MOTION OF THE PACIFIC PLATE AND FORMATION OF MARGINAL BASINS

Abstract

We have examined agencies inducing flow in the sub-Pacific mantle. The capable agencies are buoyancy forces (causing convection), and passage of the tidal bulge. Bulge passage induces rotational flow dimensionally identical to that induced by buoyancy, and interacting with it. The stress imposed by bulge passage is additive across the width of the sub-Pacific flow cell, in the fashion that the buoyancy stress is additive in thickness. Each stress builds thus to values in excess of 108dyn/cmcm2, overcoming the flow resistance provided by mantle viscosity. The tidal flow contribution favors the development of the west limb of convection cells, because these represent rotational flow in the tidal direction. East limbs are inhibited.Under these forces the Pacific plate has developed as the cool, rigid surface of mantle convection surfacing in the eastern Pacific, and foundering at the western margin. The west, co-tidal limb of the East Pacific Rise now extends clear across the Pacific. The counter-tidal east limb is dwindling through its replacement by the extending west limb of the Atlantic spreading center, bearing the Americas plates.The unbroken line of gravity highs at the western Pacific margin marks excess material accumulating where the west limb of the East Pacific Rise encounters the less mobile Asian continental mass. Material added to the asthenosphere causes disruption of the Asia margin, secondary sea-floor spreading, and the formation of arc-bounded basins. At the termination of the east, counter-tidal limb (overlain by the Cocos and Nazca plates) this process is not operative. Marginal basins, if they form, are destroyed along the Andean orogenic front by the westward-advancing Americas plates.