The 36–18 Ma Indian Peak–Caliente Ignimbrite Field and Calderas

Home , Bull Valley Mountains, Chief Range, King Tuff, Markagunt Plateau, Wah Wah Mountains

The 36–18 Ma Southern Great Basin, USA, Ignimbrite Province and Flareup themed issue

The 36–18 Ma Indian Peak–Caliente ignimbrite ﬁ eld and calderas, southeastern Great Basin, USA: Multicyclic super-eruptions

Myron G. Best1, Eric H. Christiansen1, Alan L. Deino2, Sherman Gromme3, Garret L. Hart4, and David G. Tingey1 1Department of Geological Sciences, Brigham Young University, Provo, Utah 84602-4606, USA 2Berkeley Geochronology Center, Berkeley, California 94709, USA 3420 Chaucer Street, Palo Alto, California 94301, USA 4Paciﬁ c Northwest National Laboratory, Richland, Washington 99352, USA

Dedicated to J. Hoover Mackin, who initiated study of the Indian Peak–Caliente ignimbrite ﬁ eld with his rec- ognition in the 1950s that the “lava ﬂ ows” near Cedar City are actually widespread ignimbrites, including the unusual trachydacitic Isom-type tuffs and the colossal Needles Range monotonous intermediates.

ABSTRACT of meters. Outfl ow ignimbrite sequences com- sibly kindred, phenocryst-rich latite- prise as many as several cooling units from andesite ignimbrite with an outfl ow volume The Indian Peak–Caliente caldera complex different sources with an aggregate thickness of 1100 km3 was erupted at 22.56 Ma from a and its surrounding ignimbrite fi eld were locally reaching a kilome ter; sequences are concealed source caldera to the south. a major focus of explosive silicic activity in almost everywhere conformable and lack 2. Trachydacitic Isom-type tuffs. Also rela- the eastern sector of the subduction-related substantial intervening erosional debris and tively uniform but phenocryst poor (<20%) southern Great Basin ignimbrite province angular discordances, thus manifesting a with plagioclase >> clinopyroxene ≈ ortho- during the middle Cenozoic (36–18 Ma) lack of synvolcanic crustal extension. Fallout pyroxene ≈ Fe-Ti oxides >> apatite. These

ignimbrite ﬂ areup. Caldera-forming activity ash in the mid-continent is associated with alkali-calcic tuffs are enriched in TiO2, K2O,

migrated southward through time in response two of the super-eruptions. P2O5, Ba, Nb, and Zr and depleted in CaO, to rollback of the subducting lithosphere. Ignimbrites are mostly calc-alkalic and MgO, Ni, and Cr, and have an arc chemical Nine partly exposed, separate to partly over- high-K, a refl ection of the unusually thick signature. Magmas were erupted from a con- lapping source calderas and an equal num- crust in which the magmas were created. cealed source immediately after and just to ber of concealed sources compose the Indian They have a typical arc chemical signature the southeast of the multicyclic monotonous Peak–Caliente caldera complex. Calderas and defi ne a spectrum of compositions that intermediates. Most of their aggregate out- have diameters to as much as 60 km and are ranges from high-silica (78 wt%) rhyolite to fl ow volume of 1800 km3 was erupted from fi lled with as much as 5000 m of intracaldera andesite (61 wt% silica). Rhyolite magmas 27.90 to 27.25 Ma. Nothing like this couplet tuff and wall-collapse breccias. were erupted in relatively small volumes of distinct ignimbrites, in such volumes, have More than 50 ignimbrite cooling units, more or less throughout the history of activ- been documented in other middle Ceno- including 22 of regional (>100 km3) extent, ity, but in a much larger volume after 24 Ma zoic volcanic fi elds in the southwestern U.S. are distinguished on the basis of stratigraphic in the southern part of the caldera complex, where the ignimbrite fl areup is manifest. position, chemical and modal composition, creating ~10,000 km3 of ignimbrite. Magmas were created in unusually thick 40Ar/39Ar age, and paleomagnetic direction. The fi eld has some rhyolite ignimbrites, crust (as thick as 70 km) where large-scale The most voluminous ash fl ows spread as far the largest of which are in the south and were inputs of mantle-derived basaltic magma as 150 km from the caldera complex across a emplaced after 24 Ma. But the most distinc- powered partial melting, assimilation, mix- high plateau of limited relief—the Great Basin tive attributes of the Indian Peak–Caliente ing, and differentiation processes. Dacite and altiplano, which was created by late Paleozoic fi eld are two distinct classes of ignimbrite: some rhyolite ignimbrites were derived from through Mesozoic orogenic deformation and 1. Super-eruptive monotonous intermediates. relatively low-temperature (700–800 °C), crustal thickening. The resulting ignimbrite More or less uniform and unzoned deposits water-rich magmas that were a couple of log fi eld covers a present area of ~60,000 km2 of dacitic ignimbrite that are pheno cryst rich units more oxidized than the quartz-fayalite- in east-central Nevada and southwestern (to as much as ~50%) with plagioclase > bio- magnetite (QFM) oxygen buffer at depths of Utah. Before post-volcanic extension, ignim- tite ≈ quartz ≈ hornblende > Fe-Ti oxides ± ~8–12 km. In contrast to these “main-trend” brites had an estimated aggregate volume sanidine, pyroxene, and titanite; apatite magmas, trachydacitic Isom-type magmas of ~33,000 km3. At least seven of the largest and zircon are ubiquitous accessory phases. were derived from drier and hotter (~950 °C) cooling units were produced by super-erup- These tuffs were deposited at 31.13, 30.06, magmas originating deeper in the crust (to as tions of more than 1000 km3. The largest, and 29.20 Ma in volumes of 2000, 5900, and deep as 30 km) by fractionation processes in at 5900 km3, originally covered an area of 4400 km3, respectively, from overlapping, andesitic differentiates of the mantle magma. 32,000 km2 to outfl ow depths of hundreds multicyclic calderas. A unique, and pos- “Off-trend” rhyolitic magmas that are both

Geosphere; August 2013; v. 9; no. 4; p. 864–950; doi:10.1130/GES00902.1; 78 ﬁ gures; 8 tables; 6 supplemental ﬁ les. Received 16 January 2013 ♦ Revision received 13 February 2013 ♦ Accepted 16 February 2013 ♦ Published online 13 June 2013

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younger and older than the Isom type but of the explosive activity in the Great Basin constitute the Indian Peak–Caliente ignimbrite possessed some of their same chemical char- was the Indian Peak–Caliente caldera com- fi eld, the subject of this article. acteristics possibly refl ect an ancestry involv- plex from which over 50 eruptions (Table 1) As is true for volcanism generally through- ing Isom-type magmas as well as main-trend broadcast ash fl ows to at least 150 km distant out the Great Basin, caldera-forming eruptions rhyolitic magmas. in southwestern Utah and southeastern Nevada in the Indian Peak–Caliente ignimbrite fi eld Andesitic lavas extruded during the to cover a present area of ~60,000 km2. The migrated southward through time (Best and fl areup but mostly after 25 Ma constitute aggregate volume of the eruptions is estimated Christiansen, 1991). (In our usage, the Great a roughly estimated 12% of the volume of to be 32,600 km3; at least seven had volumes Basin [Fig. 1] encompasses western Utah and silicic ignimbrite, in contrast to major vol- of more than 1000 km3 (Table 2) and, thus, nearly all of Nevada, rather than just the smaller canic fi elds to the east, e.g., the Southern qualify as super-eruptions (Miller and Wark, hydrographic basin.) It is generally agreed that Rocky Mountain fi eld, where the volume of 2008; de Silva, 2008; volumes are corrected this southward sweep in volcanism through- intermediate-composition lavas exceeds that for an assumed uniform 50% east-west crustal out the Great Basin was a result of southward of silicic ignimbrites. extension post-dating volcanism; see below). steepening in dip, or rollback, of a formerly The eruptions created six partly exposed, “fl at” subducting oceanic lithosphere that had INTRODUCTION mostly overlapping source calderas to the extended far inland from the continental margin north and three to the south (Fig. 2). Some during the early Cenozoic. Volcanic rocks older The southern Great Basin ignimbrite prov- calderas are nested, or multicyclic. Several than ca. 18 Ma in the Great Basin bear an arc ince (Fig. 1) comprises on the order of 250 additional source calderas have not been accu- chemical signature indicative of their subduc- cooling units of silicic ash-fl ow tuff and 43 rately located because of concealment beneath tion-related origin. Younger volcanic rocks lack at least partly exposed calderas formed dur- younger deposits; some were engulfed in this characterizing signature and were formed ing the middle Cenozoic ignimbrite fl areup in younger calderas. The calderas and their sur- during a subsequent extensional tectonic regime southwestern North America. A major focus rounding related outfl ow ignimbrite sheets supplanting the earlier subduction regime.

Figure 1. Middle Cenozoic, southern Great Basin ignimbrite province in Nevada and Indian Peak- Caliente southwestern Utah resulting Silicic Ignimbrite Andesite Lava Western NV Central NV from the 36–18 Ma ignimbrite caldera caldera caldera fl areup (modifi ed from Stewart 42° and Carlson, 1976). The prov- California ince is divided into three parts: Nevada Utah the Western Nevada (NV) fi eld and calderas (blue), the Central 86 Sri = 0.706 Nevada fi eld and calderas (red), Sr/ 87 Salt Lake City and the Indian Peak–Caliente GREAT Elko fi eld and calderas (green). The western edge of the Precam- SIERRA BASIN brian continental basement is 40° indicated by the dashed initial 87 86 Sr/ Sr = 0.706 line (modifi ed Reno Austin from Wooden et al., 1999). Just Ely to the east, the yellow band Marysvale marks the approximate position of the western lip of the middle Cenozoic Great Basin altiplano (Best et al., 2009). The 38° Indian Peak–Caliente and Cen- NEVADA tral Nevada ignimbrite fields Ton opah field developed on this high plateau. COLORADO The Marysvale volcanic field Miles St. George and calderas (black) that are 050 100 not part of the southern Great Arizona Basin ignimbrite province and 050 100 PLATEAU lie to the east on the western Kilometers Las Vegas margin of the Colorado Pla- 36° teau are shown here to empha- 120° 118° 116° 114° 112° size the contrasting dominance of andesitic lavas over silicic ignimbrites.

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TABLE 1. STRATIGRAPHIC, COMPOSITIONAL, AND CHRONOLOGIC DATA FOR IGNIMBRITES IN THE INDIAN PEAK–CALIENTE FIELD Modal Chemical Age Stratigraphic ignimbrite unit LS composition composition (n) Caldera Tuff of Tepee Rocks (1) V Xr; q > p ≈ s > b > h > t R 76.3 18.1 ± 0.7 (1)* (C) Hiko Tuff (2) 0 Xr; p > q ≈ s > b > h > t R 69.3–73.9 18.51 ± 0.05 (2) (C) Delamar Racer Canyon Tuff (12) R Xr; p ≈ q > s > b > h > t D–R 68.4–77.6 (C) Telegraph Draw Upper cooling unit 18.57 ± 0.03 (1) Lower cooling units 18.88 ± 0.06 (1) 18.85 ± 0.03 (1) Harmony Hills Tuff (2) H Xr; p >> b > h ≈ q ≈ px A (M?) 61.3–63.9 22.56 ± 0.11 (2) p (C) Condor Canyon Formation Bauers Tuff Member (1) B Xp; p > s > b > cpx R 70.3–73.5 23.04 ± 0.11 (2) (C) Clover Creek Swett Tuff Member (1) T Xp; p >> b R 69.8–73.5 24.15 ± 0.10 (2) p (C) Leach Canyon Formation, tuff (1) E Xr; p ≈ q ≈ s > b ≈ h > t R 71.7–76.5 24.03 ± 0.01 (2) (C) Isom Formation I Xp; p >> px (IP) Hole-in-the-Wall Tuff Member (1) Z R 69.6–72.9 24.55 ± 0.12 (2) p Hamlight Tuff Member (4) Y TD 64.8–67.7 Fourth cooling unit 24.75 ± 0.13 (1) p Second cooling unit 24.63 ± 0.17 (1) p First cooling unit (sanidine) 24.91 ± 0.05 (1) First cooling unit (plagioclase) 25.10 ± 0.70 (2) p Bald Hills Tuff Member (4?) X TD 63.7–70.0 Uppermost dated cooling unit 27.25 ± 0.09 (4) p Intermediate cooling unit 27.59 ± 0.15 (1) p Intermediate cooling unit 27.36 ± 0.12 (1) p Lowest dated cooling unit 27.90 ± 0.09 (3) p Ripgut Formation, tuff member (3) U Xp; p >> s ≈ b ≈ h > q R 72.5–77.7 28.96 ± 0.05 (2) (IP) Mt. Wilson 28.99 ± 0.10 (2) p Petroglyph Cliff Ignimbrite (1) P Xp; p >> px D–TD 62.1–68.8 29.1† (IP) Blind Mountain Lund Formation, tuff member (1) L Xr; p > q ≈ b ≈ h > s > t D(M) 64.1–71.3 29.20 ± 0.08 (2) (IP) White Rock Silver King Tuff (1) K Xr; p > q > b > s ≈ h > t D–R 67.2–71.7 29.40 ± 0.06 (2) (IP) Ryan Spring Formation Mackleprang Tuff Member (1) M Xp; p >> b > q R 70.1–-72.3 30.01 ± 0.09 (4) p (IP) Tuff of Deadman Spring (1) D Xr; p > q > s > b R 73.8–76.6 30.00 ± 0.10 (1) (IP) Kixmiller Ryan Spring Formation Greens Canyon Tuff Member (5) N Xp; p >> b R 71.0–72.0 30.13 ± 0.13 (2) p (IP) Wah Wah Springs Formation, tuff (1) W Xr; p > h > b > q ≈ px D(M) 62.2–70.2 30.06 ± 0.05 (11) p (IP) Indian Peak Cottonwood Wash Tuff (1) C Xr; p > q ≈ b ≈ h > px D(M) 61.0–68.3 31.13 ± 0.09 (4) p (IP) Lamerdorf Tuff (3) F Xp; p >> b > h R 69.1–72.5 (IP) Third cooling unit 31.90 ± 0.16 (2) p Second cooling unit 32.09 ± 0.10 (4) p Marsden Tuff (3) A Xp; q > p > b, s? R 75.5–77.1 (IP) Sawtooth Peak Formation, tuff (1) S Xr; p > q > s ≈ b > h R 70.1–71.0 33.5 ± 1.2§ (IP) Tunnel Spring Tuff (3) Q Xr; q > s > p > b R 76.0 35.26 ± 0.03 (1) Crystal Peak Formation of The Gouge Eye (many) G Xp; p, q, b, s variable D–R 69.1–71.5 36.02 ± 0.20 (1) p (IP) The Gouge Eye

SUMMATION: >51 cooling units; 22 regional units exposed in more than one mountain range; 9 exposed calderas Note: Stratigraphic ignimbrite unit column: Maximum number of cooling units observed in parentheses. Table does not include numerous small cooling units associated with non-caldera forming eruptions of the 25-21 Ma Blawn Formation. LS—letter symbol for the unit in variation diagrams. Modal composition: Xr—crystal-rich (>20 % phenocrysts in rock, not corrected to dense-rock equivalence); Xp—crystal-poor (<20 % phenocrysts in rock); p—plagioclase; s—sanidine; q—quartz; b—biotite; h— hornblende, px—pyroxene; cpx—clinopyroxene; t—titanite. Chemical composition column: R—rhyolite; D—dacite; TD—trachydacite; A—andesite; M—monotonous 40 39 intermediate; numbers indicate wt % SiO2. Age column: in Ma by the Ar/ Ar method, referenced to 28.20 Ma for the Fish Canyon sanidine; by laser fusion of sanidine except where labeled by “p” for plateau method on plagioclase. If number of analyses is more than one (n>1), cited age is weighted mean. One sigma uncertainty. Caldera name of individual caldera in the Caliente complex (C) and Indian Peak complex (IP). *Preliminary 40Ar/39Ar age on biotite by L.W. Snee cited in Rowley et al. (1995). †Stratigraphically constrained age between the 29.0 Ma Ripgut and the 29.20 Ma Lund. §K-Ar age on biotite; also 33.6 ± 1.8 Ma ﬁ ssion track age on zircon (Best and Grant, 1987).

The calc-alkaline, subduction-related ignim- was from 27.90 to 27.25 Ma. Yet another distinct rhyolitic activity shifted southward until ca. brites and minor contemporaneous lavas in the phenocryst-rich latite-andesite ignimbrite was 32 Ma with eruption of seven more cooling units Indian Peak–Caliente ignimbrite fi eld range in erupted at 22.56 Ma with an apparent volume whose total volume is ~700 km3. Source cal- composition from andesite to rhyolite (Table 1), of 2200 km3. No other part of the southern Great deras were apparently engulfed within younger like those in other middle Cenozoic volcanic Basin ignimbrite province nor other volcanic large calderas. Several ash-fl ow cooling units of fields in southwestern North America. But fi elds in southwestern North America involved rhyolite and one of dacite totaling ~2400 km3 two compositionally distinct classes of ignim- in the middle Cenozoic ignimbrite fl areup, to were erupted ca. 30–29 Ma within and periph- brite resulted from three super-eruptions of our knowledge, has such compositionally dis- eral to the very large, 31–29 Ma monotonous phenocryst-rich, relatively uniform dacite, tinct ignimbrites in such volumes (Table 2). intermediate calderas. or monotonous intermediate, magma followed Prior to the distinct tandem monotonous Following a lull in activity of 3 m.y. after by voluminous eruption of higher-temperature, intermediate and trachydacitic eruptions, the emplacement of trachydacitic tuffs, major cal- drier, near-liquidus trachydacitic magma. The long-lived ignimbrite activity in the Indian dera-forming eruptions shifted to the south into burst of three super-eruptions of 2000, 5900, Peak–Caliente ignimbrite fi eld began at ca. the Caliente area (Fig. 2). Three eruptions at ca. and 4400 km3 occurred at 31.13, 30.06, and 36 Ma with eruption of small volumes of rhyo- 24–23 Ma and several from 18.88 to18.51 Ma 29.20 Ma, respectively, while eruption of possi- lite from two small calderas to the north of the were all of rhyolite and had an aggregate vol- bly as much as 3600 km3 of trachydacitic magma main Indian Peak caldera cluster. Precursory ume of 10,000 km3.

866 Geosphere, August 2013

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e Contemporaneous lavas in the 36–18 Ma d d l e r a Indian Peak–Caliente ignimbrite ﬁ eld are of c u a o t r t

n minor volume compared to that of ash-ﬂ ow n d i o m m m m m m m m m m e c d 0 0 0 0 0 0 0 0 0 0 v tuff. As ﬁ rst revealed in the compilation of r + e 0 0 0 0 0 0 0 0 0 0 e f 6 3 3 1 3 4 3 3 4 4 f m m m m s

ll tuff in older ll tuff Stewart and Carlson (1976) for the approxi- u ow u ﬁ s s s s s s s s s s b t 0 0 i i i i i i i i i i - 0 s o ow ponded in older ow ponded in older 0 0 a 4 5 s a h h h h h h h h h h s r 1 1 r t mately similar 34–17 Ma time frame (Fig. 1), a c c c c c c c c c c d 4 e o m ll tuff inside older e n , t s s a a a a a a a a a a n i i d d e l m 0 l p p p p p p p p p p 3 a f f 0 the volume of intermediate composition lavas, a f f s 0 a o o o o o o o o o o m , , 0 0 c s u u c 0 s s s s s s s s s s i i t i t i i i i i i i 2 3 ow 0 0 , m e , a , , 1 2

r mainly andesitic, constitutes only a fraction of m o n e e e e e e e e e e e e 2 t 1 1 ; ; k s s s s s s s s s s s s C n 0 i d n n c s s p p p p p p p p p p p p i l l 0 e o o the volume of silicic ignimbrites. From another a a a a a a a a a a a a i i r h e e l l l l l l l l l l l l 8 t t m t l l l l l l l l l l l l u d d a a o o o o o o o o o o o o h l l 0 o o o t perspective, no major composite, or strato-, c c c c c c c c c c c c t m i u u 0 ------n c c M M w 0 l l 0 e e e e e e e e e e e e , o r r r r r r r r r r r r f f a a 0 volcanoes existed prior to or during most of d 1 c p p p p p p p p p p p p o o c c 1 ; e ; t t t t t t t t t t t t r e e 2 3 2 2 s s s s s s s s s s s s u m ow g g the ignimbrite fl areup. In striking contrast, l l l l e e e e e e e e e e e e r o a a e e e e k k k k k k k k k k k k t o r r f d d d d c c c c c c c c c c c c n i i i i i i i i i i i i i e e outfl intracaldera + contoured outfl Indian Peak depression White Rock caldera intracaldera tuff + thick outfl intracaldera tuff Kixmiller depression + 50 m thickest outfl intermediate-composition lavas dominate over o o o o o n v v h h h h h h h h h h h h M A T T T T M C T T T M T M T T T T U A silicic ignimbrite in the two contemporaneous volcanic fi elds to the east on the margins of

) the Colorado Plateau. In the Marysvale ﬁ eld 0 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 5 0 0 % 5 2 5 5 5 5 5 5 2 5 5 5 5 5 5 5 2 5 1 ( est. on the west margin, lavas are ten times more

Uncertainty voluminous than ignimbrites (Cunningham et al., 2007). In the Southern Rocky Mountain * * ﬁ eld on the east margin, lavas are 1.7 times ) 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 0 0 0 0 0 0 0 5 0 0 0 4 8 0 0 8 6 0 5 0 9 0 1 2 6 4 600 50 Uniform 300 m of caldera-ﬁ 3 4 2 6 8 2 2 3 1 2 1 4 more voluminous (Lipman, 2007). In these , , , , , , , , (km 2 5 4 3 2 3 4 1 volume total unit

Preferred two ﬁ elds, stratovolcanoes dominated the landscape prior to the explosive eruptions of silicic valent diameters are calculated assuming a circular shape. (C)–Caliente caldera

structural caldera margin defined by ring fault is unknown. For definition of model magmas. Truly, the Indian Peak–Caliente ) 0 0 0 0 0 0 0 0 0 0 0 0 5 0 3 0 0 0 0 5 0 0 0 4 5 9 3 2 5 ignimbrite fi eld is just that, and resulted from 0 2 1 3 1 8 3 6 2 1 1 5 , , , , , (km 1 2 1 1 1 a great fl areup in explosive activity exceeding Model 1 tuff volume tuff of ignimbrite; at least 7 super-eruptions Pre-collapse

3 extrusion of andesitic lava ﬂ ows.

PLAN OF ARTICLE 0 6 4 2 9 2 0 4 1 1 2 4 (km) diameter

Equivalent This article consists of two main parts dealing with the basic stratigraphy, composition, chronology, and correlation of the ignimbrite ) 0 5 0 2 0 0 units and their dimensions and source calderas 0 5 , , area (km 1 1 caldera in the Indian Peak–Caliente ignimbrite fi eld; a Structural brief description of contemporaneous lava fl ows follows. ? ? ? ) 0 0 0 5 0 0 2 The fi rst main part of the article deals with 0 0 6 5 0 0 0 1 2 4 0 , , area (km 2 2 36.02–24.55 Ma ignimbrites and the Indian Peak caldera

Topographic Topographic caldera cluster displayed in Figure 2. This part

y of the article is a major update of the summary e l e l n w i n y a k k a k

a by Best et al. (1989a) and revises some strati- o V E r e t a c r s k l e n D e o e i e e r a l u l n g

P graphic relations, presents data on new ignim- i e R h i C W o u p P ) ) ) ) ) ) ) r n P m t e ) ) ) o M t l a x e a i n P P P P P P P i r

i brite units not recognized earlier, and reﬁ nes the C C C h I I I I I I I a G v t h ( ( ( u d t ( ( ( ( ( ( ( g d K o u o n s l e e n i ) W l l I o y distribution and thickness of ignimbrites as well h C M r e P ) ) B S I T T ) ) ( C P ) P ) I I ) n P C i ( as the margins of calderas. We describe in detail ( I P ( P I ( C ) I ( ( ( P

I marginal segments of two of the largest calderas ( and propose interpretive models for their complex origin during subsidence. Many local inter- TABLE 2. DIMENSIONS OF IGNIMBRITE UNITS AND ASSOCIATED CALDERAS IN THE INDIAN PEAK–CALIENTE IGNIMBRITE FIELD CALDERAS IN ASSOCIATED AND 2. DIMENSIONS OF IGNIMBRITE UNITS TABLE 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 5 0 0 0 0 0 6 2 2 3 0 5 3 5 2 6 2 4 9 7 7 3 6 7 4 2 1 1 1 1 1 1 1 pretations and conclusions drawn two decades (km) Caldera

Equivalent ago in our published geologic maps have been unit diameter changed in the light of newly developed regional 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ) 2 5 0 0 0 5 0 5 0 0 0 0 0 0 0 0 0 0 0 0 0 5 7 0 0 6 0 5 0 2 0 9 0 0 3 6 4 0 2 5 4 concepts for the entire Indian Peak–Caliente , , , , , , , , , , , , , , , , , Unit area 1 7 2 2 5 3 3 8 2 2 2 3 1 3 1 1 3 (km 2 1 1 2 1 1 3 caldera complex and its relation to other parts

l of the southern Great Basin ignimbrite province. l h s d a s g g e a f e n n The outline of this ﬁ rst part is essentially strati- k W f i i d i y n - r i n r l s a g W l v o e p E o l p g i e n i C i y d y h graphic, beginning with the oldest ignimbrite S n S d r t e P H n f o n h g - a n p r g n h a r o a p n n h y u i All dimensions corrected for 50% uniform east-west extension. See Table 1 for additional information on ignimbrite units. Equi Table All dimensions corrected for 50% uniform east-west extension. See i o u t n S a a p y

- units. However, because the ignimbrites are of n C w l C d l o o e K s t e e r o m l n g m r r l W e t o d h r u G t e t k d o o o e e o s n c d m e t r g h three distinct compositions—rhyolite, dacite, e c r s t a r c u w t e n a n m v I H Note: *Refer to Table 7. Table *Refer to p a l a a e o a i a w a a i e h u e u a SUMMATIONcalderas exposed km 9 32,600 T D C S L Isom Formation M Greens Canyon 2,000 50 W T L B H R Ignimbrite unit M complex; (IP)–Indian Peak caldera complex. Topographic caldera area lies within the topographic margin where known or Topographic complex; (IP)–Indian Peak caldera complex. L Hiko 8,000 100 (C) Delamar 300? 20 850 1,700 25 Thickest pre-collapse isopach is 400 m S S volume calculations see Figure 4 and text. Uncertainty estimate for the preferred is our best approximation. P R and trachydacite—that are intermingled with

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Figure 2. The Indian Peak– Caliente caldera complex and

its surrounding ignimbrite Caldera Provo

fi eld (blue shade) lie across the Utah 0255075100 Nevada Nevada-Utah state line. Most 40° Km of the fi eld lies in the eastern Great Basin but extends on the east into the area of the Marys- vale volcanic fi eld whose cal- Austin Eureka deras are shown in the previous Delta figure. The caldera complex Central Nevada Ely ignimbrite field consists of the 36–29 Ma Indian 39° Baker Peak caldera cluster (CC) to the Richfield north and the 23–18 Ma Cali- Lund ente cluster to the south. The Indian Peak - ignimbrite fi eld is delineated by Marysvale the outermost limit of exposed Caliente Milford ignimbrite field outfl ow sheets surrounding the Beaver two caldera clusters. To the west, Indian Peak CC outlined by a thin black line, is 38° Tonopah Pioche the Central Nevada ignimbrite Panguitch fi eld surrounding the associated Modena Caliente Cedar caldera complex delineated by City Rachel the outermost limit of exposed outfl ow sheets. West of the Cen- Alamo Caliente CC tral Nevada caldera complex are NV St. George Utah eastern calderas of the Western 37° Nevada ignimbrite fi eld (in red; CA Arizona Henry and John, 2013). 116° 114° 112°

respect to age (Table 1), we fi rst describe all of their extensive 1:24,000-scale geologic map- Mark Hudson (2012, personal commun.) and the rhyolite tuff units from oldest to youngest, ping, mostly under the auspices of the U.S. Geo- 40Ar/39Ar chronology by Alan Deino (2013, per- then, second, all of the dacites, and, third, the logical Survey (e.g., Rowley et al., 1995). sonal commun.). All 40Ar/ 39Ar ages cited herein two trachydacite formations. Brief comments on petrogenesis of the mag- are referenced to 28.20 Ma for the Fish Canyon In the second main part of the article, the mas are included where appropriate. Sample sanidine. Caliente caldera cluster to the south and younger numbers, their stratigraphic unit assignment, 24.03–18.51 Ma ignimbrites are described in and locations are in Supplemental File 11. DELINEATION OF THE INDIAN PEAK- stratigraphic order. All of these units are rhyo- Chemical and modal analyses are in Supple- CALIENTE CALDERA COMPLEX AND lite, except for the latite-andesite Harmony Hills mental Files 22 and 33, respectively; for analyti- IGNIMBRITE FIELD Tuff emplaced at 22.56 Ma. For this region, we cal methods see Supplemental File 44. Paleo- relied heavily on the published research of other magnetic data for the Indian Peak–Caliente fi eld Although previously treated as separate enti- geologists, especially Peter Rowley, through has been provided by Sherman Gromme and ties (e.g., Best et al., 1989a; Rowley et al., 1995), the Indian Peak and Caliente fi elds and their associated source calderas are here combined 1Supplemental File 1. Zipped fi le containing an Excel table of data for samples from the Indian Peak– Caliente fi eld and a PDF fi le of references cited in the table. If you are viewing the PDF of this paper or read- into one unifi ed whole. The 36–29 Ma Indian ing it offl ine, please visit http://dx.doi.org/10.1130/GES00902.S1 or the full-text article on www.gsapubs.org Peak calderas in the north are separated from the to view Supplemental File 1. 23–18 Ma Caliente calderas in the south by a 2Supplemental File 2. Zipped fi le containing an Excel table of chemical composition of rocks from the gap of ~25 km (Fig. 2, Table 1). No large-scale Indian Peak–Caliente fi eld and a PDF fi le of references cited in the table. If you are viewing the PDF of this geologic maps cover this gap, but the 1:250,000- paper or reading it offl ine, please visit http://dx.doi.org/10.1130/GES00902.S2 or the full-text article on www .gsapubs.org to view Supplemental File 2. scale map of Ekren et al. (1977; see also Rowley 3Supplemental File 3. Zipped fi le containing an Excel table of modal composition of ignimbrites in the et al., 1995) shows it is underlain by variably Indian Peak–Caliente fi eld and a PDF fi le of references cited in the table. If you are viewing the PDF of this altered Late Oligocene–Miocene lava fl ows and paper or reading it offl ine, please visit http://dx.doi.org/10.1130/GES00902.S3 or the full-text article on www tuffs and younger, basin-fi ll sedimentary rocks .gsapubs.org to view Supplemental File 3. 4Supplemental File 4. PDF fi le of analytical methods: modal and chemical composition. If you are viewing deposited south of Panaca, Nevada. (Fig. 3 is the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/10.1130/GES00902.S4 or the full-text an index map to geographic place names cited article on www.gsapubs.org to view Supplemental File 4. in this article. For clarity, these place names are

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Indian Peak–Caliente ignimbrite ﬁ eld

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W NEVADA UTAH NEVADA " Limestone ge Hills an " Snake Range R Ursine Panaca Summit Wilson Creek Condor Canyon "

Mountains Atlanta Figure 3. Index map of geographic places cited in this article. Figure Fortification Pioche ills H Caliente Panaca " Range Clover Mt. Wilson Mt. Lake Kixmiller Summit Valley

Eye Bristol Highland Fairview Range

Schell Crekk Range Gouge The Range Range Ely Springs Delamar

Range Mountains Dry Lake Valley Lake Dry Ely " Grassy

Mountain Egan Range Range Pahroc North E " E E E 115° South Pahroc Range Lund Mountain Silver KingSilver Alamo " River White Narrows " center Seaman volcanic volcanic Hiko Hancock Hancock Summit Currant " ′ ′ 39° 38° 38°30 37°30

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omitted from following fi gures.) Source cal- at ~37°55′ N, 115°9′ W, is a mostly dacitic of explosive silicic activity in the Indian Peak– deras for ca. 24 Ma ignimbrites are likely con- stratovolcano that was active at ca. 27 Ma, was Caliente fi eld. Like the Crystal Peak caldera, cealed in this gap, or possibly the southern part 10 km in diameter, and had a restored volume the source caldera of the Pancake Summit Tuff of these sources were engulfed in the younger of ~20 km3. also lies appreciably north, by ~60 km, of the Caliente calderas. The northern limit of the Indian Peak– main caldera cluster in the Central Nevada fi eld. A further reason for combining the Indian Caliente fi eld cannot be set on compositional Accordingly, we include the Tunnel Spring Tuff Peak and Caliente fi elds is the substantial over- grounds as for its southern limit because calc- and its source caldera within the Indian Peak– lap of the ignimbrite outfl ow sheets from the alkaline ignimbrites possessing an arc chemical Caliente ignimbrite fi eld. two caldera clusters (refer to later fi gures in this signature occur far to the north. Rather, the limit We separate the Indian Peak–Caliente ignim- article and to Best et al., 2013, their fi g. 7). is geographic. In easternmost central Nevada, brite fi eld from the contemporaneous Marysvale The unity of the 36–18 Ma Indian Peak– more than 60 km to the north beyond the Indian volcanic fi eld on the western margin of the Colo- Caliente caldera complex and ignimbrite fi eld Peak calderas, there are only scattered remnants rado Plateau (Fig. 1; Cunningham et al., 2007) is consistent with its evolutionary parallelism to of small- to modest-volume silicic ash-fl ow tuffs because of an 80 km gap between the calderas of the Central Nevada caldera complex and ignim- (Hose et al., 1976; Gans et al., 1989; Grunder, the two fi elds, even though ignimbrite outfl ow brite fi eld to the west, where calc-alkaline sub- 1995); none are known to occur in the Indian sheets from them overlap. The Marysvale fi eld duction-related ignimbrite eruptions took place Peak–Caliente fi eld. Among these tuffs is the has relatively smaller calderas and ignimbrites from 36–18 Ma calderas (Fig. 2). Arc vol- phenocryst-rich rhyolite Charcoal Ovens Tuff constitute only one-tenth the volume of inter- canism associated with this caldera complex that has conspicuous titanite phenocrysts (Hose mediate composition lavas and volcanic debris was succeeded by non-arc, alkalic activity from et al., 1976) and a 40Ar/39Ar age on sanidine fl ows in coalescing composite volcanoes, one of 15.3 to 7.5 Ma to the south in the Southwest- of 35.82 ± 0.11 Ma (sample ELY-1; A. Deino, which is located in the 80 km gap between the ern Nevada volcanic fi eld (Sawyer et al., 1994). 2013, personal commun.). An additional 30 km two fi elds. A gap of ~20 km separates the southernmost or so to the north of the Charcoal Ovens out- caldera in the former from the northernmost crops is the southern outfl ow margin of the ca. DIMENSIONS OF IGNIMBRITES caldera in the latter. In the Caliente area, sev- 35 Ma Kalamazoo Tuff, a phenocryst-poor, eral relatively small-volume ignimbrites that normally zoned rhyolite-dacite that has an esti- Without a knowledge of the volume of ignim- were deposited ca. 16–12 Ma generally lack mated volume of more than 500 km3 (Gans brites, as well as their composition and age, the defi nitive arc chemical signature possessed et al., 1989). We do not consider these distant as detailed in this article, it is impossible to by the 36–18 Ma ignimbrites derived from ignimbrites to be part of the Indian Peak– fully understand the evolution of the explosive the Indian Peak–Caliente caldera complex. Caliente ignimbrite fi eld. magma system from which they were derived. The younger ignimbrites have, instead, affi ni- In western Utah, 130 km to the north of the For this reason, we devote the following para- ties with the alkalic late Miocene and younger Indian Peak calderas, is the east-west–trending graphs to ignimbrite dimensions. bimodal basalt-rhyolite assemblage, including Tintic–Deep Creek magmatic belt of granitic The area of a particular unit (Table 2) was the Kane Springs Wash ignimbrites and associ- intrusions and volcanic rocks (Stewart and Carl- calculated within the zero isopach using ArcGIS ated caldera source to the south (Rowley et al., son, 1976; Hintze and Kowallis, 2009) that was software (Environmental Systems Research 1995; Nealey et al., 1995). determined to be active from 39 to 32 Ma based Institute [ESRI]; www.esri.com/software/arcgis) Although there is an overlap of the larger chiefl y on 40Ar/39Ar age determinations. This and corrected for an assumed uniform 50% ignimbrite outfl ow sheets derived from the belt is not deemed a part of the Indian Peak– east-west crustal extension post-dating vol- Indian Peak–Caliente caldera complex and from Caliente fi eld. Between this magmatic belt and canism (Appendix). From published geologic the Central Nevada caldera complex to the west the Indian Peak–Caliente caldera complex are maps, supplemented by fi eld measurements, (Fig. 2; see also Best et al., 2013, their fi gures 6 three centers of magmatic activity, including more than 800 determinations of the thick- and 7; Supplemental File 55), these two caldera 28–17 Ma granitic intrusions in the Mineral ness (in meters) of individual ignimbrite units complexes are separated by an east-west gap of Range 15 km east of Milford and 34–21 Ma were made in the Indian Peak–Caliente fi eld ~60 km, which is more than twice the separa- granitic intrusions and andesitic extrusions to (Supplemental File 66; see also Sweetkind and tion of the Indian Peak and Caliente calderas. the west in the southern San Francisco and cen- du Bray, 2008). Most thicknesses (in meters) These contrasting separations, together with the tral Wah Wah Mountains and Shauntie Hills. of outfl ow sheets were measured at sites where lack of any major, caldera-forming ignimbrite The third center is the wholly concealed Crys- older and younger deposits are exposed to con- erupted in this 60 km gap, provide further justifi - tal Peak caldera in the northernmost Wah Wah strain the entire cooling unit; erosion of any cation for the unity of the Indian Peak–Caliente Mountains (Steven, 1989) ~50 km northeast of of the upper part of the sheet prior to deposi- caldera complex and keeping it separate from the Indian Peak–Caliente complex. The Tunnel tion of the younger unit is assumed to be nil. the Central Nevada caldera complex. Spring Tuff (Bushman, 1973), which originated A zero thickness is recorded where the particu- The Seaman volcanic center (Fig. 3; du Bray, from this caldera, has an age of 35.26 ± 0.03 Ma lar unit is absent in the stratigraphic interval 1993) is the only focus of silicic volcanism and an estimated volume of only 50 km3. where it would be expected to lie. At sites where between the Central Nevada and Indian Peak– Younger ignimbrites derived from sources to the top, or bottom, or both, of the unit are not Caliente caldera complexes. This small center, the south in the Indian Peak caldera complex overlie the Tunnel Spring Tuff. Despite having a 6Supplemental File 6. Zipped fi le containing an 5Supplemental File 5. PDF fi le of critical data volume less than an order of magnitude smaller Excel fi le of thickness of ignimbrites in the Indian for outfl ow ignimbrite cooling units in select strati- than that of the 35.30 Ma Pancake Summit Tuff Peak–Caliente fi eld and a PDF fi le of references graphic sections. If you are viewing the PDF of this cited in the table. If you are viewing the PDF of this paper or reading it offl ine, please visit http://dx.doi in the Central Nevada fi eld, the Tunnel Spring paper or reading it offl ine, please visit http://dx.doi .org/10.1130/GES00902.S5 or the full-text article on Tuff is a similar phenocryst-rich, high-silica .org/10.1130/GES00902.S6 or the full-text article on www.gsapubs.org to view Supplemental File 5. rhyolite that is among the earliest expressions www.gsapubs.org to view Supplemental File 6.

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exposed, the measured thickness is prefi xed by Model 1 No caldera volume included a greater-than symbol (>). At sites where the Pre-collapse tuff unit thickness is somewhat variable the upper limit is prefi xed by a less-than symbol (<). In some places, nearby differing measured thick- Ring fracture zone nesses overlap at the map scale on which they of concealed caldera are shown, in which case the upper and lower values are indicated (e.g., 180–235); if the range is less than a few tens of meters, a single average Model 2 value is prefi xed by a tilde (~). Flat-floor (piston) caldera Thicknesses were plotted on a base map for Pre-collapse tuff each unit and contoured by eye as isopach lines Intracaldera tuff Syn-collapse tuff at appropriate intervals. Because of the pro- Pre-collapse tuff gressive smoothing of the landscape during the ignimbrite fl areup, contours for most units were Ring fracture zone fairly regular, but maps of older units reveal of caldera local paleohills and paleovalleys. Magma For the ignimbrites in the Indian Peak– Caliente fi eld, we base volume estimates on the Figure 4. Schematic models four ideal models shown in Figure 4; results of used in calculating volumes of the calculations are in Table 2. ignimbrite units. See text for Model 3 Asymmetric (trapdoor) and piecemeal caldera In Model 1, we ignore any intracaldera vol- explanation. Pre-collapse tuff ume and from the isopach lines calculate the Intracaldera tuff Syn-collapse tuff volume of the pre-collapse tuff using a triangu- Pre-collapse tuff lated irregular network (TIN) in ArcGIS to fi t Tilted, irregular floor the lines. Model 1 is most useful for ignimbrite units in which (1) the source caldera is partly Ring fracture zone or wholly concealed, or (2) so little of the cal- of caldera Magma dera margin is known that it could not be drawn with any degree of certainty, or (3) the caldera margin is relatively certain but the thickness of the intracaldera tuff inside its associated source Model 4 Subsidence (after Lipman, 1997, his figure 1) caldera is unknown, or (4) the thickness of the intracaldera tuff is represented by only one Unfilled collar height very minimal value, e.g., 200 m. This method Intracaldera Subsidence depth Landslides was also used for some ignimbrites with iden- tuff = Structural caldera Intracaldera tuff + tifi ed caldera margins for comparison with the floor unfilled collar height estimates by alternate models. For Model 1 Ring fracture zone of caldera ignimbrites, the estimated thickness at the cal- Subsidence depth dera rim, typically 300–400 m, was extrapo- Magma lated across the entire area of the caldera. In cases for which Model 1 was used, the calculated volume of the pre-collapse tuff sheet is shown in Table 2 and the preferred volume of Central Nevada fi eld, the volume of the out- mum found for the intracaldera deposit. Iso- the total ignimbrite unit is double this value. fl ow ignimbrite sheet of the Windous Butte pach lines within the caldera are nested to make The doubling factor stems from the concept of Formation is ~1400 km3 and that of the intra- a steep caldera wall and the volume calculated Lipman (1984) that the volume of the outfl ow caldera tuff at least 3000 km3. In the Western using a triangular irregular network (TIN) in tuff beyond the source caldera is equivalent to Nevada fi eld (Henry and John, 2013), volumi- ArcGIS fi tted to isopachs drawn for outfl ow and that within the caldera. For the super-eruptions nous ignimbrites within the adjacent 34.0 Ma intracaldera tuffs. Eruptions of several hundred of the Lund and Wah Wah Springs ignimbrite Caetano and 24.9 Ma Fish Creek Mountains to thousands of cubic kilometers from large cal- units, this approach appears to work well and calderas have almost no outfl ow counterparts, deras (>15 km diameter) typically yield intra- yields volume estimates comparable to other though erosion of the outfl ow deposits cannot caldera deposits that are kilometers thick (e.g., methods. However, doubling is known to yield be discounted. Salisbury et al. (2011) noted that Acocella, 2007). Lipman (1997, p. 210) stated incorrect volumes in some instances. For exam- in the Altiplano-Puna volcanic complex in the that the “best solutions for total subsidence ple, the outfl ow tuff of Deadman Spring and central Andes where only limited relief exists depths at large ash-fl ow calderas are typically the intracaldera ignimbrite within its Kixmiller within ignimbrite deposits, outfl ow:intracaldera at least 3–4 km.” In the Western Nevada fi eld caldera source have volumes of 20 and appar- ratios are as much as 1:5. (Henry and John, 2013), four exceptionally ently 180 km3, respectively. Even if these two In Model 2, we assume that the caldera fl oor well-exposed calderas—Caetano, Elevenmile values might be considerably in error, there is subsided as a simple piston and that the intra- Canyon, Job Canyon, and Poco Canyon—all no doubt as to their nonequivalence. In the caldera tuff has a uniform thickness—the maxi- reveal more than 4 km of subsidence. In the

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Central Nevada fi eld, two calderas—Cathedral COMPOSITION OF THE INDIAN and shoshonitic (Fig. 5). Nonetheless, these Ridge and Williams Ridge—have exposed PEAK–CALIENTE IGNIMBRITES AS latter ignimbrites, like all of the 36–18 Ma ash- minimum intracaldera thicknesses on the order A WHOLE fl ow tuffs and lavas in the southern Great Basin of 2000 m. ignimbrite province, possess an arc chemical In Model 3, we use a similar approach to A series of variation diagrams summarizes signature manifesting their subduction kinship. Model 2 but assume a perhaps more realis- the composition of 36–18 Ma ash-fl ow tuffs Although a few samples fall in the anorogenic tic, nonuniform thickness for the intracaldera in the Indian Peak–Caliente ignimbrite fi eld or within-plate field, they nonetheless show ignimbrite, which is contoured to repre- (Figs. 5 and 6). See Table 1 for a list of letter the negative Nb anomaly characteristic of arc sent asymmetric subsidence using observed symbols used in Figures 5 and 6 and follow- rocks (Fig. 5). thicknesses. ing fi gures. Representative chemical analy- On element variation diagrams, dacite (C, Model 4 is the method used by Lipman ses of the major ignimbrite units are listed in K, L, W) and more evolved rhyolite (S, A, E, 3 (1997) to estimate the volume of the 5000 km Table 3. D, O, U) ignimbrites plot in rather tight, con- Fish Canyon Tuff, among others. Actually, it tinuous linear clusters and constitute what we is the volume of the magma erupted from the call “main-trend” tuffs (Fig. 6). Trachydacitic subterranean chamber and does not use the vol- Chemical Composition Isom-type tuffs (I and P) plot well off the main

ume of the outﬂ ow, or pre-collapse, tuff in the trend because of relatively greater TiO2, K2O, calculation. Instead, the volume estimate is The total alkalies-silica classiﬁ cation diagram and Zr (and Ba, Y, Nb, not shown) and lower

derived from the area of the structural caldera shows that most ignimbrites in the Indian Peak– CaO (and MgO). The low-silica, high-K2O, inside its ring fault multiplied by the subsidence Caliente fi eld are rhyolite and dacite (Fig. 5). rhyolitic Condor Canyon (B, T), Ryan Spring depth, which is assumed to be uniform across A few samples of the Wah Wah Springs (letter (M, N), and Lamerdorf (F) ignimbrites also lie the caldera. The subsidence depth is the thick- symbol W) and Cottonwood Wash (C) monot- to varying degrees off trend, especially with ness of the intracaldera tuff plus the unfi lled col- onous intermediates are andesite whereas all regard to Zr. These off-trend ignimbrites also lar height (Fig. 4), which is equivalent to what analyzed samples of the Harmony Hills (H) are have slightly lower 87Sr/86Sr ratios than their we refer to as caldera-fi lling, or post-collapse, andesite and latite, making Harmony Hills the main-trend counterparts (Fig. 6D).

tuff. Subtracted from the volume is a value for only such maﬁ c ignimbrite of regional extent Of all diagrams, that for Zr-TiO2 (Fg. 6C) the estimated amount of wall-collapse breccia in the middle Cenozoic southern Great Basin most clearly distinguishes among the ignimbrite incorporated into the intracaldera deposit. ignimbrite province, so far as we are aware. units, including the petrographically and other- Several factors impact the accuracy of esti- Most of the Isom-type tuffs (I) are trachydacite wise chemically similar trachydacitic Isom-type

mates of ignimbrite volumes, including: but some are low-silica, K2O-rich rhyolites. The cooling units (P, X, Y, Z). 1. Uncertainties in the perimeters of outfl ow Isom-type designation stems from the occur- deposits and calderas and thickness of ignim- rence of most of these ignimbrites in the Isom Sr Isotopic Composition brite within them. Formation. 2. Correction for east-west, mostly post- Most ignimbrites are calc-alkalic and calcic, Table 4 gives the Sr isotopic compositions of ignimbrite crustal extension. The 50% value magnesian, and high-K but the Isom-type and volcanic rocks from the Indian Peak–Caliente used throughout this article could be in error, some of the Condor Canyon Formation tuffs fi eld. These data, together with those from Unruh but an inventory of north-south versus east-west (B and T) are alkalic and alkalic-calcic, ferroan, et al. (1995) for volcanic rocks and shallow outfl ow-sheet dimensions indicate this value is reasonable (see Summary and Conclusions section below). The assumption of uniform extension throughout the whole Indian Peak–Caliente ignimbrite fi eld cannot be justifi ed in detail, but is adopted as a matter of expedience because of Figure 5 (on following page). Chemical classifi cation diagrams for ignimbrites in the Indian the lack of explicit quantitative information on Peak–Caliente fi eld. All analyses in this fi gure as well as in all other composition diagrams individual strain domains within the fi eld. in this article are of bulk tuff samples, unless otherwise noted. The key to letter symbols is 3. The doubling of the pre–caldera collapse, in Table 1. The relatively alkalic trachydacitic Isom-type tuffs of the Isom Formation (I) or outfl ow, ignimbrite volume to obtain the total and of the Petroglyph Cliff Ignimbrite (P) as well as the rhyolitic Bauers (B) and Swett (T) for the unit in Model 1 where little or no data tuffs comprise off-trend ignimbrites, shown in red; the Mackleprang, Greens Canyon, and are available on the source caldera. The ratio of Lamerdorf ignimbrites, also in red, are less extreme in composition. For further defi nition intracaldera to outfl ow volume can be expected of off-trend ignimbrites see the next two fi gures. (A) Total alkalies-silica diagram and Inter- to range widely. Examples cited above indicate national Union of Geological Sciences classifi cation fi eld boundaries (Le Maitre, 1989) used ratios >1, so that the assumption of equivalent throughout this article. Many analyzed samples that have perturbed alkali concentrations volumes (ratio of 1) for may err conservatively. are omitted from this diagram. Two samples of ash-fl ow tuff in the formation of The Gouge Hence, ignimbrite volumes cited in this arti- Eye (G) appear to have perturbed, low alkali contents but are shown here as they are the cle are, at best, working rough estimates, subject only available analyzed samples for that unit. (B) Modifi ed alkali-lime index and (C) ferroan-

to modifi cation as more data become available. magnesian classifi cations of Frost et al. (2001). FeO* is total Fe. (D) K2O-SiO2 classifi ca- As order-of-magnitude estimates—that is, thou- tion of Le Maitre (1989) and Ewart (1982). (E, F) Diagrams showing the arc signature in sands versus hundreds of cubic kilometers—we Indian Peak–Caliente ignimbrites. In E, three tectonic regimes are from Pearce et al. (1984). feel they are meaningful. An approximate and Although some samples plot in the “within plate” fi eld, the spidergram (shown in F) reveals subjective uncertainty for each volume estimate that all ignimbrites possess clear negative Nb anomalies typical of subduction-related rocks. is listed in Table 2. Primitive mantle composition is from McDonough and Sun (1995).

