Structure Judge, S.A., Elliot, D.H., Wilson, T.J., and Foland, K.A. Geology of the Intermountain West 2024 Volume 11 deposition of the unit. Unfortunately, the age of the Cra- zy Hollow Formation is poorly constrained. Previous age dating includes three stratigraphically disparate air- fall tuffs from the underlying Green River Formation in Sanpete Valley that yielded biotite ages of about 46.4 to 43.1 Ma by 40Ar/39Ar analysis (Sheliga, 1980). The Au- rora ash-flow tuff gives a minimum age for monocline flexure of 38.0 ± 0.2 Ma (Figure 8), with fold initiation preceding ash-flow deposition. This Aurora ash-flow tuff age is slightly older than ages for the volcanic/volcaniclastic Moroni Forma- tion of the Cedar Hills and south of Thistle farther to the north, both near the northern end of the Wasatch monocline. K/Ar ages between 30.6 ± 1.1 Ma to 37.8 ± 2.2 Ma on biotite, plagioclase, and hornblende sep- arates, were obtained by Witkind and Marvin (1989), whereas 40Ar/39Ar dates of 34.3 ± 0.3 Ma on biotite from an ash-flow tuff and 37.5 ± 0.1 Ma on amphibole from a sandstone were obtained by Albrecht (2001). The horn- blende analyzed by Witkind and Marvin (1989) came from an ash-flow tuff south of Thistle, but its strati- graphic position is not recorded. The detrital amphibole (Albrecht, 2001) only gives a maximum age of deposi- tion of the Moroni Formation. Constenius et al. (2003) reported two sanidine dates from pumice clasts at the base of the Moroni Formation south of Thistle. These are 34.4 ± 0.1 Ma and 34.6 ± 0.1 Ma and are similar to the biotite date obtained by Albrecht (2001) for an ash- flow tuff. End of Sevier Contraction Sevier orogenesis impacted the Sanpete Valley re- gion, from the San Pitch Mountains in the west to the Wasatch Plateau in the east. Contractional events, in the form of a north-northeast-trending, west-vergent monocline (the Rock and Dry monocline) and sever- al thrust faults, are exposed along the eastern margin of the San Pitch Mountains (Kilmer, 1988; Judge et al., 2011). Many previous workers attributed formation of the Rock and Dry monocline to contractional events associated with the final stages of the Sevier orogeny (e.g., Spieker, 1946; Standlee, 1982; Lawton, 1985; Vil- lien and Kligfield, 1986; Kilmer, 1988). Lawton and Weiss (1999) and Weiss and Sprinkel (2002) mapped the area in 7.5-minute quadrangles and suggested that the Rock and Dry monocline was a fault-propagation fold that formed during the Late Eocene to Oligocene or post-Early Eocene, respectively. Two authors here- in (Judge et al., 2011) undertook a modern structural analysis of the Rock and Dry monocline, following ini- tial work done by Kilmer (1988). Structural work on the Rock and Dry monocline focused on the near-continuous exposures of units in- volved in monocline flexure, with a focus on dip do- main stations along cross-strike transects in the Paleo- gene Flagstaff Limestone (Figure 3). Kinematic data and analyses of these mesoscopic structural features (stylo- lites and teeth, calcite veins, and thrust faults) of the Rock and Dry monocline indicated that an east-west compression (trend 278° to 098° [N. 82° W. to S. 82° E.]) preceded flexure and continued as the monocline devel- oped. This maximum compression direction is sub-per- pendicular to the trend of the monocline. West-ver- gent thrust faults with an average orientation of 020°, dip 42° SE. (N. 20° E., dip 42° SE.) along east-dipping bedding planes in the Jurassic Twist Gulch Formation place the Twist Gulch over the Campanian-Paleocene North Horn Formation. The back thrusts caused local- ized flexure and uplift of Paleogene strata in the area into a fault-propagation fold. These thrust-fold spatial associations and the evidence for layer-parallel com- pression clearly link Rock and Dry monocline forma- tion with forced-folding associated with west-vergent thrusting and regional compression of the Sevier thrust belt. The Rock and Dry monocline, therefore, was de- veloped in the latest stage of back thrusting associated with the Gunnison thrust system in the easternmost Se- vier thrust belt (Kilmer, 1988; Judge et al., 2011). The timing of the Rock and Dry monocline has regional tectonic implications. Because the Paleocene to Lower Eocene Colton