Salt Tectonics and Seafloor dynamics
Numerous normal faults, expressing thin-skinned salt tectonics, are detected along the Levant continental slope,15–30 km west of the Israeli coast. Previous studies estimated that this faulting initiated during Late Pliocene to Early Pleistocene, which leaves an uncertainty of about 50% in the total duration of active faulting. The purpose of this study is to improve the dating of the onset of faulting, and to reconstruct displacement rates with time. This will allow better understanding of salt tectonics as well as better assessment of geohazards. To increase dating resolution, we use 2D and 3D seismic material integrated with well logs and cuttings to combine bio- and seismo-stratigraphic data and to divide the Plio-Quaternary section offshore Israel to 4 depositional units: Pliocene (5.33–2.6 Ma); Gelasian (2.6–1.8 Ma); Calabrian-Ionian (1.8–0.35 Ma); and Ionian-Holocene (< 0.35 Ma). Then, we measure displacement of dated horizons, reconstruct displacement history along faults, and identify syn-tectonic depositional units that thicken towards growth faults.
Intense salt deformation in the Levant Basin in the middle of the Messinian Salinity Crisis
While numerous studies have shown that salt related deformation in the Levant Basin began in the Late Pliocene or Early Pleistocene, here we show that the first salt related deformation event occurred 3–4 myr earlier, in the middle of the Messinian Salinity Crisis. Considering that the entire crisis lasted only about 650 kyr and that halite deposition in the deep basin may have lasted only ∼50 kyr, this deformation event must have been very short. At some point after deposition of nearly half of the evaporitic sequence, the upper 200 m thick clastic-rich layer glided downdip and formed a series of steep contractional ridges on the deep basin floor. However, unlike the recent salt motion, which is derived from northwestward tilting of the Levant continental margin towards the Cyprus Arc and by basinward progradation of the Nile-derived overburden, the short intra-Messinian deformation event is enigmatic. It predates the Nile-derived overburden and its direction does not match northwestward tilting. We postulate that it may reflect the uplift of the Carmel block northeast of the study area and possibly the entire north Levant coast. In a wider view, intra-Messinian deformation is a circum-Mediterranean phenomenon, possibly reflecting reorganization of the Africa–Eurasia boundary.
Published paper: Gvirtzman, Z., Reshef, M., Buch-Leviatan, O., Ben-Avraham, Z., 2013,
Intense salt deformation in the Levant Basin in the middle of the Messinian Salinity Crisis,
Earth and Planetary Science Letters, vol. 379, 109-119. https://doi.org/10.1016/j.epsl.2013.07.018
Bathymetry of the Levant basin: interaction of salt-tectonics and surficial mass movements
A new high resolution bathymetric map of the Levant Basin between Israel and the Eratosthenes Seamount reveals previously undetected folds, faults and channels. The map facilitates a regional map-view analysis of structures that were previously examined only in cross section. The systematic mapping of morpho-structural elements in the entire basin is followed by a kinematic interpretation that distinguished between two main processes sculpting the seabed from bottom and top: salt tectonics and sediment transport. We show that the contractional domain related to salt tectonics is far more complex than previously thought. Ridges, previously interpreted as contractional folds are, in fact, surficial undulations of the seabed reminiscent of sediment waves. Moreover, other folds previously interpreted as downdip contraction of the westward gliding Plio-Quaternary section are, in some parts of the basin, caused by updip climbing of this section eastwards as a result of the regional pattern of salt flow away from the Nile Cone. In the context of sediment transport, we show that the northern Sinai continental slope is covered by a dense net of turbidite channels, whereas the Levant slope has no channels at all. Particularly interesting is the Levant Turbidite Channel, described and named here for the first time. This feature begins at the southeastern corner of the Mediterranean at water depths of ~1100 m, continues along the valley between the Sinai and Levant slopes, and reaches the deepest part of the basin, inwater depths of ~2500 m, northeast of the Eratosthenes seamount. However, this prominent feature cannot be explained by the current drainage, consisting of two minor rivers that enter the basin at that point, and thus most likely reflects periods of wetter climate and/or lower sea-level, when these rivers were more active and possibly connected to the submarine channel system.
Published paper: Gvirtzman, Z., Reshef, M., Buch-Leviatan, O., Groves-Gidney, G., Karcz, Z., Makovsky, Y., & Ben-Avraham, Z., 2015. Bathymetry of the Levant basin: interaction of salt-tectonics and surficial mass movements.
Marine Geology, 360, 25-39. https://doi.org/10.1016/j.margeo.2014.12.001 .
