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Home , Minarets (California), Tuff

Middle Cretaceous ash-flow tuff and -collapse deposit in the Minarets Caldera, east-central Sierra ,

R. S. FISKE Department of Mineral Sciences, Smithsonian Institution, NHB-119, Washington, D.C. 20560 O. T. TOBISCH Earth Science Board, University of California, Santa Cruz, California 95064

ABSTRACT areas in the Pendant, east-central , is un- derlain by a thick and massive sequence of metavolcanic rocks no- A 2.3-km section of ash-flow tuff and associated caldera-collapse ticeably less deformed than those immediately to the east. Fiske and deposit, representing the extrusive of part of the Sierra Nevada Tobisch (1978) interpreted these rocks to be remnants of a caldera-fill batholith, is totally exposed from its floor to its top in the Minarets complex deposited in what they called the Minarets Caldera. Later Caldera, east-central Sierra Nevada. Rapid burial and subsequent work by us, reported here, clarifies basic stratigraphic relations in this hornblende hornfels facies resulted in remarkable pres- complex. ervation of primary textures and structures, despite the development of The rocks of the Minarets Caldera are noteworthy because they cleavage domains in parts of the caldera fill; late Tertiary uplift and provide a superbly exposed record of large-scale caldera formation erosion have produced a rugged terrain where every meter coeval with mid-Cretaceous volcanism in the east-central Sierra Ne- of section is available for study. Large-scale caldera-filling eruption of vada. The caldera rocks have yielded mid-Cretaceous radiometric ash-flow tuff was interrupted by emplacement of a wedge-shaped mass ages (Table 1, samples 1-3), suggesting that they may be genetically of caldera-collapse deposit as much as 2 km thick, whose volume ex- related to nearby granitoids of the Shaver, Buena Vista, and Merced ceeded 70 km3. Individual clasts in the caldera-collapse deposit range to Peak intrusive sequences (Stern and others, 1981; Bateman, 1992). as much as 1.8 km across and include a wide variety of andesitic to rhyolitic To date, only a few localities of mid-Cretaceous volcanic rocks and related volcaniclastic rocks, remnants of a precaldera volcanic have been recognized in the Sierra Nevada. Kistler and Swanson field that was probably much more extensive than the caldera itself. The (1981) reported a 100 Ma volcanic neck 8 km northeast of the Minarets caldera-fill sequence rests with angular unconformity on a rugged sur- Caldera; Saleeby and others (1990) described extensive exposures of face eroded into older volcanic rocks; the sequence is capped by bedded 110-100 Ma silicic volcanic rocks in the Kings Canyon area, 60 km to volcaniclastic rocks, including delicately laminated tuffs of probable the south; and Saleeby and Busby-Spera (1986) noted similar rocks in caldera lake origin. The total aerial extent of the Minarets Caldera is not the Lake Isabella area near the southern end of the Sierra Nevada. It known, but the area studied, plus scattered pendants of ash-flow tuff is probable that much of the Sierra Nevada batholith was mantled by and associated volcaniclastic rocks to the west, defines a 30- x 22-km coeval volcanic rocks in the mid-Cretaceous, but only scattered rem- elliptical area that may approximate its original shape. The caldera fill nants of this remain. is invaded by a body of monzonite porphyry, locally miarolitic, Of all of these remnants, however, those contained in the Min- that was probably emplaced during an episode of caldera resurgence. arets Caldera are unique for their superb exposures and remarkable preservation of original volcanic features. Initially altered by caldera- INTRODUCTION related fluids (Hanson and others, 1993), these rocks were rapidly buried to depths of several kilometers and heated by emplacement of Huber and Rinehart (1965) recognized that the rugged terrain nearby granitoids, producing fine-grained groundmass textures that comprising , Banner Peak, the Minarets, and nearby for the most part faithfully preserve original volcanic features. Late

TABLE 1. PRELIMINARY U/Pb AGES FROM THE MINARETS CALDERA AND ASSOCIATED ROCKS

Sample* Concentration (ppm) Atomic ratios Ages (Ma) »ft 204pb ™pb ^Pb mPb ^Pb ^Pb OTPb 2«Pb »spb mPb ^Ih

