Wyoming Province Research Articles

Geochemical and geochronologic evidence for a contiguous northeastern Wyoming Province

The extent and nature of the Wyoming Province, an Archean craton in southwestern Laurentia, are poorly understood due to limited exposure between spatially isolated basement-cored uplifts. This lack of exposure has led to debate about whether the northeastern Wyoming Province is underlain by contiguous Archean crust or Proterozoic rocks and suture zone associated with the Trans-Hudson orogeny. To assess these models we analyzed samples recovered from drill cores in the buried northeastern Wyoming Province that straddle the proposed Proterozoic suture. Whole-rock geochemical and Nd-Pb-O isotopic data suggest the rocks formed by partial melting of > 3.5 Ga hydrated mafic to tonalitic sources similar to elsewhere in the northern Wyoming Province. Zircon U-Pb dates record Mesoarchean magmatism and metamorphism (3.0–2.8 Ga). Published whole-rock Rb-Sr, hornblende and biotite K-Ar, and apatite fission track dates suggest these rocks have not been heated above 300 °C since 2.5–2.1 Ga. Geophysical potential field data are consistent across northeastern Wyoming contrasting with major discontinuities associated with documented Proterozoic orogens on all other margins of the Wyoming Province. Hence, geochemical, isotopic, and geochronologic data along with geophysical imaging can be most simply interpreted in terms of continuous Archean crust in the northeastern Wyoming Province. A geophysically-imaged reflector east of the Bighorn Mountains may juxtapose Archean terranes with similar ages and sources, similar to the boundary between the Montana metasedimentary terrane and Beartooth-Bighorn magmatic zone in the northwestern Wyoming Province. This work emphasizes the value of integrating geologic and geophysical constraints to constrain Archean provinces and their tectonic evolution.

Abundant 2480 Ma Detrital Zircons in ∼1740 Ma Vishnu Schist in Northwestern Arizona: Derivation From the Matachewan Large Igneous Province?

Detrital zircon grains in the ∼1740–1750 Ma Vishnu Schist and similar rock units in northwestern Arizona consist of up to 30% grains dated by U-Pb isotopic analysis at 2470–2490 Ma. These zircon grains are distributed over ∼40,000 km2 and define an age peak at 2480.0 ± 27.3 Ma (2SE). These grains have yielded unusually consistent 207Pb/206Pb dates, with generally smaller analytical uncertainty and greater concordance to ideal U-Pb evolution than grains of other ages. A weighted mean age of 2480 ± 0.9 Ma (2SE) for this zircon population reflects consistent analytical results and high analytical precision but not the accuracy of the age. The source of these zircons has not been identified. To better characterize the unidentified source, we analyzed 45 of these grains for trace and rare-earth elements by laser-ablation mass spectrometry and scanned 16 grains with an electron microprobe to identify mineral inclusions. Mass spectrometer determinations of Sc/Yb and Nb/Sc support derivation from an oceanic-island igneous source. Electron microprobe scans revealed quartz in 5 of 16 grains, indicating a felsic source. The low variability in 207Pb/206Pb dates and a generally linear relationship between U and Th support zircon derivation from a single igneous unit or closely related set of units without xenocrystic zircons. A literature search for other zircon populations with similar age and U/Th ratios identified ∼2480 Ma zircons in a Mesoproterozoic(?) metapsammite and conglomerate in southwestern Montana. This sandstone was deposited near the margin of the Wyoming craton and contains almost entirely 2400–3600 Ma zircons, unlike zircon grains in Vishnu Schist which include a large population of 1730–1900 Ma zircons. From this relationship we infer that the 2480 Ma zircons in both areas were derived from a source in the Wyoming craton. We conclude that the 2480 Ma Vishnu zircons were derived from a felsic batholith that formed above and from hotspot magma related to the ∼2450–2480 Ma Matachewan Large Igneous Province, that this batholith formed by mixing between a mantle-derived hotspot magma and assimilated Archean continental crust, and that the source rock was emplaced during initial rifting between the Wyoming craton and the Superior province.

Strong Physical Contrasts Across Two Mid‐Lithosphere Discontinuities Beneath the Northwestern United States: Evidence for Cratonic Mantle Metasomatism

AbstractMid‐lithosphere discontinuities are seismic interfaces likely located within the lithospheric mantle of stable cratons, which typically represent velocities decreasing with depth. The origins of these interfaces are poorly understood due to the difficulties in both characterizing them seismically and reconciling the observations with thermal‐chemical models of cratons. Metasomatism of the cratonic lithosphere has been reported by numerous geochemical and petrological studies worldwide, yet its seismic signature remains elusive. Here, we identify two distinct mid‐lithosphere discontinuities at ∼87 and ∼117 km depth beneath the eastern Wyoming craton and the southwestern Superior craton by analyzing seismic data recorded by two longstanding stations. Our waveform modeling shows that the shallow and deep interfaces represent isotropic velocity drops of 2%–8% and 4%–9%, respectively, depending on the contributions from changes in radial anisotropy and density. By building a thermal‐chemical model including the regional xenolith thermobarometry constraints and the experimental phase‐equilibrium data of mantle metasomatism, we show that the shallow interface probably represents the metasomatic front, below which hydrous minerals such as amphibole and phlogopite are present, whereas the deep interface may be caused by the onset of carbonated partial melting. The hydrous minerals and melts are products of mantle metasomatism, with CO2‐H2O‐rich siliceous melt as a probable metasomatic reagent. Our results suggest that mantle metasomatism is probably an important cause of mid‐lithosphere discontinuities worldwide, especially near craton boundaries, where the mantle lithosphere may be intensely metasomatized by fluids and melts released by subducting slabs.

