Within extended orogens, records that reflect the driving processes and dynamics of early extension are often overprinted by subsequent orogenic collapse. The Copper Mountains of northeastern Nevada preserve an exceptional record of hinterland extensional deformation and high-elevation basin formation, but current geochronology and thermochronology are insufficient to relate this to broader structural trends in the region. This extension occurred concurrent with volcanism commonly attributed to Farallon slab removal. We combine thermochronology of both synextensional hanging-wall strata and footwall rocks to comprehensively evaluate the precise timing and style of this deformation. Specifically, we apply (U-Th)/(He-Pb) double dating of minerals extracted from Eocene–Oligocene Copper Basin strata with multi-mineral (U-Th)/He and 40Ar/39Ar thermochronology of rocks sampled across an ∼20 km transect of the Copper Mountains. We integrate basement and detrital thermochronology records to comprehensively evaluate the timing and rates of hinterland extension and basin sedimentation. Cooling and U-Pb crystallization ages show the Coffeepot Stock, which spans the width of the Copper Mountains, was emplaced at ca. 109–108 Ma, and then cooled through the 40Ar/39Ar muscovite and biotite closure temperatures by ca. 90 Ma, the zircon (U-Th)/He closure temperature between ca. 90 and 70 Ma, and the apatite (U-Th)/He closure temperature between 43 and 40 Ma. Detrital apatite and zircon (U-Th)/(He-Pb) double dating of late Eocene fluvial and lacustrine strata of the Dead Horse Formation and early Oligocene fluvial strata of the Meadow Fork Formation, both deposited in Copper Basin, shows that Early Cretaceous age detrital grains have a cooling history that is analogous to proximal intrusive rocks of the Coffeepot Stock. At ca. 38 Ma, cooling and depositional ages for Copper Basin strata reveal rapid exhumation of proximal source terranes (cooling rate of ∼37 °C/m.y.); in these terranes, 8–12 km of slip along the low-angle Copper Creek normal fault exhumed the Coffeepot Stock in the footwall. Late Eocene–early Oligocene slip along this fault and an upper fault splay, the Meadow Fork fault, created a half graben that accommodated ∼1.4 km of volcaniclastic strata, including ∼20 m of lacustrine strata that preserve the renowned Copper Basin flora. Single-crystal sanidine 40Ar/39Ar geochronology of interbedded tuffs in Copper Basin constrains the onset of rapid exhumation to 38.0 ± 0.9 Ma, indicating that surface-breaching extensional deformation was coincident with intense proximal volcanism. Coarse-grained syndeformational sediments of the Oligocene Meadow Fork Formation were deposited just prior to formation of an extensive regional Oligocene–Miocene unconformity and represent one of the most complete hinterland stratigraphic records of this time. We interpret this history of rapid late Eocene exhumation across the Copper Mountains, coeval volcanism, and subsequent unconformity formation to reflect dynamic and thermal effects associated with Farallon slab removal. The final phase of extension is recorded by late, high-angle normal faults that cut and rotate the early middle Miocene Jarbidge Rhyolite sequence, deposited unconformably in the hanging wall. These results provide an independent record of episodic Paleogene to Miocene exhumation documented in Cordilleran metamorphic core complexes and establish that substantial extension occurred locally in the hinterland prior to province-wide Miocene extensional break-up.
In the hinterland of the Sevier orogenic belt, from eastern Nevada to western Utah, the potential links between Eocene magmatism, extension, and sedimentation are widely debated (Fig. 1; Armstrong, 1982; Axen et al., 1993; McGrew and Snee, 1994; Brooks et al., 1995; Rahl et al., 2002; Henry, 2008; Druschke et al., 2009a). Hinterland deformation in this area associated with Miocene Basin and Range extension is well documented (e.g., Colgan et al., 2010; Brueseke et al., 2014; Lund Snee et al., 2016; Camilleri et al., 2017), but the importance of an earlier, Eocene or Oligocene, phase of extension is supported by relatively few studies (e.g., Axen et al., 1993; Potter et al., 1995; Rahl et al., 2002; Cline et al., 2005; Long et al., 2018). This uncertainty reflects the absence of quantitative constraints on both the timing and magnitude of Eocene and Oligocene extensional deformation in the Cordilleran hinterland (Henry et al., 2011). During the Paleozoic and Mesozoic, the Sevier hinterland region underwent protracted contractional deformation (Trexler and Nitchman, 1990; Miller and Hoisch, 1995; DeCelles, 2004; Trexler et al., 2004; Long et al., 2014) to form a belt of 50–60-km-thick crust (Coney and Harms, 1984; McGrew et al., 2000) that supported a Paleogene orogenic plateau with elevations of ≥2 km by the Late Cretaceous (Snell et al., 2014) and up to 3.5 km by the late Oligocene (Thorman et al., 1991; Axelrod, 1997; Camilleri and Chamberlain, 1997; Best et al., 2009; Cassel et al., 2014, 2018). From the Paleogene to the early Miocene, a chain of metamorphic core complexes formed across the hinterland, from British Columbia to Arizona, some of which brought middle and lower crustal rocks from depths of ∼35–40 km to the surface (Armstrong, 1982; Dokka et al., 1986; Hodges et al., 1992; McGrew and Snee, 1994; Henry et al., 2011; Hallett and Spear, 2014). Well-documented thermochronologic trajectories of multiple Cordilleran metamorphic core complexes suggest that initial metamorphic core complex exhumation occurred during the Late Cretaceous and/or Paleogene (Snoke and Miller, 1988; Wright and Snoke, 1993; Wells, 1997; McGrew et al., 2000), potentially aided by the onset of magmatism (MacCready et al., 1997; Bendick and Baldwin, 2009; Smith et al., 2014, 2017; Hallett and Spear, 2015). However, this metamorphic core complex record of partial melting, crustal flow, and exhumation is not clearly reflected in the stratigraphic record (Colgan et al., 2010; Lund Snee et al., 2016; Smith et al., 2017; Canada et al., 2019). Outside or peripheral to the metamorphic core complexes, existing structural histories of Paleogene hinterland extension also commonly lack a well-preserved complementary stratigraphic record or constraints on the magnitude and/or timing of deformation (e.g., Camilleri and Chamberlain, 1997; Long et al., 2015; Pape et al., 2016). These uncertainties present a considerable obstacle for deciphering the spatial partitioning of strain at this time and its implications for the tectonic evolution of the North American Cordilleran hinterland.
