Extension of the Anaconda Metamorphic Core Complex: 40Ar

Home , Anaconda Range, Elkhorn Mountains, Flint Creek Range, Mount Powell (Colorado)

Extension of the Anaconda metamorphic core complex: 40Ar/39Ar thermochronology and implications for Eocene tectonics of the northern Rocky Mountains and the Boulder batholith

David A. Foster1, Warren C. Grice Jr.1,*, and Thomas J. Kalakay2 1DEPARTMENT OF GEOLOGICAL SCIENCES, P.O. BOX 112120, UNIVERSITY OF FLORIDA, GAINESVILLE, FLORIDA 32611, USA 2DEPARTMENT OF GEOLOGY, ROCKY MOUNTAIN COLLEGE, 1511 POLY DRIVE, BILLINGS, MONTANA 59102, USA

ABSTRACT

Thermochronologic data deﬁ ne the extension and exhumation history of the Anaconda metamorphic core complex and have implications for the Eocene tectonic setting of the northern Rocky Mountains. The 40Ar/ 39Ar data indicate that relatively rapid extension on the Anaconda detachment started at ca. 53 Ma and continued through ca. 39 Ma. Apatite ﬁ ssion-track data reveal that faulting and exhumation of the footwall continued until ca. 27 Ma. The average displacement rate on the Anaconda detachment was on the order of 1 mm/yr between ca. 50 and 39 Ma based on the lateral gradient in mica 40Ar/39Ar ages in the direction of fault slip. The total displacement along the Anaconda detachment in Eocene and Oligocene times is estimated to be ≥25–28 km based on reconstruction of the Cretaceous Storm Lake Stock with its detached roof, which is now exposed in the Deer Lodge Valley. Extension exhumed crust from ~12 km depth and exposed middle-greenschist-facies mylonites in the easternmost part of the Anaconda complex footwall. On a regional scale, the Anaconda detachment dips east beneath the Cretaceous Boulder batholith, indicating that the batholith and the Butte mineralization were transported east in the hanging wall. The Ana- conda metamorphic core complex formed at the transition between the Cordilleran hinterland and the foreland at the same time as extension occurred in the Bitterroot and Priest River metamorphic core complexes but exhumed a shallower part of the Eocene crustal section than the contemporaneous complexes to the west.

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INTRODUCTION of hyperextended terrains exposed from western Montana to northeastern Washington and British Columbia (Fig. 1B). Unlike the other core com- Eocene metamorphic core complexes are a signifi cant tectonic ele- plexes, the Anaconda complex is located at the easternmost margin of the ment of the northern Rocky Mountains, United States, and southern Cordilleran metamorphic-plutonic hinterland. Published geochronology Canadian Cordillera. Exposure of the core complexes resulted from and thermochronology indicate that regional extension and core complex widespread extension that began between 55 and 53 Ma during the fi nal exhumation began in the early to middle Eocene at ca. 54–52 Ma, coin- stages of or immediately after shortening ended in the Cordilleran thrust cident with onset of Challis-Colville-Kamloops-Absaroka magmatism belt (e.g., Constenius, 1996; Foster et al., 2007). Several hypotheses have (Armstrong and Ward, 1991; Morris et al., 2000; Foster et al., 2001, 2007; been proposed to explain the onset of rapid Eocene extension and associ- Breitsprecher et al., 2003; Haeussler et al., 2003). ated magmatism in the northern Cordillera including: orogenic collapse, as theno spheric upwelling within a slab window, rapid rollback of the Far- GEOLOGICAL BACKGROUND allon slab, regional transtension associated with northward motion of the Kula plate, or accretion of the Siletzia terrane (Armstrong et al., 1977; The Anaconda metamorphic core complex is located along the east- Severinghaus and Atwater, 1990; Morris et al., 2000; Vanderhaeghe and ern edge of the Cordilleran hinterland in western Montana (O’Neill et al., Teyssier, 2001; Breitsprecher et al., 2003; Haeussler et al., 2003; Foster 2004; Foster et al., 2007). This extensional terrain is south of the Lewis et al., 2001, 2007; Humphreys, 2009a). The 40Ar/39Ar thermochronologic and Clark fault zone, east of the Idaho batholith and Bitterroot metamor- data from the Anaconda metamorphic core complex in western Montana phic core complex, west of the Boulder batholith, and within the west- (Fig. 1) have implications for the timing, duration, and rate of extension ern part of the Helena salient of the Cordilleran thrust belt (Fig. 1) (Foster in the easternmost core complex in the Cordillera, as well as the Eocene et al., 2007). The Anaconda complex is composed of three structural- tectonic setting in the northern U.S. Cordillera. metamorphic domains: (1) a metamorphic-plutonic footwall exposed in the The Anaconda metamorphic core complex (O’Neill et al., 2004; Fos- Anaconda and Flint Creek Ranges (Figs. 2 and 3), (2) a low-grade hanging ter et al., 2007), along with the Bitterroot, Priest River, Clearwater, Shus- wall exposed along the western edge of and within the Deer Lodge Valley wap, Kettle, Okanagan, and Walhalla core complexes, constitutes a belt (Fig. 2), and (3) a brittle-plastic detachment fault system exposed along the eastern fl anks of the Anaconda and Flint Creek Ranges (Figs. 2 and 3). *Current address: ExxonMobil Corporation, 800 Bell Street, Houston, Texas Footwall rocks of the Anaconda complex are made up of Late Cretaceous 77002, USA. to Eocene granitic plutons intruded into metamorphosed Mesoproterozoic

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detachment Clearwater Clearwater Columbia River Basalt River Columbia A N 45° complex and other Eocene metamorphic core complexes. The core complexes are shaded gray, with names corresponding to the abbrev to with names corresponding shaded gray, are complexes The core complexes. core complex and other Eocene metamorphic A. the map in the Anaconda metamorphic core complex. The box shows the area of the map in Figure 2. (B) Inset map showing a larger area depic area a larger (B) Inset map showing 2. of the map in Figure the area shows The box complex. core Anaconda metamorphic the Figure 1. (A) Tectonic map of the northern U.S. Rocky Mountains showing major Phanerozoic structures and tectonic elements in t and tectonic structures major Phanerozoic Mountains showing Rocky U.S. map of the northern Tectonic (A) 1. Figure

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113°W 112°W v v v v Cenozoic sediments N 10 km v & volcanic rocks v v v v v v Eocene v v intrusive rocks RS v v Flint Range Creek v Cretaceous v v v v v v v volcanic rocks v Rock v PP v v Cretaceous v intrusive rocks MPP Deer v Creek Valley t Lodge v v l Proterozoic (Belt), u n a Deer Lodge Valley e F Paleozoic & Mesozoic d e LC d k

a DF02-118 sedimentary rocks H

Valley L Philipsburg SP A ben v Anaconda Eocene mylonite DF02-114 Lowland v SLP HL Creek Gra v faults: Boulder batholith detachment 46˚ v Butte N A' v v normal CJ thrust other

exploration well PB sample location Anaconda Range

Big Hole Valley Pioneer 113°W Range 112°W A Anaconda A' Amoco detachment DF02-114 Amoco Deer Lodge Arco A Jacobson Valley HLF #1 #1 Depth (km) v 0 v v v 0 v v v v v v v v v v v v 5 km BOULDER 5 km

10 km Anaconda BATHOLITH 10 km meta-Belt & K-T intrusions mylonit 15 km e 15 km 0km 10 km Figure 2. Geologic map and cross section of the central part of the Anaconda metamorphic core complex, Boulder batholith, and adjacent regions. The map was compiled from Emmons and Calkins (1913), Lewis (1998), Lonn et al. (2003), O’Neill et al. (2004), Foster et al. (2007), and Vuke et al. (2007). Locations of samples used for 40Ar/39Ar analyses that are not plotted on Figure 4 are also shown. Cretaceous intrusive rock at location DF02-114, in the hanging wall of the Anaconda detachment, is coarse-grained granodiorite interpreted to be the detached top of the Storm Lake pluton in the footwall. Abbreviations: SP—Sapphire pluton; CJ—Chief Joseph pluton; SLP—Storm Lake pluton; PB—Pioneer batholith; PP—Philipsburg batholith; MPP—Mount Powell pluton; RS—Royal Stock; HL—Hearst Lake plutonic suite; LC—Lost Creek Stock; HLF—Hidden Lake fault. The box shows the area of Figure 4.

