Researchers Sift Evidence Concerning Plate Boundary
Ancient Plate Boundary Exhumed
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A series of papers presented at the annual conference of the American Geophysical Union in San Francisco could shed new light on the geological processes by which continents grow, as well as on the nature of the structures, known as faults, that lie along plate boundaries.
For the past two years, a team of National Science Foundation (NSF)-supported scientists has used seismic techniques along a network of deep mountain fjords near the border of British Columbia and southeast Alaska to study what may be part of an ancient boundary between two tectonic plates: the Kula plate, which disappeared under Alaska about 50 million years ago, and the North American plate.
"Scientists from various earth-science disciplines have assembled evidence on the nature of the rocks and rock formations found deep beneath the surface of the earth," says Leonard Johnson, director of NSF's continental dynamics program, which funded the research. "As they present and discuss their findings, they will be attempting to draw some conclusions about whether they have, in fact, located the exhumed portion of a major ancient strike-slip fault."
At the AGU conference, the researchers will assemble the sometimes conflicting evidence gathered from their various disciplines, and attempt to decide whether, in fact, they have found the boundary. If it can be demonstrated that the zone crossed by the seismic section represents the deep portions of an ancient strike-slip fault zone, now exhumed by erosion, this region will provide unprecedented opportunities to study the geological processes that occur in the lower crust during strike-slip faulting. Understanding the growth of continents and other large-scale structures, such as mountain ranges, is of more than passing scientific interest. The relative motions of continental plates along fault lines create earthquakes. Knowing more about the nature of these faults could help scientists better understand earthquake activity.
Aboard the research vessel Maurice Ewing in 1994, earth scientists used ultrasound techniques to create a three-dimensional image of British Columbia and southeast Alaska along the network of fjords that crosses the mountains along the northwestern continental margin. The ship navigated the fjords Portland Canal, Dixon Entrance and Clarence Strait, towing a 2.5-mile cable of sound-wave sensors called hydrophones and an array of air guns. The project was coordinated by Lincoln Hollister, a geoscientist at Princeton University in New Jersey.
The air guns created bubbles by rapid release of compressed air. These bubbles produced sound waves that traveled to the ocean bottom and into the rock below. The sound waves were reflected back from the rock formations from up to 40 kilometers below the beds of the fjords to the sensors towed behind the ship. Measuring the refraction of the sound waves through the rock bed gave the researchers information about the physical properties of the layers through which the sound had traveled.
Editors: The AGU conference takes place Sunday, December 15, through Thursday, December 19, at the Moscone Center in San Francisco. The papers from the Accrete project will be delivered on Sunday, beginning at 8:30 a.m. in Room 125. Many of the presenters will be available also at the poster session that begins at 1:30 p.m. in Hall D.
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