**5. Discussion**

between the actual basin depth and the modeled basin depth were -800 m, +400 m, and -300

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

In the Kawabata stage, we attempted to restore six sedimentary basins, "A", "B", "D", "E", "F1" and "F2" (Figure 5E), and the result is shown in Figure 10E. The right lateral movements of fault zones, (including the T2 fault, Rumoi-ShinTotsu tectonic line, Onishika-chikubetsu fault, Tenpoku fault and the Horonobe fault), were required in order to restore these six sedimentary basins. The amount of movement of each fault is determined by trial and error

From the amount of displacement on each fault plane, it is estimated that a horizontal

As described above, these lateral motions could restore the basic distribution pattern of six sedimentary basins formed in the Kawabata stage. However, the whole distribution pattern of sedimentary basins in this stage could not be restored. After repeated trial and error, it was found that the reverse motion of two fault zones, (including the Hidaka-north fault and Hidaka-south fault), is necessary to restore the whole distribution pattern of the sedimentary basins in this stage (Figure 10F). In fact, such a reverse motion is required to successfully restore the whole basin distribution pattern. The amount of reverse motion required is shown in Table

Information on the depth of the sedimentary basins "A", "B", "D", "E", "F1" and "F2" is obtained from geological observations, and depths are found to reach 2000 m, 4000 m, 4000 m, 3500 m, 2000 m and 2000 m. In our model, the depths of modeled sedimentary basins "A", "B", "D", "E", "F1" and "F2" reached 1100 m, 1800 m, 1600 m, 1900 m, 1300 m and 800 m. The differences between the actual basin depth and the modeled basin depth were -900 m, -2200 m, -2400 m, -1600 m, -700 m, and -1200m in basins "A", "B", "D", "E", "F1" and "F2", respec‐ tively. Although the whole basin distribution pattern is good, the differences between the actual basin depth and the modeled basin depth are large in all the basins. If the lateral or reverse motion of each fault zone was increased to adjust the depth component of basins, it

would not be possible to restore the spatial distribution patterns of the basins.

movement reaching about 240 km occurred during this tectonic stage.

m for basins "B", "E" and "F", respectively.

and is shown in Table 2 and as follows:

**2.** Rumoi-ShinTotsu tectonic line zone: 44 km

**3.** Onishika-chikubetsu fault zone: 21 km

**1.** T2 fault zone: 98 km

**4.** Tenpoku fault zone: 41 km

**5.** Horonobe fault zone: 36 km

**1.** Hidaka-north fault zone: 2.5 km

**2.** Hidaka-south fault zone: 2.5 km

2 and as follows:

**4.5. Kawabata stage (15-12 Ma)**

As described in the previous sections within this paper, the distribution patterns of sedimen‐ tary basins restored by dislocation modeling are very similar to the actual distribution patterns of the sedimentary basins formed in each stage, although depth differences between the actual sedimentary basins and the restored sedimentary basins occurred because of the dislocation plane based on the linear elasticity. From these results, it is suggested that almost all the sedimentary basins in central Hokkaido can be explained as pull-apart basins, caused by rightlateral fault motions during the Paleogene. The results also show that sedimentary basins formed during the Kawabata stage were formed by a combination of right-lateral fault motions and reverse fault motions located at the western margin of the Hidaka Mountains.

Although we have simplified the distributions of each fault zone (Figure 9), it would be difficult for contiguous fault zones to move independently, simultaneously, as different faults, namely a reverse fault and a lateral fault, under their arrangement as shown in Figure 9. Consequently, we suggest that the Kawabata stage should be divided into two stages, namely the "early stage" and the "late stage", from the viewpoint of the stress field or fault motion. By considering the continuity of tectonics or the stress field, it is found that the lateral movements were made in the early Kawabata stage and that the reverse movements were made in the late Kawabata stage.

