INDEPENDENCE DIKES AND MAFIC ROCKS OF THE EASTERN SIERRA

1998 Geological Society of America Cordilleran Section Field Guidebook (DRAFT 1-20-98)

April 4-6, 1998

Co-Leaders:
BRIAN S. CARL, Department of Geology, University of North Carolina, Chapel Hill, NC 27599-3315, and White Mountain Research Station, 3000 E. Line St., Bishop, CA 93514
ALLEN F. GLAZNER, Department of Geology, University of North Carolina, Chapel Hill, NC 27599-3315
JOHN M. BARTLEY and DAVID A. DINTER, Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112-1183
DREW S. COLEMAN, Department of Earth Sciences, 685 Commonwealth Avenue, Boston University, Boston, MA 02215

Table of Contents:
STOP 1: Fry Mountains, Mojave Desert
STOP 2: Spangler Hills
STOP 3: Poison Canyon, southern Argus Range
STOP 4: Aberdeen Mafic Sill Complex, Owens Valley
STOP 5: Santa Rita Flat Mylonite Zone, Owens Valley
STOP 6: Woods Lake pluton, Onion Valley Road (Independence)
STOP 7: Alabama Hills, Owens Valley (Lone Pine)
OPTIONAL STOP 8: Pine Creek Canyon, Owens valley (Bishop)
OPTIONAL STOP 9: Dolomite, southern Inyo Mountains
Acknowledgements

Link to IDS Photo List
Field Guide Figure Captions
References

The Independence dike swarm

Introduction

The Independence Dike Swarm (IDS) has attracted widespread attention since its first published description 37 years ago (Moore and Hopson, 1961; Smith, 1962). Mapping by J. G. Moore and C. A. Hopson revealed a dense set of northwest-striking dikes in the high country of the central Sierra Nevada northwest of the town of Independence, and G. I. Smith found a similar swarm near Searles Lake. It soon became clear that these swarms were continuous (Smith, 1962). Additional field work and geochronology by Chen and Moore (1979) and James (1989) extended the swarm both north and south of the type locality. The swarm is now recognized to extend over 600 km along strike, from the northern Sierra Nevada and White Mountains to southeasternmost California (Fig. 1).

Several aspects of the swarm make it a valuable stratigraphic and temporal marker in regional tectonic studies. In particular, the IDS crosses several tectonic province boundaries, individual dikes consistently strike northwest, and the Jurassic part of the swarm intruded during a brief interval around 148 Ma. Many tectonic studies of southern and eastern California use these properties to advantage (e.g. Smith, 1962; Karish et al., 1987; Glazner et al., 1989; Dunne and Walker, 1993; Schermer, 1993; Martin et al., 1993; Dunne et al., 1994; Schermer and Busby, 1994; Ron and Nur, 1996).

In this guide we discuss recent work by our group on the age, deformation, and tectonic significance of the IDS. The guide includes a roadlog for a 3-day trip to examine dikes and related mafic plutons in the Mojave Desert, Owens Valley, and eastern Sierra Nevada.

Significant New Results From This Study

On this field trip we will examine Independence dikes at various localities and exposure levels, present new structural and geochronological data, and discuss the swarm's origin. The most important findings of our study to date are:

1) Not all Independence dikes are Late Jurassic; many are Cretaceous (Coleman et al., 1994, 1998). Supporting evidence includes Cretaceous U/Pb ages of some Independence dikes and dated Cretaceous plutons that are cut by Independence dikes in the type locality of the swarm (STOP 6). Cretaceous Independence dikes are probably related to mafic complexes in the eastern Sierra (STOPS 4,8; e.g., Coleman et al., 1995; Sisson et al., 1996).

2) Offset wall rock markers in the Sierra Nevada indicate that net displacement vectors across dikes trend broadly north-south rather than northeast-southwest, i.e., oblique rather than perpendicular to dike walls. However, at least some such dikes initially opened perpendicular to their walls, implying that the dikes were initiated as tensile fractures but then accommodated wall-parallel sinistral shear. The lack of a solid-state fabric in some dikes with pronounced sinistral separations implies that at least some of the dike-parallel sinistral shear occurred during dike intrusion.

3) Many dikes contain an oblique petrographic or magnetic fabric that is consistent with sinistral oblique opening (STOP 3, 6). We interpret the oblique fabrics to have formed as a result of sinistral shear along the subduction boundary that was partitioned into the Late Jurassic magmatic arc.

4) In the eastern Sierra, broadly north-striking reverse-sense mylonitic shear zones both crosscut and are crosscut by dikes, indicating that the shear zones formed during dike intrusion (STOP 6). Contraction during emplacement of the IDS may have occurred within a sinistral transpressive strain regime (Carl et al., 1996a), although more quantitative strain data are needed to substantiate this interpretation (see further discussion below). Emplacement of a dike swarm within a contractional strain regime strongly contrasts with the conventional view that dike swarms are extensional in origin.

Characteristics of the Independence Dike Swarm

Extent

The northernmost accepted exposures of the swarm (Fig. 1) have been found within the Mt. Morrison quadrangle in the Sierra Nevada (Rinehart and Ross, 1964) and in the Blanco Mountain quadrangle in the White Mountains (Nelson, 1966; Ernst, 1997). The swarm crosses the Mojave Desert to its southernmost exposure in the Chuckwalla Mountains (James, 1989; Davis et al., 1994). Dikes correlated with the IDS occur north of the Mt. Morrison quadrangle in the Snow Lake pendant of the central Sierra Nevada, but these dikes likely formed much farther to the south and were translated by Cretaceous dextral faulting to their present location (Lahren et al., 1990; Schweickert and Lahren, 1990). Independence dikes may occur as far north as Lake Tahoe (M. Lahren, personal communication) and as far south as northern Mexico (James, 1989).

Age

Moore and Hopson (1961) distinguished two distinct periods of plutonism in the Mt. Pinchot quadrangle northwest of Independence, based on whether or not the plutons contain Independence dikes. They interpreted early lead-alpha and K-Ar dates to indicate that the dikes and host plutons all are Cretaceous. U-Pb dating of three silicic dikes by Chen and Moore (1979) established a latest Jurassic age of 148 Ma, and James (1989) determined ages near 148 Ma for silicic IDS dikes in the Mojave Desert and eastern Transverse Ranges. The seemingly narrow age range of the swarm makes it a valuable temporal marker and has been used to infer ages of deformation up and down eastern California (e.g., Lahren et al., 1990; Schweickert and Lahren, 1990; Walker et al., 1990; Walker and Martin, 1991; Dunne and Walker, 1993; Stephens et al., 1993; Dunne et al., 1994; Davis et al., 1994).

