Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) detrital zircon age data and detailed mapping of metasedimentary and metavolcanic pendants in the central Sierra Nevada are used to examine the age and origin of the metasediments, and to search for evidence of the location and history of the Cretaceous Mojave–Snow Lake fault. Quartzites from the Snow Lake, Benson Lake, May Lake, and Quartzite Peak pendants yield age spectra that best match Neoproterozoic to Ordovician passive-margin strata, thus supporting the presence of displaced passive-margin strata now preserved in Sierran pendants. Sediments at Cinko Lake, Strawberry Mine, and NE of Snow Lake are interpreted to be Early Jurassic to Early Cretaceous and marine, and probably not equivalents of the Fairview Valley Formation at Black Mountain as previously interpreted. Without this correlation, the suggested 400 km of displacement along the Mojave–Snow Lake fault is unconstrained, the exact location of origin for these passive-margin and Jurassic marine metasediments is uncertain, and the nature of the contact between these two sediment packages, which has been suggested to be an angular unconformity, is speculative and deserving of a more detailed evaluation. The timing of displacement along the inferred dextral Snow Lake fault is constrained to be between ca. 145 Ma, the maximum depositional age of Jurassic strata at Cinko Lake, and 102 Ma and 103–108 Ma, the age of the oldest intruding pluton and the youngest volcanic rocks juxtaposed along the stratigraphic break with adjacent eugeoclinal rocks.
During the Mesozoic, the Cordilleran Sierran batholith, California, was characterized by a long deformational history punctuated by short periods of voluminous magmatism (Coleman and Glazner, 1997). The emplacement of enormous volumes of magma (Fig. 1), transferring mass and heat throughout the crustal column, obliterated much of the pre- and synmagmatic rock record, including information about tectonic histories. Cretaceous intrabatholithic, dextral, strike-slip faults were probably active due to oblique subduction of the Farallon and Kula plates beneath North America (Engebretson et al., 1985; Busby-Spera and Saleeby, 1990; Kistler, 1993). Previous workers, using stratigraphic, paleontologic, compositional, and isotopic data, have attempted to constrain the location and history of these faults (e.g., Kistler, 1993; Lahren et al., 1990; Schweickert and Lahren, 1993). However, preserved exposures of these faults are rare, and therefore information about prebatholithic fault tectonics in the Sierra Nevada batholith, and the origin of highly disrupted host rock remnants (referred to as roof pendants and interplutonic screens), remains speculative.
We used structural and stratigraphic analyses combined with laser ablation ICP-MS U/Pb geochronology analyses of detrital zircons to investigate the origin of quartzite-rich, metasedimentary pendants and interplutonic screens northwest, west, and south of the Cretaceous Tuolumne batholith in the central Sierra Nevada (Figs. 1 and 2). These metasedimentary remnants were originally classified as pendants of the Kings Sequence (Bateman and Clark, 1974). Bateman and Clark (1974) and Saleeby et al. (1978) defined the King Sequence as Lower Mesozoic quartz-rich metasedimentary and silicic metavolcanic rocks, which contained sparse Late Triassic and Early Jurassic fossils, but also included other units of variable age and generally unknown origin. Lahren and Schweickert (1989) first correlated strata from the Snow Lake pendant (Fig. 2) with Precambrian to Ordovician passive-margin rocks (inner shelf, Death Valley facies; Fig. 3) of the Mojave Desert region based on stratigraphic, structural, and chemical studies. They included the Snow Lake pendant and at least eight other metasedimentary pendants to the south (including the ones studied in this paper) as part of the Snow Lake block, and suggested that much of the Kings Sequence consists of Precambrian to Paleozoic rocks, a view with which geologists working on the Kings Sequence in the southern Sierra have disagreed (Saleeby and Busby, 1993).
Lahren and Schweickert (1989) proposed that the Snow Lake block—a crustal slice that may be underlain by Precambrian sialic crust—was transported northward ∼400 km from the Mojave Desert region (Death Valley facies exposures near Black Mountain by Victorville; Fig. 3) into what is now the central Sierra Nevada by movement on a Cretaceous intrabatholithic strike-slip fault named the Mojave–Snow Lake fault (MSLF; Figs. 1 and 3). A detrital zircon provenance study by Grasse et al. (2001) conducted on two different stratigraphic units of the Snow Lake pendant found similar detrital zircon age distributions and overlap between analyses from rocks of the Snow Lake pendant and Neoproterozoic and Cambrian passive-margin strata in the Mojave region, supporting the Mojave–Snow Lake fault hypothesis.
Although the Mojave–Snow Lake fault may have been a significant structure in the Cretaceous North American Cordillera, no direct fault trace has been found. Some evidence for strike-slip faulting has been suggested to be related to the Mojave–Snow Lake fault south of our study area in the Boyden Cave and Mineral King pendants (Fig. 1; Schweickert and Lahren, 1991; Lahren et al., 1990). However, this interpretation and other issues with the Mojave–Snow Lake fault hypothesis, such as the amount of displacement along this structure, have been controversial. For example Saleeby and Busby (1993) argued that a significant structure that could represent the trace of the Mojave–Snow Lake fault does not exist in the Boyden Cave or Mineral King pendants (Fig. 1). They furthermore suggested that the so-called Snow Lake block is unlikely to be an intact crustal block since the proposed Mojave–Snow Lake fault trace cuts across major petrologic and geochemical features of the batholith. Saleeby and Busby (1993) concluded based on their structural analysis that no more than 200 km of horizontal displacement along the AIB (axial batholithic break) would be needed to explain the position of passive-margin pendant rocks at the present location (Fig. 3). The Mojave–Snow Lake fault of Lahren and Schweickert (1989) coincides with the AIB and also the IBB2 (intrabatholithic break 2) proposed by Kistler (1993; Figs. 1 and 3). The IBB2 was identified based on apparent offset of the Sri 0.706 line, which also suggests only ∼200 km of displacement. A volcanic petrography and geochemistry provenance study on basinal volcaniclastic and tuffaceous sedimentary rocks of the Lower to Middle Jurassic Sailor Canyon Formation north of our study area concluded that 200 km of offset along the Mojave–Snow Lake fault is sufficient to move these sediments back to volcanic centers exposed in western Nevada north of 38°N from which the Sailor Canyon Formation was most likely derived (Lewis and Girty, 2001). Wyld and Wright (2001, 2007) supported several hundreds of kilometers of displacement along the Mojave–Snow Lake fault, which they suggested was coeval and likely connected to the Salmon River suture zone in western Idaho via an Early to mid-Cretaceous western Nevada shear zone, forming part of a major crustal boundary in western North America.
According to Lahren and Schweickert (1989), displacement must have occurred between 150 Ma, the approximate age of mafic dikes in the Snow Lake pendant interpreted as Independence dikes (Lahren et al., 1990), which were emplaced before the northward displacement of the Snow Lake block, and the emplacement of younger plutons at 80–110 Ma, which overprinted the trace of the Mojave–Snow Lake fault.
To identify the age and origin of metasedimentary rocks in the central Sierra Nevada and to test the Mojave–Snow Lake fault hypothesis, we present new detrital zircon age data from six metasedimentary pendants at Cinko Lake, Snow Lake, Benson Lake, May Lake, Quartzite Peak, and Strawberry Mine (Fig. 2). We reanalyzed zircons from the Snow Lake pendant originally analyzed by Grasse et al. (2001), and passive-margin strata from the Mojave Desert, central Nevada, and Sonora (Mexico), originally sampled and analyzed by Stewart et al. (2001), Gehrels and Dickinson (1995), and Gehrels and Stewart (1998), to test the passive-margin affinity of the pendant rocks in this northern part of the Snow Lake block (Fig. 3). We also tested (using data presented by Stone et al., 2005) the hypothesis that younger metasediments (herein named the “Jurassic cover sequence”), which overlie the passive-margin rocks along a contact interpreted to be an angular unconformity in the Snow Lake pendant (Fig. 4), are equivalents of the Fairview Valley Formation at Black Mountain near Victorville (e.g., Lahren and Schweickert, 1989). To provide constraints on the Mojave–Snow Lake fault location and the timing of fault displacement, we additionally dated zircons from Cretaceous to Jurassic metavolcanic rocks in the Strawberry Mine area and east of the Cinko Lake metasedimentary pendant (Figs. 2 and 5; Huber et al., 1989; Wahrhaftig, 2000), as well as two plutons, the granodiorite of Harriet Lake and the quartz diorite of Cinko Lake (Fig. 5; Thompson et al., 2007), that were emplaced at the boundary between the metasedimentary and metavolcanic rock pendants at Cinko Lake.