From a geological viewpoint, it has been illustrated that the building of the Hidaka Mountains was caused by reverse fault motions (e.g., [45]), and the timing of this event has been consid‐ ered as being during the late Miocene or around 13 Ma (e.g., [45, 69]). This geological view supports our results and ideas, and our results also support the tectonics constructed based on geological data. Hence, we suggest that the boundary of the late Kawabata stage is around 13 Ma.

In Figures, we show the total vertical displacement field calculated from the vertical displace‐ ment field in each stage (Figures 10). The vertical displacement fields shown in Figures 11 are normalized by the maximum value of absolute values of the total vertical displacement field restored by dislocation modeling. Thus, the displacement fields shown in Figures 11 do not have a unit.

Figure 11A illustrates the normalized displacement field map that the negative displacement areas are shown in gray. This shows the distribution of the subsurface sedimentary basins restored in this study, and the distribution is seen to be similar to the distribution of actual buried sedimentary basins formed during the Paleogene (Figure 4). Figure 11B is the normal‐ ized total vertical displacement pattern restored in this study. From this figure, it is found the deepest sedimentary basin restored is located at the center of central Hokkaido. In actual depth distribution, the basin "C" is not the deepest basin. However, this sedimentary basin has a depth reaching 6000 m and is large and deep basin.

than -100 mGal (area III in Figure 7) consists of a very thick sedimentary layer of 8 km, and the depth of the Moho discontinuity in this area is more than 30 km (e.g., [70, 71]), and that the negative gravity anomaly which reaches -20 mGal (area I in Figure 7) consists of a sedimentary layer of several kilometers thickness, and a Moho discontinuity of around 30 km in depth (e.g., [70, 71]). Consequently, we understand that the conspicuous gravity lows in areas I and III are caused by a thick sedimentary layer and a deep Moho. In contrast, the Bouguer gravity anomaly in area II is not negative, in spite of an area of subsidence of several kilometers depth. In actual fact, using receiver function analysis [71], it is reported that the depth of the Moho discontinuity in this area ranges from a depth between 26–31 km. The Moho in area II is about

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

We made a simple subsurface structure model consisting of sedimentary layer, crust and mantle, based on information of subsurface structures mentioned above. We then calculated the gravity anomalies along a D-D' profile with assumption density contrasts of sedimentary

Figure 12 shows the simple subsurface structure model and the estimated gravity anomaly along the D-D' profile. The assumed subsurface structure model explains the Bouguer gravity anomalies very well, in spite of the model being very simple, and leads us to assume that the conspicuous subsidence at the surface would induce the crustal thinning-mantle uplift, via isostasy. Subsidence at the surface, caused by frequent lateral motions during the Paleogene, as shown and discussed in this paper, has reached to several km. In this study, right lateral motions of the crust reaching about 1000 km were required, in order to restore the sedimentary basins distributed in central Hokkaido, as shown in the previous section. Although the correct depths of sedimentary basins were not shown in our dislocation modeling because the model was based on linear elasticity, it is expected that deeper basins would be formed in the actual crust by lateral motions. Consequently, it is considered possible that conspicuous subsidence caused by a large lateral motion would induce the crustal thinning-mantle uplift. Or, since this area experienced a tension stress field over a period of about 35 million years in spite of the local stress caused by the pull-apart basin formation, this tension stress field might induce the crustal thinning-mantle uplift viscoelastically over a very long time scale. In either case, it is necessary to reconsider the estimated mass deficiency and volume of the sediment in area II

from the Bouguer gravity anomalies, by correcting the effect of mantle uplift.

In this study, we suggested one tectonic model to explain the distribution of sedimentary basins located in central Hokkaido and formed between 48 Ma and 12 Ma. We then discussed the characteristics of the Bouguer gravity anomalies based on the tectonic model. In future studies, we will attempt to estimate the subsurface structure more accurately, and discuss the tectonics through simulation procedures considering a more realistic behavior of the crust and

mantle (e.g., FEM, FDM, DEM and others), based on the model shown in this paper.

and -500 kg/m3

. We employed the two dimen‐

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

295

6 km shallower than in areas I and III.

layer-crust and crust-mantle being -300 kg/m3

sional Talwani's method [72] in our calculations.

