slab suction

-->Fun geology term for the day: slab suction (bonus term: slab penetration).

There are two giant plateaus on our planet: one in Tibet, the other in the central Andes of South America. Despite sharing roughly similar crustal thicknesses and extreme average elevations, the two orogens (mountain building zones) have drastically different plate tectonic settings: continental collision between India and Eurasia in Tibet and oceanic subduction beneath the South American continent. A new hypothesis described by Faccenna et al. (2013), however, suggests that these two mountainous regions are ultimately formed by the same process: slab suction. The idea begins with the observation that the two plateaus are situated above regions of cold dense material that are thought to be sinking into the deep mantle. These two sinking regions are said to be the downward return of giant convection cells involving the entire (upper and lower) mantle. Both of these convection cells share the large, upwelling column (“superswell)” of hot, deep mantle beneath Africa. 

The downwelling beneath each orogen creates a suction that generates forces on the overriding plates dragging India into Eurasia and the Nazca plate into South America. Without this suction, Faccenna et al. (2013) argue that the other forces driving plate movement (slab pull, ridge push, and plume push) would be insufficient to support the large-scale mountain building. The non-suction forces may be more important in driving upper mantle convection cells and smaller-scale mountain building. The full mantle convection cells may ultimately be driven by the suction generated by deep-sinking subducted lithospheric plates that accumulated at the base of the lower mantle, before reaching a critical mass and penetrating the lower mantle at ~ 65-55 Ma (Tibet) and ~45 Ma (Andes).

Slab suction dragging plates together. From figure 7. Faccenna et al. 2013
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Citation: Faccenna, C., T. W. Becker, C. P. Conrad, and L. Husson, 2013, Mountain building and mantle dynamics, Tectonics, 32, 80–93, doi:10.1029/2012TC003176.