Eocene Paleolatitude of the Kitakami Massif in Northeastern Japan

The geologic basement of the Japan Arc consists primarily of rocks formed by subduction-related processes. Mesozoic to Cenozoic accretionary complexes are the vast majority of the subduction-related products, but there are also many fragments of ancient continents or volcanic islands or seamounts of different sizes, characterized by lithologies and geologic histories distinct from surrounding rocks. The South Kitakami Massif in northeastern Japan is typical of those allochthonous units. Early paleomagnetic studies demonstrated that the sedimentary rocks of the South Kitakami Massif have completely been remagnetized by intrusion of Cretaceous plutons. Recently, shallow paleomagnetic inclinations were reported from early Cretaceous and early Paleogene volcanic rocks of the Northern Kitakami Massif, leading to an interpretation that the whole Kitakami Massif was located at low latitude in the Cretaceous to early Paleogene. Here we present new paleomagnetic results from the Northern Kitakami Massif. Dacite dikes of Eocene age (ca. 44 Ma) and baked country rocks were sampled at 16 sites. The dikes possess vertical or subvertical intrusion surfaces, suggesting little or no significant tectonic tilt. We obtained for the first time an Eocene reliable paleomagnetic direction for the Kitakami massif; a positive baked contact test proves the primary nature of remanence. A suggestion that pre-Miocene rocks of northeastern Japan have completely been remagnetized can be eliminated. The mean direction is characterized by a westerly declination, indicating large counterclockwise rotation. Miocene rotational motion of the northeastern Japan Arc associated with backarc opening can account for this westerly direction. The mean inclination is statistically indistinguishable from that expected at the present latitude of the studied area, indicating little or no significant latitudinal motion. This finding provides a constraint on the suspected northward motion; the Kitakami Massif has not suffered latitudinal motion since at least Eocene time.