Three-dimensional seismic structure of the Dragon Flag oceanic core complex at the ultraslow spreading Southwest Indian Ridge (49°39′E).
Résumé
The Southwest Indian Ridge (SWIR) is an ultraslow spreading end-member of mid-ocean ridge
system. We use air gun shooting data recorded by ocean bottom seismometers (OBS) and multibeam
bathymetry to obtain a detailed three-dimensional (3-D) P wave tomographic model centered at 49390E
near the active hydrothermal ‘‘Dragon Flag’’ vent. Results are presented in the form of a 3-D seismic
traveltime 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 (7.0–8.0
km) toward the segment ends (3.0–4.0 km). This variation is dominantly controlled by thickness
changes in the lower crustal layer. We interpret this variation as due to focusing of the magmatic activity
at the segment center. The across-axis velocity model documents a strong asymmetrical structure
involving 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 vertical
velocity gradient. We infer that this OCC may be predominantly made of gabbros. We suggest that
detachment faulting is a prominent process of slow spreading oceanic crust accretion even in
magmatically robust ridge sections. Hydrothermal activity at the Dragon Flag vents is located next to the
detachment fault termination. We infer that the detachment fault system provides a pathway for
hydrothermal convection.
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