Geosphere; February 2008; v. 4; no. 1;
p. 75-106; DOI: 10.1130/GES00116.1
© 2008 Geological Society of America
Magmatic and tectonic evolution of the Caetano caldera, north-central Nevada: A tilted, mid-Tertiary eruptive center and source of the Caetano Tuff
David A. John1,
Christopher D. Henry2 and
Joseph P. Colgan3
1 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA
2 Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada 89557, USA
3 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA
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ABSTRACT
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The Caetano Tuff is a late Eocene, rhyolite ash-flow tuff that crops out within an 
90-km-long, east-west–trending belt in north-central Nevada, previously interpreted as an elongate graben or "volcano-tectonic trough." New field, petrographic, geochemical, and geochronologic data show that: (1) the east half of the "trough" is actually the Caetano caldera, formed by eruption of the Caetano Tuff at 33.8 Ma and later structurally dismembered during Miocene extension; (2) the west half of the trough includes both the distinctly younger and unrelated Fish Creek Mountains caldera (ca. 24.7 Ma) and a west-trending paleovalley partly filled with outflow Caetano Tuff; and (3) the Caetano Tuff as previously defined actually consists of three distinct units, two units of the 33.8 Ma Caetano Tuff and an older (34.2 Ma) tuff, exposed north of the Caetano caldera, herein named the tuff of Cove Mine.
Miocene extensional faulting and tilting has exposed the Caetano caldera over a paleodepth range of >5 km, from the caldera floor through post-caldera sedimentary rocks, providing exceptional constraints on an evolutionary model of the caldera that are rarely available for other calderas. The Caetano caldera filled with more than 4 km of intracaldera Caetano Tuff, while outflow tuff flowed west and south of the caldera, primarily down Eocene paleovalleys. Caldera fill consists of two units of Caetano Tuff. The lower compound cooling unit is as much as 3600 m thick and is separated by a complete cooling break from a 500–1000-m-thick upper unit that consists of multiple, thin, ash flows interbedded with sedimentary deposits. Multiple granite porphyries, including the 25-km2 Carico Lake pluton, intruded and domed the center of the caldera within 0.1 Ma of caldera formation; one of these porphyries is associated with pervasive argillic and advanced argillic alteration of the western half of the caldera. All exposed caldera-related rocks are rhyolites or granites (71–77.5 wt% SiO2). Caldera collapse was significantly greater than the thickness of caldera fill and created a topographic depression that served as a depocenter until at least 25 Ma, filling with nearly 1 km of sediments and distally derived, ash-flow tuffs.
The caldera is presently exposed in a series of 40–50°, east-tilted blocks bounded by north-striking, west-dipping normal faults that formed after 16 Ma. Slip on these faults accommodated 100% E-W extension, making the restored Caetano caldera 20 km east-west by 10–18 km north-south. The estimated volume of intracaldera Caetano Tuff is, therefore, 840 km3, and the minimum estimated total eruptive volume is 1100 km3. Although the Caetano magmatic system was probably too young to supply heat for nearby Carlin-type gold deposits in the Cortez district, earlier nearby magmatic activity may have contributed to formation of these deposits. Reconstruction of the late Eocene, pre-Caetano caldera geologic setting, immediately prior to caldera formation, indicates that the Cortez Hills and Horse Canyon Carlin-type deposits formed at 
1 km depths.
Keywords: calderas • ash-flow tuff • magma resurgence • Basin and Range Province • extensional tectonics • Carlin-type gold deposit
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INTRODUCTION
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The Caetano Tuff in north-central Nevada is one of the volumetrically largest manifestations of vigorous mid-Tertiary (ca. 43–19 Ma) magmatism dominated by voluminous caldera-forming ash flow eruptions (Lipman et al., 1972; Best et al., 1989; Christiansen and Yeats, 1992). Carlin-type gold deposits in northern Nevada, which are among the largest gold deposits in the world and help make Nevada the second largest gold producer in the world (Price and Meeuwig, 2006), formed during this magmatism between 42 and 30 Ma (Hofstra et al., 1999; Fig. 1). Widespread tuffs that issued from the calderas can be dated with great precision, and, together with younger volcanic and sedimentary rocks, they provide key markers for determining the timing and magnitude of magmatism and extension relative to the formation of Carlin-type gold deposits (e.g., Cline et al., 2005). The Caetano Tuff is a regionally widespread, late Eocene, ash-flow tuff in north-central Nevada (Fig. 1; Masursky, 1960; Gilluly and Masursky, 1965; Stewart and McKee, 1977). It is the only extensive, mid-Tertiary volcanic unit between the Tuscarora Mountains, 120 km to the north of exposures of the Caetano Tuff (Castor et al., 2003; Henry, 2008), and the vast expanse of Oligocene and Miocene calderas >75 km to the south (Fig. 1; Best et al., 1989; Ludington et al., 1996). The Caetano Tuff and related rocks thus offer one of the few windows into the history of Cenozoic magmatism, extension, and ore deposit formation in the adjacent Battle Mountain-Eureka trend, which boasts several world-class, Carlin-type gold deposits.
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Figure 1. Map showing mid-Cenozoic (43–19 Ma) volcanic rocks and intrusions in northern Nevada and calderas (modified from Ludington et al. [1996]) and volcano-tectonic troughs of Burke and McKee (1979). Box shows outline of Figure 2. B—Battle Mountain; BM—Bald Mountain; C—Cortez Range; CA—Clan Alpine Range calderas; CW—Cowboys Rest; D—Desatoya Mountains calderas; F—Fish Creek Mountains caldera; S—Shoshone Range; SC—Stillwater caldera complex; T—Toiyabe Range; TM—Tuscarora volcanic field; TR—Tobin Range.
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The Caetano Tuff mostly occupies a west-trending belt 40 km long by 10–18 km wide that has been described as the eastern half of a fault-bounded, volcano-tectonic trough (Masursky, 1960). As originally defined, the trough was inferred to extend 90 km west from Grass Valley on the west side of the Cortez Range to Pleasant Valley on the west side of the Tobin Range (Figs. 1 and 2; Burke and McKee, 1979). This inferred structural control on the distribution of the tuff implied a late Eocene to early Oligocene episode of north-south extension not recognized elsewhere in the northern Great Basin. We interpret these relations as two middle-Tertiary calderas modified by Miocene Basin and Range extension. The thickest exposures(>3.4 km) of the Caetano Tuff are in the northern Toiyabe Range a few kilometers southwest of the Cortez and Cortez Hills Carlin-type gold deposits and 10 km south of the Pipeline and Gold Acres Carlin-type gold deposits (Fig. 2). Rhyolite dikes of similar composition but slightly older age than the Caetano Tuff are exposed in the Cortez Mine, where they have been variably interpreted to both pre-date and post-date formation of Carlin-type ores (Wells et al., 1969; Rytuba, 1985; McCormack and Hays, 1996; Mortensen et al., 2000).
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Figure 2. Generalized geologic map of the Caetano and Fish Creek Mountains calderas, showing distribution of the Caetano Tuff and tuff of Cove Mine and geochemical and geochronologic samples of this study. Geology modified from digital county geologic maps (Hess and Johnson, 1997) based on geologic maps for Lander, Churchill, Pershing, Humboldt, and Eureka Counties. CH—Cortez Hills deposit; CLV—Carico Lake Valley; GC—Golconda Canyon; HC—Horse Canyon mine; RM—Red Mountain; TYR—Toiyabe Range; WP—Wilson Pass.
