Middle Miocene to Recent Exhumation of the Slate Range, Eastern California, and Implications for the Timing of Extension and the Transition to Transtension

Middle Miocene to recent exhumation of the Slate Range, eastern California, and implications for the timing of extension and the transition to transtension

J. Douglas Walker, Tandis S. Bidgoli, Brad D. Didericksen*, Daniel F. Stockli†, and Joseph E. Andrew Department of Geology, University of Kansas, Lawrence, Kansas 66045, USA

ABSTRACT sional belt at the latitude of Las Vegas, Nevada, 2005) considered the Furnace Creek fault to be may have been stretched as much as 300% since active from ca. 9 to 5 Ma, and that this fault is a New mapping combined with fault-slip and ca. 16 Ma (e.g., Wernicke et al., 1988; Niemi component of the dextral regime (Fig. 1). Snow thermochronological data show that Middle et al., 2001). The earlier deformation resulted and Lux (1999) interpreted the change to dex- Miocene to recent extension and exhumation in primarily east-west extension across the tral deformation in Death Valley to have started of the Slate Range, eastern California, is pro- region (Wernicke et al., 1982, 1988), initiat- ca. 11 Ma (Fig. 1). Other studies that focus on duced by the active Searles Valley fault sys- ing ca. 15 Ma, as inferred by the ages of early different areas interpret younger ages for this tem and the Slate Range detachment, an older basin deposits of the Panuga (Snow and Lux, change to transtension. Hodges et al. (1989) Middle Miocene low-angle normal fault. Off- 1999) and Eagle Mountain formations (Niemi placed the transition as ca. 3.7 Ma in Pana- set Middle Miocene rocks record a combined et al., 2001) of the northern Death Valley area, mint Valley (Fig. 1). Monastero et al. (2002) ~9 km of west-directed extension over the and of the Panamint Valley volcanic sequence considered this transition to have occurred ca. past ~14 m.y. for the fault zones. (U-Th)/He in the Argus, Panamint, and Slate Range region 3–2 Ma in the Indian Wells Valley based on apatite cooling ages of samples from the (Fig. 1; Andrew and Walker, 2009). Peak exten- mostly subsurface stratigraphic data. Bellier and central and southern Slate Range indi- sion, however, probably occurred after 15 Ma Zoback (1995) considered the transition to have cate that footwall cooling began ca. 14 Ma; (Snow and Wernicke, 2000, p. 687) and lasted occurred in the past 300 k.y. using Holocene we interpret this as the age of initiation of until Late Miocene time; the locus of the major fault scarp slip data. In Stockli et al. (2003), a motion on the Slate Range detachment. This deformation is interpreted to be east of Pana- general westward migration of the transition timing is consistent with inferences made mint Valley. was reported, starting at 6 Ma and progressing using stratigraphic and structural criteria. Later deformation is primarily transtensional westward to 4 Ma, based on fault kinematic and Data from the northern Slate Range show in character, expressed as dextral-oblique nor- thermochronometric data from the Fish Lake that rapid fault slip began along the Searles mal faults and strike-slip faults active in the Valley–White Mountain area. Thus, the precise Valley fault ca. 4 Ma; data from the central region from the Spring Mountains to the eastern timing and associated spatial patterns of the and southern Slate Range can be interpreted fl ank of the Sierra Nevada. At this latitude, this onset of signifi cant dextral deformation remain as indicating cooling at 5–6 Ma. This tim- later deformation belt is variously referred to uncertain. The transition from east-west exten- ing correlates to the results of nearby stud- as the eastern California shear zone or Walker sion to northwest-directed oblique extension is ies, suggesting a strain transition in the sur- Lane belt, and affects the western half of the apparently complex in both timing and location. rounding area between ca. 6 and 3 Ma. The older Basin and Range extensional province. The resolution of this problem requires more data collected are most consistent with a Atwater and Stock (1998) interpreted a complete documentation of the age of transten- westward migration in the locus of transten- change in plate motions ca. 10 Ma, from a sional slip on specifi c structures. sional deformation, and show that the initia- more westward to a more northward motion This paper aims to more closely bracket the tion of that deformation commonly lags the of the Pacifi c plate relative to North America, initiation of extension and to better defi ne the timing predicted by plate reconstructions by and associated this change to the transition to age of the transition from extension to transten- a few million years. dextral-transtensional deformation. Geological sion for the Slate Range and adjacent Searles evidence indicates a more complex deforma- Valley, which are west of Panamint and Death INTRODUCTION tional history. Mahan et al. (2009) suggested Valleys (Fig. 1). To do this we present new initiation of dextral transtension along the State- structural interpretations tied to three thermo- The Miocene Basin and Range extensional line fault in southern Nevada ca. 5 Ma (Fig. 1). chronologic transects from the northern, cen- province and the latest Miocene to recent east- Wernicke et al. (1988; and subsequently Snow tral, and southern Slate Range. The northern ern California shear zone–Walker Lane transten- and Wernicke, 2000; McQuarrie and Wernicke, transect (approximately along cross-section

*Present address: Platte River Associates Inc., Houston, Texas 77270, USA †Present address: Jackson School of Geosciences, University of Texas, Austin, Texas 78712, USA

Geosphere; April 2014; v. 10; no. 2; p. 276–291; doi:10.1130/GES00947.1; 9 ﬁ gures; 3 tables; 1 supplemental ﬁ le. Received 13 May 2013 ♦ Revision received 24 August 2013 ♦ Accepted 12 February 2014 ♦ Published online 17 March 2014

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CM = Cottonwood Mountains Nevada EIF = Eastern Inyo Fault Test Site FCF = Furnace Creek fault Region GF = Garlock fault HMF = Hunter Mountain fault

118° W 118° OVF = Owens Valley fault 37° N NB = Nova Basin Inyo Mountains EIF NDVF NDVF = Northern Death Valley fault Nevada California SDVF = Southern Death Valley fault SLF = State Line fault SV CM OVF SNFF = Sierra Nevada Frontal fault SV = Saline Valley TPF = Towne Pass Fault HMF TPF FCF SLF Spring Mountains

Panamint Valley NB Death Valley Panamint Range

Argus Range

Sierra Nevada

SNFF 36° N SDVF

Slate Range 116° W 116°

Study Area (Figure 2) Kilometers GF 0255012.5

Figure 1. Digital elevation model of study area and surrounding region. Warm colors correspond to the mountain ranges and cool colors to the basins. Extensional and transtensional structures that are part of the eastern Cali- fornia shear zone are indicated.

A–A′ in Fig. 2) trends east-west across the along the northern transect consist of 100 Ma lava ﬂ ows, pumaceous epiclastic rocks, and northern Slate Range, near where the south- Stockwell diorite, 159 Ma Copper Queen andesitic to dacitic laharic deposits (Smith et al., west-striking Manly Pass fault joins the north- alaskite, and 166 Ma Isham Canyon granite 1968; Andrew and Walker, 2003, 2009; Dider- striking Searles Valley fault; the central transect (Moore, 1976; Dunne and Walker, 2004) con- icksen, 2005). A similar volcanic and sedimen- (along cross-section B–B′ in Fig. 2) crosses the taining pendants of Paleozoic and Mesozoic tary sequence is also found to the northwest in Slate Range just north of Tank Canyon; and the metasedimentary rocks. The central and south- the Argus Range (Moore, 1976; Andrew and southern transect (cross-section C–C′ in Fig. 2) ern transects cross Jurassic plutonic rocks that Walker, 2003, 2009), to the east in the Panamint parallels Layton Canyon. This work comple- contain pendants of Jurassic metavolcanic and Range (Johnson, 1957; Andrew and Walker, ments studies of late Cenozoic deformation in metaepiclastic rocks (Smith et al., 1968; Dunne 2009) and Owlshead Mountains (Davis, 1988; the southern and northern Slate Range (Walker and Walker, 2004). These assemblages repre- Davis and Fleck, 1977; Luckow et al., 2005), et al., 2005; Numelin et al., 2007; Andrew and sent portions of the Mesozoic Sierran magmatic and to the southeast in the Quail Mountains Walker, 2009). arc and the Paleozoic continental passive mar- (Muehlberger, 1954; Andrew, 2002), although gin. There is an ~85 m.y. time gap between the thicknesses vary greatly between the sections to GEOLOGIC SETTING youngest Cretaceous intrusive rock and Middle the east and south of the study area. Miocene sedimentary-volcanic units, marked The relatively uniform nature of the Mio- The Slate Range trends north-south and the by an extensive erosion surface (Moore, 1976; cene sections in the Slate Range and nearby southern two-thirds of the range is bound on Andrew and Walker, 2003, 2009). Miocene areas implies that the basal nonconformity was its western ﬂ ank by a west-dipping, low- to rocks dip eastward, and comprise a ca. 14.5– relatively planar (Andrew and Walker, 2009), moderate-angle normal fault zone (Fig. 2; 12.5 Ma sequence consisting of basal sandstone although the thickness of the lowest Miocene Moore, 1976; Smith et al., 1968; Walker et al., and conglomerate with locally interbedded sedimentary deposits varies from 0 to 30 m. 2005; Numelin et al., 2007). Pre-Cenozoic rocks limestones overlain by basaltic rocks, rhyolite The thickest sections were deposited against

