Triassic Caldera at Tioga Pass, Yosemite National Park, California: Structural Relationships and Significance

Triassic caldera at Tioga Pass, Yosemite National Park, California: Structural relationships and significance

Richard A. Schweickert* Department of Geological Sciences, University of Nevada, Reno, Nevada 89557 Mary M. Lahren }

ABSTRACT INTRODUCTION basis of Rb-Sr whole-rock dating (Kistler and Swanson, 1981). However, field relationships de- A Middle or Late Triassic volcanic vent Metavolcanic and metasedimentary rocks of scribed here show that the caldera fill is of Triassic structure, named the Tioga Pass caldera, is ex- Triassic and Jurassic age occur widely within age. In addition, we summarize evidence that the posed near the eastern boundary of Yosemite roof pendants of the eastern Sierra Nevada in Late Triassic Lee Vining Canyon pluton may rep- National Park, California. The caldera and re- California. These rocks, together with coeval in- resent the subvolcanic magma chamber that was lated volcanic and plutonic rocks—part of an trusions, are generally regarded as remnants of an closely related to the evolution of the caldera and early Mesozoic continental-margin magmatic early Mesozoic continental-margin magmatic arc eruption of the 222 Ma ash-flow tuff. arc in east-central California—formed prior to that was engulfed by younger plutons of the Cre- A preliminary report of some of the relation- or during an episode of contractional deforma- taceous Sierra Nevada batholith (Busby-Spera, ships described here appeared in a field trip tion in the arc. Field relationships show that a 1984, 1988; Fiske and Tobisch, 1978, 1994; guidebook (Schweickert and Lahren, 1993a), but widespread 222 Ma rhyolitic ash-flow tuff was Schweickert, 1978; Schweickert and Lahren, this paper presents additional field and age data erupted as an extensive outflow sheet during 1993a, 1993b). The volcanic sections in many and more complete interpretations of the rocks of the formation of the caldera. The Late Triassic areas comprise thick sequences of lava flows, ash the Tioga Pass caldera. Lee Vining Canyon pluton may represent the flows, and interbedded sedimentary rocks. Few Ancient calderas may be recognized through subvolcanic magma chamber that was par- volcanic eruptive centers have been identified, some combination of key features (Lipman, tially evacuated during the eruption of the ash- however, and relationships between volcanic and 1984), including (1) the presence of precollapse flow tuff. The caldera wall is now exposed as a intrusive rocks are largely unknown. Several pos- volcanic rocks, (2) the structural and/or topo- highly irregular boundary between prevolcanic sible Triassic calderas were previously reported graphic boundary of the caldera, (3) possible vents basement and intracaldera rocks that formed in the Mineral King pendant in the southern from which extensive ash flows were erupted, by a combination of initial caldera collapse and Sierra Nevada (Busby-Spera, 1984); these are the (4) the associated regionally extensive ash flow, subsequent intracaldera intrusive and extru- only source vents known until now. A much (5) the caldera floor, (6) the caldera-fill sequence sive events. Intracaldera rocks include a thick younger Cretaceous caldera related to evolution with associated collapse breccias, ash flows, in- section of Triassic metasedimentary and of plutons of the Sierra Nevada batholith was also trusions, and sedimentary fill, and (7) intrusions metavolcanic rocks on Gaylor Peak, together reported by Fiske and Tobisch (1978, 1994). that represent the subvolcanic magma chamber with the Dana sequence on Mount Dana. This paper reports new field evidence from the that fed the eruption of ash-flow tuff. Even after All of the Triassic rocks of the Saddlebag Saddlebag and northern Ritter Range pendants in deformation and metamorphism, many of these Lake pendant later underwent strong defor- the east-central Sierra Nevada (Fig. 1) for linkage features should still be recognizable. mation and metamorphism involving folding between a Triassic ash-flow tuff, its source caldera and thrusting during Middle Jurassic time. and vent area, and its underlying subvolcanic in- REGIONAL SETTING The caldera fill is now exposed in the lower trusion. In this area, detailed geologic mapping has plate of an east-vergent Jurassic thrust, which revealed major components of what we interpret to Metavolcanic and metasedimentary rocks of emplaced lower Paleozoic through Jurassic(?) be a caldera and an associated vent complex that Triassic and Jurassic age near the eastern edge of metasedimentary and metavolcanic rocks developed in the Triassic, not long after the initia- Yosemite National Park occur mainly within the structurally above the caldera fill. tion of east-directed subduction along the western Saddlebag Lake and northern Ritter Range pen- The results of this study indicate that margin of North America. We provide evidence dants (Fig. 1). These two pendants form a con- caldera formation may occur in a contrac- that this volcanic structure is the source area from tinuous, elongate remnant of pre-Cretaceous tional arc setting. Structural and strati- which an extensive, 222 Ma ash-flow tuff was metamorphic rocks along the eastern edge of the graphic relationships described here may also erupted. The vent complex is located adjacent to a Late Cretaceous Tuolumne Intrusive Suite. provide clues to recognition of other caldera thick sequence of Upper Triassic metavolcanic and Tioga Pass, along Highway 120, is generally and vent complexes in highly deformed metasedimentary rocks (Schweickert and Lahren, taken as the boundary between the Saddlebag metavolcanic sequences in the western United 1989, 1993a). Distinctive rocks that we interpret as Lake pendant to the north and the Ritter Range States and elsewhere. caldera fill and that are known in part as the Dana pendant to the south. sequence (Kistler, 1966a, 1966b; Russell, 1976) The oldest rocks within the pendants consist *E-mail: [email protected]. were previously assigned a Cretaceous age on the of eugeoclinal metasedimentary rocks known

GSA Bulletin; November 1999; v. 111; no. 11; p. 1714–1722; 3 figures.

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Figure 1. Tectonic sketch map of the Saddlebag Lake pendant and part of the northern Ritter Range pendant, eastern Sierra Nevada, Cali- fornia. Diagonal lines delineate the Lee Vining Canyon pluton, which is interpreted as the subvolcanic magma chamber beneath a Triassic caldera. GP—Gaylor Peak. Filled circles represent blocks of chert near base of caldera fill described in text. Inset shows location of the study area.

