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LITHOSPHERE | Volume 5 | Number 3 | www.gsapubs.org 321

INTRODUCTION

The Tehachapi Mountains trend southwest-ward from the southern Sierra Nevada Range to the San Emigdio Range, which lies adjacent to the San Andreas fault in central California (see Fig. 1). Both the Tehachapi Mountains and Sierra Nevada expose plutonic rocks that were emplaced as part of the Sierra Nevada batholith as the Farallon plate was subducted beneath western North America during Mesozoic time (Hamilton, 1969; Stern et al., 1981; Chen and Moore, 1982; Saleeby et al., 2008). Although the plutonic rocks in the Tehachapi Moun-tains are genetically related to the batholiths of the Sierra Nevada complex, they appear to have different postemplacement histories, with the Tehachapi Mountains having undergone a substantially greater amount of exhumation since emplacement (Pickett and Saleeby, 1993; Saleeby et al., 2007). One of the puzzles in try-ing to understand the differences in exhuma-tional histories of the Tehachapi Mountains and the Sierra Nevada is the nature of the bounding faults: the Sierra Nevada is tilted to the west

along the Sierra Nevada escarpment, a late Mio-cene down-to-the-east and right-lateral normal fault system (e.g., Unruh et al., 2003), whereas the major structure bounding the Tehachapi Mountains on the southeastern margin is the Garlock fault, a major left-lateral strike-slip fault (Davis and Burchfi el, 1973). The timing of the initiation and the evolution of the Garlock fault are not well constrained. In this study, we have obtained detailed low-temperature thermo-chronologic data from a north-south transect of the southernmost Sierra Nevada as it transitions into the Tehachapi Mountains. These data pro-vide us with new insights into the timing and evolution of the southernmost Sierra Nevada and allow us to propose a model for the initia-tion of the Garlock fault.

GEOLOGICAL SETTING

Rocks now at the surface in the Tehachapi Mountains have been exhumed from much greater depths than rocks in the rest of the Sierra Nevada. Detailed pressure-temperature-time (P-T-t) analyses of rocks in the Tehachapi Mountains document pluton emplacement between 115 and 99 Ma at depths of 24–35 km (Pickett and Saleeby, 1993; Saleeby et al.,

2007). In the Sierra Nevada to the north of the Tehachapi Mountains, the maximum depth of exhumation decreases over a distance of ~100 km before reaching an average maximum depth of 5–12 km for the remainder of the Sierra Nevada (Evernden and Kistler, 1970; Ague and Brimhall, 1988).

The difference in exhumation between the Tehachapi Mountains and Sierra Nevada is attributed to the formation of a lateral ramp in the subduction zone beneath the southern Sierra Nevada in Late Cretaceous time (e.g., Saleeby, 2003). To the south of this lateral ramp, the sub-duction zone fault fl attened and remained shal-low, whereas to the north, it maintained a steeper trajectory into the mantle. As the result of fl at-slab subduction in the south, tectonic erosion of the mantle lithosphere occurred (Ducea and Saleeby, 1996, 1998), followed by underplating of a subduction zone accretionary assemblage (Jacobson et al., 2007, and references therein). In the Tehachapi Mountains, the Rand schist, a remnant of the accretionary assemblage, occurs in fault contact with the base of the lower-crustal plutonic rocks (Sharry, 1981; Ross, 1989; Wood, 1997). Rapid exhumation of the Rand schist to depths of ~12 km had occurred by 80 Ma (Saleeby et al., 2007). Much of the

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

*Current address: Department of Geology, Univer-sity of California, Davis, California 95616, USA.

For permission to copy, contact [email protected] | © 2013 Geological Society of America

BLYTHE AND LONGINOTTI

322 www.gsapubs.org | Volume 5 | Number 3 | LITHOSPHERE

postemplacement exhumation of the eastern Tehachapi Mountains may have been accommo-dated on a Late Cretaceous–Paleocene detach-ment fault in the western Tehachapi Mountains that appears to be related to the southern Sierra detachment system (Wood and Saleeby, 1997; Chapman et al., 2010).

The northern and central Sierra Nevada, along with the Central Valley to the west, pres-ently behave as a coherent microplate (e.g., Argus and Gordon, 1991; Wernicke and Snow, 1998; Dixon et al., 2000) bounded on the east by the Sierra Nevada escarpment and on the west by the San Andreas fault. Westward tilting and exhumation of the Sierra Nevada micro-plate occurred along a series of down-to-the-east normal faults of the Sierra Nevada escarp-ment (Bateman and Wahrhaftig, 1966). The Sierra Nevada escarpment is commonly recog-nized as the westernmost edge of the Basin and Range Province. The normal faults forming the escarpment appear to have become active in the past ~10–5 m.y., although constraints

on the exact timing are poor (Unruh, 1991; Wakabayashi and Sawyer, 2001; Monastero et al., 2002). Faults of the Sierra Nevada escarp-ment also accommodate right-lateral slip on the Eastern California shear zone, a component of the North American–Pacifi c plate boundary with 20%–25% of the total plate motion (e.g., Bennett et al., 2003).