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New field, petrographic, geochemical, and geochronologic data for the Caetano Tuff and related intrusive rocks are presented in this paper and in a companion paper (Colgan et al., 2008) that bear on the origin and source of the Caetano Tuff and the tectonic evolution of the surrounding area, including post-ore (<34 Ma) deformation of major nearby Carlin-type gold deposits. We have remapped the caldera, including caldera margins, the caldera floor, resurgent intrusions, and intracaldera stratigraphy (Plate 1). These data show that tuffs previously correlated with Caetano Tuff consist of two distinct, ash-flow tuff units erupted ca. 400 ka apart: (1) the tuff of Cove Mine, a slightly older and more mafic outflow tuff that mostly crops out north of the Caetano caldera and erupted from an unidentified source, and (2) multiple cooling units of Caetano Tuff that fill the highly extended, structurally dismembered Caetano caldera and flowed primarily south and west of the caldera (Fig. 2). Reconstruction of the Caetano caldera provides a strain marker for constraining later extension, and the companion paper (Colgan et al., 2008) uses the reconstructed caldera to document Late Cenozoic extension and the regional implications of this extension on adjacent Carlin-type gold deposits.
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Plate 1. Colgan, J.P., Henry, C.D., and John, D.A., Geologic map and cross sections of the Caetano caldera, Lander County, Nevada, scale 1:100:000. If you are viewing the PDF of this paper or reading it offline, please visit http://dx.doi.org/10.1130/GES00115.S1 or the full-text article on www.gsajournals.org to view Plate 1.
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METHODS OF STUDY
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Major objectives of this study included (1) distinguishing between caldera versus volcano-tectonic trough origins for the "Caetano trough"; (2) in the case of a caldera origin, determining the timing and duration of ash-flow eruption, caldera collapse, and resurgent doming; (3) determining whether previously mapped "Caetano Tuff" (including intracaldera and outflow tuff) is all the same tuff; and (4) constraining the timing of post-caldera events, including the timing of major E-W extension. To accomplish these objectives, we compiled a 1:100,000-scale geologic map of the caldera (Plate 1), based on new geologic mapping of key localities at 1:24,000 scale, and we conducted petrographic, geochemical, and geochronologic analyses of samples collected throughout the caldera and surrounding region (Fig. 2; Appendices 1A, 1B, and 21).
To aid in correlation of the widely distributed tuffs, 100 thin sections of previously mapped Caetano Tuff and related intrusive rocks were examined petrographically, and modal analyses were made for 75 samples. Modally analyzed samples were divided into groups of intracal-dera Caetano Tuff, extracaldera Caetano Tuff, intrusive rocks, and the tuff of Cove Mine as described in the next section.
Seventy-two whole-rock samples of the Caetano Tuff, tuff of Cove Mine, and intrusive rocks in Carico Lake Valley were analyzed for major and trace elements by XRF (X-ray fluorescence) techniques (Appendix 1A and 1B). Most tuff samples were devitrified and densely welded, and we did not try to separate the strongly flattened, crystal-rich pumice blobs. Our new chemical analyses were combined with 25 previously published analyses in Roberts (1964), Gilluly and Masursky (1965), Stewart and McKee (1977), Doebrich (1995), Gonsior (2006), and M.G. Best (2004, written commun.). About 20% of all analyzed samples were strongly hydrothermally altered, as indicated by the presence of hydrothermal minerals (e.g., calcite or kaolinite) or by high SiO2 or K2O and/or low Na2O contents; these analyses were discarded. The remaining, relatively unaltered samples were divided into groups of intracaldera Caetano Tuff, extracal-dera Caetano Tuff, intrusive rocks, and the tuff of Cove Mine using the same divisions as for the modal data.
Published K-Ar dates from the Caetano Tuff range from 31.3 ± 0.6 to 35.3 ± 1.1 Ma (Sloan et al., 2003) and are insufficiently precise to address major objectives of this study. Single crystal 40Ar/39Ar dating of sanidine provides highly precise, reproducible ages that can distinguish volcanic events separated by as little as 100,000 yr at the ca. 34 Ma age of the Caetano Tuff(s) (Deino, 1989; McIntosh et al., 1990; Henry et al., 1997). Therefore, we determined 15 new, sanidine, single-crystal 40Ar/39Ar ages for previously mapped Caetano Tuff and a related pluton (Table 1). We also determined 40Ar/39Ar ages for intracaldera Fish Creek Mountains Tuff 15 km west of the Caetano caldera (Fig. 2) and for a tuff that resembles the Caetano Tuff from Bald Mountain 100 km to the east (Fig. 1). Analytical techniques and data are provided in Appendix 2.
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GEOLOGY OF THE CAETANO CALDERA
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Due to large magnitude (100%) east-west extension along west-dipping normal faults in the middle Miocene (Colgan et al., 2008), the Caetano caldera is exceptionally well exposed in a series of east-tilted fault blocks. The entire stratigraphy of the caldera fill is exposed (Fig. 3), as well as a wide range of pre- and post-caldera rocks, thereby allowing a more complete understanding of caldera evolution than seen in most calderas. Numerous caldera-related structural features are evident, including the caldera floor and margins, mesobreccias and megabrec-cias, resurgent intrusions, and post-collapse, caldera-filling sediments. This section emphasizes caldera-related rocks but also briefly summarizes pre- and post-caldera rocks. Detailed field descriptions of caldera stratigraphy, structure, and hydrothermal features are presented in subsequent sections.
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Figure 3. Generalized composite stratigraphic section for the Caetano caldera and post-caldera deposits filling the caldera. Unit symbols correspond to units in Plate 1. QTs—Quaternary and late Tertiary surficial deposits and basin fill; Ts—tuffaceous sedimentary rocks, sandstone, and conglomerate, undivided; Tbm—Bates Mountain Tuff; Tad—andesite and dacite lava flows; Tcs—Post-Caetano Tuff sedimentary rocks within the Caetano caldera; Tcb—megabreccia blocks and mesobreccia lenses in Caetano Tuff; Tcu—upper unit of the Caetano Tuff; Tcl—lower unit of the Caetano Tuff; Tcc—Caetano Tuff, undivided; Tci—granite porphyry intrusions related to Caetano Tuff; Tog—older gravel and conglomerate; Pzu—Paleozoic rocks, undivided.
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Pre-Cenozoic Basement Rocks
Complexly deformed Paleozoic sedimentary rocks form the basement beneath the Caetano caldera. Lower Paleozoic siliciclastic rocks of the upper plate of the Roberts Mountains alloch-thon probably underlie most of the caldera. These rocks structurally overlie lower plate Neopro-terozoic-Devonian, carbonate-rich, continental-shelf sedimentary rocks. The two sequences were superimposed along the Roberts Mountains thrust during the Late Devonian-Early Mississippian Antler orogeny (Roberts et al., 1958). The Roberts Mountains allochthon is overlain unconformably by Pennsylvanian-Permian clastic and carbonate rocks of the Antler overlap sequence (Roberts, 1964), which is exposed locally along the south margin of the caldera in the Shoshone Range (Plate 1; Moore et al., 2000) and underlies the caldera floor near Caetano Ranch in the Toiyabe Range (Fig. 4A). The Late Jurassic (ca. 158 Ma) granodioritic Mill Canyon stock intrudes the Paleozoic rocks in the Cortez Range just east of the caldera (Plate 1).
