Hildreth Jvgr04 Longvalley

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Volcanological perspectives on Long Valley, Mammoth Mountain,

and Mono Craters: several contiguous but discrete systems

Wes Hildreth*

U.S. Geological Survey, Volcano Hazards Team, MS-910, Menlo Park, CA 94025, USA

Abstract

The volcanic history of the Long Valley region is examined within a framework of six successive (spatially discrete) foci of

silicic magmatism, each driven by locally concentrated basaltic intrusion of the deep crust in response to extensional unloading

and decompression melting of the upper mantle. A precaldera dacite field (3.52.5 Ma) northwest of the later site of Long

Valley and the Glass Mountain locus of >60 high-silica rhyolite vents (2.2 0.79 Ma) northeast of it were spatially and

temporally independent magmatic foci, both cold in postcaldera time. Shortly before the 760-ka caldera-forming eruption, the

mantle-driven focus of crustal melting shifted f 20 km westward, abandoning its long-stable position under Glass Mountain

and energizing instead the central Long Valley system that released 600 km3 of compositionally zoned rhyolitic Bishop Tuff

(760 ka), followed by f 100 km3 of crystal-poor Early Rhyolite (760650 ka) on the resurgent dome and later by three

separate 5-unit clusters of varied Moat Rhyolites of small volume (527101 ka). West of the caldera ring-fault zone, a fourth

focus started upf

160 ka, producing a 10

20-km array of at least 35 mafic

vents that surround the trachydacite/alkalicrhyodacite Mammoth Mountain dome complex at its core. This young 70-vent system lies west of the structural caldera and

(though it may have locally re-energized the western margin of the mushy moribund Long Valley reservoir) represents a

thermally and compositionally independent focus. A fifth major discrete focus started up by f 50 ka, 2530 km north of

Mammoth Mountain, beneath the center of what has become the Mono Craters chain. In the Holocene, this system advanced

both north and south, producingf 30 dike-fed domes of crystal-poor high-silica rhyolite, some as young as 650 years. The

nearby chain of mid-to-late Holocene Inyo domes is a fault-influenced zone of mixing where magmas of at least four kinds are

confluent. The sixth and youngest focus is at Mono Lake, where basalt, dacite, and low-silica rhyolite unrelated to the Mono

Craters magma reservoir have erupted in the interval 14 to 0.25 ka. A compelling inference is that mantle-driven magmatic foci

have moved repeatedly, allowing abandoned silicic reservoirs, including the formerly vigorous Long Valley magma chamber, to

crystallize. A 100-fold decline of intracaldera eruption rate after 650 ka, lack of crystal-poor rhyolite since 300 ka, limited

volumes of moat rhyolite (most of it crystal-rich), absence of postcaldera mafic volcanism inside the structural caldera (or north

and south adjacent to it), low thermal gradients inside the caldera, and sourcing of hydrothermal underflow within the western

array well outside the ring-fault zone all suggest that the Long Valley magma reservoir is moribund.Published by Elsevier B.V.

Keywords: magmatism; rhyolites; calderas; Long Valley; Mammoth Mountain; volcanic fields

0377-0273/$ - see front matter. Published by Elsevier B.V.doi:10.1016/j.jvolgeores.2004.05.019

* Tel.: +1-650-329-5231; fax: +1-650-329-5203.

E-mail address:[email protected] (W. Hildreth).

www.elsevier.com/locate/jvolgeores

Journal of Volcanology and Geothermal Research 136 (2004) 169 198


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Fig. 1. Regional location map for Long Valley caldera and contemporaneous volcanic fields within and just east of the Sierra Nevada in east-

central California. Heavy dashed lines enclose main areas with numerous volcanic vents of Pliocene and Quaternary age (50 Ma). Near 36th

parallel, Coso and Kern Plateau (KP) volcanic fields have both Pliocene and Quaternary vents, while Panamint Valley (PV) field is Pliocene

only. Near 37th parallel, Kings River (KR) and Saline Range (SR) volcanic fields are Pliocene while Big Pine (BP) field is Quaternary. Near

38th parallel, a Pliocene volcanic field continuous from San Joaquin River (SJ) to Adobe Hills (AH) has abundant vents for basalt,

trachyandesite, and K-rich mafic lavas; near its center, silicic magmas of a more restricted region close to Long Valley caldera (hachured

enclosure) began erupting f 3.5 Ma, continuing to the present, as detailed in the text and subsequent diagrams. Bold solid lines are faults of

Pliocene-to-Quaternary age, many with both normal and strike slip displacement, representing encroachment of Basin and Range

transtensional tectonics (contemporaneous with the volcanism) upon the Sierra Nevada (Mesozoic) batholith province. White areas are

alluvium-filled basins. Main sources for this diagram:Moore and Dodge (1980);Bacon and Duffield (1981);Duffield and Bacon (1981);Novak

and Bacon (1986);Ross (1970);Gilbert et al. (1968).

W. Hildreth / Journal of Volcanology and Geothermal Research 136 (2004) 169198 171


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Fig. 2. Outline map of Long Valley caldera and adjacent Glass Mountain, Mammoth Mountain, and Mono-Inyo systems, adapted fromBailey

(1989). Shown for the caldera are its topographic margin (dashed), ring-fault zone (RFZ; dotted), and limit of structurally uplifted resurgent dome

(RD; dash-dot). The caldera moatis the physiographically low, annular region of the caldera floor that surrounds the resurgent dome, separates it

from the caldera wall, and conceals the structural ring-fault zone. Four sets of intracaldera rhyolitevents (clustered by both age and location) are

identified in inset. Dome 7403 in NE moat is early postcaldera rhyodacite. Distribution of precaldera Glass Mountain rhyolite lavas and thick

pyroclastic apron is shown in pink. Mammoth Mountain trachydacite rhyodacite dome complex is in green, and the array of contemporaneous

mafic vents around it is patterned grey; the numerous vents are located in Fig. 5.For Mono-Inyo chain, more than 30 rhyolite vents are exposed;

all but a few are Holocene lava domesthe youngest toward the north and south ends. Place name abbreviations: CM = Crater Mtn;

DC = Deadman Creek dome; DM = Deer Mtn dome; EQD= Earthquake dome; GC = Glass Creek dome; IC = Inyo Craters (phreatic); JLB = June

Lake basalt vent; LM = Lookout Mtn; ML= Mammoth Lakes downtown; NC = North Coulee; OD = Obsidian Dome; PB = Punch Bowl;

PC = Panum Crater; SC = South Coulee; WB = Wilson Butte. Selected faults (after Bailey, 1989) named: ACF= Alpers Canyon fault;

BMF = Black Mountain fault; FLF = Fern Lake fault; HCF = Hilton Creek fault; HSF = Hartley Springs fault; SLF= Silver Lake fault.

W. Hildreth / Journal of Volcanology and Geothermal Research 136 (2004) 169198172


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crystal-poor; a dozen have 68% phenocrysts, but the

great majority have 0 5%. Phenocryst abundances,

species, and compositions resemble those of the

evolved, first-erupted part of the zoned Bishop Tuff,most units having quartz, sanidine, plagioclase, bio-

tite, allanite, zircon, apatite, and FeTi oxides. The

means of generating and sustaining crystal-poor high-

silica rhyolite for 1.4 Myr are addressed below in

Section 7.

3. Climactic eruption and caldera formation

The caldera-forming eruption of the Bishop Tuff at

760 ka began as a plinian outburst along or near the

Hilton Creek fault in the south-central part of what

soon became the caldera (Hildreth and Mahood,

1986). The roof of the growing chamber, then about

5 km deep (Wallace et al., 1999), ultimately failedcatastrophically, releasing f 600 km3 of gas-rich

rhyolitic magma, compositionally and thermally

zoned (Hildreth, 1979), i n a virtually continuous

eruption about 6 days long (Wilson and Hildreth,

1997), thereby permitting 23 km subsidence of the

roof, creating the caldera. About half the Bishop Tuff

volume was emplaced radially as a set of sectorially

distributed ignimbrite outflow sheets along with con-

current plinian and coignimbrite fallout. The other

half ponded inside the subsiding caldera, where

welded intracaldera ignimbrite is as thick as 1500 m

Fig. 3. Postcaldera Long Valley rhyolites, simplified from Bailey (1989).Estimated position of main ring-fault zone is 1 to 5 km inboard of

topographic margin, which receded by syncollapse landsliding and subsequent erosion. Early Rhyolite (760650 ka) lavas in red and tuffs in

yellow are cut by numerous faults associated with structural uplift. Black stars indicate 13 Early Rhyolite vents exposed (as well as vents for

younger rhyolites). Three clusters of Moat Rhyolite lavas crop out in north (527481 ka, orange), southeast (362288 ka, green), and west

(161101 ka, blue); all are crystal-rich except three (of the five) units in the southeastern cluster, which are phenocryst-poor rhyolite lavas

(distinguished in unpatterned pale green). Arrows generalize lava flow directions. Place name abbreviations: CD = Casa Diablo geothermal

plant; DCD = Dry Creek dome; DM = Deer Mtn; GD = Gilbert Dome; HCF = Hot Creek flow; LM = Lookout Mtn; MK = Mammoth Knolls (two

domes); ND = North dome; Ski = Mammoth Mtn ski complex; WMC = West Moat Coulee. Drillholes mentioned in text are: LVEW= Long

Valley Exploratory Well; SR = Shady Rest; others are named as designated on mapCP-1, M-1, PLV-1, PLV-2, 44-16, 66-29, and Inyo-4.



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and was subsequently buried by 500 800 m of

postcaldera rhyolite tuffs, lavas, and sedimentary fill

(Bailey, 1989). Pumice clasts in the Bishop Tuff are

zoned from 1 to 25 wt.% phenocrysts and define acompositional continuum in the range 7873% SiO2and 2600 ppm Ba (Hildreth, 1979). The main suite

of white pumice is accompanied by a sparse popula-

tion of crystal-poor grey pumice that extends the

range to 65% SiO2 and to 1350 ppm Ba (Hildreth

and Wilson, in review).

