Nature Notes; Rio Grande Rift

When it comes to the geology of North America, it doesn’t take a scientist to see the West is where the action is.

Across this mountainous land, tectonic forces that shaped the landscape are still at work. The Trans-Pecos region of Texas is showcases one major transformation, the Rio Grande Rift.

The Rio Grande established its course, spanning from the Rocky Mountains to the Gulf of Mexico, during the last two million years. For geologists, though, the river is a recent development. Why? There are older forces at work.

Jesse Kelsch is a geology professor at Sul Ross State University in Alpine. “The river was the last thing to have happened after the long-term formation of the rift basins themselves,” Kelsch said, “and so, I don’t give it much thought.”

The rupture that provided a route for the river began more than 35 million years ago.

Beginning more than 100 million years ago, a plate located off the Pacific Coast, the Farallon Plate, began to collide with North America. The colliding plate added land to the continent. But the Farallon Plate was also subducted, meaning it drove itself beneath North America. It was a grinding collision. Rock on the continent was compressed and warped.

Finally, when almost all of the Farallon Plate was chewed up, the pressure was relieved and western North America began to stretch out, extending toward the Pacific Plate. That extension continues today.

As it stretches, the crust of the continent has faulted and split, creating mountain chains and basins.

For reasons that are still uncertain, the Colorado Plateau has proved resistant to the stretching, the four-corners area of New Mexico, Arizona, Utah and Colorado. The Rio Grande Rift developed between that stable plateau and the similarly resistant rock of the Midwest.

“So with that extension focus starting at the western edge of North America,” Kelsch explained, “it’s being pulled in the direction of the Pacific Plate, the Colorado Plateau is kind of being pulled along too, and that has the effect of creating the rift between the Colorado Plateau and the rest of the North American craton, the old stable crust, the High Plains, and so there’s this north-south trending gash, basically, that’s a down-dropped basin, and that’s the Rio Grande Rift.”

The gash is discreet and narrow in its northern stretches. In the Trans-Pecos, the rift spreads out into multiple faults. The rifting began to create basins here about 23 million years ago.

The most striking expression in West Texas is what is known as the “Sunken Block,” visible in Big Bend National Park. With the volcanic rock of the Chisos Mountains in its center, the Sunken Block is bounded on its eastern edge by the escarpments of the Dagger, Dead Horse and Del Carmen mountains. The Mesa de Anguila, through which Santa Elena Canyon passes, forms part of the western edge.

“Well,” Kelsch said, “the best place to see that is when you go to Santa Elena Canyon. What you’re looking at – that big wall, through which the river comes – that’s a fault that bounds the western edge of the Sunken Block. That’s the best topographic expression of extension in the park.”

The pace has slowed, but the Rio Grande Rift has more tearing to do. Now remember the Atlantic Ocean began as a rift in the ancient super-continent of Pangaea. Ultimately, the Colorado Plateau may be the shore of a new ocean, where the Rio Grande flows today.

“This rift is a nice modern example of how ocean basins form,” Kelsch explained. “We haven’t made two new plates yet, but if it continues, then everything west of the Rio Grande rift, and everything east of that, would be two separate plates, and then the water would come in and make a new ocean.”

A skinny “proto-ocean,” similar to the Red Sea, could form here… within 20 million years.

West Texas may be removed from the hustle of modern life, but when it comes to the Earth’s fundamental transformations, it is a center of activity.

Nature Notes is underwritten by the Dixon Water Foundation and is produced by Marfa Public Radio, in cooperation with the Sibley Nature Center in Midland, Texas. This episode was written by Andrew Stuart.

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