Geomagnetic and gravity modeling of the fore-arc shelf of North-east Japan

Abstract

Northeast Japan is one of the continental slivers of the Eurasian Continent rifted by the Miocene fan-shaped genesis of the Japan Sea. A pair of longitudinal geomagnetic anomalies are traced along the Pacific coast of Northeast Japan. Basement core samples obtained from MITI Kesennuma-Oki well drilled on the southernmost part of anomaly belt are examined. The basement, which is identified as biotite granite and characterized by sheared fabric, shows early Cretaceous radiometric ages. Through rock-magnetic experiments, it was confirmed that the granitic basement holds thermoremanent magnetization (TRM) strong enough to cause the observed anomaly with a minor contribution from induced magnetization. The carrier of TRM is multi-domain titanomagnetite. On the basis of experimental results, two-dimensional geomagnetic and gravity modeling for semi-infinite magnetized polygons have been constructed along two E–W seismic lines on the fore-arc shelf of Northeast Japan. Vector components of magnetization are tuned through fitting anomaly peaks caused by basement topography, delineated by seismic survey. As for the northern part of study area, the inclination of westerly deflected magnetization vector should be 30° or smaller, the amount required to restore the Cretaceous Northeast Japan to the south of Southwest Japan. The gravity model for the same lines as the geomagnetic modeling suggests that fore-arc basement off the northern Kitakami Terrane consists of granitic rocks, except for a conspicuous basalt, whereas the non-magnetic basement off the southern Abukuma Terrane consists of a variety of blocks of different density. The anomalous direction of remanent magnetization and variety in gravity data are linked to NNW–SSE sinistral transcurrent faults activated throughout the Paleogene.