Zvi Ben-Avraham

The structure of the Dead Sea basin

Abstract The Dead Sea basin is located along the left-lateral transform boundary between the Arabian and Sinai plates. Its structure and history are known from surface geology, drilling, seismic reflection and other geophysical data. The basin comprises a large pull-apart, almost 150 km long and mostly 8–10 km wide, which is flanked by a few kilometres wide zones of normal faulting. The basin formed at about 15 Ma or earlier, close to the beginning of the transform motion, and it reached about half its present length before the end of the Miocene. A strong negative gravity anomaly records a thick sediment basin fill: > 5 km under half its length, reaching a maximum of ≥ 10 km. The fill includes a few km of salt (ca. 6-4 Ma) which forms several diapirs. At any one time large parts of the basin subsided simultaneously, but the site of fastest subsidence seems to have shifted northward. Sedimentation rates reached at least hundreds of metres per million years or more in the Miocene, and ≥ 1 km/Myr in later periods. The basin structure is dominated by longitudinal faults: intrabasinal faults which delimit the pull-apart and which are the extensions of the major strike-slip faults north and south of the basin, and normal faults which extend along the basin margins. The latter faults express a small component of extension across the basin, whereas the pull-apart resulted from the much larger lateral motion along the basin. In addition, transverse faults divide the pull-apart into several segments with somewhat different histories. The pull-apart grew by becoming longer parallel to the transform motion. At shallow levels this was probably achieved by normal slip on transverse listric faults while the fill between them was little deformed. The crust under the basin was stretched and thinned during basin lengthening, which caused its subsidence. Basin formation was accompanied by uplifting of its flanks by ≥ 1 km. Sparse igneous activity occurred along the basin and its flanks. Its presence suggests a thermal anomaly in the lower lithosphere beneath the basin and adjacent parts of the transform.

Transform-normal extension and asymmetric basins: An alternative to pull-apart models

Continental transforms are commonly associated with relatively small scale pull-apart basins. However, much larger scale basins, which are not consistent with the pull-apart model, commonly exist adjacent to transforms. Structures associated with several basins along the Dead Sea rift and El Pilar fault illustrate this. These basins are remarkably asymmetric, bounded by essentially linear transform segments on one side and subparallel normal faults on the other, suggesting simultaneous strike-slip motion and transform-normal extension. This observation, which is incompatible with classical faulting theory, can be explained along divergent plate boundaries if the transform fault is much weaker than the adjacent crust. Analysis of the orientation of the horizontal principal-stress direction near a weak transform fault embedded in an otherwise strong crust shows that convergent and divergent plate motion results in a stress field characterized by nearly fault-normal compression and extension, respectively. Structural styles along the San Andreas, Dead Sea, and El Pilar transform systems show that predictable changes between these two states have occurred through time along strike of the transforms.

Early tectonic extension between the Agulhas Bank and the Falkland Plateau due to the rotation of the Lafonia microplate

Abstract Along the Southeast African continental margin, a deep sedimentary basin and marginal ridge bordering the Agulhas Fracture Zone formed coevally with South American counterparts underlying the Falkland Plateau. Extension in both the Southern Outeniqua and the Falkland basin is related to large-angle ( ∼ 100°) rotation of the Lafonia (Falkland Islands) block in Middle-Late Jurassic times, during attempted propagation of the proto-Indian Ocean across the active circum-Pacific margin of Gondwanaland. In the Early Cretaceous, the Jurassic microplate boundaries were partly reactivated, and partly transected by a right-lateral shear along the ∼ 1200 km long Falkland-Agulhas transform fault. Preceded by substantial vorticity of the surrounding crustal blocks, this boundary segment separating the South American and African plates was probably generated by complex geodynamic interactions between oceanic plate subduction, continental to back-arc rift propagation, and the impingement of deep mantle plumes on the lithosphere.

Development of asymmetric basins along continental transform faults

Abstract Many basins associated with wrench or transform faults are asymmetrical, both longitudinally and laterally; the sense of basin asymmetry changes along the strike of transform faults. Geophysical data from the Dead Sea rift and other transforms indicate that wrench-induced asymmetric basins are bounded on only one side by a transform fault. This implies that, during the evolution of such basins, maximum compression was parallel to the transform. The direction of extension can change during the geologic evolution of a basin. A possible explanation of this observation takes the strength of the fault into into consideration. In the case of a weak fault in a strong crust, the horizontal principal stresses would probably rotate to orientations parallel and perpendicular to the fault, thus minimizing the shear stress on the main fault. Basins formed by regional extension, such as in the East African rift system, sometimes show similar features. Recent studies have shown that these basins often undergo oblique extension. This suggests that the asymmetry in these basins may result from simultaneous strike-slip motion and fault-normal extension, similar to the way asymmetry is produced in large transform basins. Periods of oblique extension can alternate with periods of orthogonal extension. The asymmetry formed during periods of oblique extension is often preserved in the stratigraphic record.

