and U.S.
ArmyEarth Science Applications, National Training Center, Fort Irw
and U.S.
Army
1 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025
2 U.S. Geological Survey, University of Nevada, Reno, NV 89557
3 Dept. Of Geology, Western Washington University, Bellingham, WA 98225
4 Science Applications International Corporation, Golden, CO
ABSTRACT
Detailed geologic mapping west of Tiefort Mountain in the southwestern
part of Fort Irwin, California, has located several previously unmapped
faults and clarified the stratigraphy of Quaternary deposits. The age constraints
for Quaternary units provides a basis for assessing the ages of last movement
for several faults that cut some deposits and are overlapped by others.
Quaternary deposits closely match those previously described and dated in
other parts of the Mojave Desert and southern Great Basin, permitting general
age designations for the deposits. Relative-age criteria include: 1) characteristics
of microtopography, particularly of channels and interfluves, 2) size of
surface clasts, 3) degree of soil development, 4) degree of desert pavement
development, and 5) degree of desert varnish coating clasts. Widespread
units range from middle Pleistocene to latest Holocene, and locally distributed
units extend back to the latest Miocene. Several newly discovered faults
lie within blocks outlined by previously identified major faults; these
new faults generally accumulated less separation than the major faults.
Most major and minor faults underwent last major movement during the late
Pleistocene or earliest Holocene, but one fault shows evidence for significant
movement during the middle Holocene. A fracture 3 km long developed along
another fault during 1993. This fracture is attributed to aseismic creep
induced by increased stress from the Landers (1992) and related earthquakes.
INTRODUCTION
Neogene strike-slip faults have been recognized in the western Mojave
Desert (Fig. 1) for several decades, but critical information such as the
amount, rate, and timing of movement has remained elusive for most faults.
As a result, models describing the tectonic behavior of the western Mojave
Desert during the Neogene are under-constrained. This study provides preliminary
data for one aspect of faulting that is crucial for the models<the age
of the most recent movement on the faults.
Figure 1. Location of Fort Irwin region in north-central Mojave Desert. Major faults of southern California are shown. Shaded area represents the Eastern California shear zone and faults farther west in the Mojave Desert. Heavy gray line is the approximate zone of ground rupture from the Landers and related earthquakes.
This study is derived from detailed field investigations that were conducted
to describe the bedrock and surficial materials of Fort Irwin National Training
Center (Yount and others, 1994, and unpubl. data, 1993). We describe faults
in an area centered on the main logistics base for Fort Irwin, which was
constructed on a broad valley herein informally termed "Fort Irwin
basin" that lies in the Fort Irwin and Langford Well U.S. Geological
Survey 7.5-minute quadrangles. Findings from past geologic mapping of this
area (Byers, 1960; Jennings and others, 1962) have been updated by several
recent preliminary studies by Schermer and others (1991), Ford and others
(1992), Schermer (1993), Sabin and others (1993, 1994), and Keith and others
(1994).
We conclude that characteristics of geomorphic surfaces and their underlying
deposits indicate that the deposits span much of the Quaternary, and use
these ages to constrain the recency of activity on faults. Although this
chronostratigraphy is imprecise in certain respects, it restricts the most
recent major movement of many faults to the Pleistocene.
GEOLOGIC SETTING
A family of mainly northwest-striking strike-slip faults spans the western
Mojave Desert, where the faults cut remnants of Paleozoic and Proterozoic
rocks, Mesozoic granitoids, and Miocene volcanic rocks. Dokka and Travis
(1990a, b) termed the eastern part of this strike-slip realm the Eastern
California shear zone, noting that the zone of faults accommodated part
of the net slip between the Pacific and North American plates during the
Neogene. They built upon previous syntheses and models such as Dibblee (1961),
Garfunkel (1974), and Dokka (1983) that described several kilometers of
separation on most faults and proposed that some blocks bounded by the faults
rotated as fault separation accumulated (Valentine and others, 1993). The
faults are inferred to be late Miocene to Quaternary in age by many workers
because they cut early and middle Miocene volcanic sequences in many places.
In general, sedimentary sequences younger than the Miocene volcanic rocks
are poorly dated resulting in little precise information on times that faulting
was initiated, slip rates, and times that faulting ended.
