Sierra Nevada–Eastern Tehachapi Mountains Constrained by Low-Temperature Thermochronology: Implications for the Initiation of the Garlock Fault

Home , Sierra Nevada Batholith, Tehachapi Mountains

Exhumation of the southern Sierra Nevada–eastern Tehachapi Mountains constrained by low-temperature thermochronology: Implications for the initiation of the Garlock fault

A.E. Blythe and N. Longinotti* GEOLOGY DEPARTMENT, OCCIDENTAL COLLEGE, LOS ANGELES, CALIFORNIA 90041, USA

ABSTRACT

New apatite and zircon fi ssion-track and apatite (U-Th)/He data from nine samples collected on a north-south transect across the southern Sierra Nevada–eastern Tehachapi Mountains constrain the cooling and exhumation history over the past ~70 m.y. The four northernmost samples yielded zircon and apatite fi ssion-track ages of ca. 70 Ma, indicating rapid cooling from ~250 °C to <60 °C (6–8 km of exhumation) at that time. Four of the fi ve southernmost samples yielded slightly younger zircon fi ssion-track ages (57–46 Ma) and apatite fi ssion-track ages (21–18 Ma); the fi fth southern sample (from a lower elevation) yielded an apatite fi ssion-track age of ca. 11 Ma. Eight of the nine samples yielded apatite (U-Th)/He ages; these ranged from 60 to 9 Ma, with the youngest ages from the southernmost samples. Inverse thermal history models developed from the data reveal two major stages of cooling for the area, with an initial major cooling event ending at ca. 70 Ma, followed by 50 m.y. of thermal stasis and a second major cooling event beginning at 20 Ma and continuing to the present. The data are consistent with northward-directed tilting and exhumation beginning at 20 Ma, probably as the result of north-south extension in the Mojave Desert on an early strand of the Garlock fault with down-to-the-south offset. A third minor phase of rapid exhumation beginning at ca. 10 Ma is suggested by the data; this may indicate the beginning of left-lateral slip on the Garlock fault.

LITHOSPHERE; v. 5; no. 3; p. 321–327; GSA Data Repository Item 2013181 | Published online 20 April 2013 doi:10.1130/L252.1

INTRODUCTION along the Sierra Nevada escarpment, a late Mio- 2007). In the Sierra Nevada to the north of the cene down-to-the-east and right-lateral normal Tehachapi Mountains, the maximum depth The Tehachapi Mountains trend southwest- fault system (e.g., Unruh et al., 2003), whereas of exhumation decreases over a distance of ward from the southern Sierra Nevada Range the major structure bounding the Tehachapi ~100 km before reaching an average maximum to the San Emigdio Range, which lies adjacent Mountains on the southeastern margin is the depth of 5–12 km for the remainder of the Sierra to the San Andreas fault in central California Garlock fault, a major left-lateral strike-slip Nevada (Evernden and Kistler, 1970; Ague and (see Fig. 1). Both the Tehachapi Mountains and fault (Davis and Burchfi el, 1973). The timing of Brimhall, 1988). Sierra Nevada expose plutonic rocks that were the initiation and the evolution of the Garlock The difference in exhumation between the emplaced as part of the Sierra Nevada batholith fault are not well constrained. In this study, we Tehachapi Mountains and Sierra Nevada is as the Farallon plate was subducted beneath have obtained detailed low-temperature thermo- attributed to the formation of a lateral ramp in western North America during Mesozoic time chronologic data from a north-south transect of the subduction zone beneath the southern Sierra (Hamilton, 1969; Stern et al., 1981; Chen and the southernmost Sierra Nevada as it transitions Nevada in Late Cretaceous time (e.g., Saleeby, Moore, 1982; Saleeby et al., 2008). Although into the Tehachapi Mountains. These data pro- 2003). To the south of this lateral ramp, the sub- the plutonic rocks in the Tehachapi Moun- vide us with new insights into the timing and duction zone fault fl attened and remained shal- tains are genetically related to the batholiths evolution of the southernmost Sierra Nevada low, whereas to the north, it maintained a steeper of the Sierra Nevada complex, they appear to and allow us to propose a model for the initia- trajectory into the mantle. As the result of fl at- have different postemplacement histories, with tion of the Garlock fault. slab subduction in the south, tectonic erosion the Tehachapi Mountains having undergone of the mantle lithosphere occurred (Ducea and a substantially greater amount of exhumation GEOLOGICAL SETTING Saleeby, 1996, 1998), followed by underplating since emplacement (Pickett and Saleeby, 1993; of a subduction zone accretionary assemblage Saleeby et al., 2007). One of the puzzles in try- Rocks now at the surface in the Tehachapi (Jacobson et al., 2007, and references therein). ing to understand the differences in exhuma- Mountains have been exhumed from much In the Tehachapi Mountains, the Rand schist, a tional histories of the Tehachapi Mountains and greater depths than rocks in the rest of the remnant of the accretionary assemblage, occurs the Sierra Nevada is the nature of the bounding Sierra Nevada. Detailed pressure-temperature- in fault contact with the base of the lower- faults: the Sierra Nevada is tilted to the west time (P-T-t) analyses of rocks in the Tehachapi crustal plutonic rocks (Sharry, 1981; Ross, Mountains document pluton emplacement 1989; Wood, 1997). Rapid exhumation of the *Current address: Department of Geology, Univer- between 115 and 99 Ma at depths of 24–35 Rand schist to depths of ~12 km had occurred sity of California, Davis, California 95616, USA. km (Pickett and Saleeby, 1993; Saleeby et al., by 80 Ma (Saleeby et al., 2007). Much of the

