Fig. 21 Along-track magnetic anomaly profiles north of the Joban Seamounts. The area is shown
in Fig. 3 . Positive anomalies are shown as shaded . The scale bar near the upper-left corner is for
the amplitude of magnetic anomalies. Bold lines represent magnetic anomaly lineations. Dotted
lines distinguish possible pseudofaults caused by a propagating spreading ridge
The most remarkable result about magnetic anomaly lineations in this study is
the discovery of magnetic anomaly lineations north the Joban Seamounts with a
strike different from that of magnetic anomaly lineations in the other areas (Fig. 21 ).
There are not any remarkable topographic structures near the junction between the
regular and irregular lineations which would indicate any post-spreading, off-ridge
tectonic event. This implies that these lineations formed during a reorganization of
the plate boundaries between the Pacific and Izanagi plates. Nakanishi et al. ( 1989,
1991 ) identified similar curved magnetic anomaly lineations and Nakanishi et al.
( 1991 ) speculated that the curved lineations are due to a propagating spreading
ridge. The identifications in this study support that speculation. The identifications
indicate that rift propagation stopped around chron M10 (134 Ma).
The discontinuity of the magnetic anomaly lineations around the Joban
Seamounts is also a remarkable result. Increased density of the geomagnetic data