MAGNETIC AND GRAVITY CONSTRAINTS ON FORE-ARC UPPER CRUSTAL STRUCTURE AND COMPOSITION, OFFSHORE NORTHEAST JAPAN

Marine magnetic and gravity data from the northeast Japan forearc offe r insight to the subsurface structure, density and magnetization from which geologic interpretations and tectonic reconstructions can be mad e. Positive marine magnetic anomalies, on-land geology, drill hole dat a, and 2-1/2-dimensional models reveal that Kitakami plutons and possi bly their associated volcanic rocks constitute part of the modem forea rc basement and lie 100-150 km further east than previously thought. A method to create magnetization and density contrast maps was employed to produce a three-dimensional picture of the forearc basement rock p roperties averaged over a 14-km thickness. Comparisons of the magnetiz ation map with drill hole information suggest that Cretaceous andesite s may contribute to the observed positive aeromagnetic anomalies and t herefore, may be more magnetic than the sparse on-land rock property m easurements suggest. The lower magnetization values for the area south of 40-degrees-N latitude than for the area to the north may indicate that the andesites are not present in the south and erosion was deeper there. The gravity data primarily reflect variations of density withi n marine sedimentary rocks and crustal thickness.