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as the Palmetto Formation (Schweickert and topic of this paper. Although all metamorphic Schweickert, and Walker, unpub. data) intrude Lahren, 1984, 1987, 1993a; Greene, 1995), rocks described herein have been intensely de- lower Paleozoic rocks and metavolcanic rocks in which are of early Paleozoic age (Fig. 1). These formed and have undergone metamorphic recrys- the eastern parts of the pendants (Fig. 1). Triassic rocks are unconformably overlain in parts of tallization in the hornblende hornfels facies, and Jurassic(?) thrust faults have imbricated the the Saddlebag Lake pendant by Lower Trias- protolith names are used in this paper to empha- metamorphic and plutonic rocks in the pendants sic(?) clastic sedimentary rocks. Upper Triassic size original stratigraphic, extrusive, and intru- (Schweickert and Lahren, 1987; Schweickert and and Jurassic metavolcanic and metasedimen- sive relationships. Volcanic rock names such as Lahren, 1993a; Greene, 1995). tary rocks of the Koip sequence form a thick, dacite and rhyolite are based on hand-sample and west-facing section that rests upon the older thin-section criteria. In this paper, we use the TIOGA PASS CALDERA rocks (Fig. 1) (Kistler, 1966a, 1966b; Brook, term “ash-flow tuff” rather than “ignimbrite” to 1977; Keith and Seitz, 1981; Bateman et al., describe pyroclastic deposits, to honor the prior Rocks we associate with the Tioga Pass 1983; Lahren et al., 1984; Schweickert and use of this term in the region. caldera are well exposed along the Yosemite Lahren, 1984, 1987, 1993a; Greene, 1995; Huber Two large plutons of Late Triassic age—the National Park boundary on the flanks of Gaylor et al., 1989). Lee Vining Canyon and Wheeler Crest plutons Peak and Mount Dana, northwest and southeast, Upper Triassic metavolcanic and metasedi- (Kistler, 1966b; Chen and Moore, 1982)—and respectively, of the Tioga Pass entrance station mentary rocks in the Tioga Pass area are the main the Middle Jurassic Mono Dome pluton (Lahren, to Yosemite National Park (Figs. 1 and 2). Both

B 69 A

79 Gaylor Peak thrust 52 Gaylor Peak

85 55 Gaylor Yosemite Lake National 75 Park 76 entrance 85 72 station Tioga Tioga Pass Caldera 30 77 120 60 69 Pass 65 49 47

67 42

57 52 S3 (S1-Koip) 49 60 49 S0xS3(S1-Koip) A' Tioga 30 S 0 Lake Bennettville 49 S0xS1

Boundary of 73 caldera-fill rocks B' 71

ake ag L dleb Sad N To

0 0.5 1 km 120

Figure 2. Geologic map of the Tioga Lake–Gaylor Peak area near Highway 120. Map is shown with north toward lower right to provide oblique cross-sectional view of the northern part of the Tioga Pass caldera. Map legend and cross sections A–A′ and B–B′ are shown in Figure 3. Unit of undifferentiated sandstone, siltstone, and conglomerate near north edge of map includes upper Paleozoic(?) to Lower Triassic(?) sedimentary

units. A pre-Mesozoic fold and foliation (S1) are shown in the Palmetto Formation.