The Late Cretaceous to Eocene Cordilleran hinterland is often described as a low exhumation, eroding bedrock highland with low (∼2 km) structural relief (Armstrong, 1968; Coney and Harms, 1984; Vandervoort and Schmitt, 1990; Thorman et al., 1991; Long, 2012; Best et al., 2016; Camilleri et al., 2017), where high-magnitude (>1 km throw) surface-breaching normal faults may have been rare until the Miocene (Long, 2019). By combining lower bounds for paleoelevation estimates of 2.0 ± 0.2 km from leaf physiognomic methods in Copper Basin (Wolfe et al., 1998) with upper bound elevation estimates of ∼4.2 km (Mix et al., 2011; Chamberlain et al., 2012), Chamberlain et al. (2012) suggested that middle Eocene topographic relief of up to 2.2 km existed in northeastern Nevada. Based on the depth of inferred paleovalleys, Henry (2008) reached an estimate of up to 1.6 km of Eocene relief across this same area, concluding that this reflects deep incision into Paleozoic basement, which was potentially augmented by limited Eocene extension. A pre-Miocene phase of extension is supported by isolated Cretaceous–Eocene hinterland basins and angular unconformities documented within Paleogene strata (Brooks et al., 1995; Dubiel et al., 1996; Mueller et al., 1999; Satarugsa and Johnson, 2000; Henry et al., 2001; Haynes, 2003; Cline et al., 2005; Henry, 2008; Druschke et al., 2009b). Widespread hinterland sedimentation occurred during the Eocene, primarily within several lacustrine-dominated basins that have been largely interpreted to signify extensional basin formation at relatively high elevations (Axelrod, 1966, 1996; Rahl et al., 2002; Haynes, 2003; Druschke et al., 2009a). In east-central Nevada, Cretaceous to middle Eocene alluvial and lacustrine rocks of the Sheep Pass Formation are scattered across a 15,000 km2 area adjacent to the Snake Range metamorphic core complex (Druschke et al., 2009b). Deposition of these sediments is interpreted to reflect accommodation formed during 4 km of throw along the Ninemile fault system, but this deformation is broadly attributed to episodic Late Cretaceous–Oligocene extension (Druschke et al., 2009b). To the north, fluvial and lacustrine rocks of the Elko Formation cover a 23,000 km2 area adjacent to the Ruby Mountains–East Humboldt Range metamorphic core complex (Coats, 1987; Haynes, 2003) but are not unequivocally diagnostic of deposition within an extensional basin (Henry, 2008; Smith et al., 2017). Elko Formation strata are interpreted to be locally tilted 10°–15° to the southeast prior to eruption of ca. 40–38 Ma ignimbrites, which is inferred to indicate extension contemporaneous with late Eocene magmatism (Brooks et al., 1995; Henry and Faulds, 1999; Haynes, 2003; Hickey et al., 2005). The degree of discordance between Elko Formation strata and overlying volcanic rocks is interpreted to vary across northeastern Nevada (Brooks et al., 1995). This may partly relate to inclined fabrics created through tractional aggradation in pyroclastic debris flows, which may be misinterpreted as bedding (Branney and Kokelaar, 2002). Similar angular unconformities (10°–15° westward tilting) in proximal volcanic rocks of the Robinson Mountain volcanic field are interpreted by Lund Snee et al. (2016) to represent a maximum of 1 km of extension. This limited and often enigmatic record of Paleogene crustal deformation across the hinterland hinders development of a clear understanding of the relationship between Eocene accommodation and crustal extension.
Copper Basin, located in the Copper Mountains of northeastern Nevada, is characterized as a half graben that contains ∼1.4 km of middle Eocene to early Oligocene volcaniclastic and alluvial-fluvial strata that accumulated during 8–12 km of displacement along the northeast-trending Copper Creek normal fault (Fig. 2; Coats, 1964; Axelrod, 1966; Rahl et al., 2002). Here we present precise measurements of the timing of extension in the Copper Mountains and the implications of this deformation for Eocene tectonics in the Cordilleran hinterland. This is accomplished by integrating (1) multi-mineral 40Ar/39Ar (hornblende, biotite, muscovite, and K-feldspar) and (U-Th)/He thermochronology (zircon, titanite, and apatite) of igneous and metamorphic rocks sampled across a structural transect of the Copper Mountains (Fig. 1) with (2) (U-Th)/(He-Pb) double dating of detrital apatite and zircon grains from late Eocene–early Oligocene Copper Basin strata (Fig. 2); double dating provides both a U-Pb crystallization age and (U-Th)/He cooling age for each individual grain. These methods show agreement between source exhumation and basin sedimentation trends and permit evaluation of the potential association of hinterland deformation with magmatism.
Volcanism migrated to the southwest across a >200 km transect of northeastern Nevada from ca. 44 Ma to ca. 38 Ma (Henry and John, 2013) and across a >110 km transect of central Nevada during the ca. 36–18 Ma Great Basin ignimbrite flare-up (Best et al., 2009; Henry and John, 2013). The comparatively slow migration of volcanism across central Nevada (∼11 km/m.y. compared to ∼31 km/m.y. for northeastern Nevada; Henry et al., 2012), in addition to the widespread volcanism in this area, may reflect a major change in subduction geometry (Best et al., 2016) and/or a transition from predominantly slab rollback volcanism to mantle lithosphere delamination (Best et al., 2009, 2016; DeCelles et al., 2009). Calderas associated with this volcanism erupted up to 3000 km3 of silicic volcanic rocks, some of which can be geochemically correlated over areas exceeding 55,000 km2 (Best et al., 2009; Cassel et al., 2009; Henry et al., 2012; Henry and John, 2013). In northeastern Nevada, the uneven spatial distribution of these volcanic rocks indicates many of them were deposited in preexisting valleys (Henry, 2008). 40Ar/39Ar geochronology of abundant tuffs exposed within Copper Basin (Fig. 2; e.g., Rahl et al., 2002; Henry et al., 2011; Smith et al., 2017) facilitates basin-wide correlation of strata and depositional age assessment for sediment lag-time calculations.
Lag time is defined as the difference between the cooling and depositional ages of detrital minerals and can therefore be used to provide quantitative constraints on exhumation trends (e.g., Brandon and Vance, 1992; Garver et al., 1999; Ruiz et al., 2004; Saylor et al., 2012; Thomson et al., 2017; Canada et al., 2019). By combining cooling histories of detrital minerals with depositional ages and bedrock thermochronology of rocks exposed across the Copper Mountains, we constrain the timing and magnitude of Eocene extension in the central Cordilleran hinterland and evaluate the response of this high-elevation landscape to magmatism induced by removal of the Farallon slab and/or progressive delamination of the mantle lithosphere.
