Episodic volcanism in the Miocene southwestern Nevada volcanic field: Revised stratigraphie framework, 40Ar/39Ar geochronology, and implications for magmatism and extension

DAVID A. SAWYER U.S. Geological Survey, M.S. 913, Denver, 80225 R J FLECK 1 M A LANPHERE J U.S. Geological Survey, M.S. 937, 345 Middlefield Road, Menlo Park, 94025 R. G. WARREN j" EES-1, M.S. D462, Los Alamos National Laboratory, Los Alamos, 87545 D. E. BROXTON MARK R. HUDSON U.S. Geological Survey, M.S. 913, Denver, Colorado 80225

ABSTRACT the SWNVF occurred during eruption of the ic time. Volcanism and extension in the Paintbrush and Timber Mountain Groups, SWNVF area were broadly concurrent, but The middle Miocene southwestern Ne- when over 4500 km3 of metaluminous in detail they were episodic in time and not vada volcanic field (SWNVF) is a classic ex- magma was erupted in two episodes within coincident in space. ample of a silicic multicaldera volcanic field 1.35 m.y., separated by a 750 k.y. magmatic in the Great Basin. More than six major gap. Peralkaline and metaluminous mag- INTRODUCTION formed between >15 and 7.5 Ma. matism in the SWNVF overlapped in time The central SWNVF caldera cluster consists and space. The peralkaline Tub Spring and The southwestern Nevada volcanic field of the overlapping Silent Canyon caldera Grouse Canyon Tuffs erupted early, and the (SWNVF) is an outstanding example of a complex, the Claim Canyon caldera, and the peralkaline Thirsty Canyon Group tuffs and multicaldera silicic volcanic field. The Timber Mountain caldera complex, active Stonewall Flat erupted late in the his- SWNVF has been the focus of intensive sci- from 14 to 11.5 Ma and centered on topo- tory of the SWNVF, flanking the central, entific study over the past 35 yr, largely as a graphic Timber Mountain. Locations of volumetrically dominant peak of metalumi- result of underground nuclear testing at the calderas older than the Claim Canyon nous volcanism. Magma chemistry transi- Nevada Test Site (NTS) and nuclear-waste caldera source of the Tiva Canyon Tuff are tional between peralkaline and metalumi- repository studies at Yucca Mountain. It is uncertain except where verified by drilling. nous magmas is indicated by petrographic well exposed due to Basin and Range fault- Younger peralkaline calderas (Black Moun- and chemical data, particularly in the ing and dissection in an arid environment, tain and Stonewall Mountain) formed overlapping Grouse Canyon and Area 20 and it is well known geometrically due to northwest of the central SWNVF caldera calderas of the Silent Canyon caldera hundreds of deep drill holes (>600 m cluster. We summarize major revisions of complex. depth). The history of geologic investiga- the SWNVF stratigraphy that provide for Volcanism in the SWNVF coincided with tions of the volcanic field was recently sum- correlation of flows and small-volume the Miocene peak of extensional deforma- marized by Byers and others (1989). Geo- tuffs with the widespread outflow sheets of tion in adjoining parts of the Great Basin. chronology of the volcanic rocks of the field the SWNVF. Although regional extension was concurrent was first determined by K-Ar methods (Kist- New laser fusion 40Ar/39Ar isotopic ages with volcanism, it was at a minimum in the ler, 1968). We report initial results of a new central area of the SWNVF, where synvol- generation of isotopic ages determined by are used to refine and revise the timing of 40 39 eruptive activity in the SWNVF. The use of canic faulting was dominated by intra- the laser-fusion Ar/ Ar method (Dal- high-sensitivity mass spectrometry allowed caldera deformation. Significant stratal tilt- rymple and Duffield, 1988) for the principal analysis of submilligram-sized samples ing and paleomagnetically determined SWNVF stratigraphic units, and we use with analytical uncertainties of ~0.3% (la), dextral shear affected the southwestern these results to confirm significant revisions permitting resolution of age differences as margin of the SWNVF between the Paint- in the stratigraphic framework of the small as 0.07 Ma. These results confirm the brush and Timber Mountain caldera epi- SWNVF and precisely define episodes of revised stratigraphic succession and docu- sodes. Larger magnitude detachment fault- major eruptive activity. Source calderas for ment a pattern of episodic volcanism in the ing in the Bullfrog Hills, southwest of the the major ash-flow sheets are described for SWNVF. Major caldera episodes (Belted central SWNVF caldera cluster, followed calderas that are well known on the basis of Range, Crater Flat, Paintbrush, Timber the climactic Timber Mountain caldera ep- outcrop or drill-hole data. Finally, we sum- Mountain, and Thirsty Canyon Groups) isode. Postvolcanic normal faulting was marize the implications of the high-preci- erupted widespread ash-flow sheets within substantial to the north, east, and south of sion ages for the magmatic evolution of the 100-300 k.y. time spans, and pre- and post- the central SWNVF caldera cluster, but the SWNVF, and the temporal and spatial dis- caldera erupted within 100-300 k.y. of central area of peak volcanic activity re- tribution of extensional deformation rela- the associated ash flows. Peak volcanism in mained relatively unextended in postvolcan- tive to SWNVF magmatism.

Geological Society of America Bulletin, v. 106, p. 1304-1318, 6 figs., 3 tables, October 1994.

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TABLE 1. SUMMARY OF MAJOR STRATIGRAPHIC UNITS OF THE SOUTHWESTERN NEVADA VOLCANIC FIELD

Assemblage Current name Age Estimated Old Volcanic center symbol (Ma) erupted magma (previous usage) volumes (km3)

Ts Stonewall Flat Tuff 125 Stonewall Mountain volcanic center Civet Cat Canyon Member 40 Spearhead Member 80 Tt Thirsty Canyon Group 300 Thirsty Canyon Tuff Black Mountain caldera Gold Flat Tuff 20 Gold Flat Member Trail Ridge Tuff 50 Trail Ridge Member Pahute Mesa Tuff 100 Pahute Mesa Member Rocket Wash Tuff 9.4 100 Rocket Wash Member Tf Fortymile Canyon assemblage 140 Diverse vent areas Beatty Wash Formation* 110 of Beatty Wash Tm Timber Mountain Group 2275 Timber Mountain Tuff Timber Mountain Caldera Complex Ammonia Tanks Tuff 11.45 900 Ammonia Tanks member Ammonia Tanks caldera Rainier Mesa Tuff 11.6 1200 Rainier Mesa Member Rainier Mesa caldera Tp Rhyolite of the Loop 12.5 40 Paintbrush Group 2270 Paintbrush Tuff Tiva Canyon Tuff 12.7 1000 Tiva Canyon Member Claim Canyon caldera Yucca Mountain Tuff 25 Yucca Mountain Member Claim Canyon caldera? Pah Canyon Tuff 35 Pah Canyon Member Uncertain Topopah Spring Tuff 12.8 1200 Topopah Spring Member Uncertain Ta Calico Hills Formation* 12.9 160 Tuffs and lavas of Calico Hills, Area 20 Tw Wahmonie Formation 13.0 90 Wahmonie Tc Crater Flat Group 880 Crater Flat Tuff Silent Canyon caldera complex Prow Pass Tuff 45 Prow Pass Member Bullfrog Tuff 13.25 650 Bullfrog Member (and Uncertain Stockade (Wash Tuff) Area 20 caldera* Tram Tuff 170 Tram Member Prospector Pass caldera complex? Tb Belted Range Group 350 Belted Range Tuff Dead Horse Flat Fm.* 13.5 120 Tuff and lava of Dead Horse Grouse Canyon Tuff 13.7 Flat/volcanics of Saucer Mesa bedded member 210 Grouse Canyon Member Grouse Canyon calderat Comendite of Split Ridge 13.85 Tunnel bed 5 20 Rhyolite of Split Ridge Tr Lithic Ridge Tuff 14.0 250 Uncertain Lava of Tram Ridge 14.0 60 Quartz lava and unit C tuff Tn Tunnel Formation* 50 Tunnel beds 3 and 4 Tu Tub Spring Tuff 14.9 130 Tub Spring Member Uncertain To Tuff of Yucca Flat 15.1 50 Uncertain Redrock Valley Tuff 15.25 360 Uncertain

Note: No cntiy in "Old" column indicates that current usage is unchanged. *Newly defined with formai stratigraphie name. ^Th e Area 20 and Grouse Canyon calderas only compose the Silent Canyon caldera complcx.

REVISED SWNVF STRATIGRAPHIC flows and nonwelded tuffs are associated are described next in older to younger se- FRAMEWORK with their regional welded ash-flow tuff quence and related to their respective vol- sheets. Second, these well-defined, region- canic centers (Table 1). Many units of the SWNVF were system- ally correlative ash-flow sheets (members of Major ash-flow sheets of the SWNVF atically described by Byers and others the former formations) are raised to forma- erupted predominantly from calderas (Ta- (1976a), who discussed in detail only the vol- tion rank and given the rank term Tuff. For- ble 1); lava flows and minor pyroclastic de- canic rocks they interpreted to be related to mation rank units are the Tub Spring (now posits erupted from numerous smaller the Timber Mountain-Oasis Valley caldera separated from the Belted Range Group), vents. General consensus exists about the complex. Older metaluminous stratigraphie Grouse Canyon, Tram, Bullfrog, Prow Pass, location and approximate geometry of the Units were described in Carr and others Topopah Spring, Pah Canyon, Yucca Silent Canyon caldera complex, the Claim (1986), but no comprehensive stratigraphie Mountain, Tiva Canyon, Rainier Mesa, Am- Canyon caldera, the Timber Mountain discussion of the older and younger peral- monia Tanks, Rocket Wash, Pahute Mesa, caldera complex, and the Black Mountain kaline units of the SWNVF and their rela- Trail Ridge, and Gold Flat Tuffs. Civet Cat caldera (Fig. 1). Sources for units older tion to the metaluminous units has been Canyon and Spearhead Members of the than the Tiva Canyon Tuff are completely published. The revised stratigraphy of the Stonewall Flat Tuff, Wahmonie Formation, buried and are documented only where re- SWNVF summarized in Table 1 is based on Lithic Ridge Tuff, tuff of Yucca Flat, and vealed by extensive drilling, as for the Si- our new petrologic and geochronologic Redrock Valley Tuff retain the same rank as lent Canyon caldera complex in the sub- studies. assigned in the earlier literature (Byers and surface of Pahute Mesa (Orkild and The stratigraphie changes (Table 1), first, others, 1976a; Noble and others, 1984; Carr others, 1969; Ferguson and others, 1994). raise formally defined formations of previ- and others, 1986). Newly defined formal Caldera sources for Topopah Spring, ous usage (Belted Range, Crater Flat, stratigraphie units are the Beatty Wash For- Tram, Lithic Ridge, Tub Spring, and Paintbrush, Timber Mountain, and Thirsty mation, Calico Hills Formation, Dead Redrock Valley Tuffs remain poorly Canyon) to group rank. Petrographically, Horse Flat Formation, and Tunnel Forma- known, though various source areas have geochemically, and temporally related lava tion. The stratigraphie units of the SWNVF been postulated (Byers and others, 1976a;