Belt Supergroup and Middle Cambrian to Cretaceous shelf-platform strata granodiorite and early to middle Eocene granitic plutons, which intruded (Figs. 2 and 4; Fig. DR11) (Emmons and Calkins, 1913, 1915; Desmarais, deformed Belt Supergroup and metamorphosed Middle Cambrian strata 1983; Heise, 1983; Wallace et al., 1992; Lonn et al., 2003; Grice, 2006). (Fig. 4; Fig. DR1 [see footnote 1]) (Desmarais, 1983; Wallace et al., In the Flint Creek Range, footwall rocks are composed of granodiorite 1992; Lonn et al., 2003; O’Neill et al., 2004; Foster et al., 2007). Upper- to granite plutons of the Late Cretaceous Mount Powell batholith, Royal amphibolite-facies metamorphism and nappe-style folding (Fig. 3C) of Stock, and Lost Creek Stock. These plutons intruded, deformed, and the Belt and Cambrian strata occurred in Late Cretaceous time, with peak metamorphosed Middle Cambrian to Cretaceous strata and in a few areas metamorphic temperatures (>650–700 °C) accompanying intrusion of metamorphosed Belt strata (Emmons and Calkins, 1913, 1915; Allen, quartz diorite–granodiorite plutons at ca. 78–75 Ma, based on U-Pb zircon 1966; Hyndman et al., 1982; Lonn et al., 2003; O’Neill et al., 2004). In data (Grice, 2006). Cretaceous metamorphism and deformation took place the Anaconda Range, the footwall is largely Late Cretaceous diorite to at pressures of 4.6–6.0 kbar based on metamorphic thermobarometry of garnet-bearing metapelitic rocks (Grice, 2006). The hanging wall of the Anaconda core complex is made up of an 1GSA Data Repository Item 2010208, geologic map of the NE Anaconda Range, tabulated 40Ar/ 39Ar data, and 40Ar/ 39Ar age spectra, is available at www.geosociety array of asymmetric fault-bounded basins containing unmetamorphosed .org/pubs/ft2010.htm, or on request from [email protected], Documents Cenozoic clastic, volcaniclastic, and volcanic strata. These strata are Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA. exposed in the Deer Lodge Valley and preserved in a reentrant between

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Figure 3. (A) Photograph of the footwall and hanging wall of the Anaconda metamorphic core complex looking WNW into the Mill Creek Valley. (B) Pho- tograph showing a listric normal fault cutting greenschist-facies mylonite looking north at the canyon wall of the Mill Creek Valley. (C) Photograph of the north wall of the Mill Creek Valley showing a large Cretaceous nappe deﬁ ned by metamorphosed Belt Supergroup and Cambrian strata. The ductile strain in the nappe occurred at upper-amphibolite conditions at ca. 75 Ma. (D) Photograph of late-stage semibrittle extensional shear bands cutting greenschist-facies mylonite and ultramylonite in the Mill Creek Valley (view is to the north). (E) Photograph of the south side of the Clear Creek Valley showing listric normal faults that cut through and sole into the greenschist-facies mylonites beneath the Anaconda detachment (view to the south).

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A 113°15′W 46°7.5′N 0 1 2 km N SL-38

DF02-116a WG04-052 Ug-1

WG04-114 WG04-033

WG04-100 DF02-120 WG04-101

WG04-138 WG04-089 WG04-092 WG04-112 WG04-109 meta-Cambrian Anaconda beds Eocene Dacitic dike meta-Missoula Group mylonite Late Hearst Lake gd meta-Helena Fm. Cretaceous Hearst Lake gr meta-Ravalli Group ductile s.z. ME-6 WG04-103 Storm Lake stock meta-Greyson Fm. Sample location

normal detachment fault thrust antiform fault fault

B 64 51- 48 47 46 40 46°7.5′N 74 0 1 2 SL-38 N bt 79.0 ± 1.2 km 39 kfs (LT) 56.9 ± 1.6 DF02-116a bt 39.6 ± 2.3 WG04-052 ms 40.5 ± 2.0 bt 64.1 ± 1.0 Ug-1 bt 40.7 ± 0.7 bt 51.5 ± 0.8 WG04-114 bt 74.0 ± 0.9 WG04-033 bt 48.5 ± 0.6

DF02-120 WG04-101 WG04-100 ms 50.4 ± 0.8 bt 38.5 ± 0.7 bt 49.2 ± 0.9 ms 49.6 ± 0.6 WG04-138 ms 46.1 ± 0.6 WG04-112 WG04-089 WG04-092 bt 52.8 ± 0.7 WG04-109 bt 47.6 ± 0.6 ms 45.5 ± 0.6 bt 47.1 ± 0.7 ms 56.9 ± 0.7 ms 48.0 ± 0.6 Sample location with data (age in Ma) Late Cretaceous ME-6 WG04-103 Eocene mylonite ductile shear zone bt 48.6 ± 1.0 ms 47.3 ± 1.1 ms 51.5 ± 1.2 normal fault detachment fault thrust fault antiform 113°15′W

Figure 4. (A) Geologic sketch map of the northern Anaconda range (simpliﬁ ed from the map in Fig. DR1 [see text footnote 1]) showing sample locations. The two colors for the Storm Lake Stock are for the granodiorite and quartz diorite compositions. Abbreviations: gd—biotite granodiorite; gr—two-mica granite; sz—shear zone. (B) Map of the same part of the footwall of the Anaconda metamorphic core complex with 40Ar/39Ar cooling ages (errors are 2σ). The thick black lines are “contours” of the cooling ages for muscovite and biotite. The 40Ar/39Ar ages for biotite and muscovite are concordant for each location in the eastern two thirds of the footwall, because of rapid cooling during Eocene time. Abbreviations: ms—muscovite; bt—biotite; kfs (LT)—K-feldspar low-temperature plateau ages; TF—total fusion.

