CENOZOIC SOLDIERS PASS VOLCANIC FIELD, CENTRAL UTAH—IMPLICATIONS for the TRANSITION to EXTENSION-RELATED MAGMATISM in the BASIN and RANGE PROVINCE by Eric H

CENOZOIC SOLDIERS PASS VOLCANIC FIELD, CENTRAL UTAH—IMPLICATIONS FOR THE TRANSITION TO EXTENSION-RELATED MAGMATISM IN THE BASIN AND RANGE PROVINCE by Eric H. Christiansen1, Nichelle Baxter1, Thomas P. Ward1, Elizabeth Zobell1, Matthew R. Chandler1, Michael J. Dorais1, Bart J. Kowallis1, Donald L. Clark2

ABSTRACT The late Cenozoic transition from subduction-related to extension-related volcanism is recorded in the Soldiers Pass volcanic field of the southern Lake Mountains, north-central Utah. The Soldiers Pass Formation (new formal name) is a Paleogene (35–33 Ma) suite of intermediate to silicic volcanic rocks interstratified with and overlain by lake and hot-spring deposits. In ascending order, its volcanic units include the trachydacite tuff member, Chimney Rock Pass Tuff Member, breccia member, and andesite member. Nearly horizontal lake and hot-spring deposits (White Knoll Member) are interlayered with and cap the volcanic strata. The volcanic rocks are very high-K, magnesian rocks with large negative Nb anomalies on normalized trace-element diagrams. Mineral compositions show they crystallized at high oxygen and water fugacities. Magma mixing is evidenced by high concentrations of compatible elements in the intermediate composition rocks, plagioclase and sanidine compositions and textures, and disequilibrium mineral assemblages. In short, the Paleogene suite has the characteristics of magmas formed at continental subduction zones. There is no structural evidence of extension during the eruption of the Paleogene suite. Following a lull in volcanic activity of about 14 million years, the 19 Ma Mosida Basalt (new formal name) erupted as one of the oldest basaltic magmas in the eastern Great Basin. This mildly alkaline, potassic trachybasalt has phenocrysts of olivine (Fo60), plagioclase (An65), and clinopyroxene. Trace-element patterns lack large negative Nb and Ti anomalies, consistent with a nonsubduction origin. Tectonic discrimination diagrams also imply a within-plate, alkalic character. We conclude that it is one of the oldest asthenosphere-derived magmas in the Great Basin, but low Mg/(Mg+Fe), Ni and Cr concentrations, and relatively Fe-rich olivine compositions show that it is not primary. This transitional magmatic sequence is probably the result of the progressive foundering of a shallowly dipping subducting slab that began during the Eocene below this part of the Great Basin. Foundering produced widespread dehydration of the subducted lithosphere and generated voluminous arc like magma that intruded, hybridized, and differentiated in the crust. Compensating inflow of asthenospheric mantle beneath the Great Basin, along with`1 the development of a transform boundary and lithospheric extension, eventually resulted in decompression melting of the mantle by 19 Ma. The Mosida Basalt has not been tilted but the Lake Mountains horst is bounded on the east and west by normal faults.

INTRODUCTION The onset of lithospheric extension has both structural and magmatic aspects. Some aspects of the struc- The Basin and Range Province of the western United tural transition and many aspects of the magmatic transi- States is one of Earth’s most studied regions of continen- tion (from subduction related to extension related vol- tal rifting, and yet the timing and character of the onset canism) are recorded in the southern part of the Lake of Cenozoic extension are still controversial. Some struc- Mountains of western Utah, which lie near the eastern tural studies have concluded that extension in the Great border of the Great Basin (figure 1). In the Soldiers Pass Basin part of the Basin and Range Province is as old as volcanic field, a Paleogene suite of silicic ignimbrites 41–37 Ma (Rahl and others, 2002) or 32–30 Ma (e.g., and intermediate-composition lavas is interlayered with Axen and others, 1993). Others have concluded that sig- lake and hot spring deposits (figure 2). Following a hia- nificant extension on large, high-angle normal faults tus in magmatism, a mafic lava erupted as one of the old- started much later. For example, Parry and Bruhn (1986) est basaltic magmas in the eastern Great Basin. concluded that the Wasatch fault has been active since Our study aims to answer questions about the timing about 17–18 Ma. and nature of the onset of extension in the Great Basin 1 Brigham Young University, Department of Geological Sciences, Provo, Utah 84602 [email protected] 2 Utah Geological Survey, P.O. Box 146100, Salt Lake City, Utah 84114-6100 2 Central Utah - Diverse Geology of a Dynamic Landscape

Figure 1. Index map of north-central Utah. Rectangle shows location of Soldiers Pass volcanic field (figure 2) in the southern Lake Mountains. The Lake Mountains form a horst between grabens in Utah and Goshen Valleys on the east and Cedar Valley on the west.

Figure 2. North map: Generalized geologic map of the Soldiers Pass volcanic field (modified from Proctor, 1985; Biek and others, 2006; Clark and others, 2006). 2007 UGA Publication 36 - Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C. Jr., editors 3

