Insights from the Alabama Hills into Mesozoic Magmatism and Tectonics in Eastern California

Since the Triassic, the Sierra Nevada batholith and Owens Valley in eastern California have experienced a long and varied history of dextral and sinistral shear, compression, and extension [1–8]. Modern deformation is localized in the complex Owens Valley fault zone on the western margin of the Basin and Range Province, where recent earthquakes have included both dominantly right-lateral (1872 M_W 7.5–7.9 Owens Valley event; [9, 10]) and dominantly normal displacement events (2020 M_W 5.8 Lone Pine event; [11]). However, it has been more difficult to decipher the degree of deformation in Owens Valley’s geologic past despite excellent bedrock exposures on both sides of the valley.

Initial correlations of bedrock units in Owens Valley relied on a partial understanding of those units’ distributions and structure. Early work on the distinctive Independence dike swarm, which is present on both sides of Owens Valley, suggested that there has been at most a few km of lateral offset because the dike swarm appeared to strike across the valley uninterrupted [12]. Jurassic plutons were identified at similar latitudes in the ranges on both sides of the valley, suggesting little offset [13]. However, later work revealed the Independence dike swarm to be regionally extensive and more compositionally diverse than Moore and Hopson’s [12] assessment [14–16]. In addition, Jurassic plutons were later found to be common on both sides of Owens Valley such that they could not be distinctive piercing points for determining offset (e.g., [17, 18]). Therefore, early correlations across the valley were incomplete or nonunique. Recent workers have instead found the magnitude of dextral offset across Owens Valley to be on the order of 65–75 km. Stevens et al. [5] correlated Devonian submarine channel deposits in the Sierra Nevada and the Inyo Mountains, suggesting 65 km of dextral offset across the northern Owens Valley. The most distinctive marker across Owens Valley, the approximately 83.5 Ma Golden Bear-Coso dikes, also supports 65 km of dextral offset (Figure 1) [19].

Despite advances in correlating other units across Owens Valley and the similarities in the magnitudes of their offset (i.e., 65–75 km), the Independence dike swarm has remained a rather enigmatic marker, although the swarm has been much better characterized recently. A close examination of the dike swarm’s structure revealed systematic patterns in cumulative dike thicknesses at outcrops in the Sierra Nevada and the White, Inyo, and Coso Ranges [14]. The measurements revealed that the greatest dilations of wall rocks by diking are in the southern Coso Range (north of Volcano Mtn.; Figure 1), the northern Alabama Hills, and the eastern Sierra Nevada batholith near Mt. Prater [14]. The Coso Range and Alabama Hills dike localities are approximately 75 km apart, broadly in agreement with other offset markers across the valley. However, the Alabama Hills and Mt. Prater localities are themselves separated by about 55 km, suggesting the Alabama Hills block could be offset relative to the Sierra Nevada batholith, thus allowing for as much as 130 km of cumulative offset across Owens Valley since the Late Jurassic [14].

A large amount of dextral offset between the Alabama Hills and the Sierra Nevada batholith based on the Independence dike swarm introduces mismatches for other bedrock units in the Alabama Hills and Sierra Nevada. Middle Jurassic metavolcanic rocks exposed in the northern Alabama Hills are interpreted to belong to a linear belt of similar rocks extending into the Sierra Nevada, suggesting little offset between the two blocks (Figure 1) [20]. The Alabama Hills Granite (85 Ma; [21]) has been linked petrographically and geochronologically to the adjacent Mount Whitney Intrusive Suite (MWIS) in the batholith (88–83 Ma; [21–25]), implying no lateral offset between the Alabama Hills block and the batholith since the Late Cretaceous. However, the petrographic link is nonunique (c.f., [26]), and the U-Pb geochronologic data are multigrain zircon analyses with no physical or chemical abrasion pretreatment. This limits the precision and accuracy of the U-Pb data as the bulk fractions could be compromised by inheritance of xenocrystic grains or by unmitigated Pb-loss, as observed elsewhere in the Sierra Nevada batholith (e.g., [27, 28]).

To address the relationship between the Alabama Hills and the Sierra Nevada batholith and assess the potential for offset between them, we present new chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon geochronology and whole rock chemical and radiogenic isotopic analyses. The data permit correlation of the Alabama Hills Granite with nearby plutons in the eastern Sierra Nevada batholith. Similar geochemical and geochronologic characteristics for the newly-recognized Kearsarge plutons allow us to assess dextral offset between the Sierra Nevada block, the Alabama Hills, and rocks on the eastern side of Owens Valley. The data also have implications for the distribution and style of middle Cretaceous magmatism in the Sierra Nevada.

Cretaceous plutonic rocks in the eastern Sierra Nevada batholith are characterized by individual plutons and large, zoned intrusive suites that are part of the Sierra crest magmatic event [17, 29]. The batholith experienced rapid growth from 98-83 Ma through the assembly of multiple 800–1500 km² suites, with each suite composed of older granodiorites and younger granites [21, 29–33]. The MWIS follows this pattern; the marginal granodiorites of Lone Pine Creek and Sugarloaf have yielded bulk zircon U-Pb ID-TIMS ages of ~88–87 Ma, whereas the more silicic Paradise and Whitney plutons in the center of the suite were slightly younger (86–83 Ma; [21, 34]). Zircon U-Pb analyses by CA-ID-TIMS have broadly corroborated the age pattern of Chen and Moore [21], although mitigating for Pb-loss has shifted the age range to be slightly older: 90.6–84.8 Ma (Figure 2; [28, 31]).