This simple story was upset when Coleman et al. (1994) determined the first U-Pb ages on mafic dikes from the IDS. Some dikes in the type locality are Late Cretaceous (90-100 Ma; Table 1; STOP 6). Further, in at least three localities in the Mt. Pinchot quadrangle (Moore, 1963), Independence dikes cut plutons that are now known to be Cretaceous (STOPS 4,8; Carl et al., 1996b; Coleman et al., 1998). Dikes exposed northwest of Bishop in Pine Creek Canyon also are Late Cretaceous (Coleman et al., 1994, 1998). Cretaceous ages on several dated mafic dikes led to the uncomfortable fear that all mafic Independence dikes are Cretaceous, but several U-Pb ages near 148 Ma have recently been determined for mafic IDS dikes (Table 1; e.g., Coleman et al., 1994, 1998; R. Whitmarsh and J. D. Walker, unpublished).

The Cretaceous ages raise an important question: how can Jurassic dikes be distinguished from Cretaceous ones? Preliminary data indicate that dikes with a clear sinistral solid-state fabric (see below) are uniformly Jurassic. All known Cretaceous mafic dikes occur in the central Sierra Nevada between Independence and Pine Creek. These dikes lie near and strike toward Late Cretaceous mafic plutons (see STOPS 4,8), and locally contain a dextral solid-state fabric.

Petrography

Dike compositions vary along the length of the swarm. Dikes are dominantly bimodal, either silicic or basaltic (Moore and Hopson, 1961), but many intermediate types occur as well (e.g., McManus and Clemens-Knott, 1997). Dike textures range from glassy and aphyric to medium-grained porphyritic (>30% phenocrysts). Toward the northern end of the swarm, especially in the Inyo Mountains and Sierra Nevada, Independence dikes are dominantly mafic (90% in the Mt. Pinchot 15' quadrangle). Composite dikes also are present as coplanar intrusions (up to 8 or more) of varying composition and as enclave-rich dikes (>50% enclaves; Carl et al., 1997).

Deformation

Fault offsets

Because the IDS is a fairly linear feature, it has been used to determine strike-slip offsets across major faults in eastern California. Moore and Hopson (1961) estimated no more than several km net offset across Owens Valley based on continuity of the swarm. In a now-classic paper, Smith (1962) used the swarm to estimate approximately 65 km of sinistral offset across the Garlock fault. Glazner et al. (1989) and Martin et al. (1993) showed that dikes in the northeastern Mojave Desert line up with dikes southeast of Barstow if several tens of km of dextral offset related to Miocene extension are removed (Fig. 1). James (1989) suggested that dikes in the San Gabriel Mountains, west of the San Andreas fault, might correlate with those in the Mojave Desert after removal of slip on the San Andreas fault.

Vertical-axis rotation

Smith (1962) noted that dikes south of the Garlock fault strike 20°-45° more clockwise (northerly) than those north of the Garlock fault. He assumed that the dikes were originally parallel and attributed the strike discrepancy to clockwise rotation of the Mojave block relative to the Basin and Range. Ron and Nur (1996; but see also Howard et al., 1997) used dike orientations to infer relatively minor rotation of different blocks in eastern California. James (1989) noted that dikes of the IDS generally do not show the extreme rotations inferred from paleomagnetic studies of Tertiary volcanic rocks (e.g., Valentine et al., 1993). This discrepancy may result from sampling of different domains (e.g., Tertiary cover generally obscures pre-Tertiary dikes) or from problems with the paleomagnetic data that may include inadequate structural correction and inadequate averaging of secular variation. However, Independence dikes may not be recognized as such if they do not strike northwest. For example, B. Cox (personal communication, 1991) noted that northeast-striking diorite dikes southeast of Barstow (near Daggett Ridge; Dibblee, 1970) could be clockwise-rotated Independence dikes. Combined rotations studies of both Tertiary and pre-Tertiary markers are needed to resolve these discrepancies.

Deformation of individual dikes

Independence dikes contain an oblique foliation which consistently strikes counterclockwise relative to the dike's strike. We have identified two manifestations of this oblique fabric. In the eastern Sierra, a mesoscopic solid-state foliation indicates sinistral shear along the margins of many Independence dikes (Moore and Hopson, 1961; Carl et al., 1995). This feature is recognized in dikes over a >60 km distance from the Woods Lake area (Mt. Pinchot 15' quadrangle) to the Tungsten Hills near Bishop. Evidence for sinistral ductile deformation also has been found along the margins of some Independence dikes in the Coso Range (Whitmarsh et al., 1996), although we have not studied the extent and nature of this deformation. In the Sierra, the foliation resembles an asymmetric sigmoidal foliation concentrated along a dike's margins and sometimes extends across the entire width of a dike (Fig. 2). Moore and Hopson (1961) elegantly described this foliation as a "reverse integral sign." Visible deformation is largely confined to the dike's margins, generally affecting host granitic wall rock only near dike jogs (Carl et al., 1995). Silicic and mafic dikes contain mylonitic features including S-C composite foliations and rotated phenocrysts with tails. Plagioclase and amphibole are ductilely deformed in these rocks. A weakly deformed silicic dike is found along the roadcut at STOP 6 (Onion Valley Road), but the best examples of ductilely deformed dikes are found in the High Sierra. Ductilely deformed dikes appear limited to the eastern Sierra and possibly the western Inyo Mtns (STOP 5).

Farther to the south, Independence dikes contain a cryptic foliation defined by anisotropy of magnetic susceptibility (AMS). AMS was measured to look for preferred orientations that might record magmatic flow kinematics. Magnetic lineation (maximum susceptibility direction) generally is interpreted to record the flow direction, and magnetic foliation the flow plane (ref). AMS data obtained from Independence dikes in the southern Argus range and Spangler Hills reveal that the magnetic foliation is consistently oriented counterclockwise relative to dike walls (Fig. 3; STOP 3). We interpret this fabric to indicate that dikes opened obliquely to their walls (Dinter et al., 1996a).

Key Questions Regarding the Independence Dike Swarm

What tectonic setting does the swarm represent?

