Download Magmatism and tectonics in a tilted crustal section through a

The Geological Society of America
Field Guide 11
2008
Magmatism and tectonics in a tilted crustal section through a
continental arc, eastern Transverse Ranges and
southern Mojave Desert
Andrew P. Barth
Department of Earth Sciences, Indiana University–Purdue University, Indianapolis, Indiana 46202, USA
J. Lawford Anderson
Department of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA
Carl E. Jacobson
Department of Earth and Atmospheric Sciences, Iowa State University, Ames, Iowa 50011, USA
Scott R. Paterson
Department of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA
Joseph L. Wooden
U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA
ABSTRACT
This field guide describes a two-and-one-half day transect, from east to west
across southern California, from the Colorado River to the San Andreas fault. Recent
geochronologic results for rocks along the transect indicate the spatial and temporal
relationships between subarc and retroarc shortening and Cordilleran arc magmatism. The transect begins in the Jurassic(?) and Cretaceous Maria retroarc fold-andthrust belt, and continues westward and structurally downward into the Triassic to
Cretaceous magmatic arc. At the deepest structural levels exposed in the southwestern part of the transect, the lower crust of the Mesozoic arc has been replaced during
underthrusting by the Maastrichtian and/or Paleocene Orocopia schist.
Keywords: California, structural geology, petrology, geochronology, tectonics
OVERVIEW
Achieving the goal of understanding the geodynamic evolution of a convergent continental margin arc requires an understanding of the interplay between magmatic and tectonic processes
through time. How did shortening in the southwestern North
American Cordillera relate in time and space to arc magmatism?
Advances in geochronology applied to regional field and petrologic studies in an exhumed arc and its associated thrust belts in
southern California are illuminating the timing of underthrusting
and shortening relative to voluminous arc magmatism.
Cordilleran foreland shortening and arc magmatism were
broadly contemporaneous, but their relative timing remains
poorly known. As a result, it has long been argued whether
Barth, A.P., Anderson, J.L., Jacobson, C.E., Paterson, S., and Wooden, J.L., 2008, Magmatism and tectonics in a tilted crustal section through a continental arc,
eastern Transverse Ranges and southern Mojave Desert, in Duebendorfer, E.M., and Smith, E.I., eds., Field Guide to Plutons, Volcanoes, Faults, Reefs, Dinosaurs,
and Possible Glaciation in Selected Areas of Arizona, California, and Nevada: Geological Society of America Field Guide 11, p. 101–117, doi: 10.1130/2008.
fld011(05). For permission to copy, contact [email protected]. ©2008 The Geological Society of America. All rights reserved.
101
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Barth et al.
shortening leads to, or is the structural response to precursory
magmatic thickening in the arc. As higher precision U-Pb ages
become more readily available for more plutons in the Cordilleran arc, it seems likely that batholith emplacement was an episodic phenomenon superimposed on a longer-term background
magmatic flux. In a like manner, more precise geochronologic
control on evolution of the foreland fold-and-thrust belt is necessary to better establish the orogen-wide stress field over long time
scales, and thus clarify whether foreland shortening is a cause or
consequence of convergent margin arc magmatism.
This field trip offers an overview of the tectonic evolution
of a portion of the retroarc region and a tilted section through the
crust of the Cordilleran arc. An east to west transect will afford
us a view of arc tectonics in the Cretaceous, and a top-down view
of variations in the composition and emplacement style of Mesozoic igneous rocks in a tilted arc section. Comparison of the plutonic and shortening records allows us to relate deformation in
time and space to the progress of arc magmatism.
The tilted arc section was likely created during shallow
underthrusting of oceanic lithosphere beneath this long-lived
Mesozoic arc, and so the timing of this underthrusting event is key
to understanding arc extinction and exhumation. In the Orocopia
Mountains, exhumation has exposed detached plates of moderate
and deep structural levels of the arc. These are underlain by Orocopia Schist, which is part of the Pelona-Orocopia-Rand schist
subduction complex underplated beneath southern California and
western Arizona during the Laramide orogeny. Here we will consider both the underplating and exhumation history of the schist,
as well as geologic relations within the overlying crystalline and
supracrustal rocks of native North America.
On this trip, we will follow an east to west transect from
the retroarc fold-and-thrust belt along the Arizona–California
border, westward into the Mesozoic arc. Along this transect, the
timing of Mesozoic plutonism is now reasonably well characterized using U-Pb geochronology, which has also been applied
to estimating the timing of shortening in the retroarc and subarc
regions. At this latitude, the Mesozoic plutonic record incorporates overlapping segments of Permo-Triassic, Middle Jurassic, Late Jurassic and Late Cretaceous arc segments. As we
progress further to the west into the core of the arc, we will
see a region of the arc that enjoyed shallow underthrusting and
consequent extreme extension during and following the early
Cenozoic Laramide orogeny. The result of this variable extension is regional west-side-up tilting that allows us a view of
depth-dependent changes in arc magmatism and processes of
magma transport and emplacement.
DAY 1
Directions to Stop 1
Depart Blythe, traveling west on Interstate 10 (I-10). Exit Mesa
Road [UTM E 0710924 N 3721114 (NAD 27 CONUS)]. Continue west on Black Rock Road to Stop 1 [0708035 3721024].
Stop 1. Mule Mountains Thrust Zone and McCoy
Mountains Formation
This stop finds us in the south-central part of the McCoy
basin, a west-northwest–trending basin in western Arizona and
southeastern California. The later structural evolution of the
basin is characterized by crystalline thrust sheets of opposing
vergence, primarily exposed along the margins of the basin. The
basin margins are visible as Middle Jurassic plutonic rocks thrust
over Jurassic volcanic rocks in the Mule Mountains visible to
the south, and Proterozoic basement and Paleozoic cratonal cover
in the Maria Mountains visible to the northeast (Stone, 2006;
Fig. 1). At this location, we can see rocks characteristic of the
basin and its margins; Jurassic metavolcanic rocks of the Dome
Rock sequence are thrust northward over clastic sediments of the
Jurassic(?) and Cretaceous McCoy Mountains Formation.
The earlier evolution of the McCoy basin is controversial, depending on interpretation of the origin of the 4–7.5 km
of clastic sediment of the McCoy Mountains Formation that
have yielded very few, problematic fossils. An inferred Jurassic depositional age for the McCoy Mountains Formation led to
an early hypothesis that during most of its evolution the basin
was bounded by sinistral transtensional faults of the hypothetical
Mojave-Sonora megashear (Harding and Coney, 1985; Saleeby
and Busby-Spera, 1992; Anderson and Nourse, 2005). Alternatively, an inferred Cretaceous depositional age (Reynolds et al.,
1986; Stone et al., 1987; Tosdal, 1990; Tosdal and Stone, 1994)
led to the hypothesis that at least some basin sedimentation was
associated with shortening in the Maria fold-and-thrust belt.
Figure 1. Simplified cross section (adapted from Stone, 2006) of the McCoy Mountains Formation (gray shaded) in the McCoy basin. Tu—undifferentiated Tertiary; Mg—Mesozoic granitic and gneissic rocks; Jv—Jurassic volcanic rocks; Pz—Paleozoic and Triassic(?) sedimentary rocks;
Xg—Proterozoic crystalline rocks.
