Thermochronometrically constrained anatomy and evolution of a Miocene extensional accommodation zone and tilt domain boundary: The southern Wassuk Range, Nevada

Authors


Corresponding author: K. E. Gorynski, Department of Geological Sciences, University of Kansas, 1475 Jayhawk Blvd., Lawrence, KS 66044, USA. (kyle.gorynski@encana.com)

Abstract

[1] Apatite (AHe) and Zircon (ZHe) (U-Th)/He thermochronometric data from the southern Wassuk Range (WR) coupled with 40Ar/39Ar age data from the overlying tilted Tertiary section are used to constrain the thermal evolution of an extensional accommodation zone and tilt-domain boundary. AHe and ZHe data record two episodes of rapid cooling related to the tectonic exhumation of the WR fault block beginning at ~15 and ~4 Ma. Extension was accommodated through fault-block rotation and variably tilted the southern WR to the west from ~60°–70° in the central WR to ~15°–35° in the southernmost WR and Pine Grove Hills, and minimal tilting in the Anchorite Hills and along the Mina Deflection to the south. Middle Miocene geothermal gradient estimates record heating immediately prior to large-magnitude extension that was likely coeval with the extrusion of the Lincoln Flat andesite at ~14.8 Ma. Geothermal gradients increase from ~19° ± 4°C/km to ≥ 65° ± 20°C/km toward the Mina Deflection, suggesting that it was the focus of Middle Miocene arc magmatism in the upper crust. The decreasing thickness of tilt blocks toward the south resulted from a shallowing brittle/ductile transition zone. Postmagmatic Middle Miocene extension and fault-block advection were focused in the northern and central WR and coincidentally moderated the large lateral thermal gradient within the uppermost crust.

1 Introduction

[2] The onset of large-magnitude extension in the Basin and Range province was intimately associated with Cenozoic magmatism. Major igneous activity often immediately precedes major extension [Gans et al., 1989; Gans and Bohrson, 1998]. The uninterrupted progression of volcanism to extension suggests that upper-crustal deformation is partly a response to its thermal weakening during magmatic advection [Kusznir and Park, 1987; Stockli et al., 2002; Surpless et al., 2002]. Magmatism is also spatially and temporally related to the formation of tilt-domain boundaries and accommodation zones between extended terrains, either preceding their formation or occupying permeable fault and fracture conduits [Aldrich, 1986; Axen, 1998; Faulds and Varga, 1998; Rowley, 1998; Stewart, 1998]. These relationships are well expressed throughout the dextral Walker Lane belt where Miocene volcanism [e.g., Christiansen et al., 1992] peaked immediately prior to Middle Miocene extension [e.g., Dilles and Gans, 1995; Stockli et al., 2002; Surpless et al., 2002], and spatially overlaps with strike- and oblique-slip transfer zones [Stewart, 1998]. The Walker Lane belt has the added complication that there were major crustal boundaries/preexisting structures present in the area that correspond to, and may have influenced the formation of accommodation zones [Oldow, 1992]. We explore in this paper specifically the thermal evolution of a Middle Miocene accommodation zone, south of the Wassuk Range (WR) of the Walker Lane belt. Focusing on preexisting weaknesses in the crust, accommodation zones may play a major role in the localization of magmatism, volcanism and heating prior to extension and subsequently influence the degree of syn-extensional heat transfer by controlling the architecture and structural evolution of extended terrains.

2 Geologic and Tectonic History

2.1 The Central Walker Lane Belt

[3] The central Walker Lane belt [Stewart, 1980, 1988, 1992] is a northwest-trending belt of northwest-striking, Cenozoic-age, strike-slip, transtensional and extensional structures that lies between the highly extended Basin and Range Province to the east and the stable Sierra Nevada block to the west (Figure 1). The belt is developed in a complex geologic collage of Mesozoic to Miocene rocks that predate its initiation. Triassic-Lower Jurassic metasedimentary and metavolcanic rocks that were intruded by Middle Jurassic-Cretaceous granitic plutons and silicic volcanic rocks [Dilles and Wright, 1988; Dilles and Gans, 1995] underlie much of the area. Late Cretaceous-Paleogene erosion resulted in the formation of a prominent, laterally continuous unconformity (ca. 40 Ma) across Mesozoic intrusive and associated wall rocks throughout western Nevada and eastern California [Van Buer et al., 2009]. Throughout the Walker Lane belt, a sequence of Cenozoic volcanic, tuffaceous, and siliciclastic rocks overlie the Paleogene nonconformity. The basal nonconformity is overlain by Oligocene rhyolite ignimbrites (e.g., Mickey Pass Tuff and Singatse Tuff) and Middle Miocene andesites (e.g., Lincoln Flat, Kate Peak, and Alta lithologies) [Thompson, 1956; Proffett and Proffett, 1976; Dilles and Gans, 1995]. Oligocene ignimbrites show large variations in thickness and are generally confined to paleovalleys [Henry, 2008]. Middle Miocene andesites commonly overlie the Oligocene ignimbrites and nonconformably overlie the basement where the ignimbrites are absent. Miocene, siliciclastic rocks (e.g., Wassuk Group) deposited within fault-bounded basins overlie Middle Miocene andesites [Axelrod, 1956; Gilbert and Reynolds, 1973]. Gently tilted Late Miocene basaltic andesites overlie, often in slight angular unconformity, the sedimentary rocks of the Wassuk Group [Gilbert and Reynolds, 1973; Dilles and Gans, 1995].

Figure 1.

Major late Cenozoic faults (modified after Stewart et al. [1977], Oldow [1992], Reheis and Sawyer [1997], Stockli et al. [2003]) for the central Walker Lane belt. The west-tilted WR footwall and the east-tilted White Mountains footwall are separated by the east-west trending faults of the Mina Deflection. Insert map shows the location of the study area with respect to the pre-Cenozoic crustal boundary defined by the 87Sr/86Sr 0.706 line [Kistler and Peterman, 1973; Kistler et al., 1991].

[4] Structurally, the central Walker Lane belt is characterized by coeval and kinematically linked northerly striking normal, northeast-striking sinistral, and northwest-striking dextral-slip faults (Figure 1) [Stewart, 1988; Hardyman and Oldow, 1991, Oldow, 1992; Stewart, 1998; Wesnousky, 2005a, 2005b]. Two distinct phases of Cenozoic deformation are recognized throughout the central Walker Lane belt and involve the episodic occupation and reactivation of Cenozoic structures [e.g., Dilles and Gans, 1995; Stockli et al., 2002; Surpless et al., 2002; Stockli et al., 2003]. The first is faulting that began between 15 and 10 Ma [Hardyman and Oldow, 1991] and resulted from east-west directed back-arc extension [Stewart, 1998]. In the Basin and Range Province, moderate- to large-magnitude extension during the Middle to Late Miocene was mostly accommodated through slip on low-angle detachments, but in the central Walker Lane belt deformation was accommodated by large degrees of fault-block rotation (40–70°) along high-angle normal faults [e.g., Proffett, 1977; Dilles and Gans, 1995; Surpless et al., 2002]. Strike-slip faults that initiated during this phase are interpreted as transfer and accommodation structures because their orientation is orthogonal to the regional structural grain and roughly parallel to the Middle to Late Miocene extension direction [Liggett and Childs, 1977; Guth, 1981; Wernicke et al., 1984; Stewart, 1998].

[5] A second episode of deformation began in the Late Miocene-Pliocene, which was the result of far-field stresses along the Pacific Plate margin [Atwater and Stock, 1998; Faulds and Henry, 2008] and resulted in the formation of a belt of northwest-oriented strike-slip and transtensional faults. A prominent right step occurs in the Walker Lane belt in the Excelsior region (Figure 1) and is marked by a number of east-northeast-trending sinistral-slip faults. This step that corresponds with the Mina Deflection [Ryall and Priestley, 1975], transfers displacement between misaligned northwest-striking transcurrent faults to its north and south [Oldow, 1992; Oldow et al., 1994; Stockli et al., 2003; Oldow et al., 2008]. The Mina Deflection forms a dextral antithetic accommodation zone between oppositely dipping fault blocks of the WR and White Mountains to its north and south, respectively [Stockli et al., 2003; Tincher and Stockli, 2009] (Figure 1), and was established by the middle Pliocene.

2.2 Wassuk Range and Pine Grove Hills

[6] The WR is ~90 km long and is in the footwall of a north-south-striking, east-dipping normal fault (Figures 1 and 2). Extension of the range has exhumed thick sections (≤ 10 km) of granitic crust and overlying Tertiary cover. In the southern WR and Pine Grove Hills (PGH), granitic basement rocks are nonconformably overlain by the Middle Miocene Lincoln Flat andesite and Middle-Late Miocene sedimentary and volcanic rocks of the Wassuk Group, deposited in fault-bounded basins [Dilles and Gans, 1995; Surpless et al., 2002]. The extrusion of the Lincoln Flat andesite at 14.8–15.1 Ma immediately precedes extension in the northern and central WR, and its attitude reflects the cumulative Neogene tilting of the WR footwall [e.g., Dilles and Gans, 1995; Stockli et al., 2002; Surpless et al., 2002].

Figure 2.

Simplified geologic map of the southern WR and Anchorite Hills, showing west tilted blocks of Mesozoic basement and overlying Tertiary cover. Four tilt blocks are recognized in the southern WR and are defined as regions of consistent dip along strike. (U-Th)/He sample locations are shown and sampled the three northernmost tilt blocks, and the PGH to the west. Cross-section lines (A-A′, B-B′, C-C′) are shown for Figure 6.

