Mediterranean Ridge Research Articles

Imaging crustal discontinuities and the downgoing slab beneath western Crete

The migration of teleseismic receiver functions yields high-resolution images of the crustal structure of western Crete. Data were collected during two field campaigns in 1996 and 1997 by networks of six and 47 short-period three-component seismic stations, respectively. A total of 1288 seismograms from 97 teleseismic events were restituted to true ground displacement within a period range from 0.5 to 7 s. The application of a noise-adaptive deconvolution filter and a new polarization analysis technique helped to overcome problems with local coda and noise conditions. The computation and migration of receiver functions results in images of local crustal structures with unprecedented spatial resolution for this region. The crust under Crete consists of a continental top layer of 15–20 km thickness above a 20–30 km thick subducted fossil accretionary wedge with a characteristic en echelon fault sequence. The downgoing oceanic Moho lies at a depth of 40–60 km and shows a topography or undulation with an amplitude of several kilometres. As a consequence of slab depth and distribution of local seismicity, the Mediterranean Ridge is interpreted as the recent accretionary wedge.

The Augias megaturbidite in the central Ionian Sea (central Mediterranean) and its relation to the Holocene Santorini event

Abstract A thick mud turbidite, up to 24 m in thickness (termed the “Augias turbidite”) is decribed from the Ionian and Sirte Abyssal Plains and the adjacent slopes of the Calabrian Rise and the Mediterranean Ridge. The material of the megaturbidite is derived from the Gulf of Sirte (revealed by its high aragonite content) and, through erosion, from the Sirte Rise. The shallow water sediments making up the turbidite were probably mobilized by a tsunami caused by the Minoan Santorini collapse around 3500 yr B.P. The volume of the mobilized sediments is calculated as a minimum of 65 km3. Isolated turbidites of the same age deposited on the upper slope of the Mediterranean Ridge may consist of tsunami-mobilized shallow water material as well as of earthquake-triggered sediment from the flanks of local depressions.

Deep-sea tsunami deposits triggered by the explosion of Santorini (3500 y BP), eastern Mediterranean

Abstract The collapse of the Santorini caldera after the catastrophic eruption of the Bronze Age, 3500 y BP, caused a tsunami wave that had catastrophic effects in the Ionian basin, including its deepest parts. Pelagic turbidites of local origin were deposited on the bottom of small perched basins of the Southern Calabrian, Western and Central Mediterranean Ridges (type A homogenite) whereas a megaturbidite of distal origin, presumably triggered by the tsunami wave hitting the shoreline of the Sirte Gulf, was deposited on the Ionian and Sirte Abyssal Plains, extending eastwards as far as the Western Herodotous Trough depositing a thick, acoustically transparent layer (type B homogenite). Three core transects crossing the deformation front of the Mediterranean Ridge are presented and discussed. A fourth transect of giant piston cores was collected on the abyssal plains located to the south of the Mediterranean Ridge. All those to the west of the collision zone contain the Holocene homogenite with a thickness in excess of 20 m, whereas the cores taken from the Herodotous Abyssal Plain east of the collision zone are devoid of the homogenite. Sedimentological analyses were performed on the only giant core that penetrated the sandy base of the homogenite and the underlying pelagic sediments of late Pleistocene (last glacial) age. The African provenance of this typical type B homogenite is corroborated by shallow-water fauna derived from the North African shelf. No sedimentological characteristics peculiar to tsunamiites are observed in the deep-sea homogenite of the eastern Mediterranean.