**Figure 11.** Total vertical displacement pattern. The displacement field was calculated by adding up the vertical dis‐ placements field restored in each stage by the dislocation modeling. A: Gray shows negative displacement areas, and indicates the distribution of the subsurface sedimentary basins restored in this study. B: Total vertical displacement pattern normalized by the maximum value of absolute values of the total vertical displacement field.

From results shown in Figures 10-11 and the discussion above, we conclude that the sedimen‐ tary basins formed from 48 to 12 Ma in central Hokkaido can be explained by the formation of pull-apart basins, due to right lateral motions (before 13 Ma), and by reverse motions of the Hidaka-North fault zone and the Hidaka-South fault zone after 13 Ma. Namely, although the right-lateral motions were predominant from 48 Ma to 13 Ma, the reverse motions were dominant in 13 Ma. This leads us to expect a significant change in the regional tectonic stress field during this stage. Although a change of the collision direction of the Northeast Japan arc and the Kurile arc could be cited as a possibility for this, a more accurate and quantitative future investigation is required.

We here reconsider the Bouguer gravity anomaly (Figure 6 and 7) and subsurface structures in viewpoint of tectonics mentioned above. As described in the section on the Bouguer gravity anomaly, gravity low area less than 20 mGal corresponds to the area where the Paleogene strata distribute under the surface. It is known that the southern negative gravity anomaly less than -100 mGal (area III in Figure 7) consists of a very thick sedimentary layer of 8 km, and the depth of the Moho discontinuity in this area is more than 30 km (e.g., [70, 71]), and that the negative gravity anomaly which reaches -20 mGal (area I in Figure 7) consists of a sedimentary layer of several kilometers thickness, and a Moho discontinuity of around 30 km in depth (e.g., [70, 71]). Consequently, we understand that the conspicuous gravity lows in areas I and III are caused by a thick sedimentary layer and a deep Moho. In contrast, the Bouguer gravity anomaly in area II is not negative, in spite of an area of subsidence of several kilometers depth. In actual fact, using receiver function analysis [71], it is reported that the depth of the Moho discontinuity in this area ranges from a depth between 26–31 km. The Moho in area II is about 6 km shallower than in areas I and III.

We made a simple subsurface structure model consisting of sedimentary layer, crust and mantle, based on information of subsurface structures mentioned above. We then calculated the gravity anomalies along a D-D' profile with assumption density contrasts of sedimentary layer-crust and crust-mantle being -300 kg/m3 and -500 kg/m3 . We employed the two dimen‐ sional Talwani's method [72] in our calculations.

Figure 12 shows the simple subsurface structure model and the estimated gravity anomaly along the D-D' profile. The assumed subsurface structure model explains the Bouguer gravity anomalies very well, in spite of the model being very simple, and leads us to assume that the conspicuous subsidence at the surface would induce the crustal thinning-mantle uplift, via isostasy. Subsidence at the surface, caused by frequent lateral motions during the Paleogene, as shown and discussed in this paper, has reached to several km. In this study, right lateral motions of the crust reaching about 1000 km were required, in order to restore the sedimentary basins distributed in central Hokkaido, as shown in the previous section. Although the correct depths of sedimentary basins were not shown in our dislocation modeling because the model was based on linear elasticity, it is expected that deeper basins would be formed in the actual crust by lateral motions. Consequently, it is considered possible that conspicuous subsidence caused by a large lateral motion would induce the crustal thinning-mantle uplift. Or, since this area experienced a tension stress field over a period of about 35 million years in spite of the local stress caused by the pull-apart basin formation, this tension stress field might induce the crustal thinning-mantle uplift viscoelastically over a very long time scale. In either case, it is necessary to reconsider the estimated mass deficiency and volume of the sediment in area II from the Bouguer gravity anomalies, by correcting the effect of mantle uplift.

