A FIELD INVESTIGATION OF THE CLARK MOUNTAIN FAULT COMPLEX, SAN BERNARDINO COUNTY, CALIFORNIA

Jessey, David. R., Tarman, Don. W., Waki, Miho, and Baltzer, Suzanne M.,

Geological Sciences Department, California Polytechnic University - Pomona

ABSTRACT

The Clark Mountain fault (CM) was first described by Hewett (1956). Despite decades of study, the geometry, kinematics and even the existence of some components of the CM fault remain the subject of debate. Our research indicates that the complex is comprised of two parallel to subparallel faults striking roughly north-south along the eastern flank of Clark Mountain and Mohawk Ridge. The westernmost of the two faults juxtaposes hanging wall Cambrian Bonanza King and footwall rocks of variable lithology and stratigraphic position (lower Cambrian to Proterozoic). The fault plane dips gently to the west (10-30°) and is characterized by a lack of brecciation in both footwall and hanging wall rocks adjacent to the fault. Northeast verging, shallowly northwest plunging, small-scale folds with amplitudes of cm's to a meter or two are common in the footwall rocks. The general character of rocks associated with this fault is markedly similar to that of the Keystone thrust in the Spring Mountains. This fault is unquestionably the Keaney-Mollusk Mine fault identified by Burchfiel and Davis (1971). The second major fault lies at a variable distance (up to a few hundred meters) to the east in the footwall of the thrust. The fault is poorly exposed, but where observed generally dips steeply (50-80°) to the west. The footwall is comprised of Precambrian metamorphic basement while the hanging wall consists of brecciated lower Cambrian Zabriskie and Carrara Formations. Slickenfiber analysis of footwall basement rocks by Rodman (1989) indicates northeast-southwest extension. Our investigations are consistent with the interpretation of normal movement along this fault. Detailed mapping shows the high angle fault to be locally over-ridden by the thrust. We are lead to the conclusion that extension predates compression. Our data best fits a model of pre-thrust extension (the Clark Mountain fault and Ivanpah faults), perhaps associated with back-arc development. This was followed by northeast-directed thrusting.

Introduction

The field study area lies immediately west of the Molycorp Mountain Pass Rare Earth Mine, San Bernardino County California. It extends from the east end of Mohawk Ridge (Microwave Tower Hill) north for a distance of 10 miles to the Mesquite Mountains. Much of the study area lies within the East Mojave Scenic Preserve. Access is by a series of drill roads and 4-WD drive tracks. Topography varies from low hills (Chaos Hill) to rugged mountains (Clark Mountain). Burchfiel and Davis (1971) were the first to suggest that the faults within the study area were a part of the much larger Mesozoic Sevier Fold and Thrust Belt, a north-south belt of crustal shortening extending from southern California to, at least, Alberta.

 

Previous Work

Dixon Hewett was the first geologist to study the east Mojave in detail. Much of his work centered on 1500 square miles of southern California and western Nevada termed the Ivanpah Mining District. Hewett's work, although not published until 1956, was largely completed before World War II. To his credit, Hewett recognized many of the structural and stratigraphic relationships that make the study area so unique. Our research investigates an area that has remained controversial since Hewett's first map was published.  Hewett mapped a fault on the east side of Clark Mountain as a normal fault (Clark Mountain Fault) based on the structural relationship.  His map depicts Cambrian Goodsprings Dolomite (now subdivided into several units including the Bonanza King Formation), overlying basement gneiss.  In the 1970's, Burchfiel and Davis proposed that this fault should be remapped as a decollement thrust fault, naming it the Keaney-Mollusk Mine thrust. In a 1988 field guide they offered several lines of evidence to support their hypothesis, the most compelling based upon mapping in the Mescal Range south of Mountain Pass. In the Mescal Range, Hewett's Clark Mountain normal fault can be seen to place Bonanza King Formation (Cambrian) over Cretaceous volcanics and Jurassic Aztec Sandstone. In 1989, Elizabeth Rodman studied a small portion of the Clark Mountain fault complex. She recognized the presence of not one, but two north-south striking faults lying nearly side-by-side. She interpreted the eastern-most fault as a thrust and the western fault as a normal fault.  We have reexamined the area mapped by Rodman and extended our mapping both north and south of her study area.