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Figure 4. Photographs showing pre-Caetano caldera geology. (A) Chert-pebble conglomerate underlying caldera floor near Caetano Ranch in the northern Toiyabe Range. Rocks are thought to be part of the Pennsylvanian-Permian Antler Overlap sequence. Hammer is 46 cm long. (B) Middle Tertiary conglomerate forming caldera floor on northwest side of the Toiyabe Range. Well-lithified, non-calcareous conglomerate contains clasts of Paleozoic quartzite, chert, and argillite, Mesozoic(?) granite and diorite, and several textural types of Tertiary flow-banded rhyolite (Tr) up to 1.5 m in diameter. (C) View looking south along the crest of the north end of the Fish Creek Mountains. Questa in foreground is formed by flat-lying tuff of Cove Mine that fills a paleovalley. Higher part of range in background is comprised of Fish Creek Mountains Tuff that fills the younger Fish Creek Mountains caldera. (D) View north of Horse Mountain, Wilson Pass, and north margin of the Caetano caldera. Horse Mountain composed of Paleozoic quartzite and argillite (Pz). Caldera-bounding fault lies at base of talus slopes. Low area of Wilson Pass composed of poorly exposed mesobreccia (Tcb; Fig. 6D). Densely welded intracaldera Caetano Tuff (Tcc) forms ridge in foreground and dips 40° east (right).
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Cenozoic Pre-Caetano Tuff Rocks
Few Cenozoic rocks predating the Caetano Tuff are exposed in the vicinity of the Caetano caldera. Gravel deposits overlie Paleozoic basement on both the east and west ends of the caldera and locally form the caldera floor in the northern Toiyabe Range (Plate 1; Fig. 4B). These gravels are thought to fill a west-trending paleovalley now largely obscured by the Caetano caldera; evidence for this interpretation is discussed in a later section. Andesite or dacite lava flows are present locally between the gravels and the Caetano Tuff in the northern Toiyabe Range, and thin andesite or basalt flows underlie the caldera southwest of Wilson Pass in the Sho-shone Range. Andesite flows interbedded with tuffaceous sedimentary rocks underlie outflow Caetano Tuff on the east side of the Fish Creek Mountains just west of the caldera (Fig. 2). Rhyolite dikes dated at 35.2 ± 0.2 Ma (U-Pb zircon, Mortensen et al., 2000) and a small rhyolite dome (K-Ar ages of 35.2 ± 1.1 Ma (sanidine) and 35.3 ± 1.2 Ma (biotite), Wells et al., 1971) intrude and overlie Paleozoic sedimentary rocks in the Cortez and Toiyabe Ranges just north of the caldera (Plate 1). Blocks of these rhyolites occur as breccia in the Caetano Tuff in the Toiyabe Range.
Caetano Tuff and Related Rocks
The Caetano Tuff is a phenocryst-rich, rhyolite ash-flow tuff widely exposed in north-central Nevada (Fig. 2; Gilluly and Masursky, 1965; Stewart and McKee, 1977; Burke and McKee, 1979; Gonsior, 2006). Petrographic, geochemical, and geochronologic data show that the Caetano Tuff as portrayed by Stewart and McKee (1977) consists of two distinct units: (1) an older outflow tuff on the north side of the caldera, erupted at ca. 34.2 Ma (probably from a northern source), and herein referred to as the tuff of Cove Mine, and (2) the main caldera-filling Caetano Tuff and related outflow tuff on the south and west sides of the caldera that erupted at ca. 33.8 Ma (Fig. 2). The two tuffs are in contact only near the north margin of the caldera southwest of Wilson Pass in the Shoshone Range, where 50–100 m of the tuff of Cove Mine overlies Tertiary basalt flows and is, in turn, overlain by intracaldera Caetano Tuff. Intrusive rocks related to the Caetano Tuff magmas intrude and locally deform and hydrothermally alter the central and western parts of the caldera (Plate 1).
Tuff of Cove Mine
The tuff of Cove Mine is named for prominent exposures at the north end of the Fish Creek Mountains, where a compound cooling unit of ash-flow tuff 200 m thick fills a paleo-valley that extends from the northern tip of the range to the Cove Mine (Figs. 2 and 4C; Stewart and McKee, 1977; Emmons and Eng, 1995). Other exposures of tuff that we correlate with the tuff of Cove Mine include outcrops in the south part of Battle Mountain (Roberts, 1964; Doebrich, 1995), Mill Canyon in the Shoshone Range (Gilluly and Gates, 1965), and the northwest corner of the Shoshone Range (John and Wrucke, 2003). The tuff of Cove Mine also forms part of the caldera floor and underlies intracaldera Caetano Tuff along the northwest edge of the caldera near Wilson Pass (Plate 1). The phenocryst-rich tuff of Cove Mine resembles the Caetano Tuff but is slightly older and more mafic with a greater abundance of mafic mineral phenocrysts (especially hornblende) and has a lower overall silica content.
Caetano Tuff
The Caetano Tuff is herein restricted to thick exposures of crystal-rich, rhyolite ash-flow tuff within the Caetano caldera (described below) and outflow tuffs south of the caldera in the Toiyabe and Shoshone Ranges, west of the caldera on the east side of the Fish Creek Mountains, and in Golconda Canyon in the Tobin Range (Fig. 2; Stewart and McKee, 1977; Gonsior, 2006). Gravels in the southwestern Cortez Range (unit Tog, Plate 1) also contain blocks of Caetano Tuff. The outflow tuffs are correlated with the Caetano Tuff on the basis of petrographic characteristics, geochemistry, and/or geochronology. We divide the intracaldera Caetano Tuff into two major units, separated by a complete cooling break and locally by thin sedimentary deposits. The lower unit is a single, compound, cooling unit as much as 3600 m thick in the northern Toiyabe Range. The upper unit consists of several, thin, cooling units interbedded with volcaniclastic sedimentary rocks and has a maximum exposed thickness of 1000 m. The thickness of outflow Caetano Tuff varies widely, reflecting deposition over paleotopography, primarily into paleovalleys cut into the Eocene landscape. Outflow tuff is several hundred meters thick in the Toiyabe Range, 30 km south of the caldera and in Golconda Canyon, 40 km west of the caldera (Fig. 2).
Intracaldera Caetano Tuff is mostly densely welded and crystal rich, containing 35–50 vol% phenocrysts as much as 5 mm in maximum dimension. Quartz, plagioclase, and sanidine generally form >90% of the total phenocrysts (Figs. 5 and 6). Quartz phenocrysts commonly are dark gray to black (smoky) and partly resorbed. Total mafic mineral content generally is <4%. Biotite is the main mafic mineral, with trace amounts of hornblende present locally. Euhedral allanite crystals as much as 1 mm long, apatite, and zircon are common accessory phenocryst phases.
