Eruptive History and Structural Development of the Toquima Caldera Complex, Central Nevada

Home , Mount Jefferson (Nevada), Toquima Range

Eruptive history and structural development of the Toquima caldera complex, central Nevada

DAVID R. BODEN Department of Geology, Stanford University, Stanford, California 94305

ABSTRACT

The Toquima caldera complex, located in the Toquima Range of central Nevada, consists of three overlapping to nested calderas. The Moores Creek caldera is the largest (~30 by 20 km); it formed -27.2 Ma in response to eruption of the high-silica rhyolite tuff of Moores Creek. Because of recurrent volcanic activity and subsequent basin- range faulting, only the northern segment of the Moores Creek caldera is preserved; its eastern and western margins are downfaulted below valley fill, and its southern part was obscured by collapse of the Mount Jefferson caldera. Eruption of the tuff of Mount Jefferson resulted in collapse of the 18- by 20-km Mount Jefferson caldera [ Silicic Ignihibrites -26.5 Ma. The ash-flow tuff exposed at Round Mountain is a silicic -J Austin' outflow-facies equivalent of the compositionally zoned (76-67 wt % Other Volcanic Roqks Si02) intracaldera tuff of Mount Jefferson. Pyroclastic eruptive activity in the complex concluded -23.6 Ma with formation of the compar- /

INTRODUCTION Figure 1. Regional location map of the Toquima caldera complex, showing outlines of the individual eruptive centers. Other major geo- Voluminous and areally extensive mid-Tertiary ash-flow tuff covers logic features shown include the Northumberland and Manhattan cal- much of central Nevada, but the location, configuration, and evolution of deras (NC, MC, respectively) and the Toiyabe-Kawich lineament most source areas remain dimly understood. This is because detailed geo- (TKL). Upper box shows the location of the enlarged area with re- logic studies in the region are few, eruptive centers have been obscured by spect to the west-northwest-trending belt of Oligocene to early Mio- recurrent volcanic activity, and late Tertiary and Quaternary basin-range cene volcanic rocks, dominated by silicic ash-flow tuff. MVF denotes faulting has disrupted and truncated parts of the source areas. the Marysvale volcanic field.

Geological Society of America Bulletin, v. 97, p. 61-74, 9 figs., 2 tables, January 1986.

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Tre j i- . Trail Canyon MSI i * <• i * caldera Tt yp Ttb

Mt.Jefferson ISP caldera Tju

STjr,

+++++++4 , Contact Moores Cree^k caldera f+Tmc V» + + + Fault: ball on downdropped side; dashed where approximately located. ^,-oTcc Cauldron boundary: ball on downthrown ». ;.o ; side of margins reactivated by basin Unknown it ; • ^-range faulting: rectangle on • Tis." caldera side. calderas •jO^ Strike and dip of foliation '•Tdb

- Horizontal eutaxitic foliation Qd Alluvium,Colluvium,and Fanglomerate H4" Location of K-Ar sample (Table 2)

•-t Topographic wall "TrcJ, tuffs of RoadCanyon (23-22Ma)

tuff of Trail Cyn (23.6 Ma) Ttl-local capping porphyritic lava ¿TdPÍ» megabreccia of Dry Canyon (32.3 Ma) Tt-upper and lower members undifferentiated Ttb-caldera collapse breccia Porphyritic toaphyric plugs and llava-chocked ignimbrite feeder vents tuff of Ryecroft Canyon (25.0 Ma)

tuff of Mt. Jefferson (26.4 Ma) V V s / Granitic rocks of Shoshone Mountain isTjCÈ^Tic— discontinuous capping cooling units Kg's •Tju- Tju- upper member WiFTI Tjl- lower member: solid triangles denote caldera fi'f ' j :ollapse breccias Pzs; lower Paleozoic sedimentary , | ; J Tjr- outflow Round Mountain member (26.7 Ma) rocks M Pzb; severely brecciated Paleozoic + +4 + + y rocks and megabreccia. tuff of Moores Creek (27.2 Ma)

its own story to tell, involving not only caldera-related processes, but regional lithospheric magmatism and tectonism, as well. tuff of Corcoran Canyon (27.5 Ma) This report chronicles the eruptive history and structural development of the Toquima caldera complex—a major ignimbrite source area located in northern Nye County, Nevada, -400 km east of Reno (Fig. 1). tuff of LoganSpring (29.6 Ma) PK8 The complex is situated in the Toquima Range and consists of three overlapping to nested calderas of Moores Creek (27.2 Ma), Mount Jeffer- The development of calderas reflects a complex feedback between son (26.5 Ma), and Trail Canyon (23.6 Ma) (Fig. 2). The town and Au-Ag magmatic and tectonic processes. Their evolution is influenced by the mine of Round Mountain are located in the southwest corner of the study pre-existing structural fabric of the basement, state of crustal stress at the area. Data for this summary are derived from detailed mapping (1:24,000 time of development, thermal and mass flux through the lithosphere, and scale), K-Ar dating, petrography, and selected whole-rock chemical depth and residence time of magma prior to eruption. As a result, each has analyses.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/1/61/3445131/i0016-7606-97-1-61.pdf by guest on 25 September 2021 Figure 2. Generalized geologic map of the Toquima caldera complex. Inset shows the inferred configurations of the nested Moores Creek (MC), Mount Jefferson (MJ), and Trail Canyon (TC) calderas prior to the onset of basin-range block faulting.

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TABLE 1. CALDERAS AND RELATED ROCK UNITS OF THE TOQUIMA PREVIOUS WORK CALDERA COMPLEX, CENTRAL NEVADA

Early work in and around the Toquima caldera complex concen- Caldera Eruptive Unit Age SiOjStt Volume 3 trated on describing the geology and mineralization of various mines, (m.y.)' (km )

including Round Mountain (Ransome, 1909; Ferguson, 1921) and Jeffer- Moores Creek Tuff of Moores Creek 23.6 ± 0.4 76.0-72.5 X« son Canyon and Gold Hill (Krai, 1951). Ferguson and Cathcart (1954) j ' Tuff of Mount Jefferson published a geologic map of the 30-minute Round Mountain quadrangle. Mount Jefferson 1 Upper member 1 intra. 26.4 ± 0.5 75.7-67.7 -250 1 Lower member f caldera Kleinhampl and Ziony (1985) reviewed the geology and compiled a re- 1i Round Mountain mbr 26.7 ± 0.6 77.5-73.8 >10 connaissance geologic map of northern Nye County that depicts general Trail Canyon Tuff of Trail Canyon 27.2 ± 0.6 78.0-76.8 >(00 rock relations wi:hin the Toquima caldera complex. A generalized geo-