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Quaternary alluvium, fanglomerate, lacustrine, eolian sand, and evaporite deposits

~15-12.8 Ma (Walker et al., 2003) sedimentary and volcanic rocks

Neogene fanglomerate, volcanic, and sedimentary rocks

Granite , undated. Possibly a phase of the Copper Queen alaskite or of the Stockwell diorite Manly Pass fault (~100 Ma) Stockwell diorite Slate Range zone detachment (151 +/- 1 Ma) Biotite granite AA'

349,24

333,18 Late Jurassic tuffaceous schist 365,30 (~153-149 Ma) Metamorphosed andesite and volcaniclastic rocks

(~159 Ma) Copper Queen alaskite and (~152 Ma) granite or alaskite

Searles Valley (~166 Ma) Isham Canyon quartz monzonite and correlated units from fault zone CopperCanyon Queen Southern Panamint Valley fault the central and east side of the Slate Range

Searles Valley BB'35°45'0"N (184-148 Ma) Mixed plutonic complex of monzonite, hornblende diorite, Tank Canyon leucogranite, granodiorite

(184.5 +/- 6 Ma) Quartz monzonite, granodiorite, alaskite

Mesozoic plutons, undifferentiated New York Canyon Searles Valley C C' Paleozoic and Triassic fault zone Layton sedimentary rocks Canyon Slate Range detachment Precambrian rocks

Searles Valley - Manly Pass fault zones

Slate Range detachment

Mesozoic faults in the Slate Range

km 117°15'0"W 117°7'30"W

Figure 2. Simpliﬁ ed geologic map of the Slate Range with locations of major fault systems and zones. Black dots correspond to the locations of our thermochronology samples. Arrows with corresponding orientations show the locations of the northward projection of the Miocene nonconformity (2σ orientations) used in calculating extension magnitudes (see text for discussion). Geology from this study, Smith et al. (1968), Walker et al. (1994, 2002), Andrew (2002), Dunne and Walker (2004), and Andrew and Walker (2009).

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subvertical buttress nonconformities and deposi- The geometry and kinematics of these faults are erate (Fig. 4C). This fanglomerate is undated, tional half-grabens (Andrew and Walker, 2009). critical to establishing paleodepth estimates for but the deformed nature of it along with poorly The nonconformity on the eastern fl ank of the thermochronology samples and are examined in exposed but east-tilted bedding suggest that it is central Slate Range appears to be a laterally con- more detail in the following. older than the nearby 4–5 Ma relatively untilted tinuous surface dipping between 14° and 25° east- basalt fl ows in the Argus Range (Andrew and ward (calculated using three point solutions; Table Faulting History of the Central Slate Range Walker, 2009). 1). Volcanic rocks of the same age in the northern The Searles Valley fault zone as defi ned in Slate Range and in the southern Panamint Valley The Slate Range detachment is currently Walker et al. (2005) is a curviplanar fault zone region are tilted to moderate east-northeast dips inactive, rotated to near horizontal dips, and is striking 155°–180° and dipping 15°–35° to the and are locally capped by rock avalanche depos- offset across the Searles Valley fault zone (Figs. west-southwest in the southern and central Slate its, suggesting that the most signifi cant tilting 2 and 3). The fault is interpreted to have initi- Range (Fig. 2). The dip steepens to ~50° as the event occurred in the Pliocene (Andrew, 2005). ated as a roughly north- to northwest-striking fault is traced northward into the north-central Bedding above the nonconformity in the Pana- normal fault that dipped westward between 25° Slate Range. Fault-slip data for the southern and mint Valley area averages 35° and ranges from and 50° (Didericksen, 2005). The interpreted central Slate Range, measured along the fault 20° to 50° (Andrew and Walker, 2009). initial dip is derived from its apparent 25° cut- zone in the Mesozoic bedrock, indicate that off angle with Miocene strata east of the break- overall motion is west directed (Didericksen, STRUCTURAL SETTING away in the footwall (Fig. 4A); it cannot be any 2005). This structure continues to the northern steeper than 50°, the cutoff angle of Miocene portion of the central Slate Range, where it The structural geology presented here is based rocks in fault horses in the hanging wall. The was mapped as the Manly Pass fault by Moore on our detailed geologic mapping and previous Slate Range detachment probably initiated dur- (1976). As noted herein, we consider the Manly work by Moore (1976) and Smith et al. (1968). ing Miocene deposition because the volcanic Pass and Searles Valley faults to constitute a Structural interpretations come mostly from sequences (ca. 12–15 Ma) contain interbedded single segmented fault system. South of cross- the mapping of Andrew and Walker (2009; see rock avalanche deposits, some basal conglom- section line A–A′ (Fig. 2), the Manly Pass fault also Walker et al., 2005) for the northern Slate erates, and locally consist almost entirely of is indistinguishable from the Searles Valley Range, and Didericksen (2005) and Numelin et footwall-derived clasts, indicative of signifi cant fault; north of A–A′ it bends to the northeast, al. (2007) for the central Slate Range. topographic relief (see additional discussion in dips moderately to gently northwest, and has There are two Tertiary-age extensional fault Andrew and Walker, 2009). Defi nitive Pliocene left-lateral oblique normal motion (Walker et systems in the Slate Range: the Slate Range and younger rocks are not found in the hang- al., 2005; Andrew and Walker, 2009). The fault detachment fault and the active Searles Val- ing wall of the Slate Range detachment, sug- continues northeastward into Panamint Valley, ley and Manly Pass faults (Fig. 2). The Searles gesting that the fault was uplifted and inactive where it intersects with the southern Panamint Valley and Manly Pass fault zones constitute before Pliocene volcanism began in the nearby Valley fault zone and the Panamint detachment a younger, segmented fault system (Walker et Argus Range (Andrew and Walker, 2009). In (Walker et al., 2005; Andrew and Walker, 2009). al., 2005; Numelin et al., 2007) that we refer the central Slate Range, Middle Miocene units We interpret these relationships to indicate to as the Searles Valley fault zone (SVFZ). The in the hanging wall dip eastward as much as that the Slate Range detachment is an older Slate Range detachment is the older structure, 50° into a 2–3-m-thick zone of fault gouge, structure related to Middle Miocene, west- because it is variously exposed in the footwall which grades into brecciated Jurassic basement directed extension in the Death Valley region. and hanging wall of the Searles Valley fault (Figs. 2 and 4B). The detachment in the north- The Searles Valley fault zone is a younger struc- and the hanging wall of the Manly Pass fault ern Slate Range is exposed west of the Manly ture related to transtension because it (1) clearly (Fig. 2; Andrew, 2005; Andrew and Walker, Pass fault; there are several fault splays that cuts the Slate Range detachment; (2) appears 2009; Didericksen, 2005). Geologic relation- dip 5°–10° westward, cutting Mesozoic plu- to be moving with active fault systems (Walker ships from the Layton Canyon area indicate that tonic rocks. A several-meter-thick gouge zone et al., 2005; Numelin et al., 2007); and (3) is collectively the Searles Valley fault and Slate separates the Mesozoic plutonic rocks from coincident with arrays of earthquake foci (e.g., Range detachment accommodate a minimum hanging wall faulted and brecciated Miocene Unruh and Hauksson, 2009). of ~9 km of horizontal extension in an apparent basaltic-andesite lava fl ows and an overlying In the following section we present preex- westward direction (Fig. 3; Didericksen, 2005). carbonate-cemented footwall-derived fanglom- tensional paleodepth estimates for our rock samples, which are critical to properly evaluating the thermal histories of the Slate Range TABLE 1. STRUCTURAL DATA USED TO ESTIMATE detachment and Searles Valley fault systems. THE PRESENT-DAY ORIENTATION OF MIDDLE These estimates are independent of the cool- MIOCENE PREEXTENSIONAL PALEOSURFACES ing ages, which are discussed herein. The ther- Strike Dip Measurement surface (°) (°) mochronologic data better delineates the tim- 335 14 TPS—nonconformity ing of initiation and exhumation of structures 350 16 TPS—nonconformity described here. 325 16 TPS—nonconformity 350 18 TPS—nonconformity 008 25 TPS—nonconformity Preextensional Paleodepth Estimates 010 43 bedding—ash deposit ~50 m thick 335 40 bedding—limestone ~50 m thick 325 35 bedding—limestone ~50 m thick The preextensional paleodepths of rock sam- 347 23 bedding—calcareous sandstone ~5 m thick 345 25 bedding—limestone ples in the Slate Range are essential to placing the 018 25 bedding—calcareous sandstone thermochronologic data into a cooling history Note: TPS—three point solution. (Stockli, 2005). Middle Miocene sedimentary

