Convection currents in the mantle, still mainstream?

My post to GEO-TECTONICS List (Tectonics & structural geology discussion list ).
Date: Fri, 5 Apr 2013 07:11:08 +0300

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The phrase "North America then began moving westward, pushed by the spreading of the Atlantic Ocean to its east." caught my eye in:

< http://www.nature.com/news/how-the-west-was-built-1.12724 > Alexandra Witze (03 April 2013). How the West was built. Nature|News.

Interestingly, I thought, NOT convection currents in the mantle moved North America westward, but the very spreading of the Atlantic Ocean pushed it. What's the driving force behind the process? Well, let's look at the supplied link:

< http://www.nature.com/nature/journal/v496/n7443/full/nature12019.html > Karin Sigloch, Mitchell G. Mihalynuk. Intra-oceanic subduction shaped the assembly of Cordilleran North America. Nature 496, 50–56 (04 April 2013) doi:10.1038/nature12019

Read on vertical slabs:

"Beneath North America these slab relics are massive, almost vertical walls extending from 800 to 2,000 km in depth, in depth, and typically 400-600 km wide. (Fig. 1.Supplementary Fig.1)" 

Well, let's go to the Supplementary. Read on material in the transition zone being smeared out:

"This view shows most clearly the almost vertical geometry of the deep slab walls below ~800 km depth, the segmentation of the Mezcalera and Angayucham walls, and their clear spatial separation from the crescent -shaped Cascadia Root and slab C2 further west. By contrast, material in the transition zone is smeared out laterally (yellow, green, blue shades). The vertical walls carry the geometric signature of intra-oceanic trenches, which can and do remain stationary over long periods, whereas the shallower slabs were deposited into a trench dragged along by the migrating continent. "

Great, shallower material was dragged along by the migrating continent, but how about deep slab walls? Why didn't they move with convection currents in the mantle? The slab walls' depth 2000km had been quite comparable to to the distance from the wall to the spreading ridge of the Atlantic Ocean on the course of the 150 Ma. The convection currents in the mantle should definitely disturb the walls; they did not.

No convection currents to drive the process, then what else to do the job? Hotspot frame? Some tens thousands km line of some km height speedy stream at the given mantle viscosity looks to require energy budget well above visible small fraction of Watt per square meter.   

Thank you.

Sergey D. Sukhotinsky.

http://divergent-boundaries.blogspot.com/

http://weblogs.asp.net/SergeyS

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