Frontal accretion along the western Mediterranean Ridge: the effect of Messinian evaporites on wedge mechanics and structural style

In the context of the IMERSE project, several crossings of the deformation front of the western Mediterranean Ridge were made in the region of the Sirte Abyssal Plain, the Messina Abyssal Plain and the intervening region. In this paper, we present seismic images and interpretations across the deformation front, with particular emphasis on the role the Messinian evaporites have played in controlling the accretionary tectonics of the thin frontal portion of the wedge. The seismic images show that the basal detachment generally is located at the base of the evaporites. From a consideration of the mechanics of the wedge, for both Coulomb and plastic rheologies, we show that the low wedge taper (c. 2°) requires that the detachment is characterised by extreme fluid overpressuring (within 2% of lithostatic in places) and that the basal yield stress (less than 1 MPa) is lower than that of a wet salt décollement zone. This supports the seismic interpretation that the detachment occurs in overpressured sediments beneath the impermeable evaporites. Lateral variations in the accretionary style can be related to lateral variations in evaporite thickness, the effectiveness of the evaporite as an impermeable seal and to local relief on the subducting plate; these factors control the escape of fluids from beneath the evaporites and hence fluid pressure and basal yield stresses.