[7] The WR underwent two episodes of extension and rapid exhumation as recorded by apatite fission track and apatite (AHe) and zircon (ZHe) (U-Th)/He thermochronologic data from its eastern flank [Stockli et al., 2002; Surpless et al., 2002; Stockli, 2005; Krugh, 2008]. Extension and exhumation in the northern and central WR at ~12 to 15 Ma was accommodated along east-dipping, high-angle normal faults, which rotated the WR crustal blocks and associated faults to the west (< 60°) [Dilles and Gans, 1995; Surpless, 1999; Stockli et al., 2002; Surpless et al., 2002]. Middle Miocene geothermal gradient estimates of 27° ± 5°C/km from the central WR are higher than those of the Sierra Nevada (< 15°–20°C/km) [Stockli, 1999] to which the WR was attached, and indicate an increase in the geothermal gradient immediately prior to Middle Miocene extension [Stockli et al., 2002; Surpless et al., 2002]. A second episode of exhumation initiated at ~4 Ma and is related to active, high-angle faulting along the present-day range front in the central WR [Stockli et al., 2002; Surpless et al., 2002], and transtensional deformation at the southernmost WR at the Whisky Flat pull-apart structure [Stockli et al., 2003; Krugh, 2008]. Mio-Pliocene deformation in the WR is synchronous with the development of pull-apart structures at the northern end of the White Mountains [Stockli et al., 2003; Tincher and Stockli, 2009] and normal and transcurrent faults in the Mina Deflection [Stewart, 1988; Hardyman and Oldow, 1991; Oldow, 1992; Oldow et al., 2001], and correlate with a period of structural reorganization throughout the central Walker Lane belt [Oldow et al., 2008].

3 Methodology

3.1 Tilting Histories

[8] Mapping of the pre- and syn-extensional Tertiary cover sequence of the WR constrains the Neogene deformation and tilting [e.g., Gilbert and Reynolds, 1973; Proffett, 1977; McIntyre, 1990; Dilles and Gans, 1995; Surpless et al., 2002]. The most complete history of extension and fault-block rotation is recorded by differentially tilted Middle Miocene andesites and younger sedimentary and volcanic rocks of the Wassuk Group, preserved in fault-bounded basins west of the WR. Two basins are defined by the present-day Coal and Fletcher valleys and lie immediately west of the southern WR. These basins are bounded to their west by east-dipping, high-angle normal faults along the range front of the PGH (Figure 2). Although Quaternary sediments cover much of the Tertiary section in Fletcher Valley, a rather continuous and folded Tertiary section is preserved in Coal Valley. This area was previously mapped by Gilbert and Reynolds [1973], who focused on the distribution of Middle Miocene andesites (i.e., Lincoln Flat), and the Aldrich Station, Coal Valley and Morgan Ranch Formations of the Wassuk Group. To better bracket the temporal and lateral variability of tilting, our mapping further divided the Wassuk Group into thinner, continuous, and more easily recognizable packages of sedimentary and volcanic rocks, and their distributions were mapped along the western flank of the southern WR (Figures 3 and 4).

Figure 3.

Geologic map of the Mt. Grant and Coryville blocks from the southern WR, and the nonconformably overlying tilted Tertiary section in the Coal and Fletcher Valleys to their west. The variably tilted Tertiary section in Coal Valley, west of the Mt. Grant block, was used to constrain the timing and magnitude of Middle Miocene tilting related to WR footwall rotation. Range-front faults to the east of the WR footwall were modified after Hinz et al. [2010] and illustrate a number of Pleistocene transtensional faults that occupy a prominent right en-echelon step in the range front, and tilt-block boundary. The tilt-block boundaries are defined by northeast-striking scissor faults (SF) that accommodated differential tilting between the Mt. Grant and Coryville blocks, and the Coryville and Lucky Boy blocks (off the map). A shallow, east-dipping, scoop-shaped normal fault (NF) on the western flank of the Mt. Grant block repeats the Mesozoic and Tertiary section.

Figure 4.

Geologic map of Tertiary sedimentary and volcanic rocks in Coal Valley between the southern WR and PGH footwalls. Tertiary units dip toward the west and record the progressive tilting of the southern WR footwall and are used to calculate the pre-extensional paleodepths for footwall AHe and ZHe samples. A transect of samples were collected for 40Ar/39Ar dating and are shown on the map.

3.2 (U-Th)/He Thermochronology

[9] (U-Th)/He thermochronometry is a widely applied thermochronometric technique and has been utilized in a number of studies of extensional terrains [e.g., Stockli, 2005, and references therein] to determine the onset of rapid fault-block exhumation related to large-magnitude extension and to evaluate the thermal state of the crust immediately prior to extension [e.g., Fitzgerald and Gleadow, 1990; Howard and Foster, 1996; Miller et al., 1999; Foster and John, 1999; Stockli et al., 2002; Surpless et al., 2002]. (U-Th)/He dating is based on the radioactive decay of 235U, 238U, 232Th, and 147Sm to radiogenic 4He, and the subsequent loss of 4He during thermally controlled diffusion. Because the diffusion domain is the crystal itself, the temperature at which 4He is lost is controlled primarily by mineralogy.

[10] Apatite helium (AHe) thermochronometry is the most utilized (U-Th)/He thermochronometer because of its well-defined diffusion kinetics [e.g., Farley, 2000] and applicability to the study of low-magnitude thermal events in the uppermost crust [e.g., Stockli et al., 2000]. Diffusion of 4He out of the apatite crystal is variable over a range of temperatures from ~40° to ~80°C, termed the helium partial retention zone (PRZ) [Wolf et al., 1996, 1998; House et al., 1999; Stockli et al., 2000], but is completely lost by diffusion at temperatures above ~80°C and completely retained at temperatures below ~40°C. The onset of exhumation is recorded by the rapid cooling of fault blocks through the AHe PRZ. AHe ages for samples that resided below the ~80°C isotherm prior to exhumation, and above the ~40°C isotherm after exhumation, now reflect the exhumation rate and timing. Cooling of the fault block below ~40°C preserves the fossil AHe PRZ. The fossil AHe PRZ can be recognized as a spectrum of ages that decrease with increasing paleodepth that are bounded by inflection points in an AHe age vs. paleodepth plot. A new PRZ develops below the fossil PRZ and its depth is related to the geothermal gradient. The preservation of multiple fossil PRZs in the exhumed fault blocks allows for the analysis of multiple exhumation events and changing geothermal gradients through time.

[11] The zircon helium (ZHe) thermochronometer, although less utilized, has been applied to a number of studies of extensional terrains [e.g., Reiners et al., 2000; Stockli, 2005; Bargnesi et al., 2012] and is important in constraining the thermal evolution of footwall blocks at temperatures between ~140° and ~200°C [Reiners et al., 2002, 2004; Wolfe and Stockli, 2010]. The ZHe PRZ is somewhat poorly defined because of the less well-behaved diffusion kinetics of zircon [Reiners et al., 2002, 2004], and the infrequent preservation of the ZHe PRZ that requires exhumation on the order of ~5–8 km, assuming a geothermal gradient of ~25°C/km and a surface temperature of 10° ± 5°C. Together the AHe and ZHe thermochronometers cover the thermal evolution of individual samples through a temperature range of ~40°–200°C and record thermal processes in the upper ~1–8 km of the crust, again assuming a geothermal gradient of ~25°C/km and a surface temperature of 10° ± 5°C [e.g., Stockli, 2005].

3.3 Pre-Extensional Paleodepth Estimates

[12] Placing thermochronological samples in an accurate paleodepth reference frame is imperative to our interpretation of the pre-extensional thermal state of the crust. In this study, we measure paleodepth by adding the structural depth, measured orthogonal to the Paleogene nonconformity, to the thickness of the pre-extensional overlying sedimentary and volcanic rocks. Structural depth is measured here by flattening the Paleogene nonconformity and then measuring downward from the overlying, westward-dipping, Lincoln Flat andesite [e.g., Stockli et al., 2002; Surpless et al., 2002; Stockli et al., 2003]. This method generates obvious uncertainties by assuming a lack of topography on the Paleogene nonconformity, and neglects tilting or folding resulting from deformation other than fault-block rotation. Nevertheless, topographic relief on the Paleogene nonconformity was likely minor as evident from its planar nature [e.g., Mancktelow and Grasemann, 1997], and would only significantly affect paleodepth estimates for samples at the shallowest structural levels [e.g., Stockli et al., 2002]. Finally, errors of ± 5° were added to westward-tilted fault blocks to account for uncertainties of measuring fault-block rotation.

[13] The pre-extensional overburden above the Paleogene nonconformity must also be calculated to determine a sample's paleodepth. The simplest way to determine overburden is by measuring the pre-extensional stratigraphic thickness above the nonconformity, which in the WR this is the thickness of Oligocene conglomerates and ignimbrites, and the overlying Miocene Lincoln Flat andesite. Overburden can also be estimated by projecting model-derived, geothermal gradient estimates up through the crustal section, and then calculating the vertical distance between a thermally defined surface at 10 ± 5°C and the Paleogene nonconformity [e.g., Stockli et al., 2002; Stockli, 2005].

3.4 Pre-Extensional Geothermal Gradient Estimates

[14] Coupling of a geochronometrically constrained tilting history with thermochronologic data provides an opportunity to study the structural and thermal evolution of the upper crust prior to and during extension [e.g., Howard and Foster, 1996; Foster and John, 1999; Stockli et al., 2002; Stockli, 2005]. Described below, this study applies the methods utilized by Stockli et al. [2002] and outlined by Stockli [2005] to estimate geothermal gradients.

[15] The AHe and ZHe PRZs are important in the formation of thermal markers in the crust, where their upper and lower boundaries are defined by isotherms and are recognized by inflections in AHe and ZHe vs. paleodepth plots. Estimates of the geothermal gradient are acquired by determining the temperature difference between the paleosurface and the observed inflection points for the ZHe (~140°C and ~200°C) and AHe (~40°C and ~80°C) PRZs. Similarly, the distance between the top and the bottom of the PRZs can be used to estimate the geothermal gradient, but errors are usually larger due to the small change in temperature and thinness of the PRZs in the isotherm perpendicular direction. Additional error in geothermal gradient estimates may arise from variability in 4He diffusion, and therefore the thermal definitions of the PRZs, due to varying grain sizes and cooling rates [e.g., Farley, 2000, 2002]. A second method for estimating the geothermal gradient uses inverse-model-derived time-temperature histories for individual samples to estimate their temperature at any given time. The distribution of sample temperatures with depth show a linear relationship, and the slope of a best-fit line, through these points, yields the geothermal gradient [e.g., Stockli et al., 2002; Stockli, 2005].