Trace metal signatures of eastern Mediterranean sapropels

Sapropel samples of Pliocene to Pleistocene age recovered during ODP Leg 160 from Sites 964 (Ionian Basin), 969 (Mediterranean Ridge), 966, 967 (top and northern slope of the Eratosthenes Seamount, south of Cyprus) and 968 (Cyprus margin) were analysed for major and trace elements. Eastern Mediterranean sapropels are characterized by high organic carbon and sulphur contents and by significant enrichments in several redox-sensitive and/or sulphide-forming trace metals (Bi, Cd, Co, Cr, Cu, Mo, Ni, Re, Sb, Tl, U, V). Enrichment factors relative to ‘average shale’ are comparable to those found in CTBE black shales. The Re content is extremely high in the sapropels studied with maximum values up to 1000 ng/g. Re/Mo ratios averaging 2.7×10 −3 are close to the seawater value. High trace metal enrichments and Re/Mo ratios close to the seawater value point to an anoxic water column during sapropel formation. Additionally, enhanced Ba concentrations in the sapropels support the assumption that bioproductivity was also high during these episodes. Trace metal signatures of sapropels originating from different drill sites on a transect across the eastern Mediterranean are not only influenced by water depth but also by their location in the eastern Mediterranean. Metal enrichments seem to be higher at the western sites compared with the eastern sites. This probably reflects a change in the circulation pattern during periods of sapropel formation. Pliocene sapropels exhibit elevated trace metal contents in comparison with Pleistocene examples. Therefore, different palaeoceanographic conditions may have developed during the Pleistocene compared with the Pliocene. Altering palaeoceanography was probably controlled by climatic change, which may be related to the onset of glaciation of the northern Hemisphere. Decreasing summer precipitation throughout the late Pliocene and the Pleistocene may also have influenced sapropel deposition.

Chemical cycles in Pliocene sapropel-bearing and sapropel-barren eastern Mediterranean sediments

High-resolution (3–4 cm) investigations of major and minor elements were carried out on sediment cores from ODP Leg 160 Sites 964 (Ionian Basin), 969 (Mediterranean Ridge), 967 and 966 (northern slope and top of Eratosthenes Seamount, south of Cyprus). At Site 967, element/Al ratios display pronounced cyclic records that are related to astronomically-forced climatic variations. Sediments from the top of Eratosthenes Seamount (Site 966) show a similar pattern to those from Site 967, but they are more disturbed by bioturbation and also strongly influenced by long-term variations in sediment dispersal. Sediments from Site 969 display similar trends to those from Site 964, i.e. they are characterized by higher Mg/Al, slightly higher K/Al and lower Ti/Al ratios during sapropel formation. Higher Ti/Al ratios at Site 969 compared with sediments from Site 964 may be explained by a westward transport of material from a source in the east, where Ti/Al ratios are highest (Sites 966 and 967). Elevated Mg/Al ratios at Site 969 during sapropel intervals are due to river-derived material, transported through the Kithira Strait between Peloponnesus and Crete from the northwestern Aegean Sea, which originates from ultramafic rocks. A change in intermediate water currents (current reversal?) during sapropel formation may explain the enhanced supply of Mg-rich and the relatively diminished supply of Ti-rich material. We propose that, during deposition of the reddish intervals at Site 969, a change in circulation did not occur, because a drastic decrease in Ti/Al ratios is not evident. In these sediments Ba peaks, indicative of former sapropels, are not present. Ba enrichments in the same sapropel-barren intervals at Site 967 (northern slope of Eratosthenes Seamount) suggest that, during such intervals, the ‘burn-down’ process was more effective than during the sapropel-bearing intervals. Mn contents in all investigated Pliocene sediments correlate with the carbonate contents. Higher Mn contents of the sapropel-barren sediments suggest that, during time intervals of sapropel formation, either Mn accumulation at the margins of the permanently oxic upper water column, or Mn export into the western Mediterranean, might have occured. The latter could only be explained by sub- or anoxic conditions in large parts of the water column.

Origin of mud breccia from the Mediterranean Ridge accretionary complex based on evidence of the maturity of organic matter and related petrographic and regional tectonic evidence