The Landers earthquake on June 28, 1992 and related preshocks and aftershocks
caused fault rupture in a huge area south of Fort Irwin (Hauksson and others,
1993), leading to a model for the development of a new major fault system
crossing previously known faults of the Eastern California shear zone (Nur
and others, 1993). Faults in the Eastern California shear zone can be separated
into domains on the basis of strike. The largest domain has faults that
strike mainly northwest and have right-lateral movement. The other domain
has faults that strike mainly west and have left-lateral movement. A boundary
between the two domains lies in western Fort Irwin (Dokka and Travis, 1990a;
Schermer and others, 1991; Luyendyk and others, 1993; Schermer, 1993), roughly
along the west side of the Fort Irwin basin map area (Fig. 2). However,
this boundary is complex in detail; it is evident on the scale of the Eastern
California shear zone (Fig. 1) but less so at larger scales. Much of the
central part of the Eastern California shear zone is seismically active,
including the area of ground ruptures in 1992 (Fig. 1). Scattered seismicity
extends to the eastern part of the zone, making careful assessment of the
recency of movement on faults necessary to assess seismic rupture, shaking,
and related hazards.
Figure 2. Generalized map of faults, physiography, and cultural
features, southwestern Fort Irwin. Main post is patterned area in center
of Fort Irwin basin. Gray circles represent approximate locations of deep-space
antennas operated by NASA. Faults from Byers (1960), Jennings and others
(1962), Dokka (1992), Schermer (1993), Yount and others (in press), this
study. Click on the image to see the full-size figure.
Cenozoic volcanic rocks at Fort Irwin that provide timing constraints on
the initiation of faulting include thick sections of Miocene (19 to 12 Ma)
basalt and rhyolite flows (Sabin and others, 1993, 1994; Keith and others,
1994; E.R. Schermer, unpubl. data, 1994) and areally limited latest Miocene
(~5.6 Ma) basalt flows (Yount and others, 1994; E.R. Schermer, unpubl. data,
1994). Sedimentary sequences locally bracket the youngest basalt flows but
are not directly dated. In general, the Miocene extrusive piles appear to
predate the strike-slip faulting (Schermer, 1993), whereas the latest Miocene
basalt flows are spatially associated with the faults, which may have formed
conduits for magma transport (Byers, 1960).
During the last few million years, the present geomorphology of the Training
Center was established. Broad alluvial fans lead from mountain fronts to
wide basins that in some cases contain playas. In some places the geomorphology
is controlled by youthful faulting and uplift; in other places features
such as pediments and domes may indicate a tectonically more stable geomorphology.
QUATERNARY GEOMORPHOLOGY AND STRATIGRAPHY
The geomorphology of the region around Fort Irwin forms part of the basis
for assigning ages to deposits and assessing recent tectonism, and contains
many elements of desert landscapes. The classic bolson (Peterson, 1981),
with closed internal drainages feeding from exposed bedrock mountain ranges
via alluvial fan systems to a flat-floored valley bottom occupied in part
by a dry lake or playa, is typified by the basins containing Bicycle Lake
and, to some degree, Langford Well Lake. These bolsons, as well as the semi-bolson
of Fort Irwin basin, contain deposits exhibiting geomorphic characteristics
suggesting that ages for the deposits span most of the Quaternary.
Undated gravel lying above latest Miocene basalt in the Coyote Ridge area
(Figure 2) encloses paludal deposits that probably once occupied the axis
of a bolson. These gravel and paludal deposits are Pliocene and early Pleistocene
in age on the basis of their highly dissected morphology and relations to
latest Miocene basalt flows. Overlying deposits are probably entirely Quaternary
in age (Table 1).
Ages of the Quaternary deposits are derived from several characteristics
based on a widely applicable model for soil development in arid locations
and by comparison with other Mojave Desert sites. The following description
of the model is brief and does not treat many complex, but essential, elements
in the evolution of soils and geomorphology. The model describes alteration
of geomorphic features after deposition ceases. Channels dissect and erode
the feature and sheetwash moves fine material into small channels, smoothing
out the microtopography. Desert pavements develop on the smoothed surfaces
and soils form in response to weathering of the stable surface and eolian
influx. Desert varnish builds on surface clasts. As the surface becomes
deeply dissected, side-slopes along incised channels expand, and the original
fan surface begins to erode. Ultimately, the entire landform becomes so
eroded that the original form is difficult to discern. Although this evolution
of depositional features allows relative dating of deposits in many areas,
other areas such as pediments and desert domes were formed by erosional
processes. Ages of such erosional features are limited by the ages of the
oldest overlapping deposits.
