Synextensional Dike Emplacement Across the Footwall of a Continental

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Research Paper THEMED ISSUE: Active Margins in Transition—Magmatism and Tectonics through Time: An Issue in Honor of Arthur W. Snoke

GEOSPHERE Synextensional dike emplacement across the footwall GEOSPHERE; v. 13, no. 6 of a continental core complex, Chemehuevi Mountains, doi:10.1130/GES01402.1 southeastern California 10 figures; 3 tables; 2 supplemental files Justin S. LaForge1, Barbara E. John1, and Craig B. Grimes2 CORRESPONDENCE: justinlaforge@gmail.com 1Department of Geology and Geophysics, University of Wyoming, P.O. Box 3006, Laramie, Wyoming 82071-3006, USA 2Department of Geological Sciences, Ohio University, 208 Clippinger Laboratories, Athens, Ohio 45701-2979, USA CITATION: LaForge, J.S., John, B.E., and Grimes, C.B., 2017, Synextensional dike emplacement across the footwall of a continental core complex, Cheme huevi Mountains, southeastern California: Geosphere, ABSTRACT INTRODUCTION v. 13, no. 6, p. 1867–1886, doi:10.1130/GES01402.1. We characterize the composition, timing, geometry, and deformation Exposures of Oligocene–Miocene metamorphic core complexes along Received 20 July 2016 style of the syntectonic Miocene Chemehuevi dike swarm exposed in the the Colorado River extensional corridor (CREC; eastern Basin and Range Revision received 20 July 2017 Accepted 24 August 2017 footwall of the regionally developed low-angle Chemehuevi detachment Province, USA) provide a natural laboratory to study the evolution of struc- Published online 26 October 2017 fault system (southeastern California, USA). Our data support mafic to felsic tures associated with large-magnitude crustal extension (Fig. 1; Howard dike emplacement from ~1.5 ± 1 to 3.8 ± 1 m.y. after initiation of regional and John, 1987). These core complexes offer a wealth of information on extension (ca. 23 Ma), followed by rapid slip and denudation with minor extensional processes from inception to cessation of large-slip normal magmatism. Pb/U zircon ages indicate intermediate to felsic dike emplace- faults, often including associated magmatism (e.g., John, 1987b; Lister and ment adjacent to the Mohave Wash fault, part of the regional fault system, Davis, 1989; Campbell-Stone and John, 2002). The original orientation of as it was active across the upper limit of the brittle-plastic transition, from large-offset low-angle normal faults in the CREC (and elsewhere) remains 21.45 ± 0.19 to 19.21 ± 0.15 Ma. Intermediate to felsic dikes are undeformed the subject of significant debate due to the apparent incompatibility with at structurally shallow levels (<9 km minimum paleodepth), but are rotated Andersonian mechanics (Axen, 2007). The early evolution of such sys- and locally folded, and host a well-developed mylonitic foliation and linea- tems across the brittle-plastic transition are complex and influenced by a tion at deeper structural levels (≥9 km paleodepth), even where the country spectrum of competing processes, including fracture and brittle faulting, rock is nonmylonitic. Syntectonic mafic to intermediate dikes were emplaced plastic deformation, fluid flow, and commonly magmatism (Parsons and into the footwall, hanging wall, and fractures and cataclasites hosted in the Thompson, 1993; Faulkner et al., 2010). We focus on constraining the timing Mojave Wash fault zone. Dike emplacement therefore occurred into and ad- and style of intrusive magmatism in the early evolution of a metamorphic jacent to a low-angle normal fault zone during its early history across the core complex. upper brittle-plastic transition, with dikes locally composing as much as 25% To better understand the evolution of dike emplacement and tectonic ex- of the footwall adjacent to fault zone, and <2% of the total extension re- tension in low-angle normal fault systems across the brittle-plastic transition, gionally. The predominant east-west and northeast-southwest orientations this study documents the syntectonic Miocene Chemehuevi dike swarm, ex- of dikes within the swarm are unique to this core complex, and differ from posed in the Chemehuevi Mountains, southeastern California (Fig. 1). Here the predicted emplacement orientation for northeast-directed extension and we concentrate on the relation between the dike swarm and the gently north- other complexes in the region. Dikes have moderate to subvertical dips at east-dipping Miocene Mohave Wash fault (MWF), a subdetachment hosted the highest crustal levels (domains 1–3), and are subhorizontal in the deep- in the footwall to the large slip (>18 km) Chemehuevi detachment fault (CDF; est exposures of the fault system (domains 4 and 5), where they host my- John, 1987a). The MWF accommodated only 1–2 km of northeast-directed off- lonitic fabrics. The elemental geochemistry of the Chemehuevi dike swarm set at paleodepths of ~5 to >10 km across the base of the seismogenic zone, is similar to that of local volcanism exposed in tilted hanging-wall blocks to and was passively denuded by the structurally higher Chemehuevi detach- the regional fault system and regional intrusive magmatism, and the swarm ment fault, preserving the early-slip deformation history (John, 1987b). Based For permission to copy, contact Copyright is proposed to have fed the now rootless volcanic systems as part of the on geochronologic and thermochronologic constraints from previous studies Permissions, GSA, or [email protected]. regional magmatic system. (John and Foster, 1993; Foster and John, 1999), we compare characteristics

GEOSPHERE | Volume 13 | Number 6 LaForge et al. | Synextensional dike emplacement across the footwall of a continental core complex Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/13/6/1867/3990639/1867.pdf 1867 by guest on 02 October 2021 Research Paper

AB114° 34′30″ 114° 34′30″ NV UT A′′′ A′′′ CA A′′ A′′ Area of AZ A, B A′′ A′′ Colorado River extensional A′ 5 corridor A′ 5 domain domain 200 km

Colorado Colorado 4 4 domain domain

Sedimentary rocks and deposits (Quaternary and Paleogene- 3 Rive River Neogene) 3 domain domain r A′ Volcanic and sedimentary rocks A′ 2 (Miocene and Oligocene) 2 domain Undeformed Chemehuevi Mountain domain

plutonic suite (Cretaceous) 400°C 1 Mylonitic gneiss (Proterozoic and 1 domain Cretaceous) domain Gneiss and granitoid rocks (Proterozoic)

Contact 350° Normal fault C

Chemehuevi detachment fault 30 250°C Mohave Wash fault 0° A A C Devils Elbow fault 200°

34° 30′ 34° 30′

0 5 0 5 km km

Figure 1. (A) Simplified geologic map of the Chemehuevi Mountains. (B) Simplified map showing the distribution of paleoisotherms in the footwall to the Chemehuevi detachment fault system at initiation of extension ca. 23 Ma (reconstruction from 40Ar/39Ar and fission track data from footwall samples; modified from John and Foster, 1993). Structural domains separated by dashed black lines. Lines A–A’, A’–A’’, and A’’–A’’’ indicate the location of cross section shown in Figure 6.

of the dike swarm in 5 northwest-striking structural domains perpendicular GEOLOGIC SETTING to the regional extension direction, in an effort to understand dike emplacement over 23 km of exposed footwall during early slip (Fig. 1). We use this The CREC lies within the southern North American Basin and Range Province documentation along with related studies to summarize a comprehensive tec- (Howard and John, 1987). It is a 50–100-km-wide region of extension that con- tono-magmatic history of the Chemehuevi Mountains core complex during tinues from Las Vegas to Mexico, in part along the Colorado River (Fig. 1). The Miocene extension. CREC underwent at least ~100% to as much as 400% northeast-directed Mio-

