Active tectonic morphology and submarine deformation of the northern Gulf of Eilat/Aqaba from analyses of multibeam data

A high-resolution marine geophysical study was conducted during October-November 2006 in the northern Gulf of Aqaba/Eilat, providing the first multibeam imaging of the seafloor across the entire gulf head spanning both Israeli and Jordanian territorial waters. Analyses of the seafloor morphology show that the gulf head can be subdivided into the Eilat and Aqaba subbasins separated by the north-south-trending Ayla high. The Aqaba submarine basin appears starved of sediment supply, apparently causing erosion and a landward retreat of the shelf edge. Along the eastern border of this subbasin, the shelf is largely absent and its margin is influenced by the Aqaba Fault zone that forms a steep slope partially covered by sedimentary fan deltas from the adjacent ephemeral drainages. The Eilat subbasin, west of the Ayla high, receives a large amount of sediment derived from the extensive drainage basins of the Arava Valley (Wadi ’Arabah) and Yutim River to the north–northeast. These sediments and those entering from canyons on the south-western border of this subbasin are transported to the deep basin by turbidity currents and gravity slides, forming the Arava submarine fan. Large detached blocks and collapsed walls of submarine canyons and the western gulf margin indicate that mass wasting may be triggered by seismic activity. Seafloor lineaments defined by slope gradient analyses suggest that the Eilat Canyon and the boundaries of the Ayla high align along north- to northwest-striking fault systems—the Evrona Fault zone to the west and the Ayla Fault zone to the east. The shelf–slope break that lies along the 100 m isobath in the Eilat subbasin, and shallower (70–80 m isobaths) in the Aqaba subbasin, is offset by approx. 150 m along the eastern edge of the Ayla high. This offset might be the result of horizontal and vertical movements along what we call the Ayla Fault on the east side of the structure. Remnants of two marine terraces at 100 m and approx. 150 m water depths line the southwest margin of the gulf. These terraces are truncated by faulting along their northern end. Fossil coral reefs, which have a similar morphological appearance to the present-day, basin margin reefs, crop out along these deeper submarine terraces and along the shelf–slope break. One fossil reef is exposed on the shelf across the Ayla high at about 60–63 m water depth but is either covered or eroded in the adjacent subbasins. The offshore extension of the Evrona Fault offsets a fossil reef along the shelf and extends south of the canyon to linear fractures on the deep basin floor.     

Origin and role of major strike-slip transfers during plate collision in the central Mediterranean

Variations in the crustal structure along the northern African plate margin have caused different modes of collision with Eurasia. Lateral density variations along the central Mediterranean collision zone are expressed in a change of the angle of the downbending African Plate and lead to the formation of strike-slip transfers in these transition zones that are roughly perpendicular to the trend of the collisional zone. In some cases these transfer zones are developed into hinge faults, while in others they can be developed into transform faults. This process governs the segmentation of the collision zone in the central Mediterranean region south of the Maghrebian thrust belt in Tunisia and Sicily through the Calabrian Arc to the northeastern Hellenic Arc, extending further to the Cyprian Arc and to the Taurus-Zagros chain.     

Moveout approximation for horizontal transversely isotropic and vertical transversely isotropic layered medium. Part I: 1D ray propagation‡

Anisotropy in subsurface geological models is primarily caused by two factors: sedimentation in shale/sand layers and fractures. The sedimentation factor is mainly modelled by vertical transverse isotropy (VTI), whereas the fractures are modelled by a horizontal transversely isotropic medium (HTI). In this paper we study hyperbolic and non‐hyperbolic normal reflection moveout for a package of HTI/VTI layers, considering arbitrary azimuthal orientation of the symmetry axis at each HTI layer. We consider a local 1D medium, whose properties change vertically, with flat interfaces between the layers. In this case, the horizontal slowness is preserved; thus, the azimuth of the phase velocity is the same for all layers of the package. In general, however, the azimuth of the ray velocity differs from the azimuth of the phase velocity. The ray azimuth depends on the layer properties and may be different for each layer. In this case, the use of the Dix equation requires projection of the moveout velocity of each layer on the phase plane. We derive an accurate equation for hyperbolic and high‐order terms of the normal moveout, relating the traveltime to the surface offset, or alternatively, to the subsurface reflection angle. We relate the azimuth of the surface offset to its magnitude (or to the reflection angle), considering short and long offsets. We compare the derived approximations with analytical ray tracing.     

Slowness-domain kinematical characteristics for horizontally layered orthorhombic media. Part II: Pre-critical slowness match

Part II of this paper is a direct continuation of Part I, where we consider the same types of orthorhombic layered media and the same types of pure-mode and converted waves. Like in Part I, the approximations for the slowness-domain kinematical characteristics are obtained by combining power series coefficients in the vicinity of both the normal-incidence ray and an additional wide-angle ray. In Part I, the wide-angle ray was set to be the critical ray (‘critical slowness match’), whereas in Part II we consider a finite long offset associated with a given pre-critical ray (‘pre-critical slowness match’). Unlike the critical slowness match, the approximations in the pre-critical slowness match are valid only within the bounded slowness range; however, the accuracy within the defined range is higher. Moreover, for the pre-critical slowness match, there is no need to distinguish between the high-velocity layer and the other, low-velocity layers. The form of the approximations in both critical and pre-critical slowness matches is the same, where only the wide-angle power series coefficients are different. Comparing the approximated kinematical characteristics with those obtained by exact numerical ray tracing, we demonstrate high accuracy. Furthermore, we show that for all wave types, the accuracy of the pre-critical slowness match is essentially higher than that of the critical slowness match, even for matching slowness values close to the critical slowness. Both approaches can be valuable for implementation, depending on the target offset range and the nature of the subsurface model. The pre-critical slowness match is more accurate for simulating reflection data with conventional offsets. The critical slowness match can be attractive for models with a dominant high-velocity layer, for simulating, for example, refraction events with very long offsets.     

