11:30 AM - 11:45 AM
[SCG52-14] Spatial heterogeneity of fault structures in the continental crust before seafloor expansion in the Yaeyama Rift of south Okinawa Trough
Keywords:Okinawa Trough, back-arc basin, normal fault, East China Sea, magmatism
1. Introduction
Approximately five percent of the earth's surface is comprised of backarc basins.The Wilson cycle, which explains the birth and extinction of tectonic plates, does not include the backarc basins because its formation process, especially its dynamic characteristics, has not been clarified yet. There has been a long debate about the driving force of the backarc rifting, and two models, the corner-flow model and the slab-pull model, have been considered as primary mechanism. Once the backarc seafloor expansion occurs, the deformation experienced just before the expansion is disturbed. So, it is difficult to recognize the stress state before the expansion.
In this study, we focus on the Okinawa Trough, which is a unique backarc basin in the world characterized as an early stage of continental rifting yet to reach the stage of seafloor spreading. We research the southern part of the Okinawa Trough (around the Yaeyama Rift), where the seafloor is the deepest, faults develop densely, and magma rises from the subsurface. We examine the mechanical characteristics based on the geological structures.
2.Data and analysis method
We used the seafloor bathymetry and seismic reflection data obtained during the Hakuho Maru KH-21-3 cruise conducted in 2021(Otsubo et al., 2021) and bathymetric data of the GEBCO, compiled bathymetric data of the JAMSTEC and AIST(Misawa et al., 2020). The topographic images around the Yaeyama Rift were created using the GMT (Generic Mapping Tools) software (Wessel et al. 2019). Based on the images, we detected step structures and then measures their spacing and strike. And we calculated the thickness of the mechanical layer by applying the average spacing of the structures to equation suggested by Soliva et al. (2006).
3.Results and Discussion
We identified 157 step structures, most of which have the E-W orientation. The number of identified structures was almost the same on the northern and southern side of the Yaeyama Rift. These structures are classfied into six groups for comparison, and it was found that the preffered strike orientations in the eastern, central, and western areas were different. In the middle part of the northern side, we recognized the structures with a curved geometry. An average spacing of the structures is ~1.76 km, with the average values on the north side, but differences were observed on the south side. The difference between the smallest and largest values was as much as ~2.9 times. These differences may indicate the spatial difference in stresses, physical properties and process of magma upwelling at a stage before the seafloor spreading in the southern part of the Okinawa Trough.
Approximately five percent of the earth's surface is comprised of backarc basins.The Wilson cycle, which explains the birth and extinction of tectonic plates, does not include the backarc basins because its formation process, especially its dynamic characteristics, has not been clarified yet. There has been a long debate about the driving force of the backarc rifting, and two models, the corner-flow model and the slab-pull model, have been considered as primary mechanism. Once the backarc seafloor expansion occurs, the deformation experienced just before the expansion is disturbed. So, it is difficult to recognize the stress state before the expansion.
In this study, we focus on the Okinawa Trough, which is a unique backarc basin in the world characterized as an early stage of continental rifting yet to reach the stage of seafloor spreading. We research the southern part of the Okinawa Trough (around the Yaeyama Rift), where the seafloor is the deepest, faults develop densely, and magma rises from the subsurface. We examine the mechanical characteristics based on the geological structures.
2.Data and analysis method
We used the seafloor bathymetry and seismic reflection data obtained during the Hakuho Maru KH-21-3 cruise conducted in 2021(Otsubo et al., 2021) and bathymetric data of the GEBCO, compiled bathymetric data of the JAMSTEC and AIST(Misawa et al., 2020). The topographic images around the Yaeyama Rift were created using the GMT (Generic Mapping Tools) software (Wessel et al. 2019). Based on the images, we detected step structures and then measures their spacing and strike. And we calculated the thickness of the mechanical layer by applying the average spacing of the structures to equation suggested by Soliva et al. (2006).
3.Results and Discussion
We identified 157 step structures, most of which have the E-W orientation. The number of identified structures was almost the same on the northern and southern side of the Yaeyama Rift. These structures are classfied into six groups for comparison, and it was found that the preffered strike orientations in the eastern, central, and western areas were different. In the middle part of the northern side, we recognized the structures with a curved geometry. An average spacing of the structures is ~1.76 km, with the average values on the north side, but differences were observed on the south side. The difference between the smallest and largest values was as much as ~2.9 times. These differences may indicate the spatial difference in stresses, physical properties and process of magma upwelling at a stage before the seafloor spreading in the southern part of the Okinawa Trough.