Sevier Belt Research Articles

The real McCoy: A record of deep‐water basin deposition in southwestern North America during the Cretaceous

AbstractThe McCoy Mountains Formation (McMF) in southern California–Arizona preserves an anomalously thick record of sedimentation during the Mesozoic at a critical time when western North America experienced contrasting tectonic events related to intracontinental rifting along the Mexican Border rift system and consolidation of the North American Cordilleran system. The spaciotemporal interactions among these events and the development of the McCoy basin challenge our understanding of the evolution of the southern extent of North America. At its type locality in the McCoy Mountains, the McMF consists of ~ 7 km of low‐grade metasedimentary rocks, originally interpreted as meandering fluvial to alluvial‐fan deposits. Uncertainty in the initial timing of sedimentation in the McCoy basin has resulted in multiple tectonic models. We measured ~ 7160 m of detailed stratigraphy and present new sedimentological and detrital zircon results showing that the McCoy basin was occupied by deep‐water turbidite systems. These systems deposited an upward‐coarsening succession of fine‐ to coarse‐grained detritus during the Cretaceous (ca. 137–70 Ma). Provenance data indicate that the McCoy basin received sediment from Proterozoic basement rocks and metamorphosed Palaeozoic to early Mesozoic sedimentary units. These source rocks are equivalent to the stratigraphy found in the Grand Canyon and Colorado Plateau regions and were likely shed from the southward‐advancing Maria fold‐thrust belt and possibly the southern Sevier belt in southern Nevada and California. These results, combined with subsidence curves typical of foreland basins, favour deposition within a subaqueous flexural foreland basin system. The presence of a Cretaceous foreland basin this far southwest challenges previously proposed models and suggests that the contractional tectonic regime associated with the North American Cordillera extended into the southwestern most United States during the Early–Late Cretaceous.

Middle Albian provenance, sediment dispersal and foreland basin dynamics in southwestern Montana, North American Cordillera

AbstractThe Lower Cretaceous Blackleaf Formation in southwestern Montana records sedimentation in the Idaho‐Montana retroforeland basin of the North American Cordillera. Regional‐scale sedimentology suggests that during Albian time southwestern Montana was partially flooded by an early marine incursion of the Western Interior Seaway during deposition of the Blackleaf Formation. We use sandstone petrography, large‐n (n = 600) U‐Pb detrital zircon geochronology and mixture modelling to determine the provenance of these strata. Our analysis suggests three distinct provenance groups: Group 1 sandstones occur in the eastern region of the study area, are quartz‐rich and have zircon age‐probability peaks of ca. 110, 160, 420–450, 1050 and 1160 Ma; these sandstones match with a primarily Appalachian provenance. Group 2 sandstones occur in the western region of study area, are lithic‐rich and have peaks of ca. 110, 160, 1780, 1840, 1920, 2080 and 2700 Ma; the primary source for these sandstones was exhumed lower‐middle Palaeozoic strata from the Idaho sector of the Sevier belt. Group 3 sandstones occur in the western region of the study area, are lithic‐rich and have prominent peaks of ca. 115, 170, 430, 600, 1085, 1170, 1670 and 1790 Ma; the primary source for these sandstones was exhumed Triassic‐upper Palaeozoic strata from the Idaho sector of the Sevier belt. Our provenance data record a sharp change that coincides with the western shoreline of the seaway, and we infer that it may indicate the position of an irregular, submarine forebulge depozone influenced by dynamic subsidence during a period of reduced thrusting in the adjacent fold‐thrust belt. Albian‐aged sediments in southwestern Montana were delivered by rivers with headwaters in the Sevier belt as well as transcontinental river systems with headwaters in eastern North America. In southwestern Montana, west‐flowing transcontinental fluvial systems were flooded by the Western Interior Seaway as it encroached from the north.

