International Geology Review Unroofing History of Alabama And

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Unroofing history of Alabama and Poverty Hills basement blocks, Owens Valley, California, from apatite (U-Th)/He thermochronology Guleed A. H. Ali a; Peter W. Reiners a; Mihai N. Ducea a a Department of Geosciences, University of Arizona, Tucson, AZ, USA

To cite this Article Ali, Guleed A. H., Reiners, Peter W. and Ducea, Mihai N.(2009) 'Unroofing history of Alabama and Poverty Hills basement blocks, Owens Valley, California, from apatite (U-Th)/He thermochronology', International Geology Review, 51: 9, 1034 — 1050 To link to this Article: DOI: 10.1080/00206810902965270 URL: http://dx.doi.org/10.1080/00206810902965270

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Unrooﬁng history of Alabama and Poverty Hills basement blocks, Owens Valley, California, from apatite (U–Th)/He thermochronology Guleed A.H. Ali*, Peter W. Reiners and Mihai N. Ducea

Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA (Accepted 11 April 2009)

Most of the 150 km-long Owens Valley of east-central California, the westernmost graben of the Basin and Range Province and location of the active right-lateral Owens Valley Fault Zone, is filled with more than 2–3 km of Neogene sediments. Two prominent but structurally enigmatic basement blocks, the Alabama Hills and Poverty Hills, rise from the floor of the southern part of the valley. The late Cenozoic tectonic origin of these basement blocks is not known, but previously published hypotheses include: (1) transpressional uplifts; (2) down-dropped normal fault blocks; and (3) giant landslides from adjacent ranges. We measured apatite (U–Th)/He ages on 15 samples from the Alabama and Poverty Hills to understand the history of shallow crustal exhumation of these structures, and to potentially correlate them to rocks from adjacent ranges. Apatite He ages for the Alabama and Poverty Hills range from ,63 to 52 and ,61 to 39 Ma, respectively, with one sample from the Alabama Hills yielding an older weighted mean age of 79.3 ^ 2.2 Ma (2s). These ages are similar to those measured at elevations 1.5–3.0 km higher in both the adjacent Sierra Nevada and White/Inyo Mountains. Together with a lack of evidence for transpressional structures and the presence of extensive weathering of an ancient inherited surface on top of the Alabama Hills, these results are most consistent with an origin as a down-dropped normal fault block. A structural reconstruction using tilt-corrected southern Sierra Nevada age–elevation correlations requires 2.6 km of vertical and 1.5 km of eastward motion for the Alabama Hills along the Sierra Nevada Frontal Fault. Although we cannot conclusively rule out a landslide origin, our preferred explanation for the Poverty Hills data is that it is a down-dropped normal fault block that was transported from the Inyo Mountains along a right-lateral fault possessing a significant normal component. Keywords: Alabama Hills; Poverty Hills; Owens Valley; (U–Th)/He thermochronology; Sierra Nevada

Introduction Downloaded By: [Canadian Research Knowledge Network] At: 23:39 2 March 2010 The Alabama and Poverty Hills are enigmatic outcrops of crystalline basement rocks in the Owens Valley, between the Sierra Nevada and the White/Inyo ranges in east-central California. Most of the Owens Valley is covered and ﬁlled with Neogene and Quaternary sediments and volcanics to minimum depths of ,2400–2700 m (Kane and Pakiser 1961; Pakiser et al. 1964), except where the Alabama and Poverty Hills’ structure outcrops (Gillespie 1991). The origin and tectonic history of both the structures were studied in the early seismic surveys by Gutenberg et al. (1932), and various hypotheses have been proposed for their origins. Competing models include landslides or down-dropped normal fault blocks (Gutenberg et al. 1932; Richardson 1975; Bishop and Clements 2006), as well as

*Corresponding author. Email: [email protected]

ISSN 0020-6814 print/ISSN 1938-2839 online q 2009 Taylor & Francis DOI: 10.1080/00206810902965270 http://www.informaworld.com International Geology Review 1035

the flower structures formed along localized transpressional step-overs associated with the Sierra Nevada Frontal Fault Zone (SNFFZ) and Owens Valley Fault Zone (OVFZ), respectively (Pakiser et al. 1964; Taylor 2002). Fundamentally, these basement blocks have either been uplifted from the valley floor along transpressional faults, or were down-dropped from the neighbouring high-elevation ranges along normal faults or as landslides. In this study, we attempt to constrain the shallow crustal exhumation history of rocks in these two basement blocks using apatite (U–Th)/He thermochronology. To first order, apatite He ages provide estimates of the timing of cooling through closure temperatures of ,50–708C, corresponding to the timing of exhumation within ,2–3 km of the surface. In this context, apatite He ages may allow for discrimination between tectonic models for the Alabama and Poverty Hills that require either recent exhumation from depth along the local transpressional uplifts (which would predict relatively young ages compared with adjacent footwall blocks), or down-dropping of normal fault hanging walls or landslide origins, which would predict older ages, similar to those in the adjacent ranges (e.g. House et al. 1997; Lee et al. 2009).