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Tehachapi Mountains include north-vergent reverse faults and folds with minor shortening, as well as two sets of Neogene–Quaternary normal faults striking north and northwest (Ross, SierraSierra 1989; Malin et al., 1995; Mahéo et al., 2009). The southeast side the Tehachapi Mountains 36°N NevadaNevada is bounded by the active left-lateral northeast- trending Garlock fault (Davis and Burchﬁ el, 1973), which has accumulated up to 64 km of offset (Smith, 1962). The best estimates for the timing of initiation of strike-slip offset on the RidgecrestRidgecrest Garlock fault come from paleomagnetic studies and correlations of volcanic rocks that suggest

Z it occurred between ca. 17 and 14 Ma (Burbank KCFKCF F F N and Whistler, 1987; Monastero et al., 1997). SNFFZS The timing of surface uplift of the Sierra Nevada is the subject of much debate, with geo- lt F au morphic evidence suggesting that the range has W k F Maricopa WWFW oc reached its highest elevation in the past 10 m.y. sub-basin arl TehachapiTehachapi GarlockG Fault (e.g., Huber, 1981; Unruh, 1991; Wakabayashi MojaveMojave and Sawyer, 2001; Figueroa and Knott, 2010), and other lines of evidence (primarily apatite i ap [U-Th]/He analyses and stable isotope paleoel- 35°N ach eh evation data) suggesting that the range was sub- TehachapiT tns MMtns StudyStudy AreaArea stantially higher during the Late Cretaceous– early Cenozoic than it is now (e.g., House et al., SanSan SanS Andreas Fault 1998, 2001; Mulch et al., 2006; Cassel et al., an A EmigdioEmigdio ndre 2009). Geomorphic studies also suggest that as F 0 20 km MtnsMtns ault there is a greater amount of late Cenozoic surface uplift in the southern part of the Sierra Nevada (e.g., Clark et al., 2005; Figueroa and 119°W 118°W Knott, 2010) than farther to the north. Recent Figure 1. Digital elevation model of the southern Sierra Nevada and Tehachapi Mountains, south- modeling of low-temperature thermochronol- central California, showing major faults (SNFFZ—Sierra Nevada frontal fault zone, KCF—Kern Canyon ogy from the southern Sierra Nevada suggests fault, and WWF—White Wolf fault). The red box indicates the study area, which is shown in Figure 2. that the region was higher during the Late Cre- taceous, lost elevation through the Tertiary, but has undergone ~2 km of surface uplift in the postemplacement exhumation of the eastern on the exact timing are poor (Unruh, 1991; past 20 m.y. (McPhillips and Brandon, 2012). Tehachapi Mountains may have been accommo- Wakabayashi and Sawyer, 2001; Monastero et Previous studies of the Sierra Nevada docu- dated on a Late Cretaceous–Paleocene detach- al., 2002). Faults of the Sierra Nevada escarp- mented cooling and exhumation events predat- ment fault in the western Tehachapi Mountains ment also accommodate right-lateral slip on ing the formation of the Sierra Nevada frontal that appears to be related to the southern Sierra the Eastern California shear zone, a component fault system and the Garlock fault. Apatite detachment system (Wood and Saleeby, 1997; of the North American–Pacifi c plate boundary fi ssion-track (AFT) and zircon fi ssion-track Chapman et al., 2010). with 20%–25% of the total plate motion (e.g., (ZFT) and apatite (U-Th)/He analyses from the The northern and central Sierra Nevada, Bennett et al., 2003). central and northern Sierra Nevada ranged from along with the Central Valley to the west, pres- In the southern Sierra Nevada and Tehachapi 91 to 30 Ma, bracketing batholith emplacement ently behave as a coherent microplate (e.g., Mountains, basement faulting is more compli- and cooling, as well as exhumation associated Argus and Gordon, 1991; Wernicke and Snow, cated, probably as the result of being located with the Laramide orogeny (Dumitru, 1990; 1998; Dixon et al., 2000) bounded on the east above the lateral ramp in the Late Cretaceous House et al., 1998; Cecil et al., 2006). Zircon and by the Sierra Nevada escarpment and on the subduction zone (Saleeby et al., 2009). The apatite (U-Th)/He ages from the southern Sierra west by the San Andreas fault. Westward tilting north-striking Kern Canyon fault bisects the Nevada were similar (ca. 85–31 Ma), with the and exhumation of the Sierra Nevada micro- southern Sierra Nevada and has active down- exception of three younger apatite ages of ca. 15, plate occurred along a series of down-to-the- to-the-east motion, controlling the western tilt 17, and 28 Ma, which were interpreted to have east normal faults of the Sierra Nevada escarp- of the southern Sierra Nevada (Saleeby et al., been reset via thermal blanketing by sediments ment (Bateman and Wahrhaftig, 1966). The 2009; Nadin and Saleeby, 2010). The northern in local basins (Mahéo et al., 2009; Chapman Sierra Nevada escarpment is commonly recog- margin of the Tehachapi Mountains is bounded et al., 2012). The southern Sierra Nevada and nized as the westernmost edge of the Basin and by the northeast-trending White Wolf fault (see Tehachapi fi ssion-track and (U-Th)/He analyses Range Province. The normal faults forming Fig. 1), which has left-lateral and down-to-the- we present here are younger, clearly document- the escarpment appear to have become active west offsets (Davis and Lagoe, 1988; Saleeby ing exhumation resulting from post-Laramide in the past ~10–5 m.y., although constraints et al., 2012). Structures mapped within the tectonic events.