Proterozoic to Phanerozoic Tectonism in Southwestern Montana Basement Ranges Constrained by Low Temperature Thermochronometric Data

AbstractCrystalline basement rocks of southwestern Montana have been subjected to multiple tectonothermal events since ∼3.3 Ga: the Paleoproterozoic Big Sky/Great Falls orogeny, Mesoproterozoic extension associated with Belt‐Purcell basin formation, Neoproterozoic extension related to Rodinia rifting, and the late Phanerozoic Sevier‐Laramide orogeny. We investigated the long‐term (>1 Ga), low‐temperature (erosion/burial within 10 km of the surface) thermal histories of these tectonic events with zircon and apatite (U‐Th)/He thermochronology. Data were collected across nine sample localities (n = 55 zircon and n = 26 apatite aliquots) in the northern and southern Madison ranges, the Blacktail‐Snowcrest arch, and the Tobacco Root uplift. Our zircon (U‐Th)/He data show negative trends between single aliquot date and effective uranium (a radiation damage proxy), which we interpreted with a thermal history model that considers the damage‐He diffusivity relationship in zircon. Our model results for these basement ranges show substantial cooling from temperatures above 400°C to near surface conditions between 800 and 510 Ma. Subsequent Phanerozoic exhumation culminated by ∼75 Ma. Late Phanerozoic cooling is coincident with along‐strike Sevier belt thin‐skinned thrusting in southeastern Idaho, and older than exhumation in basement‐involved uplifts of the Wyoming Laramide province. Our long‐term, low‐temperature thermal record for these southwestern Montana basement ranges shows that: (a) these basement blocks have experienced multiple episodes of upper crustal exhumation and burial since Archean time, possibly influencing Phanerozoic thrust architecture and (b) the late Phanerozoic thick‐skinned thrusting recorded by these rocks is among the earliest thermochronologic records of Laramide basement‐involved shortening and was concomitant with Sevier belt thin‐skinned thrusting.

Early Cretaceous Provenance, Sediment Dispersal, and Foreland Basin Development in Southwestern Montana, North American Cordillera

AbstractThe Lower Cretaceous Kootenai Formation in western Montana records the onset of foredeep sedimentation in the Idaho‐Montana retroforeland basin of the North American Cordillera. Contrasting models for sediment dispersal during Early Cretaceous time have been proposed. We use sandstone petrography, large‐n (n = 600) detrital zircon U‐Pb geochronology, and mixture modeling to determine the provenance of the Kootenai Formation. These new data combined with additional data from the literature suggest three discrete provenance signatures. Type I sandstones are quartz‐rich with U‐Pb age peaks at ca. 158, 238, 410, 600, and 1,035 Ma; they were sourced from exhumed Jurassic continental strata and transported northward by axial drainages. Type II sandstones are lithic‐rich with U‐Pb age peaks at ca. 154, 166, 1,840, 1,920, 2,080, and 2,700 Ma; they were deposited by transverse rivers that drained exhumed lower‐middle Paleozoic strata from the Sevier belt. Type III sandstones contain feldspars and minor volcanic lithics with U‐Pb age spectra that are dominated by ca. 112 and 162 Ma ages; they were transported by transverse drainages that connected the hinterland of the orogen with the western Montana foreland. Based on provenance analysis of the Kootenai Formation, subtle Early Cretaceous thick‐skinned deformation in southwestern Montana did not exhume basement nor did it partition the foreland into locally sourced depocenters. Sediment sources for the western Montana foreland are consistent with those to the north in Alberta and different than southern Idaho. These data also provide insight into the translational history of terranes in the hinterland of the orogen during this time.

Groundwater mixing in an alkaline paleolake: Eocene Green River Formation, Wyoming

Tufa in the Little Mesa area of the northern Bridger Basin has been interpreted to record carbonate deposition via subaqueous and subaerial springs emanating near the shoreline of Eocene Lake Gosiute. Sedimentary facies record an overall transgression, culminating with mound structures that reach up to 9 m in height and 40 m in diameter. Mounds exhibit a strong positive, linear covariance between δ13C and δ18O, defining a slope of ~1. Similar trends occur in many other paleolake deposits, where they are interpreted to reflect changes in evaporation, atmospheric CO2 exchange, and organic matter burial. However, δ13C and δ18O in this study also covary strongly with 87Sr/86Sr, a new finding that is inconsistent with previously proposed mechanisms. We conclude that Little Mesa isotopic trends reflect mixing of groundwater with low 87Sr/86Sr, δ18O and δ13C and lake water with opposite characteristics. Low 87Sr/86Sr in groundwater likely resulted from interaction with marine carbonate strata within the Sevier fold and thrust belt to the west, whereas drainage from Precambrian-cored uplifts that bounded Lake Gosiute to the north, east, and south was responsible for higher lake water ratios.Little Mesa carbonate facies are all less radiogenic than any time-equivalent facies near the center of the basin, implying horizontal and vertical gradients in Lake Gosiute 87Sr/86Sr. Previous studies have shown that the lowest 87Sr/86Sr in basin center deposits correspond to lake highstands. Results of this study support the hypothesis that climatic modulation of surface runoff and spring emanations from the Sevier belt were principally responsible for precessional-scale expansions and contractions of Lake Gosiute. More broadly, groundwater discharge may represent an important but underappreciated contributor to covariance between 87Sr/86Sr ratios, δ13C and δ18O in closed paleolake systems.