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areas expose sequences of metavolcanic and they are truncated at the caldera wall (Fig. 2). The lower part made up of silicic ash-flow tuffs and metasedimentary rocks that we interpret to be rhyolitic-dacitic ash-flow tuff, the lowest unit in andesite flows, a middle part consisting of vol- caldera fill and associated intrusive rocks. The ex- the Koip, is a thin unit that rests directly on the caniclastic sandstone, and an upper part content and distribution of these rocks suggest that Palmetto Formation where unit 1 is absent. Erup- sisting of interbedded silicic ash-flow tuffs and the caldera originally had a minimum diameter tion of this unit, which represents direct evidence andesite flows (Figs. 2 and 3). of about 7 km, which is close to the median of precollapse volcanism, signaled the develop- Lower Part. The lower part of the section is diameter for Quaternary calderas reported by ment of a magmatic system at shallow depths. poorly preserved and is extensively intruded by Walker (1984). The stratigraphically higher rhyolitic ash-flow plutonic and hypabyssal rocks. It is about 520 m tuff of Saddlebag Lake (unit 4) (Figs. 1 and 2) is thick. The lowest exposed unit is rhyolitic ash- General Features an extensive outflow sheet that is coeval with flow tuff near the southwest shore of Tioga Lake. some of the caldera-margin intrusions, as shown Its thickness is unknown because it is intruded An outstanding exposure of the north wall of later, and was erupted from fissures along the by the granodiorite of Tioga Lake on the east and the proposed caldera is preserved between caldera wall during caldera collapse. by Triassic(?) diorite and quartz monzodiorite on Gaylor Lake and Tioga Lake, northwest of High- In the following sections, we describe first the the west. This unit is overlain by a unit com- way 120 (Fig. 2), where a 1.1-km-thick section caldera-fill sequence, then relationships along the posed of massive andesitic flows and breccias, of Triassic metavolcanic and metasedimentary north wall of the caldera, the outflow sheet of with several thin, interlayered ash-flow tuffs and rocks is nested within rocks of the lower Paleo- ash-flow tuff, and finally the eruptive vent itself. minor volcanic conglomerate. The uppermost zoic Palmetto Formation along a nearly vertical Later, we present an interpretation of evolution of unit in the lower section is another rhyolitic ash- contact with more than 0.5 km of vertical relief. the caldera. flow tuff. These ash-flow tuffs in the lower part A variety of breccias and hypabyssal intrusive of the caldera-fill sequence are petrographically units separate the Triassic section from the base- Intracaldera Fill similar to the outflow sheet and, together with ment rocks. Stratified units of the Triassic sec- the lava flows, were probably erupted more or tion, here interpreted as caldera fill, strike at Gaylor Peak Section. We interpret a 1.1-km- less concurrently with the coeval outflow sheet, nearly right angles toward the contact with the thick sequence of sedimentary and volcanic the rhyolitic ash-flow tuff of Saddlebag Lake. Palmetto Formation (Fig. 2). The caldera wall is rocks on the eastern slope of Gaylor Peak and on We consider them to be cogenetic with the out- now exposed as a highly irregular boundary be- Mount Dana to represent the sedimentary fill of flow sheet (discussed later). The fact that much tween prevolcanic basement and intracaldera the Triassic Tioga Pass caldera (Figs. 2 and 3). of the section is more mafic than the outflow rocks. In the Jurassic, the wall was folded during Brook (1977) noted that these rocks have enig- sheet is consistent with observations of Tertiary regional contractional deformation. matic stratigraphic and structural relationships calderas such as the 11 Ma Timber Mountain North of the caldera wall, precaldera base- and are unknown elsewhere in the Saddlebag caldera in Nevada, where more mafic tuffs have ment includes part of the lower Paleozoic Pal- Lake pendant to the north. Our mapping of the preferentially ponded in the collapsing caldera metto Formation and overlying Mesozoic units, area for more than 40 km along strike shows that (Byers et al., 1976). the latter overlapping the age of the caldera. The these particular units are highly localized and Caldera-collapse breccia is absent within the Palmetto Formation is overlain unconformably only occur on Gaylor Peak and Mount Dana. On exposed lower part of the caldera-fill sequence by the following Mesozoic units: (1) a discon- Gaylor Peak, these rocks comprise a west-dipping between Gaylor Peak and Tioga Lake. Although tinuous, thin, sandstone-siltstone-conglomerate section, whose lowest exposed part occurs along thick accumulations of collapse breccia are typi- unit; (2) a 30–40-m-thick rhyolitic-dacitic ash- the southwestern shore of Tioga Lake and in cal of the fill within many calderas, some flow tuff unit that is poor in quartz phenocrysts; road cuts along Highway 120. The base of the calderas with intracaldera ash-flow accumula- (3) the conglomerate of Cooney Lake, a later- section is not exposed because the lowest part tions of 1 km or more have been reported to be ally continuous, 150-m-thick unit of conglom- of the caldera fill is extensively intruded by devoid of collapse breccias (Lipman, 1984). In erate and sandstone; (4) the rhyolitic ash-flow andesite porphyry, diorite, and Cretaceous grano- these cases, caldera subsidence is regarded to tuff of Saddlebag Lake, a laterally continuous, diorite of Tioga Lake (Kistler, 1966a, 1966b; have been slow during the ash-flow eruption, and 100-m-thick unit, and (5) andesitic breccia and Bateman et al., 1993; Lahren, Schweickert, and