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A WNW trending lineated topographic depression (Magellan Trough) and associated magnetic anomaly lineations (Magellan lineation set) were revealed in the basin northeast of the Magellan Rise. We identified symmetric magnetic anomaly lineations from M9 (121 m.y.) to Mll (126 m.y.)of Early Cretaceous age with a symmetric axis at the Magellan Trough and concluded that the Magellan Trough is a remanent spreading center which ceased its opening at the latter half of M9 time. The Magellan lineation set fans drastically to the southeast, and it represents an asymmetric spreading system. The Magellan spreading system has two separate rotation poles for the northern flank lineations and the southern flank ones, and both poles lie on the extension of the Magellan Trough. The amplitudes of the magnetic anomalies decrease toward the poles, possibly owing to the contamination of the magnetized blocks as a function of decreasing spreading rate. The Magellan spreading system shows unusually rapid rotation rates of 4.2ø/m.y. for the northern flank and 3.1ø/m.y. for the southern flank, which together with the proximity of the rotation poles suggest unstableness of the spreading system. This unstable spreading may have shortened the life of the Magellan spreading system. Such an unstable spreading system could have been generated in order to balance changes in movements of the Phoenix, Farallon, and Pacific plates. The spreading center may have jumped to the northeast at M9 time as part of its evolution as the Pacific-Farallon ridge. Thus the Magellan Trough may be a fossil Pacific-Farallon ridge and the northern flank of the Magellan set a piece of captured Farallon plate. The bathymetric chart by Winterer [1973] shows a NW with low flanking ridges on both sides and extends more than trending lineated depression in the basin northeast of the 300 km from 173øW to 176øW with a width of 5-10 km. The Magellan Rise in the Central Pacific Basin. Larson et al. [ 1972] reported NW trending magnetic anomaly lineations in this area, north of the Phoenix lineations, and noted their fanshaped appearance and close approximation of the trends with the bathymetric contours of this area. The magnetic anomaly lineations subsequently were identified southward from M4 (117 m.y.) to M12 (128 m.y.) by Larson [1976]. Recent detailed data of bathymetric, seismic, geomagnetic, and gravimetric records obtained durin•g three cruises of the R.V. Hakurei-rnaru by the Geological Survey of Japan from 1975 to 1977 (GH75-4, GH76-1, and GH77-1 cruises) [Mizuno and Chujo, 1975; Mizuno and Moritani, 1977; Moritani, 1979] revealed detailed and characteristic features of the lineated depression and the associated magnetic anomaly lineations (Figures 1 and 2). W e call the lineated depression the Magellan Trough and the associated magnetic anomaly lineations the Magellan lineation set because of the proximity of the Magellan Rise. Combined old and recent data and well-preserved magnetic anomalies led us to a successful revised identification and detection of several ...
A WNW trending lineated topographic depression (Magellan Trough) and associated magnetic anomaly lineations (Magellan lineation set) were revealed in the basin northeast of the Magellan Rise. We identified symmetric magnetic anomaly lineations from M9 (121 m.y.) to Mll (126 m.y.)of Early Cretaceous age with a symmetric axis at the Magellan Trough and concluded that the Magellan Trough is a remanent spreading center which ceased its opening at the latter half of M9 time. The Magellan lineation set fans drastically to the southeast, and it represents an asymmetric spreading system. The Magellan spreading system has two separate rotation poles for the northern flank lineations and the southern flank ones, and both poles lie on the extension of the Magellan Trough. The amplitudes of the magnetic anomalies decrease toward the poles, possibly owing to the contamination of the magnetized blocks as a function of decreasing spreading rate. The Magellan spreading system shows unusually rapid rotation rates of 4.2ø/m.y. for the northern flank and 3.1ø/m.y. for the southern flank, which together with the proximity of the rotation poles suggest unstableness of the spreading system. This unstable spreading may have shortened the life of the Magellan spreading system. Such an unstable spreading system could have been generated in order to balance changes in movements of the Phoenix, Farallon, and Pacific plates. The spreading center may have jumped to the northeast at M9 time as part of its evolution as the Pacific-Farallon ridge. Thus the Magellan Trough may be a fossil Pacific-Farallon ridge and the northern flank of the Magellan set a piece of captured Farallon plate. The bathymetric chart by Winterer [1973] shows a NW with low flanking ridges on both sides and extends more than trending lineated depression in the basin northeast of the 300 km from 173øW to 176øW with a width of 5-10 km. The Magellan Rise in the Central Pacific Basin. Larson et al. [ 1972] reported NW trending magnetic anomaly lineations in this area, north of the Phoenix lineations, and noted their fanshaped appearance and close approximation of the trends with the bathymetric contours of this area. The magnetic anomaly lineations subsequently were identified southward from M4 (117 m.y.) to M12 (128 m.y.) by Larson [1976]. Recent detailed data of bathymetric, seismic, geomagnetic, and gravimetric records obtained durin•g three cruises of the R.V. Hakurei-rnaru by the Geological Survey of Japan from 1975 to 1977 (GH75-4, GH76-1, and GH77-1 cruises) [Mizuno and Chujo, 1975; Mizuno and Moritani, 1977; Moritani, 1979] revealed detailed and characteristic features of the lineated depression and the associated magnetic anomaly lineations (Figures 1 and 2). W e call the lineated depression the Magellan Trough and the associated magnetic anomaly lineations the Magellan lineation set because of the proximity of the Magellan Rise. Combined old and recent data and well-preserved magnetic anomalies led us to a successful revised identification and detection of several ...
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