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A B 10 Indian Peak-Caliente 11 I ignimbrites n = 458 I I I B BB I I I I III IBBBB A I 8 I I II I I I BBBBT B TB D I I III I I I TTBTTI B A 10 I I I III IIII I T T D DUU I I II I I I III B D EEDE Trachydacite I I I II I B Rhyolite I I I II I MBE UD E V IIII I I I BB II I I I FBB D E U II IIII I I I BBBIBBBBBBB II I I I I B OT EE E U E R I II I II I II B TTB BB I I I MM E E D I I I I I TT T 6 I I I NE U 9 I I I I I M TTE BT I II U OM OO U DDD I I I B F M I A I I O OT E FE O A Q U II I I BO M E D P FO M F F PI I E E D E Alkalic I P O NSFNFK O (wt %) I T E U E DE I L E 2 I P IO M EN E UD DUAU P P R F LFF F 8 I I II U OM E D DE I P RLO T Latite F O U E V 4 KWK L L R OO NF O D L L P W W WW K LL P P O FSN U E O-CaO (wt %) L P L L L K KS L PW R L T F F DE 2 Alkali- CW W LLL O + K P P F DD U LLCL LL 2 L W P LF F F E O L L W LCLCLC K K S G C L CLWL CL L TW A R calcic WLWCPL L WLWC L K KK S S 7 H CPC W L LW W L WKWFK K Q U P LLLL LL LL LL K Na W LCCW L LW LL W K HH H W WPWLCPL LL WLC L LL K S O+K C WCLL W H W CLLWLLLWL LCL L KK 2 H W WCPWWL WWK KK P CWCCCCWCWC C CWWC LLW K S L 2 CCWCPCWWC WLWCCWC C WCCCCCCCWWLW WL CLL L K KLKKS CCCCCL LW L LK C CCPWL WWC LWWWCC WWK L S Na H CWLCCCCWWCLWW K WW WWLWWL K Calc- HH PC CCWWC L W 6 WPC L CLWWC L L KK H WCCWWC WL CWWCL G W W WWW W K CK C WCPWWCW LWL W WW K G C W PW W W WW LL alkalic W WW 0 W 5 Andesite Dacite G Calcic 4 -2 59 61 63 65 67 69 71 73 75 77 59 61 63 65 67 69 71 73 75 77 SiO (wt %) SiO 2 (wt %) 2 C 1.0 D 8 II I I II 7 I II I I 0.9 I IIIIII I I I I I I I I I I M I U I I I D I I I I I UU I I I I II I I II M I III I I I D P P P I I II S BT BB 6 II I I III I MI F B TB Ferroan I UBT U I I I IIII I I IBBBBBBTBB BTD D LI I B B B BB III I IIIIIII BI B B B D EU IIPI P L I FFB B E D D R I PI III F MBIB TTBI B D V 0.8 P I L I II I IK F B D E I P B O E EE UU P I L IL KKS KM BBTTEB DED E D Shoshonitic I I I I M NTET E DE U I LL PI I I I K LFL KLOS F MBTBE 5 I I I I I O OT TE DD U E A CCIPCL L IPL LPLWLL II UK LLKKF F SF BEN FTE U Q T L MEE E DDEU EE P CC C CLP LI I I KL W KB NT OB D E P W LI OLOFSFOE U CC L CLI W L I O MF S BNK T ED A I L I W LO LWOMBTNO F P LWL CLL CL CLLCWLCL WK O F E T D P L L PKLLFLWFKF F E U O A R C CCCC WWLWCL LLC LLL K CR O E DE P LWC L W WL PU KLL OWKSFN K 0.7 C LCWC CWL C LL LWW GO D 4 H CWL WCWLWWLLWCLWCWCLWCWK RL K Q CCWLCWWC WLWWWL LKRO WWO CPCWWL WWCLWCWPL LCWC CK KK KK R A C C W C WWK WW E O (wt %) HW LWCWWC CWPLLWWLPLL LWL I KL W W L 2 CWWWL CCWLLLL W K S HC WHWWWW C WWLW W G HH WWPWWCWCWCWCCCCCLCCW L LC S G W W W K WC C K H WH L L E CHC CCCP L WCL L LL K R K S W WW WW WW O 3 W WP L WC WWC W W O R E A High-K W W W G 0.6 W O U W W L C L WW W W C C W W 2 W Fe*O/(FeO* + MgO) (wt %) Magnesian Medium-K 0.5 1 Low-K 0.4 0 59 61 63 65 67 69 71 73 75 77 59 61 63 65 67 69 71 73 75 77

SiO2 (wt %) SiO2 (wt %) E 1000 F 1000 Continent-continent Indian Peak-Caliente Indian Peak-Caliente collision ignimbrites ignimbrites

100 A-type or Within plate

100 10 Rb (ppm) Rock/Primitive Mantle Rock/Primitive Continental arc 1.0

Isom Fm. Isom Fm. All other All other 0.1 10 Rb Ba Th U K Nb La Pb Ce Sr Nd P Sm Zr Ti Y 10 100 1000 Y + Nb (ppm) Figure 5.

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A 1.2 B 7 P Indian Peak-Caliente I ignimbrites n = 405 I I 6 P 1.0 I I WW HH C P P I I I W C W C P P CWWW H P P I P I I CWCCWC H P P P PII IIIII II I W WCWCWCWCCLCC P P P I II I 5 C WWCLLCC P I I II I WWWWCWCLWWCCLCCCCL I P P IIIIIII WCWWWWWLC L L H PP P 0.8 C H P IIIPIII P I C WW C C L L II C WWCW LLWCLLWCWL H H LC L III I I WWCLCW LLKLLLL L C HC CCL CCCC II I 4 W WWLKKLWLLKLLCL P WWC C LLCWLLC L L I I WCWC LLLLLWLKL LL P WW WCCWWCW LLLLCCLLLLLLK I C CGC KL WL P I I WCWWWCWCWWCLWCL CLL L I S W W LLWLK WL L P I WCWCWCCWCWW LWW LWLL L K L S S O KKLK P I 0.6 WCCWWWWCWLL LLLL LLWK F I RSW WLKLL P I W WCW LLL K K KFKIKFIF W R K K P I (wt %) W L KK W WWWCWWCLL LL L K I FF I 3 R T L FWL P I 2 C W F C WWCC KKGLLOK K FFI CaO (wt %) L F FF II CWWW U LL L KK F IIK B ENOG SOL O FKF I IIII WC CWCR LWL L F EEM O OFF F IP I I II I

TiO O E U I W W O BE N OBO M I I I I O WW SMT O E BOU M O K F I I I I II I 0.4 R SSOSTMSL T 2 EO B T T F III IIIIIIII II I I TWB O T A DME I I I II I I OO BNMG TTUTTT DD DB I IIII M OBBBEBEBBEBRB E U E UD BBBB T T I I II I I B BNBEBBEB BBTOW Q D EE B BBTTT N MBBEEE E D D UDUD EE E BBBE II I I D D 1 V E TB DU EE E E DU 0.2 E R DDU R D EEAE AA D DQUU U UUDDV AAU 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.0 TiO2 (wt %) 59 61 63 65 67 69 71 73 75 77

SiO2 (wt %) C 700 X D 0.714 Isom Formation X Indian Peak andesite Z Hole-in-the-Wall hA 24 Ma lava 600 Y Hamlight X X XX X lA Lund S X Bald Hills X X X XX XXX X X wA Wah Wah Springs eA A X X X X 0.712 FF XX X X X X eA Escalante F F X XXXX XX eA F 500 X XXX XX X FZ X X XY X C C F XXXXXX Y Caliente C C Z X initial Z Y A Andesite lava D D

F X Sr 400 ZZZ 0.710 s Intermediate stocks eA K K FZ Y 86 L P Y cR Rhyolite lavas W WW s M W U FFF PYY YY P LLLL W U

U Sr / R T T P P

T P 87 Zr (ppm) TT F P 300 BBBB T PP P P BBBBBB MM P 0.708 BTBM K P lA I I I R M L L K KL I cR R cR V G LGOKKK LKLLLLLLL LLL H N N S LLKKLKKL KKLKLKLKLLIL LLLLLL C wA s E Os O 200 N U O O K LLKLLLL L H CH s MEE OSS WS WCWC CWCCCCCHCCCCHHHC hA s B BS DDDE S CWWCRWTWOPWCWCWCWIPTWIPTWWIPCWCIPWWCCWCWCWIICWWCICCWCCC A A EE OOWO WC OPWWOPCWCCWgWCWWWWWWWWCW DDD EDUE E C CWC WW W 0.706 DD AE OW 100 VUU EE UUE AAQ 0 0.704 0.0 0.2 0.4 0.6 0.8 1.0 1.2 45 50 55 60 65 70 75 80 SiO (wt %) TiO 2 (wt %) 2

Figure 6. Variation diagrams for ignimbrites in the Indian Peak–Caliente ﬁ eld. (A) TiO2-SiO2. (B) CaO-TiO2. (C) Zr-TiO2. (D) Initial Sr isotopic ratios of volcanic and shallow intrusive rocks. Key to most letter symbols is in Table 1, but note in diagrams A and B, the entire Isom Formaion is represented by the letter symbol I, whereas in diagram C, symbols are for individual members of the Isom Formation: Bald Hills (X), Hamlight (Y), and Hole-in-the-Wall (Z). Note that off-trend Isom-type tuffs are clearly distinct from the “main trend” rhyolites and dacites (see also Figs. 5 and 7). Also note that in C, Cottonwood Wash and Wah Wah Springs monotonous intermediates are tightly

clustered from 0.50 to 0.75 wt% TiO2 at ~170 ppm Zr with the Lund (L) just above, interspersed with the modest-volume, dacitic Silver King (K). The initial Sr isotopic ratios shown in D include data from Unruh et al. (1995) for the Caliente caldera complex.

intrusions from the Caliente caldera complex , of the magmas. Basement rocks are poorly Oligo cene (Coleman and Walker, 1992). Nelson are plotted in Figure 6D. Initial Sr isotopic exposed in the Basin and Range province, but it et al. (2002) reported the isotopic composition ratios range from 0.7064 for the 18 Ma andesite is presumed to be underlain by the Proterozoic of Proterozoic igneous and metamorphic rocks lava of Buckhorn Spring of the Caliente caldera Mojave province (Whitmeyer and Karlstrom, (n = 6) from the Santaquin complex (~200 km complex to 0.7124 for the Sawtooth Peak For- 2007). Wright and Wooden (1991) estimated northwest of Indian Peak). For these rocks, mation—the oldest rhyolite for which we have the Sr isotopic composition of the Proterozoic the Sr isotopic ratios today range from 0.708 isotopic data. basement in the Mojave province to range from to 0.720 with an outlier at 0.752. The aver- The high 87Sr/86Sr ratios show that most of ~0.710 to 0.735. Precambrian gneiss and schist age value is 0.720. Crustal xenoliths (n = 23) the magmas do not have sources solely in the from the Mineral Mountains, ~100 km east brought up in the Navajo minettes on the Colo- mantle; signiﬁ cant proportions of old conti- of the center of the Indian Peak caldera, had rado Plateau have 87Sr/86Sr ratios ranging from nental crust have been incorporated into most very high 87Sr/86Sr ratios of 0.852–0.726 in the 0.704 to 0.767 with two outliers above 0.800.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/4/864/3346014/864.pdf by guest on 29 September 2021 Indian Peak–Caliente ignimbrite fi eld ) 4 1 1 2 ll fi MIN-13 Caldera continued ( 1 1 ow 1 2 1E Lund Lund Outfl COYOT- 3 0 1 2 2A Cliff Cliff WHEAT- Petroglyph 2 4 1 2 Cliff Cliff clast 1CCB Glassy WEEP- Petroglyph 6 5B part Upper MTWILS- 4 5A Indian Peak caldera complex lower Ripgut Ripgut Typical MTWILS- 0 3 1 3 Hills Bald 4 CU 4th of 0 3 1 2 4 CU 1st of Hamlight 5 5 2 Hole-in- the-Wall the-Wall 5 3 1 2 Leach Canyon 5 2 Lower Upper Leach Canyon 1 7 2 Basal vitrophyre CND-1QV WHRN-1E CND-11B CND-8E CND-8H CND-8B 9 1 1DU Upper Bauers Swett ALAMO- 1 7 1 1 Hills Harmony Caliente caldera complex TABLE 3. CHEMICAL COMPOSITION OF REPRESENTATIVE ROCKS FROM THE INDIAN PEAK–CALIENTE VOLCANIC FIELD ROCKS FROM COMPOSITION OF REPRESENTATIVE 3. CHEMICAL TABLE Hiko Intra- caldera 87-1231 CND-1U 8 1 1B Racer Canyon HEBRON- 5 1 0.20 0.39 0.440.04 0.76 0.08 0.28 0.15 0.36 0.26 0.22 0.05 0.31 0.04 0.52 0.04 0.89 0.09 0.80 0.09 0.14 0.21 0.35 0.18 0.89 0.01 0.83 0.08 0.68 0.26 0.57 0.23 0.19 0.16 1A 77.6312.25 68.91 15.40 71.72 14.30 61.30 16.36 72.58 13.92 72.53 14.42 75.26 12.93 72.76 13.48 71.21 14.83 66.49 15.09 68.10 15.74 77.73 13.06 72.50 14.15 65.90 15.39 65.95 15.67 65.29 15.71 68.37 15.23 Racer Canyon HEBRON- * 1.63 3.05 2.71 5.86 1.73 1.67 1.51 2.12 2.55 4.30 3.71 0.88 2.48 4.71 4.94 4.99 4.22 3 3 5 2 O 2.89 4.36 3.33 3.62 3.67 3.68 3.52 2.99 3.90 4.03 3.61 2.38 3.61 3.03 2.97 2.80 2.42 2 O 2 O 2 OO 4.28 3.15 4.41 3.23 5.77 5.50 4.77 4.99 5.63 5.26 5.86 5.02 4.20 4.53 5.41 4.03 3.86 2 2 2 c b Sample MnOMgOCaONa 0.06 0.36 0.66 0.07 1.28 3.30 0.03 0.82 2.08 0.02 3.10 5.48 0.05 0.28 1.66 0.05 0.44 1.32 0.06 0.35 1.34 0.02 0.78 2.46 0.05 0.14 1.08 0.04 0.52 3.17 0.07 0.24 1.69 0.08 0.10 0.59 0.08 0.45 2.10 0.09 1.43 3.77 0.07 0.78 3.14 0.08 1.41 4.82 0.06 1.74 3.37 K Al P TotalLOIAnalytical Total 100.13 elements (ppm) Trace 100.61 100.00 100.00 99.19 1.64 100.00 99.04 2.17 100.00 99.23 100.00 1.31 100.00 99.01 1.23 100.00 99.74 100.00 0.76 99.35 100.00 100.28 2.82 100.00 99.45 100.00 0.68 100.00 99.76 2.94 100.00 100.00 0.60 100.00 99.87 1.98 100.00 100.00 0.79 100.56 99.41 2.65 99.28 1.60 0.99 1.17 1.22 Notes Basal CU CU Top SiO TiO Fe Cr911417366474 2061216 Ni55 18100002263001513 Unit Major oxides (wt%) m6811581012459887 CeNdSm7668910105 62 24 61 26 94 33 83 42 118 46 108 45 72 30 77 34 118 46 118 48 183 66 77 26 96 120 31 93 43 101 41 87 37 34 S V 244797536191440453754468668 ZnGaRbNbBa 18La 14 32305241586236416970110496773595335 141 35 15 113 18 218 150 1164 8 140 54 952 19 17 191 21 954 15 156 9 866 29 15 175 1248 15 14 176 13 565 16 29 189 17 828 14 157 32 1219 16 10 230 1062 72 18 1413 187 12 62 17 913 164 12 36 13 945 164 16 1097 44 15 136 47 1145 79 124 18 958 43 127 59 16 912 17 62 20 17 58 19 13 13 P Cu78 3248295888111081713 SrYZr 1812161921251316282532223032302320 104 129 657 149 404 180 825 179 246 270 249 316 160 138 273 193 235 403 415 370 323 511 105 99 296 200 445 345 366 288 532 217 473 207 ThU 44547575 547563356 16 10 20 15 33 21 30 24 29 20 43 25 24 20 13 22 19

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/4/864/3346014/864.pdf by guest on 29 September 2021 Best et al. 4 0 8 1 . 1 1 2 Peak Basal Sawtooth vitrophyre 5 Lower CU Basal vitrophyre HFW-1AV LUND-1L SWP-6V Lamerdorf Marsden ) 2 7 3 8 ow . 2 0 1-24 Wash Outfl continued MWASH- Cottonwood *—total iron. CU—cooling unit; LOI—Loss on ignition. 3 0 7 1 O 0 . 2 2 1 Wash 1-5P-1 pumice . Fe Cognate Cottonwood GOUGEWL- 3 8 1 6 . 2 0 1-9 ow east Distal Springs Wah Wah PANGNW- outfl 4 7 0 5 . 2 1 60-2A MIN-8- Springs porphyry Wah Wah Granodiorite ) 6 8 2 7 . 2 0 Springs continued MIN-3D Wah Wah Intracaldera 9 4 0 3 . 3 ow 3 Basal outfl 153-3A Springs evolved HFW-8- Wah Wah 7 GLE-6- 98-1XB Springs Cognate inclusion Wah Wah Indian Peak caldera complex ( 5 4 2 ow west Distal outfl Springs Wah Wah 0 8 1 1 . 3 1 Intra- Spring caldera Deadman 6 0 7 8 . 1 2 1 Basal Greens Canyon MLLR-5V BAILEY-5D BRS-5V vitrophyre 7 Basal 61-2V MIN-8- vitrophyre Mackleprang TABLE 3. CHEMICAL COMPOSITION OF REPRESENTATIVE ROCKS FROM THE INDIAN PEAK–CALIENTE VOLCANIC FIELD ( ROCKS FROM COMPOSITION OF REPRESENTATIVE 3. CHEMICAL TABLE 4 7 2 2 . 2 1 1E King Silver SILVRWL- uorescence spectrometry at Brigham Young University, Provo, Utah, except for sample 87-1231 which is from Nealey et al. (1995) University, Young uorescence spectrometry at Brigham 1 5 9 1 . 2 0.59 0.59 0.420.18 0.34 0.15 0.15 0.09 0.54 0.06 0.57 0.03 0.37 0.15 0.65 0.17 0.55 0.12 0.63 0.17 0.78 0.13 0.54 0.17 0.56 0.19 0.11 0.13 0.40 0.06 0.01 0.11 2 lava 68.1114.96 69.20 15.02 70.77 15.39 71.25 15.13 74.79 13.82 67.20 15.14 66.66 15.09 70.21 14.06 64.16 15.59 66.67 14.85 64.33 15.52 63.14 15.83 67.41 15.32 70.64 14.91 76.97 13.09 70.64 14.97 70-1 Ring Lund fracture WRP-7- Analyses by X-ray fl * 4.12 3.83 2.41 2.20 1.38 4.85 4.75 3.13 5.57 4.83 5.51 6.20 4.42 2.81 0.75 3.26 3 3 5 2 O 2.80 2.92 3.60 3.17 2.49 2.27 2.63 1.92 3.16 3.25 2.68 3.19 2.98 3.00 3.20 3.03 2 O I 2 O 2 OO 3.98 3.47 4.43 4.36 5.61 4.00 3.95 3.66 3.13 4.00 3.71 3.27 3.83 4.52 4.92 3.23 Note: 2 2 2 b m O r 5 1 6 1 265430033 2 1 473 Cr 5 90894131113105 1Ni405 6 1 226251433303023133 Ce116100140102718310977848687798019471120 Nd 39 34 45 42 21 31 40 24 38 35 40 36 35 56 28 48 L Analytical Total 99.55 elements (ppm) Trace ScV 99.23 80633227129480541141111121207533942 9 99.23 99.12 5 99.66 7 99.40 100.19 3 99.08 99.73 4 99.94 12 99.20 99.22 12 6 100.01 17 98.94 13 17 15 12 9 3 7 b28 30106341 6 30159 13201110161314118 ZnGaRbNb12188 Ba 62La 19 143 61 18 135 1048 47 63 150 15 891 165 41 49 15 999 260 36 79 976 18 145 45 587 61 16 139 771 52 48 16 182 759 27 53 14 123 648 56 66 18 147 705 55 46 120 18 699 145 50 42 17 713 143 70 43 638 17 199 51 42 765 17 150 1162 54 45 19 95 626 31 44 11 1876 51 82 17 38 55 MnOMgOCaONa 0.07 1.63 3.56 0.07 1.13 3.62 0.06 0.60 2.23 0.07 0.74 2.68 0.10 0.55 1.08 0.06 1.74 4.05 0.07 1.89 4.22 0.05 3.53 2.95 0.10 2.74 4.73 0.08 2.18 3.46 0.08 2.29 5.08 0.09 2.37 4.93 0.06 1.55 3.76 0.05 1.00 2.44 0.02 0.43 0.49 0.06 0.85 3.45 TiO Fe K P Unit Al Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 S u73 1112221176 1 2 517181223262017127 Cu17139 SrYZr 18192722301715172118202219432224 520 237 466 220 357 274 383 212 93 138 432 170 421 166 292 154 514 169 509 151 506 159 392 196 348 161 323 419 101 64 642 189 See Table 1 for additional information and Supplemental File 4 (see footnote 4) analytical precision accuracy. Table See ThU 556 5367856 45 4 821 26 26 30 20 31 25 27 31 20 24 18 23 25 36 13 22 P Notes SiO Sample Major oxides (wt.%)

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/4/864/3346014/864.pdf by guest on 29 September 2021 Indian Peak–Caliente ignimbrite ﬁ eld error 2 standard Sr 86 initial Sr/ 87 Sr 86 Sr/ 87 measured Sr 86 Rb/ 87 Sr (ppm) Rb (ppm) Age (Ma) 2 SiO (wt%) —whole rock composition. *Symbols used in Figure 6D to represent these samples. 2 Caldera complex Material Composition TABLE 4. Sr ISOTOPIC COMPOSITION OF SELECTED IGNEOUS ROCKS FROM THE INDIAN PEAK–CALIENTE CALDERA COMPLEX THE INDIAN PEAK–CALIENTE CALDERA COMPOSITION OF SELECTED IGNEOUS ROCKS FROM 4. Sr ISOTOPIC TABLE IP—Indian Peak caldera complex; Ca—Caliente complex. SiO Note: LAM-9-71-2 Escalante DeserteATET-9-77-4BLAM-9-71-2 Escalante MIL-1HFW-2V Escalante DesertHAM-3D IPLUND-1USWP-9-3 eA Escalante Desert Lamerdorf Lamerdorf Marsden Lava Sawtooth Peak IP eA F F S IP Andesite A Lava IP IP IP IP 60.8 Lava Andesite 32 Tuff Tuff Tuff Tuff 78.9 60.0 Andesite 32 984.9 Rhyolite 0.232 Rhyolite Rhyolite 58.3 Rhyolite 78.0 0.7123 32 0.7121 69.9 510.0 72.5 0.000010 70.6 77.1 0.442 32 62.0 32 33.5 0.7117 32.50 612.0 179.2 0.7115 108.7 141.5 133.0 0.293 0.000009 446.2 500.5 0.7100 306.2 39.1 1.162 0.627 0.7099 1.338 9.840 0.000007 0.7122 0.7127 0.7123 0.7168 0.7116 0.7124 0.7117 0.000018 0.7122 0.000011 0.000019 0.000118 BAN-11CND-1PVTRC-2DTRC-2VTHREE-3DTHREE-3D Hornblende Andesite Leach CanyonMTWILS-5B Isom: Bald HillsMTWILS-5A hA Isom: Bald HillsELYS-1JD Isom: Bald Hills Isom: Bald HillsSTM-8-130-4 Lund Ripgut ECOYOT-1E RipgutMIN-8D X IPMIN-8V X X Petroglyph CliffMLLR-2V X LundSILVRWL-2A CaCOYOT-1D IPBAILEY-5E Lund P IP Lava IP L UBAILEY-5A Lund Silver King IP Lund UATL-4D Silver KingSILVRWL-1D-8 Tuff Deadman SpringHAM-6V IP Tuff Deadman SpringBRN-1PC IP IP Plagioclase Springs Wah Wah L Tuff TuffBRN-1PB IP Latite KBRN-3 Mackelprang D LGOUGE-2V D K W Springs L Wah Wah L Rhyolite Trachydacite Springs Wah Wah Trachydacite IP Ttuff Lava Tuff Springs Wah Wah Trachydacite Trachydacite IP Tuff 55.9 Cottonwood Wash IP W M IP Springs Wah W Wah IP IP IP 69.9 IP W IP 68.2 69.0 24 69.9 67.7 C Trachydacite Tuff 27.6 W Andesite 27.6 24.1 IP IP Rhyolite Tuff IP 27.6 27.6 132.8 Tuff Rhyolite Tuff IP Tuff Tuff Tuff 214.6 213.9 Tuff 4.2 67.2 Tuff IP 247.5 200.0 714.4 IP 60.1 Pumice clast 2273.0 385.7 72.4 133.5 29.1 0.537 Dacite Tuff Pumice clast Tuff 297.5 387.2 77.4 0.005 1.610 29.20 4.635 Dacite Rhyolite 0.7068 29.0 155.0 2.408 1.494 Dacite Rhyolite 29.0 0.7078 Dacite Dacite Tuff 0.7083 0.7088 Dacite 0.7066 Lava Rhyolite 40.7 Dacite 0.7087 0.7083 164.0 366.0 0.7078 0.000013 0.7076 0.7072 Dacite 65.3 187.0 0.7078 0.7077 0.000010 0.000022 850.3 0.000020 Rhyolite 73.8 69.6 1.224 Dacite 296.0 0.000028 0.000020 64.7 29.20 75.4 105.0 Basaltic andesite 62.8 68.8 0.139 73.8 0.7089 64.8 1.603 30.02 29.40 64.8 128.1 Dacite 5.155 29.20 30.02 66.8 0.7078 0.7084 30.06 29.40 0.7097 30.06 203.3 29.20 121.3 72.3 56.2 29.20 0.000014 67.1 0.7116 0.7077 258.3 118.0 546.2 0.7091 30.06 120.9 157.6 151.4 114.8 0.000012 166.3 173.6 464.2 0.7094 30.01 30.5 0.000019 30.06 0.679 582.8 165.0 67.8 0.000057 81.4 540.7 425.5 3.389 0.756 404.9 609.2 151.4 507.6 0.7095 139.0 0.586 61.6 9.191 0.647 31.13 1.072 419.0 1.082 0.7117 0.7102 0.545 0.7093 0.948 276.0 0.7095 411.0 0.7142 0.7096 0.000012 0.7103 0.893 0.7102 0.7099 147.0 626.5 0.7099 0.7095 0.7096 1.588 0.7092 0.000039 0.000014 0.977 0.7103 0.7093 0.7099 0.7099 0.7094 0.285 0.7092 0.000012 0.7092 295.0 0.000103 0.000012 0.7101 0.000016 0.000017 0.000011 0.7098 0.7095 0.000020 0.7072 1.440 0.7094 0.000013 0.7094 0.7071 0.000019 0.000013 0.7116 0.000011 0.7110 0.000018 HAM-1VBRN-2PBRN-2P Cottonwood Wash Cottonwood Wash Cottonwood Wash C C C IP IP IP Pumice clast Tuff Plagioclase Dacite Dacite Dacite 69.4 69.4 66.5 31.13 31.13 31.13 160.0 122.0 2.6 297.0 1043.0 1.559 357.0 0.007 0.7117 0.988 0.7110 0.7109 0.7114 0.000019 0.7109 0.7110 0.000010 0.000013 CND-1U Harmony Hills H Ca Tuff Andesite 61.3 22.56 95.5 821.3 0.337 0.7075 0.7074 0.000020 CHIEF-1HU Bauers B Ca Tuff Rhyolite 71.0 23.04 204.0 251.3 2.349 0.7075 0.7068 0.000020 Sample Stratigraphic unit Symbol*

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The average value, minus the outliers, is 0.725. A 60 Thus, the volcanic rocks from the Indian Peak– Indian Peak-Caliente ignimbrites n = 371 Caliente complex with initial ratios higher than 50 B B 0.710 could include substantial crustal contami- B B R E nants ranging from 50% to as much as 100% 40 D B AV KKK depending upon the composition of the initial R DD K RKK B B B E KK K B BBB V V V E EE O O E B mantle-derived magmas and of the contaminat- EDO BB C C R R E BBBB 30 R O R E B B ing crust. R E E E E E B B E E RE B However, there is little correlation between RE O O VR RRE DR OO EEOO R OR R RRRE EOOE O O E the Sr isotopic composition of the volcanic rocks 20 R R R E E E V V RO ORD OOE ORO R O U U and their silica concentrations. In fact, ratios for RO OO E R R O O Sanidine (% phenocrysts) R mafi c Oligocene rocks (<60 wt% SiO ) span S R OGOE 2 10 G OL R S LL OOO O nearly the entire isotopic range (Fig. 6D); Sr S E E RLLLLL K U LLKLLLLLLRLL isotope ratios range from 0.7064 (the Buckhorn H L LHS LLLK U L KCCLKCLCKGLCLCLHLKLCLHCWLHCHHHIIHH UIIIIIIIII II III Spring andesite) or 0.7071 (basaltic andesite 0 CCCCWWWWAAWCWWHWCWWWWCWLLWCWWWLWCLWCCWLLHWWCCWHWWWCLHWHCWWHWHWWHHTTFFFFFMMTTFFTNMMMTTNTTTTTFTTTTTNTT TTTT T 020406080100 BRN-3) to 0.7121 (andesite LAM-9-71-2). Plagioclase (% phenocrysts) Rather, initial Sr isotope ratios are highest for oldest rocks regardless of silica content. Thus, B 30 KO lavas and tuffs of the ca. 32 Escalante Desert S Formation and the 33.5 Ma rhyolitic ignimbrite R of the Sawtooth Peak Formation with ratios C of 0.7099 to 0.7124 have incorporated large 20 L T T C H H H M amounts of ancient continental crust. Igneous O H G T CLL H T rocks associated with the 24–22.56 Ma Caliente K L H HH N O O C HHH MTTT caldera complex have distinctly lower isotopic E KK C OC HHH F NTT R KWLCL WK TN EKK K K L CLOKC H H MM T T T compositions of 0.7066–0.7074 regardless of OO OK CCLCWCLC L HH F TTTTT K K ORLWLLWWOCWL COCLCH H MT 10 R RRO E SLSOLCLLCWBWKWWW U silica content. OD RO ROG LCRBCLHEWLLCBWCWLLWLCWWOW F TT RER BOWLWLOL WLWLLWWCLBW U O R V OSER R LLCLBWCWWCLRWEWBW E F T Biotite (% phenocrysts) R R E R OOB E OWEBBOLCW U F EERRRE R O BO O B B WBCWWWWW F R R D BBB WCOWEW Modal Composition ER R RO R BO O EBB F RV OO E EERR EORB E A B U RV EDER E E E EE G U RADDVE EV E E BAB V VE B All ignimbrites contain biotite phenocrysts, 0 R IIIIIIIIIII II III except for the Isom-type tuffs (Fig. 7; Table 1). 0 102030405060708090100 Small concentrations of Fe-Ti oxides, essen- Plagioclase (% phenocrysts) tially magnetite and minor ilmenite, are ubiqui- C tous in the ignimbrites, as are trace amounts of W apatite and zircon. W 30 WW Only about half of the ignimbrites contain OW W W 40 39 WW WW suffi cient sanidine that can be dated by Ar/ Ar WWWWW WW analyses. Notably, most phenocryst-poor, low- HWC W W WWW WLWLWW W silica rhyolites, as well as trachydacitic Isom- CL WW 20 C LWL L BWW WLWW type tuffs, which together compose the off-trend C CWCWWCWL WW LCC LC WWW class of ignimbrites, lack sanidine; however, C LC LCC WW C CCWL L U LL C L C the off-trend sanidine-bearing Bauers is an LL CLC C L CL CLLL H H L LCLL exception. L LLLLL 10 C HC CHH H F F Regional rhyolite ignimbrites older than L LLH CH L H H H C HHH HHF 29 Ma include the phenocryst-rich, main-trend Hornblende (% phenocrysts) KO L EO HH FFF R SK O GO COHO UF R K SOR ECO KHK H Deadman Spring (D) and Sawtooth Peak (S) O RE R KOEKR O O R F E R E RV OOOEOKOKO KOO KOO E E R OREERRE ER EROERSROKEOEOERKEKR K which contain all three felsic phenocrysts (pla- 0 V R RVVRDRADDDEDVOVRREEERERRERVEEORRREORROEBBBERRBBDG BBBAABOBSBBBERBGBBBBBIITTUIIIIIMUIITII ITNMIMMMTTNTTTT ITTIITTNTTTUTT gioclase, sanidine, and quartz) in addition to 0 20406080100 biotite. Other older rhyolites—Lamerdorf (F), Plagioclase (% phenocrysts) Mackleprang (M), Greens Canyon (N), and Ripgut (U)—as well as the younger Swett (T) Figure 7. Modal proportions of phenocrysts in ignimbrites in the Indian Peak–Caliente fi eld. and Bauers (B) are phenocryst poor and contain Key to letter symbols is in Table 1. Inclined dashed lines show 100% of the x plus y pheno- mostly, or only, plagioclase and biotite as sili- cryst proportions. Off-trend ignimbrites, shown in red, mostly plot apart from the main-trend cate phenocrysts; none of this group of off-trend ignimbrites. (A) Sanidine-plagioclase. In the Bauers ignimbrite (letter symbol B), nearly all of rhyolite tuffs contains quartz phenocrysts. Other the phenocrysts are sanidine and plagioclase. (B) Biotite-plagioclase. Nearly all of the pheno- younger rhyolites—Leach Canyon (E), Racer crysts in the low-silica, off-trend rhyolite Swett (T), Mackleprang (M), Greens Canyon (N), and Canyon (R), and Hiko (O)—are main trend, Lamerdorf (F) ignimbrites are biotite and plagioclase. (C) Hornblende-plagioclase. The Wah are phenocryst rich, and contain all three felsic Wah Springs ignimbrite (W) contains proportionately more hornblende than all other tuffs and pheno crysts plus biotite and hornblende. this phenocryst plus plagioclase constitute most of the total phenocrysts in the ignimbrite.

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Main-trend dacite ignimbrites, most of INDIAN PEAK CALDERA COMPLEX large White Rock caldera. These two calderas which are the voluminous and distinct monoto- are the sources of the super-eruptive 30.06 Ma nous intermediates, are phenocryst rich and Exposed calderas composing the Indian Peak Wah Wah Springs and the 29.20 Ma Lund contain plagioclase as the major phenocryst caldera complex were created from ca. 30 to monotonous intermediates, respectively. Based together with lesser biotite, hornblende, and 29 Ma as the result of explosive eruptions from on the distribution and thickness of the outﬂ ow minor pyroxene; quartz occurs in most samples a long-lived crustal magma system (Table 1). ignimbrite of the 31.13 Ma Cottonwood Wash whereas sanidine is absent in the Wah Wah This complex constitutes the northern of the monotonous intermediate, it too was the result Springs (W) and Cottonwood Wash (C) ignim- pair of caldera clusters composing the greater of a super-eruption (Table 2) and its caldera was brites. The Wah Wah Springs, as a unit, has Indian Peak–Caliente caldera complex. The engulfed by one or both of the younger calderas; the highest hornblende-to-biotite ratio of any roughly elliptical Indian Peak caldera complex no evidence has been found for any intracaldera ignimbrite of which we are aware in the middle lies across the Utah-Nevada state line and spans facies. Overlapped by the westernmost seg- Cenozoic of the Great Basin. The andesite- across ﬁ ve mountain ranges east-west over ment of the White Rock caldera, and extending latite Harmony Hills (H) is mineralogically like a present distance of 115 km (Figs. 2 and 8); somewhat beyond, is the older and smaller Kix- the dacite tuffs. compensating for 50% post-caldera extension, miller caldera related to eruption of the 30.00 The off-trend trachydacitic Isom-type tuffs the east-west dimension is 77 km. The north- Ma rhyo lite tuff of Deadman Spring. Nested (Isom Formation and Petroglyph Cliff Ignim- south, unextended dimension of the complex entirely within the older White Rock caldera brite) are another distinct class of as many as is ~65 km. is the small Mount Wilson caldera that was the nine cooling units; they contain no sanidine, Following precursory rhyolitic eruptions source of the 29.0 Ma rhyolite tuff of the Ripgut quartz, hornblende, or biotite and have plagio- of modest volume (Sawtooth Peak, Marsden, Formation. Little is known of this small caldera clase and subordinate clino- and orthopyrox- Lamerdorf), the Indian Peak caldera probably because of incomplete mapping and conceal- enes and Fe-Ti oxides as their only pheno- engulfed their sources. This caldera is in turn ment beneath younger deposits on the south, crysts. largely eclipsed on the southwest by the equally southwest, and southeast. Offset ~5 km from

38°30′ Km Post-Ripgut 02.5 5 10 15 20 volcanic rocks

Miles Ripgut Formation 29.0 Ma 05102.5 (Mt. Wilson caldera)

Post-collapse caldera fill tuff Nevada Intracaldera tuffs Utah & breccias Fig. 13 and 39 Precollapse Lund Tuff 29.20 Ma White Rock caldera White

38°15′

HD Fig. 44

NM Fig. 58 38° Blind Mountain NM intrusive complex

Silver King Tuff 29.40 Ma SD

HD tuff of Deadman Spring 30.00 Ma (Kixmiller caldera)

Post-collapse caldera fill tuff Pre-Wah Wah Springs SD volcanic rocks Intracaldera Wah Wah Springs Fm Paleozoic & Mesozoic sedimentary rocks Precollapse 30.06 Ma

Indian Pk caldera Pk Indian Wah Wah Springs Fm 37°45′ 114°45′ 114°30′ 114°15′ 114° 113°45′ 113°30′

Figure 8 (on this and following two pages). Indian Peak caldera complex. All three maps are shown on the same base map and scale. (A) Generalized geologic map compiled mostly from published maps. Many faults are omitted for clarity. NM—not mapped; HD and SD— Highland and Stampede detachments, respectively (e.g., Axen et al., 1988).

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the approximately located southwestern margin The northern margin of the Indian Peak caldera intruded by granodiorite porphyry. The north- of these overlapping calderas in the complex is complex and its constituent calderas is clearly ern half of the collar zone lies in a transecting the very small (3 km diameter) and partly buried defi ned, and the eastern and western less so. The low area in the central Needle Range between Blind Mountain caldera source of the 29.1 Ma southern margin is hidden by younger deposits what is designated on post–A.D. 1974 maps Petroglyph Cliff Ignimbrite. The source caldera but a distinct gradient in the Bouguer gravity fi eld as the Mountain Home Range to the north and for the 30.01 Ma Mackleprang Tuff Member of provides insight into its position (Fig. 8C). the Indian Peak Range to the south (Fig. 3). the Ryan Spring Formation is entirely buried Basin-and-range faulting and tilting of horst On maps published before ca. 1974 these two beneath younger deposits at the southern end of blocks subsequent to formation of the calderas ranges were designated as the Needle Range. Pine Valley in the eastern segment of the Indian have revealed the internal structure of the cal- Where we refer to both ranges the older appel- Peak–Caliente caldera complex. The source of deras (Fig. 8A). This is especially true of the lation Needle Range will be retained for brevity. the voluminous trachydacitic tuffs of the Isom northeastern sector of the Indian Peak cal- Decades of usage distinguishes the geographic Formation emplaced mainly from 27.90 to dera where the east-tilted Needle Range horst place Needle Range from the stratigraphic unit 27.25 Ma cannot be located with certainty. It exposes the collar zone of wall-collapse brec- Needles Range (see Historial section below). is shown to lie to the southeast of the complex cias and caldera-fi lling tuff that lies between Additional features of the Indian Peak caldera where it is entirely concealed beneath the broad the topographic margin and the ring-fault complex include: (1) resurgent uplift in the two alluvial valley of the Escalante Desert and fl ank- structural margin; to the southeast, intercaldera larger calderas; (2) downsagging of the northern ing younger lava fl ows (Fig. 8). tuff and breccia that is 4–5 km thick has been and eastern perimeter of the Indian Peak caldera

38°30′ Utah Nevada ^ ^ BE AFF

′ ^ 38°15 ^ ^ ^

^ ?

? ^ ?

38°

Km t y Source area for 02.5 5 10 15 20 v i G r r a Isom Formation a d i g e n t i n 27.90–24.55 Ma Miles 05102.5 ? ? 37°45′ 114°45′ 114°30′ 114°15′ 114° 113°45′ 113°30′

Kixmiller caldera Blind Mountain caldera Mt. Wilson caldera White Rock caldera 29.20 Ma Indian Peak caldera 30.06 Ma Source caldera area 30.00 Ma 29.1 Ma Topographic margin Topographic margin 30.01 Ma 29.0 Ma ^ Ring intrusion Ring fault Ring fault ^ Ring fault lava and intrusions (Deadman Spring) (Petroglyph Cliff) (Ripgut) (Lund) (Wah Wah Springs) (Mackleprang)

Figure 8 (continued). (B) Caldera margins. The unit erupted from each caldera is indicated in parentheses in the ﬁ gure explanation. On the map, BE—Brent Energy well; AF—Apache Frontier Federal No. 23-13 well.

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in addition to deep subsidence of its central part in the vicinity of the Indian Peak caldera com- cation Range (Fig. 9). The upper part of the for- along ring faults, and development of an exten- plex occurred at two small centers to the north, mation is mostly a sequence of numerous, thin, sive layer of collar-zone breccia; and (3) ring- at Crystal Peak and in the Fortifi cation Range. weakly to moderately welded ignimbrites that fault extrusions in the White Rock caldera. contain upwards of 75% clasts of pumiceous, Fortifi cation Range felsitic, and massive vitrophyric rhyolite that RHYOLITE IGNIMBRITES >29 MA The oldest known explosive activity took are as much as 25 cm in diameter. This part of ERUPTED FROM THE INDIAN PEAK place 15 km north of the northwestern margin the formation was likely the result of explosive CALDERA COMPLEX of the Indian Peak caldera complex, deposit- fragmentation of the growing extrusive dome, a ing locally thick ejecta that are exposed to the remnant of which is a mass of lava and auto- Centers North of the Indian Peak Caldera north of a small rhyolite lava dome (Loucks clastic debris ~250 m thick and 1.5 km in diam- Complex et al., 1989). These silicic volcaniclastic rocks eter located at the north end of The Gouge Eye. that were deposited directly on Paleozoic sedi- Underlying the effusive dome is a sequence of Because of the general pattern of southward- mentary rocks are here designated the formation generally loosely welded lapilli ash-fl ow tuffs sweeping volcanism in the Great Basin, the ear- of The Gouge Eye, so named for a topographic and minor bedded tuffs emplaced by surge, plin- liest expressions of explosive silicic volcanism scallop in the western escarpment of the Fortifi - ian, and epiclastic processes that were the result

38°30′ mGal

Utah –100 Nevada

–150

–200

–250

38°15′

38° 00′

Source for Isom Formation y i t 27.9–24.55 Ma a v Km g r G 02.5 5 10 15 20 r a d i e n t i n

Miles 05102.5 ? ? 37°45′ 114°45′ 114°30′ 114°15′ 114° 113°45′ 113°30′

Kixmiller caldera Blind Mountain caldera Mt. Wilson caldera White Rock caldera 29.20 Ma Indian Peak caldera 30.06 Ma Source caldera area 30.00 Ma 29.1Ma Topographic margin Topographic margin 30.01 Ma 29.0 Ma Ring fault Ring fault (Deadman Spring) (Petroglyph Cliff) (Ripgut) (Lund) (Wah Wah Springs) (Mackleprang)

Figure 8 (continued). (C) Bouguer gravity compiled by PACES (Pan American Center for Earth and Environmental Sciences) and downloaded from http://research.utep.edu/Default.aspx?alias=research.utep.edu/paces. Note the relatively lower gravity within the caldera complex, the southern margin of which is delineated by a gradient in gravity (see B). The Kixmiller caldera appears to be asymmetric, or hinged, so that its southwestern part has less low-density intracaldera ﬁ lling. The concealed source of the Isom Formation ignimbrites, which might lie in the Escalante Desert, has no apparent gravity expression.

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39° E E E NV UT Tunnel Spring Tuff (35.26 Ma) Formation of The Gouge Eye (36.02 Ma) ! 0 >15 ^ Lava dome/vent >25 570? ! ! >120 ! 310 ! ! ! ! >7 0 ! 0 0-135 Crystal Peak caldera 230 150 ! ! 1590 0 ! The Gouge Eye ! >1300!^ caldera

0 !60 ′ E 100 E 38°30 ADJ 0 E ! 0 0 185 ! ! ! ! 0 BE Sawtooth Peak Formation ~250 0 ! ! 00! (33.5 Ma) 0 3 ! 300 0 ! 200? 0 ! ! Younger

Indian Peak caldera: ! Topographic rim 0 0 ! <200?

! <200 ! ~3? 0 5 10 15 20 25 ! <150 ! Km 200 0 0 0 5 10 15 ! 38° E Miles E E

115° 114° 113°

Figure 9. Distribution and thickness (in meters) of early rhyolite ignimbrites and their caldera sources (dashed lines) in the Indian Peak– Caliente ﬁ eld. Source of the Sawtooth Peak Formation is unknown but was probably engulfed at 30.06 Ma in the younger Indian Peak caldera. Star marks location of the lava dome overlying the vent from which the formation of The Gouge Eye was erupted. In this map and all other maps in this article that display the distribution of ignimbrite, zeros denote sites where none of the particular unit is exposed between older and younger deposits. BE—Brent Energy well; ADJ—Amoco Dutch John Unit No. 1 well.

of explosive venting of the more volatile-rich Indian Peak rhyolite ignimbrites upper part of the magma chamber. U Ripgut Formation 12 Ryan Spring Formation The lava dome–vent complex lies on the M Mackleprang Tuff Mbr. southern margin of a source caldera, inside of N Greens Canyon Tuff Mbr. 11 D Tuff of Deadman Spring which the volcaniclastic deposits aggregate to F Lamerdorf Tuff as much as 1590 m thick. Distal extra-caldera A Marsden Tuff 10 S Sawtooth Peak Fm. deposits of the formation that are 185–250 m Figure 10. Total alkalies-silica Q Tunnel Spring Tuff thick were found in well cuttings in Lake Valley. diagram for rhyolite ignim- G Fm. of The Gouge Eye 9 M brites older than 28 Ma in the F M A The total volume of the formation calculated by M D D 3 O (wt %) U D 2 N U U Model 2 in Figure 4 is ~100 km . Indian Peak–Caliente ﬁ eld. Two 8 U MM D D D DAU F U analyzed samples of ignim brite NF D U Phenocrysts make up less than 15% of the FSFN D O + K F F DD U 2 F F F ignimbrites and consist of variable proportions in the formation of The Gouge 7 F A U

Na S Q of plagioclase, quartz, and biotite; sanidine Eye (G) likely have perturbed S S S Rhyolite is found in some samples as well as apparent low alkali contents. 6 Dacite xenocrysts of pyroxene and hornblende. G Chemically, the lava dome is a rather ordi- 5 G nary rhyolite with ~74 wt% silica and 8 wt% total alkalies. Two analyses of the tuffs show 4 perturbed concentrations of alkalies but silica 59 61 63 65 67 69 71 73 75 77 ranging from 69 to 72 wt% (Figs. 5, 6, and 10). SiO2 (wt % )

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Armstrong (1970) cited a K-Ar age of 34.2 ± (Figs. 5, 6, and 10). Relative to other tuffs of (Best et al., 1987a). Due west across Hamlin 0.8 Ma (corrected for new decay constants) on comparable silica content, the Sawtooth Peak Valley in the southern White Rock Mountains

biotite from the basal part of the oldest major ash- has somewhat greater CaO, Fe2O3, and Sr and a 7-km-long north-to-south exposure of Cam- fl ow unit. Our 40Ar/39Ar plateau age on plagio- has the highest Ba of any rhyolite ignimbrite brian rocks is overlain by Sawtooth Peak tuff clase from a younger tuff is 36.02 ± 0.20 Ma. in the Indian Peak–Caliente fi eld. The high Ba as thick as 150 m, but near the southern end of concentrations probably refl ect extensive dif- the exposure, younger rocks lie directly on the Crystal Peak ferentiation in the magma before the onset of Cambrian rocks (Keith et al., 1994). Fifty kilometers northeast of the main Indian sanidine crystallization, after which lesser con- A possible correlative of the Sawtooth Peak Peak caldera complex is the small Crystal Peak centrations developed in the residual melt. The ignimbrite is the petrographically similar tuff caldera which was the source of the Tunnel Sawtooth Peak tuff also has the highest initial of Tower Point in the southeastern Wah Wah Spring Tuff (Bushman, 1973; Steven, 1989). 87Sr/86Sr ratio (0.7124) of any rock in the Indian Mountains (Hintze et al., 1994b). The small iso- Identifi cation of the wholly concealed, hypo- Peak–Caliente fi eld (Table 4), indicating this lated exposures which are as thick as 200 m lie thetical caldera is based on gravity data and on early magma assimilated signifi cant amounts of in the same stratigraphic position as the Saw- the distribution and thickness of the ignimbrite, continental crust. tooth Peak tuff, i.e., between the Marsden Tuff which crops out in a 35-km-long paleovalley The only comparable phenocryst-rich rhyo- and Eocene and Jurassic rocks. that includes Crystal Peak where the Tunnel litic ash-fl ow tuff in the northern part of the The Sawtooth Peak was deposited on an Spring Tuff is 310 m thick (Fig. 9). Exposures at Indian Peak–Caliente fi eld is the 30.00 Ma, high- irregular erosion surface carved into the terrain this edifi ce are famous for the exceptional cav- silica rhyolite tuff of Deadman Spring (Table 1). of older rocks and appears to lie mostly in three ernous weathering (McBride and Picard, 2000). The fi ve other regional rhyolite ignimbrite cool- more or less east-west–trending paleovalleys The pre-collapse ignimbrite volume is estimated ing units in the fi eld that are less than 29 Ma are and is thin to absent between them (Fig. 9). to be 25 km3 (Table 2). A sanidine 40Ar/39Ar age off trend, phenocryst poor, and lacking in quartz We roughly estimate its volume at 150 km3. on the tuff is 35.26 ± 0.03 Ma. The weakly and usually sanidine. Other phenocryst-rich, No caldera is exposed, having been engulfed welded, phenocryst-rich, high-silica (76.8 wt%) main-trend rhyolite ignimbrites (Leach Canyon, in younger calderas in the Indian Peak Range rhyolite tuff contains conspicuous clasts to as Racer Canyon, and Hiko) were not emplaced to vicinity. Assuming an equivalent volume in the much as 2 m in diameter of carbonate rock and the south until after 24 Ma. concealed caldera gives a total of 300 km3 for abundant small lapilli of pumice. The tuff has the unit (Table 2). about 30% phenocrysts, consisting mostly of Distribution and Source doubly terminated quartz, lesser sanidine and The Sawtooth Peak ignimbrite occurs chiefl y Escalante Desert Group plagioclase, and a trace of biotite. in the Needle Range as a compound cooling unit (Fig. 9). It is ~300 m thick in a paleo valley at In early work on older volcanic rocks east of Sawtooth Peak Formation Sawtooth Peak (Fig. 11) just beyond the topo- the Indian Peak caldera complex, Grant (1978) graphic margin of the Indian Peak caldera where named and described the Escalante Desert The next eruptive activity took place in the several meters of a near-basal black vitrophyre Formation as consisting of phenocryst-poor, northeast sector of the Indian Peak caldera are exposed. The unit pinches out within ~9 km lithic-rich rhyolite ignimbrites. Later, Best and complex, creating ignimbrite of the Sawtooth to the north (Best et al., 1987b), and then recurs Grant (1987) formally named two constituent Peak Formation (Conrad, 1969; Best and Grant, again in a 60-m-thick section ~5 km farther members of the formation—the Marsden Tuff 1987). A K-Ar age of 33.5 ± 1.2 Ma on biotite north. About 12 km southwest of Sawtooth Member and the younger Lamerdorf Tuff Mem- and a fi ssion-track age of 33.6 ± 1.8 Ma on zir- Peak, altered rocks as thick as 200 m were ber; the latter unit was originally named and con have been determined on the same sample designated as Sawtooth Peak tuff by Best et al. described by Campbell (1978). Best and Grant (Best and Grant, 1987). (1987b) but, on reexamination of this terrain, (1987) also expanded the formation to include Three samples of the Sawtooth Peak con- the unit does not appear to be present among local informal rhyolite and andesite lava fl ow tain 44%–39% phenocrysts (dense rock basis), andesitic lavas and other rock types. Still far- members that interfi nger with the ignimbrites consisting of plagioclase (49%–41%), quartz ther south within the Indian Peak caldera (Fig. and an overlying, mostly epiclastic unit, the (41%–24%), sanidine (12%–7%), biotite 8A) in a structurally high, apparently resurgent Beers Spring Member, which was fi rst named (10%–8%), hornblende (5%–2%), and opaque block exposing the caldera fl oor, the Sawtooth and defi ned by Conrad (1969). phases (4%–2%). Samples of the Sawtooth Peak tuff is as much as 200 m thick but locally Subsequent analytical work has disclosed Peak plot on the rhyolite-dacite fi eld boundary pinches out on the underlying Paleozoic rocks that the Marsden and Lamerdorf, although both

Figure 11. Panoramic view looking west at a thick (300 m), Sawtooth Peak Fm. Sawtooth Peak Fm. compound cooling unit of the Pz Sawtooth Peak Formation draped over a paleohill of Ordo- vician carbonate rocks (Pz) on the east side of the central Needle Range. The Sawtooth Peak geographic feature is on the left.

