Frontal accretion along the western Mediterranean Ridge: the e¡ect of Messinian evaporites on wedge mechanics and structural style T.J. Reston  , R. von Huene, T. Dickmann, D. Klaeschen, H. Kopp GEOMAR Research Centre, Wischhofstrasse 1^3, D24148 Kiel, Germany Received 6 May 1997; received in revised form 7 May 1999; accepted 19 November 2001 Abstract 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 de ¤collement 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. ß 2002 Elsevier Science B.V. All rights reserved. Keywords: Mediterranean Ridge; accretionary wedges; basal detachment; £uid pressure; Messinian 1. Introduction It is near the deformation front that the depth to the basal detachment and hence the stratigra- phy of much of an accretionary wedge are estab- lished. As it is in the ¢rst 20 km or so of the wedge that frontal accretion takes place, the structure of this region provides information on the mechanics of accretion. Furthermore, it has generally been acknowledged that most pore £u- ids are mobilised not far from the deformation front (e.g. Moore and Vrolijk, 1992). Hence a study of the deformation front and areas immedi- ately arcward provides information vital for an understanding of wedge evolution and mechanics. The Mediterranean Ridge has formed by con- vergence of Africa and the Aegean region and the subduction of the African Plate beneath Eurasia: sediments have been scraped o¡ the subducting African plate and piled up in front of the Aegean 0025-3227/02/$ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII:S0025-3227(02)00173-1 * Corresponding author. E-mail address: treston@geomar.de (T.J. Reston). Marine Geology 186 (2002) 59^82 www.elsevier.com/locate/margeo