The PM tephra bed, a thick (～10 m), felsic tephra deposit in the Pliocene sedimentary sequence in the Himi area, has been sampled for determining its paleomagnetic direction. Ten oriented cores were collected from the uppermost fine-grained vitric ash portion at one locality, and stepwise alternating-field and thermal demagnetizations were performed for 20 cylindrical specimens in order to isolate remanent magnetization components. More than half of the specimens had a single magnetic component with a northerly and down direction close to the present geomagnetic field direction (i.e., normal polarity). However, six samples, which also provided a normal polarity linear component, displayed a directional change along a great circle during stepwise demagnetization, indicating the presence of another higher coercivity/unblocking temperature component. Application of the great circle method disclosed the component with a SSW and up direction (i.e., reversed polarity). A previous study has reported that the PM tephra bed is normally magnetized; however, the author interprets that the normal polarity direction is most likely a secondary component and the primary magnetization has reversed polarity. This interpretation is concordant with tephrostratigraphic investigations suggesting that the PM tephra bed is correlated to the reversely magnetized tephras found in several areas of central Japan.
富山県氷見地域に分布するPMテフラ層の古地磁気方位
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