Western Idaho Shear Zone Research Articles

Strain and kinematics within the Salmon River suture zone and western Idaho shear zone, Idaho, USA: Exploring the contribution of ductile stretching to mass transfer and exhumation in fold-thrust and transpressional systems

Measuring penetrative strain is critical for understanding the 3D strain field of structural systems. Here, we investigate ductile strain and kinematics in two Cordilleran structural systems in north-central Idaho: the Salmon River suture zone (SRSZ), which is a west-vergent ductile fold-thrust system that accommodated shortening associated with terrane collision between ∼144 and 105 Ma, and the north-striking, subvertical western Idaho shear zone (WISZ), which accommodated dextral-transpressional shearing between ∼105 and 86 Ma. We collected finite strain data from stretched clasts in three SRSZ thrust sheets, which define 56–87% average thrust-parallel stretching and 35–48% average thrust-normal thinning. Thrust-parallel stretching contributed >27 km of cumulative displacement to the up-dip portion of the fold-thrust system, comparable to the 34 km of total thrust displacement estimated at down-dip levels. In the WISZ, we documented dextral kinematics in lineation-normal planes, and we measured boudinaged and folded granitic dikes to estimate late-stage (∼91-86 Ma) strain, which yielded 65% minimum lineation-parallel stretching and 50% minimum east-west shortening. Subvertical stretching in the WISZ accommodated >9–10 km of exhumation relative to the Idaho batholith to the east. The SRSZ and WISZ both demonstrate the 1st-order importance of ductile stretching for accommodating the large-scale transfer of mass and exhumation in fold-thrust and transpressional systems.

Two phases of Cretaceous dextral shearing recorded in the plutonic rocks of NW Nevada (USA): A tectonic link between intra-arc shearing in the Sierra Nevada and Idaho batholiths

Abstract The Cretaceous intrusive units of the Sahwave and Nightingale ranges in northwestern Nevada, USA, located between the Sierra Nevada and Idaho batholiths, represent a critical segment of Cretaceous arc magmatism. U-Pb zircon age dating shows that the older, 104 Ma Power Line intrusive complex is dominantly granodioritic in composition, while the younger 94–88 Ma Sahwave Range intrusive suite (the Juniper Pass, Bob Springs, and Sahwave plutons) is similar in composition (tonalite to granodiorite) and age to the plutons of the Tuolumne intrusive suite of the east-central Sierra Nevada batholith. We present new field measurements, microstructural observations, and anisotropy of magnetic susceptibility analyses of the Power Line intrusive complex and Sahwave Range intrusive suite. The Power Line intrusive complex is characterized by a vertical, N–S-striking, solid-state foliation and down-dip lineation. Evidence of dextral shearing is observed on subhorizontal planes that are perpendicular to the lineation, which is consistent with pure shear-dominated transpression. This fabric is similar in style and timing to both the western Idaho shear zone of the Idaho batholith and mid-Cretaceous shear zones of the central Sierra Nevada. The plutons of the Sahwave Range intrusive suite are not affected by the pure shear-dominated transpressional fabric observed in the Power Line intrusive complex, which indicates that this deformation ceased by ca. 94 Ma. Rather, the Juniper Pass pluton contains an E–W-striking magmatic foliation fabric that rotates to a steep NW–SE-striking, solid-state foliation in the younger Sahwave pluton. These fabrics are strikingly similar to fabrics in the Tuolumne intrusive suite, Sierra Nevada, California, USA. Recent work in the western Idaho shear zone also indicates that late-stage deformation occurred there until ca. 85 Ma. Therefore, the intrusions of northwestern Nevada provide a tectonic link between the Sierra Nevada and Idaho batholiths, which suggests that two distinct phases of mid-Cretaceous, transpressional deformation occurred in at least three magmatic arc segments of the western U.S. margin.

