*2.1.3. Episodic change of deformation mode*

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

**Figure 2.** Incipient activity of the Median Tectonic Line (MTL). (a) Paleoreconstruction of the eastern Eurasian margin in the Cretaceous and early Paleogene stage [5]. (b) Distribution of the Cretaceous Izumi Group deposited in a series of pull-apart basins [11]. (c) Geologic features showing sinistral motion of the Median Tectonic Line in the Kii Peninsu‐ la. Metamorphic grade in the Sanbagawa belt is after Wang and Maekawa [12]. A star shows the Funaokayama bar in the River Kinokawa, where a remarkable gap in metamorphic grade was confirmed [14]. Mapped areas are shown in Figure 1

Provenance studies of the recent clastics on the northern flank of the MTL support the theory of a strong contraction phase in the Kii Peninsula. In some areas of the Pleistocene exposure, the frequent influx of schist gravels, apparently derived from the Sanbagawa belt in the Outer Zone (Figure 1), has been confirmed by many researchers (e.g., [22-24]). In contrast, Pleistocene sediments around Osaka Bay are lacking in such components in spite of the fact that the aforementioned metamorphic unit is widely distributed in the Kii Peninsula. The authors submit a hypothesis that the strong contraction phase provoked an inversion of the Cretaceous Izumi sedimentary basin along the MTL trace, and an E-W barrier (Izumi Mountains) pre‐ vented northward sediment transport through the late Quaternary.

Visualization of subsurface structures in the forearc region implies that the contraction event has a broader impact upon the basin formation/deformation processes of southwest Japan. Takano et al. [25] stated that the Kumano-nada basin (Figure 1) suffered from an episode of contraction around the early Pleistocene, which became dormant in later periods. Thus the

**Figure 4.** Geologic interpretation of a N-S depth-converted seismic profile across the Median Tectonic Line in the Kii

Peninsula. Location of seismic line is shown in Figure 2. Original seismic data is after Yoshikawa et al. [21]

**Figure 3.** Recent active trace (red line) of the Median Tectonic Line around the Kii Peninsula, compiled after Yoshika‐ wa et al. [20,21] and an active fault database [19]. See Figure 1 for mapped area. Base maps in upper and lower

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frames show geomagnetic anomaly [16] and geology [9], respectively

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 195

**Figure 3.** Recent active trace (red line) of the Median Tectonic Line around the Kii Peninsula, compiled after Yoshika‐ wa et al. [20,21] and an active fault database [19]. See Figure 1 for mapped area. Base maps in upper and lower frames show geomagnetic anomaly [16] and geology [9], respectively

**Figure 4.** Geologic interpretation of a N-S depth-converted seismic profile across the Median Tectonic Line in the Kii Peninsula. Location of seismic line is shown in Figure 2. Original seismic data is after Yoshikawa et al. [21]

Provenance studies of the recent clastics on the northern flank of the MTL support the theory of a strong contraction phase in the Kii Peninsula. In some areas of the Pleistocene exposure, the frequent influx of schist gravels, apparently derived from the Sanbagawa belt in the Outer Zone (Figure 1), has been confirmed by many researchers (e.g., [22-24]). In contrast, Pleistocene sediments around Osaka Bay are lacking in such components in spite of the fact that the aforementioned metamorphic unit is widely distributed in the Kii Peninsula. The authors submit a hypothesis that the strong contraction phase provoked an inversion of the Cretaceous Izumi sedimentary basin along the MTL trace, and an E-W barrier (Izumi Mountains) pre‐

**Figure 2.** Incipient activity of the Median Tectonic Line (MTL). (a) Paleoreconstruction of the eastern Eurasian margin in the Cretaceous and early Paleogene stage [5]. (b) Distribution of the Cretaceous Izumi Group deposited in a series of pull-apart basins [11]. (c) Geologic features showing sinistral motion of the Median Tectonic Line in the Kii Peninsu‐ la. Metamorphic grade in the Sanbagawa belt is after Wang and Maekawa [12]. A star shows the Funaokayama bar in the River Kinokawa, where a remarkable gap in metamorphic grade was confirmed [14]. Mapped areas are shown in

vented northward sediment transport through the late Quaternary.

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

Figure 1

Visualization of subsurface structures in the forearc region implies that the contraction event has a broader impact upon the basin formation/deformation processes of southwest Japan. Takano et al. [25] stated that the Kumano-nada basin (Figure 1) suffered from an episode of contraction around the early Pleistocene, which became dormant in later periods. Thus the MTL seems to have a much more complicated activation history than the common theory would suggest. This history may be a key to reconstructing the motion of the Philippine Sea Plate, which is not determined from global plate kinematics.

