**4. Development of the Osaka Basin**

**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

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

sediment supply is a key to describe development processes of the tectonic basin.

**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

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.

**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]

#### **4.1. Paradox of basin formation at a confining bend of a fault**

Figure 11 delineates the deformation scheme at stepping parts of strike-slip faults. Generally speaking, a depression is formed at a right-stepping part of a dextral fault (Figure 11a), whereas an upheaval is formed at a left-stepping part of a dextral fault (Figure 11b). The MTL and the Arima-Takatsuki Tectonic Line (Figure 11c) are considered to act as a confining left-step of the regional dextral fault. However, geologic information shows that the Osaka Basin is a site of Quaternary basin formation. In order to solve the paradox, Kusumoto et al. [23] executed dislocation modeling for assessment of the vertical displacement at a complex termination of a strike-slip fault. They found that actual basin morphology could be restored by introducing reverse motions to secondary faults as shown in Figure 11c. The simulated deformation field predicts that a relative basement high, which corresponds to the Uemachi Basement High (UBH) in Figure 6, emerges within a depression surrounded by the modeled active faults.

**4.2. Seismic interpretation**

deformation.

Figure 10 for line location

Figure 12 is an E-W seismic reflection profile across the northern part of the Osaka Plain (land area of the Osaka Basin) [24]. Although the internal structure of the sedimentary basin should be discussed with detailed stratigraphic control utilizing borehole information in the future, it is noted that faults within the profile show normal and reversed displacements in the western and the eastern portions, respectively. A close-up of the western portion is characterized by a master normal fault accompanied by a collapsed anticline on its downthrown side. It coincides with the area of relative upheaval in the numerical modeling (shown by warm-colored portions within the fault-bounded basin in Figure 11c), and accords with the actual extensional structure, which is generally expected around an area of upheaval in the modeling of crustal

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**Figure 12.** A depth-converted seismic profile (Osaka-Suzuka) crossing the northern part of the Osaka Plain [24]. See

Seismic data indicate a large diversity in structural attitudes in the Osaka Basin. Figure 13 is an E-W seismic profile across the southern part of the Osaka Plain [25]. Remarkable vertical displacement with a steep gradient of the Bouguer gravity anomaly is observed at the easternmost part of the profile, and interpreted as the southern part of the Ikoma fault system (Figure 11c). A strong reflection in the eastern part of the basin is correlated with the Miocene volcanic surface based on surface geology along the survey line. It is noteworthy that a unit

**Figure 11.** Architecture of numerical modeling of the formation of the Osaka Basin [23]. (a) Normalized vertical dis‐ placement at a releasing bend of a strike-slip fault. (b) Normalized vertical displacement at a confining bend of a strike-slip fault. (c) Dislocation model of the Osaka Basin. Red lines are major faults adopted for the modeling. Warm and cold color gradations indicate upheaval and subsiding areas, respectively

## **4.2. Seismic interpretation**

**4.1. Paradox of basin formation at a confining bend of a fault**

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Figure 11 delineates the deformation scheme at stepping parts of strike-slip faults. Generally speaking, a depression is formed at a right-stepping part of a dextral fault (Figure 11a), whereas an upheaval is formed at a left-stepping part of a dextral fault (Figure 11b). The MTL and the Arima-Takatsuki Tectonic Line (Figure 11c) are considered to act as a confining left-step of the regional dextral fault. However, geologic information shows that the Osaka Basin is a site of Quaternary basin formation. In order to solve the paradox, Kusumoto et al. [23] executed dislocation modeling for assessment of the vertical displacement at a complex termination of a strike-slip fault. They found that actual basin morphology could be restored by introducing reverse motions to secondary faults as shown in Figure 11c. The simulated deformation field predicts that a relative basement high, which corresponds to the Uemachi Basement High (UBH) in Figure 6, emerges within a depression surrounded by the modeled active faults.

**Figure 11.** Architecture of numerical modeling of the formation of the Osaka Basin [23]. (a) Normalized vertical dis‐ placement at a releasing bend of a strike-slip fault. (b) Normalized vertical displacement at a confining bend of a strike-slip fault. (c) Dislocation model of the Osaka Basin. Red lines are major faults adopted for the modeling. Warm

and cold color gradations indicate upheaval and subsiding areas, respectively

Figure 12 is an E-W seismic reflection profile across the northern part of the Osaka Plain (land area of the Osaka Basin) [24]. Although the internal structure of the sedimentary basin should be discussed with detailed stratigraphic control utilizing borehole information in the future, it is noted that faults within the profile show normal and reversed displacements in the western and the eastern portions, respectively. A close-up of the western portion is characterized by a master normal fault accompanied by a collapsed anticline on its downthrown side. It coincides with the area of relative upheaval in the numerical modeling (shown by warm-colored portions within the fault-bounded basin in Figure 11c), and accords with the actual extensional structure, which is generally expected around an area of upheaval in the modeling of crustal deformation.

**Figure 12.** A depth-converted seismic profile (Osaka-Suzuka) crossing the northern part of the Osaka Plain [24]. See Figure 10 for line location

Seismic data indicate a large diversity in structural attitudes in the Osaka Basin. Figure 13 is an E-W seismic profile across the southern part of the Osaka Plain [25]. Remarkable vertical displacement with a steep gradient of the Bouguer gravity anomaly is observed at the easternmost part of the profile, and interpreted as the southern part of the Ikoma fault system (Figure 11c). A strong reflection in the eastern part of the basin is correlated with the Miocene volcanic surface based on surface geology along the survey line. It is noteworthy that a unit showing similarity in reflection pattern with the volcanic rocks is buried on the upthrown side of a reverse fault around the western part of the section. As the unit is accompanied by a positive gravity anomaly [21] and geomagnetic anomaly [26], we will construct threedimensional models for the subsurface seismic unit.

**Figure 13.** A depth-converted seismic profile (Yamatogawa South) crossing the southern part of the Osaka Plain [25] without vertical exaggeration. See Figure 10 for line location

**Figure 14.** Three-dimensional gravity and geomagnetic modeling of subsurface structure in the southern Osaka Plain.

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An integrated geophysical study of sedimentary basins was executed on an active plate margin. Volumes of conspicuous depressions upon both terminations of the Median Tectonic Line (MTL), a dextral bisecting fault of the southwest Japan arc, were estimated by means of gravimetric methods. The western end of the MTL and its secondary faults constitute a releasing step and form a gigantic composite depression of the Beppu-Iyo Basin and has been developed since the Pliocene. Sedimentological and chronological investigation revealed that the major constituent, the Iyonada Sea depression, was rapidly buried during the Quaternary by clastics derived from different geologic terrains. On the other hand, the eastern end of the MTL is a site of basin formation (Osaka Basin), even though the fault architecture is regarded as a confining step. Numerical modeling showed that a combination of major strike-slip and secondary reverse faults can provoke complicated ups and downs within an area surrounded by faults. The stress regime predicted through the modeling of the vertical displacement was concordant with the deep structure of the basin visualized by seismic interpretation. Although the present study area is not accompanied by sufficient geologic evidence of a deep basin interior provided by drilling survey, geomagnetic anomaly modeling successfully delineated a buried volcanic unit, which was anticipated from the viewpoint of regional geology.

See Figure 10 for mapped area

**5. Summary**
