Sevier Thrust Belt Research Articles

Zircon (U-Th)/(He-Pb) double-dating constraints on the interplay between thrust deformation and foreland basin architecture, Sevier foreland basin, Utah

AbstractThe Cretaceous Cordilleran foreland basin strata exposed in the Book Cliffs of eastern Utah and western Colorado have motivated important concepts linking thrust belt deformation and foreland basin evolution largely on the basis of sequence stratigraphy, stratal architecture, and sediment provenance evolution. However, these methods and approaches generally cannot provide critical insights into the temporal or causal linkages between foreland basin architecture and thrust belt deformation. This is in part due to discrepancies in age resolution and lack of evidence with which to directly couple sediment supply and basin-fill evolution to thrust belt unroofing. New detrital zircon (DZ) geothermochronometric data from Upper Cretaceous proximal to distal foreland basin strata in the Book Cliffs provide new quantitative insights into sediment origin and dispersal in relation to thrust belt deformation and exhumation. Detailed DZ U-Pb and (U-Th)/He double dating reveals that the Book Cliffs foredeep detritus was mainly delivered by transverse routing systems from two major sources: (1) Neoproterozoic and Lower Paleozoic strata from the central Utah Sevier thrust belt, and (2) Permian–Jurassic and synorogenic Cretaceous strata recycled from the frontal part of the thrust belt. A dramatic increase in Sierran magmatic arc and Yavapai-Mazatzal DZ U-Pb ages, as well as Paleozoic DZ He ages, in the deeper marine portions of the foreland basin points to axial fluvial and littoral sediment input from the Sierran magmatic arc and Mogollon highland sources. Both transverse and axial transport systems acted contemporaneously during eastward propagation of the Late Cretaceous thrust belt. DZ He depositional lag time estimates reveal three distinct exhumation pulses in the Sevier thrust belt in the Cenomanian and Campanian. The exhumation pulses correlate with shifts in sediment provenance, dispersal style, and progradation rates in the foreland basin. These new data support conceptual models that temporally and causally link accelerated exhumation and unroofing in the thrust belt to increases in sediment supply and rapid clastic progradation in the foreland basin.

Laramide Orogenesis Driven by Late Cretaceous Weakening of the North American Lithosphere

AbstractThis paper investigates the causes of the Late Cretaceous transition from “Sevier” to “Laramide” orogenesis and the spatial and temporal evolution of effective elastic thickness (EET) of the North American lithosphere. We use a Monte Carlo flexural model applied to 34 stratigraphic profiles in the Laramide province and five profiles from the Western Canadian Basin to estimate model parameters which produce flexural profiles that match observed sedimentary thicknesses. Sediment thicknesses come from basins from New Mexico to Canada of Cenomanian–Eocene age that are related to both Sevier and Laramide crustal loads. Flexural models reveal an east‐to‐west spatial decrease in EET in all time intervals analyzed. This spatial decrease in EET may have been associated with either bending stresses associated with the Sevier thrust belt, or increased proximity to attenuated continental crust at the paleocontinental margin. In the Laramide province (i.e., south of ~48°N) there was a coeval, regional decrease in EET between the Cenomanian–Santonian (~98–84 Ma) and the Campanian–Maastrichtian (~77–66 Ma), followed by a minor decrease between the Maastrichtian and Paleogene. However, there was no decrease in EET in the Western Canada Basin (north of ~48°N), which is consistent with a lack of Laramide‐style deformation or flat subduction. We conclude that the regional lithospheric weakening in the late Santonian–Campanian is best explained by hydration of the North American lithosphere thinned by bulldozing by a shallowly subducting Farallon plate. The weakening of the lithosphere facilitated Laramide contractional deformation by focusing end‐loading stresses associated with flat subduction. Laramide deformation in turn may have further reduced EET by weakening the upper crust. Finally, estimates of Campanian–Maastrichtian and Paleogene EET are comparable to current estimates indicating that the modern distribution of lithospheric strength was achieved by the Campanian in response to flat subduction.