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phenocryst-poor and lithic-rich rhyolite ignim- clase, possible sanidine, and less biotite; the Formation but here it both overlies and under- brites, have contrasting modal and especially abundance of phenocrysts appears to increase lies the Lamerdorf Tuff. Still farther northwest, chemical compositions (Table 3; Figs. 5, 6, and upward in the unit. in a graben bounded on the south by the Ryan 10) and, therefore, were probably derived from Freshest samples of the Marsden Tuff are Spring fault and on the north by the outer ring different magma sources and evolved along highly evolved, high-silica rhyolite (Figs. 5, 6, fault of the Indian Peak caldera, cooling units

contrasting paths; the Marsden is a main-trend and 10) and have among the lowest TiO2, CaO, of the Marsden are interlayered with debris-

rhyolite whereas the Lamerdorf is an off-trend Fe2O3, Sr, and Zr of any tuff in the Indian Peak– fl ow deposits, each layer being no more than a rhyolite. Caliente fi eld. Moreover, the Marsden has a few meters thick. These deposits have a sandy Here, we elevate the stratigraphic status of high 87Sr/86Sr ratio like the Sawtooth Peak tuff matrix between angular to subangular clasts less the Marsden, Lamerdorf, and Beers Springs (Table 4; Fig. 6D). than 1 m across of Paleozoic rock and minor to formation rank and the Escalante Desert to At its type section in the southeastern Wah altered andesitic rock. This tuff–debris fl ow group rank. The Marsden and Lamerdorf Tuffs Wah Mountains (Hintze et al., 1994b), the Mars- sequence is 300–400 m thick, continues for are described in the following sections while the den is 110 m thick (Fig. 12) and consists of three ~2 km southeast of the Ryan Spring fault, and informal andesite and rhyolite lava-fl ow forma- similar cooling units. Metasedimentary xenoliths is overlain by very poorly exposed deposits of tions are described at the end of this article. locally constitute upwards of one-half the tuff apparently monolithologic breccias of the same The Beers Spring Formation is a discontinu- and are as much as 20 cm across. The ignimbrite Ordovician units that are exposed to the north- ous sequence of well-sorted, poorly bedded, thickens to 300 m a few kilometers to the northwest. North of the graben, the Marsden sequence green to brown sandstone that is as thick as west, pinches out farther north, but reappears in and overlying breccias of Ordovician rocks are 400 m in the central Needle Range and southern a section more than 70 m thick ~20 km beyond absent and instead of this sequence lying on the Wah Wah Mountains (Best et al., 1987b, 1987d), that, all in the Wah Wah Mountains. Ordovician terrain, there are landslide breccias which is essentially its only area of occurrence. The Marsden is also exposed in the central composed mostly of clasts of Cotton wood Wash Grains of feldspar and pyroxene are locally Needle Range in a continuous belt ~8 km long Tuff related to the collapse of the Indian Peak visible in hand sample. Locally, the unit also trending northwest from the inner ring fault of caldera. includes: (1) poorly cemented gravel of pebble- the Indian Peak caldera (Fig. 13; Best et al., Best and Grant (1987, p. 16) postulated that to boulder-size clasts of carbonate rock, locally 1987b). Near the ring fault, north of Indian Peak this Marsden sequence and its overlying brec- also of quartzite and andesitic rock; (2) loosely (Fig. 14), intact exposures of the Marsden are cias were deposited at the northern margin of welded, crystal-poor tuff, a single sample of nonexistent because of closely spaced fractures a postulated Pine Valley caldera that was the which has 12% phenocrysts consisting of 55% resulting in angular rubble. This terrain may source of the Marsden ignimbrite. The northern plagioclase, 13% quartz, 18% sanidine, 10% contain unexposed faults and, in the absence of structural margin of this caldera was alleged to biotite, 3% amphibole, and 1% Fe-Ti oxides; either stratigraphic markers or a conspicuous be the northern-bounding fault of the graben and (3) volcanic debris-fl ow deposits with clasts foliation, an accurate thickness determination described in the previous paragraph, beyond of greenish-brown porphyritic andesitic rock. cannot be made; the unit here is likely hundreds which no Marsden occurs. Judging from the dis- of meters, possibly 1000 m, thick. To the north- placement of Paleozoic strata, this fault appears Marsden Tuff west of the ring fault, increasing amounts of to have had ~600 m of normal displacement This pale gray to locally pale green, orange, poorly exposed, gray to green, fairly well-sorted associated with collapse of the putative Mars- and pink ignimbrite is loosely compacted and sandstone made up of volcanic detritus overlies den caldera. Farther to the southeast, the propor- contains as much as 50% lapilli of rarely fl at- the Marsden ignimbrite. This sandstone closely tion of tuff to epiclastic deposits increases and tened pumice; consequently, the very pheno- resembles the sandstone in the Beers Spring the latter disappear. Within this area (4 km from cryst-poor tuff is typically massive. Unlike most other ignimbrites in the Indian Peak–Caliente fi eld, the Marsden contains a varied and commonly abundant xenolithic assemblage of Marsden Tuff NV UT (ca. 32.5 Ma) (1) rare, small lapilli of andesitic rock and (2) more common, larger, gray carbonate rock, ! 0 ′ 0 0 E ! 38°30 0 ! green phyllite, and white, red, pink, and purple ! quartzite whose source is likely the thick section

of underlying late Proterozoic metasedimen- 100s ! 0 ! <70 ! tary rocks. 300 0 No precise isotopic age has been determined 0 ! ! 100

for the Marsden because of the small size (gen- ! 300 ! ! ! 0 erally <1 mm) and paucity (only a few percent 0 ! 110 0 0 ! of the whole rock) of the phenocrysts and espe- 38° E cially the widespread and pervasive alteration

that has typically left quartz as usually the only 010203040 0 Km ! preserved phenocryst. The quartz grains are not 01020 xenocrysts disaggregated from the abundant Miles quartzite clasts because they are unstrained 114°30′ 113°30′ and possess embayments and glass inclusions characteristic of a magmatic heritage. In addi- Figure 12. Distribution and thickness (in meters) of the Marsden tion to quartz, other phenocrysts include plagio- Tuff. Location of source caldera is unknown.

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113°57′30″ TERTIARY Post-Escalante Desert rocks 26 38° 22′ 30″ Tped Quaternary Tped Escalante Desert Group

Tel Tes Lamerdorf Tuff Sandstone

24 Pz 31 113° 38° Tmr Rhyolite lava 55′ 20′ 00″ 00″ O u Breccia t Tmb e r r i n g f a 25 Tm Tuff Pz u l t Marsden units Tm Tped 33 21 Pz Quat- PALEOZOIC Tm ernary Ryan fault Undifferentiated Pz Tmb Spring Pz sedimentary rocks Tm ring 35 R y an 55 Sp 38° 27 20′ Tel Tmb Tped 00″ 54 Strike and dip in sedimentary Quaternary 24 rocks and compaction Tm Tes foliation in ash-flow tuffs Pz Tes

38 High-angle fault Tes Ball and bar on downdropped side Quat- Tes Tel 38° 45 ernary 17′ 30″ Tes 55 Tm Tel Pz Tmr

In ne Tped r Pz

46 Tm

Quaternary Tm Pz

r ev Tm er

se 113°

′

Tmr 52

″

Tm 30

N Tmr 38° ring 17′ ″ - 30 fault

Figure 13. Generalized geologic map of the Marsden Tuff Tped units and other parts of the Escalante Desert Group in 0 1 2 the central Needle Range lying in the collar zone of the kilometers Indian Peak caldera between the topographic margin ″ 0 1 2 (not shown but at about 38° 21°55 N) to the north and Indian miles the inner reverse ring fault to the south (modiﬁ ed from Peak Best et al., 1987b; see also Fig. 39A).

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Figure 14. Panoramic view looking south across a foreground of poorly exposed but likely thick (1000? m), light- colored Marsden Tuff. In the distance is Indian Peak (elevation 2984 m) underlain by darker-colored, caldera- collapse, or intracaldera, lithic Wah Wah Springs ignimbrite and intercalated wall-collapse breccias. See also Figures 13 and 39. Tallest trees in the foreground are ~4 m.

the inner ring fault of the Indian Peak caldera), orange, red, and purple. Contributing to this ignimbrites (Figs. 9, 12, and 15). It consists of a very phenocryst-poor rhyolite lava fl ow lies mottling are lapilli of compacted light-colored as many as three simple cooling units whose within the Marsden ignimbrite. No more can pumice and of angular, dark-colored, altered total thickness is as much as 100 m on the west be said of the postulated Pine Valley caldera, andesitic rock that constitute as much as one- side of the Mountain Home Range. Weighted because of overprinting and engulfment by the fourth of the tuff. Compared to the Marsden mean plagioclase 40Ar/39Ar ages on the second younger Indian Peak caldera and lack of expo- Tuff, phenocrysts in the Lamerdorf are larger (to and third cooling units are 32.09 ± 0.10 Ma and sures in alluvial valleys to the west. Moreover, 3 mm) and more abundant (9%–24% of rock). 31.90 ± 0.16 Ma, respectively. An unusually apparently near-source Marsden containing Plagioclase is by far the dominant phenocryst thick section of ~900 m occurs as a paleovalley large quartzite clasts at the type section in the (73%–83%) followed by biotite (4%–14%), deposit in the central Wah Wah Mountains. This southeastern Wah Wah Mountains presents fur- hornblende (3%–10%), Fe-Ti oxides (2%–7%), paleovalley is ~6 km wide and, in addition to ther ambiguity regarding a source caldera. and clinopyroxene (1%–3%). Unlike the Mars- the Lamerdorf, includes an intercalated ande- Overlooking possible thick sections of the den, quartz is absent and sanidine occurs only sitic lava fl ow ~100 m thick, less than 70 m of Marsden engulfed in the younger Indian Peak in trace amounts in some samples. Anhedral Marsden ignimbrite, and underlying local lenses caldera, its volume is estimated at 260 km3, clinopyroxene present in a few samples may be of conglomerate as thick as 80 m of Paleozoic including ignimbrite assumed to be present in xenocrystic, as may be some of the hornblende. rock clasts. The Lamerdorf pinches out on the concealed caldera (Table 2); the true volume Some stratigraphic sections indicate a subtle the north side of the paleovalley but persists is likely more. An alternate view has the ignim- upward diminution in the proportion of mafi c as a 70–170-m-thick sheet to the south and a brite confi ned mostly to two, more or less east- phenocrysts. 30–100-m-thick sheet ~50 km to the northwest. west paleovalleys in about the same position as The Lamerdorf is a rather distinct low-silica The Lamerdorf Tuff thus appears to have accu- those that are possible for the Sawtooth Peak rhyolite (Fig. 10) that has, relative to other tuffs mulated on a somewhat fl attened terrain beyond

ignimbrite. If this is actually the situation, less of comparable silica content, high TiO2, Ba, and, the central Wah Wah Mountains paleovalley. tuff would have been engulfed in the younger especially, Zr (Fig. 6C). Chemically, this off- There may be a possible extension of this paleo- Indian Peak caldera than estimated from the trend rhyolite resembles some of the younger valley as far as 80 km to the east-southeast in contours in Figure 12 and the total unit volume Isom-type tuffs. Nonetheless, its 87Sr/86Sr ratio what is now the High Plateaus where a possible could be reduced to roughly half. is relatively high (0.712) compared to the tuffs correlative phenocryst-poor, densely welded of the Isom Formation (0.708; Fig. 6D); this tuff that underlies the Wah Wah Springs For- Lamerdorf Tuff suggests that its parental magma assimilated mation has a biotite K-Ar age of 31.9 ± 0.5 Ma This lapilli ash-ﬂ ow tuff is partially to densely more old continental crust than the younger (unit Tvl in Anderson et al., 1990). welded, locally possesses a near-basal black to Isom magmas. The volume of the Lamerdorf Tuff is esti- dark-brown vitrophyre, and, where devitriﬁ ed, The Lamerdorf Tuff has a larger areal extent mated at 180 km3, inclusive of that assumed to is distinctively mottled in shades of gray, brown, than the older Sawtooth Peak and Marsden occur in a concealed source caldera (Table 2).

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be xenocrysts derived by disaggregation of the E E 39° Lamerdorf Tuff common xenoliths of Wah Wah Springs dacite (32.09- 31.90 Ma) tuff. However, the Greens Canyon has no horn- 0 ! 0 blende, an abundant phenocrystic constituent in 0 ! ! ! <30 the Wah Wah Springs. Microprobe analyses of 0 ! 30 ! Greens Canyon plagioclases reveals that some 0 ! E 38°30' ! ! 43 <100 0 small cores are as calcic as An70–80 whereas 0 ! ! ~70 ! most of the grains are An36–64, thus overlapping ! ! 100! 900 ~100 ~200 ! the An content of plagioclases in the Wah Wah <80 ~30 ! Springs. However, they have distinctly lower ! ! 150 0 ! 0 10? ! 0 E ! E ! >15? concentrations of the K-feldspar component in 38° ! ! 0 0 ? accordance with crystallization from the cooler 0 ! Greens Canyon rhyolitic magma. 010 30 50 Km Greens Canyon Tuff Member 0102030 NV UT Miles 37°30' E E The Greens Canyon superfi cially resembles the older Marsden Tuff except that the lithic 114° 113° lapilli and small blocks are commonly of red Figure 15. Distribution and thickness (in meters) of the Lamerdorf Wah Wah Springs tuff rather than the sedimen- Tuff. Location of source caldera is unknown. tary assemblage that is typical of the Marsden. Most outcrops of the Greens Canyon have holes a centimeter or so in diameter surrounded by Ryan Spring Formation which are almost entirely confi ned within the haloes that are lighter in color than the surround- older Indian Peak caldera where they constitute ing pale orange to pinkish-brown matrix; these Following emplacement of the Lamerdorf post-collapse caldera-fi lling tuff (Fig. 17). Only holes apparently formed by weathering out of Tuff, two super-eruptions produced the Cotton- three occurrences of the Mackleprang and none reacted xenoliths of unknown character. Small wood Wash and Wah Wah Springs monotonous of the Greens Canyon have been found beyond lapilli of andesitic rock are visible in thin sec- intermediates. They were followed by eruption the Indian Peak caldera. tions. Also unlike the Marsden, the Greens Can- of modest volumes of the Ryan Spring Forma- Weighted mean 40Ar/39Ar ages of plagioclases yon lacks quartz phenocrysts but does similarly tion (Tables 1 and 2). It consists of two tuff for the Greens Canyon of 30.13 ± 0.13 Ma and contain plagioclase, biotite, and Fe-Ti oxides members of phenocryst-poor, low-silica rhyo- the overlying Mackleprang of 30.01 ± 0.09 Ma that are generally less than 1.5 mm. Another lite—the older Greens Canyon and the younger indicate their deposition very soon after the cre- contrast with the main-trend Marsden is the fact Mackleprang—as well as local andesite and ation of the Indian Peak caldera and eruption that the Greens Canyon is an off-trend rhyolite rhyolite lava fl ows (see end of this article) and of the dacitic Wah Wah Springs ignimbrite at and is not nearly as chemically evolved (Figs. 5, sedimentary deposits (Best and Grant, 1987; 30.06 ± 0.05 Ma (Table 1; taking into account 6, and 10) with, for example, higher concentra-

Best et al., 1989a). These two members are well analytical uncertainties, these ages are con- tions of CaO, TiO2, and Zr. exposed in a stratigraphic section on the west sistent with the stratigraphic relations of the In the west-central Needle Range (Best et al., side of the central Needle Range at Ryan Spring units). The initial 87Sr/86Sr ratios of the Mackel- 1987b) near Ryan Spring, the Greens Canyon in the collar zone of the Indian Peak caldera. prang and Wah Wah Springs are indistinguish- is a moderately welded and entirely devitriﬁ ed They have identical modes (Fig. 16) and similar able (Table 4), providing another link between simple cooling unit ~500 m thick. To the west, chemical compositions (Figs. 5, 6, and 10) but a their magmas. in the northeastern White Rock Mountains (Best different appearance in hand sample. We considered the possibility that the dated et al., 1989d), as many as ﬁ ve cooling units, No compositional zoning is evident in our plagioclases in the Greens Canyon, which has some with prominent, dark-brown, near-basal limited sampling of either of the two units only small, sparse grains of this phase, might vitrophyres, lie in what appears to be a partially fault-bounded basin between the topographic wall and resurgently uplifted block of the Indian Figure 16. Modal proportions Peak caldera; if not repeated by unrecognized of phenocrysts in ignimbrites 100 Ryan Spring Formation faulting, this compound cooling unit sequence is of the Ryan Spring Formation. Mackleprang Tuff Member n = 6 ~650 m thick. Basal parts of individual cooling In this diagram and all such 80 Greens Canyon Tuff Member n = 4 units contain compacted glassy orange or black diagrams in this article, the pumice lapilli; several meters of thinly bedded proportion of total phenocrysts 60 sandstone are locally intercalated between them in the whole-rock sample (not whereas a greater thickness of sandstone over- corrected to dense rock equiva- 40 lies the member to the southwest. The Greens lence) is shown in the far right- Mode (vol %) Canyon is only 20 m thick to the northeast. hand column labeled “pheno.” 20 A 1300 m section of the Ryan Spring ignim- Plag—plagioclase; Qtz—quartz; brite farther northeast south of Atlanta was San—sanidine; Bio—biotite; 0 mapped as entirely Greens Canyon by Willis Hb—hornblende; Px—pyrox- Plag Qtz San Bio Hb Px Opaq Pheno et al. (1987); however, subsequent examination ene; Opaq—opaque minerals. reveals at least the top portion of this thick sec-

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Ryan Spring Formation NV UT ! Mackleprang Tuff Member (30.01 Ma) ( Greens Canyon Tuff Member (30.13 Ma)

! 0 38°30′ E E

! ( ~650 ~650 ! 100 ( 20 0 ( 500 0 ! ! ! ! ( <60 <70 0 650 !

O 0 l !<30 d !( e 0 r

In 200 d ia 50 400 n ! P 500 e ak c 50 al ( 0 de ! 0 ra <50 to ! po gra 38° E phi E c ma rgin Mackleprang 0 ! 0-20 caldera ! ! 0

10 ! 0 !

01020304050 Km 0102030 Miles

115° 114°

Figure 17. Distribution and thickness (in meters) of the Greens Canyon and Mackleprang Tuff Members of the Ryan Spring Formation. The approximately located and concealed Mackleprang caldera is inferred from the occurrence of unusually thick (1100–1400 m) sections of the overlying Lund ignimbrite.

tion is Mackleprang. The variations in thickness (Fig. 6). From the older Greens Canyon Tuff source caldera in which the Lund ponded. The of the Ryan Spring ignimbrites possibly refl ect the Mackleprang is distinguished by slightly caldera shown in Figure 17 (not to be confused relief on fault blocks on the fl oor of the resur- larger (<2 mm) and relatively euhedral plagio- with a postulated Pine Valley caldera source gent Indian Peak caldera. clase phenocrysts, commonly by more abundant for the Marsden Tuff above) is delineated on The source of the Greens Canyon lies within fl attened pumice lapilli that impart a more pro- the west by a rhyolite lava that appears to be the older Indian Peak caldera, but we are nounced foliation, and larger, more abundant of about Mackleprang age (Best et al., 1987a). unaware of direct evidence defi ning its location. xenoliths of varicolored andesitic rock. A small The Mackleprang caldera is assumed to be fi lled Because the eruption followed closely after the amount of quartz phenocrysts occurs in the with an equivalent volume of ignimbrite as the collapse of the caldera, further subsidence might ignimbrite near Atlanta. In thicker stratigraphic estimated outfl ow, giving a total volume for have occurred, rather than the creation of an sections the Mackleprang appears to be a com- the unit of 480 km3 (Table 2). independent source caldera. pound cooling unit whereas thinner sections are For the volume of the Greens Canyon Tuff a simple cooling unit. Tuff of Deadman Spring Member, we multiply its area of exposure by the A thick (500 m) section of the Mackleprang average thickness of 300 m, obtaining 600 km3 ignimbrite occurs south of the resurgent uplift The phenocryst-rich rhyolite tuff of Dead- (Table 2). of the Indian Peak caldera in the Indian Peak man Spring (Taylor, 1990) was deposited at Range (Fig. 17), where it is capped by several 30.00 ± 0.10 Ma and consists of a thick intra- Mackleprang Tuff Member meters of sandstone. Other post–caldera-fi lling caldera deposit in the Fairview Range and thin- The mottled Mackleprang is similar to sections to the north are thinner, suggesting the ner outfl ow to the southwest (Fig. 18). Strati- the older Lamerdorf Tuff but can be distin- source of the Mackleprang lies in the southeast- graphic relations are somewhat uncertain but guished by stratigraphic position and by the ern sector of the caldera. Because the thickness the Deadman Spring appears to underlie the lack of hornblende phenocrysts. Chemically, of the younger Lund ignimbrite is as much as Mackleprang Tuff Member of the Ryan Spring the Mackleprang differs from the Lamerdorf 1400 m in hills in southernmost Pine Valley, Formation but it certainly overlies Wah Wah

in that it has much lower TiO2, Zr, and Nb this area is believed to harbor the Mackleprang Springs ignimbrite.

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The source of the tuff of Deadman Spring is Tuff of Deadman Spring the Kixmiller caldera that apparently encom- (30.00 ± 0.10 Ma) passes all of the Fairview Range (Figs. 8 and ^ Ring intrusion 0 18; Best et al., 1998). This caldera formed sev- ! 0 ! eral kilometers west of the slightly older Indian ′ E 38°30 Peak caldera but was later partially engulfed in the younger White Rock caldera when the Lund

0 magma erupted at 29.20 Ma. The Fairview Range 0 ! ! >2000 comprises horst-and-graben blocks of Dead- ! man Spring and younger rocks as well as Paleo- 2000 zoic rocks in the south. The sharply deﬁ ned Kixmiller 500 ! caldera 1000 northern margin of the Kixmiller caldera is a Figure 18. Distribution and <380 ! west-southwest–striking, steeply dipping ring thickness (in meters) of the tuff 50 ^ ! of Deadman Spring and location ~30 fault through Kixmiller Summit between the of the apparently asymmetric 60 Fairvew Range and the Grassy Mountain mass E ! 0 Kixmiller caldera source (dot- 38° of Paleozoic rocks to the north (Fig. 19). A sin- 0 100 gle exposed slab of Wah Wah Springs outﬂ ow ! ! ted where concealed beneath 0 100 ! tuff, ~300 m long and 20 m thick, embedded in alluvium). <120 ! 0 the Deadman Spring tuff and lying just inboard ! 0 of the southwestern segment of the ring fault is ! the only manifestation of a wall-collapse land- 0 ! slide deposit. The intracaldera tuff appears to be at least 2000 m thick just south of the ring fault but decreases southward to less than 380 m at

0 01020 the southwest end of the Fairview Range. Just ! Miles 37°30′ to the south, the tuff thins markedly to only a 010203040 few tens of meters across what we have inter- Km preted to be a caldera margin. It could be an ill-defi ned topographic wall or a hinge of a trap- ′ 115° 114°30 door-like depression. A granitic porphyry with an incremental-heating 40Ar/39Ar age of 30.04 ± 0.07 Ma on biotite is believed to be a small, The Deadman Spring outfl ow is a moderately phenocrysts. A compaction foliation is seldom near–ring-fault intrusion marking this southern to loosely welded, simple cooling unit. One sam- evident because of very rare lapilli of pumice caldera margin (Best et al., 1998). Although the ple has 27% phenocrysts that include 47% plagio- and only sparse biotite and an abundance of rela- age is identical to that of the tuff of Deadman clase, 30% quartz, 17% sanidine, 5% biotite , and tively equant felsic pheno crysts; the tuff has a Spring, it should be noted that some such bio- 1% opaque grains. The compound cooling unit lava-like appearance in outcrop. One sample has tite ages are anomalously old compared to sani- that comprises the 2000-m-thick intracaldera 41% phenocrysts that include 35% plagioclase, dine ages in the same rock. The bulk chemical deposit is densely welded throughout and is only 32% quartz, 23% sanidine , 9% biotite, and 1% composition of the porphyry has similarities to very locally vitrophyric. In some places it has opaque grains. Chemically, the Deadman Spring that of both the Deadman Spring and the Lund, alternating layers a few centimeters thick made is a relatively silica-rich rhyolite (73.8%–76.6%; whose caldera source margin also lies close of fi ner (<1 mm) and coarser (2–3 mm) felsic Figs. 5, 6, and 10). to the porphyry intrusion. Small-scale silver

Intracaldera Deadman Spring

Ring fault Pz

Figure 19. Panoramic view looking west at the Fairview Range (highest peak at 2400 m), underlain by 2000 m of east-dipping intracaldera tuff of Deadman Spring. To the north is Grassy Mountain underlain by Paleozoic (Pz) rocks. The ring fault of the Kixmiller caldera strikes westerly in the low divide between the ranges. Valley ﬂ oor lies at less than 1950 m.

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mineralization and widespread silicifi cation of rounded weathered out, reacted(?) xenoliths of the resurgent uplift in the caldera and along its carbonate rocks is spatially associated with the unknown character. In some outcrops, the Rip- northern topographic margin. Thicker sections granitic porphyry (Best et al., 1998). gut superfi cially resembles the Greens Canyon that consist of two or three simple cooling units As an alternate interpretation for the south- Tuff Member of the Ryan Spring Formation but are locally separated by several tens of meters of ern Kixmiller caldera margin, Ekren and Page the Ripgut mostly contains fewer phenocrysts loosely consolidated sediment. The only expo- (1995) suggested the abrupt change in thickness (Figs. 16 and 20). In compacted vitrophyre, sure outside the White Rock caldera is a simple of the tuff marks the margin of an east-trending only ~2% of the tuff consists of small (~1 mm) cooling unit less than 120 m thick ~2.5 km “volcanic trough” that was subsiding during pheno crysts that are mostly plagioclase and southwest of Modena. deposition; they relate it to the east-striking Blue lesser sanidine, biotite, and Fe-Ti oxide. Only a Ekren et al. (1977) noted the presence Ribbon lineament of Rowley et al. (1978) that few of the plagioclase phenocrysts in a sample of a thick monolithologic breccia of dacitic extends into central Utah and connects with the are euhedral; the remainder are anhedral and “Needles Range tuff” north of Mount Wilson in Warm Springs lineament in central Nevada. The may be xenocrystic, derived by disaggregation the Wilson Creek Range that they believed to easterly striking faults that are associated with of clasts of dacite tuff. be an intracaldera deposit. This, together with near-vertical dips and easterly striking compac- other features considered to indicate a caldera, tion foliation in tuffs in the southern Fairview Distribution and Source led them to refer to the area as the “Mount Wil- Range are an unusual aspect of this part of the Nearly all of the Ripgut ignimbrite is found son volcanic center.” Subsequently, we found Indian Peak caldera complex (see discussion in within the White Rock caldera (Figs. 8 and an area of ~3 km2 of breccias of the Lund tuff Best et al., 1998). 21). The unit thins and locally pinches out over lying atop intact Lund on the northwestern We estimate the volume of the Deadman Spring ignimbrite from Model 3 in Figure 4 to be ~200 km3, ~90% of which is intracaldera Ripgut Formation 80 Ignimbrite n = 4 tuff that lies within the asymmetric, or trapdoor, + Fiamme caldera. Figure 20. Modal proportions of phenocrysts in tuff of the Ripgut 60 Ripgut Formation Formation. Fiamme occur in the uppermost part of the intracal- 40 Ignimbrite of the Ripgut Formation overlies dera formation on Mount Wil- the Lund monotonous intermediate. The name son where the host ignimbrite is Mode (vol %) is taken from Ripgut Springs on the east side of relatively more phenocryst rich 20 the White Rock Mountains where a surround- and less evolved. ing fence of sharpened poles embedded in the ground protects the water source for range 0 stock; any large animal attempting to com- Plag Qtz San Biot Hb Px Opaq Pheno promise the integrity and purity of the springs would suffer the intended consequence! Most of the Ripgut Formation consists of pumice-rich Ripgut Formation, tuff member NV UT rhyolite ignimbrite but the unit also includes one (29.0 Ma) 0 small dike of similar composition, intracaldera ! 0 ′ E ! ~30 E breccias made of clasts of Lund and Wah Wah 38°30 ! 0 Springs tuffs, and tuffaceous and conglomeratic ! ! 400 sandstone as much as 130 m thick (Willis et al., 0 ! ?? !! 0 ! 60 ! 1987). Somewhat rounded clasts in the con- 2000 600 glomerate are of Wah Wah Springs and Greens Mt. Wilson ! caldera <2 ! <2? !>650 ! 0 Canyon tuffs. segment ! E ! <75 38° ! Duplicate sanidine and plagioclase ages have 0 200 ! Older weighted means of 28.96 ± 0.05 Ma and 28.99 ± 0 ! White Rock 0 <120 0.10 Ma, respectively (Table 1). ! caldera Above a meter or so of uncompacted and margin 0 loosely welded lapilli tuff, the Ripgut ignim- ! 01020304050 brite grades upward from a densely welded and Km ′ E compacted black to dark-brown massive glass 37°30 0102030 Miles 0 to devitrifi ed, densely welded brown lapilli tuff ! to a loosely welded, uncompacted light-brown 115° 114° lapilli tuff at the top. Pumice lapilli typically constitute about one-third of the tuff and in Figure 21. Distribution and thickness (in meters) of the tuff mem- devitrifi ed parts are varicolored—black, gray, ber of the Ripgut Formation. Drastic variations in thickness in the brown, orange, and white—within a single out- White Rock Mountains refl ect deposition on block-faulted, resur- crop. Xenoliths of Lund and Wah Wah Springs gently uplifted older White Rock caldera where the tuff constitutes tuffs as much as 10 cm in diameter are locally post-collapse, partial caldera-fi lling tuff. Only a short segment of evident, as are light-colored haloes that sur- the margin of the Mount Wilson caldera source is exposed.

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slopes of Mount Wilson; Lund clasts are as are not xenocrysts disaggregated from the com- (1981) designated such voluminous phenocryst- much as 1 m across and lie in a matrix of Ripgut mon xenoliths of Lund tuff. This less-evolved rich dacite ignimbrites “monotonous interme- tuff or comminuted Lund (Willis et al., 1987). tuff hosts dark-brown fi amme several centime- diates,” citing, as examples, among others, the Nearby, clasts of Wah Wah Springs and Paleo- ters long of compacted pumice that contains 8% Monotony Tuff in the Central Nevada fi eld and zoic quartzite and carbonate rock occur within phenocrysts as large as 3 mm across of euhedral the Needles Range tuffs, by which the three in the Ripgut. These breccias confi rm the presence plagioclase plus minor biotite and lesser quartz, the Indian Peak–Caliente fi eld were formerly of the Mount Wilson caldera source for the Rip- sanidine, hornblende, Fe-Ti oxides, and a trace designated (see historical note below). As a gut ignimbrite. East of the large area of breccia of aggregated pyroxene (Fig. 20). Chemically, distinct end-member in the broad spectrum is an elongate east-west mass of subvertically the fi amme are trachydacite (Fig. 10), contain of ash-fl ow deposits now known in the geo-

fl ow-layered rock of Ripgut character that we 68.3 wt% SiO2, 0.5 wt% TiO2, and 335 ppm Zr, logic record, monotonous intermediates gen- interpret to be a dike possibly emplaced along and resemble the Lamerdorf Tuff and Hole-in- erally lack the pronounced systematic modal the caldera ring fault. Reconnaissance mapping the-Wall Tuff Member of the Isom Formation. and chemical zoning seen in many other and reveals a thickness of ~2000 m of Ripgut tuff Because the Ripgut ignimbrite was erupted typically smaller ignimbrite deposits. Because underlying Mount Wilson and overlying thick soon after the Lund, and from within the source the actual extent of compositional zoning in intracaldera Lund in the lower foothills of the caldera of the Lund, most of the erupted, highly monotonous intermediates and compositional northwestern part of the range (Fig. 22). evolved Ripgut magma might have been a late gradients in the pre-eruption magma cham- The exposed volume of 800 km3 of the tuff differentiate of the residual unerupted Lund bers were not fully understood when Hildreth calculated by Model 2 in Figure 4 is a mini- magma. Niobium concentrations, however, are (1981) recognized this distinct class of ignim- mum for the unit because the thick intracaldera high, which leads us to conclude this cannot brite, we will devote some attention to these deposit is truncated 4 km south of Mount Wilson be true. The lower part of the erupted Ripgut aspects below. by a major east-west–striking fault system that magma chamber was apparently invaded by and All together, the three super-eruptive monoto- has downdropped lower Miocene rhyolite lavas mixed with magma similar to that which formed nous intermediates constitute a unique attribute and tuffs on the south, concealing the southern the Isom and Lamerdorf ignimbrites, possibly of the Indian Peak–Caliente fi eld not found, sector of the caldera (Figs. 8A and 21). Likewise triggering the eruption. to our knowledge, in other volcanic fi elds in to the east, but here overlying Miocene rhyolites southwestern North America where the middle obscure the caldera. SUPER-ERUPTIVE MONOTONOUS Cenozoic ignimbrite fl areup is expressed. In INTERMEDIATES the Great Basin, the only other monotonous Zoned Magma Chamber intermediate is the Monotony Tuff. Beyond the Most of the exposed Ripgut ignimbrite is a Three of the largest cooling units in the Great Basin, the only other is the Fish Canyon very phenocryst-poor (<2%), highly evolved Indian Peak–Caliente ignimbrite fi eld (Table 2) Tuff in the Southern Rocky Mountain fi eld rhyolite with silica ranging upwards to 77.7 are of relatively uniform, phenocryst-rich (Lipman , 2007). wt%; it has the highest concentration of Nb dacite; they include the 31.13 Ma Cottonwood Before crustal extension, individual monoto- (35–47 ppm) of any ignimbrite in the Indian Wash, the 30.06 Ma Wah Wah Springs, and the nous intermediates in the Indian Peak fi eld Peak–Caliente fi eld, as well as among the low- 29.20 Ma Lund. With volumes of 2000, 5900, cropped out over as much as 32,000 km2 est Zr (mostly ~100 ppm) (Figs. 5, 6, and 10; and 4400 km3, respectively, they easily fall in (Table 2) as simple outfl ow cooling units tens Table 3). However, the uppermost exposed the super-eruptive category of Miller and Wark to hundreds of meters thick. Normal zoning in tens of meters of the 2000-m-thick intracaldera (2008) and de Silva (2008). As will be detailed welding, compaction, devitrifi cation, and vapor- deposit on the southeast slope of the southeast below, their tightly overlapping source calderas phase crystallization (Ross and Smith, 1961) is peak of Mount Wilson (Fig. 22) is chemically indicate a recurrently eruptive, or multicyclic, typical; they are relatively low-grade non-rheo-

less evolved (72.5 wt% SiO2); phenocrysts are magma system beneath the Indian Peak caldera morphic ignimbrites in the grade continuum of larger (to 2 mm) and more abundant (12%) and complex. Branney and Kokelaar (1992). Outﬂ ow sheets include hornblende and a little clinopyroxene. Because of their relatively uniform compo- have a near-basal black vitrophyre as much as Analyses of the hornblende grains indicate they sition and intermediate silica content, Hildreth 10 m thick (Fig. 23). Xenoliths of other rock

Mt. Wilson

Figure 22. Panoramic view looking east at the northern Wilson Creek Range. The upper half of the range beneath Mount Wilson (2838 m) is underlain by 2000 m of lighter-colored intracaldera Ripgut tuff. Underlying it all the way to the valley ﬂ oor (less than 1950 m) and to the left margin of the photo is the darker-colored intracaldera Lund tuff (see Fig. 8A).

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Figure 23. Panoramic view looking west at Toms Knoll at ~39° N, 113°45′ W. Craggy red-brown exposures of Cotton- Wah Wah Springs vitrophyre wood Wash Tuff on the left are overlain by the near-basal, Cottonwood Wash black vitrophyre of the Wah Wah Springs ignimbrite that appears as a thin line at the top of the knoll and to the right. This outﬂ ow couplet of the two super-eruptive monotonous intermediates (“Needles Range” tuffs) without any intervening deposit is exposed over thousands of square kilometers in the eastern Great Basin.

types are notably absent in these monotonous and designated the lowermost two composed status and made the older Cottonwood Wash intermediates, except for the intracaldera tuff of of phenocryst-rich dacite as members of the Tuff and the younger Wah Wah Springs Forma- the Wah Wah Springs Formation. Poorly bedded Needles Range Formation. This formation was tion constituent formations in the Needles Range surge deposits a couple of meters thick are very subsequently redefi ned by Best et al. (1973) and Group; they also included a third, still younger locally exposed beneath the Cottonwood Wash later by Best and Grant (1987) who elevated its phenocryst-rich dacite tuff designated the Lund and Wah Wah Springs ash-fl ow tuffs as much as 90 km north of the caldera complex (Fig. 24). Precursory plinian deposits seem to be lacking, but we acknowledge that typically poorly or nonwelded pyroclastic material beneath the resistant overlying densely welded ignimbrite and above the underlying welded tuff of an older unit is very seldom exposed. Densely welded, pore-free monotonous intermediates contain as much as ~50% phenocrysts that consist of mostly plagioclase with lesser biotite, hornblende, quartz, and magnetite, much smaller concentrations of pyroxene and ilmenite, and trace amounts of zircon, apatite, and sulfi des. We have verifi ed sanidine only in the Lund ignimbrite, although some Figure 24. Basal contact of workers have reported some in the Cottonwood the Cottonwood Wash Tuff Wash and Wah Wah Springs. The Lund also has on south fl ank of Toms Knoll. trace amounts of titanite. Phenocrysts lie in a Hammer lies at the contact vitroclastic matrix of high-silica rhyolite glass between massive ash-fl ow tuff (Table 5; Christiansen, 2005). Mineral thermo- and an underlying crudely bed- barometers indicate pre-eruption magmas had ded surge deposit that is ~1 m temperatures of ~740–830 °C at pressures thick (bracketed by red lines). of 2.0–2.5 kbar, corresponding to a depth of Fluvial sandstone and con- 7–9 km. Oxygen fugacities were ~2 log units glomerate of rounded pebbles above the QFM buffer and magmas were not of Paleozoic rock underlie the water saturated (Maughan et al., 2002; Woolf, surge beds. See preceding fi g- 2008). Monotonous intermediates are domi- ure caption for location. nantly high-K dacite, magnesian, and calc- alkalic or alkalic (Fig. 6).

HISTORICAL NOTE: NEEDLES RANGE TUFFS

A turning point in the earliest pioneering work on ignimbrites in southwestern Utah was the study by Mackin (1960; see also Anderson et al., 1975, p. 13–16, for a comprehensive history going back to the late 1800s). He recognized a sequence of several cooling units in the Iron Springs mining district west of Cedar City

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TABLE 5. COMPOSITION OF GLASS IN MONOTONOUS INTERMEDIATES FROM THE INDIAN PEAK VOLCANIC FIELD Unit Cottonwood Wash Cottonwood Wash Cottonwood Wash Cottonwood Wash Wah Wah Springs Wah Wah Springs Wah Wah Springs Lund Emplacement Fallout tuff Pumice Surge Ignimbrite Surge Ignimbrite Pumice BRN-1PC Ignimbrite Samples 3 1 1 4 1 6 1 4 Spots 38 12 19 44 38 38 10 8

SiO2 77.34 77.22 77.30 77.35 77.48 77.75 78.38 78.09 TiO2 0.12 0.11 0.11 0.12 0.16 0.16 0.13 0.07 Al2O3 12.60 12.66 12.54 12.62 12.34 12.65 12.06 12.72 Fe2O3 0.86 0.69 0.78 0.56 0.86 0.77 0.39 0.57 MnO 0.05 0.00 0.00 0.00 0.00 0.04 0.04 0.03 MgO 0.11 0.07 0.09 0.08 0.13 0.11 0.12 0.05 CaO 0.93 0.66 0.95 0.82 0.89 0.93 0.77 0.98

Na2O 2.37 2.67 2.73 2.56 2.73 2.33 2.01 2.54 K2O 5.62 5.91 5.49 5.91 5.40 5.26 6.09 4.95 Normalized total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 Analytical total 96.77 95.94 97.17 96.47 96.37 98.25 96.22 98.39 Note: Electron microprobe analyses, reported in weight percent normalized to 100%. Samples—number of different samples analyzed. Spots—number of separate analyses.

Formation in the group. Apparently the fi rst to relation—together with trace amounts of zir- and abundant biotite), zircon morphology, and recognize these three distinct dacite cooling units con, apatite, sulfi des, and very rare orthopyrox- age of 30.90 ± 0.22 Ma as the Cotton wood in the eastern Great Basin was Conrad (1969), ene are found in the Cottonwood Wash (Ross Wash (Kowallis et al., 2005). Our age of the a student of Mackin, who mapped them in the et al., 2002). upper ash of the Whitney Member of the Brule Needle Range. He referred to the middle unit as Formation (White River Group; Tedford et al., the Wah Wah Springs but did not name the old- Distribution, Volume, and Source 1996; LaGarry, 1998; Larson and Evanoff, est and youngest units, which were named later 1998) in western Nebraska of 31.29 ± 0.17 Ma, by Best et al. (1973). Geologic maps and strati- The terrain on which the Cottonwood Wash as well as the compositions of biotites and graphic literature dealing with the volcanic geol- outfl ow ignimbrite was deposited had some local hornblendes, match those of the Cottonwood ogy of southwestern Utah over the past decades relief that was not entirely smoothed by earlier Wash (Blaylock, 1998). Thicknesses of the commonly refer to these phenocryst-rich dacite ignimbrites (Fig. 25). For example, in the south- Utah and Nebraska fallout deposits are as much ignimbrites as “Needles Range tuffs,” following ern Fairview Range in Nevada the Cottonwood as 0.5 m and 2 m, respectively. If a narrow the original designation of Mackin (1960). We Wash is as thick as 280 m but pinches out a few elliptical area with axes of 1300 and 250 km now refer to these three distinct ignimbrites as kilometers to the northwest over a pile of lava is drawn to encompass the three fallout sites, monotonous intermediates. (Best et al., 1998). Southward, the Cottonwood the calculated area, minus that of the Cotton- Wash thins and pinches out over a northeasterly wood Wash outfl ow ignimbrite of 12,000 km2 COTTONWOOD WASH TUFF trending highland of Paleozoic rocks and then (Table 2), multiplied by an average tuff thick- reappears southward. A similar, but more sub- ness of 1 m yields a very conservative volume This oldest monotonous intermediate depos- dued, pinchout is evident in the Wah Wah Moun- of ~240 km3. A similarly narrow elliptical area ited at 31.13 ± 0.13 Ma is known only as an tains in Utah at ~38°23′ N. To the south, the unit of fallout occurred for the 18 May 1980 erup- outfl ow sheet; no caldera and associated intra- is as much as 170 m thick but pinches out a few tion of Mount St. Helens (Washington State; caldera deposit have been found. In addition kilometers to the southwest over a paleohill of Houghton et al., 2000). However, more equant to its stratigraphic position, this unit is distin- Mesozoic and Paleozoic rocks. Mindful of these and larger fallout distributions have been docu- guished from other monotonous intermediates irregularities in the depositional surface, we mented for the 760 ka Bishop Tuff erupted with which it is coextensive over broad areas of estimate the volume of the outfl ow Cottonwood from the Long Valley, California, caldera (Izett the Indian Peak fi eld (Figs. 2, 23, and 25) by the Wash to be 1000 km3 (Table 2). et al., 1970) and the 640 ka eruption of the Lava presence of uncommonly large books of biotite No underlying plinian deposits have been Creek B Tuff from the Yellowstone, Wyoming, as much as 8 mm in diameter and similarly large, found but at four sites surge beds no more than caldera (Izett and Wilcox, 1982). The latter ash embayed quartz phenocrysts. Some quartz and 2 m thick are exposed (Fig. 24; see also Ross deposit covers an area of 4,000,000 km2. plagioclase grains in tuffs are smaller broken et al., 2002). We previously postulated (e.g., Best et al., phenocrysts, or phenoclasts, blown apart during Distal fallout deposits add hundreds of cubic 1989a) a concealed source caldera for the eruption (Best and Christiansen, 1997), which, kilometers to the erupted Cottonwood Wash Cotton wood Wash in the area between the at least in part, may account for the generally ejecta. As much as 9 m of thick-bedded pumi- Mountain Home and Fortifi cation Ranges and larger size (<9 mm) of these phases in pumice ceous tuff in southwestern Nevada ~200 km south of the Snake Range. However, low hills compared to tuff (<5 mm). Plagioclases in tuff southwest of the caldera has a modal compo- of unaltered limestone lie here, leaving insuf- commonly have inclusions of melt as well as of sition (including the presence of biotite books fi cient space for a caldera that, judging from all other phases present in the tuff; plagioclases to as much as 5 mm in diameter) and age the outfl ow volume, would have a diameter of with diverse mineral inclusions might be restite consistent with that of the Cottonwood Wash ~25 km or more. This inconsistency, together grains. Although about equally abundant as bio- (Barnes et al., 1982). Fine-grained, co-ignim- with our updated distribution and thickness data tite (Figs. 7 and 26), euhedral hornblendes are brite deposits occur in northeastern Utah and as for the outfl ow deposit, lead us to now believe rarely visible in hand sample because of their far as western Nebraska. The ash of Diamond that the caldera was engulfed in younger cal- small size (<1 mm). Lesser amounts of mag- Mountain Plateau along the south fl ank of the deras. The probable area in which the caldera netite, ilmenite, and clinopyrox ene—some Uinta Mountains in northeastern Utah has a lies in the Wilson Creek Range and White Rock mantled by hornblende in an apparent reaction similar mineral assemblage (including large Mountains, shown in Figure 25, coincides with

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39°30′ E 0 E E ! Cottonwood Wash Tuff NV UT Corrected (31.13 Ma) 0 for 50% ! extension 0 0 ! !

50 km 46, P ! 10 !

39° E E E 40 55 0 ! ! ! P <60 ! Figure 25. Distribution and ! 5 ! 0 thickness (in meters) of the ! ! 100? 30 <70 ! Cottonwood Wash Tuff. The 140 ! ! 60 0 P ! dotted outline is the maximum ! ! 120 ! 220 ! 200 allowable perimeter for a con- P ! 38°30′ 200! E 150 ! 0 E ~200 <33 ! cealed source caldera based on ! ! ! ! ~210 45 known geology. P—site where ! ! 0 0 Probable 45 cognate pumice inclusions were ! ~50 ! ! <10 source ! collected. Inset map shows out- ~20 170 ! area ! 0 100 0 line of the outﬂ ow sheet and 280 ! ! ! ! probable source area after com- 300 0 160 ! ! 100 200 ! 0 0 pensation for an assumed uni- 160 ~10 ! 0 ! 25 ! form 50% east-west extension. 38° E ! E E 0 0 ! >2 ! 0 ! ! 45 0 >75 ! ! ! 0

0 ! <100 !

0 20406080 Km 0 ! 37°30′ E E 02040 0 Miles !

115° 114° 113°

the lowest Bouguer gravity anomaly in the est δ18O of the monotonous intermediates, with WARM-2, and GOUGEWL-1-5 sites, and the δ18 Indian Peak caldera complex (Fig. 8C). Assum- O quartz of 9.8–10.4 ‰. remainder more evolved with >67.5 wt% SiO2

ing the volume of hidden tuff inside the buried Two samples of ignimbrite plotting in the and <0.58 wt% TiO2. The more-evolved pumice caldera is at least equivalent to that of the pre- andesite fi eld were collected a meter or so from clasts occur at all sites, both at the base and top collapse tuff (Model 1 in Fig. 4), a total volume the base of sections north of the source; samples of the stratigraphic sections. The more-evolved of 2000 km3 is estimated for the Cottonwood higher in these sections contain more silica. pumices contain rare sanidine, plagioclase Wash (Table 2). But this might be a minimum Otherwise, no systematic lateral or vertical phenocrysts with more sodic rims, and slightly value for the total eruptive volume because of compositional zoning is evident in the ignim- more Fe-rich biotites and hornblendes than sam- the signifi cant amount of distal fallout ash. brite deposit. ples of tuff whereas the less-evolved pumices Cognate pumice inclusions to as much as plot with less-evolved tuffs. Composition and Implications 25 cm in longest dimension were collected from The spatial variations in composition of the Cottonwood Wash outfl ow sheet at sites ignimbrite and cognate pumice clasts have no On many chemical variation diagrams (Figs. north of the probable source of the tuff (Fig. consistent explanation for withdrawal from a 5 and 6), element concentrations for the Cotton- 25). Chemical analyses were made of inclusions simply zoned, pre-eruption magma chamber. In wood Wash Tuff overlap those for the Lund and from the base of the outfl ow sheet at proximal whatever manner the evacuation of the chamber especially the Wah Wah Springs ignimbrites. sites ATL-1-70-1 and GOUGEWL-1-5 and at progressed, there appears to have been compo-

For the Cottonwood Wash, SiO2 ranges from the distal BRN-2 and WARM-2 sites, and of sitionally contrasting parts.

