Rapid middle Miocene extension and unroofing ... - Wiley Online Library

TECTONICS, VOL. 29, TC6022, doi:10.1029/2009TC002655, 2010

Rapid middle Miocene extension and unroofing of the southern Ruby Mountains, Nevada Joseph P. Colgan,1 Keith A. Howard,1 Robert J. Fleck,1 and Joseph L. Wooden2 Received 28 December 2009; revised 31 August 2010; accepted 16 September 2010; published 31 December 2010.

[1] Paleozoic rocks in the northern Ruby Mountains

were metamorphosed during Mesozoic crustal shortening and Cenozoic magmatism, but equivalent strata in the southern Ruby Mountains were never buried deeper than stratigraphic depths prior to exhumation in the footwall of a west dipping brittle normal fault. In the southern Ruby Mountains, Miocene sedimentary rocks in the hanging wall of this fault date from 15.2 to 11.6 Ma and contain abundant detritus from the Paleozoic section. Apatite fission track and (U‐Th)/ He samples of the Eocene Harrison Pass pluton record rapid cooling that peaked ca. 17–15 Ma, while apatite fission track data from Jurassic plutons east and west of the southern Ruby Mountains indicate near‐surface temperatures (4000 m if no fault separates this section from previously recognized Tertiary strata to the west. The depositional basement to this sequence is unknown, but a drill hole (Frontier Federal 16‐5, Figure 6) ∼9 km north of the area shown in Figure 8b bottomed in Pennsylvanian limestone overlain by Tertiary rocks inferred to correlate with the Indian Well Formation. 3.3. Toyn Creek and Cedar Mountain: Reinterpretation of the Cedar Mountain Klippe [25] About 400 m of coarse (clasts up to 1.2 m) conglomerate dipping 6°–38° NE is exposed in roadcuts west of the Harrison Pass pluton (Figures 7e and 7f). This deposit coincides with a poorly exposed unit mapped by Willden

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COLGAN ET AL.: SOUTH RUBY MOUNTAINS

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Figure 9. Alternative interpretations of the Cedar Mountain klippe. (a) As mapped by previous workers (Qa, alluvium; Tg, Harrison Pass pluton; Dc, Devonian carbonate). (b) As mapped in this study (units and colors are same as in Figures 2 and 4). and Kistler [1969] as “older [Quaternary] gravels” before roadcuts revealed its internal character. If the conglomerate underlies the entire 3–4 km northward extent of that map unit, then projection of an average ∼25° dip indicates a thickness up to 1.5–2 km. Clasts consist of granite and granodiorite typical of the Harrison Pass pluton, limestone (probably derived from Cambrian and Ordovician rocks), rare (Cambrian?) quartzite and marble, and sparse porphyry (Eocene pluton‐related dikes?). This clast assemblage suggests a source mainly from the Harrison Pass pluton and country rocks along its southern margin. Two replicate sanidine splits from a 1–2 m thick tephra bed in the lowest exposed conglomerate (Figures 7f and 9b) (sample JC06‐ HP110) yielded 40Ar/39Ar laser fusion ages of 11.58 ± 0.048 Ma and 11.61 ± 0.054 Ma (weighted mean of six multigrain laser fusion analyses from each split) (Table 1). The exposed conglomerate section is bounded by two high‐ angle faults (Figures 4 and 9b). The eastern fault is a down‐ to‐the‐west normal fault that dips 57°, exhibits gouge and down‐dip slickenlines, and places the Miocene conglomerates against brecciated granite crushed to pebble‐cobble size and affected by propylitic alteration. We infer that this breccia is part of the west dipping Ruby detachment and that the Miocene sedimentary rocks are in the hanging wall of

