Sallyport - The Magazine of Rice University - Summer 2002

Rock of Ages - Through the Sallyport
Spring 2002
VOL.58, NO.4
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
http://www.rice.edu/sallyport/2002/summer/sallyport/rockofages.html (1 of 2) [10/30/2009 10:49:37 AM]