Abstract

New geologic mapping, structural data, and 40Ar/39Ar geochronology document early Miocene sedimentation and volcanism and Neogene deformation in the Calico Mountains, located in a complexly deformed region of California's central Mojave Desert. Across most of the Calico Mountains, volcaniclastic sediments and dacitic rocks of the Pickhandle Formation accumulated rapidly between ca. 19.4 and 19 Ma. Overlying fine-grained lacustrine beds (here referred to as the Calico Member of the Barstow Formation) are bracketed between ca. 19 and 16.9 Ma, and are thus older than the type section of the Barstow Formation in the Mud Hills. Several 17.1–16.8 Ma calc-alkaline dacite domes intrude the Calico Member and represent a previously unrecognized volcanic episode in this region.

In the southern Calico Mountains, the Calico fault (part of the Eastern California shear zone) forms a west-northwest–striking, transpressional restraining bend with ∼3 km of right-lateral slip and perhaps 1 km of reverse (north side up) throw distributed on two main fault strands. Part of the Calico fault appears to have originated as an early Miocene normal fault that unroofed metavolcanic basement rocks in the footwall and created a hanging-wall basin in which Pickhandle Formation strata accumulated. This extensional slip must have largely ceased prior to deposition of the Calico Member, which unconformably overlies the Pickhandle Formation north of the Calico fault and directly overlies metavolcanic rocks south of the Calico fault. Deposition of the Pickhandle Formation and at least part of the Calico Member was coeval with rapid unroofing of the central Mojave metamorphic core complex, yet extension in the Calico Mountains is minor and is overprinted by dextral faulting and transpression.

Calico Member beds north of the Calico fault are intensely folded into numerous east-west–trending, upright anticlines and synclines that represent 25%–33% (up to ∼0.5 km) north-south shortening. Folds are detached along the base of the Calico Member and thrust over the Pickhandle Formation, which dips homoclinally ∼15–30°S to SE. The geometry and distribution of folds are most compatible with localized transpression between the Calico Member and the Pickhandle Formation within a positive flower structure. Transpressional folding and faulting in the Calico Mountains postdate the ca. 17 Ma dacite intrusions and appear to be largely restricted to the area along the Calico fault restraining bend.

INTRODUCTION

The central Mojave Desert region in southern California records a complex deformation history that includes Cenozoic extension, contraction, and strike-slip faulting. Early Miocene detachment faulting and extensional basin development generally preceded transform-dominated tectonics related to the Pacific–North American plate boundary, yet the timing, magnitude, and tectonic significance of these disparate modes of deformation remain controversial (see Glazner et al., 2002, for a review).

The central Mojave metamorphic core complex exposes a low-angle normal fault (the Waterman Hills detachment fault) that juxtaposes tilted early Miocene volcanic and sedimentary rocks in the hanging wall against variably mylonitized basement rocks in the footwall. Based on apparent offsets of pre-Tertiary markers, several workers (Glazner et al., 1989; Walker et al., 1990; Martin et al., 1993) proposed that 40–60 km of northeast-directed normal slip occurred along the Waterman Hills detachment fault. The distribution of extension is controversial. Dokka (1989) argued that regional extension occurred within an east-west–trending belt across most of the Mojave Desert region. In contrast, Glazner et al. (2002) suggested that extension was largely confined to an ∼25-km-wide area centered around the central Mojave metamorphic core complex. Currently there is no strong consensus on the precise timing of extension in the central Mojave Desert. A few lines of evidence suggest that deformation associated with the central Mojave metamorphic core complex occurred between ca. 24 and 19 Ma. First, a dacite dike in the Mitchel Range and the Waterman Hills granodiorite are interpreted to have been emplaced synkinematically into the footwall of the central Mojave metamorphic core complex (Walker et al., 1990; Fletcher and Bartley, 1994); these intrusions have zircon U-Pb ages of 23.0 ± 0.9 Ma and 21.9 ± 0.8 Ma, respectively (Walker et al., 1990, 1995). Second, the Pick-handle Formation volcanic and sedimentary rocks in the hanging wall of the central Mojave metamorphic core complex are interpreted as synextensional deposits ranging in age from ca. 24 to 19 Ma (Fillmore and Walker, 1996). Younger (ca. 17–13 Ma) fine-grained lacustrine rocks of the Barstow Formation are considered postextensional deposits. Thermochronologic data of mylonitic rocks from the Mitchel Range and Hinkley Hills indicate that the footwall of the central Mojave metamorphic core complex underwent rapid cooling (50–100 °C/m.y.) between ca. 21 and 17.5 Ma (Gans et al., 2005). This episode of cooling is interpreted to reflect exhumation of the footwall during slip on the Waterman Hills detachment fault.

Strike-slip faulting associated with the Eastern California shear zone appears to have been the dominant mode of postextensional deformation in the Mojave Desert region. Northwest-trending right-lateral faults are ubiquitous and accommodate a small percent of the relative motion between the Pacific and North American plates (Dokka and Travis, 1990b). East- to northeast-trending left-lateral faults are also common, particularly in the northeastern Mojave Desert. The cumulative amount of northwest-directed dextral shear across the region is probably on the order of 50–75 km (Dokka and Travis, 1990a; Glazner et al., 2002). It is unclear when this faulting began, but some indirect evidence suggests that northwest-trending dextral faults may have locally been active as early as 19 Ma (Bartley et al., 1990). Several strike-slip faults are considered to be active now (Jennings, 1994).

Neogene shortening in the Mojave Desert region has primarily been attributed to local transpression along northwest-striking dextral faults (e.g., Dibblee, 1980b, 1994), or regional north-south contraction (Bartley et al., 1990; Linn et al., 2002). The most common types of contractional structures are approximately east-west–trending folds, many of which occur in Miocene lacustrine rocks. Folds are widespread across the region, suggesting that contraction is a regional phenomenon (Bartley et al., 1990). The magnitude of shortening represented by these folds is not well documented, and the timing of folding is unclear. Gently folded Quaternary gravels indicate that some folding is related to active strike-slip faults, and north-south shortening may play an important role in present-day strain accumulation across the Eastern California shear zone (Oskin et al., 2007). The goal of this study is to understand the stratigraphic and structural evolution of the Calico Mountains, which arguably best exemplify synextensional deposition and transpression in the central Mojave Desert.

GEOLOGIC OVERVIEW OF THE CALICO MOUNTAINS

Located ∼15 km northeast of Barstow, the Calico Mountains form a 15-km-long, northwest- trending range composed primarily of early Miocene sedimentary and volcanic rocks in the upper plate of the central Mojave metamorphic core complex (Fig. 1). Dacite and coarse volcaniclastic sedimentary rocks of the Pick-handle Formation compose most of the northern and central Calico Mountains (Fig. 1). The type locality of the Pickhandle Formation is in the northwestern Calico Mountains, where the south- to southwest–dipping section is ∼1500 m thick (McCulloh, 1952; Dibblee, 1994; Fig. 1). Overlying the Pickhandle Formation are fine-grained lacustrine rocks generally considered part of the Barstow Formation and referred to in this study as the Calico Member of the Barstow Formation. These lacustrine rocks are intruded by several dacite domes that form most of the peaks in the southeastern Calico Mountains.

The dominant structure in the Calico Mountains is the Calico fault, a northwest-trending right-lateral fault with a maximum displacement of ∼10 km in the Rodman Mountains (∼30 km southeast of the Calico Mountains; Dibblee, 1964; Glazner et al., 2000). In the southern Calico Mountains, the Calico fault strikes west-northwest and forms a 10-km-long restraining bend (Fig. 1). Lacustrine rocks north of the Calico fault are folded into numerous anticlines and synclines, making the Calico Mountains one of the type localities for Neogene contractional deformation in the central Mojave Desert. The overall east-west trend of the folds and their proximity to the restraining bend in the Calico fault have led most geologists to interpret the folding as transpressional (e.g., Tarman and McBean, 1994; Dibblee, 1994; Glazner et al., 1994).