steep, free-standing caldera walls have not devel- lava flows with minor volcaniclastic strata. Walker, unpub. data). The highest exposed part oped (see also Walker, 1984). Units 1, 2, and 3 predate formation of the of the caldera fill lies immediately east of the Middle Part. Volcaniclastic sandstone, which caldera, and units 2, 3, 4, and 5 form part of the summit of Gaylor Peak, where it is cut by the composes the 290-m-thick middle part of the Koip sequence of Kistler (1966a). Most con- Gaylor Peak thrust. Since the base and top of the caldera-fill section, varies from massive to tacts between these units are sharp and regular, caldera fill are not exposed, and cleavage in the parallel- and cross-laminated; rare cross-lamina- and all but unit 1 are rather continuous over the rocks is subparallel to bedding (as described tion indicates that the section faces southwest. entire extent of Figure 1 (about 40 km). The under Structural Relationships), the 1.1 km thick- The sandstones, although strongly recrystal- basal contact of unit 2, the rhyolitic-dacitic ash- ness is a minimum value. lized, are all lithic rich with volcanic-rock frag- flow tuff, is an angular unconformity, and the Field relationships (discussed later) indicate ments, and quartz content varies from about 5% basal contacts of the conglomerate of Cooney that the sedimentary and volcanic units of the to 20%. Parallel lamination suggests that these Lake and the rhyolitic ash-flow tuff of Saddle- caldera-fill sequence are younger than the con- units were deposited in water, either in caldera bag Lake are probably disconformities. glomerate of Cooney Lake, and we interpret lakes or in marine conditions. We interpret the units north of the caldera wall them to be younger than the outflow sheet. Thus, Upper Part. The 300-m-thick upper part of as follows: The rhyolitic-dacitic ash-flow tuff the caldera fill can also be considered part of the the caldera-fill section consists of a lower dacitic to (unit 2) and the conglomerate of Cooney Lake lower portion of the Koip sequence. rhyolitic ash-flow tuff, a middle unit of andesitic (unit 3) that overlie the Palmetto outside the The section on the eastern slope of Gaylor flows and breccias, and an uppermost unit of caldera must predate caldera collapse, because Peak can be divided into three main parts: A dacitic ash-flow tuff. Numerous hypabyssal in-

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Cenozoic cover SW NE Cretaceous granite B Gaylor Peak thrust B' Koip sequence

Andesitic breccia Tr-Jr?

Rocks of Tioga Pass caldera

Andesite Dacite breccia porphyry

Quartz Diorite monzodiorite

Rhyolitic- Andesite Volcani-

Triassic dacitic ash- clastic flow tuff sandstone SW Gaylor Peak thrust Caldera fill NE 222 Ma rhyolitic ash- flow tuff of Saddlebag A A' Lake Conglomerate of Cooney Lake Rhyolitic-dacitic ash- flow tuff

Undiff. sandstone, siltstone, conglomerate Palmetto Formation with calc-silicate Paleozoic interbeds 0 1000 2000 3000 4000 5000 ft Contacts 0 1 km Thrust faults High-angle faults

Figure 3. Cross section A–A′ of the Tioga Pass caldera and B–B′ of the area just north of the caldera. Explanation includes units shown in Figure 2. Structural relationships indicate that gently inclined folds in the Palmetto Formation predate the unconformable Mesozoic section.

trusions occur in this section. These units may described by Kistler (1966a, 1966b), Russell stone, but also includes minor lacustrine(?) signify a return to mainly volcanic input to the (1976), and Greene (1995) and have been limestone in its upper part. Extensive sills and caldera, possibly from scattered intracaldera remapped by us. They compose a section that, podlike intrusions of andesite porphyry occur in vents, or, alternatively, some of these tuffs could although gently folded, dips and tops generally the lower and middle parts of the section, and have come from nearby eruptive centers. Some to the southwest and is of comparable thickness these rocks are especially prominent north of of these volcanic rocks may actually be contem- to that of the section described above. All con- Mount Dana in Glacier Canyon. poraneous with volcanic rocks overlying the ash- tacts between the Dana sequence and other units We have mapped large, 10–30-m-length flow tuff of Saddlebag Lake north of the caldera. are either faults or intrusive contacts (Russell, blocks of metachert of the Palmetto Formation However, this possibility cannot be evaluated 1976; Greene, 1995). near the base of the exposed section in Glacier because a large mass of dacite breccia south of According to Russell (1976), the Dana se- Canyon, immediately north of the summit of Gaylor Lake separates the upper part of the quence includes several lower units, aggregat- Mount Dana (Fig. 1). The contact relationships caldera fill from other parts of the Koip sequence ing about 250 m thick, of silicic to intermediate of these blocks are equivocal because of the to the north (Fig. 2). tuff and volcanic breccia. These are overlain by presence of younger intrusions, but they may be Mount Dana Section. Metavolcanic and argillite, tuff, and minor volcaniclastic sand- wall-rock blocks in megabreccia or collapse metasedimentary rocks of the Dana sequence on stone in the middle part, totaling about 645 m breccia that slumped off the caldera wall during Mount Dana, 1–2 km southeast of the Gaylor thickness. The upper part of the Dana sequence, initial caldera collapse. Alternatively, these Peak area, lie on strike with the units described with a preserved thickness of about 60 m, con- chert masses may be preserved remnants of the above (Fig. 1). These rocks were mapped and sists mainly of volcanic tuff and quartzose sand- precollapse caldera floor.