After a brief introduction of the regional geology and pendant lithologies, we present LA-ICP-MS zircon age distribution spectra for samples from the studied pendants, compare these to potential correlative strata, and discuss possible alternative interpretations. We show that our data are consistent with the hypothesis that passive-margin strata exist within the Sierra Nevada batholith and that they were probably transported northwestward by a major Cretaceous intrabatholithic strike-slip fault of large displacement. However, the correlation of the Jurassic cover sequence overlying the passive-margin strata to the Fairview Valley Formation at Black Mountain near Victorville is questionable, and thus so is the estimated 400 km of displacement along the Mojave–Snow Lake fault suggested by this correlation. We also offer detailed constraints on the timing of displacement using zircon age data from the Snow Lake cover sequence, the metavolcanic rocks, and plutons near the metasedimentary pendants at Cinko Lake, discuss possible interpretations for the nature of contacts between these different units, and further constrain the likely location of the Mojave–Snow Lake fault trace in the Tuolumne batholith area (Fig. 2).
The examined metasedimentary pendants, which have formerly been assigned to the Kings Sequence (Bateman and Clark, 1974) and then later considered to be part of the Snow Lake block (Lahren and Schweickert, 1989), are composed of passive-margin deposits that are highly deformed and metamorphosed to upper-amphibolites-facies conditions (Rose, 1957; Foley et al., 2007). They are situated in a belt sandwiched between Lower Paleozoic eugeoclinal (deep water) deposits of the eastern and western Sierran metamorphic belts: these are the Shoo Fly complex—a belt of Cambrian to Ordovician quartzites, phyllites, and chert—in the Western metamorphic belt, and the Roberts Mountains allochthon, exposed in eastern Sierran pendants (Figs. 2 and 3). The Roberts Mountain allochthon is a sequence of sediments that were thrust over the Precambrian to Paleozoic passive-margin belt during the Late Devonian to early Mississippian Antler orogeny (e.g., Stevens and Greene, 1999). Remnants of the Roberts Mountain allochthon in pendants along the eastern Sierra (e.g., Northern Ritter Range pendant; Figs. 1 and 3) are composed of chert, siliceous argillite, and slate that overlie Cambrian to Permian argillites interbedded with limestones and calcareous sandstones, representing a transitional passive-margin–deep-water slope facies of the Mount Morrison block (Greene et al., 1997; Stevens and Greene, 1999). The closest sequence of Neoproterozoic to Cambrian quartzite-rich passive-margin deposits of the Inyo and Death Valley facies are exposed 50–200 km to the east and southeast of the studied pendants (Fig. 3). The Inyo and Death Valley facies rocks are just a small part of an enormous passive-margin belt striking north-south throughout western North America and south to Sonora, Mexico. The Ordovician Quartzite of Sierra Lopez (Stewart et al., 1990), part of the Sonoran miogeocline sequence, was analyzed for comparison to the metasedimentary pendant rocks in the axial part of the Sierra Nevada (Fig. 3).
GEOLOGICAL SETTING AND LITHOLOGIES
Cinko Lake Pendant and Surrounding Metavolcanic and Plutonic Rocks
The ∼0.5 km2 Cinko Lake pendant (Fig. 5) is dominantly composed of centimeter- to decimeter-thick, fine-grained, rose and light- to dark-gray quartzites, metasiltstones, and calcareous metasiltstones (Fig. 9A). These are interlayered with millimeter to centimeter metapelitic laminae and bands, and centimeter- to decimeter-thick, fine-grained, green, brown, and gray calc-silicate layers. Dolomite and decimeter-scale, fine-grained amphibolite and hornblende gneiss layers occur locally. The Cinko Lake pendant reveals a complicated folding pattern with generally tight folds, and we have found evidence for three to locally four deformation phases. The thickness of the original sedimentary section is unknown. Sample JT14 for detrital zircon analyses was taken from a quartzite in the eastern margin of the pendant (Fig. 5).
East of the Cinko Lake pendant, a belt of metavolcanic rocks is exposed called the Piute Meadows pendant, and it may show correlations to the Saddlebag Lake pendant to the southeast (Fig. 2). The westernmost extent of these metavolcanic units is located just a few hundred meters east of the Cinko Lake pendant, and is there composed of metavolcanic rocks that are locally interlayered with siliciclastic, calc-silicate, and marble layers, which are very different from the well-laminated and multiply deformed strata in the Cinko Lake pendant (Fig. 5). The metavolcanic rocks include centimeter to decameter beds of andesitic to rhyolitic crystal and lapilli tuffs, lithic tuffs, and local flow units (Huber et al., 1989; Wahrhaftig, 2000). Local interbeds of volcaniclastic and metasedimentary units with graded beds and cross-bedding indicate top-to-the-west younging, but switch to top-to-the-east in the easternmost metavolcanic units (Foley et al., 2007). The metavolcanic rocks are more mafic in composition farther to the east, with basalt flows occurring within the volcaniclastic package. These units are now tilted to near vertical dips and are internally strained, preserving a well-developed mineral foliation and lineation and local intrafolial folds. The metavolcanic rock samples near Cinko Lake (CL01A, CL02A, HL016) are situated 400 m to 2 km east of the Cinko Lake metasedimentary pendant (Fig. 5).
The Cinko Lake pendant is predominantly surrounded by the granodiorites of Harriet Lake and Fremont Lake (Fig. 5; Huber et al., 1989). The Harriet Lake granodiorite and the Cinko Lake quartz diorite were dated with chemical abrasion isotope dilution–thermal ionization mass spectrometry (ID-TIMS) U/Pb zircon geochronology at the Berkeley Geochronology Center in collaboration with R. Mundil and J. Matzel. Both plutons intruded along the boundary between the Cinko Lake metasedimentary pendant and the metavolcanic rock package to the east, and thus represent one possible location of the Mojave–Snow Lake fault (Fig. 2). If true, the pluton ages represent minimum age constraints on the activity of the Mojave–Snow Lake fault. The coarse-grained Cinko Lake quartz diorite revealed a crystallization age of 95.2 ± 0.2 Ma, and the medium-grained Harriet Lake granodiorite was 101.8 ± 0.2 Ma (Fig. 5; Thompson et al., 2007).
Snow Lake Pendant
In the Snow Lake pendant (7 km2; Fig. 4), Lahren and Schweickert (1989) and Lahren et al. (1990) distinguished five lithologic units, which they correlated with Neoproterozoic to Middle Cambrian passive-margin strata and overlap sequence at Quartzite Mountain near Victorville in the San Bernardino Mountains (Fig. 3).
From base to top, the five units include (1) quartzite and feldspathic quartzite with intercalations of quartz–mica schist, calc-silicate schist, micaceous quartzite, and marble (correlative with the Stirling Quartzite), which Grasse et al. (2001) referred to as the “lower quartzite”; (2) quartzite with intercalations of micaceous feldspathic quartzite that contain evidence for the trace fossil Skolithos, schist, calc-silicate schist, and marble (correlative to the Wood Canyon Formation); (3) light-colored, vitreous quartzite and feldspathic quartzite (correlative to the Zabriskie Quartzite), which Grasse et al. (2001) referred to as the “upper quartzite”; and (4) calc-silicate schist with layers of quartz–mica schist, quartzite, and marble (correlative to the Carrara Formation). Grasse et al. (2001) reported apparent age groups from 12 analyzed zircons from the lower quartzite unit and 25 zircons from the upper quartzite at 1.00–1.13 Ga, 1.41–1.44 Ga, 1.64–1.68 Ga, and 1.74–1.80 Ga (Fig. 10). We additionally sampled and analyzed one quartzite sample each for comparison from the other two sections at Snow Lake interpreted by Lahren and Schweickert (1989) as Carrara Formation (SLP-21) and the upper part of the Wood Canyon Formation (SLP-13; Fig. 4).
