Paleomagnetic results from the eastern Caliente- Enterprise zone, southwestern Utah: Implications for initiation of a major Miocene transfer zone

Michael S. Petronis1,*, Daniel K. Holm2, John W. Geissman3, David B. Hacker2, and Billie J. Arnold2 1Environmental Geology, Natural Resource Management Department, New Mexico Highlands University, Las Vegas, New Mexico 87701, USA 2Department of Geology, Kent State University, Kent, Ohio 44242, USA 3Department of Geosciences, University of Texas at Dallas, ROC 21, 800 West Campbell Road, Richardson, Texas 75080-3021, USA

ABSTRACT large and statistically signifi cant. For exam- defi ne the CEZ after Axen (1998) as the region ple, site P-18 from the Bauers Tuff yields an encompassing all transverse structures whether The Miocene Caliente-Enterprise zone R = –61.1° ± 5.3° and F = –0.6° ± 5.0°. Rela- or not they include evidence for counterclock- (CEZ) in southwestern Utah (USA) is a tive to the expected Miocene direction, in situ wise vertical axis rotation. Displacement trans- 20–50-km-wide east-northeast–trending left- paleomagnetic data from the Iron Axis lac- fer systems, or accommodation zones, are often lateral transfer zone that displaces north- coliths, specifi cally the Three Peaks laccolith, subvertical fault systems that transfer displace- south–trending crustal blocks of the eastern yield a mean that is discordant in declination, ment from one region of the crust to another Basin and Range Province to the west. Pre- with estimated R = –22.2° and F = –8.8° val- (Moustafa, 1976; Bosworth, 1985, 1986; Lister vious paleomagnetic results from the central ues. These rotation and fl attening estimates, et al., 1986; Rosendahl, 1987; Chapin, 1989; and western CEZ show signifi cant counter- although consistent with the overall data set Faulds et al., 1990; Faulds and Varga, 1998). clockwise vertical axis rotations of strike- from volcanic rocks, must be considered of Transfer zones accommodate or partition strain slip–bounded fault blocks, and these rotation lesser quality, as we are unable to accurately into areas of heterogeneous extension, and often estimates vary in magnitude both across and correct these data for possible effects of local are not simple strike-slip or oblique-slip fault along the strike of the zone. Results of recent tilting. If the rotation estimate is viable, then systems, but tend to be associated with diffuse detailed geologic mapping and new geo- we suggest that this component of deforma- belts of magmatism and, at times, signifi cant chronologic data in the area east of previous tion involves much of the upper crust, and vertical axis rotation of fault-bound structural studies allow us to extend paleomagnetic stud- we furthermore propose that the boundary blocks (Faulds and Varga, 1998; Hudson et al., ies into the easternmost CEZ. New paleomag- of the eastern CEZ extends farther east than 1998; Petronis et al., 2002, 2007, 2009). The netic results include data from 4 regionally previously envisioned, to within a few kilome- CEZ is a sinistral transfer zone that has under- extensive latest Oligocene to early Miocene ters of the breakaway with the Colorado Pla- gone vertical axis rotation of spatially vari- (ca. 24–22 Ma) ignimbrite sheets and from 3 teau. The transitional zone between the east- able, yet systematic magnitudes (Hudson et al., ca. 22–20 Ma Iron Axis laccoliths. These data ern CEZ and Colorado Plateau is therefore 1998). Emplaced within the CEZ are numerous reveal signifi cant magnitudes and similar spa- abrupt and occurs within a narrow zone near 22–20 Ma laccoliths of the Iron Axis magmatic tially variable components of counterclock- Cedar City, Utah. province, a northeast-trending igneous feature wise vertical axis rotation. Rotation estimates in southwestern Utah characterized by several from the ignimbrites are assessed relative to INTRODUCTION early Miocene laccolithic intrusions, including what we interpret to be a nonrotated to only the Pine Valley megalaccolith and extensive minimally rotated reference section just north Since the middle Cenozoic, continental litho- latest Oligocene to earliest Miocene regional of the Colorado Plateau (Grass Valley) and spheric extension in the southwestern United ignimbrites (Hacker et al., 1996, 1999; Axen, from several low-extension areas in southeast States has resulted in the relative westward 1998). Previous paleomagnetic studies in the Nevada. Accepted tilt-corrected paleomag- transport of crustal material away from unex- CEZ concentrated on rocks from the central netic data from sites away from the reference tended stable crust of the Colorado Plateau and western part of its extent, predominantly areas are discordant in declination from the (Fig. 1). In general, the transition zone separat- because those areas were already mapped (Hud- expected individual ash-fl ow tuff directions, ing the extended and unextended regions trends son et al., 1998). This study focuses on the more with rotation (R) and fl attening (F) estimates north-south; however, in southwestern Utah the recently mapped areas to the east and southeast that range from R = –2° to –84° and F = +15° transition zone swings nearly 90° and trends (Hurlow, 2002; Rowley et al., 2006; Biek et al., to –14°. Many of the rotation estimates are roughly east-west, forming a major displace- 2009). We report paleomagnetic data from late ment transfer system known as the Caliente- Oligocene and Miocene volcanic and intrusive *Corresponding author e-mail: mspetro@nmhu Enterprise zone (CEZ) (Anderson and Mehnert, rocks from the eastern CEZ, which is the area .edu. 1979; Hudson et al., 1998; Fig. 2A). Here we thought to record the earliest phase of deforma-

Geosphere; June 2014; v. 10; no. 3; p. 534–563; doi:10.1130/GES00834.1; 14 fi gures; 5 tables. Received 11 June 2012 ♦ Revision received 26 August 2013 ♦ Accepted 2 April 2014 ♦ Published online 25 April 2014

534 For permission to copy, contact [email protected] © 2014 Geological Society of America

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active during vertical axis rotation, resulting in 118W 114W as much as 45°–85° of counterclockwise rota- tion, as evidenced by paleomagnetic data from Northern Basin the volcanic rocks in the area (Hudson et al., and Range Provence 1998). The central section of the CEZ overlaps the Caliente caldera complex, the eruption his- 40N tory of which lasted from ca. 23 to 12 Ma. The Caliente caldera is centered in eastern Nevada; UTAH the eastern edge of the caldera extends to within CALIFORNIA NEVADA 25 km of Enterprise, Utah. Early eruptions appear to have occurred before major exten- Sierra Nevada sion to the north or south during the Miocene, whereas the latter part of the caldera’s activ- CEZ ity occurred coincident with large-magnitude San Andreas Fault extension and detachment faulting to the north and south (Axen, 1998). The western section Central Basin of the CEZ is defi ned by the Pahranagat fault and Range Provence Colorado Plateau 36N zone (Tschanz and Pampeyan, 1970), which is composed of three northeast-striking sinistral ARIZONA faults and includes the area north of the fault. Southern Basin Pacific Ocean These faults have been interpreted to accommo- and Range Provence date varying degrees of extension that was trans- ferred from the Dry Lake Valley–Delamar Val- ley area to the Desert Valley west of the Sheep Range, where discrete faults of the system end Figure 1. Simplifi ed map of the central part of the western United (Liggett and Ehrenspeck, 1974; Wernicke et al., States showing the major tectonic provinces in the northern Basin 1984; Hudson et al., 1998). Areas north of the and Range Province. The trace of the San Andreas fault (dashed Pahranagat fault zone were also shown by Hud- line) is after Stewart (1988). CEZ—Caliente-Enterprise zone. Star son et al. (1995, 1998) to have rotated >15° represents the approximate location of the southern Delamar– counterclockwise. Meadow Valley Mountains and Condor Canyon reference areas. This area provides reference locations to base vertical axis rotation STRATIGRAPHY, LACCOLITHS, estimates for the Harmony Hills and Bauers Tuffs. AND GEOCHRONOLOGY

Ignimbrite Stratigraphy tion within the zone (Hacker, 1998). New paleo- the early Miocene. This has been interpreted magnetic data from the eastern part of the CEZ to refl ect increased crustal ductility beneath the Cenozoic volcanic rocks, dominated by allow us to assess the amount and variation of zone as the result of the ca. 23–12 Ma magma- Oligocene to Miocene ash-fl ow tuffs and lava vertical axis rotation where the CEZ interacts tism within the Caliente caldera complex (Hud- fl ows, as well as intrusive rocks, are abundant with the Iron Axis laccolith province along the son et al., 1998), in addition to the extension that and well exposed in southeastern Nevada and western margin of the Colorado Plateau and to occurred to the north of the adjacent Colorado southwestern Utah. The volcanic stratigraphy better defi ne the eastern extent of the zone. Plateau during the middle Miocene to Holocene of the eastern segment of the CEZ is, in part, (Axen, 1998). Two episodes of major Miocene the result of Iron Axis magmatic activity that GEOLOGIC SETTING to Holocene extension probably occurred north produced an assortment of ash-fl ow tuffs, lava of the CEZ and led to the current arrangement fl ows, and allochthonous gravity slide masses The CEZ trends east-northeast across the of basins and ranges. To the south, the Mio- related to the rapid emplacement and subsequent approximately north-south structural grain of the cene to Holocene extensional history was high- eruption of some of the Iron Axis laccoliths eastern Basin and Range Province for ~220 km lighted by a single, large-magnitude extensional (Hacker, 1998; Hacker et al., 2002). In addition, west from Cedar City, Utah, into southeastern event on three west-rooted detachment faults far-traveled ash-fl ow tuffs from the Caliente Nevada (Fig. 2A). This structural zone, mani- (Axen, 1998). caldera that predated and postdated Iron Axis fested as a right-stepping jog of the Colorado The eastern section of the CEZ is just south magmatism were deposited in the area, and Plateau, separates the northern Basin and Range of the Escalante Desert, where it forms a tilt bimodal volcanism continued throughout the Province from the narrower, but more highly domain boundary (Fig. 2A). Large-magnitude late Cenozoic and was accompanied by regional extended, central Basin and Range Province counterclockwise vertical axis rotations have extensional faulting. To assess the regional pat- (Wernicke et al., 1988) and coincides with the been recorded in the westernmost part of this tern of vertical axis rotation, we focused our northern boundary of the so-called amagmatic eastern section of the CEZ (Hudson et al., sampling on four readily identifi able ignim- corridor (Eaton, 1982) of the central Basin and 1998). Here, Miocene volcanic rocks located brites: the Leach Canyon Tuff, the Bauers Tuff Range Province. Throughout its Cenozoic his- south of the Escalante Desert are intersected Member of the Condor Canyon Formation, the tory, the CEZ was a zone of distributed sinistral by north-northwest– to west-northwest–strik- Harmony Hills Tuff, and the ash-fl ow tuff mem- shear that grew longitudinally westward after ing dextral fault systems that were probably ber of the rocks of Paradise. The ignimbrites are

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A

Figure 2 (on this and following two pages). Simplifi ed geologic maps of southeastern Utah. (A) Digital elevation model with simplifi ed geology and traces of Tertiary faults of the eastern Caliente-Enterprise zone (after Utah Geological Survey ArcGIS State Geographic Informa- tion System Database, SGID. U500.GeologicFormations and SGID.U500.GeologicFaults 1993; http:// gis .utah .gov /data/). Laccoliths: TP—Three Peaks; GM—Granite Mountain; IM— Iron Mountain; SM—Stoddard Mountain; PV—Pine Valley.

lithologically distinctive and of the appropri- Oligocene calc-alkaline andesite to rhyolite Mesozoic and Cenozoic sedimentary strata at ate age to have undergone the main phases of ash-fl ow tuffs of the Wah-Wah Springs Forma- upper crustal levels in this area (Mackin, 1960; Cenozoic deformation in the area (Table 1). The tion (ca. 30 Ma), Isom Formation (ca. 27 Ma), Blank and Mackin, 1967; Blank et al., 1992; Leach Canyon and Bauers Tuffs originated from and Quichapa Group (ca. 24–22.5 Ma) were Rowley et al., 1995; Rowley, 1998). The Three the Caliente caldera complex more than 100 km erupted over the area from sources to the north- Peaks, Granite Mountain, and Iron Mountain to the northwest; the Harmony Hills Tuff origi- west and west. This sequence of pre–Iron Axis laccoliths were emplaced into gently tilted lime- nated from the nearby Bull Valley Mountains; volcanic rocks overlies fl uvial and lacustrine stone strata of the Jurassic Carmel Formation or and the rocks of Paradise tuff erupted from the sedimentary rocks of the late Paleocene–Oligo- the Temple Cap Formation. Emplacement into Pinto Peak intrusion (Hacker et al., 2002). cene Claron Formation, which unconformably carbonate strata of the Carmel Formation pro- overlies Cretaceous and Jurassic sedimentary duced several replacement magnetite, hematite, Iron Axis Laccoliths rocks deformed during the Sevier orogeny. The hemo-ilmenite, and ilmeno-hematite deposits igneous rocks are part of the middle Cenozoic that were extensively mined and together con- The Iron Axis laccolithic group consists of a calc-alkaline igneous sequence that spans much stitute the largest iron ore production in the series of early Miocene calc-alkaline hypabys- of the western United States. These intrusions western United States (Mackin, 1960; Blank sal intrusions and associated volcanic rocks. The are considered to be associated with oblique and Mackin, 1967; Barker, 1995). The studied three laccoliths from which we collected paleo- convergence during subduction of oceanic intrusions are quartz monzonite to granodio- magnetic data (Three Peaks, Granite Mountain, lithosphere beneath western North America that rite porphyries with phenocrysts of plagioclase Iron Mountain) are part of a latest Oligocene produced large fl uxes of mantle-derived mafi c (andesine-labradorite), biotite, hornblende, to earliest Miocene laccolith swarm that strad- magma intruded into the overlying continental and/or pyroxene (diopsidic augite), and mag- dles the western edge of the Colorado Plateau lithosphere (Johnson, 1991; Nelson and David- netite in a groundmass (~33%–50% total vol- in southwestern Utah and are exposed ~30 km son, 1998; Rowley, 1998). Emplacement of ume) of very fi ne grained quartz and potassium west of Cedar City (Fig. 2A). Prior to laccolith the Iron Axis laccoliths was focused along one feldspar (Petronis et al., 2004). Mafi c nodules emplacement, several large-volume, regional or more Sevier-age thrust faults that displace are rare and generally <5 cm along their long

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Figure 2 (continued). (B) Ash- fl ow tuff site locations and refer- ence site locations for the Leach Canyon Tuff and rocks of Para- dise tuff. HM—Harmony Hills; PP—Pinto Peak; MM—Moun- tain Meadows; BVM—Bull Valley Mountains; GV—Grass Valley; PVL—Pine Valley lac- colith.

