Miocene Slip History of the Eagle Eye Detachment Fault, Harquahala

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RESEARCH ARTICLE Miocene slip history of the Eagle Eye detachment 10.1002/2016TC004241 fault, Harquahala Mountains metamorphic core Key Points: complex, west-central Arizona • Total displacement on the Eagle Eye detachment fault is ~44 ± 2 km Michael G. Prior1, Daniel F. Stockli1, and John S. Singleton2 • Apatite and zircon (U-Th)/He slip rates along the Eagle Eye detachment are 1Department of Geological Sciences, University of Texas at Austin, Austin, Texas, USA, 2Department of Geosciences, ~6.6 + 7.8/À2.3 km/Myr and ~6.6 + 7.1/À2.0 km/Myr, respectively Colorado State University, Fort Collins, Colorado, USA • Active extension along the Eagle Eye detachment fault from ~21 ± 1 Ma to ~14 Ma Abstract The structural and thermal evolution of major low-angle normal faults in the Colorado River extensional corridor has been a controversial topic since the pioneering studies of metamorphic core Supporting Information: complexes in the early 1980s. We present new geo-thermochronometry data from the Harquahala Mountains • Supporting Information S1 in west-central Arizona to determine the timing of extension, displacement magnitude, and slip rates along the • Table S1 Eagle Eye detachment fault (EED) during large-magnitude Miocene extension. Zircon and apatite (U-Th)/He • Table S2 • Table S3 data (ZHe and AHe, respectively) from 31 samples along a ~55 km extension-parallel transect indicate active • Table S4 slip along the EED occurred between ~21 ± 1 Ma and ~14 Ma. The spatial extent of ZHe ages and exhumation • Table S5 of the zircon partial retention zone indicated ~44 ± 2 km of total displacement, whereas lithologic similarity

Correspondence to: and identical U-Pb ages between correlated footwall rocks in the Little Harquahala Mountains and breccia M. G. Prior, clasts at Bullard Peak in the NE Harcuvar Mountains indicated ~43–45 km of displacement across the EED. AHe [email protected] and ZHe data indicated slip rates of ~6.7 + 7.8/À2.3 km/Myr, and ~6.6 + 7.1/À2.0 km/Myr, respectively, both consistent with the duration and displacement estimates. The EED initiated as a listric fault with an ~34 ± 9° dip Citation: that decreased to ~13 ± 5° below ~7 km depth. Secondary breakaway development and footwall exposure Prior,M.G.,D.F.Stockli, andJ.S.Singleton occurred by ~17 Ma, during active EED slip. Lithologic and geo-thermochronometric offset constraints show (2016), Miocene slip history of the Eagle Eye detachment fault, Harquahala excellent agreement and provided a rare opportunity to fully resolve the timing, rates, and total displacement Mountains metamorphic core complex, magnitudes along a major continental detachment fault. west-central Arizona, Tectonics, 35, doi:10.1002/2016TC004241. 1. Introduction Received 18 MAY 2016 Accepted 18 JUL 2016 Low-angle normal faults (detachment faults) are major extensional structures that accommodated large- Accepted article online 22 JUL 2016 magnitude displacements (greater than or equal to tens of kilometers) and exhumed midcrustal rocks within Cordilleran metamorphic core complexes from British Columbia to Sonora, Mexico [Coney, 1980]. The significant crustal strain and large-magnitude displacements accommodated by detachment fault systems within a variety of extensional tectonic regimes underscores the importance of fully understanding their geometric, thermal, and temporal evolution. Many fundamental concepts regarding the structural evolution of detachment faults and metamorphic core complexes were developed in the Colorado River extensional corridor (CREC) [Lister and Davis, 1989; Spencer and Reynolds, 1991; John and Foster, 1993] and subsequently applied to understanding detachment faulting within numerous extensional settings such as the Greek Cycladic islands [Lister et al., 1984; Brichau et al., 2006, 2010], the Appenines [Collettini et al., 2006], and the Rio Grande rift [Ricketts et al., 2015]. Low-temperature thermochronometry has been an important tool for quantifying the timing and magnitudes of exhumation and cooling along detachment faults within these varied extensional tectonic regimes [e.g., Stockli, 2005, and references therein]. Numerous studies within the Colorado River extensional corridor of west-central Arizona and eastern California (Figure 1) have focused on determining slip rates, timing of extension, and displacement magnitudes within Cordilleran metamorphic core complexes [Reynolds and Spencer, 1985; Howard and John, 1987; Davis and Lister, 1988; Spencer and Reynolds, 1991; Foster and John, 1999; Brady, 2002; Singleton et al., 2014]. Despite these previous investigations, the estimates for total displacement magnitudes and slip rates within individual core complexes of the CREC still vary significantly, largely due to the absence of unambiguous offset structural, lithological, and/or thermochronometric markers. The robust quantification of displacement magnitudes, slip rates, and the timing of extension along low-angle normal faults is crucial to understanding how detachment fl ©2016. American Geophysical Union. faults in uence the thermal structure of continental crust during extension. This study presents new geo- All Rights Reserved. thermochronometric data from the footwall and hanging wall of the Eagle Eye detachment fault system in

PRIOR ET AL. HARQUAHALA MOUNTAINS MIOCENE SLIP 1 Tectonics 10.1002/2016TC004241

Figure 1. Simpliﬁed geologic map of the lower Colorado River extensional corridor of west-central Arizona and southeastern California (modiﬁed from Spencer and Reynolds [1989], Richard et al. [2000], and Jennings [1977]).

the Harquahala Mountains metamorphic core complex. We integrated these new data with additional lithologic constraints to provide new insights into the slip history and thermal conditions along large- displacement detachment faults. Quantifying the displacement history and geometric evolution of the Eagle Eye detachment contributes new insights on the classic CREC detachment fault system by constraining initial and evolving fault geometries, synextensional deposition and displacement of hanging wall rocks, footwall surface exposure and fault slip rates, which are all key to further understanding how low-angle normal faults evolve within varied extensional tectonic settings.

2. Geologic Background 2.1. Colorado River Extensional Corridor The Colorado River extension corridor (CREC) in the central Basin and Range province contains extensive exposures of highly extended continental crust characterized by numerous type examples of Cordilleran metamorphic core complexes [Coney, 1980]. Metamorphic core complexes within the CREC of west-central Arizona and eastern California contain low-angle normal faults (detachment faults) that exhumed midcrustal mylonites during the Miocene (Figure 1) [Rehrig and Reynolds, 1980; Spencer, 1984; Davis et al., 1986; Howard and John, 1987; Spencer and Reynolds, 1991; Singleton et al., 2014]. The Harquahala Mountains metamorphic

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core complex is the southernmost exposure of Tertiary mylonites exhumed along the regional corrugated detachment fault system connecting the lower plate of the Chemehuevi, Whipple, Buckskin-Rawhide, Harcuvar, and Harquahala Mountains within the Whipple tilt domain (Figure 1) [Spencer and Reynolds, 1991]. Metamorphic core complexes within the CREC and the Whipple tilt domain have an elongate topographic expression of ~ N60°E that is aligned subparallel to the regional extensional direction of ~ N50° ± 10°E based on mylonite lineations and synextensional dikes (Figure 1) [Richard et al., 1990; Singleton, 2013]. The extension-parallel topographic trend of metamorphic core complexes is distinct from the trend of fault blocks exhumed by high-angle “Basin and Range style” faulting. The prominent topographic expression reﬂects the corrugated geometry of the regionally correlative detachment fault system that links individual core complexes within the Whipple tilt domain [Rehrig and Reynolds, 1980; Spencer and Reynolds, 1991]. Published displacement estimates for the Buckskin-Rawhide detachment fault have varied from as much as ~70 km [Spencer and Reynolds, 1991] to more recent estimates of ~40–50 km [Singleton et al., 2014]. Displacement estimates within the adjacent Harcuvar Mountains are ~40–50 km [Reynolds and Spencer, 1985] based on a hypothesized lithologic correlation. We further test the original hypothesis of Reynolds and Spencer [1985] to determine displacement magnitude along the Eagle Eye detachment fault, which is correlative with the Bullard detachment fault that bounds the Harcuvar Mountains. South of the Harquahala Mountains distributed high-angle normal faults in the Big Horn Mountains predominate and mark the southern boundary of the Whipple tilt domain [Spencer and Reynolds, 1991]. The location of the Harquahala Mountains metamorphic core complex makes this an excellent location to evaluate changes in detachment geometry, timing of extension, and displacement magnitudes at the southern boundary of this major tilt domain.

2.2. Harquahala Mountains Metamorphic Core Complex The Harquahala Mountains metamorphic core complex is bound to the northeast by the Eagle Eye detachment fault, which exhumed primarily Proterozoic and Mesozoic crystalline basement rocks during Miocene extension [Richard et al., 1990]. The Eagle Eye detachment fault is exposed at Eagle Eye Peak (Figure 2) at the northeast end of the Harquahala Mountains and along the southern flank of the range where it is buried beneath Quaternary deposits. At Eagle Eye Peak the Eagle Eye detachment fault zone consists of chloritized mylonitic gneiss, separated from the overlying Miocene hanging wall basalt and sandstone by an ~ 1 m thick chlorite-smectite gouge zone [Haines and van der Pluijm, 2012]. Additional zones of chloritic breccia on the tops of Harquahala, Brown’s, and Stone Corral Mountains (Figure 2) are typical of rocks within 100 m below the Eagle Eye detachment fault [Richard et al., 1990]; assuming these chloritic breccias were formerly a single continuous zone, it indicates the Eagle Eye detachment fault dips ~5°NE and projects just above these NE ridges of the Harquahala Mountains [Richard et al., 1990]. Miocene mylonitic fabrics are only present within ~15 km of the NE tip of the Harquahala Mountains [Richard et al., 1990] and largely confined to the region east of Stone Corral [Pollard et al., 2013] (Figure 2). The relatively limited extent of Tertiary mylonites in the Harquahala Mountains is distinct from the Harcuvar or Buckskin-Rawhide Mountains where Tertiary fabrics are present across a large portion of the entire extent of the exhumed footwall (Figure 1). The limited exposure of Tertiary mylonite has previously been used to infer the Harquahala Mountains represent a structurally higher level of the Whipple tilt domain detachment fault system and do not accommodate enough displacement to exhume ductile rocks [Rehrig and Reynolds, 1980; Richard et al., 1990]. The footwall of the Harquahala and Little Harquahala Mountains defines an ~55 km elongate topographic ridge that trends ~ N55–60°E, approximately parallel to the regional extensional direction of ~ N50 ± 10° defined by Tertiary mylonitic lineations [Richard et al., 1990]. Several thick-skinned thrust faults formed during Mesozoic shortening within the Maria fold and thrust belt [Spencer and Reynolds, 1990] are preserved within the footwall of the Eagle Eye detachment fault. Thrust faults within the Harquahala Mountains and the Little Harquahala Mountains structurally juxtapose granitic and metamorphic Proterozoic rocks along with Paleozoic metasedimentary rocks and Mesozoic igneous rocks [Richard et al., 1987, 1990] (Figure 2). Tertiary footwall volcanic rocks consist of shallowly west dipping basaltic and andesitic flows primarily exposed in the western Little Harquahala Mountains (Figure 2). A small ~4 km2 exposure of the hanging wall at Eagle Eye Peak consists of basaltic flows and breccias and intercalated coarse-grained sandstones. Abundant mafic dikes are exposed throughout the majority of the Harquahala Mountains, along with older felsic and pegmatite dikes (Figure 2). Mafic dikes strike approximately NW-SE and typically dip ~60–70° northeast [Richard et al., 1990]. A single hornblende 40Ar/39Ar date of ~22.3 ± 2.0 Ma [Richard et al., 1994] from a