61.0 to 68.3 wt%, TiO2 from 0.5 to 0.8 wt%, inclusions from near the top of the sheet at prox- Several of the cognate pumices have more-

and Fe2O3 from 4.2 to 6.8 wt% (Fig. 27). Of all imal site GOUGEWL-3 and distal site KNOLL- evolved bulk compositions than the overall the monotonous intermediates, the Cottonwood 1-38-1 (see Supplemental File 1 [see footnote 1] population of tuff samples (Fig. 27), which is Wash has the highest 87Sr/86Sr ratio (0.711; Fig. for locations). Pumices can be divided into two consistent with fractionation or elutriation of 6D) and must have incorporated signifi cant pro- groups (Fig. 27): fi ve less evolved with <66.3 high-silica rhyolite vitroclasts from the ash fl ow

portions of Proterozoic crust. Hart (1997) also wt% SiO2 and >0.63 wt% TiO2 that occur at the during eruption and emplacement. Nonetheless, showed that the Cottonwood Wash has the high- base of the outﬂ ow sheet at the ATL-1-70-1, some pumices are as little evolved as some tuffs,

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A 80 indicating that not all of the chemical variation Cottonwood Wash Tuff + Ignimbrite n = 34 in tuffs is the result of fractionation. Moreover, Pumice inclusions n = 12 the more-evolved pumice clasts also have more- 60 evolved mineral compositions than ignimbrites. Pumices have similar total phenocryst concentrations as tuffs (Fig. 26A; adjustment to a dense 40 Figure 26. (A) Modal propor- rock equivalent does not change this similarity);

tions of phenocrysts in the Mode (vol %) this also supports the lack of major fraction- Cottonwood Wash ignimbrite 20 ation of glass shards in the Cottonwood Wash and cognate pumice inclu- ash fl ows. sions. Most modal analyses were made by Keryn Tobler 0 WAH WAH SPRINGS FORMATION Plag Qtz San Bio Hb Px Opaq Pheno Ross. (B) Modal proportions of biotite and hornblende in the B 25 This unit was named by Mackin (1960) for Indian Peak phenocryst-rich dacite ignimbrites n = 153 Cottonwood Wash and other its occurrence near the large complex of Wah L Lund phenocryst-rich dacite ignim- 0.6 1.0 K Silver King Wah Springs on the eastern fl ank of the Wah Wah 20 brites in the Indian Peak fi eld. C L W Wah Wah Springs Mountains (Figs. 28 and 29; see also Best et al., C Cottonwood Wash Samples of the Cottonwood C 1973, Figures 4 and 5). The unit includes an K L L L Wash have highly variable 15 Hb/Bio = 0.47 C informal non-lithic outfl ow tuff member and, at K K C C ratios of hornblende to biotite. KK C C W L W its Indian Peak caldera source, an intra caldera KK KKK K C L C L K L L C The ratio for the Wah Wah L LLC LCC C W L K K C L LC L C W W WW member, which includes lithic dacite tuff, wall- 10 K L L L C W WWW Springs is >1. LL LL CWCCWWL W W C W W C L LL CL C WL WWLLW WWWWWW W collapse breccia, and intrusive granodiorite WCLL L C WC WC L WWW W L L C CL WWW W WW porphyry (Skidmore et al., 2012). Because we

Biotite (% phenocrysts) W L W W W W W W W CWW W 5 C W W now realize that some of the non-lithic outﬂ ow W ignimbrite occurs inside the topographic margin of the caldera as slabs in wall-collapse breccias, 0 it is more accurate to refer to it as pre–caldera 0 5 10 15 20 25 30 35 collapse tuff and the intracaldera tuff as caldera- Hornblende (% phenocrysts) collapse tuff. The Wah Wah Springs ignimbrite shares compositional aspects with other monotonous A 9 intermediates (Figs. 5–7). Like the Cottonwood Trachydacite Rhyolite Wash, some samples are andesite but unlike that 8 TT unit some ignimbrite samples are more silica B T C C B B rich and barely reach the rhyolite ﬁ eld (Fig. 30). C B B 7 CC CC CC CC Although other element concentrations are simi- O (wt %) C CC C B 2 CC C C C CCBCCCCB C C C CC CCC C CC B lar between these two tuffs, there are some sig- 6 C BC C

O + K C niﬁ cant differences. One intriguing distinction 2 Cottonwood Wash Tuff B of the Wah Wah Springs (including ignimbrite, Na Dacite C Ignimbrite n = 46 5 Pumice clasts n = 14 cognate inclusions, and intracaldera granodiorite Andesite T top of unit B bottom of unit porphyry) from other monotonous intermediates 4 is the unusually high concentration of Cr (Fig. 59 61 63 65 67 69 71 73 75 77 30C); only the phenocryst-rich, andesite-latite SiO (wt %) 2 Figure 27. Chemical variation Harmony Hills Tuff has still more Cr. The Wah B 7 diagrams for the Cottonwood Wah Springs can generally be distinguished in Cottonwood Wash Tuff Wash ignimbrite and cognate outcrop because of the presence of conspicu- C Ignimbrite n = 46 C C pumice clasts. (A) Total alka- Pumice clasts n = 14 ously large hornblende phenocrysts that are more B T top of unit lies-silica. (B) Fe2O3-TiO2. abundant than biotite (Figs. 26B, 31, and 32), an 6 C C B bottom of unit C C C C CCCC CC B aspect that is unique among all middle Cenozoic C CC C C C C C C C C C Great Basin ignimbrites of which we are aware. B C C C BBC C Whereas other monotonous intermediates con- C (wt %) C C 3 5 C tain proportionately more quartz pheno crysts, O 2 C C CCC T T upwards of 25%, the Wah Wah Springs contains Fe C C CC less than 10% and in some samples it is absent. B T 4 Sanidine is notably absent throughout the ignim- B B B brite unit but is found in one cognate pumice B B clast described below.

3 The weighted average age of the formation 0.3 0.4 0.5 0.6 0.7 0.8 based on 16 published K-Ar determinations TiO2 (wt %) (corrected to new decay constants as needed)

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Andesitic lava

Three Creeks

Lund

Wah Wah Springs

Paleozoic sedimentary rocks

Figure 28. Panoramic view looking south at Wah Wah Springs (marked by slender poplar trees) with Wallaces Peak in the background. At this type section (38°28.7′ N, 113°29.8′ W), the pre–caldera collapse (outﬂ ow) ignimbrite of the Wah Wah Springs Formation lies on Paleozoic carbonate rocks and very local Cottonwood Wash Tuff (behind peak) and is overlain by thin Lund tuff and thicker Three Creeks Tuff (formerly designated the Wallaces Peak Tuff), which was derived from a source caldera in the Marysvale volcanic ﬁ eld ~100 km to the east. See Best et al. (1973) for geologic map.

on biotite and hornblende from outfl ow tuff Sections are as thick as 500 m immediately volcanic rocks of Wales Canyon, and the tuff is 30.1 ± 0.3 Ma (Best and Grant, 1987). north of the topographic margin of the caldera of Dog Valley. The latter unit is clearly older, Eleven 40Ar/39Ar analyses of plagioclase in source in the central Needle Range and 460 m at 33.6 Ma, than the Wah Wah Springs whereas seven outfl ow and one intracaldera samples to the east in the Wah Wah Mountains. Both sec- the former two units that locally interfi nger with yield a weighted mean age of 30.06 ± 0.05 Ma tions appear to lie in paleovalleys. In the central one another are of roughly the same age as the (Table 1). Needle Range (Best et al., 1987b), the outfl ow Wah Wah Springs. Caskey and Shuey (1975) tuff is synformal with a northerly trending axis, obtained reconnaissance paleomagnetic direc- Distribution of the Outfl ow Tuff Member, whereas in the Wah Wah Mountains, the tuff tions from four sites (their numbers 224, 225, or Pre–Caldera Collapse Ignimbrite completes the fi lling of a deep easterly trend- 226, 227) in what they thought was an upper unit ing paleovalley occupied by older tuff deposits in their Clear Creek tuff, as the Three Creeks (Abbott et al.,1983; see also Figs. 9 and 12). was then designated. The sites in this tuff are Correlation of the Wah Wah Springs through- Beyond these paleovalleys, sections are as thick widely distributed in the east-central part of the out the vast outfl ow sheet based on stratigraphic, as 375 m. Red Ridge 7.5-minute quadrangle where the tuff petrographic, and compositional attributes is On the western margin of the High Plateaus is less than 50 m thick, and are located either on confi rmed by its reversed paleomagnetic direc- in south-central Utah, a thick pile of 34–33 Ma or north of the hinge zone of the obscure trap- tion at 30 distributed sites (S. Gromme and andesitic and dacitic lavas and tuffs in the door caldera that was the source of the younger M. Hudson, 2006, personal commun.); the other Marysvale fi eld (Hintze et al., 2003; Steven Three Creeks Tuff Member (Steven et al., two monotonous intermediates in the Indian et al., 1979) prevented accumulation of the dis- 1979). Taken together, these paleomagnetic Peak–Caliente fi eld are normally magnetized. tal outfl ow in that area, but Wah Wah Springs data are perfectly representative of those we The outfl ow (Fig. 29) was the most exten- ash fl ows traveled farther east around the north have obtained from the Wah Wah Springs sites sive at 32,000 km2 and had the greatest volume and south sides of the pile. On the north of the to the west. Hornblende phenocrysts in our of 3000 km3 (Table 2) of any ignimbrite in the pile, three stratigraphic units of phenocryst-rich sample REDR-CS227 have the same compo- Indian Peak–Caliente ignimbrite fi eld, and as dacite tuff that are compositionally and petro- sition as those in the Wah Wah Springs and well in the Central Nevada fi eld. It is found over graphically similar to the Wah Wah Springs are unlike hornblendes in other ignimbrites in a present north-south distance of 240 km and an outfl ow tuff overlie a heterogeneous sequence the central and eastern Great Basin, includ- east-west distance of 370 km from the High Pla- of tuffs and lava and volcanic debris fl ows. ing other monotonous intermediates (Fig. 33). teaus of south-central Utah westward into south- These three units are the Three Creeks Tuff 40Ar/39Ar ages on plagioclase from our samples central Nevada. Member of the Bullion Canyon Volcanics, the REDR-CS226 and REDR-CS227 are 29.82 ±

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Corrected for Wah Wah Springs, ignimbrite (30.06 Ma) 0 0 50% EW Indian Peak caldera structural margin ! ! extension 46 E Indian Peak caldera topographic margin E ! E

55 ! 0 ! ~60 0 100 km ! >20 ! 0 ! 0 ! ! ! >20 39° E EEE E >20 >70 61 !<40 2-10 ! ! ! ! 18 >15 <60 ! ! ! <30 60? 35 ! ! ! 30 <50 <70 ! ! >140 ! ! 0? 20 ! ! <60 ! ! ! 60 <270 ! ! ! >300 0 270 ! ~250 ! E E ! 340 E E ! ! ! 0 100 106 ! ! >2100 250 0 >60 ! ! ! 130 500 ! 75 ! ! 400 ! ! <250 ~3500 460 ! ~30 ~5000 ! 375 ! 170 ! 15 16 4000 ! 75 ! ~4000 ! ! <120 ! 0 300 ! 1000 ! ! 300 0 ! 270 15-25 ! !>500 ! <135 ! ! 250 <250 ! 200 ! <40 E E ! E ! 38° >260 E 100 ! ! ! !15 10? ! ! 170 125 <120 30 ! 200 ! ! ~30 >80 ! 0-5 0 >50 100 ! ! ! 120 ! ! 0 ! ! 0 0 0 ! 0 0 ! 0 0-20 E ! 0 ! E E EE17 ! ! 10 ! 0 ! 0-9 0 20 40 60 80 100 15 ! !0-6 ! 0 Km 0 0 20 40 60 ! Miles NV UT

116° 115° 114° 113° 112° Figure 29. Distribution and thickness (in meters) of Wah Wah Springs ignimbrite. Topographic and structural margins of the Indian Peak caldera source are well exposed in the north, less certain in the east, and speculative in the west and south. The 1000 m isopach is drawn to coincide with the structural margin of the caldera.

0.16 and 29.87 ± 0.15 Ma, respectively. Hence, limestone and sandstone interfi nger between commun.; also Best and Williams, 1997), the we do not doubt the existence of the Wah Wah ignimbrite cooling units all the way up section pre–caldera collapse ignimbrite pinches out Springs north of Marysvale, as well as else- from the Wah Wah Springs to the Bald Hills over Cambrian carbonate rocks. where on the western margin of the High Pla- Tuff Member of the Isom Formation (Table 1), teaus. Parenthetically, we note that apparently whereas conglomerate beds of rounded Paleo- Mid-Continent Fallout Ash Deposit the most widespread of the Three Creeks cool- zoic rock lie between cooling units below the ing units occurs as far west as the Wah Wah Wah Wah Springs (Scott et al., 1994). This Some of the fi ne fallout (locally reworked) Mountains (Fig. 28; Abbott et al., 1983; Best sequence of interbedded sedimentary rock and ash deposits in western Nebraska (Tedford and Grant, 1987) where it lies between the ignimbrite has an aggregate thickness of about et al., 1996; LaGarry, 1998; Larson and Evanoff, Lund and older Isom tuffs (Table 1) as well as 1 km, with about one-half of each, and was 1998) might have originated from the Wah Wah local andesitic lavas and, thus, has an age of deposited in a basin that apparently existed for Springs super-eruption. A candidate correlative ca. 28 Ma. at least 5 m.y. The rounded clasts in the con- of the Wah Wah Springs is one of three ash beds The distal outfl ow sheet of the Wah Wah glomerate appear to have been derived from a that constitutes the widespread Nonpareil (NP) Springs south of the Indian Peak caldera in nearby long-lasting highland and are not a result ash zone. In a 20-m-thick section of this ash westernmost Utah is as thick as 17 m and either of erosion off contemporaneously created fault zone studied by Tedford et al. (1996, p. 317), lies conformably above a thick sequence of lake blocks. Farther north in the North Pahroc Range, the lowest of three distinct ash beds has reverse sediments and conglomerates (Hintze et al., beds of ash-fall and reworked tuff ~5 m thick polarity, like that of the Wah Wah Springs tuff, 1994a) or is conformably interbedded within lie between the Cottonwood Wash and the Wah whereas the overlying two ash layers are nor- a sequence of lacustrine limestones (Blank, Wah Springs tuffs and have a “Needles Range” mally polarized. They also indicate (p. 314) 1959). In distal sections southwest of the caldera modal composition (Swadley et al., 1994). that at Roundtop an 40Ar/39Ar age of 30.05 ± source, outfl ow tuff is as thick as 120 m in the Proximal to the caldera source in the area of 0.19 Ma was obtained on biotite by Swisher and North Pahroc Range where beds of lacustrine Condor Canyon (Gary J. Axen, 1989, personal Prothero (1990) on an ash correlated with the

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A 9 from those in other hornblende-bearing dacite ignimbrites in the Indian Peak fi eld as well as Rhyolite ignimbrites in the Central Nevada fi eld. 8 Further study of the Nonpareil ashes is clearly OP I W warranted. Nonetheless, we feel confi dent, con- gOP WOP WW 7 WW WW W W sidering the vast volume and areal extent of the O (wt %) W W Wg W 2 I W W W WW IPIPWIP OP Wah Wah Springs ignimbrite, that hundreds of I WWWI W IP I I WWIP WW WW W WWW cubic kilometers of fallout, presumably fi nely O + K 6 W W 2 W WWW W IP Wah Wah Springs Formation WW WWW sorted co-ignimbrite, ash was deposited from Na W W Intracaldera Member g Granodiorite porphyry n = 2 this super-eruption in the mid-continent, as for 5 Dacite I Tuff n = 6 the Cottonwood Wash discussed above. IP Cognate inclusion n = 6 W Outflow Tuff Member n = 57 Andesite OP Cognate inclusion n = 4 Lithic Intracaldera Tuff Member, or 4 59 61 63 65 67 69 71 73 75 77 Caldera-Collapse Ignimbrite SiO 2 (wt %) B 7 Unlike the lithic-free, pre–caldera collapse Figure 30. Chemical varia- ignimbrite, the caldera-collapse Wah Wah tion diagrams for the Wah Springs contains lapilli and rare blocks as W 6 W I I Wah Springs Formation and WWW much as a meter in diameter of varied volcanic WIWWW W WWWW I other phenocryst-rich dacite WW I rocks that generally constitute less than 10%, WWWW WWWI W but locally make up as much as about half, of and andesitic tuffs. (A) Total WgW W 5 WWIPW IP W OP WWIPIP IP the tuff (Fig. 34). In most places in the Indian alkalies-silica for the Wah Wah W WWg IPOP

(wt %) OPWWOPW Peak Range, most of the lithic clasts are dark- Springs Formation. (B) Fe O - 3 OPW 2 3 W OP O W

2 W colored, essentially aphyric volcanic rocks that TiO2 for the Wah Wah Springs 4 Fe W Wah Wah Springs Formation resemble rhyolite lavas of the Escalante Desert Formation. (C) Cr-SiO2 for W W Intracaldera Member Group; fewer clasts are of Cottonwood Wash monotonous intermediates and g Granodiorite porphyry n = 2 Silver King and Harmony Hills 3 I Tuff n = 5 tuff. In the northern White Rock Mountains, IP Cognate inclusion n = 6 lapilli of sedimentary rock are common. These Tuffs. Cognate inclusions for W Outflow Tuff Member n = 58 the Wah Wah Springs Forma- OP OP Cognate inclusion n = 8 older rock fragments were apparently caught up tion are included (see diagrams 2 in the pyroclastic mass as it vented along the A or B for key to additional 0.30.4 0.5 0.6 0.7 margin of the collapsing caldera. None of this letter symbols). TiO2 (wt %) lithic ash-fl ow tuff is known to occur beyond the topographic margin of the caldera, imply- C 50 H Harmony Hills Tuff n = 5 ing ash fl ows venting along the ring fracture K Silver King Tuff n = 18 H had insuffi cient energy to surmount this barrier. HH H Monotonous intermediates 40 H W Pumice lapilli are obvious in many places and, L Lund n = 57 W W WW W Wah Wah Springs n = 63 although commonly densely compacted, the WWW W WWW IP C Cottonwood Wash n = 46 WW WWW WW ignimbrite lacks an easily discerned foliation. WWWW WW 30 WWWW WIPW W W WWIPOiPWg OP W An additional distinguishing aspect of the cal- W W W Wg W W W W W OP C W W IPW W dera-collapse ignimbrite is its widespread and IPIP WOPWW W LCLC C CW W Cr (ppm) C OP C CC CC L L W variably intense, generally propylitic, alteration. 20 C CCCCCL LLL CCL W OP CC L CLLLLL W L W C CC LCLCLLL L The associated intrusive granodiorite porphyry LC LCLL LL LLL CL C CC L CCL LC L L C LCLC L is also so altered. Because variable degrees of L L L 10 L L C L alteration have affected nearly all samples of the K K LLK LKK K KK KK K ignimbrite, exact comparisons of modal and L KLK L K LL KK chemical composition with the pre-collapse tuff 0 K 59 61 63 65 67 69 71 73 75 77 can be somewhat uncertain; however, chemical

SiO2 (wt %) variation diagrams (Fig. 30) indicate it overlaps the less silica-rich, pre–caldera collapse ignimbrite whereas modal compositions of the two

uppermost layer (NP3). This age has not been in the ignimbrite in the Great Basin are about overlap completely (Fig. 31A). adjusted to the Fish Canyon reference standard ten times larger. The proportions of different The lithic-rich ignimbrite was sampled for

of 28.20 Ma. A sample (AJ-94) of the NP3 ash types of crystals are entirely consistent with paleomagnetic direction at two sites, 0.8 km from Swisher and Prothero’s Roundtop locality the Wah Wah Springs; especially noteworthy is east-northeast of Indian Peak by Shuey et al. was kindly provided by James B. Swinehart. As the greater amount of hornblende than biotite. (1976, their site WW 135) and 9 km southeast of expected for distal fallout ash, the sample has Microprobe analyses of the hornblendes in the Atlanta, Nevada by S. Gromme and M. Hudson abundant, very ﬁ ne glass particles (and plenti- sample reveal that they are indistinguishable (2006, personal commun., their site B98-8; our ful interspersed secondary carbonate) in which from those in the Wah Wah Springs ignimbrite petrologic sample ATL-33 was collected at this sparse but larger crystals to as much as 0.4 mm for all of the nine elements analyzed (Fig. 33). site). The directions are identical but signiﬁ -

are embedded; for comparison, phenocrysts Moreover, the NP3 hornblendes are distinct cantly steeper in inclination and more easterly

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A Wah Wah Springs Formation and PANGNW-1) were analyzed to evaluate + Intracaldera Tuff Member n = 12 vertical zoning in proportions among pheno- 60 Outflow Tuff Member n = 47 crysts. No systematic modal variation in any Weighted average of 83 analyses of outflow tuff in the easternmost Great Basin and High phenocryst was found in the PANGNW-1 sec- Plateaus of Utah (Anderson et al.,1975) tion. But, despite considerable scatter, the other 40 three sections reveal poorly defi ned zoning with the proportion of plagioclase increasing up section (Fig. 35B). In the SILVRWL-1D and Mode (vol %) FRSC-8 sections, adjustment of total pheno- 20 cryst concentrations to a dense rock equivalent revealed no systematic variation. Figure 31. Modal proportions Analyzed samples of intracaldera, or caldera- of phenocrysts in the Wah Wah 0 collapse, ignimbrite are among the least evolved Springs Formation. (A) Intra- Plag Qtz San Bio Hb Px Opaq Pheno of the Wah Wah Springs data set (Fig. 30) but caldera (caldera-collapse) and some outfl ow samples are equally so. The latter outfl ow (pre–caldera collapse) B 80 Wah Wah Springs Formation are mostly in the SILVRWL-1D, LUND-10, and tuffs. (B) Cognate pumice inclu- Cognate inclusions PANGNW-1 sections. sions and intracaldera grano- + Intracaldera tuff host n = 11 Outflow tuff host n = 14 The HFW-8-153-3 section that comprises diorite porphyry. 60 R Rhyolite in outflow host n = 1 the lower several meters of an ~250-m-thick Intracaldera granodiorite porphyry outfl ow sheet is atypical in several respects. R Whereas ash-fl ow tuff beneath a near-basal 40 vitrophyre is typically poorly welded and compacted and almost never exposed, in this section, Mode (vol %) less than one-third meter of tuff beneath the 3 m 20 of black vitrophyre, and above the non-exposed R R R base, is moderately welded, is glassy red-brown, R and contains an unusually small proportion R R (14%, DRE [dense rock equivalent]) of unusu- 0 R Plag Qtz San Bio Hb Px Opaq Pheno ally small (<1 mm) phenocrysts. This basal quartz- and clinopyroxene-free tuff (HFW-8-

153-3A) is the most evolved (70.2 wt% SiO2;

in declination than directions in the pre–caldera No systematic chemical zoning is evident in 0.37 wt% TiO2) of any Wah Wah Springs tuff collapse ignimbrite. Before tilt corrections, bulk rock samples from the FRSC-8 and LUND- and plots barely into the ﬁ eld of rhyolite on the these two directions were closer to that of the 10 sections whereas the other three disclose total alkalies-silica diagram (Fig. 30A). The

mean Wah Wah Springs, which suggests that zoning in, for example, SiO2 and TiO2 (Fig. overlying black vitrophyre (HFW-8-153-3B; there was probably resurgence within the Indian 35A). However, the zoning is inconsistent from 24% phenocrysts) is only slightly less evolved

Peak caldera before the included thick pile one section to another; the HAM-10 section in at 70.1 wt% SiO2 whereas the overlying red- cooled below its Curie temperature. the Mountain Home Range shows up-section brown devitriﬁ ed tuff (HFW-8-153-3C; 30%

decreases in SiO2 and increases in TiO2 whereas phenocrysts) immediately above the vitrophyre

Compositional Zonation the SILVRWL-1D and PANGNW-1 sections is a more normal dacite with 68.4 wt% SiO2. west and east of the caldera are the reverse. In addition to the HFW-8-153-3 section, Ignimbrite Bulk rock samples from four stratigraphic sec- other samples of near-basal, densely welded, To evaluate vertical zoning in the Wah Wah tions (SILVRWL-1D, HFW-8-153-3, FRSC-8, black vitrophyres have total phenocryst concen- Springs pre–caldera collapse ignimbrite, fi ve complete stratigraphic sections were sampled, including: (1) a 130-m-thick section (SILVRWL- 1D) ~45 km west of the caldera source at the south end of the Egan Range, (2) a 340-m-thick Figure 32. Photomicrograph proximal section (HAM-10) on the west side of of densely welded Wah Wah the Mountain Home Range north of the caldera, Springs vitrophyric tuff, show- (3) a 106-m-thick section (FRSC-8) at the Wah B ing abundant plagioclase, Wah Springs type locality (Fig. 28) on the east less hornblende, and still less fl ank of the Wah Wah Mountains northeast of opaque Fe-Ti oxides, biotite (B), the caldera, (4) a 270-m-thick section (LUND- Q quartz (Q), and clino pyrox ene Q C 10) at the southeast end of the Wah Wah (C). Welding has virtually oblit- Mountains, and (5) a distal 45-m-thick section erated margins of glass shards (PANGNW-1) north of Panguitch, Utah, in the in the pore-free matrix. High Plateaus. A partial basal proximal section B B (HFW-8-153-3) was sampled on the east fl ank of the Mountain Home Range. 1 mm

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A 0.8 phenocryst poor (6%, DRE) (Figs. 30, 31, and Wah Wah Springs tuff and correlatives 36). Because most of the phenocrysts appear 0.7 Wah Wah Springs to be resorbed, or are anhedral, the possibility Wah Wah rhyolite arises that the rhyolite pumice might be a piece Nonpareil ash Marysvale field of fused Precambrian roof rock from above the 0.6 Lund magma chamber caught up in the early erupting magma. However, its initial 87Sr/86Sr composition of 0.7094 (Table 4) is identical to that of the 0.5 Cottonwood Wash Wah Wah Springs ignimbrite (0.7093–0.7095) Mg / (Mg+Fe) and inconsistent with this origin; thus, it appears Central Nevada field 0.4 Lunar Cuesta to represent a small parcel of more-evolved Upper Shingle Pass magma from near the top of the chamber. Pro- Lower Shingle Pass Monotony portions of phenocrysts in the rhyolite pumice 0.3 Hot Creek Palisade Mesa are similar to less-evolved dacite tuff, except for Windous the presence of substantial quartz and euhedral Stone Cabin 0.2 sanidine that has a relatively high Or content 5.8 6.0 6.2 6.4 6.6 6.8 7.0 7.2 similar to that of sanidine in the Lund ignim- Si (atoms per forumula unit) 2.3 brite (see below). Other phenocryst composi- B tions, such as that of hornblende (Fig. 33), are 2.1 like those in less-evolved Wah Wah Springs tuff. Lund Glass in the pumice is slightly more evolved 1.9 than that in tuff (Table 5). All together, the compositions of phases in the rhyolite pumice indi- 1.7 cate it is likely the low-temperature part of the main dacitic magma body and probably differ- 1.5

Al (apfu) entiated from that part of the Wah Wah Springs 1.3 magma chamber. Data from the dacite inclusions indicate that Wah Wah Springs tuff 1.1 Cottonwood and correlatives the main part of the pre-eruption chamber pos- Wash Wah Wah Springs sessed compositional gradients. Inclusions 0.9 Wah Wah rhyolite hosted in intracaldera tuff have 65.7–66.1 wt% Nonpareil ash Marysvale field SiO2 whereas those in pre–caldera collapse, or 0.7 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 outfl ow, tuff contain slightly more at 67.1–67.6 Ti (atoms per forumula unitu) wt%; these contrasts indicate the upper, early erupted part of the chamber had as much as 2 Figure 33. Comparative compositions of hornblende phenocrysts. wt% more silica. Cognate inclusions hosted See text for discussion. Diagram shows analyses of hornblendes in in intracaldera ignimbrite tend to have less Wah Wah Springs ignimbrites, in Wah Wah Springs cognate rhyo- plagioclase and more quartz than the outfl ow- lite pumic sample BRN-1PC, in Nonpareil fallout ash in western hosted inclusions, whereas proportions of other Nebraska, and in sample REDR-CS227 of Wah Wah Springs tuff in pheno crysts overlap (Fig. 31B). Plots of quartz, the Marysvale volcanic fi eld. These are compared with hornblendes plagio clase, and biotite in the entire population in the Cottonwood Wash and Lund ignimbrites (elliptical fi elds) and of ignimbrite and inclusion samples disclose no ignimbrites in the Central Nevada fi eld (Fig. 2). systematic patterns whereas plots of quartz versus hornblende and quartz versus clinopyroxene reveal subtle correlations (Fig. 37). Hornblende correlates negatively with respect to quartz, a trations as low as 25%, but ranging more or less The existence of compositional gradients in the pattern also seen in the Lund (Maughan et al., continuously to as high as 50% (adjusted to a chamber is supported by our observations on 2002, their fi gure 5). Clinopyroxene correlates dense rock equivalent; see Fig. 36). As well, the cognate inclusions. positively with respect to quartz. Table 6 shows lowest parts of the Wah Wah Springs outfl ow that two pumices from the GOUGE-3P site at sheet commonly have relatively smaller pheno- Cognate Inclusions the top of the outfl ow sheet have the lowest total crysts compared to overlying parts. Smaller, Cognate pumiceous inclusions were col- quartz plus clinopyroxene (0.4%) whereas the less abundant phenocrysts are conceivably the lected at six sites (Table 6; for locations see highest totals (8%–14%) occur at the top of the result of dynamic processes at the base of Supplemental File 1 [see footnote 1]). Inclu- intracaldera deposit at site GLE-6-98-1X. The the pyroclastic fl ow as it moved across the depo- sions from all sites in the outfl ow and intra- intracaldera granodiorite porphyry exposed sitional surface (e.g., Branney and Kokelaar, caldera ignimbrites are phenocryst-rich dacites, south of Indian Peak plots within the higher 2003, p. 29–30). On the other hand, the fewer, but one small (4 cm) pumice lapilli (sample quartz-plus-clinopyroxene range. These data smaller phenocrysts in the lowest parts of the BRN-1PC) collected from the non-welded base imply that the late-erupted and lower part of the Wah Wah Springs outfl ow sheet might refl ect of the distal outfl ow is unique in being rhyolite Wah Wah Springs magma chamber had slightly

a gradient in the pre-eruption magma chamber. (73.8 wt% SiO2 and 0.31 wt% TiO2) and very less silica but a greater proportion of quartz

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and clinopyroxene, apparently at the expense of plagioclase and hornblende. Pumices at the MLLR-6-63 site at the top of the intracaldera host were apparently extracted from only the upper part of the chamber (0.2%–2.0% quartz plus clinopyroxene) whereas other pumices constitute a mixed population from different levels. Experiments by Clemens and Wall (1981) on a granitic composition that is quite close to the most silica-rich Wah Wah Springs (except for

half as much CaO and slightly more K2O) reveal that the crystallization fi eld of quartz broadens at higher pressure and lower water concentration in the system. Johnson and Rutherford (1989) showed the same for the dacite Fish Can- yon magma. Cognate inclusions from the top of the intracaldera deposit are mostly less vesicular (measured densities 1.59–2.33 g/cm3, average 2.0) than ones from the outfl ow sheet (0.88–1.81 g/cm3, average 1.4) (Table 6), indicating lesser concentration of volatiles in the corresponding deepest magma erupted. Figure 34. Outcrop of lithic-rich caldera-collapse, or intracaldera, Wah Wah Springs ignim- Summary brite just inside the ring fault of the Indian Peak caldera north of Indian Peak. Dark- The main dacitic part of the pre-eruption Wah colored, aphyric and phenocryst-poor volcanic rock (near hammer) is the most common Wah Springs magma chamber apparently pos- lithic clast type. Clasts of greenish gray Cottonwood Wash that have large and abundant sessed gradients in silica concentration of ~2 phenocrysts are visible at right of the photograph. Hammer head is 20 cm long. wt% and in proportions of phenocrysts. The more silica-rich, upper part of the chamber had Figure 35. Diagrams showing A 0.8 little or no quartz and clinopyroxene and the less the character of zoning in strati- Wah Wah Springs Formation, Outflow Tuff Member Samples in 3 stratigraphic sections silica-rich, lower erupted part had as much as graphic sections in the Wah Wah 0.7 14% quartz plus clinopyroxene combined and Arrows point Springs outfl ow tuff. (A) TiO2- less hornblende and volatiles. The uppermost SiO . Squares are samples at 1, up-section level of the main part of the chamber had smaller 2 0.6 8, 20, 36, 54, 82, 104, and 126 m and lesser concentrations of total phenocrysts.

above base of 130-m-thick sec- (wt %) Apparently, a relatively very small differenti- 2 tion SILVRWL-1D at the south 0.5 ated cap on the main dacitic chamber consisted TiO end of the Egan Range, Nevada, SILVRWL-1D of a phenocryst-poor rhyolite that contained 74 PANGNW-1 at 38.34° N, 114.97° W. Pluses 0.4 wt% silica and somewhat similar proportions of HFW-8-153-3 are samples at the base and 9, FRSC-8 phenocrysts of basically the same compositions 22, and 38 m above the base of 0.3 as the dacitic part, but with additional sanidine. distal section PANGNW-1 north 61 63 65 67 69 71 The more silica-rich, phenocryst-poorer upper- of Panguitch, Utah, at 37.97° N, SiO2 (wt %) most level of the magma chamber, including the 112.44° W. Triangles are samples rhyolite cap, comprised but a few percent of B 7 at 1, 4, and 15 m above the base the whole chamber. of proximal 250-m-thick section 6 Fractionation of fi ne particles of high-silica HFW-8-153-3 in the southern rhyolite glass from the ash fl ows appears to Mountain Home Range, Utah, at 5 have had limited infl uence on compositional 38.58° N, 113.86° W. (B) Modal variations in the Wah Wah Springs ignimbrites. proportions of opaque phases 4 Although chemical compositions of some versus plagioclase. × symbols ignimbrite samples arguably refl ect this process are samples at the base and 4, 3 (Figs. 30A, 30C), modal data on a smaller set 16, 42, 74, and 100 m above the of samples discount a role. Total phenocryst base at the 106-m-thick FRSC-8 Opaques (% phenocrysts) concentrations on a dense rock equivalent basis 2 Arrows point type section of the Wah Wah up-section in the cognate inclusions that range from 33%– Springs outfl ow tuff just south 1 48% overlap the most phenocryst-rich tuffs (Fig. of Wah Wah Springs (Fig. 28). 54 56 58 60 62 64 66 68 36). Limited fractionation is not necessarily Other symbols as in A. Plagioclase (% phenocrysts) contradicted by the presence of fi ne, glass-rich

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fallout ash in Nebraska, which represents the distal, winnowed fraction of the co-ignimbrite 60 Wah Wah Springs Formation portion of the eruption. Intracaldera Member 50 V Vitrophyre tuff n = 3 V INDIAN PEAK CALDERA I Other intracaldera tuff n = 1 OP OIP V OV g Granodiorite porphyry n = 1 OO IPIPV OO O IP Cognate inclusion n = 5 g OV V OI On the basis of regional geologic relations, OP IPO OV O 40 Outflow Tuff Member IPOP V Shuey et al. (1976) suggested that the source V Near-basal vitrophyre n = 11 OO O Other outflow tuff n = 22 of the Needles Range tuffs, and the Wah Wah O OP Cognate inclusion n = 5 OPV V Springs in particular, might lie in the vicinity of O 3C V 30 O OV the Indian Peak Range. Mapping by Grant (1979) V and Best et al. (1987b) revealed a sequence 3BVV O of ignimbrite and intercalated breccias that is 20 HFW - 8 - 153 3500 m thick faulted against an apparently thick 3A mass of Marsden Tuff in the vicinity of Indian O Percent total phenocrysts (DRE) Peak (Figs. 14, 38, and 39); these relations deﬁ ne 10 the ring fault and northeastern structural margin OP of the Indian Peak caldera. Southward from the Glass ring fault, breccias are composed of clasts of the 0 Cottonwood Wash and Lamerdorf ignimbrites as 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 well as crystal-poor andesitic and rhyolite rocks TiO2 (wt %) that resemble those of the Escalante Desert. Clasts of Paleozoic rock and the Marsden Tuff Figure 36. Concentration of TiO versus total phenocrysts in the Wah Wah Springs For- appear to be absent here. 2 mation. Phenocryst concentrations have been adjusted to a dense rock equivalent (DRE) The thickness of the caldera-collapse ignim- of 2.50 g/cm3, which is the density measured in pore-free vitrophyres. Volumes of samples brite inside the structural margin cannot be deter- for density calculation were measured by a laser scanner. For samples HFW-8-153-3A, 3B, mined directly because of the lack of internal and 3C see Figure 35 and text for discussion. Glass is vitroclasts in surge and ignimbrite stratigraphic markers, which precludes deter- (Table 5). mination of displacements on normal faults

TABLE 6. CRITICAL DATA FOR COGNATE INCLUSIONS IN THE WAH WAH SPRINGS TUFF AND FOR THE INTRACALDERA GRANODIORITE PORPHYRY

SiO2 TiO2 Density Total phenocryst Quartz Clinopyroxene Inclusion Sample number Host tuff wt% wt% g/cm3 DRE vol% vol% 1 BRN-1PB Base of distal outfl ow 67.2 0.50 0.83 43.1 0.0 0.2 2 BRN-1PC* Base of distal outfl ow 73.8 0.31 1.51 5.9 16.1 0.0 3 BRN-1PD Base of distal outflow 67.2 0.57 0.4 0.2 4 BRN-1PH Base of distal outfl ow 67.5 0.52 5 BRN-1PI Base of distal outfl ow 68.0 0.51 6 GOUGE-3PA 1.45 35.3 0.3 0.1 GOUGE-3PB 1.45 30.3 0.0 0.4 average Top of proximal outfl ow 67.1 0.51 1.45 32.8 0.2 0.3 7 GOUGE-3P12 0.88 50.3 4.5 1.6 GOUGE-3P13 44.6 6.4 2.6 average Top of proximal outfl ow 67.6 0.53 0.88 47.5 5.5 2.2 8 HAM-9-129-1PA 1.78, 1.81 39.9 3.0 0.6 HAM-9-129-1PB 42.5 5.0 0.4 HAM-9-129-1PC 35.8 3.6 0.5 HAM-9-129-1PE 41.8 2.7 0.5 average Upper proximal outfl ow 67.6 0.53 1.80 40 3.6 0.5 9 MLLR-6-63-1PA 1.59, 1.63 39.4 0.0 0.5 MLLR-6-63-1PB 42.9 0.0 0.2 MLLR-6-63-1PC 36.6 1.9 0.1 MLLR-6-63-1PD 36.0 1.6 0.1 MLLR-6-63-1PE 41.5 0.7 0.1 average Top of intracaldera 66.4 0.52 1.61 39.3 0.8 0.2 10 MLLR-6-64-2XA* Top of intracaldera 2.17 47.7 8.1 3.5 MLLR-6-64-2XB* 2.28 average 65.7 0.62 2.22 47.7 8.1 3.5 11 GLE-6-98-1X Top of intracaldera 66.1 0.60 10.2 3.8 12 GLE-6-98-1XA Top of intracaldera 66.1 0.58 2.33 44.5 8.6 5.1 13 GLE-6-98-1XB Top of intracaldera 66.7 0.57 2.24 41.0 10.1 2.9 14 GLE-6-98-1XC Top of intracaldera 66.6 0.57 1.96 40.8 5.6 2.5 15 GLE-6-98-1XD Top of intracaldera 66.2 0.57 2.27 44.8 6.8 2.1 MIN-8-60-2A Granodiorite porphyry 66.7 0.55 42.9 8.4 3.0 Notes: GLE-6-98-1X samples are vesicle-poor pumice clasts. Numbers under Quartz and Clinopyroxene are modal proportions as percentage of total phenocrysts. DRE—Total volume percent of phenocrysts on a dense rock basis, here assumed to have a density of 2.5 g/cm3 except for BRN-1PC which was assumed to have a density of 2.35 g/cm3. *Small samples, less than 10 cm3 .

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A 45 repeating parts of the pile. But because the bottom Wah Wah Springs Formation Intracaldera Member of the pile, which has been tilted by basin-and- 40 g Granodiorite porphyry range faulting and by resurgence, is not exposed I Tuff n = 10 IP Pumice inclusion n = 1 and the top has been eroded off, we assume that 35 IV Vesicle-poor pumice inclusion n = 6 the unknown amount of missing section counter- O O Outflow Tuff Member n = 42 O balances the amount of section possibly repeated 30 OP Pumice inclusion n = 13 O I O O O by faulting. The apparent thicknesses measured O O O O OI O OO I OP I in four places from geologic maps and shown in 25 OPO IP O OP OP IVI OP O I O I I Figure 29 are 2100, 3500, 4000, and 5000 m. O OP O O OP O O OP O OP 20 OPOP O IV O In the following sections we trace the evi- O O O OO I O OO O g Hornblende (% phenocrysts) OOP OI IV IV dence for and the character of the Indian Peak IV O O O 15 IV caldera, mostly along its margin in a clock- wise direction, beginning in the central Needle OP 10 Range. Reference to Figures 3, 8, and 29 will 024681012 Quartz (% phenocrysts) aid the reader in this circumnavigation.

Central Needle Range Collar Zone B 6 Because of fault-related tilting subsequent to 5 IV volcanism, the internal stratigraphy and structure of the northeastern sector of the Indian Peak 4 caldera are well exposed in the central Needle IV IV Range (Figs. 8 and 39; see also Best et al., 3 g 1987a, 1987b; Best and Grant, 1987, p. 10–13; IV OP IV Best et al., 1989b, p. 121, especially their ﬁ g- O O O 2 OIV O ure R32). In this sector, the collar zone of the O O O OP O caldera lies between its topographic margin on OI I O O O O OO I the northwest and its structural margin, marked Clinopyroxene (% phenocrysts) O I 1 I OI OP O OOO O O O OOPO I O by the inner ring fault, on the southeast. This OPOI O OP OP O I OPOOPO OOO collar-zone terrane extends ~11 km parallel to 0 OPOOPO IP O I 024681012 the range and is composed of the pre-caldera Quartz (% phenocrysts) section of Paleozoic rocks and an overlying Marsden sequence, described above, overlain Figure 37. Modal proportions of quartz and other phenocrysts in ignimbrites and in turn by a caldera-collapse breccia layer as cognate pumice inclusions of the Wah Wah Springs Formation. Modes of cognate thick as ~700 m, described in detail below. Post- rhyolite pumice sample BRN-1PC (0.0% quartz, 0.0% clinopyroxene, 14.5% collapse, caldera-ﬁ lling tuffs of the Ryan Spring hornblende) are omitted. (A) Hornblende-quartz. (B) Clinopyroxene-quartz. Formation thicken from zero at the topographic

Figure 38. Panoramic view looking easterly at the inner reverse ring fault of the Indian Peak caldera ~2 km northwest of Indian Peak (Fig. 14) . Darker-colored intracaldera Wah Wah Springs tuff on the south is juxtaposed against lighter colored Marsden ignimbrite on the north. From this perspective, the fault appears to dip at a moderate angle to the south, but see Figure 39. Photo in Figure 34 was taken at the base of the knob on the right.

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113°57′30″ 38°22′30″ Of TERTIARY Oh Tw 26 Tc 38° Andesitic 22′ Ts Sandstone ″ OCn graphi lava m 30 Topo c ar Quaternary gin Ti Isom tuff

OCn Ts 22 Twb Tl Lund tuff Co Tl Tr 24 Ti Ts Ryan Spring tuff 31 113° Tr 38° OCn 55′ 20′ Cw 00″ 00″ 45 O Co 25 u Ct Twb t e r r 23 Twp Granodiorite i n Cw porphyry g 43 f a 25 Of u l OCn t Twb Tl 33 21 TwbTTwwb Breccia Tw Tr Oh Twb Lithic intracaldera Of Quaternary Twl 35 Ryan 32 tuff Of Spring fault Ted Tw 38 75 Tl ing Twb 35 Tw Pre-collapse tuff 55 pr Tw R y an S Twb Wah Wah Springs Formation Wah Wah 50 50 38° Of 20′ Tr 00″ Quaternary 54 24 25 Tr Tc Cottonwood Wash tuff Twb 40 OCn 50 Oh Tw 40 45 Ted 33 38 Tl Tl Escalante Desert Group Co 80 Ted 70 40 33 Cu 70 85 Twb 75 Quat- Tr 38° 45 ernary 17′ Tr ORDOVICIAN- 30″ CAMBRIAN 54 36 55 Undifferentiated OCn 0 - C sedimentary Cu Co rocks Tw Inn Tr er OCn OCn Co 46 Twb Tw

Quaternary Twp Co Cu Twb

Twp r ev er

Twl se

113°

Ted Tr 52′

Twp ″ 30

N Twb 38° ring 17′ ″ Figure 39 (on this and following page). Eastern seg- - f 30 ault ment of the Indian Peak caldera in the east-tilted Twl Twp Twb central Needle Range. (A) Geologic map of the collar zone that lies between the topographic mar- 44 gin to the north and the inner reverse ring fault to the south (modiﬁ ed from Best et al., 1987b, which 0 1 2 26 should be consulted for details of stratigraphic kilometers units; see also Fig. 8). The same strike-and-dip Twl 0 1 2 18 symbol is used for bedding in Cambrian–Ordovi- Indian miles cian sedimentary strata and for compaction folia- Peak tion in ignimbrites.

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Indian Peak NNW Indian Peak Feet Ryan Inner reverse 2984 m 10,000 Outer ring fault

Andesitic Spring N 38˚15’ ring Tl 8,000 lava fault fault Tw Twl Tl Tl Tr 6,000 Tw Tr Twl Twp Tr Twb Twp 4,000 Tc Oh Pz Pz Twb Tw Ted 2,000 Pz Ted Escalante Desert Group Sea Tc Cottonwood Wash Tuff (31.13 Ma) (~32 Ma) Pz king, level Oh NW-stri Pz Pz ing Pz Twp 2,000 Oh Paleozoic sedimentary rocks; E-dipp Ordovician House Limestone Pz 4,000 Pz Feet Caldera SSE Arrowhead Topographic Meters Pass Tr Tl 3,000 ring fault margin of Twb White Rock caldera Twl Twl Tl 2,400 Twp Pz Twl Twl Isom tuff 1,800 Twp Twb Tr Andesitic lava Twl 1,200 Intrusive Buried margin of Twb Caldera collapse Intracaldera Pre-caldera Lund tuff Twpwp granodiorite Twl TwTw Tl Indian Peak caldera breccia (schematic) lithic tuff collapse tuff (29.20 Ma) porphyny 0 123miles Wah Wah Springs Formation Tr Ryan Spring tuff and lava 30.0 Ma) 012 3 4 kilometers

Figure 39 (continued). (B) Generalized north-northwest to south-southeast cross section along the crest of the Needle Range showing the internal structure of the Indian Peak caldera (modiﬁ ed from Best et al., 1987a, 1987b). No vertical exaggeration. The left half of the section north of Indian Peak (taken from A) has been restored to its conﬁ guration prior to a ~30° eastward tilt by post-volcanic basin-and-range faulting. Note the patterned Ordovician marker unit (Oh; also shown on geologic map). The southern topographic margin of the caldera is obscured by the northern topographic margin of the younger White Rock caldera.

margin of the caldera to as much as 500 m north Caldera-Collapse Wall-Breccia Layer outcrops reveal more or less planar seams that of Indian Peak. The overlying, caldera-fi lling The northwestern two-thirds of the breccia are as much as a few meters square and gen- Lund ignimbrite is uniformly ~600 m thick layer in the collar zone (Twb in Fig. 39A) is erally <1 cm thick in which a substantial pro- throughout the eastern Indian Peak Range south composed mostly of a matrix-supported, mono- portion consists of broken grain fragments of of the topographic margin, but 55 m thick north lithologic breccia made of Cottonwood Wash submicron dimensions (Fig. 41). These seams of it; thus, Lund ash fl ows barely spilled over the ignimbrite that has a pervasive cataclastic fabric of ultracataclasite (Snoke et al., 1998, p. 8) lie topographic margin. Overlying the Lund north down to sub-phenocryst scale (Fig. 40). Clasts on the upper parts of outcrops of brecciated and south of the topographic margin, the Isom in outcrop have local, and commonly multi- Cottonwood Wash. In places, seams lie on two ignimbrite is uniformly tens of meters thick. ple, slickenside surfaces but no through-going near-orthogonal sides of the outcrop, or a thin, The topographic margin of the Indian Peak interclast shears are evident. Also included in branching subsidiary seam penetrates several caldera trends easterly for several kilometers the breccia layer are local zones of brecciated centimeters into the exposed block. Variably across the central Needle Range. Immediately Lamerdorf Tuff and andesitic lava, the latter cataclastic Wah Wah Springs and Cottonwood west of Sawtooth Peak, the margin is marked by likely derived from the post–Cottonwood Wash, Wash tuffs commonly lie on opposite sides of outcrops of brecciated Cottonwood Wash Tuff, pre–Wah Wah Springs lava dome on the topo- a seam with the former generally on top of the shown as unit Twib in Best et al. (1987b; see graphic margin of the caldera (Fig. 39). Near the outcrop. Thin laminae within the ultracata clasite also next section). A narrow exposed segment top of the layer are slabs of lithic-free, pre–cal- seams appear isotropic under cross-polarized of topographic-wall rocks, chiefl y brecciated dera collapse Wah Wah Springs; these slabs are light and some parts appear to be glass that con- Cottonwood Wash, is offset several kilometers shown in Figure 39 but some are too small to tains scattered minute grain fragments. Because to the south of the peak by a major, south-strik- appear in the cross section. No internal defor- ultracataclasite and pseudotachylyte are com- ing, basin-and-range(?) fault. The topographic mation is evident in the larger slabs. monly intimately associated and are considered margin continues for about 4 km to the east to form a continuum as extreme brittle commi- between alluvium and overlying caldera-fi lling Seams of Ultracataclasite in the Breccia Layer nution leads into frictional melting under very Lund. Reexamination of this narrow 4-km-long Cataclastic Cottonwood Wash still constitutes high strain rates (e.g., Magloughlin and Spray, segment, which is as little as ~7 km from the most of southeastern third of the breccia layer, 1992; Spray, 1995; Snoke et al., 1998), it is pos- southern terminus of the collar breccia layer at 2–2.5 km north of the inner ring fault. However, sible that the glassy seams are the quenched the inner ring fault, has revealed rocks similar to near the top of the layer, beneath the overlying product of frictional melting. However, because those in the breccia layer. caldera-fi lling Ryan Spring ignimbrites, many none of the devitrifi cation textures typically

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Figure 40. Cataclastic breccia composed entirely of fragments of Cottonwood Wash ignimbrite in the collar-zone breccia layer of the Indian Peak caldera (Fig. 39). This outcrop just south of Ryan Spring shows the matrix-supported character of the breccia and wide range of clast size. Some clasts have slickensides on one or more sides. Hammer is 30 cm long.

seen in pseudotachylyte exist, we tentatively the collar zone, only of the overlying Cotton- First of all, we speculate that a brecciating conclude that the glass represents segregated wood Wash and Lamerdorf tuffs? Why is there layer of the Cottonwood Wash tuff and thin- and fused vitroclasts from the host tuff. is so little of the pre–caldera collapse Wah Wah ner underlying Lamerdorf moved southward Springs ignimbrite in the breccia layer, given and down on a weak, enabling detachment sur- Origin of the Breccia Layer and Evolution that a thickness as much as 500 m is exposed face—such as provided by the sandstone and of the Northeastern Collar Zone immediately north of the topographic margin debris-ﬂ ow deposits that are locally exposed at Although caldera collapse was undoubt- (Fig. 29)? Why does no lithic Wah Wah Springs the top of the Marsden sequence. edly involved in the creation of the remarkably occur in the collar zone, as might be expected Further speculation regarding the setting in extensive breccia layer and its local seams of if the collar zone were downsagging to gener- which the layer originated as well as the evo- ultracataclasite near the ring fault, we are uncer- ate sufﬁ cient slope to cause extensional breakup lution of the whole collar zone follows from tain of many details regarding the mechanics of and brecciation of the Cottonwood Wash and a wide range of independent observations on their origin. It appears unlikely that the entire other rock units? Although the upper contact of calderas, subsidence into other types of evacu- layer represents a single massive landslide the breccia layer with the overlying Ryan Spring ating subsurface voids, and experimental stud- calved off the topographic-wall margin that now Formation is presently ~200 m higher near the ies using scaled analog models (e.g., Acocella lies 11 km outboard from the structural-margin inner ring fault than near the topographic mar- et al., 2012; Acocella, 2007; Branney, 1995; ring fault. As kilometers of caldera subsidence gin, this slope must have reversed since the layer Burchardt and Walter, 2010; Howard, 2010; was taking place, why was there no apparent originated, presumably as a result of resurgent Kennedy et al., 2004; Roche et al., 2000). All breakup of the Marsden or Paleozoic section in uplift of the caldera. of these studies reveal that following downsag,

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A B 2 cm WWS

WWS

Ultracataclasite Ultracataclasite

CW CW

C D

Cataclastic Wah Wah Springs WWS Clast

Shattered plagioclase

Ultracataclasite

Inj ec CW clast tion vein

Plagioclase with Cataclastic sub-parallel Cottonwood curved fractures Wash

“Ladder” micro-structure in quartz

“Ladder” micro-structure in quartz 0 25 0 25 CW clast mm mm

Figure 41. Ultracataclasite seams in brecciated ignimbrite, collar-zone breccia layer of the Indian Peak caldera. (A) Barely visible, more or less planar, sub-horizontal, dark-gray seam of ultracataclasite lies between a discontinuously exposed scab of less-cataclastic Wah Wah Springs (WWS) on top and cataclastic Cottonwood Wash (CW) below. Hammer is 30 cm long. (B) Hand sample (MIN-8-107-12) showing a 3–4-cm-thick layered seam of variably cataclastic tuff sandwiched between less-deformed Wah Wah Springs (WWS) and Cottonwood Wash (CW). A thin dark-brown ultracataclasite lamina near the bottom of the seam is broken along small thrust faults. (C, D) Photomicrograph and annotated sketch of sample MIN-8-107-3 showing a seam of ultracataclasite between cataclastic Cottonwood Wash and Wah Wah Springs ignimbrites. Darker-brown parts of ultracataclasite that appear optically isotropic are of minute, uniform-sized grain fragments. The “ladder” microstructure is manifest throughout the quartz phenocrysts (identiﬁ ed by green color) under cross-polarized light as very faint, ill-deﬁ ned, sub-parallel bands (“ladders”) that contain perpendicular zones of subtly contrasting extinction not extending into adjacent bands.