Diversion and morphology of submarine channels in response to regional slopes and localized salt tectonics, Levant Basin
In the Levant Basin, submarine channels are abundant around the Nile deep-sea fan (NDSF), an area which is also affected by salt tectonics related to the Messinian salt giant. Here we focus on the relationship between submarine channels and obstacles formed by salt tectonics. Initially, we use methods developed for terrestrial morphological analysis and quantify channel sinuosity, width and slope in search for consistent relationships between morphometric parameters and channel response to obstacles. However, this traditional analysis did not yield robust conclusions. Then, we apply two new morphometric parameters suggested here to express the distortion of channels by obstacles: incident angle (a), defined as the acute angle between the regionally influenced channel direction and the strike of the tectonic obstacle and diversion angle (W), defined as the angle between the direction of the regional bathymetric slope and the average direction of the channel. These parameters illustrate the influence of the regional-scale basin geometry and the superimposed tectonic-influenced seabed patterns, on channel development. We found hyperbolic relationships between incident angle (a) and diversion angle (W) in which channels flowing approximately parallel (a ≈ 0°) to tectonic folds are (obviously) not diverted; channels nearly orthogonal (a z 90_) to obstacles, crosscut them right through and, again, not diverted much. In contrast, channels with a general direction diagonal to the obstacles (a ≈ 40°), are diverted by ten degrees (W ≈ 10°). This diversion accumulates along large distances and significantly influences the regional development of channels around the NDSF. Noteworthy, this phenomenon of channel diversion, indirectly deteriorate normal slope-sinuosity relationships known from terrestrial studies. In light of these findings, we suggest that these new parameters can be applied to other basins, where submarine channels interact with seabed obstacles.
Published paper: Zucker E., Gvirtzman Z., Steinberg J., Enzel Y., 2017,
Diversion and morphology of submarine channels in response to regional slopes and localized salt tectonics, Levant Basin, doi.org/10.1016/j.marpetgeo.2017.01.002
Chronology of post-Messinian faulting along the Levant continental margin and its implications for salt tectonics
Numerous normal faults, expressing thin-skinned salt tectonics, are detected along the Levant continental slope,15–30 km west of the Israeli coast. Previous studies estimated that this faulting initiated during Late Pliocene to Early Pleistocene, which leaves an uncertainty of about 50% in the total duration of active faulting. The purpose of this study is to improve the dating of the onset of faulting, and to reconstruct displacement rates with time. This will allow better understanding of salt tectonics as well as better assessment of geohazards. To increase dating resolution, we use 2D and 3D seismic material integrated with well logs and cuttings to combine bio- and seismo-stratigraphic data and to divide the Plio-Quaternary section offshore Israel to 4 depositional units: Pliocene (5.33–2.6 Ma); Gelasian (2.6–1.8 Ma); Calabrian-Ionian (1.8–0.35 Ma); and Ionian-Holocene (< 0.35 Ma). Then, we measure displacement of dated horizons, reconstruct displacement history along faults, and identify syn-tectonic depositional units that thicken towards growth faults.
Our analysis shows that during the Pliocene (duration of 2.7 my) faulting activity was minor. In the Gelasian (duration of 0.8 my) faulting activity peaked alongside huge slumping. In the past 1.8 my, faulting and slumping had both decreased, but they are still active today. The observation that the onset of faulting significantly predates the progradation of the shelf edge to its present location indicates that differential loading was probably not the first trigger that initiated salt tectonics offshore Israel. The association of intense faulting and slumping in the Gelasian is interpreted as a result of basinwards tilting due to regional inland uplift and basin subsidence. Yet positive feedback between faulting and slumping cannot be ruled out. The presence of fault scars on the present seafloor, where current sedimentation rate is similar and even faster than the average displacement rate, apparently indicates that faulting is dominated by sudden seismic ruptures rather than by continued creeping. Hence, considering sedimentation rate of about 50 m/my, a 1m high fault scarp should be roughly younger than about 20 ky.
Published paper: Elfassi, Y., Gvirtzman, Z., Katz, O. and Aharonov, E. (2019). Chronology of post-Messinian faulting along the Levant continental margin and its implications for salt tectonics. Marine and Petroleum Geology. https://doi.org/10.1016/j.marpetgeo.2019.05.032
Salt tectonics in the Eastern Mediterranean Sea: Where a giant delta meets a salt giant
The circum-Nile deformation belt (CNDB) demonstrates the interaction between a giant delta and a giant salt body. The semi-radial shape of the CNDB is commonly interpreted as the product of salt squeezing out from under the Nile Delta. We demonstrate, however, that this is not the dominant process, because the delta and its deep-sea fan do not reach the deep-basin salt. The distal part of the deep-sea fan overlies the edge of the salt giant, but squeezing this edge (<150 m thickness) should have had only little effect on the regional salt tectonics. Only on the easternmost side of the deep-sea fan, toward the Levant Basin, does the squeeze-out model work. Here, the delta front reaches the thick salt layer and differential loading promotes basinward salt flow, even upslope. On the western side of the delta, downslope gliding of the sediment-salt sequence toward the Herodotus Basin is driven by the elevation gradient toward the deepest part of the basin. Our analysis shows that salt squeezing by differential loading was previously overestimated in the Eastern Mediterranean and raises the need to carefully map the boundary of salt basins prior to any interpretation. This conclusion is especially relevant in young basins where deltas and shelves have not propagated far enough into the basin.