logic map of the Northumberland caldera (McKee, 1974) includes the •Weighted mean iige . * XRF bulk tuff analyses. northern half of tlie Toquima caldera complex. Marvin and others (1973) and Silberman and others (1975) published K-Ar ages on rock samples collected from Mount Jefferson and Round Mountain, respectively. More recently, Tingley and Berger (1985) reported on the geology meso- and megabreccias (terminology after Lipman, 1976a). Small, and geochemistry of lode deposits in the Round Mountain Au-Ag mine. aphyric to porphyritic, flow-layered, intrusive plugs occur locally. Bedded, Detailed geologic: relations in the vicinity of the mine are depicted in the tuffaceous sedimentary rocks are relatively minor, and airfall tv.ffs and 7.5 minute Round Mountain quadrangle (Shawe, 1981a). Results of a lavas are surprisingly scarce. regional gravity survey by Snyder and Healey (1983) show the Toquima Eight major volcanic units, ranging in age from 32 to 22 m.y., are caldera complex to be outlined by a multilobed gravity low. Harrington exposed in and around the Toquima caldera complex (Fig. 2). Three (1983) reported on the geology of a part of the Mount Jefferson caldera. units—the tuffs of Moores Creek, Mount Jefferson, and Trail Canyon— Detailed investigations of alteration and mineralization in the active were erupted from the complex (Table 1). Other map units include the open-pit mine at Round Mountain are reported by Mills (1984), Sander megabreccia of Dry Canyon and tuffs of Logan Spring, Corcoran (Canyon, and Mills (1984), and Mills and Sander (in press). Ryecroft Canyon, and Road Canyon, all having their sources elsewhere. The 32-m.y.-old megabreccia of Dry Canyon is exposed just north- GEOLOGIC S ETTING east of Round Mountain (Fig. 2). It is inferred to mark the eastern fringe of a caldera that lies largely buried by valley fill to the west. The 29.5-m.y.- The Toquima caldera complex lies near the middle of an extensive old tuff of Logan Spring is exposed only in a small area in upper Road west-northwest-trending belt of Oligocene and early Miocene volcanic Canyon; its source is unknown, and it is unclear whether the tuff is an rocks, consisting mainly of silicic ash-flow tuff (Stewart and others, 1977; outflow or intracaldera accumulation. The 27.7-m.y.-old tuff of Corcoran Stewart and Carlson, 1978). This belt is -200 km wide and extends -800 Canyon is 1 km thick along the southeastern margin of the study area and km from the Marysvale volcanic field in south-central Utah on the southeast may be a near-source accumulation, but subsequent volcanic activity, to an ill-defined termination in west-central Nevada or east-central Cali- hydrothermal alteration, and basin-range faulting make accurate recon- fornia on the northwest (Fig. 1). As such, it rivals the great fields of struction of its source difficult, if not impossible. It was probably derived ash-flow tuff of Ihe Sierra Madre Occidental in Mexico (Clabaugh, 1979; from a volcanic center now largely buried by valley fill to the east. The tuff Swanson and McDowell, 1984) and of the central Andes (Baker, 1981). of Ryecroft Canyon, on the other hand, appears to be a distal outflow This once-continuous volcanic belt has been fragmented into north- correlative with the tuff of Arc Dome, which erupted from a volcanic northeast-trending ranges and valleys of the Basin and Range province. center in the southern Toiyabe Range (G. Brem, 1983, personal com- The Toquima caldera complex lies near the central part of the Toquima mun.). These two units are correlated on the basis of strikinglj' similar Range, a north-northeast-trending, gently west-tilted horst block. Two phenocrystic assemblages and proportions and identical K-Ar ages of-25 other volcanic centers are exposed in the Toquima Range: the 32-m.y.-old m.y. (D. John, 1985, personal commun.; Boden, unpub. data). For the Northumberland caldera (McKee, 1974) and the 25-m.y.-old Manhattan same reasons, two units from the tuff of Road Canyon are correlated with caldera (Shawe., 1981b) (Fig. 1). To the west and east lie the boldly cooling units 3 and 4 of the Bates Mountain Tuff, whose ages are 23 m.y. 1 expressed, but also gently west tilted, Toiyabe and Monitor Ranges, re- and 22 m.y., respectively (Sargent and McKee, 1969). These two units are spectively. Like the Toquima Range, these ranges host very thick accumu- interpreted as extensive outflow sheets whose sources lie north of the lations of ash-flow tuff from sources reported by Brem and Snyder (1983) Toquima caldera complex. and Brem and others (1985) and understudy in the Toiyabe Range by G. Brem (1984, personal commun.) and in the Monitor Range by W. ERUPTIVE CENTERS Keith (1983, personal commun.) and R. Hardyman (1984, personal commun.). Recognizing calderas in the faulted and eroded Basin and Range Volcanic rocks of the Toquima complex unconformably overlie Late province is difficult because of broken exposures and postcollapse struc- Cretaceous granitic rocks of Shoshone Mountain and strongly deformed tural modifications. At the Toquima caldera complex, little remai ns of the lower Paleozoic sedimentary rocks and their metamorphosed equivalents original morphologic characteristics of calderas (Fig. 3), that is, a high- (Fig. 2). Rocks of late Paleozoic, Mesozoic, and early Tertiary age are standing topographic wall surrounding a subcircular depression. Instead, absent and weru either never deposited or were eroded prior to onset of their existence is based on the following. mid-Tertiary volcanism. 1. Coarse lithic debris entrained in an ash-flow tuff. These debris zones form by cavitation of oversteepened caldera walls during collapse VOLCANIC SEQUENCE

'Gromm6 and others (1972) subsequently redefined cooling unit 4 of the Bates Volcanic rocks of the Toquima caldera complex consist almost en- Mountain Tuff as the tuff of Clipper Gap based on its distinct areal distribution and tirely of variably welded, crystal-rich, ash-flow tuff and discontinuous younger age relative to the other cooling units.

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SE Mt. Jefferson Inferred position of B Bend in Section Section A-A burled SE margin of MJC. 12,5000 - Tic \ji° 12,500' 10,000'- 10,000' > V '' 'tpB 5,000' V\ Til ' Vf/ Til 5,000' Tiu v-'--- / / •• •.. + + Inferred » + + + >-r/+ + Tmc + + + ^ 1 S.L._ + f< I structural Deeply eroded | - S.L. Basin-range faulted structural "S* margin of V structural margin of the' Basin~range faulted v margin of Mt. Jefferson caldera TC caldera Moores Creek caldera western margin of Trail Canyon caldera

Figure 3. Geologic cross sections through the Toquima caldera complex. Scale and symbols are same as used in Figure 2. No vertical exaggeration.

and are termed "caldera-collapse megabreccia" (clasts >1 m) or "meso- tuff is restricted to narrow septa and films between clasts. To the southeast, breccia" (clasts < 1 m) (Lipman, 1976a). in vicinity of upper Road Canyon, the structural margin is not exposed. 2. Occurrence of thick ash-flow tuff (> 1 km). As a result of subsi- Instead, the contact between the tuff of Moores Creek and the tuff of dence concurrent with eruption, the intracaldera tuff is typically an order Logan Spring and variably brecciated Paleozoic sedimentary rocks dips of magnitude thicker than its outflow counterpart (Christiansen, 1979). moderately westward (-25°). These relations are interpreted to represent a 3. Partial preservation of a topographic wall or exposure of the part of the topographic wall of the Moores Creek caldera. ring-fracture zone (structural margin). Slumping of oversteepened, caldera- The precise location of the southern margin of the Moores Creek bounding, fault scarps can produce a topographic wall larger in diameter caldera cannot be determined, but it is believed to be near the contact than the structural margin. Dips of the topographic walls can range from between Cretaceous granitic rocks of Shoshone Mountain and Paleozoic nearly vertical, as at Crater Lake (Bacon, 1983), to 8° to 10° in some of metasedimentary rocks in Jefferson Canyon; this is approximately coinci- the Oligocene calderas in the San Juan volcanic field of southwestern dent with the southern margin of the Mount Jefferson caldera. This inter- Colorado (Lipman, 1975, 1976b). In more deeply dissected calderas, the pretation is based on the transition in Jefferson Canyon from competent topographic wall may be represented by listric normal faults that merge granitic rocks to comparatively weak sedimentary rocks, which forms a downward into the structural margin of the caldera; the latter is a vertical pre-existing structural discontinuity that could help localize both tectonic or steeply dipping, arcuate fault or several arcuate faults that can be locally and volcanic structures. Extrapolation of the known northern and inferred intruded by dikes or plugs as exemplified in the Lake City and Bennett southern margins of the Moores Creek caldera results in an originally Lake calderas (Lipman, 1976b; Lambert, 1974, respectively). northwest-elongate caldera, measuring -30 by 20 km. Tuff of Moores Creek. The intracaldera tuff of Moores Creek is Moores Creek Caldera restricted to the northern part of the Toquima caldera complex and is presently exposed over -35 km2 (Fig. 2). No outflow remains exposed. The Moores Creek caldera, formed -27.2 Ma upon eruption of the The interpretation that the tuff is an intracaldera accumulation in its own tuff of Moores Creek, makes up the northern part of the Toquima caldera caldera is based on the following. (1) A thickness of -1 km, with base complex (Fig. 2). It is the oldest and least well preserved of the three unexposed and top eroded, is consistent with an intracaldera accumula- calderas. The northern sector is well exposed, but its eastern and western tion. (2) The presence of entrained clasts and megaclasts (measuring up to margins are faulted below valley fill, and its southern part is obscured by several hundred metres across) in the tuff along its northern extent is the Mount Jefferson and Trail Canyon calderas. The northern structural characteristic of caldera-collapse breccias. The incidence and size of these margin is marked by a south-facing arcuate zone of severely brecciated clasts diminish southward. These relations, together with a weighted mean Paleozoic rocks that bound the tuff of Moores Creek. The arcuate breccia K-Ar age of 27.2 ± 0.6 m.y. (Table 2, no. 15), argue that the tuff of zone is as much as 0.5 km wide and is intruded by an unbrecciated rhyolite Moores Creek is not outflow from either the 32-m.y.-old Northumberland plug with an age of 26.6-26.5 ± 0.5 m.y. (Table 2, no. 19). A large caldera (McKee, 1974) or from the 26.5-m.y.-old Mount Jefferson caldera exposure of Paleozoic sedimentary rocks on the northwestern margin of (see discussion below). the Moores Creek caldera is interpreted as megabreccia in which the host The tuff of Moores Creek is characterized by conspicuous, light co-

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TABLE 2. K-Ar AGES AND ANALYTICAL DATA

Rock unit Sample Map Material Wtfg) K2O%" 40 A. Arrad Age (m.y.)t Age (m.y.)§ Age (m.y.)" Comments no. no. xl0"10mol/g % ±2o ±2o ±>2(7

139-RRC 1 Sanidine 0.2459 7.44 2.382 68.0 22.1 ± 0.3 Correlative to cooling unit 4 of Bates Mm. Tuff (Sargent and McKee, 1969) Tuffs of Road Canyon 048-MC 2 Sanidine 0.2545 11.96 4.096 51.2 23.6 ± 0.4 Informally called "tuff of Pipe Organ Sp ring"

514-ADW 3 Sanidine 0.2216 10.88 3.702 59.9 23.5 ± 0.4 Vitrophyric zone in lower member

Tuff of Trail Canyon 497-TC 4 Sanidine 0.2070 11.14 3.790 64.0 23.5 ± 0.4 Top of upper member

