**4. Discussion**

#### **4.1. Comparison with Dickinson's forearc basin classification scheme**

Dickinson's simple classification scheme for forearc basin morphology [1] is based on the basin filling conditions and sectional basin configurations basically controlled by trench slope break height (Figure 15). Since the basin filling condition comprises two classes: underfilled and overfilled, and the sectional basin configuration comprises four classes: sloped, ridged/ terraced, ridged/shelved and ridged/benched, depending on the trench slope break height, forearc basins can be classified into eight different types in the Dickinson's classification scheme [1] (Figure 15). According to our analysis results, the Eocene Ishikari–Sanriku-oki forearc basins can be categorized into the "emergent ridged", "overfilled shelved" to "benched" types (Figure 15), as it is interpreted that the trench slope break was uplifted and emerged, and the major sedimentary environments were mostly near the sea level except partly developed braided rivers in an elevated setting. On the contrary, the Pleistocene Tokaioki–Kumano-nada forearc basins can be categorized into the "overfilled sloped", "underfilled submerged ridged" to "overfilled deep marine terraced" types (Figure 15), as the estimated trench slope break was submerged and low, and the major sedimentary environments were submarine fans and muddy slope to basin floor.

Variation in Forearc Basin Configuration and Basin-filling Depositional Systems… http://dx.doi.org/10.5772/56751 19

**Figure 15.** Dickinson's forearc basin classification chart on the basis of basin filling conditions and sectional basin con‐ figuration. Modified after [1]. TSB: trench slope break.

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

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 (Figures 16, 17).

#### *4.2.1. Trench slope break development*

**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].

Dickinson's simple classification scheme for forearc basin morphology [1] is based on the basin filling conditions and sectional basin configurations basically controlled by trench slope break height (Figure 15). Since the basin filling condition comprises two classes: underfilled and overfilled, and the sectional basin configuration comprises four classes: sloped, ridged/ terraced, ridged/shelved and ridged/benched, depending on the trench slope break height, forearc basins can be classified into eight different types in the Dickinson's classification scheme [1] (Figure 15). According to our analysis results, the Eocene Ishikari–Sanriku-oki forearc basins can be categorized into the "emergent ridged", "overfilled shelved" to "benched" types (Figure 15), as it is interpreted that the trench slope break was uplifted and emerged, and the major sedimentary environments were mostly near the sea level except partly developed braided rivers in an elevated setting. On the contrary, the Pleistocene Tokaioki–Kumano-nada forearc basins can be categorized into the "overfilled sloped", "underfilled submerged ridged" to "overfilled deep marine terraced" types (Figure 15), as the estimated trench slope break was submerged and low, and the major sedimentary environments were

**4.1. Comparison with Dickinson's forearc basin classification scheme**

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

submarine fans and muddy slope to basin floor.

**4. Discussion**

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 the basin filling depositional systems tend to be fluvial to bay systems if sediment supply is enough. Dickinson [1] suggests that the trench slope break development is strongly related to differences in plate subduction conditions such as accretional prism formation and tectonic erosion.

arrays around the subbasins, which indicate a strike-slip fault system consisting of main faults and splay faults, and set them in the model. When right-lateral motion occurred along the main faults, then the subbasins corresponding to the Sorachi and Yubari subbasins were properly simulated in the modeling [30]. This result suggests that the forearc basin segmentation was

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21

Consequently, strike-slip tectonics is also one of the crucial factors to determine basin config‐ uration and depositional system distributions in a forearc zone (Figures 16, 17). Figure 17 demonstrates schematic diagram showing type variations of forearc basins as functions of trench slope break development, arcward compression and strike-slip movement. In addition to the Dickinson's forearc basin classification scheme (Figure 15), this study delineates that both arcward compression and strike-slip movement are crucial factors in forearc basin classification. In case arcward compression is intense due to trench slope break evolution, a confined shrinking or trough-fill type forearc basin can be formed, as seen in Stages 2 and 3 in the Tokai-oki–Kumano-nada forearc basins (Figures 12, 13, 14). In case strike-slip movement is dominant, a segmented marine or non-marine forearc basins can be formed, as seen in the Sorachi and Yubari subbasins (Figures 5, 10). When strike-slip movement is intense, the forearc

caused by strike-slip tectonics along the forearc zone.

basin can be transformed into a fragmented strike-slip basin.

**Figure 16.** Controlling factors on variation in forearc basin configuration and depositional systems.

In addition to the height of a trench slope break, related arcward suppression accompanied with the trench slope break uplift is also regarded as an important factor to control basin deformation as seen in the Tokai-oki–Kumano-nada forearc basin (Figure 13).

