Medicine Hat Block Research Articles

A Reference Model of Crustal Thickness and Vp/Vs of Western Canada

AbstractThe Canadian Cordillera marks a transition region from the current plate boundary through the Phanerozoic Cordilleran orogen to the Precambrian cratons. Knowledge of the subsurface structure of western Canada has been greatly advanced by seismological investigations during the past two decades, pioneered by the Lithoprobe project and, more recently, by regional passive seismic arrays. In this study, we construct a new model (WCANM22) of crustal thickness and P‐ to S‐wave velocity ratio, or Vp/Vs, by compiling receiver function data from 473 stations and existing constraints from over 2,600‐km long active source experiments. Our model covers a broad swath (about 1/4) of the land area of North America (105°–140°W, 48°–72°N) and shows an overall flat Moho beneath the Cordillera with an average depth of ∼36 km and a standard deviation of 3 km across orogenic belts. This study provides a comprehensive catalog of Vp/Vs in western Canada and reveals a moderate correlation between Poisson's ratio and the age of crustal domains. The average Vp/Vs values are 1.72, 1.79, and 1.82 for the Phanerozoic Cordillera, Proterozoic Cratons, and Archean‐aged Medicine Hat Block, respectively, suggesting continued modifications to crustal composition through episodic tectonothermal events. This distinct trend in western Canada sheds new light on the debated role of secular changes in the composition of continental crust.

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.

The Great Falls Tectonic Zone after the assembly of Laurentia: evidence for long-term tectonic stability from xenolith apatite

Combined U-Pb geochronology, and trace element analysis (Th, U, Y, Sr, rare earth elements: REE) by LA-Q-ICPMS and halogen chemistry (F, Cl, OH) by SEM-EDX are applied to apatite from a suite of amphibolite- and granulite-facies xenoliths from the Bearpaw Mountains (Montana, USA), to constrain cooling of Paleoproterozoic mid- to lower crust. Xenoliths hosting fluorapatite and hydroxyfluorapatite were emplaced at 54 to 50 Ma within K-rich, host lamprophyres (“minettes”). Granulite xenoliths yield apatite with highly dispersed U-Pb isotopic compositions. But discordia fit through their youngest populations yield Neoproterozoic-Cambrian (c. 650–500 Ma) lower intercepts. A single amphibolite xenolith from a shallower depth contains apatite with older single-grain apatite ages in the range of 1200–890 Ma. These lower intercept ages are much younger than previously reported zircon U--Pb ages from the same samples, which register a thermal maximum in the Palaeoproterozoic (c. 1700–1800 Ma). Trace element analysis of apatite confirms that most grains did not undergo retrogressive recrystallization. Additionally, there is a statistically significant link between apatite size and single grain ages in at least one sample. Our interpretation is that these apatite U-Pb ages represent slow cooling through the apatite Pb partial retention zone (c. 375–450 °C). Whether these data represent a distinct cooling event, or simple protracted cooling from a Palaeoproterozoic thermal peak, cannot be strictly constrained by these data. However, previous evidence from rutile U-Pb in xenoliths from nearby locations implies slow cooling from a Palaeoproterozoic peak. If a slow cooling scenario is assumed, by comparing apatite cooling ages to previous results from Ti-in-zircon thermometry on metamorphic zircon from the same xenoliths, a cooling rate of 0.14--0.33 °C/Myr can be estimated for these granulite xenoliths. Our apatite data thus contribute to a growing consensus on the geodynamic history of the northern Wyoming Craton and Medicine Hat Block, wherein lithospheric stability was maintained on a giga-year timescale from a granulite-facies metamorphic event associated with the assembly of Laurentia up until Laramide orogenesis.

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.