PREVIOUS WORK

The geology of the Calico Mountains was mapped by McCulloh (1952, 1960, 1965) and later by Dibblee (1970). Prior to this investigation, the fine-grained lacustrine section in the Calico Mountains was correlated with the type locality of the Barstow Formation in the Mud Hills (McCulloh, 1952; Dibblee, 1980a). Reynolds (2000) argued that a sequence of three marker beds suggest a stratigraphic correlation between the lacustrine sections in the Calico Mountains and the Mud Hills. However, the age of fine-grained lacustrine sedimentation in the Mud Hills Barstow Formation is bracketed between ca. 17 and 13 Ma (MacFadden et al., 1990), which is distinctly younger than what we determine to be the age of lacustrine beds in the southern Calico Mountains. In this paper, the lacustrine section in the Calico Mountains is referred to as the Calico Member of the Barstow Formation.

Fletcher (1986) mapped a 6 × 1 km area along the Calico fault west of Calico Ghost Town at a scale of 1:4800, focusing primarily on silver mineralization. Based on the apparent offset of two similar mineralized deposits, he estimated that 1.9–3.2 km of right-lateral offset existed along the Calico fault.

METHODS

Detailed geologic mapping provided the basis for understanding the stratigraphy and structural geology of the southern Calico Mountains. The folded lacustrine section in the center of the study area (Plate 1; Fig. 2) was mapped at a scale of 1:6000; surrounding areas were mapped at 1:12,000. Axial traces of all folds with amplitudes ≥1 m were mapped, and the orientations of fold axes and axial surfaces were determined by measuring bedding orientations around the hinge of each fold and axial trace orientations on profile views of folds.

The 40Ar/39Ar geochronology was used to determine the ages of Miocene sedimentation and volcanism in the Calico Mountains. Detailed step-heating experiments (typically 7–12 steps) were performed on pure separates of plagioclase, biotite, and whole rock from 11 different volcanic samples 01(Table 1). Errors reported on plateau ages are ±2σ. The major and trace element geochemistry of selected volcanic rocks was determined by X-ray fluorescence (XRF).

STRATIGRAPHY OF THE CALICO MOUNTAINS

Pickhandle Formation and Metavolcanic Basement Rocks

The Pickhandle Formation consists primarily of coarse-grained volcaniclastic deposits and silicic volcanic rocks that are generally interpreted to have accumulated in extensional basins during rapid slip along the Waterman Hills detachment fault (e.g., Fillmore and Walker, 1996). The thickest and most widespread occurrence of the Pickhandle Formation in the central Mojave Desert is in the Calico Mountains. In the northern part of the study area, the Pickhandle Formation forms a gently southeast- to south-dipping homocline, and is composed primarily of volcaniclastic sandstone, tuff breccia, and dacite domes (Plate 1). Most of the Pickhandle Formation beds are channelized and were likely deposited by alluvial or fluvial processes.

A distinct characteristic of the Pickhandle Formation in the southern Calico Mountains is that it primarily contains clasts of dacite. In addition, a significant part of the Pickhandle Formation consists of biotite ± hornblende dacite domes and flows (unit Tpd; Plate 1). In the study area the thickest exposed section of Pickhandle sedimentary rocks is only ∼300 m because dacite domes are widespread lower in the section. Some of these domes intrude Pick-handle sedimentary beds, and others are depositionally overlain by volcaniclastic intervals. The association of domes, flows, volcaniclastic breccias, and sandstone beds suggests that the Pickhandle Formation in the southern Calico Mountains represents a dacite lava-dome field and associated pyroclastic and epiclastic apron deposits.

Immediately south of the Calico fault, rocks resembling the Pickhandle Formation do not exist, as the fine-grained lacustrine section (Calico Member) directly overlies metavolcanic rocks and conglomerates and breccias composed largely of pre-Tertiary detritus (Fig. 3). The metavolcanic basement rocks range from basalt to rhyolite, but most are andesitic. The green-schist facies metamorphism that affected these rocks preserves volcanic textures, but resulted in widespread growth of metamorphic epidote, chlorite, and albite. It is unclear whether these metavolcanic rocks are part of the Jurassic Side-winder Formation (e.g., Schermer and Busby, 1994) or the upper Paleozoic Coyote Group described by McCulloh (1952). Similar rocks have been described in the Elephant Mountain area to the west-southwest (Fig. 1; Cox and Wiltshire, 1993).

Nature of the Pickhandle Formation–Calico Member Contact

The contact between the Pickhandle Formation and directly overlying Calico Member is generally conformable, and maroon sandstone beds of the Pickhandle Formation appear to grade up into the white-gray volcaniclastic sandstone beds at the base of the fine-grained lacustrine section. However, on a larger scale, homoclinally southeast-dipping Pickhandle strata strike obliquely to the Calico beds, and the amount of section in the uppermost Pickhandle Formation subunit Tpsu varies from ∼170 m to <30 m beneath the contact with the Calico Member (Plate 1). Volcaniclastic sandstone beds at the base of the Calico Member are consistently present above the contact east of Calico Ghost Town (Fig. 1), so there does not appear to be any section omitted from the base of the Calico Member. These stratigraphic relationships indicate that the bedding-subparallel Pickhandle Formation–Calico Member contact is a subtle angular unconformity. The geometry of this unconformity has most likely been modified by transpressional movement along the contact.

40Ar/39Ar Geochronology

Previous geochronologic studies of the Pickhandle Formation in other parts of the central Mojave Desert suggest that volcanism and coarse volcaniclastic sedimentation occurred between ca. 24 and 19 Ma (Burke et al., 1982; Fillmore and Walker, 1996). Three new 40Ar/39Ar dates on dacite from the Pickhandle Formation in the Calico Mountains range from 19.35 ± 0.15 to 19.0 ± 0.1 Ma. An exogenous dacite dome at the base of the Calico Member near Old Borate yielded 40Ar/39Ar plateau ages of 19.13 ± 0.04 Ma and 19.0 ± 0.1 Ma on biotite and plagioclase, respectively (Plate 1; 01Table 1). Detritus of this dome is present in beds at the base of the lacustrine section, indicating that the oldest part of the Calico Member is ca. 19 Ma or younger. A compositionally different dacite dome in the north-central part of the study area yielded a plagioclase plateau age of 19.0 ± 0.1 Ma (Plate 1; 01Table 1). Locally this dome appears to have intruded Pickhandle sedimentary beds ∼150 m beneath the contact with the Calico Member. In the northwestern Calico Mountains, a dacite flow near the base of the Pickhandle Formation along Fort Irwin Road yielded a plateau age of 19.35 ± 0.15 Ma on plagioclase. There is ∼1 km of volcanic and volcaniclastic sedimentary rocks between the top of this dacite flow and the base of the overlying Barstow Formation (McCulloh, 1952, 1960). Assuming that the uppermost Pickhandle Formation here is ca. 19 Ma, as it is in the Mud Hills (MacFadden et al., 1990) and eastern Calico Mountains, then the Calico Mountains were the site of very rapid volcaniclastic sedimentation (∼2.5 mm/yr) from ca. 19.4 to 19.0 Ma. This rapid sedimentation is most likely a product of both abundant dacitic volcanism and extensional basin development.

Calico Member of the Barstow Formation

The Calico Member of the Barstow Formation consists primarily of siltstone, sandstone, and limestone (Plate 1; Fig. 4). Lateral and vertical facies changes are common, although some distinct groups of beds can be mapped for a distance of ∼4 km (Plate 1). We have divided the internal stratigraphy of the Calico Member north of the Calico fault into different subunits based on distinct lithostratigraphic and color characteristics (Plate 1; Fig. 4). In the center of the study area the Calico Member is ∼375 m thick (near cross-section C–C'; Plate 1; Fig. 4). East of Old Borate, exposure of the section is not as complete, but the thickness appears to be at least 450 m. South of the Calico fault, the minimum thickness of the fine-grained lacustrine section is ∼300 m. The Calico Member north of the fault decreases in thickness westward toward Calico Ghost Town. For example, a distinct set of brown sandstone beds that is ∼50 m thick and ∼130 m stratigraphically above the top of the Pickhandle Formation near Mule Canyon is only ∼20 m thick and 35–40 m above the Pickhandle Formation near Calico Ghost Town (Fig. 4).

