Cooling and Exhumation of the Southern Idaho Batholith

Home , Sawtooth Range (Idaho), White Cloud Mountains

THEMED ISSUE: EarthScope IDOR project (Deformation and Magmatic Modification of a Steep Continental Margin, Western Idaho–Eastern Oregon)

Cooling and exhumation of the southern Idaho batholith

A.K. Fayon1,*, B. Tikoff 2, M. Kahn2, and R.M. Gaschnig3,† 1DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF MINNESOTA TWIN CITIES, MINNEAPOLIS, MINNESOTA 55455, USA 2DEPARTMENT OF GEOSCIENCE, UNIVERSITY OF WISCONSIN–MADISON, MADISON, WISCONSIN 53706, USA 3SCHOOL OF EARTH AND ATMOSPHERIC SCIENCES, GEORGIA INSTITUTE OF TECHNOLOGY, ATLANTA, GEORGIA 30332, USA

ABSTRACT

We conducted a (U-Th)/He zircon thermochronology study of the southern part of the Idaho batholith (central Idaho, USA) to constrain cooling through ~200 °C and exhumation of the batholith. Samples were collected adjacent to the Idaho-Oregon (IDOR) seismic transect and at localities where U-Pb zircon, geochemical, and fabric analyses were conducted. The rocks affected by the western Idaho shear zone and associated border zone suite of the batholith cooled through the closure temperature for He in zircon prior to ca. 60 Ma, before or during emplacement of the voluminous Atlanta lobe. In contrast, the Atlanta lobe (Atlanta peraluminous suite) records a relatively constant cooling rate, in which the (U-Th)/He zircon ages are systematically ~30 m.y. younger than the U-Pb zircon ages. We interpret this data to reflect post-magmatic isobaric cooling with little or no unroofing. The only deviation from a smooth regional cooling pattern occurs near Sawtooth Valley, where samples from the Sawtooth Range on the west side of the valley show distinctly younger ages than those from the White Cloud Peaks to the east. We interpret this difference to reflect recent cooling and exhumation associated with extensional deformation. The regionally consistent pattern of cooling and hence exhumation indicates that the current exposure level of the Idaho batholith was <5 km deep (assuming a geothermal gradient of 40 °C/km) at 50 Ma during the initiation of Challis magmatism. Our data are consistent with the existence of a crustal plateau during formation of the Atlanta lobe of the Idaho batholith.

LITHOSPHERE; v. 9; no. 2; p. 299–314 | Published online 13 January 2017 doi:10.1130/L565.1

INTRODUCTION There are additional features that distinguish the Idaho batholith from the other Cordilleran batholiths. First, the Idaho batholith is notable due to A diagnostic attribute of the North American Cordillera is the presence its relative structural and bulk geochemical homogeneity (e.g., Gaschnig of large igneous batholiths that intruded the continental margin from ca. et al., 2011; Byerly et al., 2016). Second, the Idaho batholith is the only 125 Ma to 50 Ma (e.g., Ducea, 2001; DeCelles et al., 2009; Gehrels et large batholith of the North American Cordillera that intrudes entirely into al., 2009; Miller et al., 2009; Gaschnig et al., 2010; Paterson and Ducea, North American crust, and it is located farther east than other subduction- 2015; Premo et al., 2014) (Fig. 1). These batholiths—Coast Mountains, related batholiths in the North American Cordillera (e.g., Armstrong et Idaho, Sierra Nevada, and Peninsular Range—are typically thought to have al., 1977; Burchfiel et al. 1992). Third, the batholith is dominated by resulted from eastward-dipping subduction from the Pacific basin under the two-mica–bearing granites; its sources are predominantly recycled Pre- western margin of North America. The timing and geochemistry of these cambrian crust (Gaschnig et al., 2011), and its zircons show pervasive igneous rocks provide a framework for the growth of North America and xenocrystic components (e.g., Chase et al., 1978; Bickford et al., 1981; the influence of pre-existing structures on subsequent tectonism. Further, Toth and Stacey, 1992; Foster and Fanning, 1997; Gaschnig et al., 2010, constraining the cooling and exhumation of batholiths provides a record 2013). Fourth, Giorgis et al. (2005) hypothesized that a ~80–100-km-wide, of orogenic crust stabilization through time (e.g., DeCelles et al., 2009). subduction-related magmatic arc was active from ca. 125 to 91 Ma and was The age and petrogenesis of the Idaho batholith (central Idaho, USA) located entirely west of the current Idaho batholith; this arc was shortened has recently been constrained through detailed U-Pb geochronology (Gas- to a ~8-km-wide zone by subsequent tectonism associated with the west- chnig et al., 2010) and isotope geochemistry (Gaschnig et al., 2011). ern Idaho shear zone (see also Gaschnig et al., 2017). The other coastal Gaschnig et al. (2010) subdivided the batholith into five main pulses batholiths lack this type of major shear zone with intense shortening on of magmatism: the border zone suite, the early metaluminous suite, the their margins, suggesting that Cretaceous igneous rocks in Idaho might Atlanta peraluminous suite, the late metaluminous suite, and the Bitterroot also record fundamentally different intrusive and cooling paths, reflect- peraluminous suite (Fig. 2). Geochemical analyses of the Atlanta peralu- ing a different tectonic environment relative to other coastal batholiths. minous suite, which forms the bulk of the geographically defined Atlanta In this contribution, we provide 46 new (U-Th)/He zircon data that help lobe of the Idaho batholith, indicate that it is largely a result of crustal constrain the post-intrusion cooling and exhumation of the Idaho batho- melting, unlike any other batholith of the North American Cordillera. lith. These (U-Th)/He zircon data were co-located with geochronology and geochemistry sites of Gaschnig et al. (2010, 2011) and Braudy et al. Basil Tikoff http://orcid.org/0000 -0001 -6022 -7002 (2016) and the fabric studies of Byerly et al. (2016). This coordination was *Corresponding author: [email protected] †Present address: Department of Environmental, Earth, and Atmospheric Sci- made possible through the Earthscope IDOR project, which—in addition ences, University of Massachusetts Lowell, Lowell, Massachusetts 01852, USA to active-source (Davenport et al., 2017) and passive-source (Stanciu et

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/2/299/1001583/299.pdf by guest on 29 September 2021 FAYON ET AL.

Z CANADA Wallowa terrane USA WIS Accreted Terranes Riggins NorthNorth Fig. 2 America McCall Baker Yellow Pine 500 km 119° terrane Lemhi Whit Sa R wtooth Rang a e Cloud nge Challis intrusive Izee “terrane” and extrusive rocks Olds Ferry Pe Atlanta lobe aks Basin terrane e and Range Idaho batholith SRSZ Boise normal faults IDOR line intrusive rocks

OR ID Sawtooth Valley Pioneer core complex 43° N Undifferentiated 0 100km igneous and meta- N 119W° morphic rocks

Figure 1. Generalized geologic map of the Idaho batholith and accreted terranes (after LaMaskin et al., 2011). The location of the Idaho batholith is shown relative to tectonic elements of the North American Cordillera and other major Cretaceous-age batholiths of western North America (inset). Grid pattern shows extent of muscovite-bearing granite belt. The east-west–oriented dashed line indicates the position of the IDOR active-source seismic line (Davenport et al., 2017). The north-south–oriented dashed line indicates the position of the western Idaho shear zone (WISZ) within the broader Salmon river suture zone (SRSZ), shown by the bracket.

al., 2016) seismic surveys—funded integrated structural, geochronologi- along the margins of the batholith and younger ages at lower elevations cal, and geochemical data collection and interpretation. Together, these and in close proximity to Challis suite exposures. The Challis magmatic data allow us to provide temperature-time (T-t) paths documenting the event succeeded Idaho batholith emplacement and continued for ~10 m.y. evolving thermal structure of the batholith and its margins. These data resulting in elevated thermal gradients during the Eocene. Based on their indicate distinct patterns of cooling across the Idaho batholith, with sys- results, Sweetkind and Blackwell (1989) concluded that the batholith tematic differences in cooling within the Atlanta lobe, along the western cooled below ~110 °C by ca. 50 Ma and has undergone ~3 km of erosion Idaho shear zone, and in the Sawtooth Valley region. Within the Atlanta since 10 Ma. Sweetkind and Blackwell (1989) inferred that hydrothermal peraluminous suite, which constitutes the majority of the Atlanta lobe, the activity affected the apatite fission-track ages, particularly in the deeply (U-Th)/He zircon ages are ~30 m.y. younger than the U-Pb zircon ages. incised valleys, resulting in young ages. The presence of a relationship between age and elevation in the Atlanta Other studies focusing on the cooling of the Idaho batholith have lobe indicates that the (U-Th)/He zircon cooling ages reflect post-mag- considered the evolution or exhumation of the deep to mid-crust along matic isobaric cooling, with little or no tectonic unroofing. The western low-angle detachment faults, forming metamorphic core complexes (e.g., Idaho shear zone exhibits systematically older (U-Th)/He zircon ages than Silverberg, 1990; Foster et al., 2001; Foster and Raza, 2002; Vogl et al., the adjacent Atlanta lobe, with a west-to-east younging trend. Finally, 2012). Foster et al. (2001) documented magmatism, extensional deforma- on the eastern edge of the batholith, there is a distinct step in regional tion, and exhumation of the Bitterroot metamorphic core complex occur- cooling patterns in the vicinity of the Sawtooth Valley (Fig. 1), which we ring in regions of thickened crust in the northeastern section of the Idaho interpret as recent extensional deformation resulting in the exhumation of batholith, within the Bitterroot lobe. Another metamorphic core complex the Sawtooth Range (Fig. 1). Overall, the pattern of cooling is consistent in the vicinity of the Idaho batholith is the Pioneer metamorphic core com- with formation of a Cretaceous–Paleogene crustal plateau in central Idaho. plex, located east of the Idaho batholith (Fig. 1; O’Neill and Pavlis, 1988; Silverberg, 1990; Vogl et al., 2012). Vogl et al. (2012) have constrained the PREVIOUS WORK ON THE COOLING AND EXHUMATION OF timing of the syn-extensional Pioneer intrusive suite to 50–48 Ma, similar THE IDAHO BATHOLITH to that of the early phases of Challis magmatism (Gaschnig et al., 2010). More recent studies have focused on the cooling and exhumation of Previous studies on the cooling and exhumation of the Idaho batholith the border zone suite adjacent to the western Idaho shear zone (Fig. 1; document variable cooling as a function of position within the batholith. Giorgis et al., 2008; Braudy et al., 2016; Gaschnig et al., 2017; Montz Earlier studies (Criss et al., 1984; Sweetkind and Blackwell, 1989) show and Kruckenberg, 2017). U-Pb monazite ages of ca. 90 Ma were obtained older ages obtained along the margins, while younger ages were observed within a migmatite domain immediately east of the western Idaho shear within the central part of the batholith. Criss et al. (1982) obtained K-Ar zone (Montz and Kruckenberg, 2017). Within the western Idaho shear cooling ages as old as 95 Ma along the margins. In the central portions of zone, Giorgis et al. (2008) documented that cooling below the 40Ar/39Ar the batholith, cooling ages are significantly younger, ca. 35 Ma. Sweetkind biotite closure temperature (~350 °C) and the apatite fission-track anneal- and Blackwell (1989) observed a similar pattern in apatite and zircon ing temperature (<120 °C) occurred between 85–70 Ma and ca. 40 Ma, fission-track ages. They reported older apatite fission-track ages (>45 Ma) respectively. The 40Ar/39Ar ages from within the western Idaho shear zone

300 www.gsapubs.org | Volume 9 | Number 2 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/2/299/1001583/299.pdf by guest on 29 September 2021 Cooling and exhumation of the southern Idaho batholith | THEMED ISSUE

73.1 ID MT 64.9 71.1 63.1 42.7

70.9 43.8

51.9 75 76.2 29.6 69.5 49.4 56.8 29.5 34.2 48.8 87.7 67.1 29.3 65.2 57.2 39.7 65 34.2 48.8 36.8 32.1 68.5 63.2 34.5 20.2 64.2 40.5 44.5

26.6. 35.4 40.5 N 38.8 40.7 43.3 63.6

Pliocene and younger Atlanta peraluminous sediment suite

Miocene-Pleistocene Early metaluminous volcanics suite

Eocene Challis Border zone suite volcanics

Early Eocene Challis Idaho batholith Suture zone suite intrusive rocks Undifferentiated Jurassic-Cretaceous Faults intrusive rocks

43.8 Sample location Paleozoic sedimentary and w/ (U-Th)/He metasedimentary rocks zircon age 050 100 Kilometers Proterozoic metasedimentary rocks

Figure 2. Geologic map of the study area (central Idaho) showing various phases of magmatism. Sample locations are shown along with the mean (U-Th)/He zircon age (in Ma) for each sample (after Gaschnig et al., 2010). Geologic map is reproduced from Lewis et al. (2012). ID—Idaho; MT—Montana.

