Early Cretaceous paleomagnetic results of the Sasayama Group, southwest Japan

Abstract

A paleomagnetic study of terrestrial sediments was performed to acquire reliable Cretaceous paleomagnetic data for the inner arc of the southwestern Japanese islands (southwest Japan). Samples of red bed sedimentary rocks from the Early Cretaceous (approximately 110 Ma) Sasayama Group were collected from 28 sites. A high-temperature magnetization component with unblocking temperatures of up to 690°C was isolated from all the sites. The site-mean directions of the component fail the fold test, and incremental unfolding for the tilt yielded an optimum mean direction (D = 77.0°, I = 51.8° with α₉₅ = 2.9°) that passes the fold test, yielding a paleomagnetic pole at 28.0° N, 203.0° E with A₉₅ = 3.5°. Previously reported site-mean directions of the Sasayama basin from six sites were incorporated with those of this study to provide a grand-mean direction for the Sasayama Group, yielding an optimum mean direction through incremental unfolding (D = 75.8°, I = 53.0° with α₉₅ = 2.8°) and a corresponding paleomagnetic pole (29.4° N, 202.2° E with A₉₅ = 3.4°). This pole shows a paleomagnetic rotation of 64.9°±6.1° with respect to the coeval pole for East Asia, with a negligible latitudinal difference of 4.6°±4.9°.