We determine the relative ages of the features and their underlying deposits
by: 1) in-filling of bar and swale microtopography (Ritter, 1987), 2) depth
and pattern of incision of channels that erode into the landform, 3) degree
of flatness or roundness of interfluves, 4) grain-size and weathering of
surface clasts (McFadden and others, 1989), 5) degree of soil development
(Gile and others, 1966; McFadden and others, 1989; Reheis and others, 1989),
6) degree of development of interlocking desert pavement, and 7) degree
of desert varnish cover of surface clasts (McFadden and others, 1989). Table 1 presents generalized observations of these
criteria for the alluvial fan and wash units of the Fort Irwin basin and
vicinity.
Age estimates for these units are taken from studies (Table
1) where independent dating of Quaternary units has been possible (Wells
and others, 1984; Taylor, 1986; Reheis and others, 1989), as well as by
comparison with described and dated strata at sites in the Providence Mountains
(McDonald and McFadden, 1994) and Cajon Pass (Harrison and others, 1994).
Younger alluvium and wash deposits (Qya, Qyw) probably are less than 8,000
years old. Intermediate alluvium (Qia) is between 20,000 and 180,000 years
old. Older alluvium and wash deposits (Qoa, Qow) are probably greater than
250,000 years old. Oldest alluvium (QToa) is probably greater than 500,000
years old. Additional age resolution is provided by subdividing younger
and intermediate units on the basis of soil and geomorphic development (Yount
and others, 1994).
Geomorphic features commonly reflect the environment of deposition and erosion,
the materials the features are cut into or built upon, and the recency and
types of tectonic processes. Also relevant are the climates and microclimates
during deposition and erosion. Granite domes or desert domes (Davis, 1933,
1938) are formed in tectonically stable areas underlain by Cretaceous granites
in the south (Noble Dome; Fig. 2). Pediments built on granitoids also are
extensive throughout the southern part of the map area. Locally, more rugged
topography in the south is developed on Jurassic granitic and metamorphic
rocks, which are more resistant than Cretaceous granites. More deeply eroded
topography developed on granites and Tertiary volcanic rocks in the central
and northern areas, implying more recent tectonism there. At Coyote Ridge,
all rocks are deeply eroded as a consequence of recent reduction of base
level, probably a result of uplift of the ridge relative to the bolson axis.
FAULT DESCRIPTIONS
Descriptions of fault features and relations of faults with Quaternary
deposits that identify the last pronounced movement on the fault are given
in this section and summarized in Table 2.
We describe west- and northwest-striking faults, along with related minor
faults, sequentially from south to north, and then describe faults with
unusual north and northeast strikes. Faults are shown and labelled in Figure
3.
Figure 3. Index map for faults (heavy lines) and physiographic
features (green shaded areas) in the Fort Irwin basin area. Letter and number
labels refer to faults and sites described in text. Red zone is 1993 fracture.
Click on the image to see the full-size figure.
Coyote Lake fault
The west-striking Coyote Lake fault was mapped and described by Byers (1960)
as a combination of a few surface exposures and a continuation of a subsurface
feature defined by gravity anomalies to the west (McCulloh, 1953, 1960)
across a region of undisturbed deposits. Within the area we mapped, it is
marked by exposures of sheared basement rocks and truncation of overlying
older gravel (Fig. 3; A). Bedrock is faulted against intermediate alluvium,
and bedrock fragments in gouge zones are rotated into parallelism with the
fault plane. Holocene alluvium (Qya3) is not cut by the fault. Other sites
of scarps in alluvial materials are not on Byers fault trace, but are shown
as part of the fault in Figure 3. All scarps face north and faults probably
all dip south, indicating a reverse component of up-to-the-south on the
presumably left-lateral fault (Byers, 1960). One measured fault plane dips
moderately south (at site A) and faults dip steeply south at Jack Spring
(Fig. 3). Late Miocene or Pliocene gravel south of the fault is broadly
folded, but the north trends on these fold axes are difficult to reconcile
with deformation associated with movement on the Coyote Lake fault. Scarps
bounding gravel deposits west of site A, such as at Jack Spring, appear
to be similar to relations described above but a lack of exposures limits
our knowledge of the age of movement on the faults. Taken together, faults
in this zone describe a sinuous map pattern with strikes varying from N70°W
through west to S55°W. We ascribe this trace to the Coyote Lake fault,
rather than the straight trace inferred by Byers. Intermediate-alluvial
(Qia) veneers lie on desert domes and extensive pediments that grade smoothly
to both sides of the Coyote Lake fault, suggesting that the region has been
relatively stable during late Pleistocene time. If this is the case, the
latest faulting at site A that cuts intermediate alluvium may be relatively
minor and younger than the penultimate major movement on the Coyote Lake
fault.
