Discordant Tidal Characteristics Between the Lithosphere and the Asthenosphere as a Possible Contributing Plate-Driving Mechanism

Two small but repetitive processes associated with earthtides are studied as possible contributing plate-driving mechanisms. The first process is a mechanical ratcheting effect caused by the upward intrusion of asthenospheric material into the fractionally relaxed discontinuities at mid-ocean ridges during the expansion phase of the tidal cycle. Considered here, is that while the earthtide bulge is of magnitude .25m at the equator, the tidal bulge for the much less viscous asthenosphere would be significantly larger were it not constrained by the slower swelling lithosphere. Because the constraining tensile force cannot be easily transmitted across discontinuities in the plates, the resulting increased upwards pressure during high earthtide translates into a small thrusting force acting to separate the plate faces and permitting the intrusion of new material. Such a process repeated with each tidal cycle can produce a ridge-push that ratchets the plates apart and might assist to initiate the subduction process. An intrusion layer of just 20 microns per cycle can produce a plate separation velocity of order 3 cm/year. The second plate driving process results from the difference in the earthtide lag angles between the lithosphere and asthenosphere. Due to the rigidity of the lithosphere, the tidal bulge on the surface of the earth is observed to lag behind the earth-moon axis by roughly 0.3 degrees. Considered here is how a likely smaller lag angle of the less viscous asthenosphere produces an asymmetrical pressure bulge on the bottom of the lithosphere. This asymmetric pressure, relative to the prolate axis of the surface, results in a small but constant net westward torque acting on the bottom of the lithosphere. Such a process might be significant enough to contribute to the observed westward migration of the plates.