Smaller plutons are interspersed among the large Late Cretaceous suites in the eastern part of the batholith, especially north and east of the MWIS (Figures 1 and 2). Most plutonic rocks outside the named Cretaceous suites in the eastern Sierra Nevada are Cretaceous or have been interpreted as Cretaceous because they are not cut by the Jurassic Independence dike swarm [35]. Some smaller individual plutons, such as the ca. 95 Ma McDoogle pluton [36] and the ca. 92 Ma mafic complex of Onion Valley [37, 38], were intruded during the Sierra crest magmatic event and are contemporaneous with the large-zoned suites [36, 37]. Others, however, are older than the Late Cretaceous magmatic event, including the Bullfrog pluton exposed along the Sierra crest north and east of the MWIS (Figure 2). Bulk U-Pb zircon ID-TIMS geochronology suggested an age of 103 Ma for the Bullfrog pluton [21]; one single-grain CA-ID-TIMS analysis yielded an age of $100.2±0.3Ma$ [31]. On the Sierran range front, the granitic Independence pluton has yielded ages of 112 Ma by bulk zircon geochronology [21] and $102.5±0.2Ma$ by physical abrasion zircon U-Pb ID-TIMS [19]. Several intrusions exposed between the Bullfrog and Independence plutons, including the granitic Diamond and Sardine plutons and the granodioritic Dragon pluton, have approximate ages from 106 to 102 Ma acquired by bulk ID-TIMS (Figure 2; [21, 34, 35, 39]). Numerous other nearby plutons are presumed to be Cretaceous but are not dated [35].

The Alabama Hills Granite, located in Owens Valley about 3 km east of the Late Cretaceous MWIS, has also yielded Late Cretaceous ages (Figure 2) [21]. The pluton is an equigranular granite with a color index below 10 [25, 26] exposed as distinctive inselbergs and ridges over approximately 30 km². One sample dated by bulk zircon ID-TIMS U-Pb geochronology yielded a concordant age of 85 Ma [21], and another sample gave a biotite K-Ar date of 82 Ma [23]. On its eastern margin, the Alabama Hills Granite intrudes metamorphosed Middle Jurassic volcanic rocks [20, 25, 26] that are pervasively intruded by the Late Jurassic Independence dike swarm (~148 Ma; [40, 41]). The Alabama Hills Granite is not cut by the dike swarm, consistent with a Cretaceous age for the granite.

Several regional and local structures bear on the history of the tectonic block whose only bedrock exposure is in the Alabama Hills [42–44]. Long-lived contractional deformation occurred on the eastern edge of the batholith from before 188 Ma to after 140 Ma, accommodated by the 150 km-long east Sierran thrust system [5, 45]. The east Sierran thrust system is well documented in the southern Inyo Mountains, where it is 18 km wide, and may also be present in metamorphosed volcanic rocks in the far northeast part of the Alabama Hills [45]. Dikes of the Late Jurassic Independence dike swarm record synmagmatic sinistral shear, suggesting the swarm invaded a regional sinistral shear system ca. 148 Ma [3]. This agrees with hot spot reconstructions showing left-oblique North America–Farallon convergence in the Jurassic [2].

There are several ductile shear zones recognized in and near the field area dating from the Early and/or Late Cretaceous. Near the Sierra Nevada crest, the steeply dipping Sawmill Lake shear zone records east-side-up, east-west contraction along a ~35–50 km-long structure [36, 46]. The available evidence suggests the shear zone postdates Late Jurassic Independence dikes and was active until assembly of the late synkinematic McDoogle pluton ca. 95 Ma [36, 46]. Similar fabrics in the Oak Creek pendant may correlate with the Sawmill Lake shear zone and are interpreted to postdate 110 Ma [39]. The Sawmill Lake shear zone may be a younger expression of deformation related to the east Sierran thrust system [36, 46].

Dextral ductile deformation has been documented near the field area in the transpressional Santa Rita Flat shear zone and the proto-Kern Canyon fault zone. The Santa Rita Flat shear zone is exposed on the east side of Owens Valley at the base of the Inyo Mountains (Figure 1); it is interpreted to have been active during the Late Cretaceous [47]. Bartley et al. [14] suggested the Santa Rita Flat shear zone belongs to a regionally extensive series of dextral-transpressive ductile shear zones (e.g., [48]) active from 80 to 70 Ma. They hypothesized that these shear zones accommodated ~65 km of dextral shear across Owens Valley; this structure has also been called the Tinemaha-proto Owens Valley fault [44]. Within the Sierra Nevada batholith, the proto-Kern Canyon fault zone is a complex feature striking north-south for 130 km, terminating in the north at the MWIS (Figure 1). However, its significance to this study’s field area may be small owing to the near-zero displacements at its northern terminus [49].

The modern Owens Valley hosts three major faults—the Sierra Nevada frontal fault, the Owens Valley fault, and the West Inyo fault—that have been interpreted as initiating deformation in the middle Miocene (e.g., [50]), although estimates for activation on these faults vary from early Eocene to the late Miocene [51, 52]. The Sierra Nevada frontal fault, Owens Valley fault, and smaller Lone Pine fault bounding the east side of the Alabama Hills together accommodate dextral-dominated transtensional strain, similar to the strain pattern in the broader eastern California shear zone [4, 53]. Apatite (U-Th)/He thermochronology suggests that the Alabama Hills block has been displaced ~2.6 km vertically from Mt. Whitney along the Sierra Nevada frontal fault since the early Miocene [22, 52]. Approximately 10 km of dextral offset in Owens Valley is estimated to have accumulated since the late Pliocene [1, 14].

