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1. Mantle flow distribution beneath the California margin

   https://doi.org/10.1038/s41467-020-18260-8  

   Barbot, Sylvain (September 2020, Nature Communications)

   Abstract Although the surface deformation of tectonic plate boundaries is well determined by geological and geodetic measurements, the pattern of flow below the lithosphere remains poorly constrained. We use the crustal velocity field of the Plate Boundary Observatory to illuminate the distribution of horizontal flow beneath the California margin. At lower-crustal and upper-mantle depths, the boundary between the Pacific and North American plates is off-centered from the San Andreas fault, concentrated in a region that encompasses the trace of nearby active faults. A major step is associated with return flow below the Eastern California Shear Zone, leading to the extrusion of the Mojave block and a re-distribution of fault activity since the Pleistocene. Major earthquakes in California have occurred above the regions of current plastic strain accumulation. Deformation is mechanically coupled from the crust to the asthenosphere, with mantle flow overlaid by a kinematically consistent network of faults in the brittle crust. 

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2. Upper mantle tomography beneath the Pacific Northwest interior

   https://doi.org/10.1016/j.epsl.2020.116214  

   Stanciu, A. Christian; Humphreys, Eugene D. (June 2020, Earth and Planetary Science Letters)

   null (Ed.)
3. Shear Wave Splitting and Mantle Flow Beneath Alaska

   https://doi.org/10.1029/2019JB018329  

   McPherson, A_M; Christensen, D_H; Abers, G_A; Tape, C. (April 2020, Journal of Geophysical Research: Solid Earth)

   Abstract Shear wave splitting is often assumed to be caused by mantle flow or preexisting lithospheric fabrics. We present 2,389 new SKS shear wave splitting observations from 384 broadband stations deployed in Alaska from January 2010 to August 2017. In Alaska, splitting appears to be controlled by the absolute plate motion (APM) of the North American and Pacific plates, the interaction between the two plates, and the geometry of the subducting Pacific‐Yakutat plate. Outside of the subduction zone's influence, the fast directions in northern Alaska parallel the North American APM direction. Fast directions near the Queen Charlotte‐Fairweather transform margin are parallel to the faults and are likely caused by the strike‐slip deformation extending throughout the lithosphere. In the mantle wedge, fast directions are oriented along the strike of the slab with large splitting times and are caused by along‐strike flow in the mantle wedge as the slab provides a barrier to flow. South of the Alaska Peninsula, the fast directions are parallel to the trench regardless of sea floor fabric, indicating along strike flow under the Pacific plate. Under the Kenai Peninsula, the complex flat slab geometry may cause subslab flow to be parallel to Pacific APM direction or to the North America‐Pacific relative motion. 

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4. Anomalous mantle transition zone beneath the Yellowstone hotspot track

   https://doi.org/10.1038/s41561-018-0126-4  

   Zhou, Ying (May 2018, Nature Geoscience)
5. Azimuthal Anisotropy of the Crust and Uppermost Mantle Beneath Alaska

   https://doi.org/10.1029/2020JB020076  

   Feng, L.; Liu, Chuanming; Ritzwoller, M. H. (December 2020, Journal of Geophysical Research: Solid Earth)

   Abstract This study presents an azimuthally anisotropic shear wave velocity model of the crust and uppermost mantle beneath Alaska, based on Rayleigh wave phase speed observations from 10 to 80 s period recorded at more than 500 broadband stations. We test the hypothesis that a model composed of two homogeneous layers of anisotropy can explain these measurements. This “Two‐Layer Model” confines azimuthal anisotropy to the brittle upper crust along with the uppermost mantle from the Moho to 200 km depth. This model passes the hypothesis test for most of the region of study, from which we draw two conclusions. (a) The data are consistent with crustal azimuthal anisotropy being dominantly controlled by deformationally aligned cracks and fractures in the upper crust undergoing brittle deformation. (b) The data are also consistent with the uppermost mantle beneath Alaska and surroundings experiencing vertically coherent deformation. The model resolves several prominent features. (1) In the upper crust, fast directions are principally aligned with the orientation of major faults. (2) In the upper mantle, fast directions are aligned with the compressional direction in compressional tectonic domains and with the tensional direction in tensional domains. (3) The mantle fast directions located near the Alaska‐Aleutian subduction zone and the surrounding back‐arc area form a toroidal pattern that is consistent with mantle flow directions predicted by recent geodynamical models. Finally, the mantle anisotropy is remarkably consistent with SKS fast directions, but to fit SKS split times, anisotropy must extend below 200 km depth across most of the study region. 

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