The 36–18 Ma Southern Great Basin, USA, Ignimbrite Province and Flareup Themed Issue

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

Introduction: The 36–18 Ma southern Great Basin, USA, ignimbrite province and ﬂ areup: Swarms of subduction-related supervolcanoes

Myron G. Best1, Eric H. Christiansen1, and Sherman Gromme2 1Department of Geological Sciences, Brigham Young University, Provo, Utah 84602, USA 2420 Chaucer Street, Palo Alto, California 94301, USA

ABSTRACT chronologic, and paleomagnetic data on the fi rst drew attention to this burst of explosive unusually plentiful and voluminous ignim- silicic volcanism and Coney (1978) named it During the middle Cenozoic, from 36 to brites in the southern Great Basin ignimbrite the ignimbrite fl areup. Large parts of Nevada, 18 Ma, one of the greatest global expressions province. These data permit rigorous corre- Utah, Colorado, New Mexico, Arizona, and of long-lived, explosive silicic volcanism lations of the vast outfl ow sheets that span northwestern Mexico are blanketed to depths of affected a large segment of southwestern between mountain-range exposures across hundreds, and locally thousands, of meters by North America, including central Nevada intervening valleys as well as correlation of silicic ash fl ows erupted from caldera-forming and southwestern Utah in the southern the sheets with often-dissimilar accumula- magma systems (Fig. 1; see also Best and Chris- Great Basin. The southern Great Basin tions of tuff within dismembered source cal- tiansen, 1991, their fi gure 4; Best et al., 1989b, ignimbrite province, resulting from this deras. Well-exposed collar zones of larger their table 1). Many magma systems were active fl areup, harbors several tens of thousands calderas reveal complex wall-collapse brec- for several million years and were of batholithic of cubic kilometers of ash-flow deposits. cias. Calculated ignimbrite dimensions in scale, manifested by the thousands of cubic They were created by more than two hun- concert with precise 40Ar/39Ar ages provide kilometers of magma ejected in single erup- dred explosive eruptions, at least thirty of insights on the growth and longevity of the tive events. The 36–18 Ma subduction-related which were super-eruptions of more than colossal crustal magma systems. Exactly how ignimbrite fl areup near the southwestern margin 1000 km3. Forty-two exposed calderas are these subduction-related magma systems of the North American continental plate is the as much as 60 km in diameter. As in other were sustained for millions of years to create only one of its magnitude known anywhere in parts of southwestern North America multicyclic super-eruptions at a particular the world of Mesozoic or Cenozoic age that is affected by the ignimbrite fl areup, rhyolite focus remains largely unanswered. What not related to continental breakup (Bryan et al., ash-fl ow tuffs are widespread throughout factors created eruptive episodes lasting mil- 2010; Cather et al., 2009). The magnitude and the southern Great Basin ignimbrite prov- lions of years separated by shorter intervals brevity of the 18 m.y. fl areup are all the more ince. However, the province differs in two of inactivity? What might have been the remarkable when it is realized that volcanism in significant respects. First, extrusions of role played by tears in the subducting plate southwestern North America related to subduc- contemporaneous andesitic lavas were mini- focusing a high rate of mantle magma fl ux tion has persisted more or less continuously and mal. Their volume is only about 10% of the into the crust? What role might have been diachronously for some 200 m.y. ignimbrite volume. Unlike other contem- played by an unusually thick and still-warm In the southern Great Basin ignimbrite prov- poraneous volcanic fi elds in southwestern crust inherited from earlier orogenies? Are ince of Nevada and southwestern Utah (Fig. 2), North America, only a few major composite the numerous super-eruptions, especially of on the order of 250 explosive events during the (strato-) volcanoes predated and developed the unusual monotonous intermediates and ignimbrite fl areup resulted in at least 70,000 km3 during the fl areup. Second, the central sec- succeeding trachydacitic eruptions, during of ash-fl ow deposits. At least 30 of the events tor and especially the eastern sector of the the Great Basin ignimbrite fl areup simply a created volumes of more than 1000 km3, thus province experienced super-eruptions of result of the coupling effect of high mantle- qualifying as super-eruptions (Rampino and relatively uniform, crystal-rich dacite mag- magma fl ux and a thick crust, or did other Self, 1992; de Silva, 2008; Miller and Wark, mas; resulting deposits of these monoto- factors play a role? 2008). Forty-two exposed calderas have diam- nous intermediates measure on the order eters ranging upwards to 60 km. of 16,000 km3. Following this 4 m.y. event, INTRODUCTION Several outstanding problems are associated very large volumes of unusually hot and dry with the southern Great Basin ignimbrite prov- trachydacitic magmas were erupted. These The middle Cenozoic (36–18 Ma) ignim- ince and its investigation. Not the least of these two types of magmas and their erupted brite fl areup ranks as a premier volcanic event problems is its vast size. Ash-fl ow deposits are volumes are apparently without parallel in in southwestern North America and is one of found from the foothills of the Sierra Nevada the middle Cenozoic of southwestern North the greatest global manifestations in the terres- eastward across the Sierra and then across the America. trial rock record of long-lived explosive silicic entire Great Basin into the western Colorado A fundamental goal of this themed issue is volcanism in a continental-margin volcanic Plateau—a current distance of ~900 km (Fig. 1). to present basic stratigraphic, compositional, arc. Lipman et al. (1971) and Noble (1972) Fallout ash deposits occur at least as far as

Geosphere; April 2013; v. 9; no. 2; p. 260–274; doi:10.1130/GES00870.1; 7 ﬁ gures. Received 9 October 2012 ♦ Revision received 12 December 2012 ♦ Accepted 20 December 2012 ♦ Published online 6 March 2013

CA 40° SIERRA NEVADA NE NV UT Figure 1. Middle Cenozoic vol- Great canic fields in southwestern

North America in which silicic CO ash-flow deposits are promi- Basin nent, manifesting the ignimbrite fl areup. The Marysvale COLORADO fi eld has relatively little ignim- Marysvale Southern brite compared to andesitic 35° Rocky Mtn lava (see Fig. 2) whereas the AZ Southern Rocky Mountain PLATEAU fi eld and Mogollon-Datil fi eld (and the area to the west) have Mogollon-Datil large volumes of ignimbrite as well as andesitic lava. The huge middle Cenozoic Sierra Madre Occidental ignimbrite fi eld of NM 30° northwestern Mexico covers an area of at least 300,000 km2 to a thickness locally in excess of 1 km (e.g., Ferrari et al., 2007; Swanson et al., 2006). The fi eld “...probably displays the largest Si erra Madre Occidental continuous ignimbrite expanse in the world” (McDowell and Clabaugh, 1979, p. 116). Only 25° MEXICO a few percent of the estimated 350–400 calderas have been recognized to date. CA—Cali- fornia; NV—Nevada; UT— Utah; CO—Colorado; NE— Nebraska; AZ—Arizona; NM— New Mexico. Figure modifi ed from Garrity and Soller (2009). 20°

0 200 400 miles

0 200 400 600 kilometers

120° 115° 110° 105° 100°

western Nebraska. Many decades of fi eld work many more have yet to be discovered. Thermo- In this introduction to the themed issue of by numerous geologists have established most barometric interpretations of phase assemblages Geosphere, our intent is to provide an over- of the basic stratigraphy of much of the province to elucidate intensive variables in magma view of the 36–18 Ma southern Great Basin and many hundreds of samples have provided systems have been carried out on only a few ignimbrite province as a whole. After briefl y insights into the composition and radiometric systems . Only reconnaissance isotopic analyses summarizing the geologic setting of the Great chronology of the ignimbrites. But unresolved have been made; far more data will be required Basin and its volcanism, highlighting previ- correlation problems abound. Many eruptions to provide meaningful syntheses of magma evo- ous research, we then discuss the ignimbrites have yet to be characterized in any detail with lution in individual caldera-forming magma and their associated source calderas, and con- respect to dimensions, age, and composition. systems as well as the origin of the Great Basin clude with a statement of the scope of this Many calderas have barely been identifi ed and magmatic regime as a whole. themed issue.

Geosphere, April 2013 261 Best et al.

Indian Peak- Caliente Silicic Ignimbrite Andesite Lava Western NV Central NV caldera caldera caldera

42° California Nevada Utah

86 Sri = 0.706 Sr/ 87 Salt Lake City GREAT Elko

SIERRA BASIN 40°

Reno Austin Ely Marysvale

38° NEVADA Ton opah field COLORADO Miles St. George 050 100 Arizona 050 100 PLATEAU Kilometers Las Vegas 36° 120° 118° 116° 114° 112°

Figure 2. Middle Cenozoic southern Great Basin ignimbrite province in Nevada and southwestern Utah resulting from the 36–18 Ma ignimbrite fl areup (modifi ed from Stewart and Carlson, 1976). The province is divided into three parts: the Western Nevada fi eld and calderas (blue), the Central Nevada fi eld and calderas (red), and the Indian Peak–Caliente fi eld and calderas (green). The western edge of the Precambrian continental basement is indicated by the dashed initial 87Sr/86Sri = 0.706 line (modifi ed from Wooden et al., 1999). Just to the east, the yellow band denotes the approximate position of what we interpret to be a topographic barrier on the western lip of the middle Cenozoic Great Basin altiplano (see Fig. 3). The Marysvale volcanic fi eld that is not part of the southern Great Basin ignimbrite province and lies to the east on the western margin of the Colorado Plateau is shown here to emphasize the contrasting dominance of andesitic lava over silicic ignimbrite in this fi eld. NV—Nevada.

