Mediterranean Ridge Research Articles

Prehistoric mega-tsunami in the eastern Mediterranean and its sedimentary response

The volcanic island of Santorini that belongs to the trench-arc-backarc Aegean system of the eastern Mediterranean underwent a catastrophic eruption in the Bronze age (circa 3500 y BP), with caldera collapse and creation of a tsunami wave that was funnelled westward. The sedimentary response to this exceptional event in the deep sea was the deposition of resediments called Homogenites. The data set available after almost thirty years of active research consists of 60 sediment cores, including several giant piston cores up to 30 meters long, raised from water depths ranging from 4100 to 2500 m in discrete parts of the Ionian Sea. Two types of Homogenite are recognized. Type A Homogenite consists of pelagic turbidites typically recorded in the Calabrian Ridge and in the western Mediterranean Ridge accretionary prisms, characterized by the «cobblestone topography», only in lows or basinal settings. Their stratigraphic position consistently above the Holocene sapropel S-1 and the depositional base are interpreted as the result of liquefaction of the unconsolidated pelagic sediments draping the nearby slopes caused by the tsunami waves, followed by downslope movement and settling. Type B Homogenite is a megaturbidite of African provenance that expanded throughout the Messina and Sirte abyssal plains originating an up to 24 m thick surficial acoustically-transparent layer calibrated by coring. Type B Homogenite has a coarse and thick sandy base consisting of bioclasts deriving from the continental shelf of North Africa and is interpreted as triggered by the tsunami wave reaching the Sirte Gulf. The Dec. 26, 2004 megatsunami that hit the coastlines of SE Asia provides a real world example of how sediment dispersal can be triggered by a series of gigantic waves. Megaturbidites similar in thickness to Type B Homogenite have been recorded in all the abyssal plains of the Mediterranean, both west and east of the Ionian basin, but their age is older (late Pleistocene) and their emplacement occurred during low sea-level stands corresponding to the last glaciation. The occurrence of a Holocene megabed, postdating the Climatic Optimum, is a paradox in a paleoclimatic scenario. An exceptional event, as the megatsunami originated by the collapse of the Santorini caldera is required.

Mud volcanoes and mud domes of the Central Mediterranean Ridge: Near-bottom and in situ observations

The first high-resolution mapping of mud volcanoes and mud domes of the Central Mediterranean Ridge (Eastern Mediterranean) presented here is based on successive in situ observations from the Nautile submersible [MEDINAUT (1998) and NAUTINIL (2003) surveys] and near-bottom side-scan sonar data (MEDINETH cruise, 1999). Data were obtained over two types of clay-kinetic-related features previously identified south of Crete: the Olimpi field mud volcanoes and the Southern belt mud domes, characterized by highly contrasting morpho-acoustic characteristics. Using the new data we can better define the morphological and backscatter characteristics of both mud volcanoes and mud domes and illustrate their similarities and differences; and establish ground truth, in terms of the presence or not of mud flows, diagenetic carbonate pavements, active seepage and macro-and microbiology. This study reveals strong contrasts between: (1) large mud volcanoes, made of successive mud flows, and associated with diagenetic carbonates and fluid venting activity, and (2) smaller mud domes, characterized by steep slopes affected by sedimentary instabilities, and without any evidence of mud flow, specific fluid seepage activity, authigenic carbonate pavement, or biologic communities. From these results we demonstrate a strong variability of clay-kinetic structures from the central domain of the Mediterranean Ridge accretionary complex to its northern thrust boundary against the Cretan continental backstop. From an integration of the high-resolution results to the Mediterranean Ridge geologic and structural settings, a qualitative model is finally proposed to explain the mud volcano and mud dome emplacement.