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Mean Age (Ma) 10 0 0 2500 5000 7500

Miocene nonconformity A ~150m projected A’ (from south at 349°) 2000 Miocene 2000 ~125m SRD SVFZ 7 6 54 1500 hanging wall 5 51 1500 4 52 ? klippe 2 3 1000 8 1 PRZ-P 4849 ~300m 1000 50 25 ? 26 24 +/- 6°

Elevation (m) Elevation 500 500 ? ? 0 SRD 0 0 2500 5000 7500 Horizontal distance (m) B 60 AHe age 50

20 Mean Age (Ma) 10

0 02500 5000 7500

B B’ 2000 Miocene 2000 40 SRD 39 nonconformity 1500 1500 SVFZ 35 36 38 30 29 33 28 1000 37 31 32 27 1000 Jc 500 500

Elevation (m) Elevation PRZ-P PRZ-M 0 0 0 2500 5000 7500 Horizontal distance (m)

Figure 3 (Continued on following page). Cross sections across the Slate Range based on work of Smith et al. (1968), Andrew and Walker (2009), and this study. (A) Northern. (B) Central. (C) Southern. Samples, shown as black dots, are projected in the plane of the cross section. In the projected view, samples may be above or below topography of the section line. Number above each sample corresponds to sample numbers referred to in Tables 2 and 3. Green dashed lines show the projected Miocene nonconformity (lower) and preextensional surface datum (upper). Dashed orange line is the approximate base of the Miocene and/or Pliocene partial retention zones (hachured areas). Mean (U-Th)/He ages are plotted as a function of horizontal distance above each cross section. Errors shown at 1σ standard deviation. PRZ-M—Miocene partial retention zone; PRZ-P—Pliocene zone; SRD—Slate Range detachment; SVFZ—Searles Valley fault zone. Locations of section lines are shown in Figure 2.

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C 70 60 AHe age ZHe age 50 40 30 20 Mean Age (Ma) 10 0 02500 5000 7500

C C’ 2000 Miocene SRD 2000 hanging wall klippen Miocene 1500 1500 nonconformity SVFZ SRD 14 10 11 13 SRD 15 16 SRD 1000 3 4 7 8 9 12 1000 1 2 500 Jurassic plutonic rock Jurassic plutonic rock 500 Elevation (m) Elevation PRZ-P PRZ-M 0 0 02500 5000 7500 Horizontal distance (m) Figure 3 (Continued).

and volcanic rocks nonconformably overlie sandstones and limestones could collectively they are computed using the Miocene noncon- basement along the eastern ﬂ ank of the Slate be as much as 50 m thick, and conglomerate formity. The removal of hanging-wall rocks by Range, providing a preextensional paleohori- sections could be <30 m thick; overlying pyro- the Slate Range detachment as well as possible zontal datum (Fig. 4A). However, the topogra- clastic rocks may be as thick as ~30 m; and the footwall tilting makes estimates of depths for phy of the nonconformity, thickness of volca- capping basaltic and andesitic volcanic section later Miocene to recent time less certain as most nic and sedimentary overburden, and potential is commonly >50 m thick and locally may be of the samples are below the detachment (Figs. errors in post–14 Ma tilt corrections need to be as thick as 100 m. Thus, an overburden value of 3 and 6). We assume that the main eastward tilt- considered in order to accurately estimate the ~200 m is added to all sample paleodepths. This ing of this section occurred during the Pliocene paleodepths of thermochronology samples (e.g., thickness is at the high end of measurements in due to recent motions on the Searles Valley and Stockli et al., 2002). The thickness of basal sedi- Andrew and Walker (2009), in which values of Panamint Valley fault systems, although it is ments overlying the nonconformity provides ~100–200 m for the section in the northern Slate possible that some tilting accompanied move- evidence for only minor, short-wavelength topo- Range were presented. A difference of 100 m, ment on the detachment: we present informa- graphic relief (~30 m) prior to the deposition of however, has little effect on the reconstructions, tion herein that supports former interpretation the remainder of the Middle Miocene sedimen- and is minor compared to the other uncertain- for timing of the tilting. tary and volcanic sequence. It is unlikely that ties. Although not seen or preserved in the Slate For the northern Slate Range, the paleohori- these small-amplitude variations perturbed the Range sections, correlative rocks thicken to zontal and/or nonconformity reference plane thermal structure (e.g., Stockli, 2005). >700 m to the south and east in the southern was projected northward to the latitude of the The orientation of the paleohorizontal and/ Panamint Range and Brown Mountains areas. thermochronologic transect as Miocene units or nonconformity surface in the footwall block These values and orientations bracket are not present to the east of the Manly Pass of the Slate Range detachment and Searles Val- paleodepths in the central and southern Slate fault in this area (Figs. 2 and 3). The projection ley fault zone was calculated as the mean of 11 Range relatively well (Figs. 2 and 3) because was made in the direction of mean strike from measurements made in Middle Miocene units Miocene strata directly overlie the basement the northernmost exposure of the nonconfor- on the east-central ﬂ ank of the central Slate rocks in the line of section. Assuming an ~25° mity (Fig. 2) at an elevation of ~510 m. In addi- Range, yielding a strike of 349° ± 16° and dip dip for this surface gives paleodepth values tion, there are three small faults that cut across of 24° ± 6° (2σ) (Fig. 5; Table 1). The preex- ranging from 0.3 to 3.2 km. This estimate is or project into the area. These were initially tensional volcanic and sedimentary overburden possible because the range can be considered ignored in estimating the paleodepths. above the Miocene nonconformity at the onset a simple tilted fault block, thus eliminating the For all three areas, paleodepth estimate errors of faulting was estimated based on the preserved need to consider the hinge position for tilting are primarily a function of the uncertainty in the stratigraphic thickness. Our observations and (e.g., Armstrong et al., 2003). These depths timing and amount of eastward footwall tilting those of Smith et al. (1968) suggest the basal apply to the area in Middle Miocene time since and, for the northern transect, the projection of

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the paleohorizontal surface northward ~6 km. Timing of tilting is important as rotation of the A block changes the estimated depths. If tilting is prior to cooling age, then depth estimates would become shallower. Uncertainties for tilt correction of the central (B–B′) and southern (C–C′) transects mainly result from the variation in dip of the nonconformity and the assumption that the block behaves in a rigid manner, and only amount to a few hundred meters for this transect (with probable uncertainty of 500 m for the deepest samples). For the northern transect, projection uncertainties are computed using the Basal Miocene Nonconformity extreme high and low values for dip and strike of the section. This results in a possible variation of as much as 1.5 km of depth (2σ). (This uncertainty is not plotted in any ﬁ gures but is included in the paleogeothermal gradient calcu- lation.) Because the sample locations are mostly adjacent to or east of the projected breakaway for the Slate Range detachment (Figs. 3 and 6), we assume that tilting was minor prior to move- ment on the Searles Valley fault zone. Struc- tural reconstructions based on interpretations of B Miocene Strata the thermochronologic data will help evaluate this uncertainty. The paleodepth estimates for Gouge samples 1, 2, 8, and 37 (located in the hanging wall of the Searles Valley fault zone) were not calculated. Thermochronologic data give some information about their possible reconstructed positions (Fig. 6; see the following detailed discussions for how positions were determined).

Jurassic Rocks (U-Th)/He THERMOCHRONOMETRY

To better understand the timing and magnitude of fault motion, we analyzed 43 samples from the Slate Range using (U-Th)/He West fault breccia & East C gouge of Miocene basaltic-andesite late Miocene(?)-Pliocene Figure 4. (A) Miocene volcanic rocks crop- footwall-derived fanglomerates ping out over Jurassic granite and metavolcanic rocks on the eastern ﬂ ank of the Slate Range north of Layton Pass. The tilted nonconformity here appears to be a rotated gen- fault gouge of tle erosional surface with beds steepening Jurassic Copper upsection to higher dips in the dark volca- Queen alaskite nic rocks. Field of view is ~800 m. Panamint Range is in background. Panoramic view is toward the northeast. (B) Slate Range detachment west-southwest of Layton Pass. Miocene strata dip ~45° at top of hill, an ~2-m-thick ground zone is beneath (tan layer), and brecciated Jurassic metaplu- fault gouge of Cretaceous tonic rocks are at base. White box is next to Jurassic Copper Stockwell diorite ~2-m-tall person for scale. (C) Slate Range Queen alaskite 1 meter detachment fault and splay faults in the hanging wall of the Searles Valley normal fault, just north of Copper Queen Canyon.