Eastern Transverse Ranges and southern Mojave Desert
103
Recent petrography, zircon geochronology and geochemistry of McCoy sandstones provide new limits on the timing and
origin of McCoy basin fill (Barth et al., 2004). Detrital zircon
populations in sandstones vary systematically with stratigraphic
height (Fig. 2) and sandstone composition. Minimum detrital zircon ages of 116 and 109 Ma in the lower part of basal sandstone
member 2 demonstrate that >90% of the formation was deposited in post-Aptian, middle Early to middle Late Cretaceous
time. Comparison of detrital zircon ages to sandstone compositions indicates that sand was derived from both sides of the basin,
and that deposition was synchronous with volcanism to the west
and shortening and exhumation in the fold-and-thrust belt. These
results support the hypothesis that the McCoy basin originated as
a retroarc foreland basin during Cretaceous thrusting. This result
is significant because the McCoy basin then links shortening and
foreland basin sedimentation in the Maria fold-and-thrust belt to
contemporaneous retroarc shortening in the southern Sevier belt
to the north and the Sonoran thrust belt to the south, recording
regional crustal shortening synchronous with voluminous upper
crustal arc magmatism.
Directions to Stop 2
Depart stop, return east on Black Rock Road to I-10.
Continue west on I-10. Exit Eagle Mountain Road [0643527
3730127]. Continue north on Eagle Mountain Road to Stop 2
[0643247 3738425].
Stop 2. Middle Jurassic Eagle Mountain Pluton, Overlain
by Pliocene Alkali Basalt
Figure 2. Stratigraphic column for the McCoy Mountains Formation
in its type section in the McCoy Mountains (Barth et al., 2004). Symbols to the right of the column show sample locations and U-Pb zircon
minimum detrital ages.
Middle to upper crustal portions of two juxtaposed Mesozoic
magmatic arcs are exposed in the area being investigated in this
field trip, one being the Mojave province of the Cordilleran orogen and the other being the “exotic” or “suspect” Tujunga terrane
(also termed the “San Gabriel Terrane”), that, in part, comprises
the Transverse Ranges, including the San Gabriel Mountains, and
which continues southeastward into the Mojave Desert to include
desert mountain ranges south of exposures of the McCoy Mountains Formation, including the Chuckwalla, Little Chuckwalla,
Chocolate, Pinto, Eagle, and Mule Mountains. The Tujunga terrane, a fault-bounded block underlying 21,000 km2 of southern
California and adjacent Arizona, has been termed a suspect terrane due to its distinctive crystalline units that appear to have no
correlative ties to native North America, including its exposure of
1190 Ma anorthosite (Barth et al., 2001a). The Tujunga terrane is
everywhere allochthonous. Crystallization thermobarometry of
dated plutons within the terrane has indicated that much of its
apparent “suspect” nature stems from its partial derivation from
the middle crust.
Batholiths of the central and western Mojave have been
studied by Coleman and Walker (1992), Coleman et al. (1992),
and Miller and Glazner (1995). Large intrusions also comprise
the central and eastern Mojave Desert and early studies include
those of Anderson and Rowley (1981), Beckerman et al. (1982),
Miller et al. (1982), Howard et al., (1987), John (1987), Miller
et al. (1990), Anderson and Cullers (1990), Miller et al. (1992),
Anderson et al. (1992), Young et al., (1992), Miller and Wooden
(1994), Gerber et al. (1995) and Mayo et al. (1998). The Mojave
Desert region also contains other basement terranes of uncertain
origin, including the Joshua Tree terrane. Bender et al. (1993)
have concluded that the Joshua Tree terrane is contiguous with
the Mojave and that the allochthonous Tujunga terrane represents a displaced, middle crustal section of the Mojave block.
Mesozoic plutons of the Tujunga terrane (Barth et al., 1990;
Barth et al., 1995) bear strong isotopic and elemental affinities
to the plutons of the eastern Mojave, including essentially identical time-transgressive and compositional changes in magmatic
arc construction, including both Early Proterozoic and Mesozoic
plutons, thus suggesting that the Tujunga terrane is a displaced
portion of the Mojave province (Bender et al., 1993; Anderson et
al., 1992; Anderson et al., 1993).
Mesozoic batholiths of the Mojave Desert and the Tujunga
terrane occur in three distinct pulses (Fig. 3). Scattered Triassic intrusions, often distinctly K-feldspar megacrystic, were
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15
Number of Analyses
Mojave U-Pb Plutonic Ages
Tujunga
Mojave
Enclave suites
10
5
0
50
100
150
200
250
Age (Ma)
Figure 3. Three pulses of Mesozoic batholith emplacement in the Mojave Desert and the Tujunga terrane.
emplaced between 250 and 207 Ma (Barth and Wooden, 2006).
After a magmatic lull of ca. 40 Ma, Jurassic magmatism led to
widespread intrusion between 165 and 143 Ma. The Independence
dike swarm was emplaced at 149 Ma late in this magmatic epoch
(Carl and Glazner, 2002). Following another magmatic lull, one
of 45 Ma, renewed magmatism in the mid Cretaceous progressed
to a peak of activity at 82–72 Ma. Similar-aged magmatic episodes occur throughout the Cordilleran orogen of western North
America, but nowhere else in the orogen is there the range of
crustal depth of pluton emplacement as seen in the Mojave province, including age-correlative units in the Transverse Ranges.
Mid-crustal plutons emplaced at depths greater than 20 km occur
both in allochthonous sheets in Mesozoic thrust fault complexes
and in the lower plates of Cenozoic metamorphic core complexes
(Anderson, 1996). Interestingly, all Mesozoic plutons of the
Mojave, regardless of age, were derived from high fO2 magmas,
as shown by their high magnetite content. Low fO2 granitic magmas typically have ilmenite as the principle Fe-Ti oxide phase as
found for considerable eastern portions of the Peninsular Ranges
batholith (Shaw et al., 2003)
Triassic Plutonism
Several Triassic plutons occur in southern California
including in the San Gabriel and Mule Mountains of the Tujunga
terrane and the Granite I, Little San Bernardino and San Bernardino Mountains. U-Pb (zircon) age determinations range
from 250 to 207 Ma (Barth et al., 1990; Miller, 1977; Frizzell et
al., 1986). (Note: there are four named Granite Mountains in the
Mojave Desert. What we term Granite I borders the San Bernardino Mountains and Granite II in the central Mojave Desert
is adjacent to the Providence Mountains.) The plutons of both
the Mojave province and the Tujunga terrane share many common megascopic and petrologic attributes making them much
different from those of Jurassic and Cretaceous age. Metaluminous, high K2O + Na2O, and often of low silica (Fig. 4), the
plutons are principally of monzonite and quartz monzonite
with lesser amounts of monzodiorite and diorite. Most are
alkalic, with a shoshonitic affinity. High abundances of Ba, Sr
(>1000 ppm) (Fig. 5), and light rare earth elements are also
characteristic as first recognized by Miller (1977). Megacrystic K-feldspar is common. High feldspar content is reflected by
low abundances of Fe, Mg, and Ti. The principal mafic minerals
are hornblende and biotite ± clinopyroxene and garnet. The garnet coexisting with hornblende in the San Gabriel Mountains
Mount Lowe intrusion (Tujunga terrane) and the quartz monzonite pluton of Granite I Mountains (Mojave province), is exceptionally enriched in grossular and andradite components. Barth
(1990) has documented that the Mount Lowe was emplaced in
the middle crust based on calculated pressures of 5.5–7.0 kbar,
consistent with several mineralogical attributes of deep-seated
crystallization, including markedly aluminous hornblende (to
>11% Al203), calcic garnet, magmatic epidote, and siliceous
primary muscovite.
