**5. Tectonic basin formation in and around Lake Biwa: Chronological development and mechanism**

m drilling cores at the rivermouth of the Yasu River [29]; [30]; [31]. The gravel of the uppermost part of the Kobiwako Group in Kosei Area (western side of present Lake Biwa) constitutes sediments of the last Actual I stage. These gravels were the first sediments from present western Hira Mountains. Subsequently, the crustal movements along the Katata Fault (southern part of Biwako-seigan Fault zone) became more active. Thereafter, in actual II stage, the area of actual northern lake ("Hokko") basin of present Lake Biwa began to subside rapidly.

**Figure 11.** Basement and Pliocene-Quaternary sediment distribution in and around Lake Biwa and Ise Bay region. "Kinki Triangle" region is shown in inset figure. Paleogeographical regions such as "Chubu Tilting Block", Lake Tokai, Paleo-lake Biwa, and Paleo-lake (bay) Osaka are shown. MTL: Median Tectonic Line, ATL: Arima-Takatsuki Tectonic Line, I: Ikoma Fault, H: Hanaore Fault, B: Biwako-seigan Fault zone, Ya: Yanagase Fault, Ic: Ichishi Fault, Yo: Yoro Fault

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

The process of basin transition in and around Paleo-Lake Biwa is summarized as follows:

migrated gradually to the north in Older Stage I.

Stage II.

Biwa basin formation.

**1.** In the early stage, the sedimentary basin appeared in the southern area of the basin, and

**2.** Subsequently, the sedimentary basin migrated northwestward on a large scale in Older

**3.** The sedimentary basin migrated gradually to the west. Finally, it is divided by the structural movements in N-S trend accompanied rapid subsidence of the eastern area (rapid upheaving of western area) in Actual stage I and Actual stage II of the present Lake

As described previously, the geohistory in central Kinki region had two conspicuous episodes of ca 3.0 Ma and of ca 1.2-1.5 Ma. A similar pattern of basin transition was revealed as recognized in both sedimentary basins of Lake Tokai and Paleo-lake Biwa (Figure 14), which suggests that the changes of the tectonic stress state were common throughout that province. The sedimentary basins before ca 3.0 Ma (Stage 1 of Lake Tokai and older I stage of Paleo-lake Biwa) are characterized mainly by E-W arrangement of depressional zone and their northward migration. Those characteristics might be attributable to the upheaval of the southern area under the tectonic stress state in N-S direction. However, in the middle of Stage 1, Lake Tokai received a large amount of gravel supply from the east, which was related to the movements of "Chubu Tilting Block". Kuwahara[33] discussed tectonism of eastern area of Ise Bay (the area from Nohbi Plain to mountains). He stated that this area had received tilting movement since Pliocene and this movement formed the topographic contrast between the sedimentary

**Figure 13.** Paleogeographic map of the central Kinki Region including Paleo-Lake Biwa and Lake Tokai area [32] e: & f: Older stage II of Paleo-Lake Biwa and Stage II of Lake Tokai, g: final stage of Older stage II of Paleo-Lake Biwa and Stage II of Lake Tokai

basin of Lake Tokai and eastern upheaving area. He called this tilting block as "Chubu Tilting Block". A large amount of gravel supply suggests the beginning of new structural control superposing the older one. At ca 3.0 Ma, the Lake Tokai sedimentary basin migrated north‐ westward.

of the eastern area (Figure 14). Conclusively we proposed an idea that the deposition in the central part of Kinki region since the Pliocene was under the influence of two superposed tectonic stress states represented by upheaving of the southern area and tilting of the eastern area. Hitherto, Huzita [34] have stated that the tectonic stress state change since Miocene took place in the Inner zone of southwestern Japan during the time of Pliocene and Pleistocene. This change from N-S compressional stress state to E-W one was conspicuous. Therefore, the results of the present work are not contradictory to that idea. In fact, the results confirm the chrono‐ logical setting and detailed process of tectonosedimentary turnover since the Pliocene.

Tectonic Basin Formation in and Around Lake Biwa, Central Japan

http://dx.doi.org/10.5772/56667

223

**Figure 14.** Tectonic implications in central Kinki Region including Paleo-Lake Biwa and Lake Tokai area [32]

**6. Present Lake Biwa Basin and tectonosedimentary implications**

Interruptions in the sedimentary record are commonly regarded as evidence of tectonic changes in a lake basin. Major unconformities exist in the core lithology and seismic profiles at the B Horizon separating the P Bed from the basement rocks and at the Z Horizon between

Subsequently, the sedimentary basin of Lake Tokai migrated to the west as a whole, although slightly northward, because of active movement of "Chubu Tilting Block" under the stress state of the E-W trend. However, during the same time interval, the sedimentary basin of Paleolake Biwa migrated gradually northward by the upheaval of southern area. At the time of ca 1.2 Ma, the sedimentary basin of Lake Tokai became to extinct, accompanying the upheaving of the Suzuka and Yoro mountains with a N-S structural trend. It was peculiar that the sedimentary basin of Paleo-lake Biwa transferred its position northwestward. Thereafter, it migrated westward gradually by tilting of the eastern area. This fact shows the origination of conspicuous movement under E-W tectonic stress in the Ohni Basin.

In this way, the sedimentary basin migration in Lake Tokai and Paleo-lake Biwa is commonly explained by the hypothesis of interaction between upheaving of the southern area and tilting


**Figure 14.** Tectonic implications in central Kinki Region including Paleo-Lake Biwa and Lake Tokai area [32]

basin of Lake Tokai and eastern upheaving area. He called this tilting block as "Chubu Tilting Block". A large amount of gravel supply suggests the beginning of new structural control superposing the older one. At ca 3.0 Ma, the Lake Tokai sedimentary basin migrated north‐

**Figure 13.** Paleogeographic map of the central Kinki Region including Paleo-Lake Biwa and Lake Tokai area [32] e: & f: Older stage II of Paleo-Lake Biwa and Stage II of Lake Tokai, g: final stage of Older stage II of Paleo-Lake Biwa and

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

Subsequently, the sedimentary basin of Lake Tokai migrated to the west as a whole, although slightly northward, because of active movement of "Chubu Tilting Block" under the stress state of the E-W trend. However, during the same time interval, the sedimentary basin of Paleolake Biwa migrated gradually northward by the upheaval of southern area. At the time of ca 1.2 Ma, the sedimentary basin of Lake Tokai became to extinct, accompanying the upheaving of the Suzuka and Yoro mountains with a N-S structural trend. It was peculiar that the sedimentary basin of Paleo-lake Biwa transferred its position northwestward. Thereafter, it migrated westward gradually by tilting of the eastern area. This fact shows the origination of

In this way, the sedimentary basin migration in Lake Tokai and Paleo-lake Biwa is commonly explained by the hypothesis of interaction between upheaving of the southern area and tilting

conspicuous movement under E-W tectonic stress in the Ohni Basin.

westward.

Stage II of Lake Tokai

of the eastern area (Figure 14). Conclusively we proposed an idea that the deposition in the central part of Kinki region since the Pliocene was under the influence of two superposed tectonic stress states represented by upheaving of the southern area and tilting of the eastern area. Hitherto, Huzita [34] have stated that the tectonic stress state change since Miocene took place in the Inner zone of southwestern Japan during the time of Pliocene and Pleistocene. This change from N-S compressional stress state to E-W one was conspicuous. Therefore, the results of the present work are not contradictory to that idea. In fact, the results confirm the chrono‐ logical setting and detailed process of tectonosedimentary turnover since the Pliocene.