Geological Society of America Bulletin, October 1994 1305

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ne*»' 116°00'W

Figure 1. Location of calderas of the southwestern Nevada volcanic field (SWNVF) and index map to ge- ographic features. Known 37»15'N calderas of the SWNVF are the Timber Mountain-Claim Can- yon-Silent Canyon caldera complexes and the Black Mountain caldera. The Timber Mountain caldera complex was the source of the Rainier Mesa Tuff and the Ammonia Tanks Tuff. The Tiva Canyon Tuff was erupted from the Claim Can- yon caldera. The Silent Canyon caldera complex consists of an 37W earlier Grouse Canyon caldera and a younger Area 20 caldera, source of the Bullfrog Tuff. The Trail Ridge Tuff was probably erupted during the final col- lapse of the Black Mountain caldera, but the caldera may have largely formed in re- sponse to the eruption of the Pahute Mesa Tuff. Geographic localities mentioned in the text are abbreviated as follows: CM, — 36°45' Chocolate Mountain; OM, Oa- sis Mountain; TH, Transvaal Hills; TM, Timber Mountain.

drill hole peralkaline caldera metaluminous caldera or volcano other bedrock • UE-20f

Christiansen and others, 1977; Carr and indicate that the Grouse Canyon and Tub formation base overlies the top of the Tub others, 1986). Spring peralkaline ash-flow sheets are not Spring Tuff, and the top underlies the base The oldest well-characterized metalumi- comagmatic (Sawyer and Sargent, 1989). of the bedded member of the Grouse Can- nous ash-flow sheets in the SWNVF are the The source of the Tub Spring Tuff is un- yon Tuff on the east side of Rainier Mesa Redrock Valley Tuff and the tuff of Yucca known, and intracaldera Tub Spring has not (bed 5 of Gibbons and others, 1963). The Flat (Byers and others, 1976a; Carr and oth- been penetrated in Pahute Mesa, although a Tunnel Formation consists of a diverse se- ers, 1986). The oldest peralkaline units in few drill holes penetrate the stratigraphic quence of bedded and nonwelded tuffs, pre- the SWNVF include the Tub Spring Tuff, level of the Tub Spring. dominantly rhyolitic in composition. Drill previously the Tub Spring Member of the The Tunnel Formation, as defined here, is hole UE-12nl4 (Fig. 1), from a depth of Belted Range Tuff (Sargent and others, divided into two informal subunits, beds 3 351-508 m, is a reference section for the 1964). We remove this unit from the Belted and 4, that are widely used in the Rainier Tunnel Formation; the type section extends Range Group because it is 1.2 m.y. older Mesa area and are equivalent to tunnel beds about 1500 m east-northeast from than the Grouse Canyon Tuff and because 3 and 4 of the Oak Spring and Indian Trail UE-12nl4, across the east face of Rainier as much as 200 m of predominantly meta- Formations as used by Gibbons and others Mesa. Tephras in the upper part of the Tun- luminous bedded and nonwelded tuffs of the (1963) and Hansen and others (1963). The nel Formation at Rainier Mesa probably Tunnel Formation separate the Tub Spring name is derived from the extensive series of correlate with rhyolitic to dacitic eruptive and the Grouse Canyon Tuffs at Rainier tunnels used for underground nuclear test- and intrusive units described below as lava Mesa. Petrologic and geochemical criteria ing on the east side of Rainier Mesa. The of Tram Ridge. Lithic Ridge Tuff (Carr and

1306 Geological Society of America Bulletin, October 1994

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others, 1986) stratigraphically overlies the Pahute Mesa (Fig. 1), where the formation Caldera sources for other units of the Tunnel Formation in the Shoshone Moun- overlies the Grouse Canyon Tuff and under- Crater Flat Group are less well known. The tain area (Fig. 1). The lava of Tram Ridge is lies the Bullfrog Tuff. The Dead Horse Flat northern half of the proposed Prospector a sequence of lava flows and related tuff and Formation includes the extracaldera lavas Pass caldera complex (near Tram Ridge, intrusive rocks that is present beneath Lithic exposed north of Pahute Mesa that overlie Fig. 1) is reasonably supported by field and Ridge Tuff at Tram Ridge (Fig. 1) and as the Grouse Canyon Tuff that were termed geophysical evidence (Snyder and Carr, dikes around Crater Flat (quartz latite of volcanics of Saucer Mesa by Sawyer and 1984) and may have been the source of the Carr and others, 1986). The lava and tuffs Sargent (1989). Tram Tuff (Carr and others, 1986). The correlate with the biotite-hornblende rhyo- The Bullfrog Tuff is the ash-flow sheet as- Prow Pass Tuff does not have an identified lite west of Split Ridge (Byers and others, sociated with the younger Area 20 caldera source, and it may be small enough (Carr 1976b) and unit C tuff in the subsurface of (Fig. 1) of the Silent Canyon caldera com- and others, 1986) not to have erupted from Yucca Mountain (Warren and others, plex (Sawyer and Sargent, 1989); the intra- a caldera. 1984). caldera facies was termed the lithic-rich ash- The Wahmonie Formation (Poole and The Silent Canyon caldera complex in- flow tuff by Orkild and others (1969) and is others, 1965) is a sequence of and cludes the oldest calderas in the SWNVF interleaved with thick megabreccia lenses. lava flows and related volcaniclastic that are confidently identified. It is com- The intracaldera Bullfrog is more than 600 sedimentary rocks and tephras that erupted pletely buried by younger deposits but co- m thick in three drill holes, including from the Wahmonie volcano (Fig. 1) south- incides with a major gravity low (Healey, UE-20f (1859-2522 m depth; Ferguson and east of the Timber Mountain caldera com- 1968; Kane and others, 1981; Healey and others, 1994). Boundaries of the Area 20 plex (Frizzell and Shulters, 1990; Broxton others, 1987). Existence of this geophysically caldera (Minor and others, 1993) are fixed and others, 1989). Biotite-rich mafic tephras defined caldera complex has been verified by drill holes east and north of drill hole that erupted from the Wahmonie volcano by more than 70 deep (>600 m depth) drill UE-20f, but evidence for the caldera south form a widespread, distinctive marker be- holes in Pahute Mesa (Ferguson and others, and west of UE-20f and UE-18r (Fig. 1) is tween Crater Flat Group tuffs and Calico 1994). The composite topographic wall of concealed by younger units and not deline- Hills Formation tuff. This marker is com- the Silent Canyon caldera complex is ex- ated by drilling southwest of the NTS mon in Yucca Flat and Rainier Mesa, is ex- pressed at the surface by inward dips of boundary. Gravity data (Kane and others, posed west of Yucca Flat and east of Yucca younger Timber Mountain tuffs that were 1981) suggest that the Area 20 caldera ex- Mountain (Broxton and others, 1993), but is plastered on the walls of the older caldera tends south under the Timber Mountain absent to the northwest in Pahute Mesa complex (Orkild and others, 1969; Byers caldera complex. The distribution of thick (Ferguson and others, 1994). and others, 1976b). The Silent Canyon Calico Hills lavas and tephras south of Tim- The Calico Hills Formation combines the caldera complex formed by two separate ber Mountain and at Yucca Mountain is ev- tuffs and lavas of Calico Hills (significant at caldera collapses (Sawyer and Sargent, idence of petrochemically related post- Yucca Mountain) with the tuffs and lavas of 1989): an initial collapse of the northeastern Area 20 caldera volcanism and supports the Area 20 (significant at Pahute Mesa). These caldera of the complex related to eruption interpretation of Area 20 caldera collapse two sequences are stratigraphically and pet- of the Grouse Canyon Tuff, and a younger extending at least partly beneath Timber rologically equivalent (Warren, 1983) and collapse of the Area 20 caldera caused by Mountain (Warren, 1983). The Stockade resulted from Crater Flat Group post- eruption of the Bullfrog Tuff (Fig. 1). Wash Tuff (Byers and others, 1976a) is pet- caldera volcanism. Throughout the SWNVF rographically similar to both the outflow fa- The Grouse Canyon caldera was the they lie stratigraphically between the Wah- cies of the Bullfrog Tuff at Yucca Mountain source of the peralkaline Grouse Canyon monie Formation (or the Crater Flat Group and the intracaldera Bullfrog Tuff on Pahute Tuff. Comendite of Split Ridge, previously where Wahmonie is absent) and the Paint- Mesa. The Stockade Wash Tuff and the called the rhyolite of Split Ridge (Orkild and brush Group. The type locality for the Cal- Bullfrog Tuff at Yucca Mountain may be others, 1969; Sawyer and Sargent, 1989), is a ico Hills Formation is the northwestern Cal- separate outflow lobes of a composite ash- petrologically related precursor lava to the ico Hills (Orkild and O'Connor, 1970); flow sheet that represent slightly different Grouse Canyon Tuff. The bedded peralka- reference sections are drill holes USW G-2, parts of the compositional zonation in a line tuff beneath the welded part of the at 518-824 m depth at Yucca Mountain single magma chamber. The paleomag- Grouse Canyon Tuff is the bedded member (Warren and others, 1984), and UE-20f, at netic signatures of these two geograph- of the Grouse Canyon Tuff, replacing tunnel 899-1324 m depth at Pahute Mesa (Fergu- ically discrete ash-flow tuffs are identical bed 5 of previous usage (Table 1). Following son and others, 1994). The Calico Hills For- and distinct from a typical Miocene direc- Grouse Canyon eruption, the northeast Si- mation consists of rhyolite lava flows, tion (Hudson and others, 1994), consistent lent Canyon caldera complex was filled and domes, and nonwelded ash-flow and bedded with eruption at the same time. For these overflowed by postcollapse lava flows. This tuffs. It has a maximum thickness of 1306 m reasons, we correlate the Stockade Wash sequence of peralkaline lava flows and re- and commonly is >600 m thick in western Tuff with the Bullfrog Tuff, an important lated tuffs, previously termed the lava and Pahute Mesa drill holes (Fig. 6, Ferguson unit in the Yucca Mountain area (Warren, tuff of Dead Horse Flat (Orkild and others, and others, 1994); in the Yucca Mountain 1983; Broxton and others, 1989; Scott, 1969), are now assigned a formal stratigraph- area it is 50-300 m thick (Warren, 1983; 1990). A caldera source for the Bullfrog ic rank as the Dead Horse Flat Formation, Warren and others, 1984; Broxton and Tuff in Crater Flat (Carr and others, 1986) named for an open meadow in the Dead others, 1993). has been contested (Scott, 1990) and is in- Horse Flat topographic quadrangle. The consistent with evidence for an Area 20 Paintbrush Group tuffs and lavas are one type section is drill hole UE-19c (depth 725- caldera source. of the most widespread and voluminous 2249 m, Ferguson and others, in press) in caldera-related assemblages in the SWNVF.

