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[SCG55-12] Late Cenozoic shortening deformation in the back-arc of Northern Honshu, Japan
Keywords:Shortening deformation, back-arc, NE Japan arc, late Cenozoic, style of deformation
1. Introduction Style of shortening deformation and its special distribution were investigated mainly surface geology. Recent surveys on the crustal structure by seismic profiling provided the new image of style of crustal deformation. Here, I present the late Cenozoic shortening deformation in the back-arc of northern Honshu, Japan, based on the data of crustal structures obtained by seismic reflection profiling.
2. Tectonic Inversion Common feature of shortening deformation is a tectonic inversion of normal faults as reverse faults. Normal faulting occurred in the stage of opening the Sea of Japan (20-15 Ma). The basin inversion was found in the Sea of Japan area (Okamura et al., 1995) and followed in the forearc region, such as Kitakami rever valley area (Sato et al., 2004; Kato et al., 2006). In the stretched continental crust in the back-arc, failed rift zones were developed along the coastal area of northern Honshu, such as Akita-Yamagata and Niigata-northern Fossa magna area. The failed rifts are characterized the floored mafic igneous rocks and thick sedimentary fill. The boundary between intruded mafic rich crust and normal continental crust shows dipping surface toward outside of the rift zone. This surface reactivated as a thrust during the later shortening deformation stage. Due to thrusting, a fold-and-thrust belt was developed in the rift-fill sediments (Sato, 2014). Tectonic inversion is commonly observed in the forearc to back-arc except for the Ou back bone range. The Ou backbone range is located along the present volcanic front of NE Japan arc. The Ou backbone range forms a pop-up structure bounded by reverse fault (Sato et al., 2002). To find out the evidence of normal faulting before reverse faulting in those faults is difficult. The suitable mechanism for the origin of this uplift zone is the thermal regime beneath the backbone rage under compressional stress regime suggested by Shbazaki et al. (2008). The volcanic front has been stayed along the backbone range since 13 Ma.
3. Plate boundary hypothesis along the eastern margin of the Sea of Japan Degree of shortening deformation in Late Cenozoic has been studied through surface geology (Otsuka, 1938; Matsuda et al., 1987; Sato, 1989). On shore area, the degree of shortening deformation increase toward back-arc and shows its maximam along the failed rift zone. The height of shoreline at 125 ka (Kobayashi and Machida eds., 2001) shows concordant results with the amount of shortening deformation. Nakamura and Uyeda (1980) described the stress gradient in overriding plate. However, it is very difficult to explain the shortening deformation in the northern Honshu. Nakamura (1983) proposed the new plate boundary along the eastern margin of the Sea of Japan. This was supported the occurrence of 1983 Nihonkai chubu earthquake, and 1993 Hokkaido Nansei-oki earthquake. As the large earthquakes (> M 7.5) in the 20th century lined up along the eastern part of the Sea of Japan, this plate boundary has been widely accepted. However, it is discussed that there are no corresponding faults to the plate boundary faults (Okamura et al, 1999). The new seismic survey by JAMSTEC, revealed that the 1983 Nihonkai-chubu earthquake occurred along the boundary between oceanic curt and Honshu continental crust (No et al., 2014). East-dipping thrust was formed by necking of continental crust and uplift of mafic magma. Modeling of seismogenic source fault has been made using the seismic reflection data acquired by JAMSTEC an ERI, Univ. Tokyo (Sato et al., 2021) shows large number of active faults distributing in the eastern margin of the Sea of Japan, where the stretched continental crust area. It shows that the strain is distributed in the wide area of the back-arc continental crust and to define the plate boundary is very difficult. As it is shown by the 2024 Noto earthquake, the large earthquake (>M7.5) occurs not along the plate boundary. The size of size of source faults reflects the amount of stretching during the opening of the Sea of Japan.
2. Tectonic Inversion Common feature of shortening deformation is a tectonic inversion of normal faults as reverse faults. Normal faulting occurred in the stage of opening the Sea of Japan (20-15 Ma). The basin inversion was found in the Sea of Japan area (Okamura et al., 1995) and followed in the forearc region, such as Kitakami rever valley area (Sato et al., 2004; Kato et al., 2006). In the stretched continental crust in the back-arc, failed rift zones were developed along the coastal area of northern Honshu, such as Akita-Yamagata and Niigata-northern Fossa magna area. The failed rifts are characterized the floored mafic igneous rocks and thick sedimentary fill. The boundary between intruded mafic rich crust and normal continental crust shows dipping surface toward outside of the rift zone. This surface reactivated as a thrust during the later shortening deformation stage. Due to thrusting, a fold-and-thrust belt was developed in the rift-fill sediments (Sato, 2014). Tectonic inversion is commonly observed in the forearc to back-arc except for the Ou back bone range. The Ou backbone range is located along the present volcanic front of NE Japan arc. The Ou backbone range forms a pop-up structure bounded by reverse fault (Sato et al., 2002). To find out the evidence of normal faulting before reverse faulting in those faults is difficult. The suitable mechanism for the origin of this uplift zone is the thermal regime beneath the backbone rage under compressional stress regime suggested by Shbazaki et al. (2008). The volcanic front has been stayed along the backbone range since 13 Ma.
3. Plate boundary hypothesis along the eastern margin of the Sea of Japan Degree of shortening deformation in Late Cenozoic has been studied through surface geology (Otsuka, 1938; Matsuda et al., 1987; Sato, 1989). On shore area, the degree of shortening deformation increase toward back-arc and shows its maximam along the failed rift zone. The height of shoreline at 125 ka (Kobayashi and Machida eds., 2001) shows concordant results with the amount of shortening deformation. Nakamura and Uyeda (1980) described the stress gradient in overriding plate. However, it is very difficult to explain the shortening deformation in the northern Honshu. Nakamura (1983) proposed the new plate boundary along the eastern margin of the Sea of Japan. This was supported the occurrence of 1983 Nihonkai chubu earthquake, and 1993 Hokkaido Nansei-oki earthquake. As the large earthquakes (> M 7.5) in the 20th century lined up along the eastern part of the Sea of Japan, this plate boundary has been widely accepted. However, it is discussed that there are no corresponding faults to the plate boundary faults (Okamura et al, 1999). The new seismic survey by JAMSTEC, revealed that the 1983 Nihonkai-chubu earthquake occurred along the boundary between oceanic curt and Honshu continental crust (No et al., 2014). East-dipping thrust was formed by necking of continental crust and uplift of mafic magma. Modeling of seismogenic source fault has been made using the seismic reflection data acquired by JAMSTEC an ERI, Univ. Tokyo (Sato et al., 2021) shows large number of active faults distributing in the eastern margin of the Sea of Japan, where the stretched continental crust area. It shows that the strain is distributed in the wide area of the back-arc continental crust and to define the plate boundary is very difficult. As it is shown by the 2024 Noto earthquake, the large earthquake (>M7.5) occurs not along the plate boundary. The size of size of source faults reflects the amount of stretching during the opening of the Sea of Japan.