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JOURNAL
OF GEOPHYSICAL
RESEARCH, VOL. 85, NO. BS, PAGES 2381-2404, MAY
10, 1980
Late Cenozoic Volcanism, Geochronology,and Structureof the Coso Range,
Inyo County, California
WENDELL A. DUFFIELD, CHARLES R. BACON, AND G. BRENT DALRYMPLE
U.S. GeologicalSurvey,Menlo Park, California 94025
The Coso Range lies at the westedge of the Great Basin, adjacentto the southernpart of the Sierra
Nevada. A basementcomplexof pre-Cenozoicplutonic and metamorphicrocksis partly buried by --35
km3 of late Cenozoicvolcanicrocksthat wereeruptedduringtwo periods,as definedby K-At dating:
(1) 4.0-2.5 m.y., ~31 km3 of basalt,rhyodacite,dacite,andesite,and rhyolite,in descending
orderof
abundance,and (2) _<1.1m.y., nearly equal amountsof basalt and rhyolite, most of the rhyolite being
_<0.3m.y. old. Vents for the volcanicrocksof the youngerperiod are localizedon and near a horst of
basementrockswithin a concavitydefinedby the distributionof ventsof the older period.The alignment
of many ventsand the presenceof a considerablenumberof roughlynorth-trendingnormal faultsof late
Cenozoic age reflect basin and range tectonicsdominated by roughly east-westlithosphericextension.
Fumaroles,intermittently active thermal springs,and associatedaltered rocksoccurwithin and immediately eastof the centralpart of the field of Quaternaryrhyolite, in an area characterizedby variousgeophysicalanomaliesthat are evidently related to an activehot-watergeothermalsystem.This systemapparentlyis heatedby a reservoirof silicicmagmaat _>8-kindepth,itselfproducedand sustainedthrough
partial melting of crustal rocksby thermal energy containedin mantle-derivedbasalticmagma that intrudesthe crustin responseto lithosphericextension.
INTRODUCTION
The Coso volcanic field, of late Cenozoic age, forms a
patchy veneer over part of the Coso Range in Inyo County,
scribedseveralmammalianfossillocalitiesin sediment-filled
basinsadjacent to the range. Roquemore[1977] reported on
tuffaceouslacustrinedepositsof the range. Babcockand I•ise
California(Figure 1). Approximately35 km3 of volcanicrock, [1973] and Babcock[1975, 1977]outlined the stratigraphyand
mostlylava flowsand domes,covers•400 km2 of the range. geochemistryof certain volcanicrocks.Geologic,geophysical,
The youthfulnessof someof the volcanicrocksand the pres- and hydrologic studiesof adjacent basins [von Huene, 1960;
ence of fumaroles,intermittently active hot springs,and asso- Moyle, 1963;Zbur, 1963;Healy and Press,1964;Pakiser et al.,
ciated hydrothermallyaltered rock promptedthe selectionof 1964; Dutcher and Moyle, 1973] have provided a framework
this area for study by the U.S. Geological Survey in its Geo- for researchin somesurroundingareas.
Interest in the local geothermal systemhas prompted sevthermalResearchProgram.In conjunctionwith ongoingand
planned investigationsby workersat universitiesand at the eral studies. Frazer et al. [1943], followed by Austin [1964],
China Lake Naval Weapons Center, within whose bounds Austin and Pringle [1970], and Austin et al. [1971], described
most of the volcanic field lies, a comprehensiveprogram of the principal fumarolic areas and general geothermal phegeologic,geophysical,geochemical,geodetic,and hydrologic nomena. Preliminary geophysicalassessmentof the geotherstudieswas formulated in 1974;severalprogressreports have mal systemincluded a surveyof seismicnoise [TeledyneGeobeen published and are cited below. This report summarizes tech, 1972], electrical resistivitystudies[Furgerson,1973], and
our presentknowledgeof the geologyof the area, basedprin- infrared photography [Koenig et al., 1972]. A low-altitude
cipally on informationgainedfrom field mapping,rock chem- aeromagneticsurvey [Fox, 1978a], further resistivity studies
istry, K-Ar dating, and study of thin sections;it also synthe- [Fox, 1978b],geologicand alteration mapping [Hulen, 1978],
sizesour understandingof the geothermalsystemon the basis and analysisof logsand cuttingsfrom the bore hole CGEH-1
of all types of data now available and is intended to provide [Galbraith, 1978] were performed by the University of Utah
Research Institute Earth Science Laboratory under contract
backgroundfor the related papersin this issue.
Earlier work in the Coso Range included several topical
studies.Ross and Yates [1943] and Dupuy [1948] described
from theU.S.Department
of Energy.
The current program of study includesexpandedelectrical
[Jackson
et al., 1977; Jackson and O'Donnell, 1980; Towle,
mercury occurrencesthat were once worked commercially
from altered rocks in a few fumarolic areas. Power [1958, 1980] and seismic[Combs, 1975; Combsand Rotstein, 1976;
1959] and geologistsof Lucius Pitkin, Inc. [1976], examined Reasenberget al., 1980; I4•alterand I4•eaver,1980; I4•eaverand
the generalgeologyand uranium mineralizationin the north- Hill, 1978/1979; Youngand I•ard, 1980] studies,gravity and
westernpart of the range. Chesterman[1956] describedlate aeromagnetic surveys [Plouff and Isherwood, 1980], leveling
Cenozoic pyroclastic deposits, some of which have been and geodimetersurveys(B. E. Lofgren, oral communication,
mined for pumice intermittently for many years. Schultz 1977), measurement of heat flow [Combs, 1980], geologic
[1937] studieda volcaniclastic-rich
sequenceof sedimentary mapping [Duffieldand Bacon, 1977, 1980;Roquemore,1980],
rocks on the northwest flank of the range and named it the K-Ar dating of volcanic rocks [Lanphereet al., 1975; Duffield
Coso Formation. Everndenet al. [1964] determined K-Ar ages and Bacon, 1977;Table 1, this paper], obsidianhydration-rind
for volcanic rocks within and immediately above the Coso as dating [Friedman, 1976; Table 3, this paper], geochemical
age control for its mammalian fossils.Von Huene [1971] de- studyof volcanicrocks[Baconand Duffield, 1976;Baconet al.,
1979a],geochemicalstudy of geothermalfluids [A. H. TruesThis paper is not subjectto U.S. copyright.Publishedin 1980 by dell, oral communication,1977; Fournier et al., 1980], and a
the American GeophysicalUnion.
summary of basichydrologic data [Moyle, 1977]. Before these
Paper number 9B 1076.
2381
2382
DUFFIELD ET AL.: COSO GEOTHERMAL AREA
Pleistocenerhyolite as an index to the possiblevolume of underlying unerupted rhyolitic magma in the upper 10 km of
crust and estimatedthe presentassociatedheat contentto be
1.57x 10• J. Subsequent
geologicandgeophysical
investiga-
.o,•
%
',•.
,,,
•,
•
• J
•
-
• ,,
tions indicate a somewhatsmaller and deepermagma reservoir and thus a lower heat content [Baconet al., 1980].Either
estimate,especiallywhen viewedin contextwith the presence
of young rhyolite and basalt, suggestsa substantialcrustal
thermalanomalythat may supporta geothermalsystemat exploitable depth.
K-AR
360
,• % ,•
•
t •
)•",•
t
I
%
........
'• •t •'1
INDIAN
WELLS
I
117o45 '
Fig. 1. Index maps showinglocationand physiographyof Coso
DATING TECHNIQUE
Samplesfor K-Ar radiometric dating were coRRected
from
outcropsat the localities shown in Figure 2; these samples
usuallyconsistedof trimmed unweathered'pieces
of the bulk
rock. For specimensof silicicpyroclasticrocks(localities43,
44, and 48 in Table 1), pumicelapilli were handpickedat the
outcropand cleanedin the laboratoryto removefragmentsof
foreignmaterialbeforemineral separation.
Where the grain size permitted, biotite, plagioclase,sanidine, or hornblendewere separatedby using standardmagnetic, heavy liquid, and shape-dependent
methods.Feldspar
concentrates
weretreatedwRha colddilutesolutionof HF,
sonically cleaned in distilled water and rinsed with distilled
water to remove adhering fragmentsof glassand other mineral grains. Basalt and andesitesampleswere examined in
recentstudies,we had little detailedknowledgeabout the age, thin section to ensure that they met the usual criteria for
stratigraphicsequence,and volume of late Cenozoicvolcanic whole-rock K-Ar dating [Mankinen and Dalrymple, 1972].
rocksof the Coso Range and their relation to the geothermal Suitable whole-rock sampleswere crushedto either 0.5-1 or
system.A relativelycoherentpictureof two successive
periods 1-2 mm, dependingon grain size.Aliquantsweretaken with a
of volcanismduring the past •4 m.y. is now evident, and the samplesplitterfor potassiumand argon analyses;the aliquant
geothermal system apparently derives its heat principally for potassiumwaspulverizedto <75/•m beforeanalysis.
from magma of the later period.
Potassium(Table 1) was measuredon two aliquantsof each
Most well-known high-temperaturegeothermal fields lie sample, using lithium metaborate fusion followed by flame
within or adjacent to zonesof late Cenozoicvolcanism,where photometry[Suhr and Ingamells, 1966;Ingamells,1970]. For
risingmagma has transportedthermal energyfrom the mantle whole-rock samples,duplicate analyseswere made of each
and lower crustto exploitablenear-surfacedepths.In conti- aliquant, sothat a total of four measurementswastaken.
nental areas, silicic magma appearsto be the most effective
Radiogenic4øAr(Table 1) wasmeasuredby isotopedilution
causeof suchnear-surfacethermal anomalies,presumablybe- massspectrometryaccordingto the techniqueof Dalrymple
causeit tends to becomelodged in shallow,long-lived reser- and Lanphere[1969]. Two different massspectrometers
were
voirs more commonly than mafic magma [Smith and Shaw, usedfor massanalyses,one a Nier-type 60ø, 15.24-cm-radius
1975].Silicicvolcanicrocksof the earlier periodof volcanism instrument with analog data acquisition, the other a highin the Coso Range are Pliocenein age, and thus any coeval speed90ø, 22.86-cm-radiusinstrumentwith five faraday cup
shallow plutons are probably too old to serveas active heat ion collectors.The high-speedinstrument employs a dedisourcesfor the presentgeothermalsystem.However, silicic cated minicomputer for control, simultaneousisotopicdata
volcanicrocksof the later period, which consistof a clusterof acquisition,and data reduction[Sherrilland Dalrymple,1979;
late Pleistocenerhyolite domes and flows, are consideredto Staceyet al., 1978].
have eruptedfrom a crustalmagmareservoirthat may still exMore than one argon measurementwas made for mostsamist [Baconet al., 1980],and that probablyhasbeenthe princi- ples.The ages(Table 1) were calculatedfrom the meanpotaspal crustalsourceof heat for the geothermalsystem.The surf- sium and weighted-meanargon results,where weightingis by
icial geothermal phenomena and geophysical anomalies the inverseof the variance.This procedureallowsargon data
localizedwithin and adjacentto the area underlainby Pleisto- of varyingquality,i.e., varyingradiogenic•øArpercentage,
to
cene rhyolite presumablyreflectthe generalvertical projec- be used without the less precise results having a distion of a magmareservoirinferredto underliepart of the rhy- proportionateeffecton the calculatedage.
olite field [Baconet al., 1980].
Age calculations(Table I and Figure 3) were made by usThe youthfulnessof the later period of volcanismsuggests ing the most recent decay and abundanceconstantsrecomthat additional eruptionsmay occur in the near geologicfu- mended by the International Union of Geological Sciences
ture, althoughthis possibilityis clearlyspeculative.Much late Subcommission
on Geochronology[$teigerand Jager, 1977].
Pleistocenebasaltis solittle erodedthat someflowscan easily All previouslypublishedK-Ar agesreferred to in this paper
be picturedmovingdown and pondingin the valleysand ba- have been recalculated with these constants,and the reader is
sinsthat they now occupy;most of the rhyolite domesand cautioned to consider the effect of these new constants when
flows are similarly youthful in appearance.Smith and Shaw comparingour resultswith older data.
[1975, 1979] used the volume and geographicdistributionof
Figure 3 presentsa summaryof the 'best'or most probable
Range and adjacentareas.
DUFFIELDET AL.: COSOGEOTHERMAL
AREA
2383
1 17045 '
36 o15'
4-
4-
4-
4+
-e-
44-
+
4-
4-
44-
4-
44-
-e-
+\+
ROSE
L LE¾
360/
COSO
ß
ß
ß
BASIN
++++++
+% +
+
+
N
0
5 KILOMETERS
I
I
WELLS
EXPLANATION
IQuaternary
alluvial,
fluvial,
playa,and wind-blown
deposits
ßi
•
::):•.......•" r. ,, ', I
ß^j Pliocene
volcanic
rocks
and
• '••'1intercalated
Coso
Formation
4- + + +
Pre-Cenozoicgraniticand
•.+ + + +
metamorphic
rocks
:. flows,
concentric
dotspyroclastic
deposits,scattereddots
leistocene
sedimentary
rocks
of the White Hills
ß
K-Arage
Fault, dotted where concealed
and dashed where uncertain.
Ball and bar on downthrown side
Generalized
attitude
of sedimentary
rocksand Pliocene basalt flows
&
Chemical
analysis
Fig.2. Geologic
mapofCoso
volcanic
field,generalized
afterDu27ield
andBacon
[1977,
1980],
withlocalities
of K-Ar
datedandchemically
analyzed
samples.
SeeFigure
8 forlocalities
of dated
rhyolite
domes
andflows.
CP,Coso
Peak;
LCF,Lower
Cactus
Flat;UCF,UpperCactus
Flat;SP,SilverPeak;
CHS,Coso
HotSprings;
SM,Sugarloaf
Mountain;
VB;VolcanoButte;VP, VolcanoPeak;AL, AirportLake;andLL, LittleLake.
DUFFIELD ET AL.: COSO GEOTHERMAL AREA
2384
TABLE
1.
