Western Idaho Research Articles

The utility of statistical analysis in structural geology

Recent advances in statistical methods for structural geology make it possible to treat nearly all types of structural geology field data. These methods provide a way to objectively test hypotheses and to quantify uncertainty, and their adoption into standard practice is important for future quantitative analysis in structural geology. We outline an approach for structural geologists seeking to incorporate statistics into their workflow using examples of statistical analyses from two locations within the western Idaho shear zone. In the West Mountain location, we test the published interpretation that there is a bend in the shear zone at the kilometer scale. Directional statistics on foliations corroborate this interpretation, while orientation statistics on foliation-lineation pairs do not. This discrepancy leads us to reconsider an assumption made in the earlier work. In the Orofino location, we present results from a full statistical analysis of foliation-lineation pairs, including data visualization, regressions, and inference. These results agree with thermochronological evidence that suggests that the Orofino area comprises two distinct, subparallel shear zones. The R programming language scripts that were used for both statistical analyses can be downloaded to reproduce the statistical analyses of this paper.

Predicting Spatial Risk of Wolf-Cattle Encounters on Rugged and Extensive Grazing Lands

Author(s): Clark, Patrick E.; Chigbrow, Joe; Johnson, Douglas E.; Williams, John; Larson, Larry L.; Roland, Tyanne | Abstract: Cattle grazing lands in the mountainous western United States are rugged, complex, and extensive. Terrain, vegetation, and other landscape features vary greatly across space. Risk of wolf-cattle encounters and potential for depredation loss certainly differ spatially as consequence of this variability. Yet, our understanding of this spatial risk is quite poor and this knowledge gap severely hampers our abilities to manage wolf-livestock interactions and mitigate conflicts. During 2009-2011, a research study was conducted at four study areas (USFS cattle grazing allotments) in western Idaho to evaluate and predict risk of wolf-cattle encounters. Each year, a random sample of 10 lactating beef cows from each study area was instrumented with GPS collars that logged positions at 5-minute intervals throughout the summer grazing season. Cattle resource selection was modeled using these GPS data and negative-binomial regression. An existing model was used to classify habitats within the study areas in terms of probability of use by wolves as rendezvous sites. Efficacy of this model was confirmed using scat, telemetry, and rendezvous site data. Spatial overlaps in the predicted selectivity of wolves and cattle were assessed and study area landscapes were then classified into five encounter-risk classes (very low to very high). Concurrent wolf and cattle GPS tracking data were used to document wolf-cattle encounters and thus evaluate the accuracy of this classification. About 94% of observed wolf-cattle encounters occurred within either the high or highest encounter-risk classes. Areas classified to the highest risk class were located on smooth, relatively flat slopes in concave terrain (e.g., stream terrace meadows) but not all were associated with surface water. Having this predictive understanding of where wolf-cattle encounters are most likely to occur will allow livestock producers and wildlife managers to more effectively apply resources, husbandry practices, and mitigation techniques to reduce conflict.

FiDO

Homes located on tribal lands, particularly in rural areas of the United States, continue to lack access to broadband Internet and cellular connectivity [19]. Inspired by previous observations of community content similarity in tribal networks, we propose FiDO, a community-based Web browsing and content delivery system that takes advantage of user mobility, opportunistic connectivity, and collaborative filtering to provide relevant Web content to members of disconnected households via opportunistic contact with cellular base stations during a daily commute. We evaluate FiDO using trace-driven simulations with network usage data collected from a tribal-operated ISP that serves the Coeur d’Alene Indian Reservation in Western Idaho. By collecting data about household Web preferences and applying a collaborative filtering technique based on the Web usage patterns of the surrounding reservation community, we are able to opportunistically browse the Web on behalf of members of disconnected households, providing an average of 69.4 Web pages (all content from a specific URL, e.g., “http://gis.cdatribe-nsn.gov/LandBuyBack/”) crawled from 73% of their top 10 most visited Web domains (e.g., “cdatribe-nsn.gov” or “cnn.com/”) per day. Moreover, this content is able to be fetched and pushed to users even when the opportunistic data rate is limited to an average of only 0.99 Mbps (σ = 0.24 Mbps) and the daily opportunistic connection time is an average of 45.9 minutes (σ = 2.3 minutes). Additionally, we demonstrate a hybrid “search and browse” approach that allocates a percentage of opportunistic resources to the download of user-specified content. By dedicating only 10% of opportunistic windows of connectivity to the download of social media content, 51% of households were able to receive all of their daily expected social media content in addition to an average of 55.3 Web pages browsed on their behalf from an average of 4 different Web domains. Critically, we demonstrate the feasibility of a collaborative and community-based Web browsing model that extends access to Web content across the last mile(s) using existing infrastructure and rural patterns of mobility.