REGIONAL GEOLOGIC SETTING OF strict hydrographic sense, for both convenience ing the early Cenozoic (late Paleocene–Eocene) THE GREAT BASIN and brevity. had developed a gentle slope incised into the Prior to late Cenozoic extension, folding and western margin of the altiplano; this slope was Overview thrusting during multiple late Paleozoic to ear- underlain by Paleozoic–Mesozoic oceanic rock liest Cenozoic orogenies (e.g., DeCelles, 2004; assemblages, in part accreted onto the continent. Strictly speaking, the Great Basin of Nevada Dickinson, 2006) had thickened the crust in the East of the Precambrian edge, a thick section and western Utah (Fig. 2) is an area of internal area of the Great Basin to as much as 70 km; we of Phanero zoic–latest Proterozoic continental- drainage within the broad northern Basin and call the resulting high plateau the Great Basin margin carbonate rocks, shales, and sandstones Range province, which has been created by altiplano (Fig. 3; Best et al., 2009) because of its overlies earlier Proterozoic crust composed block faulting during still-ongoing east-west pre-extensional tectono-magmatic resemblance of medium- to high-grade schist, gneiss, and crustal extension. However, tributaries to the to the present-day central Andean Altiplano migmatite and intruded granitic rocks (Stewart, Colorado River, which empties into the Paciﬁ c (e.g., Kay and Coira, 2009). West of the edge 1980). Intermittent and diachronous subduc- Ocean, drain the southeastern segment of of the Precambrian continental margin in the tion-related magmatism occurred during the Nevada and adjacent Utah in an area we herein Great Basin, manifest by the initial 87Sr/86Sr = Mesozoic. include as a part of the Great Basin. Thus, we 0.706 line (Fig. 2), headward erosion by rivers Following a lull in magmatic activity of use the term Great Basin more liberally than in a emptying into the ancestral Paciﬁ c Ocean dur- tens of millions of years, time-transgressive

262 Geosphere, April 2013 Introduction: The 36–18 Ma southern Great Basin, USA, ignimbrite province and ﬂ areup

California Nevada Utah

Great Basin “altiplano”

Wet western slope Dry plateau (3–4 km high)

Line of section Deeply incised, tuff-filled valleys Poorly integrated drainage Topographic Ignimbrite flattened surface 4 barrier CNCC IPCC 2

Elevation (km)Elevation 0

20 Phanerozoic Sierra Nevada Oceanic 40 accreted terranes batholith Proterozoic crust lithosphere ? ? ?

Depth (km) 60 Subcontinental lithospheric mantle

Figure 3. Conceptual east-west cross-section though the middle Cenozoic orogenic plateau, or Great Basin altiplano, at ~38.5° N showing the unusually thick crust, especially beneath the Indian Peak–Calieinte caldera complex (IPCC). The crust probably was somewhat thinner beneath the Central Nevada caldera complex (CNCC) and thinner still farther west on the western slope of the altiplano beneath the West- ern Nevada volcanic fi eld. The western Great Basin is underlain by Phanerozoic accreted terranes whereas the eastern part is underpinned by felsic Proterozoic basement. Note the change in vertical scale at sea level. Figure modifi ed from Best et al. (2009, their fi gure 17).

subduction-related, or arc, magmatism resumed than ca. 20 Ma were created during subduction ignimbrite fl areup based on their present dimen- at ca. 45 Ma in what became the northern Great of oceanic lithosphere beneath the continent; sions, (2) the character of the land surface over Basin and swept at a diminishing speed south the resulting volcanic rocks possess a charac- which the ash fl ows moved and accumulated, and southwestward into southern Nevada, where terizing arc chemical signature (e.g., Fig. 4). and (3) the crustal environment in which silicic it stalled in the mid-Miocene at ca. 15 Ma (Best Basalt, as defi ned by the International Union ignimbrite-forming magmas were created. and Christiansen, 1991, their fi gure 2). The of Geological Sciences (Le Maitre, 1989), is Although some workers (e.g., Zoback et al., sweeping magmatism is widely believed to have essentially absent. After ca. 20 Ma, relatively 1981; Gans et al., 1989; Constenius et al., 2003) developed as a result of progressive steepening, more-alkaline silicic magmas developed and have claimed broad synchroneity of middle or rollback, of the subducting oceanic Farallon were accompanied by basalt, which appeared in Cenozoic volcanism and extension in the Great slab from its prior fl at confi guration because signifi cant volumes after ca. 17–15 Ma, such as Basin, we fi nd that little and usually only local of its decreasing age upon entry beneath the in the central Nevada rift (Zoback and Thomp- extension occurred during the 36–18 Ma ignim- continent and a slowing rate of convergence son, 1978) and on the western and eastern mar- brite fl areup; nearly all of the regional tectonic (Severing haus and Atwater, 1990). In western gins of the Great Basin (e.g., Nelson and Tingey, extension took place signifi cantly later than the Utah and eastern Nevada, this sweeping mag- 1997). An arc geochemical signature is absent fl areup, resulting in the current basin-and-range matism is expressed by more-or-less separate, in these rocks less than 20 Ma that typically topography (e.g., Stewart, 1998). Early work in sub-parallel, roughly east-west belts of volcanic occur in bimodal suites. This absence refl ects local metamorphic core complexes had led to rocks and minor granitic intrusions (e.g., Stewart derivation from magmas not related to subduc- the concept of substantial extension in the mid- and Carlson, 1976; Christiansen and Yeats, tion but instead generated in an extensional dle Cenozoic, and this concept was extrapolated 1992; Rowley, 1998; Rowley and Dixon, 2001). tectonic regime (e.g., Christiansen et al., 2007; to the entire Great Basin. However, in at least In western Nevada, these belts are less distinct. John, 2001). one of these metamorphic complexes, the Snake By about an order of magnitude, the greatest vol- Range east of Ely, Nevada, fi ssion-track dat- ume of volcanic rocks occurs in a belt extending Crustal Extension ing has revealed that large-magnitude extension from the Marysvale volcanic fi eld on the western in the Early Miocene at 17 Ma had been “seri- margin of the Colorado Plateau westward across Occurrence Mostly After the ously underestimated” relative to the originally the southern Great Basin to Reno, Nevada, and Ignimbrite Flareup conceived middle Cenozoic (Late Eocene– beyond into the Sierra Nevada (Fig. 2). Exclud- The time interval during which the orogeni- Early Oligocene) extension (Miller et al., 1999, ing the Marysvale fi eld, which is dominated by cally thickened crust beneath the Great Basin p. 902). On the basis of the sedimentary and intermediate-composition lavas, the belt in the altiplano in the early Cenozoic was subse- low-temperature thermochronologic record of southern Great Basin is composed mostly of quently thinned by extensional faulting to its upper-crustal extension in a 200 km east-west silicic ash-fl ow tuffs produced during the middle current ~30 km thickness has been controver- transect from 118°00′ to 115°30′ W between Cenozoic, 36–18 Ma, ignimbrite fl areup, the sial. Did the extension take place before, during, ~40°00′ and 40°30′ N, Colgan and Henry (2009; topic of this themed Geosphere issue. or after the middle Cenozoic ignimbrite fl areup? see also Henry et al., 2011) concluded that little Since the classic works of Lipman et al. (1971, Resolution of this question signifi cantly impacts extension took place until ca. 17–10 Ma when 1972) and Christiansen and Lipman (1972), it three facets of our investigation of the ignim- major extension occurred. In an east-west tran- has been recognized that calc-alkaline rhyolitic brite province: (1) calculation of the areas and sect from ~113°30′ to 117°20′ W (western- to andesitic magmas in the Great Basin older volumes of ash-fl ow deposits created during the most Utah to central Nevada) between 39° and

Geosphere, April 2013 263 Best et al.