Syncontractional deposition of the Cretaceous Newark Canyon Formation, Diamond Mountains, Nevada: Implications for strain partitioning within the U.S. Cordillera

Abstract The timing of deformation and deposition within syntectonic basins provides critical information for understanding the evolution of strain in mountain belts. In the U.S. Cordillera, contractional deformation was partitioned between the Sevier thrust belt in Utah and several structural provinces in the hinterland in Nevada. One hinterland province, the Central Nevada thrust belt (CNTB), accommodated up to ∼15 km of shortening; however, in most places, this deformation can only be bracketed between Permian and Eocene. Cretaceous deposits of the Newark Canyon Formation (NCF), which are sparsely exposed along the length of the CNTB, offer the opportunity to constrain deformation timing. Here, we present mapping and U-Pb zircon geochronology from the NCF in the Diamond Mountains, which demonstrate deposition of the NCF during proximal CNTB deformation. Deposition of the basal NCF member was under way no earlier than ca. 114 Ma, a tuff in the middle part of the section was deposited at ca. 103 Ma, and the youngest member was deposited no earlier than ca. 99 Ma. Intraformational angular unconformities and abrupt along- and across-strike thickness changes indicate that NCF deposition was related to growth of an east-vergent fault-propagation fold. Clast compositions define unroofing of upper Paleozoic sedimentary rocks, which we interpret as the progressive erosion of an anticline ∼10 km to the west. CNTB deformation was contemporaneous with shortening in the Sevier thrust belt, which defines middle Cretaceous strain partitioning between frontal and interior components of the Cordillera. Strain partitioning may have been promoted by renewed underthrusting during a period of high-flux magmatism.

Orogenic link ∼41°N–46°N: Collisional mountain building and basin closure in the Cordillera of western North America

AbstractPolyphase structural mapping and mineral age dating across the Salmon River suture zone in west-central Idaho (Riggins region; ∼45°30′N, ∼117°W–116°W) support a late Mesozoic history of penetrative deformation, dynamothermal metamorphism, and intermittent magmatism in response to right-oblique oceanic-continental plate convergence (Farallon–North America). High-strain linear-planar tectonite fabrics are recorded along an unbroken ∼48 km west-to-east transect extending from the Snake River (Wallowa intra-oceanic arc terrane; eastern Blue Mountains Province) over the northern Seven Devils Mountains into the lower Salmon River Canyon (ancestral North America; western Laurentia). Given the temporally overlapping nature (ca. 145–90 Ma) of east-west contraction in the Sevier fold-and-thrust belt (northern Utah–southeast Idaho–southwest Montana segment), we propose that long-term terrane accretion and margin-parallel northward translation in the Cordilleran hinterland (∼41°N–46°N latitude; modern coordinates) drove mid- to upper-crustal shortening >250 km eastward into the foreland region (∼115°W–113°W). During accretion and translation, the progressive transfer of arc assemblages from subducting (Farallon) to structurally overriding (North American) plates was accommodated by displacement along a shallow westward-dipping basal décollement system underlying the Cordilleran orogen. In this context, large-magnitude horizontal shortening of passive continental margin strata was balanced by the addition of buoyant oceanic crust—late Paleozoic to Mesozoic Blue Mountains Province—to the leading edge of western Laurentia. Consistent with orogenic float modeling (mass conservation, balance, and displacement compatibility), diffuse dextral-transpressional deformation across the accretionary boundary (Salmon River suture: Cordilleran hinterland) was kinematically linked to eastward-propagating structures on the continental interior (Sevier thrust belt; Cordilleran foreland). As an alternative to noncollisional convergent margin orogenesis, we propose a collision-related tectonic origin and contractional evolution for central portions of the Sevier belt. Our timing of terrane accretion supports correlation of the Wallowa terrane with Wrangellia (composite arc/plateau assemblage) and implies diachronous south-to-north suturing and basin closure between Idaho and Alaska.

Geochemical evidence for an orogenic plateau in the southern U.S. and northern Mexican Cordillera during the Laramide orogeny

AbstractPrevious studies of the central United States Cordillera have indicated that a high-elevation orogenic plateau, the Nevadaplano, was present in Late Cretaceous to early Paleogene time. The southern United States Cordillera and northern Mexican Cordillera share a similar geologic history and many of the same tectonic features (e.g., metamorphic core complexes) as the central United States Cordillera, raising the possibility that a similar plateau may have been present at lower latitudes. To test the hypothesis of an elevated plateau, we examined Laramide-age continental-arc geochemistry and employed an empirical relation between whole-rock La/Yb and Moho depth as a proxy for crustal thickness. Calculations of crustal thickness from individual data points range between 45 and 72 km, with an average of 57 ± 12 km (2σ) for the entire data set, which corresponds to 3 ± 1.8 km paleoelevation assuming simple Airy isostasy. These crustal thickness and paleoaltimetry estimates are similar to previous estimates for the Nevadaplano and are interpreted to suggest that an analogous high-elevation plateau may have been present in the southern United States Cordillera. This result raises questions about the mechanisms that thickened the crust, because shortening in the Sevier thrust belt is generally not thought to have extended into the southern United States Cordillera, south of ∼35°N latitude.