Figure 2. South map: Generalized geologic map of the Soldiers Pass volcanic field (modified from Proctor, 1985; Biek and others, 2006; Clark and others, 2006). using the petrochemistry of volcanic rocks. We focus on magmatism swept across what is now the eastern Great evidence for a petrologic transition from subduction- to Basin, and was later supplanted by a younger suite of extension-related magmatism. Using new geochemical bimodal basalt and rhyolite (Best and Christiansen, data and 40Ar/39Ar ages, we show that extension-related 1991). The volcanic products of these episodes form magmatism began about 19.5 Ma in this region. some of the low hills south of Soldiers Pass and constitute the Soldiers Pass volcanic field. We show below that the petrologic transition is contemporaneous with the GEOLOGIC SETTING onset of normal faulting that created the Basin and Range Province. The Lake Mountains form a horst bounded by Soldiers Pass lies in the southern Lake Mountains of the Cedar Valley graben on the west and the composite north-central Utah (figure 1). The pass separates the main Utah-Goshen Valley graben on the east. The Lake mountain range from a series of low hills to the south. Mountains are the easternmost range in this part of the The sedimentary rocks of the southern Lake Mountains Great Basin (figure 1) and lie about 30 km west of the are dominated by shallow-marine limestone and sand- province-bounding Wasatch fault zone. stone of Mississippian to Pennsylvanian age. South of The Soldiers Pass volcanic field lies between two of Soldiers Pass sedimentary rocks exposed in the low hills Utah’s major volcanic fields and their associated mining are as old as Silurian. These sedimentary rocks were districts: the Bingham district to the north and the Tintic folded into a broad syncline in the core of the range and district to the south. The Bingham area was volcanically smaller, tighter folds formed to the south and east of active between 39 and 33 Ma (Babcock and others, 1997; Soldiers Pass (Biek and others, 2006; Clark and others, Deino and Keith, 1997). The Tintic district is 30 km 2006). Folding was presumably related to the Sevier south of Soldiers Pass (Morris and Lovering, 1979) and orogeny of Cretaceous to early Paleogene age (DeCelles, was volcanically active from about 39 to 32 Ma (Clark, 2004). Beginning in the Eocene, intermediate to silicic 2003; Moore and others, this volume). No mineralization 4 Central Utah - Diverse Geology of a Dynamic Landscape is known in the Soldiers Pass area; the only known tions and members is depicted on several detailed geo- hydrothermal deposits are veins and irregular masses of logic maps, except for a small unmapped area in the coarse brown calcite that cut the Paleozoic limestones Goshen Pass quadrangle (figure 2; Proctor, 1985; Biek (Hoffman, 1951). and others, 2006; Clark and others, 2006). Formalizing these mappable rock units will aid in reconstructing the geologic history, making regional correlations, and in CENOZOIC STRATIGRAPHY land use planning. New lithologic and stratigraphic data, 40Ar/39Ar Background and Stratigraphic Nomenclature ages, mineral chemistry, and whole-rock geochemical 40 39 2 data for each unit are given below. Ar/ Ar ages were The Soldiers Pass volcanic field covers about 85 km determined at the New Mexico Geochronology Research extending from near Soldiers Pass on the north, south- Laboratory and are reported in table 1. Chemical anal- ward along the southern Lake Mountains between Cedar yses were performed at Brigham Young University. A Valley (west) and Utah-Goshen Valley (east) to Chimney description of the geochemical techniques used and X-ray Rock Pass in the northernmost East Tintic Mountains fluorescence analyses of international reference materials (figure 2). This volcanic field encompasses parts of the is available at http://www.geology.byu.edu/faculty/ehc/. Soldiers Pass, Goshen Valley North, Goshen Pass, and Representative analyses are given in table 2 and a larger ′ Allens Ranch 7.5 quadrangles. Volcanic rocks from the whole-rock data set is included in Biek and others (2006). Tintic volcanic field overlap in age with those of the Soldiers Pass volcanic field in Chimney Rock Pass. The Soldiers Pass Formation volcanic and sedimentary rocks of this area were originally mapped in detail in the late 1940s to early 1950s, The Paleogene rocks of the southern Lake Mountains mostly by individuals affiliated with Brigham Young are included in the Soldiers Pass Formation (new formal University (Rigby, 1949; Stringham and Sharp, 1950; name), named for exposures in and near the pass. The Bullock, 1951; Hoffman, 1951; Smith, 1951; Williams, formation consists of four volcanic members and an 1951; Ornelas, 1953). Their map units included geo- interstratified lacustrine-fluvial deposit (figure 3). Bul- graphic names that are not on modern maps, including lock (1951) grouped the volcanic rocks in the area with the Fox Hills, Mosida Hills, and Selma Hills. Informal the Salt Lake Formation, which is now known to be rock-type names of Tertiary age were typically applied younger. (basalt, limestone, clay, pumice, tuff breccia, and undif- ferentiated volcanics). However, Bullock (1951) and Trachydacite Tuff Member Ornelas (1953) grouped some of the volcanic and sedimentary rocks in the area with the Salt Lake Formation, The oldest volcanic unit in the southern Lake which is now known to be younger (Miocene to Mountains is a densely welded, phenocryst-rich ignim- Pliocene). Proctor and others (1956) and Proctor (1985) brite we informally name the trachydacite tuff member. subsequently remapped the Selma Hills area (Allens The type locality is 3 km southeast of Soldiers Pass at Ranch quadrangle), which includes our basalt, limestone, 40.170° N. and 111.950° W. The trachydacite tuff crops and rhyolite tuff units. The recent interim maps of the out only in a small area (less than 4 km across) near the Soldiers Pass quadrangle (Biek and others, 2006) and type locality (figure 2). Its maximum exposed thickness Goshen Valley North quadrangle (Clark and others, is 20 m. The tuff lies unconformably on folded Missis- 2006) used informal names for the various Cenozoic rock sippian limestone and is overlain unconformably by the units. newly named Chimney Rock Pass Tuff Member. We We herein propose new formal and informal names obtained a single-crystal laser fusion 40Ar/39Ar age on for the Cenozoic rock units of the Soldiers Pass volcanic biotite of 34.79 Ma (table 1), and an 40Ar/39Ar age of field. We name the lower part of this section the Soldiers 34.7 ± 0.2 Ma is reported by Moore and others (this vol- Pass Formation and divide it into five members, in ume) for this unit. ascending order: trachydacite tuff member, Chimney Phenocrysts comprise as much as 30% of the trachy- Rock Pass Tuff Member, andesite member, breccia mem- dacite tuff with plagioclase > biotite > amphibole > Fe Ti ber, and White Knoll Member. We consider the trachy- oxides. Accessory minerals include apatite, zircon, and dacite, andesite, and breccia members to be informal very rare barite. The phenocrysts are set in a black, glassy members due to their limited lateral extent, and consider groundmass of rhyolitic composition. Plagioclase crys- the more laterally extensive Chimney Rock Pass Tuff and tals are strongly zoned and range widely from An40 to White Knoll Members to be formal members. We name An85. The plagioclase compositions form a bimodal dis- the uppermost rock unit (formation) of the volcanic field tribution. Plagioclase grains also display unique sieve the Mosida Basalt. Because these formations and mem- textures wherein resorbed calcic plagioclase is healed by bers are largely nonstratiform bodies, no type section is more sodic plagioclase. The compositions and textures designated; however, type localities of representative are both strong evidence that magma mixing occurred outcrops within the volcanic field (type area) are pre- shortly before eruption. The amphiboles are tscher- sented below and on figure 2. The extent of these forma- makitic and Fe-poor (Fe/[Fe+Mg] ~0.30 to 0.35) like the 2007 UGA Publication 36 - Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C. Jr., editors 5 8 Tst 0.10 1.72 trachydacite 9 Tsc 0.08 0.60 34.73 34.79 11 Tsc 0.07 1.68 Chimney Rock Pass Tuff Soldiers Pass Formation 22 Tsc 0.07 6 Tsa 4.20 9 Tsb 0.65 3.13 107.72 1.52 breccia andesite 9 Tb 0.17 3.65 19.65 33.73 34.90 34.70 34.70 Ar as determined by isochron analysis 36 Ar/ Basalt Mosida 40 9 Tb 0.14 1.41 19.47 1365.4 1367.3 1393.6 1370.0 1392.9 1390.7 1392.2 1388.8 40.17759 40.15062 40.15045 40.15850 40.20338 40.15518 40.06028 40.15631 111.97360 111.98955 111.99398 111.97190 111.97778 111.97959 112.02833 111.97585 Step heating Step heating Step heating Step heating Laser fusion Laser fusion Laser fusion Laser Groundmass Groundmass Groundmass Groundmass Sanidine Sanidine Sanidine Biotite isochron age isochron age isochron age spectrum single crystals single crystals single crystals step heating