Factors controlling dike injection

Most authors interpret the IDS to have formed in response to extension perpendicular to the swarm (e.g. Chen and Moore, 1979, 1982; Karish et al., 1987; James, 1989; Schermer, 1993; Schermer and Busby, 1994; Davis et al., 1994). Other proposed origins include formation within a transtensional, obliquely convergent margin (James, 1989), and dikes formed above expanding plutons (Hopson, 1988).

Field evidence supporting an extensional setting for the latest Jurassic is found in the central and southern Mojave Desert, where dikes apparently intruded along normal faults (Karish et al., 1987; Schermer 1993). Independence dikes also are the same ages as and spatially associated with volcanogenic rocks of the Upper Sidewinder series in the west-central Mojave Desert (Table 2; Karish et al., 1987; Schermer, 1993; Schermer and Busby, 1994). Sidewinder rocks are interpreted to have formed in an extensional environment (Karish et al., 1987; Schermer, 1993).

Chen and Moore (1979) suggested that the IDS intruded along fractures formed by regional extension. They cited as supporting evidence the observation that dikes within the swarm are parallel to the overall trend of the swarm. Previously published maps of the swarm show dikes subparallel to the overall trend of the swarm (e.g. Chen and Moore, 1979; James, 1989; Davis et al., 1994). However, this depiction of the swarm is inaccurate; dikes north of the Garlock fault strike 10-20° counterclockwise to the overall trend of the swarm (Fig. 4). This evidence suggests that Independence dikes intruded oblique dilatant fractures within a regional-scale sinistral shear zone (Fig. 5; Glazner et al., 1997, and in review). We have found additional evidence for sinistral oblique opening of Independence dikes in the Sierra (STOP 6; Glazner et al., 1997, and in review).

We also present evidence (STOP 6) that mylonitization occurred along reverse-sense shear zones and closely overlapped in time with dike intrusion. A major difficulty of emplacing the Independence dike swarm at the same time as reverse-sense mylonitization is that it appears to require dilation in the same direction as the mylonitic contraction. One solution that we have proposed above is that dikes intruded Riedel-style fractures that opened obliquely to their margins in a direction perpendicular to the shortening direction (Glazner et al., 1997, and in review). Partly on this basis, we proposed that the dikes and mylonite zones are expressions of a crustal-scale sinistral shear zone along the Late Jurassic magmatic arc. However, the mechanical feasibility of forming dikes that open obliquely to their margins is uncertain, and there is clear field evidence that some dikes with sinistral net opening vectors initially opened perpendicular to their margins.

Therefore, we do not rule out intrusion along oblique fractures but also consider an alternative shear-zone model in which dikes intruded in local domains of transtension along the crustal-scale shear zone and, similarly, contractional mylonite zones formed in local domains of transpression (Fig. 6). Such domains of transtension and transpression are expected along a complex and anastomosing shear zone similar to the modern Pacific/North American plate boundary (e.g., Sylvester, 1988). As displacement across an anastomosing shear zone proceeds, the areal pattern of transpression and transtension changes and individual rock volumes will experience mutual overprinting of contractional and dilational strains. In this model, the overall kinematics of the crustal-scale shear zone need not be transpressional or transtensional but could be neutral (see STOP 6 discussion).

Even if the presence of late Jurassic sinistral shear is accepted, numerous questions remain unanswered. If fluctuations between transtension and transpression were controlled by shear-zone irregularities, what was the length scale of the irregularities and how might that relate to the map pattern of the dike swarm? Why do dike orientations vary? If the stress field varied in map view, how might it have varied with depth and how would that be expressed in characteristics of the dike swarm? Why are dikes and their host rocks not mylonitized (or only minimally so) farther south along the arc? Why was the swarm intruded over such a short interval of time (although some evidence suggests that the IDS may have intruded over a longer interval, >10 Ma: R. Whitmarsh and J.D. Walker, unpublished data; Whitmarsh, 1998; Table 1). Did the shear zone exist only as long as dikes intruded? We have yet to answer these questions satisfactorily.

Plate setting; Nevadan orogeny

IDS intrusion coincided with a number of interesting plate-scale events in the latest Jurassic, although the exact relationship between these events and the IDS remains uncertain. Chen and Moore (1979) first noted that IDS intrusion coincided with an interval of relatively little plutonic activity, a magmatic "lull" in the arc's history (but this is not true in the Mojave Desert nor Inyo Mountains; Schermer, 1993; Dunne and Walker, 1993; Glazner et al., 1994). Why did arc magmatism manifest itself in the form of a dike swarm with (apparently) few associated plutons? The IDS may be related to the Nevadan orogeny (e.g., Bateman and Clark, 1974; Tobisch et al., 1987; Edelman, 1991) which occurred just before and during IDS intrusion. The swarm's short life span could be related to a discrete collisional event, such as that proposed for formation of structures in the western Sierra and Klamath Mountains attributed to the Nevadan orogeny (e.g., Ingersoll and Schweickert, 1986; Harper et al., 1994). Major changes in plate motions also may have caused or contributed to formation of the swarm. Paleomagnetic data record an abrupt change in the apparent polar wander path for North America in the latest Jurassic ("J2 cusp" of May et al., 1986; May and Butler, 1989) that is inferred to represent rapid northward acceleration of the plate. Intrusion of the IDS and several other northwest-striking swarms closely coincides with the J2 cusp (Wolf and Saleeby, 1995). Plate reconstructions (Page and Engebretson, 1984; Engebretson et al., 1985) support a rapid shift in movement of the North American plate at this time. Opening of the Gulf of Mexico could also be involved in IDS formation (e.g., Davis et al., 1994; Fackler-Adams et al., 1997).

Why are dikes preferentially deformed relative to weaker(?) wall rocks?

The shear fabric recorded in deformed rocks in the central Sierra clearly formed in the solid state, based on its continuation into wall rocks and on petrographic data. However, rock deformation experiments indicate that, at the relevant P-T conditions, the mainly mafic to intermediate IDS dikes should have been as much as an order of magnitude stronger than the quartz-rich granitoids that form much of the wall rock (e.g., Kirby and Kronenberg, 1987). Indeed, the common field relation in granitoid gneiss terranes is that mafic to intermediate rocks are boudined within more ductile granitoids. This presents a rock-mechanical paradox. Thermal weakening related to dike emplacement cannot account for the fabric relations unless unusually high strain rates (essentially seismic events) occurred. One possible explanation may be hydrolytic weakening concentrated along dike margins.

What controlled dike orientation and location?

