Abstract
Geologic structure was discussed on a seismic profile of Asaka-Konosu-Oura transect in the western part of the Kanto Plain, central Japan. A general model of stratigraphy and geologic structures, established based on surface geology in the Kanto District, was applied to subsurface geology. Both surface and subsurface geology can be divided into three stratigraphic units as "N.8 deposits (ca. 16.5-15.0 Ma)", "post N.8 deposits (ca. 15.0-3.0 Ma)" and the Kazusa-Shimosa Groups (<3 Ma). The "N.8 deposits" is composed of latest Early Miocene to earliest Middle Miocene submarine fan and overlying half-graben fill deposits. The "post N.8 deposits" is interpreted as Middle Miocene intra-arc basin deposits, which widely cover underlying strata unconformably (Niwaya Unconformity). The Kazusa-Shimosa Groups are latest Pliocene to Quaternary fore-arc basin deposits, which cover the "post N.8 deposits " horizontally. The base of this unit (Kurotaki Unconformity) is traced throughout transect. The basement structures underneath of the Kanto Plain are characterized by southerly-tilted basement blocks, which are cut by northward-dipping large normal faults. These structures were probably formed under extension stress field during Japan Sea opening (ca. 19-15 Ma). As metamorphic rocks in the Yoshimi Hills have been located at the crest of tilted-block since Miocene, such basement rocks are cropped out within the Quaternary sediments of the Kanto Plain.