Copper Mountains Geology
The Copper Mountains preserve Neoproterozoic to early Paleozoic metasedimentary rocks that record prolonged passive margin sedimentation, tectonic burial, and Cenozoic extension (Rahl et al., 2002; Crafford, 2008; Linde et al., 2016). This includes phyllite and quartzite of the Neoproterozoic McCoy Creek Group and Neoproterozoic–Cambrian Prospect Mountain Quartzite (Misch and Hazzard, 1962; Woodward, 1967), which form the main bedrock lithologies exposed adjacent to Copper Basin (Fig. 1). Rocks of this age are sparsely exposed across northeastern Nevada, but equivalent strata are inferred to be 3 km thick beneath >5 km of Paleozoic strata in the southern Ruby Mountains (Colgan et al., 2010; Pape et al., 2016) and ∼3 km thick in the Schell Creek Range of east-central Nevada (Young, 1960; Dechert, 1967; Norman, 2013). In the Copper Mountains, these metasedimentary rocks are overlain by ∼1.6 km of shale and limestone of the Ordovician Tennessee Mountain Formation (Bushnell, 1967). Crafford (2008) assigned these strata to the Nolan Belt Domain because they exhibit a complex polyphase deformation history that is distinct from proximal rocks of the same age. Ordovician strata are unconformably overlain by ∼1.3 km of conglomerate and limestone of the Pennsylvanian to Permian Sunflower Formation (Bushnell, 1967), deposited within the Antler overlap assemblage, and Mississippian–Permian strata that were emplaced along the Golconda thrust around Early Triassic time (Silberling, 1975; Crafford, 2008). In the Copper Mountains, this sequence of Proterozoic–late Paleozoic passive margin strata is intruded by rocks associated with the Cretaceous Coffeepot intrusive suite, which locally cut Paleozoic thrust faults to the north of Copper Mountain (Fig. 1; McGrew et al., 2000; Rahl et al., 2002).
The Coffeepot Stock is one of the largest backarc plutons exposed in northeastern Nevada and part of a series of east-west–trending 110–75 Ma Cretaceous granitoids that are exposed near the Nevada-Idaho border (Fig. S5 in the Supplemental Materials1; Seymour, 1980; Stewart and Carlson, 1981; Miller et al., 1990; du Bray, 2007). Over 90% of the total volume of the stock consists of coarse-grained biotite quartz monzonite, monzogranite, and granodiorite that are locally cut by late-stage alaskitic, aplitic, and pegmatitic dikes and veins (Seymour, 1980; Rahl et al., 2002; Table S1 and Fig. S1). Despite the homogeneity of the stock and lack of assimilated material, geochemical data indicate an inward progression of fractional crystallization that implies crystallization was time-transgressive (Seymour, 1980). At the contact with the Tennessee Mountain Formation, amphibolite-facies metamorphism from localized metasomatism formed a broad contact aureole: the Tennessee Mountain skarn (Lapointe et al., 1991; Rahl et al., 2002). The mineralogy of this skarn is attributed by Coats and McKee (1972) to the relatively high content of hyperfusible material and the high intrusion temperature of Coffeepot Stock magma. Rahl et al. (2002) used high-grade mineral assemblages in this skarn to estimate peak metamorphic pressures of <375 MPa as well as muscovite-bearing granitoids with fibrolitic sillimanite in the structurally deepest eastern portion of the pluton to record emplacement pressures of ≥220 MPa, indicating emplacement depths of 8–14 km.
The Copper Mountains are separated from Copper Basin by the east-dipping, low-angle Copper Creek and Meadow Fork normal faults (Figs. 2 and 3; Rahl et al., 2002). The Copper Creek fault (CCF) places Cambrian to Ordovician Tennessee Mountain Formation strata in contact with Neoproterozoic–Lower Cambrian rocks of the McCoy Creek Group and Prospect Mountain Quartzite (Rahl et al., 2002). The CCF fault zone is characterized by mylonitized rocks that are locally brecciated and contain slickenlines that imply dip slip toward the southeast (Rahl et al., 2002). The Meadow Fork fault (MFF) splays off the CCF and separates Meadow Fork Formation strata from the Tennessee Mountain Formation. In the northern part of Copper Basin, Tennessee Mountain Formation strata contain a skarn associated with a granitoid intrusion, both of which were likely transported along the CCF from the Tennessee Mountain area (Fig. 1; Rahl et al., 2002).
Stratigraphy of Copper Basin
Copper Basin consists of ∼1.4 km of middle Eocene to early Oligocene strata that accumulated in the hanging wall of the Meadow Fork fault between ca. 43 and 29 Ma (Fig. 4; Coats, 1964; Henry, 2008). The age model for these strata integrates new single-crystal sanidine 40Ar/39Ar geochronology (Henry, 2008; Smith et al., 2017) and legacy biotite 40Ar/39Ar ages (Rahl et al., 2002) recalculated to the 28.201 Ma age for Fish Canyon Tuff sanidine (Fig. 2; cf. Smith et al., 2017). The middle to late Eocene Dead Horse Formation is composed of ∼1 km of volcaniclastic strata that are dominated by ignimbrites and ash-fall tuffs deposited unconformably on Paleozoic limestone (Fig. 4; Rahl et al., 2002). The upper portion of the Dead Horse Formation contains laterally discontinuous beds of tuffaceous sandstone and pebble-cobble conglomerate with planar-tabular and trough cross-bedding and thin pebble lags (Fig. 5). These beds are often scoured at the base but also contain rippled tops preserved in thin interbeds of siltstone and shale (Fig. 5). These characteristics are indicative of channel and overbank deposits of braided streams (e.g., Cassel et al., 2012), which are broadly correlative to basal Elko Formation facies, deposited as paleovalley fill (Henry, 2008; Smith et al., 2017). Conglomerate clast lithology counts show coarse-grained detritus was primarily derived from volcanic and proximal Precambrian–early Paleozoic metasedimentary rocks (Fig. 4). Paleocurrent measurements taken near the Meadow Fork Formation contact suggest that paleoflow was predominantly oriented toward the south during the late Eocene (mean trend 187°; Fig. 5). A thin (<25-m-thick) section of tuffaceous siltstone and claystone near the top of the Dead Horse Formation preserves abundant Eocene paleoflora assemblages (e.g., Axelrod, 1966, 1997), including conifer and alder species (Fig. 5) that are interpreted to signify paleoelevations ranging from 1.1 to 2.8 km based on a range of paleoclimate techniques (Axelrod, 1996; Chase et al., 1998; Wolfe et al., 1998). Mix et al. (2011) used δ18O values of smectites extracted from Dead Horse Formation strata to estimate hinterland elevations of up to ∼3.4 km by the late Eocene. In comparison, by using δD values of volcanic glasses from across eastern Nevada and western Utah, Cassel et al. (2014, 2018) estimate Eocene and early Oligocene hinterland plateau elevations of 2.7–3.1 km. Two tuff samples collected from the Dead Horse Formation contain glass with δD values of −90.6‰ ± 4.0‰ and −100.7‰ ± 2.5‰, indicating evaporative enrichment of meteoric waters in a fluctuating profundal lacustrine environment (cf. Smith et al., 2017; Cassel and Breecker, 2017).
