1000 0 1000 2000 3000 4000 5000 6000 7000 FEET

1 .5 0 1 KILOMETER

SCALE 1:24 000

CONTOUR INTERVAL 40 FEET

NATIONAL GEODETIC VERTICAL DATUM OF 1929

1 1/2 0 1 MILE

12'30" 10' 122°07'30"

12'30" 10' 122°07'30"

37°25'

37°30'

122°15'

122°15'

37°25'

27'30"

37°30'

27'30"

GEOLOGIC SUMMARY

The northwest-trending Chocolate Mountains are situated along the northeastern margin 

of the southern Salton Trough (figs. 1, 2a). The Chocolate Mountain Aerial Gunnery Range 

occupies most of the 75-km-long part of the Chocolate Mountains that lies between Salt 

Creek to the north and California State Highway 78 to the south. Mapping studies in the 

Chocolate Mountains within the gunnery range are few (Dillon, 1976; Powell and others, 

1994; Powell, 1995; Powell and Fleck, 2008) and this study was conducted in cooperation 

with the U.S. Navy (Naval Facilities Engineering Command Southwest, San Diego, California) and U.S. Marine Corps (Range Management Department, Marine Corps Air Station, 

Yuma, Arizona).

Crystalline basement rocks in the Chocolate Mountains range in age from early Proterozoic to middle Cenozoic. Early and middle Proterozoic metamorphosed sedimentary and 

plutonic rocks include sillimanite-biotite-quartz feldspar gneiss, layered biotite-quartz-feldspar gneiss, biotite-quartz-feldspar augen gneiss, and largely undeformed late Proterozoic 

anorthosite and syenite. These rock types, which crop out as dispersed domains in the 

Chocolate Mountains, are remnants—along with more extensive domains observed in the 

Eastern Transverse Ranges to the north and in the San Gabriel Mountains to the northwest—of an originally more continuous assemblage that has been dextrally displaced along 

strands of the San Andreas Fault System (Powell, 1981, 1993).

Mesozoic rocks, including both metamorphic and plutonic rocks, are abundantly exposed 

in approximately the southern third of the map area and occur as scattered pendants and 

structural outliers in the northern two thirds of the map area. Plutonic and metamorphosed 

plutonic rocks comprise four suites: (1) scattered small bodies of foliated rocks of the Mount 

Lowe intrusive suite of Barth and Ehlig (1988) (unit ml) that occur as inclusions within 

Tertiary hypabyssal rocks in the northern two thirds of the mountain range and are incorporated in Mesozoic orthogneiss in the southern third; (2) Jurassic mafic intrusive rocks (unit 

Jmi), predominantly hornblendite and hornblende gabbro and hornblende- and biotite-hornblende-bearing diorite; (3) coarse-grained Jurassic biotite-hornblende porphyritic plutonic 

rocks (unit Jm) that span a compositional range of monzodiorite, granodiorite, and quartz 

monzonite; and (4) Cretaceous and (or) Jurassic leucocratic granitic orthogneiss (unit KJlg) 

and biotite-hornblende-quartz-feldspar orthogneiss (unit KJog).

As with the Proterozoic rocks, the domains of Mount Lowe intrusive suite in the Chocolate Mountains are remnants of a much larger paleogeologic domain, a very large, zoned 

pluton that was disrupted by the evolving San Andreas Fault System (Silver, 1968, 1971; 

Ehlig, 1981; Dillon and Ehlig, 1993; Powell, 1993). The most extensive remnants of this 

pluton now crop out in the San Gabriel Mountains to the northwest across the San Andreas 

Fault and in the Little Chuckwalla Mountains to the northeast.

Four other metamorphic suites that are wholly or partially Mesozoic originated, at least 

in part, as supracrustal sedimentary and (or) volcanic strata. The first unit, muscovite-quartzfeldspar phyllite, crops out sparsely in the southwest corner of the map area and is likely a 

metamorphosed sedimentary or volcanic part of the Jurassic Winterhaven Formation (unit 

Jw). The second, in the southern third of the Chocolate Mountains, is biotite-quartz-feldspar 

gneiss (unit KJbg) that may be paragneiss spatially associated with the Cretaceous and (or) 

Jurassic orthogneiss suite (units KJlg and KJog). The third unit is paragneiss (unit gn) 

that contains rare quartzite, marble, and metamorphosed conglomerate that are certainly 

formed from sedimentary protoliths. Finally, the Cenozoic and Mesozoic Orocopia Schist 

(Dillon, 1976; Haxel, 1977; Haxel and Dillon, 1978; Haxel and others, 2002) is predominantly muscovite-quartz-plagioclase quartzofeldspathic schist, likely metamorphosed turbiditic greywacke and mudstone. Subordinate rock types include chlorite-albite greenschist, 

quartzite, and marble representing metamorphosed basalt, chert, and limestone, respectively. 

In the Orocopia Mountains to the northwest of the Chocolate Mountains and in the Trigo 

Mountains to the east of the southeastern Chocolate Mountains, Orocopia Schist has yielded a 

few zircons that give Eocene dates (two in the Orocopia Mountains and four in the Trigo 

Mountains). These Eocene zircon dates are identified as outliers to the much larger number of 

zircons that yield Late Cretaceous dates in Orocopia, Chocolate, and Trigo Mountains and it 

has not been resolved whether these young dates represent the age of detrital zircons or record 

metamorphism of the Orocopia Schist (Jacobson and others, 2011; see also, Jacobson and 

others, 2017; Seymour and others, 2017). 

Much of the northern two-thirds of the Chocolate Mountains in the map area is underlain 

by a composite batholith of late Oligocene and lower Miocene plutonic and hypabyssal rocks. 

Satellite bodies of these rocks also crop out in the southern third of the map area, including 

the monzogranite of Mount Barrow of Dillon (1976). Plutonic rocks in the batholith range in 

composition from mafic to felsic, including gabbro, diorite, quartz diorite, and granodiorite 

(collectively, unit Tqd), monzogranite (unit Tmg), and monzogranite and syenogranite (units 

Tgr and Tlg). The sequence of intrusion of the plutonic rocks progressed from mafic through 

felsic. Hypabyssal rocks include dacitic, rhyodacitic, and rhyolitic feldspar porphyry units 

(Tfpi, Trfp, and Tqp, respectively).

Cover strata in the Chocolate Mountains consists of sections of Tertiary sedimentary and 

volcanic strata of various ages that typically occur in isolated domains. In various combinations, mechanisms responsible for isolation of these domains include deposition in different 

settings, structural disruption during or subsequent to deposition, isolation by subsequent 

magma intrusion, and burial by younger deposits. A discontinuous edifice of Tertiary volcanic 

rocks crop out on both the northeastern and southwestern flanks of the Chocolate Mountains 

and represents the extrusive manifestation of the Tertiary composite batholith. Volcanic rocks 

include dacitic, rhyodacitic, and rhyolitic tuffs, welded tuffs, and silicic flows. Published ages 

for the rocks in this volcanic edifice in the Chocolate Mountains and to the southeast are 

largely late Oligocene and early Miocene (Crowe, 1978; Crowe and others, 1979; Needy and 

others, 2007). Our previously unpublished 40Ar/39Ar dates of 25.8±0.1 to 21.9±1.1 Ma and 

17.9±2.9 Ma on tilted volcanic rocks in the southern Chocolate Mountains just north of 

Highway 78 (tables 1, 3) are consistent with zircon U-Pb dates of 24.1 to 23.2 Ma on similar 

rocks just to the southeast of the highway (Needy and others, 2007).

