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Quaternary Geology and Neotectonics of the Pinto Mountain Fault, Mojave Desert, Southern California

R. FORREST HOPSON, Geologist

Photo 1.View to the west across Morongo Valley toward Big Morongo Canyon, a linear valley (arrow) along the western part of the Pinto Mountain Fault. Mt. San Gorgonio and the San Bernardino Mountains in background. Photo by R. F. Hopson.

INTRODUCTION

An overview of the Quaternary geology and neotec-tonics of the Pinto Mountain Fault is presented in this article. In addition I include a discussion on the

age of this fault, tectonic geomorphic evidence for Quaternary slip and the seismic activity that has recently occurred near this fault. Potential seismic activity of the Pinto Mountain Fault is also briefly discussed. Much of this discussion is based on my own geological mapping and observations (Hopson, 1994; 1996) as well as from other workers cited below.

Previous work

The Pinto Mountain Fault was first recognized and named by Hill (1928). A number of workers who have either mapped the geology of the area or investigated the Pinto Mountain Fault include the following: Bader and Moyle (1960), Rogers (1961), Dibblee (1967a, 1967b, 1968b, 1975, 1982a, 1982b, 1992), Rasmussen & Associates (1977, 1990), Bacheller (1978), Bryant (1986), Grimes (1987, 1992), Howard and Allen (1988), Earth Systems Consultants (1992), Matti and others (1992), Powell (1993), Richard (1993), Howard (in press), and Howard and others (in press).

REGIONAL GEOGRAPHIC AND GEOLOGIC SETTING

The Pinto Mountain Fault is one of the most prominent east-trending geographic features in the southern California desert. The fault passes through the communities of Morongo Valley, Yucca Valley, Joshua Tree, and Twentynine Palms

just north of Joshua Tree National Park, and separates parts of two geomorphic provinces, the eastern Transverse Ranges and the Mojave Desert (Figure 1). It extends from near Mt. San Gorgonio (Photo 1) eastward along the north side of the

Figure 1. Generalized map of southern California showing the eastern California shear zone and geomorphic provinces (Mojave Desert—purple outline; Transverse Ranges—gold outline). Stars indicate epicenters of the 1992 Landers and Joshua Tree earthquakes. Modified after Richard (1993).

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EXPLANATION FOR FIGURES 2-5

Eolian (wind-blown) sand

Alluvium, Basin Fill deposits (undifferentiated)

Lacustrine (lake) sediments

Campbell Hill Formation

Twentynine Palms Formation

Pioneertown Basalt

Old Woman Sandstone

Predominately Mesozoic granitic rocks with Pre-cambrian Pinto gneiss and Paleozoic (?) metasediment

Strike-slip fault, dashed where approx-imately located, ? where queried, dotted where covered. Hatchures represent downdropped block of Oasis of Mara scarp.

Thrust fault, barbs on upthrown block

Anomalous ridge Trench location

Pleistocene (undifferentiated)

Arkose sandstone of Grimes (1987)

FAULT STUDY

R1= Rasmussen & Assoc. (1977)

R2= Rasmussen & Assoc. (1990)

E = Earth Systems Consultants (1992)

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eastern Transverse Ranges, which include the Little San Bernardino and Pinto mountains.

The Little San Bernardino and Pinto mountains have low to moderate relief, and east-sloping, plateau-like summits that are uplifted erosion surfaces of an ancient Tertiary plain (Dibblee, 1982a). Alluvium-filled valleys surround the Pinto Mountains, which on their north and south fronts have steep, deeply scalloped escarpments that rise abruptly from the valley floor. Queen Mountain is the highest peak in the Pinto Moun-tains at 1,731 meters (m).

Sweeping northward from the Pinto Mountains is a broad, triangular allu-vium-filled basin referred to as Morongo Basin. Located within this basin are the Bartlett Mountains, Copper Mountain, Donnell Hill, Campbell Hill and several playas or dry lakebeds. Alluvium is up to 3,100 m thick in Morongo Basin.

