Wyoming Province Research Articles

Origin of thick, first-cycle quartz arenite successions: Evidence from the 1.7 Ga Ortega Group, northern New Mexico

The 1700-Ma-old Ortega Group in northern New Mexico represents a > 1000-m-thick succession of metamorphosed quartz arenites and subordinate pelites. Published ages of detrital zircon grains from the Ortega Group indicate that the sediments were derived mainly from 1760-1700-Ma-old volcano-plutonic basement rocks and not from the Archean Wyoming Province to the north. Evidence for recycling within the Ortega Group also is lacking. Consequently, the Ortega Group quartz arenites are considered to be first-cycle sediments. The compositional and textural maturity of these sandstones is attributed to intense chemical and mechanical weathering. Abnormally high Al contents of pelites within the Ortega Group imply intense chemical weathering in the provenance similar to present-day low-relief tropical regions. Quartz arenites in the Ortega Group were deposited exclusively within a shallow-marine shelf environment influenced by tide, storm and wave processes. Prolonged sediment transport, especially on the tide-dominated inner shelf, in conjunction with inferred gradual subsidence optimized mechanical destruction of labile grains and rounding of quartz grains. Although the maturity of sediments in the Ortega Group can be accounted for, the mode of origin of > 850 m of quartz arenites remains enigmatic. Thick quartz arenite sequences of global distribution in the Phanerozoic coincide with first-order eustatic sea-level rises. World-wide occurrences of 1700-Ma-old, thick quartz arenite shelf successions suggest that the Ortega Group may have accumulated in response to a similar major eustatic sea-level rise.

Evidence for an Early Archean component in the Middle to Late Archean gneisses of the Wind River Range, west-central Wyoming: conventional and ion microprobe U-Pb data

Gneissic rocks that are basement to the Late Archean granites comprising much of the Wind River Range, west-central Wyoming, have been dated by the zircon U-Pb method using both conventional and ion microprobe techniques. A foliated hornblende granite gneiss member from the southern border of the Bridger batholith is 2670±13 Ma. Zircons from a granulite just north of the Bridger batholith are equant and faceted, a typical morphology for zircon grown under high grade metamorphic conditions. This granulite, which may be related to a second phase of migmatization in the area, is 2698±8 Ma. South of the Bridger batholith, zircons from a granulite (charnockite), which is related to an earlier phase of migmatization in the Range, yield a discordia with intercept ages of about 2.3 and 3.3 Ga. However, ion microprobe analyses of single zircon grains indicate that this rock contains several populations of zircon, ranging in age from 2.67 to about 3.8 Ga. Based on zircon morphology and regional geologic relationships, we interpret the data as indicating an age of ≃3.2 Ga for the first granulite metamorphism and migmatization. Older, possibly xenocrystic zircons give ages of ≃3.35, 3.65 and ≃3.8 Ga. Younger zircons grew at 2.7 and 2.85 Ga in response to events, including the second granulite metamorphism at 2.7 Ga, that culminated in the intrusion of the Bridger batholith and migmatization at 2.67 Ga. These data support the field and petrographic evidence for two granulite events and provide some temporal constraints for the formation of continental crust in the Early and Middle Archean in the Wyoming Province.

Archean rocks of the Black Hills, South Dakota: Reworked basement from the southern extension of the Trans-Hudson orogen