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Topopah Spring Tuff directly overlies bed- Rainier Mesa caldera. The dome was later others, 1977) are inadequate to confirm its ded rhyolite tuffs of the Calico Hills Forma- truncated and down-dropped on the east existence (Noble and others, 1991); the tion at Yucca Mountain and at most local- by formation of the Ammonia Tanks gravel-filled basin in Oasis Valley cannot be ities south of Timber Mountain. A caldera caldera. The Rainier Mesa caldera struc- unequivocally linked to caldera collapses source for the Topopah Spring Tuff is buried tural boundary is buried but can be delim- from eruption of the Ammonia Tanks, Tiva and its location uncertain. The Pah Canyon ited by geophysical and drill-hole data Canyon, or Rainier Mesa Tuffs. A lap-out of Tuff may have been too small (Byers and (UE-18r); the margin (Fig. 1) approxi- Ammonia Tanks Tuff against precaldera others, 1976a) to have formed a caldera; no mately coincides with the inferred buried rocks at Oasis Mountain (Byers and others, separate source has been identified. The cauldron boundary on the Timber Moun- 1976b) is probably related to paleotopogra- younger of the widespread Paintbrush tain caldera geologic map (Byers and oth- phy generated by syn-volcanic tectonism in Group ash-flow sheets, the Tiva Canyon ers, 1976b; Minor and others, 1993). the Oasis Valley area (Minor and others, Tuff, was erupted from the Claim Canyon Intracaldera Ammonia Tanks Tuff forms 1993). Proposed caldera walls on the west caldera (Byers and others, 1976a). The topographic Timber Mountain and was orig- side and north of Oasis Mountain (Byers Claim Canyon caldera (Fig. 1) is bounded by inally identified as the dome of the Timber and others, 1976b; Noble and others, 1991) precaldera rhyolite tuffs and lavas of the Mountain caldera (Byers and others, do not continue into the exposed Rainier Calico Hills Formation to the southeast, and 1976a); it is, more specifically, the resurgent Mesa and Ammonia Tanks topographic by precaldera tuffs and lavas of the Crater dome of the Ammonia Tanks caldera (Mi- walls near Timber Mountain (Fig. 1) accord- Flat Group to the southwest. Only part of nor and others, 1993). The structural margin ing to existing evidence. Aeromagnetic data the caldera is exposed because much of the or ring-fault zone of the Ammonia Tanks (Kane and others, 1981; U.S. Geological caldera collapsed into the younger Timber caldera (Fig. 1) is exposed on the east mar- Survey, 1979) indicate that the arcuate neg- Mountain caldera complex to the north gin of Timber Mountain. The margin was ative magnetic anomaly associated with re- (Byers and others, 1976a). The boundary depicted as a tuff dike zone on the Timber versely magnetized intracaldera Rainier illustrated is approximately the structural Mountain caldera geologic map (Byers and Mesa Tuff does not extend west to Oasis boundary of the resurgent intracaldera block; others, 1976b); its continuity as a ring-fault Mountain. This anomaly is the basis for the the topographic wall is not exposed (Chris- zone around the Ammonia Tanks resurgent inferred (though buried) western boundary tiansen and Lipman, 1965). The presence of dome is clearly demonstrated by subregional of the Rainier Mesa caldera illustrated in thick intracaldera Tiva Canyon Tuff, a re- gravity and aeromagnetic anomalies (Kane Figure 1. Because no unit can uniquely be versely magnetized unit, is corroborated by and others, 1981). attributed to it, we recommend that the con- a negative aeromagnetic anomaly centered cept of an Oasis Valley caldera segment be Previous studies (Byers and others, 1976a, on Chocolate Mountain (Fig. 1); a local abandoned and disassociated from the well- 1976b, 1989) depicted only the geometry of gravity high is consistent with the exposed characterized Timber Mountain caldera an "approximate outer cauldron boundary" Claim Canyon caldera being a fragment of complex formed by eruption of the Ammo- and a "partial buried inner cauldron fault" the resurgent dome (Kane and others, nia Tanks and Rainier Mesa Tuffs. New sub- for the Timber Mountain caldera complex. 1981). The Yucca Mountain Tuff, directly surface data will be required to unambigu- As has been documented for calderas in the beneath the Tiva Canyon, is an early small- ously establish the origin of Oasis Valley. and many other lo- volume eruption of the uppermost high-sil- calities (Lipman, 1984), calderas have a col- The newly defined Beatty Wash Forma- ica rhyolite part of the Tiva Canyon magma lapse boundary or topographic wall outside tion is part of an informal sequence of units, chamber (Broxton and others, 1989), prob- of the ring-fault or structural margin. The the Fortymile Canyon assemblage. Assign- ably from the same area of the Claim Can- composite topographic wall of the Timber ment of rhyolite lava flows and tuffs previ- yon caldera. Mountain caldera complex (Minor and oth- ously called the rhyolite lavas of Fortymile Climactic ash flows of the Timber ers, 1993) is exposed around 200° of the Canyon on the south side of the Timber Mountain Group were erupted from the calderas (Fig. 1); it is covered by younger Mountain caldera complex (Byers and oth- Timber Mountain caldera complex. The units on the northwest side of the complex ers, 1976a, 1976b) has been uncertain be- complex consists of two overlapping, re- (south and west of drill holes UE-18r and cause field relations to the major ash-flow surgent calderas: an older caldera formed UE-20f and the NTS boundary; Byers and sheets of the Timber Mountain and Paint- by eruption of the Rainier Mesa Tuff, and others, 1976b). The distinction between the brush Groups are lacking. Most of these a younger, nested caldera formed by Ammonia Tanks and Rainier Mesa topo- units can be correlated with the Timber eruption of the Ammonia Tanks Tuff (Mi- graphic walls is uncertain along most of the Mountain and Paintbrush Groups based on nor and others, 1993). Intracaldera Rain- length of this contact. The two collapse fea- detailed petrography and chemistry (War- ier Mesa Tuff and caldera-collapse brec- tures can be clearly distinguished only east ren and others, 1988). We restrict the For- cias are exposed in the Transvaal Hills, and south of the Transvaal Hills (Fig. 1), tymile Canyon assemblage to the sequence west of Timber Mountain (Byers and oth- where the Ammonia Tanks topographic wall of rocks stratigraphically above the Ammo- ers, 1976a, 1976b). These exposures and swings north to the east of the Transvaal nia Tanks Tuff and related lavas of the Tim- their inferred subsurface extensions coin- Hills and the Rainier Mesa topographic wall ber Mountain Group, and beneath the per- cide with aeromagnetic lows (Kane and trends west to the south of the Transvaal alkaline units of the Thirsty Canyon Group. others, 1981), consistent with the reversed Hills (Minor and others, 1993) before being This assemblage is a diverse sequence of remanent magnetic polarity of the Rainier covered by younger gravels in Oasis Valley. rhyolite lava flows and domes, related Mesa Tuff (Bath, 1968). We interpret the Geologic data previously cited as evi- tephra deposits, minor ash-flow deposits, Transvaal Hills as the western part of a dence for an Oasis Valley caldera segment and basaltic and other mafic lavas erupted larger resurgent dome formed within the (Byers and others, 1976a; Christiansen and from a variety of relatively small-volume

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TABLE 2. REPRESENTATIVE ^Ar/^Ar ANALYSES 1993) is >50 km northwest of the central