Basin morphology along the MTL shows a spatial variation under the Quaternary transient tectonic stress. Figure 5 presents a plan view of an active trace of the westernmost part of the MTL (upper) and the deep structure of recent sedimentary basins developed along the active segment, which is interpreted from reflection seismic data [26] (lower). It is obvious that the active MTL has a releasing bend around the Beppu Bay where countless secondary tensile faults develop. The volume of the pull-apart basin is estimated based on gravity data in this book (Itoh, Y., Kusumoto, S. and Takemura, K.). Deep structures interpreted from two seismic profiles with no vertical exaggeration indicate the following characteristics. (1) The youngest structural trend is a bunch of high-angle faults (with a so-called 'flower structure') implying strike-slip motion on the MTL fault system. (2) Temporal transition of the active fault trace is inferred from the migration of depocenters of the sedimentary basins. (3) Low-angle detach‐ ment in the acoustic basement, which was regarded as a material boundary in the upper crust [26], is clearly reactivated in an extensional sense as shown by the dragging deformation of the adjacent Plio-Pleistocene sediments.

A previous study [27] attributed the along-arc difference in deformation style (east, contrac‐ tion; west, extension) to counterclockwise rotation of the forearc sliver in response to the relative motion of the Philippine Sea and Pacific Plates, and the backarc spreading of the Okinawa Trough. Further quantitative investigation of the three-dimensional structure of the island arc crust is necessary for constructing a probable tectonic model.

## **2.2. Niigata-Kobe Tectonic Zone (NKTZ)**

Based on geodetic analyses, Sagiya et al. [28] described a NE-SW zone of deformation in southwest Japan (Figure 1), and named it as the Niigata-Kobe Tectonic Zone (NKTZ). It is characterized by right-lateral shear deformation [29], and obliquely crosses over the Itoigawa-Shizuoka Tectonic Line (ISTL; Figure 1) with pure reverse motions. As shown by Nakajima and Hasegawa [30], the NKTZ is a deeply rooted crustal weakness accompanied by a P-wave velocity anomaly in the mid-crust. Iio et al. [31] argued that the high water content of the lower crust, linked to dehydration of the subducting slab, is responsible for the formation of such a weak zone.

Paleomagnetic studies have shown that the NKTZ is not a short-lived feature but contributes to cumulative deformation of the island arc. Itoh et al. [32] compiled reliable paleomagnetic data around the eastern part of southwest Japan, and confirmed significant clockwise rotation on the NKTZ during the Quaternary. They pointed out that similar rotational events were identified on both flanks of the ISTL, and stated that the two crossing tectonic zones with different deformation senses may be alternately activated in response to fluctuation of the regional tectonic stress, which is a theory to comprehend the paradox of a geophysicallyassessed low activity level of the ISTL showing geologic significance.

**2.3. Echizen-Shima Tectonic Line (ESTL)**

ter Yusa et al. [26]

Huzita [33] stated that a triangular portion in the central Kinki district is characterized by intensive deformation and basin formation, and named it the 'Kinki Triangle'. The southern border of this tectonic area coincides with the MTL, and its western border roughly corre‐ sponds to the NKTZ. The tectonic context of the eastern border, however, has not been clearly discussed. Here, the authors attempt to redefine a tectonic line from the viewpoint of consis‐

**Figure 5.** Upper: Westernmost part of active trace of the Median Tectonic Line after [9] and [27] with seismic line map [26]. Lower: Reinterpreted depth-converted seismic profiles without vertical exaggeration. Original seismic data is af‐

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Motion of Crustal Blocks http://dx.doi.org/10.5772/56588 197

tency in deformation trend including the forearc and backarc regions.

