11:00 AM - 11:15 AM
[SCG50-08] Tectonic development in the Median Tectonic Line and surrounding regions during the Late Cretaceous to Paleogene periods: Spatiotemporal development
★Invited Papers
Keywords:Median Tectonic Line, The Late Cretaceous to Paleogene, Kinematics, Oblique subduction, Low-angle normal faulting, Sinistral transpression
The tectonic development of the Median Tectonic Line (MTL) during the period from the Late Cretaceous to Paleogene has been studied for a long time, and a synthesis on it was given in the Special Issue entitled “Median Tectonic Line of Southwest Japan” published by the Geological Society of Japan in 1980. Here, it has been summarized that the proto-MTL was originated as mylonitic shear zones in the Ryoke granitoids in the Late Cretaceous (Kashio phase), and then the Uppermost Cretaceous Izumi Group was deposited in pull-apart basins formed at the north side of MTL. Subsequently, the Sambagawa metamormorphic rocks in the outer belt was greatly elevated, and juxtaposed against the Ryoke granitoids and Izumi Group before the deposition of the Eocene Kuma Group (the formation of the MTL sensu stricto at the so-called Ichinokawa phase). Further, it has been believed that the Uppermost Cretaceous Izumi Group thrusted over the Eocene Kuma Group at the Tobe phase in the Eocene.
The tectonic development of the MTL had not been studied much since then until Takeshita and co-workers made new discoveries about it after 1990s. Although the fault rocks and kinematics of the MTL at the Kashio phase in the Late Cretaceous have been intensely investigated by e.g. Hara et al. (1986) and Takagi (1986), the kinematics of the MTL during the Paleogene period has remained to be unclear. This is not only because the kinematics of the so-called Ichinokawa phase has not been described at the type locality, but also because most of the Kuma Group has been found to be the Early Miocene, not Eocene in age (Kashima and Takechi, 1996; Narita et al., 1999; Takeshita et al., 2000). In the following, the tectonic development of the MTL from the Late Cretaceous to Paleogene will be summarized including the new discoveries, including some geophysical implications.
(a) Kashio phase in the Late Cretaceous
It has been known that large-scale sinistral shearing occurred along the proto-MTL based on the foliation deflection and microstructures in mylonites (e. g. Hara et al., 1980; Takagi ,1986). Similarly, a top-to-the-west sense of shear was recorded in the Sambagawa metamorphic rocks (e. g. Takeshita and Yagi, 2004). The movement around the MTL can be ascribed to the oblique subduction of the Izanagi plate in the NNW direction relative to the NNE-SSW trending proto-Japanese arc.
(b) So-called Ichinokawa phase at c. 59 Ma in the Paleocene
It has been discovered by Kubota and Takeshita (2008) for the first time that the Ichinokawa phase is characterized by top-to-the-north, large-scale normal faulting based on the occurrence of a few hundred meter thick boudinaged belt and the north-vergent drape folds in the Izumi Group at the north side of the MTL (Kubota and Takeshita, 2008). On the other hands, in the Sambagawa metamorphic rocks, not only numerous low angle normal faults developed at the Ichinokawa phase, but also it has been found by seismic reflection studies (Sato et al., 2005; Ito et al., 2009) that the original shape of the MTL itself was a low-angle north dipping fault. This low-angle normal faults, which are common to those seismogenic faults in Italy, perhaps resulted from the anisotropic ductile strength of the Sambagawa metamorphic rocks, which are more prone to ductile flow parallel to the foliation.
(c) pre-Tobe phase at c. 47 Ma in the Eocene
The Kumamoto earthquake in 2016 in fact made it clear that the MTL has been still fairly active during the Quaternary period. It should be known however that the position of the active MTL systems does not coincide with the one of the MTL sensu stricto, but located in the Izumi Group at the north of the MTL (e. g. Okada, 1973). While there has been no information on when and under what kinematics the active MTL systems were originally formed, it has been newly discovered that the faults systems were originally formed under the setting of sinistral transpression at c. 47 Ma in the Eocene (Kubota et al., 2020). This phase of movement of the MTL has been named as the pre-Tobe phase, because the Tobe thrusting has been now found to occur at 15 Ma (Takeshita et al., 2000).
The tectonic development of the MTL had not been studied much since then until Takeshita and co-workers made new discoveries about it after 1990s. Although the fault rocks and kinematics of the MTL at the Kashio phase in the Late Cretaceous have been intensely investigated by e.g. Hara et al. (1986) and Takagi (1986), the kinematics of the MTL during the Paleogene period has remained to be unclear. This is not only because the kinematics of the so-called Ichinokawa phase has not been described at the type locality, but also because most of the Kuma Group has been found to be the Early Miocene, not Eocene in age (Kashima and Takechi, 1996; Narita et al., 1999; Takeshita et al., 2000). In the following, the tectonic development of the MTL from the Late Cretaceous to Paleogene will be summarized including the new discoveries, including some geophysical implications.
(a) Kashio phase in the Late Cretaceous
It has been known that large-scale sinistral shearing occurred along the proto-MTL based on the foliation deflection and microstructures in mylonites (e. g. Hara et al., 1980; Takagi ,1986). Similarly, a top-to-the-west sense of shear was recorded in the Sambagawa metamorphic rocks (e. g. Takeshita and Yagi, 2004). The movement around the MTL can be ascribed to the oblique subduction of the Izanagi plate in the NNW direction relative to the NNE-SSW trending proto-Japanese arc.
(b) So-called Ichinokawa phase at c. 59 Ma in the Paleocene
It has been discovered by Kubota and Takeshita (2008) for the first time that the Ichinokawa phase is characterized by top-to-the-north, large-scale normal faulting based on the occurrence of a few hundred meter thick boudinaged belt and the north-vergent drape folds in the Izumi Group at the north side of the MTL (Kubota and Takeshita, 2008). On the other hands, in the Sambagawa metamorphic rocks, not only numerous low angle normal faults developed at the Ichinokawa phase, but also it has been found by seismic reflection studies (Sato et al., 2005; Ito et al., 2009) that the original shape of the MTL itself was a low-angle north dipping fault. This low-angle normal faults, which are common to those seismogenic faults in Italy, perhaps resulted from the anisotropic ductile strength of the Sambagawa metamorphic rocks, which are more prone to ductile flow parallel to the foliation.
(c) pre-Tobe phase at c. 47 Ma in the Eocene
The Kumamoto earthquake in 2016 in fact made it clear that the MTL has been still fairly active during the Quaternary period. It should be known however that the position of the active MTL systems does not coincide with the one of the MTL sensu stricto, but located in the Izumi Group at the north of the MTL (e. g. Okada, 1973). While there has been no information on when and under what kinematics the active MTL systems were originally formed, it has been newly discovered that the faults systems were originally formed under the setting of sinistral transpression at c. 47 Ma in the Eocene (Kubota et al., 2020). This phase of movement of the MTL has been named as the pre-Tobe phase, because the Tobe thrusting has been now found to occur at 15 Ma (Takeshita et al., 2000).