South Noble Dome fault
The south Noble Dome fault lies 1.5 km north of the Coyote Lake fault (Fig.
3; B). It strikes generally east-northeast and undergoes kilometer-scale
undulations like those of the Coyote Lake fault (Fig. 4). Along most of
its 6-km length it is marked by a north-facing hill front underlain by bedrock,
from which stubby alluvial fans of intermediate age have been built. In
most places these fans are not cut by the fault but were deposited against
strongly-sheared bedrock. At one location, the intermediate, but not young
(Qya2), alluvium is cut by the fault. As with the Coyote Lake fault, these
relations may indicate that most fault movement predated the intermediate
deposits, with local, minor reactivation during latest Pleistocene time.
The north-facing hill fronts suggest an up-to-the-south component of movement
on this fault.
North Noble Dome fault
The north Noble Dome fault (Fig. 3; C) lies 3.2 km north of the Coyote Lake
fault and 1.2 km north of the south Noble Dome fault. It strikes roughly
west but bends at a right step about at its midpoint (Fig. 4). The western
segment is represented by an alignment of north-facing mountain fronts that
may indicate up-to-the-south movement. The eastern segment is defined by
breccia and gouge within granite and shows about 110 meters of left-lateral
displacement of a gabbro dike. A parallel breccia zone lies about 200 m
north of the eastern segment of the north Noble Dome fault. Typical surface
exposures of the eastern fault zone consist of linear subdued topography,
with the trough generally trending N85°E, and 2 to 10 m wide. The surface
of the materials in the zone is marked by grüs that is generally lighter
in color than surrounding rocks. Materials in the fault zone consist of
brecciated granite and dike rocks, with clay seams occurring along many
fractures. Fracture spacing is less than 3 cm through most of the zone and
numerous orientations of fractures are observed. Most fracture surfaces
are not lineated, but striae measured on fractures approximately parallel
with the overall zone (N85°E, 70° N.) at one locality are within
23° of horizontal, and plunge both east and west. Aerial photographs
taken in 1947 show no evidence that the fault cut intermediate alluvial
deposits, the oldest Quaternary deposits present along its trace.
Figure 4. Photographs of cracks along north Noble Dome fault,
taken Dec. 15, 1993. A. Crack in gravelly alluvium. B. Crack in gravelly
alluvium. Tire track for scale. [Click on the photos for larger images]
During late October, 1993, we observed open fractures along 2.4 km of the
fault. Following two small rainfalls that substantially degraded the exposed
fractures, on December 19, 1993 we identified and mapped another 0.6 km,
making the fractured zone at least 3 km long. The open fractures were approximately
centered on the right step (Fig. 4). Fracture intensity, as measured by
percentage of the length of the fault zone containing one or more fractures,
varied directly with the width of fractures. At the center of the fractured
zone, virtually no unbroken tracts of alluvium existed, whereas near the
western and eastern extremes of the cracked interval, long stretches of
undeformed alluvium lay between cracks. Likewise, cracks were as much as
1 cm wide in the center of the zone and were only a few mm wide at the ends
of the zone. Fractures were observed in a variety of surficial materials
ranging from clay, silt, sand, and gravel (to 8 mm). In most cases, surficial
materials are less than 1 or 2 m thick. Fractures formed in both moderately
lithified and unlithified materials, but fresh cracks were not discerned
in rock. In detail, fractures underwent 2-8 cm bends (Fig. 5), with no preferred
stepping, but the zones of fracturing were remarkably linear and confined
to the breccia zone in granite. Fractures stepped en echelon both left and
right, although the major steps consistently were to the right at the bend
in the fault about midway along its length.
Figure 5. Photograph of cracks stepping down to the right (north)
along north Noble Dome fault, taken Dec. 15, 1993. Knife lies on upthrown
block just south of fracture. Fracture partly degraded compared to condition
in October 1993. [Click on the photo for a larger image]
Sense of separation was not well constrained, but a few matched features,
such as pebbles and sharp indentations, across the fractures indicated small
components of left-lateral and normal movement; any net movement was probably
less than a few mm. An average vector of separation in the central part
of the fracture plunges roughly 3° to N5°W. At one location, where
the fractured colluvial soils lay on a steeper slope than other places (~15%
slope perpendicular to the fault plane), as much as 2 cm of normal displacement
occurred (Fig. 6). Two fracture traces existed along part of this segment,
each with normal displacement. The normal displacement was probably accommodated
by lateral movement of colluvium.