Formation (Figure 3) is the youngest unit visibly affected by Rock and Dry mono- cline folding, flexure and amplification of the monocline occurred after deposition of at least the lower approxi- mate 60 m (197 ft) of the Colton. If the overlying Green River Formation, exposed only to the west of the Rock and Dry monocline at higher elevations, is also involved in that flexure, then the Green River ash beds dated at about 46 to 43 Ma (Sheliga, 1980) provide a maximum 13 The Wasatch Monocline, Central Utah—A Pre-Basin and Range Extensional Structure Judge, S.A., Elliot, D.H., Wilson, T.J., and Foland, K.A. Geology of the Intermountain West 2024 Volume 11 age of post-Early Eocene for the contractional Rock and Dry monocline. Considering a younger age limit for the Green River Formation of about 42 Ma, then this back thrusting likely occurred prior to about 41 Ma. It was the youngest phase of Sevier orogenesis in this region. Pre-Monocline Extension The limb of the Wasatch monocline is dominat- ed by normal faults, both large-scale antithetic faults and hundreds of small-scale mesoscopic faults (Judge, 2007). These faults, some of which are accommoda- tion features resulting from flexure, document multiple slip events and divide the limb into several distinctive north-south-trending structural domains. Although strike-slip faults occur locally, normal faults are by far the most abundant and pervasive structures. Reverse or thrust faults are nearly absent along the monocline summit and limb, as are tectonic stylolites (Judge, 2007). In addition to the abundant normal faults, each structural domain exposes prominent smaller-scale, mesoscopic joints and calcite veins within Paleogene units. Data from the systematic joints and veins (i.e., at- titude, spacing, density, morphologic, and mineralogic characteristics) were collected along several cross-strike transects for a kinematic analysis across the monocline. Simple fold test analyses of the joints and veins show that these fractures formed prior to the limb rotation of the Wasatch monocline (Judge, 2007). Therefore, a tensional stress regime in the Sanpete Valley region pre-dated monocline flexure. This tensional regime suggests that the Wasatch monocline formed in a dif- ferent and younger tectonic regime than the Sevier-in- fluenced Rock and Dry monocline, just west across the valley (Judge, 2007). Based on the dated unconformable relations discussed previously, the change to a tension- al regime occurred in the Eocene prior to monocline flexure. Evidence of Pre-Basin and Range Extension In central Utah, there is no evidence of Lara- mide-style, basement-cored uplifts distinct to other re- gions influenced by contractional tectonics. The long- lived compressional Sevier stress regime that produced localized Rock and Dry monocline back thrusting was replaced by a tensional stress regime in the Eocene. Cordilleran pre-Middle Miocene extension is well doc- umented, with some evidence for extension as early as the Eocene (e.g., Burchfiel et al., 1992; Axen et al., 1993; Murphy et al., 1998; Rowley et al., 1998). There is am- ple evidence of mid-Cenozoic extension in Utah. To the north of our study area, Constenius (1996) and Con- stenius et al. (2003) have documented extension in the Uinta arch region and northward along the Wyoming border. This period of extension has been attributed to gravitational collapse following cessation of Sevier contractional tectonism (Constenius, 1996; Vogel et al., 2001; Constenius et al., 2003). To the west and south of our study area, geologic contacts in Sevier Valley show evidence for regional tectonic extension (Cline and Bartley, 2007). On a broader scale, Constenius et al. (2003) pointed out that this mid-Cenozoic extension is recorded regionally by the metamorphic core complex- es in northeastern Nevada and adjacent Idaho. In par- ticular, the Snake Range detachment has been dated as no older than ca. 35 Ma, the age of the regionally exten- sive Kalamazoo ash-flow tuff deposited on a subdued landscape (e.g., Gans et al., 1989; Miller et al., 1999). During the mid-Cenozoic, there was extensive volcanism in Utah, linked with the transition from re- gional contraction to extension (e.g., Constenius 1996; Constenius et al., 2003). The