4 Structural Results

[16] The southern portion of the WR (Figures 1 and 2) is particularly important in our understanding of the structural and thermal characteristics of a terminating normal fault and tilt-domain boundary. In map view, the width of the southern WR lessens from ~20 km in the area south of Walker Lake to ~9 km in the Anchorite Hills, and is associated with a similar southward decrease in fault-block tilting (from > 60° to < 10°), and exposed paleodepths. Abrupt tilt discontinuities occur at northeast-striking scissor faults that gradually lessen tilting and topography of the southern WR, which itself terminates at a northeast-striking sinistral-slip fault along the northern Mina Deflection (Figures 1 and 2). Four tilt blocks are recognized in the southern WR and are defined by regions of consistent dip and are separated by northeast-striking scissor or sinistral-slip faults expressed as en-echelon right steps along the modern range-front fault system. Each of the tilt blocks are described in detail below and are, from north to south: the Mount Grant, Coryville, Lucky Boy and Anchorite Hills blocks [Gorynski, 2011] (Figure 2).

4.1 Mount Grant Block

[17] The Mt. Grant block exposes a ~9.5 km crustal section comprised primarily of Mesozoic quartz monzonite and Triassic to Jurassic metavolcanic rocks and is structurally similar to the central WR [e.g., Stockli et al., 2002; Surpless et al., 2002]. North-northwest-striking, high-angle, east-dipping normal faults run along the entire eastern flank of the Mt. Grant block (Figures 2 and 3). Mt. Grant is the highest point in the WR at almost 3.5 km in elevation and is topped by steeply, northwest-dipping (60°–70°), Miocene andesites, presumably the Lincoln Flat andesite, and deposits of the Wassuk Group (Figure 3). The overlying Tertiary units strike to the northeast and are oriented oblique to the north-northwest-trending range front; therefore, exposed paleodepths increase toward the southeast corner of the Mt. Grant block. A low-angle, scoop-shaped normal fault dips to the east and dissects the Mt. Grant block repeating the Mesozoic and Tertiary section to the west (Figure 3). In the Coal Valley, west of the Mt. Grant block, the entire Mesozoic through Tertiary section is preserved between the WR and PGH to the west (Figures 2 and 4) and has been mapped previously by Gilbert and Reynolds [1973] and Anderson et al. [2012].

[18] In Coal Valley, the entire Tertiary section has been folded into a gentle, west-northwest-trending syncline (Figure 4). Miocene andesites and Wassuk Group rocks are younger toward the west and their dips shallow up section. 18.6 ± 0.3 Ma tuffs [Gorynski, 2011] and ~14.8–15.1 Ma Hornblende andesites of Lincoln Flat [Gilbert and Reynolds, 1973] nonconformably overlie Mesozoic crystalline rocks of the southern WR and dip 60–75° to the west; where in faulted contact they locally dip as little as 17–35°. The Wassuk Group overlies the Lincoln Flat andesite and has dips ranging from ~5–60° to the west. Interbedded tuffs dated by 40Ar/39Ar from the Wassuk Group bracket much of the Middle Miocene WR tilting history (Table 1 and Figures 5 and 6). A tuff dipping ~10° to the west yielded a 40Ar/39Ar plateau age of 11.33 ± 0.07 Ma and overlies slightly older tuffs with 40Ar/39Ar plateau ages of 11.74 ± 0.07 Ma and 11.59 ± 0.09 Ma that dip ~35° to the west (Figures 4 and 6), thereby constraining much of the Miocene tilting to the interval from ~11 to 15 Ma. An abrupt increase in dip occurs up section where 30–50° dipping andesitic and tuffaceous sandstones from the Morgan Ranch Formation appear to be obliquely thrusted to the east. Farther to the west a north-south-trending, east-dipping normal fault juxtaposes the uppermost Wassuk Group against Mesozoic granites of the PGH.

Table 1. Coal Valley 40Ar/39Ar Age Data
IDLaser Watts40Ar39Ar38Ar37Ar36ArCa/K%39Ar%40ArAge (Ma)±1σ
  1. Samples were irradiated for 20 h using Cadmium shielding at the USGS TRIGA reactor in Denver, CO.

  2. Sanidine from the Fish Canyon Tuff was used as the neutrol fluence monitor with a reference age of 28.201 Ma [Kuiper et al., 2008]. Decay constants and isotopic abundances after Min et al. [2000].

09WR97, Biotite, J ( X 10-3) ± 1 s = 4.385 ± 0.003; Integrated Age: 13.21 ± 0.08 Ma; Plateau Age: 13.05 ± 0.09 Ma
A0.1027.560.870.040.080.090.1790.51.43.392.73
B0.3016.264.500.080.140.030.0592.651.114.540.44
C0.5037.3416.990.270.270.020.0319.981.714.150.12
D0.7042.3923.740.360.120.010.01013.896.113.530.08
E0.9041.2823.320.360.070.010.00613.694.513.180.08
F1.1041.1423.650.360.030.010.00313.895.413.080.09
G1.3058.3433.450.520.040.010.00219.594.613.000.06
H1.5046.7426.850.410.010.010.00015.695.213.060.08
I1.7031.8818.240.280.010.010.00110.693.412.860.11
09WR98, Biotite, J ( X 10-3) ± 1 s = 4.385 ± 0.003; Integrated Age: 11.43 ± 0.09 Ma; Plateau Age: 11.59 ± 0.09 Ma
A0.1059.1916.870.270.090.130.0106.834.69.570.14
B0.3071.5414.570.250.350.170.0475.930.111.640.19
C0.4056.9914.200.230.580.120.0805.736.311.500.17
D0.4525.3010.000.150.110.030.0224.059.511.870.21
E0.5023.439.190.140.100.030.0213.759.011.870.28
F0.7037.5013.510.220.210.060.0315.452.311.450.17
G0.9046.1217.610.280.230.070.0257.154.811.310.13
H1.1057.8720.820.330.300.090.0288.452.711.560.11
I1.3068.4526.960.410.390.100.02910.958.511.700.09
J1.5056.3924.180.370.180.070.0149.862.411.480.09
K1.7058.9630.400.460.130.050.00912.376.411.690.08
L1.9051.5628.630.430.030.030.00211.683.011.780.08
M2.1019.8010.410.160.010.020.0014.274.611.180.18
N2.3015.786.780.110.010.020.0032.755.110.110.28
O2.507.043.700.050.000.000.0001.582.212.330.51
09WR99, Biotite, J ( X 10-3) ± 1 s = 4.385 ± 0.003; Integrated Age: 11.78 ± 0.07 Ma; Plateau Age: 11.78 ± 0.06 Ma
A0.1011.363.120.040.040.020.0221.157.616.520.61
B3.0018.6110.260.140.070.010.0143.783.511.950.18
C0.5031.6819.700.280.080.010.0087.193.611.870.09
D0.7036.0322.560.310.100.010.0088.193.411.760.08
E0.9038.8924.580.340.220.010.0178.892.911.580.08
F1.1035.6622.310.320.450.010.0398.093.111.730.08
G1.3033.3620.880.300.650.010.0617.592.311.640.09
H1.5037.9623.440.331.570.010.1318.491.811.730.08
I1.7041.9125.020.352.500.020.1969.089.611.830.08
J1.9042.9726.530.382.350.010.1749.590.111.510.07
K2.1043.1627.450.381.750.010.1259.993.211.550.07
L2.3032.4220.700.300.770.010.0727.495.111.750.09
M2.5024.5815.890.220.310.000.0385.7100.012.190.11
N3.0024.2415.850.220.100.000.0135.797.411.750.12
09WR100, Biotite, J ( X 10-3) ± 1 s = 4.385 ± 0.003; Integrated Age: 11.28 ± 0.07 Ma; Plateau Age: 11.33 ± 0.07 Ma
A0.108.023.820.050.060.010.0321.658.89.740.47
B0.3038.6424.200.330.170.010.01410.389.511.270.08
C0.4017.3010.600.160.230.010.0434.584.610.890.19
D0.5013.068.020.110.290.000.0713.489.211.460.23
E0.6014.068.890.130.240.010.0523.888.611.060.20
F0.7019.9212.580.180.410.010.0645.490.611.320.15
G0.8018.5511.930.160.280.000.0455.192.511.350.16
H0.9017.9511.470.160.340.000.0594.993.311.520.16
I1.0022.1414.100.200.580.010.0806.091.211.290.13
J1.2022.8714.350.200.520.010.0716.192.011.560.13
K1.4036.8323.150.321.770.010.1509.988.911.160.08
L1.60110.1170.300.995.670.030.15830.191.611.320.03
M2.0031.8120.310.291.960.010.1898.793.211.510.10
N2.500.410.220.000.000.000.0020.159.38.698.21
09WR104, Biotite, J ( X 10-3) ± 1 s = 4.349 ± 0.002; Integrated Age: 12.78 ± 0.07 Ma
A0.3020.1212.050.190.120.010.01917.386.011.240.12
B0.5015.8410.980.170.160.000.02815.8106.712.030.12
C0.7010.567.440.120.080.000.02110.799.911.090.19
D0.9010.407.350.120.110.000.03110.6101.111.190.18
E1.107.675.460.080.030.000.0097.8106.611.710.24
F1.306.284.480.070.100.000.0446.496.310.560.29
G1.5010.187.310.110.020.000.00510.5109.111.880.19
H1.708.886.350.100.000.000.0019.199.410.880.22
I2.006.154.390.070.000.000.0006.3110.412.110.32
J2.505.233.750.060.000.000.0005.4118.512.930.34
09WR105a, Biotite, J ( X 10-3) ± 1 s = 4.349 ± 0.004; Integrated Age: 17.9 ± 0.03 Ma; Plateau Age: 18.6 ± 0.03 Ma
A0.10102.1611.460.220.080.270.01416.821.715.220.28
B0.3064.9110.110.170.070.140.01314.835.217.800.24
C0.5061.6711.260.180.050.120.00916.542.318.240.23
D0.0017.962.630.050.030.040.0263.833.317.890.78
E0.9043.965.870.100.080.100.0288.632.619.190.41
F1.1050.286.530.110.180.120.0559.530.318.370.38
G1.3060.837.970.150.310.140.07711.631.118.700.33
H1.5093.0912.590.233.030.210.47218.432.218.740.23
09WR105b, Biotite, J ( X 10-3) ± 1 s = 4.349 ± 0.004; Integrated Age: 18.3 ± 0.04 Ma; Plateau Age: 18.6 ± 0.03 Ma
A0.0110.240.770.020.000.030.0002.114.114.851.86
B0.108.250.740.020.000.020.0002.017.515.401.83
C0.2013.241.470.030.000.030.0004.023.616.681.07
D0.309.721.240.020.020.020.0323.426.316.181.16
E0.5013.972.210.040.010.030.0086.136.918.350.70
F0.7513.852.480.040.030.030.0276.842.018.440.58
G1.0023.344.430.070.000.040.00212.144.818.590.37
H1.2516.583.190.050.000.030.0038.746.318.940.46
I1.6022.854.780.080.020.040.00813.149.118.470.33
J2.0026.584.400.070.090.060.03812.137.617.900.38
K2.3018.003.010.050.050.040.0308.341.719.590.52
L2.6011.502.140.040.110.020.1025.944.118.670.71
M3.009.792.050.030.150.020.1435.646.617.540.70
N3.506.541.540.020.140.010.1734.258.719.570.92
O4.005.251.270.020.080.010.1313.558.919.171.09
P4.503.150.740.010.020.000.0612.063.221.091.83
Figure 5.