Six samples were collected from the Alabama Hills for geochronological and geochemical analyses (Table 1; Figure 2). Sample AH14-01 was collected from a diabase dike in the northern part of the volcanic complex of the Alabama Hills. Samples AH14-03, RP15-01, RP15-03, and RP15-04 were taken from the main body of the Alabama Hills Granite with the aim of maximizing spatial distribution of samples. Samples are equigranular, although RP15-03 and RP15-04 featured alkali feldspar crystals up to 1 cm long with rapakivi texture. Sample RP15-02 was collected from a prominent hill mapped as a fine-grained facies of the Alabama Hills Granite [26]. Only fresh samples were used for whole rock geochemical and isotopic analyses.

We analyzed two samples from the central part of the Bullfrog pluton collected by Wenner and Coleman [54]. Both are leucocratic granites. Sample GP98-5 was used for U-Pb zircon CA-ID-TIMS geochronology, and sample KL98-1 was analyzed for whole rock Sr isotopic composition.

Geochronology was performed by CA-ID-TIMS on either a VG Sector-54 or IsotopX Phoenix-X62 in the Department of Earth, Marine, and Environmental Sciences at the University of North Carolina at Chapel Hill following the sample preparation and analytical methods of Frazer et al. [27] and Gaynor et al. [55]. Zircons for each sample were thermally annealed for 48 hours at 900°C then chemically abraded in HF and HNO₃ acids for 16 hours at 220°C [28, 56]. Individual abraded grains were selected under a binocular microscope for dissolution and isotope dilution using an in-house mixed ²⁰⁵Pb-²³³U-²³⁶U tracer (GS-1) after Parrish and Krogh [57]. Dissolution and chemical purification methods for U and Pb were modified after Krogh [58] and Parrish [59]. Data processing and age calculations were completed using Tripoli and ET_Redux [60, 61] and include 2σ analytical uncertainties only. Corrections for initial Th/U disequilibrium were made in ET_Redux (e.g., [62, 63]). Corrections for samples RP15-01 through RP15-04 were made assuming the measured whole rock Th/U value for each sample approximates the magmatic Th/U value; this adjustment increased the ages of individual zircons by up to 100 ka. Sample GP98-5 was corrected using published whole rock Th/U data [54]. Samples without compositional data were corrected with the following Th/U values and rationales: AH14-01 : 3.5 (average of Independence dike samples AH95-8B, AH95-8C, and ID95-11 of Glazner et al. [16]; AH14-03 : 4.4 (average of samples RP15-01, RP15-03, and RP15-04 from this study)).

Whole rock aliquots of samples RP15-01 through RP15-04 were crushed to powder in a SPEX Shatterbox® alumina swing mill and analyzed at Actlabs (Ontario, Canada). Samples were dissolved by fusion in a lithium metaborate/tetraborate mixture. Major elements and Ba, Sr, Y, Zr, Sc, Be, and V were analyzed by inductively-coupled plasma-optical emission spectroscopy (ICP-OES), with remaining trace elements and rare earth elements (REE) analyzed by ICP-MS. Uncertainties (±2σ relative) for major elements were less than 2% for all oxides except MgO (3%), MnO (5%), and P₂O₅ (16%). Trace elements analyzed at Actlabs that are reproducible at better than 2 ppm (±2σ absolute) include Ag, Be, Cs, Ge, Hf, Sb, Sn, Ta, Th, Tl, U, and W. Other elements are reproducible within 3 ppm (Co, Mo, Nb, and Sc) and 4 ppm (Ga, Y), and others have higher uncertainties, in parentheses in ppm: Pb (9); Cu (14); Rb and Zr (15); V (19); Ni (23); Sr (25); Ba (31); Zn (42); Cr (49). Rare earth elements reproducible (±2σ absolute) at 0.2 ppm or better are Eu, Ho, Lu, Tb, and Tm; REE reproducible at 0.2-0.5 ppm or better are Dy, Er, Gd, Pr, Sm, and Yb; others are reproducible at higher uncertainties, in ppm: Nd (1.2); La (1.5); Ce (2.1).

Whole rock isotope geochemistry analyses involved dissolving powders in HF and HNO₃ in Teflon® (Parr) dissolution vessels at 180°C for 48-72 hours. Following dissolution, samples were dried down then fluxed in 6 M HCl for 24 hours. Samples were subsequently aliquoted for Sr and Nd purification by ion exchange column chromatography. Strontium was purified using Sr-spec cation exchange resin after the methods of Lundblad [64] and loaded on single Re filaments with a TaF₅-H₃PO₄ mixture. Neodymium was purified through a three-stage column chemistry procedure after Harvey and Baxter [65] and loaded on single Re filaments in a Ta₂O₅-H₃PO₄ slurry. Strontium isotopic analyses were accomplished on a VG Sector-54 TIMS, and Nd isotopes were analyzed on an IsotopX Phoenix X62 TIMS at the University of North Carolina at Chapel Hill. Strontium was analyzed as a metal in dynamic multicollector mode with $Sr88=3V$ (10¹¹ Ω resistors); Nd was analyzed as an oxide in dynamic multicollector mode with $Nd142O16=1V$ (10¹¹ Ω resistors). Strontium and Nd isotope ratios were corrected for mass fractionation assuming exponential fractionation behavior; Sr isotopic ratios were normalized to $Sr86/Sr88=0.1194$⁠; Nd isotopic ratios were normalized to $Nd146/Nd144=0.7219$⁠. Replicate analyses of the NBS 987 Sr standard yielded $Sr87/Sr86=0.710265±0.000015$ (2σ; $n=26$⁠). Replicate analyses of the Nd standard JNdi yielded $143Nd/144Nd=0.512098±0.000012$ (2σ; $n=18$⁠). Isotopic data for the Alabama Hills samples were corrected to initial values using elemental concentration values and an initial age of 102 Ma (this study). Initial ⁸⁷Sr/⁸⁶Sr for sample KL98-1 of the Bullfrog pluton was calculated using published Rb and Sr concentration data [54] and an initial age of 101 Ma. Initial Nd isotopic values (εNd_i) relative to chondritic uniform reservoir (CHUR) were calculated assuming present day $Nd143/Nd144CHUR=0.512638$ and $Sm147/Nd144CHUR=0.1967$⁠.