As noted earlier, IDS dikes appear initially to have opened perpendicular to their walls, but their net opening vectors are generally sinistral oblique. The former observation is consistent with the dikes having formed perpendicular to the least compressive stress as is normally the case (e.g., Anderson, 1951). The second observation suggests that the principal axes of finite strain related to dike emplacement are oriented counterclockwise relative to the principal stress axes. A pronounced discordance between stress and strain axes indicates a noncoaxial strain path, i.e., there was a significant component of simple shear in the bulk strain field. We interpret the Independence dike swarm to have formed as tension gashes in a regional zone of sinistral shear along the Jurassic arc (Fig. 6).

The intimate association of dike emplacement with sinistral shear suggests that the shear regime was transtensional. However, as noted earlier, north-striking thrust-sense mylonite zones also both crosscut and are crosscut by dikes, suggesting that sinistral shear, dike injection, and east-west contraction all affected the same rock volume at about the same time. This complex overprinting is explicable if the dikes were emplaced in an anastomosing crustal-scale shear zone in a uniform field of regional east-west compression in which the maximum horizontal stress, sH, trended roughly E-W and was subequal to the vertical stress, sv (Fig. 6). In segments of the anastomosing zone that were oriented at a high angle to sH, sH was locally greater than sv, resulting in a thrust stress regime. Along segments of the shear zone oriented at a low angle to sH, sH was locally less than sv, resulting in a transtension accommodated by dike injection perpendicular to sh, the least horizontal stress, and sinistral shearing of the dikes and their wall rocks. As displacement accumulated across the shear zone as a whole, its irregular boundaries and undeformed enclaves among the anastomosing branches would have moved with respect to each other. This would result in individual rock volumes passing with time from transpressional to transtensional domains, resulting in the observed complex overprinting relations.

Whether the crustal-scale shear zone accommodated overall transtension, transpression, or nearly pure strike slip, depends on the relative magnitudes of dilation across dikes vs. contraction across thrust-sense mylonite zones. As yet we are unable to quantify the strains sufficiently to answer this question.

It is curious that Cretaceous dikes in the swarm generally parallel Jurassic ones. In Woods Lake basin dikes of each age are strictly parallel and can only be distinguished by differences in deformation and by U-Pb dating (Table 1). This raises the interesting question of what ultimately controls dike orientation. Coleman et al. (1998) suggest that, because dike orientation is so consistent from Jurassic through Cretaceous time despite highly variable plate convergence angles across that time period, dike and pluton orientation could not be controlled by interactions between the subducting slab and overriding plate. Rather, they suggest that dike orientation is controlled by the orientation of the free-margin of the overriding plate, or that Cretaceous dikes intruded along fractures established during emplacement of the Jurassic dikes.

What is the petrogenesis of the swarm?

What does its chemistry tell us about the nature of its origin? There have been few geochemical studies of the IDS, in part because the swarm is locally highly altered. Is there a coeval magmatic (plutonic) arc genetically linked to the swarm (Hopson, 1988)? Evidence for this during the Jurassic is sparse in the Sierra Nevada, but more common in the Mojave Desert (e.g., Glazner et al., 1994). Cretaceous Independence dikes in the east central Sierra Nevada are almost certainly related to relatively abundant coeval mafic plutons (Table 3) and can, in some cases, be mapped into coeval mafic plutons (Coleman et al., 1998). Did coeval plutons form in response to the same tectonic event that triggered formation of the swarm? Is style of magmatism (dikes vs. plutons) affected by changing tectonic settings? Why do coeval Cretaceous mafic rocks occur as steeply-dipping dikes and plutons composed of nearly horizontal sills at the same structural level? Field and geochemical data indicate that the swarm is bimodal in composition (Moore and Hopson, 1961; Smith, 1962; Chen and Moore, 1979). Is this bimodality related to the swarm's tectonic origin?

Acknowledgements

This work was graciously supported by a National Science Foundation grant (EAR-9526803) to Allen F. Glazner and grants to Brian S. Carl from the Martin Fund (University of North Carolina), White Mountain Research Station (University of California), and Geological Society of America. Thanks to Max Woodbury, Kathileen Davidek, Brian Coffey, and Kent Ratajeski for valuable field assistance. Kings-Canyon Sequoia National Park and Inyo Nationa Forest Service staff provided information and permits to the high country. Special thanks to the staff of the White Mountain Research Station in Bishop for providing palatial accommodations, incredible cuisine and a friendly home away from home, and especially Scott Hetzler for his welcome voice on the amateur airwaves while in the remote Sierra.

Road Log

Each day will involve one or more hikes of moderate length. Participants should bring water bottles, sturdy shoes, hat, sunscreen, etc.

DAY 1

Depart at 7:30 AM from the California State University Long Beach Activities Pyramid located at the north end of the campus. The pyramid is a large blue structure (not easily missed) and can be accessed via Atherton Avenue. The roadlog begins at the turnoff to Bear Valley Road located off I-15 near Apple Valley. This is reached from Long Beach by driving north via Highway 605, east on I-10, and then north on I-15, a total distance of about 90 miles (Fig. 7).

[Note: Directions to individual stops have been deleted from this web page. See published guidebook for road log details]

STOP 1. Fry Mountains

What to see: 1) Numerous unusually alkalic Independence dikes emplaced at shallow depths in the south-central part of the swarm (Fig. 1). 2) Foliation defined by color banding or penetrative fracturing of dikes, rarely flanked by a fabric in the host wall rock. Discussion questions: How does foliation form in a dike? Do deformed dike margins reflect regional strain? How does evidence for crustal extension during IDS emplacement in the central Mojave Desert relate to evidence for transpression at the same time in the Sierra Nevada?

The stop involves a short hike along the powerline road that crosses the Fry Mountains across the strike of the IDS. Begin where the road turns away from the powerline near the lowest support tower (Fig. 9). A blue-gray fine-grained felsic dike ("latite" of Dibblee, 1964a,b; "comendite" of Karish et al., 1987) is exposed at the corner of the first large bend where the road begins to turn back toward the powerline. The dike contains a fabric defined by alternating gray/white (quartz/feldspar?) bands that is oriented 125° 52° SW, subparallel to the contacts. Continue several hundred meters further along the road, uphill toward the next support tower, crossing numerous mafic and felsic dikes. From the tower, proceed southward along a >4 meter-wide mafic dike that is intensely fractured parallel to its margins. The dike strikes nearly north-south but resembles nearby NW-striking mafic dikes. How did this fabric form? The dike also contains network veining by thin felsic segregations. Enclave-rich composite dikes are found in the Fry Mountains; the closest lies <1 km northeast along the road (Fig. 9).

