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Down and under in L.A.

Down and unde in L.A.

In a million years, the land under Los Angeles may slide beneath the land to the north, shoving the city itself up the side of the San Gabriel Mountains. So says geophysicist Marcia McNutt of the Massachusetts Institute of Technology, who has evidence that southern California gradually is pushing itself northward.

The process of subduction, by which one of the earth's crustal plates pushes underneath another, is commonly observed in the ocean. The continents, on the other hand, have been considered too buoyant to undergo subduction on a large scale. However, in the May 10 JOURNAL OF GEOPHYSICAL RESEARCH, McNutt and MIT geologist Barbara Sheffels report evidence that subduction is occurring in California, at the point north of Los Angeles where the San Andreas Fault runs east and west.

McNutt and Sheffels report that there is an unusually high arch in the gravitational profile for the southern side of the fault in this area. The mountains are not massive enough to account for this feature, so the geogolists conclude that the earth's mantle is pulling down on the southern side.

This subduction most likely is caused by the bend in the fault line, McNutt says. Along most of the San Andreas, the North American and Pacific plates slip alongside one another. At the east-west bend, the motion continues, but it causes one plate to move under the other, she explains.

McNutt notes that the amount of slippage between the Pacific and North American plates has been calculated at about 60 millimeters a year. If the subduction has been occurring for about 14 million years--the age of the mountains in the area--it would follow that the southern plate has pushed 240 kilometers under the northern plate, she says.
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Title Annotation:geological research
Publication:Science News
Date:Jun 21, 1986
Words:293
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