Berkeley 3-D Isotropic and Anisotropic S-velocity Model of North American Upper Mantle, Updated to June 2008

Huaiyu Yuan and Barbara Romanowicz

Figure 2.54: Figure 3-D isotropic Vs (A-H) and radial anisotropy Xi (I-L) model of North America upper mantle.
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We present here the isotropic and radial anisotropy part of our 3-D North American upper mantle shear wave velocity model. This new model is constructed starting from a new global 1D reference Earth model, based on a combination of seismic and mineral physics data (Cammarano and Romanowicz, 2007), which is smooth at 220 km, and a crustal correction approach that takes into account the non-linearities involved (Lekic and Romanowicz, submitted).

North American Isotropic Vs and Radial Anisotropy model

The pronounced findings of the Vs model include: 1) high velocity cratonic roots extend to 200-250 km depth range (A-E); 2) at shallow depth ($<$ 200km; A-D) low velocities flank the cratonic region from the west and the south, with a sharp transition along the Rocky Mountain Front; 3) a high velocity feature exists from 450 to 600 km (H) in transition zone beneath the western US; and 4) small scale ($~$100km) velocities correlate with surface geological features, e.g., the Rio Grande rift, Colorado Plateau, and the Eastern Snake River Plain in the western US.

The radial anisotropy model (I-L) is plotted with respect to an isotropic reference. The dominant feature above 200 km is the faster Vsh than Vsv or Xi$>$1 everywhere. Abnormal Xi is spotted beneath the northeastern Pacific rise, the Rio Grande Rift, the Nevada Basin and Range, and the central Trans-Hudson Orogen, suggesting Vsv is faster in those regions. At $>$300km, most of the radial anisotropy has disappeared, except beneath the western US where Xi $<$1, indicating a probable deep origin of the anomaly. A negative Xi anomaly is observed in the vicinity of the East Pacific Rise at all depths (I to L), suggesting a probable deep ($>$ 300km) origin of this anomaly.


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