Geologic map of the east-central Meadow Valley Mountains, and implications for reconstruction of the Mormon Peak detachment, Nevada

The role of low-angle faults in accommodating extension within the upper crust remains controversial because the existence of these faults markedly defies extant continuum theories of how crustal faults form, and once initiated, how they continue to slip. Accordingly, for many proposed examples, basic kinematic problems like slip direction, dip angle while active, and magnitude of offset are keenly debated. A well-known example is the Miocene Mormon Peak detachment and overlying Mormon Peak allochthon of southern Nevada (USA), whose origin and evolution have been debated for several decades. Here, we use geologic mapping in the Meadow Valley Mountains to help define the geometry and kinematics of emplacement of the Mormon Peak allochthon, the hanging wall of the Mormon Peak detachment. Pre-exten­sion structural markers, inherited from the east-vergent Sevier thrust belt of Meso­zoic age, are well suited to constrain the geometry and kine­matics of the detachment. In this study, we add to these markers a newly mapped Sevier-­age monoclinal flexure preserved in the hanging wall of the detachment. The bounding axial surfaces of the flexure can be readily matched to the base and top of the frontal Sevier thrust ramp, which is exposed in the footwall of the detachment to the east in the Mormon Mountains and Tule Springs Hills. Multiple proxies for the slip direction of the detachment, including the mean tilt direction of hanging wall fault blocks, the trend of striations measured on the fault plane, and other structural features, indicate that it is approximately S77°W (257°). Given the observed structural separation lines between the hanging wall and footwall, this slip direction indicates 12–13 km of horizontal displacement on the detachment (14–15 km net slip), lower than a previous estimate of 20–22 km, which was based on erroneous assumptions in regard to the geometry of the thrust system. Based on a new detailed map compilation of the region and recently published low-temperature thermochronologic data, palinspastic constraints also preclude earlier suggestions that the Mormon Peak allochthon is a composite of diachronously emplaced, surficial landslide deposits. Although earlier suggestions that the initiation angle of the detachment in the central Mormon Mountains is ∼ 20°–25° remain valid, the geometry of the Sevier-age monocline in the Meadow Valley Mountains and other structural data suggest that the initial dip of the detachment steepens toward the north beneath the southernmost Clover Mountains, where the hanging wall includes kilometer-scale accumulations of volcanic and volcaniclastic strata.

Shallow-crustal metamorphism during Late Cretaceous anatexis in the Sevier hinterland plateau: Peak temperature conditions from the Grant Range, eastern Nevada, U.S.A.

Documenting spatio-temporal relationships between the thermal and deformation histories of orogenic systems can elucidate their evolution. In the Sevier hinterland plateau in eastern Nevada, an episode of Late Cretaceous magmatism and metamorphism affected mid- and upper-crustal levels, concurrent with late-stage shortening in the Sevier thrust belt. Here, we present quantitative peak temperature data from the Grant Range, a site of localized, Late Cretaceous granitic magmatism and greenschist facies metamorphism. Twenty-two samples of Cambrian to Pennsylvanian metasedimentary and sedimentary rocks were analyzed, utilizing Raman spectroscopy on carbonaceous material, vitrinite reflectance, and Rock-Eval pyrolysis thermometry. A published reconstruction of Cenozoic extension indicates that the samples span pre-extensional depths of 2.5–9 km. Peak temperatures systematically increase with depth, from ∼100 to 300 °C between 2.5 and 4.5 km, ∼400 to 500 °C between 5 and 8 km, and ∼550 °C at 9 km. The data define a metamorphic field gradient of ∼60 °C/km, and are corroborated by quartz recrystallization microstructure and published conodont alteration indices. Metamorphism in the Grant Range is correlated with contemporary, upper-crustal metamorphism and magmatism documented farther east in Nevada, where metamorphic field gradients as high as ∼50 °C/km are estimated. These data have implications for localized but significant thermal weakening of the plateau crust, including attaining temperatures for quartz plasticity at depths of ∼5–6 km, and the potential for partial melting possibly as shallow as ∼12–15 km. Thermal weakening may have contributed to a slowing of shortening rates documented in the Sevier thrust belt at this latitude at ca. 88 Ma, by locally inducing mid- and lower-crustal ductile thickening in the hinterland.