The 1732-km depression called Long Valley

caldera owes its dimensions and the physiography of

its walls to large-scale syneruptive slumping (cf. Lip-

man, 1997) and to subsequent secular erosion. As

inferred from gravity, drillholes, and vent distribution

(Kane et al., 1976; Carle, 1988; Suemnicht and Varga,

1988; Bailey, 1989), the ring-fault zone(Figs. 2, 3)

outlining the area of steep structural collapse of the

cauldron (roof) block into the magma reservoir

encloses a subsided oval 12 22 km (f 220 km2),

roughly 55% of the 400-km2 floor of the topographic-

hydrographic basin conventionally portrayed as Long

Valley caldera. The magma chamber, of course, had to

be somewhat wider than the roof plate that sank into

it. Nonetheless, clarity in definition of the structural

caldera can help avoid misleading conceptualizations.

For example, the Inyo rhyolites are commonly said tohave invaded the caldera 650 years ago and Mammoth

Mountain is said to straddle the caldera rim. In reality,

both are extracaldera volcanoes, compositionally and

spatially independent of the Long Valley reservoir

(see Sections 5, 6 below).

Seismic refraction profiles (Hill, 1976; Hill et al.,

1985) and gravity models (Kane et al., 1976; Carle,

1988) indicate that the caldera fill thickens substan-

tially toward the north and east, as confirmed in

drillholes that show the intracaldera Bishop Tuff

thickening from 0.9 to 1.2 km centrally to >1.4 km

in the eastern third of the caldera (Bailey, 1989). What

fractions of the deepening may reflect precalderatopography, differential magma withdrawal, or tilting

of the cauldron block during collapse remain uncer-

tain. The Hilton Creek fault (Fig. 2, where the caldera-

forming eruption began) clearly had several hundred

meters of NE-facing precaldera relief, and where the

caldera center is now, a north-sloping ramp (Bailey,

1989)probably separated the en-echelon Hilton Creek

and Hartley Springs faults(Fig. 2).Greater subsidence

in the north and east is also consistent with strati-

graphic and petrological evidence that the final erup-

tive packages of the Bishop Tuff, which preferentially

flowed toward those sectors, were withdrawn from

deeper, hotter levels of the magma reservoir(Hildreth,

1979; Hildreth and Mahood, 1986; Wilson and Hil-

dreth, 1997; Wallace et al., 1999; Hildreth and Wil-

son, in review).

4. Postcaldera eruptive history of Long Valley

proper

Compositions of postcollapse eruptive units (750

to 100 ka) that vented inside or near the calderas ring-fault zone(Fig. 3)are consistent with derivation from

a reorganized, convectively mixed, and thermally

restructured Long Valley magma reservoir. Composi-

tions of silicic units farther west are not. The post-

caldera units of Long Valley compositional affinity

(Fig. 4) are (1) rhyodacite Dome 7403, (2) the

voluminous crystal-poor Early Rhyolites, and (3)

three sets of Moat Rhyolites (Bailey, 1989)the

North-central rhyolite chain, the Southeastern rhyolite

Fig. 4. Compositional contrasts between Long Valley and Mammoth suites. Symbols in inset are for eruptive units discussed in text. LateBishop

Tuff is the set ofIg2ash-flow packages (ofWilson and Hildreth, 1997), representing magma that issued from the zoned reservoir on the final 2

days of the 6-day-long caldera-forming eruption. Early Bishop Tuff (EBT), representing the first three quarters of the 600 km 3 of magma

withdrawn in that eruption, is more homogeneous and barely distinguishable from high-silica rhyolite products of precaldera Glass Mountain

(GM) and of (largely Holocene) Mono Craters (MC), as grouped in the small shaded fields. (a) Total alkalies vs. SiO 2 (wt.%), showing

Mammoth Mountain, two sets of western dacites, and crystal-poor Inyo-fp magmas to be more alkalic than the Long Valley suite, which

includes Bishop Tuff, Early and Moat Rhyolites, and most crystal-rich Inyo-cp magmas. (b) Zr vs. Ba (ppm) for same samples as in top panel,

showing relative Zr enrichment of Mammoth suite. West Moat Rhyolites plot in two groups, the higher Zr+ Ba (lower SiO2) group representing

West Moat Coulee and Deer Mountain(Fig. 3).SE Moat Rhyolites also fall in two groups (Fig. 3),the crystal-rich lavas having f 75.5% SiO2and the crystal-poor ones f 76%. The three Inyo-other domes, which predate the 650-year-old eruption, are labelled: ND =North Deadman

Dome (46 ka);WB = Wilson Butte (1.3 ka); CD = tiny Cratered Dome(post-WB). Data from Hildreth and Wilson (in prep.), Cousens (1996),

Heumann (1999), Sampson and Cameron (1987),Bailey (1978; 2004),Metz and Mahood (1991),Kelleher and Cameron (1990).



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cluster, and the West Moat rhyolites. The caldera

moat is aphysiographicterm for the annular trough

in resurgent calderas (Smith and Bailey, 1968) that

separates the central structural uplift from the calderawall; typically the site of postcaldera sedimentation

and ring-fracture eruptions, the moat conceals and is

broader than the structural zone of caldera ring faults.

4.1. Rhyodacite Dome 7403

In the NE corner of the caldera floor, a small

rhyodacite lava dome stands alone at the foot of the

Glass Mountain scarp (Figs. 2, 3). Only 110 m high

and f 0.01 km3 in volume, this glassy, columnar

dome, subcircular in plan, is not a downfaulted

precaldera mass but an unequivocally postcaldera

eruptive unit.Bailey (1989)suggested that its slender

glassy columns reflect eruption into a Pleistocene

intracaldera lake. The dome is compositionally homo-

geneous (67.8% SiO2with 1550 ppm Ba and 820 ppm

Sr) but unique in being the only non-rhyolite post-

caldera eruptive unit from the Long Valley reservoir.

Rich in small euhedral plagioclase and hornblende

phenocrysts, it also contrasts with Glass Mountain,

Bishop Tuff, and early postcaldera rhyolites (Section

4.2) in having hornblende (instead of biotite or

pyroxene) as the mafic silicate phase. The lava iscompositionally somewhat like rare dacite pumice

ejected toward the end of the Bishop Tuff eruption

(Fig. 4), probably withdrawn from some depth be-

neath the rhyolite reservoir.

An attempt was made to determine its age by40Ar/39Ar dating its clean euhedral plagioclase. Al-

though it contains no obvious xenocrysts, excess Ar

was indicated by an erratic incremental-fusion spec-

trum. A few accordant steps in the middle of the Ar-

release spectrum suggest an early postcaldera age. An

attempt to date its tiny acicular hornblende euhedra isunderway.

4.2. Early Rhyolite (ER)

WhatBailey et al. (1976)termed theEarly Rhyolite

(Fig. 3) consists of f 100 km3 of fairly uniform,

phenocryst-poor rhyolite (74 75% SiO2) that erupted

during the 100,000-year interval following caldera

collapse. This enormous volume, thicker than 600

m, is as great as that of precaldera Glass Mountain

and an order of magnitude greater than the total of all

subsequent Long Valley rhyolites erupted in the last

half-million years. Released in scores of separate

eruptions from at least 13 exposed vents (Bailey,1989), the Early Rhyolite (ER) includes at least 14

exposed lava flows (and domes), several more inter-

sected by drilling, and a predominance of varied tuffs

(fallout and pyroclastic-flow deposits, nonwelded,

welded, and reworked) that make up about three

quarters of the ER assemblage. Eight lava flows (but

no tuffs) have been KAr dated (Mankinen et al.,

1986), ranging from 751F16 ka to 652F 14 ka.The

ER extends far beyond its outcrop area (Fig. 3), as

documented in numerous wells (Suemnicht and

Varga, 1988; Bailey, 1989). At least 622-m thick near

its center of outcrop, the ER assemblage is still >350

m thick where deeply buried in the SE moat and 230-

to 537-m thick in wells in the west moat.

Because no correlative layers of distal ash are

reported outside Long Valley, it seems likely that

individual eruptions of ER tephra, though numerous,

were subplinian and modest in volume. This might be

interpreted to mean that the residual rhyolite magma

had been relatively depleted in volatiles during the

caldera-forming eruption, but on the other hand, the

observation that three quarters of the ER is pyroclastic

and nearly aphyric indicates that the ER magma waswater-rich. Perhaps, the abundance of medium-scale

ER eruptions reflected relative ease of magma escape

through the downfaulted and broken roof plate, there-

by aborting by frequent eruptive release (and perhaps

also by passive degassing) any postcaldera recurrence

of severe gas overpressure.

Compositions of ER are similar in most respects to

the last-erupted part of the zoned Bishop Tuff, but for

a few elements (e.g., Zr and Ba) ER extends the range

of Bishop zoning(Fig. 4).Phenocryst contents of ER

are low, only 03%, compared to 1525% in thedirectly preceding, last-erupted part of the Bishop

Tuff. The dominating minerals in the Bishop Tuff,

sanidine and quartz, are absent in ER, and the sparse

crystals present are all new, euhedral, and unresorbed.

