[S-CG55] Ocean Floor Geoscience

[SCG55-P22] Seismic velocity structure at 37ºE in the southwest Indian Ridge

*Hayato Onuki¹, Toshinori Sato², Kimihiro Mochizuki³, Masanao Shinohara³ (1.Graduate of Science and Engineering, Chiba university, 2.Graduate school of Science, Chiba university, 3.Earthquake Research Institute, University of Tokyo)

Keywords:ultra-slow spreading ridge, southwest Indian Ridge, Marion hotspot

At the oceanic ridges, there is uplift of mantle materials to fill the space between the plates, forming new oceanic crust and lithosphere. The structure of the oceanic crust produced is known to vary significantly depending on the spreading rate. For example, in the East Pacific Ridge as fast spreading ridges, the ridge axis is gently rising and the two-dimensional mantle upwelling produces a homogeneous crust. The crust thickness is 6-7 km. On the other hand, in slow spreading ridge, such as the Mid-Atlantic Ridge, median rift valleys develop along the ridge axis, the mantle upwelling pattern becomes three-dimensional and the crust is heterogeneous. The thickness of the crust is considered to be thinner than that in fast spreading ridges. However, it has recently become clear that it is not only the spreading rate that determines the diversity of the ridges, but the balance between the spreading rate and the supply of melt. It has been reported that even in regions with slow spreading rates, thicker-than-normal crusts, such as those found in fast spreading ridges, are formed on ridges where hotspots exist nearby; conversely, in regions with the same spreading rate but low melt supply, oceanic crust production is poor, large normal faults develop, and seafloor spreading is carried out by exposing mantle material to the seafloor. In order to investigate how the structure beneath the ridges changes depending on parameters other than the spreading rate, a crustal structure survey was conducted on the KH07-4 research cruise of the R/V Hakuho Maru of JAMSTEC in January 2008 at the southwest Indian Ridge, around 37ºE using 10 ocean bottom seismometers and air guns. The southwest Indian Ridge is classified as an ultra-slow spreading ridge with spreading rate of about 15 mm/yr. Marion hotspot exists to the south-east of the survey area, and the existence of two or three segments is suggested in the survey area. It is also expected that the melt supply differs from north to south or east to west of the ridge axis, and that differences in melt supply may be responsible for structural differences.
In the analysis, we first determined the two-dimensional structure under the survey line using the PMDM method (Sato and Kennett, 2000). Using the 3D model created from the obtained 2D model as the initial model, we used FAST (Zelt and Barton, 1998) to obtain the 3D P-wave velocity structure. The number of P-wave travel times was 3878, the RMS was reduced from 149 ms to 51 ms. The χ² of the final model was 1.07.
In the final model, a region of slower seismic velocity structure than the surrounding area was identified on the north-eastern side of the survey area to a depth of 5-7 km. This area is located near the segmental topography extending east-west in the survey area, and it is considered that melt is supplied to this segment from the hotspot.

Acknowledgments
We thank the captain and crew of JAMSTEC's R/V Hakuho Maru for their support. We thank Mr. Takayuki Kitamura whose Master thesis at Chiba University was used in our analysis.