Structure of the upper mantle in the north-western and central United States from USArray S-receiver functions

Abstract. We used more than 40 000 S-receiver functions recorded by the USArray project to study the structure of the upper mantle between the Moho and the 410 km discontinuity from the Phanerozoic western United States to the cratonic central US. In the western United States we observed the lithosphere–asthenosphere boundary (LAB), and in the cratonic United States we observed both the mid-lithospheric discontinuity (MLD) and the LAB of the craton. In the northern and southern United States the western LAB almost reaches the mid-continental rift system. In between these two regions the cratonic MLD is surprisingly plunging towards the west from the Rocky Mountain Front to about 200 km depth near the Sevier thrust belt. We interpret these complex structures of the seismic discontinuities in the mantle lithosphere as an indication of interfingering of the colliding Farallon and Laurentia plates. Unfiltered S-receiver function data reveal that the LAB and MLD are not single discontinuities but consist of many small-scale laminated discontinuities, which only appear as single discontinuities after longer period filtering. We also observe the Lehmann discontinuity below the LAB and a velocity reduction about 30 km above the 410 km discontinuity.

Synorogenic extension localized by upper-crustal thickening: An example from the Late Cretaceous Nevadaplano

Synconvergent extension within orogenic systems is often interpreted as gravitational spreading of thickened crust or as a response to thrust belt dynamics. However, the processes that spatially localize extension during orogenesis are not fully understood. Here, a case study from the United States Cordillera demonstrates that localized upper-crustal thickening can exert a first-order control on the spatial location of synorogenic extension. The Eureka culmination, a 20-km-wide, north-trending anticline with 4.5 km of structural relief in the hinterland plateau of the Sevier orogenic belt (or “Nevadaplano”) in eastern Nevada was deformed by two sets of north-striking normal faults that pre-date late Eocene volcanism. (U-Th)/He and fission-track thermochronology data collected from Paleozoic quartzite in the footwalls of two normal faults demonstrate rapid (10 °C/m.y.), Late Cretaceous to Paleocene (75–60 Ma) cooling, which we interpret as tectonic exhumation during extension, and which was concurrent with late-stage shortening in the frontal Sevier thrust belt. This example illustrates that structural and topographic relief generated within zones of localized upper-crustal thickening can spatially focus extension during orogenesis, and adds to a growing body of evidence that Late Cretaceous–Paleocene extension in the Nevadaplano occurred at both mid- and upper-crustal levels.

An upper-crustal fold province in the hinterland of the Sevier orogenic belt, eastern Nevada, U.S.A.: A Cordilleran Valley and Ridge in the Basin and Range

Overprinting Cenozoic extension hinders analysis of Cordilleran contractional deformation in the hinterland of the Sevier thrust belt in Nevada. In this study, a 1:250,000 scale paleogeologic map of eastern Nevada, showing spatial distributions of Paleozoic–Mesozoic rocks beneath a Paleogene unconformity, is combined with dip magnitude maps for Paleozoic–Mesozoic and Tertiary rocks, published sedimentary thickness records, and a published reconstruction of extension, in order to define and regionally correlate thrust faults and folds, and estimate the pre-extensional amplitude, wavelength, and limb dips of folds. A new structural province, the Eastern Nevada fold belt, is defined, consisting of a 100-km-wide region containing five first-order folds that can be traced for map distances between 100 and 250 km and have amplitudes of 2–4 km, wavelengths of 20–40 km, pre-extensional limb dips typically between 10° and 30°, and deform rocks as young as Jurassic and Early Cretaceous. No regional-scale thrust faults or decollement horizons breach modern exposure levels in the Eastern Nevada fold belt. First-order folds of the Eastern Nevada fold belt are interpreted to have formed above a deep (≥10 km below the Paleogene unconformity), blind decollement or shear zone, perhaps the westward projection of the master decollement of the Sevier thrust belt. Three hinterland structural provinces, the Central Nevada thrust belt, Western Utah thrust belt, and the intervening Eastern Nevada fold belt, collectively record low-magnitude (a few tens of kilometers), upper-crustal shortening that accompanied Cretaceous translation of the Cordilleran passive-margin basin ∼220 km eastward during the Sevier orogeny. Low deformation magnitudes in the hinterland are attributed to the rheological competence of this thick basin.