For the plutons in the San Bernardino and adjacent Granite I Mountains, Miller (1978) has argued that the magmas were
derived from a large ion lithophile element-enriched, quartz
eclogite source. The Mount Lowe intrusion is an immense, batholithic-sized (>300 km2), zoned plutonic complex that occurs in
the San Gabriel Mountains and in correlative exposures east of
San Andreas fault (Chocolate, Little Chuckwalla, Mule, and Trigo
Mountains). Based on a broad database of elemental and Sr, Pb,
and Nd isotopic data, Barth and Ehlig (1988) and Barth et al. (1990)
have argued that the marginal zone of the intrusion was derived
from low degrees of partial melting of an eclogitic and enriched
subcontinental lithospheric source with virtually no input of continental material, whereas the central zone formed originally in a
similar manner but with considerable crustal enrichment.
Jurassic Plutonism
Magmatic arc construction in the southern Cordillera and in
the Tujunga terrane became a major feature of the orogen by the
mid-Jurassic. The plutons are largely metaluminous, typically
contain hornblende ± clinopyroxene and include gabbro, diorite, quartz monzodiorite, quartz monzonite, quartz syenite, and
syenogranite. The granitic rocks are coarse grained and seriate to
porphyritic with large, lavender-colored K-feldspar phenocrysts
(Tosdal et al., 1989).
The plutons have metasomatic effects not encountered in
intrusions of Triassic or Cretaceous age. Zones of albitization
occur in the Bristol, Ship, and Marble Mountains plutons. Based
on stable isotopic data of the altered rocks, Fox and Miller (1990)
have interpreted the fluids to be of meteoric origin. Replacement
deposits of massive magnetite occur at upper crustal contacts
with Paleozoic marbles in the Providence and Eagle Mountains,
which have had considerable historical mining interest. Hall et al.
(1988) present isotopic data supporting a magmatic origin of the
fluids leading to these iron deposits.
Crystallization thermobarometry shows a range of determined emplacement depth (Fig. 6). Upper crustal complexes
occur in the Providence, Marble, Bristol, Chuckwalla, and Eagle
Mountains (1–3 kbar). In contrast, deep-seated Jurassic complexes
Eastern Transverse Ranges and southern Mojave Desert
12
105
80
Cretaceous Plutons
Cretaceous Plutons
75
70
8
alkaline
SiO 2 (wt %)
K 2O + Na 2O (wt. %)
10
6
2
45
55
60
CA
Mojave
Tujunga
Tujunga
subalkaline
50
60
55
Mojave
4
65
50
65
70
75
TH
45
0.5
80
0.6
0.7
SiO 2 (wt %)
0.8
0.9
1.0
FeO*/(FeO*+MgO)
12
80
Jurassic Plutons
Jurassic Plutons
75
70
alkaline
SiO 2 (wt %)
K 2O + Na 2O (wt. %)
10
8
6
Mojave
65
60
CA
Mojave
55
Tujunga
Tujunga
4
subalkaline
50
TH
2
45
50
55
60
65
70
75
45
0.5
80
0.6
0.7
0.8
0.9
1.0
0.9
1.0
FeO*/(FeO*+MgO)
SiO 2 (wt %)
80
12
Triassic Plutons
Triassic Plutons
75
10
SiO 2 (wt %)
K 2O + Na 2O (wt. %)
70
8
alkaline
6
65
60
Mojave
subalkaline
4
55
Tujunga
CA
Mojave
50
2
45
50
55
60
65
SiO2 (wt %)
70
75
80
45
0.5
Tujunga
TH
0.6
0.7
0.8
FeO*/(FeO*+MgO)
Figure 4. Comparison of the silica content with alkali and iron enrichment in Mesozoic plutons in the Mojave Desert and Tujunga terrane.
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Barth et al.
10000
and 206Pb/204Pb defines a strikingly linear array, suggesting 1.6–
1.7 Ga crustal rocks were an important component of the source
region (Anderson et al., 1990; Fox and Miller, 1990; J.L. Wooden,
unpublished data). Young et al. (1992) modeled the origin of plutons in the Granite and Bristol Mountains by partial melting of
hydrous and enriched mantle coupled with variable assimilativefractional crystallization involving ~10% Proterozoic crust.
Triassic
Sr (ppm)
1000
Jurassic
100
10
1
45
Whipple K
Other Mojave K
Tujunga K
Mojave J
Tujunga J
Mojave Tr
Tujunga Tr
50
55
Cretaceous
60
65
70
75
80
SiO2 (wt %)
Figure 5. Decrease in mean Sr content as a function of age of Mesozoic plutons.
have been identified the Granite II and Cargo Muchacho Mountains (6–7 kbar) (Anderson, 1996, and references therein).
Compositionally, these plutons are unlike those of Cretaceous or Triassic age but no significant difference occurs across
the inferred position of the Mojave-Tujunga terrane boundary.
Total alkalis are elevated, Fe/Mg ratios are intermediate (transitional between calc-alkaline and tholeiitic), and Sr abundances
are moderate. Alkali-lime indexes require the existence of both
calc-alkalic and alkali-calcic members. Limited isotopic data
show δ18O values clustering near 8‰, Sri ranging from 0.706 to
0.709, and εNdt from −6 to −9 (Fig. 7). Covariation of 207Pb/204Pb
Cretaceous Plutonism
After extended post-Jurassic magmatic lull, voluminous
Cretaceous igneous activity in both the Tujunga terrane and the
Mojave region began at ca. 95 Ma and reached a peak at 82–
72 Ma (Miller et al., 1982; Beckerman et al., 1982; Wright et al.,
1986, 1987; Anderson et al., 1990).
As found for the Jurassic plutons, the depths of emplacement
of the Cretaceous vary widely (Anderson, 1996). Mid-crustal
intrusions have been found in the San Gabriel, Old Woman,
Granite II, Chemehuevi, and Whipple Mountains (Anderson et
al., 1988). Plutons in Joshua Tree National Park were largely
intruded at shallow crustal levels, but include a mid-crustal section (Needy et al., 2006); upper crustal intrusions have been identified in the Teutonia and Cadiz Valley batholiths, the Chuckwalla
Mountains, and the Sacramento Mountains core complex.
Compositionally, and by rock type, the Cretaceous suites of
the Mojave and the Tujunga terrane are distinct from the magmatic activity of earlier Mesozoic intrusions. None are alkalic.
