July 2001

From University of Michigan

New technique dates faults near earth's surface

ANN ARBOR---A new approach developed by scientists at the University of Michigan and ExxonMobil Upstream Research Company allows direct dating of faults---surfaces along which rocks break and move---near Earth's surface. A report on the work appears in the July 12 issue of the journal Nature.

Dating shallow faults is essential to understanding the evolution of Earth's crust, the interactions among the plates that make up Earth's surface, and the processes by which faults are activated and reactivated, explains Ben van der Pluijm, professor of geological sciences at U-M.

For some time, scientists have been able to directly determine the ages of deeper rocks, but until now the age of shallow crustal faults could only be inferred through indirect dating methods---by studying the ages of fossils in associated deposits, for example.

Such estimates can suggest only a broad age range spanning many million years, not a precise age. The technique described in Nature, however, narrows down the age to within a couple of million years---practically pinpoint accuracy in geologic terms.

The researchers used the new method, which combines several approaches, to carefully analyze clays from near-surface faults in the Canadian Rocky Mountains. "That's an extremely well-studied area geologically, but there have been few reliable absolute ages on the faulting," says van der Pluijm. As a result, "we have not been able to get a firm handle on how fast processes like mountain building occur, when old faults stop being active and when new ones kick in, and the link between global plate tectonic processes and their surface expression," he says.

That kind of information has more than academic value, says van der Pluijm: "If you remember the stories about earthquakes in California, they often occur along faults we hadn't really seen active before. It's not because they weren't there; they just hadn't been activated in recent record." While the new method probably never will help scientists predict exactly when earthquakes will strike again at a specific fault, it should provide more general insights into the fault processes involved.

"If we understand more about the rates of these processes, we'll understand more about recurrence in general, and we'll get a better understanding of the mechanical behavior of the outer part of Earth," says van der Pluijm. "So we might ultimately get a better handle on the activation and reactivation of faults."

Van der Pluijm collaborated on the research with U-M research scientist Chris M. Hall, and Peter J. Vrolijk, David R. Pevear and Michael C. Covey of ExxonMobil Upstream Research Co. in Houston, Texas.

The work was supported by the National Science Foundation and Exxon Production Research Co.











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