Isotope hydrology of early paleogene Lake Flagstaff, central Utah: Implications for Cordilleran evolution

Lacustrine sediments from Paleogene foreland basins of the North American Cordillera serve as powerful integrators of information on climatic and tectonic conditions, but episodes of hydrologic connection between basins may confound straightforward interpretations. We use stable isotope ratios of carbon and oxygen in authigenic carbonates of the Flagstaff Formation in central Utah to investigate the history of the lake system which produced these sediments. Flagstaff limestone and dolostone exhibit isotopic compositions consistent with having formed from the same fluid at surface conditions, indicating high evaporative concentration was unnecessary to drive dolomite precipitation. Isotopic data are consistent with the Flagstaff Formation having been produced by a large lake which occupied its basin from the late Paleocene into the early Eocene. Lake Flagstaff was hydrologically distinct from Lake Uinta to the north, but may have been connected to Lake Claron in the south. Paleoaltimetry estimates suggest that the mean elevation of Lake Flagstaff9s catchment in the Sevier Highlands did not exceed 1.5 km. The growing evidence from stable isotope data that Cordilleran elevations were relatively low in the early Paleogene is in stark contrast to structural and sedimentological evidence for a high Nevadaplano and a topographically rugged Sevier thrust belt.

Isotope Hydrology of Early Paleogene Lake Flagstaff, Central Utah: Implications for Cordilleran Evolution

Lacustrine sediments from Paleogene foreland basins of the North American Cordillera serve as powerful integrators of information on climatic and tectonic conditions, but episodes of hydrologic connection between basins may confound straightforward interpretations. We use stable isotope ratios of carbon and oxygen in authigenic carbonates of the Flagstaff Formation in central Utah to investigate the history of the lake system which produced these sediments. Flagstaff limestone and dolostone exhibit isotopic compositions consistent with having formed from the same fluid at surface conditions, indicating high evaporative concentration was unnecessary to drive dolomite precipitation. Isotopic data are consistent with the Flagstaff Formation having been produced by a large lake which occupied its basin from the late Paleocene into the early Eocene. Lake Flagstaff was hydrologically distinct from Lake Uinta to the north, but may have been connected to Lake Claron in the south. Paleoaltimetry estimates suggest that the mean elevation of Lake Flagstaff9s catchment in the Sevier Highlands did not exceed 1.5 km. The growing evidence from stable isotope data that Cordilleran elevations were relatively low in the early Paleogene is in stark contrast to structural and sedimentological evidence for a high Nevadaplano and a topographically rugged Sevier thrust belt.

A comparison of the effectiveness of clast based finite strain analysis techniques to AMS in sandstones from the Sevier Thrust Belt, Wyoming

Geological strain analysis of sedimentary rocks is commonly carried out using clast-based techniques, which rely on assumptions regarding clast shapes and orientations as well as distances between clast centres. To evaluate the accuracy of these methods in low strain regimes we have compared strain analysis estimates to the anisotropy of magnetic susceptibility (AMS) in sandstone samples from the Wyoming salient. The Cordilleran Mountain Belt of North America is one of the world's classic foreland fold and thrust belts. The Sevier Belt represents the thin-skinned front of this orogenic event characterized by thrust faults and folds that shortened and translated sequences of Devonian to Cretaceous strata eastward. Deformation increases westward providing an ideal laboratory and geological setting to explore the potential correlation of anisotropy of magnetic susceptibility (AMS) to clast-based strain analyses. Studies attempting to define the relationship between AMS and finite strain have been in vogue since the link between layer parallel shortening and AMS was first established. This relationship, despite proven strong correlations between the AMS tensors and tectonic directions, is generally complicated by competing sub-fabrics as well as the various magnetic anisotropy properties of the minerals contributing to the AMS fabric. Even with these complications, we conclude that AMS is much more sensitive to incipient tectonic fabrics than clast based methods.

Fluvial deposition during transition from flexural to dynamic subsidence in the Cordilleran foreland basin: Ericson Formation, Western Wyoming, USA

AbstractThe Ericson Formation was deposited in the distal foredeep of the Cordilleran foreland basin during Campanian time. Isopach data show that it records early dynamic subsidence and the onset of basin partitioning by Laramide uplifts. The Ericson Formation is well exposed around the Rock Springs uplift, a Laramide structural dome in southwestern Wyoming; the formation is thin, regionally extensive, and does not display the wedge‐shaped geometry typical of foredeep deposits. Sedimentation in this area was controlled both by activity in the thrust belt and by intraforeland tectonics. The Ericson Formation is ideally situated both spatially and temporally to study the transition from Sevier to Laramide (thin‐ to thick‐skinned) deformation which corresponded to the shift from flexural to dynamic subsidence and the demise of the Cretaceous foreland basin system. We establish the depositional age of the Ericson Formation as ca. 74 Ma through detrital zircon U–Pb analysis. Palaeocurrent data show a generally southeastward transport direction, but northward indicators near Flaming Gorge Reservoir suggest that the intraforeland Uinta uplift was rising and shedding sediment northward by late Campanian time. Petrographic data and detrital zircon U–Pb ages indicate that Ericson sediment was derived from erosion of Proterozoic quartzites and Palaeozoic and Mesozoic quartzose sandstones in the Sevier thrust belt to the west. The new data place temporal and geographic constraints on attempts to produce geodynamic models linking flat‐slab subduction of the oceanic Farallon plate to the onset of the Laramide orogenic event.