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Figure 2. Geologic map of the Harquahala Mountains metamorphic core complex and the Little Harquahala Mountains; simpliﬁed after Richard et al. [1994]. See Table 1 for zircon and apatite summarized (U-Th)/He data and Table S5 for 40Ar/39Ar data.

representative mafic dike in the Arrastre Gulch (Figure 2) area suggests many of these mafic dikes are early Miocene [Richard et al., 1990]. Based on the consistent ~60–70°NE dip of mafic dikes Richard et al. [1990] interpreted ~30° of horizontal axis rotation within the Harquahala Mountains footwall after ~22 Ma, assuming vertical dike intrusion. The current ~5° dip of the Eagle Eye detachment, combined with ~30° of rotation was used to infer an original, preextensional northeast dip of ~35° [Richard et al., 1990]. Paleomagnetic work by Livaccari and Geissman [2001] suggested 10 ± 5° of horizontal axis rotation that would restore the Eagle Eye detachment to an original dip of ~20°. Minor low-angle normal faulting is present along the Tenahatchipi Pass fault in the western Harquahala Mountains and in parts of the Little Harquahala Mountains (Figure 2). The NW-SE striking oblique-slip Harquahala, Brown’s, and Dushey Canyon faults within the Harquahala Mountains crosscut the Eagle Eye detachment fault and control the locations of prominent NW-SE canyons (Figure 2). Steeply dipping postdetachment faults cumulatively have ~2.5 km of apparent dextral slip and 1.3 km of apparent SW down slip [Richard et al., 1990]. These postdetachment faults have a similar trend to mapped faults within the Buckskin-Rawhide Mountains that offset the topographic trend of that range [Singleton, 2015]. The Dushey Canyon fault (Figure 2) is along strike with the oblique reverse Lincoln Ranch fault in the Buckskin Mountains, suggesting faults within the Harquahala Mountains may have a more complex postdetachment slip history. The difference between the trend of the topographic range and mylonitic lineations in the Harquahala Mountains may be a result of late-stage dextral displacement along

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postdetachment faults [Singleton, 2015], but these faults are not considered inﬂuential to the slip history of the Eagle Eye detachment fault discussed herein.

3. Methodology 3.1. Sampling Strategy Twenty-nine footwall samples were collected for (U-Th)/He dating along an ~55 km transect parallel to the topographic range trend from Eagle Eye Peak in the northeast to the Little Harquahala Mountains in the southwest (Figure 2). Footwall samples are primarily Proterozoic and Mesozoic granitic intrusions and orthog- neiss. The majority of samples (n = 24) were collected parallel to the central corrugation axis, with an additional five samples up to 7 km orthogonal to the transect line. Sample density is highest near the Eagle Eye detachment fault and the northeastern Harquahala Mountains where the Eagle Eye detachment is inferred to project just above the ridge crests of Harquahala, Brown’s, and Stone Corral Mountains (Figure 2) [Richard et al., 1990]. A sample of coarse-grained sandstone and upper plate nonmylonitic gneiss were collected from the hanging wall at Eagle Eye Peak to compare upper and lower plate thermal histories across the Eagle Eye detachment fault. Detrital zircon U-Pb dating was conducted on two hanging wall samples of Eagle Eye Peak sandstone along with 40Ar/39Ar geochronology of a single whole-rock basalt sample (EEB3) (Figure 2) to determine the timing of hanging wall sedimentation and volcanism relative to active displacement along the Eagle Eye detachment fault. Zircon U-Pb geochronology was conducted on footwall samples LHQ1 and HQ14 within the Western and Little Harquahala Mountains (Figure 2) and two hanging wall samples at Bullard Peak in the Harcuvar Mountains (Figure 3) to determine if specific lithologic units are correlative and can be used as a displacement marker for slip along the Eagle Eye detachment fault, following the hypothesis of Reynolds and Spencer [1985]. The rhyolite ash flow tuff stratigraphically below unit Tcb was also dated to provide a maximum age for deposition of the Bullard Peak sediments (Figure 3).

3.2. (U-Th)/He Thermochronometry This study utilized zircon and apatite (U-Th)/He thermochronometry to determine the cooling and exhumation history of the Harquahala Mountains and the displacement history of the Eagle Eye detachment fault. Low-temperature thermochronometry is a widely employed technique for quantifying the cooling and exhumation history of rocks and has been applied to a variety of extensional settings [e.g., Stockli, 2005, and references therein]. (U-Th)/He thermochronometry is based on temperature-dependent volume diffusion and retention of radiogenic helium within a mineral lattice. (U-Th)/He ages record the bulk thermal history of a sample as it cools and begins to retain radiogenic helium. For fast-cooled samples (≥10°C/Myr) the simplified concept of a closure temperature can be applied [Dodson, 1973]. For a 10°C/Myr cooling rate and standard crystal sizes between ~60 and 100 μm, the apatite closure temperature (Tc) is nominally ~60–75°C, with complete retention of helium below ~40°C [Wolf et al., 1996, 1998; Farley, 2000; Reiners and Farley, 2001]. The zircon (U-Th)/He closure temperature is ~180 ± 10°C for a 10°C/Myr cooling rate and an effective grain radius of ~60 μm[Reiners et al., 2002, 2004; Wolfe and Stockli, 2010]. All (U-Th)/He analyses were performed on single-grain aliquots at the University of Texas at Austin using a diode laser to degass 4He for measure- ment using isotope dilution and a Blazers Prisma QMS-200 quadropole mass spectrometer. Apatites and zircons are spiked with a 7 N nitric solution enriched in 235U, 230Th, and 149Sm after complete degassing. Zircon aliquots undergo additional dissolution with hydrofluoric and nitric acid in high-pressure digestion vessels; all solutions are analyzed for 238U, 235U, 232Th, and 147Sm with a Thermo Element 2 inductively coupled plasma mass spectrometer (ICP-MS). Raw (U-Th)/He ages are corrected for alpha-particle ejection using the Ft correction factor of Farley et al. [1996]. A standard error of 8% for zircon and 6% for apatite is applied to each aliquot based on the internal reproducibility of Fish Canyon Tuff zircon and Durango apatite standards. Most samples consist of four to five individual aliquots, with up to nine aliquots per sample. A total of 156 zircons and 118 apatites was analyzed from 31 samples (Tables 1, S1, and S2). Apatite grains were excluded from mean sample age calculations on the basis of old ages that are clear outliers. Anomalously old ages are most likely due to U and Th-rich microinclusions not visible under magnification. Seven zircon aliquots that yielded erroneous negative values due to analytical errors such as broken or lost grains are excluded from com- piled data tables. See Table S1 for reduced (U-Th)/He data from all individual aliquots. Six mean AHe sample ages were excluded from the sample transect since they produced ages older than the respective ZHe sample ages. Microinclusions are the most likely cause of the observed age inversion as no high-uranium

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Figure 3. Geologic map of the Bullard Peak region at the northeastern end of the Harcuvar Mountains modiﬁed after Reynolds and Spencer [1984]; 1:24,000 scale. Samples for U-Pb correlation are shown with yellow circles. Geologic symbols are the same as in Figure 2.

rims were measured during U-Pb dating of zircons from the same samples, which could have resulted in underestimated Ft correction and anomalously young ZHe ages [Hourigan et al., 2005].

3.3. U-Pb & 40Ar/39Ar Geochronology Laser ablation ICP-MS U-Pb dating was performed at the University of Texas at Austin using a Photon Machines Analyte G2 Excimer laser coupled to a single collector Thermo Element 2 magnetic sector ICP-MS. Zircon U-Pb analyses were done on unpolished tape mounts using a laser spot diameter of 30 μm and reduced using Iolite and the VizualAgeDRS [Petrus and Kamber, 2012] that operates on an IGOR Pro plat- form. GJ1 zircon was used as a primary standard (~601 Ma) [Elhlou et al., 2006] and PAK1 (~43 Ma) was used as an in-house secondary standard for independent veriﬁcation of ages. U-Pb concordia plots and weighted mean calculations were generated using the Isoplot v 3.75 visual basic add-in [Ludwig, 2003]. Errors for all U-Pb analyses are reported at 2σ with ~30–40 individual zircon analyses per sample for igneous crystallization ages; 145 and 113 zircons were analyzed for detrital samples EESS1 and EESS2, respectively, in order to capture all age modes that constitute ≥5% of the zircon population [Vermeesch, 2004]. Whole-rock 40Ar/39Ar geochronology was conducted on Eagle Eye Peak basalt sample EEB3 (Figure 2) to constrain the timing of hanging wall deposition and subsequent faulting. 40Ar/39Ar analysis was performed

at the U.S. Geological Survey (USGS) Argon Lab in Denver, using a 25 W CO2 laser to step heat sample EEB3 (19 steps) after neutron irradiation at the USGS TRIGA reactor. Liberated gas was puriﬁed and analyzed for masses 40Ar through 36Ar using a Thermo Scientiﬁc ARGUS VI mass spectrometer. Data were corrected for blanks, nucleogenic interferences, radioactive decay, and mass discrimination.

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Table 1. Summary of (U-Th)/He Data From the Harquahala Mountains Metamorphic Core Complexa UTM Coordinatesb

Sample Lithology Northing Easting Distance (km) Mean ZHe Age Max 2SE nc Mean AHe Age Max 2SE nc

