10:45 AM - 11:00 AM
[HGM03-07] Why is Lake Biwa there?
Keywords:Lake Biwa, slab configuration, large-scale landform, dislocation model
Lake Biwa, located in the northern part of Shiga Prefecture, is the largest lake in Japan. It is considered that it was originally formed around Iga-Ueno about 4 million years ago, when the Kobiwako (Paleo-Biwa lake) Group began to be deposited, and then gradually moved northward.
A well-known cause of the formation of Lake Biwa is that Lake Biwa locates on the footwall of a reverse fault. Indeed, the depth of Lake Biwa is asymmetrical, shallow in the east and deep in the west, which can be well explained by the reverse fault movement of the Biwako-Seigan fault zone, together with the uplift of the Hira Mountains.
However, while there are many reverse faults in the Kinki region, Lake Biwa is the only one. In other words, even if the "the footwall of the reverse fault" is one of the important reasons for the existence of Lake Biwa, other important reasons are clearly missing. We consider there are two other important reasons.
One reason is that Lake Biwa locates in the subsidence zone of central Japan, which passes from Ise Bay to Wakasa Bay. The cause of this subsidence is due to the subduction of the Philippine Sea slab with a ridge-like shape just beneath it (Fukahata and Mori, 2022, 2023, JpGU). In short, the subsidence can be theoretically explained using the dislocation model of subduction zones (Matsu'ura & Sato, 1989, GJI; Fukahata & Matsu'ura, 2016, GJI). Although the regional subsidence is disturbed by the uplift of the Yoro Mountains and the Suzuka Mountains, the rias coast of Wakasa Bay and the existence of the Nobi Plain suggest the significant subsidence of this region, which is supported by the negative free-air gravity anomalies.
Another important reason is that Lake Biwa locates on the extension of the Seto Inland Sea, which is the so-called central lowland zone. The central lowland zone, formed between the fore-arc mountains and volcanic arcs, is widely observed in island arcs, such as Cascadia and Chile, where slabs are subducted at relatively low angles. The lowland belt from the Kitakami River to the Abukuma River in the Tohoku district is also a central lowland zone. The central lowland zone can be reproduced by the dislocation model of subduction zones through numerical simulations. In the Pliocene, the Paleo Setouchi Lowland is thought to have extended across the Kinki region to the area of the Nobi Plain (Ichihara, 1966, Chikyu-kagaku). As mentioned above, Lake Biwa was born within this lowland zone. Because the Kii Mountains locates above the valley of the Philippine Sea slab, it has now been uplifted violently, which disturbed the structure of this central lowland zone and have moved Lake Biwa to the north.
A well-known cause of the formation of Lake Biwa is that Lake Biwa locates on the footwall of a reverse fault. Indeed, the depth of Lake Biwa is asymmetrical, shallow in the east and deep in the west, which can be well explained by the reverse fault movement of the Biwako-Seigan fault zone, together with the uplift of the Hira Mountains.
However, while there are many reverse faults in the Kinki region, Lake Biwa is the only one. In other words, even if the "the footwall of the reverse fault" is one of the important reasons for the existence of Lake Biwa, other important reasons are clearly missing. We consider there are two other important reasons.
One reason is that Lake Biwa locates in the subsidence zone of central Japan, which passes from Ise Bay to Wakasa Bay. The cause of this subsidence is due to the subduction of the Philippine Sea slab with a ridge-like shape just beneath it (Fukahata and Mori, 2022, 2023, JpGU). In short, the subsidence can be theoretically explained using the dislocation model of subduction zones (Matsu'ura & Sato, 1989, GJI; Fukahata & Matsu'ura, 2016, GJI). Although the regional subsidence is disturbed by the uplift of the Yoro Mountains and the Suzuka Mountains, the rias coast of Wakasa Bay and the existence of the Nobi Plain suggest the significant subsidence of this region, which is supported by the negative free-air gravity anomalies.
Another important reason is that Lake Biwa locates on the extension of the Seto Inland Sea, which is the so-called central lowland zone. The central lowland zone, formed between the fore-arc mountains and volcanic arcs, is widely observed in island arcs, such as Cascadia and Chile, where slabs are subducted at relatively low angles. The lowland belt from the Kitakami River to the Abukuma River in the Tohoku district is also a central lowland zone. The central lowland zone can be reproduced by the dislocation model of subduction zones through numerical simulations. In the Pliocene, the Paleo Setouchi Lowland is thought to have extended across the Kinki region to the area of the Nobi Plain (Ichihara, 1966, Chikyu-kagaku). As mentioned above, Lake Biwa was born within this lowland zone. Because the Kii Mountains locates above the valley of the Philippine Sea slab, it has now been uplifted violently, which disturbed the structure of this central lowland zone and have moved Lake Biwa to the north.