Effects of Wolf Presence on Daily Travel Distance of Range Cattle

The presence of gray wolves (Canis lupus) can directly and indirectly affect beef cattle (Bos taurus) production on rangelands of the Northern Rocky Mountains. While fairly extensive knowledge exists for the direct effects of wolf predation threat (e.g., cattle death and injury losses, elevated stress), our understanding of wolf-caused changes in cattle behavior and the associated cascade of potential indirect effects on cattle resource selection, diet quality, activity budgets, and energetic relationships is still largely in its infancy. We investigated whether wolf presence affected the daily travel distance of Global Positioning System (GPS)−collared cattle under a replicated, Impact-Control study conducted in western Idaho and northeastern Oregon during 2008−2012. Cattle in three Control (Oregon) study areas, where wolf presence was consistently low, traveled farther per day (13.7 ± 0.396 SE km day−1) than those in three Impact (Idaho) study areas (11.4 ± 0.396 SE km day−1) with moderate to high wolf presence. At Control study areas, cattle traveled farthest per day in July (13.2 ± 0.355 SE km day−1) and were least mobile in October (11.8 ± 0.365 SE km day−1), but daily travel distances were similar across all months for cattle in Impact study areas. This observational study provides evidence suggesting cattle in mountainous grazing areas alter their spatial behavior in response to gray wolf presence. These behavioral changes have energetic consequences that could potentially impact cattle productivity and ranch economics. Additional research into the activity budget and resource selection responses of these collared cattle is required to better understand the specific mechanisms behind these daily travel distance results.

MORPHOMETRIC AND TECHNOLOGICAL ATTRIBUTES OF WESTERN STEMMED TRADITION PROJECTILE POINTS REVEALED IN A SECOND ARTIFACT CACHE FROM THE COOPER'S FERRY SITE, IDAHO

In western North America, the Western Stemmed Tradition (WST) is contemporaneous with, but technologically different from, the Clovis Paleoindian Tradition as initially defined from the Great Plains and American Southwest. The foundational principles of WST lithic technology have not been as clearly delineated as those of their fluted and unfluted Paleoindian Tradition technological counterparts, largely due to the paucity of extensive WST lithic assemblages recovered from intact buried contexts. Recent excavations at the Cooper's Ferry site, located in western Idaho, revealed a stratified series of WST components spanning the late Pleistocene to early Holocene periods. The study of these components offers a unique opportunity to evaluate current expectations about WST lithic technology. Here, we describe the discovery, context, and contents of a new cache of 14 WST projectile points from the Cooper's Ferry site that provide clues about WST lithic reduction patterns and the design of early stemmed projectile points. We employ several novel methods of lithic analysis based on three-dimensional digital scanning technology and geometric morphometry and, in doing so, seek to demonstrate new ways of studying stone tools through the use of next-generation methods of lithic analysis applied to exploring the poorly known technological details of the WST.

