Prologue: Tectonic Context of the Study Area

Pull-apart basins are formed at releasing morphologies of strike-slip faults. They have an important role in transferring mass at oblique convergent margins, at which strong simple shear provoked by a subducting slab eventually generates an arc-bisecting strike-slip crustal break [1]. Due to the terminal propagation of a group of sub-parallel transcurrent faults, the depocenter of a pull-apart basin migrates along the forearc and is progressively filled by terrigenous sediments [2]. The EW-trending Median Tectonic Line (MTL) active fault system in southwest Japan (Figure 1) is a typical arc-bisecting fault driven by oblique subduction of the Philippine Sea Plate through the late Quaternary [4]. The western part of the regional dextral fault is accompanied by releasing bends and steps, and previous studies [3, 5, 6] revealed the dimensions and buildup sequence of a pull-apart sag near northwest Shikoku Island (Figures 1 and 2).

Figure 1.

Index maps for the northwestern Pacific margin and Southwest Japan after [3]. The open arrow denotes the late Quaternary relative motion of the Philippine Sea plate [4].

Figure 2.

(a) Results of volumetric study of the Quaternary basin around the northwestern coast of Shikoku Island [5]. Contours in the index map (left) are Bouguer anomalies at 5-mGal intervals, and numbered lines are for 2D gravity anomaly modeling. (b) Hypothetical process of pull-apart basin formation on the northwestern coast of the Shikoku Island [3, 6]. See Figures 1 and 2a (left) for the location.

In this book, we concentrate on the Beppu Bay basin in the central part of Kyushu Island (Figure 2a and 3). It is located near the western termination of the MTL active fault system and occupies the eastern part of the volcano-tectonic depression of the Hohi Volcanic Zone [8], as shown in Figure 3. Although the basinal area has an absence of deep boreholes that could constrain the stratigraphic framework, its structural outline and evolutionary process have been addressed based on gravimetric analysis [9] and numerical modeling [10], which has an extraordinary advantage to figure out dimensions and structural buildup within a volcanic basin. Deep architecture of the basin shown by modeling studies has been confirmed by reflection seismic interpretations [7, 8, 10]. The previous works have suggested that the architectural characteristics of the basin are governed by a temporal transition of the active segments of the MTL. As it is an interesting case of a tectonically controlled estuary system, the authors introduced the latest technology of 3D high-resolution seismic (3D-HRS) surveying. 3D-HRS has the ability to visualize exquisite shallow fractures and sedimentary facies that have direct linkage with the late Pleistocene MTL transition and eustatic sea-level changes that are recorded around the bay, respectively. After geological and geophysical reviews in Chapters 1 and 2, the methodology and effect of the 3D-HRS is fully described in Chapter 3. Then, we demonstrate the notable stress gradient around the pull-apart basin, which provokes the Pleistocene depocenter migration and delineate the subsurface paleo-drainage system by means of a conventional 2D seismic survey [7] and the seismic attributes of the 3D-HRS dataset, respectively, in Chapter 4. Finally, our achievement and perspective for future work is summarized in Epilogue.

Figure 3.

Extent, geologic features, and gravity anomaly around the Hohi volcanic zone modified from [7]. See Figure 1 for the mapped area.