The Little Elk and the Bear Mountain terranes on the margin of the Precambrian core of the Black Hills provide the only known exposures of Archean rocks between the Archean Wyoming and Superior provinces. The Little Elk terrane consists of a supracrustal sequence dominated by biotite-feldspar gneiss (meta-arkose or metatuff) that is intruded by the slightly metaluminous to peraluminous, calc-alkaline Little Elk Granite, which has a U-Pb intercept age of 2,549 ± 11 m.y. A biotite-plagioclase schist (metagraywacke) dominates the Bear Mountain terrane and is intruded subparallel to foliation by the peraluminous granite at Bear Mountain. An isochron calculated from six severely discordant U-Pb zircon analyses yields an age of 2,393 ± 230 m.y. The Archean terranes participated in the Trans-Hudson orogenic event, suggested by a published Rb-Sr age for the Little Elk Granite (∼1,850 m.y.), 1.87-1.97 Gaigneous activity in the Black Hills, and the similarity in orientation of the Proterozoic deformational fabric with geophysical anomalies interpreted to be associated with the Trans-Hudson orogen. The metamorphic conditions estimated for the Archean rocks may be the result of a thermal event that produced the Proterozoic Harney Peak Granite and thermal high northeast of Lead, South Dakota. The unexposed Precambrian basement is probably dominated by granites similar to the Little Elk Granite and supracrustal rocks consisting of continental margin deposits, based on the REE contents of the Archean schists and on the heavy-mineral assemblages in, the uraniferous nature of, and the metasedimentary clasts in, the lower Proterozoic Box Elder Creek Formation. The ages and the relatively long-lived crustal sources for these Archean granites, and the similarity of the inferred basement characteristics to those of rocks in the Wyoming province, suggest that the Black Hills Archean rocks may be the easternmost exposures of this province. The probable Trans-Hudsonian reworking of the Black Hills Proterozoic rocks and their similarity to those in northern Saskatchewan imply that the Black Hills occupies a tectonic position comparable to that of rocks described from the Cree Lake Zone of Saskatchewan.

Meteoric depositions on a forest in southern Sardinia (Italy)

A strontium and neodymium isotopic investigation of plagioclase-rich cumulate rocks in the 1.43 Ga Laramie anorthosite complex (LAC) provides new insights into the evolution of magma chambers and underlying magma plumbing systems during the crystallization of Proterozoic anorthosites. The 725 km2 LAC intruded the boundary between Archean rocks of the Wyoming Province in the north and Proterozoic island arc terranes in the south, the Cheyenne belt, at pressures of 3–4 kilobars. Initial strontium and εND isotopic ratios from the three large composite anorthositic intrusions (Poe Mountain, Chugwater, and Snow Creek) and late troctolitic intrusions range from 0.7034–0.7055 and +0.9 to −4.9, respectively. The isotopic ratios in the cumulates vary as a result of (1) the isotopic composition of the parental mantle-derived high-Al gabbroic magmas, (2) the amount of contamination by Archean rocks during ascent through the crust, and (3) mixing of replenishing magmas and resident magmas with different isotopic compositions in the magma chamber at the final level of emplacement.Strontium isotopic and normative An variations across the 5–7 km thick stratigraphic section of layered plagioclase-rich cumulates in the 200 km2 Poe Mountain anorthosite indicate that multiple inputs of magma are needed to construct even relatively small intrusions in Proterozoic anorthosites. The earliest and stratigraphically lowest intrusion was emplaced as a plagioclase-rich magma that crystallized large volumes of compositionally homogeneous anorthosite. At least three subsequent chamber-wide episodes of magma replenishment, followed by mixing, are indicated by abrupt shifts in ISr and/or normative An in the stratigraphically higher levels of the Poe Mountain anorthosite. Strontium and neodymium isotopic disequilibrium between a high-Al clinopyroxene megacryst and the layered cumulates is consistent with a high-pressure origin for Al-rich pyroxene megacrysts in Proterozoic anorthosites. The megacryst crystallized in a magma chamber at pressures of 10–12 kilobars from relatively uncontaminated basaltic parent magma (high-Al gabbro) and was transported through the crust in progressively contaminated anorthositic magma.The range of observed isotopic compositions in the anorthositic rocks is nearly identical to that of high-Al gabbroic dikes in the LAC, supporting the proposition that high-Al gabbros are parental to anorthosite (Mitchell et al., 1995). The isotopic data require that the anorthositic parental magmas were contaminated during ascent through the crust by Archean orthogneisses and/or metapelitic rocks. Decreasing ISr and increasing εNd with a relative decrease in age of the intrusions (Snow Creek → Poe Mountain/Chugwater → troctolites) indicates that the magma conduit became increasingly insulated from crustal contamination over time.This study indicates that the strontium and neodymium isotopic systems can be used to distinguish between processes that occurred at lower crustal/upper mantle pressures, during the ascent of magma diapirs, and within individual magma chambers in Proterozoic anorthosites, and underscores the need for stratigraphic control when addressing the origin of plagioclase-rich cumulates.