Geology and in situ stress of the MH-2 borehole, Idaho, USA: Insights into western Snake River Plain structure from geothermal exploration drilling

Project HOTSPOT, the Snake River Scientific Drilling Project (International Continental Scientific Drilling Program), tested for deep geothermal resources and examined the petrology of volcanic rocks with three drillholes in the central and western Snake River Plain (western USA). The MH-2 drillhole targeted fractured crystalline and hydrothermally altered basalt in the area of the Mountain Home Air Force Base (Idaho) to a total depth of 1821 m. At 1745 m depth the drillhole encountered flowing artesian hydrothermal fluids of at least 150 °C. We integrate geological analyses of core, image log, and borehole geophysical data, and in situ stress analyses to describe the structural environment that produces permeability for artesian flow. The rocks in the lower 540 m of the drillhole consist of basalt flows as much as 30 m thick, altered basalt, and thin sedimentary horizons. The mechanical stratigraphy is defined by nine mechanical horizons that are in three ranges of rock strength on the basis of experimentally determined strength data, core logging, and geophysical log signatures. Hydrothermal alteration products and mineralization in the core are associated with three highly faulted sections; the lowermost section is associated with the zone of flowing thermal water. Shear slip indicators on faults observed in core indicate slip ranging from pure strike slip to normal failure mechanisms in the stronger horizons. The borehole breakouts indicate that the maximum horizontal stress, SH, is oriented 047° ± 7°, and drilling-induced tensile fractures indicate that SH is oriented at 67° ± 21°. The in situ stress orientations exhibit little variation over the depth of the measured interval, but the SH magnitude varies with depth, and is best explained by an oblique normal fault stress regime. The geomechanical model indicates that if pore pressures at depth are elevated above the normal hydrostatic gradient, as observed here, the system has the potential to deform by mixed normal and strike-slip failure. Our observations and interpretations suggest that the MH-2 borehole was drilled into oblique normal faults that intersect a buried 300°-trending fault block masked by the basaltic volcanic complex. These data indicate that the transition from the central to western Snake River Plain is characterized by complex structures developed in response to a transitional stress state related to Snake River Plain and western Basin and Range stress regimes. The western Basin and Range stress and tectonic regime may extend from northern Nevada into western Idaho and may enhance the potential for geothermal resources by creating interconnected fracture and fault-related permeability at depth.

Magma‐facilitated transpressional strain partitioning within the Sawtooth metamorphic complex, Idaho: A zone accommodating Cretaceous orogen‐parallel translation in the Idaho batholith

Structural data from metasedimentary rocks and geochronologic data from intrusive rocks in the Idaho batholith provide evidence for the relationship between deformation and magmatism in the northern U.S. Cordillera. The Sawtooth metamorphic complex (SMC), Idaho, is exposed as an inlier in the central Idaho batholith and contains strongly deformed metasedimentary and intrusive rocks. Geologic mapping reveals north-south-striking, alternating contraction- and shear-dominated domains across strike. The contraction-dominated domains consist of centimeter- to tens of meter-scale, shallowly to steeply plunging upright folds with subhorizontal lineations. The shear-dominated domains are characterized by highly strained subvertical foliations, subhorizontal lineations, and syntectonic intrusive sheets. Pervasive S-C structures, winged porphyroclasts, and asymmetric folds indicate dextral strike-slip shearing. The fabrics in the two types of domains are structurally compatible and are interpreted to be broadly synchronous. This work suggests that the SMC structures represent a wrench-dominated transpressional zone, in which the regional strain partitioned into the contraction- and shear-dominated domains and the partitioning was facilitated by emplacement of syntectonic magmas. Zircon U-Pb data of the syntectonic intrusions indicate that the SMC transpressional deformation occurred mainly between ca. 95-92 Ma and ca. 84 Ma, and had ended by ca. 77 Ma. The transpressional deformation in the SMC and the western Idaho shear zone (WISZ) were kinematically compatible and partially coeval. This suggests that the SMC and WISZ represent a regional transpression system and that crustal deformation inboard of the continental margin may have contributed to the northward orogen-parallel translation of accreted terranes during the Late Cretaceous.

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

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 component 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.

