Paleomagnetism of the Akiyoshi limestone. Evidence for secondary magnetization.

Abstract

The Akiyoshi limestone group is an accreted reef complex originated on oceanic basaltic seamounts. The direction value of the natural remanent magnetization (NRM) of the Akiyoshi limestone is not adequate to deduce where the limestone was deposited, because the NRM seems to be secondary (a remagnetized value). The mean in situ direction, namely the remagnetized direction, of the limestone is 49.90° in declination and 61.1° in inclination (Fisher statistic parameters: k=90.0 and α95=4.1°) for fifteen sites with the precision parameter k>10.0. This mean direction is almost the same as the in situ directions of welded tuffs from four formations distributed around the Akiyoshi Plateau, which have been already reported by previous study. These welded tuffs have fission track ages between 85 and 94Ma. The remagnetized direction is also rather similar to the well-known paleomagnetic direction for Southwest Japan before about 15Ma. These correspon-dences suggest that the Akiyoshi limestone and the welded tuffs were remagnetized by some event after about 90Ma and before 15Ma. Results of thermal demagnetization and observation in thin sections of the limestone suggest that dominant magnetic particle is magnetite, and that the limestone may not have suffered high temperature (>500°C) alteration. This implies the presence of a low temperature chemical event.