A Supplement to Magnetic Anomaly of the Japan Basin.

Abstract

Magnetic and seismic reflection data of the Japan Basin with a good quality of ships positioning were selected to make a magnetic anomaly and reflective basement contour maps. The magnetic contour map reveals a number of magnetic anomaly lineations. We identified one additional lineation group trending N70°-75°E in the western side of the two another lineations trending N30°-35°E, N65°E formerly identified by other researches. The trends of these anomalies are qualified to be real ones by use of modified semblance analyses. Age identification of these magnetic anomaly lineations is achieved by introducing a basement magnetization model based on the Cenozoic magnetic reversal time-scale referring to the age constraints of other works. Albeit the Japan Basin has a smooth topography of 3, 000-3, 700 m water depth, the reflective basement map shows fairly rugged topography. The reflective basement topography has no correlation to the magnetic anomaly patterns of the present study area. Therefore we adopt an assumption of a flat magnetic basement model referring to recent seismic refraction studies of this area. Using variable half spreading rates, layer thickness, intensity of natural remanent plus induced magnetization, and skewness parameters, a possible range of ages of the formation of these magnetic anomalies is determined. Among a number of possibilities, we assign the age of these anomalies to 13-24 Ma. The older age 22-24 Ma and a half spreading rate 8.0 cm/yr are the best fit to the eastern area. The central area shows an age 22-24 Ma and a slower half spreading rate 7.5 cm/yr. The youngest age 13-15 Ma, and faster half spreading rate 10.0 cm/yr fit to the western anomaly group. We conclude that the formation of the Japan Basin proceeded toward west changing its spreading direction.