VOL. 81, NO. 5 JOURNAL OF GEOPHYSICAL RESEARCH FEBRUARY 10, 1976

Volcanism, Structure,and Geochronologyof Long Valley Caldera, Mono County, California

RoY A. BAILEY

U.S. GeologicalSurvey, Reston, Virginia 22092

G. BRENT DALRYMPLE AND MARVIN A. LANPHERE

U.S. GeologicalSurvey, Menlo Park, California 94025

Long Valley caldera, a 17- by 32-km elliptical depressionon the east front of the Sierra Nevada, formed 0.7 m.y. ago during eruption of the Bishoptuff. Subsequentintracaldera volcanism included eruption of (1) aphyric rhyolite 0.68-0.64 m.y. ago during resurgentdoming of the caldera floor, (2) porphyritic hornblende-biotiterhyolite from centersperipheral to the resurgentdome at 0.5, 0.3, and 0.1 m.y. ago, and (3) porphyritic hornblende-biotiterhyodacite from outer ring fractures0.2 m.y. ago to 50,000 yr ago, a sequencethat apparently records progressivecrystallization of a subjacentchemically zoned magma chamber. Holocene rhyolitic and phreatic eruptions suggestthat residual magma was present in the chamber as recentlyas 450 yr ago. Intracaldera hydrothermalactivity beganat least0.3 m.y. ago and was widespreadin the caldera moat; it has sincedeclined due to self-sealingof near-surfacecaldera sediments by zeolitization, argillization, and silicificationand has becomelocalized on recentlyreactivated north- west-trendingSierra Nevada frontal faults that tap hot water at depth.

INTRODUCTION concentrates were treated with a dilute HF solution to remove small bits of attached glassand fragments of other mineral In the westernUnited States,only three calderasare known grains. Obsidian used for dating was totally unhydrated and to be large enoughand youngenough to possiblystill contain not devitrified. Small blocks sawed from many of the hand residual magma in their chambers:the Vailes caldera (•1.1 specimenswere used for dating. For samplesthat contained m.y. old) in north central New Mexico; the Long Valley cal- phenocryststhe obsidianwas crushedto between2 and 4 mm, dera (•0.7 m.y. old) in central eastern California, and the and pure glassfragments were handpickedfor analysis.Basalt Yellowstone caldera (•0.6 m.y. old) in northwesternWyom- and andesite were examined in thin sections to be sure that ing. Detailed mapping of the Vailes caldera [Smith et al., 1961, they met the usual criteria for whole-rock K-Ar dating 1970] and the Yellowstone caldera [Christiansenand Blank, [Mankinen and Dalrymple, 1972]. 1969; U.S. GeologicalSurvey, 1972;Keefer, 1972] has contrib- Argon was measuredby isotope dilution massspectrometry uted much to the understanding of caldera evolution and usingan aSArtracer and extraction techniques,mass analysis, mechanisms.Long Valley caldera (Figures 1 and 2) heretofore and data reduction proceduresdescribed by Dalrymple and has not beenmapped in detail, although a number of excellent Lanphere[1969]. Potassiumwas determinedby usingthe lith- studiesof the general geologyof the region have been made, ium metaboratefusion techniqueand flame photometry [Suhr notably by Gilbert [1938, 1941], Gilbert et al. [1968], Kistler and Ingamells, 1966; Ingamells, 1970]. Errors given in Table 1 [1966a, b], Rinehart and Ross [1957, I964], and Huber and and in the text are standard deviationsof analytical precision, Rinehart [1967]. Inclusionof Long Valley as one of the target estimatedby usingthe method of Cox and Dalrymple [1967]. areas of the U.S. Geological Survey Geothermal Research The analytical data and calculated K-Ar ages are given in Programhas provided an opportunityto apply a wide rangeof Table 1 and discussedin the appropriate sectionsalong with geologic,geochemical, and geophysicaltechniques to thestudy the stratigraphy,petrology, and structure.A summary of the of a large, active geothermal system;it also has provided the age data, showing stratigraphicrelations and relevant statis- first opportunityto synthesizethe volcanichistory of this fasci- tical data, is presentedin Figure 4. nating area. This paper is a preliminary report of the geologic and chronological studies based on mapping and sampling GEOLOGIC SETTING carried on during the summers of 1972 and 1973. Geologic mappingand petrologicstudies are continuing.The petrologic Long Valley calderais at the eastbase of the Sierra Nevada, model presentedin the summary is based primarily on field 50 km northwestof the town of Bishop and 30 km south of studies,supplemented by preliminarypetrography and limited Mono Lake (Figure 1). The calderais an elliptical depression preexistingpetrochemical data, and shouldbe consideredten- about 32 km from eastto westby 17 km from north to south;it tative. has an area of about 450 km2 (Figure 2). The easternhalf of the caldera, Long Valley proper, is a broad, crescenticgrass- K-AR DATING TECHNIQUE and sage-coveredvalley of low relief, at an elevationnear 2070 Samplesfor age measurementwere collectedfrom outcrops m (6800ft). The westernhalf of the calderais a forestedarea of at locationsshown on Figure 3. When it was possible,biotite higher relief at an averageelevation of 2440 m (8000 ft); it or sanidine in the size range of 149-420 um was separatedby forms the 'Mammoth embayment,' a prominent reentrant in usingstandard magnetic and heavyliquid techniques.Sanidine the Sierra Nevada range front near the town of Mammoth Lakes.In the westcentral part of the calderaa group of faulted Copyright¸ 1976by the American Geophysical Union. and dissectedhills risesto 2590-m (8500 ft) elevation. Between 725 726 BAILEYET AL.: LONGVALLEY SYMPOSIUM

119ø15 ' 119o00 '

oO.

lO 20 KI LOMETRES i

ß . ß

38o00 , • MonoLake

ADOBE

VALLEY

37045 ,

LONG

VALLEY

37030 ,

VALLEY © Bishop

118o45 ' 118030 , 118015 '

EXPLANATION

Bishop Tuff Holocene rhyolite and rhyodacite Mesozoicplutonic and metamorphicrocks

Rhyoliteof GlassMountain Postcaldera basaltic rocks Alluvium and glacial deposits

Tertiary volcanic rocks Postcalderarhyolite and rhyodacite

? Normal fault Geophysicalboundary of Mono Basin Mono Craters ring fracture zone and Long Valley caldera after Pakiser of Kistler (1966b) Ball and bar on downthrown side (1961) Fig. 1. Indexmap and generalized geologic map of the LongValley-Mono basin area. BAILEY ET AL.: LONG VALLEY SYMPOSIUM 727

Fig. 2. Shaded relief map of Long Valley caldera. these central hills and the. caldera walls is an annular moat, 200-250 m crop out in the west wall of the caldera, as well as which is drained on the north by Deadman Creek and the along part of the north wall. Thinner, lessextensive sequences upper Owens River and on the south by Mammoth Creek and of flows from small local centers occur also on the east rim Hot Creek. The walls of the caldera are well defined and rise south of Glass Mountain and on the south rim on McGee steeplyon all sidesexcept the southeast.On the south the wall Mountain. Rhyodacitesoverlie the basaltic rocks on the west risesprecipitously 1000 m to remnants of a late Tertiary ero- rim and form the main mass of San Joaquin Mountain and sion surface [Rinehart and Ross, 1964, Plate 1; Curry, 1971] at Two Teats, the dissectedremnants of a large Pliocenevolcano an elevation of 3050 m (10,000 ft) on the shouldersof McGee northwest of the caldera. Three additional massesof rhyoda- and Laurel mountains. In the west the wall rises 500 m to the cite, the largestof which is Bald Mountain, occur on the north easternslopes of San Joaquin Mountain and Two Teats. On rim. the north and northeast the wall is formed by the steep south Abundant basaltic inclusions found in postsubsidencein- face of Bald Mountain and of Glass Mountain, which rises tracaldera rhyolitessuggest that the thick sequencesof basaltic 1200 m to an elevation of about 3350 m (11,000 ft). The east rocks on the west and north walls extend well into the caldera wall, in contrast, is a low terraced escarpment with a max- area. In contrast, rhyodacite inclusions are rare in the post- imum relief of only 250 m, and the southeastwall is little more caldera rhyolites, suggestingthat no comparable thicknessof than an arch that merges with the surface of the Volcanic rhyodacite occurs within the caldera. The available evidence Tableland, which slopes gently southeast toward Bishop. indicates that no great accumulation or edifice of precaldera The pre-Tertiary basementrocks in the immediate vicinity volcanic rocks existed within the calde a area, as is true of of Long Valley caldera are Jurassic and Cretaceous gran- many other resurgent cauldrons [Smith and Bailey, 1968; odiorites and granites of the Sierra Nevada batholith and Smith et al., 1970]. The wide and sporadic distribution of Paleozoic and Mesozoic metamorphic rocks of the Mount precaldera volcanic centerssuggests that these Tertiary rocks Morrison and Ritter range roof pendants [Bateman et al., are not directly related to the Long Valley magma chamber, 1963; Rinehart and Ross, 1964] (Figure 3). Overlying the base- although they may representan early part of the volcaniccycle ment rocks on an erosion surface of moderate relief are late that culminated with the voluminous rhyolites of Long Valley. Tertiary volcanic rocks, mainly basalt, andesite,and rhyoda- These Tertiary volcanic rocks range in age from 3.2 to 2.6 cite. m.y. old. Units in the vicinity.of Long Valley that havebeen Basalt and andesite flows with a cumulative thickness of dated by the K-Ar method include (1) the basalt at Old Main- 728 BAILEY ET AL.: LONG VALLEY SYMPOSIUM

119000 ' 50' 40' 37050 '

45'

%

I I LVCH-1 !

40'

Mammotl" •)5 ß 31 Lakes )22 Diablo

emmmmmm S17

•mmmm Crowlej/

35'

0 2 4 6 8 10 KM I i i i i i

E X P L A N A T I O N J JAlluvium, glacial deposits, andcaldera fill J• •i Holocenerhyolite-rhyodacite Volcanicvents m rhyodacite fßo basalt-andesiterhyolite • Latebasaltic rocks 3. K-Arsample locality

_,,,; .,',,:• Rimrhyodacites Drill hole

"'""..;:•"'" Moat rhyolites I Direction of dip of strata Earlyrhyolites flows:coarse dotted •....•.>:.... • tuffs:finedotted Generaldirection of -•-•-•,• Bishop Tuff flowage of lava

Normal fault- ball and bar ::•(/•••--•Rhyoliteof Glass Mtn { tuffs:dom efl coarse ows: flinedin eli ne d on downthrown side • Tertiaryvolcanic rocks Outline of Long Valley }./• Jurassic-Cretaceousgraniticrocks caldera floor .• Paleozoic-Mesozoicmetamorphic rocks

Fig. 3. Generalizedgeologic map of Long Valley caldera. BAILEY ET AL.: LONG VALLEY SYMPOSIUM 729

