GEOMAGNETIC AND GRAVITY MODELING OF THE FORE-ARE SHELF OF NORTHEAST JAPAN

Northeast Japan is one of the continental slivers of the Eurasian Cont inent rifted by the Miocene fan-shaped genesis of the Japan Sea. A pai r of longitudinal geomagnetic anomalies are traced along the Pacific c oast of Northeast Japan. Basement core samples obtained from MITI Kese nnuma-Oki well drilled on the southernmost part of anomaly belt are ex amined. The basement, which is identified as biotite granite and chara cterized by sheared fabric, shows early Cretaceous radiometric ages. T hrough rock-magnetic experiments, it was confirmed that the granitic b asement holds thermoremanent magnetization (TRM) strong enough to caus e the observed anomaly with a minor contribution from induced magnetiz ation. The carrier of TRM is multi-domain titanomagnetite. On the basi s of experimental results, two-dimensional geomagnetic and gravity mod eling for semi-infinite magnetized polygons have been constructed alon g two E-W seismic lines on the fore-are shelf of Northeast Japan. Vect or components of magnetization are tuned through fitting anomaly peaks caused by basement topography, delineated by seismic survey. As for t he northern part of study area, the inclination of westerly deflected magnetization vector should be 30 degrees or smaller, the amount requi red to restore the Cretaceous Northeast Japan to the south of Southwes t Japan. The gravity model for the same lines as the geomagnetic model ing suggests that fore-are basement off the northern Kitakami Terrane consists of granitic rocks, except for a conspicuous basalt, whereas t he non-magnetic basement off the southern Abukuma Terrane consists of a variety of blocks of different density. The anomalous direction of r emanent magnetization and variety in gravity data are linked to NNW-SS E sinistral transcurrent faults activated throughout the Paleogene. (C ) 1998 Elsevier Science Ltd. All rights reserved.