Calico Hills Formation lava/sanidine SWNVF caldera cluster. Distal Stonewall Field sample number: RWI9f2-m Irradiation laboratory number LXXXVI1:A22; J = 0.003590 tuff overlaps the Thirsty Canyon Group tuffs at Black Mountain. 36 3 Sample Percent 36Ar-Ca 40Ar/39Ar 37Ar/39Ar Ar/ "Ar '"Ar/3®Ar Age number 40Ar, (Ma) 40Ar/39Ar GEOCHRONOLOGY 90Z0582 87.6 1.29 2.2829 0.05373 0.00097 12.90 ± 0.08 90Z0583 77.4 0.54 2.5735 0.04612 0.00197 12.85 ± 0.08 90Z0584 86.6 0.47 2.3218 0.02106 0.00105 12.98 ± 0.08 Ages reported here were determined at 90Z0585 80.2 0.53 2.4910 0.03815 0.00167 12.90 ± 0.08 the U.S. Geological Survey laboratories in 90Z0586 0.00178 12.86 ± 0.08 79.1 0.62 2.5171 0.04768 40 39 90Z0587 71.8 0.23 2.7693 0.02589 0.00265 12.83 ± 0.08 Menlo Park, California. Two Ar/ Ar Weighted mean age and uncertainty (Ma) methods (described in the Appendix) were 12.89 ± 0.03 s.e.m: 0.02 MSWD: 0.458 used: laser fusion determination of small Grouse Canyon Tuil7sani(line sanidine samples and incremental heating Field sample number: DS18al Irradiation laboratory number XCIII:47; J = 0.004055 determination of larger biotite samples. Twenty-six sanidine samples were analyzed Sample Percent ÄAr-Ca 4llAr/3,Ar 37Ar/39Ar 36Ar/39Ar 4(1 Ar/39 Ar Age by the laser fusion 40Ar/39Ar technique number 4"Ar- (Ma) (Dalrymple and Duffield, 1988; Dalrymple, 91Z0410A 90.3 1.60 2.0854 0.03953 0.00066 13.72 ± 0.09 1989). Analytical results for two represen- 91Z0410B 88.4 1.67 2.1246 0.05067 0.00082 13.69 ± 0.10 91Z0410C 89.4 1.69 2.1063 0.04626 0.00074 13.72 ±0.10 tative samples are listed in Table 2: RW19f2, 91Z0410D 90.3 1.94 2.0693 0.04759 0.00066 13.62 ± 0.12 a Calico Hills Formation sanidine from a 91Z04I0E 79.5 0.71 2.3878 0.04309 0.00163 13.84 ± 0.14 91Z0410F 88.9 1.45 2.1031 0.04140 0.00077 13.63 ± 0.11 rhyolite lava north of Timber Mountain, and Weighted mean age and uncertainty (Ma) DS18al, Grouse Canyon Tuff sanidine irra- 13.70 ±0.04 s.e.m: 0.03 MSWD: 0.407 diated and analyzed twice. Two biotite sam- Irradiation laboratory number CII:A24; J = 0.002975 ples (Wahmonie Formation lava and dacite lava of Tram Ridge, Fig. 2) were analyzed by 36 39 40 39 Sample Percent 36Ar-Ca 40Ar/39Ar 37Ar/39Ar Ar/ Ar Ar/ At Age 40 39 number JIIAr* (Ma) the incremental heating Ar/ Ar tech- nique (Dalrymple and Lanphere, 1971; Lan- 91Z1035A 98.5 9.31 2.6145 0.04548 0.00013 13.76 ± 0.08 phere, 1988). These biotites yielded undis- 91Z1035B 97.9 6.56 2.6190 0.04482 0.00018 13.70 ± 0.09 91Z1035C 97.9 3.07 2.6171 0.01977 0.00017 13.70 ± 0.09 turbed age plateaus containing >90% of the 91Z1035D 87.4 1.36 2.9642 0.06386 0.00126 13.85 ± 0.09 91Z1035E 97.5 6.79 2.6182 0.05635 0.00022 13.64 ± 0.10 total gas and have atmospheric argon inter- 91Z1035F 99.2 14.93 2.5990 0.03647 0.00007 13.78 ± 0.09 cepts on 36Ar/40Ar-39Ar/40Ar isochron dia- 91Z1035G 95.3 3.24 2.6823 0.05112 0.00042 13.67 ± 0.09 91Z1035H 96.3 1.95 2.6727 0.02349 0.00032 13.76 ± 0.09 grams. Weighted mean age and uncertainty (Ma) 40 39 13.74 ± 0.03 s.e.m: 0.02 MSWD: 0.553 We determined Ar/ Ar ages for the Pooled weighted mean age and uncertainty (Ma) largest volume, most widespread units of the 13.72 ±0.03 MSWD: 0.498 SWNVF (Table 3). All reported ages are Note: MSWD is the mean square of weighted deviates; s.e.m. is the standard error of the mean; and J is the neutron flux rounded to ±50 ka because additional re- parameter. sults for the same units and further refine- ments of the monitor ages may affect the final ages by as much as 10-50 k.y. due to cumulative J-curve calibration uncertainties vent complexes. The Beatty Wash Forma- southeast margin of the Timber Mountain and within-unit and within-sample age vari- tion is the most significant of these post- caldera complex (Byers and others, 1976b). ations. Sample locations and detailed ana- Timber Mountain units and consists of rhyo- The Black Mountain caldera, just west of lytical data for samples other than those lite lava domes and related pyroclastic the Silent Canyon caldera complex (Fig. 1), listed in Table 2 and Figure 2 are available deposits in the moat of the Ammonia Tanks produced the youngest widespread ash-flow from the first author upon request. Results caldera. The type area is upper Beatty Wash sheets in the SWNVF area. Those sheets for four samples of Topopah Spring Tuff, on the Topopah Spring NW quadrangle make up the several moderate-volume per- three samples each of Ammonia Tanks, (Christiansen and Lipman, 1965); the for- alkaline ash-flow tuffs (Christiansen, 1979; Rainier Mesa, and Tiva Canyon Tuffs, and mation also includes the related tuif of Cut- Noble and others, 1984): the Rocket Wash, two samples of Bullfrog Tuff are reported as off Road to the west of the Timber Moun- Pahute Mesa, Trail Ridge, and Gold Flat error-weighted pooled averages for the ab- 40 39 tain caldera complex adjoining Oasis Valley Tuffs (Table 1) of the Thirsty Canyon solute age and uncertainty. All Ar/ Ar (Byers and others, 1976b). The Fortymile Group. Individual collapse boundaries have ages were measured relative to a monitor Canyon assemblage also includes such not been identified due to a lack of dissec- age of 513.9 Ma for MMhb-1 (Lanphere and widely dispersed units (Minor and others, tion of the caldera. The composite topo- others, 1990; Dalrymple and others, 1993). 40 39 1993) as the rhyolite of Rainbow Mountain graphic wall of the Black Mountain caldera The Ar/ Ar results presented here agree 40 39 in the Beatty area (Maldonado and Haus- (Fig. 1) is mantled by Trail Ridge Tuff (Mi- well with Ar/ Ar determinations recently back, 1990), the rhyolite of West Cat Can- nor and others, 1993), indicating that col- published for a few SWNVF units (Haus- yon west of Timber Mountain, and the mafic lapse was syn- or pre-Trail Ridge in age. back and others, 1990; Noble and others, lavas at and the rhyolite The Stonewall Mountain volcanic center 1991), when recalculated to the same mon- lavas of Shoshone Mountain along the (Noble and others, 1984; Minor and others, itor ages.

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24 ~i—I—I—I—I—I—I—I—I—I I—I I—I I I I I r ing all older volcanic rocks up through the WAHMONIE FORMATION LAVA Paintbrush Group (Table 1), a Timber FB25a6 BIOTITE Mountain Group episode, and a younger 18 (but inaccurately dated by K/Ar) Thirsty É 13.01 + 0.10 "H . Canyon Group and episode.

39 young sanidine ages exhibit no simple rela- Ar released, cumulative percent tionship to sanidine chemical composition and are not as common as would be ex- Figure 2. 40Ar/39Ar plateau age spectra for biotite from FB25a6, Wahmonie Formation pected if due to a standard analytical prob- lava, and from RW19f9, dacite lava of Tram Ridge. lem. A relationship between alkali feldspar structural state and concordance of K/Ar and 40Ar/39Ar age determinations may ex- DISCUSSION certainty. Because the interval between plain the aberrant ages, as the most strongly eruptions was nonuniform—in fact dis- exsolved sanidines have the most discordant Comparison of New 40Ar/39Ar and tinctly episodic (as detailed below)—the ac- ages (P. W. Lipman, U.S. Geological Sur- Previous K/Ar Results tual overlap of uncertainties is even greater. vey, 1969, written commun.; F. W. McDow- Consequently, conventional K/Ar geochro- ell, 1989, written commun.). More than half Volcanic rocks of the SWNVF have been nology discriminates only three episodes of of K/Ar biotite age determinations agree the subject of many conventional K/Ar geo- SWNVF volcanism: an early episode includ- chronology studies since the early 1960s. Over 150 K/Ar determinations have been made on SWNVF units (Kistler, 1968; Mar- TABLE 3. 40Ar/39Ar AGE DETERMINATIONS vin and others, 1970,1973,1989; Marvin and Unit Sample Mineral/N Weighted s.e.m. Uncertainty Cole, 1978; Weiss and others, 1989; Noble mean age and others, 1984, 1991; R. J. Fleck, unpub. (Ma) data; and F. W. McDowell, University of Spearhead Member N80A sanidine/5 7.5 0.01 0.03 , 1989, written commun.). Typical re- Rocket Wash Tuff Age 11 sanidine/4 9.4 0.03 0.04 ported analytical uncertainties for K/Ar ages Ammonia Tanks Tuff Pooled average of 3 sanidines/19 11.45 0.03 0.03 Rainier Mesa Tuff Pooled average of 3 sanidines/16 11.6 0.03 0.03 average about 3% (Kistler, 1968; Tabor and Rhyolite of Loop POG2bll sanidine/6 12.5 0.02 0.03 Tiva Canyon Tuff Pooled average of 3 sanidines/20 12.7 0.03 0.03 others, 1985) and thus represent absolute Topopah Spring Tuff Pooled average of 4 sanidines/21 12.8 0.02 0.03 age uncertainties of 0.25-0.50 m.y. for the Calico Hills Formation RW19f2-m sanidine/6 12.9 0.02 0.04 Wahmonie Formation FB25a6 biotite-plateau/8 13.0 0.09 0.10 7.5-15.25 Ma rocks of the SWNVF. Even if Bullfrog Tuff Pooled average of 2 sanidines/13 13.25 0.02 0.04 Dead Horse Flat Formation DS19dl5 sanidine/5 13.5 0.02 0.02 the -100 units of the SWNVF, or the 20 Grouse Canyon Tuff DS18al sanidine/14 13.7 0.02 0.04 major ash-flow sheets, or the 13 petrologic Comendite of Split Ridge DS19f9 sanidine/5 13.85 0.01 0.02 Lithic Ridge Tuff TSV-417B sanidine/5 14.0 0.07 0.06 assemblages (Table 1) were distributed Lava of Tram Ridge RW19f9 biotite-plateau/7 14.0 0.35 0.13 Tub Spring Tuff DS15e6 sanidine/5 14.9 0.01 0.04 evenly across this time period, the ages of Tuff of Yucca Flat FB16a3 sanidine/6 15.1 0.05 0.06 units would be unresolvable given a 3% un- Redrock Valley Tuff Age 13 sanidine/6 15.25 0.06 0.06

1310 Geological Society of America Bulletin, October 1994

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a) T O Sanidlne S 4 - t O Biotite z 3- 2000 Peralkaline I Young ad 0 2(- K/Ar ages K/Arages Metaluminous rhyolites tr 1 UJ m UJ 1 ^ OOHJ O oca Metaluminous mafic lavas z

80 85 90 95 97 99 101103 107 5 100 < 2 CONVENTIONAL K/Ar AGE OF UNIT (PERCENT) 38 s 1000 — ••"Ar/ Ar AGE OF UNIT s Figure 3. Comparison of laser fusion o 40Ar/39Ar ages with previous conventional UJ K-Ar ages for stratigraphic units of the cfcc southwestern Nevada volcanic field. K/Ar UJ data are from published sources listed in text and from unpublished data provided by F. W. McDowell (1989, written commun.).

9 8

within 1% of the new 40Ar/39Ar ages, but about 30% of the K/Ar biotite ages are >3% PRE- older. A probable explanation for these b) VOLCAN IC older ages is that they are contaminated by SYN-VOLCANIC POST-VOLCANIC EXTENSION EXTENSION xenocrysts due to fusion of large (>1 g) bulk 15 Ma EXTENSION UASMa samples.