cene stretching. Extension was accommodated along a system of low-angle BACKGROUND normal faults that cut downsection eastward from the headwall breakaway in the Old Woman–Piute Mountains and root under the Hualapai Mountains and Paleogene–Neogene Magmatism in the Colorado River Colorado Plateau in Arizona (Fig. 1; Howard and John, 1987; Davis and Lister, Extensional Corridor 1988; Spencer and Reynolds, 1991). These faults initially cut to paleodepths of 10–15 km and are estimated to have accommodated ~50 km total unidirectional The history of the CREC includes extensive Oligocene–Miocene magma- top-to-the-northeast slip (040°–060°), often juxtaposing volcanic and sedimen- tism during extension. Voluminous extrusive magmatism is documented to tary rocks against mid-crustal basement (Howard and John, 1987; Miller and have propagated northward through the CREC, mimicked by a similar geo- John, 1999; Lister and Davis, 1989). In the center of the CREC, isostatic dom- graphic pattern of brittle normal faulting ~1–4 m.y. later (Gans et al., 1989; Gans ing exposes the well-known metamorphic core complexes in the Sacramento, and Bohrson, 1998; Faulds et al., 1999). Following cessation of major fault slip, Chemehuevi, and Whipple mountains (Fig. 1), which offer a natural laboratory extrusive magmatism resumed at a significantly reduced level (Howard and for studying low-angle normal fault–related processes. John, 1987; Gans and Bohrson, 1998; Miller and John, 1999), highlighting a The regionally developed detachment system exposed in the Chemehuevi temporal and geographic pattern characteristic of active rifting, with the bulk of Mountains comprises northeast-dipping, imbricate low-angle Miocene normal magmatism facilitating brittle failure of the upper and middle crust (Gans et al., faults (Fig. 1A; Howard and John, 1987; John, 1987a, 1987b). Three low-angle 1989). Extrusive magmatism in the CREC also records an evolution in composi- normal faults are exposed for more than 23 km in the downdip direction: the tion during extension; the oldest volcanic deposits comprise a heterogeneous most shallow Devil’s Elbow fault, the regionally developed CDF (>18 km of off- suite of basaltic, intermediate, and minor felsic rocks, whereas the youngest set), and the structurally deepest small offset (1–2 km) MWF (John, 1987b). Horn- volcanism is basaltic or bimodal in composition (Gans et al., 1989; Gans and blende, biotite, and alkali feldspar 40Ar/39Ar plateau ages combined with apatite Bohrson, 1998; Foster and John, 1999). Although a significant volume of the fission-track thermochronology constrain the thermal structure of the CDF foot- documented syntectonic magmatism in the CREC is extrusive, plutons and wall at initiation of extension 23 ± 1 Ma, from <150° in the southwest to >400 °C dikes were also emplaced prior to, during, and after peak slip on low-angle in the northeast (Fig. 1), with an initial dip between 15° and 30° northeast (John, normal faults associated with the core complexes (Davis et al., 1982; Camp- 1987a; John and Foster, 1993; Foster and John, 1999). Peak slip rates constrained bell and John, 1996; Campbell-Stone et al., 2000; Campbell-Stone and John, by geochronology allied with sedimentation patterns in the hanging wall to the 2002; Singleton and Mosher, 2012). Dikes characteristic of the early syntectonic CDF range from 7 to 8 mm/yr between 19 and 15 Ma (Miller and John, 1999); the magmatism in the CREC are exposed in the Whipple, Mohave, Chemehuevi, majority of slip was accomplished by ca. 13.9 Ma based on diminished sedimen- and Sacramento mountains (Howard and John, 1987; John, 1987b; Lister and tation rates (Miller and John, 1999), with cessation by 11.6 Ma based on coeval Davis, 1989; Nielson and Beratan, 1995; Sherrod and Nielson, 1993). dikes and plugs that cut the CDF (John, 1987b; John and Foster, 1993). Bedrock in the Chemehuevi Mountains is dominated by crystalline base- Dike Emplacement in Metamorphic Core Complexes ment with minor sedimentary and volcanic deposits. Mid-crustal crystalline rocks cut by the CDF include heterogeneous Proterozoic amphibolite gneiss Studies of synextensional dikes associated with metamorphic core com- and migmatite, isotropic granitic rocks of the Cretaceous Chemehuevi Moun- plexes can offer a wealth of kinematic information, and have proposed that dikes tains Plutonic Suite, and Proterozoic, Cretaceous, and Miocene dikes and small may influence low-angle normal fault evolution. Dikes commonly intrude as plugs (Fig. 1; Howard and John, 1987; John, 1987a, 1987b, 1988). Although the mode I fractures parallel to the greatest principal stress and orthogonal to the history of the Miocene Chemehuevi dike swarm was not a focus of previous least principal stress, highlighting the regional stress field during their emplace- studies, some dikes in the suite are known to be Miocene based on preliminary ment (Anderson, 1951). In extensional stress regimes with the greatest principal 40 39 Ar/ Ar ages, composition, and morphology, having accommodated minor stress (σ1) vertical, dikes have characteristic strikes orthogonal to the extension (≤2%) northeast-directed extension (John and Foster, 1993). The flanks of the direction and dip steeply (Anderson, 1951). Dikes can therefore be used as indica- range host Oligocene to Miocene volcanic rocks and locally derived Paleogene tors of the magnitude of postemplacement block rotation (Howard, 1991; Wong to Quaternary sedimentary deposits in the hanging wall to both the Cheme and Gans, 2008; John and Cheadle, 2010) and to determine fault offset (e.g., John, huevi and Devil’s Elbow faults (Miller and John, 1999). The volcanic succes- 1987b; Howard and John, 1987). Dikes with anomalous orientations have invoked sion includes an ~500-m-thick basal sequence of mafic and intermediate lavas discussion related to evolving stress fields during metamorphic core complex (lower volcanic and sedimentary section of Sherrod and Nielson, 1993) erupted development (Spencer, 1985). Dike emplacement has also been proposed to between ca. 22.0 and 18.78 ± 0.02 Ma (Miller and John, 1999), the 0–60-m-thick facilitate initiation of low-angle normal faults by stress reorientation, or effective 18.78 ± 0.02 Ma Peach Spring Tuff (Miller and John, 1999; Ferguson et al., 2013), weakening of the host rock. Collectively, dikes may facilitate initiation of a gently and minor rhyolite ash flows and basaltic lavas deposited between 15.54 ± 0.03 dipping fault just above its upper termination or tip, by rotating the greatest prin- and 13.9 ± 0.1 Ma (Miller and John, 1999). cipal stress away from vertical (Parsons and Thompson, 1993).

114° 34”30” METHODS tion including basalt, trachybasalt, basaltic trachyandesite, trachyande BJ13Ch-21, 22a CG14Ch-123, 132 BJ14Ch-4, 10, 14b, 20b site, trachydacite, dacite, and rhyolite (50.9–80.5 wt% SiO ; Table 1; Fig. 2). Jl14Ch-2 2 BJ13Ch-4 Field Investigations We refer to these compositions as mafic (<52% SiO2), intermediate (>52% BJ13Ch-1 CG13Ch-33 CG13Ch-38 CG13Ch-37 to <63% SiO2), intermediate-felsic (>63% to <69% SiO2), and felsic (>69% CG13Ch-46, 49, 53, 54, 55, 56 Outcrop and macroscopic investigations documenting relations between SiO2; Fig. 2). All dikes with <56 wt% SiO2 are calc-alkalic; those with higher

ol the MWF and Chemehuevi dike swarm were completed by measuring struc-

l oor silica range from calc-alkalic to alkali-calcic (Fig. S2 [see footnote 1]). All

r aado

d o tural orientations and dike thickness, and documenting the geometry and dikes are magnesian (Fig. S2 [see footnote 1]); dikes with <64 wt% SiO2 are River macroscopic textures of syntectonic dikes. Volume percentages of dikes were metaluminous, and those with >64 wt% SiO2 are peraluminous (Fig. S2 estimated by transects in the field, measuring cumulative dike thickness di- [see footnote 1]), and commonly host mica (either biotite and/or musco- vided by total transect distance. Representative samples were collected for vite) and rarely garnet. Geographically, mafic to intermediate-felsic dikes geochemical and geochronologic analysis to constrain both compositional BJ14Ch-30 CG13Ch-5,6,7 are present throughout the footwall to the detachment system; dikes with BJ13Ch-3 variation and age (Tables 1 and 2; Supplemental Fig. S11). CG13Ch-70 >70% SiO2 are characteristic of and restricted to the easternmost domains (4 and 5, Fig. 3). Geochemistry Major and trace element oxides show typical continuous differentiation

trends for compatible and incompatible elements (Harker, 1909). TiO2, CaO,

X-ray fluorescence analyses of 27 dikes for SiO2, TiO2, Al2O3, FeO, MnO, Al2O3, FeOtot, MgO, and P2O5 all decrease as K2O and Na2O increase with in- 34° 30’

0 5 MgO, Ca2O, Na2O, K2O, P2O5, and trace elements Ba, Sr, and Zr were conducted creasing SiO2 wt% (Table 1; Figs. 4A–4C; Fig. S3l [see footnote 1]). Ba remains Kilometers on the Rigaku Supermini200 at Ohio University. Concentrations were deter- at similar concentrations over the expanded range of silica concentrations (Fig.