The Pinto Mountain Fault is the northernmost fault of a set of east-trending, left-lateral strike-slip faults that controls the east-west physiographic grain of the eastern Transverse Ranges (Powell, 1993). North of the Pinto Mountain Fault, northwest-trending, right-lateral strike-slip faults are the predominant set of faults. These fault sets form a broad network of mostly strike-slip faults, called the eastern California shear zone (ECSZ), (Dokka and Travis, 1990; Dokka, 1992) that extend north from the San Andreas Fault near Indio, across the Mojave Desert to the Garlock Fault (Figure 1). The ECSZ accounts for an estimated 9-14 percent of right-lateral slip occur-ring along the Pacific-North American plate boundary and has accumulated a total of about 65 kilometers (km) of right-lateral slip in the last 10 million years (Dokka and Travis, 1990).

Pre-Tertiary crystalline basement rocks and Quaternary sediments are cut by the Pinto Mountain Fault. The basement rocks form rigid continental crust beneath the eastern Transverse Ranges and Mojave Desert and consist of Precambrian gneiss, Paleozoic marine metasedimentary rocks, and Mesozoic plutonic and volcanic rocks. Dike swarms of presumably late Jurassic

and possibly younger microdiorite and rhyolite intrude this basement complex.

The 1 billion-year-old-Precambrian gneiss and Paleozoic marine metasedi-mentary rocks are roof pendants in Mesozoic batholithic rocks. The Meso-zoic batholithic rocks vary from horn-blende diorite to monzogranite. The quartz monzonite of Twentynine Palms, which is a part of this batholithic belt, may represent the eastern edge of a belt of magmatism associated with the onset of batholithic emplacement above a subduction zone in western North America.

QUATERNARY DEPOSITS

Quaternary deposits have been map-ped in the area along the Pinto Moun-tain Fault and are shown in Figures 2-5. The oldest of these are Pleistocene deposits determined from fossil age correlations and radiometric age dates. Fault deformation of these older Pleis-tocene rock units, as well as Holocene deposits along the Pinto Mountain Fault, is clearly indicated; however, late-Holocene sediments conceal a significant portion of this fault.

In the absence of radiocarbon dating or fossil evidence, differentiating Qua-ternary deposits into Pleistocene and Holocene in the Mojave Desert is based on degree of cementation; soil profile development; extent and development of desert-pavement* surfaces and rock varnish color (Christenson and Purcell, 1985; Dohrenwend and others, 1991). Pleistocene deposits are com-monly well-cemented and have well-developed B horizons with argillic (Bt) and calcic (Bk) soil horizons. They also have well-developed desert pavement surfaces, dark rock varnish, and red soil matrix. In comparison, use-ful criteria for distinguishing Holocene deposits in the Mojave Desert are bar-and-swale morphology, and unvar-nished to poorly developed rock varnish surfaces.

The Quaternary geology along the Pinto Mountain Fault was mapped on a regional scale by Dibblee (1967a, b, 1968b). Mapping on a local scale and descriptions of the Quaternary geology for areas near Twentynine Palms and

* Terms in boldface type are defined on page 11.

4

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Figure 2. General map of Quaternary geology and faults along the western part of the Pinto Mountain Fault. Modified after Bortugno and Spittler (1986) and Grimes (1987).

Figure 3. General map of Quaternary geology and faults along the Pinto Mountain Fault between Joshua Tree and Indian Cove. Modified after Bacheller (1978) and Bortugno and Spittler (1986).

Figure 4. General map of Quaternary geology at the inter-section of the Pinto Mountain and Mes-quite Lake faults. Modified after Bacheller (1978), Bortugno and Spittler (1986), Howard (in press), and Howard and others (in press).

Figure 5. General map of Quaternary geology along the eastern extension of the Pinto Mountain Fault. Modified from Howard and Allan (1988) and Howard (in press).

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Yucca Valley were done by Bacheller (1978) and Grimes (1987) respectively. Bacheller (1978) characterized two Pleistocene sedimentary deposits near Twentynine Palms and informally named them the Twentynine Palms and Campbell Hill deposits. I have included these deposits in my detail maps of the area in Figures 2-5. Bacheller (1978) was also able to distinguish Holocene deposits from Pleistocene deposits.

Twentynine Palms Deposit

The Twentynine Palms deposit mapped by Bacheller (1978) is largely alluvial fan and partially lacustrine or lake deposits derived from the Pinto Mountains. Equivalent deposits were mapped along the front of the Pinto Mountains just east of Twentynine Palms by Howard and others (in press). A layer of airfall volcanic ash that occurs near the base of the Twentynine Palms deposit is correlated with the Bishop Tuff (Bacheller, 1978), an ash that erupted from the Long Valley caldera in the eastern Sierra Nevada, and radio-metrically dated at 730,000 years before present (Mankinen and others, 1986). This suggests the Twentynine Palms deposit is Pleistocene.

