Retardation at the lithospheric/asthenospheric interface
The quantification of this mechanism has been very largely neglected. Richardson et al. (1979) and Richardson (1992) have suggested that the restraint to basal movement is 10-2 MPa (0.1 bar) or less per unit area. These estimates have been derived to fit the limitations set by the ridge-push/trench-pull mechanisms, and are, we suggest, not appropriate values.
The total resistance of basal slip will, of course, be related to the whole area of the plate and, because the restraint provided by the LVZ is a viscous one, it will also be related to the velocity of plate motion. If the plate is large and the motion relatively rapid, the total resistive force presented by this basal restraint will not be trivial! We shall discuss this topic later in this chapter and present what we consider to be more realistic values for the basal restraint to plate motion than those cited above.
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