The Calico Member is generally considered part of the middle Miocene Barstow Formation, which is ∼1000 m thick at its type locality in the Mud Hills (Dibblee, 1968; Woodburne et al., 1990). The age of the Barstow Formation and potential stratigraphic correlations are important because the Barstow Formation in the Mud Hills provides the basis for the Barstovian land- mammal age. Although stratigraphic correlations between the Mud Hills and Calico Mountains have been suggested (Reynolds, 2000), there are a few clear lithostratigraphic differences between the Calico Mountains lacustrine rocks and the type section of the Barstow Formation. Lacustrine rocks in the Calico Mountains contain less granitic detritus than the Mud Hills Barstow Formation, and the prominent water-laid ash-fall tuff beds in the Mud Hills are not present in the Calico Mountains. Sandstone beds in the Calico Mountains are typically dominated by dacitic detritus, and rare ash-rich beds are all strongly reworked. In addition, borates are present in the upper part of the lacustrine section in the Calico Mountains (Fig. 4), whereas the Mud Hills Barstow Formation apparently lacks borate mineralization.

The new 40Ar/39Ar data in this study 01(Table 1) bracket the age of the Calico Member north of the Calico fault primarily between ca. 19 and 17 Ma, demonstrating that this section is older than the fine-grained Barstow Formation in the Mud Hills. However, the Calico Member overlaps in time with the Owl Conglomerate Member at the base of the Barstow Formation (Dibblee, 1968; MacFadden et al., 1990). We propose that the fine-grained lacustrine section in the Calico Mountains be given a new designation, Calico Member of the Barstow Formation, to indicate that the Calico Mountains lacustrine rocks are not age correlative to fine-grained lacustrine rocks at the type locality of the Barstow Formation.

Dacite of the Yermo Volcanic Center

Dacite domes and dacite breccias that are younger than the Calico Member form an approximately east-west–trending belt that covers ∼20 km2 in the southeastern Calico Mountains (Plate 1). This dome field is designated as the Yermo volcanic center (after the nearby town of Yermo; Fig. 1). The domes intrude Calico beds that are generally steepened and baked (hardened and oxidized) against the margins of the domes. At least 15 different dacite intrusions are present (Fig. 1). Coarse, clast-supported, monolithologic breccias that overlie the Calico Member are compositionally identical to these dacite intrusions and were most likely shed from the domes as rock avalanche deposits and block and ash flows. In some areas it is difficult to distinguish these breccia deposits from the brecciated portions of the domes. Some of the breccia sheets appear to be as thick as 150 m, but these sheets are concentrated around domes and do not appear to be laterally extensive.

The dacites of the Yermo volcanic field are mineralogically and geochemically homogeneous. All contain phenocrysts of plagioclase (10%–22%) and hornblende (4%–7%), and some contain sparse (1%–3%) biotite phenocrysts. The XRF analyses of eight samples from the Yermo volcanic center indicate an IUGS classification range from dacite to trachydacite (Fig. 5), with a narrow range of SiO2 content (65.2–67.7 wt%) and generally calcalkaline geochemical signatures (Singleton, 2004).

The Yermo volcanic center was active during the final stages of lacustrine sedimentation preserved in the Calico Member of the Barstow Formation. Clast-supported dacite breccia sheets (unit Tdbbc) are locally interbedded with shale in the upper part of the lacustrine section (Plate 1; Fig. 4). The clasts in this breccia are compositionally indistinguishable from the Yermo domes that intrude the Calico Member and do not resemble dacite from the Pickhandle Formation. Assuming that this breccia sheet was shed from a Yermo dacite dome, at least 60–80 m of fine-grained lacustrine beds were deposited west and east of the Yermo volcanic center after the initiation of volcanic activity. It appears that dacitic volcanism locally shut off lacustrine sedimentation in the southeastern Calico Mountains, but sedimentation distal to the Yermo volcanic center continued and may have been synchronous with deposition the Barstow Formation beds in the Mud Hills.

40Ar/39Ar Geochronology

Nine mineral separates from seven different Yermo dacite units yielded 40Ar/39Ar plateau ages that range from 16.8 ± 0.1 to 17.1 ± 0.1 Ma (Plate 1; 01Table 1), indicating that the Yermo volcanic center was short-lived. This ca. 17 Ma volcanic activity has not previously been recognized in the central Mojave Desert. Prior to this study, it was thought that most volcanism in the Barstow area occurred predominantly between 24 and 20 Ma and had ceased by ca. 18 Ma (Glazner et al., 2002).

These new 40Ar/39Ar data provide a clear upper age bracket on lacustrine sedimentation in the southern Calico Mountains. North of the Calico fault, a dacite dome that intrudes the upper part of the Calico Member yielded plateau ages of 16.8 ± 0.1 and 16.9 ± 0.1 Ma on a whole-rock separate and plagioclase, respectively. A clast from dacite breccia that caps the Calico Member near Old Borate yielded a plateau age of 16.9 ± 0.2 Ma (plagioclase). This particular breccia layer contains clasts of baked shale, indicating that the breccia clast age is younger than fine-grained sedimentation. Two dacite domes that intrude the Calico Member near Sunrise Canyon yielded ages of 17.0 ± 0.1 and 17.1 ± 0.1 Ma. Two dacite intrusions south of the Calico fault yielded ages of 17.1 ± 0.1 and 16.9 ± 0.1 Ma. These ages indicate that the most of the exposed lacustrine rocks in the southern half of the Calico Mountains are older than ca. 17 Ma, although post–17 Ma lacustrine sedimentation continued both east and west of the Yermo volcanic center.

STRUCTURAL GEOLOGY

The Miocene rocks in the southern Calico Mountains record a complex structural history that involves extension, strike-slip faulting, and shortening. Northwest-striking, high-angle (≥45°) normal faults are common in the Pickhandle Formation (Fig. 6). However, strike-slip and transpressional faulting strongly overprint the extensional fault system and represent the dominant style of deformation in the study area. Northwest-striking dextral and oblique dextral faults are particularly common. The most significant structure in the area is the Calico fault, which strikes west-northwest and forms a left bend in the southern Calico Mountains. Based on the orientation of this apparent restraining bend, the Calico fault is thought to accommodate trans-pressional deformation, although the amounts of heave and throw along this segment of the fault have not been previously estimated. Dibblee (1994) suggested that this restraining bend was formed by slip on the Manix fault, a major east-northeast–striking sinistral fault that is inferred to intersect the Calico fault at the southeastern end of the Calico Mountains (Fig. 1).

The Calico Mountains are probably best known for the folded lacustrine rocks that are spectacularly exposed along Mule Canyon Road and in the parking lot of the Calico Ghost Town (Plate 1; Fig. 7). These folds are located north of the restraining bend in the Calico fault and have been cited as an example of contraction associated with a strike-slip fault (Tarman and McBean, 1994; Dibblee, 1994; Glazner et al., 1994).

Faults

The majority of faults in the Calico Mountains strike northwest and dip steeply (Fig. 6). These northwest-striking faults include normal faults in the Pickhandle Formation and oblique dextral faults throughout the study area. East- to northeast-striking faults are also present and typically have oblique sinistral slip. The intensity of faulting is strongly dependent on rock type; the Pickhandle Formation and the Yermo dacite rocks are highly faulted, whereas faults are rare within the Calico Member (Plate 1). Northwest-striking dextral faults that cut the Pickhandle Formation and Yermo dacite appear to die out abruptly in the Calico Member. Individual Calico beds that show little or no evidence of brittle offset persist along strike for >2 km (Plate 1).

Dextral and oblique dextral-reverse shearing are clearly the dominant modes of brittle deformation. The Calico fault system accounts for at least 3 km of dextral slip (see following), and the cumulative amount of dextral shear in the study area (across a northeast transect from C–E'; Plate 1) is estimated to be ∼4.1 km. The largest northeast-trending fault in the study area is an oblique sinistral-reverse fault with ∼650 m apparent heave. Other east- to northeast-trending faults appear to have small offsets, and the cumulative amount of sinistral slip across the study area is probably ≤750 m. Locally the Yermo volcanic center is shortened by the combination of dextral slip along the Calico fault and sinistral slip along the largest east- northeast–trending fault. However, the dominance of dextral and dextral-reverse shearing over sinistral shearing indicates that conjugate strike-slip faulting does not play a major role in north-south shortening. In addition, although the east- to northeast- trending sinistral faults are oriented as a conjugate set to the dextral faults, dextral faulting appears to have postdated most of the sinistral faulting. The Calico fault and Southern Calico fault are not cut by any east- or northeast- trending faults, whereas the Calico fault cuts three northeast-trending faults (including the largest oblique sinistral fault; Plate 1). Outside of the Calico fault system, east- to northeast-trending faults are commonly cut by northwest-trending dextral faults (Plate 1). These timing relationships suggest that sinistral and dextral faulting were generally not mutually active.