The lithosphere–asthenosphere boundary and the tectonic and magmatic history of the northwestern United States

This study explores the properties of the lithosphere–asthenosphere boundary (LAB) and other shallow mantle discontinuities across the diverse geologic provinces of the northwest United States. Sp phases were used to image three-dimensional discontinuity structure by common conversion point stacking with data from 804 temporary and permanent broadband stations, including 247 from EarthScope's USArray Transportable Array. Substantial variations in mantle discontinuity structure are apparent over a variety of spatial and temporal scales. To the west of the Sevier Thrust Belt, a coherent Sp negative phase coincides with the LAB depth range inferred from tomographic models. To the east, within the stable craton, multiple negative phases typically occur in the high velocity lithospheric layer, although in places a single mid-lithospheric discontinuity is imaged. Sub-cratonic LAB phases are often absent, indicating an LAB velocity gradient that is distributed over >50 km and that is consistent with effects of temperature alone. Where weak and intermittent LAB phases appear, they suggest more vertically localized velocity gradients produced by other factors such as bulk composition, volatile content, or contrasts in grain size or melt. In the tectonically active west, a positive Sp phase at depths consistent with the base of the asthenospheric low velocity zone in tomography models is intermittently observed. Beneath magmatic provinces in the west, the LAB Sp discontinuity deepens by ∼10 km from the High Lava Plains, where magmatism has occurred from 0–10.5 Ma, to the northern region of the Columbia River Basalts, which has been magmatically quiet since 15 Ma. Here we suggest a model in which the negative LAB velocity gradient is created by a layer of partial melt ponding beneath a solidus-defined boundary. This model predicts that higher temperatures associated with more recent magmatism would result in a shallowing of the intersection of the geotherm with the solidus. Beneath the Yellowstone Caldera, the absence of an LAB Sp phase suggests that the contrast in seismic velocity between the lithosphere and asthenosphere has been erased by intrusion of partial melt and heat into the lithosphere.

Detrital zircon geochronology of Cordilleran retroarc foreland basin strata, western North America

AbstractWe present a compilation of 8717 U‐Pb analyses from 95 detrital zircon samples of Jurassic‐Eocene North American Cordillera foreland basin strata. Of these samples, 30 are new and previously unpublished. Variation in detrital zircon age spectra between samples records erosion or recycling of basement and cover rocks within the Cordilleran orogenic wedge. Each sample can be classified into one of six major provenance groups, whose age spectra suggest derivation from (1) Mesozoic eolianites of the western United States, (2) Paleozoic passive margin strata of the western United States, (3) Paleozoic passive margin strata of western Canada, (4) the Mogollon Highlands, (5) the Cordilleran magmatic arc, or (6) Yavapai‐Mazatzal Province crystalline basement rocks. Referencing these provenance interpretations to their location and stratigraphic deposition age produces a detailed spatial and temporal record of sediment dispersal within the foreland basin system. Late Jurassic provenance is dominated by recycling of Mesozoic eolianites from sources in the Sevier thrust belt. Cretaceous‐Eocene provenance is dominated by recycling of the passive margin, with increasing complexity upsection. We interpret that this provenance transition records a basin‐wide unroofing sequence. A composite age‐probability plot of 1539 young (<250 Ma) detrital zircons reveals at least four age‐abundance peaks that we interpret to represent periodic high‐flux magmatism in the Cordilleran arc.

