Introduction: Earthscope IDOR Project (Deformation and Magmatic Modification of a Steep Continental Margin, Western Idaho–Eastern Oregon) Themed Issue

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

Introduction: EarthScope IDOR project (deformation and magmatic modification of a steep continental margin, western Idaho–eastern Oregon) themed issue

B. Tikoff1, J. Vervoort2, J.A. Hole3, R. Russo4, R. Gaschnig5, and A. Fayon6 1DEPARTMENT OF GEOSCIENCE, UNIVERSITY OF WISCONSIN-MADISON, 1215 W DAYTON STREET, MADISON, WISCONSIN 53706, USA 2SCHOOL OF THE ENVIRONMENT, WASHINGTON STATE UNIVERSITY, PO BOX 642812, PULLMAN, WASHINGTON 99164, USA 3DEPARTMENT OF GEOSCIENCES, VIRGINIA POLYTECHNIC INSTITUTE AND STATE UNIVERSITY, DERRING HALL, 1405 PERRY STREET, BLACKSBURG, VIRGINIA 24061, USA 4DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF FLORIDA, 241 WILLIAMSON HALL, GAINESVILLE, FLORIDA 32611, USA 5DEPARTMENT OF ENVIRONMENTAL, EARTH AND ATMOSPHERIC SCIENCES, UNIVERSITY OF MASSACHUSETTS LOWELL, 1 UNIVERSITY AVENUE, LOWELL, MASSACHUSETTS 01854, USA 6DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF MINNESOTA TWIN CITIES, 310 PILLSBURY DRIVE SE, MINNEAPOLIS, MINNESOTA 55455, USA

ABSTRACT

The EarthScope IDOR (Idaho-Oregon) project was designed to investigate the formation of the steep accretionary boundary associated with the Salmon River suture zone and western Idaho shear zone (WISZ), in western Idaho and eastern Oregon (western USA). The research was designed to test three hypotheses concerning the present geometry of the boundary: (1) a crustal detachment; (2) a lithospheric detachment; and (3) a curved original geometry for the WISZ. The data presented in this volume indicate that none of these original three models is correct. Rather, the WISZ continues vertically downward through the entire crust, its steep orientation consistent with a significant compo- nent of strike-slip and/or transpressional tectonism. Moreover, the western Idaho shear zone is extremely well preserved despite abundant subsequent magmatism (Idaho batholith, Challis event, Columbia River Basalt Group) and tectonism (Cretaceous contraction, Eocene extension, Miocene–present extension). The ten research papers in this volume report on various aspects of this history. Four papers address the kinematics and tectonic history of rocks spatially associated with the WISZ, including its along-strike northward continuation. Two papers address the tectonic history of the accreted terranes, including geochemical arguments for different lithospheres sampled during magmatic episodes. Three papers document deformation, exhumation, and emplacement rates in the Idaho batholith. The final paper constrains the modern crustal architecture along a transect from the accreted terranes in the west to cratonic North America in the east. Together, these studies constrain the tectonic history of a subvertical tectonic boundary in the North American Cordillera.

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ACCRETED TERRANE-CRATON BOUNDARY IN WESTERN Following Giorgis et al. (2007), we refer to the Salmon River suture IDAHO: MOTIVATION FOR THE IDOR EARTHSCOPE PROJECT zone as the zone of western Idaho that records the collision of the accreted terranes of the Blue Mountains with North America. The western Idaho Understanding the growth and modification of the western North shear zone (WISZ), in contrast, is a younger shear zone within the over- American continental margin has been a long-standing goal in the geologi- all Salmon River suture zone. Although accretion occurred by the Late cal science community. The continental margin evolution in the western Jurassic–Early Cretaceous (Walker, 1986; Selverstone et al., 1992; Snee United States was dominated by active plate boundary processes, includ- et al., 1995; LaMaskin and Dorsey, 2016), the accretionary boundary ing subduction, terrane accretion and translation, distributed deformation, was significantly modified by the mid-Cretaceous subvertical WISZ (e.g., and voluminous magmatism. McClelland et al., 2000; Giorgis et al., 2005). Unusually sharp isotopic The Salmon River suture zone of western Idaho is an archetypal example gradients (Sr, Nd, O) representing the edge of continental lithosphere cor- of a steep terrane boundary of the North American Cordillera (Fig. 1). The respond to the surface location of the WISZ (Tikoff et al., 2001). Structural Salmon River suture zone juxtaposes accreted terranes (island arcs of the studies in the WISZ suggested dextral transpression with strong east-west Blue Mountains) and Precambrian North America. Hamilton (1963, 1969), shortening (Giorgis and Tikoff, 2004; Giorgis et al., 2005). working in the region around Riggins, Idaho, was the first to notice a spa- The EarthScope IDOR (Idaho-Oregon) project was designed to unravel tially abrupt change in both age and rock composition along this zone. Since the tectonic history along this boundary. This project was submitted to that time, the region has also been noted for sharp isotopic gradients (Sr, Nd, the EarthScope program of the National Science Foundation (primary O) (e.g., Armstrong et al., 1977; Fleck and Criss, 1985, 2004; Manduca et investigators: Tikoff, Hole, Russo, and Vervoort). Specifically, the IDOR al., 1992) interpreted to represent the edge of continental mantle lithosphere. project investigated the tectonic history of western Idaho and eastern Ore- gon, including collision (e.g., Salmon River suture zone), syncollisional Basil Tikoff http://orcid.org/0000 -0001 -6022 -7002 to postcollisional transpressional deformation (e.g., WISZ), voluminous

