5:15 PM - 6:45 PM
[SCG48-P18] The Fluctuation of Mid-Ocean Ridge Processes along the Vulcan Transform in the Southern Ocean – MOWALL-Vulcan
Keywords:mid-ocean ridge process, oceanic crust, transform fault
We conducted a systematic mapping and rock sampling along the Vulcan Transform in the Southern Ocean as part of the MOWALL (Moho Observation along Transform Fault WALLs) project. The MOWALL project aims to reveal the temporal fluctuations of mid-ocean ridge processes and their causes. The survey was conducted during the R/V Hakuho-maru KH-19-6 cruise, and preliminary results were reported at the JpGU 2021 annual meeting. This presentation provides a detailed analysis of bathymetry, gravity, and magnetic anomalies, along with geochemical and isotopic characteristics and age data of recovered rock samples.
The Vulcan Transform fault is part of the Antarctica-Sul Plates boundary and offsets the ridge axes by approximately 120 km in an east-west direction. Based on the global plate model, the full seafloor spreading rate here is ultra-slow, at 15-16 mm/yr (MORVEL, DeMets et al., 2010). Therefore, from 15 Ma to the present, oceanic crust was expected to be exposed along the transform fault wall. We conducted multibeam bathymetry and scalar/vector magnetic field observations along this fault and collected rock samples from the ridge axis to 115 km off-axis.
The Vulcan Transform is a right-stepping fault offsetting the N-S trending ridge axial valley segments. We conducted 130 km long E-W trending survey lines in the southern segment along the transform. The northern segment was also partly mapped. A domed structure that appears to be a detachment surface was discovered at the inside corner of the ridge-transform intersection, suggesting a recent melt-poor environment. Small volcanic cones are distributed on the axial floor. The depth is relatively shallow, about 30 to 50 km from the axis, and longer-wavelength N-S lineaments are developed. In far-off-axis areas, a typical abyssal hill morphology is dominant. We calculated the mantle Bouguer gravity anomaly (MBA) using satellite gravity anomaly and newly collected bathymetry. The inside corner shows a positive MBA, supporting the existence of a detachment fault exposing lower denser material in the shallow part. We also calculated the residual MBA considering the plate cooling effect and estimated the crustal thickness variation. The crust thickness fluctuates between 2 – 6.5 km, but no remarkable periodicity is observed. Magnetic anomaly patterns along two survey lines south of the transform correlate well and can be considered to record the seafloor spreading history. The amplitude of the anomaly is large from the axis to 40 km off-axis and decreases in far off-axis regions. We conducted forward modeling of the magnetic anomaly to estimate the spreading rate. The rock age data based on each analysis in 49Ar/39Ar age of basaltic samples and zircon U-Pb age of gabbroic rock samples were used to constrain the modeling. The synthetic anomaly well explains the observation when we assume a half rate of 7 mm/yr (same as in the global model) before 8 Ma and an increase of the rate to 15 mm/yr after 5.5 Ma. This is likely interpreted as 1) a misfit of the global model due to a lack of enough field data in the Southern Ocean and 2) asymmetric spreading in recent years. Rock samples were recovered in the rift valley in the southern segment and along the south wall of the transform at approximately every 30 km. We collected basalt and gabbroic rock samples from five stations. Basalts from each location record unique source characteristics. Temporal variation in the source has been recognized but may not be systematically periodic. We will reveal the fluctuations of structure and composition of the oceanic crust along the ultra-slow ridge system and discuss the relationship among the fluctuation, the source mantle heterogeneity, and plate motion.
The Vulcan Transform fault is part of the Antarctica-Sul Plates boundary and offsets the ridge axes by approximately 120 km in an east-west direction. Based on the global plate model, the full seafloor spreading rate here is ultra-slow, at 15-16 mm/yr (MORVEL, DeMets et al., 2010). Therefore, from 15 Ma to the present, oceanic crust was expected to be exposed along the transform fault wall. We conducted multibeam bathymetry and scalar/vector magnetic field observations along this fault and collected rock samples from the ridge axis to 115 km off-axis.
The Vulcan Transform is a right-stepping fault offsetting the N-S trending ridge axial valley segments. We conducted 130 km long E-W trending survey lines in the southern segment along the transform. The northern segment was also partly mapped. A domed structure that appears to be a detachment surface was discovered at the inside corner of the ridge-transform intersection, suggesting a recent melt-poor environment. Small volcanic cones are distributed on the axial floor. The depth is relatively shallow, about 30 to 50 km from the axis, and longer-wavelength N-S lineaments are developed. In far-off-axis areas, a typical abyssal hill morphology is dominant. We calculated the mantle Bouguer gravity anomaly (MBA) using satellite gravity anomaly and newly collected bathymetry. The inside corner shows a positive MBA, supporting the existence of a detachment fault exposing lower denser material in the shallow part. We also calculated the residual MBA considering the plate cooling effect and estimated the crustal thickness variation. The crust thickness fluctuates between 2 – 6.5 km, but no remarkable periodicity is observed. Magnetic anomaly patterns along two survey lines south of the transform correlate well and can be considered to record the seafloor spreading history. The amplitude of the anomaly is large from the axis to 40 km off-axis and decreases in far off-axis regions. We conducted forward modeling of the magnetic anomaly to estimate the spreading rate. The rock age data based on each analysis in 49Ar/39Ar age of basaltic samples and zircon U-Pb age of gabbroic rock samples were used to constrain the modeling. The synthetic anomaly well explains the observation when we assume a half rate of 7 mm/yr (same as in the global model) before 8 Ma and an increase of the rate to 15 mm/yr after 5.5 Ma. This is likely interpreted as 1) a misfit of the global model due to a lack of enough field data in the Southern Ocean and 2) asymmetric spreading in recent years. Rock samples were recovered in the rift valley in the southern segment and along the south wall of the transform at approximately every 30 km. We collected basalt and gabbroic rock samples from five stations. Basalts from each location record unique source characteristics. Temporal variation in the source has been recognized but may not be systematically periodic. We will reveal the fluctuations of structure and composition of the oceanic crust along the ultra-slow ridge system and discuss the relationship among the fluctuation, the source mantle heterogeneity, and plate motion.