Bent incised valley formed in uplifting shelf facing subduction margin: case study off the eastern coast of the Boso Peninsula, central Japan

Abstract

Abstract The Boso Peninsula is located in central Japan near the junction of the subduction boundary of three tectonic plates. A forearc basin has been developing there since 3 Ma and has been uplifting since 1 Ma. The basal surface of the Holocene deposits in the offshore area was investigated based on a seismic survey and is very similar to the adjacent land areas (the Iioka Plateau, the Kujukuri Plain and the Kazusa Hills). The basal surface in the Kujukuri Plain and its corresponding offshore area contains many incised valleys. Most of them extend southeastward, parallel to the direction from the hinterland to the ocean, but one incised valley (Kujukuri-oki Buried Valley) lies perpendicular to the others. A buried terrace is located SE of the valley and along the area where mudstone (of the Kiwada Formation) is distributed. The present observations indicate that differential erosion formed the terrace, after which the valley bent to follow the terrace. The rivers tend to be perpendicular to the strike of the sediment in the forearc basin owing to tectonic movement. Thus, the valley must have been incised into the underlying strata with a perpendicular strike and may have become bent in uplifting forearc basins.