Crustal imprints of Precambrian orogenesis in western Laurentia

Abstract

Crustal low‐velocity zones (LVZs) have been reported in active orogens such as the Himalayas and the Andes but rarely in stable cratonic regions. In this study, we provide compelling evidence for a significant midcrustal LVZ beneath eastern‐central Alberta, an integral part of the Precambrian Canadian Shield covered by thick Phanerozoic sedimentary deposits. This 200 km wide, over 10 km thick midcrustal LVZ is well resolved by shear velocity inversions using P‐to‐S receiver functions from more than 4600 earthquakes. It is generally overlain by a high‐velocity upper crust in the depth range of 8–15 km, especially in western‐central Alberta, which coincides with the previously documented Winagami reflection sequence. We interpret the LVZ to be of granitic composition, potentially in connection with the crystallization of partially molten crust during the Paleoproterozoic eon. In addition to the Precambrian tectonic history of western Laurentia, which featured plate convergence conducive to crustal melting, our crustal model is further supported by (1) a moderate spatial correlation between the LVZ and heat flow, and (2) shear velocities consistent with that of granite. The well preserved Winagami reflection sequence and the LVZ are potential evidence of distinct episodes of magmatism and crust modification in the Precambrian basement of the Western Canada Sedimentary Basin. The existence of a broad crustal LVZ suggests extensive subduction, orogenesis, and crustal melting during the Precambrian assembly of the North American craton.

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

“…Maps of the shear wave velocity structure in the upper (~8 km) and middle crust (~17 km) are shown in Figure . Generally, crustal shear velocities range from 3.4 to 4.0 km/s, which are consistent with the shear velocity estimates from inversions of receiver function data (Chen et al, ). Both Vsv and Vsh models reveal similar velocity variations across the study region: in the shallow crust, predominantly low shear velocities (up to −6%) are present in the two models beneath the foreland basin with extensive sediment deposition, whereas the velocities reverse sign in the upper crust and reach a maximum amplitude of 4% beneath the southeastern Canadian Cordillera and the Proterozoic terranes in northern Alberta.…”

“…The elongated section of high SV velocities in the middle crust beneath the Foreland Belt is much more pronounced in our model than that of the earlier study using a similar approach (Gu & Shen, ), which we attribute to differences in spatial resolution between these two studies. 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, ).…”

“…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.…”

“…During the past 10 years, increased availability of broadband seismic data has enabled a new appraisal of the tectonic history beneath the Cordillera‐Craton boundary region. Regional receiver functions presented evidence for a midcrustal low‐velocity zone (LVZ) in central Alberta in connection with Paleoproterozoic plate convergence and syncollisional/postcollisional partial melting (Chen et al, ). This finding is supported by the below‐average shear velocities from seismic interferometry (Gu & Shen, ).…”

Discriminating induced seismicity from natural earthquakes using moment tensors and source spectra

“…Blue and red lines denote P wave and S wave velocity values, respectively [modified from Rebollar et al ., ]. (b) S wave velocity model at NOR stations used in equation with finer details in the upper crustal layers [ Chen et al ., ].…”