The K2O abundance is lower (at medium to high K) and most
plutons are relatively silicic (>66 wt% SiO2) and calc-alkaline.
Two-mica granites are common, as are metaluminous hornblende-biotite-sphene granodiorites. Sr contents are usually less
than 800 ppm, except for the calcic and Sr-rich granitoids of the
Whipple core complex. Available isotopic data are currently limited to plutons in the Whipple, Chemehuevi, San Gabriel, and
Old Woman Mountains and include δ18O values from 7 to 9‰,
Mid-crustal plutons
Mojave Plutons (Hbld - Plag)
Axtel qd
10
Cargo Muchacho qm
granite solidus
Pressure (kb)
8
San Gabriel grd
tonalite solidus
Granite J spgr
Granite K gr
6
Intermediate-level
29 Palms qm
Queen Mtn grd
4
Sacramento grd
Rock Spgs qmd
2
0
600
Shallow intrusions
Eagle Mtns grd
War Eagle qd
650
700
750
Temperature °C
800
850
War Eagle grd
Mid Hills gr
Providence qm
Figure 6. Crystallization thermobarometry for Mesozoic plutons.
Eastern Transverse Ranges and southern Mojave Desert
10
Stop 3. Late Jurassic Granite of Cottonwood Intruding
Proterozoic Gneiss
tle A
Man
5
rra y
Subcontinental lithosphere
CHUR
0
Triassic
Jurassic
Cretaceous
εNd i
-5
-10
-15
Proterozoic crust
-20
0.700
107
0.705
0.710
0.715
87Sr/86Sr
i
0.720
0.725
Figure 7. Initial Sr versus epsilon Nd in Mesozoic plutons.
Sri from 0.706 to 0.711 (to 0.719 for the strongly peraluminous
granites of the Old Woman Mountains), and εNdt from −10 to −17
(John and Wooden, 1990; Anderson and Cullers, 1990; Barth et
al., 1995; Miller et al., 1990). Tested models show a broad range
of magma origins for the above complexes, including significant
derivation from heterogeneous Proterozoic crust, but with some
variable input of radiogenic mantle and/or mafic crust mixed with
a Proterozoic component.
The main compositional difference between the Cretaceous
plutons is seemingly related to depth of emplacement. While the
shallowly emplaced (<12 km) complexes tend to be fundamentally granitic and often leucocratic, deeper Mesozoic plutons in
both areas (Tujunga and Mojave) tend to be more mafic including
metaluminous diorite to quartz diorite and marginally metaluminous to calcic peraluminous tonalite and granodiorite. Most of the
shallow plutons appear to have been largely crustal derived. In
contrast, the origin of plutonism in the deeper complexes is interpreted to be more variable, including (1) derivation from Proterozoic crust, including granites of the Chemehuevi and Old Woman
Mountains (John and Wooden, 1990; Miller et al., 1990), (2) derivation from mafic crust with significant felsic crustal interaction,
including calcic granitoids of the Whipple core complex (Anderson and Cullers, 1990) and (3) mantle-derived, such as the Josephine tonalite of the San Gabriel Mountains (Barth et al., 1995).
Directions to Stop 3
Depart stop, return south on Eagle Mountain road to I-10.
Continue west on I-10. Note that Chiriaco summit [0618500
3725100] is the last fuel before driving through Joshua Tree
National Park. Exit Cottonwood Springs Road [0611140
3725050]. Continue north on Cottonwood Springs Road, past the
park entrance [0611170 3726600] to Stop 3 [0610180 3731570].
We have now traveled ~35 km west-southwest, and are now
west of the main body of the Middle Jurassic arc visited at Stop 2.
As noted there, Mesozoic batholiths of the Mojave Desert and the
Tujunga terrane were emplaced in distinct Triassic, Jurassic and
Cretaceous pulses. As we move west, we move both structurally
deeper and outboard through the arc. At this stop we are structurally deeper, but still at relatively shallow crustal levels. Here we
can see a Late Jurassic pluton intruding Proterozoic crystalline
basement that underlies supracrustal sequences to the east. Here
the pluton is composed of felsic biotite granite cut by mafic dikes,
characteristic of the relatively outboard, Late Jurassic intrusions
of the Jurassic batholithic pulse (Barth et al., 2008).
Directions to Stop 4
Depart stop, continue north on Cottonwood Springs Road.
Note that the park Visitor Center [0608950 3734665] issues permits to pass through the park, and is also the last water source in
the park. Here you can also purchase a comprehensive guide to
park geology (Trent and Hazlett, 2002). Note also that there is
currently reservable group camping at Cottonwood Campground,
located just east of the Visitor Center. Continue north on Cottonwood Springs Road to Stop 4 [0611577 3738280].
Stop 4. Late Jurassic and Late Cretaceous Granite Plutons
This stop finds us in the northern Cottonwood Pass area of
the southcentral part of Joshua Tree National Park, between the
Hexie Mountains to the west and the Eagle Mountains to the east.
Here we have a 360° view of major plutonic rock units that characterize the central part of the park. In this region we are west
of the main body of the Middle Jurassic arc, where the PermoTriassic, Late Jurassic and Late Cretaceous arcs overlap. Dated
plutons in this area include the Triassic Munsen Canyon pluton
(Barth and Wooden, 2006), the Late Jurassic Cottonwood pluton (Barth et al., 2008) and the Late Cretaceous Porcupine Wash
and Smoke Tree Wash plutons (Fig. 8). Compositions of plutons
in central Joshua Tree National Park illustrate regional secular
compositional variation in arc plutons, here expressed in these
comparatively homogeneous plutons. Permo-Triassic plutons are
typically composed of alkalic monzodiorite to monzonite, and
true granite is relatively rare. Rock compositions are characterized by high Sr and low Rb/Sr. Late Jurassic and Late Cretaceous
plutons are typically calc-alkalic granodiorite to granite, and have
lower Sr and exhibit more linear Rb/Sr covariation (Fig. 9).
The Smoke Tree Wash pluton is an excellent example of
a discordant, relatively homogeneous pluton that characterizes
the central part of the tilted section. It has at least three different phases: the most abundant are coarse and medium-grained
phases of k-feldspar, plagioclase, quartz, biotite ± hornblende
felsic granodiorite to granite. K-feldspars tend to form larger
108
Barth et al.
Figure 8. Simplified geologic map of plutonic units in the Cottonwood Pass area, adapted from Jennings (1967)
and Powell (2001a, 2001b).
400
29Palms
350
Queen
Eagle
Rb (ppm)
300
White
250
Palms
200
Oasis
150
100
50
0
0
500
1000
Sr (ppm)
1500
Figure 9. Secular compositional
variation among relatively homogeneous middle to upper crustal
Mesozoic plutons in the eastern
Transverse Ranges (Brand, 1985;
Mayo et al., 1998; Palmer, 2005;
K. Howard, unpublished data).
Eastern Transverse Ranges and southern Mojave Desert
phenocrysts in the medium grained phase. There is also a slightly
more mafic, medium grained granodiorite, discontinuously
exposed near the contact with the host gneisses that is also richer
in microgranitoid enclaves. All three units are cut by aplitic to
pegmatitic, probably radial dikes that also extend into the host
and discordantly cut host fabrics. These granitic dikes are variable in composition some with large k-feldspars, some with two
micas and a few with rare garnet.
