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Unconformity‐related uranium (URU) deposits in the Athabasca Basin, Canada, include the highest grade, large tonnage deposits in the world. Recent studies of the Patterson Lake corridor uranium deposits suggest a deep heat source enhances hydrothermal fluid flow and incursion of basinal brines along brittlely reactivated ductile shear zones. However, the spatial extent of the ore systems and lower crustal features capable of driving the hydrothermal cells remained largely unresolved. Three‐dimensional electrical conductivity models derived from inversion of magnetotelluric data and coincident gravity modeling identified a voluminous conductivity anomaly and corresponding gravity low related to a belt of high‐heat producing intrusions that dips below the deposit. Shallow conductive bodies are identified below the deposits and similarly prospective corridors. The volume and location of the intrusive bodies may have provided a source of radiogenic heat that helped drive remobilization of fluids along reactivated structures, redefining the spatial extent of URU systems. Plain Language Summary: The western Athabasca Basin, straddling the border of Saskatchewan and Alberta in Canada, is home to some of the highest‐grade, large tonnage uranium deposits in the world. Recent studies have suggested a spatial connection between uranium ore systems that are found near the earth's surface and geological processes occurring deep within the crust. However, the processes connecting the lower crust to these shallow deposits is poorly understood. This study used 3D models created from magnetotelluric data, a passive a geophysical technique that measures the earth's natural electric and magnetic fields, to image the earth from surface to a depth of 40 km. We identified a voluminous igneous intrusion that extends from the surface to great depths in the earth's crust. We propose that this intrusion may have contributed to the uranium endowment of the basin, and that radiogenic heat from the large igneous body helped circulate uranium‐bearing fluids through the shallow crust along reactivated geologic structures. The modeling, when combined with other geoscientific datasets, showcases the role of high‐heat producing granitic intrusions in driving the hydrothermal fluid cells that formed the deposits. Key Points: Integrated resistivity—gravity modeling over the Arrow uranium deposit in CanadaMagnetotelluric inversion images deep‐seated thermal source and fluid pathways that formed some of the highest‐grade uranium depositsHolistic ore system model illustrates relationship between crustal ore system and lower crustal resistivity anomalies [ABSTRACT FROM AUTHOR]

Middle and lower orogenic crust exposed at the Earth's surface represent an exceptional opportunity to investigate accretionary and collisional processes in various tectonic settings. In particular, deciphering their metamorphic history in terms of evolving P-T-t-D paths enables an assessment of lower crustal behavior and the role of granulitic terranes throughout orogeny. In this paper, we use field work, average P-T thermobarometry, phase equilibria modeling, and zircon and monazite U Pb dating coupled with detailed Lu Hf and Sm Nd garnet dating to investigate the style, the timing, the duration, the extent, and the significance of metamorphism in the lower plate of the Torngat Orogen, a transpressional orogen that is part of the larger-scale Paleoproterozoic Trans-Hudson Orogen in Canada. Supracrustal rocks in the region record clockwise P-T paths with peak conditions gradually increasing from 8.7 kbar at 815 °C to 11 kbar at 915 °C from west to east, and homogeneous near-solidus retrograde conditions at ~6–8 kbar and ~ 775–850 °C. Lu Hf garnet geochronology indicates that prograde metamorphism was ongoing at c. 1885 Ma, marking the onset of crustal thickening earlier than previously thought. Zircon and monazite U Pb ages suggest protracted growth during melt crystallization along the retrograde metamorphic path between c. 1859 and 1811 Ma. The minimum duration of anatexis is estimated to be ~50 Myr. We show that the metamorphic effect of the Torngat orogeny is wider than previously thought, resulting in a large mid-pressure, high-temperature domain with widespread anatexis. Our dataset provides an example of the utility of multi-geochronometer dating for deciphering the metamorphic history of granulitic terranes. • We quantify the metamorphic conditions of the exhumed middle crust of the Paleoproterozoic Torngat Orogen • We combine field work, petrography, mineral chemistry, thermometry, phase equilibria modeling, and multi-system dating • The onset of continental collision is estimated at c. 1895 Ma and was followed by protracted anatexis over 50 to 80 Myr. • We discuss the geological significance of garnet Lu-Hf and Sm-Nd dating in long-lived granulite terranes [ABSTRACT FROM AUTHOR]

