Bokelmann, G.H.R., Silver, P.G., 2000, Mantle variation within the Canadian
Shield: Travel times from the APT89 portable broadband transect, Journal
of Geophysical Research, 105, 579-605
We report travel times from the Archean-Proterozoic Transect 1989.
This type of data set recorded by a transect of portable
broadband instruments allows us to make inferences
about mantle structure in the region between
the Wyoming Craton and the Superior Province of the Canadian Shield.
With station separations of 50 to 100 km and frequencies up to 5 Hz
the resolution of lateral changes is increased by nearly an order of magnitude
over previous studies to a scale that allows us to study the
relation between
velocity variation in the continental upper mantle, surface geology,
tectonic features and age provinces.
Travel times of direct and later phases are obtained from waveform matching.
The values are corrected for crustal contribution and
inverted for the vertical path upper mantle delay dt_UM
under each station as well as the azimuthal dependence of this quantity.
The prominent feature in the upper mantle delays dt_UM is
the variation by at least 1.5 s for S, much of which
occurs over a narrow zone of just a few hundred kilometers width.
This suggests a major lateral upper mantle
transition which does not coincide with the surface geological
edge of the Canadian Shield but is located within the shield.
This same transition is also observed in shear wave splitting delay times.
Surprisingly, however, the P delays do not exhibit a corresponding variation.
We address this apparent contradiction and show how it may be explained
in conjunction with anisotropy in the subcontinental lithosphere.
A simplified thermal model of the lithospheric transition zone, in which
temperature controls the degree of crystallographic alignment and thus
seismic anisotropy, predicts this phenomenon.
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