Formation of lower to middle crust of the Wyoming Craton, Montana (USA), using evidence from zircon Hf-O isotopic and trace element compositions

Coupled oxygen-hafnium isotope and trace element geochemical data were obtained using thirty eight previously dated zircon grains extracted from five mafic to intermediate crustal xenoliths of the Wyoming Craton (Montana, USA). Xenoliths include mid to lower crustal (642–817 °C and 3.5–9.4 kbar) mafic granulites and amphibolites with dominantly Mesoproterozoic (1772–1874 Ma) and minor Paleoproterozoic to Late Archean (2004–2534 Ma) 207Pb/206Pb zircon ages. Zircon oxygen isotope data indicate derivation from melts in equilibrium with a mantle source that interacted with limited supracrustal material (δ18O = 4.4–5.7‰), as well as the incorporation of supracrustal fluids or melts into mantle source regions (δ18O = 6.0–8.1‰). The small within-sample isotopic variability suggests that primary zircon did not exchange with isotopically distinct fluids or melts after initial formation. Initial zircon Hf isotopic values are highly variable across all xenoliths (εHf = +3.7 to −17.6), consistent with protolith derivation from mantle sources that incorporated evolved, unradiogenic material or were modified by subduction-related fluids. Within a single granulite xenolith, two zircon types are recognized based on CL imagery, Hf isotopes and U-Pb ages (Type I and Type II). Type I magmatic zircons show dispersed ages (ca. 1700–2534 Ma) and unradiogenic initial Hf (εHf = −17.6 to −1.5, 176Hf/177Hf = 0.281074–0.281232). The spread in ages and initial εHf, but narrow range in initial 176Hf/177Hf, points to variable Pb loss in response to dissolution-recrystallization of pre-existing zircon. Type II metamorphic zircon yields a younger Proterozoic population (ca. 1700–2155 Ma) with more radiogenic initial Hf relative to Type I zircon (εHf = −7.9 to +1.4, 176Hf/177Hf = 0.281427–0.281578); this type represents newly grown metamorphic zircon that formed in the solid-state and incorporated Zr and Hf from pre-existing zircon and silicate matrix/metamorphic phases. REE patterns from all xenoliths are steep and positively sloping without discernible HREE depletion relative to LREE, implying zircon crystallization/recrystallization in the absence of garnet. Negative Eu anomalies signify simultaneous zircon and feldspar crystallization. Solid-state recrystallization may have lead to variations in LREE, Eu and Ce in certain xenoliths. Xenoliths containing magmatic zircon (1834 ± 19 Ma) with mantle-like δ18O (4.4–5.5‰) and radiogenic initial εHf (−2.3 to +3.7) likely formed through crystallization of melts derived from a mantle source that incorporated minor amounts of subducted sedimentary/supracrustal material. Proterozoic (1874 ± 8 Ma) xenoliths with elevated δ18O (6.0–7.0‰) and unradiogenic initial εHf (−8.2 to −9.6) within magmatic zircon represent melt products of subduction-induced melting and metasomatism of the overlying mantle wedge in the vicinity of the northern GFTZ. Older (ca. 2534 Ma) xenoliths containing zircons with elevated δ18O (6.4–7.2‰) and unradiogenic εHf (up to −17.6) represent crystallization of protolith magmas extracted from a mantle source metasomatized by subduction-derived fluids and melts in the Late Archean or earlier. Zircon geochronology and isotope systematics within Mesoproterozoic xenoliths support a model of ocean-closure and subsequent continental collision between the Medicine Hat Block and Wyoming Craton, resulting in the formation of subduction-related melts at ca. 1834–1874 Ma, followed by ca. 1770 Ma collision-related metamorphism thereafter.