The fifth unit, the Jurassic cover sequence, is interpreted to overlie the others on an angular unconformity with a basal conglomerate (Fig. 4). This contact is fairly discrete (Fig. 9B), is locally stepped, and the inferred basal metaconglomerate is clast supported and contains angular clasts of mostly Stirling Quartzite and less commonly of a brown weathered lithology of questionable composition (Fig. 9C). The Jurassic cover sequence is dominantly composed of thinly bedded to laminated metasiltstones and calc-silicate schists with local layers of marble (Lahren and Schweickert, 1989), similar to the metasediments at Cinko Lake located just to the NE of the Snow Lake pendant. Locally, flute and ripple marks are preserved. The Jurassic cover sequence at Snow Lake has been interpreted to be part of an overlap sequence correlative with the Fairview Valley Formation at Victorville and was originally thought to be Triassic in age (Lahren and Schweickert, 1989). Based on more recent data, the Fairview Valley Formation near Victorville is now known to be Early Jurassic or younger in age (Schermer et al., 2002; Stone et al., 2005). We analyzed one metasiltstone sample (SLP-5) from the Snow Lake cover sequence above the inferred unconformity to identify the maximum deposition age and to test the hypothesized correlation to the Fairview Valley Formation at Victorville.
All strata in the Snow Lake pendant have undergone at least four deformation phases, including, from oldest to youngest, (1) isoclinals folds, (2) W-vergent folds, (3) NE-vergent folds and thrust faults, and (4) NW-trending upright folds (Lahren and Schweickert, 1989). According to Lahren and Schweickert (1989), the overlap sequence was deposited on older strata that had already been folded and thrust faulted at least once.
A granite and co-magmatic gabbroic complex south of Bigelow Lake, as well as felsic and mafic dikes intruding the main pendant body in the southwestern part, yielded U/Pb zircon TIMS ages ranging from 147 to 153 ± 2–4 Ma (Lahren et al., 1990). No known Independence dikes cut the Jurassic cover sequence. We hypothesize that dikes intruding the cover sequence in the northeastern part of the pendant may be derived from the Harriet Lake pluton, which turns out to be important for the interpretation of our results (Fig. 4).
Fossils in the Snow Lake Pendant
During a field trip to the Snow Lake pendant in summer 2008, led by M. Lahren and R. Schweickert (University of Nevada, Reno [UNR]), Mueller and Memeti found a 20-cm-diameter ammonoid fossil in float close to the base of the calcareous metasiltstone in the cover sequence immediately above the unconformity after Lahren and Schweickert (1989). Mueller also discovered an ∼30-cm-thick in situ metasiltstone layer ∼5–10 m structurally above the angular unconformity near the SLP-5 sample location, containing, for the high Sierra Nevada, an unusual abundance of fossils. According to Kathleen Ritterbusch (University of Southern California), who examined digital images of these fossils, they are of marine invertebrates that lived on and above the seafloor of a continental shelf. The fossil assemblage includes two ammonoid specimens, several crinoids, and many bivalves. The two ammonoid fossils found include a specimen that reveals the outer wall of a platyconic ammonoid shell ∼20 cm in diameter (Fig. 9D). Moderate ornamentation includes evenly spaced concave ribs. At least five whorls are present and are moderately evolute, leaving a wide umbilicus. This shell morphology was interpreted by Westermann (1996) as demersal. Crinoids appear as star-shaped segments, which are part of the disarticulated skeletons of epibenthic filter-feeders. The bivalve fossils are of delicate pectiniform shells with very fine radial striae. These filter-feeding mollusks likely lived on the seafloor, attached to bottom substrate by byssus threads. The descriptions of the ammonoid and other fossils and their communal occurrence suggest that this fossil assemblage is marine and of Early Jurassic age (Kathleen Ritterbusch, University of Southern California, 2009, personal commun.).
Benson Lake Interplutonic Screen
The ∼8 km2 Benson Lake pendant (Fig. 6) is primarily composed of centimeter- to meter-thick beds of vitreous, fine- to dominantly coarse-grained, white quartzite (Figs. 9E and 9F). The thickest coherent quartzite package with millimeter-scale thin interlayers of metapelite (biotite schist) is 300–400 m thick (Fig. 6). At several locations, sedimentary structures such as cross-bedding and troughs are preserved in the white quartzite (Fig. 9F). The quartzite is intercalated with millimeter-thick to several tens-of-meter-thick layers of mica schist and calc-silicate schist. The thickest metapelitic package is ∼150–250 m thick and occurs on the west side of the pendant (Fig. 6). Centimeter-thick to a few-meter-thick layers of marble and hornblende gneiss are locally exposed. This package of metasedimentary rocks shows evidence for five deformation phases, the most prominent of which formed a pendant-scale tight antiform with a moderately NW-plunging fold axis (see also Fig. 9E). The Benson Lake pendant is bordered along its northeastern margin by the 95–93 Ma Kuna Crest granodiorite unit of the Tuolumne batholith and along its southwestern margin by the 102 Ma El Capitan granite. Sample BPM-314 and RE242 were collected from thick layers of extremely pure quartzite NW and SE of Benson Lake (Fig. 6).
May Lake Interplutonic Screen
The ∼1.5 km2 May Lake pendant (Fig. 7), similar to the Benson Lake pendant, is dominated by centimeter- to meter-thick, vitreous, fine- to dominantly coarse-grained, white quartzites. These are interlayered in up to 250-m-thick quartzite packages with millimeter-scale thin biotite-schist laminae. In more heterogeneous packages, the quartzites are finer grained and interlayered with centimeter- to several-meter-thick meta-psammopelitic and metapelitic layers, the latter of which are a maximum of 100 m thick. Where calc-silicate layers occur, they are centimeter to meter thick and often boudinaged. Marble layers, more common than in the Benson Lake pendant, are particularly common in the southern part of the pendant and have tens to hundreds of meter thickness. In the northern section of the pendant, marble occurs only locally in centimeter- to meter-thick layers similar to the calc-silicates.
The May Lake pendant has been intensely deformed by five deformation phases, which transposed and disrupted bedding. Large-scale, tight folds with moderately to the northeast plunging axes are the most apparent structures on the geologic map. Sample ML-369 was sampled from a pure quartzite layer from the central part of the interplutonic screen, and ML-459 was taken from a metasiltstone outcrop at the northern end of the pendant just south of May Lake (Fig. 7).
The ∼0.3 km2 Quartzite Peak pendant (Fig. 2) is the largest of a number of small metasedimentary bodies that lie along the western and northern margin of the plutonic (Mount Clark) host rock reentrant into Early Cretaceous units of Yosemite Valley intrusive suite and Cretaceous Red Peak Granodiorite at the southern end of the Tuolumne batholith (Peck, 1980; Karpowicz, 2004). The Quartzite Peak pendant is dominated by biotite schist and fine-grained gray quartzite with thin micaceous layers. Strong foliation in these rocks is axial-planar to tight and isoclinal folds of compositional layering. The other small metasedimentary bodies consist of marble, calc-silicate rock, silicic hornfels to quartzite, and biotite schist. In the biotite schist, early tight folds are locally coaxially refolded, and abundant thin (<1 cm) veinlets may represent local partial melts (R.B. Miller, 2007, personal commun.). Karpowicz (2004) noted similarities between these rocks and those in the May Lake pendant. Due to the small size and lack of detailed mapping of the Quartzite Peak and other bodies, we do not present a separate geologic map for these rocks. The analyzed sample SN7131 from Quartzite Peak is a quartzitic biotite schist.
Strawberry Mine Pendant and Nearby Metavolcanic Rocks
The Strawberry Mine pendant is composed of metasedimentary rocks of probably Early Jurassic age that are entrained by a sequence of meta-igneous rocks of middle Cretaceous age (Nokleberg, 1981; Fig. 8) at the southern end of the 97.1 ± 0.7–98.5 ± 0.3 Ma Jackass Lakes pluton (McNulty et al., 1996). The metasedimentary units consist of calc-silicate hornfels, marble, and biotite hornfels derived from protoliths of marl, limestone, and calcareous shale, respectively (Fig. 8; Nokleberg, 1981). Nokleberg (1981) subdivided these rocks into six lithologically distinct stratigraphic units, which are, from youngest to oldest: (1) upper biotite hornfels, (2) upper quartz-plagioclase hornfels, (3) lower biotite hornfels, (4) middle quartz-plagioclase hornfels, (5) diopside-plagioclase hornfels and marble, and (6) lower quartz-plagioclase hornfels (Fig. 8). Units 1–3 are dominantly thin bedded. Units 4–6 are thin to thick bedded, including the marble, which in unit 5 is 1–3 m thick. Units 3 and 6 additionally preserved relict graded bedding. A poorly preserved pelecypod was found in float of the quartz-plagioclase unit, and was identified as a Mesozoic bivalve, possibly Inoceramus pseudomytiloides of Early Jurassic age (Nokleberg, 1981). The meta-igneous rocks consist of metamorphosed shallow intrusive rocks of granodioritic to granitic compositions.