AnalyticalDataandCalculated
Potassium-Argon
AgesonVolcanicRocksof theCosoVolcanicField
Argon
10040Arrad
Calculated
40Arrad,
Locality*
28
29
Sample
75G301
75G304
Material
basalt
basalt
K20,'['%
1.356+ 0.004(4)
1.738+ 0.005(4)
3O
31
32
75G305
75G306
75G308
basalt
basalt
basalt
1.789+ 0.010(4)
1.610ñ 0.003(4)
1.312+ 0.004(4)
33
75G309
basalt
1.530+ 0.009(4)
34
35
36
37
38
75G310
75G313
75G314
75G315
75G318
basalt
basalt
basalt
0.820+ 0.005(4)
1.124+ 0.005(4)
1.525ñ 0.007(4)
Weight,g
0.222
1.0
0.312
1.6
14.052
6.29
4.1
14.912
4.84
5.6
19.561
18.995
20.230
1.029
1.126
1.638
8.6
5.0
3.8
21.858
2.118
29.248
2.317
11.4
29.693
2.395
21.2
21.589
0.222
26.381
0.220
18.393
19.752
2.6
1.07 ñ 0.14
3.0
15.169
0.529
7.7
25.247
0.505
7.6
40.58
43.1
41
42
andesite
1.713ñ 0.007(4)
8-196-2
8-199-6
basalt
basalt
1.318ñ 0.012(4)
0.857ñ 0.004(4)
45
13-108-1
biotite
plagioclase
biotite
sanidine
basalt
46
47
13-111-1
13-111-5
basalt
basalt
0.708ñ 0.004(4)
0.543ñ 0.003(4)
0.582ñ 0.004(4)
48
13-113-6
49
13-113-15
biotite
plagioclase
basalt
7.00(2)
0.518(2)
1.042ñ 0.005(4)
50
13-113-17
basalt
0.726ñ 0.002(4)
43
9-8-11
9-85-2
27.3
9.6422
21.163
6.706
9.218
19.5
28.0
3.53 ñ 0.17
3.60 ñ 0.08
19.000
9.312
47.1
20.110
8.700
25.5
19.990
8.794
42.3
19.991
6.941
37.1
19.688
6.925
33.7
20.458
4.188
15.8
15.225
5.202
8.2
22.975
2.004
41.34
54.88
7.347
2.4
16.0
9.709
1.151
5.863
75G316
biotite
hornblende
53
54
55
75G317
9-6-10
13-110-1
1-101-11
biotite
57
58
1-103-3
1-100-1
8-197-4
43.2
15.406
7.358
29.9
18.711
10.584
10.413
3.572
3.374
3.480
20.7
14.6
11.7
2.824
18.8
33.45
2.203
4.686
4.1
24.4
6.7
8.38(2)
1.036(2)
1.96
6.108
16.000
8.63(2)
5.9
66.2
60.4
16.000
4.569
19.530
3.076
16.3
17.627
3.169
16.4
0.10
0.70
O. 13
0.18
3.54 ñ 0.08
3.66 ñ 0.08
3.67 ñ 0.16
2.46 ñ 0.98
3.04 ñ 0.20
3.75 + 0.32
3.09 ñ 0.09
3.64ñ0.11
3.50 ñ O. 19
4.31
4.15
3.37
3.31
ñ
ñ
ñ
+
0.27
0.21
0.07
0.42
2.95 + 0.13
3.10 ñ 0.22
7.9
1.845
65.78
37.7
2.306
67.12
28.9
5.643
10.29
23.2
8.706
16.7
4.711
30.90
27.8
5.208
31.69
33.6
2.98 + 0.12
5.50 ñ 0.08
6.79 + 0.23
2.51 ñ 0.05
sanidine
9.21(2)
6.701
51.8
0.587 ñ 0.018
8.24(2)
2.007
37.64
29.5
3.33 ñ 0.20
1.930
2.713
44.60
39.30
28.3
28.7
2.110
43.91
40.9
1.816
41.44
39.5
biotite
basalt
basalt
sanidine
60
8-126-1
1-99-1
basalt
andesite
61
13-43-13
biotite
59
7.540
10.053
ñ
ñ
ñ
ñ
biotite
plagioclase
56
3.1
1.977
3.835
15.522
6.076
52
2.210
6.475(2)
0.458(2)
7.645(2)
12.315(2)
1.401ñ 0.005(4)
10.228
51
0.234 ñ 0.022
3.42
2.20
4.23
5.91
1.813
8-195-2
1.08 ñ 0.06
1.695
7.848
basalt
9.9
1.770
8.24 (2)
8-193-2
0.399 ñ 0.045
0.486 + O.108
1.07 ñ 0.12
0.188 ñ 0.035
19.620
39
2.06 ñ 0.34
2.4
0.696(2)
1.320ñ 0.015(4)
1.786ñ 0.004(4)
Age,$m.y.
0.140 ñ 0.089
18.325
plagioclase
1.236ñ 0.006(4)
4øArtot•
20.134
biotite
basalt
x 10-12mol/g
8.00(2)
0.740(2)
1.166ñ 0.002(4)
0.608ñ 0.001(4)
10.97(2)
0.894ñ 0.005(4)
1.566ñ 0.034(4)
10.682
3.674
41.8
10.113
3.595
21.7
18.501
7.003
23.2
15.607
14.591
6.031
6.061
51.1
46.7
16.308
7.317
2.9
15.970
7.298
3.7
3.046
16.48
71.1
3.282
16.53
48.7
20.001
11.552
10.414
8.56(2)
7.800
3.357
4.455
4.160
30.8
6.0
3.827
7.6
31.40
24.5
3.66 + 0.07
3.39 ñ 0.06
3.64 ñ 0.08
8.33 ñ 0.98
1.04 ñ 0.02
3.46 ñ 0.07
1.75 ñ 0.10
2.54 ñ 0.05
DUFFIELD
ET AL..'COSOGEOTHERMAL
AREA
2385
TABLE 1. (continued)
Argon
4øArr•,
Locality*
62
Sample
Material
1-88-2
andesitc
K20,T%
1.962+ 0.014(4)
Weight,
g
x 10-•2 mol/g
10.091
9.916
63
8-191-1
basalt
0.883+ 0.002(4)
Calculated
4øArtot•
11.468
25.7
11.263
26.9
13.046
4.087
11.7
15.521
4.736
45.9
Age,:•
m.y.
4.02+ 0.06
3.56q-0.10
*See Figures2 and 8.
•Meanandcalculated
standard
deviation.
Number
ofmeasurements
in parentheses.
$h•+ X•'= 0.581
x 10
-1,XO
--4.692
x 10
-iøyr-I, 4øK/K
= 1.167
x 10
-4mol/mol.
Errors
areestimated
standard
deviations
[Cox
andDal-
rymple,
1967].
Wheremorethanonemeasurement
wasmade,
calculated
ageisa meanweighed
byinverse
ofvariance
ofindividual
measurements.
ageof eachdatedunit,alongwith theprobabilityof any two [DuffieMand Smith, 1978].Strand lines in Pleistocenebasalt at
unitsdifferingin age;probabilities
werecalculated
by usinga AirportLake (dry) alongthe southmarginof the range(FigMaclaurinexpansionof the probabilitydensityfunctionfor a ure 2) are mute remindersof a Wisconsin(?) pluvial period,
normal distribution.
whichwasa muchwettertime than that characterized
by the
GENERAL
severalcentimetersof annual rainfall of the presentregime.
This currentarid climatemay bear criticallyon the availabil-
GEOLOGY
The CosoRange lies at the westedgeof the basinand range
physiographicprovince. It is boundedby the Sierra Nevada
on the west, Owens Valley on the north, the Argus Range on
the east, and Indian Wells Valley on the south (Figure 1).
North- to northwest-trendingelongatedshapesof the Sierra
Nevada, the Argus, and other nearby rangesreflect roughly
east-west tectonic
extension
that favors formation
of north-
trending range-boundingnormal faults [Wright, 1976]. The
CosoRange is more nearly amoeboidin plan view, with irregular and locally ill-defined boundaries,especiallyalong its
east and northeast sides, a shape that apparently reflects a
complextectonichistory.
The Coso Range is underlain principally by Mesozoicplutonsand subordinatemetamorphicrocks(Figure 2). Pliocene
and Pleistocenevolcanic and sedimentary rocks bury the
basementcomplex over much of the range, but this cover is
generallylessthan a few tens of metersthick. Contactsbetweenthe basementand volcanicrocksare widely exposedat
the marginsof and within the volcanicfield,whereinliersprotrudethroughthe thin volcanicsequence.
Moderately outward dipping sedimentaryrocks flank the
range around most of its periphery.Theserocks,which range
in age from Miocene to Pleistoceneand vary in lithology from
coarseelasticcontinentalfaciesagainstand near the range to
fine-grainedtuffaceouslacustrinebedsfarther away, recorda
historywithin the range of relative mountain uplift and concomitant volcanism during depositionof sediment [Giovannetti, 1979; Bacon et al., 1979b].The exposurealong normal
faults of youngergently dipping, late Pleistoceneor Holocene
alluvial depositsin range-flankingbasinssuggestscontinued
relativeuplift of the rangeinto Holocenetime; a currenthigh
level of seismicity[Combsand Rotstein,1976; Weaverand Hill,
1978/1979; Walter and Weaver, 1980] may indicate ongoing
uplift.
The Coso Range apparently never sustainedglaciers,but
considerablerunoff associatedwith late Cenozoicglaciationin
the neighboringSierra Nevada as far north as Mono Lake was
channeledsouth through a downfaultedtrough betweenthe
Sierra Nevada and adjacentranges[Snyderet al., 1964].Near
Little Lake at the southwestedgeof the CosoRange (Figure
2) this runoff haserodeda 160-m-deepcanyonthat waspartly
filled by basalticlava flows twice during late Pleistocenetime
ity andrecharge
of fluidsin the geothermal
system,although
little is knownin detail aboutthe localgroundwater[Moyle,
1977].Evidencepresentedbelowindicatesthat duringat least
part of late Pleistocene
time, presumablyduringone or more
pluvial episodes,thermalspringsdepositedtravertineand, locally, siliceoussinter in areaswhere no presentsurfaceflow
occurs.Changingclimate may have repeatedlyaffectedthe
availabilityof anddepthto groundwater
duringgrowthof the
field of late Pleistocenerhyolite that is a surfacemanifestation
of the inferredcrustalmagmaticheat sourcefor the geothermal system.
DESCRIPTION OF THE ROCKS
Pre-Cenozoic
Rocks
Pre-Cenozoicrocksof the CosoRange are importantto
geothermalstudiesbecausethe geothermalsystemexistsessentiallywithin them. In decreasing
orderof abundance,the
basement
complexconsists
of graniticplutons,dioriticto gabbroic plutons, and metamorphicrocks. Delineation of con-
tactswithinthebasement
andof relativeagesof theplutonsis
beyondthescope
of thisstudy;in general,
however,
maficplutons and metamorphicrocksare most abundant in the south-
ern andeasternpartsof the range,andgraniticplutonsdominateelsewhere.
The plutonsconstitute
part of the composite
SierraNevadabatholith.The CosoRangeisstructurally
separatedfrom the main bodyof the batholithalongthe Sierra
Nevadafault zone,but an east-west-trending
gravityhigh
crossing
the fault zonesouthof Little Lake [PlouffandIsherwood,1980,Figure5] suggests
subsurface
lithologiccontinuity
whereit is not obscured
by thicksedimentary
fill.
Agesof plutonsin the CosoRangeare inferredto range
frommiddleto lateMesozoic.
A K-Ar ageof 89.0_+2.6m.y.
has beenobtainedfor biotitefrom a graniticplutonin the
northwestern
part of the range(W. E. Hall, written communi-
cation,1974).Near the northeastboundaryof the range,• 1
km southof Darwin, a concordantU-Pb zirconage of 156
m.y. wasreportedby Chenand Moore [1979,Table 3, number
12]for 'quartzmonzoniteof theCosopluton.'
The arealdistributionof a swarmof Mesozoicdikessuggeststhat mostgraniticplutonsin the CosoRangeare late
Mesozoic,whereasmost of the mafic plutonsand metamorphic rocks are somewhat older. The dikes are northwest-
2386
DUFFIELD ET AL..' COSO GEOTHERMAL AREA
MAP
BEST.
AGE
SYMBOLS
LOCALITIES
GEOLOGIC
UNIT (106
yEARs)
Qbv
2at
basaltflow
0.039+-0.033
Qr
26t
rhyolite
flow
0.044
-+0.022
Qr
17 t
rhyolitedome
0.057-*0.016
Qr
4 •
ryholitedome
0.072*-0.031
Qr
Qr
Qr
1t
24t
25+
rhyolite
dome
rhyolite
flow
rhyolite
dome
0.088-+0.038
0.090-+0.025
Qr
18
rhyolite
dome
0.090(h•)
Qr
27t
rhyolit{
dome
0.093ñ0.026
Qr
16t
rhyolite
dome
0.099-*0.072
rhyolite
dome
0.101ñ0.033
--
0.081
*-0.008
Qr
6t
Qbe
28
basalt flow
Qr
20
rhyolite
flow
Qbh
34
basaltflow
0.188ñ0.035
Qbd
36
basaltflow
0.234ñ0.022
Qr
10
rhyolite
dome
0.235 (hyd)
Qr
14•
rhyolite
dome
0.244+_0.028
Qr
22
rhyolite
dome
0.265 (hyd)
Qr
15+
rhyolite
dome
0.293_+0.035
0.140_.0.089
0.16 (hyd)
Qr
13
rhyolitedome
Qbl
30
basaltflow
0.399_+0.045
0.30 (h•)
Qbl
31
basaltflow
0.486-+0.108
Qr
5
rhyolitedome
0.56-+0.24
Qr
53
rhyol
ite dome
0.587_+
0.018
Qr
58
rhyolitedome
Qbx
32
basaltflow
1.04_+0.02
1.07_*0.12
Qbc
35
basalt flow
1.07-+0.14
Qbm
33
basaltflow
1.08_+0.06
QTbr2
,,
60
andesite
flow
1.75t0.10
Tbr
29
basaltflow
2.06 -*0.34
Trd
51,52,61
rhyodacite
flow
2.52ñ0.05
Tbp
50
basalt
flow
2.98ñ0.12
Tcp
43,48
Tm
49
Tc
44
Tbp
3+
rhyodacite
aidallpumice 2.99ñ0.20
basalt flow
rhyolite
ash-flow
basaltflow
daciteplug(?)
3.10ñ0.22
3.14-*0.15
3.32ñ0.10
Td
54
Td
37
Tbp
59
basaltflow
3.46ñ0.07
Td
55
dacite flow
3.50ñ0.15
Tbp
46
basalt
flow
3.50ñ0.19
Tbp
38
basaltdike
3.53:•0.17
Ta
40
andesiteflow
3.54•:0.08
dacite flow
3.33-+0.20
3.40-+0.10
Tbu
63
basaltflow
3.56•:0.10
Tbc
39
basaltflow
3.60ñ0.08
Tbc
45
basaltflow
3.64:•0.11
Ta
56
basaltflow
3.64ñ 0.08
Tbp
Tm
41
42
basalt
flow
basaltflow
3.66•:0.08
3.67ñ 0.16
Ta
62
andesite
flow
4.02•:0.06
Fig. 3. Summaryof geochronology
of theCosovolcanicfield,basedon K-Ar andobsidianhydrationfind determinations and field relations.For many units, tabulatedage is statisticallyweightedmean of multiple K-Ar agesreportedin
Table 1. In rare instances(e.g.,locality51), K-Ar agedeterminationsare in conflictwith independentagedata and have
beenomittedfrom calculationof the weighted-meanage. Someobsidianhydrationfind data (hyd) arejudged more accurate than K-Ar determinations(e.g., locality 13). Seetext for further discussion.