The late Eocene to early Oligocene Meadow Fork Formation consists of ∼400 m of coarse-grained fluvial and alluvial strata and interbedded tuffs that rest conformably above Dead Horse Formation strata (Fig. 4; Coats, 1964; Henry, 2008). The Meadow Fork Formation is distinguished by poorly sorted, non–traction-structured pebble-cobble conglomerate beds that contain angular to subrounded clasts of marble, quartzite, phyllite, and granite up to 1 m in diameter (Fig. 6). All of the strata within Copper Basin thicken to the northwest, toward the Meadow Fork fault, but this is most pronounced within the coarse-grained clastic strata of the Meadow Fork Formation. These stratal geometries and facies characteristics are interpreted to signify deposition in debris-flow–dominated alluvial fans that transported local detritus from the relatively steep northwestern margin of the basin.
MATERIALS AND METHODS
Detrital Zircon and Apatite (U-Th)/(He-Pb) Double Dating
Detrital (U-Th)/(He-Pb) double dating refers to the process of obtaining a U-Pb crystallization age and a (U-Th)/He cooling age for the same detrital grain (Reiners et al., 2005). This combined history of crystallization and cooling permits discrimination between potential sources with similar U-Pb age populations as well as quantification of source exhumation rates (Saylor et al., 2012; Painter et al., 2014; Thomson et al., 2017). Here we apply (U-Th)/(He-Pb) double dating of both detrital zircon and detrital apatite to determine the provenance of detrital minerals and assess the timing of high-magnitude extensional deformation (full analytical data and procedures listed in Supplemental Materials [footnote 1]). Apatite dating is not typically used for detrital studies, because apatite has low U concentrations, high concentrations of common Pb, and low resistance to physical and chemical abrasion in comparison to zircon (Thomson et al., 2012). Copper Basin strata, however, contain large, euhedral, and inclusion-free detrital apatite grains suitable for double dating. The apatite (U-Th)/He system has a substantially lower closure temperature (∼70 °C; Farley, 2000) than the zircon (U-Th)/He system (∼180 °C; Wolfe and Stockli, 2010), making it an excellent complementary technique for deciphering near-surface processes.
Detrital apatite grains were analyzed at the UTChron facility at the University of Texas at Austin by laser ablation split stream–inductively coupled plasma mass spectrometry (LASS-ICPMS), which simultaneously generates precise U-Pb isotopic ratios and trace-element concentrations (Kylander-Clark et al., 2013; Sections 1 and 4 of Supplemental Materials [footnote 1]). These were used to determine sediment provenance in an area with geochemically and geochronologically distinct source areas (Fig. S5). Apatite U-Pb ages are inferred to be equivalent to the lower intercept of a chord fit to all apatite grain ages on a Tera-Wasserburg projection (Tera and Wasserburg, 1972; Kirkland et al., 2018). Since the amount of 204Pb is generally significant in apatite, this graphical approach allows estimation of an age of initial crystallization using only 207Pb/206Pb and 238U/206Pb ratios, assuming original isotopic homogeneity and no subsequent disturbance of the system (Tera and Wasserburg, 1972; Ludwig, 1998). Apatite U-Pb ages were also determined using the 204Pb correction of Stacey and Kramers (1975), where 207Pb/206Pb ages are used as an initial estimate for model 204Pb composition (Fig. S4). Correction of 206Pb, 207Pb, and 208Pb peaks was completed by measurement of 202Hg to correct for 204Hg isobaric interference and application of Stacey and Kramers (1975) common lead model. We then used the iterative process of Thomson et al. (2012) to calculate new 206Pb/238U ratios and ages. 204Pb-corrected ages yield a weighted-mean age that is within 2σ uncertainty of the lower intercept age of the Tera-Wasserburg projection. During detrital apatite ablation, simultaneous analysis of the same ablation volume, using a secondary ICPMS, measured 43Ca, 44Ca, 87Sr, 89Y, 137Ba, 139La, 140Ce, 141Pr, 146Nd, 147Sm, 153Eu, 157Gd, 159Tb, 163Dy, 165Ho, 166Er, 169Tm, 172Yb, 175Lu, 232Th, and 238U to evaluate potential correspondence between grain age, grain trace element geochemistry, and source geochemistry (see Supplemental Materials).
Following U-Pb geochronology, zircon and apatite grains from each major grain age population, defined as including three or more grains whose ages overlap given the analytical uncertainties (Saylor et al., 2012; Thomson et al., 2017), were picked from mounts and analyzed using (U-Th)/He thermochronology following the procedures of Wolfe and Stockli (2010) at the University of Texas at Austin (see also Section 1 in Supplemental Materials [footnote 1]). Apatite, zircon, and titanite extracted from granitoid samples in the Copper Mountains (Table 1) were analyzed for (U-Th)/He thermochronology following the procedures of Wolfe and Stockli (2010) at the University of Kansas (Section 5 in Supplemental Materials).
40Ar/39Ar Thermochronology and U-Pb Geochronology
Igneous and metamorphic rocks were sampled for 40Ar/39Ar thermochronology along a transect across the Copper Mountains to constrain the timing of plutonic emplacement, the cooling history of footwall rocks, and depositional ages within Copper Basin (Figs. 3 and 4). Incremental-heating and single-crystal laser-fusion experiments were conducted at Ohio State University on a range of minerals extracted from these rocks (i.e., hornblende, muscovite, biotite, and K-feldspar). Full results of these analyses are reported in Table 2, and methods are discussed in detail in Section 1 of Supplemental Materials (footnote 1; see also Section 6). For incremental-heating experiments, all reported plateau ages are >70% concordant and contain >50% cumulative 40Ar released within the plateau fraction. For single-crystal laser-fusion experiments, weighted-mean ages were calculated using all analyses that attained >90% 40Ar released. Ages were calculated using the 28.201 Ma age for Fish Canyon Tuff sanidine (FCs) and the equations of Kuiper et al. (2008) and Renne et al. (1998).
Crystallization ages were determined using zircon U-Pb geochronology (Fig. 7) for samples within the western fault block (980730-5), the eastern fault block (000714-1), and the displaced cupola within the northern part of Copper Basin (970721-4C) (Figs. 1 and 2). Zircon geochronology was completed using high-resolution, laser ablation–inductively coupled plasma mass spectrometry (HR-LA-ICPMS) at the UTChron facility at the University of Texas at Austin (analytical data and procedures listed in Supplemental Materials [footnote 1]).
Zircon U-Pb Geochronology
Figure 7 shows concordia diagrams of zircon U-Pb ages. Most zircon grains have concordant ages that imply mean crystallization ages for the Coffeepot Stock between 108.4 ± 0.5 Ma and 111.0 ± 0.5 Ma (Fig. 7), which is in excellent agreement with prior estimates inferred from hornblende and biotite 40Ar/39Ar geochronology (109–108 Ma; Rahl et al., 2002). One sample taken at the southeastern extent of the pluton (000714-1) contains a bimodal distribution of Cretaceous and Jurassic (176–157 Ma) ages that we infer to reflect inheritance (Fig. 7). This interpretation is supported by the uneven profile of 238U/206Pb ratios during laser ablation; these ratios show Cretaceous rim and Jurassic core ages for many of the Jurassic age zircon grains.