Nokleberg (1981) identified at least three deformation phases of regional deformation in the Strawberry Mine pendant, which resulted, from oldest to youngest, in (1) NE- to E-W–trending folds of likely Middle Jurassic age, which are only found in the metasedimentary sequence, (2) folds and faults with 335° trends, which also include the meta-igneous rocks and were contemporaneous with middle Cretaceous volcanism and plutonism, and (3) folds, faults, and foliations trending 270°–295° of likely middle to Late Cretaceous age. The metasedimentary sample JT09 that we analyzed for the Strawberry Mine pendant is from the middle quartz-plagioclase hornfels unit (unit 4, Fig. 8).
The metavolcanic rocks surrounding the Strawberry Tungsten Mine pendant are intercalated with Upper Jurassic metasedimentary rocks and Lower Cretaceous felsic to intermediate granitoid rocks (Fig. 8; Nokleberg, 1981). The analyzed metavolcanic samples JT06 and JT13 are derived from an assemblage composed of mostly rhyolitic to dacitic tuff and tuff breccias, and lesser lava flows (Fig. 8; Huber et al., 1989).
LA-ICP-MS U/Pb ZIRCON GEOCHRONOLOGY
Analytical Methods and Samples
We conducted U-Pb geochronology on detrital zircons from metasedimentary samples and zircons from the volcanic rocks using the laser ablation–multicollector–inductively coupled plasma–mass spectrometer (LA-MC-ICP-MS) at the Arizona LaserChron Center, Tucson, Arizona. Zircons were separated and processed with standard preparation techniques (Arizona LaserChron Center, 2010) from two 1–2 kg samples each. For each metasedimentary and metavolcanic sample, 100 and 50 zircon grains, respectively, were randomly selected, although zircons containing inclusions or fractures were generally avoided. To ensure a statistically significant comparison of ages (Gehrels et al., 2006; Gehrels et al., 2008) between our new samples and those reported in previous studies dated with the ID-TIMS method (only small number of zircon ages), we reanalyzed samples of passive-margin strata from previous studies (see following). Crystallization ages were calculated from the weighted mean of 206Pb/238U ages for younger than 1000 Ma grains and of 206Pb/ 207Pb ages for older than 1000 Ma grains (Gehrels et al., 2008). The uncertainty of the weighted mean incorporates the scatter and precision of the individual analyses, and it is typically ±1%–2% (2σ), with a mean square of weighted deviates (MSWD) of ∼1. The systematic errors are typically ∼1% (2σ), yielding a total error for most igneous ages of ∼2% (2σ). Analyses that were >30% discordant (by comparison of 206Pb/238U and 206Pb/207Pb ages) or >5% reverse discordant were discarded during data reduction. Normalized age probability plots are used for the illustration and interpretation of detrital zircon ages (Arizona LaserChron Center, 2010; Figs. 10, 11, and 12; see GSA Data Repository for auxiliary data table1).
Table 1 summarizes the number of zircons analyzed for each group of samples and location and the major and minor age peaks each sample yielded in our analyses. The metasedimentary samples from the studied pendants can be divided into at least two different groups based on their age distribution patterns: while samples from Cinko Lake (JT-14), the cover sequence in the northeastern part of the Snow Lake pendant (SLP-5), and the Strawberry Mine pendant (JT09) mostly reveal Phanerozoic zircon ages, all other metasedimentary samples failed to yield any significant (≥2 zircon grains) age peaks younger than 1 Ga (Figs. 10 and 11; Table 1, sections A and B).
Detrital Zircon Data from Passive-Margin Strata in the Mojave Region, Death Valley Region, Central Nevada, and Sonora (Mexico)
To test Lahren and Schweickert's (1989) and Grasse et al.’s (2001) hypothesis that Snow Lake and nearby pendants described here represent displaced Upper Proterozoic to Cambrian passive-margin units of the Death Valley facies in the Mojave Desert near Victorville, and to find best-fit zircon age spectra for detrital zircon age data collected from the other metasedimentary pendants, we reanalyzed detrital zircons from samples originally dated with the ID-TIMS by Stewart et al. (2001) of the Wood Canyon Formation in the Death Valley region (WC#1) and the Mojave (WC#2), the Zabriskie Quartzite from the Mojave region (Zab2), and the Eureka Quartzite from central Nevada described in Gehrels and Dickinson (1995). The Middle to Late Ordovician Quartzite of Sierra Lopez (n = 92) from Gehrels and Stewart (1998), an equivalent unit of the Eureka Quartzite, is located in Sonora (Mexico) just south of the interpreted position of the Mojave-Sonora megashear (Anderson and Schmidt, 1983; Stewart et al., 1990). Both samples contain the same Peace River Arch signature, with major peaks at ca. 1900 and 2700 Ma (Table 1, section C; Fig. 10). Stirling Quartzite (ST1) age results for additional comparison are from Barth et al. (2009; Figs. 3 and 10; Table 1, section C). All reanalyzed samples show consistent detrital zircon age spectra compared to the spectra obtained by the ID-TIMS method in the previous studies; however, the larger number of zircons analyzed here added more detail to the age spectra by revealing additional intermediate and smaller peaks important for the correlation process, and, in some cases, shifted age peaks by 50–100 Ma. For example, the main age peak for the Zabriskie Quartzite (Zab2) in Stewart et al. (2001) is close to 1800 Ma, whereas our most recent analysis indicates the same peak to be closer to 1700 Ma.
Stirling Quartzite from the San Bernardino Mountains
A recent study of the Proterozoic crustal evolution of Southern California by Barth et al. (2009) presented new ion microprobe ages from a yellow-tan, cross-bedded to ripple-laminated, mature quartzite from the Big Bear Lake area in the San Bernardino Mountains (same sample as in Stewart et al., 2001; Fig. 3), interpreted to be equivalent to the upper part of the Stirling Quartzite. At least one quartzite unit at the Snow Lake pendant has been petrographically and stratigraphically interpreted to be Stirling Quartzite from the San Bernardino Mountains (Fig. 4). Other siliciclastic metasedimentary packages stratigraphically underlying the passive-margin strata are the Big Bear and the Pinto Mountain Groups from the Pinto and Eagle Mountains (Barth et al., 2009). The Pinto Mountain Group includes basal conglomerate, mature and cross-bedded quartzites, pelitic rocks, laminated Fe-rich rocks, and dolomites. Detrital zircon spectra suggest that the Pinto Mountain metasediments were deposited at the earliest at 1630 Ma, defined by the youngest zircon ages found, and thus are Mesoproterozoic in age. These sediments are characterized by a major zircon age peak at ca. 1650–1800 Ma, and smaller peaks formed by Archean zircons (Barth et al., 2009). The overlying siliciclastic Big Bear Group has four major age peaks at ca. 1250 Ma, 1400 Ma, 1650 Ma, and ca. 1850–1900 Ma (see zircon age spectra in Barth et al., 2009). Although much older in age than the passive-margin strata, the Big Bear and Pinto Mountain Group cannot be ruled out as potential sedimentary equivalents of the metasediments exposed in pendants of the Tuolumne batholith area based on lithologic characteristics and detrital zircon age spectra.
The metavolcanic rocks just east of the Cinko Lake pendant and north of the Tuolumne batholith, which were mapped as Jurassic by Huber et al. (1989) and were suggested to be Cretaceous by Wahrhaftig (2000), yielded mid-Cretaceous age peaks (Table 1, section D; Figs. 5 and 12). We interpret the youngest age peaks as the likely eruption ages, and the older ages as ages derived from incorporated xenocrystic zircons. These results show that the metavolcanic rocks must have been deposited and deformed after 103–108 Ma, and are close in age to the plutons now intruding them (Fig. 5; Foley et al., 2007; Thompson et al., 2007).
The two metavolcanic samples at the Strawberry Mine pendant yielded mid-Cretaceous zircon age distributions similar to the Cinko Lake volcanic rocks (Table 1, section D; Fig. 12). The likely extrusion age for these metavolcanic rocks is 96–100 Ma and thus coeval with the crystallization age of the adjacent Jackass Lakes pluton and enclosed metavolcanic pendants (McNulty et al., 1996; A. Barth, 2008, personal commun.).
