M. Fournier

ABSTRACT Several types of mass failures are observed along the Owen ridge (NW Indian ocean) using multibeam bathymetry, acoustic imagery and sediment echosounder. The Owen ridge is associated with the Owen fracture zone, a 800 km-long active fault system which accommodates the strike-slip motion between the Arabia and India plates. Mass failures mobilize a pelagic cover and display a large variety of features along the three parts of the Owen ridge, from cohesive to desintegrative flows. We present a complete morphometric analysis of submarine landslides, and provide a synthetic map of the different types of sediment destabilization along the ridge. Spectacular instability scars, which could have removed up to 45 km3 of material, were evidenced on the southern part of the ridge. Such volumes are unexpected in a sedimentary environment dominated by slow pelagic sedimentation rate. The spatial variation of failure morphology seems to be strongly related to the topography of the basement. The detailed re-analysis of seismic lines collected during the ODP Leg 117 reveals the recurrence and the sporadic repartition of mass wasting events along the southern fragment of the ridge since its uplift in the early Miocene. Earthquakes are more frequent than slides along the southern ridge, excluding seismicity as a unique triggering process. We propose that mass wasting frequency is mainly constrained by the slow pelagic sedimentation rates.