Campbell Hill Deposit

Bacheller's (1978) Campbell Hill sedimentary unit, informally named after exposures at Campbell Hill just east of Twentynine Palms, is primarily com-posed of lacustrine and fluvial deposits. These deposits are derived largely from the San Bernardino Mountains and adja-cent areas (Bacheller, 1978; Howard and others, in press), and exposures are mainly confined to the Morongo Basin. This deposit forms many of the low hills just north of the Pinto Mountains between Twentynine Palms and Yucca Valley, including Campbell Hill, Donnell Hill, and a linear ridge south of Copper Mountain. The Campbell Hill deposit is Pleistocene. This is based on its Rancho-labrean mammalian fauna that is probably less than 500,000 years old (Bacheller, 1978).

Undifferentiated Pleistocene Deposits

Grimes (1987) mapped and de-scribed three probable Pleistocene deposits that are included as undiffer-

entiated Pleistocene in my maps. Two of these are fanglomerate deposits, each containing a particular clast com-position, one quartzite, the other basalt. The quartzite clasts originate from Paleozoic rocks in the southern San Bernardino Mountains and are set in a matrix of reddish-brown arkosic sand-stone. The basalt clasts, set in a reddish-brown silty sand, silt and clay matrix, are probably derived from two different sources, the Pioneertown basalt and amphibole-bearing basalt flows at Ante-lope Creek in the eastern San Bernar-dino Mountains. The third deposit is an older alluvium. It's reddish-brown silty, massive, poorly indurated sand that varies from 1.5 to 18 m thick.

Probable older Pleistocene or latest Tertiary fanglomerate deposits comprise Burnt Mountain, a prominent elongate hill a few kilometers south of Yucca Valley (Figure 2). This deposit contains granite and gneiss clasts and is tilted and dissected. The age of these deposits is suggested by their degree of deforma-tion and erosion.

Holocene Deposits

At least three kinds of Holocene (less than 11,000 years old) deposits are recognized in Morongo Basin and along the Pinto Mountain Fault: fluvial (mostly alluvial-fan and streamwash), lacustrine (playa), and eolian (wind-blown) (Bacheller, 1978; Grimes, 1987; Howard and Allen, 1988; Howard and others, in press). The presence of late-Holocene sedimentary deposits along the Pinto Mountain Fault indicates that modern sedimentation is occurring.

Holocene fluvial deposits occur as alluvial fans and stream wash deposits along the front of the Pinto Mountains. They are identified by the bar-and-swale geomorphic surfaces and are made of poorly sorted sand, gravel, cobbles and boulders (Howard and Allen, 1988; Howard and others, in press). The flu-vial deposits are shown as Basin Fill deposits in Figures 2-5. The lacustrine or playa deposits are recognized in Yucca Valley and Dale Lake (Bacheller, 1978; Grimes, 1987). They are charac-terized by grayish brown fine-grained sand, silt and clay, and lack shoreline features. Eolian deposits are made

of loose, fine- to medium-grained wind-blown sand. These deposits form both active and stabilized dune fields that pile up against the eastern Pinto and Sheep Hole mountains (Howard and Allen, 1988; Tchakerian, 1992).

PINTO MOUNTAIN FAULT

The Pinto Mountain Fault is the long-est (approximately 110 km) east-trending fault in the southern Mojave Desert. Its western terminus meets the north branch of the San Andreas Fault (also called the Mission Creek Fault) (Dibblee, 1967a; 1975) and is considered to extend east to meet the northwest-trending Sheep Hole Fault. Surface fault breakage has been well documented between its inter-section with the Mission Creek Fault and the Mesquite Fault; however, east of the Old Dale site its surface trace is not exposed (Figure 5). This most eastern extension is considered buried; the only evidence of its existence is the linear escarpment along the northern Pinto Mountains (Powell, 1993).

