Nicolas Lyberis

Subsidence in the Gulf of Suez: implications for rifting and plate kinematics

The Gulf of Suez is a Neogene rift which has evolved as one arm of the Sinai triple junction together with the Gulf of Aqaba and the Red Sea. Backstripping of well data in the central and southern Gulf of Suez shows three distinct phases of tectonic subsidence. During the initial stage of opening of the rift, the rate of subsidence was very low. In the late Burdigalian (Rudeis fm.) there is a rapid increase in subsidence. By the Middle Miocene, the subsidence had slowed again. Extending the backstripping to a two-dimensional cross-section of the Gulf of Suez allows better quantitative estimates of extension. Correcting for the large regional uplift (~ 1100 m on the Egyptian side) is critical for obtaining accurate values for the extension. Uplift of the rift decreases the net tectonic subsidence by over one fourth, relative to that predicted by uniform extension. The total extension at the latitude of Gebel Zeit (28° N) is ~ 30 km, which corresponds to 32–36 km at the triple junction. Approximately 13 to12 of the extension occurred during the rapid subsidence of the second opening phase. Slower extension continued for the rest of the Miocene and throughout the Plio-Pleistocene. Stress directions calculated from microstructures exhibit several directional phases to the opening of the Gulf of Suez. These data, together with constraints from the other arms of the Sinai triple junction, can be combined into a scenario for the kinematic evolution of the region. At first, the Gulf of Suez is the northward continuation of the Red Sea, and both open at ≈ N30°. The first subsidence phase in the Gulf of Suez represents an initial startup of the rifting with very low amounts of extension. This geometry continued through the main phase of Suez extension. Subsequently, the Gulf of Aqaba formed as a strike-slip boundary. As the Arabia-Africa motion transferred to the Gulf of Aqaba, the direction of extension in the Gulf of Suez rotated clockwise towards perpendicular extension and slowed. The post-Miocene shift to oblique opening in the Gulf of Aqaba and the Dead Sea transform is the result of a minor change in the pole of opening. This change is below the current resolution of the data for the Gulf of Suez, which is probably opening at < 1 mm/yr.

The East Anatolian Fault : an oblique collisional belt

Abstract The tectonic setting for the area around the Arabia-Africa-Anatolia triple junction is described from combined Landsat-SPOT satellite image analysis and field observations. Since the Late Miocene the motion along the East Anatolian Fault generated major structures corresponding to shortening with a sinistral-slip component. The 150 km wide area of the triple junction is highly deformed by folding and thrusting, distributed mainly on the border of the Anatolian block. In addition, kinematic reconstructions confirm that the Arabia-Anatolia relative motion is both sinistral and convergent. The sinistral strike-slip faults of the East Anatolian Fault zone are of second-order and local consequences of the N-S Arabia-Anatolia collision.

Tectonic evolution of the Gulf of Suez and the Gulf of Aqaba

Abstract The tectonic evolution of the Gulfs of Suez and Aqaba can be determined using fault slip data. Early opening of the Suez rift in the lower Miocene resulted from NNE-SSW extension, oblique to the rift trend. Subsequent tectonic events began in the Late Burdigalian and were associated with ENE-WSW extension which determined the shape of the Suez rift. In the Gulf of Aqaba the movements are younger. The Late Miocene motion is associated with a strike-slip stress pattern (040° direction of extension associated with a 130° compression) which produced the left lateral motion between the Arabian plate and the Sinai peninsula. Since the end of the Miocene the faulting is the result of an E-W extension, which indicates a rotation of the regional stress pattern in the vicinity of the transform fault.

Strain distribution over the east Mediterranean ridge: A synthesis incorporating new Sea-Beam data

Abstract We present the results of several multi-narrow-beam (Sea-Beam) traverses of the Mediterranean ridge on board R.V. “Jean Charcot”. These results can be considered to be complementary, in resolution and coverage, to the previously published long-range side-scan sonar (Gloria) results. A characteristic of the Mediterranean ridge is the presence over about half of its surface of an extensive fold system affecting the Messinian and Plio-Quaternary cover, which is dissected by conjugate strike-slip faults. The resolution of Sea-Beam was used to map the main characteristics of folding and faulting. This analysis, together with previously available data, enables us to define the strain pattern over the Mediterranean ridge. We thus now have an overall view of the strain pattern from a marginal basin (Aegea) through the Hellenic trench and adjacent sea-floor to the opposite continent. The bottom of the Hellenic trench is a boundary between an extensional domain to the north and a compressional one to the south. The southern domain is strongly affected by the presence of the Lybian promontary. It is related to the formation of an accretionary ridge due to the inability of the upper 3–4 km of sediments to subduct. This ridge is similar to the accretionary prism but it is much wider with respect to its height, probably because of the existence of a decollement over a level of overpressure. The Hellenic trench then is in the situation of a fore-arc basin. Its origin is due to the fact that the Aegean continental slope is not vertical but rather has a small 4° dip. As a result, a sedimentary wedge, symmetric but shorter than the main accretionary wedge develops over the buried portion of the slope. It is characterized by landward thrusts. Such an origin probably applies to fore-arc basins in general.

