Discovery of stagnant lithosphere says less mixing occurs in Earth

July 02, 2001

CHAMPAIGN, Ill. -- The discovery of a large amount of subducted lithosphere beneath the Fiji Islands suggests that the mixing of Earth's mantle caused by plate tectonics occurs less than previously thought, so large volumes of primordial mantle may still exist, University of Illinois researchers say. According to plate tectonics, hot material rises to Earth's surface where it cools and eventually sinks back down to be recycled, a process called subduction. But fundamental questions remain - such as how fast this cycle occurs, and to what extent the material is mixed.

"Wholesale penetration of subducted lithosphere would destroy the primordial mantle," said Wang-Ping Chen, a UI professor of geophysics and a researcher at the Mid-America Earthquake Center. "Mixing isn't as severe as once thought: A lot of material may never enter the deep Earth."

As reported in the June 29 issue of the journal Science, Chen and doctoral candidate Michael Brudzinski used seismic wave speeds and earthquake ruptures to investigate the nature of subducting lithosphere and deep earthquakes near Fiji. At this region, the cold Pacific plate is plunging beneath the Indo-Australian plate at nearly 20 centimeters per year - five times faster than movement along the San Andreas fault in California.

In addition to the usual pattern of deep earthquakes along most subduction zones, the researchers identified a group of deep earthquakes off to the side. "This expansive cluster of earthquakes extended hundreds of kilometers above and to the west of the active subduction zone," Chen said, "making it difficult to connect these outboard earthquakes with the actively subducting lithosphere." The most plausible explanation, Chen said, is a large slab no longer attached to the subducting lithosphere. "This detached slab may be from a previous episode of subduction along a nearby trench about 5 to 8 million years ago. The subduction zone then shifted its geometry, leaving behind a large remnant."

If similar slabs exist elsewhere, a substantial amount of subducted material may never penetrate deep within Earth, thereby preserving a significant primordial component of the mantle, Chen said. "We are not saying that subducted lithosphere never goes to great depth - but the extent of such deep recycling is much less than some researchers believe."

Chen and Brudzinski also pointed out that seismic wave speeds, alone, are an inadequate indicator of the temperature of the material through which the waves pass. The chemical composition and crystal structure of the rocks also must be carefully considered.

"Temperature and pressure change the crystal structures of rock below certain depths," Chen said. "We found that seismic waves traveling through the cold, remnant slab have about the same speed as in the surrounding mantle material, which has already changed to a denser crystal structure."

Cold temperature of the subducting lithosphere hinders its transition to a denser material, a mechanism that triggers deep earthquakes in laboratory experiments, Chen said.

University of Illinois at Urbana-Champaign

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