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caldera collapse is accompanied by develop- Marsden, Lamerdorf) might have impacted the Cottonwood Wash fails and brecciates in the ment of an annular collar zone bounded by evolution of the collar zone. downward-sloping hanging-wall block. Lamer- an inner, bell-shaped reverse ring fault and a Figure 43A represents the situation before dorf Tuff overlying the hypothetical detach- complementary curved outer, normal ring fault; collapse. The thickness of the Cottonwood ment layer at the top of the Marsden also slides together these two faults defi ne a downward- Wash Tuff is shown to increase somewhat downward and brecciates, as does Escalante tapering, triangular annulus (Fig. 42) whose southward, consistent with a postulated source Desert rhyolite lava (see Fig. 39). All of these apex meets the margin of the erupting magma to the southwest where it was engulfed into the brecciating units slide into the accumulating, chamber. In plan view, these bounding faults Indian Peak or overlapping White Rock caldera. thickening pile of intracaldera ejecta—forming, form a set of segmented arcuate faults. For A further and intuitively reasonable assump- in part, mappable lenses—and contribute to the model experiments in which chambers have a tion is that surface tumescence resulting from lithic component of it. In the deeper levels of horizontal diameter much larger than thickness, growth of the Wah Wah Springs magma cham- the breccia layer, suffi cient load exists to create Roche et al. (2000) found that the initial subsid- ber in the shallow crust caused the pre–caldera intense cataclasis at the base of sliding blocks. ence involves downsag, which continues with collapse ignimbrite to pinch out southward; the Boiling magma continues to ascend along the displacement on the normal and reverse faults, fl at surface in the section refl ects initial down inner reverse ring fault; we speculate it might whose dips at the surface are as little as 50° fl exure after it erupted. have propped up the Paleozoic- and Marsden- but steepen downward. Initially, a reverse fault Figure 43B shows a small increment of bounded margin of the hanging-wall block so develops at the margin of the chamber followed downsag accompanied initial subsidence of the it did not fail and fragment. Magma must, now, by a normal fault on the outside, or a normal caldera on the inner reverse ring fault shown erupt through the thickening accumulating pile fault followed on the outside by a reverse fault. dashed in A. The inner reverse and outer nor- of intracaldera ejecta, thereby consuming energy In all cases, subsidence is asymmetric, with the mal ring faults converge to the margin of the and constraining the lithic ash fl ows within the maximum on the side of the fi rst reverse fault. erupting magma chamber, shown at a depth of topographic margin of the caldera. Incremental As subsidence increases, (1) successive reverse more than 7 km, as indicated by the pressure failure and brecciation of the Cottonwood Wash ring faults develop stepwise outward into the of equilibration of the phase assemblage in the layer toward the topographic margin of the cal- annulus as (2) extension also progresses out- Wah Wah Springs ignimbrite (Wolff, 2008). As dera is accomplished by activation of succes- ward in the annulus, (3) marker layers rotate a result of the subsidence of the footwall cal- sive reverse faults stepping in that direction that down and into the depression, and (4) blocks dera block, the unstable edge of the layer of results in concomitant extension of the collar- break away and slide into the depression. Cottonwood Wash Tuff fails and brecciates; zone, hanging-wall block. Additional faults Roche et al. (2000, p. 410) speculated that the the potential void along the reverse ring fault is stepping farther to the north-northwest that are topographic margins of many large calderas fi lled with boiling magma that erupts through activated in the next cross section are dashed. coincide with the outer limit of extension. the brecciating mass, entraining fragments into Figure 43D shows that a total of 4500 m of The evolution of the collar zone of the Indian the ejecta. A major proportion of the lithic clasts subsidence has occurred as the eruptive activ- Peak caldera is shown schematically in the ideal- in the caldera-collapse tuff north of Indian Peak ity ends. The last ~2000 m of movement takes ized cross sections of Figure 43 that are essen- are of aphyric lava of the Escalante Desert type place on faults in the northern half of the collar tially coincident with the plane of the section in (Fig. 34), indicating venting of boiling magma zone, mostly on a normal fault located ~3 km Figure 39B. Our speculative model is designed through a large mass of this rock at depth. south of the topographic margin and labeled the to be consistent with the fi ndings described in Figure 43C indicates that 2000 m of further outer ring fault in Figure 39. Paleozoic sedi- the previous paragraph and with our observa- subsidence of the caldera has occurred, sche- mentary strata have dropped down to the south tions on the Indian Peak caldera. We do not matically, by additional downsag but mostly apparently by ~2600 m, based on projection of know how the engulfed older source calderas by movement on the two reverse faults located a marker unit across the fault. The strata have of certainly the Cottonwood Wash and possibly to the north-northwest of the inner ring fault, also been rotated almost 50° from an easterly of earlier rhyolite ignimbrites (Sawtooth Peak, which are dashed in B. Part of the layer of the strike and north dip of ~30° to a northwesterly

Figure 42. Fault-bounded, tri- Normal ring fault angular annulus that develops A B Normal in the collar zone of subsiding Reverse ring fault calderas as magma is with- ring faults drawn from the underlying 1 2 3 magma chamber. (A) Sche- matic perspective view. The inner reverse ring fault is shaded. Dips of both reverse and normal faults at the surface range from 50° to 85° but steepen downward. (B) Cross section through the right-hand Reverse ring fault side of the annulus showing that as subsidence progresses additional reverse faults (1, 2, 3) develop sequentially step-wise toward the normal fault within the annulus as it experiences radial extension. Sagged marker layer is downdropped and stretched within the extending annulus. Modiﬁ ed from Roche et al. (2000, their ﬁ gures 10a and 9c).

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A Andesitic strike and east dip of ~60°. (Compensation has NNWlava SSE w w ccbeen made in these measurements for the east m m c tilt of the range by post-volcanic basin-and- Pz Pz range faulting.) About 600 m of displacement on this fault occurs as the sequence of Mars- B den ignimbrites, debris ﬂ ows, and breccias NNW SSE of Paleozoic rocks, described above, accu- w c m wl mulates south of the fault, and 200 m of dis- Pz c Pz Pz m e Pz placement occurs after the caldera-ﬁ lling Ryan s

r s t e l Pz Pre-collapse Hypothetical normal v u Spring and Lund ignimbrites are emplaced. e a w r f Potential void Wah Wah e g r This leaves possibly ~700 m of throw dur- n

u i r Springs tuff ring fault t

F ing subsidence of the Indian Peak caldera and Cottonwood c Wah Wah Springs Wash Tuff during which time an equivalent thickness of Collapse breccia Marsden and caldera breccia is deposited. Further brec- m (schematic) Lamerdorf Tuffs ciation of the Cottonwood Wash layer occurs Paleozoic Lithic intracaldera back to the andesitic lava dome, the south part Pz Magma wl rocks tuff of which also collapses and brecciates. Slabs C of the tapered wedge of pre–caldera collapse NNW Wah Wah Springs ignimbrite slide downward SSE w c on the brecciating Cottonwood Wash. More m Pz magma invades along the inner reverse ring Pz wl Future m fault and already initiated resurgent uplift of Pz m Pz c the collar zone block. Note that no lithic Wah Pz ? faults Pz m Wah Springs ash fl ows advance beyond the Pz midpoint of the collar zone. Figure 43E (cf. Fig. 39B) indicates still more D NNW magma intrudes beneath the southern half of the collar zone, causing resurgent uplift by reversal w w w SSE w of displacement along the three inner ring faults m Pz Pz m ornamented with half arrows. Paleomagnetic m wl data discussed above are consistent with resur- Pz m 2 Pz gence before the intracaldera tuff cooled below its Curie temperature. The magma beneath the km Magma c resurgent dome solidifi es as granodiorite por- 0 ? m phyry. Note that hypothetical, lithic caldera- 0 2 Pz collapse Wah Wah Springs ignimbrite has been Future Indian eroded off the resurgently uplifted southern Peak SSE half of the collar zone, in agreement with its E observed absence there. As much as 1100 m NNW m wl w r of post-collapse, caldera-fi lling Ryan Spring l+r l+r Pz m and Lund ignimbrites accumulate in the caldera Pz m m Pz moat between the topographic margin and resur- Pz Pz gent intracaldera pile. Pz ? Caveat. Our proposed evolution of the north- Lund g l eastern segment of the Indian Peak caldera col- tuff c m lar zone with its unusually broad breccia layer Ryan Spring r ? satisfi es the constraints of the observed geology tuff as well as observations on naturally occurring, Wah Wah Springs ? Pz g granodiorite smaller-scale subsidence overlying voids and porphyry on experimental studies. However, we note that our proposed evolution for the Indian Figure 43. (A–D) Idealized cross sections showing the catastrophic evolution of the northeast- Peak caldera fi nds no support in the Caetano ern collar zone sector of the Indian Peak caldera and the origin of the breccia layer. Sections and Stillwater calderas in the Western Nevada strike from 38°15′ N latitude toward the north-northwest along the spine of the Needle Range fi eld (Henry and John, 2013), from which and are positioned at the north-northwest end of Figure 39A and B. The plane of the cross sec- more than 1000 km3 of ejecta were vented. tions is not perpendicular to all of the collar-zone faults and movement along them as well as Post-collapse tilting that has exposed 4000– the movement of landslides and brecciating masses may be out of the plane. (E) Generalized 5000-m-thick cross sections through the cal- present-day cross section showing deeper interpreted geology than in Figure 39B. Horizontal deras reveals subvertical to steeply inward- and vertical scales (shown in D) are the same in all sections (A–E). See text for explanation. dipping master ring faults and topographic Note that these cross sections correspond to the left side of the collar annulus in Figure 42. margins less than 1 km outboard of the ring

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fault. The contrast between these calderas and structural block and subsequent erosion prior to tain quadrangle (Keith et al., 1994; Figs. 8, 29, our interpretation of the northeastern collar deposition. The post-caldera, 500-m-thick Ryan and 44), no wall-collapse breccia is exposed, but zone of the Indian Peak is striking. Spring tuff is draped across the northern end less than 250 m of lithic Wah Wah Springs over- of the ring fault and the rocks to the north and lies older volcanic rocks deposited on Cambrian Indian Peak Range Segment south. However, this unit pinches out 7 km to the limestone. Drastic changes in thickness of the north and for several kilometers to the south it is lithic tuff across a fault indicate displacement Southward of Indian Peak for ~9 km (Figs. 8 <50 m thick and generally absent. on the fault after its deposition followed by ero- and 39B) are intrusive masses, likely connected sion before the younger Ryan Spring ignimbrite at depth, of granodiorite porphyry (Grant, Segment in the Southwestern Wah Wah was emplaced. These relations are interpreted to 1979; Best et al., 1987a). Although propyliti- Mountains and West to the State Line have resulted from resurgent uplift in the Indian cally altered, this porphyry has the distinct Wah Peak caldera. Wah Springs modal and chemical composition, including relatively high Cr. For another 9 km On the west side of the southern Wah Wah Nevada Segment southward of the largest intrusion and exposed Mountains in hills at the south end of Pine Valley, over ~5 km2, the sequence of lithic tuff and Best et al. (1987d) found no wall-collapse brec- minor wall-collapse breccia is tilted to the south cias but mapped ~1 km2 of lithic tuff (unit Twi Ursine Area from 20° to 65° over the intrusion. on their geologic map) like that which occurs At the south end of the Wilson Creek Range, At Arrowhead Pass (Best et al., 1987a), an within the Indian Peak caldera in the Indian over a distance of ~6 km northeast of Ursine, ignimbrite-breccia sequence ~4000 m thick to Peak Range to the northwest; the tuff is at least reconnaissance mapping discloses a stratigraphic the north is juxtaposed across an easterly strik- 500 m thick and mostly is in fault contact with section that consists of Cambrian limestone over- ing fault against a section of Paleozoic sedimen- Lund and Mackleprang ignimbrites. Reexami- lain by a few hundred meters of Escalante Desert tary rocks. We interpret this pre–Ryan Spring nation of variably altered exposures in low hills andesite lavas and Cottonwood Wash and Wah fault to be a ring fault that marks the southern to the southeast, which were designated as out- Wah Springs ignimbrites; this section is typical structural margin of the caldera. Overlying the fl ow, or pre–caldera collapse, Wah Wah Springs of those outside the Indian Peak caldera margin. Paleozoic section is a south-dipping sequence (unit Two) on the geologic map, reveals that However, to the west of the major Meadow Val- no more than 200 m thick of the Sawtooth Peak they actually contain a few percent of clasts to ley Wash through Ursine, this pre-Lund volcanic ignimbrite overlain by less than 400 m of lithic as much as 0.3 m across of aphyric red volcanic section is entirely absent and the Cambrian strata Wah Wah Springs followed by ignimbrites and rock and should, therefore, also be considered are overlain by roughly 500 m, or more, of Lund lava fl ows of the caldera-fi lling Ryan Spring as caldera-collapse Wah Wah Springs. The unit tuff that has local black vitrophyre near its base. Formation. The relatively thin, lithic Wah Wah pinches out southward on a paleohill of Meso- The underlying limestone is variably bleached Springs continues southward for several kilome- zoic and Paleozoic rocks (Fig. 8). We interpret and locally intensely silicifi ed for several meters ters before disappearing beneath a 1500-m-thick this area of lithic Wah Wah Springs to lie within below the contact. We believe this area north- section of Lund and Isom ignimbrites on the the southeastern collar zone of the Indian Peak west of Ursine lies in the Indian Peak caldera margin of the younger White Rock caldera (see caldera between the ring fault and an obscure collar zone and the missing pre-Lund volcanic below). No wall-collapse breccias associated topographic margin to the east and south. The units became detached from the sequence to the with the subsidence of the Indian Peak caldera relatively large areal extent of the southeastern northeast and slid to the north into the caldera are exposed in this southern collar zone of the collar zone sector of the caldera is related in part before deposition of the Lund; downsag in this caldera like those in the northern collar zone. to post-volcanic extension. part of the collar zone could have facilitated the The topographic margin of the Indian Peak South of the Indian Peak Range, the position slippage. caldera must lie farther south, buried beneath of the caldera margin lacks geologic constraints younger deposits. That the Wah Wah Springs because of burial beneath younger deposits, Window Area in the Southwestern ignimbrite south of the ring fault at Arrowhead but gravity data (Fig. 8C) indicate no apparent Wilson Creek Range Pass is not pre-collapse tuff but caldera-collapse extension into the Escalante Desert. West of the Five to ten kilometers to the northwest of tuff is indicated by the presence of lithic clasts desert, 25–20 Ma lava fl ows further conceal Ursine on the western fl ank of the southern and especially by the occurrence within it in the caldera margin. Wilson Creek Range, more than 260 m of non- the western foothills of the range of a one-half In the southern White Rock Mountains along lithic, pre-collapse Wah Wah Springs is exposed. square kilometer exposure of altered and brecci- the Utah-Nevada state line, in the Rice Moun- The southwestern caldera margin likely passes ated Paleozoic rocks of unknown stratigraphic identity (Best et al., 1987a). Drilling has shown that these Paleozoic rocks are underlain by altered volcanic rocks (Gerald Park, consulting Figure 44 (on following page). Geologic map of an area in the southern White Rock Moun- geologist, Salt Lake City, Utah, 1992, personal tains that includes most of the Rice Mountain quadrangle (Keith et al., 1994) and the south- commun.) and are, therefore, not an in situ ernmost part of the map by Best et al. (1989d). See text for discussion and also Figure 8A caldera fl oor but a slide mass. for location. A resurgently and recurrently uplifted segment of both the White Rock and Relations near Arrowhead Pass (1) as detailed Indian Peak calderas is bounded on the north by a high-angle, easterly striking normal fault above, indicate that the Indian Peak caldera inside (near 38°7′30″ N, truncated on the east by a north-striking basin-and-range fault) and on its southeastern margin did not subside nearly as the south by thick Ripgut Formation tuff and overlying lower Miocene lavas and tuffs (Ty). much as in its northeastern sector, that is, caldera Frame on the left is an enlargement of the dashed-rectangle area in the main map. Ball and subsidence was asymmetric; and (2) are con- bar is on the downdropped block of a high-angle fault. Strike-and-dip symbols for compac- sistent with resurgent uplift of an intracaldera tion foliation in ignimbrite have fi lled triangles.

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TERTIARY 114° 6′ 114° 1′ Ty Younger volcanic rocks 20 Tl Ty Tl Ti Ti

Ti Isom tuff Ty Tg Ty Tl Tg Ripgut tuff Tl Ty Ty Tl Lund tuff Quaternary

Ty Ryan Spring tuff Tr 17

Lithic intracaldera Twl 38° Wah Wah Springs tuff Tl 7′ 30″ 10 Tg Tc Cottonwood Wash tuff Ti 12 C Escalante Desert Tea Tl andesitic lava

Tsp Sawtooth Peak tuff Ty Quaternary 33 CAMBRIAN Tg C Sedimentary rocks Tl

24 Ti Ti 10 20 Tc Tc 38° Tsp 28 5′ Ty Tea Tg Twl 0″ Tea Tc 29 Q C Tl

Twl Tc Tea Twl Tg 23 5 Tc 70 Tc Ty Tg 20 Twl 30 Tsp Ty

Tr 31 8 65 9 Tea Ty Ty Tl 38° Tea ′ 28 2 Tg 30″ Tc 23 Tg Twl Tg 30

Ty 27 18 15 Ty

0 0.5 1 Tg

kilometer 012

0 0.5 Ty 10 kilometers 0 1 2 mile 38° 0′ miles

Figure 44.

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between this area and the lithic Wah Wah Springs exposed in the Rice Mountain quadrangle (Fig. 44). The caldera margin must also lie between this area of exposure and one farther to the northwest, ~17 km northwest of Ursine, where reconnaissance mapping reveals a 5 km2 window of generally poorly exposed volcanic rocks amongst alluvium and surrounded on vir- Lithic tually all sides by early Miocene rhyolite lavas Wah and tuffs. On the west side of the window, appar- Wah Springs tuff ently in situ, mostly unaltered Paleozoic carbonate rocks are overlain to the east by Cotton wood Wash Tuff. Still farther east is a northerly trend- Non-lithic ing, 100-m-long by tens of meters wide train of Wah W boulders of white Eureka Quartzite of Ordovi- ah Springs tuff cian age and minor gray carbonate rocks lying Lithic W in alluvium. The nearest exposures today of the ah W quartzite lie 45 km to the north, at the north end ah Springs tuff of the range at Atlanta (Willis et al., 1987). It is unlikely the boulders are a part of the alluvial debris, but instead represent erosionally resistant fragments of wall breccia shed off the caldera margin from the west that are imbedded in weathered unexposed caldera-collapse ignimbrite; such wall breccia is well exposed 2 km Figure 45. Outcrop of a slab of non-lithic Wah Wah Springs ignimbrite encased in lithic Wah southeast of Atlanta, as described below. Beyond Wah Springs at 38°5.8′ N, 114°19.8′ W on the southwest fl ank of the Wilson Creek Range. this train of apparent wall-breccia boulders are This outcrop is interpreted to be wall-collapse breccia near the southwestern margin of the exposures of non-lithic Wah Wah Springs tuff Indian Peak caldera. Note 36-cm-long hammer for scale in the lower left of the outcrop. with randomly oriented compaction foliation that appear to represent chaotic blocks of a megabreccia. In one outcrop, a slab ~0.5 m thick of non-lithic Wah Wah Springs is embedded in 1998). This erroneous conclusion was based Wah Wah Springs, or with the altered tuff occur- lithic Wah Wah Springs (Fig. 45). Extending on a correlation of a local plagioclase-pyrox- ing as thin septa within the brecciated rock. for ~2 km east of this intracaldera assemblage ene tuff with the caldera-collapse Wah Wah These exposures represent the broken caldera is a moderately north-dipping section of Lund Springs. However, the local tuff has a different wall that was invaded by or fell into the Wah ignimbrite that appears to be a few hundred composition and paleomagnetic direction than Wah Springs ejecta. About 9 km south-southeast meters thick and disappears to the east beneath the Wah Wah Springs and our age on sample of the Atlanta mine, and farther into the caldera, fl at-lying Isom ignimbrite and overlying rhyo- GRASSY-1-213-2 of 33.94 ± 0.18 Ma is much a slab more than 1 km in diameter of broken litic tuff and lavas. Locally, at the unconform- too old. Silurian dolomite (unit Twbd in Fig. 46A) over- able contact between the Lund and the Isom, Geologic relations in the northern Wilson lies slightly altered caldera-collapse lithic Wah is another outcrop area of only a few hundred Creek Range (Willis et al., 1987) further constrain Wah Springs. Nearby, smaller remnant slabs of meters square of Wah Wah Springs megabrec- the position of the Indian Peak caldera margin. brecciated Ordovician Eureka Quartzite rest on cia that here is a heterogeneous assortment of West of the Atlanta mine, the caldera margin is tuff that is more than 2100 m thick. lithologically contrasting megablocks of igne- apparently eclipsed by the younger White Rock For several kilometers to the southeast, the ous rocks of unknown stratigraphic identity in a caldera (see below). Isolated exposures of altered Indian Peak caldera margin is obscured by lithic Wah Wah Springs matrix. ignimbrite—apparently both Wah Wah Springs younger volcanic rocks, but critical relations Considering the position of this window of and Lund—lie south of a kilometer-square area reappear in the northern White Rock Mountains older rocks in the central part of the eclipsing of rhyolite lava that is about the same age as the (Fig. 46B). Here, in the “moat” between the younger White Rock caldera (see below), we formation of The Gouge Eye to the northwest topographic margin and the resurgent uplifted interpret the entire window as an exposure of in the Fortifi cation Range. This rhyolite lava interior of the caldera, upwards of 700 m of post- the fl oor of the resurgent uplift of this caldera was erroneously assigned to the younger 30 Ma collapse, caldera-fi lling Ryan Spring and Lund that reveals a segment of the margin of the older Ryan Spring Formation (Willis et al., 1987) but ignimbrites covers the structural margin ring Indian Peak caldera. fi ssion-track dating of zircon yields signifi cantly fault. The thickness of the resurgently tilted lithic older ages of 34.8 ± 3.2 Ma (sample ATL-1-70-3) Wah Wah Springs tuff appears to be on the order Fairview and Northern Wilson Creek Ranges and 35.6 ± 3.3 Ma (HOR-1-70-4) (Kowallis and of 5000 m; although this extraordinary thickness West of the northern Wilson Creek Range Best, 1990). might be the result of repetition by undetected and across Lake Valley in the northern Fair- About 2 km southeast of the Atlanta mine, faults, its base is not exposed, and hence these view Range (Figs. 8 and 29), a westernmost some exposures consist of breccia of silicifi ed two factors may counterbalance. No wall-col- segment of the Indian Peak caldera is no longer Ordovician and Silurian sedimentary rock clasts lapse breccias occur in this intracaldera section believed to be present, as once thought (Best et al., in a matrix of intensely argillized, quartz-free that is ~10 km from the structural margin.

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A NORTH INDIAN PEAK CALDERA SOUTH Feet TOPOGRAPHIC Meters 8000 MARGIN Twbq 2500 Twbq Twbd Tr Twbd 2000 6000 SOd Twit Twit Oq Ordovician Hypothetical 1500 limestone Cambrian rocks ring fault 4000 1000 TrTwit Twbd Twbq SOd Oq One mile

Ryan Spring Intracaldera Breccia of Breccia of Silurian- Ordovician One km Formation tuff dolomite quartzite Ordovician Eureka tuffs WAH WAH SPRINGS FORMATION dolomite Quartzite

NORTH INDIAN PEAK CALDERA WHITE ROCK CALDERA SOUTH Feet White Rock Meters 10000 TOPOGRAPHIC Peak Tp 3000 TOPOGRAPHIC MOAT RESURGENT UPLIFT MARGIN 8000 MARGIN Hamlin Valley Tp Tlt Tg 2000 6000 ? Tlt Tlp Two Tlt Tr 4000 Twit F Tc RI 1000 F R AUL

A I N 2000 N U PALEOZOIC G ROCKS G L T

T 0 0 0 miles 2

Tc Twit Two Tr Tlt Tlp Tg Tp 0 kilometers 2 Cottonwood Wah Wah Springs Formation Ryan Spring Lund Formation tuff (Tlt) Ripgut Post-Ripgut tuffs Wash Tuff intracaldera (Twit) and Formation and intrusive porphyry (Tlp) Formation outflow (Two) tuffs

Figure 46. Generalized north-south cross sections of the northern margin of the Indian Peak caldera. No vertical exaggeration. See also Figure 8. (A) From Atlanta south-southeastward to 38°22.5′ N, 114°15′ W (Willis et al., 1987). Modifi ed from Best et al. (1989b, their fi gure R38). (B) Hamlin Valley into the northern White Rock Mountains (Best et al., 1989d). Modifi ed from Best et al. (1989b, their fi gure R33).

The northern margin of the Indian Peak cal- 2000 km2 prior to extension. The position of the north of Ursine and then swings northwest up dera trends east-southeast across Hamlin Valley deeply subsided, structural caldera inside the Lake Valley. into the central Needle Range. inner reverse ring fault, whose area was one- Because of the large dimensions of the Wah half that within the topographic margin, is well Wah Springs ignimbrite and its Indian Peak Dimensions of the Indian Peak constrained in the Indian Peak Range, but else- caldera source, and with the uncertainties in the Caldera and Volume of the Wah Wah where is less certain. In the north, the ring fault margins of the latter, we employ four models Springs Ignimbrite may lie close to the exposed topographic wall, to estimate the volume of the ash-fl ow deposit and is shown coincident in Figure 29 just north (Fig. 4 and Tables 2 and 7). Our intent is to fi nd As drawn in Figures 8 and 29 from the of the intracaldera section southeast of Atlanta some agreement amongst the different model evidence cited in the preceding paragraphs, that is more than 2100 m thick. Southwestward estimates because suffi cient data exist to make the Indian Peak caldera spans four moun- into the Wilson Creek Range, the position of valid comparisons. tain ranges. The topographic margin extends the structural margin is drawn conservatively; In Model 1, the volume of the contoured pre– ~40 km north-south and the area within it was the topographic margin is believed to lie just caldera collapse ignimbrite is 3000 km3, using

TABLE 7. VOLUMES OF THE WAH WAH SPRINGS AND LUND IGNIMBRITE UNITS ESTIMATED BY DIFFERENT METHODS Structural Topographic Ignimbrite Ignimbrite caldera caldera unit area volume Ignimbrite (Caldera) km2 km2 km2 km3 Model Notes Lund (White Rock) 12,000 4,600 1 Pre-collapse tuff doubled; thickest 500 m 12,000 5,400 2 Contoured unit with 2.5 km in structural caldera 12,000 4,600 3 Contoured unit with 2.0 km maximum in asymmetric structural caldera Lund magma 4,100 4 Structural caldera and 2.9 km subsidence minus intracaldera breccia Preferred values 1,500 2,000 12,000 4,400 Average of Models 1, 3, and 4

Wah Wah Springs (Indian Peak) 32,000 6,000 1 Pre-collapse tuff doubled; thickest 400 m 32,000 6,200 2 Contoured unit with 3.65 km in structural caldera 32,000 5,700 3 Contoured unit with 4.0 km maximum asymmetric structural caldera Wah Wah Springs magma 5,900 4 Structural caldera with 6.1 km subsidence minus intracaldera breccia Preferred values 1,000 2,000 32,000 5,900 Average of Models 1, 2, 3, and 4 Note: Refer to Figure 4 for description of different models for calculating unit volume.

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a 400 m maximum thickness across the future of a uniform plate, or piston, multiplied by the amount of exposed sediment seems problem- caldera. The volume of the pre-collapse ignim- structural caldera area yields the erupted magma atic. Breaching of the topographic margin of brite within the topographic margin of the future volume. For the subsidence in the Indian Peak the caldera may have allowed eroded rock to caldera is 800 km3 because the area is 2000 km2 caldera, we use the maximum thicknesses of the be fl ushed out of the depression, but no signifi - (Table 7), making the volume of the outfl ow intracaldera tuff, 5000 m, and for the “unfi lled cant sediment is known between the Wah Wah beyond the margin 2200 km3. These fi gures collar height” of Lipman (1997), the 1100 m Springs and younger tuffs beyond the caldera. assume no tumescence, or doming, of the land thickness of the post-collapse, caldera-fi lling The location of the caldera on the Great Basin surface prior to eruption of the Wah Wah Springs Ryan Spring and Lund tuffs deposited in the col- altiplano (Best et al., 2009, p. 617) where little ash fl ows resulting from growth of the underlying lar zone in the central Needle Range. Southwest precipitation apparently occurred might account shallow crustal magma chamber, as was assumed of Atlanta, no Lund is exposed but the Ryan for the sparse erosional sediment; such is the in Figure 43A. We note that there is no basis in Spring is 1300 m thick (Willis et al., 1987). The case on the modern central Andean plateau. fact for this intuitively reasonable assumption of Model 4 calculation yields 6100 km3 for the vol- surface tumescence. In his review of Tertiary cal- ume of the erupted Wah Wah Springs magma. SILVER KING TUFF deras in western North America, Lipman (1984, Because the intracaldera pile includes wall-col- p. 8813) concluded that evidence for infl ation “is lapse breccias as well as ignimbrite, the actual The 29.40 ± 0.06 Ma Silver King Tuff lies elusive.” In the well exposed, essentially three- thickness of the ignimbrite is some lesser value stratigraphically above the rhyolite tuff of Dead- dimensional exposures of the Caetano caldera in than 5000 m. Using the formulation of Lipman man Spring (Table 1) in the Fairview Range in the Western Nevada fi eld (Henry and John, 2013) (1997, his appendix 1) for the collar-rock breccia the northwestern Indian Peak–Caliente fi eld there is no direct evidence for pre-collapse dom- volume (Cv) collapsed into an equivalent circu- (Fig. 47; Best et al., 1998). The Silver King ing. If, say, 100 m of tumescence did occur over lar structural caldera from an equivalent circular was previously designated as an informal strati- the caldera area with negligible downsag, then topographic caldera, we calculate ~200 km3, or graphic unit (Maughan et al., 2002), but here we the 3000 km3 volume would be reduced by less ~3% of the magma volume, reducing the erupted make it a formal unit. than 100 km3. An additional uncertainty in these magma volume to 5900 km3. As with other phenocryst-rich dacitic tuffs in calculations is the unknown thick volume of Wah All four models yield similar volumes, rang- the southern Great Basin ignimbrite province, Wah Springs ignimbrite fi lling the older engulfed ing from 5700 to 6200 km3 (Table 7). These the Silver King contains abundant plagioclase Cottonwood Wash caldera. In any case, doubling two extreme values are for asymmetric and uni- and lesser hornblende, biotite, quartz, sanidine, the volume of the pre–caldera collapse ignim- form caldera collapse models, 3 and 2, respec- and magnetite (Fig. 48), and trace amounts of brite, according to the concept of Lipman (1984), tively. From the foregoing discussion of the ilmenite, apatite, and zircon. A small amount yields a total volume for the Wah Wah Springs Indian Peak caldera, there is little doubt that it (<2%) of titanite is characteristic of this ignim- ignimbrite of 6000 km3. collapsed asymmetrically, so the 6200 km3 vol- brite. Hornblende is distinctly less abundant In Model 2, the volume of the contoured tuff ume is un realisti cally high. Interestingly, the than biotite (ratio <0.47 in Fig. 26B). Chemi- beyond the structural caldera is added to the vol- 6000 km3 volume derived from the doubling cally, the average Silver King is more evolved ume of the intracaldera tuff within it, assuming assumption is not signifi cantly different from than most samples of monotonous intermediates an average thickness of 3650 m and a simple volumes found by the other models. The aver- (Figs. 5, 6, 27A, 30A, and 49). A more signifi - piston geometry; this estimate yields a total of age of the four models is 5900 km3, which we cant difference from the three super–monoto- 6200 km3 for the entire unit. take as our preferred total volume for the Wah nous intermediates in the Indian Peak fi eld is The volume estimate for Model 3 is similar Wah Springs ignimbrite (Table 2). This preferred the modest areal extent and order-of-magnitude but assumes a more realistic asymmetric cal- volume of 5900 km3 equals the estimated vol- lesser volume of the Silver King, estimated by dera in which the intracaldera tuff ranges from ume of the erupted magma, which would argue Model 1 in Figure 4 to be ~350 km3 (Table 1). a thickness contour of 1000 m coincident with that not accounting for the wall-collapse brec- No direct evidence of a source caldera for the southern structural margin to a maximum cia in Models 2 and 3 is counterbalanced by the the Silver King ignimbrite has been found. It is contour of 4000 m inside the caldera (Fig. 29); ignored fallout-ash volume. debatable whether the considerable thickness of this estimate yields a total of 5700 km3 for the the unit, as much as ~1400 m, within the older entire unit. Dearth of Sediment Inside the Kixmiller caldera source of the tuff of Deadman In Models 2 and 3, we have not compensated Indian Peak Caldera Spring in the Fairview Range represents post- for the volume of wall-collapse breccia within collapse caldera fi ll or is a syn-collapse deposit the intracaldera tuff. But the fallout ash depos- A puzzling aspect of the Indian Peak caldera within a Silver King caldera nested in the older ited beyond the outfl ow sheet to as far as western is the apparent dearth of eroded rock debris Kixmiller caldera. Widespread areas in the Nebraska which can easily amount to hundreds shed off the topographic wall and resurgent southwestern Fairview Range of brecciated of cubic kilometers might counter balance the uplift into the depression. Despite the fact that and cataclastically deformed Silver King tuff, subtracted wall-collapse breccia. the caldera-fi lling Ryan Spring and Lund tuffs which were previously interpreted to be related Model 4 in Figure 4 is an entirely different are as much as 1100 m thick in the Indian Peak to the collapse of the younger source caldera approach that avoids some of the uncertainties Range segment, the total thickness of observed of the Lund tuff (Best et al., 1998), could have of the previous estimates; it was employed by sediment is generally no more than a few tens taken place during or shortly after emplacement Lipman (1997) to determine the 5000 km3 vol- of meters. The sediment is typically crudely of the Silver King at its source. ume of magma erupted from the La Garita cal- bedded, but well sorted, sandstone that occurs The loosely welded ash-fl ow tuff underlying dera represented in the Fish Canyon Tuff in the between cooling units of the Ryan Spring For- the Lund in the northern White River Narrows Southern Rocky Mountain volcanic fi eld. This mation and locally overlying this unit and below is not the Silver King; its paleomagnetic direc- volume estimate is based on a simplifi ed geom- the Lund. Although the volume of tuff eroded tion (sample 9P058, S. Gromme and M. Hudson , etry in which the assumed symmetric subsidence off the resurgent uplift is unknown, this small 2006, personal commun.) is signifi cantly dif-

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as well as small lava domes and possible shallow Silver King Tuff 0 intrusions along the caldera ring fracture. ! (29.40 Ma) 0 The Lund ignimbrite (Maughan et al., 2002) ! 0 Figure 47. Distribution and 38°30′ E ! is petrographically and compositionally similar 0 thickness (in meters) of the ! to the other monotonous intermediates but can Silver King Tuff. Ash flows usually be distinguished in the fi eld by the pres- 2 170 accumulated to thickness of ! ! 65 ence of trace amounts of titanite phenocrysts; 10–30 and 75 m to the east of a 50 ! however, as this phase is commonly obliterated ! 200~1400 Older pile of andesitic lavas in the ! by even slight alteration, other criteria must be >180 Kixmiller northern North Pahroc Range. ! caldera 0 employed to distinguish the unit. These criteria 100 ! Location of the source caldera include: ! 10-30 is unknown but possibly lies 38° E 1. Typically smaller mafi c phenocrysts, gen- 0 0 within the older Kixmiller cal- ! erally no more than 2 mm, in contrast to larger 0 ! dera that was partly eclipsed ! ! 75 0 mafi c phenocrysts in the other two monotonous by the younger White Rock cal- ! intermediates, especially the biotites in the 0 0102030 0 dera (see Figs. 8 and 51). Km ! Cotton wood Wash and the more conspicuous 01020 Miles abundant hornblendes in the Wah Wah Springs (Figs. 26 B, 32, and 50). ′ ′ 115°30 114°30 2. Generally more quartz and presence of sanidine in most samples. Sanidine has not been 70 Silver King Tuff n = 16 verifi ed in the Cottonwood Wash and Wah Wah 60 plus < 2 % titanite Springs ignimbrites. Although petrographically and modally simi- 50 lar to the slightly older Silver King Tuff, the hornblende/biotite ratio of the Lund (>0.6) exceeds 40 Figure 48. Modal proportions of that of the Silver King (<0.5) (Fig. 26B). The Sr phenocrysts in the Silver King isotopic composition of the Lund is also slightly 30 Tuff. lower than that of the Silver King (0.7092 ver- Mode (vol %) 20 sus 0.7099; Table 4). Paleomagnetic analysis shows the Silver King to be reversely magnetized 10 instead of normal as for the Lund. These two cooling units are in direct contact in the Fairview 0 Plag Qtz San Biot Hb Px Opaq Pheno Range and nearby locales, and in other places a thin andesitic lava separates them. Samples of the Lund ignimbrite range across 9 the dacite fi eld and slightly into the rhyolite Rhyolite fi eld; SiO2 ranges from ~64 to 71 wt% (Figs. 5, 6, and 49). Relative to the Cottonwood Wash 8 Trachydacite and Wah Wah Springs ignimbrites, the Lund X Latite L has more TiO2, Sr, Ba, and Zr at a given silica L L L LL L L content. The Lund ranges to lower silica con- 7 L L KK L HH H L L L L K K H H L LL LL L LX LL K tents than the Silver King and has distinctly O (wt %) L LLL L L KK Figure 49. Total alkalies-silica 2 LL K L L L LKKLKK higher concentrations of P2O5, Fe2O3, Sc, and L LL L K L diagram for Silver King, Lund, LL X K O + K 6 L LL X L L KK V at similar TiO2 (Maughan, 1996). Because and Harmony Hills ignimbrites. 2 K K Na K some element variation diagrams show separate LL H Harmony Hills Tuff n = 6 but sub-parallel trends for the Lund and Silver Andesite Dacite 5 Lund Formation King tuffs, it is unlikely that the Lund magma X Ring-fault lava n = 4 L Tuff n = 55 evolved from the same system as the 0.2-m.y.- K Silver King Tuff n = 19 older Silver King. 4 The relatively large variation in the propor- 59 61 63 65 67 69 71 73 75 77 tions and compositions of phenocrysts in the SiO2 (wt %) Lund (Fig. 50), as well as the order-of-magnitude variation in some of their ratios, refl ect ferent and the modal composition also differs. LUND FORMATION compositional heterogeneity in the pre-eruption Instead, the phenocryst assemblage is similar to magma chamber (Maughan et al., 2002, p. 139); that of the lava fl ows erupted from the 29–27 Ma Ignimbrite of this formation resulted from the increase in sanidine and quartz relative to Seaman volcanic center (duBray, 1993) ~13 km the youngest, at 29.20 ± 0.08 Ma, of the three declining mafi c phases is that expected in a to the west. This pre-Lund ignimbrite may rep- super-eruptions of monotonous intermediates in crystallizing magma. The variation cannot be resent a small-volume explosive eruption during the Indian Peak fi eld. The Lund Formation also entirely the result of emplacement processes, the development of that volcano. includes local intracaldera wall-collapse breccias e.g., fractionation of fi ne vitroclasts.

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70 Segment in the Northern Wilson Creek Lund Formation ignimbrites n = 41 Range and White Rock Mountains 60 plus trace of titanite In the low foothills of the northwestern Wil- 50 son Creek Range north of Mount Wilson (Fig. Figure 50. Modal proportions 22), lenses of bleached Paleozoic carbonate of phenocrysts in ignimbrites 40 rock fragments interﬁ nger with Lund tuff in an of the Lund Formation. Most 30 almost continuously exposed sequence 2500 m

analyses were made by Larissa Mode (vol %) thick (Willis et al., 1987). As is commonly the LeVitre Maughan. 20 case in thick sections of tuff that lack stratigraphic markers, there may be some repetition 10 by unrecognized normal faults; however, the base of the section is cut off by the transect- 0 Plag Qtz San Bio Hb Px Opaq Pheno ing younger Mount Wilson caldera. One breccia lens has pieces of andesitic lava that were probably derived from the andesite fl ows at Despite these overall modal variations in the Although the Lund is 230 m thick in Condor the base of the ignimbrite sequence overlying Lund tuff, no consistent spatial pattern of varia- Canyon (G.J. Axen, 1989, personal commun.), the Pennsylvanian–Permian Ely Limestone in tion in the proportions of phenocrysts can be immediately to the north in the southwestern the divide between Grassy Mountain and Fair- discerned laterally in the outfl ow sheet or in corner of the Rose Valley quadrangle (Best and view Range to the west across Lake Valley (Fig. four vertically sampled stratigraphic sections. Williams, 1997) the ignimbrite pinches out over 17; Best et al., 1998). Patches of ash-fl ow tuff However, an upward decrease of ~10% in the a highland of Cambrian rock that apparently and minor breccia lenses surrounded by alluvial phenocryst/glass ratio in two of the sections may extended to the west. cover occur northward for ~12 km to the approxi- account for subtle and inconsistent reverse chem- The present east-west extent of the Lund mately located northern margin of the intra caldera ical zonation manifest in decreasing MgO and ignimbrite is more than twice that of the north- deposit. In this area west of the Atlanta mine, the

TiO2 and increasing K2O. The average outfl ow south extent, exceeding that which would have margin of the White Rock caldera appears to seems to have signifi cantly fewer phenocrysts, resulted solely from crustal extension (an merge with the margin of the older Indian Peak on a dense rock basis, than the intracaldera tuff assumed uniform 50%) after emplacement. caldera (Fig. 8). Partially fi lling in the north- (34% versus 51%), which would be consistent The topographic wall on the older Indian Peak ern part of the White Rock caldera southwest with a pre-eruption chamber possessing deeper, caldera apparently mostly blocked dispersal of the Atlanta mine is ~400 m of ignimbrite of the more crystal-rich magma. The modal proportion of Lund ash fl ows to the north. Similarly, the Ripgut Formation. of quartz in the outfl ow is lower (9% versus 17%) Indian Peak topographic wall that is postu- For ~22 km southeast of the Atlanta mine, the and the total mafi cs higher (32% versus 25%) lated to lie in Lake Valley (Fig. 29) could have caldera margin is concealed beneath a tableland than in the intracaldera tuff. Because the outfl ow prevented dispersal of thick ash fl ows to the of early Miocene rhyolite tuffs and lava fl ows and intracaldera tuffs are chemically indistin- west. An alternate factor that possibly infl u- (Willis et al., 1987). guishable there is, thus, a puzzling disconnect enced the relatively large east-west distribu- Beyond and to the southeast of the tableland, between the elemental and modal composition of tion of the Lund is obscure extensional faults in the northern White Rock Mountains (Fig. the Lund ignimbrite (Maughan et al., 2002). of the same orientation that had possibly cre- 46B; Best et al., 1989d), more than 900 m of None of the Lund contains lithic clasts except ated a concealed graben where the ignimbrite Lund tuff is banked against thick lithic Wah Wah for at one locale north of Ursine noted above. was deposited. Although this implied stress Springs ignimbrite, defi ning the topographic The general absence of lithic fragments in the fi eld—north-south extension—is seemingly margin of the White Rock caldera. As much as caldera-collapse Lund is unlike in the older, at variance with prevailing wisdom for the 600 m of Ripgut tuff and more than 350 m of lithic intracaldera Wah Wah Springs monoto- middle Cenozoic Great Basin, it is consistent the overlying Isom tuff constitute post-caldera nous intermediate. with widespread east-west dikes of about this fi ll. In the southern White Rock Mountains, in The thickest exposed sections of intracaldera age (Best, 1988). North-south extension is the Rice Mountain 7.5-minute quadrangle (Fig. Lund ignimbrite occur in the northwest sector also evident in the late Oligocene–early Mio- 44; Keith et al., 1994), faulted and tilted Lund of the White Rock caldera source, whereas the cene, west-northwest–striking normal faults is unconformably overlain by Ripgut tuff. To thickest outfl ow lies to the east of the caldera in the High Plateaus of south-central Utah the south of this resurgent uplift of the White (Fig. 51). Dispersal of pre-collapse ash fl ows (Anderson, 2001). Rock caldera, the Ripgut thickens to greater appears to have been constrained by their erup- than 650 m. tion from the eastern sector of the caldera source WHITE ROCK CALDERA which was located, at least in part, within the Southeastern Segment of the incompletely fi lled depression of the older In contrast to the well-exposed internal struc- White Rock Caldera Indian Peak caldera. Consequently, 300–350 m ture of the Indian Peak caldera in the Needle of outfl ow ignimbrite is exposed in the southern Range, no similar deep exposures exist for the The caldera margin continues southeastward Wah Wah Mountains whereas just to the west White Rock caldera, the source of the Lund across Hamlin Valley into the southwestern within the Indian Peak depression the Lund is ignimbrite. The following paragraphs describe Indian Peak Range. Here, a post-Lund andesite 450 to as much as 1400 m thick, the latter thick- successive segments of the caldera in a clock- lava dome that is 400 m thick and 2 km in diam- ness refl ecting accumulation within the nested wise direction, beginning with the best-exposed eter is overlain by as much as 900 m of Isom Mackleprang caldera. northern part (Figs. 8 and 51). ignimbrite that thins abruptly to less than 250 m

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/4/864/3346014/864.pdf by guest on 29 September 2021 Indian Peak–Caliente ignimbrite ﬁ eld Km Miles 0 ! 0 ! 100 km 30 <60 ! ! source. source. gure is in gure E E E 113° E E ows. The zero Corrected for 50% extension for Corrected 60 ! 0 0 ! ! 30 0204060 ! 02040 0 ! 0 300 ! ! 500 ! ! ! ~630 ! 0-12 ! 0 ! ~1400

~200 !

120? ! 350 1000

~1100

400

800

300 600 500 ! 0 ! ~30 !