Extending the realm of Archean crust in the Great Falls tectonic zone: Evidence from the Little Rocky Mountains, Montana

Two prominent features separate the Archean Wyoming and Hearne cratons: the Paleoproterozoic Great Falls tectonic zone (GFTZ) and the Medicine Hat block (MHB), neither of which is well defined spatially because of Phanerozoic sedimentary cover. Based on limited data, the MHB is thought to be a structurally complex mix of Archean (2.6–3.1 Ga) and Proterozoic (1.75 Ga) crust, but is recognized primarily by its geophysical signature, and its influence on the geochemistry of younger igneous rocks. Similarly, the GFTZ was recognized on the basis of broad differences in geophysical patterns, isopachs of Paleozoic sedimentary sections, and lineaments; however, juvenile arc rocks in the Little Belt Mountains (LBM) and strongly overprinted Archean rocks in southwestern Montana show it to be a dominantly Paleoproterozoic feature. The Little Rocky Mountains (LRM) of Montana provide access to exposures of the northeastern-most Precambrian crust in the MHB-GFTZ region. U/Pb ages of zircons from Precambrian rocks of the LRM range from 2.4 to 3.3 Ga, with most ages between 2.6 and 2.8 Ga. Whole-rock analyses yield Sm-Nd TDM from 3.1 to 4.0 Ga and initial εNd(T) values calculated at U-Pb zircon crystallization ages range from −0.9 to −10.5, indicating significant contributions from older Archean crust. The high proportion of 2.6–2.8 Ga U/Pb ages differentiates LRM crust from arc-related Paleoproterozoic magmatic rocks exposed in the LBM to the southwest. The age and isotopic composition of the LRM gneisses are similar to crust in the northern Wyoming Province (2.8–2.9 Ga), but Paleoproterozoic K-Ar cooling ages suggest crust in the LRM experienced the Paleoproterozoic metamorphism and deformation that characterizes the GFTZ. Consequently, its history differs markedly from the adjacent Beartooth-Bighorn magmatic zone of the northern Wyoming Province, which does not record Paleoproterozoic tectonism, but has a strong correlation with the Montana metasedimentary terrane that was strongly overprinted during the Paleoproterozoic Great Falls orogeny that defines the GFTZ. The LRM, therefore, likely provides a unique, and perhaps the only, opportunity to characterize Archean crust of the MHB.

Finite-frequency P-wave tomography of the Western Canada Sedimentary Basin: Implications for the lithospheric evolution in Western Laurentia

The lithosphere beneath the Western Canada Sedimentary Basin has potentially undergone Precambrian subduction and collisional orogenesis, resulting in a complex network of crustal domains. To improve the understanding of its evolutionary history, we combine data from the USArray and three regional networks to invert for P-wave velocities of the upper mantle using finite-frequency tomography. Our model reveals distinct, vertically continuous high (>1%) velocity perturbations at depths above 200km beneath the Precambrian Buffalo Head Terrane, Hearne craton and Medicine Hat Block, which sharply contrasts with those beneath the Canadian Rockies (<−1%) at comparable depths. The P velocity increases from −0.5% above 70km depth to 1.5% at 330km depth beneath southern Alberta, which provides compelling evidence for a deep, structurally complex Hearne craton. In comparison, the lithosphere is substantially thinner beneath the adjacent Buffalo Head Terrane (160km) and Medicine Hat Block (200km). These findings are consistent with earlier theories of tectonic assembly in this region, which featured distinct Archean and Proterozoic plate convergences between the Hearne craton and its neighboring domains. The highly variable, bimodally distributed craton thicknesses may also reflect different lithospheric destruction processes beneath the western margin of Laurentia.

Large variations in lithospheric thickness of western Laurentia: Tectonic inheritance or collisional reworking?