Geologic background Owens Valley (Figure 1) is the westernmost graben of the Basin and Range Province, and is morphologically subdivided into northern and southern parts; the features studied here are in the southern part. Early rifting since 8–10 Ma (Jayko 2009) created ,2–4km of vertical relief from the Sierra Nevada and White/Inyo crests to the valley ﬂoor (Bishop and Clements 2006). To the west, elevations along the Sierra Nevada range from ,1500 to 4400 m, and show similar elevations to the east along the Inyo/White Mountains (Gillespie 1991). The peak of the Sierra Nevada, also the highest elevation in the coterminous USA, is Mt Whitney (4421 m). The Alabama Hills is adjacent to the steep escarpment at the base of Mt Whitney, which is controlled by an extensive regional fault system. Prominent faults in the Owens Valley include range-bounding normal faults, grouped as the SNFFZ, the NNW-striking OVFZ, and some subsidiary NNE-striking oblique strike-slip normal faults such as the Deep Springs Fault (Figure 1).

Faults The OVFZ is an ,100 km-long right-lateral oblique strike-slip fault in east-central California that cuts the Owens Valley asymmetrically, and plays an important role in the Downloaded By: [Canadian Research Knowledge Network] At: 23:39 2 March 2010 processes related to strain accumulation and transfer in the Eastern California Shear Zone (Frankel et al. 2008). A combination of faults grouped into the Eastern California Shear Zone (including the OVFZ) and the Walker Lane Shear Zone, accommodate ,25% of the modern Paciﬁc–North American relative plate motion (Flesch et al. 2000; Bennett et al. 2003; Hammond and Thatcher 2004). The southern OVFZ begins near the southern edge of the Owens Lake, strikes north–northwest through the valley, bounds the Alabama Hills to their east, and terminates just north of the Poverty Hills near the town of Bishop. Geodetic data constrained by block models show slip rates between 5 and 7 mm/yr (Gan et al. 2000; McClusky et al. 2001, Miller et al. 2001). These rates are approximately three times higher than the long-term geologic slip rates determined from palaeoseismic studies (e.g. Lubetkin and Clark 1988; Beanland and Clark 1994; Lee et al. 2001b). Earthquake cycle effects and viscoelastic rheology of the lower and upper mantle may explain the apparent discrepancies between geodetic and palaeoseismic slip rates (Dixon et al. 2003). 1036 G.A.H. Ali et al. Downloaded By: [Canadian Research Knowledge Network] At: 23:39 2 March 2010

Figure 1. Digital elevation map of the central and south Owens Valley showing modiﬁed quaternary fault traces of the OVFZ, Sierra Nevada Fault Zone, White Mountain Fault Zone, northern Inyo Mountain Fault, southern Inyo Mountain Fault, and Deep Springs Fault with locations dated using apatite (U–Th)/He thermochronology (US Geologic Survey 2006; Slemmons et al. 2008).

Holocene lateral slip rates of the OVFZ were studied using offset abandoned Tioga outwash fans and palaeoseismic trenches that resulted in different slip rates of 1.8 ^ 3to 3.6 ^ 0.2 mm/yr (Lee et al. 2001b) and 2.0 ^ 1 mm/yr (Beanland and Clark 1994), respectively. Holocene displacement rates determined from a fault strand near the town of Lone Pine Range from 2.2 to 4.4 mm/yr with a 0.4–1.3 mm/yr down-to-the-east International Geology Review 1037

component (Lubetkin and Clark 1988). This particular strand of the OVFZ, which is known as the Lone Pine Fault, is located just east of the Alabama Hills and shows a well-preserved 6.5 m-high scarp from the 1872 Owens Valley Mw 7.5–7.75 earthquake (Lubetkin and Clark 1988; Ellsworth 1990; Beanland and Clark 1994). This down-to-the- east normal oblique rupture produced the largest known earthquake in the Basin and Range Province, and the third largest in the continental US history (Beanland and Clark 1994; Bacon and Pezzopane 2007). New estimates consistent with long-term geologic records indicate Mw 7.8–7.9, which would make this rupture even larger than the 1906 San Francisco earthquake (Hough and Hutton 2008). Just south of the Poverty Hills, the OVFZ deviates to the NNE where its trace is no longer observed, but a primary strand is observed to continue NW of the Poverty Hills, forming an apparent transpressional zone along the left stepover (Martel 1989; Taylor 2002). Approximately, 15–45% of the total OVFZ slip, or 0.7 mm/yr, is transferred NNE through the Deep Springs Fault to the dextral NNW-striking eastern Death Valley Fault Zone (Lee et al. 2001a). The remaining total slip from the OVFZ may continue north and east of the Poverty Hills, as it transfers into the White Mountain Fault Zone via a right stepover that cuts through the Papoose Canyon volcanic sequence of the Big Pine Volcanic ﬁeld (M.N. Ducea et al., in preparation). Other studies used the Alabama Hills to demonstrate slip along the OVFZ since the Late Cretaceous. Dikes of the Late Jurassic Independence dike swarm span across the Owens Valley, and over 600 km through the Sierra Nevada and the southern Mojave Desert region (Carl and Glazner 2002). The dike swarm intruded the Alabama Hills and showed a zircon U–Pb age of 148 Ma (Chen and Moore 1979). Studies from the Independence dike swarm as well as Permian–Triassic pendant rocks and Cretaceous dikes show evidence for 65 ^ 5 km of offset along the OVFZ since the Middle to latest Triassic (e.g. Coleman et al. 2000; Stevens and Stone 2002; Glazner et al. 2005; Kylander- Clark 2005). This indicates that a signiﬁcant portion of this dextral slip along the Owens Valley region precedes major extension in the modern Owens Valley as early as 83 Ma (Kylander-Clark et al. 2005; Frankel et al. 2008). The two range bounding faults are the SNFFZ and the Inyo Mountain Fault. The normal oblique SNFFZ strikes NNW and dips ,65–758 to the east and is active since 9.5–10.5 Ma (Kane and Pakiser 1961; Beanland and Clark 1994; Jayko 2009). Slip rates differ slightly between the southern, central, and northern SNFFZ, but the southern SNFFZ is more pertinent to this study because it is located at the latitude of this study. Average late Pleistocene to Holocene vertical slip rates from the southern SNFFZ determined by Downloaded By: [Canadian Research Knowledge Network] At: 23:39 2 March 2010 10Be from the offset alluvial fan surfaces from the southern SNFFZ range from 0.2 to 0.3 mm/yr (e.g. Le et al. 2007). The Inyo Mountain Fault is located at the western base of the Inyo Mountains, and terminates just southeast of the Poverty Hills. A slip along the southern Inyo Mountain Fault demonstrates a normal oblique dextral sense of motion (Bacon et al. 2005). In this study, Bacon et al. (2005) used 14C dating of the alluvial fan scarps to constrain the Holocene and the latest Pleistocene normal oblique dextral slip along the southern Inyo Mountain Fault between 0.1 and 0.3 mm/yr.