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the Anaconda and Flint Creek Ranges (O’Neill et al., 2004; Foster et middle-greenschist-facies mylonitic shear zone of stretched two-mica al., 2007) (Fig. 2). The stratigraphically lowest rocks in these basins are granite, biotite granite, granodiorite, and mylonitic micaceous quartzite moderately west tilted (~50°–60°), poorly sorted, and poorly consolidated (Emmons and Calkins, 1913; Kalakay et al., 2003; O’Neill et al., 2004, conglomerates, sandstones, breccias, and megabreccias (Kalakay et al., Foster et al., 2007; Grice, 2006). Fractured K-feldspar porphyroclasts in 2003; O’Neill et al., 2004). These strata grade upward into progressively the granitoids are encased by a matrix of plastically deformed quartz. The less tilted (~0°–25°) felsic lava fl ows, tuffs, and volcaniclastic deposits micaceous quartzite exhibits unannealed quartz ribbons with undulatory of the Eocene Lowland Creek volcanic fi eld (ca. 53–49 Ma; Dudas et al., extinction and mica fi sh (Fig. 5). These metamorphic textures are indica- 2010). The upward decrease in the tilt of these basin-fi ll strata indicates tive of deformation at temperatures less than ~400–450 °C (Passchier and deposition synchronous with extension. Trouw, 2005). Greenschist mylonites exhibit shallow-plunging mineral Metamorphic and plutonic rocks of the footwall are juxtaposed with stretching lineations and kinematic indicators, which show top-to-the- the hanging-wall rocks along an east-dipping, low-angle, brittle-plastic east-southeast (102°–110°) sense of motion (Kalakay et al., 2003; O’Neill detachment system, which shows top-to-the-east-southeast displacement et al., 2004; Grice, 2006). Strain in the greenschist mylonites is heteroge- (Emmons and Calkins, 1913; O’Neill and Lageson 2003; Kalakay et al., neous and distributed into 0.1–2-m-thick zones of ultramylonite alternat- 2003; O’Neill et al., 2004; Foster et al., 2007). The Anaconda detach- ing with ~5–15-m-thick zones of mylonite and protomylonite (Fig. 5) and ment has a mapped strike length of at least 100 km from the northern some bands of pseudotachylyte (Kalakay et al., 2003; Foster et al., 2007). Flint Creek Range to the southern Anaconda Range (Kalakay et al., 2003; The mylonites exposed in the northeastern Anaconda Range are cut by an O’Neill et al., 2004; Foster et al., 2007). array of closely spaced, east-dipping brittle normal faults. Many brittle The Anaconda detachment is characterized by greenschist-facies faults are listric and become subhorizontal with depth and parallel to ultramylonite, ultramylonite, pseudotachylyte, and overprinting brittle normal mylonite zones in the granitoids (Figs. 3B and 3E). Slickenline striations faults (Figs. 3 and 5). Along the eastern fl ank of the northeastern Anaconda on the brittle fault surfaces show top-to-the-east-southeast (100°–110°) Range, the detachment is characterized by a 300–500-m-thick lower- to displacement parallel to the stretching direction in the greenschist-facies mylonites (Kalakay et al., 2003). Exposures of the brittle-plastic detachment are not continuous along strike in the Anaconda and Flint Creek Ranges because segments have been removed by erosion, cut out by younger brittle normal faults, and covered by hanging-wall fault slivers or thick talus (Foster et al., 2007). Isolated exposures of the detachment along eastern fl anks of the central and southern Anaconda Range are characterized by low-grade mylonitic two-mica granites and granodiorite cut by a series of east-dipping, northeast-trending brittle normal faults similar to those found in the northeastern Anaconda Range (Wallace et al., 1992). Along the eastern fl anks of the Flint Creek Range, greenschist-facies mylonite is found locally in the Lost Creek Stock and metasedimentary rocks equivalent to the Belt and Middle Cambrian section (Allen, 1966; Lonn et al., 2003; O’Neill et al., 2004). These mylonites are cut by high-angle normal faults similar to those found in other parts of the detachment (O’Neill et al., 2004). Along the eastern fl anks of the Flint Creek and Anaconda Ranges, the detachment dips gently (~10°–30°) beneath the Deer Lodge Valley. The gentle dip of the detachment is also revealed by industry exploration wells, which intersected greenschist mylonite at the base of the Tertiary basin fi ll in the western Deer Lodge Valley at depths of ≤5 km (Fig. 2; McLeod, 1987). The downward projection of the low-angle detachment is aligned with subhorizontal seismic refl ectors beneath the Boulder batholith (Vejmelek and Smithson, 1995), suggesting that the detachment shal- lows with depth and continues to the east (Fig. 2; Foster et al., 2007). This consistent shallow dip along with the listric faults soling into the shear zone is consistent with the deeper parts of the Anaconda detachment origi- nating at low angles within the brittle-plastic transition. The detachment is not well exposed along the western margin of the Anaconda core complex. The trace of the detachment is inferred in several places by the juxtaposition of brittle faulted upper-plate rocks with plastically deformed metamorphic and plutonic rocks. The western part of the detachment probably originated as a series of east-dipping listric normal faults east of a breakaway zone that is inferred to have been located east of the Georgetown thrust and is either no longer exposed or was removed Figure 5. (A) Field photograph of greenschist-facies mylonite and by erosion (O’Neill et al., 2004; Foster et al., 2007). Upper-amphibolite- ultra mylonite developed within Eocene granite from the eastern facies mylonite and extreme attenuation of footwall strata in the western part of the Mill Creek Valley (orientation: SSE to the right side of part of the complex footwall (O’Neill et al., 2004) are related to Cretaceous the photo). (B) Photomicrograph showing mica fi sh that grew in Cambrian quartzite mylonite beneath the Anaconda detachment deformation and are not Eocene structures (Grice et al., 2005); the Creta- (orientation: SSE to the right side of the photo). ceous fabrics are locally overprinted by the Eocene brittle-plastic fabrics.

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40Ar/39Ar THERMOCHRONOLOGY Florida following analytical procedures described by Foster et al. (2009). The 40Ar/39Ar data are summarized in Table 1, age spectra along with Samples were collected for 40Ar/39Ar analysis from lower-plate rocks inverse isochron plots are shown in Figure DR2 (see footnote 1), complete along a transect parallel to the slip direction on the Anaconda detachment step-heating data are presented in Table DR1 (see footnote 1), and inter- in the northeastern Anaconda Range (Fig. 4). All samples were collected preted 40Ar/39Ar ages are plotted on Figure 4. as close to the level of detachment as possible. Samples include: (1) high- grade Mesoproterozoic Belt Supergroup and Middle Cambrian metamor- RESULTS phic rocks; (2) granodiorite of the Late Cretaceous Storm Lake Stock; and (3) granite-granodiorite intrusions of the Eocene Hearst Lake suite The biotite 40Ar/ 39Ar ages (Table 1; Fig. DR2 [see footnote 1]) deﬁ ne a (Table 1). Two samples were collected from the upper plate in the Deer lateral gradient across the lower plate where ages become younger toward Lodge Valley: (1) biotite-hornblende granodiorite (DF02-114) that is cor- the ESE (Fig. 4). In the westernmost part of the footwall, biotite ages related with the Storm Lake Stock in the footwall (Fig. 2), and (2) crystal- from the Storm Lake Stock range from ca. 79 to 64 Ma (samples SL-38, lithic rhyolitic tuff (DF04-113) of the Eocene Lowland Creek volcanic WG04-114, and WG04-052). East and south of the Storm Lake Stock, sequence. One sample was collected from greenschist-facies biotite gran- biotite 40Ar/ 39Ar ages become abruptly younger. Biotite from sample ite mylonite of the Lost Creek Stock in the lower plate on the eastern Flint WG04-112, a muscovite- and biotite-bearing quartzite gneiss, yielded a Creek Range (Fig. 2). 40Ar/39Ar cooling age of 52.8 ± 0.7 Ma. East of the Storm Lake Stock, in the central part of the exposed footwall, biotite 40Ar/39Ar ages from METHODS two-mica granite, biotite granite, and a dacite dike (samples WG04-109, WG04-101, ME-6, WG04-092, and WG04-033) range from ca. 52 to Biotite, muscovite, and K-feldspar were separated using standard den- 47 Ma. Biotite from a deformed Cretaceous quartz diorite sill (Ug-1) gave sity and magnetic techniques, followed by handpicking to >99% purity. a 40Ar/ 39Ar age of 50.5 ± 0.8 Ma. In the easternmost part of the north- The mineral separates were irradiated at the Radiation Center at Oregon eastern Anaconda Range, biotite from two-mica granite (DF02-116a) and State University and underwent 40Ar/39Ar analyses at the University of biotite granite (DF02-120) mylonites yielded ages of ca. 41–39 Ma.