1718 Geological Society of America Bulletin, November 1999

Although lateral changes are probable, we geneous and consists of several bodies of differ- and the dacite breccia are gradational or inter- suggest that the Dana sequence and the Gaylor ent ages and origins. Much of the breccia con- finger vertically and laterally (Fig. 2), suggesting Peak section are correlative. The lower part of the tains irregular fragments, 1–20 cm in maximum that the two types of intrusions may locally be section on Gaylor Peak may correlate broadly dimension, of light gray dacite and flow-banded contemporaneous. Finally, in some places, with the lower units of Russell (1976) on Mount rhyolite. These fragments are enclosed within a andesite porphyry has a lobate, pillowed contact Dana. The middle parts of both sections consist matrix that weathers dark gray to black and con- against dacite breccia, and angular fragments of mainly of well-bedded and parallel-laminated sists of loose crystals of quartz and plagioclase the andesite porphyry have spalled off into the sedimentary rocks, including volcaniclastic sand- and metavolcanic rock fragments, all set in a re- dacite breccia, forming an intrusive breccia. stone. The upper section on Gaylor Peak, with crystallized mat of quartz and plagioclase. The Interpretation of Relationships among Units abundant ash-flow tuff and andesitic rocks, is breccia locally contains small (several centime- along the Caldera Wall. Contact relationships poorly represented on Mount Dana, where Russell ters across) clasts of andesite porphyry and are complex and contradictory; they suggest (1976) reported only 60 m of section. quartz monzodiorite. Near Gaylor Lake, the brec- that multiple intrusions of andesite porphyry Depositional Environments of Sedimentary cia contains blocks or megaclasts of andesite as and dacite breccia of various ages exist. Early Caldera Fill. Depositional environments for much as 20 m in length. Locally, fragments of andesite porphyry is coeval with the dacite both the Gaylor Peak and Mount Dana sections Triassic(?) quartz monzodiorite as much as 10 breccia and may therefore have been intruded may have been either shallow marine or lacus- cm across occur within the dacite breccia, and in concurrently with initial caldera collapse. Mul- trine, on the basis of the dominance of parallel one place, the dacite breccia forms a thin (2–3 cm tiple intrusions or extrusions of dacite breccia lamination and laterally continuous units in thick) intrusive tongue into an earlier sill of were emplaced along the caldera margin at vari- clastic sedimentary rocks, together with the pres- quartz monzodiorite. Rhyolite and dacite frag- ous times. Sometime during the evolution of the ence of thin limestone units on Mount Dana. ments within the dacite breccia commonly show caldera, texturally similar andesite porphyry internal isoclinal folds of flow layering and also was intruded into both volcanic and sedi- Intrusive and Extrusive Relationships along highly digitated margins, indicating that they mentary intracaldera fill. the North Wall of the Caldera were still in a plastic state when incorporated within the breccia. Outflow Sheet Three types of metamorphosed intrusive rocks The northern contact of the dacite breccia and a distinctive breccia unit in the Gaylor Peak (Fig. 2) cuts discordantly across layering in the Rhyolitic Ash-Flow Tuff of Saddlebag area appear to be closely related to the caldera Palmetto Formation, the overlying rhyolite-dacite Lake. A regionally extensive outflow sheet of margin and separate the stratified caldera fill ash-flow tuff, and the conglomerate of Cooney rhyolitic ash-flow tuff, named the rhyolitic ash- from the lower Paleozoic Palmetto Formation Lake (units 2 and 3), suggesting that the dacite flow tuff of Saddlebag Lake, is exposed (Brook, and overlying units. These units near the caldera breccia is in part intrusive. South of Gaylor Lake, 1977; Keith and Seitz, 1981; Bateman et al., margin include quartz monzodiorite and diorite, the dacite breccia also appears to intrude andesite 1993; Schweickert and Lahren, 1993a) north and andesite porphyry, and dacite breccia. Multiple and ash-flow tuff of the caldera-fill sequence. south of the Tioga Pass caldera (Figs. 1 and 2). intrusions of each type may exist in the area, and Because of the lack of wall-rock fragments, the Evidence discussed later shows that this unit was age relationships among them are complex. In dacite breccia is interpreted to be a primary pyro- erupted from a fissure near the caldera margin addition, the rhyolitic ash-flow tuff of Saddlebag clastic breccia that was directly related to the during caldera collapse. This rhyolitic ash-flow Lake (Figs. 2 and 3; described later) appears to eruptive process, rather than due to gravitational tuff is very distinctive and weathers white to have been erupted from a vent in this area. Rela- slumping of the caldera wall. Parts may be pyro- light gray. It contains abundant, conspicuous, tionships between the sedimentary caldera fill clastic lag breccia, whereas other parts are clearly broken phenocrysts of quartz and minor plagio- and the intrusions are poorly exposed and ambig- intrusive. These types of deposits have been noted clase and alkali feldspar phenocrysts together uous. We interpret the stratified fill to postdate at Miocene calderas in Nevada and elsewhere with collapsed pumice fragments that commonly part of the intrusive suite, as the caldera-collapse (Shawe, 1981, Lipman, 1984; Walker, 1984). are 2 to 3 cm in length. Locally, the tuff also con- event was necessary to allow the accumulation of Andesite Porphyry. Andesite porphyry forms tains lenses (or blocks) as much as 10 m in the caldera fill. Some of the intrusions of andesite very large sheetlike and crosscutting bodies that length of dark gray, andesitic ash-flow tuff. We porphyry postdate the fill. both inflate the stratigraphic section of the have noted no systematic changes in size or Small Intrusions. Small, foliated quartz caldera fill and separate parts of the caldera fill abundance of pumice or rock fragments in the monzodiorite intrusions, which resemble younger from the dacite breccia (Figs. 2 and 3). The ash-flow tuff with distance from the caldera. Cretaceous intrusions described later, occur andesite porphyry contains small, 0.3 cm, white The rhyolitic ash-flow tuff everywhere over- within the lowest exposed part of the caldera fill plagioclase phenocrysts within a dark gray to lies the conglomerate of Cooney Lake and under- near Tioga Lake (Fig. 2). In addition, dark, green- black, aphanitic groundmass. lies a thick succession of andesitic volcanic rocks ish-gray, foliated diorite forms several small Contradictory intrusive relationships suggest that makes up most of the Koip sequence. We sheetlike intrusions near and west of Tioga Lake. that the andesite porphyry may include several have traced this unit 25 km north from Gaylor Contact relationships between the intrusions are discrete intrusions and may overlap in age with Lake, where it occurs in several different thrust unclear. However, both of these types of deformed the dacite breccia. For example, in one area (as sheets within the Saddlebag Lake pendant. A intrusions are probably of Triassic age because shown in Fig. 2), dacite breccia truncates a dike sample of this unit near the north shore of they predate other Triassic units described later. of andesite porphyry. Together with the observa- Cooney Lake has yielded a U-Pb zircon age of Dacite Breccia. A distinctive unit of dacite tion that clasts of andesite porphyry occur locally 222 + 5 Ma (Fig. 1) (Schweickert and Lahren, breccia forms a highly irregular mass between in the dacite breccia, this intrusive relationship 1987). The ash-flow tuff has a maximum thick- Gaylor Lake and Tioga Lake, along the north indicates that some of the andesite porphyry ness of about 100 m in areas north of Gaylor wall of the caldera (Figs. 2 and 3). intrusions predate the dacite breccia. Elsewhere, Lake, and it appears to thin gradually northward The dacite breccia unit is extremely hetero- however, contacts between the andesite porphyry and more abruptly southward. The unit 2 km

