Abstract

The hot-spot hypothesis of Wilson and Morgan and the recent success in worldwide correlation of marine magnetic anomalies by Larson and his co-workers have enabled us to depict the Mesozoic and Cenozoic movements of plates of the Pacific Ocean. The inferred movements show a good correlation with the tectonic history of Japan. The mid-oceanic Kula-Pacific Ridge probably collided with Japan and then descended beneath it in Late Cretaceous time. The succeeding events were similar to those that occurred during the descent of the East Pacific Rise (Farallon-Pacific Ridge) beneath western North America in middle Tertiary time. The similarity of the sequence of events that took place during ridge descent in Japan and western North America suggests that these events are genetically related to ridge descent.

The first event was high-pressure metamorphism; it occurred before ridge descent, probably because of the high rate of plate convergence. This metamorphism halted about 30 m.y. before ridge descent, possibly because of the increase in temperature of the plate toward the ridge axis. For several tens of millions of years during ridge descent, magmatism occurred in an unusually wide area on the continental side of the subduction zone. This could be ascribed to the high temperature of the subducted “plate” near the ridge axis and presumably also to the activity of the submerged ridge axis. Finally, in both cases, the continental plate was subjected to tension after ridge descent. This resulted in generation of the Japan Sea and the extensional structure of the Basin and Range province of western North America. The formation of the Median Tectonic Line of southwest Japan may also be a result of the ridge submergence.

According to Morgan, the Pacific plate changed its movement direction in middle Cenozoic time. This probably caused new orogenies in northeast Japan and in the eastern and southern margins of the Philippine Sea leading to the formation of the Izu-Bonin, Mariana, Yap, and Palau arcs. The western Philippine Basin was formed probably by spreading at a ridge within it.

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