1 275.5 5.93 153.9 0.00485 0.12022 0.35497 98.3 99.5 129 98.6 2 782.1 13.53 337.0 0.00100 0.06635 0.18151 101.1 108.4 104.1 3 781.3 13.13 339.3 0.00093 0.06268 0.17505 99.2 101.1 98.6 4 526.8 14.40 369.0 0.00174 0.07450 0.28380 144.0 143.7 140 140.6 5 413.6 10.82 247.2 0.00292 0.09181 0.29740 131.8 132.2 131.6

Note: sample locations are shown in Figure 2. •Sample 1, ash-flow tuff, from massive unit 1.5 km west of westernmost Davis Lakes; Sample 2, clast of tuff in caldera-collapse deposit, 3.4 km southwest of Mount Ritter summit; Sample 3, quartz monzonite porphyry of Shellenbarger Lake, 0.4 km east of Shellenbarger Lake; Samples 4 and 5, thick-bedded tuff near western margin of Bench Canyon zone, 0.7 and 0.8 km, respectively, southwest of Hemlock Crossing. Analyses obtained in 1974-1977 by T. W. Stem, U.S. Geological Survey, on bulk zircon separates from 50- to 100-kg samples. In the text, samples 1-3 and 4-5 are referred to as mid-Cretaceous and Lower Cretaceous, respectively, because the uncertainties in the radiometric ages are largely indeterminate. The fact that the ^VbP^V ages are within -1-2 m.y. of the 206Ph/2BU ages in four out of the five samples, however, suggests that the radiometric ages are likely to fall within the broad system/period designations given above (compare Stem and others, 1981, p. 5).

Geological Society of America Bulletin, v. 106, p. 582-593, 9 figs., 1 table, May 1994.

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rocks are bounded on the east and northeast by a near-vertical fault zone that changes strike through about 80° of arc. We interpret this curved fault zone to mark the original caldera boundary, complicated by later normal faulting and postcaldera intrusions. A large mass of diabase porphyry postdates faulting and obscures the fault zone south of Ediza Lake (Fig. 2); the elongate outcrop of this mass suggests that its configuration may have been influenced by the pre-existing fault zone. Rocks of the caldera fill are truncated on the west by the Bench Canyon shear zone, where high-angle reverse motion has brought lower Cretaceous volcaniclastic rocks into contact with the caldera fill. Still farther to the west, small pendants of mid-Cretaceous ash- flow tuff and associated volcaniclastic rocks are associated with co- eval granitoids (Stern and others, 1981, Table 1). Peck (1983, p. 4) suggested that these intrusive and extrusive rocks "may represent the preserved lower part of a caldera." We agree with this possibility and have drawn a heavy dotted and dashed line around a 30- x 22-km elliptical area that encloses all known pendants in the Ritter Range area containing rocks likely deposited in the caldera environment (Fig. 1), but it is uncertain whether this line encloses a single caldera or a complex of several such structures.