A 1000 brite sequences are faulted and tilted as a result Continent-continent collision of post-deposition extension. (2) Many ignimbrite outfl ow sheets (Figs. 5 and 6) have a vast areal extent. Older ash- Anorogenic or within plate fl ow tuffs do show evidence of accumulation in paleovalleys and restriction by topographic highs, but as the ignimbrite fl areup progressed, 100 the topography was generally smoothed by the

Rb (ppm) deposition of hundreds of meters of ignimbrite Figure 4. Diagrams showing in outfl ow areas surrounding source calderas. the arc chemical signature in Volcanic arc Had there been signifi cant topography, younger 36–18 Ma ash-fl ow tuffs in the Central Nevada and ash fl ows would not have been dispersed so middle Cenozoic Great Basin Indian Peak–Caliente widely from their sources and in a generally ignimbrite province. (A) Dia- ignimbrites n = 889 diminishing thickness away from the source as gram discriminating among 10 we have documented. Analysis of the east-west three tectonic regimes accord- 10 100 1000 versus north-south dimensions of ten major ing to Pearce et al. (1984). Y + Nb (ppm) 31–18 Ma outfl ow sheets in the eastern Great Although a few samples fall Basin reveals no indication of extension during in the anorogenic or within- their deposition; older sheets are as extended as plate field, they nonetheless 1000.0 younger ones (Best et al., this themed issue [a], show an arc characteristic in B. B their table 8). (B) Spidergram for all ignim- (3) On a hilly depositional surface, non-hor- brites. The trace-element pat- izontal compaction foliations in the ignimbrites terns display obvious negative 100.0 would have increased the between-site disper- Nb anomalies typical of sub- sions of paleomagnetic directions and adversely duction-related rocks. Primi- impacted the success of the paleomagnetic cor- tive mantle composition from relations documented in Gromme and Hudson McDonough and Sun (1995). 10.0 (this themed issue). (4) Evidence for the continued existence during the ignimbrite fl areup of unusually thick Rock/Primitive Mantle 1.0 and, therefore, unextended crust (Fig. 3), which had formed by contractile deformation during Central Nevada and Indian Peak–Caliente earlier orogenies in the Great Basin area, is ignimbrites n = 889 furnished by middle Cenozoic andesitic lavas. 0.1 Comparison of the composition of these Great Rb Ba Th U K Nb La Pb Ce Sr Nd P Sm Zr Ti Y Basin lava samples (n = 376; Barr, 1993) with a worldwide database (n > 6000) of recent andesitic arc lavas whose composition correlates with known crustal thickness indicates the Great 40° N, Smith et al. (1991) determined that the outfl ow ignimbrite sheets are lacking. Perusal of Basin crust was probably as thick as 60–70 km extension resulted from mostly Early Miocene the maps by Stewart and Carlson (1976) show- in its eastern part and thinned somewhat west- (23 Ma) and younger faulting. McQuarrie and ing the distribution of Cenozoic rock types in ward (Best et al., 2009). The absence of basalt Wernicke (2005, their table 1) found that most Nevada reveals that sedimentary deposits are until after ca. 20 Ma is another indication of an of the extension in an east-to-west transect virtually nonexistent for the 34–17 Ma ignim- unusually thick crust in the Great Basin dur- from 111°47′ to 117°23′ W between 40°20′ brite fl areup time period, with the few existing ing the middle Cenozoic; mantle-derived basalt and 38°40′ N occurred mostly after ca. 18 Ma. deposits mostly inside calderas. Sedimentary magmas were not extruded because of the long Although these transects lie for the most part to deposits are more widespread from 43 to 34 Ma path distance through the thick crust where frac- the north of the southern Great Basin ignimbrite and became abundant after 17 Ma, when sig- tionation to less-mafi c daughters and assimila- province (Fig. 2), we believe it is reasonable to nifi cant crustal extension began. In numerous tion of silicic material intervened. extrapolate the timing of extension southward conformable stratigraphic sections of 36–18 Ma Local extension did occur during the fl areup into the province. ash-fl ow tuffs that we have examined (for loca- and is manifest in angular discordances between Four independent lines of evidence support tions of most of these, see Best and Christian- deposits in and near centers of volcanism and the absence of signifi cant regional extension sen, 1991), intercalated epiclastic sediment such plutonism as a result of shallow-crustal magma during the ignimbrite fl areup in the southern as might have been shed off nearby uplifts is intrusion, doming, caldera collapse and resur- Great Basin, which would have resulted in mostly restricted to the older parts of the sec- gence, and depositional onlap. However, inter- fault-related topography and fault-tilted strata tions. Numerous examples of conformable polation and extrapolation of local deformation, and deposition of syntectonic erosional debris sequences of ignimbrites can be seen in photo- whatever its cause, through time and space (e.g., between ignimbrites: graphs throughout this themed issue of Geo- Gans et al., 1989) do not constitute evidence for (1) Substantial and widespread angular dis- sphere. Throughout the southern Great Basin large-magnitude, regional tectonic extension in cordances and erosional-debris deposits between ignimbrite province, entire conformable ignim- the southern Great Basin ignimbrite province as

264 Geosphere, April 2013

Introduction: The 36–18 Ma southern Great Basin, USA, ignimbrite province and ﬂ areup ada

tah Provo

Lovelock U Nev 40°

Haskell PPeakeak

Reno AustinAustin EurekaEureka Delta Ely CentralCentral U 39° Yerington NevadaNevada CC Baker WesternWestern Richfield NevadaNevada Lund

HawthorneHawthoorne Marysvale WB Milford

Beaver W M Indian Peak CC 38° Tonopah S P L Pioche I Panguitch Caldera Modena Caliente Cedar City 87Sr/86Sri = 0.706 Rachel Caliente CC Alamo N Topographic barrier ev California ada 0255075100 St. George Utah 37° Km Arizona

120° 118° 116° 114° 112° Figure 5. Map distinguishing the three fi elds in the 36–18 Ma southern Great Basin ignimbrite province. Each fi eld is delineated by the outermost limit of exposed ignimbrite outfl ow sheets that surround their associated source calderas. In the eastern Great Basin, the Indian Peak–Caliente caldera complex and its surrounding ignimbrite fi eld (blue shade) lie across the Nevada-Utah state line. The caldera complex (blue lines) consists of the Indian Peak caldera cluster to the north and the Caliente cluster to the south. To the west is the Central Nevada caldera complex and its surrounding ignimbrite fi eld (black outline). These two fi elds lay on the middle Cenozoic Great Basin altiplano and to the east of a topographic barrier, or drainage divide (yellow band), on its western lip (Fig. 3; see also Best et al., 2009). The topographic barrier apparently blocked the westward dispersal of all but one known ash fl ow from the Central Nevada calderas located to the east. Although the caldera sources (red lines) for ignimbrites in the Western Nevada fi eld (pink shade) are not cleanly separated from the Central Nevada calderas, their outfl ow sheets are, for the most part. Most ash fl ows from Western Nevada caldera sources west of the barrier were not dispersed eastward but fl owed to the west, in part via stream channels on the western slope of the altiplano that extended across what is now the Sierra Nevada and its western foothills (Fig. 3; Henry and Faulds, 2010). CC—caldera complex. The western edge of the Precambrian continental basement is indicated by the dashed initial 87Sr/86Sri = 0.706 line (modifi ed from Wooden et al., 1999). Letters inside calderas stand for the tuffs that erupted from them: U—Underdown; WB—Windous Butte; M—Monotony; P—Pahranagat; S—Shingle Pass; W—Wah Wah Springs; L—Lund; I—Isom. ash fl ows were emplaced during the fl areup. This but our measurement of their present-day cross- the northern part or beyond. To justify extrapo- conclusion is in opposition to the recent views of sectional length compared to the palinspastically lation of the strains from these more northerly Humphreys (2009, his fi gure 7) and Bryan et al. restored section indicates 101 km of overall transects southward, we note the results of a (2010, p. 208) who indicate synchroneity of the extension, or 43%. In their 200-km-long east- paleomagnetic study by Hillhouse and Gromme ignimbrite fl areup with crustal extension. west transect in northeastern Nevada, Colgan (2011). They demonstrated that the pole of rota- and Henry (2009; see also Henry et al., 2011) tion of the Sierra Nevada plutonic block during Amount of Extension estimated ~50–60 km of extension across the the Cenozoic was located near the north geo- The amount of extension after deposition of 200 km transect, or 33%–43%. McQuarrie and graphic pole. Hence, the extension of the Great ignimbrites signifi cantly impacts calculations of Wernicke (2005, their table 1) found in their Basin was quasi-rectilinear so that the amount their areas and volumes in this themed issue. We 287-km-long, east-west transect from the Utah- of strain as measured in kilometers is uniform have not attempted to make detailed palinspastic Nevada state line to 117°23′ W the extension is north to south within most of the Great Basin analyses of the amount of extensional strain in 74 km, or 35%. In western Utah, to 111°47′ W, ignimbrite province. the areas over which the southern Great Basin estimates of extension are less certain, ranging Based on these investigations, authors of this ignimbrites are exposed (Figs. 2 and 5) but rely from 75% to 30%. themed issue have adopted values of 50% uni- instead on the fi ndings of other workers. These three transects more-or-less corre- form extension for the Indian Peak–Caliente fi eld In their 335-km-long, east-west transect from spond with the east-west extent of the central (Best et al., this themed issue [a]) and 40% for easternmost Utah to central Nevada, Smith and eastern sectors of the southern Great Basin the Central Nevada fi eld (Best et al., this themed et al. (1991) cited an overall extension of 55%, ignimbrite province (Fig. 5), but lie mostly in issue [b]). For the Indian Peak–Caliente fi eld,

Geosphere, April 2013 265 Best et al.

