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
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.
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.
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.
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
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.
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.
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.
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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