We thus recognize that there is a continuous belt of NNW-SSE–striking mid-Cretaceous metavolcanic rocks just east of the metasedimentary pendants of the Snow Lake block, which originally were interpreted to be Jurassic (Fig. 2; Huber et al., 1989; Wahrhaftig, 2000), and are overlying the Jurassic sediments on the Snow Lake block passive-margin units. This mid-Cretaceous metavolcanic sequence is intruded by contemporaneous granodiorite plutons, and is consistent with an overall westward decrease in age of Mesozoic volcanic rocks in the central Sierra.
INTERPRETATION AND DISCUSSION
Our detrital zircon data from quartzites from pendants at Snow Lake, Benson Lake, May Lake, and Quartzite Peak yielded exclusively Proterozoic and Archean ages ranging from 1.0 to 3.3 Ga (Table 1, section A; Fig. 10). The age distribution and peak pattern of the normalized age probability plots suggest a maximum sedimentation age in the Neoproterozoic or early Paleozoic. Zircon ages and age distribution patterns are reproducible in pendants when tested, as shown by the similarity of detrital zircon age spectra from two quartzite samples from both the Benson Lake and May Lake pendant (Fig. 10). The fact that samples were taken from different locations within each pendant (Figs. 6 and 7) implies that both samples in each pendant were derived from the same stratigraphic unit and/or source region. However, lithologic variation and variation in the details of detrital zircon data argue that these pendants contain rocks from a variety of stratigraphic units, as suggested by Lahren and Schweickert (1989), in the Snow Lake pendant.
The metasedimentary pendants near the Tuolumne batholith are undoubtedly tectonically out of place, since this belt of typical passive-margin, shallow-water strata (miogeocline) presently lies between western and eastern belts of Paleozoic basinal sedimentary rocks (eugeocline), namely the Shoo Fly Complex and the Roberts Mountain allochthon–El Paso terrane, respectively (Figs. 2 and 3). Therefore, there is no question that a tectonic break—no matter if called the AIB (Saleeby and Busby, 1993), IBB2 (Kistler, 1993), or Mojave–Snow Lake fault (Lahren and Schweickert, 1989)—existed in the form of a strike-slip fault between the “Snow Lake block” and the Roberts Mountain allochthon (Fig. 3). Presumably, a second fault also occurred to the west of the Snow Lake block as well. Next, we attempt to make stratigraphic correlations of metasediments exposed in several pendants around the Tuolumne batholith with potential areas of origin to the south using lithology and detrital zircon age spectra, narrow down the probable location of the fault trace in the Tuolumne batholith area using that information (Fig. 2), and propose a time span of fault activity based on age dating of plutons and metavolcanic rocks from the pendants.
Correlation of the Metasedimentary Pendants at Snow Lake, Benson Lake, May Lake, and Quartzite Peak
The study by Grasse et al. (2001) was the first to use detrital zircon age spectra to test Lahren and Schweickert's (1989) stratigraphic correlation of the Snow Lake pendant strata with passive-margin rocks of the Death Valley facies in the Victorville area, San Bernardino Mountains (Fig. 3). However, detrital zircons were studied from only the Snow Lake pendant and none of the other pendants of the Snow Lake block. Furthermore, all samples from other studies (including Grasse et al., 2001; Stewart et al., 2001; Gehrels and Stewart, 1998; Gehrels and Dickinson, 1995) that we used for comparison applied the time-consuming ID-TIMS technique, which means that the number of detrital zircon age analyses per sample was small and thus potentially not sufficient to detect all characteristic major and minor age groups (Gehrels et al., 2000). Our study was thus designed to test the origin of the pendants and the Mojave–Snow Lake fault hypothesis with larger detrital zircon data sets obtained by LA-ICP-MS.
A visual comparison of our and Grasse et al. (2001) ages from the Snow Lake pendant and our zircon age results from the Benson Lake, May Lake, and Quartzite Peak pendants with reference data from the surrounding “terranes” such as the Roberts Mountains allochthon (Gehrels et al., 2000), the Shoo Fly Complex (Harding et al., 2000), and data from the passive-margin strata of the Mojave region, Nevada, and Sonora, Mexico, described previously herein, suggest that the Shoo Fly Complex and the Roberts Mountains allochthon in general can be excluded as a potential correlation for the Snow Lake block. The Shoo Fly Complex and Roberts Mountains allochthon have distinctive age distribution patterns containing Paleozoic and Precambrian zircon age peaks, which are missing in the Snow Lake strata, confirming the assessment made by Grasse et al. (2001, see also their fig. 3). Phanerozoic zircons are entirely absent, and zircon ages younger than 1 Ga are very rare in the analyzed age spectra of these four pendants.
To quantitatively evaluate our comparison of normalized (relative) age probability plots from the studied metasedimentary pendants to passive-margin strata in the Mojave Desert (Fig. 10), we ran all data sets through three Excel macros that are descriptive methods for comparing zircon ages from different samples. The “K-S (Kolmogorov-Smirnoff)” test evaluates whether two age spectra are statistically similar using the K-S statistics. The “degree of overlap” test measures the degree to which two samples contain the same zircon ages, independent of their relative abundances, whereas the “degree of similarity” test analyzes whether the proportions of overlapping ages are similar (Gehrels, 2000; Arizona LaserChron Center, 2010). Table 2 summarizes the best fit for all metasedimentary samples from the Snow Lake, Benson Lake, May Lake, and Quartzite Peak pendants using all three methods.
We note that careful visual comparisons reveal some mismatch of age peaks (<100 Ma); however, our results are supported by statistical analyses. In general, the zircon data from passive-margin strata break down into two groups: The data for the Zabriskie Quartzite, Wood Canyon Formation, and Stirling Quartzite (ZWS) define three major peaks at ca. 1100 Ma, 1400 Ma, and 1700 Ma. In contrast, the Harmony B, Sierra Lopez, and Eureka quartzites (HSE) define two major peaks at ca. 1850 Ma and 2700 Ma.
Snow Lake, Benson Lake, and Quartzite Peak Pendants
Results of the combined visual and statistical comparisons of data from the Snow Lake, Benson Lake, and Quartzite Peak pendants with detrital zircon age distribution spectra obtained from the region shown in Figure 3 yielded best matching results with the detrital zircon age distribution pattern obtained from Neoproterozoic to Ordovician passive-margin strata of the ZWS (Zabriskie Quartzite, Wood Canyon Formation, and Stirling Quartzite) group (Fig. 10; Table 2). We interpret metasedimentary strata from (1) the Snow Lake pendant as representing the Stirling (Neoproterozoic) through Carrara (Cambrian) formations, supporting Lahren and Schweickert's (1989) hypothesis (Fig. 4), (2) metasedimentary strata at Benson Lake as Neoproterozoic to Cambrian Wood Canyon Formation or Zabriskie Quartzite, and (3) metasedimentary rocks at Quartzite Peak as Cambrian Zabriskie Quartzite (Figs. 10 and 13; Table 2).
May Lake Pendant
The two May Lake pendant samples have visually almost identical zircon age spectra and seem to best match the normalized probability plot for the Eureka Quartzite (Figs. 3, 10, and 13; Tables 1 and 2). However, four other potential tectonic assemblages contain a typical ca. 1.8 Ma peak Peace River Arch signature, which we discuss as a potential match next:
(1) The detrital zircon age spectra for samples of white, clean quartzite from the May Lake pendant closely match detrital zircon age distribution pattern for the Ordovician Eureka Quartzite, similar to the other studied pendants that correlate with passive-margin strata (Fig. 10; Tables 1 and 2). In addition, most of the southern part of the May Lake pendant is composed of marble and calc-silicate layers that have undergone several phases of deformation and folding (Fig. 7). At this point, we can only speculate that these marble outcrops could potentially represent strata of the stratigraphically underlying Lower to Middle Ordovician Pogonip Group or the Upper Ordovician Ely Springs Dolomite (Stevens, 1986; Figs. 10 and 13).
(2) LA-ICP-MS analyses for the Quartzite of Sierra Lopez of Gehrels and Stewart (1998) from Sonora (Mexico) show that this quartzite contains the same zircon age populations as the Eureka Quartzite, thus qualifying as another possible match for May Lake quartzites. However, we also recognize that the 2720 Ma peak is significantly larger and more prominent than in the May Lake zircon age spectra, making the Quartzite of Sierra Lopez a less likely correlation for the May Lake pendant (Fig. 10; Table 2).
