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Within the earth's upper mantle, an eastward-flowing mainstream is postulated from the character of asymmetric fracture zones and the difference in crustal response to westward versus eastward underthrusting along active subduction zones. Coupled to convection cells beneath the oceanic ridges, the mainstream forms a pattern of migrating zones of primary and secondary mantle upwelling which, in a kinematic model, indicates (1) that a form of asymmetric sea-floor spreading has created the small ocean basins behind island arcs associated with westward-underthrust subduction zones; (2) that the depth to the base of convection varies systematically eastward from ridge to ridge, being shallowest beneath the East Pacific Rise and deepest beneath the Indian Ocean ridge; (3) that econdary zones of mantle upwelling are located along the leading edges of eastward-migrating convection systems and underlie such features as the East African rift system; and (4) that plates descend into the mantle along subduction zones in response to the removal of material from below.
In the model, the base of the mainstream forms an unbroken spheroid with its poles of rotation near the earth's geographic poles. The maximum velocity of the mainstream is therefore latitude-dependent. This postulated configuration of the mainstream suggests that directional flow within the mantle may be the result of rotation of the base of the upper mantle at an angular velocity which is slightly higher than that of the lithosphere. To produce a 40-cm/year mainstream velocity, the rotational velocity of the lithosphere must be slowed relative to the lower mantle by one part in 10 billion. This slowage would increase the length of day by only 0.5 millisecond per century, or about a quarter of the amount of increase in length of day attributable to tidal forces.
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