Western Mediterranean Ridge mud belt correlates with active shear strain at the prism-backstop geological contact

A high-resolution swath-mapping survey conducted in the deep waters of the eastern Mediterranean Sea allowed mapping of active faults and mud volcanism along a sizable portion of the Mediterranean Ridge. Active shear is localized at the prism-backstop con- tact, a major dextral flower structure and a site of massive mud expulsion. We investigate the relationship between the mud output rate and horizontal strain rate by combining the mud volume estimate from sea-bottom reflectivity with kinematic modeling based on far- field global positioning system data and local fault and strain patterns. We find a direct correlation between maxima of mud output and maxima of the shear component of strain at the backstop contact. Mud volcanism may reflect the abundance of solid (mud) and fluid (methane) sources combined with a favorable tectonic regime established at the prism-backstop contact in post-Pliocene time, in relation to plate tectonic changes.

Indications of low macrobenthic activity in the deep sediments of the eastern Mediterranean Sea

The fluxes and budget of organic matter from the oligotrophic surface waters of the eastern Mediterranean to the deep waters are poorly known, and little information is available on past and present macrobenthic activity on the sea floor. Evidence of macrobenthic activity can be direct, through recovery of living organisms or their autochthonous skeletal remains, or indirect, through bioturbation and trace fossils. The evidence of biological activity in deep eastern Mediterranean sediments has been evaluated and compared through 210 Pb profiles from box-cores and study of dredge samples from sites on Medina Rise (1374 m water depth), the Messina Abyssal Plain (4135 m) and several sites along the Mediterranean Ridge, SW and S of Crete (1783 to 3655 m). All these sites are remote from the continental shelves, so the biological benthic activity is expected to depend primarily on primary production from surface waters. The results show that present-day macrobenthos and trace fossils are generally scarce, especially at depths > 2500 m. This observation is supported by surface sediment 210 Pb excess distributions that show a surface mixed layer (SML) 2500 m. The historical layer of some box-cores and the Pleistocene hardgrounds collected in the Cleft area (Mediterranean Ridge) do, however, record a macrobenthic activity that is apparently more intense than at present, which may be related to higher primary production of the Pleistocene glacial intervals. In contrast with most areas of the present-day deep eastern Mediterranean which depend on surface primary production based on photosynthesis, a relatively dense and diversified macrobenthic community based on chemosynthesis has been recognised at depths > 1100 m on the Napoli Dome mud volcano in the Olimpi area, and on the Kazan and other mud volcanoes in the Anaximander Mountains.

Structural setting and tectonic control of mud volcanoes from the Central Mediterranean Ridge (Eastern Mediterranean)

Based on a recent marine geophysical data set, including swath bathymetry, acoustic imagery and six-channel seismics, recorded over a large area of the Mediterranean Ridge (MR) in early 1998 during the Prismed 2 survey, this paper presents a study of the various relationships observed between tectonic features cutting across the Central Mediterranean Ridge accretionary wedge and massive mud expulsions (known as mud volcanoes), identified over large areas of the ridge. Regional mapping of two of the mud volcano fields previously only partly investigated (Olimpi and United Nations Rise) revealed the presence of many new mud expulsion centres and a third new mud volcano field has been identified. All the mud features show great variability in morphology, size and backscatter strength of their surrounding mud flows. Based on their contrasting morpho-acoustic characteristics, we propose a classification into three main groups of mud construction: (1) “mud volcanoes”, these consist of subcircular and prominent reliefs, often associated with high backscatter mud flows; (2) “mud domes”, similar to mud volcano, but smaller, these occurrences correspond to weakly reflective mud constructions; (3) “mud plateaus” represent a third category which appears as wide, often highly reflective and rather flat mud extrusions. From all available data, an attempt to explain the different mud ascent processes and driving forces is discussed, in relation to the initial collision structural setting of the Central Mediterranean Ridge. Within this area, most of the mud constructions have been observed to be associated with tectonic features and, in particular, with strike-slip faulting for the first time. As a hypothesis, we propose in this paper two different ascent processes to explain the contrasting mud constructions: (1) “extrusion” for the mud volcanoes and plateaus and (2) “intrusion” for the mud domes, connected to two different mud levels controlled by the crustal geometry of this pre-collision area and especially the southwards extension of the Cretan continental crust below the Mediterranean Ridge accreted sediments.