**Figure 11.** Total vertical displacement pattern. The displacement field was calculated by adding up the vertical dis‐ placements field restored in each stage by the dislocation modeling. A: Gray shows negative displacement areas, and indicates the distribution of the subsurface sedimentary basins restored in this study. B: Total vertical displacement

From results shown in Figures 10-11 and the discussion above, we conclude that the sedimen‐ tary basins formed from 48 to 12 Ma in central Hokkaido can be explained by the formation of pull-apart basins, due to right lateral motions (before 13 Ma), and by reverse motions of the Hidaka-North fault zone and the Hidaka-South fault zone after 13 Ma. Namely, although the right-lateral motions were predominant from 48 Ma to 13 Ma, the reverse motions were dominant in 13 Ma. This leads us to expect a significant change in the regional tectonic stress field during this stage. Although a change of the collision direction of the Northeast Japan arc and the Kurile arc could be cited as a possibility for this, a more accurate and quantitative

We here reconsider the Bouguer gravity anomaly (Figure 6 and 7) and subsurface structures in viewpoint of tectonics mentioned above. As described in the section on the Bouguer gravity anomaly, gravity low area less than 20 mGal corresponds to the area where the Paleogene strata distribute under the surface. It is known that the southern negative gravity anomaly less

pattern normalized by the maximum value of absolute values of the total vertical displacement field.

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

future investigation is required.

In this study, we suggested one tectonic model to explain the distribution of sedimentary basins located in central Hokkaido and formed between 48 Ma and 12 Ma. We then discussed the characteristics of the Bouguer gravity anomalies based on the tectonic model. In future studies, we will attempt to estimate the subsurface structure more accurately, and discuss the tectonics through simulation procedures considering a more realistic behavior of the crust and mantle (e.g., FEM, FDM, DEM and others), based on the model shown in this paper.

stress field during this stage, its source should be investigated accurately and quantita‐

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

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

297

**4.** The distribution pattern of sedimentary basins restored in this study is similar to the actual distribution of buried sedimentary basins formed during the Paleogene in this area. Finally, we discovered the following two points relating to the Bouguer gravity anomaly in

**5.** A gravity low area less than 20 mGal corresponds to the area where the Paleogene strata

**6.** The Bouguer gravity anomalies in the gravity low belt less than 20 mGal can be roughly explained by the subsurface structure model that the mantle around the center of the belt was lifted upwards. Although it is considered that the conspicuous subsidence caused by large lateral motion would induce the crustal thinning-mantle uplift, this possibility

This study was partially supported by the First Bank of Toyama Scholarship foundation and a Grants-in-Aid for Scientific Research (No. 21671003). We are most grateful to Ana Pantar and

, Osamu Takano3

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

2 Department of Physical Science, Graduate School of Science, Osaka Prefecture University,

[1] Aydin A, Nur A. Evolution of Pull-apart Basins and Their Scale Independence. Tec‐

and Machiko Tamaki4

should be discussed more accurately and quantitatively in the future.

tively in the future.

are distributed under the surface.

Book Editors for their editorial advices and cooperation.

, Yasuto Itoh2

3 JAPEX Research Center, Japan Petroleum Exploration Co. Ltd., Japan

central Hokkaido:

**Acknowledgements**

**Author details**

Japan

**References**

Shigekazu Kusumoto1

4 Japan Oil Engineering Co. Ltd., Japan

tonics 1982; 1 91-105.

**Figure 12.** Two-dimensional gravity modelling along D-D' profile in Figure 7. Blue circle shows measured Bouguer gravity anomalies and red solid line shows calculated values. Bottom figure shows the density structure. Density con‐ trasts of sedimentary layer-crust and crust-mantle were assumed to be -300 kg/m3 and -500 kg/m3, respectively.
