INTERACTION OF CHEMICALLY STRATIFIED SUBDUCTED OCEANIC LITHOSPHERE WITH THE 660 KM DISCONTINUITY

Subducted oceanic lithosphere is composed of two chemically distinct l ayers, namely basaltic (crustal) and peridotitic (mantle) layers. Rece nt mineral physics observations suggest that the crustal and mantle co mponents of subducted oceanic lithosphere have distinct densities and viscosities. The deformation of such stratified oceanic lithosphere as it collides with the 660 km discontinuity is examined using a simple analytical model. It is shown that the separation of the crustal compo nent can occur through a small scale folding (10-20 km) if the viscosi ty of the crustal (garnetite) layer is significantly higher than that of the surrounding mantle (similar to 10(21) Pa s) but is less than si milar to 10(23) Pa s, leading to a garnet-enriched transition zone beh ind subduction zones.