In the southern Sierra Nevada and Tehachapi Mountains, basement faulting is more compli-cated, probably as the result of being located above the lateral ramp in the Late Cretaceous subduction zone (Saleeby et al., 2009). The north-striking Kern Canyon fault bisects the southern Sierra Nevada and has active down-to-the-east motion, controlling the western tilt of the southern Sierra Nevada (Saleeby et al., 2009; Nadin and Saleeby, 2010). The northern margin of the Tehachapi Mountains is bounded by the northeast-trending White Wolf fault (see Fig. 1), which has left-lateral and down-to-the-west offsets (Davis and Lagoe, 1988; Saleeby et al., 2012). Structures mapped within the

Tehachapi Mountains include north-vergent reverse faults and folds with minor shortening, as well as two sets of Neogene–Quaternary nor-mal faults striking north and northwest (Ross, 1989; Malin et al., 1995; Mahéo et al., 2009). The southeast side the Tehachapi Mountains 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 Garlock fault come from paleomagnetic studies and correlations of volcanic rocks that suggest it occurred between ca. 17 and 14 Ma (Burbank and Whistler, 1987; Monastero et al., 1997).

The timing of surface uplift of the Sierra Nevada is the subject of much debate, with geo-morphic evidence suggesting that the range has reached its highest elevation in the past 10 m.y. (e.g., Huber, 1981; Unruh, 1991; Wakabayashi and Sawyer, 2001; Figueroa and Knott, 2010), and other lines of evidence (primarily apatite [U-Th]/He analyses and stable isotope paleoel-evation data) suggesting that the range was sub-stantially higher during the Late Cretaceous–early Cenozoic than it is now (e.g., House et al., 1998, 2001; Mulch et al., 2006; Cassel et al., 2009). Geomorphic studies also suggest that there is a greater amount of late Cenozoic sur-face uplift in the southern part of the Sierra Nevada (e.g., Clark et al., 2005; Figueroa and Knott, 2010) than farther to the north. Recent modeling of low-temperature thermochronol-ogy from the southern Sierra Nevada suggests 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 past 20 m.y. (McPhillips and Brandon, 2012).

Previous studies of the Sierra Nevada docu-mented cooling and exhumation events predat-ing the formation of the Sierra Nevada frontal fault system and the Garlock fault. Apatite fi ssion-track (AFT) and zircon fi ssion-track (ZFT) and apatite (U-Th)/He analyses from the central and northern Sierra Nevada ranged from 91 to 30 Ma, bracketing batholith emplacement and cooling, as well as exhumation associated with the Laramide orogeny (Dumitru, 1990; House et al., 1998; Cecil et al., 2006). Zircon and apatite (U-Th)/He ages from the southern Sierra Nevada were similar (ca. 85–31 Ma), with the exception of three younger apatite ages of ca. 15, 17, and 28 Ma, which were interpreted to have been reset via thermal blanketing by sediments in local basins (Mahéo et al., 2009; Chapman et al., 2012). The southern Sierra Nevada and Tehachapi fi ssion-track and (U-Th)/He analyses we present here are younger, clearly document-ing exhumation resulting from post-Laramide tectonic events.

Sierra Sierra NevadaNevada

Study AreaStudy Area

SNFF

Z

SNFF

Z

Garlock Fault

Garlock Fault

San Andreas Fault

San Andreas Fault

WW

FW

WF

Tehachapi

Tehachapi

Mtns

Mtns

San San EmigdioEmigdio

MtnsMtns

KCFKCF

TehachapiTehachapi

MojaveMojave

RidgecrestRidgecrest

Study Area

SNFF

Z

Garlock Fault

San Andreas Fault

WW

F

119°W 118°W

36°N

35°N

Tehachapi

Mtns

Sierra Nevada

0 20 km

Tehachapi

Mojave

Ridgecrest

San Emigdio

Mtns

Maricopasub-basin

KCF

Figure 1. Digital elevation model of the southern Sierra Nevada and Tehachapi Mountains, south-

central California, showing major faults (SNFFZ—Sierra Nevada frontal fault zone, KCF—Kern Canyon

fault, and WWF—White Wolf fault). The red box indicates the study area, which is shown in Figure 2.


Southern Sierra Nevada–Tehachapi Mountains exhumation | RESEARCH

METHODS

Nine samples were collected for this study from a 10-km-long north-south transect across the Tehachapi Mountains (see Fig. 2). Eight of the samples were collected very close to the north-south line; the northernmost sample was collected a few kilometers to the west of the line of transect. The elevation of the samples increased from north to south: the two southern-most samples are from the tops of the highest peaks, Double Mountain and Tehachapi Moun-tain, at elevations of ~2450 m. The total eleva-tion spanned for the nine samples was ~800 m.

Fission-track and (U-Th)/He thermochronol-ogy were used to evaluate the low-temperature cooling history of the nine samples. These two methods, when applied to the minerals zircon and apatite, can document sample cooling his-tory from ~250°C to 40 °C. For a region with a geothermal gradient of ~25–28 °C/km depth (Brady et al., 2006), this range of temperatures is equivalent to depths between ~10 and 1 km, assuming a mean annual surface temperature of ~10–15 °C.