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METHODS 35°6’N

Nine samples were collected for this study from a 10-km-long north-south transect across TK-2 the Tehachapi Mountains (see Fig. 2). Eight of TK-1 A 68.0 the samples were collected very close to the ----- 62.4 71.2 north-south line; the northernmost sample was 48.5 59.6 collected a few kilometers to the west of the line of transect. The elevation of the samples TK-3 increased from north to south: the two southern- 70.8 most samples are from the tops of the highest 71.0 32.8 peaks, Double Mountain and Tehachapi Moun- TK-4 tain, at elevations of ~2450 m. The total eleva- 68.4 tion spanned for the nine samples was ~800 m. 67.9 33.8 Fission-track and (U-Th)/He thermochronology were used to evaluate the low-temperature TK-9 cooling history of the nine samples. These two 54.9 methods, when applied to the minerals zircon TK-7 20.5 35°3’N 56.6 and apatite, can document sample cooling his- 21.4 Sample Location 19.1 tory from ~250°C to 40 °C. For a region with TK-8 TK-7 12.9 a geothermal gradient of ~25–28 °C/km depth Sample # ------56.6 (Brady et al., 2006), this range of temperatures ZFT age 11.1 19.1 AFT age 12.6 is equivalent to depths between ~10 and 1 km, 12.9 A(U-Th)/He age TK-6 assuming a mean annual surface temperature of 46.2 ~10–15 °C. 0 2 km 20.2 TK-5 Fission tracks are linear zones of damage in 10.0 53.9 crystals or glass formed by the spontaneous fi ssion B 18.2 of 238U. The tracks are visible and can be counted 118°33’W 118°30’W 9.0 under an optical microscope if they are etched with acids or bases. Fission tracks are unstable North South at elevated temperatures, and the crystal lattice repairs itself, which is manifested as a shorten- A B ing in length of the fi ssion tracks. This process 2400 is referred to as annealing (e.g., Naeser, 1979). ELEV. 2100 The range of temperatures over which annealing (m) 1800 occurs is dependent on the mineral, and in the 1500 case of the most commonly used mineral, apa- Figure 2. Digital elevation model of the southern Sierra Nevada and Tehachapi Mountains, showing tite, the chemical composition. At cooling rates of sample locations (yellow stars), and zircon fi ssion-track, apatite fi ssion-track, and (U-Th)/He ages, ~10 °C/m.y., newly formed tracks in fl uorine-rich respectively. The elevation profi le for line A-B is shown below the map, with sample locations and apatites (~95% of all apatites) shorten and disap- elevations projected onto the line. pear at temperatures >120 °C, and they remain long at temperatures <60 °C (e.g., Ketcham et al., 1999). Between 120 and 60 °C, the tracks shorten (U-Th)/He thermochronometry is based on in Tables A2 (AFT and ZFT), and A3 (apatite in length at a rate proportional to the temperature; the release of He in the crystal lattice during the [U-Th]/He) in the supplemental materials (GSA this range is referred to as the partial annealing decay of U and Th. As with the retention of fi s- Data Repository).1 The data, which consist of zone (Gleadow and Fitzgerald, 1987). The “age” sion tracks, He accumulation in the crystal lattice seven zircon and nine apatite fi ssion track and of the sample and its distribution of track lengths is dependent on ambient temperature. At average He analyses, show a distinct pattern with respect can be used to reconstruct the thermal history of geologic cooling rates, He is retained in apatites to sample location, generally becoming younger the sample within the partial annealing zone (e.g., at temperatures <40 °C and completely lost at to the south, and younger with increasing eleva- Ketcham, 2005). temperatures >80 °C; this range of temperatures tion (see Fig. 3A). For the mineral zircon, fi ssion tracks will corresponds to the helium partial retention zone The four northernmost samples (TK-1 begin to anneal at temperatures of ~180 °C and (Wolf et al., 1996, 1998; Farley, 2002). A closure through 4) yielded three ZFT and four AFT ages, +7.5 +12.3 will be totally annealed at ~350 °C at average geo- temperature of 70 °C is generally used to inter- ranging from 71.2 ( /–6.8) to 67.9 ( /–10.4) Ma. logic cooling rates (Tagami, 2005, and references pret apatite (U-Th)/He ages (Farley, 2002). therein), but variations in annealing rate occur in different zircons, possibly as the result of dam- FISSION-TRACK AND (U-Th)/He ANALYSES 1GSA Data Repository Item 2013181, data tables with age by alpha particles in older zircons with high zircon and apatite fi ssion-track analyses, (U-Th)/He analyses, and HeFTy model inputs, is available at U concentrations (e.g., Garver, 2002). A generic Table 1 is a summary of all thermochrono- www.geosociety.org/pubs/ft2013.htm, or on request closure temperature of ~250 ± 50 °C is commonly metric ages obtained for the nine study sam- from [email protected], Documents Secretary, used to interpret zircon fi ssion-track ages. ples. Complete analytical data can be found GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA.