TABLE 1. 40Ar/39Ar DATA FROM THE ANACONDA METAMORPHIC CORE COMPLEX Sample Rock type* Latitude Longitude Elevation Mineral* Age Error§ % 39Ar# MSWD Comments (°N) (°W) (m) (Ma)†§

Northeastern Anaconda-Pintlar Range SL-38 Hb bt granodiorite 46°06′06″ 113°16′32″ 2311 bt 79 1.2 78 2.02 hb 102.5 12.8 65 144.5 Excess argon; age is unreliable kfs 56.9 1.6 9 4.98 Age of low-temperature steps WG04-114 Hb bt granodiorite 46°04′25″ 113°15′48″ 2503 bt 74 0.9 94 0.69 WG04-112 Micaceous quartzite 46°03′26″ 113°14′43″ 2646 ms 56.9 0.7 81 0.34 bt 52.8 0.7 100 0.63 WG04-052 Quartz diorite 46°05′09″ 113°13′46″ 2754 bt 64.1 1.0 88 1.43 WG04-033 Dacite dike 46°04′29″ 113°10′50″ 2583 bt 48.5 0.6 94 0.99 WG04-109 Two mica granite 46°02′35″ 113°10′43″ 2878 ms 48 0.6 87 1.75 bt 47.6 0.6 100 1.6 WG04-101 Two mica granite 46°03′39″ 113°10′01″ 3073 ms 49.6 0.6 78 0.34 bt 49.2 0.9 93 1.25 WG04-100 Pegmatitic dike 46°03′36″ 113°09′52″ 2927 ms 50.4 0.8 95 2.63 ME-6 Two mica granite 46°02′25″ 113°10′15″ 2732 ms 51.5 1.2 79 0.31 bt 48.6 1.0 95 1.74 Ug-1 Mylonitic quartzite 46°04′32″ 113°08′55″ 2610 bt 50.5 0.8 92 0.1 WG04-103 Mylonitic quartzite 46°01′16″ 113°10′04″ 2524 ms 47.3 1.1 80 0.91 Error plateau WG04-092 Bt granite 46°03′34″ 113°08′25″ 2896 bt 47.1 0.7 78 3.01 WG04-089 Garnet leucogranite 46°02′56″ 113°08′18″ 2646 ms 45.5 0.6 100 1.43 WG04-138 Mylonitic quartzite 46°03′37″ 113°06′14″ 3098 ms 46.1 0.6 99 0.19 DF02-116a Mylonitic two mica 46°05′29″ 113°02′05″ 2573 ms 40.5 2.0 65 2.21 granite bt 39.6 2.3 70 1.36 bt 40.7 0.7 100 – Total fusion age DF02-120 Mylonitic bt granite 46°03′25″ 113°01′38″ 2229 bt 38.5 0.7 100 – Total fusion age Other areas DF02-118a Mylonitic bt granodiorite 46°11′49″ 112°59′11″ – bt 38.8 1.6 72 0.82 Lost Creek mylonite DF02-113 Rhyolite tuff 46°01′11″ 112°59′03″ – bt 53.7 1.4 66 0.71 Lowland Creek volcanics DF02-114 Bt hb granodiorite 46°04′24″ 112°56′22″ – bt 76.3 1.1 95 0.57 From upper plate – Mill Creek Note: MSWD—mean square of weighted deviates for plateau age. *Mineral abbreviations: bt—biotite, ms—muscovite, hb—hornblende, kfs—K-feldspar. †Weighted plateau age unless noted otherwise in comments. §2σ errors. #% 39Ar—percent of 39Ar used in weighted plateau age calculation.

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Muscovite 40Ar/39Ar ages (Table 1; Fig. DR2 [see footnote 1]) also DISCUSSION become younger to the ESE across the lower plate. In the west, muscovite from WG04-112 gave a 40Ar/39Ar age of 56.9 ± 0.7 Ma. To the east, The 40Ar/39Ar cooling ages provide constraints on the cooling and muscovite from a two-mica granite (samples WG04-109, WG04-101, exhumation history of the Anaconda metamorphic core complex, and the ME-6), pegmatite dike (WG04-100), garnet-bearing leucogranite dike timing of extension. The 40Ar/39Ar ages for coexisting muscovite and bio- (WG04-089), and mylonitic muscovite-bearing quartzite (samples tite from every location in the footwall, with the exception of the western- WG04-103 and WG04-103) gave ages ranging from ca. 52 to 46 Ma. most part of the footwall, are concordant within error. This indicates that In the eastern part of the footwall, muscovite from a two-mica granite rapid cooling took place through the argon closure temperature interval of mylonite (DF02-116a) yielded a cooling age of 40.5 ± 2.0 Ma. muscovite (~450–350 °C) and biotite (380–330 °C) (closure temperatures Furnace step heating of K-feldspar from granodiorite of the Storm for rapid cooling; e.g., McDougall and Harrison, 1999) for all of the cen- Lake Stock (SL-38) gave a saddle-shaped age spectrum indicative of tral and eastern parts of the footwall in Eocene time. For example, samples excess argon (e.g., Foster et al., 1990). Despite the presence of excess WG04-101 and WG04-109 cooled through this temperature interval in 40Ar in the K-feldspar, some useful age information is deduced from the less than 1 m.y., or at a rate of >100°/m.y. (Fig. 6, path 2). Eocene granitic low-temperature steps. From ~2% to 20% 39Ar released, the spectrum is rocks (e.g., DF02-116: U-Pb zircon age of 53 ± 1 Ma; Foster et al., 2007) characterized by a saw-tooth pattern, where older apparent ages alter- within the greenschist-facies mylonite in the eastern part of the footwall nate with slightly younger ages. These steps correspond to isothermal also cooled through the muscovite and biotite closure intervals at this heating steps at temperatures from 550 to 750 °C. Harrison et al. (1994) rapid rate, but nearly 10 m.y. after WG04-101 (Fig. 6, path 3). demonstrated that excess Ar released in the relatively older isothermal Based on the 40Ar/39Ar data, the western part of the Anaconda foot- steps is commonly derived from fl uid inclusions, and that the second iso- wall was below 300–250 °C before middle Eocene time (Fig. 6, path 1). thermal step often provides meaningful age information. Removal of the Results from this part of the footwall record relatively rapid Late Cre- older isothermal steps over the low temperature portion of the age spec- taceous cooling of the 75 ± 1 Ma Storm Lake Stock (U-Pb zircon age; tra gives an error plateau age of 57 ± 2 Ma (Fig. DR2 [see footnote 1]). Grice et al., 2005; Grice, 2006), followed by slower cooling until early Hornblende-biotite granodiorite (DF02-114) from the upper plate Eocene time (Fig. 6, path 1). The K-feldspar low-temperature release east of the northeastern Anaconda Range yielded a biotite age of 76.3 data from the Storm Lake Stock sample show that the western part of the ± 1.1 Ma. Crystal-lithic tuff sample DF02-113 yielded a biotite age footwall had cooled through the temperature interval of ~250–200 °C by of 53.7 ± 1.4 Ma. Mylonitic biotite granite (DF02-118) from the Lost ca. 55–57 Ma. Collectively, these 40Ar/39Ar data from the western part Creek Stock in the eastern Flint Creek Range gave a biotite age of 38.8 of the footwall require that the amphibolites-facies metamorphism of ± 1.6 Ma. the footwall rocks was Late Cretaceous in age and not associated with Eocene extension. FISSION-TRACK ANALYSES The apatite fi ssion-track data from the eastern parts of the footwall in both the Anaconda and the Flint Creek Ranges defi ne the cooling his- Apatite fi ssion-track data from fi ve samples are presented in Table 2. tory to temperatures lower than those recorded by the 40Ar/39Ar data. The The fi ssion-track analyses were performed at the University of Melbourne mean track lengths of ~14 µm for the samples that give fi ssion-track ages using methods summarized by Green (1986). All samples yielded suffi - of ca. 27 Ma (Table 2) indicate that these samples record relatively rapid cient apatite grains and high enough uranium concentrations for statisti- cooling through the apatite partial annealing zone (110–60 °C) and no cally sound age populations. The apatite fi ssion-track ages for samples subsequent thermal annealing. The apparent average cooling rate between of greenschist-facies mylonite from the footwall are concordant within ca. 40 Ma and 27 Ma was therefore ~20 °C/m.y. (Fig. 6, path 3). It is pos- error at 27 ± 1 Ma and give mean fi ssion-track lengths of ~14.0 µm. One sible, however, that cooling and exhumation were not continuous from sample of granodiorite from the hanging-wall block, interpreted to be the 40 to 27 Ma at this average rate, because the rate of cooling may have detached top of the Storm Lake Stock (DF02-114), gave a fi ssion-track decreased after ca. 40–39 Ma, and then increased again at ca. 27 Ma. The age of ca. 44 Ma and mean track length of ~12.8 µm. ca. 27 Ma apatite fi ssion-track ages, therefore, either record continuous