mined using the fundamental parameters (FP) method and calibrated against 4D); Zr concentrations positively correlate with SiO2 between 50 and 62 wt% 1Supplemental Figures. Figure S1: Simplified geo- 12 U.S. Geological Survey rock standards. Relative uncertainty (accuracy and logic map of the Chemehuevi Mountains showing and negatively correlate at >62 wt% SiO2 (Fig. 4E), and positively correlate with locations from Tables 1 and 2. Figure S2: Dikes of precision) for trace elements was <15%. Sr concentrations (Fig. 4F). the Chemehuevi dike swarm plotted on MALI (alu- The Chemehuevi dike swarm is similar in major element chemistry to minum modified alkali-lime index), ASI (aluminum saturation index), and Fe index (Fe-saturation index) Geochronology Miocene dikes hosted in the adjacent Whipple and Mohave Mountains, and plots (after Frost and Frost, 2008). Figure S3: Addi- pre–Peach Spring Tuff (PST) volcanic deposits, the Swansea Plutonic Suite tional Harker variation diagrams for major element U-Pb dating and trace element geochemical analyses of zircon from five in the Buckskin Mountains, the PST, and post-PST volcanic deposits hosted oxides from the Chemehuevi dike swarm. Figure S4: felsic dikes were performed at the U.S. Geological Survey–Stanford Ion Micro in the Black Mountains (Fig. 4; Figs. S2 and S3 [see footnote 1]; Pease, 1991; Tera-Wasserburg concordia diagrams showing analyzed grains from five Chemehuevi dike swarm sam- probe Laboratory using sensitive high-resolution ion microprobe–reverse Bryant and Wooden, 2008; Pamukcu et al., 2013; McDowell et al., 2014; Gentry, ples, Chemehuevi Mountains. Please visit http://doi geometry. Recovered zircon range from large, euhedral grains with well- 2015). Trace element concentrations in Chemehuevi dikes are comparable to .org/10 .1130/GES01402 .S1 or the full-text article on defined oscillatory zoning and rare embayed cores to subhedral or subequant coeval regional magmatism, but differ from the PST ignimbrite (Fig. 4: Fig. S3 www.gsapubs.org to view the Supplemental Figures. grains with weak oscillatory or irregular zoning. We analyzed 13–29 grains [see footnote 1]; Pease, 1991; Bryant and Wooden, 2008; Pamukcu et al., 2013; ~40–400 µm in long dimension from each sample. We note that a significant McDowell et al., 2014; Gentry, 2015). Supplemental Data 1: Detailed Methods proportion of the Chemehuevi dike swarm are weakly to moderately altered Geochemistry basalt, trachybasalt, and basaltic trachyandesite, hosting no appropriate min- Geochemical data presented are from representative Miocene dikes in the Chemehuevi 40 39 dike swarm collected across the range. Samples were hand-crushed, and pulverized for 3 minutes erals for geochronology (i.e., U-Pb or Ar/ Ar); dated samples are therefore using a tungsten-carbide mill for whole-rock geochemistry. The resulting powders were fused limited to dacite and rhyolite dike compositions. Data reduction for geochro- U-Pb Geochronology into glass discs using a Li-borate flux with a flux:sample ratio of 3:1, and 1 drop of LiI non- nology follows the methods described by Ireland and Williams (2003) using wetting agent. Beads were prepared using an 11-minute cycle on a single burner XRF-Scientific the Microsoft Excel add-in programs Squid 2.5 and Isoplot 4.1 (Ludwig, 2012). U-Pb zircon dating from five representative dikes proximal to the MWF Phoenix Fusion machine, and analyzed forSiO2, TiO2, Al2O3, FeO, MnO, MgO, Ca2O, Na2O, Ti-in-zircon temperatures were calculated using measured 48Ti and the recali- in domains 1, 2, 4, and 5 shows that intermediate to felsic dikes of the K2O, P2O5, and trace elements Ba, Sr, and Zr. X-Ray Fluorescence (XRF) analyses were conducted on the Rigaku Supermini200 at Ohio University. Concentrations were determined brated method of Ferry and Watson (2007). Detailed geochronology and geo- Chemehuevi dike swarm are Miocene in age (Table 2; Fig. 5). Dated dacite using the Fundamental Parameters (FP) method (Kansai, 2008)and calibrated against 12 USGS chemistry methods are in Supplemental Data2. dikes vary in age between 21.45 ± 0.19 Ma (BJ13Ch-1, mylonitic dacite), to rock standards. BHVO-2 (basalt) was used as the running standard throughout analysis. Based on 20.94 ± 0.13 Ma (BJ13Ch-3, undeformed biotite hornblende dacite). U con- the repeated analyses of the running standard, relative uncertainty for element oxides was centrations of analyzed zircon range from 63 to 2846 ppm; most grains host typically 1-2% or less. Relative uncertainty(accuracy and precision) for trace elements was less RESULTS than 15%. These are comparable to the long-term uncertainties obtained using the Rigaku abundances of a few hundred parts per million. Errors on calculated ages Supermini200 at OU for major and trace elements. Whole-Rock Elemental Geochemistry for each sample range from >1% to <6%, and are shown as 2σ standard 2Supplemental Data. Detailed geochronology and deviation (Fig. S4 [see footnote 1]). Note that none of the dacite dikes host geochemistry methods. Please visit http://doi.org We collected and analyzed 27 dikes from southwest to northeast inherited zircon, and mean Ti-in-zircon temperatures span a narrow range /10.1130/GES01402 .S2 or the full-text article on www across the core complex footwall that show a broad range in composi- from 721 ± 16–748 ± 43 °C. In contrast, rhyolitic dikes have ages from 20.23 ± .gsapubs.org to view the Supplemental Data.

TABLE 1. GEOCHEMICAL DATA OF THE MIOCENE CHEMEHUEVI DIKE SWARM Location Major oxides Trace elements (WGS84) (wt%) (ppm) Lat Long

Sample (°N) (°W)SiO2 TiO2 Al2O3 FeOtotal MgOMnO CaONa2OK2OP2O5 TotalZrSrBa BJ13-Ch3 34°34.035′ 114°36.419′ 66.24 0.48 16.00 2.77 1.29 0.05 3.28 4.27 3.15 0.18 97.69164 9441372 BJ13-Ch4 34°40.974′ 114°29.465′ 50.70 1.47 17.73 7.28 5.15 0.14 7.83 3.92 1.14 0.39 95.74210 615651 BJ13-Ch5 34°34.301′ 114°32.674′ 67.75 0.40 16.112.330.840.052.344.872.940.1797.80 243766 1940 BJ13-Ch6 34°34.191′ 114°32.856′ 48.96 1.52 15.67 9.48 6.31 0.17 8.26 2.99 1.53 0.33 95.22145 554689 BJ14-Ch4 34°42.044′ 114°29.844′ 71.63 0.16 14.78 1.05 0.24 0.09 0.84 4.67 4.73 0.07 98.26165 1791680 BJ14-Ch10 34°42.149′ 114°29.816′ 73.08 0.11 14.20 0.84 0.19 0.07 0.97 4.05 4.48 0.07 98.0689229 1420 BJ14-Ch14b 34°41.892′ 114°30.003′ 70.93 0.30 14.80 1.48 0.63 0.02 1.52 4.34 3.89 0.10 98.01130 4551377 BJ14-Ch20b 34°42.086′ 114°29.853′ 71.20 0.27 14.96 1.67 0.73 0.03 2.09 4.29 3.21 0.10 98.54126 5491584 BJ14-Ch21 34°42.302′ 114°29.703′ 70.38 0.30 15.01 1.81 0.73 0.03 1.91 4.02 3.79 0.10 98.08132 4851155 BJ14-Ch22a 34°42.312′ 114°29.701′ 79.36 0.15 10.22 1.45 0.30 0.03 0.83 2.28 3.96 0.03 98.60174 1171153 BJ14-Ch30 34°34.309′ 114°35.210′ 56.96 1.02 15.33 5.71 4.61 0.09 5.10 2.98 3.56 0.41 95.76275 8121779 JL14-Ch2 34°41.914′ 114°29.982′ 68.41 0.34 15.56 2.12 0.66 0.07 1.83 4.49 4.13 0.16 97.78317 4981807 CG13-Ch5 34°34.850′ 114 32.767′ 47.75 1.72 14.43 9.03 7.10 0.21 7.77 3.29 1.60 0.51 93.42218 739832 CG13-Ch6 34°34.850′ 114 32.767′ 48.43 1.58 13.52 8.61 9.49 0.19 9.07 2.59 0.80 0.46 94.73185 779455 CG13-Ch7 34°34.850′ 114 32.767′ 48.60 1.72 14.43 9.17 7.80 0.19 8.24 3.22 1.64 0.51 95.52221 801785 CG13-Ch33 34°40.846′ 114°29.484′ 67.45 0.39 16.01 2.25 0.80 0.03 2.10 5.36 2.55 0.18 97.12238 661 1111 CG13-Ch37 34°40.382′ 114°29.849′ 70.70 0.09 15.57 0.72 0.20 0.02 1.20 4.85 4.28 0.06 97.6883427 1744 CG13-Ch38 34°40.369′ 114°29.883′ 59.93 0.78 17.09 4.56 2.18 0.06 3.65 5.25 2.04 0.43 95.97277 1000 1872 CG13-Ch46 34°40.203′ 114°30.059′ 74.04 0.09 14.09 0.74 0.21 0.03 1.05 4.45 3.59 0.07 98.3683325 1409 CG13-Ch49 34°40.203′ 114°30.059′ 72.70 0.10 14.39 0.85 0.22 0.04 0.96 4.59 3.61 0.07 97.5283337 1312 CG13-Ch53 34°40.193′ 114°30.062′ 50.07 1.24 13.77 7.51 8.89 0.13 8.23 2.93 2.52 0.68 95.98284 1049 1265 CG13-Ch54 34°40.193′ 114°30.062′ 51.14 1.25 14.44 7.28 7.58 0.12 7.97 3.19 2.60 0.63 96.20282 1037 1363 CG13-Ch55 34°40.193′ 114°30.062′ 49.87 1.25 14.31 7.70 7.05 0.13 6.85 3.11 1.84 0.65 92.78286 785 1112 CG13-Ch56 34°40.193′ 114°30.062′ 50.06 1.26 14.17 7.39 8.02 0.13 8.20 3.18 2.01 0.72 95.16279 1003 1234 CG13-Ch70 34°34.027′ 114°35.289′ 51.00 1.21 12.66 6.23 4.94 0.11 8.04 2.58 3.54 0.85 91.18323 1010 2477 CG14-Ch123 34°42.358′ 114°29.669′ 71.99 0.17 14.75 0.82 0.25 0.05 0.87 4.62 4.72 0.07 98.31169 1841771 CG14-Ch132 34°41.913′ 114°29.981′ 56.65 1.15 17.72 6.60 2.19 0.11 5.11 4.80 2.20 0.67 97.20357 1221 930 Note: WGS84—World Geodetic System 1984.

0.23 Ma (CG14Ch-123, mylonitic) to 19.59 ± 0.71 Ma (BJ14Ch-4, mylonitic, Dike Orientations garnet, two mica rhyolite) and 19.21 ± 0.15 Ma (BJ13Ch-5; undeformed bio tite rhyolite). We note no significant age difference between undeformed Dikes of the Chemehuevi dike swarm form sets that strike dominantly and mylonitic dikes from this data set; undeformed dikes range from 20.94 ± west-northwest, south-southwest, and east-northeast with a systematic range 0.13 to 19.21 ± 0.15 Ma, whereas those hosting a well-developed mylonitic in dip. Dikes in domains 1 and 2 (Fig. 6) form two sets striking west-northwest lineation parallel to the regional extension direction range in age from and south-southwest with moderate to steep dips. Maximum eigenvector (E1) 21.45 ± 0.19 to 19.59 ± 0.71 Ma. Although their ages overlap within error, of poles to dikes in domain 1 for the west-northwest set is 189°/42° and E1 differences in whole-rock composition and concentration of inherited and/or for the south-southwest is 106°/16°; in domain 2, the west-northwest set E1 xenocrystic zircon (absent in BJ13Ch-5, CG14Ch-123 hosts <10% xenocrystic is 199°/35° and the south-southwest set E1 is 112°/21°. We also note that the zircon with 204Pb-corrected 207Pb/206Pb ages ca. 1.7 Ga; in contrast, BJ14Ch-4 west-northwest and south-southwest dike sets of domains 1 and 2 are roughly hosts >70% inherited zircon between 1700 and 600 Ma) indicate at least 2 orthogonal to each other and when combined average an east-west strike phases of these intermediate to felsic dikes. Ti-in-zircon temperatures span (maximum E1 = 178°/44° and 182°/39°, respectively). In domain 3, subvertical a wider range from 704 ± 26 to 788 ± 26 °C, suggesting multiple phases (and dikes strike east-northeast (pole to dikes E1 = 351°/10°). In contrast, dikes in possible sources) of felsic dikes (separated by hundreds of thousands of domains 4 and 5 dip gently to the southeast while also striking east-northeast years or less). (pole to dikes E1 = 356°/56° and 338°/72°, respectively).