This fracturing almost certainly took place after the last water movement
in washes, which was probably during heavy March, 1993 rainfalls (R. Quinones,
Directorate of Public Works, Ft. Irwin, oral comm., 1993). Fracturing probably
occurred during or before a time when soils were moist, because in some
places grass appeared to have been growing in the fracture. Two small rainfalls
in December, 1993, rapidly degraded the cracks, supporting the inference
that fracturing occurred after heavy March 1993 rainfalls. We conclude that
fracturing took place between March and May, 1993.
The fracturing seems to require a tectonic origin. Non-tectonic origins
can be ruled out because (1) no nearby sedimentary basins exist that might
have deformed due to groundwater withdrawal, (2) the presence of fractures
in sand and gravel containing little clay indicates that fracturing was
not caused by desiccation, and (3) vectors include a left-lateral component.
Lack of fractures on nearby fault zones containing similar fault-zone materials
argues against dynamic shaking as a cause. The 3-km long fractures, the
jog in the fracture zone that conforms to a bend in the older breccia zone,
and the presence of lateral down-slope movement strongly suggest fracturing
caused by stress release. Between January 1, 1992 and November 1, 1993,
a few small-magnitude (<3.0) seismic events took place about 3 km distant,
and 2 or 3 events of magnitude 3 to 4 took place about 10 km distant. These
seismic events probably were insufficient to produce surface rupture. The
Landers earthquake (1992) took place before the last rainfall. The remaining,
most attractive hypothesis, is aseismic creep. Open fractures attributed
to creep were observed along 1 km in the Pine Nut Mountains, Nevada, in
1988 (Bell and Hoffard, 1990). The fractures degraded rapidly and appear
to be a good analog for those at Fort Irwin.
Main Gate Fault
The Main Gate fault is a nearly vertical breccia zone that strikes northwest
and extrapolates to the southeast to a position near the main gate of Fort
Irwin (Fig. 3; D) The fault probably belongs to the Goldstone Lake east
fault zone, which was defined by Dokka (1992) about 10 km to the northwest
(Fig. 2). Breccia in the fault is overlain by undisturbed older alluvium
(Qoa), and therefore last movement on this fault was before or during the
early Pleistocene.
A parallel inferred fault lies southwest of the Main Gate fault by about
1.5 km (Fig. 3). It is not directly exposed within the map area, but is
inferred by the linear juxtaposition of two large plutons and by small parallel
fracture zones. This fault may correspond to the Goldstone Lake west fault
zone of Dokka (1992). Studies of the fault further to the northwest are
needed. This contact between granites is overlapped by intermediate alluvium,
suggesting latest movement was Pleistocene or older.
Old Stable Fault
The Old Stable fault (Fig. 3; E) is informally applied to a set of faults
located northwest of the old stables of Fort Irwin. The fault set probably
belongs to the Goldstone Lake east fault zone (Fig. 2) of Dokka (1992).
One fault strikes north-northwest where it cuts Miocene basalt, dropping
it down to the west 40 to 80 m. Southward, this fault passes to a north
strike and splays to several strands. The curving splay of faults has relatively
youthful appearing geomorphic expression, with Holocene alluvium (Qya3)
inset against, and (or) faulted against, older alluvium. The faults appear
to have undergone both right-lateral and down-to-the-west movement at this
point. The main fault scarp is as much as 4 m high where older alluvial
fan surfaces are elevated east of the fault, but less than 1 m high where
it cuts intermediate age alluvial surfaces, a relation we take to indicate
recurrent movement on the Old Stable fault. The fault cuts intermediate
alluvium, possibly cuts older Holocene alluvium (Qya3), but does not cut
younger Holocene alluvium (Qya2). The fault set apparently last moved in
the late Pleistocene or early Holocene. We have not thoroughly investigated
faults lying about 0.5 km west of the Old Stable fault. They cut Miocene
deposits, but apparently do not cut Quaternary deposits.
Rifle Range Faults
The Rifle Range fault system (Fig. 3; F) is applied to exposures near several
rifle ranges southwest of Goldstone Road. The fault system probably belongs
to the Goldstone Lake east fault zone (Fig. 2) of Dokka (1992). Along its
northwest part, two faults cut early Miocene basalt, and the eastern of
these faults also cuts older and intermediate alluvium. At one place, a
fault scarp approximately 1 m high marks the uplift of intermediate alluvial
fan deposits on the northeast relative to older alluvial fan deposits on
the southwest. Most exposures of the eastern fault show older alluvium on
the southwest faulted against basalt on the northeast. Southeastward, the
faults splay within outcrops of Miocene volcanic rocks. Apparent cumulative
west-side-down offset of the basalt flows is as great as 150 m. The faults
are overlapped by early Holocene alluvium (Qya3), making last movement on
them latest Pleistocene or earliest Holocene.