Numbersin diagonalarray to fight indieatemaximum probability(in percent)that calculatedagesare truly differentand not due to randomerrors;probability
of)99% is indicatedby an asterisk.Map symbolsreferto Du.ffieldandBacon[1980].Daggersindicatesamplesdatedby
Lanphereet al. [1975]and recalculated
by usingmorerecentdecayconstants
of SteigerandJager[1977].
DUFFIELD ET AL..' COSO GEOTHERMAL AREA
trending,vertical to steeplydipping,and abundantin parts of
the range underlain predominantly by mafic plutons and
metamorphicrocks;elsewherethey are absent.Most dikes are
• 1-3 m thick and felsicor mafic in appearance;textureis generafly fine grained, with or without a few percentof feldspar
or biotite phenocrysts.In the southwestern
part of the range,
many dikes crop out for tens to hundredsof meters along
strike,forming parallel stripesacrossthe ruggedterrain. These
dikes are tentatively correlatedwith the Independencedike
swarm, first mapped by Moore and Hopson [1961] from the
Sierra Nevada near Independence,California, south-southeast to the Argus Range, and later mapped by Smith [1962]
farther south to the Garlock fault, acrosswhich the swarm is
apparentlyoffset64 km to the east.Moore and Hopson[1961]
showedthat the dikes predatesomeand postdateother plutons in the Sierra Nevada, a circumstancesimilar to apparent
field relationsin the CosoRange, and suggesteda Cretaceous
agefor the dikes.Chenand Moore [1979]reportedconcordant
U-Pb agesof 148& 2 m.y. for zirconfrom one dike in the Alabama Hills and two dikes in the Argus Range. Our tentative
correlation implies a comparable age for dikes of the Coso
Range and implies considerablygreater width for the Independencedike swarm [Chen and Moore, 1979, Figure 1], at
leastlocally, than previouslymapped.
East-west-trendingvertical to steeplydipping dikes intruding granitic plutons locally in the northern part of the range
are a few meters to 10 m thick and consistof homogeneousappearing granitic rock containing several percent of conspicuouspotassiumfeldsparphenocrysts1-3 cm in diameter
and quartz phenocrysts0.5-1 cm in diameter. These dikes
2387
ning and an assumedbulk densityof 1 g/cm3. We consider
our estimateddimensionsof the original deposit to be conservative,sothat the calculatedvolumeof •3 km3 of erupted
magmamay be somewhatsmallerthan the actualeruptedvolume. The volume of magma erupted to create a •3-m.y.-old
rhyolite air-fall pumice and ash-flow tuff, which is the only
other pyroclasticunit of notable volume, is estimatedat • 1
km•; but this figure is speculativebecauselittle is known of
the originalextentof the unit.
Our calculationssuggestthat •35 km• of lava has been
eruptedduring the past4 m.y. (Figure 4). Most of this volume,
•31 km•, was emplaced4-2.5 m.y. agoduring accumulation
of the Pliocene volcanic sequence.Generally speaking, the
proportionof silicicrocks(dacite,rhyodacite,and rhyolite) in
this sequenceincreasedas the Pliocenefield evolved.Ages of
someof the youngestPliocenedacite are looselyconstrained,
but emplacementof the widespreadrhyodaciteair-fall pumice
and overlyingrhyodaciteand daciteflowsmarksthe end of silicic eruptionsin the 4- to 2.5-m.y.-old volcanic episodeand
accountsfor •60% of all silicic lava from that period. Similarly, the emplacementof mostrhyolite of the Pleistocenevolcanicsequencesince•0.25 m.y. agodefinesan increasingproportion of silicic volcanism and an increasing average
volume-rate of eruption of rhyolite during this later volcanic
period. The volume ratio of Pleistocenebasalt to rhyolite is
-2:3.
Pliocene
Volcanic Rocks
Pliocene volcanic rocks and their vents crop out in an arcuate band extending from Haiwee Reservoir on the northprobably are Late Cretaceous,youngerthan dikes that we west,throughCosoPeak near the northeastmargin of the volcorrelate with the Independenceswarm, and may be coeval canicfield, to Volcano Butte at the southeastmargin (Figures
with petrographically identical dikes associated with the 2 and 5). The oldest units are basalt flows whose vents are
Mount Whitney pluton nearby in the Sierra Nevada (J. G. widespreadthroughoutmuch of the area underlain by PlioMoore, oral communication,1978).
cenelavas.After emplacementof theseflows,a variety of rock
typesfrom basaltto dacite,comprisinga continuumof magVolumeof VolcanicRocks
matic compositions
in termsof SiO2content(C. R. Baconand
The volume of volcanicrocksin the CosoRange was calcu- W. A. Duffield, unpublisheddata, 1979)was erupted.Most of
lated with referenceto the geologicmap of DuffieM and Bacon the andesite and dacite of this continuum is localized at rela[1980].Most eruptionsproducedlava flowsor domes,with or tively large polygeneticvolcanic centers,while basalt, probwithout minor pyroclasticdebris,and the area presentlycov- ably reflectingmore widely distributedvents and relatively
ered by the flow and dome units is accuratelyknown from low viscosity,remainswidespread,similar to the earlier basalt.
About 3 m.y. ago the sequenceof basalt, andesite,and daplanimetricmeasurements.
The thicknessof mostflowscan be
reasonablyestimatedfrom cross-sectional
exposures;burial of cite was blanketed by a few to severalmeters of rhyodacite
lavasor their removal by erosioncausessomeerror in the cal- air-fall pumice (localities 43 and 48, Figure 2) that was
culation.
Gener?lly
speaking,
theamount
of'missing'
material eruptedfrom a vent near the westmargin of the volcanicfield.
is greatestfor the oldestunitsand, if accountedfor, would fur- A thick accumulationof rhyodaciteash-flowtuffsoverliesthis
ther enhancethe predominanceof Plioceneover Pleistocene pumicein and near the vent area, and the pumiceis underlain
volcanicrocksevident in the presentlyexposedunits (Figure near its vent by rhyolite air-fall pumice, a relation that sug4). For a conservativeestimateof volumewe adjustedonly for gests repeated tapping of a silicic magma reservoir. Subobviouserosionand burial but made no adjustmentsfor the sequently,basalt, andesite,and rhyodacite lava flows were
vesicularityof flows and domes.The estimatedvolume of rel- emplaced locally over the rhyodacite air-fall pumice. Vents
atively minor amountsof pyroclasticdebrisin vent areaswas for these flows are scattered over much of the Pliocene volreducedby 50%to accountfor porosity.
canic field, and in a crude way, ventsfor the more siliciclavas
The relatively voluminous pyroclastic rocks merit addi- occurtoward the inner perimeterof the arcuatefield (Figure
tional discussion.Minimum original distribution and thick- 5). Rhyodacite flows lie only on and adjacent to Haiwee
nessof a •3-m.y.-old unit of rhyodacite air-fall pumice are Ridge at and near the vent area of the underlyingsilicicpyrodeterminablefrom remnantsthat suggestboth an exponential clastic rocks.
decrease in thickness with distance from the source vent and a
virtual restrictionto a •90 ø sectoropeningto the east-southeastfrom the vent area. The original depositpresumablyalso
thinned toward the lateral marginsof this sector.Accordingly,
volume was computedfrom an exponentialfunction fitted to
the thicknessof remnants,with an adjustmentfor lateral thin-
Our subdivision
of the volcanic rocks into various rocks
typesis basedon the weight percentof SiO2:basalt,48-54%;
andesite, 54-60%; dacite, 60-65%; rhyodacite, 65-70%; and
rhyolite, 70-77%. This subdivisioncorrelateswell with initial
rock names based solely on featuresobservablein the field
and in thin sections[Duffieldand Bacon,1977, 1980];a more
2388
DUFFIELD
ET AL..' COSO GEOTHERMAL
AREA
lO
Si02, wt.%
Ry= rhyolite
Rd= rhyodacite
76-77
6.5-70
-8
60-6.5
D = dacite
A = andesite
.54-60
B = basalt
48 -54
RK
R,
yx
4
3
2
Age, m.y.
Fig. 4. Rockvolumeandcomposition
versusage,plottedat 0.25-m.y.intervals.Data fromFigure3 and Table 1, supplementedby field relationsand unpublishedchemicalanalyses(C. R. Baconand W. A. Duffield, 1978).Seetext for discussion of volume calculations.
sophisticated
treatmentof the rock chemistryis in prepara- as thosedescribedby Goff[1977]. Most of the step-faultedtertion.
rane eastof CosoHot Springsis underlain by this basalt;elsewhere the Pliocene basalt tends to be relatively rich in SiO•
the Pliocenevolcanicrocks.Vents, now markedby moder- (locality 2, Table 2) and exhibits neither dikytaxitic texture
ately to highlyerodedcindercones,apparentlyfed oneor sev- nor vesiclesheetsand cylinders.
eral 2- to 5-m-thickflowsthat spreadlaterallyasfar asseveral
The many exposedcontactsat the baseof the volcanicsekilometersand accumulatedlocally in aggregatesectionsat quenceindicatethat mostof the basaltwasemplacedonto relleastasthickas 120m. Thickestsections
are exposedin scarps atively subduedterrain.Many flowsnow cap upfaultedridges
of normal faultseastof CosoHot Springs(Figures2 and 6). and floor downfaultedvalleys. Locally, field relationsin the
Most of the basaltcontainsseveralpercentof plagioclase
and easternpart of the volcanic field suggestrelief of a few hunolivinephenocyrsts;
someflowscontainclinopyroxene
pheno- dred meterson the surfaceof pre-Cenozoicrocksbeneaththe
crystsas well, and a few near CosoPeak containonly olivine lava flows,but the great lateral extent of theserelatively thin
and clinopyroxene
phenocrysts.
Somerelativelyhigh SiO2ba- flows arguesagainstwidespreadruggedterrain at the onsetof
Basalt. Basalt is the most widespreadand voluminousof
saltcontainsresorbed,sievedplagioclase
and roundedquartz
grainsjacketedwith brownglassandgreenclinopyroxene.
Chemicallyanalyzedsamplesare generaflyolivine-normative alkali basalt (C. R. Baconand W. A. Duttield, unpublished data, 1979). Relatively low SiO2 basalt (locality 1,
Table 2) is characterizedby pahoehoeflow surfaces,clotsof
plagioclaseand olivine, ophitic groundmassclinopyroxene,
diktytaxitictexture,andvesicle-rich
sheetsandcylinders,such
volcanism.
The age of the Pliocenebasaltflowsis partly constrainedby
that of the overlyingrhyodaciteair-fall pumice, -3 m.y. old.
K-Ar agesfor the underlyingbasaltrange from 2.98 +_0.12 to
3.67 +_0.16 m.y. (localities50 and 42, Table 1), and mostages
are between-3.7 and 3.3 m.y. (localities3, 39, 41, 45, 46, 56,
and 59, Table I and Figure 3). The absenceof weathered
zonesbetweenflows and the preservationof relatively fragile
DUFFIELD ET AL..' COSO GEOTHERMAL AREA
2389
age [Du•eld and Bacon,1980]hasyieldedno materialjudged
freshenoughfor K-Ar dating.
Andesire. Andesiteof Plioceneage is lessvoluminousand
placement.
One sampleof basaltfrom the lowestexposedflow in a can- less widespreadthan basalt. Accumulationstens of meters
yon (locality 5'7, Table 1) yielded a K-Ar age of •8 m.y., thick occur(1) near the north end of Haiwee Ridge, (2) 2 km
whichis probablyincorrectin the absenceof independentevi- northeastof Haiwee Spring, and (3) 5 km west of Volcano
dencethat would indicate sucha significantlygreater age for Butte (Figure 5). Andesitenear Haiwee Spring and Volcano
this basalt than for flows elsewhere in the Coso volcanic field.
Butteformspartsof complexpolygeneticvolcanoescharacterNo Pliocenebasalt has been found to overlie the rhyoda½ite ized by andesiteflows,daciteflowsand plugs,and mixed anair-fall pumice,but at the southwestedgeof the volcanicfield desite and dacite cinders(Figure 6). Simpler vents that fed
a basalticflow, fed by one of a clusterof four cinder cones only andesiteflowsoccurnear the north end of Haiwee Ridge,
whosemorphologiessuggestcloselysimilar ages,yields a K- '7 km east of Coso Hot Springs,and 5 and '7 km east of VolAr ageof 2.06 +_0.34 m.y. (locality29, Table 1). Other basalt cano Butte [Du•eld and Bacon, 1980].From generalappearmappedas youngerthan the air-fall pumiceand Pliocenein ances in the field and in thin sections,Du•eld and Bacon
pahoehoesurfacesin this basalt are evidencethat no protractedperiodof volcanicinactivityintervenedduringits em-
, conoco,
SpringsI D
•
Sugarloaf •,•
•
Mtn.•
Red
Rß
Hill
=
...
•
•,•le •
KM •
•
•
•//• •
Volcano
_.• ./Peak
//////llJxx%x
Fig. 5. Distribution of eruptiveventsand rhyolite domesand flowsmappedby Du•eld and Bacon[1977, 1980]in Coso
volcanicfield. Age assignments
basedon K-Ar datessupplementedby field relations.Late Plioceneor early Pleistocene
ventsindicatedby asterisk.B, basalt;A, andesite;D, dacite;Rd, rhyodacite.
2390
DUFFIELD
....,•.
ET AL.: Coso
GEOTHERMAL AREA
•.•;•;
......
.%.
,.......
.•%.,•
&-...
•,,.
,•.•.•;.-......
. "::':'..•3
:(•
....:,.,
.
.......v
.?..•."•:•, ,• •'
....
:........
..
ß
....
.'•
-.--
;;;-...
,.-,•:•,•
:,,.•
..;q
.......
•
........
4•..
Fig. 6. Obliqueaerialvieweastshowing
mostof CosoRange.Notestep-faulted
terranein middleandrhyolitedomes
andflowsdottingforeground.
Partof ArgusRangevisibleto far right,background.
Dark line trendingnorth-northeast
at
topis probablyshadowof condensation
trail of aircraft.Areashownabout40 km wide.ComparewithFigures2 and5.
[1977] originaRy describedthe flows east of Volcano Butte as
basalt,but recentlycompletedchemicalanalyses(C. R. Bacon
and W. A. Duffield, unpublisheddata, 1979)indicateandesire
accordingto the classificationusedhere.
The an&site contains as much as 15% phenocrysts of
plagioclaseand green clinopyroxene, generaRy with minor
olivine and, rarely, orthopyroxene.Most flows also contain resorbed (?), sieved plagioclaseand accessoryquartz grains
jacketed by brown glassand green clinopyroxene.The andesiregenerally appearsmore stronglyporphyritic than the basalt, partly becausemany an&site flowscontain more phenocrysts than the basalt but also because a relatively finer
grained groundmasshighlightsthe phenocrystsin the andesire.The SiO2 content of the variousan&site flows spansthe
entire range of the classificationusedhere. One analysisthat
andesite flows, one unit demonstrablyoverlying the pumice
and another inferred to be youngerthan the pumice [DuffieM
and Bacon, 1980],appear, from degreeof erosion,to be older
than this dated flow. From this evidence, these undated units
are probablyyoungerthan ---3m.y. and older than ---1.8m.y.