Ar ages of volcanic units from the Soldiers Pass volcanic field, central Utah

39 Ar inter 299.6 ± 1.2 298 ± 3.1 304.4 ± 3.0 288 ± 62 -6 Ar/ 36

40 Ar/ Ar inter = intercept value of Steps 2 σ Age (Ma) MSWD Member Sample NumberLatitude (°N) SP-3303 SP-4003 SP-3205 SP-4103 SP-603A SP-1603B AR-105 SP-1903 Formation Longitude (°W) Map Unit Method 40 J x 10 Material 36 = error of the mean, multiplied by the root of the MSWD, incorporates uncertainty in J factors and irradiation correction uncertainties Ar/ Notes: MSWD = mean weighted standard deviation 2 σ FC-2 (Fish sanidine Canyon Tuff) used Assigned as age flux = monitor. 28.02 Ma Analyses by New Mexico Geochronology Research Laboratory Location using WGS84 reference frame Table 1. Table 40 6 Central Utah - Diverse Geology of a Dynamic Landscape

Table 2. Average compositions of volcanic units from the Soldiers Pass volcanic field, central Utah

Formation Mosida Basalt Soldiers Pass Formation

Chimney Rock Member breccia andesite Pass Tuff trachydacite Map Unit Tb Tsb Tsa Tsc Tst Rock type trachybasalt shoshonite andesite rhyolite tracydacite Age (ma) 19.6 33.7 34.5 34.7 34.7 Samples (n) 12 78 78

SiO2 48.07 51.27 55.87 71.87 63.72

TiO2 2.64 1.42 1.03 0.22 0.79

AI2O3 16.40 15.02 15.02 13.08 16.66

Fe2O3 12.01 9.71 8.01 1.44 3.81 MnO 0.15 0.12 0.13 0.03 0.06 MgO 4.40 3.63 4.23 0.47 1.07 CaO 8.29 9.74 7.73 1.71 4.09

Na2O 3.31 3.42 3.46 2.58 2.86

K2O 2.24 2.40 2.40 6.02 4.61

P2O5 0.68 0.53 0.36 0.04 0.19 Subtotal 98.19 97.25 98.24 97.47 97.86 LOI 1.30 2.66 1.70 3.45 2.33 Total 99.49 99.02 99.52 100.92 100.19

V 237 228 179 18 60 Cr 23 221 193 46 Ni 18 63 52 24 Cu 28 33 36 36 Zn 95 90 85 27 60 Ga 22 20 20 14 20 Rb 43 54 58 242 153 Sr 983 932 897 230 674 Y 25 26 18 20 34 Zr 264 225 218 152 346 Nb 38 12 11 15 15 Ba 930 1226 1208 971 1753 La 51 57 40 52 66 Ce 133 142 105 102 140 Nd 49 51 38 33 56 Sm 11 12 10 6 13 Pb 8 12 14 29 20 Th 54 6 22 13 U33 3 86

Notes: Major oxides reported in weight percent and trace elements reported in parts per million (ppm) by X-ray fluorescence spectrometry at Brigham Young University. LOI is loss on ignition at 1000°C for 4 hours. Refer to Biek and others (2006) for complete data set. 2007 UGA Publication 36 - Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C. Jr., editors 7

Figure 3. Stratigraphic relations and ages of rocks in the Soldiers Pass volcanic field, southern Lake Mountains. All units except the Mosida Basalt belong to the Soldiers Pass Formation. Vertical scale is not linear and shows only stratigraphic position and interfin- gering relations. biotites (Fe/[Fe+Mg] ~0.29 to 0.34). High concentrations collected from the type locality at Soldiers Pass yielded 40 39 of TiO2 (~5.2 wt. %) in the biotite and compositions of an Ar/ Ar age of 34.2 Ma ± 0.2 (Moore and others, co-existing magnetite-ilmenite and hornblende-plagio- this volume). Three new laser fusion 40Ar/39Ar sanidine clase show that eruption temperatures (870°C to 900°C) ages are listed in table 1. All three ages agree very well and oxygen fugacities (2.3 log units above the QFM with one another and yield an age of 34.7 Ma for the [quartz-fayalite-magnetite oxygen buffer]) were high Chimney Rock Pass Tuff. (Ward, 2006). Pumice clasts constitute about 20% of the rock and The high alkali content (7.5 wt. % K2O+Na2O; fig- are as much as 1 m across (figure 6). Lithic fragments are ure 4) sets the trachydacite tuff apart from most of the locally abundant and also reach extreme sizes (as much large-volume dacitic ignimbrites that erupted during the as 1 m across) in the central part of the mapped area near Oligocene in the Great Basin (see, for example, L. Black Point (figure 2). The lithic inclusions are mostly Maughan and others, 2001). The trachydacite tuff mem- fragments of fine-grained volcanic rock. The location of ber has rather high FeO/(FeO+MgO) ratios (figure 4) and the vent is unknown, but the size of the lithic fragments straddles the ferroan-magnesian dividing line of Miya- near Black Point suggests that this is a near-vent facies. shiro (1974). The tuff is also calc-alkalic and very high- Lithic (and pumice) fragments are smaller in outcrops in K (figure 4). The trace-element patterns for several sam- the Chimney Rock Pass area (typically less than 3 cm ples of the tuff are given on figure 5 and show that the across), but the mineral assemblage and elemental com- tuff is especially enriched in Ba and Zr. It also has the position are indistinguishable from the outcrops to the characteristic Nb and Ti depletions of subduction-zone north. magmas. The phenocrysts in the Chimney Rock Pass Tuff Member and in its pumice clasts consist of plagioclase > Chimney Rock Pass Tuff Member sanidine > quartz > biotite > Fe-Ti oxides, together with The Chimney Rock Pass Tuff Member of the glassy, bubble-wall shards and angular pumiceous clasts Soldiers Pass Formation (new formal name) is a non that have prominent tube vesicles. Zircon and apatite are welded, pumice rich rhyolitic ignimbrite with about 20% accessory minerals. The plagioclase is typically reverse- phenocrysts. The light-colored (various shades of white, ly zoned (An25 to An55). Sanidine also varies widely in pink, tan, gray, or orange) tuff is prominently exposed at composition (Or50 to Or75) and is Ba-rich. The biotite the type locality in Chimney Rock Pass (40.062° N. and Fe/(Fe+Mg) ratio is about 0.40, higher than that of biotite 112.037° W.) in the central part of the Allens Ranch 7.5′ in the trachydacite tuff member, but still rather low for quadrangle and at a reference locality in the “pumice pit” rhyolite, and indicative of high oxygen fugacities. just west of Soldiers Pass (figures 2 and 6). The maxi- The Chimney Rock Pass Tuff Member is very high- mum exposed thickness of the Chimney Rock Pass Tuff K2O, calc-alkalic, magnesian (figure 4), and composi- Member is about 20 m. The tuff is distributed discontin- tionally similar to other rhyolite ignimbrites from the uously from the type locality northward to Soldiers Pass, Great Basin. Bulk samples of the tuff have silica contents a distance of about 20 km. In Chimney Rock Pass, the that range from 71% to 75%, but pumice clasts range tuff overlies another rhyolitic tuff with prominent quartz more widely in composition (to as low as 65% SiO2) with phenocrysts mapped as the Packard Quartz Latite high alkali concentrations that make them trachydacites (Proctor, 1985), a more voluminous rhyolitic tuff found (figure 4). Trace-element patterns in the ignimbrite are in the northern East Tintic Mountains (Morris and marked by deep Nb and Ti anomalies (figure 5). Strong Lovering, 1979; Moore and others, this volume). In the depletions of Ba, Sr, and P are most likely the result of Soldiers Pass area, the Chimney Rock Pass Tuff Member extensive sanidine, plagioclase, and apatite fractionation unconformably overlies eroded knobs of the trachydacite from a less-evolved parent. tuff member and is overlain by sedimentary rocks of the The variety of compositions found in the pumice White Knoll Member. A sanidine separate from a sample fragments (figure 4) and complex zoning relations in the 8 Central Utah - Diverse Geology of a Dynamic Landscape