TABLE 2. SHRIMP-RG ISOTOPIC AGE DATA, CHEMEHUEVI DIKE SWARM (CHEMEHUEVI MOUNTAINS) 207Pb corrected ages T Concentrations Atomic ratios (Ma) (°C) Grain U Th 206Pb 238U/206Pb 207Pb/206Pb 207Pb corrected 206Pb/238U 206Pb/238U Weighted mean age Mean T sample (ppm) (ppm) (ppm) f206 (absolute error) (absolute error) (absolute error) (1σ) (2σ)(1/1) (°C) CG14Ch-123 mylonitic rhyolite from domain 5 (34°42.358’N 114°29.669’W)* 1.1 328 268 0.87 0.053 324.8 ± 7.5 0.047 ± 0.005 0.00308 ± 0.00002 19.80 ± 0.47 20.23 ± 0.23 757788 ± 26 2.1 133 165 0.36 0.284 320.6 ± 6.6 0.049 ± 0.007 0.00311 ± 0.00002 20.02 ± 0.45 MSWD = 0.78 837 3.1 143 54 37.01 –0.215 3.3 ± 0.1 0.103 ± 0.001 0.301578 ± 0.0024 1699.10 ± 37.34 832 4.1 273 160 0.73 0.022 320.3 ± 3.5 0.047 ± 0.005 0.00312 ± 0.00001 20.09 ± 0.25 760 5.1 82 72 0.22 0.230 317.9 ± 6.5 0.048 ± 0.009 0.00314 ± 0.00002 20.20 ± 0.47 779 6.1 107 81 0.28 1.125 320.2 ± 9.5 0.056 ± 0.009 0.00309 ± 0.00003 19.88 ± 0.63 773 7.1 438 207 1.19 0.615 313.1 ± 4.8 0.051 ± 0.004 0.00317 ± 0.00002 20.43 ± 0.33 816 8.1 11392 0.31 –1.285 312.6 ± 8.7 0.036 ± 0.007 0.00324 ± 0.00003 20.85 ± 0.61 777 9.1 201 197 0.56 –1.058 313.1 ± 4.3 0.038 ± 0.006 0.00323 ± 0.00002 20.77 ± 0.31 772 10.1 271 327 0.72 0.439 323.4 ± 4.9 0.050 ± 0.006 0.00308 ± 0.00002 19.84 ± 0.33 806 11.1 99 95 0.26 1.304 320.2 ± 10.2 0.057 ± 0.010 0.00308 ± 0.00003 19.84 ± 0.68 772 12.1 264 163 0.68 –0.039 331.9 ± 2.4 0.046 ±0.005 0.00301 ± 0.00001 19.40 ± 0.19 763 13.1 90 80 0.25 0.786 311.2 ± 4.7 0.053 ± 0.010 0.00319 ± 0.00002 20.52 ± 0.40 804 14.1 63 89 0.17 –0.426 319.5 ± 6.6 0.043 ± 0.010 0.00314 ± 0.00002 20.23 ± 0.48 784 BJ13Ch-1 mylonitic biotite dacite from domain 4 (34°40.367’N 114°30.028’W)* 1.1 155 108 0.44 1.25 295.9 ± 8.5 0.056 ± 0.006 0.003337 ± 0.0000321.48 ± 0.64 21.45 ± 0.19 785748 ± 45 2.1 11248 0.31 1.75 302.8 ± 6.3 0.060 ± 0.008 0.00324 ± 0.00002 20.88 ± 0.48 MSWD = 1.5730 3.1 450 579 1.30 0.54 295.8 ± 6.3 0.051 ± 0.003 0.00336 ± 0.00002 21.64 ± 0.47 774 4.1 131 146 0.37 0.54 300.9 ± 8.4 0.051 ± 0.006 0.00331 ± 0.00003121.27 ± 0.61 774 5.1 11381 0.35 0.97 276.9 ± 2.4 0.054 ± 0.006 0.00358 ± 0.00001 23.02 ± 0.27 794 6.1 448 499 1.28 0.32 299.0 ± 6.5 0.049 ± 0.003 0.00333 ± 0.00002 21.45 ± 0.47 745 7.1 92 27 0.26 2.79 296.7 ± 4.2 0.069 ± 0.008 0.00328 ± 0.00002 21.09 ± 0.37 657 8.1 770 1015 2.20 0.53 299.4 ± 6.3 0.051 ± 0.005 0.00332 ± 0.00002 21.38 ± 0.46 728 9.1 474 254 1.36 0.72 297.0 ± 3.5 0.052 ± 0.003 0.00334 ± 0.00001 21.51 ± 0.27 725 10.1 228 176 0.65 0.09 302.9 ± 6.5 0.047 ± 0.005 0.00330 ± 0.00002 21.23 ± 0.47 784 11.1 240 136 0.68 –0.52 305.7 ± 13.7 0.042 ± 0.005 0.00321 ± 0.00005 21.16 ± 0.96 761 12.1 11885 0.33 4.02 291.6 ± 12.4 0.078 ± 0.008 0.00321 ± 0.00005 21.18 ± 0.93 808 13.1 290 236 0.83 1.87 293.2 ± 5.6 0.061 ± 0.004 0.00350 ± 0.00002 21.54 ± 0.43 731 14.1 123 75 0.34 3.14 301.1 ± 10.9 0.071 ± 0.007 0.00322 ± 0.00004 20.70 ± 0.77 771 15.1 261 150 0.79 –0.34 285.5 ± 3.6 0.044 ± 0.004 0.00351 ± 0.00002 22.62 ± 0.30 735 16.1 52 37 0.15 1.74 296.1 ± 6.6 0.060 ± 0.0110.00332 ± 0.00003 21.36 ± 0.56 807 17.1 209 108 0.61 –0.30 295.4 ± 7.5 0.044 ± 0.004 0.00339 ± 0.00003 21.85 ± 0.56 780 18.1 176 102 0.52 0.85 290.8 ± 7.9 0.053 ± 0.005 0.00341 ± 0.00003 21.94 ± 0.62 744 19.1 361 162 1.03 –0.29 302.2 ± 5.7 0.044 ± 0.003 0.00332 ± 0.00002 21.35 ± 0.41 733 20.1 161 107 0.47 –0.10 294.5 ± 7.1 0.046 ± 0.005 0.00340 ± 0.00003 21.87 ± 0.55 764 21.1 2846 6244 9.90 4.02 237.0 ± 4.8 0.078 ± 0.013 0.00405 ± 0.00003 26.05 ± 0.69 786 22.1 268 8 0.77 –0.53 300.9 ± 7.5 0.042 ± 0.004 0.00334 ± 0.00003 21.50 ± 0.54 622 23.1 213 91 8.49 4.28 20.6 ± 1.8 0.087 ± 0.001 0.04640 ± 0.00132 292.50 ± 25.52 733 24.1 66 35 0.19 3.12 295.9 ± 9.2 0.071 ± 0.0110.00327 ± 0.00004 21.07 ± 0.72 791 25.1 11779 0.33 2.54 294.6 ± 12.5 0.067 ± 0.007 0.00331 ± 0.00005 21.29 ± 0.92 754 26.1 274 190 0.74 0.53 314.6 ± 4.5 0.051 ± 0.004 0.00316 ± 0.00002 20.35 ± 0.31 679 27.1 161 107 0.47 –0.33 296.4 ± 4.8 0.044 ± 0.005 0.00338 ± 0.00002 21.78 ± 0.38 773 28.1 81 50 0.37 74.49 47.6 ± 23.0 0.636 ± 0.270 0.00536 ± 0.00243 34.48 ± 48.90 729 (continued)