Intrusive and depositional constraints on the Cretaceous tectonic history of the southern Blue Mountains, eastern Oregon

We present an integrated study of the postcollisional (post–Late Jurassic) history of the Blue Mountains province (Oregon and Idaho, USA) using constraints from Cretaceous igneous and sedimentary rocks. The Blue Mountains province consists of the Wallowa and Olds Ferry arcs, separated by forearc accretionary material of the Baker terrane. Four plutons (Lookout Mountain, Pedro Mountain, Amelia, Tureman Ranch) intrude along or near the Connor Creek fault, which separates the Izee and Baker terranes. High-precision U-Pb zircon ages indicate 129.4–123.8 Ma crystallization ages and exhibit a north-northeast–younging trend of the magmatism. The 40Ar/39Ar analyses on biotite and hornblende indicate very rapid (<1 m.y.) cooling below biotite closure temperature (∼350 °C) for the plutons. The (U-Th)/He zircon analyses were done on a series of regional plutons, including the Lookout Mountain and Tureman Ranch plutons, and indicate a middle Cretaceous age of cooling through ∼200 °C. Sr, Nd, and Pb isotope geochemistry on the four studied plutons confirms that the Izee terrane is on Olds Ferry terrane basement. We also present data from detrital zircons from Late Cretaceous sedimentary rocks at Dixie Butte, Oregon. These detrital zircons record only Paleozoic–Mesozoic ages with only juvenile Hf isotopic compositions, indicating derivation from juvenile accreted terrane lithosphere. Although the Blue Mountains province is juxtaposed against cratonic North America along the western Idaho shear zone, it shows trends in magmatism, cooling, and sediment deposition that differ from the adjacent part of North America and are consistent with a more southern position for terranes of this province at the time of their accretion. We therefore propose a tectonic history involving moderate northward translation of the Blue Mountains province along the western Idaho shear zone in the middle Cretaceous.

A strong contrast in crustal architecture from accreted terranes to craton, constrained by controlled-source seismic data in Idaho and eastern Oregon

Crustal structure was derived from EarthScope Idaho-Oregon (IDOR) controlled-source seismic data across the Precambrian continental margin in the Idaho and Oregon region of the U.S. Cordillera. Refraction and wide-angle reflection traveltimes were inverted to derive a seismic velocity model that constrains the contact between oceanic accreted terranes and craton. The seismic data reveal that the boundary is a near-vertical, through-going feature of the crust, represented by the transpressional western Idaho shear zone (WISZ). The WISZ separates crust with different seismic velocities at all depths, implying a contrast in lithology, and extends to an ∼7 km offset of the Moho. The thinner, ∼32-km-thick accreted terrane crust to the west is characterized by faster seismic velocities that correspond to an intermediate composition. We interpret a high-velocity layer below a high-amplitude seismic reflection as mafic magmatic underplating associated with the feeder system of the Columbia River Basalts. The cratonic crust east of the WISZ is 37–40 km thick, with a felsic composition to ∼29 km subsurface depth, underlain by an intermediate-composition layer above the Moho. The strong contrasts in lithology and crustal thickness across the WISZ have influenced subsequent magmatism and extension in the region. The northwestern extent of the Archean Grouse Creek cratonic block beneath the Atlanta lobe of the Idaho batholith is interpreted based on continuity of crustal architecture in the seismic model. The velocity structure and crustal thickness east of the WISZ are consistent with the Atlanta lobe melting within a thickened crust.

Cretaceous partial melting, deformation, and exhumation of the Potters Pond migmatite domain, west-central Idaho

The Potters Pond migmatite domain (PPMD) is a heterogeneous zone of migmatites located ∼10 km southwest of Cascade, Idaho, within the western Idaho shear zone (WISZ). The PPMD is the only known exposure of migmatites within the WISZ over its ∼300 km length, occurring where the shear zone orientation changes from 024° south to 005° north of the migmatite domain. Structural mapping within the PPMD has identified multiple generations of migmatite with varied structural fabrics. Leucosome layers were sampled from distinct migmatite localities and morphologies (e.g., metatexite and diatexite) to determine the timing and duration of partial melting in the PPMD. U-Pb age determinations of zircon by means of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) document two periods of protracted migmatite crystallization during the Early and Late Cretaceous. Early Cretaceous (ca. 145–128 Ma) migmatite crystallization ages are coeval with the collision and suturing of oceanic terranes of the Blue Mountains province with North America and the formation of the Salmon River suture zone (SRSZ). Migmatite crystallization ages from ca. 104–90 Ma are associated with Late Cretaceous dextral transpression in the WISZ. Field observations and geochronology of crosscutting leucosome relationships are interpreted to record deep crustal deformation and anatexis associated with formation of the SRSZ, subsequently overprinted by solid-state deformation and renewed anatexis during the evolution of the WISZ. These data are the first direct evidence of the synmetamorphic fabric related to the SRSZ east of the initial Sr 0.706 isopleth and indicate that the WISZ is a temporally distinct overprinting structure.