Sample AH14-01 was collected from a diabase dike that cuts the metavolcanic rocks in the northern Alabama Hills. Four concordant zircon analyses yield a total 2σ analytical uncertainty range (e.g., [55]) of 152.0–150.7 Ma (Table 2). Zircon U-Pb analyses from the Alabama Hills Granite (AH14-03, RP15-01, RP15-03, and RP15-04) and Bullfrog pluton (GP98-5) are concordant within uncertainty. Sample AH14-03 had the oldest age spectrum, with a range of 103.1–102.2 Ma (⁠$n=7$⁠). Seven zircons from sample RP15-03 yielded a nearly identical spectrum (103.0–102.1 Ma). Sample RP15-04 ranges 102.8–102.0 Ma (⁠$n=4$⁠). Sample RP15-01 has three overlapping grains (102.6–101.2 Ma), although that range is encompassed by one zircon with high analytical uncertainty (Figure 3). One younger grain from RP15-01 has an age of $99.5±0.3Ma$⁠. Excluding the two youngest grains from sample RP15-01, 20 individual zircon analyses from the Alabama Hills Granite span 1.3 Ma from 103.1 to 101.8 Ma. Seven zircons analyzed from Bullfrog pluton sample GP98-5 yield a range of 101.3–100.5 Ma.

Samples of Alabama Hills Granite have 73–77 wt% SiO₂ and a narrow range of major element concentrations, with the exception of sample RP15-02 from the fine-grained facies, which has lower Fe₂O₃ (total), MnO, MgO, CaO, TiO₂, and higher K₂O (Table 3). Trace element concentrations are variable (Figure 4). Relative to the equigranular facies, sample RP15-02 is depleted in Ba, Sr, Y, and Zr and enriched in Rb and U. All samples contain more than 13 ppm Nb and have Y/Sr ratios above 0.1 (Figure 4).

Samples from the equigranular facies of the Alabama Hills Granite have La_N/Lu_N ratios near 10 and Gd_N/Lu_N near 1 (Figure 5), resulting in relatively flat chondrite-normalized heavy REE patterns (subscript “N” indicates concentration normalized to chondritic values of McDonough and Sun [66]). The same samples also have $Eu/Eu∗$ (calculated as $EuN/SmN×GdN1/2≈0.4$⁠, whereas sample RP15-02 has no discernable Eu anomaly. Sample RP15-02 has low REE concentrations and a distinct scoop-shaped REE pattern with low La_N/Lu_N and Gd_N/Lu_N.

The 2σ analytical uncertainties of whole rock samples were smaller than the 2σ reproducibility of replicate aliquots of Sr and Nd isotopic standards (Table 4). Thus, the 2σ reproducibility of the standards is used for sample comparison. Initial Sr isotopic ratios (⁸⁷Sr/⁸⁶Sr_i) among the Alabama Hills Granite samples range from 0.703025 to 0.705816. The equigranular samples have more restricted ⁸⁷Sr/⁸⁶Sr_i values (0.705167 to 0.705816), whereas RP15-02, with $Rb/Sr=11.75$⁠, yielded $S87r/S86ri=0.703025$⁠. Sample KL98-1 from the Bullfrog pluton has $S87r/S86ri=0.705826$ and is indistinguishable from Alabama Hills Granite sample RP15-01 within uncertainty. Initial Nd isotopic compositions from four Alabama Hills Granite samples are restricted (⁠$εNdi=−2.4$ to -3.1), with samples RP15-01, RP15-03, and RP15-04 indistinguishable within uncertainty (Table 4; Figure 6).

Modern CA-ID-TIMS zircon U-Pb data for intrusive rocks have become difficult to interpret geologically, especially without additional information such as zircon trace element or Hf isotopic data (e.g., [67, 68]). However, our interpretations do not depend on calculated weighted mean ages to define precise temporal distinctions between samples. Weighted mean ages are subject to complications including subtle inheritance of antecrysts (e.g., [69]) and Pb-loss that has not been completely mitigated by chemical abrasion (e.g., [27, 55, 70]). Therefore, we use the total 2σ analytical uncertainty range of Th-corrected ²⁰⁶Pb/²³⁸U ages from individual samples and assume the age range captures the time during which the magmas solidified (e.g., [55]; Table 1; Figure 3).

Zircon U-Pb data for the Bullfrog pluton and a diabase dike in the northern Alabama Hills are similar to previously published data for those units. The data for the dike indicate that it is part of the Late Jurassic Independence dike swarm, which has a nominal age of approximately 148 Ma and may contain dikes as old as $152±3Ma$ ([41]; unpub. data by J.M. Mattinson, E.R. Schermer, and C.J. Busby cited in [71]). Our age range for the Bullfrog pluton (101.3–100.5 Ma) is slightly younger than the legacy 103 Ma bulk zircon ID-TIMS age [21] and is similar to a single grain analysis of $100.2±0.3Ma$ by CA-ID-TIMS [31].