Structural observations clearly separate wall rock deformation from pluton emplacement. The host gneisses comprise both
paragneisses and orthogneisses, all strongly foliated with the
dominant foliation parallel to the axial planes of isoclinal folds.
These folds and foliation are refolded by locally developed more
open folds with variable orientations. All of these structures are
clearly cut by the pluton and by dikes emanating from the pluton.
Magmatic fabrics in the pluton are present, but weak. Orientations are quite variable but there is some suggestion that both the
magmatic foliation and locally more intense magmatic lineation
have fairly shallow orientations. We don’t yet know if this reflects
a regional fabric such as that seen in the host or if the shallow
orientation reflects the presence of a nearby roof. The contact
with the host is very discordant, takes numerous decameter-scale
steps, and is bordered by xenoliths of host gneiss in the pluton,
some of which are clearly rotated and stoped blocks. The contact
has a fairly steep dip on the sides of the pluton but then rolls over
into a more gently dipping contact in locations that we interpret
to be close to the roof of the pluton.
Directions to Stop 5
Depart stop, continue north on Cottonwood Springs Road to
Stop 5 [0613737 3744989].
Stop 5. Late Cretaceous Granodiorite: Overview of
Pinto Basin
At this brief stop in the northern part of the broader Cottonwood Pass area, we can examine somewhat more mafic granite and granodiorite of Late Cretaceous age, in comparison to
the relatively leucocratic granites at the last two stops (Fig. 8).
This stop also provides a scenic overview of the topography and
geology of the eastern part of the National Park. To the north
is Pinto Mountain, composed of Mesoproterozoic sedimentary rocks overlying Paleoproterozoic gneiss and metagranite.
The eastern side of this ridge and the lower, dark hills to the
right (east) are composed of Jurassic granitic rocks of the Pinto
Mountains, similar to those described at Stop 2. In the foreground is Pinto Basin, eroded out of fractured rock along the
(transtensional?) Blue Cut fault (Powell, 1981). To the northeast
across Pinto Basin are the northern Eagle Mountains, composed
of rocks similar to the Pinto Mountains, but the low dark hills
at the foot of the Eagle Mountains are alkali basalts of Pliocene
age. The distinctly lighter-colored range on the skyline in the
background is the Coxcomb Mountains (also seen from Stop
109
2), composed of Late Cretaceous granitic rocks intruding the
McCoy Mountains Formation.
Directions to Stop 6
Depart stop, continue north on Pinto Basin Road to Stop 6
[0591190 3760244].
Stop 6. Late Jurassic White Tank Granite Intruding
Paleoproterozoic Gneiss
The plutons we will see on this field trip, and in all arcs on
Earth, show highly variable characteristics. We should thus not
be surprised to find that there are more than one means by which
plutons are emplaced. As we evaluate different emplacement
models, we need to keep in mind the following:
1. Plutons grow incrementally or continuously over some
duration of time. Thus “emplacement” also occurs over
some length of time and may or may not be an episodic
process.
2. There is widespread evidence that host is displaced by
multiple processes during chamber construction and
that these processes vary with depth, distance from the
pluton, and time.
3. As younger magma pulses arrive in a growing chamber,
the “emplacement problem” may largely involve the
displacement of older magmatic pulses rather than the
original host.
4. Some “host-rock” transfer processes, such as stoping
may remove evidence of earlier processes.
5. The displacement near plutons of regional pre-emplacement markers is an invaluable tool for determining the
general direction that host rock is displaced during
emplacement.
On this trip we will see two general types of plutons (1) roughly
circular (in map view) bodies with steep walls that typically cut
discordantly across pre-existing structures; and (2) more gently
dipping sheeted bodies, often deformed in the magmatic or subsolidus state, that are typically concordant to structures in the host,
but on close inspection may have some discordant margins. At
this stop we are viewing the southeast margin of the White Tank
pluton, an excellent example of the first style of pluton. Here the
pluton contact has a fairly steep dip (but when mapped in detail is
somewhat irregular) and on a regional scale is very discordant to
structures in the host gneisses. Locally these structures bend into
parallelism with the margin. But only very minor emplacementrelated strain is seen in the contact aureole. Elsewhere we find
xenoliths of the host gneisses in the pluton, some which may be
rafts (unrotated and only slightly displaced xenoliths), and others
that are clearly rotated stoped blocks. Host xenoliths are not volumetrically common in the plutons and rapidly decrease in size and
number with distance from the pluton margin.
One of the exciting new observations from these plutons is
that they have geochronologic and/or geochemical data suggesting
110
Barth et al.
that they are made up of more than one batch of magma. Unfortunately internal contacts between these pulses have not yet been recognized and examined in detail, information needed to fully understand how these bodies grew to their present size and shapes.
Even so we can use the shape of the plutons and nature of
their contacts with host to consider emplacement mechanisms.
It is clear that stoping did affect the final characteristics of these
contact aureoles and thus we have probably lost information
about previously operating emplacement mechanisms. We will
discuss the pros and cons of emplacement models such as diapirs,
dike fed balloons, sheeted dike complexes, punch laccoliths, and
incrementally grown laccoliths and lopoliths at this locality.
End of Day 1.
DAY 2
Today we will take three moderate hikes to view rock units
of the Keys View quadrangle (Fig. 10), situated along the boundary between the middle and lower parts of the tilted crustal section that is the focus of this trip. In this quadrangle, the terrane of
Paleoproterozoic gneissic rocks intruded by relatively homogeneous plutons gives way, across a relatively abrupt boundary, to
a northwest-trending terrain of layered crystalline rocks lacking
discrete mappable plutons such as those we visited on Day 1. This
terrain was originally mapped as Precambrian(?) gneissic rocks,
and later interpreted as a largely late Mesozoic gneissic complex
with pervasive brittle deformation (Powell, 1981). More recent
geochronologic work (Needy et al., 2006) shows that this terrain
is largely composed of Jurassic and Cretaceous foliated igneous
rocks characterized by an intrusive geometry of concordant to
slightly discordant meter to decimeter thick sheets, typically dipping moderately north and east. Reconnaissance petrographic
work suggests that intrusive sheets record a broad compositional
spectrum of intrusions, but that the volumetrically dominant
components are tonalite-granodiorite and biotite ± muscovite
± garnet granite sheets. Thermobarometric results suggest that
components of this heterogeneous sheeted complex crystallized
at 15–22 km, ~5 km deeper than plutons exposed to the east.
Similarly layered complexes, recording similar paleodepths, are
exposed for an additional 100 km to the northwest (Powell, 1993;
Barth et al., 2001b). These results, and the widespread nature of
these sheeted rocks, suggest that the sheeted complex represents
exposure of the mid-crustal counterpart of the ignimbrites and
discordant plutons typical of the upper crust of the Cordilleran
continental margin arc at this latitude.
Regionally extensive (10s to 100s of km long) magmatic
“sheeted complexes” occur in other arcs and/or orogenic belts.