SYM, FC BH, YF 8000 & UJ 5 7000 Figure 4. Episodes of volcanism in the 3 southwestern Nevada volcanic field, (a) Ap- o> 6000 proximate erupted magma volume for each < s group or assemblage plotted against the age eg range. Group or assemblage symbols and 5000 erupted magma volumes are from Table 1. 4000 IS Area under the curves is not proportional to CL volume, as width of the bars is solely a func- CC 3000 tion of the age range of the assemblage, (b) UJ UJ Cumulative erupted magma volume and Si 2000 rates of material erupted for volcanic as- s semblages of the southwestern Nevada vol- Z3 5 1000 canic field (SWNVF). Top of figure super- D U imposes episodes of extension and relative ^ / magnitude of extension during extensional 16 15 13 12 10 episodes. Abbreviations for extensional do- AGE (Ma) mains described in text and illustrated in Figures 4b and 6 are SYM, southern Yucca Mountain; FC, Fluorspar Canyon; BH, Episodicity of Caldera Volcanism (1976a), Broxton and others (1989), and Bullfrog Hills; and YF, Yucca Flat. Group Sawyer and Sargent (1989) or are compiled or assemblage symbols as listed in Table 1. The high-resolution 40Ar/39Ar ages reveal from geologic map data (Ekren and others, Rates of material erupted are derived for the first time a pattern of episodic vol- 1971; Frizzell and Shulters, 1990) for units graphically and listed above dashed lines. canic activity in the SWNVF. This pattern is not cited in the previous sources. Volume Average eruptive rate for the SWNVF over accentuated when erupted magma volumes estimates are at best semiquantitative be- 3 the 7 m.y. histoiy of the field is ~110 km / are considered (Fig. 4a). Magma volume es- cause of the limitations of the calculation 100 k.y. timates (Table 1) are from Byers and others method (known area of exposure multiplied

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by estimated average thickness). Volumes of Fig. 4a), widespread Calico Hills lava vents displays a similar episodicity in major major ash-flow sheets older than the Tiva (12.9 Ma), and Paintbrush Group calderas caldera pulses (Mcintosh and others, 1992), Canyon Tuff have an additional significant and lava vents. The peak of early SWNVF but caldera sources there had lateral migra- uncertainty in that intracaldera tuff is not volcanism was eruption of Paintbrush tions, which contrast with sequential exposed, and thus intracaldera tuff volumes Group tuffs and lavas, including the Topo- eruptions from a single area of overlapping are not estimated. Volumes of the intra- pah Spring Tuff (12.8 Ma) and Tiva Canyon calderas in the SWNVF (Warren, 1983). caldera Bullfrog and Grouse Canyon Tuffs Tuff (12.7 Ma), within 100 k.y. The rapid The San Juan volcanic field, like the Mo- have been estimated because they are pen- rate of magma extrusion, about 2250 km3/ gollon-Datil, is composed of several over- etrated by several drill holes in the subsur- 100 k.y. (Table 1; Fig. 4b), was the highest lapping clusters of calderas, each of which face of the Silent Canyon caldera complex. during the lifetime of the SWNVF. might be considered analogous to the 40 39 Despite these volumetric uncertainties, the Lava flows that were petrographically SWNVF. Ar/ Ar geochronologic data for general time-volume relationship is decid- similar to the Timber Mountain Group the central San Juan caldera cluster (Lan- edly episodic (Fig. 4a). magma (for example, rhyolite of the Loop, phere, 1988) indicate a peak volcanic Eruption of the Paintbrush and Timber Tables 1 and 3) were first erupted at ca. 12.5 eruption rate and frequency (the four mid- Mountain Group magmas was the climax Ma, but between 12.45 and 11.7 Ma no dle, out of six, caldera eruptions were about of volcanism in the SWNVF. Over 4500 eruptions occurred (Fig. 4a) in the SWNVF. 250 k.y. apart) comparable to the SWNVF. km3 of magma was erupted in two distinct This 750 k.y. magma gap probably repre- The waxing, waning, and eventual extinction and subequal episodes separated by a sents the time of magma generation and ac- of silicic volcanism in the SWNVF contrasts magmatic gap of 750 k.y. Each episode cumulation in magma chambers prior to the with the time-predictable migration of silicic lasted 150 k.y. or less (Fig. 4a). Volume of climactic eruption of the Timber Mountain volcanism along the Yellowstone-eastern the remaining units of the SWNVF was Group tuffs. Precursory eruptions of rhyo- Snake River Plain hot-spot track (Pierce dwarfed by the magma erupted as the four lite lava began ca. 11.7 Ma and were fol- and Morgan, 1992). large ash-flow sheets of the Paintbrush lowed by the first of two major Timber and Timber Mountain Groups. The cumu- Mountain Group ash-flow sheets, the Rain- Magmatic Evolution of the SWNVF: lative volume of magma erupted prior to ier Mesa Tuff, at 11.6 Ma. Rhyolite lava The Relationship of Peralkaline and the climactic eruptions of the Paintbrush eruptions continued after eruption of the Metaluminous Volcanism in Time and 3 Group was about 2500 km erupted over Rainier Mesa Tuff and preceded eruption of Space 3.3 m.y. (Fig. 4b). The volume erupted af- the second major Timber Mountain ash- ter the Timber Mountain Group was 600 flow sheet, the Ammonia Tanks Tuff, at Magma composition varied along with 3 km over 4 m.y. during the waning of 11.45 Ma. The Timber Mountain eruptions eruptive volume during the lifetime of the SWNVF volcanism. represent the second major peak of volcan- SWNVF. Peralkaline magmas were erupted The rate of material erupted gradually in- ism in the SWNVF, and time-averaged at several time intervals interspersed with 3 creased with time from the initiation of eruption rates on the order of 900 km /100 eruption of the dominant metaluminous SWNVF at ca. 16 Ma to the eruption of the k.y. (Fig. 4b) characterized a 250 k.y. period. magmas, and source areas for the peralka- Paintbrush Group between 12.8 and 12.7 Significant basalt was first erupted in the line magmas overlapped in part the meta- Ma. The first major episode of volcanism SWNVF during Timber Mountain time. luminous sources (Figs. 1 and 5a), in con- occurred between 15.25 and 14.9 Ma when Ash flows and postcaldera moat lava flow trast to previous interpretations (Byers and the Redrock Valley Tuff (15.25 Ma), tuff of eruptions related to the Ammonia Tanks others, 1976a; Christiansen and others, Yucca Flat (15.1 Ma), and the Tub Spring Tuff followed soon after; rhyolite lava flows 1977; Noble and others, 1991). Earliest vol- Tuff (14.9 Ma) were erupted. The rate of and tuffs erupted primarily west of the canism in the SWNVF produced mainly eruption was much lower between 14.9 and caldera. metaluminous ash-flow tuffs, including the 14.0 Ma during the accumulation of tephras Local rhyolite lava flows and more wide- Redrock Valley Tuff and the tuff of Yucca and distal ash-flow tuffs of the Tunnel For- spread basalt eruptions continued after Flat; late in the earliest volcanic episode, mation (Fig. 4a). During this early volcan- 11.45 Ma during waning SWNVF volcanism. however, peralkaline Tub Spring Tuff was ism, magma erupted at a time-averaged rate A 2 m.y. gap separates peak Timber Moun- erupted. Zirconium concentrations (Fig. 5a) 3 of about 50 km /100 k.y. (Fig. 4b). At 14.0 tain ash-flow eruptions and the next caldera- are high in the peralkaline units, whereas Ma, metaluminous lava of Tram Ridge forming eruptions of the Black Mountain modal plagioclase is very low to absent erupted from many centers, followed soon caldera (Fig. 4a). Units of the peralkaline (Fig. 5b). Large-volume metaluminous vol- after by the related Lithic Ridge Tuff. Thirsty Canyon Group, ash-flow sheets that canism began again at 14.0 Ma (Fig. 4a) with Eruption of the peralkaline Grouse Canyon total about 300 km3 in volume, were erupted the eruption of the lava of Tram Ridge and Tuff (13.7 Ma; Fig. 4a) caused the initial from the Black Mountain caldera after 9.4 the Lithic Ridge Tuff; these units have typ- caldera collapse in the Silent Canyon Ma. The Stonewall Mountain caldera ical low zirconium contents and high modal caldera complex (Fig. 1), subsequently filled erupted almost 2 m.y. later than the Black plagioclase contents (Figs. 5a and 5b). Per- by lavas and tuffs of the peralkaline Dead Mountain caldera (Spearhead Member, 7.5 alkaline comendite of Split Ridge lava, Horse Flat Formation until 13.5 Ma. Be- Ma). Eruption rates (15 km3/100 k.y.) were Grouse Canyon Tuff (the largest peralkaline tween 14.0 and 12.8 Ma the rate of volcanic low during this waning stage of SWNVF ash-flow sheet in the SWNVF), and Dead 3 eruption increased to about 160 km /100 k.y. volcanism (Fig. 4b). Horse Flat lavas were erupted between (Fig. 4b), and the greatest number of sepa- 13.85 and 13.5 Ma. Episodic volcanic activity observed in the rate eruptive centers were active. Centers SWNVF is similar to some multicaldera Oli- The Grouse Canyon Tuff and lavas of the active during this time include the sources of gocene to Quaternary volcanic fields but Dead Horse Flat Formation ponded in the the Crater Flat Group tuffs (ca. 13.25 Ma), contrasts with temporal patterns observed in northeastern caldera of the Silent Canyon the Wahmonie andesitic volcano (13.0 Ma; others. The Mogollon-Datil volcanic field caldera complex and were subsequently

1312 Geological Society of America Bulletin, October 1994

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a) b) 1250 EJ Peralkaline • Metaluminous Tm-xr rioo 1000 Tr • Tw TI To Tp-xr g •75 ~ •750 Tn c E Tal? Tm Q. Tc I Ô Tmn r-50 § Tn4 Tb -500 fi Tw T„ Tr TS Tun Tmn TmJ' To •25 .2 • -250 Tt ¥ D DO D Tu Tb 0- Ta •JT P - I I -r~ T I -T" 15 14 13 12 11 10 15 14 13 12 11 10 Age (Ma) Age (Ma) C) d)

0.8 I--« Tb Tn To ® ç --8 h 0.6 J: D rffl 0D O Tma •o o Tc Tmn •0.4 ® 1 Tmrl Tf —io Z Tp A CP To-Tpt ¡5 D D h05 % Ta --12 Tw[ Tb Tu • - -14 -r T T- "T" T~ -0.0 -T T ~r 15 14 T 12 11 10 15 14 13 12 11 10 Age (Ma) Age (Ma) Figure 5. Ranges for selected chemical, petrographic, and isotopic characteristics of assemblages and units of the southwestern Nevada volcanic field (SWNVF) through time, (a) Ranges of zirconium contents for whole-rock samples, (b) Ranges of median proportions of plagioclase phenocrysts among felsic phenocrysts in units of each assemblage, as determined by petrography, (c) The variation of Mg# [Mg/(Mg + total Fe)] in biotite or olivine, (d) The range of analyzed neodymium isotopic compositions for each assemblage, coeval basalts, and selected units (Tegtmeyer, 1990; Tegtmeyer and Farmer, 1990; Farmer and others, 1991). Symbols from Table 1 except for Tm-xr, Timber Mountain Group, crystal-rich tops of ash-flow sheets; Tmn, transitional Timber Mountain Group lavas (Broxton and others, 1989); Tp-xr, Paintbrush Group, crystal-rich tops of ash-flow sheets; Tma, Ammonia Tanks Tuff; Tmr, Rainier Mesa Tuff; Tpt, Topopah Spring Tuff; and Tn4, Tunnel Formation subunit 4j. Hachured pattern for peralkaline units; metaluminous units unpatterned.