Biotite 40Ar/39Ar release spectra for interbedded tuffs in the variably tilted Tertiary sections in Coal Valley west of the southern WR. Sample locations are shown in Figure 4. 40Ar/39Ar ages were used in collaboration with geologic mapping (Figure 4) to constrain the progressive tilting of the southern WR footwall.

Figure 6.

Simplified cross-sections through the Mt. Grant (A-A′), Coryville (B-B′), and Lucky Boy (C-C′) blocks of the southern WR (Figure 2). Dotted lines on the fault blocks denote the pre-extensional paleodepths. The WR fault blocks are west tilted and bounded along their eastern flank by high-angle normal faults. ~60°–70° of footwall rotation in the Mt. Grant block is represented by the similarly west-tilted Paleogene nonconformity and overlying 14.8–15.1 Ma Lincoln Flat andesite. Tilting of the Mt. Grant footwall is much greater than that of the Coryville (~35°) and Lucky Boy (<20°) blocks and therefore exhumed much greater Middle Miocene pre-extensional paleodepths. Range-front faults in the Coryville block have been modified after Hinz et al. [2010].

4.2 Coryville Block

[19] Neogene tilting and extension in the Coryville block exposed a ~8 km thick crustal section. The block is bounded to the north and south by northeast-striking scissor faults and to its east by north-northwest-striking, east-dipping normal faults (Figures 2, 3, and 6). The range front of the Coryville block is structurally more complicated than that of the Mt. Grant block and is defined by a complex network of normal and transtensional faults [e.g., Hinz et al., 2010]. Many of the range-front structures developed within a pull-apart zone [Bell and Hinz, 2010; Hinz et al., 2010; Moeck et al., 2010; Shoffner et al., 2010] at the northeastern corner of the Coryville block, marked by a ~1–2 km right step in the range front (Figure 3). Quaternary fault scarps along the range front penetrate into basement and gradually step down basement topography into the present-day Walker Lake basin (Figure 6).

[20] In Fletcher Valley, immediately west of the Coryville block, the Lincoln Flat andesite nonconformably overlies Mesozoic basement of the WR (Figures 2 and 3). The Lincoln Flat andesite dips consistently along the western flank of the Coryville block at ~35° to the west. Although Quaternary alluvium covers most of the Tertiary section in Fletcher Valley, calcareous siltstones from the lowermost Aldrich Station Formation (9.3–13 Ma) of the Wassuk Group [Axelrod, 1956; Gilbert and Reynolds, 1973] are exposed where they dip ~30–35° to the west and overlie the Lincoln Flat andesite. An east-dipping normal fault that dissects the Mt. Grant block to the north is projected into Fletcher Valley and likely offsets the Tertiary section.

4.3 Lucky Boy Block

[21] The Lucky Boy block is bounded to the north and south by northeast-striking scissor and sinistral-slip faults, respectively, and to east by high-angle, east-dipping normal faults. The < 4 km thick crustal section in the Lucky Boy block is primarily composed of Cretaceous granite and overlying Miocene andesite. The gently dipping (≤ 15°) Lincoln Flat andesite is found on top of a few peaks throughout the Lucky Boy block where it nonconformably overlies Mesozoic basement and also forms a ~3–9 km long dip slope along its northwestern flank (Figure 6) . Aside from the Lincoln Flat andesite and a few flat-lying Neogene-Quaternary basalt plugs and lava fields, the remaining Tertiary section is covered by Quaternary alluvium. In contrast to the Mt. Grant and Coryville blocks to the north, the strike of the range front of the Lucky Boy block varies by almost 90°. The strike of the northernmost Lucky Boy block range front matches that of the entire southern WR and is oriented north-northwest, but in the south turns ~90° clockwise over a distance of ~3–5 km (Figures 1 and 2). This smooth curvilinear fault trace forms the Pliocene-age Whisky Flat pull-apart structure, and demarks the northern extent of the Mina Deflection [Stockli et al., 2003]; it is the structural equivalent to the Queen Valley pull-apart structure (Figure 1) at the northern terminus of the White Mountains [e.g., Stockli et al., 2003; Tincher and Stockli, 2009].

4.4 Anchorite Hills Block

[22] The Anchorite Hills block is composed of Mesozoic granite nonconformably overlain by Miocene andesites (Lincoln Flat?), ignimbrites and siliciclastic rocks that are overlain in areas by Neogene-Quaternary basalts [Stewart, 1980; Stewart et al., 1984]. Tertiary volcanic and sedimentary rocks are variably inclined, and although predominantly dipping toward the west, their attitude appears to be controlled by local structures that are unrelated to WR tilting. The Anchorite Hills are bound to the south by the left-lateral Anchorite fault zone that strikes to the northeast and extends from along the eastern flank of the southern WR, through Anchorite Pass, and into a diffuse zone of fault splays and lineaments in the northern Mono Lake Basin [Wesnousky, 2005a]. The deepest structural levels are recognized as areas of exposed Mesozoic basement and are only present at the southeastern corner of the Anchorite Hills block. Thermochronometric data from Mesozoic intrusive rocks exposed along the Anchorite Hills range front record the Late Cretaceous-Paleogene denudation of Sierran intrusive and associated wall rocks, but contain no signal of Middle Miocene exhumation [Krugh, 2008].

4.5 Pine Grove Hills Block

[23] The PGH are a ~30 km long, northeast-striking range that lies immediately west of the Mt. Grant block. An east-dipping, high-angle normal fault along its eastern side has exhumed a ~2.5 km thick crustal section of Mesozoic basement and overlying Tertiary cover. Oligocene silicic ignimbrites and Miocene andesites dip ~5°–30° to the west [Gilbert and Reynolds, 1973], and appear to have been tilted prior to the deposition of the 12–8 Ma Wassuk Group [Dilles and Gans, 1995]. Despite their latitudinal contiguity, tilting between the PGH and Mt. Grant blocks are dissimilar and tilting of the PGH block more closely resembles that of the Coryville block.

5 Thermochronometric Results

[24] Samples were collected from the southern WR and PGH, and were taken from outcrops of unaltered and unweathered Mesozoic granite and metavolcanic rocks which yielded good, datable apatite and zircon crystals. Locations are shown on Figure 2 and data are given in Tables 2 and 3.

Table 2. Wassuk Range Footwall ZHe Sample Locations and Ages
SampleLatitudeLongitudeElevation (m)Age (Ma)+/– (8%)U (ppm)Th (ppm)Sm (ppm)He (nmol/g)aFTbESR
  • a

    Ft, 4He ejection correction factor.

  • b

    ESR, equivalent spherical radius.

