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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).
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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.
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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
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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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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.
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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-
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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
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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
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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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