TABLE 1. K-Ar Age Data on VolcanicRocks From LongValley Caldera, California

Argon

1O0 4øAr,aa Sample Map No. Material 4øArraa, CalculatedAge,•' No. (Figure 3) Rock Type Dated K20*, wt % Weight,g 10-•2mol/g 4øArtota• 106yr 72G001 1 Aphyricrhyolite Obsidian 5.10(2) 3.470 5.189 43.5 0.675ñ 0.016 3.461 4.948 29.1 72G002 2 Biotiterhyolite Obsidian 5.24 (2) 4.228 5.125 53.4 0.660 ñ 0.027 4.188 5.070 16.5 72G003 3 Pyroxenerhyolite Obsidian 5.25 ñ 0.06 (4) 3.870 5.199 16.7 0.664 ñ 0.029 4.075 5.131 34.6 72G004 4 Pyroxenerhyolite Obsidian 5.22 (2) 4.426 5.312 34.7 0.670 ñ 0.014 4.442 5.057 57.0 72G005 5 Hornblende-biotiterhyolite Sanidine 10.96(2) 5.828 5.263 48.2 0.324ñ 0.010 72G006 6 Pyroxenerhyolite Obsidian 5.02 (2) 4.089 5.583 46.4 0.73 ñ 0.016 4.197 5.280 45.3 72G007 7 Hornblende-biotiterhyolite Sanidine 10.88 (2) 12.182 1.724 64.4 0.106 ñ 0.003 72G008 8 Hornblende-biotiterhyolite Sanidine 11.24(2) 5.080 1.330 9.1 0.094 ñ 0.006 5.872 1.590 33.5 72G009 9 Hornblende-biotiterhyolite Sanidine 11.32(2) 6.755 1.735 38.8 0.103 ñ 0.002 6.112 1.744 34.2 72G010 10 Aphyric rhyolite Obsidian 5.07 (2) 3.430 5.145 43.9 0.673 + 0.014 4.458 4.967 53.0 72G011 11 Hornblende-biotiterhyolite Sanidine 10.64 (2) 6.544 7.293 40.4 0.468 ñ 0.010 7.160 7.440 40.7 72G012 12 Hornblende-biotiterhyolite Sanidine 10.45(2) 5.799 8.023 62.3 0.509 ñ 0.01! 5.158 7.730 76.0 72G013 13 Aphyric rhyolite Obsidian 5.14 (2) 4.932 4.992 54.5 0.658 ñ 0.014 5.384 5.036 41.6 72G014 14 Pyroxenerhyolite Obsidian 5.15 (2) 3.955 4.958 57.4 0.634 ñ 0.013 3.908 4.725 34.5 72G015 15 Basalt Whole-rock 1.654(2) 10.578 0.367 3.1 0.149 ñ 0.070 72G016 16 Basalt Whole-rock 1.663(2) 11.078 0.372 8.8 0.151 + 0.027 72G017 17 Hornblende-biotiterhyolite Sanidine 10.63(2) 4.347 1.890 24.8 0.113 + 0.004 12.901 1.728 32.2 73G001 18 Andesite Whole-rock 2.13 + 0.01 (4) 9.102 0.700 2.8 0.222 + 0.080 73G008 19 Basalt Whole-rock 2.00 + 0.00 (4) 8.379 0.428 8.8 0.145 + 0.015 73G009 20 Basalt Whole-rock 1.63 ñ 0.05 (8) 12.256 0.271 13.3 0.104 ñ 0.011 14.187 0.201 4.3 73G010 21 Andesite Whole-rock 3.01 ñ 0.01 (4) 14.401 12.787 70.2 2.87 ñ 0.09 73G012 22 Basalt Whole-rock 2.26 ñ 0.06 (4) 5.837 0.482 5.5 0.126 ñ 0.025 7.784 0.372 4.7 73G014 23 Basalt Whole-rock 1.82 ñ 0.01 (4) 12.806 0.148 4.3 0.062 ñ 0.013 11.736 0.194 3.5 73G016 24 Biotite rhyolite Sanidine 9.75 ñ 0.26 (4) 1.053 27.749 61.6 1.92 ñ 0.05 73G017 25 Hornblende-biotiterhyolite Sanidine 11.34ñ 0.03 (4) 4.084 6.029 55.5 0.349 ñ 0.006 ' 3.433 5.812 58.2 Biotite 8.93 ñ 0.06 (4)1 1.337 6.645 4.7 0.503 ñ 0.105 73G018 26 Rhyodacite Biotite 8.60 (2) 1.688 2.212 4.1 0.145 ñ 0.029 1.995 1.422 2.8 73G019 27 Rhyodacite Biotite 7.12 (2) 1.897 1.460 2.6 0.138 ñ 0.079 73G021 28 Basalt Whole-rock 1.57 ñ 0.01 (4) 11.928 0.211 5.7 0.093 ñ 0.0• 10.930 0.225 4.8 73G023 29 Rhyodacite Sanidine 7.76 (2) 2.251 0.582 9.2 0.050 ñ 0.010 73G024 30 Rhyodacite Biotite 8.22 (2) 1.841 1.436 2.4 0.118 ñ 0.048 73G025 31 Andesite Whole-rock 2.63 ñ 0.01 (4) 7.543 0.366 7.1 0.083 ñ 0.010 9.418 0.320 14.8

*Mean and, where more than two measurementswere made, standarddeviation. Number of measurementsis in parentheses. •'X• = 0.585 x 10-xø yr -x, X• = 4.72 x 10-xø yr -x, 4øK/K = 1.19 x 10-4 moi/moi. Wheremore than one measurementwas made on a sample, the agegiven is the weightedmean; weighting was by the inverseof the variance.Errors given are estimatesof the standarddeviation of analyti- cal precision [Cox and Dalrymple, 1967]. moth Mine and (2) andesitefrom San Joaquinridge, both 3.1 + 0.1 m.y. old [Dalrymple, 1964a] and 2.7 + 0.1 m.y. old + 0.1 m.y. old [Dalryrnple, 1964a];(3) basaltsin the Benton [Curry, 1966]. range that erupted from a cone on the northeast rim of the RHYOLITES OF GLASS MOUNTAIN caldera3.2 + 0.1 m.y. ago [Dalryrnple,1964a]; (4) basaltin the Owens River gorge, 3.2 + 0.1 m.y. old [Dalryrnple,1963]; (5) The earliest volcanic rocks that can be related to the Long andesitein the northwestwall, 2.87 + 0.09 m.y. old (location Valley magma chamber are the rhyolites of Glass Mountain 10; location numbersrefer to map locationsin Figure 3 and (Figure 3). Glass Mountain is a thick accumulation(> 1000 m) Table 1); (6) basalt on McGee Mountain, 2.6 + 0.1 m.y. old of domes, flows, and shallow intrusionsflanked by extensive [Dalryrnple,1963]; and (7) quartz latites from Two Teats, 3.0 fans of pyroclastic deposits that consist of pumice and ash 730 BAILEY ET AL.: LONG VALLEY SYMPOSIUM

(location16), indicatethat GlassMountain wasbuilt by erup- ROCKLOCATION 06 UNIT NO. ( YEARS) tions that extendedover a period of at least 1 m.y. 23 O. 062 -+ O. O13 , BISHOPTUFF 31 • .0.083 -+ 0.010 ,,, 28 0.093 -+ 0.012 The Bishop tuff, a voluminous rhyolite ash flow sheet, •: w 20 0.104 +- 0.011 erupted about 0,7 m.y. ago [Dalrympleet al., 1965]from vents

>- 22 0.126 +- 0.025 now buried within Long Valley caldera [Gilbert, 1938, pp.

• • 19 0 145 -+ 0.015 1859-1860]. From thesevents, ash flows spreadradially south-

15 0.149 -+ 0.070 east through the low area now occupiedby the Owens River, north over the west shoulder of Glass Mountain into Adobe 16 0.151 -+ 0.027 Valley, northwest over Deadman Summit into Mono basin, 18 0.222 -+ 0.080 , and westover the low passnow occupiedby Mammoth Moun- 29 0.050 -+ 0.010 ,,, tain into the valley of the Middle Fork of the San Joaquin •_• 30 0.118 -+0.048 River (Figure 1). The most extensiveexposure of Bishoptuff is ,-,-n 27 0 138 -+ 0 079 o ß ß on the Volcanic Tableland southeastof the caldera [Gilbert, • 26 0.145 -+ 0.029 1938; Sheridan, 1965]. Lesser exposuresoccur in the south- 8 0.094 -+ 0.006 easternpart of the Adobe Valley and in the southernpart of 9 0.103 -+ 0.002 the Mono basin [Gilbert, 1938], where the sheetis extensively 7 O. 106 -+ 0.003 covered by pumice from the Mono craters. Small erosional • 17 0.113 +- 0.004 remnantsalso cling to the walls of the Middle Fork of the San c> -,- S17 0.28 -+ 0.03 Joaquin near Reds Meadow [Huber and Rinehart, 1967]. •- 5 0.324 -+ 0.010 The Bishoptuff is a crystal-richrhyolite tuff that containsup o 25 0.349 -+ 0.006 to 30% phenocrystsof quartz, sanidine, plagioclase,biotite, 11 0.468 -+ 0.010 and Fe-Ti oxides [Gilbert, 1938; Sheridan, 1965]. Available 12 0.509 +_ 0.011 chemical data [Bateman, 1965, Table 21; Sheridan, 1965,Table

BIOTITE 2 0.660 + O. 027 5] indicatethat the Bishoptuff differsonly slightlyin composi-

14 0.634 +- 0.013 tion from top to bottom and is not a strongly differentiated ,,, i •n zw 3 0.664 -+0.029 sheet.According to Sheridan[1968] the tuff exposedin the lower OwensRiver gorgeis a doublecooling unit, which in- • • 4 0.670+_0.014 -,- 6 0.731 -+ 0.016 dicatesthat it was emplacedduring two eruptivepulses. Gilbert[.1938, p. 1833]estimated that the Bishoptuff covers >- 13 0.658 -+ 0 014 an area of 1040-1150 km •' and has a volume of about 140 km 3. ,,, >' 10 0.673 -+ 0.014 His estimate,however, did not includethe largevolume of tuff 1 0.675 -+ 0.016 buried.within the caldera.Although Bishop tuff is not exposed BISHOP TUFF 0.708 -+ 0.015 anywhere within the caldera, blocks of the tuff occur as acci- PRE-

CALDERA 24 1.92 -+0.05 , dentalinclusions in early postcalderarhyolite tuffs exposed in ROCKS 21 2.87 -+ 0.09 , the central part of the caldera,clearly indicating that Bishop tuff underliesthe tuffs at depth [Bailey, 1973]. Most of the Fig. 4. Maximum probability, in percent,that the calculatedages inclusionsare of denselywelded tuff, suggestingthat the in- of any two volcanic units are truly different and not the result of randomerrors in the analyticalprocedure. A probabilityof more than tracalderaBishop tuff is probablyquite thick. By analogywith 99% is indicatedby an asterisk. intracalderawelded tuffs exposedin older, more deeplydis- sectedresurgent cauldrons [Rattd and Steven,1967; Byers et al., 1968;Lipman and Steven, 1970; Lipman et al., 1973;R. L. falls,small ash flows, blocky Pelean avalanche deposits, and Smith and R. A. Bailey, unpublisheddata, 1970], the in- epiclasticconglomeratic sediments [Gilbert, 1941; Rinehart and tracalderaBishop tuff is probablyof the orderof 1000m thick. Ross, 1957]. Dissectedremnants of the flankingfans extend Seismicrefraction studies of the caldera[Hill, 1976]suggest a northeastinto Adobe Valley and southalong the eastside of thicknessof 1000-1500m. By usinga thicknessof 1000m and the caldera. Similar fans probably extendedsouthwest from thearea of thesubsided cauldron block (350 km •' as defined by Glass Mountain, but the southwest half of the mountain has theinferred position of themain boundary faults in thecaldera been downfaultedalong the calderaboundary faults and is moat), the volumeof intracalderaBishop tuff is estimatedto now buriedbeneath the sedimentaryand volcanicfill of Long be about350 km 3. Thus the total volume of Bishopash flows is Valley. The ventsand intrusivecenters of Glass Mountain are of the order of 500 kma, two-thirds of which accumulated well exposedin its southface. They are arrangedin an arc within the subsidingcaldera. approximatelyparallel to the northeastwall of Long Valley The volumeof Bishopash that wascarried away by winds caldera,which suggests that their locationwas controlled by and thinly dispersedover much of the westernUnited Statesis an incipientcaldera ring fractureand that theyrepresent early difficultto estimateon the basisof presentlyavailable data. leakagefrom the Long Valley magmachamber. However,using (1) thedistribution of Bishopash indicated by The rhyolitesof GlassMountain are aphyricto sparsely Izett et al. [1970]and (2) one-halfthe maximumthickness (3 porphyritic,and, accordingto Nobleet al. [1972],are highly m) of Bishoppumice fall in thevicinity of thecaldera and (3) differentiated,peraluminous, and high in silica(containing assuminglogarithmic thinning of the ashdownwind, we calcu- about77%). Two K-Ar ages,one of 0.90 + 0.10m.y. obtained latea volumeof 300km a. Although this estimate seems surpris- on sanidinefrom obsidiannear the top of Glass Mountain ingly large,we believeour methodof calculationyields a [Gilbertet al., 1968,p. 302]and another of 1.92+ 0.05m.y. on conservativefigure. Furthermore, the relativeproportion of sanidinefrom a biotite rhyolite low on the southwestside ash fall to ashflows (3: 5) doesriot seemunreasonable when it BAILEY ET AL.: LONG VALLEY SYMPOSIUM 731