Oroclines: Thick and thin

An orocline is a thrust belt or orogen that is curved in map-view due to it having been bent or buckled about a vertical axis of rotation. Two distinct types of oroclines are recognized: progressive and secondary. Progressive oroclines are restricted to the scale of a thrust sheet to thrust belt, are thin-skinned, and develop during thrust sheet emplacement. Secondary oroclines are larger, occurring at the scale of an orogen, and are plate-scale features that affect crust and lithospheric mantle. Unlike progressive oroclines, which develop during initial orogenesis and in response to the same orogen-perpendicular stress responsible for thrust sheet emplacement, secondary oroclines are extra-orogenic, developing after initial orogenesis and in response to an orogen-parallel principal compressive stress that is oriented at a high angle to the stress responsible for orogen development. We present case studies of the Wyoming Salient, a progressive orocline that characterizes the Sevier thrust belt of the western United States, and the coupled Cantabrian and Central Iberian oroclines, which are linked secondary oroclines affecting the Variscan orogen of Iberia. The vertical-axis rotations involved in progressive and secondary orocline formation are most readily quantified through paleomagnetic analysis. Detailed three-dimensional palinspastic restoration that incorporates translation rotation and strain can distinguish the role, if any, of primary curvature in progressive oroclines. The use of tectonic vectors, such as paleocurrent directions, provides a means of recognizing and characterizing the initial geometry of secondary oroclines. Because secondary oroclines involve the entire lithosphere, detailed studies of coeval metamorphism and magmatism provide a means of constraining the fate of the mantle lithosphere during oroclinal buckling.

Provenance of the Paleogene Colton Formation (Uinta Basin) and Cretaceous–Paleogene provenance evolution in the Utah foreland: Evidence from U-Pb ages of detrital zircons, paleocurrent trends, and sandstone petrofacies

The fluviodeltaic Colton Formation (Late Paleocene–Early Eocene) forms a lobate depositional system that prograded from the south into the Laramide Uinta Basin of northeastern Utah (United States) with a preserved sediment volume of ∼3000 km3 and a maximum thickness of ∼1000 m. Joint consideration of detrital zircon ages, paleocurrent trends, and sandstone petrofacies permits an assessment of Colton provenance relations in the context of evolving Cretaceous–Paleogene sedimentation in the Utah foreland. Grains with U-Pb ages younger than 285 Ma derived from the Cordilleran magmatic arc form ∼50% of the detrital zircons in arkosic Colton sand, and were transported ∼750 km to the Uinta Basin from the Mojave segment of the arc by the California paleoriver. Colton sedimentation was the Paleogene culmination of a persistent pattern of Cretaceous sediment transport northward, subparallel to the Sevier thrust front, to supplement east-directed sediment delivery to the retroarc foreland from the Sevier thrust belt. The ratio of longitudinally to transversely derived sediment was enhanced in foreland strata after Laramide deformation produced intraforeland uplifts that screened the foreland belt from Sevier sources. The relative abundance of arc-derived detrital zircons that were contributed to strata of the Utah foreland increased in late Campanian time and remained high into Eocene time. Detrital zircon populations in Paleogene forearc strata of southern California are compatible with coeval derivation of arc-derived detritus in the forearc sands and the Colton backarc sand from a common paleodrainage divide crossing the Mojave region to connect hinterland Nevadaplano and Mexicoplano uplands to the north and south.

Regional structure and kinematic history of the Cordilleran fold-thrust belt in northwestern Montana, USA

The Cordilleran thrust belt of northwestern Montana (United States) has received much less attention than its counterparts in the western interior of USA and Canada. The structure of the thrust belt in this region is well preserved and has not been strongly overprinted by Cenozoic extension, providing an opportunity to reconstruct its geometry and to relate it to the foreland basin system. The thrust belt in this region consists of a frontal part of highly deformed Paleozoic, Mesozoic, and Paleocene sedimentary rocks, and a western region dominated by a >15-km-thick succession of Proterozoic Belt Supergroup strata underlain by faults of the Lewis thrust system. The frontal part can be subdivided into the foothills and the Sawtooth Range. At the surface, the foothills show deformed Mesozoic and Paleocene rocks; at depth, refl ection seismic data indicate numerous thrust faults carrying Paleozoic strata. The Sawtooth Range, south from the Lewis thrust salient, is defi ned by steeply dipping imbricate thrusts that detach at the basal Cambrian stratigraphic level. The Sawtooth Range plunges northward beneath the Lewis thrust salient and diverges into a pair of independent thrust systems that form the Flathead and Waterton duplexes in Canada. The relatively minor internal deformation in the western part of the thrust belt resulted from the great rheological strength of the Belt Supergroup rocks and initial high taper of the preorogenic stratigraphic wedge. A new ~145-km-long balanced cross section indicates ~135 km of shortening, a value similar to that in the southern part of the Canadian thrust belt. Previous work and new conventional and isotopic provenance data from the foreland basin and U-Pb ages from crosscutting intrusive rocks establish a preliminary kinematic model for this segment of the Cordilleran thrust belt. The emerging pattern is a relatively simple forelandward progression of thrusting events. Most shortening in the Lewis thrust system, Sawtooth Range, and foothills occurred roughly between mid-Campanian and Early Eocene time (ca. 75‐52 Ma), yielding a shortening rate of ~5.9 mm/yr. This pattern differs from the pattern of shortening in the better known Sevier thrust belt to the south, where regional far-traveled Proterozoic quartzitebearing thrust sheets were mainly active during Early Cretaceous time. From Middle Eocene to Early Miocene time, this sector of the Cordillera collapsed, generating a number of extensional depocenters.

