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The multidisciplinary research described in this book has elucidated the entire evolutionary process of a tectonic basin, Beppu Bay, located at the termination of a large strike-slip fault bisecting the southwestern Japan arc.
The Median Tectonic Line (MTL) active fault system in southwest Japan is a typical arc-bisecting fault driven by oblique subduction of the Philippine Sea Plate through late Quaternary period. At the western termination of the MTL active fault system, the Beppu Bay basin in central Kyushu Island has been developing since the Pliocene as the eastern part of a volcano-tectonic depression of the Hohi Volcanic Zone (HVZ), of which the outline and dimensions (more than 5000 km3) were delineated by means of gravimetric analyses.
The HVZ is a box-shaped graben buried by Plio-Pleistocene volcaniclastic materials. Although its genesis is often attributed to N-S breakup of the continental crust of Kyushu, recent horizontal movements detected by GNSS-based analysis have contradicted this hypothesis. Instead, the HVZ is related to the extrusion tectonics provoked by westward indentation of the forearc sliver of southwest Japan according to intermittent dextral slips on the MTL, which has been active as a right-lateral fault system under the control of west-northwestward convergence of the Philippine Sea Plate since the late Pleistocene. The right-stepping configuration of the dextral fault resulted in the emergence of an active pull-apart basin around Beppu Bay.
The basement architecture of the HVZ, including Beppu Bay, was reproduced using dislocation modeling. Development of the HVZ is divided into three stages: formation of a half-graben (Stage I, Pliocene), formation of the initial pull-apart basins due to activation of right-lateral faults (Stage II, early Quaternary), and growth of the pull-apart basins due to changes in the active area of the right-lateral fault (Stage III, middle to late Quaternary). During the course of optimization, a two-layer model of density contrast (basement and sedimentary layer) was introduced around the study area. The calculated gravity anomaly caused by the basement topography matched well with observed anomalies. Thus, the geological evolutionary model of the HVZ is endorsed from a geophysical viewpoint.
High-resolution 3D seismic (3D-HRS) survey data were acquired in Beppu Bay. The survey area was a rectangle of 6 km by 3 km (18 km2). The compact 3D-HRS data acquisition system was developed for detailed understanding of geologic structures of the shallow subsurface, and it comprises short streamer cables, small high-frequency seismic sources, and onboard equipment. As the survey area is congested with many ships at all times, data acquisition was conducted using a small vessel equipped with approximately 100-m-long streamer cables. During the acquisition of seismic data, the streamer spread was automatically optimized by the steering devices mounted on the tail floats of the cables, to ensure that the towing interval of the streamer cables remained in a specified range. After a series of data processing steps (pre-stack noise attenuation, multiples removal, velocity analysis, and footprint removal), seismic attribute calculations (similarity and thinned fault likelihood) were conducted to provide a quantitative basis for the interpretation of geologic features. The 3D-HRS method has much higher resolution than existing 2D seismic sections. In particular, it effectively extracted spatial discontinuities such as faults and fractures. The 3D view of the final migration results delineates an exquisite subsurface shallow structure with seafloor topography. In the vertical sections and time slices, a spatially continuous fault distribution is clearly observed.
Conventional 2D seismic and the latest 3D-HRS data were jointly used to decipher the subsurface structure and sedimentary facies in the Beppu Bay basin. On the 2D sections, a series of high-angle faults having a flower-like appearance extend near the southern bay coast and are interpreted as active traces of the laterally moving MTL. A releasing bend of the MTL forms a pull-apart sag around the bay bottom accompanied by numerous normal fractures at shallow depths, whereas a compressional bulge is emerging on the southern side of the bay, suggesting complicated and transient stress states during the late Quaternary. Such a tectonic context brought about westward migration of depocenters within the bay. Based on the general development history of the HVZ, the lower, middle, and upper seismic horizons are assigned to 5–6 Ma (initial stage of the HVZ), 0.7, and 0.3 Ma, respectively. We identified three auxiliary reflectors in the upper part of the sediment pile and correlated them with oxygen isotope stages 15–11. On such a stratigraphic basis, paleoenvironments in the latest Pleistocene are discerned by means of an amplitude RMS attribute analysis of 3D-HRS data. This method successfully delineated a river channel buried during the last glacial period, which is probably connected with the present Onogawa River. Structural interpretation of the 3D-HRS data indicates that a part of the ancient channel was cut by superficial fractures that are still growing.
It is noted that Beppu Bay is not an ideal place to execute such a comprehensive study. It has not been studied with deep boreholes, which can provide a firm stratigraphic basis for its burial history. Also, frequent influx of volcaniclastic debris degrades the seismic data quality. Vigorous activity on many faults obscures the detailed structural features due to spatiotemporal variance in the stress-strain state. We, however, succeeded in presenting a real image of tectonic basin development in this book based on integrated research. We hope our work stands as a precedent for the further challenge of understanding mobile belt frontiers, where intensive movements hinder efforts to decipher the Earth’s evolutionary processes.