Fission tracks are linear zones of damage in crystals or glass formed by the spontaneous fi ssion of 238U. The tracks are visible and can be counted under an optical microscope if they are etched with acids or bases. Fission tracks are unstable at elevated temperatures, and the crystal lattice repairs itself, which is manifested as a shorten-ing in length of the fi ssion tracks. This process is referred to as annealing (e.g., Naeser, 1979). The range of temperatures over which annealing occurs is dependent on the mineral, and in the case of the most commonly used mineral, apa-tite, the chemical composition. At cooling rates of ~10 °C/m.y., newly formed tracks in fl uorine-rich apatites (~95% of all apatites) shorten and disap-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 in length at a rate proportional to the temperature; this range is referred to as the partial annealing zone (Gleadow and Fitzgerald, 1987). The “age” of the sample and its distribution of track lengths can be used to reconstruct the thermal history of the sample within the partial annealing zone (e.g., Ketcham, 2005).

For the mineral zircon, fi ssion tracks will begin to anneal at temperatures of ~180 °C and will be totally annealed at ~350 °C at average geo-logic cooling rates (Tagami, 2005, and references therein), but variations in annealing rate occur in different zircons, possibly as the result of dam-age by alpha particles in older zircons with high U concentrations (e.g., Garver, 2002). A generic closure temperature of ~250 ± 50 °C is commonly used to interpret zircon fi ssion-track ages.

(U-Th)/He thermochronometry is based on the release of He in the crystal lattice during the decay of U and Th. As with the retention of fi s-sion tracks, He accumulation in the crystal lattice is dependent on ambient temperature. At average geologic cooling rates, He is retained in apatites at temperatures <40 °C and completely lost at temperatures >80 °C; this range of temperatures corresponds to the helium partial retention zone (Wolf et al., 1996, 1998; Farley, 2002). A closure temperature of 70 °C is generally used to inter-pret apatite (U-Th)/He ages (Farley, 2002).

FISSION-TRACK AND (U-Th)/He ANALYSES

Table 1 is a summary of all thermochrono-metric ages obtained for the nine study sam-ples. Complete analytical data can be found

in Tables A2 (AFT and ZFT), and A3 (apatite [U-Th]/He) in the supplemental materials (GSA Data Repository).1 The data, which consist of seven zircon and nine apatite fi ssion track and He analyses, show a distinct pattern with respect to sample location, generally becoming younger to the south, and younger with increasing eleva-tion (see Fig. 3A).

The four northernmost samples (TK-1 through 4) yielded three ZFT and four AFT ages, ranging from 71.2 (+7.5/

–6.8) to 67.9 (+12.3/

–10.4) Ma.

TK-1-----62.448.5

TK-268.071.259.6

TK-370.871.032.8 TK-4

68.467.933.8

TK-954.920.521.4

TK-756.619.112.9 TK-8

------11.112.6

35°6’N

35°3’N

118°33’W 118°30’W

TK-646.220.210.0

TK-553.918.2 9.0

North South

2400210018001500

ELEV. (m)

A

B

BA

0 2 km

TK-756.619.112.9

Sample #ZFT ageAFT ageA(U-Th)/He age

Sample Location

Figure 2. Digital elevation model of the southern Sierra Nevada and Tehachapi Mountains, showing

sample locations (yellow stars), and zircon fi ssion-track, apatite fi ssion-track, and (U-Th)/He ages,

respectively. The elevation profi le for line A-B is shown below the map, with sample locations and

elevations projected onto the line.

1GSA Data Repository Item 2013181, data tables with zircon and apatite fi ssion-track analyses, (U-Th)/He analyses, and HeFTy model inputs, is available at www.geosociety.org/pubs/ft2013.htm, or on request from [email protected], Documents Secretary,

GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA.



Te

mp

era

ture

(°C

)

C. TK-2

0

20

40

60

80

100

2 4 8

Ag

e (

Ma

)

Distance from North to South (km)10

-2 -9

D. TK-4

90 80 70 60 50 40 30 20 10 0

140

120

100

80

60

40

20

0

100

G. TK-7

Time (Ma)

90 80 70 60 50 40 30 20 10 0

140

120

100

80

60

40

20

0

100

TK-1 -3,4 -6,5-7,8

0 6

E. TK-5

Time (Ma)

90 80 70 60 50 40 30 20 10 0

140

120

100

80

60

40

20

0

100

ZirconFT Age

ApatiteFT Age

Apatite(U-Th)/He

Age

Te

mp

era

ture

(°C

)

A. Sample Ages versus N-S Location

90 80 70 60 50 40 30 20 10 0

140

120

100

80

60

40

20

0

100

B. TK-1

90 80 70 60 50 40 30 20 10 0

140

120

100

80

60

40

20

0

100

F. TK-6

Time (Ma)

90 80 70 60 50 40 30 20 10 0

140

120

100

80

60

40

20

0

100

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:

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

Figure 3. (A) Individual sample ages for all zircon

and apatite fi ssion-track and apatite (U-Th)/He

analyses are plotted with respect to their loca-

tion along north-south line A-B. HeFTy ther-

mal models are shown for samples: (B) TK-1,

(C) TK-2, (D) TK-4, (E) TK-5, (F) TK-5, and (G) TK7.

The black triangles and squares on these plots

are the measured apatite (U-Th)/He and fi ssion-

track ages, respectively, for each sample. Table

A4 (supplementary data [see text footnote 1])

lists the model inputs used. The shaded regions

on the plots represent envelopes of thermal

histories with “acceptable” (light-gray) and

“good” (medium-gray) statistical fi ts. The black

line represents the “best” statistical fi t. See

text for further discussion.



