**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 the Q Bed and the P Bed (Figure 3). The alternating characteristics of the S, R, and Q beds indicate short-term cycles in the depositional environment. These cycles may represent periods of uplift of surrounding areas, with subsequent erosional readjustments, or periods of subsidence of the lake basin with subsequent infilling. A study of the lithology and chronology of the Kobiwako Group, the Plio-Pleistocene freshwater sediments distributed around Lake Biwa, reveals that the history of this basin is divisible into two main stages; the Older and the Actual [27]. The deposits of the Older Stage are mainly distributed southward of the present Lake Biwa. Accordingly, the sedimentary depocenter of the Older Stage was located south‐ ward of the modern Lake Biwa. Between the Older Stage and the Actual Stage, the depocenter evidently shifted northward to its present location, as indicated by the stratigraphy and chronology of the Kobiwako Group, creating the basin of modern Lake Biwa. Large amounts of gravel began to deposit at the northern part of the Older Stage Lake Biwa basin starting about 1.5 Ma. This occurrence is thought to record the beginning of the migration of the lake depocenter. The R bed corresponds to the sediments of the early stage of Actual Stage. The upper part of the S bed and the entire T bed represent the Actual Stage. The high and constant sedimentation rates within the drilling sequences in present Lake Biwa represent a time of rapid infilling. Rapid subsidence was necessary for the deposition of the continuous sequence in the present Lake Biwa with a sedimentation rate of 0.57 m/kyr [13] and inferred a subsidence rata of 0.74 m/kyr from the data of age of T bed and water depth at the 1400 m drilling site. Reconstruction of these tectonic implications cannot be done competently from data from one core site, and we must use surrounding geological data and geophysical data.

Figure 15 shows the first-order horizontal derivative of the Bouguer gravity anomalies (the horizontal gradient of gravity anomaly) more than 2 mGal/km and contour interval is 1 mGal/ km. The horizontal gradient of gravity anomaly (Δ*ghg*) is defined as the following equation.

$$\Delta \mathbf{g}\_{hg} = \sqrt{\left(\frac{\partial \mathbf{g}\left(\mathbf{x}, \mathbf{y}\right)}{\partial \mathbf{x}}\right)^2 + \left(\frac{\partial \mathbf{g}\left(\mathbf{x}, \mathbf{y}\right)}{\partial \mathbf{y}}\right)^2} \tag{1}$$

**Figure 15.** First order horizontal derivative of Bouguer gravity anomalies more than 2 mGal/km. Contour intervals is 1 mGal/km. MTL: Median Tectonic Line, ATL: Arima Takatsuki Tectonic Line, R-A: Rokko-Awaji Fault zone, I: Ikoma Fault zone, K: Kambayashi Fault, H: Hanaore Fault, B: Biwako-seigan Fault zone, Ya: Yanagase Fault zone, Yo: Yoro Fault

Tectonic Basin Formation in and Around Lake Biwa, Central Japan

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

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The present Lake Biwa is a tectonic basin under the E-W compressional stress state. Distribu‐ tion of active faults is characterized by the western boundary of the lake (Biwako-seigan Fault zone; Hira, Katata Faults with N-S direction) and the northeast region of Lake (Yanagase Fault etc). Activity of faults (mainly Biwako-seigan Fault zone) of more than 1 m/kyr along the west

Basement topography revealed by the seismic reflection survey shows alignment of the valley and range with a N-S direction. The "A" horizon (bottom of T bed since about 0.43 Ma) topography of the seismic reflection profile revealed its tilting topography from east to west. The sedimentary record from present Lake Biwa by deep drilling includes a shift of the lake depocenter from farther to the south to its current location. The sedimentary record shows

side of Lake Biwa is important for the present lake basin formation.

zone

**7. Summary**

Here, g(*x, y*) are gravity anomaly data given on *xy* mesh with a constant interval.

The horizontal gradient of gravity anomaly emphasizes shorter wavelength signals of subsurface structures. Therefore, it is a good indicator of a conspicuous density change and/or a large fault. High gradient anomalies of greater than 2 mGal/km correspond well with large faults and tectonic lines such as the Rokko-Awaji fault zone, Arima-Takatsuki tectonic line, Ikoma fault, Kanbayashigawa fault, Hanaore fault, Biwako-seigan fault and others. They have NE-SW and E-W trends in their strike directions. However, distribution of high gradient anomalies around the Lake Biwa is extremely complex. It is difficult to find a specific trend in the strike directions of the horizontal gradient of gravity anomaly. As one reason, it can be inferred that they reflect subsurface structures caused by extreme crustal activities including faulting during the Quaternary.

**Figure 15.** First order horizontal derivative of Bouguer gravity anomalies more than 2 mGal/km. Contour intervals is 1 mGal/km. MTL: Median Tectonic Line, ATL: Arima Takatsuki Tectonic Line, R-A: Rokko-Awaji Fault zone, I: Ikoma Fault zone, K: Kambayashi Fault, H: Hanaore Fault, B: Biwako-seigan Fault zone, Ya: Yanagase Fault zone, Yo: Yoro Fault zone
