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Crustal links record plate motions.

The seafloor has been likened to a giant tape recorder, because as it is churned out conveyor-belt-style at mid-oceanic ridges, it becomes imprinted with the earth's changing magnetic field. The resultant "magnetic stripes" that line the ocean floor enable scientists to reconstruct the past positions of the continents as they, and the plates upon which they sit, move around the globe.

But these magnetic lines are not the only oceanic record of relative plate motions. REcent studies by Brian E. Tucholke and Hans Schouten at Woods Hole (Mass.) Oceanographic Institution indicate that the motions are reflected on a much finer scale in the structure of fracture zones that cut across ocean basins, perpendicular to mid-oceanic ridges. While Tucholke and Schouten have focused on one fracture zone, they suspect the structures of all fracture traces are very similar. What's more, the researchers think they see evidence in fracture zone structure for global changes in plate motions every few million years. They presented their findings at the recent meeting of the Geological Society of America in Orlando, Fla.

Using seismic reflection and bathymetry (seafloor depth) data, Tucholke and Schouten examined the detailed structure of two 700-kilometer-long segments situated on each side of the Mid-Atlantic Ridge. The researchers located spots where the fracture zone had changed direction or where it had been blocked -- perhaps by the upwelling of molten rocks at times when changes in plate motion caused the crust near the ridge axis to be stretched out. They found that the kinks and bends on the western segment correlated remarkably well with similar structural changes in the eastern limb.

Tucholke and Schouten also compared a 450-km-long Kane segment straddling the ridge with similar segments in the Pacific and Indian oceans. "There are plate motion changes recorded in all of these oceans at roughly the same times -- at about 4.5 million years, 2.5 million years and 1 million years," says Tucholke. "So what we're seeing at the Kane fracture zone is a global response."

Tucholke expects that more detailed studies will show crustal structure to be an extremely precise indicator of changes in plate motion. But what causes these changes is anyone's guess. "Perhaps the plates move like bumper cars and get hung up every 2 million to 3 million years," says Tucholke. "Where and why this happens is the $64,000 question."
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Author:Weisburd, Stefi
Publication:Science News
Date:Nov 16, 1985
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