The ca. 2.0–1.8Ga tectonic assembly of Laurentia provides a record of complex processes resulting in amalgamation of distinct lithospheric domains. In global and continental-scale teleseismic tomographic models, however, the subcontinental lithosphere beneath western Laurentia appears to have a deceptively simple structure that lacks a clear correlation with mapped crustal domains. Here we present a new shear-velocity model of the upper mantle beneath western Laurentia through Rayleigh-wave tomography, using data from several newly deployed broadband seismic arrays. Our models show prominent heterogeneities that appear to correlate well with crustal domains and other geophysical observations. The tomographic results delineate high-velocity keel-shaped anomalies beneath the Archean Hearne Province and the Paleoproterozoic Buffalo Head Terrane; these features are inferred to extend to depths of up to 260km and likely represent ancient thick cratonic roots, whereas relatively thin lithosphere characterizes the adjacent Wabamun domain and Medicine Hat Block. A regional isostatic residual gravity anomaly in the foreland of the Cretaceous–Paleocene southern Canadian Rockies coincides with an area of inferred thick lithosphere in the Hearne Province, suggesting that along-strike variations in flexural rigidity correlate with lithospheric thickness. Taken together, our results suggest that high-amplitude basal topography of the lithosphere–asthenosphere boundary beneath cratons reflects a complex lithospheric evolution that combines effects of both tectonic inheritance and collisional reworking.

Precambrian Crustal Evolution in the Great Falls Tectonic Zone: Insights from Xenoliths from the Montana Alkali Province

Zircon U-Pb ages, Lu-Hf data, and whole-rock Sm-Nd data from xenoliths in the Eocene Grassrange diatremes of central Montana reveal the presence of Proterozoic-Archean crust at depth in the Great Falls tectonic zone. The data suggest that the Proterozoic granitoids are not part of the juvenile Little Belt magmatic arc but likely involved melting of Neoarchean crust after closure of the Little Belt ocean. Zircons in the Archean granites suggest their source(s) was part of the Medicine Hat block, not the northern Wyoming craton, and that the samples represent crustal material that was reworked during the Paleoproterozoic collision.

Structure and tectonics of the northwestern United States from EarthScope USArray magnetotelluric data

The magnetotelluric component of the EarthScope USArray program has covered over 35% of the continental United States. Resistivity tomography models derived from these data image lithospheric structure and provide constraints on the distribution of fluids and melt within the lithosphere. We present a three-dimensional resistivity model of the northwestern United States which provides new insight into the tectonic assembly of western North America from the Archean to present. Comparison with seismic tomography models reveals regions of correlated and anti-correlated resistivity and velocity that help identify thermal and compositional variations within the lithosphere. Recent (Neogene) tectonic features reflected in the model include the subducting Juan de Fuca–Gorda plate which can be traced beneath the forearc to more than 100 km depth, high lithospheric conductivity along the Snake River Plain, and pronounced lower-crustal and upper-mantle conductivity beneath the Basin and Range. The latter is abruptly terminated to the northwest by the Klamath–Blue Mountains Lineament, which we interpret as an important structure during and since the Mesozoic assembly of the region. This boundary is interpreted to separate hot extended lithosphere from colder, less extended lithosphere. The western edge of Proterozoic North America, as indicated by the Cretaceous initial 87Sr/86Sr = 0.706 contour, is clearly reflected in the resistivity model. We further image an Archean crustal block (“Pend Oreille block”) straddling the Washington/Idaho border, which we speculate separated from the Archean Medicine Hat block in the Proterozoic. Finally, in the modern Cascades forearc, the geometry and internal structure of the Eocene Siletz terrane is reflected in the resistivity model. The apparent eastern edge of the Siletz terrane under the Cascades arc suggests that pre-Tertiary rocks fill the Washington and Oregon back-arc.

