A New Appraisal of Lithospheric Structures of the Cordillera‐Craton Boundary Region in Western Canada

Abstract

The Western Canada Sedimentary Basin marks a boundary zone between the Precambrian North American craton and the Phanerozoic Cordillera. Its crystalline basement has documented more than 3 billion years of evolution history of western Laurentia. Here we conduct a high‐resolution survey of the mantle P and S wave velocities using finite‐frequency tomography. Our models show pronounced eastward increases of 4% P and 6% S wave velocities beneath the foreland region, which define a sharp seismic Cordillera‐Craton boundary. In the cratonic region, distinctive high‐ (>2%) velocity anomalies representing depleted mantle lithospheres are well correlated with major Precambrian crustal domains. The largest lithosphere thickness contrast coincides with the Snowbird Tectonic Zone, where the Hearne province extends down to ~300 km, nearly 100 km deeper than the Proterozoic terranes in northern Alberta. In the latter region, a pronounced cylindrical negative velocity anomaly extends subvertically from 75 to ~300‐km depth, which potentially results from significant tectonothermal modifications during subduction and/or plume activities. At the basin scale, mantle velocities show no apparent correlations with surface heat flux, suggesting a minimum mantle contribution to the regional thermal variability. Furthermore, the long‐wavelength isostatic gravity correlates negatively with the velocities, which confirms that the melt extraction from Precambrian cratons is responsible for the formation of highly depleted mantle lithospheres. Moreover, our model reveals the increased concentrations of kimberlites and lamproites near the zones of high horizontal velocity gradients. The distinct spatial pattern may reflect either preferential formation or eruption of potentially diamondiferous rocks at lithospheric weak zones near the western margin of Laurentia.

Shear Velocity and Radial Anisotropy beneath Southwestern Canada: Evidence for Crustal Extension and Thick‐Skinned Tectonics

“…We detect upper crustal LVZs in central Alberta, which were suggested earlier through independent analyses of receiver functions (Chen et al, ) and noise correlation tomography (Gu & Shen, ). These basin‐scale LVZs are rarely observed in stable cratons, and their presence could signify residual imprints of ancient orogenic processes surrounding the STZ and Vulcan Structure (Chen et al, , ; Gu et al, ). Low isotropic velocities of similar strengths in the mid‐to‐lower crust beneath the Cordillera are further suggested by ambient noise data (Dalton et al, ), which contrasts with the moderately high velocities of the upper crust in the intact Archean and Proterozoic accreted domains.…”

“…This extensive deformation history is highlighted by the presence of three proposed tectonic discontinuities known as the Great Slave Lake Shear Zone, Snowbird Tectonic Zone (STZ), and Vulcan Structure (Figure ), respectively, from north to south. We refer the reader to a more detailed review of the regional tectonics (Chen et al, ; Gu et al, ).…”

The Upper Mantle Structure of Northwestern Canada From Teleseismic Body Wave Tomography

“…Furthermore, fast-axis orientations remain parallel to the faults at uppermost mantle depths suggesting that Journal of Geophysical Research: Solid Earth 10.1029/2019JB018837 the Tintina and the Denali Faults shear features extend well into the NCC lithosphere (Audet et al, 2016;McLellan et al, 2018), but such studies do not yet fully incorporate seismic data from the EarthScope USArray-Transportable Array stations across Alaska and Yukon. Seismic tomography at various resolution scales generally reveals low seismic velocities within the Cordillera lithosphere transitioning to high velocities within the cratonic lithosphere across a boundary roughly coinciding with the surface expression of the Rocky Mountain Trench, which is the southern extension of the Tintina Fault in the Southern Canadian Cordillera (Bao et al, 2014;Bedle & van der Lee, 2009;Chen et al, 2018;Frederiksen et al, 1998Frederiksen et al, , 2001McLellan et al, 2018;Mercier et al, 2009;Schaeffer & Lebedev, 2014;van der Lee & Frederiksen, 2005;Zaporozan et al, 2018). Regional seismic tomography studies conducted over Alaska, USA, show numerous details on the underlying lithosphere.…”

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

“…Over the past few decades, the Alberta Basin has been one of the most intensively studied regions thanks to oil and gas exploration, Lithoprobe active-source experiments (e.g., Clowes et al, 2002;Cook, 1995;Eaton et al, 2000;Gorman et al, 2002;Hope et al, 1999;Ross et al, 2000), and, more recently, broadband seismic surveys (Bao & Eaton, 2015;Chen et al, 2015;Dalton et al, 2011;Gu et al, 2011Gu et al, , 2015Gu et al, , 2018. These studies offer compelling evidence for intricate domains below the sedimentary basin, which largely support an earlier-proposed regional Proterozoic tectonic framework (e.g., Pilkington et al, 2000;Ross et al, 1991;Villeneuve et al, 1993) and a Cenozoic sharp structural gradient from the stable continental cratons east of the Alberta Basin to the Canadian Cordillera (e.g., Chen et al, 2017Chen et al, , 2018Nettles & Dziewoński, 2008;van der Lee & Frederiksen, 2005). Recent seismological evidence further suggests that much of the past and ongoing deformation has been engraved onto the underlying lithosphere and upper mantle, contributing to mantle fabrics that are largely parallel to the present-day absolute plate motion (APM) directions based on surface wave tomography (e.g., Bao et al, 2016;Gung et al, 2003;Marone & Romanowicz, 2007;Yuan & Romanowicz, 2010) and shear wave splitting observations (e.g., Bastow et al, 2011;Courtier et al, 2010;Currie et al, 2004;Gu et al, 2011;Liu et al, 2014;Shragge et al, 2002).…”

“…Since early 2006, the regional seismic station coverage in Alberta has improved substantially through the establishment of the Canadian Rockies and Alberta Network (hereafter CRANE), the first semiuniform broadband seismic array in Alberta and parts of Saskatchewan, Canada (Gu et al, ). Continuous seismic signals from this array have laid the groundwork for a number of recent findings pertaining to regional seismicity and crust/mantle structures (e.g., Bao et al, ; Chen et al, , ; Gu et al, ), some of which are considered in this analysis for the assessment of past and ongoing lithosphere deformation processes beneath the Alberta Basin.…”