Stratigraphy

Rocks exposed in the study area range in age from Proterozoic through Devonian.  The basement is comprised of hornblende-biotite gneiss with lenses of quartzite.  Age of the basement rocks is uncertain, but may be similar to the 1.7 Ga Fenner Gneiss to the southwest of the field area. Pegmatite dikes and a younger alkali granite have cut the gneiss. Both predate intrusion of a 990 Ma carbonitite complex at Mountain Pass (Beckerman, et. al., 1982). Proterozoic rocks were not differentiated during our mapping.

Proterozoic rocks are in fault contact with Lower Cambrian units throughout the field area. Cambrian Wood Canyon Formation outcrops locally. It is comprised of interbedded coarse, arkosic sandstones and polymict pebble conglomerates. It is differentiated from the overlying Zabriskie Quartzite, by poor sorting, lack of cross-bedding and the rusty, red-brown color on outcrop. Maximum outcrop thickness of the Wood Canyon rarely exceeds thirty meters. Zabriskie Quartzite conformably overlies the Wood Canyon. The Zabriskie provides an excellent marker horizon, forming prominent ridges. It is well sorted, often cross-stratified and more highly indurated than the underlying Wood Canyon.  Maximum thickness of the Zabriskie locally exceeds 150 meters, but is generally 50 meters or less.

Middle Cambrian Carrara Formation overlies the Zabriskie. The lithology and thickness of the Carrara is highly variable within the field area. The variable thickness is a consequence of the fault contact with the overlying Bonanza King Formation. Nowhere in the map area is the contact conformable, the variation in thickness due to faulting out of the upper Carrara.  Lithology varies both up section and along strike. To the south, shale and siltstone predominate with lesser silty carbonate. Further north, the Carrara is comprised largely of silty limestone. Locally the Carrara has been weakly metamorphosed to slate/hornfels and schist.


Figure 1.  Geologic map of the Clark Mountain fault complex.

a.
 

                       b.

Figure 2. (a) Cross section A-A' across the east end of Microwave Tower Hill. (b) Cross section B-B' along the crest of Chaos Hill.
The contact between the Carrara and middle Cambrian Bonanza King is marked by a gently west-dipping fault. The Bonanza King forms prominent outcrops throughout the map area. It is markedly banded, comprised of alternating blue and light gray layers of limestone and dolomitic limestone. There is little or no evidence of metamorphism, even adjacent to a granitic intrusive at the Pacific Fluorite Mine. The Bonanza King is conformably overlain by Cambrian Nopah Formation and Ordovician Pogonip Group.  Neither is shown on our field map as outcrops lie to the west of the mapped area along the steep slopes near the summit of Clark Mountain (both are the subject of ongoing field mapping). On Mohawk Hill Devonian Sultan lies above the Pogonip and at the summit of Clark Mountain and in the Mesquite Mountains both Mississippian Monte Cristo and Pennsylvanian Bird Springs were mapped by Hewett (1956). The western (updip) contact of the latter three formations is marked by the Mesquite Pass thrust fault.

 

Figure 3. Keaney-Mollusk Mine thrust fault, Microwave Tower Hill.  Upper plate rocks are Cambrian Bonanza King Formation (Cbk), lower plate is Precambrian gneiss (pCg).

 

Structure

Figure 1 presents the results of our field mapping. Stratigraphic relationships are generally straightforward in the southern portion of the map area. Figure 2a is a cross section through the Microwave Tower Hill (A-A') at the west end of Mohawk Ridge. Bonanza King Formation can be seen in fault contact with the underlying Proterozoic gneiss (Figure 3). The fault plane dips west at 20-30°.  Rocks along the fault plane are unbrecciated.  Although the stratigraphic relationship, younger rocks (Cambrian Bonanza King) over older rocks (Proterozoic gneiss) suggests a normal fault we conclude this fault is a thrust fault, the Keaney-Mollusk Mine (KMM) thrust proposed by Burchfiel and Davis (1971).  We base this conclusion on mapping by Burchfiel and Davis (1988) south of I-15.  There, the continuation of the KMM fault southward from Microwave Tower Hill can be seen to juxtapose upper plate Bonanza King and lower plate Cretaceous Delfonte Volcanics.  Furthermore, the geometry of this fault is markedly similar to that of the Keystone thrust in the southern Spring Mountains.

