10:00 AM - 10:15 AM
[SCG55-16] Spatial heterogeneity of crustal extensional deformation in the Yaeyama Rift and Yonaguni Rift in the southern part of the Okinawa Trough
Keywords:backarc basin, extension, normal faulting, crustal thinning, magmatic intrusion, rifting
1. Introduction
Backarc basins are important tectonic settings for understanding the evolution of the lithosphere, changes in plate motions and the dynamics of plate subduction. However, the mechanisms of their development, particularly the mechanical characteristics during the early stages of formation, has yet to be understood. We focus on the spatial heterogeneity of fault developments in the shallow crust (less than 2 km depth) at a rift scale ranging from 1 to 10 km just before the initiation of backarc spreading. Fault development at this rift scale provides new insights into the evolution and dynamics of fault systems by examining fault interaction and the mechanics of segment linkage in detail. The Okinawa Trough is a globally unique backarc basin that is still in the stage of continental rifting and has not yet reached seafloor spreading. The Yaeyama Rift (YaR) and the Yonaguni Rift (YoR) in the southern part of the Okinawa Trough are believed to have formed through north-south extensional deformation from approximately 0.1 Ma to the present, as suggested by Sibuet et al. (1998). However, the mechanical structures accommodating this extensional deformation, such as normal faults, and their spatial distribution have not been thoroughly investigated. Since these rifts are thought to have formed simultaneously, a comparative study is essential for understanding their development. Therefore, this study, compares the two rift zones in the early stage of back-arc spreading and examine the spatial distribution characteristics of normal faults, including their extent, density, and number.
2. Data and Methods
In this study, we primarily used bathymetric and seismic reflection data collected during the KH-23-11 cruise (Otsubo et al., 2024) conducted by the R/V Hakuho-Maru around the YaR and YoR in the Southern Okinawa Trough from December 2023 to January 2024. We identified normal fault-related structures on the seafloor and analyzed their spatial distribution using the bathymetric maps and seismic reflection profiles obtained along multiple survey lines crossing the rifts.
3. Results and Discussion
Bathymetric maps identified scarps indicating significant differences in the height of the seafloor surface, appearing as lineaments approximately 5–10 km long. Additionally, these scarps were observed in seismic reflection profiles, with some bounded by normal faults near the rifts. Based on these results, two major points were discussed.
Discussion 1: Formation of normal fault systems in the YaR and YoR
The orientations of normal fault in the both rifts align with the north-south extensional stress regime of the southern Okinawa Trough, which has been ongoing since approximately 0.1 Ma (Sibuet et al., 1998). Seismic reflection profiles show that trough-fill sediments thicken towards the rift axis. Some normal faults extend to the seafloor, deforming the seafloor surface, suggesting that these faults are active. This indicates that the normal faults around both rifts formed as a result of ongoing extensional stress leading to rifting.
Discussion 2: Differences in shallow crustal deformation between the YaR and YoR
Owing to the activity of normal faults, the both rifts exhibits graben morphology along the rift axis. Small-scale normal fault systems in the shallow crust developed nearly symmetrically across the rift axis in both rifts. However, differences in deformation structures between the two rifts were identified. Bathymetric maps and seismic reflection profiles show that the YaR has a greater number and higher density of normal faults compared to the YoR. Additionally, in the YoR, active normal faults are distributed over a broader north-south range, and the amount of seafloor subsidence associated with rifting is relatively smaller. These differences suggest variations in deformation styles of the shallow crust, potentially reflecting differences in mechanical rigidity (or softness) in response to extensional deformation.
Backarc basins are important tectonic settings for understanding the evolution of the lithosphere, changes in plate motions and the dynamics of plate subduction. However, the mechanisms of their development, particularly the mechanical characteristics during the early stages of formation, has yet to be understood. We focus on the spatial heterogeneity of fault developments in the shallow crust (less than 2 km depth) at a rift scale ranging from 1 to 10 km just before the initiation of backarc spreading. Fault development at this rift scale provides new insights into the evolution and dynamics of fault systems by examining fault interaction and the mechanics of segment linkage in detail. The Okinawa Trough is a globally unique backarc basin that is still in the stage of continental rifting and has not yet reached seafloor spreading. The Yaeyama Rift (YaR) and the Yonaguni Rift (YoR) in the southern part of the Okinawa Trough are believed to have formed through north-south extensional deformation from approximately 0.1 Ma to the present, as suggested by Sibuet et al. (1998). However, the mechanical structures accommodating this extensional deformation, such as normal faults, and their spatial distribution have not been thoroughly investigated. Since these rifts are thought to have formed simultaneously, a comparative study is essential for understanding their development. Therefore, this study, compares the two rift zones in the early stage of back-arc spreading and examine the spatial distribution characteristics of normal faults, including their extent, density, and number.
2. Data and Methods
In this study, we primarily used bathymetric and seismic reflection data collected during the KH-23-11 cruise (Otsubo et al., 2024) conducted by the R/V Hakuho-Maru around the YaR and YoR in the Southern Okinawa Trough from December 2023 to January 2024. We identified normal fault-related structures on the seafloor and analyzed their spatial distribution using the bathymetric maps and seismic reflection profiles obtained along multiple survey lines crossing the rifts.
3. Results and Discussion
Bathymetric maps identified scarps indicating significant differences in the height of the seafloor surface, appearing as lineaments approximately 5–10 km long. Additionally, these scarps were observed in seismic reflection profiles, with some bounded by normal faults near the rifts. Based on these results, two major points were discussed.
Discussion 1: Formation of normal fault systems in the YaR and YoR
The orientations of normal fault in the both rifts align with the north-south extensional stress regime of the southern Okinawa Trough, which has been ongoing since approximately 0.1 Ma (Sibuet et al., 1998). Seismic reflection profiles show that trough-fill sediments thicken towards the rift axis. Some normal faults extend to the seafloor, deforming the seafloor surface, suggesting that these faults are active. This indicates that the normal faults around both rifts formed as a result of ongoing extensional stress leading to rifting.
Discussion 2: Differences in shallow crustal deformation between the YaR and YoR
Owing to the activity of normal faults, the both rifts exhibits graben morphology along the rift axis. Small-scale normal fault systems in the shallow crust developed nearly symmetrically across the rift axis in both rifts. However, differences in deformation structures between the two rifts were identified. Bathymetric maps and seismic reflection profiles show that the YaR has a greater number and higher density of normal faults compared to the YoR. Additionally, in the YoR, active normal faults are distributed over a broader north-south range, and the amount of seafloor subsidence associated with rifting is relatively smaller. These differences suggest variations in deformation styles of the shallow crust, potentially reflecting differences in mechanical rigidity (or softness) in response to extensional deformation.