024-MC 5 Sanidine 0.1877 11.03 3.814 69.2 23.9 ± 0.5 Tuff matrix of megabreccia along southern margin of Trail Canyon cauldron

Tuff of Ryecroft Canyon 005-MC 6 Sanidine 0.2314 11.32 4.103 59.1 25.0 ± 0.5 Quartz-rich upper part Believed correlative to tuff of Arc Dome of Toiyabe Range

Tuff of Mount Jefferson / 657-SC-S 7 Sanidine 0.1911 11.06 4.158 68.3 25.9 i 0.5 Upper member, south side of Mount Jerferson 1 657-SC-B 7 Biotite 0.2880 8.12 3.129 67.4 26.6 i 0.6

Upper member < 641-NPC 8 Sanidine 0.2501 11.01 4.235 69.6 26.5 ± 0.5 Top of upper member on summit of Mount Jefferson

\ 618-SS 9 Sanidine 0.2074 10.78 4.024 59.8 25.8 ± 0.5 Poorly welded ignimbrite interbedded t iffaceous sedimentary rocks capping Mount Jefferson tuff

Lower member 451-ACK 10 Sanidine 0.1777 10.83 3.958 42.8 26.3 ± 1.0 Top of lower member, north side of Mount 0.1746 10.84 4.290 31.7 Jefferson

723-SP-S 11 Sanidine 0.2294 11.07 4.256 37.7 26.5 ± 0.4 Densely welded interior

723-SP-B 11 Biotite 0.2087 8.47 4.698 57.9 27.2 ± 0.6

720-SP-S 12 Sanidine 0.1847 9.53 3.704 75.9 26.8 ± 0.6 Basal vitrophyte

Round Mountain ^ . 720-SP-B 12 Biotite 0.2494 7.50 2.767 75.9 25.5 ± 0.6 member

217-SP 13 Sanidine 0.1448 10.63 4.350 34.6 27.3 ± 1.1 Lower pari of densely welded zone 0.1644 10.68 4.073 35.8 K-Ar-RM-2 14 Sanidine 0.1896 11.18 4.349 50.1 26.8 ± 0.5 Densely welded zone on fringe of hydr sthermally altered area. In thin section, sanidine is K-Ar-RM-2 14 Biotite 0.1744 8.36 3.267 80.5 24.4 t 0.6 fresh, but biotite shows dissem. opaque oxides along cleavage planes and grain rims

Tuff of Moores Creek 367-RC-S 15 Sanidine 0.2841 11.20 4.431 32.4 27.3 ± 0.5 Basal vitrophyre on caldera in upper Road Canyon 367-RC-B 15 Biotite 0.2219 8.09 3.168 83.1 27.0 ± 0.8

Tuff of Corcoran Canyon 550-CC 16 Sanidine 0.1866 10.90 4.388 59.7 27.7 ± 0.7 Biotite-rich upper part

Tuff of Logan Spring 368RC 17 Sanidine 0.5120 9.81 2.168 31.9 29.6 ± 0.6 Basal vitrophyre overlying Paleozoic std. rocks

Megabreccia of Dry 701-DC 18 Biotite 0.2670 7.78 3.646 58.5 32.3 ± 0.7 Local vitrophyric zone in welded tuff matrix of Canyon megabreccia

380-MZ-S 19 Sanidine 0.1711 10.73 4.26 20.4 Porphyritic, flow-banded plug intruded along 0.1606 10.79 4.03 25.2 26.6 ± 1.1 northern structural margin of the Moores Creek cauldron

380-MZ-B 19 Biotite 0.2400 8.35 3.213 73.6 26.5 ± 0.7 Intrusive plugs 527-CC 20 Sanidine 0.1853 10.93 3.437 61.9 21.7 ± 0.4 Boldly outcropping plug intruded along margin of Trail Canyon cauldron

490-NBC-S 21 Sanidine 0.2141 11.40 3.914 68.6 23.7 ± 0.4 Small, porphyritic, flow-banded plug intruded along northwestern structural margin of Mount 490-NBC-B 21 Biotite 0.2323 8.48 3.102 58.7 25.2 ± 0.5 Jefferson cauldron

'Mean of two aliquots, cixept analyses by Geochron Labs where actual value of each split is reported. K determined, in alt cases, by lithium metaborate fusion technique and flame photometry. 10 10 -4 ^Constants used: X€ = 0.S81 X 10" yr"', kg = 4.962 X Kr yr~', and '^K/K = 1.167 X 10 . Ar extraction and measurement conducted at the Branch of Isotope Geology, U.S. Geological Survey, Menlo Park, CA. Standard deviation of precision calculated using equation 2 of Mahoodand Drake (1982). r extraction and measirement conducted at Stanford University and at the Branch of Isotope Geology, U.S. Geological Survey, respectively. Same decay constants and error analysis as used in ^note. ""Age determinations pel formed by Geochron Labs, Cambridge, Mass. Same decay constants as used in ' note except 4®K/K = 1.19 X 10"4. Reported age is the weighted mean of two Ar extractions and measurements per sample. Emir assigned to age determination is the sum of 1% of the calculated age plus the product of the calculated age and the percent error in the ^ArV^^K ratio; the latter reflects a composite standard deviation calculated from a large pool of in-house age dt terminations grouped according to specific concentrations of K and Ar (T. Bills, written comm., 1983).

lored, porphyritic fiamme, as much as 30 to 40 cm long. Both pumice and bly; the sandine:plagioclase ratio remains at about 2:1 over thick sections matrix contain similar amounts and proportions of phenocrysts. This rela- of the rock unit. The uniform phenocryst proportions are also paralleled by tionship suggests little elutriation of fine ashy material, possibly because relatively uniform chemical composition (Fig. 5). The tuff of Moores

sorting processes, that normally enrich the matrix in crystals, were retarded Creek is a high-silica rhyolite throughout (76.8% to 78.0% Si02). by high rates of deposition within the caldera. Phenocrysts comprise about Caldera Resurgence. Because little of the Moores Creek caldera is 15% to 20% of the rock and consist of sanidine, quartz, plagioclase, minor preserved, its post-collapse volcanic history is difficult to assess. Lxal 15° biotite, and trace opaque oxides (Fig. 4). Although total crystal content to 20° northward dips in the tuff of Moores Creek could reflect tilti ng from varies with stratigraphic position, phenocryst proportions change negligi- central resurgent uplift, because much younger basin-range faulting usually

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% Phenocrysts % Mafics Points Sample Counted No. 1.0 20 30 40 4 6 8 10 ' ' ' ' < ' 'I 'I ' I I I ' • I I I • I • l • I Capping Lava 1164 511PB i T 1204 497XC 400m IUpper 2-400 1276 517ADW BASE IBA: 1160 508PB Xop o fx. S07PB 5 -350m 1266 Member 1112 501XC J LOW TL * IL CB 1210 205BC _— 1217 202-1MRC 1139 284PC r^rÜ 1162 294-1PC i. XOP 1205 293PC 5 -1150m 313-1PCK _ I LOW 1107

RIDDLE 1162 723SP 2 ~250m 1210 721SP [BASI mM 2937 725SP

® J HIGH I^IV/I Pasco 1247 324PC m ° -500m i ' ' Canyon 1284 322PC S I tow Section 1160 321PC O T- Moores ox

•ÇLAG;, TSAN M äBIOXS PYX PYX* 9^, tmmV

Figure 4. Modal variations of phenocryst proportions (normalized 100% lithic-free) with stratigraphie position in the tuffs of Moores Creek, Mount Jefferson, and Trail Canyon. Note overlap and continuation of trends in phenocryst percentages and proportions between the outflow Round Mountain member and intracaldera members of the tuff of Mount Jefferson. By contrast, the tuff of Moores Creek is characterized by relatively constant phenocryst percentages over thick stratigraphie intervals.

