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07/12/94

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U.S.-Russian project to profile North American-Asian continental link

STANFORD -- On all the maps, North America is North America and Russia is Russia, and the two never appear to meet. But to a geologist, northern Alaska and eastern Russia are all part of the Arctic-Alaska Plate, one partly submerged isthmus of continental crust that links Eurasia to North America and separates the Pacific from the Arctic Ocean.

This summer, U.S. and Russian earth scientists are taking advantage of melted Cold War barriers, and the visit of a unique research ship to Alaskan waters, to make a geophysical profile of that continental link, from ocean to ocean and 30 kilometers down to the base of the earth's crust.

Led by geologist Elizabeth Miller and geophysicist Simon Klemperer, the three-year project is funded by more than $1 million in grants from the National Science Foundation and the U.S. Geological Survey.

The goal is to begin to unravel a number of scientific mysteries about the plate tectonics of one of the most poorly understood regions of the world. The scientists want to find out how Alaska was formed; how the structure of the Alaskan peninsula continues in the rocks and faultlines of the Russian Far East; how deposits of minerals like gold and oil have formed in the region. The larger question to be answered is how the plates of the North American and Eurasian continents moved and reshaped themselves around the Arctic Basin at the crown of the planet.

Klemperer's team will make a 2,000-mile voyage in the University National Oceanographic Laboratory System's ship, the R/V Maurice Ewing, trailing a 2-mile-long listening cable. Using airguns towed behind the ship, the researchers will bounce acoustic signals off rock layers down as far as the base of the earth's crust beneath the continental shelf. They will record the time that it takes those signals to bounce back to the listening cable and to seismographs on land, and use the data to make a map of the rock formations below.

Miller leads an international team of geologists who will collect data on land, cataloging rock types and geologic formations to coordinate with the data collected by the ship. Among Miller's collaborators will be top experts from the Far East Branch of the Russian Academy of Sciences in Magadan. The team will work in Alaska, on St. Lawrence Island in the Bering Strait, and at remote sites on Russia's Chukotka Peninsula.

Combining their data with observations by scientists from the U.S. Geological Survey and several other universities, Miller and Klemperer will produce the first cross-sectional profile across the continental bridge.

How the Aleutians formed

As this story goes to press, the Ewing has sailed on the first leg of its Alaska voyage, making soundings on a zigzag path through the Aleutian Islands. Klemperer and Stanford graduate student Moritz Fliedner are using a small airplane to follow the ship's course. They fly between the volcanoes and land on remote island sites to place seismic recording equipment that picks up reflected signals from the ship's airguns.

Explains Klemperer, "New continental crust is being formed by the active volcanoes of the Aleutian island arc, from magma melted as the Pacific oceanic crust dives northward beneath the Alaskan continental margin, subducting into the earth's mantle.

"The seismic recordings we are making in the Aleutian island chain will reveal the structure and physical properties along the volcanic arc. In particular, we hope to detect variations between the volcanoes that are built on oceanic crust in the western half of the island chain, and volcanoes of the eastern islands and Alaskan peninsula that are built on continental crust."

How the lower crust works

One of Klemperer's main interests in the project is to learn more about the behavior of the deep crust as continental plates collide and slide under one another, and as packages of rock called terranes are sliced off to form features on land. Klemperer is also a leader of projects using acoustic signals and seismographs to profile the earth's deep crust at the point where the Indian continent slides under the Himalayas of Asia, and to profile the Pacific Plate as it slides past the North American plate in Northern California.

"It's very hard to look at how the lower crust works," Klemperer says. "We do that indirectly most of the time. Here, we may find out how the crust is made by the process of terrane accretion, then modified by stretching and extension. Alaskan geology changes from east to west - on land the Brooks Range of Alaska is up to 8,000 feet high, yet offshore it's underwater. We want to find out why."

