Committed to a Fault

Ajeet Johnson is digging into Central Oregon's violent past

As an undergraduate, Ajeet Johnson (left) worked with Andrew Meigs to study the ages of fault lines. Research, says Meigs, requires students to think differently. “If I tell you that something is one way, you’re not supposed to nod your head and say ‘yes.’ You’re supposed to say, ‘why do you know that?’” (Photo: Conner Burke)

Skiing and rock climbing just weren’t enough. Growing up in Central Oregon’s spectacular landscape, Ajeet Johnson challenged the backcountry of the Cascades. She pulled herself hand-over-hand up Smith Rock and carved down slopes at Mt. Bachelor, but over time, she became curious about the forces that shaped the terrain and will influence its future.

Over the last four years, Johnson has gone from jamming her boots into toeholds and plowing through deep powder to mapping data and measuring fault lines. She received scholarship support for her research and graduated with a bachelor’s in geosciences from Oregon State University last summer. Today, she is pursuing her master’s at OSU.

Her wonderment at the origins of mountains has morphed into a question that has concerned geologists for decades: Why does the expanding Basin and Range region of the American West – one of the most geologically active in the continental United States – come to an abrupt end in the area east of Bend known as the High Lava Plains?

The answers could have implications for Central Oregon’s future. Population has grown faster (73 percent between 1995 and 2007) here than in any other part of the state. The area has seen more than 75 volcanic events over the past 10,000 years, and while the region’s unusual geology provides a source of geothermal energy, it also poses a continuing risk of earthquakes. South of Bend, cinder cones and the 17-square-mile-wide Newberry Crater are reminders of a violent past.

Johnson has focused her research on the Brothers Fault Zone, a complex of relatively young, one- to 10-kilometer-long cracks in the Earth’s surface that run from near Bend toward southern Idaho. Conventional wisdom among geologists is that the age of a fault relates to its length and the differences in height (what geologists call “displacement”) of adjacent terrain. The problem is that, east of Bend, ancient lava flows cover hundreds of square miles, obscuring faults and complicating analysis of their ages.

In 2007, Johnson started measuring fault lines, distinguishing between those that are partially buried and those that are not. She measured the elevations of hundreds of points along the tops and bottoms of slopes. Using a geographic information system, she analyzed data to see if a standard method would yield ages that were consistent with other evidence.

She received support for her research from the Mark W. Chambers Undergraduate Research Grant in the Department of Geosciences and from the Undergraduate Research, Innovation, Scholarship and Creativity fund in the OSU Office of Research.

In a March 2008 presentation at a Geological Society of America meeting in Las Vegas, Nevada, Johnson reported her findings. In short, her analysis showed two results. For faults that cut across rocks older than 7 million years, growth is revealed by the length and height of the fault scarp, or adjacent slope. Faults that cut younger rocks, however, do not show this relationship. Linking among faults and burial of the landscape by lava flows obscure the fault topography.

For her master’s research, Johnson plans to continue studying the forces at work under Central Oregon. Questions remain, she says, about this transition zone between the Basin and Range to the south and the Yakima Folds to the north on the Columbia River Plateau. “The Basin and Range is spreading and is thought to be pivoting from a point in Eastern Washington or Idaho,” says Johnson. “What we’re learning is important for the growing population here and for educating our future scientists.”

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