Abstract
Samples of the Middle Miocene （～14.3 Ma) Kumano Acidic Igneous Rocks (KAIRs) collected at eight sites were measured for determining a paleomagnetic direction. Stepwise demagnetization with both alternating-field and thermal methods successfully isolated two remanent magnetization components, high-temperature (high-T) or high-coercivity (high C) component and low T or low C one, from natural remanent magnetization in many specimens. The Konogi Rhyolite, a thick pile of rhyolite flows at the lower part of the KAIRs, has an easterly deflected direction of normal polarity, while welded tuff and granite porphyry at the middle and upper part respectively possess a steeply inclined, southwesterly-deflected direction of reversed polarity. These characteristics of paleomagnetic direction are basically the same as reported previously (Tagami, 1982). The clockwise-deflected direction was considered by the previous study as representing a clockwise rotation of the southwest Japan arc after the emplacement of the KAIRs. The present study, however, interprets that the clockwise-deflected direction is of non tectonic origin because there are many lines of paleomagnetic evidence suggesting the termination of the clockwise arc rotation by 15 Ma. The clockwise-deflected direction of the KAIRs would have been acquired in an extraordinary geomagnetic field during a polarity transition or excursion. Furthermore, the similarity of the direction of the tuff and granite porphyry to that of the Muro Pyroclastic Flow Deposit, also dated at about 14.3 Ma, implies that these units are genetically associated with each other.