is compared with the ratio (1:1) of the Mazama ash to its timingof subsidenceestablished by K-Ar dating.Bishop tuff associatedCrater Lake ash flows [Williams and Goles, 1968], inclusionsin the early postcalderarhyolite tuffs, which are at especiallywhen one takesinto accountthe likelihoodthat the least500 m thick, indicatethat Bishoptuff has definitelysub- Crater Lake eruption had a strongervertical component. sided to a considerabledepth within the depression(a min- The foregoingvolumes of ash and tuff, when they are re- imum of 800 m belowthe generallevel of Bishoptuff on the duced by appropriatefactors for their porosity,indicate that northcaldera rim). K-Ar dating(Figure 4) of the Bishoptuff the total volume of magma ejected during eruption of the (0.71 m.y.) and the earliestintracaldera rhyolites (0.73-0.68 Bishoptuff was of the order of 600 km•. m.y.) indicatesthat this subsidenceoccurred within a very Hildreth and Spera [1974] report on the basis of miner- short time followingeruption of the Bishoptuff. Thus sub- alogicaland geochemicalstudies that the initial ash falls of the sidencecan clearlybe associatedwith eruptionof the Bishop Bishoptuff eruptedat temperaturesof 745øC under a pressure tuff and is not the result of tectonic movements, which have of lessthan 2 kbar and that the ash flows erupted at 800øC beencontinuously active in the generalarea over a much underpressures of greaterthan 3 kbar. Thesedata suggestthat longertime at much slowerrates. the roof of the chamberwas at a depth of about 6 km at the The minimum amount of subsidence of the cauldron block time of initial outburst and that the last ash flows came from a is about 1000m, indicatedby the topographicrelief between depth of about 10 km. Thesedata are in fair agreementwith the floor of Long Valleyand the shouldersof GlassMountain evidence to be discussedbelow, i.e., that the maximum amount and remnantsof the Tertiary erosionsurface on the southrim of calderacollapse associated with the eruption of the Bishop of the caldera.The averagethickness of low-densitymaterials tuff was about 3 km. in the eastern half of the caldera beneath Long Valley, in- K-Ar ageson samplesfrom three exposuresof the Bishop dicatedby thegravity model of Kaneet al. [1976,Figure 5] and tuff range from about 0.68 to 0.74 m.y. [Dalrymple et al., by the seismicrefraction study of Hill [ 1976],is about 3000 m. 1965], but none of the individual measurementsare signifi- Of this thickness,500-1000 m is probablydownfaulted pre- cantly different from the weightedmean age of 0.708 + 0.015 calderarhyolite of GlassMountain. The calderafill, including m.y. at the 95% level of confidence.Both the K-Ar agesand the Bishoptuff, in LongValley is therefore about 2000-2500 m the paleomagneticdata, from weldedparts of the Bishoptuff, thick, so the maximumsubsidence in the easternpart of the are consistentwith the interpretationthat this ash'flowsheet caldera is of the order of 3000 m. was emplacedwithin a time span of no more than a few The topographicrelief on the wallsin the westhalf of the centuries[Dalryrnple et al., 1965]. caldera,about 300-600 m, is considerablyless than in the east. Part of this differencemay be accountedfor by the infill of CALDERA SUBSIDENCE youngbasaltic lavas in thewestern moat, but they probably do As a result of eruption of the Bishop tuff the Long Valley notgreatly exceed 200 m in thickness.The averagethickness of magma chamber was partially emptied, and its roof con- low-densityrocks in the westernpart of the caldera,indicated sequentlycollapsed. Collapse probably took place along ar- by Kaneet al. [1976,Figure 5] and Hill [1976],is about 1700 cuate ring faults, althoughfew of theseare now exposed.One m, of which about 200 m probably is precalderavolcanic outer ring fault, with a maximum displacementof 250 m, can rocks,judging from exposuresin the westand northwalls of be traced for a distanceof 12 km along the east wall of the the caldera.Thus the calderafill, includingBishop tuff, in the caldera (Figure 3), but the gravity gradient on the eastern westprobably is about1500 m, andthe total subsidenceis of margin of the caldera[Pakiser, 1961;Pakiseret al., 1964;Kane the order of 2000 m. Subsidence in the west therefore is about et al., 1976] requiresmuch greaterdisplacement, and the main 1000 m less than in the east. boundary fault must be buried in the caldera fill to the west. This difference in subsidencein the east and west is espe- The main boundaryfault probably lieswell within the caldera ciallyevident in the gravitymaps of Kaneet al. [1976,Figures moat, and the presentdiameter of the calderaundoubtedly has 2, 3], a conspicuousfeature of whichis a northwest-trending been enlarged by slumping of the initially unstablewalls and gradienttransecting the middleof the caldera.This gradient by subsequenterosion. coincideswith the northwardprojection of the Hilton Creek AlthoughGilbert [1938, 1941 ] recognizedLong Valleyas the fault, a major active Sierra Nevada frontal fault, and is sourceof the Bishoptuff and suggestedthat mostof the recent thoughtto reflecteither precaldera displacement of the base- faulting in the depressionwas a result of extrusionof a large ment (down to the east) on the intracalderasegment of the volume of magma from beneathit, the calderaorigin of Long fault or, as suggestedby Hill [1976], differentialdisplacement Valley has been questionedby some investigators,probably on the fault duringcaldera collapse. It is alsoevident from the becausethe boundary faults are largely covered and actual gravityand seismicrefraction studies that thecaldera fill thick- displacementon them cannot be measured. The gravity ensmarkedly to the north, suggestingthat the floor tilted in studiesof Pakiser [1961] and Pakiseret al. [ 1964]lent support that direction during subsidence. to the caldera origin by more clearly defining its oval shape The total volume of caldera subsidence can be estimated and establishingthat it containeda great thicknessof low- from the gravitymodel and the presenttopography. The vol- densityfill. Pakiser(in the work by Rinehartand Ross[1964]) ume of the presenttopographic depression, based on recon- elaborated on the mechanismof eruption and collapse,but structionof the precalderatopography, is about200 kma. The becausehe overestimatedthe volume of fill [Kane et al., 1976], volumeof the subsurfacedepression calculated from the grav- he concluded that volcanism alone could not account for the ity model[Kane et al., 1976,Figure 3] is about800 km a [Muf- amount of subsidence,and consequentlyhe consideredLong fler and Williams,1976]. Probably about one quarterof this Valley a volcano-tectonicdepression. Recognition of a resur- volumeis subsidedprecaldera volcanic rocks; hence the sub- gent dome within the depression[Smith and Bailey, 1968] surfacevolume occupied by the Bishoptuff andyounger rhyo- providedindirect evidence for calderaorigin, but perhapsthe lite tuffs and lavas is about 600 kma, and the total combined most convincingevidence is that providedby the inclusionsof subsurfaceand topographicsubsidence is of the orderof 800 Bishoptuff in the early rhyolitesin the depressionand by the kma. About 75% of this volume can be accountedfor by the 732 BAILEY ET AL.; LONG VALLEY SYMPOSIUM estimated600 km3 of magma ejected during eruption of the age of 0.73 m.y. on pyroxenerhyolite (location 6), which at the Bishoptuff, and possiblyan additional5-10% can be accounted 95% confidence level is significantlyolder than the ages of for by minor subsidenceaccompanying eruption of post- either of the three dated aphyric rhyolites(locations 1, 10, and caldera tuffs and lavas (40-80 km3 of magma). Thus with 13). None of the K-At agesof the early rhyolites are signifi- presently available data, 80% or more of the estimatedvolume cantly older than the age of the Bishoptuff. The data indicate of subsidencecan be attributedto eruptionof the Bishoptuff that (1) the early rhyoliteswere erupted during a span of no and associated intracaldera volcanic rocks. more than 100,000 yr and perhapsas little as 40,000 yr after eruption of the Bishoptuff and collapseof the caldera and (2) EARLY RHYOLITES the pyroxenerhyolites may have beencontemporaneous in part K-At ages of 0.73-0.63 m.y. indicate that almost immedi- with the aphyric and biotite rhyolites. ately after subsidence,eruptions resumedwithin the caldera. RESURGENT DOMING During this time crystal-poor rhyolite tuffs, domes, and flows accumulated on the floor to a thickness of at least 500 m. Contemporaneouswith emplacementof the early rhyolites, These rocks, informally designatedthe 'early rhyolites,' are the west central part of the caldera floor was uplifted and exposedin tilted and uplifted fault blocksin the centralpart of deformed into a subcircularstructural dome. This resurgent the caldera. dome [Smith and Bailey, 1962, 1968] is 10 km in diameter and The tuffs contain abundant inclusionsof Bishoptuff and consists of a mosaic of fault-bounded blocks that rise 500 m basaltand, lesscommonly, fragments of graniticand metamor- above the surrounding moat. The dome is transectedby a 5- phicrocks, indicating that all of theserock types occui' within km-wide complexlyfaulted keystonegraben that trendsnorth- the cauldron block. Many of the rhyolite tuffs show varying west. The early rhyolitesexposed in the dome are tilted radi- degreesof reworking by water: low-amplitude cross-bedding, ally outward as much as 30 ø, and detailed mapping of flow sorting, and grading. Although ripple marks are absent, the foliationsand lineations[Chelikowsky, 1940; Bailey, 1974] tuffs were very likely deposited, at least partly, in an early shows that most of the extrusive units flowed radially away formed caldera lake. from the geometriccenter of the dome.•Evidencethat the The rhyolite domesand flows associatedwith the tuffs typi- domeis the resultof positivecentral uplift rather than differen- cally contain abundantjet black obsidian and have lessthan tial collapseof themoat is asfollows: (1)' lake terraces tilted 3% phenocrystsof quartz, plagioclase, biotite, and hyper- outward on the dome at elevationsas much as 35 m higher sthene. The low crystal content of these lavas indicates that than the highest terrace on the caldera walls; (2)scattered they erupted at near-liquidustemperatures. They also appear beachpebbles as much as 80 m higher on the dome than on the to have been unusuallyfluid for rhyolites, as some units less calderawalls; and (3) 1- to 2-m-diameterblocks of granitic and than 50 m thick flowed as far as 6 km. Three mineralogical metamorphic rocks, interpreted as ice-rafted glacial erratics facieshave beenmapped [Bailey, 1974]; in order of increasing from the Sierra Nevada [Rinehartand Ross, 1964,p. 68], 200 m phenocrystcontent and, in general, decreasingage, they are: higher on the dome than the highest terrace on the caldera (1) aphyric rhyolite, (2) pyroxene rhyolite, containing walls and 60 m higher khanthe lowest part of the caldera rim phenocrystsof quartz, plagioclase, hypersthene,and Fe-Ti during early postcalderatime. Thesefeatures also indicatethat oxide, and (3) biotite rhyolite, containing phenocrystsof the resurgent dome was an island in a caldera lake during quartz, plagioclase, hypersthene, biotite, and Fe-Ti oxide. much of its history (see Figure 5). Scarcephenocrysts of fayalite and augitealso have beenidenti- The orientation of the keystone graben in the resurgent fied in some samplesof biotite rhyolite. Available chemical dome is undoubtedly controlled by the dominant northwest data [Rinehart and Ross, 1964, Table 9; Jack and Carmichael, trend of regional structuresalong the east front of the Sierra 1968, Table 1; R. A. Bailey, unpublisheddata, 1974] indicate Nevada. The eccentric location of the resurgent dome is that the early rhyolitestypically contain about 75% silica and thought to be controlledby linesof structural weaknesswithin that the three faciesare not appreciablydifferent chemically, the Mount Morrison roof pendant,which probably continues suggestingthat the successionrepresents progressive crystalli- in the basementnorthwestward through the cauldron block to zation of the same magma. exposuresof metamorphic rock on the north wall of the cal- Twelve eruptive centers for the early rhyolites, many of dera near Big Springs.Several major precalderafaults within them alignedon and offsetby northwest-trendingfaults, are and bounding the roof pendant, the Hilton Creek, McGee exposedin the central part of the caldera. Additional vents Mountain, and Laurel-Convict faults [Rinehart and Ross, probably are buried in the caldera moat. Kane et al. [1976] 1964,Plate 1] (Figure 3), undoubtedlycontributed to the weak- interpretthe arcuatechain of secondarylows in their gravity ness of this zone. model as local accumulationsof low-densitymaterials. These Decrease in the amount of radial tilting of progressively lows quite possiblyrepresent accumulations of coarsepumice youngerflow units on the dome suggeststhat uplift was wan- around early rhyolite vents on outer caldera ring fractures. ing toward the closeof eruption of the early rhyolites.How- Evidencethat early rhyolitesare buriedin the moat include(1) ever, no units between0.63 and 0.51 m.y. old have beendated, lithic aphyric rhyolite debrisin the phreaticdeposits of the so the time of termination of doming is not preciselyknown. Inyo craters;(2) aphyric rhyolite tuff uplifted on the side of a The top of a 0.51-m.y.-old hornblende-biotiterhyolite dome large rhyodacitedome in the southwestmoat; and (3) pyrox- (location 12) on the north flank of the resurgentdome slopes ene rhyolite beneath younger basalts in the northwest moat gently to the north, but whether this is due to posteruption at 200 m depthin U.S. GeologicalSurvey heat-flow hole (Fig- tilting or to slumpingduring extrusion onto a preexistingslope ure 3, DC). is uncertain. A 2280-m (7500 ft) lake terrace that laps against K-At measurementson 8 of the 12 early rhyolite domesand this dome does not appear to be tilted, nor do the many other flows give ages of 0.73-0.63 m.y. (Table 1 and Figure 4). lower terracesand strandlinesdown to 2140 m (7000 ft) on the With only one exception the ages are consistentwith the north slope of the resurgentdome. Thus doming,which began mappedstratigraphic succession: the exceptionis the apparent shortly after collapseof the cauldron, continued until about BAILEY ET AL.: LONG VALLEY SYMPOSIUM 733