These include plagioclase, orthopyroxene, Fe Ti

oxides, and (in some units) rare biotite, as well as

traces of apatite, zircon, and pyrrhotite(Bailey, 1978,

1989). The contrast in crystal content between late

Bishop Tuff and (compositionally similar) postcol-

lapse ER might reflect (1) wholesale resorption of



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crystals in unerupted rhyolite magma during convec-

tive reorganization of the postcollapse reservoir, ow-

ing to heating by (and limited mixing with) hotter

dacitic or mafic magma drawn up from deeper levels;or (2) pressure-release melting of the crystals in

rhyolite magma that convectively rose several kilo-

meters to replace the topmost zone of the reorganized

chamber; or (3) resorption of phenocrysts in such

magma (previously deeper and water-undersaturated)

drawn to the top of the partially evacuated chamber

and subsequently saturated with water, owing to

bubble ascent and concentration near the roof of

aqueous gas exsolved from still-deeper (untapped)

parts of the reservoir during the climactic depressur-

ization; or (4) similar resorption caused by CO2exsolution (thereby raising H2O activity) during as-

cent of gas-saturated but formerly CO2-richer rhyolite

magma from deeper in the reservoir; or (5) concen-

tration at the top of the chamber (in response to such

depressurization and reorganization) of interstitial

melt expelled from a great reservoir of crystal mush

that had underlain the zoned Bishop Tuff magma that

erupted. The mush model is discussed in Section 7,

below. Similarities in composition (Fig. 4) and in

temperature(Bailey, 1978; Hildreth, 1979; Heumann,

1999) between ER and late Bishop Tuff suggest that

mixing with hotter deeper magma was limited, whilethe relatively elevated Ba content of ER (Fig. 4)

suggests contributions either from dacitic magma or

from resorption of sanidine-rich cumulates. Basaltic

enclaves (49% SiO2) that reflect mafic recharge have

been found in only one lava (680 ka) among the many

ER-eruptive units (Bailey, 2004).

4.3. In what sense was the structural uplift

resurgent?

Intracaldera resurgence was defined bySmith andBailey (1968) as structural uplift of the caldera floor

by renewed buoyancy or intrusion of the viscous

magma remaining in the postcollapse reservoir; but

the term has sometimes been inappropriately conflated

with postcalderaeruptiveactivity that may or may not

accompany such uplift. Bailey et al. (1976) showed

that structural uplift at Long Valley was largely

contemporaneous with the 100-kyr interval of ER

eruptions and was probably largely over by f 500

ka. Some ER vents lie along or close to faults

associated with the uplift, but the relative timing of

individual eruptive units and the offset on long-active

faults is seldom clear. The roughly circular area of

uplift (Fig. 2) is f 10 km across and dips radiallyoutward at 1025j(Bailey, 1989). Lookout Mountain

(677692ka), an ER cone in the NW moat, is outside

the uplift (Figs. 2, 3), as are thick sections of ER

concealed beneath other sectors of the moat.

Thehigh point of the uplift is Gilbert Dome (2626

m asl; Fig. 3), and if ER thickness is similar to that

(622 m) in the Long Valley Exploratory Well (LVEW)

f 2 km south, then the top of the subjacent Bishop

Tuff would be f 2000 m asl, probably its maximum

intracaldera elevation. This is 261 m higher than the

top of the Bishop Tuff in the LVEW (at a site down-

faulted within the medial graben; Fig. 3), 4 6 9 m

higher than in well 44 16 in the west moat, and

575 m higher than in well 6629 in the SE moat

(Suemnicht and Varga, 1988; Bailey, 1989; McCon-

nell et al., 1995). It seems likely that the fluidized

primarysurfaceof the Bishop Tuff that ponded inside

the caldera was virtually horizontal at the close of its

eruption. Therefore, even though part of the excess

elevation of the resurgent dome owes to the construc-

tional pile of proximal ER, and part to differential

compaction of the Bishop Tuff (which is much thicker

in the low eastern third of the caldera; Hill, 1976;Bailey, 1989), doming of the top surface of the Bishop

Tuff clearly demonstrates central uplift of at least 400

m. Most of the uplift appears to have been during ER

time, but what fraction may have continued episodi-

cally is not clear. The 400-m total uplift in roughly

100 kyr can be compared with f 1 m of renewed

uplift in the last 25 years (Savage and Clark, 1980;

Langbein, 2003), 10 times greater than the earlier rate.

Drillcore from the LVEW (virtually central to the

uplift) revealed in the 1.2-km-thick Bishop Tuff some

10 phenocryst-poor intrusions, apparently sill-like andnot present in wells drilled peripheral to the uplift

(McConnell et al., 1995). Compositionally, the sills

are Ba-rich rhyolite much like the ER, and with a

cumulative thickness off 330 m, they could account

for most of the resurgent uplift. For a 10-km-wide

domical uplift of 400 m, the apparent volume of

inflation is about 10 km3, merely 10% of the volume

of ER erupted. The conventional model that a resur-

gent residual magma chamber buoyantly upwarps the

cauldron block by reinflation or upward stoping is,



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therefore, for Long Valley, no more compelling than a

model of central uplift by injection of shallow sills or

laccoliths into the thick intracaldera fill(McConnell et

al., 1995).The fault system on the resurgent dome is domi-

nated by NNW trends essentially parallel to those of

rangefront faults north and south of the caldera (Figs.

2, 3), butalsoparallel to the strike of steeply dipping

structures and bedding in the metamorphic basement

rocks (Rinehart and Ross, 1964). Radial and dome-

concentric faults are inconspicuous. Thus, the struc-

ture of the uplift appears to have been influenced more

by (1) regional precaldera structures and (2) suscep-

tibility of the shallow, subhorizontally layered Bishop

Tuff to sill injection rather than by chamber-wide

buoyancy. Location of the uplift nonetheless surely

reflects the main locus of the reorganized postcaldera

magma reservoir, which so voluminously supplied the

ER. For the last half-million years, however, despite

paths provided by the complex fault system(Fig. 3),

there have been no further eruptions on the resurgent

dome.

4.4. North-central rhyolite chain

The earliest of three clusters of post-resurgence

rhyolites thatBailey (1989) termed Moat Rhyolite isa NW-trending chain of five units (orange in Fig. 3)

crossing the NE sector of the resurgent dome, therefore

not really in the caldera moat at all nor aligned along

the ring-fault zone. In contrast to the voluminous ER,

all are phenocryst-rich and of small eruptive volume,

totaling f 1 km3. About 100 jC lower in FeTi-oxide

temperature than the nearly aphyric ER, the north-

central rhyolites are rich in quartz, plagioclase, sani-

dine, hornblende, and biotite. SiO2contents (7475%)

are similar to ER, but K2O (4.7%) and Ba (680715

ppm) contents are significantly lower than ER(Fig. 4),probably as a result of sanidine fractionation.

The four extrusive units yielded sanidine K Ar

ages of 527F 12, 523F 11, 505F 15, and 481F10

ka(Mankinen et al., 1986),thus potentially spanning

an eruptive interval 46F 22 kyr long. The fifth and

SE-most member of the chain(Fig. 3)is a granophyric

intrusion in well CP-1 (Suemnicht and Varga, 1988),

similar to the lavas in composition (74.2% SiO2) and

mineralogy. Mafic enclaves (53.5% SiO2; Bailey,

2004) occur in lava and agglutinate of the NW-most

vent of the chain, but none have been found in any

younger Long Valley rhyolite.

4.5. Southeastern rhyolite cluster

After an apparent hiatus off 120 kyr, another set

of rhyolites erupted over an interval as long as f 75

kyr, from a cluster of five vents (green inFig. 3) in the

calderas low SE moat. Two of these vents arguably

extend the trend of the north-central chain just dis-

cussed, and two clearly lie along the ring-fault zone

(Fig. 3), the others inboard. The extensive (12 km2)

Hot Creek flow and the two small eastern lavas are

quartz-free and crystal-poor (13% feldspars, biotite,

cpx, FeTi oxides), whereas the central pair (striped

green in Fig. 3) of the cluster are phenocryst-rich

hornblende-biotite rhyolites like the north-central

chain. All five, however, have f 76% SiO2 and

500700 ppm Ba (Fig. 4). Altogether, the five add

up to only f 1.5 km3, the Hot Creek flow being most

of it. All have been dated (Mankinen et al., 1986;

Heumann, 1999): sanidine yields 362F 8 ka for the

northernmost lava and 333F 10 ka for the south-

central lava, both crystal-rich. For the three pheno-

cryst-poor units, sanidine gave 329 F 23 ka for the NE

lava and 329F 3 ka for the SE lava, and for the Hot

Creek flow obsidian gave 288F 31 ka (possibly tooyoung owing to Ar loss from glass?). Whatever

process promoted reversion to crystal-poor rhyolite

at about 330 ka, it was unique in the post-ER evolution

of the Long Valley magma reservoir, because all other

Long Valley rhyolites (527 to 100 ka) are rich in

phenocrysts. Thermal rejuvenation by basalt injection

is an unlikely explanation for these low-temperature

rhyolites because the crystal-poor units are marked by

small euhedral phenocrysts and lack xenocrysts or

partly resorbed relicts of an earlier generation. A more

likely process, high-silica melt extraction from crystal-rich felsic mush is discussed in Section 7.

4.6. West moat rhyolites

After another hiatus of about 150 kyr, a third

cluster of moat rhyolites (blue inFig. 3)erupted west

of the resurgent domefour small lava domes and the

extensive West Moat Coulee (8.5 km2; as thick as 574

m;Benoit, 1984). The coulee represents f 4 km3 of

rhyolite lava but the four domes add up to only f 1



11/30

km3 more. Four of the rhyolite vents are along the

ring-fault zone, while Deer Mountain lies f 2.5 km

outboard of it (Fig. 3). All five have been dated

(Mankinen et al., 1986; Heumann, 1999; Ring,2000). Oldest is the West Moat Coulee at 161 F 2

ka, while the Dry Creek dome, the two Mammoth

Knolls, and Deer Mountain yield overlapping ages in

the range 11597 ka. All are phenocryst-rich (20

30% quartz + sanidine + plagioclase + biotite + horn-

blende + FeTi oxides), low-temperature rhyolites, but

chemically they are of two kinds (Fig. 4): Deer

Mountain and the coulee have high Ba (700 860

ppm) and only 72 73% SiO2, whereas Dry Creek

dome and the Mammoth Knolls are more evolved,

with 7677% SiO2 and lower Ba (110200 ppm).

