海洋底拡大説について

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History and evidences of the sea floor spreading theory are reviewed with special interest in its influences on both the land geology and the theory of development of island arcs. The concept of the sea floor spreading was proposed by R.S. DIETZ and H.H. HESS at early 1960. This was derived through an attempt to interpret sea-floor bathymetry, age and distribution of sea mounts and sediments, and age of mid-oceanic ridges. Present seafloor spreading hypothesis is based on extensive geophysical investigation on and near the ridge axis : topography, heat flow, gravity anomaly, crustal structure, sediment distribution, and geomagnetic anomaly. Especially, the survey of magnetic anomalies on the ridges revealed that they have parallel and symmetrical distribution along the ridge axis. F.J. VINE and D.H. MATTHEWS proposed at 1963 a hypothesis to interpret this characteristic pattern ; this pattern is a result of normal and reversed (thermo-remanent) magnetization of basaltic rocks which underlain the oceanic bottom, and this in-' verse magnetization is a result of the Earth's magnetic field reversals. Correspondence of the sequence of the Earth's field reversals and the magnetic anomaly pattern on the ridges gives both sea floor spreading rates and, as a consequence, isochrons of the ocean floor. Seismicity and seismic mechanism at the ridge and fracture zone can be predicted by this hypotheses. Geophysics at trenches, however, indicates more complicted features than at the ridges. For example, it seems that thickness and structure of sediments do not suport this hypotheses ; seismology and gravity are concordant with this ; but relation of magnetic anomaly with trenches are obscure. A "rotation pole" can be defined by trend of ridge-axis and fracture zone or by spreading rate for each ridge. It is interesting that this "pole" is nearly coincide with the present geomagnetic pole position and ocean floor may be opening or closing with a constant angular rate.
1969-07-25

海洋底拡大説について

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