Faults within the Pickhandle Formation

Most faults in the Pickhandle Formation strike northwest, dip ≥45° (dominantly to the southwest), and have downdip striae (Fig. 6B). Offset markers and fault plane kinematic indicators (e.g., Reidel shears) suggest that most of these dip-slip faults have normal offset, although the amount of normal throw along individual faults is typically <50 m and often only a few meters. The total magnitude of northeast-southwest extension calculated from fault dips and apparent offsets is ∼5% (∼100 m over a distance of ∼2 km). Northwest-striking normal faults are rare in the Calico Member and Yermo dacite, suggesting that much of this faulting occurred prior to ca. 19 Ma and was related to extension in the central Mojave metamorphic core complex. However, the gentle south and southeast tilting of Pickhandle Formation strata is not compatible with northeast-directed extension, suggesting that the tilting may be the composite effect of both extensional and transpressive deformation.

Of measured fault planes with downdip striae, ∼40% also have subhorizontal striae (Fig. 6B). On fault planes where the relative timing of slip could be determined, dextral movement overprints dip-slip (mostly normal) movement. These relationships are consistent with the idea that dextral faulting related to the Eastern California shear zone was superimposed on early Miocene northeast-southwest extension.

Calico Fault

The Calico fault is one of the largest strike-slip faults in the Mojave Desert. In the Rodman Mountains (∼30 km southeast of the Calico Mountains) the Calico fault has a maximum displacement of 9.8 km (Dibblee, 1964; Oskin et al., 2007). Displacement decreases to the northwest, and the Calico fault appears to break into several strands in the Mud Hills (Fig. 1). The segment of the fault northwest of the restraining bend in the Calico Mountains was estimated to have 1.9–3.2 km of dextral slip based on the apparent offset of the Waterloo and Langtry barite-silver deposits (Fletcher, 1986). In the eastern Mud Hills the main strand of the Calico fault appears to have 1.3 km of slip, based on the apparent offset of a dacite dome at the top of the Pickhandle Formation.

In the study area the Calico fault cuts lacustrine rocks of the Calico Member, domes of the Yermo volcanic center, and, in the southeastern end of the range, metavolcanic rocks (Plate 1). Two major strands of the Calico fault system exist along the restraining bend, including a poorly exposed fault that flanks the southern edge of the Calico Mountains east of Calico Ghost Town (Plate 1). This fault is referred to as the Southern Calico fault. The fault previously identified by McCulloh (1965) and Dibblee (1970) as the main Calico fault is referred to as the Calico fault in this study.

Geometry and Kinematics

Southeast and northwest of the study area, the Calico fault has an average strike of ∼N35–40W (Fig. 1). Along the restraining bend in the southern Calico Mountains, the Calico fault has an average strike of ∼N70W and dips 45°–70°NNE (Plate 1; Fig. 6C). Fault plane striae on the Calico fault and subsidiary fault strands consistently rake obliquely from the west-northwest, indicating oblique dextral-reverse slip (Fig. 6C). The reverse (north side up) component of slip can also be inferred from the north-dipping, overturned beds on the south side of the Calico fault, west of Calico Ghost Town (Plate 1). This fault geometry and fault slip data are compatible with a transpressional strain regime in which the principal shortening direction is approximately north-south. Northwest of the restraining bend, the Calico fault dips ≥70°NE and has subhorizontal striae, suggesting that the transpressional slip regime is primarily restricted to the restraining bend in the southern Calico Mountains (Fig. 6C). The offset Pickhandle Formation–Barstow Formation contact in the easternmost Mud Hills does not appear to be uplifted across the Calico fault, providing further evidence that transpressional shortening across the Calico fault is localized along the restraining bend. However, reverse slip has also been reported along the Calico fault in the Rodman Mountains (Glazner et al., 2000; Oskin et al., 2007), indicating that locally the Calico fault accommodates shortening without the presence of a restraining bend or stepover.

Fault striae on the Southern Calico fault also rake obliquely from the west-northwest, but the dip of this fault varies from near vertical to 48°N over short distances, and near Mule Canyon Road the fault makes an abrupt bend (Plate 1). This irregular geometry suggests that the Southern Calico fault may have been an older strand of the Calico fault system that became inactive and was deformed.

Offset

The clearest offset markers along the Calico fault are found between Mule Canyon and Ghost Town Road. A distinctive 17.1 ± 0.1 Ma dacite dome south of the main Calico fault can be matched to a dacite dome north of the fault at Camp Rock, ∼1.2 km to the east-southeast (Plate 1). These domes contain 20%–25% phenocrysts, including 3%–5% hornblende, 2%–4% biotite, and ∼1% anhedral quartz. No other dacite rocks near the Calico fault have more than 1% biotite phenocrysts or more than a trace amount of quartz. Both offset domes along the Calico fault intrude a calcareous interval within the Calico Member, and on their eastern margin are in intrusive (?) contact with a distinctive hydrothermally altered dacite (Plate 1). This older dacite is highly fractured and weathered and has a distinct sea-green color from pervasive celadonite alteration (unit Tdc). The northwestern margins of both intrusive domes are faulted against crudely stratified, purple dacite breccia that appears to be interbedded with lacustrine rocks on the south side of the Calico fault (Plate 1). Two sets of northeast-striking faults that are cut by the Calico fault northwest of the offset domes are likely the same (Plate 1). These distinct sequences of rocks on opposite sides of the Calico fault match well if 1.1–1.2 km of right-lateral slip along the Calico fault is restored. East of Mule Canyon, the amount of slip most likely increases due to additional northwest-striking faults that either merge with or are cut by the Calico fault. A northeast-striking, northwest-dipping fault along Mule Canyon Road south of the Calico fault may be the offset portion of a similarly oriented fault north of the Calico fault, which would indicate ∼1.5–1.6 km of cumulative right-lateral slip (Plate 1).

Exposure south of the Southern Calico fault is poor, but two distinctive dacite domes crop out along opposite sides of the fault east of Ghost Town Road and west of Mule Canyon Road (Plate 1). Mineralogically, these domes are similar to most dacite of the Yermo volcanic center (∼5% hornblende phenocrysts, <1% biotite, no quartz), but both domes are distinct in outcrop because of their patchy celadonite alteration and liesegang banding. Assuming that these domes are the same, the southern Calico fault has 1.8–1.9 km of right-lateral slip. Thus, the Calico fault system in the southern Calico Mountains has 3 ± 0.1 km of cumulative right-lateral slip distributed between two faults, and restoring this right-lateral slip concentrates the Yermo dacite dome field into an approximately east-west–trending ellipse. This offset estimate is similar to the 1.9–3.2 km of offset estimated by Fletcher (1986) along the Calico fault northwest of the study area. However, an offset of 3 km should be considered a minimum estimate because additional strands of the Calico fault may exist under alluvium southwest of the Calico Mountains.

The amount of reverse (north side up) slip along the Calico fault system is ambiguous. Based on average slickenline rakes, the main strand of the Calico fault should have 0.2–1 km of reverse slip, corresponding to ∼100–500 m of approximately north-south shortening. The Southern Calico fault should have 0.7–1.3 km of north-side-up throw, but the amount of horizontal shortening across this fault is unknown due to the variable dip of the fault plane. Thus, if slip vectors measured along the Calico fault system are representative of the entire slip history of the fault, the cumulative amount of reverse slip along the Calico fault system is ∼1–2 km. However, the Calico fault juxtaposes fine-grained lacustrine rocks <500 m thick that are presumably part of the same section, suggesting that reverse slip along this strand is <500 m. Oblique dextral-reverse slip vectors may only reflect the most recent movement along the Calico fault system. It is possible that the Manix fault bent the Calico fault into a restraining bend orientation after the Calico fault had been active as a more northwest-trending, purely strike-slip fault (Dibblee, 1994). Accordingly, 1–2 km would be an overestimate of the cumulative amount of reverse slip along the Calico fault system. Given that the amount of reverse slip along the Calico fault is most likely <500 m, the total amount of north-side-up slip along the Calico fault system is probably closer to 1 km, corresponding to ∼500 ± 150 m of north-south shortening.

