**2.1. Geophysical background**

structure of the Takayama basin, and changes in the declination of the thermoremanent magnetization, can be restored by considering the cumulative activity of two right-lateral faults (the Enako Fault and the Makigahora Fault) and a reverse fault (the Harayama Fault). In addition, they showed that the reverse fault can be divided into two segments that moved independently with a time lag between their active periods. Because this modeling technique can take into account the type and amount of fault motion, we can reflect the history of fault activity based on certain geological evidence, including paleomagnetic studies, in the numer‐

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

This modeling technique has also been applied to the formation of pull-apart basins located in Hokkaido by Itoh et al. [37] and Tamaki et al. [38]. Tamaki et al. [38] found that a strike-slip fault motion reaching 30 km, is required to restore the distribution and volume of the Minami-

Here, we describe the disadvantages of the dislocation modeling defined using elasticity, and the solutions for the dislocation plane are given as a range of the linear elasticity. Since fracture, flow and other non-linear phenomena seen in the general solid material are not considered in the model, it is difficult to directly compare the amount of displacement between the modeled structure and the actual structure in long-time scale modeling. In general, dislocation modeling (including visco-elasticity effects) is often employed in discussions on crustal movements over a long time-scale such as on a geological time scale (e.g., [18, 39, 40]). In addition, Finite Element Modeling (FEM), Finite Difference Modeling (FDM) and Discrete Element Modeling (DEM) can simulate the formation processes of sedimentary basins or the building processes of mountains over a geological time-scale and can quantitatively discuss the mechanisms of their

As mentioned above, dislocation modeling defined using a range of the linear elasticity has disadvantages in the ability to directly compare the amount of displacement between the modeled structures and the actual structures. However, when we simply discuss the essential aspects of tectonics from the distribution pattern of structures caused by fault motions, dislocation modeling is a very useful tool because it provides the pattern of displacement using

The aims of this study are to simplify the complex formation processes of sedimentary basins through numerical simulations and to show that the simplification enables us to estimate deductively which processes cause materialization. Central Hokkaido was selected as the field in which to achieve these aims, as many sedimentary basins are distributed in this area.

As will be described later in the paper, many sedimentary basins were formed from 48 to 12 Ma in central Hokkaido, and it is difficult to discuss their formation processes using only observational data because of their complex distribution both at and below the surface. In order to simplify the complex formation processes of sedimentary basins, we attempted to restore sedimentary basins using the advanced technique already mentioned, and we evaluated the

fault type and the amount of movement required to form these sedimentary basins.

ical modeling, and can discuss the tectonics in detail.

Naganuma Basin located in southern central Hokkaido.

formation on a time axis (e.g., [41-43]).

easy calculations.

**1.3. Aims of this study**

Hokkaido is located on the North American plate, at a junction of the Northeast Japan arc and the Kurile arc (Figure 3). Using recent GPS observations, an east-west compressive strain field has been observed in the northern part of Hokkaido, and this strain field is considered to be caused by the convergence of the Eurasia plate with the northern part of Hokkaido functioning as a part of the plate boundary (e.g., [44]).

**Figure 3.** Location map of our study area. Hokkaido is located on the North American plate, at a junction of the North‐ eastern Japan arc and the Kurile arc. Gray dashed line indicates the old plate boundary between the Eurasian and North American Plates. Rectangular area by gray thin line indicates the study area of Itoh and Tsuru [58].

Although the present plate boundary between the North American plate and the Eurasian plate exists in the Sea of Japan, it is known that the plate boundary was located in central Hokkaido at around 13 Ma. This period of time corresponds to the stage when the uplifting of the Hidaka Mountains began (e.g., [45]). This tectonic framework is controlled by the dextral oblique collision between the Eurasian and North American Plates and the oblique subduction of the Pacific Plate beneath the Kurile Trench. It is considered that the Kurile arc migrated into the southwestward as a forearc sliver by the oblique subduction of the Pacific Plate and that the Hidaka Mountains would be formed by collision of the Kurile arc and the Northeast Japan arc (e.g., [46-48]).