Cooling and exhumation of the southern Idaho batholith

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.

Historic Distribution of Mountain Quail in the Pacific Northwest

Mountain quail (Oreortyx pictus) are among the least studied of the North American quails. The prehistoric and early historic distributions of this bird are uncertain. In the Pacific Northwest, mountain quail were first recorded by Lewis and Clark in 1806 near the Columbia River adjacent to the Cascade Range in Oregon. Written evidence relating to the original distribution of mountain quail in this area indicated that the birds were found from the Oregon Coast Range to the Cascades along the Columbia River and southward. Translocations of birds into this region began in 1860 and continued for several decades, which further confused the historic status. Eventually, mountain quail were distributed from southern British Columbia throughout Washington and into western Idaho and eastern Oregon by the early 20th century. Archeological evidence revealed it is possible that mountain quail existed in west-central Idaho, likely as refugia populations, 700 to 1000 years ago. Populations in Idaho and the interior Columbia River Basin have declined substantially during the past several decades. Similar declines have not been observed in the Pacific Northwest (western Oregon) or in the humid coastal region of western California.

Timing and deformation conditions of the western Idaho shear zone, West Mountain, west-central Idaho

The western Idaho shear zone is a major, lithospheric-scale structure separating accreted terranes of the Blue Mountains from continental North America. We document the occurrence of the western Idaho shear zone in West Mountain, west-central Idaho. Rocks deformed by the western Idaho shear zone at West Mountain are dominantly orthogneisses, although exposures on West Mountain containing screens of metamorphosed sedimentary rocks are also present. Steeply E-dipping, N-NNE–oriented foliations and downdip lineations characterize the fabric in the orthogneisses, consistent with dextral transpressional kinematics. The foliation orientation changes from 005° to 024° from the northern to the southern part of the field area, and this is interpreted to reflect a primary along-strike variation in the orientation of the western Idaho shear zone. The westernmost unit in West Mountain (Four Bit Creek tonalite) has a U-Pb zircon age of 101 ± 3.0 Ma, yet it is only weakly deformed. We interpret this unit to have been emplaced pretectonically, thus constraining the initiation of the western Idaho shear zone. The youngest unit at West Mountain is the undeformed Rat Creek granite (88.2 ± 3.3 Ma). U-Pb analyses of zircons from orthogneisses at West Mountain span ages of 111–91 Ma, indicating both precursory and continuous magmatism coeval with western Idaho shear zone deformation. Two Lu-Hf garnet isochron ages, 97.3 ± 0.7 Ma and 99.5 ± 1.4 Ma, are interpreted to indicate peak metamorphism during western Idaho shear zone deformation. Geochemical analyses suggest that the westernmost exposed orthogneiss units are dominantly derived from continental material in West Mountain, and yet there is also evidence for a component of accreted terrane rocks at depth east of the western Idaho shear zone.