Garlic Spring Fault
The Garlic Spring fault (Fig. 3; G) consists of a northwest-striking linear
breccia and gouge zone (Byers, 1960), and subparallel splays bordering a
ridge of bedrock (Beacon Hill). Near Garlic Spring, an alignment of springs,
seeps, and spring deposits mark the fault. This fault zone does not appear
to cut intermediate alluvium. Yount and others (1994) extrapolated the fault
zone northwest into Fort Irwin basin on the basis of two small hills exposing
basement rocks near the center of Fort Irwin basin, which may form higher
topography on basement northeast of the fault, like relations farther southeast
near Garlic Spring.
Figure 6. Photograph (taken Dec. 12, 1993) of calcite-cemented
fault plane cutting intermediate alluvium. Fault is a splay of Garlic Spring
fault. Plane dips toward the camera and to the right (northeast) 70°
and displays shallowly south-plunging striae. View to west-northwest; shovel
for scale. [Click on the photo for a larger image]
We have documented several subparallel splays of the Garlic Spring fault
on the basis of subtle scarps in alluvium, several of which apparently cut
various intermediate alluvial deposits. These short fault segments are of
uncertain significance. One larger and more obvious splay (Fig. 3; H) can
be mapped almost continuously from its departure from the main fault to
a parallel position 200 m to the northeast. This splay has prominent aligned
spring deposits adjacent to it, which interfinger with intermediate alluvium,
and the fault cuts and deforms intermediate alluvium in excellent exposures.
A carbonate-cemented fault plane (Fig. 7) dips northeast 70° and displays
shallowly south-plunging (~10°) striae; it cuts the intermediate alluvium.
Overlying early Holocene deposits (Qya3) are unfaulted. Intermediate and
older alluvium within 15 m of the fault is folded to easterly dips, suggesting
deformation with a component of shortening across the splay. A small right
step in the topographic front northeast of the Garlic Spring fault coincides
with the location of this right-stepping splay. A much larger 1-km right
step of the topographic front takes place a little farther northeast along
the fault. The step may be due to a step in the Garlic Spring fault system,
but no faults are evident in gully exposures close to the mountain front.
Movement on the splay of the Garlic Spring fault is late Pleistocene or
earliest Holocene.
Bicycle Lake Fault Zone
The Bicycle Lake fault zone (Fig. 3; I) is an east-striking fault zone with
left-lateral sense of separation as mapped by Byers (1960). A segment east
of the map area (Fig. 2) lies south of Tiefort Mountain where it is bounded
on the north by a Quaternary basin and on the south by Jurassic granitoids.
It offsets 5.6 ± 0.2 Ma basalt left-laterally by 3 to 8 km, and offsets
a band of marble by 2.9 km (~3.5 km if drag is taken into account; Yount
and others, 1994). Within the map area, the fault bifurcates and brackets
Beacon Hill, with main faults lying along the north (south margin of Bicycle
Lake) and southwest sides of Beacon Hill (Fig. 4). These segments appear
to comprise a stepover of about 3.5 km to the right. Within the Beacon Hill
block between the two main strands, we mapped several faults (Fig. 3; 1-5)
that appear to show both strike-slip and reverse separation. The presence
of youthful Quaternary basins north of the Bicycle Lake fault and, in particular,
Bicycle Lake itself, suggest that there is youthful up-to-the-south movement
on this northern fault strand as well as the left-lateral movement seen
farther east. The Bicycle Lake fault appears to cut Pleistocene (Qia) surfaces
immediately east of the map area. The northern strand by Bicycle Lake does
not cut Holocene alluvium (Qya1 and Qya2)and therefore last movement on
was approximately middle Holocene or older.
Faults that cut southern Beacon Hill and border the southwest front of Beacon
Hill consist of discrete zones of gouge and breccia. Along the southwest
front of Beacon Hill, offset unconformities indicate both southwest and
northeast sides down on various faults. There, older alluvium is faulted
and fractured, but intermediate alluvium is not, so latest movement occurred
prior to the late Pleistocene. Faults within the Beacon Hill block (Fig.