 

Figure 4. KMM thrust exposed north of Chaos Hill.  Hanging wall is Cambrian Bonanza King Formation (Cbk), footwall Cambrian Carrara Formation (Cc).

Father to the north (Cross Section B-B') (Figure 2b), geologic relationships become more complex.  A second major fault appears to the east of the KMM thrust.  This fault lies beneath the upper plate of the KMM along Mohawk ridge.  Westward erosion of the upper plate block exposes the second fault east of Clark Mountain.  The hanging wall of the KMM thrust remains Bonanza King, as it does throughout the study area.  However, footwall rocks are comprised of lower Paleozoic clastics; the Carrara, Zabriskie and Wood Canyon Formations (Figure 4).  The lower Paleozoic clastic units comprise the hanging wall of the second fault; Precambrian gneiss comprises the footwall.  This fault plane is poorly exposed, but in scattered prospect pits it can be seen to dip steeply (70-80°) to the west.  Footwall rocks are heavily brecciated and altered.  Rodman (1989) preformed slickenfiber analysis on basement rocks in the footwall of this fault.  Her data suggests northeast-southwest extension.  Therefore, this second fault most likely is a normal fault.  Hewett, (1956) first mapped a normal fault, he termed the Clark Mountain fault, trending roughly north-south in this area.  We have chosen to retain his terminology and term the second fault the Clark Mountain (CM) fault.

Locally, the KMM thrust bifurcates generating a series or smaller sub-plates.  This relationship is especially well developed one kilometer north of the Colosseum Mine.  Here, a series of parallel fault planes repeat section and result in one of the rare instances of the classic older over younger relationship.  Upper plate Zabriskie Quartzite has overridden lower plate Carrara Formation. 

West of Chaos Hill and north of the Colosseum Mine the Carrara Formation is tightly folded (Figure 5).  Stereoplots of fold data consistently show the folds to be northwest vergent.  We suggest the folds developed when the upper plate of the KMM thrust moved northeastward over the ductile shales of the lower plate Carrara Formation. 

Field mapping has recently commenced in the Mesquite Mountains to the north of the Clark Mountains.  Preliminary reconnaissance indicates the KMM thrust can be extended into the Mesquite range.  The stratigraphic relationships are somewhat different than those in our map area.  Carrara Formation appears to lie within the upper plate of the KMM thrust while the lower plate consists of lower to middle Paleozoic units. The Clark Mountain fault has not been mapped but structural relationships are complicated by Cenozoic volcanism and limited exposures.  Hewett (1956) suggests that the Clark Mountain and Ivanpah faults converge in the Mesquite Mountains.  We find the Ivanpah fault to be present in the Mesquite Range, but its relationship to the CM fault remains problematic.

 

Figure 5.  Small-scale folds in the Carrara Formation near Benson Mine.

 

 Conclusions

Our field investigation has shown that the Clark Mountain fault complex is comprised of two subparallel faults of markedly dissimilar character.  The easternmost of the two faults, the Clark Mountain fault (CM fault), juxtaposes Precambrian basement and lower Paleozoic clastic sedimentary rocks.  It dips steeply to the west and is characterized by extensive brecciation of hanging wall rocks.  Slickenfiber data for footwall rocks suggests extension.  We interpret this fault as a normal fault with the west side down.  The age of this fault is difficult to constrain.  South of I-15, the South fault (correlative with the CM fault) (Burchfiel and Davis, 1988) cuts Delfonte volcanics.  These rocks have been dated at 100 Ma (Fleck et al, 1994) providing a maximum age for the fault.  The Delfonte volcanics have an average composition approximating an andesite/dacite.  Such rocks would normally be emplaced in an extensional arc or back-arc environment.  Perhaps the CM fault and others such as the Ivanpah fault provided the conduit for extrusion of the volcanics.  However, extension must have continued after the cessation of volcanism to yield the observed structural relationship.