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Figure 5. Histograms of modal data for the Caetano Tuff, tuff of Cove Mine, and Caetano caldera intrusive rocks. All analyses represent point counts of thin sections. Sample 06-DJ-13 is tuff that forms the caldera floor near Wilson Pass and is correlated with the tuff of Cove Mine. (A) Total phenocryst content. Lithic fragments were not counted. (B) Total mafic mineral phenocryst content.
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Figure 6. Ternary plot showing relative modal abundances of quartz, plagioclase, and K-feldspar phenocrysts in the Caetano Tuff, tuff of Cove Mine, and Caetano caldera intrusive rocks. All analyses represent point counts of thin sections.
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The lower unit of Caetano Tuff (map unit Tcl, Plate 1) is a compound cooling unit of relatively homogeneous, generally densely welded rhyolite and high-silica rhyolite ash-flow tuff (Fig. 7A). A 10–20-m-thick basal vitrophyre is preserved along the caldera floor in the northern Toiyabe Range (Fig. 7B), and numerous thin vitrophyric zones are irregularly distributed throughout the tuff in this part of the caldera (Gilluly and Masursky, 1965). Many of these vitrophyric zones envelop beds of mesobreccia or zones of tuff rich in lithic clasts (Fig. 7C), similar to vitrophyres quenched against mesobreccia in calderas in San Juan volcanic field (Hon and Lipman, 1989; Lipman, 2000, p. 27 and Fig. 7 therein). Nearly all other exposures of the intracaldera tuff are devitrified, and tuff in the western half of the caldera is hydrothermally altered. Clasts in the tuff include Paleozoic quartzite, chert, argillite, and limestone, granitic rocks, and Tertiary rhyolites, dacites, and andesites. Limestone, granite, and rhyolite clasts were only observed in the northern Toiyabe Range, near outcrops of the same rocks outside the caldera. Lithic content of the tuff varies significantly—Gilluly and Masursky (1965) described conglomerate beds in the tuff. These beds actually are zones of lithic-rich tuff (lag deposits) or non-tuffaceous mesobreccia that locally contain blocks of pre-caldera rocks as much as 5 m across (Fig. 7C). The pumice content of the tuff varies significantly, but nearly everywhere the crystal-rich pumice are strongly flattened (Fig. 7B) and generally <15 cm in maximum dimension.
The upper Caetano Tuff (map unit Tcu, Plate 1) lies above a pronounced welding break at the top of the lower unit (Fig. 8A) and locally is marked at its base by 5 m of finely laminated, tuffaceous siltstone and sandstone. The upper unit consists of numerous, thin, ash flows inter-bedded with volcaniclastic siltstone, sandstone, and pebble conglomerate (Fig. 8B). Many of the ash flows are poorly welded and have undergone vapor-phase alteration, although thin, densely welded vitrophyres are present locally. The upper unit is widely exposed in the western half of the caldera (Plate 1), but both the upper and lower units are pervasively hydrothermally altered throughout these exposures and not everywhere distinguished on Plate 1. Exposures of relatively unaltered upper unit are best seen south of Rocky Pass along the crest and on the east side of the ridge running south toward Red Mountain and in the low hills northwest of Tub Spring (Fig. 8C). In this area, the upper unit consists of multiple, thin, poorly to moderately welded ash flows locally containing abundant blocks of the densely welded lower unit (Fig. 8D). Thin (5–10 m) zones of more densely welded tuff within the poorly welded tuff indicate that the upper unit is composed of multiple, thin, cooling units that are petrographically and geochemically similar to the main (lower) unit.
Outflow tuffs correlated with the Caetano Tuff are found to the west and south sides of the caldera and as blocks in a gravel deposit (unit Tog, Plate 1) in the Cortez Range. They are generally moderately to densely welded and lithic poor, commonly with vitrophyric zones, notably in exposures south of the caldera. These tuffs are petrographically and geochemically similar to intracaldera Caetano Tuff (Figs. 5 and 6). In contrast, samples of the tuff of Cove Mine (previously mapped as Caetano Tuff) collected from the north side of the caldera in the northern Fish Creek Mountains, Battle Mountain, and northern Shoshone Range (Fig. 1) generally have significantly greater total mafic mineral (6–10%) and plagioclase contents than the intracaldera tuff. The tuff of Cove Mine typically contains 5–8 vol% biotite and 1–2% hornblende, compared to 1–3% biotite and 0–0.5% hornblende in the Caetano Tuff.
New 40Ar/39Ar dates, together with petrographic and geochemical data, demonstrate that the Caetano Tuff as previously mapped consists of two distinct tuffs that erupted at ca. 34.2 and 33.8 Ma. Nine samples of intracaldera and outflow Caetano Tuff were analyzed, including three samples from the northern Toiyabe Range that span nearly the entire stratigraphic thickness (>3.4 km) of intracaldera tuff, two samples from the lowest exposed intracaldera tuff at Moss Creek Canyon and south of Rocky Pass, and two samples of outflow tuff, one from 2 km southwest of the southwestern corner of the caldera and one from 45 km west of the caldera at Golconda Canyon (Fig. 2). They yielded ages ranging from 33.71 ± 0.07 to 33.85 ± 0.09 Ma (Table 1), with a mean and standard deviation of 33.79 ± 0.05 Ma. Ages from the two outflow samples fall within the range of the intracaldera samples (Table 1). A sample of pyroclastic-fall tuff in gravel in the Cortez Range dated at the U.S. Geological Survey (USGS) in Menlo Park is 33.97 ± 0.20 Ma. This age overlaps at 2
with the Caetano Tuff ages determined at New Mexico Tech, and, thus, it may be related to the Caetano eruption, but this comparison and one for a sample of the tuff of Cove Mine from the northernmost Shoshone Range may be subject to a slight interlaboratory bias.
Ages of four samples from the tuff of Cove Mine, including one from the floor of the Caetano caldera near Wilson Pass (Fig. 2), range from 34.21 ± 0.10 (Wilson Pass) to 34.23 ± 0.09 Ma, with a mean and standard deviation of 34.22 ± 0.01 Ma. Figure 9 illustrates the distinct age difference between the Caetano Tuff and the tuff of Cove Mine, consistent with it being exposed below the Caetano Tuff. A sample of the tuff of Cove Mine dated at the USGS in Menlo Park is 34.45 ± 0.08 Ma. An additional sample from Caetano-like tuff collected at Bald Mountain 100 km east of the caldera (Fig. 1) yielded an age of 35.10 ± 0.06 Ma; thus, this sample is not related to either the Caetano Tuff or the tuff of Cove Mine.
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Figure 9. 40Ar/39Ar single-crystal data in age probability diagrams of two samples each of Caetano Tuff (05-DJ-14, 05-DJ-27) and tuff of Cove Mine (06-DJ-13, 05-DJ-8) showing distinctly different ages of each. Unfilled data points were not used in age calculation.