All fi ssion-track errors quoted are 95% con-fi dence intervals. The AFT and ZFT ages are statistically the same, which is consistent with rapid cooling from ~250 °C to 110 °C at ca. 70 Ma. In general, the apatite (U-Th)/He ages from these samples were slightly younger, ranging from 58.9 to 33.5 Ma.

The fi ve southern samples (TK-5 through 9) yielded consistently younger ages. Four of the fi ve samples yielded ZFT ages, which ranged from 56.6 (+5.4/

–4.9) to 46.2 (+5.2/

–4.7) Ma. All fi ve of

the samples yielded AFT ages: four of the AFT ages were tightly clustered, with ages ranging from 20.5 (+2.3/

–2.0) to 18.2 (+2.5/

–2.2) Ma. A single

sample, from the lowest elevation of the fi ve southern samples, yielded a younger AFT age of 11.1 (+3.4/

–2.6) Ma. Four of these samples yielded

apatite (U-Th)/He ages, which ranged from 21.4 ± 2.36 Ma to 9.0 ± 1.27 Ma; these ages became progressively younger toward the south.

Six samples yielded AFT and (U-Th)/He ages as well as >100 AFT length analyses. For these samples, thermal history models were derived using HeFTy (Ketcham, 2005). HeFTy is an inverse modeling program that uses both AFT and (U-Th)/He analyses to calculate the range of possible thermal histories permitted by the data. For individual samples, AFT length and Dpar (diameter of tracks measured parallel to crystallographic c-axis on the exposed apa-tite surface) measurements are entered, as well as individual grain track count measurements and crystallographic c-axis orientations. The user specifi es initial constraints on the thermal history (as boxes spanning a range of times and temperatures), and the program compares the measured values of sample ages and track length distributions with those predicted by individual cooling paths for statistical fi t. The time-temperature histories for individual sam-ples are shown in Figures 3B–3G.

INTERPRETATION

The data obtained in this study are consistent with two separate major periods of exhumation-induced cooling, the fi rst at ca. 70 Ma and the second beginning at ca. 20 Ma and continuing through the present, as well as an overall tilting of the range to the north. The ca. 70 Ma cool-ing phase is seen predominantly in the northern part of our fi eld area, and the ca. 20 Ma phase is observed in the southern part. The northernmost samples document nearly 200 °C of cooling at ca. 70 Ma (Figs. 3B and 3C). These data are con-sistent with the results of Saleeby et al. (2007) from the central to western Tehachapi Moun-tains (a few kilometers west of our study area) that indicated substantial cooling from >700 °C beginning at ca. 95 Ma through ~150 °C by

80 Ma. The data presented here indicate that this regional cooling event continued until ca. 70 Ma, when temperatures of ~60–40 °C were reached. We interpret this cooling to have been a continuation of the rapid exhumation associated with the removal of the mantle lithosphere and subsequent underplating of the Rand schist in response to Laramide fl at-slab subduction.

The thermal models indicate that the area then remained stable (with no signifi cant heating or cooling) from ca. 70 to 20 Ma. An increase in cooling rate at 20 Ma is indicated by both the pattern of AFT ages (see Fig. 3A) and the thermal models for samples TK-5 and TK-7 (Figs. 3D and 3E). This distinct 20 Ma cooling event has not been documented previously in low-temperature thermochronometric data from the Tehachapi Mountains or Sierra Nevada. The total amount of cooling at 20 Ma for the south-ern part of the region was ~60–70 °C, which corresponds to ~2.1–2.8 km of exhumation for a geothermal gradient of ~25–28 °C/km depth.

Regional tilting is interpreted to explain the patterns seen in the HeFTy thermal models (for example, TK-2 was substantially cooler at 20 Ma than TK-4), as well as the apatite (U-Th)/He ages, which become progressively younger to the south. The difference in temperature at 20 Ma for the northern and southernmost samples is ~100 °C (compare Figs. 3B and 3D). If the modern geothermal gradient of ~25–28 °C/km depth is assumed to have been stable over the past 20 m.y., the ~100 °C temperature variance is equivalent to ~3.6–4 km of differential exhu-mation. This pattern is consistent with tilting of a fault block to the north around a horizontal east-west axis, with greater erosion occurring in the south, thereby exposing sample sites with

younger thermochronometric ages, as would be expected in the footwall of an east-west–striking normal fault.

North-south extension was previously docu-mented in the Mojave Desert between ca. 24–22 and 18 Ma (e.g., Glazner et al., 2002, and refer-ences therein), but it appeared to be relatively restricted to an area to the northwest of Barstow, California (~100 km east of our study area). Recent work, however, suggests that extension may have occurred over a wider area. Early to middle Miocene extension and rapid subsid-ence have been documented in the Maricopa subbasin on the northern fl ank of the Tehachapi Mountains (e.g., Goodman and Malin, 1992). Immediately to the south and north of the fi eld area (see Fig. 1), Neogene–Quaternary north-west-striking normal faults were recognized (Mahéo et al., 2009, and references therein). In a paleoseismic study (McGill et al., 2009), an active normal fault was documented within a few hundred meters of the Garlock fault ~35 km to the northeast of our fi eld area, and inferred to be the range-bounding structure.