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.

Pressure-temperature-time paths from the Funeral Mountains, California, reveal Jurassic retroarc underthrusting during early Sevier orogenesis

Abstract New metamorphic pressure-temperature (P-T) paths and Lu-Hf garnet ages reveal a temporal correlation between Middle to Late Jurassic retroarc underthrusting and arc magmatism in southwestern North America. P-T paths were determined for 12 garnet porphyroblasts from six samples from the Chloride Cliff area of the Funeral Mountains in southeastern California. The composite path shows a pressure increase from 4.2 to 6.5 kbar as temperature increased from 550 to 575 °C, followed by a pressure decrease to 5.1 kbar during a further increase in temperature to 590 °C. Lu-Hf garnet ages from a pelitic schist (167.3 ± 0.7 Ma) and a garnet amphibolite (165.1 ± 9.2 Ma) place these P-T paths in the Middle Jurassic. We interpret the near-isothermal pressure increase portion of the P-T path to have developed during thrust-related burial, similar to lower grade rocks at Indian Pass, 8 km to the southeast, where garnet P-T paths show a pressure increase dated by the Lu-Hf method at 158.2 ± 2.6 Ma. We interpret the pressure decrease portion of the composite P-T path from the Chloride Cliff area to reflect exhumation contemporaneous with cooling in the Indian Pass area documented from muscovite 40Ar/39Ar step-heating ages of 152.6 ± 1.4 and 146 ± 1.1 Ma. The conditions and timing of metamorphism determined for the Indian Pass and Chloride Cliff areas, and isogradic surfaces that cut across stratigraphy, support the interpretation that the strata were dipping moderately NW during metamorphism, parallel to the thrust ramp that buried the rocks. Burial likely resulted from top-SE motion along the Funeral thrust, which was later reactivated as a low-angle normal fault with opposite motion to become the currently exposed Boundary Canyon detachment that was responsible for Miocene and possibly older exhumation. The part of the burial history captured by garnet growth occurred ∼6 m.y. before the 161 Ma peak of high-flux magmatism in the arc. Burial was contemporaneous with metamorphic ages from the western Sierra Nevada metamorphic belt, with the possible timing of accretion of arc terranes in northern California, and with the initiation of Franciscan subduction. Burial ages are also similar in timing with generally E-W crustal shortening in the retroarc that produced the East Sierra thrust system, the Luning-Fencemaker fold and thrust belt, the possible early history of the Central Nevada thrust belt, and the western thrusts of the southern Sevier belt. The timing of tectonic burial documented in this study and of high-flux magmatism in the arc supports the interpretation that the development of a coherent arc-trench system in the Early Jurassic resulted in the underthrusting of melt-fertile material beneath the arc along west- to northwest-dipping faults such as the Funeral thrust in the Jurassic, which penetrated the basement to the west as well as the roots of the magmatic arc, leading to increased magmatism.

Surface fluids in the evolving Sevier fold–thrust belt of ID–WY indicated by hydrogen isotopes in dated, authigenic clay minerals

Hydrogen isotopic composition (δD) and 40Ar/39Ar geochronological data of neomineralized clay minerals from fault rocks and cleavage seams in the Sevier belt record primary involvement of surface-sourced fluids during Cretaceous–Paleogene mountain building events. δD values of authigenic clays range from −78.8±1.7‰ to −97.2±3.2‰. Given temperature constraints of mineral formation, we calculate δD values of mineralizing fluids as ∼−63 to −96‰, which fall within the range of δD values of Cretaceous–Paleogene surface waters in the region. The potential infiltration and presence of surface fluids at depths of several kilometers during deformation implies that pore spaces and fractures acted as conduits for surface water. Uplift of thrust sheets generated topography that favored meteoric recharge and flow of fluids into margins of the foreland basin, which were subsequently incorporated into the propagating thrust wedge. This is in contrast to the hypothesis that the release of overpressured metamorphic fluids dominates during fold–thrust belt evolution. As a result of shortening and tectonic uplift, fluid flows through basin sediments toward the foreland basin as meteoric waters recharge sediments at the basin margins. This study shows that stable isotopic compositions of shallow, deformation-related clay minerals are robust and may be used alongside other surface fluid proxies to probe the origin and composition of ancient fluids in orogenic systems.

