A Late Cretaceous paleomagnetic pole from Koto rhyolite, southwest Japan: Implications for eastern margin deformation of Asia

Abstract

A paleomagnetic pole was obtained from Late Cretaceous rhyolitic ash flow tuffs and associated ring dikes, which are collectively called the Koto rhyolite in the eastern part of southwest Japan. Characteristic remanent magnetizations were isolated from 33 sites through thermal demagnetization. Normal and reversed polarity magnetizations were found and are mutually antipodal. Although the magnetization directions become more dispersed after tilt correction, the clearly defined polarity sequence and the positive contact test between ash flow tuffs and ring dikes ensure that the characteristic magnetizations of ash flow tuffs predate the intrusion of associated ring dikes. Since no regional tilting is expected, we regard the in situ magnetization directions to represent the Late Cretaceous (70 Ma) paleomagnetic field (D = 84.6°, I = 57.5°, α95 = 5.5°) in eastern southwest Japan. The paleomagnetic pole is not significantly different from the coeval poles of western southwest Japan. This result gives a further support to a microplate model for the Miocene clockwise rotation of southwest Japan. By canceling out the effect of the Miocene rotation, we were able to obtain a paleomagnetic pole of 70 Ma (219.7°E, 63.0°N, α95 = 12.2°) for southwest Japan when the Japan Arc was incorporated in the eastern margin of Asia. Paleomagnetic poles of Late Cretaceous age from Korea, south China coast, and restored southwest Japan are statistically indistinguishable from each other but significantly differ from that of the continental interior of east Asia. The observed rotation of about 20° probably results from shear deformation exerted by northeast‐trending strike‐slip faults along the eastern continental margin of Asia.