STRUCTURAL ASPECTS OF THE CALDERA FILL

The caldera fill is tilted to the southwest, best shown by the 8° dip of its base near Ediza Lake and the 25°-60° dips of the overlying volcaniclastic rocks near Twin Island Lakes (Fig. 2). Tectonic cleav- age in these rocks is generally absent, but isolated domains of weakly Figure 1. Geologic map of the Minarets Caldera and nearby areas: to moderately developed cleavage are present (Tobisch and Fiske, ccd, caldera-collapse deposit; af, ash-flow tuff and associated rocks in- 1982, and unpub. observations). In the field, the cleavage is defined by terpreted to have been deposited in a caldera environment; c, mid- preferred alignment of the long of lithic lapilli (and/or Cretaceous granitoids approximately coeval with the caldera-fill rocks; fragments), which becomes discernible at relatively low values of and ovr, older volcaniclastic rocks. Undesignated areas are chiefly strain (that is, ~15%-20% shortening, Tobisch, 1984, Fig. 4). At still younger granitoids of the Sierra Nevada batholith. The Bench Canyon lower values (<15% shortening), however, field observation of pre- shear zone (heavy wavy lines) separates caldera-fill deposits described in ferred alignment of lapilli becomes problematic. The average com- this report from areas to the west and southwest where similar rock paction strain normal to bedding is about 10% in undeformed lithic types (af) are preserved in scattered pendants. Heavy dashes mark ap- lapilli tuffs from magmatic arc environments (Tobisch and others, proximate boundary fault along the northeast caldera margin; heavy 1977, Table 1), and it is possible that a substantial part of the caldera dots depict speculative outline of entire caldera complex. Geology is fill may have absorbed a small additional component of tectonic strain simplified from Huber and others (1989), Huber and Rinehart (1965), but not enough to alter primary textural relationships. and this study. In addition, many exposures of nonwelded ash-flow tuff display a defined by flattened lapilli, but it is very difficult to establish whether this planar structure is (1) a primary compaction Tertiary uplift and Quaternary erosion have produced a rugged, highly foliation (collapse of glassy pyroclasts during cooling) that was sub- dissected terrain having more than 1.2 km of local relief; glacially sequently tilted to high dips, (2) a primary flow foliation that devel- polished surfaces reveal original volcanic textures and structures in oped varying dips as the ash-flow deposit compacted upon a topo- intricate detail. The resulting terrain provides an extraordinary cross graphically irregular caldera floor (for example, Chapin and Lowell, section of a batholith-related caldera complex from its floor to its top. 1979), (3) a tectonic cleavage contemporaneous with domains of Because the emphasis of this report is on the volcanic history of cleavage found in lapilli tuffs as described above, or (4) some com- these rocks, and because original textures and structures are so well bination of these. With the possible exception of the consistently ori- preserved, we use rock names such as rhyolite and tuff breccia that ented foliation in the ash-flow tuff just south of Davis Lakes (Fig. 2), best reflect protolith characteristics, despite the fact that most of the which may represent a relatively large domain of predominantly tec- caldera fill has been deformed and metamorphosed to the hornblende tonic-induced fabric, we suspect that the origin of the observed foli- hornfels facies (S. S. Sorensen, 1993, personal commun.) ation varied from place to place, reflecting the combination of the primary and tectonic processes described above. ORIGINAL CALDERA CONFIGURATION At the western edge of the mapped area (Fig. 2), rocks of the caldera fill are deformed by the Bench Canyon shear zone (McNulty, Remnants of the Minarets Caldera form the southwestern part of 1991). The boundaries of this shear zone are not sharply defined, the Ritter Range Pendant in the east-central Sierra Nevada batholith because the intensity of cleavage decreases gradually away from its (Fig. 1). The eastern part of the caldera is well defined by the thick axis. Preliminary estimates of strain (Tobisch and Fiske, 1982) from accumulation of ash-flow tuff and a caldera-collapse deposit; these stations along the North Fork of the San Joaquin River are relatively

Geological Society of America Bulletin, May 1994 583

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Figure 3. Panoramic view of part of the Ediza Lake basin looking northwestward, showing Mount Hitter (R), Banner Peak (B), and the caldera boundary fault zone (white arrow). Base of the caldera-fill sequence shown approximately by the lower dashed line (black arrow); gradational contact between ash-flow tuff and overlying caldera-collapse deposit shown by upper dashed line.

along this contact are steep-sided paleogullies, some with a local relief near the pinchout of the collapse deposit southwest of Davis Lakes, of 10-15 m, filledwit h cobble conglomerate. The cobbles, as much as the preserved ash-flow tuff is about 2.2 km thick. The northward 40 cm in diameter, consist chiefly of massive and flow-banded acidic thinning of the caldera-collapse deposit, and the complementary lavas and lesser amounts of welded tuff and lapilli tuff and fine tuff thickening of the ash-flow tuff in the same direction, results in a rea- breccia; all of this material was presumably eroded from a largely vanished volcanic field whose local remnants here underlie the caldera fill. Noteworthy are scattered cobbles of coarsely crystalline granitoid. Apparently, nearby parts of the ancestral Sierra Nevada had been uplifted and eroded to a depth sufficient to unroof this gran- itoid by about 100 Ma. The roughness of the gullied surface and the coarseness of the cobble conglomerate suggest that vigorous erosion was underway at the time of caldera formation. 1500- & A A A AÛ^AA Ash-Flow Tuff A A A AAA y 3 A deposit of ash-flow tuff, as much as 2.2 km thick and ranging .y 1000-\s A A ¿^AAA A A % in composition from rhyolite (Si02 = 73%) at its base near Ediza Lake CL to (Si02 = 66%) at its top near Twin Island Lakes, forms the dominant unit in the Minarets Caldera. With the exception of one ffli^^ffîn A possible 30-m-thick flow unit capping the deposit west of Twin Island Lakes, the unit appears to be massive and may have been the product A ffl^ûAA of a single large eruption, possibly from a zoned reservoir. In the absence of detailed geochemical transects through the ash-flow tuff, a suggestion of systematic upward compositional variation Adj|A A comes from specific gravity data collected at ten stratigraphic levels A

