**3. Pleistocene Tokai-oki–Kumano-nada forearc basins**

## **3.1. Geologic setting and stratigraphic framework**

The Pleistocene Tokai-oki–Kumano-nada forearc basins were developed in the forearc zone between the SW Japan Arc and the Nankai Trough subduction zone (Figures 2A, 2C). On the contrary to the sporadic developments of forearc basins during the late Paleogene and early Neogene time, thick sedimentary packages of the Late Pliocene to Pleistocene Tokai-oki– Kumano-nada forearc basins widely developed in this forearc zone. This section picks up the major basin-filling sediments equivalent to the Late Pliocene to Early Pleistocene Kakegawa Group (Atsumi-oki Group [21]) and Middle Pleistocene Ogasa Group (Hamamatsu-oki Group [21]; Figure 11) to examine the basin filling conditions and basin configurations. The Kakegawa Group unconformably overlies the underlying units with a certain time gap, indicating the different phase of forearc basin tectonics, and the Ogasa Group unconformably overlies the Kakegawa Group, indicating a tectonic event between depositions of the two groups. The study area is set between the Present continental slope toe and the trench slope break zone, which covers the Tokai-oki, Atsumi-oki and Kumano-nada areas (Figure 2C). From the standpoints of sequence stratigraphy and sedimentology, the targeted forearc sediments are divided into seventeen depositional sequences: Sequence Kg-a to -h and Og-a to –i, based on reflection termination patterns on the seismic sections and facies succession patterns on the well successions [22, 23](Figure 11). The major depositional system of the whole interval is a submarine fan turbidite system [22, 23].

Islands functioned as fixed feeder systems, along which submarine fans were formed in the forearc basins (Figure 12). These facies maps also suggest that submarine-fan architecture was intermittently transformed through time (Figure 12)[22, 26]; from a braided channel-dominat‐ ed condition (Stage 1 represented by the map of Sequence Kg-a), through a small fan-domi‐ nated condition with shrinking separated small basins (Stage 2 represented by the map of Sequence Kg-e), and a trough-fill turbidite-dominated condition (Stage 3 represented by the map of Sequence Og-e), to a channel-levee system-dominated condition (Stage 4 represented by the map of Sequence Og-h). Although the submarine-fan architecture was transformed temporary, some spatial differences in depositional patterns between the Tokai-oki, Atsumioki and Kumano-nada areas can also be recognized (Figure 12), possibly resulting from forearc

**Figure 11.** Litho- and sequence stratigraphic framework of the latest Pliocene to Pleistocene successions in the Tokai-

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To examine the relationships between the changes in the submarine-fan depositional styles and basin configuration, which may be indicating the background tectonics, we investigated seismic sections transecting the Tokai-oki–Kumano-nada forearc basins. Figure 13 depicts the interpreted cross sections with depositional stage division characterized by the different submarine-fan depositional styles as mentioned above. The interrelationships between the geologic structures and sediment thickness change shown on these cross sections reveal that the depositional stage division can be connected with tectonic phases that created specific

basin segmentation and sediment supply variation.

oki–Kumano-nada forearc basins. Modified after [22, 23].

**3.3. Transformation of basin configuration and background tectonics**

## **3.2. Transformation of depositional styles**

Takano et al. [22] demonstrated a series of facies maps in the Tokai-oki–Kumano-nada forearc basins for each depositional sequence unit for the interval equivalent to the Kakegawa and Ogasa Groups (Figure 12). These facies maps were created on the basis of seismic facies information plotted on the seismic survey line maps as well as some exploration well data (Figure 2C) [22–25]. These facies maps clearly show the depositional patterns of submarine fans, indicating that quite a few numbers of submarine canyons from the main land of Japanese


Inside the forearc basin, major depositional systems were bay to fluvial systems without any wave-influenced facies. In response to relative sea level changes, transgression and regression repeated, and the major depositional system alternated between a fluvial system-dominated condition and a bay system-dominated condition. Because of the existence of marine sedi‐ ments, it is estimated that there were an inlet interconnecting between the open sea and the inside of the forearc basin, through which the seawater came into the inside of the forearc basin. Our forearc setting model also demonstrates forearc basin segmentation, reflecting the fact that the Eocene Ishikari–Sanriku-oki forearc basins were segmented into 50 to 150 km long subbasins aligned along the forearc extension (Figure 2B). As described above, the segmented subbasins show a different subsidence pattern and different sediment thickness for each