Metamorphic P–T paths of Archean granulite facies metasedimentary lithologies from the eastern Beartooth Mountains of the northern Wyoming Province, Montana, USA: constraints from quartz-in-garnet (QuiG) Raman elastic barometry, geothermobarometry, and thermodynamic modeling

Abstract. Metamorphic pressure and temperature (P–T) paths in late-Archean high-grade rocks of the eastern Beartooth Mountains of Montana (USA), a portion of the Wyoming Province, are established by a combination of imaging, analytical, and modeling approaches. Garnet inclusion mechanical and chemical thermobarometry, applied to several granulite-facies migmatites and an iron formation, is particularly useful in constraining the prograde P–T conditions. Quartz-in-garnet (QuiG) elastic Raman barometry was used on quartz inclusions in garnet for all samples studied. For a smaller subset of four representative samples, QuiG constraints were used in conjunction with Ti-in-quartz (TitaniQ) and Ti-in-biotite (TiB) thermometry to establish unique prograde inclusion entrapment P–T conditions. Ti measurements of garnet hosts and cathodoluminescence (CL) imagery of inclusion and matrix quartz grains were employed to check for Ti loss/diffusion. Lastly, inclusion studies were supplemented with thermodynamic modeling and matrix chemical thermobarometry to examine garnet nucleation temperatures and peak metamorphic conditions. Disagreement between the volume strain and elastic tensor methods used to calculate quartz inclusion pressures implies that quartz inclusions studied are under strong differential strain. Prograde entrapment results from the two inclusion thermobarometry pairs used are distinct: 0.55–0.70 GPa and 475–580 ∘C (QuiG–TitaniQ) versus 0.85–1.10 GPa and 665–780 ∘C (QuiG–TiB). Garnet modal isopleth modeling indicates that the majority of garnet growth occurred at ∼ 450–600 ∘C, implying that P–T conditions of garnet growth are interpreted to be most reliably represented by QuiG–TitaniQ inclusion thermobarometry. Normal distributions of calculated QuiG inclusion pressures and the concentration of mineral inclusions in garnet cores suggest that the majority of garnet inclusions were entrapped during a single stage of porphyroblast growth. A general lack of evidence from CL imagery for post-entrapment mechanical or chemical modifications to quartz inclusions suggests that quartz inclusions used to calculate entrapment P–T largely preserve their initial entrapment conditions. Biotite inclusions preserve higher temperatures than quartz inclusions in the same garnets, likely due to Fe–Mg exchange with garnet hosts that allowed Ti content of biotite to change after entrapment. Pseudosection modeling and matrix chemical thermobarometry of multiple, independent lithologies examined during inclusion studies suggest a range of peak granulite facies conditions of ∼ 0.50–0.70 GPa and 730–800 ∘C. Peak metamorphic P–T modeling work from this study, together with evidence of regional amphibolite facies overprinting in immediately adjacent samples, indicates an overall clockwise metamorphic P–T path with nearly isobaric prograde heating to peak temperatures. Interpreted P–T path reconstructions are consistent with metamorphism developed in a more modern-style continental arc subduction zone and are observed in portions of the northern Wyoming Province as exemplified by metasupracrustal lithologies of the eastern Beartooth Mountains.

The disparities in thickness between the thermal and seismic lithosphere beneath the global Precambrian cratons and its geodynamic implications

Cratons play an important role in the evolution of Earth and usually exhibit extremely thick and strong lithosphere roots and low surface heat flow. Most cratons remain in a stable state throughout their long evolutionary process. However, the stability of a few cratons was lost, the thickness of the lithosphere was greatly reduced, and surface heat flow greatly increased. Significant thinning of the lithosphere is one of the most obvious features of craton destruction. However, even within the same area, the lithospheric thickness values estimated from different methods can vary a lot. The thickness disparities between the thermal and seismic lithosphere remain unclear. On the basis of the latest global heat flow data and crustal stratification information, we calculated the thickness of the thermal lithosphere of cratons globally and conducted a detailed comparison to that of the seismic lithosphere. We found that 1) the thickness of the thermal lithosphere of most cratons is more than 150 km, while that of the Bundelkhand, Singhbhum, and upper Yangtze craton ranges from approximately 120–150 km. The thickness of the eastern North China craton, lower Yangtze craton and Wyoming craton ranges from only approximately 80–100 km. 2) For most cratons, the thickness of the seismic lithosphere is tens of kilometers (30–141 km) larger than that of the thermal lithosphere, except for the Wyoming craton, eastern North China craton, upper Yangtze craton, and Dahawar craton. 3) The part between the bottom boundaries of the seismic and thermal lithosphere can be considered as the rheological boundary layer (RBL), which is related to the craton stability. The thicker the RBL is, the higher the craton stability. 4) Due to plate subduction, the water content in the upper mantle is high, the viscosity is low, and the thickness of the RBL of cratons is small.