200

0 0

570 ! 55 100 ! ! ~600 uence on the dispersal of Lund ash fl >100 E E E E 0 114° ! 1000 ! ^ ! ! NV UT >900 ^ 150 ! 450 ! 85 ^ ! 2000 0 >600 ! 230 ! ! ! ^ 0 ! >230 ^ 0 ! >2500 ects resurgent uplift and erosion. A preliminary version of this fi preliminary A uplift and erosion. ects resurgent 0 ! 0-40 >1600 ! ! 10 ! ^ 0 ! 10-30 ! 0 ! 0 10 60 ! ! ! 0 ! 20 E E E ! 115° 20 ! 0 ! 100 ! 30 ! 0 ! 0 ! 0 0 ! ! Lund ring-fault lava Lund White Rock caldera topographic margin Rock caldera topographic White Rock caldera structuralWhite margin margin caldera topographic Older Indian Peak E E Figure 51. Distribution and thickness (in meters) of ignimbrite of the Lund Formation and the margin of the White Rock caldera 51. Distribution and thickness (in meters) of ignimbrite the Lund Formation margin Figure Indian Peak caldera is also shown because of its infl margin of the older Topographic Maughan et al. (2002). thickness of the Lund in the southern Wilson Creek Range refl Creek Wilson thickness of the Lund in southern E E E 116° ^ 0 Lund ignimbrite (29.20 Ma) ignimbrite Lund ! ′ ′ 39° 38° 38°30 37°30

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to the northeast, hence, beyond the topographic appears to be caldera related, a mélange that Hills (Axen, 1998; Axen et al., 1988; Burke margin of the White Rock caldera. Outside the overlies the Wah Wah Springs ignimbrite con- and Axen, 1997; Rowley et al., 1994; G.J. Axen, White Rock caldera but within the northern sists of pieces of somewhat altered Lund vitro- 1989, personal commun.). The latest period collar zone of the Indian Peak caldera (Figs. 8 phyre and cataclastically deformed granitic rock involved normal faulting after the mid-Miocene and 51), the Lund section between older and and rare altered Wah Wah Springs(?) as well as that formed horsts and grabens, or the present- younger deposits is 570 m thick. abundant crystal-rich rhyolite of unknown strati- day ranges and valleys. Mid-Miocene (ca. Farther east within the collar zone between the graphic identity. The mélange, which is overlain 15 Ma) faulting is dominated by the Highland structural and topographic margins of the Indian by a thick sequence of younger andesitic and detachment (Fig. 8A) that is exposed west of the Peak caldera, unusually thick sections of Lund dacitic lava fl ows, is interpreted to be a land- Pioche Hills and has more than 10 km of west- tuff occur in the hills at the south end of Pine Val- slide deposit that involved some stratigraphically ward displacement. Late Oligocene, synvolcanic ley. Three stratigraphic sections have thicknesses unrecognized rock units on the fl oor of the White (ca. 29–27 Ma) faulting is implied by contrasts of 630, 1100, and 1400 m (Best et al., 1987d) Rock caldera and has been exposed at the surface in dip of less than 30° of Oligocene and early but these are approximate as sections are incom- by considerable resurgence and erosion. Miocene ignimbrite sheets. Evidence for pre- plete without exposed bases and tops; sections For ~12 km farther northwest along the volcanic extension is represented by closely might also have unrecognized faults. Not far to southern Wilson Creek Range, thick deposits spaced normal faults and east-west–trending the north (Abbott et al., 1983) and east (Hintze of lower Miocene rhyolite tuff and lava con- tear faults that are found in the footwall of et al., 1994b) the Lund is only 200–500 m thick ceal older units. But in the western foothills the Highland detachment and that sole into the (Fig. 51). We believe the unusually thick Lund the Lund is mostly absent between patches of Stampede detachment, according to Axen et al. marks a caldera source (Fig. 8) of the immedi- older outfl ow Wah Wah Springs and younger (1988); this detachment is a zone of bedding- ately older Mackleprang ignimbrite. Isom ignimbrites, presumably because the intra- parallel faults between the middle Pioche Shale To the west, the roughly east-west–trending caldera Lund was removed by erosion off the and lower Highland Peak Formation. The Stam- southern margin of the White Rock caldera is resurgent uplift of the caldera prior to deposi- pede detachment cuts through the Pioche Hills concealed beneath younger deposits and is con- tion of the Isom. and was recognized by Tschanz and Pampeyan strained only by a gradient in Bouguer gravity About 17 km northwest of Ursine is the win- (1970), who believed it to be an early Tertiary that limits the southern margin of the greater dow area in the southwestern Wilson Creek (“Laramide”) regional thrust sheet. Indian Peak caldera complex (Fig. 8C). Range (Fig. 8A) along the margin of the Indian Signifi cant aspects of the earlier geologic Peak caldera described above. For ~2 km east of studies and large-scale mapping that bear on the Southern Wilson Creek Range Segment the intracaldera Wah Wah Springs assemblage elucidation of the White Rock caldera margin exposed in this window is a moderately north- are summarized in the following paragraphs. More than 15 km north of the approximately dipping section of Lund tuff that is possibly as Especially pertinent is the work of Tschanz located southern caldera margin is an exposed much as 700 m thick and disappears beneath et al. (2009) done in the mid-1900s but recently area of the resurgent fl oor of the White Rock fl at-lying Isom tuff and overlying rhyolitic tuff edited and made available to us by D.R. Shawe; caldera. The southern part of this area, imme- and lavas. Locally at the top of the exposed sec- this work contains detailed observations of the diately north of Ursine, occupies the northwest tion of Lund and at the unconformable contact extensive underground mine workings. corner of the map by Williams et al. (1997), between it and the overlying Isom is an exposure whereas the remaining part is covered only of a heterogeneous assortment of lithologically Faulting and Brecciation by unpublished reconnaissance mapping. The contrasting megablocks in a lithic Wah Wah Faulting in the generally gently dipping Cam- western half of this area consists of a northerly Springs matrix. We interpret the entire window brian rocks in the Pioche Hills is unusually trending synclinal outcropping of Cambrian area to be a segment of the resurgently uplifted intricate and complex, in marked contrast to the carbonate rocks (see maps of Tschanz and Pam- fl oor of the White Rock caldera that includes the “simple structure” of the nearby Highland Range peyan [1970] and Ekren et al. [1977]). These margin of the older Indian Peak caldera. to the west and the Bristol Range to the north- rocks are overlain by a compound cooling unit west. Faults that cause bedding offset have nor- of rather densely welded Lund ignimbrite more Pioche Hills Segment mal extensional displacements, but it is common than 600 m thick. The Cambrian rocks are vari- for a fault to have been offset by a cross fault, ably silicifi ed along the depositional contact We believe the Pioche Hills horst and fl ank- after which block movements continued on both. with the overlying Lund tuff, which is locally ing grabens represent a signifi cant defi ning Relative ages of thrust faults, bedding faults, and a vitrophyre. The Lund here is atypical in that segment of the southwestern collar zone of the normal faults are diffi cult to evaluate, but in most it contains sparse lapilli of red aphanitic vol- White Rock caldera (Fig. 8A). places the evidence is confl icting or at least con- canic rock. Within 2 km east of Ursine, the Exposures in the relatively small, ~10 × 4 km, fusing. In many places contemporaneity is indi- Lund grades locally into zones where clasts of anomalously northwest-trending horst consist cated and one type of fault may bend and merge older rock, as much as a meter across (including of a sequence of Cambrian sedimentary rocks with a fault of another system. In one instance, as Wah Wah Springs and Ryan Spring tuffs as well almost 2000 m thick (Fig. 52A; see also Park many as four cross-cutting generations of faults as andesitic rock), are embedded in a matrix of et al., 1958; Tschanz and Pampeyan, 1970). can be documented that have different orienta- comminuted Lund tuff. The Pioche Hills have been the site of signifi - tions and senses of movement. Late deformation About 5 km north of Ursine in the northern cant zinc, lead, silver, and manganese mining was so intense and widespread that it likely reac- part of the area, the synclinal terrain of Cambrian (Gemmill, 1968); these ore deposits are possi- tivated and/or reoriented earlier faults. rocks and overlying thick intracaldera Lund are bly related, at least in part, to the Lund magma Everywhere associated with the complex faults truncated by a high-angle east-west fault that system and attendant caldera collapse. are zones of breccia that occur in all lithologic is on trend with the marked eastward jog in Four periods of extensional faulting are rec- units—limestone, dolomite, shale, and quartzite. Meadow Valley Wash. North of this fault, which ognized in the region encompassing the Pioche The breccia zones can be as much as hundreds

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SW NE

Highland Peak Limestone Stampede Lund tuff Chcl Chisholm Shale detachment Lyndon Limestone Wah Wah Springs and Cp Pioche Shale Cottonwood Wash tuffs

e t ul Cpm Prospect Mountain Quartzite tur a u f F ing r

B SW

Chcl NE ∆ ∆ Lund tuff Cp ∆∆ ∆ Wall-collapse breccia mostly of Cambrian rocks Cpm 2 ∆ ∆

Wah Wah Springs and Figure 52. Sequence of north- Future faults Cottonwood Wash tuffs km east-southwest cross sections schematically illustrating the evolution of the southwest- 0 ern collar-zone margin of the 024 km White Rock caldera where the northwest-trending Pioche Hills C horst later developed by basin- SW WHITE ROCK CALDERA and-range faulting. Intercalated Chcl breccias in the Lund tuff consist NE ∆∆ of fragments of Paleozoic rock. Cp ∆∆ Wall-collapse breccia Intercalated breccias and ∆ ∆ ∆ Vertical extent and thickness of Cpm ∆ ∆ ∆ Lund tuff the Wah Wah Springs tuff ﬁ s- sure ﬁ lling is exaggerated. See ∆ ∆ ∆ ∆ ∆

text for explanation. ∆ ∆

e Wah Wah Springs and g n Cottonwood Wash tuffs a Chcl r lt e u r a tu f Cp u t F n Wah Wah ro F f Springs tuff u Cpm f t r u fissure filling o r n e t r f a a n u g lt e

D PIOCHE HILLS

Quaternary Chcl Quaternary SW NE ∆ ∆ Cp Cpm ∆ ∆ ∆ Intercalated breccias and Wah Wah ∆ Lund tuff Springs tuff fissure filling ∆

of meters thick and continuous along strike for Shale is puzzling. But, locally, the shale unit is Structure kilometers with only minor interruptions and highly sheared. The almost complete absence of The overall structure of the Pioche Hills offsets. In many places, both on the surface folding and crumpling in the Pioche Hills, even shown in the east-west cross section in Park et al. and underground, breccia is all that remains of in shales, and the very widespread brecciation (1958, their section CC′) and Tschanz and Pam- beds of quartzite hundreds of meters thick. Why of quartzite and carbonate rocks are thought to peyan (1970, their plate 4) is a system of nor- deformation caused massive quartzite to shatter indicate that the faulting took place under com- mal faults that have downdropped subhorizontal rather than to be taken up in the overlying Pioche paratively light loads, or shallow burial. strata across the hills in a synformal manner.

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However, underground workings in Tschanz all of the three phenocryst-rich dacite ignim- than a few hundred meters beneath alluvium the et al. (2009) reveal a more complex, even cha- brites—Wah Wah Springs, Cottonwood Wash, drills commonly encountered Cambrian lime- otic, structure than portrayed in this generalized and Lund. Although they report that pieces of stone that overlay welded tuff, or alternations of published section. This general pattern of domi- welded tuff can be found on the mine dumps, the two, which Tschanz and Pampeyan (1970) nantly downward displacements inward toward we were unable to fi nd any with diligent search- referred to as “megabreccia” (see cross sections the center of the horst is suggestive to us of that ing. Tschanz et al. (2009) wondered whether in Gemmill, 1968; Park et al., 1958; Tschanz seen in some extensional grabens in the Basin the “dikes” might represent vents for eruption et al., 2009). Our examination of the few avail- and Range province (e.g., Effi moff and Pine- of the welded tuff sheets exposed, for example, able drill cuttings of tuff revealed them to be zich, 1986; Liberty et al., 1994). It is unlike the in Condor Canyon to the southeast. This may be of Wah Wah Springs and Lund. One especially outward and downward displacements marginal the case for “dikes” of the dacitic Lund that was deep hole on the west fl ank of the hills encoun- to horsts typically seen in the province. derived from a magma chamber whose margin, tered these megabreccias overlying a thick sec- In contrast to the relatively fl at strata in the in our interpretation, lies under the Pioche Hills. tion of volcanic rocks that continued to a depth fault synform, bedding in the Prospect Moun- However, our examination of all that remains of more than 760 m. “Megabreccia” was not tain Quartzite in the eastern part of the Pioche exposed of the Slaughterhouse Canyon “dike” reported in the underground workings in the Hills has a moderate east dip that we interpret of welded Wah Wah Springs lends little support Pioche Hills horst. to be the result of initial downsagging along the for the feeder “dike” interpretation, for the fol- “Megabreccias” in the fl anking graben were margin of the White Rock caldera to the north- lowing reasons: interpreted by early workers to have resulted east as magma began to be withdrawn from the 1. The eutaxitic foliation is perpendicular in from thrust faulting or from gravity sliding, con- underlying chamber (Fig. 52A). On the other places to the contact with the Cambrian coun- ceivably off a rising horst. On the other hand, we hand, G.J. Axen (2010, personal commun.) try rock; this fact, together with the crushed believe they are entirely consistent with inter- believes this discordance in dip resulted from and broken character of the rock in the “dike,” cala tion of ignimbrites and wall-collapse brec- faulting related to the Stampede detachment. seems inconsistent with it being a feeder vent cias in the collar zone of the White Rock caldera. The anomalous northwest-southeast trend (cf. Ekren and Byers, 1976), unless breccia- of the relatively small Pioche Hills horst may tion somehow occurred after the “dike” was Summary and Interpretations refl ect the local structural grain in this part of emplaced. Salient geologic attributes of the Pioche Hills, the Great Basin; the northwest-trending Kern 2. A feeder dike of Wah Wah Springs tuff is as contained in the early reports, include the fol- Mountains east of Ely may be another example. unlikely 25 km southwest of the margin of its lowing: G.J. Axen (2010, personal commun.) suggested Indian Peak caldera source (Fig. 8). 1. Unusual northwest-southeast trend of the the anomalous range trend is the result of verti- 3. No thermal alteration, such as bleaching, Pioche Hills horst. cal-axis rotational displacement that opened of the adjacent limestone wall rock is apparent, 2. Complex and intricate faulting. the valley to the south, a possibility that could as would be expected in a hot 8-m-wide feeder 3. Widespread, pervasive brecciation, includ- be tested by paleomagnetic data. Yet another “dike.” ing of thick-bedded quartzites. explanation, which we favor, is that the unusual For these three reasons, we believe the “dike” 4. Generalized synformal structure in fl at- trend of the Pioche Hills horst, compared to the originated as a deposit of ignimbrite capping lying strata in the core of the Pioche Hills horst; typical north-south trend of larger ranges in the the Cambrian rocks that was drawn into a the structure in underground mine workings Great Basin, simply refl ects a controlling infl u- dilatant tensional fi ssure in the Cambrian rock appears to be more chaotic. ence by marginal faults along the White Rock during deformation, such as described by Lip- 5. A surface exposure of a dilatant fi ssure caldera. man (1964) for a similar occurrence in south- fi lling of welded Wah Wah Springs tuff embed- ern Nevada. It is likely that at least some of the ded in broken Cambrian rocks, and by impli- Ignimbrites and “Dikes” “dikes” of welded tuff in underground workings cation, additional such features in the under- Although Tschanz et al. (2009) noted the are also dilatant fi ssure fi llings. ground workings. Underground occurrences existence of welded andesite and dacite ash- Vikre and Browne (1999) listed K-Ar ages of Lund tuff could be either fi ssure fi llings or fl ow tuff in the Pioche Hills, only three surface on biotites from two underground vitrophyre feeder dikes. exposures totaling no more than 0.01 km2 are dikes and the Slaughterhouse Canyon Wah Wah 6. “Megabreccias” of alternating welded tuff shown on the geologic map (Park et al., 1958). Springs dike; the three ages (29.2 ± 1.2, 28.3 ± and Cambrian limestone and of limestone over- One in the northwest is Cottonwood Wash Tuff 0.8, 28.8 ± 0.9 Ma) are within analytical error lying tuff in grabens fl anking the central horst. whereas the other two are of Wah Wah Springs of one another and have a weighted average of Some structural aspects are possibly related ignimbrite. One of the exposures of the Wah 28.6 ± 0.5 Ma, which seems more consistent to regional extensional tectonism. For example, Wah Springs is a 100-m-long “dike” as much with the Lund (29.20 Ma) rather than the Wah G.J. Axen (2010, personal commun.) noted that as 8 m wide exposed near the crest of the range Wah Springs (30.06 Ma). However, these K-Ar some steep faults with fairly large stratigraphic at the head of Slaughterhouse Canyon near ages on biotites cannot be compared directly separations in the Pioche Hills die out as they coordinates 16,500 N and 10,000 E on Park with the 40Ar/39Ar ages on feldspars reported in approach the Pioche Shale; he believes this et al. (1958). The “dike” rock is crushed and the this article, which were determined in a different geometry is consistent with soling of the steep broken rock along its borders contains angular analytical facility. faults into the Stampede detachment. Overprint- fragments of limestone up to 20 cm in diameter. ing by extensional basin-and-range faulting is “Dikes” of welded tuff have also been found in Drill-Hole Information an additional complexity. the underground workings. Descriptions and Numerous exploratory churn-drill holes However, considered all together, we believe a few available photomicrographs of the tuffs drilled in the late 1940s and early 1950s sought the six salient geologic aspects of the Pioche in the Pioche Hills (Tschanz et al., 2009) are extensions of the ore bodies in the grabens Hills are entirely consistent with collapse along consistent with the occurrence of any one or fl anking the Pioche Hills horst. At depths of less the southwestern collar-zone margin of the

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White Rock caldera. Our interpreted evolution by the remnant of the east-dipping downsag. 40Ar/39Ar age of biotite compared to that of sani- of the caldera margin, shown schematically in This sort of structure, called a “keystone gra- dine in the same sample (e.g., Salisbury et al., Figure 52, was inspired by the work of Branney ben” by Branney (1995, e.g., his fi gure 5), is 2011), this porphyry may instead be co-genetic (1995, especially his fi gures 1, 2, and 5), Roche drawn to correspond to the northeast-southwest with the Lund. et al. (2000), and Burchardt and Walter (2010) cross section of the Pioche Hills shown in Park and profi ted from suggestions by Gary J. Axen et al. (1958, their section CC′) and in Tschanz Fairview Range and Daniel R. Shawe. Our interpretation follows and Pampeyan (1970, their plate 4). The rapid the same sort of caldera-related extensional col- unconfi ned collapse during the geologically Additional direct evidence for the White Rock lapse that we believe took place in the north- instantaneous subsidence of the caldera imparts caldera margin is in the Fairview Range 30–40 km eastern collar zone of the Indian Peak caldera a signifi cant amount of observed complexity to north-northwest of Pioche where lenses of brec- (Fig. 43), but differs in some details. Roche the extensional faulting as well as widespread cia of older rocks, chiefl y tuff of Deadman et al. (2000) and Burchardt and Walter (2010) brecciation that is overprinted on the basic Spring, are intercalated within a section of Lund used natural examples in concert with analogue synformal structure. Rapid, large-magnitude that is more than 800 m thick on the east fl ank model experiments to demonstrate that caldera unconfi ned collapse differs from deformation in of the range (Best et al., 1998). Similar breccias, subsidence commonly begins as a downsag typical extensional tectonic regimes of more or but not in contact with Lund, occur elsewhere in but displacement generally progresses along less confi ned and slow incremental movement. the range. Some fault blocks expose greater than outward-stepping and outward-dipping reverse Signifi cantly, dilatant fi ssures would draw in 1600 m of Lund without intercalated breccias, as faults whose dips steepen with depth; then, as both the older, cold Wah Wah Springs as well well as sections of Deadman Spring and Silver displacement continues, peripheral extensional as the newly deposited Lund, creating “dikes.” King ignimbrites more than 1000 m thick. All fractures develop in the hanging-wall block and Steep topographic walls would collapse to cre- of these thick sections of tuff were likely depos- propagate downward to form inward-dipping ate lenses of breccia mingled within the thick ited within their own source calderas or in older normal faults that merge at depth with the earlier intracaldera Lund. depressions nested within the northwestern seg- reverse faults and take up continued displace- In Figure 52D, later basin-and-range faulting ment of the Indian Peak caldera complex. ment (Figs. 42 and 43). drops grabens fl anking the central Pioche Hills In Figure 52A, as magma begins to be with- horst, where the preserved keystone graben of Magmatism Post-Dating Collapse of drawn from the underlying Lund chamber, a Cambrian strata is now exposed. “Megabreccias” the White Rock Caldera downsag develops along the caldera margin. We of interbedded Lund and wall-collapse breccia interpret the east-dipping Prospect Mountain in these fl anking grabens were intersected in Six, mostly small (<1 km diameter) lava Quartzite to be evidence for this downsag, but, exploratory drill holes. fl ows and a vent complex appear to mark the as noted above, this dip could be related to the northern ring fault of the White Rock caldera pre-caldera Stampede detachment and related Caldera-Related(?) Magmatism (Fig. 51) and crop out less than 4 km inboard extensional faulting, which, for simplicity, is and Mineralization (mostly south) of the exposed topographic mar- shown here only schematically and is omitted in In their work on the ore deposits in the Pioche gin of the caldera. The lava fl ows are porphy- succeeding parts of the fi gure. Caldera-collapse mining district that includes the Highland and ritic, holocrystalline to glassy, and mostly fl ow Lund ignimbrite begins to pond within the cal- Bristol Ranges to the west of the Pioche Hills, layered. Outer contacts of the smaller ones are dera depression. Vikre and Browne (1999) documented two concealed beneath alluvium, leaving open the In Figure 52B, further eruption of Lund ejecta periods of magmatic activity and mineraliza- possibility that they could be shallow intru- results in further downsagging as well as initial tion, namely, Cretaceous at 100–90 Ma and late sions. The large (1 × 1.5 km) lava fl ow in the reverse movement on an outward-dipping ring Oligocene at 29–27 Ma (biotite K-Ar). The lat- Wilson Creek Range ~5 km south-southwest fault. The hanging-wall block, whose unmodi- ter period is roughly that of the Lund activity, of Atlanta (Willis et al., 1987) is a rhyolite fi ed form is indicated by the blue dashed line, with due allowance for different methods of (Fig. 49) that is more evolved than the Lund experiences extensional deformation strain, dating, and carries the implication that localiza- ignimbrite samples but plots along the same including fracturing and fi ssuring (long dashed tion of the Cu-rich Oligocene ores could have element-variation trends. Three samples from lines), to compensate for the wedge-shaped been related to the extensional structures along ring-fault lavas in the White Rock Mountains potential void along the fault. A scallop of the the caldera margin as well as to the underlying (Best et al., 1989d) are dacitic like the Lund. steep topographic margin of the hanging-wall Lund magma system. Apart from the ages on The easternmost mass has a fi ssion-track age block collapses into landslides that cascade “dikes” in the Pioche Hills indicated above, late on zircon of 27.6 ± 2.5 Ma (Kowallis and Best, into the deepening depression as more Lund is Oligocene magmatism is manifest by the small 1990). The vent complex in the western White deposited, creating intercalations of wall-col- Ida May dike of altered granitic porphyry in the Rock Mountains has a bedded surge deposit at lapse breccia of Cambrian limestone (and minor northern Bristol Range, just to the west of the the base that grades upwards into massive tuff Wah Wah Springs ignimbrite) within the Lund. hypothesized western margin of the White Rock containing clasts as much as 1 m in diameter of In Figure 52C, considerable further sub- caldera; this dike has a K-Ar age on seri cite Lund-like vitrophyre and capped by a mass of a sidence of the caldera fl oor during continuing of 26.7 ± 0.7 Ma. Farther north, in the south- similar tuff. Small blocks of granodiorite and eruption produces large displacement along the ern Fairview Range (Best et al., 1998), a gra- feldspar-quartz-biotite-hornblende gneiss also reverse ring fault. The large potential void above nitic porphyry with a 40Ar/39Ar age on biotite occur in the massive tuff. The lava fl ow in the the fault is compensated for by large-magnitude of 29.84 ± 0.07 Ma is spatially associated with Fairview Range differs from the other ring-fault unconfined collapse, a major component of silver mineralization. This porphyry was linked occurrences in being larger (~2.5 km diameter) which is a synformal structure within the hang- (above) with the tuff of Deadman Spring and and having conspicuous phenocrysts of horn- ing-wall block that is bounded on the southwest formation of the Kixmiller caldera. On the other blende (lava fl ow of Chokecherry Spring in Best by a major normal fault and on the northeast hand, owing to the commonly recognized older et al., 1998). Nonetheless, its stratigraphically

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constrained age and chemical composition are account for the hypothetical intracaldera deposit or 6% of the magma volume, reducing the consistent with a co-genetic relation to the Lund yields 4600 km3. erupted magma volume to 4100 km3. magma system (Maughan, 1996). For the White Rock caldera used in Models Models 2–3 ignore the volume of fallout ash 2–4, we assume the concealed inner ring fault surely deposited beyond the Lund ignimbrite White Rock Caldera Summary marking the margin of the structural caldera as well as the volume of wall-collapse breccias lies a uniform 5 km inside the topographic mar- within the intracaldera tuff; these two volumes On the basis of geologic relations in the sev- gin, which is the entire perimeter of the caldera might counterbalance. eral sites just described, we construct the out- shown in Figures 8 and 51. The 5-km-inboard In summary, the estimated volumes using line of the White Rock caldera shown in Figures assumption is constrained by the positions of Models 1, 3, and 4 give values of 4600, 4600, 8 and 51; the topographic caldera could be as ring-fault lavas. The area of the so-defi ned struc- and 4100 km3, respectively, whereas Model 2 much as 60 km north-south and have an equiva- tural caldera is 1500 km2. In Model 2, we use a gives signifi cantly more at 5400 km3. Our pre- lent diameter of ~50 km after correction for uniform thickness of 2500 m of caldera-collapse ferred total volume of the Lund ignimbrite is extension (Table 2). Lund and add this volume (3750 km3) to that 4400 km3. Although less is known of the internal char- of the pre–caldera collapse ignimbrite, yielding acter of the White Rock caldera relative to the 5400 km3. In Model 3, the contribution of con- ISOM-TYPE TUFFS Indian Peak caldera, some comparisons can be toured caldera-collapse tuff (maximum thick- made. Both show clear evidence for resurgent ness 2000 m) in an asymmetric caldera has been Whereas rhyolite ignimbrites occur through- uplift after collapse. Both disclose only little added to the pre-collapse tuff, giving 4600 km3. out the 36–18 Ma period of activity in the intracaldera sediment; only 130 m was found in It should be noted that the assumed asymmetry southern Great Basin ignimbrite province, the caldera-fi lling Ripgut Formation in one part of the caldera is based on the thickest exposed super-eruptions of monotonous intermediates of the White Rock caldera. Signifi cant differ- caldera-collapse tuff lying in the northern sector are restricted to a brief time period of 31.13– ences between the two calderas include: of the caldera whereas the thickest pre–caldera 29.20 Ma in the Indian Peak caldera complex 1. Ring-fault magmatic activity is expressed collapse tuff lies to the east of the caldera. and to 27.57 Ma in the Central Nevada complex. in the White Rock caldera whereas in the Indian In Model 4, to evaluate the amount of sub- Immediately following these monotonous inter- Peak caldera post-collapse magmatic activity sidence of the caldera fl oor, we use the thick- mediate eruptions, numerous cooling units of is manifest by resurgent intrusion of grano- nesses of the caldera-collapse Lund plus the unique trachy dacitic ignimbrite were erupted diorite porphyry in the eastern core of the cal- overlying caldera-fi lling Ripgut and Isom, for from both caldera complexes. These typically dera; this large post-collapse porphyry may which there are four constraints. In the southern densely welded, mostly relatively thin cooling be a factor in the widespread alteration of the Indian Peak Range, the southeastern extremity units were designated as the Isom compositional Wah Wah Springs intracaldera tuff, in contrast of the White Rock caldera at its topographic type by Best et al. (1989b), but because their to the general absence of alteration in the intra- margin is fi lled by a 400-m-thick, post-Lund characteristics go beyond composition, and for caldera Lund. lava dome and ~650 m of overlying Isom tuff, brevity, they are referred to here as Isom-type 2. Comparison of Figures 29 and 51 reveals a making the total caldera fi ll thickness 1050 m. tuffs. They contain less than 15% phenocrysts, wider collar zone between the topographic and In the Fairview Range, the Lund is more than mostly plagioclase, together with lesser clino- structural margins of the Indian Peak caldera 800 m thick. (A greater than 1600-m-thick sec- and orthopyroxene and magnetite, which are than inferred for the White Rock caldera, imply- tion in another fault block has no intercalated commonly aggregated (Fig. 53). Ilmenite and ing a greater amount of subsidence resulting in breccias and could be fi lling an older depression apatite are rare and most samples lack zircon. a greater retreat of the topographic wall in the related to eruption of the Silver King or Dead- Very sparse grains of sanidine, quartz, amphi- former. This contrast is evident in the estimates man tuffs.) In the central White Rock Moun- bole, and biotite occur inconsistently and appear of subsidence for the Indian Peak caldera of tains, the Lund is greater than 900 m thick and to be, at least in part, xenocrystic, possibly 6100 m (see above) and for the White Rock cal- the overlying caldera-fi lling Ripgut and Isom derived from the older monotonous intermedi- dera of 2900 m (see below). tuffs are 600 m and more than 350 m, respec- ates such as the Lund, which occurs only very 3. A possible consequence of the greater tively, giving a subsidence greater than 1850 m. locally as xenoliths. Lapilli of andesitic rock are subsidence of the Indian Peak caldera is the The fourth constraint is in the northern Wilson common. abundance of lithic clasts in the intracaldera or Creek Range, north of Mount Wilson, where In the Indian Peak–Caliente fi eld, Isom- collapse ignimbrite in it relative to the intracal- 2500 m of Lund is exposed and, to the north, type tuffs comprise a single cooling unit of the dera Lund tuff in the White Rock caldera that is ~400 m of Ripgut tuff, giving ~2900 m of sub- 29.1 Ma Petroglyph Cliff Ignimbrite and nine mostly lacking in lithics. sidence. Following the approach used by recognized cooling units constituting three tuff Lipman (1997), and assuming 2900 m of sub- members of the 27.90–24.55 Ma Isom Formation Volume of the Lund Ignimbrite sidence across the 1500 km2 area of the struc- (Table 1; another Isom-type cooling unit with an tural caldera, we determine the erupted volume age of ca. 23 Ma and apparently of local origin in To estimate the total volume of the Lund of the Lund magma to be 4350 km3. Because the the Northern Pahroc Range in the southwestern ignimbrite we use the same multi-model intracaldera pile includes wall-collapse breccias sector of the fi eld [Scott et al., 1992] is not dis- approach (Fig. 4 and Tables 2 and 7) as for the as well as ignimbrite, the actual thickness of the cussed further here). During the 4.6 m.y. interval Wah Wah Springs. latter is some lesser value than 2900 m. Using of Isom-type eruptions only one other ignimbrite In Model 1, the volume of the contoured, pre– the formulation of Lipman (1997, his appen- was deposited—the rhyolitic Ripgut–—shortly caldera collapse Lund exclusive of the source cal- dix 1) for the collar-rock breccia volume (Cv) after the Petroglyph Cliff. The sources of the dera, but including the thick accumulation within collapsed into an equivalent circular structural Petroglyph Cliff Ignimbrite and the Isom For- the older Indian Peak and nested Mackleprang caldera from an equivalent circular topographic mation lie just beyond the northwest and the depressions, is 2300 km3. Doubling this value to caldera, we calculate a maximum of ~250 km3, southeast margins of the Indian Peak–Caliente

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eral elements as in other Isom-type tuffs (Figs.

5, 6, and 54); for example, TiO2, Ba, and Zr are lower, as are total alkalies so that most samples Figure 53. Photomicrograph of are dacite and less are trachydacite, andesite, Isom-type tuff, showing sparse and latite. isolated phenocrysts of euhedral Although not dated isotopically, the 29.1 Ma to anhedral plagioclase and age of the Petroglyph Cliff is constrained by its opaque magnetite anhedra stratigraphic position between the 29.20 Ma together with aggregates of Lund and the 29.0 Ma Ripgut (Table 1). plagio clase, pyroxene, and magnetite (upper left and lower Stratigraphy right) in a matrix of fl attened vitroclasts. Preliminary correlation of the Petroglyph Cliff Ignimbrite in the fi eld is based on its 1 mm unique physical appearance but is confi rmed by chemical composition, stratigraphic position, and paleomagnetic direction. In the Ely Springs caldera complex, respectively (Fig. 8). The 4.6 turally distinct “blobs” (Fig. 55). Phenocrysts, Range and at its White Rock Spring type section m.y. Isom-type time interval corresponds to the constituting less than 10% of this clast type, ~6 km north of the White River Narrows (Fig. time for production of dacite-rhyolite magmas in consist of tabular plagioclase (87%), pyroxene 57; see also Supplemental File 5 [see footnote the crust to shift southward from the Indian Peak (10%), and Fe-Ti oxides (3%) in a vesicle-free, 5]; du Bray and Hurtubise, 1994) it is a simple focus to the Caliente (Fig. 2). black glassy matrix. These blobs are especially cooling unit. At the Petroglyph Cliff locale at On many chemical variation diagrams, the conspicuous in the intracaldera unit in the Blind the north end of the narrows, the ignimbrite lies Petroglyph Cliff Ignimbrite and the three mem- Mountain caldera source and at its type local- below ground level and the petroglyph carvings bers of the Isom Formation are distinct from one ity at the north end of White River Narrows, are in the overlying 27.57 Ma Monotony Tuff another (Figs. 5, 6C, and 54). but are also seen in the Fairview Range. Their derived from the Central Nevada caldera com- shapes are consistent with incorporation into plex (Fig. 2). Just north of the petroglyphs and PETROGLYPH CLIFF IGNIMBRITE the ash fl ow in molten form. Chemically, the between the southward-dipping Monotony and blobs are somewhat variable in composition and Lund tuffs is a 3-m-thick bed of well-sorted, Like other Isom-type tuffs, the Petroglyph range from andesite to trachydacite (Figs. 54 fi ne, dark-gray semi-consolidated ash of the Cliff Ignimbrite (Cook, 1965; originally desig- and 56). Similar blobs characterize the dacitic Isom type. How this apparently correlative ash- nated as the White Rock Spring Ignimbrite by San Jose ignimbrite in northern Peru (Longo, fall deposit relates to the thicker (15 m) Petro- Martin, 1957) is generally partially to densely 2006) and have been reported in the Grizzly glyph Cliff Ignimbrite in the same stratigraphic welded and sparsely porphyritic, but is readily Peak Tuff in southwestern Colorado (Fridrich position 4 km to the north is not clear because of distinguished from them by unusually abundant and Mahood, 1987). discontinuous exposures. clasts of both cognate and foreign heritage; the The bulk chemical composition of the Petro- The Petroglyph Cliff Ignimbrite has been unit is basically a tuff breccia. In many outcrops glyph Cliff Ignimbrite is not as extreme for sev- found throughout the North Pahroc Range where there is a continuum in fragment size from the fi nest ash to the largest clast, ~25 cm, so there is no clearly distinguishable matrix, implying a 11 low-energy eruptive process such as dome col- X Y Y X Rhyolite lapse creating a block-and-ash fl ow. Striking 10 XXX X X X X Z XX X X X Z contrasts in the color of the fragments—shades Trachydacite YYXXXXXX X X Z XYX YYX XXX X Z Z of orange, red, purple, brown, gray, and black— 9 Y X X X XX XYXXX lend a distinct mottled appearance to outcrops. X X Z X XY Z X P X Most of the clasts appear to be cognate and Latite P 8 X X XX X some clasts are themselves pieces of eutaxitic X Isom-type tuffs O (wt %) 2 P P P Isom Formation tuff that contain discernible clasts. Angular P P P Z Hole-in-the-Wall lapilli of purple-gray porphyritic andesitic rock 7 P P P Tuff Mbr. n = 7 O + K

are common. Pale orange to yellow haloes sur- 2 P P Y Hamlight Tuff round holes that are fi lled with fi ne unidenti- Na 6 P Member n = 10 fi ed material. The abundance of clasts and the Dacite X Bald Hills Tuff potential for their disaggregation prior to fi nal Andesite Member n = 52 5 emplacement of the rock preclude an accurate P Petroglyph Cliff Ignimbrite n = 16 determination of phenocryst proportions. The diffi culty of excluding foreign material during 4 sample preparation likely compromises the true 59 61 63 65 67 69 71 73 75 77

chemical composition. SiO2 (wt %) The most distinctive type of cognate clast, not seen in other Isom-type tuffs, is rounded, tex- Figure 54. Total alkalies-silica diagram for Isom-type tuffs.

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AABB

Figure 55. Slabs of Petroglyph Cliff Ignimbrite displaying non-vesicular, porphyritic, petrographically identical “magma blobs.” (A) Intra- caldera unit in the Blind Mountain caldera (see Figs. 57 and 58) contains abundant dark-gray blobs in a pink clastic matrix. Some of the blobs have lighter-colored, more devitrifi ed cores. Key near the bottom of the slab is 5 cm long. (B) Upper oxidized part of the outfl ow cooling unit at the north end of the White River Narrows contains abundant fl attened blobs, some of which have less oxidized, light to dark gray interiors. Numerous, smaller, lighter-gray lapilli are of limestone; one near the head of the hammer is bleached white. Hammer is 36 cm long.

it is overlain by another Isom-type cooling unit A 700 Petroglyph Cliff Ignimb. called the upper member of the Petroglyph Cliff 600 Cognate blobs Andesite dike Ignimbrite by Scott et al. (1992, 1994, 1995a, Lamerdo 500 Quartz diorite 1995b). However, because this younger unit has Isom Fm.

a distinctly different paleomagnetic direction, is 400 r f Figure 56. Variation diagrams not everywhere petrographically like the Petro-

Zr (ppm) showing the composition of glyph Cliff, and is separated from it by bedded 300 the Petroglyph Cliff Ignim- ash or by a more widespread andesitic lava ﬂ ow, Rhyolite 200 Andesite lavas Monotonous Int. brite and its included cognate we do not include it as a part of the Petroglyph blobs and the composition of Cliff Ignimbrite. 100 an andesite dike and quartz The Petroglyph Cliff has a markedly lobate 0 diorite intrusions within the distribution north and southwest of the source 50 55 60 65 70 75 80 SiO (wt%) Blind Mountain caldera, com- caldera (Fig. 57); it is not found to the east 2 pared with the composition across Lake Valley in the Wilson Creek Range of other rocks in the Indian nor to the west in the Bristol Well quadran- B 10 Peak-Caliente ﬁ eld. The Petro- gle (Page and Ekren, 1995). Isom-type tuffs 9 glyph Cliff magma apparently in the latter area probably correlate with the 8 followed a differentiation path Bald Hills Tuff Member of the Isom Forma- vas 7 from andesite to a composition tion, according to our chemical data. In the Andesite la 6 similar to that of some parts of Coyote Spring quadrangle to the west, Ekren 5 the Isom Formation. (A) SiO - and Page (1995) mapped a compound cool- Int. 2 CaO (wt%) 4 Zr. (B) TiO -CaO. Int.—inter- ing unit of what they designated Petroglyph Monot 2 3 mediates. Cliff Ignimbrite. However, the paleomagnetic 2 te direction of this unit (site 4L073, S. Gromme Isom Fm. 1 Rhyoli Lamerdorf and M. Hudson, 2006, personal commun.) 0 differs from the Petroglyph Cliff, as does the 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 lithology. TiO 2 (wt%)

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the range front (Fig. 58). Inside this Blind Moun- 0 38°30′ Petroglyph Cliff ! tain caldera, a mass of variably metamorphosed Ignimbrite (29.1Ma) Devonian limestone is intruded by granite, quartz diorite, and porphyritic plagioclase-pyroxene 0 ! 0 ! andesite; one mass of andesite forms a ring dike 0 ! adjacent to the caldera margin. Lenses of steeply ~50 ! dipping, foliated Petroglyph Cliff Ignimbrite, 0 0 Figure 57. Distribution and ! ! which contain an abundance of the distinctive thickness (in meters) of the 0 black blobs (Fig. 55), trend parallel to the long ! <170 Petroglyph Cliff Ignimbrite 38° E ! 10 Blind Mountain axis of the exposed caldera and likely represent ! and location of the small Blind 15 ~15! caldera feeder vents; they are no more than 170 m thick. ! ~60! Mountain caldera source where ! 0 85 ! Using the isopach map of Figure 57 and

the intracaldera tuff is no more ! 50 0 Model 1 of Figure 4, the volume of the Petro- 35 !0 than 170 m thick. glyph Cliff Ignimbrite is estimated to be 40 km3. ! ~45 ISOM FORMATION 0 ! 0102030 37°30′ E Km 0 01020 Mackin (1960) realized that some dark-col- ! Miles ored, phenocryst-poor rocks that occur as widespread and generally thin lava-like deposits in ′ 115° 114° 30 the eastern Great Basin and High Plateaus of south-central Utah (Fig. 59) are ash-fl ow tuffs, not lava fl ows as believed by earlier workers. Blind Mountain Caldera corrected for new decay constants), which is He named these densely welded cooling units analytically the same as the Petroglyph Cliff. the Isom Formation. The formation is the basis The igneous center at Blind Mountain on the However, Johnson (1972) obtained a (corrected) of the designation “Isom-type tuffs” indicated southwestern fl ank of the Bristol Range (Figs. age on hornblende of 35.8 ± 3.3 Ma but this age above. Anderson et al. (1975) recognized three 57 and 58) is believed to be the source of the is signifi cantly older than ages on nearby intru- tuff members of the formation in the High Pla- Petroglyph Cliff Ignimbrite. Page and Ekren sions and lava fl ows (Best et al., 1989b, their teaus; in ascending stratigraphic order they are (1995) recognized that the dioritic intrusions in fi gure 3) and we consider it spurious. the Blue Meadows, the Bald Hills, and the Hole- the center have modal compositions like Isom- Detailed mapping by G.J. Axen (2010, per- in-the-Wall. The Blue Meadows (Fryman, 1987) type tuffs and considered it to be a local source sonal commun.) has disclosed that the igneous is found only in the Markagunt Plateau in cen- for some of them. A K-Ar age on hornblende center surrounded by Cambrian sedimentary tral Utah and is not considered further here. The (plus minor biotite and chlorite) from an associ- rocks is a semi-elliptical segment of a caldera other members are more widespread regional ated granite is 28.4 ± 0.9 Ma (Armstrong, 1970; exposed for ~2 km in longest dimension along ignimbrites, especially the Bald Hills. In our mapping we have not distinguished between the Bald Hills and the Hole-in-the-Wall because of 114°37′30″ 114°36′10″ the similar modal composition and characteris- 38°1′10″ 01Mile tics of their constituent cooling units in the fi eld, Є except where other rock units intervene (e.g., in 01Figure 58. Geologic map of the Tqd the Rose Valley area; Best and Williams, 1997) Kilometer Blind Mountain caldera. Gen- Ta or where paleomagnetic data are available to Ta eralized from a more detailed distinguish the reversely magnetized Hole-in- D Ta Є 1:12,000-scale map kindly pro- Ta Tp 82 the-Wall from the normally magnetized Bald Tp vided by Gary J. Axen, who Tp Hills. In Condor Canyon, just southwest of the Tp indicates that the southwest Tqd 66 probable source area of the Isom Formation, an Ta Tp Ta half of the caldera had been additional sequence of normally magnetized Ta cut off by the Highland detach- TERTIARY 45 D cooling units of the Isom type has been found D 84 Є ment, before basin-and-range Tqd Quartz lying between the 27.90–27.25 Ma Bald Hills diorite Ta faulting dropped it as a gra- and the 24.5 Ma Hole-in-the-Wall Tuff Mem- Tg Tp ben. Note the steeply dipping Tg Granite D bers (Table 1). This proximal sequence (Fig. feeder dikes of Petroglyph Cliff D 60), referred to as the tuff member of Hamlight Ta Andesite 38°00′ Ignimbrite, ring dike of por- Tp D Canyon by Scott et al. (1995a), but here formal- Quaternary Tp phyritic plagioclase-pyroxene Petroglyph Cliff ized as the Hamlight Tuff Member, is also petro- Tp Tqd D andesite, intrusions of granite Ignimbrite graphically indistinguishable from the other Є and quartz diorite, and vari- DEVONIAN two members of the Isom Formation; absent ably metamorphosed Devonian D Sedimentary rocks stratigraphic control, only paleomagnetic or limestone. chemical analyses distinguish the Hamlight. CAMBRIAN The four Hamlight cooling units are presumed Є Sedimentary rocks Ring fault Sample site to have also originated from the same source

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A Isom Formation ! Hole in the Wall Tuff Member (24.55 Ma) 38°30′ E E Hamlight Tuff Member (24.91 - 24.75 Ma)

Figure 59. Distribution and thickness (in meters) of the Isom For- mation and the location of a possible source area in the Escalante E E E 0-3 ! Desert. (A) Hole-in-the-Wall and Hamlight Tuff Members. (B) Isom 38° ! 5 0-8 Formation undivided, including all three tuff members but con- ! <130 ! Source 38 ! 0 100 area ! 3-18 sisting mostly of the Bald Hills Tuff Member. Sites with no indi- ~14 ! ! ! ! 50 150 0 cated thickness are where paleomagnetic samples of the Isom were 0 30 12 ! ! ! 0 collected . ′ 0 E E E 37°30 !

010 30 50 0 Km ! 0 ! 0102030 NV UT Miles 37° E 115° 114° 113°

B Isom Formation undivided NV UT (27.90–24.55 Ma) 0 !

6 0 ! 0 ! ! ! 6 0-6 ! 12 60 E 38°30′ E 5 E ! E ! 4 Km 10-20 ! ! Corrrected for 28 0-5 ! 50% extension ! 0-5 ! 135 100 10 20 ! ! ! ! ! 0 >350 ! 20 0-20 10 ! ! ! ! 10 35 <400 100 ! ~50 ! 600 >150 105 0 ! <100 ! ! ! ! E 38° E ! E ~800 E 100 45 ! >5 ! ! ! ! ~105 8? 8 ! ~50 ! ! 15 ! Source ! 410 148 0-6 0-25 ! ! ! area 50 ! ! ! ! ! <35 ! ~5 0 ! 45-100 ! <65 <200 400 365 130 ! ! ! ! 300 ! <73 >230 0 200 12 ! 0 ! ! >100 ! <20 100 ! ′ ! ! 0 37°30 E E 35 ! 0 E 6 ! 24 20? 20? ! ~30 ! ! ! 0 ! ! ! 0 0? 0 ! 0 ! 020406080 ! Km 0 0 ! ! 02040 Miles 37° 115° 114° 113°

as the rest of the formation (Fig. 59; see also Most outcrops contain lighter-colored lenses brite that separated and buoyed intervening below). as much as a meter in diameter but usually less less-gaseous layers during emplacement of The generally cliff- or ridge-forming cool- than 2 cm thick whose parallelism imparts a the relatively thin ignimbrites. According to ing units of the Isom Formation are relatively foliation to the tuff. Some of these lenses may these workers, these gassy zones facilitated thin, typically less than 10 m to as thick as be compacted pumice that are devitrifi ed and laminar fl ow, allowing the mass to travel great 20 m, densely welded, and usually occur in dark replaced by vapor-phase minerals. But others, distances. Chapin and Lowell (1979) came to shades of purple, red, and brown. A black, near- called “lenticules” by Mackin (1960) and the same conclusion in their study of lenticles basal vitrophyre a few meters thick commonly Anderson and Rowley (2002), are believed to in the Wall Mountain Tuff of south-central underlies the devitrifi ed facies. be devitrifi ed gas-rich portions of the ignim- Colorado.

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Leach CCa nyon

H am li g Hol ht e-in-th e-Wa ll

Lava

Figure 60. Panoramic view looking north at exposures of the Hamlight and Hole-in-the-Wall Tuff Members of the Isom Formation in Condor Canyon. The relatively thinner four Hamlight cooling units in the photo center (the upper two units are a doublet of thin ledges) are overlain by a prominent cliff of the Hole-in-the-Wall Tuff Member and underlain by andesitic lava. Person (in circle) at the bottom of the outcrop of the second Hamlight unit provides scale. For description of the entire stratigraphic section of 20 ignimbrite units in Condor Canyon see Supplemental File 5 [see footnote 5].

