SUBMARINE GEOLOGY AND POST-OPENING TECTONIC MOVEMENTS IN THE SOUTHERN REGION OF THE SEA OF JAPAN

The submarine geologic structure of the southern region of the Sea of Japan was analyzed on the basis of seismic reflection profile data and bottom samples. The seismic reflection patterns depict sedimentary layers with well-developed onlap and toplap terminations and truncations by submarine erosion. The planes marked by series of onlap, toplap, and erosional truncations enabled us to define three stratigraphic groups composed of six formations in this region. They are the Kasumioki Group, the Hamasakaoki Group and the Tottorioki Group in ascending order. Detailed analysis of the seismic reflection profiles reveals subtle structures of these sedimentary units. The oldest Kasumioki Group is folded and faulted, forming submarine ridges. The Hamasakaoki Group mainly occurs in the depressions between these ridges. The youngest Tottorioki Group covers widely both of the older formations with a pronounced marine onlap. Fossil evidence indicates that the depositional ages of the Kasumioki Group, the Hamasakaoki Group and the Tottorioki Group are Early Miocene to Earliest Pliocene, late Early to Late Pliocene, and Late Pliocene to Holocene, respectively. These observations provide the basis to understand two main structural features in this region; (1) E-W trending folds, and (2) northwestward tilting blocks. The former construct a fold zone composed of two to four ranges of anticlines that have steeper southern flanks truncated by reverse faults. The latter are shown by slightly-inclined floors formed by submarine erosion, step-like asymmetric basins bounded by NE-SW trending reverse faults, and onlap-toplap sedimentation patterns. The lateral change in bed-thickness, sedimentation patterns, and age of submarine sediments indicate that the E-W trending folds were formed in the early Middle or Late Miocene to the Early Pliocene by N-S compression, whereas the northwestward tilt blocks were formed in the late Early to Late Pliocene by NW-SE compression. The sedimentary records in this region dictate an abrupt change in tectonic setting in the late Early Pliocene. This change is interpreted to show that the Sea of Japan has been closing since then due to either the reactivation of the Amurian Plate, or the directional change of the Philippine Sea Plate motion.