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TABLE 1. SUMMARY OF ZIRCON AND APATITE FISSION-TRACK AND APATITE (U-Th)/He ANALYSES FOR THE TEHACHAPI MOUNTAIN TRANSECT Sample no. Lat. (°N) Long. (°W) Elev. (m) ZFT AFT AHe

From north to south: ′ ″ ′ ″ +9.8 TK-1 35°05 37 118°33 05 1646 – 62.4 ( /–8.5) 48.5 (± 0.59) ′ ″ ′ ″ +9.0 +7.5 TK-2 35°04 45 118°29 41 1737 68.0 ( /–8.0) 71.2 ( /–6.8) 59.6 (± 1.77) ′ ″ ′ ″ +11.3 +15.6 TK-3 35°03 58 118°29 11 1890 70.8 ( /–9.8) 71.0 ( /–12.8)32.8 ′ ″ ′ ″ +13.0 +12.3 TK-4 35°03 46 118°28 48 2012 68.4 ( /–10.9) 67.9 ( /–10.4) 33.8 (± 0.24) ′ ″ ′ ″ +12.6 +2.3 TK-9 35°02 49 118°29 00 2438 54.9 ( /–10.3) 20.5 ( /–2.0) 21.4 (± 2.36) ′ ″ ′ ″ +5.4 +3.2 TK-7 35°02 28 118°29 30 2286 56.6 ( /–4.9) 19.1 ( /–2.8) 12.9 (± 1.68) ′ ″ ′ ″ +3.4 TK-8 35°02 23 118°29 08 2225 – 11.1( /–2.6) 12.6 (± 4.26) ′ ″ ′ ″ +5.2 +2.9 TK-6 35°02 03 118°29 11 2438 46.2 ( /–4.7) 20.2 ( /–2.5) 10.0 (± 0.05) ′ ″ ′ ″ +5.9 +2.5 TK-5 35°01 59 118°29 08 2438 53.9 ( /–5.3) 18.2 ( /–2.2) 9.0 (± 1.27)