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Caetano Intrusive Rocks
Several bodies of granite porphyry intrude the central and west-central parts of the caldera (Plate 1). The largest intrusion is the 25 km2 Carico Lake pluton that intrudes the center of the caldera in Carico Lake Valley. The Carico Lake pluton consists of 55–65 vol%, 1–5 mm phenocrysts of smoky quartz, sanidine, plagioclase and 3–4% biotite and hornblende in a microcrystalline (0.05–0.1 mm) groundmass of quartz and feldspar. Sparse sanidine phenocrysts as much as 2 cm long are poikilitic and contain numerous plagioclase inclusions. Small miaro-litic cavities are common. The pluton locally is strongly flow banded (Fig. 7D). The modal and chemical compositions of the pluton are similar to the Caetano Tuff, and it yielded a 40Ar/39Ar age of 33.78 ± 0.05 Ma, analytically indistinguishable from the Caetano Tuff that it intrudes (Table 1). The geochronologic data indicate that the maximum time between ash-flow eruption-caldera collapse and emplacement and cooling of the intrusion could therefore not have been more than ca. 0.1 Ma. The pluton appears to have domed the surrounding Caetano Tuff (as described below), and we interpret it as a resurgent intrusion of the magma that formed the Caetano Tuff.
The Redrock Canyon pluton intrudes the upper unit of the Caetano Tuff across the western part of the caldera between Redrock Canyon and Carico Lake Valley (Plate 1). It is exposed as scattered, strongly altered intrusive bodies that crop out in fault-bounded blocks, and it is inferred to be more extensive in the subsurface. The pluton is a medium-grained granite porphyry containing 30%, 1–5 mm phenocrysts of rounded smoky quartz, tabular sanidine, and altered plagioclase and biotite in a felsite ground-mass now altered to kaolinite and quartz.
A small, fine-grained, holocrystalline granite porphyry intrudes the north margin of the caldera at the north end of Carico Lake Valley and is interpreted as a ring-fracture intrusion. It consists of 45 vol% phenocrysts composed of subhedral to euhedral plagioclase, K-feldspar, and dark-gray, rounded to strongly resorbed quartz in a microcrystalline felsic groundmass. The intrusion contains sparse, white K-feldspar phenocrysts as much as 1 cm long. Mafic phenocrysts comprised of biotite, opaque minerals, and trace amounts of hornblende form 3% of the intrusion. Sparse, small (<1 mm) miarolitic cavities are present.
Geochemistry of the Caetano Tuff, Related Intrusive Rocks, and the Tuff of Cove Mine
The Caetano Tuff and the tuff of Cove Mine are subalkaline rhyolites using total alkali-silica relations and the IUGS (International Union of the Geological Sciences) chemical classification (Fig. 10; Irvine and Baragar, 1971; Le Bas et al., 1989). Normalized silica contents range from 68.7 to 77.5 wt% SiO2. Five samples of the Carico Lake pluton and the ring-fracture intrusion have 71.3–73.3 wt% SiO2. Total alkali (Na2O + K2O) contents of all samples are mostly between 7.5 and 8.5 wt%.
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Figure 10. Silica variation diagrams for whole-rock samples of the Caetano Tuff, tuff of Cove Mine, and Caetano caldera intrusive rocks. Major elements normalized to 100% volatile free. See text for data sources. (A) Al2O3-SiO2; (B) Fe2O3T-SiO2; (C) MgO-SiO2; (D) CaO-SiO2; (E) Na2O+K2O-SiO2; (F) TiO2-SiO2; (G) P2O5-SiO2; (H) Ba-SiO2; (I) Zr-SiO2.
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The tuffs clearly separate into two compositional groups with different trends of major and trace elements: (1) high-silica intracaldera and extracaldera Caetano Tuff from west, south, and east sides of the caldera, and (2) more mafic, lower silica tuff of Cove Mine from the north side of the caldera (Fig. 10). The tuff of Cove Mine has notably higher Mg, total Fe, Ti, and P contents and lower Al, Ba, and Zr contents relative to the Caetano Tuff at the same silica content. These chemical data corroborate the separation of the extracaldera tuffs into two groups defined by their petrographic and modal characteristics and 40Ar/39Ar ages, and suggest that extracaldera tuffs from the west, south, and east sides of the caldera are related to intracal-dera Caetano Tuff.
Chemical analyses of intrusive rocks in Carico Lake Valley are generally similar to the intracal-dera Caetano Tuff (Fig. 10). Silica contents of the intrusive rocks are similar to the mafic parts of the intracaldera tuff, and major and trace element contents generally lie on the same compositional trends as the intracaldera tuffs, consistent with them being genetically related. The Redrock Canyon pluton is pervasively altered, and we have no chemical analyses of it.
Chemically analyzed samples of intracaldera Caetano Tuff can be projected onto cross sections to estimate their relative stratigraphic position within the caldera and allow examination of general compositional trends within intracal-dera tuff (Fig. 11). The intracaldera tuff reaches a maximum observed thickness of 3400 m in the northern Toiyabe Range (just south of the Copper Fault, Plate 1), but neither the upper unit nor the caldera floor are exposed in this section. We assume that the caldera floor is very close to the exposed base of this section, consistent with exposures a few km to the south in the foot-wall of the Caetano Ranch Fault (Plate 1). We next assume that the cooling break between the upper and lower units is just above the top of the exposed section, buried beneath younger sediments in Grass Valley; this yields a thickness of 3600 m for the lower unit. No single fault block exposes both the caldera floor and the top of the upper unit; therefore, we estimate the stratigraphic position of our samples relative to either the cooling break between the upper and lower units or the exposed caldera floor. They are plotted on Figure 11 relative to their inferred stratigraphic height (in meters) above the caldera floor. Varying the assumed thickness of the lower cooling unit will thus stretch the vertical axis of the Figure 11, but will not change the relative position of the plotted samples.
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Figure 11. Plot of silica content versus stratigraphic position (height above caldera floor) for intracaldera Caetano Tuff. See text for description of how stratigraphic position was calculated and assumptions inherent in these estimates.
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The basal (vitrophyric) Caetano Tuff exposed along the caldera floor in the northern Toiyabe Range is high-silica rhyolite containing 76.2–76.4 wt% SiO2. The lower 2800 m of the intra-caldera tuff shows little compositional zoning with high silica contents (76.0–77.7 wt% SiO2) throughout. Samples collected near the top of the lower unit in the northern Toiyabe Range, Tub Spring, Rocky Pass, and Carico Lake Valley areas all have markedly lower silica contents, 71.8–73.5 wt% SiO2, than samples from the lower part of the tuff. Silica contents of the upper unit range from 71.9 to 75.6 wt%, overlapping silica contents of the upper part of the lower unit and extending to higher values. Silica content increases abruptly 1.5–2 wt% from the lower to upper units in the Rocky Pass section.
Combined stratigraphic and geochemical data for intracaldera Caetano Tuff and related intrusive rocks suggest four major trends. (1) Thick, relatively homogeneous, high-silica rhyolite (76–78 wt% SiO2) forms most of the intracal-dera tuff. (2) More mafic rhyolite compositions (72–74% SiO2) comprise the upper 700 m of the lower unit, indicating overall normal compositional zoning upward in this single compound cooling unit. (3) The upper unit tends to be more silicic than the upper part of the lower unit, but overall it displays a wider range of compositions, probably reflecting smaller volume, less widespread (?) eruptions. (4) The Carico Lake pluton and the ring-fracture intrusion have compositions (71–73% SiO2) overlapping to slightly more mafic than the upper part of the lower unit, suggesting that these intrusions represent residual, deeper (?) parts of the magma that erupted forming the lower Caetano Tuff.