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122° 118° 114° 110° Canada A Cenozoic, undivided 48° Cretaceous plutonic belts

Washington B Wrangellia terrane and metamorphic Oregon BM rocks of the Washington Cascades Montana 44° Wyoming Cretaceous Ochoco and 44° Hornbrook basins WISZ Cretaceous subduction assemblages Idaho CCoastoast RangeRange OphioliteOphioli and Great VVaalleylley (forearc basin)basin) Sequence 40° 440°0° Cordilleran Outboard exotic arc terranes Thrust Belt ColoradoColorado plplateauateau NorthNorth American frinfringing-arcgi terranes

Ne UtahUtah Pre-Cretaceous subductionsubd complexes vad Arrizonaizona 36° a 36°36° Paleozoic andand MesozoicMesoz eugeoclinal basinbasin assemblagesass LateLate ProterozoicProterozoic toto PermianP miogeoclinalmiogeoclinal rocksrocks 100 0 100 200 300 km California 32° 32°32° NNorthorth AmericanAmerican cratocraton 122° 118° 114° 110°

3 B 0 100 km Blue Mountains plutons 119° W 7 9 Volcanic, sedimentary 46° N Melange, schist Sedimentary, volcanic Volcanic, sedimentary SalmonRiver metamorphic N Blue Mountains Chief Joseph Dike Swarm 5 8 Province terranes Idaho batholith Z

S Challis volcanics I Challis magmatic belt W Proterozoic platform sed Paleozoic platform sed 4 Cenozoic sediment

119° 1 Braudy et al. 2 2 Montz & Kruckenberg 6 10 3 Schmidt et al.

1 4 Giorgis et al. Idaho Batholith 5 Kurz et al. Atlanta Lobe 6 Gaschnig et al., 2017 WA MT 7 Byerly et al.

OR 8 Fayon et al. ID Sr 0.706 9 Gaschnig et al., 2016 CA NV UT Oregon Idaho 43° N 10 Davenport et al.

Figure 1. (A) Location of the study area in the western United States. WISZ—Western Idaho shear zone; BM—Blue Mountain terranes (diagram originally from Wyld et al., 2006). (B) Regional geological map of eastern Oregon and western Idaho (sed—sedimentary). The boxes indicate the areas of study for the publications within this themed issue.

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Cretaceous and Miocene magmatism (e.g., Idaho batholith, Columbia River basalt flows), and active extensional deformation. In particular, work A. OBSERVATIONS concentrated on the role of the steep accreted terrane-craton boundary in Idaho and Oregon as a major controlling feature for both subsequent OREGON IDAHO deformation (e.g., Tikoff et al., 2001; Giorgis et al., 2006, 2008; Lund et al., 2008) and magmatism (e.g., Carlson and Hart, 1987; Leeman et al., 1992). Miocene Cretaceous The following are the three overarching questions driving the EarthScope Sr 0.706 isopleth Sr 0.706 isopleth IDOR project. 1. How do fundamental lithospheric boundaries, such as the juxtaposi- WEbasalt granite tion of continental and oceanic lithosphere at the WISZ, guide subsequent deformation and magmatism? WISZ 2. How do magmatism and extensional deformation cause modifica- ? tion (destabilization) of the continental lithosphere, and at what lithospheric levels? 3. What are the implications of this destabilization for magmatism, B-D. PROPOSED MODELS deformation, and evolving lithospheric strength? B. Crustal Detachment (Leeman et al., 1992) THREE REJECTED MODELS

