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

**Figure 5.** Isopach map of the Iyonada Basin based on 2D gravity anomaly modeling, indicating basement structure of

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

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

> <sup>1</sup> (,) <sup>2</sup> *M g x y dxdy*


¥

p*G*

the Iyonada Sea. See Figure 3 for mapped area. Grid shows data points for volumetric analysis

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

**2.2. Osaka Basin**

in this book) in a following section.

with a 5 km interval (Figure 6) by Gauss's theorem [15]:

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 an adjoining onshore sedimentary unit.

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 activity of the MTL running along the southern margin of the Iyonada Sea [18]. The Middle Member of the Gunchu Formation contains a considerable amount of crystalline schist gravels that were derived from the Sanbagawa metamorphic belt [19]. Its sedimentological descrip‐ tion, however, has not been reported. Therefore the authors present the results of our prelimi‐ nary geologic survey and chronological analysis in the following sections.

in response to changes in lithology. The content of schist gravel is related to sediment supply from the Outer Zone, a geologic zone on the southern side of the MTL. The paleocurrent direction, based on the imbrication structure of gravels, also shows considerable variation. It seems that the E-W elongate trough of the Iyonada Sea was alternately buried by the westward and northward influx of clastics. Further facies analysis would help provide an understanding

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263

**Figure 8.** Sedimentological description of the Pleistocene Gunchu Formation. Left: Lithologic column of the Middle Member of the Gunchu Formation. Center: Gravel composition of selected horizons. Right: Rose diagrams showing paleocurrent directions (downcurrent directions) of the selected horizons measured using imbricate structure of grav‐

of the sedimentary system of the pull-apart basin.

els. Number of data is 100 for each measured horizon

**Figure 7.** Geological index around the eastern part of the Iyonada Sea [16]


**Table 1. Fission-track age of granite pebbles contained in basal part of the Gunchu Formation***r* is correlation coefficient between ρs and ρu. P (χ<sup>2</sup>) is the probability of obtaining χ2-value for ν degrees of freedom (where ν = No. of crystals - 1). IS means internal surface

#### **3.1. Provenance and sedimentary structure**

Figure 8 shows results of the geologic survey of the Middle Member of the Gunchu Formation along the coastal section. It is clear that the composition of gravels fluctuates along the section in response to changes in lithology. The content of schist gravel is related to sediment supply from the Outer Zone, a geologic zone on the southern side of the MTL. The paleocurrent direction, based on the imbrication structure of gravels, also shows considerable variation. It seems that the E-W elongate trough of the Iyonada Sea was alternately buried by the westward and northward influx of clastics. Further facies analysis would help provide an understanding of the sedimentary system of the pull-apart basin.

activity of the MTL running along the southern margin of the Iyonada Sea [18]. The Middle Member of the Gunchu Formation contains a considerable amount of crystalline schist gravels that were derived from the Sanbagawa metamorphic belt [19]. Its sedimentological descrip‐ tion, however, has not been reported. Therefore the authors present the results of our prelimi‐

nary geologic survey and chronological analysis in the following sections.

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

**Figure 7.** Geological index around the eastern part of the Iyonada Sea [16]

**Spontaneous track Total U count** *r* **P (χ2)**

**Table 1. Fission-track age of granite pebbles contained in basal part of the Gunchu Formation***r* is correlation coefficient between ρs and ρu. P (χ<sup>2</sup>) is the probability of obtaining χ2-value for ν degrees of freedom (where ν = No. of

Figure 8 shows results of the geologic survey of the Middle Member of the Gunchu Formation along the coastal section. It is clear that the composition of gravels fluctuates along the section

**ρs (cm-2)** *N***<sup>s</sup> ρu (cm-2)** *N***<sup>u</sup>** granite zircon 30 1.11×107 4938 3.48×108 154845 0.856 0 280 77.9±6.1IS

**(%)**

**U (ppm)** **Age±1σ (Μa)**

**Method**

**sample mineral No. of**

crystals - 1). IS means internal surface

**crystals**

**3.1. Provenance and sedimentary structure**

**Figure 8.** Sedimentological description of the Pleistocene Gunchu Formation. Left: Lithologic column of the Middle Member of the Gunchu Formation. Center: Gravel composition of selected horizons. Right: Rose diagrams showing paleocurrent directions (downcurrent directions) of the selected horizons measured using imbricate structure of grav‐ els. Number of data is 100 for each measured horizon

#### **3.2. Chronology**

Kitabayashi et al. [20] executed fission-track dating of an ash layer intercalated in the Lower Member of the Gunchu Formation, and obtained an age of 2.2 Ma. Thus the subsidence and burial of the huge depression of the Beppu-Iyo Basin seems to be a recent event, maybe in response to an accelerated slip rate on the MTL. From the viewpoint of sediment provenance, we executed fission-track dating for pebble samples. It is noted that the Lower Member of the Gunchu Formation is lacking in schist gravels, and is characterized by sporadic granite pebbles. Table 1 and Figure 9 suggest that the granite pebbles were derived from the Cretaceous Ryoke intrusive rocks, which are distributed on the northern side of the MTL (Figure 7). Cessation of sediment supply from the northern terrane is thought to reflect entrapment of clastics in a deepening tectonic basin, and the succeeding emergence of voluminous schist gravels may correspond to an episodic uplift of the forearc sliver. Thus the drastic change in the pattern of sediment supply is a key to describe development processes of the tectonic basin.

**4. Development of the Osaka Basin**

In contrast to the simple half-graben on the western end of the MTL, the Osaka Basin at the eastern end is characterized by a complicated subsurface morphology reflected in the pattern of the gravity anomaly [21,22] (Figure 10). We argue a mechanism of basin formation based on numerical modeling, describe the seismic reflection profile showing the deformation pattern during the development of the basin, and interpret the origin of a concealed geologic unit on the basis of gravity and geomagnetic anomaly modeling in the following sections.

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265

**Figure 10.** Geologic and geophysical database of the Osaka Basin (mainly for land area). Gravity contour is compiled

after Nakagawa et al. [21] and Komazawa et al. [22]

**Figure 9.** Result of fission-track dating of granite pebbles contained in the lowermost part of the Gunchu Formation. See Table 1 for detailed analytical data