Deep Probe: imaging the roots of western North America

Analysis of the Lithoprobe Deep Probe and Southern Alberta Refraction Experiment data sets, focusing on the region between Deep Probe shots 43 and 55, has resulted in a continental-scale velocity structural model of the lithosphere of platformal western Laurentia reaching depths of ~150 km. Three major lithospheric blocks were investigated: (i) the Hearne Province, a typical continental Archean cratonic province lying beneath the Western Canada Sedimentary Basin; (ii) the Wyoming Province, an even older block of Phanerozoic-modified Archean crust with an enigmatic lower lithosphere; and (iii) the Yavapai–Mazatzal Province, Proterozoic terranes underlying the Colorado Plateau and Southern Rocky Mountains. In this study, the northern two of these regions are investigated with a modified ray-theoretical traveltime inversion routine that respects the spherical geometry of the Earth. The resulting crustal velocity structure, combined with supporting geological and geophysical data, reveals that the Medicine Hat block (MHB), lying between the Hearne and Wyoming provinces, is a third independent Archean crustal block. The subcrustal lithosphere along the profile is homogeneous in velocity structure, but two significant northward-dipping reflectors are apparent and interpreted as relic subduction zones associated with sutures between the three Archean blocks. The Hearne crust is typical of an Archean shield or platform both in its thickness of 34–50 km and its seismic velocity structure. The crust of the Archean MHB and Wyoming Province, which ranges in thickness from 49 to 60 km, includes a 10–30 km thick high-velocity layer, interpreted to be Proterozoic in age. Such a feature is unexpected beneath Archean crustal provinces, but if the region is considered to be the remanent marginal portion of a larger Archean continent, then the interpreted Proterozoic underplating and lack of an Archean lithospheric root can be explained. The variable topography along the reflective upper and lower boundaries of this layer, especially within the MHB, suggests considerable variability in its emplacement and subsequent tectonic history.

Crustal velocity structure from SAREX, the Southern Alberta Refraction Experiment

Lithoprobe's Southern Alberta Refraction Experiment, SAREX, extends 800 km from east-central Alberta to central Montana. It was designed to investigate crustal velocity structure of the Archean domains underlying the Western Canada Sedimentary Basin. From north to south, SAREX crosses the Loverna domain of the Hearne Province, the Vulcan structure, the Medicine Hat block (previously considered part of the Hearne Province), the Great Falls tectonic zone, and the northern Wyoming Province. Ten shot points along the profile in Canada were recorded on 521 seismographs deployed at 1 km intervals. To extend the line, an additional 140 seismographs were deployed at intervals of 1.25–2.50 km in Montana. Data interpretation used an iterative application of damped least-squares inversion of traveltime picks and forward modeling. Results show different velocity structures for the major blocks (Loverna, Medicine Hat, and Wyoming), indicating that each is distinct. Wavy undulations in the velocity structure of the Loverna block may be associated with internal crustal deformation. The most prominent feature of the model is a thick (10–25 km) lower crustal layer with high velocities (7.5–7.9 km/s) underlying the Medicine Hat and Wyoming blocks. Based on data from lower crustal xenoliths in the region, this layer is interpreted to be the result of Paleoproterozoic magmatic underplating. Crustal thickness varies from 40 km in the north to almost 60 km in the south, where the high-velocity layer is thickest. Uppermost mantle velocities range from 8.05 to 8.2 km/s, with the higher values below the thicker crust. Results from SAREX and other recent studies are synthesized to develop a schematic representation of Archean to Paleoproterozoic tectonic development for the region encompassing the profile. Tectonic processes associated with this development include collisions of continental blocks, subduction, crustal thickening, and magmatic underplating.

Integrated teleseismic studies of the southern Alberta upper mantle

This paper presents results from a teleseismic experiment conducted across the Hearne Province in south-central Alberta. Data from an array of nine portable broad-band seismographs deployed along a 500 km NW–SE array have been supplemented with recordings from two Canadian National Seismograph Network stations. P-wave delay times from 293 earthquakes have been inverted for upper-mantle velocity structure below the array. The recovered model reveals high velocities beneath much of the southern Hearne Province to depths of 200–250 km, which are interpreted as deep-seated lithospheric structure. Contrary to recent tectonic models, these results suggest that the Hearne lithosphere has remained intact. In particular, it appears unlikely that evidence for extensive, lower crustal melting derives from lithospheric delamination. However, the results admit the possibility that high mantle conductivity, as revealed in magnetotelluric studies, originates through small volumes of connected hydrous minerals or other conductive species introduced during subduction. Decreased upper-mantle velocities at the northern end of the Medicine Hat block also pose challenges for the interpretation of differential subsidence across the region which may manifest distant forcing due to more recent subduction. Multievent SKS-splitting analysis yields an average polarization direction that is broadly consistent with both the orientation of fossil strain fields, related to ~ 1.8 Ga NW–SE shortening, and North American absolute plate motion. Moho depth estimates from receiver functions are fairly uniform (~ 38 km) beneath northern stations but show crustal thickening (&gt;40 km) within the Medicine Hat block to the south and are consistent with values from active-source profiling.

