Paleomagnetic direction of the Lower Miocene Hokusetsu Subgroup in the Shitara area, Aichi Prefecture, and implications for the formation of the cuspate geologic structure in central Honshu

Abstract

Collision between the eastern part of southwestern Japan (SWJ) and the Izu−Bonin Arc caused lateral bending of pre-Miocene geologic belts and boundary faults in central Honshu, including the Median Tectonic Line (MTL), producing a prominent cuspate structure. Paleomagnetic studies suggest that these geologic belts and the MTL were linear prior to that collision; however, a number of previously determined paleomagnetic directions have significant uncertainties and thus provide only poor constraints on the timing and amount of rotation that occurred during collision. Here, we present the results of a paleomagnetic study of sediments from the Hokusetsu Subgroup in Shitara, on the western limb of the cuspate structure, with the aim of obtaining a precise paleodirection. Samples were collected from 14 sites within the uppermost Kadoya Formation, with principal component analysis of the results of stepwise demagnetization providing reliable site-mean directions for 6 of these sites. Rock magnetic experiments determined that magnetite is the main remanence carrier, and these directions pass a fold test, indicating that magnetization predated tilting at 15 Ma and is probably primary. The resultant overall mean direction has a NE declination: D (declination) = 30.1°, I (inclination) = 48.3°, α₉₅ (95% confidence limit) = 3.9°. The most likely cause of paleomagnetic deflection from the north is the clockwise rotation of SWJ relative to the Asian continent during the Early to Middle Miocene. The angle of deflection measured in these sediments is smaller than within the main part of SWJ, suggesting counterclockwise rotation in the western limb of the cuspate structure relative to the main part during or after clockwise rotation of SWJ. The Shitara paleodirection is deflected 25° counterclockwise relative to the strike of the nearby MTL; this is also the case for the Chichibu area of the eastern limb, indicating that the MTL had the same strike direction in these two areas in the late Early Miocene.