“outflow CENTRAL NEVADA FIELD alley” INDIAN PEAK - CALIENTE FIELD 18 Fraction ? ? Hiko Racer Canyon N U (Several cooling units) 20 E T V A A H D 22 A Harmony Hills Pahranagat Bauers Swett Buckwheat Rim 24 Leach Canyon & Buckskin Point Hole-in-the-Wall Clipper Gap Hamlight (4 cooling units) Lunar Cuesta, Goblin Knobs 26 Z Shingle Pass Y, H

Age (Ma) X V R Bald Hills (4? cooling units) 28 Monotony Ripgut Petroglyph Cliff Palisade Mesa & Lund Silver King Hot Creek Canyon Deadman Spring & Ryan Spring 30 Wah Wah Springs Hoodoo Canyon & Black Rock Summit Cottonwood Wash Windous Butte 32 Lamerdorf (at least Cottonwood Canyon ? Marsden 3 cooling units) ?

34 Sawtooth Peak Pritchards Station Pancake Summit Tunnel Spring 36 Stone Cabin (3 cooling units) The Gouge Eye

118 117 116 115 114 113 112 Longitude West (degrees)

Figure 6. East-west extent of 36–18 Ma major ash-fl ow tuff cooling units in the Central Nevada and Indian Peak–Caliente ignimbrite fi elds. Note that, because of the general southward sweep of source activity, older tuff units at the bottom of the diagram chiefl y occur in the northern part of the fi elds and younger units in the south. Thus, the diagram may be roughly considered as an inverted map with south at the top; this view is more accurate for the Indian Peak–Caliente fi eld than for the Central Nevada fi eld. Also note overlapping outfl ow sheets in the two fi elds in “outfl ow alley” (see also Fig. 7). Omitted are several small scattered cooling units of 27–23 Ma Isom-type tuffs in the Central Nevada fi eld and many small 24–18 Ma Blawn ash-fl ow tuffs in the Indian Peak–Caliente fi eld. To prevent overcrowding in the left part of the diagram, some unit names are designated by red letters, as follows: V—tuff of Pott Hole Valley; R—tuff of Orange Lichen Creek; Z, Y, H, X—Upper Tuff, Tikaboo Tuff, Hancock Tuff, and Lower Tuff Members, respectively, of the Shingle Pass Formation. The age of the Marsden Tuff of the Escalante Desert Group in the lower right is only approximate. See also Table 1 in Best et al. (a, b, this themed issue). the 50% extension is consistent with the east- It should be emphasized that the amount of be justifi ed in detail, but is a convenience we west versus north-south dimensions of ten major strain in adjacent structural domains varies sig- have adopted in the absence of explicit quantita- 31–18 Ma ignimbrite outfl ow sheets (Best et al., nifi cantly. Smith et al. (1991) noted that in their tive information on individual strain domains in this themed issue [a], their table 8). Interestingly, transect, extension ranged from ~110% in east- the fi elds; the uniform assumption is more likely in contrasting the relatively greater amount of ern Nevada, to ~40% in central Nevada, and nil valid for larger ignimbrite sheets. extension for the Indian Peak–Caliente fi eld with in between as well as in easternmost Utah. Col- Because of their deposition during the the slightly lower value for the Central Nevada gan and Henry (2009, p. 939) found that in their middle Cenozoic in irregular stream valleys on fi eld, we note that, on the basis of much less transect, extension was strongly partitioned into the western slope of the Great Basin altiplano information at the time, Proffett (1977; see also highly extended domains (50%–100% or more (Fig. 3) and due to deformation related to the Henry and Faulds, 2010, their fi gure 12) sug- strain) separated by essentially undeformed Walker Lane, volume estimates for most ignim- gested that 30%–100% extension was likely in crustal blocks. Hence, the assumption of uni- brites in the Western Nevada fi eld are very the western and eastern parts of the Great Basin form extension throughout the entire Central approximate (see Henry and John, this themed but only 10%–15% in the central. Nevada and Indian Peak–Caliente fi elds cannot issue, for details).

266 Geosphere, April 2013 Introduction: The 36–18 Ma southern Great Basin, USA, ignimbrite province and ﬂ areup

PREVIOUS RESEARCH ON fi rst described by Mackin are exposed. In other Askren et al. (1997) and Barr (1993; see THE SOUTHERN GREAT BASIN early work, but independent of Mackin, ignim- also Best et al., 2009) documented the chemi- IGNIMBRITE PROVINCE brites in the Grant Range, Nevada, were mapped cal composition of andesitic lava fl ows coeval and described by Scott (1965, 1966). with the Great Basin ignimbrites. Isotopic stud- Working in the Iron Springs mining district Beginning in 1966, E. Bart Ekren and other ies of volcanic rocks include Scott et al. (1971), of southwest Utah in the 1950s, J. Hoover geologists of the U.S. Geological Survey with the Larson and Taylor (1986), Unruh et al. (1995), Mackin (1960) applied a new paradigm fos- Nevada Test Site project systematically mapped Hart (1997), and Hart et al. (1997). tered by Howell Williams and others to the ignimbrite terranes over thousands of square volcanic rocks, realizing that they were ignim- kilometers, including thick accumulations in the THE SOUTHERN GREAT BASIN brites, rather than lavas as previously believed. Central Nevada caldera complex (Ekren et al., IGNIMBRITE PROVINCE Mackin gave stratigraphic names to several ash- 1974; see also Best et al., this themed issue [b]). fl ow deposits in the southeastern Great Basin, Gromme et al. (1972) demonstrated the utility of Delineation which he characterized as extensive, datable paleomagnetism for correlation of dismembered sheets constituting instantaneous time hori- ash-fl ow sheets. These and other fi eld investiga- The southern Great Basin ignimbrite province zons of use in elucidating the structural evolu- tions, supplemented by extensive K-Ar dating shown in Figure 2 comprises a swath of mostly tion of the province. (While doing geophysical throughout the Great Basin (e.g., Marvin et al., mountain-range exposures of silicic ignimbrite fi eld work as an intern in the summer of 1958, 1973), led to a series of province-wide, small- and lesser rhyolitic to andesitic lava extending the senior author encountered Williams and scale maps compiled for four time periods of through the Sierra Nevada and eastward across Mackin in a jeep on a narrow dirt track in the the Cenozoic by Stewart and Carlson (1976; the Great Basin to the Colorado Plateau. The hills west of Milford, Utah [Fig. 5]. We moved see also Sargent and Roggensack, 1984). These Marysvale volcanic fi eld that lies on the western our vehicles aside, exchanged greetings, and maps delineated some of the then-known major margin of the Colorado Plateau is not included went our separate ways.) Two of Mackin’s stu- caldera sources but were remarkable in show- in the ignimbrite province but is shown in Fig- dents, Earl Cook (1965; also unpublished data) ing, for the fi rst time and with unusual clarity, ure 2 to emphasize the dominance of interme- and Paul Williams (1967), with the fi nancial the middle Cenozoic (36–18 Ma) Great Basin diate-composition lavas over silicic ignimbrite assistance of oil companies, successfully cor- ignimbrite fl areup and the much less volumi- (Cunningham et al., 2007). The southern Great related ignimbrite sheets over broad areas from nous, contemporaneous andesitic lavas. Basin ignimbrite province excludes ignim- southwestern Utah into central Nevada where a In a paper that was prescient for its time and brites lacking an arc chemical signature that newly discovered oil fi eld was hosted in part in based on only sparse data, Noble (1972) also are younger than 18 Ma in age. These tuffs lie ash-fl ow tuffs. They used phenocryst types and perceived the ignimbrite fl areup in the south- in northwestern Nevada (e.g., McDermitt area; proportions and other petrographic features as western United States and recognized, for the Rytuba and McKee, 1984), in the Southwest- well as stratigraphic position in order to over- Great Basin, that (1) younger ignimbrites were ern Nevada volcanic fi eld between Tonopah come the discontinuity of outcrops resulting deposited on a surface of little relief, (2) sig- and Las Vegas (Sawyer et al., 1994), and in the from dismemberment by post-emplacement nifi cant crustal extension forming the basin- southern part of the Caliente caldera cluster and basin-and-range faulting. These workers drew and-range topography began after ca. 17 Ma, farther south (Fig. 5; Rowley et al., 1995). We the fi rst isopach maps of many of the ignimbrite and (3) only minor amounts of andesitic lavas also exclude subduction-related silicic ignim- units. Cook (1965) estimated that the volume of were extruded during the ignimbrite fl areup. In brites emplaced ca. 41–35 Ma over a present ignimbrite (uncorrected to dense rock equiva- addition, he concluded that (4) the southward area of ~10,000 km2 in the Independence and lent and for crustal extension) in eastern Nevada sweep of arc volcanism was the result of steep- Tuscarora Mountains of northeastern Nevada and southwestern Utah exceeded 20,000 km3 ening of the subducting oceanic lithosphere, and mostly north of Elko (e.g., Henry and Boden, and noted that phenocryst-rich ignimbrites tend (5) a very signifi cant amount of mantle magma 1999) and ca. 39–32 Ma ignimbrites exposed to be thicker than phenocryst-poor ones. He also was necessary to drive the generation of silicic over a present area of ~7000 km2 in west-central astutely perceived that the ash-fl ow sheets were crustal magma systems producing the fl areup. Utah and adjacent east-central Nevada (Stewart emplaced in a relatively brief time interval— Preliminary overviews of the Central Nevada and Carlson, 1976; Hintze and Kowallis, 2009). the ignimbrite fl areup—and that their sources and Indian Peak caldera complexes and their These distinctly separate areas of subduction- migrated southward through time; these two associated ash-fl ow tuffs have appeared in Best related ignimbrite are only one-tenth the area concepts were later confi rmed over a larger part et al. (1989a, 1989b, 1993) and John (1994) of the ~162,000 km2 over which the products of of the Great Basin by Armstrong et al. (1969). while Nealey et al. (1995), Scott et al. (1995), the 36–18 Ma southern Great Basin ignimbrite Williams (1967) also described, in thorough and Rowley et al. (1995) have described ignim- fl areup are found. detail for his time, the petrography and den- brites associated with calderas in the Caliente sity variations of several ignimbrite cooling cluster (Fig. 5). Three Ignimbrite Fields Making Up units in the southeastern Great Basin. In other Early work on the Western Nevada fi eld was the Province work done under Mackin’s tutelage, Richard focused in the area near Reno and included Blank (1959) mapped and described the vol- Bonham (1969), Bingler (1978), Proffett The southern Great Basin ignimbrite province canic rocks in the Bull Valley district of south- (1977), and Proffett and Proffett (1976). Pub- is readily divided into three contrasting geologic west Utah, Omar Conrad (1969) worked in the lications on other areas include Stewart et al. parts or sectors: eastern, central, and western central Needle Range, and John Anderson and (1977), Ekren et al. (1980), and Robinson and (Figs. 2 and 5). In the eastern sector, calderas Peter Rowley (see references in Best et al., this Stewart (1984). Since these pioneering efforts, astride the Utah-Nevada state line and surround- themed issue [a]) mapped many quadrangles in many geologic maps and topical reports have ing associated outfl ow ignimbrite sheets are the Iron Springs district and the nearby High been published, as referenced in Henry and John designated as the Indian Peak–Caliente caldera Plateaus of central Utah where ignimbrite units (this themed issue). complex and ignimbrite fi eld (Best et al. [a], this