Stratigraphy time period of 160,000 years. Only a slightly of 27.59 ± 0.15 Ma. The lowest of three cooling longer time period separates the youngest Ham- units in a proximal section 148 m thick east of A summary of the stratigraphy and occur- light from the Hole-in-the-Wall Tuff Member. the source area has an age of 27.60 ± 0.14 Ma. rence of the Isom units, based primarily on their The only other known occurrence of the Based on the dated units in the two sequences of paleomagnetic directions as known in the early Hamlight Tuff Member is in the North Pahroc three units, it appears, therefore, that the Bald 1990s, is provided by Scott et al. (1995a). Here, Range (Supplemental File 5 [see footnote 5]; Hills consists of at least four cooling units, the we add further pertinent details. For reasons that Scott et al., 1992) where two Isom-type cooling youngest and oldest dated ones being exposed will become apparent, we discuss the members units with a total thickness of 30 m lie in the in Condor Canyon. These four cooling units of the Isom Formation from youngest to oldest. same stratigraphic interval and have the same were emplaced over a time interval of 0.65 m.y., paleomagnetic direction as the Hamlight cool- longer than the duration of emplacement of the Hole-in-the-Wall Tuff Member ing units in Condor Canyon. four Hamlight cooling units. There was a lull in This reversely magnetized ignimbrite was eruptions in the Indian Peak–Caliente fi eld of emplaced at 24.55 ± 0.12 Ma as a single simple Bald Hills Tuff Member 2.34 m.y. between the Bald Hills activity and cooling unit. Thickest sections (50–150 m) of This is the oldest and by far the most exten- the much less voluminous Hamlight and Hole- the member occur west and southeast of the sive member of the Isom Formation (Fig. 59). in-the-Wall eruptions that together lasted 0.36 Escalante Desert. Because of its similarity to It consists of at least four cooling units, and m.y. (Table 1). other Isom cooling units in the fi eld, it has likely perhaps more in some places near the source; Paleomagnetic data and stratigraphic rela- been included with the Bald Hills Tuff Member they are everywhere superposed directly atop tions of dated units indicate that the western- in many places and has been verifi ed by strati- one another without intervening deposits. Distal most outcrop of the extensive Bald Hills units graphic position and paleomagnetic and/or occurrences are of a single cooling unit. In addi- is at Hancock Summit in eastern Nevada (Fig. chemical analyses at only eleven sites (Fig. 59A). tion to age and stratigraphic position, the Bald 59B) where it is exposed as a 6-m-thick simple Hence, its true areal extent and volume estimated Hills is chemically (e.g., Fig. 6C) and paleo- cooling unit of black vitrophyre lying between by Model 1 (Fig. 4) are unknown but were at magnetically distinct from younger members. the Monotony Tuff (27.57 Ma) and the Lower least 5900 km2 and 600 km3, respectively. 40Ar/39Ar analyses of plagioclase from the Tuff Member of the Shingle Pass Formation three exposed Bald Hills cooling units in Con- (26.98 Ma), both of which had sources in the Hamlight Tuff Member dor Canyon yield a weighted mean age of Central Nevada caldera complex (Fig. 2). Addi- In Condor Canyon, this 130-m-thick middle 27.90 ± 0.09 Ma for the lowest unit and 27.25 tional exposures of a thin simple cooling unit member of the Isom Formation comprises four ± 0.09 Ma for the uppermost unit; analyses of of Isom-type tuff have been found for ~100 km cooling units that are superposed directly atop the middle unit yielded stratigraphically incon- northward at nearly the same longitude. The one another; they underlie the Hole-in-the-Wall sistent ages. Samples from two other locales most easterly exposures of the Bald Hills are in Tuff Member and are underlain by local ande- have intermediate and indistinguishable ages the High Plateaus in south-central Utah (Kurlich sitic lava fl ows (Fig. 60; Supplemental File 5 between the lowest and uppermost units in Con- and Anderson, 1997; Anderson et al., 1990), [see footnote 5]). 40Ar/39Ar ages on the cooling dor Canyon, indicating they might be the same making the present east-to-west extent ~250 km, units range from 24.91 to 24.75 Ma (Table 1), cooling unit: A single distal cooling unit at the or 170 km after correction for post-deposition indicating emplacement over a relatively brief south end of the Fortifi cation Range has an age extension. The most northerly certain occur-

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rence of the Bald Hills is a single cooling unit exposed rocks are post-Isom lavas several hun- Whether one or more of these four cooling units 5 m thick at the south end of the Fortiﬁ cation dreds of meters thick (Fig. 8A; Best, 1987) and resulted from super-eruption of at least 1000 km3 Range (Loucks et al., 1989); its age of 27.59 Ma where no evidence for a fault-bounded caldera cannot be answered without further study. was indicated above. Southernmost (<37.5° N exists. On the northeastern margin of this area, latitude) exposures of the Isom Formation on in the southern Indian Peak Range (Best et al., Composition of Isom Formation either side of the Utah-Nevada state line (e.g., 1987a), the map relations described above for Ignimbrites Hintze et al., 1994a) are not distinguished on the southeastern margin of the White Rock cal- a member basis so the true, present north-to- dera could equally be considered as evidence The modal composition of each of the three south extent of the Bald Hills Tuff Member is for the northeastern margin of a source caldera members appears to be uniform and similar uncertain but could be ~170 km. Its extension- for the Isom Formation; especially noteworthy to the other two members (Fig. 61). The only corrected area of exposure is 21,000 km2. in this regard is the drastic thinning of the Isom difference might be a tendency for the Hole-in- tuff toward the northeast. the-Wall to have a little less pyroxene than the Source On the basis of the equilibration pressure of other two older members. The lowest Hamlight the phenocryst assemblage, Isom-type magmas cooling unit contains a few sanidines that do not Anderson et al. (1975, p. 18; see also Best appear to have erupted from a depth of ~30 km appear to be xenocrystic, based on their strati- et al., 1989a) suggested that the source of the (see below), which is at least three times deeper graphically consistent 40Ar/39Ar age (Table 1). Isom Formation lay to the southeast of the Indian than that from which the rhyolite and dacite cal- Densely welded, near-basal vitrophyres of three Peak caldera complex in the Escalante Desert dera-forming magmas erupted in the Great Basin. Bald Hills cooling units sampled in the White where thickest sections occur in surrounding With the magma erupting from a chamber beneath Rock Mountains show an upward increase in hills (Fig. 59B). The aggregate thickness of the a 30-km-thick roof and the chamber itself of per- the concentration of total phenocrysts (5.0% to Bald Hills and Hole-in-the-Wall Tuff Members haps a similar horizontal diameter, it might be 8.4% to 14.3%) but the proportions of constitu- is 365 m to the southeast of the desert and of all thought that a fault-bounded depression would ent phenocrysts reveal no consistent variation. three members of the formation is ~480 m to the not have developed; instead, the surface might In contrast to the apparent uniformity in west in Condor Canyon; both sites lie outside only have sagged downward as the magmas modal composition of the three members of the of the Indian Peak caldera complex delineated were withdrawn. However, in the model experi- formation, their bulk chemical compositions are in Figure 8 and do not, therefore, represent ments of Roche et al. (2000) fault-bounded cal- distinctly different in some variation diagrams unusual thicknesses ponded within older caldera deras develop for roof aspect ratios (thickness / (Figs. 6C and 54). Especially noteworthy are the

depressions. However, thickness of 350–800 m width) to as much as 4.5. Surface subsidence distinctly lower concentrations of TiO2, Fe2O3,

occur as caldera ﬁ ll within the older White Rock diminishes relative to the magma drawdown as and Sr and higher SiO2 in samples of the more caldera to the northwest of the desert whereas the ratio increases; but for an aspect ratio of 1, evolved Hole-in-the-Wall, all of which are low- regional thicknesses of 100 m or so occur just which might be approximately the case for the silica rhyolite, in contrast to samples of the other beyond the caldera margin. Isom caldera, the subsidence and drawdown are two older members that are less evolved and Cuttings from three drill holes in the Escalante about the same. Gravity data (Fig. 8C) provide nearly all trachydacite. The Hole-in-the-Wall as Desert offer only inconclusive support for the no constraint on the location of an Isom caldera. well as the Hamlight have less Zr than most of presence of a buried source caldera. A well just the Bald Hills samples. The Hamlight samples south of the Beryl railroad siding reveals only Volume mostly overlap the Bald Hills but lie at their high

several meters of Isom beneath hundreds of end for TiO2, SiO2, Fe2O3, CaO, and Sr. There is

meters of the Bauers ignimbrite (Table 1) and The combined volume of all of the outﬂ ow an increase in Al2O3/CaO ratios from the Bald above several hundred meters of a sequence of cooling units of the Isom Formation is estimated Hills and Hamlight to the more evolved Hole- andesitic and carbonate rock before bottom- at 2100 km3. Doubling this outﬂ ow volume in-the-Wall, which is consistent with fractional ing at a depth of 1862 m. A well 8 km east of according to Model 1 in Figure 4 gives a total crystallization of a less evolved Bald Hills–type Table Butte passed through the Leach Canyon volume for the formation of 4200 km3 (Table 1). magma to the Hole-in-the-Wall, but the 2.7 m.y. ignimbrite from ~1384 to 1686 m depth and then At least four cooling units of the Bald Hills Tuff age difference precludes such a simple interpre- through a few tens of meters of siltstone before Member together had a volume of ~3600 km3. tation; a closed magma body would not persist bottoming in the Isom at a depth of 1750 m. A 5644-m-deep well 3 km northwest of Table Butte discloses no Isom tuff between 200 m of the Leach Canyon and hundreds of meters of Isom Formation 80 Hole-in-the-Wall Tuff Member n = 3 underlying Lund, Wah Wah Springs, and Lamer- Average of 10 analyses (Anderson et al., 1975) dorf tuffs. Thousands of meters of Isom tuff was + Hamlight Tuff Member n = 3 anticipated in these wells, if indeed the source 60 Bald Hills Tuff Member n = 7 caldera is concealed beneath the desert, but was Average of 40 analyses (Anderson et al., 1975) not found. However, there is the possibility that Figure 61. Modal proportions the sequence of andesitic and carbonate rock in of phenocrysts in the Isom For- 40

the Beryl well and the Lund, Wah Wah Springs, mation. Mode (vol %) and Lamerdorf tuffs found in the well northwest 20 of Table Butte actually represent wall-collapse breccias in a concealed Isom caldera. We cannot rule out a source to the west of 0 the Escalante Desert in an area where the only Plag Qtz San Bio Hb Px Opaq Pheno

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that long. Moreover, the Hamlight magma that time relations and 87Sr/86Sr ratios suggest the For example, rhyolites from the Caliente com- is only slightly older than the Hole-in-the-Wall following scenario: With continued inﬂ ux of plex have Sr isotope ratios that are lower than does not have the proper composition to be maﬁ c mantle-derived magmas into the crust, 0.7085 whereas all rhyolites from Indian Peak parental to it because, for example, Zr concen- less additional silicic magma could be created are higher than this, ranging to as high as 0.712

trations are too low and TiO2 is too high in the by partial melting because felsic components (Table 4, Fig. 6D). This difference implies less Hamlight (Fig. 6). in the now less-fertile magma source volume crustal contamination of magmas in the Caliente Relative to rocks of comparable silica con- had been largely extracted during generation complex or a contrast in the source and compo- centration (mostly 66–69 wt%), alkali-calcic of the preceding three super-eruptive monoto- sition of the mantle magma component.

trachydacitic Isom-type tuffs have higher Al2O3, nous intermediate magmas (Cottonwood Wash, It might be expected that some degree of

TiO2, K2O, P2O5, Ba, Rb, Ce, Zn, Zr, Y, and Th Wah Wah Springs, Lund). Instead, accumulat- inheritance or commonality among the chemi- and generally higher Nb, Y, and U. On the other ing masses of mantle-derived magmas evolved cal compositions of the rhyolite ignimbrites hand, they have distinctly lower concentrations into andesitic derivatives that upon further frac- would be evident because of their eruption from of many compatible constituents such as CaO, tional crystallization created the near-liquidus a focused magma system in the crust. However, MgO, Sr, Ni, Cr, and V. trachydacitic Isom-type magmas, into which representative analyses (Fig. 62B) reveal that little or no sialic crustal components could be the fi ve rhyolite ignimbrites evolved along dis- Origin of Magmas mixed or assimilated. As a result, initial 87Sr/86Sr tinctly separate but essentially parallel trends, ratios are distinctly lower in both the Petroglyph lacking any simple age progression. Isom magmas were less crystallized, drier, and Cliff (0.7084) and the Isom (Bald Hills Member, Williams (1967, p. 118) concluded that the hotter compared to most rhyolite and dacite mag- 0.7077) than any of the preceding intermediate Leach Canyon, Swett, and Bauers ignimbrites, mas in the Great Basin as indicated by the sparse to felsic magmas erupted in the fi eld (Fig. 6D). and possibly the Hiko Tuff, originated in “a phenocrysts that include pyroxenes rather than The Isom-type magmas could not ascend buoy- large caldera-like igneous complex center- the more common hydrous assemblage of bio- antly nor by diking through the residual mag- ing about the town of Caliente.” He based this tite and hornblende. Phase equilibria calculated mas under the Indian Peak caldera complex but conclusion chiefl y on the fact that the thickest using MELTS (Ghiorso and Sack, 1995) also were able to erupt just beyond on the northwest sections of these tuffs surround this area. Recon- indicate a greater depth of phenocryst equilibra- and southeast margins of the complex. naissance work by Noble et al. (1968) and Noble tion. An average Isom-type composition at ~8 kb and McKee (1972) further targeted the Caliente (~30 km depth), 950 °C, oxygen fugacity fi xed topographic depression as an ignimbrite source. IGNIMBRITES OF THE CALIENTE at QFM + 1, and 2 wt% H O yields the phase Ekren et al. (1977) produced the fi rst geologic 2 CALDERA COMPLEX assemblage seen in the Isom-type ignimbrites, map of the Caliente caldera complex and, on i.e., clinopyroxene, orthopyroxene, plagio clase, the basis of geologic, gravity, and aeromagnetic and magnetite. Lower-pressure assemblages Following voluminous eruptions of monoto- data, expanded its extent to the east and west lack clinopyroxene and may have quartz or sani- nous intermediate and subordinate rhyolite mag- and shifted its center southward. Larger-scale dine. Fractionation of this high-pressure phase mas from the multicyclic calderas of the Indian mapping by Peter Rowley and associates, sum- assemblage develops high concentrations of Peak complex, generation of rhyolite magmas marized in Rowley et al. (1995), extended the

many incompatible elements, including K2O, shifted southward tens of kilometers while the margin of the complex still farther south as well

without strong enrichment in SiO2. Isom-type magmas were being erupted from as east into Utah (Fig. 2). They recognized the The intimate spatial and temporal association 27.90 to 24.55 Ma. The new focus of eruptive Clover Creek caldera source of the Bauers and of Isom-type tuffs in some near-source occur- activity after ca. 24 Ma lay in the multicyclic the Delamar caldera source of the Hiko in the rences with high-K pyroxene andesite lava Caliente caldera complex where overlapping, complex. Rowley et al. (2008) later identifi ed fl ows, some of which also have high Zr con- or at least nearby, calderas developed (Fig. 2). the Telegraph Draw caldera source for the Racer centrations, indicate a kindred relationship. We Over the next 5.5 m.y., fi ve signifi cant erup- Canyon in the complex. No geologic evidence envisage fractionation of pyroxenes, plagioclase, tions of rhyolite magma took place, creating, has been found for caldera sources of the other and minor Fe-Ti oxides and apatite from ande- from oldest to youngest, the Leach Canyon, ignimbrites, but their locations can be roughly sitic parent magmas at depths of near 30 km in the Swett, Bauers, Racer Canyon, and Hiko ignim- approximated on the basis of the distribution crust to yield the near-liquidus Isom-type mag- brites (Table 1). An additional eruption of the and thickness of the outfl ow deposits. mas. Slightly different parental magmas, envi- unique andesite-latite Harmony Hills ignimbrite According to Rowley et al. (1995, 2001), the ronments of differentiation, and possible extent occurred at 22.56 Ma. Four of these six erup- 80 × 35 km Caliente caldera complex evolved of contamination with crustal wall rock led to tions were of super magnitude (>1000 km3), from 23 to 13 Ma during regional tectonic different erupted compositions. including the Harmony Hills. The dominance of extension in the area. East-striking syn-volcanic Immediately following deposition of the rhyolite over dacitic ignimbrites in the Caliente fracture systems, or transverse zones, bound the Lund, which is the youngest monotonous inter- complex represents an inversion of what had margins of the elongate caldera complex. mediate in the Indian Peak fi eld, the small-vol- taken place before 28 Ma in the Indian Peak Local ash-fl ow tuff and local tuff breccia ume Petroglyph Cliff Ignimbrite was erupted caldera complex when and where monotonous as thick as 500 m of the Rencher Formation from its Blind Mountain source caldera just intermediates dominated eruptive activity. are exposed in the Bull Valley and Pine Valley west of the northwestern margin of the Indian Another distinctive compositional difference Mountains in the southwestern corner of Utah Peak caldera complex. A little more than one between the volcanic rocks of the two caldera (Cook, 1957; Blank, 1959, 1993; Blank et al., million years later, voluminous eruptions of complexes is found in their 87Sr/86Sr ratios; for a 1992; Rowley et al., 2006; Biek et al., 2009). the Isom Formation cooling units began from a given silica content, the initial Sr isotope ratios This unit has an age of 22.2 Ma, placing it concealed source on the opposite, or southeast- of rocks erupted from the Caliente complex are between the Harmony Hills and the Racer Can- ern, side of the caldera complex. These space- lower than those from the Indian Peak complex. yon Tuffs (Table 1; the Rencher is not listed in

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A 11 be a product of a more or less continuous erup- Caliente rhyolite ignimbrites V Tuff of Tepee Rocks n = 1 tive event during which, possibly as a result of O Hiko Tuff n = 11 10 vent collapse, the later-erupted ejecta entrained R Racer Canyon Tuff n = 3 Condor Canyon Fm. BB more wall-rock fragments. Although the pro- B BB B Bauers Tuff Mbr. n = 19 BBBB BBBB B TTB BB portions of phenocrysts in the two members are T Swett Tuff Mbr. n = 8 TT T 9 TE BT quite similar, the older Narrows tuff tends to O (wt %) E Leach Canyon Fm. n = 17

2 O E E E E E have a lower proportion of plagioclase and bio- O E E E E O E O + K 8 O E V tite and more sanidine and quartz than the Table 2 R OO O O E Na R E Butte (Fig. 63); hence, considered together, the O R two members exhibit normal zoning. 7 Dacite Rhyolite Based on the general pattern of southward Figure 62. Chemical variation migration of the inception of volcanism through diagrams for rhyolite ignim- 6 time in the region, the source for the Leach Can- brites younger than 25 Ma 59 61 63 65 67 69 71 73 75 77 yon likely lies south of the Indian Peak caldera in the Indian Peak–Caliente SiO2 (wt %) complex. However, no evidence for a fault- fi eld. (A) Total alkalies-silica. bounded caldera created upon super-eruption (B) TiO2-Fe2O3. Note separate B 4 of the exposed 1800 km3 of outfl ow ejecta has but parallel trends and normal Caliente caldera complex rhyolite ignimbrites been found. Compilation of thicknesses of the zonation in all but the Bauers R Leach Canyon by Williams (1967, his fi gure 9) Tuff Member of the Condor 3 R O O clearly targeted a source around Caliente and Canyon Formation. O OO our updated compilation does the same (Fig. OO EEE T 64). Younger lava fl ows and sedimentary depos- RO B Arrows point

(wt %) E O E O T in up-section its in the Panaca Basin (Rowley and Shroba, 3 2 E B B T T O EEO B TTT direction 2 E B EB BBBB T 1991) conceal any direct evidence of a source R E BBBBBB TT Fe E EE B T V Tuff of Tepee Rocks n = 1 in this area. Outcrops of the upper Table Butte E EE O Hiko Tuff n = 11 member near the northeast margin of the basin 1 E R Racer Canyon Tuff n = 4 V Condor Canyon Formation near Ursine reveal the “largest known lithic B Bauers Tuff Mbr. n = 21 T Swett Tuff Mbr. n = 11 fragments” (Williams, 1967, p. 116); the Nar- E Leach Canyon Fm. n = 17 rows member is most densely welded in Utah 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 where a near-basal vitrophyre occurs in some places. South of Caliente, younger calderas in TiO2 (wt %) the Caliente caldera complex have likely buried at least the southern part of the source caldera of this table). The Rencher is chemically an ande- between the two members was indicated by Wil- the Leach Canyon ignimbrite. site-dacite (60–64 wt% silica) and lacks quartz liams (1967), an aspect observed by us in Con- The western distribution of the tuff beyond and sanidine, but is otherwise similar to the dor Canyon (Fig. 60). In quadrangles northwest its presumed source area was infl uenced by phenocryst-rich Harmony Hills. The Rencher of Cedar City, Rowley (e.g., 1976) distinguished piles of andesitic lava fl ows. In the Deadman is considered to be a small-volume, local erup- the upper Table Butte as a less resistant tuff that Spring NE quadrangle (Swadley et al., 1994) tive facies of the Bull Valley intrusion, one of contains as much as 10%–15% dark red and pur- at 37°59′ N and 114°52′30″ W, the Leach Can- several shallow monzonitic intrusions along the ple aphanitic volcanic fragments underlain by yon is absent and only several meters of the “Iron Axis” trending west-southwest of Cedar the more resistant, moderately welded Narrows older Isom and Petroglyph Cliff are draped on City that have associated iron ore deposits (Biek that contains less than 2% of the same type of top of a stack of lava fl ows as thick as 600 m. et al., 2009). The Rencher is not considered fur- lithic fragments. Although not optimal magnetic The Leach Canyon is also absent to the south ther here. recorders, the two members have similar paleo- in the Pahroc Spring SE quadrangle (Swadley magnetic directions. The two members appear to and Rowley, 1994) at 37°32′ N and 114°47′ W LEACH CANYON FORMATION

Ignimbrite of the 24.03 ± 0.01 Ma Leach Canyon Formation is rather ordinary rhyolite Leach Canyon Formation (plus trace of titanite) 60 that consists of slightly more phenocrysts of + Table Butte Tuff Member n = 14 plagioclase than subequal amounts of sanidine X Average of 8 analyses (Anderson et al., 1975) and quartz; lesser biotite, hornblende, and Fe-Ti X Narrows Tuff Member n = 14 Figure 63. Modal proportions of oxides; and trace amounts of pyroxene, titanite, 40 Average of 12 analyses phenocrysts in the Leach Canyon (Anderson et al., 1975) zircon, and apatite (Figs. 62 and 63). Conspicu- Formation. Data from Williams ous lapilli of pumice are typical. Mode (vol %) X (1967), Rowley et al. (1995), and The Leach Canyon was subdivided by Wil- 20 X Anderson et al. (1975). liams (1967; see also Mackin, 1960; Cook, 1965; X Rowley et al., 1995) into two units, an older X X X Narrows Tuff Member and an overlying Table 0 X Butte Tuff Member. No deﬁ nite cooling break Plag Qtz San Biot Hb Px Opaq Pheno

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NV UT ! Leach Canyon Tuff 0 ! 60 E 38°30' (24.03 ± 0.01 Ma) E E 0 0 ! ! 0 12 0 ! ! !

0 ! 0 <75 ! ! ~5 ! 0 ! ~135 0 0 ! ! ! 0 ! 90 ! 38° E 0 E ! E ! <120 <200 >130 ! ! ! 60 140 105 180 ! ! ! >60 200! 36 >80 ! 250 ! 160 ! ! 0 >200 200! >25 ! ! ~60 ! ! 120 <150 ! >20 ! ! ! 150 18 ! 0 143 ! 0 ! ! ! Possible source 200 >200 ! !0 100 area 37°30' E 0 E ! 75 E <180 >120 ! 152 Corrected for 50% ! ! 30 extension 350 ! ! ! 40 100 ! >40 ~7 200 ! >60 ! 0 >6 01020304050 ! Km 100 0 10 20 30 ! 50 km 37° Miles E E E

115° 114° 113°

Figure 64. Distribution and thickness (in meters) of the Leach Canyon Formation. Dashed line indicates the outer perimeter of the possible area within which a source caldera of unknown size is likely to be concealed beneath younger deposits. The caldera margin could possibly extend as far west as the contemporaneous stratovolcano to the west marked by 0 and 100 m isopachs.

where a cluster of andesitic-dacitic stratovol- and Zr but low CaO and Sc (Figs. 5, 6, and 62). tain moderate to abundant lapilli and blocks canoes lie between the Lower Tuff Member of In some element variation diagrams, Condor of pumice. Chemical and modal compositions the 26.98 Ma Shingle Pass Formation and the Canyon samples are relatively tightly clustered are reminiscent of low-silica rhyolite tuffs Swett Tuff Member of the Condor Canyon For- and lie apart from other main-trend ignimbrites (Lamerdorf and Ryan Spring) erupted from mation (Table 1); thus, they essentially bracket of the Indian Peak–Caliente ﬁ eld. Each member the Indian Peak caldera complex except for the the time of eruption of the Leach Canyon contains 8%–23% phenocrysts, mostly plagio- general absence of lithic clasts in the Condor ignimbrite. It is possible that these stratovol- clase and lesser biotite, but no quartz (Fig. 65); Canyon tuffs. canoes of about the age of the Leach Canyon the Bauers also has abundant sanidine and trace Condor Canyon cooling units are com- lie on the west margin of the source caldera. amounts of clinopyroxene. Both members con- monly relatively thin and moderately to densely Taking into account an assumed equivalent volume of ignimbrite in the concealed source caldera as occurs as outflow (Model 1 in Fig. 4), the total volume of the Leach Canyon Condor Canyon Formation 80 + Bauers Tuff Member n = 22 is 3600 km3. X Average of 13 analyses Figure 65. Modal proportions (Anderson et al., 1975) of phenocrysts in the Condor 60 Swett Tuff Member n = 25 CONDOR CANYON FORMATION X Average of 9 analyses Canyon Formation. The major (Anderson et al., 1975) difference between the Bauers The Condor Canyon Formation (Table 1; 40 and the Swett Tuff Members Mackin, 1960; Cook, 1965; Williams, 1967; Mode (vol %) X is the absence of sanidine in Rowley et al., 1995) consists of two similar, the Swett. Data from Williams low-silica, off-trend rhyolite ignimbrites (Fig. 20 X (1967), Rowley et al. (1995), 62), the older Swett Tuff Member and the order- X and Anderson et al. (1975). X of-magnitude-larger Bauers Tuff Member. They 0 X X X have relatively high concentrations of alkalies Plag Qtz San Biot Hb Cpx Opaq Pheno

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welded, and thus commonly crop out as ledges. Thicker, proximal outfl ow sections are simple cooling units and display conspicuous secondary zonation (Williams, 1967). A black, near- basal vitrophyre a few meters thick is locally Figure 66. Bauers ignimbrite present and grades into an overlying zone of displaying strongly flattened densely welded, devitrifi ed tuff that is shades light-colored pumice clasts. Out- of brown, red, and purple. The gradation is mani- crop is in Condor Canyon. Keys fest by an upward increase in the concentration on the right are each 5–7 cm of red-brown spherulites as much as several long. centimeters in diameter. This zone of gradational devitrifi cation is overlain, in places with a rather sharp contact, by a strongly foliated zone (Fig. 66) that contains lighter-colored material in disc shapes; these discs are ~5 cm thick and locally as much as 2 m in diameter and in the Bauers compose upwards of one-half of the cooling unit. Some of these discs could be fl attened There is no direct evidence for a source cal- A small segment of the Clover Creek source collapsed pumice clasts that have devitrifi ed dera of the Swett; it was either largely engulfed caldera of the Bauers lies near Caliente in the and been replaced to varying degrees by vapor- in the Bauers source caldera (see below) or is northern sector of the Caliente caldera complex phase alkali feldspar, cristobalite, and tridymite, buried to the north of Caliente in the Panaca (Figs. 2 and 67B; Rowley et al., 1994). The but others could be lenticules as described above Basin that is fi lled with late Miocene–Pliocene intracaldera tuff is a compound cooling unit for the Isom Formation. The strongly foliated sedimentary deposits. A possible intracaldera more than 400 m thick that contains as much zone grades upward within several centimeters sequence more than 200 m thick of volcanic as 20% pumice clasts as long as 20 cm and as into a zone of massive tuff lacking the discs and minor sedimentary units that underlies much as 30% angular, mostly cognate volcanic whose matrix is rich in vapor-phase crystals. In the Bauers Tuff Member is exposed in the rock fragments as much as 12 cm across. The thinner, more distal stratigraphic sections, these southwest corner of the Panaca Summit quad- intracaldera tuff is intruded by a fl ow-foliated, zones are not as well developed and the foliated rangle (Williams et al., 1997) at 37°47′ N and hypabyssal intrusion of similar composition zone is absent. 114°13′45″ W. This sequence, whose base is and overlain by a sequence more than 150 m Though compositionally similar, 40Ar/39Ar not exposed, consists of ~100 m of intermedi- thick of tuffs and volcanic debris fl ows. An east- ages indicate the two members are more than a ate-composition lava fl ows, beds of intervening west–trending caldera margin is speculated to lie million years apart (Table 1). The Bauers has sedimentary rock, and lithic- and pumice-rich concealed beneath younger deposits to the north- a weighted mean age on sanidine of 23.04 ± ignimbrite cooling units, one of which has east and west of the intracaldera rocks (Rowley 0.11 Ma. Rowley et al. (1994) reported a sanidine the paleomagnetic direction of the Swett (site et al., 1992). age by L.W. Snee of 23.13 ± 0.1 Ma on the intra- 2P130, S. Gromme and M. Hudson, 2006, per- Of the ignimbrites derived from the Caliente caldera Bauers intrusion (see below). Duplicate sonal commun.). This sequence has not been caldera complex, the Bauers has the largest analyses of plagioclase from one sample of the seen anywhere else between the Leach Canyon outfl ow distribution at 23,000 km2 (Table 1 and Swett from Condor Canyon yielded a weighted and the Bauers where the Swett normally lies. Fig. 67B). But throughout most of this area it mean age of 24.15 ± 0.10 Ma. The Swett is very is no more than 50 m thick. In only three sites close in age to the underlying Leach Canyon at Bauers Tuff Member is the outfl ow as much as 200 m thick north 24.03 ± 0.01 Ma. and east of the caldera source. There is a hint The Bauers ignimbrite contains conspicuous of east-trending paleovalleys in the distal east- Swett Tuff Member phenocrysts of sanidine in addition to slightly ern part of the outfl ow sheet; paleotopography greater amounts of plagioclase and much less in the Condor Canyon area also infl uenced its The distribution of the Swett is quite irregular biotite, Fe-Ti oxides, and trace amounts of thickness. in the northeast (Fig. 67A). This unusual out- clinopyroxene (Fig. 65). The sparse feldspar Total volume of the Bauers ignimbrite, crop pattern might refl ect the general thinness of phenocrysts in the Bauers are conspicuously including assumed equivalent amounts as out- the unit and some erosion after its emplacement. euhedral (Best and Christiansen, 1997, their fi g- fl ow and within the caldera, is 3200 km3, second At least some of this pattern could be the result ure 1) relative to other crystal-poor ignimbrites only to the Leach Canyon in volume among the of topographic control at the time of deposition in the Great Basin. Aggregates of plagioclase ignimbrites deposited after 24 Ma. by northeast-southwest–trending paleovalleys and smaller mafi c phenocrysts are common. carved into the older Leach Canyon tuff. The Rowley et al. (1995) indicated that the Bauers HARMONY HILLS TUFF southeastern paleovalley could have developed outfl ow deposit (Fig. 67B) is reversely zoned,

on thicker parts of the Leach Canyon tuff (Fig. which we confi rmed. TiO2 and Fe2O3 decline The Harmony Hills Tuff (Mackin, 1960; 64) that experienced relatively greater compac- from the bottom to the top of the unit (Fig. 62B). Cook, 1965; Williams, 1967; Rowley et al., tion. Control by faulting cannot be ruled out. In three stratigraphic sections there is an upward 1995) generally occurs as a simple cooling unit The volume of the outfl ow Swett is estimated decrease in the plagioclase-to-sanidine ratio, throughout most of its 13,000 km2 extent. at 200 km3 and the total volume of the unit from an average of 2.0 to 1.4, and the amount In the middle Cenozoic southern Great including that assumed to be hidden in the con- of mafi c phenocrysts diminishes somewhat Basin ignimbrite province, the Harmony cealed caldera is 400 km3. upwards as well. Hills is the most mafi c and poorest in silica

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A Condor Canyon Formation 50 km Swett Tuff Member (24.15 Ma) 0 ′ E NV UT ! 38°30 0 Corrected for 50% ! 0 ! extension 3! 0 ! 050-5 0 ! Figure 67. Distribution and thickness (in meters) of the Condor ! ! 0 0 ! Canyon Formation. (A) Swett Tuff Member. A concealed small ! 0-15 0 0 0 0 ! ! E ! E E source caldera possibly lies within the dashed area. (B) Bauers Tuff ! 38° 0-6 ! ! 70 ! 140 10-15 ! ! Member. Sites with no indicated thickness are where paleomagnetic ! ! ! 50 0 !17 ! 10 90! ! ! 0 ! 100 ! ~25 ! 0 samples of the Bauers were collected. Only a small segment of the 100? 30 0-15 ! 0 ! 70 ! 0-10 ! 20 ! margin of the Clover Creek caldera source is exposed. Possible ! 0 0-20! 0 ′ E ! source E ! E 37°30 ! <15 0 ! ~50 area ! ! 0 0 0 ! 0204050 25 Km 0 0 ! 0 ! 0102030 ! 0 Miles 37° 115° 114° 113° B E 39° Condor Canyon Formation E NV UT E Bauers Tuff Member (23.04 Ma)

0 10 ! !

0 ! 10 6 ! ! ′ 50 km 38°30 E E E ! E 3 2-5 ! ! Corrected for 50% ~10 ! 0-3 <30 <15 extension <10 25 ! ! 0 ! ! ! 0 ! ~15 20 ! <40 ! 10 ! ! 0 <30 0-8 ! >50 ! ! ! ~20 ! 38° E E 20? 10 60 ! E ! ! E ! >55 120 ! 100 ~17 ! ! ! <25 26 <200 ! 65 ! 0-15 ! ! >40 ! 11 180 ! ~30 140 <130 ! ! <15 0 ! ! <125 ! ! ! ! 100 55 ~40 ! ! ! ! <125 ! <140 ! ? ! ? Clover Creek 70 2? ! ! ! ! 0 >400 50 ! 0 ! <60 caldera ! <55 ! ′ E ! 55 60 37°30 E 400 segment E ! E >20 ! ! ~60 ! ! <215 61 ~42 ! ! ! ~30 12 >170 200 ! ! ! 0 24 ! 100 >60 ! 0 1020304050 Km ! 0 3-12 ! ! 0 0102030 Miles 37° E E E E

116° 115° 114° 113°

of major regional ignimbrites. Six available Tuff (Fig. 27A). The Harmony Hills contains lesser biotite, hornblende, quartz, clinopyrox- analyses are relatively tightly clustered near relatively high concentrations of MgO, CaO, ene, and Fe-Ti oxides, and a trace of sanidine

the junction of the dacite, latite, and ande- Fe2O3, Sr, and Cr (Figs. 5, 6 and 30C). Thirty (Figs. 68 and 69). site ﬁ elds in Figure 49, but lie mostly in the modes indicate the ignimbrite is exception- We have no data on cognate inclusions nor on latter two. Analyses overlap with the least- ally phenocryst rich, with as much as 58% possible systematic zoning in the outﬂ ow sheet. evolved samples of the Cottonwood Wash phenocrysts, including abundant plagioclase, Whether it should be classed as an unusually

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80 RACER CANYON AND HIKO TUFFS Harmony Hills Tuff n = 30 The Racer Canyon and Hiko Tuffs (Blank, 60 1959; Rowley et al., 1979; Dolgoff, 1963) are products of eruptions from calderas on the east Figure 68. Modal proportions and west sides of the Caliente caldera complex of phenocrysts in the Harmony 40 resulting in dispersal of the ash ﬂ ows chieﬂ y, but Hills Tuff. Data from Williams not entirely, into Utah and into Nevada, respec- (1967) and Rowley et al. (1995). Mode (vol %) tively (Figs. 71 and 72). 20 Stratigraphy

In most places, the Hiko is a single simple 0 cooling unit whereas the mostly older Racer Can- Plag Qtz San Biot Hb Px Opaq Pheno yon locally consists of multiple cooling units. At sites within a few tens of kilometers on either side of the Utah-Nevada state line the mafi c monotonous intermediate remains to be may be the result of accumulation as caldera fi ll. two units overlap. The unusually thick section evaluated. Nearby exposures are less thick. of cooling units of rhyolite ignimbrites near Duplicate 40Ar/39Ar ages on plagioclase from These apparent near-source occurrences of Panaca Summit, originally mapped as all Racer a sample in Condor Canyon yield a weighted the Harmony Hills delineate an area within the Canyon Tuff (Williams et al., 1997), has now mean age of 22.56 ± 0.11 Ma. Caliente caldera complex where the source cal- been divided into 390 m of the Hiko Tuff above dera probably lies (Figs. 2 and 70). This inte- a prominent layer of vitrophyre (paleomagnetic Distribution, Source, and Volume grated area reconciles the two confl icting pro- sample 0P284 in Gromme et al., 1997) and posed sources for the Harmony Hills near the 485 m of the underlying Racer Canyon Tuff. No direct evidence has been found for a Bull Valley Mountains in Utah and the south- The section more than 150 m thick south- source caldera of the Harmony Hills Tuff. eastern part of the Caliente caldera in Nevada. west of Modena, Utah (Best, 1987), may be all Whether it lies in Utah or Nevada has been con- The dimension of a caldera enclosed within Racer Canyon whereas just to the south, Siders troversial for decades (e.g., Blank, 1959; Ekren the source area is not unreasonable, given that (1991) described a lower cooling unit less et al., 1977; Scott et al., 1995a). the volume of the outfl ow tuff is 1100 km3. than 122 m thick of the Racer Canyon that is In the southwestern corner of Utah, in the Doubling this volume gives 2200 km3 for the overlain by beds of fi ne sediment and then an eastern Bull Valley Mountains, proximity to the approximate total volume of the unit. upper thinner (<30 m) weakly welded tuff that source is suggested by the presence of two cooling units separated by 0.5 m of fi nely bedded ash and sandstone. Northwest of the Bull Valley Mountains, Rowley et al. (2007) documented a thickness of at least 275 m of the Harmony Hills (Fig. 70) and an unusual abundance (as much as 20% of the rock) of collapsed pumice clasts as long as 0.3 m. To the southwest, Anderson and Hintze (1993) also reported a near-source character for the Harmony Hills Tuff, including an unusual thickness of at least 330 m, the presence of two cooling units, zones rich in large cognate Figure 69. Harmony Hills Tuff pumice clasts (to as much as 0.3 m), and as showing its phenocryst-rich much as 5% inclusions of andesite lava rock in character. Coin (2 cm in diam- the lower and upper parts of the unit. eter) lies on the compaction In Nevada, in the southern Clover Mountains, foliation expressed by oriented Ekren et al. (1977) found more than 100 m of biotite phenocrysts. Remainder Harmony Hills Tuff that contains basketball- of the outcrop is a broken face size pumice clasts, which diminish rapidly in oblique to foliation. size and abundance southward. In the southern part of the Delamar caldera source of the Hiko Tuff (see below), a deposit of Harmony Hills breccia that appears to have been derived from a thick unit at the time the caldera collapsed was mapped by Rowley et al. (1995). To the north of the Clover Mountains, Rowley et al. (1994) documented ~170 m of Harmony Hills in the older Clover Creek caldera (Fig. 67B); this thickness

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Corrected for NV UT 0 Harmony Hills Tuff ! 50% extension (22.56 Ma)

0 ! ~45 ! 0 50 km ! 0 ! 6 <180 ! 38° E ! E 0 ! E 0 20 ! ! 85 ! >25 0 ! ! 40 10 ! ! 0 0-6 ! 3-11 >30 <150 ! ! 130 80 ! ! <15 ! <35 ! ! 100 95 100 ! >70 ! 125 170 200 ! 30 ! 0 ! 92-107 ! !

! 100 0 Probable >30 ! E ! 70 E 37°30' 0-10 source area E ! 0 ! ! <275 ! ! 0 100 ! ! >100 ! <330 ! 300 ~100 ! 70 ! !

<80 75 ! ! ! <80 ! 0 1020304050 120 ! Km 0102030 Miles 37° E E E

115° 114° 113°

Figure 70. Distribution and thickness (in meters) of the Harmony Hills Tuff. Dotted outline encloses an area within which a source caldera is probably located. To the west of the postulated source area, a thick, northeast-trending pile of rhyolitic lava domes and ﬂ ows at the stratigraphic level of the Harmony Hills restricted its distribution (Scott et al., 1995a).

has less quartz and sanidine than the average Although considerable work has been done ing, 1987) where it consists of two cooling units Racer Canyon; we consider this thinner tuff to on these two similar rhyolite ignimbrites (e.g., totaling ~300 m thick; the upper unit appears to be Hiko. Much farther south, Anderson and Rowley et al., 1995; Gromme et al., 1997), full be the most widespread and voluminous. The Hintze (1993) reported a section 230 m thick understanding of their stratigraphic relations, Hiko commonly weathers into bulbous granite- of Hiko but Rowley et al. (2007) instead corre- distribution, and compositional variations is like outcrops (Fig. 73) controlled by joint sets. lated this section with the Racer Canyon. Still incomplete. Further paleomagnetic and chrono- Locally, a vitrophyre occurs near the base of farther south, Hintze et al. (1994a) and Hintze logic analyses will be needed to distinguish the unit as well as fl attened pumices and sparse and Axen (1995) documented 155 m and them throughout their extent in the eastern Great xenoliths of volcanic and sedimentary rock. 50 m, respectively, of Hiko Tuff but admit the Basin (Figs. 71 and 72). Available modal data do not unequivocally possibility that some or all is Racer Canyon. distinguish between the Racer Canyon and Hiko This latter correlation seems more reasonable Petrography and Composition because of overlap in proportions of phenocrysts because the source of the Racer Canyon lies (Rowley et al., 1995, their fi gure 15; Gromme just to the north and is closer than the source The Racer Canyon Tuff is loosely to moder- et al., 1997, their fi gure 3). But in an attempt to of the Hiko (see caldera sources below). To ately welded, is tan to light gray, and consists possibly clarify differences we have plotted Hiko the east, sections are 90–335 m thick but lie in most places of multiple cooling units (for a modes from samples well into Nevada and Racer geographically between sections 50 m thick thorough description of the Racer Canyon see Canyon modes from samples well into Utah, both or less. Siders et al. (1990) and Rowley et al. Siders et al., 1990). Pumice and lithic clasts con- from Rowley et al. (1995); these selected samples (1995) pointed out that the Racer Canyon was stitute 10%–20% of the tuff. avoid uncertain stratigraphic identities near the deposited on uneven terrain created by syn- The generally more densely welded, gray state line as just described. These modes (Fig. 74) eruptive faulting as well as by earlier shallow to brown Hiko Tuff in most places outside its show differences in the relative proportions of laccolith emplacement that created local topo- caldera source appears to be a single cooling felsic phenocrysts. The younger Hiko has more graphic domes. unit except in the South Pahroc Range (Mor- plagioclase than quartz whereas in the Racer

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NV UT 0 Racer Canyon Tuff ! (18.88–18.57 Ma) 38° E E

0 !

0? ! 50 km 0 ! Corrected for

! 0 50% extension 485 ! 0? ! ! >10 370 400 ! >150? ! ! 0 0? ! 260 ! ! <122 25 ! 300

! 50 <250 ! Telegraph Draw ! 90 caldera 200 400 >450 ! 37°30' E E

300 200 <230 ! 100 0 155? !

0 50? 01020 ! Miles

010203040 Km 37° 114° 113°

Figure 71. Distribution and thickness (in meters) of the Racer Canyon Tuff. The western and southwestern margin of the Telegraph Draw caldera source is conjectural, as are the isopachs on the outﬂ ow sheet.

Canyon the proportions of these two phenocrysts on the Fe2O3 versus TiO2 diagram that we have Age and Paleomagnetism are similar. Proportions of quartz and sanidine are used to distinguish many of the tuffs from the 40 39 similar in the Hiko whereas in most samples of region (Fig. 62B). For a given TiO2 content, Ar/ Ar ages of sanidine and paleomagnetic the Racer Canyon quartz exceeds sanidine. the Racer Canyon Tuff has ~0.2 wt% more data provide insight into the stratigraphic rela-

An additional challenge in the fi eld is distin- Fe2O3 than the Hiko. Moreover, the Racer tions of the Racer Canyon and Hiko Tuffs. guishing between the Racer and the Leach Can- Canyon Tuff has the highest Fe/Ti ratios of In the South Pahroc Range, a slight difference yon ignimbrites, especially the lower, Narrows the silicic units erupted from the Indian Peak in the paleomagnetic directions of the two Hiko member of the latter because of nearly identical or the Caliente caldera complexes. The Racer cooling units was interpreted by Hudson et al. appearance and modal composition (Rowley Canyon and Hiko Tuffs are rather different (1998) to represent an elapsed time of as little as et al., 1995, their fi gures 6 and 15); their modes from the older tuffs of the Quichipa Group— one or two centuries between their emplacement. include trace amounts of titanite, which is gen- the Leach, Swett, and Bauers. Most signifi - A sample of the Hiko in Condor Canyon in erally not present in most silicic Great Basin cantly, the younger tuffs are less alkaline (Fig. eastern Nevada is 18.47 ± 0.04 Ma whereas a tuffs and, therefore, serves as a useful tool in 62A). Most samples of the three older units sample in the North Pahroc Range is 18.56 ±

distinguishing most stratigraphic units. How- also have signiﬁ cantly higher Al2O3/CaO 0.04 Ma. These samples have a reverse paleo- ever, the Leach Canyon is signiﬁ cantly older, ratios than the Racer Canyon and Hiko Tuffs. magnetic polarity (Gromme et al., 1997). Row- lying below the Harmony Hills Tuff and Condor The thick section (>450 m) of the Racer Can- ley et al. (1995) reported identical preliminary Canyon Formation, so that the Leach Canyon yon south of the Enterprise Reservoir displays plateau ages by L.W. Snee on sanidine of 18.5 ± can usually be recognized as such on strati- strong normal zoning (Fig. 62); the base of 0.1 Ma for Hiko ignimbrite and a presumed co- graphic grounds, except in terranes of complex the unit is a high-silica rhyolite (77.6 wt%) genetic rhyolite dome on the rim of the source faulting or isolated outcrop. whereas the top is a high-silica dacite (68.9 caldera (see below). Chemically, the Racer Canyon and Hiko are wt%). The Hiko is less strongly zoned, from In the section of twelve cooling units of the also very similar but they form distinct trends ~74 to 69 wt% silica. Racer Canyon south of the Enterprise Reservoir

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! NV UT 0 38° E E 0 ! Hiko Tuff (18.51 Ma) >80 ! >85 >27 ! ! 30 km 0 ! >80 ! ! Corrected for 50% extension 390 ! <50 ! 300 <30 45 200 ! >100 ! !

30 !

>400 Delamar ! ! ? 400 caldera 37°30' ! E E 300 ? ! ! ! <370 300

! ! <280 ! ! ! 200

! ! ! ! ! 100

! 180 0 !

! 0102030 ~45 ! Km ! 0 01020 Miles 37° E

114° 115°

Figure 72. Distribution and thickness (in meters) of the Hiko Tuff. Sites with no indicated thickness are where paleomagnetic samples of the Hiko were collected. The eastern and southeastern margin of the Delamar caldera source is speculative. The zero isopachs of the Hiko and the Racer Canyon (previous ﬁ gure) are poorly known because of the lack of constraining younger deposits.

(northeast corner of Goldstrike quadrangle; Source Calderas Canyon Tuff totaling at least 450 m thick are Rowley et al., 2007), an upper unit is reversely exposed (Rowley et al., 2007); this thick sec- magnetized and has an age of 18.57 ± 0.03 The poorly exposed source of the Racer tion south of the Enterprise Reservoir and its whereas the lowest exposed cooling unit is nor- Canyon Tuff is designated the Telegraph Draw abrupt southward thinning mark the south- mally magnetized and has an age of 18.85 ± caldera by Rowley et al. (2008). Near 114° W, eastern margin of the caldera. A largely coin- 0.03 Ma (Gromme et al., 1997). Rowley et al. the northern sector of the caldera (Fig. 71) is cident younger caldera apparently combines (1995) reported a similar age of 19.0 ± 0.1 Ma expressed by intercalated wall-collapse brec- with the Telegraph Draw caldera to produce a by L.W. Snee from the same section. Normally cias in ignimbrite that has pumice clasts to as large negative gravity anomaly in the eastern magnetized Racer Canyon cropping out on the much as 0.6 m in diameter. In the southeastern part of the Caliente caldera complex (Rowley highway between Panaca and Modena near sector, about twelve cooling units of Racer et al., 2008). the state line has an age of 18.88 ± 0.06 Ma. Hence, available data indicate the multiple cooling units of the Racer Canyon are mostly older than the Hiko, but the youngest is about Figure 73. Outcrop of the pheno- the same age as the Hiko. cryst-rich Hiko Tuff along The paleomagnetic direction of a sample from Nevada State Highways 25 and the lowest part of the Racer Canyon at Panaca 93, ~5 km east of Hiko, Nevada. Summit is reversely magnetized whereas the Bulbous erosional forms are immediately overlying part of the unit is nor- similar to those of weathered mally magnetized. Thus, the sampled sequence granite. Bushes are 1–2 m tall. of Racer Canyon cooling units reveal two paleomagnetic reversals, i.e., R → N → R.

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A 60 volume (Table 1). The total volume of the Racer Racer Canyon Tuff (plus trace of titanite) 3 n = 37 Canyon Tuff so determined is 1100 km . 50 Average of 14 analyses Multiple Racer Canyon eruptions that resulted (Siders and Shubat, 1986) in as many as twelve cooling units zoned from 40 high-silica rhyolite to high-silica dacite contrasts with eruption of no more than two cool- 30 ing units of the zoned, low-silica (74–69 wt%) rhyolite Hiko (Fig. 62). The mostly older Racer Mode (vol %) 20 Canyon and the Hiko also exhibit inter-unit normal compositional zonation. Because the exten- 10 sion-corrected distance between their sources at opposite ends of the Caliente caldera complex 0 was no more than 8–10 km (Figs. 71 and 72), it Plag Qtz San Biot Hb Px Opaq Pheno is possible that their two zoned magma chambers were connected; venting began at the east B Hiko Tuff n = 35 end, withdrawing evolved high-silica rhyolite 60 plus <1% titanite magma from the top of the chamber, followed by deeper withdrawal of less-evolved magma at Figure 74. Modal proportions both ends. of phenocrysts in the (A) Racer 40 The Racer Canyon and Hiko were the termi- Canyon Tuff, (B) Hiko Tuff, nal large-scale eruptions of the long-lived 36 to and (C) tuff of Tepee Rocks. 18 Ma Indian Peak–Caliente magma system. Mode (vol %) Resulting calc-alkaline arc rocks possess pro- Data for latter two units from 20 Rowley et al. (1995). nounced Nb-Ta-Ti anomalies on normalized diagrams and plot in the Pearce et al. (1984) continental arc ﬁ eld (Fig. 75). Thereafter, from 0 ca. 16 to 12 Ma, mostly more-alkaline, within- Plag Qtz San Biot Hb Px Opaq Pheno plate magmas lacking strong arc signatures were explosively erupted from the Caliente C 60 Tuff of Tepee Rocks magma system and the Kane Springs Wash plus trace of titanite system to the southwest (Nealey et al., 1995). n=7 The 15–14.2 Ma local tuff of Kershaw Canyon in the Caliente caldera complex has a transi- 40 tional composition, but it is not considered here because of its younger age. To the west, in the Central Nevada ignimbrite

Mode (vol %) fi eld, the high-silica, phenocryst-rich rhyo- 20 lite Fraction Tuff is similar in age (18.57 Ma) and composition to the Racer Canyon and Hiko Tuffs, including the presence of titanite. It is also the terminal calc-alkaline, arc-type 0 eruption in the fi eld and contrasts with the Plag Qtz San Biot Hb Px Opaq Pheno younger more alkaline activity to the south in the Southwest Nevada volcanic fi eld (Sawyer et al., 1994). Ekren et al. (1977) discovered that the source Tuff that possesses a subvertical foliation defi ned caldera of the Hiko Tuff constitutes the western by fl attened pumice clasts (Rowley et al., 1995, TUFF OF TEPEE ROCKS lobe of the Caliente caldera complex (Figs. 2 and p. 60). The margin of the caldera consists of com- 72). Rowley and Siders (1988) named this source plex faults that are believed to have been active Rowley et al. (1995) reported that the small, the Delamar caldera. As documented by Swadley during its development. weakly welded, high-silica rhyolite tuff of Tee- and Rowley (1994), the Delamar caldera is The outfl ow Hiko occurs as a thick lobe pee Rocks that overlies the Hiko Tuff has an age expressed by more than 400 m of relatively extending for at least 50 km southwest of the of 18.1 ± 0.7 Ma and may have been erupted from pumice-rich tuff, which in its upper part occurs in Delamar caldera and a smaller lobe extending to a source to the east of the Delamar caldera. It multiple cooling units that interfi nger with lenses the northeast (Fig. 72). resembles the Hiko in many respects, but is more of caldera-collapse breccias, chiefl y of andesitic evolved chemically and modally, although the fl ow rocks. A lava dome similar in composition Interpretations and Speculations proportions of phenocrysts are highly variable to the Hiko Tuff lies on the topographic margin (Fig. 74C). It may represent a very late differ- of the caldera. An eruptive vent 2 km in diam- The Hiko Tuff qualifi es as a super-eruption entiate of the residual Hiko magma. Chemically, eter for the tuff lies on the southwestern margin of 1700 km3 if its largely concealed intracaldera it belongs with the group of 36–18 Ma arc-type of the caldera; the vent is fi lled with mostly Hiko volume is assumed equivalent to the outfl ow ignimbrites (Fig. 75; Nealey et al., 1995).

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A 1000 Oligocene lavas of the ﬁ rst two time periods have been described by Best et al. (1989a), Askren (1992), and Askren et al. (1997) whereas Best et al. (1987c) have described the Miocene 100 lavas in the third period. For major- and trace- element analyses of maﬁ c lavas for all three time periods see Best et al. (2009).

10 34–31 Ma Lavas

Andesitic Youngest arc rhyolite ignimbrites Rock/Primitive Mantle Extrusion of andesite and lesser low-silica in Caliente caldera complex 1 dacite lavas at 34–31 Ma (E in Fig. 77) around Tuff of Kershaw Canyon (ca. 15 Ma) Tuff of Tepee Rocks (18.1 Ma) all but the south fl ank of the Indian Peak caldera Hiko Tuff (18.51 Ma) complex (Fig. 76) began when the fi rst regional Racer Canyon Tuff (18.88 - 18.57 Ma) rhyolite ignimbrites were emplaced from sources 0.1 within the complex, i.e., the Sawtooth Peak and Rbb Ba Thh U K Nbb La Pbb Ce Sr Ndd P Sm Zr Ti Y Marsden tuffs. The greatest volume of lavas com- B 1000 prises remnants of small andesite-dacite stratovolcanoes immediately to the west of the Indian Continent-continent Peak caldera complex near Silver King Moun- collision tain (Ekren et al., 1977) and 30–60 km to the northeast of the complex. Lavas near Silver King Anorogenic Mountain that underlie the 31.69 Ma Windous or Within plate Butte Formation have not been mapped nor OO O OOO studied in detail; distal flows may extend OO 100 southward into the North Pahroc Range (Scott O et al., 1995b). Northeast of the caldera com- Rb (ppm) Volcanic arc plex, 34 Ma lavas, debris fl ows, and breccias Youngest arc rhyolite ignimbrites are 360–600 m thick (Best et al., 1989c; Hintze in Caliente caldera complex et al., 1984) and contain phenocrysts of plagio- Tuff of Kershaw Canyon (ca.15 Ma) clase, hornblende, Fe-Ti oxides, and biotite and Tuff of Tepee Rocks (18.1 Ma) less-common quartz and clinopyroxene; lavas O Hiko Tuff (18.51 Ma) in the east are associated with Pb-Ag-Cu min- Racer Canyon Tuff (18.88 - 18.57 Ma) eralization. Small intrusions of diorite exposed 10 in the low pass across the Wah Wah Mountains 10 100 1000 northwest of Wah Wah Springs (Hintze, 1974) Y + Nb (ppm) might represent intrusive counter parts of lavas Figure 75. Chemical variation diagrams showing the arc signature to the east. Southwest of Milford , 32 Ma lavas of some younger ignimbrites erupted from the Caliente caldera com- are ~400 m thick and contain phenocrysts of plex. See also Figures 5E and 5F. Data for tuffs of Kershaw Canyon plagioclase, two pyroxenes, and Fe-Ti oxides and Tepee Rocks from Nealy et al. (1995). (A) Normalized trace ele- (Best et al., 1989c). ment diagram with primitive mantle composition from McDonough Much smaller erosional remnants of the 32– and Sun (1995). (B) Tectonic discrimination diagram according to 31 Ma andesite lava flow formation of the Pearce et al. (1984). Escalante Desert Group occur in three places in and around the Indian Peak caldera complex: 1. Near Atlanta, Nevada, immediately north of the northern margin of the complex fl ow rem- LAVAS IN THE INDIAN PEAK– the ignimbrite fl areup until after ca. 20 Ma and nants are less than 200 m thick (Willis et al., CALIENTE FIELD even younger in southwest Utah. 1987); one fl ow has a stratigraphically con- The lavas are described in the following sec- strained age between 31.1 and 31.7 Ma. Rhyolitic to andesitic lava flows were tions for three time periods: (1) 34–31 Ma, prior 2. In the southern Wah Wah Mountains, extruded from numerous vents from 34 to 18 Ma to eruption of the very large-volume monoto- duplicate 40Ar/39Ar analyses of plagioclase from in the Indian Peak–Caliente fi eld (Fig. 76) dur- nous intermediates in the Indian Peak caldera sample LAM-1C collected at the top of the ing the time of explosive silicic activity. Minor complex; (2) 31–26 Ma, contemporaneous with 100–200-m-thick fl ow between cooling units of volumes of granitic magma were intruded dur- eruption of the monotonous intermediates and the Lamerdorf Tuff (Table 1; Fig. 15) yielded a ing this activity. It is especially noteworthy that trachydacitic Isom-type tuffs; and (3) 25–18 Ma stratigraphically consistent weighted mean age no true basalt magma (International Union of after eruptive activity in the Indian Peak caldera of 31.98 ± 0.16 Ma. Geological Sciences classifi cation of Le Maitre, complex and during ignimbrite eruptions in the 3. In the southern White Rock Mountains, as 1989) was extruded in the Great Basin during Caliente caldera complex. much as 350 m of andesite lavas lie between the

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Nevada Utah

38°30′

Indian Peak caldera complex

Seaman Range 38° center

Caliente caldera complex 37°30′

NevadaNevada Utah

MapMap

05 1020304050 Km 37° 115° 114°30′ 114° 113°30′ 113°

Indian Peak–Caliente volcanic field 34–31 Ma 31–26 Ma 25–18 Ma Calderas/Sources Intermediate composition lavas

Figure 76. Intermediate-composition lava ﬂ ows in the Indian Peak–Caliente ﬁ eld within 40–50 km of the outer margin of the caldera complex. The possible source of the Isom ignimbrites concealed in the Escalante Desert extends southeast of the main part of the Indian Peak caldera complex. Data are from published maps and, in Nevada, from Crafford (2007).