Pretranspressional Slip on the Calico Fault

The significant stratigraphic mismatch of pre-lacustrine rocks across the Calico fault in the southeastern Calico Mountains strongly suggests there was uplift on the south side of the fault prior to dextral and/or transpressional movement. Metavolcanic basement rocks underlie the Calico Member south of the Calico fault, whereas a thick section of Pickhandle Formation underlies the Calico Member north of the fault. Calico beds across the fault are most likely correlative, indicating that either metavolcanic basement rocks were unroofed of Pickhandle deposits prior to the deposition of the Calico Member (beginning ca. 19 Ma), or that the area south of the Calico fault was a structural high during Pickhandle deposition. We favor the interpretation that an early Miocene proto-Calico fault was a basin-bounding, northeast- to north-northeast–dipping normal fault that enabled >1 km of Pickhandle Formation rocks to accumulate to the north. The present ∼45°–70°NNE dip of the Calico fault is consistent with the inferred dip of this hypothetical proto-Calico normal fault, and in one location in the southeastern Calico Mountains, the main Calico fault plane has downdip striae that are overprinted by striae from oblique dextral-reverse movement. The sense of slip on the downdip striae is unclear, but this older dip-slip movement may record early Miocene normal faulting. Northwest of the restraining bend in the Calico Mountains, Pickhandle Formation rocks are present on both sides of the Calico fault (Fig. 1), suggesting that only the restraining bend segment of the fault was a Pickhandle basin-bounding normal fault.

Folds

Folds are common structures across the Mojave Desert, particularly in Miocene lacustrine rocks (e.g., folds in the Calico Mountains, Alvord Mountains, Lead Mountain area, Mud Hills, Kramer Hills, and Black Canyon area; for locations see Bartley et al., 1990). In most of these areas the age and structural significance of folding are poorly understood. The Calico Member rocks in the southern Calico Mountains expose some of the tightest folds in the central Mojave Desert (Fig. 7). This folding is generally thought to be related to transpression along the Calico fault (Tarman and McBean, 1994; Dibblee, 1994; Glazner et al., 1994), yet based on field work to the west of Calico Ghost Town, Weber (1976) proposed that folding was due to south-directed gravitational sliding of the Calico Member off of the underlying Pickhandle Formation. This interpretation of folding was supported in part by Tarman and McBean (1994), who noted that west of Calico Ghost Town, a south-dipping polished plane separates the Pickhandle Formation from Calico Member, and that some beds appear to be missing from the base of the Calico Member, suggesting that gravity slumping may have formed some folds. However, Tarman and McBean (1994) argued that the upright axial surfaces of many of the folds are not compatible with gravity folding. Another possible explanation for folding is that forceful emplacement of dacite domes into the Calico Member ca. 17 Ma folded the lacustrine rocks.

Folding in the Calico Mountains is primarily restricted to the Calico Member north of the Calico fault restraining bend. Folds are detached from the underlying Pickhandle Formation, which homoclinally dips ∼15–30°S to SE beneath the Calico Member (Plate 1; Fig. 8A). Calico beds south of the Calico fault are steeply tilted but not folded into the same scale of anticlines and synclines that characterize deformation in the Calico beds north of the Calico fault (Fig. 8).

Geometry

The geometry of folds in the Calico Member varies considerably, but several important generalizations can be made. Most folds have steeply dipping axial surfaces (>75°) and shallowly plunging axes that trend east-west ±30° (Fig. 9). One exception to this fold orientation is a small set of north-south–trending folds on the east side of Mule Canyon Road (Plate 1; Fig. 9). These north-south folds plunge into the core of an east-west–trending syncline, suggesting that they have been refolded by the syncline. Folds are cylindrical and systematically oriented on short length scales, although fold axis orientation, axial surface attitude, and interlimb angle commonly change along the axial trace of a fold (Plate 1; Fig. 9).

One of the most distinct characteristics of the Calico folds is their relatively small size. The largest anticlines and synclines have amplitudes up to ∼30 m, but the majority of folds have amplitudes of 2–12 m. Most fold sets have wavelengths <100 m, and axial traces are mostly ≤0.75 km long. These small-scale folds are very different than isolated, kilometer-scale folds such as the Barstow syncline in the Mud Hills (Dibblee, 1968), the Box Canyon syncline in the Rodman Mountains (Dibblee, 1964), and the Lenwood anticline near Barstow (Dibblee, 1967).

Many of the folds have a clear north-vergent asymmetry. Although axial surfaces dip nearly equally to the north and south (Fig. 9), north-dipping limbs are usually shorter than south-dipping limbs (Plate 1, Fig. 2). In certain areas folds are symmetric, but south vergence is rare. The majority of folded sandstone, limestone, and chert beds are classified under Ramsay's class 1B (parallel folds), whereas low competence shale layers are commonly thickened in the hinge area, producing class 2 and class 3 folds (Ramsay, 1967; Fig. 7). Angular, chevron-like fold hinges are common in competent beds (Fig. 7). The presence of some parasitic folds and the strong planar mechanical anisotropy in the Calico Member indicate that folding was accommodated largely by flexural slip. Most folds have an interlimb angle between ∼45° and 90° (average = ∼60°–70°). Based on eight restorable cross sections of the Calico beds, the average amount of north-south horizontal shortening represented by the Calico folds is ∼25%–30% (see Singleton, 2004, for details). The total shortening due to folding between the Calico fault and the Pickhandle Formation is ∼150–500 m.

Map View Patterns

In the eastern half of the study area, folds parallel the contact with the dacite domes and breccias of the Yermo volcanic center (Plate 1; Fig. 9). This map view pattern is particularly evident north of the volcanic center, where axial traces form a sigmoidal shape that is parallel to the overall contact with the dacite domes and breccias. East of cross section C–C', axial traces wrap around the western edge of the Calico Member–dacite breccia contact, trending approximately northeast-southwest to the northwest of the contact and northwest-southeast to the southwest of the contact (Plate 1; Fig. 9). West of the Yermo volcanic center, axial traces are consistently more east-west oriented; thus the map view geometry of the folds appears to have been influenced by the presence of the Yermo volcanic center. The map view pattern of axial traces is also reflected by the contact with the underlying Pickhandle Formation (Plate 1; Fig. 9). North of the Yermo volcanic center, the Pickhandle Formation–Calico Member contact also forms a sigmoidal shape that is parallel to the contact with the Yermo dacite rocks.

Axial surface attitudes vary across the study area, but folds with similar axial surface orientations generally occur in groups (Fig. 9). There does not appear to be a systematic spatial pattern of north-dipping or south-dipping axial surfaces (Fig. 9). Neither fold interlimb angles nor the amount of shortening due to folding vary systematically across the study area (Fig. 9). The folds with the largest average interlimb angles (75°–90°) that represent the smallest percent of north-south shortening (20%–23%) are near Calico Ghost Town at the western end of the folded Calico Member (Fig. 9). The tightest folds (average interlimb angle = 45°–60°) occur in an ∼1.5-km-long belt where folds trend northeast-southwest to east-west between the contacts with the underlying Pickhandle Formation and overlying dacite breccias (Fig. 9). These folds represent 30%–40% shortening. Between Mule Canyon and Odessa Canyon Roads the amount of north-south shortening ranges from 27% to 33%, and the amount of north-northeast–south-southwest shortening near Old Borate is 24%–29%. Along transects where folding accounts for ∼500 m north-south shortening (near C–C'; Plate 1; Fig. 2), folding appears to die out near the Pickhandle Formation–Calico Member contact, suggesting that 500 m is an approximate upper limit on shortening due to folding.

Stratigraphic Controls

Stratigraphic controls on folding are significant in the Calico Mountains. The most obvious contrast in deformation can be seen between the Pickhandle Formation and the Calico Member. The Pickhandle Formation is clearly not folded, whereas Calico beds stratigraphically 5–15 m above the Pickhandle Formation are folded into a series of anticlines and synclines (Plate 1; Figs. 2 and 8). Thus, there is a detachment horizon between the top of the Pickhandle Formation and lower part of the Calico Member. The homogeneous, massively bedded Pickhandle Formation lacks the marked planar mechanical anisotropy of the thinly bedded lacustrine section and was rheologically less able to deform by folding.

