Seismic anisotropy in the Earth's upper mantle is generally attributed to lattice-preferred orientation of anisotropic crystals in minerals such as olivine and pyroxene (e.g. Nicolas and Christensen, 1987) resulting from rock deformation in past and present mantle flow. Under continents, seismic anisotropy results from a combination of frozen-in lithospheric anisotropy from past deformation processes, shear coupling between the lithosphere and asthenosphere, and current flow in the asthenosphere (Park and Levin, 2002). Characterizing seismic anisotropy and relating it to past and present geodynamic processes thus provides insights into our understanding of the driving mechanisms of plate tectonics and lithospheric evolution.
The North American (NA) continent is in many ways an ideal target for this type of study due to its rich tectonic history (Figure 2.19; e.g. Hoffman, 1989; Thomas, 2006; Whitmeyer and Karlstrom, 2007). In this report, we show strong anisotropic layering in the stable North American craton upper mantle, inferred from our updated 3-D upper mantle SVEMum_NA2 model (Yuan et al., 2010). This layering of anisotropy in the craton enables us to distinguish three domains of azimuthal anisotropy, two in the lithosphere and one in the asthenosphere (Yuan and Romanowicz, 2010). In particular, the boundary between the two lithospheric layers roughly corresponds to the negative velocity jump detected in receiver function studies (Yuan et al., 2010).
Moreover, a change in direction of the fast axis of azimuthal anisotropy at mid-lithospheric depths clearly defines two layers (Layer 1 and 2; Figure 2.20) within the cratonic lithosphere, separated by a boundary with large lateral variations in depth. Layer 1 is thick under the central part of the craton and tapers off at its boundaries with Paleozoic provinces (e.g. Figure 2.19). The thickest parts of Layer 1 are found in regions affected by orogenies in the Archean (e.g. the Trans-Hudson orogen). Layer 1 also thins in the Mid-continental rift zone (Figure 2.20a). The lateral variations in the thickness of Layer 1 are in good agreement with geochemical estimates from xenolith studies for the most depleted part of the craton, as defined in terms of Mg # (Figure 2.20c; Griffin et al., 2004)
We suggest that receiver functions and long range seismic profiles preferentially detect the transition between the ancient Archean lithosphere (Layer 1) and the subsequently accreted thermal boundary layer (Layer 2). The details of this transition and its precise nature are likely to be complex, as indicated by the fine layering documented by long range seismic profile studies (Thybo, 2006), and may involve stacks of thin, low-velocity layers marking the trace of partial melting and dehydration (Mierdel et al., 2007), possibly at the top of oceanic lithosphere that was welded onto the bottom of Layer 1. It could also result from kimberlite accumulation (Sleep, 2009) if the strong, chemically distinct, Archean Layer 1 acts as a barrier to their further ascent. Note that this mid-lithospheric anisotropic boundary zone must be a sharp high-to-low velocity horizon since it produces converted phases seen in receiver function studies, but it is barely detectable by isotropic velocity tomography, although we have noted the presence of a local minimum in the depth profile of shear velocity in some parts of our model. On the other hand, the LAB under cratons is likely more gradational, as it is hard to detect with receiver functions, which is consistent with a largely thermal, anisotropic boundary that likely does not involve any significant compositional changes or partial melting.
Here, by using an approach based on seismic azimuthal anisotropy, we have documented the craton-wide presence of a mid-lithospheric boundary, separating a highly depleted chemical layer of laterally varying thickness, from a less depleted deeper layer bounded below by a relatively flat LAB (Figure 2.21). The change of fast-axis direction of azimuthal anisotropy with depth is a powerful tool for the detection of lithospheric layering under continents. Our study indicates that the ``tectosphere'' is no thicker than 200-240 km and that its chemically depleted part may bottom out around 160-170 km.
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