New data for the Alabama Hills Granite indicate that it was assembled from approximately 103–102 Ma, with one $99.5±0.3Ma$ zircon from sample RP15-01 that we interpret to reflect Pb-loss (Figure 3). The new age is 17–18 Ma older than the legacy 85 Ma age and suggests the young age obtained for bulk zircon fractions by Chen and Moore [21] was due to pervasive, unmitigated Pb-loss. Zircon Pb-loss in bulk ID-TIMS data has been recognized using modern techniques elsewhere in the Sierra Nevada batholith (e.g., [27, 72, 73]). However, there are not any clear commonalities between these geographically and compositionally distinct units, suggesting that the fidelity of legacy bulk zircon data can only be assessed by redating those same units using modern methods. Geochronology performed on bulk zircon fractions should be used cautiously in interpretations of crystallization ages.

Each bedrock map unit exposed in the Alabama Hills can be used as a piercing point to interpret potential offset between the Alabama Hills block and adjacent bedrock exposures in the Sierra Nevada. Bartley et al. [14] allowed the possibility that the Late Jurassic Independence dike swarm was dextrally offset by up to 55 km between the Alabama Hills and the Sierra Nevada batholith (Figure 1). However, Dunne et al. [20] assigned the Middle Jurassic metavolcanic rocks in the Alabama Hills, which are intruded by the dikes, to a linear belt of “arc-core” rocks that extends into the Sierra Nevada batholith, implying the Alabama Hills block has been stationary with respect to the batholith since approximately 170 Ma [74]. Similarly, the previously published age of 85 Ma for the Alabama Hills Granite led Ali et al. [22] to conclude the pluton was part of the adjacent MWIS [21], suggesting little or no offset. Our new data for the Alabama Hills Granite and Bullfrog pluton allow us to refine the position of the Alabama Hills block relative to the Sierra Nevada batholith during the Mesozoic.

The zircon U-Pb data for the Alabama Hills Granite demonstrate that it is substantially older than the MWIS as dated by CA-ID-TIMS (Figure 2; 90.6–84.8 Ma; [28, 31]). This precludes a genetic link between them. Instead, the Alabama Hills Granite has similar zircon U-Pb ages and trace element and isotope geochemistry to the Bullfrog and Independence plutons (Figures 4–6), suggesting they were generated broadly contemporaneously from a similar source in the middle Cretaceous. A genetic connection between these plutons also seems likely because their ages, geochemistry, and isotopic compositions are atypical of other Cretaceous plutonic rocks exposed along the eastern margin of the Sierra Nevada batholith (Figures 2 and 4–6).

The Alabama Hills Granite is presently exposed 10-30 km south of the bulk of the Bullfrog and Independence plutons, allowing up to 30 km of dextral offset between the Alabama Hills block and the Sierra Nevada batholith since 103 Ma (Figures 1 and 2). However, the 83.5 Ma Golden Bear granitic dike is exposed 10 km north of the Alabama Hills in the Bullfrog and Independence plutons [19, 34, 35], but it is not exposed in the Alabama Hills, suggesting the Alabama Hills block is offset 10 km or less relative to the rest of the batholith (Figures 1 and 2). Large-scale, middle-to-Late Cretaceous dextral shear zones that should be present to accommodate transport of the Alabama Hills block are also not observed on the Sierran range front but are well-documented within the interior of the batholith (e.g., [7, 75, 76]). Thus, we suggest that dextral offset between the Alabama Hills block and the main batholith since 103 Ma is probably limited to 10 km or less.

Our suggestion that the Alabama Hills block has moved less than 10 km since 103 Ma does not preclude earlier dextral offset between the batholith and the Alabama Hills allowed by potential offset of the Late Jurassic Independence dike swarm [14]. If such offset occurred, the data suggest it must have accumulated between 151 and 103 Ma. However, this also seems unlikely because there are no Late Jurassic to Early Cretaceous dextral shear zones documented in the Sierra Nevada, and Farallon-North America convergence was likely sinistral during at least part of that period [2, 3, 77, 78]. Therefore, we suggest that there has been little lateral offset of Late Jurassic Independence dikes between the Alabama Hills and the Sierra Nevada batholith.

Middle Cretaceous silicic plutons on both sides of Owens Valley support the suggestion that there has been 65 to 75 km of dextral slip across the Owens Valley inferred from offset of Paleozoic submarine deposits [5] and the approximately 83.5 Ma Golden Bear and Coso dikes [19]. Kylander-Clark et al. [19] examined the leucogranite of Cactus Flat, which is intruded by the Coso dikes (Figure 1), and found the pluton to be approximately 102 Ma, with $S87r/S86ri=0.705974$⁠. Those data are similar to the Bullfrog and Independence plutons, leading Kylander-Clark et al. [19] to propose a connection among the three. We also note that the Cactus Flat pluton has trace element characteristics that are similar to the Alabama Hills Granite and the Bullfrog and Independence plutons, including high Y/Sr, a negative Eu anomaly (⁠$Eu/Eu∗≈0.4$⁠), and a flat HREE pattern, further supporting a connection between the Cactus Flat pluton and the 103–100 Ma rocks on the west side of Owens Valley. The data for the plutons alone support 50–100 km of dextral offset since the middle Cretaceous, with the large range owing to uncertainty in the initial spatial relationship between the plutons and their host tectonic blocks, both horizontally and vertically (e.g., [16]). Therefore, combining the data for the Sierran middle Cretaceous plutons, the Alabama Hills Granite, the leucogranite of Cactus Flat, and Late Jurassic Independence dikes [14] suggest approximately 65–75 km of dextral slip has accumulated across Owens Valley since the middle Cretaceous.

