**4.2. Late Ishikari stage (45–40 Ma)**

In the late Ishikari stage, sedimentary basins "A" and "C" (Figure 5B) were restored (Figure 10B). The right lateral movements of fault zones (including the T1 fault, T2 fault, Onishika fault and the Hidaka-North fault) were required in order to restore these two sedimentary basins. The amount of movement of each fault was determined by trial and error (see Table 2) and is as follows.


From these amounts of displacement on each fault plane, it is estimated that during this tectonic stage, a horizontal movement reaching about 123 km occurred.

From geological observations, sedimentary basin "C" is known to be the largest basin and reaches a depth of 2800 m. The depth of the model basin in our modeling reached 1800 m. Sedimentary basin "A" reaches a depth of about 400 m, and the modeled basin corresponding to this had a depth of 500 m. The differences between the actual basin depth and the modeled basin depth were -1000 m and +100 m in basins "C" and "A", respectively. The restored subsidence amount of basin "C" was smaller than the actual basin depth. If the lateral motions of the Hidaka-North fault zone and Onishika fault zone were increased to adjust to the depth component of basin "C", it was not possible to restore the spatial distribution patterns of the basins.

#### **4.3. Horonai stage (40 Ma–32 Ma)**

In the Horonai stage, we attempted to restore six sedimentary basins, "A", "B", "C", "D", "E" and "F" (Figure 5C). Results are shown in Figure 10C. The right lateral movements of fault zones (including the T1 fault, T2 fault, the Rumoi-ShinTotsu tectonic line, Onishika fault, the Tenpoku fault, Horonobe fault and Hidaka-North fault) were required in order to restore these six sedimentary basins. The amount of movement of each fault was determined by trial and error and is shown in Table 2 and as follows:

**1.** T1 fault zone: 14 km

both depths of restored basins (modeled basins) corresponding to these basins are 1000m as a result of dislocation modeling. Differences between the actual basin depth and the modeled basin depth, namely "the modeled basin depth–actual basin depth," were +400 m, +400 m and +500 m for basins "C", "A" and "B", respectively. The amount of restored subsidence may be

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

Here, we assigned the right lateral motion to each fault as mentioned above, in order to restore the spatial patterns of basin distribution. If the amount of lateral motion of each fault is reduced to adjust to the depth component of each basin, it is not possible to restore the spatial distri‐

In the late Ishikari stage, sedimentary basins "A" and "C" (Figure 5B) were restored (Figure 10B). The right lateral movements of fault zones (including the T1 fault, T2 fault, Onishika fault and the Hidaka-North fault) were required in order to restore these two sedimentary basins. The amount of movement of each fault was determined by trial and error (see Table 2) and is

From these amounts of displacement on each fault plane, it is estimated that during this

From geological observations, sedimentary basin "C" is known to be the largest basin and reaches a depth of 2800 m. The depth of the model basin in our modeling reached 1800 m. Sedimentary basin "A" reaches a depth of about 400 m, and the modeled basin corresponding to this had a depth of 500 m. The differences between the actual basin depth and the modeled basin depth were -1000 m and +100 m in basins "C" and "A", respectively. The restored subsidence amount of basin "C" was smaller than the actual basin depth. If the lateral motions of the Hidaka-North fault zone and Onishika fault zone were increased to adjust to the depth component of basin "C", it was not possible to restore the spatial distribution patterns of the

In the Horonai stage, we attempted to restore six sedimentary basins, "A", "B", "C", "D", "E" and "F" (Figure 5C). Results are shown in Figure 10C. The right lateral movements of fault zones (including the T1 fault, T2 fault, the Rumoi-ShinTotsu tectonic line, Onishika fault, the Tenpoku fault, Horonobe fault and Hidaka-North fault) were required in order to restore these six sedimentary basins. The amount of movement of each fault was determined by trial and

tectonic stage, a horizontal movement reaching about 123 km occurred.

a little large.

as follows.

basins.

bution patterns of the basins.

**1.** T1 fault zone: 14 km

**2.** T2 fault zone: 25 km

**3.** Onishika fault zone: 48 km

**4.** Hidaka-North fault zone: 36 km

**4.3. Horonai stage (40 Ma–32 Ma)**

error and is shown in Table 2 and as follows:

**4.2. Late Ishikari stage (45–40 Ma)**


From the amount of displacement of each fault plane, it is estimated that a horizontal move‐ ment reaching about 342 km occurred during this tectonic stage.

From geological observations, it is known that depths of the sedimentary basins "A", "B", "C", "D", "E" and "F" reach to 3500 m, 1200 m, 1200 m, 600 m, 300 m and 1500 m, respectively. In our model, depths of modeled sedimentary basins "A", "B," "C", "D", "E" and "F" reached 1000 m, 600 m, 1800 m, 1100 m, 900 m and 1000 m, respectively. The differences between the actual basin depth and the modeled basin depth were -2500 m, -600 m, +600 m, +500 m, +600 m, and -500 m in basin "A", "B", "C", "D", "E" and "F", respectively.

#### **4.4. Minami-Naganuma stage (34–20 Ma)**

We attempted to restore sedimentary basins "B", "D", "E" and "F" in the Minami-Naganuma stage (Figure 5D), and the results are shown in Figure 10D. The right lateral movements of fault zones, (including the T2 fault, the Rumoi-ShinTotsu tectonic line, Tenpoku fault and the Horonobe fault), were required in order to restore these four sedimentary basins.

In this stage, Tamaki et al. [38] have restored already the Minami-Naganuma basin (corre‐ sponding to basin "B": pull-apart basin) located in south central Hokkaido. We referred to their results and determined the amount of movement of each fault by trial and error. The amount of fault movement is shown in Table 2 and as follows:


From the amounts of displacement on each fault plane, it is estimated that horizontal move‐ ment reaching about 200 km occurred during this tectonic stage.

From geological observations it is known that the depth of sedimentary basins "B", "E" and "F" reached 2000 m, 300 m and 1500 m, respectively. As already mentioned, the maximum depth of basin "D" is unknown. In our model, the depths of the modeled sedimentary basins "B", "D", "E" and "F" reached 1200 m, 1100 m, 700 m and 1200 m, respectively. The differences between the actual basin depth and the modeled basin depth were -800 m, +400 m, and -300 m for basins "B", "E" and "F", respectively.

**5. Discussion**

stage.

13 Ma.

have a unit.

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

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

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

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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

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

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

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.

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

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 and is shown in Table 2 and as follows:


From the amount of displacement on each fault plane, it is estimated that a horizontal movement reaching about 240 km occurred during this tectonic stage.

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 2 and as follows:


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.
