ALBERT

Description: Seismic reflection observations from the IMERSE cruise have confirmed the presence of at least two localised thick evaporite basins on the crest and Inner Plateau of the Mediterranean Ridge accretionary complex. The discovery is supported by interval velocity results from both near-normal incidence reflection data and wide-angle seismic data. These evaporite basins are located north of a deep trough known as the Cleft and at a currently active thrust zone south of the Cleft, on the crest of the Mediterranean Ridge. The polarity of the seismic reflections associated with the base of the evaporite shows a phase reversal relative to the seafloor. This high amplitude phase reversed reflection indicates a decrease in velocity between the evaporite and the underlying sediment. The existence of these thick evaporite deposits suggests that the distribution of Messinian salt is much thicker (∼1.8–2 km) on parts of the Ridge crest than initially perceived by previous investigations. We propose that these thick evaporite deposits were deposited in localised basins on the Ridge, formed in response to pre- and syn-Messinian tectonics. The evaporites on the Inner Plateau basin most likely represent the infill of a Messinian forearc basin. The existence of local deep evaporite basins on the crest of the Ridge at depths of 2.7–3.3 km below sea level at the present day supports the hypothesis that the Mediterranean Sea level must have dropped by around 3 km below its present sea level during the Messinian salinity crisis.