Dike compositions in the Fry Mountains include felsic, intermediate, basaltic andesite, basaltic, and composite dikes, but the majority are felsic or basaltic. The high alkali content is reflected in alkali amphiboles, e.g., riebeckite-arfvedsonite and kaersutite (Karish et al., 1987). Dikes here are shallow-level examples of Independence dikes and resemble dikes exposed in the Ord and Rodman Mountains to the north and west (Dibblee 1964a, 1964b) that cut nearly coeval volcanic rocks of the Upper Sidewinder series (e.g., Schermer and Busby, 1994). Dikes that feed lava flows of the Upper Sidewinder volcanic series in the Ord Mountains (Karish et al., 1987; Schermer and Busby-Spera, 1990; Schermer, 1993) demonstrate the shallow emplacement of these Independence dikes. Dikes in this area mainly strike 320° and are interpreted to have formed during extension orthogonal to the swarm (Karish et al., 1987; Schermer, 1993), based on the observation that the dikes both were injected along and are cut by normal faults in the Upper Sidewinder volcanic series. These relations contrast with field evidence in the eastern Sierra Nevada for east-west contraction during Independence dike intrusion.

Sinistral fabrics in dikes in the eastern Sierra (e.g., Carl et al., 1995) prompted us to search for similar fabrics farther south along the IDS. We have found only very limited wall rock deformation adjacent to dikes in the Fry Mountains and its origin is not completely understood. Karish et al. (1987) described a "fine-scale protoclastic lamination, parallel to, and usually adjacent to, the dike margins " of silicic dikes but did not propose an origin for this feature. Wall rock adjacent to two silicic dikes in the Fry Mountains appears mylonitized (best observed in thin section), but this fabric could have formed as a result of wall rock interaction with a highly viscous silicic melt, and not as a result of regional strain concentrated along dike margins, as we infer for dikes in the Sierra. No similar fabric has been found along the margins of mafic dikes in the Fry Mountains We interpret the alternating blue-white foliation within felsic dikes as magmatic flow banding.

On the drive northward along Highway 247 toward Barstow, we cross the proposed Mojave Valley Fault (Martin et al., 1993; Fig. 1), which separates an area to the north that was highly extended in the Miocene from a relatively unextended area to the south. Up to 60-70 km of dextral displacement across this proposed fault is supported by offset of the western limit of the IDS and several other tectonostratigraphic markers (Martin et al., 1993). Extensional slip of > 40 km across the Waterman Hills detachment fault exhumed a high-grade Mesozoic metamorphic complex intruded by a Miocene granite pluton (Walker et al., 1990, 1995). The Mojave Valley fault may continue eastward for several 100 km and link the central Mojave metamorphic core complex to the Colorado River extensional corridor.

In the Iron Mountains southwest of Barstow (Fig. 1), the Jurassic Hodge volcanic series is cut by a weakly deformed 148 Ma gabbroic pluton (Boettcher, 1990; Boettcher and Walker, 1993). The pluton commonly is cumulate in texture and may be genetically related to the IDS. Other 148 Ma plutons (Table 3) are uncommon along the swarm but several are present in the Mojave Desert (Boettcher, 1990; Boettcher and Walker, 1993; Miller and Glazner, 1995), the Inyo Mountains (Dunne et al., 1990), and the eastern Sierra (Frost and Mattinson, 1993). The plutons are dominantly mafic in composition. Chen and Moore (1979, 1982) proposed that the IDS intruded during a magmatic "lull" in Jurassic arc history. Hopson (1988) instead suggested that Independence dikes originated from a coeval magmatic arc beneath the swarm but the coeval plutons remain largely unexposed. As more IDS-age plutons are recognized, the "lull" hypothesis seems less tenable.

North of Johannesburg we cross the Garlock fault (Fig. 1). Smith (1962) inferred 65 km of left-lateral displacement across the Garlock fault from lateral offset of the IDS from the Granite and Avawatz Mountains, south of the fault and far to our east, to the Spangler Hills, our next stop.

STOP 2. Spangler Hills

What to see: Mafic and felsic Independence dikes near the center of the swarm. Discussion questions: What is the significance of bimodal dike compositions? How are Independence dikes of differing compositions genetically related? How do dikes intrude plutons that are scarcely older than themselves?

Walk eastward from the parking area to a granite dike (IDS Photo 1) 2-3 m thick. The western contact of the dike with the host monzonite (338°63° NE) is well exposed. Splays or sheeted fingers up to 10 cm thick extend from the western contact subparallel to the contact. Grain size within the dike varies. The dike is fractured along internal planes of foliation (flow banding?) that are oriented 345°53° NE, i.e., subparallel to the wall. Several thinner dikes, including a <0.5 m aplite, branch from the main dike. One branch strikes 285° and intersects a nearby diorite dike, but it is not clear if it cuts it. The diorite dike lies about 50 m to the east and is at least 6 m thick. It is largely homogeneous except for sparse mafic enclaves that contain abundant 1-2 mm acicular amphibole crystals in a finer-grained matrix. The dike is only slightly altered and contains amphibole and plagioclase phenocrysts in a fine grained groundmass. Dike margins here are typically poorly exposed, and the western contact of the diorite dike is visible only for about 1 meter. Within 10-20 cm of the contact, a network of fractures appears to be filled with amphibole crystals. The fractures are commonly spaced 2-10 cm apart, and the dike margin is irregular at this scale.

Dikes in the Spangler Hills are dominantly bimodal in composition, but intermediate dikes also are found and a few are composite. Independence dikes generally are thicker at this latitude than to the north or south and range up to 60 m (Smith, 1962). Mylonitic fabrics have not been identified along the margins of Spangler Hills dikes. Several important ages have been obtained from rocks in the Spangler Hills. Chen and Moore (1979) dated a felsic dike in the eastern Spangler Hills (148±2 Ma; Table 1) and its slightly older host pluton (149 Ma; Table 3). A similar relationship also has been identified in the Coso Range 40 km to the north (Whitmarsh, 1996), where northwest-striking dikes cut a 149 Ma hypabyssal complex. Mafic dikes have not been dated in the Spangler Hills, but mafic dikes in the Coso Range have been dated at 148 Ma (Whitmarsh, 1998; Table 1). Felsic dikes in the Spangler Hills petrographically resemble a pluton in the northwest Spangler Hills near this location. If the pluton is similar in age, it is one of the few felsic plutons coeval with the IDS (Table 3).