axis. The intrusions yielded K-Ar mineral dates 2004; Uehara et al., 2010; Petronis et al., 2011). and the formation of abundant ilmenite lamellae and 40Ar/39Ar mineral age spectra determination The distributions of opaque grains in the Iron with a classic trellis oxidation exsolution pattern values of 22–20 Ma (Armstrong, 1972; Hacker Axis laccoliths are somewhat typical of many (e.g., similar to the Widmanstatten texture often et al., 1996; McKee et al., 1997; Rowley et al., felsic to intermediate intrusions where sub- seen in iron meteorites) (Figs. 3C, 3D). The 2006), indicating a 2 m.y. period of Iron Axis equant grains (50–100 μm) occur predominantly oxy-exsolved lamellae show sharp, well-defi ned magmatic activity (Petronis et al., 2004). as interstitial material in the silicate framework contacts with their titanomagnetite hosts and in volume concentrations of 0.5%–2.0% (Fig. appear smooth in outline. The ilmenite lamellae Magnetic Oxide Petrology of the 3A). The opaque grains are typically Fe-Ti are typically concentrated along cracks, around Iron Axis Laccoliths oxides and closely associated spatially with bio- silicate inclusions, and along the titanomagne- tite and hornblende crystals, a textural relation- tite grain boundaries (Fig. 3D). These textural The magnetic oxide mineralogy of the Iron ship seen in other shallow igneous intrusions features are often interpreted as evidence that Axis laccoliths provides important information (Speer and Becker, 1992; Stevenson et al., 2007; low-temperature subsolidus oxidation and not on the cooling rate, temperature, and chemistry Petronis et al., 2011). The grains occur as roughly magmatic oxidation exsolution is responsible of the magma and can refl ect the igneous and equidimensional crystals to 100 μm in size and for the formation of ilmenite lamellae from a hydrothermal history as revealed by their com- also as very fi ne material distributed through- primary titanomagnetite phase (Lindsley, 1991, position, morphology, and textural relationships out the groundmass (Figs. 3A, 3B). The Fe-Ti and references therein). to the silicate mineral phases (Lindsley, 1991; oxides include an abundance of titanomagnetite Petrographic analysis, paleomagnetic, and Speer and Becker, 1992, and references therein). and titanomaghemite that in some cases have rock magnetic data indicate there are at least In turn, combining these observations with altered to hemo-ilmenite or ilmeno-hematite, two separate magnetic mineral assemblages detailed rock magnetic data provides a powerful and rare unoxidized magnetite (Fig. 3C). Many that contribute to the remanent magnetization tool to fully characterize the cooling history and relatively coarse magnetite grains show evi- of each intrusion; it is interesting that one of postemplacement alteration of the intrusions dence of apparent high-temperature oxidation the magnetic mineral assemblages dominates (e.g., Alva-Valdivia et al., 2001; Lagrou et al., exsolution (Buddington and Lindsley, 1964) at each site. The fi rst assemblage is charac-

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Figure 2 (continued). (C) Iron Axis laccolith site locations. Paleo- magnetic data for the Iron Axis Intrusions are referenced to a late Miocene expected fi eld direction (declination, D = 358°,

inclination, I = 58°, α95 = 6.3°; Mankinen et al., 1987). EMH— Eight Mile Hills; SH—Swatt Hills; AR—Antelope Range; HM— Harmony Mountains. The coordinate system used was UTM Zone 12, Northing and Easting.

terized by nearly pure magnetite to low-Ti axis of the silicate minerals that are host to these slow cooling (Carmichael, 1961), which is titano magnetite. The minerals associated with grains, most commonly biotite and hornblende unexpected considering the fi eld evidence for magnetite-dominated sites are hematite, plagio- (Fig. 3C). The presence of ilmeno-hematite very shallow emplacement of the laccoliths clase, quartz, biotite, epidote, and hornblende. with hemo-ilmenite lamellae is unusual for (e.g., abundant drusy or miarolitic cavities). Magnetite grains typically follow grain bound- rocks of this composition, possibly indicating The coarse size of the ilmeno-hematite phase aries, especially as a product of oxidation of hornblende (Fig. 3B). The amount of hematite TABLE 1. NEW AND PUBLISHED AGE DETERMINATIONS FROM replacement varies from site to site. In some IGNIMBRITES AND LACCOLITHS SAMPLED IN THIS STUDY cases, there is almost no hematite present, yet at Age some sites it constitutes as much as 50% of the Formation (Ma) Method Reference oxide area. The second magnetic mineral assem- Previously published age determinations Iron Axis laccoliths 21.96 ± 0.11 to 20.20 ± 0.05 40Ar/39Ar Rowley et al. (2006); Biek et al. (2009) blage is dominated by ilmeno-hematite (Fig. 3C). Rocks of Paradise 21.93 ± 0.15 40Ar/39Ar Hacker et al. (1997, 2002) The minerals characteristic of the ilmeno-hema- Harmony Hills Tuff 24.4–20.3 K-Ar Best et al. (1989) Bauers Tuff 22.8 40Ar/39Ar Rowley and Siders (1988) tite–dominated sites include plagioclase, quartz, Leach Canyon 24.0 40Ar/39Ar Rowley et al. (1995); Scott et al. (1995) biotite, white mica, pyroxenes, chlorite, and This study Technique hornblende. The ilmeno-hematite crystals con- Harmony Hills 22.03 ± 0.15 40Ar/39Ar Plateau age tain lamellae of hemo-ilmenite (ferroan ilmenite) Rencher Formation 21.83 ± 0.17 40Ar/39Ar Plateau age that are mostly aligned parallel with the long 21.46 ± 0.40 40Ar/39Ar Isochron age

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/3/534/3333171/534.pdf by guest on 01 October 2021 Paleomagnetic results from the eastern Caliente-Enterprise zone, SW Utah B m Hb m μ μ 5 50 Mt Hb low cooling (Carmichael, 1961), Il B ective Fe-Ti oxide grain exhibiting an ective Fe-Ti pattern (e.g., Widmanstatten texture). P— texture). Widmanstatten pattern (e.g., n with the silicate matrix. The slightly lower The slightly lower n with the silicate matrix. Hb Mt ection grains are Fe-Ti oxide grains set against Fe-Ti ection grains are Mt Il Mt Il Hb B D P m C just above their Curie temperature. (D) A highly refl A (D) Curie temperature. C just above their m ° μ μ 100 300 Mt Mt P Mt ne-grained apparently randomly distributed titanomagnetite. (B) Relatively euhedral magnetite crystals in ne-grained apparently B Hb Mt B P Ilhm B Mt P B P Mt Ilhm Mt Mt P Hb ectance silicate grains. (A) High concentrations of fi ection grains in close association with titanomagnetite grains are ilmeno-hematite. The presence of ilmeno-hematite suggests s The presence ilmeno-hematite. ection grains in close association with titanomagnetite are Ilhm A C plagioclase feldspar; B—biotite; Hb—hornblende; Mt—magnetite (titanomagnetite); Il—ilmenite; Ilhm—ilmeno-hematite. Figure 3. Backscatter electron microprobe images of select samples from the Iron Axis laccoliths. The bright, high index of refl Axis laccoliths. the Iron images of select samples from microprobe electron 3. Backscatter Figure refl the lower close spatial association with hornblende. (C) High concentrations of coarse-grained titanomagnetite grains in associatio index of refl between 570 and 600 large size indicates an exsolution temperature and their oxidation oxidation exsolution and the formation of abundant ilmenite lamellae with a classic trellis high-temperature apparent

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indicates an exsolution temperature between standing of the timing of large magnitude defi ned plateau age (8 steps constituting 91% 570–600 °C, just above the Curie temperature pyroclastic volcanism in the area, we obtained of the total 39Ar released) of 22.03 ± 0.15 Ma of phases of this composition, likely allowing 40Ar/39Ar incremental release age determina- (Fig. 4). This date is indistinguishable from a the ilmeno-hematite grains to acquire a thermo- tions on plagio clase from two tuff units previ- 21.93 Ma 40Ar/39Ar biotite date reported for the remanent magnetization (TRM) and retain a ously dated by the K/Ar method. Separates immediately overlying ash-fl ow tuff member of remanence that is comparable in direction with were analyzed using standard procedures at the rocks of Paradise (Hacker et al., 1996, 2002). those sites dominated by magnetite-titano- the University of Nevada, Las Vegas Center for We also obtained a plateau age of 21.83 ± 0.17 magnetite assemblages (Carmichael, 1961). Geochronology (methods described and ana- Ma (4 steps, 55% total 39Ar) and an associated lytical data reported in Cornell, 2005; Cornell concordant isochron age of 21.46 ± 0.40 Ma Geochronology et al., 2001). The age of the Harmony Hills on the Rencher Formation (Fig. 4). This new Tuff (Quichapa Group), a key unit deformed age is consistent with a 21.93 Ma 40Ar/39Ar age Historically, K/Ar age determinations have by all intrusions and emplaced across the CEZ, reported by Hacker et al. (1996) for the rocks provided the main age constraints on the Ceno- has been only poorly defi ned by 6 previously of Paradise tuff directly underlying the Rencher zoic volcanic stratigraphy and Iron Axis mag- reported K/Ar dates ranging from ca. 24.4 to Formation. Collectively, these data better defi ne matism of this region. To improve our under- 20.3 Ma (Best et al., 1989). We obtained a well- the age of key units sampled for this study

A Harmony Hills Tuff B Rencher Formation 50 40 to 152.99 +/– 4.35 Ma

40 Plateau age = 22.03 +/– 0.15 Ma Plateau age = 21.83 +/– 0.17 Ma 91.2% of39 Ar 55.8% of39 Ar Steps 3 through 10 30 Steps 5 through 8 30 Age (Ma) Age (Ma) 20 20

A-1 KSU-003-plagioclase 10 B-1 KSU-004-plagioclase 10 0 20 40 60 80 100 39 020406080100 Fraction Ar Released Fraction39 Ar Released 0.006 Age = 20.99 +/– 0.17 Ma Age = 21.46 +/– 0.40 Ma 0.0032 40Ar/ 36 Ar = 286 +/– 9.5 40Ar/ 36 Ar = 196.0 +/– 55.0 MSWD = 1.7 MSWD = 1.9 Steps 2 through 8 Steps 5 through 9 0.0024 0.004

0.0016 Ar/ Ar 36 40 Ar/ Ar

36 40 0.002

0.0080

A-2 B-2 0.0000 0.000 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.00 0.04 0.08 0.12 39Ar/ 40 Ar 39Ar/ 40 Ar

Figure 4. 40Ar/39Ar age determinations from the Harmony Hills Tuff and Rencher Formation. Ar/Ar release spectra (–1) and (–2) isochron plot on plagioclase separates obtained from the Harmony Hills Tuff and Rencher Formation. (A) The Harmony Hills Tuff yields a well- defi ned plateau age with 8 steps constituting 91% of the total 39Ar released to yield 22.03 ± 0.15 Ma. (B) The Rencher Formation yields a plateau age of 21.83 ± 0.17 Ma defi ned by 4 steps constituting 55% total 39Ar released and an associated concordant isochron age of 21.46 ± 0.40 Ma. MSWD—mean square weighted deviation.

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(Table 1) and show that widespread explosive sive ash-fl ow tuff (Wells and Hillhouse, 1989; they were available for sampling in close prox- magmatism took place in the study area over a Byrd et al., 1994; Petronis and Geissman, 2008; imity to each other and because the nearby Stod- short time interval. Sussman et al., 2011). Paleomagnetic investiga- dard Mountain laccolith shows little evidence of tions of ignimbrites reveal that individual cool- vertical axis rotation relative to the expected MOTIVATION AND REFERENCE ing units typically yield an instantaneous record Miocene fi eld direction (Petronis et al., 2004). LOCATION of the geomagnetic fi eld (e.g., Reynolds, 1977; Paleomagnetic data from the Stoddard Moun- Geissman et al., 1982; Wells and Hillhouse, tain laccolith yielded a time-averaged rema- The purpose of this study is to expand upon 1989; Byrd et al., 1994). Thus, combining nence direction (declination, D = 351.1°, incli- α the research of Hudson et al. (1998) in a part of results from a small number of discrete ash-fl ow nation, I = 57.8°, 95 = 3.7°) that is statistically the CEZ in southwestern Utah. Until recently, tuffs will not provide an adequate long-term indistinguishable from a late Miocene expected α this area was poorly studied due to the limited average of the geomagnetic fi eld. Consequently, fi eld direction (D = 358°, I = 58°, 95 = 6.3°; availability of geologic maps documenting fi eld our analysis of the data from the ignimbrites Mankinen et al., 1987; Fig. 5) implying little, if relations in the easternmost part of the zone. we studied involved comparing results from any, vertical axis rotation of the reference area Hudson et al. (1998) suggested that the CEZ each tuff to data obtained from what we inter- (rotation, R = –6.9° ± 11.3°; fl attening, F = 0.2° did not extend much farther than ~10 km east pret to be internally coherent reference sec- ± 7.1°). In Grass Valley, the average dip of the of Newcastle (Fig. 2A). Many of our sampling tions. Two reference areas were chosen (Figs. ignimbrite deposits, based on the orientation of sites, however, are located along the far eastern 1 and 2A). One area encompasses the south- eutaxitic fabrics and contacts between units, is margin of the zone in an area that exhibits a pat- ern Delamar and Meadow Valley Mountains ~5° (Rowley et al., 2006). Given the low dips of tern of faulting very similar to that of the cen- and farther north in Condor Canyon in south- these units and the proximity of this section to tral and western part of the CEZ (Butler et al., eastern Nevada (Fig. 1) (Hudson et al., 1995, the stable Colorado Plateau margin, we assume 2001). Hudson et al. (1998) documented some 1998). This area provides reference locations that these rocks underwent minimal vertical axis counterclockwise vertical axis rotation within to base vertical axis rotation estimates for the rotation and that the current dip of the deposits the Harmony Mountains and Desert Mount Harmony Hills and Bauers Tuffs. The locations is related to recent extension responsible for the areas between Newcastle and Cedar City and were selected because the southern Delamar and present physiography of the region. reported localized counterclockwise rotations of Meadow Valley Mountains are within a zone of All paleomagnetic data from the ignim- 20°–45° ~10 km south of Cedar City (Fig. 2A). mild extension (Wernicke et al., 1988; Scott brites, following correction for local tilt based This study spans an ~35 km × 70 km area north et al., 1995) and Condor Canyon, southeast of on orientation of eutaxitic structures, contacts of the early Miocene Pine Valley megalaccolith Pioche, Nevada, provides exposures of numer- between volcanic units, or stratifi cation in over- in the eastern CEZ. The Pine Valley intrusion is ous Oligocene to early Miocene ash-fl ow tuffs. lying or underlying volcaniclastic sedimentary within the stable unextended Colorado Plateau, Hudson et al. (1998) showed that the combined deposits, are compared to data from these refer- and therefore serves as a marker for the southern results from the Delamar and Meadow Valley ence sections (Fig. 5). The Grass Valley refer- boundary of the CEZ in this region. Mountains and Condor Canyon provide a time- ence directions for this study were established in averaged paleomagnetic result that indicates no the Leach Canyon Tuff and the rocks of Paradise Paleomagnetism Reference Section signifi cant rotation with respect to the North tuff and the Bauers and Harmony Hills Tuffs are American craton. Our second reference area is referenced to the Delamar and Meadow Val- The ignimbrites sampled in this study were located in Grass Valley, just west of the Stod- ley Mountains and Condor Canyon (Hudson erupted across the Pine Valley region and adja- dard Mountain laccolith (Fig. 2A). We selected et al., 1998). The reference directions for each cent areas and were subsequently faulted, with Grass Valley as a reference area for the rocks of tuff were computed by averaging several mean the formation of numerous structural blocks Paradise tuff and Leach Canyon Tuff because directions for sites within the reference area during Cenozoic extension and the development of the CEZ. Observed paleomagnetic declina- tions that deviate or are discordant from some N expected value can provide rotation estimates in either an absolute or a relative framework. Abso- Figure 5. Equal area projection lute rotation determinations require a result that of reference section corrected adequately samples the geomagnetic fi eld over group mean paleomagnetic a suffi ciently long time. For example, a slowly directions from the Bauers cooled, large volume intrusion or a sequence of Tuff, Harmony Hills Tuff, numerous basalt fl ows showing multiple polar- Leach Canyon Tuff, and rocks of Paradise tuff. Also shown ity zones and directional dispersion that is con- + sistent with an adequate sampling of paleosecu- are group mean results from the lar variation is typically regarded as capable of Stoddard Mountain (Petronis Harmony Hills Tuff averaging paleosecular variation. If signifi cant et al., 2004) and Iron Axis lac- Bauers Tuff coliths (this study). Solid sym- time has elapsed, the result is then compared Rocks of Paradise Tuff with a suffi ciently robust estimate of the time- bols are projections onto lower averaged fi eld (paleomagnetic poles) based hemisphere; open symbols are Leach Canyon Tuff on independent paleomagnetic data from the projections onto upper hemi- Stoddard Mountain respective craton. Relative rotation determina- sphere. Laccolith tions, however, require sampling a single, later- Iron Axis Laccoliths ally extensive datum such as a regionally exten-

