Ridge Push and Gravitational Head

Thanks Percy. Since this is a provisional thread I gather I should just edit the first post?The slab-pull view was justified as being called the primary mechanism from a fairly impressive couple of graphs of plate velocity vs length of the ridge and length of the subduction.There was little correlation between plate velocity and the length of MORs but there was a readily apparent positive correlation between the length of subduction zones and the velocity of the plate. So slab pull looks like the dominant force.Maybe slab pull is more important, but it's not the whole story. My structural text (Structural Geology, by Davis and Reynolds) says that based on data on the current stress fields of world (from Zoback, 1992), ridge push is the primary compressive force on midplate North America today. And this force is apparently driven by topography through a process called "gravitational head"--the force of gravity on MORs and also high mountain regions like the Himalayas, the Andes, and even the Rockies does not outpace the bouyancy force--which is somehow increased by plate-driving forces?--in the mantle and this causes extension. Evidence was cited that ridge-push is real (shallow earthquakes in the crust just off the ridges that represent compression) and a value of about 20-30 MPa of stress was ascribed to the process.Writing this summary makes me feel like I understand it better than the last I time I read it through, so I think I'll ask a more general question: is this most of the story, or are there other components that should be included on ridge-push? Does this extension at the MOR in any way pump additional magma in?{Note from Adminnemooseus - MOR = mid ocean ridge}This message has been edited by gene90, 09-14-2005 05:35 PMThis message has been edited by Adminnemooseus, 09-16-2005 01:08 AM

Thanks to the mods. I suppose if it doesn't get any bites you can retire it. I have noticed that sea-floor spreading still seems to be current topic in some threads.

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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.

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I am having a conceptual problem here. How would the 'pull' not have to be propagated across the plate?