The study area was chosen because prior work had suggested that it con- WEWISZ tained an attribute that is unique in the North American Cordillera, i.e., an ID batholith offset vertical, lithospheric-scale boundary (Fig. 2). This vertical geometry detachment would allow investigation of the effect of different lithospheric layers on WISZ deformation and magmatism. Cretaceous intrusive rocks record a whole- Moho lithosphere vertical boundary beneath western Idaho (e.g., the exposed WISZ; Manduca et al., 1992); however, the mantle isotopic signature of Miocene basaltic rocks (Hart, 1985; Carlson and Hart, 1987; Hart and C. Deep Detachment Carlson, 1987; Leeman et al., 1992) indicates that the edge of continental mantle is ~120 km to the east of the exposed WISZ in eastern Oregon (Fig. 2). This observation led Leeman et al. (1992) to hypothesize that the WE subvertical WISZ, i.e., the boundary between oceanic arc and continental ID batholith lithosphere, is cut and offset along a Sevier orogeny detachment (Fig. 2). WISZ Moho This interpretation was supported by geophysical work (Evans et al., 2002). Miocene magmatism, including the extrusion of the voluminous Colum- t detachmen bia River basalt flows, appears to be localized on a sharp boundary in the mantle beneath eastern Oregon (Carlson and Hart, 1987), while extensional deformation is localized on the steep crustal boundary in western Idaho (Fig. 2). Confirmation and elaboration of this model required geophysical D. No Detachment (Smith, 1981) imaging to determine whether the offset exists, at what lithospheric level it occurs, and how later geological events modify the regional lithospheric WE structure. Therefore, our three proposed models addressed different aspects ID batholith of an offset (Fig. 2): (1) the crustal detachment model of Leeman et al. (1992) (Fig. 2B); (2) a possible detachment within the lithospheric mantle WISZ Moho (Fig. 2C); or (3) a curved WISZ (Fig. 2D). In the last model, contractional deformation in the Sevier fold and thrust belt is driven by emplacement of the voluminous granites of the Idaho batholith (e.g., Smith, 1981). We did not know what to expect for a crustal geometry along the accreted terrane-craton boundary in western Idaho–eastern Oregon. As a result of the data presented in this volume and published elsewhere (e.g., E. IDOR MODEL Stanciu et al., 2016), none of the three models originally proposed is Plateau formation (e.g. Byerly et al., viable. The primary reason to reject these models is that the geophysical 2016; Fayon et al., 2017)

Figure 2. Illustrations from the National Science Foundation EarthScope WEOlds ID batholith proposal for the IDOR (Idaho-Oregon) project. (A) Observations of broad Baker Ferry geochemical trends across the arc-continent boundary. WISZ—western Moho Idaho shear zone. (B–D) Three models that possibly explain the crustal ? geometry inferred from geochemical work on igneous rocks (e.g., the Sr 0.706 line). (E) The result, constrained by geophysical and geochronologi- Terrane boundary from geochemistry From seismic constraints cal data, indicates that the western Idaho shear zone continues through (Kurz et al., 2016) (Davenport et al., 2017) the entire crust, and offsets the Moho by ~7 km.