Alabama Hills The ,65 km2 Alabama Hills comprise a shallow, Late Cretaceous composite intrusive suite, ranging in composition from monzogranite to granite, which was emplaced in a Jurassic metasedimentary and metavolcanic framework (Richardson 1975; Dunne and Walker 1993; Dunne et al. 1998; Stone et al. 2001). The zircon U–Pb ages of the intrusive 1038 G.A.H. Ali et al.

rocks are 85 Ma (Chen and Moore 1982), similar to those of the Mt Whitney Intrusive Suite at 87–83 Ma (Chen and Moore 1982), and the biotite K–Ar ages are 82 ^ 0.5 Ma (Evernden and Kistler 1970). The zircon U–Pb ages from the Middle Jurassic metarhyolite tuff in the Alabama Hills have been dated at 167 ^ 2 Ma (Dunne et al. 1998) and 170 ^ 4 Ma (Dunne and Walker 1993). The Alabama Hills are bounded by the OVFZ to the east, and by the SNFFZ and the eastern escarpment of the Sierra Nevada, near the base of Mt Whitney, to the west (Stone et al. 2001). The southern limit of the Alabama Hills is shown by sharp contrasts in gravity data that truncate the Alabama Hills just north of the Owens Lake from Kane and Pakiser (1961); the structure also extends ,15 km north in the subsurface, approximately twice its visible extent (Kane and Pakiser 1961). The seismic evidence from Pakiser et al. (1964) indicates that short faults orthogonal to the OVFZ cut the Alabama Hills, and are most likely responsible for gradually greater subsurface depths of the top of the structure to the north. Geomorphologically, the Alabama Hills block is characterized by a weathered surface of varnished tors and inselbergs (Nichols et al. 2006). Nichols et al. (2006) used 10Be from quartz to determine erosion rates of 0.005–0.02 mm/yr from inselbergs and the pediments in the Alabama Hills, with erosion rates increasing from high inselbergs and low inselberges to the pediment surfaces. Minimal erosion of the high inselbergs indicates greater surface stability compared to low inselbergs and pediment surface, which erode slightly faster, but are still indicative of low-erosion environments similar to other arid climates (Nichols et al. 2006). This study also noted the possibility that the modern topographic features in the Alabama Hills may be representative of a relict surface, as previously suggested by Richardson (1975). The petrologic differentiation trends of the Alabama Hills granitoids are also similar to those in the nearby Sierra Nevada to the west, and distinct from those of plutons from the Inyo Mountains to the east (Richardson 1975). Based on thermochronologic and petrographic analyses (e.g. Evernden and Kistler 1970; Richardson 1975; Chen and Moore 1979, 1982; House et al. 1997), both the Alabama Hills and the adjacent Mt Whitney Intrusive Suite were emplaced and exhumed at similar times. The Mt Whitney Intrusive Suite is comprised of three granitic plutons that were emplaced from 88 to 83 Ma at a depth of less than 10 km (Hirt 2007).