08WR0138.4291–118.6311177065.35.24972332.0653.90.8385
08WR0238.4208–118.6204181259.74.84785761.2780.90.8481
08WR0338.3883–118.6319205612.71.04702130.612.30.7344
08WR0438.3898–118.6243201113.41.13133212.3335.80.89114
08WR0538.3909–118.6156194612.31.06881751.035.70.7852
08WR0638.3971–118.6071187018.81.54811761.8010.60.8060
08WR0738.4023–118.5995178766.95.44281601.1739.80.8267
08WR0838.5982–118.7513166625.22.04111651.1610.00.8382
08WR0938.6022–118.7480159739.13.14531951.749.70.7650
08WR1038.6019–118.7469155535.12.86212791.1414.50.7853
08WR1138.5837–118.7501194850.74.19653800.9319.90.7444
08WR1238.5897–118.7432188660.14.86473963.3935.00.7958
08WR1338.5937–118.7409188146.23.75512902.8919.80.7854
08WR1438.5952–118.7400178850.94.14942521.9618.80.7854
08WR1538.5969–118.7373170347.53.85252831.1261.70.8478
08WR1638.5923–118.7344159353.74.35503020.9530.10.7958
08WR1738.5734–118.7126140821.61.710284621.1133.00.8164
08WR1838.5466–118.7119150015.91.36622232.306.00.7750
08WR1938.5452–118.7012142412.61.010272691.397.00.7650
08WR2038.5441–118.6993141216.51.35322210.894.20.7548
08WR2238.5214–118.6795141476.36.14792251.5123.70.7853
08WR2338.5147–118.7025172862.35.08902870.6826.90.7547
08WR2438.5150–118.6979174358.34.79553260.9884.40.8372
08WR2538.5167–118.6986166459.64.86892531.0829.00.7853
08WR2638.5237–118.6927153468.55.519203382.8366.90.7750
08WR2738.5223–118.6850146972.85.86583331.1470.60.8164
08WR2938.4872–118.7419194927.02.23511140.7516.20.8170
08WR3038.4865–118.7287187320.11.6257770.762.80.7650
08WR3138.4866–118.7143181829.72.414057513.1114.20.7650
08WR3238.4826–118.7058174414.21.1269670.796.00.8165
08WR3338.4844–118.6980176518.01.4201772.821.70.7548
08WR3438.4826–118.6857165614.81.26853050.907.60.7854
08WR3538.4845–118.6739158721.51.7288770.607.40.7957
08WR3638.4842–118.6629152552.04.23591572.3221.80.8162
08WR3738.5685–118.791134428.80.7143152898.9217.40.7859
08WR3838.5608–118.7876331070.15.6241991.3618.20.7958
08WR4138.5464–118.8007295258.84.73091673.6212.10.7753
08WR4338.5081–118.8076283669.95.69125423.7253.50.7753
08WR4438.4992–118.8070283382.46.64933510.8328.60.7649
08WR4538.4902–118.8061290889.47.211166542.5543.20.7445
08WR4638.4841–118.8027293670.35.68034182.8071.50.8166
08WR4738.4788–118.8033284664.35.17273951.8139.80.7956
08WR4838.4801–118.7971272854.84.415575876.7760.00.7957
08WR4938.4766–118.7836241768.15.48783382.3233.60.7751
08WR5238.5693–118.7830302874.86.02151141.7911.10.7854
08WR5438.5723–118.7758279768.15.4120550.5710.30.8266
08WR5538.5764–118.7671253356.14.54483995.8118.60.7652
08WR5638.5773–118.7647245055.14.44713342.7512.10.7547
08WR5738.5799–118.7611230655.14.417047305.3850.20.7547
08WR5838.5818–118.7567215253.14.26634264.1023.00.7650
08WR5938.5860–118.7496197362.15.011464606.6064.70.7854
08WR6038.4407–118.7461234625.12.06462030.5814.20.8164
08WR6138.4460–118.7810311749.64.012236442.3959.60.7958
08WR6238.4617–118.7585297646.53.715497016.1449.20.7754
08WR6338.4597–118.7548284762.55.08953320.6751.70.8370
08WR6438.4545–118.7514265042.43.411353230.8528.30.7750
08WR6538.4544–118.7445255336.02.98833801.0532.10.8055
08WR6638.4540–118.7403244925.42.06932690.6328.70.8062
08WR6838.4600–118.7226225522.11.84381890.744.40.7547
08WR6938.4644–118.7120213616.71.39243330.5316.70.8059
08WR7038.5334–118.7138164655.44.47023892.6514.90.7344
08WR7138.5278–118.7086166258.84.73551820.7921.30.7957
08WR7238.5226–118.7041168853.74.33031241.1422.00.8167
08WR7338.5040–118.6854170346.03.75441650.6825.40.8162
08WR7438.5000–118.6866179734.02.75441650.6831.10.7958
08WR7538.4938–118.6782164628.22.3298800.265.80.7853
08WR7638.4589–118.6742179212.11.0301990.574.20.8163
08WR7738.4443–118.6548178657.34.6107612882.8628.60.7445
08WR7838.4422–118.6459170966.45.34722082.5964.60.8270
08WR7938.5892–118.7233144236.22.99164880.9717.80.7650
08WR8038.5802–118.7189142220.51.69014671.4016.20.8162
08WR8138.5118–118.6831152658.14.610683731.0040.60.7850
08WR8238.4618–118.6920203412.91.08382330.529.70.8058
08WR8538.4673–118.6694172211.580.94081491.333.00.7649
08WR8738.4376–118.6554175257.434.65672492.2140.30.8165
08WR8838.4322–118.6628181857.894.64422291.5742.30.8271
09WR9038.6477–118.7687133965.945.39033741.3650.80.7854
09WR9138.3679–118.5920188514.181.110842280.3516.20.8163
09WR9238.3713–118.6385233218.601.57452160.859.20.7852
09WR9338.3678–118.6262224417.021.47061551.0112.40.7954
09WR9438.3680–118.6170254719.331.55481780.188.70.7957
09WR9538.3688–118.6068202415.11.210712770.9715.20.7958
09WR10238.4996–118.6633154151.04.13951210.4922.70.8163
09PGH0138.5039–118.9472179661.04.96732730.1217.30.7441
09PGH0338.5047–118.9452172438.53.113233380.2823.60.7546
Table 3. Wassuk Range Footwall AHe Sample Locations and Ages
SampleLatitudeLongitudeElevation (m)Age (Ma)+/– (6%)U (ppm)Th (ppm)Sm (ppm)He (nmol/g)aFTbESR
  • a

    Ft, 4He ejection correction factor.

  • b

    ESR, equivalent spherical radius.

08WR0138.4291–118.631117709.90.63279460.1370.6444
08WR0238.4208–118.620418128.20.52672290.2040.6884
08WR0338.3883–118.631920564.80.33453200.0720.6472
08WR0438.3898–118.624320114.60.31333930.0200.6162
08WR0538.3909–118.615619465.00.32639240.0730.6777
08WR0638.3971–118.607118705.10.32242400.0730.6576
08WR0738.4023–118.599517876.40.42257280.1170.6781
08WR0838.5982–118.751316664.20.34611.5160.6746
08WR0938.6022–118.748015974.90.3121280.0320.7243
08WR1038.6019–118.746915555.90.41717440.0090.5633
08WR1138.5837–118.750119488.40.519295240.0140.5855
08WR1238.5897–118.7432188611.40.7922390.0170.6139
08WR1338.5937–118.7409188113.10.8922450.0210.6139
08WR1438.5952–118.7400178812.10.71331500.0360.6341
08WR1538.5969–118.737317038.20.5525420.0050.5556
08WR1638.5923–118.734415938.30.5918350.1050.6544
08WR1738.5734–118.712614082.60.21726220.0260.7068
08WR1838.5466–118.711915004.50.3203670.0310.6772
08WR1938.5452–118.701214244.20.31510590.0250.6772
08WR2038.5441–118.699314124.40.31919700.0140.6239
08WR2238.5214–118.6795141447.72.9818470.1320.6239
08WR2338.5147–118.7025172814.20.92547760.2110.6675
08WR2438.5150–118.6979174310.40.62040740.1050.6675
08WR2538.5167–118.6986166413.30.81941780.0900.6166
08WR2638.5237–118.6927153444.12.63025730.4540.6469
08WR2738.5223–118.6850146976.64.62617350.7210.6745
08WR2838.4821–118.7547208411.30.73052440.1640.6470
08WR2938.4872–118.741919498.80.51826260.0860.6675
08WR3038.4865–118.728718735.40.31933280.0680.6670
08WR3138.4866–118.714318189.20.62331350.1920.7188
08WR3238.4826–118.705817444.20.33241320.0830.6979
08WR3338.4844–118.698017654.30.32340180.0510.6573
08WR3438.4826–118.685716563.60.28272380.2140.7086
08WR3538.4845–118.673915874.70.31727260.0500.6676
08WR3638.4842–118.6629152542.72.6108220.0760.6648
08WR3738.5685–118.791134425.50.39831090.0330.6748
08WR4238.5352–118.8101279156.43.41955450.2290.5836
08WR4338.5081–118.8076283658.13.536821060.3340.6038
08WR4638.4841–118.8027293655.63.32050930.3400.6541
08WR4738.4788–118.8033284625.21.536210.0220.6850
08WR4838.4801–118.7971272842.52.51213630.0910.6542
08WR4938.4766–118.7836241715.60.92115850.1060.7049
08WR5438.5723–118.7758279715.60.9252780.0570.6342
08WR6038.4407–118.7461234611.60.71417650.1520.7153
08WR6138.4460–118.7810311715.70.92529470.0650.5936
08WR6238.4617–118.7585297616.01.01425890.1940.6746
08WR6338.4597–118.7548284714.30.92748330.4220.6948
08WR6438.4545–118.7514265013.70.82224870.0700.6645
08WR6538.4544–118.7445255312.50.82337740.2050.6950
08WR6638.4540–118.7403244911.60.72547610.1070.6341
08WR6838.4600–118.722622557.90.53236580.1220.6849
08WR6938.4644–118.712021365.90.48571450.2940.6847
08WR7038.5334–118.7138164666.24.01121360.6970.6240
08WR7138.5278–118.7086166244.52.71428360.4380.6442
08WR7338.5040–118.685417038.90.51826580.1500.7153
08WR7438.5000–118.686617977.80.53541470.1490.6847
08WR7538.4938–118.678216466.90.4613160.2090.8289
08WR7638.4589–118.674217923.10.2910130.0610.7975
08WR7738.4443–118.6548178627.81.739234762.6060.6647
08WR7838.4422–118.6459170933.02.0712130.5670.7974
08WR7938.5892–118.723314422.80.23147140.0220.6645
08WR8038.5802–118.718914224.30.352120.0570.8181
08WR8138.5118–118.6831152610.10.62544580.2320.6951
08WR8238.4618–118.692020344.50.32533180.0840.6950
08WR8538.4673–118.669417223.30.2181820.0940.7768
08WR8738.4376–118.655417524.00.2812130.0840.7976
08WR8838.4322–118.662818183.60.21021170.1280.8080
09WR9038.6477–118.7687133911.80.72653190.0500.5935
09WR9138.3679–118.592018852.80.271150.0880.8398
09WR9238.3713–118.638523325.30.33248210.0760.6341
09WR9338.3678–118.626222442.80.22643180.0790.7154
09WR9438.3680–118.617025473.20.22949150.1150.7358
09WR9538.3688–118.606820243.00.22745130.1080.7256
09WR10138.5013–118.662914766.30.41619220.0530.6746
09WR10238.4996–118.663315416.70.41013210.0390.6746
03WL4038.3442–118.599419384.170.3      
03WL4138.3606–118.610524943.200.2      
03WL4238.3554–118.606324033.300.2      

5.1 Mt. Grant Block

[25] ZHe ages from the Mt. Grant block range between ~70 and 12.6 Ma and show an overall younging trend toward the southeast (Figure 6). An apparent Middle Miocene ZHe PRZ is evident by the presence of two inflection points at ~42 and ~15 Ma and with paleodepths of 6.1 ± 0.5 km and 7.7 ± 0.7 km, respectively (Figure 7). ZHe ages above the PRZ decrease with increasing depth from ~70 to ~42 Ma recording Late Cretaceous-Paleogene cooling at a rate of ~2°–2.5°C/Ma related to the cessation of Sierran arc magmatism and subsequent denudation (Figure 7). Apparent ZHe ages below the PRZ record the onset of rapid cooling related to fault-block exhumation at ~12–15 Ma. Assuming a mean annual surface temperature of 10° ± 5°C, the depth of the upper and lower isotherms defining the ZHe PRZ together, yield an average Middle Miocene geothermal gradient of ~21.5° ± 2°C/km.

Figure 7.

Plots of zircon (U-Th)/He age vs. Middle Miocene pre-extensional paleodepth for the Mt. Grant, Coryville and Lucky Boy blocks of the southern WR. At the shallowest depths in the Mt. Grant block, Late-Cretaceous to Paleogene ZHe cooling ages record the erosional denudation and cooling of the Sierra Nevada batholiths after the end of Sierran arc magmatism at a rate of < 2°–2.5°C/Ma. The onset of fast-paced exhumation at ~12–15 Ma is evident throughout the WR and is represented here by invariant ages at the deepest structural paleodepths. The location of the exhumed fossil Miocene ZHe PRZs are highlighted and yield Middle Miocene pre-extensional geothermal gradient estimates that increase toward the Lucky Boy block at the southernmost WR.

