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

Abstract

Origin of a conspicuous longitudinal geomagnetic anomaly on the fore-arc of northeast Japan, Kitakami magnetic belt (KMB), is investigated on the basis of fission-track dating, rock magnetic experiments, reflection seismic interpretation and potential field modeling. Radiometric ages suggest that the basement core samples obtained from the Numanohata SK-2D (southern Hokkaido) and Ministry of International Trade and Industry Kesennuma-Oki (Offshore Honshu) boreholes are part of the voluminous Cretaceous plutons in northeast Japan. Two-dimensional gravity models on fore-arc seismic profiles show that acoustic basement with geomagnetic anomaly has densities comparable with 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. Conspicuous paired anomaly is caused by consistent normal remanence of the granitic basement acquired during the Cretaceous long normal chron (C34n). As a result of geomagnetic modeling, optimized direction of the remanence is characterized by westerly deflection and abnormally shallow inclination (<30°), which requires to restore the Kitakami massif (northern part of northeast Japan) to the south of southwest Japan. Significant northward translation 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 off the Abukuma massif (southern part of northeast Japan), which does not show geomagnetic anomaly, consists of varied density blocks that are bounded by transcurrent faults.