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(Tables 2A, 2B; Fig. 5). With the exception of opportunity for high-precision estimates of METHODS the Leach Canyon Tuff, each ignimbrite at the declination discrepancies across the eastern reference section has a unique characteristic CEZ; any deviation of the ChRM at individual Field Methods remanent magnetization (ChRM) that is well sites compared to the reference section, when defi ned and of low dispersion. The average ref- inclinations are essentially identical, is inter- Samples were drilled at each site over an erence directions, following correction of dip, preted as refl ecting vertical axis rotation. The area of ~10 m2, and for stratifi ed materials for the ignimbrites are as follows. The Harmony Leach Canyon Tuff yields a mean direction of at least 5 strike and dip measurements were Hills Tuff yields a mean direction of D = 352.1°, D = 066.0°, I = –10.7°, also consistent with an obtained for each fl ow. All samples were col- I = 50.3°, the Bauers Tuff D = 349.3°, I = 44.3°, intermediate polarity state. The directional data lected using a portable gasoline-powered and the rocks of Paradise tuff D = 198.3°, I = from the Leach Canyon Tuff are highly variable drill with a nonmagnetic diamond tip drill bit 20.1° (Table 2C; Fig. 5). The ChRM of the Har- across the study area and we place little confi - (Fig. 2A). Paleomagnetic data from 61 ignim- mony Hills and Bauers Tuffs are interpreted to dence in the overall results from this ignimbrite brite sites were collected along 25–35-km- be of normal polarity. The rocks of Paradise tuff (see following). All paleomagnetic data from wide (i.e., varying width along the traverse) is interpreted to be of intermediate polarity as the Three Peaks, Granite Mountain, and Iron north-south transverse north of the Pine Val- the virtual geomagnetic pole (VGP) derived Mountain laccoliths are compared to the middle ley Mountains (Fig. 2B). Paleomagnetic data from the ChRM mean direction is >45° from Miocene expected fi eld direction (D = 358°, I = from the intrusions were collected from sites α the North American paleomagnetic pole posi- 58°, 95 = 6.3°; Mankinen et al., 1987) and pro- across the best exposed and accessible parts of tions for early Miocene time. The well-defi ned vide an absolute rotation estimate relative to this the laccoliths (Fig. 2C). The site mean results ChRM of these three ignimbrites affords the direction. for the ignimbrites and intrusions are shown in

TABLE 2A. GRASS VALLEY REFERENCE PALEOMAGNETIC SITES D I Paleomagnetic (in situ) (in situ) Strike, dip D I Northing α sampling site N/No R 95 k (°) (°) (°) (corrected) (corrected) UTM Zone 12 Easting Leach Canyon Tuff GV-4 8/8 7.85 8.3 46.0 65.3 –7.6 092, 7NE 66.0 –10.7 4146572 281352 Rocks of Paradise tuff GV-1 8/10 7.94 5.0 122.3 193.7 13.4 092, 4NE 193.9 17.3 4147776 282018 GV-9 9/9 8.93 4.7 120.0 197.6 34.4 036, 5SE 194.5 32.7 4146211 282405 GV-10 7/7 6.94 7.4 82.8 202.0 24.2 036, 5SE 199.9 22.9 4146098 282476 GV-11 9/9 8.86 6.9 56.2 201.7 31.3 036, 5SE 198.8 29.9 4146098 282476 GV-12 10/10 9.96 3.3 221.3 197.9 9.9 036, 5SE 197.1 8.3 4146098 282476 GV-13 8/8 7.98 2.6 441.7 205.6 10.0 036, 5SE 204.8 9.1 4146098 282476 α Note: N/No—ratio of samples used (N) to samples collected (No) at each site; R—resultant vector length; 95—95% confi dence interval about the estimated mean direction, assuming a circular distribution; k—best estimate of (Fisher) precision parameter; D (in situ)—in situ declination; I—inclination. Strike and dip of unit are based on compaction fabric, fl ow contacts, or fi amme. D, I (corrected)—stratigraphically corrected. Coordinates—Northing, Easting, UTM (Universal Transverse Mercator), Zone 12, WGS84 (World Geodetic System 1984).

TABLE 2B. SOUTHERN DELAMAR MOUNTAINS, MEADOW VALLEY MOUNTAINS, AND CONDOR CANYON REFERENCE PALEOMAGNETIC SITES D I Paleomagnetic (in situ) (in situ) Strike, dip D I α sampling site N/No R 95 k (°) (°) (°) (corrected) (corrected) Lat Long Harmony Hills B90-2 8/8 7.98 2.9 374.0 348.8 49.3 275, 4N 349.9 45.4 37.083 114.867 B90-5 8/8 7.98 2.7 415.0 353.0 53.2 355, 5E 359.7 53.1 37.208 114.958 8P377* 8/8 7.99 2.5 509.0 315.9 47.8 341, 26E 347.3 51.9 37.859 114.325 Bauers Tuff B90-4 8/8 7.97 3.8 211.0 343.4 40.6 000, 9E 351.2 42.6 37.209 114.958 OPO49 3/3 3.00 4.8 671.0 339.1 57.4 303, 12NE 350.9 49.4 37.135 114.796 OP033 8/8 7.99 2.2 608.0 347.0 52.9 303, 12NE 355.9 44.0 37.135 114.796 8P369* 8/8 7.98 2.9 369.0 319.2 43.8 330, 25NE 343.5 43.3 37.858 114.327 α Note: Site data after Hudson et al. (1998). N/No—ratio of samples used (N) to samples collected (No) at each site; R—resultant vector length; 95—95% confi dence interval about the estimated mean direction, assuming a circular distribution; k—best estimate of (Fisher) precision parameter; D, I (in situ)—in situ declination and inclination. Strike and dip of unit are based on compaction fabric, fl ow contacts, or fi amme. D, I (corrected)—stratigraphically corrected. Latitude and longitude of the site—North American Datum 27. *Sites located in Condor Canyon.

TABLE 2C. FORMATION MEAN PALEOMAGNETIC REFERENCE DIRECTIONS D I α Formation S/So R 95 k (°) (°) Reference* Harmony Hills Tuff 3/3 2.9897 8.9 194 352.1 50.3 Hudson et al. (1998) Bauers Tuff 3/4 2.9943 6.6 352 349.3 44.3 Hudson et al. (1998) Leach Canyon Tuff 1/1 7.852 8.3 46.0 66.0 –10.7 This study Rocks of Paradise tuff 6/6 5.909 9.1 65.6 198.3 20.1 This study α Note: S/So—ratio of sites used (S) to sites collected (So) at each reference location; R—resultant vector length; 95—95% confi dence interval about the estimated mean direction, assuming a circular distribution; k—best estimate of (Fisher) precision parameter; D, I—declination and inclination. *Source used to calculate reference location.

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Figures 6 and 7, respectively, and associated Ignimbrites rocks of Paradise tuff. Twelve sites were elimi- statistics are listed in Tables 3 and 4, respec- nated from further consideration because they tively. All laboratory and rock magnetic meth- Of the 61 sites, 49 yield readily interpretable did not yield stable end-point behavior and/or ods are shown in Appendix 1 and Appendix 2 results. These include 13 sites from the Harmony had high within-site dispersion (see following). and the rock magnetic results are presented in Hills Tuff, 16 sites from the Bauers Tuff, 11 sites Of the 49 sites considered acceptable for further Appendix 3. from the Leach Canyon Tuff, and 9 sites from the analysis, seven were established in the Grass Valley reference locality (Fig. 2B; Table 3); thus 42 sites in the ash-fl ow tuffs provide relative N N rotation estimates with respect to the reference A B locations across the CEZ. Bauers Harmony Hills Laccoliths

A total of 82 paleomagnetic sites were estab- lished in the Three Peaks, Granite Mountain, Iron Mountain laccoliths (Fig. 2C). The site mean results are shown in Figure 7 and associ- ated statistics are listed in Table 4. These include 46 sites from the Three Peaks laccolith, 14 sites N from the Granite Mountain laccolith, and 22 sites D from the Iron Mountain laccolith. Of the 82 sites C + collected, 53 sites were used to calculate a grand mean for the 3 intrusions (Table 4). From the 46 Leach Canyon sites analyzed in the Three Peaks laccolith, 39 yield readily interpretable results with 35 sites Rocks of used to calculate a group mean result. Seven Paradise sites were rejected as they did not yield stable end-point behavior and were likely lightning + struck. The remaining four sites excluded from the group mean have well-defi ned site mean directions, but they are separated by greater than two angular standard deviations from the overall Figure 6. Lower hemisphere equal area projection of corrected site mean paleomagnetic group mean of the other 35 sites. For the Gran- directions for each ignimbrite. Black star is reference section group mean (see Fig. 5; Table ite Mountain sites, eight sites yield interpretable 2B). Black arrow indicates rotation relative to the reference location. Solid symbols are pro- data with seven sites used to calculate a group jections onto lower hemisphere; open symbols are projections onto upper hemisphere. See mean for the intrusion. The six rejected sites did Figure 9 for the map-view spatial distribution of the rotation estimates. not yield stable end-point behavior and showed

N N N A B C D=291.9, I=57.6, R=10.0 α K=11.0,95 =15.2, N=11

Svgp =26.6

X X X D=332.9, I=51.7, R=34.36 D=353.2, I=45.2, R=6.69 α α K=52.9,95 =3.4, + + K=22.5,95 =14.1, +

N=35, Svgp =13.1 N=7, Svgp =16.3

Figure 7. Lower hemisphere equal area projection of in situ site mean paleomagnetic directions for the Iron Axis laccoliths. D—declination;

I—inclination; R—rotation; F—fl attening; R—resultant vector; k—best estimate of the precision parameter; α95—95% confi dence ellipse

