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The Rio Grande Rift: A Continent "Stretched Like Taffy", Feb. 23, 2005

Magma body beneath Socorro

Right: Conceptual view of the magma body beneath Socorro.

by Karl Hill, NMSU

NMSU CONTACTS: Karl Hill, University Communications, (505) 646-1885, khill@nmsu.edu; Dr. James Ni, Physics, (505) 646-1920, jni@nmsu.edu
NM TECH CONTACTS: George Zamora, Public Information Office, (505) 835-5617, gzamora@admin.nmt.edu; Dr. Richard Aster, Earth and Environmental Science, (505) 835-5924, aster@ees.nmt.edu

SOCORRO, N.M., Feb. 23, 2005 – Using a vast array of seismic instruments reaching nearly 600 miles across the desert Southwest, scientists from New Mexico, Texas and Arizona have produced the most detailed images yet of the geology up to hundreds of miles beneath the Rio Grande Rift.

The story the pictures tell, in the words of one of the collaborators, is that of a continent being slowly “stretched like taffy” over the past 30 million years.

“There has been a very symmetrical thinning of the earth’s crust” along the Rio Grande Rift, said Richard Aster, professor of geophysics at New Mexico Institute of Mining and Technology.

This extension is about four times wider in the lower crust – about 20 miles down – than it is at the surface of the rift, where the Rio Grande flows southward from southern Colorado through New Mexico, said James Ni, professor of physics at New Mexico State University.

A rift is a narrow opening of the earth’s crust caused by lateral stretching.

Much as medical doctors can make pictures of their patients’ internal organs using Computed Tomography (CT) scans and ultrasound technology, the scientists were able to produce high-resolution images of the crust and upper mantle of the Rio Grande Rift using seismic instruments placed across the surface of the rift. Their findings are described in an article in the Feb. 24 issue of the journal Nature.

The linear array of seismic instruments – 54 of them – was placed along a 600-mile line from near Pecos, Texas, to near Lake Powell in the Four Corners area of Utah. The line dissected New Mexico diagonally, from southeast to northwest, crossing the Rio Grande just south of Belen near the center of the state.

The instruments were put into place between July and November 1999 and the last station was removed in May 2001. For more than a year and a half, any earthquake in the world above about magnitude 5.6 sent detectable waves through the lithospheric structures beneath the instruments. Seismic waves change in speed and character as they pass through structures of different compositions, so the readings allowed the scientists to put together what Ni called “the best, most detailed two-dimensional images to date, all the way down to the upper mantle about 670 kilometers or 400 miles down.”

The deep-imaging seismic experiment, which stretched from the edge of the Great Plains region in west Texas to the Colorado Plateau, is known as the Colorado Plateau/Rio Grande Rift Seismic Transect Experiment, or La RISTRA.

“The Rio Grande Rift is the defining topographic feature of our region,” Aster said. “There are only a few clear places on the planet where narrow rifts are being pulled apart by tectonic forces, and this is one of the classic examples.”

The scientists wanted to learn how the rift formed and what the underlying mantle and crust look like today. They found a uniform, symmetrical stretching of the crust, unlike the asymmetrical rift other scientists had theorized.

Because of the thinning of the crust, the partial-melt zone of the upper mantle is closer to the surface than normal, which has stimulated recent volcanic activity in the region. Ni noted that relatively recent lava flows exist at the edges of the Rio Grande Rift, on the east near Carrizozo and on the west near Grants.

But the Rio Grande Rift shows a history of less volcanic activity than other rifts in the world. It’s likely that temperatures or water content in the mantle beneath other rifts are higher than those below the Rio Grande Rift, Ni said.

Ni said RISTRA data, which the scientists have been analyzing for four years, suggest that the thinning of the crust along the Rio Grande Rift followed the subduction, or sinking, of an oceanic plate known as the Farallon Plate, which started about 36 million years ago. As the Farallon Plate disappeared beneath the North American continental plate, he said, convection forces caused the rift to stretch and thin.

At the same time, the entire region, which had been a high plateau, gradually dropped in elevation. The Rio Grande Valley in southern New Mexico, which today is about 4,000 feet above sea level, may once have been about 10,000 feet, Ni said.

Aster said a characteristic feature of the Rio Grande Rift is the large amount of sediment. The mountains that rise up on each side of the rift, such as the Sandia Mountains near Albuquerque, “are up to their necks in alluvium left by erosion,” he said. “If you didn’t have so much sediment, you would have an enormously deep, Death Valley-like feature going through the middle of New Mexico.”

The deep deposits of alluvium in the rift were crucial to the formation of aquifers that provide much of the state’s water supply, he noted.

This research was supported by National Science Foundation grants, the Los Alamos National Laboratory Institute for Geophysics and Planetary Physics, the NMSU Arts and Sciences Research Center, and the New Mexico Tech Geophysical Research Center.

The seismic recorders used in RISTRA were drawn from a large pool of NSF instruments managed by the Incorporated Research Institutions for Seismology through its instrument center at New Mexico Tech.

Aster noted that the RISTRA team and its temporary seismic stations were welcomed across a huge swath of the Southwest by a diverse group of landowners and managers interested in promoting science, including the Navajo Nation, Laguna and Isleta Pueblos, the Sevilleta National Wildlife Refuge of the U.S. Fish and Wildlife Service, the Bureau of Land Management, the N.M. State Land Office, the U.S. Forest Service and many private landowners.

The lead author of the Nature paper, David Wilson, recently received his Ph.D. at New Mexico Tech and now is a postdoctoral researcher at the University of Texas at Austin. Other scientists collaborating on the research are Steve Grand, Wei Gao and Paresh Patel of the Jackson School of Geosciences at the University of Texas at Austin; W. Scott Baldridge of the Earth and Environmental Sciences Division of Los Alamos National Laboratory; Steve Semken of the Department of Geological Sciences at Arizona State University; and Michael West, who worked with Ni as a postdoctoral researcher at NMSU and now is an assistant professor at the University of Alaska at Fairbanks.

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