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N 5 min at 1050 °C using a continuous-mode 20W individual analyses (see Supplemental File1). In Nd-YAG (neodymium-doped yttrium aluminum most cases (n = 38), 3 or more replicates were garnet) laser (House et al., 2000). Extracted He used to calculate the mean; however, 8 of the gas was spiked with 3He, purifi ed using a getter- 46 reported ages were calculated using fewer ing and cryogenic gas system, and measured on than three replicates. These samples contained a quadrupole mass spectrometer. Degassed apa- outlier analyses that were excluded from our tite grains were then dissolved (in their platinum calculated mean ages. A signifi cant portion (n = packets) in a spiked HNO solution in prepara- 25) of our reported mean ages had outliers that Axial Mean = 3 349°+/-16°, 24°+/-6° tion for U, Th, and Sm determinations using were excluded from our calculations. Of the 195 inductively coupled plasma mass spectrometry individual analyses completed, 41 (or 20%) of (ICP-MS). Zircon grains were heated for 10 min the aliquots were excluded. Outliers were deter- at 1300 °C. After laser degassing, zircon grains mined, in most instances, as ages that were more were removed from their platinum packets and than 2 standard deviations from the mean. Anal- dissolved using standard U-Pb double pressure- yses with a large number of reextractions during vessel digestion procedures. Following disso- laser He degassing were also excluded from the N = 11 lution, samples were spiked and analyzed for determined means, as this usually points to the U, Th, and Sm using ICP-MS. Laboratory and presence of unseen mineral or fl uid inclusions Figure 5. Equal-area projection showing the analytical work was performed at the Isotope within the grain (House et al., 1999). The data distribution of oriented planes within the Geochemistry Lab at the University of Kansas were evaluated for other possible trends that Miocene sedimentary and volcanic sequence and at the (U-Th)/He Thermochronology Lab at might explain the high degree of scatter in our (Table 1). The axial mean (349° ± 16°, 24° ± the University of Texas at Austin. ages. No apparent correlation was identifi ed 6°; bold line) of these planes should approx- A major challenge in (U-Th)/He dating of between (U-Th)/He dates and effective U con- imate the Middle Miocene nonconformity apatite is the presence of small U- and Th- centrations that would suggest radiation damage or preextensional paleosurface used in bearing impurities or inclusions such as zircon effects on closure temperatures (Shuster et al., paleodepth reconstructions within a 2σ con- and monazite microlites (House et al., 1999; Lip- 2006; Flowers et al., 2007). Similarly, we did fi dence interval (gray area; data are given polt et al., 1994). These impurities can bias the Ft not observe grain size effects in our data (Farley, in Table 1). correction (geometrical and statistical correction 2000; Reiners and Farley, 2001). Although we factor; see following), modify diffusive behavior cannot determine the exact cause of the scatter by changing the He concentration gradient, and/ in our data, we suspect that microscopic min- thermochronometry (Fig. 2). The (U-Th)/He or incompletely dissolve during dilution of the eral or fl uid inclusions (e.g., House et al., 1999),

method is a well-established and effective tech- grain in HNO3. The latter is particularly prob- inhomogeneous parent isotope distributions nique for directly dating the exhumation and lematic because a signifi cant portion of the mea- (e.g., Hourigan et al., 2005), and/or He implan- cooling of footwalls of major normal faults (e.g., sured He is parentless, resulting in anomalously tation from adjacent minerals (e.g., Spiegel et Axen et al., 2000; Stockli et al., 2000; Ehlers older ages (Farley and Stockli, 2002). In order to al., 2009) may be the source of our problems. et al., 2003; Armstrong et al. 2004), as well as minimize these problems, inclusion-free grains Propagated analytical uncertainties for indi- the initiation of transtensional structures (e.g., were hand-picked from separates with a 160X vidual analyses are ~3%–4% (2σ). However, Stockli et al., 2003; Lee et al., 2009; Mahan et Nikon stereo petrographic microscope under analytical uncertainties generally underestimate al., 2009). The method is based on the produc- crossed-polarized light before being loaded into the reproducibility of a sample because uncer- tion of 4He through the decay of 235U, 238U, 232Th, the Pt packets for analysis. tainties related to the Ft correction and U and and 147Sm. However, 4He retention in minerals is During α decay, a portion of the 4He pro- Th distributions are diffi cult to quantify. Com- temperature dependent. In apatite, 4He is com- duced is ejected out of the grain. This effect is mon practice in (U-Th)/He dating is to apply a pletely lost by thermally activated volume dif- the single greatest impediment to high- precision percentage error to individual analyses based fusion above ~80 °C and mostly retained below (U-Th)/He ages in common accessory miner- on the reproducibility of laboratory standards: ~40 °C (Wolf et al., 1996, 1998; House et al., als (Farley et al., 1996). Unaccounted-for 4He 6% for apatite and 8% for zircon (Farley et al., 1999; Stockli et al., 2000). For zircon, helium ejected from the grain will result in a younger 2001; Reiners et al., 2002). For our mean ages, is retained over a higher temperature range, (U-Th)/He age. The loss of α particles is esti- we report the error as the standard deviation from 190 to 130 °C (Reiners et al., 2002, 2004). mated by using Ft, which is based on the (1σ) of our grain population for a given sample. These temperatures defi ne thermal sensitiv- dimensions of the apatite grain and an assumed However, 4 samples (BDCSR28, BDCSR 36, ity windows or partial retention zones (PRZs) homogeneity of parent abundance throughout (Wolf et al., 1998) that can be used to recon- the crystal (Farley and Stockli, 2002; Farley et struct temperature-time paths or apparent age- al., 1996). Apatite grains were measured using paleodepth and/or elevation trends for a range of computer software calibrated to the microscope 1Supplemental fi le. The fi le includes two data common accessory minerals (e.g., apatite, zir- through a mounted digital camera. The Ft cor- tables: Table DR1: Apatite (U-Th)/He data for the Slate Range, and Table DR2: Zircon (U-Th)/He con, titanite), ideal for evaluating upper crustal rection works best for euhedral grains >100 um data for the Slate Range. The supplemental data processes (see summary in Stockli, 2005). in width. Running several aliquots per sample can be found online at http://dx.doi.org/10.1130/ Apatite and zircon separates from rock sam- helped to deal with inherent correction errors GES00947.S1 or with the full-text article on www. ples were prepared by standard mineral separa- resulting from the smaller size and imperfect gsapubs.org. The data is also available at http:// tion techniques. Each single-grain or multigrain shape of many of the analyzed grains. www.geochron.org/dataset/html/geochron_data- set_2013_09_24_IU9f6, http://www.geochron.org/ apatite or zircon sample was loaded into a 1 mm The resulting (U-Th)/He mean ages shown dataset/html/geochron_dataset_2013_07_31_hXt54, platinum packet. Apatite grains were heated for in Tables 2 and 3 were calculated from several and http://dx.doi.org/10.1594/IEDA/100424.

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8 500

2266 1000 1 2 51 48499 484 5500 2255 PRZ-P 5544 52 1500 7 52 3 6

5 2000 Meters SRD 4 1

D 2500

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R 2Fault Fault 3Fault

S MP-SVFZ ? 3000 B WE

Miocene rocks 0

27 29 28 500 38 30

39 1000

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36 2000 35 Meters 2500 SRD 33 32 PRZ-P 3000 31 37 3500

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C WE

Miocene rocks 0 500 16 PRZ-M 1000 15

14 1500 ck ro 13 ic on 2000 lut c p 11 si 12 ras 10 Ju 2500 SVFZ 9 8 PRZ-P Meters 7 3000 ck ro ic ton 4 plu 3500 ic 3 ss SRD ura 2 J 40000 1 4500

5000

Figure 6. Reconstructed sample positions for the thermochronology transects. (A) Northern. (B) Central. (C) Southern. Abbreviations and line coloring are as in Figure 3. Sample numbers refer to Tables 2 and 3. MP—Manly Pass.

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12LC09, and 12LC13) are single grain ages; young 4.2 Ma to as old as 51.8 Ma, but no sys- ages come from the western part of the tran- therefore, we show the error in the mean age as tematic relationship between cooling age and sect (Table 2; Fig. 3B). The three westernmost the standard error. elevation is observed. The youngest cooling samples from the Searles Valley fault footwall ages along this transect are located in the foot- have ages varying from ca. 6 to 9 Ma. Higher in RESULTS wall immediately east of the Manly Pass fault, the range, 2 samples give consistent ages of ca. clustering at ca. 4 Ma (Table 2; Fig. 3A). The 14 Ma. Samples then increase in apparent age The results for the northern, central, and samples increase in apparent age to east (Table eastward over the range (Table 2; Fig. 3B). The southern thermochronology transects are dis- 2; Fig. 3A). Some of the oldest apparent ages oldest apparent ages are located on the eastern cussed separately because of structural and age are located west of the fault, ranging from 28.8 slope, from samples located only a few hundred differences between them. to 51.8 Ma (Table 2; Fig. 3A). meters beneath the projection of the Miocene nonconformity, and have ages of ca. 45–50 Ma Northern Transect Central Transect (Table 2; Fig. 3B).