about the mean; Svgp—angular dispersion of the virtual geomagnetic poles. (A) Three Peaks laccolith (35N); R = –22.2° ± 6.4° and F = 8.8° ± 4.3°. (B) Iron Mountain laccolith (4R,7N); R = –54.4° ± 28.4° and F = –2.6° ± 14.3°. (C) Granite Mountain laccolith (7N); R = –4.8° ± 21.6° and F = 12.8° ± 15.3°). Blue star with ellipse—group mean result; Red X with ellipse—expected fi eld direction.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/3/534/3333171/534.pdf by guest on 01 October 2021 Petronis et al. F) F Δ Δ (±°) R, Δ F (°) R Δ (±°) R (°) Easting WGS1984 Northing Rotation (R) and fl attening (F) and associated error estimates ( Rotation (R) and fl UTM Zone 12 ) d e t on compaction fabric, fl ow contacts, or fi amme. D, I (corrected)—stratigraphically ow contacts, or fi on compaction fabric, fl c e r r o c ( DI F error (see Petronis et al., 2009). Δ —95% confi dence interval about the estimated mean direction, assuming a circular distribution; —95% confi 95 α (°) Strike, dip ) u t i s n i ( —resultant vector length; DI R k 95 α R TABLE 3. PALEOMAGNETIC DATA AND ROTATION ESTIMATES FROM THE IRON AXIS REGION, SOUTHWESTERN UTAH THE IRON FROM ESTIMATES AND ROTATION DATA 3. PALEOMAGNETIC TABLE name N/No Formation N/No—ratio of samples used (N) to collected (No) at each site; *Site rejected, low-confi dence rotation estimates. Statistically signifi cant fl attening (F) estimate >10° beyond the cant fl dence rotation estimates. Statistically signifi *Site rejected, low-confi Note: —best estimate of (Fisher) precision parameter. D, I—declination and inclination (in degrees). Strike dip of unit are based —best estimate of (Fisher) precision parameter. are after Beck (1980) and Demarest (1983) with respect to the Grass Valley ignimbrite section. See text for discussion. are after Beck (1980) and Demarest (1983) with respect to the Grass Valley corrected. Site location coordinates: UTM, Zone 12, WGS 84 (UTM—Universal Transverse Mercator; WGS—World Geodetic System 1984). Mercator; WGS—World Transverse corrected. Site location coordinates: UTM, Zone 12, WGS 84 (UTM—Universal Holt Canyon sites HC-4* Leach Canyon 8/8 7.944 5.0 122.2 67.9 –56.4 070, 6NW 76.9 –56.1 4160028 269004 10.9 12.1 –45.4 7.0 P-1 Paradise 8/8 7.975 3.3 282.1 180.5 14.2 280, 7NE 180.1 21.1 4151504 280272 –18.2 7.6 –1.0 14.3 Pine Valley sites Pine Valley GV-2GV-3 Harmony Hills Bauers 7/10 6.95 9/9 5.5 8.76 122.4 9.1 13.8 33.1 63.3 092, 340.9 4NE 45.4 12.4 092, 7NE 59.4 343.1 4147776 38.8 282018 4146572 20.3 281352 12.3 –6.2 –9.1 8.2 5.5 5.5 9 HC-1*HC-2HC-3* Bauers Bauers Leach Canyon 8/8 8/8 8/8 7.979 7.765 7.992 3.0 10.5 332.7 1.9 28.9 252.4 857.7 273.7 –64.8 300.5 –32.1 340, 25SW 47.7 059, 25SE 150.0 323, 282.5 9SW –89.0 –16.1 292.2 4159064 43.6 4158396 4158570 269744 269784 84.0 –66.8 269509 71.2 –57.1 7.9 –78.3 5.3 60.4 5.4 11.3 0.7 5.1 Paleomagnetic sampling site P-11* Leach Canyon 6/6 5.992 2.6 664.7 51.4 –40.4 071, 35NW 83.8 –42.6 4158146 276896 17.8 8.5 –31.9 6.6 Pinto sites k PV-1PV-3PV-4PV-5PV-6 Leach Canyon Harmony Hills Harmony Hills 7/9 Harmony Hills 8/9 Bauers 9/9 6.936 10/10 7.645 8.897 9.939 6.3 13.2 6/6 6.8 5.3 92.2 18.6 5.963 112.2 101.6 67.3 12.0 –18.3 5.8 358.5 13.6 50.4 308, 5NE 134.2 57.5 61.1 308, 5NE 103.2 344.5 075, 075, 6NW 6NW –20.5 14.3 356.7 33.0 9.3 45.9 51.6 058, 4142491 5NW 55.7 343.7 4142580 4144552 278188 28.2 4144426 278309 37.2 276546 4144372 276424 22.2 8.3 4.6 17.2 276379 17.4 –9.8 12.9 12.3 –5.6 13.8 4.4 –1.3 –5.4 7.5 9.4 6.4 5.6 16.1 6.9 Grass Valley sites Grass Valley P-17*P-18 Leach Canyon Bauers 8/9 7.989 8/8 1.9 7.994 748.3 1.6 327.8 1250.6 –87.9 280.6 014, 21SE 287.8 55.2 –67.4 046, 12NE 288.2 4162737 44.9 276887 4163142 221.8 10.5 276957 –56.7 –61.1 5.1 5.3 –0.6 5.0 P-4P-5*P-10 Bauers Canyon Leach Bauers 9/9 7/8 8.970 8/8 6.937 3.2 7.993 262.6 5.3 1.7 138.7 110.1 1027.4 –37.5 349.8 314.2 298, 50SW –5.7 70.8 96.9 46SW 298, 062, 33NW –37.3 343.0 324.6 29.0 4152513 38.4 4152457 279945 4145601 30.9 279928 273356 –6.3 8.4 –24.7 –26.6 5.7 5.0 7.5 15.3 5.9 8.4 5.6 HC-5HC-6* Bauers Harmony Hills 11/11 10.983 9/9 1.9 8.972 576.1 3.1 37.5 281.1 54.9 267.8 347, 50SW 60.1 188.1 063, 6NW 73.8 276.4 57.2 4160691 4160555 268463 –164.0 268695 9.6 –72.9 –23.5 6.8 5.3 –12.9 4.6 P-14P-15P-16* Harmony Hills Bauers Leach Canyon 10/10 8/10 9.968 7.940 8/8 2.9 4.5 278.0 7.971 131.6 305.9 3.6 113.0 243.6 8.8 –63.7 303.8 028, 33SE 035, 25SE 308.4 11.8 118.2 055, 34SE 41.4 –39.0 296.1 4160560 4161392 42.8 276607 276659 4160804 –43.7 48.2 276481 7.3 9.0 –53.2 –28.3 8.9 5.9 7.6 7.3 1.5 5.7 P-2P-3 Paradise Harmony Hills 16/17 15.901 8/9 7.913 2.8 160.1 6.3 350.9 79.3 4.8 183.9 300, 50SW 22.9 334.6 063, 17SE 40.3 181.5 8.1 4152364 4152181 279970 –17.5 279877 7.2 –16.8 10.0 16.9 12.0 7.4 6.4 PV-7*PV-10PV-11PV-12PV-13 BauersPV-14 ParadisePV-15 Harmony Hills Bauers Canyon Leach 9/9 Bauers 8/8 7/7 Harmony Hills 7/8 8.983 7.737 6.960 8/8 8/9 6.921 2.4 11.2 8/8 4.9 7.969 7.620 7 482.9 25.5 150.2 7.984 11.8 3.6 332.8 74.6 192.7 2.7 227.3 2.7 20 51.5 14.7 429.8 93.3 353.4 22.5 329, 8NE 45.0 –16.9 329, 8NE 35.7 067, 13.2 8NW 342.6 306, 190.9 51.7 9NE 292, 11NE 41.8 50.3 1.5 352, 60SW 20.1 356.4 96.0 352, 60SW 302.8 15.3 –21.6 4145415 319.5 25.9 4158110 55.2 4143692 34.5 4144878 4144507 273381 276851 4147580 4147691 305690 –9.5 273396 273415 –7.4 268009 9.9 9.4 30.0 268012 8.2 7.1 –49.3 –29.8 10.7 0.0 9.2 19.0 0.0 6.2 29.0 6.4 –1.5 5.1 –4.9 14.7 18.4 8.7 15.0 9.8 8.4 6.9 5.0 GV-7GV-8 Bauers Harmony Hills 7/9 7/9 6.99 6.96 2.5 4.9 587.1 155.6 11.0 357.8 44.2 32.7 036, 5SE 036, 5SE 15.6 0.5 46.1 35.7 4146211 4146399 282405 282307 18.9 8.4 7.2 7.2 6.1 8.6 6.8 7 P-12P-13 Bauers Bauers 7/7 8/8 6.986 7.987 2.9 2.4 440.7 542.8 330.7 319.8 70.8 60.8 036, 30NW 352, 19SW 316.6 299.9 41.9 47.8 4159630 4160128 276774 276831 –32.7 –49.4 5.5 5.7 2.4 –3.5 5.5 5.0 P-19*P-21 Leach Canyon Harmony Hills 8/9 8/10 7.995 7.959 1.5 5.0 1464.8 144.7 31.6 276.5 –63.5 43.6 315, 16NE 17NE 318, 13.8 292.6 –78.5 53.1 4165890 4166400 278017 279002 –52.2 –59.5 11.9 9.0 –67.8 –2.8 4.8 7.0 PV-16PV-17PV-18PV-19 Bauers Harmony Hills Harmony Hills Canyon Leach 8/9 8/9 7/9 8/9 7.972 7.939 6.986 7.972 3.5 5.3 2.9 3.5 247.8 112.2 419 249 13.7 13.6 338.3 67.2 76.9 61.1 47.0 –19.8 16NW 053, 029, 15NW 354.5 16NW 053, 079, 17NW 353.0 335.2 54.9 83.0 54.0 31.4 –19.5 4144644 4144718 4144607 4144480 266466 266453 266435 266343 2.4 –14.1 0.9 –17.0 8.3 6.3 9.3 7.1 –4.6 12.9 –3.7 –8.8 4.9 13.1 5.5 5.5

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TABLE 4A. PALEOMAGNETIC DATA FROM THE THREE PEAKS LACCOLITH Paleomagnetic VGP Northing Easting α sampling site N/No R 95 k DILat Long UTM - Zone 12 WGS1984 Method 1 9/9 8.9 4.1 140.7 330.9 41.5 61.69 136.22 4183179 307504 1 2 10/10 — 8.2, 12.8 22.7, 1.6 321.4 61.9 60.29 176.86 4183189 307514 2 3 9/12 8.8 6.0 66.6 328.8 44.0 61.15 141.75 4182320 307144 1 4 4/6 4.0 7.9 102.8 339.7 52.8 73.00 148.77 4181825 306744 1 5 10/12 9.9 3.8 144.4 335.4 48.0 67.86 141.48 4181228 306806 1 6† 8/12 7.6 9.6 30.0 346.9 25.6 63.33 95.95 4180722 306604 1 7 9/10 8.9 6.5 56.6 346.0 53.9 78.28 145.85 4180221 306626 1 8† 8/14 6.9 8.2 41.0 339.4 6.2 50.81 100.32 4179942 306238 1 9 17/22 16.9 4.5 137.1 328.6 53.3 64.35 158.03 4179474 305653 1 10 10/14 9.9 5.9 60.4 341.7 54.7 75.12 153.50 4183509 307615 1 11 9/13 9.0 2.0 580.0 354.9 59.5 85.10 194.04 4183943 307911 1 12 10/17 9.7 7.1 42.7 309.3 60.0 51.09 177.88 4184020 308660 1 13 8/10 7.9 5.7 84.5 334.9 65.3 69.11 192.91 4183390 308630 1 14 8/11 7.9 6.9 57.5 338.3 55.3 72.55 157.82 4183874 309295 1 15 19/19 18.0 5.4 78.4 313.1 38.9 46.93 149.11 4184490 308010 1 16 5/10 5.0 8.7 63.2 332.3 53.6 67.37 156.54 4182755 308226 1 17 8/8 — 6.9, 13.4 31.2 342.9 46.0 72.44 126.48 4181890 308323 2 18 9/12 8.9 4.6 111.6 326.6 51.4 62.19 157.62 4182563 308122 1 19 17/18 16.9 2.6 187.1 320.6 48.7 56.50 154.91 4182553 308131 1 20 9/11 8.9 6.0 65.4 318.9 58.9 58.06 173.45 4181650 307585 1 21 12/12 — 6.4, 9.3 24.3, 1.4 311.7 60.1 52.89 177.49 4181012 307439 2 22 10/10 10.0 3.3 192.3 346.2 49.5 76.54 128.74 4180580 308233 1 23 9/10 — 6.0, 9.6 33.5, 1.8 353.0 48.2 79.91 103.91 4180162 307054 2 24*————————4179228 305430 — 25 6/14 5.9 8.9 47.8 350.3 45.3 76.57 106.90 4184056 307215 1 26*————————4183469 306707 — 27 9/13 8.9 5.1 90.4 345.8 38.2 69.85 108.05 4182991 306435 1 28 ————————4182991 306435 — 29 10/10 10.0 2.1 466.0 326.9 49.0 61.60 151.02 4182991 306435 1 30*————————4182991 306435 — 31† 12/13 11.9 5.2 65.0 10.5 52.4 80.34 0.88 4182651 306258 1 32 12/12 11.9 6.6 40.3 316.7 43.8 51.62 151.38 4179562 306036 1 33 ————————4179562 306036 — 34 15/15 — 8.8, 10.3 9.9, 0.5 310.4 48.1 48.20 160.17 4179562 306036 2 35*————————4181075 305336 — 36*————————4181424 305553 — 37 9/11 8.7 12.2 16.6 356.3 62.8 82.68 225.25 4181953 305817 1 38 9/9 — 6.9, 11.0 28.0, 1.7 352.2 49.3 80.26 110.46 4182435 306238 2 39 10/11 10.0 2.4 369.4 328.7 43.4 60.83 141.03 4182441 306572 1 40 10/13 — 6.3, 6.8 25.8, 0.3 320.6 52.2 57.68 160.31 4180735 308487 2 41 7/10 — 5.3, 6.4 51.3, 0.7 338.1 54.4 72.19 155.04 4181669 308763 2 42† 7/10 — 6.8, 8.7 39.8, 0.9 340.5 13.6 54.68 101.52 4179742 307394 2 43 9/10 — 6.9, 7.2 25.0, 0.2 322.5 58.2 60.69 171.07 4179815 303821 2 44 14/19 13.9 2.8 191.3 339.9 57.8 74.15 166.33 4181464 306568 1 45 14/14 — 5.3, 6.0 30.3, 0.5 329.1 35.3 57.68 131.43 4181146 306128 2 46 12/14 11.9 3.5 141.7 338.8 39.4 66.30 122.87 4180703 306120 1 α Note: N/No—ratio of samples used (N) to samples collected (No) at each site; R—resultant vector length; 95—95% confi dence interval about the estimated mean direction, assuming a circular distribution; k—best estimate of (Fisher) precision parameter. D, I—in situ declination and inclination (in degrees). VGP Lat, Long—latitude and longitude of the site mean virtual geomagnetic pole position. Coordinates: UTM, Zone 12, WGS 84 (UTM—Universal Transverse Mercator; WGS—World Geodetic System 1984). Method: 1—least squares fi t (Kirschvink, 1980), 2—remagnetization circle analysis (McFadden and McElhinny, 1988). Dashes indicate rejected site mean data. *Rejected: did not yield stable end-point behavior. †Outliers: >18° from the estimated group mean. §High dispersion at the site level.

evidence of signifi cant hydrothermal altera- coliths differs from the Miocene age expected anhysteretic remanent magnetization (ARM), tion. The remaining one site rejected yielded a direction for this area based on the paleomag- (3) direct current (DC) acquisition of a satura- well-defi ned reverse polarity direction, but upon netic pole estimates for North America for this tion isothermal remanent magnetization (IRM), inversion through the origin, plotted greater than time period (Mankinen et al., 1987). (4) DC demagnetization of the saturation IRM to two angular standard deviations from the overall yield a backfi eld IRM (coercivity of remanence), group mean. Of the 22 sites from the Iron Moun- Rock Magnetism and (5) AF demagnetization of saturation isother- tain laccolith, 16 yield stable end-point behavior mal remanent magnetization (SIRM). The rock with well-defi ned magnetization directions with To characterize the magnetic mineralogy, we magnetic methods are discussed in Appendix 2. 11 sites used to calculate a group mean direc- conducted a suite of standard rock magnetic tion. The six rejected sites did not yield stable experiments with the principal goal of identi- RESULTS end-point behavior and the fi ve sites excluded fying the mineral phases that carry the over- from the group mean calculation were consid- all remanence and the quantity, composition, General Demagnetization Behavior ered outliers because they were greater than two domain state, and grain size of the magnetic angular standard deviations from the overall phases present. These tests included: (1) analysis Demagnetization response during progressive group mean for the intrusion. Collectively, the of low-fi eld susceptibility versus temperature, AF demagnetization for most samples is gener- grand mean calculated for the Iron Axis lac- (2) alternating fi eld (AF) demagnetization of ally well behaved, depending on rock type, and

