HR: 1340h
AN: T13A-0434    [Abstracts]
TI: Variations and anisotropy of the elastic thickness of the lithosphere determined by the wavelet method: Examples from the Canadian Shield.
AU: Audet, P
EM: paudet@eos.ubc.ca
AF: Dept Earth and Ocean Sciences University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4 Canada
AU: * Mareschal, J
EM: jcm@olympus.geotop.uqam.ca
AF: GEOTOP-UQAM-McGill University of Quebec at Montreal, POB 8888, sta. "downtown", Montreal, QC H3C3P8 Canada
AB: Different spectral methods have been used to estimate the elastic thickness and the mechanical properties of the lithosphere. We have used a wavelet transform to compute the local variations of the coherence between Bouguer gravity and topography in eastern Canada. The isotropic coherence is calculated by averaging the wavelet spectra from optimally overlapping 2-D Morlet wavelets having an isotropic spectral enveloppe in adjacent directions within 180 °, defining the so-called 'fan' wavelet. The local isotropic wavelet coherence is inverted to obtain local estimates of the elastic thickness (T_e) of the lithosphere. We calculate the anisotropic coherence by spatially averaging adjacent local wavelet spectra obtained from the rotation of the Morlet wavelet. The anisotropic direction of maximum observed coherence is diagnostic of the direction of preferred isostatic compensation, or the direction of mechanically weak lithosphere. We have carried out extensive tests on synthetic topography and Bouguer gravity data sets to verify that: (1) the wavelet method can recover T_e for simple models with either homogeneous or spatially variable rigidity patterns; and that:(2) the method can determine azimuthal variations in the 2-D coherence for homogeneous models with anisotropic T_e. We have then used real data from the Canadian Shield to infer the variations in T_e and the anisotropy of the coherence. The relative variations in T_e agree remarkably well with our previous studies where we used the maximum entropy method to determine the elastic thickness [ Audet & Mareschal, 2004a]. The wavelet transform gives T_e values between 20 and 90 km. T_e is generally high (>70 km) throughout eastern Canada. Lower values (30-50 km) are found around Hudson and James Bay, and near the Abitibi subprovince. High values are found within Hudson Bay, which is consistent with the previous studies. The main difference between this study and the one by Audet & Mareschal [2004a] is the absence of a low T_e region in the southeastern Churchill Province. While T_e was poorly estimated by both the maximum entropy and multitaper methods in the Appalachians [ Audet & Mareschal, 2004b], the wavelet method yields values ranging from 60 to 80 km. The direction of maximum coherence obtained from the wavelet method is also consistent with our previous results obtained with the multitaper method and shows that the weak mechanical axis is perpendicular to the fast seismic axis where seismic anisotropy has been detected. Audet, P., & Mareschal, J.C., 2004a, Variations in elastic thickness in the Canadian Shield, Earth Planet. Sci. Lett., 226, 17-31, doi:10.1016/j.epsl.2004.07.035. Audet, P., & Mareschal, J.C., 2004b, Anisotropy of the flexural response of the lithosphere in the Canadian Shield, Geophys. Res. Lett., 31, L20601, doi:10.1029/2004GL021080.
DE: 3280 Wavelet transform (3255, 4455)
DE: 8138 Lithospheric flexure
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Tectonophysics [T]
MN: Fall Meeting 2005