Based on our data, we suggest that the nor-mal fault responsible for the 20 Ma exhuma-tional event in our study lies to the south of our study area and may be coincident with the active Garlock fault in this region, making it diffi cult to recognize. We suggest that north-south exten-sion occurred on a proto–Garlock fault. We speculate that the restricted location of exten-sion in this region controlled the location of the Garlock fault as it transitioned into a through-going strike-slip fault. A schematic model for this scenario is shown in Figure 4A.

Finally, there is weak evidence for a renewed pulse of cooling at ca. 12–10 Ma. This evidence

20 Ma: North-South Extension

Central SAF

Elsinore F

ault?

12 to 10 Ma: Initiation of Garlock Fault

SGF

Garlock F

Central SAF

Proto - Southern SAF

Garlock F

Proto -SGF

MOJAVE MOJAVE

Tehachapis

Tehachapis

A B

Figure 4. Proposed tectonic evolution for the southern Sierra Mountains and eastern Tehachapi

Mountains, showing (A) north-south extension at 20 Ma, and (B) the initiation of the Garlock fault

at ca. 12 Ma. SAF—San Andreas fault; SGF—San Gabriel fault. The black lines with hachures are

proposed normal faults with the northern Mojave faults idealized from Glazner et al. (2002).



includes the four apatite (U-Th)/He ages from the southernmost part of the transect, which ranged from 12.9 to 9.0 Ma, as well as the 11.1 Ma AFT age from TK-8; the overlap of these fi ve ages suggests a more rapid pulse of cooling, although in general, the samples were at temperatures that were too low at this time to accurately bracket this event. We suggest that isolated strands of the Garlock fault, with normal offsets on them, linked together and became a through-going strike-slip fault at that time (see Fig. 4B).

CONCLUSIONS

The low-temperature thermochronometric data presented here, from the eastern Southern Sierra and Tehachapi Mountain transect, are in general signifi cantly younger than ages obtained in previous studies from the Tehachapi Moun-tains to the west and the Sierra Nevada to the north (Mahéo et al., 2009; Saleeby et al., 2007; Clark et al., 2005). Two very distinct phases of cooling are evident in the data. The earli-est cooling phase, at ca. 70 Ma, documented by the northern four samples, appears to be a continuation of the major exhumation and cool-ing event associated with emplacement of the Sierra Nevada batholith and fl attening of the subduction zone beneath the southern Sierra and Tehachapi Mountains. The second phase of cooling, at ca. 20 Ma, is robustly documented by the southernmost samples from the study area, and is attributed to >2 km of exhumation associ-ated with extension along an east-west normal fault along the southern boundary of the sample area. The data also appear to suggest a third, less-distinct cooling event at 10 Ma, which may represent the time at which the Garlock fault transitioned to being a through-going left-lateral strike-slip fault.

ACKNOWLEDGMENTS

Samples used in this study were collected by Sopurkh Khalsa, as part of his senior research project at the Univer-sity of Southern California, and A. Blythe. Mineral separates were processed by Apatite to Zircon, Inc. The fi ssion-track microscope and stage uses the software program FTStage to aid in analyses (Dumitru, 1993). The (U-Th)/He analyses were obtained by N. Longinotti during her participation in the National Science Foundation–sponsored HeDWaAZ course run by P. Reiners at the University of Arizona, with the assis-tance of S. Thompson. N. Longinotti was partially supported by a summer research grant from Occidental College. J. Wros assisted us with fi gures, and M. Rusmore helped with a fi nal read-through of the manuscript. Reviews by J. Saleeby and D. Walker substantially improved this manuscript and are very much appreciated. We thank E. Kirby for his assistance with the manuscript through the review process.

REFERENCES CITED

Ague, J.J., and Brimhall, G.H., 1988, Magmatic arc asym-metry and distribution of anomalous plutonic belts in the batholiths of California: Effects of assimilation, crustal thickness, and depth of crystallization: Geologi-cal Society of America Bulletin, v. 100, p. 912–927, doi:10.1130/0016-7606(1988)100<0912:MAAADO>2.3.CO;2.

Argus, D.F., and Gordon, R.G., 1991, Current Sierra Nevada–North America motion from very long baseline interfer-ometry: Implications for the kinematics of the western United States: Geology, v. 19, p. 1085–1088, doi:10.1130/0091-7613(1991)019<1085:CSNNAM>2.3.CO;2.

Bateman, P.C., and Wahrhaftig, C., 1966, Geology of the Sierra Nevada, in Bailey, E.H., ed., Geology of Northern Cali-fornia: California Division of Mines and Geology Bul-letin 190, p. 107–172.

Bennett, R.A., Wernicke, B.P., Niemi, N.A., Friedrich, A.M., and Davis, J.L., 2003, Contemporary strain rates in the northern Basin and Range Province from GPS data: Tec-tonics, v. 22, 1008, doi:10.1029/2001TC001355.

Brady, R.J., Ducea, M.N., Kidder, S.B., and Saleeby, J.B., 2006, The distribution of radiogenic heat production as a func-tion of depth in the Sierra Nevada batholith, California: Lithos, v. 86, p. 229–244, doi:10.1016/j.lithos.2005.06.003.