volcaniclastic Aurora stra- ta and our new age at Black Cap Mountain likely reflect this regional framework. We establish a 38 Ma age for the Aurora ash-flow tuff. This age is slightly older than the ages of 33.5 to 36.6 Ma given for emplacement of the Keetley Volcanics, which are a part of the Wasatch intrusive belt and located in the central Wasatch Range, east-southeast from Salt Lake City (Vogel et al., 2001; Constenius et al., 2003). The age for the Aurora ash-flow tuff also is slightly older than the age assigned to tephras in the Tibble half graben on the south flank of the Uinta arch (Constenius et al., 2003). The oldest tephra from basin-fill in the Tibble half graben has been dated at 36.4 ± 0.2 Ma using a biotite 40Ar/39Ar plateau age (Vo- gel et al., 2001). Finally, the Marysvale volcanic field lies 65 km (40 mi) to the south-southwest of our study area; however, there is no indication that its initial volcanism, which is accepted at 32 Ma (Steven et al., 1978; Cun- ningham et al., 2007), was active at the time of Aurora 14 The Wasatch Monocline, Central Utah—A Pre-Basin and Range Extensional Structure Judge, S.A., Elliot, D.H., Wilson, T.J., and Foland, K.A. Geology of the Intermountain West 2024 Volume 11 ash-flow tuff deposition. For central Utah, Constenius (1996) and Conste- nius et al. (2003) proposed that, south of 40° N. latitude, extension began at about 39 Ma; they bracketed exten- sion regionally based on the relationships between the ages of the youngest foreland basin deposits, Paleogene volcanic/volcaniclastic deposits, and synextensional basin-fill. Constenius et al. (2003) showed a series half grabens bounded by normal faults in their work on the Charleston-Nebo salient of the Utah thrust belt to the north of this study. In these half grabens, pre-extension strata have rollover fold geometries and underlie basin deposits characterized by symmetrical stratal growth. One of the obvious extensional structural features of Sanpete Valley is the Gunnison fault (also called the Valley fault and the Wales fault; see Fong, 1995; Lawton and Weiss, 1999; Weiss and Sprinkel, 2002; Main, 2015), which forms the structural boundary between Sanpete Valley and the San Pitch Mountains (Figure 1). The fault is an east-dipping normal fault with several thousand meters of offset. From well data, Sprinkel (1994) shows Paleogene units with 945 m (3100 ft) of stratigraphic offset between the San Pitch Mountains and the under- lying Sanpete Valley. The Gunnison fault’s 60° east dip at the surface becomes listric at depth below Sanpete Valley and is responsible for the formation of a half gra- ben beneath the valley; the San Pitch Mountains are its horst block to the west (e.g., Fong, 1995; Lawton and Weiss, 1999; Schelling et al., 2007; Main, 2015). Weiss and Sprinkel (2002) suggested that early movement along the fault might have occurred during the Mio- cene. Here we suggest that the formation of the Sanpete half graben, a result of movement along the Gunnison fault, could have been initiated at the same time (ca. 39 Ma) as proposed by Constenius et al. (2003) for the re- gion immediately to the north. The Gunnison fault re- mains in a tensional stress regime, as recent alluvium north of the Rock and Dry monocline is cut by the fault, and fault scarps are evident in outcrop (e.g., Fong, 1995; Lawton and Weiss, 1999; Weiss and Sprinkel, 2002). Formation of the Wasatch Monocline During Extension We document a narrow time window between con- tractional deformation and extensional deformation between about 41 to 38 Ma, which agrees with the pro- posed timing of early extension for central Utah (Con- stenius, 1996; Constenius et al., 2003). Therefore, we argue that the Wasatch monocline formed in an exten- sional setting. There is a spatial proximity between the Wasatch monocline and the subsurface west-dipping Ephraim fault interpreted from seismic data (Moulton, 1976; Schelling et al., 2007). The Ephraim fault has typically been interpreted as a normal fault that formed as part of the eastern boundary of the Middle Jurassic Sanpete-Se- vier rift. Some recent interpretations, however, suggest post-Jurassic reactivation of the Ephraim fault during Neogene Basin and Range extension (e.g., Schelling et al., 2007; Frank