4.7. The postcaldera Long Valley magma chamber

Although Glass Mountain rhyolites totaled f 100

km3, Bishop Tuff rhyolite f 600 km3, and Early

Rhyolites f 100 km3, the 15 eruptive units of post-

650-ka Long Valley rhyolite add up to only 7 or

8 km3less thanthat released at Novarupta (Alaska)

in 1 day in 1912 (Fierstein and Hildreth, 1992). The

scarcity of tuff accompanying the three sets of moat

rhyolite lavas suggests that any lost fallout volume is

small. Despite the near-verticalstructural grain of thestratified metamorphic rocks (Rinehart and Ross,

1964) that compose much of the foundered cauldron

block, and despite the complex system of faults trans-

ecting the resurgent dome(Fig. 3),there has not been

a single eruptive leak on the uplift itself in the last

half-million years.

Additional evidence suggesting that the Long Val-

ley magma chamber may have largely crystallized

includes:

(1) Volumetric eruption rate of postcaldera rhyolite

wasf

1 km

3

/kyr in ER time but has beenf

0.01km3/kyr since 650 ka, a hundredfold decline.

(2) Most of the 15 moat rhyolites (and all those

younger than f 300 ka) were crystal-rich, low-tem-

perature magmas, suggesting that active separation of

melt from crystal mush has ceased.

(3) Although teleseismic arrival-time tomography

(Dawson et al., 1990; Wieland et al., 1995)appears to

have identified diffuse low-velocity anomalies in the

mid-crust, high-resolution tomography based on local

earthquakes (Kissling, 1988; Romero et al., 1993)

found no distinct low-velocity bodies in the upper

crust beneath Long Valley caldera.

(4) Present-day geothermal fluids beneath the south

and southeast moat are supplied by eastward under-flow from western areas of younger magmatism

outside the structural caldera, not from beneath the

immediately adjacent resurgent dome (Sorey et al.,

1991; Romero et al., 1993; Pribnow et al., 2003).

(5) The 3-km-deep LVEW, virtually centered on

the resurgent dome, is isothermal at 100 jC over its

bottom 1000 m, requiring an astonishingly steep

thermal gradient if there were residual 700 jC rhyo-

litic magma in the upper crust beneath it.

(6) Although self-sealing might isolate hydrother-

mal convection cells promoting cooling at still deeper

levels,Sorey et al. (1991)pointed out that deep wells

elsewhere on the resurgent dome have kilometer-scale

segments with near-linear thermal gradients near 40

jC/km, suggesting only modest conductive heat

flowinconsistent with survival of a subjacent up-

per-crustal magma chamber.

(7) Fluid-inclusion and oxygen-isotope studies of

hydrothermally altered material from deep in LVEW

(McConnell et al., 1997; Fischer et al., 2003) identi-

fied a high-temperature (300350 jC) paleo-hydro-

thermal system that appears to have died out soon

after 300 ka (Sorey et al., 1991)perhaps not coin-cidentally the age of the last crystal-poor intracaldera

rhyolites.

(8) If basaltic resupply to the roots of the subcal-

dera rhyolitic reservoir (Lachenbruch et al., 1976) had

prolonged rhyolite crystallization to the present, it

would seem anomalous that such postcaldera mafic

magma has erupted nowhere immediately north or

south of the structural caldera. Postcaldera eruptions

of mafic and intermediate magma (most or all younger

than 200 ka) are limited to a NS belt west of the

structural caldera(Figs. 2, 5)where they are likely tohave influenced the thermal state of the Long Valley

rhyolite reservoir only marginally.

In summary, the compelling inference is that the

formerly vigorous Long Valley magma chamber is

moribund. If the recent (19792003) 80-cm uplift of

the resurgent dome(Hill et al., 2002, 2003),attributed

to an inflation source at a depth of 67 km(Langbein,

2003), was caused by intrusion of a mafic dike, this

would provide further evidence that the subcaldera

rhyolitic reservoir, now penetrable, has crystallized.



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5. Western postcaldera magmatism

Because drilling(Suemnicht and Varga, 1988) has

shown the ring-fault zone to lie as far as 5 km insidethe western wall of the topographic depression(Figs.

2, 3, 5), all of the following eruptive groups vented

outside the structural caldera: (1) the Mammoth

Mountain dome complex; (2) >35 mafic scoria cones

and lavas; (3) three crystal-poor dacite lavas periph-

eral to Mammoth Mountain; and (4) a chain of four

hybrid dacite lavasthat crosses the northwest wall of

the caldera(Fig. 5). Products of the 60-odd vents are

magmatically unrelated to the residual Long Valley

reservoir (Fig. 4), and most or all are younger than

about 200 ka. Of the many units erupted west of the

ring-fault zone, only Deer Mountain (101F 8 ka) and

the coarsely porphyritic rhyolite that mingled syner-

uptively with the 650-year-old Deadman Creek and

Glass Creek (Inyo) domes (Sampson and Cameron,

1987) appear to represent residual or rejuvenated

Long Valley magma.

5.1. Mammoth Mountain dome complex

Mammoth Mountain is a silicic dome cluster at the

focus of a peripheral array of roughly contemporane-

ous mafic vents(Fig. 5),in the same sense as Adams,Hood, Mazama, Newberry, Medicine Lake, Shasta, or

the Lassen domefield in the Cascades or the San

Francisco Peaks in Arizona. The eruptive volume of

Mammoth Mountain is relatively small (4F 1 km3),

though with 750 m of relief the edifice is imposing

because draped over the high basement rim of the

Long Valley depression. Although only f 13 vents

are exposed (Fig. 5), the edifice consists of at least

2530 overlapping domes and flows of trachydacite

and alkalic rhyodacite (6571% SiO2)(Bailey, 1989,

2004). They define a compositional continuum (Fig.

4), but products with < 70% SiO2 are phenocryst-

rich (hornblende + biotite + plagioclase + FeTi oxi-

desFpyroxeneFNa-sanidine) and those with z

70% SiO2 are generally crystal-poor (with the samesuite but generally including Na-sanidine and

quartz). The Mammoth Mountain compositional

array (Fig. 4) is distinct from those of Long Valley

a nd t he M on o C ra te rs c ha in (Kelleher and

Cameron, 1990; Ring, 2000; Bailey, 2004).

Radiometric ages determined for Mammoth Moun-

tain include eight units K Ar dated in the 1970s

(Mankinen et al., 1986) and nine units dated by40Ar/39Ar incremental heating (Ring, 2000). Many

of the KAr ages were for biotite separates shown

by Ring (2000) to contain excess Ar, thus giving

systematically older ages than coexisting feldspar. The

commonly cited 25050 ka range of eruptive activity

for Mammoth Mountain is therefore too long. Reli-

ably precise ages for lavas (and a pumice fall) from

most sectors of the edifice, and from top to toe, range

from 111F 2 to 57F 2 ka. The Earthquake Dome (2

km NE of Mammoth Mountain; Fig. 5) is of similar

age (86F 2 ka) and composition (crystal-rich trachy-

dacite; 66.4% SiO2), thus magmatically related to the

main edifice as a flank dome. Magmatic eruptive units

older than 111 ka might well be buried within the

edifice, but there are none younger than 57 F 2 ka.The 40Ar/39Ar ages suggest that more than half the

bulk of Mammoth Mountain erupted in the interval

6757 ka(Ring, 2000).

Not only is Mammoth Mountain far outside the

Long Valley ring-fault zone, neither is it magmatically

related to the Mono-Inyo chain, as sometimes

asserted. (1) Mammoth Mountain is wholly older than

the Mono-Inyo chain, the southern 13 km of which

(Fig. 5) propagated episodically toward Long Valley

only during the Holocene(Bursik and Sieh, 1989).(2)

The trachydacites and alkalic rhyodacites of Mam-

Fig. 5. Mammoth Mountain and its mafic periphery. Topographic margin of caldera basin and ring-fault zone (RFZ) as in Figs. 2 and 3.Vent

symbols identified in inset. Additional silicic vents are concealed within the Mammoth Mountain edifice. Nearly all vents in the array depicted

are monogenetic and erupted after 200 ka, but farther east there are no vents younger than f 300 ka, neither inside nor outside the caldera. The

400-ft contours show that 11,053-ft Mammoth Mountain is a modest edifice (4F 1 km3) built atop a high basement ridge. Placename

abbreviations as inFig. 3; in addition, CC = Crystal Crag; DP = Devils Postpile; HL = Horseshoe Lake; MP= Mammoth Pass; MR = Mammoth

Rock; MS = Minaret Summit; ND = North Deadman Dome; PB = Pumice Butte; RC = Red Cones; RM = Reds Meadow. Three largest domes of

rhyolitic Inyo chain are Deadman Creek (DC), Glass Creek (GC), and Obsidian Dome (OD); two slightly older mini-domes adjacent to GC are

Cratered Dome (CD) and a southerly one unnamed. Vent alignments marked by dashed lines are NW wall hybrid dacites, mafic units along Fern

Lake fault zone (FLFZ), and Red Cones. Drillholes mentioned in text include Long Valley Exploratory Well (LVEW) and Shady Rest (SR); four

others are named as labelled on map.



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15/30

layer of cinders midway through the pile suggests

proximity to a buried vent.

A third well 3 km farther SE (Figs. 3, 5; PLV-1;

Benoit, 1984) encountered no mafic lavas at all in

penetrating 687 m of 161-ka Moat Rhyolite lava and

tuff that rests directly on ER, indicating that the mafic

eruptions in (at least that part of) the west moat took

place afterf 160 ka. This inference is supported by

data for a fourth well, PLV-2(4 km N ofPLV-1;Figs.

3, 5;Benoit, 1984), where 169 m of mafic lavas (f 8

flows with soils and ash intercalated) overlie an 86-m

package of Moat Rhyolite lava and tuff (not exposed

at the surface), which again rests directly upon ER.

Finally, and remarkably, a fifth well (Shady Rest;Figs.

3, 5; Wollenberg et al., 1987) f 1.5 km east of

Mammoth Knolls, at the foot of the resurgent dome,

penetrated till, Moat Rhyolite, and Early Rhyolite,

finding no mafic lavas at all. Farther east, none of the

Fig. 6. Outline map of six successive magmatic foci in the Long Valley region. #1 encloses area of precaldera dacite (PCD) vents (3.5 2.5 Ma).