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southwest of Mount Dana is only about 15 m of the caldera are roughly centered above the constraints discussed later indicate that the subsi- thick, and it pinches out locally near the Dana pluton. The pluton consists of medium-grained dence (or collapse) event was closely followed by Fork of the Tuolumne River, although thick sec- biotite granite with equigranular and mega- contractional deformation in the pendant, rather tions of the ash flow are exposed as far south as crystic phases (Bateman, 1992). Chen and Moore than extensional normal faulting. No evidence of Gem Lake in the Ritter Range pendant (our un- (1982) reported discordant U-Pb zircon data normal faulting or fault-related structures has been published mapping), 5 km south of Figure 1. from this pluton that they interpreted to indicate observed near or at the base of the Koip sequence. These occurrences indicate that the ash flow orig- an age of ca. 210 Ma. The 207Pb/206Pb ages on Second, as discussed earlier, the volcanic and inally had a minimum along-strike continuity of their samples range from 220–234 Ma, suggest- sedimentary deposits nested within the Palmetto at least 50 km. ing that the igneous age of this pluton could be Formation are very localized and restricted in ca. 220 ± 5 Ma and thus coeval with the Tioga occurrence. No volcanic and sedimentary units Eruption Vent along the Caldera Wall Pass caldera. The spatial relationships (Fig. 1), similar to those at Gaylor Peak and Mount Dana composition, and radiometric age of the pluton have been recognized anywhere else beneath the Exposures on the east side of Gaylor Lake suggest to us that the pluton originated as the ash-flow tuff outflow sheet within the northern show that the rhyolitic ash-flow tuff of Saddlebag magma chamber that fed the eruption of the rhy- Ritter Range pendant and the Saddlebag Lake Lake was erupted from a vent along the north olitic ash-flow tuff of Saddlebag Lake. pendant, even though the rhyolitic ash-flow tuff wall of the Tioga Pass caldera and that the ash of Saddlebag Lake has been traced for more than flow is contemporaneous with or younger than Younger Cretaceous Intrusion 45 km along strike. In all other areas studied, the some of the dacite breccia and andesite porphyry ash-flow tuff rests concordantly upon the con- in that area. North of Gaylor Lake, the rhyolitic The Cretaceous granodiorite of Tioga Lake glomerate of Cooney Lake and the more discon- ash-flow tuff concordantly overlies the conglom- (Kistler, 1966a, 1966b; Bateman et al., 1993; tinuous lower ash-flow tuff. erate of Cooney Lake (Figs. 2 and 3). Within Bateman, 1992), is a medium-grained biotite- In addition, the upper plate of the Gaylor Peak about 200 m of the lake, the basal contact of the hornblende quartz monzodiorite that weathers thrust (described later) contains exposures of ash-flow tuff continues smoothly southward and dark gray. This unit crops out mainly near Tioga rocks downdip from the exposures of the pro- overlies dacite breccia along a sharp, planar con- Lake, where it intrudes both the Palmetto posed caldera fill themselves. In the upper plate tact. Locally, along this stretch, a dark, fine- Formation and the lower part of the exposed of the thrust, the conglomerate of Cooney Lake grained chilled zone developed in the rhyolitic caldera fill. West of Tioga Lake, outer borders of and the ash-flow tuff of Saddlebag Lake rest rock; in other places, the two units appear to inter- this intrusion grade progressively over a few directly upon Palmetto Formation. These expo- penetrate. In some localities, the rhyolitic ash- meters into quartz monzodiorite porphyry and sures indicate that the volcanic structure described flow tuff contains clasts of the dacite breccia, and andesitic porphyry, both of which closely resem- here had a rather limited extent downdip, parallel locally the rhyolitic material has spalled angular ble the older, foliated quartz monzodiorite and to the bounding fault, unlike a riftlike depression, fragments into the cooler, but possibly still liquid, andesite porphyry intrusions within the caldera which would be expected to extend indefinitely matrix of the dacite breccia. Clasts of andesite fill. For these reasons, we previously interpreted parallel to the bounding fault. porphyry 3–8 cm across also occur locally within this pluton to be of Triassic age, perhaps flooring Finally, the thickness and widespread distri- the rhyolitic ash-flow tuff, indicating that some the Triassic caldera (Schweickert and Lahren, bution of the outflow sheet indicate it formed andesite porphyry predates the ash-flow tuff. 1993a). However, new detailed field mapping from a relatively large-volume eruption, which At the edge of Gaylor Lake, the contact turns and U-Pb zircon dating have shown that this would have been most consistent with a caldera- sharply to the east, and the rhyolitic ash-flow tuff intrusion is Cretaceous in age as previously collapse event caused by the eruption. forms a vertical, dikelike intrusive body (an reported (Kistler, 1966a; Bateman et al., 1993). Of the criteria for recognizing ancient calderas ignimbrite dike) immediately east of the lake The Cretaceous intrusion locally cuts discor- cited in the beginning of this paper, we have iden- (Fig. 2). The ignimbrite dike forms an irregular dantly across foliated, Triassic(?) quartz monzo- tified all seven. One associated feature that is tongue, 50 m thick, that appears to intrude (and to diorite and andesite porphyry along the north poorly represented is collapse breccia. However, have been extruded from within) the dacite wall of the caldera. Our new U-Pb zircon ages Walker (1984) and Lipman (1984) have described breccia and grades upward into the extrusive for three samples from the Tioga Lake intrusion cases of slow or incremental caldera subsidence in rhyolitic ash-flow tuff of Saddlebag Lake. This give an age of ca. 96 Ma (Lahren, Schweickert, which extensive collapse breccias did not form. intrusive part of the rhyolitic ash-flow tuff con- Walker, and Girty, unpublished data). For these reasons, the caldera interpretation is tains phenocrysts of plagioclase and quartz; the best explanation for the intrusive and extru- plagioclase is more abundant than in the over- Alternate Interpretations for the Caldera sive relationships described here. lying ash-flow tuff. We interpret this intrusive tongue to have formed from vesiculated and frag- A possible alternate interpretation for the fea- Sequence of Caldera-Forming Events mented rhyolitic magma in a fissure vent within tures described here is that the stratified volcanic the partly coeval dacite breccia. The dike directly and sedimentary rocks west of Tioga Lake and on Relationships described above suggest the fol- fed an eruption column that produced the outflow Mount Dana represent the fill of a fault-bounded lowing sequence of events before and during the sheet during the collapse of the caldera. volcano-tectonic depression or rift rather than a evolution of the caldera (Figs. 2 and 3): (1) caldera. In this interpretation, the rhyolitic ash- Localized or distal eruption and deposition of Subvolcanic Magma Chamber flow tuff of Saddlebag Lake would have been rhyolitic-dacitic ash-flow tuff (lowest part of the erupted from a fissure along the bounding fault of Koip sequence), signaling the initial development The Lee Vining Canyon pluton is a large, the depression, perhaps following subsidence of of a shallow magmatic system now exposed as felsic intrusion that lies east of, and structurally the floor of the depression. the Lee Vining Canyon pluton. (2) Deposition of underlies, the rocks of the Tioga Pass caldera This alternate interpretation does not seem conglomerate of Cooney Lake upon a surface of (Kistler, 1966b; Bateman, 1992; Fig. 1). Rocks consistent with several observations. First, timing low relief across the entire area; the duration of