LITHOSPHERE | Volume 9 | Number 2 | www.gsapubs.org 301

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/2/299/1001583/299.pdf by guest on 29 September 2021 FAYON ET AL.

are generally consistent with regional data (e.g., Snee et al., 1995) that as the mean and standard deviation of three single-grain ages, unless oth- indicate increasingly younger ages from west to east within the western erwise noted. We report the ages and standard deviations as strict averages Idaho shear zone (Fig. 1). to the nearest ±0.1 m.y., although we acknowledge that the true analytical We present new (U-Th)/He zircon data across the western Idaho shear precision is likely ±1–2 m.y. Ages range from Late Cretaceous to Early zone and Idaho batholith to constrain the cooling and exhumation of the Miocene (Table 1; Figs. 2, 3). Projecting all ages onto a latitudinal transect batholith relative to the IDOR seismic transect. The T-t history—from reveals a pattern in the cooling data, with the western and eastern margins crystallization to <200 °C—has not been considered in the context of new of the batholith cooling before the interior of the Atlanta lobe (Fig. 3). The divisions of the Idaho batholith based on U-Pb geochronology and geo- data also show variability in the vicinity of the Sawtooth Valley. chemistry (Gaschnig et al., 2010, 2011) and new seismic data (Davenport et al., 2017) acquired through the EarthScope IDOR project. Combining Western Idaho Shear Zone thermochronometric data with new seismic models (Davenport et al., 2017) and structural and geochronologic data (Braudy et al., 2016; Byerly Samples within the western Idaho shear zone near McCall and Rig- et al., 2016; Gaschnig et al., 2017) gives a broader understanding of the gins, Idaho, range in age from ca. 76 to 63 Ma. The variability of ages is crustal growth and stabilization of the northern North American Cordillera. a function of position relative to the boundary with the Atlanta lobe to the east; ages decrease with increasing proximity to the Atlanta lobe (Fig. 3). ZIRCON (U-Th)/He DATING METHODS Near McCall, ages range from 76.2 ± 3.1 (sample 10RMG009) to 63.2 ± 2.5 Ma (07RMG47). Along strike northward within the western Idaho (U-Th)/He zircon thermochronometry is based on the formation and shear zone near Lava Buttes (Fig. 2), ages are slightly younger ranging retention of He in zircon and provides the time at which rocks cool below from 69.2 ± 2.4 (13AF03) to 63.1 ± 2.7 Ma (13AF04). These results ~130–250 °C (e.g., Reiners, 2005). Closure temperature variability is a document cooling below ~200 °C by ca. 63 Ma (Fig. 3) and agree with function of grain size and cooling rate, with larger grain sizes and faster previously published 40Ar/39Ar ages (Giorgis et al., 2008). Samples col- cooling rates corresponding to higher closure temperatures. Recent inves- lected in accreted terrane rocks exposed near Riggins yield slightly older tigations on the (U-Th)/He system in zircon have revealed the relationship ages of 73.1 ± 14.6 (13AF06) and 71.1 ± 12.9 Ma (13AF10). However, between radiation damage, ages, and cooling rates. Accumulated radiation both ages are consistent with other cooling ages within the western Idaho damage in zircon can result in either inhibited or enhanced diffusion (e.g., shear zone. The large standard deviations on these ages reflect the spread Nasdala et al., 2004; Reiners, 2005; Guenthner et al., 2013; Ketcham et in the single-grain ages for these two samples. al., 2013). Diffusion is inhibited when the alpha damage acts as a trap for The single-grain age-eU patterns for samples from within the western He, preventing the atom from readily diffusing through the lattice. The net Idaho shear zone show no variation in age as a function of eU concentra- result is a higher closure temperature and an older apparent cooling age. tion (Fig. 4). Single-grain ages range between 60 and 85 Ma with a range Conversely, as the amount of accumulated radiation damage increases, of eU concentration from 100 to 1200 ppm (Figs. 4B, 4C). This trend the alpha tracks form a connected network thereby providing fast diffu- is consistent with rapid cooling through the He retention temperature in sion pathways for He. This results in a lower closure temperature and a zircon (e.g., Guenthner et al., 2013). younger apparent cooling age. Modeling of these different scenarios has allowed for more detailed interpretation of (U-Th)/He zircon ages. The Atlanta Lobe closure temperatures predicted by the age-eU (effective Uranium) model of Guenthner et al. (2013) are 140–220 °C. At higher alpha doses the clo- Immediately east of the western Idaho shear zone (east of longitude sure temperature decreases, resulting in a younger apparent cooling age. 116°W), rocks cooled below ~200 °C ~7–30 m.y. after those rocks exposed Samples used in this study were obtained by Gaschnig et al. (2010, within the western Idaho shear zone. Adjacent to the western Idaho shear 2013) and Braudy et al. (2016); supplemental samples were collected at zone, the Atlanta lobe yields (U-Th)/He zircon ages of 57.2 ± 1.7 (sample strategic locations relative to the EarthScope IDOR active-source seismic 98IB53), 49.4 ± 3.4 (10RMG44), and 36.8 ± 6.5 Ma (07RMG46). Further survey (Fig. 2; Davenport et al., 2017). Supplemental samples were pro- east into the Atlanta lobe, ages vary and are as young as 20.2 ± 1.4 Ma cessed at the University of Minnesota using standard mineral separation (13AF14) in the Sawtooth Range adjacent to Sawtooth Valley. The overall techniques—milling, water, heavy liquid, and magnetic separation. Zircon age pattern shows no distinct variation with elevation, but there is a variability grains were analyzed and selected for clarity, morphology, and size at with longitude, with younger ages in the center of the transect and older ages University of Arizona (USA) Radiogenic Helium Dating Laboratory using toward the margins (Fig. 3). This pattern is also evident in the U-Pb zircon a Leica MZ16 stereo-zoom microscope. Three grains per sample were age distribution. The average (U-Th)/He age for the Atlanta lobe is 43 Ma. imaged and measured for alpha-ejection corrections (Farley et al., 1996; The eU concentrations for samples from the Atlanta lobe range from

Farley, 2002) and packed in Nb tubes. Grains were lased using a CO2 laser ~100 to 2400 ppm, yet there is no correlation between eU concentrations for 12 min for the first extract, followed by 15 min re-extractions until the and ages obtained (Fig. 4). If eU concentration affected the (U-Th)/He age, He extracted was <1% of the total He. Gas concentrations were measured younger ages would be expected for samples with higher concentrations. using a quadropole mass spectrometer; U, Th, and Sm were analyzed on a This pattern is not observed. Rather, some samples with higher eU con- high-resolution Element 2 inductively coupled plasma-mass spectrometer centrations yield older ages (Fig. 4). Further, we interpret these samples (ICP-MS) using methods outlined in Reiners and Nicolescu (2006). Ages as recording slow cooling in contrast to the rapid cooling observed in the are reported in Table 1 and shown in Figures 2 and 3. western Idaho shear zone. A rapid cooling event would result in similar ages regardless of eU concentrations. COOLING OF THE SOUTHERN IDAHO BATHOLITH: RESULTS Sawtooth Valley The 46 new (U-Th)/He zircon data presented here illustrate the variability of cooling and hence exhumation recorded by the Idaho batholith as Samples collected in the Sawtooth Range and White Cloud Peaks west exposed in the vicinity of the IDOR seismic surveys. All ages are reported and east of the Sawtooth Valley, respectively, yield ages ranging from

302 www.gsapubs.org | Volume 9 | Number 2 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/2/299/1001583/299.pdf by guest on 29 September 2021 Cooling and exhumation of the southern Idaho batholith | THEMED ISSUE ) .2 .7 .4 .0 4. 6 2.5 3.7 4. 6 2.3 5.2 0.1 1. 5 3.1 6. 7 12.9 (Ma ) St Dev continue d ( 0. 94 3. 11 3. 12 9. 22 0. 52 71.1 63.2 65.2 64.9 67.1 69.5 75.0 59.1 76.2 67.2 (Ma ) Mean ag e § .1 .0 .1 .8 .8 .9 .9 .8 .0 .9 .8 .0 .8 1. 27 1.0 1. 0 0.9 1. 37 1.0 0. 8 1.0 1. 0 0.9 1.2 1. 0 1.5 0. 96 1.3 1.3 1.4 1. 06 1.2 1.3 0. 9 1.4 1.4 0.8 1.4 0. 74 1.5 0. 6 1.5 0.8 0. 7 1.3 1.0 1.4 (Ma ) Erro r 5. 41 7. 31 2. 61 7. 10 0. 40 4. 80 3. 60 0. 20 2. 01 0. 60 7. 70 1. 31 9.5 0. 80 69.8 63.1 73.6 60.7 86.3 65.7 57.4 69.5 75.5 62.8 63.4 69.6 68.3 65.5 64.4 68.6 75.5 68.0 65.7 67.5 67.6 74.9 75.1 58.9 75.0 41.1 74.3 37.8 80.1 42.4 73.2 70.8 77.2 date (Ma ) Corrected 2. 17 6. 86 8. 28 9. 15 2. 06 6. 96 3. 56 8. 16 4. 07 4. 36 8. 75 6. 87 0. 85 0. 84 53.2 51.9 54.7 51.0 59.8 51.1 43.2 54.3 61.3 50.5 51.9 60.7 52.9 49.2 49.8 55.5 61.0 50.8 55.7 60.1 55.0 60.7 59.9 47.2 63.4 33.8 54.3 27.7 56.1 33.2 53.8 53.7 56.8 (Ma ) Raw date e .8 26 .7 04 .7 05 .6 83 .8 65 .8 85 .8 45 .8 04 .7 55 .7 34 .8 44 .8 15 .7 63 Ft 0.73 0.82 0.75 0.84 0.66 0.78 0.75 0.78 0.82 0.81 0.82 0.87 0.77 0.75 0.81 0.81 0.81 0.75 0.85 0.89 0.81 0.82 0.80 0.81 0.85 0.83 0.74 0.74 0.71 0.8 65 0.79 0.74 0.76 0.74 3 30 00 40 14 39 70 08 79 70 03 32 46 76 75 12 He 111 13 20 12 00 10 50 12 80 159 162 168 125 122 126 20 50 129 25 20 167 244 19 6 154 277 196 30 70 176 170 43 30 17 8 132 41 70 123 272 153 20 90 4 (nmol/g) 93 39 07 01 61 0 14 19 79 51 38 43 83 59 48 61 20 96 71 57 62 79 62 43 53 59 10 64 91 11 11 eU 210 190 251 22 12 151 173 24 73 122 14 69 10 08 17 83 17 6 143 38 51 106 12 71 53 52 184 (ppm) 90 69 13 24 06 38 18 13 97 63 06 Th 08 51 57 01 516 542 549 390 315 23 98 395 49 97 428 60 72 463 81 72 527 716 497 836 42 19 578 22 532 481 91 02 415 28 53 382 891 501 21 51 340 1081 1312 1604 1087 1055 (ppm) 05 58 16 65 39 69 44 71 78 80 U 09 01 39 33 47 34 33 23 565 13 31 587 21 62 608 89 08 425 23 82 341 436 442 478 556 34 73 739 71 46 510 28 62 850 597 28 31 542 494 449 35 71 407 63 56 934 526 25 41 362 11 11 11 1354 1645 (ppm) l 83 89 32 56 10 43 69 59 72 22 71 38 3 18 1 22 86 293 382 268 18 9 226 19 8 241 42 52 267 43 35 224 53 46 439 29 18 439 38 2 362 344 528 28 44 320 24 11 225 25 7 346 40 0 25 39 165 27 91 216 225 270 19 41 194 (µm ) 1 2 21 8 81 31 0 72 02 82 4 71 03 32 0 r 53 57 60 43 46 58 52 71 56 78 43 79 48 64 48 49 49 66 92 53 52 51 51 66 61 59 38 50 32 36 38 37 (µm ) ABLE 1. ZIRCON (U-Th)/He D ATA T † 6. 74 3. 23 3. 33 3. 32 5. 03 2. 63 8.9 5. 54 8.6 7.6 6. 43 5. 94 6. 23 5. 03 4.4 4. 23 4.0 2. 07 9. 34 5.4 7.3 5. 64 4.8 (µg ) 19.9 17.1 25.4 12.2 12.0 38.3 15.3 47.7 60.9 18.4 21.5 18.4 15.8 15.8 27.0 81.2 14.2 15.8 10.8 12.0 27.6 27.2 16.0 12.7 Mass zr3 zr2 zr1 zr3 zr2 zr3 zr1 zr2 zr3 zr1 zr2 zr3 zr1 zr2 zr3 zr1 zr2 zr3 zr1 zr3 zr2 zr2 zr1 zr1 zr3 zr3 zr2 zr2 zr1 zr1 zr3 zr3 zr2 zr2 zr1 zr1 zr4 zr4 zr3 zr3 zr3 zr 23 zr2 zr2 zr1 zr1 zr1 Grain 84 6 878 868 170 (m) 1300 1575 1538 2257 161 5 2305 1521 2156 2214 1596 1337 s Elevation (°N) 5.25512 3.68403 Latitude 45.254 32 44.95907 45.25205 44.08955 45.42367 44.28507 44.33624 45.23703 44.42589 44.44122 45.26395 45.26357 44.43941 169 197 1 127 54 1527 (°W) 6.18527 5.2786 94 16.1 16.21 16.1 16.1 16.20927 16.33488 16.10825 16.32702 16.12923 16.10758 16.23504 16.10893 16.26923 11 11 Longitude -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 1- 9- 1 estern Idaho shear zone in the vicinity of Lava Butte estern Idaho shear zone in the vicinity of McCall, ID Sample number 13AF28 13AF10 07RMG47 13AF06 10NB22 13AF05 10RMG01 13AF04 10NB376 13AF03 10RMG010 13AF02 13AF0 10RMG009 07RMG2 W W Atlanta lobe

