Integrated description of deformation modes in a sedimentary basin; A Case study around a shallow drilling site in the Mizunami area, eastern part of southwest Japan

Ito, Yasuto*; Amano, Kenji  ; Kumazaki, Naoki*

Tectonic episodes in a sedimentary basin are described on the basis of an integrated study combining reflection seismic interpretation, drilling survey and paleomagnetism. A shallow inclined borehole penetrated a fault shown by reflection seismic and geological surveys in the Mizunami area, the eastern part of southwest Japan. Paleomagnetic measurements were executed on core samples successfully oriented using side-wall image logging of structural attitude. At six horizons, we confirmed stable characteristic remanent magnetization (ChRM) through thermal and alternating field demagnetization tests, which is carried by magnetite with minor amount of high coercivity minerals as revealed by experiment of stepwise acquisition of isothermal remanent magnetization. After correction of multi-phase deformation inferred from borehole structural analysis, ChRM directions are combined with previous data and confirm easterly deflection reflecting coherent clockwise rotation of the arc before the Middle Miocene. Compilation of reliable paleomagnetic data describes differential rotation of the eastern part of southwest Japan raised by collision of the Izu-Bonin arc since the Middle Miocene.