Fault and salt tectonics in the southern Dead Sea basin

Abstract The sedimentary fill of the southern Dead Sea basin contains large amounts of salt and halokinetic features. In this study, the relationship between these halokinetic features and major faults was investigated. The basis for the interpretation was seismic reflection profiles from the southern Dead Sea basin. The seismic profiles were used to construct a series of two-way time structural maps and two-way time isochore maps for the various reflectors and sedimentary sequences in the area; such maps are published for the first time in the Dead Sea basin. Electrical and geophysical logs from wells in the area were used to relate the mapped reflectors and sequences with geological units. The seismic profiles give evidence to the largest subsidence that had occurred during the late Pliocene and that the amount of subsidence was much larger in the deeper part of the basin than in the median block. This difference in subsidence is partly a result of salt withdrawal. There are two ways to explain the present day structures of the southern Dead Sea basin. Either (1) basement faulting took place before salt deposition or (2) basement faulting took place during or after salt deposition. In this research, a combination of the two is favored. In particular the influence of the movement on the longitudinal fault separating the deeper part of the basin from the median block (Sedom fault) on salt flow is outlined. The much larger offset on the Sedom Fault in the northern part of the area is thought to be the reason for salt piercing only in this part of the area. It is also suggested that the Sedom Fault has experienced strike–slip movements and not only vertical movements. A ridge, the Neot Hakikar Ridge, is shown to divide the southern basin into two sub-basins. The presence of this ridge and the salt tectonics triggered motion on a listric fault (Amazyahu Fault) in the early Pleistocene. The listric curvature of the Amazyahu Fault induced antithetic faulting recognized in the Pleistocene sediments. Earlier studies have suggested that the depocenter migrated to the north. The isochore maps, however, show that locally the depocenter shifted to the southeast. This shift is probably caused by the movement along the Amazyahu Fault and the withdrawal of salt. A much larger amount of salt than previously thought is identified and it is suggested that salt reaches as far south as the Iddan Fault.

Structure and tectonics of the Agulhas-Falkland fracture zone

Abstract The ∼1200-km-long Agulhas-Falkland Transform developed during the Early Cretaceous break-up of West Gondwanaland. On the African Plate, the continent bordering the Agulhas Fracture Zone extends between the Agulhas Bank and the South Tugela reentrant. It is divided into four distinct parts: Mallory Trough segment (I), Diaz Ridge segment (II), East London segment (III) and Durban segment (IV), from southwest to northeast. Each segment differs from the ohters in its physiography and in the nature of the continent-ocean crustal boundary. In segment I, a wedge-like, eastward-narrowing deep-water basin separates the steep continental slope from a marginal fracture ridge of probable volcanic origin along the transform fault trace. Along segment II the South African continental slope is deeply embayed and a great thickness of sedimentary strata has been ponded behind a buried ridge along the northern side of the fracture-zone trace, above a probable fragment of oceanic crust representing a northerly extension of the Jurassic Falkland Plateau Basin. In segment III, the continental margin is steep, especially along the middle slope; it lies directly along the transform trace, but conspicuously rugged along the continental slope, due to the effects of submarine canyoning and sediment slumping. In segment IV, the continent-ocean boundary also lies directly along the transform fault trace, and the margin shows evidence of seaward tilting above landward-dipping faults along the middle or lower slope. The segmentation is inherited from Jurassic structural fabrics formed prior to Early Cretaceous transform motion, when tectonic rotation of microplates caused rifting and partial oceanization along the future Agulhas-Falkland megafault trajectory.

The evolution of marginal basins and adjacent shelves in east and southeast Asia

Abstract The structural elements on the shallow (Sunda Shelf) and deep seas of east and south—east Asia are interpreted as the result of past interaction between lithospheric plates. During the Mesozoic the western Pacific Ocean and the eastern Indian Ocean were parts of the Tethys Sea and were moving to the north relative to Antarctica. A Mesozoic ridge system trending east—west produced east—west trending magnetic anomalies throughout the entire area. The ridge system was bisected by large north—south transform faults which divided the eastern Indian Ocean—western Pacific Ocean into sub-plates traveling at different speeds. The Mesozoic evolution of the Sunda Shelf and the deep seas resulted from such horizontal differential movement in a north—south direction. During Late Cretaceous—Eocene the various segments of the spreading ridge gradually submerged beneath the deep sea trenches to the north, causing a gradual change in the direction of motion of the Pacific plate. The change in motion of the Pacific plate resulted in the separation between the Pacific and the eastern Indian Ocean plates, the formation of large northeast—southwest tectonic elements on the Sunda Shelf and elsewhere in south—east Asia, the formation of the western Philippine Basin and the rapid northward motion of Australia. The only remnant of the Mesozoic ridge system exists today at the western Philippine Basin.

