Paleomagnetism of the Miocene Inase volcanic rocks at the western margin of the Kitakami Mountains in Northeast Japan: Block rotations?

Highly scattered remanent magnetization directions were obtained from middle Miocene andesites of the Inase volcanic rocks on the western margin of the Kitakami Mountains, Northeast Japan. Oriented rock samples of lavas and intrusions were collected from 22 sites, and standard paleomagnetic analyses, including careful demagnetization experiments, were conducted to obtain reliable characteristic remanent magnetizations (ChRMs). Demagnetization data indicate that the main magnetic mineral that carries ChRMs is magnetite or Ti-poor titanomagnetite. Of 20 site-mean ChRM directions, 19 were successfully corrected for gentle post-emplacement tilt. Both positive and negative inclinations were determined, and though they show considerable scatter in declinations, many are northerly to easterly when directions with negative inclinations are flipped. Compared with the declinations, the scatter in inclinations is relatively small, and the inclinations lie along a small circle with an almost vertical axis. The mean inclination of ~47° is similar to the expected middle Miocene inclination value in Inase. A plausible explanation for the scattered directions aligning along the small circle is vertical-axis tectonic rotation, possibly related to island arc-scale crustal deformation. A recent study hypothesizes that, in the middle Miocene, the crust beneath the backarc side of Northeast Japan broke into several domino-like crustal blocks in a dextral shear regime along the eastern margin of the Japan Sea. The vertical-axis rotations identified in the ChRM data from Inase, which is located near the eastern end of the crustal blocks, may have resulted from rotations of these larger crustal dominos.