5:15 PM - 6:30 PM
[SCG45-P01] Temporal variation of crustal accretion along the Vulcan Transform, the Southern Ocean: Preliminary results of KH-19-6 cruise
Keywords:mid-ocean ridge process, transform fault, oceanic crust
The Vulcan transform is a 120 km long, east-west trending strike-slip boundary between the Antarctic and Sur plates. The ridge axis offsets right-stepped, and the Sur plate moves westward at 15-16 mm/yr. relative to the Antarctic plate (MORVEL, DeMets et al., 2010). R/V Hakuho-maru KH-19-6 cruise conducted geophysical mapping along with this transform and conducted five dredge hauls along Antarctic side of transform wall from the present-day ridge axis to 120 km off-axis, that roughly corresponds 15 Ma crust assuming global plate motion model. As part of the project MOWALL (Moho Observation along transform fault WALLs), we try to investigate the temporal variation of the mid-ocean ridge process and the relationship with mantle heterogeneity by systematic rock sampling along with long oceanic transform, where the whole crustal section is exposed in chronological order. In this presentation, we present the preliminary analysis of geophysical data collected during the cruise.
The axial valley of the southern ridge segment shows an asymmetric structure. In the western part of the axial valley, ridge-parallel, inward-facing fault scarps develop to the ridge flank. In the eastern part, contrary, relatively smooth, doomed high forms the deeper part of the axial wall, and a large volcanic peak continues eastward. The doomed high is likely a detachment surface suggesting magma-starved spreading. A steep and oblique lineament develops about 20 km east of the axis. The abyssal hills are well-ordered east of 17°W, showing typical magmatic seafloor.
The magnetic profiles along the TF were also collected, to obtain the exact history of the spreading rate. Our forward modeling (2D, assuming constant thickness magnetization layer draped the observed bathymetry) provides two possible scenarios of spreading. The preliminary result of age dating of dredged rock samples possibly supports one of these two models, assuming the change of spreading rate at 2~3 Ma. We also calculate mantle Bouguer anomaly (MBA) using newly collected multibeam bathymetry. Anomalous high positive MBA appears around the eastern axial wall, suggesting the exposure of high-density material. The variation of crustal thickness is also estimated from gravity anomalies. The result indicates that the thickness of crust shows a million-year order fluctuation, reflecting the temporal variation of melt supply at this ridge segment.
The axial valley of the southern ridge segment shows an asymmetric structure. In the western part of the axial valley, ridge-parallel, inward-facing fault scarps develop to the ridge flank. In the eastern part, contrary, relatively smooth, doomed high forms the deeper part of the axial wall, and a large volcanic peak continues eastward. The doomed high is likely a detachment surface suggesting magma-starved spreading. A steep and oblique lineament develops about 20 km east of the axis. The abyssal hills are well-ordered east of 17°W, showing typical magmatic seafloor.
The magnetic profiles along the TF were also collected, to obtain the exact history of the spreading rate. Our forward modeling (2D, assuming constant thickness magnetization layer draped the observed bathymetry) provides two possible scenarios of spreading. The preliminary result of age dating of dredged rock samples possibly supports one of these two models, assuming the change of spreading rate at 2~3 Ma. We also calculate mantle Bouguer anomaly (MBA) using newly collected multibeam bathymetry. Anomalous high positive MBA appears around the eastern axial wall, suggesting the exposure of high-density material. The variation of crustal thickness is also estimated from gravity anomalies. The result indicates that the thickness of crust shows a million-year order fluctuation, reflecting the temporal variation of melt supply at this ridge segment.