Figure 4. (A) The Soldiers Pass volcanic field includes a Paleogene sequence of shoshonites-andesites-trachydacites and rhyolites that contrasts with the Neogene trachybasalts (classification of LeBas and others, 1986). Gray line separates alkaline from subalka- line Hawaiian rocks (Macdonald, 1968). (B) Most of the volcanic rocks of the Soldiers Pass volcanic field are calc-alkalic with the exception of the Mosida Basalt of Neogene age (classification of Frost and others, 2001). (C) The shoshonite breccia member is the only Paleogene unit that is not distinctly magnesian (classification of Miyashiro, 1974). FeO* = total Fe as FeO. (D) Paleogene and Neogene rocks are high-K2O to shoshonitic in composition (classification of LeBas and others, 1986). Refer to Biek and others (2006) for the analyses. feldspars conclusively show that a Ba-rich trachydacitic As the two magmas mixed, the sanidine and plagioclase magma mixed and partially equilibrated with rhyolite from the Ba poor magma crystallized more Ba rich rims, magma shortly before eruption. We have found several whereas plagioclase derived from the Ba rich magma populations of feldspars in the tuff and pumice clasts. Ba- crystallized Ba poor rims. The magmas must have been rich calcic plagioclase cores have sodic rims with lower only partially mixed at the time of eruption, as evidenced Ba; in other grains, Ba-poor sodic plagioclase is mantled by the range of compositions among the pumice frag- by Ba-rich, more calcic plagioclase similar in composi- ments. The trachydacitic pumice clasts represent the tion to the cores of the other grains. On the other hand, more mafic end member, which was rich in Ba, whereas sanidine grains are reversely zoned, with Ba-rich rims. the most silicic pumice clast probably represents the We interpret this to signify that an originally Ba poor, more silicic end member, which was poor in Ba. The tuff evolved rhyolite was crystallizing phenocrysts of Ba samples represent thorough mixing of small pyroclasts as poor sanidine and plagioclase (along with quartz, biotite, a result of fragmentation during eruption of both end and Fe-Ti oxides). Shortly before eruption, a more Ba members. Other evidences for magma mixing in the rich, trachydacitic magma mixed with the rhyolite Chimney Rock Pass Tuff Member are calculated temper- magma. Because no normally zoned sanidine (with high ature differences for the high-Ba sanidine-plagioclase concentrations of Ba) was found, it is likely that the more pairs (815°C to 830°C) and the low-Ba sanidine plagio- Ba rich magma only had phenocrysts of plagioclase (plus clase pairs (700°C to 720°C), as well as the difference in Ba-rich biotite and Fe-Ti oxides) at the time of mixing. temperatures calculated from Ba-rich (790°C) and Ba- 2007 UGA Publication 36 - Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C. Jr., editors 9

Figure 5. Trace-element patterns for the Paleogene volcanic rocks of the Soldiers Pass Formation (A, B, and C) are significantly dif- ferent from those of the Neogene Mosida Basalt (D), which are relatively smooth and have only small negative Nb-anomalies. Normalization values and patterns for mid-ocean ridge and ocean island basalt from McDonough and Sun (1995).