results in east or west-directed tilting. Intrusion of a 26.5-m.y.-old rhyolite to more passive lava effusion as more crystal-rich, less volatile-charged plug along the northern structural margin indicates that at least some magma welled up into the vent. The gradational contacts and similar post-collapse, ring-fracture volcanism occurred, but it is uncertain as to phenocrystic assemblages between flow-layered lava in the plug vents and whether there was general caldera resurgence or simply isolated igneous tuff of Mount Jefferson argue that they are oogenetic. A K-Ar age of 25.2 activity. ± 0.5 m.y. for one such plug (Table 2, no. 21) is only slightly less than that for the tuff of Mount Jefferson (see discussion below), which supports field Mount Jefferson Caldera and mineralogical relations that such plugs are genetically related to the tuff. Eruption of the tuff of Mount Jefferson resulted in collapse of the Along the north and east side of the Mount Jefferson caldera, the Mount Jefferson caldera. The Mount Jefferson caldera occupies the core bounding ring fault is exposed and is intruded by a small rhyolite plug. The of the Toquima caldera complex and appears to be eccentrically nested fault juxtaposes the lower tuff of Mount Jefferson on the south against the within the Moores Creek caldera (Fig. 2). The caldera measures ~ 15 by tuff of Moores Creek on the north (Fig. 2). Next to the ring fault, discon- 20 km, being slightly elongated to the northwest. tinuous tongues of coarse debris are entrained in the lower tuff of Mount The structural margin of the Mount Jefferson caldera is outlined by Jefferson. Some clasts are composed of the welded pumice lapilli tuff of seven small, aphyric to porphyritic plugs which measure no more than a Moores Creek. few hundred metres across and are ovoid to elongate in plan. Field, tex- In Jefferson Canyon, on the southwestern side of the Mount Jefferson tural, and mineralogical relations suggest that some of the plugs repre- caldera, the tuff of Mount Jefferson is juxtaposed against variably meta- sent lava-choked, ash-flow-tuff, feeder vents (Boden, unpub. data). Near an morphosed Paleozoic sedimentary rocks along the locally well-exposed inferred feeder vent, the tuff of Mount Jefferson contains both small (<~5 Jefferson Canyon fault. This fault was active at the time of caldera col- cm), pink fiamme and dark-colored, porphyritic pumice bombs. Closer to lapse, because landslide breccias in the tuff are more abundant and thicker the vent, the compaction foliation steepens, and the porphyritic pumice next to the fault than farther away. Although this fault consistently dips bombs become more abundant and stretched, and they ultimately coalesce 50° to 55° northeast at the surface, drill-hole data indicate that the dip into more or less continuous-flow layering in the core of the vent. Attend- lessens with depth to 40° to 45°. This listric geometry is interpreted to have ing this change, phenocrysts become larger and less broken. These rela- formed by slumping along the topographic wall. Similar faulting geometry tions appear to chronicle the transition from explosive pyroclastic venting is observed in the topographic walls of the Timber Mountain caldera in

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Rb(ppm) Sr(ppm) Zr(ppm) Ba(ppm) SiO, (wt.%) TiO,(wt.%) ~750m c Lava o>, c <0 Upper o Member

Lower o Member

3 T 200 300 50 550 100 150 200 250 200 700 1200 1700 65 70 75 80 0.10 0.30 1650m

c Upper o CO Member

3 Mountain Member

-500m

d> Pasco I O Canyon to CD k- Section, O J -200m ». fMoores ® Creek 3 Section

300 50 550 100 150 200 250 200 7001200 1700 65 70 75 80 0.10 0.30 0.50

Figure 5. Selected chemical variations with stratigraphic position in the tuffs of Moores Creek, Mount Jefferson, and Trail Canyon. Analyses are of same samples as used in Figure 4, plus an additional analysis from lower in the Pasco Canyon section of the tuff of Moores Creek. Note overlap and continuation of chemical trends between the Round Mountain and intracaldera members of the tuff of Mount

Jefferson. Chemical values determined by wavelength dispersive XRF analysis. Si02 and Ti02 normalized to 100% anhydrous.

Nevada (Byers and others, 1976) and of the Aso caldera in Japan densely welded rhyolitic ash-flow tuff that forms subdued outcrops on (Yokoyama, 1969). The structural margin in vicinity of Jefferson Canyon rounded, featureless hills. Scattered knobby outcrops generally mark the is believed marked by a crystal-poor, flow-layered rhyolite plug located occurrence of large, brecciated lithic blocks as much as several tens of about 1 km inboard of the Jefferson Canyon fault (Fig. 2). metres across. These clasts occur throughout the lower member but are Tuff of Mount Jefferson. The tuff of Mount Jefferson is largely largest and most abundant in the lower parts. Near the top of the lower restricted to within the Mount Jefferson caldera, where it makes up the member, discontinuous horizons of tuffaceous sandstone and landslide prominent massif of Mount Jefferson (Fig. 6). The tuff consists of lower mesobreccia are intercalated with moderately welded ash-flow tuff. In and upper intracaldera members and an outflow Round Mountain places, soft-sediment deformational structures (for example, sandstone member. dikes, flame structures, and convoluted folding) are present, suggesting Intracaldera Members. The lower and upper members are exposed these discontinuous sedimentary layers are waterlaid rather th£.n surge over -120 km2 and have a combined thickness of at least 2 km, yielding a deposits. minimum intracaldera volume of -250 km3. Comparing crystal content The overlying upper member, unlike the lower member, crops out of fiamme (-25 vol%) with that of the matrix (-35-40 vol%) indicates boldly and consists of devitrified, maroonish-brown, densely welded, that -30% of the ash was elutriated, which effectively increases the min- crystal-rich, rhyolitic to rhyodacitic ash-flow tuff. Its depositional thickness imum eruptive volume to -350 km3. The contact between the lower and increases from -800 m in the northern part of the caldera to at least 1,200 upper members is not well exposed but appears more or less conformable m in the southern part (Fig. 3, cross section B-B'). In contrast with the based on the regular map pattern and analytically indistinguishable K-Ar lower member, entrained breccia zones are quite sparse and ha ve been ages (Table 2, nos. 8,10). The intracaldera tuff has a weighted mean K-Ar found only in proximity to the Jefferson Canyon fault. The contact be- age of 26.4 ± 0.5 m.y. tween the upper and lower members is not well exposed, but it appears Gentle, west-southwest tilting along range-bounding faults has ex- more or less conformable on the basis of the regular map pattern of the posed the lowei member of the tuff of Mount Jefferson. Its maximum contact and the analytically indistinguishable K-Ar ages for the two exposed thickness is 1,300 m. The lower member consists of moderately to members (Table 2, nos. 8, 9).

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<5 cm across, typical of fiamme throughout the upper member. Relative to the smaller fiamme and crystal-shard matrix, the large porphyritic fiamme are impoverished in sanidine and quartz and enriched in plagioclase and ferromagnesian minerals. They also resemble fiamme near the lava- choked, ash-flow-tuff, feeder vents described above. Similar pumice bombs are exposed atop the Topopah Spring Member of the Paintbrush Tuff in the Nevada Test Site (Lipman and others, 1966) and in the upper part of Unit S of the Loma Seca Tuff in Chile (Hildreth and others, 1984). They are interpreted as samples of a degassed, less silicic zone of magma tapped near the end of an eruption. In some of the deeper grabens, a veneer of bedded tuffaceous sandstone, siltstone, and interbedded, poorly welded, lithic-lapilli ignimbrites overlies the sequence of welded cooling units. A K-Ar age on sanidine of 25.8 ± 0.5 m.y. from one of the intercalated, poorly welded ignimbrites (Table 2, no. 10) is analytically indistinguishable from the ages on the tuff Figure 6. View looking east-northeast across Round Mountain of Mount Jefferson. (foreground) to south summit of Mount Jefferson (3,640 m). The The intracaldera members are mineralogically and compositionally massively welded intracaldera upper member of the tuff of Mount zoned. Bulk-tuff crystal content increases uniformly upward from -18 Jefferson (Tju) has an exposed thickness in excess of 1 km in this view vol% in the lower member to slightly more than 40 vol% at the top of the and is capped locally by one of the upper cooling units (Tjc). The upper member (Fig. 4). Over this same interval, the plagioclase:sanidine outflow Round Mountain member (Tjr) is currently mined for Au and ratio increases from -1:1 to >4:1, and the mafic content increases by almost an order of magnitude. Apatite, zircon, and allanite occur in trace Ag. amounts throughout, although the incidence of allanite diminishes upward. Chemically, the intracaldera unit is zoned from high-silica rhyolite

(75.7% Si02) in the lower member to rhyodacite (67% SiC>2) at the top of the upper member (Fig. 5). The large porphyritic fiamme in the capping

cooling units of the upper member, at 65.3% Si02, represent the least silicic component erupted. Round Mountain Member. The Round Mountain member is restricted to the southwestern corner of the study area and is exposed over only -5 km2 (Fig. 2). Its original distribution may have been much greater, but rock units correlatable with the Round Mountain member have not been recognized in the neighboring Toiyabe Range (G. F. Brem, 1983, personal commun.). In the study area the Round Mountain member wedges out depositionally and erosionally eastward against pre-Tertiary rocks along the lower flanks of Shoshone Mountain and is buried by valley fill to the west. The outflow member takes its name from a partially silicified knob that is currently being mined for Au-Ag. Drill-hole data indicate that the member thickens to -500 m below the western end of the Round Mountain Mine. Several K-Ar ages on sanidine and biotite from vitrophyric and from devitrified, but unaltered, samples of the Round Figure 7. View of capping cooling units (Tjc) overlying upper Mountain member range from 27.3 ± 0.6 m.y. to 25.5 ± 0.6 m.y. (Table intracaldera member of the tuff of Mount Jefferson (Tju). East side of 2 2), with a weighted mean of 26.7 ± 0.6 m.y. south summit of Mount Jefferson forms high point on skyline. Cap- On Round Mountain, welding is zoned from poor at the base to ping cooling units are set off by light colored poorly welded basal dense in the middle to poor at the top. The upper, poorly welded zone was zones that grade upward into darker densely welded interiors. Exist- conformably capped, before recent mining, by a thin (< 10 m) sequence of ence of the capping cooling units helps to define a complex system of tuffaceous sedimentary rocks and intercalated ash-flow tuff and surge de- horsts and graben in the otherwise massive upper member. posits. In addition to welding zones, the Round Mountain member is zoned mineralogically and compositionally. From the lower, poorly welded to the central welded zone, whole-rock crystal content increases Grabens in the summit region of Mount Jefferson preserve several from -7 vol% to -40 vol% (Fig. 4). Crystal content of pumice, over the thin (10-15 m), discontinuous cooling units at the top of the upper same interval, ranges from nil to -15 vol%, suggesting that >50% of the member. The capping cooling units are set off by ledge-forming, light ash was elutriated. In contrast to the tuff of Moores Creek, its pumice colored, poorly welded zones (Fig. 7); where these are lacking, units are fiamme are small and uniform, averaging <2.0 cm. As in the intracaldera difficult or impossible to distinguish. Thin cooling units atop the otherwise members, apatite, zircon, and allanite are present in trace amounts. The massively welded upper member reflect longer time intervals between Round Mountain member ranges from high-silica rhyolite (77.5% Si02) in emplacement of successive ash flows of smaller volume during the flagging stages of a major eruption. In the welded interiors of the capping cooling 2One biotite age of 24.4 ± 0.6 m.y. is notably discordant with that of its units, dark colored porphyritic fiamme, measuring up to 30 cm across, sanidine mate (26.8 ± 0.5 m.y.; Table 2, no. 14). Consequently, the biotite age was occur in association with light colored, less crystal-rich fiamme, averaging omitted in calculating the weighted mean age.

