The University Record, August 13, 1997

Stress from plate collisions travels through continents

This limestone outcropping in the eastern mid-continent of the United States was one of many sampled in the U-M study. Although the layers of limestone are flat and seemingly undisturbed, they contain stress-induced deformations in microsco pic grains of calcite created when the Appalachians formed about 300 million years ago. Photo by K.C. Lohmann

By Sally Pobojewski
News and Information Services

When continental plates come together to form mountain ranges, the impact from the collision bends microscopic grains in rocks more than 1,200 miles away, according to an article published in the Aug. 1 issue of Science.

"Classic plate tectonics theory maintains that all activity is concentrated at the plate margins, but our evidence shows that seemingly quiet mid-continent areas are highly sensitive recorders of plate tectonic activity," says Ben A. van der Pluijm, professor of geological sciences.

When examined under a microscope, calcite grains in limestone samples collected by van der Pluijm and his colleagues at more than 70 North American mid-continental locations all showed the characteristic bending or deformation pattern produced in rocks under shearing stress. Intensities of stress recorded in the limestone samples varied directly with distance from the Appalachian and Rocky Mountain ranges.

"These grains of calcite are like stress gauges placed in the Earth long ago," van der Pluijm says. "Since each deformation pattern corresponds to a specific stress level and orientation, these patterns preserve a permanent record of the effects of plate collision and mountain-building over long periods of geologic time."

The U-M researchers collected samples from limestone deposited between 500 million and 200 million years ago-a period of time when the North American mid-continent often was covered with shallow seas. Samples were taken at regular intervals along two transects or straight lines. One sampling line extended from the Rocky Mountains in Wyoming into Minnesota. The second line, which replicated results from a previous study, originated in the Appalachian Mountain range in Tennessee and extended into Indiana .

"Surprisingly, deformation patterns from both transects were identical, even though the Appalachian range on the eastern side of the North American continent differs dramatically in age and style of mountain formation from the Cordilleran range on the western side," van der Pluijm says. "Mountain belts appear to act as 'stress filters' absorbing any variations in plate margin conditions before transmitting stress into plate interiors."

While the orientation of the calcite grain stress indicators did not change along either transect, the level of stress recorded decreased steadily with distance from the mountain range. Stress levels recorded in Wyoming nearest the Rockies, for example, were about five times greater than those recorded in western Minnesota.

The research was conducted by van der Pluijm; John Craddock, associate professor of geology at Macalester College; and U-M undergraduate student Brita Graham and graduate student John H. Harris.

The research project was funded by the Pew Charitable Trust, the Blandin Foundation, the Huron Mountain Wildlife Foundation and the National Science Foundation.

Graham participated in the project through LSA's Undergraduate Research Opportunity Program, which facilitates undergraduate involvement in faculty research.