HQ1 Jurassic granite 256213 3721002 52.00 46.0 6.49 (5/5) 19.4 1.56 (4/4) LHQ2 Cretaceous granite/granodiorite 253267 3729615 50.80 47.7 3.42 (5/5) 17.5 1.21 (3/5) HQ2 Med-grained granite 259893 3730741 44.10 54.7 4.38 (4/8) 34.8 6.57 (3/6) LHQ1 Jurassic porphyritic qtz-monzonite 258968 3734954 43.40 38.0 3.14 (5/5) - - - HQ3 Med-coarse grained granite 260976 3731678 42.30 43.2 4.00 (3/4) 32.6 1.86 (5/5) HQ14 Porphyritic granodiorite 261936 3739482 38.60 20.4 1.46 (5/5) 23.0 3.56 (4/5) HQ4 Med-grained granite 265245 3735329 37.00 31.8 2.94 (4/4) 21.2 1.79 (2/4) TPF1 Muscovite granite 269671 3736778 32.64 22.0 3.29 (3/5) - - - TPF3 Tenahatchipi gneiss 269368 3736834 32.60 21.8 1.57 (5/5) 25.0d 2.38 (4/4) TPF4 Socorro Suite biotite granite 270417 3736758 32.06 19.7 1.79 (5/5) - - - HQ5A Coarse-grained granite; altered 272667 3738207 29.30 17.9 2.48 (4/6) - - - HQ5B Med-grained granite 272674 3738600 29.10 18.9 1.54 (4/5) 24.5d 2.80 (3/4) HQ16 Proterozoic mafic gneiss 276975 3738256 25.50 19.5 1.56 (4/4) 19.9 1.07 (5/5) HQ18 Porphyritic Harquahala granite 276003 3740790 25.25 17.8 1.42 (4/5) - - - HQ17 Foliated granitoid 275472 3739818 25.18 20.3 2.30 (5/5) 17.1 5.22 (3/4) HQ6 Proterozoic granitic gneiss 276434 3742924 22.00 17.6 1.14 (6/6) 17.5 0.93 (4/5) HQ7 Fine-grained granite, protomylonitic 280675 3742855 19.80 15.4 2.79 (3/5) 21.6d 2.09 (2/5) HQ8NW Granite w/ musc + grt 280302 3747194 17.90 15.8 3.15 (4/5) 25.0 d 2.12 (4/5) HQ8 Proterozoic fine-grained granite 282829 3743557 17.50 16.3 1.09 (6/6) 18.1 2.80 (2/5) HQ8SE Medium-grained granitoid 289032 3737335 16.30 14.9 1.29 (5/5) 26.5 d 3.18 (1/1) HQ9 Med-grained biotite granitoid 284301 3744554 15.50 13.4 0.93 (6/6) 13.2 4.05 (3/4) HQ10 Med-grained granite w/musc + grt 289043 3747746 10.00 15.8 1.33 (5/6) 16.5 2.19 (4/13) HQ15 Proterozoic gneiss 289477 3749355 9.22 16.9 1.36 (4/4) 21.1 d 1.51 (3/5) HQ11 Med-grained granite; weak foliation 294043 3749607 4.75 15.2 1.23 (7/7) 17.3 1.13 (5/10) SC6 Stone Corral Granite, mylonitic 296564 3749610 3.20 16.0 1.33 (6/6) 13.5 1.60 (5/5) SC1 Stone Corral Granite,protomylonitic 297114 3750058 1.50 16.1 1.16 (5/5) 15.9 1.90 (4/4) HQ12 Stone Corral Granite,protomylonitic 297559 3751265 1.15 15.4 0.95 (9/9) 12.6 0.86 (2/5) HQ13B Quartzofeldspathic mylonite 298320 3751957 0.15 15.2 1.10 (5/5) - - - HQ13 Quartzofeldspathic mylonite 298588 3751815 0.005 16.2 1.16 (5/5) 16.9 0.91 (5/5) Hanging Wall EED1 Proterozoic gneiss 298662 3752136 -0.01 45.3 8.06 (5/5) - - EESS1 Coarse-grained sandstone 298704 3752093 -0.02 78.5 35.40 (5/5) 19.6 1.77 (4/4) aThe 156 total zircon aliqouts, seven excluded as neg. ages or unreasonably old ages; 118 total apatite aliquots, 81 included; see text and supporting information tables for individual aliquot data. bUniversal Transverse Mercator (UTM) Coordinates in WGS84. cFour fifths indicates four out of five grains analyzed were used to calculate mean ages: SE = standard error, see Tables S1 and S2 for details. dExcluded ages: AHe ages older than ZHe.

4. Results 4.1. Footwall (U-Th)/He Thermochronometry The extensional and thermal history of the Harquahala Mountains was interpreted from 31 new zircon and 24 apatite mean (U-Th)/He ages (ZHe and AHe, respectively) determined from 29 footwall samples and 2 hanging wall samples. ZHe mean ages plotted versus distance from the Eagle Eye detachment fault display three distinct trends that closely conform to predicted thermochronometer patterns in extensional settings and low-angle normal fault systems (Figure 4) [Fitzgerald et al., 1991, 2009; John and Foster, 1993; Reiners et al., 2000; Stockli, 2005]: (1) Samples farthest from the Eagle Eye detachment fault, within the Little Harquahala Mountains, yielded the oldest ZHe ages of ~46 Ma to 54 Ma that define a relatively flat age profile from ~52 to 42 km southwest of the Eagle Eye detachment fault; (2) an ~10 km long middle segment of ZHe ages that decrease from ~38 Ma to ~22 Ma and typically display more variable intra-sample aliquot ages; and (3) 22 samples within ~34 km southwest of the Eagle Eye detachment fault that define a linear trend of ZHe ages correlated with distance from the Eagle Eye detachment fault (Figure 4). The second and third transect segments are clearly distinguished by a break in slope along the ZHe age profile at ~34 km southwest of the Eagle Eye detachment fault (Figure 4). Samples from within ~5 km of the Eagle Eye detachment fault yielded the youngest ZHe ages of ~15–16 Ma (Table 1).

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Figure 4. Mean zircon and apatite (U-Th)/He ages from the Harquahala and Little Harquahala Mountains transect plotted against distance west of the Eagle Eye Peak detachment fault. (a) Mean zircon and (b) mean apatite (U-Th)/He ages plotted versus distance from the Eagle Eye detachment fault; mean (U-Th)/He age error bars shown at two standard errors. Hanging wall samples shown with gray circles. The interpreted break-in slope that signals the onset of extension is shown by the gray dashed line.

Samples from ~44 to 42 km southwest of the Eagle Eye detachment fault produced the oldest AHe ages of ~35–33 Ma (Figure 4). AHe ages decrease to ~21 Ma at ~38 km southwest of the Eagle Eye detachment fault with a break in slope located at ~40 km, ~6 km farther southwest than in the ZHe data (Figure 4). The AHe profile is less well defined than in the ZHe age profile since six samples did not yield useable apatite and six AHe mean sample ages were excluded that yielded ages older than mean ZHe age from the same sample. Mean AHe sample ages were included when younger than, or within 1σ, of the mean ZHe age. AHe ages within ~40 km from the Eagle Eye detachment fault range from ~21 to 20 Ma, decreasing to ~13 to 14 Ma for samples closest to the Eagle Eye detachment fault at Eagle Eye Peak (Figure 4). Sample HQ1 at ~52 km from the Eagle Eye detachment fault and LHQ2 at ~51 km yielded AHe ages of 19.4 ± 1.6 Ma and 17.5 ± 0.4 Ma, respectively. HQ1 and LHQ2 are the most distant AHe samples within the transect and are significantly younger than the ~35–33 Ma AHe ages of samples that are ~42–44 km southwest of the Eagle Eye detachment fault (Figures 2 and 4) (Table 1). ZHe data from samples HQ1 and LHQ2 did not produce any Miocene ages. The discrepancy between zircon and apatite (U-Th)/He ages from samples HQ1 and LHQ2 suggests something other than fault slip along the Eagle Eye detachment fault affected the bulk AHe age of these samples.

4.2. Hanging Wall Geo- and Thermochronometry (U-Th)/He dating of a small exposure (<5m2) of hanging wall Proterozoic gneiss (EED1), directly above the Eagle Eye detachment fault zone yielded a ZHe age of 45.3 ± 9.0 Ma (Figure 4 and Table 1). The ZHe age of EED1 is most similar to footwall samples from ~55 to 40 km southwest of the Eagle Eye detachment fault in the Little Harquahala Mountains. Eagle Eye Peak sandstone sample EESS1 shows a large spread in individual ZHe detrital aliquot ages (~122, 101, 84, 67, and 17 Ma) (Table S1) that record signiﬁcantly older cooling events and a single Miocene cooling age. In contrast, detrital AHe aliquots (n = 4) from EESS1 yielded a mean age of 19.6 ± 1.8 Ma with no pre-Miocene cooling ages (Table S1). Detrital zircon U-Pb dating was conducted on two sandstone samples from Eagle Eye Peak and age modes were evaluated using kernel density estimation plots generated with DensityPlotter [Vermeesch, 2012]. Both sandstone samples (EESS1 and EESS2) show prominent Proterozoic and Oligo-Miocene age modes, as well as a small Jurassic mode (Figure 5). Oligo- Miocene modes in both samples show a predominance of 23–25 Ma zircons with several U-Pb ages as young as ~19 Ma; EESS1 also includes a single grain with an ~17 Ma U-Pb age (Figure 5). Combined (U-Th)/(He-Pb) double dating was used to investigate the signiﬁcance of the single Miocene ZHe age from EESS1. A total of

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Figure 5. Kernel density estimation (KDE) plots of detrital zircon U-Pb dates from Eagle Eye Peak sandstone samples. Inset KDE plots show Miocene age mode. KDE plots were generated using DensityPlotter [Vermeesch, 2012].

13 zircons was selected from the three EESS2 age modes for ZHe dating to further differentiate possible source regions for Eagle Eye Peak sandstones (Table 2). Seven aliquots yielded ages of ~58–116 Ma, whereas six aliquots yielded ZHe ages of ~17–19 Ma (Figure 6 and Table 2). Four zircons with Proterozoic U-Pb ages produced Miocene ZHe ages, and two zircons produced Oligo-Miocene 206Pb/238U and (U-Th)/He ages (Figure 6 and Table 2).

4.3. U-Pb and 40Ar/39Ar Geochronology Zircon U-Pb geochronology data from the Little Harquahala, western Harquahala Mountains, and Bullard Peak in the northeastern Harcuvar Mountains were used to correlate distinctive porphyritic igneous rocks in the Eagle Eye detachment fault footwall to sedimentary deposits preserved within the hanging wall at Bullard Peak (Figure 7). Zircon U-Pb ages are reported as a weighted mean when distinctly clustered concordant population exists and as concordia intercept ages otherwise (Table S3). Many samples retain zircons with likely inherited Proterozoic 206Pb/238U ages, even when the majority of grains plot as much younger

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Table 2. Eagle Eye Peak Sandstone (U-Th)/He-(U-Pb) Double Datesa Aliquot ZHe Age ZHe SE 8% 207/235 Age 207/235 Age 2SE 206/238 Age 206/238 Age 2SE 207/206 Age 207/206 Age 2SE

EESS2_9 73.7 5.9 1422.1 7.4 1424.0 11.0 1425.0 18.0 EESS2_15 17.2 1.4 26.1 0.9 24.3 0.5 199.0 69.0 EESS2_27 77.6 6.2 1407.0 13.0 1399.0 26.0 1444.0 25.0 EESS2_41 17.6 1.4 157.9 6.0 155.4 2.9 188.0 74.0 EESS2_55 17.3 1.4 22.9 0.9 23.3 0.6 16.0 80.0 EESS2_64 116.2 9.3 1723.7 9.0 1729.0 16.0 1717.0 18.0 EESS2_66 18.0 1.4 1649.1 8.9 1602.0 15.0 1723.0 17.0 EESS2_72 19.8 1.6 1681.0 16.0 1642.0 28.0 1740.0 23.0 EESS2_84 63.0 5.0 1677.0 11.0 1616.0 19.0 1741.0 16.0 EESS2_87 19.3 1.5 1714.6 5.9 1702.0 11.0 1727.0 15.0 EESS2_90 58.8 4.7 1395.8 8.8 1381.0 14.0 1430.0 18.0 EESS2_96 60.9 4.9 1404.2 8.1 1397.0 15.0 1420.0 14.0 EESS2_101 106.5 8.5 1429.0 7.1 1430.0 13.0 1426.0 15.0 aSE = standard error.

concordant ages. Sample LHQ1 from the Little Harquahala Mountains (Figure 2) yielded a weighted mean 206Pb/238U age of 164.3 ± 1.4 Ma (Figure 8). Isachsen et al. [1999] previously reported an isotope-dilution thermal ionization mass spectrometry lower concordia intercept age of 163.2 ± 2.9 from within the same unit as LHQ1 but interpreted a potentially older age of 163–166 Ma. Sample HQ14 from the western Harquahala Mountains (Figure 2) yielded a weighted mean 206Pb/238U age of 185.2 ± 3.0 Ma (Figure 8). A combined multi- clast sample (AR1), consisting of approximately seven subangular, cobble-boulder clasts of Bullard Peak breccia unit Tcb [Reynolds and Spencer, 1984] (Figure 7) yielded a weighted mean 206Pb/238U age of 164.4 ± 1.1 Ma (Figure 8). Sample AR6 (Figure 7) from a large (4-5 m) monzodiorite block within unit Tcb yielded concordant zircon U-Pb ages that range from ~160 Ma to ~220 Ma (Figure 8). Analysis of the 10 youngest zircons yielded a weighted mean 206Pb/238U age of 167.0 ± 2.6 Ma, whereas the youngest five zircons yield a weighted mean 206Pb/238U age of 164.0 ± 3.5 Ma, respectively (Figure 8). Sample AR4 from rhyolitic ash flow tuff unit Tt [Reynolds and Spencer, 1984] that underlies unit the Tcb breccia yielded a weighted mean 206Pb/238U age of 22.6 ± 0.2 Ma (Figure 8). Whole-rock 40Ar/39Ar dating of Eagle Eye Peak basalt flows (Figure 2), unit Tb of Richard et al. [1994], produced an integrated mean age of 17.3 ± 0.4 Ma. A preferred 40Ar/39Ar plateau age of 17.1 ± 0.1 Ma was calculated using Isoplot [Ludwig, 2003] from a consistent plateau of 13 steps that accounts for 84.9% of released 39Ar (Figure 9). See Table S5 for raw isotopic abun- dances and calculated results.