[26] AHe ages from the Mt. Grant block also get younger toward the southeast where Mio-Pliocene paleodepths are greatest at ~3.4 km. A few exceptions to this overall younging trend are from samples that were collected directly beneath the Lincoln Flat andesite and a few along the present-day range front that respectively experienced reheating by overlying volcanic flows or migrating hydrothermal fluids along range-front structures. Two inflection points in the AHe vs. Mio-Pliocene paleodepth profile are recognized (Figure 8) and occur at ~15 Ma and ~1.2 km and ~4 Ma and ~2.6 km. These inflection points define the location of the upper and lower limits of the Mio-Pliocene AHe PRZ defined by the ~40°C and ~80°C isotherms, respectively. An AHe age of 15.6 ± 1.2 Ma above the Mio-Pliocene PRZ and invariant AHe ages of ~4 Ma below the Mio-Pliocene PRZ record the onset of rapid cooling related to the exhumation of the southern WR. The measured depth of the upper and lower limits of the Mio-Pliocene AHe PRZ below the Paleogene nonconformity provides a mean, Mio-Pliocene geothermal gradient estimate of ~32° ± 9°C/km.

Figure 8.

Plots of apatite (U-Th)/He age vs. Mio-Pliocene pre-extensional paleodepth for the Mt. Grant, Coryville and Lucky Boy blocks of the southern WR. Two episodes of fast-paced exhumation are recognized in the Coryville block by invariant ages above and below the Mio-Pliocene AHe PRZ, and occurred at ~12–15 Ma and ~3–5 Ma. Although poorly constrained in the Mt. Grant and Lucky Boy blocks, the location of the inferred exhumed fossil Mio-Pliocene AHe PRZs are highlighted and yield pre-extensional geothermal gradient estimates, which increase toward the Lucky Boy block at the southernmost WR.

5.2 Coryville Block

[27] Both ZHe and AHe ages from the structurally intact Coryville block get younger with increasing paleodepth (Figures 6, 7, and 8). However, samples to the east along the range front were collected from structurally detached fault blocks and yielded older ages analogous to those from the uppermost crustal levels on the western side of the Coryville and Mt. Grant blocks (Figure 6). The southernmost transect is the most complete and has the smallest sample spacing of the two transects and will be the primary data set for the Coryville block. Two inflection points are recognized in the ZHe age vs. Middle Miocene paleodepth profile at ~35–40 Ma and ~3.5 km, and ~12–14 Ma and ~5.8 km, and define the upper and lower limits of the Middle Miocene ZHe PRZ (Figure 7). In the entire Coryville block, ZHe ages above the PRZ are Cretaceous to late Eocene in age and young toward the southeast and likely record cooling related to the cessation of the Sierran Magmatic arc and erosional denudation associated with the formation of the Paleogene nonconformity surface. Below the PRZ, ZHe ages are ~12–14 Ma and record the onset of rapid cooling. The depth of the ~140° and ~200°C isotherms yields an average Miocene geothermal gradient estimate of ~36° ± 2°C /km.

[28] AHe ages from the northwestern corner of the Coryville block are Paleocene to Oligocene in age, and are the result of heating and cooling throughout the Tertiary. Three inflection points are recognized in the AHe age vs. Mio-Pliocene paleodepth profile from the northernmost transect and occur at ~15 Ma and ~0.7 km, ~15 Ma and ~1 km, and at ~3.5–5 Ma and ~2.4 km. Only two of the inflection points are recognized in the southernmost transect (Figure 8) and are well defined at ~15 Ma and ~0.7 km, and ~3–4 Ma and ~2 km. These two inflection points define the upper and lower limits of the Mio-Pliocene AHe PRZ (Figure 8) and are further defined by the ~40°C and ~80°C isotherms thereby yielding a mean geothermal gradient estimate of ~39° ± 7°C/km.

5.3 Lucky Boy Block

[29] Sample transects were only collected between ~1.5 and ~4 km Middle Miocene paleodepth, and did not reach the Paleogene nonconformity (Figure 6). In general, both ZHe and AHe ages from the Lucky Boy block young toward Whisky Flat in the southeast, where the southern WR-front fault trace becomes curvilinear and strikes toward the northeast. A small population of Paleogene ZHe cooling ages is present along the range front and is not represented anywhere else in the Lucky Boy block, even on its westernmost margins, and therefore likely sampled structurally detached slivers of the southern WR. ZHe ages from the structurally intact Lucky Boy block show little variation, and all range between ~20 and 12 Ma (Figures 6 and 7) and likely define the lowermost portion of the ZHe vs. Middle Miocene paleodepth profile described in the Mt. Grant and Coryville blocks to the north, and probably resided directly below the Middle Miocene ZHe PRZ. The middle transect, which sampled Middle Miocene paleodepths of ~1.5–3.5 km, may arguably have preserved the lowermost portion of the PRZ at a depth of ~2.2 km where the shallowest ZHe ages approach 20 Ma (Figure 7). These transects provide us with a lowermost constraint on the depth of the Middle Miocene ~200°C isotherm, and therefore estimates for a geothermal gradient of ≥76° ± 5°C/km; these estimates are significantly higher than those for the Coryville and Mt. Grant blocks to the north.

[30] AHe ages from the Lucky Boy block are similarly invariant with respect to the ZHe data set and range between ~2.8 and ~6.4 Ma at Mio-Pliocene paleodepths of ~0.6–2.3 km (Figures 6 and 8). These samples represent the deepest portion of the AHe vs. Mio-Pliocene paleodepth profile, as seen in the Mt. Grant and Coryville blocks, and likely resided below the Mio-Pliocene AHe PRZ and therefore below ~80°C. A very slight inflection point in the middle vertical transect occurs at ~3 Ma and 0.7 km, and may mark the depth of the Mio-Pliocene ~80°C isotherm (Figure 8). Although poorly constrained, these data provide Mio-Pliocene geothermal gradient estimates of ≥ 80° ± 26°C/km, despite the absence of a complete, preserved, Mio-Pliocene AHe PRZ.

5.4 Pine Grove Hills Block

[31] Only three samples were collected from Mesozoic granites from the eastern flank of the PGH (Figure 2). In the sample area, Middle Miocene andesites are not in depositional contact with Mesozoic basement and therefore paleodepth estimates are difficult. However, 35° west-dipping Middle Miocene andesites, approximately 8 km north of the study area provide some constraints on the degree of tilting where east-dipping normal faults exposed a ~2.5 km thick crustal section. (U-Th)/He samples were only collected from the deepest structural paleodepths and do not sample the entire section. The three ZHe cooling ages from the PGH are Paleocene and Eocene in age (Figure 9) and their cooling was therefore unaffected by Miocene and younger extension. AHe ages, however, are invariant at ~12.5–13 Ma and likely represent the structurally lowest portion of the Miocene AHe age vs. paleodepth profiles described in the Mt. Grant and Coryville blocks and are possibly associated with rapid cooling during exhumation of the PGH fault block (Figure 9). Even though AHe samples resided below the PRZ, no geothermal gradient estimates can be made for the PGH since Neogene tilting and therefore paleodepths are poorly constrained. Unlike the central and southern WR, Mio-Pliocene cooling is not recorded in the PGH AHe data set.

Figure 9.

Plots of zircon (dark gray) and apatite (light gray) (U-Th)/He ages vs. depth below the Paleogene nonconformity for samples from the PGH. Only three samples were collected from the PGH block, and sampled the deepest exposed pre-extensional paleodepths. The three AHe ages shown likely resided below a Middle Miocene AHe PRZ and record the onset of fast-paced footwall exhumation at ~12 Ma, which is slightly younger than seen in the WR to the east. The PGH data set does not record any cooling associated with exhumation during Mio-Pliocene transtension.

5.5 Inverse Modeled Time-Temperature Histories

[32] The Helium Modeling Program (HeMP) [Hager and Stockli, 2009] was used to model individual sample data sets with ZHe and AHe age data from the southern WR and PGH. Inverse modeling through HeMP is practically identical to Ketcham’s [2005] HeFTy software in that it solves the 4He production and diffusion equations using a series of user-constrained, but random, Monte Carlo-type simulated time-temperature (T-t) paths. Model (U-Th)/He ages for each sample are then statistically compared to laboratory-derived ages to evaluate the compatibility of the associated model-generated T-t path. The advantage to HeMP relative to other He modeling programs is its ability to simultaneously model AHe and ZHe age data from an individual sample, thereby expanding the thermochronometrically constrainable thermal history. Inverse models were used to constrain the Cretaceous and younger cooling history of individual samples from a range of structural paleodepths in the southern WR (Figure 10). Additionally, paleotemperatures derived from modeled T-t histories in collaboration with paleodepth calculations were used to estimate both the pre-extensional geothermal gradient and volcanic overburden [e.g., Stockli et al., 2002; Stockli, 2005].

Figure 10.

Apatite and zircon (U-Th)/He inverse time-temperature history modeling results showing good (dark gray) and acceptable (light gray) fits for individual samples from the shallowest and deepest structural levels in the Mt. Grant and Coryville blocks, and from just the deepest levels in the Lucky Boy and PGH blocks. Modeling results from the shallow structural levels in the Mt. Grant and Coryville blocks record protracted cooling during the Late Cretaceous-Paleogene related to the erosional denudation of the Sierra Nevada batholith, and associated Mesozoic wall rock; and a second cooling event at ~12–15 Ma related to the exhumation of the WR footwall. Time-temperature history models for the deeper structural levels in the southern WR record a three-stage thermal history with two episodes of fast-paced cooling starting at ~15 and ~4 Ma, separated by a period of isothermal holding. Thermal modeling results for the PGH indicate that this fault block only experienced limited Middle Miocene cooling starting at ~12 Ma, and is devoid of any Mio-Pliocene and younger cooling.