To advance the understanding of crustal deformation and earthquake hazards in Canada, we analyze seismic and geodetic data sets and robustly estimate the crust strain accumulation and release rate by earthquakes. We find that less than 20% of the accumulated strain is released by earthquakes across the study area providing evidence for large‐scale aseismic deformation. We attribute this to glacial isostatic adjustment (GIA) in eastern Canada, where predictions from the GIA model account for most of the observed discrepancy between the seismic and the geodetic moment rates. In western Canada, only a small percentage (<20%) of the discrepancy can be attributed to GIA‐related deformation. We suspect that this may reflect the inaccuracy of the GIA model to account for heterogeneity in Earth structure or indicate that the present‐day effect of GIA in western Canada is limited due to the fast response of the upper mantle to the deglaciation of the Cordillera Ice Sheet. At locations of previously identified seismic source zones, we speculate that the unreleased strain is been stored cumulatively in the crust and will be released as earthquakes in the future. The Gutenberg‐Richter model predicts, however, that the recurrence interval can vary significantly in Canada, ranging from decades near plate boundary zones in the west to thousands of years in the stable continental interior. Our attempt to quantify the GIA‐induced deformation provides the necessary first step for the integration of geodetic strain rates in seismic hazard analysis in Canada. Plain Language Summary: We took advantage of the increasing density of Global Navigation Satellite System and seismic stations across Canada to perform a detailed investigation of the strain buildup and release rate by earthquakes. Our results indicate that strain release rates by earthquakes are slower than the strain accumulation rates except at locations where earthquakes are generated due to tectonic and/or man‐made activities. We compare our results to the estimated rate of strain accumulation due to postglacial rebound and found that the postglacial rebound model can satisfactorily explain our observation in eastern Canada but not in western Canada. Consequently, we infer that the effect of the postglacial rebound in western Canada may be short lived or the model used is less accurate. We investigate the possibility that strain is cumulatively stored in the crust and can be released by future earthquakes. Our results reveal that the recurrence interval of a major earthquake (magnitude ≥ 6) can vary significantly in Canada, ranging from decades near plate boundary zones in the west to thousands of years in the stable continental interior. Our study demonstrates the advantage of jointly analyzing seismic and geodetic data sets to obtain a more complete picture of crustal deformation and potential seismic hazard. Key Points: Analysis of seismic and geodetic data sets across Canada reveals that only 20% or less of the accumulated strain is released by earthquakesGlacial isostatic adjustment model can account for most of the discrepancy between the seismic and geodetic moment rates in eastern Canada, but not in western CanadaThe recurrence time of large earthquakes in Canada varies from decades near the plate boundary to millenniums in the plate interior [ABSTRACT FROM AUTHOR]

The Grenville Province forms the exhumed remnants of a 1.1 Ga collisional orogeny that telescoped an older continental margin. Terranes with distinct crustal formation ages can be mapped using Nd isotopes, revealing a ramp-flat thrust structure. The ramp is identified by the presence of retrogressed eclogites, and its trajectory is refined using Nd model ages. The main allochthon is locally overlain by the Parry Sound klippe, but is also underlain by a tectonic duplex. Northwest-directed nappes represent remnants of a corrugated thrust sheet, but a ring-shaped remnant was also preserved where the thrust sheet was down-buckled under the dense rocks of Parry Sound domain. [ABSTRACT FROM AUTHOR]