Burbank, D.W., and Whistler, D.P., 1987, Temporally con-strained tectonic rotations derived from magneto-stratigraphic data: Implications for the initiation of the Garlock fault, California: Geology, v. 15, p. 1172–1175, doi:10.1130/0091-7613(1987)15<1172:TCTRDF>2.0.CO;2.

Cassel, E.J., Graham, S.A., and Chamberlain, C.P., 2009, Cenozoic tectonic and topographic evolution of the northern Sierra Nevada, California, through stable iso-tope paleoaltimetry in volcanic glass: Geology, v. 37, p. 547–550, doi:10.1130/G25572A.1.

Cecil, M.L., Ducea, M.N., Reiners, P.W., and Chase, C.G., 2006, Cenozoic exhumation of the Sierra Nevada, California, from (U-Th)/He thermochronology: Geological Soci-ety of America Bulletin, v. 118, no. 11/12, p. 1481–1488, doi:10.1130/B25876.1.

Chapman, A.D., Kidder, S., Saleeby, J.B., and Ducea, M.N., 2010, Role of extrusion of the Rand and Sierra de Sali-nas schists in Late Cretaceous extension and rotation of the southern Sierra Nevada and vicinity: Tectonics, v. 29, TC5006, doi:10.1029/2009TC002597.

Chapman, A.D., Saleeby, J.B., Wood, D.J., Piasecki, A., Farley, K.A., Kidder, S., and Ducea, M.N., 2012, Late Cretaceous gravitational collapse of the southern Sierra Nevada batholith, California: Geosphere, v. 8, p. 314–341, doi:10.1130/GES00740.1.

Chen, J.H., and Moore, J.G., 1982, Uranium-lead isotopic ages from the Sierra Nevada batholith: Journal of Geophysical Research, v. 87, p. 4761–4784, doi:10.1029/JB087iB06p04761.

Clark, M.K., Maheo, G., Saleeby, J., and Farley, K., 2005, The non-equilibrium landscape of the southern Sierra Nevada, California: GSA Today, v. 15, no. 9, doi:10:1130/1052–5173.

Davis, G.A., and Burchfi el, B.C., 1973, Garlock fault: An intra-continental transform structure, southern California: Geological Society of America Bulletin, v. 84, no. 4, p. 1407–1422, doi:10.1130/0016-7606(1973)84<1407:GFAITS>2.0.CO;2.

Davis, T.L., and Lagoe, M.B., 1988, A structural interpreta-tion of major tectonic events affecting the western and southern margins of the San Joaquin Valley, California, in Graham, S.A., ed., Studies of the Geology of the San Joaquin Basin: Society of Economic Paleontologists and Mineralogists, Pacifi c Section, v. 60, p. 65–87.

Dixon, T.H., Miller, M., Farina, F., Wang, H., and Johnson, D., 2000, Present-day motion of the Sierra Nevada block and some tectonic implications for the Basin and Range Province, North American Cordillera: Tectonics, v. 19, p. 1–24, doi:10.1029/1998TC001088.

Ducea, M.N., and Saleeby, J.B., 1996, Buoyancy sources for a large, unrooted mountain range, the Sierra Nevada, California: Evidence from xenolith thermobarometry: Journal of Geophysical Research, v. 101, p. 8229–8244, doi:10.1029/95JB03452.

Ducea, M.N., and Saleeby, J.B., 1998, The age and origin of a thick mafi c-ultramafi c keel from beneath the Sierra Nevada batholith: Contributions to Mineralogy and Petrology, v. 133, p. 169–185, doi:10.1007/s004100050445.

Dumitru, T.A., 1990, Subnormal Cenozoic geothermal gradi-ents in the extinct Sierra Nevada magmatic arc: Conse-quences of Laramide and post-Laramide shallow-angle subduction: Journal of Geophysical Research, v. 95, no. B4, p. 4925–4941, doi:10.1029/JB095iB04p04925.

Evernden, J.F., and Kistler, R.W., 1970, Chronology of Emplacement of Mesozoic Batholithic Complexes in

California and Western Nevada: U.S. Geological Society Professional Paper 623, 42 p.

Farley, K.A., 2002, (U-Th)/He dating: Techniques, calibrations, and applications: Mineralogical Society of America Reviews in Mineralogy and Geochemistry, v. 47, p. 819–844, doi:10.2138/rmg.2002.47.18.

Figueroa, A.M., and Knott, J.R., 2010, Tectonic geomorphol-ogy of the southern Sierra Nevada Mountains (Cali-fornia): Evidence for uplift and basin formation: Geo-morphology, v. 123, p. 34–45, doi:10.1016/j.geomorph.2010.06.009.

Garver, J.I., 2002, Discussion: Metamictization of natural zir-con: Accumulation versus thermal annealing of radio-activity-induced damage: Contributions to Mineralogy and Petrology, v. 143, p. 756–757.

Glazner, A.F., Walker, J.D., Bartley, J.M., and Fletcher, J.M., 2002, Cenozoic evolution of the Mojave block of south-ern California, in Glazner, A.F., Walker, J.D., and Bartley, J.M., eds., Geologic Evolution of the Mojave Desert and Southwestern Basin and Range: Geological Society of America Memoir 195, p. 19–41.

Gleadow, A.J.W., and Fitzgerald, P.G., 1987, Uplift history and structure of the Transantarctic Mountains: New evidence from fi ssion track dating of basement apa-tites in the Dry Valleys area, southern Victoria Land: Earth and Planetary Science Letters, v. 82, p. 1–14, doi:10.1016/0012-821X(87)90102-6.