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TABLE 4B. PALEOMAGNETIC DATA FROM THE GRANITE MOUNTAIN AND IRON LACCOLITHS Paleomagnetic VGP Northing Easting Peak fi eld or α sampling site N/No R 95 k DILat Long UTM Zone 12 WGS1984 Method temperature Treatment Granite Mountain laccolith 3 7/9 6.8 5.5 106.0 357.2 57.1 87.77 163.82 4176088 302113 1 120 mT AF 4 8/8 7.9 6.5 64.4 353.6 46.1 78.65 96.70 4175848 301639 1 540 °C AF-TH 5 8/8 7.9 6.5 64.1 352.3 30.9 68.13 86.70 4175743 301255 1 120 mT AF 6 7/8 6.9 5.1 141.9 338.6 18.7 56.12 106.74 4175736 300752 1 570 °C AF-TH 7† 8/9 7.9 3.5 218.4 2.8 –23.5 –40.21 242.96 4176165 300438 1 560 °C AF-TH 8 8/9 7.9 4.6 128.7 0.5 58.7 87.98 257.56 4176086 300742 1 120 mT AF 9 5/8 4.9 10.3 55.9 343.8 39.1 69.26 113.41 4176642 301180 1 570 °C AF-TH 14 6/6 5.9 6.3 115.8 25.8 59.6 69.79 320.02 4177981 302145 1 570 °C AF-TH Iron Mountain laccolith 3 9/9 9.0 3.4 230.9 49.1 –77.8 20.60 227.80 4169974 290311 1 630 °C AF-TH 9 8/9 7.9 8.1 47.5 89.3 –50.0 17.63 182.21 4166510 291900 1 570 °C AF-TH 10 8/10 7.9 6.1 82.3 288.7 55.8 34.43 178.87 4166797 292033 1 640 °C AF-TH 15 8/10 8.0 3.6 242.8 330.1 46.2 63.05 143.75 4167280 290560 1 570 °C AF-TH 16† 7/8 6.7 13.2 21.8 120.8 24.9 –14.98 306.50 4168648 290318 1 590 °C TH 19 7/10 6.9 9.1 44.7 301.0 33.0 35.22 152.84 4167731 290436 1 120 mT AF 20† 9/10 8.9 5.2 98.1 39.8 54.2 57.96 329.13 4168323 291082 1 570 °C AF-TH 21† 8/10 7.9 7.8 51.3 14.4 74.0 65.51 263.90 4166571 290109 1 120 mT AF 22 3/9 3.0 18.6 45.1 306.9 62.3 49.86 182.59 4166985 290205 1 610 °C TH 23T† 5/9 4.9 15.3 25.9 91.9 –14.9 6.10 161.43 4167423 290013 1 590 °C TH 24 5/10 5.0 4.5 291.4 331.9 44.7 63.81 139.59 4167873 289785 1 630 °C TH 25 6/6 5.9 8.7 60.1 338.5 51.8 71.72 147.04 4167461 289729 1 630 °C TH 28§ 6/12 5.0 32.6 5.2 233.3 27.8 –17.68 192.08 4167236 289516 1 575 °C TH 32§ 6/9 5.6 20.3 11.9 299.0 –12.5 –18.39 312.90 4168247 288905 1 590 °C TH 33 5/6 4.9 9.7 63.4 204.3 –36.0 62.58 9.40 4169311 289431 1 680 °C TH 34 9/10 8.9 6.9 56.0 93.5 –62.9 27.36 193.04 4169316 290134 1 570 °C AF-TH 35 9/10 8.9 6.5 62.8 84.2 –60.9 20.28 194.44 4168974 289725 1 570 °C AF-TH 36† 5/8 4.8 15.6 25.0 199.5 –52.3 73.46 346.66 4170427 291153 1 640 °C AF-TH α Note: N/No—ratio of samples used (N) to samples collected (No) at each site; R—resultant vector length; 95—95% confi dence interval about the estimated mean direction, assuming a circular distribution; k—best estimate of (Fisher) precision parameter. D, I—in situ declination and inclination (in degrees). VGP Lat, Long— latitude and longitude of the site mean virtual geomagnetic pole position. Coordinates: UTM—Universal Transverse Mercator; WGS—World Geodetic System 1984. Method: 1—least squares fi t (Kirschvink, 1980), 2—remagnetization circle analysis (McFadden and McElhinny, 1988). Peak fi eld or temperature is that at which the characteristic remanent magnetization was isolated. For sites that respond with AF(alternating fi eld) demagnetization alone or TH (thermal) demagnetization near 580 °C, the remanence is likely carried by titanomagnetite. For sites that required TH demagnetization above 580 °C, the remanence is likely carried by ilmeno-hematite or titanomaghemite. *Rejected: did not yield stable end-point behavior. †Outliers: >18° from the estimated group mean. §High dispersion at the site level.

characterized by high-quality results (Fig. 8). demagnetization contain magnetite-titanomag- representative specimens to 500 °C did not iso- Of the 61 sites established in the ignimbrites, netite as the principal magnetic phase carrying late any additional magnetization components 49 sites yield readily interpretable demagne- the remanence of these rocks, consistent with the and reduced the intensity to <10% of the NRM tization data (Table 3). The rejected sites had rock magnetic results (Appendix 3). In contrast, value. The Harmony Hills Tuff in the reference α high within-site dispersion ( 95 > 15°, k < 15) those sites that required both AF and TH demag- area yields well-grouped, low-coercivity north- and did not yield interpretable results; these netization to isolate the ChRM are dominated by northeast declination and steep positive incli- sites are not discussed further. In general, most ilmeno-hematite with hemo-ilmenite lamella as nation demagnetization data that decay along samples contain a single ChRM that is well the principal magnetic phase (Fig. 8B). Typical a roughly linear path to the origin (Fig. 8A). grouped at the site level, but some samples also maximum laboratory unblocking temperatures The data are well grouped at the site level with contain additional magnetizations that are read- for the ilmeno-hematite-dominated sites are <25% of the NRM intensity remaining after ily removed by 20 mT or by 300 °C; this behav- ~555 °C, with a range of maximum laboratory AF treatment to 120 mT. For higher coercivity ior is generally restricted to the Iron Mountain unblocking temperatures of 530–570 °C. samples, TH demagnetization to 585 °C yielded and Granite Mountain laccoliths. We interpret no change in direction data. The estimated these low-coercivity, randomly oriented mag- Ash-Flow Tuffs site mean data vary in direction systematically netization components as viscous overprints across the sampled region (Fig. 6). The Bauers (VRM). After removing the VRM, the ChRM, The Leach Canyon Tuff yields a low-coerciv- Tuff yields high-coercivity north-northwest which we interpret as the primary TRM, decays ity, generally single component magnetization declination and steep positive inclination data in along a roughly univectorial path to the origin that is well grouped at the within-site level (Fig. the reference area, although >50% of the NRM with <10% of the natural remanent magnetiza- 8A). The ensemble of estimated site mean direc- intensity remains after AF treatment to 120 mT tion (NRM) intensity remaining after treatment tions is highly variable, in particular in inclina- (Fig. 8A). Selected treatment of high-coercivity (Fig. 8A). For the Iron Axis laccoliths, 82 sites tion values, across the sampled region and the specimens with TH demagnetization to 575 °C were analyzed with 53 sites used to calculate pattern of estimated site mean directions does results in a roughly univectorial decay path to grand mean directions for each of the laccoliths. not reveal a readily identifi able trend (Fig. 6). the origin and does not isolate any additional Some sites responded well to AF demagnetiza- Median destructive fi eld values range from 10 magnetization components. The estimated tion, whereas other sites required a combination to 20 mT with <25% of the NRM intensity site mean data vary in direction systematically of AF and thermal (TH) demagnetization to iso- remaining after AF treatment to 120 mT (Fig. across the sampled region (Fig. 6). The rocks of late the ChRM. The sites that responded to AF 8A; Appendix 3). Thermal demagnetization of Paradise tuff in the reference location yields a

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/3/534/3333171/534.pdf by guest on 01 October 2021 Paleomagnetic results from the eastern Caliente-Enterprise zone, SW Utah 140 50 120 80 ated by site number, (e.g., GV2), ated by site number, AF TH HC3-b GV2-f Leach Canyon 20 Harmony Hills 400 100 540 1 tick = 10-3 A/m 565 NRM 585 10 mbrite diagrams. sent the projections onto true vertical plane. sent the projections 1 tick = 10-4 A/m NRM C. Typically, AF and TH demagnetization results from two from TH demagnetization results and AF Typically, C. ° AF GV2-b 575 555 Harmony Hills 10 NRM 1 tick = 10-3 A/m 50 20 100 80 500 TH HC3-g 400 Leach Canyon 1 tick = 10-4 A/m 200 NRM NRM collected in the eastern rocks ed demagnetization diagrams (Zijderveld, 1967; Roy and Park, 1974) for 10 1 tick = 10-4 A/m 20 20 10 50 100 5 AF P2-a AF Paradise NRM GV7-g Bauers 50 1 tick = 10-3 A/m 80 120 140 ). Representative in situ modifi ). Representative N Up S Down 575 540 TH GV7-g Bauers 400 Horz W eld (AF) demagnetization steps are given in mT and thermal (TH) demagnetization steps are in and thermal (TH) demagnetization steps are given in mT eld (AF) demagnetization steps are 1 tick = 10-3 A/m TH 200 P2-b on this and following two pages two on this and following NRM Paradise 1 tick = 10-4 A/m NRM 585 555 method of treatment (AF or TH), and rock type. Intensity (A/m) is shown along one axis for each sample. (A) Representative igni each sample. (A) Representative type. Intensity (A/m) is shown along one axis for TH), and rock or (AF method of treatment specimens from the same site are shown for comparison. NRM—natural remanent magnetization; Horz—horizontal. Diagrams are design magnetization; Horz—horizontal. Diagrams are comparison. NRM—natural remanent shown for the same site are specimens from Caliente-Enterprise zone. Solid symbols represent the projections onto the horizontal (true vertical) plane; open symbols repre the projections Caliente-Enterprise zone. Solid symbols represent Alternating fi 500 A 8 ( Figure

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/3/534/3333171/534.pdf by guest on 01 October 2021 Petronis et al. NRM 1 tick = 47.0-3 A/m AF-TH IM3-C 10 Iron Mountain 50 200C 500C 400C 80 120 630C 650C 595C 680C 570C AF-TH 550C IM15-E NRM iron Mounatin 500C 50 525C 1 tick = 36.1-3 A/m 400C 20 120 540C ). (B) Representative Iron Axis laccolith diagrams. Iron ). (B) Representative 530C 120 520C 50 300C 80 20 continued 1 tick = 10-3 A/m TP4-b AF-TH Three Peaks 10 Figure 8 ( Figure NRM 110 80 100 50 20 10 AF NRM TP9-e 1 tick = 10-4 A/m Three Peaks N Up S Down 90 50 20 10 5 AF TP33-a NRM 1 tick = 10-2 A/m W Horz Three Peaks B

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/3/534/3333171/534.pdf by guest on 01 October 2021 Paleomagnetic results from the eastern Caliente-Enterprise zone, SW Utah 120 80 50 200C 20 450C 550C 10 525C 570C AF-TH GM14-L Granite Mounatin 1 tick = 53.4-3 A/m NRM NRM 10 1 tick = 56.3-3 A/m 25 400C 120 50 525C 80 570C 200C ). (C) AF-TH GM9-L Granite Mountain continued 500C 300C 25 120 Figure 8 ( Figure 10 AF GM6-J Granite Mountain 5 1 tick = 1.80 A/m NRM N Up S Down 640C 550C 600C AF-TH IM10-I Iron Mountain 500C 1 tick = 63.5-3 A/m W Horz 50 NRM 120 C 80 20

