Rock of Ages

California Ranges May Be Millions of Years Older than Once Thought

A new analysis of the topography of California indicates that the coastal ranges might be three million years or more older than previously estimated. And the good news is that one aspect of the plate movement that could cause earthquakes there is occurring more slowly, potentially resulting in fewer or less severe disturbances.

Until recently, the movement of continental plates has been measured within deforming zones—the areas where plates or segments of the Earth’s crust and mantle slide past one another while also pushing together or pulling apart. Now, using several years of data from satellites, lasers, and radio telescopes, a research team from Rice University and NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, has studied changes outside California’s deforming zone.

“We used more accurate data than obtained within the deforming zone itself,” says Richard Gordon, the W.M. Keck Professor of Earth Science at Rice, who collaborated with Donald Argus at JPL. “The measurements within the deforming zone don’t capture all the motion of the plates, some of which occurs offshore and underwater.”

The researchers specifically looked at changes where the Pacific plate and the adjacent Sierran plate meet. Much of the California coast rests on the Pacific plate, most of the continental United States rides on the North American plate, and the smaller Sierran plate serves as a buffer between the two.

Gordon and Argus estimate that along the San Andreas fault system, the Pacific plate slides horizontally past the Sierran plate at about 39 millimeters (1.5 inches) per year, which is significantly more than previous estimates of 34 millimeters (1.3 inches). But in addition to sliding horizontally along a fault, plates also push against each other in an action called convergence, which, in California, results in the region’s coastal ranges. The researchers found that the convergence there is about 3.3 millimeters (.13 inch) per year, a far lower rate than the previous estimates of 15 millimeters a year (.6 inch). They also discovered that north of San Francisco, the Pacific and Sierran plates are slowly pulling apart at a rate of 2.6 millimeters (.1 inch) per year.

“People have assumed that the convergence rate was much higher than what we found,” says Gordon, who used to hike in the foothills of these coastal ranges as a child in San Jose. “If the rate is lower, it takes more time to build the mountains to their present size.” Gordon estimates California’s coastal mountains to be three million to six million years old—quite a leap from the previous estimates of one million to three million years.

The researchers also studied the relationship between the degree of convergence and the degree of stable sliding along the San Andreas fault and nearby fault lines. Low convergence rates are associated with stable sliding and result in fewer earthquakes that tend to be of lesser magnitude, while high convergence rates have the opposite effect.

The research was funded as part of NASA’s Earth Science Enterprise, a long-term research effort focusing on the effects of human-induced and natural changes on the global environment.

—B. J. Almond