These volcanic rocks are, at least in part, younger than and locally overlap the oldest 

sedimentary rocks, which comprise scattered remnants of redbed sections that consist of 

sedimentary megabreccia, fluvial sandstone and conglomerate, and lacustrine mudstone. 

Sections of still younger Tertiary cover strata consist of andesite, dacite, and basalt flows that 

are interlayered with sedimentary strata. Our K-Ar dates on basalt flows interbedded with 

sedimentary strata in the southeastern Chocolate Mountains lie in the range of 10 to 7 Ma 

(tables 1, 3), again consistent with, but perhaps a bit younger than previously published K-Ar 

and 40Ar/39Ar dates in the range of 13 to 9 Ma (Haxel and Dillon, 1973; Dillon and Ehlig, 

1993; Needy and others, 2007).

Surficial deposits are predominantly Quaternary sediment deposited in piedmont alluvial 

and debris-flow aprons that flank the Chocolate Mountains. These apron deposits interfinger 

with braided-stream deposits along Salt Creek and lacustrine deposits on the floor of the 

Salton Trough.

Structure in the Chocolate Mountains manifests as late Oligocene to middle Miocene and 

possibly earlier extensional tectonism that culminated in exhumation of Orocopia Schist by 

tectonic denudation (Powell, 1995; Jacobson and others, 2007). In its early stages, tectonism 

was accompanied by sedimentation and by voluminous magma generation, which produced a 

batholithic-to-volcanic edifice. The principal structural feature is a complexly faulted, 

northwest-trending array of en echelon antiforms that runs the length of the range and 

continues southeast into Arizona and northwest into the Orocopia Mountains. In the anticlinorium core, Orocopia Schist is intruded by a late Oligocene composite batholith of mafic to 

felsic plutons. A succession of tectonic plates separated by detachment faults overlies the 

schist and plutons. The structurally lowest fault is ductile and juxtaposes mylonite against the 

schist. Three higher faults, all brittle, vertically stack plates of (1) Mesozoic orthogneiss, (2) 

little-deformed Triassic and Jurassic plutonic rocks, Proterozoic gneiss and anorthosite, and 

dacitic to rhyolitic late Oligocene hypabyssal intrusive rocks, and (3) moderately to steeply 

tilted supracrustal sedimentary and volcanic rocks.

Field relations and age data allow us to bracket sequential stages in the late Oligocene to 

middle Miocene (from about 28 to 13 Ma) magmatic-tectonic evolution of the Chocolate 

MESOZOIC AND PROTEROZOIC IGNEOUS AND METAMORPHIC ROCKS

Mylonite (Tertiary and (or) Cretaceous)—Mylonite, ultramylonite, and mylonitic gneiss and schist

Orocopia Schist (Mesozoic)—Metamorphosed sedimentary and volcanic rocks 

that display prominent schistosity. Predominantly biotite-muscovite-microcline-quartz-plagioclase (albite or oligoclase) quartzofeldspathic schist and 

metasandstone metamorphosed from turbidite section of graywacke 

sandstone and mudstone. Subordinate quartzite, marble, greenschist, 

serpentinite, and talc-actinolite schist that typically are associated spatially 

with one another and occur as isoclinally folded and discontinuous layers 

within quartzofeldspathic schist. Distribution of this assemblage of subordinate metavolcanic and metasedimentary rock types in the map area indicates 

that they occur structurally high within the Orocopia Schist, near the fault 

boundary between Orocopia Schist and tectonically superposed Proterozoic 

and Mesozoic crystalline rocks. Compositional layering is likely transposed 

bedding. Inverted metamorphic grade ranges from albite-epidote-amphibolite 

to amphibolite facies. In Mammoth Wash area, contains bodies of metamorphosed hornblende diorite containing zircons that have yielded a U-Pb date of 

163 Ma (Mukasa and others, 1984). Contact relation between metamorphosed 

diorite and quartzofeldspathic schist is equivocal (Jacobson and others, 

2000). Sedimentary zircons from quartzofeldspathic schist in the Chocolate 

Mountains have yielded U-Pb dates as young as Late Cretaceous (Jacobson 

and others, 2000, 2011; Grove and others, 2003). Larger bodies of subordinate rock-types mapped include:

Quartzite and marble (Mesozoic)—Laminated spessartite-muscovite quartzite 

and very thin-layered limestone marble and calc-silicate rock. Metamorphosed from protoliths of chert and siliceous limestone, respectively

Greenschist (Mesozoic)—Dark greenish-gray albite-epidote-actinolite±chlorite 

schist. Metamorphosed from basalt protolith. Jurassic metamorphosed diorite 

in the Mammoth Wash area may be related petrogenetically to the metamorphosed basalt

Leucogranite (Cretaceous or Jurassic)—Foliated biotite-bearing leucocratic 

granitic rock; aplitic, granitic, and pegmatitic; in places contains muscovite 

and (or) garnet

Orthogneiss (Cretaceous or Jurassic)—Predominantly microcline-quartz-biotite-hornblende-plagioclase quartz dioritic gneiss, commonly exhibits 

evidence of propylitic alteration, including chlorite, epidote, and saussuritized 

plagioclase. Other rock types included in unit are gneissic diorite and 

amphibolite

Biotite gneiss (Cretaceous or Jurassic)—Gneissic biotite-quartz-feldspar 

granodiorite and (or) monzogranite

Winterhaven Formation(?) (Jurassic)—Muscovite-quartz-feldspar phyllite; 

light gray and likely metasedimentary and metavolcanic (see also Dillon, 

1976, p. 203)

Porphyritic monzodiorite, granodiorite, and quartz monzonite (Jurassic)—Hornblende- and biotite-bearing; quartz-poor; typically porphyritic; 

characterized by lavender-tinted phenocrysts of alkali feldspar. 