Crustal structure beneath the Western Canada Sedimentary Basin: constraints from gravity and magnetic modelling

Regional gravity and magnetic anomalies that originate from crystalline basement rocks extend over many parts of the Western Canada Sedimentary Basin. Although these potential-field anomalies provide a basis for tectonic subdivisions of the basement crust, most previous interpretations of these features have been largely qualitative in nature. This study focuses on numerical simulation and quantitative interpretation of five regional potential-field anomalies in Alberta, Canada, for which independent constraints on crustal structure are available from Lithoprobe seismic and electromagnetic studies. The Kimiwan High (~55°N, 116°W) is a roughly 250 km long linear magnetic high. Seismic profiles across this anomaly provide evidence for a crustal-scale extensional fault system that offsets the Winagami reflection sequence, a series of mid-crustal sills. We find that the magnetic anomaly can be modelled as either a 15–40 km wide zone of moderate positive susceptibility (4.5 × 10–3 SI units) in the hanging wall of the detachment (5–17 km depth), or as a narrower (5–10 km), steeply dipping zone of high susceptibility (2.5 × 10–2 SI units) in the footwall (16–32 km depth). We interpret the former scenario as indicative of an extensive zone of alteration above the fault, whereas the latter could represent a decapitated granitic pluton that correlates with magmatic rocks farther north. To the southeast, the Thorsby Low (~53°N, 114°W) is a sinuous, 400 km long magnetic low and gravity gradient trend that appears to be a splay of the much more extensive Snowbird tectonic zone. Previous seismic interpretations across the Thorsby Low indicate that it coincides with a 10 km offset in the Moho. Our results show that this abrupt change in crustal thickness is consistent with, but not required by, the gravity signature of this feature. The northeast-striking Red Deer High (~53°N, 112°W) is a narrow magnetic anomaly in central Alberta with variable intensity along strike. Previous magnetotelluric studies suggest that the Red Deer anomaly is closely associated with a linear, highly conductive body in the upper basement. Our modelling results confirm the shallow depth of the causative body and suggest an eastward dip that is consistent with published seismic interpretations. Near the eastern border of Alberta, the 300 km long Eyehill High (~52°N, 110°W) is a prominent north-striking magnetic anomaly adjacent to the western hinterland of the Trans-Hudson Orogen. Combined gravity and magnetic modelling show that this feature occurs near the western boundary of a large block of dense material in the lower crust. The magnetic anomaly can be simulated by a near-vertical dyke-like body in the upper crust. Lastly, the Vulcan structure (~50.5°N, 112°W) forms the ~400 km long, northern boundary of the Archean Medicine Hat block. It is defined by a sinuous east-trending magnetic anomaly and gravity low. The wavelength and polarity of the magnetic anomaly (positive in the north), coupled with the coincident gravity low, are most simply explained by a mid-crustal low-density body with significant remanent magnetization oriented antiparallel to the present-day field.