[33] Structurally shallow samples from the southern WR were modeled to constrain the earliest thermal evolution of the upper crust. Inverse T-t histories show two phases of cooling starting with a period of slow cooling from ~85 to ~40 Ma, and followed by a rapid-cooling event between ~12–15 Ma (Figure 10). Protracted cooling throughout the Late Cretaceous and Paleogene are recorded in the modeling data and are consistent with the development of the Paleogene nonconformity during erosional unroofing of the Sierra Nevada over the same time span [e.g., Christiansen et al., 1992, Van Buer et al., 2009]. Inverse T-t histories from samples along the range front represent the deepest structural levels of the WR and record an episodic two-stage thermal history characterized by rapid cooling at ~12–15 Ma, followed by a period of isothermal holding, and finally renewed cooling at ~3–5 Ma (Figure 10). These events are consistent with the faulting history derived from the tilting of the Tertiary section west of the southern WR, and inflections in the AHe and ZHe vs. paleodepth plots. T-t histories from the PGH to the west also record a two-stage thermal history starting with protracted cooling throughout the Late Cretaceous and Paleogene associated with the erosional unroofing of the WR region, followed by rapid cooling at ~12 Ma and isothermal holding till the present day. The similar T-t histories shared by the PGH and the shallowest structural levels in the southern WR suggest that the area immediately west of the WR was devoid of any Mio-Pliocene and younger exhumation and extension.

[34] Model-derived Middle Miocene geothermal gradients increase southward and are ~19° ± 4°C/km in the Mt. Grant block, ~29° ± 4°C/km in the Coryville block, and ≥ 65° ± 20°C/km in the Lucky Boy block (Figures 11 and 12) and are in agreement with geothermal gradient estimates gleaned from the positions of the Middle Miocene ZHe PRZ (Figure 7). The geothermal gradient estimates are used to determine overburden and when projected upward, intersects the thermally defined surface at ~0.8 km above the Paleogene nonconformity. Overburden estimates of ~0.8 km agree with other mapped [e.g., Surpless, 1999] and thermally determined [Stockli et al., 2002] regional thickness estimates for the Lincoln Flat andesite.

Figure 11.

Plots of model temperature vs. pre-extensional paleodepth for both the Mt. Grant and Coryville blocks immediately prior to middle Miocene and Mio-Pliocene extension. The geothermal gradient can be estimated by the slope of a linear best-fit line through the data sets, and can further be projected to a thermally defined surface at 10° ± 5°C to estimate Middle Miocene pre-extensional overburden. Model-derived geothermal gradient estimates corroborate geothermal gradient estimates calculated from the depth of the ZHe and AHe PRZs (Figures 6 and 7) and similarly illustrate an elevated geothermal gradient in the southernmost WR. In the Mt. Grant block, an increase in the geothermal gradient of ~8° ± 7°C/km is attributed to footwall advection during Middle Miocene extension. An apparent deviation from linearity in the 6 Ma geothermal gradient plot for the Coryville block is likely an artifact of poorly constrained samples that primarily do not lie within the Mio-Pliocene AHe PRZ.

Figure 12.

Tilting and geothermal gradient histories for the southern WR. Tilting history is illustrated by plots of a stratigraphic unit's 40Ar/39Ar age vs. dip and shows a southward decrease in cumulative Miocene tilting and fault-block rotation. Model-derived geothermal gradient estimates are also plotted below against the position of preserved isotherms (white data points) defined by inflections in the AHe and ZHe Age vs. paleodepth plots (Figures 6 and 7). Together the diagrams show an increased geothermal gradient in the southernmost WR, which is inversely correlated with degree of footwall rotation. Modern geothermal gradient of ~35°C/km in the Basin and Range province [Lachenbruch and Sass, 1980] is shown for reference.

[35] Mio-Pliocene geothermal gradients also show a similar increase toward the southernmost WR, and are ~27° ± 4°C/km in the Mt. Grant block, ~28° ± 2°C/km in the Coryville block, and ≥ 65° ± 8°C/km in the Lucky Boy block. Temperature estimates from the Mt. Grant and Coryville blocks are within error of the present-day Basin and Range geothermal gradient (35° ± 5°C/km) [Lachenbruch and Sass, 1980]. The Mio-Pliocene geothermal gradient in the Mt. Grant block exhibits a significant increase (8° ± 7°C/km) since the middle Miocene and follows a period of large-magnitude extension, exhumation, and tilting. Indiscernible middle Miocene to Mio-Pliocene footwall advection in the southernmost WR correlates with decreasing amounts of Middle Miocene extension, exhumation, and tilting (Figure 12).

6 Discussion

6.1 Faulting and Tilting History of the Southern Wassuk Range

[36] Integrated structural and (U-Th)/He thermochronological data constrain the thermal and structural evolution of the southern WR and PGH prior to and during separation from the eastern Sierra Nevada. AHe and ZHe age data indicate that the southern WR underwent three episodes of cooling since the Late Cretaceous, associated first with unroofing during the formation of the Paleogene nonconformity, followed by two episodes of Neogene extension and footwall exhumation. This thermal history is in good agreement with the 40Ar/39Ar-constrained tilting history of overlying Tertiary cover in the Coal and Fletcher Valleys, and Neogene extension documented in the central WR [McIntyre, 1990; Dilles and Gans, 1995; Surpless et al., 2002; Stockli et al., 2002; Stockli, 2005].

[37] Late Cretaceous through Paleogene cooling was related to the erosional unroofing of the Sierra Nevada block and is recorded in AHe and ZHe ages from the shallowest structural levels in the Mt. Grant and Coryville blocks in the southern WR (Figures 7 and 8), and in ZHe data at all structural levels from the PGH (Figure 9). The most complete record of Cenozoic cooling is preserved in the Mt. Grant block, where ZHe ages from the upper ~6 km of section cover most of the Paleogene and record a cooling rate of < 2°–2.5°C/Ma (Figure 7), which is consistent with a period of deep erosion following the cessation of Sierran magmatism at ca. 85 Ma and prior to the development of the Paleogene nonconformity surface at ca. 40 Ma [Van Buer et al., 2009].

[38] Miocene and younger cooling ages result from the tectonic exhumation of the southern WR and PGH. An episode of Middle Miocene east-west extension is recorded across the entire WR [e.g., Surpless et al., 2002; Stockli et al., 2002] and to the west in the northern Singatse Range [Proffett, 1977; Proffett and Dilles, 1984; Dilles and Gans, 1995], Grey Hills [Surpless et al., 2002], and PGH. AHe and ZHe data indicate the onset of rapid fault-block exhumation at ~12–15 Ma in the southern WR and ~12.5 to 13 Ma in the PGH to the west (Figures 7, 8, and 9). Extension was accommodated primarily by fault-block rotation along a horizontal, north-northwest-trending axis and variably tilted the southern WR and PGH. Tilting was greatest in the Mt. Grant block to the north (60–70°) and matched the tilting history of the central WR [e.g., Stockli et al., 2002; Surpless et al., 2002]. Degree of footwall rotation decreases dramatically toward the south to ~35° in the Coryville block and PGH, and ~15–20° in the Lucky Boy block (Figure 2). East-northeast-striking scissor faults in the southern WR formed the boundaries between tilt blocks and accommodated differential tilt between blocks. Despite their similarities in tilting, exhumation of the southern WR was much greater than that of the PGH and is evident by the presence of young (≤ 15 Ma) ZHe ages along the entire WR front, and nonreset (≥ 35 Ma) ZHe ages in the PGH.

[39] The westernmost Mina Deflection likely developed in response to the onset of east-west extension in the east-tilted White Mountains at ~12 Ma [Stockli et al., 2003], and accommodated a polarity switch with the west-tilted WR. Interestingly, a number of right en-echelon steps in the southern WR front that developed in response to differential tilting also appear to gradually mitigate misalignment between the strike of the range-front normal faults bounding the east side of the WR and the west side of the White Mountains, respectively. Nevertheless, the southern WR is a terminating normal-fault system in the sense that it dies into a transverse-strike and oblique-slip fault zone [e.g., Faulds and Varga, 1998], which like many throughout the Basin and Range are characterized by a disrupted structural and topographic grain, changes in the faulting and fracture density, and the absence of major tilted blocks [e.g., Slemmons, 1967; Stewart, 1980; Duebendorfer and Black, 1992].

[40] The second episode of rapid exhumation that began at ~3 to 5 Ma developed in response to right-lateral transtensional deformation in the Walker Lane belt. Mio-Pliocene transtension was accommodated primarily by dextral-oblique slip along the southern WR front. A number of Mio-Pliocene pull-apart structures also developed at this time and are highlighted by the distribution of young (< 5 Ma) AHe ages at en-echelon steps in the range front and at Whisky Flat. This episode of transtension is contemporaneous with the development of pull-apart structures along the northern White Mountains [Stockli et al., 2003; Tincher and Stockli, 2009] and normal and transcurrent faults in the Mina Deflection [Stewart, 1988; Hardyman and Oldow, 1991; Oldow, 1992; Oldow et al., 2001; Rood et al., 2011], and a period of structural reorganization throughout the central Walker Lane belt [Oldow et al., 2008]. Unlike the central and southern WR [e.g., Stockli et al., 2002; Surpless et al., 2002], the PGH did not experience any Mio-Pliocene deformation, as evident from the absence of Mio-Pliocene and younger cooling ages, and suggests that at this latitude the WR defines the western limit of Mio-Pliocene right-lateral, Walker Lane belt transtension.

[41] Anderson et al. [2012] present a completely different model for the Cenozoic tectonic evolution of the WR based on the mapping of the same 40Ar/39Ar constrained Tertiary section in Coal Valley. They propose that the faulting and folding of the Mesozoic and Tertiary section in Coal Valley was the result of east-west extension and north-south shortening between 10 and 7.6 Ma associated with the formation of the Walker Lane belt – Eastern California shear zone. Additionally, they conclude that exhumation of the WR was the result of mild flexural upwarping in the Middle Miocene associated with magmatic inflation resulting in minimal Neogene extension, fault-block rotation, and tilting of the Tertiary section. Their tectonic interpretations invoke complex midcrustal flow processes to explain geometries which can arise simply from differential tilting of fault blocks in the WR and PGH. Furthermore, their interpretation of the timing of deformation is based solely on the 40Ar/39Ar age data of units which predate and do not constrain the timing of Neogene tilting and disregards data from a number of studies documenting the timing, accommodation, and geometry of Miocene E-W extension via fault-block rotation in the WR [e.g., Proffett, 1977; McIntyre, 1990; Dilles and Gans, 1995; Stockli et al., 2002; Surpless et al., 2002; Krugh, 2008; Gorynski, 2011] and throughout the Basin and Range [e.g., Stewart, 1998; Miller et al., 1999; Stockli et al., 2003; Colgan et al., 2006, 2008; Fosdick and Colgan, 2008; Tincher and Stockli, 2009]. Most importantly, after palinspastic reconstruction of the tilted WR fault blocks the thermochronologic data presented here agree very well with documented Cenozoic cooling events in the western Basin and Range.