5. Thermochronometric Constraints on Timing, Magnitude, and Slip Duration Along the Eagle Eye Detachment Fault (U-Th)/He data from the Harquahala Mountains transect preserve a break in slope that is interpreted as the base of the exhumed zircon partial retention zone and fully reset ages that record cooling due to slip on the Eagle Eye detachment fault. The preserved base of the zircon partial retention zone was used to infer the Figure 6. Double-dated zircon (U-Th)/(He-Pb) ages from Eagle Eye sand- onset of extension and a minimum stone sample EESS2. See Table 2 for individual zircon aliquot data. displacement magnitude for the

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Figure 7. Field photos of rock samples used for the lithologic and geochronologic correlation. (a) Sample LHQ1 porphyritic monzodiorite from the Little Harquahala Mountains; (b) AR1 clasts of porphyritic monzodiorite within angular clast-supported breccia Tcb [Reynolds and Spencer, 1984]; (c) sample AR6 megablock within Tcb breccia, ﬁeld notebook for scale; and (d) portion of megabreccia block showing pervasive internal fracturing or crackle/jigsaw breccia texture.

Eagle Eye detachment fault [Fitzgerald et al., 1991, 2009; John and Foster, 1993; Foster and John, 1999; Reiners et al., 2000; Stockli et al., 2000, 2002; Stockli, 2005]. The slope break in ZHe and AHe age data occurs between ~21 ± 1 Ma (Figure 4), indicating slip along the Eagle Eye detachment fault began to exhume rocks from below the zircon partial retention zone (>180°C) at this time. There is no evidence that the onset of extension and rapid cooling within the Harquahala Mountains is related to plutonism. Maﬁc dikes are abundant throughout the footwall, but the Harquahala Mountains do not contain any Miocene plutons that could have signiﬁcantly heated or weakened the crust, priming it for extension and mylonitization [e.g., Keith et al., 1980; Lister and Baldwin, 1993]. Approximately 14 Ma AHe ages closest to the Eagle Eye detachment fault indicate active slip until at least this time. The ~21 ± 1 Ma timing for initial fault slip and the youngest AHe ages of ~14 Ma indicate ~ 8 Myr of active slip along the Eagle Eye detachment fault during the Miocene. The ~14 Ma estimate for cessation of extension is a maximum value since (U-Th)/He ages reset by fault slip could be present along the downdip, and subsurface, continuation of the Eagle Eye detachment fault. A minimum displacement magnitude along the Eagle Eye detachment fault was interpreted from the location between (U-Th)/He samples that are interpreted to have resided in the partial retention zone and those fully reset during detachment slip. The base of the exhumed partial retention zone is ~34 km southwest of

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Figure 8. U-Pb concordia plots from samples used to geochronologically correlate footwall rocks from the Harquahala Mountains to Bullard Peak clasts. All error ellipses are shown at 2σ. See Table S3 for reduced U-Pb data.

the Eagle Eye detachment fault (Figure 4), indicating a minimum of ~34 km of displacement during Miocene extension. Exhumation of rocks that resided at shallower levels than the zircon closure temperature depth (~7 km) when fault slip began must also be included to determine a total displacement magnitude. An additional 8–12 km of displacement is required for a fault dip of 60–35° within the upper 7 km of the crust and a geothermal gradient of 25°C/km, indicating 44 ± 2 km of total displacement along the Eagle Eye detachment fault based on thermochronometric considerations. The mean ZHe age of 45.3 ± 9.0 Ma from hanging wall sample EED1 at Eagle Eye Peak is similar to ~43–55 Ma ZHe ages from the Little Harquahala Mountains that are now ~42–52 km southwest of the Eagle Eye detachment. Sample EED1 is interpreted as a hanging wall basement exposure of the same Proterozoic gneiss in the footwall at Eagle Eye Peak. The preservation of pre-Miocene ZHe ages in sample EED1 indicates that these hanging wall rocks were shallower than the zircon partial retention zone depth at the inception of slip along the Eagle Eye detachment fault at ~21 ± 1 Ma, as were samples from the Little Harquahala Mountains. The current separation of ~42–52 km between sample EED1 and footwall samples with similar ZHe ages further supports the thermochronometric estimate of ~44 ± 2 km and the 40–50 km displacement estimate for the Harcuvar Mountains [Reynolds and Spencer, 1985]

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Figure 9. 40Ar/39Ar release spectrum for Eagle Eye Peak whole-rock basalt sample EEB3. Steps 6–18, shown in red, yielded ~85% of 39Ar and yield a plateau age of 17.1 ± 0.1 Ma. 40Ar/39Ar was performed at the USGS Argon lab in Denver; see Table S5 for complete 40Ar/39Ar data.

and Buckskin-Rawhide Mountains [Singleton et al., 2014]. If the EED1 exposure is actually a large megabreccia block within hanging wall sedimentary rocks, the ZHe age of ~45 Ma indicates it was most likely derived from ~42 to 52 km southwest of the Eagle Eye detachment fault (Figure 4). If the megabreccia block was deposited prior to slip along the Eagle Eye detachment fault, it could still be used as a supporting displacement estimate assuming minimal transport distances. The similar displacement estimates within metamorphic core complexes of the CREC (~40–50 km) suggest variations in the extent of Miocene mylonitization between different metamorphic core complexes are not controlled by the amount of displacement magnitude but instead by either the initial detachment fault dip or the presynextensional thermal conditions of the crust. The Eagle Eye detachment fault exhumed relatively shallow crustal levels based on the limited extent of Tertiary mylonitic fabrics and likely initiated at ~35° [Richard et al., 1990], whereas the footwall of the Buckskin-Rawhide detachment fault is pervasively mylonitized and likely initiated at ~33–64° [Singleton and Mosher, 2012]. It is possible the relatively shallow level of crustal exhumation in the Harquahala Mountains compared to other metamorphic core complexes such as the Buckskin-Rawhide Mountains is controlled by the initial detachment fault, although the ~21 Ma Swansea Plutonic Suite [Bryant and Wooden, 2008; Singleton and Mosher, 2012; Singleton et al., 2014] could have significantly altered the thermal conditions in the Buckskin-Rawhide footwall and facilitated pervasive mylonitization. Plutonism may influence the degree of footwall mylonitization but does not significantly affect thermochronometric slip rate calculations [Brichau et al., 2006]. Miocene plutonism may influence footwall mylonitization, but it is absent in many metamorphic core complexes such as the Whipple, Harcuvar, and Harquahala Mountains, indicating the initial detachment fault dip controls the varying levels of footwall mylonitization. Our new thermochronometry data are used to further evaluate the original geometry of the Eagle Eye detachment fault in subsequent sections.

6. Correlation of Footwall and Hanging Wall Units We correlated lithologic units now offset by the Eagle Eye detachment fault to determine a displacement estimate independent of thermochronometry data. Offset markers of displacement in the CREC are rare but have been used to estimate displacement magnitudes along different sections of the CREC detachment fault system [Reynolds and Spencer, 1985; Davis and Lister, 1988; Singleton et al., 2014]. Reynolds and Spencer [1985] originally hypothesized a lithologic correlation between very distinctive plutonic footwall rocks in the Little Harquahala Mountains (Figure 2) and lithologically similar clasts contained in unit Tcb at Bullard Peak (Figure 3) [Reynolds and Spencer, 1984]. Unit Tcb consists of a lower conglomerate, a middle sedimentary breccia member, and an overlying conglomerate containing clasts of Mesozoic metasedimentary rock, with a total thickness of ~600 m [Reynolds and Spencer, 1984; Roddy et al., 1988]. The middle sedimentary breccia of unit Tcb is clast-supported, angular, pebble-boulder breccia primarily comprised of porphyritic diorite to monzodiorite clasts, typically in a matrix of comminuted clast fragments and coarse sand. Unit Tcb also contains very large megabreccia clasts that are up to ~20 m across, highly internally fractured, and commonly display jigsaw or crackle breccia textures (in the sense of Yarnold, 1993) (Figure 7). We tested the correlation of Reynolds and Spencer [1985] using zircon U-Pb geochronology of igneous footwall rocks and hanging wall sedimentary breccia to determine if breccia clasts are sourced from exposures in the Little Harquahala Mountains or western Harquahala Mountains. Assuming minimal transport distances for the sedimentary

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breccia, these units can act as an offset marker to determine a displacement magnitude for the Eagle Eye detachment fault that is independent of thermochronometric data.

Distinctive footwall rocks mapped as unit Jg2 in the Little Harquahala Mountains and western Harquahala Mountains by Richard et al. [1994] consist of porphyritic monzodiorite, diorite, and granodiorite that are lithologically unique from other igneous rocks mapped in the region [Reynolds and Spencer, 1985]. Porphyritic monzodiorite footwall sample LHQ1 from the Little Harquahala Mountains (Figure 2) contains abundant potassium feldspar megacrysts up to 3 cm long (Figure 7), whereas the western Harquahala footwall sample HQ14 (Figure 2) is more equigranular diorite and lacks signiﬁcant amounts of potassium feldspar phenocrysts. The weighted mean 206Pb/238U ages of LHQ1 and HQ14 are 164.3 ± 1.4 and 185.2 ± 3.0 Ma, respectively (Figure 8). Breccia clasts within unit Tcb at Bullard Peak [Reynolds and Spencer, 1984] are composed of porphyritic monzodiorite (sample AR1) and a more equigranular monzodiorite (sample AR6). Sample AR1 consists of several individual clasts from unit Tcb that yielded a weighted mean 206Pb/238U age of 164.4 ± 1.1 Ma. LHQ1 and AR1 are interpreted as the same lithologic unit based on mineralogical and textural similarity and nearly identical 206Pb/238U ages. Hanging wall sample AR6 is from an ~20 m (maximum dimension) block of heavily fractured monzodiorite within Tcb that is texturally and mineralogically similar to LHQ1 and AR1 but with less abundant potassium phenocrysts (Figure 7). The large, heavily fractured monzodiorite blocks are large clasts within the Tcb sedimentary breccia and were previously interpreted as landslide blocks by Reynolds and Spencer [1985]. The weighted mean of AR6 zircon 206Pb/238U ages did not give a clear age since concordant ages range from ~160 to 220 Ma (Figure 8), whereas the 10 youngest zircons yielded a 206Pb/238U age of 167.0 ± 2.6 Ma, and the youngest ﬁve zircons yielded a 206Pb/238U weighted mean of 164.0 ± 3.5 Ma. The weighted mean ages of these youngest zircons overlap within error of the 206Pb/238U ages of LHQ1 and AR1 (Figure 8). The large spread in concordant ages from AR6 is attributed to xenocrystic contam- ination from older igneous rocks, with the youngest 206Pb/238U ages interpreted as the most accurate crystallization age of AR6, indicating AR6 is the same lithologic unit as LHQ1 and AR1. U-Pb data from distinctive porphyritic rocks clearly demonstrate sample HQ14 in the western Harquahala Mountains does not match the 206Pb/238U age signature of Bullard Peak breccia clasts, and the source of Bullard Peak breccia must be unit