The northern transect crosses the Manly Pass The central transect crosses the Searles Val- Southern Transect fault and consists of 15 samples of medium- ley fault and consists of 14 samples of Copper grained Stockwell diorite from both the footwall Queen alaskite and Jurassic metadiorite (Figs. 2 The southern transect crosses the Searles Val- and hanging wall (Figs. 2 and 3). Hanging-wall and 3). Only one sample was collected from the ley fault and consists of 14 samples of Jurassic rocks are complicated in that they come from hanging wall of the fault zone; all other samples diorite, leucogranite, and metavolcanic rocks horses within the hanging wall of the offset Slate are from the footwalls of the Searles Valley fault (Figs. 2 and 3). Two samples were collected Range detachment. Samples were collected and Slate Range detachment. from the hanging wall of the fault zone; all other approximately parallel to the west-directed Table 2 shows that apatite (U-Th)/He mean samples are from the footwall of both the Sear- motion (Walker et al., 2005) and were taken in ages from the central transect range from as les Valley fault and Slate Range detachment. ~100 m vertical increments in order to represent young 6.0 Ma to as old as 50.1 Ma. Like the Although there are 14 samples in the southern the entire footwall section (Fig. 3; Table 2). northern transect, no systematic relationship transect, only 7 contained inclusion-free apatite Table 2 shows that apatite (U-Th)/He mean between cooling age and elevation is observed grains. Table 2 shows that the resulting mean ages ages from the northern transect range from as (Table 2). Samples with the youngest cooling range from as young 5.2 Ma to as old as 26.0 Ma.

TABLE 2. SUMMARY OF (U-TH)/HE APATITE DATA FOR THE SLATE RANGE Longitude Latitude Elevation Mass U Th Sm He Mean St.Dev.† Sample (W) (N) (m) (µg) Ft* (ppm) (ppm) (ppm) Th/U (ncc/mg) age (Ma) (Ma) Replicates Northern Transect (section A–A′) BD04CSR-8 117.2649 35.8494 840 0.57 0.56 46.9 68.8 – 1.49 223.0 51.8 0.6 3 BD04CSR-1 117.2627 35.8487 900 1.02 0.61 15.1 32.8 – 2.10 74.5 43.1 6.3 5 BD04CSR-2 117.2574 35.8468 1010 1.60 0.69 97.7 106.7 – 1.13 304.9 28.8 3.4 4 BD04CSR-3 117.2528 35.8496 1090 1.22 0.66 11.3 19.1 – 1.72 9.1 7.3 0.7 5 BD04CSR-4 117.2485 35.8498 1190 1.55 0.64 11.6 22.3 – 2.43 6.6 4.6 0.9 6 BD04CSR-5 117.2450 35.8488 1290 1.80 0.64 6.1 16.9 – 2.79 3.3 4.2 0.4 3 BD04CSR-6 117.2430 35.8474 1400 1.23 0.62 11.2 33.0 – 3.04 5.4 4.2 1.9 3 BD04CSR-7 117.2403 35.8454 1500 0.85 0.64 45.4 31.2 – 0.66 39.2 9.5 2.3 4 BD04CSR-54 117.2369 35.8433 1460 1.88 0.66 34.2 39.3 – 1.10 37.0 10.6 1.4 4 BD04CSR-51 117.2289 35.8403 1390 1.28 0.59 61.5 100.7 – 1.63 125.2 20.6 1.0 4 BD04CSR-52 117.2268 35.8396 1310 3.09 0.67 7.5 21.0 – 2.84 10.8 10.8 1.4 7 BD04CSR-4849 117.2124 35.8405 950 3.20 0.67 19.3 44.1 – 2.31 50.9 21.0 0.9 3 BD04CSR-50 117.2092 35.8402 810 5.70 0.75 8.6 16.9 – 1.93 16.7 14.9 0.7 4 BD04CSR-25 117.2062 35.8393 700 3.88 0.68 20.8 33.4 – 1.61 44.6 20.0 3.6 4 BD04CSR-26 117.2019 35.8400 600 2.27 0.64 92.1 67.7 – 0.74 326.0 39.0 3.5 3 Central Transect (section B–B′) BDCSR27 117.1536 35.7456 800 0.68 0.59 6.0 34.0 – 10.93 40.6 44.4 7.5 4 BDCSR28 117.1568 35.7442 920 1.30 0.68 1.6 15.8 – 9.69 20.0 45.0 – 1 BDCSR29 117.1608 35.7440 1000 2.28 0.67 7.0 35.4 – 5.39 56.2 45.4 5.4 5 BDCSR30 117.1638 35.7436 1100 4.88 0.73 5.1 16.1 – 3.25 37.6 50.1 4.2 5 BDCSR-31 117.2270 35.7581 800 1.20 0.66 20.0 95.3 – 4.66 31.1 8.8 1.2 3 BDCSR-32 117.2196 35.7571 900 0.50 0.54 20.2 48.8 – 3.07 13.6 6.8 1.4 2 BDCSR33 117.2161 35.7558 1010 0.83 0.61 25.9 62.6 – 2.50 18.0 6.0 0.9 3 BDCSR-35 117.2094 35.7495 1200 1.57 0.61 23.2 57.2 – 2.54 33.4 12.4 4.0 3 BDCSR36 117.2010 35.7488 1303 0.90 0.63 26.0 61.7 – 2.37 42.3 13.6 – 1 BDCSR-37 117.2332 35.7561 710 3.47 0.69 20.8 79.1 – 3.81 76.2 23.7 6.0 3 BDCSR38 117.1767 35.7439 1400 0.65 0.58 26.8 83.9 – 3.14 135.2 42.1 7.4 2 BDCSR39 117.1866 35.7472 1440 1.10 0.60 28.2 111.6 – 3.92 140.0 34.5 6.5 6 BDCSR40 117.1936 35.7459 1320 0.60 0.56 36.6 125.6 – 3.27 138.0 27.8 6.0 6 Southern Transect (section C–C′) 12LC07 117.1985 35.6792 800 1.13 0.60 49.7 49.8 26.7 1.00 1.1 5.6 0.4 2 12LC08 117.1951 35.6804 850 1.41 0.61 27.7 35.0 28.7 1.27 0.6 5.2 0.5 4 12LC09 117.1913 35.6801 850 1.83 0.64 31.3 60.3 66.9 1.92 1.1 6.8 – 2 12LC11 117.1848 35.6797 900 1.64 0.63 57.7 41.9 59.1 0.67 1.5 6.2 0.7 3 12LC13 117.1745 35.6818 980 1.61 0.61 16.7 32.4 12.0 1.94 1.1 13.7 – 1 12LC14 117.1673 35.6802 1020 0.74 0.53 6.9 45.2 65.3 6.41 0.7 12.7 0.8 2 12LC16 117.1472 35.6843 1000 1.96 0.65 30.0 46.2 24.6 1.50 3.9 26.0 3.2 3 Note: Individual analyses are given in Tables DR1 and DR2 in the Supplemental File and on-line (see text footnote 1); dash—not measured. *Ft is the alpha ejection correction of Farley et al. (1996). †St. Dev.—standard deviation.

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TABLE 3. SUMMARY OF (U-TH)/HE ZIRCON DATA FOR THE SLATE RANGE Longitude Latitude Elevation Mass U Th Sm He Mean age St.Dev.† Sample (W) (N) (m) (µg) Ft* (ppm) (ppm) (ppm) Th/U (nmol/g) (Ma) (Ma) Replicates Southern Transect (section C–C′) 12LC01 117.2191 35.6778 675 6.12 0.77 659.4 253.5 5.7 0.39 151.8 50.1 5.8 3 12LC02 117.2159 35.6784 700 2.88 0.72 663.6 257.2 5.0 0.38 147.2 52.1 5.1 3 12LC03 117.2129 35.6787 700 4.79 0.75 771.4 225.8 1.0 0.30 149.2 44.6 8.4 3 12LC04 117.2092 35.6798 750 5.90 0.76 644.5 355.5 0.7 0.56 135.4 45.3 2.6 3 12LC07 117.1985 35.6792 800 6.00 0.76 1171.5 312.8 0.8 0.26 257.4 50.7 7.0 3 12LC09 117.1913 35.6801 850 14.93 0.83 433.1 146.5 0.6 0.34 113.8 54.3 0.0 2 12LC10 117.1880 35.6803 900 5.86 0.77 45.8 35.7 0.5 0.76 11.7 51.3 3.6 3 12LC11 117.1848 35.6797 900 4.72 0.75 367.8 125.3 0.6 0.34 81.6 51.0 2.0 3 12LC12 117.1812 35.6792 950 7.17 0.78 190.8 221.3 1.0 1.19 46.4 46.1 3.5 3 12LC15 117.1551 35.6823 1070 5.63 0.76 381.4 114.4 0.7 0.31 92.4 55.2 3.1 3 12LC16 117.1472 35.6843 1000 4.35 0.75 195.9 135.6 0.7 0.69 52.5 56.6 2.9 3 Note: Individual analyses are given in Tables DR1 and DR2 in the Supplemental File and on-line (see text footnote 1). *Ft is the alpha ejection correction of Farley et al. (1996). †St. Dev.—standard deviation.