LITHOSPHERE | Volume 9 | Number 2 | www.gsapubs.org 303

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/2/299/1001583/299.pdf by guest on 29 September 2021 FAYON ET AL. ) .4 .3 .2 0. 6 5. 6 0.9 3. 6 6.5 6.7 2. 9 0.8 1.7 0.3 5. 3 0.9 4. 6 5.8 (Ma ) St Dev continue d ( 3. 8 9. 43 6. 62 9. 7 1. 96 50.6 38.8 42.7 36.8 34.2 43.3 34.5 65.0 40.7 35.4 29.5 63.6 (Ma ) Mean ag e § .0 .6 .6 .6 .6 .8 0. 84 0. 9 0. 8 1. 0 0.7 0.6 0.6 0. 9 0. 8 0.5 0.6 0. 8 0.5 1. 14 1. 1 0.7 0.6 1. 2 0.6 1. 1 0.4 0.5 0. 6 0.6 0. 6 1.0 0.6 0. 42 1.0 0. 5 1.0 0.6 0. 5 0. 73 0.6 0.6 0.7 0. 6 0.6 0. 5 0.6 0.9 1. 0 1. 05 1.0 1.1 (Ma ) Erro r 3. 9 6.3 6. 71 6. 50 4. 7 1. 9 0. 10 4. 60 3. 90 0. 90 43.1 44.3 49.4 37.9 39.6 45.8 38.6 34.4 38.9 43.9 31.8 49.2 45.5 44.1 41.4 53.0 33.0 46.1 28.2 33.6 34.6 63.0 35.2 23.9 66.2 27.5 65.7 41.0 28.3 45.1 40.6 36.3 40.4 39.3 34.5 29.9 35.4 56.9 24.7 58.5 65.8 date 68.0 (Ma ) Corrected 1. 44 3. 44 8. 65 5. 14 4. 04 4. 94 4. 04 6. 03 7. 33 5. 45 32.7 36.7 36.1 27.6 29.1 32.1 24.7 29.7 29.7 29.7 26.6 35.5 29.7 32.8 30.1 37.6 21.7 35.2 21.6 26.8 29.0 52.2 27.7 20.3 56.7 20.9 53.9 32.0 24.1 34.0 30.2 31.8 29.4 31.7 30.5 23.9 27.2 39.5 15.9 46.2 53.6 57.0 (Ma ) Raw date e .7 23 .6 83 .7 63 .7 73 .8 43 .8 63 .7 52 .8 12 .6 93 Ft 0.76 0.83 0.73 0.7 33 0.74 0.74 0.71 0.64 0.86 0.77 0.68 0.84 0.74 0.66 0.74 0.73 0.71 0.66 0.77 0.76 0.80 0.84 0.83 0.79 0.85 0.86 0.76 0.82 0.78 0.85 0.76 0.75 0.88 0.73 0.81 0.89 0.80 0.77 0.70 0.64 0.79 0.82 0.84 70 5 50 60 1 1 7 20 3 36 00 50 36 63 97 76 69 92 93 78 48 52 76 34 28 73 69 69 97 42 67 28 73 He 11 16 4 28 50 172 163 193 209 18 70 149 28 70 274 171 318 207 23 0 13 00 190 183 16 10 260 4 (nmol/g) 95 9 19 18 1 60 76 96 78 43 98 34 86 34 72 17 22 65 69 111 21 51 eU 10 22 12 21 103 16 06 174 63 01 284 17 3 221 776 12 5 36 69 103 445 208 359 185 21 75 21 29 535 24 41 251 33 0 18 88 173 164 287 18 41 145 148 18 91 532 111 1 ) (ppm) 76 27 61 50 52 44 90 17 52 78 28 08 47 Th 32 25 216 374 435 406 136 42 82 870 20 4 425 272 86 06 905 25 4 885 101 910 650 479 407 56 48 374 397 85 71 63 01 520 424 621 504 78 02 721 1 1309 1350 continued 1049 1000 133 6 1080 1692 1062 (ppm) ( 36 97 83 37 83 85 59 05 U 50 54 84 18 27 92 239 398 56 05 48 94 476 429 48 44 922 183 23 4 445 39 83 50 34 296 954 51 71 969 144 30 32 82 87 709 528 47 24 448 400 435 588 458 891 32 33 69 16 629 84 3 841 11 1 1 11 11 1333 1368 120 31 144 1 1018 1036 1770 (ppm) 1 l 57 68 66 67 34 52 23 34 13 94 94 48 51 19 73 19 9 28 5 19 5 170 181 390 223 347 21 44 245 188 15 9 132 254 253 251 20 51 234 231 28 1 35 2 280 23 2 272 253 34 8 174 24 45 172 26 08 32 06 460 29 0 305 177 29 6 260 306 (µm ) 02 3 0 61 91 31 61 91 81 3 32 12 41 72 8 61 r 60 38 38 38 73 41 59 36 36 36 28 40 49 66 72 63 46 85 66 76 39 57 44 65 39 81 36 52 50 90 47 43 31 45 56 64 (µm ) † .5 ABLE 1. ZIRCON (U-Th)/He D ATA 7. 34 4. 03 3. 23 3. 13 4.8 4.4 1. 92 4.7 2. 23 1. 22 7.0 1. 82 4. 9 1. 32 5.7 4.3 3. 23 1.8 7. 74 6. 54 7.4 1.0 6. 74 8.9 6. 5 9.0 5. 93 4.8 4. 73 4.1 9.6 3.2 1. 72 T 1 (µg ) 11 18.9 38.1 22.3 19.8 19.4 17.1 34.3 28.0 29.5 15.9 27.0 60.9 12.8 10.4 68.4 10.8 14.7 22.7 Mass zr3 zr2 zr2 zr1 zr3 zr1 zr3 zr2 zr2 zr1 zr3 zr3 zr1 zr2 zr2 zr3 zr1 zr1 zr3 zr2 zr2 zr1 zr4 zr1 zr3 zr3 zr2 zr2 zr3 zr1 zr1 zr3 zr2 zr2 zr1 zr3 zr1 zr3 zr2 zr2 zr3 zr1 zr3 zr1 zr2 zr2 zr1 zr3 zr1 zr3 zr2 zr1 Grain 979 (m) 1419 1680 2333 1689 2333 1777 1468 121 8 1777 1859 1208 162 1 1600 2255 2091 1785 Elevation 16 (°N) 4.64159 5.16353 3.47128 4.29224 Latitude 44.0504 43.591 44.96267 43.60373 45.16353 44.37438 44.17221 44.07081 44.26681 43.78053 43.75253 44.35758 43.44953 5.4927 (°W) 5.6272 44 5.3849 44 5.3849 4 5.9521 2 4.3465 34 5.6405 84 14.8595 15.5381 15.50292 15.89086 15.24409 14.75722 14.95833 15.24416 15.23559 14.43998 11 11 11 11 11 11 11 Longitude -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 E- B- 2- 4- 1- 5- 6 6- Sample number 10NB356 07RMG5 10NB355 07RMG35 10NB355 07RMG46 10RMG4 07RMG43 07RMG45 07RMG4 07RMG58 07RMG2 07RMG36 10RL89 07RMG28 07RMG5 07RMG40