STOP 3. Poison Canyon

What to see: 1) Mafic dike that contains oblique anisotropy of magnetic susceptibility (AMS) foliation. 2) Variably altered aphyric to porphyritic mafic dikes. Discussion questions: What is the origin of oblique AMS fabric in Independence dikes?

This area lies in the Pleistocene drainage that carried water from China Lake to Searles Lake, and green sediments from ancestral Searles Lake cover the surrounding countryside. From the parking area, cross the drainage and walk along the northern side of Poison Canyon for several hundred meters to inspect several mafic dikes cutting Mesozoic granite and granodiorite. Dikes are variably altered and range in thickness from <1m to 10 m. One dike contains enigmatic epidote "spherules" up to several 10's of cm in diameter. The well-exposed dike margins are unmylonitized but commonly contain fractures (spaced at 10 cm) subparallel to the contact. Phenocryst contents in the dikes range from near 0 to 30%. Several Poison Canyon dikes have been cored for paleomagnetic study.

The most intriguing feature of the mafic dikes here is not visible in outcrop. The dike exposed at the western end of Poison Canyon strikes 303°, is at least 3 km long, and ranges from a few to >14 m across (Dinter et al., 1996a). More than 40 drill cores collected at 1-2 meter intervals along 5 traverses across the dike were analyzed for anisotropy of magnetic susceptibility (AMS). The AMS fabric in this dike is defined by shape preferred orientation of titanomagnetite grains and by the planar alignment of these grains between plagioclase and hornblende. Both textures appear to be inherited from magmatic flow. Magnetic lineation is variably developed but, where well defined, plunges steeply, which we interpret to indicate that magma injection was mainly upward rather than lateral in this area.

The surprising result is that magnetic foliation is uniformly oriented 10-40° clockwise from the dike walls (Fig. 3), irrespective of sample location within the dike (Dinter et al., 1996a). An oblique igneous foliation may result from laminar shearing of magma against the walls during injection, but the obliquity should reverse across the midplane of the dike. Such symmetrical flow foliation patterns have been observed in some dikes (B. Housen, personal comm., 1996), but AMS data from the Poison Canyon dike clearly differ from this pattern. Dinter et al. (1996a) interpreted the oblique fabric to have formed during sinistral dike opening. A similar fabric was found in >30 other dikes in the Coso and Argus ranges to the north (Dinter et al., 1996b). Other field evidence supporting synintrusive sinistral opening of dikes is found along mafic dikes at the northern end of the swarm in the eastern Sierra, where markers are displaced sinistrally across otherwise undeformed dikes (see above; Glazner et al., 1997, and in review).

Geochemistry of Spangler Hills , Poison Canyon dikes

McManus and Clemens-Knott (1997) conducted a geochemical study of dikes exposed in the Spangler Hills and southern Argus Range, including a sample of the intensively drilled dike in Poison Canyon just described. This is the only study yet to address directly the genetic relationships among individual Independence dikes. McManus and Clemens-Knott (1997) confirmed that the dikes are dominantly bimodal but also found intermediate compositions. They interpreted the wide bulk-rock compositional range to have formed by fractional crystallization of plagioclase and hornblende±pyroxene. Characteristic reversals in geochemical trends on Harker diagrams and relatively low d18O values support this interpretation. However, significant hydrothermal activity altered the compositions of dikes following formation. In contrast, Farber et al. (1989) found compelling evidence for magma mixing in a suite of IDS-age alkalic plutons in the Chuckwalla Mountains, southern Mojave Desert, and inferred that to be the main mechanism by which intermediate compositions were produced. Magma mixing may also be responsible for intermediate compositions observed in composite dikes along the swarm (Carl et al., 1997). Further geochemical and isotopic analyses are required to test whether both processes were involved.

DAY 2

STOP 4. Cretaceous mafic rocks in the Sierran range front west of Aberdeen

What to see: Mixing and mingling features among mafic and felsic rocks of the Cretaceous Aberdeen Mafic Sill Complex (AMSC). Discussion questions: What is the areal extent of Cretaceous Independence dikes? Are mafic complexes like the AMSC their source? Why are the orientations of the Cretaceous dikes similar to those of Jurassic dikes?

The AMSC (Bradford et al., 1994; Bradford, 1995) is exposed along the eastern Sierran range front below a striking light-on-dark contact at about 3200 m elevation that is easily visible from Highway 395. At STOP 4 north of Goodale Canyon, mafic to felsic rocks of the AMSC are intimately mingled at all scales (Fig. 13). The rocks form a northwest-striking outcrop 300 m long by 50 meters thick. Petrographic features in the body are typical of the sill complex and include mafic enclaves with chilled margins, mutually crosscutting mafic and silicic dikes, cuspate-lobate contacts (IDS Photo 2), and hybrid rocks. The mafic rocks contain layering from <1 cm to >10 cm in thickness that is especially strong near the contact with granite at the highest point on the ridge. More spectacular examples are found several km to the southwest of this location at the mouth of Goodale Canyon (Fig. 13), where mafic sills dip gently westward beneath the Sierra Nevada. The mafic sills range from 1 cm to >10 meters thick and continue southward along the rangefront for several km.

These rocks are part of a suite of Cretaceous (90-96 Ma) mafic complexes exposed in the eastern Sierra (e.g. Frost and Mattinson, 1993; Coleman et al., 1995; Sisson et al., 1996). The complexes that have been studied in some detail include the AMSC (Fig. 13; Bradford et al., 1994; Bradford, 1995), the Onion Valley complex (Sisson et al., 1996), and a mafic body 20 km to the north of here (in the Lamarck granodiorite: Coleman et al., 1992, 1995). Dates from Cretaceous Independence dikes match closely the ages of the mafic complexes (Table 1; Coleman et al., 1994, 1998); for example, a mafic dike that cuts pendants rocks within the Woods Lake pluton 8 km to the southwest of Stop 4 yielded an U/Pb age of 94±1 Ma (Coleman et al., 1994; Table 1). Also, a Cretaceous Independence mafic dike in Pine Creek is close in age to a mafic body near Pine Lake (STOP 9; Coleman et al., 1998; Frost and Mattinson, 1988). Sills of the AMSC project below Sierran granitoid plutons that are intensely intruded by the IDS, and we propose that many Cretaceous Independence dikes emanated from similar mafic sill complexes.

