Avihu Ginzburg

The Afar Depression: transition between continental rifting and sea-floor spreading

Abstract Detailed geophysical and geological studies were undertaken in the 1970s in the Afar Depression, Ethiopia, in order to study the processes of continental break-up occurring in this unique setting; the Afar is located between the Ethiopian and Somali Plateaus and the Red Sea. We have re-evaluated the available seismic refraction and gravity data using modern interpretation techniques and incorporated new information derived from magnetic, gravity, seismicity and seismic refraction data. The results of the re-evaluation indicate that the Afar Depression is a transition zone between the continental rifts of Kenya and the present ongoing sea-floor spreading of the Red Sea and the Gulf of Aden. In the Depression itself the crust is greatly attenuated and is underlain by a low velocity (7.4–7.5 km/s), high temperature upper mantle material. However, it is still a continental crust which thins to the northeast and east towards the Red Sea and the Gulf of Aden. It is not a newly formed oceanic crust as has been suggested in the past. The trends of the areas of crustal thinning are indicated by the gravity anomalies. The trend of the Ethiopian Rift reaches South Afar where the trend of crustal thinning changes and becomes parallel to the Red Sea trend. It is offset en echelon from the spreading area of the Red Sea, which is marked by an alignment of gravity maxima. The Gulf of Aden trend continues west towards the Ethiopian Rift. The gravity trends coincide with areas of maximum crustal thinning and may thus suggest a possible triple junction in development.

Crustal structure of the Levant Basin, eastern Mediterranean

Abstract A seismic refraction/wide-angle reflection experiment was undertaken in the Levant Basin, eastern Mediterranean. Two roughly east–west profiles extend from the continental shelf of Israel toward the Levant Basin. The northern profile crosses the Eratosthenes Seamount and the southern profile crosses several distinct magnetic anomalies. The marine operation used 16 ocean bottom seismometers deployed along the profiles with an air gun array and explosive charges as energy sources. The results of this study strongly suggest the existence of oceanic crust under portions of the Levant Basin and continental crust under the Eratosthenes Seamount. The seismic refraction data also indicate a large sedimentary sequence, 10–14 km thick, in the Levant Basin and below the Levant continental margin. Assuming the crust is of Cretaceous age, this gives a fairly high sedimentation rate. The sequence can be divided into several units. A prominent unit is the 4.2 km/s layer, which is probably composed of the Messinian evaporites. Overlying the evaporitic layer are layers composed of Plio–Pleistocene sediments, whose velocity is 2.0 km/s. The refraction profiles and gravity and magnetic models indicate that a transition from a two layer continental to a single-layer oceanic crust takes place along the Levant margin. The transition in the structure along the southern profile is located beyond the continental margin and it is quite gradual. The northern profile, north of the Carmel structure, presents a different structure. The continental crust is much thinner there and the transition in the crustal structure is more rapid. The crustal thinning begins under western Galilee and terminates at the continental slope. The results of the present study indicate that the Levant Basin is composed of distinct crustal units and that the Levant continental margin is divided into at least two provinces of different crustal structure.

A seismic refraction study of the north basin of the Dead Sea, Israel

A seismic refraction experiment involving the use of 9 OBS and 11 portable seismic land stations was conducted along a profile in the north basin of the Dead Sea. The seismometers were deployed along a north-south profile in the lake and the adjacent land area. The interpretation of the recorded seismograms indicates the presence of two large Pliocene salt diapirs in the young basin fill. The basement lies at a relatively shallow depth (6–8 km) under the north basin. Further south along the profile a major fault affecting the basement was detected. The apparent sense of the faulting is down to the southeast, representing the boundary of the south basin. This major faulting with a downthrow of some 4–5 km has depressed the crystalline basement and the overlying Cretaceous and pre-Cretaceous sediments. The faulting was followed by the deposition of over 8 km of Recent and Tertiary sediments resulting in a 14 km thick sedimentary sequence in the south basin of the Dead Sea.

The structure of the crust and upper mantle in the Dead Sea rift

Abstract The previously published results of a deep seismic refraction study of the Dead Sea—Gulf of Elat rift show crustal thinning underneath the rift and the presence of a 5 km thick velocity transition zone in the lower crust along the rift. The structural interpretation of the first-arrival data was revised using the detailed velocity-depth distribution. The revised crustal thicknesses are 35 km near Elat and 27 km, 160 km south of Elat. The crustal thinning and the presence of the velocity transition zone are interpreted as being the result of intrusion of upper mantle material into the lower crust, possibly representing the initial shape of the processes which have been active further south in the Red Sea since earlier times.

Changes in the crust and in the sedimentary cover across the transition from the Arabian Platform to the Mediterranean Basin: evidence from seismic refraction and sedimentary studies in Israel and in Sinai

Abstract A seismic refraction study of Sinai and the Negev region indicates a change in the thickness of the crust from more than 40 km in the northeastern Sinai and central Negev to about 18 km towards the Mediterranean, of which one half is sediments and one half is a crystalline crust. The sedimentary cover changes in thickness from more than 2 km in northeastern Sinai and central Negev to about 9 km near the Mediterranean. This change is interpreted as the transition from a continental to an apparently oceanic crust. Along the present coastline of the Levant and of Sinai there is a hinge-belt in which changes in thickness and environments of deposition of the sediments were observed. There is a transition from shallow shelf and platform sediments, through shelf-edge reefs and high-energy sediments to continental-slope environments. The thickness changes also indicate a basinwards shift, with time, of the axis of maximum thickness, which is interpreted as a progressive construction of continental shelves on pre-existing continental slopes. These coinciding features along the hinge-belt are interpreted as the fossil continental margin of the Arabian Platform bordering the Tethys Ocean.

