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Geologists Try to Map Under the Sierra : Science: Researchers hope a series of strategically placed explosions will yield information on why the mountains are rising.

September 25, 1993|KENNETH REICH | TIMES STAFF WRITER

LONE PINE, Calif. — An elaborate attempt to map the geologic underpinnings of the Sierra Nevada and discover why the peaks are rising moved into a new stage this month with a series of small explosions over a wide area.

Scientists from Caltech and Stanford, Princeton and Duke universities, the Navy, Lawrence-Livermore Laboratory and the U.S. Geological Survey set off the charges every 25 miles from north of Bishop to the Garlock Fault east of Mojave, 250 miles south.

Later, they set off similar blasts on an east-west axis, from the Death Valley west across the Sierra and the San Joaquin Valley to Coalinga.

Energy waves from the explosions were recorded as they passed through rock strata and faults by hundreds of seismographs set up by the scientists, who used utility vehicles, helicopters and pack animals to cover the rough terrain.

One purpose of the study was to map deeply buried thrust faults that could cause big earthquakes. But other important goals were to delineate the boundary between the Earth's crust and mantle, the line known as the Moho, about 20 miles underneath the surface, and determine the mechanics causing Sierra peaks to rise.

The explosions were part of a three-year-long series of experiments, which are being coordinated this summer by a Princeton geophysicist, Robert Phinney, under a $180,000 grant from the National Science Foundation.

In July, Phinney and five Princeton students put 31 tiny seismic stations, powered by solar panels, in remote areas from Darwin, east of Lone Pine in Inyo County, across the Owens Valley and the High Sierra. Through November they are recording the passage of earthquake waves at multiple angles through the Moho and other strata.

In a third experiment, a consortium of scientists, including some from UC Riverside and San Diego State, will examine the origin of the Sierra uplift by taking measurements designed to detect fluctuations in local magnetic fields. This is a way of identifying areas of higher conductivity, heat sources and heat flows from the mantle through the crust that may help explain why the mountain range is rising. It may also indicate ore deposits that are being formed over very long periods of time.

Steve Park, associate professor of geophysics at UC Riverside, said that 10 million years ago "the area now occupied by the Sierra Nevada consisted of rolling hills and rivers crossing from east of the present mountain range into the San Joaquin Valley."

"Subsequently, the granite has risen rapidly to form the mountains we see today," Park said. "This study is designed to determine what caused this rapid uplift. Is it simply the buoyancy of the granite itself or (are) the partially molten rocks from the mantle pushing up the mountains?

"During the same time that the Sierra Nevada were rising rapidly, the crust to the east in Death Valley was stretching and thinning," he added. "One of our goals is to determine if this stretching is related to the uplift of the mountains."

A recent daylong expedition by Phinney and an assistant, Chris Parker, near Darwin showed how the seismic station phase of the study is conducted.

Under a burning sun at about 5,000-foot elevation, Phinney and Parker performed a task they do weekly--copying seismic information collected at each station onto disks that can be analyzed by their computers in a house Phinney rented in Independence, 15 miles north of Lone Pine.

Late the night before, Phinney was at the house poring over data from hundreds of tiny earthquakes recorded by the stations, which are clustered near Darwin, at Horseshoe Meadows in the High Sierra and at Mineral King on the western edge of the range.

Later, the data from all 31 stations showing how earthquake waves reflect off various rock strata allows drawing of what Phinney acknowledges will be a fuzzy map of the Moho and other subterranean structures in the Sierra-Owens Valley region. Collecting data from each station is hard work, often entailing miles of walking over steep or rough terrain.

Near Darwin, where the stations are 100 yards or so off a well-graded road, the Princeton professor acknowledged that he has only modest hopes of coming to definitive conclusions.

"We are simply trying to establish what's there," Phinney said. "In this case, when we're so totally ignorant about the architecture (of geological structures) underneath California's surface, whatever we learn will at least influence the way people think about these questions."

For instance, he said, "the Sierra is a classic question. With increasing elevation comes a thickening of the Earth's crust. We may see that in the maps we will draw. But what causes these peaks to go up where they clearly are going up?"

Phinney said that the plate tectonics theory--so much in vogue among geologists--under which great blocks of the crust are seen as floating on the mantle, does not explain all details of the movements in this region.

"We are examining the complications" that allowed the Sierra to move from a location near present-day Las Vegas millions of years ago to its present position, he said.

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