Constraints on the post-orogenic tectonic history along the Salmon River suture zone from low-temperature thermochronology, western Idaho and eastern Oregon

ABSTRACTThe abrupt boundary between accreted terranes and cratonic North America is well exposed along the Salmon River suture zone in western Idaho and eastern Oregon. To constrain the post-suturing deformation of this boundary, we assess the cooling history using zircon and apatite (U–Th)/He thermochronology. Pre-Miocene granitic rocks, along a regional transect, were sampled from accreted terranes of the Blue Mountains Province to cratonic North America (Idaho batholith). Each sample was taken from a known structural position relative to a paleotopographic surface represented by the basal unit of the Miocene Columbia River basalts. An isopach map constructed for the Imnaha Basalt, the basal member of the Columbia River Basalt Group (CRBG), confirms the presence of a Miocene paleocanyon parallel to the northern part of Hells Canyon. The (U–Th)/He zircon dates indicate mostly Cretaceous cooling below 200°C, with the ages getting generally younger from west to east. The (U–Th)/He apatite dates indicate Late Cretaceous–Paleogene cooling, which post-dates tectonism associated with the western Idaho shear zone (WISZ). However, (U–Th)/He apatite dates younger than the Imnaha Basalt, with one date of 3.4 ± 0.6 Ma, occur at the bottom of Hells Canyon. These young (U–Th)/He apatite dates occur along the trend of the Miocene paleocanyon, and cannot be attributed to local exhumation related to faults. We propose that burial of Mesozoic basement rocks by the Columbia River basalts occurred regionally. However, the only samples currently exposed at the Earth’s surface that were thermally reset by this burial were at the bottom of the Miocene paleocanyon. If so, exhumation of these samples must have occurred by river incision in the last 4 million years. Thus, the low-temperature thermochronology data record a combination of Late Cretaceous–Paleogene cooling after deformation along the WISZ that structurally overprinted the suture zone and Neogene cooling associated with rapid river incision.

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.

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.

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.

Westward Growth of Laurentia by Pre–Late Jurassic Terrane Accretion, Eastern Oregon and Western Idaho, United States: A Reply

Previous article No AccessWestward Growth of Laurentia by Pre–Late Jurassic Terrane Accretion, Eastern Oregon and Western Idaho, United States: A ReplyTodd A. LaMaskin and Rebecca J. DorseyTodd A. LaMaskin1. Department of Geography and Geology, University of North Carolina at Wilmington, 601 South College Road, Wilmington, North Carolina 28403-5944, USA*Author for correspondence; e-mail: [email protected]. Search for more articles by this author and Rebecca J. Dorsey2. Department of Geological Sciences, University of Oregon, 1272 University of Oregon, Eugene, Oregon 97403-1272, USA Search for more articles by this author PDFPDF PLUSFull Text Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 124, Number 1January 2016 Article DOIhttps://doi.org/10.1086/684120 Views: 206Total views on this site Citations: 5Citations are reported from Crossref HistoryReceived September 04, 2015Accepted September 09, 2015Published online December 16, 2015 © 2015 by The University of Chicago. All rights reserved.PDF download Crossref reports the following articles citing this article:Todd A. LaMaskin, Jonathan A. Rivas, David L. Barbeau, Joshua J. Schwartz, John A. Russell, Alan D. Chapman A crucial geologic test of Late Jurassic exotic collision versus endemic re-accretion in the Klamath Mountains Province, western United States, with implications for the assembly of western North America, GSA Bulletin 134, no.3-43-4 (Jul 2021): 965–988.https://doi.org/10.1130/B35981.1Paul M. Bremner, Mark P. Panning, R. M. Russo, Victor Mocanu, A. Christian Stanciu, Megan Torpey, Sutatcha Hongsresawat, John C. VanDecar, Todd A. LaMaskin, D. A. Foster Crustal Shear Wave Velocity Structure of Central Idaho and Eastern Oregon From Ambient Seismic Noise: Results From the IDOR Project, Journal of Geophysical Research: Solid Earth 124, no.22 (Feb 2019): 1601–1625.https://doi.org/10.1029/2018JB016350B. Tikoff, J. Vervoort, J.A. Hole, R. Russo, R. Gaschnig, A. Fayon Introduction: EarthScope IDOR project (deformation and magmatic modification of a steep continental margin, western Idaho–eastern Oregon) themed issue, Lithosphere (Feb 2017): L628.1.https://doi.org/10.1130/L628.1N. Braudy, R.M. Gaschnig, D. Wilford, J.D. Vervoort, C.L. Nelson, C. Davidson, M.J. Kahn, B. Tikoff Timing and deformation conditions of the western Idaho shear zone, West Mountain, west-central Idaho, Lithosphere (Dec 2016): L519.1.https://doi.org/10.1130/L519.1Tracy L. Vallier, Keegan L. Schmidt, Todd A. LaMaskin Geology of the Wallowa terrane, Blue Mountains province, in the northern part of Hells Canyon, Idaho, Washington, and Oregon, (): 211–249.https://doi.org/10.1130/2016.0041(07)