#2 encloses area of >60 Glass Mountain (GM) vents for high-silica rhyolite (2.20.79 Ma). #3 is bounded by ring-fault zone of Long Valley

caldera (LVC), which collapsed on eruption of the Bishop Tuff (0.76 Ma). #4 encloses the trachydacite-rhyodacite Mammoth Mounta in (MM)

center (110 57 ka) and its peripheral array off 35 mafic vents (1608 ka). #5 encloses area above teleseismically anomalous domain(Achauer

et al., 1986)in mid-crust beneath central core of (f 500.65 ka) Mono Craters (MC); arrows depict late Holocene propagation of dike-fed chains

of rhyolite domes north and south of the core. #6 is the youngest focus (14 to 0.25 ka) at Mono Lake (not dealt with in detail in this paper),

including vents at Black Point and both islands (Lajoie, 1968; Stine, 1987; Bailey, 1989; Kelleher and Cameron, 1990).Faults as in Fig. 2.



16/30

many wells on the resurgent dome (or south and east

of it) have encountered mafic lavas. Vents for such

lavas are limited to the west moat and areas still

farther west and southwest(Fig. 5).Volumes of mafic lavas erupted are hard to recon-

struct owing to burial in the west moat and to glacial

erosion elsewhere, but most units are small. The most

voluminous mafic units appear to be the andesites of

Mammoth Pass and Devils Postpile and the pair of

lava aprons just west of Lookout Mountain (Bailey,

1989), all four of which had volumes in the range

0.51 km3 (assuming average thicknesses in the

range 50100 m). On the floor of the San Joaquin

canyon, the severely glaciated basalt of The But-

tresses, now a 1.5-km2 remnant as thick as 120 m,

might once have been three times as big, thus likewise

as voluminous as 0.5 km3. The Pumice Butte vent

cluster south of Mammoth Mountain adds an addi-

tional f 0.2 km3. As f 45 km2 of the west moat

appears to be underlain by mafic lavas, where drilling

suggests an average thickness of 200F 50 m, their

total volume there might be roughly 9 km3. If the

thickness of the mafic lavas (f 16 km2) that flowed

down the south moat averages f 50 m, they add < 1

km3 to the total mafic volume. In summary, then,

although not well constrained, the total volume of

mafic magma erupted from the 35-vent array sur-rounding Mammoth Mountain is probably >10 km3

but is unlikely to exceed 15 km3.

Ages of eruption of the many postcaldera mafic

units remain inadequately known and are the object

of ongoing work. The only patently postglacial unit

among them is the basalt of Red Cones, f 8.5 ka

(M. Bursik, unpubl. data). KAr ages for nine units

(plus several duplicates) were published by Manki-

nen et al. (1986), and 40Ar/39Ar ages for nine more

were determined byRing (2000).Sixteen of the ages

fall between 160F

2 and 65F

2 ka. The nominallyoldest determination is a whole-rock K Ar age of

228F 82 ka for an andesite vent f 500 m SE of

Mammoth Knolls (Fig. 5), but this low-precision

result needs to be verified. The youngest is 31F 2

ka for the basaltic apron south of Crestview near the

calderas NW wall. The 9-km-long basalt tongue in

the north moat gave a KAr age of 108 F 12 ka, and

the stack of 3 south-moat lava flows near Casa

Diablo yield 40Ar/39Ar ages in the range 160 98

ka(Ring, 2000), apparently surmounting the contam-

ination problems that had plagued previous attempts

to date them by KAr(Mankinen et al., 1986). Four

mafic lavas from the Inyo-4 drillhole were dated by40

Ar/39

Ar incremental heating (Vogel et al., 1994).Two near the top of the 319-m stack of 26 flows

gave reasonable plateau ages of 161F14 and

151F17 ka, consistent with the likelihood that the

stack banks against the nearby West Moat rhyolite

coulee (161F 2 ka; Ring, 2000), beneath which

drillhole PLV-1 showed mafic lavas not to be present.

The two samples near the bottom of Inyo-4, however,

gave low yields of radiogenic Ar and highly dis-

turbed spectra (owing to excess Ar or recoil or both)

that were interpreted to yield a combined age

(415F 53 ka) that, pending verification, should not

be accepted.

Whether all 35 mafic eruptive units are younger

than 160 ka remains to be determined but seems

possible. Some have considered the undated basalt of

The Buttresses in the San Joaquin canyon to be

much older (e.g., Cousens, 1996), but Bailey

(2004) pointed out that its dike-fed vent complex

is on the present-day canyon floor, andBailey (1989)

showed that it directly underlies the dacite of Rain-

bow Falls, which is now well dated at 97F 1 ka

(Ring, 2000).

The youngest mafic products actually erupted inthe Long Valley area are andesitic enclaves in the 650-

year-old Inyo Domes, found only in the coarsely

porphyritic mixing member present in the Glass Creek

and Deadman Creek domes (Varga et al., 1990). It

may be, however, that dike ascent such as fed the

mafic vent array for the last 160 ka is likewise

responsible (though as yet unerupted) for (1) ongoing

upper-crustal seismicity and CO2 discharge beneath

Mammoth Mountain(Hill, 1996);(2) numerous long-

period volcanic earthquakes at focal depths of 10 25

km in a cluster that extends 10 km WSW frombeneath Mammoth Mountain to the Devils Postpile

area (Pitt et al., 2002), spatially coinciding with the

vent array in that sector; and (3) the ESE-striking

array of late Holocene phreatic craters at the north toe

of Mammoth Mountain(Bailey, 1989).

Compositionally, the peripheral mafic array ranges

continuously from trachybasalt to trachyandesite (47

to 58.5% SiO2), plus 3 dacites to be discussed in

Section 5.3. Nearly all are mildly to transitionally

alkalic, except the Holocene basalt of Red Cones,



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which is subalkalic (Vogel et al., 1994; Cousens,

1996; Bailey, 2004). The general alkalinity of the

mafic magmatic rocks is consistent with such material

having provided a parental contribution to the con-temporaneous trachydacite to alkali-rhyodacite suite

of Mammoth Mountain. In contrast, silicic rocks of

the generally older Long Valley suite (Glass Mountain

through Moat Rhyolites) and the younger Mono

Craters suite are subalkalic(Fig. 4).

The only fairly primitive eruptive units in the mafic

array are the basalts of The Buttresses and of Horse-

shoe Lake, each in the range 47 49% SiO2 with

f 10% MgO and 160210 ppm Ni, but these are

very enriched in Ba, Sr, and LREE, consistent with

their alkalinity(Cousens, 1996).The subalkalic basalt

of Red Cones, with 50% SiO2, 8% MgO, and 120

ppm Ni, is the only other relatively primitive Quater-

nary basalt so far recognizedin the array. The 30-odd

additional samples analyzed (Cousens, 1996; Bailey,

2004), which representf 20 separate eruptive units,

contain only 2.5 7% MgO and < 100 ppm Ni, as do

lava flows in the Inyo-4 drillhole(Vogel et al., 1994).

Like many intracontinental mafic lavas, these have

high 87Sr/86Sr (0.70520.7067) and low 143Nd/144Nd

(0.5124 0.5128) (n =35; Cousens, 1996). There is

extensive xenocrystic and chemical evidence for

crustal assimilation by many of the mafic units(Man-kinen et al., 1986; Vogel et al., 1994; Cousens, 1996;

Ring, 2000), but even the more primitive basalts

appear also to contain a contribution from enriched

mantle lithosphere (Nielsen et al., 1991; Cousens,

1996). The relative contributions of partial melts of

mafic to silicic crustal rocks and of enriched upper

mantle needs clarification.

5.3. Crystal-poor dacites peripheral to Mammoth

Mountain

Three widely separated eruptive units of pheno-

cryst-poor dacite lava lie near the foot of Mammoth

Mountain (Fig. 5). (1) The dacite of Rainbow Falls

(67% SiO2) is a 6-km-long glaciated coulee on the

floor of the San Joaquin canyon that erupted f 2 km

SW of the toe of Mammoth Mountain. (2) The dacite

of upper Dry Creek (67.8% SiO2), also glaciated and

partly concealed by basalt and surficial deposits,

crops out from 1 to 3 km north of the base of

Mammoth Mountain. Both have f 5% phenocrysts

of plagioclase>opxf cpx>FeTi oxides. They yield

ages of 97F 1 and 103F 9 ka, respectively (Ring,

2000; Mankinen et al., 1986). (3) The undated dacite

of McCloud Lake crops out f 1 km south of thebase of Mammoth Mountain as glaciated lava rem-

nants resting on granitic basement along the up-

thrown side of a N-striking fault (Bailey, 1989).

With only 1 2% plagioclase>hornblendeF sparse

cpx, it is even crystal-poorer than the other two.

Their relatively alkalic compositions suggest that the

three crystal-poor dacites are related to the adjacent

Mammoth Mountain system, perhaps as interstitial

melts that separated from trachyandesitic crystal

mush. In contrast to the next set of dacites discussed,

they lack obvious xenocrysts or detectable evidence

for mixed parentage.

5.4. Northwest wall hybrid dacite chain

Crossing the NW wall of the topographic caldera

basin is a NW-aligned chain (Fig. 5) of four crystal-

rich silicic lavas (61 66% SiO2) called olivine-

bearing quartz latite by Rinehart and Ross (1964),

quartz latite by Bailey (1989), and hybrid

dacites by Bailey (2004). The chain includes two

small domes (each < 0.005 km3) and twomodest lava

flows (each f 0.04 km3

). Ring (2000) and Bailey(2004) provide petrographic evidence for complex

magma mixing and phenocryst disequilibrium, in-

volving (1) trachydacite similar to that of Mammoth

Mountain, (2) rhyolite containing quartz and K-rich

sanidine similar to Long Valley rhyolites, and (3)

mafic enclaves and derivative xenocrysts of olivine,

cpx, and calcic plagioclase. 40Ar/39Ar ages deter-

mined by Ring (2000) are 40F 1 and 39F 1 ka for

the NW pair and 30F 1 and 27F 1 ka for the SE pair

of lavas comprising the chain, all four ages being for

K-rich sanidine. Sorey et al. (1991) reported that thepresent interval of hot-spring discharge in Long Val-

leys south moat began f 40 ka and is thought to be

supplied by underflow in shallow aquifers from an

unidentified deeper heat source in the west moat. The

timing may be a coincidence but, if the 40-ka start-up

time is accurate, intrusions associated with the hybrid

dacite chain are more plausible a heat source than

Mammoth Mountain (dormant since 57 ka) or the

650-year-old Inyo dike (only 7-m thick; Eichelberger

et al., 1985).