Build-up of multiple volatiles in Earth's continental keels: Implications for craton stability

Elevated contents of structurally-bound hydrogen in olivine, introduced to cratonic mantle during metasomatism, are widely believed to decrease the effective viscosity so that it is similar to the underlying asthenosphere (1018–1019 Pa⋅s) and susceptible to basal erosion. Nevertheless, large variations exist in H contents of mantle olivines from the Kaapvaal, Siberia, Slave, Tanzania and Wyoming cratons (0 to 320 ppmw) and the role of water as the main driver of rheological weakening is contentious. While recent experimental studies have shown that olivine has the capacity to host large quantities of fluorine, as well as hydrogen, the magnitude of this has not yet been established for cratonic mantle.Our new dataset for Kaapvaal craton peridotites shows that the style of metasomatism influences the addition of H2O and F to cratonic mantle. Silicic fluids derived from subducted slabs, and responsible for the pervasive orthopyroxene enrichment observed in the Kaapvaal (and many of the world's other cratons), deliver significant quantities of H2O but lesser amounts of F, whereas proto-kimberlite melts transport high quantities of both H2O and F. Kaapvaal mantle olivines are major hosts of both H2O (up to 105 ppmw) and F (up to 180 ppm), and we propose that a major increases in the bulk H2O (∼30%) and F (∼65%) of the Kaapvaal craton occurred over a short (20 Ma) time interval between the two main pulses of Cretaceous kimberlite emplacement in southern Africa.By combining the thermal structure of individual cratons with corresponding H2O data for olivine, we show that the effective viscosity contrast at the base of the mechanical boundary layer and asthenosphere below the Kaapvaal, Slave and Siberia cratons varies from 2 to 14. Some of the lowest effective viscosities (3.5×1017 Pa⋅s) occur in H2O-rich olivine in Siberia sheared peridotites from the base of the craton and are consistent with highly-localised metasomatism. More viscous peridotites (up to 1.5×1021 Pa⋅s) were entrained from the thermal boundary layer beneath the Kaapvaal and Tanzania cratons but residence times in this region are short due to convective overturn (<100 Ma) and this dehydrated mantle would offer only temporary protection to cratonic keels. Our viscosity estimates are based on the H content of olivine and, while the additional effect of structurally bound F in olivine is currently uncertain, we anticipate that it will have a similar effect on mantle rheology to H, so that the base of the mechanical boundary layer beneath cratons at the time and location of kimberlite generation is close to the tipping point for instability.The rapid build-up of volatiles associated with pervasive kimberlite activity may have been the catalyst for lithospheric thinning on the southern margin of the Kaapvaal craton, but more localised pulses of kimberlite activity occurred over longer time intervals in the Siberia and Slave cratons and so had a less profound effect on their stability. A prolonged and widespread subduction-flux of H2O to the Wyoming and N. China cratons, and subsequent sub-solidus partitioning in olivine, may have been the driver for the rheological weakening that ultimately led to delamination.

Eclogite Varieties and Their Positions in the Cratonic Mantle Lithosphere beneath Siberian Craton and Archean Cratons Worldwide

The pressure-temperature (PT) conditions and position of different groups of eclogites in the sub-cratonic lithospheric mantle (SCLM) worldwide were established using clinopyroxene Jd-Di and garnet thermobarometry. Beneath Siberia, Fe-eclogites found within the 3.0–4.0 GPa formed in Early Archean times. In the Middle and Late Archean, eclogites were melted during and after subduction. High-Mg eclogites (partial melts or arc cumulates) are related to low-T (LT) geotherms. Melt-metasomatized eclogites trace a high-temperature (HT) geotherm. Eclogitic diamond inclusions from Siberia mostly belong to the middle SCLM (MSCLM) part. Ca-rich eclogites from Precambrian Indian kimberlites are located in the MSCLM. In Phanerozoic time, they were located in the lithosphere base. In Proterozoic South Africa, Ca-rich eclogites and grospydites occur within 4.0–5.0 GPa and HT eclogite and diamond inclusions from the Premier pipe trace a HT geotherm at depths of 7.0–4.0 GPa, showing an increase in Fe upwards in the mantle section. Similar trends are common for eclogites worldwide. In the Wyoming craton, kimberlites captured eclogite xenoliths from the 4.0–2.5 GPa interval. Mantle eclogites have clinopyroxenes and garnet trace element patterns with high (La/Yb)n determined by KDs with melts and are magmatic. Flatter and bell-like REE patterns with Eu anomalies, HFSE troughs, and U and Pb peaks, are common for clinopyroxenes from MORB-type “basaltic” eclogites. High-Mg eclogites show less fractionated incompatible element branch in patterns. LILE-enrichments and HFSE troughs are typical for kyanite-bearing eclogites. Clinopyroxenes from diamond-bearing eclogites show lower REE, troughs in Nb and Zr, and peaks in Pb and U concentrations, compared to barren eclogites with round smooth trace element patterns and small depressions in Pb and Ba.