The Pinto Mountain Fault has accum-ulated a maximum of 16 km of left-lateral displacement (Dibblee, 1968a Hopson, 1996). Of this, 9 km occurred since Pleistocene time based on offset of the Pleistocene quartzite- and basalt-bearing fanglomerates (Dibblee, 1968a; Grimes, 1987). The accumulated 16 km of displacement is based on fault restor-ation of the Proterozoic and Mesozoic crystalline basement rocks—specifically, the realignment of the quartz monzon-ite of Twentynine Palms, monzogranite of Queen Mountain, the zone of Pinto gneiss intruded by monzogranite of Queen Mountain, White Tank mon-zogranite, and the igneous and meta-morphic complex of the Little San Bernardino Mountains.

Evidence that points to the age of initiation of the Pinto Mountain Fault is not well defined; however earliest move-ment may be no older than 7.3 million years (Grimes, 1987). This is indicated by deformed arkose and conglomerate deposits identified by Grimes (1987). Bedding units in these rocks are vertical- dipping (Photo 2), cut by the Pinto Moun-tain Fault between Yucca and Morongo valleys and have been correlated with the Old Woman Sandstone named by

6 CALIFORNIA GEOLOGY

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linear trough or valley

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TECTONIC GEOMORPHOLOGY

Tectonic geomorphology involves the study of the earth's landforms

resulted from tectonic processes such as faulting, folding and uplift. In order to unravel an area's tectonic history by studying its landforms, one must also understand the weathering process or the amount and rate of degradation the surface has undergone. The degree to which these landforms or construc-tional features have been obscured can be used to determine their age relative to other landforms that have lesser or greater amount of degrada-tion, provided one takes into account the relative resistance to erosion of the landform's material (Yeats and others, 1997). In this way, tectonic geomor-phology can yield insights about move-ment along faults, which includes the nature, timing, and distribution of fault-ing. Landforms identified along the Pinto Mountain Fault and useful geo-

morphic indicators for identification and characterization of the activity along the fault are described below. The block diagram illustrates some of these landforms (Figure 6).

Fault Scarps

Fault scarps are steep slopes or cliffs formed directly by movement along a fault; they represent the exposed surface of the fault before it is modified by erosion. Generally the less eroded or more 'fresh' the scarp is, the more recently it has moved. Fault scarps developed in Quaternary alluvium at several locations along the Pinto Mountain Fault. The best preserved scarp is at the Oasis of Mara in Twentynine Palms, the location of the Joshua Tree National Park visitor center. The scarp is about 2.5 m high and

1 km long and occurs in alluvium of probable Holocene age. Other, less prominent scarps, occur in Pleistocene alluvium near Copper Mountain and in Morongo Valley.

continued on page 8...

Figure 6. Block diagram showing some typical landforms suggestive of recent movement along active faults. Modified after Clark (1973).

Dibblee (1967a) for exposures found a few kilometers to the north (Grimes, 1987). The Old Woman Sandstone underlies the 7.3-million-year old Pioneertown Basalt.

The Pinto Mountain Fault appears to cut the northwest-trending faults in the Mojave Desert with two possible exceptions: the Johnson Valley Fault and the Mesquite Lake Fault. Aftershock epicenters indicate the right-lateral strike-slip Johnson Valley Fault propa-gated south across the Pinto Mountain Fault, identified as the Eureka Peak Fault, during the June 1992 Landers earthquake near Yucca Valley. This is suggested by two things: the Johnson Valley Fault's alignment with the Eureka Peak Fault and by focal mechanisms that indicate events from this sequence extend southward across the Little San Bernardino Mountains to the San Andreas Fault. The focal mechanisms exhibit right-lateral strike-slip faulting along steeply dipping north- to north-northwest-striking faults (Hauksson and others, 1993). The Mesquite Lake Fault offsets the Pinto Mountain Fault 1.5 km based on geo-logic mapping (Bacheller, 1978). Its southern extension, south of the Pinto Mountain Fault, is interpreted to be the west-dipping Twenty-nine Palms Mountain Fault (Figure 4), which uplifts and thrusts Twentynine Palms Mountain eastward (Howard and others, in press).

Photo 2. Arkose alluvial sandstone and gravel beds adjacent to the Pinto Mountain Fault near Yucca Valley. These beds have been tilted to vertical and may repre-sent the first motion on the Pinto Mountain Fault. Professor Perry L. Ehlig for scale. Photo by R. F. Hopson.

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Photo 3. View is to the northeast. Donnell Hill, a pressure ridge along the Pinto Mountain Fault near Twentynine Palms. The Pinto Mountain Fault forms the change in slope of Donnell Hill. Another pressure ridge, Campbell Hill, uplifted by the Mesquite Lake Fault, is the low, highly dissected hill in the distance on right. Photo by R.F. Hopson.

continued from page 7...