TABLE 2. APATITE FISSION-TRACK DATA Sample Number Standard track Fossil track Induced track Uranium Chi square Fission-track Mean track Std. number of grains density density density content probability age*† length† dev. (×106 cm–2) (×105 cm–2) (×106 cm–2) (ppm) (%) (Ma) (µm) (µm) DF02-114 Mill Creek Granodiorite 7 1.081 3.475 1.623 18 85 44 ± 4 12.8 ± 0.2 1.9 (upper plate) (3495) (149) (696) (88) DF02-116A Clear Creek Granite 30 1.092 0.365 0.308 4 7 25 ±3 (3495) (82) (692) DF02-117A Clear Creek Granodiorite 25 1.104 1.257 0.97 11 58 27 ± 2 14.0 ± 0.3 1.9 (3495) (181) (1397) (47) DF02-119B Race Track Monzodiorite 20 1.127 7.06 5.408 60 64 28 ± 1 14.0 ± 0.1 1.1 Creek (3495) (671) (5140) (100) DF02-120A Mill Creek Granite 30 1.138 1.083 0.887 10 30 27 ± 3 (3495) (127) (1040) Note: Parentheses show number of tracks counted and measured. Standard and induced track densities were measured on mica external detectors (g = 0.5), and fossil track densities were measured on internal grain surfaces. Ages were calculated using the zeta method (384 ± 5) for dosimeter glass CN-5. Complete sample location details are in Table 1. *All ages are central ages. †1σ error used.

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1000 1000 the eastern part of the complex due to slip on the detachment was greater than 350–400 °C. This temperature gradient implies that the Anaconda 900 Intrusion of 900 Storm Lake granodiorite detachment originally dipped east at an angle less than 15° for any reason- able range of paleogeothermal gradients (20–50 °C/km) (e.g., Foster and 800 800 Emplacement of John, 1999). 40 39 700 Eocene granitoids 700 Published K-Ar and Ar/ Ar data further defi ne the thermal history of the Anaconda core complex for areas outside the northeastern Anaconda 600 600 Range. Granitic rocks of the Royal Stock and Mount Powell batholith, Eocene granitoids cool high-T/ low-P 3 metamorphism from the footwall in the Flint Creek Range, gave mica cooling ages of to ambient temperature 500 500 ca. 65–62 Ma (Marvin et al., 1989), indicating cooling through ~300 °C by 2 1 400 Paleocene time. The Philipsburg batholith, a large granodiorite intrusion Cooling greenschist- D C 400 facies mylonite A G in the western Flint Creek Range, yielded hornblende and biotite K-Ar Temperature (°C) 300 300 ages of ca. 77–72 Ma (Hyndman et al., 1972), indicating rapid cooling to below 300 °C in Late Cretaceous time. Foster and Raza (2002) reported B 200 4 200 an apatite fi ssion-track age of ca. 57 Ma for the Philipsburg batholith, indicating the intrusion had cooled below 110 ± 10 °C by Paleocene time. E 100 H 100 These thermochronologic data are consistent with the interpretation that 0 0 the Philipsburg batholith lies west of the original breakaway zone of the 010 20406080100 30 50 70 90 Anaconda detachment and was intruded at very shallow depths (O’Neill Time (Ma) et al., 2004; Foster et al., 2007). In the southern Anaconda Range, the majority of the lower plate con- Figure 6. Plot of temperature-time paths for three locations in the lower sists of Eocene biotite ± muscovite granitoids and dacite dikes similar to plate of the Anaconda metamorphic core complex and one location in those exposed in the northeastern Anaconda Range. These intrusions have the hanging wall. Cooling path 1 is for Storm Lake Stock (75 ± 1 Ma U-Pb zircon age) and metamorphic rocks in the western part of the footwall. mica cooling ages from 60 to 50 Ma (Desmarais, 1983), which indicate Cooling path 2 is for metamorphic rocks and Cretaceous leucogranite that the lower plate in southern Anaconda Range area had cooled through in the central part of the footwall. Cooling path 3 is for Eocene plutons ~300 °C by the early to middle Eocene. The Chief Joseph batholith, within the greenschist-facies mylonite in the eastern part of the foot- located to the west of this area, has hornblende and biotite cooling ages wall. Cooling path 4 is for granodiorite in the hanging wall east of the from ca. 75 to 60 Ma. Foster and Raza (2002) reported apatite fi ssion- northeastern Anaconda Range. The letters depict example mineral cool- track ages of ca. 40–30 Ma for the southern Anaconda Range. Together, ing ages: (A) biotite from WG04-114; (B) K-feldspar low-temperature steps these data indicate cooling through 550–350 °C in the Late Cretaceous from SL-38; (C) muscovite and biotite from WG04-110; (D) muscovite and biotite from DF02-116; (E) apatite fi ssion-track from DF02-116; (G) biotite to late Paleocene and then through 110 ± 10 °C in late Eocene to early from DF02-114; and (H) apatite fi ssion-track from DF02-114. Oligocene time. A more comprehensive thermochronological data set will need to be collected from this area for direct comparison with our results from the northeastern Anaconda Range.