Since, consequently, it is an important area for understanding characteristics and mechanism of collision zone, numerous geophysical surveys (e.g., seismic prospecting, gravity surveys, electromagnetic surveys) have been carried out around the Hidaka Mountains in order to obtain information regarding the subsurface structures and to apply such knowledge to a tectonic discussion of the Mountains and Hokkaido (e.g., [49-54]). Using these surveys, subsurface structures which indicate a collision between the Northeast Japan arc and the Kurile arc have been obtained, and have contributed to tectonic discussions. However, in contrast, geophysical studies in the sedimentary basin area in central Hokkaido are limited in number.

#### **2.2. Geological and tectonic background**

The geological characteristics in Hokkaido are that Cenozoic strata consist of island-arc-trench systems of the Northeast Japan arc and Kurile arc, and that each Cenozoic strata distributed in the Northeast Japan arc and the Kurile arc appear in the western half and eastern half area of Hokkado, respectively. This characteristic also appears in Neogene and Quaternary strata, volcanoes and their products, and topography (e.g., [41]).

2800 m and 400 m. Itoh and Tsuru [58] identified a NNW-SSE trending deformation zone bounded by large transcurrent faults including T1 and T2 later describing from seismic reflecting data in the northern part of the Northeast Japan forearc (Figure 3) and their right lateral motions have been indicated by the clockwise rotation of Paleogene marine sediments and by paleogeographic reconstruction. Since, as already mentioned, the present study area (the western half of Hokkaido) has same Cenozoic strata distributed in the Northeast Japan arc, study area of Itoh and Tsuru [58] and our study area are geologically continuous in the Paleogene time. Consequently, it is expected that a right lateral motion of the crust was

**Figure 4.** Distribution of the Paleogene strata. Green and blue areas indicate distribution areas of the Paleogene sedi‐

Numerical Modeling of Sedimentary Basin Formation at the Termination of Lateral Faults in a Tectonic Region…

http://dx.doi.org/10.5772/56558

279

mentary layer under and on the surface, respectively. (After Kurita and Hoyanagi [56])

The Horonai stage is the sedimentation stage of the Horonai Formation, the Tappu Group, the Sankebetsu Formation and the lower Magaribuchi Formation. This stage corresponds to the Eocene and the early Oligocene. In this stage, sedimentary basins "A", "B", "C", "D", "E" and "F" were formed (Figure 5C), and from well data their depths are estimated to be 3500 m, 1200 m, 1200 m, 600 m, 300 m, and 1500 m, respectively. It is expected that a right lateral motion was dominant in this stage, because right lateral motion was also dominant in the Eocene and

The Minami-Naganuma stage is the sedimentation stage of the upper Magaribuchi Formation, the Minami-Naganuma Formation, the Horomui Formation, and the upper Sankebetsu Formation. This stage corresponds to the late Oligocene and the early Miocene. In this stage, sedimentary basins "B", "D", "E" and "F" were formed (Figure 5D), and from well data their depths (B, E and F) are estimated to be 2000 m, 300 m and 1500 m, respectively. The maximum depth of basin "D" is unknown because of lack of the well data and/or of outcrop section of the whole Minami-Naganuma Fromation. Itoh et al. [37], Tamaki et al. [38] and Itoh and Tsuru

dominant.

the late Oligocene (see below).

Figure 4 shows the distribution of the Paleogene strata in a north-south direction in central Hokkaido. This distribution traces the old plate boundary. This N-S elongation area is included in the Ishikari-Teshio Belt that is underlain by the Cretaceous Yezo Group, and is regarded as a typical sequence in a forearc basin setting [55]. It is known that the Paleogene sedimentary strata were deposited during the early Eocene and Oligocene in almost the entire region (e.g., [56]).

In the study area, sedimentary basins and sedimentary layers were formed during 48–12 Ma and have been complexly distributed. They have been divided into 5 stages according to their formation: The Ishikari stage (48–40 Ma), The Horonai stage (40–32 Ma), The Minami Naga‐ numa stage (34–20 Ma), and The Kawabata stage (15–12 Ma). The Ishikari stage is divided into early (48–45 Ma) and late (45–40 Ma) stages. The shape of the sedimentary basin and the distribution of sedimentary layers are shown in Figure 5.

It is known that the Ishikari Group differentially subsided and was then divided into several components [57]. Based on detailed sedimentological studies, Takano and Waseda [57] also points out that the rate of subsidence accelerated during deposition of the Ishikari Group.

