FATE OF OCEANIC PLATES DESCENDING INTO THE EARTHÕS MANTLE
YOSHIO FUKAO
The EarthÕs activities, such as earthquakes and volcanic eruptions, have been interpreted in terms of plate tectonics, according to which the Earth is covered by a dozen of lithospheric plates with a thickness of about 100km. Oceanic plates are created at mid-oceanic ridges, spread away from there and are subducted into the EarthÕs mantle at deep-sea trenches. Upon subduction an oceanic plate slips against a continental plate to case great earthquakes. The hypocentral distribution of deep earthquakes delineates the subducted part of an oceanic plate (=slab) now descending through the upper mantle with its bottom at the 660km discontinuity, across which seismic wave speeds increase abruptly.
Until 1990, little had been known about subducted slabs at greater depths and, for this reason, about the dynamic processes occurring under the plate tectonic regime. Recent development in seismic tomography has begun resolving this problem. According to these studies, subducted slabs tend to stagnate in the transition zone in a depth range from 400 to 1000km, either above or below the 660km discontinuity.
Stagnant slabs have been reported in many of the well-developed subduction zones along the Circum Pacific, including South America, Caribbean region, Aleutian, Southern Kuril, Japan, Izu-Bonin, Indonesia and Tonga, where descending slabs tend to deflect to shallower angles or even to flatten subhorizontally, suggesting strong resistance in the transition zone against descending motion of a slab. Subducted slabs tend to lose their seismic signatures at depths less than 1400km, although some exceptions have been reported e.g. in Central America.
The concept of stagnant slab is a key to understand the mantle convection that drives plate tectonics in the upper mantle. It is now almost certain that vertical current of mantle convection tends to be stagnant within the transition zone, causing both spatial and temporal complexities in convecting pattern, thus yielding profound effects on plate tectonics. One of our efforts in this direction is to develop a geophysical network (Ocean Hemisphere Network) in the western Pacific, where downwelling of convection occurs most extensively.