Goodman, E.D., and Malin, P.E., 1992, Evolution of the south-ern San Joaquin Basin and mid-Tertiary transitional tectonics, central California: Tectonics, v. 11, p. 478–498, doi:10.1029/91TC02871.

Hamilton, W.B., 1969, California and the underfl ow of Pacifi c mantle: Geological Society of America Bulletin, v. 80, p. 2409–2430, doi:10.1130/0016-7606(1969)80[2409:MCATUO]2.0.CO;2.

House, M.A., Wernicke, B.P., and Farley, K.A., 1998, Dating topo-graphic uplift of the Sierra Nevada, California, using apa-tite (U-Th)/He ages: Nature, v. 396, p. 66–69, doi:10.1038/23926.

House, M.A., Wernicke, B.P. and Farley, K.A., 2001, Paleo-geomorphology of the Sierra Nevada, California, from (U-Th)/He ages in apatite: American Journal of Science, v. 301, p. 77–102, doi:10.2475/ajs.301.2.77.

Huber, N.K., 1981, Amount and Timing of Late Cenozoic Uplift and Tilt of the Central Sierra Nevada, California—Evi-dence from the Upper San Joaquin River Basin: U.S. Geological Survey Professional Paper 1197, 28 p.

Jacobson, C.E., Grove, M., Vucic, A., Pedrick, J.N., and Ebert, K.A., 2007, Exhumation of the Orocopia schist and asso-ciated rocks of southeastern California; relative roles of erosion, synsubduction tectonic denudation, and middle Cretaceous extension, in Cloos, M., et al., eds., Convergent Margin Terranes and Associated Regions: A Tribute to W.G. Ernst: Geological Society of America Special Paper 419, p. 1–37.

Ketcham, R.A., 2005. Forward and inverse modeling of low-temperature thermochronometry data, in Reiners, P., and Ehlers, T., eds., Low-Temperature Thermochronol-ogy: Reviews in Mineralogy and Geochemistry, v. 58, p. 275–314.

Ketcham, R.A., Donelick, R.A., and Carlson, W.D., 1999, Vari-ability of apatite fi ssion-track annealing kinetics: III. Extrapolation to geologic time scales: The American Mineralogist, v. 84, p. 1235–1255.

Mahéo, G., Saleeby, J., Saleeby, Z., and Farley, K.A., 2009, Tectonic controls on southern Sierra Nevada topog-raphy, California: Tectonics, v. 28, TC6006, doi:10.1029/2008TC002340.

Malin, P.E., Goodman, E.D., Henyey, T.L., Li, Y.G., Okaya, D.A., and Saleeby, J.B., 1995, Signifi cance of seismic refl ec-tions beneath a tilted exposure of deep continental crust, Tehachapi Mountains, California: Journal of Geophysi-cal Research, v. 100, no. B2, p. 2069–2087, doi:10.1029/94JB02127.

McGill, S.F., Wells, S.G., Fortner, S.K., Kuzma, H.A., and McGill, J.D., 2009, Slip rate of the western Garlock fault, at Clark Wash, near Lone Tree Canyon, Mojave Des-ert, California: Geological Society of America Bulletin, v. 121, p. 536–554, doi:10.1130/B26123.1.

McPhillips, D., and Brandon, M.T., 2012, Topographic evolu-tion of the Sierra Nevada measured directly by inver-sion of low-temperature thermochronology: Ameri-



can Journal of Science, v. 312, p. 90–116, doi:10.2475/02.2012.02.

Monastero, F.C., Sabin, A.E., and Walker, J.D., 1997, Evidence for post–early Miocene initiation of movement on the Garlock fault from offset of the Cudahy Camp Formation, east-central California: Geology, v. 25, no. 3, p. 247–250, doi:10.1130/0091-7613(1997)025<0247:EFPEMI>2.3.CO;2.

Monastero, F.C., Walker, J.D., Katzenstein, A.M., and Sabin, A.E., 2002, Neogene evolution of the Indian Wells Val-ley, east-central California, in Glazner, A.F., Walker, J.D., and Bartley, J.M., eds., Geologic Evolution of the Central Mojave Desert and Southern Basin and Range: Geological Society of America Memoir 195, p. 199–228.

Mulch, A., Graham, S.A., and Chamberlain, C.P., 2006, Hydro-gen isotopes in Eocene river gravels and paleoeleva-tion of the Sierra Nevada: Science, v. 32, p. 167–181.

Nadin, E.S., and Saleeby, J.B., 2010, Quaternary reactiva-tion of the Kern Canyon fault system, southern Sierra Nevada, California: Geological Society of America Bul-letin, v. 122, p. 1671–1685, doi:10.1130/B30009.1.

Naeser, C.W., 1979, Fission track dating and geologic anneal-ing of fi ssion tracks, in Jager, E., and Hunziker, J.C., eds., Lectures in Isotope Geology: Berlin, Springer-Verlag, p. 154–169.

Pickett, D.A., and Saleeby, J.B., 1993, Thermobarometric con-straints on the depth of the exposure and conditions of plutonism and metamorphism at deep levels of the Sierra Nevada batholith, Tehachapi Mountains, Califor-nia: Journal of Geophysical Research, v. 98, p. 609–629, doi:10.1029/92JB01889.