Testing the early Paleoproterozoic connection of the Superior and Wyoming cratons with geochronology and geochemistry

Paleogeography is critical as a backdrop for environmental changes, yet the Paleoproterozoic configuration of Archean cratons remains controversial. The Snowy Pass Supergroup (SPS), a Paleoproterozoic succession on the SE Wyoming craton (Medicine Bow Mountains, MBM), has been correlated with the Huronian Supergroup (HS) of the Superior craton, suggesting deposition in the same basin, and indicating that these cratons were adjoined in the Paleoproterozoic Superia supercraton. We present U-Pb volcanic and detrital zircon data for the underlying Phantom Lake Metamorphic Suite (PLMS), SPS, and Snowy Pass Group (SPG; Sierra Madre, SM) and U-Pb zircon ages for two mafic sills and Baggot Rocks granite. The PLMS contains a statistically significant mode of ca. 2.45 Ga zircon dates in the SM; zircons of this age also occur in one to two grain quantities in the PLMS in the MBM. The Baggot Rocks granite and Colberg Metavolcanics of the PLMS also yielded the ca. 2.45 Ga U-Pb zircon ages, indicating the early Paleoproterozoic age for this suite and the ca. 2.45 Ga extensive, rift-related mafic magmatism on the southern margin of the Wyoming craton, allowing correlation with the basal HS units of the Superior craton. Zircon age spectra for lithostratigraphically correlative units on the Wyoming and Superior cratons show strikingly similar, primarily Archean patterns, which, along with paleocurrent directions, establish provenance of both cratons for the shared basin; ca. 2.55 and 2.3 Ga modes are common for the uppermost formations. Fine-grained siliciclastic rocks throughout the SPS yield similar TDM and εNd ages as well as μ and κ values to the HS, further supporting similar provenance for the two supergroups. Mafic sills in the MBM yielded U-Pb zircon dates of 2220 ± 8 Ma and 2094.3 ± 1.6 Ma that closely correspond to the age of the Nipissing mafic intrusions in the HS and the Cauchon Lake dikes in the NW Superior craton. Correlation between the HS and SPS breaks down in the upper SPS at the Nash Fork Formation, which bears detrital zircon age modes at ca. 2.2 and 2.1 Ga, whereas deposition of the HS terminated before ca. 2.22 Ga when the Nipissing dikes intruded and shortly after a ca. 2.31 Ga volcanic event recorded by the Gordon Lake and Bar River formations. The matching detrital zircon dates to a globally unique, ca. 2.3 Ga, mode first appeared in the Lookout Schist of the SPS and steadily decreased up-section through the upper SPS, pointing to a similar volcanic source. Maximum depositional ages for the uppermost formations of the SPS and SPG, the purportedly correlative Nash Fork and Slaughterhouse formations, are at 2081 ± 10 Ma and 2334 ± 20 Ma, respectively. The former detrital zircon mode corresponds to that recorded by the basal Animikie basin units (Denham Formation of the Mille Lacs Group in MN and East Branch Arkose of the Dickinson Group, MI) and are linked to rifting, breakup, and ultimate separation of the Wyoming and Superior cratons at ca. 2075 Ma.

The Neoarchean and Paleoproterozoic crustal evolution of the Clearwater block, northwestern Laurentia: Implications for the assembly of supercontinents