Jg2 in the Little Harquahala Mountains, located ~43–45 km from the current position of the EED (Figure 2). The maximum depositional age of unit Tcb is 22.6 ± 0.2 Ma based on the 206Pb/238U age of the underlying rhyolitic ash ﬂow tuff (Unit Tt) [Reynolds and Spencer, 1984] (Figure 8). Previous age determinations for unit Tt include a K-Ar biotite age of 23.9 ± 0.9 Ma [Brooks, 1984] and a whole-rock K-Ar age of 15.8 ± 0.4 Ma [Scarborough and Wilt, 1979]. Pervasive K-metasomatism within upper-plate rocks in the region [Roddy et al., 1988] likely accounts for the unreliable K-Ar ages. Related exposures of Oligocene-Miocene sedimentary breccia in the Artillery Mountains to the north (Figure 1) have been previously interpreted as rock avalanche

deposits [Spencer et al., 1989; Yarnold, 1993, 1994]. The current geographic separation of unit Jg2 in the Little Harquahala Mountains and the Bullard Peak breccia can be used as an offset marker for the maximum displacement magnitude along the Eagle Eye detachment. This approach assumes the sedimentary breccia was deposited prior to slip along the Eagle Eye detachment fault and that transport distance was negligible. Since we can only constrain the maximum depositional age of ~22–23 Ma (Figure 8) it is possible the Bullard Peak breccia was deposited after an unknown amount of displacement had already occurred. Substantial runout distances are possible for large rock avalanche deposits [Spencer et al., 1989; Yarnold, 1993, 1994] and are empirically related to the volume of the deposit [Topping, 1993; Fryxell and Deubendorfer, 2005]. The largest megabreccia block observed is ~400 m2 at the surface. Although the subsurface dimension is unknown, ~3 m of exposure is preserved within a drainage at Bullard Peak. If the maximum subsurface dimension is taken as ~10 m then the total volume is ~4000 m3. The runout distance the estimated megabreccia block volume as well as 5 and 10 times this volume indicate runout distances of ≤1 km based on the empirical calibration of Topping [1993]. The current separation of ~44 ± 1 km is a maximum estimate for displacement along the Eagle Eye detachment, but any overestimation is likely negligible given the small estimated runout distance inferred from the megabreccia block volume. The inferred runout distance of ≤ 1 km for the Bullard Peak sedimentary breccia suggests deposition proximal to elevated topo- graphy in an initial breakaway zone of the Bullard-Eagle Eye detachment within the Little Harquahala Mountains. The new maximum age constraint for unit Tcb (Figure 8) indicates predetachment extension accompanied by topographic relief development that produced coarse sedimentary breccia that was subsequently displaced along the Eagle Eye detachment ~1 Myr after deposition initiated.

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7. Discussion 7.1. Timing of Slip Along the Eagle Eye Detachment Fault The preserved base of the exhumed apatite and zircon partial retention zones from the footwall of the Harquahala Mountains indicates extension along the Eagle Eye detachment fault initiated at ~21 ± 1 Ma. Rapid extension was active from ~21 ± 1 Ma to at least ~14 Ma, based on the youngest AHe ages proximal to the Eagle Eye detachment fault (Table 1). Our new timing and displacement estimates for the Eagle Eye detachment fault indicate detachment slip began at ~21 ± 1 Ma in the Harquahala Mountains, similar to estimates of ~23 Ma in the Sacramento Mountains [Carter et al., 2006], ~22 Ma in the Chemehuevi Mountains [John and Foster, 1993; Foster and John, 1999], ~23–21 Ma in the Whipple Mountains [Nielsen and Beratan, 1995], and ~21–20 Ma in the Buckskin-Rawhide Mountains [Singleton et al., 2014]. The inception of detachment slip at ~21 ± 1 Ma just postdates the Bullard Peak sedimentary breccia maximum depositional age of ~22.6 Ma indicated by underlying ash flow tuff sample AR4 (Figure 6). Coarse sediments were likely derived from topographic development associated with normal faulting as early as ~23 Ma then subsequently displaced by slip along the Eagle Eye detachment fault. 7.2. Displacement Magnitude Along the Eagle Eye Detachment Fault A new total displacement magnitude of ~44 ± 2 km is presented for the Eagle Eye detachment fault based on footwall (U-Th)/He thermochronometry and geochronologic correlation of distinct lithologies preserved in the footwall and hanging wall of the Eagle Eye-Bullard detachment fault system. The spatial extent of fully reset Miocene ZHe ages records a minimum of ~34 km of displacement along the Eagle Eye detachment fault. Exhumation of the zircon partial retention zone in the breakaway zone requires ~8–12 km of displacement, assuming a 60°–35° dip of the Eagle Eye detachment fault and a geothermal gradient of 25°C/km, in addition to the ~34 km minimum estimate derived from reset (U-Th)/He ages (Figure 3). Metamorphic core complexes within the regionally correlative detachment fault system of the Whipple tilt domain record similar displacement magnitudes of ~40–50 km [Reynolds and Spencer, 1985; Singleton et al., 2014]. Our new ~44 ± 2 km displacement estimate for the Eagle Eye detachment fault indicates there is no gradual decrease in detachment slip at the southern boundary of the Whipple tilt domain. The abrupt transition from large magnitude detachment displacement along the Eagle Eye detachment to the distributed high-angle normal faulting within the Bighorn, Belmont, and Vulture Mountains [Spencer and Reynolds, 1991; Stimac et al., 1994] supports the interpretation that the Eagle Eye detachment fault acted as a transfer fault along the southern flank of the Harquahala Mountains [Richard et al., 1990]. The Bighorn, Belmont, and Vulture Mountains may act as an accommodation zone between the Eagle Eye detachment fault and the low-angle normal faults that exhumed the White Tank and South Mountains to the southeast. 7.3. Slip Rates Along the Eagle Eye Detachment The primary assumption for calculating thermochronometric slip rates is that thermochronometers record cooling directly related to slip along a fault, such that any correlation between distance and the cooling age of a sample can be interpreted as a direct estimate of fault slip rates. Extensional thinning of the hanging wall will increase the effective vertical cooling rate and systematically overestimate fault slip rates. Restored cross sections through the Big Horn and Belmont Mountains indicate ~24–63% of hanging wall extension just south of the Harquahala Mountains [Stimac et al., 1994] (Figure 1). Fault slip rates are not likely to be significantly overestimated with this ~24–63% of hanging wall extension since ~200% extension is needed to produce an ~50% overestimation [Wells et al., 2000]. Assuming cooling ages represent the time a sample passes through a particular “closure” isotherm, the depth to the isotherm must be fixed but not necessarily flat [Ketcham, 1996] in order to calculate a fault slip rate. Errors in calculated fault-slip rates due to rising isotherms are smaller for lower closure temperature systems [Brichau et al., 2006] and Ketcham [1996] demonstrated that with slip rates of 5–10 km/Myr an ~120°C isotherm reaches a static position within ~1–2 Myr. The significantly lower closure temperature of the AHe system (~70°C) is unlikely to underestimate fault slip rates, whereas ZHe ages (closure temperature ~180°C) would only produce errors of up to 5% [Ketcham, 1996]. Structural repetition of the footwall due to incisement [e.g., Lister and Davis, 1989] could replicate age versus distance patterns and lead to an overestimation of slip rates [e.g., Stockli et al., 2006], but this pattern is not observed in our high sample density transect, which lacks a clearly repeated age versus distance pattern. Three samples from the middle of the transect do produce a similar pattern as described previously (Figure 10) but are located from the flanks and center of the transect where this pattern does not provide

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Figure 10. Zircon and apatite (U-Th)/He slip rate plots. Slip rates were interpreted from linear regression slopes calculated in Isoplot [Ludwig, 2003] using mean ages and two standard errors. Slip rates are calculated at 95% conﬁdence interval. Gray symbols indicate the two samples from the ﬂanks of the Harquahala Mountains.

information on fault slip rates. Lithologic contacts and thrust faults are well preserved in the Harquahala Mountains compared to many other metamorphic core complex footwalls (Figure 2) and show no evidence for significant footwall incisement or synextensional faulting that could cause a misinterpretation of slip rates. The two southwesternmost samples show significantly younger AHe ages compared to the ZHe ages from the same samples, which suggests the AHe ages may have been reset by later high angle normal faulting. Although faulting could have reset these AHe samples, it does not affect slip rate interpretations since these two samples would have already resided at shallow crustal levels prior to extension along the Eagle Eye detachment fault. (U-Th)/He data from the lower age versus distance segment within the Harquahala Mountains sample transect (Figure 3) were used to calculate time-averaged slip rates along the Eagle Eye detachment fault [Foster et al., 1993; John and Foster, 1993; Foster and John, 1999]. Correlation between (U-Th)/He ages and distance from the Eagle Eye detachment was tested using linear regression of fully reset ZHe and AHe cooling ages within the Harquahala Mountains. Linear regressions for ZHe and AHe data were calculated using the robust regression method in Isoplot [Ludwig, 2003] (Figure 10). Mean sample ages are reported at two standard errors with fault slip rates reported at a 95% confidence interval (Figure 10). The maximum value between two different standard error calculations was applied to each mean age (see Tables S1 and S2 for details). (U-Th)/He data do not show a significant deviation from the expected linear correlation with distance from the fault, other than scatter inherent to the (U-Th)/He system. The larger percentage of excluded AHe aliquots is based on clear outliers that typically have pre-Miocene ages (Table S1). We interpret the good linear fits of (U-Th)/He data (Figure 10) as a relatively constant time-averaged slip rate during Miocene extension. Linear regression of ZHe and AHe data provides good fits and is most simply explained by a single time-averaged slip rate along the Eagle Eye detachment fault. Linear regression of 22 ZHe mean sample ages yielded fault slip rates of 6.6 + 7.1/À2.0 km/Myr along the Eagle Eye detachment fault (Figure 10). Fault slip rates calculated from 13 AHe mean sample ages yielded a rate of 6.7 + 7.8/À2.3 km/Myr (Figure 10). (U-Th)/He derived slip rates agree well with the total displacement determination of ~44 ± 2 km and ~6–8 Myr of extension based on the inferred start of slip at ~21 ± 1 Ma and the interpreted end of extension at ~14 Ma. AHe and ZHe rates of ~6.6 km/Myr (Figure 10) for ~6–8 Myr indicates ~46 ± 7 km of total displacement. The similarity of calculated fault slip rates highlights the strength of using multiple thermochronometers and any available geologic constraints when attempting to calculate reliable slip rates along low-angle normal faults. Previous studies by Carter et al. [2004, 2006] interpreted a change in fault slip rates within the CREC at ~15 Ma based on two-stage AHe age trends in the Sacramento and Harcuvar Mountains. Two significant