TABLE 2. SHRIMP-RG ISOTOPIC AGE DATA, CHEMEHUEVI DIKE SWARM (CHEMEHUEVI MOUNTAINS) (continued) 207Pb corrected ages T Concentrations Atomic ratios (Ma) (°C) Grain U Th 206Pb 238U/206Pb 207Pb/206Pb 207Pb corrected 206Pb/238U 206Pb/238U Weighted mean age Mean T sample (ppm) (ppm) (ppm) f206 (absolute error) (absolute error) (absolute error) (1σ) (2σ)(1/1) (°C) BJ13Ch-3 undeformed, biotite dacite from domain 1 (34°33.935’N 114°36.419’W)* 1.1 266 220 0.75 0.39 301.8 ± 3.0 0.050 ± 0.004 0.00330 ± 0.00001 21.24 ± 0.24 20.94 ± 0.13 N.D. 721 ± 16 2.1 320 245 0.92 –0.16 300.3 ± 6.0 0.045 ± 0.004 0.00334 ± 0.00002 21.46 ± 0.44 MSWD = 1.10 685 3.1 511 413 1.46 0.33 299.5 ± 1.8 0.049 ± 0.003 0.00333 ± 0.00001 21.41 ± 0.15 708 4.1 344 716 0.94 1.23 308.8 ± 17.33 0.056 ± 0.003 0.00320 ± 0.00006 20.59 ± 1.16 1102 5.1 225 149 0.63 1.28 302.2 ± 4.3 0.056 ± 0.005 0.00327 ± 0.00002 21.03 ± 0.32 673 6.1 303 227 0.87 0.98 296.8 ± 7.2 0.054 ± 0.004 0.00334 ± 0.00003 21.47 ± 0.53 663 7.1 337 219 0.97 –0.14 297.8 ± 3.2 0.045 ± 0.004 0.00336 ± 0.00001 21.64 ± 0.25 655 8.1 173 68 0.48 0.13 309.2 ± 12.0 0.047 ± 0.005 0.00323 ± 0.00004 20.79 ± 0.82 655 9.1 345 294 0.93 0.00 317.5 ± 3.6 0.046 ± 0.004 0.00315 ± 0.00001 20.28 ± 0.25 687 10.1 336 341 0.94 1.87 302.2 ± 4.8 0.061 ± 0.004 0.00325 ± 0.00002 20.90 ± 0.35 700 11.1 203 133 0.58 1.38 2966 ± 10.2 0.057 ± 0.005 0.00332 ± 0.00004 21.40 ± 0.75 682 12.1 287 221 0.78 18.16 259.4 ± 13.6 0.190 ± 0.024 0.00316 ± 0.00006 20.31 ± 1.30 686 13.1 186 94 0.52 –0.39 310.6 ± 4.5 0.043 ± 0.005 0.00323 ± 0.00002 20.80 ± 0.32 684 14.1 263 137 0.73 0.19 309.9 ± 2.2 0.048 ± 0.004 0.00322 ± 0.00001 20.73 ± 0.18 694 15.1 229 122 0.66 0.55 295.3 ± 4.9 0.051 ± 0.004 0.00337 ± 0.00002 21.67 ± 0.38 650 16.1 227 114 0.62 0.43 314.3 ± 9.3 0.050 ± 0.004 0.00317 ± 0.00003 20.39 ± 0.61 692 17.1 168 73 0.47 –0.17 306.8 ± 4.9 0.045 ± 0.004 0.00327 ± 0.00002 21.01 ± 0.36 673 18.1 199 115 0.56 0.25 306.1 ± 4.6 0.048 ± 0.004 0.00326 ± 0.00002 20.97 ± 0.34 705 19.1 149 63 0.42 –0.15 302.1 ± 3.5 0.045 ± 0.005 0.00331 ± 0.00001 21.33 ± 0.29 675 20.1 256 121 0.71 0.18 307.9 ± 6.7 0.048 ± 0.004 0.00324 ± 0.00002 20.86 ± 0.46 690 21.1 194 91 0.52 0.96 317.7 ± 8.8 0.054 ± 0.005 0.00312 ± 0.00003 20.06 ± 0.57 686 22.1 239 92 0.66 0.04 311.0 ± 7.5 0.047 ± 0.004 0.00321 ± 0.00003 20.69 ± 0.51 675 23.1 159 73 0.45 0.66 303.6 ± 7.7 0.052 ± 0.006 0.00327 ± 0.00003 21.06 ± 0.56 686 24.1 165 75 0.43 2.05 321.7 ± 11.2 0.063 ± 0.006 0.00304 ± 0.00003 19.60 ± 0.70 671 25.1 147 61 0.42 0.79 301.0 ± 9.1 0.053 ± 0.005 0.00330 ± 0.00003 21.21 ± 0.66 654 26.1 374 374 1.05 0.57 303.1 ± 5.2 0.051 ± 0.003 0.00328 ± 0.00002 21.11 ± 0.37 697 27.1 193 82 0.54 1.51 303.3 ± 2.2 0.058 ± 0.005 0.00325 ± 0.00001 20.90 ± 0.21 684 28.1 171 73 0.48 –0.15 3069 ± 2.4 0.045 ± 0.005 0.00326 ± 0.00001 21.00 ± 0.21 661 29.1 297 161 0.84 0.81 2998 ± 6.3 0.053 ± 0.004 0.00331 ± 0.00002 21.30 ± 0.46 693 BJ13Ch-5 undeformed rhyolite from domain 2 (34°34.301’N 114°32.674’W)* 1.1 273 271 0.67 1.60 341.9 ± 11.0 0.059 ± 0.005 0.00288 ± 0.00003 18.52 ± 0.60 19.21 ± 0.15 657704 ± 35 2.1 295 363 0.75 1.22 334.4 ± 4.9 0.056 ± 0.008 0.00295 ± 0.00002 19.01 ± 0.34 MSWD = 1.5682 3.1 136 147 0.34 1.64 3413 ± 14.1 0.059 ± 0.009 0.00288 ± 0.00004 18.55 ± 0.80 681 4.1 123 189 0.31 –0.29 338.7 ± 7.3 0.044 ± 0.006 0.00296 ± 0.00002 19.07 ± 0.44 713 5.1 261 318 0.67 0.86 331.1 ± 2.3 0.053 ± 0.004 0.00299 ± 0.00001 19.27 ± 0.17 715 6.1 249 323 0.58 1.13 365.4 ± 2.5 0.055 ± 0.005 0.00271 ± 0.00001 17.42 ± 0.17 725 7.1 215 200 0.56 2.25 322.9 ± 5.5 0.064 ± 0.007 0.00302 ± 0.00002 19.48 ± 0.38 N.D. 8.1 167 197 0.41 0.55 347.5 ± 4.9 0.050 ± 0.007 0.00286 ± 0.00002 18.42 ± 0.30 794 10.1 139 186 0.36 1.68 321.3 ± 2.5 0.059 ± 0.007 0.00306 ± 0.00001 19.69 ± 0.22 752 11.1 419 599 1.11 –0.27 325.8 ± 2.1 0.044 ± 0.003 0.00308 ± 0.00001 19.81 ± 0.15 727 12.1 200 195 0.52 0.87 330.0 ± 2.5 0.053 ± 0.005 0.00300 ± 0.00001 19.33 ± 0.19 687 13.1 160 203 0.41 1.85 330.8 ± 19.1 0.061 ± 0.013 0.002967 ± 0.0000619.10 ± 1.15 682 14.1 300 380 0.77 0.65 334.1 ± 2.2 0.051 ± 0.004 0.00297 ± 0.00001 19.10 ± 0.16 721 15.1 261 223 0.69 0.18 323.0 ± 3.6 0.048 ± 0.004 0.00309 ± 0.00001 19.90 ± 0.20 661 16.1 150 132 0.40 0.42 323.2 ± 6.9 0.050 ± 0.004 0.00308 ± 0.00002 19.80 ± 0.48 692 17.1 161 132 0.41 4.92 321.5 ± 2.5 0.085 ± 0.007 0.00296 ± 0.00001 19.03 ± 0.24 693 18.1 135 111 0.34 2.97 334.0 ±7.7 0.070 ± 0.007 0.00290 ± 0.00002 18.69 ± 0.47 685 (continued)

The orientations of many dikes within the Chemehuevi Mountains parallel agree with previous studies in that mafic dikes are the most volumetrically sig- preexisting anisotropies within the country rock. Dikes in domain 1 commonly nificant, especially in the center of the range (domain 3; John and Foster, 1993). parallel quartz-sericite joints associated with late-stage cooling of the Cheme- The geometry of dikes also varies across the range. In domains 1–3, dikes huevi Mountains Plutonic Suite (John and Foster, 1993; John and Mukasa, are planar and commonly traceable for kilometers. In these domains, dikes 1990). In domains 4 and 5, dikes locally intrude moderately south-dipping brit- with mafic to intermediate compositions truncate intermediate-felsic and felsic tle to mylonitic shear zones. There is no evidence that significant rotation of dikes, indicating that mafic to intermediate dikes are relatively younger (Fig. dikes took place outside of localized ductile deformation, as presented in this 7A). Commonly, the MWF truncates dikes at high angles; in some instances, study, and minor isostatic doming related to denudation concentrated on the dikes terminate within meters of the fault zone, and/or are deflected in the flanks of the core complex (John and Foster, 1993). extension direction (Fig. 7B). Dikes in domains 4 and 5 can be planar in outcrop (Fig. 7C), but locally define open to isoclinal folds (Fig. 7D) and have un- Dike Abundance and Geometry dulatory geometries (Fig. 7E). Dikes in these eastern domains are commonly hosted in centimeter-thick, southeast-dipping shear zones that accommodated Dikes hosted in the footwall to the Chemehuevi detachment fault, proximal top-to-the-southeast displacement (normal-sense deflection) of metamorphic to the MWF, are centimeter- to multimeter-thick planar intrusions that extend foliation (Fig. 7F). for meters to kilometers along strike (Fig. 7). This study provides an upper bound on dike abundance with intermediate to felsic Miocene dikes compos- Fault Zone Dikes ing as much as 25% of the footwall, observed in domains 4 and 5. Previous studies estimated dikes to compose <1%–2% of the CDF footwall overall and Mafic to intermediate dikes are common within the damage zone of the >10% in the center of the range (John and Foster, 1993). The relative abundance MWF, and show extreme variability in their shape, thickness, and orientation. of dikes of different compositions also varies spatially across the Chemehuevi These dikes vary in thickness from centimeters to meters, and vary in shape Mountains, from southwest to northeast; mafic to intermediate dikes are the from subparallel walled to irregular (Fig. 8). Like mafic to intermediate dikes most abundant in domains 1–3, whereas intermediate-felsic to felsic dikes are outside the damage zone, the age of these dikes is unconstrained due to ex- relatively more abundant in domains 4 and 5 (Fig. 3). The results of this study treme greenschist facies alteration, but are coeval with MWF slip based on a

14 Mafic IntermediateIntermediate- Felsic felsic 12 Trachyte Rhyolite 10 Trachy- O (wt% ) 2 andesite Trachydacite Figure 2. Total alkali versus silica (after Le K Bas et al., 1986) plot showing the range 8 Basaltic