Coarse-grained porphyritic plutonic unit spans compositional range indicated 

in unit name

Mafic intrusive suite (Jurassic)—Mafic and ultramafic intrusive rocks that 

include hornblende gabbro, anorthosite, hornblendite, and diorite

Mount Lowe intrusive suite of Barth and Ehlig (1988) (Triassic)—Light-colored monzodiorite that contains large phenocrysts of hornblende and alkali 

feldspar in a feldspar-rich matrix; quartz typically less than 10 percent; also 

contains phenocrysts of sphene and, in places, garnet. Occurs as inclusions 

within Tertiary hypabyssal rocks in northern part of Chocolate Mountains and 

is co-mingled intrusively and (or) tectonically with Mesozoic orthogneiss in 

the southern part; dikes and sills reported by Dillon (1976) not observed 

during this study

Gneiss (Mesozoic and (or) Proterozoic)—Interlayered orthogneiss; light-gray, 

coarse-grained, muscovitic and feldspathic quartzite; garnet-muscovite-quartz±kyanite±sillimanite pelitic gneiss; and calc-silicate gneiss. As 

mapped, also may include metamorphosed Paleozoic rocks

Syenite, anorthosite, and anorthositic gabbro (middle Proterozoic)—Syenite 

consists predominantly of mesoperthite, plagioclase, minor quartz, and uralite 

and biotite pseudomorphous after orthopyroxene. Anorthosite and anorthositic gabbro predominantly contain andesine and uralite that is pseudomorphous after orthopyroxene. These rocks occur in northeasternmost Chocolate 

Mountains in small, isolated domain that is offset sinistrally along Salt Creek 

Fault from larger domain in southern Orocopia Mountains

Gneiss (early Proterozoic)—Consists of paragneiss and orthogneiss: 

coarse-grained sillimanite-biotite-quartz-feldspar gneiss; laminated 

biotite-quartz-plagioclase-alkali feldspar gneiss; amphibolite; light-colored 

biotite-quartz-feldspar granitic gneiss; dark-colored biotite-quartz-plagioclase-alkali feldspar augen gneiss that contains megacrysts as long as 3 cm 

of alkali feldspar and that originated as rapakivi-textured porphyritic granodiorite to monzogranite

EXPLANATION OF MAP SYMBOLS

Contact—Solid where location is accurate; dotted where location is concealed

Faults—Solid where location is accurate; dotted where location is concealed; 

queried where identity is questionable

Normal fault—Ball and bar on downthrown block; lineation on fault 

surface—showing bearing and plunge

Strike-slip fault—Arrows show relative motion 

Detachment fault—Boxes on upper plate

Folds—Dashed where identity and existence certain, location approximate

Plunging anticline—anticlinorium; axial plane trace, approximately located;

 large arrowhead shows direction of plunge

Dike—Igneous; intrusive; hypabyssal

PLANAR POINT FEATURES

Strike and dip of bedding

Inclined

Strike and dip of primary igneous foliation

Inclined

Strike and dip of metamorphic foliation

Inclined

Vertical

SAMPLE LOCATIONS

Geochronological data—sample location

DESCRIPTION OF MAP UNITS

SURFICIAL DEPOSITS

Younger alluvial and debris-flow deposits (Quaternary)—Sand, gravelly sand, 

and sandy gravel; deposited in washes and piedmont alluvial fans

Eolian deposits (Quaternary)—Windblown sand

Lacustrine deposits (Quaternary)—Mudstone and silty sandstone that contain 

channel deposits of sand and gravelly sand, locally fossiliferous; likely part 

of Brawley Formation

Older alluvial and debris-flow deposits—Quaternary. Unit includes upper and 

middle subunits mapped within the gunnery range and a lower subunit that 

crops out along Salt Creek just north of the gunnery range boundary

Older alluvial and debris-flow deposits, upper unit (Quaternary)—Sandy 

gravel and gravelly sand; deposited in piedmont alluvial fans. Geomorphic 

surface developed on deposits is typically smooth, well-developed desert 

pavement characterized by interlocking pebbles and dark desert varnish. 

Pavement underlain by pale brown, calcareous, vesicular, loessy windblown 

silt (pedogenic Av horizon); in turn underlain by reddened (7.5YR, Munsell 

colors), clayey pedogenic Bt horizon; in turn underlain by chalky, well 

cemented Bk horizon

Older alluvial and debris-flow deposits, middle unit (Quaternary)—Predominantly sand and pebbly sand that contains scattered cobbles; locally, 

contains bouldery beds; typically very thin- to thin-bedded; channel 

cross-bedding is common. Unit is typically very well cemented and 

commonly is capped by remnants of a roughly planar geomorphic surface, 

deeply incised, that represents an old soil stripped to the Bk horizon. In some 

wash walls along Salt Creek just north and outside of the Chocolate Mountain Aerial Gunnery Range, unit overlies exposures of a lower, more steeply 

tilted, and very well cemented unit of bouldery, sandy gravel

TERTIARY SEDIMENTARY AND VOLCANIC STRATA

Sedimentary rocks younger (Tertiary)—Fluvial sandy gravel interbedded with 

middle and late Miocene basalt (unit Tb). Includes conglomerate of Bear 

Canyon of Dillon (1976). As mapped, unit may also include younger 

Miocene or Pliocene deposits

Basalt (Tertiary)—Flows of olivine basalt interlayered within Tertiary sedimentary rocks (unit Tsy). Five samples from basalt flows exposed in southernmost part of this study area have yielded K-Ar dates ranging from about 11 to 

7 Ma, averaging 9.3±0.1 Ma (tables 2, 3). Previously published dates from 

basalts in Peter Kane Mountain area to the south across State Highway 78 

have yielded K-Ar dates in the range of 13.4 to 9.6 Ma (Crowe and others, 

1979; Needy and others, 2007)

Dacite (Tertiary)—Massive, gray, biotite microphenocrysts in siliceous matrix; 

resistant cliff-former; overlies and may be in part intrusive into volcanic and 

sedimentary rocks (unit Tvs)

Andesite (Tertiary)—Occurs as flows in Tabaseca Tank area in northeasternmost 

Chocolate Mountains

Volcanic, volcaniclastic, and sedimentary rocks (Tertiary)—Interlayered silicic 

flows, tuffs, volcaniclastic rocks, and volcanic-clast conglomerate. Flows are 

massive to flow-banded rhyolite and (or) rhyodacite; phenocrysts consist of 

quartz, biotite, and feldspar in brown or pinkish-gray or maroon aphanitic 

matrix. Tuff beds include pyroclastic welded tuff and unwelded, white to 

greenish-gray, thin-bedded to massive lithic airfall tuff that contains clasts of 

comagmatic silicic flow rocks. Discontinuous layers of obsidian vitrophyre 

occur locally. Conglomerate contains clasts of volcanic rocks and, in places, 

crystalline-basement rocks. As mapped, may include intrusive volcanic and 

hypabyssal rocks. Tilted volcanic rocks in the southern Chocolate Mountains 

just north of Highway 78 have yielded 40Ar-39Ar dates of 25.8±0.1 to 

21.9±1.1 Ma and 17.9±2.9 Ma (tables 1, 3)

Sedimentary rocks, older (Tertiary)—Sedimentary rocks that consist of breccia, 

conglomerate, sandstone, mudstone, limestone, and tuff. Clastic rocks 

typically are redbeds. Breccia and conglomerate beds contain clasts of 

Proterozoic and Mesozoic gneissic and plutonic units and contain no clasts of 

Orocopia Schist or Tertiary magmatic rocks. Likely correlative with the upper 

Oligocene to lower Miocene Diligencia Formation of Crowell (1962) in the 

Orocopia Mountains

TERTIARY HYPABYSSAL ROCKS

Porphyry rocks, undivided (Tertiary)

Quartz porphyry—Phenocrysts of quartz and feldspar in an aphanitic felsic 

matrix; leucocratic. Occurs in swarm of orange-weathering dikes at north end 

of Chocolate Mountains

Pyroxene dacite—Flow-banded dacite that contains sparse microphenocrysts 

of pyroxene, hornblende, and plagioclase (andesine)