The Ishikari stage is the sedimentation stage of the Ishikari Group, corresponding to the Eocene and is divided into early and later stages according to the sedimentation style. Sediments in the early Ishikari stage are distributed shallowly and widely (Figure 5A). In this stage, the sedimentary basins "A", "B" and "C" were formed (Figure 5A), and from well data their depths are estimated to be 600 m, 500 m and 1000 m, respectively. Sediments in the later Ishikari stage are distributed deeply and narrowly (Figure 5B). In this stage, the sedimentary basins named "A" and "C" were formed (Figure 5B), and from well data their depths are estimated to be

of the Pacific Plate beneath the Kurile Trench. It is considered that the Kurile arc migrated into the southwestward as a forearc sliver by the oblique subduction of the Pacific Plate and that the Hidaka Mountains would be formed by collision of the Kurile arc and the Northeast Japan

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

Since, consequently, it is an important area for understanding characteristics and mechanism of collision zone, numerous geophysical surveys (e.g., seismic prospecting, gravity surveys, electromagnetic surveys) have been carried out around the Hidaka Mountains in order to obtain information regarding the subsurface structures and to apply such knowledge to a tectonic discussion of the Mountains and Hokkaido (e.g., [49-54]). Using these surveys, subsurface structures which indicate a collision between the Northeast Japan arc and the Kurile arc have been obtained, and have contributed to tectonic discussions. However, in contrast, geophysical studies in the sedimentary basin area in central Hokkaido are limited in number.

The geological characteristics in Hokkaido are that Cenozoic strata consist of island-arc-trench systems of the Northeast Japan arc and Kurile arc, and that each Cenozoic strata distributed in the Northeast Japan arc and the Kurile arc appear in the western half and eastern half area of Hokkado, respectively. This characteristic also appears in Neogene and Quaternary strata,

Figure 4 shows the distribution of the Paleogene strata in a north-south direction in central Hokkaido. This distribution traces the old plate boundary. This N-S elongation area is included in the Ishikari-Teshio Belt that is underlain by the Cretaceous Yezo Group, and is regarded as a typical sequence in a forearc basin setting [55]. It is known that the Paleogene sedimentary strata were deposited during the early Eocene and Oligocene in

In the study area, sedimentary basins and sedimentary layers were formed during 48–12 Ma and have been complexly distributed. They have been divided into 5 stages according to their formation: The Ishikari stage (48–40 Ma), The Horonai stage (40–32 Ma), The Minami Naga‐ numa stage (34–20 Ma), and The Kawabata stage (15–12 Ma). The Ishikari stage is divided into early (48–45 Ma) and late (45–40 Ma) stages. The shape of the sedimentary basin and the

It is known that the Ishikari Group differentially subsided and was then divided into several components [57]. Based on detailed sedimentological studies, Takano and Waseda [57] also points out that the rate of subsidence accelerated during deposition of the Ishikari Group. The Ishikari stage is the sedimentation stage of the Ishikari Group, corresponding to the Eocene and is divided into early and later stages according to the sedimentation style. Sediments in the early Ishikari stage are distributed shallowly and widely (Figure 5A). In this stage, the sedimentary basins "A", "B" and "C" were formed (Figure 5A), and from well data their depths are estimated to be 600 m, 500 m and 1000 m, respectively. Sediments in the later Ishikari stage are distributed deeply and narrowly (Figure 5B). In this stage, the sedimentary basins named "A" and "C" were formed (Figure 5B), and from well data their depths are estimated to be

arc (e.g., [46-48]).

**2.2. Geological and tectonic background**

almost the entire region (e.g., [56]).

volcanoes and their products, and topography (e.g., [41]).

distribution of sedimentary layers are shown in Figure 5.

**Figure 4.** Distribution of the Paleogene strata. Green and blue areas indicate distribution areas of the Paleogene sedi‐ mentary layer under and on the surface, respectively. (After Kurita and Hoyanagi [56])

2800 m and 400 m. Itoh and Tsuru [58] identified a NNW-SSE trending deformation zone bounded by large transcurrent faults including T1 and T2 later describing from seismic reflecting data in the northern part of the Northeast Japan forearc (Figure 3) and their right lateral motions have been indicated by the clockwise rotation of Paleogene marine sediments and by paleogeographic reconstruction. Since, as already mentioned, the present study area (the western half of Hokkaido) has same Cenozoic strata distributed in the Northeast Japan arc, study area of Itoh and Tsuru [58] and our study area are geologically continuous in the Paleogene time. Consequently, it is expected that a right lateral motion of the crust was dominant.

