As spreading at known interarc basins occurs to the west of westward-dipping subduction zones, and movement of the lower plates is also to the west, it is suggested that all plates move chiefly westward. Rates of motion are equal to a net difference between eastward and westward transport on semidiurnal tidal bulges. Eastward-dipping subduction zones at the leading edges of plates that are primarily continental are overridden by less dense material and are forced to step westward. Yet lines of downbending of westward-dipping subduction zones are fixed with respect to the lower mantle, and interarc basins result from westward movement of their adjacent plates. These fixed subduction zones can be used to calibrate previously measured movement between plates; they suggest that the westward transport imparted during each tidal cycle is as much as 0.3 mm.

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