Scarps about 2 m high occur in the Twentynine Palms deposit along splays just north of the Pinto Mountain Fault (Hopson, 1994).

Pressure Ridges

Pressure ridges are hills formed along a strike-slip fault because of transverse pressure and shortening that occurs at restraining bends along the fault or between different strands of the fault. Several pressure ridges are recognized along the Pinto Moun-tain Fault. One pressure ridge, Donnell Hill made of the Campbell Hill deposit (Photo 3), is between the Pinto Moun-tain Fault and a northwest-striking fault, suggesting that uplift may be related to interaction between the two faults. Other pressure ridges occur at Copper Mountain and at the intersection

Linear Ridges

Linear ridges and linear valleys refer to long narrow features that can be used to identify potential fault activ-ity. Linear ridges may be the result of either compression or lateral offset along a strike-slip fault. Linear valleys occur along strike-slip faults where bedrock weakened by fault action is more easily eroded, and are not necessarily indicators of recent move-ment. Two linear ridges occur along the Pinto Mountain Fault. The linear ridge at the east end of Morongo Valley is especially well preserved (Photo 4). It is about 1 km long and made of Cretaceous igneous and metamorphic rocks overlain by Terti-

of the Pinto Mountain and Mesquite Lake faults. These pressure ridges are made of Pleistocene deposits includ-ing those of the Campbell Hill and Twentynine Palms (Figure 4).

Photo 4. Linear ridge adjacent to the highway at the east end of Morongo Valley. The Pinto Mountain Fault trends along the base of the ridge. Photo by R.F. Hopson.

NEOTECTONICS

Neotectonics is the study of the faults and the deformational history of the earth's crust that has occurred in Post-Miocene time. Neotectonic defor-mation is unmistakable along the Pinto Mountain Fault from its intersection with the San Andreas Fault to Twenty-nine Palms where Bryant (1986) identi-fies the fault as forming a broad, dis-tributive zone of strike-dip normal faults.

Pleistocene Displacements

Nine km of displacement has oc- curred along the Pinto Mountain Fault

since the deposition of lower Quater-nary or Pleistocene age fanglomerates (Dibblee, 1968a; Grimes, 1987). This is indicated by matching the quartzite and basalt clasts found in these fanglom-erates with their nearest source expo-sures farther west. The quartzite was eroded from probable Paleozoic age rocks near the headwaters of Big Morongo and Mission creeks on the southeast slopes of the San Bernardino Mountains (Grimes, 1987; Dibblee, personal communication, 1992). Quartzite clasts were washed down Big Morongo Creek and deposited across the Pinto Mountain Fault where they were offset 9 km to the east. The basalt

clasts are two types, indicating they were derived from different sources. Olivine basalt clasts come from the Pioneertown basalt (Figure 2) that forms prominent mesas near Pioneertown. The other type is an amphibole-bearing basalt clast and was derived from basalt flows near Antelope Creek in the San Bernardino Mountains (Grimes, 1987). This fanglomerate also demonstrates a minimum of 9 km of left slip. Other basalt clasts, though rare, occur in the Pleistocene sediments of Campbell Hill near Twentynine Palms (Bacheller, 1978).

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Photo 5. View is to the east. Shutter ridge along the Pinto Mountain Fault near its inter-section with the Mesquite Lake Fault. The Pinto Mountain Fault strikes along the base of the shutter ridge. The Sheep Hole Mountains are in the distance. Photo by R.F. Hopson.

Photo 6. View is to the northeast from the Pinto Mountains. Anomalous ridges along the Pinto Mountain Fault in Twentynine Palms (arrows).The Pinto Mountain Fault cuts left to right in the foreground. Photo by R.F. Hopson.

ary sandstone and stream gravels (Grimes, 1987). The only linear valley along the Pinto Mountain Fault is Big Morongo Canyon (Photo 1) and is approximately 6 km long.

Shutter Ridges

Shutter ridges are formed from ridges that have been displaced along a strike-slip fault that is traversing a ridge-and-valley topography, with the displaced part of the ridge "shutting in" the adjacent valley or drainage (Figure 6). Shutter ridges are most prominent where the Pinto Mountain and Mes- quite Lake faults intersect (Photo 5). One less prominent shutter ridge oc- curs near the mouth of Big Morongo

Fault-line Saddle

Fault-line saddles occur where a fault crosses a drainage divide or ridge. The soft rock along the fault is more readily eroded creating a saddle in the slope's profile. These features are use- ful for identifying the surface trace of a fault. One fault-line saddle occurs on the Pinto Mountain Fault at the west end of Big Morongo Canyon (Figure 2).