119'00' 50' 40'

Hot Creek

37'40'

Crow/eyLake

0 5 10 KM I • I

Fig. 5. Maximumextent of PleistoceneLong Valley Lake and distribution of hydrothermalactivity. The heavysolid linerepresents remnants of highest(2320 m (7600ft)) laketerrace; heavy dashed line, inferred position of highestshoreline and maximumextent of lake;triangles, active fumaroles; solid circles, active hot springs;dotted area, distribution of fossil gasvents and hydrothermalalteration; and solid thin lines, recentlyactive faults. Contour intervalis --•150 m (500 ft).

0.63 m.y. ago but probablydid not persistmuch beyond0.51 the three groups erupted about 0.5, 0.3, and 0.1 m.y. ago in m.y. ago. clockwisesuccession around the resurgentdome. These rocks are temporally, spatially, and petrographicallyanalogous to MOAT RHYOLITES the ring domesof the Vailes caldera, New Mexico [Smith and After resurgence,coarsely porphyritic hornblende-biotite Bailey, 1968;Doell et al., 1968],and their distributionis prob- rhyolite erupted from three groups of vents in the caldera ably controlled by ring fractures that bound the resurgent moat. These rhyolites,informally designatedthe 'moat rhyo- dome. The presenceof hydrous minerals and the greater ve- lites,' are highly pumiceous and contain as much as 20% sicularityof the moat rhyolitessuggest that their extrusionand phenocrystsof hornblende,biotite, quartz, s•nidine, and reopeningof the ring fracturesmay have been causedby the plagioclase.A singlechemical analysis from a domein the west concentrationof volatiles accompanyingcrystallization of the moat indicates that they have silica contents of about 72% magma and consequentincrease in volatile pressurein the [Rinehartand Ross,1964, Table 1]. They typicallyform steep- chamber.The apparent0.2-m.y. periodicityof the groupsmay sided domes or thick flows of limited extent. Their crystal be a measureof the time requiredto build up sufficientpres- content and geomorphic form indicate that they were more sure to reopen the ring fractures. viscousand probably erupted at lower temperaturesthan the The restriction of the centers to three distinct areas in the early rhyolites. ring fracturezone may be due to the fact that in theseareas the The oldesthornblende-biotite rhyolites erupted on the north ring fracturesintersect major northwest-trendingprecaldera flank of the resurgentdome where two of four centershave faults within the cauldron block. This suggestionis further been dated at 0.509 4- 0.011 m.y. (location 12) and 0.468 4- supported by the northwest alignment of some of the domes 0.010 m.y. (location 11). A secondgroup of five eruptedin the within the groups.In the northerngroup, four ventsare spaced southeast moat, three of which have been dated at 0.349 4- at 1.5-km intervalson a northwesttrend that may be either an 0.006 m.y. (location 25), 0.324 4- 0.010 m.y. (location 5), and arc concentricto the margin of the resurgentdome or a north- 0.28 4- 0.03 m.y. (S17 of Doell et al. [1966]). A third group of west extension of the Hilton Creek fault. The five vents of the four, with K-Ar agesof0.113 4- 0.004 m.y. (location 17), 0.106 southeastgroup appear to be randomly distributed,but they 4- 0.003 m.y. (location 7), 0.103 4- 0.002 m.y. (location 9), and also are in the generalarea of intersectionof the Hilton Creek 0.094 4- 0.006 m.y. (location 8), occursin the west moat. Thus fault with the ring fracture zone. The three main domesof the 734 BAILEY ET AL.: LONG VALLEY SYMPOSIUM westerngroup are aligned north-northwestparallel to minor It is probably not accidental that' the two largest intracalderafaults that are in line with the Hartley Springs accumulationsof rhyodacite, Mammoth Mountain and the fault northwestof the caldera.The fourth domein thisgroup, Deadman Creek domes, occur where the caldera rim is inter- Deer Mountain, although isolated from the other three, is sectedby a north-trendingfracture systemthat also localized aligned with the younger Inyo domes,which also trend north- the more widely distributed, contemporaneousbasalts de- northwestparallel to the Hartley Springsfault. scribedin the following section.The intersectionof this frac- ture systemwith the outer calderarim fracturesundoubtedly RIM RHYODACITES was a zone of weaknessthat preferentiallychanneled magma Hornblende-biotiterhyodacites, less silicic and richer in to the surface.However, the fact that one and possiblytwo crystalsthan the hornblende-biotiterhyolites of the caldera other rhyodacite centers occur along the north wall of the moat, occur at three localities on the caldera rim or in the caldera,well to the eastof this fracturezone, suggests that the outer part of the calderamoat: (1) Mammoth Mountain on rhyodacitesare geneticallyrelated to the Long Valley magma the southwest,(2) Deadman Creek on the northwest,and (3) chamberand not to the more deeplyderived basalts. the foot of Glass Mountain on the northeast. A fourth un- Four rhyodaciteshave been dated in this study, two of ventedrhyodacite (?) intrusionprobably occurs at the foot of which are among the youngestin the Mammoth Mountain Bald Mountain on the north, where cementedconglomeratic complex.One from the summit dome (location 29) has an sedimentscapped with travertinehave beenpunched upward apparent sanidineage of 0.050 + 0.010 m.y., and one from a into a steep-sidedstructural dome about 1 km in diameter.It is flow midway down the northern flank (location 30) givesa difficultto accountfor this structureother than by intrusionof biotite age of 0.118 + 0.048 m.y. Two additionalrhyodacites a smalligneous plug. Because of theirperipheral location and near Mammoth Mountain, a dome (location 27) and a flow semiarcuatedistribution, these rhyodacites are informally des- (location26), give K-Ar ageson biotitesof 0.138 + 0.079m.y. ignatedthe 'rim rhyodacites.' and 0.145 + 0.029 m.y., respectively.Previously published K- Mammoth Mountain, the largest and most imposingof Ar ages from Mammoth Mountain include one of 0.148 + theserhyodacite centers, is a complexcumulo-volcano, con- 0.049 m.y. on biotite [Curry, 1971] and two on the samedome, sistingof manysuperimposed domes and shortthick flows. At 0.18 + 0.09 m.y. on biotite [Huber and Rinehart, 1967] and least 10 major eruptivevents can be identifiedon the moun- 0.37 + 0.04 on plagioclase[Dalrymple, 1964a]. Petrographic tain, most of which occur within an arcuatezone parallel to studiesindicate that the plagioclasephenocrysts in the Mam- the caldera wall. The northeast face of the mountain has been moth Mountain rocks have had complexhistories and prob- oversteepenedby glaciationand alsoin part by faulting.Dis- ably have xenocrysticcores. Preliminary results of K-Ar dat- placementof lavasacross a conspicuouscleft high on the ing of coexistingbiotite and plagioclasefrom other Mammoth northeastside of the mountainsuggests that the northeasthalf Mountain rocksshow that K-Ar agesof plagioclaseare more of the mountain has subsided(see section on postcalderatec- variable and consistentlyolder than thoseon coexistingbio- tonic activity). tite, suggestingthat biotite agesmore closelyreflect eruptive The lavas of Mammoth Mountain have silica contents rang- ages.If so,the rockson Mammoth Mountain probablyare no ing from 74 to 66% [Huberand Rinehart, 1967, pp. D14-D15, older than 0.18 + 0.09 m.y. Figure5]. Recentmapping by R. P. Koeppen(personal com- These age data indicate that Mammoth Mountain was ac- munication,1973) showsan upwardsuccession from biotite tive for a minimum of about 100,000yr and that rhyodactic rhyolite through biotite-augiterhyodacite to hypersthene-volcanism was partly contemporaneouswith the western hornblenderhyodacite, indicating a trendtoward more mafic group of moat rhyolites,as well as with the basaltsdescribed compositionswith time and suggesting that the lavas may have below. eruptedfrom a high-leveldifferentiated cupola. LATE BASALTIC VOLCANISM The rhyodacitesof DeadmanCreek were mapped by Rine- hartand Ross [1964] and Huber and Rinehart [1967] as 'olivine- The westmoat of LongValley caldera contains many basal- bearingquartz latite.' Theserocks comprise three small lava tic flows and cindercones that are part of a more extensive domes and a flow. The three domes are hornblende-biotite chain of mafic volcanicrocks extendingfrom southwestof rhyodacitesmineralogically similar to thoseof Mammoth Mammoth Mountain 45 km northward into Mono basin. This Mountain and have silica contents of 63-67%. The associated chainincludes the trachybasalticrocks of the DevilsPostpile flow, the only unit that contains olivine, has a lower silica [Huberand Rinehart, 1967],the trachyandesitecinder cones content, 59-61% (N. K. Huber and C. D. Rinehart, personal and flows of the June Lake area [Putnam, 1949], and the communication,1974), and is mineralogicallyheterogeneous, sublacustrine cinder cone of Black Point on the north shore of with olivine,augite, hypersthene, biotite, hornblende,plagio- Mono Lake [Christensenand Gilbert, 1964; Lajoie, 1968]. clase, sanidine,and quartz crystals,all in varying stagesof These rocks appear to have erupted from a north-trending reactionwith the enclosingbrown glass. Such relations suggest fracturesystem consisting of north-northwest-trendingen ech- that the rock is the productof intermixingof rhyodaciticand elonsegments that parallelthe eastfront of the SierraNevada. basalticliquids, a distinctlikelihood since the K-Ar data in- Chemicallyand mineralogically, the rocksare similar to Ceno- dicate that basaltsand rhyodaciteswere erupted contem- zoic maficrocks that occurthroughout the Basinand Range poraneously in the western moat. province [Leemanand Rogers, 1969], and presumablythey The small rhyodacitedome at the baseof Glass Mountain have a similar origin. has beendescribed by Gilbert [1941, p. 799] as 'andesite,'but In general, the agesof the rocks decreasenorthward in the its mineralogyindicates that it is more silicicthan typical chain. The oldest rocks are from the Devils Postpilearea, andesitesof the region. The rock contains abundant small where K-Ar measurementson two differentplagioclase sepa- crystalsof hornblendeand plagioclasein a light gray glassy rates from the same samplegive ages of 0.63 + 0.35 m.y. matrix; its compositionprobably does not differ much from [Huberand Rinehart, 1967] and 0.94 + 0.16 m.y. [Dalrymple, the rhyodacitesof Deadman Creek and Mammoth Mountain. 