Like the other transects, no systematic relationship samples are found within the hanging wall of the record rapid exhumation ca. 4 Ma. Based on between apparent age and elevation is observed fault (Fig. 3A). Clearly, any offsets from faults the proximity of these samples to the Manly (Table 2). Samples with the youngest cooling ages within the cross section must be accounted for Pass fault (Figs. 3A and 6A), these invariant come from the western part of the transect, east of in order to restore all of the samples to their (U-Th)/He ages are interpreted to be related the both the Searles Valley fault and Slate Range true preextensional paleodepths. Our initial to fault slip along that structure. The invari- detachment, ranging from 5.2 to 6.8 Ma (Table 2; interpretations ignored footwall faults, restoring ant ages indicate that fault slip was signifi cant Fig. 3C). Farther east, 2 samples give consistent samples to paleodepths relative to our projection enough to cool samples from just below the ages of ca. 13 Ma (Table 2; Fig. 3C). The oldest of the Miocene nonconformity. This yielded a zero retention isotherm (>~80 °C) to <40 °C. apparent age is from a sample located a few hun- clear trend of age with depth, but resulted in a We therefore interpret the Manly Pass segment dred meters below the projection of the Miocene somewhat jagged PRZ. We then made small of the Searles Valley fault zone to have initiated nonconformity (Table 2; Fig. 3C). adjustments to the paleodepths by using the ca. 4 Ma. Progressively older apparent ages at Zircon (U-Th)/He mean ages from the south- cooling ages to create a smooth PRZ to infer structurally shallower depths record residence ern transect are shown in Table 3. Only 11 of offset across these structures (Figs. 3A, 6A, and in the apatite He PRZ prior to early Pliocene the 14 samples had useable zircon grains. The 7). Our interpretation based on cooling ages is exhumation (Fig. 7). Exhumation and cooling mean ages range from as young as 44.6 Ma to that the western 2 faults have ~125 and ~150 m directly attributed to the earlier Slate Range as old as 56.6 Ma; most samples cluster around of down-to-the-west throw, and the eastern fault detachment is not observed in the data. The pro- 51 Ma. No systematic relationship between the has ~300 m of down-to-the-east throw at the jected north-northwest strike of the Slate Range age of samples and their elevation was observed eastern fl ank of the northern-central Slate Range detachment and structural reconstructions sug- in the data. (Fig. 3A); therefore, the down-to-the-east and gests that the breakaway for this fault was west down-to-the-west components are essentially or above most samples in this transect (Fig. 6A). INTERPRETATION OF THE (U-Th)/He the same. This means that these faults have little This may indicate that the effect of motion on THERMOCHRONOLOGIC DATA effect on our depth estimates in the western part this structure was not in a position to cause a of the transect, and the relatively minor offsets signifi cant change in the thermal structure. Plotting the apparent (U-Th)/He ages against restored on these structures do not quantitatively Based on our paleodepth reconstructions, the paleodepth provides a basis for interpreting the affect our conclusions. However, samples in the relative position of the 80 °C isotherm is con- cooling and exhumation history of the Slate hanging wall of the Manly Pass fault are par- strained to be between 1.8 and 2.0 by samples Range with respect to the mapped faults and ticularly important, as they can be used to con- 6 (shallowest Pliocene sample) and 7 (deepest faulting history of the area (Fig. 6). It is impor- strain Pliocene slip on that structure and the top Miocene sample). Although the relative position tant to note that the paleodepth interpretations of the Pliocene PRZ. These samples (1, 2, 3, and is tightly determined, our structural restoration come from basic structural mapping of the area, 8) were restored by adjusting the depths of the (using all estimated errors) places the preex- whereas the temperature and PRZ interpretations sample until they intersected the extrapolated tensional depth of the 80 °C isotherm at 1.8 ± come solely from the (U-Th)/He age data. Thus, trend of data from the footwall of the Manly 1.5 km. Using an assumed mean annual surface the two data sets are independent, although we Pass fault, through the Pliocene PRZ (Fig. 7). temperature of 10 ± 5 °C (Stockli et al., 2000), use the apparent age data to make some adjust- After restoring the samples to their the preextensional geothermal gradient is 40 ± ments to the depths in the northern profi le. paleodepths, the apatite (U-Th)/He data from 30 °C/km. The uncertainty based on absolute the northern Slate Range exhibit systematic paleodepths is obviously quite large, but it is the Northern Transect apparent age versus paleodepth patterns indica- most precise estimate that can be obtained with tive of rapid footwall exhumation of a shal- the available structural data. The northern transect is somewhat compli- low crustal block (House et al., 1999; Reiners The ~40 °C/km geothermal gradient is cated in that three mapped faults cross or project et al., 2000; Armstrong et al., 2003; Stockli, elevated compared to average continental geo- into the footwall of the Searles Valley fault zone 2005; Stockli et al., 2000, 2002). Apparent ages thermal gradients of ~25 °C/km; however, local (Fig. 3A). The amount slip on these structures decrease with increasing paleodepths from 51.8 volcanic activity supports high heat fl ow and is unknown. An additional complication for this to 4.3 Ma at ~2.3 km paleodepth (Fig. 7). Invari- an elevated geothermal gradient in the Late transect is that three of the oldest (U-Th)/He ant ages at the structurally deepest paleodepths Miocene and Pliocene Slate Range (Andrew and

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0.0 are somewhat simpler than for the northern one Basal Miocene nonconformity because almost all of the samples reside within the minimally faulted footwall to the Searles Unreset Valley fault zone and Slate Range detachment 0.5 (Figs. 3, 6B, and 6C). However, interpretation 8 of the reconstructed cooling ages is signiﬁ cantly more complicated. Zircon (U-Th)/He ages from Uncertainty ~40°C isotherm the southern transect cluster at ca. 51 Ma, sug- 1.0 gesting that there was an exhumation event at 26 1 2 that time (Fig. 8; Table 2). While this exhumation event is signiﬁ cantly older than the Mio- 51 4849 Pliocene cene and Pliocene fault history, this age is an 1.5 50 PRZ important constraint for interpreting the apatite 52 data (see following). 54 25 3 Apatite (U-Th)/He apparent ages increase 7 ~80°C isotherm Uncertainty with decreasing paleodepth from ca. 5 Ma to 2.0 ca. 51 Ma (Fig. 8). The oldest apatite samples 6 from the central transect (samples 27–30, 38, 5 Onset of rapid Pliocene cooling and 39) are similar in age to zircons from the 4 at ~4 Ma southern transect, suggesting that these rela-

Restored Middle Miocene Paleodepth (km) Restored 2.5 tively old apatite samples resided near the top of a PRZ, and are either unreset or only mini- Pliocene geothermal gradient mally reset. Below 1.0 km (depth of sample 39), = 40+/-30°C/km apatite ages get progressively younger. Figure 8 3.0 shows a possible interpretation of data, where 0 5 10 15 20 25 30 35 40 45 50 55 60 a PRZ is identifi ed between 0.4 and 2.5 km. (U-Th)/He Age (Ma) Below 2.5 km, (U-Th)/He ages form a cluster at ca. 6 Ma, suggesting rapid exhumation at that Figure 7. Apatite (U-Th)/He ages versus Middle Miocene paleodepth for the northern tran- time (Fig. 8). Using this paleodepth reconstruc- sect (A–A′). Error bars are 1σ standard deviations. Labeled sample numbers refer to Tables tion, the relative position of the 80 °C isotherm 2 and 3. Hachured area shows uncertainty in position of the 40 °C and 80 °C isotherms. is constrained to be between 2.2 and 2.5 km by sample 12LC11, the shallowest ca. 6 Ma sample, and sample 35, the deepest sample defi ni- tively within the PRZ. Using an assumed mean Walker, 2009). It is interesting that this value is is ~50° to the west in the northern-central Slate annual surface temperature of 10 ± 5 °C (Stockli similar to estimates of the geothermal gradient Range. This dip constrains the amount of heave et al., 2000), the calculated Late Miocene geo- in the Wassuk Range (Stockli et al., 2002) and on the fault over the past 4 m.y. to ~1 km. This thermal gradient is 30 ± 4 °C/km. to modern geothermal gradients of ~35 °C/km yields an average west-directed extension rate Although the pattern of cooling ages and cal- in the western Basin and Range province (e.g., of ~0.25 mm/yr since the early Pliocene on the culated geothermal gradient, using the above Lachenbruch and Sass, 1980). Unfortunately, basal fault of the system. This rate is the same interpretation, are broadly similar to those of the uncertainties in our data make drawing conclu- as the slip rate of 0.21–0.35 mm/yr inferred for northern transect (Fig. 7), it implies that exhu- sions about the thermal structure of the Slate the Searles Valley fault to the south by Numelin mation along Searles Valley fault zone initiated Range preextensional upper crust diffi cult. et al. (2007) over latest Pleistocene to Holocene at 6 Ma in the central and southern parts of the Using the cooling ages, we can estimate the time. Because that fault dips shallowly, it slip Slate Range, and at 4 Ma in the north. Fault ini- throw, fault slip, and west-directed heave or rate is approximately the same as the east-west tiation at 6 Ma is not improbable, because exten- extension on the Manly Pass segment over the extension rate. For this reason, it appears that the sion is known to have occurred in the Death Val- past ~4 m.y. Our principal estimate using sam- modern and time-averaged rates on the Searles ley area at that time (e.g., Topping, 1993; Snow ple 2 does not directly rely on paleodepth, but Valley fault zone are similar along its extent. It and Lux, 1999). However, Pliocene cooling and rather restoration of cooling ages in cross sec- is important to note that this estimate applies to fault histories are much more common for this tion. The dip slip estimated here is ~1.5 km. This the basal fault strand of the Searles Valley fault part of the Basin and Range (e.g., Hodges et al., is the amount of offset of sample 2 to restore it zone. There is signifi cant distributed faulting in 1989; Snyder and Hodges, 2000; Monastero to the appropriate age horizon of the PRZ. This the hanging wall of this structure (not sampled et al., 2002; Stockli et al., 2003; Andrew and estimate is similar to the ~2 km magnitude of by this estimate or that of Numelin et al., 2007) Walker, 2009; Lee et al., 2009). The interpre- the dip slip estimated along cross-section A–A′ that probably amounts to ~3.5 km of additional tation shown in Figure 8 also ignores a group by restoring the Middle Miocene rocks in the regional extension (Andrew and Walker, 2009). of samples between 1.7 and 2.2 km that cluster hanging wall along the fault to the projected between 12 and 14 Ma, coincident with the tim- Middle Miocene nonconformity in the footwall Central and Southern Transects ing of initiation for the Slate Range detachment (although this interpretation is complicated by (Andrew and Walker, 2009). For these reasons, uncertain effects of the Slate Range detach- Paleodepth reconstructions of (U-Th)/He we prefer an alternative interpretation of our ment). The present dip of the Manly Pass fault ages from the central and southern transects reconstructed samples, shown in Figure 9. In this

