**4.1. Characteristics of gravity anomaly**

We show a Bouguer gravity anomaly map for our study area in Figure 1. This Bouguer gravity anomaly map is based on gravity mesh data [8], and the Bouguer density is 2670 kg/m3 .

In this region, positive gravity anomalies are dominant, and there are conspicuous posi‐ tive anomalies over the Pacific Ocean and the Japan Sea. The Bouguer gravity anomaly of the Japan Sea side is relatively flat, while the Pacific Ocean side has a large gradient (Fig‐ ure 1). The Bouguer gravity anomaly (Δg*B*) in a marine area is generally positive in an area with a deep water, this is inferred form the Bouguer gravity anomaly given by the following (e.g., [38]).

$$
\Delta \mathbf{g}\_B = \Delta \mathbf{g}\_F - 2\pi \mathbf{G} \left(\rho\_w - \rho\right) \mathbf{D} \tag{1}
$$

Here, Δg*F*, *D* and *G* are the free-air gravity anomaly, the depth of water and the universal gravitational constant, respectively; *ρw* and *ρ* are water density and surface crust density, and generally *ρw* < *ρ*. Consequently, it is expected that these positive gravity anomaly areas have deep water. In fact, the areas correspond to the subduction zone along the Nankai Trough and the back-arc basins in the Japan Sea. There are negative gravity anomalies indicating the existence of subsidence structures between these positive gravity anomalies, and the subsi‐ dence structures forming the negative anomalies would be due to intra-arc basins.

These negative anomalies correspond to the active tectonic zone during the Quaternary called the 'Kinki Triangle' [33], and it is divided into the Osaka Bay and Lake Biwa areas. Negative gravity anomalies around Osaka Bay and the Lake Biwa reach -15 mGal and -60 mGal, respectively.

It is known that negative gravity anomalies in the Osaka Bay area can be explained by sediments accumulated in and around Osaka Bay (e.g., [39,40]), and these negative gravities are divided by some active faults (Figure 1). In contrast, it is known that negative gravity anomalies in the Lake Biwa area can not be explained by the distribution of soft sediments in the lake (e.g., [41]). Nishida et al. [41] have suggested that depression of the Conrad surface or the existence of very low-density materials due to faulting is necessary to explain the gravity low reaching -60 mGal.

Figure 6 depicts the first order horizontal derivative of the Bouguer gravity anomalies larger than 2 mGal/km that is shown by color gradation with an interval of 1 mGal/km. The first order horizontal derivative of the Bouguer gravity anomalies is defined by the following equation.

$$
\sqrt{\frac{\left\|\text{cg}\left(\mathbf{x},\mathbf{y}\right)\right\|^2}{\left\|\mathbf{x}\right\|^2} + \left[\frac{\left\|\text{cg}\left(\mathbf{x},\mathbf{y}\right)\right\|^2}{\left\|\mathbf{y}\right\|}\right]^2} \tag{2}
$$

Here, g(*x, y*) is the gravity anomaly on *xy* mesh data at a constant interval. Since the first order horizontal derivative of the Bouguer gravity anomaly emphasizes the shorter wavelength signals of subsurface structures, it is a good indication of a conspicuous density change and/or a large fault.

In Figure 6, high gradient anomalies greater than 2 mGal/km, except in the Lake Biwa area, have the same direction (roughly parallel to the Nankai Trough), and most of them in the land area correspond well with large faults or tectonic lines (Figure 1). The distribution of high gradient anomalies around Lake Biwa is very complex, and it could be considered that they reflect subsurface structures caused by extreme crustal activity including faulting during the Quaternary.

**Figure 6.** First order horizontal derivative of Bouguer gravity anomalies larger than 2 mGal/km, shown by color grada‐ tion with an interval of 1 mGal/km

#### **4.2. Development of subordinate structure**

As shown in the previous section, gravimetric analysis indicates that the crust of the Kinki domain is damaged under the influence of the complicated activity of surrounding tectonic zones. Numerous faults provoke the formation of intra-arc basins, among which Lake Biwa **Figure 7.** a) Bouguer gravity anomaly around the Osaka Bay at 2 mGal contour interval. The Bouguer density is 2670 kg/m3. Green grid shows domains for calculation of sediment thickness [42]. (b) Altitude of basement around the Osa‐

Neotectonic Intra-Arc Basins Within Southwest Japan — Conspicuous Basin-Forming Process Related to Differential

Motion of Crustal Blocks http://dx.doi.org/10.5772/56588 201

ka Bay inferred from gravity data [45]

Neotectonic Intra-Arc Basins Within Southwest Japan — Conspicuous Basin-Forming Process Related to Differential Motion of Crustal Blocks http://dx.doi.org/10.5772/56588 201

Here, g(*x, y*) is the gravity anomaly on *xy* mesh data at a constant interval. Since the first order horizontal derivative of the Bouguer gravity anomaly emphasizes the shorter wavelength signals of subsurface structures, it is a good indication of a conspicuous density change

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

In Figure 6, high gradient anomalies greater than 2 mGal/km, except in the Lake Biwa area, have the same direction (roughly parallel to the Nankai Trough), and most of them in the land area correspond well with large faults or tectonic lines (Figure 1). The distribution of high gradient anomalies around Lake Biwa is very complex, and it could be considered that they reflect subsurface structures caused by extreme crustal activity including faulting during the

**Figure 6.** First order horizontal derivative of Bouguer gravity anomalies larger than 2 mGal/km, shown by color grada‐

As shown in the previous section, gravimetric analysis indicates that the crust of the Kinki domain is damaged under the influence of the complicated activity of surrounding tectonic zones. Numerous faults provoke the formation of intra-arc basins, among which Lake Biwa

and/or a large fault.

