January 19, 1999

150-million-year-old sunken slab beneath Siberia
ANN ARBOR—Scientists from the University of Michigan and Utrecht University have located a piece of Earth's ancient history buried 1,550 miles below its surface.

Some 200 million years ago, during the Jurassic period when dinosaurs roamed the world, it was part of a now-extinct ocean bed. Today it rests at the bottom of a churning layer of superheated rock that makes up the Earth's mantle. It is the oldest section of subducted lithosphere ever identified.

"Originally this piece of Earth's crust was located at the bottom of the Mongol-Okhotsk Ocean separating what is now Siberia and Mongolia," said Rob Van der Voo, U-M professor of geological sciences. "As the Siberian and Mongolian continental plates converged between 200 million and 150 million years ago, this material was forced down or subducted deep into the Earth. It has been sinking ever since at an average rate of one centimeter (about one-half inch) per year."

Van der Voo and Utrecht University scientists Wim Spakman and Harmen Bijwaard used seismic tomographic imaging to identify the slab among a "graveyard" of slab remnants in the mantle beneath Siberia's Lake Baikal. Results of their study are published in the Jan. 21 issue of Nature. The study is significant because it coonfirms that subducted slabs do eventually reach the bottom of the mantle, according to Van der Voo. The study also illustrates how seismic tomography can provide valuable data to validate theoretical models of what's happening inside our planet.

"Greater understanding of deep Earth dynamics will help scientists understand the Earth's internal engine which drives the global convection system and the movement of continental plates," Van der Voo said.

The Earth's crust is broken up into eight major segments or plates and about a dozen minor plates—all moving over the planet's surface at a rate of several inches per year. Continental plates move because they are pushed and pulled by convection currents in the Earth's mantle, which is located just beneath the continental and oceanic plates that make up the Earth's thin crust. When plates collide, one is forced down beneath the other into the mantle creating what geologists call a subduction zone. Because subducting slabs are colder and denser than surrounding mantle material, they tend to sink.

Van der Voo and his colleagues selected the Lake Baikal area for their study because it is the site of an ancient, well-documented subduction zone and is located in a part of the world with many earthquakes and an extensive network of seismic monitoring stations.

Seismologists study the time and trajectory of sound waves traveling through the Earth from earthquake sites to seismic monitoring stations to learn more about the temperature, density and composition of the mantle layer. Acoustic energy travels through different types of mantle material at different speeds. The warmer the material, the longer it takes for sound energy to pass through. Using supercomputers to analyze the travel times and paths of thousands of seismic waves, seismologists can create a three-dimensional image, similar to a CAT scan, of the Earth's interior.

Seismic tomographic images in Van der Voo's study clearly show the slab of subducted material descending to the bottom of the mantle layer. "We didn't expect to see such a strong signal at these depths," Van der Voo added. "Whether its visibility is the result of differences in temperature, composition, pressure or a combination of these remains unclear."

The fact that the slab is located almost directly below the site of its original subduction indicates that the Siberian continental plate has moved very little in the last 150 million years. North America, on the other hand, is steadily moving toward Asia at a rate of about 1.5 inches annually.

"Eurasia will be the next supercontinent," Van der Voo said. "If North America continues drifting west, the Pacific will gradually shrink and eventually the two continents will merge into one."

The research was supported by Utrecht University, the Netherlands Organization for Scientific Research and the U.S. National Science Foundation.

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