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Figure 8. Schematic diagram illustrating eruptive/collapse his- minimal, but post-collapse ring-fracture and intracaldera volcanism and tory of the Mount Jefferson caldera. Rock patterns and symbols are intrusion are significant, the term "filled caldera" has been applied (Noble, same as those used in Figure 2. A. Initial eruptive outburst and deposi- 1968). Actually, the two types are surface manifestations of the same tion of the outflow Round Mountain member (Tjr); shortly thereafter, process (compare Bailey, 1976; and Marsh, 1984). Whether a caldera major caldera collapse occurred, ponding most of the eruptive prod- resurges by doming, by post-collapse igneous activity, or by broad regional ucts as the lower intracaldera member of the tuff of Mount Jefferson uplift depends on such factors as structural integrity of the caldera block, (Tjl). B. Eruption of the upper intracaldera member (Tju) into an viscosity of the magma, and volume and rate of cooling of the magma existing caldera inhibited outside dispersal without requiring appreci- system. As proposed by Elston (1984, p. 8744), the term "resurgence" able caldera collapse to promote ponding. C. During the waning should be perhaps redefined, "to shift emphasis from doming to its under- stages of eruption, several successive ash flows of small volume cap a lying cause, i.e., one or more magma surges that followed the initial southwestward-thickening prism of Tju. caldera-forming eruption." In this paper, the term "resurgence" encom- ^ passes structural uplift or doming and post-collapse igneous activity. Although some resurgence undoubtedly occurred in the Mount Jef- ferson caldera, strong resurgence is not evident, on the basis of the following.

the lower part to rhyolite (73.8% Si02) in the central, densely welded zone 1. Postcollapse igneous activity in the Mount Jefferson caldera was (Fig. 5). The upper, poorly welded zone is too altered for a reliable less than in well-described resurgent calderas in the Oligocene and Mio- chemical analysis. cene San Juan and Marysvale volcanic fields (Steven and Lipman, 1976; Stratigraphic Relations. Several lines of evidence indicate that the Steven and others, 1984). Considering the >2.5 km of exposed structural Round Mountain member is an outflow-facies equivalent of the intracal- relief at Mount Jefferson, it might be argued that the postcaldera extru- dera tuff of Mount Jefferson. (1) Trends in modal mineralogy and chemi- sions have been eroded. It is surprising, then, that no resurgent intrusions cal composition of the lower and upper members of the tuff of Mount or stocks, typical of other deeply dissected resurgent calderas, such as the Jefferson overlap and extend those developed in the Round Mountain Lake City and Grizzly Peak calderas (Lipman, 1976b; Fridrich and Ma- member (Figs. 4 and 5). (2) The intracaldera tuff of Mount Jefferson and hood, 1984, respectively), are exposed. Round Mountain member both contain <2-cm aphyric to crystal-poor 2. The lack of an angular discordance between 26.4 m.y.-old tuff of pumice and identical trace phases of apatite, zircon, and allanite. (3) K-Ar Mount Jefferson and 23- to 22-m.y.-old tuff of the Road Canyon indicates ages for lower and upper tuff members of Mount Jefferson and the Round that little or no tilting occurred between deposition of the two units. Uplift Mountain member are analytically indistinguishable. without tilting, that is, piston resurgence (Bonham and Noble, 1982), is Collapse History. Collapse of the Mount Jefferson caldera is illus- possible but does not appear to have been important based on the local trated schematically in Figure 8. In this scenario, the Round Mountain preservation of tuffaceous sedimentary rocks and poorly welded ash-flow member represents an early outflow-facies equivalent of the intracaldera tuff that conformably cap the tuff of Mount Jefferson inside the caldera. If lower tuff member of Mount Jefferson. Collapse of the intracauldron there had been significant resurgent uplift of any style, erosion would have block, however, appears to have occurred relatively early during eruption, quickly removed such a weakly indurated capping veneer, as occurred in because collapse breccias are restricted mainly to the lower member. Early the resurgent uplifts of the Creede (Steven and Ratte, 1965), Valles collapse and a low eruptive column (as suggested by the lack of coeval (Smith and others, 1970), Calabozos (Hildreth and others, 1984), and airfall tuff and apparent limited distribution of the Round Mountain Long Valley calderas (Bailey and others, 1976). member) may have inhibited outflow dispersal of all but the earliest erupted tuff. A pause in the eruption and collapse is suggested by the Trail Canyon Caldera presence of discontinuous horizons of tuffaceous sedimentary rocks near the top of the lower tuff member of Mount Jefferson. This pause, however, The 23.6-m.y.-old Trail Canyon caldera, measuring 8 by 10 km, is was brief as indicated by: (1) the interfingering nature and gradational the smallest of the three volcanic centers of the Toquima caldera complex. contacts between sedimentary horizons and the ash-flow tuff, (2) the con- It formed along the southeastern margin of the Mount Jefferson caldera tinuous compositional trends between lower and upper members of (Fig. 2) in response to eruption of the tuff of Trail Canyon. A conspicuous, Mount Jefferson tuff, and (3) the analytically indistinguishable K-Ar ages north- to northeast-trending arcuate fault scarp marks the western margin of the lower and upper members. of the Trail Canyon caldera (Fig. 9). Although no caldera-collapse brec- Additional collapse occurred during eruption of the upper member cias are exposed along this fault at current erosional levels, the greater based on entrained landslide breccias in the vicinity of the Jefferson Can- offset of the tuff of Trail Canyon (>2,600 m), relative to the overlying 23 yon fault and accumulation of 800 m to > 1 km of ash-flow tuff. Collapse to 22-m.y.-old tuff of Road Canyon (>1,400 m), indicates that the during eruption of the upper member appears to have been asymmetrical, fault was active during collapse. The fault's distinct arcuate trend, more- having been most pronounced in the southwestern part of the caldera, over, contrasts sharply with the orthogonal map patterns of younger where there are collapse breccias and the upper member is thickest. The basin-range faults. Renewed movement occurred in response to basin- presence of capping cooling units in both the north and south parts of the range extension, which reactivated the original caldera structure. caldera indicates that the variation in thickness of the upper member is Along the southern margin of the caldera, collapse breccias are ex- depositional (Fig. 3, cross section B-B'). posed where they overlie Paleozoic sedimentary rocks and older ash-flow Caldera Resurgence. Caldera resurgence, sensu stricto, describes tuff. Three small, porphyritic, flow-layered plugs mark an arcuate seg- structural uplift or doming of the intracaldera block as part of the caldera ment of the buried structural margin of the Trail Canyon caldera. All three cycle (Smith and Bailey, 1968). In those calderas where resurgence is plugs bear phenocrysts and trace minerals similar to the upper part of the