Metamorphic terranes, isotopic provinces, and implications for crustal growth of the western United States

Simplified maps of the western Cordillera, indicating distribution of metamorphic facies and times of principal recrystallization have been compiled. Despite rearrangement by transcurrent shuffling, general trends are recognizable. Narrow belts of Phanerozoic high‐P blue schist, and serpentinized peridotite lie outboard from broad, penecontemporaneous high‐T metamorphosed calcalkaline magma‐intruded belts bordering the Pacific Ocean, and both are disposed approximately parallel to the present‐day North American margin; they represent recrystallized products of mid‐Paleozoic and younger subduction zones, Andean‐type margins, and exotic island arcs. In contrast, dominant metamorphic mineral parageneses within the continental interior are spatially associated with chiefly mesozonal granitic plutons. Glaucophane schists, eclogites, and large tracts of ophiolitic ultramafics are absent, in part due to selective overprinting of old accretionary margin lithotectonic assemblages by subsequent orogenies, high‐T recrystallization, and anatexis. For example, at depth within the California Coast Ranges, high‐P phases are currently being destroyed; farther inland, a general paucity of oceanic petrotectonic assemblages is evident, possibly reflecting the differential sinking of dense basaltic/peridotitic units during thermal softening. Preserved metamorphic field gradients reflect the magnitude of subsequent uplift and erosion as well as original PT conditions. Late Mesozoic to mid‐Cenozoic recrystallization accompanied earlier crustal thickening + ductile deformation, succeeded by later extension + brittle faulting + mylonitization; Mesozoic‐Cenozoic metamorphism involved cratonal rocks in the Mojave‐Sonoran Desert, but in Nevada and to the north, eugeoclinal sections were recrystallized. Metamorphic intensity decreases eastward toward the Phanerozoic hinge line. Basement rocks in the Rockies display the effects of shallow level crustal deformation but much less widespread evidence of post‐Proterozoic plutonism and regional metamorphism. Initial Pb, Nd, and Sr isotopic compositions and crystallization ages of igneous rocks, in comparison with times of intrusion, recrystallization, and metamorphic facies distributions are compatible with a geologic scenario involving gradual, proximal growth of the ancient continent chiefly southward from the Archean Wyoming craton during early and mid‐Proterozoic time, followed by late Proterozoic‐Phanerozoic westward development. Accretion involved the incorporation of some exotic, mostly oceanic, outboard terranes. However, continental growth in the western U.S. Cordillera during both Proterozoic and Phanerozoic time periods appears to have been due chiefly to partial fusion and magma ascent above subducting paleo‐Pacific lithospheric plates, combined with metamorphic (+ sedimentary) reworking in the forearc + trench + back arc setting, rather than resulting from the amalgamation of preexisting ancient continental fragments.

Tectonothermics of the North American great plains basement

Heat flow and heat generation data from the Superior, Churchill and Wyoming Provinces of western North America contribute to our understanding of the nature of the crust beneath the sedimentary cover of the Great Plains. A major structural boundary defined by gravity gradient data (Thomas et al., 1987) splits the terrains between the cratons into two distinct geothermal provinces. The Wyoming and Churchill Provinces are characterised by higher heat flow than the Superior Province and the eastern part of the intervening Hudsonian terrain, the Proterozoic Mobile Belt (PMB). Heat flow in the PMB is typical of that of the Superior craton. This suggests, on the basis of a model by Ballard and Pollack (1987), that the PMB is underlain by Archaean cratonic lithosphere. The similarity in heat flow between the PMB and the Superior craton suggests that the crust of the PMB is akin to that of the Superior craton rather than to that of the more radiogenic Wyoming and Churchill cratons. Geochronologic, gravity, aeromagnetic and geothermal data suggest that the structural boundary defining the western boundary of the PMB is also the eastern boundary of the Wyoming craton. Geothermal data alone suggest that the Wyoming craton might extend as far north as the cratonic segment of the Churchill Province, or that the Wyoming and Churchill Provinces may gradually merge together.