Royse, Jr., Chevron, written communi- cation, 2020). This suggests a model of the monocline as a forced fold above a west-dipping subsurface nor- mal fault, analogous to folds modeled by Withjack et al. (1995). The age data reported here, however, indicates monocline formation in the Eocene, associated with pre-Basin and Range extension. An alternative model is development of the mono- cline as a rollover fold associated with a half graben bound to the west by the listric, east-dipping Gunnison fault that forms the Sanpete half graben (Schelling et al., 2007). Analogous rollover folds in extensional settings (see models of Groshong, 1989; Schlische, 1995; Poblet and Bulnes, 2005; Withjack and Schlische, 2006; Uzke- da et al., 2014) have many of the same general features as those of the Wasatch monocline: (1) a lower limb that is either horizontal or dips slightly toward the ma- jor fault; (2) synthetic and antithetic faults displacing the fold limb; and (3) the presence of a shoulder graben near the anticlinal hinge of the monocline (Withjack et al., 1995). Rollover folds exhibit axial traces parallel to subparallel to the strike of major listric normal faults (Schlische, 1995). In the case of the Wasatch monocline, its axial trace (020° to 030° in the northern segment, 000° to 005° in the southern segment) is subparallel to the trace of the Gunnison fault, which is segmented and varies from 005° to 030°. We prefer the rollover model given the newly doc- umented timing of monocline development during the regional development of extensional half grabens during 15 The Wasatch Monocline, Central Utah—A Pre-Basin and Range Extensional Structure Judge, S.A., Elliot, D.H., Wilson, T.J., and Foland, K.A. Geology of the Intermountain West 2024 Volume 11 Sevier orogenic collapse, as documented by Constenius et al. (2003). Future modeling work is planned to test compatibility of these extensional folding mechanisms with the architecture of the Wasatch monocline. CONCLUSIONS The purpose of this research is to better constrain the age of Wasatch monocline flexure and to improve our understanding of the geologic history of central Utah regarding the change from the Sevier compres- sional stress regime to the pre-Basin and Range ten- sional stress regime. Three lines of evidence provide in- formation about the timing and affiliated stress regime of monocline flexure: (1) the transition from contrac- tion in the Rock and Dry monocline to extension and development of the Wasatch monocline in a short time window in the Eocene, (2) paleoflow patterns of Paleo- gene fluvial systems that crossed central Utah without deflection by topography at the site of the monocline, and (3) the age of a volcaniclastic unit in the formation of Aurora deposited as a wedge onlapping the Wasatch monocline during flexure. Biotite from an ash-flow tuff at the base of the onlapping formation of Aurora at the southern end of the Wasatch monocline yielded a best estimate 40Ar/39Ar age of 38.0 ± 0.2 Ma. Thus, the Wasatch monocline flexure was initiated after the middle Eocene (using Sanpete Valley Green River Formation biotite dates) but before the upper Eocene ash-flow tuff at the base of the Aurora. In contrast to both Sevier deformation and classical Laramide monocline formation, the Wasatch mono- cline is interpreted here to have most likely formed as a rollover fold associated with a half graben in an ex- tensional setting. Because the Wasatch monocline doc- uments early extensional tectonics in the region, the development of the monocline must post-date Sevier orogenesis. This interpretation of the Wasatch mono- cline supports Weiss and Sprinkel (2002) and more re- cent work in Sanpete and Sevier Valleys by Cline and Bartley (2007), Schelling et al. (2007), and Main (2015). If Wasatch monocline development occurred during this tensional stress regime, then it is one of the first documented occurrences of pre-Basin and Range ex- tension affecting Sanpete-Sevier Valley and extends the region of extension related to gravitational collapse of the Sevier thrust belt defined by Constenius (1996) and Constenius et al. (2003) in northern Utah. ACKNOWLEDGMENTS This research was supported by funds from the American Association of Petroleum Geologists, the Colorado