Crustal geometry and tectonic evolution of the Archean crystalline basement beneath the southern Alberta Plains, from new seismic reflection and potential-field studies

Deep seismic reflection profiling of the southern Alberta crystalline basement, namely the Archean Medicine Hat block (MHB), has revealed that the crust is characterized by west- and southwest-dipping reflection fabrics, a layered lower crust, and geometric cutoffs that are interpreted to be remnants of a complex tectonic history. Upper to middle crustal dipping reflections are interpreted to be associated with Archean rocks drilled below the Western Canada Sedimentary Basin and their geometry is consistent with east- and northeast-verging imbrication. Layered lower crustal reflections of the MHB are interpreted as Early Proterozoic delaminated Loverna block crust and injected mafic material, based on geometric relationships, xenolith dating, and gravity modelling. The reflection Moho is diffuse below the MHB at ~15 s two-way time (~47 km). This is similar to estimates based on adjacent seismic refraction data and is up to ~8 km deeper than regions north of the MHB. The MHB crust is interpreted to have undergone two episodes of crustal deformation in the Precambrian: (1) Late Archean formation of the MHB crust by continental collision of two Archean domains along a crustal-scale ramp; and (2) Early Proterozoic crustal shortening associated with the formation of the Vulcan structure, the delamination of the Loverna block crust, and postcollisional injection of mafic melts beneath the MHB crust. Radiometric dating of lower crustal xenoliths suggests a significant Early Proterozoic thermal event in the deep crust beneath the MHB that is coeval with orogenic activity in the Trans-Hudson Orogen and the East Alberta Orogen.

Seismic imaging of the upper mantle beneath the Rocky Mountain foreland, southwestern Alberta

The Vibroseis Augmented Listen Time (VAuLT) experiment is a special seismic-reflection survey designed to image the fine-scale structure of the continental upper mantle of the Rocky Mountain foreland in southwestern Alberta to depths of 200 km or more. Two mutually perpendicular profiles were acquired across and within the Vulcan structure, a roughly east-west-trending tectonic belt in the crystalline basement beneath the Western Canada Sedimentary Basin that separates the Medicine Hat block from the Loverna block. Relative-amplitude-preserving processing procedures were developed to estimate the seismic-signal-penetration limit, which varies between 100 and 220 km depth. Amplitude-decay analysis and Q estimation show that a seismically unreflective zone within the Vulcan structure is not caused by inadequate signal penetration. This blank zone is interpreted as part of an intrusive complex that has overprinted the preexisting structural fabric. Unlike most other parts of Alberta, the reflection Moho is indistinct and the uppermost mantle (45–60 km depth) is reflective, particularly for source–receiver offsets &gt;10 km. South-dipping reflectivity in the lower crust and upper mantle beneath the Loverna block and northern Vulcan structure gives way to subhorizontal reflectivity beneath the Medicine Hat block. We interpret this reflectivity as compositional layering and (or) zones of ductile deformation that were previously part of the mafic lower crust, but that have now undergone metamorphic transformation to eclogite. The deepest observed reflection is an isolated, gently north-dipping event at ~120 km depth.

Minette bodies and cognate mica-clinopyroxenite xenoliths from the Milk River area, southern Alberta: records of a complex history of the northernmost part of the Archean Wyoming craton

Minettes exposed in southern Alberta near the Milk River are the northern outliers of the Eocene Sweet Grass Hills igneous complex of the Montana alkalic igneous province. These minettes often contain coarse-grained xenoliths of phlogopite + clinopyroxene ± apatite. The parent magmas of the minettes were generated at pressures [Formula: see text]17 kbar in equilibrium with clinopyroxene + phlogopite ± olivine. Fractional crystallization and mixing provided a spectrum of evolved minettes and cumulates, the latter of which were sampled by subsequent minette magmas as xenoliths. Two xenoliths were dated at 49.0 ± 0.8 Ma and 52 ± 1.7 Ma. The host dyke of the latter xenolith gave an age of 50 ± 0.3 Ma. The minettes and their xenoliths have overlapping values of 87Sr/86Sri, εNdT, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb, similar to those of alkaline igneous rocks from farther south in the Montana alkalic igneous province. The Sweet Grass Hills lie north of the Great Falls Tectonic Zone, previously interpreted as a Proterozoic suture zone separating the Archean Medicine Hat block from the Archean Wyoming craton to the south. Geochemical data for the Milk River minettes provide evidence for a history of the mantle underneath the Medicine Hat block, similar to that found previously for mantle-derived rocks of the Wyoming craton, including a significant Proterozoic mantle enrichment event. Given this similarity, we suggest that the Wyoming craton extends into southern Alberta, and that the Great Falls Tectonic Zone does not represent a Proterozoic suture of two Archean blocks.