Geochemistry and petrogenesis of early Proterozoic amphibolites, west-central Colorado, U.S.A.

Early Proterozoic supracrustal rocks exposed in west-central Colorado include amphibolites intimately associated with felsic volcanics and associated volcaniclastic sediments. Three successions are recognized. The Dubois succession (1770-1760 Ma) is comprised chiefly of amphibolites and felsic volcanics. The amphibolites exhibit low concentrations of light REE and HFSE and are broadly similar in composition to tholeiites from immature island arcs and, in some respects, to NMORB. The Cochetopa succession (1745-1730 Ma) includes large amounts of felsic volcanics and volcaniclastic sediments together with amphibolites that exhibit variable but slight incompatible-element enrichment similar to basalts from evolved island-arc systems and associated back-arc basins. The Black Canyon succession is coeval with the Cochetopa succession and is comprised chiefly of volcaniclastic sediments with minor amphibolite. These amphibolites exhibit significant incompatible-element enrichment similar to basalts from continental-margin arc systems. Geochemical modeling indicates that the amphibolites evolved by closed-system fractional crystallization of depleted (for the Dubois succession) to variably enriched parental basalts (for the Cochetopa and Black Canyon successions), chiefly at shallow depths. Incompatible-element distributions in the amphibolites indicate that mantle sources changed with time from relatively depleted (1770-1760 Ma) to variable but slightly enriched (1745-1730 Ma). A provisional model calls upon the Dubois succession forming in an island-arc system followed by collision with the Archean Wyoming Province at ∼ 1750 Ma. The Cochetopa succession was deposited in an intra-arc basin developed within a new continental-margin arc, and the Black Canyon succession formed in a back-arc basin associated with this arc.

Evidence for Archean subduction and crustal recycling, Wyoming province

The Beartooth Mountains of the northern Wyoming province contain a suite of incompatible-element-enriched amphibolites of andesitic composition that contains both calcalkalic and tholeitic varieties. These amphibolites formed about 2800 Ma and show trace-element and lead isotopic signatures that suggest an origin similar to some modern andesites produced in convergent margin environments. In particular, these rocks show depletions of the high-field-strength elements (e.g., Nb, Ti, Zr) and enriched Pb isotopic compositions indicative of recycling of Pb from a crustal reservoir into the mantle. These data and other geologic considerations suggest that the northern Wyoming province may have been the site of a Late Archean convergent continental margin.

Pb, Sr, and Nd isotopic compositions of a suite of Late Archean, igneous rocks, eastern Beartooth Mountains: implications for crust-mantle evolution

A series of compositionally diverse, Late Archean rocks (2.74–2.79 Ga old) from the eastern Beartooth Mountains, Montana and Wyoming, U.S.A., have the same initial Pb, Sr, and Nd isotopic ratios. Lead and Sr initial ratios are higher and Nd initial ratios lower than would be expected for rocks derived from model mantle sources and strongly indicate the involvement of an older crustal reservoir in the genesis of these rocks. Crustal contamination during emplacement can be ruled out for a variety of reasons. Instead a model involving subduction of continental detritus and contamination of the overlying mantle as is often proposed for modern subduction environments is preferred. This contaminated mantle would have all the isotopic characteristics of mantle enriched by internal mantle metasomatism but would require no long-term growth or changes in parent to daughter element ratios. This contaminated mantle would make a good source for some of the Cenozoic mafic volcanics of the Columbia River, Snake River Plain, and Yellowstone volcanic fields that are proposed to come from ancient, enriched lithospheric mantle. The isotopic characteristics of the 2.70 Ga old Stillwater Complex are a perfect match for the proposed contaminated mantle which provides an alternative to crustal contamination during emplacement. The Pb isotopic characteristics of the Late Archean rocks of the eastern Beartooth Mountains are similar to those of other Late Archean rocks of the Wyoming Province and suggest that Early Archean, upper crustal rocks were common in this terrane. The isotopic signatures of Late Archean rocks in the Wyoming Province are distinctive from those of other Archean cratons in North America which are dominated by a MORB-like, Archean mantle source (Superior Province) and/or a long-term depleted crustal source (Greenland).