## *4.2.2. Balance between basin accommodation and sediment supply*

Even in a fully uplifted trench slope break setting, a condition under minor sediment supply or relatively rapid subsidence causes a deeper marine forearc basin. The Ishikari–Sanriku-oki forearc basins maintained a balanced condition between the amount of sediment supply and the basin accommodation space, causing a thick accumulation of fluvial to bay sediments. Accordingly, it is suggested that the balance between sediment supply and forearc basin accommodation created by a trench slope break barrier and basin subsidence [28] (total subsidence) can be a crucial controlling factor not only on the forearc basin filling conditions such as underfilled and overfilled conditions but also on the variation of depositional systems. Dickinson [1] suggests that the underfilled types mostly occur along an island arc with a small amount of sediment supply, whereas the overfilled types mostly occur along a continental arc with a large amount of sediment supply.

## *4.2.3. Strike-slip movement related to basin segmentation*

Our examination results suggest that a forearc zone is commonly segmented into subbasins. The Ishikari–Sanriku-oki forearc basins were segmented into 50 to 150 km long subbasins aligned along the forearc extension (Figure 2B). The Tokai-oki–Kumano-nada forearc basins are also possible to have been segmented as suggested by Sasaki et al. [29] and as seen in the facies maps (Figure 12), in which the sedimentary packages tend to be segmented into the Tokai-oki, Atsumi-oki and Kumano-nada possible subbasins. As described above, the segmented subbasins show a different subsidence pattern and sediment thickness for each subbasin (Figures 7, 13), and differential subsidence within a subbasin is characteristically observed (Figures 6, 7).

As a possible formation mechanism of forearc segmentation, Dickinson [1] suggests strike slip tectonics along a forearc zone, induced by oblique plate subduction underneath a forearc zone. As many of plate subduction direction at the convergent margin tend to be not complete normal direction to the subduction trench, oblique plate subduction is quite common. The oblique subduction may induce a strike-slip motion of forearc sliver and basin segmentation as seen in the Sumatra forearc and Aleutian forearc [1].

To examine the effect of strike-slip motion on the forearc basin segmentation, Kusumoto et al. [30] conducted dislocation modeling for basin segmentation, using the Sorachi and Yubari subbasins as examples. Dislocation modeling is to simulate basin dislocation by fault move‐ ment with the assumption of a homogeneous elastic body. Kusumoto et al. [30] picked up fault arrays around the subbasins, which indicate a strike-slip fault system consisting of main faults and splay faults, and set them in the model. When right-lateral motion occurred along the main faults, then the subbasins corresponding to the Sorachi and Yubari subbasins were properly simulated in the modeling [30]. This result suggests that the forearc basin segmentation was caused by strike-slip tectonics along the forearc zone.

the basin filling depositional systems tend to be fluvial to bay systems if sediment supply is enough. Dickinson [1] suggests that the trench slope break development is strongly related to differences in plate subduction conditions such as accretional prism formation and tectonic

In addition to the height of a trench slope break, related arcward suppression accompanied with the trench slope break uplift is also regarded as an important factor to control basin

Even in a fully uplifted trench slope break setting, a condition under minor sediment supply or relatively rapid subsidence causes a deeper marine forearc basin. The Ishikari–Sanriku-oki forearc basins maintained a balanced condition between the amount of sediment supply and the basin accommodation space, causing a thick accumulation of fluvial to bay sediments. Accordingly, it is suggested that the balance between sediment supply and forearc basin accommodation created by a trench slope break barrier and basin subsidence [28] (total subsidence) can be a crucial controlling factor not only on the forearc basin filling conditions such as underfilled and overfilled conditions but also on the variation of depositional systems. Dickinson [1] suggests that the underfilled types mostly occur along an island arc with a small amount of sediment supply, whereas the overfilled types mostly occur along a continental arc

Our examination results suggest that a forearc zone is commonly segmented into subbasins. The Ishikari–Sanriku-oki forearc basins were segmented into 50 to 150 km long subbasins aligned along the forearc extension (Figure 2B). The Tokai-oki–Kumano-nada forearc basins are also possible to have been segmented as suggested by Sasaki et al. [29] and as seen in the facies maps (Figure 12), in which the sedimentary packages tend to be segmented into the Tokai-oki, Atsumi-oki and Kumano-nada possible subbasins. As described above, the segmented subbasins show a different subsidence pattern and sediment thickness for each subbasin (Figures 7, 13), and differential subsidence within a subbasin is characteristically