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predominantly low-coercivity northeast decli- do not preserve a geologically stable remanent netic data from the 11 sites used to calculate a nation and shallow positive inclination that is magnetization. Host rock obtained within a few group mean from the Iron Mountain laccolith readily removed by 10 mT, leaving a character- centimeters of any contact with intrusive rock yield a mean direction of west-northwest decli- istic southwest declination and shallow positive carries a magnetization having a direction simi- nation and moderate positive inclination (D = α inclination direction. Median destructive fi eld lar to that of the laccolith. The demagnetization 303.6°, I = 60.6°, 95 = 12.9°, k = 14.9, N = ranges from 10 to 50 mT (Fig. 8A). The remain- response of the host rock, however, is erratic, 4R, 7N) that is statistically distinct from the ing component, defi ned by ~30% of the NRM lacking any stable end-point behavior, for any expected Miocene direction with an inferred R intensity, decays along a near linear path to the samples obtained beyond a few centimeters from of –54.4° ± 28.4° and F = –2.6° ± 14.3° (Fig. origin and is well grouped at the site level with the contact. We interpret these results, although 7B). The dispersion of the VGPs from Iron <10% of the NRM intensity remaining after AF somewhat tenuously, to indicate that the lacco- Mountain is 26.6°, which is well beyond the treatment to 120 mT. TH demagnetization to liths preserve a geologically stable remanence predicted range for the site (see following). The 500 °C of duplicate specimens did not isolate and that host rock at least immediately adjacent Iron Mountain data do not pass the reversal test any additional magnetization components. to the intrusion margin acquired a magnetization at 95% confi dence (McFadden and McElhinney, parallel to that of the laccolith. 1990). Paleomagnetic data from the 7 accepted Three Peaks, Granite Mountain, and Iron sites from the Granite Mountain laccolith yield a Mountain Laccoliths Three Peaks Results group mean direction of north-northwest decli- nation and moderate positive inclination (D = α The paleomagnetic behavior of the Iron Axis Paleomagnetic data from the 35 accepted 353.2°, I = 45.2°, 95 = 14.1°, k = 22.5, N = laccoliths can be readily divided into two types: sites normal polarity yield an in situ group mean 7N) that is statistically indistinguishable from (1) a well-defi ned response to AF demagnetiza- direction of northwest declination and moder- the expected Miocene direction with an inferred tion over a broad range of peak fi elds, and (2) a ate positive inclination (D = 332.9°, I = 51.7°, R of –4.8° ± 21.6° and F = 12.8° ± 15.3° (Fig. α very limited response to AF demagnetization, 95 = 3.4°, k = 52.9). If we can assume that 7C). The dispersion of the VGPs from Granite thus requiring TH demagnetization to isolate the magnetization characteristic of the Three Mountain is 16.3°, which is within error of the the ChRM. In sites that responded well to AF Peaks laccolith adequately averages a Miocene predicted range for the site (see following). demagnetization, the principal mineral assem- geomagnetic fi eld, the group mean result dif- blage carrying the ChRM is dominated by fers, in both declination and inclination, from DISCUSSION magnetite and titanomagnetite that are partially the expected Miocene direction (D = 358°, I = oxidized to hematite, as shown by backscatter 58°) (Fig. 7A) with an inferred rotation (R) and Interpretation of the Paleomagnetic Data electron microprobe images (Figs. 3A, 3B, 3D). fl attening (F) estimate (Beck, 1980; Demarest , The second group of sites, all from parts of the 1983) of R = –22.2° ± 6.4° and F = 8.8° ± 4.3° With the exception of the Leach Canyon Three Peaks laccolith, is characterized by a very relative to expected Miocene age directions Tuff, each tuff in the study area has a charac- limited response to progressive AF demagneti- based on the synthetic pole and North American teristic magnetization that is distinct in direc- zation, yet complete laboratory unblocking in paleomagnetic pole for the Miocene (Mankinen tion and, overall, the inclination of the charac- thermal demagnetization by ~555 to 580 °C, et al., 1987; Besse and Courtillot, 2002). Using teristic magnetization maintains a consistent although a few sites required TH demagneti- paleosecular variation (PSV) models (e.g., value with respect to the paleohorizontal, an zation to 680 °C to fully isolate the ChRM. In Merrill and McElhinny, 1983) for the average observation that is consistent with previous sites characterized by very high coercivities and paleolatitude of the study area (~37.7°N), the observations of the paleomagnetics of ash-fl ow that required TH demagnetization, the principal VGP angular standard deviation is predicted to tuffs (e.g., Geissman et al., 1982; Wells and mineral phase is ilmeno-hematite with hemo- be ~15.8°. The VGP angular standard deviation Hillhouse, 1989; Byrd et al., 1994; Best et al., ilmenite lamella (Fig. 3C). of the 35 sites in the Three Peaks laccolith is 1995; Hudson et al., 1994), and is considered a 13.1°, which is within error of the predicted requirement in using paleomagnetic data from Field Tests PSV dispersion range and is consistent with the ignimbrites to estimate magnitudes of vertical sampling of suffi cient fi eld variations. The lack axis rotation. Therefore, the data from dispersed Where fi eld exposures allowed, several baked of reverse polarity magnetizations in the Three sites from a regionally extensive ignimbrite, contact tests (Everitt and Clegg, 1962) were Peaks laccolith may simply refl ect the fact that when accurately referenced to the paleohorizon- attempted between the host Jurassic rocks and the early Miocene (22–20 Ma) is characterized tal, can be compared to estimate relative verti- the intrusions. A positive test is characterized by several normal and reverse polarity chrons cal axis rotations with respect to the reference by the remanence direction of the igneous rock (e.g., Cande and Kent, 1995) and magnetiza- location (Fig. 1). In principle, data from all tuffs, being acquired by the host rock and progres- tion blocking took place during a single normal regardless of their age, exposed at the same sively dominated by the ChRM of the host rock polarity chron. We argue that the magnetiza- locality should yield similar relative rotation (if any) as a function of proximity to the igneous tion characteristic of the Three Peaks laccolith estimates if rotation postdated their emplace- contact. All contact tests failed to show a grada- provides an adequate average of the paleomag- ment. The paleomagnetic data from the ignim- tional change between a magnetization consis- netic fi eld. brites (excluding the Leach Canyon Tuff) across tent with an expected Miocene direction for the the eastern CEZ reveal an orderly and predict- laccoliths and that of the Jurassic host rocks. We Granite Mountain and able pattern of discordant directional data with attribute this result to the poor paleomagnetic Iron Mountain Results respect to the reference location sites (Fig. 9). character of the host rock. Sites in the host rock The rock magnetic data and highly dispersed located well away from intrusion contacts did not Of the 36 sites analyzed from the Iron Moun- paleomagnetic results from the Leach Canyon yield stable end-point behavior, and we interpret tain (N = 22) and Granite Mountain (N = 14), Tuff indicate that this unit is not reliable to use this result to indicate that the Jurassic host rocks 24 yield readily interpretable results. Paleomag- to estimate vertical axis rotation across the study

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Figure 9. Summary diagram of all paleomagnetic data from the eastern Caliente-Enterprise zone. Each arrow represents a location where several paleomagnetic sites from the same tuff or intrusion were averaged together when the results between sites in the area were essen- tially uniform. Any deviation of the arrow from north indicates a vertical axis rotation. The paleomagnetic data from the Iron Axis lac- coliths are referenced to the Miocene expected direction and the ignimbrite data are with respect to fi xed locations in Grass Valley or the southern Delamar and Meadow Valley Mountains and farther north in Condor Canyon in southeastern Nevada. See text for discussion, Table 2A for paleomagnetic data from each reference site (means of sites are in Table 2B), Figure 2 for a description of symbols and site locations, and Tables 3 and 4 for mean rotation and fl attening estimates from each site. The rotations from Hudson et al. (1998) for Bauers Tuff and Harmony Hills Tuff sites 10 km south of Cedar City are attributed to local drag adjacent to a buried sinistral fault west of the Hurricane fault (Anderson and Mehnert, 1979). In the southwestern part of the map, north of Veyo, Hudson et al. (1998) reported numer- ous sites that show rotation; we have omitted most of these for clarity. The coordinate system used is UTM Zone 12, Northing and Easting.

area. All VGPs determined from directions from Typically, the data from the three tuffs sam- east, including the Pinto, Holt Canyon, and New- the Leach Canyon Tuff are more than 45° from pled in stratigraphic succession at the same castle Reservoir localities, exhibit a progressive North American paleomagnetic poles for the general locality yield internally consistent decli- northward increase in the magnitude of counter- early Miocene, and it is likely that this deposit nation and inclination results as well as similar clockwise rotation (Fig. 9). The paleomagnetic recorded rapid changes of the geomagnetic magnitudes of rotation, although there are some data from all locations north and northwest of fi eld during a transition or other form of high- inconsistencies, largely restricted to the Leach the Grass Valley reference area are discordant amplitude fi eld event (Verosub and Banerjee, Canyon Tuff (Fig. 8). In the southern part of the in declination from the individual tuff expected 1977; Merrill and McFadden, 1994). In addi- study area, which includes the Pine Valley and directions (Table 3). The rotation and fl attening tion, the low coercivity of the remanence typi- central regions, the site mean data are all essen- estimates (Beck, 1980; Demarest , 1983) range cal of each site (Appendix 3) further indicates tially concordant with reference directions for from R = +38° to –84° and F = +15° to –14° that the results from the Leach Canyon Tuff are each ash-fl ow tuff unit, thus showing little evi- (most F values are considerably lower and not not reliable (e.g., Geissman et al., 1982; Hudson dence of vertical axis rotation. In contrast, site statistically signifi cant). Of the 49 site mean et al., 1998). mean data from tuffs exposed to the north and directions, 10 yield rotation estimates that we

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consider to be of relatively low confi dence. The unusual fault geometry, although we suspect this plex magnetization acquisition histories and no low-confi dence sites include seven from the conclusion to be tenuous (e.g., Lamb, 1987). longer contain a primary ChRM component. Leach Canyon Tuff, two from the Bauers Tuff, The paleomagnetic data from 53 accepted The 35 accepted sites from the Three Peaks and one from Harmony Hills Tuff. We consider sites in the Iron Axis laccoliths, all located laccolith yield an in situ group mean direction low-confi dence rotation estimates to be associ- northeast of the Newcastle Reservoir sampling of northwest declination and moderate positive α ated with sites that yield a statistically signifi cant locality, yield a combined overall in situ grand inclination (D = 332.9°, I = 51.7°, 95 = 3.4°, F estimate that is 10° greater than the delta F mean direction of northwest declination and k = 52.9, N = 35N) that is discordant to the error (see Petronis et al., 2009) (Table 3). The moderate positive inclination (D = 331.3°, I = expected Miocene direction with an inferred R α F values for data from the identical ignimbrite 53.4°, 95 = 4.3°, k = 21.6). This approach to = –22.2° ± 6.4° and F = 8.8° ± 4.3°. This result should be ~0°, assuming that the ChRMs at each evaluating the paleomagnetic data from the is consistent with previously published rota- site are accurately referenced to the paleohori- three laccoliths we studied is likely inappropri- tion estimates from nearby ash-fl ow tuffs (e.g., zontal and have been adequately isolated during ate, but serves as an initial examination. Several estimates of R = –10° to –20° from the Bauers progressive demagnetization (e.g., Wells and individual site results from the Iron Mountain and Harmony Hills Tuffs near Desert Mound, Hillhouse, 1989). One explanation for fl atten- and Granite Mountain laccoliths are less robust, although some sites are not rotated) (Figs. 7A ing values that are signifi cantly greater than their given the pervasive alteration of these laccoliths, and 9; Hudson et al., 1998). Efforts to estimate associated error is that the data are inadequately and we place less confi dence in the rotation esti- vertical axis rotation based on paleomagnetic referenced to the paleohorizontal. Other sources mates based on average mean directions from data from intrusions are compromised because of error include not fully isolating the ChRM of these intrusions. Therefore, we base our rotation of the inherent inability to accurately reference the individual samples at a site, lightning strikes, estimate for the area northeast of the Newcastle the data to the paleohorizontal, considering the unusual rock magnetic properties of a particu- Reservoir exclusively on the results from the possibility that postmagnetization acquisition lar phase (Geissman et al., 1982), or inclination Three Peaks laccolith. local tilting has changed the original direction of fl attening associated with the development of a The lesser quality of the paleomagnetic magnetization in the pluton (e.g., Beck, 1980). strong compaction foliation (e.g., Uyeda et al., results from Granite Mountain and Iron Moun- In the case of intrusions that have a relatively 1963; Gattacceca and Rochette, 2002). An addi- tain intrusions may refl ect the fact that each uniform cooling history, and do not represent tional explanation for this phenomenon is that intrusion is pervasively hydrothermally altered. substantially deep crustal levels, paleomagnetic the ignimbrite acquired a remanence over a pro- Most sites contain multiple magnetization com- data that are discordant from expected fi eld tracted period of time, possibly while the geo- ponents that are isolated across a spectrum of directions of comparable age can be reconciled magnetic fi eld direction was rapidly changing. It peak laboratory unblocking temperatures and in an infi nite combination of rotations about is likely that a combination of these factors con- applied fi elds. A few sites, however, respond axes of any orientation. Realistically, how- tributes to the low confi dence estimates for these to AF demagnetization and often yield a single ever, the tilting of crustal materials generally is 10 sites (Table 3), especially those in the Leach component magnetization that decays linearly accommodated by rotation about horizontal to Canyon Tuff. With the exception of the Leach to the origin with <10% of the NRM remain- subhorizontal tilt axes. To investigate the pos- Canyon Tuff, for the ignimbrites sampled, after ing after treatment to 120 mT peak fi elds after sibility of whole-scale tilting of the Three Peaks structural correction, the overall dispersion of a weak VRM is removed by 20 mT (Fig. 8B). laccolith, we considered structures in the gen- site mean directions is far greater in declination These sites yield directional data similar to the eral area that could have accommodated a pos- than in inclination (Fig. 6), as would be expected results from the Three Peak laccolith and Curie sible post–22 Ma tilt of the structural blocks in if a single, sheet-like deposit, with an overall point estimates consistent with magnetite and/or the area and thus the likely tilt direction and uniformly directed magnetization, is fragmented titanomagnetite as the dominant magnetic phase plausible magnitude of tilt. and distorted by different magnitudes of ver- (Table 5; Appendix 3). We argue that the sites The Three Peaks laccolith is bounded on the tical axis rotation (e.g., Hillhouse and Wells, that respond well to AF demagnetization and east by the East fault, an east-side-down normal 1991; Petronis et al., 2009). The paleomagnetic yield directional data consistent with the Three fault, and on the west by the Northwest Intru- data from the ignimbrites, when compared with Peaks laccolith carry a ChRM that is likely a sive fault, a west-dipping normal fault (Rowley respective data from the reference sections, indi- primary TRM. Samples from sites that require et al., 2006). Motion along the Northwest Intru- cate an organized pattern of vertical axis rotation a combination of AF and TH demagnetization sive fault likely tilted the footwall, composed of (Fig. 9; Table 3), in that rotation magnitudes often reveal the presence of a high-coercivity Cretaceous Iron Springs Formation strata that vary from <1° for sites located in the south to a phase (likely ilmeno-hematite or hematite) that strike northeast and dip northwest at 20°–30°. maximum of –84° for sites located in the north, decays to the origin at peak temperatures rang- It is likely that the stratal orientation refl ects a near Newcastle (Fig. 9). Four sites yield clock- ing from 590 to 660 °C and yields directions combination of pre–22 Ma deformation-asso- wise vertical axis rotations, when compared similar to those from the Three Peaks intrusion. ciated Sevier-age shortening and late Cenozoic with respective data from the reference section. It is probable that for these sites, we are success- Basin and Range extension. Strata adjacent to Three of these sites are from the Leach Canyon fully isolating the primary ChRM of the rocks, the intrusion show steep (>45°) west dips likely Tuff, and we question the overall reliability of which has been partially overprinted by a mag- refl ecting deformation of the country rock asso- this pyroclastic deposit to provide any form of netization associated with late-stage hydrother- ciated with intrusion of the Three Peaks lacco- representative geomagnetic fi eld direction. The mal alteration. Unfortunately, many of the sites lith. The Three Peaks laccolith could also have clockwise rotation estimate from the Bauers Tuff analyzed from the Granite Mountain and Iron been tilted to the northwest along with adjacent is statistically insignifi cant (R = 8.0° ± 6.6°), Mountain laccoliths yielded a complex demag- Cretaceous strata; however, we believe this yet we cannot entirely discount the possibility netization response refl ecting overlapping coer- unlikely, considering that the laccolith is located that some areas may have undergone localized civity spectra and mixed polarity results at the in a structural horst (Rowley et al., 2006). Strati- clockwise or very minimal rotation, and these site level. We interpret this demagnetization fi ed materials from other structural horst blocks could refl ect local crustal heterogeneities and/or response to indicate that these sites have com- in the region show no preferred tilt orientation