Scientific Society, the Geological Society of America, The College of Wooster, The Ohio State Uni- versity Graduate School, the Society for Sedimenta- ry Geology, and Sigma Xi. Special thanks to field and lab assistance from J. Olney (Hillsborough Communi- ty College), D. Caccamise (NOAA National Geodetic Survey), F. Hubacher (The Ohio State Univerity), K. Lewandowski (Eastern Illlinois University), and to T. Brunner (College of Wooster) and J. Robison (College of Wooster) for graphical design assistance. The authors enjoyed conversations on the geology of central Utah with W. Ausich (The Ohio State University), J. Collinson (The Ohio State University), L. Krissek (The Ohio State University), D. Sprinkel (Utah Geological Survey), C. Summerson (The Ohio State University), and G. Willis (Utah Geological Survey). Thanks to R. Allmendinger (Cornell University) for use of Stereonet. This manu- script has benefited greatly from two anonymous and insightful reviews. Their time and effort on the manu- script is much appreciated. REFERENCES Albrecht, J.L., 2001, The paleovolcanological setting of the Moroni Formation, central Utah: Columbus, The Ohio State University, M.S. thesis, 204 p. Anderson, R.E., and Barnhard, T.P., 1986, Genetic relation- ships between faults and folds and determination of Lar- amide and neotectonic paleostress, western Colorado Plateau-transition zone, central Utah: Tectonics, v. 5, p. 335–357. Anderson, R.E., Diehl, S.F., and Barnhard, T.P., 2001, Age and style of deformation and stratal thinning at the tran- sition, Wasatch Plateau to Great Basin, central Utah: Rocky Mountain Geology, v. 36, p. 49–80. Axen, G.J., Taylor, W.J., and Bartley, J.M., 1993, Space-time patterns and tectonic controls of Tertiary extension and 16 The Wasatch Monocline, Central Utah—A Pre-Basin and Range Extensional Structure Judge, S.A., Elliot, D.H., Wilson, T.J., and Foland, K.A. Geology of the Intermountain West 2024 Volume 11 magmatism in the Great Basin of the western United States: Geological Society of America Bulletin, v. 105, p. 56–76. Baer, J.L., 1976, Structural evolution of central Utah—Late Permian to Recent, in Hill, J.G., editor, Geology of the Cordilleran hingeline: Rocky Mountain Association of Geologists Guidebook, p. 37–45. Bump, A.P., 2003, Reactivation, trishear modeling, and folded basement in Laramide uplifts—implications for the ori- gins of intra-continental faults: GSA Today, v. 3, p. 4–10. Burchfiel, B.C., Cowan, D.S., and Davis, G.A., 1992, Chap- ter 8—tectonic overview of the Cordilleran orogen in the western United States, in Burchfiel, B.C., Lipman, P.W., and Zoback, M.L., editors, The Cordilleran orogen— conterminous U.S.: Geological Society of America, The Geology of North America Volume G-3, p. 407–477. Cline, E.J., and Bartley, J.M., 2007, Nature of the Cenozo- ic-Mesozoic contact in Sevier Valley and tectonic impli- cations, in Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C., Jr., editors, Central Utah—diverse geology of a dynamic landscape: Utah Geological Association Publication 36, p. 1–29. Constenius, K.N., 1996, Late Paleogene extensional collapse of the Cordilleran foreland fold and thrust belt: Geologi- cal Society of America Bulletin, v. 108, p. 20–39. Constenius, K.N., Esser, R.P., and Layer, P.W., 2003, Ex- tensional collapse of the Charleston-Nebo salient and its relationship to space-time variations in Cordilleran orogenic belt tectonism and continental stratigraphy, in Raynolds, R.G., and Flores, R.M., editors, Cenozoic systems of the Rocky Mountain region: Society for Sed- imentary Geology (SEPM), Rocky Mountain Section, p. 303–343. Cunningham, C.G., Rowley, P.D., Steven, T.A., and Rye, R.O., 2007, Geologic evolution and mineral resources of the Marysvale volcanic field, west-central Utah. in Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C., Jr., editors, Central Utah—diverse geology of a dynamic landscape: Utah Geological Association Publication 36, p. 143–161. Davis, G.H., 1978, Monocline fold pattern of the Colorado Plateau, in Matthews, V., III, editor, Laramide folding associated with basement block faulting in the Western United States: Geological Society of America Memoir 151, p. 215–233. Davis, G.H., 1999, Structural geology of the Colorado Pla- teau