The Cretaceous geological evolution of the Canadian Arctic was marked by voluminous magmatism comprising Canada's portion of the High Arctic large igneous province (HALIP) that is thought to have resulted from a mantle plume head. This magmatism is largely recorded as extensive Early Cretaceous lavas of the Isachsen Formation, Late Cretaceous continental flood basalts of the Strand Fiord Formation, and an extensive network of dykes and sills forming their plumbing systems. Axel Heiberg Island near South Fiord contains a small, structurally complex portion of the overall network of HALIP tabular intrusions, from which samples from the South Fiord intrusions and Isachsen Formation basaltic lavas were analysed to better understand their petrogenesis. Specifically, we apply trace-element ratios together with Sm-Nd isotope systematics in order to identify processes that shaped the chemical evolution of the South Fiord intrusions and Isachsen Formation lavas, to identify (1) mantle source chemistries and (2) the role of crustal assimilation. On the basis of Sm-Nd isotopic results, South Fiord intrusion magmas were derived from a homogeneous mantle source whereas a more heterogeneous source is invoked for the Isachsen Formation lavas. Furthermore, modelling with Th/ La, Nb/U, Ba/Th and Ba/Nb suggests that South Fiord intrusive magmatism was contaminated by sedimentary rocks from the Sverdrup Basin. Conversely, a trend towards high Ba/Th in Isachsen Formation lavas suggests a subducted sediment component derived from extinct subduction zones. We surmise that (1) South Fiord intrusive rocks are geochemically distinct from the Isachsen Formation lavas and (2) the HALIP mantle plume head intersected and incorporated sediments from ancestral subduction zones to the present-day Aleutian and Alaska subduction zones to produce the Isachsen Formation flows, whereas the South Fiord intrusions (and correlated Strand Fiord magmatism) were generated from plume material that interacted with upper crustal sedimentary rocks. [ABSTRACT FROM AUTHOR]

Most studies of the seismic structure of continental crust assume that the wave speeds are isotropic at seismic wavelengths. The ability to measure surface wave propagation speed from the cross-correlation of ambient seismic noise provides new opportunities to image the crust and uppermost mantle. We investigate radial anisotropy in the continental crust of northwestern Canada from group-velocity curves of Love and Rayleigh waves obtained from ambient-noise cross-correlation. We test the null hypothesis that the Love and Rayleigh group-speed curves can be simultaneously fit by an earth model containing isotropic seismic velocities throughout the crust. Group velocity is predicted for 200 000 one-dimensional earth models, which are generated by randomly varying the crustal shear velocity and radial anisotropy within a prescribed range. The goodness-of-fit of the predictions is assessed by comparison with two sets of observed dispersion curves that correspond to two tectonically distinct terranes: the Archean/early Proterozoic craton and the transition from craton to Cordillera. The majority of best-fitting models contain VSH > VSV (4–5 per cent) in the middle crust. The finding that the middle/lower crust is seismically anisotropic across a large swath of northwestern Canada, combined with recent observations of anisotropic crust in much of the western United States, suggests that anisotropy may be ubiquitous in the continental crust. [ABSTRACT FROM AUTHOR]

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The crustal structure of the NE Flemish Cap margin was determined along a 460-km-long refraction/wide-angle reflection seismic transect (FLAME Line) to define the thickness, structure and composition of the crust and uppermost mantle along the line. A P-wave velocity model was developed from forward and inverse modelling of dense airgun shots recorded by 19 ocean bottom seismometers. A coincident multichannel seismic profile was used to guide the modelling as reflections could be identified down to Moho. The model displays a sediment cover of up to 3.6-km-thick, subdivided into three layers with velocities of 1.8-1.9 km s, 2.8-3.1 km s and 4.7-4.8 km s. For the western part of the FLAME Line over Flemish Cap, the P-wave velocity model displays an up to 32-km-thick, three-layer continental crust. The continental crust has velocities of 5.8-6.1 km s, 6.3-6.45 km s and 6.65-6.85 km s and thicknesses of about 5 km, 7 km and 20 km in the upper, middle and