The second major fault lies to the west of the CM fault, but locally overrides the CM fault.  The fault plane dips gently to the west.  Rocks associated with the fault lack brecciation.  The hanging wall is comprised of Cambrian Bonanza King Formation while the footwall is composed of lower Cambrian/Precambrian rocks.  Based on observations of faults throughout the eastern Mojave, we believe this fault to be a thrust fault.  Its geometry and appearance are remarkably similar to the Keystone thrust well exposed along Keystone Wash in the Spring Mountains.  The KMM thrust has been mapped for nearly ten miles along strike and is known to continue southward in the Mescal Range (Burchfiel and Davis, 1988) and northward into the Mesquite Mountains (Tarman, Jessey, Waki, unpublished data).  As such, it represents a major crustal break.  Unfortunately, the low angle of the fault plane precludes any estimate of displacement along the fault, but most certainly it is on the order of kilometers.  Age of the KMM thrust is more tightly constrained.  Since it overrides the CM fault it most be younger than that fault and the 100 ma Delfonte volcanics.  In the Mescal Range, the fault is cut by a granitic intrusion dated at 83 Ma (Walker, et. al., 1995). This suggests the KMM thrust was active during the mid-Cretaceous, between 83 and 100 Ma. Curiously, near the Colosseum mine a granitic intrusive is seen to cut the fault (see field map).  Although this intrusive has not been dated, a nearby stock of similar granitic composition yielded an age of 100 Ma (Sharp, 1984). Does this indicate different periods of activity for segments of the fault?  Sharp (1984) was also the first to suggest that the KMM thrust may have been reactivated during the Cenozoic as a normal fault.  In the Clark Mountains, our mapping to date does not support this hypothesis. 

REFERENCES CITED

Beckerman, G. M., Robinson, J.P., and Anderson, J.L., 1982, The Teutonia Batholith: A large intrusive complex of Jurassic and Cretaceous age in the eastern Mojave Desert: in E.G. Frost and D.L. Martin eds., Mesozoic-Cenozoic Tectonic Evolution of the Colorado River Region, California, Arizona, and Nevada: San Diego, Calif., Cordilleran Publishers, p. 205-221.

 

Burchfiel, B.C., and Davis, G.A., 1988, Mesozoic thrust faults and Cenozoic low-angle normal faults, Eastern Spring Mountains, Nevada, and Clark Mountains Thrust Complex, California: in D.L. Weide and M.L. Faber eds., This Extended Land Fieldtrip Guidebook: Las Vegas, Nevada, Cordilleran Publishers, p. 87-106.

 

Burchfiel, B.C., and Davis, G.A., 1971, Clark Mountain thrust complex in the Cordillera of southeastern California: Geologic summary and field trip guide: California University, Riverside Campus Museum Contribution, no. 1, p. 1-28.

 

Fleck, Robert, Mattinson, J.M., Busby, Cathy J., Carr, M.D., Davis, G.A., and Burchfiel, B.C., 1994, Isotopic complexities and the age of the Delfonte volcanic rocks, eastern Mescal Range, southeastern California: Stratigraphic and tectonic implications: Geological Society of America Bulletin, v. 106, p. 1242-1253.

 

Hewett, D.E., 1956, Geology and mineral resources of the Ivanpah Quadrangle, California and Nevada: U.S. Geological Survey Professional Paper 275, 172 p.

 

Rodman, E.J., 1989, A detailed structural analysis of the Clark Mountain fault in SE California: [M.S. Thesis] University of California-Davis, 76 p.

 

Sharp, J.E., 1984, A gold mineralized breccia pipe complex in the Clark Mountains, San Bernardino County, California; in J. Wilkins Jr. ed., Gold and silver deposits of the Basin and Range Province, western USA: Volume XV, Arizona Geol. Soc. Digest, Tucson AZ, p. 119-139.

 

Walker, J.D., Burchfiel, B.C., and Davis, G.A., 1995, New age controls on initiation and timing of foreland belt thrusting in the Clark Mountains, Southern California: Geological Society of America Bulletin, v. 107, p. 742-750.