(3) The quartzite of the Pinto Mountain Group in the San Bernardino Mountains is also associated with some carbonate rocks (Barth et al., 2009) and has a similar zircon age distribution pattern to the May Lake pendant. However, if May Lake sediments were equivalents of the Pinto Mountain Group, then they would be much older (Mesoproterozoic) than the surrounding pendants. Furthermore, characteristic Pinto Mountain group strata, such as the basal conglomerate and Fe-rich rocks (Barth et al., 2009), are missing at May Lake, making the Pinto Mountain Group a less likely match. However, we cannot completely rule it out due to the good match in detrital zircon age spectra (see fig. 12 inBarth et al., 2009), which could be potentially tested by means of Hf analyses of zircon.
(4) The zircon age patterns from quartzite samples from the May Lake pendant also match reference data from some units within the Roberts Mountains allochthon (mostly with Harmony B, see also Fig. 10; Gehrels et al., 2000). Lahren and Schweickert (1994) reported a potential eugeoclinal assemblage at Sachse Monument pendant just west of the main pendant exposures at Snow Lake, raising the possibility of Roberts Mountains allochthon–type assemblages occurring in western Sierra pendants. However, the quartzites at May Lake are of shallow-water origin and are associated with thick marble layers. Any chert-like deposits are absent.
The statistical tests—especially the K-S test—suggest the best correlation with Harmony B of the Roberts Mountains allochthon (Table 2). Note, however, that the overlap and similarity with Eureka Quartzite are statistically high as well. The difficulty of correlating the May Lake pendant, which shows typical Ordovician detrital zircon age spectra, with any other Ordovician deposits is that during Middle Ordovician time, a highstand sea level and longshore currents deposited sands from the Peace River Arch over a wide area of the Cordilleran margin (Gehrels and Dickinson, 1995). This led most of the Ordovician strata at the Cordilleran margin to have similar detrital zircon age distributions and thus nonunique detrital zircon age spectra characteristic of any specific location in western North America. However, based on the lithologic and stratigraphic characteristics of the May Lake deposits, and since nearby pendants correlate with Neoproterozoic to Cambrian passive-margin strata, we favor the interpretation that the quartzite at May Lake is an equivalent of the Ordovician Eureka Quartzite.
Correlation of the May Lake pendant rocks with Ordovician Eureka Quartzite has significant implications since this correlation places a definite limitation on where the Snow Lake block rocks may be derived from. West of the Sierra Nevada, Eureka Quartzite occurs only north of the Garlock fault and is not exposed anywhere in the Mojave Desert area (including the San Bernardino Mountains) due to nondeposition or pre-Devonian erosion (Burchfiel and Davis, 1981).
In spite of some uncertainties raised by the statistical tests (Table 2), we suggest that, based on the normalized probability plots (Fig. 10) and the geologic relationships and stratigraphy studies (Lahren and Schweickert, 1989), our suggested correlations are robust given the existing database. This also is a good reminder that although the western North American database for detrital zircon ages is growing quickly, we need a larger density of analyzed strata to improve correlations. For example, detrital zircon age spectra from the White and Inyo Mountains are completely missing, which could not be tested as potential correlative units in this study. Data for comparing the Carrara Quartzite from the Snow Lake pendant (SLP-21) with equivalents in the Mojave Desert are also missing.
We conclude that given the current incomplete detrital zircon age data pool, the best existing match of the detrital zircon age spectra for some of the passive-margin strata in the examined pendants remains the passive-margin strata exposed in the San Bernardino Mountains (Fig. 13). For example, the sample Wood Canyon Formation (WC#1) from Stewart et al. (2001) from Pahrump Valley has very different zircon age distributions than the age spectra derived from samples also characterized as Wood Canyon Formation from the San Bernardino Mountains (Stewart et al., 2001; WC#2; Fig. 10). For the May Lake pendant, similarities to detrital zircon age distribution patterns derived from the Eureka Quartzite in the Mojave region are greater than with age spectra from the Ordovician passive-margin Quartzite of Sierra Lopez in Sonora (Mexico). We will, however, be eager to see if equally robust matches occur with other passive-margin strata elsewhere in the Inyo or Death Valley sequences as data from these areas become available, and if we can get more robust correlations by adding geochemical data to the age determinations.
Correlation of the Metasedimentary Pendants at Cinko Lake, Northeastern Snow Lake Area, and Strawberry Tungsten Mine
The metasediments at Cinko Lake, from the Strawberry Mine pendant, and the Jurassic cover sequence at Snow Lake yielded younger zircons of Mesozoic to Paleozoic age. The Cinko Lake sample clearly differs from the other two metasediments, which contain almost entirely Jurassic zircons, therefore implying a Late Jurassic–Early Cretaceous maximum sedimentation age of <145 Ma (Fig. 11). No “Independence dikes,” such as those interpreted to occur in the southwestern part of the Snow Lake pendant (Lahren et al., 1990), were found in the Cinko Lake pendant. The youngest possible deposition age is constrained by intrusive relationships with the 101.8 ± 0.2 Ma Harriet Lake pluton (Fig. 5).
The Strawberry Mine sediments and the Jurassic sediment overlap at Snow Lake have closely matching zircon age spectra with high abundances in mid-Jurassic and Devonian zircons. While the Strawberry Mine metasediment contains a few minor peaks of Precambrian grains, the sample from the Snow Lake cover sequence additionally yielded a major peak at 599 Ma (Table 1; Fig. 11). We interpret the maximum sedimentation age for the Strawberry Mine sediment to be <181 Ma and the minimum age to be 96–103 Ma, the age range of the dated volcanic rocks (Fig. 12). This is consistent with the finding of an Early Jurassic bivalve at Strawberry Mine (Nokleberg, 1981). The Snow Lake cover sequence shows its youngest significant age peak at 372 Ma (Devonian) and contains several single zircons of Early Jurassic (188 Ma) to Carboniferous age. The Early Jurassic zircon age is here also compatible with the age of the marine fossils (K. Ritterbusch, 2009, personal commun.), making the Early Jurassic the maximum and most likely deposition age for the metasedimentary cover sequence at Snow Lake.
We conclude that Jurassic sediments occur in the studied pendants and are thus distinctly different from the much older passive-margin strata, which are shown to be overlain by these younger sediments at Snow Lake. Our data suggest that both the marine metasediments containing fossils at Snow Lake and Strawberry Mine are correlative, representing an Early Jurassic cover sequence. The sediments at Cinko Lake are younger and at most Late Jurassic to Early Cretaceous in age. The Cinko Lake strata could thus be interpreted as the top of the stratigraphic sequence, given the similar lithologies and deformation record that the Cinko Lake pendant share with the cover sequence at Snow Lake and Strawberry Mine. The latter outcrops may be representing the bottom of the cover sequence and thus the oldest part of the section. Alternatively, the Cinko Lake sediments may be unrelated to the Snow Lake or Strawberry Mine cover sequence strata and may represent a tectonic sliver or may be part of the Cretaceous volcanic package to the east (Fig. 5). The differences in detrital zircon age spectra could be explained through either hypothesis: (1) the source for the zircons caught in the Cinko Lake sediment changed through time from the Early to the Late Jurassic–Early Cretaceous compared to the other exposures, or (2) the Cinko Lake sediments are unrelated to the other two cover sequence outcrops and may have been deposited at a different location.
Constraints Based on Deformation Records
All metasedimentary pendants that we tentatively correlate with passive-margin strata (Snow Lake, Benson Lake, May Lake, and Quartzite Peak) are reported to have evidence for four overprinting deformation phases, with a locally occurring fifth deformation phase at Benson Lake and May Lake. However, the Jurassic marine strata at Cinko Lake, Snow Lake, and Strawberry Mine record only three to locally four deformation phases, and the metavolcanic rocks only have regionally one and locally two. This implies that all three belts have internally consistent numbers of deformation phases and that the hypothesis that the Cinko Lake pendant is part of the metavolcanic sequence cannot be correct and instead must be considered as a younger part of the Early Jurassic cover sequence at Snow Lake and Strawberry Mine. Lahren and Schweickert (1989) suggested that the Jurassic cover sequence at Snow Lake was deposited on already deformed passive-margin rocks and then was subsequently deformed with the passive-margin rocks as one sediment package. Alternatively, the passive-margin rocks and the Snow Lake cover sequence could have been juxtaposed after both sequences underwent ductile deformation independently, which would explain some of the characteristics of the contact at Snow Lake (Figs. 9C and 9D). The question of whether or not the contact between both deformed sequences has displacement along it is crucial for the interpretation of the Mojave–Snow Lake fault, and will be discussed later herein.
Are Jurassic Metasediments Correlative to the Fairview Valley Formation Type Locality at Black Mountain?