Figure 6. The Chimney Rock Pass Tuff is a rhyolitic tuff with abundant pumice and lithic clasts. In this photo, pumice clasts as much as 10 cm across are angular and have extensive networks of tube vesicles. Photograph is from the reference locality at the “Pumice Pit” near Soldiers Pass. 10 Central Utah - Diverse Geology of a Dynamic Landscape poor (758°C) biotites (Baxter, 2006). The fact that the stone; it is unconformably overlain by a much younger Ba-rich minerals produced temperatures higher than pahoehoe lava flow—the Mosida Basalt. The sedimenta- those of the low-Ba minerals indicates that a hotter tra- ry unit is as much as 100 m thick. Bedding ranges from chydacitic magma produced the higher-Ba feldspars (and laminated to medium to indistinct. Locally, thin pyro- biotite), and a cooler rhyolitic magma produced the clastic-fall beds, altered to clay, are interstratified with feldspars (and biotite) with less Ba. the limestone. Fossils of terrestrial plants are found at a few sites. We interpret this assemblage to have been Andesite Member deposited in and along the shores of a shallow lake. In addition, some outcrops contain vertical laminae of We informally name a flow-layered, medium gray travertine and algal laminations suggestive of spring lava flow with no phenocrysts and platy fractures the deposits (Bullock, 1951) near the margin of the lake (fig- andesite member of the Soldiers Pass Formation from ure 7). ledgy outcrops in the southern part of the Soldiers Pass The numerical age of the White Knoll Member is quadrangle (figure 2). The type locality is Black Point given by the radiometric age of the coeval shoshonitic (40.178° N. and 111.972° W.). Outcrops cover an area of 2 breccia member, which is 33.7 Ma (table 1). The ages of about 0.5 km . The source vent is not known but is prob- the underlying andesite member (~35 Ma) and overlying ably near Black Point because of the limited extent of the Mosida Basalt (19.5 Ma) yield a consistent, but much flow. The exposed maximum thickness is about 12 m. looser, constraint. The lava unconformably overlies the Chimney Rock Pass Carbonate lake sediments are common in the Tuff Member and is locally overlain by the White Knoll Paleogene of central Utah and nearby regions (Witkind Member. The andesite is a small, isolated lava flow and and Marvin, 1989; Nutt and Howard, 2003). Morris and was not part of a large stratovolcano complex like those of similar age in the Oquirrh Mountains to the north (D.T. Lovering (1961) correlated the rocks of the White Knoll Maughan and others, 2001) and the East Tintic Member with a similar rock unit, the Sage Valley Mountains to the south (Moore and others, this volume). Limestone, which also consists of lacustrine limestone A new, but imprecise, 40Ar/39Ar age for a ground- and claystone and is exposed 50 km to the south. The mass separate from this lava flow is 34.9 ± 4.2 Ma (table Sage Valley Limestone lies beneath the 37.4 Ma tuff of 1). Considering the large error, this age is consistent with Little Sage Valley and above the 38.6 Ma Chicken Creek the 34.7 Ma age of the Chimney Rock Pass Tuff which Tuff in Sage Valley itself (Clark, 2003). According to underlies the andesite flow. McKee and others (1993) Witkind and Marvin (1989), the Sage Valley Limestone reported a younger K-Ar age of 32.6 ± 1.0 Ma for this also lies beneath the approximately 34 Ma Packard unit. Both dated samples are from the type locality. Quartz Latite (which they correlate with the Fernow This small lava flow is fairly homogeneous andesite Quartz Latite) in the East Tintic Mountains. Lacustrine with about 57% SiO2 and plots near the dividing line for limestone and shaley limestone of similar character are trachylavas on the total alkali versus silica diagram (fig- also interstratified with Paleogene volcanic rocks in the ure 4). Like other rocks in the Soldiers Pass volcanic East Tintic Mountains (Moore and others, this volume), field, the lava is high-K, calc-alkalic, and magnesian (fig- but their relationship to the White Knoll Member is ure 4). These characteristic are held in common with presently unclear. We speculate that a high, broad “alti- many other intermediate-composition lavas of middle plano” that developed during the Sevier orogeny persist- Cenozoic age from the eastern Great Basin (Barr, 1993). ed through the Paleogene and may have been dotted with Incompatible trace-element patterns have prominent neg- large and small lakes. Several modern orogenic plateaus ative Nb and Ti anomalies, and enrichments in Rb, Ba, U, have poorly integrated drainage networks with common and K. lakes (e.g., Lake Titicaca and the salars of Bolivia and northern Argentina). Alternatively, the lakes may have White Knoll Member formed as a result of volcanic damming of local streams, as on the southeast flank of the Bingham volcanic com- The most extensive Cenozoic bedrock unit in the vol- plex (Biek, 2006). canic field is a light-colored (various shades of white, gray, tan, yellow, orange, pink) package of lacustrine Breccia Member limestone with thin interbeds of pale-orange to gray claystone (beds of altered volcanic ash), fluvial sandstone A distinctive member of the Soldiers Pass Formation and conglomerate that we formally name the White is a shoshonitic flow breccia we informally name the Knoll Member of the Soldiers Pass Formation from out- breccia member (figure 8). It is exposed near Goshen crops on White Knoll (40.164° N. and 111.962° W.). A Pass and northeastward to White Knoll (figure 2). The reference locality lies in a gully near Soldiers Pass type locality is in the low hills at 40.149° N. and (40.188° N. and 111.963° W.). It outcrops as ledges and 111.996° S., about 0.5 km east of Goshen Pass. The brec- weathered slopes over an area of at least 30 km2. In the cia member laterally interfingers with and is partially Soldiers Pass volcanic field, the nearly horizontal White overlain by the carbonate and clastic sediments of the Knoll Member is draped on paleotopography formed on White Knoll Member. eroded Paleogene volcanic rocks and Paleozoic lime- The breccia member crops out as slopes, ledges, 2007 UGA Publication 36 - Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C. Jr., editors 11

Figure 7. The White Knoll Member of the Soldiers Pass Formation is composed of dom- inantly lacustrine carbonate sediments. (A) Light-colored (yellows and tan) limestones are thinly laminated, vuggy, and have highly regu- lar bedding. (B) Locally, carbonate beds are cross-cut by nearly vertical laminations sug- gesting that some of the unit was deposited as tufa around hot springs on the margin of a freshwater lake. These outcrops are in a drainage 1.25 km southeast of Soldiers Pass.

rounded knobs, and small cliffs covering an area of a few we conclude it was produced when a shoshonitic lava km2 (figure 2). The unit varies greatly in thickness to as flow entered a shallow lake. The vent for the flow has not much as 50 m, replacing an equivalent section of sedi- been identified but must have been near the present out- mentary beds in the White Knoll Member. Some outcrops. A new 40Ar/39Ar isochron age on the freshest crops consist of dark, vesicular, fine grained lava with a groundmass material we could separate is 33.73 ± 0.65 trachytic to felty groundmass of plagioclase, olivine (typ- Ma (table 1). The large uncertainties in the ages of the ically altered), and Fe-Ti oxides, but most of the unit is a andesite and breccia members allow for them to be carbonate impregnated brecciated shoshonitic lava (fig- coeval, but stratigraphic relations clearly show that the ure 8). Fragments of brecciated lava are as much as 1 m breccia member is younger. across and commonly have reddened, quenched, vesicu- The composition of the breccia member differs in lar rinds and more massive crystalline interiors. The frag- many respects from the older andesite member, even ments are commonly supported by coarse-grained, vuggy though their outcrop areas are similar. Samples of the calcite. Locally, rounded pillow structures are apparent lava breccia are all shoshonitic (figure 4) and have lower along with the more common angular blocks. The brec- silica but similar total alkali concentrations when com- cia grades laterally into layers of scoria fragments (about pared with the andesite member. The silica content of the 1 cm or less across). Because the breccia is interbedded shoshonite is among the lowest 3% of all the intermedi- with the sedimentary strata of the White Knoll Member, ate-composition lavas of middle Cenozoic age from the 12 Central Utah - Diverse Geology of a Dynamic Landscape

A Figure 8. Shoshonite lava comprises the breccia member of the Soldiers Pass Formation which interfingers with and is partially overlain by the lacustrine sediments of the White Knoll Member. (A) Much of the unit is autoclastic breccia impregnated with light-colored carbonate. Fragments of brecciated lava are more than 1 m across. This outcrop from the type area near Goshen Pass is about 10 m high. (B) The fragments in the breccia commonly have reddened, quenched, vesicular rinds, and more massive crystalline interiors. They are surrounded by thin layers of coarse grained, vuggy calcite. Calcite crystals point inward. Locally, rounded pillow structures are apparent but blocks are more common. Apparently the breccia formed when a shoshonitic lava flow entered a shallow carbonate-saturated lake. Lava quenched and frag- mented as it interacted with the lake. Heating of the lake water caused precipitation of massive amounts of carbonate.