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evidence clearly shows that the accreted terrane-craton boundary, cur- Salmon River suture zone deformation on the cratonic side of the arc- rently defined by WISZ, continues vertically through the crust and off- continent boundary. sets the Moho by ~7 km. This result is clearly demonstrated by both the The deformation in the Syringa embayment, along the 90° bend of active source seismic data (Davenport et al., 2017) and the IDOR Project the Sr 0.706 isopleth in central Idaho, is the topic of a paper by Schmidt broadband seismic results (Stanciu et al., 2016). This interpretation is also et al. (2016). Field-based and microstructural analyses indicate reverse corroborated by the study of Kurz et al. (2016), who maintain that the (northeast side up) shear along the Ahsahka shear zone, which is present in isotopic break, recorded by the Miocene basaltic rocks (e.g., Hart, 1985; this region. This result is corroborated using the crystallographic vorticity Carlson and Hart, 1987), indicates the limit of the Olds Ferry magmatic axis method of Michels et al. (2015) on quartzites that indicates horizontal arc relative to the other terranes of the Blue Mountains. The Olds Ferry vorticity vectors. Schmidt et al. (2016) also present U-Pb zircon data that terrane shows more enriched compositions in relation to the isotopic suggest that similar age deformation occurs in the Ahsahka shear zone and data (e.g., Sr, Nd, and Pb), and thus reflects interaction with continental the WISZ located to the south. Finally, Schmidt et al. (2016) document material. The extent of the Olds Ferry terrane also appears to demarcate the presence of two right-lateral, transpressional, northeast-trending fault the region of mafic underplating, as observed in the IDOR geophysical and/or shear zones in the Syringa embayment, the Limekiln and Mount data (e.g., Stanciu et al., 2016; Davenport et al., 2017). Idaho deformation zones. These features modify and offset the original The proposed model, based on the IDOR work, is shown in Figure orientation of Ahsahka shear zone and the WISZ, and were active until 2E. We first summarize the contributions in this volume that resulted in ca. 54 Ma. The Limekiln and Mount Idaho deformation zones were also this new view. reactivated after eruption of the Miocene Columbia River Basalt. The transpressional nature of the WISZ is the subject of the paper by CONTRIBUTIONS IN THIS VOLUME Giorgis et al. (2016). The fabrics of the WISZ contain a steeply dipping foliation and a downdip lineation, and it was previously suggested that There are ten papers on various aspects of the Salmon River suture this pattern reflects transpressional deformation (e.g., Giorgis et al., 2005). zone and surrounding areas included in this volume. Although many Giorgis et al. (2016) first use a series of well-exposed outcrops with late- papers are from researchers funded through the EarthScope IDOR proj- stage dikes to document boudinage and folding consistent with dextral ects, there are also independent researchers with important contributions transpressional kinematics. An estimate of kinematic vorticity, using tails from this region. The first four papers concentrate on the WISZ, including on porphyroclasts within the shear zone, suggests a convergence vec- its along-strike extension to the north. The following two papers con- tor oriented 60° or slightly higher. These methods are supplemented by centrate on the tectonic history of the accreted terranes. The next three microstructural analyses of deformed quartzites on West Mountain, includ- papers document deformation, exhumation, and emplacement rates in ing the crystallographic vorticity axis method of Michels et al. (2015). the Idaho batholith. The final paper describes the results from the active These analyses indicate subvertical vorticity axes, when normal faulting source seismic study that spans the entire region. The locations of all of is restored, and dominantly prism slip. These data are compared to the study areas presented in this volume are shown in Figure 1. the data in Schmidt et al. (2016), who utilized the same methods. Together, Braudy et al. (2016) concentrate on the WISZ in the location where the these data are consistent with a ~060° orientation of convergence, which shear zone is transected by the active seismic line (West Mountain, Idaho; leads to dextral transpressional deformation in West Mountain and con- Davenport et al., 2017). Braudy et al. (2016) document a primary bend vergent deformation in the Ahsahka shear zone. in the shear zone, from a north-south–oriented foliation observed in the Kurz et al. (2016) use exposures in Hells Canyon and elsewhere in the north to a 020-oriented fabric observed to the south. The latter orientation Blue Mountains terrain to compare the Wallowa and Olds Ferry magmatic is consistent with that observed in the WISZ south of the western Snake arcs; their isotopic (Sr, Nd, and Pb) evidence indicates that the Olds Ferry River Plain in the Owyhee Mountains (Benford et al., 2010). The mapping arc is isotopically enriched, consistent with its formation as a fringing in Braudy et al. (2016) also determined the presence of a metasedimentary continental arc. In contrast, the Wallowa terrane is isotopically depleted screen within the WISZ. These metasedimentary exposures provide the and likely reflects a more outboard oceanic position. New U-Pb zircon first estimates of pressure and temperature conditions within the WISZ of data provide the geochronological context for the isotopic data. Based ~4.4 kbar and ~730 °C. New U-Pb zircon geochronology data constrain on new results, Kurz et al. (2016) argue that the three different crustal the initiation deformation in the WISZ to ca. 103 Ma, and Lu-Hf garnet columns (from northwest to west-east, Wallowa, Olds Ferry, and North geochronology indicates that peak metamorphism during WISZ defor- America) have been connected to distinct mantle reservoirs since the mation occurred ca. 98 Ma. Geochemistry and zircon xenocrysts also early Mesozoic; in their model, the west to east spatial variability in the suggest deeper crustal imbrication that formed during terrane accretion isotopic compositions sampled by Neogene volcanic rocks results from prior to WISZ deformation. this lithospheric structure. This model contrasts sharply with the inter- Montz and Kruckenberg (2017) report on the Potters Pond migmatite pretation of Leeman et al. (1992), who suggested that that the edge of domain, which occurs near the change in orientation of the WISZ on continental mantle is ~120 km to the west of the exposed WISZ. The Kurz West Mountain, Idaho. Their structural mapping indicates two distinct et al. (2016) interpretation resolves the issue of the isotopic compositions periods of protracted migmatite formation that are constrained by U-Pb of the Neogene volcanic rocks in a manner consistent with the seismic zircon analysis. The first episode, in the Early Cretaceous, occurred dur- results of Davenport et al. (2017). ing known deformation in the Salmon River suture zone. The second Gaschnig et al. (2017) examine Cretaceous igneous rocks and sedi- episode, in the mid-Cretaceous, occurred during dextral transpressional ments on the southernmost part of the exposed Blue Mountain terranes deformation on the WISZ. The data of Montz and Kruckenberg (2017) in easternmost Oregon. A series of four plutons intrude the Blue Moun- are the first direct confirmation of a two-stage model for deformation in tain terranes on or near the Conner Creek fault, which separates the Izee western Idaho, with an earlier Salmon River suture zone overprinted by a and Baker terranes. These plutons are dated from 129.4 to 123.8 Ma later WISZ (e.g., McClelland et al., 2000). Furthermore, their study docu- and crosscut the fault and constrain the timing of motion. The (U-Th)/ ments synmetamorphic fabric development east of the initial Sr isotope He zircon studies on these plutons indicate a middle Cretaceous cooling (87Sr/86Sr = 0.706 isopleth), thus providing the first direct evidence for age through ~200 °C. Gaschnig et al. (2017) also present a detrital zircon