Dacite. Dacite occurs as parts of polygenetic volcanoes
northeastof Haiwee Spring and along the south edge of the
Pliocene volcanic
field from Volcano
Butte westward
and as
two conspicuous,partly overlappingflows 3 km northeastof
Coso Hot Springs(Figure 5). In the polygeneticvolcanic centers,dacite flows,shaRowintrusivemasses,and pumice are interlayered with andesiteflows and cinders.Dacite in these associationscommonlycontainsas much as 30 volume percent
of rounded, fine-grained porphyritic andesiticto basaltic inclusions from a few millimeters
to several meters in size. Field
is representative
of flowsintermediate
in SiO2contentis re-
and petrographicevidencesuggeststhat at least someof these
ported in Table 2 (locality 3).
rocksrepresentmechanicalmixtures of relatively lessviscous,
The oldest dated sample of an&site, with a K-Ar age of hotter mafic magma with dacite magma. Intricately shaped
4.02 _+0.06 m.y., was obtained from a flow southeastof Volcontactsbetweenthe two magma typesare apparent in large
cano Butte (locality 62, Table 1). Andesire overlies basalts outcrops, in hand samples, and also in thin sections;some
with K-Ar agesof 3.50 4-_0.19 (locality46, Table 1) and 3.64 4-_ contactzonesseveralmillimeterswide betweenrock types are
0.08 m.y. (locality 56, Table 1), just northeast of Haiwee gradational in color and phenocrystcontent.
Spring and west of Volcano Butte, respectively.Near Haiwee
The dacitecontains10-30%phenocrystsof plagioclasewith
Spring, andesirefrom near the bottom of the exposedsection various combinations of quartz, clinopyroxene, orthoyieldsa K-Ar age of 3.54 _+0.08 m.y. (locality 40, Table 1), es- pyroxene,amphibole,and biotite; minor magnesianolivine is
sentiaRycoeval with the underlyingbasalt. Andesireof volca- present in some flows, and accessoryzircon, sphene, and
noesthat erupted both dacite and an&site flows underliesthe opaque oxides are common. A chemical analysis of dacite
basaltthat is stratigraphicaRy
below the widespread---3-m.y.- from Volcano Butte is presentedin Table 2 (locality4).
old rhyodaciteair-faR pumice;thusthe age range of the andeMineral concentratesfrom three samplesof dacite coRected
sire beneath this pumice is likely similar to that of the associ- at widely separatedlocalitiesin the polygeneticvolcaniccenated basalt.
One an&site flow stratigraphicaRyabovethe pumiceyields
a K-Ar age of 1.74 _+0.10 m.y. (locality 60, Table 1). Other
tersyieldedweightedmeanK-At agesof 3.50 _+0.15 (locality
55, Figure 3), 3.40 _+0.10 (locality 37, Figure 3), and 3.33 +_
0.20 m.y. (locality 54, Figure 3), essentiaRycoeval. These K-
DUFFIELD
ET AL..' COSO GEOTHERMAL
Ar agesfor dacite are nearly the sameas most agesfor andesite and basalt, which also underlie the rhyodacite air-fall
pumice.
Field relations suggestthat two dacite flows northeast of
Coso Hot Springsare slightlyolder than the rhyodacitepumice. These flows, which were erupted from vents now marked
by erodedpilesof cinders~ 1.5km apart (Figure 5), advanced
~2 km eastward over an alluvial fan composedof granitic
clasts,a local faciesof the Coso Formation [Du•eld and Bacon, 1977, 1980; this paper, Figure 2]. The rhyodacite air-fall
pumice is intercalatedwithin this alluvial fan just a few tens
of meters from where it appearsto overlie the younger flow;
the older flow overlies alluvial material containing rhyolite
pumicesimilarto that belowthe rhyodacitepumiceat Haiwee
Ridge. We found no samplesof theselavas suitablefor K-Ar
dating. Considerablefault displacement[Duj•eld and Bacon,
1977, 1980]and the presenceof a 150-m-deepcanyoneroded
acrossthe flows suggestan age near 3 m.y. by comparison
AREA
2391
where mass slippage and concomitant sorting have resulted
from depositionon surfacesexceedingthe angle of repose
[Duffieldet al., 1979].
Remnantsof the rhyodacitepumice depositcontain larger
clastsand are generally thicker from southeastto northwest
acrossthe volcanic field. Maximum exposedthicknessin the
westis about 12 m near the southend of Haiwee Ridge, where
the base of the section is covered. Across the volcanic field
about 30 km to the east-southeast the thickest remnant is
about 5 m. Moreover, the size of scattered, dense lithic clasts
increases from a few millimeters
in the east to as much as 1.5
m near Haiwee Ridge. Areal trends of this coarseningand
thickening converge toward a vent area at the south end of
Haiwee Ridge (Figure 5), wherethe air-fall pumiceformspart
of a complex sequenceof silicic pyroclasticflow and fall deposits.
Pumice lumps contain several percent phenocrysts of
plagioclase,green hornblende, and brown biotite, as well as
with units dated elsewhere in the volcanic field.
minor quartz, clinopyroxene,orthopyroxene,and pink zircon
Rhyodacite. Rhyodacite occurs both as the widespread set in extremelyvesicularcolorlessglass.The bulk rock conair-fall pumice mentionedaboveand as lava flows.The pum- tains 65.?% SiO2 (68.2% on an anhydrous basis) and is
ice blanketed the Coso Range and adjacent areas for an un- rhyodaciteaccordingto the compositionalclassificationused
known distance during a violent eruption, or pene- here. The weighted-meanage, calculatedfrom K-Ar determicontemporaneouseruptions,~3 m.y. ago (localities43 and 48, nationson mineral concentratesfrom samplescollectedin two
Table 1; Duj•eld et al. [1979]).The depositstypically consistof widely separatedareas(localities43 and 48, Figure 3), is 2.99
a massiveaccumulation of grayish-white subangularlapilli. _ 0.20 m.y.
Well-developed reverse-gradedbeds are present locally,
Rhyodacitelava flows cap parts of Haiwee and adjacent
TABLE 2. ChemicalAnalysesand CIPW Norms of Samplesof All Major VolcanicRock Typesof the CosoVolcanicField
Locality*
I
SiO2
AI•O•
Fe203
49.5
16.9
CO•
2.5
6.2
7.4
10.2
3.5
0.6?
0.25
0.21
1.3
0.30
0.12
0.04
Total
99.09
FeO
MgO
CaO
Na20
K20
H2O+
H20TiO2
P20•
MnO
Q
hy
ol
mt
2
52.7
18.3
3.1
4.8
5.5
8.1
4.3
1.4
0.26
O. 16
1.5
0.38
0.11
0.04
100.65
......
............
4.01
30.03
28.82
16.29
2.30
11.57
3.68
8.25
36.30
26.44
8.73
8.22
3.77
4.48
3
57.0
16.8
1.8
4.5
5.4
8.2
3.6
1.9
0.42
0.24
0.98
0.34
0.08
0.00
4
61.6
16.5
1.1
3.1
4.3
5.0
4.0
2.6
0.63
O. 12
0.74
0.22
0.05
0.06
5
65.7
15.5
6
67.0
15.8
ap
2.50
0.72
0.09
74.0
13.1
1.5
1.6
1.3
3.5
3.4
3.1
3.7
0.53
0.54
0.15
0.04
0.01
1.0
1.5
1.1
2.9
3.9
3.5
2.2
0.21
0.44
0.15
0.03
0.00
0.45
0.20
0.10
0.84
3.1
5.2
3.4
0.44
0.09
0.04
0.04
0.03
49.4
16.5
2.7
7.3
6.1
10.3
3.5
0.9?
0.39
O.12
1.9
0.36
0.14
0.06
100.02
100.57
99.73
101.03
99.74
4.79
11.24
25.61
23.55
35.31
......
15.48
34.10
19.53
2.91
13.08
0.59
19.02
29.86
16.94
.........
4.28
0.70
21.25
33.91
13.78
1.04
31.62
26.99
3.82
4.11
0.27
.........
5.78
29.85
26.65
17.93
1.36
9.88
1.49
0.53
11.16
30.28
23.93
11.51
13.11
...............
2.59
1.61
2.26
0.10
2.84
0.90
0.09
8
101.26
..................
il
7
1.85
0.80
...
1.42
0.53
0.14
1.07
0.37
0.02
0.86
0.37
...
0.18
0.10
0.07
3.95
9
10
52.35
16.41
76.4
12.56
2.59
6.17
5.90
7.95
3.89
1.78
O. 17
0.06
1.79
0.58
0.15
0.02
0.35
0.60
0.02
0.44
4.32
4.58
0.28
0.09
0.09
<0.01
0.02
...
99.81
99.76
32.85
10.56
33.06
22.15
10.91
11.38
3.36
3.77
27.23
36.78
1.36
0.68
0.40
...
0.51
3.41
1.38
0.05
0.17
<0.02
-..
.........
3.64
0.86
0.14
Norms calculatedfrom analysesnormalizedto total of 100%.Samples1-8 analyzedat RestonRapid Rock AnalysisLaboratoryof the U.S. Geological Survey by methodsdescribedunder 'Single solution'by Shapiro[1975];analysts:K. Coates,Z. A. Hamlin, N. Skinner, and H. Smith.
Sample9 analyzedat DenverRockAnalysisLaboratoryof the U.S. GeologicalSurveyby methodsdescribed
by Peck[1964];analyst:E. L. Brandt
(analysis
courtesy
ofH.G.Wilshire).
Sample
10analyzed
atUniversit,
y ofLancaster,
GreatBritain,
bywet-chemical
methods
(analysis
courtesy
of
R. Macdonald).
*See Figure 2.
2392
DUFFIELD ET AL.: COSO GEOTHERMAL AREA
Pumice in the ash-flowtuff contains3% phenocrystsof sanidine, plagioclase,quartz, and biotite in a colorlessglassmatrix; this pumice is easily distinguishedin the field from the
rhyodaciteair-fall pumiceby its lighter color and by the absenceof hornblende.Pumice of the ash-flow is considerably
more silicic than that of the air-fall rhyodacite (localities 5
and 7, Table 2).
The weighted mean of K-At ageson sanidine and biotite
from the rhyolite pumice is 3.14 +_0.15 m.y. (locality 44, Figure 3), coevalwith the weighted-meanage of the rhyodacite
air-fall pumice within the uncertaintiesof the K-Ar ages.
area.
Field evidence suggeststhat the air-fall depositsare stratiMany of the rhyodacitelava flows may have been erupted graphically above the ash-flowdepositsand that both occur
from the samevent area that producedthe silicic pyroclastic within the upper part of the CosoFormation.
rocks because the flows are thickest and most abundant at
The rhyolite ash-flowtuff may have been erupted from the
those localities.Steeply dipping, concentricflow banding in same vent that was the sourceof the rhyodacite air-fall dethe northern and northeasternparts of the flow field probably posit. Exposuresof the tuff are localized northwestof this
marks subsidiaryvents, and faulting and erosion may have vent, and no other silicicvent of appropriateage is found in
renderedother ventsunrecognizable.
the area. Severalmetersof rhyolite basesurge(?) depositsand
The rhyodacite lava flows contain •20% phenocrystsof beds of air-fall pumice petrographicallyidentical to the ashplagioclase,hornblende, biotite, opaque oxides, and quartz, flow pumice underlie the rhyodaciteair-fall pumice within its
with accessoryclinopyroxene,orthopyroxene,apatite, zircon, vent complexat the southend of Haiwee Ridge.
and sphenein a groundmassof colorlessperlitic glass.The avPleistocene Volcanic Rocks
erage SiO2 content from severalanalysesof the rock is •69%
Pleistocenevolcanismwas characterizedby the eruption of
on an anhydrousbasis(locality 6, Table 2); individual determinationsrangeabout 1.5%aboveand belowthis value (C. R. basalt and rhyolite. The youngestlavas of intermediatecomBacon and W. A. Duflield, unpublisheddata, 1979).The least positionsconsistof andesite-1.8 m.y. in age (locality 60, Figsilicic flows are very similar in major-element compositionto ure 3) and are describedin the sectiontitled 'PlioceneVolcanic Rocks.'
The Pleistocene
volcanic
rocks were all
the widespreadrhyodaciteair-fall pumice.
Everndenet al. [1964] reported a K-Ar age of 2.1 m.y. for emplacedduring the past 1.1m.y.
We are unable to decipherthe detailed sequenceof erupbiotite from a rhyodacitelava flow (sample KA1026, called
andesiteby them) at the northwestmargin of the flow field; tion of the Pleistocenevolcanicrocks.Generafly speaking,bathey also reported a potassiumcontent of 6.00% for the bio- salt is demonstrablyolder than nearbyrhyolite, but locallythe
tite, a rather low value that suggestssome alteration. We re- emplacementsequenceis complex;near Volcano Peak, basalt,
port K-Ar agesof 5.50 _+0.08 and 6.79 _+0.23 m.y. (locality rhyolite, basalt,rhyolite,and basaltwere eruptedsuccessively
51, Table 1) for biotite and hornblende concentrates,respec- within a 4-km2 area [DuffieMand Bacon,1977, 1980].Basalt
tively, from a samplewe collectedat the samelocality; these was apparently erupted both first and last during Pleistocene
time, when a minimum of 19 basaltcinder conesand 38 rhyoagesareconsiderably
olderthanthose
onstratigraphically
lower units, a situationthat we are unable to explain. Biotite lite domes and flows were formed. All of the Pleistocene vents
from samples collected at widely separated localities else- lie within the concavitydefined by the arcuate zone of Pliocwhere in the rhyodaciteflow field yielded K-At agesof 2.51 _+ ene ventsand lava flows(Figures2 and 5).
Basalt. Basalt vents occur near and at the east, south, and
0.05 (locality 52, Table 1) and 2.54 +_0.05 m.y. (locality 61,
Table 1), with a weighted-meanage of 2.52 + 0.05 m.y. (local- west marginsof the rhyolite field (Figure 5). Most vents are
ities 52 and 61, Figure 3). We prefer this age becauseit ac- marked by moderately eroded to little-eroded cinder cones,
cords with stratigraphyand the timing of all other events in and each vent appearsto have fed one to five flowsaveraging
the history of the volcanic field, as deduced from geologic a few metersin thickness.Superposedflowsshowno evidence
mapping and K-At agesof many other volcanicunits. More- of interveningperiodsof weatheringor erosionand form secover, this age implies a reasonableperiod of time for the ero- tions as thick as 15 m or, locally, 70 m where ponded.
All the Pleistocenebasaltis porphyritic and containsvarysion of canyonsin the underlying silicic pyroclasticrocks,
ing proportionsof phenocrysts
of olivine, plagioclase,clinopywhich are •3 m.y. old.
