Huaiyu Yuan and Barbara Romanowicz
Since the pioneering work by Vinnik (1984) and Sliver (1988), seismic anisotropy has been extensively used to infer upper mantle strain induced fabrics caused by the past and present time mantle deformation processes (e.g. Park and Levin, 2002). Successful correlation of the mapped seismic anisotropy field with many tectonic phenomenon in North America (e.g., edge flow of the continent keels (Fouch et al., 2000); lithospheric drip (West et al., 2009); toroidal flow (Zandt and Humphreys, 2008); fossil slab stacking (Bostock 1998; Snyder 2003, 2008; Gorman et al., 2002; Mercier et al., 2009); rifting (Gok et al., 2003); hotspot tracks (Walker 2004; Schutt et al., 1998; Eaton and Federiksen 2007); lithosphere thickening (Deschamps et al. 2008); and lithosphere-asthenosphere-boundary topography (Plomerova et al., 2002)) has greatly improved our understanding of the dynamics and revolution of the continent.
Using long-period surface waveform tomographic inversion, here we show that the seismic anisotropy beneath North American cratons is strongly stratified, characterized by abrupt changes of fast velocity symmetry axis with depth. At the lithosphere and asthenosphere boundary (LAB), the observed craton-wide change of the anisotropy fast axis towards the absolute plate motion direction defines an anisotropic LAB of the North American upper mantle.
We perform a two-stage inversion to acquire the 3-D azimuthal anisotropy structure in the North American Continent upper mantle. First we simultaneously invert over 150000 3-component long period surface wave fundamental and overtone waveforms to obtain the isotropic Vs and radial anisotropy Xi structure. Then we further perturb the optimal Vs and Xi model (shown in the next research contribution) from the first step for two azimuthal 2-psi coefficients Gc and Gs. Station averaged shear wave splitting measurements are added, which greatly improves the inversion resolution to the deeper mantle (Marone and Romanowicz, 2007).
Figure 2.50 shows the inverted azimuthal anisotropy structure at selected depths. Those depths are chosen to present upper and lower lithosphere and asthenosphere depths based on estimates from other studies, including seismic velocity, petrology and lithosphere electrical resistivity (e.g., van der Lee, 2002; Griffin et al., 2004; Darbyshire et al., 2000, 2007; Snyder 2008; Chen et al ., 2008; McKenzie and Priestley 2008). At shallow depth (70 km, Figure 2.50a), the craton fast axis is generally plate motion direction parallel, except beneath the east central US and the Labrador Sea/Baffin Island, where the fast axis is pointing north. In the lower lithosphere (150km), the craton region has a nearly uniform north-pointing fast axis, with exclusions of east-west striking fast axis beneath North Dakota Trans-Hudson Orogen and northeast Grenville province. At deeper depth (250km), the fast axis aligns everywhere to the plate motion direction. These images thus demonstrate stratified anisotropic layers beneath the continent, with two lithospheric layers clearly presented in some of the cratons.
To further constrain the change between the lithosphere and asthenosphere anisotropy, we look for the gradient of the fast velocity axis near the cratonic LAB depths estimated from other studies, with combined constraints from our isotropic velocity Vs and radial anisotropy Xi (see Eaton (2009) for various seismic definitions of the LAB). Results shown in Figure 2.51 are derived from our tomographic inversion results only.
Azimuthal anisotropy at selected depths. Sticks on each sub-plot show both the fast velocity symmetry axis direction and the anisotropy strength. The deviation of the inverted fast axis from the absolute plate motion direction (Gripp and Gordon, 2002) is drawn in the background such that blue regions show a fast axis parallel to the absolute plate motion direction and red regions perpendicular to it. The Rocky Mountain front (green curve) separates the stable cratons and active west Cordillera.
Our anisotropic LAB is consistently uniform in the depth range of 180-240 km beneath the cratons, except the Wyoming craton, whose LAB is substantially shallower (Figure 2.51a). A depth profile across the major cratons of the study area clearly shows a two layer anisotropy domain in the Archean craton lithosphere, and the abrupt transition of the fast axis to the plate motion direction parallel across the LAB. The unique two-layer pattern of the lithospheric anisotropy may suggest multiple-stage continent lithosphere formation processes (e.g., Arndt 2009). Alternatively, Thybo (2006) suggests a ubiquitous low velocity zone at 100 km depth due to lowered solidus of normal mantle rocks by the presence of limited amounts of fluids. This is consistent with the velocity drop around 100km depth in our isotropic Vs image (upper right panel in Figure 2.50b); however causes of the anisotropy change associated with the low velocity zone are unknown. The anisotropy stratification changes when going into the late Proterozoic Grenville and even younger Appalachian orogens, reflecting somehow different lithosphere formation processes in this area.
Overall, our tomographic inversion reveals a layered anisotropic cratonic upper mantle. The strong lithospheric layering suggests different stages of the cratonic formation. Detecting anisotropy changes from surface wave tomography makes it possible to obtain large scale and high resolution LAB measurements at once.
We thank the IRIS DMC and Canadian National Data Center for providing the waveform data. This project is supported by NSF EAR-0643060.
LAB estimates. A cratonic cross section through the yellow line is shown in b. From top to bottom, isotropic Vs variation, radial anisotropy (with respect to isotropy), deviation of the fast axis direction from the plate motion direction, and the absolute value of the inverted fast axis direction. Black dotted line is the LAB measurements shown in a. TH, Trans-Hudson Orogen, MR, Mid-continent Rift, Gren, Grenville Orogen, and App, Appalachian Orogen.
References are listed in the Section 27.5.
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