Allochthonous Archean basement in the northern East Humboldt Range, Nevada

Archean basement rocks occur in the core of a fold nappe in the northern East Humboldt Range, Nevada. The Archean rocks consist primarily of plutonic orthogneiss and metasedimentary paragneiss. Both units contain numerous amphibolitic bodies interpreted as mafic intrusions. U-Pb zircon studies indicate that the orthogneiss unit is at least 2520 ±110 Ma and therefore is an example of an Archean granitoid. These data extend the known distribution of Precambrian basement rocks into northeastern Nevada and provide new constraints on the delineation of the Archean Wyoming province. The basement rocks are surrounded by upper amphibolite facies, metasedimentary rocks correlated with late Precambrian to Devonian(?) rocks of the eastern Great Basin miogeoclinal sequence. The contact between the Archean basement rocks and the metasedimentary mio geoclinal sequence is interpreted as a premetamorphic and prefolding, low-angle fault. The age and prefolding geometry of this low-angle fault are currently poorly constrained, but it may record important Mesozoic crustal shortening in the hinterland of the Sevier orogenic belt. The age of nappe formation is not yet fully established, but at least some isoclinal folds in the area were formed in the Tertiary and are kinematically linked to a major extensional deformation.

Crustal genesis on the Oregon Plateau

The areal and temporal distribution and the chemical and isotopic characteristics of Cenozoic volcanic rocks from the Oregon Plateau reflect the changing tectonic conditions affecting this area over the last 20 m.y. Following subduction zone related calc‐alkaline volcanism, flood basalt eruptions occurred in the time interval 18–14 Ma. This primarily basaltic volcanism is interpreted to be the result of a major shift in spreading orientation of the Juan de Fuca ridge that resulted in changes in the geometry of subduction against the Pacific Northwest. The compositions of the mid‐Miocene flood basalts show signs of extensive crystal fractionation, in some cases accompanied by crustal contamination. Roughly 11 m.y. ago, basalt eruptions showed an increasing tendency toward smaller volumes and less evolved compositions. This implies that as extension in the Oregon Plateau proceeded, the crust became less of an impediment to the passage of relatively unfractionated mantle melts. Initial Sr, Nd, and Pb isotopic compositions of all the basalts show more correlation with the geographic position of eruption than with the composition of the lava. Primary control over the isotope systematics appears to be due to changes in the age and chemical characteristics of the crust and mantle throughout this area. These observations suggest that an area of some 130,000 km2 of crust and basalt depleted subcontinental mantle has been added to northwestern North America over the last 20 m.y. The intensity of basalt volcanism as manifest in the Columbia River Group and in the Oregon Plateau does not appear to reflect the presence of a “hot spot” beneath the area. Both the areal and temporal distribution of the lavas and their geochemical characteristics are more easily reconciled with the suggestion that this crustal section grew by “back arc” spreading in a manner analogous to the formation of the marginal basins of the western Pacific. This spreading initiated along the western border of the Archean Wyoming craton with the location of mantle ascent, melt production, and extraction controlled by the changing geometry of subduction, the existence of a zone of lithospheric weakness, and the presence of a thick, low‐density, subcontinental mantle “keel” existing beneath the Archean craton in this area.

Proterozoic accretionary tectonics at the southern margin of the Archean Wyoming craton