Rhyolite. Rhyolite cropsout locallyalongthe westflank of foxerie, and opaque oxidesthat make up as much as 30% of
Haiwee Ridge as a 10- to 20-m-thick unwelded pumiceous someflows. Plagioclasecrystalsas large as 5 cm occurin some
ash-flowtuff that is intercalatedwith sedimentaryrocksof the flows,noticeablyin lavas at the southmargin of the volcanic
Coso Formation and may have in part been emplacedin a field [Duffield and Bacon, 1980], but most phenocrystsare no
shallowlake. Stronglydeformedlacustrinebedsat the baseof more than a few millimeters long. Many flows contain corthe ash-flowtuff suggestthe presenceof wet, plasticsediment roded plagioclaseand quartz crystalsas long as severalmilliduring emplacement.Layering in the lower part of the ash- meters, commonly associatedwith pebble- to cobble-sizedinflow tuff may record local lake depth, above which the unit is clusionsof granitic rocks.Chemical analyses(localities8 and
massiveand structureless,
typicalof many unweldedash-flow 9, Table 2; C. R. Baconand W. A. Duffield (unpublisheddata,
tuffs. Inclusions as much as 1 m wide of lacustrine beds within
1979); Babcock and Wise [1973]) show that the Pleistocene
the tuff are typically surroundedby zonesseveralcentimeters lavas are alkali basalts.
wide enrichedin vapor-phasemineralson the wallsof pumice
K-At agesfor the Pleistocenebasaltrange from 1.08 + 0.06
vesicles,an effectthat is probablydue to water bakedfrom the (locality 33, Figure 3) to 0.039 + 0.033 m.y. (locality 2a, Figsedimentby heat from the coolingash-flow.
ure 3). Three dated flows(localities32, 33, and 35, Figure 3)
ridges.Such capsaverageabout 25 m thick, but lava ponded
locally to at least 200 m thick in a canyon eroded in silicic
pyroclasticrockson the southwestflank of Haiwee Ridge. At
the summit of the ridge in the vent area of thesepyroclastic
rocks, the rhyodacite flows and associatedpyroclasticdebris
are altered, so that the rocks have a bleachedto oxidized appearance;many of the joint surfacesare coated with opal.
This alteration may record the effectsof a hydrothermal system active during the waning stagesof a Pliocenesilicic volcanic centerat the southend of Haiwee Ridge. No evidenceof
a presentlyactivehydrothermalsystemhas beenfound in this
DUFFIELDET AL.: Coso GEOTHERMALAREA
and possiblya fourth similarly eroded undated flow were
erupted-1.1 m.y. ago at the west,south,and eastmarginsof
what later becamethe rhyolite field. Contemporaneouslywith
this early basalt in terms of K-Ar age, a single rhyolite dome
(locality 58, Figure 3) was emplacedat the northeastmargin
of the rhyolite field. After an apparentlull in eruptive activity,
nine or more vents fed basalt flows during the past •0.5 m.y.
(localities2a, 28, 34, 36, 30, and 31, Figure 3; Duffieldand Bacon [1977, 1980]).Other undatedbasaltmay be older than 0.5
m.y., althoughmostundatedflowsare much closerto 0.5 than
to 1.1 m.y. old on the basisof degreeof erosion.
Pahoehoe and aa surfaces on many basalt flows appear
little eroded, and the paths of most flows were at least partly
guided by stream channels(Figure 7). A major north-south
2393
Well-bedded pyroclastic deposits of obsidian, pumiceous
rhyolite, and minor amountsof basementrocks form partial
to completerings around somedomes(Figure 7). Some pyroclastic beds contain accretionary lapilli, and other are downbowed
beneath
cobble-
to boulder-sized
clasts of basement
rocks. These depositsevidently record repeated explosions
just prior to dome emplacement.Well-exposedexplosiondebris associatedwith one dome (locality 18, Figure 3) was deposited partly against and upon an adjacent older dome; a
crater that was formed during generation of these deposits
transectsthe older dome to exposeobsidianbeneath the original pumiceouscarapace.
The diameter of most explosionringsis ---600m, and crests
rise from several meters to 30 m above the surroundingterfiver channel at the southwestedge of the volcanic field near rain. Domes that apparently lack such deposits may have
Little Lake was partly filled by basalt twice during late overridden and completely buried or bulldozed them, alPleistocenetime [DuffieM and Smith, 1978]. Stream channels though it is uncertainwhether eraplacementof each dome was
have been reestablishedon or adjacent to most intracanyon preceded by formation of an explosion ring. However, a
flows.
mantle of beddedpyroclasticdebriscoveringalmostthe entire
Rhyolite. Rhyolite forms steep-sicJedextrusions, most 5 x 18 km area of the rhyolite field and consistingof material
lying atop a roughly 5 x 18 km horst of pre-Cenozoic base- similar to that in the explosionrings indicateswidespreadexment rocks (Figures 2 and 6). Most of the rhyolite occursas plosiveactivity associatedwith dome eraplacement.The origiisolated,nearly symmetricaldomes (Figure 7); some rhyolite nal mantling deposit probably averaged 1-2 m in thickness
forms flows as long as 3 km. Severaldomesand flows overlap within the rhyolite field and thinned rapidly at the marginsof
to form compositebodies,the largestof which is named Sug- the field; some of this deposit has been reworked and transarloaf Mountain (Figure 6). The most northwesterly and ported, partly to basinswithin the rhyolite field and partly to
southwesterly domes are almost completely covered by flanking basins,so that many inliers of basementrocksare exyounger volcanic rocks. One isolated dome lies at the north- posedthroughout the field [Duffeld and Bacon, 1977, 1980].
east margin of the rhyolite field within the horst-bounding Widely distributed coarseash- to lapilli-sized obsidian fragfault zone.
mentswith hydrated surfaces,littering the ground as far as 20
With the exceptionof this most northeasterlydome, which km to the east of the rhyolite field, are evidencethat some exis the oldestdome in the rhyolite field, the surfacesof the rhy- plosionswere relatively violent. Continuousblanketing layers
olite bodies consistmostly of looseblocks of perlitic, moder- of explosion-deriveddebrisare, however,restrictedto the rhyately pumiceous,rhyolite glassthat forms part of an original olite field.
Nearly all the Pleistocenerhyolite is remarkably uniform in
carapace; partly devitrified, commonly spherulitic or lithophysal obsidian protrudes through this carapace on some appearance.One of the domes(locality 53, Table 1, and Figdomesand flows.On other bodies,obsidianis exposedonly in ure 8) contains ---7% phenocrysts;the rest contain <2% and
roadcutsor is still entirely buried beneaththe pumiceouscara- most of them <<1% phenocrysts,including some or all of the
pace,evidenceof their youthfulness.
phasesquartz, sanidine,oligoclase,titanomagnetite,ilmenite,
Fig. 7. Oblique aerial view to north of 400-m-wide rhyolite dome (0.081 + 0.008 m.y.; locality 1, Figure 3) surrounded
partly by concentricridgeof explosiondebrisand partly by basaltlava flow (0.039 + 0.033m.y.; locality 2a, Figure 3) from
Volcano Peak, just out of view at bottom.
2394
DUFFIELD ET AL.?COSO GEOTHERMALAREA
117o50 '
117o45 '
I
360
05'
/
/
/
/
A
360
Fig.8. Generalized
geologic
mapshowing
principal
geothermal
areas
andPleistocene
rhyolite.
Heatflow(triangles)
in
heatflowunits(HFU) [fromCombs,
1980,Figure11].A 1477-m-deep
drillholeis adjacent
to sitewithHFU of 15.F,
fumarole
area;heavysolidlines,faults,barandballondownthrown
side;dashed
lines,heatflowcontours;
hachures,
outlineforareasof internaldrainage
(modified
afterMoyle[1977]);
53,rhyolitelocality.
biotite,hornblende,
clinopyroxene,
orthopyroxene,
andfayal- con et aL, 1979a].Details of rhyolite chemistrywill be pubite. The salic phasesare by far the most abundant. Some lished later.
domescontainscattered
inclusions
of vesicular
basaltand,less
K-Ar ageshavebeendeterminedfor 18rhyolitedomesand
commonly,graniticrock as largeas 20 cm.
flows(Table 3 and Figure 3; Lanphereet ai. [1975]);no mateMany chemicalanalyses(C. R. Bacon,P. A. Baedecker, rial suitablefor radiometricdatingis exposedon otherdomes.
W. A. Duffield,R. Macdonald,andR. L. Smith,unpublished One dome (locality58, Table I and Figure 8) that yieldeda
data)underscore
thepetrographic
homogeneity
of therhyolite K-Ar age of 1.04 :!: 0.02 m.y. is judged to be the oldestbedomes and flows; the SiO2 content varies within <1% on an causethe pumiceouscarapacehas been completelyeroded
anhydrous
basis.Only one analysisof rhyolite(locality10, from it, so that a deeperlevel of devitrifiedrock is exposed.
Table 2) is givenhere.Significantdifferences
in the contentof The nextoldestK-Ar age(0.99:!:0.20m.y.,locality13,Table
many minor and trace elements allow the extrusionsto be 3 and Figure 8; Lanphereet ai. [1975],sample13) is almost
groupedin severalsetsthat are believedto reflecteruptionof certainlyin error.This domeis little eroded,is similarin apdiscrete
batches
of magmafroma singlemagmareservoir
[Ba- pearanceand chemicalcomposition[Baconet ai., 1979a]to
DUFFIELD
ET AL..' COSO GEOTHERMAL
AREA
2395
many other domesthat have yielded K-Ar agesof ~0.25 m.y., TABLE 3. Obsidian Hydration-Rind Ageson PleistoceneRhyolite
of the Coso Volcanic Field
and is far lesserodedthan the dome 1.04 + 0.02 m.y. in age.
The obsidiandated at 0.99 + 0.20 m.y. may contain inherited
Locality*
HydrationAge,• m.y.
K-Ar Age,m.y.
nøAr.The remainingK-At agesare all <0.6 m.y.;noneof the
updateddomesappear,from their degreeof erosion,to be any
older than this. The youngestage,0.044 + 0.022 m.y. (locality
26, Figures 3 and 8), is for the youngestpart of Sugarloaf
Mountain, which is littered with pyroclasticdebris presumably erupted during an explosivephase that precededemplacementof a youngernearbydome.
Friedman[1976](seeTable 3) measuredthe thicknessof hydrated rinds on obsidian fragmentsfrom 13 extrusionsand
calculated obsidian hydration ages, 10 of which are concordant or nearly so with the correspondingradiometricages.
Of the discordantages,two (localities5 and 25, Table 3) are
consideredless reliable than the correspondingK-At ages,
and one (locality 13, Table 3) is consideredmore reliable.
These judgments are based on field relations, including the
relative degreeof erosion,and agessuggestedby all other KAr and hydration rind determinations.Multiple K-At determinations are currently being made on severalobsidiansamples to improvethe precisionof existingagesfrom-singledeterminations,and ongoingstudy of trace- and minor-element
analysesthat appear to show a time-relatedchemicaltrend
[Baconet at., 1979a]may help unravel the detailed sequence
of rhyolite emplacement.Lanphereet at. [1975] concludedon
the basisof their reconnaissance
study that most domesand
flows are younger than 0.15 m.y. Our study reinforces this
general conclusionand suggests
that perhapsfive domesare
older than ~0.3 m.y.
17
4
6
26
18
18
24
5
20
14
25
10
22
13
13
The CosoFormation. The oldestsedimentaryrocksin the
area constitute the Coso Formation. This formation, defined
by Schultz[1937], is exposedalong the west and north flanks
of the Coso Range and dips away from the range at _<25ø.
About 100-200 m of sectionis exposed,but the base of the
formation is not exposedin its thickest parts. Rock types
range from alluvial fan deposits,lapping onto pre-Cenozoic
basementrocks exposedin the range, to sandy arkosic beds
and fine-grained,tuffaceouslacustrinematerial at increasing
distancesfrom the range.
In addition to a general coarse- to fine-grained facies
changeoutward from the range,the Cosois increasinglytuffaceoussouthwardalong the west flank of Haiwee Ridge. A
rhyolite ash-flowtuff, describedin the sectiontilted 'Pliocene
VolcanicRocks,'is sandwichedwithin the CosoalongHaiwee
Ridge; at the south end of the ridge the formation consists
wholly of silicicvolcanicrocksthat mark the silicicvent area,
alsodescribed
above.Eruptionsfrom thisvent areamay have
generatedmostof the volcanicdebrisin lacustrinebedsof the
Coso Formation
to the north.
Power[1958, 1959],who haspublishedthe mostdetailed descriptionof the CosoFormationto date, characterizedthe sequence as '... a heterogeneousassemblageof rock types
which records repeated volcanism, uplift, and erosion on
Haiwee Ridge, and almost continuousdepositionin Owens
Valley' [Power, 1958, p. Ill. Becausethe Coso as originally
definedby Schultz[1937] includesa variety of rocksthat reflect varied modes and environmentsof deposition, further
subdivision of this sequence would be desirable. Stinson
[1977a, b], who recentlymappedseveralfacies,delineatedthe
ñ
ñ
ñ
ñ
0.016
0.031
0.033
0.022
<0.1
0.088 ñ 0.038
0.56 ñ 0.24
<0.2
0.244 ñ 0.028
0.090 ñ 0.025
<0.3
ND•
0.99 ñ 0.20
*See Figure 8.
'•From Friedmanand Obradovich[1979].
•ND, not determined.
pyroclasticrocksmarking the silicicvent at the southend of
Haiwee Ridge as a separateunit.
Du.ffield and Bacon [1977, 1980] mapped an alluvial fan
facies of the Coso Formation in the easternpart of the volcanic field (Figure 2). Correlation of thesedepositswith the
Coso Formation is basedon interlayeringof the ~3-m.y.-old
rhyodaciteair-fall pumice(locality48, Figures2 and 3) and of
rhyolite pumicethat is petrographicallyidenticalto the rhyolite pumice in the silicic vent complex at the south end of
Haiwee Ridge and to the rhyolite pumice of the ~3-m.y.-old
ash-flow
SedimentaryRocks
0.057
0.072
0.101
0.044
0.065
0.055
0.065
0.070
0.110
0.150
0.140
0.260
0.260
0.260
0.250
0.290
0.370
0.360
0.580
tuff in the Coso Formation
on the west flank
of
Haiwee Ridge (locality 44, Figure 3). These alluvial fan depositsconsistof clastsof pre-Cenozoicbasementrocks.They
thickenfrom eastto westand are characterizedby gravel and
scatteredcobbleson the eastto gravel, cobbles,and scattered
bouldersas large as 2 m in the most westerlyexposuresat the
east margin of the horst on which the Pleistocenerhyolites
lies. Similar alluvial depositsand associatedrhyodacitepumice crop out locally in a seriesof en echelongrabenthat nearly
bisectthe Coso Range on the north from Coso Hot Springs;
thesedepositsare interpretedas coevalwith the Coso Formation. Apparently, an elevated terrane of pre-Cenozoic basement rocks between Haiwee Ridge and the east margin of
what later becamethe rhyolite field shed alluvial debris both
west and east contributingto the depositionof the Coso Formation.
