How the West Was Stretched

A team of researchers including Scripps Institution of Oceanography, UC San Diego geophysicist Dave Stegman has created a model to explain the forces that gave rise to the the Great Basin that covers most of Nevada over the past 30 million years.

The research, led by Monash University’s Wouter Schellart, was published in the July 16 issue of the journal Science.

The new theory extends the theory of plate tectonics – a kinematic description of plate motion without reference to the forces behind it – with a dynamical theory that provides a physical explanation for both the motions of tectonic plates as well as motion of plate boundaries. The new findings have implications for how scientists understand the geological evolution of Earth, and in particular, the tectonic evolution of western North America, in the past 50 million years.

At the beginning of the sequence 50 million years ago, the Farallon plate was sinking under the North American continent, pushing into it and building mountain ranges such as the Rocky Mountains. The Farallon plate was subducting as a coherent plate along the entire length of North America from Alaska down past Mexico. Then around 30 million years ago, a dramatic segmentation occurred when the mid-ocean ridge collided with the continent, effectively splitting the 10,000-kilometer (6,200-mile)-long plate into two smaller plates. This gave rise to the San Andreas Fault that now connects the northern and southern portions of the former plate, the Juan de Fuca plate off Washington state and the Cocos plate off Central America. 

Since then, the force of the Farallon plate has begun to ebb as the San Andreas plate boundary replaced the subducting plate boundary over nearly the entire length of California.  Meanwhile, western North America has been extending towards the west with the Basin and Range Province that includes the Great Basin of Nevada and Utah as the most obvious signature of the expansion. While scientists have accepted that the events took place over the course of 50 million years, attempts to explain how the movement happened had been lacking.

”The tectonics indicate a gradual shift from compression and mountain-building in the North American continent 50 million years ago to widespread extension and volcanism over the past 30 million years at the same time the Farallon plate evolved into two plates each less than 3,000 kilometers (1,860 miles) long,” said Stegman. “The results of our computer models show that long plates push into continents and small plates retreat away from continents. We’re essentially connecting the dots.”

The computer model was developed by Stegman, along with team members Schellart, Rebecca Farrington, Justin Freeman, and Louis Moresi from Monash University.

“In some ways, plate tectonics is the surface expression of dynamics in the earth’s interior but now we understand the plates themselves are controlling the process more than the mantle underneath,” Stegman said. “It means Earth is really more of a top-down system than the predominantly held view that plate motion is being driven from the bottom-up.”

This discovery explains why the Australian, Nazca, and Pacific plates move up to four times faster than the smaller African, Eurasian, and Juan de Fuca plates.

Stegman said his team’s conclusions could be an important breakthrough in the field of geodynamics, which attempts to use models of plate tectonics to explain the complex forces behind tectonic events over millions of years. The team at the new Deep Earth Virtual Lab (DEVL) at Scripps is planning more advanced studies using a National Science Foundation (NSF) grant awarded in June. The NSF Teragrid grant allows the team access to 2.5 million hours of computing time at the Texas Advanced Computing Center, home to Ranger, the world’s 11th-most powerful computer. Ranger has 433 Teraflops of computational capability.

“While the models are highly simplified at this point, they help build the foundation of understanding from which we can rapidly progress towards more realistic models,” said Stegman.

— Robert Monroe