Poverty Hills

Downloaded By: [Canadian Research Knowledge Network] At: 23:39 2 March 2010 The Poverty Hills is a low-relief (,300 m) basement block, comprising approximately 13 km2 of Mesozoic granodiorite and Palaeozoic metasedimentary rocks (Bateman 1965; Nelson 1966). It is located ,46 km north of the Alabama Hills along the strike of the OVFZ, near an apparent ,3 km left stepover (Martel 1989). Granodiorite rocks were categorized as equivalent to the Tinemaha granodiorite in the nearby Inyo Mountains, which yields concordant zircon U–Pb ages of 146 Ma (Chen and Moore 1979). The metasedimentary rocks in the western portion of the block were correlated with the Keeler Canyon Formation and Rest Spring Shale based on petrographic, stratigraphic, and palaeontologic features (Hoylman 1974). As well as in the Poverty Hills, both of these units and the Tinemaha granodiorite are exposed about 20 km to the southeast, in the northern Inyo Mountains. At this location, the Jurassic intrusive is known as the Santa Rita Flat (SRF) pluton. The SRF pluton is a porphyritic granodiorite (Hoylman 1974), which intruded carboniferous metasedimentary rocks from the Keller Canyon Formation and Rest Spring International Geology Review 1039

Shale, as well as Silurian to Ordovician sedimentary strata in the central Inyo Mountains (Ross 1965). Of these three formations, both the Keeler Canyon Formation and Rest Spring Shale are also found in the Poverty Hills. This observation has led others to suggest that the Poverty Hills is sourced from the SRF pluton (e.g. Hoylman 1974; Bishop and Clements 2006). Based on a gravity study, Kane and Pakiser (1961) hypothesized that the Poverty Hills originated as a gravity slide from the Sierra Nevada to the west, despite the fact that the Poverty Hills more closely resemble the Palaeozoic geology and subsequent intrusive history of the Inyo Mountains to the east. Alternatively, Martel (1989) and Taylor (2002) argued that the OVFZ in the region of the Poverty Hills displays a left stepover, consistent with a local transpressional regime. Based on a detailed structural analysis, Taylor (2002) interpreted the Poverty Hills as a positive ﬂower structure that formed since 1.7 Ma. By contrast, Bishop and Clements (2006) argued for a landslide origin, based on the sedimentary structures analogous to sturzstroms (long-run-out rock avalanches) seen beneath the basement rocks of the block at one location near the Tinemaha campground.

Samples and methods Apatite He ages were determined on three to eight multiple single-grain aliquots from nine samples of the monzogranite or granodioritic rocks from the Poverty Hills and six from the Alabama Hills (Table 1). Samples from the Poverty Hills were collected over a horizontal distance of 1 km and ,150 m of the vertical relief. The samples from the Alabama Hills were collected over 8 km and ,900 m of the vertical relief. Analytical procedures are described in detail elsewhere (Reiners et al. 2004). Mineral separation was performed by standard crushing, sieving, and magnetic and density separation procedures. Grains with minimum half-widths of 60 mm and predominantly unbroken, euhedral forms, were selected for the analysis. Although inclusion-free grains are preferred for these types of analyses, many of the samples from these units, especially those from the Poverty Hills, yielded apatite with inclusions of unknown composition. This may account for at least some of the age scatter among multiple replicates from single samples. The grains were loaded into ,0.8 mm Nb tubes, and heated by an Nd:YAG laser. The liberated He was cryogenically puriﬁed and concentrated and measured by a 3He-isotope dilution using the quadrupole mass spectrometry. U, Th, and Sm in each grain were measured by isotope dilution using HR-ICP-MS, and standard alpha ejection corrections (e.g. Farley et al. 1996) were applied to each grain. Downloaded By: [Canadian Research Knowledge Network] At: 23:39 2 March 2010

Results With the exception of two samples, weighted mean apatite He ages of each replicate analysis from the Alabama Hills range from 60 to 69 Ma (Table 1; Figure 2). The easternmost sample yielded a weighted mean age of 52.4 ^ 2.6 Ma (2s), and one sample in the middle of the transect yielded a weighted mean age of 79.3 ^ 2.2 Ma (2s). The weighted mean ages of the Poverty Hills samples are 39–64 Ma (Table 1; Figure 3) and, as in the case of the Alabama Hills, show no correlation with elevation. Although some samples yielded relatively reproducible single-grain ages (e.g. PH1D and PH8D, 32.26 ^ 3.5 (2s), and 54.05 ^ 3.2 (2s), respectively), several Poverty Hills samples yielded highly scattered replicate ages, with one or more signiﬁcant outliers, and large SEs, which may be attributable to the low U–Th concentrations, U–Th-rich inclusions, and He implantation from neighbouring phases. Downloaded By: [Canadian Research Knowledge Network] At: 23:39 2 March 2010 1040 Table 1. Single-grain apatite (U-Th)/He data.