33.5 Ma Sawtooth Peak and 31.13 Ma Cotton- Rhyolite dating of zircons revealed ages of 36–32 Ma wood Wash ignimbrites in the resurgently The rhyolite lava fl ow formation of the (Kowallis and Best, 1990). On the east side of uplifted fl oor of the caldera complex (Fig. 44; Escalante Desert Group is generally nonvesicu- the caldera complex, in the southern Wah Wah Keith et al., 1994). lar and is exposed over an area of only ~10 km2 Mountains, fi nely fl ow-layered, locally glassy Local volcanic debris fl ows tens of meters mostly on the north and east sides of the Indian fl ows are as thick as 250 m and occupy a similar thick that contain clasts of andesitic rock occur Peak caldera complex. Oldest fl ows are more or stratigraphic interval as the andesite lava fl ow in the Beers Spring Formation of the Escalante less contemporaneous with the Marsden Tuff. formation of the Escalante Desert Group (Best, Desert Group and at the top of the Marsden On the north side of the complex, west of Atlanta 1987; Best et al., 1987d). The variegated colors sequence, as described above, in the central (Fig. 76), altered fl ow remnants were originally as well as the size and abundance of phenocryst Needle Range; these debris fl ows testify of a mapped as the 30 Ma Ryan Spring Formation are similar to those of the Lamerdorf Tuff, as nearby unexposed extrusive mass. (Willis et al., 1987) but subsequent fi ssion-track are the types of phenocrysts, viz., plagioclase,

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10 2. Lava fl ows and fl ow breccias of horn- Figure 77. Total alkalies- Trachydacite E blende-bearing andesite and latite (H) that are as 9 B silica diagram for lavas in the Latite E E thick as several hundred meters occur north and L B B B B Indian Peak–Caliente fi eld. For H E south of Modena (Siders, 1985). Sparse pheno- 8 H B L E L E L E E 25–18 Ma lavas: B—Blawn For- H LBB I crysts include conspicuous hornblende together BH B BBB S E E E mation; S—dacite of Spanish 7 B BH E with minor plagioclase, pyroxene, and biotite. Shoshonite B i I E BBB HS E George Spring; H—ca. 24 Ma O (wt %) B B H Critical stratigraphic relations are lacking to fi x 2 BE I 6 EEH I I E B Hiii IE E hornblende-phyric andesite and B I I i i i H I the age of these fl ows but one K-Ar hornblende B I EE BiBHiiiiE i E B O + K I E Rhyolite 2 B B E latite lavas; L—“latite” lavas 5 E E B E E age is 24 Ma. East of Lund, the hornblende- B EE E I Dacite Na B E IEE E containing phenocrysts of bio- B E IE bearing Mount Dutton Formation is mostly a tite, pyroxene, and plagioclase. 4 Lava flows in the Indian vent-facies, volcanic debris fl ow as thick as Andesite Peak-Caliente volcanic field For 31–26 Ma lavas: I—con- Basaltic B, S, H, L 25–18 Ma n = 55 600 m and probably emplaced ca. 26–22 Ma temporaneous with Isom Forma- 3 andesite I,i 31–26 Ma n = 38 E 31–34 Ma n = 26 (Rowley, 1978). The mostly younger Horse Val- tion; i—contemporaneous with 2 ley Formation, also a stratovolcanic complex, monotonous intermediates. 52 54 56 58 60 62 64 66 68 70 72 74 76 78 appears to be more silicic, but neither unit has SiO2 (wt %) been chemically analyzed. 3. A large area of latite and trachydacite lava fl ows and fl ow breccias (L) as thick as 1500 m biotite, and locally amphibole. Farther north The largest silicic body is a dominantly emplaced ca. 22 Ma are exposed north and west in the Wah Wah Mountains, in the deep paleo- dacitic stratovolcano ~10 km in diameter with of Modena. Phenocrysts make up ~10% of the valley fi lled with thick Lamerdorf Tuff (Fig. 15), a volume of less than 20 km3 that developed rock and include prominent biotite, plagioclase, an unusual low-silica rhyolite lava at the base of at ca. 28 Ma (du Bray, 1993) in the Seaman minor clinopyroxene, and local hornblende the volcanic pile above local conglomerate has Range midway between the Central Nevada (Best, 1987; Williams et al., 1997). altered phenocrysts of platy plagioclase as long and the Indian Peak–Caliente caldera com- 4. The most widespread but not the largest as 1 cm and lesser smaller unidentifi able rem- plexes. Granitic to dioritic intrusions in the 29.1 volume of the intermediate-composition lava nants of mafi c phases (Abbott et al., 1983, their Ma Blind Mountain caldera (Figs. 8 and 58) fl ows are those assigned to the informal mafi c unit Toa designated as “older andesite”; alkali are too small to be shown in Figure 76. In the fl ow member of the Blawn Formation (B) on concentrations are perturbed in our sample Fairview Range, a dacite lava was apparently geologic maps of the Indian Peak caldera com- LAM-9-18-2). A petrographically and chemi- extruded on the margin of the White Rock cal- plex and the southern Wah Wah Mountains and cally very similar fl ow lies directly on Paleozoic dera (Best et al., 1998, their unit Tch). A small Shauntie Hills (e.g., Best et al., 1987c). Flows rocks just beyond the northwest margin of the rhyolite lava in the southern Indian Peak Range range from ca. 25 to 21 Ma (Best et al., 1989c). caldera complex in the western Fairview Range (Best et al., 1987a, their unit Trr) lies between Some sequences are hundreds of meters thick. (Best et al., 1998, their unit Tb, our sample the Wah Wah Springs and Ryan Spring ignim- Typical phenocrysts include plagioclase, clino- FAIRV-3-160-1). brites. West of Milford are small 31–30 Ma plu- pyroxene, Fe-Ti oxides, and combinations of A perlitic rhyolite lava fl ow hundreds of tons of granitic rock. lesser orthopyroxene, hornblende, and either meters thick was encountered between dacitic olivine or biotite. Chemically, the mafi c fl ow “Needles Range” tuffs and Cambrian carbon- 25–18 Ma Lavas member ranges widely and includes trachy- ate rocks in two adjacent deep wells in Pine dacite, andesite, basaltic andesite, and, most Valley at ~38°32′45″ N and 113°43′00″ W (see Figure 76 displays a blossoming of andesitic common, latite (Barr, 1993; Best et al., 2009). Hintze and Davis, 2003, p. 257, for data on one and latitic lava extrusions from ca. 25–18 Ma The large area in Utah south of 37°30′ N of these wells). as explosive silicic activity waned in the south- on the southern margin of the Caliente caldera Four small plutons of granitic rock northwest ern part of the Indian Peak caldera complex and complex (Fig. 76) comprises intermediate-com- of Milford, Utah, that may be connected at depth progressed farther southward into the Caliente position stratovolcano deposits of lava and vol- were intruded at ca. 31 Ma (Best et al., 1989c). caldera complex and surrounding areas. On the canic debris fl ows as thick as 1200 m emplaced These plutons are associated with porphyry Cu northwest margin of the Escalante Desert, four mostly between the Leach Canyon and Racer mineralization. types of chemically rather similar intermediate- Canyon tuffs (Table 1; see also Rowley et al., composition lava fl ows have been recognized 2007). Westward into Nevada, this outcrop area 31–26 Ma Lavas based on phenocryst assemblage; chemical is drawn from the generalized 1:250,000-scale distinctions are not clearly evident (Fig. 77; Lincoln County map of Ekren et al. (1977). No Andesitic to dacitic lava and debris fl ows Best, 1987): chemical analyses are available for this large extruded during the monotonous intermediate 1. The most restricted in areal extent is the area of lavas. and trachydacitic ignimbrite activity constitute dacite of Spanish George Spring (S in Fig. 77), To the southeast of the Escalante Desert, small exposures within the Indian Peak caldera a lava dome complex ~5 km in diameter and no the northeast-trending, so-called “Iron Axis” complex and to the southwest (Fig. 76). The more than 0.5 km thick, in which ~20% of the (Shubat and Siders, 1988; Blank et al., 1992; most voluminous occurrence of andesitic rocks rock is composed of blocky plagioclase grains Rowley et al., 2008; Biek et al., 2009) is com- is in the latter area, where they are remnants of as much as 3 cm across; smaller lesser pheno- posed of locally Fe-mineralized granitic lacco- a volcanic pile that is as thick as 1200 m lying crysts include biotite, hornblende, quartz, Fe-Ti liths emplaced at 22–21 Ma. South of these between the 30.06 Ma Wah Wah Springs and oxides, and, in some samples, a trace of titanite, intrusions is the very large, 20 Ma laccolith con- 29.1 Ma Petroglyph Cliff ash-fl ow tuffs (Swad- clinopyroxene, and sanidine. Stratigraphic rela- stituting the Pine Valley Mountains (not shown ley et al., 1994). tions indicate an age of ca. 25 Ma. in Fig. 76).

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North of the Escalante Desert within the ter (du Bray, 1993)—between the two caldera systems were operative in the unusually thick Indian Peak caldera complex and to the east of complexes. To the east, the circumscribed crust, the masses of low-density silicic magma, it is a broad area in which 23–20 Ma rhyolite area merges into the Marysvale volcanic fi eld or possibly a widespread layer of crustal partial to dacite lava fl ows were extruded from many (Fig. 1). melt, effectively blocked the ascent of denser local vents (Best et al., 1987c); most of these are Generalized, extension-corrected areas of andesitic magmas. The circumscribed area too small to be shown in Figure 76. High-silica, lavas for the three time frames are, from oldest over which volumes are compared might corre- locally topaz-bearing, rhyolite extrusions of the to youngest: 500, 300, and 2400 km2. Multi- spond to the greater source region, or “sphere Blawn Formation (B in Fig. 77; Willis, 1985; plied by the approximate average thickness for of infl uence,” in the crust where magmas were Christiansen et al., 1986, 2007b) were com- each outcrop area, the volumes are 120, 35, and being generated, and from this region magmas monly preceded by explosive venting of pyro- 1200 km3. Before comparing these volumes with collected and ascended to beneath and erupted clasts of the same composition that formed local those for silicic ignimbrites, some sort of correc- from the caldera complex. aprons of tuff; none of these eruptions were of tion must be made for the lavas buried beneath Why the extruded volume of andesitic lava suffi cient volume to create a caldera. alluvium in the broad valleys. This correction blossomed after 25 Ma is uncertain. Because The only expression of effusive activity after would likely double the lava volumes. Another ~12,000 km3 of contemporaneous, mostly rhyo- 20 Ma and contemporaneous with the eruption correction might be made for lavas buried within lite ignimbrite was erupted from the Caliente of the Hiko–Racer Canyon Tuffs are small 18 calderas. The tilted Needle Range and White calderas, an underlying melt region must still and 19 Ma rhyolite lavas south of Modena and Rock Mountains and resurgent cores of the have existed to provide buoyancy blocking for west of the Delamar Mountains. Indian Peak and White Rock calderas expose andesitic magmas. Although the demise of arc partial intracaldera volcanic sections above magmatism wasn’t until ca. 18 Ma as plate sub- Volumes of Andesitic Lavas Paleozoic rocks. Had major stratovolcanoes duction ceased, early effects of crustal exten- existed within the calderas, distal volcanic debris sion might have been operative to facilitate, via The compilation of Cenozoic volcanic rocks fl ows would be exposed in the extracaldera sec- fracturing and diking, the ascent of andesitic in the Great Basin by Stewart and Carlson tions; however, only minor such fl ows occur in magmas. (1976) clearly showed the subordinate volume the central Needle Range, as indicated above. The voluminous 25–18 Ma lavas, as well as of andesitic rocks relative to contemporaneous Nonetheless, major piles (stratovolcanoes?) of shallow intrusions, include more-extreme com- silicic ignimbrites for the 34–17 Ma time period lava are likely buried in the Caliente caldera positions, as manifest in silica and total alkali (Fig. 1). We have made minor adjustments from complex. Doubling the andesitic rock volumes contents (Fig. 77), than in older lavas in the geologic maps published in the decades since for the fi rst two time periods and tripling it for Great Basin as a whole (Best et al., 1989b). A this compilation, especially by Crafford (2007), the third gives a total of ~4000 km3 for all three comprehensive interpretation of these time-vol- and have cast lava occurrences into the three time periods, which is ~12% of the total volume ume-composition changes is beyond the scope narrower time frames as outlined above to gen- of contemporary silicic ignimbrite (32,600 km3; of this article, but they obviously correspond to erate Figure 76. This new compilation allows Table 2). Remarkably, for the 31–26 Ma time the changing tectonic regime of this part of the quantifi cation the volumes of andesitic fl ow frame when 18,000 km3 of monotonous interme- Great Basin near the beginning of the Miocene rocks for comparison with the volume of con- diate and Isom-type tuffs was erupted, less than as volcanism related to subduction was sup- temporaneous ignimbrites. Because of sparse 100 km3 of andesitic lavas have been inventoried. planted by volcanism related to tectonic exten- chemical data and a range of apparent composi- sion (Christiansen et al., 2007a). Fundamental tions in many occurrences as described in pub- Signifi cance of Lavas shifts in the nature of the mantle magma input lications, it is impossible to cleanly distinguish accompanied these changes at the surface. An andesitic and latitic rocks having <63 wt% silica Despite the several uncertainties in our esti- additional factor might be the closer proximity from dacitic rocks containing more silica. mate of the volume of andesitic lavas, there can to the northeast-trending margin of the Great To further constrain the intended volumetric be no question of its small fraction relative to Basin where the thickened and pre-warmed comparison, we have circumscribed the lava fl ow the colossal mass of ignimbrite. By way of com- crust thinned into the Colorado Plateau (Figs. activity within an arbitrary distance of 40–50 km parison with contemporaneous volcanic fi elds, 1 and 76). from the outer perimeter of the Indian Peak and the Marysvale to the east on the western margin Caliente caldera complexes. This distance corre- of the Colorado Plateau (Fig. 1) has an order SUMMARY sponds to the diameter of the larger calderas and of magnitude more intermediate-composition limits the activity to within all but the most distal lava than ignimbrite (Cunningham et al., 2007) General reaches of the larger ignimbrite outfl ow sheets. and the Southern Rocky Mountain fi eld on the To the north of the circumscribed area, the next eastern margin of the plateau has 1.7 times more The 36–18 Ma Indian Peak–Caliente caldera nearest andesitic lavas are ~125 km distant and lava than ignimbrite (Lipman, 2007). complex and its surrounding ignimbrite fi eld ca. 36 Ma in age (Gans et al., 1989). To the Although a huge input of mantle-derived was a major focus of subduction-related, explo- south of the Caliente caldera complex, andesitic basalt magma into the crust was necessary sive silicic volcanism in the southern Great lavas more than 18 Ma are only present within to provide energy and mass to drive crustal Basin ignimbrite province during the middle less than 20 km of the complex. In the other magma systems generating the silicic ignim- Cenozoic ignimbrite fl areup. The 32,600 km3 two quadrants, the perimeter is more arbitrary. brite magmas, only a relatively small volume of silicic ash-fl ow tuff was produced by con- To the west, the circumscribed area overlaps by of derivative andesitic magma penetrated all tinuously evolving, southward-migrating, ~25 km a similarly circumscribed area for the the way through the crust to the surface. And crustal magma systems in unusually thick and Central Nevada caldera complex (Fig. 2); the this penetrating volume was apparently small- pre-warmed crust into which invading mantle- overlap encompasses the only locus of silicic est during the time period from 31 to 26 Ma. derived basaltic magmas furnished heat as volcanism—the 20 km3 Seaman volcanic cen- Hence, while the ignimbrite-forming magma well as mass. These powering magmas were

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generated during southward “rollback” of the Conformable sequences of outfl ow cooling were of the distinctive monotonous intermedi- subducting oceanic lithosphere beneath the units are commonly hundreds of meters thick ate magmas composed of relatively uniform continental margin (see Best and Christiansen, and lack intervening erosional debris depos- phenocryst-rich dacite that erupted at 31.13 Ma 1991, their fi gure 2). its, thus testifying to tectonic quiescence and (Cotton wood Wash), 30.06 Ma (Wah Wah Twenty-two mapped regional ignimbrite units absence of large-magnitude crustal extension Springs), and 29.20 Ma (Lund) in volumes of (generally >100 km3 each) that are exposed in during most of the ignimbrite fl areup. ~2000, 5900, and 4400 km3, respectively. After more than one mountain range have been cor- At least seven super-eruptions having vol- this tremendous burst of activity from a multi- related over a present area of ~60,000 km2 on umes of more than 1000 km3 each took place cyclic locus, almost 4000 km3 of trachydacitic the basis of geologic mapping, position in strati- from 31.13 to 18.51 Ma. Super-eruptions of magma (Bald Hills) was erupted from 27.90 graphic sequence, composition, paleomagnetic rhyolite magma occurred after 24 Ma whereas to 27.25 Ma to the southeast of the locus. An direction, and 40Ar/39Ar age. Runout distances of smaller rhyolite eruptions took place earlier, eruption farther south of 2200 km3 of pheno- ash fl ows were as great as 150 km. to 36 Ma (Fig. 78). Three super-eruptions cryst-rich andesite-latite magma (Harmony

Indian Peak - Caliente ignimbrite field

Rhyolite: Main trend, phenocryst-rich B Bauers C Cottonwood Wash D Deadman Spring Rhyolite: Main trend, phenocryst-poor G The Gouge Eye GC Greens Canyon Rhyolite: Off trend, phenocryst-poor H Hiko HH Harmony Hills I Isom Zoned dacite-rhyolite L Lund LD Lamerdorf Figure 78. Time-volume-com- LC Leach Canyon position synopsis of ignimbrites Dacite (HH = andesite-latite) M Mackleprang in the Indian Peak–Caliente MD Marsden Trachydacite R Ripgut ignimbrite ﬁ eld. The width of RC Racer Canyon a panel is the range in weight S Swett SK Silver King percent silica from Table 1. 6000 SP Sawtooth Peak Cooling units of the trachyda- W Wah Wah Springs cite Bald Hills Tuff Member of W the Isom Formation and their cumulative volume are represented by a single panel. The 4000

signiﬁ cantly younger Hole-in- ) 3 the-Wall Tuff Member is shown L HH I separately. A single panel for the Lamerdorf and Racer Can- Volume (km Volume B yon tuffs represents multiple H cooling units emplaced over a 2000 C detectable age span. Note the overwhelming dominance of phenocryst-rich dacites, i.e.,

monotonous intermediates, and S trachydacite relative to rhyolite 0 SK erupted before 27 Ma in the 60 Indian Peak caldera complex RC and the less dominant reversal I 64 after 24 Ma in the Caliente cal- LC 20 dera complex. GC M SP 68 LD G 24 SiO D R 2 (wt%) 72 MD 28

76 Age (Ma) 32

80 36

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Hills) occurred at 22.56 Ma. The volumes and With these caveats in mind, it is of interest phenocryst-rich rhyolite magma at the end of compositions of these monotonous intermedi- to use the outfl ow sheets as strain markers by the ignimbrite fl areup. ate, trachydacitic, and andesite-latite eruptions comparing the present-day ratios of east-west to The earliest explosive eruptions, in the are without parallel elsewhere in the southern north-south dimensions of the larger ignimbrite northern part of the fi eld, totaled no more than Great Basin ignimbrite province and in other sheets with their dimensions after compensating ~300 km3 of rhyolite magma. The 36.02 Ma for- well-documented volcanic fi elds in southwest- for three different amounts of uniform crustal mation of The Gouge Eye comprises a small ern North America where the middle Cenozoic extension (Table 8). Although only ten sheets lava dome and surrounding related pyroclastic ignimbrite fl areup is expressed. were examined, the dimensional ratios indicate deposits that apparently accumulated mostly The apparent lack of significant Plinian that, after compensation for an arbitrary uni- within a small asymmetric caldera located deposits associated with the monotonous inter- form 35% extension, sheets are still, on aver- immediately north of the Indian Peak caldera mediates and the lack of evidence for signifi cant age, somewhat elongate east-west. Correction complex. The formation is exposed only in the fractionation of fi ne glass particles in the Wah for 65% extension is too much as most sheets Fortifi cation Range but well cuttings have been Wah Springs indicate ash fl ows were erupted at are more elongate north-south. For 50%, or found in adjacent Lake Valley. Fifty kilometers high rates, “boiling over” from the vent, rather slightly less, extension, the sheets are, on aver- northeast of the caldera complex the 35.26 Ma than by collapse of high-standing eruptive age, equidimensional, supporting our use of the Tunnel Spring Tuff was deposited in a paleo- columns. 50% value throughout this article for correction valley around its small concealed Crystal Peak Eruptions created nine at least partly exposed of areas and volumes of tuffs (Table 1). Even if caldera source. Sources of the larger 33.5 Ma calderas as much as 60 km in diameter in the the proper value for the amount of extension has Sawtooth Peak Formation, undated Marsden unextended north-south dimension that are been applied as a correction, an individual sheet Tuff, and 32.09–31.90 Ma Lamerdorf Tuff fi lled with as much as 5–6 km of syn-collapse may still be non-equidimensional because of the are concealed beneath younger deposits and intracaldera tuff and post-collapse caldera-fi ll- factors controlling sheet shape listed above; but were likely engulfed, at least in part, in younger ing tuff. Additional source calderas are buried for several sheets these factors cancel out in the calderas in the northeastern part of the caldera beneath younger deposits in nested, multicyclic average. complex. The phenocryst-rich Tunnel Spring caldera clusters. Collar zones between inner If crustal extension had accompanied the and phenocryst-poor Marsden Tuffs are high- reverse and outer normal ring faults in two of the ignimbrite fl areup in the Indian Peak–Caliente silica rhyolites whereas the other three ignim- largest calderas reveal complex evolution during fi eld, as advocated by some geologists, then brites are low-silica rhyolites. caldera collapse that involved extensional fault- present-day dimensional ratios would be larger Following this precursory rhyolitic activity, ing and brecciation of wall rock; these two cal- for the older sheets and smaller for the younger. three super-eruptions of phenocryst-rich dacite deras also display evidence for resurgence and The absence of such temporal variation in the occurred at 31.13, 30.06, and 29.20 Ma, creat- post-collapse intrusive activity. ratios in Table 8 is an argument for the lack of ing monotonous intermediate ignimbrites of the extension during most of the ignimbrite fl areup. Cottonwood Wash Tuff, Wah Wah Springs Dimensional Aspects of Ignimbrite Formation, and Lund Formation whose Outfl ow Sheets: Strain Markers Individual Ignimbrite Units and volumes were ~2000, 5900, and 4400 km3, Source Calderas: A Unit-By-Unit respectively. Hundreds of cubic kilometers of Dispersal of ash fl ows from their sources to Chronologic Summary Cottonwood Wash and Wah Wah Springs fall- create ignimbrite outfl ow sheets is infl uenced by out ash occur as far as Nebraska. The Cotton- eruption dynamics and landscape topography; From 36.02 Ma to 27.25 Ma, eruptions in wood Wash and Lund ignimbrites appear to be on the Great Basin altiplano (Best et al., 2009) the Indian Peak–Caliente fi eld (Table 1; Fig. unzoned; limited variations in their composition where the Indian Peak–Caliente fi eld is located, 78) were essentially continuous, with breaks of are likely the result of slight contrasts in compo- only minimal relief existed when ash fl ows, no more than about 1 m.y. A lull in activity sition of parcels of magma within the pre-erup- especially the younger ones, were broadcast of 2.3 m.y. followed and another lull of 3.6 m.y. tion chamber and, for the Cottonwood Wash, across the landscape. Present outcrop dimen- after 22.56 Ma before the culminating eruptions not of substantial fractionation of vitroclasts in sions of outfl ow sheets were also controlled from 18.9 to 18.5 Ma of many cooling units of their ash fl ows. On the other hand, the Wah Wah by subsequent erosion and by east-west crustal extension subsequent to their emplacement in the Indian Peak–Caliente fi eld. Perusal of the TABLE 8. RATIOS OF EAST-WEST TO NORTH-SOUTH DIMENSIONS OF maps in this article that display the distribution LARGE IGNIMBRITE SHEETS IN THE INDIAN PEAK FIELD of the ignimbrite deposits clearly refl ect the Age Ignimbrite (Ma) At present 35% extension 50% extension 65% extension influence of east-west crustal extension on Hiko 18.51 1.4 1.0 0.9 0.8 the present shapes of the larger outfl ow sheets Racer Canyon 18.88–18.57 1.5 1.1 1.0 0.9 for which adequate information is available. Harmony Hills 22.56 1.6 1.2 1.1 1.0 Bauers 23.04 1.3 1.0 0.9 0.8 For all sheets except the Cottonwood Wash, Swett 24.15 1.5 1.1 1.0 0.9 their present east-west dimension exceeds their Leach Canyon 24.03 1.4 1.0 0.9 0.8 north-south. Caldera-fi ll deposits whose distri- Isom 24.55–27.90 1.6 1.2 1.1 1.0 Lund 29.20 2.0 1.5 1.3 1.2 bution was governed by an older depression, Wah Wah Springs 30.06 1.5 1.1 1.0 0.9 such as the Lund, and small deposits, such as the Cottonwood Wash 31.13 1.0 0.7 0.7 0.6 Average ratio 1.48 1.09 0.99 0.89 Petroglyph Cliff Ignimbrite, which are exposed Average without Lund 1.42 1.04 0.96 0.86 only in a restricted area, do not necessarily show Note: Outfl ow tuff dimensions measured from the zero isopach for the present (taken from maps in this article) a similar infl uence or amount of extension on and corrected for three different amounts of east-west, post-ignimbrite extension. The Lund tuff erupted within the their distribution. older Indian Peak caldera, which infl uenced its more elongate east-west distribution.

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Springs magma chamber possessed signifi - remainder of this depression, and spilled over of at least four cooling units of the Bald Hills cant gradients in chemical composition—a few in thick accumulations to the east but progres- Tuff Member that were emplaced from 27.90 weight percent more silica and more volatiles at sively less to the south, west, and north. As to 27.25 Ma over a broad area of ~21,000 km2. the top; in the main dacitic part of the chamber much as 2500 m of intracaldera Lund ignimbrite After an eruptive lull of 2.3 m.y. as many as four there were fewer and smaller phenocrysts near and lesser wall-collapse breccias plus 1000 m cooling units of the Hamlight Tuff Member the top and the proportion of quartz plus clino- of post-collapse, caldera-fi lling tuffs defi ne the were deposited from 24.91 to 24.75 Ma, appar- pyroxene increased from about nil to 14% in the northern sector of the White Rock caldera. The ently only to the west of the concealed source. deepest erupted level as a result increased pres- Pioche Hills horst exposes a complexly faulted Following the Hamlight eruptions, at 24.55 Ma, sure. Comparison of compositions of ignimbrite and intensely brecciated sequence of Cambrian a cooling unit of the Hole-in-the-Wall Tuff and cognate clasts indicate minimal fractionation rocks that is interpreted to have resulted from Member was emplaced east and west of the of vitroclasts in the Wah Wah Springs ash fl ows. local collapse-related extension in the south- source; this is a chemically distinct low-silica The caldera source of the Cottonwood Wash western segment of the White Rock caldera col- rhyolite. Tuff was apparently engulfed in the younger lar zone between hypothesized inner reverse and Thereafter, explosive activity shifted farther to Indian Peak caldera or overlapping White outer normal ring faults. Like the older Indian the south where it was dominated by eruption of Rock caldera, sources of ash-flow tuffs of Peak caldera, the White Rock caldera experi- rhyolitic magmas rather than the dacitic magmas the Wah Wah Springs and Lund Formations, enced resurgent uplift, but unlike in the Indian that were dominant in the north. Although the respectively. Peak no major intracaldera intrusion is exposed; sources of the next two ignimbrites have not been Tilting of the Needle Range by post-vol- instead, late, post-collapse magmatic activity is located, they must lie between the Indian Peak canic extensional faulting has provided an manifest by several small extrusions and pos- caldera complex to the north and the Caliente exceptional exposure of the internal struc- sible shallow intrusions of dacitic lava along the caldera complex to the south, and possibly were ture and stratigraphy of the northeastern sec- hypothetical concealed ring fault. engulfed, at least in part, within the latter com- tor of the Indian Peak caldera that includes a After the Ryan Spring, explosive eruptions plex. Ash-fl ow tuffs of the 24.03 Ma Leach 3500-m-thick section of lithic caldera-collapse over the next million years occurred in the north- Canyon Formation compose a normally zoned, ignimbrite and intercalated breccia deposits western sector of the Indian Peak caldera com- somewhat phenocryst-rich rhyolite deposit with inboard of the inner reverse ring fault. In the plex. The 30.00 Ma tuff of Deadman Spring is a volume of 3600 km3. Ignimbrites of the Con- 11-km-wide collar zone between this ring fault the only phenocryst-rich, high-silica rhyolite in dor Canyon Formation deposited next con- and the topographic margin, a layer of cata- the complex. The source of this 180 km3 ignim- sist of two generally simple cooling units, the clastic wall-collapse breccia as thick as 600 m brite was the modest-size asymmetric Kixmiller 400 km3 Swett Tuff Member and the larger underlies 1100 m of post-collapse, caldera- caldera that formed several kilometers west of 3200 km3 Bauers Tuff Member deposited at fi lling tuff. The breccia layer was apparently the slightly older Indian Peak caldera and was 23.04 Ma. Both are low-silica, phenocryst-poor, formed by incremental collapse accompany- later partially engulfed in the younger White off-trend rhyolites that generally plot apart from ing downward displacement on progressively Rock caldera. The source of the 350 km3 pheno- other Great Basin ignimbrites on many variation outward-stepping reverse faults and on a north- cryst-rich dacitic Silver King Tuff that was diagrams and share some of the characteristics

bounding normal fault in the extending annu- emplaced at 29.40 Ma has not been verifi ed but of the Isom-type tuffs, e.g., high K2O, TiO2, Ba, lar collar zone. The breccia layer is made all its localization in the northwestern part of the and Zr and low CaO. But they contain biotite as the more unusual by the presence of seams of fi eld and great thickness—as much as 1400 m— the major mafi c phenocryst rather than pyrox- ultracataclasite that testify of extreme shearing within the Kixmiller caldera indicate a nearby enes. The source of the Bauers was the Clover in the collapsing milieu of pre-collapse Cotton- source, or one possibly nested within this ear- Creek caldera that is poorly exposed in the north- wood Wash and Wah Wah Springs tuffs slid- lier structure. The oldest Isom-type tuff is the ern sector of the Caliente caldera complex. This ing into the deepening depression. Apparently small—40 km3—unusually clast-rich Petro- source is marked by hundreds of meters of clast- immediate resurgent uplift of this northeastern glyph Cliff Ignimbrite that was erupted at ca. rich intracaldera tuff comprising numerous cool- caldera sector accompanied a major intrusion 29.1 Ma from the small Blind Mountain caldera ing units and by a hypabyssal intrusion of similar of a granodiorite porphyry of the same compo- located ~5 km beyond the southwestern margin composition. sition as the Wah Wah Springs ignimbrite. of the White Rock caldera. The 800 km3 ignim- One-half million years after deposition of The lower part of the caldera-fi lling tuff in brite volume of the 29.0 Ma Ripgut Forma- the Bauers, a super-eruption of 2200 km3 cre- the Indian Peak caldera comprises the pheno- tion constitutes caldera fi ll in the White Rock ated the Harmony Hills Tuff at 22.56 Ma. The cryst-poor, low-silica rhyolite tuffs of the Ryan depression. Nested within this older caldera is volume of this ignimbrite and its andesite-latite Spring Formation whose volume is estimated the Mount Wilson caldera source of the Rip- composition are apparently unique in all of the at ~1000 km3. Multiple cooling units of the gut, but only its northern part is exposed. The Great Basin; it may be of the same genre as the 30.13 ± 0.13 Ma Greens Canyon Tuff Member 2-km-thick intracaldera tuff is normally zoned older monotonous intermediates. The Harmony from an undisclosed source accumulated mostly from high-silica, very phenocryst-poor rhyolite Hills is not only unusually mafi c but is also in the northern part of the collar zone. An equiv- to a less evolved rhyolite that contains fi amme unusually phenocryst rich—phenocrysts make alent volume of the 30.01 Ma Mackleprang of trachydacite compositionally like the Lamer- up as much as 58% of the tuff. The Harmony Tuff Member had a source caldera in the south- dorf ignimbrite. Hills apparently had a source in the central part eastern collar zone that was buried beneath an Relatively dry, alkaline, high-temperature of the Caliente caldera complex where it is con- unusually thick accumulation—as much as magmas forming the 4200 km3 ignimbrites of cealed by younger deposits. 1400 m—of the younger Lund ignimbrite. the trachydacitic Isom Formation were then The last major eruptions of subduction- Lund ash fl ows apparently mostly vented erupted from a concealed source just beyond related magmas from the Caliente caldera com- from within the southeastern sector of the older the southeast margin of the Indian Peak cal- plex followed a period of inactivity of ~3.6 m.y., Indian Peak caldera at 29.20 Ma; they fi lled the dera complex. Most of this volume is made up creating two phenocryst-rich ignimbrites. The

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18.88 to18.57 Ma Racer Canyon Tuff com- Y, and U, but distinctively lower concentrations and David Davis at the Nevada Bureau of Mines and prises numerous cooling units totaling 1100 of many compatible elements including MgO, Geology made well cuttings available for our examina- km3 that are normally zoned from high-silica CaO, Sr, Ni, Cr, and V for the same range of tion. Jim Toy and Virgil Frizzell in Caselton allowed us to examine and sample well cuttings from grabens rhyolite to high-silica dacite exposed chiefl y in silica as in the dacites and rhyolites. These fl anking the Pioche Hills. Michael Dorais performed Utah near the Telegraph Draw caldera source magmas equilibrated at depths of ~30 km with electron microprobe analyses and Kathleen Robertson in the eastern sector of the caldera complex. The an assemblage of plagioclase, two pyroxenes, helped draw some of the illustrations. 18.51 Ma, 1700 km3 Hiko Tuff is generally a and Fe-Ti oxides. Relatively small volumes of Because we have conducted almost no fi eld work in the Caliente caldera complex and little with the single, normally zoned, rhyolite cooling unit silica-poor, off-trend rhyolitic magmas appar- associated ignimbrites, we relied heavily on the pub- exposed principally in Nevada near the Dela- ently originated by combination of the main- lished and unpublished research of other geologists, mar caldera source in the western sector of the spectrum magmas with those of the Isom type. including John Anderson, Richard Blank, Earl Cook, Caliente caldera complex. Relatively high initial 87Sr/86Sr ratios in Gary Dixon, Bart Ekren, Robert Scott, Paul Williams, A very small volume of the nonwelded, high- the lavas and especially the ignimbrites indi- and, especially, Peter Rowley through their extensive 1:24,000-scale geologic mapping, mostly under the silica rhyolite tuff of Tepee Rocks that is found cate magmas were not derived solely from the auspices of the U.S. Geological Survey. Except for as caldera-fi lling in the Delamar caldera is the mantle , but refl ect varying proportions of old Scott and Ekren, these geologists were mentored as last manifestation of arc magmatism at 18.1 Ma. continental crust combined with the mantle- students by J. Hoover Mackin, who laid the ground- derived basaltic magmas. Older lavas and work for the volcanic history of the southeastern Great Basin and adjacent High Plateaus of Utah through his Magma Genesis ignimbrites, especially the trachydacitic Isom seminal investigation of the Iron Springs mining dis- types, have higher ratios than younger, testify- trict west of Cedar City (Mackin, 1960). The relatively small volume, ~300 km3, of ing to the gradual diminution of fertile felsic We are indebted to Robert Biek, Matthew Brueseke, andesitic lava fl ows extruded before 25 Ma crust added into the magmas. Anita Grunder, and Peter Rowley for their construc- probably refl ects the existence of a widespread In short, we conclude that the southern Great tive comments on an earlier version of this article. Financial support for the Great Basin project was zone of silicic magma in the deeper crust that Basin ignimbrite province with its colossal vol- provided by the National Science Foundation through effectively blocked the ascent of the more dense ume of erupted silicic magmas owes it origin grants EAR-8604195, 8618323, 8904245, 9104612, magmas. After 25 Ma, the onset of crustal exten- to distinctive tectono-magmatic conditions and 9706906, and 0923495 to M.G. Best and E.H. Chris- sion apparently allowed these mafi c magmas to processes related to thick pre-heated felsic crust tiansen. The U.S. Geological Survey and Nevada Bureau of Mines and Geology supported quadrangle ascend through fractures so that thousands of and rollback of a subducting slab of oceanic litho- mapping. The continuing fi nancial and material cubic kilometers of lavas were extruded. This sphere. Consequently, this province developed assistance of Brigham Young University is gratefully transition in tectonic regime was ultimately along a convergent margin during subduction, in acknowledged. complete after 18 Ma when extensional geo- contrast to other large silicic igneous provinces chemical signatures are evident in the volcanic that developed during continental breakup. APPENDIX. DETERMINATION OF THE AMOUNT OF POST-VOLCANIC EXTENSION rocks of the Great Basin. IN THE INDIAN PEAK–CALIENTE FIELD In the southern Great Basin ignimbrite prov- ACKNOWLEDGMENTS ince, 36–18 Ma ignimbrites possess typical arc In an east-west transect from ~113°30′ to 117°20′ W chemical signatures, indicating their subduc- Our understanding of the geology of the Indian longitude (westernmost Utah to central Nevada) tion-related heritage. Magmas had unusually Peak caldera complex profi ted from the collaborative between 39° and 40° N latitude, Smith et al. (1991) mapping of Jeffrey Abbott, Kerry Grant, Jeff Keith, determined an overall extension of 55% that resulted high concentrations of K, indicating their origin Mark Loucks, Hal Morris, Margo Toth, Van Williams, from mostly early Miocene (23 Ma) and younger in unusually thick continental crust, likely to as Julie Willis, and, especially, Lehi Hintze and some 600 faulting. (Our measurement of their present-day cross- much as 70 km thick. The absence of extruded geology students at Brigham Young University in con- sectional length compared to the palinspastically basalt and the limited volume of andesitic lavas junction with their summer fi eld courses from 1967 restored section gives 44% overall extension.) How- to 1997. We gratefully acknowledge their very sig- ever, the amount of extension in this transect varied during the ignimbrite fl areup are also a conse- nifi cant contributions. The fi eld work of three of these greatly in different domains, from ~110% in eastern quence of the unusually thick crust. The gen- students—Richard Holmes, Kim Sullivan, and Jack Nevada to ~40% in central Nevada and nil in between eral southward migration of eruptive sources Rogers—was instrumental in the discovery of three and in westernmost Utah. Although this transect lies through time refl ects the southward rollback of calderas in the Indian Peak complex. Others worked just north of the Indian Peak–Caliente ignimbrite fi eld the subducting plate beneath the continent. An out details of the petrology—Larissa Maughan, Keryn (Fig. 2), it is reasonable to extrapolate the same over- Tobler Ross, Kurtus Woolf, Garret Hart, and Glenn all amount of extension southward. This extrapolation unusually high infl ux of mantle magma into the Blaylock. Many geologic maps were published under follows from the paleomagnetic observation that the unusually thick crust was necessary to create the auspices of and with the fi nancial support of the Sierra Nevada block moved uniformly westward rela- the colossal volume of erupted silicic magma. U.S. Geological Survey’s Richfi eld 2° National Ura- tive to the Great Basin about a pole of rotation near the The main spectrum of magmas ranges from nium Resources Evaluation Project under the direction geographic north pole (Hillhouse and Gromme, 2011). of Thomas A. Steven. Tom’s patient encouragement, McQuarrie and Wernicke (2005, their table 1) high-silica rhyolite (78 wt% SiO2) to high- mentoring regarding ash-fl ow tuffs and calderas, as have tabulated the amount of extension, mostly after silica andesite (61 wt% SiO2) and includes well as his ability to expedite government publications ca. 18 Ma, within individual strain domains between huge monotonous intermediates. These mag- is gratefully acknowledged. H. Richard Blank, Jr., fi rst 40°20′ and 38°40′ N in an east-to-west transect from mas equilibrated under water-rich conditions at provided a Bouguer gravity map of the Indian Peak 114°7′ to 117°23′ W. Strains are signifi cantly greater temperatures of <830 °C and depths of ~7–9 km caldera complex that served to constrain its margin. In on two adjacent structural domains in their transect, the 1970s, Kerry Grant, Ralph Shuey, and Rick Cas- namely, the Sevier Desert detachment in western Utah with an assemblage of plagioclase, quartz, bio- key assisted in establishing the volcanic stratigraphy and the Snake Range décollement in easternmost tite, and Fe-Ti oxides with or without horn- in southwestern Utah. Bart J. Kowallis, Ted McKee, Nevada. Because interpretations of these structures blende, sanidine, pyroxene, zircon, apatite, and and Harald H. Mehnert provided crucial isotopic ages. are controversial, relevant confl icting information is titanite. Hotter (~900 °C) and drier trachydacitic Daniel R. Shawe made available important unpub- provided as a basis for corrections to the areal extents lished work on the Pioche Hills. Gary J. Axen furnished of ash-fl ow sheets in the Indian Peak–Caliente fi eld magmas of the Isom type contained unusually copies of his unpublished geologic maps. Michael that lies principally to the south of these two domains. high concentrations of TiO2, K2O, P2O5, Ba, Rb, Laine, Thomas Dempster, and Thomas Chidsey at The 40 km of extension claimed by many geolo- Ce, Zn, Zr, Y, and Th, and generally higher Nb, the Utah Geological Survey Core Research Center gists (e.g., McQuarrie and Wernicke, 2005) for the

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Sevier Desert detachment in west central Utah has Anderson, J.J., 2001, Late Oligocene–early Miocene normal Best, M.G., and Christiansen, E.H., 1997, Origin of broken been challenged by, among others, Hintze and Davis faulting along west-northwest strikes, northern Marka- phenocrysts in ash-fl ow tuffs: Geological Society of (2003) and Wills et al. (2005). They question the gunt Plateau, Utah, in Erskine, M.C., Faulds, J.E., America Bulletin, v. 109, p. 63–73, doi:10.1130/0016 very existence of the detachment—interpreted from Bartley, J.M., and Rowley, P.D., eds., The Geologic -7606(1997)109<0063:OOBPIA>2.3.CO;2. Transition, High Plateaus to Great Basin—A Sym- Best, M.G., and Grant, S.K., 1987, Stratigraphy of the vol- seismic refl ection data—and argue that the avail- posium and Field Guide: The Mackin Volume: Utah canic Oligocene Needles Range Group in southwestern able information may permit substantially less total Geological Association Publication 30, Pacifi c Section Utah and eastern Nevada: U.S. Geological Survey Pro- extension, to as low as 6 km across the Sevier Desert. American Association of Petroleum Geologists Publi- fessional Paper 1433-A, 28 p. McBride et al. (2010) re-analyzed the seismic refl ec- cation GB 79, p. 401–418. Best, M.G., and Williams, V.S., 1997, Geologic map of the tion data and concluded it is consistent with either Anderson, J.J., and Rowley, P.D., 2002, The Oligocene Isom Rose Valley quadrangle, Lincoln County, Nevada: substantial or minimal extension. Formation, Utah-Nevada: A regional ash-fl ow tuff U.S. Geological Survey Geologic Quadrangle Map For more than two decades, B.P. Wernicke and sheet containing fl uidal features of a lava fl ow: Geo- GQ-1765, scale 1:24,000. associates (e.g., Lewis et al., 1999, p. 50) have spec- logical Society of America Abstracts with Programs, Best, M.G., Shuey, R.T., Caskey, C.F., and Grant, S.K., v. 34, no. 3, p. A-9. 1973, Stratigraphic relations of members of the ulated that the Snake Range décollement is a major Anderson, J.J., Rowley, P.D., Fleck, R.J., and Nairn, A.E.M., Needles Range Formation at type localities in south- low-angle extensional shear zone that underlies sev- 1975, Cenozoic Geology of Southwestern High Pla- western Utah: Geological Society of America Bulletin, eral ranges in the eastern Great Basin and cuts deep teaus of Utah: Geological Society of America Special v. 84, p. 3269–3278, doi:10.1130/0016-7606(1973)84 into the upper crust. Bartley and Wernicke (1984) Paper 160, 88 p. <3269:SROMOT>2.0.CO;2. estimated 60 km of displacement on the décollement Anderson, J.J., Rowley, P.D., Machette, M.N., Decatur, S.H., Best, M.G., Grant, S.K., Hintze, L.F., Cleary, J.G., Hutsin- between the Egan Range and the Confusion Range. and Mehnert, H.H., 1990, Geologic map of the Never- piller, A., and Saunders, D.M., 1987a, Geologic map of McQuarrie and Wernicke (2005, their table 1) indi- shine Hollow area, eastern Black Mountains, southern the Indian Peak (southern Needle) Range, Beaver and cated 79 km of extension over the same interval. In Tushar Mountains, and northern Markagunt Plateau, Iron Counties, Utah: U.S. Geological Survey Miscella- Beaver and Iron Counties, Utah: U.S. Geological Sur- neous Investigation Series Map I-1795, scale 1:50,000. contrast to the Wernicke model, Gans and Miller vey Miscellaneous Investigation Series Map I-1999, Best, M.G., Hintze, L.F., and Holmes, R.D., 1987b, Geo- (1983), Miller et al. (1983), and Gans et al. (1985) scale 1:50,000. logic map of the southern Mountain Home and north- concluded that the décollement is a broadly domical Anderson, R.E., and Hintze, L.F., 1993, Geologic map of the ern Indian Peak Ranges (central Needle Range), Beaver brittle-ductile transition zone overlying a metamor- Dodge Spring quadrangle, Washington County, Utah, County, Utah: U.S. Geological Survey Miscellaneous phic core complex and that the décollement does not and Lincoln County, Nevada: U.S. Geological Survey Investigation Series Map I-1796, scale 1:50,000. extend more than 60 km in any direction. Restoration Geologic Quadrangle Map GQ-1721, scale 1:24,000. Best, M.G., Mehnert, H.H., Keith, J.D., and Naeser, C.W., of tilted fault slices in the brittle hanging wall to their Armstrong, R.L., 1970, Geochronology of Tertiary igne- 1987c, Miocene magmatism and tectonism in and near pre-deformation confi guration indicates as much as ous rocks, eastern Basin and Range province, west- the southern Wah Wah Mountains, southwestern Utah: ern Utah, eastern Nevada, and vicinity, U.S.A: Geo- U.S. Geological Survey Professional Paper 1433-B, 47 p. 500% extension whereas the ductile footwall has been chimica et Cosmochimica Acta, v. 34, p. 203–232, Best, M.G., Morris, H.T., Kopf, R.W., and Keith, J.D., stretched at least 330%. This deformation, along with doi:10.1016/0016-7037(70)90007-4. 1987d, Geologic map of the southern Pine Valley area, slip on major faults to the north, constitutes a unifi ed Askren, D.R., 1992, Origin of andesites interlayered with Beaver and Iron Counties, Utah: U.S. Geological Sur- extensional system with at least 15 km of slip. Gans large-volume felsic ash-fl ow tuffs in the western United vey Miscellaneous Investigations Series Map I-1794, and Miller (1983, p. 136) believed that similar low- States [Ph.D. thesis]: Athens, University of Georgia, scale 1:50,000. angle faults and large stratal rotations are characteris- 276 p. Best, M.G., Christiansen, E.H., and Blank, H.R., Jr., 1989a, tic of the Egan and Schell Creek Ranges where 95 km Askren, D.R., Roden, M.F., and Whitney, J.A., 1997, Petro- Oligocene caldera complex and calc-alkaline tuffs (238%) of extension occurred in a 40-km-wide corri- genesis of Tertiary andesite lava fl ows interlayered of the Indian Peak volcanic fi eld, Nevada and Utah: with large-volume felsic ash-fl ow tuffs of the western Geological Society of America Bulletin, v. 101, dor. Fifty kilometers of extension took place between USA: Journal of Petrology, v. 38, p. 1021–1046, doi: p. 1076–1090, doi:10.1130/0016-7606(1989)101<1076: the Schell Creek and Confusion Ranges. 10.1093/petroj/38.8.1021. OCCACA>2.3.CO;2. Accepting maximum extensions of 40 and 79 km Axen, G.J., 1998, The Caliente-Enterprise zone, southeast- Best, M.G., Christiansen, E.H., Deino, A.L., Gromme, C.S., on the Sevier Desert detachment and Snake Range ern Nevada and southwestern Utah, in Faulds, J.E., and McKee, E.H., and Noble, D.C., 1989b, Eocene through décollement, respectively, as cited in McQuarrie and Stewart, J.H., eds., Accommodation Zones and Trans- Miocene volcanism in the Great Basin of the western Wernicke (2005, their table 1), we calculate there is fer Zones: The Regional Segmentation of the Basin and United States, in Chapin, C.E., and Zidek, J., eds., 145 km (64%) of extension over a present-day dis- Range: Geological Society of America Special Paper Field Excursions to Volcanic Terranes in the western tance of 372 km from 111°47′ to 116°7′ W, which 323, p. 181–194. United States, Volume II: Cascades and Intermoun- Axen, G.J., Lewis, P.R., Burke, K.J., Sleeper, K.G., and tain West: New Mexico Bureau of Mines and Mineral is the longitudinal span of the Wah Wah Springs Fletcher, J.M., 1988, Tertiary extension in the Pioche Resources Memoir 47, p. 91–133. tuff—the largest sheet in the Indian Peak fi eld. For area, Lincoln County, Nevada, in Weide, D.L., and Best, M.G., Lemmon, D.M., and Morris, H.T., 1989c, Geo- the Caliente ignimbrite fi eld the extension is 109 km Faber, M.L., eds., This Extended Land: Geological logic map of the Milford quadrangle and east half of (75%) over ~254 km from 112°30′ to 115°30′ W. But Excursions in the Southern Basin and Range: Las Vegas, the Frisco quadrangle, Beaver County, Utah: U.S. Geo- if we assume more conservative displacements of, University of Nevada, Geological Society of America, logical Survey Miscellaneous Investigation Series Map say, 10 and 20 km, respectively, for the two structural Cordilleran Section, Field Trip Guidebook, p. 3–5. I-1904, scale 1:50,000. domains, the extensions for the two fi elds are reduced Barnes, H., Ekren, E.B., Rodgers, C.L., and Hedlund, D.C., Best, M.G., Toth, M.I., Kowallis, J.B., Willis, J.B., and Best, to 83 km (29%) and 62 km (32%), respectively. As a 1982, Geologic and tectonic maps of the Mercury V.C., 1989d, Geologic map of the northern White Rock quadrangle, Nye and Clark Counties, Nevada: U.S. Mountains–Hamlin Valley area, Beaver County, Utah, compromise and for convenience, as well as to be con- Geological Survey Miscellaneous Investigations Map and Lincoln County, Nevada: U.S. Geological Survey sistent with dimensions of ignimbrite deposits cited in I-1197, scale 1:24,000. Miscellaneous Investigations Series Map I-1881, scale earlier publications (e.g., Best et al., 1989a), we use a Barr, D.L., 1993, Time, space, and composition patterns of 1:50,000. value of 50% uniform extension for the Indian Peak middle Cenozoic mafi c to intermediate composition Best, M.G., Hintze, L.F., Deino, A.L., and Maughan, L.L., and Caliente fi elds. This value is consistent with the lava fl ows of the Great Basin, western U.S.A. 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