Published paper: Zucker, E., Gvirtzman, Z., Steinberg, J., and Enzel, Y., 2020, Salt tectonics in the Eastern Mediterranean Sea: Where a giant delta meets a salt giant. Geology, v. 48, p. 134–138, https://doi.org/10.1130/G47031.1
When Two Salt Tectonics Systems Meet: Gliding Downslope the Levant Margin and Salt Out‐Squeezing From Under the Nile Delta
At present, salt tectonics in the Levant Basin comprises extension along the continental slope and folding and thrusting in the deep basin. However, here we show that the shape, the age, and the amplitude of deformation in the extensional and contractional domains do not fit the paradigm of “updip extension and downdip contraction.” Deep basin shortening is not parallel to continental margin extension; it is younger and stronger. We propose that while extension of the Levant continental slope is an expression of basinward gliding, the deep basin shortening belongs to the circum‐Nile deformation belt (CNDB) propelled by out‐squeezing of salt from under the Nile delta. However, despite the independent driving forces, the two deformation systems seem to affect each other. In the southern part of the basin where the two systems meet, the NW gliding of the continental slope is restrained by the NE motion of the CNDB and vice versa. Interestingly, the beginning of folding along the CNDB (~2 Ma) coincides with the establishment of a continental shelf offshore Israel. We thus propose that these two apparently independent phenomena express maturity of the Nile delta that had reached a size and shape sufficient to trigger salt out‐squeezing and to form a continuous continental shelf from the Nile to Israel. This continuity allowed shelf alongshore sediment transport that further propagated the Israeli shelf northward and westward.
Initiating salt tectonics by tilting: Viscous coupling between a tilted salt layer and overlying brittle sediment
Salt basins often exhibit a shelf/slope region of extension and a deeper domain of contraction. The upslope normal faults in such salt tectonics systems are often associated with the pinchout edge of the buried salt layer. Although the spatial correlation between the normal faults and the salt pinchout was previously observed, the mechanism was not fully explained. The Levant basin, which is a young and mildly deformed salt basin with a relatively thin overburden, provides an opportunity to analyze a simple salt tectonics system driven by basin margin tilt, and the formation of the normal faults. This work presents analytical and numerical modeling of the coupled viscous salt and overlying visco-plastic sediment layer. Results suggest the viscosities of both the sediment overburden and salt, as well as their thicknesses, control the deformation of the coupled layers. The visco-plastic deformation mechanism explains quantitatively the position of faulting and observations of the temporal evolution of brittle deformation in the Levant basin margin. It predicts that the largest stress in the overburden arises above the salt edge, driving normal faulting at that location. Our model also places quantitative constraints on the effective viscosity of the overburden (5*1019–5*1020 Pa s), which is consistent with experimentally-determined creep laws. Our results contribute to the understanding of halokinematics in salt basins during the early stages of deformation and will allow better assessment of geological hazards related to salt related deformation.
Classifying marine faults for hazard assessment offshore Israel:a new approach based on fault size and vertical displacement
For many countries, the methodology for offshore geohazard mitigation lags far behind the well-established onshore methodology. Particularly complicated is the assessment of fault hazard in the marine environment. The determination of whether a fault is active or not requires ultrahigh-resolution seismic surveys and multiple coring and, unfortunately, frequently ends with uncertain results. Moreover, if a pipeline must cross a fault, it is not enough to determine whether the fault is active; slip rates are needed for resistant planning.Here we suggest a new approach for fault hazard assessment for the master planning of infrastructure. We provide planners a way to choose a route that will cross the least hazardous faults; these faults will then be investigated in site-specific surveys for slip rates that will allow seismic design. Instead of following the onshore practice that is hard to implement in the marine environment, we suggest taking advantage of the marine environment where seismic data are commonly better in quantity and quality. Based on existing industrial 3D seismic surveys, we measure for each fault in the study area the amount of its recent (in our specific case, 350 ka) vertical displacement and the size of its plane. According to these two independently measured quantities, we classify the faults into three hazard levels. This allows planners to choose infrastructure routes that cross the least hazardous faults at an early stage of planning and direct them to sites that need further investigation.
Our case study is the Israeli continental slope, where numerous salt-related, thin-skinned, normal faults dissect the seabed, forming tens of meters high scarps. A particular hazardous zone is the upper slope south of the Dor disturbance, where a series of big listric faults rupture the seabed in an area where the sedimentation rate is 4 times faster than the vertical displacement rate. We suggest that this indicates exceptionally fast creep, seismic rupture, or rapid tremor and slip episodes.