The tectonic diversity of the North American continent makes it an ideal region to investigate the structure and dynamics of the continental upper mantle. Investigations of timely geophysical questions, such as the relation to geological age of the variations in the lithospheric thickness, the relation of upper-mantle anisotropy to present day asthenospheric flow and past tectonic events, the nature and strength of the lithosphere/asthenosphere coupling and the driving mechanisms of plate motions, are contingent upon obtaining high-resolution 3-D tomographic models of the isotropic and anisotropic mantle structure of the continent.
In the framework of non-linear asymptotic coupling theory (NACT; Li and Romanowicz, 1995), we had developed a regional 3-D tomographic model of the upper mantle beneath North America that includes both isotropic S velocity structure as well as radial and azimuthal anisotropy (Marone et al., 2007; Marone and Romanowicz, 2007). This model was constructed from a joint inversion of fundamental and higher mode surface waveforms together with constraints on azimuthal anisotropy derived from SKS splitting measurements. The model showed evidence for the presence of two layers of anisotropy beneath the stable part of the North American continent: a deeper layer with the fast axis direction aligned with the absolute plate motion, and a shallower lithospheric layer with north pointing fast axis likely showing records of past tectonic events. Under the tectonically active western US, where the lithosphere is thin, the direction of tomographically inferred anisotropy is stable with depth and compatible with the absolute plate motion direction.
Our published regional 3-D tomographic model (Marone et al., 2007; Marone and Romanowicz, 2007), however, is based on pre-EarthScope data (before 2003). And since it was our first regional tomographic attemp using NACT, the model has large horizontal resolution, i.e., 400- and 800-km for the isotropic (S) and anisotropic (X) models, respectively. Therefore, during last year, we concentrated mostly on improving the model horizontal resolution by filling the station and event distribution gaps, i.e., adding newly deployed permanent and temporary stations and selectively collecting data from the NE and South back-azimuthal quadrants. Our first set of updated models, with horizontal resolution of 200- and 400-km for S and X respectively, has been presented at 2007 Fall AGU meeting and 2008 IRIS workshop (Figure ).
Our isotropic and anisotropic shear-wave images are consistent with our published model, but show greater details beneath the cratonic upper mantle, benefitting from the augmented waveform datasets from the US array and newly deployed IRIS PASSCAL and Canadian arrays. For example, our isotropic shear-wave velocity model sees a high velocity “curtain” at 250-400 km depth range beneath central North America. This feature is also seen by higher resolution P-wave tomography studies (e.g., Li et al., 2007, Ren et al., 2007; Sigloch et al., 2008). Our updated radial anisotropy and azimuthal anisotropy images confirm the distinct upper mantle anisotropic domain beneath North America. This continent-wide multiple-layer upper mantle anisotropic domain has also been reported from many studies, e.g., in Rayleigh wave imaging (Deschamps et al., 2008; Chen et al., 2007; Snyder and Bruneton, 2007), in multiple-layer anisotropy modeling (Snyder et al., 2003; Currie et al., 2004; Fox and Sheehan, 2005; Frederiksen et al., 2007; Yuan et al., 2008), and in receiver functions (Levin and Park, 1999). Deschamps et al. (2008) speculates that the north trending lithospheric LPO was frozen into the lithosphere during northward drift of the North American plate during Mesozoic times. Some new but minor features appear in the shallow upper mantle in our anisotropic models, correlating well with the surface expression of some past and ongoing NA tectonic events.
Currently we are working on improving our 3-D velocity model from the following aspects. First, we are collecting all the EarthScope data and broadband data from the Canadian National Data Center, which will greatly improve our data coverage and hence tomographic resolution, especially beneath the western US. To account for the irregular ray density brought in by the new datasets, we will densify our model spacing in the relevant regions (e.g., beneath western U.S.), which means that higher horizontal resolution ( 100 km) will be reached in those regions. Second, we are introducing a new 1-D reference model in our inversion. Our previous 1-D reference model, the Preliminary Reference Earth Model (PREM), has a 220-km velocity jump, which is, however, not a ubiquitous feature of the Earth (e.g., Nettles and Dziewonski, 2008; Kustowski et al., 2008). We are switching from the PREM model to a new 1-D reference model developed at UC, Berkeley (Lekic and Cammarano, pers. Comm.) that does not preserve the 220-km discontinuity. Lastly, we are incorporating into our tomographic model the upper mantle discontinuities. These upper mantle discontinuities, serving as either boundaries separating distinct upper mantle anisotropy domains (e.g., the LAB) or isotropic velocity gradients, are better constrained by other techniques, such as teleseismic receiver functions. We are currently exchanging data with research groups at Brown and Rice Universities: the LAB from those groups will be incorporated into our tomographic inversion as a priori constraints; in return, our new 3-D tomographic results will provide a better velocity model for the receiver function depth migration.
We thank the IRIS DMC and Canadian National Data Center for providing the waveform data. Figures are prepared using GMT (Wessel and Smith, 1998). This project is supported by NSF EAR-0643060.
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