Geosphere, April 2013 267 Best et al. themed issue). This complex is composed of drawn. Only a few major composite volcanoes over less silicic, or andesitic, compositions. The three, at least partly exposed, caldera segments or stratovol canoes existed in the southern Great silicic ignimbrites in the Great Basin gener- in the Caliente cluster in the south and six in Basin ignimbrite province prior to the ignim- ally contain phenocrysts in some combination the Indian Peak cluster to the north. The Indian brite fl areup and few during most of its activity. of plagioclase, sanidine, quartz, biotite, horn- Peak–Caliente caldera complex is separated by In striking contrast, intermediate-composition blende, and Fe-Ti oxides. Pyroxenes and titanite ~100 km from fi ve small calderas in the Marys- lavas dominate over ignimbrite in the two con- occur in a few tuffs whereas zircon and apatite vale volcanic fi eld (Cunningham et al., 2007) temporaneous volcanic fi elds to the east on the are ubiquitous in trace amounts. These phases east of the Great Basin on the western margin of margins of the Colorado Plateau (Fig. 1). In the equilibrated at depths of 7–12 km and tempera- the Colorado Plateau. In the central sector of the Marysvale fi eld on the western margin, lavas are tures of ~700–800 °C (Best et al. [a], this themed Great Basin province, eleven partly exposed and an order of magnitude greater in volume than issue). Drier, higher temperature (~950 °C) relatively more widely scattered source calderas silicic ignimbrite (Cunningham et al., 2007). In trachydacitic magmas contained sparse pheno- constitute the Central Nevada caldera complex the Southern Rocky Mountain fi eld on the east- crysts of plagioclase, two pyroxenes, and Fe-Ti (Best et al. [b], this themed issue). Because of ern margin, lavas are 1.7 times more voluminous oxides equilibrated at greater depths, perhaps the wide dispersal of ash fl ows from the two cal- than silicic ignimbrite (Lipman, 2007). In these as much as 30 km. dera complexes, ignimbrite outfl ow sheets that two fi elds (Fig. 1), stratovolcanoes dominated The relatively minor middle Cenozoic lavas compose their ignimbrite fi elds substantially the landscape prior to the explosive eruptions are principally high-K andesite and lesser dacite, overlap in an “outfl ow alley” (Fig. 6). Disper- of silicic magmas. Clearly, the middle Ceno- latite, and rhyolite (Barr, 1993; Best et al., 2009). sal of ash fl ows to the west from the Central zoic southern Great Basin ignimbrite province However, beginning at roughly 20 Ma lava com- Nevada calderas was apparently almost com- is aptly named and, in addition, justifi es our use positions were increasingly broader, include pletely blocked by a north-south topographic of ignimbrite fi eld rather than the more general basalt, and are locally bimodal basalt-rhyolite barrier, or drainage divide, on the western lip volcanic fi eld for its parts. suites accompanying the demise of plate subduc- of the Great Basin altiplano (Figs. 3 and 5; The second signifi cant attribute of the south- tion and transition into an extensional regime; Best et al., 2009). Calderas to the west of the ern Great Basin ignimbrite province was super- arc geochemical characteristics disappear. barrier and their associated ignimbrite deposits eruptions of relatively uniform, phenocryst-rich constitute the Western Nevada fi eld (Henry and dacite, or monotonous intermediate, magmas IGNIMBRITES John, this themed issue). With three exceptions over 4 m.y. followed by voluminous eruptions of Defi nitions in the northern part of the fi eld, ash-fl ows from unusual, higher temperature, drier trachydacitic Western Nevada calderas were apparently not magmas over a similar time span. These tan- In this themed issue, the synonymous terms dispersed east of the topographic barrier but dem eruptions had their greatest expression in ignimbrite and ash-fl ow tuff will be used inter- instead traveled westward, in part down stream the Indian Peak fi eld where eruptions of 2000, changeably, or informally and for brevity, tuff. channels draining the western slope of the alti- 5900, and 4400 km3 of monotonous intermedi- Outfl ow ignimbrite is deposited beyond its plano in what is now the western Great Basin ates at 31.13, 30.06, and 29.20 Ma, respectively, source caldera, in large part, we believe, prior to and Sierra Nevada (Best et al., 2009, their fi g- were followed by at least nine eruptions totaling initiation of caldera collapse. However, some of ures 14 and 17; Henry and Faulds, 2010). 4200 km3 of trachydacitic ignimbrites from 27.90 this early tuff is deposited within the area that to 24.55 Ma. In the Central Nevada fi eld these subsequently collapses; pre–caldera collapse Signifi cant Attributes of the Southern tandem eruptions were less voluminous, includ- ignimbrite is, thus, a more inclusive term. The Great Basin Ignimbrite Province ing one monotonous intermediate ignimbite of outfl ow or, pre–caldera collapse, ignimbrite typi- 4500 km3 at 27.57 Ma followed by eruption of cally consists of a simple ash-fl ow cooling unit, The southern Great Basin ignimbrite province ~600 km3 of trachydacitic magma from ca. 27 to or locally a compound cooling unit, and ranges in shares many aspects with other volcanic fi elds 23 Ma. Such compositions and colossal eruptive thickness from a meter or so to as much as a few involved in the middle Cenozoic ignimbrite volumes appear to be without parallel elsewhere hundreds of meters, depending on magnitude of fl areup in southwestern North America (Fig. 1) in the middle Cenozoic of the southwestern topographic relief in the pre-eruption landscape, including recurrent explosive eruptions of rhyo- United States. In the Western Nevada fi eld, data on proximity to the source, and on volume of litic magma, some of super magnitude, from are incomplete, but nothing like the monotonous the eruption. Cooling units display zonal varia- long-lived shallow-crustal silicic magma sys- intermediates to the east are known and only a tions in welding, compaction, devitrifi cation, tems and collapse of multicyclic calderas, some very small volume of the high tempera ture, dry and vapor-phase crystallization as described by tens of kilometers in diameter. However, two trachydacitic magmas were erupted. Smith (1960). Over the course of the ignimbrite attributes of the southern Great Basin ignimbrite Ignimbrites and lavas older than ca. 18 Ma fl areup, the altiplano in what is now the central province are especially signifi cant, seemingly in the Great Basin are magnesian (calc-alkalic) and eastern sectors of the southern Great Basin requiring special tectono-magmatic conditions and possess an arc geochemical signature (Fig. 4) province was progressively smoothed as succes- for its origin and evolution, as discussed in Best resulting from subduction-related magma sys- sive outfl ow tuff sheets were deposited (Fig. 3; and Christiansen (this themed issue). tems. As expected from the unusually thick crust Best et al., 2009). This resulted in, for example, First is the relatively small volume (generally in which they were spawned, magmas were accumulation of many conformable ignimbrite ~10%) of intermediate-composition, mostly mostly high-K. Magma systems in the southern layers to depths of several hundreds of meters in andesitic, lava fl ows relative to the volume of Great Basin ignimbrite province contrast sharply the “outfl ow alley” between the Central Nevada contemporaneous ignimbrites; this attribute with those in typical continental-margin volcanic and the Indian Peak–Caliente caldera complexes was obvious in the seminal space-time-com- arcs that are founded on thinner crust in their (Figs. 5, 6, and 7). In the western Great Basin, position compilation by Stewart and Carlson much greater eruptive volume and correspond- ash fl ows were commonly confi ned to drainage (1976) of middle Cenozoic volcanic rocks in ing much longer repose times between eruptions channels on the western slope of the altiplano the Great Basin, from which our Figure 2 was as well as their dominance of rhyolite and dacite (Henry and Faulds, 2010).