Tectonic evolution of the Syringa embayment in the central North American Cordilleran accretionary boundary

The Syringa embayment (Idaho, USA) is in the Mesozoic accretionary margin of western North America, where the north-south–oriented lithospheric boundary bends abruptly to an east-west orientation near the 46th parallel. New geologic mapping, structural analysis, and laser ablation–inductively coupled plasma–mass spectrometry U-Pb zircon age data constrain the origin and prolonged evolution of this embayment. We agree with previous workers that the Syringa embayment may have initiated as an inherited Proterozoic rift boundary, producing a kink in the north-south–oriented continent margin. Structural analysis on the northwest-oriented Ahsahka shear zone that is adjacent to the accretionary boundary indicates dominantly reverse, southwest-vergent motion, as confirmed by crystallographic vorticity axis analysis on quartzites. New U-Pb zircon age dating brackets the age of deformation along the Ahsahka shear zone to between ca. 116 and 92 Ma. These dates indicate that deformation on the Ahsahka shear zone occurred simultaneously with deformation on the western Idaho shear zone to the south. Consequently, the Ahsahka and western Idaho shear zones are a continuous, albeit kinked, mid-Cretaceous shear zone system that maintained kinematic compatibility during oblique dextral convergence along the margin. The Syringa embayment has an additional younger structural history, specifically movement on a pair of northeast-trending dextrally transpressive structural zones, the Limekiln and Mount Idaho zones. The Mount Idaho deformation zone truncates the Ahsahka–western Idaho shear zone. Following truncation, continued orthogonal contraction was partitioned to the northeast of the Ahsahka shear zone in the northwest-trending Clearwater zone, which is bracketed by existing age data to ca. 73–54 Ma.

Kinematic and vorticity analyses of the western Idaho shear zone, USA

The western Idaho shear zone (WISZ) is a Late Cretaceous, mid-crustal exposure of intense shear localized in the Cordillera of western North America. This shear zone is characterized by transpressional fabrics, i.e., downdip stretching lineations and vertical foliations. Folded and boudinaged late-stage dikes indicate a dextral sense of shear. The vorticity-normal section is identified by examining the three-dimensional shape preferred orientation of feldspar populations and the intragranular lattice rotation in quartz grains in deformed quartzites. The short axes of the shape preferred orientation ellipsoid gather on a plane perpendicular to the vorticity vector. In western Idaho this plane dips gently to the west, suggesting a vertical vorticity vector. Similarly, sample-scale crystallographic vorticity axis analysis of quartzite tectonites provides an independent assessment of vorticity and also indicates a subvertical vorticity vector. Constraints on the magnitude of vorticity are provided by field fabrics and porphyroclasts with strain shadows. Together these data indicate that the McCall segment of the WISZ displays dextral transpression with a vertical vorticity vector and an angle of oblique convergence ≥60°. North and south of McCall, movement is coeval on the Owyhee segment of the WISZ and the Ahsahka shear zone. Together, the kinematics of these shear zones are consistent with northeast-southwest–directed convergence. Plate motion in this orientation acting on a curved plate boundary could have produced pure shear–dominated transpression in the Owyhee (α = 40°) and McCall (α = 60°) segments of the WISZ, while causing reverse-sense shearing (α = 90°) in the Ahsahka shear zone.

Isotopic compositions of intrusive rocks from the Wallowa and Olds Ferry arc terranes of northeastern Oregon and western Idaho: Implications for Cordilleran evolution, lithospheric structure, and Miocene magmatism

Late Paleozoic and Mesozoic intrusive rocks from the Wallowa and Olds Ferry arc terranes of the Blue Mountains Province, Oregon-Idaho, provide constraints on the paleogeographic and tectonic setting of magmatism preserved in both arcs. Sr, Nd, and Pb isotopic data show that the Wallowa terrane represents an isotopically depleted, juvenile intra-oceanic island arc. By contrast, isotopic data for intrusive rocks of the Olds Ferry arc are more isotopically enriched, and thereby establish a clear distinction between the two arcs. This distinction strengthens paleogeographic interpretation of the Olds Ferry terrane as a fringing continental arc, and it provides a basis for correlation to other inboard Cordilleran arc terranes including Quesnellia and Stikinia. The Wallowa terrane is by contrast more similar geologically and isotopically to the outboard Insular terranes. These isotopic data also constrain interpretations of regional lithospheric architecture. Isotopic profiles generated orthogonal to the inferred Wallowa–Olds Ferry terrane boundary and the western Idaho shear zone show abrupt increases in initial 87 Sr/ 86 Sr that mark the transitions between three geochemically distinct lithospheric columns. West-to-east spatial variability in the isotopic compositions of Neogene volcanic rocks is explained by the partial melting of these three geochemically distinct mantle reservoirs coupled to their respective crustal columns since the early Mesozoic, rather than alternative models of lithosphere-scale decollement offset during Sevier shortening. The inherited arc-related mantle of the Olds Ferry arc may also have played a primary role in the petrogenesis of distinctive Neogene low-K, high-alumina olivine tholeiites of the High Lava Plains.