Seismic study of the crust of the northern Red Sea and Gulf of Suez

We report the results of fifteen Expanding Spread Profiles (ESPs), and a seismic wide angle reflection-refraction Une, performed during March–April 1986, in the Gulf of Suez and the Egyptian part of the northern Red Sea area (north of 25°N). Four 16.4 air guns were used as a sound source on board R.V. “Le Suroit” and a 96-channel 2.4-km long streamer was towed by a supply vessel, the “Whity Tide”. Most of the profiles show good crustal reflection and refraction arrivals and often good Moho arrivals obtained for a distance of 80 to 100 km. We present the results of X-T and τ-p analysis, obtained by a velocity inversion performed in the τ-p plane and by ray-tracing modeling of both the τ-p and the X-T sections. The velocity models are computed for planar and linear gradient velocity layers. The northernmost part of the Red Sea appears to be characterized by a continental type crust, extremely thinned (β ≥ 4), lying at a mean depth of 7–8 km, whereas the Moho is at a mean 14–15 km depth. The southern part shows a seismic velocity structure of an oceanic type, except in the 40 km closest to the coastline. In both parts, seismic waves progressively get more attenuated with distance from the shore to the axial zone, which is presently tectonically active. The difference between the northern continental and southern oceanic zone is related to the termination of the Levant Fault. The northern continental area appears to represent the termination of the Levant Fault as a zone of distributed deformation.

Tectonic evolution of the North Aegean trough

Summary Geological field data are used together with interpretation of Landsat images, results obtained during a Jean Charcot cruise in 1972, and other available seismic reflection profiles, to discuss the tectonic evolution of the North Aegean trough. This bathymetric trough is located over a narrow sedimentary basin with up to 6 km of sediment connected to two NNW-SSE sedimentary basins: the Thermaïkos and Kavala basins. The prevailing tectonic regime is extensional associated with strike-slip, with no evidence of compression as confirmed by seismo-tectonic observations. This regime has dominated the evolution of the North Aegean, as the prolongation of the North Anatolian fault since the Tortonian, However, the present eastern bathymetric trough does not have the same tectonic origin. The main extensional zone apparently lies to the north of it and continues eastward into the Thrace basin up to the Maritsa fault.

From subduction to transform motion: a seabeam survey of the Hellenic trench system

Abstract Preliminary results of a multi-narrow beam survey of the Hellenic trench system, in the Eastern Mediterranean, are presented. The southwestern Ionian branch is divided in small basins, partly filled with Pleistocene sediments. The morphology suggests that the basins are deformed by a compressional stress acting roughly perpendicularly to the trench along N50°E. This direction is the direction of the regional slip vector of the shallow thrust-type earthquakes. The structure of the southeastern Pliny-Strabo branch is quite different. Narrow en-e´chelon slots, oriented N40°E, have been mapped within the main troughs oriented N60°E. The regional earthquake slip vector is also oriented along N40°E. We conclude that the Hellenic trench system is an active subduction system, dominated by thrust along the Ionian branch and by transform motion along the Pliny-Strabo branch.

The origin of the West Spitsbergen Fold Belt from geological constraints and plate kinematics: Implications for the Arctic

Abstract The West Spitsbergen Fold Belt, which extends for 300 km along the western margin of Svalbard and is up to 80 km wide, has much in common with foreland fold and thrust belts. The near-foreland segment of the fold belt exhibits ramp-flat thrust trajectories whilst structurally higher nappes are typified by more listric thrusts. Higher nappes to the west contain imbricated Carboniferous and basement rocks showing that the latter were actively involved in the fold belt deformation and a minimum of 80 km total shortening perpendicular to the western margin of Svalbard is estimated. The early stages of the Eurekan deformation in North Greenland can be linked to that of the West Spitsbergen Fold Belt and the combined shortening across the two fold belts may exceed 80 km. In Ellesmere Island Eurekan structures are distributed in an arc-like belt which records between 50 to 100 km of shortening since the Late Cretaceous. Kinematic reconstructions suggest that before the opening of the Eurasian Basin and Norwegian-Greenland Sea (Chron 25), Svalbard was linked to North America. In the Late Cretaceous-Palaeocene interval the motion across the Greenland-Svalbard margin, was mainly convergent giving rise to the West Spitsbergen Fold Belt and the Eurekan structures of North Greenland. The dextral separation of Greenland and Svalbard in post-Chron 24 time was accompanied by extension followed by pure extension in post-Chron 13 time.

Subsidence history of the North Aegean Trough

Summary The subsidence of the North-Aegean trough is examined quantitatively using available geophysical data, the emphasis being on thermal and gravity computations. The two dimensional thermal calculations incorporate conduction as well as convection with lateral variations, the effect of the sedimentary cover being treated as a perturbation. The data can be explained within the framework of the homogeneous stretching model with a maximum stretching factor of about 3.5. The stretched lithosphere has a very small flexural parameter so that local compensation prevails. The effect of conduction is sufficiently important to prevent large-scale melting of the mantle, thus making the transition to oceanic accretion rather improbable. The narrowness of the zone of extreme stretching, compared to the widths now observed on most continental margins where comparable stretching values have been observed, may be due to the presence of an earlier deep lithospheric fault which enabled the strain to concentrate along its path.

The Hellenic margin from eastern Crete to Rhodes: Preliminary results

Abstract Complementary data on previously surveyed areas have been obtained and more than 3000 km of continuous seismic profiles along the eastern Hellenic continental margin, between eastern Crete and Rhodes, have been made. Among the important preliminary observations, we note: the relatively great thickness, due to probable coarse rapid fill, of the margin sedimentary cover, which is also intensively affected by extensional faulting. The presence of a well-defined sedimentary basin (lying between the Strabo- and Pliny Trench), where evaporitic strata exist but where the acoustic facies also suggest coarse fill, is probably derived from surrounding topographical highs exposed to erosion during the Messinian. Finally we observe a progressive and perplexing change of the Strabo Trench structural pattern from west to east, which indicates a more complex tectonic regime than we expected. We speculate that at the southern margin of the Rhodes Basin, northward thrusting may now be occurring.