Abstract Drilling the Milano mud dome situated in the Olimpi field on the northern flank of the Mediterranean Ridge accretionary complex has documented episodic eruptive activity over the last 1 to >1.5 million years. Mud extrusion is related to backthrust faulting due to subduction of the Africa Plate beneath a relatively rigid backstop of Cretan continental crust. The mud dome is mainly composed of mud breccia with up to 65% of polymictic clasts embedded in a clayey matrix. Previously published results from petrography and new maturity data of both clasts and mud matrix presented here allow us to distinguish between two different areas of provenance: (1) A northern province, characterised by a trimodal maturity distribution of organic material, was found for the majority of the mud breccia matrix samples studied. Similarly, some of the clasts studied show the three vitrinite-reflectance populations. These are carbonate and sandstone clasts, inferred to have been eroded from the upper nappes of the Cretan thrust stack after exhumation in the late Miocene, as well as mudstone clasts. Clay mineralogy of these clasts coincides with earlier results on seafloor samples of a northern provenance, recovered in the Hellenic Trench. (2) A southern provenance is proposed for a second group of clasts with a bimodal maturity distribution, not showing the second population (0.64–0.82% Rm). These clasts are litharenites, calcilutites and calciturbidites of a proposed southern provenance, i.e. North Africa. The presence of huminite in all clast and matrix samples studied (0.26–0.45% Rm) is indicative of a shallow burial history. Estimates regarding the depth of mobilisation of the mud using the regional thermal gradient indicate burial not deeper than 2 km below the seafloor. Organoclastic matrix components mirror the northern clast provenance so that, prior to subduction and subsequent extrusion, deposition of the mud in the Northeastern Mediterranean can be inferred. We propose a model in which sediment from the north was trapped within the forerunner of the Pliny trench; this sediment was deposited above the backstop of the subduction complex, which was later overridden by the accretionary prism as a result of incipient collision; density inversion forces then favoured uprise and extrusion of underconsolidated sediments as mud volcanoes. The mud matrix was probably largely derived from a Messinian succession, although alternatives remain possible.

Synchronous and velocity‐partitioned thrusting and thrust polarity reversal in experimentally produced, doubly‐vergent thrust wedges: Implications for natural orogens

A sandbox analog modeling research program was used to study the detailed evolution of doubly‐vergent thrust wedges. High‐resolution multilayers of thin alternating sand and mica layers were used in contractional Coulomb wedge experiments to simulate deformation of anisotropic, brittle upper crustal strata in doubly‐vergent orogens. Experiments incorporating syntectonic sedimentation in foreland basins and in piggyback basins were also carried out. Our laboratory models evolved in two main stages: (1) initial high‐velocity thrusting in the retrowedge and high‐frequency together with low displacement folding and thrusting in the prowedge; and (2) low‐frequency, high‐displacement synchronous thrusting in the prowedge and low‐velocity thrusting in the retrowedge. Transition from stage I to stage II occurred when the growing wedges reached the critical height at which they behaved as a backstop for further prowedge accretion. Addition of syntectonic sediments increased the persistence of stage I and triggered out‐of‐sequence thrusting in the axial zone of the experimental orogens. Thrust motion was stick‐slip. Retrovergent thrusting occurred along a long‐lived ramp whose lower tip was located at the subduction slot. Provergent kink bands nucleated at the subduction slot in stage I. In contrast, during the second stage of wedge evolution, kink bands nucleated in a piggyback fashion in the foreland far from the subduction slot and then evolved into high‐displacement faults that remained active up until the end of the experiments, at progressively decreasing rates of thrusting. The axial zones of the model wedges were characterized by fast uplift rates during stage I due to backward translation of the belt along the retrovergent, long‐lived ramp and due to the localization of deformation close to the subduction slot. Outward migration of the deformation front in the prowedge region during stage II caused the progressive decrease in the rate of the wedge uplift, until it eventually stopped. Analytical models to quantify the Coulomb behavior in the wedges validated the reversal of thrust polarity with increasing shortening, triggered by the buildup of the topographic load during deformation. This thrust polarity reversal highlights problems with the classic concept of back thrusting as a reflection of a significant change in the deformation regime. Our results compare well with the kinematic evolution of natural accretionary prisms and of thrust‐and‐fold belts such as the Lesser Antilles Arc System, the Mediterranean Ridge, and the Pyrenees.

Biomarker evidence for widespread anaerobic methane oxidation in Mediterranean sediments by a consortium of methanogenic archaea and bacteria. The Medinaut Shipboard Scientific Party.

Although abundant geochemical data indicate that anaerobic methane oxidation occurs in marine sediments, the linkage to specific microorganisms remains unclear. In order to examine processes of methane consumption and oxidation, sediment samples from mud volcanoes at two distinct sites on the Mediterranean Ridge were collected via the submersible Nautile. Geochemical data strongly indicate that methane is oxidized under anaerobic conditions, and compound-specific carbon isotope analyses indicate that this reaction is facilitated by a consortium of archaea and bacteria. Specifically, these methane-rich sediments contain high abundances of methanogen-specific biomarkers that are significantly depleted in (13)C (delta(13)C values are as low as -95 per thousand). Biomarkers inferred to derive from sulfate-reducing bacteria and other heterotrophic bacteria are similarly depleted. Consistent with previous work, such depletion can be explained by consumption of (13)C-depleted methane by methanogens operating in reverse and as part a consortium of organisms in which sulfate serves as the terminal electron acceptor. Moreover, our results indicate that this process is widespread in Mediterranean mud volcanoes and in some localized settings is the predominant microbiological process.