region of southern Utah, with special emphasis on deformation band shear zones: Geological Society of America Special Paper 342, 157 p. Davis, G.H., and Bump, A.P., 2009, Structural geologic evo- lution of the Colorado Plateau, in Kay, V.A., and Dick- inson, W.R., editors, Backbone of the Americas—shal- low subduction, plateau uplift, and ridge and terrane collision: Geological Society of America Memoir 204, p. 99–124. Davis, S.J., Mulch, A., Carroll, A.R., Horton, T.W., and Chamberlain, C.P., 2009, Paleogene landscape evolution of the central North American Cordillera—developing topography and hydrology in the Laramide foreland: Geological Society of America Bulletin, v. 121, no. 1/2, p. 100–116. DeCelles, P.G., and Coogan, J.C., 2006, Regional structure and kinematic history of the Sevier fold-and-thrust belt, central Utah: Geological Society of America Bulletin, v. 118, no. 7/8, p. 841–864. Dickinson, W.R., Klute, M.A., Hayes, M.J., Janecke, S.U., Lundin, E.R., McKittrick, M.A., and Olivares, M.D., 1988, Paleogeographic and paleotectonic setting of Lara- mide sedimentary basins in the central Rocky Mountain region: Geological Society of America Bulletin, v. 100, p. 1023–1039. Dickinson, W.R., Lawton, T.F., Pecha, M., Davis, S.J., Gehrels, G.E., and Young, R.A., 2012, Provenance of the Paleogene Colton Formation (Uinta Basin) and Cretaceous-Paleo- gene provenance evolution in the Utah foreland—evi- dence from U-Pb ages of detrital zircons, paleocurrent trends, and sandstone petrofacies: Geosphere, v. 8, no. 4, p. 854–880. Dutton, C.E., 1880, Report on the geology of the High Pla- teaus of Utah with atlas: Washington D.C., Department of the Interior U.S. Geographical and Geological Survey of the Rocky Mountain region, 307 p. Erb, E.E., Jr., 1971, Geology of the volcanic rocks north of Salina Canyon, Salina, Utah: Columbus, The Ohio State University, M.S. thesis, 104 p. Erslev, E.A., and Rogers, J.L., 1993, Basement-cover geome- try of Laramide fault-propagation folds, in Schmidt, C.J., Chase, R.B., and Erslev, E.A., editors, Laramide base- ment deformation in the Rocky Mountain foreland of the Western United States: Geological Society of Ameri- ca Special Paper 280, p. 125–146. Fograscher, A.C., 1956, The stratigraphy of the Green River 17 The Wasatch Monocline, Central Utah—A Pre-Basin and Range Extensional Structure Judge, S.A., Elliot, D.H., Wilson, T.J., and Foland, K.A. Geology of the Intermountain West 2024 Volume 11 and Crazy Hollow Formations of part of the Cedar Hills, central Utah: Columbus, The Ohio State University, M.S. thesis, 88 p. Foland, K.A., Elliot, D.H., Fleming, T.H. and Heimann, A., 1993, Potassium-argon dating of fine-grained basalts with massive Ar loss; application of the 40Ar/39Ar tech- nique to plagioclase and glass from the Kirkpatrick Ba- salt, Antarctica: Chemical Geology, v. 107, p. 173–190. Fong, A.W., 1995, Geologic map of the Fountain Green South quadrangle, Sanpete and Juab Counties, Utah: Utah Geological Survey Miscellaneous Publication Map MP 95-1, 18 p., 2 plates, scale 1:24,000. Gans, P.B., and Mahood, G.A., and Schermer, E., 1989, Syn- extensional magmatism in the Basin and Range Prov- ince—a case study from the eastern Great Basin: Geo- logical Society of America Special Paper 233, p. 1–53. Groshong, R.H., 1989, Half-graben structures—balanced models of extensional fault-bend folds: Geological Soci- ety of America Bulletin, v. 101, p. 96–105. Hintze, L.F., and Kowallis, B.J., 2021, Geologic history of Utah: Brigham Young University Department of Geo- logic Sciences Special Publication 10, 266 p. Judge, S.A., 2007, The origin and evolution of the Wasatch monocline, central Utah: Columbus, The Ohio State University, Ph.D. dissertation, 396 p. Judge, S.A., Wilson, T.J., and Kilmer, D.S., 2011, Kinemat- ic analysis of a fault-related monocline, Sevier thrust belt (San Pitch Mountains), central Utah, in Sprinkel, D.A., Yonkee, W.A., and Chidsey, T.C., Jr., editors., Sevi- er thrust belt—northern and central Utah and adjacent areas: Utah Geological Association Publication 40, p. 73–87. Keating, D.P., Fischer, M.P., and Blau, H., 2012, Physical modeling of deformation patterns in monoclines above oblique-slip faults: Journal of Structural Geology, v. 39, p. 37–51. Kelley, V.C., and