Other Geomorphic Features

Low anomalous ridges 1 km long are formed from antiformal structures that are parallel or subparallel to the Pinto Mountain Fault (Photo 6). These structures may be pressure ridges or elevated areas of bedrock concealed by surface deposits. Canyon (Hopson, 1994). Shutter

ridges along the Pinto Mountain Fault are made of Mesozoic and Precam-brian basement rocks capped by Pleistocene alluvium.

Holocene Displacement

The geomorphic features found along the Pinto Mountain Fault show evidence that suggests Holocene movement. However, late Holocene movement is not indicated because the fault is cov-ered in many places by late Holocene deposits. In addition, no direct dating of the Holocene deposits displaced by the Pinto Mountain Fault has been done. Therefore incremental displacement of the Pinto Mountain Fault is poorly constrained and Holocene or recent activity of the fault has been investigated only to a limited degree. However, a number of consulting reports cite offset

Holocene alluvium exposed in several trenches between Yucca Valley and Twentynine Palms. For example, Rasmussen & Associates (1977) report trench exposures in Joshua Tree that show late Pleistocene alluvium juxta-posed against unconsolidated deposits thought to be Holocene (Figure 3). In this trench, the Holocene sand is faulted against tilted brown fine-grained sand and gravel and coarse-grained white sand with caliche-filled cracks that appear to be Pleistocene. Exposed in trenches in Twentynine Palms are prob-able Holocene sediments that consisted of warped fine sand and silt layers with minor gravel truncated by the Pinto

Mountain Fault (Rasmussen & Associ-ates, 1990) (Figure 4). Earth Systems Consultants (1992) reported that uncon-solidated to slightly consolidated sand, silt, and gravel cut by the Pinto Moun-tain Fault were exposed in a trench in Twentynine Palms (Figure 4). Exposed in a second trench, Earth Systems Con-sultants recovered an Indian arrowhead a few feet below the surface in sag pond deposits cut by the fault. This is a very significant find. If the arrowhead was in situ (recovered from sediments undisturbed by human activities), it proves the sediment was deposited in Holocene times. Holocene fault activity is also evident owing to the lack of

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well-developed soil profiles in the basin fill (Rasmussen & Associates, 1977, 1990; Bryant, 1986; Earth Systems Consultants, 1992), lack of desert pavement and rock varnish on geomor-phic surfaces, and the presence of well-developed scarps (Bacheller, 1978; Hopson, 1994). There is little surfi-cial evidence, however, that the Pinto Mountain Fault has been active during late Holocene time.

Slip Rate

The slip rate is poorly constrained, but is estimated to be 0.3-5 mm/yr based on the offset Pleistocene fanglom-erate (Peterson and Wesnousky, 1994). However, it is difficult to be certain of the slip rate because the age of the Pleistocene fanglomerate is not well determined. Knowing the age of the fanglomerate is crucial for determining the slip rate.

RELATIONSHIP TO THE LANDERS—BIG BEAR

EARTHQUAKE SEQUENCE

The Pinto Mountain Fault transects faults that ruptured during the 1992 Joshua Tree and Landers earthquakes (collectively called the Landers earth-quake sequence) (Hart and others,

1993; Hauksson, and others, 1993). It is possible that the Pinto Mountain Fault influenced the character of the Landers earthquake sequence (Johnson and others, 1994). The earthquake sequence began with the Joshua Tree foreshock in April that had a moment magnitude (Mw) of 6.1; the main shock and aftershocks were centered just south of the Pinto Mountain Fault. The Landers June 28 earthquake was centered north of the fault and ruptured northward (Figure 1). Fault plane solutions indicate a right sense of slip for the Joshua Tree event (Hauksson and others, 1993) although total slip displacement is unknown because no evidence for ground rup-ture was observed after the earthquake (Rymer, 1992). The Landers main shock was centered about 10 km north of the Pinto Mountain Fault on the Johnson Valley Fault near Landers. The aftershock pattern to this event extended north from the main shock epicenter, and south, crossing the Pinto Mountain Fault to the San Andreas Fault (Hauksson and others, 1993). South of the Pinto Mountain Fault, the Johnson Valley Fault aligns with the Burnt Peak and Eureka Peak faults where they were exposed by ground rupturing after the Landers earthquake (Treiman, 1992). Hopson (1994, 1996) speculated that the Johnson Valley Fault

may be propagating across the Pinto Mountain Fault and will eventually con-nect with the Eureka Peak Fault.