304 www.gsapubs.org | Volume 9 | Number 2 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/2/299/1001583/299.pdf by guest on 29 September 2021 Cooling and exhumation of the southern Idaho batholith | THEMED ISSUE .0 .3 .1 .2 .7 .4 .7 1.4 3.1 7.5 1.7 5.9 5.8 2.3 (Ma) St Dev 40. 52 32. 12 20.2 68. 52 33.3 64. 24 29.3 57.2 48. 85 87. 75 50.2 34.2 44. 53 29.6 (Ma) Mean age § .5 .6 .5 .5 .0 .0 .0 .1 .9 .8 .7 .7 .7 .3 .8 .3 0.6 0.5 0.3 0.3 0.3 0.5 0.4 0.6 0.4 0.8 0.4 1. 0 0.8 0.5 2.2 0.7 0.5 0.7 0.4 0.6 2. 1 0.4 0.5 0.7 0.4 0.4 2. 5 (Ma) Error 5.7 38. 20 41.6 41. 60 29. 50 33. 00 33.8 18.6 21.2 20.8 70. 61 66. 31 34.5 68. 61 29.8 35.5 67. 21 25.0 59. 40 55.9 24.9 54. 90 66. 1 58.4 38.0 43. 80 83. 9 54.4 35.3 47. 60 43. 30 52.7 87. 52 28.0 48. 70 43.5 81. 9 27.0 39.4 41.5 31.2 89. 42 date 30.5 (Ma) Corrected 33.1 34.5 33.4 19.2 22.0 25.4 16.2 16.5 16.8 51.8 53.7 28.8 57.7 22.3 28.3 59.0 20.3 50.0 48.2 21.4 42.2 54.3 49.5 33.3 35.1 63.5 42.3 28.7 37.5 28.7 37.3 64.5 20.7 36.5 36.1 61. 4 23.1 32.8 28.8 26.7 68. 4 25.1 69. 29 (Ma) 1); those beginning with “98IB” are from Gaschnig Raw date e Ft 0.86 0.84 0.80 0.65 0.66 0.75 0.87 0.78 0.81 0.74 0.81 0.84 0.84 0.75 0.80 0.88 0.81 0.84 0.78 0.86 0.77 0.82 0.71 0.88 0.80 0.75 0.83 0.81 0.79 0.67 0.86 0.75 0.74 0.75 0.85 0.75 0.86 0.84 0.70 0.86 0.77 0.83 0.72 4 5 1 67 60 75 55 73 80 38 56 24 25 He 11 21 132 309 249 520 491 458 151 189 257 134 197 313 141 140 172 184 358 281 134 157 199 128 257 239 133 136 149 134 105 4 1098 schnig et al. (201 (nmol/g) .—st andard deviation of the mean. Ga 55 80 68 95 11 eU 121 290 175 793 959 184 187 306 214 126 109 234 167 133 139 199 201 376 928 133 310 423 405 176 266 107 377 146 158 181 166 106 133 120 ) 1355 1069 1004 1453 4028 (ppm) Th 709 435 634 319 928 510 363 415 630 476 410 446 706 580 678 438 798 724 174 427 123 356 130 369 432 259 322 141 330 158 257 1 1 1 1589 1343 4692 3883 continued 3106 1539 1896 2491 1247 1340 1043 5245 (ppm) ( U 738 478 678 346 560 393 440 685 515 441 478 752 628 709 457 871 823 468 185 381 403 448 302 361 166 361 186 279 1 1657 1384 5010 4134 3341 1725 2121 2833 1000 1335 1558 1269 1218 1080 6192 (ppm) l 368 323 275 155 120 203 331 215 235 234 253 306 359 298 264 485 242 367 377 255 214 269 339 371 219 258 182 336 196 178 148 178 188 179 298 172 407 238 133 303 229 300 158 (µm) 3 1 9 7 9 5 2 1 3 2 9 8 7 8 1 0 3 1 6 r 38 81 45 53 60 36 48 80 54 73 76 43 55 66 87 47 51 35 38 59 69 65 75 58 (µm) . † ABLE 1. ZIRCON (U-Th)/He D ATA 2. 02 1. 92 5.5 8.0 1.9 5. 33 7.1 1.1 7. 3 7. 03 7.2 8. 34 2. 42 3.2 4. 83 4.9 5. 44 5. 14 2. 63 7. 04 3. 63 T 1 (µg) 1 45. 88 21. 76 12. 14 39.3 12. 65 20.4 24. 76 56. 3 12.7 26. 26 36.9 27.1 15. 1 27.0 51.0 10. 85 15.9 18.9 26.3 18.2 22.4 13.8 Mass zr3 zr2 zr1 zr3 zr2 zr1 zr3 zr2 zr1 zr3 zr2 zr3 zr1 zr2 zr1 zr3 zr3 zr2 zr2 zr2 zr3 zr1 zr1 zr1 zr2 zr5 zr3 zr3 zr1 zr3 zr2 zr4 zr2 zr2 zr1 zr3 zr3 zr1 zr1 zr2 zr2 zr1 zr1 Grain Atlanta lobe, Challis) 958 (m) 1606 1668 2459 2786 2575 2915 2338 1955 1870 2053 2502 1992 2446 ective uranium; Ft—alpha ejection correction of Hourigan et al. (2005); St. dev ff Elevation (°N) Latitude 44.0369 44.15025 44.15202 44.18338 44.04743 44.18165 44.21889 44.18603 44.26778 44.23125 44.04617 44.42514 44.173712 44.323673 1 . (°W) 14.6404 14.6282 15.9161 14.756 11 15.14777 15.14645 15.05597 15.06092 15.05495 14.91238 15.08847 14.68433 15.19254 13.53705 1 1 1 1 1 Longitude -1 -1 -1 -1 -1 -1 -1 -1 -1 lley in the vicinity of Lemhi Range lley – White Cloud Peaks (Early metalumious suite, Va alley-Sawtooth Range (Atlanta lobe, Challis) Va 4- 1A : r—average radius; l—average length; eU—e 5- 4- 3 2 1- 1- analytical uncertainty σ Zr-based mass. 1 Note *Samples beginning with “10” are from Braudy et al. (2016) and Gaschnig (2017); those “07RMG” † § Sample number 13AF2 13AF2 13AF14 13AF2 13AF13 13AF2 98IB53 et al. (2010); those beginning with “13AF” and “Z14BT” were collected for this study 13AF16 Z14BT1 98IB68 13AF1 13AF2 10RL899 Z14BT1 Sawtooth V Sawtooth Pahsimeroi

LITHOSPHERE | Volume 9 | Number 2 | www.gsapubs.org 305

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/2/299/1001583/299.pdf by guest on 29 September 2021 FAYON ET AL.

WISZ Idaho batholith 120 WISZ-Lava Buttes 90 Atlanta lobe Sawtooth Valley 80 100 Lemhi Range ) 70

80 60

Lemhi Range 50

60 40

30 Zircon age (Ma) 40 (U-Th)/He zircon age (Ma 20 White Cloud Peaks (U-Th)/He single-grain age 10 20 (U-Th)/He mean age A U-Pb zircon age 0 (Gaschnig et al., 2010; 0 1000 2000 3000 4000 5000 6000 7000 Sawtooth Range Braudy et al., 2016) 0 116.5 116.0 115.5 115.0 114.5 114.0 113.5 113.0 Longitude (degrees west) 90 B

Figure 3. (U-Th)/He and U-Pb zircon age as a function of longitude (central 80 Idaho); ages are projected onto 44°N latitude, along the IDOR active-source seismic survey line (Davenport et al., 2017). Sample locations are shown ) 70 in Figure 2. Vertical black lines represent errors on the mean age. U-Pb 60 ages are from Gaschnig et al. (2010) and Braudy et al. (2016). Dashed line represents the boundary between the western Idaho shear zone (WISZ) 50 and the Idaho batholith. Samples from the Sawtooth Range, White Cloud Peaks, and Lemhi Range are outlined. 40 10RMG009 13AF01 30 10RMG010 13AF02

(U-Th)/He zircon age (Ma 10NB22 13AF03 20 10RMG011 13AF04 10NB376 13AF05 20.2 ± 1.4 to 68.5 ± 2.1 Ma (Kahn, 2014). Younger ages were obtained 10 13AF06 from higher elevations in the Sawtooth Range, whereas older ages were 13AF10 0 obtained from high elevations in the White Cloud Peaks. Samples from 0 200 400 600 800 1000 1200 1400 1600 1800 this region yield ages with the highest variability. Ages obtained for the Sawtooth batholith, an Eocene Challis intrusion 13AF21 13AF11 that cross-cuts the Atlanta lobe exposed in the Sawtooth Range, are 20.2 ± 90 Z14BT14 13AF16 1.4 (sample 13AF14) and 29.3 ± 7.5 Ma (13AF16). The early metalumi- 13AF22 13AF13 80 nous suite in the Sawtooth Range yields a slightly older age of 33.3 ± 3.1 13AF23 13AF14 Ma (13AF13), whereas the same phase of the batholith exposed at high 70 13AF24 13AF25 elevations in the White Cloud Peaks yields ages of 68.5 ± 2.1 (13AF23) 60 and 64.2 ± 4.2 Ma (13AF22). Lower-elevation samples include a Challis volcaniclastic rock dated at 44.5 ± 3.7 Ma (13AF21) and a granite related 50 to the Atlanta lobe dated at 48.8 ± 5.7 Ma (Z14BT14). 40 Certain samples from the Sawtooth Valley have very high eU concentrations, exceeding 6000 ppm (Fig. 4C). With such high concentra- 30 tions, the expectation is a lower closure temperature and a younger age. (U-Th)/He zircon age (Ma) 20 However, the data reveal a trend of slight increase in age with increasing 10 eU concentrations (Fig. 4C). Evaluating the single-grain ages for sample C 0 13AF11, the youngest age is obtained from the grain with the lowest eU 0 1000 2000 3000 4000 5000 6000 7000 concentration. Therefore, the young ages obtained are not likely a result eU concentration (ppm) of radiation damage, but do record recent cooling below ~200 °C. Figure 4. Age versus effective Uranium (eU) plots (central Idaho). Each symbol represents single-grain ages for one sample. (A) All samples for Lemhi Range this study. Atlanta lobe and western Idaho shear zone (WISZ) data exhibit a range of ages, from ca. 20 Ma to 80 Ma, and eU concentrations <2000 The oldest age obtained along the IDOR transect is located beyond ppm. (B) Samples from the western Idaho shear zone–Lava Buttes area. the eastern edge of the batholith within the Basin and Range region. A The eU concentrations range from 200 to 1200 ppm, and single-grain ages Proterozoic quartzite from the easternmost margin in the Lemhi Range, are between 60 and 80 Ma. There is no trend of increasing or decreasing near Pahsimeroi Valley, yields an age of 87.7 ± 5.4 Ma (Z14BT11). This age with change in eU, consistent with rapid cooling through the closure temperature for He in zircon (Guenthner et al., 2013). (C) Samples from age indicates that the Proterozoic rocks in the eastern Lemhi range cooled the Sawtooth Valley area. The Sawtooth batholith zircons yield the high- during the Sevier orogeny and were not thermally perturbed by subsequent est eU concentrations, including one grain with >6000 ppm, yet this age Atlanta lobe or Challis magmatism. is slightly older than other ages obtained for the region.

306 www.gsapubs.org | Volume 9 | Number 2 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/2/299/1001583/299.pdf by guest on 29 September 2021 Cooling and exhumation of the southern Idaho batholith | THEMED ISSUE

MODELING OF THERMOCHRONOLOGY DATA U-Pb zircon ages (Gaschnig et al., 2010), we modeled the cooling paths for the Atlanta lobe. The Cape Horn summit sample (07RMG56) constraints We used HeFTy inverse modeling software (Ketcham, 2005) to deter- are T = 700–750 °C between 70 and 80 Ma and T = 300–400 °C at 40–50 mine the best-fit T-t paths for the (U-Th)/He ages obtained. We concen- Ma, based on a U-Pb zircon age of 71.9 ± 2.7 Ma (Gaschnig et al., 2010) trated on specific locations along boundaries between different structural/ and a 40Ar/39Ar biotite age of 46.9 ± 0.2 Ma (Byerly et al., 2016), respec- magmatic domains or areas that exhibited changes in cooling patterns. The tively. We modeled this sample using three different scenarios. In the first HeFTy program tests T-t paths based on the input of independent constraints. case, the model determines best-fit T-t paths based on simple cooling from Here we used geochronologic (Giorgis et al., 2008; Gaschnig et al., 2010; crystallization temperatures through the Ar closure temperature in biotite Braudy et al., 2016), thermochronologic (Giorgis et al., 2008; Braudy et (Fig. 5C). Because of the sample’s close proximity to Challis-age rocks, al., 2016), and petrologic (Dutrow et al., 2014) data as model constraints. we test two further scenarios in which heating from Challis magmatism Specifically, U-Pb zircon ages and40 Ar/39Ar ages provide first-order age is introduced (Figs. 5D, 5E). Each model predicts acceptable T-t paths constraints for the HeFTy modeling. For the U-Pb zircon ages, we used a given the (U-Th)/He single-grain ages. Regardless of constraints used, range of ages based on all results from that particular intrusive suite (e.g., this sample cooled below 200 °C by 40 Ma (Figs. 5D, 5E). border zone suite), as given by Gaschnig et al. (2010). For the 40Ar/39Ar The second sample from the Atlanta lobe with an age of 39.7 ± 5.3 Ma ages, we use a range of ages based on the closure temperatures for both (10RL896) is located near the Boise Basin dike swarm at the southeast hornblende and biotite from all nearby samples of the relevant intrusive corner of Deadwood (Fig. 2). The T-t constraints are T = 700–750 °C suite (Giorgis et al., 2008; Braudy et al., 2016). We applied the Guenthner between 70 and 80 Ma and T = 300–400 °C at 65–75 Ma, based on a et al. (2013) He diffusion model, which accounts for eU concentrations, U-Pb zircon age of 71.9 ± 2.7 Ma (Gaschnig et al., 2010) and a 40Ar/39Ar for each sample modeled. Each model result is based on 10,000 paths. The biotite age of 69.0 ± 0.2 Ma, respectively (Byerly et al., 2016). The best- acceptable and good-fit paths calculated along with the mean and best-fit T-t fit T-t paths represent rapid cooling through the Ar closure temperature paths are shown in Figures 5 and 6. Results clearly demonstrate the different for biotite, followed by slow cooling from ca. 65 Ma to present, passing thermal histories recorded by different domains within the Idaho batholith. through the He closure temperature in zircon at ca. 35 Ma (Fig. 5F).

