AEROMAGNETIC EVIDENCE FOR A BURIED EARLY CRETACEOUS MAGMATIC ARC, NORTHEAST JAPAN

Positive aeromagnetic anomalies, recent drilling, and models construct ed from these data delineate the plutonic roots of the Early Cretaceou s Kitakami magmatic are in northeast Japan. Buried plutons, mostly off shore, produce belts of positive magnetic anomalies. These anomalies a nd magnetotelluric data suggest that the plutons form a batholith 70-1 20 km wide, nearly 800 km long, and 10-15 km thick. The batholith may mark the location of the main Kitakami are. Most of the exposed Kitaka mi plutons are 2-20 km in diameter; some are 3 km thick. The small plu tons line up along NW trending faults; some may have been satellite ve nts that tapped into the magma supply of the main are. The batholithic roots of the main are now compose almost half of the modern Japan for earc basement. Steep magnetic gradients, offset anomalies, and basin s tratigraphy portray extensive faulting of the Kitakami batholith durin g oblique subduction in the Late Cretaceous and rifting in the Miocene . The eastern boundary of the Kitakami batholith lies between 90 and 1 40 km west of the modern trench, much closer than the 300-km distance between the active are and trench. The Early Cretaceous forearc basin and accretionary prism may underlie the modern forearc basin east of t he batholith, but clear evidence is lacking. Much of the Early Cretace ous margin, including most of its forearc therefore is missing. How th e material was removed is unknown: it could have been strike-slip faul ted, eroded by subduction-related processes, or both.