Cause of the rupture and distribution of broken submarine carbonate chimneys in the Gulf of Cádiz (southwestern Spain)

Abstract

During the 2000 and 2001 R/V Cornide de Saavedra cruises a vast field of hydrocarbon-derived carbonate chimneys was discovered and sampled along the Cádiz Contourite Channel and Guadalquivir Diapiric Ridge. Photographs taken on board using an underwater camera revealed a spectacularly high density of pipe-like chimneys lying on the sea floor, though some of them were also found protruding from muddy sediment. The major axes of the fallen chimneys, which can be as long as 1 m, exhibit a regular spatial distribution showing a conspicuous NW-SE trend. Furthermore, their basal morphology shows a characteristic angular breakage associated to a flexo-traction process. Both the uniform distribution of the major axis and the basal morphology of the chimneys imply that the cause of the rupture could be related either to strong bottom sea currents or to inertia forces associated with oscillatory ground motion due to seismic shaking. The preferential orientation of the broken chimneys, their mechanical properties, the geological and topographical conditions of the area in relation to seismic ground motion amplification and the flow speed of the Mediterranean Outflow Water (MOW) along the Cádiz Contourite Channel, suggest that the most probable cause of the chimneys’ rupture was an earthquake. The minimum magnitude of this seismic event is estimated to be more than 7.0, the causative fault being located directly under the chimney field or as far as 30e60 km away. The chimneys’ rupture took place between ca 5 ka to Present.