Scientists Say State Is Stretching

At the rate of an inch of east-west growth every 40 years, you will probably not notice the change. It took five years’ data from a high-precision statewide network of Global Positioning System instruments to detect the movement.

But Vaughn and Portales really do seem to be moving apart, according to Colorado geophysicist Henry Berglund. Ditto Gallup and Albuquerque.

Scientists have long had a pretty good idea something like this was happening, because of the way the Rio Grande Rift splits the state down the middle. The rift, a tear in the Earth’s crust, defines the state’s central mountain chain and the valley the Rio Grande now follows. So scientists expected Rio Rancho, on the west side of the rift, to be pulling away from Albuquerque on the east.

But until scientists began collecting data from their monitoring network back in 2006, they had no idea how widespread the stretching was, Berglund said. Berglund, a student at the University of Colorado when the research was done, is lead author of a paper outlining the findings in a recent issue of the journal Geology.

Instead of two rigid chunks of Earth’s crust being torn apart at the rift, with the movement focused there, the new data suggest New Mexico and much of the territory that surrounds it is more like a rubber sheet, stretching uniformly, from Texas and Oklahoma to Arizona.

It is that relatively uniform stretching that caught scientists off guard.

“We didn’t expect it to be so spread out,” said University of Colorado geophysicist Anne Sheehan, who was Berglund’s faculty adviser and a member of the research team.

“It’s surprising,” said University of New Mexico geophysicist Mousumi Roy, who also worked on the project, “because given the geology, we weren’t expecting it.”

The stretching and straining of Earth’s crust is a well-known phenomenon at the edge of continental plates, the big pieces of Earth’s crust that are always slowly slipping and sliding around the globe, driven by movement of the mantle below. In places like California, where the famous San Andreas Fault is actively on the move, the results can be dramatic and easy to detect. Similar monitoring networks have found much more pronounced movement in seismically active places like Japan and New Zealand, Sheehan said.

But what happens in continental interiors has been more of a puzzle. There is movement there, too, but it is slower and more mysterious.

The scientists began deploying their GPS network back in 2006, and now have 25 stations in Colorado and New Mexico. The stations use the same technology found in the navigation devices common in new cars today, determining where you are by reading signals from a constellation of satellites. But the systems used by the scientists are far more precise, Berlund said.

The discovery poses a puzzle, Sheehan said: What forces are driving the stretching?

One possibility, Sheehan said, is an upwelling in the mantle, the gooey region on which the Earth’s rocky crust rides. Mantle movements are often fingered to explain movements of the crust above. Another possibility is the crust itself sagging down and stretching in response to past mountain-building episodes, she said.

The scientists plan to leave the GPS network in place to look for changes in the stretching over time, Berglund said. They also hope, with more time, to determine whether Earth’s crust is rising or falling across New Mexico.