O + trachy- in whole-rock composition (anhydrous) 2 andesite for dikes of the Chemehuevi dike swarm, Na hosted in the footwall to the Chemehuevi 6 Trachy- detachment fault. Black circles are dikes basalt without a mylonitic fabric; red circles are Dacite dikes that host a penetrative L (linear) > S Andesite (foliation) mylonitic fabric. Regional data 4 Basaltic Basalt andesite are plotted as gray symbols. Terminology used in the text includes mafic (<52 wt%

SiO2), intermediate (>52 to <63 wt% SiO2), 2 intermediate-felsic (>63 to <69 wt% SiO2), 40 45 50 55 60 65 70 75 80 and felsic (>69 wt% SiO2). Raw data are shown in Table 1. PST—Peach Spring Tuff. SiO 2 (wt%) Chemehuevi dike swarm (mylonitic) Chemehuevi dike swarm (nonmylonitic) Mohave Mountains dike swarm (Pease, 1991) Chambers Well dike swarm (Gentry, 2015) Pre- and post-PST volcanics (McDowell et al., 2014) PST intracaldera and outflow (Pamukcu et al., 2013) Swansea Plutonic Suite (Bryant and Wooden, 2008)

70 )

Figure 3. Whole-rock SiO2 (wt%) versus

65 (wt% 2 longitude for dikes from the Chemehuevi O dike swarm collected across the footwall 60 Si in the regional slip direction. Mafic to 55 intermediate-felsic composition dikes are exposed throughout the range, whereas dikes with >70% SiO are only noted in the 50 2 structurally deepest exposures of the foot- 45 wall (northeast). 114° 38′ 114° 36′ 114° 34′ 114° 32′ 114° 30′ 114° 28′ Longitude (W)

Chemehuevi dike swarm (mylonitic) Chemehuevi dike swarm (nonmylonitic)

20 8 3.0 A B C 18 2.5 6 2.0 16 4 1.5 (wt%) (wt% ) 3 2 O (wt% )

O 14 2 2 1.0 Ti O K Al 2 12 0.5

10 0 0.0 45 55 65 75 85 45 55 65 75 85 45 55 65 75 85

SiO2 (wt%) SiO2 (wt%) SiO2 (wt%) 10000 800 800 D E F

600 600 ) )

1000 ) (ppm 400 (ppm 400 Zr Ba (ppm Zr 100 200 200

10 0 0 45 55 65 75 85 45 55 65 75 85 10 100100010000

SiO2 (wt%) SiO2 (wt%) Sr (ppm) Chemehuevi dike swarm (mylonitic) Chemehuevi dike swarm (nonmylonitic)

Mohave Mountains dike swarm (Pease, 1991) Figure 4. Selected trace element plots and Harker variation diagrams showing whole-rock chemistry (normalized to an anhydrous Chambers Well dike swarm (Gentry, 2015) basis) for selected elements from the Chemehuevi dike swarm and regional data (gray). PST—Peach Spring Tuff. Pre- and post-PST volcanics (McDowell et al., 2014) PST intracaldera and outflow (Pamukcu et al., 2013) Swansea Plutonic Suite (Bryant and Wooden, 2008)

number of crosscutting relations, including (1) mafic dikes intrude open frac- magmatic textures (Fig. 9A). These textures are characterized by an aphanitic tures (Fig. 8A) and cataclasites (Fig. 8B) within the damage zone with locally groundmass hosting randomly oriented euhedral to subhedral phenocrysts of preserved chilled margins (Fig. 8C); (2) mafic dikes host clasts of cataclasite quartz, feldspar, biotite, and hornblende in felsic dikes, and feldspar, hornblende, (Fig. 8B), yet show repeated fracture themselves (Figs. 8A, 8C); and (3) dikes pyroxene, and oxides in mafic to intermediate dikes. In contrast, all interme- interfinger with quartz-epidote veins within and adjacent to the MWF damage diate to felsic dikes in eastern domains (4 and 5) are L (linear) > S (foliation) zone (Fig. 8D). These relations are common in domains 1–3 but rare to absent tectonites with fine-grained or porphyroclastic textures (including porphyro- in domains 4 and 5, and indicate episodic and repeated fracturing and magma- clasts of feldspar, amphibole, and mica). Aligned phyllosilicates and quartz and tism at structurally shallow depths within the MWF zone during slip. feldspar ribbons define the tectonic fabric (Figs. 9B–9D). Lineations hosted in mylonitic dikes trend northeast-southwest, with the maximum eigenvector ori- Dike Textures and Fabrics ented 231°/9°, parallel to the regional extension direction (Fig. 6; John, 1987a). The present-day gentle southwest plunge of this mylonitic lineation is likely a Magmatic and solid-state fabrics are preserved in dikes of the Chemehuevi result of minor denudation-related footwall uplift and rotation. Mylonitic foli- dike swarm, and vary by position in the detachment footwall downdip. Dikes ations hosted by the dikes parallel dike margins, and are dominantly subhori- exposed in the western domains (1–3) preserve fine-grained or porphyritic zontal to moderately south dipping (Figs. 9C, 9D).

Mean = 21.45 ± 0.19 [0.88%] 2σ Mean = 20.23 ± 0.23 [1.1%] 2σ MSWD = 1.5, probability =0.063 MSWD = 0.78, probability =0.87

Figure 5. Weighted mean Th-corrected 206Pb/238U ages of zircon from five Miocene dikes of the Chemehuevi dike swarm (domains 1, 2, and 5). Each bar represents the Th-corrected 206Pb/238U date from a single spot analysis, showing 1σ uncertainty. Analyzed grains in Table 2 interpreted to be inherited xenocrystic zircons are ex- cluded. MSWD—mean square of weighted Mean = 20.94 ± 0.13 [0.64%] 2σ MSWD = 1.10, probability =0.33 deviates. Mean = 19.21 ± 0.15 [0.79%] 2σ MSWD = 1.5, probability =0.11

Mean = 19.59 ± 0.71 [3.6%] 2σ MSWD = 0.23, probability =0.88

Poles to dikes Domain 1 Domain 2 Domain 3 Domain 4 Domain 5

Set 1 Set 2 Set 1 Set 2

n =114 n = 63 n = 104 n = 66 n = 38 n = 42 n = 38 E1 = 189°/42° E1 = 106°/16° E1 = 199°/35° E1 = 112°/21° E1 = 351°/10° E1 = 356°/56° E1 = 338°/72° Meters A A′ A′′A′′′ 1000 MWF 1000 CDF 500 500 °C Sea level 200 Sea level C °C °C –500 250 ° 300 350 400 °C –500

Domains 4 and 5 Lithologies Quaternary sediments

Miocene Chemehuevi Dike Swarm Mylonitic lineations hosted in dikes Cretaceous Chemehuevi plutonic suite n=47 Proterozoic gneiss and migmatite E1 = 231°/9°

Figure 6. Cross section through the Chemehuevi Mountains (looking northwest, normal to the slip direction) showing generalized rock types, isotherms at fault initiation, structural domains, and schematic dike orientations. Wavy lines in Proterozoic gneiss and migmatite indicate the approximate orientation of metamorphic and tectonic fabrics. Stereonets show poles to dikes and mylonitic lineations contoured with an interval of 2 and signifi cance level of 3 and include the maximum eigenvector (E1; from Stereonet v 9.3.0; Allmendinger, 2015). Dike orientations plotted in domains 1–3 are Miocene but potentially include some similar oriented Cretaceous dikes indistinguishable in the field. In domains 1 and 2, set 1 includes dikes that strike within 45° of 105° or 285°; set 2 represents all other orientations. Only Miocene dike orientations are plotted in domains 4 and 5. Mylonitic lineations hosted in Miocene dikes are shown for domains 4 and 5; mylonitic foliations are parallel to dike orientations. All data are shown in their present-day orientation (unrotated). CDF—Chemehuevi detachment fault; MWF—Mohave Wash fault.

The Chemehuevi dike swarm shows a systematic increase in mylonitic DISCUSSION fabric intensity northeastward (downdip) in intermediate to felsic composition dikes. This progression in macroscopic fabric development is mimicked Evidence for an Elevated Brittle-Plastic Transition by the country rock, but the transition between country rock hosting unde- in Synextensional Dikes formed versus mylonitic fabrics occurs at the deepest exposed structural levels in the northeast (John, 1987b; LaForge, 2016). None of the dikes in This study highlights distinctions in orientation, geometry, and internal fab- domains 1–3 host mylonitic fabrics. However, equivalent dikes in domain 4 ric between dikes of similar composition and age at different structural levels host a penetrative mylonitic foliation and lineation; the country rock only of the low-angle normal fault system. These observations indicate that syntec- rarely hosts localized zones of centimeter-thick mylonitic deformation in tonic intermediate to felsic dikes deformed with an elevated brittle-plastic tran- quartz (Fig. 9C). In domain 5, both dikes and their country rock host mylonitic sition (BPT) relative to the surrounding country rock. The BPT in quartz and feld- fabrics of similar orientation; recrystallized mineral phases in county rock are spar associated with these dikes is expressed geographically between domains limited to meter-thick zones of quartz and phyllosilicates (Fig. 9D; John and 3 and 4 (~18 km downdip from the westernmost exposure of the MWF, at ~9 km Mukasa, 1990). minimum paleodepth), based on penetrative mylonitic fabrics hosted in inter-

A LANF Slip B LANF Slip

intermediate-felsic dike

mafic dike

mafic dikes

3m 40 cm

C LANF Slip D LANF Slip Figure 7. Dikes hosted in country rock ad- amphibolite gneiss jacent to the Mohave Wash fault (MWF). (Proterozoic) LANF—low-angle normal fault. (A) Parallel- amphibolite gneiss walled, nonmylonitic mafic dikes that cut (Proterozoic) an intermediate-felsic dike (domain 1). mylonitic felsic dike (Miocene) (B) Nonmylonitic mafic dike pinching out near the basal MWF contact (domain 2). (C) Mylonitic felsic dike cutting metamor- iate ne) phic foliation in domain 4. (D) Folded mylo folded intermed dike (Mioce nitic intermediate-felsic dike (domain 4). -felsic (E) Undulatory geometry of a mylonitic felsic dike hosted in mylonitic gneiss representative of domain 5. (F) Relict folded shear zone intruded by a mylonitic Mio- fold axial trace cene dike (domain 4). 1m 1m

A E LANF Slip F LANF Slip

amphibolite gneiss (Proterozoic)

mylonitic amphibolite gneiss (Proterozoic) mylonitic felsic mylonitic felsic dike (Miocene) dike (Miocene)

1m 20 cm

A LANF Slip B LANF Slip

mafic dike MWF zone (cataclasite)

MWF zone

mafic dike

Figure 8. Annotated photographs of fault- zone dikes within the damage zone to the Mohave Wash fault (MWF). LANF—low- angle normal fault. (A) Irregular mafic dike 15 cm 10 cm intruded into fractures. (B) Irregular mafic dike intruded into a cataclasite in domain 2 hosting chilled margins. (C) Mafic dike C LANF Slip D with an offset chilled margin hosting open fractures in domain 2. (D) Mafic dike and quartz-epidote vein interfingering with a fractured, offset chilled margin sharp to gradational contact in the basal position of the damage zone.