Thick-skinned tectonics in a Late Cretaceous-Neogene intracontinental belt (High Atlas Mountains, Morocco): The flat-ramp fault control on basement shortening and cover folding

Most of the structural studies of the intracontinental High Atlas belt of Morocco have dealt with the central part of the belt, whose basement does not crop out. Here we study the Alpine deformation of the North Subatlas Zone, which is the part of the Western High Atlas (WHA) Paleozoic Massif that involves both Paleozoic basement units and remnants of their Mesozoic-Cenozoic cover formations. Our aim is to better constrain the geometry and kinematics of the basement faults during the Alpine shortening. Based on detail mapping, satellite imagery and field observations, we describe an array of sub-equatorial, transverse and oblique faults between the WHA Axial Zone and the Haouz Neogene basin. They define a mosaic of basement blocks pushed upon one another and upon the Haouz basement along the North Atlas Fault (NAF). The Axial Zone makes up the hanging-wall of the Adassil-Medinet Fault (AMF) south of this mosaic. The faults generally presents flat-ramp-flat geometry linked to the activation of multiple décollement levels, either within the basement where its foliation is subhorizontal or within favourable cover formations (Jurassic evaporites, Lower Cretaceous silty red beds, Upper Cretaceous evaporitic marls, Neogene basal argillites). The occurrence of the North Atlas detachment (NAD) allowed folded pop-up units to develop in front of the propagating NAF. Shortening began as early as the Campanian-Maastrichtian along the AMF. The direction of the maximum horizontal stress rotated from NNE-SSW to NNW-SSE from the Maastrichtian-Paleocene to the Neogene. The amount of shortening reaches 20% in the Azegour transect. This compares with the shortening amount published for the central-eastern High Atlas, suggesting that similar structures characterize the Paleozoic basement all along the belt. The WHA thick-skinned tectonics evokes that of the frontal Sevier belt and of the external Western Alps, although with a much minor pre-inversion burial.

Timing of exhumation, Wheeler Pass thrust sheet, southern Nevada and California: Late Jurassic to middle Cretaceous evolution of the southern Sevier fold-and-thrust belt

Zircon (U-Th)/He (ZHe) thermochronologic data for 46 samples from the Wheeler Pass thrust sheet, which carries thick passive-margin strata and is discontinuously exposed across the southern part of the Sevier fold-and-thrust belt, record Late Jurassic cooling related to exhumation during early thrust slip, and local middle Cretaceous cooling related to footwall basement imbrication. Within the frontal part of the Wheeler Pass sheet exposed in the northwest Spring Mountains, ZHe ages decrease from ca. 160 to 140 Ma over a paleodepth interval of ∼6 to 9 km in Devonian to Lower Cambrian strata, interpreted to record exhumation and enhanced cooling above a frontal ramp, followed by slow cooling. A similar pattern of ZHe ages occurs over a more limited paleodepth range exposed within the Resting Spring Range. To the south in the Nopah Range, ZHe ages of ca. 140 Ma in Lower Cambrian strata record early cooling, followed by slow cooling until ca. 100 Ma. ZHe ages in underlying Paleoproterozoic basement rocks decrease from ca. 100 to 85 Ma over a paleodepth interval of ∼9 to 12 km, interpreted to record uplift, exhumation, and cooling during footwall basement imbrication as slip was transferred eastward onto the Keystone thrust system. Late Jurassic slip on the Wheeler Pass thrust overlapped with deformation along the NW-trending East Sierran thrust system, early hinterland crustal thickening, and growth of the Sierra magmatic arc, and may have been correlative with early deformation in the Central Nevada thrust belt. Slip on the Wheeler Pass thrust preceded major slip on other thrust sheets that carried thick passive-margin strata in more northern parts of the Sevier belt, probably reflecting the influence of different initial widths and tapers of the passive-margin sedimentary wedge.

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.