slip on the Anaconda detachment or a second stage of exhumation associ- Timing and Rate of Extension ated with normal faulting and opening of the Deer Lodge Valley. The cooling history of Cretaceous granodiorite (DF02-114) within The 40Ar/39Ar cooling ages obtained from samples collected from the hanging-wall block east of the NE Anaconda Range is remarkably extensional fault blocks and exhumed metamorphic core complex foot- similar to that of the Storm Lake Stock in the footwall (Fig. 6). This dis- walls may be used to determine the onset of extension if the base of the placed block cooled below biotite argon closure in Late Cretaceous time, partial retention zone for a thermochronologic system is identifi ed within and more slowly though the apatite partial annealing zone in Eocene time the fault block (John and Foster, 1993; Foster and John, 1999; Stockli, based on the ~12.8 µm mean fi ssion-track length. 2005). The partial retention zone corresponds to an interval of crustal Approximate contours of biotite and muscovite 40Ar/39Ar ages from the depths specifi c to each thermochronometer where progressively higher Anaconda core complex are shown in Figure 4. The “contours” represent temperatures result in only partial retention of radiogenic 40Ar (or radio- the time when lower-plate rocks in the respective locations cooled through genic 4He for U-Th/He thermochronology) (Fig. 7). For biotite, the argon ~450–330 °C (closure temperatures for rapidly cooled micas; e.g., Lister partial retention zone occurs between ~250 and 330 °C. At pre-extension and Baldwin, 1996; McDougall and Harrison, 1999). These data reveal depths deeper than the partial retention zone, temperatures are too hot for a preserved Paleocene–early Eocene thermal gradient across the lower radiogenic 40Ar to be retained prior to the onset of extension and cooling plate and an eastward progression in the timing of rapid cooling. The mica (e.g., John and Foster, 1993). At pre-extension depths shallower than the 40Ar/39Ar ages highlight that the western part of the lower plate cooled partial retention zone, the crust is cool enough to allow retention of radio- though ~350–300 °C (muscovite and biotite closure temperatures for genic 40Ar, and rocks containing K-bearing minerals record 40Ar/39Ar ages slow cooling; McDougall and Harrison, 1999) in Late Cretaceous to early related to earlier cooling events. Within the partial retention zone, temper- Paleocene time (Fig. 6), whereas the easternmost lower plate remained atures progressively increase, giving rise to a progression from near argon above ~450–350 °C until the late Eocene, as indicated by the mica ages closure at shallower depths to almost complete loss of daughter isotopes of ca. 41–39 Ma. Cooling of the western part of the footwall though mica prior to extension at deeper levels. K-bearing minerals residing within closure predated the development of the Anaconda detachment. In fact, the partial retention zone record “mixed ages” upon cooling (e.g., Foster cooling of the western area due to tectonic exhumation beneath the Ana- and John, 1999). At the onset of extension, rapid exhumation and cooling conda detachment was restricted to less than 200 °C, based on the K-feld- of rocks immediately below the base of the partial retention zone—the spar data. In contrast, the total amount of cooling of the rocks exposed in depth transition between no argon retention and partial retention—quench

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Depth (km) 30° C/km extension accommodated by slip on the Anaconda detachment continued 0 0 at least into late Eocene time. The inverse of the slope on the age-distance plot for muscovite and Biotite ages biotite 40Ar/39Ar cooling ages ≤51 Ma (Fig. 9) provides the basis to esti- 5 older than extension 150 mate the rate of slip on the Anaconda detachment (Foster et al., 1993; Ket- cham, 1996; Foster and John, 1999; Brichau et al., 2006). We calculated inverse slopes of the biotite and muscovite 40Ar/39Ar ages using the least- 10 BiotiteBBiootitete partialp parr ala rer retentiontioon zonzzone: zoone:: mmixedxxedddag agageses 300 squares regression option in Isoplot v. 3.09a (Ludwig, 2004). Dacite dike Biotite age zero before extension sample WG04-033 was excluded from the calculation because the dike 15 450 was intruded late in the history of extension. The biotite and muscovite data give extension rates of 0.93 ± 0.33 km/m.y. and 0.87 ± 0.48 km/m.y. eroded hanging wall Biotite ages older than extension (2σ), respectively (Fig. 8). The larger error of the muscovite slip rate is the Age gradient gives rate of 40 39 extension result of scatter in the muscovite Ar/ Ar ages at ~10–11 km and relatively large error in age of sample DF02-116 at ~19 km. The muscovite and biotite data give similar values, suggesting that the average rate of slip on the Anaconda detachment was ~0.9 km/m.y. This average rate does not formerforformerormeer bbiotite oto e partialp parar ala account for any increase or decrease in slip rate between 51 and 39 Ma. retentionretentior entiononn zzone onon A displacement rate of ~0.9 km/m.y. is 5–10 times slower than Miocene slip rates for detachments in the southern Basin and Range Province (e.g., Harcuvar and Buckskin detachments [Foster et al., 1993; Carter et al., 2004]; Chemehuevi detachment [John and Foster, 1993; Foster and John, Figure 7. Sketch depicting the exhumation of the argon partial reten- 1999; Carter et al., 2006]; Raft River detachment [Wells et al., 2000]), but tion zone for biotite due to normal slip on a detachment fault following similar to that estimated for the Ruby detachment in the central Basin and concepts summarized in John and Foster (1993), Foster and John (1999), Range (Gifford et al., 2007). Wells et al. (2000), and Stockli (2005).

Magnitude of Extension

minerals that previously retained no radiogenic argon, thereby recording Displacement on the Anaconda detachment may be constrained by the onset of extension (Foster and John, 1999; Stockli, 2005). reconstructing the granodiorite phase of the Storm Lake Stock with cor- A plot of muscovite and biotite 40Ar/39Ar cooling ages against distance relative Cretaceous granodiorite within a detached block to the east in the along a section in slip direction (105°) of the Anaconda detachment is Deer Lodge Valley (Fig. 2). The granodiorite in both areas is unfoliated given in Figure 8A. This diagram was constructed by orthogonally pro- medium- to coarse-grained biotite ± hornblende granodiorite with concor- jecting sample locations to the transect line (Fig. 8B) and includes an dant Late Cretaceous biotite 40Ar/ 39Ar cooling ages (samples DF02-114 applied error of ±1 km to account for projection errors and elevation (e.g., and WG04-114; Table 1), consistent post-Cretaceous cooling histories Foster and John, 1999; Brichau et al., 2006). (Fig. 6), and complementary major- and trace-element concentrations As summarized already, the mica 40Ar/39Ar cooling ages become (Grice, 2006). Restoring the displaced top of the Storm Lake Stock gives younger to the ESE. There is a rapid decrease from Late Cretaceous ages between 25 and 28 km of heave on the Anaconda detachment. This value of (≥74 Ma) to early Eocene ages (ca. 53 Ma) over a distance of ~5 km heave is greater than the amount indicated by the slip rate of ~0.9 km/m.y. and then a much more gradual decrease to late Eocene ages farther ESE between 53 and 39 Ma (~13 km), but is consistent with this slip rate if (ca. 40–39 Ma). The change in the slope of mica cooling ages at ~5 km the Anaconda detachment continued to be active until ca. 25–27 Ma as corresponds to a quenched paleoisotherm marking the base of the biotite indicated by the apatite ﬁ ssion-track data. partial retention zone, or ~330 °C (Foster and John, 1999; Stockli, 2005). Late Cretaceous metamorphic rocks beneath the detachment were Samples to the ESE of this point were at higher temperatures prior to the metamorphosed at pressures of 4–6 kbar or depths of ~12–18 km, based onset of exhumation and cooling of the lower plate. The top of the partial on metamorphic thermobarometry data (Grice, 2006). Throw on the Ana- retention zone lies to the west of this point, but east of sample WG04-114 conda detachment since the Late Cretaceous, therefore, cannot have been from the Storm Late Stock, which gave a biotite cooling age of ca. 74 Ma. more than 12–18 km, although the amount in Eocene time may have been The mica cooling age at the base of the Eocene partial annealing zone signiﬁ cantly less due to erosional exhumation of the Cretaceous metamor- is between ca. 54 and 52 Ma, which we interpret to be the time that tec- phic and plutonic complex prior to Eocene extension. tonic exhumation began to rapidly cool the footwall beneath the Anaconda detachment. Eocene granodiorite of the Hearst Lake suite was intruded Relationship to Regional Extension into the eastern part of the footwall at 53 ± 1 Ma (Foster et al., 2007) and was subsequently overprinted by greenschist-facies fabrics. Intrusion of Extension in the Anaconda metamorphic core complex was coeval the granodiorite was presumably related to the change in tectonic setting with early Eocene extension throughout the northern U.S. Rocky Moun- that initiated regional extension starting at 54–53 Ma (Foster et al., 2001, tains and Canadian Cordillera, and very similar in timing to the Bitterroot 2007). An early Eocene age for the onset of extension is also consistent metamorphic core complex (Foster et al., 2007). The extension direction with the age of oldest Lowland Creek volcanic rocks (ca. 53 Ma; Dudas in the Bitterroot complex (100°–110°; Hyndman and Myers, 1988; Fos- et al., 2010; this study). The ca. 41–39 Ma mica cooling ages obtained ter, 2000; House et al., 2002) is identical to the extension direction in the from the greenschist mylonite in the easternmost lower plate record when Anaconda core complex. this part of the lower plate cooled through ~350 °C and was exhumed Eocene extension within the Bitterroot mylonite initially took place at through the brittle-plastic transition. These cooling ages indicate that upper-amphibolite-facies conditions (Foster et al., 2001), at temperatures