Magnitudes and spatial patterns of erosional exhumation in the Sevier hinterland, eastern Nevada and western Utah, USA: Insights from a Paleogene paleogeologic map

The hinterland of the Sevier orogenic belt in Nevada and western Utah (United States) has been interpreted as an ancient high-elevation orogenic plateau, or Nevadaplano, that collapsed extensionally during Tertiary time. To illustrate the preextensional structural relief of this region, a new paleogeologic (or subcrop) map showing the distribution of Neoproterozoic to Triassic rocks exposed beneath a regional Paleogene unconformity is presented here. The map area extends between the traces of the westernmost major Sevier thrust system and the Roberts Mountains thrust. Three across-strike subcrop domains can be defined: (1) Cambrian to Mississippian subcrop levels in the leading part of the Delamar–Wah Wah–Canyon Range (DWC) thrust sheet, indicating high (as much as 7 km) structural relief; (2) a broad region of eastern Nevada and westernmost Utah devoid of surface-breaching thrust faults, with Mississippian–Triassic subcrop levels, indicating low (2 km) structural relief; and (3) subcrop levels varying between Neoproterozoic and Permian in the central Nevada thrust belt (CNTB), indicating high (as much as 8 km) structural relief. Using published isopach maps of depositional thickness of Neoproterozoic to Triassic sedimentary rocks, the paleogeologic map has been converted into a map that contours the thickness of rock eroded between the top of the Triassic section and the Paleogene subcrop level, and thus illustrates magnitudes and spatial patterns of synorogenic erosional exhumation. Characteristic exhumation magnitudes for the three subcrop domains are: (1) 4–8 km in the leading part of the DWC thrust sheet; (2) 4–10 km, 2 km, and 4–6 km in the southern, central, and northern parts of the CNTB, respectively; and (3) 1–3 km in the intervening low-relief region. Isolated exposures of southern CNTB structures can be correlated by their subcrop and exhumation patterns into two through-going thrust systems that connect with structures of the Sevier thrust belt in southern Nevada. This supports previously suggested correlations, and implies a direct structural link between these two thrust systems. Subcrop and exhumation patterns do not reveal a surface-breaching thrust trace that would represent a southern continuation of the Windermere thrust south of the Pequop or Ruby Mountains. Thus, if the Windermere thrust model is correct, this implies either termination at a lateral structure such as a tear fault, or a transition to a blind geometry. The ∼2 km structural relief that characterizes much of the Sevier hinterland indicates that the majority of high-magnitude (>1–2 km throw), regionally distributed, surface-breaking normal faulting that dismembered the orogenic highland and produced the high structural relief observed today had to be post-Oligocene. Although the traces of 70 normal faults that are overlapped by the Paleogene unconformity are identified, the throw on nearly all of these structures is limited to a maximum of 1–2 km. This further highlights the paradox of extensive Late Cretaceous and Paleogene exhumation of mid-crustal rocks now exposed in metamorphic core complexes without corresponding high-magnitude upper crustal extension. The close spatial association of high exhumation magnitudes with the hanging walls of major thrust faults suggests that erosional exhumation is a response to relief generation accompanying contractional deformation in orogenic plateaus. The lack of significant along-strike exhumation variability within much of the Sevier hinterland implies that relief generation was relatively uniform along strike. The steep across-strike exhumation gradient between the leading part of the DWC sheet and the low-exhumation region to the west is interpreted as the result of shielding of the western area from headward erosion by long-lived uplift, erosion, and relief development through passive eastward translation of the DWC thrust sheet and growth of antiformal culminations at depth. A significant difference between the Sevier and central Andean orogenic plateaus is emphasized here. Much of the interior part of the Sevier plateau was an eroding highland, composed of preorogenic rocks, with low structural relief, while the interior part of the Andean plateau consists of a variably deformed and exhumed, synorogenic hinterland basin as much as 12 km thick, with very high structural relief. Thus, comparison of the structural level of the stratigraphic contact between preorogenic and synorogenic rocks indicates a difference in rock uplift between the interior parts of these two plateaus of as much as 14–15 km.