Paleomagnetism of Quaternary volcanics in the Izu peninsula and adjacent areas, Japan, and its tectonic significance.

Abstract

A paleomagnetic investigation was made on the Quaternary volcanics in the Izu Peninsula and the adjacent Ashitaka and Izu-Oshima areas located on the northern tip of the Philippine Sea plate, central Japan. This study evaluates the deformation associated with the collision of the Izu block with Honshu. About 900 volcanic rock samples ranging in age from 0.2 to 1.5Ma were measured. Alternating field demagnetization indicates that these samples have stable natural remanent magnetizations. Fine grained titanomagnetite was identified as the carrier of the remanent magnetizations by thermomagnetic analysis and microscopic observation. The paleomagnetic directions from both Ashitaka Volcano and Izu-Oshima island, which are located to the north and to the east of the Izu Peninsula, respectively, are not significantly different from that of the present axial geocentric dipole field. In contrast, the mean directions from the northwestern Izu Peninsula show on average 10° clockwise deflections, whereas the directions from the northeastern to southern Izu Peninsula show 8° to 25° counterclockwise deflections. These deflections of paleomagnetic directions are probably caused by tectonic deformations since 1.5Ma. We propose a simple tilting model to explain these paleomagnetic directions.