Constraints on mantle anisotropy beneath Precambrian North America from a transportable teleseismic experiment

Data from the APT89 transportable teleseismic experiment was examined to characterize the variations in shear‐wave splitting along a 1500 km traverse in North America (see Silver et al, 1993, this issue). Three geologic terranes were sampled: the western Superior Province of the Canadian Shield, the Trans‐Hudson Orogen and the Wyoming Craton. The primary goal was to determine the extent to which the anisotropy was controlled by Archean and Proterozoic tectonic episodes. Of the large suite of teleseismic events recorded, 5 were suitable for determining the fast polarization direction ϕ and delay time δt from the phases SKS and SKKS. The resulting observations reveal two important features. First, the values of ϕ display good geologic coherence. Stations along the northern part of the array in the exposed shield reveal a consistent ENE direction for ϕ that is generally parallel to exposed geologic structures. Within the part of the Superior Province buried beneath Phanerozoic cover, ϕ rotates to a NE‐SW azimuth as do geophysical indicators of geologic fabric. Moving onto the Trans‐Hudson, ϕ changes abruptly to a more EW azimuth. The stations within the Wyoming Craton suggest values with a more northerly direction although the waveforms from these stations are more complex. Second, δt, which ranges between 0.40s and 1.75s, shows systematic variations across the traverse. The largest delay times (approaching 2 s) are found in a 250 km wide band from Red Lake, Ontario to the U.S‐Canada border. δt is reduced to about 1 s or less both north and south of this region. For the stations with detectable splitting, we find an inverse relationship between S isotropic station delays derived from the portable data set and δt, which is most easily explained by the anisotropy being primarily localized in the fast, presumably lithospheric mantle. The splitting observations overall suggest that mantle anisotropy along this traverse is dominated by Precambrian ‘fossil’ anisotropy that is preserved in the subcontinental lithosphere.