A. Sample Ages versus N-S Location 100 TK-1 -2 -3,4 -9 -7,8 -6,5 Figure 3. (A) Individual sample ages for all zircon and apatite fi ssion-track and apatite (U-Th)/He 80 analyses are plotted with respect to their loca- Zircon tion along north-south line A-B. HeFTy ther- FT Age mal models are shown for samples: (B) TK-1, 60 (C) TK-2, (D) TK-4, (E) TK-5, (F) TK-5, and (G) TK7. Apatite The black triangles and squares on these plots FT Age are the measured apatite (U-Th)/He and fi ssion- track ages, respectively, for each sample. Table 40 Age (Ma) Apatite A4 (supplementary data [see text footnote 1]) (U-Th)/He lists the model inputs used. The shaded regions Age on the plots represent envelopes of thermal 20 histories with “acceptable” (light-gray) and “good” (medium-gray) statistical fi ts. The black line represents the “best” statistical fi t. See 0 text for further discussion. 0 246 8 10 Distance from North to South (km)

B. TK-1 C. TK-2 D. TK-4 0 0 0

20 20 20

40 40 40

60 60 60

80 80 80

100 100 100 Temperature (°C) 120 120 120

140 140 140 100 90 80 70 60 50 40 30 20 10 0 100 90 80 70 60 50 40 30 20 10 0 100 90 80 70 60 50 40 30 20 10 0 E. TK-5 F. TK-6 G. TK-7 0 0 0

20 20 20

40 40 40

60 60 60

80 80 80

100 100 100 Temperature (°C) 120 120 120

140 140 140 100 90 80 70 60 50 40 30 20 10 0 100 90 80 70 60 50 40 30 20 10 0 100 90 80 70 60 50 40 30 20 10 0 Time (Ma) Time (Ma) Time (Ma)

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All fi ssion-track errors quoted are 95% con- 80 Ma. The data presented here indicate that younger thermochronometric ages, as would be fi dence intervals. The AFT and ZFT ages are this regional cooling event continued until ca. expected in the footwall of an east-west–striking statistically the same, which is consistent with 70 Ma, when temperatures of ~60–40 °C were normal fault. rapid cooling from ~250 °C to 110 °C at ca. 70 reached. We interpret this cooling to have been a North-south extension was previously docu- Ma. In general, the apatite (U-Th)/He ages from continuation of the rapid exhumation associated mented in the Mojave Desert between ca. 24–22 these samples were slightly younger, ranging with the removal of the mantle lithosphere and and 18 Ma (e.g., Glazner et al., 2002, and refer- from 58.9 to 33.5 Ma. subsequent underplating of the Rand schist in ences therein), but it appeared to be relatively The fi ve southern samples (TK-5 through 9) response to Laramide fl at-slab subduction. restricted to an area to the northwest of Barstow, yielded consistently younger ages. Four of the The thermal models indicate that the area California (~100 km east of our study area). fi ve samples yielded ZFT ages, which ranged then remained stable (with no signifi cant Recent work, however, suggests that extension +5.4 +5.2 from 56.6 ( /–4.9) to 46.2 ( /–4.7) Ma. All fi ve of heating or cooling) from ca. 70 to 20 Ma. An may have occurred over a wider area. Early to the samples yielded AFT ages: four of the AFT increase in cooling rate at 20 Ma is indicated by middle Miocene extension and rapid subsid- ages were tightly clustered, with ages ranging both the pattern of AFT ages (see Fig. 3A) and ence have been documented in the Maricopa +2.3 +2.5 from 20.5 ( /–2.0) to 18.2 ( /–2.2) Ma. A single the thermal models for samples TK-5 and TK-7 subbasin on the northern fl ank of the Tehachapi sample, from the lowest elevation of the fi ve (Figs. 3D and 3E). This distinct 20 Ma cooling Mountains (e.g., Goodman and Malin, 1992). southern samples, yielded a younger AFT age of event has not been documented previously in Immediately to the south and north of the fi eld +3.4 11.1 ( /–2.6) Ma. Four of these samples yielded low-temperature thermochronometric data from area (see Fig. 1), Neogene–Quaternary north- apatite (U-Th)/He ages, which ranged from 21.4 the Tehachapi Mountains or Sierra Nevada. The west-striking normal faults were recognized ± 2.36 Ma to 9.0 ± 1.27 Ma; these ages became total amount of cooling at 20 Ma for the south- (Mahéo et al., 2009, and references therein). In progressively younger toward the south. ern part of the region was ~60–70 °C, which a paleoseismic study (McGill et al., 2009), an Six samples yielded AFT and (U-Th)/He corresponds to ~2.1–2.8 km of exhumation for active normal fault was documented within a ages as well as >100 AFT length analyses. For a geothermal gradient of ~25–28 °C/km depth. few hundred meters of the Garlock fault ~35 km these samples, thermal history models were Regional tilting is interpreted to explain the to the northeast of our fi eld area, and inferred to derived using HeFTy (Ketcham, 2005). HeFTy patterns seen in the HeFTy thermal models (for be the range-bounding structure. is an inverse modeling program that uses both example, TK-2 was substantially cooler at 20 Based on our data, we suggest that the nor- AFT and (U-Th)/He analyses to calculate the Ma than TK-4), as well as the apatite (U-Th)/He mal fault responsible for the 20 Ma exhuma- range of possible thermal histories permitted ages, which become progressively younger to tional event in our study lies to the south of our by the data. For individual samples, AFT length the south. The difference in temperature at 20 study area and may be coincident with the active and Dpar (diameter of tracks measured parallel Ma for the northern and southernmost samples Garlock fault in this region, making it diffi cult to crystallographic c-axis on the exposed apa- is ~100 °C (compare Figs. 3B and 3D). If the to recognize. We suggest that north-south exten- tite surface) measurements are entered, as well modern geothermal gradient of ~25–28 °C/km sion occurred on a proto–Garlock fault. We as individual grain track count measurements depth is assumed to have been stable over the speculate that the restricted location of exten- and crystallographic c-axis orientations. The past 20 m.y., the ~100 °C temperature variance sion in this region controlled the location of the user specifi es initial constraints on the thermal is equivalent to ~3.6–4 km of differential exhu- Garlock fault as it transitioned into a through- history (as boxes spanning a range of times mation. This pattern is consistent with tilting of going strike-slip fault. A schematic model for and temperatures), and the program compares a fault block to the north around a horizontal this scenario is shown in Figure 4A. the measured values of sample ages and track east-west axis, with greater erosion occurring Finally, there is weak evidence for a renewed length distributions with those predicted by in the south, thereby exposing sample sites with pulse of cooling at ca. 12–10 Ma. This evidence individual cooling paths for statistical fi t. The time-temperature histories for individual samples are shown in Figures 3B–3G. AB20 Ma: 12 to 10 Ma: INTERPRETATION North-South Extension Initiation of Garlock Fault