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continental geothermal gradients of ~25 °C/ km Central (Tank Canyon) Transect Apatite and signifi cantly higher than the ~15 °C/km Southern (Layton Canyon) Transect Apatite geothermal gradient documented for the nearby Inyo (Lee et al., 2009) and White Mountains Southern (Layton Canyon) Transect Zircon (Stockli et al., 2003). Although local volcanic activity is consistent with relatively high heat 0.0 fl ow and an elevated geothermal gradient dur- Basal Miocene nonconformity 27 ing the Miocene in the Slate Range (Andrew 28 and Walker, 2009), the high magnitude of the 0.5 29 30 ~40°C isotherm geothermal gradient suggests that there may be Uncertainty 38 a problem with our absolute paleodepths. This 12LC16 12LC16 1.0 is confi rmed when evaluating the position of the Late Miocene 39 12LC15 40 °C isotherm in both Figures 8 and 9 at 0.4 km, 40 which is an unrealistically shallow depth. This 1.5 PRZ suggests there was additional overburden in the

12LC14 36 central and southern parts of the Slate Range not accounted for in our reconstructions. Miocene 2.0 12LC13 12LC12 sections to the east in the Panamint Range are 35 ~80°C isotherm typically 500 m thicker; if this is applied to the central and southern transects, then the 40 °C 2.5 12LC11 12LC11 33 isotherm would have been at a more reasonable 32 12LC10 12LC09 12LC09 depth of ~1 km. 3.0 12LC08 12LC07 Extension across the central and southern 12LC07 transects is somewhat larger; we see the effects of both the Miocene and Pliocene to recent fault 31 3.5 Rapid cooling systems. In the Layton Canyon area, we estimate

Restored Middle Miocene Paleodepth (km) Restored at ~6 Ma 12LC04 a minimum of ~9 km of east-west extension; 4.0 12LC03 4 km of this is from the Slate Range detachment 12LC02 (the distance between the westernmost klippe 12LC01 and its likely footwall equivalent to the east) 4.5 and 5 km is from the Searles Valley fault (see 0 5 10 15 20 25 30 35 40 45 50 55 60 Andrew and Walker, 2009). A signifi cant com- (U-Th)/He Age (Ma) ponent of the apparent horizontal displacement across the central Slate Range results from the Figure 8. (U-Th)/He ages versus Middle Miocene paleodepth for the central (B–B′) and post-Miocene tilting of a moderately dipping southern (C–C′) transects. Error bars are 1σ standard deviations. Labeled sample numbers Slate Range detachment fault to low dips. This refer to Tables 2 and 3. Hachured area shows uncertainty in position of the 40 °C and 80 °C tilting appears to have occurred during initiation isotherms. PRZ—partial retention zone. of the SVFZ; it occurred after the slip on the Slate Range detachment and before 4 Ma, the age of untilted basalt fl ows in the nearby Argus Range (Andrew and Walker, 2009). If there is interpretation, two PRZs are identifi ed from the to be the most reliable time of initiation of fault- a general westward-moving wave of the initia- data. The fi rst is evident between 0.4 and 1.6 km ing. For the central transect, motion on the fault tion of transtensional deformation, then exten- (Fig. 9). Below 1.6 km, ages cluster between was probably not suffi cient to expose rocks that sional faults in the Death Valley extensional 12 and 14 Ma, recording an exhumation event were originally below the PRZ for the Plio- system may have been active before transten- at that time (Fig. 9). We interpret these samples cene (Figs. 3, 6, and 9). In contrast, paleodepth sion began in the Slate Range (ca. 11 Ma; Snow to record cooling related to motion on the Slate reconstructions of the data from the south- and Lux, 1999; ca. 8 Ma; Topping, 1993). The Range detachment (consistent with the work ern transect suggest that samples 12LC03 and Death Valley extensional fault system dips to the of Andrew and Walker, 2009). While these 12LC04 resided below the Pliocene PRZ (Fig. west beneath the Panamint Range and probably samples were exhumed and cooled through the 6C). Unfortunately this could not be confi rmed also below the Slate Range; therefore, displace- apatite He PRZ in the Miocene, structurally because inclusion-free apatite grains were not ment on these systems could have led to similar deeper samples yield younger apparent ages identifi ed within these samples. amounts and orientations of tilted Miocene units (between ca. 5 and 9 Ma). Although the data Structural restoration using our preferred of the hanging walls of the Panamint and Slate are somewhat clustered, they show inconsistent interpretation places the preextensional depth ranges before transtensional deformation began age to paleodepth scatter. We therefore interpret of the Middle Miocene 80 °C isotherm at 1.6 ± in this area. these rocks to have resided in a second PRZ dur- 0.2 km (Fig. 8). Based on an assumed mean ing Late Miocene to early Pliocene time; they annual surface temperature of 10 ± 5 °C (Stockli DISCUSSION were then exhumed and cooled during Pliocene et al., 2000), the preextensional geothermal gra- motion on the Searles Valley fault. We accept dient is 45 ± 9 °C/km. The 45 °C/km geother- Our age of ca. 4 Ma for cooling of the Sear- the ca. 4 Ma age from the Manly Pass segment mal gradient is elevated compared to average les Valley fault zone footwall rocks provides an

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continued northward through the Towne Pass Central (Tank Canyon) Transect Apatite area between the Panamint and Cottonwood Southern (Layton Canyon) Transect Apatite mountains. Although all these scenarios are possible, we favor the last for several reasons. Southern (Layton Canyon) Transect Zircon First, the errors seem reasonably computed and 0.0 the ages are reproducible. Second, other studies Basal Miocene nonconformity show that rapid cooling is associated with fault 27 28 initiation (see Stockli, 2005). Third, there is evi- 0.5 ~40°C isotherm 29 30 dence for signiﬁ cant deformation in the Towne Uncertainty Pass area at the time motion began on the Sear- 38 12LC16 12LC16 les system. Snyder and Hodges (2000) reported 1.0 Miocene PRZ 39 12LC15 a large increase in the sedimentation rate for the 40 Nova basin starting ca. 4 Ma and continuing to ca. 3 Ma; they also reported that faulting in the 1.5 ~80°C isotherm 12LC14 area is distributed in a complex manner between 36 Rapid cooling the Emigrant detachment and other structures 2.0 12LC13 at ~14 Ma over this time interval. Thus, it is our interpre- 12LC12 35 tation that the Panamint-Searles system initi- ~40°C isotherm ated ca. 4 Ma, and that for the ﬁ rst 1 m.y. of its 2.5 12LC11 12LC11 33 history fed deformation northward through the 32 Pliocene 12LC10 Emigrant and Towne Pass systems. This ceased 12LC09 12LC09 3.0 12LC08 PRZ ca. 3 Ma when deformation in northern Pana- 12LC07 12LC07 mint Valley was transferred northwestward into ~80°C isotherm Saline Valley via the Hunter Mountain fault. 3.5 31 If these interpretations are correct for the timing of extension and the initiation of transten-