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5.5. Vent alignments

Much attention has been paid the 8-km-long NS

alignment of Holocene Inyo rhyolite domes andcraters (Miller, 1985; Section 6 below). Other vent

alignments (Figs. 2, 5) include: (1) the NW-wall

chain of hybrid dacites just discussed, which strikes

N45jW; (2) the principal vent array of Mammoth

Mountain, which strikes N60jW; (3) the array of

late Holocene phreatic craters that strikes N70jW

across the north toe of Mammoth Mountain (Bailey,

1989); (4) a chain of five phenocryst-poor mafic

vents that strikes N45jW from the west moat along

the Fern Lake fault zone (Fig. 5; Bailey, 1989); (5)

clusters of phreatic craters aligned roughly NW

along a fault zone just west of Deer Mountain

(Mastin, 1991); (6) the chain of 500-ka rhyolites

that strikes N45jW across the northeast slope of the

resurgent dome (Figs. 2, 3); and (7) fault-influenced

alignments of ER vents that trend N2040jW across

the resurgent dome (Fig. 3). Predominance of north-

westerly alignments is probably related to the NNW

trend of the rangefront fault system and to the

roughly parallel strike of near-vertical bedding and

structures in the metamorphic basement (Rinehart

and Ross, 1964).

Many vents for the southeast- and west-moatrhyolite groups (Figs. 2, 3) appear to be arranged,

however, along or adjacent to the buried ring-fault

zone, as likewise are early postcaldera Dome 7403

and ER Lookout Mountain. As Deer Mountain dome,

however, which is the only moat rhyolite clearly

outsidethe ring-fault zone, lies 3 5 km NW of coeval

100-ka moat rhyolites, its intrusive feeder may also

have been influenced by the NW-trending basement

structures. On the other hand, a line connecting the

Red Cones vent pair (the only Holocene basaltic

eruptive unit) strikes N25j

E (Fig. 5), presumablythe orientation of a mutual feeder dike. This is roughly

parallel to the NNE trend of a possible dike inferred to

have been emplaced beneath nearby Mammoth

Mountain during an extended earthquake swarm in

1989(Hill et al., 1990).

The NS alignment of the Inyo chain (N7jW for

the three 650-year-old domes) is apparently unique.

Its feeder dike may have been controlled, at least in

the shallow crust, by the Hartley Springs fault system,

which swings to a nearly southerly trend as it transects

the caldera wall (Fig. 2; Bailey, 1989; Bursik et al.,

2003).

The major concentrations of magmatism in the

Long Valley area, however, have been expressed bynon-linear, roughly equant domains marked by clus-

ters of scattered eruptive vents (Fig. 6): (1) precaldera

dacite and andesite vents concentrated just northwest

of the later site of the caldera, but nowhere else in the

region (Bailey, 1989). (2) the Glass Mountain con-

centration of >60 non-aligned high-silica rhyolite

vents 3 8 km outside the ring-fault zone; (3) the

Bishop Tuff-Early Rhyolite-Moat Rhyolite sequence

that issued from a reservoir ovoid in plan view

beneath the central part of the caldera; and (4)

trachydacitic Mammoth Mountain and its surrounding

array of >40 mafic and dacitic vents. Relative to these

major long-lived domains, each the surface expression

of a large volume of mantle and deep-crustal partial

melting, the local alignments are shallow second-

order features. Only in the continuous linear array of

>40 rhyolitic vents composing the young Mono-Inyo

chain has shallow magma ascent been utterly domi-

nated by upper-crustal tectonics.

6. Mono-Inyo chain

Extending 25 km north from the NW corner of

Long Valley, the Mono-Inyo chain (Fig. 2) is a

sickle-shaped single-file alignment of rhyolite vents,

mostly of Holocene age. The Mono chain, forming

the arcuate segment of the sickle, consists off 28

domes and coulees, several associated explosion

craters and ejecta rings, and an extensive apron of

pumiceous fall, flow, and reworked deposits (Bailey,

1989; Bursik and Sieh, 1989). As conventionally (but

arbitrarily) designated, the Inyo chain refers to the

rectilinear handle of the sickle, the segment repre-sented by an additional seven rhyolite domes (and

several phreatic craters) that strikes south from where

the arcuate segment impinges on the rangefront fault

system (Fig. 2). Continuity of the chain of virtually

contiguous Holocene rhyolite vents demands that the

Mono-Inyo chain represent in some sense a coherent

magmatic system. Farther north, in and adjacent to

Mono Lake, however, a cluster of young basalt-

dacite-rhyodacite vents (Fig. 6; not dealt with in this

paper), is compositionally different (Lajoie, 1968;



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Stine, 1987; Bailey, 1989; Kelleher and Cameron,

1990) and is best regarded as a magmatic subsystem

independent of the rhyolitic reservoir that feeds the

Mono-Inyo chain.

6.1. Mono Craters chain

For the Mono chain, all lavas but one are high-

silica rhyolite (75.477% SiO2). Major element (and

most trace-element) contents are quite similar to

those of Glass Mountain and the early Bishop Tuff

(Fig. 4), the principal compositional feature distin-

guishing the Mono domes being somewhat higher

FeO* content (1.0 1.3 wt.%). Half of the Mono

domes have 03% phenocrysts and the rest 3 8%

(Wood, 1983; Kelleher and Cameron, 1990). The

exception is an undated crystal-rich rhyodacite

(68% SiO2), substantially older than any exposed

Mono-Inyo rhyolites. All but four off 27 rhyolite

lavas are of Holocene age; three are f 13 ka and one

f 20 ka (Wood, 1983; Bursik and Sieh, 1989).

Single-crystal 40Ar/39Ar ages for sanidine from pre-

Holocene rhyolitic ash layers intercalated with lacus-

trine silts of Mono Lake, however, suggest that as

many as 15 explosive eruptions took place before 20

ka, with a few as old as 5055 ka(Chen et al., 1996;

Kent et al., 2002). This appears to require that one orseveral older explosive vents be concealed by the

Mono domes currently exposed (Bursik and Sieh,

1989).

It was speculated that the arcuate trend of the Mono

chain is controlled by a Mesozoic structure (Kistler,

1966; Bailey, 1989), but the exposure is inadequate to

verify the implausibility of the suggestion. More

attractive is the proposal by Bursik and Sieh (1989)

that the arcuate alignment represents the extensional

margin of a pull-apart basin between NNW-trending

oblique-slip faults having a dextral component.Straightening of the Holocene chain where the arcuate

segment meets the rangefront fault zone (Fig. 2) is

consistent with fault control of shallow dike propaga-

tion farther southward; the east-dipping Hartley

Springs fault zone, which remains active (Bursik et

al., 2003), has dropped the Bishop Tuff f 135 m

down to the east, thus yielding a 760-kyr average

vertical displacement off 0.18 m/kyr.

Estimates of the magma volume erupted from the

Mono chain range from f 5 km3 (Wood, 1983)to 8.5

km3 (Bursik and Sieh, 1989), the amount of lost and

concealed pyroclastic deposits being the main uncer-

tainties. Lava volume is f 4 km3, the largest units

being the North and South Coulees at f 0.5 km3

each. Wood (1983) pointed out a 4-fold increase in

volumetric eruption rate atf 3 ka, an earlier Holo-

cene rate off 0.2 km3/kyr jumping to f 0.8 km3/kyr

for the last three millennia and coinciding with a

switch from crystal-poor to virtually aphyric rhyolite.

The four southernmost Mono domes are younger than

5 ka, and South Coulee (Fig. 2) is part of the 1.3-ka

South Mono eruptive episode (Bursik and Sieh,

1989). The youngest Mono eruptions apparently is-

sued from a 6-km-long dike that released thecomplex

North Mono episode 660F 20 years ago (Sieh and

Bursik, 1986), which included f 0.22 km3 of pyro-

clastic fall and flow deposits and five separate lavas

(0.44 km3), including North Coulee and Panum Cra-

ter, all at the north end of the chain(Fig. 2).There has

thus been a tendency for the Mono chain to propagate

both northward and southward in the late Holocene

(Fig. 6). This tendency continued with southward

propagation (Mastin, 1991; Bursik et al., 2003) of

the Inyo dike, serial eruptions of which (in the mid-

14th century) followed the North Mono eruption by at

most a few years (Miller, 1985; Sieh and Bursik,

1986).

6.2. Inyo chain

The 10-km-long Inyo chain(Figs. 1, 4)consists of

7 rhyolitic lava domes, several phreatic craters, and a

modest composite apron of pyroclastic fall and flow

deposits(Miller, 1985; Sampson and Cameron, 1987).

The oldest unit is North Deadman dome (f 0.04 km3;

75% SiO2), undated but probably mid-Holocene (46

ka), followed by Wilson Butte (f 0.05 km3; 77%

SiO2), which erupted about the same time as the 1.3-ka South Mono episode. Both domes are crystal-poor

rhyolite. Wilson Butte is similar compositionally and

petrographically to the Mono domes and would cer-

tainly be considered a Mono dome were it not for the

45j change in trend of the chain (Fig. 2). North

Deadman dome is compositionally intermediate (Fig.

4; Sampson and Cameron, 1987) between Wilson

Butte and the crystal-poor lower-silica rhyolite that

dominated the youngest Inyo eruptive episode 650

years ago. Two crystal-poor mini-domes (each



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f 0.001 km3) just north and south of the large Glass

Creek dome(Fig. 5)have 7374% SiO2 and erupted

after Wilson Butte but prior to the major 14th century

Inyo eruption, the products of which they composi-tionally resemble(Sampson and Cameron, 1987).