MWF zone mafic dike

quartz-epidote mafic dike vein

5cm 8cm

mediate to felsic dikes only in domains 4 and 5, and their absence in domains weaker than monomineralic aggregates. Alternatively, enhanced fluid activity 1–3. The BPT of quartz hosted in country rock is structurally deeper, between in dikes may aid in mylonitic fabric development (Selverstone et al., 2012). domains 4 and 5 (John, 1987b; LaForge, 2016). This evidence substantiates that We suggest that residual heat from dike emplacement during extension may the MWF was active across the upper limit of the BPT (John and Foster, 1993), have facilitated deformation in the solid state as dikes cooled. We also propose with a shallowed BPT in synextensional intermediate to felsic dikes. that ambient heat of the footwall also played a role in deformation. The lack We suggest that dikes deformed within an elevated BPT resulting from a of deformation in intermediate to felsic dikes at shallow structural levels (do- combination of characteristics unique to dikes in this system, including the mains 1–3) suggests that higher ambient temperature in the footwall downdip fine-grained polymineralic texture, enhanced fluid activity, and elevated tem- allowed dikes to deform plastically. This deformation could have been accom- peratures relative to the footwall. Studies of naturally deformed mylonites plished by (1) dikes cooling slower at deeper structural levels while undergoing (Stünitz and Fitzgerald, 1993; Kilian et al., 2011; Sullivan et al., 2013; Oliot et al., differential stress forming a mylonitic fabric or (2) hotter ambient temperatures 2014), and experimental formed mylonites (Bos and Spiers, 2001; Holyoke in deep structural levels after cooling to background temperatures, allowing and Tullis, 2006a, 2006b) indicate that fine-grained, polyphase aggregates are the rheologically distinct dikes to form a mylonitic fabric. While the exact

A B LANF Slip

Figure 9. Internal fabrics of dikes adjacent to the Mohave Wash fault (MWF). LANF— low-angle normal fault. (A) Undeformed 2 cm 2 cm porphyritic intermediate dike in domain 2. (B) Lineation-parallel and foliation-perpendicular surface of a mylonitic felsic dike in domain 4. (C) Mylonitic dike and unde- C LANF Slip D LANF Slip formed country rock contact in domain 4. (D) Mylonitic fabric in Miocene dike (shown with dashed lines) and gneissic country rock in domain 5. mylonitic felsic dike (Miocene)

mylonitic felsic non-mylonitic amphibolite dike (Miocene) gneiss (Proterozoic)

mylonitic amphibolite gneiss (Proterozoic)

4 cm 40 cm

mechanisms that facilitated the elevated BPT in intermediate to felsic dikes are orthogonal to the extension direction (Anderson, 1951), as they do in the New- unclear, the dikes represent a distinct component of the system that localized berry Mountains (Spencer, 1985), Whipple Mountains (Gans and Gentry, 2016), plastic strain during the early evolution of this low-angle normal fault system. Harquahala Mountains (Richard, et al., 1990), and Buckskin Mountains (Single- ton, 2015). Large populations of Miocene dikes in the Chemehuevi Mountains strike east-west or northeast-southwest, parallel or oblique to the extension Comparison of the Chemehuevi Dike Swarm Orientation direction. Although the west-northwest dike sets in domains 1 and 2 are close to Local Dike Swarms to extensional-orthogonal (E1 of poles to dikes trend within 26° and 36° of be- ing orthogonal extension direction, respectively, in domain 1 and 2), they are The orientations of dikes in the Chemehuevi dike swarm are not as ex- still oblique to the extension direction of 45°–55° northeast. Oblique or parallel pected for the regional northeast-directed extension, but are similar to swarms dikes to extension the direction are more comparable to dikes in the Homer of dikes exposed elsewhere in the CREC north of the Chemehuevi Mountains. Mountains, Piute Range, and northwestern Sacramento Mountains, which Assuming no preexisting anisotropies, dikes are predicted to intrude roughly host east-west–trending dikes (Spencer, 1985).

East-west–oriented dikes in the Homer Mountains area occur in structural footwall rotation to be minor, and this scenario is therefore less likely (John domains of synformal warping near the detachment breakaway (Spencer, and Foster, 1993). In domains 4 and 5, dikes intruded moderately dipping shear 1985). Spencer (1985) argued that synformal down-warping creates compres- zones hosted in Proterozoic gneiss, and subsequently rotated into parallelism sive stresses that overwhelm regional extensional stress during denudation, with the overriding low-angle normal faults, an effect that is also displayed in shifting the local extension direction to north-south, accounting for east-west– the transitions between domains 3, 4, and 5. With increasing structural depth oriented dike emplacement. However, the Chemehuevi Mountains are hosted in the footwall to the fault system, dikes are closer to horizontal; in domains in an antiformal uplift geographic-structural domain in the CREC (Spencer, 3, 4, and 5 the maximum eigenvectors of poles to dikes plunge 10°, 56°, and 1985), where dikes are predicted to be north-south or northwest-southeast, 72°, respectively. and therefore do not clearly fit this explanation. We propose that dikes in the Chemehuevi Mountains were likely controlled by preexisting weaknesses and minor localized ductile rotation rather than the contemporaneous stress Comparison of the Chemehuevi Dike Swarm to Local Magmatism state. In domains 1–3 Cretaceous joints (noted in John, 1987a, 1987b; John and Foster, 1993) likely controlled the original orientations of the two sets of dikes The Miocene Chemehuevi dike swarm correlates both temporally and hosted in the Chemehuevi Mountains Plutonic Suite. Alternatively, if we con- chemically with local and regional extrusive magmatism. The earliest pulse sider west-northwest dikes to represent an extension-orthogonal, originally of the Chemehuevi dike swarm (Table 3) was emplaced during early extension vertical set, they would have rotated to the southwest around a horizontal axis between 21.45 ± 0.19 and 19.21 ± 0.15 Ma, and shortly thereafter magmatism by >40°. This rotation varies significantly from other data constraining CDF overlaps in age with the ca. 22–18.8 Ma lower volcanic section of Miller and

TABLE 3. MIOCENE MAGMATISM AND FAULTING IN THE CHEMEHUEVI MOUNTAINS Ages Intrusive magmatic pulse Relative volume (Ma) Dating method CompositionOutcrop style 21.45 ± 0.19 to Parallel-walled dikes offset by MWF, mylonitic U/Pb in zircon Felsic to intermediate 19.20 ± 0.11 deep in footwall First Large Parallel-walled dikes in MWF footwall and Intermediate-mafic During MWF slip Crosscutting relationships unfractured to fractured irregular dikes in to mafic the fault zone Postdates CDF slip; Localized flows and plugs that intrude or Second Minor Whole-rock K/Ar Mafic* 11.6 ± 1.2 to 11.1*,† overlie and fuse CDF cataclasites*

Extrusive magmatic pulse Thickness in CDF hanging wall Ages Dating method CompositionOutcrop style Stratigraphic correlation to nearby volcanism dated by fission track Lower volcanic section § §,# Mafic to intermediate§ In tilted CDF hanging wall§ 500 m to 1 km thick ca. 22 to 18.8 in zircon and K/Ar in whole- rock and biotite samples In tilted CDF hanging wall, stratigraphically Peach Spring Tuff 0–60 m thick§ 18.78 ± 0.02** 40Ar/39Ar Rhyolite ignimbrite above and dipping 5°–15° gentler than the lower volcanic section§ 15.54 ± 0.03 to Bimodal; Post–Peach Spring Tuff N.D. 40Ar/39Ar Rare and interbedded in sedimentary rocks§ 13.9 ± 0.1§ mafic and felsic§

Fault Fault initiation Period of rapid slip Peak slip rate CessationTotal slip MWF 23 ± 1 Ma† N.D.N.D. ca. 19 Ma ~2 km* ~7.5 ± 0.5 mm/yr§; CDF 23 ± 1 Ma† 19 to 15†, § ca. 11.6 ± 1.2 Ma*>18 km§ accelerated slip at 15 ± 1 Ma†† Note: MWF—Mohave Wash fault; CDF—Chemehuevi detachment fault; N.D.—no data. *From John (1987b). †From John and Foster (1993). §From Miller and John (1999). #From Sherrod and Nielson (1993). **From Ferguson et al. (2013). ††From Carter et al. (2006).