Contemporary kinematics of the southern Aegean and the Mediterranean Ridge

SUMMARY This study focuses on the kinematics of the southern Aegean and the Mediterranean Ridge (MR). A quantification of the deformation of the MR is essential for both evaluating physical models of accretionary wedges in general and for obtaining a self-consistent model of the surface deformation over the entire Nubia‐Eurasia (NU‐EU) plate boundary zone in the eastern Mediterranean. Previous kinematic studies have not properly considered the deformation field south of the Hellenic arc. Although this study focuses on the deformation field of the MR, we also discuss the kinematics of the southern Aegean, because the geometry and movement of the Hellenic arc determine to a large extent the kinematic boundary conditions for kinematic studies of the MR. We calculate a continuous velocity and strain rate field by interpolating model velocities that are fitted in a least-squares sense to published Global Positioning System (GPS) velocities. In the interpolation, we use information from a detailed data set of onshore and offshore active faulting to place constraints on the expected style and direction of the model strain rate field. In addition, we use the orientations of tracks left by seamounts travelling into the wedge to further constrain the offshore deformation pattern. Our model results highlight the presence of active shear partitioning within the Mediterranean ridge. High compressional strain rates between the ridge crest and the deformation front accommodate approximately 60‐ 70 per cent of the total motion over the wedge, and the outward growth rate of the frontal thrust is ∼ 4m m yr −1 . Strain partitioning within the wedge leads to 19‐23 mm yr −1 of dextral motion at the wedge‐backstop contact of the western MR, whereas the Pliny and Strabo trenches in the eastern MR accommodate 21‐23 mm yr −1 of sinistral motion. The backstop of the western MR is kinematically part of the southern Aegean, which moves as a single block [the Aegean block (AE)] at 33‐34 mm yr −1 in the direction of S24 ◦ W ± 1 ◦ towards stable Nubia (NU). Our model confirms that there is a clear divergence between the western and eastern Hellenic arc and we argue for a causal relation between the outward motion of the arc and the gradient in the regional geoid anomaly. Our results suggest that a significant driving source of the surface velocity field lies south of the Hellenic arc and only for the southeastern Aegean could there be some effect as a result of gravitational collapse associated with density differences within the overriding plate.

The August 27, 1886 earthquake in Messenia (Peloponnesus) and reported flames over the Ionian Sea—a Mediterranean Ridge gas escape event?

The 1960 bathymetric map of Pfannenstiel showed a submarine volcano west of the Peloponnesus which could not be confirmed by modern bathymetric surveys. Pfannenstiel's interpretation was supported by a report of flames on the sea in 1886. It is most likely that the strong earthquake on August 27, 1886, triggered a submarine mud volcano eruption and gas escapes on the Mediterranean Ridge (MR), and burning gas was observed from several places on land and on sea.

Seismicity of the Hellenic subduction zone in the area of western and central Crete observed by temporary local seismic networks

Temporary local seismic networks were installed in western Crete, in central Crete, and on the island Gavdos south of western Crete, respectively, in order to image shallow seismically active zones of the Hellenic subduction zone. More than 4000 events in the magnitude range between −0.5 and 4.8 were detected and localized. The resulting three-dimensional hypocenter distribution allows the localization of seismically active zones in the area of western and central Crete from the Mediterranean Ridge to the Cretan Sea. Furthermore, a three-dimensional structural model of the studied region was compiled based on results of wide-angle seismics, surface wave analysis and receiver function studies. The comparison of the hypocenter distribution and the structure has allowed intraplate and interplate seismicity to be distinguished. High interplate seismicity along the interface between the subducting African lithosphere and the Aegean lithosphere was found south of western Crete where the interface is located at about 20 to 40 km depth. An offset between the southern border of the Aegean lithosphere and the southern border of active interplate seismicity is observed. In the area of Crete, the offset varies laterally along the Hellenic arc between about 50 and 70 km. A southwards dipping zone of high seismicity within the Aegean lithosphere is found south of central Crete in the region of the Ptolemy trench. It reaches from the interface between the plates at about 30 km depth towards the surface. In comparison, the Aegean lithosphere south of western Crete is seismically much less active including the region of the Ionian trench. Intraplate seismicity within the Aegean plate beneath Crete and north of Crete is confined to the upper about 20 km. Between 20 and 40 km depth beneath Crete, the Aegean lithosphere appears to be seismically inactive. In western Crete, the southern and western borders of this aseismic zone correlate strongly with the coastline of Crete.