This slip was likely accommodated on a regional dextral shear zone in Owens Valley; parts of this regional Owens Valley shear zone may be preserved in the Santa Rita Flat shear zone, among other fragments (Figure 1; [14, 47]). Bartley et al. [14] suggested 5–10 km of dextral slip across Owens Valley has occurred since the Pliocene. This accords with the current tectonic regime, where about 20–25% of the total relative motion between the North America and Pacific plates is accommodated by dominantly right-lateral, minor transtensional motion across the eastern California shear zone–Walker Lane region ([1, 79], and references therein). Thus, the remaining 55–70 km of dextral offset across Owens Valley occurred earlier, perhaps during the Late Cretaceous–Paleogene Laramide orogeny when Farallon-North America convergence had a significant right-oblique component [2, 14, 77]. The timing, magnitude, and tectonic setting of Late Cretaceous–Paleogene Owens Valley offset agree with the estimated distance (50+ km) and timing (ca. $77±5Ma$⁠) of dispersal of crustal fragments from the nearby southern Sierra Nevada; these fragments were displaced during rapid unroofing and gravitational collapse of the southern batholith [80].

The similarities in ages, isotopic compositions, and trace element abundances among the Alabama Hills, Bullfrog, Independence, and Cactus Flat plutons suggest they were generated together from a common source. We informally refer to this group of plutons as the Kearsarge plutons, after numerous geographic features in the Sierra Nevada bearing that name, including Kearsarge Pass and Kearsarge Peak (Figure 2). The Kearsarge plutons are characterized by U-Pb zircon crystallization ages from 103 to 100 Ma, $S87r/S86ri=0.7045$ to 0.7060, $εNdi≈−2.4$ to -4.5 (Table 1; Figure 6; [19, 31, 54]), and zircon δ¹⁸O (δ¹⁸O_zrc) values from 4.61 to 6.41‰ [81]. Note that owing to its very low calculated ⁸⁷Sr/⁸⁶Sr_i, we do not consider the Rb-Sr system for sample RP15-02 to have been closed since crystallization, and the sample likely gained Rb and/or lost Sr to open system processes (Figure 6; e.g., [82]). It is possible the Rb-Sr systematics of some other Kearsarge samples have also been disturbed to a lesser degree, but in the absence of evidence to support such disturbance, we prefer to include their calculated ⁸⁷Sr/⁸⁶Sr_i values in our definition of the Kearsarge plutons.

The Kearsarge plutons’ isotopic characteristics suggest several other nearby plutons on the Sierra crest could correlate with the suite, including the Diamond, Dragon, and Sardine plutons (Figure 2; [35, 39, 81]). Bulk zircon U-Pb data also suggest these plutons have middle Cretaceous ages [21, 39], although modern geochronology is required to confirm such an assignment. A link between the Diamond and Sardine plutons and the Kearsarge plutons suggests their previous bulk zircon U-Pb ID-TIMS data could have been compromised by inheritance (Figure 2; [39]), as in the Independence pluton [19, 21].

The radiogenic and O isotopic compositions in the Kearsarge plutons are slightly more primitive than the enriched mantle sources proposed for the nearby 92 Ma mafic complex of Onion Valley (⁠$S87r/S86ri=0.7065$⁠, $εNdi≈−4.3$⁠; [37, 38]; Figures 2 and 6) and the 94–92 Ma Lamarck Granodiorite (⁠$S87r/S86ri=0.7064$⁠, $εNdi≈−4.5$⁠; $δ18Ozrc≈5.7‰$⁠; [81, 83]). Both of these Late Cretaceous intrusions are interpreted as representing nearly 100% new additions to the crust from the mantle [29, 37, 38, 81, 83]. Wenner and Coleman [54] investigated several mafic–felsic plutonic suites in the eastern Sierra Nevada batholith using Sr, Nd, and Pb isotopic data and also hypothesized that most eastern Sierra granites were ultimately sourced from enriched mantle (Figure 6). Indeed, the mafic-granodioritic Dragon pluton may correlate with the Kearsarge plutons and has $S87r/S86ri=0.70566$ [39]; the similar Sr isotopic composition in mafic to felsic rocks in the Kearsarge plutons suggests little involvement of ancient crust in their source, similar to interpretations for other mafic–felsic suites in the eastern Sierra [29, 37, 54, 83, 84]. We also note that basalt flows from the Quaternary Big Pine volcanic field, including xenolith-bearing flows near the Oak Creek pendant and the Independence pluton, yield Sr and Nd isotopic compositions broadly similar to the Kearsarge and other eastern Sierran suites (Figure 6; [85, 86]). Thus, we suggest this group of Kearsarge plutons is like others in the eastern Sierra Nevada and was principally derived from an enriched mantle source.