throughout the unit. The upward increase in average specific gravity 2.6 2.65 2.7 (Fig. 4) is compatible with progressive upward compositional change Specific Gravity from rhyolite to rhyodacite. A simple cooling history is implied by the presence of only one zone of dense welding forming the lower 200 m Figure 4. Upward increase of ash-flow tuff specific gravity shown of the unit; much of the rest of the unit is only slightly welded, and an by measurements at ten stratigraphic levels along transect about 1 km extensive zone of vapor-phase crystallization is located in the area south of section A-A' in Figure 2. Note that the 1.2-km thickness of east of Mount Ritter and Banner Peak. caldera-collapse deposit has been graphically removed from this figure. The lower and upper parts of the ash-flow tuff have an aggregate The dots show the mean values of fieldmeasurement s made on ten large thickness of 1.1 km along section A-A' (Fig. 2), and the intervening hand samples using a portable specific-gravity balance developed by caldera-collapse deposit is about 1.2 km thick. Farther to the north, J. G. Moore (1965, written commun.).

586 Geological Society of America Bulletin, May 1994

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sonably consistent 2.2- to 2.3-km combined thickness of these two placed by an avalanche sufficiently thick and energetic to enclose units composing the caldera fill in the area studied. and rotate the 800-m-long slab. Monolithologic clasts of crystal- The densely welded vitrophyre at the base of the ash-flow tuff line marble as much as 30 cm in diameter are conspicuous locally, north of Ediza Lake preserves excellent vitroclastic texture, defined but it is curious that we nowhere observed marble or calcareous by collapsed shards and aligned pumice shreds. Interestingly, this tuff as interbeds in the abundant clasts of bedded volcaniclastic texture is difficult to recognize in thin section and virtually impossible material. In addition, no clasts of coarsely crystalline granitoid (of to see on freshly broken surfaces; in contrast, it is easily seen by the type found in the conglomerate at the base of the caldera fill) hand-lens inspection of smooth, slightly weathered surfaces on gla- were observed in the caldera-collapse deposit. cially polished outcrops. Phenocrysts of potassium 0.5-3.0 Most of the caldera-collapse deposit is massive, but at two iso- mm in diameter are scattered evenly through the rock; quartz phe- lated places we found crudely stratified interbeds of lapilli tuff and tuff nocrysts are rare. Lithic inclusions, mostly clasts of rhyolite to an- breccia 1-6 m thick; these lenses grade laterally and vertically into the desite 0.5-10.0 cm in diameter, make up 5%-20% of the rock. surrounding breccia, suggesting that the avalanche that formed the The vitrophyre in the Ediza Lake area grades upward, deposit may actually have consisted of a few closely spaced flow units through a transition interval of about 100 m, to a zone of vapor- or pulses. These lenses are located just east and west of Mount Ritter, phase crystallization; this tuff consists mostly of pumice shreds, where the bedding attitudes are plotted within the caldera-collapse phenocrysts of potassium feldspar, vitric ash, and scattered lithic deposit in Figure 2. inclusions 1-10 cm in diameter. In places, the percentage of lithic The caldera-collapse deposit grades vertically into the underlying inclusions increases to 20%-30% of the rock, making it difficult to and overlying ash-flow tuff, generally over tens or hundreds of meters distinguish the ash-flow tuff from the finer-grained facies of the of section. This prompted us to show this contact by color gradation overlying caldera-collapse deposit. Much of the vapor-phase zone in Figure 2, rather than by a traditional dashed or dotted line. Near the is porous and vuggy. Small spherical masses of microcrystalline base of the steep cliff on the east side of Banner Peak, for example, quartz, presumably pseudomorphs after cristobalite, occur indi- the two units are gradational through a zone at least 200 m thick. The vidually and in chains along the walls of many vugs in the area top of the collapse deposit near Twin Island Lakes grades upward into east of Mount Ritter. a conspicuous zone of massive lithic lapillistone 100 m thick, which in turn passes upward into the overlying ash-flow tuff. About 100 m Caldera-Collapse Deposit higher stratigraphically (just west of Twin Island Lakes), lenses of tuff breccia as much as 3 m thick and 50 m long are enclosed in the over- Coarse tuff breccia, interpreted to be the result of large-scale lying ash-flow tuff, suggesting that smaller amounts of lithic-rich ma- collapse from the walls of the subsiding caldera, is spectacularly well terial continued to avalanche from the caldera walls after the main exposed in the rugged terrain that includes Mount Ritter, Banner mass of the collapse deposit came to rest. Peak, and the Minarets. The deposit ranges in thickness from about Three kilometers southeast of Mount Ritter, the base of the 2 km southwest of Ediza Lake, to 1.2 km along section A-A' in Fig- caldera-collapse deposit cuts abruptly downsection, truncating the ure 2, and to zero where it pinches out on the slopes southwest of lower part of the ash-flow tuff and the precaldera rocks of the Ediza Davis Lakes. If an average thickness of 1.4 km is assumed throughout Lake basin. It is possible that this relation resulted from the down- its 50 km2 outcrop area east of the Bench Canyon shear zone, a min- cutting effect of one of the later avalanche pulses, perhaps compli- imum volume of at least 70 km3 is indicated. cated by changing failure patterns of the caldera wall and irregular Much of the deposit is composed of a wide array of volcanic paleotopography. In places, angular pieces of ash-flow tuff lithologi- clasts that float in a matrix of finer, nonwelded tuff breccia or lapilli cally identical to the ash flow near the base of the caldera fill occur as tuff containing varying amounts of pumice. The clasts range in diam- clasts in the overlying caldera-collapse deposit (Fig. 8). This relation- eter from a few centimeters to about 1.8 km and include blocks and ship also supports the notion that the caldera collapse was a complex slabs of lava ranging in composition from to rhyolite and in event, possibly involving the disruption of lower parts of the caldera texture from massive, flow-banded, to scoriaceous (Fig. 5). Pieces of fill and incorporation of some of this material into later slumping interbedded tuff, lapilli tuff, and finetuf f breccia are common as shred- events. Another 1-2 km to the southeast, near , parts of ded masses or in irregular, taffy-like forms. In places, irregular blobs the breccia interfinger with jostled blocks of bedded tuff and lapilli tuff of flow-banded and autobrecciated andesite several hundred meters to form a chaotic terrain, whose map pattern is shown only schemat- across convey the impression that partly molten masses of magma ically in Figure 2. The blocks in this terrain are as much as 500 m were somehow included in the caldera-collapse breccia. We did not across and possibly represent large parts of the caldera wall in the observe the gross shattering (without disaggregation) of clasts de- process of disaggregating and avalanching into the caldera. scribed in the Grizzly Peak Caldera, Colorado (Fridrich and others, The caldera-collapse deposit becomes conspicuously finer 1991), but many larger clasts are fractured and invaded by dike-like grained toward the northwest, prompting us to avoid using the widely masses of the surrounding volcaniclastic matrix (Fig. 6). used name caldera collapse breccia (Lipman, 1976) as a descriptive Some of the lithic clasts are sufficiently large to be mapped term for the deposit. North of Lake Catherine, for example, it is separately (Fig. 2). The 1.8-km clast south of Dead Horse Lake difficult to distinguish the finer-grained facies from lithic-rich parts of contains a coherent sequence of andesite lava flows and inter- the enclosing ash-flow tuff, and in places the contact between these bedded volcaniclastic material that presumably represents a large facies has been shown interpretatively in Figure 2. The northward disrupted piece of a pre-existing . A slab of inter- diminution of clast diameter, the thinning of the entire deposit in the bedded tuff and lapilli tuff east of Twin Island Lakes is nearly 800 same direction, as well as the chaotic terrain near Minaret Lake de- m long and dips westward at 47° (Fig. 7). The host breccia in this scribed above, suggest that at least part of the collapse deposit orig- area dips westward at 25°-30° and is only about 1.3 km thick, inated by failure of the caldera wall southeast or south of the mapped indicating that this part of the caldera-collapse deposit was em- area shown in Figure 2.