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and broad implications for detachment fault slip in the CREC result from this interpretation (1) initially slow (~2.6 km/Myr) slip rates increase at ~15 Ma, interpreted from a change in the AHe age versus distance slope [Carter et al., 2004], and (2) post- 15 Ma slip rates that were interpreted to dramatically increase to ≤30 km/Myr in the Harcuvar Mountains based on a nearly flat AHe age versus distance trend [Carter et al., 2004]. If these two interpretations are valid then the same general relationships should be present in the Harquahala Mountains since the continuation of the Bullard-Eagle Eye Figure 11. Initial dip estimate for the Eagle Eye detachment fault at the detachment fault system is not sepa- inception of Miocene extension. Zircon and apatite (U-Th)/He ages and rated by any known major structure biotite 40Ar/39Ar ages between 20 and 22 Ma were corrected for their (Figure 1). It is noteworthy that our paleodepth at the start of Miocene extension and used to infer the original dip of the Eagle Eye detachment fault using geothermal gradients ranging ZHe data show a relatively shallow from 20 to 40°C/km and a surface temperature of 10°C. Angles in the lower age versus distance profile, similar right are shown at the same vertical exaggeration as graph. to the post ~15 Ma AHe data from Carter et al. [2004]. Using AHe data from Carter et al. [2004] and the same data treatment from this study, we calculated a time-averaged slip rate of ~6.7 + 43.3/À2.2 km/Myr. Although the 95% confidence interval allows a wide range of slip rates, the mean rate (Figure S1) is in good agreement with our AHe and ZHe slip rate calculations from the Harquahala Mountains and previous apatite fission-track slip rate determinations from the Harcuvar Mountains of 6.7 ± 1.6 km/Myr [Foster et al., 1993].

7.4. Initial Geometry of the Eagle Eye Detachment Fault The initial dip of the Eagle Eye detachment was inferred from a range of geothermal gradients and multiple thermochronometer samples [John and Foster, 1993; Foster and John, 1999] from the Harquahala Mountains footwall that record cooling ages of ~20–22 Ma (Figure 11). Three biotite 40Ar/39Ar ages located between ~8 and 13 km southwest of the Eagle Eye detachment fault record ages of ~20.0–21.4 Ma [Richard et al., 1990]. ZHe samples TPF1, TPF3, and TPF4, all ~32 km southwest of the Eagle Eye detachment fault produced ages between ~19.7 and 22.0 Ma (Table 1). AHe ages from samples HQ14 and HQ4 record ages of 20.9 ± 5.7 Ma and 21.2 ± 0.1 Ma, respectively. HQ14 is ~39 km southwest of the Eagle Eye detachment fault, whereas HQ4 is ~37 km from the Eagle Eye detachment fault. Closure temperatures for the biotite 40Ar/39Ar, ZHe, and AHe systems are taken to be 320 ± 40°C [Harrison et al., 1985; Grove and Harrison, 1996], 180 ± 10°C [Reiners et al., 2002, 2004; Wolfe and Stockli, 2010], and 75 ± 10°C [Wolf et al., 1996, 1998; Farley, 2000; Reiners and Farley, 2001], respectively. A range of geothermal gradients from 20°C/km to 40°C/km and a mean surface temperature of 10°C was used to calculate the paleodepth of respective thermochronometers at ~21 Ma which were then plotted versus distance from the Eagle Eye detachment fault to determine the initial dip of the detachment fault at the inception of Miocene extension (Figure 11). The paleodepth of samples at ~21 Ma is best correlated with distance by fitting two segments that correspond to a varied dip angle along a listric Eagle Eye detachment fault (Figure 11). AHe and ZHe samples are best fit with a fault dip between 34 ± 9° in the upper ~7 km of the crust, whereas the fault segment separating ZHe and biotite 40Ar/39Ar ages is best fit with a fault dip of 13 ± 5° (Figure 11). The current ~5° dip of the Eagle Eye detachment and paleomagnetic evidence for ~10 ± 5° of horizontal axis footwall rotation [Livaccari and Geissman, 2001] suggest an initial detachment dip of 10–20°. If the Eagle Eye detachment did initiate at an initial angle of ~20°, it suggests the amount of footwall rotation inferred from the orientation of mafic dikes [Richard et al., 1990] is not valid. Mafic dikes within the Eagle Eye detachment fault footwall dip ~60–70°NE suggesting ~30° of footwall rotation and an initial fault dip of ~35° assuming dikes intruded vertically [Richard et al., 1990]. Paleomagnetic data from

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Livaccari and Geissman [2001] were determined from regions that yielded fully reset ZHe ages that, according to our initial fault dip reconstruction, would have been initially located along the shallow 13 ± 5° dipping section of the listric Eagle Eye detachment fault, explaining why they record less rotation than mafic dikes. Although our listric geometry and the location of paleomagnetic data can explain the rotation estimates of Livaccari and Geissman [2001], there is no systematic change in the dip of mafic dikes [Richard et al., 1990] that would be expected from a listric geometry and permit a full reconciliation of paleomagnetic and dike orientation estimates. 7.5. Synextensional Surface Exposure of the Harquahala Mountains Footwall The presence of Miocene detrital ZHe cooling ages within the Eagle Eye Peak sandstone suggests surface exposure of the Harquahala metamorphic core complex during extension along the Eagle Eye detachment fault, likely caused by initiation of a secondary breakaway that exposed the Harquahala Mountains footwall. Detrital zircons with ~17 Ma ZHe ages within the Eagle Eye Peak sandstones indicate zircons with young cooling ages were actively being exhumed and deposited into hanging wall basins with very short lag times. Zircon (U-Th)/He aliquots with ~17 Ma ZHe ages were measured from grains with ~25 Ma 206Pb/238U ages as well as grains with Proterozoic 206Pb/238U ages (Figure 6). Zircons with ~25 Ma 206Pb/238U ages are likely derived from volcanic rocks in the hanging wall of the Eagle Eye detachment fault, whereas zircons with Proterozoic 206Pb/238U ages and ~17 Ma ZHe ages are abundant throughout the Harquahala Mountains footwall (Table 1). Detrital zircons with Proterozoic crystallization ages and ~17 Ma ZHe ages indicate surface exposure of the Harquahala Mountains footwall likely occurred by ~17 Ma. If young ZHe ages correspond to footwall rocks in the Harquahala Mountains, it implies rapid cooling of the footwall and very short ~1 Myr lag times for sediments at Eagle Eye Peak. It is possible zircons with ~17 Ma ZHe ages and Proterozoic U-Pb ages are derived from other Proterozoic exposures, but since this would not explain the young ZHe age signature indicative of rapid exhumation, the proximal Harquahala Mountains are considered the most likely source. Eagle Eye Peak sandstone sample EESS2 (Figure 2) was collected approximately tens of meters away from any basalt contact, which is significantly farther than the centimeter-scale distances where thermal resetting from basalt flow emplacement has been observed [Cooper et al., 2011]. Although ~17 Ma ZHe ages are similar to the 17.1 ± 0.1 Ma 40Ar/39Ar plateau age of the Eagle Eye Peak basalt (Figure 9), we do not attribute cooling ages to thermal resetting from basalt flows. Apatite (U-Th)/He ages from the same sandstone sample record ~19 ± 1 Ma cooling ages, with no pre-Miocene ages (Table S2). If thermal resetting from basalt flow emplacement occurred at ~17 Ma, AHe ages would be reset more readily and record ~17 Ma ages not older ~19 Ma AHe ages. New geothermochronometric constraints from synextensional sedimentary rocks within the Harquahala Mountains indicate that surface exposure of footwall rocks predated the final cessation of detachment faulting along the EED and likely resulted from initiation of a secondary breakaway at ~17 Ma. Dorsey and Becker [1995] previously interpreted surface exposure of the Whipple Mountains footwall by ~14.5 ± 1 Ma based on mylonitic clasts preserved in growth strata in the upper plate of the Whipple detachment. Singleton et al. [2014] also interpret development of a secondary breakaway in the eastern Bouse Hills by ~16.5 Ma. The new geothermochronometry results of this study, taken together with other interpretations from nearby metamorphic core complexes, indicate that surface exposure of the footwall during active extension is typical within the CREC and likely within large magnitude extensional regimes in general. 7.6. Extensional Evolution of the Harquahala Mountains Metamorphic Core Complex The Miocene extensional evolution of the Harquahala Mountains is reconstructed using the displacement estimates from lithologic offset constraints and new (U-Th)/He thermochronometry data (Figure 12). Three displacement markers are shown along the schematic cross section: (1) the location of the southwesternmost fully reset ZHe ages, (2) the southwesternmost extent of Tertiary mylonitic fabrics in the footwall, and (3) the current Eagle Eye detachment fault position now located ~45 km from the initial position (Figure 12). All displaced markers maintain consistent separation throughout the reconstruction. We depict an initial listric fault geometry with a shallow ~35° dipping segment that decreases to ~20° between ~ 7 km depth and the approximate depth of the brittle-ductile transition (~15 km) and Miocene mylonitization (Figure 12). Bullard Peak sedimentary breccia is depicted to have been eroded from a topographic high within an initial

breakaway zone that exhumed footwall porphyritic monzodiorite (Jg2) in the Little Harquahala Mountains at ~22–23 Ma, based on the maximum depositional age of Bullard Peak sedimentary rocks (Figure 8). Coarse-

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Figure 12. Schematic geometric reconstruction of the Harquahala Mountains metamorphic core complex before and after Miocene extension. Initial conﬁguration of the listric Eagle Eye detachment fault with an ~35° dipping fault that shallows to ~20° below the zircon partial retention zone. Three key displacement markers are depicted by numbered circles: (1) location of westernmost reset ZHe ages, (2) westernmost extent of Tertiary mylonitic fabric, and (3) location of the structurally deepest exposure of the Eagle Eye detachment fault. All markers maintain constant separation during the reconstruction.

grained breccia deposits (unit Tcb) [Reynolds and Spencer, 1984] were subsequently displaced by the Eagle Eye detachment fault to their current location at ~42–46 km away at Bullard Peak. Continued displacement along the Eagle Eye detachment fault and subsequent erosion is interpreted to have removed any original sedimentary rocks deposited in the breakaway zone, leaving only footwall source rocks exposed. Progressive isostatic flexure and back rotation in the footwall [e.g., Buck, 1988] of ~30° during fault slip resulted in secondary breakaway initiation at ~17 Ma and rotation of the Eagle Eye detachment fault to the current ~5° dip (Figure 12). The post detachment slip configuration of the Harquahala Mountains at ~14 Ma depicts the synextensional ~17 Ma Eagle Eye Peak basalt (Figure 9) and sandstones in their pre- erosional configuration, at the same position along the cross section as Bullard Peak breccia in the adjacent Harcuvar Mountains (Figure 12). Timing estimates for the Eagle Eye detachment fault indicate extension began at ~21 ± 1 Ma in the Harquahala Mountains and between 23 and 21 Ma across the CREC [John and Foster, 1993; Nielsen and Beratan, 1995; Foster and John, 1999; Carter et al., 2006; Singleton et al., 2014]. Together, these timing constraints indicate extension across the regionally correlative CREC detachment fault system initiated somewhat synchronously over a distance of ~200 km. Our ~44 ± 2 km displacement estimate for the Eagle Eye detachment fault is also very consistent with displacement estimates of ~40–50 km of displacement within the Harcuvar Mountains [Reynolds and Spencer, 1985] and the Buckskin-Rawhide Mountains [Singleton et al., 2014]. No southward decrease in displacement magnitude occurred in the Whipple tilt domain, and extension along the CREC detachment fault system was both consistent temporally and with respect to displacement magnitudes.

