Basin dynamics and subsidence history of the Nobi plain, central Japan, revealed by 600m-deep core analyses and deep seismic reflection survey

Authors: Toshihiko Sugai (Univ. Tokyo) and Yuichi Sugiyama

Abstract: Coupling the integrated core analyses of 601m-deep GS-NB-1 with E-W and N-S deep seismic profiles, over 10 km long in total, we demonstrated the evolutionary history of the Nobi tilted basin whose western margin is bounded by the Yoro fault system. Well-traceable reflection surfaces at several tens meters intervals are seen at the stratigraphic horizon less than 600m deep at the eastern edge of the E-W profile. These reflectors are correlative with interbedded marine mud beds and non-marine gravely beds whose depositions were controlled by glacial eustacy and basin subsidence. Considering the decompacted thicknesses of the GS-NB-1 core based on the bulk density-depth relations for fine sediments of the core, we estimated the tectonic subsidence rate as 0.67 m/ky at the core site, which rate is constant for the last 0.9 m.y. The average subsidence rate at the depocenter attains to 1.1 m/ky and mean tilting rate of the plain is estimated at 0.86×10-4 /ky for the last 0.9 m.y., suggesting 1,150 to 2,300 years recurrence interval of the Yoro fault. The activity of the Yoro fault is likely to have started about 2 Ma, accelerating over the period from 2 to 1 Ma, and attained to the maximum speed in the last 1 m.y.

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