Detrital Zircon and Apatite U-Pb Geochronology and Trace-Element Chemistry
Detrital zircon U-Pb geochronology of four sandstone samples from the Dead Horse Formation and one sandstone sample from the Meadow Fork Formation shows an up-section transition from Eocene volcaniclastic to Cretaceous plutonic grain age populations (Fig. 8; Table S2 [footnote 1]). Samples from the upper part of the Dead Horse Formation (NV12-181CB, NV12-169CB, and NV12-159CB) with maximum depositional ages ranging from 39.4 ± 0.2 Ma to 37.1 ± 0.1 Ma are composed of >99% Eocene volcanic grains (Figs. S2 and S3). A sample taken from near the top of the Dead Horse Formation (NV12-176CB) has a bimodal distribution of 45–35 Ma volcanic grains (79%) and 111–101 Ma plutonic grains (17%). A sample taken from the base of the Meadow Fork Formation (NV12-162CB) includes 43–34 Ma (4%), 115–100 Ma (61%), and 130–116 Ma (32%) detrital zircon grain ages (Fig. 8). The largest of these detrital zircon U-Pb age groups (115–100 Ma) is consistent with crystallization ages obtained for the Coffeepot Stock, but the source of the older (130–116 Ma) zircon grain ages is uncertain; it may reflect an early phase of crystallization within the Coffeepot intrusive suite. This sudden introduction of plutonic detritus at the contact of the Dead Horse Formation and Meadow Fork Formation is also reflected in conglomerate clast lithology counts throughout Copper Basin, which show the Meadow Fork Formation consists of ∼50% granitic clasts and <10% volcanic clasts (Figs. 4 and 6).
Lower intercepts on Tera-Wasserburg plots, calculated by regressing 129 apatite grains from these same samples, yield mean U-Pb ages of 106 ± 9 Ma and 108 ± 10 Ma with a mean square of weighted deviates (MSWD) of <2 (Fig. 9); these ages are within 2σ uncertainty of the ca. 109 Ma crystallization age of the Coffeepot Stock (Fig. 7). We therefore interpret these ages to represent the time of apatite crystallization within the Coffeepot Stock, suggesting the stock was emplaced at relatively shallow depths. Rare-earth element (REE) concentrations and patterns are similar for all apatite grains from both detrital samples (Fig. 9), but observed variations in trace-element composition likely reflect evolution in the parental melt composition or late crystallization from residual melts (Belousova et al., 2002). The low slope of the REE pattern, low Ce/Yb ratios, and heavy rare-earth element (HREE) enrichment are all indicative of a granitic source (Belousova et al., 2001, 2002). The weak negative Eu anomaly indicates less fractionation and that the ratio of plagioclase to potassium feldspar in the host rocks was relatively high (Belousova et al., 2002; Kirkland et al., 2018), which is consistent with chemical analyses of the Coffeepot Stock (i.e., Seymour, 1980; Section 2 in Supplemental Materials [footnote 1]). As discussed above, detrital zircon U-Pb geochronology reveals Copper Basin samples yield two significant age populations that represent volcanic (45–35 Ma) and plutonic (130–100 Ma) detritus. Since volcanic grains have statistically equivalent crystallization and cooling ages (Ruiz et al., 2004; Saylor et al., 2012), the plutonic age population was targeted for (U-Th)/He analysis to assess source exhumation trends.
(U-Th)/He and 40Ar/39Ar Thermochronology
Several samples of the Coffeepot Stock were analyzed for zircon, titanite, and apatite (U-Th)/He thermochronology following the procedures of Wolfe and Stockli (2010) at the University of Kansas. In the western fault block, these samples have zircon (U-Th)/He (ZHe) ages that range from ca. 89.2 to 69.6 Ma and apatite (U-Th)/He (AHe) ranging from ca. 52.2 to 27.0 Ma (Fig. 3; Table 1). Both ZHe and AHe ages generally decrease toward the west, where a steep (∼75° dip) normal fault bounds the western margin of the pluton. These ages suggest that this fault was active during Cretaceous–Oligocene exhumation of the western part of the Coffeepot Stock. In the eastern fault block, between the Bruneau Valley fault and Copper Creek fault, ZHe ages decrease from 53.7 ± 3.2 Ma in the west to 37.2 ± 1.9 Ma in the east (Figs. 2 and 3). This progression of ages records footwall exhumation along the Copper Creek fault and tilting along the Bruneau Valley fault. In the northern part of Copper Basin, a sample of the allochthonous plutonic body has a ZHe age of 81.3 ± 3.8 Ma and an AHe age of 32.6 ± 2.0 Ma, which are consistent with Coffeepot Stock cooling ages near Tennessee Mountain, where this body is inferred to have originated (Figs. 1–3; cf. Rahl et al., 2002).
40Ar/39Ar thermochronology of a range of minerals separated from the same samples as Rahl et al. (2002), as well as several new samples, yields new constraints on the thermal history of footwall rocks exposed in the Copper Mountains. Single-crystal K-feldspar 40Ar/39Ar analyses yield weighted-mean ages of 70.73 ± 1.41 Ma and 71.87 ± 1.65 Ma for samples collected in the eastern fault block (Table 2), indicating cooling through the ∼200 °C K-feldspar closure temperature during the latest Cretaceous (Fig. 3; Reiners and Brandon, 2006). A sample collected near the western margin of the eastern fault block, however, has multi-crystal muscovite and biotite 40Ar/39Ar ages of 91.37 ± 0.64 Ma and 55.39 ± 2.39 Ma, respectively, suggesting slower cooling through the ∼310 °C biotite closure temperature (Fig. 3; Reiners and Brandon, 2006). In Copper Basin, a tuff located near the base of the Meadow Fork Formation yields a biotite plateau age of 32.92 ± 0.29 Ma and is separated by ∼150 m of coarse-grained alluvial strata from a tuff with a biotite 40Ar/39Ar age of 29.77 ± 0.53 Ma (Figs. 2 and 4). These ages yield maximum age constraints for the final period of deposition within Copper Basin.