We report new coupled zircon U-Pb age and Hf isotope data, along with whole-rock Hf-Nd isotope data for 20 samples from the Priest River and Clearwater complexes—Neoarchean and Paleoproterozoic basement exposures in northern Idaho, northwest Laurentia. The new zircon data reveal two well-defined periods of magmatism at ∼ 2.66 and ∼ 1.86 Ga in each complex, with no evidence of any intervening or older magmatic events. Positive zircon initial εHf values of the Neoarchean rocks (+1 to +3) indicate juvenile magmas derived from a depleted mantle source. The Paleoproterozoic rocks, in both complexes, share a much wider range of initial zircon εHf values (−7 to +6) which suggests magmas derived from a depleted mantle source intruded and assimilated pre-existing Neoarchean continental crust. The shared age and isotope compositions in the Priest River and Clearwater complexes suggest that the two complexes likely belong to a single crustal block—the Clearwater block.Based on these new zircon U-Pb ages and Hf isotopic data of the Neoarchean and Paleoproterozoic rocks, the Clearwater block is shown to be distinct from the nearby Wyoming Province, the Medicine Hat block, and components of the Great Falls tectonic zone. We also report new zircon Hf isotope data for the ∼ 1.58 Ga Laclede orthogneiss in the Clearwater block, which along with published U-Pb and Hf isotope constraints, support a paleogeographic connection between the Gawler craton, East Antarctica, and northwestern Laurentia during ∼ 1.9 Ga to ∼ 1.6 Ga. Comparison with other examples of co-existing Neoarchean and Paleoproterozoic igneous rocks present in other continents suggests that the global record of 2.7–2.5 Ga magmatism often represents new crustal additions from a depleted mantle reservoir. The record of 2.0–1.7 Ga rocks also represent additions of juvenile magma, but with significantly larger contributions from pre-existing continental crust. In many cases the 2.0–1.7 Ga crust preserved on Earth today was added during the assembly of the supercontinent Nuna.

Geophysical extent of the Wyoming Province, western USA: Insights into ancient subduction and craton stability

AbstractA new 3-D resistivity model, estimated from inversion of magnetotelluric data, images crustal and upper-mantle structure of the Wyoming Province and adjacent areas. The Archean province is imaged as a coherent resistive domain, in sharp contrast to active tectonic domains of the western U.S. Prominent high-conductivity belts define the northern, eastern, and southern margins of the Wyoming Province and are interpreted as sutures marking the remnants of Paleoproterozoic orogens. The model results suggest the northern boundary of the Wyoming Province is located 150 km south of its traditional placement and adjacent to a composite orogen separating the Wyoming Province and Medicine Hat block. The eastern province boundary is clearly imaged along the Black Hills, whereas the western margin is obscured by Cenozoic extension and magmatism. An internal boundary within the Wyoming Province is interpreted to represent a Neoarchean suture; in stark contrast to Proterozoic sutures, though, it is not marked by a high-conductivity belt. This difference in conductivity is speculated to reflect changes in the subduction process through time. The absence of high-conductivity along Archean sutures appears to be global in nature and related to reduced continental freeboard in the Archean which limited continental weathering and the delivery of carbon-rich sediments to the seafloor. Although the entire Wyoming Province has been proposed to have undergone lithospheric modification that lessened its stability, the resistivity model suggests a thick lithospheric root remains in place except along its western margin. These results suggest that Archean cratons may be more resistant to lithospheric modification by influx of heat and fluids associated with extension and plumes than previously thought, and that metasomatism does not necessarily weaken the lithosphere and set a craton on the path to destruction.

Provenance of Devonian–Carboniferous strata of Colorado: The influence of the Cambrian and the Proterozoic

ABSTRACTWe report new LA-ICP-MS U–Pb detrital zircon ages and sedimentary petrology of silty to sandy limestones and dolostones, as well as calcareous to dolomitic sandstones of the Devonian–Carboniferous (Mississippian) Chaffee Group. We also report new detrital zircon ages from the late Cambrian Sawatch Quartzite, and a U–Pb zircon crystallization age on a late Mesoproterozoic (1087.9 ± 13.5 Ma) granitoid of underlying basement from the Eagle Basin of northwest Colorado. Grain populations in the Chaffee Group are mostly bimodal. More than 84% of zircons centered around a Paleoproterozoic (ca. 1.78 Ga) mode typical of the Yavapai province that forms much of the basement of Colorado and an early Mesoproterozoic (ca. 1.42 Ga) mode typical of A-type granites that intrude this region. A notable late Mesoproterozoic (ca. 1.08 Ga) mode exists in some Chaffee samples, giving those samples a trimodal detrital zircon age distribution. These bipartite or tripartite detrital zircon age modes exist in Cambrian, Devonian, and Carboniferous strata from paleogeographically adjacent successions, but the correlation between the Chaffee zircons is highest with the region’s basal Cambrian sandstones of the Sawatch Quartzite, Flathead Sandstone, and Ignacio Quartzite, which have similar (ca. 1.08 Ga, 1.43 Ga, 1.70 Ga, respectively) zircon populations, and a paucity of &amp;gt; 1.8 Ga grains. This similarity suggests that most grains in the Chaffee Group derive from recycling of these basal sandstones, and that little sediment was derived directly from thenexposed Precambrian basement highs, from the Wyoming craton to the north, or from Paleoproterozoic arcs and orogens to the west and northeast. Minor Mesoarchean to early Paleoproterozoic (ca. 3.00 to 2.40 Ga) grains exist in the Chaffee Group, an attribute shared by the Late Ordovician Harding Sandstone of Colorado’s Front Range, but that is absent from the region’s underlying Cambrian sandstones—suggesting some recycled mixture of Cambrian and Ordovician sedimentary rocks. No near-depositional age grains are present in the Chaffee Group. The youngest grain is Early Devonian (~417 Ma), &amp;gt; 45 million years (m.y.) older than these strata. Additionally, Paleozoic grains are extremely uncommon (&amp;lt; 0.1%; n = 2,927 grains).