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8. Conclusions This study provides new data on the timing, rates, and displacement magnitude along the Eagle Eye detachment fault and utilized rare, but crucial, constraints such as preserved thermochronometric and lithologic markers to determine the extensional history of the Harquahala Mountains metamorphic core complex. Extensional deformation within the Harquahala Mountains initiated by ~23 Ma based on the presence of coarse clastic sediments preserved at Bullard Peak and was followed by rapid detachment fault slip from ~21 ± 1 to ~14 Ma. The spatial extent of reset ZHe ages indicated a minimum displacement estimate of ~34 km, which in addition to the ~8–12 km of displacement required to exhume the zircon partial retention zone, yielded ~44 ± 2 km of total Miocene displacement along the Eagle Eye detachment fault. Geochronologic correlation of offset lithologic marker units in the Little Harquahala Mountains and Bullard Peak indicated ~43–45 km of displacement along the Eagle Eye detachment fault. Both displacement estimates are in very good agreement, indicating ~44 ± 2 km of slip along the Eagle Eye detachment fault, high- lighting the utility of combining complementary methods to quantify displacement along low-angle normal faults. The Eagle Eye detachment fault initiated at a low angle and originally listric geometry, with an ~34 ± 9° dipping segment within the upper 7 km of crust, decreasing to ~13 ± 5° between ~7 and ~12–16 km (Figure 11). Secondary breakaway development at ~17 Ma resulted in surface exposure of the footwall and synextensional deposition of Eagle Eye Peak sediments during active slip along the Eagle Eye detachment fault. ZHe data yielded fault slip rates of 6.6 + 7.1/À2.0 km/Myr, whereas AHe data indicate slip rates of 6.7 + 7.8/À2.3 km/Myr (Figure 10). Both the ZHe and AHe rates agree well with ~6–8 Myr of extension and an estimated displacement magnitude of ~44 ± 2 km. The Harquahala Mountains provided an excellent opportunity to fully constrain the initial geometric and thermal conﬁguration along the Eagle Eye detachment fault and quantify slip rates, displacement and extension duration at the southern end of the Whipple tilt domain [Spencer and Reynolds, 1991] within the CREC. Our new constraints from the Harquahala Mountains, combined with previous studies in the region, indicate that total displacement magnitudes (~40–50 km) and the timing of large-magnitude extension was fairly uniform across the ~200 km swath of metamorphic core complexes from the Sacramento to the Harquahala Mountains (Figure 1).

Acknowledgments References This research was partially supported by a William R. Muehlberger Graduate Brady, R. J. (2002), Very high slip rates on continental extensional faults: New evidence from (U-Th)/He thermochronometry of the Buckskin – Fellowship in Structural Geology/ Mountains, Arizona, Earth Planet. Sci. Lett., 197(1-2), 95 104. Tectonics to M. Prior. We thank Alison Brichau, S., U. Ring, R. A. Ketcham, A. Carter, D. Stockli, and M. Brunel (2006), Constraining the long-term evolution of the slip rate for a major – MacNamee for field assistance; the extensional fault system in the central Aegean, Greece, using thermochronology, Earth Planet. Sci. Lett., 241(1-2), 293 306. Stockli research group for laboratory Brichau, S., S. Thomson, and U. Ring (2010), Thermochronometric constraints on the tectonic evolution of the Serifos detachment, Aegean – assistance and geologic discussion, Sea, Greece, Int. J. Earth Sci., 99(2), 379 393. especially Spencer Seman, Edgardo Brooks, W. E. (1984), Volcanic stratigraphy of part of the McLendon volcano, Anderson Mine area, Yavapai County, Arizona, U.S. Geol. Surv. Pujols, and Nikki Seymour. Mike Cosca Open File Rep., 84-350, 42. and the USGS Denver 40Ar/39Ar lab are Bryant, B., and J. L. Wooden (2008), Geology of the northern part of the Harcuvar complex, west-central Arizona, U.S. Geol. Surv. Prof. Pap. thanked for basalt analysis funded by 1752, 1-52. – JSG Off-Campus Research Funds. This Buck, W. R. (1988), Flexural rotation of normal faults, Tectonics, 7, 959 973, doi:10.1029/TC007i005p00959. research benefited from discussions Carter, T. J., B. P. Kohn, D. A. Foster, and A. J. W. Gleadow (2004), How the Harcuvar Mountains metamorphic core complex became cool: – with W. M. Behr, R. A. Ketcham, Evidence from apatite (U-Th)/He thermochronometry, Geology, 32(11), 985 988. J. E. Spencer, and M. L. Wells. G. Axen Carter, T. J., B. P. Kohn, D. A. Foster, A. J. W. Gleadow, and J. D. Woodhead (2006), Late-stage evolution of the Chemehuevi and Sacramento — and E. M. Duebendorfer provided detachment faults from apatite (U-Th)/He thermochronometry Evidence for mid-Miocene accelerated slip, Geol. Soc. Am. Bull., 118(5-6), – thorough and constructive reviews that 689 709. significantly improved this manuscript. Collettini, C., N. De Paola, R. E. Holdsworth, and M. R. Barchi (2006), The development and behaviour of low-angle normal faults during – Supplementary data are available in Cenozoic asymmetric extension in the Northern Apennines, Italy, J. Struct. Geol., 28(2), 333 352. Tables S1–S5. Coney, P. J. (1980), Cordilleran metamorphic core complexes: An overview, in Cordilleran Metamorphic Core Complexes, Memoir, vol. 153, edited by M. L. Crittenden, P. J. Coney, and G. H. Davis, pp. 7–31, Geol. Soc. Am, Boulder, Colo. Cooper, F. J., M. C. van Soest, and K. V. Hodges (2011), Detrital zircon and apatite (U-Th)/He geochronology of intercalated baked sediments: A new approach to dating young basalt flows, Geochem. Geophys. Geosyst., 12, Q07003, doi:10.1029/2011GC003650. Davis, G. A., and G. S. Lister (1988), Detachment faulting in continental extension: Perspectives from the southwestern U.S. Cordillera, Geol. Soc. Am. Spec. Pap., 218, 133-160 pp. Davis, G. A., G. S. Lister, and S. J. Reynolds (1986), Structural Evolution of the Whipple and South Mountains Shear Zones, Southwestern United-States, Geology, 14(1), 7–10. Dodson, M. H. (1973), Closure temperature in cooling geochronological and petrological systems, Contrib. Mineral. Petrol., 40, 259–274. Dorsey, R. J., and U. Becker (1995), Evolution of a large Miocene growth structure in the upper plate of the Whipple detachment fault, northeastern Whipple Mountains, California, Basin Res., 7, 151–163. Elhlou, S., E. Belousova, W. L. Griffin, N. J. Pearson, and S. Y. O’Reilly (2006), Trace element and isotopic composition of GJ-red zircon standard by laser ablation, Geochim. Cosmochim. Acta, 70(18), A158. Farley, K. A. (2000), Helium diffusion from apatite: General behavior as illustrated by Durango fluorapatite, J. Geophys. Res., 105, 2903–2914, doi:10.1029/1999JB900348.