the detachment. The western fault is a down‐to‐the‐west normal fault that dips 60° and places unconsolidated alluvial deposits down against the Miocene conglomerate; it is aligned with a topographic scarp on the south side of Toyn Creek and may have Quaternary displacement. Displacement on both of these faults is unknown, but assumed to be small in section A‐A′ (Figure 10). [26] The Cedar Mountain klippe (Figure 4) was mapped by Willden and Kistler [1969, 1979] as Devonian carbonate rocks (Nevada Formation and Devil’s Gate Limestone) thrust eastward over the Eocene Harrison Pass pluton along a low‐angle fault [Willden et al., 1967]. Subsequent workers reinterpreted Cedar Mountain as a klippe in the hanging wall of a down‐to‐the‐west normal fault [Armstrong, 1972; Snoke and Howard, 1984; Blackwell et al., 1985; Reese, 1986; Hudec, 1990; Burton, 1997]. We interpret the carbonate outcrops at Cedar Mountain as megabreccia blocks within the Miocene sedimentary section described above rather than as an intact piece of Paleozoic basement (Figure 9b versus Figure 9a). Float in the western half of the mapped “klippe” includes limestone and quartzite typical of Cambrian rocks, tuffaceous sandstone, conglomerate, chert, dacite, basalt, and granite; we interpret this assemblage as Tertiary conglomerate underlying or

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Figure 10. Geologic cross sections of the southern Ruby Mountains (section lines shown in Figure 4). Colors and symbols are shown in Figure 2. interfingered with the megabreccia blocks. Fossiliferous limestone of the Devil’s Gate Limestone and finely banded dolomite typical of the Nevada Formation are recognizable (with bedding preserved) in the larger outcrops within the main “klippe” [Willden and Kistler, 1979], although many

of these blocks are shattered and recrystallized like those at Mitchell Creek and Ruby Wash. [27] No two previous studies have mapped the extent and internal structure of the Cedar Mountain klippe the same way (Figure 9a), with the exception of the normal fault that

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bounds it on the east side. This fault dips 0°–20° west, has unequivocal Harrison Pass granite in its footwall, and is marked by a greenish zone of chloritic breccia several meters thick [Burton, 1997] (also this study). We interpret this fault as the Ruby detachment, which crops out 5 km to the north along strike (Figure 7e). In contrast, the highly irregular trace of the western margin and outline of the main “klippe” indicates an essentially horizontal basal contact (Figure 9a), with many small “fingers” extending downhill (Figure 9a) that we interpret as carbonate clasts eroding downhill on the land surface (rather than a highly irregular fault surface). Similarly, we interpret the many small fault‐ bounded outliers of carbonate rock mapped by other workers below the main klippe on the north and south sides (Figure 9a) as blocks or lenses of carbonate rock within the Miocene sedimentary section.

4. Structure of the Southern Ruby Mountains [28] Here we outline the cross‐sectional structure of the range in the area covered by Figure 4, beginning with the youngest structures and working back to the geometry of the range at the time of emplacement of the Eocene Harrison Pass pluton. The emplacement depth of the pluton, and thus the depth from which it was subsequently exhumed, is critical for interpreting the thermochronologic data presented in section 6. 4.1. Structure of Ruby Valley and the Ruby Valley Fault Zone [29] The high‐angle, down‐to‐the‐east Ruby Valley fault zone bounds the east side of the Ruby Mountains (Figure 4). Subsurface geometries of cross sections B‐B′ and C‐C′ (Figure 10) across Ruby Valley are loosely constrained by depth‐to‐basement derived from models of the isostatic residual gravity anomaly presented by Berger [2006], using methods described by Ponce [2004] and Watt and Ponce [2007]. [30] Along B‐B′, the gravity model indicates a deep, steep‐sided basin whose edges are not coincident with bedrock outcrops but rather “stepped out” beneath Ruby Valley. Computed depth‐to basement suggests ∼2 km of low‐density fill along the line of section [Berger, 2006]. The modeled western side of the basin, which we infer to be the basin‐bounding fault, dips ∼68° east. The modeled eastern side dips 53° west, too steep to merely be the down‐tilted “passive” side of the basin, and is likely also a fault. Between the faults, we assume slightly shallower basin fill than indicated by modeled depth to basement, which, if taken literally, indicates nothing but low‐density fill between the two faults and leads to an unrestorable fault geometry. This problem may be resolved in part by the presence of the lower‐than‐average density Paleozoic rocks at depth—the model assumes a uniform basement density of 2.67 g/cm3, but borehole measurements of the Mississippian Chainman shale (inferred to underlie Ruby Valley at depth on section B‐B′) indicate a density of 2.5 g/cm3 [Satarugsa and Johnson, 2000]. Even using the geometry in Figure 10b, however, it is unlikely that both the east and west dipping faults moved together, because their intersection at depth