268 Geosphere, April 2013 Introduction: The 36–18 Ma southern Great Basin, USA, ignimbrite province and ﬂ areup

Figure 7. Panoramic views of conformable stratigraphic sequences of outfl ow ignimbrite sheets in “outfl ow alley” between the Central Nevada (CN) caldera complex to the west and the Indian Peak–Caliente (IPC) caldera complex to the east (Fig. 5). (A) Late-afternoon view toward the northeast of the Golden Gate Range (~115°19′ W, 38°13.5′ N) composed of a stack of ten ignimbrite sheets totaling ~500 m in thickness (for additional information see Best et al., [b], this themed issue, Supplemental File 4). Photograph kindly provided by Wanda J. Taylor. From oldest upwards above Paleozoic rocks, the outfl ow sheets are: 31.13 Ma Cottonwood Wash and 30.06 Ma Wah Wah Springs (~20 m and ~30 m thick, respectively; both derived from IPC but concealed behind the low hill in left foreground); 29.4 Ma Silver King (100 m, IPC); 29.20 Ma Lund (50 m, IPC); 27.57 Ma Monotony (55 m, CN); 27.16 Ma Lower Tuff Member (40 m), 26.82 Ma Hancock Tuff Member (116 m), and 26.36 Ma Upper Tuff Member (30 m; all three members of the Shingle Pass Formation, CN); 23.04 Ma Bauers (15 m, IPC); and 22.93 Ma Pahranagat (60 m, CN). (B) View toward the north from U.S. Highway 93 of the south end of the North Pahroc Range ~70 km south-southeast of the Golden Gate Range (for additional information see Best et al., [a], this themed issue, Supplemental File 5). Tilted mesa on right exposes nine cooling units, including (Scott et al., 1992): ca. 27.3 Ma upper Bald Hills Tuff Member (45 m) and 24.55 Ma Hole-in-the-Wall Tuff Member (15 m, both members of the Isom Formation, IPC); 24.01 Ma Leach Canyon (95 m, IPC); 24.15 Ma Swett (60, IPC); local trachydacite tuff (10 m); 23.04 Ma Bauers (60 m, IPC); 22.93 Ma Pahranagat (10 m, CN); 22.56 Ma Harmony Hills (30 m, IPC); and 18.51 Ma Hiko (100 m, IPC). Mesa on left exposes the same sequence with an additional intervening three cooling units of the Shingle Pass (140 m, CN) and another trachydacite tuff (15 m). Complete section a few kilometers to north that includes six additional ignimbrites below the Bald Hills totals ~900 m thick.

Intracaldera ignimbrite is deposited, com- Caldera-fi lling ignimbrite is deposited in a Correlation monly as a compound cooling unit, within its pre-existing and unrelated caldera from a source source caldera as it is collapsing, as described either within or outside the caldera. In the for- To reconstruct the full geographic extent and below. Caldera-collapse ignimbrite is a synony- mer case, the ash fl ows are essentially confi ned volume of a particular ignimbrite unit deposited mous term if none of it is deposited outside the within the depression and the resulting deposit in a single eruptive event, it is necessary to cor- caldera. can be hundreds of meters thick. relate exposures separated by post-emplacement

Geosphere, April 2013 269 Best et al. faulting, erosion, and younger deposits. Corre- of which we are aware: the tuff of Clipper Gap tions of stratigraphic units, whereas similarity of lation between mountain ranges across alluvial in the Central Nevada fi eld and the Nine Hill directions provides only permissive evidence of valleys is an obvious challenge but within a Tuff in the Western Nevada fi eld. Ignimbrites of a correlation. single range, exposures of a particular tuff unit the Wah Wah Springs Formation in the Indian Obviously, no single correlation tool is gen- may be separated in one way or another. Lat- Peak–Caliente fi eld are unusual in containing erally suffi cient to validate a match between eral variations in the character of an ignimbrite more hornblende than biotite. separate exposures of a particular cooling unit hamper correlation; a case in point is contrasts Vapor-phase and other post-emplacement because two or more different units can possess between intracaldera and outfl ow ignimbrites alteration of tuffs preferentially destroys some similar attributes. The greater the number of deposited during a single eruptive event. phenocrysts. Titanite and mafi c minerals are independent criteria employed, the more cred- Several methods (Hildreth and Mahood, most commonly susceptible; only quartz is ible is the correlation. 1985) can be marshaled to correlate, with vary- universally immune. Ignimbrites of the horn- ing degrees of certainty, discontinuous expo- blende-rich Wah Wah Springs Formation and CALDERAS sures of a particular ignimbrite unit. As a case the titanite-bearing Lund Formation, both in study for an unusually challenging correlation, the Indian Peak–Caliente fi eld, have commonly Because all topographic expression of cal- Best et al. (1995) investigated the ignimbrite been misidentifi ed in hand samples because dera depressions has been obliterated by post- of the Pahranagat Formation in the Central of the destruction of these diagnostic phases. volcanic basin-and-range faulting, erosion, and Nevada volcanic fi eld using stratigraphic posi- Nonetheless, unless alteration is extreme, burial beneath valley fi ll, recognition of middle tion, chemical and modal composition, precise pseudomorphs of critical phases can be recog- Cenozoic source calderas for Great Basin ignim- 40Ar/39Ar ages, and paleomagnetic direction. nized in thin section. brites must be based on other criteria. (Some The Pahranagat outfl ow sheet is vertically and Similarly as for modes, few ignimbrite units geologists designate deeply eroded ignimbrite laterally zoned in composition and is found in possess unique bulk chemical compositions. sources as “cauldrons” or “caldrons”; how- widely scattered exposures over a present area Clearly, magmatic and eruptive processes ever, following Lipman [2000, p. 644], we have of 26,000 km2 where it has been previously des- repeatedly combined through time and space to adopted the designation caldera.) Exposed seg- ignated by four different stratigraphic names yield compositionally similar tuffs. Many units ments of calderas in uplifted and tilted mountain in different areas. Additional names have been overlap to some degree for virtually all of 30 blocks reveal critical details of the internal struc- applied to the intracaldera ignimbrite inside the analyzed elements, again limiting the usefulness ture and stratigraphy to depths of several kilo- Kawich caldera source. of chemical composition in correlation. Unique meters as, for example, in the Caetano caldera Position in stratigraphic sequence is the most for the entire Great Basin is the petrographically and the Stillwater caldera complex, both in the basic correlation tool in the fi eld. Modal compo- distinct Nine Hill Tuff that has unusually high Western Nevada fi eld (John et al., 2008; John, sition—types and proportions of phenocrysts— concentrations of both Nb (26–33 ppm) and Zr 1995; Henry and John, this themed issue). together with other petrographic criteria, such as (360–430 ppm) (Deino, 1985, 1989). Among Caldera complex, as used in this themed characteristics of phenocrysts (e.g., size), abun- the compositionally similar monotonous inter- issue of Geosphere, denotes a cluster of source dance and nature of lithic and pumice clasts, and mediates, the Wah Wah Springs has unusually calderas for the sequence of ignimbrites in the character of welding, devitrifi cation, and vapor- high concentrations of Cr. Indian Peak–Caliente and Central Nevada vol- phase crystallization, can also be used advan- Chemical composition of constituent pheno- canic fi elds (Fig. 5). Most of the calderas in each tageously in the fi eld. However, modes and crysts is a very useful correlation tool, but one complex are partially overlapping, or nested the other criteria are compromised by compo- used only sparingly by authors of this themed (multicyclic) within one another, but some may sitional variations in an ignimbrite. In the Pah- issue. lie apart. In the Central Nevada fi eld, a mostly ranagat and Windous Butte tuffs, for example, The superb precision (±0.0x Ma, one sigma) exposed caldera lies well to the north of the the quartz/plagioclase ratio ranges over almost of the 40Ar/39Ar single-crystal, laser-fusion dat- main caldera cluster (Fig. 5) but is included in an order of magnitude and variations in other ing technique has made it a standard tool in the Central Nevada caldera complex because mineral proportions are substantial. correlation of pyroclastic deposits. Deino (this its associated ignimbrite outfl ow sheet occurs Another drawback for the use of modal com- themed issue) describes its application to the in sequences with those derived from the main position in correlation is the fact that nearly all Great Basin ignimbrites. Even though most age caldera complex. middle Cenozoic Great Basin ignimbrites con- determinations, especially on sanidine, clearly In some instances, the Bouguer gravity fi eld tain combinations of phenocrysts of plagioclase, distinguish among units in a particular ignim- is useful in delineating the relatively lesser den- sanidine, quartz, biotite, and Fe-Ti oxides with brite fi eld, some units that are distinctly differ- sity of the fi lling ignimbrite or underlying plu- or without hornblende; only by accurate point ent cooling units in a stratigraphic sequence can tonic granitic rock as, for example, in the Indian counting is it possible to distinguish, in some have the same age within analytical uncertainty; Peak caldera (Best et al. [a], this themed issue, cases, among so many similar ignimbrite units. an example is the compositionally similar their fi gure 8C). Only a few of the more than 200 ignimbrite tuffs of Lunar Cuesta and Goblin Knobs in the The approximate location of a particular cal- cooling units possess a unique modal compo- Central Nevada fi eld that have separate source dera in some cases can be inferred from the dis- sition. One is the Lower Tuff Member of the calderas and are distinguished by contrasting tribution and thickness of its associated outfl ow Shingle Pass Formation in the Central Nevada paleomagnetic directions. tuff. However, a potential pitfall exists because, fi eld having a phenocryst assemblage of sani- Paleomagnetism, as just indicated, is a pow- although a few calderas are positioned more- dine, plagioclase, quartz, biotite, and, especially, erful tool for correlation of ash-fl ow deposits. or-less centrally within the area of distribution, Fe-rich olivine that is not found in any other of Gromme et al. (1972) and Gromme and Hudson others are decidedly eccentrically positioned, the investigated tuffs. Three feldspar pheno- (this themed issue) describe its application to the especially in the Western and Central Nevada cryst types (sanidine, anorthoclase, plagioclase) Great Basin ignimbrites. Signifi cant differences fi elds (Fig. 5). An example in the latter fi eld is occur in only two middle Cenozoic ignimbrites among paleomagnetic directions negate correla- the 22,500 km2 outfl ow of the Windous Butte

