Stratified Seismic Anisotropy beneath the East Central United States

Stratified Seismic Anisotropy beneath the East Central United States Rayleigh wave azimuthal anisotropy (yellow bars) at 28, 55, and 140 s. The orientation and size of the bar show the direction of fastest propagation of Rayleigh waves at the period and the amplitude of the anisotropy, respectively. Also plotted are main tectonic boundaries, past and present absolute plate motion, and previous anisotropy measurements. (a) At 28 s, the Rayleigh-wave fast-propagation direction beneath orogenic provinces is roughly parallel to the Grenville and Appalachian fronts, as well as to Pn fast- propagation direction. (b) At 55 s, the fast-propagation azimuth is close to the direction of the NNW drift of the North American plate during the Mesozoic. (c) At 140 s, the fast-propagation direction is parallel to the current absolute plate motion, as well as to most fast-propagation directions inferred from shear-wave splitting observations in the area.
A key problem in seismology is to resolve the radial distribution of azimuthal anisotropy beneath continents. Seismic anisotropy can be related to rock deformation that occurs during various geodynamical events, and can thus be used to better understand the deformation history of continental lithosphere. Array studies of azimuthal variations in surface-wave phase velocity are well suited to recover the radial distribution of anisotropy, provided that dispersion curves can be measured in a broad enough period range. Using about 20000 records from the IRIS database, we measured broadband inter-station dispersion curves of the Rayleigh-wave phase velocity and built an anisotropic Rayleigh-wave model that clearly resolves the radial distribution of azimuthal anisotropy beneath the East-central United States (31°-41° N and 82°-92° E) [Deschamps et al., 2008a]. We then estimated the amplitude of VS-anisotropy as a function of depth, from the lower crust to the upper asthenospheric mantle (≤400 km) [Deschamps et al., 2008b]. Beneath the orogenic terrains (south and east of the Grenville front), distinct pat- terns can be identified in three period ranges (Figure 1), suggesting that anisotropy is radially distributed within three distinct layers. At 20-35s (sampling the upper lithosphere), anisotropy is strong (~ 1% of the regional average isotropic velocity), with a direction of fast propagation sub-parallel to orogenic fronts. At 45-60s (sampling the lower lithosphere), anisotropy is moderate (~ 0.5%), and the direction of fast propagation is parallel to the reconstructed motion of the North-American plate 160- 125 Ma ago. Finally, around 140s (sampling the upper asthenosphere), anisotropy is strong again (> 1%), and the direction of fast propagation is in good agreement with SKS splitting and with the absolute plate motion of North America. Beneath the cratonic terrains (north and west of the Grenville front), only the anisotropy at 140s, likely related to the current motion of the North American plate, is clearly visible. This distribution suggests the following scenario: ca 270 My ago, the upper lithosphere deformed during the final stages of the Appalachian orogeny (for instance, due to lateral extrusion), leading to fabrics that are now frozen and seen by Rayleigh wave azimuthal anisotropy at 20-35s. After the orogen, the hotter, softer, lower lithosphere recorded the motion of the North-America plate during a brief laps of time (160-125 Ma), resulting in fabrics that are now observed at 45-60s. Finally, at present time, the asthenospheric flow induces deformations at the top of the asthenosphere, which are mapped around140s.
</p><p>Deschamps, F., S. Lebedev, T. Meier, et J. Trampert, 2008a. Azimuthal anisotropy of Rayleigh-wave phase velocities in the east-central United States, Geophys. J. Int., 173, 827-843, DOI:10.1111/j.1365-246X.2008.03751.x.
</p><p>Deschamps, F., S. Lebedev, T. Meier, et J. Trampert, 2008b. Stratified seismic anisotropy reveals past and present deformation beneath the east-central United States, Earth Planet. Sci. Lett., 274, 489-498, doi:10.1016/j. epsl.2008.07.058.</p>


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