The Crustal Structure of the Southwest Indian Ridge(49°17'E–50°49'E) ——3D wide-angle seismic experiment

JiabiaoLi, AiguoRuan, XiongweiNiu, Minhui Zhao,Xuelin Qiu, Y. John Chen,Hanchao Jian

1) Second Institute of Oceanography, State Oceanic Administration, Hangzhou 3100122) South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301

3)Institute of Theoretical and Applied Geophysics, School of Earth and Space Science, Peking University, Beijing 100871 

TheSouthwest Indian Ridge (SWIR) is characterized by an ultraslow spreading rate, thincrust, and extensive outcrops of serpentinizedperidotite. Here we present a 3-D wide-angle seismic experiment carried out in 2010using forty OBSs at the SWIR segments that hoststhe active Dragon Flag hydrothermal vent(37°47'S, 49°39'E) and a volcano area. The main results are asfollows: 

(1) Along the ridge axis(segment 27-28) oceanic layer 2 is of roughly constant thickness and steep velocity gradient, underlainby a layer 3 with variable thickness and low velocity gradient. The crustal thickness varies from5 km beneath nontransform discontinuities (NTDs) up to -10 km beneath the volcanic center; themelt supply is focused in segment centers despite a small NTD between adjacent segments.

(2) Near Dragon Flag hydrothermal vent the crustal thickness, defined as the depth to the 7 km/s isovelocity contour, decreases systematically from the center segmentof 28toward its2 ends and dominantly controlled by thicknesschanges in the lower crustal layer. We interpret this variation as due to focusing of the magmatic activityat the segment center. The across-axis velocity model documents a strong asymmetrical structureinvolving oceanic detachment faulting. A locally corrugated oceanic core complex (Dragon Flag OCC)on the southern ridge flank is characterized by high shallow crustal velocities and a strong verticalvelocity gradient. We infer that this OCC may be predominantly made of gabbros and suggest thatdetachment faulting is a prominent process of slow spreading oceanic crust accretion even inmagmatically robust ridge sections. 

(3) Our results reveal a low velocity anomaly (-0.6 km/s) in the lower crust beneath the central volcano(segment 27), suggesting the presence of partial melt, which is accompanied by an unusually thick crust(10.2km). We conclude that the extremely magmatic accretion is due to localized melt flow towards the segment 27 center, which was enhanced by the significant along-axis variation in lithosphere thickness at the ultraslow spreading Southwest Indian Ridge.


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[3] Jiabiao Li, HanchaoJian, Yongshun John Chen, Satish C. Singh, AiguoRuan, XuelinQiu, Minhui Zhao, Xianguang Wang, XiongweiNiu, Jianyu Ni, and Jiazheng Zhang.Seismic observation of an extremely magmatic accretion at the ultraslow spreading Southwest Indian Ridge. Geophysics Research Letters, 2015, 42,2656-2663.doi:10.1002/2014GL062521