Transverse faults and segmentation of basins within the Dead Sea Rift

Abstract The Dead Sea rift is a large transcurrent fault system extending from the Red Sea to Turkey. Several morphotectonic depressions of various sizes exist along its length. Most of them are divided into several sedimentary basins which are in turn divided into smaller units by transverse faults. Several transerve faults extend beyond the boundaries of the basins into areas which are otherwise unaffected by the rifting activity. In some cases the transverse faults are active at present as strike slip faults. In the Dead Sea area where multichannel seismic profiles are available across several transverse faults, there are indications that some of these faults have changed their mode of activity from normal to strike-slip faults during the evolution of the basins. The apparent uniform spacing between transverse faults (20–30 km) indicates that their location was, probably, not dictated by the location of oversteps in the en-echelon system of longitudinal faults. Rather, they may have formed to accommodate the internal deformation of the rapidly subsiding basins.

Gas hydrate and mud volcanoes on the southwest African continental margin off South Africa

Widespread occurrence of bottom-simulating reflectors (BSRs) has been detected in multichannel seismic profiles on the upper continental slope in the southern periphery of the Orange River delta, probably indicating the presence of large quantities of gas hydrate in this area. This report is the first to show the presence of BSRs on seismic records on the southwest African continental margin south of the Walvis Ridge. Another remarkable feature in the area is the occurrence of a large number of mud volcanoes. The distribution of the BSRs and the location of the mud volcanoes are controlled by the locations of active faults. The gas hydrate in this region may consist of a mixture of microbial and thermogenic gas, whereas much of the gas flowing through the mud volcanoes probably originated from deep-seated Aptian source shales.

Post-Eocene seismic stratigraphy of the deep ocean basin adjacent to the southeast African continental margin: a record of geostrophic bottom current systems

Abstract A high-resolution seismic-reflection survey of the Transkei Basin and Natal Valley permits the first recognition of three major reflectors that mark basin-wide unconformities across the continental rise and deep abyssal plain off the southeast African continental margin. Reflector O marks a change in acoustic reflectivity, coincident with a change in sedimentary bedforms from generally parallel bedding below to large-scale lenticular and clinoform shapes above. Reflector O probably marks the onset of cold, abyssal current circulation around the Eocene–Oligocene boundary. The overlying O sequence records deposition of a contourite drift (Oribi Drift) by northeast flowing abyssal currents at ∼4000 m water depths along the continental rise of the northeastern Agulhas Fracture Zone. This water depth is shallower than present-day Antarctic Bottom Water (AABW). The M reflector unconformity (possibly lower Middle Miocene) marks seafloor erosion in 4500 m water depth in the Transkei Basin and the cessation of drift construction along the continental rise. Above reflector M in the abyssal plain, a contourite drift (M-Drift) records deposition from an east-flowing bottom current in a location similar to, but slightly shallower than present-day AABW. The stagnation of bottom current activity in the northern Natal Valley and/or a rapid influx of sediment accumulation is marked by M sequence turbidite sediments (the Mzimkulu apron) deposited against and burying the Oribi Drift on the continental rise. Reworking of M sequence sediment along the continental rise to form low mounds (M4) and sediment waves in the northern Natal Valley indicate that a shallow, bottom current flowed at depths of 3800 to 3600 m. The coeval current-molding of the slope and abyssal plain indicates a two-layered structure of the bottom water may have commenced in the Miocene. Reflector P is the most pronounced unconformity in the deep abyssal plain, where it truncates M Sequence reflectors, and marks the base the Agulhas Drift which stands approximately 200 m above the surrounding seafloor. The P Sequence sedimentation is estimated to have begun in the Pliocene prior to or concurrent with an expansion of Southern and Northern polar ice-caps. Major slumping of the continental slope in the Natal Valley also began at this time, probably triggered by a combination of onland neotectonic activity and erosion of the base of the slope by vigorous bottom currents (possibly North Atlantic Deep Water, NADW).