270 Geosphere, April 2013 Introduction: The 36–18 Ma southern Great Basin, USA, ignimbrite province and ﬂ areup

Formation, which is not found in a 180° arc caldera-fi lling deposits that lap unconformably Deino (this themed issue) presents the results northwest-west-south-southeast of the Williams against older rocks on the topographic margin. of 40Ar/39Ar laser fusion analyses of 200 sepa- Ridge caldera source. Caldera-fi lling tuff derived from sources rates of sanidine and lesser plagioclase prepared Unusually thick ignimbrite (e.g., several hun- within or outside the caldera after its collapse as from whole-rock samples; sanidines establish dreds of meters) by itself is not necessarily an well as locally derived lava fl ows can partially the chronology of the Central Nevada fi eld and indication of accumulation within its associated to completely fi ll the topographic depression. plagioclases the Indian Peak–Caliente fi eld. source caldera. Outfl ow, or pre–caldera col- Intracaldera epiclastic deposits of sandstone and These highly precise ages generally have analyti- lapse, ignimbrites can be as thick as 500 m, such conglomerate as well as lacustrine limestone that cal uncertainties (one sigma) of 0.05–0.11 m.y. as the Wah Wah Springs north of its Indian Peak are produced by weathering and erosion of the on sanidines from a single tuff sample in which caldera source (Best et al. [a], this themed issue) caldera escarpment can serve as caldera markers six grains were fused. Uncertainties are greater and the Windous Butte north of its Williams where thick and well exposed. However, such for plagioclases. Replicate analyses from several Ridge caldera source (Best et al. [b], this themed deposits are generally sparse or absent in Great samples of the same ignimbrite unit yield uncer- issue). Caldera-fi lling tuff can accumulate to Basin calderas, possibly as a result of the arid tainties of as little as 0.02 m.y. Such precision thicknesses of several hundred meters within an conditions on the altiplano. not only provides a powerful tool for correlation older and unrelated caldera depression. Exposures of comagmatic intrusions are rare of ignimbrites found in exposures separated by The most defi nitive evidence for a caldera in Great Basin calderas. Examples include the wide distances but also yields a tight chronology is a very thick, locally upwards of 3–5 km, granodiorite porphyry that domed the intra- of evolving magma systems. intracaldera deposit that commonly occurs as caldera tuff of the Wah Wah Springs Formation Gromme and Hudson (this themed issue) remnants in uplifted mountain blocks. Such in the Indian Peak caldera (Best et al. [a], this present the results of analyses of natural rema- deposits consist of a compound cooling unit themed issue). In the Western Nevada fi eld, nent magnetization of ignimbrites in more than or multiple cooling units of intracaldera tuff, the Carico Lake granite porphyry intrudes and 450 sites across the southern Great Basin ignim- together with intercalated lenses of wall- domes the Caetano Tuff inside its source caldera brite province. Signifi cant differences among collapse breccia. The intracaldera tuff that (John et al., 2008) and intrusions occur in the paleomagnetic directions negate correlations of accumulated in the deepening depression dur- Stillwater caldera complex (Henry and John, this stratigraphic units, whereas similarity of direc- ing continuing explosive eruption has usually themed issue). Post-caldera lava domes positions provide permissive evidence for correla- suffered variable but pervasive hydrothermal tioned along the ring fracture—another facet tion. In addition to its utility as a correlation alteration as a result of slow cooling of the of the classic Valles caldera cycle (Smith and tool, paleomagnetism confi rms the deposition of massive thickness in the presence of volcanic Bailey , 1968)—are only locally developed, such the ash-fl ow tuffs on a surface of limited relief fl uids. The intercalated lenses of breccia are as in the White Rock caldera source of the Lund in the central and eastern parts of the ignimbrite landslide deposits of older rock that intermit- ignimbrite in the Indian Peak–Caliente fi eld. province on the Great Basin altiplano. In some tently sloughed off the steep and unstable Evidence for resurgent uplift of caldera fl oors cases, ignimbrites with analytically indistin- topographic escarpment bordering the caldera has been found for some calderas where it has not guishable ages nonetheless possess contrasting as its fl oor subsided along the circumscribing been obscured by subsequent dismemberment paleomagnetic directions resulting from shifts ring fault (Lipman, 1976). Some landslides and uplift by basin-and-range block faulting. in the geomagnetic fi eld that have been rapid traveled as much as a few kilometers into the with respect to the analytical uncertainty of depression. The breccias range from abundant SCOPE OF THIS THEMED ISSUE the dating. lithic clasts of older rock of lapilli size within OF GEOSPHERE Best et al. (a, b, this themed issue) build a the tuff to lenticular masses meters to hundreds basic foundation of stratigraphic, compositional, of meters thick made entirely of fragments Much has been published regarding other dimensional, chronologic, and paleomagnetic of older rock. Megabreccia of automobile- to volcanic fi elds and calderas in the southwestern data for the more than 100 ignimbrite cooling house-size blocks are not uncommon. In some United States manifesting the middle Cenozoic units in the eastern and central sectors of the calderas, such as the Big Ten Peak (Bonham ignimbrite fl areup (Fig. 1), including the South- ignimbrite province (Indian Peak–Caliente and and Garside, 1979; Keith, 1993) in the Central ern Rocky Mountain volcanic fi eld in south- Central Nevada ignimbrite fi elds, respectively). Nevada caldera complex, kilometer-size slabs western Colorado, which has been extensively Many hundreds of samples were collected in of more-or-less intact rock lie within intra- studied by P.W. Lipman and associates (e.g., these two fi elds; 830 chemical analyses are caldera tuff. Some intracaldera as well as out- Lipman, 2007), and the Mogollon-Datil fi eld reported as well as 960 modal analyses. Data fl ow tuffs contain fragments of deeper-crustal in southwestern New Mexico, which has been reveal that these two sectors of the ignimbrite rock than those in the caldera wall. Such brec- studied by, among others, McIntosh et al. (1992). province harbor apparently the largest source cias of older Paleozoic and underlying plutonic Although a brief overview of the volcanic rocks calderas and ignimbrite deposits, including the and metamorphic rock near the margins of in the Great Basin was published more than two super-eruptive monotonous intermediates and calderas in the Toquima Range are interpreted decades ago (Best et al., 1989b), as well as work voluminous trachydacitic tuffs not seen in other by Shawe and Snyder (1988) to have formed by on some individual ignimbrite units and their contemporaneous fi elds in southwestern North eruptive processes rather than by landsliding. caldera sources, no comprehensive, up-to-date, America. Because of the caving of unstable caldera integrated treatment has been devoted to the Henry and John (this themed issue) describe walls, the outermost topographic margin of a voluminous 36–18 Ma southern Great Basin ignimbrites and calderas in the Western Nevada caldera can lie as much as several kilometers out- ignimbrite province as a whole. The following fi eld on the western slope of the Great Basin side its structural margin, defi ned by the arcuate articles of this themed issue of Geosphere aim altiplano where ash fl ows were constrained in ring fault that bounds the down-dropped caldera to rectify that defi ciency, thus characterizing large part in stream valleys. Several correlations fl oor. Where erosion has not cut too deeply, this the anatomy of the vast ignimbrite province and are suggested, linking ignimbrites exposed in the topographic rim can be manifest by younger, highlighting its unusual attributes . western part of the fi eld with caldera sources as