Quaternary.

tion with an interval of 1 mGal/km

**4.2. Development of subordinate structure**

**Figure 7.** a) Bouguer gravity anomaly around the Osaka Bay at 2 mGal contour interval. The Bouguer density is 2670 kg/m3. Green grid shows domains for calculation of sediment thickness [42]. (b) Altitude of basement around the Osa‐ ka Bay inferred from gravity data [45]

synchronous acceleration of subsidence on the both flanks of the basement-high (Figure 8b). Similar events of crustal deformation are also confirmed in the Osaka Bay area. Itoh et al. [43] and Inoue et al. [44] indicated that a N-S warping (2 in Figure 7a) emerged around the mid-Quaternary and acted as a sedimentation divide in Osaka Bay. Basement altitude estimated from gravity (Figure 7b; [45]) implies that other subordinate structures emerged synchronous with the basin development. Thus, the differential motion of crustal blocks in a damage zone is closely related with complicated basin formation and conspicuous environmental changes.

Neotectonic Intra-Arc Basins Within Southwest Japan — Conspicuous Basin-Forming Process Related to Differential

Motion of Crustal Blocks http://dx.doi.org/10.5772/56588 203

A summary of basin-forming processes in an island arc was presented in connection with the development of a damaged area on an active plate margin. The Kinki district in southwest Japan has been a site of vigorous basin formation since the Pliocene. An accelerated Quaternary strain rate around the area is generally interpreted as a result of compressive stress linked to the westerly subduction of the Pacific Plate. Recent geodetic analyses demonstrated a NE-SW tectonic zone (Niigata-Kobe Tectonic Zone), which is an oblique trend of the geologicallydetected active structure with a N-S azimuth (the Itoigawa-Shizuoka Tectonic Line). Based on the contrast in fault architecture and the subsurface structures depicted using geophysical methods, the authors define another cross-arc structural component, the Echizen-Shima Tectonic Line. Forearc deformation closely linked to activity of this tectonic line is discussed in a chapter of this book [46]. Westerly subduction of the Philippine Sea Plate has provoked the transcurrent motion of the forearc sliver and active faulting upon the along-arc Median Tectonic Line. Surrounded by these regional tectonic zones, the Kinki district is studded by countless subordinate faults and suffers from differential motion of crustal blocks, which

The authors are grateful to A. Noda for his constructive review. The early version of our

and Shigekazu Kusumoto3

1 Graduate School of Science, Osaka Prefecture University, Osaka, Japan

2 Graduate School of Science, Kyoto University, Kyoto, Japan

**5. Summary**

results in great diversity of basin formation.

, Keiji Takemura2

manuscript was greatly improved based on his comments.

**Acknowledgements**

**Author details**

Yasuto Itoh1

**Figure 8.** a) Sediment thickness diagram from the late Pliocene to early Pleistocene (from top of the basement to the Ma 2 marine clay intercalated in the Osaka Group) [42]. See Figure 7 for plan view and nomenclature of the analyzed domains. (b) Interval subsidence rates through the late Quaternary in the Osaka Plain [42]. See Figure 7 for locations of the selected boreholes

and Osaka Bay are the largest and most important for understanding the paleoenvironmental changes in southwest Japan. Takemura and others in this book present a comprehensive history of the Lake Biwa sedimentary basin.

It is noted that the majority of the subordinate faults have a N-S azimuth (Figure 1). Their activity results in the formation of N-S warping zones within the island arc as shown in Figure 7. Based on detailed well stratigraphy, Itoh et al. [42] showed that the largest warping in the Osaka sedimentary basin (Uemachi basement-high; 1 in Figure 7a) has been developing since the late Pliocene (Figure 8a), and episodically grew around 550 kyr, which is shown by synchronous acceleration of subsidence on the both flanks of the basement-high (Figure 8b). Similar events of crustal deformation are also confirmed in the Osaka Bay area. Itoh et al. [43] and Inoue et al. [44] indicated that a N-S warping (2 in Figure 7a) emerged around the mid-Quaternary and acted as a sedimentation divide in Osaka Bay. Basement altitude estimated from gravity (Figure 7b; [45]) implies that other subordinate structures emerged synchronous with the basin development. Thus, the differential motion of crustal blocks in a damage zone is closely related with complicated basin formation and conspicuous environmental changes.
