| Summary: | 碩士 === 國立海洋大學 === 應用地球物理研究所 === 87 === The magnetic lineation ages in the east offshore area of Taiwan were suggested to be between 35-45 m.y.b.p. (magnetic lineations 14-19). A new combined magnetic data in the Taiwan offshore areas, including the 1996 Taiwan -France cooperated ACT cruise data, suggested that the E-W magnetic lineations are widely spread in the east and south of Taiwan offshore area. Our study is aiming to better understand the magnetic lineations ages, and their tectonic evolution.
The E-W lineations represent a N-S spreading history in which the new oceanic crust were magnetized by different magnetic inclinations. This has increased the difficulty to model the magnetic lineations. In order to solve this problem, we introduced the "Phase Shift modelling" method. The characteristics of magnetic anomalies in the east and south of Taiwan offshore areas are different : one has an amplitude of about 300 nT and the other only 100 nT. This is probably due to the east plate is exposing in the front of Ryukyu subduction system while the south plate has subducted beneath the Luzon arc system. However, the best ages fit in east plate are in about 37-45 m.y.b.p. This suggest that the east plate was part of the remanent West Philippine Basin which continued to spread along the Central Basin Ridge, after the Pacific plate changed its motion in about 45 m.y.b.p. with a much lower half rate about 23 mm/yr. The best ages fit in south plate crust are in about 15-22 m.y.b.p. The half spreading rate is about 27 mm/yr. This suggest that the south plate was part of the remanent South Chinese Sea plate. Our interpretation may be different with the new tectonic evolution suggested by Sibuet et al. However, both works are important to improve our understanding of the formation of Taiwan and South China Sea.
More efforts have been devoted into the studies of collision history of Taiwan since 5 m.y.b.p. However, the pre-5 m.y. geohistory is not clear. We intend to tackle this puzzle with the magnetic data as well as other geological and geophysical evidences.
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