s Sampled Corrected (2-sig err) Radius Mass U Th 4He Sample Lat (N) Long (W) Elevation (m) grain U (pg) Th (pg) Raw age FT age (Ma) (Ma) (mm) (mg) (ppm) (ppm) Th/U (nmol/g) Alabama Hills samples AH1 368 360 08.800 1188 100 22.000 1730 Weighted mean age: 68.9 Ma AHG1B 83.6 341 57.0 0.781 73.1 3.6 67.3 6.16 13.6 55.4 4.2 8.28 Weighted SE (2s): 2.1 Ma AHG1C 42.8 125 48.0 0.764 62.9 4.0 60.3 4.91 8.7 25.4 3.0 3.84 AH1F 27.3 113 46.0 0.662 69.5 3.5 40.8 1.62 16.8 69.6 4.3 8.32 AH2 368 350 48.200 1188 090 54.300 1673 Weighted mean age: 60.4 Ma AHG2A 32.7 156 42.0 0.723 58.0 3.8 53.8 2.79 11.7 56.0 4.9 5.72 Weighted SE (2s): 1.6 Ma AHG2B 29.2 140 43.3 0.706 61.4 3.9 48.5 2.49 11.7 56.1 4.9 5.88 AH2A 17.3 98.5 34.5 0.589 58.5 5.9 32.0 0.95 18.2 103.4 5.8 8.01 AH2B 12.2 61.1 35.6 0.604 59.0 4.3 34.8 0.92 13.2 66.5 5.2 5.61 AH2C 21.2 97.0 40.5 0.663 61.0 3.6 37.0 3.05 7.0 31.8 4.7 3.19 Ali G.A.H. AH2D 20.5 78.8 38.0 0.662 57.4 5.8 41.3 1.36 15.0 57.8 4.0 5.93 AH2E 10.4 49.9 40.0 0.619 64.7 4.1 36.5 0.78 13.4 64.1 4.9 6.22 AH3 368 350 46.800 1188 080 05.700 1494 Weighted mean age: 63.5 AHG3A 41.7 208 49.6 0.731 67.8 3.7 53.8 3.32 12.6 62.6 5.1 7.39 Weighted SE (2s): 1.6 Ma AHG3B 49.6 262 48.1 0.731 65.8 3.9 3.32 15.0 79.2 5.4 8.82 al. et AH3B 28.7 115 46.9 0.696 67.3 3.6 47.8 2.08 13.7 55.1 4.1 6.83 AH3C 28.7 142 40.7 0.698 58.4 3.5 43.5 2.42 11.9 59.0 5.1 5.73 AH3D 31.2 139 40.7 0.683 59.5 3.4 47.3 2.59 12.0 53.6 4.6 5.47 AH4 368 350 08.300 1188 060 56.600 1417 Weighted mean age: 79.3 Ma AH4A 16.2 81.2 55.2 0.651 84.8 4.1 39.8 1.38 11.7 58.8 5.1 7.70 Weighted SE (2s): 2.2 Ma AH4C 10.4 58.1 45.3 0.580 78.1 4.1 30.8 0.95 11.0 61.3 5.7 6.29 AH4D 16.6 85.1 46.3 0.614 75.4 3.5 35.0 0.82 20.3 103.9 5.3 11.28 AH5 368 350 13.500 1188 060 04.300 1316 Weighted mean age: 64.2 Ma AHG5A 46.8 253 46.1 0.731 63.0 3.9 55.0 3.20 14.6 79.7 5.6 8.40 Weighted SE (2s): 2.0 Ma AH5A 18.5 101 40.8 0.702 58.1 3.7 52.3 1.55 12.0 64.9 5.6 6.06 AH5B 13.1 61.5 39.3 0.641 61.3 4.1 38.8 1.29 10.1 47.5 4.8 4.57 AH5C 13.6 68.6 46.8 0.614 76.2 4.2 35.8 1.02 13.3 67.3 5.2 7.45 AH6 368 350 22.300 1188 040 56.100 1197 Weighted mean age: 52.4 Ma AH6A 7.68 39.3 30.4 0.582 52.2 5.6 32.8 0.76 10.0 51.4 5.3 3.67 Weighted SE (2s): 2.6 Ma AH6B 12.2 58.1 31.2 0.571 54.6 4.6 32.8 0.96 12.7 60.3 4.9 4.57 AH6E 23.4 110 34.7 0.679 51.1 3.8 41.0 2.48 9.4 44.5 4.8 3.77 Downloaded By: [Canadian Research Knowledge Network] At: 23:39 2 March 2010

Table 1 – continued

s Sampled Corrected (2-sig err) Radius Mass U Th 4He Sample Lat (N) Long (W) Elevation (m) grain U (pg) Th (pg) Raw age FT age (Ma) (Ma) (mm) (mg) (ppm) (ppm) Th/U (nmol/g) Poverty Hills samples PH1 378 030 18.300 1188 140 50.400 1400 Weighted mean age: 39.9 Ma PHG1A 18.9 89.1 21.3 0.605 35.2 3.8 35.3 0.68 28.0 131.9 4.8 6.89 Weighted SE (2s): 1.5 Ma PHG1C 27.0 81.6 23.5 0.561 41.9 3.8 29.5 1.01 26.8 80.8 3.1 5.90 PH1A 10.6 24.9 20.0 0.619 32.2 9.6 36.3 0.93 11.4 26.9 2.4 1.94 PH1B 85.9 168 29.2 0.693 42.2 3.8 43.0 2.29 37.6 73.4 2.0 8.73