Great Basin analyzed by Barr (1993). Samples of the mas (figures 4 and 9). Apparently, the andesite and flow have relatively low MgO and high Fe2O3 concen- shoshonite were formed and evolved independently. In trations and are ferroan (figure 4), unlike the other common with other Paleogene volcanic rocks from the Paleogene volcanic rocks in the Soldiers Pass volcanic Soldiers Pass area, the shoshonite of the breccia member field. The shoshonite breccia member has higher light has a deep Nb anomaly and is enriched in soluble ele- REE and Y concentrations (figure 5), as well as higher Ti, ments such as Ba, Rb, K, and Pb (figure 5 and table 2), Fe, Ca, and P than the andesite member (figure 9 and elements known to be transported in subduction-zone table 2). In spite of its lower silica concentrations, the fluids. shoshonite has lower MgO. However, it has distinctly higher concentrations of other compatible elements Mosida Basalt including V, Cr, and Ni (figure 9 and table 2) compared to the older andesite. This shows that the andesites are Neogene magmatism is represented here by the not differentiates of a magma similar to the shoshonites. Mosida Basalt (new formal formation name). This mafic Moreover, the andesite does not lie on mixing lines lava flow typically lies atop the White Knoll Member of between the shoshonite and any of the more silicic mag- the Soldiers Pass Formation, but in places it directly 2007 UGA Publication 36 - Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C. Jr., editors 13

Figure 9. Silica variation diagrams display the compositional variety of the rocks in the Soldiers Pass volcanic field. Refer to Biek and others (2006) for the analyses. 14 Central Utah - Diverse Geology of a Dynamic Landscape overlies the breccia member. No bedrock units overlie DISCUSSION the basalt, but it is locally mantled by Quaternary deposits from Lake Bonneville, and by colluvial and allu- Geochemical Comparisons vial sediments. The type locality for the unit is 2 km south of Soldiers Pass at 40.180° N. and 111.974° W. Two distinctive suites of volcanic rocks erupted dur- (figure 2). ing the Cenozoic history of the Soldiers Pass volcanic Scattered erosional remnants of the pahoehoe flow field: a Paleogene calc-alkalic suite and a younger show its original length was probably at least 15 km (fig- Neogene alkalic suite. The Paleogene suite (35–33 Ma) ure 2). The flow is locally vesicular to scoriaceous and includes the trachydacite and rhyolite tuffs, as well as caps ridge tops to form blocky ledges and small cliffs. intermediate-composition lavas. These calc-alkalic units The flow has a maximum thickness of about 35 m. No are high-K to shoshonitic and have low Nb, Ti, and LREE concentrations (figures 4 and 5). The Fe-Ti oxide source vent is exposed, but based on flow morphology it compositions, low Fe/Mg ratios of mafic silicates, and was probably located near Soldiers Pass. Two new 40 39 the magnesian character of the rocks (except the Ar/ Ar isochron ages of 19.47 ± 0.14 and 19.65 ± 0.17 shoshonitic breccia member) show that these magmas Ma were obtained from the groundmasses of two sam- crystallized at high oxygen fugacities, several log units ples (table 1). McKee and others (1993) previously above the quartz-fayalite-magnetite oxygen buffer. The reported a whole-rock K-Ar age of 17.0 ± 0.5 Ma. The presence of amphibole and biotite are evidence that the lava flow is thus Neogene (Miocene) in age and signifi- more silicic magmas were also fairly hydrous. Magma cantly younger than the next older volcanic unit. mixing is evidenced by high concentrations of compati- The dark lava is porphyritic with common phe- ble elements (Mg, Cr, Ni) in the andesites and shoshon- nocrysts (10% to 20%) of plagioclase > clinopyroxene > ites, and by complex sanidine and plagioclase textures olivine > Fe-Ti oxides, all set in a fine grained ground- and compositions in the more silicic magmas. Finally, the mass. Plagioclase shows oscillatory zonation from anor- trace-element patterns are marked by deep negative Nb thite-rich cores to more sodic rims (An72 to An58). and Ti anomalies, and high concentrations of soluble Clinopyroxene varies slightly in composition (Fe/[Fe+ metals (Ba, Rb, Sr, K, and Pb). All of these characteris- Mg] ~0.22 to 0.28) and is commonly found in glomero- tics are common in magmas formed at continental sub- crystic clusters with plagioclase. Olivine is weakly zoned duction zones. Consequently, on the discrimination dia- and ranges from Fo68 to Fo60, excluding narrow Fe-rich grams of Müller and Groves (2000), the Paleogene suite rims crystallized after eruption, but it is commonly falls in the continental arc fields, as a result of low TiO2 altered to iddingsite. and Nb/Zr ratios (figure 10). In the discrimination dia- The Mosida Basalt is chemically distinctive from the grams for silicic magmas of Pearce and others (1984), Paleogene series. On the total alkali versus silica diagram both of the Paleogene silicic units plot as volcanic arc (figure 4), it plots as a potassic trachybasalt (or magmas. absarokite). It is also slightly silica undersaturated and In contrast, the Neogene Mosida Basalt has elemen- more sodic than any of the other volcanic rocks, although tal and mineralogical characteristics of within-plate it also has very high K2O concentrations (figure 4). Even basalt, typical of rift and mantle-plume associations (fig- though the flow has lower silica than any of the other vol- ure 10). Silica-undersaturated lavas are uncommon in canic units, it also has lower concentrations of compati- other tectonic settings, but are common in continental ble elements like Cr, Ni, and Cu (and MgO in some sam- rifts and above oceanic as well as some continental ples) than the older intermediate-composition rocks (fig- hotspots. On the discrimination diagrams of Müller and ure 9 and table 2). It is distinctly enriched in Nb, Ti, and Groves (2000), the Mosida Basalt plots in the within- plate fields as a consequence of high TiO2 and Nb/Zr Zr compared to the Paleogene suite. The low MgO (<5 ratios (figure 10). The normalized trace-element pattern wt. %), Mg/(Mg+Fe) (0.42), Ni (<30 ppm), and Cr (<50 of the trachybasalt lacks a large negative Nb anomaly or ppm), coupled with Fe-rich olivine, show that the lava is a significant Ti anomaly (figure 5). Overall, the trace-ele- not primary. The mafic magma must have experienced ment pattern of the trachybasalt is similar to ocean island extensive fractionation en route to the surface. basalts except for enrichments of Ba, U, and K. Compared to the Paleogene rocks, the Mosida Basalt In a survey of mafic lavas erupted during the Neo- has a much smoother trace-element pattern (figure 5), gene from the Basin and Range Province, Fitton and oth- with only small negative Nb and Ti anomalies. Moreover, ers (1991) identified several key characteristics of mag- concentrations of fluid-soluble elements (Rb, Ba, and Pb) mas probably derived by partial melting of the asthenos- are significantly lower than in the volcanic rocks of the pheric mantle during extension. These features included Soldiers Pass Formation. As a result, the trace-element silica-undersaturated compositions, high La/Ba ratios, pattern of the Mosida trachybasalt is much more like and low La/Nb and Rb/Nb ratios (figure 11). The Mosida ocean island basalts (figure 5); ocean island basalts typi- Basalt is similar, though not identical, to these so-called cally have smooth patterns but their Rb, Ba, U, and K “Basin and Range” basalts. All of the “asthenospheric” concentrations are lower than found in the Mosida lavas identified by Fitton and others (1991) were younger Basalt. than 5 Ma and most were found far from the margins of 2007 UGA Publication 36 - Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C. Jr., editors 15