The CIELO Seismic Experiment

Abstract The Crust and lithosphere Investigation of the Easternmost expression of the Laramide Orogeny was a two-year deployment of 24 broadband, compact posthole seismometers in a linear array across the eastern half of the Wyoming craton. The experiment was designed to image the crust and upper mantle of the region to better understand the evolution of the cratonic lithosphere. In this article, we describe the motivation and objectives of the experiment; summarize the station design and installation; provide a detailed accounting of data completeness and quality, including issues related to sensor orientation and ambient noise; and show examples of collected waveform data from a local earthquake, a local mine blast, and a teleseismic event. We observe a range of seasonal variations in the long-period noise on the horizontal components (15–20 dB) at some stations that likely reflect the range of soil types across the experiment. In addition, coal mining in the Powder River basin creates high levels of short-period noise at some stations. Preliminary results from Ps receiver function analysis, shear-wave splitting analysis, and averaged P-wave delay times are also included in this report, as is a brief description of education and outreach activities completed during the experiment.

Structural analysis of the Casper Mountain fault zone and area, Wyoming surrounding area, Wyoming: Implications for Laramide kinematics and structural inheritance across the Wyoming Province

Casper Mountain is an E–W trending anticlinal structure that is bound on the north by the oblique-slip Casper Mountain fault. The fault is postulated to reflect preexisting Precambrian structure/fabrics that were reactivated and/or guided deformation during the Laramide orogeny. A structural analysis of the fault zone and surrounding area was conducted to confirm this hypothesis, and to garner insight into both Precambrian origins and Laramide kinematics. Surface and subsurface data for structural analysis was collected and synthesized from numerous published sources along the proposed deformation corridor that roughly coincides with the Oregon Trail structural belt of central Wyoming. The Casper Mountain fault zone is characterized by an E–W rectilinear zone of en échelon, steeply inclined faults. The Casper Mountain fault strikes E–W with smaller faults in the zone striking N65°E. Folds trend to the WNW and are left-stepping. Foliations in Precambrian rocks of Casper Mountain are oriented subparallel to the Casper Mountain fault. The North Granite Mountains fault zone is located due west of Casper Mountain and is similarly oriented E–W with associated faults striking NE, NW/SE, and ENE/WSW, off the dominant master fault. Curvilinear, left-stepping, en échelon folds trend to the northwest and are truncated on the south by the North Granite Mountains fault. Faults in basement rocks of the Popo Agie Primitive Area of the central Wind River Mountains are characterized by moderate to high-angle faults striking E–W, NNW, and NE that coincide with mapped surface lineaments and fabric data. Fabric data suggest that Laramide deformation along the Casper Mountain fault was guided by Precambrian anisotropies. Surface and subsurface mapping of the fault zone and the deformation corridor to the west indicate that the Casper Mountain and North Granite Mountains faults are part of a basement-rooted system (wrench fault) that likely extends westward into the Popo Agie Primitive Area. Here, the steeply inclined (75–90°) proposed master fault is exposed within a WNW-striking corridor of faults that sinistrally offset steeply dipping, NE-striking Proterozoic diabase dikes. The dikes likely intruded older faults that are antithetic to the WNW-striking faults. Other faults strike to the NNW and have shallower dips of 45–65°. These three directions of anisotropy (WNW, NE, and NNW) are proposed to have formed from SW–NE-directed subduction along a long-lived, Neoarchean, active continental margin.

The peridotite deformation cycle in cratons and the deep impact of subduction

Xenoliths play a crucial role in interpretation of mantle deformation and geochemistry. The classic work of Mercier and Nicolas (1975) introduced the concept of the peridotite deformation cycle, which connected observed microstructures to a physical sequence of deformation. We revisit Mercier and Nicolas' original concept, bringing in new constraints using large area EBSD maps and associated microstructural datasets, analysis of water contents in nominally anhydrous minerals, and trace element chemistry of pyroxenes and garnets. We apply these techniques to a well-characterized suite of peridotite xenoliths from the Eocene-age Homestead and Williams kimberlites in the northwestern Wyoming Craton. Pyroxene water content and trace element mineral chemistries reveal ubiquitous hydrous metasomatism beneath the craton, most likely linked to the Cenozoic Laramide Orogeny. Homestead xenoliths primarily exhibit coarse protogranular and equigranular textures, B-type olivine fabrics, and generally elevated mineral water contents compared to Williams. Xenoliths from Williams are strongly deformed, with porphyroclastic and transitional textures containing annealed olivine tablets, mostly A-type olivine fabrics, and generally lower mineral water contents. As a whole, mantle from Homestead to Williams reflects a cratonic scale deformation cycle that likely initiated in Laramide times and lasted until the end of orogeny in the Eocene. At Williams, evidence for a rapid deformation “sub-cycle” within the main deformation cycle is preserved in the tablet-bearing xenoliths, corresponding to the enigmatic “transitional” texture of Mercier and Nicolas (1975). Our results suggest that this texture reflects interruption of the main deformation cycle by processes possibly related to a rapidly forming lithospheric instability and generation of the kimberlite magma – offering a new interpretation of this ambiguous peridotite texture. Collectively, our results incorporate typically disparate geochemical and textural datasets on xenoliths to shed new insights into how metasomatism, volatiles, and deformation are connected in the deep cratonic lithosphere.