Ross, D.C., 1989, The Metamorphic and Plutonic Rocks of the Southernmost Sierra Nevada, California, and their Tec-tonic Framework: U.S. Geological Survey Professional Paper 1381, 159 p., 2 plates.

Saleeby, J., 2003, Segmentation of the Laramide slab—Evi-dence from the southern Sierra Nevada region: Geo-logical Society of America Bulletin, v. 115, p. 655–668, doi:10.1130/0016-7606(2003)115<0655:SOTLSF>2.0.CO;2.

Saleeby, J., Farley, K.A., Kistler, R.W., and Fleck, R., 2007, Thermal evolution and exhumation of deep-level

batholithic exposures, southernmost Sierra Nevada, California, in Cloos, M., Carlson, M.D., Gilbert, M.C., Liou, J.G., and Sorenson, S.S., eds., Convergent Mar-gin Terranes and Associated Regions: A Tribute to W.G. Ernst: Geological Society of America Special Paper 419, p. 39–66, doi:10.1130/2007.2419(02).

Saleeby, J.B., Ducea, M.N., Busby, C.J., Nadin, E.S., and Wetmore, P.H., 2008, Chronology of pluton emplace-ment and regional deformation in the southern Sierra Nevada batholith, California, in Wright, J.E., and Sher-vais, J.W., eds., Ophiolites, Arcs, and Batholiths: A Tribute to Cliff Hopson: Geological Society of America Special Paper 438, p. 397–427.

Saleeby, J.B., Saleeby, Z., Nadin, E., and Maheo, G., 2009, Step-over in the structure controlling the regional west tilt of the Sierra Nevada microplate: Eastern escarpment system to Kern Canyon system: Interna-tional Geology Review, v. 51, p. 634–669, doi:10.1080/00206810902867773.

Saleeby, J., Le Pourhiet, L., Saleeby, Z., and Gurnis, M., 2012, Epeirogenic transients related to mantle lithosphere removal in the southern Sierra Nevada region, Califor-nia: Part I. Implications of thermomechanical modeling, in Saleeby, J., ed., Geodynamics and Consequences of Lithospheric Removal in the Sierra Nevada, California: Geosphere, v. 8, themed issue, p. 1286–1309, doi:10.1130/GES00746.1.

Sharry, J., 1981, The Geology of the Western Tehachapi Mountains, California [Ph.D. thesis]: Cambridge, Mas-sachusetts, Massachusetts Institute of Technology, 215 p., 3 plates.

Smith, G.I., 1962, Large lateral displacement on the Garlock fault, California, as measured from offset dike swarms: The American Association of Petroleum Geologists Bul-letin, v. 46, p. 85–104.

Stern, T., Bateman, B.A., Morgan, A., Newell, M.F., and Peck, D.L., 1981, Isotopic U-Pb Ages of Zircon from the Gran-itoids of the Central Sierra Nevada, California: U.S. Geo-logical Survey Professional Paper 1185, 17 p.

Tagami, T., 2005, Zircon fi ssion track dating and application to fault studies, in Reiners, P., and Ehlers, T., eds., Low-

Temperature Thermochronology: Reviews in Mineral-ogy and Geochemistry, v. 58, p. 95–122.

Unruh, J.R., 1991, The uplift of the Sierra Nevada and implica-tions for late Cenozoic epeirogeny in the western Cor-dillera: Geological Society of America Bulletin, v. 103, p. 1395–1404, doi:10.1130/0016-7606(1991)103<1395:TUOTSN>2.3.CO;2.

Unruh, J.R., Humphrey, J., and Barron, A., 2003, Transten-sional model for the Sierra Nevada frontal fault system, eastern California: Geology, v. 31, p. 327–330.

Wakabayashi, J., and Sawyer, T.L., 2001, Stream incision, tec-tonics, uplift, and evolution of topography of the Sierra Nevada, California: The Journal of Geology, v. 109, p. 539–562, doi:10.1086/321962.

Wernicke, B., and Snow, J.K., 1998, Cenozoic extension in the central Basin and Range: Motion of the Sierran–Great Valley block: International Geology Review, v. 40, p. 403–410, doi:10.1080/00206819809465217.

Wolf, R.A., Farley, K.A., and Silver, L.T., 1996, Helium diffusion and low temperature thermochronometry of apatite: Geochimica et Cosmochimica Acta, v. 60, p. 4231–4240, doi:10.1016/S0016-7037(96)00192-5.

Wolf, R.A., Farley, K.A., and Kass, D.M., 1998, A sensitivity analysis of the apatite (U-Th)/He thermochronom-eter: Chemical Geology, v. 148, p. 105–114, doi:10.1016/S0009-2541(98)00024-2.

Wood, D.J., 1997, Geology of the Eastern Tehachapi Moun-tains and Late Cretaceous–Early Cenozoic Tectonics of the Southern Sierra Nevada Region, Kern County, Cali-fornia [Ph.D. thesis]: Pasadena, California, California Institute of Technology, 287 p.

Wood, D.J., and Saleeby, J.B., 1997, Late Cretaceous–Paleo-cene extensional collapse and disaggregation of the southernmost Sierra Nevada batholith: International Geology Review, v. 39, p. 973–1009.

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