Figure 10. Tectonic discrimination diagrams show that Paleogene volcanic rocks are most like volcanic arc magmas related to subduction, and that Neogene lavas are most similar to those formed in within-plate settings (rift and hot spot). (A) and (B) for silicic rocks (classification of Pearce and others, 1984) and (C) and (D) for mafic rocks (classification of Müller and Groves, 2000). The shaded areas in (A) and (B) show the composition of contemporaneous Miocene rhyolite lavas from the eastern Great Basin (Christiansen and others, 1986). the Basin and Range Province. tion of continental crust. Both fractional crystallization In addition, the Mosida Basalt is similar to three and assimilation played important roles in the evolution other Miocene lava flows from the Great Basin analyzed of the Mosida Basalt. by Barr (1993). These lavas erupted in central Nevada Rift-related mafic lavas are commonly found in and are 15.8 to 20.4 Ma. In addition to having trace-ele- bimodal associations with rhyolite; associations with ment patterns similar to ocean island basalts with low intermediate compositions are rare. This bimodal pattern Rb/Nb ratios, two of the Nevada basalts are silica-under- emerges during the Miocene in the eastern Great Basin saturated like the Mosida Basalt. Because they have (figure 11). The Miocene silicic rocks (e.g., topaz-bear- many of the characteristics of within-plate basalts, we ing rhyolites and granites; Christiansen and others, 1986, conclude that they represent the oldest asthenosphere 1988) also have trace-element characteristics of within- derived (nonsubduction) magmas in the Great Basin. plate magmas, typical of rift and mantle plume associa- However, we re-emphasize that the Mosida Basalt is tions (figure 10). The oldest of these silicic rocks are the not primary as shown by its low MgO (<5 wt. %), Ni rhyolites of the Spor Mountain Formation and the (<30 ppm), and Cr (<50 ppm) and relatively Fe-rich Sheeprock granite, both of which are 21 Ma and lie 120 olivines (

Figure 11. Subduction-related magmatism was gradually replaced by intraplate magmatism in the eastern Great Basin. Gray sym- bols are lava compositions from elsewhere in the eastern Great Basin. Sources of data are listed in Christiansen and others (in press). (A) Silica concentrations in the most mafic lavas became lower and intermediate composition lavas became increasingly rare, pro- ducing a bimodal silica distribution by 20–15 Ma. CRP Tuff is the Chimney Rock Pass Tuff. (B) Rb/Nb ratios became lower with time showing that Nb anomalies gradually disappeared in the mafic rocks. Rb/Nb ratios for ocean island basalts are shown for reference.

Tectonic History ated with distinctive mineralization—Cu, Au, Ag, Pb, Zn—characteristic of subduction settings worldwide. We have attempted to use these petrologic observa- Composite volcanoes and caldera complexes were the tions to construct a regional tectonic synthesis. Figure 12 principal volcanic features. This episode marks the onset outlines our interpretation of the tectonic evolution of the of the ignimbrite flare-up in the region. western United States (see also Best and Christiansen, The volcanic units in the Soldiers Pass Formation 1991; Christiansen and others, in press). Before about 45 (35–33 Ma) are apparently part of this volcanic episode Ma, shallow subduction of oceanic lithosphere beneath and appear to represent a volcanic center separate from the western United States shut off magmatism over a the nearby East Tintic and Bingham volcanic fields. They broad area and caused widespread folding, thrust fault- are similar in age and composition to the younger vol- ing, and crustal thickening during the Nevadan, Sevier, canic series at Bingham (Deino and Keith, 1997; D.T. and Laramide orogenies (figure 12a). A high orogenic Maughan and others, 2001; Biek, 2006) and to the mag- plateau underlain by thick continental crust probably mas erupted from stratovolcanoes and small calderas that formed in what is now the eastern Great Basin (DeCelles, developed in what are now the East Tintic Mountains 2004). Between 45 Ma and about 22 Ma (figure 12b), the (Morris and Lovering, 1979; Clark, 2003; Moore and shallowly subducting oceanic slab dropped away from others, this volume). The major- and trace-element char- the lithosphere (slab rollback). The immersion of the slab acteristics, mineral compositions, and the oxidized, wet into the hotter asthenosphere caused it to dehydrate over nature of the magmas, along with their low Fe/Mg ratios a broad area and induced the formation of magmas with (in whole rocks and in mafic silicates) are similar to other subduction-zone signatures far from the coastal trench. Paleogene silicic rocks in the Great Basin and consistent The rollback of the slab appears to have started beneath with a subduction-zone origin. Montana and progressed southward as an east-trending Volcanism may have dammed local drainages to cre- bend or a series of tears in the plate propagated in that ate the freshwater lake that deposited the carbonates and direction. This is evidenced by the southward-moving associated clastic sediments in the White Knoll Member front of calc-alkalic magmatism that swept across the of the Soldiers Pass Formation. Biotite-bearing clay lay- western United States during this time period (Best and ers within the carbonate sequence are probably distal Christiansen, 1991). These magmas are andesites to rhy- records of tephra-fall deposits of eruptions elsewhere in olites with strong subduction-related geochemical signa- the Great Basin. tures—high concentrations of fluid-soluble elements The subsequent eruption of the Mosida Basalt her- (Ba, Rb, K, Pb) and low concentrations of insoluble Nb alded a significant change in the magma-generating and Ti (Barr, 1993). The magmas are also hydrous and processes in western Utah. The basaltic rocks lack the oxidized—features inherited from dehydration of the prominent negative Nb anomalies so typical of the older underlying slab of oceanic lithosphere. Moreover, this volcanic rocks. In addition, the lava flow is silica-under- suite of Paleogene magmas in the Great Basin is associ- saturated and more similar to ocean island basalts than to 2007 UGA Publication 36 - Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C. Jr., editors 17