The Pleistocene Tokai-oki–Kumano-nada forearc basins were developed in the forearc zone between the SW Japan Arc and the Nankai Trough subduction zone (Figures 2A, 2C). On the contrary to the sporadic developments of forearc basins during the late Paleogene and early Neogene time, thick sedimentary packages of the Late Pliocene to Pleistocene Tokai-oki– Kumano-nada forearc basins widely developed in this forearc zone. This section picks up the major basin-filling sediments equivalent to the Late Pliocene to Early Pleistocene Kakegawa Group (Atsumi-oki Group [21]) and Middle Pleistocene Ogasa Group (Hamamatsu-oki Group [21]; Figure 11) to examine the basin filling conditions and basin configurations. The Kakegawa Group unconformably overlies the underlying units with a certain time gap, indicating the different phase of forearc basin tectonics, and the Ogasa Group unconformably overlies the Kakegawa Group, indicating a tectonic event between depositions of the two groups. The study area is set between the Present continental slope toe and the trench slope break zone, which covers the Tokai-oki, Atsumi-oki and Kumano-nada areas (Figure 2C). From the standpoints of sequence stratigraphy and sedimentology, the targeted forearc sediments are divided into seventeen depositional sequences: Sequence Kg-a to -h and Og-a to –i, based on reflection termination patterns on the seismic sections and facies succession patterns on the well successions [22, 23](Figure 11). The major depositional system of the whole interval is a

Takano et al. [22] demonstrated a series of facies maps in the Tokai-oki–Kumano-nada forearc basins for each depositional sequence unit for the interval equivalent to the Kakegawa and Ogasa Groups (Figure 12). These facies maps were created on the basis of seismic facies information plotted on the seismic survey line maps as well as some exploration well data (Figure 2C) [22–25]. These facies maps clearly show the depositional patterns of submarine fans, indicating that quite a few numbers of submarine canyons from the main land of Japanese

**3. Pleistocene Tokai-oki–Kumano-nada forearc basins**

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

**3.1. Geologic setting and stratigraphic framework**

submarine fan turbidite system [22, 23].

**3.2. Transformation of depositional styles**

subbasin.

**Figure 11.** Litho- and sequence stratigraphic framework of the latest Pliocene to Pleistocene successions in the Tokaioki–Kumano-nada forearc basins. Modified after [22, 23].

Islands functioned as fixed feeder systems, along which submarine fans were formed in the forearc basins (Figure 12). These facies maps also suggest that submarine-fan architecture was intermittently transformed through time (Figure 12)[22, 26]; from a braided channel-dominat‐ ed condition (Stage 1 represented by the map of Sequence Kg-a), through a small fan-domi‐ nated condition with shrinking separated small basins (Stage 2 represented by the map of Sequence Kg-e), and a trough-fill turbidite-dominated condition (Stage 3 represented by the map of Sequence Og-e), to a channel-levee system-dominated condition (Stage 4 represented by the map of Sequence Og-h). Although the submarine-fan architecture was transformed temporary, some spatial differences in depositional patterns between the Tokai-oki, Atsumioki and Kumano-nada areas can also be recognized (Figure 12), possibly resulting from forearc basin segmentation and sediment supply variation.

## **3.3. Transformation of basin configuration and background tectonics**

To examine the relationships between the changes in the submarine-fan depositional styles and basin configuration, which may be indicating the background tectonics, we investigated seismic sections transecting the Tokai-oki–Kumano-nada forearc basins. Figure 13 depicts the interpreted cross sections with depositional stage division characterized by the different submarine-fan depositional styles as mentioned above. The interrelationships between the geologic structures and sediment thickness change shown on these cross sections reveal that the depositional stage division can be connected with tectonic phases that created specific

geologic structures related to basin configuration (Figures 13, 14). Since the Stage 1 sediments show a mostly uniform thickness and a braided channel-dominated condition, the forearc basin during Stage 1 (Late Pliocene to earliest Pleistocene) is interpreted to have been a gently inclined, sloped basin without major topographic undulation, which is characteristic of an incipient phase of forearc basin development [27]. Stage 2 (Early Pleistocene) is interpreted as a compressional stress stage with trench slope break uplift, since only limited synclinal areas contain thick sediments, and the depositional areas shrunk continuously. Stage 3 (Middle Pleistocene) can be a relaxing phase, which induced subsidence of folded forearc basins, since the sedimentation is characterized by trough-fill (syncline-fill) turbidite systems and the depositional territory became wider gradually. Stage 4 (Middle to Late Pleistocene) can be a compressional stress stage again, as trench slope break prominently uplifted as shown on the