Plate convergence in the Cyprean Arc

Abstract The Cyprean Arc is a part of the plate boundary between Afro-Arabia and Eurasia in the Eastern Mediterranean. A re-examination of the geophysical properties of the Cyprean Arc together with an analysis of multichannel seismic reflection profiles now available in the Cyprean Arc area provides a clearer view of the tectonic setting and processes in this region. Seismic activity and a gravity anomaly indicate that a northward subduction of oceanic material related to the African Plate beneath the Turkish Plate is the mode of convergence along the western segment of the Cyprean Arc. Subduction is interrupted due to the collision of the Eratosthenes Seamount at the central segment of the Cyprean Arc, which forms a zone of intense deformation. The zone of deformation spreads away and disappears in the eastern segment of the Cyprean Arc. No signs for deformation have been found in this portion, either in the multichannel seismic reflection profiles, in the seismicity or in the bathymetry. The change in the mode of convergence and in the type of deformation along the Cyprean Arc is induced by crustal differences in the underthrusting plate. Interruption of subduction activity in one segment of the arc also affects the geometry of nearby segments and the processes over there.

European Geotraverse: A seismic refraction profile across the Ligurian Sea

Abstract A seismic refraction profile was shot in the Ligurian Sea between Italy and Corsica in September 1983. The profile, which is part of the southern portion of the European Geotraverse, was shot using small dynamite charges, and the seismograms were recorded by ocean bottom seismometers and land stations, resulting in a number of partially overlapping reversed profiles. The refraction data were interpreted by means of the velocity-intercept and ray-tracing methods, using seismic reflection derived velocity-depth information for the sedimentary section. Refraction velocities of 4.8, 6.0 and 6.6 km s −1 were calculated for the base of the sedimentary section, crystalline basement and lower crust, respectively. An upper mantle velocity of 7.4 km s −1 was inferred using the refraction profiles recorded on Corsica. The interpretation of the data indicates a prominent high in the crystalline basement in the northern part of the profile, while the sedimentary section increases in thickness from about 1.5 km over the basement high to over 6 km between the basement high and Corsica, and includes a considerable thickness of pre-Miocene sediments. The crust-mantle boundary shows shoaling towards the centre of the profile, where a minimum crustal thickness of 16 km is reached. The model, therefore, is that of a stretched and thinned continental crust which was rifted in Oligo-Miocene times. The presence of a low-velocity upper mantle may be indicative of the present renewal of subcrustal activity under the Ligurian Sea.

Magnetic anomalies over the Central Levant continental margin

Abstract The magnetic field over the central Levant continental margin, off northern Israel and southern Lebanon, and the adjacent Levant Basin has two distinct trends. Mount Carmel and its offshore continuation (Carmel Nose), which are the surface expression of a large subbottom structure that extends from the land area across the continental shelf to the continental slope, form a dividing zone between the two magnetic trends. South of the Carmel structure the magnetic field trends east-west, while north of the Carmel structure it trends northeast and north-northeast. Several pronounced magnetic anomalies exist mainly north of the Carmel structure, the majority of which trend north-northeast and northeast, parallel and sub-parallel to the trend of the magnetic field in this area. Some also trend northwest, perpendicular to the trend of the magnetic field. In several cases the magnetic anomalies indicate large lithological elements which continue from land to sea. Gravity and seismic refraction data show that the two magnetic domains north and south of the Carmel structure are associated with areas of different crustal structure. South of the Carmel structure the continetal-oceanic crustal transition zone is located beyond the continental margin at the base of the continental slope, while north of the Carmel structure it is located under the continental shelf, near the shore. On land, there are also differences in the structure of the crust north and south of the Carmel structure, the crust being much thinner north of the structure than south of it. We suggest that some of the large magnetic anomalies off the Central Levant were formed during the rifting phase of the eastern Mediterranean.

Gravity and density distribution in the dead sea rift and adjoining areas

Abstract The results of seismic refraction profiles along the Dead Sea—Gulf of Elat graben show a thinning of the crust along the Gulf of Elat and from the Negev towards the Mediterranean. Density distribution in the crust and upper mantle was computed from two dimensional gravity models. The models were constrained by the seismic results and by the observed Bouguer gravity. The computed gravity models along the Dead Sea—Mediterranean and Gulf of Elat-Mediterranean profiles show a thinning of the crust in the rift area. From the Central Negev towards the Mediterranean there is a considerable thinning of the crystalline crust accompanied by an increase in the thickness of the low density sedimentary section. Along the rift the thinning of the crust towards the southern end of the Gulf of Elat, detected by the seismic study, is confirmed by the gravity data. It was found necessary, however, to introduce a low density wedge in the upper mantle along the Gulf of Elat in order to explain the relative amplitudes of the Bouguer anomalies in the northern and southern part of the rift valley. This low density wedge is probably connected with the presence of high temperature low density upper mantle which is related to the invasion of upper mantle material into the crust in the southern part of the Gulf of Elat graben.

Seismic measurements in the southern Dead Sea

Abstract A seismic refraction profile and several seismic CDP reflection lines were recently occupied in the southwestern part of the Dead Sea. The seismic data, which are of good quality, give a clear picture of the structure of the area. The western flank of the rift comprises a series of step faults, downthrown to the east with a total throw of some 7 km at which depth the Cretaceous base of the post-Cretaceous fill is located. On the east—west lines the base of the fill dips to the east while on the north—south lines this complex dips to the south with a change in direction of dip being evident in the southern portion of this profile. The post-Cretaceous sediments reach a maximum thickness of 7 km but may be even thicker eastward near the main eastern rift fault. These sediments are gently folded, possibly due to differential compaction and are dislocated by small-magnitude adjustment faults. Lateral transition from bedded layers of salt in the graben fill to a diapir is clearly seen.