Western Idaho Shear Zone Sawtooth Valley

We modeled T-t paths for two samples from within the western Idaho Samples from the Sawtooth Valley area yield the largest differences shear zone, samples 10RMG011 (Braudy et al., 2016; Byerly et al., 2016) in ages obtained in the study, varying from ca. 20 to 68 Ma. We model and 13AF03 (this study). The protolith for 10RMG011 is from the suture samples from the early metaluminous suite as exposed in the Sawtooth zone suite (Braudy et al., 2016). The sample is modeled using two T-t Range (sample 13AF13) and White Cloud Peaks (13AF23), as well as the constraints (Fig. 5A). The high-temperature constraint of 700–750 °C at Eocene Sawtooth batholith (13AF16). Sample 13AF13 is from a high ele- a time of 84–93 Ma is based on a 93 Ma U-Pb zircon age from Braudy vation in the Sawtooth Range near the contact of the Idaho batholith with et al. (2016). A second constraint of 300–400 °C at 72–84 Ma is applied the Sawtooth batholith. This sample contains hornblende and is therefore based on an 40Ar/39Ar age on the same sample (Byerly et al., 2016). The likely part of the early metaluminous suite. We present three models for resulting model predicts two stages of cooling. The first stage involves this sample, each with different constraints to assess which thermal his- moderate to rapid cooling event, from T > 700 °C to T < 150 °C in ~20 tory best explains the observed (U-Th)/He ages (Figs. 5G–5I). In all three m.y., corresponding to a cooling rate of ~30 °C/m.y. The second stage is models, the high-temperature constraint is the same: T = 700–750 °C at a relatively slow cooling event, from T ~150 °C to near-surface tempera- 80–100 Ma, based on the U-Pb age for the early metaluminous suite (Gas- tures, at a rate <<10 °C/m.y. (Fig. 5A). chnig et al., 2011). In the first scenario, a moderate- to low-temperature Sample 13AF03 is located near Lava Buttes within the western Idaho constraint similar to the thermal conditions of the suture zone suite within shear zone. The easternmost part of the Hazard Creek intrusive suite is the western Idaho shear zone is added (Fig. 5G). In the second model, we affected by the western Idaho shear zone and is presumed to have intruded assume the sample remained at elevated temperatures prior to the intrusion through accreted terrane material (Manduca et al., 1993; Giorgis et al., of the Sawtooth batholith. The T-t conditions for the Sawtooth intrusive 2008). This sample is modeled with constraints from Giorgis et al. (2008), event are T = 600–700 °C at 40–50 Ma (Fig. 5H; Dutrow et al., 2014). as follows: T = 700–750 °C at 90–110 Ma (U-Pb age), T = 300–500 °C at In the third scenario, we assume the sample cooled to <200 °C and was 70–80 Ma (40Ar/39Ar hornblende and biotite ages), and T = 50–120 °C at subsequently reheated by the Sawtooth batholith intrusion (Fig. 5I). Each 40–50 Ma (apatite fission-track ages) (Fig. 5B). The resulting paths are scenario can explain the observed (U-Th)/He single-grain ages for sample similar to those calculated for 10RMG011 in that two stages of cooling 13AF13. Regardless of model conditions, the early metaluminous suite as are consistent with the observed (U-Th)/He zircon ages (Fig. 5B). Dif- exposed in the Sawtooth Range did not cool below 200 °C before ca. 30 Ma. ferences are apparent in the higher-temperature part of the cooling path The White Cloud Peaks sample 13AF23 yields a (U-Th)/He age of 68.5 because the Hazard Creek suite here has an intrusive age ~15 m.y. older ± 2.1 Ma (Figs. 2, 3), which is the oldest age obtained east of the western than the border zone suite to the south. The two samples, however, cool Idaho shear zone. Using the range of ages for the early metaluminous below 300 °C at approximately the same time (ca. 80 Ma) and therefore suite given by Gaschnig et al. (2010), we constrain the high-temperature share a common T-t history from moderate to near-surface temperatures range of the T-t model at 80–100 Ma. We further constrain the model at (Figs. 5A, 5B). Regardless of location along strike, the western Idaho shear T = 300–400 °C at 75–85 Ma based on a published 40Ar/39Ar age from a zone rocks record two stages of cooling and were below 150 °C by 70 Ma. quartz monzonite exposed in the White Cloud Peaks (Taylor et al., 2007). Results indicate the sample likely cooled from T > 700 °C to T < 150 °C Atlanta Lobe along a monotonic cooling path defined by a cooling rate of ~25 °C/m.y., similar to the border zone suite within the western Idaho shear zone Byerly et al. (2016) reported two 40Ar/39Ar biotite ages for samples (Fig. 5K). These rocks were likely at or near the Earth’s surface prior to within the Atlanta lobe. Combining these data with previously published Challis magmatism.

LITHOSPHERE | Volume 9 | Number 2 | www.gsapubs.org 307

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/2/299/1001583/299.pdf by guest on 29 September 2021 FAYON ET AL.

Western Idaho shear zone A. 10RMG011 B. 13AF03 0 0 100 200 300 400 500

Temperature (°C) 600 700 700 100 80 60 40 20 0 100 80 60 40 20 0 Time (Ma)

Atlanta lobe C. 07RMG56 D. 07RMG56 E. 07RMG56F. 10RL896 0 0 0 0 100 200 300 400 500

Temperature (°C) 600 700 700 700 700 100 80 60 40 20 0 100 80 60 40 20 0 100806040200 100806040200 Time (Ma)

Sawtooth Valley - Sawtooth Range G. 13AF113AF133HH. 13AF13. 13AF13 I. 13AF13 J. 13AF16 0 0 0 0 100 200 300 400 500

Temperature (°C) 600 700 700 700 700 100 80 60 40 20 0 100 80 60 40 20 0 100806040200 100806040200 Time (Ma)

Sawtooth Valley - White Cloud Peaks 0 K. 13AF23 Weighted mean Good paths 100 calculated path 200 300 400 Best-fit path Acceptable paths 500 600 Early Metaluminous suite 700 100 80 60 40 20 0

Figure 5. HeFTy (Ketcham, 2005) model results for select samples from the western Idaho shear zone and the Atlanta lobe of the Idaho batholith. See text for constraints. The dark gray boxes indicate age constraints (U/Pb zircon, Ar/Ar biotite, (U-Th)/He zircon) used to constrain the models. Model results show that western Idaho shear zone ages are consistent with a two-stage cooling history. The Atlanta lobe can be interpreted as either one monotonic cooling history or two stages of cooling.

308 www.gsapubs.org | Volume 9 | Number 2 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/2/299/1001583/299.pdf by guest on 29 September 2021 Cooling and exhumation of the southern Idaho batholith | THEMED ISSUE

Time (Ma) indicating a relation between deformation and magmatism. Giorgis et al. 140 120 100 80 60 40 20 0 0 (2008) suggested that deformation in the western Idaho shear zone ceased at ca. 90 Ma, constrained by the presence of an undeformed, cross-cutting 50 dike in the Little Goose Creek complex. Because of the complex interplay 100 between magmatism and deformation in the western Idaho shear zone, 150 we cannot uniquely attribute our cooling ages to either effect. 200 250 Cooling in the Atlanta Lobe of the Idaho Batholith 300 Within the Atlanta lobe, samples show a diffuse pattern of increasing 350 age with elevation, consistent with cooling and erosion (Fig. 8). It should

400 be noted that all Atlanta lobe samples from the study area are projected onto a single transect line. It is therefore likely that other trends are pres- 450 ent within this population, but—given the projection—all trends are not mperature (°C) 500 visible. Nonetheless, the presence of two age populations is observed: Te 550 (1) samples from elevations between 1900 and 1000 m; and (2) samples 600 from elevations >1900 m. 650 The first population of ages consists of the majority of samples from 700 within the Atlanta lobe, located at elevations between 1900 and 1000 m. 750 Within this population, samples from higher elevations cooled through

800 the He retention temperature in zircon at ca. 60 Ma. In contrast, samples at lower elevations cooled by 40 Ma, ~20 m.y. after samples at higher elevations. The second population of ages, from elevations >1900 m, range Accreted WISZ EMS Atlanta Sawtooth Challis terrane lobe in age from 60 to 40 Ma and come exclusively from north of Yellow Pine, KEY Idaho. These samples have young ages relative to the overall age-elevation trend for the majority of Atlanta lobe samples. There are two possible reasons that these ages might be systematically younger in age. First, the Figure 6. Summary plot combining (U-Th)/He zircon ages from this samples are adjacent to the Johnson Creek–Profile Gap shear zone (Lund, study (central Idaho) with geochronologic and thermochronologic 2004). Second, there is a relatively large set of Challis-aged intrusions data from the accreted terranes (Gaschnig et al., 2017), western Idaho near the sample localities. We hypothesize that the discrepancy results shear zone (WISZ; Braudy et al., 2016; Byerly et al., 2016), and the from the presence of the nearby Challis intrusions, which likely caused Atlanta lobe of the Idaho batholith (Gaschnig et al., 2010; Byerly et al., 2016). The intrusive phases of the Idaho batholith crystallized and thermal resetting of the samples starting at ca. 50 Ma. cooled through ~200 °C at different times. By Eocene time, all phases The age-elevation correlation (Fig. 8) is based on ages from expo- had cooled below ~150 °C. EMS—Early Metaluminous Suite (EMS). sures in the interior of the Atlanta lobe except: (1) samples 98IB53 and 07RMG46; and (2) samples within the Sawtooth Range area. The latter samples are excluded from discussion of the cooling history from the main Atlanta lobe because they are affected by exhumation of the Sawtooth DISCUSSION Range, as explained below. The first two samples (98IB53 and 07RMG46) are located in the west-southwest part of the Atlanta Lobe, adjacent to Exhumation Patterns in the Western Idaho Shear Zone and the border zone suites of Gaschnig et al. (2010). These samples occur Atlanta Lobe at lower elevations and reveal older ages than the Atlanta lobe, but are similar in cooling age to the adjacent border zone suites. We interpret Cooling in the Western Idaho Shear Zone these samples to have a similar cooling pattern to the border zone suites Within the western Idaho shear zone, (U-Th)/He zircon ages clearly because they lie on the “edge” of a crustal plateau, as discussed in the decrease from west to east with no correlation to elevation (Figs. 6, 7, 8): final section of the discussion. samples near the western margin of the western Idaho shear zone yield (U-Th)/He zircon ages of ca. 76 Ma, whereas sample ages in the east are Cooling in the Western Idaho Shear Zone versus Atlanta Lobe ca. 64 Ma. We note that the spatial pattern observed in the new data mim- A first-order observation of our study is that there is a major difference ics that of both the U-Pb zircon ages (120–90 Ma; Manduca et al., 1993; between cooling in the western Idaho shear zone relative to the Atlanta Giorgis et al., 2008) and Ar-Ar hornblende and biotite ages (110–70 Ma; lobe. The western Idaho shear zone appears to have cooled through 200 °C Snee et al., 1995; Giorgis et al., 2008) from the McCall region. It does during emplacement of the voluminous Atlanta lobe to the east. This lack not, however, reflect the apatite fission-track ages within the western of thermal resetting is also consistent with the absence of Atlanta lobe Idaho shear zone, which show little spatial variation in cooling through intrusions anywhere along the extent of the western Idaho shear zone. As ~120 °C (Giorgis et al., 2008). such, the western Idaho shear zone seems to have acted both as a major We interpret this record to result from a combination of west-to-east thermal and structural barrier to subsequent magmatism. migration of magmatism and/or deformation within the western Idaho shear zone. The U-Pb zircon data indicate that there is sequential west-to- Cooling in the Sawtooth Valley Area: A Sawtooth Metamorphic east emplacement of igneous bodies, with the eastern edge of the western Core Complex? Idaho shear zone intruded by the syn-tectonic Payette River tonalite (ca. 91 Samples from the Sawtooth Valley area of the Idaho batholith record Ma; Giorgis et al., 2008). Braudy et al. (2016) documented the existence a unique history of exhumation within the largely homogeneously of syn-tectonic intrusions in West Mountain, south of McCall, ID (Fig. 1), cooled Atlanta lobe of the Idaho batholith. The north-northwest–oriented