1964b]. K-Ar ages on nine intracaldera basalts and BAILEY ET AL.: LONG VALLEY SYMPOSIUM 735 trachyandesitesrange from about 0.2 to 0.06 m.y. (Table 1, light, coarselyporphyritic hornblende-biotite rhyodacite that Figure 4). North of the caldera, the trachyandesitenear June tends to be pumiceous,and (2) dark, sparselyporphyritic Lake overliesTahoe till and is overriddenby Tioga till [Put- rhyolitic obsidian.The rhyodacitemineralogically and tex- nam, 1949];hence it is probably between75,000 and 20,000 yr turally resemblesthe rhyodacitesof Long Valley caldera, old [Curry, 1971]. The Black Point cinder cone on the north whereasthe rhyolitic obsidianresembles that of the Mono shore of Mono Lake is dated at 13,300 + 500 yr by •4C on craters. Moreover, from north to south approaching Long ostracodsfrom interbeddedsediments [Lajoie, 1968]. Valley caldera,the proportionof pumiceousrhyodacite in The K-Ar ages of the basaltic rocks within Long Valley successivedomes increasesnoticeably at the expenseof the calderaindicate that basalticvolcanism was contemporaneous obsidian,a relation that suggeststhat the Inyo domesrepre- with the younger hornblende-biotiterhyolites of the western sentthe mixingalong a north-southfissure of magmasfrom moat as well as with the rim rhyodacites.Since it is unlikely the Long Valley chamberand a chamberbeneath the ring that liquid basaltcould have penetratedany part of the rhyo- fracturezone of the Mono craters(Figure l) [Kistler,1966b, p. litic magmachamber that had not alreadysolidified, the distri- E48]. The age(about 12,000-1300yr) and frequencyof erup- bution and age of the intracalderabasaltic centers are impor- tion of the Mono craters[Dalrymple, 1967; Friedman, 1968] tant indicators of the extent of crystallization of the Long suggestthat the subjacentchamber may be activelyrising and Valley chamber. The innermost basaltic center in the western capableof feedingfurther eruptions. moat is 4 km from the caldera walls and is dated at 0.222 -1- PLEISTOCENE LONG VALLEY LAKE 0.080 m.y. (location 18). Thus at that time the chamber pre- sumablyhad solidifiedinward at least4 km from its margins After caldera subsidencethe Long Valley depressionwas and roofi The dimensionsand geometry (thickness-diameter filled with water to form PleistoceneLong Valley Lake [Mayo, ratio) of the resurgentdome [Smith and Bailey, 1968, p. 646] 1934].Although normal runoff within the drainagebasin was suggestthat at the closeof resurgenceabout 0.6 m.y. ago the probablysufficient to fill the depressionin a relativelyshort roof of the chamberprobably was not deeperthan 5 km and time,it is likelythat glacierswere present in the SierraNevada possiblywas as shallow as 2 or 3 km. Thus about 0.2 m.y. ago at that time and that runoffwas greatly accelerated by melting the chamber had congealedto a depth of 6-9 km from the causedby the blanketof hot ashlaid downduring eruption of surface. the Bishoptuff. Thiscaldera lake rose to a levelof at least2320 Of specialinterest to the glacialgeology of the area are K-Ar m (7600 ft), as indicatedby remnantsof terraceson the north- agesof three basalticflows exposedin the south moat. A till, east and east walls of the caldera (Figure 5). Terracesand namedthe Casa Diablo till by Curry [1971], is found between strandlinesare exceptionallywell developedalong the east the middle and lower of these flows. On the basisof singleK- wall, wherethey can be tracedcontinuously for 14 km and Ar agesof 0.192 q- 0.035 m.y. on the upper flow, 0.280 q- 0.067 where they have been downwarpedsouthward and locally m.y. on the middle flow, and 0.441 q- 0.040 m.y. on the lower faulted.The terracesare veneered with coarsesandand gravel, flow, Curry assignedthe Casa Diablo till an age of about 0.4 consistingpredominantly of well-roundedcobbles and pebbles m.y. and a stratigraphic position between the Sherwin till of obsidian and lithoidal rhyolite, locally cementedwith [Sharp, 1968] and the Mono basin till of possibleIllinoian age carbonate.This detrituswas derivedfrom the reworking of the [Sharpand Birman, 1963]. We have made duplicate measure- tuffs and epiclasticsediments of GlassMountain, whichare ments on the upper and lower of these three flows and ob- exposedin the eastrim and wall of the caldera.The gravels tained considerablydifferent results.On the basisof our data also contain scattered,large, angularblocks of coarselypor- the ageof Curry'sCasa Diablo till is between0.062 + 0.013 phyriticCathedral Peak-type granite and metavolcanic rocks. m.y. (location 23) and 0.126 q- 0.025 m.y. (location 22) and The only sourcefor theseblocks is in the Sierra Nevada therefore may be equivalent in age to M ono basin till. southwestof the caldera[Huber and Rinehart, 1965], so they must have beenice-rafted across Long Valley Lake. As noted HOLOCENE RHYOLITIC VOLCANISM in a precedingsection, erratics of apparentlysimilar origin are The youngestvolcanic featuresin Long Valley caldera are found on the west and south flanks of the resurgent dome the Inyo craters and domes, [Mayo et al., 1936], which are within the caldera. These are the windward flanks against alignedon an apparentnorth-trending fracture extending from which icebergsfrom the Sierranglaciers would most likely the west moat to the Mono craters along the east front of the have lodged. Sierra Nevada. The Inyo domes are five rhyolitic to Calcareouscement is common in the gravels of the lower rhyodacitic lava domes, the three largest of which are the terraces,and on the lowest terraces,dense, finely crystalline, youngest(less than 720 q- 90 yr old on the basisof "C dating calcareoustufa depositsare extensivelydeveloped. The in- [Wood, 1975]). The Inyo cratersare three phreaticexplosion :rease in the amount of cementation and in the thickness of pits on the southflank of Deer Mountain (the moat rhyolite tufa on successivelylower terracessuggests either more endur- just south of the southernmostInyo dome) and have been ing lower lake stands,increased algal activity, or progressive dated by •C at 650 q- 200 yr [Rinehartand Huber, 1965]. increasein alkalinity and salinity of the lake water with time, No primary ash or lava was expelledduring formation.of the latter possiblydue to late Pleistocenedesiccation or to in- the craters,and very probablythey werecaused by rhyodacitic creasedintroduction of alkalies and salts from hot springs magma rising to shallowdepth and flashinggroundwater to duringthe later part of theCaldera history. Absolute dating of steam. Thus it is possiblethat residual rhyodacitic magma the terraces using the hydration rind method [Friedmanand was presentin the Long Valley chamber as recentlyas 450 Smith, 1960] on obsidian pebblesis in progressand perhaps yr ago (seealso Steeplesand lyer [1976]). will providefurther information on the historyof the lake and The Inyo domesare of particular interestbecause they are its chemical evolution. chemicallyand physicallyheterogeneous [Lajoie, 1968,p. 140; Draining of the calderalake was attributed by Mayo [1934] Jack and Carmichael,1968, p. 22]. The three youngestdomes and Putnam [1960] to headward cutting by the Owens River, are fluidal mixtures of two distinctly differentrock types:(1) but it is more likely that drainagewas initiated by overflowof 736 BAILEY ET AL.: LONG VALLEY SYMPOSIUM the lake, as was believedby F. E. Matthes [seePutnam, 1960, 14) dated at 0.634 + 0.013 m.y. The pyroxenerhyolite, the p. 248]. The lake probably roserapidly, forming no terr•tces youngestdated flow on the resurgentdome, is not greatlytilted during its rise to the lowestpoint on the calderarim, which at or uplifted and is exposedat elevationsfrom 2320 m (7600 ft) that time was at about 2380-m (7800 ft) elevationand near the to 2415 m (7920 ft). The perlitized biotite rhyolite apparently presentlake outlet. It then beganoverflowing across the sur- flowedinto the lake, whereasthe overlyingpyroxene rhyolite face Of the Bishoptuff, cuttingrapidly downwardthrough the did not. Thusthe lake,probably reached its highestlevel, upper 60 m or so of nonweldedto partly welded tuff, until it presumably2380 m (7800 ft), near the time of eruptionof the reachedthe denselywelded zone at 2320 m (7600 ft), wherethe biotite rhyolite and then recededto 2320 m (7600 ft) before lake level becametemporarily stabilized.(An ancient stream eruption of the pyroxenerhyolite flow 0.63 m.y. ago. valleypreserved in the surfaceof the VolcanicTableland 20 km The approximateposition of successivelylower and younger to the southeast[Bateman, 1965, p. 167] was probably cut by lake levelsmay be inferredfrom otherdated volcanic Units thisearly outflow stream.) The 2320-m(7600 ft) levelcoincides within the caldera (Figure 6). On the north flank of the resur- with the highest terrace recognizedon the northeast caldera gent dome, lake terrace depositsat an elevation of 2290 m wall. (A figureof 2410m (7900ft) hasbeen repeatedly cited in (7500 ft) lap againstthe baseof a hornblende-biotiterhyolite the literaturefor the highestlake terrace,but the surfacerising dome (location 12) dated at 0.509 + 0.011 m.y. but do not to this elevation is an alluvial fan, not a terrace; the highest occuron an adjacentagglutinate cone and lava flow (location well-rounded beach pebblesindicating a lake strand are at 11) datedat 0.468 + 0.010m.y.; the highestterrace deposits on 2320 m (7600 ft).) Sinceinitial stabilizationof the lake, over- the latter occurat 2230 m (7300 ft). Thus the lake level appar- flow has been controlled by downwarpingand intermittent ently was near 2290 m (7500 ft) between0.51 and 0.47 m.y. ago movement on faults that bound the east front of the Sierra and recededto 2230 m (7300 ft) after 0.47 m.y. ago. Nevada and intersectthe caldera rim. (See sectionon post- Lake sedimentsabove and below the Hot Creek rhyolite caldera tectonic activity.) flow (S17 of Doell et al. [1966]) in the southeastmoat indicate That the lake reached a high level early in its history is that the lake surfacewas at about 2200 m (7200 ft) about 0.28 indicatedby the fact that many of the early rhyolitesexposed m.y. ago. Lack of evidencethat the basalt in the north moat in the resurgentdome are pervasivelyhydrated, suggesting (location 20) flowed into water indicatesthat the lake was their eruption into the calderalake. Critical in this respectare below 2135 m (7000 ft) 0.104 m.y. ago. Completedraining of two flows on the north flank of the resurgentdome, one being the lake occurredsometime within the last 0.1 m.y. (Lake a pervasivelyperlitized biotite rhyolite flow and the other an Crowleyis not a remnantof PleistoceneLong Valley Lake but overlying obsidian-bearingpyroxene rhyolite flow (location ,isimpounded behind Crowley Dam, whichwas built in 1941.)