Extrusion dynamics of mud volcanoes on the Mediterranean Ridge accretionary complex

Abstract Drilling of two submarine mud domes situated in the Olimpi field on the northern flank of the Mediterranean Ridge accretionary complex has documented episodic eruptive activity over the last 1 to >1.5 Ma. Mud extrusion is related to plate convergence between Africa and Eurasia, having caused backthrust faulting of accreted strata containing overpressured mud at depth. The domes mainly consist of mud breccia with up to 65% of polymictic clasts embedded in a clayey matrix. On the basis of modifications of Poiseuille’s and Stokes’ laws, mud extrusion rates were calculated for Milano and Napoli mud domes. Mud ascent velocities are estimated to be up to 60–300 km a −1 , and are comparable with those of silicate magmas. Using physical property, structural and flux data of the mud breccias, and compiled data from mud domes on land, the diameter of the feeder channel and the depth of origin for the overpressured muds could be reliably estimated for the first time. Feeder channels are likely to be only a few metres wide. Gas efflux estimates constrain a source depth to c. 1700 ± 50 m below sea floor in the Olimpi field, which is considerably shallower than estimations made from the thermal maturity of solid organic carbon in the mud breccias. The efflux data suggest that the overpressured muds were not mobilized at décollement depth, but at a shallower level within the accretionary prism. A comparison of mud ascent rates (as determined from Poiseuille flow) and the total volumes of mud extruded indicate that only a fraction of the time span constrained from biostratigraphic data ( c. 1 Ma) is needed to build up the Milano and Napoli mud domes. Durations of 12–58 ka of extrusive activity suggest mud volcanism here to have been a highly episodic phenomenon.

Salt tectonics in and around the Nile deep-sea fan: insights from the PRISMED II cruise

Abstract The recent PRISMED II geophysical survey has documented various styles of salt tectonics in and around the Nile deep-sea fan (Eastern Mediterranean Sea). The first main type of salt-related structures comprises listric normal growth faults and grabens, trending roughly perpendicular to the slope line of the Nile Cone. These faults and associated salt structures result from thin-skinned extension, driven by gravity gliding and spreading as a result of sediment loading of the Plio-Quaternary overburden above the Messinian evaporites, which acted as a décollement layer. The second major type of salt structures consists of lineaments that obliquely intersect the continental slope of the Nile deep-sea fan. These structures may have had some strike-slip movement, and salt diapirs grew reactively or were deformed by fault-block movement. In the western distal part of the Nile deep-sea fan, compressional tectonics of the adjacent Mediterranean Ridge caused the formation of a series of salt-cored folds and reverse faults above the Messinian evaporites. In the eastern distal part of the Nile Cone, sediment progradation progressively expelled salt northward, first forming small folds and tight diapirs, then a scarp of 400 m height around the Eratosthenes Seamount, corresponding to the basinward limit of salt deformation.

Images may show start of European‐African plate collision

Aspects of the initial stages of a collision between European and African plates may have been documented in a geophysical survey of the central Mediterranean Ridge (MR) conducted last year. The idea of an incipient collision was first suggested by Finetti [1976],and details of the seafloor and tectonic deformation along the MR, revealed for the first time in the survey, seem to point in that direction.A unique opportunity may therefore exist for studying the beginnings of such a collision—between the passive margin of a major plate (Africa) acting as a continental indenter against the active margin of another plate (Europe). More wide angle data, deep penetrating multichannel seismic data, and drilling data are crucial to better assess the nature and the architecture of the underlying lithosphere, the styles of sedimentary deformation, and the consequences on fluid releases. Such data will make it possible to establish, or reject, a geodynamic collision model.