The mafic to intermediate McDoogle pluton lies between the AMSC and a dated Cretaceous Independence dike in the Woods Lake area (Fig. 14). The pluton contains strong northwest-striking layering and is highly elongate in this direction. New U/Pb data (Carl, unpublished) indicate that the McDoogle pluton also is nearly coeval with the AMSC. The internal layering suggests that the pluton may have formed by successive dike-like intrusions (Bartley, Glazner, and Carl, unpublished mapping) that perhaps originated from mafic bodies such as the AMSC. Why Cretaceous dikes in the Sierra strike northwest, parallel to Jurassic Independence dikes, is uncertain. Perhaps Cretaceous dikes utilized fractures previously formed during the intrusion of Jurassic Independence dikes (Coleman et al., 1998).

STOP 5a,b. Santa Rita Flat pluton

What to see: 1) IDS-age(?) mylonite zone. 2) Strongly foliated felsic dikes. Discussion questions: What is the tectonic significance of the mylonite zone at Santa Rita Flat? What are the nature and origin of foliation in the dikes?

As we drive south along the dirt road at the base of the Inyo Mountains (Fig. 12), note the platy appearance of rocks to the east. Near the road, a mylonitic shear zone >5 km long deforms the northwestern edge of the Jurassic (Chen and Moore, 1982) Santa Rita Flat pluton and Independence dikes which cut the pluton (Ross, 1965).

STOP 5a. Mylonite zone in Santa Rita Flat pluton. Mylonite at this location contains a well-developed S-C composite foliation. The margins of the mylonite zone are not well-defined but the zone strikes NNW, dips steeply eastward, and contains a strong lineation oriented nearly down-dip. Shear-sense criteria including S-C composite foliation, asymmetric augen with tails, rotated phenocrysts, and mica fish, all of which typically form during ductile deformation of granitic rocks (e.g., Simpson, 1986) and indicate east-side-up sense of shear.

STOP 5b. Deformed dikes. In the lowest outcrops at the edge of the Santa Rita Flat pluton, the monzonite is not penetratively mylonitized but contains east-dipping mylonite zones 2-5 cm thick. A cataclastic fabric that is easily distinguished from the mylonites also affects the monzonite. More interesting is the pronounced foliation (IDS Photo 3) in a >4 m-wide felsic dike located close to the road where it cuts the Santa Rita Flat pluton with a strike of 350°. A shattered mafic dike that crosses directly over the top of the hill is less well-exposed but contains a similar fabric. The foliation is on a very fine scale (<1 mm), lineation is ill-defined, and the grain size is too small to deduce shear sense in the field.

The shear zone that cuts the Santa Rita Flat pluton shares many features with mylonite zones exposed in the eastern Sierra Nevada (STOP 6; Carl et al., 1996a), although the exposed dimensions of the Santa Rita Flat shear zone are greater. In the eastern Sierra, mafic Independence dikes cut mylonite zones with similar orientations and kinematics to the Santa Rita Flat zone, but the dikes are in turn mylonitically deformed. This is compelling evidence for synintrusive mylonitization (Fig. 15; Carl et al., 1996a). Latest Jurassic mylonitization in the eastern Sierra expands eastward the limit of deformation associated with the Nevadan Orogeny, previously thought to be largely limited to rocks in the western Sierra Nevada (e.g., Tobisch et al., 1987). Our tentative assignment of a latest Jurassic age to the Santa Rita Flat mylonite zone is based on its similarities to better dated mylonites in the Sierra Nevada.

STOP 6. Onion Valley Road

What to see: 1) The 165 Ma (Chen and Moore, 1982) Woods Lake pluton (WLP) cut by dikes and mylonite zones. 2) Syndeformational dikes. 3) Deformed dike margins. Discussion questions: Did mylonitization occur closely in time with dike intrusion? Is this compatible with the conventional idea that dikes intrude tensile fractures? What tectonic setting existed during IDS intrusion?

Several important structural features of the northern IDS are displayed in this roadcut (IDS Photo 4). Mylonitic shear zones exposed here appear temporally related to IDS intrusion. One mafic dike (date pending) intrudes along a west-dipping mylonite zone that cuts the WLP. The zone is 0.5 m thick and contains shear-sense criteria indicating reverse movement. The dike in turn is slightly deformed along the shear zone. The dacite dike near the center of the roadcut (U-Pb age of 148±11 Ma; Coleman et al., 1994, 1998; Table 1) contains a weak fabric near its margins. A basaltic dike cuts the dacite dike and deformed wall rock but also contains a suggestion of weak fabric along its eastern margin.

Field Relationships in the Woods Lake-Twin Lakes area, high Sierra: The best exposures of deformed dikes and mylonite zones are found in the High Sierra. To reach these exposures, however, requires strenuous hiking over some of the eastern Sierra's highest passes (Taboose Pass and Sawmill Pass trails; see Secor, 1992). Here we instead describe the most relevant observations.

At higher elevations 10 km to the west, similar relationships to those here are observed in the Jurassic (Chen and Moore, 1982) Woods Lake and Twin Lakes plutons (Fig. 14). Mafic dikes cut cm- to meter-scale mylonite zones in the plutons and themselves are deformed by 1 - >10 meter-wide NNE-striking reverse-sense shear zones with a varying sinistral strike-slip component. The majority of reverse-sense shear zones dip eastward (Fig. 15) but some west-dipping, west-side-up shear zones also have been observed.

Independence dikes exposed along the eastern Sierra between here and the Tungsten Hills (west of Bishop) commonly contain a sinistral ductile fabric along their margins. The fabric commonly is sigmoidal across the width of a dike (Fig. 2). A similar fabric has been reported along the margins of a few dikes, one dated as late Jurassic, in the Coso Range; rarely the fabric continues up to 10 cm into the wall rock in the form of a mylonitic foliation (Table 1; Whitmarsh et al., 1996; Whitmarsh, 1998).

We interpret field relations in the eastern Sierra to indicate that mylonitic shearing overlapped in time with IDS intrusion (Carl et al., 1996a). This assertion requires a more complicated process than dike intrusion into simple tensile fractures (see above: Factors controlling dike injection). Dike intrusion in a contractional environment contrasts with other proposed origins for the IDS and is unusual among dike swarms.