Intermediate feldspar porphyry—Dacitic; white feldspar phenocrysts in 

aphanitic matrix; gray to dark gray or greenish gray or maroon. Intruded by 

dikes of quartz porphyry (unit Tqp). As mapped, unit may contain volcanic 

rocks

Rapakivi feldspar porphyry—Rhyodacite porphyry containing abundant 

phenocrysts of plagioclase and rapakivi-textured orthoclase, typically 1 to 3 

cm in diameter, and less abundant phenocrysts of hornblende, biotite, and 

resorbed quartz. As mapped, unit may contain volcanic rocks

TERTIARY HYPABYSSAL AND PLUTONIC ROCKS

Porphyry and monzogranite, undivided (Tertiary)—Hornblende-biotite 

monzogranite (unit Tmg) intruded by light-colored hypabyssal porphyry (unit 

Tpu); underlies northwesternmost part of Chocolate Mountains

TERTIARY PLUTONIC ROCKS

[Composite batholith comprising a mafic through felsic suite of plutons that 

intrude Orocopia Schist]

Leucogranite (Tertiary)—Granophyric; rusty orange-brown-weathering; 

commonly intensely fractured and brecciated, resulting in cavernous weathering

Granite (Tertiary)—Light-colored biotite and hornblende-biotite monzogranite 

that has seriate texture; includes monzogranite of Mount Barrow at southeast 

end of range, which contains rapakivi-textured phenocrysts of orthoclase 

rimmed by oligoclase that range in size from 1 to 3 cm. Granite bodies at 

Mount Barrow and Salvation Pass have yielded biotite-hornblende K-Ar 

apparent ages of 24 to 21 Ma (Miller, 1974); zircons from the granite of 

Mount Barrow have yielded a U-Pb age of 24±0.2 Ma (Needy and others, 

2007)

Monzogranite (Tertiary)—Medium-colored, gray hornblende-biotite monzogranite. In plutons in central part of Chocolate Mountains, contains equant 

phenocrysts of alkali feldspar that range in size from 0.25 to 1 cm, typically 

0.5 cm; also contains mafic inclusions. Locally divided into:

Monzogranite, equigranular—Medium- to coarse-grained, nonporphyritic. 

Slight primary igneous foliation observed sparsely

Monzogranite, porphyritic—Hornblende-biotite; seriate; rapakivi-textured 

feldspar phenocrysts as large as 2 cm, but more typically 0.5 to 1 cm, set in 

fine-grained, dark-gray matrix

Quartz diorite and granodiorite (Tertiary)—Dark-colored, biotite-hornblende 

quartz diorite, tonalite, and granodiorite; fine- to coarse-grained. Slight 

primary igneous foliation observed sporadically. Contains inclusions of fineto coarse-grained gabbro and diorite; as mapped, unit contains some 

unmapped bodies of gabbro and diorite large enough to have been mapped

Qya

Qe

Ql

Qoau

Qoam

Ta

Tsy

Td

Tvs

Tso

Tmg

Tfpi

Tqp

Tpd

Trfp

Tqd

Tlg

Tgr

Tpu

KJlg

KJog

KJbg

gn

ml

Jm

gn

TKm

Jw

os

osqm

osg

Tpmgu

sa

Tb

Jmi

Tmge

Tmgp

Mountains. Our 40Ar/39Ar and K-Ar dates and published K-Ar dates (Miller, 1974) and U-Pb 

dates (Needy and others, 2007) indicate felsic plutonism occurred at 24 Ma, dacitic to 

rhyodacitic volcanism from 26 to 22 Ma and around 22 Ma and 18 Ma. These middle Tertiary 

strata are strongly tilted and are overlain by largely untilted sections of basalt (erupted from 

about 13 to 9 Ma) and conglomerate. Thus, detachment faulting in the Chocolate Mountains 

occurred between 17 Ma and 13 Ma.

The Chocolate Mountains extensional domain lies northeast of the San Andreas Fault 

(younger than about 6 Ma) and southwest of the early and middle Miocene Clemens 

Well-Fenner-San Francisquito-San Andreas strand of the San Andreas Fault System (fig. 2b). 

The spatial proximity of the extensional and strike-slip domains may be indicative of a 

genetic relation between the two (Powell, 1995)

Constrained by reconstruction of an array of paleogeologic patterns comprising Proterozoic, Mesozoic, and Paleocene through lower Miocene rocks, the approximately 300 km 

displacement on the San Andreas Fault northwest of the Garlock Fault is distributed to the 

southeast on the San Andreas Fault (about 160 km displacement, from 5 or 6 Ma to present), 

San Gabriel fault (about 40–45 km, accumulated in intervals from 12 to 5 Ma and from 5 Ma 

to present), and Clemens Well-Fenner-San Francisquito-San Andreas Fault (about 100 km, 

between about 22–17 Ma and 13 Ma) (Powell and Weldon, 1992; Powell, 1993). The youngest rocks in southern California that have been offset about 300 km across all these parts of 

the San Andreas Fault are basalt flows in the Diligencia and Plush Ranch Formations, which 

are as young as 22 Ma based on whole-rock K-Ar dates (Frizzell and Weigand, 1993). Thus, 

timing of displacement on the Clemens Well-Fenner-San Francisquito-San Andreas Fault is 

coeval with the late Oligocene-middle Miocene evolution of the highly extended terrain in the 

Chocolate Mountains.

Lack of evidence for a large-displacement dextral fault in Arizona on-trend with the 

Clemens Well Fault, the appearance of the extensional domain of the Chocolate and Orocopia 

Mountains adjacent to the Clemens Well Fault and its southeastward projection, and the 

temporal coexistence of the two tectonic features are all compatible with a strain-transfer 

mechanism whereby dextral slip on the Clemens Well Fault is accommodated to the southeast 

by hyper-extension in the Orocopia-Chocolate Mountains block (Powell, 1995). This strain 

pattern precedes later development of the West Salton detachment that was associated with 

growth of the Salton Trough and that began perhaps as early as about 10 Ma (Matti and 

Langenheim, 2008). Unlike this later strain pattern, the extensional accommodation proposed 

here was not linked to opening of the Gulf of California, but rather occurred in an extensional 

zone between the Clemens Well Fault and a reconstructed zone of sinistral shear along the 

southern boundary of the Transverse Ranges and southwestern margin of the Chocolate 

Mountains. 

The post-extensional basalt and conglomerate sections are fully offset on the modern San 

Andreas Fault, predate growth of the Salton Trough, and span much of the interval between 

cessation of the Clemens Well-Fenner-San Francisquito-San Andreas Fault, at about 13 Ma, 

and start of the San Andreas Fault at about 5 or 6 Ma. In various reconstructions of the San 

Andreas system (Dillon and Ehlig, 1993; Matti and Morton, 1993; Powell, 1993), the basalt 

center dated in this study as ranging from 11 to 7 Ma restores to a position between the basalt 

field in the Peter Kane Mountain area that has been dated as ranging from 13 and 9 Ma and 

volcanic rocks in San Gorgonio Pass that consist of 10-Ma basalt flows and andesite breccia 

(Peterson, 1975) and 6-Ma basalt flows (Matti and others, 1985).

