Seismic Reflection Profiling across the Northeast Japan arc

A series of crustal movements took place over a vast area approximately 18 million years (Ma) ago, expanding the Japan Sea and giving rise to fields formed by crustal extension. It was in these fields that the Japanese islands were formed from a part that had separated from the Eurasian continent. The formation of the Japanese islands was accompanied by large-scale igneous activity. Approximately 3 Ma ago, the Japanese islands had a stress field that changed into a compression field, which still remains today. In an attempt to find (1) differences between the respective developmental histories of geological structures on the Japan Sea side and those on the Pacific Ocean side and (2) the structural boundary between the two groups of geological structures, we conducted a seismic reflection survey along the 41st parallel of the north latitude (at the northern tip of the Northeast Japan arc). In acoustic data recorded along the survey line, we observed up to five stratigraphic successions of strata. To establish a correspondence between the acoustically observed successions and geological successions, we used the drilling operations data. Of the acoustically observed five stratigraphic successions (Unit I to Unit V), Unit IV and layers at shallower depths matched strata formed during or after the Miocene era. According to our data on deformation of sedimentary strata, faulting resulted in deformed sedimentary strata on the Japan sea side during the Miocene era, and it has maintained the deformation until the present. This deformation of sedimentary strata was traced from the Japan sea side to the western side of the extended sea area of the Natsudomari peninsula inside Mutsu Bay. In contrast, the eastern side of the Natsudomari peninsula exhibited unobstructed sedimentary structures that were formed during and after the Miocene era. We traced these smooth sedimentary structures to the Pacific Ocean side.