Paleomagnetism of Pliocene to Pleistocene lava flows in the northern part of Hyogo prefecture, Southwest Japan and Brunhes Chron paleosecular variation in Japan

Paleomagnetic investigation was performed on 26 Potassium-Argon dated Plioc ene to Pleistocene lava flows, collected at 34 sites from northern Hyogo, S outhwest Japan. Most magnetic polarities were found to be coincident with t he geomagnetic polarity timescale (GPTS), although two lava flows do not ag ree with the timescale. One of 2 lava flows incompatible with the GPTS, the Oliveine Andesite lava flow from the Oginosen volcano group dated at 0.92 +/- / - 0.05Ma, shows normal polarity. This finding is a possible evidence confirming the existence of the Santa Rosa Event proposed recently. Twenty- two paleomagnetic data sets were used to investigate paleosecular variation (PSV) having passed the following criteria: 1) characteristic components a re obtained from four specimens or more for a lava flow, 2) alpha(95) of th e lava flow mean direction is < 10 degrees and 3) the virtual geomagnetic p ole (VGP) latitude calculated is > 50 degrees or < -50 degrees. The mean VG P was calculated to be latitude = 85.5 degrees N and longitude = 285.3 degr ees E (k = 34.8 and alpha(95) = 5.3 degrees). The resultant angular standar d deviation (ASD) with respect to the geographic pole was calculated to be 14.5 degrees with the upper limit = 18.3 degrees and the lower limit = 12.1 degrees. The ASD value is almost equal to most existing results from the J apan Islands. Brunhes Chron PSV in Japan was estimated using the identical criteria for selection; 66 of 87 available data sets passed. The mean VGP w as calculated to be latitude = 89.3 degrees N and longitude = 14.4 degrees E (k = 33.1 and alpha(95) = 3.1 degrees) The resultant ASD with respect to the geographic pole was calculated to be 14.2 degrees with the upper limit = 16.1 degrees and the lower limit = 12.7 degrees. The ASD values for the p ast 3 million years in northern Hyogo and for the Brunhes Chron in Japan ar e compatible with the global trend for the past 5 million years fitted by p hysical PSV Model F and phenomenological PSV Model G. This suggests that th e PSV in Japan is not low compared with anomalously low PSV area in the Pac ific Region.