These complexes have characteristics that make them very distinct from single sheeted plutons and from adjacent and temporally overlapping magmatic belts such as the following: (1) they
often occur in the central core of orogens and/or near prominent
tectonic boundaries (2) their dimensions are much greater than
an individual pluton (often 100s of km long); (3) magmatic bodies in these complexes ALL have sheet-like or dike-like shapes
Figure 10. Simplified bedrock geologic map of the Keys View 7.5 min
quadrangle. This quadrangle encompasses the boundary between an eastern region of Paleoproterozoic gneiss and discordant plutons, typical of
most of the eastern Transverse Ranges, and the structurally lower sheeted
complex to the southwest. The Late Cretaceous Palms and Squaw Tank
granites are typical discordant plutons, and the Blue granodiorite, Stubbe
Springs and Rusty granites, and the Quail Mountain and Bighorn complexes together comprise the sheeted complex in this quadrangle.
versus the more irregular to often elliptical shapes outside them;
(4) although magmatism in these zones is typically long-lived (10s
to >100 m.y.), they ALWAYS form sheeted bodies, suggesting that
emplacement style is consistent over time; (5) they have a dramatic range in composition (gabbros to granites and cumulates to
late aplites and pegmatites) over spatially short distances that are
not organized into patterns, such as in normally or reversely zoned
plutons; and (6) sheeted plutons are usually strongly deformed in
the magmatic or solid state indicating the presence of long-lived
or recurrent deviatoric stress in the mid crust.
Eastern Transverse Ranges and southern Mojave Desert
For example, if we consider Mesozoic magmatism in the
North American Cordillera the following prominent sheeted complexes have been recognized: (1) The “Great Tonalite Sill” (really
a collection of sills or dikes) in the Coast Plutonic Complex; (2) the
Skagit Gneiss Complex in the Cascades crystalline core, Washington; (3) sheeted complexes in the Idaho suture zone; (4) a central
sheeted complex in Peninsular Ranges Batholith that extends from
southern California to at least as far south as the Sierra San Pedro
Martir; and (5) the sheeted complex in the Keys View quadrangle.
Such magmatic sheeted zones have been interpreted in a
number of ways, such as: (1) the deeper parts of magma plumbing systems that are feeding more elliptically shaped chambers at shallower levels (e.g., Cascades core); (2) magmatism
controlled by emplacement into an active fault or faults (e.g.,
Great Tonalite Sill); (3) magmatism along fundamental lithospheric-scale boundaries that may or may not be faults (e.g.,
Idaho suture zone and Peninsular Ranges Batholith examples,
interpreted to represent a transition from continental to oceanic
crust, with or without major faults); (4) dike swarms controlled
by a regional stress field; (5) sheeted bodies (whether or not
dikes) in which emplacement is strongly controlled by preexisting host rock anisotropy; (6) zones of syn-emplacement extension; and (7) combinations of the above.
Directions to Stop 7
Depart camp, travel southeast on Quail Springs Road to the
intersection with Keys View Road. Continue south on Keys View
Road to Stop 7 [0577200 3760650].
Stop 7. Hike West to View Late Cretaceous Palms Granite
and Intrusive Contact with Paleoproterozoic Gneiss
The grain size, apparent composition, and weak fabrics in
the Late Cretaceous Palms pluton do not change their general
appearance as the margin is approached, although local variations occur. The intrusive contact is often quite discordant to the
metamorphic layering and dominant foliation in the gneisses, but
there are some local zones where older structures are rotated into
subparallelism with the Palms contact. Some xenoliths (both rafts
and stoped blocks) certainly occur along the margin. The intrusive contact changes its orientation from place to place but the
overall pattern is that it dips fairly steeply to the northeast. There
is one interesting area where the Blue granodiorite has a gently
dipping upper intrusive contact with the gneisses, which is then
truncated by the fairly steeply dipping Palms contact. This is one
location that would indicate that the Palms did intrude more gently dipping sheeted intrusive rocks and gneisses.
111
Stop 8. Hike East to View Proterozoic Gneiss and Contact
with Late Cretaceous Blue Granodiorite
In the Keys View quadrangle (Fig. 10), we can examine a
wonderful transition between fairly circular (in map view) plutons
that in the field appear fairly homogeneous and weakly foliated
(Palms granite, Squaw Tank granite), to more elongate plutons
with more intense fabrics and clear evidence for multiple internal
sheets (e.g., Blue granodiorite, Stubbe Springs granite), to a very
heterogeneous sheeted complex showing a range of compositions
from gabbro to two mica granites, both Cretaceous and Jurassic
ages, and with variable fabric intensities (Quail Mountain complex, Bighorn complex).
At this stop, we will hike through the transition from Paleoproterozoic gneisses into the internally sheeted Blue granodiorite. Gneisses here are a mixture of both orthogneisses and paragneisses with local amphibolite. All are intensely deformed—we
see clear evidence for early isoclinal folds with the dominant fabric parallel to their axial planes. These early isoclines fold a previous foliation and metamorphic layering and are in turn refolded
by more open, upright folds. The regional metamorphic grade
was at least amphibolite facies. We also observe local development of andalusite, garnet, and sillimanite near the contact with
the Blue granodiorite, but more work is needed to sort out the
differences between Paleoproterozoic regional and Mesozoic
contact metamorphism.
Structures in the gneisses are discordantly intruded along a
fairly steeply dipping contact of the Blue granodiorite, although
the usual local zones of subparallelism exist. The intrusive contact takes a number of sharp steps and we do find xenoliths (some
large enough to be mapped) in the granodiorite. Along this eastern margin, the Blue granodiorite tends to be more homogeneous
(less sheeted) and has numerous mafic clots (small enclaves?)
in it. Magmatic fabrics steepen near the margin but are typically
fairly gently southwest dipping (foliation) or gently northwestsoutheast–plunging (lineation) in this body. Small compositional
and textural changes define internal sheets in the granodiorite.
More dramatic sheeting defined by 2-mica, garnet granites also
occurs. Small pods of Stubbe Springs–like granites intrude the
granodiorite, particularly as its western margin is approached.
Directions to Stop 9
Depart stop, travel west to return to Keys View Road, continue south on Keys View Road to the Keys View parking area,
Stop 9 [0575226 3754200].
Stop 9. Hike to View Sheeted Granodiorite and Granite along
Western Contact of Late Cretaceous Blue Granodiorite
Directions to Stop 8
Depart stop, continue south on Keys View Road to Lost
Horse Mine (dirt) Road, turn left and continue southeast on Lost
Horse Mine Road to parking area [0576530 3757780].
The western margin of the Blue granodiorite is often much
more cryptic because of the sheets within the granodiorite and
younger intrusions along this margin. We will hike along a transect perpendicular to this margin to examine the granodiorite
112
Barth et al.
and a number of granitic bodies intruding into the granodiorite
and end near Keys View where the midcrustal Bighorn complex
is exposed.