PH1C 42.6 120 32.7 0.675 48.4 3.8 41.5 1.41 30.3 85.4 2.9 9.01 Review Geology International PH1D 54.0 794 22.1 0.684 32.3 3.5 44.3 1.91 28.3 416.1 15.1 15.16 PH1E 96.9 150 23.4 0.635 36.8 3.8 38.8 0.97 99.5 154.5 1.6 17.27 PH1F 50.7 64.9 27.4 0.643 42.6 4.3 36.8 0.93 54.5 69.7 1.3 10.57 PH2 378 030 25.500 1188 140 34.500 1362 Weighted mean age: 54.9 Ma PHG2A 25.5 29.3 39.6 0.640 61.9 5.8 35.0 1.39 18.3 21.1 1.2 5.01 Weighted SE (2s): 2.5 Ma PHG2C 71.5 97.7 33.5 0.760 44.1 5.1 55.3 5.21 13.7 18.7 1.4 3.31 PH2C 18.9 35.6 29.1 0.736 39.5 5.0 51.3 3.63 5.2 9.8 1.9 1.19 PH2D 26.5 23.2 44.5 0.629 70.8 4.4 36.8 0.70 38.1 33.3 0.89 11.12 PH3 378 030 07.100 1188 140 49.200 1403 Weighted mean age: 54.3 Ma PHG3C 57.1 1.41 99.6 0.675 54.6 4.8 43.5 1.52 37.7 65.7 1.79 10.70 Weighted SE (2s): 4.7 Ma PH3B 0.717 0.12 8.07 0.555 66.4 29.2 30.8 0.53 1.4 15.4 11.5 1.01 PH3D 1.40 0.09 14.7 0.557 32.3 30.7 31.3 0.48 2.9 30.7 10.8 0.99 PH4 378 030 08.000 1188 140 49.200 1375 Weighted mean age: 57.7 Ma PHG4A 5.25 0.16 30.4 0.554 64.8 6.4 38.8 1.19 4.4 25.7 5.94 2.37 Weighted SE (2s): 1.8 Ma PHG4B 11.1 0.31 48.1 0.634 54.6 6.2 30.3 0.91 12.1 52.7 4.46 4.24 PHG4C 48.8 1.24 91.1 0.578 41.6 4.9 45.3 1.35 36.1 67.4 1.92 7.91 PH4A 85.8 1.82 249 0.671 62.8 3.4 45.3 3.58 24.0 69.6 2.98 9.70 PH4B 43.8 0.99 129 0.699 65.8 3.6 46.0 2.84 15.4 45.4 3.02 6.67 PH4C 21.8 0.53 68.8 0.708 46.5 4.9 33.8 1.14 19.2 60.5 3.24 5.22 PH5 378 030 09.100 1188 140 40.900 1348 Weighted mean age: 56.0 Ma PHG5A 1.21 28.8 27.0 0.615 46.7 31.4 31.3 0.71 1.7 4.0 2.43 0.39 Weighted SE (2s): 4.2 Ma PHG5B 14.4 43.8 29.9 0.579 52.6 6.7 30.5 0.51 28.2 86.1 3.13 7.92 PH5A 2.82 6.50 29.2 0.568 49.4 22.5 30.3 0.80 3.5 8.2 2.37 0.87

PH5D 12.2 20.1 36.1 0.591 59.1 5.8 32.5 0.79 15.4 25.6 1.70 4.22 1041 Downloaded By: [Canadian Research Knowledge Network] At: 23:39 2 March 2010 1042 Table 1 – continued

s Sampled Corrected (2-sig err) Radius Mass U Th 4He Sample Lat (N) Long (W) Elevation (m) grain U (pg) Th (pg) Raw age FT age (Ma) (Ma) (mm) (mg) (ppm) (ppm) Th/U (nmol/g) PH6 378 0300 14.600 1188 1400 35.600 1320 Weighted mean age: 54.3 Ma PHG6B 3.23 9.99 11.8 0.698 20.9 19.9 32.0 0.59 5.5 16.9 3.18 0.61 Weighted SE (2s): 4.6 Ma PHG6C 1.40 5.32 28.3 0.641 51.3 21.2 30.0 0.57 2.5 9.3 3.90 0.72 PH6B 2.40 11.6 16.9 0.610 25.7 31.6 41.5 1.44 1.7 8.1 4.97 0.33 PH6C 17.7 35.8 8.2 0.703 11.7 12.9 46.5 2.03 8.7 17.6 2.08 0.57 PH6E 6.64 14.5 37.4 0.567 54.7 8.5 42.0 1.64 4.0 8.8 2.24 1.25 PH6F 9.19 15.4 47.1 0.552 70.5 6.6 41.8 1.45 6.4 10.7 1.72 2.28 PH7 378 030 20.600 1188 140 31.600 1289 Weighted mean age: 64.2 Ma AH7B 2.59 13.4 31.2 0.702 73.7 9.7 46.0 3.70 34.9 31.5 0.927 12.32

Weighted SE (2s): 2.7 Ma AH7A 129 116 53.4 0.657 47.9 3.5 46.3 1.50 1.7 8.9 5.33 0.66 Ali G.A.H. AH7D 19.6 33.6 31.1 0.698 49.1 5.0 45.8 1.29 15.2 26.0 1.76 3.62 PH8 378 030 21.200 1188 140 18.400 1255 Weighted mean age: 46.5 Ma AH8A 77.6 130 29.4 0.683 42.5 3.7 46.5 1.78 43.7 73.4 1.73 9.74 Weighted SE (2s): 1.7 Ma AH8B 5.70 8.98 28.1 0.668 47.0 14.8 33.8 0.75 7.6 12.0 1.62 1.59