Possible slip triggered on the Pinto Mountain Fault during the Landers earthquake occurred in two areas. The eastern area, north of the Yucca Valley airport, was characterized by a 45 m-wide zone of generally right-step-ping cracks, but some left-stepping cracks were observed (Bryant, 1992). The deformation was generally tensional with up to 2 centimeters (cm) of exten-sion, but both left-lateral and right-lateral minor displacement occurred (Bryant, 1992; Hart and others, 1993). The second site north of the golf course in Yucca Valley had one crack with up to 6 cm of vertical offset, but may have been enhanced by downslope move-ment (Bryant, 1992).

HISTORICAL SEISMICITY

Seismic activity in the southern Mojave Desert and eastern Trans-verse Ranges is high, but none of the recorded seismicity is suggestive of left-lateral displacement along the Pinto Mountain Fault (Hutton and others, 1991). Moreover, the Pinto Mountain Fault was not seismically active in his-toric time, that is, in the last 200 years

SPRINGS—POSSIBLE FAULT INDICATORS

P indicators for fault activity are springs. 1 Active springs occur along the Pinto Mountain Fault as the result of rainfall runoff in the Little San Bernardino and Pinto mountains. This runoff infil- trates and flows northward through alluvium until it is blocked by an impervious barrier or fault gouge cre-ated by the Pinto Mountain Fault. Dammed ground water creates an elevated water table on one side of the fault and a small pool or spring at the surface.

Sherman Shady Spring and Oasis of Mara are two large springs along the Pinto Mountain Fault. Sherman Shady Spring is in Big Morongo Canyon and is marked by a thick stand of alders. The Oasis of Mara, also called the Twentynine Palms Oasis, is in Twentynine Palms at the Joshua Tree National Park visitor center. It's marked by a grove of Wash-ington fan palms (Washingtonia filifera) (Photo 7).

Photo 7. Grove of Washington fan palms at the spring, Oasis of Mara. Donnell Hill, a pressure ridge, in the background to the right. Photo by R.F. Hopson.

1 0

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(Williams and others, 1990). In general, the historic earthquakes were centered in the southern part of the Mojave Desert (with the exception of the 1927 Manix earthquake) with the most recent centered on right-lateral faults. They are the June 1992 Landers earthquake (Mw 7.3), March 1979 Homestead Val- ley earthquake (Mw 5.3), the June 1975 Galway Lake earthquake (Mw 5.1), and the November and December 1975 Goat Mountain earthquakes (Mw 2.6 and 2.3 respectively) (Hutton and others, 1991; Hart and others, 1993). Recorded earthquakes in the eastern Transverse Ranges were generally small, Mw < 1.0, and were centered on strike-slip and thrust faults (Hutton and others, 1991). Farther west, seismic events occurred along the San Andreas and Banning faults at San Gorgonio Pass, including the July 1986 North Palm Springs earthquake (Mw 5.6), centered on the Banning Fault (Hutton and others, 1991).

POTENTIAL SEISMIC ACTIVITY

Given that the Pinto Mountain Fault is one of the largest faults in the eastern Transverse Ranges, the thought of its potential for a large earthquake is sober-ing. Numerous geomorphic features indicate the Pinto Mountain Fault is a young strike-slip fault, and the presence of offset Holocene alluvium indicates the fault is active. Moreover, this fault is included in the state's Fault Evaluation and Zoning Program (Hart and Bryant, 1997), and is considered 'sufficiently active' (shows evidence of Holocene movement) in an unpublished report that evaluates the surface fault rupture hazard of the Pinto Mountain and other nearby faults (Bryant, 1986). So the question remains—if this fault has moved within the last 10,000 years, why is there a lack of historical seismic events and what is the potential for future activity? Two reasonable hypoth-eses can explain the lack of earthquakes on the Pinto Mountain Fault: 1) earth-quakes on the Pinto Mountain Fault may have a long recurrence interval, on the order of hundreds of years; 2) the Pinto Mountain Fault is becom-ing locked by north-west striking right-lateral faults in the Mojave Desert that are propagating south into the eastern Transverse Ranges. Definitive paleo- seismic data, such as radiocarbon age

dates, from Holocene sediment exposed along the Pinto Mountain Fault are lack-ing, therefore the first hypothesis is unproven. In support of the second hypothesis, epicenter locations of the Landers earthquake indicate the Johnson Valley Fault may be propagat-ing across the Pinto Mountain Fault. If this is in fact happening, the Pinto Mountain Fault may ultimately become inactive.