lower layers, respectively. The thick continental crust thins to a two-layer, 6-km-thick crust (upper layer is 5.55-6.0 km s and the layer below is 6.65-6.8 km s) over a distance of 45 km. S-wave velocities are determined in the upper layer of the thick continental crust over Flemish Cap and the transition zone by assigning Poisson's ratios in the P-wave velocity model. Comparison of calculated to observed arrival times gives a Poisson's ratio of 0.27 in the upper layer and 0.28 in the layer below, which suggests that the composition of the crust is primarily continental in both the thick crust and the thin crust of the transition zone. The thin continental crust is stretched over a width of 80 km and is underlain by a layer with velocities of 7.5-7.9 km s. We interpret this layer as partially serpentinized mantle, which is consistent with observations from the Newfoundland margin to the south. The serpentinized mantle terminates 30 km seaward of the thick continental crust. At the seaward-most end of the thin continental crust, a prominent ridge feature is observed. The seismic refraction and multichannel seismic data results indicate a mixed character between serpentinized mantle with volcanic extrusions or continental crust. The reflection seismic data show a high relief basement from the ridge feature and seaward. The FLAME Line crosses magnetic anomaly 34 and extends another ∼50 km seaward well into oceanic crust. The ridge is flanked seaward by a two-layer oceanic crust. The upper layer (Layer 2) has velocities of 4.8-5.0 km s for the landward-most 35 km of oceanic crust and 4.8-6.2 km s for the seaward 60 km. The average thickness of Layer 2 is ∼2 km. The lower layer (Layer 3) has velocities of 6.7-7.2 km s and a thickness of ∼3.5 km. The velocity model is consistent with a sharp onset of seafloor spreading seaward of the ridge feature. [ABSTRACT FROM AUTHOR]

Application of regional geophysical and geological methods throughout two decades of Canada’s Lithoprobe project provides new opportunities to analyze the Mohorovičić discontinuity (Moho) and crust–mantle transition. The transect format employed during Lithoprobe, in which 10 specified regions of Canada were targeted for approximately a decade each, between 1984 and 2003, permitted teams of scientists to focus on geological, geophysical, and tectonic issues for each transect. As a primary objective was to enhance knowledge of the structure of the crust and lithosphere, an obvious target in each transect was the nature and origin of the Moho and crust–mantle transition. Accordingly, the combined results provide new perspectives on the Moho and the relationship of the Moho to the crust–mantle transition. Perhaps the most important result is that the continental geophysical Moho is a deceptively simple feature; it has a variety of signatures at different scales that preclude a single, universally applicable interpretation. In methods that provide large-scale information, such as regional seismic studies, it is a relatively abrupt refraction velocity contrast that often displays a dramatic downward decrease in seismic reflectivity. However, its origin in a geological or tectonic sense is perhaps best determined by careful analyses of structural details near the geophysical Moho, which are complex and varied. In some areas within Canada, it appears that the geophysical Moho may be old and perhaps remains from the time the crust formed; in other areas, it appears to be a relatively young feature that was superimposed onto older crustal fabrics. L’application de méthodes géophysiques et géologiques régionales durant les deux décennies du projet canadien Lithoprobe a fourni de nouvelles possibilités d’analyser la discontinuité de Mohorovičić (Moho) et la transition croûte–manteau. Le format de transects utilisés durant Lithoprobe, selon lequel 10 régions spécifiques du Canada ont été ciblées pour environ une décennie chacune entre 1984 et 2003, a permis à des équipes de scientistes de se pencher sur les enjeux géologiques, géophysiques et tectoniques de chaque transect. Puisqu’un des principaux objectifs était de rehausser la connaissance de la structure de la croûte et de la lithosphère, une cible évidente dans chaque transect était la nature et l’origine de la transition entre le Moho et la croûte–manteau. Les résultats combinés fournissent donc de nouvelles perspectives du Moho et de la relation du Moho à la transition croûte–manteau. Le résultat peut-être le plus significatif est que le Moho