Here the Blue granodiorite is a medium-grained, hornblende
biotite granodiorite with a moderately well developed northweststriking, northeast-dipping magmatic foliation and northwest
trending lineation. Regionally, the magmatic foliation defines an
open synformal fold pattern with the northwest-southeast–trending fold axis subparallel to the mineral lineation. Locally, zones
of subsolidus deformation define steep zones of southwest-sideup shear. Bodies of 2-mica garnet granite intrude the granodiorite.
These bodies are less strongly foliated than the granodiorite and
have shapes ranging from sheet-like to blobby. Farther southwest,
granodiorites are increasingly intruded by a complex variety of
sheet-shaped bodies, locally separated by screens of orthogneiss
and paragneiss, and varying in composition from gabbros and
hornblende cumulates to 2-mica granites. Sheets vary in thickness from less than a meter to ≥10 m and generally dip moderately to the northeast. Intrusive relationships are complex, but
there is a general pattern of more mafic being intruded by more
felsic sheets. Many sheets have well-developed magmatic fabrics
locally overprinted by weak subsolidus deformation. These magmatic fabrics are more intensely developed in granodiorites and
are typically weaker in granites (Brown et al., 2006).
End of Day 2
DAY 3
In Days 1 and 2 of this trip, we traveled from east to west
through progressively deeper levels of the Cordilleran magmatic arc and its Proterozoic wall rocks. This morning we will
visit the northwestern Orocopia Mountains, which in their higher
structural levels include crustal rocks of North American affinity
similar to those viewed previously. However, the Orocopia Mountains also provide a window into an oceanic complex which was
underplated beneath much of southern California and adjoining
regions during the Late Cretaceous to early Cenozoic Laramide
orogeny. Here known as the Orocopia Schist, these rocks are part
of the larger Pelona-Orocopia-Rand Schist (Fig. 11; Haxel et al.,
2002; Jacobson, et al., 2007). Emplacement of the schists beneath
Figure 11. Geology of the Orocopia Mountains and vicinity (after Crowell, 1975). Inset shows distribution of Pelona-Orocopia-Rand Schists.
Gf—Garlock fault, LA—Los Angeles, SAf—San Andreas fault.
Eastern Transverse Ranges and southern Mojave Desert
the arc terrane represents a first order tectonic event that involved
the stripping away of the lowermost North American crust and
the entire thickness of underlying mantle lithosphere. The original
subduction thrust responsible for this event is preserved in only a
few areas. In most cases it has been replaced by syn-subduction
(i.e., Laramide age) and/or middle Cenozoic low-angle normal
faults. In the Orocopia Mountains, there is indirect evidence for
Laramide exhumation, but the primary contact between schist and
arc terrane (here referred to as the “upper plate”) is a major Miocene detachment fault (Orocopia Mountains detachment fault;
Robinson and Frost, 1996; Ebert, 2004; Jacobson et al., 2007).
Below we provide brief descriptions of the schist and upper plate.
Orocopia Schist
The schist in the Orocopia Mountains is composed dominantly of metagraywacke interpreted as trench sediment (Grove
et al., 2003; Jacobson et al., 2007). Detrital zircons indicate a
depositional age no older than ca. 70 Ma and perhaps as young
as 62 Ma (Fig. 12; but note that the schist protolith in other
113
areas is as old as 90 Ma). The schist also includes various rock
types inferred to have been scraped off the Farallon plate. These
include a few percent of metabasite with normal to enriched
mid-ocean ridge basalt composition and even lesser amounts of
Fe-Mn metachert, marble, serpentinite, and talc-actinolite rock
(Haxel et al., 2002). Hornblende 40Ar/39Ar ages of 54–50 Ma and
muscovite ages as old as ca. 50 Ma (Fig. 12) demonstrate that
underplating beneath North American crust and initial cooling
and exhumation occurred shortly after deposition of the graywacke protolith in the trench. Biotite ages of 30–20 Ma and Kfeldspar multi-diffusion domain analysis reveal a second, very
rapid, period of exhumation at ca. 24–22 Ma (Fig. 12). The latter is attributed to normal-sense slip on the Orocopia Mountains
detachment fault.
In most of the range, prograde assemblages in the schist
belong to the albite-epidote amphibolite faces. However, the
upper half of the exposed section shows extensive retrogression
to greenschist facies. We believe that this overprint is related to
the first (early Cenozoic) phase of exhumation. A thin (several m
to several 10s of m) mylonite zone right at the top of the schist is
800
700
leucogranite
intrusion
Upper Plate Rocks
an-sy unit
lg-gn unit (shallow)
lg-gn unit (deep)
accretion
of schist
Temperature (°C)
600
40
g
plutonic heatin
Orocopia Schist
mylonite
chl. zone
bio. zone
underthrusting
of schist
39
hornblende Ar/ Ar
500
Slip on CMF(?)
40
?
39
muscovite Ar/ Ar
400
40
39
biotite Ar/ Ar
300
deep
K-feldspar MDD
Slip on
OMDF
200
apatite FT
100
shallow
Maniobra Fm.
Diligencia Fm.
?
deposition
of schist
protolith
0
0
10
20
30
40
50
60
70
80
Time (Ma)
Figure 12. Generalized temperature-time paths for Orocopia Schist (left-dipping diagonal lines) and upper-plate rocks
(right-dipping diagonal lines) (from Jacobson et al. (2007)). Paths based upon apatite fission track data (boxes), detrital zircons (black circles), K-feldspar 40Ar/39Ar multi-diffusion domain analysis (snake-like bands), and hornblende
(diamonds), muscovite (pentagons), and biotite (hexagons) 40Ar/39Ar bulk closure ages. Schist versus upper-plate ages
can be identified by their proximity to the respective temperature-time bands. The exception is the apatite fission track
age denoted by the white box, which overlaps curves for both the schist and upper plate. This age result is from the
schist. Note that divergent paths have been indicated for upper-plate rocks present within the Orocopia Mountains
depending upon their proximity to the Orocopia Mountains detachment fault. CMF—Chocolate Mountains fault. See
Jacobson et al. (2007) for further explanation.
114
Barth et al.
considered to be a younger feature associated with the Orocopia
Mountains detachment fault.
Upper Plate
The upper plate includes three main lithologic units: Proterozoic gneiss, 1.2 Ga anorthosite-syenite, and 76 Ma leucogranite
(Crowell, 1962, 1975; Crowell and Walker, 1962; Barth et al.,
2001a; Jacobson et al., 2007). The gneiss is locally intruded by
the anorthosite-syenite, but in general these two units are spatially distinct. In contrast, the gneiss is intimately intruded by the
leucogranite, so we combine these two rock types in a single map
unit (leucogranite-gneiss) despite their disparate ages. In most
areas, the anorthosite-syenite unit sits structurally above the leucogranite-gneiss unit along a low-angle fault referred to as the
upper plate detachment fault (Figs. 11 and 13). Additional lowangle faults may also be present within the anorthosite-syenite
and leucogranite-gneiss units.
The anorthosite-syenite unit includes anorthosite, gabbro,
and syenite, as well as compositions (mangerite and jotunite)
intermediate between these three end members. Retrograde
metamorphism of inferred Proterozoic age is indicated by essentially universal replacement of primary igneous pyroxene by
hornblende and biotite, presence of hairline mesoperthite, and
local development of blue quartz in the more felsic units. Structurally lower levels of the anorthosite-syenite unit are intruded by
dikes and small stocks of the same leucogranite that abundantly
intrudes the gneiss.
