Abstract
Sea-bottom topography, magnetic and gravity anomaly maps were compiled for two seamounts, Smt-948 (23.7°N, 151.9°E) and Smt-1100 (22.8°N, 153.4°E) in the vicinity of Minami-Tori Sima (Marcus) Island based on survey data observed by the S/V Takuyo of the Hydrographic Department of Japan from July to August, 1985. These flat-topped seamounts are located in the Jurassic magnetic quiet zone on crust older than 150Ma. On Smt-1100, sea bottom photographs were taken with a deep-sea camera and three dredge hauls were made. Three-dimensional magnetic analyses of Smt-948 gave a paleomagnetic pole position of 57.5°N, 329.5°E with the goodness-of-fit ratio 2.83. The pole position lies in the Pacific apparent polar wander path between the Late Jurassic (150Ma) and the Late Cretaceous (65-94Ma). This result implies that Smt-948 was generated in the middle Cretaceous, a significant period after the formation of the Jurassic sea-floor. Two-dimensional modeling of magnetic and gravity anomalies across these seamounts reveal a caldera-like basement structure, suggesting a constructional origin of the flat top of the guyot. A sequence of normal and reversed magnetic polarity layers is estimated from magnetic modeling on Smt-1100.
