中西 正男

The discovery of the magnetic anomaly lineations that can give ages of ocean floor is a very important role for establishment of the plate tectonics theory. The magnetic anomaly lineations also give us information of a history of movements of the oceanic plates. However, the origin of magnetic anomaly lineations still been obscured, that is, we can not clearly answer for the following questions: How thick is the source layer of magnetic anomaly lineations? How strong is the intensity of the magnetic source layer? In this paper we examined the relevant information concerning the magnetization of the oceanic crust from studies of observed marine magnetic anomalies and from rock -magnetism of oceanic basalts to get a goal of these questions in this paper.<BR>The skewness parameter that is deduced by precise magnetic anomaly lineations is important to identify marine magnetic anomalies. The magnetic polarity transition width is also important to do, though the parameter associated with this transition width has not almost utilized in the previous works. The anomalous skewness and the skewness discrepancy are often observed over the oceans. These observations might be explained not by a single-layer model but a two-layer model for magnetic source layer. The polarity transition width is defined the width which 95.4 % of the change from normal to reversed polarity occurs within. This width increases monotonically with spreading rates of ridges and/or with ages of ocean floors. This increasing is considered to be a manifestation of a more complicated crustal source consisting of two discrete layers. The analysis of the skewness parameter and transition width strongly supports that the sourc elayer of marine magnetic anomalies has a two-layer structure. The upper layer, consisting of surface lava flows of layer 2 A and possibly the sheeted dike complex, hasdi stinct and approximately vertical magnetic in the vicinity of opposite magn etized region boundaries. The lower layer, consisting of intrusive and gabbroic layers, has the boundaries gradually sloping downward away from the spreading center.<BR>Many detailed survey are carried out to reveal the structure of magnetic source layer by the multi narrow and the deep-towed magnetometer near active ridges. Inversion of magnetic layer using results of detailed surveys concluded that the magnetic source layer near the active ridges is less than 1 km in thickness. The polarity transition width of the relatively young layer is narrower than that of older oceanic floor, and the magnetic intensity of the relatively young layers higher (more than 10A/m) than that of older one. These conclusion indicate that the magnetic source layer near the active ridg es consists of a single layer structure. It is thought that the magnetic source layer grows with ages asoceanic crust by results of analysis of skewness and polarity transition width and inversion of magnetic source layer near active ridges.<BR>Several previous paleomagnetic studies indicate that intensity of natural remanent magnetization (NRM) of basaltic rocks composing the ocean crust rapidly decreases with ages in the past 10 to 20 Ma, and gradually increases older one. This change in NRM intensity is roughly proportional to the changes in intensity of saturation magnetization of the rocks and possibly due to sea-water alteration (low-temperature oxidation) of the primary ferromagnetic minerals contained in the rocks. NRM of the oceanic rocks is initially thermoremanent magnetization acquired at the time of formation of the oceanic crust. In accordance with progressive oxidation, fraction of TRM to bulk intensity decreases, while that of the secondary magnetization increases.