A synthesis of electromagnetic studies in the Lithoprobe Alberta Basement Transect: constraints on Paleoproterozoic indentation tectonics

Abstract

Electromagnetic surveys of the crust and upper mantle underlying the Western Canada Sedimentary Basin have revealed two conductivity anomalies that are arguably related to Paleoproterozoic tectonic processes, recording a signature diagnostic of euxinic (foreland?) basin succession and perhaps defining a deep-marine transtensional basin. As well, a regionally pervasive electrical anisotropy throughout the crust crosses Proterozoic tectonic boundaries and is aligned in many areas with the regional tectonic fabric (as expressed by aeromagnetic data) created during the final stages of Proterozoic tectonic assembly of western Laurentia. There is also an order of magnitude increase in the upper mantle conductivity underlying Churchill Province Archean crust relative to that underlying the neighbouring Proterozoic crust. The interpretations of the electromagnetic observations are analogous with the Tertiary to modern-day tectonic development of southeast Asia. The foreland basin succession would reflect the closing of the South China Sea and the deep-marine basin may be analogous to the Andaman Sea and North Sumatra basin. Electrical anisotropy may be indicative of the successive lateral accretion of oceanic crust beneath the continental shelf followed by shortening. The enhanced mantle conductivity mechanism is unknown, although it may be associated with metasomatic modification of the subcontinental lithosphere subsequent to collisional thickening and followed by delamination or convective removal of some lithosphere. While the Indo-Eurasian-Laurentia tectonic analogy is incomplete, it is sufficiently compelling in explaining the electromagnetic and other data that it is a useful basis for understanding the Proterozoic assembly of Laurentia.Résumé : Des relevés électromagnétiques de la croûte et du manteau supérieur sous le bassin sédimentaire de l'Ouest canadien ont révélé deux anomalies conductrices qui peuvent être reliées aux processus tectoniques en cours au Paléo-protérozoïque, fournissant ainsi une signature diagnostique d'une succession de bassins euxiniques (d'avant-pays) et peut-être définissant un bassin marin profond de tension transversale. Aussi, une anisotropie électrique régionale péné-trant à travers toute la croûte recoupe les limites tectoniques du Protérozoïque et, dans plusieurs régions, est alignée avec la texture tectonique régionale (telle qu'exprimée par les données aéromagnétiques) créée durant les phases finales de l'assemblage tectonique de la Laurentie occidentale au Protérozoïque. Il y a aussi une augmentation d'un ordre de magnitude dans la conductivité du manteau supérieur sous-jacent à la croûte archéenne de la province de Churchill par rapport à la conductivité du manteau supérieur sous-jacent à la croûte protérozoïque avoisinante. Les interprétations des observations électromagnétiques sont analogues au développement du sud-est de l'Asie, du Tertiaire au temps présent. La succession des bassins d'avant-pays correspondrait à la fermeture de la mer de Chine méridionale et le bassin mar...

3D joint inversion of magnetotelluric and magnetovariational data to image conductive anomalies in Southern Alberta, Canada

“…Both Boerner et al (2000) and Nieuwenhuis et al (2014) reported phenomena in 2D inversions where crustal conductors moved after inclusion of the tipper data, and interpreted this as evidence of either crustal anisotropy or unstable model features. These shifting phenomena did not appear in our joint inversion result, and this would tend to indicate that the crustal conductors are stable model features.…”

“…Understanding of this basement structure is limited due to the 2-3 km thick overlying sedimentary basin (Nieuwenhuis et al, 2014). A priori knowledge is based primarily on various geophysical studies carried out in the region (Boerner et al, 1995(Boerner et al, , 2000Gorman et al, 2002;Nieuwenhuis et al, 2014). These studies have identified several conductive anomalies in the otherwise resistive crystalline basement blocks.…”

Shear Wave Splitting Discloses Two Episodes of Collision‐Related Convergence in Western North America

“…For example, observational support for the impact of the Paleoproterozoic "tectonic vise" and dual subduction Ross et al, 2000) has been elusive along the STZ. A related but unresolved issue is the origin of the similarly oriented surface plate velocities (e.g., Gripp & Gordon, 2002;Kreemer et al, 2014), stress field (e.g., Heidbach et al, 2016;Reiter et al, 2014;Wang et al, 2017), midcrustal electrical anisotropy (e.g., Boerner et al, 2000;Nieuwenhuis et al, 2014), and seismic anisotropy in the lithosphere and upper mantle (e.g., Courtier et al, 2010;Gu et al, 2011;Shragge et al, 2002). While plate boundary forces and body forces associated with the surface motion of the North American plate likely play a key role in the mantle dynamics (e.g., Bell & Bachu, 2003;Bell & Gough, 1979;Courtier et al, 2010;Currie et al, 2004;Gu et al, 2011;Reiter et al, 2014;Shragge et al, 2002), the origin of the northeast oriented directions within the crust and mantle remains enigmatic for the WCSB region.…”

Magnetotelluric imaging of anisotropic crust near Fort McMurray, Alberta: implications for engineered geothermal system development

“…These stations recorded signals from 0.001 s to 1000 s for 48 hr with a stationary remote reference station. The network was supplemented with one long period site from the Lithoprobe Alberta Basement Transect collected in the late 1990 s, which recorded signals from 1 to 10 000 s (Boerner et al 2000). Data processing followed the robust estimation of geomagnetic transfer functions as described by Egbert & Booker (1986).…”

“…The northern profile is named FMCn and is approximately 30 km long, split into FMCn east and FMCn west. The 20-km-long southern profile, named Tower Road (TR), includes one long-period MT site collected as part of the LITHOPROBE project (Boerner et al 2000).…”