Geological Society of America Bulletin, May 1994 587

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Figure 5. Clast heterogeneity in caldera- collapse deposit. (A) Shattered fine-grained (3,370-m elevation, 900 m west-south- west from Mount Ritter summit). (B) De- formed, taffy-like mass of dacite lapilli tuff en- closed in darker lapilli tuff ramifying through the outcrop (3,500-m elevation, 500 m south- east of Mount Davis summit).

Intracaldera Volcaniclastic Rocks CALDERA RESURGENCE

A sequence of bedded tuff, lapilli tuff, and tuff breccia, locally No topographic evidence for caldera resurgence is pre- displaying cross-bedding and channeling, overlies the ash-flow served, but it is possible that the porphyritic quartz monzonite of tuff of the caldera fill in the area west and northwest of Twin Shellenbarger Lake, coeval with the caldera fill (Table 1, samples Island Lakes. This material was probably eroded from nearby 1-3), represents the core of a resurgent structure. The roof of this scarps and slopes and reworked by streams on the caldera floor. pluton, well exposed in the south-central part of the mapped area, Conspicuous near the top of this sequence is an 8-m interval of is flat or gently dipping, and its contact with the overlying caldera- laminated tuff, likely deposited in the quiet waters of a caldera collapse deposit is everywhere sharp. On the eastern slopes of the lake (Fig. 9). Most of the strata in this interval are parallel or Minarets, 2-3 km southeast of Mount Ritter (and only several delicately cross-bedded and locally exhibit excellent graded bed- hundred meters from the margin of the Shellenbarger pluton), ding; small slump structures occur in places and may have been eutaxitic foliation of the ash-flow tuff displays anomalous dips and triggered by volcanogenic earthquakes. strikes (Fig. 2), possibly caused by forceful emplacement of the

588 Geological Society of America Bulletin, May 1994

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Figure 5. (Continued). (C) Pieces of flow- banded form striking tuff breccia on east face of the Minarets (3,150-m elevation on spur 300 m south of Iceberg Lake; note pencil at lower rightfo r scale). Largest block is 20 cm in diameter. (D) Piece of thin-bedded dacite tuff, its margins partly shredded, enclosed in lapilli tuff matrix (3,420-m elevation, 300 m east-southeast of Banner Peak summit).

nearby pluton. Evidence for resurgence-related deformation was DISCUSSION not recognized within the caldera-collapse deposit itself because of the scarcity of internal marker units that might be used to define As noted by Lipman (1976), in the complete absence of original such deformation. topographic expression or remnants of outflow , the best evi- Miarolitic cavities, coalesced into steeply dipping trains and dence for the existence of any large caldera, and the Minarets Caldera sheets, are locally conspicuous in the Shellenbarger pluton in the Rit- in particular, is the close association of ash-flow tuff with what can ter Pass area, 2 km south-southeast of Mount Ritter. Although not best be interpreted as caldera-collapse deposit. Even though only the proof of shallow emplacement (Candella, 1991), such cavities, along eastern part of the Minarets Caldera is preserved as a coherent unit, with the porphyritic texture of the Shellenbarger pluton, are at least a totally exposed section of its caldera fill is available for study. Fur- compatible with near-surface emplacement. The possible contempo- thermore, the Minarets Caldera is unusual in preserving excellent raneity of both the Shellenbarger pluton and its host caldera fill (Ta- outcrops of the caldera floor, as well as part of the volcaniclastic ble 1) is an added factor suggesting shallow emplacement, perhaps at sequence that caps the caldera fill. Other known mid-Cretaceous vol- depths of only a few kilometers. caniclastic sequences in the Sierra Nevada (for example, Saleeby and