Construction and preservation of batholiths in the northern U.S. Cordillera

There is a nearly continuous record of magmatism through the Late Cretaceous–early Paleogene in Idaho and adjacent areas of Oregon and Montana, including the various phases of the Idaho batholith. We suggest that much of this magmatic record, however, has been obscured by subsequent tectonic deformation, erosion, and magmatic disruption and cannibalization, the latter of which can be tracked by zircon inheritance. Specifically, a mid-Cretaceous magmatic arc was significantly deformed by the western Idaho shear zone and intruded by the 83–67 Ma Atlanta peraluminous suite of the Idaho batholith. The northern part of the Atlanta peraluminous suite was, in turn, intruded by the 65–55 Ma Bitterroot lobe of the Idaho batholith. Consequently, the present age distribution of magmatism is strongly biased toward the youngest phases of plutonism; much of the older phases were destroyed by tectonic, magmatic, and erosional processes. The destruction of granitic batholiths may characterize Cordilleran-style orogens worldwide, which can lead to significant underestimates of magmatic fluxes.

Crustal structure beneath the Blue Mountains terranes and cratonic North America, eastern Oregon, and Idaho, from teleseismic receiver functions

AbstractWe present new images of lithospheric structure obtained from P‐to‐S conversions defined by receiver functions at the 85 broadband seismic stations of the EarthScope IDaho‐ORegon experiment. We resolve the crustal thickness beneath the Blue Mountains province and the former western margin of cratonic North America, the geometry of the western Idaho shear zone (WISZ), and the boundary between the Grouse Creek and Farmington provinces. We calculated P‐to‐S receiver functions using the iterative time domain deconvolution method, and we used the H‐k grid search method and common conversion point stacking to image the lithospheric structure. Moho depths beneath the Blue Mountains terranes range from 24 to 34 km, whereas the crust is 32–40 km thick beneath the Idaho batholith and the regions of extended crust of east‐central Idaho. The Blue Mountains group Olds Ferry terrane is characterized by the thinnest crust in the study area, ~24 km thick. There is a clear break in the continuity of the Moho across the WISZ, with depths increasing from 28 km west of the shear zone to 36 km just east of its surface expression. The presence of a strong midcrustal converting interface at ~18 km depth beneath the Idaho batholith extending ~20 km east of the WISZ indicates tectonic wedging in this region. A north striking ~7 km offset in Moho depth, thinning to the east, is present beneath the Lost River Range and Pahsimeroi Valley; we identify this sharp offset as the boundary that juxtaposes the Archean Grouse Creek block with the Paleoproterozoic Farmington zone.

Paleodiet in Western Idaho: An analysis of Mid-Archaic human remains from the Braden and DeMoss sites

Two prehistoric archeological sites in western Idaho, the Braden and DeMoss sites, have produced numerous human remains dating to the Middle Holocene. As part of this study, the authors analyzed stable isotopes of carbon and nitrogen (13C and 15N) from collagen samples of these remains to reconstruct aspects of the diets of these individuals. Carbon isotope values (δ13C‰) from Braden range between −14.47 and −16.84; those from the DeMoss ranged between −19.10 and −19.13. Nitrogen values (δ15N‰) ranged between 13.7 and 18.1 Braden and between 9.13 and 9.44 at DeMoss. These results indicate that salmon played a vital dietary role for the peoples represented at Braden, whereas individuals living at DeMoss relied on a mixed terrestrial diet. Differences in these subsistence patterns are also suggested by analysis of dental calculus and pathology.