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TABLE 5. CURIE POINT ESTIMATES AND TITANIUM CONTENT laccolith is variability faulted, and that there Curie point is no compelling fi eld evidence for a regional, LT HT post–22 Ma tilt affecting the area. Nonetheless, Site (°C) (°C) Ti Content Method* Mineral phase Granite Mountain we admit that tilting may contribute to the dis- GM12 — 574 0.014 1 Mt cordant paleomagnetic data from the Iron Axis GM13 — 579 0.006 1 Mt laccoliths and that many forms of tilt correction GM14 — 574 0.014 1 Mt GM15 — 576 0.011 1 Mt applied to one or all of the data sets from the GM6 — 574 0.014 1 Mt Iron Axis laccoliths may result in an errone- GM9 — 568 0.024 1 Tmt ous estimate of vertical axis rotation of crustal Iron Mountain blocks including the laccoliths. IM1 250 563 0.032 2 Tmt† IM10 277 557 0.042 2 Tmt† IM10A — 573 0.015 1 Mt Kinematic Indicators IM11 278 557 0.042 2 Tmt† † IM12 262 558 0.041 2 Tmt The orientation of small-scale structures as IM13 — 574 0.014 1 Mt IM15 222 579 0.006 1 Mt† well as kinematic indicators on major and minor IM16 270 565 0.029 2 Tmt† structures in the study area would, ideally, aid IM17 — 568 0.024 2 Tmt IM2 — 573 0.015 1 Mt in the understanding of how the paleomag- IM20 283 589 — 1 Mg† netically observed vertical axis rotations in IM21 — 566 0.027 2 Tmt the region have been accommodated. Unfor- IM22 269 576 0.011 1 Mt† IM23 258 562 0.034 2 Tmt† tunately, very few data exist on the kinematics IM25 — 566 0.027 2 Tmt of the structures in the area. Michel-Noel et al. IM3 — 571 0.019 1 Mt (1990) provided abundant kinematic data on IM30 269 579 0.006 1 Mt† IM31 270 562 0.034 2 Tmt† faults in the Rainbow Canyon area within the IM32 — 563 0.032 2 Tmt CEZ. Rainbow Canyon is located southwest IM33 — 576 0.011 1 Mt IM34 290 562 0.034 2 Tmt† of Caliente, in eastern Nevada, ~60 km west of IM35 — 566 0.027 2 Tmt the study area. The data are from slip indicators IM36 298 571 0.019 1 Mt† collected on the Hiko Tuff (18.2 Ma) and the IM5 — 555 0.045 2 Tmt IM9 346 638 — 1 Mg† tuff of Etna (14 Ma). Here, the dominant set of Three Peaks faults strikes northwest, and striae for this fault TP23 — 546 0.060 1 Tmt set have dual modes of nearly pure dextral and TP27 — 546 0.060 1 Tmt normal sense. Oblique striae are rare. Hudson TP40 — 551 0.052 1 Tmt TP41 — 551 0.052 1 Tmt et al. (1998) interpreted the fault kinematic TP9 — 548 0.057 1 Tmt data to indicate that dextral-sense displacement Harmony Hills Tuff along northwest-striking faults was responsible PV18 299 578 0.007 2 Mt† for counterclockwise vertical axis block rota- PV11 — 566 0.027 2 Tmt tion; they argued that the separation of fault GV2 — 574 2 Mt modes into dip-slip and strike-slip end members Leach Canyon Tuff implies that the declination discordance and P17 293 534 0.079 1 Tmt† P5 — 565 2 Tmt strata dips in this area may have developed sepa- Rocks of Paradise rately in response to multiple phases of vertical GV9 330 562 0.034 2 Tmt† axis rotation and tilt, respectively. Timing rela- GV1 336 579 0.006 2 Mt† tions between dip-slip and strike-slip faulting GV11 — 576 2 Mt (Michel-Noel et al., 1990) and dip and vertical Note: After Akimoto (1962). LT—low temperature; HT—high temperature; Mt—magnetite; Tmt— titanomagnetite; Mg—maghemite. Dashes indicate no Curie point data available. axis rotation development suggests that these *1—Hopkinson peak; 2—infl ection point. deformation events probably overlapped in time † Low-temperature phase present, likely titanomaghemite or pyrrhotite. and space and may have alternated on a short time scale (Hudson et al., 1998). Hudson et al. (1998) pointed out that it is unclear if this con- and dip magnitudes that are modest (<30°). In ranging from northwest, north, to east-west and clusion can be generalized for the entire CEZ. the northern Bald Hills, ~7 km northwest of the dips ranging from 20° to 45°. To the west in the We note that fault geometries in our study area Three Peaks laccolith, Cenozoic strata typically Antelope Range, the Leach Canyon and Har- are similar to those in the Rainbow Canyon area. have a north-northeast strike with gentle east mony Hills Tuffs in different fault blocks show dips (10°–15°). To the south in the Bald Hills, strikes ranging from north-south to east-west Previous Paleomagnetic Results closer to the northwest margin of the Three and dips as much as 35° with dispersed direc- Peaks laccolith, a major northwest-striking tions. Approximately 6 km southwest of the Hudson et al. (1998) sampled lithologically structure, the Hole-in-the-Wall fault, cuts across New Castle Reservoir, Mesozoic and Cenozoic distinct early Miocene ash-fl ow tuffs of wide the structural grain of the Bald Hills (Rowley rocks are variably faulted and there is consider- spatial distribution that were involved in the et al., 2006). South of this structure is a com- able variation in the orientation of fault blocks main phase of Cenozoic deformation within plexly faulted zone with fault-bounded struc- (Rowley et al., 1988, 2006). We argue that the the CEZ; their paleomagnetic data show vary- tural blocks of varying orientations, with strikes area surrounding and including the Three Peaks ing, yet systematic degrees of counterclockwise

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vertical axis rotations throughout the CEZ, and 2009; Fig. 10). The results from this study are and Stoddard Mountain laccoliths, resulting in the results that we have obtained in this study consistent with the data of Hudson et al. (1998) components of vertical axis rotation and tilting are similar in overall pattern (Fig. 9). The in that the magnitude of counterclockwise rota- associated with regional left-lateral transten- inferred progressive increase in the magnitude tions increases to the north, from Pine Valley to sional deformation, a conclusion similar to that of counterclockwise rotation from south to Newcastle (Fig. 9). To the north, rotation esti- reached by both Hudson et al. (1998) and Axen north across the CEZ was interpreted by Hud- mates diminish to <10° in the Escalante Desert (1998). Considering a rotating block model son et al. (1998) as the result of accommodat- and defi ne the northern boundary of the shear (e.g., McKenzie and Jackson, 1986), the south- ing an increase in the magnitude of extension in zone, and to the east and northeast, data from ern boundary of the CEZ may be pinned against areas directly north and south of the zone. Hud- the Antelope Range and Desert Mound reveal a the stable and thick Colorado Plateau margin. son et al. (1998) evaluated vertical axis rotations decreasing, yet statistically signifi cant, amount To the north, the blocks became unpinned and relative to undeformed reference areas in the of rotation from west to east (see Hudson et al., allowed crust in the region to rotate in an unre- southern Delamar Mountains, Meadow Valley 1998). The data from this study allow for a more stricted fashion. This scenario implies that the Mountains, and Condor Canyon in southeast- accurate delineation of the eastern and south- Pine Valley laccolith is at the western margin ern Nevada by between-site, within-unit com- ern margin of the CEZ in that they reveal how of the Colorado Plateau, whereas the Stoddard parisons. Their study documented rotation esti- counterclockwise rotations increase from zero Mountain laccolith is positioned within the mates ranging from +18° clockwise to as high at the southern edge of the study area (north of southern extent or rather the transition zone of as –125° counterclockwise, at 95% confi dence; the Pine Valley laccolith) to a maximum of –80° the eastern CEZ, and the minimal observed rota- they found that all of the ash-fl ow tuffs exam- near Newcastle (Fig. 9). tion of the area refl ects its southern boundary ined had been rotated to some degree, with the Northeast of Desert Mound, sites in the Three being pinned against the stable Colorado Pla- largest rotations in the east-central part of the Peak laccolith yield an in situ grand mean direc- teau block. As proposed by others (e.g., Hudson CEZ and a systematic decrease to the west. Data tion (D = 332.9°, I = 51.7°) that is discordant, et al., 1998; Axen et al., 1998), we interpret the from several sections of the Bauers and Har- with inferred estimates of R = –22.2° ± 6.4° and southern boundary of the eastern CEZ to mark mony Hills Tuffs, in the eastern part of the CEZ, F = 8.8° ± 4.3°. The results from the Three Peak a fundamental change in crustal properties and show considerable variation in rotation magni- laccolith are consistent with the rotation esti- to represent a crustal boundary that Cenozoic tudes over a distance of 40 km (Fig. 9). Rotation mates (–10° to –20°) from the Bauers and Har- extension exploited as the crust extended to the estimates range from large counterclockwise mony Hills Tuffs near Desert Mound (Fig. 9), yet west away from the Colorado Plateau margin. values (–70° to –98°) near Newcastle, to mod- we emphasize that the overall reliability of this The inception of counterclockwise rotation erate values (–20° to –40°) on the eastern side result is compromised by our inability to accu- during formation of the CEZ is only broadly of the Antelope Range, to minor values (<–20°) rately reference the data to the paleohorizontal. determined to have begun after 18 Ma and near Desert Mound. Local rotations (–20° to Where the Stoddard Mountain and Pine Valley continued until after 14 Ma, based on ages of –45°) in the Bauers and Harmony Hills Tuffs laccoliths are exposed, Cenozoic faults strike rotated tuffs within the western and central CEZ are identifi ed in the South Hills, 10 km south roughly north-northwest. In the area exposing the (Hudson et al., 1998). Formation of the Caliente of Cedar City (Fig. 9). Hudson et al. (1998) laccoliths we studied, ~30 km to the northeast, caldera complex in the central CEZ was partly interpreted these rotation estimates to refl ect fault traces trend northwest to west-northwest, coeval with the formation of the CEZ, leading local drag adjacent to a buried fault west of the more typical of the zones that have been rotated previous workers to speculate on the relation Hurricane fault system (Anderson and Mehnert, within the CEZ. North of Stoddard Mountain, between magmatism and CEZ deformation 1979). Based on the small rotation estimates of a Cenozoic faults change strike over a few kilome- (Hudson et al., 1998; Axen, 1998). Hudson number of Bauers Tuff and Harmony Hills Tuff ters from north-south to north-northwest (Fig. et al. (1998) attributed the signifi cant width sites collected from the Harmony Mountains and 10). Clearly the zone between the Stoddard of the CEZ (compared to other accommoda- Desert Mound, Hudson et al. (1998) proposed Mountain and Three Peaks, Iron Mountain, and tion zones in the Basin and Range Province) to that the CEZ terminates ~10 km east of New- Granite Mountain laccoliths is marked by an enhanced ductility of the crust associated with castle, despite the existence of normal, dextral abrupt change in fault orientation. We propose coeval magmatism. In the eastern part of the and sinistral faults of dominantly northwest to that the southern boundary of the CEZ is just CEZ, an en echelon pattern of dextral faults and northeast strike (Fig. 10). The new paleomag- north of the Stoddard Mountain laccolith (Fig. laccoliths defi nes the Iron Axis magmatic prov- netic data we report from the Iron Axis lacco- 11), and attribute the change in fault strike to ince (Rowley et al., 1995) (Fig. 9). The spatial liths and the associated ignimbrites from the sur- refl ect the southernmost boundary of the zone distribution of the array of variable composition rounding area serve to modify this conclusion. of vertical axis rotation within the eastern CEZ. Iron Axis intrusions is partly controlled by older We argue here that the termination of the eastern We project this zone to the northeast beyond the northeast-striking, southeast-verging Sevier-age CEZ extends farther east and north of Cedar City. Three Peaks laccolith to where it disappears into (Late Cretaceous to early Cenozoic) thrust faults. the alluvial fi ll of the Escalante Desert, where no Iron Axis magmatism preceded formation of the Eastern Boundary of the CEZ Miocene igneous rocks are exposed. We argue eastern CEZ, as suggested by rotations of dated that the boundary of the eastern CEZ extends tuffs and the northern smaller laccoliths. Axen Geologic fault patterns in the CEZ reveal a east to the breakaway with the Colorado Plateau (1998) postulated that the emplacement of the distinct trend where northwest-southeast–ori- (Fig. 11), and hypothesize that the transitional Iron Axis laccoliths in part hindered the initia- ented strike-slip faults are more prominent in zone between the eastern CEZ and Colorado tion of the CEZ by forming a barrier that forced areas that have undergone greater magnitudes Plateau is a narrow (<~10 km wide) zone north- the zone to widen signifi cantly away from (west of counterclockwise rotation (e.g., Newcastle west of Cedar City. of) the laccoliths. Based on our new data, the area), with rotation estimates decreasing as fault The available paleomagnetic data imply that two southernmost and largest of the Iron Axis strikes are more north-south (e.g., Pine Valley; deformation in the eastern CEZ progressively laccoliths may have affected the deformational Hurlow, 2002; Rowley et al., 2006; Biek et al., fragmented the crust north of the Pine Valley response of the crust either through pluton

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Figure 10. Cenozoic Basin and Range fault map illustrating regional pattern of normal and strike-slip faults (bar on downthrown side; arrows indicate relative strike-slip motion; dotted where buried beneath basin fi ll deposits) after Hurlow (2002), Rowley et al. (2006), and Biek et al. (2009). AR—Antelope Range; C—Central; CC—Cedar City; DM—Desert Mound; EMH—Eight Mile Hills; HF—Hurricane fault; HH—Harmony Hills; P—Pinto; PV—Pine Valley; N—Newcastle. The coordinate system used is UTM Zone 12, Northing and Easting.