The amount of shortening by folding decreases in the upper ∼30–100 m of the Calico Member. The dips of beds become consistently shallower up toward the contact with the overlying dacite breccia (Fig. 10). Folds rarely exist within 15 vertical m of this contact, and shale beds typically dip <30° beneath the breccia (Plate 1). The decrease in dip in the uppermost part of the Calico Member appears to be gradual, but in one location at the south end of Old Borate Canyon, the decrease in shortening is accommodated by a discrete subhorizontal detachment ∼15 m below the Calico Member–dacite breccia contact. Below this detachment shale beds are folded into a small-scale anticline and syncline pair, whereas above the detachment beds dip shallowly toward the dacite breccia. The shallowing of dips and decrease of shortening in the uppermost part of the Calico Member appear to be restricted to beds near the Yermo volcanic center that are capped by dacite breccias (Plate 1).

Timing of Folding

The shallowing of dips in the uppermost Calico beds could be explained if sedimentation were synchronous with folding; however, fold growth strata do not appear to be present. Dacite breccias related to the Yermo volcanic center are overall conformable with the underlying Calico Member and do not cut across any fold structures. The dacite breccia sheet (Tdbbc) interbedded with shale in the upper Calico Member is clearly folded. These observations indicate that folding took place after deposition of the oldest Yermo breccias (ca. 17 Ma). Upper age constraints on folding are poor due to the lack of post–Calico Member rocks.

Interpretation of Folding

Map view patterns and stratigraphic variation. The map view pattern of fold axial traces suggests there is a geometric relationship between the folds and the dacite rocks of the Yermo volcanic center. One possible explanation for the parallelism between the axial traces and the contacts with the dacite domes and breccias is that folding was caused by the forceful intrusion of dacite into the Calico Member ca. 17 Ma. Calico beds adjacent to intrusions are steepened and generally strike parallel to the margins of the intrusions, but this deformation appears to be largely restricted to an area within ∼150 m of the intrusion contact (Plate 1). Some small-scale folds occur in beds deformed by dome emplacement, but unlike most folds in the Calico Member, the geometry of these intrusion-parallel folds is highly irregular, and the axial traces are <150 m long. Dome emplacement as a mechanism for folding also does not explain the presence of east-west–trending folds far west of the dome field, nor does this mechanism account for the overall lack of folding south of the Calico fault. In addition, evidence that folding took place following deposition of the oldest Yermo dacite breccias suggests that folding is younger than dome emplacement.

Another explanation for the overall sigmoidal pattern of fold axial traces involves the formation and subsequent rotation of originally northeast-southwest–trending folds by right-lateral shear. If folds were originally oriented more northeast-southwest, the east-west–trending folds west of cross-section C–C' (Plate 1) could have rotated clockwise due to higher dextral shear strain adjacent to the Calico fault. This model interprets the folds as wrench folds along the Calico fault. Most experimental models predict that folds associated with wrench faults will initially form at ∼45° to the trace of the master fault (i.e., perpendicular to the incremental shortening direction; Odonne and Vialon, 1983; Jamison, 1991; Tikoff and Peterson, 1998). With progressive shear, folds rotate toward parallelism with the master fault, becoming tighter as they rotate (Fig. 11). If folding in the southern Calico Mountains was produced by this mechanism, folds that parallel the trace of the Calico fault (∼N70W) should have smaller interlimb angles than the northeast-southwest–trending folds, and folds should become progressively tighter westward (closer to the Calico fault). However, there are no systematic relationships between the fold orientation, interlimb angle, and distance from the Calico fault (Figs. 9, 11, and 12). Folds that are subparallel to the Calico fault are not tighter than northeast-southwest–trending folds, and fold interlimb angles do not decrease along strike to the west (closer to the Calico fault; Fig. 12). Other observations that argue against a wrench folding model include: (1) east-west– and east-southeast–west-northwest–trending folds north of the Yermo volcanic center are not located near a zone of higher shear strain, and (2) east of cross-section C–C', northwest-southeast–trending folds are located due south of northeast-southwest–trending folds (Plate 1; Fig. 9), resulting in a map view pattern that is incompatible with progressive clockwise rotation of folds.

The most likely explanation for the map view pattern of fold axial traces is that the Yermo dacite domes and breccias resisted shortening, forcing the Calico beds to wrap around these rocks. This buttressing influence of the rigid Yermo volcanic center during north-south shortening caused folds to become oriented parallel to the overall contact of the dacite domes and breccias. The decrease in shortening in the uppermost part of the Calico Member can be similarly explained by assuming that the thick breccia sheets adjacent to domes resisted folding. Calico beds close to the dacite breccia contact were not able to fold due to the mechanical rigidity of the overlying breccias. West of cross-section C–C' (west of the Yermo volcanic center; Plate 1), folds are consistently oriented approximately east-west, and several close folds are present in the inferred uppermost part of the section. These observations are consistent with the idea that the geometry of Calico beds in this area was not influenced by dacite intrusions and breccias from the Yermo volcanic center.

The presence of a folding detachment horizon between the Calico Member and the Pickhandle Formation might be used to argue that south-directed gravity gliding was responsible for folding the Calico beds. If folding were due to the Calico Member sliding off of the south-dipping Pickhandle Formation, folds would most likely have south-vergent asymmetry and axial surfaces that dip north, away from the direction of transport. Alternatively, if gravity folding occurred when the Calico beds were still unconsolidated, folds might have highly irregular geometries that are characteristic of soft sediment deformation and slumping. None of these hypothetical geometries is consistent with folds in the Calico Mountains. Folds in the Calico Member are systematically oriented and have upright axial surfaces that dip nearly equally to the north and south (Fig. 9). Moreover, the dominant sense of fold asymmetry is north vergent, suggesting north-directed (reverse) transport above the folding detachment.

Transpression. The location of folds adjacent to a transpressional restraining bend constitutes the most basic evidence that folding is tectonic and related to the Calico fault system. Calico Member beds northwest of the Calico fault restraining bend are generally not intensely folded or overturned (Dibblee, 1970), suggesting that shortening in the southern Calico Mountains is fundamentally related to trans-pression within the restraining bend.

The Pickhandle Formation (Tp)–Calico Member (Tbc) contact represents an important break in the style and magnitude of deformation. Calico beds north of the Calico fault have undergone 25%–33% north-south shortening due to folding, whereas the Pickhandle Formation generally lacks folds and reverse faults. Slip along the Tp-Tbc contact is necessary in order to accommodate detachment folding. A few observations suggest that this contact has both a reverse and right-lateral component of movement that is compatible with transpressional slip along the Calico fault. North-south shortening would presumably have forced the Calico Member to be thrust over the Pickhandle Formation along the south-dipping contact–folding detachment. The dominant north-vergent asymmetry of folds is compatible with this sense of movement along the detachment. The total amount of north-directed reverse slip along the detachment must be at least equal to north-south shortening due to folding in the hanging wall (up to ∼500 m). In Mule Canyon, there is evidence that folding dies out just south of the contact (e.g., cross-section C–C'; Fig. 2), suggesting that the detachment is a blind fault with slip progressively decreasing northward to where the Tp-Tbc detachment becomes a normal stratigraphic contact. A right-lateral component of movement along the Tp-Tbc contact can be inferred from the change in the orientation of Pickhandle beds at the contact. West of Mule Canyon, southeast-dipping Pickhandle beds abruptly bend into parallelism with south-dipping Calico beds adjacent to the contact (Plate 1), consistent with drag due to right-lateral slip along the contact.

Along the Tp-Tbc contact to the east of Calico Ghost Town, there does not appear to be a discrete slip surface along which reverse-dextral movement occurred, suggesting that movement may have been largely distributed throughout the sandstone beds at the base of the Calico Member. West of Calico Ghost Town, the absence of the basal sandstone beds and the presence of a south-dipping, bedding-parallel fault surface between the Pickhandle Formation and the Calico Member indicate that slip along the contact–folding detachment in this area probably occurred along a discrete fault. Two sets of striae are present on this surface; one set rakes 67° from the west and a more subtle set rakes 36° from the east. The east-raking striae are compatible with dextral-reverse slip along the folding detachment. As the detachment approaches the Calico fault (e.g., near Calico Ghost Town), it steepens and is inferred to root into the main Calico fault (Fig. 2), forming a fault zone geometry that resembles a positive flower structure.