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allows little space for a hanging wall block (which would be preserved as the down‐dropped block). Rather, we suggest that the west dipping fault is an older, down‐to‐the west fault with older basin fill in its hanging wall; this fault is cut by the younger, east dipping fault. We assume this basin fill is middle Miocene based on the abundance of these rocks in and around nearby Ruby Wash (Figure 3). The thickness of fill (regardless of its age) requires a total slip on the east dipping fault of at least 2 km, and we estimate about 2400 m total (Figure 11b). We infer a small amount of west‐tilting during slip on the Ruby Valley fault, about 5° in the main range, and slightly more (10–15°) on the Ruby Valley side. Horizontal extension across these faults is ≤1 km. [31] Along C‐C′, the gravity model indicates a more gently sloping basin floor than along B‐B′, with about 1300 m of fill at its deepest point. The modeled western basin floor dips smoothly ∼15° east, comparable to the topographic east slope of the adjacent footwall block, and is therefore likely not cut by significant faults aside from the one bounding the topographic range‐front. A segment of the west dipping part of the basin floor as modeled from gravity data dips ∼52° west; which we infer to be the same west dipping fault shown on B‐B′, with less displacement here. The absence of major faults on the west side of the basin here is consistent with less vertical displacement than to the north; we estimate roughly ∼800 m total (Figure 11b). As along B‐B′, we assume a small amount of west tilting during slip on the Ruby Valley fault(s); about 5° in the main range and 5°–10° on the Ruby Valley side. Horizontal extension across these faults is ≤1 km. [32] The narrow ridge on the east side of Ruby Valley along sections B‐B′ and C‐C′ was mapped by Coats [1987] as Permian limestone (Arcturus Group and overlying Park City Group, Figure 4), which we assume are underlain here by Pennsylvanian Ely limestone as they are in the nearby Medicine Range [Collinson, 1966]. Elsewhere in the region of Figure 3, Pennsylvanian and Devonian carbonate strata are separated by up to 2000 m of Mississippian to Pennsylvanian siliciclastic rocks assigned to the Chainman Shale and Diamond Peak Formation [e.g., Smith and Ketner, 1975]. Drill holes in Huntington and Ruby Valleys (Figure 6) penetrated 330–500 m of siliciclastic rocks at this stratigraphic interval, and from these data we infer the original presence of 400–500 m of Diamond Peak and Chainman formations in the southern Ruby Mountains section (unit Mdc, Figure 10) between the combined Pilot and Joana formations (unit Mpj), and the Pennsylvanian Ely limestone. After restoring slip on the steeply east dipping faults, this stratigraphic section was used to draw sections B‐B′ and C‐C′ with the simplest geometry that would connect exposed basement on the east side of Ruby Valley to the east dipping strata in the Ruby Mountains (Figure 10b). This results in east dipping Paleozoic strata beneath Ruby Valley, broken by a west dipping, down‐to‐the‐west fault with 1.5–2 km of slip. This restoration demonstrates that there is no space between Mississippian strata in the Ruby Mountains and Permian strata on the east side of Ruby Valley for substantially a substantially thicker Paleozoic section than the Mississippian and Pennsylvanian strata typically present at this stratigraphic interval nearby

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Figure 11. Restored cross section (B‐B′) across the southern Ruby Mountains. (a) Present day, same as in Figure 10b. (b) Ruby Valley fault zone and high‐angle fault on west side of range restored. (c) Ruby detachment restored. 14 of 38