- 24oo

7800

•(14) 7600

-- 2300 (11) l

7400

(S-17) -- 22•J0 7200

• (20) 7000

I -- 2100

6800 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 MILLION YEARS Fig. 6. Elevationsand agesof PleistoceneLong Valley Lake levels.Dashed line showsgeneral rise and fall of lake level with time, basedon age of associatedvolcanic unit (solid bar). Numbersin parenthesesare localitynumbers of dated samples(Table I and Figure 3). Lengthof bar is analyticalerror of age determination. BAILEY ET AL.: LONG VALLEY SYMPOSIUM 737

LACUSTRINE SEDIMENTS ceasedsome time ago in the Little Antelope Valley area, as there are no active fumaroles nearby at present, although The sedimentswithin Long Valley have been exposedto a temperaturesare rather high (110øC) at depthsof 165-200m depth of only 50 m by the dissectionthat followed drainingof in U.S. GeologicalSurvey drill hole LVCH-6, 1 km southof the calderalake, and the deepestpenetration of them by drill- the clay quarry [Lachenbruchet al., 1976b]. ing is 305 m in U.S. Geological Survey drill hole LVCH-1 Surroundingand locally overlying these areas of acidaltera- (Figure 3). Consequently,little is known of the character of tion, the lacustrine sedimentsare thoroughly cementedwith the deepersedimentary fill, althougha largeportion of it is opalbut areotherwise unaltered. They form a hard,resistant undoubtedlylacustrine. cap rock as much as 15 m thick. The temporaland genetic The upper 50 m of sedimentexposed on the eastflank of the relation between these silica-cemented sediments and those resurgentdome are predominantlypebble conglomerate and that are argillizedand partly replacedby 0pal is unclear,but coarsetuffaceous sandstone. They contain abundantclasts of the silica-cementedsediments contain myriads of verticalpipe- early rhyolite indicating that they were derived mainly from like or honeycomblikestructures (illustrated by Cleveland erosion of the resurgentdome. The sedimentscommonly ex- [1962, Figure 8]) that have been identifiedas fossilgas vents hibit deltaic structurewith well-developedtopset and foreset (L. J.P. Muffler,personal communicatio n, 1972).These inter- beds,the latter havingdips as muchas 30ø and amplitudesof estingstructures suggest that the sedimentswere deposited as much as 10 m [Rinehartand Ross,1964, pp. 76-77, Figure over an activefumarolic area, possibly during a temporaryrise 38]. The topset beds commonly grade upward into marsh of the calderalake. Also, locally interbeddedwith thesesilica- depositscontaining or locally consistingalmost entirely of cementedsediments, mainly along faults, are now-inactive silicifiedreedlike plant stalksand rootlets.Eastward the marsh siliceoussinter deposits. and deltaicdeposits grade or interfingerwith fine-grainedbuff The wide distributionof thesefossil gas vents, ancient sinter to white diatomaceoussediments exposed in the lower parts of deposits,and areasof acid alteration(Figure 5) indicatesthat Long Valley. Ostracodsare abundant in both marsh and dia- surficialhydrothermal activity within the calderawa s more tomaceousdeposits. extensiveas well as more intensivein the past than at present, In drill hole LVCH-I the sediments below 50 m tend to be and we suspectthat parts of Long Valley were formerly as somewhatcoarser and more pumiceous,but the general lith- activeas someof the Yellowstonegeyser basins are today. The ologydoes not changeconspicuously downward, and no partic- apparentdecline and areal restriction of surficialhydrothermal ularly distinctiveunits were penetrated.The Bishoptuff was activity are probably due to the generalreduction in per- not intersected,and it undoubtedlylies at muchgreater depth. meabilityof the intracalderarocks and sedimentsby silicifica- Below 150 m the sedimentsin the drill hole are zeolitized. The tion, argillization,and zeolitization.These 'self-sealing' proc- principal zeolite is clinoptilolite, which occurs as a replace- esses[White et al., 1971]have progressed to a depthof at least ment product of glass shards and pumice; below 250 m, 300 m, as indicatedby the impermeabilityof sedimentscored heulanditeis also presentas fine crystalsin cavities.The sedi- in drill hole LVCH-1. Although in detail the interrelations mentspenetrated by the drill hole dip gently, usuallynot more betweenthese three types of'alteration appear to be complexin than 10ø, but several zones, each about 5 m thick, show the field, their generaldistribution and relativedepths suggest intenselycontorted and thrustedbedding, suggesting that pen- a zonation downward from silicificationat the surfacethrough econtemporaneouswet sedimentslumping, possibly triggered argillizationto zeolitizationat depth, This zonationis prob- by earthquakes,was an important sedimentationprocess. ably a functionmainly of increasingtemperature with depth, andthe complexities observed probably are due to fluctuations HYDROTHERMAL ACTIVITY in groundwaterlevels related to risingand falling of thecal- On the east and southeastflanks of the resurgentdome the dera lake and to recurrentfracturing of the cap rock by tec- tuffaceouslacustrine sediments are locally intenselyargillized tonic activity. as a resultof acidichydrothermal alteration by hot springand Rinehartand Ross [1964, p. 80]have pointed out that most fumarolic activity. This argillization is most intenselydevel- of the activehot springsand fumarolesare locatedon or near oped in the vicinity of Little AntelopeValley and upperLittle north- to northwest-trendingfaults. The only apparentexcep- Hot Creek, althoughsmaller areas also occur near Hot Creek, tion is the springsalong Hot Creek,which are alignednorth- Casa Diablo, and northeast of Mammoth Lakes on the south- eastalong the bottomof the gorge.These springs, however, west edge of the resurgentdome. These argillized areas have are confined within a shallow, 2-km-wide, northwest-trending not been studiedin detail as yet, but they apparentlyconsist graben,and their northeastalignment is the result of their mainly of kaolinite with tracesof montmorillonite and alunite flowingfrom the permeable, brecc{ated base of theHot Creek [Cleveland, 1962]. The kaolin depositsnear Little Antelope rhyoliteflow, whichis confinedto the gorgebottom. Signifi- Valley are of commercialgrad• and have been activelymined cantly,the hottestsprings, some of whichare boiling,occur sincethe early 1950's.In the deeperparts of the Little Ante- only nearthe two main faultsbounding the graben;the cooler lope Valley clay quarry, tuffaceousand conglomeraticsedi- springsin the centerof the grabenare probablythe resultof ments, initially consistingof clastsof lithoidal rhyolite, obsid- lateralmigration of fluid throughthe basal breccia of the flow ian, and pumice, are completelyargillized, yet bedding and or alongminor graben fractures, where mixing with coldsur- clastic textures are almost perfectly preserved.In the upper face waters occurs. part of the quarry the sedimentsare also opalized,and layers Most of the hot springsand fumarolesin LongValley areson of opal becomeprogressively thicker and more abundantup- active extensionsof the Hilton Creek fault, which suggeststhat ward. The origin of this typeof zonationhas been explained by their locationis relatedto disruptionof the 'self-sealed'moat $choenet al. [1974] as the resultof incongruentdissolution of sedimentsby thisrelatively active Sierra Nevada frontal fault. primary mineralsabove the groundwatertable by downward An apparenttemporal as well as spatial relation between fault- percolatingsulfuric acid watersgenerated by near-surfaceoxi- ing and hot springactivity was dramaticallydemonstrated dation of H2S-bearingfumarolic gas. This processapparently recently when new boiling springsand ephemeralgeysers 738 BAILEY ET AL.' LONG VALLEY SYMPOSIUM erupted along Hot Creek within hours of two earthquakeson easternescarpment is the resultof downfaultingof the region August 25, and October 17, 1973. Although the epicenters to the east during the past 3 m.y. Long Valley calderalies were 20-40 km distantand not on the Hilton Creekfault, it across the faulted front of the Sierra and is intersected on the seemslikely that tremorsdisturbed the delicateplumbing of northwestand southeastby major frontal faultsthat havebeen springson the Hilton Creek system.The activity of the new activein both precalderaand postcalderatime. Becauseit is springsand geysershas diminishedmarkedly sincetheir out- almost certain that the Long Valley magma chamberwas break, but many of the older springsalong Hot Creek and on substantiallymolten during this time, it is of interestto con- Little Hot Creek, 3.5 km to the north, have shown marked siderhow the chamberresponded to and how it may have increasesin flow, and somean increasein temperaturealso, influencedthis tectonicactivity locally. suggestingthat their channelshave been selectivelyenlarged The northwestcaldera wall is intersectedby the Hartley sincethe earthquakes.With the data of Haas [1971] the salin- Springsfault (Figure5), whichhas displaced precaldera ande- ities (•0.1%) and the geochemicallyindicated reservoirtem- sitesof theTertiary San Joaquin Mountain complex by 450m peratures (•200øC) of the new hot spring waters [Mariner and the Bishoptuff by about 300 m. The topographicrelief on and Willey, 1976] suggestthat they originatedfrom a depth of theescarpment isabout 600 m. 'This fault scarp terminates at at least 150 m. The presenceof detrital zeolite particlesin the the northwest caldera rim; southward within the westerncal- new springwaters suggests a similar minimum depth of origin, dera moat severalminor en echelonfaults project on strike as zeolitesoccur only below 150 m in drill hole LVCH-1, 1.5 with it and form an ill-defined, incipient graben that termi- km north of the new springs. natesin the 'earthquakefault' [Benloftand Gutenburg,1939] Although more sparsely distributed than in the east and near the northeast base of Mammoth Mountain. These en south, hydrothermalactivity also occurslocally in the western echelonfaults displace0.2-m.y.-old trachyandesitesby about part of the caldera. Active fumarolesand abundantacid alter- 15 m, 0.1-m.y.-old moat rhyolite by about 10 m, and 650-yr- ation occur on Mammoth Mountain [Huber and Rinehart, old Inyo crater phreatic depositsby about 5 m. Topographic 1967, p. D19], a zone of hydrothermal alteration parallels evidence also indicates that the floor of western moat east of the southwest caldera wall near the base of Mammoth these faults is about 60 m lower than to the west. However, Rock [Rinehartand Ross, 1964, p. 81], and pyritized rhyolite there is no suggestionof displacementin the westmoat of 600 occursat a depth of 210 m in U.S. GeologicalSurvey heat-flow m as on the Hartley Springsfault outsidethe caldera.Thus the drill hole DC in the northwestmoat (Figure 3). The apparent intracaldera segment or trace of the Hartley Springs fault paucity of hydrothermal activity at the surfacein the western appearsnot to have been active until fairly recent time. moat does not necessarilyindicate a lack of activity at depth The southeastcaldera rim is intersectedby the Hilton Creek becausethe thick sequenceof young basalts in the western fault, which forms an 1100-m escarpmentat the mouth of moat and the abundant subsurface flow of cold water from the McGee Creek. What part of this escarpmentcan be attributed Sierra Nevada may mask it. to postcalderamovement is not preciselydeterminable, but if Although individual hot springsand fumarolesare associ- the fault has been continuously or intermittently active, it ated locally with north- to northwest-trendingfaults and frac- probably amountsto severalhundred meters, as displacement tures, the general distribution of hydrothermal activity within on Tioga lateral moraines and outwash at the mouth of the caldera is in an arcuate zone peripheral to the resurgent McGee Creek is as much as 15 m [Putnam, 1962, p. 200]. Like dome, suggestingthat the dominant controlling structuresat the Hartley Springsfault, the Hilton Creek fault terminatesas depth are the caldera ring fractures. It is noteworthy that a major escarpmentat the caldera wall; within the caldera it fumaroles in Long Valley caldera are restrictedto fractures splintersinto severalminor divergingfaults that die out north- within the keystonegraben of the resurgentdome, whereas hot ward. The largestand most continuousof thesesplinters dis- springsare at lower elevationsmarginal to the dome wherethe placesby about 15 m 0.3-m.y.-old moat rhyolites,as well as groundwater table is near or at the surface. 0.6- to 0.7-m.y.-oldearly rhyolites.Thus the maximumage of Although fumaroles and hot springsprobably have been faulting is 0.3 m.y. However, since the intracaldera dis- active locally to a minor degreethroughout the historyof the placementabout equals that on theTioga deposits at McGee caldera, the extensivedevelopment of hydrothermalactivity in Creek, outsidethe calderait probably is very much younger lacustrinesediments that are about 0.3 m.y. old, judging from than 0.3 m.y. and may be in large part post-Tioga in age. the 0.28-m.y. age of the interbeddedHot Creek rhyolite flow, Thus both northwest and southeastof the caldera, major suggeststhat the geothermal systemmay have reachedmax- Sierra Nevada frontal faults having severalhundred meters of imum developmentat about that time. This notion seemsto be postcalderadisplacement show large and abrupt decreasesin supportedby the paucity of evidencefor hydrothermalactivity displacementat the caldera margins, and their intracaldera directly associatedwith the 0.73- to 0.63-m.y. early rhyolites continuationsapparently have been active only in very recent and the 0.5-, 0.3-, and 0.1-m.y. moat rhyolites. This implies time. Where and how this postcalderadisplacement was ac- that the geothermal system is not related to the individual commodatedwithin the caldera is an enigma. Possiblyit was postcalderaeruptive groups but to the main magmachamber, accommodated on cross-caldera faults that have since been which is a deeperand larger heat source.It also suggeststhat buried by youngerlavas and sediments,but in the centralpart developmentof the geothermalcirculatory system may have of the caldera where 0.6- to 0.7-m.y.-old early rhyolites are required considerabletime and been completedonly during exposed,east-dipping faults with significantdisplacement are the latter part of the caldera history. conspicuouslyabsent. There is, however, some evidence of postcalderamovement on the south and southwestsectors of POSTcALDERA TECTONIC ACTIVITY the caldera boundary fault. Although Pleistoceneglacial de- Matthes [1933, 1939], Christensen [1966], Bateman and positsand steepalluvial fans have buried the actual boundary Wahrhaftig [1966], and others have concludedthat the rise of fractures on all sides of the caldera, on the south wall at Laurel the Sierra Nevada crest was essentiallycompleted before the and Sherwin creeks,older morainal ridges, thought to be of advent of Pleistoceneglaciation and that developmentof the M ono basin age, are truncatedby an apparenteast-west fault BAILEY ET AL.: LONG VALLEY SYMPOSIUM 739

