Jean Mascle

Shallow structure and recent evolution of the Aegean Sea: A synthesis based on continuous reflection profiles☆

Abstract A dense grid of continuous seismic reflection profiles, partly calibrated with drilling results, has allowed construction of a structural outline of the entire Aegean Sea. In the North Aegean we observe a clear difference between the thickly sedimented northern margin basins and the steep slopes of the trough margin of the southern North Aegean. The trough itself comprises a series of separate tectonic depressions resulting from an apparent transtensional motion. The Central Aegean consists of a complex tectonic puzzle caused by the prevailing extension; incipient strike-slip faulting (sub-parallel to the North Aegean trough trend) may, however, affect its eastern (Anatolian) domain. Three main structural areas characterize the Southern Aegean. Comparable structural directions cut across this chiefly extensional basin, but one fault system may be prevailing over the others, depending on the area. The Central Cretan Sea results from the interaction of at least two distinct extensions. These were superimposed on previous, mostly E-W structural trends generated during the inception of the southern Hellenic subduction and led to the present-day predominant SW-NE trending extension. As a hypothesis we propose that the overall Late Miocene-present evolution of the Aegean Sea is directly dependent on two geodynamic processes: to the southeast of the area, collision between Arabia and Turkey leads to the lateral (westward) expulsion of the Aegean continental crust, and to the west, Hellenic subduction is still active.

Extreme efficiency of mud volcanism in dewatering accretionary prisms

Drilling results from two mud volcanoes on the Mediterranean Ridge accretionary complex as well as bottom sampling and the wealth of geophysical data acquired recently have provided fundamental knowledge of the 3D geometry of mud extrusions. Mud volcanism is generally related to buoyancy (density inversion), and is triggered by the collision of the African and Eurasian blocks, forcing undercompacted clayey sediments to extrude along faults in the central and hinterlandward parts of the prism. Volumetric estimates of extruded mud in several well-studied areas were based on pre-stack depth-migrated seismic profiles across the entire, up to >150 km wide, prism. The resulting volumes of mud were combined with ages from mud dome drilling, so that rates of mud extrusion were obtained. Subtracting the solid rock mass from the bulk mud volume using physical property data, fluid flux as a function of mud volcanism alone has been quantified for the first time. The volume of fluid extruding with the mud is found to be variable, but reaches up to 15 km3 fluid per km trench length and Ma along cross sections with abundant mud volcanoes. Such large fluid quantities in a region some 50–150 km behind the deformation front exceed estimates from those elsewhere (where undoubtedly the majority of the interstitial fluid is lost due to compaction). Such fluids near the backstop are likely to result predominantly from mineral dehydration and diagenetic reactions at depth, and consequently provide a window to understand deeper processes along the deep decollement. More importantly, the enormous rates with which such fluids and liquified mud escape along the out-of-sequence faults alter fluid budget calculations in subduction zones drastically.

FROM FORELAND TO FOREARC DOMAINS : NEW MULTICHANNEL SEISMIC REFLECTION SURVEY OF THE MEDITERRANEAN RIDGE ACCRETIONARY COMPLEX (EASTERN MEDITERRANEAN)

Abstract The Mediterranean Ridge of the Eastern Mediterranean Sea is a huge accretionary wedge emplaced as a consequence of the Africa-Aegean plate convergence, that has a rate estimated at ~40 mm/yr. Processing of recent multichannel seismic data and integration with previous results (seismic profiling, swath bathymetry, sonar) have facilitated the study of the overall deformation pattern of the area which includes three distinct major structural domains facing various forelands: 1. (1) The outer domain is bounded southward by the present-day wedge toe and towards the north by topographic slope breaks and clear changes in deformational style. Beneath the Ionian and Levantine Mediterranean Ridge branches the base of Messinian evaporites seems to act as a major decollement level. 2. (2) The central, or crestal, domain, the shallowest one, shows evidence of mud diapiric and mud volcano activity and is overthrusting northward over the inner domain. Facing the Libyan margin, this area, which displays spectacular mud volcano fields, appears much wider and shallower. 3. (3) The less-deformed inner domain is bounded northward by the Hellenic trench system. Beneath its Ionian inner area, seismic velocities characteristic of continental crust have been recorded, while within its Levantine region, it is made of a series of disconnected and uplifted basement blocks (Strabo seamounts). Along the Matapan and Pliny trenches structures which could be caused by transpressive deformation are locally observed. We believe that both the north-south structural arrangement and the clear west—east lateral variations of the Mediterranean Ridge relate to its progressive collision against the Libyan margin promontory. This event, which may have initiated ~5–6 Myr ago, may have induced: (a) a differentiated paleogeography of Messinian evaporites (later available for tectonic accretion) leading to the further lateral structural variations of the outer domain; and (b) strain partitioning between the central Mediterranean Ridge and the Hellenic trench system.