Injection of the Inyo dike (Eichelberger et al.,

1985) in the mid-14th century led to sequential

eruption of Deadman Creek, Obsidian, and Glass

Creek domes(Figs. 2,5), each preceded by substantial

pyroclastic outbursts (Miller, 1985), the last of which

was followed by phreatic eruptions at nearby Inyo

Craters (Mastin, 1991). Total volume erupted during

this episode was estimated byMiller (1985)to be 0.4

km3 of lava and 0.22 km3 of pyroclastic ejecta.

Compositionally, the mid-14th century Inyo erup-

tion was unusually complex (Fig. 4; Sampson and

Cameron, 1987; Vogel et al., 1989). In addition to

crystal-poor (2 3% crystals; finely porphyritic)

zoned rhyolite (70 74% SiO2) that dominated the

eruptive products, a very crystal-rich rhyodacite

(71.3F 1% SiO2; 25 40% crystals; coarsely por-

phyritic) piled up over the vents of the Deadman

Creek and Glass Creek domes late in their extrusive

episodes and mingled (to a limited extent) locally with

the coerupted crystal-poor magma. Evidently having

been stored separately, the crystal-rich magma was

f 100 jC cooler and chemically unrelated to thecrystal-poor one, which has much higher K, Rb, Zr,

Y, and REE (and lower Ti, Mg, Ca, and Sr) at

equivalent SiO2 contents. In addition to the silicic

magmas, andesitic enclaves (f 60% SiO2) are present

in the crystal-rich central parts of both domes (Varga

et al., 1990).

The main crystal-poor magma was itself zoned

(7074% SiO2), yielding smoothly linear composi-

tional arrays (e.g., 1.32.6 FeO* and 2651420 ppm

Ba), which are continuous but show an apparent

tendency toward volumetric bimodalism (Sampsonand Cameron, 1987; Vogel et al., 1989). To explain

the arrays, Sampson and Cameron (1987) suggested

back-mixing between two slightly zoned silicic mag-

mas that had earlier fractionated at higher pressure,

while Vogel et al. (1989) called for mixing between

dacitic and rhyolitic end-member magmas. Their

high-silica endmember is similar to Mono domes

rhyolite, and their dacitic end-member is much like

the 40-ka hybrid dacite lavas on the caldera wall, right

between the Glass Creek and Deadman Creek domes.

Bailey et al. (1976)had suggested that the crystal-

poor phase might be Mono domes magma (which is

thus partly right) and that the crystal-rich phase is

Long Valley magma (which appears to be whollyright). The compositional and petrographic similarity

of the crystal-rich Inyo phase to t he nearby Moat

Rhyolite of Deer Mountain (Fig. 4) was pointed out

bySampson and Cameron (1987). Moreover,Reid et

al. (1997) identified, in both the 100-ka Deer Moun-

tain and the 0.65-ka crystal-rich Inyo phase, zircon

populations with crystallization ages that cluster

around 230 ka. Residual or thermally rejuvenated

Long Valley magmatic mush is implicated.

The significance here is that the 14th century Inyo

eruption tapped a magma volume at the confluence

of (1) the Long Valley residue, (2) the southward-

advancing Mono domes high-silica rhyolite, (3) the

western mafic array, and (4) through the hybrid

dacite component, a contribution from Mammoth

Mountain (as well as another one from Long Valley).

It may notbe a coincidence, therefore, that seismic

refraction profiles (twice, in 1973 and 1983) identi-

fied reflections from a shallowly dipping low-veloc-

ity horizon(lens?) at a depth off 7 km beneath the

NW moat (Hill, 1976; Hill et al., 1985), virtually

adjacent to this unique magmatic confluence. Be-

cause the Inyo dike had advanced southward (Mas-tin, 1991; Bursik et al., 2003) from the northerly

domain of its Mono rhyolite component, if the

reflector does represent a magma lens, then it could

be either the hybrid dacite reservoir or the crystal-

rich Long Valley residue, or both. Equilibration

pressure calculated for the crystal-rich (Long Val-

ley-type) Inyo magma (Vogel et al., 1989), based on

Al-in-hornblende geobarometry, is 2.3F 0.5 kb,

equivalent to a depth of f 7F 1.5 km. For the

crystal-poor Inyo magma, both this method (Vogel

et al., 1989) and the water contents of melt inclu-sions trapped in phenocrysts (45 wt.% H2O) from

pumiceous Inyo fallout (Hervig et al., 1989) suggest

shallower storage (36 km), presumably north of the

caldera for the Mono component at least. Nonethe-

less, a dike is only a feeder, and it remains specu-

lative to interpret from the eruptive sequence the

preeruptive distribution of magma storage or the

withdrawal dynamics, differential transport, and na-

ture of confluence of magmas from discrete zones or

elements of the reservoir(s).



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7. Discussion

7.1. Ring-fault zone and Long Valley caldera

Because the ring-fault zone is so far inboard of the

topographic wall(Figs. 2, 3, 5), Mammoth Mountain,

much of the mafic array, and the Mono-Inyo chain all

lie well outside the structural caldera, consistent with

differences in composition, and therefore should not

be considered magmati c successors of the Long

Valley reservoir. Intersystem contiguity is recognized

only in the west, where mafic magma may have

reheated and remobilized the margin of the mushy

granitic Long Valley residue, engendering the min-

gling and mixing conspicuous in the NW moat.

The ring-fault zone is not a single fault but a buried

set of nested step-down faults, as supported by seis-

mic refraction profiles (Hill et al., 1985), gravity

modeling(Carle, 1988),and drillhole data(Suemnicht

and Varga, 1988; Bailey, 1989). The outboard location

of Dome 7403 at the calderas NE margin may reflect

such a broad zone of step faults. The zone may not be

everywhere smoothly elliptical as portrayed, especial-

ly in the west, where subsidence could have utilized

precaldera faults of the left-stepping rangefront sys-

tem. Offsets along that inherited en-echelon fault

system may have locally conveyed a jigsaw patternto the western structural margin, which might in turn

have controlled (1) the outboard dike that fed moat

rhyolite to 100-ka Deer Mountain; (2) the path of the

Long Valley-type magma that later reached the NW

moat to mix with the 4027 ka hybrid dacites and

0.65 ka Inyo rhyolite; (3) offsets on the Discovery

fault zone of Suemnicht and Varga (1988); and (4)

ascent of the deep thermal water that flows eastward

in shallow aquifers to the active geothermal areas in

south-central Long Valley.

7.2. Significance of several discrete magma systems

Starting around 4.5 Ma, extensional unloading

enhanced partial melting of the upper mantle beneath

the Long Valley region, inducing coalescence and

ascent of basaltic magmas. Distributed basaltic intru-

sion progressively warmed the lower crust and fed

many mafic eruptions scattered within a 10040-km

belt that stretched from the High Sierra to the Adobe

Hills (Fig. 1) but centered on the future site of Long

Valley. The subsequent magmatic history is funda-

mentally one of local concentrations, focussing of

basaltic injection and consequent crustal partial melt-

ing beneath particular domains within this broad belt(Fig. 6). The first focus was beneath a 20-km-wide

zone centered on what is now the NW margin of the

caldera, where numerous andesites and dacites erup-

ted from San Joaquin Mountain to Bald Mountain

(Bailey, 1989)between 3.5 and 2.5 Ma. This zone of

distributed crustal magmas failed to coalesce, became

apparently moribund after 2.5 Ma, and was marked

by no further eruptions until after f 160 ka. The

mantle-driven focus of crustal melting later shifted

f 20 km east, to Glass Mountain where at least 60

eruptions of high-silica rhyolite between 2.2 and 0.79

Ma are the evidence for growth and eventual integra-

tion of a major crustal pluton capable of sustained

fractionation of high-silica, low-Sr melt (Mahood,

1990; Metz and Mahood, 1991; Metz and Bailey,

1993). Whereas no rhyolite at all had erupted from

the earlier aborted andesite-dacite focus to the west

(Fig. 6), the thick zone of partially molten crust that

supplied Glass Mountain rhyolite eruptions for 1.4

Myr intercepted all mantle-derived magma batches

(required for its thermal sustenance), thus permitting

no basalt, andesite, or dacite to reach the surface

during its long interval of strictly high-silica rhyoliteeruptive activity.

Around the time of the last eruption of Glass

Mountain rhyolite (790 ka) but before the caldera-

forming Bishop Tuff eruption (760 ka), the mantle-

driven focus of crustal melting shifted or drifted

westward f 20 km to yet a third area, abandoning

its long-stable position beneath Glass Mountain and

thermally energizing instead a zone that became

central Long Valley, embracing the large Bishop Tuff

magma reservoir and the subsequent locus of ER

eruptions and resurgent uplift. Because there has beenno postcaldera magmatism in either the Glass Moun-

tain domain or the eastern third of the caldera (except

early Dome 7403), and because heat flow in both

areas approximates only the Basin and Range average

(Lachenbruch et al., 1976), the westward shift of

magmatic focus was evidently complete and categor-

ical. Just as during the Glass Mountain episode, no

mafic or intermediate magma reached the surface

through or peripheral to the silicic reservoir during

the central Long Valley episode (790 300 ka), al-



22/30

though a minor dacite component in the Bishop Tuff

(Hildreth and Wilson, in review) and mafic enclaves

in two small postcaldera rhyolite units (Bailey, 2004)

indicate its coexistence beneath the rhyolitic chamber.The complete absence of peripheral mafic magmatism

during the extended Glass Mountain and main Long

Valley intervals implies that intrusion of mantle-de-

rived basalt remained tightly focussed beneath these

successive domains of exceptionally voluminous rhy-

olitic magma generation. Secular westerly drift of the

mantle-derived focus of crustal intrusion helps explain

the Glass Mountain paradoxthat allknown pre-

caldera rhyolites were in the northeast, that the caldera

subsided most deeply in the northeast, but that post-

caldera eruptive and hydrothermal activity have been

entirely farther west.

