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[SGL23-P02] Atogura thrust and high-angle faults in the Yorii-Ogawa area of the Kanto Mountains
Keywords:Atogura Nappe, Atogura thrust, High-angle faults
Sanbagawa metamorphic rocks, Atogura Nappe, Chichibu accretionary complex and Miocene deposits are distributed in the northern margin of the Kanto Mountains (Figure 1). The Paleogene Atogura Nappe overlies Mikabu Unit and Chichibu accretionary complex. In the early Miocene, the Atogura and Ryoke Nappes were exposed in the northern margin of the Kanto Mountains, and the Chichibu accretionary complex was distributed to the south of the Nappes. The Ushibuseyama Nappe was formed at about 15 Ma, and then high-angle faults such as the Jyushi-Kuroya fault and the Kanai fault are activated. Reflecting the complex geological history, thrusts and high-angle faults coexist in the study area. In this article we describe the basic geological structures of the Yorii-Ogawa region.
High-angle faults mostly with near-vertical dips are observed or assumed between the Atogura Nappe and the Mikabu Unit or the Kamiyoshida Unit of the Chichibu accretionary complex (Figures 2 and 3). The Mikabu and Kamiyoshida Units have risen along the high-angle faults. For example, the Kiroko greenstone mélange is in contact with the Kamiyoshida Unit by high-angle faults, but the Kiroko greenstone mélange is partly missing in the Kibe area (Figure 4). The Kiroko greenstone mélange in the Kibe area was eroded and disappeared after the markable upward movement.
Atogura thrusts have been reported from localities X, Y, and Z at about 100-150 m elevation. At point X, the Yorii Formation and the Mikabu Unit are in contact, but the thrust is cut by an east-west high-angle fault slightly to the south of the thrust outcrop (Figure 5). No low-angle faults have been found to the south of this high-angle fault. The thrust at point Y appeared due to progress of quarrying in the Atogura Formation and is now submerged. At point Z, the Kinshozan quartz diorite is assumed to be in contact with greenstones at an inclination of about 40°S. However, all the faults examined in the surrounding area are high-angle faults.
Geological bodies of various sizes that compose the Atogura Nappe are connected to each other by high-angle faults (Figure 3). These high-angle faults of the Atogura Nappe are confirmed in many outcrops although some of them are assumed on the basis of the existence of fracture zones with almost vertical fracture surfaces and the relationship between topography and geology. In the study area, a layered structure in which multiple nappes are piled up has not been confirmed.
At localities A and B in Figure 3, the extension of the high-angle fault inside the Atogura Nappe is the boundary between the Mikabu Unit and the Atogura Nappe. The question arises as to how much the surrounding Atogura Nappe have risen along the high-angle faults in association with the uplift of the Mikabu Unit. The conclusion is that there is no geological evidence for denying or affirming the reactivation of the high-angle faults. Therefore, many high-angle faults inside the Atogura Nappe do not cut through the Atogura thrust in the geological section of Figure 3. There are two exceptions where steps of the Atogura thrust are formed.
There is a view that the Atogura thrust is widely recognized on the earth surface in the Mihina -Tochiya area. In this view, the Atogura thrust undulates greatly (Model B in Figure 6). This is unnatural. In addition, high-angle faults between the Mikabu Unit and the Atogura Nappe were confirmed at three localities, and were assumed at five localities (Figure 2).
In the northern margin of the Miocene Ogawa Basin, the boundaries between sandstones of the Kozono Formation and mudstones of the Arakawa Formation are approximately 110-140 m above sea level (red circles in Figure 3). Uplift tectonics occur uniformly in the wide region including the Paleogene Yorii Formation.
High-angle faults mostly with near-vertical dips are observed or assumed between the Atogura Nappe and the Mikabu Unit or the Kamiyoshida Unit of the Chichibu accretionary complex (Figures 2 and 3). The Mikabu and Kamiyoshida Units have risen along the high-angle faults. For example, the Kiroko greenstone mélange is in contact with the Kamiyoshida Unit by high-angle faults, but the Kiroko greenstone mélange is partly missing in the Kibe area (Figure 4). The Kiroko greenstone mélange in the Kibe area was eroded and disappeared after the markable upward movement.
Atogura thrusts have been reported from localities X, Y, and Z at about 100-150 m elevation. At point X, the Yorii Formation and the Mikabu Unit are in contact, but the thrust is cut by an east-west high-angle fault slightly to the south of the thrust outcrop (Figure 5). No low-angle faults have been found to the south of this high-angle fault. The thrust at point Y appeared due to progress of quarrying in the Atogura Formation and is now submerged. At point Z, the Kinshozan quartz diorite is assumed to be in contact with greenstones at an inclination of about 40°S. However, all the faults examined in the surrounding area are high-angle faults.
Geological bodies of various sizes that compose the Atogura Nappe are connected to each other by high-angle faults (Figure 3). These high-angle faults of the Atogura Nappe are confirmed in many outcrops although some of them are assumed on the basis of the existence of fracture zones with almost vertical fracture surfaces and the relationship between topography and geology. In the study area, a layered structure in which multiple nappes are piled up has not been confirmed.
At localities A and B in Figure 3, the extension of the high-angle fault inside the Atogura Nappe is the boundary between the Mikabu Unit and the Atogura Nappe. The question arises as to how much the surrounding Atogura Nappe have risen along the high-angle faults in association with the uplift of the Mikabu Unit. The conclusion is that there is no geological evidence for denying or affirming the reactivation of the high-angle faults. Therefore, many high-angle faults inside the Atogura Nappe do not cut through the Atogura thrust in the geological section of Figure 3. There are two exceptions where steps of the Atogura thrust are formed.
There is a view that the Atogura thrust is widely recognized on the earth surface in the Mihina -Tochiya area. In this view, the Atogura thrust undulates greatly (Model B in Figure 6). This is unnatural. In addition, high-angle faults between the Mikabu Unit and the Atogura Nappe were confirmed at three localities, and were assumed at five localities (Figure 2).
In the northern margin of the Miocene Ogawa Basin, the boundaries between sandstones of the Kozono Formation and mudstones of the Arakawa Formation are approximately 110-140 m above sea level (red circles in Figure 3). Uplift tectonics occur uniformly in the wide region including the Paleogene Yorii Formation.