The shallow structures of the Guinea and Ivory Coast-Ghana transform margins: their bearing on the Equatorial Atlantic Mesozoic evolution

Abstract Recent geological and geophysical data from a marine survey conducted off Guinea and the Ivory Coast-Ghana Region (Equatorial Atlantic) are discussed. We interpret the southern Guinean margin firstly as an area linked to the Central (Jurassic) Atlantic, and secondly as an area affected by the progressive transform opening of the Equatorial Atlantic in the Early Cretaceous. Off the Ivory Coast and Ghana we have demonstrated distinct structural domains interpreted as consequences of the progressive transform contact between the West African and Brazilian cratons, also during the Early Cretaceous. On both margins we observe a clear tectonic unconformity of probable Late Albian-Early Cenomanian age. We propose that this unconformity results from the final disruption between the two continental (West African and Brazilian) crusts. The geological structures generated along transform margins can be considered as good geological tools to constrain early phases of plate motions, particularly around the Equatorial Atlantic.

Deformational styles of the eastern Mediterranean Ridge and surroundings from combined swath mapping and seiemic reflection profiling

Abstract Recent swath mapping and seismic reflection profiling across the eastern(Levantine) branch of the Mediterranean Ridge (MR), in the eastern Mediterranean Sea, illustrate a strong variability of the deformational styles that characterize this precollisional accretionary prism. Along a north–south cross-section of the MR, a structural analysis, based on surface mapping, combined with vertical seismic reflection data, reveals two main structural domains. A southern Outer Domain consists of a series of three disconnected distinct fold belts. Folding affects a sedimentary cover which includes an approximately 2-km thick Pliocene and Quaternary wedge in this domain, resting on fairly thick Messinian evaporitic sequences that act as probable decollement layers. The MR Inner Domain includes three regions showing evidences of strong internal deformations and of numerous probable mud cones and mud flows, but no seismic evidence of significant underlying Messinian evaporites can be detected. The inner sub-regions are thrusting northwards over an area made of faulted, and locally uplifted, acoustic basement blocks that constitute the southernmost extension of the Crete continental margin, acting as a continental buttress for the MR. This innermost domain is itself structurally disconnected from the Crete continental margin by the en echelon Pliny troughs system that shows evidence of left lateral displacement. Altogether, the different structural and sedimentary cover patterns reveal a strong contrast between both MR Inner and Outer Domains, and important lateral variations within the Inner MR itself. Strike-slip faulting seams to characterizes both areas, and large-scale mud accumulations potentially exist in the northern one. Our results support a model of imbricated accretionary prisms, including, at least, two stages: (1) a pre-Messinian stage during which the Inner MR probably developed in response to northward subduction of the African lithosphere beneath southern Europe, and (2) a Messinian to post-Messinian period, during which the kinematics of the Aegean–Anatolian microplates and the presence of thick Messinian deposits became prevalent and which led to the creation of the Outer MR folded wedge piling against the previous one. In our interpretation, the occurrence of thick Messinian evaporites in this area induces important local and regional modifications of the structural pattern. Strike-slip activities reflect partitioning related to oblique subduction, and likely lateral escape of the sedimentary cover in the whole studied area.

Marine geologic evidence for a Levantine-Sinai plate, a new piece of the Mediterranean puzzle

Marine geophysical data recorded offshore Egypt illustrate the presence of an active fault belt, trending N145°E, that obliquely transects the eastern Nile deep-sea fan. This belt, more than 150 km long, consists of a series of linear transtensive faults, with an apparent right-lateral horizontal component. These fault zones bound thick-sediment-filled grabens where linear salt ridges and diapirs represent likely Messinian salt reactive response to regional transcurrent geodynamics. We infer that this tectonic belt might correspond to an offshore extension of the Gulf of Suez rift system. If our hypothesis is correct, this fault belt might represent the western boundary of a Levantine-Sinai microplate, locked between the major Arabia and Africa plates and the Anatolian-Aegean microplate.