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analysis of Late Cretaceous sedimentary rocks at Dixie Butte, Oregon. ~27 km thick, with seismic velocities that indicate intermediate to mafic These results indicate Paleozoic–Mesozoic ages with subtle, but important, composition. The Precambrian crust at the transect’s western end exhibits differences from the ages of magmatism in the Blue Mountains and WISZ slower seismic velocities, consistent with more felsic lithologies. Below region. Furthermore, radiogenic Hf isotopic compositions of these zircons or slightly eastward of the exposed portions of the WISZ, the Moho is indicate that they were derived from juvenile accreted terrane lithosphere. offset by ~7 km. This offset is interpreted to result from strike-slip motion. Together these data suggest that the source of the detrital zircons is the Davenport et al. (2017) interpret the mid-crustal reflector at ~20 km depth Insular terrane, which is inferred to have been located immediately west to indicate a melt source zone for the Atlanta lobe of the Idaho batholith. of the field area during deposition (ca. 95 Ma). On the accreted terrane side, there is a large, high-amplitude reflected Byerly et al. (2016) investigate the internal fabrics of the Idaho batho- phase ~8 km above the Moho. This reflector is interpreted as the top of lith using a combination of microstructures, shape-preferred orientation a mafic underplating layer, which is spatially coincident with the feeder of minerals, and the anisotropy of magnetic susceptibility technique. This dikes of the Columbia River Basalt Group. study utilizes the same sampling locations of the U-Pb zircon age and geochemistry studies of Gaschnig et al. (2010, 2013) and Braudy et al. (2016). PROPOSED MODEL Gaschnig et al. (2010) showed that the Atlanta lobe, which constitutes most of the central parts of the Idaho batholith, is distinctly younger than Figure 2E shows a schematic model that has emerged from both the the Bitterroot lobe found only in the northern part of the Idaho batholith. EarthScope IDOR group and other researchers working in this area. This Because the microstructures are mostly magmatic, Byerly et al. (2016) model has several distinctive aspects relative to the earlier proposed mod- utilize the existing geochronologic framework to interpret spatial and tem- els (Figs. 2B–2D). First, the WISZ continues throughout the crust and poral patterns of fabric development within the batholith. They conclude offsets the Moho; the crust is ~7 km thicker on the cratonic (east) side that the oldest igneous rocks on the western edge of the batholith contain a (Davenport et al., 2017; Stanciu et al., 2016). The presence of a crustal- consistent north-south–oriented fabric, the Atlanta lobe contains weak and scale WISZ was invoked to explain why Miocene–recent extensional nonconsistent fabric, and the young Bitterroot lobe exhibits a consistent deformation appears to be localized along this boundary (Tikoff et al., northwest-striking foliation. Byerly et al. (2016) suggest that the Atlanta 2001). Second, the vertical nature of the boundary suggests a strong com- lobe may have been intruded during a time of crustal plateau formation, ponent of strike-slip deformation, as inferred from kinematic studies of the and the lack of consistently oriented fabric resulted from emplacement WISZ (Giorgis and Tikoff, 2004; Giorgis et al., 2016). It is also consistent in a neutral tectonic stress environment (due to topographic effects), in with the overprinting concept of McClelland et al. (2000), in which the an overall contractional setting, and/or sill-like emplacement. accretionary boundary (Salmon River suture zone) is distinctly older than Fayon et al. (2017) use (U-Th)/He zircon thermochronometry to constrain the late-stage transpressional zone (WISZ). The contribution by Montz the exhumation of the southern part of the Idaho batholith. Samples were and Kruckenberg (2017) provides compelling evidence for a discrete two- taken throughout the border