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W E A 90 muscovite Exhumed biotite partial 80 retention zone Western limit of exposed Eocene mylonite 70 WG04-114 WG04-052

60 WG04112 Onset of Eocene extension Age (Ma)

40 Mica cooling age gradient due to ESE-directed tectonic unroofing 30 048121620 (A) Distance in slip direction (km) (A′)

113°15′W 46°7.5′N B 0 1 2 N km

A′

113°15′W

Figure 8. (A) Plot of mica 40Ar/39Ar cooling age against distance for sample locations along a section parallel to the slip direction of the Anaconda detachment (105°). Sample locations were orthogonally projected to section A–A′ as shown in part B. The errors on the cooling ages are ±2σ, and errors on the location are ﬁ xed at ±1 km to account for uncertainty in projection to section A–A′ and elevation. The shaded band on upper plot (A) represents the location, or paleodepth, of the partial retention zone for mica prior to Eocene slip on the detachment and exhumation. The location of the partial retention zone is also shown for reference on the map in B.

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60 Relationship to the Boulder Batholith Muscovite The Boulder batholith (Figs. 1 and 2) is a composite of >15 plutons 50 emplaced at shallow depths (<5 km) into a contemporaneous volcanic cara- pace (Tilling et al., 1968). Magmatism occurred between ca. 80 and 70 Ma, and the most voluminous plutons, including the Butte pluton (granodio- 40 Age (Ma) Slip rate = 0.9 ± 0.5 km/m.y. rite), were intruded between ca. 75 and 70 Ma (Tilling et al., 1968; Robin- son et al., 1968; Hamilton and Myers, 1974; Kalakay et al., 2001; Lund et 30 al., 2002). The Boulder batholith hosts the Butte porphyry Cu-Mo deposit, 0 2 4 6 8 10121416182022which formed between ca. 67 and 62 Ma (Lund et al., 2002). The Boulder batholith spans the western Helena salient (Fig. 1) with 60 lateral thrust ramps along the northern and southern boundaries. The Lom- Biotite bard thrust, which transported middle Proterozoic Belt rocks over Paleo-

50 zoic and Mesozoic strata, lies immediately east of the Boulder batholith (Schmidt et al., 1990; Lageson et al., 2001). In most places, the eastern contact is a steeply west-dipping mylonite zone (Rutland et al., 1989; Kal-

Age (Ma) 40 akay et al., 2001). The Boulder batholith, therefore, is bounded by thrust Slip rate = 0.9 ± 0.3 km/m.y. faults, except in the west, where it is in the hanging wall of the Anaconda detachment (Fig. 2). Seismic-refl ection studies show a highly refl ective 30 and laminated lower crust below the batholith starting at ~12–18 km depth 0 2 4 6 8 10 12 14 16 18 20 22 (Vejmelek and Smithson, 1995), which could be the downdip projection Distance in slip direction (km) of the Anaconda mylonite. Figure 9. Calculated slip rate of the Anaconda detachment based on the The 25–28 km displacement estimate for the Anaconda detachment inverse age versus distance gradient for samples that were at temperatures indicates that the Boulder batholith was translated east at least this amount higher than ~330 °C and did not retain radiogenic argon before exhumation. after emplacement. This also implies that the hydrothermal systems that produced the Butte deposit originated to the west. Late Cretaceous– Paleocene plutons in the Flint Creek and Anaconda ranges, including the Mount Powell batholiths, were intruded at deeper crustal levels than the considerably higher than the greenschist-facies conditions within the Boulder batholith (Hyndman et al., 1982), and crystallized within the age Anaconda mylonite. Overprinting deformation in the Bitterroot mylonite range of the Butte deposits (Lund et al., 2002). Equivalent-aged plutons occurred at progressively lower temperatures and produced greenschist- beneath the Deer Lodge Valley could be related to mineralization at shal- facies mylonite, ultramylonite, and shear-banded chloritic breccia (Hynd- lower levels in the Boulder batholith. man and Myers, 1988; Foster, 2000). The total amount of exhumation is greater for the Bitterroot complex than the Anaconda complex. Peak meta- Tectonic Setting morphic conditions in the Bitterroot complex took place in Cretaceous to Paleocene time and attained pressures of 6–8 kbar and 650–750 °C (House The metamorphic core complexes of the northern Cordilleran oro- et al., 1997; Foster et al., 2001). Metamorphic pressure-temperature-time gen occupy the hinterland west of the fold-and-thrust belt (Fig. 1). (P-T-t) paths in the Bitterroot complex footwall show strong decompres- These core complexes developed within either Precambrian basement, sion with upper-amphibolite-facies metamorphism and partial melting as the Mesoproterozoic Belt basin, or Phanerozoic accreted terranes. The young as 53 ± 1 Ma (Foster et al., 2001). The total amount of Eocene Bitterroot and Anaconda complexes in western Montana are within the exhumation was probably ~20–25 km in the eastern part of the core com- former Belt basin and lie over North American basement. They formed plex (Foster et al., 2001) and ~10 km in the western part of the complex after Jurassic–Cretaceous accretion of oceanic terranes and intense (House et al., 2002; Foster and Raza, 2002), owing to the asymmetry in Cretaceous–Paleocene shortening during oblique subduction of the Faral- the fault system. lon or Kula plates (Engebretson et al., 1985; Severinghaus and Atwater, U-Pb zircon crystallization ages of pre-extensional and synexten- 1990; Bird, 2002). The dominant phase of core complex extension started sional plutons in the Bitterroot footwall indicate that extension started at at ca. 54–52 Ma (Foster et al., 2001, 2007), which was within 1–3 m.y. ca. 54–53 Ma (Foster and Fanning, 1997; Foster et al., 2001, 2007), which after the end of thrusting in the Cordilleran foreland fold-and-thrust belt is identical to the estimates from the Anaconda complex based on the U-Pb at ca. 55 Ma (Harlan et al., 1988; Constenius, 1996; Sears and Hendrix, zircon crystallization age of the synextensional granodiorites of the Hearst 2004), and continued to after 40 Ma (Foster and Raza, 2002). Lake suite (Foster et al., 2007) and the 40Ar/39Ar thermochronology. Ther- Early Eocene extension throughout the northern Rocky Mountain Basin mochronologic data (40Ar/39Ar and apatite fi ssion-track data) record rapid and Range and Canadian Cordillera was coincident with the widespread exhumation of the Bitterroot complex after ca. 50 Ma (Criss and Fleck, Challis–Absaroka–Colville–Kamloops–Bitterroot–Lowland Creek–Mon- 1987; Foster and Fanning, 1997; House et al., 1997, 2002; Foster et al., tana alkalic province magmatism at 53–45 Ma (e.g., O’Brien et al., 1991; 2001; Foster and Raza, 2002), and indicate that the eastern footwall did Janecke and Snee, 1993; Foster and Fanning, 1997; Morris et al., 2000; not cool below 250 °C until ca. 40–38 Ma (Foster et al., 2001). Apatite Foster et al., 2001; House et al., 2002; Feeley et al., 2002; Feeley, 2003; fi ssion-track data suggest that exhumation of the eastern part of the com- Breitsprecher et al., 2003; du Bray et al., 2006; Dudas et al., 2010). This plex did not occur until 35–25 Ma during high-angle normal faulting and voluminous magmatism has been attributed to regional extension (Mor- opening of the Bitterroot Valley (Foster and Raza, 2002). The exhumation ris et al., 2000), subduction (Armstrong et al., 1977), or a slab window and cooling history of the Anaconda and Bitterroot complexes in terms of between the Farallon and Kula plates (Breitsprecher et al., 2003), but timing and duration of extension is, therefore, identical. regardless of the generation mechanism, it includes a trace element and

