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       Periodic violent earthquakes along the San Andreas Fault millions of years ago rotated the northwest Mojave Desert about 25 degrees clockwise, Jet Propulsion Laboratory study shows.

       Results of magnetic studies of volcanic rocks taken from total of 19 sites suggested the movement was caused by shear from the Pacific Plate sliding along the fault past the North American Plate in northwesterly direction.

       Shear is the force that causes parallel planes, such as the Earth's tectonic plates, to slide past or over one another.

       Dr. Matthew Golombek of JPL said the tectonic motion occurred in series of violent earthquakes over millions of years. Golombek and Dr. Laurie Brown of University of Massachusetts, both geologists, made the study during the past summer.

       More than 100 core samples were taken from the 20 million year old Saddleback Basalt and 56 samples were taken from the Red Buttes Quartz Basalt around Boron and Kramer Junction, Golombek said.

       The rotation occurred between 20 million and 16 million years ago, he said. Magnetic minerals line up parallel to the Earth's magnetic field to the north. Because the magnetic minerals in the rocks are now pointing about 25 degrees to the east of the north magnetic pole, scientists could determine that tectonic movement rotated the rocks, Golombek said.

       From the period of 16 million years ago to the present, there is no indication of rotation, he said. Clockwise rotation of the northwest Mojave agrees in direction with the bending of the southern Sierra Nevada mountains due to shear from the tectonic motion along the San Andreas Fault, he said.

       The rotation occurred before the formation about 10 million years ago of the Garlock Fault which separates the Mojave from the Tehachapi Mountains and runs in northeast- southwest line.

       The study was made for the Geodynamics Branch of NASA's Earth Science and Applications Division.

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10/31/86 JD
#1108