The Horonai stage is the sedimentation stage of the Horonai Formation, the Tappu Group, the Sankebetsu Formation and the lower Magaribuchi Formation. This stage corresponds to the Eocene and the early Oligocene. In this stage, sedimentary basins "A", "B", "C", "D", "E" and "F" were formed (Figure 5C), and from well data their depths are estimated to be 3500 m, 1200 m, 1200 m, 600 m, 300 m, and 1500 m, respectively. It is expected that a right lateral motion was dominant in this stage, because right lateral motion was also dominant in the Eocene and the late Oligocene (see below).

The Minami-Naganuma stage is the sedimentation stage of the upper Magaribuchi Formation, the Minami-Naganuma Formation, the Horomui Formation, and the upper Sankebetsu Formation. This stage corresponds to the late Oligocene and the early Miocene. In this stage, sedimentary basins "B", "D", "E" and "F" were formed (Figure 5D), and from well data their depths (B, E and F) are estimated to be 2000 m, 300 m and 1500 m, respectively. The maximum depth of basin "D" is unknown because of lack of the well data and/or of outcrop section of the whole Minami-Naganuma Fromation. Itoh et al. [37], Tamaki et al. [38] and Itoh and Tsuru [59] pointed out that all basins formed in the Ishikari-Teshio Belt in this stage are pull-apart basins. Tamaki et al. [38] showed that using dislocation modeling, a 30 km right-lateral strikeslip is required to restore the actual distribution and volume of the basin. Kurita and Yokoi [60] also stated that lateral faulting was dominant in forming some of the tectonic structures during the late Oligocene.

**3. Bouguer gravity anomaly**

by Komazawa [61]. The Bouguer density of 2670 kg/m3

guer density of 2670 kg/m3 was assumed. Contour interval is 10 mGal.

Numerous geological and geophysical surveys have been carried out in the Hokkaido area, and each survey has played an important role in the understanding of crustal characteristics and tectonic events in the area. In particular, seismic prospecting has proved very useful in obtaining information relating to subsurface structures. However, seismic prospecting is almost two-dimensional, and it is difficult to intuitively understand the subsurface structures as three dimensional structures, even when provided with data from more than one profile. In contrast, the characteristics of gravity anomaly maps are easy to interpret and can be used to roughly estimate three dimensional subsurface structures from the data. Figure 6 shows the Bouguer gravity anomaly map of the study area. This map is based on the gravity mesh data

Numerical Modeling of Sedimentary Basin Formation at the Termination of Lateral Faults in a Tectonic Region…

**Figure 6.** Bouguer gravity anomaly map. This map is based on the gravity mesh data by Komazawa [61], and the Bou‐

was employed.

http://dx.doi.org/10.5772/56558

281

The Kawabata stage is the sedimentation stage of the Kawabata Formation, the Ukekoi Formation, the Fureoi Formation, the Kotanbetsu Formation and the Masuporo Formation. During the Neogene, Japan was affected by the opening event of the back-arc basin of the Sea of Japan. In this stage, sedimentary basins "A", "B", "D", "E", "F1" and "F2" were formed (Figure 5E), and from well data their depths are estimated to be 2000 m, 4000 m, 4000 m, 3500 m, 2000 m and 2000 m, respectively. As mentioned above, a lateral motion of the crust was dominant during the early Neogene [37, 38, 59]. Although a building of the Hidaka Mountains in around 13 Ma has been pointed out (e.g., [45]), details are unknown.

**Figure 5.** Shapes of sedimentary basins in (A) early Ishikari stage (48-45 Ma), (B) late Ishikari stage (45-40 Ma), (C) Hor‐ onai stage (40–32 Ma), (D) Minami-Naganuma stage (34–20 Ma) and (E) Kawabata stage (15–12 Ma). Isopach maps of the Horonai stage and the Kawabata stage are after Association of Natural Gas Mining and Association for Offshore Petroleum Exploration [75].
