In order to predict the future process of crustal and lithospheric movement at the gigantic fracture zone in the Northeastern Pacific, numerical simulation of contact between two kinds of viscous fluid of different density was performed.The results suggest that there occurs downgoing motion of the higher density fluid towards the lower density fluid and that formation of a trench will appear in several tens of million years from the standpoint of both bottom topography and lithospheric structure.In their previous paper (TOMODA et al., 1983), the authors discussed the results of geophysical observations such as gravimetry, bathymetry, and geomagnetic and seismic surveys in the fracture zone area, and pointed out the possibility of generating the motion of new subduction. The outline is as follows.The age of the lithosphere south of the Mendocino Fracture Zone is 20-30 Ma. older than that north of it according to the lineation pattern of geomagnetic total force. The age discontinuity is common over the whole area of the fracture zone.Water depth south of the fracture zone is greater than that north of it over the broad area across the fracture zone. The depth offset is about 1,000 in at Free air anomaly south of the fracture zone is about 20 mgal smaller than that north of it over the broad area across the fracture zone, which means that Airy's isostasy is incomplete here. The depth of the Moho discontinuity south of the fracture zone is larger than that north of it according to seismic studies. The difference of Moho depth is difference of Moho depth is scarcely found. Residual gravity anomaly (R.G.A.) can be calculated when the free air anomaly and the crustal structure down to the Moho surface have been obtained (YOSHII, 1973). Lithosphere thickness can be estimated if R.G.A. and lithospheric age are given and the density difference between lithosphere and asthenosphere is assumed. Estimates indicate that the thickness of the lithosphere south of the fracture zone is larger than that north of it, and that the thickness difference de-183

A linear magnetic pattern, offset along fracture zones was found in the Shikoku Basin, which lies between the Izu-Bonin and northern Kyushu-Palau Ridges.This lineation, nearly parallel to the trend of the Kyushu-PalauRidge, seems to indicate that this basin originated from a type of seafloor spreading.A possible spreading center was suggested on the basis of symmetry of magnetic anomaly profiles and distribution of sediment cover in the basin. The anomaly profiles were tentatively correlated to the synthetic profile calculated from the reversal history of the earth's magnetic field during a period in Early to Middle Miocene, which gives a good fit of age to the result of the DSDP hole 297.

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