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(Figures 2 and 3). The thicknesses of the inferred units are of course uncertain, but we can rule out the presence of a 5–10 km thick thrust plate that could account for significantly deeper emplacement of the Harrison Pass pluton than indicated by its wall‐rock stratigraphy. 4.2. Geometry of the Ruby Detachment [33] On the basis of constraints from fault exposures (Figures 4 and 7e) [Burton, 1997] (also this study) and seismic data [Satarugsa and Johnson, 2000], the Ruby detachment is shown in the western part of our cross sections dipping 15°W–20°W beneath the eastern edge of Huntington Valley, where it is cut by several, high‐angle, down‐to‐the west faults. Along A‐A′, it is cut in outcrop by the eastern of the two high‐angle faults that bound the Miocene conglomerate section in roadcuts; we assume the western of the two faults cuts it as well. A drill hole (Frontier Federal 16‐5, Figure 6) about 4 km west of the range‐front (3 km south of B‐B′) encountered Pennsylvanian limestone at 1227 m, overlain by Tertiary sedimentary rocks. As the hole did not penetrate the Ruby detachment, a high‐angle fault with a minimum displacement of 800 m of vertical displacement is required to drop the detachment down beneath this drill hole (Figure 10b). [34] Drill holes in Huntington Valley (Figure 6) encountered Pennsylvanian limestone or Mississippian clastic rocks beneath Tertiary strata in the hanging wall of the Ruby detachment. Displacement of these rocks relative to their reconstructed footwall equivalents upsection from Silurian and rocks exposed east of Harrison Pass (Figure 11b) requires ∼22 km of fault slip on A‐A′. Displacement of Pennsylvanian Ely limestone underlying Tertiary rocks in Frontier Federal 16‐5 (Figure 4) requires about 15 km of fault slip along B‐B′ to displace it from reconstructed equivalent rocks in the footwall. Pennsylvanian rocks are also reported to underlie Tertiary rocks in the Anadarko Cherry Springs Federal 30‐44 well (Figure 6) south of the west end of C‐C′ (Figure 3). Assuming the Tertiary rocks in the Mitchell Creek area at the west end of C‐C′ are also deposited on Pennsylvanian rocks, 13–14 km of fault slip is required along C‐C′. For a fault dipping only 10°–20°, horizontal extension is almost the same as (93%–98% of) fault slip. On the basis of the strike of tilted units in the range, we infer that extension was subparallel to sections A‐ A′ and B‐B′ (trending ∼290°), consistent with the trend of striations on the Ruby detachment near Cedar Mountain (∼310°) and stretching lineations in mylonite on the northwestern margin of the Harrison Pass pluton (∼300°) [Burton, 1997]. 4.3. Restored Eocene Structure [35] The initial dip of the Ruby detachment and magnitude of subsequent east tilting of the southern Ruby Mountains are difficult to measure directly because no stratified, preextensional Tertiary rocks are exposed in the southern Ruby Mountains. We therefore begin by considering constraints on the Eocene land surface at about the time the Harrison Pass pluton was emplaced. Tertiary lava

