Remanent Early Cretaceous spreading center in the Central Pacific Basin

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 M11 (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 4.2°/m.y. for the northern flank and 3.1°/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.