PRIOR ET AL. HARQUAHALA MOUNTAINS MIOCENE SLIP 20 Tectonics 10.1002/2016TC004241

Farley, K. A., R. A. Wolf, and L. T. Silver (1996), The effects of long alpha-stopping distances on (U-Th)/He ages, Geochim. Cosmochim. Acta, 60(21), 4223–4229. Fitzgerald, P. G., J. E. Fryxell, and B. P. Wernicke (1991), Miocene crustal extension and uplift in southeastern Nevada: Constraints from fission track analysis, Geology, 19, 1013–1016. Fitzgerald, P. G., E. M. Duebendorfer, J. E. Faulds, and P. O’Sullivan (2009), South Virgin-White Hills detachment fault system of SE Nevada and NW Arizona: Applying apatite fission track thermochronology to constrain the tectonic evolution of a major continental detachment fault, Tectonics, 28, TC2001, doi:10.1029/2007TC002194. Foster, D. A., and B. E. John (1999), Quantifying tectonic exhumation in an extensional orogen with thermochronology: Examples from the southern basin and range province, Geol. Soc. London Spec. Publ., 154, 343–364. Foster, D. A., A. J. W. Gleadow, S. J. Reynolds, and P. G. Fitzgerald (1993), Denudation of metamorphic core complexes and the reconstruction of the transition zone, west central Arizona—Constraints from apatite fission-track thermochronology, J. Geophys. Res., 98, 2167–2185, doi:10.1029/92JB02407. Fryxell, J. E., and E. M. Deubendorfer (2005), Origin and trajectory of the Frenchman Mountain Block, an extensional allochthon in the Basin and Range Province, southern Nevada, J. Geol., 113, 355–371. Grove, M., and T. M. Harrison (1996), 40Ar* diffusion in Fe-rich biotite, Am. Mineral., 81, 940–951. Haines, S. H., and B. A. van der Pluijm (2012), Patterns of mineral transformations in clay gouge, with examples from low-angle normal fault rocks in the western USA, J. Struct. Geol., 43,2–32. 40 Harrison, T. M., I. Duncan, and I. McDougall (1985), Diffusion of Ar in biotite: Temperature, pressure and compositional effects, Geochim. Cosmochem. Acta, 49, 2461–2468. Hourigan, J. K., P. W. Reiners, and M. T. Brandon (2005), U-Th zonation dependent alpha-ejection in (U-Th)/He chronometry, Geochim. Cosmochem. Acta, 69(13), 3349–3365. Howard, K. A., and B. E. John (1987), Crustal extension along a rooted system of imbricate low-angle faults: Colorado River extensional corridor, California and Arizona, in Continental Extensional Tectonics, edited by M. P. Coward, J. F. Dewey, and P. L. Hancock, Geol. Soc., London, Spec. Publ., 28, 299–311. Isachsen, C. E., G. E. Gehrels, N. R. Riggs, J. E. Spencer, C. A. Ferguson, S. J. Skotnicki, and S. M. Richard (1999), U-Pb geochronologic data from zircons from eleven granitic rocks in central and western Arizona, Arizona Geol. Soc. Open File Rep., 99-5. Jennings, C. W. (1977), Geologic map of California: California division of mines and geology geologic data map no. 2, scale 1:750,000. John, B. E., and D. A. Foster (1993), Structural and thermal constraints on the initiation angle of detachment faulting in the southern Basin and Range: The Chemehuevi Mountains case study, Geol. Soc. Am. Bull., 105(8), 1091–1108. Keith, S. B., S. J. Reynolds, P. E. Damon, M. Shafiqullah, D. E. Livingston, and P. D. Pushkar (1980), Evidence for multiple intrusion and deformation within the Santa Catalina-Rincon-Torttolita crystalline complex, southeastern Arizona, in Cordilleran Metamorphic Core Complexes, Memoir, vol. 153, edited by M. L. Crittenden, P. J. Coney, and G. H. Davis, pp. 217–257, Geol. Soc. Am., Boulder, Colo. Ketcham, R. A. (1996), Thermal models of core-complex evolution in Arizona and New Guinea: Implications for ancient cooling paths and present-day heat flow, Tectonics, 15, 933–951, doi:10.1029/96TC00033. Lister, G. S., and G. A. Davis (1989), The origin of metamorphic core complexes and detachment faults formed during Tertiary continental extension in the northern Colorado River region, U.S.A, J. Struct. Geol., 11(1/2), 65–94. Lister, G. S., and S. L. Baldwin (1993), Plutonism and the origin of metamorphic core complexes, Geology, 21, 607–610. Lister, G. S., G. Banga, and A. Feenstra (1984), Metamorphic core complexes of Cordilleran type in the Cyclades, Aegean Sea, Greece, Geology, 12, 221–225. Livaccari, R. F., and J. W. Geissman (2001), Large-magnitude extension along metamorphic core complexes of western Arizona and southeastern California: Evaluation with paleomagnetism, Tectonics, 20, 625–648, doi:10.1029/2000TC001244. Ludwig, K. (2003), User’s Manual for Isoplot 3.00: A Geochronological Toolkit for Microsoft Excel, Berkeley Geochronology Center, Spec. Publ., vol. 5, Berkeley. Nielsen, J. E., and K. Beratan (1995), Stratigraphic and structural synthesis of a Miocene extensional terrane, southeast California and west- central Arizona, Geol. Soc. Am. Bull., 107(2), 241–252. Petrus, J. A., and B. S. Kamber (2012), VizualAge: A novel approach to laser ablation ICP-MS U-Pb geochronology data reduction, Geostand. Geoanal. Res., 36(3), 247–270. Pollard, B. M.. Singleton, J. S., and Mosher, S. (2013), Reactivation of fractures as discrete shear zones from fluid enhanced reaction softening, Harquahala metamorphic core complex, AZ, Geol. Soc. Am. Abstr. Programs, 45(7), 599. Rehrig, W. A., and S. J. Reynolds (1980), Geologic and geochronologic reconnaissance of a northwest-trending zone of metamorphic core complexes in southern and western Arizona, in Cordilleran Metamorphic Core Complexes, Memoir, vol. 153, edited by M. L. Crittenden, P. J. Coney, and G. H. Davis, pp. 131–157, Geol. Soc. Am., Boulder, Colo Reiners, P. W., and K. A. Farley (2001), Influence of crystal size on apatite (U-Th)/He thermochronology: An example from the Bighorn Mountains, Wyoming, Earth Planet. Sci. Lett., 188, 413–420. Reiners, P. W., R. J. Brady, K. A. Farley, J. E. Fryxell, B. P. Wernicke, and D. Lux (2000), Helium and argon thermochronometry of the Gold Butte block, south Virgin Mountains, Nevada, Earth Planet. Sci. Lett., 178, 315–326. Reiners, P. W., K. A. Farley, and H. J. Hickes (2002), He diffusion and (U-Th)/He thermochronometry of zircon: Initial results from Fish Canyon Tuff and Gold Butte, Tectonophysics, 349, 297–308. Reiners, P. W., T. Spell, S. Nicolescu, and K. Zanetti (2004), Zircon (U–Th)/He thermochronometry: He diffusion and comparisons with Ar-40/ Ar-39 dating, Geochim. Cosmochim. Acta, 68, 1857–1887. Reynolds, S. J., and J. E. Spencer (1984), Geologic map of the Bullard Peak-Bullard Peak Area, Eastern Harcuvar Mountains, west-central Arizona, Arizona Geol. Surv., Open File Rep. 84-4, 1:24,000 scale. Reynolds, S. J., and J. E. Spencer (1985), Evidence for large-scale transport on the Bullard detachment fault, west-central Arizona, Geology, 13, 353–356. Richard, S. M., S. J. Reynolds, and J. E. Spencer (1987), Mesozoic stratigraphy of the Little Harquahala and Harquahala Mountains, west-central Arizona, in Mesozoic Rocks of Southern Arizona and Adjacent Areas, vol. 18, edited by W. R. Dickinson and M. A. Klute, pp. 101–119, Arizona Geol. Soc. Digest, Tucson, Ariz. Richard, S. M., J. E. Fryxell, and J. F. Sutter (1990), Tertiary structure and thermal history of the Harquahala and Buckskin Mountains, west-central Arizona: Implications for denudation by a major detachment fault system, J. Geophys. Res., 95, 19,973–19,987, doi:10.1029/ JB095iB12p19973. Richard, S. M., J. E. Spencer, and S. J. Reynolds (1994), Geologic map of the Salome 30′ ×60′ Quadrangle, west-central Arizona, Arizona Geol. Surv., Open File Rep. 94-17, 1:100,000 scale.

PRIOR ET AL. HARQUAHALA MOUNTAINS MIOCENE SLIP 21 Tectonics 10.1002/2016TC004241

Richard, S. M., S. J. Reynolds, J. E. Spencer, and P. A. Pearthree (compilers) (2000), Geologic map of Arizona: Arizona geological survey map 35, scale: 1:1,000,000, Tucson, Ariz. Ricketts, J. W., K. E. Karlstrom, and S. A. Kelley (2015), Embryonic core complexes in narrow continental rifts: The importance of low-angle normal faults in the Rio Grande rift of central New Mexico, Geosphere, 11(2), 425–444. Roddy, M. S., S. J. Reynolds, B. M. Smith, and J. Ruiz (1988), K-metasomatism and detachment-related mineralization, Harcuvar Mountains, Arizona, Geol. Soc. Am. Bull., 100, 1627–1639. Scarborough, R., and J. C. Wilt (1979), A study of uranium favorability of Cenozoic sedimentary rocks, Basin and Range Province, Arizona— Part I. General geology and chronology of pre-late Miocene Cenozoic sedimentary rocks, Arizona Bureau of Geol. and Mineral Technol. Open File Rep. 79-1, 101. Singleton, J. S. (2013), Development of extension-parallel corrugations in the Buckskin-Rawhide metamorphic core complex, west-central Arizona, Geol. Soc. Am. Bull., 125(3/4), 453–472. Singleton, J. S. (2015), The transition from large-magnitude extension to distributed dextral faulting in the Buckskin-Rawhide metamorphic core complex, west-central Arizona, Tectonics, 34, 1685–1708, doi:10.1002/2014TC003786. Singleton, J. S., and S. Mosher (2012), Mylonitization in the lower plate of the Buckskin-Rawhide detachment fault, west-central Arizona: Implications for the geometric evolution of metamorphic core complexes, J. Struct. Geol., 39, 180–198. Singleton, J. S., D. F. Stockli, P. B. Gans, and M. G. Prior (2014), Timing, rate, and magnitude of slip on the Buckskin-Rawhide detachment fault, west central Arizona, Tectonics, 33, 1596–1615, doi:10.1002/2013TC003517. Spencer, J. E. (1984), Role of tectonic denudationin warpingand uplift of low-anglenormalfaults, Geology, 12(2), 95–98, doi:10.1130/0091-7613. Spencer, J. E., and S. J. Reynolds (1989), Tertiary structure, stratigraphy, and tectonics of the Buckskin Mountains, in Geology and Mineral Resources of the Buckskin and Rawhide Mountains, West-Central Arizona, Ariz. Geol. Surv. Bull., vol. 198, edited by J. E. Spencer and S. J. Reynolds, pp. 103–167, Ariz. Geol. Surv., Tucson. Spencer, J. E., and S. J. Reynolds (1990), Relationship between Mesozoic and Cenozoic tectonic features in west central Arizona and adjacent southeastern California, J. Geophys. Res., 95, 539–555, doi:10.1029/JB095iB01p00539. Spencer, J. E., and S. J. Reynolds (1991), Tectonics of mid-tertiary extension along a transect through west central Arizona, Tectonics, 10, 1204–1221, doi:10.1029/91TC01160. Spencer, J. E., M. J. Grubensky, J. T. Duncan, J. D. Shenk, J. C. Yarnold, and J. P. Lombard (1989), Geology and mineral deposits of the central Artillery Mountains, in Geology and mineral resources of the Buckskin and Rawhide Mountains, west-central Arizona, Bulletin, vol. 198, edited by J. E. Spencer and S. J. Reynolds, pp. 168–183, Arizona Geol. Surv., Tucson, Ariz. Stimac, J. A., S. M. Richard, S. J. Reynolds, R. C. Capps, C. P. Kortemeier, M. J. Grubensky, G. B. Allen, F. Gray, and R. J. Miller (1994), Geologic map and cross sections of the Big Horn and Belmont Mountains, west-central Arizona, Arizona Bureau of Geol. and Mineral Technol., Open File Rep. 94-15. Stockli, D. F. (2005), Application of low-temperature thermochronometry to extensional tectonic settings, Rev. Mineral. Geochem., 58, 411–448. Stockli, D. F., K. A. Farley, and T. A. Dumitru (2000), Calibration of the apatite (U-Th)/He thermochronometer on an exhumed fault block, White Mountains, California, Geology, 28(11), 983–986. Stockli, D. F., B. E. Surpless, and T. A. Dumitru (2002), Thermochronological constraints on the timing and magnitude of Miocene and Pliocene extension in the central Wassuk Range, western Nevada, Tectonics, 21(4), 1028, doi:10.1029/2001TC001295. Stockli, D. F., S. Brichau, T. J. Dewane, C. Hager, and J. Schroeder (2006), Dynamics of large-magnitude extension in the Whipple Mountains metamorphic core complex, Geochim. Cosmochim. Acta, 70, A616, doi:10.1016/j.gca.2006.06.1143. Topping, D. J. (1993), Paleogeographic reconstruction of the Death Valley extended region: Evidence from Miocene large rock-avalanche deposits in the Amargosa Chaos Basin, California, Geol. Soc. Am. Bull., 105, 1190–1213. Vermeesch, P. (2004), How many grains are needed for a provenance study?, Earth Planet. Sci. Lett., 224(3-4), 441–451. Vermeesch, P. (2012), On the visualisation of detrital age distributions, Chem. Geol., 312-313, 190–194. Wells, M. L., L. W. Snee, and A. E. Blythe (2000), Dating of major normal fault systems using thermochronology: An example from the Raft River detachment, Basin and Range, western United States, J. Geophys. Res., 105(B7), 16,303–16,327, doi:10.1029/2000JB900094. Wolf, R. A., K. A. Farley, and L. T. Silver (1996), Helium diffusion and low-temperature thermochronometry of apatite, Geochim. Cosmochem. Acta, 60(21), 4231–4240. Wolf, R. A., K. A. Farley, and D. M. Kass (1998), Modeling the temperature sensitivity of the apatite (U-Th)/He thermochronometer, Chem. Geol., 148, 105–114. Wolfe, M. R., and D. F. Stockli (2010), Zircon (U-Th)/He thermochronometry in the KTB drill hole, Germany, and its implications for bulk He diffusion kinetics in zircon, Earth Planet. Sci. Lett., 295,69–82. Yarnold, J. C. (1993), Rock-avalanche characteristics in dry climates and the effect of ﬂow into lakes: Insights from mid-Tertiary sedimentary breccias near Artillery Peak, Arizona, Geol. Soc. Am. Bull., 105, 345–360. Yarnold, J. C. (1994), Tertiary sedimentary rocks associated with the Harcuvar core complex in Arizona (U.S.A.): Insights into paleogeographic evolution during displacement along a major detachment fault system, Sediment. Geol., 89,43–63.

PRIOR ET AL. HARQUAHALA MOUNTAINS MIOCENE SLIP 22