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flows locally overlie Permian rocks in the northern Maverick Springs Range (just south of Ruby Wash; Figure 3). They are undated there, but McKee et al. [1971] report a 36 Ma K‐Ar age on similar rocks overlying Permian strata in the northern Butte Mountains (Figure 3), and Nutt and Hart [2004] report 40Ar/39Ar dates of 36 Ma from rhyolite lava flows overlying Mississippian and Pennsylvanian strata near Bald Mountain (Figure 3). In eastern Huntington Valley (which restores structurally above the southern Ruby Mountains), Paleogene rocks encountered in drill holes (Figure 6) range from zero to over 1 km thick and are deposited on Pennsylvanian and Mississippian strata. On the basis of these observations, Paleozoic rocks presently exposed on the east end of sections A‐A′ and B‐B′ should restore to a gentle east dip, such that Permian rocks were at the Eocene land surface on the east end of a restored section (Figure 11c) and Mississippian to Pennsylvanian rocks were at the surface on the west side. Upper Paleozoic strata were in turn overlain in the Eocene by volcanic and/or sedimentary rocks that ranged from absent to thin (100s of meters) on the east ends of sections B‐B′ and C‐C′ and thickened to the west. Restoring section B‐B′ by “untilting” it as a rigid block until it satisfies these constraints on the Eocene surface geology results the geometry shown in Figure 11c. In this model, the Ruby detachment initially dipped ∼53°W, and rocks in its footwall were rotated 42°E during fault slip (Figure 11b). Footwall rocks were then tilted ∼4° back to the west during slip on the Ruby Valley Fault zone (Figure 10a), resulting in a present dip of ∼15°W for the Ruby detachment. [36] Figure 12 shows the Harrison Pass pluton restored to shortly after it intruded but prior to slip on the Ruby detachment (ca. 35 Ma). Paleozoic strata on the southern margin of the pluton were deformed during its emplacement [Burton, 1997], but Ordovician and Silurian strata strike continuously north from section B‐B′ over the exposed top of the pluton to section A‐A′ (Figure 4). There is no major east‐west structure between sections A‐A′ and B‐B′, so we assume they restore similarly and use the Ordovician and Silurian strata overlying the pluton as a guide to its restored depth in the crust. This restoration (Figure 12) results in the Ruby detachment dipping ∼56°W, and the pluton itself tilted ∼50°E during fault slip, then tilted back ∼5°W during subsequent slip on the Ruby Valley Fault Zone. Heavy gray lines on Figure 12 show the estimated minimum and maximum depth of the top of the pluton: minimum, assuming only a thin veneer of Eocene rocks overlying Mississippian strata at the surface above the pluton (∼1200 m shallower), and maximum, assuming 1000 m of Eocene rocks overlying Permian strata at the surface above the pluton (∼900 m deeper). We thus estimate uncertainty on the restored depth of the top of the pluton to be about ±1 km, depending on the inferred thickness of the post‐Silurian Paleozoic section and overlying Eocene deposits. This uncertainty does not depend on how much the pluton is tilted (i.e., initial dip of the detachment), although the restored depth of the rest of the pluton (relative to its top) does depend on the amount of tilting. Figure 12 is consistent with Barnes et al.’s [2001]

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Figure 12. Restored cross section (A‐A′) across the Harrison Pass pluton.

petrologic interpretation that the pluton was emplaced at depths equivalent to ≤3–4 kb.

5. Age of the Harrison Pass Pluton [37] The age of the Harrison Pass pluton provides the maximum age for the onset of postintrusion slip on the Ruby detachment (the possibility of prepluton slip is considered in section 7.2), and the “start time” for time‐temperature paths derived from thermochronologic data presented in section 6. Since Wright and Snoke [1993] initially obtained a 36 Ma U‐Pb age for the pluton, Barnes et al. [2001] recognized that it consists of at least three major phases with a complex intrusive history, raising the possibility that some parts of the pluton may have intruded more recently than 36 Ma. To test this possibility, we obtained U‐Pb zircon dates by SHRIMP from each of the three major pluton phases (Figure 5) defined by Barnes et al. [2001]. A complete description of analytical methods is given in Appendix B, data from individual zircon grains is presented in Table B1, and reduced data from each sample is plotted in Figure 13. Sample H05‐RM104, from the granodiorite of Toyn Creek, yielded a U‐Pb age of 37.3 ± 0.3 Ma (12 of 15 grains) (Figure 13a). Sample JC06‐HP105, from the biotite monzogranite of Corral Creek, yielded a