Modeling deformation and salt tectonics in the eastern Mediterranean Ridge accretionary wedge

This study addresses some aspects of the kinematics and structural setting of the Mediterranean Ridge accretionary complex, particularly those aspects related to the occurrence and distribution of Messinian evaporites. The approach is by (1) identification of the geometry of deformation through reprocessing of existing multichannel seismic reflection data from the Levantine Basin, aimed at the definition of the velocity field and depth imaging; (2) physical modeling in the laboratory, aimed at the study of the kinematics of shortening of sedimentary sequences detaching on viscous decollements; and (3) comparison between the results of the two approaches. We conclude that the thick Messinian sequence of the Levantine Basin is composed primarily of salt, so that the entire evaporitic sequence can be expected to behave as a viscous material. It follows that the main detachment of the post-Messinian wedge is located at the base of the Messinian salt layer. A viscous (salt) decollement would produce thrusting trending normal to the shortening direction, but boundary conditions affect structural trends even more than stress and/or movement direction. Both strain partitioning and the formation of major strike-slip faults within the post-Messinian wedge are prevented by the high angle of convergence between the African and Aegean plates as well as by the low intraplate friction. In addition, we show that curved and anastomosing thrust fronts are reflected in the cobblestone topography of the Mediterranean Ridge. We finally postulate that extension may occur in the central Mediterranean Ridge as a result of the geometry of the plate boundaries. This extension is considered as a possible cause of mud volcanism and mud diapirism.

Receiver function study of the Hellenic subduction zone: imaging crustal thickness variations and the oceanic Moho of the descending African lithosphere

SUMMARY We use data from recently installed broad-band seismographs on the islands of Crete, Gavdos, Santorini, Naxos and Samos in the Hellenic subduction zone to construct receiver function images of the crust and upper mantle from south of Crete into the Aegean Sea. The stations are equipped with STS-2 seismometers and they are operated by GFZ Potsdam, University of Chania and ETH Zurich. Teleseismic earthquakes recorded by these stations at epicentral distances between 35° and 95° have been used to calculate receiver functions. The receiver function method is a routinely used tool to detect crustal and upper-mantle discontinuities beneath a seismic station by isolating the P–S converted waves from the coda of the P wave. Converted P–S energy from the oceanic Moho of the subducted African Plate is clearly observed beneath Gavdos and Crete at a depth ranging from 44 to 69 km. This boundary continues to the north to nearly 100 km depth beneath Santorini island. Because of a lack of data the correlation of this phase is uncertain north of Santorini beneath the Aegean Sea. Moho depths were calculated from primary converted waves and multiply reflected waves between the Moho and the Earth's surface. Beneath southern and eastern Crete the Moho lies between 31 and 34 km depth. Beneath western and northern Crete the Moho is located at 32 and 39 km depth, respectively, and behaves as a reversed crust–mantle velocity contrast, possibly caused by hydration and serpentinization of the forearc mantle peridotite. The Moho beneath Gavdos island located south of Crete in the Libyan Sea is at 26 km depth, indicating that the crust south of the Crete microcontinent is also thinning towards the Mediterranean ridge. This makes it unlikely that part of the crust in Crete consists of accreted sediments transported there during the present-day subduction process which began approximately 15 Ma because the backstop, i.e. the boundary between the current accretionary wedge of the Mediterranean ridge and the Crete microcontinent, is located approximately 100 km south of Gavdos. A seismic boundary at 32 km depth beneath Santorini island probably marks the crustal base of the Crete microcontinent. A shallower seismic interface beneath Santorini at 20–25 km depth may mark the depth of the detachment between the Crete microcontinent and the overlying Aegean subplate. The Moho in the central and northern Aegean, at Naxos and Samos, is observed at 25 and 28 km depth, respectively. Assuming a stretching factor of 1.2–1.3, crustal thickness in the Aegean was 30–35 km at the inception of the extensional regime in the Middle Miocene.

Thermal history of deep‐sea sediments as a record of recent changes in the deep circulation of the eastern Mediterranean

During three cruises of the MEDRIFF project in 1993 and 1994, 154 geothermal heat flow measurements and seven CTD profiles in the water column have been collected along a 300 km SW‐NE oriented transect traversing the Mediterranean Ridge accretionary complex. The original goal of the measurements was to identify areas of anomalous heat flow that could be interpreted as possible sites for fluid outflow. Contrary to expectations, the upper few meters of the temperature profiles in the sediments showed decreasing temperature from the seafloor down to 3 to 6 m depth indicating consistently transient temperature regimes. The only exception (positive heat flow) was found in the Sirte abyssal plain. Measurements collected in the same area at different times indicated that the thermal structure in the bottom water and sediments had changed significantly at weekly, monthly, and interannual timescales. One‐dimensional forward modeling of the conductive heat propagation into the sediment explains the observed thermal anomalies, assuming up to 0.5 K warming of the bottom waters that propagated south westward from the Matapan Trench to the crestal area of the Mediterranean Ridge. Analysis of nonsteady state thermal profiles in the upper sediment provided time information on the onset of bottom water warming, by what is now called the Eastern Mediterranean Transient (EMT). The sediment warming started in the Matapan Trench, between spring 1992 and spring 1993, and reached the Mediterranean Ridge crest in spring‐summer 1993. The average propagation velocity of the thermal perturbation along the profile is about 0.5–1.6 km d−1 (0.6–1.8 cm s−1).

The Ionian Abyssal Plain (central Mediterranean Sea): Morphology, subbottom structures and geodynamic history – an inventory

In order to understand the structure and evolution of the Mediterranean Ridge accretionary complex, it is necessary to understand the structure and history of its foreland. The Ionian Abyssal Plain is one of the varying types of foreland. The state of knowledge for that is presented. Its contour and detailed relief are described for the first time. Based on published and hitherto unpublished seismic data, information on the thickness of the Plio-Quaternary and on the Messinian evaporites are presented. Of particular interest are data concerning the pre-Messinian reflectors. They indicate a pattern of tilted blocks and horst-like features created in pre-Messinian time by tensional tectonics. Varying subsidence continued, however, during Messinian time and controlled the thickness of evaporites. At some places (e.g. Victor Hensen Seahill) vertical tectonics seem to be still active. The main tectonic structures of the Ionian Abyssal Plain are not related to the process of the present accretion and subduction at the Africa/Eurasia plate boundary but are pre-existing and should influence the internal structure of the Mediterranean Ridge which is still growing at the expense of the foreland. As a consequence of our structural evidence, we favour the following interpretation: the Ionian Abyssal Plain is not a remainder of the Jurassic Tethyan ocean but originated by extensive attenuation of continental crust.

The Mediterranean Ridge: A mass balance across the fastest growing accretionary complex on Earth

Depth migration of seismic reflection profiles across the Mediterranean Ridge accretionary complex between the African and Eurasian blocks illustrates profound variations in the geometry and internal structure along strike. Structural interpretations of four cross sections, together with bathymetric and acoustic surface information and drilling data, are used to volumetrically balance the amount of subduction versus accretion with time. Results suggest the existence of three distinct scenarios, with a jump in décollement in the west, intense backthrusting in the central part between Libya and Crete, and transcurrent tectonism in the east. The onset of accretion coincides with exhumation of thrust sheets (∼19 Ma), followed by rapid sediment accretion with thick, evaporite‐bearing incoming successions facilitating outward growth of the wedge. The minimum rate of accretion (20–25% of the total sediment supply) is observed in the central portion where the ridge suffers maximum deformation. Here the indenting leading edge of the African Plate apparently forces the sediment into subduction, or local underplating. In contrast, an estimated 40–60% of the available sedimentary input was accreted in the western domain where collision is less accentuated. The results support the hypothesis that highly destructive forearc collisional events, like slab break off and exhumation of thrust sheets, can be followed by periods of accretion and continuous growth of accretionary wedges.

Evidence of methane venting and geochemistry of brines on mud volcanoes of the eastern Mediterranean Sea

As a part of the Dutch–French MEDINAUT diving expedition in 1998, cold seeps and mud volcanoes were studied and sampled in two distinctive tectonic settings in the eastern Mediterranean Sea. The first setting was the Olimpi Mud Volcano field (OMV area), including Napoli, Milano, Maidstone and Moscow mud volcanoes, south of Crete on the Mediterranean ridge. The second setting was the Anaximander Mountains (AM area), southwestern Turkey, where Amsterdam, Kazan and Kula mud volcanoes were explored. Large methane concentrations (45–892 nmol/kg) were measured in the water column not only above mud volcanoes but also in seeps and vents along related fault systems, indicating intense degassing related to fluid circulation in sediments. The tracer results show that there is considerable variability in terms of gas seepage and matter flux between these mud volcanoes. Brine accumulations found as shallow pools on Napoli or associated with deep faults (Nadir Lake) outside mud volcanoes exhibit variable chlorinity, mineral and gas composition. The brines are significantly enriched in δ 18O relative to ambient seawater and are consistent with evaporated seawater. In the Nadir Brine Lake, the level of methane is as high as 5.93 mmol/kg, lower than the methane saturation level of 120 mmol/kg theoretically found at the salinity (120), pressure (200 bar), and temperature (13.6°C) conditions of Nadir lake. In contrast, the shallow brine pools on Napoli mud volcano (also OMV area) have methane levels of only 4.45 μmol/kg. In all brines, helium data show a clear radiogenic isotopic ratio ( R=0.06× R a), in excellent agreement with recently published data for the Urania basin. Methane to ethane ratios (>1000) and δ 13C(CH 4) values (−65.6‰PDB) indicate that the CH 4 is microbially produced. Unlike mid-ocean ridges, where abiogenic methane and helium have a common origin in the brines, the large variation in the CH 4/He ratio indicates that CH 4 and helium sources are unrelated, a fact that adds further support to the biogenic origin of methane. These results show that mud volcanoes in the eastern Mediterranean Sea are important sites of extensive biogenic methane fluxes, which are probably related to widespread occurrences of gas hydrates.

Sedimentary succession and evolution of the Mediterranean Ridge western sector as derived from lithology of mud breccia clasts

Rock fragments from mud breccia extruded by mud volcanoes, discovered during the TTR-4 cruise of the R/V Gelendzhik on the western Mediterranean Ridge, are represented by a number of different lithologies. They provide important information on the composition, genesis, and age of the deep-seated sedimentary series of the basin. Lithological description and age determination of the main rock types obtained as clasts from mud breccia were made on the basis of a study of ninety-seven samples. Depositional processes and environments of their accumulation were determined using sedimentological criteria. The sedimentary section through which mud volcanoes erupted on the western Mediterranean Ridge includes Aptian–Albian claystones, Late Cretaceous limestones, Paleogene siliciclastic rocks, and a variety of Miocene limestones and mudstones. The rock clasts studied throw insight into the evolution of the western sector of the Eastern Mediterranean Basin from, at least, Aptian time. The Aptian–Albian clayey series have been deposited in the marine conditions of a relatively narrow basin with a high input of terrigenous clayey material. During the Late Cretaceous the basin enlarged, terrigenous input into deep parts of the basin decreased, and carbonate pelagic sedimentation became prevalent. Regression in Paleogene time led to the deposition of thick terrigenous siliciclastic series of deltaic–prodeltaic sediments. Extensive turbidite systems were emplaced in the western sector of the Eastern Mediterranean during the Miocene, along with predominantly carbonate pelagic sedimentation.

Deep sea pockmark environments in the eastern Mediterranean

A great number of circular to isometric depressions have been observed on the deep water seafloor in the eastern Mediterranean by MAK-1 and ORETech deep-tow side-scan sonar during several TTR cruises of the UNESCO Floating University Program since 1993, and also by visual observations made during the late 1998 French–Dutch MEDINAUT expedition. They extend from the Cobblestone mud volcano area in the west, passing all along the Mediterranean Ridge through the Olimpi and United Nation Rise mud volcano provinces to the Anaximander Mountains and Eratosthenes Seamount to the east. Several environments in which deep water pockmarks preferentially occur in the eastern Mediterranean are: (1) active mud volcanoes such as those of the Olimpi mud field and the Anaximander mountains; (2) remnant mud volcanoes and corresponding fault systems, like pockmarks on the flanks of buried volcanoes in the Cobblestone and United Nation Rise areas; (3) most abundantly on active faults like those on the Eratosthenes Seamount, Anaximander Mountains and on the Mediterranean Ridge; and (4) submarine slumps. Some pockmarks on the Mediterranean Ridge contain brines with high gas content.

Boron and boron isotopes as tracers for diagenetic reactions and depth of mobilization, using muds and authigenic carbonates from eastern Mediterranean mud volcanoes

Abstract Authigenic carbonates and muds from six mud volcanoes in the eastern Mediterranean Sea were recovered during the French/Dutch MEDINAUT cruise utilizing the submersible Nautile in November 1998. The mud volcanoes are active seafloor vents in two areas at the plate boundary between the converging African and Eurasian Plates: the Mediterranean Ridge accretionary prism near Crete (Greece) and the Anaximander Mountains south of Turkey. B contents and δ 11 B signatures were measured with the aim of identifying the diagenetic processes and source depths of the material in the collision zone. B concentrations of the carbonate precipitates cover a range of 8–45 ppm and vary isotopically from +15.6 to +22.9‰ (corresponding to a parent solution of 34.9–42.2‰ at pH 7). Both the B-enrichment and a δ 11 B valve slightly lower than seawater suggest the mud domes originate from a moderately deep fluid source, with local admixture of seawater. B contents and δ 11 B of the mud show distinct differences between the areas: the Mediterranean Ridge mud domes have lower B contents and higher δ 11 B (average 3.9‰) compared to Anaximander Mountains mud volcanoes (δ 11 B average −0.6‰). These B results attest the release of structurally-bound B from clay mineral lattices, probably due to stronger deformation near Turkey. These mudstones, which had previously been affected by deep-seated thrusting beneath the Antalya Complex, may have been liquefied and remobilized in their present setting. By contrast, the mud on the Mediterranean Ridge represents offscraped clay-rich strata that was incorporated into the large accretionary wedge.

Grain size evidence for variations in delivery of terrigenous sediments to a Middle Pleistocene interrupted sapropel from ODP Site 969, Mediterranean Ridge

Grain size distributions of the <63-μm terrigenous fraction of closely spaced sediment samples were measured across an interrupted sapropel sequence at ODP Site 969 on the Mediterranean Ridge to explore possible climate-related changes in delivery of wind-borne particles. Size distribution patterns are generally typical of hemipelagic sediment with a subordinate amount of eolian material. However, the eolian fraction increases in the sapropel layers, which contrasts with findings from elsewhere in the Mediterranean but is consistent with other work at this location.

Late Pleistocene–Holocene planktonic assemblages in three box-cores from the Mediterranean Ridge area (west–southwest of Crete): palaeoecological and palaeoceanographic reconstruction of sapropel S1 interval

Planktonic foraminiferal and calcareous nannofossil assemblages of three box-cores from the western–central part of the Mediterranean Ridge area were investigated and correlated. In particular we focused on the most significant faunal and floral signals recorded before, during and after sapropel S1 deposition. The interval preceding S1 is characterised by an increasing trend of the two planktonic foraminiferal species, Globorotalia inflata and Truncorotalia truncatulinoides, that are usually related to a well developed cold and deep mixed layer. This change is supported also by the rarity of warm water coccolithophorid species and by the presence of the deep dwelling species Florisphaera profunda. The beginning of S1 is marked by the disappearance of G. inflata and T. truncatulinoides and by the significant increase in abundance of Globigerinoides ruber, especially the rosea variety usually considered indicative of warmer conditions. Warm water species increase also within the coccolithophorid assemblage, while the upwelling species Reticulofenestra spp. suddenly decreases and remains low across the whole sapropel interval. The sedimentary expression of the end of anoxia is the upper boundary of the oxidised level observed above S1 in the three box-cores. Across this boundary we detect a small but well defined increase of Braarudosphaera bigelowii, the decrease of warm water coccolithophorid species and the increase of Reticulofenestra spp. This boundary is marked also by the reoccurrence of the planktonic foraminiferal species G. inflata and Neogloboquadrina pachyderma dextral and by an evident decrease of G. ruber var. rosea, indicating the end of the Holocene Climatic Optimum and the beginning of a cold and wet phase, probably corresponding to the Atlantic–Subboreal continental transition.

Geodynamics along an increasingly curved convergent plate margin: Late Miocene‐Pleistocene Rhodes, Greece

Neogene‐Holocene outward migration of the absolute position of the convergent Hellenic plate boundary produced simultaneous increased curvature of the plate boundary, changing obliquity of plate convergence vectors and boundary‐parallel stretching of the forearc region. To study the effects of the plate boundary migration and curvature, a tectonostratigraphy is constructed from the middle Miocene–Pleistocene Apolakkia basin on Rhodes, whose easternmost location makes it a key island to assess the inner forearc's kinematic response to expansion of the overriding Aegean‐Anatolian block and thus obliquity of convergence with the African plate. The basin fill provides temporal and paleogeographic control to interpret its syndepositional and postdepositional structural assemblages. Five fault populations in the Apolakkia basin record two neotectonic deformation phases separated by a kinematic change at ∼4.5 Ma, both of which are consistent with outward expansion of the Aegean‐Anatolian block. The Apolakkia basin originated as a late Miocene fault wedge basin in response to syndepositional southwest‐northeast D1 extension with similar strain patterns in the adjacent offshore Hellenic inner forearc. The kinematic change at ∼4–5 Ma is attributed to a threshold of obliquity whereby the inner forearc started to experience sinistral‐oblique divergence. The Plio‐Pleistocene D2 transtensional phase reoriented the basin and resulted in combined syndepositional west‐northwest‐east‐southeast extension (283°) and 070° sinistral shear, orientations that are best attributed to simultaneous outward expansion of the Hellenic forearc, increasing curvature of the plate boundary and associated boundary‐parallel stretching of the forearc. Principal shear zones offshore also occur consistently at ∼070°, mimicking the D2 kinematic history of the Apolakkia basin and suggesting a consistent geodynamic regime throughout the inner eastern Hellenic forearc. Effects of sinistral‐oblique plate convergence along the subduction zone appear confined to the outer forearc and the accretionary prism of the Mediterranean Ridge. Thus the upper crust of the expanding Aegean‐Anatolian block behaved independently at its leading edge, and the Pliny “trench” constitutes a major boundary separating partitioned forearc slivers, a post‐4.5 Ma partitioning recorded reliably by the Pliocene fill of the Apolakkia basin.