Hotspot Frames and Shear-wave Tomography

Wagner, Forsyth, Fouch, and James (2010) have produced an intriguing model of the shear wave velocity structure beneath the northwestern US from Rayleigh wave tomography. I've calculated the predicted locus of the the Yellowstone hotspot relative to stable North America and plotted it (Fig.1) on top of their Fig. 8, (-3 percent velocity variation).

Figure 1. Wagner et al.'s (2010) Fig. 8, showing the -3 percent shear velocity anomaly, overlain by calculated locus of Yellowstone hotspot, present to 50 Ma, relative to stable North America (red line and solid circles every five m.y., calculated using Africa-hotspot parameters of Müller et al., 1993, time scale of Gradstein et al., 2005, North American-Africa parameters of Müller et al., and spline interpolation method of Pilger, 2003). The -3 percent anomaly ranges in apparent depth from ~60 to ~140 km, with the greatest apparent lateral extent at ~60 to ~80 km, from Wagner et al.'s Fig. 7. (Click graphic to enlarge, then back button to return to post.)

Note that the locus corresponds fairly well with the low-velocity anomaly. The anomaly might be interpreted to represent a combination of the Yellowstone hotspot channel formed by movement of North America to the southwest/westsouthwest over the past forty million years or so. To the west, however, it is likely that the low-velocity anomaly is also influenced by the Juan de Fuca/Farallon subduction zone and associated volcanic arc (see also Wagner et al.'s, 2003, interpretation).

For a little more completeness, I've also plotted (Fig. 2) the locus of the other hypothetical hotspot beneath the western United States, Raton, relative to stable North America, together with the reconstruction of the southwestern United States of McQuarrie and Wernicke (2005) at 36 Ma (their reconstructions are at 2 to 4 m.y. intervals between the present through 36 Ma). Note that the locus parallels, in part, the reconstructed southern edge of the Colorado plateau, between 20 and 40 Ma.

Figure 2a. Loci of Yellowstone and Raton hotspots relative to stable North America, as in Fig. 1, together with reconstructed southwestern United States and northwestern Mexico at 36 Ma., from McQuarrie and Wernicke (2005) and -3 percent velocity anomaly outline of Wagner et al. (2010, their Fig. 8).

Figure 2b. Same as Fig. 2a with addition of current fault fabric (in green) of McQuarrie and Wernicke (2005).

As the location of the inferred Raton hotspot is uncertain, a somewhat more southwesterly present location would provide an even better fit to the edge of the Plateau.

Notice that the bend in the Yellowstone locus also parallels the western part of the Snake River Plain. A slightly better fit could be made if the hotspot were located farther to the northeast, and also by taking into account Basin Range extension to the south (which might position the western part of the SRP somewhat farther east, via projection of McQuarrie and Wernicke's, 2005, reconstructions to the north).
 
Table 1

Tristan-North America Calculated Total Rotation
Age(Ma)	Longitude	Latitude	Angle|Rate
0.0	-11.117	-67.229	0.270
2.5	-16.020	-65.425	0.646
5.0	-25.340	-61.019	1.150
7.5	-32.754	-56.886	1.586
10.0	-39.278	-53.817	2.001
12.5	-46.719	-50.782	2.380
15.0	-54.305	-47.361	2.750
17.5	-60.627	-43.745	3.154
20.0	-64.785	-40.167	3.621
22.5	-66.556	-37.087	4.156
25.0	-67.030	-35.821	4.715
27.5	-67.353	-37.323	5.238
30.0	-68.483	-41.153	5.737
32.5	-71.207	-46.383	6.276
35.0	-74.764	-51.473	6.936
37.5	-77.491	-55.039	7.736
40.0	-78.222	-56.588	8.577
42.5	-76.464	-55.995	9.352
45.0	-72.610	-53.418	10.059
47.5	-67.651	-49.286	10.784
50.0	-62.973	-45.413	11.679

References
Gradstein, F. M., Ogg, J. G., & Smith, A. G., 2005, A Geologic Time Scale-2004, Cambridge Univ. Press, Cambridge, U.K. link

Gripp, A. E. & Gordon, R. G.,2002, Young tracks of hotspots and current plate velocities, Geophysical Journal International, 150, 2, 321-361. link 
McQuarrie, M., & Wernicke, B. P., 2005,  An animated tectonic reconstruction of southwestern North America since 36 Ma, Geosphere, 1, 147-172, DOI: 10.1130/GES00016.1. link
Müller, R. D., Gaina, C., Roest, W. R., & Hansen, D. L., 1999, New constraints on the Late Cretaceous/Tertiary plate tectonic evolution of the caribbean, Chapter 2,  Sedimentary Basins of the World, 4, 33-59, doi:10.1016/S1874-5997(99)80036-7, Elsevier. link

Müller, R. D., Müller, R. D., Royer, J.-Y., & Lawver, L. A., 1993, Revised plate motions relative to the hotspots from combined Atlantic and Indian Ocean hotspot tracks, Geology, v. 21, p. 275-278. link

Pilger, R. H., 2003. Geokinematics: Prelude to Geodynamics, Springer-Verlag, Berlin. link

Wagner, L., Forsyth, D. W., Fouch, M. J., & James, D. E., 2010, Detailed three-dimensional shear wave velocity structure of the northwestern United States from Rayleigh wave tomography, Earth and Planetary Science Letters, 299, 273–284, doi:10.1016/j.epsl.2010.09.005. link

Rex H. Pilger Jr., January 2, 2011