overlapped by the Area 20 caldera (Fig. 1). interaction, inheritance, or similarity in or- ber Mountain magmas (Fig. 5a) reflect ac- Metaluminous Bullfrog Tuff (13.25 Ma) of igin or evolution between the peralkaline cumulation of zircon included in cumulate the Crater Flat Group and the postcaldera Grouse Canyon magmatic system and the phenocrysts (Warren and others, 1989) and Calico Hills Formation lavas and tuffs (12.9 younger, spatially overlapping metalumi- are not due to high magmatic zirconium Ma) filled the Area 20 caldera (Warren, nous Bullfrog Tuff and Calico Hills Forma- contents. Late peralkaline volcanism oc- 1983). Peralkaline Belted Range Group and tion magma systems. Iron-enriched mafic curred with the eruption of the Thirsty Can- metaluminous Crater Flat Group and Cal- minerals also are present in the lower Topo- yon Group magmas at 9.4 Ma (Figs. 5a, 5b, ico Hills lavas have typical zirconium and pah Spring Tuff (Tpt, Fig. 5c), suggesting a and 5c), after a 2 m.y. gap in ash-flow plagioclase contents for their respective relationship between the Crater Flat/Calico caldera-forming eruptions, during which magma chemistries (Figs. 5a and 5b), but Hills magmas and the initial Paintbrush time small-volume basalts and metalumi- the mafic mineral chemistry of Crater Flat Group magmas. nous rhyolites were erupted (Fig. 4a, and Tf, and Calico Hills units are anomalously iron- Magma chemistry of both Paintbrush and Figs. 5a, 5b, and 5c). Basaltic volcanism as- rich relative to other metaluminous rocks of Timber Mountain caldera episodes is fun- sociated with the peralkaline Black Moun- the SWNVF (Fig. 5c). Iron enrichment (and damentally metaluminous, although all four tain caldera was more voluminous and wide- corresponding magnesium depletion) of bi- major ash-flow sheets are zoned from spread than with the two earlier peralkaline otite and pyroxene in these metaluminous early-erupted high-silica rhyolite to late- ash-flow tuffs. Peralkaline magmatism in the rocks is transitional to the extreme iron en- erupted moderately alkalic trachytic compo- SWNVF occurred principally during the richment (Fig. 5c) characteristic of peralka- sitions. Paintbrush Group rhyolites are early (Tub Spring Tuff and Grouse Canyon line mafic minerals (for example, fayalitic slightly more alkalic than the Timber Moun- Tuff), waxing phase of volcanism in the olivine and acmite). We interpret this pat- tain Group rhyolites. The higher zirconium SWNVF, and during the late, waning stages tern to indicate some degree of magmatic contents of crystal-rich Paintbrush and Tim- of SWNVF activity at the Black Mountain

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caldera (Vogel and others, 1983). Peralka- canism in discrete domains, and postdated characteristics: (1) small cumulative middle line SWNVF volcanism continued until the caldera volcanism. Probably most exten- to late Miocene extension of the central 7.5 Ma volcanism at the Stonewall Mountain sional deformation in the SWNVF area is SWNVF relative to adjoining parts of the volcanic center (Noble and others, 1984; middle to late Miocene (16-8 Ma) in age Great Basin (Hudson and others, 1994; Hausback and others, 1990). and thus coincides with the peak of exten- Wernicke, 1992; Axen and others, 1993) and The Nd isotopic composition of peralka- sional deformation (15-5 Ma) in the central (2) greater extension in discrete domains line magmas of the SWNVF is distinctive Basin and Range of Wernicke (1992) to the (Fig. 6) marginal to the SWNVF at different (Fig. 5d) with respect to metaluminous south. specific times. These episodic extensional magmas (Tegtmeyer, 1990; Tegtmeyer and Prevolcanic extension is known to have strains were distinctly nonuniform across Farmer, 1990; Farmer and others, 1991). affected several localities in the SWNVF the area of the SWNVF. Peralkaline units have eNd >—9, whereas area, but the timing and amount of this ex- Early synvolcanic extension affected sev- metaluminous units have eNd <-9. Meta- tension are difficult to determine because eral localities of the SWNVF. Small strains luminous of the Wahmonie vol- (1) the youngest affected units are Pennsyl- were accommodated by pre-Bullfrog (13.25 cano have the lowest eNd values (Farmer vanian, (2) structural reconstructions are Ma) normal faults and spatially limited pre- and others, 1991) in the SWNVF (Fig. 5d) complicated by Mesozoic thrust-faulting, Tiva Canyon (12.7 Ma) normal and strike- and are petrographically distinguished by and (3) exposures of pre-Tertiary rocks are slip faults northwest of Yucca Flat (Minor, their high modal plagioclase contents limited (Fig. 6). Low-angle extensional 1989). Moderate pre-Bullfrog faulting and (Fig. 5b) and calcic chemistry (Broxton and faults cut Paleozoic and older sedimentary northwest tilting also affected an area south- others, 1989). Spatially, the Wahmonie vol- rocks around the margins of Yucca Flat east (Fig. 6) of Tolicha Peak (Minor and cano is southeast of the central SWNVF (Fig. 6) at Mine Mountain (Hudson and others, 1991,1993). Large subsurface thick- caldera cluster (Fig. 1). These observations Cole, 1993; Cole and others, 1989), the CP ness changes of the lower Crater Flat Group support a derivation of Wahmonie magmas Hills (Caskey, 1991), and in the northern tuffs and the Lithic Ridge Tuff reflect growth from sources distinct from the rest of the Halfpint Range (J. C. Cole, 1993, oral com- faulting at northern Yucca Mountain (Frid- SWNVF (Broxton and others, 1989). mun.; Barnes and others, 1963,1965). Prevol- rich and others, 1994). Warren and others Basaltic eruptions, starting in the metalu- canic extension at Mine Mountain is older (1985) reported slight (2°-4°) pre-Timber minous Timber Mountain episode and con- than the Redrock Valley Tuff (15.25 Ma) Mountain northeast tilting of Paintbrush tinuing through the eruption of peralkaline and younger than Mesozoic thrusting. Gen- Group tuffs and lavas in the subsurface of Thirsty Canyon tuffs, became more volumi- tly dipping 27.3 Ma Monotony Tuff and 26.7 Pahute Mesa. Calico Hills Formation and nous late in the history of the SWNVF Ma Shingle Pass Tuff (Sargent and Orkild, Paintbrush Group tuff thicken on the west- (Figs. 4a and 5d). The Nd isotopic compo- 1973; ages from Best and Christiansen, ern, downthrown side of different faults at sitions of basalts and associated Timber 1991) and conformable 14.9 Ma Tub Spring Pahute Mesa. Slight post-Paintbrush exten- Mountain Group tuffs and post-Timber Tuff in the northern Halfpint Range depo- sion also occurred at Pahute Mesa where Mountain rhyolite lavas are similar sitionally overlie extended late Precambrian Rainier Mesa Tuff thickens over the same (Fig. 5d); most vents are south of the Silent and Paleozoic rocks. On the northwest side structures. of the Belted Range, 30°-dipping Monotony Canyon caldera complex (Fig. 1). In con- Late synvolcanic extensional deformation and Shingle Pass Tuffs overlie overturned trast, younger basalts generally coeval with is greater in magnitude (Fig. 4b) on the Paleozoic rocks at Limestone Ridge that are the peralkaline Thirsty Canyon tuffs were southwestern margins of the SWNVF. In cut by low-angle normal faults (Ekren and erupted mainly from volcanic centers on the the Fluorspar Canyon area (Fig. 6), north of others, 1971). Thus, at least in the northern north side of the SWNVF. Similarity of high Bare Mountain (Monsen and others, 1992), Halfpint and the Belted Ranges, early ex- values for eNd in young basalt and the per- the Bullfrog, Topopah Spring, and Tiva tensional deformation is pre-late Oligocene alkaline tuffs (Fig. 5d) and the geographic Canyon Tuffs are conformable and tilted in age. Evidence from a drill hole at Rainier restriction of peralkaline sources to the east ~30°-60° more than the overlying Mesa suggests that some prevolcanic exten- northern SWNVF suggest that a litho- 11.6-11.45 Ma Rainier Mesa and Ammonia sion may be pre-Late Cretaceous (Cole and spheric mantle boundary may exist between Tanks Tuffs (discordant Timber Mountain others, 1993). We find no data that support the Black Mountain and Grouse Canyon tuffs of Carr, 1990, Fig. 4). At Yucca Moun- postulated strong Paleogene extension calderas to the north and the Timber Moun- tain, Scott (1990) measured differential tilt- (Axen and others, 1993) within the area of tain caldera complex to the south. ing of 10°-20° between Tiva Canyon Tuff the central SWNVF caldera cluster (Fig. 6). and Timber Mountain Group tuffs at the Relation of Extension to SWNVF Synvolcanic normal faulting, minor strike- south end of Yucca Mountain; the Tiva Volcanism slip faulting, and tilting took place Canyon Tuff was rotated —25° clockwise throughout the main period (14-11.5 Ma) about a vertical axis, whereas rotation of the The relationship of Tertiary volcanism to of magmatism in the SWNVF (Fig. 4b), but Ammonia Tanks Tuff (6° ± 13°) was much extension in the Great Basin has been the extensional strain was modest in and near less (Rosenbaum and others, 1991; Hudson subject of much controversy. Some have as- the central SWNVF caldera cluster (Fig. 6). and others, 1994). serted that magmatism was prerequisite to Age constraints for this deformation come Postvolcanic extension (defined as younger large amounts of extension (Gans and oth- from overlap of faults in older units by than the 11.45 Ma Ammonia Tanks Tuff, ers, 1989); others have found little evidence younger units, from angular unconformities Fig. 4b) was locally large in magnitude but for significant extension during times of between units, and from thickness changes was restricted to areas marginal to the cen- peak volcanism (Best and Christiansen, of units related to growth faulting. We de- tral SWNVF caldera cluster (Fig. 6). Mod- 1991). Extensional deformation in the scribe below a composite pattern of syn- erate to large postvolcanic extension oc- SWNVF area (Figs. 1 and 6) variously pre- volcanic and postvolcanic deformation curred in the Bullfrog Hills and Fluorspar ceded volcanism, was synchronous with vol- (Fig. 4b), but with the following overall Canyon areas (Fig. 6). Extension was ex-