Clinton, N.J., 1960, Fracture systems and tectonic elements of the Colorado Plateau: University of Mexico Publications in Geology, no. 6, 106 p. Kilmer, D.S., 1988, A structural analysis of the 'S'-fold in Rock Canyon, Gunnison Plateau, central Utah: Colum- bus, The Ohio State University, B.S. thesis, 58 p. Lawton, T.F., 1985, Style and timing of frontal structures, thrust belt, central Utah: American Association of Pe- troleum Geologists Bulletin, v. 69, no. 7, p. 1145–1159. Lawton, T.F., and Weiss, M.P., 1999, Geologic map of the Wales quadrangle, Juab and Sanpete Counties, Utah: Utah Geological Survey Miscellaneous Publication 99-2, 29 p., 2 plates, scale 1:24,000. Main, J., 2015, Using travertine-cemented fault breccias to understand the architecture and history of the Gunnison fault zone, eastern Basin and Range, Utah: Columbus, The Ohio State University, M.S. thesis, 224 p. Mattox, S.R., 1987, Provisional map of the Hell’s Kitchen Canyon SE quadrangle, Sanpete County, Utah: Utah Geological and Mineral Survey Map 139, 16 p., 2 plates, scale 1:24,000. McDougall, I., and Harrison, T.M., 1988, Geochronology and thermochronology by the 40Ar/39Ar method: Oxford Monographs on Geology and Geophysics, v. 9, 212 p. McDougall, I., and Harrison, T.M., 1999, Geochronology and thermochronology by the 40Ar/39Ar method: Oxford University Press, 269 p. McGookey, D.P., 1958, Geology of the northern portion of the Fish Lake Plateau, Utah: Columbus, The Ohio State University, Ph.D. dissertation, 233 p. Miller, E.L., Dumitru, T.A., Brown, R.W., and Gans, P.B., 1999, Rapid Miocene slip on the Snake Range-Deep Creek Range fault system, east-central Nevada: Geologi- cal Society of America Bulletin, v. 111, p. 886–905. Moulton, F.C., 1976, Lower Mesozoic and Upper Paleozoic petroleum potential of the hingeline area, central Utah, in Hill, J.G., editor, Geology of the Cordilleran hingeline: Rocky Mountain Association of Geologists Guidebook, p. 219–229. (Reprint of paper published in Symposium on Deep Drilling Frontiers in the Central Rocky Moun- tains, Rocky Mountain Association of Geologists, 1975). Murphy, J.B., Oppliger, G.L., Brimhall, G.H., Jr., and Hynes, A., 1998, Plume-modified orogeny—an example from the southwestern United States: Geology, v. 26, p. 731– 734. Poblet, J., and Bulnes, M., 2005, Fault-slip, bed-length and area variations in experimental rollover anticlines over listric normal faults—influence in extension and depth to detachment estimations: Tectonophysics, v. 396, is- sues 1–2, p. 97–117. Reches, Z., and Johnson, A.M., 1978, Development of mono- clines, part II—theoretical analysis of monoclines, in Matthews, V., III, editor, Laramide folding associat- ed with basement block faulting in the western United 18 The Wasatch Monocline, Central Utah—A Pre-Basin and Range Extensional Structure Judge, S.A., Elliot, D.H., Wilson, T.J., and Foland, K.A. Geology of the Intermountain West 2024 Volume 11 States: Geological Society of America Memoir 151, p. 273–311. Ritzma, H.R., 1981, Oiling the “hingeline:” Utah Geological and Mineral Survey, Survey Notes, v. 15, no. 4, p. 1, 4–6. Rowley, P.D., Cunningham, C.G., Steven, T.A., Mehnert, H.H., and Naeser, C.W., 1998, Cenozoic igneous and tectonic setting of the Marysvale volcanic field and its relation to other igneous centers in Utah and Nevada, in Friedman, J.D., and Huffman, A.C., Jr., editors, Laccolith complexes of southeastern Utah—time of emplacement and tectonic setting workshop proceedings: U.S. Geolog- ical Survey Bulletin 2158, p. 168–201. Schelling, D.D., Strickland, D.K., Johnson, K.R., and Vrona, J.P., 2007, Structural geology of the central Utah thrust belt, in Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C., Jr., editors, Central Utah—diverse geolo- gy of a dynamic landscape: Utah Geological Association Publication 36, p. 1–29. Schlische, R.W., 1995, Geometry and origin of fault-related folds in extensional settings: American Association of Pe- troleum Geologists Bulletin, v. 79, no. 11, p. 1661–1678. Sheliga, C.M., 1980, Sedimentology of the Eocene Green River Formation in Sevier and Sanpete Counties, central Utah: Columbus, The Ohio State University, M.S. thesis, 166 p. Spieker, E.M., 1946, Late Mesozoic and Early Cenozoic his- tory of central Utah: U.S. Geological Survey Professional Paper 205-D, p. 117–161. Spieker, E.M., 1949, The transition between the Colorado Plateaus and the Great Basin in central Utah: Utah Geo- logical Society Guidebook to the Geology of Utah 4, 106 p. Sprinkel, D.A., 1994. Stratigraphic and time-stratigraphic cross sections—a north-south transect from near the Uinta Mountain axis across the Basin and Range transi- tion zone to the western margin of the San Rafael Swell, Utah: U.S. Geological Survey Map I-2184-D, 31 p., 2 plates, scale 1:500,000. Standlee, L.A., 1982, Structure and stratigraphy of Jurassic rocks in central Utah—their influence on tectonic de- velopment of the Cordilleran foreland thrust belt, in Powers, R.B., editor, Geologic studies of the Cordilleran thrust belt: Rocky Mountain Association of Geologists Guidebook, p. 357–382. Stanley, K.O., and Collinson, J.W., 1979, Depositional history of Paleocene-Lower Eocene Flagstaff Limestone and co- eval rocks, central Utah: American Association of Petro- leum Geologists Bulletin, v. 63, no. 3, p. 311–323. Steven, T.A., Rowley, P.D., and Cunningham, C.G., 1978, Ge- ology of the Marysvale volcanic field, west central Utah: Brigham Young University Geology Studies 25, part 1, p. 67–70. Stokes, W.L., 1988, Geology of Utah: Utah Museum of Nat- ural History and Utah Geological and Mineral Survey, 280 p. Uzkeda, H., Poblet, J., and Bulnes, M., 2014, Shear angle and amount of extension calculations for normal faults emanating from a detachment—implications on mecha- nisms to generate rollovers: Journal of Structural Geolo- gy, v. 67, pt. A, p. 20–36. Villien, A., and Kligfield, R.M., 1986, Thrusting and synoro- genic sedimentation in central Utah, in Petersen, J.A., editor, Paleotectonics and sedimentation in the Rocky Mountain region, United States: American Association of Petroleum Geologists Memoir 41, p. 281–307. Vogel, T.A., Cambray, F.W., and Constenius, K.N., 2001, Or- igin and emplacement of igneous rocks in the central Wasatch Mountains, Utah: Rocky Mountain Geology, v. 36, no. 2, p. 119–162. Weiss, M.P., and Sprinkel, D.A., 2002, Geologic map of the Manti 7.5-minute quadrangle, Sanpete County, Utah: Utah Geological Survey Map 188, 22 p., 2 plates, scale 1:24,000. Weiss, M.P., and Warner, K.N., 2001, The Crazy Hollow For- mation (Eocene) of central Utah: Brigham Young Uni- versity Geology Studies, no. 46, p. 143–161. Weiss, M.P., McDermott, J.G., Sprinkel, D.A., Banks, R.L., and Biek, R., 2003, Geologic map of the Chriss Canyon quadrangle, Juab and Sanpete Counties, Utah: Utah Geo- logical Survey Map 185, 26 p., 2 plates, scale 1:24,000. Willis, G.C., 1986, Geologic map of the Salina quadrangle, Sevier County, Utah: Utah Geological and Mineral Sur- vey Map 83, 20 p., 2 plates, scale 1:24,000. Willis, G.C., 1988, Geologic map of the Aurora quadrangle, Sevier County, Utah: Utah Geological and Mineral Sur- vey Map 112, 21 p., 2 plates, scale 1:24,000. Withjack, M.O., and Schlische, R.W., 2006, Models of ex- tensional fault-bend folds, in Buiter S.J.H., and Schreurs G., editors, Analogue and numerical modelling of crust- al-scale processes: Geological Society of London Special 19 The Wasatch Monocline, Central Utah—A Pre-Basin and Range Extensional Structure Judge, S.A., Elliot, D.H., Wilson, T.J., and Foland, K.A. Geology of the Intermountain West 2024 Volume 11 Publication 253, p. 285–305. Withjack, M.O., Islam, Q.T., and La Pointe, P.R., 1995, Nor- mal faults and their hanging-wall deformation—an ex- perimental study: American Association of Petroleum Geologists Bulletin, v. 79, no. 1, p. 1–18. Witkind, I.J., 1992, Paired, facing monoclines in the San- pete-Sevier Valley area, central Utah: The Mountain Ge- ologist, v. 29, no. 1, p. 5–17. Witkind, I.J., 1994, The role of salt in the structural develop- ment of central Utah: U.S. Geological Survey Profession- al Paper 1528, p. 1–145. Witkind, I.J., and Marvin, R.F., 1989, Significance of new potassium-argon ages from the Goldens Ranch and Moroni Formations, Sanpete-Sevier Valley area, central Utah: Geological Society of America Bulletin, v. 101, p. 534–548. Yin, A., 1994, Mechanics of monoclinal systems in the Col- orado Plateau during the Laramide orogeny: Journal of Geophysical Research, v. 99, no. B11, p. 22,043–22,058.