Detrital Zircon and Apatite (U-Th)/(He-Pb) Double Dating
Of the two dominant U-Pb age populations in Copper Basin samples, volcanic grains (45–35 Ma) have statistically equivalent crystallization and cooling ages (Ruiz et al., 2004; Saylor et al., 2012); so grains with U-Pb ages of 130–100 Ma were targeted to assess source terrane exhumation trends. Detrital (U-Th)/(He-Pb) double dating is critical in this circumstance to eliminate first-cycle volcanic zircon and apatite grains and to select non-volcanic detrital grains for (U-Th)/He analysis (cf. Saylor et al., 2012). Double dated detrital grains with Cretaceous U-Pb ages extracted from the uppermost Dead Horse Formation have 89.6–64.8 Ma zircon He ages and 106.5–39.8 Ma apatite He ages (Fig. 9). Seventy-one percent of detrital apatite grains from this sample have Eocene He ages. Detrital grains with Cretaceous U-Pb ages extracted from the base of the Meadow Fork Formation have 139.6–64.7 Ma zircon He ages and 21.7–15.3 Ma apatite He ages. These ages imply postdepositional heating and thermal resetting resulting from elevated near-surface temperatures, between the apatite He and zircon He closure temperatures, during the early Miocene.
Emplacement and Cooling History of the Coffeepot Stock
We divide the Copper Mountains into three structural blocks that record discrete exhumation histories, separated by the Bruneau Valley fault and the Copper Creek fault (Figs. 1–3). In addition, we infer that the entire western margin of the Coffeepot Stock is bound by a high-angle, down-to-the-west normal fault here named the Tennessee Mountain fault. Dipping ∼75° W, the Tennessee Mountain fault juxtaposes the stock against a hanging wall carrying ∼900 m of ca. 15 Ma Jarbidge Rhyolite (Bushnell, 1967; Brueseke et al., 2014). The western block also preserves a 10°–15° east-tilted unconformity on its eastern flank, overlain by at least 200 m of Jarbidge Rhyolite (Figs. 1–3). Westward projection of the nonconformity surface suggests a minimum of 2 km of throw on the Tennessee Mountain fault. Consequently, the western part of the pluton appears to be tilted gently eastward, exposing progressively deeper structural levels of the pluton west toward the Tennessee Mountain fault. Thermochronological results support this inference, showing progressively younger ages toward the west. Samples collected across the western block of the Coffeepot Stock yield hornblende 40Ar/39Ar ages of ca. 106.5–105.1 Ma, biotite 40Ar/39Ar ages of ca. 104.6–100.5 Ma, and K-feldspar 40Ar/39Ar ages of ca. 93.7–91.8 Ma (Table 2). These Coffeepot Stock 40Ar/39Ar ages are interpreted to record cooling directly following pluton emplacement in the late Early Cretaceous. We interpret the hornblende 40Ar/39Ar ages to correspond to crystallization at relatively shallow depths. Using biotite and K-feldspar closure temperatures of 310 °C and 200 °C, respectively (Reiners and Brandon, 2006), these ages correlate to an estimated cooling rate of ∼10 °C/m.y. between the biotite and K-feldspar partial retention zones. The nearly equivalent hornblende and biotite 40Ar/39Ar ages for the Coffeepot Stock show quick cooling to below ∼310 °C following crystallization, as would be expected for a relatively shallowly emplaced pluton.
To derive time-temperature (t-T) paths for the Coffeepot Stock and to test whether geologically reasonable thermal histories can satisfy AHe, ZHe, and THe data, we used the HeFTy software (Ketcham, 2005) to complete inverse models. We focused t-T modeling on the western fault block, where ZHe ages for the Coffeepot Stock decrease from 89.2 ± 4.1 Ma in the east to 69.6 ± 3.2 Ma in the west and AHe ages decrease from 42.1 ± 2.7 in the east to 27.0 ± 1.3 Ma near the Tennessee Mountain fault (Table 1). Figure 10 shows t-T paths for the western block using time constraints for cooling of the western Coffeepot Stock through the biotite 40Ar/39Ar closure and mean (U-Th)/He data for several samples collected across the block. Taken together, these samples define a protracted cooling history from the Late Cretaceous to the Oligocene, with the most prominent phase of cooling being in the Late Cretaceous following emplacement of the Coffeepot Stock. We relate a late Eocene cooling phase, evident from middle–late Eocene AHe ages of samples collected across multiple structural levels, to exhumation along the Copper Creek fault system. A later Oligocene to Miocene cooling phase is recognizable only in the AHe data from immediately beneath the Tennessee Mountain fault (sample 980730-1). The younger, 27 Ma AHe age from directly beneath the Tennessee Mountain fault could be a partial retention age, or it could record cooling associated with Oligocene extension contemporaneous with deposition of the Meadow Fork Formation farther east.
In the northern part of Copper Basin, a 109.2 ± 0.3 Ma body of granodiorite has Late Cretaceous hornblende, biotite, and K-feldspar 40Ar/39Ar ages, a ZHe age of 81.3 ± 3.8 Ma, and an AHe age of 32.6 ± 2.0 Ma, all of which are consistent with cooling ages of the Coffeepot Stock in the Tennessee Mountain area and significantly different from cooling ages in the proximal eastern block (Figs. 1 and 2). Thermal modeling indicates samples collected from this granodiorite body and from near the Tennessee Mountain skarn both had pronounced Late Cretaceous cooling (Fig. 10). This supports the interpretation of Rahl et al. (2002) that this granodiorite body represents an allochthonous slice of the Coffeepot Stock with adjacent Tennessee Mountain Formation skarn that was transported ∼10 km eastward during 8–12 km of slip along the Copper Creek normal fault. Figure 11A shows thermochronologic ages of samples collected in the footwall of the Copper Creek fault plotted against horizontal distance from the fault (e.g., Stockli, 2005). Linear regression of ZHe ages indicates a nominal slip rate of ∼0.44 mm/yr, but a large component of this slip (≥2 km) likely occurred between ca. 45 and ca. 38 Ma. The AHe age of 32.6 ± 2.0 Ma from this sample closely corresponds with the onset of coarse clastic deposition recorded by the Meadow Fork Formation in the hanging wall of the Meadow Fork normal fault. This, too, is consistent with the inference of Rahl et al. (2002) that the detached fault slice initially was transported with the hanging wall of the Copper Creek fault but was subsequently transferred to the footwall during the initiation of the Meadow Fork fault. This makes the Meadow Fork fault, together with its rotated syntectonic strata (the Meadow Fork Formation), the only clearly documented evidence of Oligocene extension in northeastern Nevada. Miocene volcanic rocks (ca. 17–15 Ma; Table 2) are deposited unconformably above both hanging-wall and footwall strata near Copper Basin (Figs. 1 and 3), providing a youngest age constraint on the timing of deformation along the Copper Creek and Meadow Fork faults.