For this project, Stanford graduate student Brian Galloway, with scientists from Russia's Institute of the Lithosphere and Institute of Oceanology, will supervise seismic signaling and data gathering as the Ewing steams from the Aleutian Islands, through the Bering Strait along the Alaska coast, past Point Barrow to the northern edge of the continental shelf and back along the coast of Russia.

Meanwhile, Miller will meet in the small East Russian town of Providenya with Russian Academy scientists Boris Natalin, a geologist now working in Turkey; Boris Sedov, a geophysicist; and Michail Gelman, an igneous and metamorphic petrologist who is an expert on the granite belts of the Russian Far East.

With Stanford graduate student Jaime Toro and Andrew Calvert, a graduate student from the University of California-Santa Barbara, they will travel by helicopter and half-track vehicle to Koolen Lake, an area of the Chukotka Peninsula known for its gneiss domes, culminations of granite and highly deformed rock that are thought to have formed from materials deep in the crust. Miller's graduate student Jeffrey Amato will collect samples of rock on St. Lawrence Island in the middle of the Bering Strait, where relatively young, 2-million-year-old volcanoes have brought samples of the crust with them as they pushed up from magmatic chambers beneath.

Analysis of the geologic formations at these and other sites in Russia and Alaska will provide "ground truth" for the acoustic data gathered by the Ewing.

The next region was Russia

Miller's interest in the international project began on a mountain peak in western Alaska, where she was studying Alaskan plate tectonics. It is thought that Alaska is formed partly from an accumulation of terranes, great islands of rock scraped off the Pacific Plate as it slides north under Alaska. In the same way, California's Sierra Nevada, Central Valley and Coast Range were formed, or arrived on the continent, at different times, and are still composed of distinctively different rocks with deep faults between them.

Miller's data about some Alaskan faults seem to tell a different story: At some time, perhaps after the accretion of terranes, the continent began to stretch apart, as it does in the Basin and Range region of Nevada and Utah.

In search of answers to geologic questions, Miller says, "you're always looking over your shoulder at the next region, because more clues may be over there." On a clear day, the next region she could see from the westernmost edge of Alaska was in Russia. In Nome, she observed a number of cross-cultural exchanges, between Alaskan and Russian Boy Scouts, Alaskan and Russian business people, and native peoples on both sides of the Bering Strait who visit each other in walrus- hide boats. "I'd always wanted to go to Russia; I decided this was the direction I wanted to go," she said.

With consultation from U.S. Geological Survey scientists who already had begun some studies in Russia, Miller began three years ago to plan the logistics of a U.S.-Russian scientific exchange. She enlisted the aid of Andrei Prokopiev, a structural geologist from Yakutsk on a fellowship at Stanford. And she joined forces with Klemperer, who has worked previously with the Ewing to profile seismically active regions like Northern California. This summer marks the ship's last scheduled North Pacific voyage for the next several years; without that voyage and the new spirit of cooperation between Russian and American scientists, the geophysics of this region might remain a mystery for some time to come.

Thanks to data gathering and data analysis from a large group of collaborators, the investigators hope they will open a data base that someday will explain the plate tectonics of this continental bridge.

In addition to the scientists named above, others contributing directly to this project include Thomas Brocher, Jon Childs, Arthur Grantz, David Scholl and Thomas Moore of the U.S. Geological Survey; Nikita Bogdanov of the Institute of the Lithosphere in Moscow; John Diebold from Lamont-Doherty Earth Observatory at Columbia University, New York; Trevor Dumitru, a postdoctoral fellow working with Stanford's fission track laboratory; David Engebretson of Western Washington State University in Bellingham; Phillip Gans of the University of California- Santa Barbara; Helios Gribidenko of the Institute of Oceanology in Moscow; Steve Holbrook of the Woods Hole Oceanographic Institution in Massachusetts; Ben Kennedy, doctoral student, and David Stone of the University of Alaska in Fairbanks; Sue McGeary of the University of Delaware; Karl Wirth of Macalester College in St. Paul, Minn.; and James Wright of Rice University in Houston.

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