**4. Basin formation and migration in central Kinki since the Pliocene**

Three sedimentary basins are arranged E-W latitudinally in the eastern part of "Second Setouchi Inland Depression of [25]: Lake Tokai, Paleo-Lake Biwa and Paleo-lake (bay) Osaka. All are filled with the Plio-Pleistocene deposits (Figure 11). They are respectively named Tokai,

**Figure 10.** Bouguer gravity anomaly map. The Bouguer density is 2670 kg/m3, and contour interval is 2 mGal

Kobiwako, and Osaka Groups. Comparing of the sedimentary basin transition between Paleo-Lake Biwa and Lake Tokai, the geohistory in central Kinki Region is known subdivisible by tectonosedimentary facies of about 3.0 Ma and 1.2-1.5 Ma (Figures 12, 13) which suggests that the change of tectonic stress state of this province took place simultaneously throughout these sedimentary basins. To complete the geohistory of Paleo-lake Biwa, patterns of sedimentary basin transition must be discussed. According to Yokoyama ([26], [27]), the geohistory of Paleo-Lake Biwa has four stages: Older I, Older II, Actual I, and Actual II. First, Paleo-Lake Biwa appeared in the Iga Basin and clay-dominant sediments were deposited in the water body ("Iga-ko") (a and b in Figure 12) [28]. In the second, the sedimentary basin center shifted its place to the north from Iga Basin to Ohmi Basin, forming a stable water body ("Sayama-ko") with massive clay deposition (c and d in Figure 12). Northward shifting of the sedimentary basin was inherited (Older II Stage) (e and f in Figure 13). During the time from Older II stage to Actual I stage, the center of the sedimentary basin migrated northwestward on a large-scale. Great amounts of gravel are represented as final sediments of Older II stage (g in Figure 13). The sedimentary basin of Actual I stage shifted its place gradually to west, accompanied by upheaval of the eastern mountain area. This explanation is supported by results of lithology, paleocurrent and sedimentological studies of the deposits of the Kosei area and data from 1,000

**4. Basin formation and migration in central Kinki since the Pliocene**

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

Three sedimentary basins are arranged E-W latitudinally in the eastern part of "Second Setouchi Inland Depression of [25]: Lake Tokai, Paleo-Lake Biwa and Paleo-lake (bay) Osaka. All are filled with the Plio-Pleistocene deposits (Figure 11). They are respectively named Tokai,

**Figure 9.** Active structure revealed by horizon A (Boundary of T and S bed) since about 0.43 Ma) of the seismic reflec‐ tion survey [19] Meshed part: depth of A horizon more than 400 m, Dotted part: depth of A horizon lower than 300 m

**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

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.

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

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 12.** Paleogeographic map of the central Kinki Region including Paleo-Lake Biwa and Lake Tokai area from the early Pliocene to 1.2 – 1.5 Ma [32] a: Initial Stage I of Lake Tokai and Initial Older Stage of Paleo-Lake Biwa, b: Stage I of Lake Tokai and Older Stage of Paleo-Lake Biwa, c: d: Older stage II of Paleo-Lake Biwa and Stage II of Lake Tokai

Tectonic Basin Formation in and Around Lake Biwa, Central Japan

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

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**development and mechanism**

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


Tectonic Basin Formation in and Around Lake Biwa, Central Japan http://dx.doi.org/10.5772/56667 221

**Figure 12.** Paleogeographic map of the central Kinki Region including Paleo-Lake Biwa and Lake Tokai area from the early Pliocene to 1.2 – 1.5 Ma [32] a: Initial Stage I of Lake Tokai and Initial Older Stage of Paleo-Lake Biwa, b: Stage I of Lake Tokai and Older Stage of Paleo-Lake Biwa, c: d: Older stage II of Paleo-Lake Biwa and Stage II of Lake Tokai
