Ridge Push and Gravitational Head

With all respect to Turcotte and Schbert, IMHO, 'push' and 'pull' are misnomers. There is no true 'push' from the ridges as most people think of a push. I know of no compressive type deformation at MORs that should develop from a true 'push'. Such a force that would move entire lithospheric plates, or even a portion of them should have some manifestation in the geologic structure or earthquake solutions.As to 'pull', I know of no conditions under which a lateral, tensional force can exerted on the oceanic lithosphere and be transmitted throughout the length of the plate from trench to ridge. This is what the lay person would think of as 'pull', as in pulling on a chain. As near as I can tell, that is not what T&S describe, however.I agree with T&S's description of the dynamic scenarios, but the terminology seems a bit misleading. And it is true that gravitational head from an elevated ridge seems small compared to the huge mass of the plate, but one must remember that we are not talking about actual elevation of the seafloor, but elevation of the gliding surface, which one would expect to be just slightly steeper because of the cooling effect. In a way, from their own description, it becomes hard to tell the 'push' from the 'pull'...

This is all very interesting, but my problem with it is twofold. One is that there is no factor for the strength and continuity of the plate. In other words, when a slab finally begins to accelerate into the asthenosphere, it tends to break away and drop like a falling icicle. THat is, there is no more 'pull'. Second, if slab pull, as most people think of it, occurs, why is there often a fore-trench bulge indicating a decelerating rather than accelerating slab at the trench. Furthermore, what does this all say about the variable slope of Benioff Zones and what does it predict for steep zones versus shallow zones? I haven't thought about this yet.In fact, I haven't thought about plate tectonics very much in decades, so my information is dated and my understanding quite rusty. Mathematical modeling is nice, but we need to look at the rocks, too.

That is the way I think of it. I do not follow T&S well enough (I just don't have time) to know if that is their concept.

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Quick summary:

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Brown University graduate student Catherine Rychert, based on high-frequency seismic wave studies, has determined that the lithosphere/asthenosphere boundry is a sharper zone of about 7 miles, as opposed to the previously thought 25 miles. The study suggests that rather than the previous long-held assumption that asthenospheric convection moves the tectonic plates around, the driving force is gravitational pulling at the subduction zones.

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I've been waiting for something like this for a long time. I haven't read the article, but my guess is that deformation results in a preferred orientation of crystals, which, if they are seismically anisotropic, should give an idea of the orientation of crystals. THis would reflect the state of strain in the asthenosphere. The only problem is that, in this scenario, slab-pull is indistinguishable from a ridge-push. What it does tell us is that a convection current model may be on the way out. If you get a link, let us know, thanks.