Structure of the Mediterranean Ridge accretionary complex from seismic velocity information

Seismic velocities obtained from ocean-bottom hydrophone, expanding spread profile and multi-channel seismic data were used to compile a velocity model for the Mediterranean Ridge along a 220-km-long transect extending from the Sirte Abyssal Plain to the Cleft region near the Hellenic Trough. A 200-300-m-thin layer of Plio-Quaternary sediments with velocities of 1800-2200 in s(-1) covers the whole Ridge. The Messinian evaporites (4000-4500 m s(-1)) occur in the southwest as a tectonically thickened layer and in a basin just northeast of the crest of the Ridge. In the intervening region however, the evaporites appear absent and the seismic velocities are generally lower. Arched reflectors, imaged in the depth-migrated section, suggest that the sediments beneath the Ridge crest belong to a Pre-Messinian accretionary wedge. Beneath the Messinian evaporites a northeastward-thinning layer of probable Tertiary sediments shows laterally increasing velocities from 3300 m s(-1) to 4600 in s(-1). Assuming that the layer thinning is caused by compaction due to increased overburden alone, we have calculated a porosity reduction from 15% to 4% and an associated fluid expulsion of 10 km(3) km(-1) along the trench axis. This corresponds to c. 60% of the initial fluid volume of an undeformed sediment column from the abyssal plain. The almost impermeable evaporitic cap over these sediments leads to high fluid pressures at the base of the evaporites, likely to make this horizon the basal decollement of the modern accretionary system. A 2.5-km-thick unit of probable Mesozoic carbonates with velocities of 4500-4600 m s(-1) is inferred at c. 8 km depth. The top of the oceanic crust occurs at a depth of about 10 km. The results from this study have widespread implications for the understanding of the regional geological history. (C) 2002 Elsevier Science B.V. All rights reserved.