Plate boundaries (red) and cratons
(tan - Proterozoic; brown - Archean)
The mechanical coupling between the asthenosphere and the lithosphere is one of the forces which drives plate tectonics. This coupling has been termed the "mantle drag" force by Forsyth and Uyeda [1975], who found that this force was small compared to other forces and that mantle drag tends to resist, rather than assist, plate motion. Their analysis, however, did not include the effect of variations in lithospheric thickness, which substantially complicate the simple picture of the asthenosphere being dragged along by an overlying, rapidly moving plate.
If the base of a plate has little or no relief, and the plate is driven by boundary forces such as slab pull, then the asthenosphere will move most quickly near the base of the plate (the plate moves the mantle). If, however, a plate is driven by internal buoyancy forces, the asthenosphere will move most rapidly at some depth within the mantle (the mantle moves the plate). In either case, lithospheric thickness variations can affect the motion of a plate by varying its interaction with the asthenosphere.
Of course, both types of forces move the plates, boundary forces as exemplified
in slab pull, and buoyancy forces as exemplified in ridge push; however,
in most plates with attached slabs, the effects of boundary forces greatly
overwhelm the effects of buoyancy forces and it is difficult to resolve
the component of buoyancy driven flow. Continental plates with no attached
slab therefore provide a unique opportunity to examine the role of buoyancy
forces in driving plate motions. In this paper, we examine the effects of
thick cratonic keels upon plate motion, and use our results to make inferences
about the nature of buoyancy driven flow in the mantle. Depending upon the
depth of the keel, it may either intersect a region of horizontal convective
counterflow, or it may remain within the region where all horizontal mantle
flow is in the upper half of the convection cell. In the former case, plate
motion is impeded by the keel, while in the latter, the keel will assist
plate motion.
(From Stoddard & Abbott, 1996).
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