1720 Geological Society of America Bulletin, November 1999

this event is unknown. (3) Intermittent or incre- tain two pre-Mesozoic fabrics and associated Saddlebag Lake, which was deposited west of the mental caldera collapse, accompanied by forma- folds (Brook, 1977; Schweickert and Lahren, present-day edge of the caldera. tion of dacite breccia, intrusion of older andesite 1987, 1993a). Both the rocks of the Koip se- The planar shape-fabric foliation that is wide- porphyries, and intrusion of ignimbrite dike lead- quence and the rocks of the Tioga Pass caldera spread in the metavolcanic rocks gradually inten- ing to eruption of the regional outflow sheet, the (the Dana sequence and the rocks on Gaylor sifies to a mylonitic or phyllonitic foliation along rhyolitic ash-flow tuff of Saddlebag Lake. (4) Peak) contain a well-developed, northwest-strik- the trace of the thrust. Although no linear fabric Gradual subsidence and infilling of caldera de- ing and steeply southwest-dipping shape-fabric elements are evident in these rocks, we infer from pression with more than 1 km of silicic ash-flow foliation and schistosity, representing strong the geometry of the hanging-wall anticline that tuff, andesitic lava, volcaniclastic sandstone, and shortening in a west-southwest–east-northeast the upper plate of the Gaylor Peak thrust moved related units. (5) Repeated intrusion of andesite direction. Nevertheless, foliation is only weakly toward the northeast and that thrust displacement porphyry along the caldera margin, crosscutting developed in some areas. accompanied the development of the regional the sedimentary caldera fill, the dacite breccia, Along the north wall of the caldera, a single northwest-striking foliation. and rhyolitic ash-flow tuff of Saddlebag Lake, cleavage in rocks of the Koip sequence continues The exact timing of the thrust imbrication and together with extrusion of late rhyolitic and smoothly southward into hypabyssal intrusions associated deformation is unknown, although in dacitic ash-flow tuffs. and into caldera-fill rocks. Because the foliation areas 5 km to the northeast of the caldera, cor- lies at a low angle to bedding within the caldera relative structures are cut by the granodiorite of Age Constraints fill, folds are uncommon, although Russell Mono Dome, which has a preliminary U-Pb (1976) mapped large upright folds in the Dana zircon age of 168 ± 2 Ma (Lahren, Schweickert, Critical relationships along the north wall of sequence on Mount Dana. A conspicuous set of and Walker, unpub. data). Northwest of Gaylor the caldera, discussed previously, show that the folds of the contact between dacite breccia and Peak, rocks as young as Early Jurassic are in- rhyolitic ash-flow tuff of Saddlebag Lake post- Palmetto Formation is present northeast of Gay- volved in the deformation (Schweickert and dates or is contemporaneous with the dacite lor Lake along the caldera wall (Fig. 2). These Lahren, 1993a), which indicates that much of the breccia. These relationships and the 222 Ma age folds probably formed because the contact was deformation of the caldera occurred in Middle of the rhyolitic ash-flow tuff of Saddlebag Lake oriented at a high angle to the plane of flattening Jurassic time. All rocks and structures are cut dis- indicate that the formation of the dacite breccia during deformation. cordantly by intrusions to the west that are part of and the caldera-collapse event occurred at ca. Rocks of the Tioga Pass caldera lie in the lower the 92–85 Ma Tuolumne Intrusive Suite (Bateman 222 Ma. The caldera fill would most likely be plate beneath the Gaylor Peak thrust (Figs. 1, 2, et al., 1982; Bateman, 1993). somewhat younger, but still Late Triassic in age. and 3). The Gaylor Peak thrust is a prominent However, Kistler (1966a) and Russell (1976) in- structure that is exposed along the east shoulder of CONCLUSIONS ferred that the Dana sequence lacks structures Gaylor Peak and areas to the north and also con- that are present in the Koip sequence, and there- tinues southeast onto the southwest flank of Mesozoic metavolcanic and metasedimentary fore they regarded the Dana sequence as Jurassic Mount Dana. On Gaylor Peak, the thrust currently rocks near Tioga Lake and on Mount Dana, along in age. In addition, Kistler and Swanson (1981) dips steeply southwest, together with the caldera- the Yosemite National Park boundary, make up reported Rb-Sr whole-rock data from rocks on fill sequence. On Mount Dana, the thrust dips the fill of a Middle or Late Triassic caldera that Mount Dana that suggested ages of 118 to vertically and is locally overturned (Greene, developed early in the history of the continental- 101 Ma. According to the structural relationships 1995), indicating that rocks of the pendant were margin magmatic arc in east-central California. (described later) and age relationships described tilted steeply westward following thrusting. The caldera lies in a lower-plate position be- here, the Dana sequence must be Triassic in age. The upper plate of the Gaylor Peak thrust con- neath the Middle Jurassic(?) Gaylor Peak thrust, We interpret the previously published Rb-Sr data sists of a very large overturned anticline cored by but its northern wall is well preserved in the steep to have little age significance for the rocks on the Palmetto Formation and outlined by the con- slopes between Gaylor Lake and Tioga Lake. In Mount Dana since the Rb-Sr isotopic system is glomerate of Cooney Lake on Gaylor Peak and this area, several hypabyssal intrusions and a dis- easily disturbed by fluid migration and thermal by the 222 Ma rhyolitic ash-flow tuff of Saddle- tinctive breccia form an irregular selvage be- effects of metamorphism (Faure, 1986). bag Lake north of Gaylor Lake (Figs. 1, 2, and 3). tween the stratified rocks of the caldera fill and The caldera-collapse event and ash-flow erup- In this thrust plate, caldera-fill rocks are absent. Paleozoic metasedimentary rocks of the Palmetto tion occurred just prior to the displacement on The hanging-wall anticline continues southeast Formation. A widespread outflow sheet, the the Lundy Canyon thrust (Fig. 1), which in- of Tioga Pass onto the southwest flank of Mount rhyolitic ash-flow tuff of Saddlebag Lake, dated cludes the rhyolitic ash-flow tuff of Saddlebag Dana and to the south (Fig. 1), where the very at ca. 222 Ma, was erupted from a vent localized Lake in both upper and lower plate. The Lundy large upright anticline and the Gaylor Peak thrust along the wall of the caldera, during the caldera- Canyon thrust was active between ca. 222 and involve, in addition, rocks of the Lee Vining collapse event. We interpret the Triassic Lee Vin- 219 Ma (Schweickert and Lahren, 1987), which Canyon pluton (Greene, 1995). We agree with ing Canyon pluton to represent the subvolcanic suggests that these eruptive events took place in Greene (1995) that this structure may be a ramp magma chamber that was partially evacuated a contractional arc setting. anticline developed above the Gaylor Peak thrust during the eruption and caldera-collapse events. as it ramped up through the footwall. This caldera-collapse event and ash-flow STRUCTURAL RELATIONSHIPS The fact that caldera-fill rocks are missing in eruption evidently took place in a contractional the upper plate of the thrust indicates that hang- arc setting, as thrust faulting occurred in the area The Mesozoic metavolcanic rocks in the main ing-wall rocks must represent part of the western between 222 and 219 Ma (Schweickert and part of the Saddlebag Lake pendant, together or downdip wall of the caldera, telescoped across Lahren, 1987). with underlying Paleozoic rocks, underwent the caldera fill by displacement on the thrust. It is interesting that the Saddlebag Lake and strong deformation during Jurassic time, and the The upper plate of the thrust contains an appre- Ritter Range pendants expose two caldera se- Paleozoic rocks of the Palmetto Formation con- ciable thickness of the rhyolitic ash-flow tuff of quences, one of middle Cretaceous age—the

