**2. Description of major tectonic zones**

### **2.1. Median Tectonic Line**

The Median Tectonic Line (MTL) is the largest crustal break in southwest Japan, which bisects the island arc into the old terranes intruded by igneous rocks in the Inner Zone and partly metamorphosed accretionary complexes in the Outer Zone (Figure 1; [9]). It has a period of activity as long as 100 m.y. and highly complicated change in slip direction. In the following sections, we present a chronicle of the MTL activity based on previous research and original interpretation of geophysical data.

Cretaceous silici-clastic rocks of the Izumi Group [11], which was deposited in an elongate basin (300 km long by 10~20 km wide) along the MTL with sinistral strike-slip movements

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 193

Wrenching deformation associated with the ancient left slip on the MTL is identified in the Outer Zone of the Kii Peninsula (Figure 2c). Wang and Maekawa [12] showed that the metamorphic grade in the Sanbagawa belt have an en echelon anticlinal trend along the MTL. The trend is a result of deformation after high-pressure metamorphism, the climax of which is assigned around the middle of Cretaceous [13]. It is noted that the most intensive postmetamorphic deformation zone does not coincide with the geologically determined MTL that runs upon the northern bank of the River Kinokawa. Hirota [14] found a remarkable discon‐ tinuity in the metamorphic grade around the Funaokayama bar within the River Kinokawa (Figure 2c), and regarded it as a tectonic block. Takasu et al. [15] argued, on the basis of chronological data, that the amalgamation of metamorphosed blocks had occurred during the late Cretaceous. A steep gradient in the gravity anomaly along the river also implies a concealed structure parallel to the surface MTL. Although the resolution is lower, the geo‐ magnetic anomaly trend (Figure 3; [16]) supports a difference in upper crustal constituents

It is accepted that the MTL has been reactivated as a right-lateral fault since the late Neogene under the influence of the oblique subduction of the Philippine Sea Plate (e.g., [17]). Nakamura et al. [6] demonstrated that the oceanic plate shifted its convergent motion counterclockwise in the Quaternary, which resulted in vigorous slips on the MTL and westward transportation of the Outer Zone (e.g., [18]). However, when compared with the older stages, geomorpho‐ logical features (e.g., [19]) suggest that the active segment of the MTL shrank during the late Quaternary. No active portion is identified in the eastern part of the Kii Peninsula (Figure 3), in which the geomagnetic anomaly contrast is also obscured. This is in contradiction to the plate subduction regime, and further study of the transient shift of MTL activity is necessary to solve this tectonic paradox. Another noteworthy point is that the MTL trace is characterized by frequent jogs and steps. A sounding survey in the Kii Channel [20] delineated a complex

Subsurface structures delineated by reflection seismic data [21] suggest a different phase of the recent activities of the MTL. Figure 4 is a N-S (normal to the MTL) seismic profile of the northern bank of the River Kinokawa. Fault morphology is classified into high-angle flower structures, implying lateral motion, and north-dipping reverse faults, reflecting a complicated slip history. Amongst the structures, the most remarkable feature is the thrust at the bottom of the Cretaceous Izumi Group. Because it is underlain by recent sediments, a strong contrac‐

(Figure 2b).

along the same line as the density contrast.

*2.1.3. Episodic change of deformation mode*

fault pattern that may cause great diversity in basin formation.

tion episode in the Quaternary should be responsible for the structure.

*2.1.2. Neotectonic activity*

**Figure 1.** An index of the neotectonic regime in southwest Japan. Base map is after Huzita [2]. Gradation on backarc shelf showing onlapping sedimentation pattern and a seismic profile (shown inset) in the area are after Itoh et al. [5]. Influx of crystalline schist gravels is shown by green areas [22-24]. (Right) Bouguer gravity anomaly map. The Bouguer density is 2670 kg/m3, and contour interval is 10 mGal. Gravity map is generated based on [8]

#### *2.1.1. Initiation of the regional fault zone*

Compiling reliable paleomagnetic data, Itoh et al. [5] reconstructed the Cretaceous to early Paleogene paleogeography around the eastern Eurasian margin (Figure 2a). They pointed out that the MTL constituted a larger fault zone together with the Central Sikhote Alin Fault, and had a left-lateral slip sense as a result of the quite rapid northerly motion of the Izanagi Plate [10]. Along the fault zone, conspicuous pull-apart basins were developed and buried by the Cretaceous silici-clastic rocks of the Izumi Group [11], which was deposited in an elongate basin (300 km long by 10~20 km wide) along the MTL with sinistral strike-slip movements (Figure 2b).

Wrenching deformation associated with the ancient left slip on the MTL is identified in the Outer Zone of the Kii Peninsula (Figure 2c). Wang and Maekawa [12] showed that the metamorphic grade in the Sanbagawa belt have an en echelon anticlinal trend along the MTL. The trend is a result of deformation after high-pressure metamorphism, the climax of which is assigned around the middle of Cretaceous [13]. It is noted that the most intensive postmetamorphic deformation zone does not coincide with the geologically determined MTL that runs upon the northern bank of the River Kinokawa. Hirota [14] found a remarkable discon‐ tinuity in the metamorphic grade around the Funaokayama bar within the River Kinokawa (Figure 2c), and regarded it as a tectonic block. Takasu et al. [15] argued, on the basis of chronological data, that the amalgamation of metamorphosed blocks had occurred during the late Cretaceous. A steep gradient in the gravity anomaly along the river also implies a concealed structure parallel to the surface MTL. Although the resolution is lower, the geo‐ magnetic anomaly trend (Figure 3; [16]) supports a difference in upper crustal constituents along the same line as the density contrast.