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Figure 11. Simplified map of Cenozoic faults and Iron Axis laccoliths in the eastern Caliente-Enterprise zone (CEZ). The southern and eastern boundaries of the eastern CEZ are after Axen (1998), Hudson et al. (1998), and this study. Based on the paleomagnetic data from this study, we argue that the boundary is between the Stoddard Mountain and Iron Mountain laccoliths and extends to the northeast beyond Cedar City. See Figure 2 caption for symbol and abbreviation expla- nations. Cenozoic faults after Winkler (1990). The coordinate system used is UTM Zone 12, Northing and Easting.

strengthening or stitching of preexisting crustal the Colorado Plateau, east and north of Cedar Rock Magnetism Laboratory using an AGICO JR6A weaknesses. We speculate that these large lacco- City, Utah. We argue that the Pine Valley and dual speed spinner magnetometer within a Magnetic Measurements Ltd. MMLFC shielded room or at liths represent the shallow-crustal expression of Stoddard Mountain laccoliths, given their size the University of New Mexico (UNM) Paleomagne- a more voluminous, deep-seated batholith per- and proximity to the stable Colorado Plateau tism laboratory with a 2G Enterprises Model 760R haps near the mid-crustal strength maximum. In margin, did not undergo deformation related to three-axis superconducting rock magnetometer with contrast, the small laccoliths to the north likely the development of the CEZ, whereas smaller an integrated alternating fi eld (AF) demagnetization do not have deep plutonic roots and were too intrusions located farther north likely had simi- system in a magnetically shielded room. Typically, between 6 and 14 specimens from each site were small to inhibit deformation, and instead were lar magnitudes of counterclockwise rotation progressively AF demagnetized in 10 to 18 steps to deformed and rotated a magnitude similar to as their near age-equivalent volcanic rocks, a maximum fi eld of 120 mT using an ASC Scientifi c that of Cenozoic volcanic rocks (Figs. 9 and 11). although the resolution of our rotation estimates D-tech 2000 AF demagnetizer at the NMHU lab or for the plutons is limited because of an inabil- the integrated 2G Enterprises AF demagnetization system at the UNM lab. Samples of high coercivity CONCLUSIONS ity to accurately restore the data to the paleo- were further treated with thermal (TH) demagnetiza- horizontal. The southernmost part of the CEZ tion until <10% of natural remanent magnetization Paleomagnetic data from volcanic and shal- remained rigid during Neogene crustal exten- (NRM) remained; typically to 590 °C for the ash- low intrusive rocks in the eastern CEZ reveal an sion and may represent a major crustal strength fl ow tuffs and to 660 °C for some intrusive igneous orderly pattern of rotation of crustal blocks pro- boundary, possibly inherited from the early rocks. To compare AF behavior, duplicate specimens of some samples were thermally demagnetized using gressing from near 0° along the southern bound- development of the western margin of North an ASC Scientifi c TD48 or Schonstedt TSD-1 thermal ary to –84° near the northern boundary. These America, in Neoproterozoic and Paleozoic time. demagnetizer. No difference in the directional data data are consistent with previous paleomagnetic between AF and TH demagnetization experiments studies that reveal similar and spatially variable APPENDIX 1. PALEOMAGNETIC METHODS was observed (Fig. 8). For most samples, a single line could be fi t to the demagnetization data and, typically, patterns of deformation in the central and west- Remanent magnetizations and standard rock mag- involved 10–15 data points. Sample MAD values for ern CEZ. We extend the eastern zone of the CEZ netic analyses were conducted either at the New Mex- linear data were typically <4°, but ranged from <1° to within a few kilometers of the breakaway of ico Highlands University (NMHU) Paleomagnetic- to 5°. In some specimens, low-coercivity, random

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with an AGICO MFK1-A kappabridge susceptibil- 600 ity meter with a CS-4 high-temperature attachment; remanence measurements were conducted with an 500 AGICO JR6A dual-speed spinner magnetometer, and 500 isothermal magnetizations were imparted using an 400 eld susceptibility versus temperature data for representative samples. (A) Results representative data for eld susceptibility versus temperature in-house built static 3 T impulse magnet at the New 400 Mexico Highlands University laboratory. Continu- 300

ous low-fi eld susceptibility versus temperature mea- 300 surements were carried out in a stepwise heating and ). Low fi cooling fashion from 25 °C to 700 °C to 40 °C in an 200 argon atmosphere using a CS-4 furnace attachment for 200 MFK1-A. These experiments allow for an evaluation 100 of the magnetic mineral composition based on Curie 100 Leach Canyon Tuff

point estimates and assist with revealing mixtures of Leach Canyon Tuff 0 magnetic phases within a given sample. Anhysteretic 0 0 0 remanent magnetization (ARM) was imparted in a DC 50 500 450 400 350 300 250 200 150 100 300 600 500 400 200 100 700 fi eld of 0.1 mT and a peak alternating fi eld (AF) fi eld 700 of 95 mT, and then AF demagnetized in ~11 steps to 600 95 mT. Isothermal remanent magnetization (IRM) 600 acquisition experiments were conducted by expos- ing the specimens to a strong magnetic fi eld with an page on this and following 500 impulse magnet from 0.005 T to saturation. Following 500 demagnetization of the ARM, IRM acquisition experi- 400 ments were conducted where samples were treated in 400 a stepwise fashion to higher fi elds until saturation. 300

IRM acquisition and backfi eld IRM acquisition are 300 measured to estimate the coercivity of remanence of the samples and to provide information on magnetic Temperature (Celsius) 200 200

grain size and composition. ARM and saturation iso- 1 ( Appendix Figure of Paradise tuff). and rocks Tuffs, the Bauers, Harmon Hills, Leach Canyon sign ignimbrite sites (two each from from thermal remanent magnetization (SIRM) demagneti- 100 zation curves that are characterized by a high coercive 100 force and high medium destructive fi eld (MDF) values Bauers Tuff Bauers Tuff 0 indicative of single domain (SD) magnetic behavior 0 0 0 60 20 80 40 80 40 (Dunlop and Özdemir, 1997). Alternatively, rapid 60 20 A 180 140 120 100 200 160

140 100 120 decay of natural remanent magnetization (NRM), SI) (E-6 Susceptibility

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B 300 900 Iron Mountain Iron Mountain 800 250 700

200 600

500 150 400

100 300

Heating 200 50 Cooling 100

0 0 0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 700 Temperature (Celsius)

500 1200 Granite Mountain Granite Mountain 450 1000 400

350 800 300

250 600

200 400 150

100 200 50

0 0 0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 700

Appendix Figure 1 (continued). (B) Results from Iron Axis laccolith sites (two each from the Iron Mountain and Granite Mountain lacco- liths). The Three Peaks laccolith yields thermomagnetic behavior similar to that of the other Iron Axis laccoliths (see Table 5).

ARM, and SIRM combined with low coercive force APPENDIX 3. ROCK MAGNETISM point of 580 °C, which decreases nearly linearly values generally indicate multidomain (MD) behavior. with increasing Ti substitution to ~–150 °C for pure Backfi eld IRM experiments were conducted by apply- Low-Field Susceptibility versus ilmenite . Curie or Neel points of other common min- ing specimens to an increasing fi eld to 1.33 T along Temperature Experiments erals include hematite (675 °C), pyrrhotite (320 °C), the –Z axis until the +Z remanence was reduced to and greigite (~330 °C) (Dunlop and Özdemir, 1997). zero (i.e., the sign changed). Finally, the samples were Continuous low-fi eld susceptibility versus tem- To avoid oxidation and chemical alteration of the again saturated in a 1.33 T fi eld and AF demagnetized. perature measurements from room temperature to magnetic phases, all experiments were conducted in The modifi ed Lowrie-Fuller test (Johnson et al., 1975) 700 °C were conducted on representative samples an inert Ar atmosphere; although it is likely that resid-

can be used to estimate the magnetic domain state. from the ignimbrites and the intrusive rocks. Curie ual O2 clinging to the grains results in some degree of This experiment was conducted on the ash-fl ow tuffs points were estimated using either the infl ection oxidation (Petronis et al., 2011). only. The results of all rock magnetic experiments are point (Tauxe, 1998) or Hopkinson peak methods Low-fi eld continuous susceptibility versus tem- provided in Appendix 3. (Moskowitz , 1981). Pure magnetite has a Curie perature experiments from the Three Peaks and

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Granite Mountain laccoliths yield a very narrow yon Tuff yields a slight increase (~10%) in susceptibil- and Taylor (1982) argued that the lower Curie point spectrum of response and reveal a slight increase in ity after complete cycling to 700 °C with fully revers- that is observed during cooling, like the higher Curie room temperature susceptibility of ~20% to ~30% ible curves (Appendix Fig. 1B). Using the Hopkinson point observed during heating, are simply an artifact after the complete cycling to 700 °C, with most sam- peak method (Moskowitz, 1981), the samples yield of the measurement procedure and are controlled by ples fully reversible (Appendix Fig. 1A). Using the inferred Curie point estimates from 534 to 565 °C, the oxygen fugacity and maximum temperature of the infl ection point method of Tauxe (1998), these rocks a temperature range consistent with moderate- to laboratory oven. The Bauers Tuff shows a moderate yield inferred Curie points from 548 to 579 °C, tem- low-Ti titanomagnetite. All curves show an increase (50%) to signifi cant (80%) increase in susceptibility peratures consistent with a low-Ti titanomagnetite to in susceptibility (bump) on heating between ~220 after complete cycling to 700 °C with nonreversible pure magnetite phase with little evidence of a second and 350 °C that is not present on the cooling curve, behavior (Appendix Fig. 1B). magnetic phase at low temperatures (Table 5). Results suggesting the presence of a second magnetic phase. from the Iron Mountain laccolith, in contrast, yield a The Harmony Hills Tuff yields little to no increase in Isothermal Remanent Magnetization (IRM) spectrum of low-fi eld continuous susceptibility versus susceptibility after complete cycling to 700 °C with Acquisition Curves and Backfi eld IRM temperature data indicating the presence of more than fully reversible curves in most cases (Appendix Fig. one magnetic phase in the majority of sites analyzed 1B). Using the infl ection point method (Tauxe, 1998), IRM and backfi eld IRM data from select specimens (Appendix Fig. 1A). All samples reveal a minor to the samples yield inferred Curie point estimates from of each of the tuff units and the Iron Axis laccoliths signifi cant increase in room temperature susceptibility 566 to 578 °C, consistent with low-Ti titanomagne- all show a very narrow range of responses (Appendix ranging from <0% to ~50% after the complete cycling tite to nearly pure magnetite. The rocks of Paradise Figs. 2A, 2B). All curves show steep acquisition with to 700 °C, with only two samples showing fully tuff shows a moderate (50%) to signifi cant (80%) near complete saturation by 0.15 T and no evidence reversible results (Appendix Fig. 1A). Using either decrease in susceptibility after complete cycling to of high-coercivity phases in applied fi elds to 2.5 T. the infl ection point (Tauxe, 1998) or Hopkinson peak 700 °C with nonreversible behavior (Appendix Fig. These results are consistent with low-Ti titanomag- method (Moskowitz, 1981), samples yield inferred 1B). Using the infl ection point method (Tauxe, 1998), netite phase of a restricted magnetic grain size, likely Curie point estimates that range from 555 to 638 °C, the samples yield inferred Curie point estimates that single domain (SD) to pseudo–single domain (PSD). with an average temperature of 570 °C, temperatures range from 562 to 579 °C, consistent with low-Ti Coercivity of remanence values in all units is <0.05 T consistent with moderate- to low-Ti titanomagne- titanomagnetite to nearly pure magnetite, although it (Appendix Figs. 2A, 2B) and distributed unblocking tite and fi ne-grained maghemite (Table 5). Several is likely that the magnetic mineralogy changed during temperatures to 580–600 °C (Fig. 8) indicate that a samples show an increase in susceptibility (bump) on the heating experiment. All curves show an increase Fe-Ti oxide phase, likely low-Ti magnetite, is the heating between ~220 and 350 °C that is not present in susceptibility on heating near 330 °C that is not main remanence carrier in the samples. on the cooling curve, suggesting the presence of a sec- present on the cooling curve, suggesting the presence ond magnetic phase (Appendix Fig. 1A). The bump of a second ferromagnetic phase. The decrease in Lowrie-Fuller Test may indicate that the oxidation of a Fe-Ti oxide phase susceptibility may refl ect that the magnetic fraction is (likely titanomaghemite) is inhomogeneous, and being altered by heating, and this alteration produces Following the modifi ed Lowrie-Fuller tests (John- during the heating experiment the mineral in some a phase with a lower susceptibility. Unfortunately, son et al., 1975), the alternating fi eld (AF) decay of fashion homogenizes to a more susceptible phase, as the nature of this alteration is poorly understood (see normalized natural remanent magnetization (NRM), refl ected by the modest increase in susceptibility (e.g., Hrouda, 2003), yet these results are typically inter- anhysteretic remanent magnetization (ARM), and Kropáček and Pokorná, 1973; Hrouda et al., 2006). preted to indicate that the primary magnetic mineral saturation isothermal remanent magnetization (SIRM) Alternatively, it is possible that some samples might is, in all cases titanomaghemite. The lower Curie was compared for representative samples from the contain a small quantity of pyrrhotite, with a Curie temperatures observed during the cooling cycle may ignimbrites (Appendix Fig. 3). This test is based on point that is between 200 and 400 °C. result from (1) the inversion of titanomaghemite to experimental observations that normalized AF demag- Low-fi eld continuous susceptibility versus tempera- titanium-poor magnetite and ilmenite during heat- netization curves of weak-fi eld thermoremanent mag- ture experiments from the four tuffs yield a spectrum ing and (2) the subsequent reduction of these phases netization (TRM) (i.e., NRM) and strong-fi eld TRM of responses that varied by rock type. The Leach Can- to form titanomagnetite. Experiments by Hammond (i.e., SIRM) have different relationships for SD and

A 1.0

0.8

HC5 - Bauers Tuff Appendix Figure 2 (on this and 0.6 following page). Representa- P21 - Rocks of Paradise tive normalized isothermal P3 - Harmony Hills Tuff remanent magnetization (IRM) acquisition and backfi eld IRM P5 - Leach Canyon Tuff 0.4 demagnetization curves. (A) All Normalized Intensity tuffs reached complete satura- tion by 0.2 T.

0.2

–0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Applied Field (Tesla)

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B 1

0.8 GM3 GM1 0.6 GM4 GM7 0.4 GM10

0.2

0

1

0.8

0.6 IM25 IM28 IM3 0.4 IM34 IM5 0.2 Normalized Intensity

0

1

0.8 TP1

0.6 TP17 TP22 TP4 0.4 TP5

0.2

0 -0.1 0.1 0.3 0.5 0.7 0.9 1.1 Applied Field (Tesla)

Appendix Figure 2 (continued). (B) All laccolith samples show steep acquisition with near complete saturation by 0.15 T.

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1.0 MD grains of magnetite (Dunlop and Özdemir, 1997). ARM Laboratory investigations typically use weak-fi eld Bauers Tuff ARM as an alternative for TRM, and we also used 0.8 SIRM

Normalized Intensity all ignimbrites is likely a ferrimagnetic phase, prob- 0.0 ably SD to PSD low-titanium magnetite, although 0.0 some variability was observed. It is probable that that 20.0 40.0 60.0 80.0 100.0 120.0 maghemite, which may display maximum laboratory unblocking temperatures above magnetite, contributes Applied Field (mT) to the remanence (Dunlop and Özdemir, 1997). 1.0 ACKNOWLEDGMENTS Harmony Hills Tuff We thank Bevan Killpack and the Pine Valley 0.8 SIRM

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