géophysique continental est une caractéristique dont la simplicité est trompeuse; il possède plusieurs signatures à différentes échelles qui empêchent une interprétation unique et universellement applicable. Lorsque les méthodes donnaient de l’information à grande échelle, par exemple les études sismiques régionales, il se présente comme un contraste de vitesse de réfraction relativement abrupt, souvent avec une diminution de réflectivité sismique vers le bas. Toutefois, son origine dans un sens géologique ou tectonique est peut-être le mieux déterminé par une analyse soignée des détails structuraux à proximité du Moho géophysique, et ces détails sont complexes et variés. Dans certains secteurs du Canada, il semble que le Moho géophysique puisse être vieux et constituer un vestige du temps de formation de la croûte; dans d’autres secteurs, il semble constituer une caractéristique jeune qui a été superposée sur des fabriques plus anciennes de la croûte. [ABSTRACT FROM AUTHOR]

Variations in crustal thickness provide important clues as to the formation of the crust, present-day isostatic equilibrium, and crustal stress. The map of the topography of the Mohorovicic discontinuity in Alberta is revised using 1900 km of reanalysed seismic reflection profiles acquired as part of the Lithoprobe Alberta Basement Transect. Time sections were depth migrated using a parallelized algorithm that accounts for steeply dipping structures. The migration process employed a geologically consistent velocity model of the metamorphic crust derived from earlier refraction experiments and constrained by compilations of high-pressure rock velocity measurements. We found that knowledge of the velocities in the sedimentary column strongly influenced the quality of the migration calculations. The Mohorovičić discontinuity is generally distinguished in these profiles, on the basis of sharp changes in reflectivity, at depths of 35–48 km. Sharp reflections from this boundary are rare. A number of geologic features are of note in these lines. A localized (~50 km extent) crustal thinning is observed in the Peace River region; this thinning is consistent with an adjacent oxygen isotopic anomaly indicative of crustal extension. In central Alberta, the Mohorovičić discontinuity topography is suggestive of a sharp jump of 10 km indicative of mantle faulting associated with the Snowbird tectonic zone. In southern Alberta, the crust thickens substantially across the Vulcan structure, with the greatest thickness correlating with the Vulcan structure itself indicating a collisional origin as noted by other authors.Des variations dans l'épaisseur de la croûte fournissent d'importants indices quant à la formation de la croûte, au présent équilibre isostatique et aux contraintes dans la croûte. La carte de la topographie de la discontinuité de Mohorovicic en Alberta est révisée en utilisant 1900 km de profils réanalysés de sismique réflexion acquis du transect Lithoprobe du socle de l'Alberta. Des sections de temps ont été migrées vers le bas en utilisant un algorithme de mise en parallèle qui tient compte des structures à fort pendage. Le processus de migration utilise un modèle de vitesse qui concorde géologiquement avec la croûte métamorphique, dérivé d'essais antérieurs de réfraction et limité par des compilations de mesures de vitesse dans les roches à haute pression. Nous avons trouvé que la connaissance des vitesses dans la colonne sédimentaire influence grandement la qualité des calculs de migration. La discontinuité de Mohorovicic se distingue habituellement dans ces profils, sur la base de changements abrupts dans la réflectivité, à des profondeurs de 35 à 48 km. Des réflexions bien définies de cette limite sont rares. Certaines caractéristiques géologiques sont dignes de mention dans ces lignes. Un amincissement localisé de la croûte (d'une étendue ~50 km) est observé dans la région de Peace River; cet amincissement concorde avec une anomalie voisine en oxygène isotopique indiquant une extension de la croûte. Au centre de l'Alberta, la topographie de la discontinuité de Mohorovicic suggère un saut abrupt de 10 km indiquant des failles dans la croûte associées à la zone tectonique Snowbird. Au sud de l'Alberta, la croûte s'épaissit considérablement en travers de la structure de Vulcan et le plus grand épaississement correspond à la structure de Vulcan, elle-même indiquant une origine de collision telle que notée par d'autres auteurs.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]