Sevier belt exhumation in central Utah constrained from complex zircon (U-Th)/He data sets: Radiation damage and He inheritance effects on partially reset detrital zircons

Research Article| March 01, 2015 Sevier belt exhumation in central Utah constrained from complex zircon (U-Th)/He data sets: Radiation damage and He inheritance effects on partially reset detrital zircons William R. Guenthner; William R. Guenthner 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA †Current address: Department of Geology, University of Illinois, Urbana-Champaign, Champaign, Illinois 61820, USA; wrg@illinois.edu. Search for other works by this author on: GSW Google Scholar Peter W. Reiners; Peter W. Reiners 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar Peter G. DeCelles; Peter G. DeCelles 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar Jerry Kendall Jerry Kendall 2ExxonMobil Exploration Company, Houston, Texas 77060, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2015) 127 (3-4): 323–348. https://doi.org/10.1130/B31032.1 Article history received: 10 Nov 2013 rev-recd: 25 Apr 2014 accepted: 29 Jul 2014 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation William R. Guenthner, Peter W. Reiners, Peter G. DeCelles, Jerry Kendall; Sevier belt exhumation in central Utah constrained from complex zircon (U-Th)/He data sets: Radiation damage and He inheritance effects on partially reset detrital zircons. GSA Bulletin 2015;; 127 (3-4): 323–348. doi: https://doi.org/10.1130/B31032.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Thermochronologic dates of partially reset detrital grains are often difficult to interpret because of the potential for variation in inherited predepositional dates and kinetic behaviors of grains from diverse source terrains. In this study, we present several examples of complex detrital zircon (U-Th)/He date distributions from sedimentary rocks that have been heated to temperatures in or near the zircon He partial retention zone, leading to several types of date-eU correlations caused by variations in predepositional inherited age and radiation-damage–induced He diffusion kinetics. These examples are from three subvertical transects in mountain ranges in central Utah: the Wasatch Range near Provo, Utah, the Oquirrh Mountains, and the Stansbury Mountains. Each range lies in the hanging wall of one of three major thrust sheets that compose part of the Charleston-Nebo salient, a segment of the Cretaceous Sevier fold-and-thrust belt. Zircon grains from two of these transects (the Stansbury and Oquirrh Mountains) show large date variations that can be at least partially understood with a radiation-damage–based model for He diffusion in zircon. We combine the output from this model with a new approach for understanding partially reset data sets that relies upon the concept of an “inheritance envelope.” The transect from the Wasatch Range is relatively simple and does not require an inheritance-based interpretation. We document a pulse of 5 km of exhumation in this range at ca. 100 Ma. Time-temperature (t-T) constraints from inheritance envelopes in the Oquirrh Mountains transect suggest a pulse of exhumation beginning at either 110 or 100 Ma. Finally, for the Stansbury transect, the inheritance envelope approach yields inconclusive results; some aspects of the model generate inheritance envelopes that match the measured data set, whereas others do not. Despite their complexity, the t-T histories for these nonideal data sets represent some of the only in situ constraints on the timing of Cretaceous exhumation of Sevier-style deformation in the U.S. Cordillera. In the case of the Oquirrh and Wasatch Range data sets, these direct measurements of thrust sheet t-T histories offer some insight into the evolution of the Sevier fold-and-thrust belt of central Utah. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Along-strike variability of thrust fault vergence

The 3D kinematic evolution of thrust systems, in which vergence changes along strike, is poorly understood. This study uses 3D seismic data from Big Piney-LaBarge field, Wyoming, to examine the geometry and kinematics of two faults at the leading edge of the Hogsback thrust sheet, the frontal thrust of the Late Cretaceous Sevier fold-thrust belt. These thrusts lie along strike of each another and share an east-vergent detachment within the Cretaceous Baxter Shale. The two thrusts verge in opposite directions: The southern thrust verges eastward forming a frontal ramp consistent with major thrusts of the Sevier belt, whereas the northern thrust verges westward to form a type 1 triangle zone with the Hogsback thrust. The thrusts have strike lengths of 5 km (3.1 mi) and 8 km (5.0 mi), respectively, and they are separated by a transfer zone of less than 0.5 km (0.3 mi) wide. Strata in the transfer zone appear to be relatively undeformed, but reflections are less coherent here, which suggests small offsets unresolved by the seismic survey. Retrodeformable cross sections and a structure contour map on the Cretaceous Mesaverde Group indicate that shortening varies along strike, with a pronounced minimum at the transfer zone and greater shortening across the northern, west-vergent thrust (610 m [2000 ft]) than across the southern, east-vergent thrust (230 m [755 ft]). Mapping of these thrusts suggests that they propagated laterally toward each other to form a type 1 antithetic fault linkage in the transfer zone. Spatial patterns expressed in seismic attributes in and near the detachment horizon, which include waveform classification and spectral decomposition, suggest that stratigraphic variations may have pinned the detachment, thus localizing the transfer zone. Thickness variations in the thrust sheet also may have influenced the thrust geometry. Our study provides an analog for analysis of similar complex contractional belts around the world.

