Exploring the North American Upper Mantle Using EarthScope Data
Dr. Huaiyu Yuan, University of California, Berkeley
Seismological constraints on the structure of the cratonic lithosphere are key to understanding its properties, origin and evolution. Using a combination of long period surface waveforms and SKS splitting measurements that benefit in particular from the EarthScope USArray TA deployment, we have developed a 3-D upper-mantle model of North America that includes isotropic shear velocity Vs, radial anisotropy, and 2-D azimuthal anisotropy. Combining these results, we infer several keyfeatures of lithosphere and asthenosphere structure:
- The combination of maps in Vs, radial and azimuthal anisotropy reflects contrasts in structure between the lithosphere and asthenosphere across the continent, and the considerable variations in the lithosphere-asthenosphere-boundary (LAB) depth, which ranges from less than 80 km in the WUS to over 200 km in the craton, with a sharp lateral gradient along the Rocky Mountain deformation front.
- Changes with depth of the fast axis direction of azimuthal anisotropy reveal the presence of two layers in the cratonic lithosphere, corresponding to the fast-to-slow discontinuity found in receiver functions.
- Below the lithosphere, azimuthal anisotropy manifests a maximum, and the fast axis of anisotropy aligns with the absolute plate motion, as described in the hotspot reference frame HS3-NUVEL 1A.
- The alignment of the fast axis of anisotropy with the NA APM between 70 and 100 km depth throughout the WUS indicates that, to first order, the uppermost part of the mantle is moving along with the NA Plate and is not strongly coupled to deeper mantle flow.
- Alternating zones of weaker and stronger radial anisotropy correlate with zones of small lateral changes in the fast axis direction of anisotropy, and faster than average Vs below the LAB, suggesting the presence of small scale convection with a wavelength of ~2000 km.
- In the western United States, complex 3-D patterns of isotropic velocity and anisotropy reflect mantle dynamics associated with the rich tectonic history of the region.
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