The most recent shifts of mantle-driven magmatic

focus(Fig. 6)have been principal elements discussed

in this paper. The fourth focus started up f 160 ka,

engendering the distributed (but compactly delimited)

array off 35 mafic vents (Figs. 2, 5), additionally

producing f 30 trachydacite rhyodacite eruptions

from the Mammoth Mountain core of the array, and

probably reenergizing the edge of the crystallizing

Long Valley reservoir to yield the 160100 ka west

moat rhyolites. The fifthmajor focus started up f 50

ka, 25 30 km north of Mammoth Mountain, beneaththe central part of what became the Mono Craters

chain. The new system progressively expanded north

and south, its eruptive frequency increasing markedly

in the mid-to-late Holocene. Because eruptive prod-

ucts along the central (arcuate) part of the Mono chain

are almost exclusively crystal-poor high-silica rhyo-

lite, nearly identical to those of long-lived Glass

Mountain, it seems reasonable to infer that a mushy

pluton capable of supplying recurrent batches of

highly evolved melt has likewise grown in the middle

crust here (as suggested by teleseismic P-wave delays;Achauer et al., 1986). Just as during the active life-

times of the Glass Mountain and Long Valley rhyolite

systems, non-rhyolitic magma is now prevented by

the Mono Craters silicic reservoir from erupting

centrally. Mafic products are recognized peripherally,

as late Pleistocene basalts at June Lake and Black

Point(Figs. 2, 6),and as mafic enclaves within three

of the 28 Mono domes (one Pleistocene rhyodacite

and two early Holocene rhyolites; Bailey, 1989;

Kelleher and Cameron, 1990).

The evidence that crustal magma systems are

energized by distributed intrusion (not underplat-

ing) of mantle-derived basalt, which in turn is not

uniformly distributed but is more intensely concen-trated in local domains, has been elaborated previous-

ly(Hildreth, 1981; Hildreth and Moorbath, 1988). The

model envisages that prolonged focussing at each

domain promotes thermal and mechanical feedback

between entrapment and crystallization of basalt,

enhancement of lower-crustal ductility and melting,

and maintenance of a buoyancy barrier. Such long-

lived focussing is intense beneath large arc volcanoes

and likewise beneath intracontinental centers like

those inFig. 6. Salient points include the following:

(1) Processes entail not just melting of older crustal

rocks but partial remelting of young mafic intrusions

and their differentiates, thermally induced by recurrent

pulses of basaltic intrusion and crystallization.

(2) The partial melting zone is not a magma

chamber but rather, a plexus of dikes, pods, and

mushy differentiated intrusions, where ductile defor-

mation promotes extraction, aggregation, and blend-

ing of varied melts.

(3) The melt-fractions in such zones wax and wane

in response to basaltic influx and to losses by ascent

of aggregated hybrids.

(4) Each focus has its own deep melting zone ofreduced density, usually impenetrable by primitive

basalts, whereas, peripheral to such foci, more prim-

itive batches (not intercepted and hybridized) can

ascend to produce monogenetic cones.

(5) Crustal thickness can be significant, by imparting

to magmas the chemical signature of pressure-depen-

dent residual phases (notably garnet) and by increasing

intracrustal path length (increasing opportunities for

hybridism), but age and composition of the varied

crustal lithologies melting are likewise important.

(6) Deep crustal melting zones feed upper-crustalmagma reservoirs (some by intermittently mobilizing

diapirically to produce mushy differentiated plutons),

and derivative mush columns consisting of cumulates

and migmatized protoliths track the ascent paths that

penetrate much of the crust.

Extension alone is insufficient to promote such a

major crustal magma system. Intensely focussed

basaltic injection into the lower crust is the key.

Contemporaneous with Long Valley magmatism,

Quaternary extension has been great in nearby Fish



23/30

Lake, Eureka, Deep Springs, Saline, Panamint, and

Death Valleys (Fig. 1), but Quaternary magmatism

has been sparse there. Merely 3040 km southeast,

Round Valley (Fig. 1) represents another left-step-ping offset in the Sierranrangefront extensional fault

system (Bateman, 1992), similar in size and tectonic

style to that at Long Valley, but it has remained

virtually nonvolcanic.

7.3. Late PleistoceneHolocene activity and current

unrest

The greater Mammoth Mountain and Mono-Inyo

systems are not magmatic daughters of Long Valley

but are, instead, new and mutual ly independent

domains of crustal melting driven by newly activated

foci of elevated melt production and ascent in the

locally subjacent mantle. After caldera collapse, there

were no mafic eruptions in or near Long Valley for

500 kyr, and the array of 35 mafic vents erupted since

f 160 ka isalmost wholly west of the ring-fault zone

(Figs. 2, 5).

The post-160-ka array of mafic vents (and the dikes

that fed them) are viewed as part of the spatially

focussed basaltic flux intrinsic to generating the

110 57 ka Mammoth Mountain silicic anomaly at

its center. Mammoth Mountain produced as many as30 silicic eruptive episodes during its f 50-kyr-long

lifetime, many of the most voluminous units having

been extruded around 67 ka(Ring, 2000),but for the

lastf 57 kyr, there have been none. Although not all

have yet been dated, the mafic vents close to Mam-

moth Mountain (with one or two exceptions) appear to

have erupted in the interval 16060 ka, thus starting

earlier but largely overlapping the 11057 ka interval

of silicic activity. Most of the trachydacite lavas that

dominate Mammoth Mountain, moreover, contain

mafic enclaves, demonstrating mafic-and-silicic mag-matic contemporaneity. Although both appear to have

ceased erupting soon after 60 ka, a clear exception is

Red Cones, a pair of early Holocene basaltic scoria

cones (and derivative lava apron) just 4 km SW of the

toe of Mammoth Mountain(Fig. 5),which might thus

signify a recent mantle-magmatic revival.

If the 1989 shallow seismic swarm directly beneath

Mammoth Mountain did represent emplacement of a

NNE-trending dike as modeled on the basis of earth-

quake distribution(Hill et al., 1990) and deformation

(Langbein et al., 1995), the dike orientation would be

athwart previous local vent alignments except thatof

the Red Cones pair, which is likewise NNE(Fig. 5). If

the 1989 dike penetrated as shallow as the 13 kmdepth estimated, this would support the inference that

the silicic reservoir had by now solidified (or, much

less likely, were shallower still). Also supporting the

notion of a local mafic-magmatic revival is the ongo-

ing sequence (beneath and southwest of Mammoth

Mountain) of long-period (LP) volcanic earthquakes

at depths of 1025 km (Hill, 1996; Pitt et al., 2002).

The LP sequence coincides areally with the mafic vent

array in the Red Cones-to-Devils Postpile region

(Figs. 2, 5), with a locally elevated extracaldera

heat-flow anomaly (Lachenbruch et al., 1976), and

with a salient in the local gravity low that extends

from the caldera as far as 5 km southwest of Mam-

moth Mountain(Carle, 1988).

The recent unrest farther east, however, including

intense seismicity in the calderas south moat and

renewed uplift of the resurgent dome (Hill et al.,

2003), is less easily reconciled with the volcanological

history. With no eruption on the resurgent dome since

650 ka and no silicic eruption in the south moat since

f 300 ka, current intrusion of rhyolite there would be

astonishing. If the inflation source modeled 6 7 km

beneath the resurgent dome(Langbein, 2003) were amafic dike, it would be the easternmost mafic event

recognized (inside or outside the caldera) in 2.5 Myr

and the first evidence for dike penetration through

much of the crystallizing silicic reservoir. Seismicity

in the south moat is consistent with displacements on

reactivated strands of the ring-fault zone (Prejean et

al., 2002), and eastward advance of a mafic dike along

that zone would not be unprecedented. The eastern-

most vent of the western mafic array, perhaps as

young as 98 ka (Ring, 2000), is merely 3 km south-

west of Casa Diablo(Fig. 5).Alternative to magmaticintrusion, however, hydrothermal processes may ulti-

mately be implicated in the current unrest.

Studies of active and fossil hydrothermal systems

in Long Valley caldera(Sorey et al., 1991; Goff et al.,

1991; McConnell et al., 1997; Farrar et al., 2003;

Fischer et al., 2003; Pribnow et al., 2003)have called

attention to two separate intervals of hydrothermal

activity and hot-spring discharge, one ending some-

time afterf 300 ka and the current one active from

f 40 ka to the present. The earlier episode ended



24/30

soon after emplacement of the southeast cluster of

moat rhyolites, which included the last crystal-poor

rhyolites ever erupted from the Long Valley reservoir.

The younger hydrothermal episode started up aboutthe time of eruption of the NW-wall dacite chain (40

27 ka), which was fed from a reservoir of hybrid

magma stored beneath the NW moat; and many of the

studies cited indeed indicate that the active hydrother-

mal areas in the south moat are supplied by shallow

aquifers that are fed in turn by deep upwelling

somewhere in the west moat. Although the 0.65-ka

Inyo dike probably mixed with the survivingmagmas

beneath the NW moat, the 7-m-thick dike (Eichel-

berger et al., 1985) is not itself an adequate heat

source. No pulse of hydrothermal activity temporally

related to either the west-moat rhyolites (160100 ka)

or the Mammoth Mountain silicic pile (11057 ka)

has been recognized.

Of the several systems (Fig. 6), the Mono-Inyo

chain appears to remain magmatically most robust, its

Holocene activity by far the most vigorous. The Glass

Mountain rhyolite system is long dead and the Long

Valley rhyolite reservoir moribund. The Mammoth

Mountain silicic system appears to have crystallized,

though renewed mafic activity in its peripheral array

would not be unexpected. As many as 20 Mono-Inyo

vents have been active in the last 2000 years, severalof the eruptions have been plinian or subplinian, and

nearly every batch of Holocene magma erupted has

been very crystal-poor, implying active separation of

high-silica melt. At least three times in the Holocene

(North Deadman dome, Wilson Butte, and the 650-

year-old Inyo dike), Mono magma has advan

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