Fate of sediment during plate convergence at the Mediterranean Ridge accretionary complex: Volume balance of mud extrusion versus subduction and/or accretion

Drilling results from two mud volcanoes on the Mediterranean Ridge accretionary complex as well as extensive geophysical surveys have provided new insights about the geometry of these domes at depth. Mud extrusion is related to buoyancy and plate convergence between Africa and Eurasia that caused back-thrust faulting of accreted strata containing overpressured mud at depth. The domes mainly consist of mud breccia formed of as much as 65% polymictic clasts embedded in a clayey matrix of presumed late Miocene age. Volumetric estimates of extruded mud in a well-studied area around the Olimpi mud-volcano field were balanced against sediment input at the deformation front. The results demonstrate that only a small fraction of rock mass having entered the subduction zone since the Messinian is needed to compensate for the mud expelled in the study area. Most of the sediment (95% or more) is either subducted or incorporated into the accretionary prism. The volume of gas expelled with the liquefied, overpressured mud was estimated to range between 1.68 × 10 6 and 2.85 × 10 7 m 3 /yr.

AVA analysis in the eastern Mediterranean

In the context of a reflection seismic AVA (Amplitude vs. Angle) analysis, data from the western Mediterranean Ridge have been analysed to obtain estimates on the reflection coefficient and the AVA gradient. The study put particular emphasis on the role of the Messinian evaporites, which display a characteristic amplitude signature on the AVA-sections. From the seismic attributes obtained from the Zoeppritz equations it could be infered that the material contrast at the salt/sediment interface of the Messinian evaporites is sufficient to generate converted waves, which are strong enough to survive the processes of NMO-correction and CMP stack. This mode conversion could subsequently be modelled by dynamic Ray-tracing to identify which events on the stacked data are likely to be based on converted waves and thus present some pitfalls for an imprudent interpreter.

Mechanisms of mud extrusion on the Mediterranean Ridge Accretionary Complex

Drilling two mud domes on the Mediterranean Ridge during ODP Leg 160 has demonstrated that the eruption of mud breccia began at least 1.5 Ma ago. An evolution through extrusive building of a cone, followed by successive eruptions of clast-bearing mud debris flows and subsequent subsidence can be deduced for both domes. Results from permeability and shear strength tests, grain size analyses, sedimentary textures, and clast provenance provide clues concerning the mechanism of mud volcanism. The collision of Africa with Eurasia resulted in backthrusting of the evaporite-dominated accretionary wedge against a rigid backstop. This allowed egress of overpressured fluid-rich mud of presumed Messinian age from the decollement, although many of the clasts may have originated from the overlying accretionary wedge.

An overview of Mediterranean Ridge collisional accretionary complex as deduced from multichannel seismic data

The Mediterranean Ridge (eastern Mediterranean) is a large accretionary complex that results from the Africa–Europe–Aegean plates convergence. Multichannel seismic data, combined with previous results showed that the ridge comprises distinct major structural domains facing different forelands: (1) An outer domain is bounded to the south by the ridge toe. Underneath the Ionian and Levantine outer Ridge, Messinian evaporites act as a major decollement level. (2) An axial, or crestal, ridge domain with mud diapiric and mud volcano activity is bounded to the north by backthrust. (3) A less tectonized inner Ridge domain, possibly a series of former forearc basins, abuts the Hellenic Trench. The ridge displays strong along-strike variations. These variations can be interpreted as consequences of an ongoing collision against the Libyan continental promontory.

Shallow structure of the eastern segment of the Mediterranean Ridge deduced from seismic and side-scan sonar data

The eastern Mediterranean Ridge reveals a peculiar feature called the “United Nations Rise”. It is notable for its complex morphology, interior structure, and mud volcanism. Its unusual structural–morphological characteristics are explained by its location at a junction of the western and eastern branches of the ridge and by the probable tectonic escape of accretionary prism sediments from the west. The geophysical data on the shallow structure of the eastern ridge branch showed some unusual structural trends, which could not be expected from the overall tectonic stress distribution. They are interpreted as resulting from the southward expansion of the Hellenic Arc.

Hydrocarbon gases in sediments and mud breccia from the central and eastern part of the Mediterranean Ridge

Microbiologically produced hydrocarbon gases (HCGs) in normal pelagic sediments have lithologically controlled distribution – their content increases sharply in organic-rich sapropels. Generally, the HCG content is low (<10000 nl/l), The amount of heavy HCGs is high (up to 60%, unsaturated HCGs dominate the saturated ones, and ethylene is prevalent. The same features were found in sediment and mud breccia from inactive mud volcanoes. Thermogenic HCG was determined in active mud volcanoes and in a high salinity fluid vent. They are characterized by: high methane concentration, δ13C(CH4) =−37.1 to −57.8%, essential ethane contents (2–7%), absence of unsaturated HCG, and the prevalence of iso-over n-butane.