Geosphere, April 2013 271 Best et al. much as 200 km farther east. Because of sub- where the Brigham Young University undergradu- province: Magmas, super eruptions, and ignimbrite stantial post-caldera tilting of ranges, the inter- ate summer fi eld geology class was working. Subse- fl are up: Geosphere. Best, M.G., Christiansen, E.H., and Blank, H.R., Jr., 1989a, nal structure of the Caetano and Stillwater quently, over the next thirty years, some 600 students mapped most of the Indian Peak caldera complex and Oligocene caldera complex and calc-alkaline tuffs calderas from fl oor to the top of the caldera- of the Indian Peak volcanic fi eld, Nevada and Utah: surrounding areas at a scale of 1:24,000. This was the Geological Society of America Bulletin, v. 101, fi lling sequence is exceptionally well exposed, basis of numerous geologic maps published by the p. 1076–1090, doi:10.1130/0016-7606(1989)101<1076: allowing an integrated elucidation of coeval U.S. Geological Survey and Nevada Bureau of Mines OCCACA>2.3.CO;2. explosive eruption, lava extrusion, caldera col- and Geology. Lehi’s continuing interest and assistance Best, M.G., Christiansen, E.H., Deino, A.L., Gromme, C.S., over four decades have been of immeasurable value. McKee, E.H., and Noble, D.C., 1989b, Eocene through lapse, plu tonism, and hydrothermal activity. As our study of ash-fl ow tuffs and calderas was Miocene volcanism in the Great Basin of the western Deino et al. (this themed issue) correlate extended farther west in the Great Basin, a valuable United States, in Chapin, C.E., and Zidek, J., eds., and describe the 25.48 Ma Nine Hill Tuff. This resource was petrographic and stratigraphic data on Field excursions to volcanic terranes in the western United States, Volume II: Cascades and Intermoun- compositionally unusual, high-silica rhyolite ignimbrite units gathered from 1955 to 1971 on some tain West: New Mexico Bureau of Mines and Mineral ignimbrite is presently exposed over an area of 40 localities by Earl F. Cook. Some of his data were Resources Memoir 47, p. 91–133. summarized in Cook (1965), but considerable addi- 2 Best, M.G., and 10 others, 1993, Oligocene-Miocene cal- ~70,000 km , one of the most extensive ash-fl ow tional unpublished archived information was provided dera complexes, ash-fl ow sheets, and tectonism in the deposits known and certainly the largest in the with the kind help of Earl H. Pampeyan and Gordon P. central and southeastern Great Basin, in Lahren M.M., Great Basin. Nine Hill ash fl ows traveled from Eaton. Additional resources without which we could Trexler J.H., Jr., and Spinosa C., eds., Crustal evolution of the Great Basin and the Sierra Nevada: Field Trip a now-concealed source east of Reno westward not have pursued our study into central Nevada were the many geologic maps by E. Bart Ekren and asso- Guidebook for the 1993 Joint Meeting of the Cordi- lleran/Rocky Mountain sections of the Geological via drainages on the western slope of the Great ciates published by the U.S. Geological Survey (see Basin altiplano all the way to what are now the Society of America, Reno, Nevada, May 19–21, 1993: listing in Best et al., [b], this themed issue). Especially Department of Geological Sciences, Mackay School of Sierra Nevada foothills in central California, as useful in eastern Nevada was Ekren et al. (1977). Mines, University of Nevada, Reno, p. 285–312. well as traveling eastward across the altiplano For guided tours of critical areas and help in sample Best, M.G., Christiansen, E.H., Deino, A.L., Gromme, C.S., into what is now eastern Nevada almost to Ely collection, we are indebted to Gary J. Axen, John M. and Tingey, D.G., 1995, Correlation and emplacement Bartley, Will Carr, Gary Dixon, Larry J. Garside, Allen of a large, zoned, discontinuously exposed ash fl ow 40 39 (Fig. 5). The origin of the unusual magma and Glazner, Richard F. Hardyman, Christopher D. Henry, sheet: The Ar/ Ar chronology, paleomagnetism, and the apparent exceptional mobility of the ash Angela Jayko, William D. Quinlivan, Robert B. Scott, petrology of the Pahranagat Formation, Nevada: Jour- nal of Geophysical Research, v. 100, p. 24,593–24,609, fl ows require innovative interpretations. Wanda J. Taylor, and Steven Weiss. E.H. McKee fur- doi:10.1029/95JB01690. Best and Christiansen (this themed issue) nished K-Ar ages on lava fl ows. Christopher D. Henry Best, M.G., Barr, D.L., Christiansen, E.H., Gromme, C.S., and David A. John furnished insights and data as well compare the southern Great Basin ignimbrite Deino, A.L., and Tingey, D.G., 2009, The Great Basin as encouragement over several years. altiplano during the middle Cenozoic ignimbrite province with other contemporaneous vol canic We have benefi tted from the continuing support of flareup: Insights from volcanic rocks: International fi elds in the southwestern U.S. as well as the our colleagues at Brigham Young University, namely, Geology Review, v. 51, p. 589–633, doi:10.1080 Neogene Altiplano-Puna fi eld in the central Michael Dorais, Garrett Hart (now at Pacifi c North- /00206810902867690. west National Lab), Jeffrey Keith, Bart Kowallis, and Best, M.G., Christiansen, E.H., Deino, A.L., Gromme, S., Andes. These comparisons with other volcanic and Tingey, D.G., (a), this themed issue, The 36–18 Ma David G. Tingey, who collected many samples, super- fi elds that experienced an ignimbrite fl areup Indian Peak–Caliente ignimbrite fi eld and calderas, vised preparation, and made chemical analyses. southeastern Great Basin, USA: Multicyclic super provide valuable insights concerning mantle For constructive comments on early drafts of this eruptions: Geosphere. magma fl ux into the crust and how east-west article, we are indebted to Peter Rowley, Gary Dixon, Best, M.G., Christiansen, E.H., Gromme, S., Deino, A.L., variations in crustal thickness across the Great Bart J. Kowallis, and Carol Frost. and Tingey, D.G., (b), this themed issue, The 36–18 Ma Valuable fi nancial support for the Great Basin proj- Central Nevada ignimbrite fi eld and calderas, Great Basin and Colorado Plateau infl uenced the ect was provided by the National Science Foundation Basin, USA: Multicyclic super eruptions: Geosphere. nature of volcanism. Based on the tectonic set- through grants EAR-8604195, 8618323, 8904245, Bingler, E.C., 1978, Abandonment of the name Hartford Hill Rhyolite Tuff and adoption of new formation names for tings of these several fi elds, ignimbrite fl areups 9104612, 9706906, and 0923495 to M.G. Best and middle Tertiary ash-fl ow tuffs in the Carson City-Silver are conceived to be the result of a steepening in E.H. Christiansen. The U.S. Geological Survey and City area, Nevada: U.S. Geological Survey Bulletin the dip of a once “fl at” subducting oceanic litho- Nevada Bureau of Mines and Geology supported 1457-D, 19 p. quadrangle mapping. The continuing fi nancial and Blank, H.R., Jr., 1959, Geology of the Bull Valley District, sphere far inland from the continental margin material assistance of Brigham Young University is Washington County, Utah [Ph.D. thesis]: Seattle, Uni- and unusually high infl ux of mantle magma into gratefully acknowledged. versity of Washington, 177 p. crust thickened by prior orogenic contraction. Additional acknowledgments are made in the indi- Bonham, H.F., Jr., 1969, Geology and mineral deposits of vidual articles in this Geosphere issue. Washoe and Storey Counties, Nevada: Nevada Bureau Striking similarities between the central and, of Mines and Geology Bulletin 70, 140 p. especially, the eastern sector of the southern Bonham, H.F., Jr., and Garside, L.J., 1979, Geology of the REFERENCES CITED Tonopah, Lone Mountain, Klondike, and northern Mud Great Basin province and the Altiplano-Puna Lake quadrangles, Nevada: Nevada Bureau of Mines fi eld emphasize the role of exceptionally thick Armstrong, R.L., Ekren, E.B., McKee, E.H., and Noble, and Geology Bulletin 92, 142 p. crust in creating gigantic bodies of monoto- D.C., 1969, Space-time relations of Cenozoic vol- Bryan, S.E., Peate, I.U., Peate, D.W., Self, S., Jerram, D.A., canism in the Great Basin of the western United States: Mawby, M.R., Marsh, J.S., and Miller, J.A., 2010, nous intermediate magma and their recurrent American Journal of Science, v. 267, p. 478–490, The largest volcanic eruptions on earth: Earth-Science super-eruption. 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