**2.1. Beppu-Iyo Basin**

The large depression of the Beppu-Iyo Basin is divided into the Beppu Bay (west) and Iyonada Sea (east) areas by a relative high of the Bouguer anomaly. Beppu Bay is a younger part of an extensive tectonic depression, the Hohi Volcanic Zone, which has been developed since the Pliocene [7]. As for the Beppu Bay basin, active trace of the MTL is terminated around southwestern coast of the bay (e.g., [7]). Right-stepping lateral faults are aligned on the northern corner of the bay, and the parallel two fault strands are connected by normal listric faults constituting a rhomboidal depression surrounded by the faults [7]. These structural patterns are characteristic for releasing bend of strike-slip fault.

The Iyonada Sea is characterized by a remarkable negative gravity anomaly [8,9], but its origin has not been discussed so far. In contrast with the Beppu Bay basin, ambiguous points remain in the formation mechanism of the extensive Iyonada Sea depression. Paired dextral fault is not identified, and the basin is not regarded as an elongate sag in an area of propagation of lateral fault terminations because the sag is buried by recent sediments (Age assignment of a sedimentary unit upon the basin margin is presented in a following section.) simultaneous with those in the western termination of the MTL (Beppu Bay; [7]).

#### *2.1.1. Beppu Bay*

subsurface constituents of the sedimentary basins. Additionally, the evolutionary processes of the basins are argued based on geologic information. This is a multidisciplinary case study of basins ahead of a comprehensive visualization of the basin interior with seismic information.

256 Mechanism of Sedimentary Basin Formation - Multidisciplinary Approach on Active Plate Margins

**Figure 1.** Index map of the studied basins. Base map shows Bouguer gravity anomaly [6]. Bouguer density is 2670 kg/m3. The northern part of the Philippine Sea Plate is depicted as a positive gravity anomaly (red-colored) portion within the Pacific Ocean. Shape of an oceanic plate is generally delineated by positive Bouguer anomaly in ocean, be‐ cause an area of deep water is accompanied by positive gravity values as a result of correction procedures (cf. Y. Itoh,

Volume estimation of the basins is necessary in order to consider the mass balance of the upper crust around the island arc. Utilizing a gravity database [6], we present the analytical results

The large depression of the Beppu-Iyo Basin is divided into the Beppu Bay (west) and Iyonada Sea (east) areas by a relative high of the Bouguer anomaly. Beppu Bay is a younger part of an extensive tectonic depression, the Hohi Volcanic Zone, which has been developed since the Pliocene [7]. As for the Beppu Bay basin, active trace of the MTL is terminated around southwestern coast of the bay (e.g., [7]). Right-stepping lateral faults are aligned on the northern corner of the bay, and the parallel two fault strands are connected by normal listric

for the Beppu-Iyo and Osaka Basins in the following sections.

K. Takemura and S. Kusumoto in this book)

**2. Volumetric analysis**

**2.1. Beppu-Iyo Basin**

Kusumoto et al. [10] determined a three-dimensional subsurface structure around Beppu Bay on the basis of gravimetric data. We adopt their structural model and estimate the volume of the sedimentary basin. Figure 2 is a compiled map of the Hohi Volcanic Zone. The volume of the basement depression is calculated by the Gauss-Legendre numerical integration [11], from depth data given on the mesh with a 10 km interval, and is 4.1×103 km3 .

**Figure 2.** Simplified geology around the Hohi Volcanic Zone including Beppu Bay. Overlain basement structural con‐ tours are based on gravity anomaly modeling after Kusumoto et al. [10]. Grid shows data points for volumetric analy‐ sis

#### *2.1.2. Iyonada Sea*

Figure 3 is a Bouguer anomaly map around the Iyonada Sea with two-dimensional gravity modeling lines (1−10). Although a previous study [9] assumed there to be three units with different densities in the basement rock, its geologic context still remains ambiguous. Therefore we adopted a simple two-layered (sediment and basement) model. We applied Talwani's method [12] in order to estimate two-dimensional subsurface structures and assumed that the each structure at both ends of each profile was infinity to the depth. As shown in Figure 4, the remarkable elongate depression has a profile common with a half-graben, implying a tensile stress state. An isopach map indicating the top basement structure (Figure 5) demonstrates that the deepest part of the basin reaches 4 km from mean sea level. The volume of the basement depression is calculated by the Gauss-Legendre numerical integration [11], from depth data given on the mesh with a 10 km interval, and is 7.2×103 km3 .

**Figure 3.** Bouguer gravity anomaly map around the Beppu-Iyo Basin [9]. Bouguer density is 2670 kg/m3. Contour in‐ terval is 5 mGal. Lines 1 to 10 are for 2-D gravity anomaly modeling in the Iyonada Sea. A, B and C show previous modeling lines [9]

**Figure 4.** Examples of Bouguer anomaly profiles calculated for the two-layer models. See Figure 3 for line location

Characteristic Basin Formation at Terminations of a Large Transcurrent Fault — Basin Configuration…

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259

*2.1.2. Iyonada Sea*

modeling lines [9]