**Figure 5.** Upper: Westernmost part of active trace of the Median Tectonic Line after [9] and [27] with seismic line map [26]. Lower: Reinterpreted depth-converted seismic profiles without vertical exaggeration. Original seismic data is af‐ ter Yusa et al. [26]

## **2.3. Echizen-Shima Tectonic Line (ESTL)**

MTL seems to have a much more complicated activation history than the common theory would suggest. This history may be a key to reconstructing the motion of the Philippine Sea

Basin morphology along the MTL shows a spatial variation under the Quaternary transient tectonic stress. Figure 5 presents a plan view of an active trace of the westernmost part of the MTL (upper) and the deep structure of recent sedimentary basins developed along the active segment, which is interpreted from reflection seismic data [26] (lower). It is obvious that the active MTL has a releasing bend around the Beppu Bay where countless secondary tensile faults develop. The volume of the pull-apart basin is estimated based on gravity data in this book (Itoh, Y., Kusumoto, S. and Takemura, K.). Deep structures interpreted from two seismic profiles with no vertical exaggeration indicate the following characteristics. (1) The youngest structural trend is a bunch of high-angle faults (with a so-called 'flower structure') implying strike-slip motion on the MTL fault system. (2) Temporal transition of the active fault trace is inferred from the migration of depocenters of the sedimentary basins. (3) Low-angle detach‐ ment in the acoustic basement, which was regarded as a material boundary in the upper crust [26], is clearly reactivated in an extensional sense as shown by the dragging deformation of

A previous study [27] attributed the along-arc difference in deformation style (east, contrac‐ tion; west, extension) to counterclockwise rotation of the forearc sliver in response to the relative motion of the Philippine Sea and Pacific Plates, and the backarc spreading of the Okinawa Trough. Further quantitative investigation of the three-dimensional structure of the

Based on geodetic analyses, Sagiya et al. [28] described a NE-SW zone of deformation in southwest Japan (Figure 1), and named it as the Niigata-Kobe Tectonic Zone (NKTZ). It is characterized by right-lateral shear deformation [29], and obliquely crosses over the Itoigawa-Shizuoka Tectonic Line (ISTL; Figure 1) with pure reverse motions. As shown by Nakajima and Hasegawa [30], the NKTZ is a deeply rooted crustal weakness accompanied by a P-wave velocity anomaly in the mid-crust. Iio et al. [31] argued that the high water content of the lower crust, linked to dehydration of the subducting slab, is responsible for the formation of such a

Paleomagnetic studies have shown that the NKTZ is not a short-lived feature but contributes to cumulative deformation of the island arc. Itoh et al. [32] compiled reliable paleomagnetic data around the eastern part of southwest Japan, and confirmed significant clockwise rotation on the NKTZ during the Quaternary. They pointed out that similar rotational events were identified on both flanks of the ISTL, and stated that the two crossing tectonic zones with different deformation senses may be alternately activated in response to fluctuation of the regional tectonic stress, which is a theory to comprehend the paradox of a geophysically-

island arc crust is necessary for constructing a probable tectonic model.

assessed low activity level of the ISTL showing geologic significance.

Plate, which is not determined from global plate kinematics.

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

the adjacent Plio-Pleistocene sediments.

**2.2. Niigata-Kobe Tectonic Zone (NKTZ)**

weak zone.

Huzita [33] stated that a triangular portion in the central Kinki district is characterized by intensive deformation and basin formation, and named it the 'Kinki Triangle'. The southern border of this tectonic area coincides with the MTL, and its western border roughly corre‐ sponds to the NKTZ. The tectonic context of the eastern border, however, has not been clearly discussed. Here, the authors attempt to redefine a tectonic line from the viewpoint of consis‐ tency in deformation trend including the forearc and backarc regions.

Itoh et al. [5] described the geologic structures of the backarc of southwest Japan. A sediment onlapping pattern depicts an inversion trend nearly normal to the elongation of the arc (Figure 1). They showed a seismic profile suggesting that the inversion developed from the Pliocene to Pleistocene. On the forearc side, the Shima Spur built up in early Quaternary [25]. These structural trends are linearly connected with onshore active faults, and constitute a regional zone of contraction. We regard it as a significant neotectonic boundary and name it the Echizen-Shima Tectonic Line (ESTL). At present, the origin of the ESTL is not fully understood. It probably has a close relation with the Miocene bending event in southwest Japan caused by the collision of the Izu-Bonin arc, the reason for this theory being that paleomagnetic studies [34,35] clarified that the hinge line of arc bending was located around the ESTL.

**4. Discussion**

following (e.g., [38]).

respectively.

low reaching -60 mGal.

**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

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

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

> D =D - - *gg G D BF w* 2pr

dence structures forming the negative anomalies would be due to intra-arc basins.

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‐

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,

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

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.

> ( ) ( ) 2 2 *gxy gxy* , , *x y* é ùé ù ¶ ¶ ê úê ú <sup>+</sup> ¶ ¶ ë ûë û

 r

( ) (1)

.

Motion of Crustal Blocks http://dx.doi.org/10.5772/56588 199

(2)