Layer-parallel shortening across the Sevier fold-thrust belt and Laramide foreland of Wyoming: spatial and temporal evolution of a complex geodynamic system

Varying patterns of layer-parallel shortening (LPS) and vertical-axis rotations from the thin-skin Sevier fold-thrust belt to the thick-skin Laramide foreland of Wyoming are quantified from integrated structural, anisotropy of magnetic susceptibility (AMS), and paleomagnetic analyses. Within the Sevier belt, widespread early LPS was accommodated by spaced cleavage, fracture sets, minor folds, and minor faults. LPS directions are subperpendicular to structural trends of systematically curved thrust sheets of the Wyoming salient, reflecting a combination of primary dispersion and secondary rotation during thrusting. Within the Laramide foreland, limited LPS was accommodated mostly by minor faults with conjugate wedge and strike-slip geometries. LPS directions in gentler fold limbs vary from perpendicular to acute with structural trends of variably oriented, anastomosing basement-cored arches. Steep forelimbs display more complex relations, including younger fault sets that developed during evolving stress states and localized vertical-axis rotations. Although internal strain is limited, weak AMS lineations defined by kinked and rotated phyllosilicates are widely developed and consistently oriented perpendicular to measured LPS directions. Palinspastically restored LPS directions, corrected for paleomagnetically determined vertical-axis rotations, vary on average from W–E in the Sevier belt to WSW–ENE in the Laramide foreland. In detail, LPS directions display deflections related to primary sedimentary wedge geometry and basement fabrics. LPS in the Sevier belt is interpreted to partly reflect stress transmitted from the hinterland through the growing orogenic wedge and topographic stress along the front of the wedge. LPS in the Laramide foreland is interpreted to partly reflect basal traction during flat-slab subduction beneath thick cratonic lithosphere, with spatial-temporal variations in stress trajectories related to basement heterogeneities and evolving fault systems. Evidence for significant late N–S Laramide shortening, as proposed by multi-stage shortening models, is not observed.

The long-term burial and exhumation history of basement blocks in the footwall of the Wasatch fault, Utah

Thermochronologic studies of the Santaquin and Farmington Canyon crystalline basement complexes, exposed in the footwall of the Wasatch fault in Utah, provide rare opportunities to investigate the long-term tectonic, burial, and exhumation history of this region. Both complexes underwent amphibolite-facies metamorphism at ∼1700 Ma, followed by a complex pressure-temperature-time history. By 740–770 Ma, exhumation had brought both complexes to the surface from a depth of ∼9–10 km (3–3.5 kbar), followed by reburial by passive margin, Oquirrh Basin, and foreland basin sedimentation from Neoproterozoic through early Cretaceous time. The final structural pathway to present-day surface exposure of both complexes began in early Cretaceous time, with crustal contraction along the Sevier belt and resultant structural stacking. Structural breaching of the thrust culminations and final cooling of the crystalline complexes occurred as a result of Tertiary through Holocene extension and accompanying normal faulting. Inferred exhumation rates for the last 10–15 my are on the order of 0.3–0.6 mm/yr, although recent slip rates across the Wasatch fault appear to be several times higher. This suggests that: (1) periods of enhanced slip on the Wasatch fault from Miocene to present time may have been punctuated by periods of quiescence; and (2) the fault now may be experiencing an episode of rapid slip. Alternatively, strain may have been partitioned into multiple fault strands at a boundary between the Provo and Nephi segments.

Effect of predeformational basin geometry in the kinematic evolution of a thin‐skinned orogenic wedge: Insights from three‐dimensional finite element modeling of the Provo salient, Sevier fold‐thrust belt, Utah