Restored Middle Miocene Paleodepth (km) Restored 12LC04 sion in the Slate Range, then regionally there 4.0 12LC03 12LC02 may not be a change from local extension to 12LC01 transtension, but instead there is a spatial wave of the local initiation of major transtensional 4.5 0 5 10 15 20 25 30 35 40 45 50 55 60 deformation structures that overprint an earlier discrete east-west extensional event, active only (U-Th)/He Age (Ma) in the Middle to Late (?) Miocene. This idea may resolve the issue of comparing timing data Figure 9. Preferred Middle Miocene paleodepth reconstruction for the central (B–B′) and derived from fault studies versus from plate tec- southern (C–C′) transects. Error bars are 1σ standard deviations. Labeled sample numbers tonic data. The plate tectonic data indicate that refer to Tables 2 and 3. Hachured area shows uncertainty in position of the 40 °C and 80 °C transtensional deformation should have begun isotherms. PRZ—partial retention zone. at 8–10 Ma (Atwater and Stock, 1998) after a period of east-west extension. Transtension in Death Valley seems to have begun at this time important regional constraint on the timing of Saline Valley (e.g., Burchfi el et al., 1987; Lee (Wernicke et al., 1988; Topping, 1993; Snow transition to transtensional deformation. Com- et al., 2009), implying that the Panamint sys- and Lux, 1999), but areas to the west (i.e., the paring the work of Mahan et al. (2009) and tem started ca. 3 Ma (see Fig. 1 for structures Panamint and Slate Ranges) were not under- Monastero et al. (2002) with this study would and locations). Alternatively, in Walker et al. going signifi cant deformation until ca. 4 Ma indicate that the onset of dextral transtensional (2005) the Searles Valley fault zone and Pana- (Hodges et al., 1989). Transtensional deforma- deformation was younger westward at this lati- mint Valley fault zones were interpreted to be tion then migrated westward in a series of jumps tude, from ca. 5 Ma in southern Nevada, to 4 Ma an integrated system of structures accomplish- to structures farther west (Hodges et al., 1989; in Searles Valley–southern Panamint Valley, to ing dextral transtensional deformation in the Andrew, 2005; Monastero et al., 2002). Trans- 3–2 Ma in Indian Wells Valley. These data there- region, implying a somewhat older age of initia- tensional faulting then began ca. 4 Ma in the fore support the idea (presented in Stockli et al., tion. This leads to several possible options for Slate Range by the initiation of the Searles Val- 2003) of a westward progression in initiation of resolving this difference in age. (1) Uncertain- ley fault zone. dextral transtension. ties on the ages interpreted from the analytical It is interesting that our ca. 4 Ma age is some- data are underestimated, and the ages would CONCLUSIONS what older than the 2.8 ± 0.7 Ma age for renewed overlap if larger and/or more reasonable errors motion on the eastern Inyo fault zone and start were assigned. (2) The rapid cooling evident Our new mapping and thermochronological of motion on the Hunter Mountain fault (Fig. in age-depth profi les does not record the onset data give important insights to the exhumation 1; Lee et al., 2009). This is signifi cant in that of faulting, but is imparted at somewhat differ- and extension of the Slate Range, California. motion on the Hunter Mountain fault is inter- ent times. (3) Slip along the Panamint-Searles The range is currently part of the eastern Cali- preted to transfer slip from the Panamint Valley system did not initially link along the Hunter fornia shear zone and Walker Lane belt and fault zone onto the eastern Inyo fault zone in Mountain fault into Saline Valley, but rather is undergoing transtension on the Searles

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Valley fault zone. Transtensional activity Jr. volume: Columbia, Maryland, Bellwether Publish- heating: Earth and Planetary Science Letters, v. 183, started ca. 4 Ma when footwall rocks of the ing, p. 393–420. p. 365–368, doi:10.1016/S0012-821X(00)00286-7. Axen, G.J., Grove, M., Stockli, D., Lovera, O.M., Roth- Johnson, B.K., 1957, Geology of a part of the Manly Peak Searles Valley fault zone cooled below the stein, D.A., Fletcher, J.M., Farley, K., and Abbott, Quadrangle, southern Panamint Range, California: apatite (U-Th)/He PRZ. P.L., 2000, Thermal evolution of Monte Blanco dome: University of California Publications in Geological Low-angle normal faulting during Gulf of Califor- Sciences Volume 30, 423 p. Apatite data from the central and southern nia rifting and late Eocene denudation of the eastern Lachenbruch, A.H., and Sass, J.H., 1980, Models of an Slate Range indicate that cooling started ca. 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Overall system, California; palinspastic evidence for low-angle along the western boundary of the Basin and Range geometry of a Neogene range-bounding fault: Journal Province and distribution of dextral fault slip rates extension during this phase was west directed, of Geophysical Research, v. 92, p. 10,422–10,426, across the eastern California shear zone: Tectonics, and has a minimum value of 4 km; this is the doi:10.1029/JB092iB10p10422. v. 28, TC1001, doi:10.1029/2008TC002295. offset distance between Miocene strata in the Davis, G.A., 1988, Enigmatic “older-over-younger” low- Lippolt, H.J., Leitz, M., Wernicke, R.S., and Hagedorn, B., angle faulting of Miocene age, central Owlshead 1994, (Uranium+thorium)/helium dating of apatite; hanging wall from the footwall nonconformity Mountains, California: Geological Society of America experience with samples from different geochemical beneath coeval rocks. Assuming that extension Abstracts with Programs, v. 20, no. 3, p. 154. environments: Chemical Geology, v. 112, p. 179–191, Davis, G.A., and Fleck, R.J., 1977, Chronology of Mio- is similar between the northern and central Slate doi:10.1016/0009-2541(94)90113-9. cene volcanic and structural events, central Owlshead Luckow, H.G., Pavlis, T.P., Serpa, L.F., Guest, B., Wagner, Range, this gives a combined total extension in Mountains, eastern San Bernardino County, California: D.L., Snee, L., Hensley, T., and Korjenkove, A., 2005, the range of ~9 km. Geological Society of America Abstracts with Pro- Late Cenozoic sedimentation and volcanism during grams, v. 9, p. 409. The timing of extension fi ts well with regional transtensional deformation in Wingate Wash and the Didericksen, B., 2005, Middle Miocene to recent faulting Owlshead Mountains, Death Valley: Earth-Science patterns. The beginning of transtensional defor- and exhumation of the central Slate Range, Eastern Reviews, v. 73, p. 177–219, doi:10.1016/j.earscirev mation is somewhat older than that in Indian California Shear Zone [M.S. thesis]: Lawrence, Uni- .2005.07.013. versity of Kansas, 104 p. Mahan, K.H., Guest, B., Wernicke, B., and Niemi, N.A., Wells Valley, but somewhat younger than in Dunne, G.C., and Walker, J.D., 2004, Structure and evo- 2009, Low-temperature thermochronologic constraints southern Nevada, suggesting that transtension is lution of the East Sierran thrust system, east cen- on the kinematic history and spatial extent of the East- tral California: Tectonics, v. 23, TC4012, doi:10 progressively younger westward. ern California shear zone: Geosphere, v. 5, p. 483–495, .1029/2002TC001478. doi:10.1130/GES00226.1. Ehlers, T.A., Willett, S.D., Armstrong, P.A., and Chap- McQuarrie, N., and Wernicke, B., 2005, An animated tec- ACKNOWLEDGMENTS man, D.S., 2003, Exhumation of the Central Wasatch tonic reconstruction of southwestern North America Mountains, Utah: 2. Thermo-kinematics of exhu- since 36 Ma: Geosphere, v. 1, p. 147–172, doi:10.1130/ mation, erosion and thermochronometer interpreta- This study was partially supported by grants GES00016.1. tion: Journal of Geophysical Research, v. 108, 2173, awarded to Walker from the Geothermal Program Monastero, F.C., Walker, J.D., Katzenstein, A.M., and doi:10.1029/2001JB001723. Offi ce at China Lake Naval Weapons Station and the Sabin, A.E., 2002, Neogene evolution of the Indian Farley, K.A., 2000, Helium diffusion from apatite: General Wells Valley, east-central California, in Glazner, A.F., EarthScope Program of the National Science Foun- behavior as illustrated by Durango fl uorapatite: Jour- et al., eds., Geologic evolution of the Mojave Desert dation. We are indebted to numerous researchers for nal of Geophysical Research, v. 105, p. 2903–2914, and southwestern Basin and Range: Geological Soci- discussions on the geology of the Panamint Valley doi:10.1029/1999JB900348. region, including George Dunne, Frank Monastero, Farley, K.A., and Stockli, D.F., 2002, (U-Th)/He dating of ety of America Memoir 195, p. 199–228, doi:10.1130/ 0-8137-1195-9.199. Jon Spencer, Eric Kirby, and Marek Cichanski. 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