Mathematics – Logic
Scientific paper
Jun 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993georl..20.1127s&link_type=abstract
Geophysical Research Letters (ISSN 0094-8276), vol. 20, no. 12, p. 1127-1130.
Mathematics
Logic
33
Anisotropy, Earth Mantle, Geological Surveys, Precambrian Period, Seismic Waves, Data Flow Analysis, North America, Tectonics
Scientific paper
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. 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. The values of the fast polarization direction (theta) display good geologic coherence. Stations along the northern part of the array in the exposed shield reveal a consistent ENE direction for theta that is generally parallel to exposed geologic structures. Within the part of the Superior Province buried beneath Phanerozoic cover, theta rotates to a NE-SW azimuth as do geophysical indicators of geologic fabric. Moving onto the Trans-Hudson, theta changes abruptly to a more EW azimuth. The delay time delta(t), which changes between 0.40 and 1.75 s, shows systematic variations across the traverse. The largest delta(t)s are found in a 250 km wide band from Red Lake, Ontario to the U.S.-Canada border. Delta(t) is reduced to about 1 s or less both north and south of this region. For stations with detectable splitting, we find an inverse relationship between isotropic station delays derived from the portable data set and Delta(t), 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.
Kaneshima Satoshi
Silver Paul G.
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