The Proterozoic Cheyenne belt, at the southern margin of the Archean Wyoming craton, consists of strongly deformed lithotectonic blocks bound by major mylonite zones. From north to south and structurally lowest to highest, these blocks include (1) Archean crystalline basement and associated early Proterozoic miogeoclinal rocks; (2) an amphibolite-grade orthogneiss terrane of unknown age containing probable rift-related mafic intrusive rocks; (3) a 1750–1790 Ma, marginal basin(?) succession consisting of upper amphibolite-grade pelitic and volcano-genie schist, associated peraluminous granite, and minor ultramafic rocks; and (4) a 1750–1790 Ma intermediate to mafic plutonic-metamorphic complex interpreted as the deep roots of an island-arc system. The earliest deformation, D1, produced a synmetamorphic, penetrative, horizontal transposition foliation and associated recumbent folds. Tectonic blocks were juxtaposed along major mylonite zones during D2. Macroscopic anil microscopic structures associated with D2 indicate northward thrusting along low-angle mylonite zones of successively deeper crustal blocks over supracrustal rocks of the Wyoming craton at about 1750 Ma. Thrusting occurred at minimum temperatures of 475 °C and produced an inverted metamorphic gradient in pelitic rocks north of the belt. Mylonite zones were subsequently steepened and reactivated under greenschist-facies conditions during a period of dextral strike slip (D3) between 1750 and 1400 Ma. Cataclasis (D4) and brecciation associated with the Laramide orogeny (D5) locally overprint earlier structures. Oblique convergence between an island arc and the Archean continental nucleus may explain accretion, crustal shortening, and subsequent strike slip. The Phanerozoic accretionary events of eastern and western North America provide an analog for this model. Presence of similar lithologies and shear zones south of the Cheyenne belt suggest that the southern margin of the Wyoming craton may have been a long-lived zone of crustal accretion.

Evolution of the Early Proterozoic Colorado province: Constraints from U-Pb geochronology

The Colorado province represents an addition of a belt of rocks more than 500 km wide to the southern margin of the Archean Wyoming craton during the Early Proterozoic, between about 1790 and 1660 Ma. Correspondence in ages between metamorphism, deformation, and plutonism; association of volcanic rocks with comagmatic calc-alkalic plutons; and lack of older basement are all consistent with the interpretation that the rocks of the province are products of arc magmatism and cannibalistic sedimentation along a convergent margin at the southern edge of the craton.

Gravity patterns and Precambrian structure in the North American Central Plains

Research Article| June 01, 1987 Gravity patterns and Precambrian structure in the North American Central Plains M. D. Thomas; M. D. Thomas 1Geological Survey of Canada, 1 Observatory Crescent, Ottawa, Ontario K1A 0Y3, Canada Search for other works by this author on: GSW Google Scholar V. L. Sharpton; V. L. Sharpton 1Geological Survey of Canada, 1 Observatory Crescent, Ottawa, Ontario K1A 0Y3, Canada Search for other works by this author on: GSW Google Scholar R.A.F. Grieve R.A.F. Grieve 1Geological Survey of Canada, 1 Observatory Crescent, Ottawa, Ontario K1A 0Y3, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information M. D. Thomas 1Geological Survey of Canada, 1 Observatory Crescent, Ottawa, Ontario K1A 0Y3, Canada V. L. Sharpton 1Geological Survey of Canada, 1 Observatory Crescent, Ottawa, Ontario K1A 0Y3, Canada R.A.F. Grieve 1Geological Survey of Canada, 1 Observatory Crescent, Ottawa, Ontario K1A 0Y3, Canada Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1987) 15 (6): 489–492. https://doi.org/10.1130/0091-7613(1987)15<489:GPAPSI>2.0.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation M. D. Thomas, V. L. Sharpton, R.A.F. Grieve; Gravity patterns and Precambrian structure in the North American Central Plains. Geology 1987;; 15 (6): 489–492. doi: https://doi.org/10.1130/0091-7613(1987)15<489:GPAPSI>2.0.CO;2 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 SocietyGeology Search Advanced Search Abstract Bouguer gravity patterns revealed in horizontal gradient images are used to locate and examine the tectonic nature of Precambrian structural boundaries buried beneath the North American Central Plains. The southern boundary of the Superior province west of the midcontinent gravity high (MGH), interpreted as a collisional suture, is relocated as much as 200 km south of previous geophysically defined positions, close to a position defined recently by using drill-hole information; east of the high, this boundary is thought to trace a 900-km arc subparallel to the MGH. The western boundary of the Superior province is regarded as a suture containing significant components of transcurrent faulting. The eastern boundary of the Wyoming province, another suture, is traced northward into the Churchill province, near the northwestern margin of a 1900–1600 Ma magmatic belt. This boundary and Proterozoic terrane to the east are transected by previously unrecognized faults hundreds of kilometres long. Gravity patterns also suggest that the North American Central Plains conductivity anomaly may not be a continuous, suture-related conductor. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Comments on “The cut and recorrelated gravity map of the United States”

