Utility of petrophysical and geophysical data to constrain the subsurface structure of the Kitakami plutons, northeast Japan

Abstract

The Geological Survey of Japan (GSJ), AIST has been systematically measuring physical properties of basement rocks in Japan for more than 30 years and has started to construct a petrophysical database called PBRock 21 (Petrophysical Database of Basement rocks in Japan for the 21st Century, http://www.aist.go.jp/RIODB/ PB-Rock21/). The petrophysical data has been integrated with aeromagnetic data to determine the subsurface structure of the Himekami and Sobanokami plutons, members of the Kitakami plutonic belt, northeast Japan. For the Himekami pluton, the result of apparent magnetization intensity mapping clearly shows zonation within subplutons as well as regional rock type differences between the North (felsic) and South (felsic and mafic complex) sub-plutons. The bottom depths of the pluton were estimated by a successive 3-D magnetic modeling to be 1.7 km (North sub-pluton) and 1.5 km below sea level (South sub-pluton), respectively. Magnetic modeling of the Sobanokami pluton, which is partly exposed at the eastern edge of the Ishinomaki Plain, implies that the main part of the pluton extends to the northwest below the surface. Its western edge is cut by the Ishinomaki-wan fault, believed to be the upward extension of the fault plane of the 2003 northern Miyagi earthquake as defined by seismic observations and reflection seismic surveys conducted after the earthquake. Integrated magnetic and gravity modeling suggests that the Kakeyama Formation, a Neogene conglomerate deposited in a half-graben along the Ishinomaki-wan fault during Miocene rifting, contains more granitic fragments toward the south than to the north in the Ishinomaki Plain.