those directly related to subduction. All of these features seem to indicate that by 19 Ma (figure 12c), alkalic basalts were derived by decompression melting during upwelling of the mantle driven by flow set up by subduction and lithospheric extension. This episode of bimodal volcanism and extension was apparently assist- ed by the progressive replacement of the convergent plate boundary with the San Andreas transform fault along part of the western margin of North America (south of the region shown in the cross sections of figure 12). About 28 Ma, the East Pacific Rise encountered the subduction zone in southern California and a triple junction migrated northward to the present location of Cape Mendocino (Atwater, 1970). This tectonic reorganization may have caused relaxation and lithospheric extension in the region behind the growing transform, including the eastern Great Basin.

CONCLUSIONS The Soldiers Pass Formation (new formal name) consists of a Paleogene suite (35–33 Ma) of trachydacitic and rhyolitic ignimbrites, intermediate-composition lavas, and sedimentary rocks separated into two formal and three informal members. The volcanic rocks have the mineralogical and chemical characteristics of magmas associated with continental subduction zones. Nearly flat-lying lake and hot spring deposits of the White Knoll Member are interlayered with and cap the uppermost volcanic rocks in the formation. The horizontality of the volcanic and sedimentary strata and lack of intervening angular unconformities yield no evidence for extension during the Paleogene magmatic episode. Following a lull in volcanic activity of about 15 million years, a mafic lava erupted as one of the oldest basaltic magmas in the eastern Great Basin. This silica undersaturated Mosida Basalt (new formal name) has a distinctive within plate character. We conclude that it is one of the oldest asthenosphere derived magmas in the Figure 12. The Cenozoic tectonic evolution of the Great Basin Great Basin, even though it is extensively differentiated. is depicted in these schematic cross sections. (A) Before 45 Rb/Nb ratios are slightly higher than typical ocean island Ma, shallow subduction of oceanic lithosphere beneath the basalt, implying that the basaltic magma was also con- western United States shut off magmatism over a broad area taminated during residence in the continental crust or in and caused contractional deformation. (B) Between 45 and about 22 Ma, the shallow slab progressively dropped away “metasomatized” lithospheric mantle. Apparently, the from the base of the lithosphere creating a southward-moving petrologic transition to extension-related magmatism in front of subduction-related, calc-alkaline magmatism that the eastern Great Basin occurred between about 21 and passed through central Utah about 35–32 Ma. Foundering or 19 Ma, and marks the point at which extension was suf- slab break-off produced widespread dehydration of the sub- ficiently pronounced to cause lithospheric thinning and ducted lithosphere and generated voluminous arc like magmas which intruded, hybridized, and differentiated in the crust. the generation of alkalic basaltic magma by decompres- Dehydration and melting were the result of inflow of asthenos- sion melting. pheric mantle. This episode is represented by the Soldiers This petrologic transition occurred at about the same Pass Formation. (C) By 21–19 Ma, counterflow of hot time as the onset of rapid unroofing of the ranges in the asthenosphere and lithospheric extension caused the lithosphere to thin sufficiently for decompression melting and eastern Great Basin. For example, Parry and Bruhn enhanced heat flow to create within-plate basaltic magmas (1986) put the initiation of the Wasatch fault at about such as the Mosida Basalt. Modified from Christiansen and oth- 17.6 Ma, based on a K-Ar age of hydrothermal sericite ers (in press). South of line shown in this section, the conver- found in the fault zone near Salt Lake City. Using zircon gent margin was replaced by a transform boundary. and apatite fission track ages, Kowallis and others (1990) concluded that significant extensional unroofing of the 18 Central Utah - Diverse Geology of a Dynamic Landscape

Wasatch Range started by at least 12 Ma. Armstrong and angle normal faults which are now obscured beneath others (2003) concurred with these conclusions but, basin-filling sediments (Zoback, 1983; Floyd, 1993; based on new apatite fission track and U-Th/He ages, Cook and others, 1997; Biek, 2004; Biek and others, suggested that an earlier episode of uplift may have start- 2006; Clark and others, 2006). This suggests that it is not ed in the late Oligocene or early Miocene. Armstrong and part of a detachment derived from the adjacent Wasatch others (2003) also correlated this uplift event with the Range, nor is it underlain by a listric normal fault. These extensional unroofing and cooling at 19(?)–15 Ma, structural relations also imply that Utah Valley is not a which was identified by Stockli and others (2001) from half-graben formed solely by displacement along the thermochronologic analysis of rocks in the Canyon Wasatch fault. Mountains to the south. Wannamaker and others (2001) and McQuarrie and Wernicke (2005) also concluded from structural data that major extension started about ACKNOWLEDGMENTS 20–18 Ma across much of the eastern Great Basin. Ren The work reported here was largely completed as a and others (1989) and Kowallis and others (1995) also mapping project conducted by the Utah Geological showed that Basin and Range extensional stresses were Survey (UGS) using STATEMAP funds (award no. not recorded in microcracks in granites until after about 05HQAG0084) from the UGS and the U.S. Geological 20 Ma. Survey, and as a series of undergraduate research projects Finally, the Paleogene and the Neogene stratified supported by Mentoring Environment Grants from rocks in the southern Lake Mountains are nearly hori- Brigham Young University (BYU). The UGS funded the zontal. This implies that the Lake Mountains horst was new 40Ar/39Ar ages. Many students in BYU field cours- not tilted during the formation of the adjacent grabens es have also helped us understand this area. The assis- beneath Utah-Goshen and Cedar Valleys (figures 1 and tance of David Tingey (BYU) in many aspects of this 2). Moreover, no angular unconformity exists between work is greatly appreciated. We thank Dan Moore (BYU- the Paleogene and Neogene rocks. Apparently, the Lake Idaho) and UGS staff Bob Biek, Grant Willis, Robert Mountains horst is bounded on the east and west by high- Ressetar, and Kimm Harty for helpful reviews. 2007 UGA Publication 36 - Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C. Jr., editors 19

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