Schultz[1937] reportedon vertebratefossilsof late Pliocene
and early Pleistoceneage in the Coso Formation, and later,
Everndenet al. [1964]reportedK-At agesof 2.3 m.y. for a tuffaceousbed in the Cosooverlyingthesefossiliferousbedsand
of 2.1 m.y. for a lava flow overlying the Coso on Haiwee
Ridge, about 13 km south of the fossil locality. As noted
above,we report a K-At age of 3.14 :t:0.15 m.y. for a rhyolite
ash-flow tuff (locality 44, Figure 3) intercalatedwith lacustrine beds of the Coso Formation
~ 10 km south of the fossil
locality and of 2.99 +_0.20 m.y. on rhyodaciteair-fall pumice
(localities43 and 48, Figure 3) near the top of the Cosoat the
southend of Haiwee Ridge. Exact stratigraphicrelationsbetween our dated samplesand the tuffaceousbed dated by
Everndenet al. [1964]are uncertain,but all samplesare probably from the upper part of the Coso Formation. Moreover,
Everndenet al. [1964, p. 177] reported only 3.21% K and 4%
radiogenicnøArin their specimen(sampleKA451) of 'biotite
2396
DUFFIELD
ET AL..' COSO GEOTHERMAL
from pumicechunksin water-laid tuff.' Theseanalyticaldata
indicate that the material was either altered or impure, evi-
dence castingconsiderabledoubt on the reportedage. Thus
the •3-m.y. age that we report suggests
that the mammalian
fossilswithin the Coso Formation are at least 0.7 m.y. older
than Everndenet al. [1964] thought.
At the northern end of Upper Centennial Flat, alluvial fan
depositsthat we correlatewith the CosoFormationare underlain by basaltdated at 3.56 +_0.10 m.y. (locality63, Figure 3).
North of our map area, in Lower Centennial Flat, this basalt
overlies lacustrine beds mapped as the Coso by $tinson
[1977b]and Hall and MacKevett[1962].
Sugarloaf,a rhyolite plug that intrudestuffaceousrocksof
the CosoFormation in the Keeler quadrangle[$tinson,1977b]
on the northwestflank of the Coso Range, has been dated at
5.7 + 0.2 m.y., and two rhyolite plugsthat showsimilar relations to the Coso 3 km northeastof Sugarloaf are dated at 6.0
+_0.1 m.y. [Baconet al., 1979b].A rhyolite dome and five as-
AREA
being eroded, generally by streams that are rejuvenated
throughoffsetalong normal faults.The apparentlyoldestfan
undergoingerosion formed >0.44 m.y. ago (average age of
samplesfrom localities30 and 31, Table 1), which is the approximate K-Ar age of overlying basalt [DuffieM and Smith,
1978]. The agesof other eroding fans are poorly known but
may form a crude continuumfrom at least 0.44 m.y. ago to
Holocene,reflectingactive tectonismthroughoutlate Quaternary time.
STRUCTURE
Evidence of late Cenozoic tectonismis widespreadin the
Coso Range. Outward dip of the Coso Formation on the west
and north flanks of the range indicatesconsiderablerelative
uplift of the mountainsafter depositionin Pliocenetime. Locally developed deformation of soft sediments,with fold axes
that parallel the strike of beds [Power, 1958], suggestssome
uplift before lithification, perhaps during and shortly after
sociated basalt flows that overlie lacustrine beds and underlie
sedimentation.Southwarddip of the somewhatyoungersediarkosicsedimentaryrocksassignedto the CosoFormation in mentary rocks along the south flank fo the range probably
the depressionbetweenthe CosoRange and Inyo Mountains recordsmountain uplift sincePleistocenetime.
have been dated at 5.9 +_0.1 m.y. and 5.3 +_0.2 to 6.0 _+0.2
Some uplift has been accommodatedby faulting. Littlem.y., respectively,by Giovannetti[1979]. These radiometric eroded fault scarpsare common in the volcanic field and in
agesdate the exposedCosoFormation as Pliocenein the area flanking basins [Roquemore,1980]. The faults may be submapped by Duffieldand Bacon[1980] and as old as Miocene divided into three sets:two with linear tracesand of regional
extent, and one with arcuate traces and of local extent, conon the northern flank of the Coso Range.
Sedimentaryrocksof the WhiteHills. A sequenceof gener- centrio about the center of the field of Pleistocenerhyolite
ally southdipping sedimentaryrocksthat is lithologicallysim- (Figure 2). Fault-controlled topography reflects principally
ilar to the Coso Formation is exposedlocally on the south the planar sets,althoughlocally valleys,saddles,and escarpflank of the Coso Range (Figure 2). The sequenceconsists ments form arcuate tracesalong curved faults. With the nopredominantly of siltstone and silty claystone interbedded table exceptionof most Pleistocenerhyolite and a few of the
with sandstone,cobble conglomerate, and tufa [yon Huene, young Pleistocenebasaltflows,all late Cenozoiclavas are bro1960]. About 70 m of sectionis exposedin fault scarpsalong ken by faults.Offsetsof alluviumand lava flowsof late Quathe southedgeof Airport Lake [Figure 2; Duffieldand Bacon, ternary age indicate youthful activity along some planar
1977, 1980], and at least 175 m of similar sedimentaryrocks faults, and the remarkably high level of local seismicity
was penetrated by a drill hole a few kilometers further south [Combsand Rotstein,1976; Weaverand Hill, 1978/1979; Wal(W. R. Moyle, Jr., oral communication,1975).The conglom- ter and Weaver, 1980] is evidence that the area remains teeeratic faciesis localized near and adjacent to hills of pre-Ce- tonically active.
nozoic basement rocks at the south end of the horst on which
the Pleistocenerhyolite lies and may representalluvial fans
that built outward from thesehills along the marginsof a lake
basin. Some coarseelasticdebris may also have been washed
in by a river that emptied into the basin near the presentexposuresof conglomerateduring part of Pleistocenetime [Duffield and Smith, 1978].
Contrastingassemblages
of mammalian fossilsindicate that
the sedimentaryrocks exposedin the White Hills are somewhat younger than lithologically similar rocks of the Coso
Formation [yon Huene, 1971]. A similar age relation is suggestedby well-rounded,reworkedsilicicpumiceof CosoFormation affinity within somebedsof the White Hills. Bedscontemaooraneous
with the CosoFormation probablyunderlie the
Airport Lake-White Hills area, where Zbur [1963] reported
~ 1070 m of basin fill above the basementcomplex,basedon
seismic refraction studies, and Plouff and Isherwood [1980]
have describedcorrespondinggravity and aeromagneticlows.
Other deposits. The youngestgroup of sedimentaryrocks
in the Coso Range includes alluvial fan deposits; fluvial
gravel, sand, and silt; wind-blown sand; and silt and clay in
local playas.Theserocksare perhapsmostnotablefor the features that they cover or obscure in older rocks but are also
sensitive indicators of relatively recent fault displacements.
Some alluvial fans are actively growing, whereas others are
Northwest-TrendingFaults
West-northwest- to northwest-trendingfaults bound the
south side of the Coso Range and are generally well developed within the southern and western parts of the range.
These faults are the local expressionof regional structureextending acrossthe Argus Range to the east [yonHuene, 1960]
into the Sierra Nevada to the west. Their profound influence
on upper crustal structure is evidenced by prominent, commonly closelyspacedcontoursof the samegeneralazimuth on
the gravity and aeromagneticmaps of Plouff and Isherwood
[1980] and on the telluric J value and apparent resistivity
maps of Jackson and O'Donnell [1980]. Linear traces across
rugged terrain indicate steepto vertical dip of the faults. Exposuresin pre-Cenozoicrocks are characterizedby zonesof
crushed material
several meters or more wide. Numerous
can-
yons as deep as 300 m that have been eroded along these
zonesin the southwesternpart of the Coso Range [DuffieM
and Bacon, 1977, 1980]impart a pronouncedcorrugatedridge
and canyonfabric to basementterraneof the southernpart of
the horston which the Pleistocenerhyolite lies. The rhyolite is
not offset by the faults, although northwestwardand northeastwardalinement of somedomessuggests
that the ascentof
rhyolitic magma was guided, at least locally, along faults in
the near-surfaceenvironment[Baconet al., 1980].
DUFFIELD
ET AL.: COSO GEOTHERMAL
West-northwest- to northwest-trendingfaults offset some
late Cenozoicvolcanicrocksby asmuch as severaltensof meters. The largest apparent displacementis seen in a 130-mhigh, north-northeast-facing
fault-line scarpin lacustrinebeds
cappedby Pleistocenebasaltalongthe southshoreof Airport
Lake (Figure 2). A northwest-trending
fault 10 km to the west
cutsPleistocenebasaltperhapsonly a few tensof thousandsof
yearsold [Du.ffieldand Smith, 1978].Some field evidencesuggeststhat west-northwest-trending
faults offsetthe Coso Formation at the south end of Haiwee Reservoir (Figure 2).
About 3 and 6 km to the north, abrupt west-northwest-trending segmentsof the otherwisegenerallynorth-trendingreservoir suggestadditional similar fault offset,althoughevidence
for interpreting the shapeof this reservoiras fault controlled
is only physiographic.
The apparentsenseof offsetalongwest-northwestto northwest faults is complex.For example,locally along the south
shore of Airport Lake, offset is apparently vertical. The en
echelonarrangementof faults ~ 10 km to the westis consistent
with right-lateral offset,and the shapeof Haiwee Reservoir,if
fault controlled,suggestsleft-lateral offset. Locally, contacts
of Mesozoicplutons underlyingthe rhyolite field show evidence of left-lateral separation.Von Huene [1960] reported
both left- and right-lateral strike-slipdisplacementsin Mesozoic plutons of the Argus Range; he also noted a dominant
northwest-trendingstructuralgrain defined by foliations in
Mesozoic plutons and older metamorphicrocks, the orientation of Mesozoic dikes, and the 'faults themselvesin the Coso
AREA
2397
to moderately dipping fault planes. Rare well-exposedfault
planesdip ~65o-70ø and exhibitdip-slipstriations.Apparent
verticaldisplacements
on individualfaultsrangefrom ~650 m
in basementrocks,through many tens of metersin Pliocene
volcanic rocks,to severalmetersor lessin Quaternary basalt
and alluvium.
First-motion
solutions for some recent earth-
quakesin the Coso Range indicatenormal displacementon
steeply dipping north- to northeast-trendingfaults [Walter
and Weaver, 1980].
Arcuate
Faults
Arcuate faults are presentin the northernand northeastern
(?) parts of the volcanicfield (Figure 2; Du.ffield and Bacon
[1977, 1980]). The physiographicexpressionof this set of
faults,as a group,is considerablylesspronouncedthan that of
the sets of linear faults, but arcuate traces in basement rocks
and Pliocene basalt are evident locally. Some cinder cones
that mark vents for Pliocene basalt lie astride arcuate faults.
Tracesof arcuatefaults are approximatelyconcentricabout
the geographiccenter of the field of Pleistocenerhyolite.
Where determinable, fault dip is 65ø-70ø inward toward the
focal area, and offsetis generallyparallel to dip. In most instances,however, the direction and amount of offset are ob-
scure,and the largestapparentoffseton a singlefracture is 30
m, withthe blocktowardthefocalareaupf•ul•ed.
In the north central part of the volcanicfield, arcuatefaults
cut Pliocene volcanic rocks across a band at least 6 km wide.
The distribution and orientation of intensely jointed to
and adjacentranges.Generallynorthwest-trending
faultsmay shearedzonesin basementrocks in adjoining areas without
have existedsinceMesozoictime and respondedto various late Cenozoic lavas suggestthat the band of arcuate faults
stress fields since then. First-motion solutions for recent earthmay extend outward severalkilometersfurther toward the
quakesin the CosoRangeindicateright-lateralstrike-slipon margin of the range. These shearedzones in the basement
northwest-trendingfaults [ Walterand Weaver,1980].
rocks are typically <1 m wide and vary from a multitude of
more or lessparallel, steeplydipping joints with or without
North-TrendingFaults
quartz veins, to brecciatedand slickensidedbasementrock
North- to north-northeast-trendingnormal faults are well and quartz veins.The zonesand componentjoints dip steeply,
developedin much of the CosoRange and give the area a ba- generally toward the field of Pleistocenerhyolite, and are
sin and range character. Linear contours of this trend and commonlydistinguishedby reddishand ocherousalteration
steepgradientsacrossit on aeromagneticmaps [Plouff and minerals;offsetgenerallycannotbe demonstrated,but steeply
Isherwood,1980]and on telluricJ value and apparentresistiv- dipping slickensidesindicate somedisplacement.Intervening
ity maps [Jacksonand O'Donnell, 1980] reflect blocks dis- areasconsistof massiveunalteredbasementrock with widely
placedby faults of this set.Pleistocene
rhyolite capsa horst spacedjoints. Individual shearzonescannot be traced more
that is boundedby faultsof thisset(Figure2), and the north- than a few tensof metersalong strike,but in the northern part
ward continuationof the fault zone at the east edge of this of the volcanicfield the zonescollectivelydefine a concentric
horstformsthe westsideof a seriesof en echelongrabenthat pattern through ~90 ø of arc focusedon the field of Pleistodividethe CosoRangeinto two terranesof roughlyequal size. cene rhyolite, much like the arcuate faults that cut Pliocene
Haiwee and adjacentnorth-trendingridgesare fault bounded lavas.The shearzonescut contactsbetweenMesozoicplutons
on at leastone side,and normalfault scarpsin many Pleisto- and may representa relatively subtle,pervasiveexpressionof
cenebasaltflowsat the southmarginof the rangeare draped the stressfield that gave rise to the better developedarcuate
by younger unfaulted flows that give vivid evidenceof the faults.
near contemporaneityof faulting and volcanism within the
Some contourson the aeromagnetic[Plouffand Isherwood,
volcanic field.
1980] and apparent resistivitymaps [Jacksonand O'Donnell,
Sinuousbut generally north-trendingnormal faults form a 1980]appearto be concentricwith arcuatefaults in the northgiant staircasesteppingdown to the westin Pliocenerocksof ern part of the range. The principal featureson these maps
the easternpart of the volcanicfield (Figures2 and 6); cu- and on the gravity map [PlouffandIsherwood,1980],however,
mulative vertical offset of at least 650 m occurs across a 9-kmare nearly perpendicularto tracesof arcuatefaults;this is eswide zone. Individual scarpsare as high as 120 m, and most peciallyapparentin the northwesternCosoRange.
A 19-km-longfault zone with an arcuatetrace cutsthe adface west;offsetappearsto decreasenorthwardand may die
out in the vicinity of Silver Peak. To the souththe fault zone jacent part of the Sierra Nevada (Figure 2; Du.ffield [1975];
extendsbeneathQuaternaryalluviumthat may maskconsid- Du.ffieldand Bacon[1977, 1980]).Faultsin this zone dip 65øerableoffsetin the underlyingrocks.