Geological Society of America Bulletin, May 1994 589

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Figure 6. Clasts in caldera-collapse de- posit showing internal shattering or complex relations with surrounding matrix. (A) Cracks in angular 6-m clast of dacite lapilli tuff in- vaded by dike-like masses of fine tuff breccia from the surrounding matrix (3,480-m eleva- tion, 700 m southeast of Mount Ritter sum- mit). (B) Part of ragged, 10-m clast of (3,380-m elevation, 900 m west- southwest from Mount Ritter summit). Note deep embayment of matrix breccia cutting di- agonally upward through end of clast.

others, 1990; Saleeby and Busby-Spera, 1986) are less well exposed shear zone totals 2.2-2.5 km; when corrections for postdepositional and have uncertain relationships to their source volcanoes. The rocks compaction and tectonic deformation are considered (Tobisch and of the Minarets Caldera, in contrast, are preserved within their source Fiske, 1982), the original thickness of the fill in this area probably volcanic structure. approached 3 km. If this same thickness is extrapolated throughout The total volume of the caldera fill is unknown, chiefly because the area outlined in Figure 1, an original intracaldera volume of about the original configuration of the caldera west of the Bench Canyon 1,500 km3 is indicated. The total volume of the caldera-forming shear zone is uncertain. It is interesting to speculate, however, on eruption would doubtless have been greater, because all outflow de- some of the implications that arise if the scattered pendants of mid- posits have been eroded away. Compared with the caldera data pre- Cretaceous ash-flow tuff and associated rocks west of this shear zone sented by Hildreth (1981) and Lipman (1984), the Minarets Caldera were originally contiguous with the caldera, and if the structure had may therefore have been a moderately large structure, comparable in an original outline as shown by the elliptical dashed and dotted line in plan-view to the Long Valley Caldera, located just 20 km to the east, Figure 1. The preserved thickness of the fill east of the Bench Canyon but considerably larger in volume of erupted material.

590 Geological Society of America Bulletin, May 1994

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Figure 6. (Continued). (C) Closer view of same basaltic andesite clast showing its locally autobrecciated interior and faintly flow banded rim (near hammer). (D) Same clast, whose nearly detached tip is invaded by light- colored lapilli tuff. Note flow-banded clast margin.

With this possibility in mind, it is interesting to consider our Tuff, erupted at Long Valley Caldera, show that the intensity of the observation that much of the ash-flow tuff overlying the basal 200-m earlier plinian phase of the eruption varied (Gardiner and others, welded tuff is either weakly welded or a zone of vapor-phase crys- 1991), and the location of the later ring-fracture eruptions changed tallization. How is it possible that the overlying hundreds of meters of with time (Hildreth and Mahood, 1986). If the same processes oper- ash-flow tuff did not retain sufficient heat to form a much thicker zone ated during the eruption at the Minarets Caldera, then much heat may of welding? We suggest that at least part of the answer may lie in the have been lost during periods of high eruption-column height, or as fact that, despite the continuous exposures in the area studied, subtle individual flow units traveled from changing eruption sites into the boundaries separating individual ash-flow units may have gone un- downgoing caldera. Remembering also that 5%-20% of the ash-flow detected. The implication is that the eruption was not a continuous tuff consists of lithic fragments that were presumably cold at the time event, and that flow boundaries have been masked by vapor-phase of incorporation, much heat may also have been dissipated in bringing crystallization and later metamorphism. Recent studies of the Bishop these inclusions up to the temperature of the tuff.