3; 1-5) are marked by breccia and gouge zones. The central strike-slip fault
does not cut older alluvium. None of the north-striking faults cut younger
alluvium; fault 2 is overlapped by intermediate alluvium, and fault 4 is
overlapped by older alluvium. Fault 5 cuts latest Miocene basalt but cannot
be mapped to exposures of alluvium. Latest movement on these faults within
the Beacon Hill block evidently occurred before the late Pleistocene, and
possibly predate the middle Pleistocene.
Coyote Canyon fault system
The Coyote Canyon fault system (Fig. 3; J) lies north of Coyote Canyon and
passes, from east to west, from the north side to the south side of Coyote
Ridge (Fig. 4). In the east, it cuts and appears to have warped Pliocene
or early Pleistocene paludal sediments. There, it cuts and significantly
displaces the entire pre-Quaternary section. The fault and its splays cut
intermediate alluvium but oldest younger alluvium (Qya3) lies undisturbed
in linear valleys formed by the fault, indicating last movement in the latest
Pleistocene or earliest Holocene. The deformation of the paludal sequence,
once probably occupying the axis of a bolson, to its present position at
the ridge crest in some places, testifies to the pronounced Pleistocene
activity on the fault.
In the west, the Coyote Canyon fault system forms a series of scarps truncating
early Holocene (Qya2, Qya3) and older surfaces. Exposures of the fault zones
show rotated clasts and nearby alluvium exhibits carbonate-cemented fractures
conjugate to the main planes. Scarps face both north and south and are not
as degraded as scarps seen along other fault systems reported in this paper.
The youngest Holocene deposits (Qya1) are not faulted. This part of the
fault last moved during the early or middle Holocene.
North-striking fault west of Painted Rock
Faults west of Painted Rock (Fig. 3; K) strike nearly north, are en echelon
right-stepping, and display down-to-the east scarps. Scarps are highest
at the center of the zone, where steep eastern fronts on hills underlain
by granite are about 10 m high. At this location, intermediate alluvium
butts against the granite (although exposures are not good enough to show
it is faulted against granite), whereas younger alluvium grades to the scarp.
Where exposed in bedrock, breccia and gouge zones as wide as 15 m dip between
46° and 71° east. Striae in the breccia zone plunge moderately to
the southeast and asymmetric fibers that grew along the slip planes indicate
normal dextral oblique movement. Near the north end of the fault, surfaces
on intermediate alluvial deposits are offset by 3 to 4 m down to the east.
This fault last moved during the late Pleistocene or earliest Holocene.
Northeast-striking Faults
The two northeast-striking fault systems are older than the Quaternary deposits
in their vicinity; each could be wholly pre-Quaternary. The fault northeast
of Noble Dome (Fig. 3; L) could be Mesozoic (Yount and others, 1994), but
in any case predates intermediate alluvium. The fault at Northwest Ridge
(Fig. 3; M) has normal separation, cuts Miocene rocks, and is overlapped
by intermediate alluvium. Thicker fill of unknown age in the northwest part
of Fort Irwin basin may indicate tilting of the basin as this fault moved
(Yount and others, 1994). The fault could be late Miocene to middle Pleistocene
in age.
Many west- and northwest-striking faults cannot be traced across a northeast-trending
linear zone passing from Painted Rock through Fort Irwin basin and west
of Bicycle Lake. No surface trace of a fault is known along this lineament,
but subsurface studies may identify its origin.
CONCLUSIONS
The last significant movement along most faults belonging to the common
west- and northwest-striking sets at southwestern Fort Irwin was during
or before earliest Holocene time, and probably during the late Pleistocene
(Fig. 7). Last movement on a north-striking fault zone also is this age.
Northeast-striking faults are older, but last movement also could be as
young as middle Quaternary in age. The Coyote Canyon fault underwent significant
movement in the latest Pleistocene, completely inverting the topography
of an early Pleistocene(?) bolson axis to that of a ridge crest. Scarps
in Holocene alluvium permit the conclusion that several meters of separation
occurred on the western part of this fault as recently as 100 yr before
present. Precise dating of the Quaternary deposits and trench studies are
needed to improve resolution on times for faulting and to establish recurrence
intervals for multiple events. The recency of faulting at Fort Irwin suggests
that seismic shaking and ground rupture hazards should strongly be considered.
Further evaluation of the timing of the fault movement and modelling of
stress and strain distribution can lead to more precise evaluations of the
potential for seismic activity.
Figure 7. Summary of age of most recent movement on faults at
southwestern Fort Irwin. Ages are given by relations with units of alluvium.
Unit symbols given in Table 1. Fault abbreviations: CL, Coyote Lake; SND,
South Noble Dome; NND, North Noble Dome; MG, Main Gate; OS, Old Stable;
RR, Rifle Range; GS, Garlic Spring; BL, Bicycle Lake; ECC, East Coyote Canyon;
WCC, West Coyote Canyon.