The north-vergent asymmetry of folds and the lack of numerous folds south of the Calico fault indicate that the Tp-Tbc contact played a fundamental role in folding. If slip along the main Calico fault was primarily responsible for folding, folds might have a south-vergent asymmetry, and lacustrine rocks south of the fault would most likely also be folded into small-scale anticlines and synclines. The presence of north-vergent folds only north of the Calico fault can be explained with a model in which north-south compression between the Calico Member and the more rigid Pickhandle Formation above a basal detachment drove folding of the lacustrine rocks. Calico beds south of the Calico fault were not thrust over a south-dipping basal detachment and therefore deformed differently than their counterparts north of the fault (Fig. 8).

DISCUSSION

Interpretive Stratigraphic and Structural History of the Southern Calico Mountains

Based on geologic mapping, field observations and 40Ar/39Ar geochronology, a schematic Neogene geologic history of the southern Calico Mountains is presented in Figure 13. Between ca. 19.4 and 19 Ma, a thick (>1 km) section of coarse volcaniclastic rocks and dacite domes and flows of the Pickhandle Formation accumulated. The absence of the Pickhandle Formation south of the Calico fault suggests a model in which a northeast- or north-northeast-dipping proto-Calico normal fault uplifted metavolcanic basement rocks in the footwall and created a Pickhandle basin in the hanging wall (Fig. 13A). Alternatively, the proto-Calico fault may have unroofed metavolcanic basement rocks of Pickhandle deposits. In either scenario, slip along this inferred proto-Calico fault must have ceased prior to the deposition of fine-grained lacustrine rocks of the Calico Member, which unconformably overlie the Pickhandle Formation north of the Calico fault and directly overlie footwall metavolcanic rocks south of the Calico fault. Pickhandle dacite domes emplaced ca. 19 Ma most likely formed topographic highs that marked the northern margin of the shallow lake in which Calico Member sediments accumulated (Fig. 13A). Both the Pickhandle Formation and at least the older part of the Calico Member were deposited during rapid slip along the Waterman Hills detachment fault (Gans et al., 2005), although most normal faulting in the southern Calico Mountains appears to predate deposition of the Calico Member.

The Yermo volcanic center became active during the waning stages of lacustrine sedimentation. Between 17.1 Ma and 16.8 Ma several dacite domes intruded Calico Member sediments north and south of the Calico fault (Figs. 13B, 13C). Coarse dacite breccias shed from the domes locally filled the shallow lake and precluded lacustrine sedimentation. East and west of the Yermo volcanic center, dacite breccia sheets are overlain by as much as 60 m of Calico Member beds, indicating that lacustrine deposition continued locally during volcanic activity.

Strike-slip and transpressional deformation in the southern Calico Mountains postdates the formation of the Yermo volcanic center (post-16.8 Ma) and reflects the dominant style of post–early Miocene deformation in the central Mojave Desert. A portion of the proto-Calico normal fault was reactivated as a dextral-reverse fault (Fig. 13D). Transpressional faulting and folding are fundamentally related to the west-northwest–striking restraining bend in the Calico fault system. It is possible that the proto-Calico fault was west-northwest striking, which would indicate that the restraining bend is an original feature of the Calico fault system. If the restraining bend is original, transpressional deformation may have initiated early in the history of the right-lateral Calico fault system, perhaps in the middle or late Miocene. Alternatively, the restraining bend may be the result of counterclockwise rotation due to movement on the east-northeast–striking, left-lateral Manix fault (Dibblee, 1994), implying that transpressional deformation may postdate some right-lateral movement on the Calico fault. Approximately 3 km of right-lateral slip and perhaps 1 km of reverse slip have occurred along two major strands of the Calico fault system.

Transpression along the Calico fault restraining bend forced the Calico Member north of the fault to detach along its base and move over the south-dipping Pickhandle Formation–Calico Member contact in a reverse-dextral sense (Fig. 13D). The thinly bedded Calico Member responded to this transpression by folding into numerous anticlines and synclines that account for 25%–33% north-south shortening. The Pickhandle Formation and dacite rocks of the Yermo volcanic center resisted folding and acted as rigid buttresses against which folds in the Calico Member developed. Post–16.8 Ma deformation within the Pickhandle Formation and Yermo dacite rocks consisted primarily of strike-slip and oblique strike-slip-reverse faulting that was broadly synchronous with folding in the Calico Member.

Miocene Stratigraphic Framework for the Central Mojave Desert

The prevailing tectonostratigraphic framework of early to mid-Miocene rocks in the central Mojave Desert is based on northeast-southwest extension associated with the central Mojave metamorphic core complex. Coarse volcaniclastic rocks of the Pickhandle Formation have been interpreted as synextensional deposits ranging in age from ca. 24 to 19 Ma (Fillmore and Walker, 1996), whereas overlying, fine-grained Barstow Formation rocks are considered postextensional deposits that infilled remnant basins (Fillmore and Walker, 1996; Ingersoll et al., 1996). This tectonostratigraphic model is flawed in detail. Thermochronologic data indicate that rapid slip on the Waterman Hills detachment fault and extensional unroofing of the central Mojave metamorphic core complex footwall occurred largely between ca. 21 and 17.5 Ma (Gans et al., 2005). Based on new geochronologic data from this study, the thick section of Pickhandle Formation in the Calico Mountains accumulated rapidly from ca. 19.4 to 19 Ma, and ∼400 m of fine-grained lacustrine rocks were deposited between ca. 19 and 17 Ma. Thus, the Pickhandle Formation and at least the older part of the Calico Member of the Barstow Formation accumulated during large-magnitude extension in the central Mojave metamorphic core complex. The association of coarse-grained volcaniclastic deposits with extensional basin development and fine-grained lacustrine deposits with postextensional sedimentation can be misleading. Assuming that extension in the central Mojave metamorphic core complex did not begin until ca. 21 Ma (Gans et al., 2005), Pickhandle Formation rocks that range in age from 24 to 21 Ma may be preextensional deposits that reflect proximity to local volcanic centers rather than extensional basin development. Similarly, the coarse breccia sheets that overlie the Calico Member in the southeastern Calico Mountains apparently postdate extension and are instead a consequence of steep volcanic topography created by the Yermo volcanic center.

Although most of the Barstow Formation was deposited after extension in the central Mojave metamorphic core complex had ended, synex-tensional, 19–17 Ma Barstow Formation lacustrine deposits appear to be more widespread than previously recognized. In the easternmost Mud Hills and northwestern Calico Mountains the Pickhandle Formation grades into comformably overlying fine-grained lacustrine beds (Singleton, 2006, personal observ.), suggesting that Calico Member sedimentation extended from the southeastern Calico Mountains to the easternmost Mud Hills (Fig. 1). Van Pelt and Gans (2005) bracketed the timing of fine-grained lacustrine sedimentation in the Lead Mountain area between ca. 19.3 and 17.2 Ma (Fig. 1), and north of Daggett Ridge several hundred meters of fine-grained lacustrine beds comformably overlie the ca. 18.5 Ma Peach Springs Tuff (Wells and Hillhouse, 1989; Dibblee, 1970). Lacustrine deposits in the central Mojave Desert are commonly assumed to correlate to the postextensional type section of the Barstow Formation in the Mud Hills, yet many of these correlations appear to be inaccurate and overlook earlier synextensional lacustrine sedimentation.

Extensional Deformation

The paucity of large normal faults and the gentle tilts of strata in the Pickhandle Formation suggest that much of the Calico Mountains behaved as a fairly coherent block during extension in the central Mojave metamorphic core complex. High-angle, northwest-striking normal faults of inferred early Miocene age are present within the Pickhandle Formation, yet northeast-southwest extension associated with these faults is minor (∼5%), and the dominant sense of normal slip (top to the southwest) is antithetic to shear across the central Mojave metamorphic core complex. In the study area the Pickhandle Formation generally dips <30°SE and S (Fig. 8A). Across the central Calico Mountains, Pickhandle beds dip <20° in various directions (Dibblee, 1970). With the exception of the section along Fort Irwin Road, the Pickhandle Formation in the Calico Mountains never dips homoclinally to the southwest, which would be expected for northeast-directed extension in the upper plate of the central Mojave metamorphic core complex.