G. L. Kinsland (Eos, August 28, 1984, p. 490) proposed that a late Proterozoic transcontinental right‐lateral strike‐slip fault system with approximately 600 km of offset exists in the United States, striking northwest from southeastern Alabama to northwestern Montana. After Kinsland corrected the proposed displacement, two aligned gravity features were noted: first, the Central North American Rift System (CNARS) and the Hartville Uplift and second, the Late Proterozoic rift passive margins of the Ouachita Mountains and the Appalachian Mountains. However, significant displacement along the proposed trace of the fault is precluded by the disruption of various terranes that would result (Figures 1a and lb). Some of these terranes include The northwestern corner of the Wyoming province (feature 4 in the figures; Dutch [1983]). The North American Central Plains conductive anomaly (NACPCA)‐Mullen Creek‐Nash Fork shear zone (feature 3; Camfield and Gough [1977]). The CNARS (feature 2), which Yarger and Lam [1983] and Yarger [1982] extended through Kansas to the Oklahoma border. Danbom et al. [1980] found gravity and magnetic evidence for a probable extension of the CNARS into northern Oklahoma.

Kinematic history of the Cheyenne belt, Medicine Bow Mountains, southeastern Wyoming

The Proterozoic Cheyenne belt marks the southern boundary of the Archean Wyoming province. Within the Medicine Bow Mountains, it consists of strongly deformed, lithologically distinct blocks bounded by mylonite zones. Detailed field and petrographic investigations of the boundary, augmented by study of microscopic kinematic indicators, allow us to refine existing models for tectonic development of the belt. Northward thrusting under amphibolite facies conditions along low-angle mylonite zones emplaced successively deeper crustal blocks over supracrustal rocks of the Wyoming province. Mylonite zones were subsequently steepened and reactivated under greenschist-facies conditions during a period of distributive dextral strike slip. We suggest oblique convergence between an island arc and the Wyoming craton as a possible mechanism for both deformational events. Accretionary events in eastern and western North America, which involved initial convergence followed by strike slip, provide a Phanerozoic analog for our model.

Precambrian basement geology of North and South Dakota

Combined analysis of drill-hole, gravity, and magnetic data indicates that the buried Precambrian basement rocks of the Dakotas can be divided into several lithotectonic terranes. Eastern North Dakota and northeastern South Dakota are underlain by Archean gneiss. Except for the Black Hills region of South Dakota, where Archean rocks are also exposed, the western third of both Dakotas is underlain mainly by Early Proterozoic gneiss and metasedimentary rocks. Part of this region is underlain by Archean crust with an Early Proterozoic tectonic overprint. A broad transition zone of strongly overprinted Archean crust occurs between the Proterozoic rocks to the west and the Archean rocks to the east. South central South Dakota is underlain by an Early Proterozoic batholith. Early Proterozoic felsic volcanic rocks occur in southeast South Dakota. The bootheel portion of South Dakota contains a diverse assemblage of basement rocks that are partly Archean in age.Churchill Province rocks of the Trans-Hudson foldbelt project into the western Dakotas. The Thompson nickel belt and the Pickwitonei gneiss belt correlate with the western and eastern halves, respectively, of the transition between Archean and Proterozoic crust, and the Archean Glennie – Hanson Lake microcontinent of the Churchill Province likely extends into western North Dakota. Archean rocks of Minnesota extend into the eastern Dakotas, and the Wyoming craton extends to the Black Hills region. The Cheyenne foldbelt projects into southwest South Dakota. The Penokean foldbelt of Michigan and Wisconsin does not extend into the Dakotas, but it most likely extends into northwest Iowa.Tectonic evolution of the Early Proterozoic terrane in the Dakotas was most likely similar to plate tectonic models for the evolution of the Trans-Hudson foldbelt in the Churchill Province. As in the Churchill Province, the western Dakotas are underlain by Early Proterozoic rocks, but it is not known whether these rocks formed as a result of rifting and subsequent closure of a once extensive Archean crust or as a result of collision of once widely separated blocks of Archean crust.

