Earth's crust in NM is stretching

ALBUQUERQUE (AP) - The Earth's crust in New Mexico is stretching at a rate of an inch of east-west growth every 40 years, researchers said in a new study.

The Albuquerque Journal reports ( http://bit.ly/yVFpYl ) that scientists have long suspected it 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 River now follows.

Colorado geophysicist Henry Berglund, the lead author of a paper outlining these findings in a recent issue of the journal Geology, said that until scientists began collecting data from their monitoring network in 2006, they had no idea how widespread the stretching was.

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, a member of the research team.

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.

In places like California, where the 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, but it is slower and more mysterious.

The scientists began deploying their GPS network in 2006 and now have 25 stations in Colorado and New Mexico.

Sheehan said one possible explanation for what's driving the stretching is an upwelling in the mantle, the gooey region on which the Earth's rocky crust rides. Mantle movements are often cited 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 to determine whether Earth's crust is rising or falling across New Mexico.