For NAU researcher, what’s found at the surface may tell a deeper story

Chloride Cliff in the Funeral Mountains

Rocks tell stories from millions of years ago, but even today new meanings can surface from a field that rests on ancient events.

Thomas Hoisch, associate dean for research and professor of geology in the College of Engineering, Forestry and Natural Sciences at Northern Arizona University, suspects that what geologists have concluded about detachment faults since their discovery 45 years ago is ready for revision, and that he is working in an ideal place—literally—to show it.

The Funeral Mountains in Death Valley National Park are “a great place to answer certain tectonic questions,” Hoisch said. Many of the rocks there “have been through quite a journey” over tens of millions of years, he said. It’s a journey from the surface to as far as 30 kilometers deep, then back again.

Questions about the mechanics of that trek are at the center of a three-year, $250,000 National Science Foundation grant. Hoisch and his collaborators intend to support their idea that the Boundary Canyon detachment fault buried, then brought back to the surface, the rocks that are found there now. He described the action of the fault as a low-angle slip—first in one direction for millions of years, then in the opposite direction.

“Detachment faults are common in the western United States” Hoisch said. “But up to this point people have not really believed that these could be reactivated burial faults. We think the fault we see at the surface in the Funeral Mountains did it all, and that this phenomenon is likely to have occurred in many other places as well.”

Hoisch said that very idea was considered early on but was rejected “for not very good reasons. Now we’re taking a second look because we have some good evidence for it.”

Much of the work will rely on scientific methods of coaxing information from rocks originally formed under cool conditions on the surface. The high temperatures and pressures that affected those rocks when they were forced deep under the surface transformed them in ways that can be analyzed and interpreted. Hoisch said that schists in particular are rocks that record their histories well through a mineral called garnet.

“Garnet dating tells us when the garnet crystallized, but there’s a lot more history than that,” said Hoisch. “There are a lot of things going on before and after the crystallization, and we can date them.”

“We see indicators associated with the deepening of the rocks and we see indicators that are younger, of the strain going in the opposite direction,” Hoisch said. “We are going to tie those indicators back to the pressure and temperature paths we come up with to see if we can complete the picture.”

Hoisch, a petrologist, performs high-precision chemical analyses of minerals in NAU’s electron microprobe laboratory. Other work for the project will be conducted by a collaborator at UNLV, and in labs at UNLV, Washington State University and the University of California, Santa Barbara.

If the researchers are able to support their idea, “It would help us to understand how these detachment faults form,” Hoisch said. “It’s been unsolved ever since they were first recognized.”