HR: 0800h
AN: T31A-1266    [Abstracts]
TI: Elastic thickness and mechanical anisotropy of the lithosphere: Implications for the depth scale of lithospheric deformations.
AU: Audet, P
EM: pascal@everest.geotop.uqam.ca
AF: GEOTOP-UQAM-McGill, University of Quebec at Montreal, P.O. Box 8888, sta. "downtown", Montreal, QC H3C3P8 Canada
AU: * Mareschal, J
EM: jcm@olympus.geotop.uqam.ca
AF: GEOTOP-UQAM-McGill, University of Quebec at Montreal, P.O. Box 8888, sta. "downtown", Montreal, QC H3C3P8 Canada
AB: We have determined the two dimensional coherence between Bouguer gravity and topography in the Canadian Shield to detect anisotropy in the flexural response of the lithosphere. We interpret the increase in the wavelength averaged coherence as indicating the direction where the lithosphere is weakest. Throughout the Canadian Shield with the exception of Hudson Bay, the flexural response of the lithosphere is strongly anisotropic. In general, this anisotropy is correlated with the geology and the weak axis is perpendicular to the main tectonic discontinuities: the Grenville Front and the Appalachian orogen in southeastern Canada, the east-west tectonic fabric of the south Superior Province, the New-Quebec and the Torngat Orogens in northern Quebec and Labrador, the Trans-Hudson Orogen in central Canada. There is also a strong correlation between the mechanical anisotropy and the seismic and electrical conductivity anisotropies where they have been observed. The weak (flexural) axis is oriented perpendicular to the seismic fast axis and the high electrical conductivity direction. In the areas of the Shield where the seismic and electrical measurements are absent, the isostatic response anisotropy is generally observed perpendicular to the boundaries between the main tectonic provinces. Shear-wave splitting results and the impedance tensor in magneto-telluric soundings mostly depend on the fossil strain recorded by the upper mantle while the flexural response is more sensitive to the mechanical properties of the crust and very shallow mantle. Thus in the regions where mechanical, electrical, and seismic anisotropies are correlated, these observations suggest that the same strain field was recorded in the crust and upper mantle during the last tectonic event. They are also consistent with the absence of major subsequent tectonic reworking. On the other hand, anisotropy is absent beneath the Hudson Bay basin, possibly because it was obliterated by a thermal perturbation preceeding the Basin subsidence.
DE: 8110 Continental tectonics--general (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8122 Dynamics, gravity and tectonics
DE: 1213 Earth's interior--dynamics (8115, 8120)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting