Abstract High resolution and multichannel seismic profiles depict the Pliocene to Recent evolution of the mud diapirism in the West Alboran Basin (WAB) and its relationship with the Miocene diapir province that occupies the WAB depocentre. During the early to middle Miocene period of basin extension (16 to 9 Ma), normal faulting triggered the diapirism from mobile overpressured shale containing olistostromes. Plio-Quaternary diapirism evolved as a second main stage of diapiric activity and developed throughout the subsequent contractive tectonic evolution of the basin (9 Ma to Holocene). Mud volcanoes, discovered to the south of the WAB, developed on the flank of Recent diapirs as a consequence of the rise of fluidized sediments through diapiric bodies and/or through fractures connecting with deeper diapirs. During the Pliocene to Recent, some diapirs stopped ascending, leading to the production of collapse structures on their tops due to lateral subsurface mud migration and/or fluid escape. Other cylindrical shaped diapirs continued rising and produced mud volcanoes on the sea floor. All the studied volcanoes seem to be currently inactive. Two major pulses of diapiric rise have been distinguished during the Pliocene to Recent contractive evolution of the basin.

ABSTRACT Two potential geologic hazards, mud volcanoes and pockmarks, have been documented with increasing frequency since worldwide requirements for offshore hazard, environmental and engineering surveys have become more exacting. Terrestrial and marine mud volcanoes, as researched in the Caspian Sea area, form a drilling hazard and at the same time present one of the most reliable indicators of hydrocarbon producing structures. Mud volcanic eruptions of plastic clays, usually containing rock fragments, are driven by thermal waters and gas (mainly methane and nitrogen). Gas which does not escape to the surface frequently charges near-surface, permeable horizons to form high pressure gas pockets which can result in blow-outs during drilling unless operations are carefully controlled. Periodic reactivation of mud-volcanic activity also forms a potential hazard to oil field structures. Foundation stability may be threatened through liquefaction and gas escaping through gryphons poses a fire hazard. An engineering geophysical survey in the Northern Aegean Sea revealed that the seafloor offshore Thasos Island contains an abundance of pockmarks. These nearly circular depressions in the silty, alluvial seafloor sediment average 20 meters in diameter and attain depths up to three meters. Analysis of the data collected led to the conclusion that the pockmarks resulted from the escape of either artesian water or gas upwards through the thick alluvial sediment of the subbottom. The evidence suggests that artesian water is the most likely cause. From an engineering hazard viewpoint, the pockmark risk for this particular case was judged to be slight regardless of which of the two origins is chosen, provided that a relief mechanism for excess pore water or gas pressure is allowed for. INTRODUCTION Occurances of mud volcanoes and pockmarks on the seafloor have been documented more and more frequently since the intensive use of side scan sonar began in the late 1960's. They have been described from both research and applied surveys. This paper will discuss the geological origin of these features and evaluate their significance as seafloor hazards based on two engineering surveys. In the case of mud volcanoes, their value as an exploration tool will be pointed out. As a basis for the discussion of mud volcanoes, the Caspian Sea occurances will be used; for pockmarks, an example from the northern Aegean Sea will be presented. MUD VOLCANOES Mud volcanoes, in contrast to magmatic volcanoes, are of limited distribution throughout the world, with occurrences having been recorded in Rumania, Italy, Iran, Iraq, New Zealand, India, the Arakan Yoma Coast, the Malaysian Islands, Java, Borneo, the Gulf of Mexico, Trinidad, Venezuela, Colombia and the USSR. However, nowhere in the world are there as many or as well documented mud volcanic occurrences as in the Azerbaijan trend which continues into the Southern Caspian area (Figure 1). Mud volcanoes are plano-conical features exhibiting relief of up to 500 meters on land. Their base diameters range from less than one to over three kilometers. Marine. mud volcanoes in the Caspian Sea are similar to their terrestrial counterparts with the exception of height.

The seafloor of the Alboran Sea reflects its complex tectonic, sedimentary, and oceanography dynamics as a consequence of the geological context, involving interaction between the Eurasian and African plates, and oceanographic context, as it is where the Atlantic and Mediterranean waters meet. Their physiography has a semi-enclosed configuration characterized by two margins (the Spanish Iberian and North Africa—mostly Moroccan margins) enclosing deep basins. Tectonic activity is mainly attested by folds and faults that predominantly affect the central and eastern seafloor sectors, as well as numerous seamounts and fluid-flow features (pockmarks, mud volcanoes, and diapirs) that dot the seafloor. The sedimentary and oceanographic processes allow us to distinctly define two principal environments in the Alboran Sea: the shallow proximal margin (continental shelf); and the deep distal margin (continental slope and base of the slope) with the adjacent sub-basins. The shelf mostly comprises prodeltaic and infralittoral prograding wedges, with local bedform fields, submarine valleys, and wave-cut terraces. Coastal and fluvio-marine sedimentary processes, acting since the last glacial period, are responsible for these features. The deep marine environment is characterised by the ubiquity of contourites, whose continuity is interrupted by turbidite systems, canyons, and landslides. The alongslope action of the Mediterranean waters and their interfaces with the Atlantic water has been the main process governing transport, seafloor reworking, and sedimentation of contourites. Mass-movement processes are responsible for the formation of: (1) turbidite systems—turbidity flows and mass flows were dominant during the last glacial sea-level lowstand, evolving to dilute gravity flows during present interglacial high stand; and (2) landslides—the main triggering factors comprising over-steepening, seismicity, under consolidation due to overpressure by interstitial fluids, stratigraphy, and high sedimentation rates. Locally, still-undetermined biological activity in the Spanish and coral activity in the Moroccan margin generated fields of mounded bioconstructions. The seafloor morphology of the Alboran Sea offers interesting clues for assessing the main potential geological hazards, with tectonic seismicity and landslides (as well as their related tsunamis) being some of the most important potential hazards affecting coastal populations. In addition, the seafloor morphology in combination with assemblages of habitat-forming species enables habitat identification and mapping.

The Mediterranean Sea ( MED-ih-tə-RAY-nee-ən) is an intercontinental sea situated between Europe, Asia, and Africa. It is surrounded by the Mediterranean basin and almost completely enclosed by land: on the east by the Levant in West Asia, on the north by Anatolia in West Asia and Southern Europe, and on the south by North Africa. To its west it is connected to the Atlantic Ocean via the Strait of th… the Ionian Basin, which is a deep and narrow oceanic basin, stretches south of Italy, Albania, and Greece and contains the Calypso Deep, also known as the Oinousses or Vavilov Deep, featuring the deepest point in the Mediterranean Sea, located in the Hellenic Trench, 62.6 kilometres (38.9 mi) southwest of Pylos, Greece, with a maximum depth of approximately 4,900 metres (16,000 ft); the Levantine Basin to the south of Anatolia separated from the Ionian Basin by the Mediterranean Ridge. The 1,300-kilometre (810 mi)-long submarine ridge running from Calabria along the south of Crete, to the southwest corner of Turkey is a 150-to-300-kilometre (93 to 186 mi)-wide curved feature, which is also known for its mud volcanoes and dome-like structures and has been the subject of studies on the Messinian salinity crisis. The Eratosthenes Seamount, a carbonate seamount is found in the Levantine basin about 100 kilometres (62 mi) south of western Cyprus. the island of Crete delineates the Levantine Basin from the Aegean Sea, which is that portion of the Mediterranean Sea north of Crete and is bordered on the east by the coast of Turkey and on the west and north by the coast of Greece. Numerous Greek islands and seamounts are located in the Aegean Sea; and the Adriatic Sea, which is northwest of the eastern Mediterranean Sea's main body, is bordered to the east by Slovenia, Croatia, Bosnia and Herzegovina, Montenegro, and Albania, and to the west and north by Italy. Until the 1960s, the Mediterranean was believed to be the primary remaining portion of the earlier (200 million years old) Mesozoic Tethys Ocean, which once encircled the Eastern Hemisphere. However, since the late 20th century, research using the theory of seafloor spreading has indicated that most of the current Mediterranean seafloor is not a portion of the Tethys sea floor. Some researchers consider the Ionian Basin, to the east of the Malta Escarpment, to be the remnant of the Tethys Ocean. Over the course of the last 44 million years, the continental plates of Africa and Eurasia have converged and receded, resulting in the current tectonically active basin and its surrounding mountain chains. According to…