70ø toward the field of Pleistocenerhyolite at Coso and are
Steepplanar scarpsin lava flowsand slightundulationsin characterizedby highly shearedrock. The fault zone is extracesacrossruggedterrain indicatenormal offseton steeply pressedphysiographicallyas saddlesover ridgesand as can-
2398
DUFFIELD
ET AL.: COSO GEOTHERMAL
AREA
liceoussinterand nearby travertine are uncertain,but the sinter, and perhaps some travertine, was deposited before
eruptionof the basalt~0.25 m.y. ago, perhapsduringthe period when severalof the older rhyolite domeswere emplaced.
Fumaroles at Devils Kitchen and the adjacent Nicol area
(Figure 8) are localizedin part of the pyroclasticexplosion
ring surroundinga rhyolite dome that yields a K-Ar age of
related to the Sierra Nevada frontal fault zone.
0.587 :t: 0.18 m.y. (locality 53, Table 1). The explosiondebris
Other late Cenozoicarcuatefaults along the eastfaceof the from the vent of this dome, the overlying rhyolitic tephra
Sierra Nevada farther to the north are closelyassociatedwith from nearby younger vents, and some adjacent basement
contemporaneous
volcanism.The Long Valley caldera fault rocks are thoroughly altered, mostly to amorphous silica;
and possibleprecursorring faults formed acrossa preexisting crustsand fracture fillings of numerous secondaryminerals,
Sierran escarpment[Bailey et al., 1976].Immediately north of including suffates,sulfur, and cinnabar, have been and are
Long Valley, a 40-km-diametersystemof ring faults, along still being deposited[Austinand Pringle, 1970].Altered rocks
part of which rhyolite domesand flowsof the Mono Craters of these fumarolic areas were once mined for mercury [Ross
were emplaced,was similarly formed acrossthe Sierran es- and Yates,1943].Minor steamis weakly emitted from the bottom of an explosioncrater that is partly filled by a rhyolite
carpment[Kistler, 1966].
Preliminary fieldwork by Du•eld [1975] suggestedthat the dome (locality 18, Figure 8) ~1 km northwest of Devils
systemof arcuate faults in the Coso Range definesa ring Kitchen and from an abandoned shallow well ~3 km west
structure, but our subsequentmapping has failed to sub- (Figure 8).
Measurementsof heat flow further outline the geothermal
stantiatethe 360ø continuitysurmisedfrom his earlier study.
Arcuate faults only partly circumscribethe volcanicfield and system.Combs[1980]calculatedheat flow valuesthat range
are best developed and most easily recognizedwhere they from a backgroundlevel of ~2 HFU, typical of much of the
Great Basin [Lachenbruchand Sass, 1978], to a maximum
trend at high anglesto the strike of planar faults.
of 23 HFU, on the basisof measurementsof temperaturegraTHE GEOTHERMAL
SYSTEM
dients in drill holes48-134 m deep. Combs[ 1980] concluded
Surficial expressionof an active geothermalsystemis con- that _<4 HFU representsa primarily conductive regime,
centrated within and immediately east of the central part of whereasa higher heat flow reflectsshallow convection.Conthe field of Pleistocenerhyolite (Figure 8). Most present-day toured heat flow for rocksto 65-m depth forms a singleclosed
activity is confined to an east-northeast-trending
zone be- high, with a maximum along the east margin of the field of
tween SugarloafMountain and Coso Hot Springs.This zone rhyolite (Figure 8); the 10-HFU contour enclosesan area of
is characterizedgeophysicallyby low telluric J value [Jackson ~41 km2,and the 5-HFU contour,~75 km2.
Chloride-rich
water was obtained from the wellhead of a
and O'Donnell, 1980]and low resistivity[Fox, 1978b]and has
been mapped as a fault by Hulen [1978]. The east-northeast 1477-m-deephole drilled entirely in pre-Cenozoicrocks[Galtrend is parallel to someleft-lateral,strike-slip,fault-planeso- braith, 1978]near the centerof the heat flow anomaly;a maximum downhole temperature of ~190øC was encountered
lutionsreportedby Walter and Weaver[1980].
One principal thermal area, Coso Hot Springs,consistsof [Fournier et al., 1980]. Chemical analysesof thermal water
fumarolesand intermittently active thermal springslocalized from this hole about 2 days before the termination of drilling
along a north-northeast-trendingzone of en echelon normal in early December 1977 are very similar to analysesof therfaults that offset alluvium at the east edge of the horst on mal water from the 125-m-deephole drilled in 1967 at Coso
which the rhyolite lies. Although pools of boiling water are Hot Springs~3.2 km to the east[Fournieret al., 1980].Water
present,surfaceflow is rare and generallyoccursonly after lo- from each hole, which contains ~3000 ppm C1, indicatesa
cal precipitation.Several abandonedwells along the fault hot-water geothermalsystemand may representa mixture of
scarpweakly emit steamthrough badly corrodedcasings.A water from a 205ø-240øC relatively deep aquifer with cool
125-m-deepwell, drilled in 1967 and still in excellentcondi- near-surfacewater [Fournieret al., 1980].
Before the 1477-m hole was drilled and implicationsof the
tion, bottomed in 142øC water containing ~3000 ppm C1
[Austinand Pringle,1970].Suchchloride-richthermalwater is chemistryof thermal water from this hole were known, Combs
generally characteristicof hot-water geothermal systems and Rotstein[1976], on the basisof the characteristicsof local
microearthquakes,
calculatedan anomalouslylow Poissonra[White et al., 1971].
About 3 km to the southalong the samefault zone a feeble tio for pre-Cenozoic basement rocks beneath the field of
fumarole producessteam in the abandoned workings of a Pleistocenerhyolite and concluded that this low ratio was
mercury prospect.Laminated siliceoussinter exposedin and probably due to steam-filledfracturesin the basementrocks,
near this prospectprovidesevidenceof formerly active ther- i.e. a vapor-dominatedgeothermalsystem.At about the same
mal springsand/or pools characteristicof high-temperature time, Jacksonet al. [1977] postulateda vapor-dominatedgeohot-watergeothermalsystems[ Whiteet al., 1971].A K-Ar age thermal systembeneaththe central part of the field of Pleistoof 0.234 :t:0.022m.y. (locality 36, Table 1) for overlyingbasalt cene rhyolite on the basis of dc resistivity measurements.
unaffectedby thermal springactivity showsthat depositionof Jackson and O'Donnell [1980] reinterpreted these resistivity
measurementsto reflect the possiblepresenceof a hot-water
the sinteroccurredby at leastthat time.
Elsewhere to the south along this same fault zone, trav- system,as evidencedby the chloride-richthermal water from
ertine forms local depositsboth as a network of veins filling the 1477-m drill hole. The apparently low Poissonratio refracturesin basementrocksat the baseof the upfaulted block ported by Combsand Rotstein[1976]may reflectintensefracand as beddedaccumulationsin alluvium adjacentto the fault turing of the basementrocks beneath the rhyolite field [Dufzone, an observationthat suggestsformerly widespreadther- fieM and Bacon, 1977]or other causesnot yet recognized.
The principal crustal heat source driving the geothermal
mal spring activity. The ages of depositionof both .the si-
yonsasdeepas 350 m that traceout ~60 ø of arc from north to
south before merging with a canyon eroded along a major
northwest-trendingfault. The trace of the arcuatefault zone is
entirely within pre-Cenozoicrocksand partly coveredby unfaulted late Quaternary deposits.Either this fault zone is genetically linked with the arcuate systemof the Coso range or
it may be explained as a local anomalousstructure,possibly
DUFFIELD
ET AL.: COSO GEOTHERMAL
systemis inferred to be a body of silicic magma from which
the Pleistocenerhyolite domesand flowswere fed and whose
top ties at least 8 km beneaththe central part of the rhyolite
field [Bacon et al., 1980]. Large magma-relatedgeothermal
systems,suchas thoseat YellowstonePark, Wyoming [Eaton
et al., 1975], The Geysers, California [lsherwood, 1976;
Steeplesand Iyer, 1976], and Long Valley, California [Bailey
et al., 1976],are characterizedby substantialnegative gravity
anomaliesand delayed arrivals of P waves from teleseisms,
features that are interpreted to reflect underlying crustal
magmaand/or anomalouslyhot rock. Geophysicalexpression
of comparablemagnitude does not exist for the magma inferred to underlie the Pleistocenerhyolite of the CosoRange,
perhapsbecausethe small sizeand midcrustaldepth of the inferred magma reservoirrender its detectionbeyond the resolution of suchmeasurements[Baconet al., 1980].Gravity data
are permissiveof sucha magmaticsystem,sinceits expression
AREA
2399
glacialstages.The presentclimatesustainsno thermalsprings
other than an ephemeralflow at CosoHot Springsfollowing
local precipitation.
DISCUSSION
The emergenceof the CosoRange and the onsetof late Cenozoicvolcanismthere appearto have beennearly contemporaneous,althoughinitial volcanismprobably predatedsubstantial relative uplift of the range. Pliocenebasalt,the oldest
volcanicrock of the area, was emplacedover basementrocks
that apparently formed terrain of gentle to moderate relief
•4.0--3.6 m.y. ago. Many of these lava flows exhibit uniform
thicknessover distancesof severalkilometersand now cap
unfaultedridgesand floor downfaultedvalleys.
By •3 m.y. ago the CosoRange clearly formed a positive
topographicfeature of moderateto substantialrelief, at which
time it was flanked by basinswhere lacustrine and alluvial
wouldbe masked-by
localeffectsof faulteduppercrustal sediments were accumulating. Power [1959] showed that
blocks[Plouffand lsherwood,1980].Seismicrefractionand lo- Haiwee Ridge was uplifted along north-northeast-trending
cal earthquakestudiesshowthat the upper crustbehavesin a normal faultswhile the CosoFormationwasbeingdeposited
brittle manner to depthsof at least8 km, a value typical of the on and adjacentto its westflank. Contemporaneously,
coarse
region [Walter and Weaver, 1980]. Application of inversion alluvial debris was depositedover part of the developing
techniquesto teleseismic
data hasoutlineda zoneof relatively Pliocenevolcanicfield to the eastand in a north-trendingselow velocity at depthsof 5 to at least 20 km, centereda few ries of en echelongrabenthat approximatelybisectthe range
kilometers southeastof Sugarloaf Mountain [Reasenberget (Figure 2). The existenceof locallyruggedterrain •3 m.y. ago
al., 1980]. The low-velocity zone is 5-10 km in diameter near is also recorded in the rhyodacite air-fall pumice that then
its top, becomingelongatedto the north-northeastand south- blanketedthe range and slid from slopesexceedingthe angle
southwestwith increasingdepth. A correspondingzone of of repose[Duffieldet al., :1979].Relative uplift of the range
high attenuationbeneath 10 km was obtainedby inversionof later than 3 m.y. ago hasexposedPlioceneand Pleistocenedeteleseismicdata by Youngand Ward [1980]. The anomalous positsin flanking basins.
body may reflect the presenceof magma or partially melted
A general concordanceof the crestof the adjacent Sierra
rock. Alternatively, a major crustal discontinuityalong the Nevada, the Argus Range, and the highestpart of the Coso
west side of the seriesof graben that bisect the range may Range suggestsa regionally extensive,gently rolling terrain
causethe geophysicalanomalies,sincethe zone more nearly prior to formationof the basinsthat now separatethe ranges.
coincideswith this feature than the axis of the rhyolite field. Possibly,this terrain extendedat least90 km eastto the Death
Nonetheless,from geologicevidence,the CosoRange is con- Valley area, about which Hildreth [1976, p. 22] statedthat Ksidereda convectivegeothermalsystemdriven by heat from Ar-dated '... gently-tiltedbasalts,distributedaround the valan underlying long-lived magma reservoirthat may still con- ley's marginsbut depositedon surfacesof only moderateretain molten or partly molten rock.
lief, suggestthat the main collapseof Death Valley took place
Surfaceexpressionof heat from the inferred magma reser- less than four million years ago.' Late Cenozoic mafic lava
voir is offset 1-2 km east of its apparentcenter;the area of flowsin the Argusand Slaterangesand Inyo Mountainsmay
highestheat flow and the principal fumaroleslie at the east recorda similargeologichistory,althoughwith rare exception
margin of the field of Pleistocenerhyolite (Figure 8). More- [e.g., Ross, 1970; Hall, 1971] radiometric agesof these lavas
over, zonesof high heat flow, anomalouselectricalresistivity, have not been determined. Numerous K-Ar dates of 5.3-6.0
and seismicnoiseextendnorth-northeast,parallel to the strike m.y. [Baconet al., 1979b;Giovannetti,1979]for volcanicrocks
of normal faults of basin and range affinity [Bacon et al., intercalatedwith and intrudingsedimentaryrocksassignedto
1980].Apparently thesefaults, especiallythosein the zone at the Coso Formation [Hall and MacKeyerr, 1962; Stinson,
the eastedge of the horston which the Pleistocenerhyolite 1977b]at the north end of the Coso Range and betweenthe
lies, are preferred channelsfor convectivegeothermalfluids CosoRange and the Inyo Mountains indicate that a basinwas
[Combs,1980].
receivingsedimentthere at leastas early as 6 m.y. ago. HowIt seemslikely that a geothermalsystemhas existedin the ever, coarseclastic rocks that might reflect high local relief
CosoRangeduringmuchor all of the time of emplacementof due to faultingappearto be confinedto sectionsthat are probthePleistocene
rhyolite;theoldestdomeis • 1 m.y.old,and ably not older than about 3 m.y.
Christensen[1966] reviewed evidence for the late Cenozoic
mostwereemplacedlater than ~0.3 m.y. ago.During the period of rhyolite emplacementthe local water table probably structural evolution of the Sierra Nevada and concluded that
fluctuatedrepeatedlyin responseto changingclimate, as re- upfaultingalong the east face of the range occurredprincicorded,for example,by glacial depositsof the Sierra Nevada pally during the past 3 m.y. Dalrymple[1964]reportedmany
(see'Summary'by Sheridan[1971])and by lacustrinedeposits mafic lava flows3.6-3 m.y. in agein the Long Valley area and
in nearby Searles Lake [Smith, 1968; Duffield and Smith, in the Sierra Nevada from Long Valley southwardto the vi1978].Accordingly,we speculatethat hot springswere rela- cinity of the Coso Range. Smith [1977; and oral communicativelyactiveduringwet periodsin contrastto the presentsitu- tion, 1978] reported an age of •3 m.y. for the oldest sedimenation, and that the sinterand travertinealongthe fault zone tary rocks of SearlesLake Basin. Similarly, Hildreth [1976]
at the eastedgeof the horston which the rhyolite lieswere de- showedthat extrapolationof the late Quaternary sedimentathat
posited during one or more pluvial periods associatedwith tion rate as definedby •4Cagesin Death Valley suggests
2400
DUFFIELD
ET AL.: COSO GEOTHERMAL AREA
rocks at the base of the sedimentarysectionin the valley are
-3 m.y. old. Thus severallines of evidencefrom scatteredlocalitiessuggestthat the CosoRange and adjacentpartsof the
Great Basin and Sierra Nevada were characterizedby the onsetof nearly contemporaneous
mafic volcanismand basin and
range faulting 4-3 m.y. ago.We view theseeventsas related to
crustal extension,which was the tectonicregime characteristic
of the Great Basinin late Cenozoictime [Wright, 1976].