Proterozoic history of the midcontinent region of North America

Age and petrographic data from the buried basement of the midcontinent region of North America, integrated with information from exposed rocks and magnetic- and gravity-anomaly maps, allow much of the Proterozoic history of the region to be assembled. The Superior craton may be traced into the subsurface on the basis of characteristic magnetic patterns and limited age data. The region between the Superior craton and the Wyoming craton to the west is evidently underlain by southerly extension of the Trans-Hudson orogen of Canada. The Penokean orogen formed on the southern margin of the Superior craton 1890–1830 Ma, but is not inferred west of northwestern Iowa in the subsurface. Between 1780 and 1720 Ma, a major orogen developed along the southern margin of the continent and is exposed in Arizona and Colorado. These rocks are volcanogenic and, for the most part, juvenile additions to the crust; they can be traced beneath the plains as far as eastern Kansas and Nebraska. Another orogen formed farther to the south about 1700–1630 Ma and is exposed in southern Arizona and New Mexico; rocks of this age and type have beer, traced as far east as central Missouri but may extend as far as central Michigan. A major geophysical feature of the midcontinent is a system of northwest-trending magnetic and gravity anomalies in Missouri, Kansas, and Nebraska; the origin of these is not currently understood. The tectonic history of the midcontinent between 1480 and 1340 Ma was dominated by extensional formation of two widespread granite-rhyolite terranes that evidently were formed from, and overlie, the orogenic provinces. The older, formed 1450–1480 Ma, underlies the eastern midcontinent, whereas the younger, formed 1340–1400 Ma, underlies the southwestern midcontinent. The latest Proterozoic events were the formation of the midcontinent rift system and the collisional Grenville and Llano provinces about 1100 Ma.

A Case for Brittle Deformation of the Basement During the Laramide Revolution in the Rocky Mountain Foreland Province

Debate over the question of whether basement rocks folded or faulted during the Laramide orogeny in the Wyoming province of the Rocky Mountain foreland has raged for many years. Proponents of one view or the other have drawn support for their case from theoretical arguments, from experimental laboratory results, from interpretations of seismic and subsurface borehole data, and from direct observation of surface outcrops and subsurface mine exposures. An analysis of the validity and applicability of these various approaches included the generation of synthetic seismic sections and an evaluation at balanced cross sections. This analysis indicated that the notion of a folded basement is supported only by the interpretation of seismic and borehole data whereas the concept of brittle deformation of the basement is supported by all of these lines of reasoning, interpretation, and observation. The conclusion is that structural basement did not fold during the Laramide orogeny.

An evolutionary model of the western churchill province and western margin of the superior province in Canada and the North-Central United States

Regional-scale geophysical information, which includes aeromagnetic, gravity, seismic refraction, multi-channel seismic reflection and electromagnetic induction data, is used to extend our knowledge of the Canadian Shield beneath the Phanerozoic Williston basin of south-central Canada and the north-central United States. A new tectonic map based on this information shows the Proterozoic Flin Flon-Snow Lake and La Ronge-Lynn Lake volcanic island arcs and their associated fore-arc (Kisseynew belt) and back-arc (Reindeer-South Indian Lakes belt) basins wedged between the Archean Superior craton on the east and the Archean parts of the Churchill and Wyoming cratons on the west. Along the western margin of the Superior craton the Thompson nickel belt, including its extension southwards beneath the Williston basin, is interpreted to have been successively the site of continental rifting and rupturing, an evolving continental margin, a continent-volcanic island arc “suture” zone and eventually a continental-scale strike-slip fault. The North American Central Plains electrical conductivity anomaly and closely related seismic low-velocity zones are explained by the presence in the lower crust of buried slices of hydrated oceanic-type material, situated within the southward extension of the Reindeer-South Indian Lakes remnant back-arc basin and adjoining tectonic units. A new plate tectonic model is proposed for this region that involves the rifting and rupturing of the Archean continents and the opening and closing of one or more oceanic basins. This model is shown to be consistent with most of the geological, geophysical and geochronological data that pertains to the Proterozoic evolution of the exposed Shield and similar geophysical data and subsurface geochronological information from further south.