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Figure 6. Map illustrating the central southwestern Nevada volcanic field (SWNVF) caldera cluster and outlining the area showing weak (<10° of tilt or vertical-axis rotation) syn- and postvolcanic deformation, and some marginal domains (enclosed by hachured lines) of moderate to large amounts of late synvolcanic (LS) and postvolcanic (PV) extension. Tilt direction of beds is indicated by Et, east tilt, and Wt, west tilt. Abbreviations for labeled domains are from Figure 4b. Additional postvolcanic domains of moderate extension that formed alluvial basins include OV, Oasis Valley; CF, Crater Flat; JF, Jackass Flats; FF, Frenchman Flat; but boundaries for these domains are not completely delimited at present. The area of prevolcanic extension at Mine Mountain is abbreviated as MnM; Nevada Test Site is abbreviated as NTS. The map is not a comprehensive depiction of all synvolcanic and postvolcanic extensional domains in the SWNVF but represents the domains that can be identified at present, relative to the central SWNVF low-strain zone.

treme in the Bullfrog Hills, where 60°-90° and slight (<2°) tilting affected the 9.4 Ma (Frizzell and Shulters, 1990, and sources east tilts are observed in conformable se- Thirsty Canyon Group (Orkild and others, cited therein). Similar alluvial basins of late quences ranging from pre-Lithic Ridge Tuff 1969). Miocene to Quaternary age are present to the Ammonia Tanks Tuff (Maldonado, Fault-controlled, post-Ammonia Tanks north (Gold Flat, Kawich Valley, and Emi- 1990), but largely predated 10 Ma latite alluvial sedimentary basins formed on the grant Valley) and south (Crater Flat, Jack- flows (Maldonado and Hausback, 1990). In margins of the SWNVF but do not overlap ass Flat, and Frenchman Flat) of the central the western Fluorspar Canyon area (Mon- the central SWNVF caldera cluster. The SWNVF (Fig. 6), but their controlling faults sen and others, 1992), Ammonia Tanks Tuff Yucca Flat basin-Halfpint Range fault sys- did not propagate across the area of peak is tilted east as much as 30°. Farther east in tem, east of the central SWNVF, is a post- volcanism and caldera formation. The cen- Crater Flat, across Yucca Mountain, and Ammonia Tanks alluvial basin filled by late tral SWNVF is one of the largest areas in the southeast of Timber Mountain, the Timber Miocene and younger sedimentary deposits Great Basin unbroken by middle Miocene Mountain Group tuffs are generally tilted (Barnes and others, 1963) to 400 m depth, and younger alluvial basins (Stewart and 10°-30° (Carr, 1990, Fig. 4). Postvolcanic ex- and locally as much as 1200 m (McKeown Carlson, 1978); the northern Basin and tension was minimal at Pahute Mesa, where and others, 1976). In Yucca Flat (Fig. 6) and Range pattern of paired late Neogene Timber Mountain tuffs tilt east <4° (Warren the Halfpint Range, units of the SWNVF north-south ranges and basins terminates at and others, 1985). Slight normal faulting are cut by similar down-to-the-east normal the north margin of SWNVF. Rainier Mesa there (offsets >60-80 m on principal faults) faults and exhibit stratal tilts of 10°-25° west Tuff is repeated in fault blocks at about the

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same elevation over an east-west distance of dation of postvolcanic (post-Ammonia within the SWNVF based upon pétro- 100 km, from the Bullfrog Hills to the Half- Tanks Tuff) extension in the central graphie, geochemical, and field geologic pint Range, with no headwall breakaway SWNVF. A strengthening of the crust dur- studies (Byers and others, 1976a; Warren zones or regions of uplifted lower plate ing synvolcanic extension, however, would and others, 1988; Broxton and others, 1989; rocks (W. B. Hamilton, 1993, written require that magmas were emplaced and Sawyer and Sargent, 1989; Minor and oth- commun.). cooled quickly relative to deformation. This ers, 1993). We have revised the stratigraphy Volcanism and extension that postdate might be possible at shallow crustal levels, as of major SWNVF units based upon this the central SWNVF have different patterns described by Davis and others (1993) for a new framework, combining major ash-flow in time and space. Christiansen and Lipman pluton that intruded and terminated slip on sheets with petrologically related lava flows (1972) suggested that extension and volcan- a coeval Miocene detachment fault in the and nonwelded tuff deposits into stratigraph- ism in the SWNVF area occurred after a eastern Mojave Desert. For synvolcanic ex- ic groups. Newly defined formal stratigraph- transition from magmatism generated by tension, silicic magma input and inflation of ic units are the Beatty Wash Formation, Cal- subduction to a regime characterized by the batholithic magma chamber roof also ico Hills Formation, Dead Horse Flat extension and bimodal basalt-rhyolite vol- may have affected the regional stress field, Formation, and Tunnel Formation. The re- canism. Magmatism and extension in the reducing the regional deviatoric stress (Par- vised stratigraphy and ages demonstrate SWNVF area also occurred as the Mendo- sons and Thompson, 1991) and decreasing that peralkaline volcanism in the SWNVF cino fracture zone migrated north as a locus regional strain. Quaternary volcanic fields was closely associated in time and space with for magmatism and extension (Glazner and that exhibit similar deformational patterns the waxing and waning of metaluminous Bartley, 1984). Detailed plate tectonic re- include the Yellowstone Plateau volcanic volcanism. Petrologic and chemical data constructions (Severinghaus and Atwater, field, where neotectonic late Pleistocene provide evidence of hybridization between 1990) indicate that during the main episode and Holocene regional normal faults splay, distinct peralkaline and metaluminous mag- of volcanism (14-11.5 Ma) the SWNVF was decrease in displacement, and terminate to mas during the 7 m.y. span of SWNVF located north of the Mendocino fracture the north approaching the 0.6 Ma third- volcanism. zone. In the SWNVF area at this time, the cycle (Pierce and Mor- Minor to moderate extension can be dem- Pacific plate was still being subducted be- gan, 1992, Plate 1), and the Long Valley- onstrated during the history of the SWNVF, neath an extending western North America, Mono Basin volcanic system (Bursik and but the slight synvolcanic extension of the analogous to the Lassen-Shasta-Medicine Sieh, 1989; Dixon and others, 1993). central SWNVF contrasts markedly with the Lake Highlands region of northern Califor- Miocene extension in the SWNVF oc- synextensional magmatism described by nia today. Younger SWNVF calderas (Black curred in a mosaic of local domains. South Gans and others (1989) for limited areas of Mountain, 9.4 Ma, and Stonewall Mountain, and west margins of the field were affected highly extended volcanic rocks in the Great 7.5 Ma) were peralkaline in composition by a dextral shear transfer zone (Rosen- Basin. Best and Christiansen (1991) con- and small in volume (Fig. 4b), and they mi- baum and others, 1991; Hudson and others, cluded that the Oligocene and early Mio- grated northwestward, consistent with a 1994) that is a northern boundary for the cene peak of volcanism in the Great Basin melting locus associated with the movement high-strain Death Valley extensional corri- correlated with only limited extension in of the Mendocino fracture zone. Late Mio- dor (Wernicke and others, 1988). In general, east-central Nevada. For the SWNVF, we cene and Pliocene silicic caldera volcanism SWNVF volcanism and regional extension extend their conclusions in that only limited in the southern Great Basin (6 Ma Silver were coeval. In detail, however, age, mag- extension occurred during peak middle Mio- Peak volcanic center) migrated west-north- nitude, and polarity of extension varied cene volcanism in the southwestern Great west toward the Pliocene and Quaternary among the more strongly deformed domains Basin. Within the SWNVF, the central area Long Valley-Mono Basin volcanic system marginal to the field. Both volcanism and of coalescing calderas and proximal caldera (Luedke and Smith, 1981), whereas younger extensional deformation in the SWNVF margins was minimally extended from mid- extension and strike-slip faulting migrated were episodic in nature as has been recog- dle Miocene to the present. Synvolcanic west toward the Death Valley region nized in the eastern Great Basin (Taylor and strain was dominated by local magma cham- (Hamilton, 1988). others, 1989). ber roof deformation or caldera subsidence In contrast to models by Gans and others (Lipman, 1984). The outer margins of the (1989), we find that the area of greatest SUMMARY SWNVF locally were more deformed in dis- magma input to upper crust—within the crete extensional domains. An inferred sub- overlapping Silent Canyon-Claim Canyon- Volcanic activity in the SWNVF, as re- caldera batholith may have acted as an up- Timber Mountain caldera complexes vealed by new high-resolution 40Ar/39Ar age per crustal buttress with respect to regional (Fig. 6)—is the area of least upper crustal determinations, was distinctly episodic: very extensional deformation. Intense middle extension. Several factors may have led to large volumes of magma (up to 2000 km3) Miocene volcanism and subjacent plutonism retardation of extension in the area of the were erupted in fairly short periods of apparently strengthened the upper crust in central SWNVF caldera cluster. The caldera <100-300 k.y. separated by longer quies- the central SWNVF area, because it stands complexes are probably rooted in batholith cent periods. For example, two episodes of today as a high plateau unbroken by re- cupolas (Lipman, 1984). After crystalliza- caldera-forming ash-flow magmatism, sepa- gional middle Miocene and younger normal tion and cooling, such batholiths could act as rated by a 750 k.y. magma gap, produced the faulting that formed young alluvial basins. buttresses (Wernicke, 1992; Hamilton and large-volume tuffs of the Paintbrush and Magmatism in the SWNVF and regional ex- Myers, 1966), strengthening the upper crust Timber Mountain Groups during a 1.35 m.y. tension, although coeval, were not geneti- against regional extensional stress. Crustal time period. New 40Ar/39Ar ages corrobo- cally linked to form a zone of highly ex- strengthening probably best explains retar- rate the stratigraphic succession of units tended upper crust. The two processes