Like nearby ranges of the Great Basin and the east face
of the Sierra Nevada, the Coso Range may be largely
shapedby north-trendingnormal faults that reflectthis crustal
extension; but unlike neighboring ranges, that are dearly
elongateparallel to the major zonesof normal faults, the outline of the Coso Range is poorly defined.This outline may in
part reflect a locally anomalousorientation of the regional
stressfield, perhaps becausethe Coso Range lies near the
fault-boundedjunction of three geologicprovinces.The outline may also in part reflectlocally developedstressesassociated with relativelyintensemagmatismbeneaththe range.
The step-faulted terrane of Pliocene rocks in the eastern
part of the volcanic field (Figure 6) is bounded on the north
and northeastby faults that may belong to the arcuate set.
Duffield [1975] interpretedthe structureof this terrane as analogousto that of the crust subsidedover a Hawaiian lava lake,
on the assumptionthat a crustal magma reservoirunderlies
the volcanicfield. Subsequentfield mapping[Duffieldand Ba-
con,1977,1980]and radiometricdating(Table 1;Figure3) indicated a more complex volcanologicand structuralhistory
than was earlier envisioned,and the contemporaneityof step
faulting with an underlying crustal magma reservoircannot
be demonstrated.Field relationsand K-At agesindicate that
thesestep faults formed since3-2 m.y. ago, the approximate
mation to the east, is tens of metersbelow its earlier level, as
indicatedby reconstruction
of its arealtopographyat the time
of fan deposition.
The Coso Range is one of many late Cenozoic volcanic
fields in the basin and range provincethat Christiansenand
Lipman [1972] describedas 'fundamentallybasaltic'in nam-
ing assemblages
of volcanicrocksthat may be characteristic
of regionsundergoingtectonicextension.The Pleistocenevolcanic rocksof the range are a classicexampleof the bimodal
basalt-rhyoliteassociationof fundamentallybasaltic suites.
The Pliocene volcanic rocks include basalt, andesite, dacite,
rhyodacite,and rhyolite and may or may not representthe
category of fundamentally basaltic suitesof rocks described
by Christiansenand Lipman [1972] as 'differentiatedalkalic
basalt.' The association of both Pliocene and Pleistocene vol-
canismin the CosoRange with crustalextensionaltectonicsis
indicatedby overlappingperiodsof eruptionand offsetalong
north-trending normal faults. First-motion solutionsfor recent earthquakes[Walter and Weaver,1980] and a study of
late Quaternarydeformation[Roquemore,1980]in the range
indicatecontinuationof thistectonicregime.
Lachenbruchand $ass [1978] and Lachenbruch[1978/1979]
correlated heat flow, crustal extension, and volcanism in the
Great Basin to show that typical heat flow there may result
from a crustal extensionrate of 1% per m.y., along with upward transportof thermal energyby intrusionto basalt from
the asthenosphereinto the lithosphere.According to their
analysis,higher heat flow impliesa higher rate of crustalextension, so that the heat flow anomalies associated with vol-
caniccentersin the CosoRangeor Long Valley imply extension rates of •10% per m.y. Weaverand Hill [1978/1979]
identifiedthe Coso area as one of unusuallyrapid extension.
age of the youngestoffsetrocks.The magma reservoirinferred The rate of crustalextensionin the CosoRange is unknown,
to underlie the field of Pleistocenerhyolite is considerably but Wright [1976] reported that late Cenozoiccrustal extensmallerin plan view [Baconet al., 1980]than envisionedear- sionalstrain in the southwesternpart of the Great Basin may
lief by Duffield [1975]. If the step faults formed over a large be as great as 50%, at least 5 times that of the northern and
crustalmagmareservoirthat predatedthe Pleistocenesystem, central parts of the Great Basin. Crustal extensionin the Coso
this magma has since cooled and solidified to become in- Range reflectedby normal faulting has been occurringfor at
distinguishable
by geophysicalmeasurementfrom the pre-Ce- leastthe past 3 m.y.
nozoic basement rocks.
In accordwith the interpretationsof Lachenbruchand $ass
The step-faultedterrane is bounded on the west and south [1978]and Lachenbruch[1978/1979],we infer that the upward
by major fault zonesthat intersectin the vicinity of the south- transportof thermal energyby emplacementof basaltmagma
westernpart of Airport Lake (Figure 2), adjacentto the south- into the lithospherecausesthe anomalouslyhigh heat flow in
eastcomer of the horst on which the Pleistocenerhyolite lies. the CosoRange. Someof this magmareachesthe surface,and
These fault zones may effectivelyuncouple the step-faulted someremains lodged within the crust,where it coolsand reterranefrom adjacentareasto the westand southand permit leasesheat that may partly melt adjacentcrustalrocksto form
downwarpingand downfaultingtoward the westand southin silicic magma. The longevity of the basaltic component in
responseto east-west crustal extension (Figure 9). Similar sucha magmaticsystemis dependentupon continuedcrustal
structuresexist nearby in the basinand rangeprovince,e.g., extension,assumingthat this extensionfavorsupward intruSaline Valley. Whatever their origin, the step faults of the sion.The longevityof the siliciccomponentis similarlyconCosoRange seemto decreasein throw to the north and to ter- trolled but dependsmore directly upon the continuedsupply
minate in the vicinity of Silver Peak againstfaults that may of heat through emplacementof basaltic magma into the
belongto the arcuateset or the northwest-trending
set (Fig- crust. In the Coso Range, Pleistocenebasalt, including the
ures2 and 6), featuresthat suggestsomecontrolon the extent youngestlavasof the entirevolcanicfield,waseruptedperiodof their developmentby one or both of thesesystems.
ically at and near the marginsof the rhyolitefield throughout
Partly fault-bounded, closed drainage basins within the the past 1.1 m.y.; this long-lived activity is inferred to reflect
field of Pleistocenerhyolite (Figure 8) suggestthat somefault the subsurfaceemplacementof basalticmagma in sufficient
offsetmay be related to the emplacementof this rhyolite. quantitiesto sustaina silicicmagma reservoirduring at least
Throw on the order of at least several meters is evident where
the last one third of this time. Petrologicand major-element
the upstream part of a south-southeast-trending
stream is chemicalhomogeneity,togetherwith an apparentlysystemdammed to form a basin partly filled with recentsedimentat atic minor-elementvariationin the Pleistocene
rhyolite, sugthe eastcentralmargin of the rhyolite field. Much of the base- geststhat a singlesilicicmagma reservoirexistedduring the
ment terrane beneath the rhyolite field, the terrane that was emplacementof most extrusions[Baconet al., 1979a].
the sourcefor alluvial fan depositsof the adjacentCosoForWe infer a similar origin for someof the Pliocenevolcanic
DUFFIELD ET AL..' COSO GEOTHERMAL AREA
2401
o
1
Fig. 9. Generalizedblock diagramillustratingstructureof step-faultedterrane.Major northwest-trending
fault zone
throughAirportLake area(seeFigure2) formssouthedgeof diagram,andpart of horston whichPleistocene
rhyolitelies
forms westedge.Only pre-Cenozoi½basementcomplexand Pliocenevolcanicrocksare shown.Step-faultedterrane interpreted to be downwarpedand downfaultedin responseto late Cenozoiccrustalextension(representedby large arrows)
anduncoupling
of terranesacross
majorfaultzonesalongAirportLakeandeastsideof horst.Verticalexaggeration,
8X.
rocksbecausebasaltwaseruptedabundantlyduring emplacement of most of the Pliocene sequence.The presenceof a
crustal,possiblychemicallyzoned,silicicmagma reservoirbeneath the southernpart of Haiwee Ridge -3 m.y. ago is suggestedby the rhyoliticto rhyodaciticpyroclasticrocksof that
age on the ridge. The Pliocenevolcanicsequence,however,
exhibits considerably greater lithologic variation than the
Pleistocenesequence,and somePliocenelavasof intermediate
compositionmay reflect fractionationin local magma reservoirs, crustal contaminationof basalticmagma, and magma
mixing. We are currently conductingchemicaland mineralogicstudiesof thesetopics.
Age-volume-compositionrelations (Figure 4) suggestthat
some contrasts between the Pliocene and Pleistocene volcanic
sequencesreflect distinct crustal magmatic events. On the
premisethat volcanicactivityservesas a reasonablemeasure
of generalcrustalmagmatism,includingcoevalplutonsbeneath volcanicfields,a •2-m.y. interval of almostno magmatism separatedPliocene and Pleistoceneeruptive episodesin
the Coso Range. This apparent lull is sufficientlylong that
possiblecrustalmagmabodiesassociatedwith Pliocenevolcanicactivity would have cooledwell belowtheir solidustemperaturesbeforethe onsetof Pleistoceneactivity [Nortonand
Knight, 1977].Accordingto the generalviewsof Lachenbruch
and Sass[1978] and Lachenbruch[1978/1979], this quiet period may reflecta period of little or no crustalextension.We
know of no independentevidencefor such timing of crustal
extensionin the Coso Range, although existing evidence is
permissiveof thisinterpretation.
The key role played by intrusionof basalticmagmain gen-
2402
DUFFIELD
ET AL.: Coso
E
SIERRA
NEVADA
SUGARLOAF
;.•,
GEOTHERMAL
AREA
Austin, C. F., W. H. Austin, and G. W. Leonard, Geothermal science
and technology:A national program, Tech. Ser. 45-029-72, 95 pp.,
China Lake Nav. Ordnance Test Sta., China Lake, Calif., 1971.
Babcock, J. W., Volcanic rocks in the Coso Mountains, California,
Geol. Soc.Amer. Abstr.Progranu, 7, 291-292, 1975.
Babcock,J. W., The late CenozoicCoso volcanic field, Inyo County,
California, Ph.D. thesis,213 pp., Univ. of Calif., Santa Barbara,
1977.
Babcock,J. W., and W. S. Wise, Petrologyof contemporaneousQuaternarybasaltand rhyolitein the CosoMountains,California, Geol.
•';l'•extension Soc.Amer. Abstr.Programs,5, 6, 1973.
Bacon, C. R., and W. A. Dufiield, Phenocrystmineralogyof Pleistocene rhyolitesand heat contentof the Coso Range geothermalsystem, California, Geol. Soc. Amer. Abstr. Programs, 8, 761-762,
l\J I l- I,l•_XI
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....... "- -,,,
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'
'"'"'""'
"''
1"',',
"- "'''';,' "-,'
•l "'••-'-'• Rhyolite
'•,
•. L•
;I "_"_•_•'_
• ¾ •,
r•ers
voir
1976.
,•1 ':?/l'-;,J';
':>/c:,.:,?,'>'/:F/..IFc,?:'i:
:'.:,1: : ',':
J/x'-/••
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,'
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,
Bacon,C. R., R. Macdonald,and J. Metz, Petrogenesis
of the Quaternary rhyolitesof the CosoRangegeothermalarea,California, Geol.
Soc.Amer. Abstr. Programs,11, in press,1979a.
Bacon, C. R., D. M. Giovannetti, W. A. Dufiield, and G. B. Dalrymple, New constraintson the age of the Coso Formation, Inyo
County, California, Geol. Soc. Amer. Abstr. Programs,11(3), 67,
1979b.
Bacon, C. R., W. A. Dufiield, and K. Nakamura, Distribution of Quaternary rhyolite domesof the CosoRange, California: Implications
for extent of the geothermalanomaly, J. Geophys.Res.,85, this issue, 1980.
Bailey, R. A., G. B. Dalrymple, and M. A. Lanphere, Volcanism,
structure, and geochronology of Long Valley Caldera, Mono
County, California, J. Geophys.
Res.,81, 725-744, 1976.
Chen, J. H., and J. G. Moore, Late JurassicIndependencedike swarm
bly plutons coeval with Pliocenevolcanic rocks;lessdensepattern
in easternCalifornia, Geology,7, 129-133, 1979.
wherepartial meltinginferredto havetakenplaceduringQuaternary.
Chesterman,C. W., Pumice, pumicite, and volcanic cindersin CaliShaded area, basalt, largely Pliocene on eastern third of section,
fornia, Calif. Div. Mines Geol.Bull., 174, 97, 1956.
mainly Pleistoceneelsewhere(seeFigure 2). Stippledarea, Pliocene
Christensen, M. N., Late Cenozoic crustal movements in the Sierra
and Pleistocenesedimentary rocks. Unpatterned area, Pleistocene
Nevada of California, Geol. Soc. Amer. Bull., 77, 163-182, 1966.
rhyolite.Thin dashedverticallines,inferredPliocenedikes;thin solid
Christiansen,
R. L., and P. W. Lipman, Cenozoic volcanism and
verticallines,inferred Pleistocenedikes;and heavy lines,faults.
plate-tectonicevolutionof the WesternUnited States,II, Late Cenozoic,Phil. Trans.Roy. Soc.London,Ser. A, 271, 249-284, 1972.
Fig. 10. Schematic
east-west
crosssectionof CosoRangethrough
SugarloafMountain;horizontalscaleequalto verticalscalewith some
exaggeration
at surface.Patternedarea,pre-Cenozoic
rocksandpossi-
erating and sustainingbodiesof silici½magma in the crustwas
recognizedby Christiansen
and Lipman [1972],Smithand Shaw
[1975], Eichelbergerand Gooley [1977], and Christiansenand
McKee [1978] and is implicit in the analysisof basin and
range heat flow by Lachenbruchand Sass[1978] and Lachenbruch [1978/1979]. Our interpretation of the Coso volcanic
field as such a system,representingcrustalextensionand attendant intrusion of mantle-derivedbasalticmagma into the
crust,is summarizedschematicallyin Figure 10. We conclude
that the Coso geothermal systemis sustainedby heat from a
crustalsili½i½
magma reservoirthat has existedfor at least the
last 0.3 m.y. and perhapsas much as 1 m.y., the approximate
age of the oldest Pleistocenerhyolite; we speculatethat this
systemmay still be expandingthroughpartial melting of the
crust by basalticmagma emplacedfrom the upper mantle in
responseto continuingcrustalextension.
Acknowledgments.Much of the field area discussedin this paper
lies within the China Lake Naval Weapons Center, and we owe considerablethanks to personnelof the naval center,especiallyCarl F.
Austin, for logisticalsupport and for accessto carry out our studies.
Julie Donnelly, Wes Hildreth, Keith Howard, Marvin Lanphere,Patrick Muffler, Glenn Roquemore,and an anonymousreviewerfor JGR
made many suggestionsthat helped improve earlier versionsof the
manuscript.Discussionswith authorsof other papersin this special
Cosoissueof JGR were helpful in interpretingmany of the resultsof
our study.
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