Forces that drive the plates
Are the plates riding passively on mantle currents?
Or do the plates themselves influence mantle flow?
Hypotheses:
1) Edge forces drive plates (Chapple and Tullis, J.G. R. 1977)
2) Mantle convection drives plates (one layer or two?)
3) Combination of both (Conrad and Lithgow-Bertelloni, Nature 2002)
Observations
Plates with large subduction slabs faster (Pacific)
Continental plates move slower than oceanic (continental keel)
Velocity independent of area (not mantle drag?)
Sinking slabs show tension and compression (not mantle downwelling).
Chapple and Tullis (1977) JGR, v. 82, p.1967
Assumptions:
Number of plates = 12
Plates move relative to hot spot frame of reference
Each plate in equilibrium (constant velocity).
Calculate driving forces:
a) ridge push (topographic high)-causes compression in plate
b) slab pull (subduction slab dense)- tension in plate
c) hot spot push?
Calculate resisting forces:
a) mantle drag (low velocity zone)
b) bending resistance
c) resistance to slab in mantle (compression in slab)
d) collision resistance
e) transform faults (minor)
Slab pull largest force: cold and more dense
Depends on length of subducting slab parallel to trench and also depth of slab
Neglect excess density of slab (olivine-spinel transition)
Mantle resists slab going down
Cross over between tension and compression in slab
Depth to cross-over point causes pull
Density of plate depends on temperature
Slabpull = sin 
g)x slab length

t slab thickness
d: depth to cross over point
: density relative to normal mantle (depends on temperature)
g: gravity
Ridge push also significant
Basal mantle shear- how large?
Small force- either resists or drive plates
Assume viscous vehavior
Basal shear stress:  = ηė
η = viscosity of low velocity zone: 1019 Pa
ė = strain rate at base of lithosphere: 10-12 s-1
(strain rate depends on with of basal shear zone)
Pa = 10 MPa = tensile strength lithosphere
Slab suction mechanism: slab detaches from plate and sinks into mantle:
Induces downward convection in mantle
Causes overlying plates to move toward subduction zone.
Slabpull and slab suction best explains plate motion (Conrad, 2002).