Three-dimensional seismic structure of the Dragon Flag oceanic core complex at the ultraslow spreading Southwest Indian Ridge (49°39′E)

International audience; The Southwest Indian Ridge (SWIR) is an ultraslow spreading end-member of mid-ocean ridgesystem. We use air gun shooting data recorded by ocean bottom seismometers (OBS) and multibeambathymetry to obtain a detailed three-dimensional (3-D) P wave tomographic model centered at 49390Enear the active hydrothermal ‘‘Dragon Flag’’ vent. Results are presented in the form of a 3-D seismictraveltime inversion over the center and both ends of a ridge segment. We show that the crustal thickness,defined as the depth to the 7 km/s isovelocity contour, decreases systematically from the center (7.0–8.0km) toward the segment ends (3.0–4.0 km). This variation is 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. We suggest thatdetachment faulting is a prominent process of slow spreading oceanic crust accretion even inmagmatically robust ridge sections. Hydrothermal activity at the Dragon Flag vents is located next to thedetachment fault termination. We infer that the detachment fault system provides a pathway forhydrothermal convection.