Role of subducted lithospheric slab in uplift and subsidence of the northwestern Pacific margins

Abstract

Vertical movements of the ocean floor and its margins at present and in the geological past were reviewed with the focus on the regions surrounding the Japanese Islands. It is emphasized that uplift and subsidence can essentially be explained by two mechanisms; one is isostatic equilibrium and the other a non-isostatic dynamic force. A typical example of the former is thermal subsidence of the ocean floor with time. An example of the latter is rapid uplift of fore-arc margins associated with large positive gravity anomalies, caused by collision and accreting subduction such as at Kikai-jima and the Okushiri Ridge. Repeated collision of the plates in the Pacific Ocean caused uplift and subsidence of the Ogasawara Islands and their fore-arc zone. The present-day Honshu Island, Japan, has an elevation of 1 000 m above sea level with large positive gravity anomalies which are maintained by a rigid, inclined slab beneath the island. Very young and embryonic back-arc basins are generally shallower than the ocean subsidence curves give and show positive gravity anomalies. Such a non-isostatic uplift is also dynamically supported by a rigid slab subducted beneath the basins. On the other hand, the Mariana Trough has normal back-arc ocean depths due to strong warping of the Mariana subducted slab which can not support the overlying arc and back-arc region. Immediately after the zones lost the support of the underlying slab, they subsided very rapidly, much faster than the normal subsidence of the ocean floor by cooling. Rapid subsidence of some rifted basins such as the initial stage of the Shikoku Basin and the Sea of Japan, demonstrated by DSDP/ODP cores, appears to have been caused by this mechanism. Inner and fore-arc zones of the NE Honshu Arc have experienced a remarkable subsidence during the Oligocene and Miocene. Distribution patterns of the horizontal stress field are synchronous with the vertical displacement of the area. It is obvious that uplift is generally associated with compression and high positive gravity, whereas subsidence is associated with extension and zero gravity anomalies. I propose that the succession of these two events can be explained by a simplified model of slab support.