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tuff of Trail Canyon. One plug has a gradational contact with the tuff of Caldera Resurgence. Resurgence of the Trail Canyon caldera also Trail Canyon that results in similar textural relations as described for a appears to have been minimal. This conclusion is based on (1) the lack of lava-choked, ash-flow-tuff, feeder vent for the tuff of Mount Jefferson. angular discordance between the tuff of Trail Canyon and overlying 23- to One of the plugs yielded a K-Ar age of 21.7 ± 0.4 m.y. (Table 2, no. 20). 22-m.y.-old tuff of Road Canyon, (2) the small volume of consang uineous Tuff of Trail Canyon. The tuff of Trail Canyon has an exposed postcollapse lavas and plugs, and (3) the inwardly dipping attitude of thickness of as much as 1,200 m, but the base is not exposed. Present compaction foliation of the intracaldera tuff of Trail Canyon. exposures cover -60 km2, and no outflow facies remain exposed. The unit consists of a lower, crystal-poor member (400 m), an upper, more Buried Calderas crystal-rich member (750 m), and a megabreccia member (120 m); the last constitutes a marginal facies of the lower member. Abundant small pumice Gravity lows suggest thicker accumulations of volcanic rocls below fiamme are a characteristic feature of the lower member. The contact valley fill adjoining the Toquima caldera complex than farther north and between lower and upper members is locally marked by a bench-forming, south, where flanking ranges have thin covers of Tertiary volcanic rocks less-welded zone. A small, porphyritic lava flow locally caps the upper (Snyder and Healey, 1983). The more pronounced gravity lows are less member. likely to be caused by greater faulting and thicker valley fill than by Relative to the upper member, the lower member tuff contains fewer downfaulted and concealed thick sequences of low-density volcanic rocks (9%-18% versus 35%-40%), smaller (<1.5 mm versus <5 mm) crystals, (Snyder, 1983). These gravity patterns support the inference that the west- and lower mafic mineral content (-1% versus -5%) (Fig. 4). A marked ern and eastern sides of the Moores Creek caldera and source areas for the compositional break separates the lower and upper members (Fig. 5). The 32-m.y.-old megabreccia of Dry Canyon and the 27.7-m.y.-old tuff of local capping pcrphyritic lava is mineralogically identical and chemically Corcoran Canyon lie, in part, below valley fill flanking the Toquima very similar to the upper member, suggesting consanguinity (Figs. 4 caldera complex. and 5). The megabreccia contains blocks of silicified, welded, ash-flow tuff REGIONAL STRUCTURAL EVOLUTION and Paleozoic shale and limestone in a host of poorly welded, propylitized, pumiceous, ash-How tuff. The low degree of welding in the tuff matrix, in The Toquima Range has undergone a complex structural history that contrast to the densely welded nature of the lower member in the interior not only modified or obscured original caldera relations but, in part, may of the caldera, is attributed to the cooling effect of the abundant lithic also have influenced their development. During most of Tertiary and fragments. The blocks of ash-flow tuff can be correlated to the biotite-rich Quaternary time, erosion or nondeposition has dominated the rock record, upper part of the 27.7-m.y.-old tuff of Corcoran Canyon and the quartz- making precise determination of timing of deformational events difficult. rich 25.0-m.y.-old tuff of Ryecroft Canyon. Lithic blocks are up to several Nonetheless, field relations and K-Ar ages of volcanic units from the field metres across and are commonly brecciated and silicified, whereas the host area and from other parts of central Nevada allow a synoptic assessment of tuff is not. regional structural development. The tuff of Trail Canyon has a weighted mean K-Ar age of 23.6 ± 0.4 m.y. (Table 2). Pre-Basin-Range Faulting

High-Angle Faults. Mapping in the Toquima caldera complex and elsewhere in central Nevada has disclosed a west- to northwest-trending structural grain of probable early to middle Tertiary age (Ekren and others, 1973a, 1973b; Quinlivan and Rogers, 1974; Quinlivan and others, 1974; Gardner and others, 1980). By contrast, late Tertiary and Quater- nary basin-range structure has a north to north-northeast trend. The older structural grain is typically manifested by high-angle faults and other features, including: (1) west-northwest-trending troughs filled with volcanic rocks (Burke and McKee, 1979; Brem and others, 1985); (2) northwestern alignment of regional structural and physiographic features, such as the Toiyabe-Kawich lineament (Fig. 1) (Kleinhampl and Ziony, 1985); (3) west- to northwest-trending aeromagnetic and gravity anomalies (Ekren and others, 1976; Snyder and Healey, 1983); (4) northwest-elongate shape of some ash-flow-tuff eruptive centers (Brem and Snyder, 1983); and (5) west-northwest trend of the entire volcanic belt extending across central Nevada (Stewart and others, 1977). Around the Toquima caldera complex, north west-trending structures Figure 9. View looking north along western margin of Trail Can- are reflected by high-angle mineralized fault zones at Round Mountain yon caldera. Flat-topped peak on skyline is south summit of Mount (Mills, 1984; Sander and Milk, 1984; Tingley and Berger, 1985; Mills and Jefferson. Qf is fanglomerate and Qc is colluvium; all other rock Sander, in press), and by the over-all northwest-elongate shape of the symbols are same as those in Figure 2. Note bounding fault of Trail Toquima caldera complex. In the Round Mountain mine, vein acularia in Canyon caldera also cuts overlying tuffs of Road Canyon and is northwest-striking faults, that cut the Round Mountain member cf the tuff marked by a prominent fault scarp cut into the upper member of the of Mount Jefferson, has a K-Ar age of 26.5 ± 0.5 m.y. (Boden and M. V. tuff of Mount J efferson (Tju). Sander, unpub. data). Some of the faults also show evidence for post-ore

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movement (M. V. Sander, 1984, personal commun.). Based on these field compositional relationships, the tuffs of Moores Creek and Mount Jeffer- and age relations, the northwest-striking faults commenced or resumed, in son are separate eruptive units, even though their K-Ar ages overlap at the part, synchronously with formation of the Toquima caldera complex, and 95% confidence level. The close spatial and temporal association of the as such, they may have facilitated the rise and eruption of magma. Moores Creek and Mount Jefferson calderas, however, suggests that they Some of the northwest-striking faults have sustained strike-slip formed above a common magma reservoir. Eruption of the tuff of Mount movement, indicated by the gently plunging axes of mullions on fault Jefferson depleted the reservoir of high-silica rhyolite magma and tapped surfaces. Large (2- to 3-m) wavelength mullions are especially well ex- deeper, less silicic levels than apparently reached during eruption of the posed along surfaces of west-northwest- to northwest-striking faults in the tuff of Moores Creek. The comparatively small Trail Canyon caldera Round Mountain open pit, where the sense of movement is right lateral. In formed -23.6 Ma along the southeastern side of the Mount Jefferson the Reveille and Hot Creek Ranges (Fig. 1), northwesterly strike-slip caldera. Eruption of the tuff of Trail Canyon reflects either a final gasp of faulting, having apparent left-lateral offset, occurred between 26 and the Moores Creek/Mount Jefferson magmatic system or the rise and 18.5 Ma (Ekren and others, 1973b; Quinlivan and Rogers, 1974). development of a separate magmatic reservoir along the southeast side of Low-Angle Faults. Tertiary low-angle normal faults with steep stra- the former system. The latter interpretation is preferred, considering the tal rotation, as described in other parts of Nevada (Anderson, 1971; Prof- 3-m.y. eruptive hiatus and distinct compositional trends between the tuffs fett, 1977; Gans and Miller, 1983), are neither evident in the study area of Mount Jefferson and Trail Canyon. In any case, the volcanic focus nor in the adjoining southern Toiyabe Range (G. F. Brem, 1983, personal shifted progressively southeastward, and caldera size diminished with time. commun.). In the Toquima caldera complex, most dips of compaction This southeastward migration of volcanism subparallels regional west- foliation are <25°. Where they are steeper, they can be related to either (a) northwest- to northwest-striking faults, suggesting fundamental structural localized syndepositional slumping, mainly near caldera margins; (b) bank- control in the rise and eruption of magma. This interpretation is corrobo- ing against old hillsides or compaction around large clasts; (c) drag along rated by noting that the northwest-trending margin of the Toquima caldera young basin-range faults; or (d) recent slumping from soil creep or frost complex lies along the projection of a major northwesterly strike-slip fault heave on steep slopes. Low-angle faults without stratal rotation, mapped in system mapped to the southeast in the Hot Creek and Reveille Ranges the Manhattan caldera (Shawe, 1981b) and in the northern Reveille (Quinlivan and Rogers, 1974; Ekren and others, 1973b; respectively). Range (Ekren and others, 1973b), probably reflect gravity slides rather Regional west-northwest-trending fault control in the development of cal- than rapid crustal extension. deras is also evident for volcanic centers in the southern Toiyabe and Shoshone Ranges (Brem and others, 1985) as well as farther west in the Desatoya and Clan Alpine Mountains and Stillwater Range (Burke and Basin and Range Faulting McKee, 1979). Age determinations, however, are too few in these areas to document any systematic spatial changes of volcanism with time. The north- to northeast-trending corrugated topography of central Nevada owes its origin to young basin-range block faulting. Mapping and Basin-range-style block faulting has subsequently disrupted the To- K-Ar dating in the Toquima caldera complex indicate that this style of quima caldera complex. The east and west margins of the Moores Creek faulting began after 22 Ma, as shown by uniform thicknesses and remark- caldera have been truncated along range-bounding normal faults, and the ably widespread distributions of the 23-m.y.-old Bates Mountain Tuff ring faults of the Mount Jefferson and Trail Canyon calderas have been (>8,000 km2) and 22-m.y.-old tuff of Clipper Gap (>6,000 km2) reactivated in part by this later faulting. A principal result of this later (Gramme and others, 1972). In the southern and western parts of Nevada, faulting is the uplift and gentle west-southwest tilting of the Mount Jeffer- the onset of basin-range block faulting can be bracketed more tightly; it son caldera, producing the prominent Mount Jefferson massif. occurred between ~7 and 11 Ma (Ekren and others, 1968; Novak, 1984; Caldera resurgence and post-collapse igneous activity are not strongly Gilbert and Reynolds, 1973; Proffett, 1977; Proffett and Dilles, 1984). developed in the Toquima volcanic complex. In the Mount Jefferson Faceted ridge spurs, fault scarps in unconsolidated alluvium, and the caldera, limited resurgence may be a consequence of ponding the bulk of broad, high interfluves and divides atop Mount Jefferson indicate that erupted material within the caldera. If so, the impetus for resurgence is differential uplift and tilting of the Toquima caldera complex are products minimized because isostatic equilibrium of the crustal column is not ap- of recent and ongoing tectonic processes. The present high-standing massif preciably perturbed. Ponding the majority of erupted material at its source of Mount Jefferson is largely a result of recent basin-range tectonism in would be favored by early collapse and a low eruptive column. Evidence which caldera structures were locally reactivated several million years after for such eruptive characteristics, albeit only permissive, includes the fol- volcanism had ceased. The dramatic effects of young basin-range faulting lowing: (1) collapse breccias are restricted mainly to the lower member of can give the spurious impression that the Toquima volcanic complex, in the tuff of Mount Jefferson; (2) only the most silicic fraction of the caldera- particular the Mount Jefferson caldera, is strongly resurgent. A similar forming eruption, that is, the Round Mountain member, is exposed outside situation is evident in the Emory caldera in southwest New Mexico, in the caldera; and (3) oogenetic air-fall tuff is absent in the study area and which at least 75% of the central uplift can be attributed to basin-range has yet to be recognized in adjoining ranges. faulting (Elston, 1984, p. 8743). For smaller calderas, such as the Trail Canyon caldera, resurgence may be limited because the rate of cooling and solidification of the magma SUMMARY AND DISCUSSION reservoir exceeds the rate of isostatic compensation. Marsh (1984) concluded that the magma reservoirs below calderas < 10 to 12 km in diame- Volcanic activity in the Toquima complex began about 27.2 Ma with ter would cool too quickly, barring new additions of heat into the roots of eruption of the high-silica rhyolite tuff of Moores Creek and formation of the system, for them to respond to crustal isostatic adjustments or other the Moores Creek caldera. About 0.6 m.y. later, collapse of the nested resurgence-inducing mechanisms. Instead, resurgence would be distributed Mount Jefferson caldera occurred in response to eruption of the high-silica over a broad area making recognition difficult, especially in faulted and rhyolite to rhyodacite tuff of Mount Jefferson. Based on field, textural, and eroded terrain as is the Basin and Range province.

