Sedimentary Facies and Plate Tectonics of Equatorial Pacific

Abstract

Facies patterns of post-Eocene equatorial Pacific pelagic sediments indicate northward motion of the Pacific plate of about 3 cm/year with respect to the earth's spin axis, whereas the age gradient of volcanoes in the Hawaiian chain indicates a northward component of motion of the plate with respect to a hot spot in the subjacent asthenosphere of about 6 cm/year. The discrepancy is ascribed to southward motion of the upper part of the asthenosphere with respect to the spin axis. To examine this possibility more closely, a theoretical isopach map is generated for post-Eocene equatorial sediments, to compare with thicknesses known from drilling and seismic studies. In the model, sediment accumulation rates at all depths are several times faster close to the equator than farther away. The sedimentation model is combined with a plate-motion model with the following elements: (a) Pacific plate moves westward away from the rise crest at 10 cm/year; (b) sea floor deepens gradually as it moves away from the rise, with constant crustal-age versus depth relation; (c) plate moves northward with respect to the earth's spin axis at 3 cm/year; and (d) hot spots in the asthenosphere, which give rise to chains of volcanoes, move southward with respect to the spin axis, with ame speed. The model isopachs correspond fairly well to the known sediment thicknesses. Details of pre-Oligocene motions of the plate versus the earth's spin axis are uncertain, but mid-Cretaceous volcanoes have been shifted about 30° north. Northwest motion of the plate over hot spots in the asthenosphere at about 5 cm/year generated seamount chains parallel with the Line Islands in the period between about 30 and 100 m.y. ago.