In the eastern fault block, a sample from the eastern margin of the Coffeepot Stock (970709-3C) has a biotite 40Ar/39Ar age of 94.21 ± 0.66 Ma and a K-feldspar 40Ar/39Ar age of 71.87 ± 1.65 Ma. Rahl et al. (2002) interpreted these younger cooling ages to imply that the eastern part of the pluton intruded deeper structural levels and cooled more slowly than the western part of the pluton, which is consistent with the greenschist-facies metamorphism and solid-state foliation that is only present in the eastern part of the pluton (Fig. S6 [footnote 1]). Samples of muscovite-biotite schist within the eastern fault block yield biotite 40Ar/39Ar ages of 88.94 ± 0.90 Ma in the east (970726-5) and 55.39 ± 2.39 Ma near the Bruneau Valley fault (980802-3A). These ages, and a muscovite 40Ar/39Ar age of 91.37 ± 0.64 Ma, support slow initial cooling within the footwall of the Bruneau Valley normal fault. Titanite (U-Th)/He (THe) and ZHe ages in the eastern block record later cooling than the corresponding thermochronometers from the western block. This area is characterized by ZHe ages of 53.7 ± 3.2 Ma in the western part of the block and 46.1 ± 3.2 Ma in the central part of the block and a THe age of 37.2 ± 1.9 Ma in the east. These ages may record cooling and unroofing associated with Eocene extension (see below).
Exhumation and Basin Formation in the Copper Mountains
Discrete changes in sedimentary facies, provenance, and cooling rates signify the onset of rapid slip along the Copper Creek and Meadow Fork fault systems. The 115–100 Ma U-Pb grain age group matches zircon U-Pb crystallization ages obtained for the Coffeepot Stock, showing that the pluton acted as a primary sediment source to the basin. Additionally, REE compositions of apatite grains from Copper Basin strata are indicative of granitic source rocks with La-enriched crystallization, and the bulk geochemistry, size, and shape of these grains are similar to apatite crystals within the Coffeepot Stock (Fig. 9; Seymour, 1980). These strata also contain abundant pebble-boulder clasts of quartz monzonite and granodiorite at multiple stratigraphic levels (Figs. 4 and 6). Confident identification of this plutonic detritus permits assessment of minimum and maximum lag-time rates for these samples.
Lag-time analysis can provide quantitative constraints on exhumation rates where precise stratigraphic (i.e., depositional) ages exist for the same samples. In Copper Basin, 40Ar/39Ar geochronology of interbedded tuffs (Figs. 3 and 4; Table 2) and detrital zircon maximum depositional ages of young (i.e., syndepositional) volcanic grains permit precise assessment of stratigraphic ages. A sample collected from the top of the Dead Horse Formation, 11 m above a tuff with an 40Ar/39Ar age of 38.95 ± 0.25 Ma (Fig. 5), contains a young population of plutonic AHe grain ages (42.5–39.8 Ma) that indicate a lag time of 2–5 m.y. and a corresponding cooling rate between 37 °C/m.y. and 14 °C/m.y. (Farley, 2000). Figure 12 shows t-T paths for this sample (NV12-176CB) using the estimated time of western Coffeepot Stock cooling through the biotite 40Ar/39Ar closure, mean ZHe and AHe grain ages of young cooling age populations, and the sample depositional age. The wide spectrum of AHe ages within this sample (106.5–39.8 Ma) implies the Copper Basin catchment may have contained sources with diverse thermal histories at this time. A sample collected from the Meadow Fork Formation, near the contact with the Dead Horse Formation, has younger AHe grain ages of 21.7–15.3 Ma that indicate postdepositional heating. This is likely a result of thermal resetting during eruption of proximal ca. 17–15 Ma Seventy Six Creek Basalt and Jarbidge Rhyolite (Fig. 4; Rahl et al., 2002; Brueseke et al., 2014). This is the stratigraphically deepest sample collected from the Meadow Fork Formation, indicating thermal resetting is a product of proximal volcanism and not heating produced by burial. Apatite extracted from a quartz monzonite clast at approximately the same stratigraphic level, near the contact of the Meadow Fork Formation and Dead Horse Formation, (010801-3) yields an AHe age of 42.8 ± 2.4 Ma (Fig. 2). The middle Eocene cooling age of this sample also suggests that the estimated rapid cooling rates during deposition of the upper Dead Horse Formation persisted during deposition of at least the lowermost Meadow Fork Formation. The similar 15.7–12.8 Ma range of AHe ages from all other detrital apatite samples (050720-11, 050724-2A, and 050720-9D) taken from the upper Dead Horse Formation and at multiple stratigraphic levels within the Meadow Fork Formation (Fig. 3; Table 1), suggests Miocene volcanism had widespread thermal effects on Copper Basin strata.
The thick accumulation of late Eocene tuffaceous material in the Dead Horse Formation supports the interpretation that an east-west paleodrainage system connected this area with proximal calderas that were likely situated near the Bull Run Mountains, currently ∼50 km to the west (Henry, 2008; Henry et al., 2011). We interpret the local accumulation of tuff and tuffaceous sediment in Copper Basin, however, primarily to accommodation generated during movement along the Copper Creek and Meadow Fork faults (Fig. 13). This fault-generated subsidence is supported by coarse-grained alluvial deposits that are conformably deposited above Dead Horse Formation strata in the Meadow Fork Formation. Chronostratigraphic correlation shows most lacustrine sedimentation within Copper Basin was confined to a narrow interval between eruption of 39.07 ± 0.25 Ma and 38.95 ± 0.25 Ma tuffs, which preceded a migration of lacustrine deposition to the northeast by ca. 38 Ma (Fig. 5). This lacustrine deposition indicates a period of increased basin subsidence and/or drainage obstruction that led to the deposition of lacustrine facies that are indicative of an overfilled lake basin, where the combined water and sediment fill rate exceeded the accommodation rate (Carroll and Bohacs, 1999). Detrital (U-Th)/(He-Pb) double dating of zircon and apatite sampled from near the top of the Dead Horse Formation shows rapid late Eocene cooling (≥14.4 °C/m.y.) and deposition of plutonic detritus in Copper Basin (Figs. 6 and 13). Detrital zircons extracted from the lower portion of the Meadow Fork Formation show cooling through the zircon (U-Th)/He partial retention zone by the Paleocene, which is consistent with cooling ages in the western part of the Coffeepot Stock (Figs. 1 and 2). In the eastern fault block, between the Bruneau Valley fault and Copper Creek fault, the Coffeepot Stock is characterized by Eocene zircon and titanite He ages that decrease from 53.7 ± 3.2–37.2 ± 1.9 Ma, which supports >8 km of slip along the Copper Creek fault during the Eocene (Figs. 11 and 12).
