Monday, July 26, 2010

Plate Motions: Is The Driver Bottom Up or Top Down?

Science Daily describes some modeling results of plate motions presented in the July 16 issue of Science: Cenozoic Tectonics of Western North America Controlled by Evolving Width of Farallon Slab.

What drives plate motion? Is is the forces of convection in the mantle below or is it the drag of the subducting plate along zones where two plate converge and one sinks below the other? The authors of the above paper suggest that it is the drag of the subducting plate that is more important.

From the press release:

Schellart and his team, including Stegman and Rebecca Farrington, Justin Freeman and Louis Moresi from Monash University, used observational data and advanced computer models to develop a new mathematical scaling theory, which demonstrates that the velocities of the plates and the plate boundaries depend on the size of subduction zones and the presence of subduction zone edges.

"The scalings for how subducted plates sink in the earth's mantle are based on essentially the same fluid dynamics that describe how a penny sinks through a jar of honey," said Stegman, who developed the computer models that helped the team reenact tens of millions of years of tectonic movement. "The computer models demonstrate that the subducted portion of a tectonic plate pulls on the portion of the plate that remains on the earth's surface. This pull results in either the motion of the plate, or the motion of the plate boundary, with the size of the subduction zone determining how much of each."

"In some ways, plate tectonics is the surface expression of dynamics in the earth's interior but now we understand the plates themselves are controlling the process more than the mantle underneath. It means Earth is really more of a top-down system than the predominantly held view that plate motion is being driven from the bottom-up."

Looking at the graphic above which shows different types of plate margins, the study in Science focused on areas of plate subduction using the Western North America as an example. Thinking about it more broadly I wonder what is happening at mid-oceanic ridges and regions where new plate boundaries are being formed like the northern part of the East Africa rift.  These are places where two plates or continental blocks are being pushed away from each other.

How much of a role does diverging convection cells in the mantle below play in these situations especially if there is no strong drag due to subduction at the other end of the plate; for example a situation like the Indian plate is experiencing today with a mature continental collisional zone at one end and a mid oceanic ridge at the other or  .. the evolving Red Sea and East African rift wherein mantle upwelling is generating tensional forces that stretches and thins and pushes away the overlying broken lithosphere blocks bottom-up a larger driver in these cases?

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