Fore-arc transcurrent faults in northeast Japan: hybrid geophysical modelsof an island-arc upper crust

Origin of a conspicuous longitudinal geomagnetic anomaly on the fore-are of northeast Japan, Kitakami magnetic belt (KMB), is investigated on the basi s of fission-track dating, rock magnetic experiments, reflection seismic in terpretation and potential field modeling. Radiometric ages suggest that th e basement core samples obtained from the Numanohata SK-2D (southern Hokkai do) and Ministry of International Trade and Industry Kesennuma-Oki (Offshor e Honshu) boreholes are part of the voluminous Cretaceous plutons in northe ast Japan. Two-dimensional gravity models on fore-are seismic profiles show that acoustic basement with geomagnetic anomaly has densities comparable w ith those of cored granites. Magnetic carrier of the granite is identified as multi-domain sized titanomagnetite. Strong thermoremanent magnetization (TRM) and high Q(n) ratios of the samples indicate significant contribution of TRM of the granitic basement as a source of observed anomaly. Conspicuo us paired anomaly is caused by consistent normal remanence of the granitic basement acquired during the Cretaceous long normal chron (C34n), As a resu lt of geomagnetic modeling, optimized direction of the remanence is charact erized by westerly deflection and abnormally shallow inclination (<30<degre es>), which requires to restore the Kitakami massif (northern part of north east Japan) to the south of southwest Japan. Significant northward translat ion since the Cretaceous may have been compensated by left-lateral motions on transcurrent faults along the Eurasian margin such as the Offshore Trend A identified in this study. Gravity modeling indicates that the basement o ff the Abukuma massif (southern part of northeast Japan), which does not sh ow geomagnetic anomaly, consists of varied density blocks that are bounded by transcurrent faults. (C) 2000 Elsevier Science B.V. All rights reserved.