zone, early metaluminous, Atlanta lobe, and step development, in addition to addressing the formation of migmatites Challis magmatic suites. These samples were colocated with, or used the in the region. Third, as discussed here, the Kurz et al. (2016) study makes zircon separates from, the samples reported in Gaschnig et al. (2010, 2011, sense of the isotopic data recorded by the Miocene basaltic rocks (e.g., 2013) and Braudy et al. (2016). Three distinct spatial domains are evident Hart, 1985; Carlson and Hart, 1987). The Miocene Columbia River basalt in their data. First, rocks affected by the WISZ exhibit a west to east young- flows apparently underplated the Olds Ferry terrane. ing in (U-Th)/He zircon ages, reflecting the spatial migration in magmatism The other aspect illuminated by IDOR research is the nature of the and/or deformation. These rocks cooled earlier than any rocks in the adja- Idaho batholith, based on the geochronological and geochemical data cent Atlanta lobe. Second, rocks from the Atlanta lobe exhibit a pattern of (Gaschnig et al., 2010, 2011, 2013, 2016). Byerly et al. (2016) and Fayon cooling ages as a function of elevation, which are interpreted to indicate et al. (2017) both indicate that the Idaho batholith likely intruded into a isobaric cooling in the absence of tectonic activity. Fayon et al. (2017) indi- crustal plateau, continuous with the proposed crustal plateau in central cate that the cooling age pattern is consistent with the Atlanta lobe forming Nevada (Nevadaplano; e.g., Wolfe et al., 1997; DeCelles, 2004). Nota- in a crustal plateau. Third, younger (U-Th)/He zircon ages in the Sawtooth bly, the WISZ is not intruded by younger (ca. 80–67 Ma; Atlanta lobe) Range indicate recent exhumation associated with extensional deformation. magmatism associated with the Idaho batholith, nor are the (U-Th)/He Gaschnig et al. (2016) examine the nearly continuous record of Cre- zircon ages in the WISZ reset by the Idaho batholith (Fayon et al., 2017). taceous–Paleogene magmatism in Idaho and interpret the relationship Together, these studies have illuminated the formation of a subverti- of core and rim ages in zircons to suggest that much of the magmatic cal boundary, one of many such boundaries that occur within the North record in Idaho has been obscured. These underrepresented episodes of American Cordillera. We hope that these studies also provide a solid foun- magmatism include the presence of (1) a middle Cretaceous magmatic dation for future work into both the tectonic evolution and geodynamic arc that was significantly shortened in an east-west direction by the WISZ; processes exemplified in the region. (2) a ca. 100–85 Ma Sierran-style arc that was disrupted, obscured, and cannibalized by the latest Cretaceous Atlanta peraluminous suite; and (3) ACKNOWLEDGMENTS a northward extension of the Atlanta peraluminous suite that was in turn A large number of people made this volume possible. First and foremost, we thank Science Editor Arlo Weil and Special Guest Editor Bernie Housen of Lithosphere for their timely assistance in disrupted, obscured, and assimilated by the younger Bitterroot metalu- making this volume happen. We thank R. Dorsey for a version of Figure 1A, which we subsequently minous and peraluminous suites. Based on this interpretation, Gaschnig edited. We thank M. Kahn for drafting final figures. We are grateful to the reviewers for all of the et al. (2016) make estimates of the magmatic flux rates for the Idaho manuscripts in this volume, most of whom were anonymous to us. The reviews were all extremely helpful and were appreciated. We acknowledge the EarthScope program for funding this often- segment of the northern Cordillera. overlooked corner of the North American Cordillera. This work was supported by National Sci- Davenport et al. (2017) report on the active source seismic data from ence Foundation grants EAR-0844260 and EAR-1251877 to Tikoff, EAR-0537913 and EAR-0844149 the EarthScope IDOR project. 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