The Kearsarge plutons have lower ⁸⁷Sr/⁸⁶Sr_i relative to suites with similar εNd_i (e.g., Lamarck Granodiorite and Onion Valley complex; Figure 6). This discrepancy may be due to cryptic disturbance of the Kearsarge Rb-Sr system, or indicates the Kearsarge mantle source had slightly lower time-integrated Rb/Sr than the enriched mantle parcels hypothesized to have sourced the other eastern Sierra suites. This supports hypotheses for a heterogeneous lithospheric mantle beneath the Sierra Nevada batholith [54, 87]. It is interesting to note that the Kearsarge plutons have ⁸⁷Sr/⁸⁶Sr_i mostly below 0.706, yet they are located more than 50 km east of the $S87r/S86ri=0.706$ isopleth (“706 line”). The 706 line has been interpreted as marking the boundary between ancient North American lithosphere on the east and younger Panthalassan lithosphere on the west [88, 89]. Exceptions to the 0.706 line have also been recently documented elsewhere in the central and eastern Sierra Nevada [54, 90–92], suggesting the lithospheric boundary may be either nonvertical (e.g., [93]) or is more diffuse than previous studies have suggested.

The distinct trace element characteristics of the high-silica Alabama Hills Granite and the Bullfrog, Independence, and Cactus Flat plutons, including HFSE, middle, and heavy REE enrichments, negative Eu anomalies, and high Y/Sr (Figures 3 and 4), suggest they were derived from a titanite-free, plagioclase-bearing source [94]. Coleman et al. [95] suggested that the distinct geochemistry of leucogranites like those of the Kearsarge plutons points to a deep-crustal source. Experimental work also supports the derivation of Sierran granites from hydrous, potassic mafic rocks in the mid- and lower-crust [96, 97]. With radiogenic and O isotope geochemistry indicating a mantle source (Figure 6; [19, 54, 81]), we suggest the granitic Kearsarge plutons were derived from juvenile plagioclase-bearing mafic rocks in the mid- to lower-crust (e.g., [54]).

Sierran leucogranite plutons have similar compositions to high-silica volcanic rocks preserved in wall rock pendants in the Sierra Nevada batholith, implying they were generated by similar processes in a deep source [95]. Therefore, the leucogranites of the Kearsarge plutons could represent intrusive equivalents to volcanic rocks. Indeed, several of the metavolcanic ash-flow tuffs and shallow sills in the Oak Creek pendant are dominantly silicic, have middle Cretaceous U-Pb bulk ID-TIMS zircon ages, and possess ⁸⁷Sr/⁸⁶Sr_i from 0.70550 to 0.70708 (Figure 1; [39]). The Oak Creek pendant is intruded by several possible Kearsarge plutons (Figure 2; [35, 39]) and paleobarometric estimates for the Independence pluton and Alabama Hills Granite range from 0.2 to 2.0 kbar [25, 82, 98], suggesting the Kearsarge plutons could be shallow subvolcanic counterparts to the middle Cretaceous volcanic rocks of the Oak Creek pendant. However, additional trace element, isotopic, and high-precision geochronologic data for the Oak Creek metavolcanic rocks are required to test such a connection to the Kearsarge plutons.

Field relationships and geochronology demonstrate that the Cretaceous Sierra Nevada batholith is broadly older in the west and younger in the east (e.g., [21, 99, 100]). The eastward-younging of ages in the Sierra has typically been attributed to shallowing of an east-dipping Farallon slab, although specific flattening mechanisms are debated [7, 21, 77, 101]. This model predicts that middle Cretaceous plutons should be located in the axial part of the batholith between Early and Late Cretaceous rocks (e.g., [102]). However, the Kearsarge plutons do not fit this pattern due to their positions 45–75 km inboard of the range axis when measured perpendicular to the approximately N20°W trend of the Cretaceous batholith (Figure 1). The nearest contemporaneous rocks dated using modern techniques are the 105–102 Ma Ash Mountain complex in the axial part of the batholith (Figure 1; [92]).

A different proposed mechanism to generate west-to-east younging in the Sierra Nevada is rollback of a west-dipping slab [103, 104]. However, west-dipping subduction would have required the Kearsarge plutons to be intruded in the forearc, where water supply is limited to low P-T sediment dewatering reactions that are not typically associated with magmatism [105]. Such trenchward magmatism would also presumably involve little contribution from the mantle, which is difficult to reconcile with the Sr, Nd, and O isotopic data for the suite (Figure 5; [54, 81]).

The Kearsarge plutons’ locations far east relative to axial middle Cretaceous Sierran rocks (Figure 1; [90, 92]) lead us to speculate that they could be an expression of backarc magmatism above an east-dipping subduction zone. Backarc magmatism has not been previously proposed in the Cretaceous Sierra Nevada batholith, although the easternmost Sierra Nevada has been referred to as “backarc” in a structural sense [106, 107]. We note that our backarc inference is principally based on the plutons’ locations, not geochemical characteristics; the high-silica granites of the Kearsarge plutons have trace element characteristics similar to other Sierran leucogranites of varying ages and arc positions [95]. The Kearsarge plutons’ isotopic compositions, while intriguingly more mantle-like than axial middle Cretaceous suites [90, 92, 108], are broadly similar to the Late Cretaceous suites that would follow the Kearsarge in the eastern Sierra (Figure 6). This suggests that the Kearsarge plutons’ backarc locations did not lead to distinct geochemical indicators; rather, their petrogenesis may have been no different than other suites in the eastern Sierra [54].

It is tempting to analogize the granitic Kearsarge plutons’ locations to silicic backarc magmatism documented in the Andes, but that magmatism has occurred much further behind the main volcanic front (up to 300 km inboard; [109, 110]). Backarc magmatism in many parts of the Andes is also interpreted to be accommodated and/or controlled by transtensional or transpressional fault zones [111, 112], which have not been observed in the Sierra Nevada during the middle Cretaceous (i.e., 110–95 Ma). However, thrust and reverse faults have also functioned as focusing structures for magmatism in the Andes [113, 114]. Such focusing thus could have played a role in the positioning of the Kearsarge suite.

