The Rio Grande Rift is a rift valley extending north from Mexico, near El Paso, Texas through New Mexico into central Colorado.^[2] The upper Rio Grande flows south following the rift valley, but did not incise it.

The Rio Grande Rift represents the easternmost mainfestation of widespread extension in the western U.S. during the past 35 million years. After the Farallon Plate subduction-associated compressive forces of the Laramide orogeny (which produced the Sangre de Cristo Mountains and many other upthrust mountain blocks visible today) ended during the Eocene Epoch, erosion of these uplands filled the area of the Raton Basin with abundant sediments. In the late Oligocene Epoch regional tensional forces became dominant and rifting was initiated as the crust and uppermost mantle began extending. The rifting produced fault zone-bounded valleys (grabens or half-grabens). A graben consists of normal faulting on each flank with the central portion downdropped. Igneous intrusions moved into the zones of weakness produced by this faulting and reached the surface in many areas as extensive volcanism along the margins of the rift. The most active rifting and associated volcanism came to an end in the late Cenozoic. The youngest eruptions in the rift region are in the Valley of Fires, New Mexico and are approximately 5400 years old.^[3][4] The Socorro, New Mexico region of the central rift hosts an inflating mid-crustal sill-like magma body at a depth of 19 km that is responsible for anomalously high earthquake activity in the vicinity, including the largest rift-associated earthquakes in historic times (two events of approximately magnitude 5.8) in July and November 1906.^[5][6][7] Earth and space-based geodetic measurements indicate ongoing surface uplift above the Socorro Magma Body at approximately 2 mm/year.^[8]

The rift is presently nearly tectonically quiescent, but significant deformation and faulting with offsets of many kilometers was responsible for its formation starting about 35 million years ago.^[9] The largest-scale manifestation of rifting involves a pure-shear rifting mechanism, in which both sides of the rift pull apart evenly and slowly, with the lower crust and upper mantle (the lithosphere) stretching like taffy.^[10][11][12] This extension is associated with very low seismic velocities in the upper mantle above approximately 400 km depth associated with relatively hot mantle and low degrees of partial melting.^[13] The upper crustal and surface manifestation of the rift is a sequence of asymmetric half-grabens that accommodate deep basins that are substantially filled with alluvium.

The Valles Caldera National Preserve is dominated by a huge caldera in the Jemez Mountains, located where the Rio Grande Rift intersects the Jemez Lineament, a linear crustal feature in the southwestern United States that may represent a suture zone from the Proterozoic accretion of North America. However, the Jemez Mountains themselves are not primarily a tectonic feature of the rift; rather, they partially overlie a range on the west side of the graben, the lower and less well-known Nacimiento Mountains. The Colorado Plateau, to the west, includes the San Juan Volcanic Field and the San Juan Mountains. The city of Albuquerque lies within the rift. The Rio Grande follows the course of the rift from southern Colorado to El Paso; it starts a southeast course out of New Mexico at the pass between the Franklin Mountains and the Sierra Juarez, which ends at the Gulf of Mexico.

[edit] See also

• Rio Grande Trail
• Potrillo volcanic field
• Geologic timeline of Western North America

[edit] References

1. ^ Wilson, David, et al., Lithospheric structure of the Rio Grande rift, Nature 433, 851 - 855 (24 February 2005); doi:10.1038/nature03297
2. ^ Keller, R., and Baldridge, W.S., Rocky Mountain Geology; March 1999; v. 34; no. 1; p. 121-130; DOI: 10.2113/34.1.121.
3. ^ Aber, James S., Rio Grande Rift, http://academic.emporia.edu/aberjame/struc_geo/rio/rio.htm Accessed 8 February 2006.
4. ^ Veatch, Steven Wade, The Rio Grande Rift, 20 March 1998, http://home.att.net/~sgeoveatch/rio_grande_rift.htm Accessed 8 February 2006.
5. ^ Reid, H.G., Remarkable earthquakes in central New Mexico in 1906 and 1907, Bull. Seism. Soc. Am., 1, 10-16, 1911
6. ^ Sanford, A.R., R.S. Balch, and K.W. Lin, A seismic anomaly in the Rio Grande Rift near Socorro, New Mexico, New Mexico Institute of Mining and Technology Geophysics Open-File Report 78, Socorro, New Mexico, 17 pp., 1995
7. ^ Schlue, J., Aster, R., Meyer, R., A lower-crustal extension to a mid-crustal magma body in the Rio Grande Rift, New Mexico, J. Geophys. Res., 101., 25,283-25,291, 1996
8. ^ Fialko, Y., and M. Simons, Evidence for on-going inflation of the Socorro magma body, New Mexico, from interferometric synthetic aperture radar imaging Geop. Res. Lett., 28, 3549-3552, 2001.
9. ^ December Geosphere media highlights
10. ^ Wilson, David, et al., Lithospheric structure of the Rio Grande Rift, Nature 433, 851 - 855 (24 February 2005); doi:10.1038/nature03297
11. ^ Fleck, John, 2005, Study Sheds Light on 30 Million Years of Rio Grande Valley Stretching, Albuquerque Journal, Electronic Ed., February 24, 2005.
12. ^ Hill, Karl, 25 February 2005, The Rio Grande Rift: a continent "stretched like taffy", NMSU News Release.
13. ^ Gao, W., Grand, S., Baldridge, S., Wilson, D., West, M., Ni, J., Aster, R., Upper mantle convection beneath the central Rio Grande rift imaged by P and S wave tomography , J. Geophys. Res., 109 (B3), B03305, doi:10.1029/2003JB002743, 2004.