REFERENCES CITED

Barth, A.P., and Ehlig, P.L., 1988, Geochemistry and petrogenesis of the marginal zone of the 

Mount Lowe Intrusion, central San Gabriel Mountains, California: Contributions to 

Mineralogy and Petrology, v. 100, p. 192–204.

Crowe, B.M., 1978, Cenozoic volcanic geology and probable age of inception of basin-range 

faulting in the southeasternmost Chocolate Mountains, California: Geological Society of 

America Bulletin, v. 89, p. 251–264.

Crowe, B.M., Crowell, J.C., and Krummenacher, D., 1979, Regional stratigraphy, K-Ar ages, 

and tectonic implications of Cenozoic volcanic rocks, southeastern California: American 

Journal of Science, v. 279, p. 186–216.

Crowell, J.C., 1962, Displacement along the San Andreas fault, California: Geological Society 

of America Special Paper 71, 61 p.

Dillon, J.T., 1976, Geology of the Chocolate and Cargo Muchacho Mountains, southeasternmost California: Santa Barbara, University of California, Ph.D. dissertation, 405 p.

Dillon, J.T., and Ehlig, P.L., 1993, Displacement on the southern San Andreas fault, in Powell, 

R.E., Weldon, R.J., II, and Matti, J.C., eds., The San Andreas Fault System: Displacement, palinspastic reconstruction, and geologic evolution: Geological Society of America 

Memoir 178, p. 199–216.

Ehlig, P.L, 1981, Origin and tectonic history of the basement terrane of the San Gabriel 

Mountains, central Transverse Ranges, in Ernst, W.G., ed., The geotectonic development 

of California; Rubey Volume 1: Englewood Cliffs, New Jersey, Prentice-Hall, p. 

253–283.

Frizzell, V. A., Jr., and Weigand, P. W., 1993, Whole-rock K-Ar ages and geochemical data 

from middle Cenozoic volcanic rocks, southern California: A test of correlations across 

the San Andreas fault, in Powell, R. E., Weldon, R. J., II, and Matti, J. C., eds., The San 

Andreas Fault System—Displacement, palinspastic reconstruction, and geologic evolution: Geological Society of America Memoir 178.

Grove, M., Jacobson, C.E., Barth, A.P., and Vućić, A., 2003, Temporal and spatial trends of 

Late Cretaceous–early Tertiary underplating of Pelona and related schist beneath 

southern California and southwestern Arizona, in Johnson, S.E., Paterson, S.R., Fletcher, 

J.M., Girty, G.H., Kimbrough, D.L., and Martín-Barajas, A., eds., Tectonic evolution of 

northwestern México and the southwestern USA: Geological Society of America Special 

Paper 374, p. 1–26.

Haxel, G.B., 1977, The Orocopia Schist and the Chocolate Mountain Thrust, Picacho-Peter 

Kane mountain area, southeasternmost California: Santa Barbara, University of California, Ph.D. dissertation, 277 p.

Haxel, G.B., and Dillon, J., 1973, The San Andreas fault system in southeasternmost California, in Kovach, R.L., and Nur, A., eds., Proceedings of the Conference on Tectonic 

Problems of the San Andreas Fault System: Stanford University Publications in the 

Geological Sciences, v. 13, p. 322–333.

Haxel, G.B., and Dillon, J., 1978, The Pelona-Orocopia schist and Vincent-Chocolate Mountain thrust system, southern California, in Howell, D.G., and McDougall, eds., Mesozoic 

paleogeography of the western United States: Los Angeles, Pacific Section, Society of 

Economic Paleontologists and Mineralogists, p. 453–470.

Haxel, G.B., Jacobson, C.E., Richard, S.M., Tosdal, R.M., and Grubensky, M.J., 2002, The 

Orocopia Schist in southwest Arizona—Early Tertiary oceanic rocks trapped or transported far inland, in Barth, A., ed., Contributions to crustal evolution of the southwestern 

United States: Geological Society of America Special Paper 365, p. 99–128.

Jacobson, C.E., Barth, A.P., and Grove, M., 2000, Late Cretaceous protolith age and provenance of the Pelona and Orocopia schists, southern California—Implications for evolution of the Cordilleran margin: Geology, v. 28, no. 3, p. 219–222.

Jacobson, C.E., Grove, M., Vućić, A., Pedrick, J.N., and Ebert, K.A., 2007, Exhumation of the 

Orocopia Schist and associated rocks of southeastern California—Relative roles of 

erosion, synsubduction tectonic denudation, and middle Cenozoic extension, in Cloos, 

M., Carlson, W.D., Gilbert, M.C., Liou, J.G., and Sorensen, S.S., eds., Convergent 

margin terranes and associated regions—A tribute to W.G. Ernst: Geological Society of 

America Special Paper 419, p. 1–37.

Jacobson, C.E., Grove, M., Pedrick, J.N., Barth, A.P., Marsaglia, K.M., Gehrels, G.E., and 

Nourse, J.A., 2011, Late Cretaceous–early Cenozoic tectonic evolution of the southern 

California margin inferred from provenance of trench and forearc sediments: Geological 

Society of America Bulletin, v. 123, no. 3/4, p. 485–506.

Jacobson, C.E., Hourigan, J.K., Haxel, G.B., and Grove, M., 2017, Extreme latest Cretaceous–Paleogene low-angle subduction: Zircon ages from Orocopia Schists at Cemetery 

Ridge, southwestern Arizona, USA: Geology, v. 45, no. 10, p. 951–954.

Jahns, R.H., ed., 1954, Geology of southern California: California Division of Mines Bulletin 

170.

Kuiper, K.F., Deino, A., Hilgen, F.J., Krijgsman, W., Renne, P.R., and Wijbrans, J.R., 2008, 

Synchronizing rock clocks of Earth history: Science, v. 320, no. 5875, p. 500–504, 

https://doi:10.1126/science.1154339.

Matti, J.C., and Langenheim, V.E., 2008, Does the West Salton detachment extend through 

San Gorgonio Pass, southern California? [abs.]: Proceedings and abstracts of the Southern California Earthquake Center Annual Meeting, v. 18, p. 140.

Matti, J.C., and Morton, D.M., 1993, Paleogeographic evolution of the San Andreas fault in 

southern California—A reconstruction based on a new cross-fault correlation, in Powell, 

R.E., Weldon, R.J., II, and Matti, J.C., eds., The San Andreas Fault system—Displacement, palinspastic reconstruction, and geologic evolution: Geological Society of America 

Memoir 178, p. 107–159.

Matti, J.C., Morton, D.M., and Cox, B.F., 1985, Distribution and geologic relations of fault 

systems in the vicinity of the central Transverse Ranges, southern California: U.S. 

Geological Survey Open-File Report 85–365, 27 p.

McIntyre, G.A., Brooks C., Compston, W., and Turek, A., 1966, The statistical assessment of 

Rb/Sr isochrones: Journal of Geophysical Research, v. 71, p. 5459–5468, https://-

doi.org/10.1029/JZ071i022p05459.

