Extrapolations from Deep Sea Drilling Project data suggest that the Pacific plate may be about 200 m.y. old at its west edge, but Cretaceous volcanism has largely buried the older record.

Post-Jurassic sedimentary facies patterns reflect the northwestward motion of the plate relative to the equatorial zone of high biological productivity, as well as progressively increasing sea-floor depths as newly formed crust moves away from the East Pacific Rise. The Early Cretaceous volcanism which inundated much of the older western part of the plate was succeeded during the rest of Cretaceous and Cenozoic time by the building and subsidence of long chains of seamounts as the plate moved northwestward, possibly over hot spots beneath the lithosphere.

A dearth of calcareous sediments of earliest Tertiary age in the central Pacific and a prominent unconformity beneath widespread middle Eocene cherts suggest a time of erosion and dissolution by especially vigorous deep currents, perhaps associated with the creation of a high-latitude circum-Antarctic Ocean.

The Pacific is about a third narrower now than during the Jurassic, owing to the opening of the Atlantic. Crustal consumption has been faster along the east side of the Pacific than on the west, resulting in a progressive eastward shift of the rise crest from its earlier more central position. Additional extra losses of old lithosphere are chargeable to Cenozoic spreading behind the western island arc systems.

End_of_Article - Last_Page 663------------