Post-Caldera Sedimentary Rocks and Ash-Flow Tuff
The depression resulting from caldera collapse served as a long-lived depocenter for sedimentary rocks and distal outflow tuffs (unit Tcs, Plate 1). The sedimentary rocks crop out extensively in the western part of the caldera and locally in the Toiyabe Range. A lower part of these sedimentary rocks consists of platy tuffaceous sandstone, siltstone, and white shale, probably deposited in a shallow lake shortly after caldera collapse and resurgence. The upper part of this unit consists of several repeated sequences of pebble conglomerate that grade upward over a few tens of meters to fine sandstone.
Interbedded with the sedimentary rocks are several ash-flow tuffs collectively referred to as the Bates Mountain Tuff, named for exposures at Bates Mountain 60 km south of the caldera (Stewart and McKee, 1968; McKee, 1968). Much has been published about the Bates Mountain Tuff in central Nevada (e.g., Sargent and McKee, 1969; Best et al., 1989), and the most recent stratigraphic subdivision recognizes four ash-flow cooling units, A through D (Grommé et al., 1972). Detailed mapping and other studies, mostly in western Nevada, show that these tuffs have distinct ages and are unrelated, and the four tuffs have been assigned individual formal and informal names (Table 1; Bingler, 1978; Best et al., 1989; Deino, 1989; Henry et al., 2004; Faulds et al., 2005). Units B (tuff of Sutcliffe), C (tuff of Campbell Creek), and D (Nine Hill Tuff) form 5–20-m-thick ledges in the caldera, although not every tuff is present in every section. Unit A (tuff of Rattlesnake Canyon) has been found only as clasts in gravel in the Cortez Range but may be present in sections not examined in detail for this study. Sanidine 40Ar/39Ar ages reported in Table 1 from Reese River Narrows and New Pass, 35 and 80 km, respectively, southwest of the Caetano caldera, range from 31.03 ± 0.07 Ma (unit A, tuff of Rattlesnake Canyon) to 25.27 ± 0.07 Ma (unit D, Nine Hill Tuff) and are representative of ages determined for the tuffs regionally (Deino, 1989; Henry et al., 2004; Faulds et al., 2005).
All four units of the Bates Mountain Tuff have been found in the Sierra Nevada in California, 300 km west of the Caetano caldera (Brooks et al., 2003; Henry et al., 2004). Unit D is particularly widespread, occurring from near Ely, Nevada, to the western foothills of the Sierra Nevada, a distance of 500 km (Deino, 1989). Unit C erupted from a caldera in the Desatoya Mountains (McKee and Conrad, 1987; Henry et al., 2004), 120 km to the southwest. Sources for the other units are unknown.
Miocene Sedimentary Rocks
Middle Miocene sedimentary rocks are exposed irregularly through the caldera (Figs. 8A and 8C). These rocks consist of fine-grained, tuffaceous sandstone and shale with abundant tephra layers, and coarser alluvialfan deposits of sandstone and conglomerate (Colgan et al., 2008). They are interpreted to have been deposited in localized hanging-wall basins during the Miocene extensional faulting that broke up the Caetano caldera. Colgan et al. (2008) report new 40Ar/39Ar ages and tephrochronologic data for tephras interbedded in these rocks, indicating deposition of the sediments mostly between ca. 16 and 12 Ma.
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CALDERA ORIGIN OF THE CAETANO TUFF
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Our reinterpretation of the Caetano trough as an ash-flow caldera has important consequences for the regional tectonic and magmatic history of the study area (Table 2). In the following section, we review field relationships from key localities where the Caetano caldera displays thick intracaldera tuff, steep caldera margins where pre-caldera and intracaldera units are abruptly truncated against each other, megabreccia and mesobreccia, a resurgent dome/intrusion, and a ring-fracture intrusion (Plate 1). These features are characteristic of well-documented, ash-flow calderas worldwide and indicate rapid eruption of ash-flow tuff from an underlying shallow magma chamber, coeval collapse of the chamber roof-caldera floor, ponding of the tuff within the caldera, and slumping of the caldera walls during and shortly following caldera collapse (Smith and Bailey, 1968; Lipman, 1984, 1997).
Red Mountain Caldera Margin
The caldera margin, thick intracaldera ashflow tuff, and coarse mesobreccia are well exposed north of Red Mountain along the south-central margin (Fig. 12). The caldera margin, which strikes west-northwest and dips steeply northward, separates Ordovician Valmy Formation on the south from interbedded intra-caldera tuff and mesobreccia on the north. Both the Valmy Formation and Caetano Tuff strike generally north to northeast, dip moderately eastward, and are truncated abruptly at the margin. Measured dips in Caetano Tuff vary from 23 to 44°, and the section is repeated by a northwest-dipping fault, so the exposed thickness is unknown but is probably 500–600 m. The tuff is mostly densely welded and strongly altered, with feldspar and biotite phenocrysts altered to kaolinite. Lithic fragments are generally sparse, but are locally abundant immediately above several mesobreccia lenses and in poorly welded tuff near the top of the ridge.
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Figure 12. Geologic map of the southern caldera margin at Red Mountain (Wood Spring Canyon 7- ' quadrangle). East-dipping Caetano Tuff and interbedded mesobreccia lenses composed of Paleozoic clasts (Fig. 13A) are abruptly truncated against Paleozoic Valmy Formation at west-northwest–striking, steeply north-dipping caldera margin. Upper mesobreccia consists of interbedded lenses of Paleozoic clast debris-flow deposits and lithic-rich Caetano Tuff (Fig. 13B). The Red Mountain fault steps abruptly westward at and probably reactivates the caldera boundary fault.
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Several lenses of mesobreccia are interbedded with tuff. They are 5–10 m thick and are composed of angular, clast- to matrix-supported clasts up to 50 cm in diameter (Fig. 13A). Most lenses are massive and unsorted, but one lens has irregular zones characterized by different clast sizes. The zones are nonplanar and commonly strongly oblique to layering in the surrounding ash-flow tuff. Clasts in the lower breccia lenses are almost entirely Paleozoic quartzite and argillite, with sparse Tertiary andesite. Upper lenses also contain clasts of Caetano Tuff and pumice up to 50 cm in diameter. Matrix in all lenses consists of finely ground Paleozoic rocks. Lithic-rich tuff overlies one of the uppermost mesobreccia lenses with sharp contact.
The capping mesobreccia (unit Tcb, Fig. 12) consists of a heterogeneous mix of massive to moderately well-bedded, very coarse to fine deposits. The massive deposits are mostly similar to the interbedded lenses but contain angular quartzite clasts up to 1 m in diameter. Lag blocks of quartzite up to 2 m in diameter were probably eroded out of breccia deposits. Sequences of coarse breccias consist of several layers, which, individually, are one to 5 m thick. A few layers show faint internal bedding, and a few layers have tuff matrix, demonstrating that they are very lithic ash-flow tuffs. Finer deposits consist of moderately to well-bedded, pebbly to coarse, volcanic sandstone, with pebbles of quartzite (Fig. 13B).