We have presented AMS evidence for oblique opening of Independence dikes in the southern Inyo Mountains and Spangler Hills (STOP 3; Dinter et al., 1996a,b). We have also found evidence in the eastern Sierra that Independence dikes opened obliquely (in a north-south direction) to their northwest-strike (Fig. 17; Glazner et al., 1997, and in review). We interpreted the oblique opening of dikes to indicate a component of Latest Jurassic sinistral shear partitioned into the magmatic arc. The sinistral fabric along the margins of deformed dikes provides support for regional sinistral shear (e.g., Carl et al., 1996a). However, the exact origin of fractures intruded by Independence dikes remains unclear.

If our interpretation of field relationships is correct, then the age of the mafic dike that is cut by the mylonite at Stop 6 is critical to determining the timing of deformation. We have not obtained reliable ages from mafic dikes from this roadcut (but we keep trying). In the Woods Lake area (Fig. 14), a mafic dike and a rhyolite dike both yielded 148±1 Ma ages (Table 1; Coleman et al., 1998). However, a mafic dike, <1 km away and similar in composition and orientation to the dated Jurassic mafic dike, yielded a Cretaceous age (94±1 Ma: Coleman et al., 1998; Table 1). The dated Jurassic dike contains a distinct sinistral fabric near its margins that is absent in the Cretaceous dike. The sinistral fabric therefore may be an important way to distinguish otherwise similar mafic Cretaceous and Jurassic dikes (Fig. 18). This method may not be reliable, however, because the degree of deformation observed in dikes ranges from none to high. In addition, Cretaceous dikes also are foliated (e.g., Fig. 19) although we interpret the fabric as igneous in origin.

DAY 3

STOP 7. Alabama Hills

What to see: 1) Independence dikes, including composite ones, cut metavolcanic rocks in the northern Alabama Hills. Discussion questions: Are these dikes part of a sheeted dike complex? What is the significance of such a feature?

A short hike takes you across-strike of Independence dikes along the prominent drainage which begins near the intersection of Moffat Ranch and Movie Roads and continues through the northern Alabama Hills (Fig. 20). Along the drainage are found examples of closely spaced subparallel mafic, intermediate, silicic and composite dikes cutting metavolcanic rocks, but dikes are dominantly either felsic (dacite) or basaltic in composition. Dacite dikes 1->3 m thick (IDS Photo 5) contain mafic enclaves, and mafic dikes contain felsic enclaves. One composite dike contains mafic "pillows" in a silicic host and cuspate/lobate margins, features recognized in composite dikes which typically form during commingling of magmas (e.g., Snyder et al., 1997). Others contain planar internal contacts. Granite xenoliths are entrained along the margins of a few dikes. Intense alteration overprints dikes and metavolcanic rocks. Narrow mafic dikes which cut metavolcanic (?) rocks and form the border phases of mafic composite dikes jog sharply along the stream bed near the east end of the traverse. In one location, one branch of a bifurcating dike jogs sharply, whereas the other branch does not.

Metavolcanic rocks exposed in the northern Alabama Hills include rhyolitic lithic ash-flow tuff, volcanogenic sedimentary cover, and hypabyssal intrusions that "take the form of north-west trending dikes and sill-like masses" (Dunne and Walker, 1993). Chen and Moore (1979) dated several 148±2 Ma felsic dikes and confirmed that Independence-age dikes cut the metavolcanics. Distinguishing metavolcanic rocks from Independence dikes is not always clear because of their similar composition and appearance. Enclaves within some vertically-oriented lithologic units range from rounded to angular making it difficult to assess whether they are intrusive or volcanic in origin. The parallel nature of lithologic contacts further obscures relative age determinations. We propose that many (if not most) of the steeply-dipping vertical lithologic units may actually comprise members of an Independence "sheeted dike complex." The abundance of end-member mafic and felsic dikes, and the presence of composite dikes, support a bimodal nature of this structure. A similar structure has tentatively been found in the Woods Lake area, where numerous mafic and dacitic Independence dikes are closely spaced (<1 m apart), or in direct contact with one another across 50 m thickness (Carl and Bartley, unpublished mapping). However, dikes in the Alabama Hills lack mylonitic fabric along their margins or within adjacent wall rock found along Sierran dikes. Perhaps this is due to the shallower level of intrusion for dikes in the Alabama Hills.

APPENDIX

We include two additional stops below which we consider important parts of our field trip, but will be unable to visit due to limited time constraints. We estimate an additional 1/2 day is required to visit these stops.

OPTIONAL STOP 8. Pine Creek

What to see: Cretaceous Independence dikes cut the Triassic Scheelite Intrusive Suite (Bateman, 1965, 1992) on the north side of the valley.

Walk around the west side of the open pits and scramble up the talus to reach the closest mafic dike. This dike is one of numerous similar dikes which strike E-W and cut Mesozoic plutons in the Pine Creek and Tungsten Hills area west of Bishop. These dikes have been mapped as part of the Independence dike swarm (Mt. Tom quadrangle; Bateman, 1965, 1992). This dike resembles dated mafic dikes in the eastern Sierra both compositionally and structurally. However, sphene from this dike yielded a U/Pb age of 94±1 Ma (Coleman et al., 1994). Many, if not all, of the mafic dikes which cut plutons in this vicinity may be Cretaceous.

Earlier (STOP 4), we suggested that Cretaceous Independence dikes may have originated from mafic intrusive complexes in the eastern Sierra. A possible source for Pine Creek dikes lies near Pine Lake, several km to the west of Pine Creek. Although no dikes have been traced directly from the pluton, the quartz diorite of Pine Lake, dated as 97.5 Ma (Frost and Mattinson, 1988: U/Pb zircon), is similar in composition and age to the dated Cretaceous dike in Pine Creek. In addition, the E-W strike of Pine Creek mafic dikes lines up closely with the mafic Pine Lake body.

OPTIONAL STOP 9. Dolomite

What to see: Dark-colored mafic Independence dikes cut Paleozoic metasedimentary rocks (carbonates) of the southern Inyo Mountains on the north side of the road.

Mafic Independence dikes along highway 136 cut strata ranging from the Ordovician Ely Springs dolomite to the Devonian Hidden Valley Formation. At the Cerro Gordo Mine, located just to the north of our stop, ore (largely silver) was extracted from the Devonian Lost Burro Formation (Merriam, 1963). Dikes that cut these metasedimentary rocks have undergone extensive alteration. It is difficult to interpret the geochemistry of such highly altered dikes (e.g., McManus and Clemens-Knott, 1997). Two parallel mafic dikes about 300 meters from the road here made a cameo appearance in the 1990 film "Tremors" (IDS Scenic Photo 10).