The gneiss is largely quartzofeldspathic but also includes
a mafic component. Metamorphism was largely to amphibolite
faces, but reached granulite facies near intrusive contacts with the
anorthosite-syenite. As in the anorthosite-syenite, most granulite
assemblages have been retrograded to amphibolite facies, which
are commonly associated with blue quartz. The leucogranite associated with the gneiss generally has an aplitic texture. It consists
mostly of subequal amounts of quartz, plagioclase, and K-Feldspar with minor biotite that is commonly retrograded to chlorite.
The various structural levels of the upper plate show
diverse cooling histories (Fig. 12). In all cases, however, the
early to middle Cenozoic history is substantially different from
that of the Orocopia Schist. It is this disparity which indicates
that the Orocopia Mountains detachment fault can be no older
than early Miocene.
Directions to Stop 10
Depart camp, head southeast on Pinto Basin Road to return to
south park entrance station, continue south on Cottonwood Pass
Road to southern park boundary and intersection with I-10. Cross
I-10 and continue heading south. In 0.4 mi [611074 3724293],
turn left onto poorly maintained paved road. In 0.05 mi [611150
3724261], turn right onto dirt road that heads SSW up the alluvial
fan toward the Orocopia Mountains. In 0.75 mi, pass power line.
In another 0.65 mi, the road enters a canyon in modest hills of
Figure 13. Stop locations, day 3. See Figure 11 for location and explanation of fault hachures. UTM 1 km
grid ticks indicated. Abbreviations: an-sy—anorthositesyenite unit, lg-gn—leucogranite-gneiss unit, os—Orocopia Schist.
porphyritic granodiorite. Our limited dating indicates a possible
Jurassic age.
The road skirts the west side of the canyon. Approximately
0.4 mi after entering the canyon [610368 3721575] take the
road that leaves the main wash heading SSW. In 0.5 mi [610174
3720855] pass a small hill of Diligencia Formation. This unit
sits unconformably upon the granodiorite we just passed in the
canyon.
The road approaches a major drainage. At 0.3 mi past the
outcrop of Diligencia Formation [610279 3720381] veer to the
right off the road traversing the alluvial fan and descend into the
wash (it is easy to miss this intersection). Drive up the wash. Pass
highly altered outcrops of leucogranite in the low walls of the
wash. These are located on the northeast side of the Clemens
Well fault, which may be a strike-slip fault with modest to large
displacement or a steepened detachment fault related to the Orocopia Mountains detachment fault and upper plate detachment
fault (Crowell, 1962, 1975; Powell, 1993; Robinson and Frost,
1996). Continuing up the wash, we cross the Clemens Well fault
and pass into the leucogranite-gneiss unit.
Stop 10 [610556 3718428]. Start of Traverse, Upper Plate
Rocks
This is as far as one can drive up the wash (~1.5 mi upstream
from our entry point into the wash). Vehicle maneuverability is
limited and for the field trip we may park a bit below this point.
From here we will take a moderate hike up the canyon
examining the leucogranite-gneiss and anorthosite-syenite units
of the upper plate and the underlying Orocopia Schist. Particular emphasis will be placed on the early Miocene fault contacts
between these units. If time permits, we will climb up to the
main ridge of the Orocopia Mountains for a view into the Salton
Trough. We will examine rocks all along the traverse, with a few
key localities broken out as stops.
Eastern Transverse Ranges and southern Mojave Desert
115
We start our hike in the leucogranite-gneiss unit. Note the
high degree of brittle deformation and hydrothermal alteration.
The leucogranite cuts most of the ductile fabric in the upper plate,
but locally shows a modest foliation itself.
Return to vehicles. Depart stop, return to I-10, east on I-10 to
Blythe, north on U.S. Highway 95 to Las Vegas.
Stop 11 [610992 3717799]. Upper Plate Detachment Fault
The National Science Foundation (EAR-0106881, 0408730 and
0711119 to APB, EAR-9902788 and EAR-0106123 to CEJ), the
National Geographic Society (7214-02), the National Park Service, the Southern California Earthquake Center, Department of
Energy, and the Joshua Tree National Park Association provided
support for this research. We are grateful to the staff at Joshua
Tree National Park for supporting our work in the park, and
to Kenneth Brown, Kristin Ebert, Nicole Fohey, Marty Grove,
Kristin Hughes, Vali Memeti, Sarah Needy, Emerson Palmer,
Jane Pedrick, Geoff Pignotta, Kelly Probst, and Ana Vućić who
collaborated with us in the research summarized here. Robert
Powell, Paul Stone, Richard Tosdal, and Dee Trent provided
guidance and many helpful discussions, and Calvin Miller and
Gene Smith provided constructive reviews of the manuscript.
This is a truly outstanding exposure of the contact between
the leucogranite-gneiss and anorthosite-syenite units (upper plate
detachment fault). The leucogranite-gneiss is exposed at the base
of the outcrop. It is overlain along a sharp, low-angle contact by
the basal part of the anorthosite-syenite unit which itself is cut by
highly sheared dikes of leucogranite. This middle slice is in turn
overlain along another low-angle fault by a structurally higher
level of the anorthosite-syenite unit without leucogranite.
Note the well developed gouge and other indicators of brittle
deformation along the various contacts, demonstrating the relatively shallow nature of this fault system. Sense-of-shear indicators are not common but generally show top-NE to top-E sense
of movement. Note the steep faults of various orientations within
the different structural plates.
Stop 12 [610947 3717648]. Orocopia Mafic Schists
Mafic Orocopia Schist with albite porphyroblasts up to 4–
5 mm in diameter. Other major prograde minerals are hornblende
and epidote (albite-epidote amphibolite facies). Secondary chlorite is widespread in thin section. Metagraywacke in adjacent outcrops is dominated by quartz, albite, and muscovite with lesser
biotite and garnet. Secondary chlorite is present in both units.
Stop 13 [611070 3717546]. Orocopia Mountains
Detachment Fault
Contact between Orocopia Schist and the part of the anorthosite-syenite unit intruded by leucogranite dikes (as seen in
the middle plate of Stop 11). Note the mylonite and asymmetric
shear bands in the schist, but brittle deformation only in the anorthosite-syenite unit. This contrast in structural behavior confirms
the thermochronologic evidence (Fig. 12) that this contact is a
normal fault.
Stop 14 [611325 3717297]. Orocopia Mountains
Detachment Fault
The contrast between brittle deformation within the upper
plate but mylonitization of the schist is similar to that seen at Stop
13. Here the upper plate is transitional in composition between
the leucogranite-gneiss and anorthosite-syenite units (i.e., it
includes rocks typical of the anorthosite-syenite suite but exhibits
an exceptionally high degree of intrusion by the leucogranite). If
time permits, we will walk up this contact to the main Orocopia
ridge. Note the mylonitic nature of the schist along the contact,
similar to the textures observed at Stop 14.
ACKNOWLEDGMENTS
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