**5. Summary**

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 three-

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

**Figure 13.** A depth-converted seismic profile (Yamatogawa South) crossing the southern part of the Osaka Plain [25]

Middle Miocene volcanic rocks, collectively named the Nijo Group, are exposed in a hilly province upon the eastern margin of the Osaka Basin. It has been studied from stratigraphic (e.g., [27]) and paleomagnetic (e.g., [28]) points of view, and regarded as a volcanic product formed just after the clockwise rotation of southwest Japan related to the opening of the Japan

We assumed that conspicuous gravity and geomagnetic anomalies in the southern Osaka Plain are caused by a buried middle Miocene volcano, and estimated its three-dimensional structure based on Talwani's methods [29,30]. Figure 14 summarizes the modeling parameters. We assumed that the body of the volcano is magnetized parallel to the Earth's axial dipole field because the majority of the Nijo Group acquired its thermoremanent magnetization during the normal polarity Chron after the Miocene clockwise rotation of southwest Japan. As shown in Figure 14, gravity and geomagnetic anomalies are successfully simulated on the assumption of a conical volcano on the base of the sedimentary basin. In the Osaka Basin, there are isolated positive Bouguer anomalies associated with magnetic signatures from place to place. Com‐ bined anomaly modeling is a useful tool to estimate the origin of subsurface units in advance

dimensional models for the subsurface seismic unit.

without vertical exaggeration. See Figure 10 for line location

of a comprehensive interpretation of seismic data.

**4.3. Anomaly modeling**

Sea.

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.