In fold‐thrust belts, sedimentary cover rocks are detached from undeformed basement and undergo crustal‐scale shortening and internal deformation. We have investigated a three‐dimensional (3‐D), nonlinear, elastic‐plastic finite element model using the restored Provo salient of the Sevier belt as our initial configuration. In the model the deformed sedimentary prism displays large‐scale geometries that are seen in many natural fold‐thrust belts (e.g., arcuate salient, wedge‐shaped cross section) and kinematics that are compatible with observations in the internal and external portions of the Provo salient; these suggest that the model can be used to predict geologic information that is generally not available from detailed observational studies in natural fold‐thrust belts (e.g., strain history, material displacements, stress conditions). The model results indicate symmetric, noncoaxial, plane strain paths with consistent stress and strain orientations and material displacement directions in the middle of the 3‐D wedge, and fully 3‐D, nonsymmetric, noncoaxial, nonplane strain paths with out‐of‐transport material displacements over the lateral boundaries. The results from test runs further suggest that oblique ramps with strike direction less than 20° from the regional transport direction behave like lateral ramps, and those with strike direction greater than 80° from the regional transport direction behave similar to frontal ramps. Oblique ramps with dips greater than 60° behave like tear faults. These variations in different parts of the wedge are caused mainly by interaction between the transport parallel motion of the moving wedge and the preexisting footwall template of ramps and flats that the wedge has to ride over during its evolution.

Three-dimensional kinematic history at an oblique ramp, Leamington zone, Sevier belt, Utah

Differences in kinematics in adjoining salients in fold–thrust belts are typically accommodated along bounding transverse zones. The Leamington zone is an oblique ramp that accommodates slip between structures in the Provo salient and the central Utah segment of the Sevier fold–thrust belt. The Leamington zone consists of the Leamington Canyon thrust, associated second-order asymmetric folds and an out-of-syncline reverse fault. The Leamington Canyon thrust is a rotated, SE-dipping thrust fault with top-to-southeast motion; it represents an oblique ramp of the folded Canyon Range thrust. Fold tightening of the syncline SE of the folded Leamington Canyon thrust trace during later emplacement of underlying structures, caused out-of-syncline reverse faulting in the fold-core. Analysis of the finite strain, minor fault populations, and conjugate fracture sets were used in interpreting the 3-D kinematic history along the Leamington zone. Restored maximum stretching directions of finite strain ellipsoids trend eastward overall. Inferred motion planes from populations of slickenlines on minor faults are consistent with overall southeastward motion. Acute bisectors of conjugate fracture sets may reflect more southward directions later in the deformation history. These superimposed deformations, with local relative chronologies of plastic and brittle structures using cross-cutting relationships, indicate that the Leamington zone area shows overall clockwise changes in kinematic directions from E to ESE to SSE during successive pulses of deformation. The temporal changes in kinematic directions along the Leamington zone most likely reflect variations in kinematics over the oblique ramp as the overall easterly displaced 3-D fold–thrust belt wedge interacted with the pre-existing oblique ramp structure, with small magnitudes of superimposed local vertical-axis rotations during later folding and fold-tightening of the Leamington Canyon thrust.

Folding by cataclastic flow: evolution of controlling factors during deformation

Folding at upper crustal levels occurs by bending of beds and flexural slip between beds. As a fold's interlimb angle decreases, changes in bed thickness and limb rotation are accommodated by various mechanisms, depending on deformation conditions. In the elastico-frictional (EF) regime, cataclastic flow may be the dominant mechanism for fold tightening. The Canyon Range (CR) syncline, located in the Sevier belt of central Utah, shows this type of deformation. The fold involves three thick quartzite units, with slight lithological variations between them. Fold tightening took place in the EF regime (<2 km overburden) by cataclastic flow, involving collective movement on a distributed network of fractures and deformation zones (DZs) from the micro- to the outcrop-scale. In detail, the degree of cataclastic deformation varies significantly across the fold due to minor variations in initial bedding thickness, grain size, matrix composition, etc. A cooperative relationship exists across different scales, and the fracture networks result in a fracture shape fabric that is relatively homogeneous at the outcrop-scale. The initial outcrop scale fracture/DZ network geometry is a product of the growth and linking of micro-scale cataclasite zones, which in turn is controlled by primary lithological variations. Once a fracture network forms, the material behavior of the fractured rock is unlike that of the original rock, with sliding of fracture-bound blocks accomplishing ‘block-controlled’ cataclastic flow. Thus, initial lithological variations at the micro-scale largely control the final deformation behavior at the largest scale. During progressive fold tightening, additional factors regulate cataclastic flow, such as fracture/DZ reactivation or healing, during folding. Although initial lithological variations in different units may produce unique network geometries, each unit's behavior may also depend upon the behavior of adjacent units. In the CR syncline, during the initial stages of cataclastic flow, the inherent nature of each quartzite unit results in unit-specific fracture network geometries. As deformation progresses, unit-specific networks begin to interact with those in surrounding units, resulting in feedback mechanisms regulating the later stages of network development. Thus, the nature of cataclastic flow changes dramatically from the initial to the final stages of folding.

