[S-CG48] Ocean Floor Geoscience

[SCG48-P12] Tectonics of the initial stage of the Shikoku Basin spreading

*Takaaki Moriguchi¹, Kyoko Okino¹, Yasuhiko Ohara^2,3,4, Yumiko Harigane⁵, Matamura Yuki⁶ (1.Atomosphere and Ocean Research Institute, The University of Tokyo, 2.Hydrographic and Oceanographic Department of Japan, 3.Japan Agency for Marine-Earth Science and Technology, 4.Nagoya University, 5.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), 6.Tohoku University)

Keywords:backarc spreading, Shikoku Basin, gravity anomaly, geomagnetism

Backarc spreading is a process in which a new oceanic floor is accreted in the extensional field behind a trench. It is known that backarc basin spreading ceases in 10- 15 million years, and another backarc spreading initiates a few million years after that. In order to understand the cycle of backarc spreading, it is essential to examine the tectonics of the initiation and termination phases of backarc spreading. In this study, We focus on the Kyushu-Palau Ridge (KPR), a paleo-island arc, and the southwestern part of the Shikoku Basin, located east of the KPR. We comprehensively interpret the geophysical characteristics of the seafloor and infer the tectonics of the initiation of the spreading of the Shikoku Basin. Bathymetric data obtained by multibeam echosounder and geomagnetic anomaly data by proton magnetometer during the cruises YK21-06S, YK22- 18S, and YK23-05S of the Japan Agency for Marine-Earth Science and Technology were mainly used for the analyses. Bathymetry data were compiled into 100 m grid data, and bathymetry maps were produced to classify the topography. The average bathymetry was approximately 5200 m, indicating that the studied area has particularly deep bathymetry among the Shikoku Basin. Four oceanic core complexes (OCCs) were identified, which are thought to form when the proportion of new ocean crust accretion is about 50% of the seafloor spreading (called M-value; Buck et al., 2005). The distribution of abyssal hills, developed during magmatically robust periods, was limited to less than 40% in M-value of the studied area. In addition, large positive mantle Bouguer anomalies of more than 150 mGal were observed throughout the southwestern Shikoku Basin, especially around the OCCs, reaching 200 mGal. The identified topography, gravity anomalies, and results of the prior seismic studies on the crustal structure suggest that melt supply was diminished, and thinner oceanic crust formed during the initiation of the spreading of the Shikoku Basin. The topography within the KPR can be divided into areas where seamounts are concentrated and areas where the topography is deep and flat. From the area where seamounts are concentrated, the island arc crust extends into the basin, and abyssal hills are continued. On the other hand, the deeper flat areas, where the original island arc crust is thought to be thinner, showed a series of distributions of depressions and OCCs. These results suggest that the preexisting crustal structures of the island arc before the backarc spreading may influence the topography of the backarc basin. The difference in the slope angles of the OCCs indicates that the direction of detachment faulting is reversed between adjacent OCCs. This suggests that ridge jumps may have occurred during the initiation of the backarc spreading. We attempted to identify the magnetic lineation patterns from the geomagnetic anomaly distribution, but it was difficult to identify the anomaly in areas other than abyssal hills. This may be due to the limited melt supply and lack of volcanism during the initiation of the backarc expansion, as well as the complexity of the seafloor structure due to the ridge jumps.