(Figure 5). Also, as noted in a precedingsection, the northeast face of Mammoth Mountain, on the southwest caldera rim, is truncatedby a fault that parallelsthe calderawall. In contrast, the east and north walls of the caldera show no evidence of postcalderamovement on the boundary fractures.These rela- tions suggestthat during early postcaldera time, when the Long Valley magma chamber was shallow and its roof rela- tively thin and weak, displacementon the SierraNevada front Mono Lake betweenthe south end of the Hartley Springsfault and the north end of the Hilton Creek fault was accommodatedalong the south and west sectorsof the caldera boundary fault. More o.e •.•:::: ß ß• .. ß recently, with increasedcrystallization of the chamber and •:::::::::::::::::: consequentthickening and strengtheningof the roof, tectonic ß ...... stresseshave begun to be transmitted through the cauldron MonoCraters ;::::::::::::::::::. ß ...... • ...... block, as indicatedby the recentdevelopment of faults cross- ß ...... e:::::::::::::: ...... ing the caldera floor on strike with the frontal faults. ß ...... ::::::: ,ß ...... This suggestionis reinforcedby analysisof the deformation ...... I•., ß...... immediatelyeast of the Sierra Nevada front. Russell[1899, p...... ::::::::::::::::::::::: ...... 302] noted that the youngestshorelines of PleistoceneMono ..• ...... ß ...... ß...• ...... ,.... :::::::::::::::::' Lake are depressedtoward the range front, and Gilbert et al. ======...... [1968, p. 313] have describedthe area betweenMono Lake and Long Valley as a broad, warped, faulted, grabenlikedepres- sion along the front. Southeastof Long Valley a similar de- pressionoccurs along the eastbase of the WheelerCrest escarp- ment, where the Bishoptuff is downwarpedas much as 300 m ©1•..... ø%.;!o into Round Valley [Bateman,1965]. Downwarping is evident ß ß ß also along the east and southeastrim and wall of the caldera ß ß ß where PleistoceneLong Valley Lake terracesare downwarped eo from 2320-m (7600 ft) elevation south of Glass Mountain to 2130 m (7000 ft) eastof Lake Crowley (Figure 5) [Rinehartand LongValley Ross, 1957; Putnam, 1960; Christensen,1966]. This zone of downwarpingalong the eastfront of the Sierra Caldera ß Nevada between Mono Lake and Round Valley (Figure 7a) constitutesa zone of backtilting or 'reversedrag' [Hamblin, 1965] that apparently developedsimultaneously with the for- mation of the Sierra escarpment.A substantialamount of this downwarpingpostdates the caldera, but it doesnot appearto be continuousthrough the caldera (Figure 7b). Its continuity may be obscuredpartly by the resurgentdome, but the dome formed during a very short time interval immediatelyfollow- ing caldera collapse, and any subsequent imposition of 200-300 m of downwarp should be discernible.The lack of -!- • :::::::::::::: evidencefor major down-to-east faulting within and the dis- • / continuity of 'reversedrag' acrossthe caldera suggeststhat the .. cauldronblock adjustedindependently of the faulting immedi- ......

......

...... ately to the north and south. If postcalderatectonic move- ...... mentswere restrictedalong the south and west sectorsof the ß:::::::::::::::::: •o caldera boundary fault, possiblythey were absorbed in the

subjacentmolten magma chamber, and any tendencyfor de- ::::::::::::::::' velopmentof 'reversedrag' within the cauldron block was impeded by hydrostaticadjustments in the magma. ß::::::::::::::: o•

SUMMARY AND DISCUSSION :::::::::::::::::::::::::

The structure,stratigraphy, and geochronologyof Long Val- Fig. 7a. Tectonicmap of Long Valley-Mono basinarea. Heavily ley caldera are summarizedin Figures8 and 9. Volcanismin dotted area is zone of 'reversedrag' on Sierra Nevada front. Lined the vicinity of Long Valley began about 3.2 m.y. ago with the area is resurgentdome in Long Valley caldera. Solid lines represent eruptionof basalt and andesitefrom widely scatteredcenters. faults (ball on downthrown side). Subsequently,3.0-2.7 m.y. ago, rhyodaciteerupted from two main areas:the Two Teats-San Joaquin Mountain and the Bald Mountain areas, respectivelywest and north of Long Long Valley calderaapparently accompanied development of Valley. This mafic to intermediatevolcanism probably oc- the escarpment,and this fundamentalchange in tectonicactiv- curred during the last major rise of the Sierra Nevada and ity may have providedthe conditionsnecessary for the gener- beforedevelopment of the easternSierra Nevada escarpment. ation and accumulationof large volumesof silicic magma. The subsequentepisode of rhyolitic volcanismassociated with Rhyolitic volcanismbegan about 1.9 m.y. ago northeastof 740 BAILEY ET AL.: LONG VALLEY SYMPOSIUM

outerarc of lesssilicic extrusions concentric with the moat rhyolitesand the early rhyolites. This concentriczonation of the postcalderaeruptives may be explainedas a consequenceof progressivedownward crys- tallization of a magma chamberthat was verticallyzoned from rhyolite in its upper part to rhyodacite in its lower part as shown in Figure 8. The continuousdecrease in silica content RESURGENT DOME with time from 75% in the early rhyolites to 64% in the rim .•...... ,•.H.•,•_. :•:•.:...... ! ...... rhyodacitesmay be inferred to reflectintermittent tapping of TECTONIC FAULTING AND "REVERSE DRAG" WITHIN CALDERA ] the chamber at progressivelygreater depthsalong ring frac- DAMPE•DBYHYDROSTATIC ADJUS'rtMENTSINMAGMA CHAMBER tures that formed successivelyoutward and extended down- ward into the congealingmelt, possiblyas a result of minor subsidence associated with the volume decrease that accom- panied its consolidation.The upper, most silicic part of the chamber (77% silica) was erupted as the rhyolite of Glass Mountain, and duringeruption of the Bishoptuff and collapse of the caldera, the chamber was drained of the zone of magma ß • WHEELER CREST •••"• ESCARPMENT containing 76% silica. The abrupt decreasein crystal content ?,,.o.,.....,:•z.. of the magma from 20-30% in the Bishop tuff to less than 3% in the immediately subsequentearly rhyolites may be the result either of phenocryst resorption caused by de- compressionof the chamber following eruption of the Bishop Fig. 7b. Schematiccross sections across the SierraNevada escarp- ment(Figure 7a) showingthe contrastin postcalderatectonic deforma- tuff or of resurgenceof hotter, near-liquidus magma from tion within Long Valley caldera (BB') and to the north (AA') and deeper in the chamber. Successivetapping of deeper levels south(CC'). Note the absenceof major tectonicfaults and associated in the chamber at later times brought progressivelymore 'reversedrag' within the caldera.The dottedline is an arbitrary refer- crystallized magma to the surface in the form of the moat ence level common to all three sections. rhyolitesand rim rhyodacites.The simultaneouseruption of rhyolitein the westmoat and rhyodaciteon the rim is possibly a function of deeperconsolidation on the outer edge of the magma chamber, which would require tapping of deeper, Long Valley. Sparselyporphyritic rhyolite,-typicallycontain- more mafic levels by outer ring fracturesand of shallower, ing 77% silica, erupted over a period of 1 m.y. and accumu- silicic levels by inner ring fractures. lated in the immense edifice of Glass Mountain with its flank- Complete consolidationof a magma chamber of this type ing pyroclasticfans. The eruptivecenters of Glass Mountain would result in a pluton zoned from granite to granodiorite are on a 13-km arcuate zone that probably coincideswith an downward.Documented examples of suchplutons are few, the early incipient ring fracture related to developmentof the more usual zonation being concentric from more mafic to Long Valley magma chamber. more silicic inward, but two notable examplescan be cited as About 0.7 m.y. ago the Long Valley magma chamber was possibleanalogues: (1) the Cruachan'granite' of the Etive and evisceratedby a series of explosiveeruptions during which Glen Coe igneousring complexes,Scotland, which is a binary about 600 km3 of coarselyporphyritic biotite rhyolite magma graniteon high peaksand adamellitein valley sidesand floors containing76% silicawas ejected,mainly as ashflows, to form [Bailey and Maufe, 1960, pp. 170, 220-221; Anderson,1937], the Bishop tuff. Contemporaneouscollapse of the roof of the and (2) the outer 'granite' of Ben Nevis, Scotland,which in a chamberresulted in formation of the Long Valley caldera,an vertical exposureof 1300 m gradesfrom granite at high levels elliptical depressionabout 15 by 30 km and 2-3 km deep. to granodioriteat low levels[Anderson, 1935, pp. 252, 264]. It Eruptionsfrom at least 12 ventsnear the centerof the caldera is noteworthy that this latter intrusion showsthe same tend- and possibly from others near its margins continued for encytoward more coarselyporphyritic textures with apparent 40,000-100,000 yr after collapse.During this time, at least 500 time as the postcalderalavas of Long Valley. m of aphyric to sparselyporphyritic rhyolite, typically con- Thermal calculations [Lachenbruchet al., 1976a] indicate taining 75% silica, accumulatedon the caldera floor. Contem- that the Long Valley magma chamber must have been re- poraneouslywith eruptionof theseearly rhyolitesthe westcen- plenishedwith heat to have sustainedvolcanism for more than tral part of the calderafloor was uplifted into a subcircular I m.y.; thus the above model is probably an oversimplifica- resurgentdome about 10 km in diameter,and along its cresta tion. Becausefurther refinementof it must await completion 5-km-wide keystonegraben formed parallel to the dominant of petrochemicaland mineralogicalstudies presently under northwest-trendingprecaldera structures of the region. way, discussionof the various possiblemechanisms for heat After resurgentdoming, coarselyporphyritic hornblende- replenishmentseems unwarranted at this time. biotite rhyolite, typicallycontaining 72% silica,erupted from During the late stagesof solidificationof the Long Valley three groupsof centersin the calderamoat peripheralto the chamber, basaltic and trachyandesiticlavas and pyroclastic centralresurgent dome. These three groups of moat rhyolites rockserupted from a systemof north-trendingfissures extend- eruptedin clockwisesuccession in the north, southeast,and ing from southof the calderathrough the westernmoat to the west at 0.2-m.y. intervals about 0.5, 0.3, and 0.1 m.y. ago. north shoreof Mono Lake. Theseeruptions, possibly triggered About 0.2 m.y. to 50,000 yr ago, coarselyporphyritic horn- by late faulting along the east front of the Sierra Nevada, blende-biotiterhyodacites containing 70-64% silicaerupted on probably tapped mafic magma from a much deeper source the southwest caldera rim and near the base of the northwest than the Long Valley chamber.A rare instanceof the inter- and north calderawalls. These rim rhyodacitesconstitute an action of thesemafic lavas with the silicicmagma of the Long BAILEY ET AL.: LONG VALLEY SYMPOSIUM 741

