Intraplate Magmatism in the Southwestern US: The Colorado Plateau Edges

Jolante van Wijk¹, W. Scott Baldridge², Jeroen van Hunen³, Saskia Goes⁴, Rick Aster⁵, David Coblentz², Steve P. Grand⁶, & James Ni⁷

¹Dept. Earth & Atmospheric Sciences, Univ. Houston, 312 Science & Research Bldg. 1, Houston, Texas 77204, USA, jwvanwijk@uh.edu

²Earth & Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA, sbaldridge@lanl.gov ; coblentz@lanl.gov

³Dept. Earth Sciences, Durham Univ., Durham DH1 3HP, UK, jeroen.van-hunen@durham.ac.uk

⁴Dept. Earth Science and Engineering, Imperial College, London SW7 2AZ, UK, s.goes@imperial.ac.uk

⁵Dept. Earth & Environmental Science & Geophysical Research Center, New Mexico Institute of Mining & Technology, Socorro, New Mexico 87801, USA, aster@ees.nmt.edu

⁶Dept. Geological Sciences, Univ. Texas, Austin, Texas 78712, USA, steveg@maestro.geo.utexas.edu

⁷Dept. Physics, New Mexico State Univ., Las Cruces, New Mexico 88003, USA, jni@nmsu.edu

This webpage is a summary of: Van Wijk, J.W., W.S. Baldridge, J. van Hunen, S. Goes, R. Aster, D. Coblentz, S.P. Grand, J. Ni, Small-scale convection at the edge of the Colorado Plateau: Implications for topography, magmatism, and evolution of Proterozoic lithosphere. Geology, 38, 611 614, doi:10.1130/G31031.1, 2010.

In the southwestern US, Cenozic magmatism is abundant. It occurs far away from the nearest plate boundary, as far east as New Mexico and Colorado (Figure 1). Some of this magmatic activity is very young. The Socorro magma body near Socorro, New Mexico, is currently developing, for example. Since about 15 Ma, much volcanic activity seems to concentrate around the edges of the Colorado Plateau. The Colorado Plateau is a tectonically stable province consisting of Proterozoic lithosphere. It has remained relatively undeformed and unaffected by recent tectonic events that formed the Basin and Range Province toward its western and southern edges, and the Rio Grande rift zone southeast of the Colorado Plateau.

Figure 1. Topographic map of Colorado Plateau region, with seismicity (red dots). CP= Colorado Plateau, BRP= Basin and Range Province, RGR= Rio Grande rift, GP= Great Plains. The black and white triangles denote the location of seismic stations of the La RISTRA seismic experiment (West et al., 2004; Wilson et al., 2005). The right panel shows ages and locations of Cenozoic volcanism in the Colorado Plateau region. The Colorado Plateau is outlined in black.

A closer look at Cenozoic magmatism in the western US reveals an interesting pattern. Magmatism was not continuous. A middle, Cenozoic pulse was separated from a younger Late Neogene-Quarternary pulse by a period of relative quiescence (Figure 2). The middle Cenozoic magmatism is intermediate to silicic and virtually absent on the Colorado Plateau. The Late Neogene-Quarternary pulse is fundamentally basaltic and present on the edges of the Colorado Plateau. We interpret these phases as resulting from separate geodynamic events and focus here on an explanation for the most recent event. Earlier studies (Humphreys et al., 2003; Christensen & Yeats, 1992; Dickinson, 1997) have documented and explained the mid-Cenozoic event.

Figure 2. K-Ar and ⁴⁰Ar/³⁹Ar igneous rocks ages in New Mexico (for source of data, see van Wijk et al., 2010).

What causes the recent pulse of volcanism on the Colorado Plateau edges? When extension of the lithosphere formed the Basin and Range Province and Rio Grande rift zone, the lithosphere in these continental rifts thinned (Wernicke et al., 1992; West et al., 2004; Wilson et al., 2005). This resulted in a variation in depth of the lithosphere-asthenosphere boundary between the Colorado Plateau and these extended areas (Colorado Plateau lithosphere is thicker than surrounding lithosphere of the extended Basin & Range and Rio Grande rift areas). This step in lithosphere thickness causes small scale convection (edge driven convection) in the upper mantle (Figure 3).

Figure 3. A small-scale convection cell develops at the edge of the Colorado Plateau. An instability grows through lateral addition of mantle lithosphere, which locally thins the lithosphere. Melting occurs at the edge of the Plateau.

Mantle lithosphere material feeds the instability (drip) that is being formed, and this process thins the mantle lithosphere at the edge of the Colorado Plateau. A convection cell develops and asthenosphere is brought up below the plateau edges. The geotherm crosses the solidus below the edges, resulting in partial melting and volcanism.

Edge-driven convection occurs when the depth of the lithosphere-asthenosphere boundary varies laterally. King & Anderson (1998) found edge driven convection at the edges of cratonic provinces, and suggested that igneous provinces such as the North Atlantic Igneous Province could be formed by this process. In our models, edge-driven convection occurs also when the depth to the lithosphere-asthenosphere boundary varies laterally only modestly, such as between the Colorado Plateau and surrounding extended provinces.

More information about this study can be found in van Wijk et al. (2010).

References

• Christiansen, R.L., and Yeats, R.S., 1992, Post-Laramide geology of the U.S. Cordilleran region, in Burchfiel, B.C., et al., eds., The Cordilleran Orogen: Conterminous U.S.: Boulder, Colorado, Geological Society of America, Geology of North America, v. G-3, p. 261–406.
• Dickinson, W.R., 1997, Tectonic implications of Cenozoic volcanism in coastal California: Geological Society of America Bulletin, 109, 936–954, 1997.
• Humphreys, E., Hessler, E., Dueker, K., Farmer, G.L., Erslev, E., and Atwater, T., 2003, How Laramide-age hydration of North American lithosphere by the Farallon slab controlled subsequent activity in the western United States: International Geology Review, 45, 575–595, doi: 10.2747/0020-6814.45.7.575.
• King, S.D., and Anderson, D.L., 1998, Edge-driven convection: Earth and Planetary Science Letters, 160, 289–296, doi: 10.1016/S0012-821X(98)00089-2.
• Van Wijk, J.W., W.S. Baldridge, J. van Hunen, S. Goes, R. Aster, D. Coblentz, S.P. Grand, J. Ni, Small-scale convection at the edge of the Colorado Plateau: Implications for topography, magmatism, and evolution of Proterozoic lithosphere. Geology, 38, 611 614, doi:10.1130/G31031.1, 2010.
• Wernicke, B., Cenozoic extensional tectonics of the U.S., in Burchfiel, B.C. et a., eds., The Cordilleran Orogen: Conterminous U.S., Boulder, Colorado, Geological Society of America, Geology of North America, v. G-3, p. 553-581.
• West, M., Ni, J., Baldridge, W.S., Wilson, D., Aster, R., Gao, W., and Grand, S., 2004, Crust and upper mantle shear wave structure of the southwest United States: Implications for rifting and support for high elevation: Journal of Geophysical Research, 109, B03309, doi: 10.1029/2003JB002575.
• Wilson, D., R. Aster, M. West, J. Ni, S. Grand, W. Gao, W.S. Baldridge, S. Semken, Lithospheric structure of the Rio Grande rift. Nature, 433, 851-855, doi:10.1038/nature03297.