In the Sierra Nevada batholith during the middle Cretaceous, the prevailing tectonic regime was compressional or weakly right-oblique according to Farallon–North America reconstructions [2, 77]. The presence of primarily contractional shear zones in the axial and eastern parts of the batholith during this time attests to this tectonic environment [7, 36]. Of particular relevance is the Sawmill Lake shear zone, a steeply dipping ductile zone of east-side-up contraction that extends for as much as 50 km along the Sierra crest, terminating in the south near the apparently undeformed ca. 102.5 Ma Independence pluton (Figures 1 and 2; [19, 35, 36, 46]). The timing of strain accumulation in the Sawmill Lake shear zone is bracketed by the ~148 Ma Independence dike swarm and the 94–92 Ma Lamarck Granodiorite; contractional deformation continued through the ca. 95 Ma crack-seal assembly of the late synkinematic McDoogle pluton [36, 115]. A steep deformational fabric in the nearby Oak Creek pendant, similar to that observed in the Sawmill Lake shear zone, began forming ca. 110 Ma (Figure 2; [39, 116]). In addition, the Sardine pluton, which intrudes the Oak Creek pendant and may be correlative with the Kearsarge plutons, is interpreted as synkinematic [39, 116]. Finally, metamorphic amphiboles from a Jurassic pluton deformed by the Sawmill Lake shear zone yield ⁴⁰Ar/³⁹Ar ages of $102±6Ma$ and $81±6Ma$ [106]. The younger age may reflect resetting by MWIS magmatism (Figure 2; [28, 31]), but the older age permits the Sawmill Lake shear zone to have been active at the same time the Kearsarge plutons intruded. Altogether, the geologic and geochronologic evidence supports the presence of the Sawmill Lake shear zone in the eastern Sierra Nevada batholith at the time Kearsarge pluton magmatism began ca. 103 Ma.

However, besides the possible exception of the Sardine pluton, no deformation from either the Sawmill Lake shear zone or other correlative shear zones of the east Sierran thrust system is observed in the Kearsarge plutons [5, 26, 34–36, 45, 46]. Thus, if Kearsarge magmas exploited reverse-sense shear zones as focused pathways (e.g., [36]), field evidence may have been overprinted (e.g., [117]) or is generally too difficult to discern owing to the Kearsarge plutons’ low color indices and mineralogical homogeneity [25, 26, 34, 35].

Alternatively, the Kearsarge pluton magmas could have followed preexisting fractures or anisotropies in the lithosphere, similar to Cretaceous dikes with similar orientations to Jurassic dikes in the Independence dike swarm [118]. In the case of the dikes, there were significant differences in plate convergence trajectories and rates in eastern California between the Jurassic and Cretaceous [2, 119]; therefore, regional strain patterns were evidently not significant in controlling dike orientation during those periods [118]. The dikes may thus have been guided instead by preexisting fractures or the orientation of the North American margin [118]. Perhaps the Kearsarge pluton magmas followed similar anisotropies; the elongated portions of the Sierran Kearsarge plutons are oriented ~325–345° [34, 35], broadly similar to the overall trend of the Independence dike swarm (~330°; [3, 15]).

New geochronologic data indicate that the Alabama Hills Granite intruded from 103 to 102 Ma, about 17 Ma earlier than indicated by previous data. The recognition of this granite as middle Cretaceous has significant implications. We suggest the Alabama Hills Granite belongs to the newly recognized and informally-named Kearsarge plutons, which comprise mostly leucogranites with ages approximately 103–100 Ma. Linking the Alabama Hills Granite with the Kearsarge plutons suggests the Alabama Hills block has moved no more than about 10 km dextrally since the middle Cretaceous and perhaps has not moved significantly since the Middle Jurassic. As such, the Alabama Hills serve as a suitable piercing point for reconstructing dextral offset across Owens Valley and suggests 65–75 km dextral offset, consistent with other estimates. The Kearsarge plutons have distinct geochemical characteristics, including high Y/Sr, high HREE concentrations, negative Eu anomalies, and isotopic compositions similar to enriched mantle (⁠$S87r/S86ri≈0.7045–0.7060$⁠, $εNdi≈−2.4$ to -4.5). Therefore, we suggest the plutons were derived from juvenile, plagioclase-bearing, and titanite-free mafic rocks in the mid- or lower-crust. The location of the Kearsarge plutons ~60 km east of other middle Cretaceous Sierran plutons suggests they could represent backarc magmatism that was focused along a preexisting shear zone or fractures.

The geochemical data used to support the findings of this study are included within the article. Additional geochronologic metadata are available in the Geochron database (geochron.org). Sample collection and storage metadata are available in the System for Earth Sample Registration database (geosamples.org).

The USGS author's contribution to this article was prepared solely by an employee of the United States federal government (i.e., the Agency) as part of the employee’s official duties and is not subject to copyright protection within the United States and is in the U.S. public domain.

The authors declare that there is no conflict of interest regarding the publication of this article.

Frazer was supported by the UNC Graduate School Dissertation Completion Fellowship and the U.S. Geological Survey (USGS) Mendenhall Research Fellowship Program. Roger Putnam generously collected samples of the Alabama Hills Granite that were critical to this work. The authors would like to acknowledge Josh Rosera who provided constructive and helpful comments that improved this manuscript.