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The Rio Grande Rift
By Brooke Molson-Moran
Emporia State University ES 767: Advanced Tectonics
April 19, 2017
Background

Divergent plate boundaries are areas of the Earth’s lithosphere where the tectonic plates are pulling apart. Usually, crustal extension occurs along mid-ocean ridges, but the Rio Grande rift is one of three places where a continental plate is pulled apart. The Rio Grande rift extends more than 1000 kilometers from central Colorado into northern Mexico. The Rio Grande River mirrors much of the rift’s axis because water follows topographic lows. Figure 1 is a photograph of the Arkansas Valley close to Leadville, Colorado and possibly the northern extent of the Rio Grande rift.

Figure 1 Looking south into the Arkansas Valley in central Colorado. Photo taken by Brooke Molson-Moran.
Figure 2 Rio Grande Rift. Photo courtesy USGS.
Figure 4 GPS monitoring station located at Great Sand Dunes National Park.  Photo courtesy Nicole Feldl.
Figure 3 Rift cross-section. Photo courtesy USGS.

References

Baldridge, W.S. and Olsen, K.H. 1989. The Rio Grande Rift.  American Scientist 77.  240-247.

Burchfiel, B.C. 1983. The Continental Crust. Scientific American 249/3. 130-142.

Cooperative Institute for Research in Environmental Sciences (CIRES). 2012. Earthquake potential in Colorado and New Mexico. Accessed online < http://cires.colorado.edu/news/earthquake-potential-colorado-and-new-mexico> April 2017.

Haddock, E. 2008. Birth of an Ocean. Scientific American 299. 60-67.

Plummer, C., McGeary, D. and Carlson, D. 2003. 9th ed. Physical Geology. McGraw Hill, New York. 574 p.

Van der Pluijm, B.A. and Marshak, S. 2004. 2nd ed. Earth Structure. W.W. Norton & Company, New York. 656 p.

Structure

Basins are distinctive features of rift systems and result from normal faulting, where the hanging wall moves downward relative to the footwall. When extensional forces pull a tectonic plate in opposite directions, a block of crust whose sides are the hanging walls may move downward and form a basin, while the flanking footwalls might be thrust upward. A basin formed this way is called a graben, meaning “ditch” in German (Plummer 2003).

Technically, the Rio Grande rift system is an asymmetric series of half-grabens. There is a normal fault on one side of the half-graben, and a sloping faulted block on the other. Moving along the length of the Rio Grande rift, the half-grabens alternate orientation (Van der Pluijm 2004).

Figure 3 is a generalized cross section of a rift system. As rifting pulls the crust apart, the breach widens and basaltic lava flows into the opening from beneath the rift. Eventually, the continental rift becomes an oceanic ridge with the same orientation. Both continental and oceanic rifts erupt the same type of basalt (Haddock 2008).

Tectonics and Volcanism

Volcanism and earthquakes are associated with the tectonic activity of the Rio Grande rift. Some of the surrounding mountains result from rift volcanism. Recent earthquakes could be caused by renewed plate motion. Figure 4 shows one of 25 GPS monitoring stations which record plate movement in order to measure if and how fast the rift is spreading. GPS uses satellite tracking to measure how far tectonic plates move. Although some geologists believed the rift was inactive, the GPS data revealed the rift is still slowly extending at a rate of 1.2 millimeters per year. Some researchers thought the true amount of movement may be higher than it is, up to 5 mm per year, but the highly accurate GPS technology proved there is indeed movement. Another relatively new finding is that the rift is much wider than previously thought, and may extend eastward into the Great Plains (CIRES 2012).

For 100 million years before rifting began, subduction off the west coast of the United States contributed to mountain-building towards the east. As the compressive forces of subduction slowed down about 40 million years ago, the continental lithosphere was rendered more pliable. As hot material rose through the mantle it pushed upward, thinning the lithosphere and creating a zone of weakness (Baldridge 1989).

The Rio Grande rift has been experiencing crustal thinning for roughly 30 million years and the most volcanism occurred in the beginning. From 18 to 10 million years ago minimal volcanism occurred. Then around 5 million years ago basalt erupted all over the rift system. Since, volcanism has been limited to the Jemez lineament, a narrow band running from central Arizona to northeastern Mexico (Baldridge 1989).

Figure 2 is a map of much of the Rio Grande rift system and highlights several of its basins. The Rio Grande rift has experienced an average of 9 kilometers of mainly vertical displacement along its faults as the grabens move downward and the surrounding blocks move upward (Baldridge 1989).

Over time, sediment accumulates in the basin as layers of mudstone, sandstone, conglomerate are interbedded with volcanic deposits. If the rift continues spreading, layers of evaporites and marine deposits may be present. The thickness of sedimentary deposits in the sedimentary basin of the Rio Grande rift varies along the rift and may be several kilometers. The weight of the sediment contributes to subsidence of the basin. Also, as the system loses heat the lithosphere cools and contracts, which also leads to subsidence. If further rifting and subsidence occurs, a new ocean will form. However, some rifts fail and do not become oceans, but merely remain geological scars.

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