The preliminary evidence we present suggests that faults last underwent
significant movement non-uniformly, with ages ranging from probably pre-Quaternary
to early Holocene. Widely developed middle and late Pleistocene landforms
in the southern part of Fort Irwin, such as the area near the Coyote Lake
and Noble Dome faults, may indicate little tectonism after this time. However,
in a few places late Pleistocene alluvium is cut by segments of faults.
We tentatively interpret these segments as representing isolated minor movement.
Farther north, splays of the Bicycle Lake fault, as well as the Rifle Range
and Old Stable faults, appear to appreciably cut late Pleistocene alluvium
and the landforms more clearly responded to the faults of this age. The
northernmost fault we have studied, the Coyote Canyon fault, displays scarps
along its west part that are younger than 8,000 years and could be as young
as one hundred years old.
Non-uniform ages for last faulting appear to hold over a wider region. More
northerly faults such as the Drinkwater, Nelson Lake, and related fault
zones apparently were not active as recently as the Coyote Canyon fault
(E.R. Schermer, unpubl. data). Farther south, faults in the Lake Manix basin
also exhibit varying ages of most recent movement. For instance, the Manix
fault appreciable displaced late Pleistocene deposits and underwent minor
historical surface rupture (McGill and others, 1988). In contrast, faults
to the north cut an old alluvium but not most alluvial deposits (Glazner
and others, 1994). The most recently active faults, such as the Manix and
Coyote Canyon faults, may accommodate most of the strain in the eastern
part of the Eastern California shear zone, essentially defining active blocks
40 km across.
Creep on the north Noble Dome fault, most likely during 1993, may have significant
implications for planning for seismic and creep-type ground-breaking events.
At this point, we cannot confidently isolate the cause of this creep event,
but two scenarios are (1) that this creep event was one of many that repeatedly
occur, and was unusual only in that it was recorded, or (2) that this creep
event was induced by recent regional stress changes such as those that may
have followed the Landers (1992) earthquake and associated shocks.
So much of the western Mojave Desert is sparsely populated that it is entirely
possible that creep-induced cracking on a continuing basis could go unnoticed.
If the cumulative slip on the creeping faults is small enough (perhaps 4
centimeters) it may not leave visible permanent records in 8,000 yr-old
alluvium. If a typical creep event is represented by the north Noble Dome
event, it results in 1 to 3 mm of slip expressed as fractures at the ground
surface. The maximum number of such creep events for the record to be unnoticed
in 8,000 yr-old alluvium is estimated to be 15 to 40 events, which corresponds
to a maximum recurrence interval of about 200 to 500 years. In this case,
chances of creep-induced cracking on any given fault at Fort Irwin is small,
although not insignificant.
If the fault creep was the result of far-field strain caused by the Landers
event, or even a new stress regime that is reorganizing faulting patterns
in the Mojave Desert (Nur and others, 1993), the creep event could represent
new fault activity and thus may be repeated by many faults at Fort Irwin
in the near future. A model of the stress field resolved on the north Noble
Dome fault as a result of all fault movements triggered by the Landers and
Big Bear earthquakes indicates that small components of left-lateral shear
stress and fault-normal tensional stress were added (R.W. Simpson, written
comm., 1994). Movements appropriate for these stresses were observed on
the north Noble Dome fracture. Evidence therefore supports the interpretation
of new activity as a result of far-field stress from the Landers event.
By this scenario, all of the Quaternary faults, and even the older faults,
at Fort Irwin are capable of creep and possibly of seismic activity. Accordingly,
it is important to acquire as many data as possible pertaining to surface
and subsurface faults in order to evaluate potential future activity and
the hazards posed by aseismic creep, surface ruptures owing to seismic events,
and groundshaking caused by nearby earthquakes. With more data available,
plans to mitigate the effects of seismic and aseismic tectonism can be developed
for critical facilities and utility corridors at Fort Irwin.
ACKNOWLEDGEMENTS
Our studies have benefitted from discussions with and information provided
by R.W. Simpson, M. Schluter, and R. Quinones. J.S. Fenton provided field
assistance. We thank reviewers J.S. Fenton, J.C. Tinsley, R.F. Yerkes, and
two anonymous reviewers for helpful comments. This paper was prepared under
Interagency Agreement between the United States Geological Survey and the
U.S. Army National Training Center, Fort Irwin (MIPR DPW-014 93).
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