LITHOSPHERE | Volume 9 | Number 2 | www.gsapubs.org 309

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/2/299/1001583/299.pdf by guest on 29 September 2021 FAYON ET AL.

average modern Idaho Batholith Western Idaho Shear Zone (WISZ) exposure level Lemhi Range Overlain &

s by CRBs 120 ˚C EM EM 200 ˚C 200 ˚C ST 15-20 Ma Cooling Exhumation Surface At Earth’

Plateau & & 120˚ C EM ~5 km 200˚ C EM 200˚ C 350˚ C Challis 43 Ma ST intrusive Cooling Exhumation Cooling Exhumation depths

High Plateau & 120˚ C 200˚ C 200˚ C 7-10 km EM 350˚ C 350˚ C Assumed 70 Ma EM Cooling Exhumation Geotherm Active Uplif t

Highland

200˚ C 12 km 18 km 350˚ C EM Al-in-hbl n 90 Ma Al-in-hbl EM Active Uplift Early Metaluminous Intrusio & Thrus t Sevier Fol d

WISZ orthogneiss sample Atlanta Lobe Paleozoic sediments in Lemhi Range Border zone and Sawtooth batholith Normal fault Early Metaluminous Suite (EM) (ST) (Sawtooth Core Complex)

Figure 7. Crustal cross-section models for 90, 70, 43, and 15–20 Ma illustrating the emplacement and exhumation of the Idaho batholith (central Idaho) as inferred from the new cooling ages. CRBs—Columbia River basalts; Al-in-hbl—Aluminium-in-hornblende barometric estimates. Gray zone indica- tions the approximate extent of the Atlanta Lobe.

Sawtooth Valley separates the Sawtooth Range on its west side from the zircon ages from Atlanta lobe in the White Cloud Peaks indicate ages White Cloud Peaks on its east side (Fig. 9). Given the consistent emplace- of ca. 50 Ma. In particular, the (U-Th)/He zircon ages from the base of ment ages of Atlanta lobe rocks in both mountain ranges, the variation the White Cloud Peaks (in Sawtooth Valley) are ~10 m.y. older than the in cooling history requires interpretation of structural features across the sample from the top of the Sawtooth Range. valley. The present geometry of Sawtooth Valley is a graben defined by There is also a different relation to Challis-aged magmatism between range-bounding faults. The Sawtooth Range is bounded on the east side the two ranges. The Sawtooth Range contains the intrusive Sawtooth by the active, east-dipping Sawtooth fault (Thackray et al., 2013). This batholith, which is Challis aged (ca. 50–45 Ma) and intruded at a depth fault is presently exposed on the west side of Sawtooth Valley. In the of ~5 km (Gaschnig et al., 2010; Dutrow et al., 2014). In contrast, the southeast corner of Sawtooth Valley, previous mapping indicates the pres- White Cloud Peaks contain dikes and volcaniclastic rocks of Challis age. ence of the west-dipping, normal Obsidian fault (Witkind, 1975; Fisher Thus, portions of the White Cloud Peaks were at or near the surface at ca. et al., 1992). We suggest that the Obsidian fault runs along the entire east 50 Ma, while the Sawtooth Range was at least 5 km deep. side of Sawtooth Valley, although it is likely inactive and its location is Two tectonic models can explain the (U-Th)/He zircon data from the cryptic on the northeast side of the valley. Sawtooth Valley region: (1) an asymmetric graben (Fig. 9A); and (2) a low- (U-Th)/He zircon ages from the early metaluminous suite in the White angle normal fault followed by an asymmetric graben (Fig. 9B). In the first Cloud Peaks indicate cooling below 200 °C significantly earlier than the tectonic model, Sawtooth Valley lies in an asymmetric graben with both same suite exposed in the Sawtooth Range. The oldest ages in the Saw- the Sawtooth Range and White Cloud Peaks exhumed along oppositely tooth Range is 40.5 ± 2.0 Ma, while the youngest is 20.2 ± 1.4 Ma at dipping, high-angle normal faults. Within the Sawtooth Range, younger low elevations in the Sawtooth Range (Table 1). In contrast, (U-Th)/He (U-Th)/He zircon ages indicate cooling and exhumation along a normal

310 www.gsapubs.org | Volume 9 | Number 2 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/2/299/1001583/299.pdf by guest on 29 September 2021 Cooling and exhumation of the southern Idaho batholith | THEMED ISSUE

cooled first because they were higher in the crustal column. In addition, A WISZ the petrological record suggest that in the Eocene the Sawtooth Range was at depth while the White Clouds Peaks were at or near the surface. 1600 80 The 48 Ma Sawtooth batholith in the Sawtooth Range intruded at ~5 km depth (Gaschnig et al., 2010; Dutrow et al., 2014), while Challis-aged

(U-Th)/He zircon volcaniclastic rocks were deposited at the Earth’s surface in the White 1200 60 Cloud Peaks. In this scenario, the Sawtooth Range cools only after low- ) angle normal faulting removes the overlying White Cloud Peaks (Fig. 9A). Then, at a later time, graben formation occurs with high-angle normal 40 800 faults on both sides of the Sawtooth Valley (e.g., Thackray et al., 2013). Age (Ma ) Elevation (m The low-angle fault is removed by erosion from on top of the Sawtooth Range and buried by a high-angle normal fault below Sawtooth Valley 400 20 Sample elevation and the White Cloud Peaks. This scenario requires significantly less throw Single-grain age (Ma) on the high-angle faults in Sawtooth Valley. Movement on the low-angle fault must have occurred during or slightly after intrusion of the ca. 48 0 0 -116.28 -116.24 -116.20 -116.16 -116.12 Ma Sawtooth batholith, consistent with studies of the nearby Pioneer core

Longitude (degrees west) complex (Vogl et al., 2012). If this second model is correct, then the Sawtooth Range represents an unrecognized metamorphic core complex in the U.S. Cordillera. The B Atlanta lobe Bitteroot metamorphic core complex occurs further north (Foster et al., 2500 2001), on the eastern edge of the Bitteroot lobe, in a structurally equivalent location. The uplift of deep rocks after Challis magmatism (e.g., Dutrow

2000 et al., 2014) and the presence of Eocene magmatism at the locus of young uplift ages are both consistent with a core-complex model.

) Given our data and the lack of corroborating structures, we cannot 1500 resolve between these two models. However, any possible model must address the observed cooling age pattern that indicates that the Atlanta

Elevation (m lobe rocks currently exposed in the Sawtooth Range were at elevated tem- 1000 peratures after the White Cloud Peaks were exhumed to shallow crustal levels. As such, we favor the low-angle fault model. 500 The Existence of a Plateau: The Atlanta Lobe as the Northern Single-grain age (Ma) 0 Extension of the Nevadaplano 0810 20 30 40 50 60 70 0

(U-Th)/He zircon age (Ma) The cooling history presented here is consistent with the formation of a Late Cretaceous–Paleogene plateau in Idaho. The cooling data alone Figure 8. Age-elevation profiles for the western Idaho shear zone (WISZ) indicate that the region was thermally stable for ~40 m.y. The self-similar (A) and the Atlanta lobe of the Idaho batholith (B). (A) (U-Th)/He zircon ages clearly decrease from west to east in the WISZ, with no correlation pattern of U-Pb and (U-Th)/He zircon ages from the Atlanta lobe sug- to elevation. This trend mimics that of the available U-Pb zircon ages. gests that the region was at a thermal steady state prior to initiation of the (B) (U-Th)/He zircon ages from the Atlanta lobe, showing a positive age- Challis magmatic event in early Eocene time and that cooling occurred elevation trend. The data in boxes are not included in the analysis, for isobarically across the entire crustal region (Figs. 7, 8). The general cor- reasons discussed in the text. relation of cooling age and elevation supports this contention. Further, it appears that this post-magmatic isobaric cooling occurred with little or no unroofing, except in the Sawtooth Valley region. The only other fault active in the Neogene. Assuming a steep geothermal gradient of 50 variations in cooling path that are younger than Early Eocene occur as a °C/km, the depth to the 200 °C isotherm would have been at 4 km prior to result of Challis magmatism. exhumation. A simple geometric analysis shows that exhumation along We hypothesize that the Atlanta lobe was emplaced into a crustal a high-angle normal fault would require ~1.6 km of throw to explain the plateau based on the cooling history described above and: (1) geochem- cooling age variability. This magnitude of slip has occurred on other nor- istry of the Atlanta lobe; (2) the lack of strong fabrics within the Atlanta mal faults adjacent to the Idaho batholith (Long Valley fault; e.g., Giorgis et peraluminous suite; (3) crustal thickness determined by seismic data; al., 2006). A more typical geothermal gradient of 25 °C/km would require and (4) the presence of detrital zircons derived from the Idaho batholith ~3.2 km of fault throw to explain the cooling age variability. The current in basins throughout the North American Cordillera. The Atlanta lobe relief between our sample in the Sawtooth Range and the valley floor is is an extensive belt of two-mica granites, which geochemical analysis 600 m, which is a minimum slip amount. Recent scarps indicate active indicates formed exclusively as a result of crustal melting (Gaschnig et fault activity along the Sawtooth fault, suggesting that normal faulting is al., 2011). Thus, the most likely timing for the formation of the crustal a viable exhumation mechanism (e.g., Thackray et al., 2013). plateau is during the intrusion of the Atlanta peraluminous suite. The Another possible model is exhumation of the Sawtooth Range along a lack of any consistently oriented tectonic fabrics within the Atlanta lobe low-angle normal fault (Fig. 9B). In this model, the White Cloud Peaks intrusions, which were emplaced during regional contraction in the Sevier restore to a position structurally overlying the Sawtooth Range. This orogenic belt, led Byerly et al. (2016) to conclude that intrusion must have model satisfies the (U-Th)/He zircon ages, as the White Cloud Peaks occurred in a “neutral” to extensional tectonic setting. They concluded

LITHOSPHERE | Volume 9 | Number 2 | www.gsapubs.org 311

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/2/299/1001583/299.pdf by guest on 29 September 2021 FAYON ET AL.

A. Asymmetric graben model KEY Sawtooth Range White Cloud Peaks 200˚C paleoisotherm at 40 Ma Sawtooth Valley low-angle fault Figure 9. Schematic cross-section younger normal fault of the Sawtooth Valley region (central Idaho) illustrating two possible models for the exhumation of the Sawtooth Range. Gray areas represent the material warmer than 200° C, and the dashed line is the 200° C isotherm, for any time slice. The solid black line is an approxima- B. Low-angle normal fault model tion of the topography at the time, while the solid gray line approxi- 50 Ma mates the current topography. The Hypothetical paleoelevation first model describes exhumation along a high-angle normal fault, 200˚C paleoisotherm which requires ≥600 m of dip slip. Modern elevation of at 50 Ma White Cloud Peaks The second model is exhumation along a low-angle normal fault as part of a metamorphic core com- Modern elevation of plex. The intrusive body is the Sawtooth Range Eocene Sawtooth batholith; the dikes show the shallow dikes and hypabyssal intrusion in the White Cloud Peaks. At 50 Ma, the White 40 Ma Cloud Peaks are located structurally above the Sawtooth Range. By 40 Ma, a low-angle normal fault has translated the White Cloud 200˚C paleoisotherm Peaks eastward. During continued extension, high-angle normal at 40 Ma faults form the Sawtooth Valley. In this model, the older detachment fault is buried beneath the Saw- tooth Valley and the White Cloud Peaks, and has been eroded above Present the Sawtooth Range.