Characteristics of Eocene Extension in the Cordilleran Hinterland
The well-preserved structural and stratigraphic evidence of late Eocene extension in the Copper Mountains of northern Nevada presented here provides one of the most complete records of synorogenic extension currently recognized in the North American Cordilleran hinterland. Lag-time analysis of double dated strata in Copper Basin reveals that rapid extension, with cooling rates of >14 °C/m.y., began by 38.0 ± 0.9 Ma. This is ∼6 m.y. after the 45–44 Ma initiation of volcanism in northern Nevada (Henry et al., 2012; Smith et al., 2017), but initial volcanism in Nevada was relatively limited in volume and spatial distribution (Henry and Boden, 1998; Henry, 2008; Smith et al., 2017). In contrast, intense magmatism associated with the proximal ca. 40.2–39.5 Ma Tuscarora volcanic field, <50 km to the southwest of Copper Basin, resulted in widespread volcanism (Henry and Boden, 1997; Henry et al., 1999; Henry, 2008) as well as emplacement of regional dike swarms and plutons (Ressel and Henry, 2006; du Bray, 2007). This peak phase of heating is also recorded by late Eocene Y-rich growth zones on metamorphic zircons in the East Humboldt Range (Hallett and Spear, 2015) and coeval Carlin-type Au mineralization across a large area of northeastern Nevada (Cline et al., 2005; Emsbo et al., 2006). Rapid extensional deformation in the Copper Mountains is contemporaneous with this peak phase of magmatism, which we interpret as an important driver for extension, both by rheological and/or thermal weakening of the lithosphere and isostatic effects of Farallon slab removal and possible subsequent delamination (Porter et al., 2016; Smith et al., 2017).
New temporal constraints reveal that high-magnitude extensional deformation in the Copper Mountains was concurrent with protracted metamorphic core complex development in the surrounding region (Fig. 14, and references therein). K-feldspar 40Ar/39Ar ages of 71.87 ± 1.65 Ma and 70.73 ± 1.41 Ma documented in the eastern part of the Coffeepot Stock are coeval with a 77.4 ± 2.8–68.2 ± 2.0 Ma cooling and melt crystallization event in the East Humboldt Range, recorded by zircon and monazite growth (Hallett and Spear, 2015). This may reflect widespread thermal effects of Late Cretaceous tectonism during orogenic plateau construction (e.g., Wells and Hoisch, 2008). The magnitude and timing of Paleogene exhumation of the Ruby Mountains–East Humboldt Range metamorphic core complex remain a matter of debate, but some studies hypothesize that initial rapid exhumation may have resulted from rollback-driven magmatism (e.g., MacCready et al., 1997; Snoke et al., 1997, 2004; Hallett and Spear, 2015; Litherland and Klemperer, 2017; Smith et al., 2017), as interpreted here. Recently documented large-magnitude late Eocene extension at Spruce Mountain (Pape et al., 2016) and in the southern East Humboldt Range (McGrew et al., 2018) supports this finding. Paleogene extensional deformation in the Copper Mountains shares several important similarities with metamorphic core complexes, including low-angle faulting, high-magnitude (>8 km) slip, and a hanging wall of syntectonic sedimentary strata faulted down against a footwall of metamorphic rocks. The footwall of the Copper Creek fault, however, is primarily composed of low-grade metasedimentary rocks rather than exhumed middle–lower crustal rocks. This comparatively muted exhumation may reflect extension in an area with preexisting crustal weaknesses developed during earlier polyphase deformation, as opposed to proximal metamorphic core complexes that formed in the foreland of the Roberts Mountains allochthon (McGrew et al., 2000). The synchronicity of high-magnitude late Eocene extension in the Copper Mountains, and likely the Ruby Mountains–East Humboldt Range, with the southward propagation of volcanism and basin formation adds to growing evidence that Farallon slab removal and/or progressive delamination of the mantle lithosphere resulted in rapid exhumation and measurable surface deformation across multiple locations in the hinterland (i.e., Smith et al., 2014, 2017; Cassel et al., 2018; Canada et al., 2019). This is in agreement with previous models that indicate thermal weakening of the upper crust can be an essential driver for extension (e.g., Coney and Harms, 1984; Gans et al., 1989; Axen et al., 1993; Wells and Hoisch, 2008; Bendick and Baldwin, 2009), especially within areas of orogenically thickened crust.
Multi-system and multi-mineral thermochronology of Cretaceous intrusive rocks and Copper Basin strata shows an excellent record of late Eocene to early Oligocene extension accommodated by movement along several high-offset normal faults across the Copper Mountains; these faults experienced up to 12 km of slip. The timing and magnitude of extensional deformation are precisely constrained by new detrital (U-Th)/(He-Pb) double dating and 40Ar/39Ar geochronology of Eocene–Oligocene strata. Detrital apatite grains derived from the Coffeepot Stock cooled through the apatite (U-Th)/He partial retention zone during the late Eocene and were deposited in Copper Basin within 2–5 m.y. This low-lag-time sedimentation reflects a 38.0 ± 0.9 Ma transition to rapid exhumation and surface-breaching extension in the Copper Mountains directly following widespread eruption of the ca. 40.2–39.8 Ma tuff of Nelson Creek and the tuff of Big Cottonwood Canyon, sourced from the Tuscarora volcanic field and proximal calderas, <50 km to the southwest of Copper Basin (Henry, 2008; Smith et al., 2017). The timing of extension in the Copper Mountains implies a close association with magmatism induced by removal of the Farallon slab and possibly subsequent removal of mantle lithosphere by delamination. Compressional stress release and advective heating of the lithosphere during associated volcanism also led to metamorphic fabric development within hinterland core complexes and heating that is inferred to reflect near-surface extensional deformation (Coney and Harms, 1984; MacCready et al., 1997; Hallett and Spear, 2014, 2015). Despite heightened gravitational potential energy since as early as the Late Cretaceous and thermal weakening during late Eocene magmatism, the Cordilleran hinterland plateau maintained high elevations (2.7–3.1 km) and experienced minor late Oligocene surface uplift (up to 500 m; Cassel et al., 2014, 2018). Surface elevations most likely decreased during widely distributed high-magnitude (>1 km throw) normal faulting in the Miocene (Dickinson, 2006; Colgan and Henry, 2009). This indicates that orogen-wide extension and lithospheric thinning are not the immediate response of all high-elevation regions, specifically not those that are underlain by rheologically strong and thick lithosphere, such as the central Andes (Long, 2012) and the Eocene Cordilleran hinterland.
National Science Foundation grant EAR-1322073 and student grants from the Geological Society of America, American Association of Petroleum Geologists, and the Society for Sedimentary Geology (SEPM) supported this research. This work greatly benefited from discussions with S. Long and C. Henry as well as field assistance from E. White, A. Wilson, and N. Seymour. We would also like to acknowledge laboratory assistance by L. Stockli, D. Barber, K. Thomson, and F. Galster. McGrew’s contributions were funded by the American Chemical Society Petroleum Research Fund (grant 40130-B8) and grants-in-aid from the University of Dayton. In addition, McGrew wishes to acknowledge the valuable contributions of undergraduate research collaborators Jeffrey Rahl (1999) and Michael Rigby (2005) and the laboratory assistance of Fritz Hubacher and Jeff Linder at the Ohio State University.