北部フォッサマグナ，中央隆起帯南部に分布する中部中新統内村累層の重複変形 ―中央隆起帯は“無褶曲地区”か？―

抄録

The Central Uplift Belt of the North Fossa Magna region, central Japan, has long been believed to be a non-folded or weakly-deformed area except for its western area. The Uchimura Formation in the southern part of this belt is mainly composed of basaltic-andesitic lavas and volcaniclastic rocks of the syn-rifting stage of Sea of Japan interfingered with clastic sedimentary rocks of the late Early to Middle Miocene in age. In addition with the precise geological surveys of this formation, We statistically evaluated the distribution patterns and dispersion-concentration ratios of the attitudes of bedding planes applying the orientation tensor method. As the results, we recognized complicated folded structures of the Uchimura Formation in contrast with previous research reports. The folded structures were produced by multi-phase deformations since the late Middle Miocene. The first phase deformation was the formation of gentle-open folds of more than several tens of meters in wavelength with non- to gently-plunging axes under a horizontal compressive regime. Then, vertical-axis rotations of gently-moderately dipping strata and overprinting of multiphase foldings in different directions resulted in the formation of complicated structures until the Late Miocene in association with a regional tectonic block rotation to form the Akaishi-Kanto Syntaxis and the Fossa Magna due to the lateral collision with the northern tip of the Izu-Bonin Arc. These folds were then re-folded during the Pliocene, probably due to the E-W horizontal compression that affected widely the back-arc side of the Northeast Japan Arc. The latest phase deformation occurred during the Middle Pleistocene, including local foldings, faultings and formations of flexures, further superimposed on the pre-existing structures. The rapid uplifting of the Central Uplift Belt to form the present-day Chikuma Mountains and the surrounding subsidences of the Suwa, Matsumoto and Ueda Basins have occurred during this latest phase deformation in association with a left-lateral motion of the Itoigawa-Shizuoka Tectonic Line on the southwest.