An inverse approach to constraining strain and vorticity using rigid clast shape preferred orientation data

We describe a three-part approach for modeling shape preferred orientation (SPO) data of spheroidal clasts. The first part consists of criteria to determine whether a given SPO and clast shape are compatible. The second part is an algorithm for randomly generating spheroid populations that match a prescribed SPO and clast shape. In the third part, numerical optimization software is used to infer deformation from spheroid populations, by finding the deformation that returns a set of post-deformation spheroids to a minimally anisotropic initial configuration. Two numerical experiments explore the strengths and weaknesses of this approach, while giving information about the sensitivity of the model to noise in data. In monoclinic transpression of oblate rigid spheroids, the model is found to constrain the shortening component but not the simple shear component. This modeling approach is applied to previously published SPO data from the western Idaho shear zone, a monoclinic transpressional zone that deformed a feldspar megacrystic gneiss. Results suggest at most 5 km of shortening, as well as pre-deformation SPO fabric. The shortening estimate is corroborated by a second model that assumes no pre-deformation fabric.

Age and structure of the Crevice pluton: overlapping orogens in west-central Idaho?

New U–Pb zircon geochronology from the Riggins region of west-central Idaho refines the timing of contractional deformation across the Salmon River suture zone (SRSZ), a broad north- to northeast-striking belt (&gt;25 km wide) of high strain recording Jura-Cretaceous island-arc–continent collision. Laser ablation – inductively coupled plasma – mass spectrometry (LA–ICP–MS) yields mid-Cretaceous crystallization ages on formerly undated plutonic rocks sampled from the Salmon River canyon. In the Crevice pluton (∼105 Ma), the development of steep to moderate northerly striking gneissic foliation (S1) was followed by tops-to-the-west slip on shallow mylonitic shear zones (S2) and brittle overprinting via systematic joints (Jn) of regional extent. Together, these structures form the pluton’s internal architecture. Subvertical gneissic foliation in the adjacent Looking Glass pluton (∼92 Ma) indicates ductile deformation was ongoing in the Late Cretaceous. Prior to this investigation, penetrative fabrics in local arc volcanogenic, plutonic, and continental rocks have been unequivocally linked to post-collisional dextral transpression on the narrow (&lt;10 km wide) western Idaho shear zone (WISZ). As an alternative to this model which requires spatially overlapping but temporally distinct orogenic belts (WISZ–SRSZ), we consider a protracted history whereby regional synmetamorphic structures accumulated over a pre-118 Ma to post-92 Ma interval without an overprinting orogen-scale ductile shear zone. In our view, a progressive deformation history more accurately accounts for the time-transgressive nature and structural continuity of fabrics observed across the arc–continent transition. This tectonic history proposed for western Idaho may be analogous to other long-lived accretionary margins in the North American Cordillera (e.g., Omineca Belt of southeastern British Columbia).

Mesozoic magmatism and deformation in the northern Owyhee Mountains, Idaho: Implications for along-zone variations for the western Idaho shear zone

The northern Owyhee Mountains of southwestern Idaho contain granitoid rocks that are the same age as the Cretaceous western border zone of the Idaho batholith to the north of the Snake River Plain. They contain a well-developed and consistently oriented 020° foliation, zircon yielding U-Pb dates of ca. 160–48 Ma, and initial 87Sr/86Sr isotopic compositions that show a steep west-to-east transition in values from 0.704 to 0.708 over a distance of ∼30 km. The rocks of the northern Owyhee Mountains are interpreted to be the southward continuation (Owyhee segment) of the western Idaho shear zone. Similar to a well-studied section of the western Idaho shear zone by McCall (McCall segment), the Owyhee segment displays steep foliation and lineation orientations, deformation of 98–90 Ma plutons, steep Sr isotopic gradients, and syntectonic tonalite intrusions. However, the Owyhee segment has three major differences from the McCall segment: (1) significantly less well-developed solid-state strain fabric foliations; (2) trend of 020° rather than 000°; and (3) a wider transition zone in initial Sr ratios from 0.704 to 0.708. We present a simple tectonic model to explain these differences, assuming a 20° along-zone difference in the initial orientation of the western margin of the Laurentia, a rigid-body collision, homogeneous material behavior, and transpressional kinematics. For the Owyhee segment, the model predicts a lower oblique-convergence angle, less convergent displacement, more dextral transcurrent displacement, and an overall lower finite strain relative to the McCall segment.