Miller, F.K., 1974, Comparison of granitic intrusions in the Orocopia and Pelona Schists, 

southern California [abs.]: Geological Society of America, Abstracts with Programs, v. 6, 

no. 3, p. 220–221.

Mukasa, S.B., Dillon, J.T., and Tosdal, R.M., 1984, A Late Jurassic minimum age for the 

Pelona-Orocopia Schist protolith, southern California [abs.]: Geological Society of 

America, Abstracts with Programs, v. 16, no. 5, p. 323.

Needy, S.K., Wooden, J.L., Barth, A.P., and Jacobson, C.E., 2007, Geochronology of igneous 

rocks in the Chocolate Mountains region as a means to interpret tectonic evolution of 

southeastern California [abs.]: Geological Society of America, Abstracts with Programs, 

v. 39, no. 6, p. 407.

Peterson, M.S., 1975, Geology of the Coachella Fanglomerate, in Crowell, J.C., ed., San 

Andreas fault in southern California: California Division of Mines and Geology Special 

Report 118, p. 119–126.

Powell, R.E., 1981, Geology of the crystalline basement complex, eastern Transverse Ranges, 

southern California—Constraints on regional tectonic interpretation: Pasadena, California Institute of Technology, Ph.D. dissertation, 441 p.

Powell, R.E., 1993, Balanced palinspastic reconstruction of pre–late Cenozoic paleogeology, 

southern California—Geologic and kinematic constraints on evolution of the San 

Andreas fault system, in Powell, R.E., Weldon, R.J., II, and Matti, J.C., eds., The San 

Andreas Fault System—Displacement, palinspastic reconstruction, and geologic evolution: Geological Society of America Memoir 178, p. 1–106.

Powell, R.E., 1995, Extensional tectonism in the Chocolate Mountains, California, and its 

relation to the early evolution of the San Andreas fault system, in Leon T. Silver 70th 

birthday symposium and celebration: California Institute of Technology Division of 

Geological and Planetary Sciences, p. 91–96.

Powell, R.E., and Fleck, R.J., 2008, Geology and tectonics of the Chocolate Mountains [abs.]: 

Proceedings and abstracts of the Southern California Earthquake Center Annual Meeting, 

v. 18, p. 139.

Powell, R.E., and Silver, L.T., 1979, Geologic analysis of ASTP photographs of parts of 

southern California, in El-Baz, F., and Warner, D. M., eds., Earth observations and 

photography: Apollo-Soyuz Test Project summary science report, v. 2: Washington, D.C., 

National Aeronautics and Space Administration NASA SP-412, p. 9-27.

Powell, R.E., and Weldon, R.J., II, 1992, Evolution of the San Andreas fault: Annual Review 

of Earth and Planetary Sciences, v. 20, p. 431–468.

Powell, R.E., Hall, R., and Dawson, M., 1994, Preliminary geologic map of the Chocolate 

Mts. and vicinity, Salton Sea, Eagle Mts., and Trigo Mts. 30' x 60' quadrangles, southern 

California [abs.]: Geological Society of America, Abstracts with Programs, v. 26, no. 2, 

p. 82.

Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T., and DePaolo, D.J., 1998, 

Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating: 

Chemical Geology, v. 145, p. 117–152, https://doi.org/10.1016/S0009-2541(97)00159-9.

Seymour, N.M., Strickland, E.D., Singleton, J.S., and Stockli, D.F., 2017, Subduction and 

metamorphism of the Orocopia Schist, northern Plomosa Mountains, west-central 

Arizona: Insights from zircon U-Pb geochronology [abs.]: Geological Society of 

America, Abstracts with Programs, v. 49, no. 6. doi: 10.1130/abs/2017AM-300952

Silver, L.T., 1968, Pre-Cretaceous basement rocks and their bearing on large-scale displacements in the San Andreas fault system, in Dickinson, W.R., and Grantz, A., eds., 

Proceedings of Conference on Geologic Problems of San Andreas Fault System: 

Stanford University Publications in the Geological Sciences, v. 11, p. 279–280.

Silver, L.T., 1971, Problems of crystalline rocks of the Transverse Ranges [abs.]: Geological 

Society of America Abstracts with Programs, v. 3, no. 2, p. 193–194.

Silver, L.T., Anderson, T.H., Conway, C.M., Murray, J.D., and Powell, R.E., 1977, Geologic 

features of southwestern North America, in Skylab explores the Earth: Washington, D.C., 

National Aeronautics and Space Administration NASA SP-380, p. 89-135.

U.S. Geological Survey, Earthquake Hazards Program, 2016, Faults, earthquake geology, and 

special earthquake studies; Salton seismic imaging—Studying earthquake hazards and 

rifting processes in the Imperial and Coachella Valleys: U.S. Geological Survey Earthquake Hazards Program webpage, accessed July 29, 2016 and February 16, 2018, at 

https://earthquake.usgs.gov/research/faults/salton.php, https://earthquake.usgs.gov/static/lfs/research/SSIP_InfoSheet.pdf.

20

20

20

57

? U

D

Geologic Map and Database of the Chocolate Mountain Aerial Gunnery Range, Riverside and Imperial Counties, California

By

Robert E. Powell, Robert J. Fleck, and Pamela M. Cossette

2018

Plotter disclaimer: This map or plate is offered as an online-only digital publication. Users should be

aware that, because of differences in rendering processes and pixel resolution, some slight distortion of

scale may occur when viewing it on a computer screen or when printing it on an electronic plotter, even

when it is viewed or printed at its intended publication scale.

Suggested citation: Powell, R.E., Fleck, R.J., and Cossette, P.M., 2018, Geologic map and database of the 

Chocolate Mountain Aerial Gunnery Range, Riverside and Imperial Counties, California: U.S. Geological

Survey Open-File Report 2018–1191, 2 sheets, scale 1: 100,000, https://doi.org/10.3133/ofr20181191. ISSN 2331-1258 (online)

https://doi.org/10.3133/ofr20181191

Prepared in cooperation with the

U.S. Navy and U.S. Marine Corps

U.S. Department of the Interior

U.S. Geological Survey

Open File Report 2018-1191

Sheet 2 of 2

Table 1. Chocolate Mountains 40Ar/

39Ar age data. [Volcanic-rock samples from volcanic and sedimentary section (unit Tvs). See table 3 for stratigraphic

positions of samples]

Sample number Mineral (method)1 Best estimate age

(Ma)2,3 MSWD4

952-19F Plagioclase (LF) 21.87±1.09 1.70

Plagioclase (IH) 22.95±0.13 1.05

952-19G Biotite (LF) 23.41±0.06 1.26

Biotite (IH) 23.42±0.12 1.90

952-19G Plagioclase (LF) 23.16± 0.15 1.83

952-20B Plagioclase (LF) 17.9±2.9 8.87

952-20C Plagioclase (LF) 25.17±0.19 0.29

Plagioclase (IH) 25.79±0.13 1.40

1

Extraction method: LF, laser fusion; IH, incremental heating.

2

Adjusted to Fish Canyon sanidine standard age of 28.198 Ma (Kuiper and others, 2008)

3

Uncertainties shown at 1 standard error of the mean.