If the fossil age interpretation and estimated maximum depositional ages for the Jurassic Snow Lake cover sequence at Snow Lake and Strawberry Mine are correct, these sediments were deposited in the Early Jurassic to Late Jurassic–Early Cretaceous after 188 Ma (to as late as <145 Ma, the maximum age at Cinko Lake), rather than in the Triassic, as originally proposed by Lahren and Schweickert (1989). More recent work on the Fairview Valley Formation at the Black Mountain type locality in the San Bernardino Mountains has suggested that the Fairview Valley Formation here also is younger than initially thought, and is likely to have been deposited in the late Early Jurassic (e.g., Schermer et al., 2002). Stone et al. (2005) proposed that the age of the Fairview Valley Formation at Black Mountain is Early Jurassic or younger, generally matching our results in the metasedimentary pendants. However, the rock types and our fossil finds at Snow Lake and the bivalve found at Strawberry Mine suggest that sediments were deposited in a marine environment, whereas the Fairview Valley Formation at Black Mountain contains abundant conglomerate and has been interpreted to be mostly nonmarine (Stone, 2006). No marine fossils have been found in the Fairview Valley Formation except for some reworked Early Triassic conodonts (Stone et al., 2005; Stone, 2006; P. Stone, 2009, personal commun.). Furthermore, we compared our detrital zircon age data from the Sierran pendants with detrital zircon age data of the Fairview Valley Formation at Black Mountain (Stone et al., 2005) and conclude that the two sedimentary sequences are probably unrelated. While the sediments in all three pendants of the Jurassic cover sequence have almost no Proterozoic zircons (Fig. 11), the Fairview Valley Formation is heavily dominated by zircons between 1600 and 1800 Ma. Only one of the three samples studied by Stone et al. (2005) yielded Mesozoic zircons, which are between 190 and 200 Ma and thus slightly older than the youngest zircons from the Jurassic cover sediments near Snow Lake, and none of the Fairview Valley Formation samples shows the 407–409 Ma peak that is revealed in the Strawberry Mine and Snow Lake pendant zircon age spectra, or the 599 Ma peak in the Snow Lake pendant sample (Fig. 11; P. Stone and A. Barth, 2009, personal commun.). Given the paleoenvironmental differences suggested by the lithologic characteristics and marine fossil findings as well as the mismatch of detrital zircon age spectra between Jurassic metasediments in the Sierran pendants and the Fairview Valley Formation type locality, we suggest that these two sedimentary sequences are not correlative.
This conclusion again focuses attention on the importance of the contact between the Snow Lake passive-margin and overlying Jurassic marine strata. At Snow Lake, this contact: (1) represents a time gap of several hundred million years (Figs. 10 and 11); (2) has an inferred “basal conglomerate” that is discontinuous along the contact (Fig. 9C) and formed in a marine section; (3) shows angular steps and contains mostly angular clasts of almost exclusively quartzite composition (Fig. 9D); and (4) shows some deflection of structures near the contact (Fig. 9C), suggesting that the contact may not be a simple depositional unconformity. More speculatively, there is the possibility that the contact at Snow Lake may be a fault contact, which raises an additional speculation of whether this contact could represent the Mojave–Snow Lake fault. Alternatively, if the contact is an unconformity, then either the Snow Lake block was in its present position before the Early Jurassic and this portion of the Sierras was under sea level during the Jurassic, or an Early Jurassic marine overlap sequence should exist in the location where the Snow Lake block originated. In the latter scenario, it is possible that the Mojave–Snow Lake fault is located between the outcrops of the Cinko Lake pendant and the metavolcanic rock package. Clearly, the contact between passive-margin strata and Jurassic sediments at Snow Lake has important implications for the origin, timing, and displacement of the Mojave–Snow Lake fault and warrants further examination.
“Mojave–Snow Lake” Fault Hypothesis
Our results indicate that at the very least the passive-margin metasediment package is clearly displaced from its origin along a fault. However, given the uncertainty of the Fairview Valley Formation constraint, the correlation of the Sierran pendants to the metasedimentary package at Black Mountain by Victorville (e.g., Lahren and Schweickert, 1989) is highly speculative. Thus, the passive-margin strata may be derived from anywhere in the Death Valley (inner shelf) or Inyo (outer shelf) facies strata exposed in central and southern California (Fig. 3). As noted above, either the marine Jurassic cover sequence was deposited on top of the passive-margin section at its location prior to fault displacement, in which case fault movement occurred after the Jurassic, or it was deposited at its current location, and any fault displacement of the passive-margin strata must have been prior to the Early Jurassic. Each scenario has fairly important tectonic implications, including in regard to the required fault displacement.
The amount of displacement on this cryptic fault has always been controversial, in part since in situ passive-margin strata are exposed along the entire length of western North America, and detrital zircon age spectra for passive-margin strata are in a cursory way similar over several hundreds (to thousands?) of kilometers. Farmer et al. (2005), for example, noted how relative probability plots of detrital zircon ages from the miogeocline in Caborca, Sonora, are very similar to the distribution pattern from passive-margin strata in the Mojave region. The closest passive-margin strata to the present-day location of the pendants are southeast of the Sierra Nevada batholith in the White–Inyo Mountains, where the miogeocline of the Inyo facies is composed of a more transitional slope facies exposing thicker horizons of quartzites and other sediment than those exposed in the shallower shelf (Death Valley facies; Stewart, 1970; Fig. 3). However, detrital zircon data from the White–Inyo Mountains are currently lacking, and it is thus not possible to make any statistical comparisons of detrital zircon populations. Our new interpretation of the Jurassic metasediments overlying the passive-margin strata implies that dextral northwestward displacement of the Snow Lake block by 400–500 km from the San Bernardino Mountains area as suggested by Lahren and Schweickert (1989), Lahren et al. (1990), and Grasse et al. (2001) is no longer required. Most other studies have suggested a smaller amount of strike-slip displacement along the Mojave–Snow Lake fault/AIB/IBB2 of ∼200 km (e.g., Saleeby and Busby, 1993; Kistler, 1993; Lewis and Girty, 2001).
Another complication is that a plethora of geophysical and mantle xenolith data show that the western Mojave region, including the San Bernardino and Shadow Mountains, is allochthonous, resting on Rand-Pelona schist, which in turn rests on underplated Farallon abyssal peridotites, supporting their displacement on regional low-angle structures in the Late Cretaceous–earliest Tertiary (Luffi et al., 2009). These low-angle displacements were additionally accompanied by ∼100 km of dextral displacement on the integrated Kern Canyon–Owens Valley transfer system (Nadin and Saleeby, 2008; Figs. 1 and 3). This suggests that working out any amount of displacement along the Mojave–Snow Lake fault between the allochthonous Snow Lake block with the allochthonous San Bernardino Mountains (even if they are correlative) is rather challenging.
Location of the Missing Fault in the Central Sierra Nevada and Timing of Displacement
Our new data in the central Sierra support the presence of three belts of rocks (passive margin, Jurassic marine sediments, and Cretaceous volcanic rocks) separated by age gaps of 40–100 m.y. These observations draw attention to the possibility that both “time gaps” may be representing faults, one of which must be the missing Mojave–Snow Lake fault. If the Jurassic overlap sequence (with the metasedimentary exposures at Cinko Lake) is unconformably overlying the passive-margin rocks, then the fault would need to be east of the overlap sequence, and potentially buried under the Cretaceous volcanic rocks, but west of the Paleozoic eugeoclinal metasediments of the Roberts Mountain allochthon (Fig. 2, area defined by dashed lines). Consequently, the location of the fault must have been located east of the Cinko Lake pendant, where a sharp break occurs between the Jurassic cover sequence and the volcanic rocks in the Piute Meadows pendant (Figs. 2 and 5). The border between these two pendants is intruded by the 102 Ma Harriet Lake granodiorite to the south, which is the oldest known pluton in the Cinko Lake area, the 95 Ma Cinko Lake quartz diorite, and the younger (<95 Ma) Fremont Lake granodiorite to the north. Detailed mapping by Thompson et al. (2007) in these plutons and surrounding metasedimentary and metavolcanic pendants showed no evidence for a large-scale dextral shear zone or fault separating the different sedimentary facies (Fig. 5). If a fault ever existed here, it has been obliterated by subsequent pluton intrusion and emplacement related deformation of the pendants.