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isotopic signature of partial melting of a lithosphere that had been metasomatized by a long history of Mesozoic subduction (e.g., Morris et al., 2000; Kamloops Feeley, 2003). Any model for the causes of Eocene extension in this region must also explain the voluminous contemporaneous magmatism. EOCENE Colville Regional Eocene extension has been attributed to changes in plate- MAGMATISM Challis boundary conditions (Haeussler et al., 2003; Foster et al., 2007) or col- & EXTENSION lapse of the orogenic pile due to signiﬁ cant partial melting of the middle Montana crust (e.g., Vanderhaeghe and Teyssier, 2001). There are several alterna- alkali province tive models for tectonic plate conﬁ gurations and dynamics due to uncer- tainties in the position of the Kula-Farallon spreading center with respect Absaroka to western North America (e.g., Atwater, 1989). Northward motion of the Kula, or the proposed Resurrection plate, would likely have caused dex- Black tral transtension inboard of the plate margin in Eocene time, explaining Hills extension if one of these two plates were adjacent to northwestern North America at the latitude of the U.S.–Canadian border, (e.g., Thorkelson Cretaceous- Eocene batholith and Taylor, 1989; Haeussler et al., 2003; Breitsprecher et al., 2003). The Eocene metamorphic southern option (Engebretson et al., 1985) for the Farallon-Kula or Far- core complex allon-Resurrection spreading center suggests that a slab window opened Eocene volcanic Colorado beneath the northern U.S. and southern Canadian Cordillera in the early rocks

to middle Eocene, resulting in the Challis-Colville-Kamloops-Absaroka strike - slip fault magmatism and extension (e.g., Breitsprecher et al., 2003). If the Farallon plate were subducting to the NE under this area in Laramide thrust fault Eocene time (Severinghaus and Atwater, 1990), then the lithosphere north Sevier thrust fault of the fl at slab segment, which extended under Colorado in Eocene time, 87Sr/86Sr 0.706 EOCENE could have been extended (Fig. 10) (Dickinson and Snyder, 1979; Sever- FLAT SLAB inghaus and Atwater, 1990; Saleeby, 2003). This is because the angle of Figure 10. Map showing the relationship between Eocene magmatism subduction was presumably progressively, or sharply, steeper to the north and extension in the northern Cordillera and shallow-angle subduction of the fl at slab and rolling back to the west. The transition to normal sub- of the Farallon slab. The base map was modifi ed from Foster et al. (2007), duction north of the fl at slab region could also explain the onset of Challis with the western margin of the continent restored to approximate an (and related) arc magmatism at ca. 53–48 Ma, as well as regional extension early Eocene position by closing up Neogene extension in the Basin and (Fig. 10). If the region north of the Eocene fl at slab had been refrigerated Range Province and Columbia embayment. The position of the Eocene and metasomatized by fl at slab subduction in Late Cretaceous–Paleocene fl at slab was inspired by Saleeby (2003) and Humphreys (2009a). The fi eld of Eocene magmatism includes the Absaroka, Challis (and related fi elds), times (Dumitru et al., 1991; Humphreys et al., 2003; Humphreys, 2009b) Colville, and Kamloops fi elds, along with plutonic rocks in the batho- and then subduction steepened as the fl at slab segment moved relatively liths (e.g., Idaho-Bitterroot batholith) and metamorphic core complexes southward, magmatism and extension would be a natural consequence of shown in Figure 1B. rising asthenosphere under western Montana, northern Idaho, Washing- ton, and southern British Columbia. Humphreys (2009a, 2009b) attributed the Challis arc and associated ca. 39 Ma. The gradient of 40Ar/39Ar muscovite and biotite cooling ages Eocene extension to the collision of the Siletzia terrane (Farallon oceanic along a cross section in the slip direction of the Anaconda detachment lithosphere) into the Columbia embayment. This hypothesis favorably gives an average displacement rate for the fault of ~0.9 mm/yr between explains the distribution and onset of Challis (and related) magmatism. 50 and 39 Ma. Additional extension and exhumation of the footwall con- The locally adakitic signatures of the magmatism (e.g., Breitsprecher et tinued into Oligocene time (27–25 Ma) based on apatite fi ssion-track al., 2003) could be attributed to heating and partial melting of the trapped data. Total displacement of the Anaconda detachment is estimated to be and partially subducted oceanic crust at the leading edge of Siletzia. Exten- 25–28 km based on reconstruction of the Cretaceous Storm Lake Stock sion in this model would be due to the weakening of the orogenic pile dur- with detached granodiorite in the hanging wall. The timing of extension ing the Eocene magmatic fl are-up, asthenospheric upwelling through a in the Anaconda complex was coincident with Eocene hyperextension in tear in the Farallon slab, and/or rollback of the slab at the establishment of the Bitterroot, Priest River, and other northern Cordillera core complexes Cascadia subduction (Humphreys, 2009b). as well as the extensive Challis volcanic arc. Regional extension and mag- Postorogenic extension north of the fl at slab segment due to rollback matism were probably caused by factors related to subduction rollback, of the Farallon plate, the collision of Siletzia, or a combination of both slab window formation, or collision of the Siletzia terrane northwest of are consistent with the majority of the data on the timing and distribution the shallow subducting segment of the Farallon slab, which did not extend of extension and magmatism in the northern Cordillera in Eocene time. north of Wyoming in Eocene time. The relationship between extension and strike-slip translation of terranes now in the northern Cordillera is also important and was somehow due to ACKNOWLEDGMENTS oblique subduction beneath North America. This research was partially supported by EDMAP grants 02HQAG0067, CONCLUSIONS 03HQAG0117, and 04HQAG0088 awarded to Kalakay; National Sci- ence Foundation grants EAR-0080086, EAR-106592, EAR-0545715, and The 40Ar/39Ar data indicate that extension in the Anaconda meta- EAR-0838476 awarded to Foster; GeoEarthScope funds awarded to Fos- morphic core complex started at 53 ± 1 Ma and continued until after ter; and awards from the Geological Society of America, Tobacco Root

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O’Neill, J.M., and Lageson, D.R., 2003, West to east geologic road log: Paleogene Anaconda metamorphic core complex: Georgetown Lake Dam–Anaconda–Big Hole Valley: North- MANUSCRIPT RECEIVED 17 DECEMBER 2009 west Geology, v. 32, p. 29–46. REVISED MANUSCRIPT RECEIVED 23 APRIL 2010 O’Neill, J.M., Lonn, J.D., Lageson, D.R., and Kunk, M.J., 2004, Early Tertiary Anaconda meta- MANUSCRIPT ACCEPTED 26 APRIL 2010 morphic core complex, southwestern Montana: Canadian Journal of Earth Sciences, v. 41, p. 63–72, doi: 10.1139/e03-086. Printed in the USA

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