2006 Volume 112 Issue 1 Pages 14-32
Tectonic boundary between Northeast (NE) and Southwest (SW) Japan arcs during Japan Sea opening is discussed based mainly on geographical and geochronological distribution of Miocene sediments in and around the Kanto district, central Japan. Geological discontinuity, such as thick marine sediments vs. volcaniclastics, and E-W vs. N-S structural trend of island arc, is recognized between the Kanto Mountains and the Ashio Mountains. The abrupt change in geological characters indicates a tectonic boundary between Miocene fore-arc region of SW Japan arc and volcanic province of NE Japan arc. This tectonic boundary can be traced to the east, where a sharp, steep, and straight-lined submarine valley is recognized. Thus the previously presumed Tonegawa Tectonic Line is re-defined as a fault which was originated during Japan Sea opening in the Early Miocene, and divided the Japanese Islands into NE and SW Japan arcs in the Kanto district.