AH8C 80.7 72.8 33.9 0.726 47.1 4.1 47.8 2.74 29.5 26.6 0.926 6.58 al. et AH8D 16.4 65.1 35.2 0.651 54.1 3.2 41.0 0.98 16.7 66.4 4.08 6.24 AHG8 50.3 155 30.0 0.578 44.3 3.8 43.0 1.79 28.0 86.4 3.17 7.91 AHG8B 99.1 286 26.4 0.632 38.4 4.7 43.3 2.19 45.2 130.5 2.96 10.92 PH9 378 030 23.600 1188 140 14.300 1232 Weighted mean age: 61.6 Ma AH9C 14.5 25.1 48.5 0.692 71.9 3.5 45.0 1.42 10.2 17.7 1.78 3.82 Weighted SE (2s): 2.2 Ma AH9B 33.6 115 40.3 0.599 59.1 4.7 44.3 1.84 18.3 62.7 3.52 7.30 AHG9A 31.3 67.6 36.4 0.720 66.0 5.0 29.3 0.83 37.7 81.4 2.21 11.29 AHG9B 20.0 64.8 40.5 0.652 65.7 5.4 37.0 0.90 22.1 71.7 3.32 8.66 International Geology Review 1043

Figure 2. Digital elevation map of the Alabama Hills and the eastern Sierra Nevada escarpment near Whitney portal with simpliﬁed quaternary fault traces of the OVFZ and the Sierra Nevada Fault Zone showing weighted average and the weighted SE (2s) of calculated apatite (U–Th)/He ages for individual samples (US Geologic Survey 2006). Downloaded By: [Canadian Research Knowledge Network] At: 23:39 2 March 2010

Discussion Apatite He ages from both the Alabama and Poverty Hills are much older than the Neogene extension that created the modern Owens Valley. If these basement blocks were exhumed by the local transpressional structures along the modern OVFZ (such as ﬂower structures proposed for the Poverty Hills; Taylor (2002)), they were not exhumed from depths greater than the ,2–3 km apatite He closure depth. Cooling ages similar to those found in the Alabama and Poverty Hills are present at higher elevations in the adjacent Sierra Nevada and Inyo Mountains, consistent with an origin as down-dropped normal fault (or landslide) blocks. House et al. (1997) showed that apatite He ages in the Sierra Nevada directly to the west of the Alabama Hills vary systematically with elevation (Figure 4), from ,23 Ma at an elevation of 2 km to ,75 Ma near the summit of Mt Whitney. Projecting these cooling age contours to the east, and 1044 G.A.H. Ali et al.

Figure 3. Digital elevation map of the Poverty Hills showing weighted average and the weighted SE (2s) of calculated apatite (U–Th)/He ages for individual samples. The fault traces are reproduced from Taylor (2002) and the US Geologic Survey (2006).

Downloaded By: [Canadian Research Knowledge Network] At: 23:39 2 March 2010 correcting for an inferred 28 westward post-Mesozoic tilting of the Sierra Nevada along the SNFFZ (House et al. 1997; Huber 1981), predicts 2.5 km of the vertical offset of the Alabama Hills (Figure 5). The Alabama Hills intrusive is highly weathered and fractured, in contrast to the granodiorite in the Mt Whitney pluton, despite the fact that they represent the same composite igneous body (Chen and Tilton 1991; Stone et al. 2001). The fracturing and subsequent preferential weathering of the Alabama Hills rocks is most likely due to the brittle deformation in the hanging wall of the postulated normal fault that produced the Alabama Hills, and is a feature observed regionally in granitoids that form hanging walls of the shallow normal faults. Contiguous geometries of the Lone Pine Creek pluton of the Mt Whitney Intrusive Suite on both sides of the SNFFZ also favour a normal fault hypothesis. Our apatite He ages from the Poverty Hills show similar ages to apatite He analyses obtained by Lee et al. (2009) from the northern Inyo Mountains. Their study showed International Geology Review 1045

Figure 4. Weighted average ages and SEs (2s) from the Alabama and Poverty Hills, and western Inyo Mountains (Lee et al. 2009) versus elevation (metres above sea level). The apatite (U–Th)/He ages and the SDs from the Mt Whitney transect (House et al. 1997) are plotted versus elevation.