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Ferguson, H. G., 1921, The Round Mountain district, Nevada: U.S. Geological Survey Bulletin 725-1, p. 383-406. Premonitory and coeval intermediate to mafic volcanism is lacking in Ferguson, H. G., and Cathcart, S. H., 1954, Geology of the Round Mountain quadrangle, Nevada: U.S. Geological Survey the Toquima caldera complex and minimal elsewhere in the central Ne- Geological Quadrangle Map GQ-49, scale 1:125,000. Fridrich, C. J., and Mahood, G. A., 1984, Reverse zoning in the resurgent intrusions of the Grizzly Peal cauldron, vada volcanic belt. This contrasts with some well-described, caldera- Sawatcb Range, Colorado: Geological Society of America Bulletin, v. 95, p. 779-787. Gans, P. B., and Miller, E. L, 1983, Style of mid-Tertiary extension in east-central Nevada: Utah Geological aid Mineral bearing volcanic fields where intermediate to mafic volcanic rocks Survey Special Studies, Guidebook, Part 1, v. 59, p. 107-160. constitute the bulk of rocks erupted, for example, the San Juan, Challis, Gardner, J. N., Eddy, A. C., Goff, F. E., and Grafft, K. S., 1980, Reconnaissance geologic map of the northern Kawich and southern Reveille Ranges, Nye County, Nevada: Los Alamos Scientific Laboratory Map LA-839G. and Marysvale volcanic fields (Steven and Lipman, 1976; Ekren, 1981, Gilbert, C. M., and Reynolds, M. W., 1973, Character and chronology of basin development, western margin of the Basin and Hardyman, 1981; and Steven and others, 1984; respectively). In cen- and Range province: Geological Society of America Bulletin, v. 84, p. 2489-2510. Grommi, C. S., McKee, E. H., and Blake, M. C,, 1972, Paleomagnetic correlation and potassium-argon datin j of middle tral Nevada, however, forerunning and coeval more mafic magma appar- Tertiary ash-flow sheets in the eastern Great Basin, Nevada and Utah: Geological Society of Ameria Bulletin, v. 83, p. 138. ently became lodged in the lower crust below an extensive zone of silicic Hardyman, R. F., 1981, Twin Peaks caldera of central Idaho: Montana Geological Society 1981 Field Conference on magma. Early eruption of intermediate to mafic magma may have been Southwest Montana: Montana Geological Society, p. 318-322. Harrington, R. J., 1983, Stratigraphy of the northern Mt. Jefferson volcanic center, south-central Nevada: Geological inhibited by an unfavorable crustal stress regime or perhaps by an insuffi- Society of America Abstracts with Programs, v. 15, p. 281. Hildreth, W., Grunder, A. L., and Drake, R. E., 1984, The Loma Seca Tuff and the Calabozos caldera: A major ash flow cient density conlrast between wall rocks and magma to foster diapiric and caldera complex in the southern Andes of central Chile: Geological Society of America Bulletin, v. 95, rise. In any case, eruption of voluminous intermediate to mafic volcanic p. 45-54. Kleinhampl, F. J., and Ziony, J. I., 1985, Geology of northern Nye County, Nevada: Nevada Bureau of Mines and rocks does not n<;cessarily have to precede or attend eruption of silicic Geology Bulletin 99A. 172 p. Krai, V. E., 1951, Mineral resources of Nye County, Nevada: Geology and Mining Series No. 50, Universitj of Nevada ash-flow tuffs or collapse of calderas, although copious intrusion of inter- Bulletin, v. 45, n. 3,220 p. Lambert, M. B., 1974, The Bennett Lake cauldron subsidence complex, British Columbia and Yukon Territo y: Geologi- mediate to mafic magma into the lower and middle crust is required. cal Survey Canada Bulletin 227, 213 p. Upman, P. W., 1975, Evolution of the Platoro caldera complex and related volcanic rocks, southeastern San Juan Mountains, Colorado: U.S. Geological Survey Professional Paper 852, 128 p. ACKNOWLEDGMENT S 1976a, Caldera-collapse breccias in the western San Juan Mountains, Colorado: Geological Society of American Bulletin, v. 87, p. 1397-1410. 1976b, Geologic map of the Lake City caldera area, western San Juan Mountains, Colorado: U.S. Geological Survey Miscellaneous Geological Investigation Map 1-962, scale 1:48,000. Trenchant reviews by C. Bacon, M. Best, M. Einaudi, W. Elston, Lipman, P. W., Christiansen, R. L., and O'Connor, T. J., 1966, A compositionally zoned ash-flow sheet in southern A. Grunder, G. Mahood, B. Mills, S. Novak, and M. Sander led to Nevada: U.S. Geological Survey Professional Paper 524-F, 47 p. Mahood, G. A., and Drake, R. E., 1982, K-Ar dating young rhyolittc rocks: A case study of the Sierra Li Prima vera, considerable improvement in the form and scope of this paper and are Jalisco, Mexico: Geological Society of America Bulletin, v. 93, p. 1232-1241. Marsh, B., 1984, On the mechanics of resurgence: Journal of Geophysical Research, v. 89, B10, p. 8245-82M. deeply appreciated. Discussions on the outcrop with G. Brem, P. Dobson, Marvin, R. F., Mehnert, H. H., and McKee, E. H., 1973, A summary of radiometric ages of Tertiary volcanic rocks in M. Einaudi, C. Fridrich, R. Hardyman, R. Harrington, R. Jeanne, D. Nevada and eastern California, Part III—Southeastern Nevada: Isochron/West, no. 6, p. 1-30. McKee, E. H., 1974, Northumberland caldera and Northumberland Tuff, in Guidebook to the geology of bur Tertiary Jones, G. Mahood, B. Mills, S. Novak, M. Sander, D. Shawe, and B. Veek volcanic centers in central Nevada: Nevada Bureau of Mines and Geology Report 19, p. 35-41. Mills, B. A., 1984, Geology of the Round Mountain gold deposit, Nye county, Nevada, in Wilkins, J., Jr., el., Gold and were most helpful. Many thanks are extended to E. H. McKee and M. A. silver deposits of the basin and range province, western U.S.: Arizona Geological Society Digest, v. 15, p. 89-99. Lanphere for providing access to the K-Ar Lab in the Branch of Isotope Mills, B. A., and Sander, M. V., Preliminary model for the geologic evolution of the Round Mountain gold ieposit, Nye County, Nevada: Engineering and Mining Journal (in press). Geology of the U.S. Geological Survey and to M. Pringle, J. VonEssen, Noble, D. C., 1968, Kane Springs Wash volcanic center, Lincoln County, Nevada, in Eckel, E. B., ed., Nevada Test Site: Geological Society of America Memoir 110, p. 109-116. and J. Saburoma::u for lending me Ar spikes and for their kind instruction Novak, S. W., 1984, Eruptive history of the rhyolitic Kane Springs Wash volcanic center: Journal of Geophysical in the use of the Ar extraction lines and mass spectrometer. J. Metz Research, v. 89, B10, p. 8603-8615. Proffett, J. M., Jr., 1977, Cenozoic geology of the Yerington district, Nevada, and implications for nature uid origin of deserves special thanks for patiently teaching me the fundamentals of Ar basin and range faulting: Geological Society of America Bulletin, v. 88, p. 247-266. Proffett, J. M., Jr., and Dilles, J. H., 1984, Geologic map of the Yerington district, Nevada: Nevada Bureau of Mines and extraction and glass blowing. I thank M. Grunder for assistance with Geology Map 77. drafting and typing. This study would not have been