The apparent lack of significant extension may be somewhat surprising given the proximity of the Calico Mountains to lower plate rocks of the central Mojave metamorphic core complex, but the geometry of Pickhandle Formation strata in several other areas is also incompatible with northeast-southwest extension. For example, Pickhandle strata in the Lead Mountain area are folded and generally strike northeast-southwest (Fig. 1; Dibblee, 1970). The Pickhandle Formation in the Gravel Hills (northwest of the Mud Hills) dips gently to the south (Dibblee, 1968). It is possible that strike-slip and/or transpressional deformation overprinted extension, or that some of these south- to southeast-dipping strata rotated counterclockwise about a vertical axis from previous southwest-dipping orientations. However, most paleomagnetic data from the central Mojave Desert suggest clockwise rotation (see Glazner et al., 2002, for a review). More detailed mapping is needed to shed light on the style and magnitude of upper plate extension in the central Mojave Desert.

Faulting

Right-lateral slip along northwest-striking faults represents the dominant style of post–early Miocene deformation across the central Mojave Desert. New field data from the southern Calico Mountains suggest that many of these faults may have complex histories that date back to at least the early Miocene. The stratigraphic mismatch of pre–Calico Member rocks across the Calico fault restraining bend cannot be explained with dextral and/or reverse slip. Prior to transpressional deformation, there appears to have been a significant normal (southwest side up) component of slip along the Calico fault. Similarly, northwest-striking, early Miocene (?) normal faults in the Pickhandle Formation are overprinted by right-lateral slip. Early Miocene northeast-southwest extension associated with the central Mojave metamorphic core complex most likely created numerous northwest-striking normal faults, many of which may have been reactivated as right-lateral faults.

Folding and Transpression

Several geologists have argued that north-south crustal contraction associated with strike-slip faulting dominates post–early Miocene deformation across most of the Mojave Desert region (e.g., Bartley et al., 1990; Glazner et al., 2002). East-west–trending folds are generally thought to be a consequence of this regional, crustal contraction, yet there are very few areas where the detailed geometry, style, and timing of folding have been documented. New geologic mapping, structural data, and interpretations presented here illustrate several important characteristics of folds in the southern Calico Mountains, including the following.

  1. The numerous approximately east-west–trending, upright folds are largely restricted to the fine-grained lacustrine beds north of the Calico fault and are detached from the underlying Pickhandle Formation. Basement rocks are clearly not involved in the folding and are instead shortened by transpressional faulting.

  2. Folding in the Calico Member represents ∼25%–33% north-south shortening (up to 0.5 km), and thus does not account for several kilometers of shortening, as proposed by Glazner et al. (1994). Shortening due to reverse slip along the Calico fault system is not as well constrained, but is estimated to be ∼0.5 km. Thus, transpressional folding and faulting across the southern Calico Mountains probably accounts for ∼1 km of shortening. This shortening most likely absorbed some of the dextral shear along the Calico fault system and may be partly responsible for the northward decrease of dextral offset along the Calico fault (Oskin et al., 2007).

  3. The map-scale geometry of folds in the southern Calico Mountains is strongly influenced by boundary conditions, including the presence of a volcanic dome field (the Yermo volcanic center) that acted as a rigid buttress and resisted folding. The interpretation of folds in the Mojave Desert should take into consideration specific boundary conditions that may affect deformation.

  4. Folding in the southern Calico Mountains postdates the formation of the Yermo volcanic center (post ca. 17 Ma) and is temporally and spatially associated with transpressional slip along the Calico fault system. However, this folding is not classic wrench folding, where folds initiate obliquely to a strike-slip fault and then rotate toward parallelism with the fault during progressive shear. Transpressional folding and faulting in the southern Calico Mountains is also inconsistent with a kinematic model in which regional-scale shortening is accommodated by conjugate dextral and sinistral faulting. Instead, folds are localized north of the restraining bend in the Calico fault system and accommodate a component of north-south shortening associated with transpressional, dextral-reverse faulting. In the Calico Mountains transpression appears to be restricted to this restraining bend region, and is not indicative of regional contraction, as proposed by Bartley et al. (1990). Although north-south contraction may be a regional phenomenon in the Mojave Desert region (Bartley et al., 1990), some of the best examples of shortening in the central Mojave Desert may reflect localized transpression more than regional contraction. For example, the Su Casa basement arch and east-west–striking overturned beds are localized adjacent to the restraining bend in the Camp Rock fault (Dibblee, 1970; Dokka, 1986). The Barstow syncline occurs in a stepover zone between the Calico fault and the Blackwater fault zone (Dibblee, 1968), and the Lenwood anticline is located along a left-stepping bend in the Lenwood fault (Dibblee, 1967; Glazner and Bartley, 1994). Shortening does not appear to be homogeneously distributed across the Mojave Desert, and localized transpression may play an important role generating contractional structures.

CONCLUSIONS

The Neogene geologic history of the Calico Mountains includes synextensional early Miocene sedimentation and volcanism, followed by transpressional faulting and folding. New 40Ar/39Ar geochronology ages indicate that most of the type section of the Pickhandle Formation accumulated rapidly between ca. 19.4 and 19.0 Ma. The lack of a thick section of Pickhandle Formation beneath the Calico Member of the Barstow Formation south of the Calico fault suggests that a northeast- or north-northeast– dipping proto-Calico normal fault unroofed metavolcanic basement rocks in the footwall and created a Pickhandle Formation basin in the hanging wall. This inferred extensional basin development must have ceased prior to the deposition of fine-grained lacustrine rocks of the Calico Member, which unconformably overlie the Pickhandle Formation north of the Calico fault and directly overlie metavolcanic rocks south of the Calico fault. The 40Ar/39Ar geochronology brackets the age of fine-grained lacustrine sedimentation north of the Calico fault between ca. 19 and 16.9 Ma, indicating that the Calico Member is older than the type section of the Barstow Formation in the Mud Hills. The Pickhandle Formation and at least the older part of the Calico Member were deposited during rapid slip along the Waterman Hills detachment fault, yet northeast-southwest extension within the Pickhandle Formation is only ∼5%. The Yermo volcanic center in the southeastern Calico Mountains was the site of calcalkaline dacite dome emplacement between 17.1 and 16.8 Ma. This dacite volcanism was active during the late stages of lacustrine sedimentation. Previous geochronologic investigations have not recognized ca. 17 Ma volcanism in the central Mojave Desert.

Strike-slip faulting and transpression are the dominant styles of post–early Miocene deformation in the southern Calico Mountains. The north-northeast–dipping Calico fault system forms a restraining bend that accommodates dextral-reverse slip. Based on the apparent offset of dacite domes of the Yermo volcanic center, the Calico fault restraining bend system has ∼3 km of right-lateral slip and perhaps 1 km of reverse slip distributed on two main faults. The cumulative amount of dextral shear across the southern Calico Mountains is ∼4.1 km.

Deformation within the Calico Member north of the Calico fault is taken up primarily by folding. Numerous approximately east-west–trending, upright folds represent 25%–33% north-south shortening (up to ∼0.5 km). Dacite domes and breccias of the Yermo volcanic center resisted folding and deformed instead by strike-slip and transpressional faulting. Folds in the Calico Member are detached from the homoclinally south- to southeast-dipping Pickhandle Formation. We interpret the Pickhandle Formation–Calico Member contact as a reverse-dextral fault zone that is part of a positive flower structure along the Calico fault restraining bend. Transpression within this flower structure was responsible for folding the Calico Member. The geometry and map view pattern of folds are not compatible with gravity folding, folding due to dacite dome emplacement, or wrench folding. Transpressional folding and faulting in the Calico Mountains is localized along the Calico fault restraining bend and is not indicative of regional north-south contraction.

If you are viewing the PDF of this paper or reading it offline, please visit http://dx.doi.org/10.1130/GES00143.S1 (Fig. S1) or the full-text article on www.gsajournals.org to view Supplemental Figure S1.

*Present address: Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712, USA

This work was funded by Rio Tinto Industrial Minerals Exploration. We thank several Rio Tinto borate exploration geologists, particularly John Reynolds, for valuable discussions on Mojave geology. Thoughtful reviews by Mike Oskin and John Fletcher have improved this manuscript and are greatly appreciated.