The data obtained in this study are consistent with two separate major periods of exhumation- Garlock F Central SAF Garlock F induced cooling, the ﬁ rst at ca. 70 Ma and the Central SAF second beginning at ca. 20 Ma and continuing Tehachapis through the present, as well as an overall tilting Tehachapis of the range to the north. The ca. 70 Ma cooling phase is seen predominantly in the northern MOJAVE MOJAVE part of our ﬁ eld area, and the ca. 20 Ma phase is Elsinore Fault? Proto - Southern SAF

observed in the southern part. The northernmost Proto -SGF samples document nearly 200 °C of cooling at ca. 70 Ma (Figs. 3B and 3C). These data are con- SGF sistent with the results of Saleeby et al. (2007) from the central to western Tehachapi Moun- Figure 4. Proposed tectonic evolution for the southern Sierra Mountains and eastern Tehachapi tains (a few kilometers west of our study area) Mountains, showing (A) north-south extension at 20 Ma, and (B) the initiation of the Garlock fault that indicated substantial cooling from >700 at ca. 12 Ma. SAF—San Andreas fault; SGF—San Gabriel fault. The black lines with hachures are °C beginning at ca. 95 Ma through ~150 °C by proposed normal faults with the northern Mojave faults idealized from Glazner et al. (2002).