U‐Pb age of 36.5 ± 0.2 (14 of 15 grains) (Figure 13b) and is thus distinctly younger than the Toyn Creek phase. Sample JC05‐HP3 was from a unit mapped as “sheets and dikes of biotite and two‐mica monzogranite” by Barnes et al [2001], although the actual sample was petrographically similar to the granodiorite of Toyn Creek and did not contain white mica. This sample contained two distinct zircon populations that yielded ages of 36.3 ± 0.4 Ma (effectively the same as the Corral Creek phase) and 38.1 ± 0.4 (the oldest zircons found in any of the three samples) (Figure 13c). One Proterozoic grain (∼1640 Ma) was found in sample JC06‐HP105, and inherited (pre‐Tertiary) cores were observed in all three samples. Although the Harrison Pass pluton does record several million years of zircon growth, the youngest zircons are 36.2–36.5 Ma, confirming that the most recent phases of the pluton were emplaced at 36 Ma.

6. Low‐Temperature Thermochronology [38] Low‐temperature thermochronology has been successfully used to date the timing of exhumation, fault slip, and extension in many parts of the Basin and Range Province. The methods most widely applied to the Ruby

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Figure 13. Concordia diagrams for U‐Pb SHRIMP analyses. Open circles omitted from age calculations.

Mountains have been K‐Ar dating of biotite and fission track dating of apatite and zircon; in this study we also apply apatite (U‐Th)/He dating. Relevant aspects of these methods are briefly reviewed here; detailed overviews of modern theory and practice can be found in the work of Reiners and Ehlers [2005]. [39] Biotite has a nominal closure temperature of 300°C– 350°C for the K‐Ar or 40Ar/39Ar systems, but it can be as high as 400°C or more depending on its composition and cooling history [Harrison et al., 1985; Grove and Harrison, 1996; McDougall and Harrison, 1999]. Because biotite is unstable during in vacuo heating, unique time‐temperature (t‐T) paths cannot be recovered from 40Ar/39Ar step‐heating experiments [McDougall and Harrison, 1999; Harrison and Zeitler, 2005], but K‐Ar and 40Ar/39Ar ages provide a minimum age for the time a sample cooled below 300°C–350°C. [40] Fission track dating is based on the spontaneous fission decay of 238U, which forms linear damage trails in the host crystal that are progressively erased (annealed) at ele-

vated temperatures [e.g., Tagami and O’Sullivan, 2005]. In apatite, fission tracks are annealed instantaneously (at geologic time scales) above 110°C–135°C, partially annealed between about 120°C and 60°C (the “Partial Annealing Zone”), and retained over geological time scales below about 60°C [e.g., Gleadow et al., 1986; Green et al., 1989]. Apatite fission track ages do not correspond to unique t‐T paths, but the distribution of confined track lengths within the sample can be inverse‐modeled to derive the t‐T path associated with the measured age [e.g., Green et al., 1989; Ketcham, 2005]. In zircon, the closure‐temperature of the fission track system varies somewhat depending on the degree of a‐damage, with “zero damage” zircons having a closure temperature of ∼340°C, compared to ∼230°C for “natural” zircon [Brandon et al., 1998; Tagami et al., 1998; Rahn et al., 2004]. Confined track lengths can be used to derive t‐T paths from zircon fission track samples [e.g., Tagami and O’Sullivan, 2005], but this is done far less commonly with zircon than with apatite.