invariant apatite He ages of 53.2 ^ 6.6 Ma across the western ﬂank of the range, and was interpreted to indicate moderate to rapid cooling and exhumation at ,54 Ma (Lee et al. 2009). Variable replicate apatite He ages of our dataset in the Poverty Hills as well as those from the northern Inyos in Lee et al. (2009) are most likely due to the poor quality of some apatite crystals (presence of inclusions; implantation from neighbouring phases) in these rocks. Lithologic contacts at the SRF pluton between the correlated granitic and the metasedimentary units from the Poverty Hills are only geometrically compatible at two elevations. At high elevations of the pluton, the granodiorite and the metamorphic units are in structural contact, but their map pattern is essentially reversed from that in the Downloaded By: [Canadian Research Knowledge Network] At: 23:39 2 March 2010 Poverty Hills. Exposures of contacts at the SRF pluton show geometries that are only compatible with the southernmost low-elevation area of the intrusive sequence. No fault traces have been mapped that cut both the granodiorite and metasedimentary units. If the southern area serves as the only proper source location, then this may represent an appropriate piercing point for the Poverty Hills basement rocks. The simplest explanation for our Poverty Hills ages is that it is a down-dropped fault block associated with an east-dipping normal fault with right-lateral oblique slip, which came from the nearby SRF pluton in the Inyo Range. Based on the older ages, we can rule out exhumation in a transpressional uplift from depths greater than 2–3 km. Exhumation from these depths would have produced partially to fully reset apatite He ages that are otherwise not observed in comparison to the northern Inyo Mountain apatite He ages (Lee et al. 2009). The two piercing points between the Poverty Hills and SRF pluton are approximately 24 km apart along the OVFZ. If we use the initiation of the Owens Valley graben between 1046 G.A.H. Ali et al.

Figure 5. WSW–ENE geologic cross section (A–A0) from the Mt Whitney peak to the Alabama Hills with apatite He ages projected onto the surface (Bateman 1965, Moore 1981). Post-Mesozoic tilt- correct apatite He ages from the Mt Whitney transect (House et al. 1997) are projected to the east to reconstruct apatite He ages from the Alabama Hills along the Sierra Nevada frontal fault. Apatite He ages presented here are weighted averages and the weighted standard errors (2s) and corrected age and errors of one standard deviation for Alabama Hills and Mt Whitney transects, respectively.

Downloaded By: [Canadian Research Knowledge Network] At: 23:39 2 March 2010 10 and 8 Ma (Jayko 2009), we establish a minimum slip rate along the OVFZ between 2.4 and 3 mm/yr since extension began. This is comparable to the estimates between 1.8 ^ 3 and 3.6 ^ 0.2 mm/yr during the Holocene by Lee et al. (2001b), and of 2 ^ 1 and 0.7–2.2 mm/yr during the late Quaternary slip rates by Beanland and Clark (1994) and Lubetkin and Clark (1988), respectively. We therefore propose that the Poverty Hills basement block originated from the Inyo Mountains and was brought into its present location via one or multiple normal faults with a signiﬁcant right-lateral oblique slip. One of the most likely candidates for the transport is the Owens Falley Fault Zone. Later uplift of this normal fault block may explain map geometries (Taylor 2002) most similar to a positive ﬂower structure.

Conclusions This study uses apatite (U–Th)/He thermochronology to constrain the shallow crustal exhumation of the Alabama and Poverty Hills. Weighted mean ages of the multiple International Geology Review 1047

single-grain replicates yielded apatite He ages of ,61–39 Ma in the Poverty Hills, and 63–52 Ma (and one anomalous age of 76.5 ^ 2.7 Ma (2s)) in the Alabama Hills (Figure 4). We interpret the Alabama Hills to represent the hanging wall of a normal fault associated with the Sierra Nevada Frontal Fault system, based on similar apatite He ages at much higher elevations in the adjacent Mt Whitney transect (House et al. 1997) and similar lithologies in the adjacent eastern Sierra Nevada. Reconstruction of the Alabama Hills to elevations along Mt Whitney with similar apatite He ages indicates at least 2.6 km of vertical and 1.5 km of eastward motion along the SNFFZ. Our apatite He ages and similar intrusive ages from the Mt Whitney plutonic suite (Evernden and Kistler 1970; Chen and Moore 1979, 1982; Chen and Tilton 1991) demonstrate that the Alabama Hills and the Mt Whitney pluton (House et al. 1997) were exhumed from shallow crustal depths contemporaneously. Because the eastern part of the southern Sierra Nevada has experienced little exhumation since the Late Cretaceous (House et al. 2001), we assume that the Alabama Hills block has been near the surface since then. Subsequent normal faulting with the Alabama Hills as a hanging wall block produced a rock appearance distinctly different from the Mt Whitney pluton, which otherwise shows a similar differentiation trend. The characteristically thick weathering mantle from the Alabama Hills was plausibly further ampliﬁed by extreme hot–cold climate transitions, minimal bedrock erosion rates, and tensional fractures associated with the extensional regime. Apatite He ages of 61–39 Ma from the Poverty Hills are similar to those from rocks in the northern Inyo Mountains (Lee et al. 2009) in a location near lithologies correlated with those in the Poverty Hills. The simplest explanation for our data and the overall geologic context is that the Poverty Hills block was transported from the Inyo Mountains along a right-lateral fault with a signiﬁcant normal component, or a combination of normal and right-lateral strike-slip faults. A likely candidate for the strike-slip motion is the OVFZ, which terminates near the Poverty Hills. Correlation of the SRF pluton and the Poverty Hills suggests that a lateral slip of ,2.4–3 mm/yr along the OVFZ, similar to the present measured rates, has been ongoing since , 8–10 Ma.

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