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includes the four apatite (U-Th)/He ages from the Argus, D.F., and Gordon, R.G., 1991, Current Sierra Nevada– California and Western Nevada: U.S. Geological Society North America motion from very long baseline interfer- Professional Paper 623, 42 p. southernmost part of the transect, which ranged ometry: Implications for the kinematics of the western Farley, K.A., 2002, (U-Th)/He dating: Techniques, calibrations, from 12.9 to 9.0 Ma, as well as the 11.1 Ma AFT United States: Geology, v. 19, p. 1085–1088, doi:10.1130 and applications: Mineralogical Society of America age from TK-8; the overlap of these fi ve ages /0091-7613(1991)019<1085:CSNNAM>2.3.CO;2. Reviews in Mineralogy and Geochemistry, v. 47, p. 819– Bateman, P.C., and Wahrhaftig, C., 1966, Geology of the Sierra 844, doi:10.2138/rmg.2002.47.18. suggests a more rapid pulse of cooling, although Nevada, in Bailey, E.H., ed., Geology of Northern Cali- Figueroa, A.M., and Knott, J.R., 2010, Tectonic geomorphol- in general, the samples were at temperatures that fornia: California Division of Mines and Geology Bul- ogy of the southern Sierra Nevada Mountains (Cali- letin 190, p. 107–172. fornia): Evidence for uplift and basin formation: Geo- were too low at this time to accurately bracket Bennett, R.A., Wernicke, B.P., Niemi, N.A., Friedrich, A.M., morphology, v. 123, p. 34–45, doi:10.1016/j.geomorph this event. We suggest that isolated strands of and Davis, J.L., 2003, Contemporary strain rates in the .2010.06.009. the Garlock fault, with normal offsets on them, northern Basin and Range Province from GPS data: Tec- Garver, J.I., 2002, Discussion: Metamictization of natural zir- tonics, v. 22, 1008, doi:10.1029/2001TC001355. con: Accumulation versus thermal annealing of radio- linked together and became a through-going Brady, R.J., Ducea, M.N., Kidder, S.B., and Saleeby, J.B., 2006, activity-induced damage: Contributions to Mineralogy strike-slip fault at that time (see Fig. 4B). The distribution of radiogenic heat production as a func- and Petrology, v. 143, p. 756–757. tion of depth in the Sierra Nevada batholith, California: Glazner, A.F., Walker, J.D., Bartley, J.M., and Fletcher, J.M., Lithos, v. 86, p. 229–244, doi:10.1016/j.lithos.2005.06.003. 2002, Cenozoic evolution of the Mojave block of south- CONCLUSIONS Burbank, D.W., and Whistler, D.P., 1987, Temporally con- ern California, in Glazner, A.F., Walker, J.D., and Bartley, strained tectonic rotations derived from magneto- J.M., eds., Geologic Evolution of the Mojave Desert and stratigraphic data: Implications for the initiation of the Southwestern Basin and Range: Geological Society of The low-temperature thermochronometric Garlock fault, California: Geology, v. 15, p. 1172–1175, America Memoir 195, p. 19–41. data presented here, from the eastern Southern doi:10.1130/0091-7613(1987)15<1172:TCTRDF>2.0.CO;2. Gleadow, A.J.W., and Fitzgerald, P.G., 1987, Uplift history Sierra and Tehachapi Mountain transect, are in Cassel, E.J., Graham, S.A., and Chamberlain, C.P., 2009, and structure of the Transantarctic Mountains: New Cenozoic tectonic and topographic evolution of the evidence from fi ssion track dating of basement apa- general signifi cantly younger than ages obtained northern Sierra Nevada, California, through stable iso- tites in the Dry Valleys area, southern Victoria Land: in previous studies from the Tehachapi Moun- tope paleoaltimetry in volcanic glass: Geology, v. 37, Earth and Planetary Science Letters, v. 82, p. 1–14, tains to the west and the Sierra Nevada to the p. 547–550, doi:10.1130/G25572A.1. doi:10.1016/0012-821X(87)90102-6. Cecil, M.L., Ducea, M.N., Reiners, P.W., and Chase, C.G., 2006, Goodman, E.D., and Malin, P.E., 1992, Evolution of the south- north (Mahéo et al., 2009; Saleeby et al., 2007; Cenozoic exhumation of the Sierra Nevada, California, ern San Joaquin Basin and mid-Tertiary transitional Clark et al., 2005). Two very distinct phases from (U-Th)/He thermochronology: Geological Soci- tectonics, central California: Tectonics, v. 11, p. 478–498, ety of America Bulletin, v. 118, no. 11/12, p. 1481–1488, doi:10.1029/91TC02871. of cooling are evident in the data. The earli- doi:10.1130/B25876.1. Hamilton, W.B., 1969, California and the underfl ow of Pacifi c est cooling phase, at ca. 70 Ma, documented Chapman, A.D., Kidder, S., Saleeby, J.B., and Ducea, M.N., mantle: Geological Society of America Bulletin, v. 80, by the northern four samples, appears to be a 2010, Role of extrusion of the Rand and Sierra de Sali- p. 2409–2430, doi:10.1130/0016-7606(1969)80[2409:MCA