Geological Society of America Bulletin, May 1994 591

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As mentioned earlier, the caldera-collapse deposit grades into the underlying and overlying ash-flow tuff. Not only is much of the ash-flow tuff choked with lithic clasts, pieces of similar ash-flow ma- terial can be found in the breccia, especially in the area southwest of Ediza Lake. We infer from these relations that collapse-deposit em- placement punctuated the eruption of ash-flow tuff, permitting the two rock types locally to be intimately intermixed. Rather than an instan- taneous, discrete event, however, the emplacement of the caldera- collapse deposit probably began gradually, peaked with the avalanch- ing of extensive parts of the caldera wall to form the main mass of the deposit, and then tapered off to form its gradational upper contact. The caldera-collapse deposit, which has the form of a giant wedge about 2 km thick in the south and pinching out toward the north, has a minimum volume of about 70 km3. The original volume of this wedge was probably much greater, because it is truncated on the west by the Bench Canyon shear zone. The local stratification within the main body of caldera-collapse deposit suggests that it may have been emplaced in several rapid pulses or avalanche units. The crosscutting relationship at the base of the collapse deposit about 3 km southeast of Mount Ritter (Fig. 2) and the incorporation of ash-flow tuff clasts within the same deposit may represent the downcutting effect of a later avalanche pulse, perhaps complicated by changing patterns of caldera collapse.

Figure 7. Slab of bedded volcaniclastic rocks, about 800 m long and 80 m thick, enclosed in the caldera-collapse deposit. View toward the southeast; the northern end of the southernmost Twin Island Lake can be seen in the lower right.

Figure 8. Angular block of vapor-phase ash-flow tuff (underlying hammer) enclosed by finer-grained lapilli tuff matrix of the caldera- fill deposit. This disrupted piece of tuff, indis- tinguishable from the vapor-phase zone near the base of the caldera fill, suggests that lower parts of the caldera fill may have been dis- rupted and redeposited during continuing caldera collapse (3,340-m elevation on spur 550 m north of Banner Peak summit).

592 Geological Society of America Bulletin, May 1994

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rensen. The laboratory assistance of Elizabeth Nielsen and Ellen O'Leary is much appreciated. Funding for our work was provided by the U.S. Geological Survey, the Smithsonian Institution, and the Uni- versity of California, Santa Cruz. We appreciate the thoughtful re- views of Victoria Avery, Cathy Busby, and Richard Hanson.

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J., 1990, Middle Cre- a light-colored, crystal-rich base to a dark, crystal-poor top (3,500-m taceous silicic metavolcanic rocks in the Kings Canyon area, central Sierra Nevada, California: elevation, 700 m northwest of northernmost Twin Island Lake). Geological Society of America Memoir 174, p. 251-270. Stern, T. W., Bateman, P. C., Morgan, B. A., Newell, M. F., and Peck, D. L., 1981, Isotopic U-Pb ages of zircon from the granitoids of the Central Sierra Nevada, California: U.S. Geological Survey Professional Paper 1185,17 p. Tobisch, O. T., 1984, Development of cleavage in lapilli-bearing volcaniclastic rock: Tectonophysics, ACKNOWLEDGMENTS v. 109, p. 309-335. Tobisch, O. T., and Fiske, R. S., 1982, Repeated parallel deformation in part of the eastern Sierra Nevada, California and its implications for dating structural events: Journal of Structural Geology, We have benefited from discussions with Gail Mahood, Tamara v. 4, p. 177-195. Tobisch, O. T., Fiske, R. S., Sacks, S., and Taniguchi, D., 1977, Strain in metamorphosed volcaniclastic Lowe, and Liz Warner, all of Stanford University, who are currently rocks and its bearing on the evolution of orogenic belts: Geological Society of America Bulletin, v. 88, p. 23-40. carrying out detailed geochemical and sedimentologic studies of Min- Tobisch, O. T., Saleeby, J. B., and Fiske, R. S., 1986, Structural history of continental arets Caldera rocks. U/Pb age determinations were kindly provided rocks, eastern Sierra Nevada, California: a case for extensional tectonics: Tectonics, v. 5, p. 65-94. by T. W. Stern, U.S. Geological Survey; information about the Bench Canyon shear zone was made available by Brendan McNulty, Uni- versity of California, Santa Cruz. We acknowledge helpful discus- sions with Charlie Bacon, Paul Bateman, Roy Bailey, Ken Cameron, MANUSCRIPT RECEIVED BY THE SOCIETY JANUARY 21,1993 REVISED MANUSCRIPT RECEIVED JULY 30,1993 Philip Candella, Brooks Hanson, Robert L. Smith, and Sorena So- MANUSCRIPT ACCEPTED AUGUST 11,1993

Geological Society of America Bulletin, May 1994 593

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