6.2 Upper-Crustal Thermal Structure of the Southern Wassuk Range

[42] The WR and PGH were part of the Sierra Nevada prior to middle Miocene extension and therefore probably experienced a similar pre-extensional thermal history and post-Cretaceous geothermal gradient of < 15°–20°C/km [e.g., Stockli, 1999; Stockli et al., 2002; Surpless et al., 2002]. ZHe data from the shallowest structural levels show that Sierran plutonic rocks and associated wall rocks cooled at a rate of < 2–2.5°C/Ma during their denudation until ~40 Ma and are in agreement with other age estimates for the development of the Paleogene nonconformity [e.g., Van Buer et al., 2009]. Thermochronologic data from the southern WR indicate an increase in the Middle Miocene geothermal gradient (> ~22°C/km) immediately prior to extension and agree with similar results from the central WR [Stockli et al., 2002; Surpless et al., 2002] and the Anchorite Hills [Krugh, 2008] that document a post-20 Ma heating event.

[43] Model-derived middle Miocene geothermal gradient estimates increase dramatically southward, from ~19° ± 4°C/km in the Mt. Grant block, to ~29° ± 4°C/km in the Coryville block, and to ≥ 65° ± 20°C/km in the Lucky Boy block. Middle Miocene heating was focused toward the south, and overprinted much of the Paleogene ZHe cooling record. Therefore, in addition to geothermal gradient estimates, the thermal structure of the southern WR can be viewed semi-quantitatively by analyzing the distribution of Paleogene ZHe ages (Figure 13): importantly, the Paleogene nonconformity does not parallel ZHe isopleths, but is instead intersected by progressively younger isopleths southward with the result that Paleogene ZHe ages are absent from the southern WR. When the southern WR is palinspastically restored to its pre-extensional configuration, Paleogene ZHe age isopleths cut up section and again are absent at all structural levels in the Lucky Boy block, corroborating a thermally elevated southern WR (Figure 14). The hot middle Miocene southern WR likely resulted from localized arc magmatism [e.g., Christiansen et al., 1992; Stewart, 1998] that occupied the structurally weak crustal boundary along the Mina Deflection [e.g., Aldrich, 1986; Rowley, 1998; Faulds and Varga, 1998]. We suggest here that the southern area may have been near the eruptive center for some of the Neogene volcanic rocks (denoted by volcano in Figure 14).

Figure 13.

ZHe age map for samples from the southern WR footwall. The Paleogene nonconformity (bold line) is intersected by progressively younger ZHe age isopleths toward the south. Oligocene and older ZHe ages are absent from the structurally intact Lucky Boy block, even for samples adjacent to the Paleogene nonconformity. Anomalously old ages along the range front mark the location of range-front normal faults and structurally displaced slivers of WR footwall.

Figure 14.

Palinspastic restoration of the southern WR footwall showing the distribution of footwall ZHe ages. ZHe age isopleths cut up section and mimic geothermal gradient trends that increase toward the south (Figure 13). ZHe age isopleths should roughly parallel the Paleogene nonconformity, under the assumption that the unconformity was a flat or slightly beveled surface and that Tertiary volcanic overburden was uniform across the region. The absence of Oligocene and older ZHe ages from the shallowest structural levels in the Coryville, Lucky Boy, and Anchorite Hills blocks of the southernmost WR suggest that the oldest thermal record of post-Cretaceous erosion and denudation was overprinted by a Miocene heating event. This heating event may have resulted from pre-extensional Miocene volcanism focused in the Mina Deflection and southern WR (denoted by volcano in figure).

[44] Mio-Pliocene geothermal gradient estimates for the southern WR also increase toward the south and substantiate that the southernmost WR remained hot until at least ~3 Ma. Deformation and igneous activity during Middle Miocene exhumation raised the geothermal gradient by as much as 8° ± 7°C/km (Figure 11) to values which are on par with the present-day geothermal gradient of 35° ± 5°C/km in the Basin and Range Province [Lachenbruch and Sass, 1980]. Middle Miocene geothermal gradients in the southern WR show an inverse correlation with Miocene and younger tilting and exhumation and consequently placate upper-crustal thermal variations between tilt blocks via fault-block advection (Figures 12 and 15).

Figure 15.

Block evolution diagrams for the southern WR footwall. (A) Extension at ~15 Ma began in the northern and central WR and progressed toward the southwest, progressively tilting footwall blocks of the southern WR and PGH. (B) A second episode of extension initiated at ~4 Ma and resulted in range-front faulting across the entire central and southern WR. Right en echelon steps along the range front are the product of middle Miocene tilting, and were the focus of Mio-Pliocene and younger transtensional deformation. The preserved Miocene ZHe and Mio-Pliocene AHe PRZs are shown on the tilted footwall blocks (B), and qualitatively illustrate the increased middle Miocene geothermal gradient toward the south (depth to the PRZ measured from the Paleogene nonconformity), and the focus of Mio-Pliocene exhumation at range-front pull aparts.

[45] A final episode of extension associated with the onset of right-lateral transtension in the Walker Lane belt initiated at ~3–5 Ma in the southern WR, and continues today [e.g., Stockli et al., 2002; Surpless et al., 2002; Oldow, 2003; Unruh et al., 2003; Surpless, 2008; Krugh, 2008; Hinz et al., 2010; Moeck et al., 2010]. Footwall advection is concentrated along en-echelon right steps at the range front and at Whisky Flat where the range front is oriented roughly orthogonal to the regional northwest-southeast extension direction (Figures 15 and 16). This is also supported by geothermal exploration studies along the southern WR front and Walker Lake basin [e.g., Gorynski et al., 2010; Kratt et al., 2010: Lazaro et al., 2010; Penfield et al., 2010] that revealed a number of thermal perturbations concentrated at Mio-Pliocene pull-apart structures [e.g., Hinz et al., 2010; Moeck et al., 2010; Gorynski et al., 2010; Gorynski, 2011].

Figure 16.

Block diagrams illustrating the structural and thermal evolution of the Mina Deflection accommodation zone, and WR and White Mountains extensional systems to its north and south, respectively. The elevated middle Miocene geothermal gradient in the Mina Deflection area may have resulted from pre-extensional volcanism that was focused along its pre-existing structural weaknesses. In the Miocene, the Mina Deflection also accommodated a polarity switch between the west-tilted southern WR and east-tilted White Mountains. The onset of “Walker Lane belt style” right-lateral shearing initiated at ~3–4 Ma throughout the central WR, and resulted in transtension across the southern WR front and the formation of pull-apart structures at right, en-echelon steps along the range front. Similarly, pull-apart structures also developed at ~3 Ma along the northern and southern margins of the Mina Deflection at Whisky Flat in the southern WR, and at Queen Valley [Stockli et al., 2003; Tincher and Stockli, 2009] in the northern White Mountains (Figure 1).

7 Conclusions

[46] The WR underwent three episodes of Cenozoic cooling: slow cooling (< 2–2.5°C/Ma) during denudation of Mesozoic basement rocks ending at ~40 Ma, rapid exhumation associated with Basin and Range extension at ~ 12–15 Ma, and rapid exhumation associated with Walker Lane belt transtension at ~3–4 Ma. The PGH also experienced a similar cooling history throughout the Paleogene and into the Miocene, but lacks a record of Mio-Pliocene transtension, suggesting that at this latitude the WR was the western boundary to early Walker Lane transtension (Figure 16).

[47] Based on the increase in calculated Middle Miocene geothermal gradients toward the southern WR and Mina Deflection, we propose that they were the focus of Middle Miocene volcanism of the Miocene magmatic arc that occupied the structurally predisposed crustal boundary demarked by the 87Sr/86Sr 0.706 line [i.e., Kistler and Peterman, 1973; Kistler et al., 1991] (Figure 16). Middle Miocene east-west extension immediately follows regional magmatic advection, which may have thermally destabilized the crust [e.g., Kusznir and Park, 1987; Stockli et al., 2002; Surpless et al., 2002]. The tilt and width of the crustal blocks in the southern WR decrease toward the south and are likely related to Middle Miocene volcanism. We interpret the southward decreasing size of fault blocks as a result of the southward increase in geothermal gradient and a shallowing of the brittle/ductile transition zone. The decrease in tilt toward the south suggests a change in strain accommodation from predominantly tilting in the north, to tilting and dike intrusion in the south. Additional strain may have also been accommodated along the eastern side of the Walker Lake basin and along strike-slip faults in the Mina Deflection which further accommodated differential tilting between the west-dipping WR and east-dipping White Mountains.

[48] The southern WR and Mina Deflection were thermally and kinematically linked and worked in tandem to raise the geothermal gradient throughout the Neogene. This detailed thermochronologic study provides a model for the thermal evolution of an accommodation zone and associated extensional system that involves a progression of magmatic and fault-block advection that mimics successions of volcanism and amagmatic extension recorded throughout the Basin and Range Province [e.g., Gans and Bohrson, 1998]. Pre-extensional magmatism and volcanism that are focused along structurally permeable accommodation zones likely thermally elevate the entire crust at depth, but show large lateral thermal variations at the upper-crustal levels, where heating is greatest near volcanic and magmatic centers. This model predicts an elevated brittle/ductile transition zone near these volcanic centers which restricts the thickness and geometry of extending tilt-blocks. Subsequent fault-block rotation, exhumation, and advection are therefore greatest at the distal portions of these extensional systems and consequently abate upper-crustal thermal disparities.

Acknowledgments

[49] This research was funded by a grant from the Navy Geothermal Program Office at China Lake, CA, and student research grants from the Geological Society of America and the American Association of Petroleum Geologists. Our work relied heavily on the intellectual and technical support of the students, staff, and researchers at the University of Kansas’ Isotope Geochemistry Laboratory; specifically, Evan Bargnesi, Edgardo Pujols, Badr Gohrbal and Roman Kislitsyn. Finally, this work significantly benefited from the thermal modeling of a rather comprehensive sample data set using the in-house HeMP software developed by Chris Hager.

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