Late Jurassic to Eocene evolution of the Cordilleran thrust belt and foreland basin system, western U.S.A.

Geochronological, structural, and sedimentological data provide the basis for a regional synthesis of the evolution of the Cordilleran retroarc thrust belt and foreland basin system in the western U.S.A. In this region, the Cordilleran orogenic belt became tectonically consolidated during Late Jurassic time (∼155 Ma) with the closure of marginal oceanic basins and accretion of fringing arcs along the western edge of the North American plate. Over the ensuing 100 Myr, contractile deformation propagated approximately 1000 kilometers eastward, culminating in the formation of the Laramide Rocky Mountain ranges. At the peak of its development, the retroarc side of the Cordillera was divided into five tectonomorphic zones, including from west to east the Luning-Fencemaker thrust belt; the central Nevada (or Eureka) thrust belt; a high-elevation plateau (the “Nevadaplano”); the topographically rugged Sevier fold-thrust belt; and the Laramide zone of intraforeland basement uplifts and basins. Mid-crustal rocks beneath the Nevadaplano experienced high-grade metamorphism and shortening during Late Jurassic and mid- to Late Cretaceous time, and the locus of major, upper crustal thrust faulting migrated sporadically eastward. By Late Cretaceous time, the middle crust beneath the Nevadaplano was experiencing decompression and cooling, perhaps in response to large-magnitude ductile extension and isostatic exhumation, concurrent with ongoing thrusting in the frontal Sevier belt. The tectonic history of the Sevier belt was remarkably consistent along strike of the orogenic belt, with emplacement of regional-scale Proterozoic and Paleozoic megathrust sheets during Early Cretaceous time and multiple, more closely spaced, Paleozoic and Mesozoic thrust sheets during Late Cretaceous–Paleocene time. Coeval with emplacement of the frontal thrust sheets, large structural culminations in Archean-Proterozoic crystalline basement developed along the basement step formed by Neoproterozoic rifting. A complex foreland basin system evolved in concert with the orogenic wedge. During its early and late history (∼155 - 110 Ma and ∼70 - 55 Ma) the basin was dominated by nonmarine deposition, whereas marine waters inundated the basin during its midlife (∼110 - 70 Ma). Late Jurassic basin development was controlled by both flexural and dynamic subsidence. From Early Cretaceous through early Late Cretaceous time the basin was dominated by flexural subsidence. From Late Cretaceous to mid-Cenozoic time the basin was increasingly partitioned by basement-involved Laramide structures. Linkages between Late Jurassic and Late Cretaceous Cordilleran arc-magmatism and westward underthrusting of North American continental lithosphere beneath the arc are not plainly demonstrable from the geological record in the Cordilleran thrust belt. A significant lag-time (∼20 Myr) between shortening and coeval underthrusting, on the one hand, and generation of arc melts, on the other, is required for any linkage to exist. However, inferred Late Jurassic lithospheric delamination may have provided a necessary precondition to allow relatively rapid Early Cretaceous continental underthrusting, which in turn could have catalyzed the Late Cretaceous arc flare-up.

Three-dimensional finite-element modeling of a thin-skinned fold-thrust belt wedge: Provo salient, Sevier belt, Utah

Three-dimensional finite-element (FE) analysis of a fold-thrust belt wedge model provides graphical and quantitative information for the strain geometry, kinematics, and mechanics of salient formation in three-dimensionalspace. A nonlinear, elastic-plastic FE model, using the initial basin configuration of the restored Provo salient, Utah, develops realistic deformation features (e.g., salient) in the deformed sedimentary prism. The three-dimensional fold-thrust belt wedge model shows different deformation characteristics (based on S 1 , γ m a x , and σ 1 orientations and material-displacement directions) in different parts of the salient, and its behavior is closely related to the preexisting template of the predeformational basin shape. The model results indicate plane-strain deformation with symmetric finite-strain geometry in the middle of the salient and nonplane-strain deformation with nonsymmetric finite-strain geometry at the lateral boundaries. Thus, conventional two-dimensional plane-strain assumptions in fold-thrust belt analysis (e.g., in balancing cross sections) are reasonable only in the middle of the salient; however, at the lateral boundaries, the results indicate fully three dimensional deformation and suggest that restorations need to be more carefully constrained in three dimensions on the basis of detailed geologic data.