The Ivory Coast-Ghana transform margin: A marginal ridge structure deduced from seismic data

Abstract The Ivory Coast-Ghana (ICG) marginal ridge is a prominent feature of the ICG transform margin and includes a fossil ridge partially buried by a thick, undeformed sedimentary cover. The fossil ICG ridge is 130 km long and 25 km wide, and towers over the adjacent rifted basin (deep Ivorian basin, DIB) and the oceanic crust by 1.3 km and more than 4 km, respectively. It formed in three successive stages. 1. (1) During the rifting of the DIB, both vertical and horizontal motions between the DIB and the South American plate varied along the plate boundary. This relative motion occurred in an accommodation zone that tilted the northern slope of the ICG ridge along en-echelon, mainly strike-slip, faults. 2. (2) After the rifting of the DIB, the relative motion remained constant along the transform plate boundary. At this time strike-slip deformation was localized into a narrow and highly deformed belt that truncated the accommodation zone. 3. (3) Finally, the transform motion occurred between the DIB and an occanic plate. Thermal exchanges between the two adjacent plates induced thermal uplift of the ICG ridge that amplified previous tectonic tilting.

Recent depositional patterns of the Nile deep-sea fan from echo-character mapping

The Nile deep-sea fan was surveyed in 1998 using swath multibeam bathymetry, backscatter imagery, and 3.5 kHz and seismic profiling. On the basis of this new data set, the fan has been divided into three main provinces: western, central, and eastern. Recent sedimentary patterns, as deduced from echo-character mapping, show that gravity-induced sedimentary deposits are predominant and are expressed through either slides, debris flows, or turbidites. Turbidity processes are particularly active in the westernmost province through a well-developed network of deep-sea channels. This network controls turbiditic flows that, subsequently, overflow levees. Mass-wasting processes are extremely efficient all over the Nile deep-sea fan. In the upper central province, a broad allochthonous tongue of mass-flow deposits, generated by slope destabilizations, may have led to channel avulsions and migrations. In the eastern province, a northwest-southeast deformed belt resulting from combined deep-seated and salt-related tectonics contains large proportions of destabilized sediments (i.e., originating from destabilization processes such as mass movements), probably settled by recent tectonic activity. This article highlights (1) the influence of both thin-skinned and thick-skinned tectonics on sedimentary distribution, and (2) the predominance of gravity-induced sediments (turbidites, slides, and mass flows). In such submarine fans, the distribution of debris flows and sedimentary deposits settled by turbidity currents appears fundamental in predicting reservoir geometry and economic potential.

Deformation of the western Mediterranean Ridge: Importance of Messinian evaporitic formations

Abstract Multichannel seismic data were acquired across several areas of the Mediterranean Ridge (M.R.) in the eastern Mediterranean Sea during the MCS Prismed survey (March 1993). The M.R. is a wide, high-standing and elongated swell, emplaced between the southern Aegean and Africa as a consequence of the long-term convergence and the ongoing collision between Europe and Africa. Sedimentary piling and associated deformation have built a thick deformed sediment wedge, the internal geological structures of which remain poorly understood. Processing of two MCS lines (PM 02 and PM 03) across the western branch of the Mediterranean Ridge reveals that Messinian evaporite-bearing formations, which were deposited within the deep Ionian oceanic basin 5 my ago, play an important part in the present structural deformation of the M.R. accretionary wedge. Within the outer M.R., the Messinian “lower evaporites” are believed to act as one of the main decollement levels, while the top of these formations may operate as a disharmonic layer below thin Plio-Quaternary sediments. The present-day M.R. toe appears to be affected by ductile-flow deformation. However, most of the accreted outer wedge is believed to have resulted from the progressive piling up and associated thrusting of Messinian and overlying Plio-Quaternary sediments. Toward the northeast (i.e., toward the central M.R. domain), a major backthrust zone indicates a structural boundary between the outer, wide and convex M.R. and its inner, rather flat, domain, which is inferred to act as a backstop.

Formation of carbonate chimneys in the Mediterranean Sea linked to deep-water oxygen depletion

Marine sediments at ocean margins vent substantial amounts of methane1, 2. Microbial oxidation of the methane released can trigger the precipitation of carbonate within sediments and support a broad diversity of seafloor ecosystems3, 4. The factors controlling microbial activity and carbonate precipitation associated with the seepage of submarine fluid over geological time remain poorly constrained. Here, we characterize the petrology and geochemistry of rocks sampled from metre-size build-ups of methane-derived carbonate chimneys located at the Amon mud volcano on the Nile deep-sea fan. We find that these carbonates comprise porous structures composed of aggregated spherules of aragonite, and closely resemble microbial carbonate reefs forming at present in the anoxic bottom waters of the Black Sea5. Using U-series dating, we show that the Amon carbonate build-ups formed between 12 and 7 thousand years ago, contemporaneous with the deposition of organic-rich sediments in the eastern Mediterranean, the so-called sapropel layer S1. We propose that the onset of deep-water suboxic or anoxic conditions associated with sapropel formation resulted in the development of intense anaerobic microbial activity at the sea floor, and thus the formation of carbonate chimneys.