Internal structure of the oceanic lithosphere at a melt‐starved ultra‐slow‐spreading mid‐ocean ridge: insights from 2‐D seismic data

International audienceExtensive outcrops of serpentinized peridotite in melt-starved spreading corridors of the ultraslow easternmost Southwest Indian Ridge are hypothesized to be due to slip on successive long-offset normal faults that alternate polarity (flip-flop detachment faults). We investigate the nature of the oceanic crust which forms under these conditions, using seismic reflection data acquired during the SISMOSMOOTH 2014 cruise. Using 3-D binning, the seismic profiles were binned elastically, while three of the profiles shot closely were merged into one to take advantage of the larger air gun source volume. Using a poststack imaging sequence, we observe several types of reflectors at crustal and infracrustal depths, in the axial valley and off-axis. Correlating our seismic observations with Residual Mantle Bouguer gravity anomalies and seafloor observations, we find that our results are explicable in the framework of the flip-flop hypothesis of detachment faulting. Reflectors imaged down to 5 km into the basement and interpreted as due to damaged zones outlining the detachment faults dip 50°at the early stages, while at late stages after developing offsets >10 km, they dip 25°. Other reflectors observed in the crust are interpreted as moderate offset (