Stagnant slabs in the upper and lower mantle transition region

We made a region-by-region examination of subducted slab images along the c ircum-Pacific for some of the recent global mantle tomographic models, spec ifically for two high-resolution P velocity models and two long-wavelength S velocity models. We extracted the slab images that are most consistent am ong different models. We found that subducted slabs tend to be subhorizonta lly deflected or flattened in the upper and lower mantle transition region, the depth range of which corresponds roughly to the Bullen transition regi on (400-1000 km). The deflected or flattened slabs reside at different dept hs, either above or across the 660-km discontinuity as in Chile Andes, Aleu tian, Southern Kurile, Japan, and Izu-Bonin; slightly below the discontinui ty as in Northern Kurile, Mariana, and Philippine; or well below it as in P eru Andes, Java, and Tonga-Kermadec. There is little indication for most of these slabs to continue "directly" to greater depths well beyond the trans ition region. Mantle downflow associated with present slab subduction appea rs to be blocked strongly to turn into predominantly horizontal flow in the transition region. Recent global tomographic models show also a group of l ithospheric slabs deeply sinking through the lower mantle, typically the pr esumed Farallon slab beneath North and Central America and the presumed Ind ian (Tethys) slab beneath Himalaya and the Bay of Bengal. These remnant sla bs are not connected to the surface plates or to the presently subducting s labs and appear to sink independently from the latter. The presence of thes e deeply sinking slabs implies that the pre-Eocene subduction occurred in m uch the same way as in the present day to accumulate slab bodies in the tra nsition region and that the consequent unstable downflow occurred extensive ly through the transition region in the Eocene epoch to detach many of the surface plates from the subducted slabs at depths and hence to cause the re organization of global plate motion.