Deep-towed side-scan survey of the United Nations Rise, eastern Mediterranean

A newly discovered area of mud volcanism, about 170 km south of Crete, in the central–eastern part of the Mediterranean Ridge, was named the “United Nations Rise” (UNR). A survey of the UNR area with the deep-towed ORE tech side-scan sonar equipped with a subbottom profiler revealed the presence of some mud volcanoes and also showed various other sea-floor features, including slumps, escarpments and pockmark-type depressions. Several of our interpretations were ground-truthed by coring. The UNR area appears to belong to the Inner deformation front of the Mediterranean Ridge.

Collision-related break-up of a carbonate platform (Eratosthenes Seamount) and mud volcanism on the Mediterranean Ridge: preliminary synthesis and implications of tectonic results of ODP Leg 160 in the Eastern Mediterranean Sea

Abstract Drilling of the Eratosthenes Seamount south of Cyprus documented incipient collision of the African and Eurasian plates. The oldest sediments recovered, mid?-Cretaceous shallow-water limestones, are overlain by Upper Cretaceous to Lower Oligocene pelagic carbonates, with several hiatuses. Following uplift, a carbonate platform was established in the Miocene; Eratosthenes was then below eustatic sea level during the Messinian desiccation crisis. The platform subsided to bathyal depths during the Lower Pliocene, associated with localized breccia deposition. Further subsidence occurred in Late Pliocene-early Quaternary, coeval with strong surface uplift of southern Cyprus. Subsidence and break-up of Eratosthenes was achieved by a combination of flexural loading and normal faulting. In addition, the Milano and Napoli mud volcanoes were drilled on the northern flank of the Mediterranean Ridge accretionary complex, south of Crete. A mainly extrusive, sedimentary origin is indicated. Multiple debris flows include clasts of sandstone and limestone of at least partly Miocene age. Both mud volcanoes are dated as τ1 Ma old and have been active episodically. Hydrocarbon gas is associated with both mud volcanoes, while methane hydrates (clathrates) exist locally at Milano. The driving force of mud volcanism is overpressuring caused by incipient plate collision. Messinian evaporites may have acted as a localized seal. Material escaped through a zone of backthrusting against rigid Cretan crust to the north.

Turbidite flux, architecture and chemostratigraphy of the Herodotus Basin, Levantine Sea, SE Mediterranean

Abstract The Herodotus Basin is the deepest part of the SE Mediterranean and receives allochthonous sediments as turbidity currents and debris flows from around its margin. During the late Quaternary, characteristic supply has been from at least four sources. These are (1) dark coloured, calcium carbonate-poor fine-grained turbidites derived from the Nile Cone to the south and south-east, (2) lighter coloured, calcium carbonate-rich, slightly coarser-grained turbidites derived from the Libyan-Egyptian shelf to the south, (3) small, light brown foraminifer-rich, muddy-silty turbidites derived from the Cyprus-Eratosthenes seamount (Anatolian Rise) carbonate shelf to the east, and (4) small localized debris flow deposits derived from the Mediterranean Ridge to the north. During the late Quaternary (0–60 ka), and specifically the period of 0–27 ka, the basin has filled predominantly with allochthonous material derived from the Nile Cone, although one megaturbidite of basinwide extent was derived from the Libyan-Egyptian shelf. Turbidites have been correlated across the Herodotus Basin using the technique of chemostratigraphy. Matching the results of geochemical analysis may show whether or not the beds in different cores were deposited by the same mass-wasting event, for individual turbidites commonly have diagnostic and unique geochemical ‘fingerprints’ in terms of major, minor and trace element composition. Sediment budgets for the three main turbidite sources are calculated. The cumulative volume of the sedimentary input for the Nile Cone- derived turbidites over the last 27 ka is c. 500 km 3 , giving an average sedimentation rate of c. 45 cm ka −1 , and a volume per unit time of 18 km 3 ka −1 . A megaturbidite, derived from the Libyan-Egyptian shelf, is of basinwide extent and has a volume of c. 400 km 3 .