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[41] The (U‐Th)/He system is based on the a‐decay of U and Th, which produces 4He that is lost by thermal diffusion at elevated temperatures [Zeitler et al., 1987; Farley, 2002]. In apatite, 4He is completely lost above about 80°C, partly retained over geologic time scales between 80°C and 40°C (the “Partial Retention Zone” or PRZ) and effectively completely retained below about 40°C [e.g., Wolf et al., 1996, 1998; Farley, 2000]. T‐t paths cannot be recovered from individual apatite (U‐Th)/ He ages, but they provide a very useful constraint on the lower temperature (4–5 km (Figure 12), the apatite fission track and (U‐Th)/He systems in all but the shallowest samples are likely to have been completely “open“ (>120°C) prior to exhumation and thus date slip on the Ruby detachment. On the other hand, little or no part of the fission track PAZ or (U‐Th)/He PRZ is likely to be preserved, limiting the information we can gather about the postintrusion (prefaulting) cooling history and geothermal gradient. To characterize the pre‐36 Ma cooling history of the crust adjacent to the Ruby Mountains, we collected additional samples from shallow Jurassic plutons in the Medicine Range to the east and the Cortez Range to the west (Figure 3) (we also collected five samples from the Piñon Range (Figure 3), three of Devonian quartzite and two of Mississippian conglomerate, but none yielded enough apatite to analyze). We also obtained six biotite 40Ar/39Ar total fusion ages from the Harrison Pass pluton in order to tie our transect to the biotite K‐Ar age contours of Kistler et al. [1981] (Figure 3), which figure prominently in previous interpretations but are not as well defined within the Harrison Pass pluton as they are within the northern part of the range. Thermochronometer age data are summarized in Table 2, analytical methods are described in Appendices A, C, D, and E, and raw data are provided in Tables A2, C1, and D1. Data and modeled time‐temperature paths are discussed in sections 6.1–6.3; tectonic interpretations based on these data are presented in the “Discussion” section. 6.1. Thermochronologic Data From the Harrison Pass Pluton [43] The Harrison Pass pluton generally yielded high‐ quality apatite–euhedral, clear, intact grains largely free of obvious inclusions or other defects, although grains from the westernmost samples were generally somewhat lower‐ quality, possibly due to proximity to the Ruby detachment and associated hydrothermal fluids. Apatite grains tended to be small, with the largest having a diameter of only 120 mm, and most being in the range of 80–120 mm. This leads to two problems with (U‐Th)/He dating, small 4He concentrations and large a‐ejection corrections. The large a‐ejection cor-

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rections (Ft 0.6–0.8) are more troublesome because no uncertainty is associated with the correction [Farley, 2002], and these probably contribute to the scatter in single‐grain ages both between and within individual samples. All samples contained abundant fresh biotite. [44] Biotite 40Ar/39Ar total fusion ages range from 33 to 30 Ma at shallower depths (close to the age of the pluton), to 24–25 Ma at deeper (9–10 km) depths (Figure 14). Apparent fission track ages from 14 samples of the Harrison Pass pluton (Table 2) ranged from about 12–25 Ma. The oldest age (24.8 ± 2.9 Ma) is from a dike just above the pluton roof on the east side of the range (sample JC05‐HP10, Figure 4). The other 13 samples ranged from 12 to 21 Ma and show a trend of decreasing age with depth at an apparent rate of ∼2 km/Ma (Figure 14). Apatite (U‐Th)/He ages from 10 of these samples ranged from about 11 to 20 Ma, with a weighted mean of 14.4 ± 0.6 Ma, and show no trend of decreasing age with depth. Another sample (JC05‐HP11) yielded single‐grain (U‐Th)/He ages ranging from 20 to 33 Ma that did not overlap at 2s uncertainty. The decrease in fission track age between the shallowest sample (JC05‐ HP10) and the next deepest cluster of samples may represent the base of the fission track PAZ (∼120°C), but otherwise the data do not show the slope breaks characteristic of either a fission track PAZ or a (U‐Th)/He PRZ on an age‐depth plot [e.g., Stockli, 2005] (Figure 14). [45] Confined track‐length data were obtained from six of the Harrison Pass pluton samples. Apatite fission track age data, confined track‐length data, and (U‐Th)/He data were also obtained from a granite boulder (derived from the Harrison Pass pluton) deposited in Miocene (11.6 Ma) conglomerate on the west side of the range (Figure 7f). Combined fission track age data, length data, and (U‐Th)/He data from these seven samples were used to model time‐temperature (t‐T) paths using the HeFTy algorithm of Ketcham [2005] (modeling parameters are given in Appendix E). For the pluton samples, models were constrained to a start time/ temperature of >300°C at 36 Ma (emplacement of the Harrison Pass pluton) and a finishing temperature of 10 ± 10°C at zero Ma. Modeled t‐T paths for these samples are shown in Figure 15a–15f. All record rapid cooling from >120°C to