Sawtooth Range Sawtooth Valley White Cloud Peaks 200˚C isotherm at present

that a crustal plateau was the most likely tectonic environment for Atlanta that Proterozoic detritus linked to unique sources in Idaho was found in lobe emplacement. Davenport et al. (2017) indicated that the base of the these basins as well as other basins in Washington (Swakane) and Alaska crust below the Idaho batholith is located at about ~43 km below current (Yakutat). These data, taken together, suggest that the early metaluminous exposure levels. Accounting for ~12 km of erosion (e.g., Jordan, 1994), suite and Atlanta lobe were at the Earth’s surface at least 20 m.y. before this estimate suggests a crustal thickness of ~55 km in central Idaho the current exposures in the Idaho batholith cooled below ~200 °C. This during Atlanta lobe emplacement. This estimate of crustal thickening pattern is consistent with continuous, slow cooling accompanied by slow is consistent with the palinspastic reconstruction of the Sevier orogeny exhumation of the highland. For example, in their study of detrital zir- hinterland (Coney and Harms, 1984). cons from the Franciscan Basin, Dumitru et al. (2013, 2015) identified a Basin evolution, however, is also critical for interpreting the Idaho clear Idaho batholith signal with zircon ages ranging from 85 to 60 Ma. batholith as a crustal plateau. The Idaho batholith is the source of major The ages of these zircons uniquely tie them to the peraluminous Atlanta detrital material throughout the North American Cordillera, including lobe, although deposition occurred while the current exposures of the the Franciscan Basin of California (e.g., Dumitru et al., 2013), the Green Atlanta lobe were at T > 200 °C. If we are correct about the geothermal River Basin of Wyoming (e.g., Chetel et al., 2011), and the Tyee forearc in gradients (a maximum of 50 °C/km), the rocks currently exposed at the coastal Oregon (e.g., Heller et al., 1985). Dumitru et al. (2016) also noted surface were at depths >4 km.

312 www.gsapubs.org | Volume 9 | Number 2 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/2/299/1001583/299.pdf by guest on 29 September 2021 Cooling and exhumation of the southern Idaho batholith | THEMED ISSUE

If Dumitru et al. (2013, 2015) have correctly identified the source of Chase, R.B., Bickford, M.E., and Tripp, S.E., 1978, Rb-Sr and U-Pb isotopic studies of the northeastern Idaho batholith and border zone: Geological Society of America Bulletin, v. 89, the sediment in a variety of Paleogene basins as the Idaho batholith, it p. 1325–1334, doi:10.1130/0016-7606(1978)89<1325:RAUISO>2.0.CO;2. suggests that the Idaho batholith was topographically high and provided Chetel, L.M., Janecke, S.U., Carroll, A.R., Beard, B.L., Johnson, C.M., and Singer, B.S., 2011, abundant source material for basin deposition. As such, a minimum of 4 Paleogeographic reconstruction of the Eocene Idaho River, North American Cordillera: Geological Society of America Bulletin, v. 123, p. 71–88, doi:10.1130 /B30213 .1. km of erosion of the top surface of the batholith must have occurred, and Coney, P.J., and Harms, T.A., 1984, Cordilleran metamorphic core complexes: Cenozoic exten- likely significantly more. Given the lack of focused tectonic unroofing dur- sional relics of Mesozoic compression: Geology, v. 12, p. 550–554, doi:10.1130 /0091 -7613 ing the Paleogene, it suggests that the entire region must have been a high (1984)12<550:CMCCCE>2.0.CO;2. Criss, R.E., Lanphere, M.A., and Taylor, H.P., Jr., 1982, Effects of regional uplift, deformation, crustal plateau. Finally, in the last 40 m.y., the Atlanta lobe was exhumed an and meteoric-hydrothermal metamorphism on K-Ar ages of biotites in the southern half additional 4 km, indicating an average exhumation rate of 1 mm/yr or less. of the Idaho batholith: Journal of Geophysical Research, v. 87, p. 7029–7046, doi:10.1029 /JB087iB08p07029. In aggregate, all of the above data support the existence of a crustal plateau Criss, R.E., Ekren, E.B., and Hardyman, R.F., 1984, Casto Ring Zone: A 4,500-km2 fossil hydro- that ultimately resulted in the formation of the Atlanta peraluminous suite. thermal system in the Challis Volcanic Field, central Idaho: Geology, v. 12, p. 331–334, doi:10.1130/0091-7613(1984)12<331:CRZAKF>2.0.CO;2. Davenport, K.K., Hole, J.A., Tikoff, B., Russo, R.M., and Harder, S.H., 2017, A strong contrast CONCLUSIONS in crustal architecture from accreted terranes to craton, constrained by controlled-source seismic data in Idaho and eastern Oregon: Lithosphere, doi:10 .1130 /L553 .1. DeCelles, P.G., Ducea, M.N., Kapp, P., and Zandt, G., 2009, Cyclicity in Cordilleran orogenic In general, cooling ages are older along the margins and younger in systems: Nature Geoscience, v. 2, p. 251–257, doi:10 .1038 /ngeo469. the interior of the Idaho batholith, consistent with results from previous Ducea, M.N., 2001, The California arc: Thick granitic batholiths, eclogitic residues, lithospheric- thermochronologic studies (Criss et al., 1984; Sweetkind and Blackwell, scale thrusting, and magmatic flare-ups: GSA Today, v. 11, no. 11, p. 4–10, doi:10 .1130 /1052 -5173(2001)011<0004:TCATGB>2.0.CO;2. 1989). (U-Th)/He zircon ages and HeFTy modeling of the data, however, Dumitru, T.A., Ernst, W.G., Wright, J.E., Wooden, J.L., Wells, R.E., Farmer, L.P., Kent, A.J.R., and illustrate the variability of cooling of the Idaho batholith as a function of Graham, S.A., 2013, Eocene extension in Idaho generated massive sediment floods into position. First, exhumation in the western Idaho shear zone differs sig- the Franciscan trench and into the Tyee, Great Valley, and Green River basins: Geology, v. 41, p. 187–190, doi:10.1130/G33746.1. nificantly from that in the Atlanta lobe of the Idaho batholith. The western Dumitru, T.A., Ernst, W.G., Hourigan, J.K., and McLaughlin, R.J., 2015, Detrital zircon U-Pb Idaho shear zone exhibits no correlation between (U-Th)/He zircon age reconnaissance of the Franciscan subduction complex in northwestern California: Inter- national Geology Review, v. 57, p. 767–800, doi:10.1080 /00206814 .2015.1008060. and present elevation. Rather, there is a west-to-east younging observed Dumitru, T.A., Elder, W.P., Hourigan, J.K., Chapman, A.D., Graham, S.A., and Wakabayashi, in the data, reflecting spatial migration in magmatism. Second, samples J., 2016, Four Cordilleran paleorivers that connected Sevier thrust zones in Idaho to de- from within the Atlanta lobe of the Idaho batholith exhibit a clear pattern of pocenters in California, Washington, Wyoming, and, indirectly, Alaska: Geology, v. 44, p. 75–78, doi:10.1130/G37286.1. cooling age as a function of elevation. This pattern applies to the majority Dutrow, B.L., Foster, D.A., Mueller, P.A., and Ma, C., 2014, New constraints on the geochro- of the batholith and suggests isobaric cooling in the absence of tectonic nology and thermochronology of the Sawtooth batholith, Idaho: Abstract V31E-4807 presented at the American Geophysical Union Fall Meeting, San Francisco, California, activity. We suggest that the distribution of cooling ages is consistent 15–19 December. with a crustal plateau that corresponds to the extent of the Atlanta lobe. Farley, K.A., 2002, (U-Th)/He dating: Techniques, calibrations, and applications: Reviews in Third, a discrete break in cooling histories occurs across the Sawtooth Mineralogy and Geochemistry, v. 47, p. 819–844, doi:10.2138 /rmg .2002.47.18. Farley, K.A., Wolf, R.A., and Silver, L.T., 1996, The effects of long alpha-stopping distances Valley, with 30–20 Ma (U-Th)/He zircon ages recorded in the Sawtooth on (U-Th)/He ages: Geochimica et Cosmochimica Acta, v. 60, p. 4223–4229, doi:10.1016 Range and 50–40 Ma (U-Th)/He zircon ages in the White Cloud Peaks. /S0016-7037(96)00193-7. Fisher, F.S., McIntyre, D.H., and Johnson, K.M., compilers, 1992, Geologic map of the Challis We attribute this difference to be the result of focused tectonic extension 1° × 2° Quadrangle, Idaho: U.S. Geological Survey Geologic Investigations Map I-1819, and speculate the presence of a core complex and associated low-angle scale 1:250,000. detachment below this section of the Idaho batholith. Foster, D.A., and Fanning, C.M., 1997, Geochronology of the northern Idaho batholith and the Bitterroot metamorphic core complex: Magmatism preceding and contemporaneous with extension: Geological Society of America Bulletin, v. 109, p. 379–394, doi:10.1130/0016 ACKNOWLEDGMENTS -7606(1997)109<0379:GOTNIB>2.3.CO;2. The (U-Th)/He zircon thermochronometry was conducted in the University of Arizona Radio- Foster, D.A., and Raza, A., 2002, Low-temperature thermochronological record of exhumation genic Helium Dating Laboratory, led by Dr. P. Reiners. We gratefully acknowledge Erin Abel and of the Bitterroot metamorphic core complex, northern Cordilleran Orogen: Tecotonophys- Uttam Chowdhury for help in the laboratory. Reviews by J. Vogl, an anonymous reviewer, and ics, v. 349, p. 23–36, doi:10.1016/S0040-1951(02)00044-6. Foster, D.A., Schafer, C., Fanning, C.M., and Hyndman, D.W., 2001, Relationships between the editor were extremely helpful in improving the presentation of the data and interpretations. crustal partial melting, plutonism, orogeny, and exhumation; Idaho-Bitterroot batholith: Funding for the (U-Th)/He zircon thermochronometry was made available through GeoEarth- Tectonophysics, v. 342, p. 313–350, doi:10.1016/S0040-1951(01)00169-X. Scope. Rock collection, sample preparation, and manuscript preparation was supported by Gaschnig, R.M., Vervoort, J.D., Lewis, R.S., and McClelland, W.C., 2010, Migrating magmatism National Science Foundation grants EAR-0844260 and EAR-1251877 to Tikoff. in the northern US Cordillera: In situ U-Pb geochronology of the Idaho batholith: Contri- butions to Mineralogy and Petrology, v. 159, p. 863–883, doi:10.1007 /s00410-009-0459-5. REFERENCES CITED Gaschnig, R.M., Vervoort, J.D., Lewis, R.S., and Tikoff, B., 2011, Isotopic evolution of the Idaho Armstrong, R.L., Taubeneck, W.H., and Hales, P.O., 1977, Rb-St and K-Ar geochronometry of batholith and Challis intrusive province, northern US Cordillera: Journal of Petrology, Mesozoic granitic rocks and their Sr isotopic composition, Oregon, Washington, and v. 52, p. 2397–2429, doi:10.1093/petrology/egr050. Gaschnig, R.M., Vervoort, J.D., Lewis, R.S., and Tikoff, B., 2013, Probing for Proterozoic and Ar- Idaho: Geological Society of America Bulletin, v. 88, p. 397–411, doi:10.1130/0016-7606 chean crust in the northern U.S. Cordillera with inherited zircon from the Idaho batholith: (1977)88<397:RAKGOM>2.0.CO;2. Geological Society of America Bulletin, v. 125, p. 73–88, doi:10.1130 /B30583 .1. Bennett, E.H., 1986, Relationship of the trans-Challis fault system in central Idaho to Eo- Gaschnig, R., Macho, A.S., Fayon, A., Schmitz, M., Ware, B.D., Vervoort, J., Kelso, P., Lamaskin, cene and Basin and Range extensions: Geology, v. 14, p. 481–484, doi:10.1130 /0091 -7613 T., Kahn, M.J., and Tikoff, B., 2017, Intrusive and depositional constraints on the Creta- (1986)14<481:ROTTFS>2.0.CO;2. ceous tectonic history of the southern Blue Mountains, eastern Oregon: Lithosphere, Bickford, M.E., Chase, R.B., Nelson, B.K., Shuster, R.D., and Arruda, E.C., 1981, U-Pb studies of doi:10.1130/L554.1. zircon cores and overgrowths, and monazite: Implications for age and petrogenesis of the Gehrels, G., Rusmore, M., Woodsworth, G., Crawford, M., Andronicos, C., Hollister, L., Patchett, northeastern Idaho batholith: The Journal of Geology, v. 89, p. 433–457, doi:10 .1086 /628607. J., Ducea, M., Butler, R., Klepeis, K., Davidson, C., Friedman, R., Haggart, J., Mahoney, B., Braudy, N., Gaschnig, R.M., Wilford, D., Vervoort, J.D., Nelson, C.L., Davidson, C., Kahn, M.J., Crawford, W., Pearson, D., and Girardi, J., 2009, U-Th-Pb geochronology of the Coast Moun- and Tikoff, B., 2016, Timing and deformation conditions of the western Idaho shear zone, tains batholith in north-coastal British Columbia: Constraints on age and tectonic evolu- West Mountain, west-central Idaho: Lithosphere, doi:10 .1130 /L519 .1. tion: Geological Society of America Bulletin, v. 121, p. 1341–1361, doi:10 .1130 /B26404 .1. Burchfiel, B.C., Cowan, D.S., and Davis, G.A., 1992, Tectonic overview of the Cordilleran oro- Giorgis, S., Tikoff, B., and McClelland, W., 2005, Missing Idaho arc: Transpressional modifica- gen in the western U.S., in Burchfiel, B.C., Lipman, P.W., and Zoback, M.L., eds., The Cor- tion of the 87Sr/86Sr transition on the western edge of the Idaho batholith: Geology, v. 33, dilleran Orogen: Conterminous U.S.: Boulder, Colorado, Geological Society of America: p. 469–472, doi:10.1130/G20911.1. The Geology of North America, v. G-3, p. 407–480. Giorgis, S., Tikoff, B., Kelso, P., and Markley, M., 2006, The role of material anisotropy in the Byerly, A., Tikoff, B., Kahn, M., Jicha, B., Gaschnig, R., and Fayon, A.K., 2016, Internal fabrics neotectonic extension of the western Idaho shear zone, McCall, Idaho: Geological Society of the Idaho batholith, USA: Lithosphere, doi:10 .1130 /L551 .1. of America Bulletin, v. 118, p. 259–273, doi:10.1130 /B25382 .1.

