1:45 PM - 3:15 PM
[SCG62-P03] Crustal structure and tectonic development of the Kanto region, Japan: revealed by seismic profiling
Keywords:Crustal structure, Integrated seismic profiling, Slab geometry, Philippine Sea Plate, Kanto region
Subduction of the Philippine Sea plate (PHS) on the Pacific plate (PAC) beneath Kanto produces unique tectonic settings. Since 2001, crustal structure and subducting slab geometry of PHS has been investigated using integrated active and passive seismic profiling. Here, we reviewed these results and characterize the crustal structure, including the geometry of subduction PHS slab, and discuss on the tectonic development of late Cenozoic.
The upper surface of PHS was imaged along the 9 seismic lines by deep seismic reflection profiling and receiver function analysis of earthquake data obtained by dense passive array. The distribution of depth to the upper surface of PHS was well defined beneath the Kanto plain (1). The upper surface of PHS forms ridge shaped geometry and an NNW-SSW trending anticlinal axis lies on the NW part of the Izu collision zone (2). The upper surface of PHS shows gentle dip in the eastern wing of the anti-form, while the western wing shows steeper angle. The western rim of the epicentral area of the 1923 Kanto earthquake coincides with the axial trace of the anti-form, suggesting that the ruptured area was controlled slab geometry. The asymmetric geometry of PHS slab can be traced around Izu peninsula. The dip of upper surface of PHS along the Suruga trough is steeper than that of the Sagami trough. The slab of PHS is traced beneath the Kofu basin at the depth of 40 km. However, its southwestward extension is not well defined beneath the NW part of the Izu collision zone.
Low velocity zone consists of Neogene accretionary body is well developed in the toe of megathrust beneath the Boso and Miura peninsula. Beneath the Hayama - Mineoka uplifted zone, a 10-km-thick Vp 4 to 5 km zone is developed. The velocity structure suggest that this low velocity zone was produced by the underplating of accretionary body. Due to the uplifting of the accretionary prism, large sedimentary basin was developed beneath Kanto plain. Off Kawasaki, maximum thickness of the basin fill reaches 6 km.
In the northern part of the Kanto plain, WNW-WSW trending, negative Bouguer anomaly zone is developed. Seismic section across this zone portrays a 4-km-thick basin fill. This zone is a Miocene back-arc rift developed in the boundary between NE and SW Honshu. Earthquake tomogram shows development of high P-wave velocity zone in the middle to lower crust, and crustal thinning (3). These features are similar to the failed rift distributed in the northern Fossa magna to Niigata basin. These rifts were formed by crustal necking and high velocity middle to lower crust represents the significant intrusion of mafic rocks associated with the opening of the Sea of Japan. Marginal fault of such rift zone commonly dips outward of the rift zone in Niigata and northern Fossa magna basin. In NW Kanto basin, basin boundary fault (Fukaya fault) is reactivated as a southward dipping reverse fault, probably represents the same feature.
Associated with the opening of the Sea of Japan, a large right-lateral tectonic movement with NS-extension occurred in the northern part of the Kanto area. WNW-ESE and NS-trending rifts were developed in the Middle Miocene. These failed rifts are marked by thick basin fill and higher P-wave velocity zone in the middle to lower crust. NNW motion of PHS formed the buoyant subduction at the Izu collision zone. NNW-trending anti-form of PHS developed during this stage (15-1 Ma). Due to the subduction, Hayama-Mineoka uplifted zone was developed as an outer ridge of accretionary prism. Since 1 Ma, change in motion of PHS from NNW to WNW, generated the change the pattern of vertical crustal movement (4), asymmetric geometry of PHS slab around the Izu collision zone.
(1) Sato, H. et al., Science, 309 (5737), 462-464, 2005. (2) Sato, H., Comprehensive report on the Special project for Earthquake Disaster Mitigation in Tokyo Metropolitan Area, 15-24, 2012. (3) Matsubara, M. et al., Tectonophysics, 710-711, 97-107, 2017. (4) Hashima, A. et al., Tectonophys., 679, 1-14, doi: 10.1016/j.tecto.2016.04.005, 2016.
The upper surface of PHS was imaged along the 9 seismic lines by deep seismic reflection profiling and receiver function analysis of earthquake data obtained by dense passive array. The distribution of depth to the upper surface of PHS was well defined beneath the Kanto plain (1). The upper surface of PHS forms ridge shaped geometry and an NNW-SSW trending anticlinal axis lies on the NW part of the Izu collision zone (2). The upper surface of PHS shows gentle dip in the eastern wing of the anti-form, while the western wing shows steeper angle. The western rim of the epicentral area of the 1923 Kanto earthquake coincides with the axial trace of the anti-form, suggesting that the ruptured area was controlled slab geometry. The asymmetric geometry of PHS slab can be traced around Izu peninsula. The dip of upper surface of PHS along the Suruga trough is steeper than that of the Sagami trough. The slab of PHS is traced beneath the Kofu basin at the depth of 40 km. However, its southwestward extension is not well defined beneath the NW part of the Izu collision zone.
Low velocity zone consists of Neogene accretionary body is well developed in the toe of megathrust beneath the Boso and Miura peninsula. Beneath the Hayama - Mineoka uplifted zone, a 10-km-thick Vp 4 to 5 km zone is developed. The velocity structure suggest that this low velocity zone was produced by the underplating of accretionary body. Due to the uplifting of the accretionary prism, large sedimentary basin was developed beneath Kanto plain. Off Kawasaki, maximum thickness of the basin fill reaches 6 km.
In the northern part of the Kanto plain, WNW-WSW trending, negative Bouguer anomaly zone is developed. Seismic section across this zone portrays a 4-km-thick basin fill. This zone is a Miocene back-arc rift developed in the boundary between NE and SW Honshu. Earthquake tomogram shows development of high P-wave velocity zone in the middle to lower crust, and crustal thinning (3). These features are similar to the failed rift distributed in the northern Fossa magna to Niigata basin. These rifts were formed by crustal necking and high velocity middle to lower crust represents the significant intrusion of mafic rocks associated with the opening of the Sea of Japan. Marginal fault of such rift zone commonly dips outward of the rift zone in Niigata and northern Fossa magna basin. In NW Kanto basin, basin boundary fault (Fukaya fault) is reactivated as a southward dipping reverse fault, probably represents the same feature.
Associated with the opening of the Sea of Japan, a large right-lateral tectonic movement with NS-extension occurred in the northern part of the Kanto area. WNW-ESE and NS-trending rifts were developed in the Middle Miocene. These failed rifts are marked by thick basin fill and higher P-wave velocity zone in the middle to lower crust. NNW motion of PHS formed the buoyant subduction at the Izu collision zone. NNW-trending anti-form of PHS developed during this stage (15-1 Ma). Due to the subduction, Hayama-Mineoka uplifted zone was developed as an outer ridge of accretionary prism. Since 1 Ma, change in motion of PHS from NNW to WNW, generated the change the pattern of vertical crustal movement (4), asymmetric geometry of PHS slab around the Izu collision zone.
(1) Sato, H. et al., Science, 309 (5737), 462-464, 2005. (2) Sato, H., Comprehensive report on the Special project for Earthquake Disaster Mitigation in Tokyo Metropolitan Area, 15-24, 2012. (3) Matsubara, M. et al., Tectonophysics, 710-711, 97-107, 2017. (4) Hashima, A. et al., Tectonophys., 679, 1-14, doi: 10.1016/j.tecto.2016.04.005, 2016.