Lahren et al. (1990) reported an age of ca. 150 Ma from mafic dikes and associated gabbroic complexes in the southwestern part of the Snow Lake pendant (interpreted as Independence dikes) that are inferred to predate displacement along the Mojave–Snow Lake fault. This, however, is inconsistent with our interpretation of cover sequence sedimentation in the Early Jurassic through Early Cretaceous (<145 Ma) because the timing of sedimentation of the uppermost part of the sequence is younger than the timing of the dated intrusions cutting across the passive-margin strata. This observation raises the following issues: (1) the dikes shown to intrude the cover sequence at Snow Lake (Fig. 4) must be younger and derived from a different source (nearby plutons?) than the dated ones intruding the passive-margin sediments in the southwestern part of the Snow Lake pendant. This could be tested through additional geochronology. (2) The “unconformity” of Lahren and Schweickert (1989) may in this case represent the Mojave–Snow Lake fault itself, agreeing with other observations (e.g., angular quartzitic clasts at base of “unconformity”). To exactly pinpoint the location of the Mojave–Snow Lake fault, we need to further examine the “unconformity” at Snow Lake, date the dikes intruding the cover sequence in the northeastern part of the Snow Lake pendant, and/or determine their structural relationship to nearby plutons.
If the trace of the Mojave–Snow Lake fault is buried east of the Cinko Lake metasediments, fault activity must have occurred between <145 Ma, the maximum deposition age of the Cinko Lake metasediments, and 101.8 Ma ± 0.2 Ma, the intrusion age of the Harriet Lake pluton (Fig. 5B). The age of the 103–108 Ma metavolcanic rocks just east of the Cinko Lake metasediments and the Harriet Lake pluton intruding the metasediments defines the youngest age limit for fault motion in this area, since neither shows evidence of dextral strike-slip motion (Foley et al., 2007). If the second hypothesis is correct and the trace of the Mojave–Snow Lake fault is located between the passive-margin deposits and the Jurassic cover sequence in the Snow Lake pendant, the fault must have operated after 145 Ma, the youngest maximum sediment deposition age at Cinko Lake, and ceased its activity before the intrusion of the Granite of Bond Pass and the 87–86 Ma Cathedral Peak granodiorite of the Tuolumne batholith (Wahrhaftig, 2000; Memeti et al., 2010). Conclusively, we suggest that based on our current data sets, the Mojave–Snow Lake fault had an approximate maximum time of activity between 145 Ma and 102 Ma, or >87 m.y., respectively (Fig. 5B).
SUMMARY AND CONCLUSIONS
Our new mapping and detrital zircon provenance study on quartzites of the Snow Lake, Benson Lake, May Lake, and Quartzite Peak pendants are most consistent with a correlation to Neoproterozoic to Ordovician passive-margin strata, specifically, the Stirling Quartzite, the Wood Canyon Formation, the Zabriskie Quartzite, the Carrara Formation, and the Eureka Quartzite. This interpretation is tentative, since it is based on only a few detrital zircon age spectra that were available for comparison from the detrital zircon age data pool for the extensive miogeocline in western North America. Age data from the White–Inyo Mountains, for example, are unavailable and may be an equally good match. The fact that remnants of passive-margin strata are exposed in pendants within the central part of the Sierra Nevada batholith, sandwiched between belts of Paleozoic basinal metasediments, makes the existence of a dextral strike-slip fault (and, most likely, a second fault of uncertain kinematics, since faults are required on each side of the Snow Lake block) highly likely.
Furthermore, we have evidence due to a recent marine fossil find in the Snow Lake pendant and detrital zircon age spectra from the Cinko Lake, the Snow Lake, and Strawberry Mine pendant that the metasediments referred to as the Snow Lake cover sequence are different from the Jurassic, nonmarine Fairview Valley Formation at Black Mountain, the type locality to which all the metasedimentary pendants of the Snow Lake block were previously correlated (e.g., Lahren and Schweickert, 1989). Instead, these sediments are Early to Late Jurassic–Early Cretaceous in age, of marine nature, and contain Devonian detrital zircon age peaks that may be a Sierran or perhaps an Antler phenomenon. Thus, a direct correlation of this unit to its location of origin remains a work in progress, but this unit may potentially correlate with other Jurassic marine sediments in the central and eastern Sierra Nevada.
Our conclusion that the Jurassic cover sequence is not the Fairview Valley Formation in the Black Mountain area by Victorville implies that the amount of 400 km of dextral strike-slip displacement is no longer needed, given that passive-margin rocks are widespread and the marine Jurassic metasediments may be derived from the central or eastern Sierra. Furthermore, the amount of displacement implied by correlations to passive-margin sedimentary sections in the San Bernardino Mountains is also speculative due to at least two concerns: (1) recent studies that consider the San Bernardino Mountains, the potential source of the pendant rocks, to be an allochthonous “terrane,” and (2) the amount of detrital zircon age data that can be used to match stratigraphic equivalents in other areas, which is too small to allow interpretations with some degree of certainty.
We have also noted that two contacts with large time gaps occur in the central Sierran pendants (between miogeocline and Jurassic cover sequence, and between cover sequence and Cretaceous metavolcanic rocks), providing an incentive to closely reexamine these contacts, including the proposed angular unconformity in the Snow Lake pendant. If we continue to accept the current interpretation that Jurassic sediments and rocks of the miogeocline are separated by a depositional contact (and not a fault contact) in the Snow Lake pendant, then we have narrowed down the location to a <10-km-wide belt. Detrital zircon spectra compared to previous TIMS ages on “Independence dikes” and field relationships suggest that the unconformity-declared contact at Snow Lake may potentially represent the missing Mojave–Snow Lake fault instead, and/or represents another fault. The timing for displacement along the trace of the Mojave–Snow Lake fault ranges between ca. 145 and 102 Ma for either location. If the Mojave–Snow Lake fault existed at the eastern location, it now is covered by Cretaceous volcanic rocks and subsequently intruded plutons. The fact that the trace of Mojave–Snow Lake fault has not been found in the field is not surprising considering the immense volume of magma emplaced in the Sierra Nevada batholith during the Mesozoic, and particularly in the Cretaceous. The fault has probably been obliterated by pluton emplacement, overprinted by subsequent deformation after the fault ceased, and/or buried during deposition of Cretaceous volcaniclastic material.
This study has implications not only for the history, location, and timing of the required dextral strike-slip faults, but for the origin of other Sierran metasedimentary pendants within the Sierran magmatic arc farther south, which have been grouped into the Kings Sequence, and more recently to the Snow Lake block. It also focuses attention on whether additional exposures of passive-margin rocks, the Jurassic marine cover sequence, and Cretaceous volcanic rocks occur in the southern Sierra (e.g., Saleeby et al., 2007). More detailed studies, including detrital zircon provenance studies of the pendants and their areas of potential origin, are underway in these areas.
This project was supported by National Science Foundation grant EAR-0537892 and U.S. Geological Survey EDMAP grants to Paterson, and the Department of Earth Sciences at the University of Southern California. Many thanks are due to Bob Miller for providing the quartzite sample from Quartzite Peak and information about the pendant, and the Arizona LaserChron Center, especially Victor Valencia and Alex Pullen, University of Arizona, for use of the LA-ICP-MS facilities and help in conducting the analyses. Roland Mundil and Jenny Matzel greatly contributed to this study financially and by providing laboratory facilities at the Berkeley Geochronology Center for obtaining precise U/Pb zircon CA-TIMS ages for the Cinko Lake quartz diorite and the Harriet Lake pluton. We thank Kathleen Ritterbusch and David Bottjer, University of Southern California, for providing a fossil identification and age estimate for the fossils found at Snow Lake. The detailed geologic mapping of the Benson Lake and May Lake pendants could not have been realized without the help of Rita Economos, Saskia Erdmann, Robert Miller, and field assistants Gayle Hough and Claire Wilke. University of Southern California undergraduate team research students, under the supervision of Scott Paterson, Lawford Anderson, and Geoff Pignotta, helped greatly in our “search for the Mojave–Snow Lake fault” during field mapping of the Cinko Lake area in summer of 2006, producing the geologic map in Figure 5. We thank Paul Stone and an anonymous reviewer for detailed reviews on the manuscript, and Jason Saleeby, Jim Wright, and Rich Schweickert for reviews of an earlier version of this paper. Andy Barth, Cal Stevens, Frank Corsetti, Greg Davis, and Marty Grove are thanked for stimulating discussions about detrital zircon provenance analyses, passive-margin strata in the Mojave Desert, and the Mojave–Snow Lake fault.