Geological Society of America Bulletin, November 1999 1721

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Minarets Caldera, which lies completely within REFERENCES CITED geochronology of metamorphosed volcanic rocks and a middle Cretaceous volcanic neck in the east-central Sierra the Ritter Range pendant (Fiske and Tobisch, Bateman, P. C., 1992, Plutonism in the central part of the Sierra Nevada, California: Journal of Geophysical Research, 1978, 1994) (just south of Fig. 1)—and the other Nevada batholith, California: U.S. Geological Survey v. 86, p. 10489–10501. Professional Paper 1483, 186 p. Lahren, M. M., Schweickert, R. A., and Girty, G. H., 1984, of Late Triassic age—the Tioga Pass caldera, Bateman, P. C., Kistler, R. W., Peck, D. L., and Busacca, A., Revised stratigraphic succession in the Saddlebag Lake described here, which overlaps the southern end 1983, Geologic map of the Tuolumne Meadows quadran- pendant (SLP), eastern Sierra Nevada, California: Geo- of the Saddlebag Lake pendant and the northern gle, Yosemite National Park, California: U.S. Geological logical Society of America Abstracts with Programs, Survey Geologic Quadrangle Map GQ-1570, scale v. 16, p. 293. end of the Ritter Range pendant. These two 1:62500. Lipman, P. W., 1984, The roots of ash flow calderas in western calderas, which are of dramatically different Brook, C. A., 1977, Stratigraphy and structure of the Saddlebag North America: Windows into the tops of granitic ages, are preserved at two different structural Lake roof pendant, Sierra Nevada, California: Geological batholiths: Journal of Geophysical Research, v. 89, Society of America Bulletin, v. 88, p. 321–334. p. 8801–8841. levels and were fortuitously exposed by a com- Busby-Spera, C. J., 1984, Large-volume rhyolite ash flow erup- Russell, S. J., 1976, Stratigraphy and structure of Mesozoic bination of Mesozoic intraarc thrusting and tions and submarine caldera collapse in the lower Meso- metavolcanic rocks in the vicinity of Mt. Dana, Yosemite zoic Sierra Nevada, California: Journal of Geophysical National Park, California [Master’s thesis]: Fresno, Cali- Cenozoic normal faulting and uplift. Research, v. 89, p. 8417–8427. fornia, California State University, Fresno, 71 p. Recognition of the Tioga Pass caldera and Busby-Spera, C. J., 1988, Speculative tectonic model for the Schweickert, R. A., 1978, Triassic and Jurassic paleogeography associated volcanic and plutonic rocks provides early Mesozoic arc of the southwest Cordilleran United of the Sierra Nevada and adjacent regions, California and States: Geology, v. 16, p. 1121–1125. western Nevada, in Howell, D. G., and McDougall, K. A., important insights into large-scale processes Byers, F. M., Jr., Carr, W. J., Orkild, P. P., Quinlivan, W. D., and eds., Mesozoic paleogeography of the western United within the Triassic convergent margin, together Sargent, K. A., 1976, Volcanic suites and related caul- States: Pacific Section, Society of Economic Paleontolo- with processes within and at the margins of drons of Timber Mountain—Oasis Valley caldera com- gists and Mineralogists, p. 361–384. plex, southern Nevada, U.S.: U.S. Geological Survey Pro- Schweickert, R. A., and Lahren, M. M., 1984, Extent of Antler calderas, and relationships between intrusive and fessional Paper 919, 70 p. and Sonoma belts, sutures, and transcurrent faults in east- extrusive rocks. It also resolves vexing strati- Chen, J. H., and Moore, J. G., 1982, Uranium-lead isotopic ern Sierra Nevada, California: Geological Society of ages from the Sierra Nevada batholith, California: Journal America Abstracts with Programs, v. 16, p. 648. graphic problems within the Saddlebag Lake of Geophysical Research, v. 87, p. 4761–4784. Schweickert, R. A., and Lahren, M. M., 1987, Continuation of pendant. The Tioga Pass caldera may serve as a Faure, G., 1986, Principles of isotope geology: New York, John Antler and Sonoma orogenic belts to the eastern Sierra model for identifying other as yet unrecognized Wiley, 464 p. Nevada, California, and Late Triassic thrusting in a com- Fiske, R. S., and Tobisch, O. T., 1978, Paleogeographic signif- pressional arc: Geology, v. 15, p. 270–273. Mesozoic or older caldera sequences within the icance of volcanic rocks of the Ritter Range pendant, Schweickert, R. A., and Lahren, M. M., 1989, Triassic caldera western Cordillera and elsewhere. central Sierra Nevada, California, in Howell, E. G., and at Tioga Pass,Yosemite National Park, CA: Structural re- McDougall, K. A., eds., Mesozoic paleogeography of the lations and significance: Geological Society of America western United States, Pacific Coast Paleography Sym- Abstracts with Programs, v. 21, p. 141. ACKNOWLEDGMENTS posium 2: Pacific Section, Society of Economic Paleon- Schweickert, R. A., and Lahren, M. M., 1993a, Tectonics of the tologists and Mineralogists, p. 209–219. east-central Sierra Nevada—Saddlebag Lake and north- Fiske, R. S., and Tobisch, O. T., 1994, Middle Cretaceous ash- ern Ritter Range pendants, in Lahren, M. M., Trexler, Our field work in the Saddlebag Lake pendant flow tuff and caldera-collapse in the Minarets caldera, J. H., Jr., and Spinoza, C., Crustal evolution of the Great has been supported by National Science Founda- east-central Sierra Nevada, California: Geological Soci- Basin and the Sierra Nevada: Cordilleran–Rocky Moun- tion grants EAR-87-07312, EAR-89-03963, and ety of America Bulletin, v. 106, p. 582–593. tain Section, Geological Society of America Guidebook, Greene, D. C., 1995, The stratigraphy, structure, and regional Department of Geological Sciences, University of EAR-91-18167. We have benefited from field tectonic significance of the northern Ritter Range pen- Nevada, Reno, p. 313–351. trips and discussions with K. Corbett, G. Dunne, dant, eastern Sierra Nevada, California [Ph.D. thesis]: Schweickert, R. A., and Lahren, M. M., 1993b, Triassic-Juras- R. S. Fiske, D. Greene, R. Hanson, J. M. Mattin- Reno, University of Nevada, 270 p. sic magmatic arc in eastern California and western Huber, N. K., Bateman, P. C., and Wahrhaftig, C., 1989, Geo- Nevada: Arc evolution, cryptic tectonic breaks, and sig- son, C. H. Stevens, R. Strobel, and O. T. Tobisch. logic map of Yosemite National Park and vicinity, Cali- nificance of the Mojave–Snow Lake fault, in Dunne, J. Douglas Walker, M. S. Wardlaw, and G. H. fornia: U.S. Geological Survey Miscellaneous Investiga- G. C., and McDougall, K. A., eds., Mesozoic paleogeog- tions Map I-1874, scale 1:125000. raphy of the western United States, Volume II: SEPM Girty have provided U-Pb dating support. We Keith, W., and Seitz, J., 1981, Geologic map of the Hoover Pacific Section, p. 227–246. appreciate the constructive reviews of an earlier Wilderness and adjacent study area, Mono and Tuolumne Shawe, D. R., 1981, Geologic map of the Manhattan quadran- version of this manuscript by R. Hanson, P. Counties, California: U.S. Geological Survey Miscella- gle, Nye County, Nevada: U.S. Geological Survey Open neous Field Studies Map MF-1185A, scale 1:62 500. File Map 81-516, Scale 1:24000. Lipman, and J. D. Walker. Kistler, R. W., 1966a, Structure and metamorphism in the Walker, G. P. L., 1984, Downsag calderas, ring faults, caldera Mono Craters quadrangle, Sierra Nevada, California: sizes, and incremental caldera growth: Journal of Geo- U.S. Geological Survey Bulletin 1221-E, p. E53. physical Research, v. 89, p. 8407–8416. Kistler, R. W., 1966b, Geologic map of the Mono Craters quadrangle, Mono and Tuolumne counties, California: U.S. Geological Survey Geologic Quadrangle Map GQ-462, MANUSCRIPT RECEIVED BY THE SOCIETY FEBRUARY 24, 1997 scale 1:62500. REVISED MANUSCRIPT RECEIVED JANUARY 6, 1999 Kistler, R. W., and Swanson, S. E., 1981, Petrology and MANUSCRIPT ACCEPTED MARCH 1, 1999

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1722 Geological Society of America Bulletin, November 1999

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