4

Mean square of weighted deviates (McIntyre and others, 1966).

Table 2. Chocolate Mountains K-Ar data from basalt samples. [Basalt samples from flows (unit Tb) interbedded with sedimentary strata (unit Tsy). See table 3 for

stratigraphic positions of samples]

Sample number Age (Ma)1 ±Age(Ma) Sample age (Ma)2

952-19A 8.757 0.148 8.73±0.11

8.692 0.162

952-19B 9.204 0.213 9.20±0.21

952-19C 9.224 0.203 9.22±0.20

952-19D 9.422 0.182 9.36±0.13

9.311 0.177

952-19E 9.388 0.139 9.39±0.14

Weighted mean age 9.12±0.12 Ma, MSWD = 3.5

1

Ages were adjusted to GA1550 biotite standard age of 98.79 Ma (Renne and others, 1998).

2

Uncertainties estimated at 1 standard error of the mean.

Table 3. Stratigraphic position of Chocolate Mountains geochronology samples. Sample Rock type Description Sequencing relations Approx.

age (Ma)

K-Ar dates on basalt samples from a stack of five basalt flows (unit Tb) interbedded within a little-deformed

section of sedimentary rocks (unit Tsy, here consisting of conglomerate of Bear Canyon of Dillon, 1976).

Average age for the five flows is 9.12±0.12 Ma.

952-19D Olivine basalt Stratigraphically highest flow Flow 5 9.36

952-19C Olivine basalt Flow 4 9.22

952-19B Olivine basalt Middle flow Flow 3 9.20

952-19A Olivine basalt Flow 2 8.73

952-19E Olivine basalt Stratigraphically lowest flow Flow 1 9.39

40Ar/39Ar dates on volcanic samples from an older, moderately to steeply tilted volcanic and sedimentary

section (unit Tvs)

952-20C Dacite flow Porphyry containing phenocrysts

of hornblende, sanidine, and

plagioclase

Apparent highest

unit in SWdipping section

25.8–

25.2

952-19G Unwelded lithic

tuff

Nearly monolithologic boulder

breccia of unwelded biotitequartz-sanidine rhyolite tuff

Apparent middle

unit in SWdipping section

23.4–

23.2

952-20B Ignimbrite Pyroclastic welded tuff Apparent lowest

unit in SWdipping section

17.9

952-19F Dacite volcanic

spine

Dacite porphyry containing

phenocrysts of quartz,

hornblende, biotite,

plagioclase, and sanidine

Intrusive into tufflahar volcanic

section that

includes units

represented by

samples 952-19G

and -20B

23.0–

21.9

ANEROZOIC

OTEROZOIC

ANEROZOIC AND (OR)

OTEROZOIC

Tsy

Tfpi

QUATERNARY

Early Miocene and 

late Oligocene

PHANEROZOIC

 JURASSIC

TRIASSIC

PROTEROZOIC

MESOZOIC

TERTIARY

CENOZOIC

Tpmgu

Tqp Tpd

Trfp

Qe

Qya

Ql

Qoau

Qoam

Tsy

Td

Tb

Ta

Tso

Tvs

Tqd

Tlg

Tgr

KJlg

KJog

KJbg

sa

gn

ml

Jmi

Jm

Jw

gn

TKm

Tpu

TERTIARY 

AND (OR)

CRETACEOUS

MIDDLE PROTEROZOIC

MESOZOIC AND (OR)

PROTEROZOIC

EARLY PROTEROZOIC

PHANEROZOIC AND (OR)

PROTEROZOIC

Late and middle 

Miocene

?

?

CRETACEOUS 

AND (OR) 

JURASSIC

CENOZOIC AND 

(OR) MESOZOIC

Tmgp

Tmg

Tmge

CORRELATION OF MAP UNITS

osqm

osg

os

SURFICIAL DEPOSITS

TERTIARY SEDIMENTARY

 AND VOLCANIC STRATA

TERTIARY HYPABYSSAL

ROCKS

TERTIARY

PLUTONIC ROCKS

MESOZOIC AND

PROTEROZOIC

IGNEOUS AND 

METAMORPHIC

ROCKS

TERTIARY HYPABYSSAL

AND PLUTONIC ROCKS

Geologic Summary

The northwest-trending Chocolate Mountains are situated along the northeastern

margin of the southern Salton Trough (figs. 1, 2a). The Chocolate Mountain Aerial

Gunnery Range occupies most of the 75-km-long part of the Chocolate Mountains that

lies between Salt Creek to the north and California State Highway 78 to the south.

Mapping studies in the Chocolate Mountains within the gunnery range are few (Dillon,

Matti, J.C., Morton, D.M., and Cox, B.F., 1985, Distribution and geologic relations of 

fault systems in the vicinity of the central Transverse Ranges, southern California: U.S. 

Geological Survey Open-File Report 85-365, 27 p.

Miller, F.K., 1974, Comparison of granitic intrusions in the Orocopia and Pelona Schists, 

southern California: Geological Society of America Abstracts with Programs, v. 6, no. 3, p. 

220-221.

Mukasa, S. B., Dillon, J. T., and Tosdal, R. M., 1984, A Late Jurassic minimum age for 

the Pelona-Orocopia Schist protolith, southern California: Geological Society of America 

Abstracts with Programs, v. 16, p. 323.

Needy, S.K., Wooden, J.L., Barth, A.P., and Jacobson, C.E., 2007, Geochronology of 

igneous rocks in the Chocolate Mountains region as a means to interpret tectonic evolution of 

southeastern California: Geological Society of America Abstracts with Programs, v. 39, n 6, p. 

407.

OLD_GEOLOGIC SUMMARY

The northwest-trending Chocolate Mountains are situated along the northeastern margin 

of the southern Salton Trough (Fig. 1). The Chocolate Mountains Aerial Gunnery Range 

occupies most of the 75-km-long part of the Chocolate Mountains that lies between Salt 

Creek to the north and California State Highway 78 to the south. Mapping studies in the 

Chocolate Mountains within the Gunnery Range are few (Dillon, 1976; Powell and others, 

1994; Powell, 1995; Powell and Fleck, 2008) and this study was conducted in cooperation 

with the U.S. Navy (San Diego, CA) and Marine Corps (Yuma, AZ).

