Gas seeps linked to salt structures in the Central Adriatic Sea Riccardo Geletti n , Anna Del Benw, Martina Busetti n , Riccardo Ramella n and Valentina Volpi n n Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS),Trieste, Italy wDipartimento di Scienze Geologiche, Ambientali e Marine (DiSGAM), University of Trieste,Trieste,Italy ABSTRACT The analyses of about 800 km of Chirp sub-bottom pro¢lers and 600 km 2 of Multibeam data acquired during the 2005 and 2007 surveys of the R/V OGS Explora, and their correlation with one new, and several public, multichannel seismic pro¢les, allow us to propose a relation between the distribution of gas seepages, fracture systems and deep salt features present in the Central Adriatic Sea. Gas seepage is evident from pockmarks on the seabed and in the shallow sub-bottom, where acoustic chimneys and bright spots have been highlighted and analyzed.The Mid-Adriatic Depression (MAD) is a distinct morphological feature in the Central Adriatic Sea elongated in a NE^SW direction.The area is a¡ected by salt doming of Triassic evaporites which cause the two main alignments of the Mid-Adriatic Ridge as far as the Palagruza High and the Jabuka Ridge.These salt tectonics have existed since, at least, Paleogene times and are still active: they characterize sectors with less resistance to deformation produced by successive regional compressive regimes that have a¡ected the area di¡erently during the di¡erent geodynamic phases. Gas-seep features are distributed preferentially above and along the fracture systems produced above and around the salt mounds. INTRODUCTION The study area is characterized by £uid leaks testi¢ed by the presence of gas-related morphologies as pockmarks, mud volcanoes, mud-carbonate mounds. Seabed pock- marks are shallow, crater-like, cone-shaped depressions on the sea£oor, related to focused £uid £ows and generally found in permeable, ¢ne-grained, soft sediments. Their formation could be associated to £uid escapes as gas erup- tions, water escaping or hydrocarbon gas migration (King & MacLean,1970; Hovland & Judd,1988).They often occur in characteristic patterns and can be found along fault trends, which is a clear indication of fault leakage (Ligten- berg, 2005). Sometimes, pockmarks are reported in seis- mically active regions, where the seabed, according to Hovland & Judd (1988), is ‘hydraulically active’due to the increase of deep £uid pressure before earthquakes. The presence of pockmarks in the Adriatic Sea was ¢rst re- ported by Van Straaten (1970) who misinterpreted them as erosional channels; successively, Stefanon (1981) hy- pothesized their presence and described the ones south- east of o¡shore Ancona (Central Adriatic Sea) and Stefa- non et al. (1983) and Stefanon (1985) described those on the north-western Adriatic shelf. Several authors have de- scribed gas seepages in the Central Adriatic Sea (i.e., Curzi & Veggiani, 1985; Mazzotti et al., 1987; Hovland & Curzi, 1989; Trincardi et al., 2004) and documented the presence of free gas at very shallow stratigraphic levels. In the Mid- Adriatic Depression some authors (Curzi & Veggiani,1985; Mazzotti et al., 1987) have observed pockmark ¢elds com- parable to those located above the salt domes of the North Sea (Hovland & Judd, 1988) and also Fer- nandez-Puga et al. (2007) correlated gas-related morphol- ogies with diapiric structures in the Gulf of Cadiz. Hovland & Judd (1988) highlighted the fact that the pock- marks, and associated subseabed columns of the Adriatic Sea periodically experience violent events, triggered by earthquakes. The mud volcanoes are sea£oor extrusions of mud ac- companied by £uids, often by methane gas, which com- monly tend to build up a solid mud deposit, which may have a conical or volcano-like shape (Hovland & Judd, 1988).The mud-carbonate mounds may have irregular, cy- lindrical or conical shape and are generally originated by carbonate concretions around sea£oor release of methane (Kopf, 2002). The aim of this paper is to identify the link between gas seepage features (such as pockmarks, mud volcanoes, mud-carbonate mounds) with the gas pulls present inner the sedimentary sequence (testi¢ed by gas chimneys and shallow bright spots) and with the deeper tectonic features as fractures and diapiric structures.The core composition, development, and distribution of these last have been pin- pointed utilizing the multichannel seismic data (MCS). The relations of the diapiric structures with shallow e¡ects on seabed morphologies have been analyzed with Correspondence: Riccardo Geletti, Istituto Nazionale di Ocea- nogra¢a e di Geo¢sica Sperimentale (OGS), Trieste, Italy. E-mail: rgeletti@ogs.trieste.it Basin Research (2008) 20, 473–487, doi: 10.1111/j.1365-2117.2008.00373.x r 2008 The Authors Journal Compilation r Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 473