Petrogenetic and tectonic interpretation of strongly peraluminous granitic rocks and their significance in the Archean rock record

Abstract Strongly peraluminous granitic rocks (SPG), defined by an aluminum saturation index ≥1.1, become abundant in the rock record in the Neoarchean. This study identifies three different varieties of Neoarchean SPG in the Archean Wyoming Province, U.S.A. These include calcic SPG, represented by the Webb Canyon Gneiss and Bitch Creek Gneiss of the Teton Range; calc-alkalic to alkali-calcic suites composed entirely of SPG, including the Rocky Ridge garnet granite gneiss of the northern Laramie Mountains and the Bear Mountain granite in the Black Hills; and calc-alkalic to alkali-calcic suites that include both weakly and strongly peraluminous granitic rocks, such as the Mount Owen batholith, Wyoming batholith, and Bears Ears granite. Although the petrogenesis of all the SPG suites involves partial melting of crustal sources, the composition of those sources, the melting conditions, and the tectonic settings vary. The calcic suites originate by dehydration melting or water excess melting of hornblende-plagioclase rocks at relatively high temperature. The suites composed entirely of SPG formed by partial melting of metasedimentary rocks by reactions involving muscovite at lower temperatures. Suites with both weakly and strongly peraluminous granite may form by partial melting of metasedimentary rocks by reactions involving biotite or by assimilation of aluminous melts of felsic crust by differentiated calc-alkalic magma. Most of the Wyoming SPG appear to have formed in collisional orogens, but SPG of the Wyoming batholith and Bears Ears granite are associated with continental arc magmatism. The appearance of SPG in the Neoarchean rock record marks the time when subduction enabled the formation of strong, thick, increasingly felsic continental crust, which in turn allowed the development of a mature, clastic sedimentary cover. Lateral movement of crustal blocks led to collisional orogeny, SPG magma genesis, and the formation of the first supercontinents.

When Wyoming Became Superior: Oblique Convergence Along the Southern Trans‐Hudson Orogen

AbstractThe Trans‐Hudson orogen (THO) is one of the best‐preserved Proterozoic orogens on Earth, largely unaffected by subsequent tectonism, yet its southern extent lies concealed beneath the North American Central Plains. A new 3D resistivity model over the southern orogen is developed and interpreted alongside borehole, potential field, and seismic reflection data. We present the first synoptic crustal view of the southern THO and a new tectonic model of the orogen. Our model reveals high‐conductivity belts marking paleo‐subduction zones while the orogen center consists of deeply exhumed relatively dense and mostly magnetic juvenile crust preserved between the deformed margins of the Wyoming and Superior cratons. Complex structure along the western margin suggests convergence began with oblique subduction and the northward transport of severed fragments of the Wyoming Province. High‐conductivity belts are in places offset from upper‐crustal geophysical boundaries, consistent with the thrusting of accreted arcs over the Archean margins during terminal closure.

Does the Owl Creek fault zone of north-central Wyoming extend to the Black Hills of South Dakota? Implications for basement architecture of the Wyoming Province

The North Owl Creek fault is an E–W-striking, basement-rooted Laramide structure located in the Owl Creek Mountains of north-central Wyoming that likely has Precambrian origins. It is defined by a rectilinear zone of deformation that extends eastward into the subsurface where it is postulated to intersect the Kaycee fault zone of the western Powder River Basin, and perhaps extends into western South Dakota along the Dewey fault zone. Several localized basement-rooted wrench zones have been identified in the foreland of the North American Cordillera; however, identification of more regional zones has been minimal. The presence of larger fault zones that cut nearly the entire Archean basement across the Wyoming Province has implications for Precambrian plate tectonics and structural inheritance in foreland basins such as the Powder River. This paper presents results of a structural analysis that tests this hypothesis.