LITHOSPHERE | Volume 9 | Number 2 | www.gsapubs.org 313

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/2/299/1001583/299.pdf by guest on 29 September 2021 FAYON ET AL.

Giorgis, S., McClelland, W., Fayon, A., Singer, B.S., and Tikoff, B., 2008, Timing of deforma- Paterson, S.R., and Ducea, M.N., 2015, Arc magmatic tempos: Gathering the evidence: Ele- tion and exhumation in the western Idaho shear zone, McCall, Idaho: Geological Society ments, v. 11, p. 91–98, doi:10.2113/gselements.11.2.91. of America Bulletin, v. 120, p. 1119–1133, doi:10 .1130 /B26291 .1. Premo, W.R., Morton, D.M., Wooden, J.L., and Fanning, C.M., 2014, U-Pb zircon geochronology Guenthner, W.R., Reiners, P.W., Ketcham, R.A., Nasdala, L., and Giester, G., 2013, Helium of plutonism in the northern Peninsular Ranges batholith, southern California: Implica- diffusion in natural zircon: Radiation damage, anisotropy, and the interpretation of zir- tions for the Late Cretaceous tectonic evolution of southern California, in Morton, D.M., con (U-Th)/He thermochronology: American Journal of Science, v. 313, p. 145–198, doi: and Miller, F.K., eds., Peninsular Ranges Batholith, Baja California and Southern Califor- 10.2475/03.2013.01. nia: Geological Society of America Memoir 211, p. 145–180, doi:10.1130 /2014 .1211 (04) . Heller, P.L., Peterman, Z.E., O’Neil, J.R., and Shafiqullah, M., 1985, Isotopic provenance of Reiners, P.W., 2005, Zircon (U-Th)/He thermochronometry: Reviews in Mineralogy and Geo- sandstones from the Eocene Tyee Formation, Oregon Coast Range: Geological Society of chemistry, v. 58, p. 151–179, doi:10.2138/rmg.2005.58.6. America Bulletin, v. 96, p. 770–780, doi:10 .1130 /0016 -7606 (1985)96 <770: IPOSFT>2 .0 .CO;2. Reiners, P.W., and Nicolescu, S., 2006, Measurement of parent nuclides for (U-Th)/He chro- Hourigan, J.K., Reiners, P.W., and Brandon, M.T., 2005, U-Th zonation-dependent alpha-ejec- nometry by solution sector ICP-MS: Arizona Radiogenic Helium Dating Laboratory Report tion in (U-Th)/He chronometry: Geochimica et Cosmochimica Acta, v. 69, p. 3349–3365, 1, 33 p., http://www.geo.arizona.edu/~reiners/arhdl/arhdl.htm. doi:10.1016/j.gca.2005.01.024. Silverberg, D.S., 1990, The tectonic evolution of the Pioneer metamorphic core complex, south- Jordan, B.T., 1994, Emplacement and exhumation of the southeastern Atlanta lobe of the central Idaho [Ph.D. thesis]: Cambridge, Massachusetts Institute of Technology, 280 p. Idaho batholith and outlying stocks, south-central Idaho [M.S. thesis]: Pocatello, Idaho Snee, L.W., Lund, K., Sutter, J.G., Balcer, D.E., and Evans, K.V., 1995, An 40Ar/39Ar chronicle of State University, 110 p. the tectonic development of the Salmon River suture zone, western Idaho, in Vallier, T.L., Kahn, M.J., 2014, Constraining exhumation of the Sawtooth and White Cloud Mountains by and Brooks, H.C., eds., Geology of the Blue Mountains Region of Oregon, Idaho, and (U-Th)/He zircon thermochronology [unpublished senior thesis]: Northfield, Minnesota, Washington: Petrology and Tectonic Evolution of Pre-Tertiary Rocks of the Blue Mountains Carleton College, 46 p. Region: U.S. Geological Survey Professional Paper 1438, p. 359–414. Ketcham, R.A., 2005, Forward and inverse modeling of low-temperature thermochronom- Stanciu, A.C., Russo, R.M., Mocanu, V.I., Bremner, P.M., Hongsresawat, S., Torpey, M.E., Van- etry data: Reviews in Mineralogy and Geochemistry, v. 58, p. 275–314, doi:10 .2138 /rmg Decars, J.C., Foster, D.A., and Hole, J.A., 2016, Crustal structure beneath the Blue Moun- .2005.58.11. tains terranes and cratonic North America, eastern Oregon, and Idaho, from teleseismic Ketcham, R.A., Guenthner, W.R., and Reiners, P.W., 2013, Geometric analysis of radiation receiver functions: Journal of Geophysical Research: Solid Earth, v. 121, p. 5049–5067, damage connectivity in zircon, and its implications for helium diffusion: The American doi:10.1002/2016JB012989. Mineralogist, v. 98, p. 350–360, doi:10.2138/am.2013.4249. Sweetkind, D.S., and Blackwell, D.D., 1989, Fission-track evidence of the Cenozoic thermal LaMaskin, T.A., Vervoort, J.D., and Dorsey, R.J., 2011, Early Mesozoic paleogeography and history of the Idaho Batholith: Tectonophysics, v. 157, no. 4, p. 241–250. tectonic evolution of the western United States: Insights from detrital zircon U-Pb geo- Taylor, C.D., Winick, J.A., Unruh, D.M., and Kunk, M.J., 2007, Geochronology and geochemistry chronology of the Blue Mountains Province, northeastern Oregon, U.S.A.: Geological of the Idaho-Montana porphyry belt, in Lund, K., ed., Earth Science Studies in Support Society of America Bulletin, v. 123, p. 1939–1965, doi:10 .1130 /B30260 .1. of Public Policy Development and Land Stewardship—Headwaters Province, Idaho and Lewis, R.S., Link, P.K., Stanford, L.R., and Long, S.P., 2012, Geologic map of Idaho: Idaho Geo- Montana: U.S. Geological Survey Circular 1305, p. 26–39. logical Survey Geologic Map 9, scale 1:750,000. Thackray, G.D., Rodgers, D.W., and Streutker, D., 2013, Holocene scarp on the Sawtooth fault, Lund, K., 2004, Geology of the Payette National Forest and vicinity, west-central Idaho: U.S. central Idaho, USA, documented through lidar topographic analysis: Geology, v. 41, Geological Survey Professional Paper 1666, 89 p. p. 639–642, doi:10.1130/G34095.1. Manduca, C.A., Kuntz, M.A., and Silver, L.T., 1993, Emplacement and deformation history of Toth, M.I., and Stacey, J.S., 1992, Constraints on the formation of the Bitterroot lobe of the the western margin of the Idaho Batholith near McCall, Idaho; Influence of a major terrane Idaho batholith, Idaho and Montana, from U-Pb zircon geochronology and feldspar Pb boundary: Geological Society of America Bulletin, v. 105, no. 6, p. 749–765. isotopic data: U.S. Geological Survey Bulletin 2008, 14 p. Miller, R.B., Paterson, S.R., and Matzel, J.P., 2009, Plutonism at different crustal levels: Insights Vogl, J.J., Foster, D.A., Fanning, C.M., Kent, K.A., Rodgers, D.W., and Diedesch, T., 2012, Timing from the ~5–40 km (paleodepth) North Cascades crustal section, Washington, in Miller, of extension in the Pioneer metamorphic core complex with implications for the spatial- R.B., and Snoke, A.W., eds., Crustal Cross Sections from the Western North American temporal pattern of Cenozoic extension and exhumation in the northern U.S. Cordillera: Cordilleran and Elsewhere: Implications for Tectonic and Petrologic Processes: Geological Tectonics, v. 31, TC1008, doi:10.1029/2011TC002981. Society of America Special Paper 456, p. 125–149, doi:10.1130 /2009 .2456(05). Witkind, I.J., compiler, 1975, Preliminary map showing known and suspected active faults in Montz, W., and Kruckenberg, S., 2017, Cretaceous partial melting, deformation, and exhumation Idaho: U.S. Geological Survey Open-File Report 75-278, 1 sheet, scale 1:500,000. of the Potters Pond migmatite domain, west-central Idaho: Lithosphere, doi:10.1130 /L555 .1. Nasdala, L., Reiners, P.W., Garver, J.I., Kennedy, A.K., Stern, R.A., Balan, E., and Wirth, R., 2004, Incomplete retention of radiation damage in zircon from Sri Lanka: The American Mineralogist, v. 89, p. 219–231, doi:10.2138/am-2004-0126. MANUSCRIPT RECEIVED 3 MAY 2016 O’Neill, R.L., and Pavlis, T.L., 1988, Superposition of Cenozoic extension on Mesozoic compres- REVISED MANUSCRIPT RECEIVED 13 OCTOBER 2016 sional structures in the Pioneer Mountains metamorphic core complex, central Idaho: Geo- MANUSCRIPT ACCEPTED 15 DECEMBER 2016 logical Society of America Bulletin, v. 100, p. 1833–1845, doi:10 .1130 /0016 -7606(1988)100 <1833:SOCEOM>2.3.CO;2. Printed in the USA

314 www.gsapubs.org | Volume 9 | Number 2 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/2/299/1001583/299.pdf by guest on 29 September 2021