We interpret the high-angle contact between Paleozoic rocks and tuff and mesobreccia to be the caldera margin. The margin originated as a fault scarp resulting from caldera collapse and, given its steepness, is probably close to the actual fault plane. However, the contact is not exposed, and debris was shed from the margin during caldera collapse, thus much of the margin is eroded topographic wall. The structurally lower, western parts of the margin probably are closer to the actual fault than are structurally higher, eastern parts. The meso-breccia lenses within the tuff probably are rock falls or avalanches from the caldera wall, and the capping mesobreccia probably is a mix of rock fall and avalanche deposits, as well as primary ash-flow tuff, debris-flow deposits, and minor fluvially reworked tuff and breccia. The paucity of coarse lithics in most intracal-dera tuff indicates that there was little erosion of the vent during tuff eruption and that most mesobreccia was deposited by abrupt rock falls that interacted little with the enclosing tuff that was being deposited. In contrast, the complex stratigraphy of the upper mesobreccia and the presence of a few layers with tuff matrixes suggest that the upper mesobreccia was deposited near the end of tuff eruption.
The caldera margin has been reactivated by, or influenced the location of, the Red Mountain fault that bounds the east side of modern Carico Lake Valley (Fig. 12). This fault, which has prominent scarps suggesting late Quaternary offset, strikes north-south but turns abruptly westward, where it intersects the caldera margin, probably following the margin. The fault turns back to a more northeasterly strike 700 m to the west.
Wilson Pass Caldera Margin
The north margin of the caldera is exposed at Wilson Pass (Fig. 14) and displays several features different from those at Red Mountain. Thick, intracaldera tuff and megabreccia are present, but the margin itself is more structurally or erosionally complicated. The caldera margin strikes west-northwest and dips steeply south. Gilluly and Gates (1965) mapped two fault strands. Paleozoic rocks exposed in or near the margin, north of the north strand, include upper plate chert, quartzite, and lesser sandstone and shale. Mesobreccia and probable megabreccia are poorly to well exposed between the two strands (Figs. 4D and 13C). Intracaldera tuff crops out south of the south strand, dips 30–45° east, and directly overlies an exposure of the caldera floor consisting of mafic lava flows and the older tuff of Cove Mine (Fig. 14). The intracal-dera tuff here is 1800 m thick. Despite its proximity to the caldera margin, the Caetano Tuff in this area contains only sparse, small lithics and no breccia lenses. The tuff is altered but less so than at Red Mountain; plagioclase is generally altered to kaolinite, and biotite is locally altered to white mica or kaolinite, but sanidine generally is unaltered.
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Figure 14. Geologic map of the northern caldera margin at Wilson Pass (Goat Peak 7-' quadrangle). Caldera boundary probably consists of two west-northwest–striking, steeply dipping faults separating Paleozoic rocks north of the northern fault, mesobreccia between the two faults, and Caetano Tuff south of the southern fault (Figs. 4D and 7D). Paleozoic rocks at Peak 7268 may be megabreccia or part of caldera wall. Caetano Tuff overlies the caldera floor consisting of tuff of Cove Mine underlain by basalt lava flows. Miocene sedimentary rocks (Tm) were deposited in the hanging wall of the middle Miocene Redrock Canyon fault (Colgan et al., 2008).
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Mesobreccia at Wilson Pass consists of blocks of variably brecciated quartzite and argillite in soil that locally contains pieces of Caetano Tuff (Fig. 4D). Breccia matrix is not exposed in this area; therefore, it is uncertain whether the tuff pieces are clasts or weathered from matrix. The largest clasts are themselves brecciated, with angular, monolithologic, clast-supported pieces of quartzite or argillite to 35 cm in an indurated, probably siliceous matrix. Well-exposed meso-breccia that consists of matrix-supported, angular to subrounded pieces of argillite, quartzite, sandstone, and chert in a finely granular matrix appears to rest depositionally on Valmy quartzite northwest of Wilson Pass. A large, coherent outcrop of chert west of Wilson Pass (Fig. 14) may be a large breccia block or an intact part of the caldera wall.
The Wilson Pass caldera margin could consist of two fault strands, as mapped by Gilluly and Gates (1965), or the northern strand could be eroded caldera wall and the southern strand closer to an actual structural margin. If the latter, mesobreccia between the two strands would be resting on intact caldera wall, similar to the well-exposed mesobreccia (Tcb 1 km northwest of Wilson Pass, Fig. 14). However, the steepness and linearity of the northern strand suggests it is a fault. The absence of lower plate Paleozoic rocks in breccia indicates that lower plate rocks were not exposed in the topographic caldera wall at the time of caldera collapse.
Northeastern Caldera Margin—The Copper Fault
The northeastern part of the caldera in the Toiyabe Range shows thick intracaldera tuff, an exposed caldera boundary fault, an upward transition from boundary fault to topographic wall resulting from major slumping of the wall, and megabreccia and mesobreccia (Fig. 15, Plate 1). The average dip of the Caetano Tuff is 40° indicating that exposed intracaldera tuff here is 3400 m thick. The base of the tuff is not exposed but is probably not far below the lowest exposed tuff given exposure of the caldera floor 7 km to the south (see section Caldera Floor at Caetano Ranch); therefore, we assign a thickness of 3600 m for the lower unit of the Caetano Tuff. Because the tuff and caldera are tilted, a transect east along the caldera margin is an oblique upward transect along the original margin.
The western half of the northeastern caldera margin mapped by us coincides with the Copper Fault of Gilluly and Masursky (1965). This western part is probably the caldera structural boundary where intracaldera tuff is faulted against Paleozoic rocks. For example, at A (Fig. 15), a planar, 58°, southward-dipping fault surface is developed in resistant Devonian Slaven Chert, which makes a low ridge on the north against less resistant tuff and breccia on the south (Fig. 16A). Chert is intensely brecciated along the fault. Caetano Tuff crops out 30 m to the south but is not exposed along the fault surface.
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Figure 16. Photographs showing the northeastern margin of the Caetano caldera. (A) Large block of Devonian Slaven Chert (Dsc) in white Caetano Tuff (Tcl) near northeastern caldera margin in northern Toiyabe Range. Low rocky ridge is Slaven Chert in caldera margin at A (Fig. 15); caldera boundary fault is developed in the chert. View looking north. (B) Steep caldera structural margin at B (Fig. 15). View looking west-northwest. (C) Close-up view of steep caldera structural margin at B (Figs. 15 and 16B). Densely welded, highly stretched Caetano Tuff is in sharp, approximately vertical contact with brecciated and recemented Devonian Slaven Chert outside caldera. (D) Caldera topographic wall at C (Fig. 15), showing megabreccia consisting of multiple blocks of Devonian Slaven Chert up to at least 50 m in diameter, locally with thin lenses of Caetano Tuff. Light-colored middle ground is lithic-rich Caetano Tuff containing clasts of chert and 35 Ma rhyolite, which crops out just to the right of the photo as megabreccia. View looking west-northwest.
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Megabreccia and mesobreccia are interbed-ded with Caetano Tuff from the Copper Fault, the northern margin, southward to near the Wenban Fault, the southern margin (Figs. 15 and 16A). Breccia near A consists of a single chert megabreccia 30 m long with brecciated margins and several lenses of mesobreccia containing clasts of Paleozoic chert, quartzite, and chert-pebble conglomerate up to 2 m in diameter (Figs. 7C, 16A, and 16B). Caetano Tuff forms vitrophyre adjacent to many of the meso-breccia lenses (Fig. 7C).