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Consequently, it is suggested that during the Pleistocene time, two compressional phases occurred in response to trench slope break uplift and arcward suppression, and the forearc

**Figure 13.** Cross sections based on the interpreted seismic sections transecting the Tokai-oki–Kumano-nada forearc basins, showing the basin deformation and background tectonics during Stages 2, 3 and 4. Traced lines on the cross sections denote sequence boundary (SB) horizons corresponding to those of Figure 14 in line colors. The section loca‐ tions (seismic survey lines) are shown on the maps in Figure 12. Seismic sections were acquired in a MITI (Ministry of International Trade and Industry of Japan) survey [24]. Large red arrows denote compression and uplift during Stage

2, subsidence during Stage 3 and uplift during Stage 4.

depositional styles were strongly controlled by these tectonic events.

section B–B' in Figure 13.

**Figure 12.** Facies maps of Sequence Kg-a, -e, Og-e and -h in the Tokai-oki–Kumano-nada forearc basins, showing the transformation of submarine-fan morphology and distributions. Modified after [22]. The mapping area is shown in Figure 2C.

geologic structures related to basin configuration (Figures 13, 14). Since the Stage 1 sediments show a mostly uniform thickness and a braided channel-dominated condition, the forearc basin during Stage 1 (Late Pliocene to earliest Pleistocene) is interpreted to have been a gently inclined, sloped basin without major topographic undulation, which is characteristic of an incipient phase of forearc basin development [27]. Stage 2 (Early Pleistocene) is interpreted as a compressional stress stage with trench slope break uplift, since only limited synclinal areas contain thick sediments, and the depositional areas shrunk continuously. Stage 3 (Middle Pleistocene) can be a relaxing phase, which induced subsidence of folded forearc basins, since the sedimentation is characterized by trough-fill (syncline-fill) turbidite systems and the depositional territory became wider gradually. Stage 4 (Middle to Late Pleistocene) can be a compressional stress stage again, as trench slope break prominently uplifted as shown on the section B–B' in Figure 13.

Consequently, it is suggested that during the Pleistocene time, two compressional phases occurred in response to trench slope break uplift and arcward suppression, and the forearc depositional styles were strongly controlled by these tectonic events.

**Figure 13.** Cross sections based on the interpreted seismic sections transecting the Tokai-oki–Kumano-nada forearc basins, showing the basin deformation and background tectonics during Stages 2, 3 and 4. Traced lines on the cross sections denote sequence boundary (SB) horizons corresponding to those of Figure 14 in line colors. The section loca‐ tions (seismic survey lines) are shown on the maps in Figure 12. Seismic sections were acquired in a MITI (Ministry of International Trade and Industry of Japan) survey [24]. Large red arrows denote compression and uplift during Stage 2, subsidence during Stage 3 and uplift during Stage 4.

**Figure 12.** Facies maps of Sequence Kg-a, -e, Og-e and -h in the Tokai-oki–Kumano-nada forearc basins, showing the transformation of submarine-fan morphology and distributions. Modified after [22]. The mapping area is shown in

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

Figure 2C.

**Figure 14.** Generalized summary chart showing the transformation of the tectono-sedimentary conditions and sub‐ marine-fan types of the Pleistocene Tokai-oki–Kumano-nada forearc basin fill. Compiled and modified after [23, 26].

**Figure 15.** Dickinson's forearc basin classification chart on the basis of basin filling conditions and sectional basin con‐

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This section attempts to discuss major controlling factors on the variation in forearc basin configurations and depositional systems on the basis of the results of the examinations above

Trench slope break is a topographic high bounding the forearc basin to a trench slope steeply dipping to the subduction zone (Figure 1). As the Dickinson's forearc basin classification places great importance [1] (Figure 15), the results of our examination also indicate that the devel‐ opment condition of a trench slope break is the most principal factor to control the forearc basin configurations and basin filling depositional systems. In case the trench slope break development is minor or moderate as seen in the Tokai-oki–Kumano-nada forearc basins, the trench slope break ridge is submerged, and the basin filling sediments tend to be deeper marine shales or turbidites. On the other hand, in case the trench slope break prominently develops as seen in the Ishikari–Sanriku-oki forearc basins, the trench slope break ridge is emerged, and

figuration. Modified after [1]. TSB: trench slope break.

*4.2.1. Trench slope break development*

(Figures 16, 17).

**4.2. Controlling factors on the variation in forearc basin styles**