1316 Geological Society of America Bulletin, October 1994

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overlapped in time and space but were not Dalrymple and Duffield (1988) and Dalrymple Barnes, H., Houser, F. N-, and Poole, F. G., 1963, Geologic map of the Oak Spring quadrangle: U.S. Geological Survey correlated in amount. (1989) described the analytical capabilities of the Map GQ-214, scale 1:24 000. 5W Ar-ion continuous laser 40Ar/39Ar laboratory Barnes, H-, Christiansen, R. L., and Byers, F. M., Jr., 1965, Ge- in Menlo Park. Analysis of sanidine generally in- ologic map of the Jangle Ridge quadrangle: U.S. Geological ACKNOWLEDGMENTS Survey Map GQ-363, scale 1:24 000. volved fusing 6-8 grains of sanidine; in a few Bath, G. D., 1968, Aeromagnetic anomalies related to remanent cases up to 15 grains of finer-size sanidine (<100 magnetism in volcanic rock, Nevada Test Site, in Eckel, J. C. Cole, P. P. Orkild, and G. A. Izett E. B., ed., Nevada Test Site: Geological Society of America mesh) were fused, and for several samples large Memoir 110, p. 135-146. provided encouragement and guidance in single crystals were analyzed. Approximate masses Best, M, G., and Christiansen, E. H., 1991, Limited extension dur- of sanidine fused in each analysis were 0.005-0.4 ing peak Tertiary volcanism, Great Basin of Nevada and the formulation of the study. We thank G. B. : Journal of Geophysical Research, v. 96, no. N8, mg. Typically, each age determination represents Dalrymple for the opportunity to work on p. 13 509-13 528. the weighted mean of 4-6 sanidine laser-fusion Broxton, D. E., Warren, R. G„ Byers, F. M., Jr., and Scott, R. B., the "GLM" at the U.S. Geological Survey in analyses (Table 3). Mass analyses were performed 1989, Chemical and mineralogical trends within the Timber Mountain-Oasis Valley caldera complex—Evidence for Menlo Park. The support of Gerry Cebula on a MAP 216 mass spectrometer at a sensitivity multiple cycles of chemical evolution in a long-lived silicic of about 2 x 10~14 mol/V. The incremental heat- magma system: Journal of Geophysical Research, v. 94, and Jack Groen for superb mineral separa- p. 5961-5986. ing/age spectrum 40Ar/39Ar technique (Dalrymple tions; of James Saburomaru, Jerry von Es- Broxton, D. E., Chipera, S. J., Byers, F. M., Jr., and Rautman, and Lanphere, 1971; Lanphere, 1988) was used to C. A., 1993, Geologic evaluation of six nonwelded tuff sites sen, and Malcolm Pringle in the lab; of Ray degas —100 mg biotite samples in a resistance- in the vicinity of Yucca Mountain, Nevada for a surface- Sabala for scientific illustrations; and from based test facility for the Yucca Mountain Project: Los heated furnace in 8-12 temperature increments; Alamos National Laboratory Report LA-12542-MS, 83 p. H. Dickensheets, R. Durham, and D. Welch temperatures were measured using a thermocou- Bursik, M., and Sieh, K., 1989, Range front faulting and volcanism ple. Argon isotopic ratios were measured on a 6 in the Mono Basin, eastern California: Journal of Geophysi- of the U.S. Air Force in providing access is cal Research, v. 94, p. 15 587-15 609. gratefully acknowledged. in. (15.2 cm) radius, 60° sector, Nier-type single- Byers, F. M., Jr., Carr, W. J., Orkild, P. P., Quinlivan, W. D„ and collector mass spectrometer. Corrections for un- Sargent, K. A., 1976a, Volcanic suites and related cauldrons Review comments by V. M. Glanzman, 36 39 40 of Timber Mountain-Oasis Valley caldera complex, south- desirable interferences of Ar, Ar, and Ar iso- ern Nevada: U.S. Geological Survey Professional Paper 919, W. D. Johnson Jr., P. P. Orkild, Marge Mac- topes caused by nuclear reactions of K and Ca 70 p. Byers, F. M., Jr., Carr, W. J., Orkild, P. P., Quinlivan, W. D., and Lachlan, Tom Judkins, Dave Schleicher, Ed were made using standard calculations (Dal- rymple and others, 1981). All 40Ar/39Ar ages have Sargent, K. A., 1976b, Geologic map of the Timber Moun- du Bray, W. J. Carr, F. M. Byers Jr., J. C. tain caldera area: U.S. Geological Survey Miscellaneous In- been measured relative to a monitor age of 513.9 vestigation Series Map 1-891, scale 1:48 000. Cole, S. A. Minor, C. J. Fridrich, C. M. Ma for MMhb-1 (Lanphere and others, 1990; Byers, F. M„ Jr., Carr, W. J., and Orkild, P. P., 1989, Volcanic centers of southwestern Nevada—Evolution of understand- Johnson, W. Mcintosh, S. Mertzman, A. G. Dalrymple and others, 1993). An age of 27.92 Ma ing, 1960-1988: Journal of Geophysical Research, v. 94, Sylvester, and J. R. LeCompte helped cor- was calculated for the internal monitor standard p. 5908-5924. Taylor Creek Rhyolite sanidine 85G003. Carr, W. J., 1990, Styles of extension in the Nevada Test Site rect many errors of commission and omis- region, southern Walker Lane belt—An integration of vol- Uncertainties in weighted means are the in- cano-tectonic and detachment fault models, in Wernicke, sion; however, the conclusions and any B. P., ed., Basin and Range extensional tectonics near the verse variance-weighted lcr standard error of the remaining errors remain the responsibility latitude of Las Vegas, Nevada: Geological Society of Amer- mean (Taylor, 1982) for multiple laser fusion ica Memoir 176, p. 283-304. of the authors. This project was funded analyses or fractions of an incremental-heating Carr, W. J., Byers, F. M„ Jr., and Orkild, P. P., 1986, Stratigraphie and volcano-tectonic relations of the Crater Flat Tuff and under Interagency Agreement DE-AI08- age spectrum and include propagation of the some older volcanic units: U.S. Geological Survey Profes- 91NV11040 between the U.S. Geological J-curve calibration uncertainty. Pooled ages for sional Paper 1323, 28 p. multiple samples of a unit are weighted means of Caskey, S. J., 1991, Mesozoic and Cenozoic structural geology of Survey and the Department of Energy, Ne- the CP Hills, Nevada Test Site, Nye County, Nevada—And several determinations. Estimates of errors as- regional implications [M.S. thesis]: Reno, University of Ne- vada Operations Office. signed to individual age measurements (internal vada, 90 p. Christiansen, R. L., 1979, Cooling units and composite sheets in errors) are made by propagating well-docu- relation to caldera structure, in Chapin, C. E., and Elston, APPENDIX: ANALYTICAL METHODS mented errors from all aspects of each analysis W. E., eds., Ash-flow tuffs: Geological Society of America (Cox and Dalrymple, 1967; Taylor, 1982). Repro- Memoir 180, p. 29-42. Christiansen, R. L., and Lipman, P. W., 1965, Geologic map of the Sanidine separates for this study were prepared ducibility of ages (external error) in highly radio- Topopah Spring Northwest quadrangle, Nye County, Ne- by Kistler (1968), Marvin and others (1970, 1973, genic samples such as sanidine and biotite is gen- vada: U.S. Geological Survey Geologic Quadrangle Map erally as good or better than internal errors unless GQ-444, scale 1:24 000. 1989), Marvin and Cole (1978), and Fred Mc- Christiansen, R. L., and Lipman, P. W., 1972, Cenozoic volcanism Dowell (University of Texas, 1989, written com- geologic factors such as contamination or argon and plate tectonic evolution of the Western United States— mun.) and were previously analyzed by the con- loss are present. The cited best estimate uncer- Part 2, Late Cenozoic: Royal Society of London Philosoph- ical Transactions, ser. A, v. 271, p. 249-284. ventional K/Ar method, were new sanidine tainty of the weighted mean is the square root of Christiansen, R. L„ Lipman, P. W„ Carr, W. J., Byers, F. M„ Jr., separates prepared at Los Alamos National Lab- the sum of the weighting factors (Taylor, 1982) Orkild, P. P., and Sargent, K. A., 1977, The Timber and represents a 1 tr (68.27%) confidence interval Mountain-Oasis Valley caldera complex of southern Ne- oratory, or were new sanidine and biotite pre- vada: Geological Society of America Bulletin, v. 88, pared at the U.S. Geological Survey (USGS) min- of the mean. These uncertainties are analogous to p. 943-959. eral separation facility in Denver, Colorado. the standard error of the mean (s.e.m., Table 3) Cole, J. C., Wahl, R. R., and Hudson, M. R„ 1989, Structural for arithmetic means except that they only include relations within the Paleozoic basement of the Mine Moun- Sanidine separates were leached with dilute HF tain block: Implications for interpretation of gravity data in followed by ultrasonic cleaning to remove volcan- internal errors and thus do not include excess dis- Yucca Flat, Nevada Test Site, in Olsen, C. W., and Carter, ic glass and other surface impurities. Only a few persion of the measured ages. External error in J. A., eds., Proceedings, Fifth Symposium on Containment of Underground Nuclear Explosions: Lawrence Liver- milligrams of sanidine were encapsulated in alu- excess of the internal estimates is alternatively in- more National Laboratory Report CONF-8909163, v. 2, minum foil for irradiation; biotite samples for in- corporated using the mean square of weighted p. 431-455. cremental heating weighed about 100 mg. The deviates (MSWD, Mclntyre and others, 1966) as Cole, J. C., Harris, A. G., Lanphere, M. A., Barker, C. E., and Warren, R. G., 1993, The case for pre-middlc Cretaceous sanidine and biotite capsules were placed in fused a measure of dispersion. Where MSWD is >1.0, extensional faulting in northern Yucca Flat, southwestern silica vials with flux monitor standards (Taylor estimated uncertainties are adjusted to incorpo- Nevada: Geological Society of America Abstracts with Pro- rate the external error component, multiplying grams, v. 25, no. 5, p. 22. Creek Rhyolite sanidine, Fish Canyon Tuff sani- Cox, A., and Dalrymple, G. B., 1967, Statistical analysis of geo- dine, and SB-3 biotite) placed at regular intervals them by the square root of MSWD (Ludwig, magnetic reversal data and the precision of potassium- and were irradiated in the USGS TR1GA reactor 1988). Resulting uncertainties produce a MSWD argon dating: Journal of Geophysical Research, v. 72, of 1.0, maintaining the weighted mean and rela- p. 2603-2614. in Denver, Colorado, using the procedures de- Dalrymple, G. B., 1989, The GLM continuous laser system for scribed in Dalrymple and others (1981) over the tive magnitudes of the error estimates for indi- 4"At/39At dating: Description and performance character- course of four irradiations (LXXXIV, LXXXVII, vidual samples. istics: U.S. Geological Survey Bulletin 1890, p. 89-96. Dalrymple, G. B., and Duffield, W. A., 1988, High precision XCIII, and CII) between 1989 and 1991; all Ar/39Ar dating of Oligocene rhyolites from the Mogollon- pooled results of multiple samples are from the Datil volcanic field using a continuous laser system: Geo- REFERENCES CITED physical Research Letters, v. 15, p. 463-466. first two irradiations. Typical irradiation time was Dalrymple, G. B., and Lanphere, M. A., 1971,4l)Ar/3,Ar technique 16 hr and resulted in total neutron doses of about Axen, G. J., Taylor, W. J., and Bartley, J. M., 1993, Space-time of K-Ar dating—A comparison with the conventional tech- 1 x 1018 nvt (nvt = neutron density X neutron patterns and tectonic controls of Tertiary extension and nique: and Planetary Science Letters, v. 12, magmatism in the Great Basin of the western United States: p. 300-308. velocity X irradiation time). Geological Society of America Bulletin, v. 105, p. 56-76. Dalrymple, G. B., Alexander, E. C., Lanphere, M. A., and Kraker,

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Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/106/10/1304/3381927/i0016-7606-106-10-1304.pdf by guest on 02 October 2021 SAWYER AND OTHERS

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1318 Geological Society of America Bulletin, October 1994

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