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 NWSE 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 200250 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 NWSE 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 (>40 km) within the Medicine Hat block to the south and are consistent with values from active-source profiling.
An array of seismographs was deployed over the central Trans‐Hudson Orogen from July 1991 to January 1992 and from October 1994 to July 1996 with the objective of characterizing subcrustal lithospheric structure in a region of diamondiferous kimberlite occurrence using tomographic imaging techniques. The two‐dimensional array was located in south central Saskatchewan and consisted of 17 stations with an average spacing of 100 km. We obtained relative travel time residuals for 321 teleseismic events and inverted them for subcrustal velocity variations. The ray coverage affords resolution from 60 to 400 km depth. Our results reveal heterogeneities in mantle velocity that deviate by up to ±1.5% from the iasp91 Earth model. The most pronounced low‐velocity anomaly is quasi‐cylindrical, 120 km in diameter and extends to ∼220 km depth. This feature is partly surrounded by a region of high velocity which penetrates to slightly greater depths. Cretaceous diamondiferous kimberlites and high concentrations of kimberlitic minerals in glacial tills occur above or near the rims of the low velocity anomalies. In addition, correlations exist between a long‐wavelength gravity low and the high‐velocity region, as well as between high heat flow and low mantle velocities in the southern portions of the study area. Taken together, these observations are consistent with the interpretation of the imaged anomalies as due to thermomechanical erosion of the lithospheric keel of the Sask craton during the Cretaceous by plume activity or Rayleigh‐Taylor‐like instability within the asthenosphere. The diamondiferous kimberlites are viewed as a direct consequence of this process. Low levels of heterogeneity below 250 km depth are interpreted to be indicative of effective homogenization in a convecting asthenosphere.
[1] The objective of this study is to characterize elastic properties, including anisotropy, at the base of the crust and uppermost mantle using the Moho P M s phase recorded in the teleseismic P coda at Canadian stations. We use linearized inverse scattering and singular value decomposition to identify those parameter combinations to which idealized teleseismic data sets are most sensitive. Five to seven independent parameter combinations are likely to be resolvable, one of which is sensitive to isotropy, whereas the remainder quantify different harmonic orders (1q, 2q, 3q) of back azimuthal response. Aside from the isotropic component which is resolved only by P-SV interactions, P-SV and P-SH conversions exhibit a redundant sensitivity to model parameters for uniform backazimuthal sampling. We use parameter combinations from the idealized, uniform backazimuthal data distribution to compare Moho anisotropy at 25 broadband stations on the Canadian landmass. The isotropic component dominates at all stations and corresponds to shear velocity contrasts ranging between 10 and 35%. Perturbations to anisotropic material property parameters are more modest, generally between 3 and 7% when consistent between SV and SH responses and in many cases suggest an anisotropic lower crust. Inconsistent responses may manifest contamination by lateral heterogeneity, upper crustal reverberations, or pervasive crustal anisotropy leading to shear wave splitting.
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