The caldera structural boundary is especially well exposed at B, 1 km east of A (Fig. 15). At this location, densely welded, highly sheared Caetano Tuff is in a sharp, vertical to steeply south-dipping contact with chert in the margin (Fig. 16B). Pumice is highly stretched, parallel to the contact, to the point the rock resembles flow-banded rhyolite (Fig. 16C). Adjacent chert is brecciated and tightly recemented. Faulting probably occurred while the tuff was still hot and could deform ductily, but adjacent chert was brittle either because it was colder or compositionally distinct (all SiO2). The attitude of Caetano Tuff away from the margin changes progressively to the normal north-northeast strike and moderate east-southeast dip, but even 50 m from the caldera margin tuff dips to the south as steeply as 87° (Fig. 15). These relationships suggest the tuff was both deposited against the chert and also downfaulted along it. Tuff deposited against chert partly stuck to it but also underwent internal shearing during caldera subsidence; tuff farther from the chert was simply dragged down to its steep southward dip with little or no shearing. The exposed contact, its topographic expression, and the steep dips in tuff show that this structural boundary dips vertically to steeply southward (Figs. 16B and 16C).
Mesobreccia lenses at the stratigraphic level of B in the central part of the caldera contain clasts of limestone and finely, sparsely porphyritic, flow-banded feldspar-quartz-biotite rhyolite, in addition to quartzite, chert, and chert-pebble conglomerate. Individual blocks are as large as 5 m. Several limestone and quartzite blocks have brecciated and recemented margins (Fig. 17A). Where exposed, matrix consists of finer clasts, but many lenses occur as trains of blocks with no exposed matrix.
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Figure 17. Photographs showing breccias and conglomerate in the Caetano caldera. (A) Large clast of brecciated, Paleozoic quartzite in lithic-rich layer in Caetano Tuff on east side of Toiyabe Range. Layer previously was mapped as conglomerate bed within the Caetano Tuff by Gilluly and Masursky (1965). (B) Mesobreccia sheet in Caetano Tuff on the southwest side of Carico Lake Valley. Mesobreccia composed of angular clasts of siliceous siltstone, quartzite, chert, and chert-pebble conglomerate up to 70 cm in diameter in a more finely clastic, non-tuffaceous matrix. Lens extends several hundred meters along strike. Hammer is 46 cm long. (C) Limestone clasts in Tertiary conglomerate underlying caldera floor near Wenban Spring, Toiyabe Range. (D) Hydrothermally brecciated Redrock Canyon pluton in low hills northwest of Carico Lake. Matrix-supported breccia consists of clasts of Redrock Canyon pluton pervasively altered to kaolinite + quartz in matrix of quartz, Fe-oxide minerals (mostly hematite), and local barite. Note larger brecciated clast in bottom of photo that has hydrothermal matrix filling fracture.
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East of B (Fig. 15), the caldera margin continues nearly due east, whereas the Copper Fault of Gilluly and Masursky (1965) turns slightly to the south to follow what we interpret as the boundary between massive, relatively lithic-poor Caetano Tuff to the south and megabreccia-rich tuff to the north. The caldera margin as mapped by us separates intact Devonian Slaven Chert on the north from megabreccia, which consists of numerous large blocks and clusters of blocks, mostly of chert, in a poorly exposed matrix of Caetano Tuff. Because chert is so much more resistant than tuff, it dominates outcrop and makes up all the hills and knobs (Fig. 16D), thus Gilluly and Masursky's (1965) interpretation that it is Paleozoic rock in place. The four large areas of upper plate Paleozoic rocks south of the caldera margin shown in Figure 15 are composite, consisting of numerous individual blocks up to at least 50 m in diameter. A few natural outcrops and several mining-related exposures show irregular "fingers" and thicker lenses of tuff through the composite blocks. Lithic-rich tuff crops out in low areas between blocks (Fig. 16D), but much of the area of unit Tcx consists of breccia blocks of all sizes in a clayey soil with grains of quartz, sanidine, and biotite from disaggregated tuff. The northern caldera margin is not exposed but can be located within a few meters in several locations where tuff gives way to coherent Paleozoic rock. This part of the northeastern caldera margin is probably the eroded topographic wall, where mega-breccia slumped from the wall during ash-flow eruption and caldera collapse.
Megabreccia consists mostly of chert, but sparsely porphyritic, flow-banded rhyolite also is common. Gilluly and Masursky (1965) mapped several rhyolite bodies as in-place intrusions in Paleozoic rock. However, the four shown on Figure 15 are mostly to entirely surrounded by Caetano Tuff, and clasts of the rhyolite are common in adjacent lithic-rich tuff. As with the chert megabreccia, several of the rhyolite blocks are probably composite.
Gilluly and Masursky (1965) mapped mega-breccia and some mesobreccia as in-place Paleozoic Valmy Formation or Slaven Chert, depending on whether the blocks were quartzite or chert, or rhyolite dikes and other mesobreccia lenses as Tertiary conglomerate. However, the large size of clasts and their interbedding or interfingering with Caetano Tuff demonstrates that they are megabreccia and mesobreccia. The distinctive clast types are consistent with outcrops in the caldera wall north of the Copper Fault and in the Cortez Range to the east. We also found limestone clasts that are probably derived from lower plate Wenban Formation, which crops out below Tertiary gravel in the Cortez Range. Lower plate rocks are recognized in breccia only in the eastern part of the caldera. Rhyolite clasts are petrographically identical to the ca. 35 Ma rhyolite dikes in the Cortez Mine and a small rhyolite dome in the Cortez Range (Plate 1). The large, east-striking body at C (Fig. 15), which is probably composite, is one of the rhyolites dated by Wells et al. (1971).
Other Mesobreccia and Megabreccia Locations
Massive to layered mesobreccia or megabreccia crops out near the caldera margin in many locations. A notable occurrence is just west of Carico Lake Valley (Plate 1). Mesobreccia composed of angular clasts of siliceous siltstone, quartzite, chert, and chert-pebble conglomerate up to 70 cm in diameter in a more finely clastic, non-tuffaceous matrix is interbedded with probably uppermost Caetano Tuff. Irregular bedding is defined by variations in clast type, size, and abundance (Fig. 17B). The mesobreccia layers form several thick sequences with minor inter-bedded Caetano Tuff; the entire mesobreccia sequence is at least 100 m thick.
Although most breccia is within 1–2 km of the caldera margin, at least one occurrence is more than 4 km from the nearest margin. Mesobreccia that crops out within the upper Caetano Tuff unit northeast of Red Mountain near Tub Spring (Plate 1) is the most distant from the caldera margin that we found. The breccia in this area contains blocks of chert up to 5 m long (Fig. 13D). This breccia was deposited very late during eruption and caldera collapse. The caldera margin may have had its greatest relief so that breccia could travel the farthest from the margin.
Upper Unit of Caetano Tuff near Tub Spring
The character of the upper unit of Caetano Tuff is best illustrated northwest of Tub Spring in the south-central part of the caldera, where the lower-upper contact is repeated by several northwest
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ABSTRACT
INTRODUCTION