Geologic Map of the Park Reservoir Quadrangle, Sheridan County, Wyoming

This project involved the construction of a detailed geologic map of the Park Reservoir, Wyoming 7.5-Minute Quadrangle (Scale 1:24,000). The Quadrangle occurs entirely in the Bighorn National Forest, which is a popular recreation site for thousands of people each year. This research advances the scientific understanding of the geology of the Bighorn Mountains and the Archean geology of the Wyoming Province. Traditional geologic mapping techniques were used in concert with isotopic age determinations. Our goal was to further subdivide the various phases of the 2.8–3.0 Ga Archean rocks based on their rock types, age, and structural features. This research supports the broader efforts of the Wyoming State Geological Survey to complete 1:24,000 scale geologic maps of the state. The northern part of the Bighorn Mountains is composed of the Bighorn batholith, a composite complex of intrusive bodies that were emplaced between 2.96–2.87 Ga. Our mapping of the Park Reservoir Quadrangle has revealed the presence of five different Archean quartzofeldspathic units, two sets of amphibolite and diabase dikes, a small occurrence of the Cambrian Flathead Sandstone, two Quaternary tills, and Quaternary alluvium. The Archean rock units range in age from ca. 2.96–2.75 Ga, the oldest of which are the most ancient rocks yet reported in the Bighorn batholith. All the Archean rocks have subtle but apparent planar fabric elements, which are variable in orientation and are interpreted to represent magmatic flow during emplacement. The Granite Ridge tear fault, which is the northern boundary of the Piney Creek thrust block, is mapped into the Archean core as a mylonite zone. This relationship indicates that the bounding faults of the Piney Creek thrust block were controlled by weak zones within the Precambrian basement rocks.

The Medicine Hat Block and the Early Paleoproterozoic Assembly of Western Laurentia

The accretion of the Wyoming, Hearne, and Superior Provinces to form the Archean core of western Laurentia occurred rapidly in the Paleoproterozoic. Missing from Hoffman’s (1988) original rapid aggregation model was the Medicine Hat block (MHB). The MHB is a structurally distinct, complex block of Precambrian crystalline crust located between the Archean Wyoming Craton and the Archean Hearne Province and overlain by an extensive Phanerozoic cover. It is distinguished on the basis of geophysical evidence and limited geochemical data from crustal xenoliths and drill core. New U-Pb ages and Lu-Hf data from zircons reveal protolith crystallization ages from 2.50 to 3.28 Ga, magmatism/metamorphism at 1.76 to 1.81 Ga, and εHfT values from −23.3 to 8.5 in the Archean and Proterozoic rocks of the MHB. These data suggest that the MHB played a pivotal role in the complex assembly of western Laurentia in the Paleoproterozoic as a conjugate or extension to the Montana Metasedimentary Terrane (MMT) of the northwestern Wyoming Province. This MMT–MHB connection likely existed in the Mesoarchean, but it was broken sometime during the earliest Paleoproterozoic with the formation and closure of a small ocean basin. Closure of the ocean led to formation of the Little Belt arc along the southern margin of the MHB beginning at approximately 1.9 Ga. The MHB and MMT re-joined at this time as they amalgamated into the supercontinent Laurentia during the Great Falls orogeny (1.7–1.9 Ga), which formed the Great Falls tectonic zone (GFTZ). The GFTZ developed in the same timeframe as the better-known Trans-Hudson orogen to the east that marks the merger of the Wyoming, Hearne, and Superior Provinces, which along with the MHB, formed the Archean core of western Laurentia.

Detrital zircon evidence for change in geodynamic regime of continental crust formation 3.7–3.6 billion years ago

The nature of the early terrestrial crust and how it evolved through time remains highly controversial. Whether conventional plate tectonics operated in the Hadean and early Archean and when it came into existence remains unclear. Here, we describe U-Pb ages, Hf isotope composition and trace element chemistry of 3.95–3.10 Ga old detrital zircons from the Singhbhum Craton in eastern India. The >3.7 Ga old zircons of this suite have crust-like Hf isotope compositions with strongly negative εHfi and their granitoid sources formed by intra-crustal reworking of a Hadean protolith that was extracted from primitive mantle at 4.4–4.5 Ga. The trace element and Hf isotope compositions of the zircons record a transition from higher Nb/Th (0.070 ± 0.010), Nb/U (0.045 ± 0.005), crust-like Hf isotope compositions, and longer crustal residence times of the protoliths prior to 3.7–3.6 Ga, to lower Nb/Th (0.032 ± 0.012), Nb/U (0.024 ± 0.009; 1σ), mantle-like Hf isotope compositions, and shorter protolith residence times post 3.7–3.6 Ga. The Nb/Th and Nb/U fractionation at 3.7 Ga seen in the detrital zircon record can be explained by transition to granitoid production at greater depths in an oceanic plateau-like regime. However, had that been the case, the crustal residence times of the protoliths of the granitoids from which the detrital zircons were sourced should have progressively increased with time, given the >1.1 billion years protracted history of granitoid magmatism in the craton, which is contrary to what is observed. We suggest that the changes in the granitoid chemistry recorded by the detrital zircons document a significant change in the depth of melting of the protoliths as well as in the tectonic setting of continental crust formation, and argue that it marks the transition to granitoid production from oceanic plateaus to arc-like tectonic environments. Broadly similar transitions at ca. 3.6 Ga have been documented in gneisses from the Acasta Gneiss Complex, the Jack Hills zircons and in detrital zircons from the Wyoming Province, which suggest that the end of the Eoarchean may have been marked by widespread transition in planetary tectonic regime.