Recent fault activity in the eastern part of the Pinto Mountain Fault is not reflected in surface geomorphic land-forms. If the steep front of the Pinto Mountains in this area is fault-controlled by the Pinto Mountain Fault, then tec-tonic activity along this stretch of the fault has apparently ceased and erosion has become the dominant process by which the mountain front is being modi-fied (Keller and Pinter, 1996).

ACKNOWLEDGMENTS

I appreciate Edward Keller, Perry Ehlig, Robert Norris, Steve Lipshie and Jonathon Matti for helpful comments

that improved this paper. Keith Howard is thanked for providing his geologic report and map of the Valley Mountain 15-minute quadrangle and Sheep Hole Mountains 30 x 60 minute quadrangle maps prior to publication. I am grateful to Gary Rasmussen of Rasmussen & Associates and to Mr. and Mrs. Paul F. Smith for allowing me to use their consulting reports on the Pinto Moun-tain Fault.

BIOGRAPHY

R. Forrest Hopson received his M.S. in geology from California State University, Los Angeles in 1996. His thesis project involved mapping the pre-Tertiary crystalline basement rocks along a 30 mile section of the western Pinto Mountain Fault to doc-ument the offsets. His research on the Pinto Mountain Fault and other sub-jects have been presented at meet-ings of the Geological Society of America and American Geophysical Union, and published in scientific journals and field trip volumes.

GLOSSARY

Argillic horizon: Soil horizon enriched in clay minerals that were moved downward by soil-forming processes. Designated as Bt horizon.

Arkose: Sandstones that contain more than 25 percent feldspar grains.

B horizon: A zone of soil accumulation that consists of a variety of material moved downward from above.

Bar-and-swale morphology: Surface morphology of young alluvial fans where ephemeral streams have deposited their bedload. The swales are stream channels between bars where sediment was deposited.

Calcic horizon: Soil horizon characterized by the accumulation of calcium carbonate that may coat soil grains or fill pores. Designated as Bk horizon.

Desert pavement: A surface of tightly-fitted pebbles formed as a result of the removal of sand grains by wind action.

Fanglomerate: Heterogeneous sedimentary material originally deposited as an alluvial fan that consolidated to form solid rock.

Focal mechanism: Determination of fault plane orientation and slip direction based on analysis of recorded first motions and/or amplitude of earthquake waves. Also called fault-plane solution.

Lacustrine: Refers to lakes or playas (desert lake basins).

Rock varnish: Brownish to purple coating of manganese oxide on rocks in desert environments.

Sag pond: A small body of water occupying an enclosed depression or sag formed where active or recent fault movement has impounded drainage.

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Nominations Sought for

ALFRED E. ALQUIST AW The California Earthquake Safety Foundation is accepting nominations for the 1999 Alfred E. Alquist Award for

Achievements in Earthquake Safety. This award recognizes individuals and/or organizations who have made outstanding contributions to seismic safety in California. Awards are given in many areas, including basic and applied research, education, volunteer services and program implementation.

Past award recipients have included elected officials, educators, engineers, architects, disaster specialists, governmen-tal advisors and businesses. One to three awards are given each year. Posthumous awards are not made.

A candidate may be nominated by an individual, a firm or an agency. Letters describing a nominee's background and accomplishments should be sent to:

California Earthquake Safety Foundation c/o George Mader

Spangle Associates 3240 Alpine Road

Portola Valley, CA 94028

Nominations must be postmarked no later than November 20, 1998.

The California Earthquake Safety Foundation was founded in 1985. It relies on tax deductible donations to support activities. It's governed by a volunteer Board of Directors. For more information, contact George Mader, Chairman, at (650) 854-6001; fax (650) 854-6070; or e-mail: mader.spanglegbatnet.com

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