Crystalline basement rocks in the Chocolate Mountains range in age from Early Proterozoic to mid-Cenozoic. Early and Middle Proterozoic metamorphosed sedimentary and 

plutonic rocks include sillimanite-biotite-quartz feldspar gneiss, layered biotite-quartz-feldspar gneiss, biotite-quartz-feldspar augen gneiss, and largely undeformed Late Proterozoic 

anorthosite and syenite. These rock types, which crop out as dispersed domains in the 

Chocolate Mountains, are remnants—along with more extensive domains observed in the 

The Chocolate Mountains extensional domain lies northeast of the post-6-Ma San 

Andreas fault and southwest of the early and middle Miocene Clemens Well-Fenner-San 

Francisquito-San Andreas strand of the San Andreas fault system (Fig. 2). The spatial 

proximity of the extensional and strike-slip domains may be indicative of a genetic relation 

between the two (Powell, 1995)

Constrained by reconstruction of an array of paleogeologic patterns comprising Proterozoic, Mesozoic, and Paleocene through lower Miocene rocks, the approximately 300 km 

displacement on the San Andreas fault northwest of the Garlock fault is distributed to the 

southeast on the San Andreas fault (ca 160 km, 0 to 5 Ma), San Gabriel fault (ca 40-45 km, 

accumulated in intervals from 0 to 5 Ma and 5 to 12 Ma), and Clemens Well-Fenner-San 

Francisquito-San Andreas fault (ca 100 km, 13 to 17-22 Ma) (Powell and Weldon, 1992; 

Powell, 1993}. The youngest rocks yet shown to be offset 300 km in southern California are 

basalt layers in the Diligencia and Plush Ranch formations, as young as 22 Ma. Thus, timing 

of displacement on the Clemens Well-Fenner-San Francisquito-San Andreas fault is coeval 

Best Estimate

Mineral (Method) Age (Ma) MSWD

952-19F Plagioclase (LF) 21.87 ± 1.09 1.70

Plagioclase (IH) 22.95 ± 0.13 1.05

952-19G Biotite (LF) 23.41 ± 0.06 1.26

Biotite (IH) 23.42 ± 0.12 1.90

952-19G Plagioclase (LF) 23.16 ± 0.15 1.83

952-20B Plagioclase (LF) 17.9 ± 2.9 8.87

952-20C Plagioclase (LF) 25.17 ± 0.19 0.29

Plagioclase (IH) 25.79 ± 0.13 1.40

Table . Chocolate Mountains 40Ar/

39

Ar Data

[LF = Laser Fusion; IH = Incremental Heating; Ages adj to

FCs =28.198 Ma]

Sample # Age (Ma) ±Age(Ma) Unit Unit Age (Ma)

952-19A 8.755 0.148 2nd Oldest? 8.73 ± 0.11

8.690 0.162

952-19B 9.202 0.212 Middle ? 9.20 ± 0.21

952-19C 9.221 0.203 2nd Youngest 9.22 ± 0.20

952-19D 9.419 0.182 Top 9.36 ± 0.13

9.309 0.177

952-19E 9.386 0.139 Bottom 9.39 ± 0.14

Table 2. Chocolate Mtns Basalt K-Ar Data (Revised*)

[* 40Ar/40K data were adjusted to GA1550 Biotite age of 98.79

Wtd Mean Age 952-19A:

Wtd Mean Age 952-19B-E:

8.7 ± 0.1 Ma

9.3 ± 0.1 Ma

Sample Rock type Description Sequencing relations Approx. Age

952-19A Olivine basalt (Tb)

Flow interlayered with Tsy, including

conglomerate of Bear Canyon of Dillon

(1976)

Second from bottom 8.73 Ma

952-19B Olivine basalt (Tb)

Flow interlayered with Tsy, including

conglomerate of Bear Canyon of Dillon

(1976)

Middle flow 9.20 Ma

952-19C Olivine basalt (Tb)

Flow interlayered with Tsy, including

conglomerate of Bear Canyon of Dillon

(1976)

Second flow from top 9.22 Ma

952-19D Olivine basalt (Tb)

Flow interlayered with Tsy, including

conglomerate of Bear Canyon of Dillon

(1976)

Top flow in stack of 5 basalt flows 9.36 Ma

952-19E Olivine basalt (Tb)

Flow interlayered with Tsy, including

conglomerate of Bear Canyon of Dillon

(1976)

Bottom flow 9.39 Ma

952-20B Ignimbrite Pyroclastic welded tuff Apparent lowest unit in SW-dipping section 17.9 Ma

952-19F Dacite volcanic spine

Dacite porphyry containing phenocrysts of

quartz, hornblende, biotite, plagioclase, and

sanidine

Intrusive into tuff-lahar volcanic section

that includes units represented by samples 

952-19G and -20B

21.9-23.0 Ma

952-19G Unwelded lithic tuff Nearly monolithologic boulder breccia of

unwelded biotite-quartz-sanidine rhyolite tuff Apparent middle unit in SW-dipping section 23.2-23.4 Ma

952-20C Dacite flow Porphyry containing phenocrysts of

hornblende, sanidine, and plagioclase Apparent highest unit in SW-dipping section 25.2-25.8 Ma

Table 3. Stratigraphic position of Chocolate Mountains Geochronology Samples

Samples from section of younger, little-deformed volcanic (Tb) and sedimentary rocks (K-Ar dates)

Samples from older, moderately to steeply tilted volcanic and sedimentary section (Tvs) ( 40 Ar- 39 Ar dates)

InProgress_GEOCHRONOLOGICAL DATA

Figure 1. Index map of southeastern California showing location of Chocolate Mountain Aerial Gunnery Range.

117° 116° 115°

34°

33°

Creek

Eastern Transverse Ranges

Peninsular Ranges

Salton Trough

Salton 

 Sea

Orocopia

Mountains

Little Chuckwalla

Mountains

Salt

Chocolate Mountains

Chocolate Mountain

Aerial Gunnery Range

Blythe

San Diego

Brawley

El Centro

Palm Springs

Indio

Yuma

San Bernardino

Mojave Desert

C A L I F O R N I A

A R I Z O N A

Hillshade image based on 30-meter 

National Elevation Dataset (NED)

Trigo Mts.

+

Peter Kane

Mountain

C

o

ol rado

River

Coachella

Canal

5

8

10

78

Figure 2a. Physiographic provinces of southern California in relation to faults of the San Andreas 

Fault System--sourced from Jahns (1954), Silver and others (1977), Powell and Silver (1979), 

U.S. Geolgocial Survey, Earthquake Hazards Program (2016), and Powell (1993).

Figure 2b. Position of Chocolate Mountains in relation to selected faults of the evolving San 

Andreas Fault System in southern California--sourced from U.S. Geological Survey, Earthquake

Hazards Program (2016), and from Powell (1993).

NASA Space Shuttle Photograph STS103-701-39-3

Monterey

Bay

Pacific

 Ocean

Gulf of

California

UNITED STATES

MEXICO

Colorado River

San Gabriel

Mountains

San Gorgonio

Pass

Chocolate Mts

+

Neversweat

Ridge

Peter Kane

 Mtn

Trigo Mts

+

Salton

 Sea

San Francisquito Fault

Clemens Well

Fault

Early & Middle Miocene

San Andreas Fault

Pliocene & Quaternary

San Andreas Fault

Pliocene & Quaternary

San Andreas Fault

Pliocene (?) 

San Andreas Fault

Late Miocene

San Andreas Fault

Fenner Fault

Garlock Fault

Pinto Mtn

Fault

Salt Creek Fault

Quaternary spreading centers

& transform faults

of San Andreas System

Salton Trough

Mojave

 Desert

Sierra Nevada

Great Valley

Transverse Ranges

Peninsular Ranges

Salinian Block

Coast Ranges

Chocolate Mts

Pacific

 Ocean

Gulf of

California

Colorado River

UNITED STATES

MEXICO

California

Arizona

Salton

 Sea