W E

Glass Mountain

77% SiO 2

... 7.5% Si02:

•..•:. -.,,,..•.. ß . . ,\' .... ß'...;72% SiO2 ...... : .- • ' 'd.3m• • :.".--:,...,•ß . ,..:.,...',...•.. • .... :. ,70%'Sio' 2...... •,.,•.:...... '"2•.,,,•,,...... O,:.'t:.•.•:..'--..'.....--•..'...... ;,,;:,:.;.-..:,.•....;..:... ß "•- ..:;..'",•...•....;..:..;.... :...,..•::.... :... ;.;...... ' ...... ::::::::::::::::::::::::::::::: .::;:i : ::•:..•...•::• •:: .•: •:: • ::."",,•.":L:.'iii•::•

;::::::::::::::::::::::::. ::::::::::::::::::::::::::::::::::::::::: ::::::: :: ::::: ::::::::::::::::::::: ::::::::::::::::: :::::::::: ::::::.",,•,•. •'::::::::::::::: :::::::::::::::::::::: :.':..•:::: ::: :::: :::: ::: :::

0 2 4 6 8 10 KM I I I I I I Fig. 8. Schematiceast-west cross section through Long Valley caldera and its subjacent magma chamber showing hypotheticalchanges in chemical compositionand depth of crystallizationwfth time. The heavily dotted part of the chamberis rhyodacitemagma; lightly dotted part is rhyolitic magma. Horizontal dotted linesshow silica gradient in the verticallyzoned chamber. Curved dashedlines show depth of crystal-liquidinterface (depth to residualmagma) at specified times (millions of years ago). Formation patterns are the same as in Figure 3, except that basementplutonit and metamorphic rocks (hachured) are undivided. The vertical scale is unspecifiedbecause of uncertaintiesin thicknessof intracalderaunits and depthsin chamber.

Valley chamber is recorded in the mineralogicallyhetero- rejuvenated segmentsof Sierra Nevada frontal faults that ex- geneousolivine-bearing rhyodacite of Deadman Creek. tend acrossthe calderaand haveruptured the self-sealedcap The most recentvolcanic rocks within Long Valley caldera rock. Although individual springsand fumaroles are located are the lnyo domes,the youngestof which are lessthan 720 yr on or near north- to northwest-trendingfaults, the general old. Thesedomes are fluidal mixturesof rhyolitic and rhyoda- distributionof hydrothermalactivity is peripheralto the resur- citic lava that have erupted on a north-trendingfissure be- gent dome, suggestingthat it is controlled at depth by the tweenLong Valley calderaand the M ono craters.We interpret calderaring fractures.These relations further suggestthat ther- them as representingthe mixing of rhyodaciticmagma from mal waters may be more extensiveat depth than is suggested the Long Valley chamberwith rhyolitic magma from a cham- by the present surfaceactivity. ber thought to underlie the Mono cratersring fracture zone. Downfaulting on the Sierra Nevada front has continued The Inyo craters,three phreatic explosion craters at the south throughout postcaldera time, but the major Sierra Nevada end of the chain of Inyo domes,were possiblycaused by the frontal faults that intersectthe caldera rim have not displaced rise of rhyodaciticmagma to very shallow depth, causingsub- the caldera floor until very recently.During early postcaldera surfacewaters to flash explosivelyto steam.The cratersmay time when the magma chamberwas shallow, movementon the be as young as 450 yr, indicatingthat residualmagma possibly front apparently was accommodatedalong the southern and was present in the Long Valley chamber that recently. The westernsectors of the caldera boundary fault. As the magma young age and frequencyof eruptionsalong the Inyo-Mono cooled and solidified, the cauldron block thickened and eventu- volcanicchain indicate an active volcaniczone; eruptionsof ally became rigid enough to transmit tectonic stresses.The similar kind and magnitudecould occur in the future. absencewithin the caldera of 'reversedrag,' presentelsewhere Throughoutmost of its history,the Long Valley calderawas along the Sierra Nevada front, suggeststhat until recently occupiedby a lake that probablyreached its highestlevel prior vertical tectonic stresses within the cauldron block have been to 0.63 m.y. ago, then overflowed at the southeastrim, and absorbedby hydrostatic adjustmentsin the subjacentmagma subsequentlywas progressivelylowered by tectonic faulting chamber. The manner in which the Sierra Nevada faults and warpingof the rim near the outlet.Calcareous tufa depos- changefrom single,continuous fractures outside the calderato its on the lower lake terracesindicate that in the latter part of branching,en echelonfractures inside suggests, however, that its history the lake was alkaline and salinein composition. the cauldron block is still considerablyless rigid than the Hydrothermalactivity within the calderaprobably reached surroundingcrust and that it may still be partially underlain maximum intensityabout 0.3 m.y. ago. The declineof surface by magma. activity since then is probably due to 'self-sealing'of the in- A tentativehistory of the rise and consolidationof the Long tracaldera rocks by silicification,argillization, and zeolitiza- Valley magma chamber can be reconstructedfrom several tion. Presentthermal activity is mainly on or near recently differentsources of information on depth. The following esti- 742 BAILEY ET AL.: LONG VALLEY SYMPOSIUM

• •o I Intracaldera •-=- Rim I basalts Inyo-Mono ites I rhyolites 0.0 GlassMountain BishopTuff•.• Early rhyolitesMoat rhyol rhyodacites I andandesites

/ ooO ß oø oz,

0.2

0.4

ß I Thi•)aper 0.6 •••• / Gilbertandothers

•I / Dalrympleandothers

0.8

! Doelland others 0.9

1.0 Dalrymple m (1967) .

1.9 Huber and Rinehart (1967)

2.0 Fig. 9. K-Ar ages and inferred geochronologyof volcanic events associatedwith Long Valley caldera. The dark bars indicate the approximate time and duration of frequent activity; lined bars with queriesindicate uncertainty.

mates,although of uncertainand variableaccuracy and sub- mingledto a minor extent with the rhyodaciticresidua in the ject to revision,are internallyconsistent and henceseem worth Long Valley chamber. summarizingat this time. Geochemicaldata [Hildreth and In conclusion,volcanological, geochronological, and struc- Spera,1974] suggest that at the time of eruptionof the Bishop tural evidenceindicates that large sourcesof heat exist at tuff 0.7 m.y. ago, the top of the chamberwas at a depthof sufficientlyshallow depth in the Long Valley-Mono craters about6 km. Structuralanalysis of theresurgent dome suggests area to be of importanceas potentialgeothermal resources. that at the closeof resurgenceabout 0.6 m.y. ago,the top of the chamberhad risento at least5-km depthand possibly to 2 Acknowledgments. We wish to thank N. K. H uber and C. D. or 3 km. By about 0.2 m.y. ago the lateral encroachmentof Rinehart for freely sharingtheir knowledgeof the area from previous basalticdikes on the chambersuggests that it had congealed studiesand for providingunpublished data and sampleinformation that greatly facilitated the early stagesof this project. We are also inwardand downward4 km to a depthof 6 to possibly9 km. gratefulto D. E. White for offeringmany helpful suggestionsduring Teleseismicand seismicrefraction studies [Steeples and lyer, the courseof the fieldwork,particularly concerning the hydrothermal 1976;Hill, 1976]suggest that an anomalouslyhot or partially activity and alteration and other featuresassociated with Pleistocene molten massstill persistsbelow 7 or 8 km. Geochemicaldata Long Valley Lake. The many exchangesof observationsin the Mam- from the Inyo domes [Carmichael, 1967] indicate that lava moth Lakesarea with S. R. Lipshieare alsogratefully_ acknowledged. Special thanks are due to R. P. Koeppen for assistancewith the from the Mono cratersmagma chambererupted under pres- mapping and samplecollecting; to J. C. Von Essen,A. H. Atkinson, suresof 6.6-2.7 kbar, which suggestsdepths of 22-6 km. B. M. Myers, S. J. Koger, L. B. Schlocker,D. A. Williams,and Thusin summary,the LongValley magmachamber appears J. Y. Saburomarufor assistancewith the K-Ar analyses,and to P. C. to have developedand risenthrough the crustover a periodof Bateman, R. L. Christiansen, M. M. Clark, N. K. Huber, S. D. McDowell, R. P. Sharp, and D. E. White for critically reviewingthe at least 1.3 m.y. (from 1.9 to 0.6 m.y. ago), during which time manuscriptand offering many constructivesuggestions. the rhyolitesof GlassMountain, the Bishoptuff, and the early rhyoliteserupted. It then apparentlyachieved isostatic equilib- REFERENCES rium in the uppercrust, probably due to lossof mass.During Anderson, J. G. C., The marginal intrusionsof Ben Nevis, the Coille the past 0.6 m.y. it has been cooling and congealingdown- LianachainComplex, and the BenNevis dyke swarm, Trans.Geol. ward, periodicallybuilding up sufficientvolatile pressureby Soc. Glasgow,19, 225-269, 1935. crystallizationto extrudethe moat rhyolitesand rim rhyoda- Anderson,J. G. C., The Etive granitecomplex, 4, Quart.J. Geol.Soc. cites.Within the last 12,000yr therehas been an insurgenceof London, 93, 487-533, 1937. new rhyolitic magma immediately to the northwest of the Bailey, E. B., and H. B. Maufe, The geologyof Ben Nevis and Glen Coe and the surroundingcountry, Mem. Geol.Surv. Scot., 1-'307, Long Valley chamberbeneath the Mono craters,and there is 1960. someevidence in the Inyo domesthat this magma has inter- Bailey,R. A., Postsubsidencevolcanism and structureof Long Valley BAILEY ET AL.: LONG VALLEY SYMPOSIUM 743

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