3-D Shear Wave Radially and Azimuthally Anisotropic Velocity Model of the North American Upper Mantle

Huaiyu Yuan, Barbara Romanowicz, Karen Fischer (Department of Geological Sciences, Brown University, Providence, Rhode Island, USA), and David Abt (ExxonMobil Exploration Company, Houston, Texas, USA)


Using a combination of long period seismic waveforms and SKS splitting measurements, we have developed a 3D upper mantle model (SAWum_NA2; Yuan et al. 2010) of North America that includes isotropic shear velocity (Figure 2.22) , with a lateral resolution of $\sim$200 km, as well as radial and azimuthal anisotropy (Figures 2.23, 2.24, respectively), with a lateral resolution of $\sim$400 km. Combining these results, we infer several key features of the lithosphere and asthenosphere structure.

Figure 2.22: Isotropic velocity ($V_s$) perturbations, plotted with respect to the North America regional average shown (Figure 3 in Yuan and Romanowicz, 2010). Black dashed line approximately delineates the continent cratonic region, which approximately follows the Rocky Mountain front to the west, and the Ouachita and Appalachian fronts to the south and east. Velocity variations are saturated at -8% to 8% scale at $\le$ 200 km depth, and -4% to 4% below 200km. The Paleozoic continent rift margin in the western US, which spatially correlates with the Sevier thrust and fold belt in the western US, is shown as a black line for reference.
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A rapid change from thin ($\sim$70-80km) lithosphere in the western US to thick lithosphere ($\sim$200 km) in the central, cratonic part of the continent closely follows the Rocky Mountain Front (RMF; thick dashed line in Figure 2.22). Changes with depth of the fast axis direction of azimuthal anisotropy reveal the presence of two layers in the cratonic lithosphere, as shown in our companion paper (Yuan and Romanowicz, 2010a), and allow us to define the lithosphere-asthenosphere boundary (LAB) throughout the craton more precisely than from isotropic velocity tomography or the analysis of receiver function data, which, on the other hand, define the LAB consistently in the western US, where the boundary is sharper. The boundary between the two lithospheric layers in the craton varies significantly in depth and may correspond to the mid-lithospheric fast-to-slow discontinuity found in receiver function studies. Lateral variations in azimuthal and radial anisotropy in the cratonic lithosphere correspond to surface geological features marking tectonic events of the past.

Figure 2.23: (a) Azimuthal anisotropy strength G. (b) Depth dependent fast axis direction. Red arrows show the absolute plate motion (APM) direction of the North American Plate, and blue arrows are those for the Pacific plate APM (Gripp and Gordon, 2002). Green dashed line approximately delineates the continent cratonic region as in Figure 2.22.
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Below the lithosphere, azimuthal anisotropy (Figure 2.23) manifests a maximum, stronger in the western US than under the craton, and the fast axis of anisotropy aligns with the absolute plate motion. In the western US, this zone is confined to between 70 and 150 km, decreasing in strength with depth from the top, from the RMF to the San Andreas Fault system and the Juan de Fuca/Gorda ridges. This result suggests that shear associated with lithosphere-asthenosphere coupling dominates mantle deformation down to this depth in the western part of the continent. The depth extent of the zone of increased azimuthal anisotropy below the cratonic lithosphere is not well resolved in our study, although it is peaked around 270 km, a robust result.

Radial anisotropy (Figure 2.24) is such that, predominantly, $\xi$1, where $\xi$=($V_{sh}$/$V_{sv}$)$^2$, under the continent and its borders down to $\sim$200 km, with stronger $\xi$ in the bordering oceanic regions. Across the continent and below 200 km, alternating zones of weaker and stronger radial anisotropy, with predominantly $\xi$1, correlate with zones of small lateral changes in the fast axis direction of anisotropy, and faster than average $V_s$ below the LAB, suggesting the presence of small scale convection with a wavelength of $\sim$2000km.

Figure 2.24: Radial anisotropy parameter $\xi$, plotted with respect to isotropy. Green dashed line approximately delineates the continent cratonic region, as in Figure 2.22. The Sevier thrust and fold belt in the western US is shown as a black line for reference.
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Yuan, H. and Romanowicz, B., Lithospheric layering in the North American Craton, Nature, 466, 1063-1068, 2010a.

Yuan, H. and Romanowicz, B., Depth Dependent Azimuthal anisotropy in the western US upper mantle, Earth Planet. Sci. Lett., doi:10.1016/j.epsl.2010.10.020, 2010b.

Yuan, H., Romanowicz, B., Fisher, K. and Abt, D., 3-D shear wave radially and azimuthally anisotropic velocity model of the North American upper mantle, Geophys. J. Int., accepted, 2010.

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