Figure 3 is a Bouguer anomaly map around the Iyonada Sea with two-dimensional gravity modeling lines (1−10). Although a previous study [9] assumed there to be three units with different densities in the basement rock, its geologic context still remains ambiguous. Therefore we adopted a simple two-layered (sediment and basement) model. We applied Talwani's method [12] in order to estimate two-dimensional subsurface structures and assumed that the each structure at both ends of each profile was infinity to the depth. As shown in Figure 4, the remarkable elongate depression has a profile common with a half-graben, implying a tensile stress state. An isopach map indicating the top basement structure (Figure 5) demonstrates that the deepest part of the basin reaches 4 km from mean sea level. The volume of the basement depression is calculated by the Gauss-Legendre numerical integration [11], from depth data

258 Mechanism of Sedimentary Basin Formation - Multidisciplinary Approach on Active Plate Margins

**Figure 3.** Bouguer gravity anomaly map around the Beppu-Iyo Basin [9]. Bouguer density is 2670 kg/m3. Contour in‐ terval is 5 mGal. Lines 1 to 10 are for 2-D gravity anomaly modeling in the Iyonada Sea. A, B and C show previous

 km3 .

given on the mesh with a 10 km interval, and is 7.2×103

**Figure 4.** Examples of Bouguer anomaly profiles calculated for the two-layer models. See Figure 3 for line location

Here, Δ*g*(*x*, *y*) is the gravity anomaly data given on an *xy* mesh with a constant interval. G, *π* and Δ*M* are the universal gravitational constant, circular constant and deficiency/excess of mass, respectively. Next, a first approximation of the volume of the sedimentary basin was calculated, from the estimated deficiency of mass by assuming a density contrast of 400

**Figure 6.** Bouguer gravity anomaly map [6] around the Osaka Basin. Bouguer density is 2670 kg/m3. Grid shows data

The present result indicates that the total volume of the sedimentary basin at the western end of the MTL is 10 times larger than that of the eastern counterpart. It may be attributed to a difference in the mechanism of basin formation between releasing and confining steps of strike-

Itoh et al. [7] demonstrated that the Hohi Volcanic Zone, including the Beppu Bay, has shifted its depocenter according to the transition of active segments of major faults, among which the MTL acted the most significant role for the development of tectonic sedimentary basin. As for the Iyonada Sea, which lacks seismic or drilling survey subsurface information, we aim at finding a temporal change in sediment supply pattern deduced from lithologic observation of

Figure 7 shows a geologic map around the eastern part of the Iyonada Sea [16]. It is known that a conspicuous sedimentary unit known as the Gunchu Formation is distributed along the northwestern coast of the Shikoku Island [17], which is a non-marine deposit containing abundant plant remains and has a steep homoclinal structure affected by the Quaternary

points for volumetric analysis. UBH is the Uemachi Basement High in the Osaka Basin

**3. Sedimentation process of the Beppu-Iyo Basin**

slip faults, which will be discussed later.

an adjoining onshore sedimentary unit.

 km3 .

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261

between sediment and basement, to be 9.1×102

kg/m3

**Figure 5.** Isopach map of the Iyonada Basin based on 2D gravity anomaly modeling, indicating basement structure of the Iyonada Sea. See Figure 3 for mapped area. Grid shows data points for volumetric analysis

#### **2.2. Osaka Basin**

The active trace of the MTL tends to shrink compared to the older phase of faulting. Its eastern termination is now around 136ºE with a length of active segment of 400 km, whereas the Cretaceous MTL as a significant geological break reached ca. 140ºE with a total length of 800 km [13]. The Arima-Takatsuki Tectonic Line (E-W trending fault on the northern flank of the Osaka Basin; Figure 1) is the unique parallel fault around the shrunken eastern termination having comparable dextral slip rate during the Quaternary [14]. Although this fault alignment should act as a confining bend, the area is characterized by recent vigorous basin formation around the Osaka Bay. We, therefore, attempt to simulate the deformation pattern introducing effect of reverse slips on secondary faults which show complex arrangement as a result of longstanding differential motion of crustal blocks (cf. Y. Itoh, K. Takemura and S. Kusumoto in this book) in a following section.

A regional structural model around the eastern termination of the MTL has not been shown because complicated basin morphology does not allow two-dimensional modeling. Thus, we estimated mass deficiency from the gravity anomaly data of the Osaka Basin given on the mesh with a 5 km interval (Figure 6) by Gauss's theorem [15]:

$$
\Delta M = \frac{1}{2\pi G} \int \int\_{-\infty}^{\infty} \Lambda g(\mathbf{x}, y) dx dy \tag{1}
$$

Here, Δ*g*(*x*, *y*) is the gravity anomaly data given on an *xy* mesh with a constant interval. G, *π* and Δ*M* are the universal gravitational constant, circular constant and deficiency/excess of mass, respectively. Next, a first approximation of the volume of the sedimentary basin was calculated, from the estimated deficiency of mass by assuming a density contrast of 400 kg/m3 between sediment and basement, to be 9.1×102 km3 .

**Figure 6.** Bouguer gravity anomaly map [6] around the Osaka Basin. Bouguer density is 2670 kg/m3. Grid shows data points for volumetric analysis. UBH is the Uemachi Basement High in the Osaka Basin

The present result indicates that the total volume of the sedimentary basin at the western end of the MTL is 10 times larger than that of the eastern counterpart. It may be attributed to a difference in the mechanism of basin formation between releasing and confining steps of strikeslip faults, which will be discussed later.