Seismic‐reflection and potential‐field studies of the Vulcan structure, western Canada: A Paleoproterozoic Pyrenees?

The Vulcan structure is a major tectonic boundary between the Medicine Hat and Loverna Blocks, Archean crustal domains that are buried beneath the Western Canada Sedimentary Basin. It is characterized by prominent east‐trending gravity and magnetic anomalies, more than 350 km in length, which cut across the potential‐field fabric of southern Alberta at a high angle. Three decades ago, the Vulcan structure was the target of one of the first deep‐crustal seismic profiles, and it has since been variously interpreted as a failed Proterozoic rift, an intraplate collisional zone, and (or) a largely amagmatic Proterozoic suture. Several recent Lithoprobe seismic‐reflection profiles, coupled with new, coincident high‐resolution gravity data, regional gravity data, and updated aeromagnetic coverage, provide the basis for a revised model of the crustal architecture in this region. We interpret the Vulcan structure to be a relatively narrow (40–70 km) axial zone of a continental collisional belt. The seismic images imply crustal delamination and southdirected subduction of the lower crust of the Loverna Block. The axial gravity low and corresponding magnetic anomaly are interpreted to originate from a large granitic pluton, while prominent gravity highs on the flanks of the structure are modeled as thrust slices of lower‐crustal and/or mantle material brought up during the collision. Cross‐cutting relationships inferred from aeromagnetic data indicate that the timing of collision postdated formation of Archean fabrics in the Medicine Hat Block but predated terminal collision in the adjacent TransHudson Orogen. The geographic extent, inferred net shortening and tectonic setting of the Vulcan structure appear to resemble the modern Pyrenees belt, although the deep structure appears to be more akin to the Scandinavian Caledonides. Either scenario is consistent with an interpretation of the Vulcan structure as the Proterozoic collisional suture between the Wyoming and Hearne Provinces of the Laurentian craton.

The HSI bright reflector: further evidence for extensive magmatism in the Precambrian of western Canada

Seismic reflection lines acquired in 1995 as part of LITHOPROBE, Canada's national geoscientific research program, revealed unusually bright reflectors of extensive lateral scale in the Archaean Medicine Hat block of southern Alberta. The herein-named Head-Smashed-In (HSI) reflector is imaged over an area of at least 6000 km2 in the upper half of the crust. The reflector appears as a mostly horizontal to subhorizontal feature. Although continuous, it has a complex and laterally variable internal structure. Comparison with a reflector of known impedance contrast in the overlying western Canadian sedimentary basin indicates a high-impedance structure to cause the reflections. Dense one-dimensional modelling is consistent with the reflector being a multilayered structure with layer thicknesses in the range of tens to hundreds of metres and lateral scales of discrete reflecting elements between tens of metres and hundreds of metres. Polarity constraints, overall geometry and reflection character strongly suggest that mafic/ultramafic horizontal sheets are the cause of HSI reflectivity. The HSI is the third major sequence of strong crustal reflectivity imaged by LITHOPROBE deep seismic data in the Precambrian crust of western Canada. All three features are very similar in overall geometry, reflector strength and character and have been interpreted as horizontal, sheetlike intrusions. Small-scale variations of the reflectivity signatures suggest a pronounced rheological heterogeneity in the upper crustal rocks within the host tectonic domains.