Wide Angle Seismic Tomography(2D/3D) Of Southwest Indian Ridge

In order to better understand the hydrothermal vents and evidence the crust structure beneath the vents and surroundings, and directly measure the crust thickness beneath the high axis to test the plate tectonics and mantle plume theory, we applied an3D array of40ocean bottom seismometers (OBS) active seismic experiment from February to March2010during the Leg6DY115-21global cruise of R/V "Dayang Yihao".Till now, the plate tectonics and mantle plume theory has been well accepted, while there are still a few but critical problems remained. And the most popular study in recent20years is on the ocean crust thicknesses and components anormal at the ultra-slow spreading ridge (full spreading speed less than20mm/yr), whose tectonic process and mechanism may renew the plate tectonics and mantle plume theory. This study located at the Southwest Indian Ridge (SWIR)50°E-Seg27and Seg28-with a ultra-slow spreading rate of14mm/yr. The most interested characteristics are there are several hydrothermal vents and spreading axis high. But the rich of hydrothermal vents needs a sufficient magma supply, which may not happen at ultra-slow spreading ridge, and this is the first question. The other one is the high axis may be effected by Marion and Crozet hotspots nearby and formed a thick crust, which match the mantle plume theory, but another explain of depleted mantle rising says no to mantle plume theory, which obtained from dredged or television-guided grabbed rocks with crust compositions missing.Base on the seismic data, after a serial of processing of shots and OBSs relocation, time correction and filter, obtained main profiles of2D and whole study area of3D P wave velocity structures by using RayInvr2D travel time inversion codes and Jive3D tomography codes, separately. The main results are as follows:Two main layers-oceanic crust2and3were found in the crust, and an oceanic crust layer2A was identified in the along axis profile with varied crust thickness; crust thickness varies along axis, with thicker crust of8-1Okm beneath NVR, while thinner (-5km) beneath NTD, and the difference mainly occurred at oceanic crust3; velocities varies along axis on the seafloor between NVR and NTD; Seg28shows topography high on the south flank, with high seafloor velocity of4.4km/s, upward Moho and thinner crust(-4-5km), while the north flank shows flat topography, low seafloor velocity (-3.2km/s) and thicker crust (-6km); Seg27shows similar structure on the two flanks, crust turns thinner with off-axis offset turn larger, and sea floor velocity is about3.2km/s; there is a low velocity zone was found beneath the center of Seg27, which has a scale of6km(NS)×10km(EW)×4km(Z).Focuing on the diverse crust structres beneath hydrothermal vents, especially the ultra-thick crust beneath center of Seg27, with a tentative thermal numerical simulation, and the difference among untra-slow spreading ridges, the detachment fault and magma chamber, we got several conclusions are following:1) hot mantle (up to1420℃) and additional melt supply resulted in ultra-thick crust developed and focused beneath segment center, which also resulted in lateral variations of velocity and density;2) distinguish crust structure was found in study area comparing with nomal ultra-slow spreading ridge, and the main feature are thick crust, non-serpentinization mantle and low velocity zone in the crust, and the hot mantle and more melt supply are also the main reason;3) special strcuture around hydrothermal vents:a) there is a detachment fault on the south flank of Seg28, with14km length and lasted about2Ma, which makes the possible active channel of the hydrothermal vent; b) the low velocity zone beneath Seg27represents a magma chamber, which may effected by hot spots or the local magma supply up, and thermal model show it will disappear and can not supply the heat for the vent, which explains why the hydrothermal vent is inactive.Highlights of this paper are following:(1) confirm the ultra-thick crust up to10km by seismic experiment;(2) discover an axial magma chamber;(3) get refraction seismic phase and velocity structure of oceanic crust layer2A on SWIR. All of the highlights are new and extraordinary in SWIR. Expetritally the confirmation of the ultra-thick crust, which saved the being threatened plume theory, and also show a bright future for the hydrothermal sulfide exploration.