possible without the Quinlivan, W. D., and Rogers, C. L., 1974, Geologic map of the Rybo quadrangle, Nye County, Nevada: U.S. Geological Survey Miscellaneous Geological Investigations Map 1-821, scale 1:48,000. generous suppori; provided by CR Exploration Co., Round Mountain, Quinlivan, W. D., Rogers, C. L., and Dodge, H. W., Jr., 1974, Geologic map of the Portuguese Mountain quadrangle, Nye County, Nevada: U.S. Geological Survey Investigations Map 1-804, scale 1:48,000. Nevada. Ransome, F. L, 1909, Round Mountain, Nevada: U.S. Geological Survey Bulletin 380, p. 44-47. Sander, M. V., and Mills, B. A., 1984, The Round Mountain gold-silver mine, Nye County, Nevada, in Liniz, J., Jr., ed., Western geological excursions, field guide, v. 3: Geological Society of America Annual Meeting, Reno, p. 176-180. REFERENCES CITED Sargent, K. A., and McKee, E. H., 1969, The Bates Mountain Tuff in northern Nye County, Nevada: U.S. Geological Anderson, R. E., 1971, Thin-skin distension in Tertiary rocks of southeastern Nevada: Geological Society of America Survey Bulletin 1294-E, p. E1-E12. Bulletin, v. 82, p. 43-58. Shawe, D. R., 1981a, Geologic map of the Round Mountain quadrangle, Nye County, Nevada: U.S. Geological Survey Bacon, C. R., 1983, Erupiive history of Mt. M&zama and Crater Lake caldera. Cascade Range, USA: Journal of Open-File Report 81-515. Volcanological and Geothermal Research, v. 18, p. 57-116. 1981b, Geologic map of the Manhattan quadrangle, Nye County, Nevada: U.S. Geological Survey Open-File Bailey, R. A., 1976, On (he mechanisms of postsubsidence central doming and volcanism in resurgent cauldrons: Report 81-516. Geological Society of America Abstracts with Programs, v. 8, p. 567. Silberman, M. L., Shawe, D. R., Koski, R. A., andGoddard, B. B., 1975, K-Ar ages of mineralization at Round Mountain Bailey, R. A., Dalrymple, G. B., and Lanphere, M. A., 1976, Volcanism, structure and geochronology of the Long Valley and Manhattan, Nye County, Nevada: Isochron/West, v. 13, p. 1-2. caldera. Mono County, California: Journal of Geophysical Research, v. 81, p. 725-744. Smith, R. L., and Bailey, R. A., 1968, Resurgent cauldrons, in Coats, R. R., and others, eds., Studies in volcanoiogy: Baker, M.C.W., 1981, The nature and distribution of Upper Cenozoic ignimbrite centers in the central Andes: Journal of Geological Society of America Memoir 116, p. 613-662. Volcanological and Geothermal Research, v. 11, p. 293-315. Smith, R. L., Bailey, R. A., and Ross, C. S., 1970, Geologic map of the Jemez Mountains, New Mexico: U.S. Geological Bonham, H. F., Jr., and Noble, D. C., 1982, A new type of resurgent caldera: Geological Society of America Abstracts Survey Miscellaneous Investigation Series, Map 1-571, scale 1:125,000. with Programs, v, 14, p. 150. Snyder, D. B., 1983, Proposed caldera structures in central Nevada inferred from gravity lows: Geological Society of Brem, G. F., and Snyder, D. B., 1983, Lithologic and gravity characteristics of the southern Pea vine volcanic center, America Abstracts with Programs, v. IS, p. 383. Toiyabe Range, Nevada: Geological Society of America Abstracts with Programs, v. IS, p. 280. Snyder, D. B., and Healey, D. L., 1983, Interpretation of the Bouguer gravity map of the Tonopah sheet: N svada Bureau Brem, G. F., Purdy, T. L., and Snyder, D. B., 1985, Oligocene and Miocene volcanic-tectonic history of the southern of Mines and Geology Report 38, 14 p. Toiyabe and Shoshone Mountains, Nevada: Geological Society of America Abstracts with Programs, v. 17, p. 343. Steven, T. A., and Lipman, P. W., 1976, Calderas of the San Juan volcanic field, southwestern Colorado: U S. Geological Burke, D. B., and McKee, E. H., 1979, Mid-Cenozoic volcano-tectonic troughs in central Nevada: Geological Society of Survey Professional Paper 958, 35 p. America Bulletin, v 90, p. 181-184. Steven, T. A., and Ratte, J. C., 1965, Geology and structural control of ore deposition in the Creede district, San Juan Byers, F. M., Jr., Carr, W J., Orkild, P. P., Quinlivan, W. D„ and Sargent, K. A., 1976, Volcanic suites and related Mountains, Colorado: U.S. Geological Survey Professional Paper 487, 87 p. cauldrons of Timber Mountain-Oasis Valley caldera complex, southern Nevada: U.S. Geological Survey Profes- Steven, T. A., Rowley, P. D., and Cunningham, C. G., 1984, Calderas of the Marysvale volcanic field, west central Utah: sional Paper 919,70 p. Journal of Geophysical Research, v. 89, B10, p. 8751-8764. Christiansen, R. L., 1979, Cooling units and composite sheets in relation to caldera structure, in Chapin, C. E., and Elston, Stewart, J. H., and Carlson, J. E., 1978, Geologic map of Nevada: U.S. Geological Survey State Map Series, scale W. E., eds., Ash-flow tuffs: Geological Society of America Special Paper 180, p. 29-42. 1:500,000. Clabaugh, S. E., 1979, Igrimbrites of the Sierra Madre Occidental and their relation to the tectonic history of western Stewart, J. H., Moore, W. J., and Zietz, I., 1977, East-west patterns of Cenozoic igneous rocks, aeromagneic anomalies, Mexico, in Chapin. C. E., and Elston, W. E., eds., Ash-flow tuffs: Geological Society of America Special Paper and mineral deposits, Nevada and Utah: Geological Society of America Bulletin, v. 88, p. 67-77. 180, p. 113-124. Swanson, E. R., and McDowell, F. W., 1984, Calderas of the Sierra Madre Occidental volcanic field, western Mexico: Ekren, E. B., 1981, Van Horn Peak—A welded tuff vent in central Idaho, Montana Geological Society 1981 Field Journal of Geophysical Research, v. 89, n. B10, p. 8787-8800. Conference on Southwest Montana: Montana Geological Society, p. 311-315. Tingley, J. V., and Berger, B. R., 1985, Lode gold deposits of Round Mountain, NV: Nevada Bureau of Mines and Ekren, E. B., Rogers, C. L., Anderson, R. E., and Orkild, P. P., 1968, Age of basin and range normal faults in Nevada Test Geology Bulletin 100,62 p. Site and Nellis Air Force Range, Nevada, in Eckel, E. B., ed., Nevada Test Site: Geological Society of America Yokoyama, I., 1969, Gravimetric studies and test drillings at three calderas in Japan: Associazione Geofisica Italiana Atti Memoir 110, p. 247-250. Convenzione Annual, v. 18, p. 659-671. Ekren, E. B., Hinrichs, E N., Quinlivan, W. D„ and Hoover, D. L., 1973a, Geologic map of the Moores Station quadrangle, Nye County, Nevada: U.S. Geological Survey Miscellaneous Geological Investigations Map 1-756, scale 1:48,000. Ekren, E. B., Rogers, C. L, and Dixon, G. I., 1973b, Geologic and Bouguer gravity map of the Reveille quadrangle, Nye County, Nevada: L .S. Geological Survey Miscellaneous Geological Investigation Map I>806,1:48,000. MANUSCRIPT RECEIVED BY THE SOCIETY APRIL 8,1985 Elston, W. E., 1984, Mid-Tertiary ash-flow tuff cauldrons, southwestern New Mexico: Journal of Geophysical Research, REVISED MANUSCRIPT RECEIVED JULY 23,1985 v. 89, B10, p. 8732-8750. MANUSCRIPT ACCEPTED JULY 25,1985

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