John (1999), exposed in the CDF hanging wall. Compositionally, mafic to inter- major and trace element geochemistry (Pamukcu et al., 2013; McDowell et al., mediate dikes associated with this pulse are similar to the lower volcanic sec- 2014; Fig. 4). We suggest that the magmatism in the Chemehuevi Mountains tion (Table 3; Miller and John, 1999); felsic dikes are not. These felsic intrusive is closely related to other magmatism in the CREC, whereas its relation to the rocks are similar in composition to the PST (Fig. 2), a unit they predate by as PST is unclear. little as 400 k.y. (Table 3). These high-silica dikes are exposed only in the northeast Chemehuevi Mountains (Fig. 3), closest to the recently recognized Peach Spring Tuff caldera, in the central Black Mountains (Ferguson et al., 2013). The Timing of Magmatism and Extension second pulse of dike emplacement includes minor 11.6 ± 1.2–11.1 Ma mafic dikes (John, 1987b) similar in composition to post-PST mafic lavas erupted in The Miocene history of extension in the Chemehuevi Mountains can be de- the CDF hanging wall between 15.5 and 13.9 Ma (Miller and John, 1999). No scribed by early brittle extension with dike emplacement and extrusive mag- intrusive rhyolitic dikes postdating deposition of the PST are observed. We matism followed by rapid brittle faulting accompanied by minor volcanism. A interpret the first pulse of the Miocene Chemehuevi dike swarm emplacement summary of inferred timing and rates of magmatism and faulting associated to represent part of the magmatic system that fed and rejuvenated a magma with the CDF system as constrained from this and other studies is presented system related to the coeval volcanic deposits hosted in the hanging wall to in Table 3 and Figure 10. the CDF, and possibly the PST. The second pulse of dike emplacement is not The syntectonic portion of the Chemehuevi dike swarm was emplaced coeval with extrusive volcanism, despite some similarities in composition. from ca. 21.5 to 19.2 Ma shortly after (~1.5 ± 1 to 3.8 ± 1 m.y.) the initiation of ex- Major and trace element geochemistry highlight similarities between the tension at 23 (±1) Ma. The first pulse of dike intrusion is distributed across the Chemehuevi dike swarm and local coeval magmatism across the CREC. The range and divided into two distinct episodes based on composition, timing, Chemehuevi dike swarm is similar to dikes in the Mohave and Whipple Moun- and emplacement and/or deformation style. The earliest dikes were interme- tains, the Swansea Plutonic Suite in the Buckskin Mountains (Pease, 1991; diate to felsic in composition, including dikes that developed a mylonitic fabric Bryant and Wooden, 2008; Gentry, 2015), and pre- and post-PST volcanism in in deep structural levels of the fault system (domains 4 and 5). The timing of the southern Black Mountains (Figs. 6 and 8; Fig. S2 [see footnote 1]). However, fabric formation is poorly constrained, but occurred in the solid state following PST-related magmatism differs from the Chemehuevi and CREC magmatism in emplacement of individual dikes before significant denudation-related cooling

8 CDF

icant Signif

4 Figure 10. Relative magmatic intrusion

(mm/yr) and/or eruption rates versus time-aver- aged slip rates for the Chemehuevi de- 2 tachment fault (CDF) and Mohave Wash

erageAv slip rate MWF Mino r fault (MWF) at the latitude of the Cheme- eruption or intrusion rate huevi Mountains plotted as a function of Estimated relative magmatic 0 time. Rates of magmatism highly generalized. MWF slip rate shown as a dotted 24 23 22 21 20 19 18 17 16 15 14 13 12 11 line; variations in slip rate of the CDF are Time (Ma) shown by the dashed line. Data presented were synthesized from multiple sources Chemehuevi dike swarm Local Volcanism outlined in Table 3.

First pulse Post Peach Spring Tuff (Bimodal) Intermediate to Felsic Peach Spring Tuff (Felsic) Mafic to Intermediate Lower Volcanic Section (Mafic to Intermediate) Second pulse Mafic

of the footwall. Mylonitization of individual felsic dikes at deep structural levels many metamorphic core complexes (Fletcher and Bartley, 1994; Singleton and is therefore bracketed as shortly after emplacement from ca. 21.5–19.2 Ma to Mosher, 2012; Singleton, 2013). These recent studies document magmatism the inception of rapid slip at or shortly after 19 Ma. The second episode of intruding into and adjacent to evolving fault systems prior to rapid slip, and dike emplacement dominated by mafic to intermediate composition dikes ob- highlight the long-standing question of the role intrusive magmatism plays in served cut felsic dikes at moderate to shallow structural depths (domains 1–3; metamorphic core complex evolution. Fig. 7A). While we do not present absolute ages of mafic to intermediate dikes, we suggest that ages cluster ca. 19 Ma because they postdate 19.2 Ma felsic dikes, and are compositionally similar to the ca. 20–19 Ma dikes in the Mohave CONCLUSIONS Mountains and local volcanic rocks as young at 18.8 Ma (Pease, 1991; Miller and John, 1999). We propose here that slip on the MWF ceased near the end This study presents data related to the synextensional mafic to felsic Mio- of emplacement of the syntectonic dike swarm ca. 19 Ma, based on mutually cene Chemehuevi dike swarm intricately involved in the early history of the crosscutting relations with fractures within the MWF fault zone that suggest CDF system, and contributes five main ideas. (1) The Chemehuevi dike swarm that fault slip and dike emplacement occurred and stopped at approximately intruded into the footwall, hanging wall, and fault zone of a low-angle normal the same time. We also note that extrusive magmatism correlative with the fault as it was active in the upper middle crust deforming with an elevated BPT. dikes spans a broader age range (ca. 22 to 18.8 Ma), suggesting that dike em- (2) Although previous studies noted that the Chemehuevi dike swarm accom- placement may overlap with the onset of extension (Fig. 10; Miller and John, modated only a minor component of the regional extension (<2% of the CDF 1999; Ferguson et al., 2013). Thermochronologic and geochronologic estimates footwall by volume), this study suggests that they are locally more volumetri- of peak slip rates (7–8 mm/yr) along the CDF postdate voluminous magmatism cally significant adjacent to a low-angle normal fault zone, composing as much between 19 and 15 Ma. These same data highlight possible accelerated slip at as 25% of the footwall near the MWF in the deepest exposures of the footwall 15 ± 1 mm/yr (Miller and John, 1999; Carter et al., 2006), with the majority of and fault system (domains 4 and 5). (3) Dike orientations are somewhat anom- slip accommodated prior to 13.9 Ma (Fig. 10; Table 3; Miller and John, 1999). alous in the Chemehuevi Mountains with large populations striking east-west Brittle extension in the upper crust continued until 11.6 ± 1.2 Ma, evidenced or southwest-northeast, oblique and parallel to the extension direction. These by a minor second pulse of mafic diking that intrudes into and above the CDF were likely controlled by preexisting fractures and minor postemplacement (Table 3; John, 1987b; John and Foster, 1993). rotation. (4) The Chemehuevi dike swarm represents part of the feeder system The temporal evolution of normal faulting accompanied by magmatism to local syntectonic volcanism, and is geochemically similar to regional mag- followed by rapid-slip faulting devoid of magmatism is similar to other highly matism. (5) The Miocene evolution of the CDF system fits a two-phase tectono- extended terranes throughout the CREC (Gans and Bohrson, 1998; Faulds magmatic evolution of other highly extended areas in the CREC and Basin and et al., 1999) and the Basin and Range Province (Gans et al., 1989) suggestive of Range Province with an initial period of faulting accompanied by magmatism an active rifting mode of crustal extension. ~1.5 ± 1 to 3.8 ± 1 m.y. after the onset of extension followed by rapid slip of rapid brittle faulting without significant magmatism.

Tectonic Significance of the Chemehuevi Dike Swarm ACKNOWLEDGMENTS Intrusive magmatism that predates rapid brittle extension represents an We acknowledge the support from National Science Foundation grants (EAR-1145183 awarded increasingly recognized component of crustal stretching in CREC core com- to John and Grimes) as primary funding for this project. LaForge also acknowledges additional financial support for field work from the American Association of Petroleum Geologists Weimer plexes. In the Whipple Mountains, the ca. 20.5–19.0 Ma Chambers Well dike Family Named Grant. We thank R. Heilbronner, H. Stunitz, R. Kilian, C. MacDonald, and J. Brown for swarm intruded prior to rapid tectonic extension beginning at 19.0–18.5 Ma useful discussions in the field and laboratory. Rose Pettiette and Connor Marr helped in the field. (Gans and Gentry, 2016). In the adjacent Buckskin-Rawhide core complex, the We thank associate editor Allen J. McGrew and reviewers John S. Singleton and Walter A. Sullivan for constructive reviews of this manuscript. We thank the Bureau of Land Management Needles ca. 22–21 Ma Swansea Plutonic Suite predates the inception of extension ca. Field Office and Ramona Daniels for logistical support and extended access to the Chemehuevi 21–20 Ma (Singleton et al., 2014). We show in the Chemehuevi Mountains, the Mountains Wilderness Area. 21.5–19.2 Ma dikes predate rapid slip ca. 19 Ma. In the past, studies have em- phasized volcanism that predates rapid slip (Gans et al., 1989; Gans and Bohr- son, 1998; Faulds et al., 1999), but as these studies show, intrusive magma- REFERENCES CITED tism that predates rapid slip may be as widespread. In addition, these studies Allmendinger, R.W., 2015, Stereonet: v. 9.3.0: Ithaca, New York, Cornell University Earth and indicate that many volcanic deposits have intrusive equivalents that intrude Atmospheric Sciences Department, http://www.geo.cornell.edu/geology/faculty/RWA /programs/stereonet.html. adjacent or directly into fault zones. Magmatism is also clearly overprinted by Anderson, E.M., 1951, The Dynamics of Faulting with Applications to Britain: Edinburgh, Oliver deformation subsequent to intrusion, forming syntectonic L > S tectonites in and Boyd, 206 p.

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