Formation of continental flood volcanism — The perspective of setting of melting

Models of continental flood basalt (CFB) formation are evaluated by examining their implications for the setting, mainly temperature and depth, of melting which is assumed to result from adiabatic decompression. Most attractive is the model of melting in upwelling bodies (probably plume heads) rising to the base of the continental lithosphere. This constrains the melting to 120–150 km or deeper (continental lithospheric thickness) and thus the plume potential temperatures to ≥ 300 °C higher than ambient mantle. The primary melts should be hot, MgO-rich, modified during ascent, and assimilate components of the lithosphere, which can provide the continental-like geochemical signature of many CFB. Circulation within the upwellings and presence of eclogite patches also influence magma generation and composition. Dehydration melting when plumes heat the lowermost lithosphere can generate CFB only if the source region contains ca. 15% hydrous minerals beneath the entire area covered by flood volcanics, which is difficult to justify. On the other hand, assimilation of “continental” chemical components from large parts of the lithosphere does not require such extreme metasomatism. Decompression melting under strongly thinned rifted lithosphere requires lower potential temperatures of the rising material and lesser modification of the primary magmas than the plume head model of CFB formation. Available observations do not support the contemporaneity of flood volcanism with rifts having the required sizes and histories, but more information is needed to further test this model. On the other hand, magma production can assist rift initiation and lithospheric rupture, so subsequent thinning can explain the common formation of volcanic rifted margins immediately following CFB emplacement. Ancient LIP should record the same processes as seen in young CFB.     

Magnetic survey of Lake Kinneret-central Jordan Valley,Israel

A magnetic survey of Lake Kinneret (Sea of Galilee) was conducted on a 1 km grid of north-south and east-west lines. The results indicate that the margins of the lake are associated with large amplitude anomalies, while the centre is quite smooth. The largest anomaly, more than 500 nT, was detected in the vicinity of the entrance of the Jordan River into the lake. Its source is interpreted to be Late Cenozoic basaltic flows. The lake's margins are associated with faults, hot springs and magnetic anomalies. A broad magnetic anomaly trending east-northeast extends from Ginosar Valley into the lake through most of the lake's width. The distribution of basalt flows of different ages and the various structures of the magnetized layers are all contributing to the magnetic anomaly pattern.     

Seismic reflection and refraction investigations of Lake Kinneret - central Jordan Valley, Israel

Seismic reflection and refraction studies in Lake Kinneret, which is located in the northern part of the Dead Sea Rift have been carried out. In the seismic reflection work several instruments including sparker, boomer and air guns were used. The acoustic penetration was limited, giving information on the uppermost sediments only. In the seismic refraction study the energy source was seismic explosives charges placed below the water table in shotholes located onshore at either end of two lines. The seismic signals were picked up by hydrophones and transmitted to the shore-based recording stations by special radio transmitters.

The seismic refraction profiles show different sedimentary structures at various parts of the lake. The layer underlying the top sedimentary sequence is of higher seismic velocity in the northern section than in the southern section. This suggests a difference in the stratigraphic section between the two parts. Unconformities and faults which account for the structures observed here probably exist under the lake.

The shallow reflection data indicate active tectonic processes in this area. Folds and faults have been observed in the uppermost sediments. The most deformed areas are along the margins but some deformation also occurs at the center of the lake in its deepest portion. The area is also seismically active.     

Volcanic gaps due to oblique consumption of aseismic ridges

The present-day consumption of oceanic ridges and other buoyant rises and fragments at circum-Pacific subduction zones, and presumably elsewhere, are closely related to existing gaps in volcanism. Examples are the gaps associated with the Nazca, Juan Fernandez, Cocos, Marcus-Necker and Louisville ridges. The buoyancy of these ridges breaks the continuity of the subducted plate, which may lead to reduced water supply required for melting of magma, and therefore create temporary volcanic gaps. The oblique consumption of these ridges causes the gap to migrate with time. This mechanism may be useful in interpreting time-space patterns of past volcanic chains associated with subduction in terms of the consumption of the disruptive oceanic plateaus and ridges.     

A case study of sub-basalt imaging in land region covered with basalt flows

In this study, a set of 20 2D seismic lines, acquired over the Golan Heights basaltic plateau, was processed and analysed. Although the data were acquired and processed by standard techniques, in some cases good-quality seismic images were obtained under several hundred metres of basalts. We describe how the seismic characteristics of the top basalt layer were defined and show the effect of the numerous widespread volcanic sources on the quality of the final images. The new data reveal the first images of the sedimentary sequence under the basalt flows, and indicate that strands of the Dead Sea Transform extend into this area. The entire region was found to be very deformed. Several attractive traps for hydrocarbon exploration were also identified on the output sections.