nas schists in Late Cretaceous extension and rotation TUO]2.0.CO;2. continuation of the major exhumation and cool- of the southern Sierra Nevada and vicinity: Tectonics, House, M.A., Wernicke, B.P., and Farley, K.A., 1998, Dating topo- ing event associated with emplacement of the v. 29, TC5006, doi:10.1029/2009TC002597. graphic uplift of the Sierra Nevada, California, using apa- Sierra Nevada batholith and fl attening of the Chapman, A.D., Saleeby, J.B., Wood, D.J., Piasecki, A., Farley, tite (U-Th)/He ages: Nature, v. 396, p. 66–69, doi:10.1038 K.A., Kidder, S., and Ducea, M.N., 2012, Late Cretaceous /23926. subduction zone beneath the southern Sierra gravitational collapse of the southern Sierra Nevada House, M.A., Wernicke, B.P. and Farley, K.A., 2001, Paleo- and Tehachapi Mountains. The second phase of batholith, California: Geosphere, v. 8, p. 314–341, geomorphology of the Sierra Nevada, California, from doi:10.1130/GES00740.1. (U-Th)/He ages in apatite: American Journal of Science, cooling, at ca. 20 Ma, is robustly documented by Chen, J.H., and Moore, J.G., 1982, Uranium-lead isotopic v. 301, p. 77–102, doi:10.2475/ajs.301.2.77. the southernmost samples from the study area, ages from the Sierra Nevada batholith: Journal of Huber, N.K., 1981, Amount and Timing of Late Cenozoic Uplift and is attributed to >2 km of exhumation associ- Geophysical Research, v. 87, p. 4761–4784, doi:10.1029/ and Tilt of the Central Sierra Nevada, California—Evi- JB087iB06p04761. dence from the Upper San Joaquin River Basin: U.S. ated with extension along an east-west normal Clark, M.K., Maheo, G., Saleeby, J., and Farley, K., 2005, Geological Survey Professional Paper 1197, 28 p. fault along the southern boundary of the sample The non-equilibrium landscape of the southern Sierra Jacobson, C.E., Grove, M., Vucic, A., Pedrick, J.N., and Ebert, area. The data also appear to suggest a third, Nevada, California: GSA Today, v. 15, no. 9, doi:10:1130 K.A., 2007, Exhumation of the Orocopia schist and asso- /1052–5173. ciated rocks of southeastern California; relative roles less-distinct cooling event at 10 Ma, which may Davis, G.A., and Burchfi el, B.C., 1973, Garlock fault: An intra- of erosion, synsubduction tectonic denudation, and represent the time at which the Garlock fault continental transform structure, southern California: middle Cretaceous extension, in Cloos, M., et al., eds., Geological Society of America Bulletin, v. 84, no. 4, Convergent Margin Terranes and Associated Regions: transitioned to being a through-going left-lateral p. 1407–1422, doi:10.1130/0016-7606(1973)84<1407: A Tribute to W.G. Ernst: Geological Society of America strike-slip fault. GFAITS>2.0.CO;2. Special Paper 419, p. 1–37. Davis, T.L., and Lagoe, M.B., 1988, A structural interpreta- Ketcham, R.A., 2005. Forward and inverse modeling of low- tion of major tectonic events affecting the western and temperature thermochronometry data, in Reiners, P., ACKNOWLEDGMENTS southern margins of the San Joaquin Valley, California, and Ehlers, T., eds., Low-Temperature Thermochronol- Samples used in this study were collected by Sopurkh in Graham, S.A., ed., Studies of the Geology of the San ogy: Reviews in Mineralogy and Geochemistry, v. 58, Khalsa, as part of his senior research project at the Univer- Joaquin Basin: Society of Economic Paleontologists p. 275–314. sity of Southern California, and A. Blythe. Mineral separates and Mineralogists, Pacifi c Section, v. 60, p. 65–87. Ketcham, R.A., Donelick, R.A., and Carlson, W.D., 1999, Vari- were processed by Apatite to Zircon, Inc. The fi ssion-track Dixon, T.H., Miller, M., Farina, F., Wang, H., and Johnson, D., ability of apatite fi ssion-track annealing kinetics: III. microscope and stage uses the software program FTStage 2000, Present-day motion of the Sierra Nevada block Extrapolation to geologic time scales: The American to aid in analyses (Dumitru, 1993). The (U-Th)/He analyses and some tectonic implications for the Basin and Range Mineralogist, v. 84, p. 1235–1255. were obtained by N. Longinotti during her participation in the Province, North American Cordillera: Tectonics, v. 19, Mahéo, G., Saleeby, J., Saleeby, Z., and Farley, K.A., 2009, National Science Foundation–sponsored HeDWaAZ course p. 1–24, doi:10.1029/1998TC001088. Tectonic controls on southern Sierra Nevada topog- run by P. Reiners at the University of Arizona, with the assis- Ducea, M.N., and Saleeby, J.B., 1996, Buoyancy sources for raphy, California: Tectonics, v. 28, TC6006, doi:10.1029 tance of S. Thompson. N. 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