As a possible formation mechanism of forearc segmentation, Dickinson [1] suggests strike slip tectonics along a forearc zone, induced by oblique plate subduction underneath a forearc zone. As many of plate subduction direction at the convergent margin tend to be not complete normal direction to the subduction trench, oblique plate subduction is quite common. The oblique subduction may induce a strike-slip motion of forearc sliver and basin segmentation

To examine the effect of strike-slip motion on the forearc basin segmentation, Kusumoto et al. [30] conducted dislocation modeling for basin segmentation, using the Sorachi and Yubari subbasins as examples. Dislocation modeling is to simulate basin dislocation by fault move‐ ment with the assumption of a homogeneous elastic body. Kusumoto et al. [30] picked up fault

deformation as seen in the Tokai-oki–Kumano-nada forearc basin (Figure 13).

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

*4.2.2. Balance between basin accommodation and sediment supply*

with a large amount of sediment supply.

observed (Figures 6, 7).

*4.2.3. Strike-slip movement related to basin segmentation*

as seen in the Sumatra forearc and Aleutian forearc [1].

erosion.

Consequently, strike-slip tectonics is also one of the crucial factors to determine basin config‐ uration and depositional system distributions in a forearc zone (Figures 16, 17). Figure 17 demonstrates schematic diagram showing type variations of forearc basins as functions of trench slope break development, arcward compression and strike-slip movement. In addition to the Dickinson's forearc basin classification scheme (Figure 15), this study delineates that both arcward compression and strike-slip movement are crucial factors in forearc basin classification. In case arcward compression is intense due to trench slope break evolution, a confined shrinking or trough-fill type forearc basin can be formed, as seen in Stages 2 and 3 in the Tokai-oki–Kumano-nada forearc basins (Figures 12, 13, 14). In case strike-slip movement is dominant, a segmented marine or non-marine forearc basins can be formed, as seen in the Sorachi and Yubari subbasins (Figures 5, 10). When strike-slip movement is intense, the forearc basin can be transformed into a fragmented strike-slip basin.

**Figure 16.** Controlling factors on variation in forearc basin configuration and depositional systems.

**Acknowledgements**

**Author details**

Osamu Takano1

**References**

52-68.

(in Japanese)

The authors are grateful to Drs. Ray Ingersoll, Cathy Busby and Paul Heller for useful suggestions on the tectonics and sedimentation of forearc basins. JAPEX, JX, JOGMEC, METI and MH21 Research Consortium kindly provided permission for data publication. This study

Variation in Forearc Basin Configuration and Basin-filling Depositional Systems…

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23

and Shigekazu Kusumoto3

3 Graduate School of Science and Technology for Research, University of Toyama, Japan

[1] Dickinson WR. Forearc basins. In: Busby C, Ingersoll RV. (eds.) Tectonics of Sedi‐

[2] Dickinson WR, Seely DR. Structure and stratigraphy of forearc regions. American

[3] Kiminami K, Kontani Y. Mesozoic arc–trench systems in Hokkaido, Japan. In: Hashi‐ moto M, Uyeda S. (eds.) Accretion Tectonics in the Circum–Pacific Regions. Tokyo:

[4] Kimura G. Cretaceous episodic growth of the Japanese Islands. Island Arc 1997; 6:

[5] Kiminami K. Cretaceous to Paleogene convergent margin. In: Niida K, Arita K, Kato M. (eds.) Regional Geology of Japan 1. Hokkaido. Tokyo: Asakura; 2010. p526-528.

[6] Ando H. Stratigraphic correlation of Upper Cretaceous to Paleocene forearc basin sediments in Northeast Japan: cyclic sedimentation and basin evolution. Journal of

was partly conducted under the MH21 Research Consortium.

\*Address all correspondence to: osamu.takano@japex.co.jp

1 JAPEX Research Center, Japan Petroleum Exploration, Japan

2 Graduate School of Science, Osaka Prefecture University, Japan

mentary Basins. Oxford: Blackwell; 1995. p221-261.

Terra Publication; 1983. P107-122.

Asian Earth Sciences 2003; 21: 919-933.

Association of Petroleum Geologists Bulletin 1979; 63: 2-31.

, Yasuto Itoh2

**Figure 17.** Schematic diagram showing type variations of forearc basins as functions of trench slope break develop‐ ment, arcward compression and strike-slip movement. Arrow direction denotes intensity of each factor.
