Y. Enzel

Late Holocene lake levels of the Dead Sea

This work presents a high-resolution lake-level record of the late Holocene Dead Sea, a hypersaline terminal lake whose drainage basin encompasses both Mediterranean and hyperarid climatic zones. The lake-level curve reflects the regional hydrologic variations in the drainage basin, which in turn represent the Levant paleoclimates. The curve is based on 46 radiocarbon ages of organic remains from well- exposed sedimentary sequences along the Dead Sea shores. These sequences record fluvial and lacustrine depositional environments. The paleolakeshores are marked by shore ridges, coarse-sand units, and aragonite crusts; in the modern Dead Sea, such features indicate the exact elevation of the shore. The late Holocene Dead Sea level fluctuated within the range of 390 to 415 m below sea level (mbsl). For most of the time the lake was below the topographic sill (402 mbsl) separating the northern and southern basins of the Dead Sea and was confined to the deep northern basin. Nevertheless, short-term rises in the late Holocene Dead Sea level caused the flooding of the shallow and flat southern basin. Highstands occurred in the second and first centuries B.C. and the fourth century A.D. during the Roman and early Byzan tine periods, respectively, in the eleventh and twelfth centuries A.D. during the Crusader period, and at the end of the nineteenth century A.D. The rises mark a significant change in the annual rainfall in the region, which likely exceeded the instrumentally measured modern average. The curve also indicates drastic drops that exposed the sedimentary sequences to erosion. The oldest and probably deepest drop in the lake level culminated during the fifteenth and fourteenth centuries B.C. after a retreat from a higher lake stand. The longest lowstand occurred after the Byzantine period and continued at least until the ninth century A.D. This arid period coincided with the invasion of Moslem-Arab tribes into the area during the seventh century A.D. The dramatic fall of the Dead Sea level during the twentieth century is primarily artificial and has been caused by the diversion of runoff water for the drainage basin, but the magnitude is not considered exceptional for the late Holocene. Although the past drops in the lake never exceeded the modern artificial drop rates, they do represent extreme arid conditions that occurred frequently over the past several thousand years.

Paleoseismic evidence for time dependency of seismic response on a fault system in the southern Arava Valley, Dead Sea rift, Israel

The Elat fault system in the southern Arava Valley (Dead Sea rift, Israel) is a complex fault zone, characterized by marginal normal faults and central sinistral strikeslip faults. Paleoseismic evidence shows that the Elat fault system has generated at least 15 earthquakes of magnitudes (M) larger than 6 during the late Pleistocene and the Holocene. At least two branches of the fault zone were tectonically active simultaneously, indicating that the seismic response over a period of 80 k.y. was time and space dependent. Late Pleistocene earthquakes displaced the surface by 1‐1.5 m; their magnitudes were between M 6.7 and M 7, and their average recurrence interval was 2.8 6 0.7 k.y. Movements along the fault system in the Holocene had a higher frequency and a recurrence interval of 1.2 6 0.3 k.y., but resulted in smaller displacement amounts (0.2‐1.3 m) and smaller earthquake magnitudes (M 5.9‐M 6.7). Historical records document the last seismic event along the Elat fault zone at ;1000 yr ago. The decrease in tectonic activity with time is inferred from the concentration of offset along the fault segments in the central part of the Elat fault zone and the decreased seismicity in the eastern and western margins. The magnitude range determined for the central zone (M 6.1‐M 6.7) was likely not high enough to activate the marginal faults. The average slip rate on the normal faults is 0.2 mm/yr. However, the slip rate has changed through time on different fault segments in the active wide shear zone and between clusters of events related to the same segment. The event-specific slip rates, therefore, have varied from 0.1 to 0.3 mm/ yr. The decrease in earthquake magnitudes with time, combined with the observations that the last large event occurred in A.D. 1068 and that no microseismicity has been detected during the past 15 yr, might signal locking of the Elat fault zone. This effect, if true, may result from episodic global reorganization of the system’s mode of strainenergy release, reflected in the configurational entropy of stress states on the fault. These results have significant implications for seismic hazard assessment in the southern Arava Valley, southern Israel, and underscore the possibility that the Elat fault may be a site of major earthquakes in the near future.

Amplified erosion above waterfalls and oversteepened bedrock reaches

�� erosion rate at the upstream end of the flow acceleration zone above a waterfall, Fr is the Froude number at this setting, and n ranges between 0.5–1.7. This amplification expression suggests that erosion at the lip could be as much as 2–5 times higher relative to erosion at a normal setting with identical hydraulic geometry. Utilizing this erosion amplification expression in numerical simulations, we demonstrate its impact on reach-scale morphology above waterfalls. Amplified erosion at the lip of a waterfall can trigger the formation of an oversteepened reach whose length is longer than the flow acceleration zone, provided incision wave velocity (Vi) at the upstream edge of the flow acceleration zone is higher than the retreat velocity of the waterfall face. Such an oversteepened reach is expected to be more pronounced when Vi increases with increasing slope. The simulations also suggest that oversteepening can eventually lead to steady state gradients adjacent to a waterfall lip provided Vi decreases with increasing slope. Flow acceleration above waterfalls can thus account, at least partially, for prevalent oversteepened bedrock reaches above waterfalls. Using the cosmogenic isotope Cl-36, we demonstrate that incision wave velocity upstream of a waterfall at the Dead Sea western escarpment is probably high enough for freefall-induced oversteepening to be feasible.

Late Pliocene and Pleistocene reversal of drainage systems in northern Israel: tectonic implications

Abstract The arching of the Galilee, northern Israel, is associated with sediment loading in the Dead Sea Transform and Rift. During the Pleistocene, the arching caused the formation of the main North–South water divide in the region and the reversal of stream flow direction. A reconstruction of a main paleochannel which drained large areas in the eastern Galilee to the Mediterranean enabled the determination of age and amplitude of the arching. This reconstruction is based on topographic analysis of thirteen sites containing fluvial remnants in the Beit-Hakerem Valley. We demonstrate that the widespread normal faulting cannot explain the present-day drainage pattern. Dating of basalt clasts from ancient alluvial remnants along the Beit-Hakerem Valley provides a maximum age limit of 1.8 Ma to the paleochannel. The Pleistocene tectonism arched the Galilee by 200 m over a wavelength of 40–60 km. A comparison between arched and unarched segments of the rifts margins indicates that fluvial and slope processes on the rift escarpment cannot explain the location and shape of the main water divide. In the Galilee, tectonism is the only factor that controls the formation, location and shape of the main water divide.

Soils as a tool for estimating ages of Quaternary fault scarps in a hyperarid environment — the southern Arava valley, the Dead Sea Rift, Israel

Abstract On the alluvial fan of Nahal Shehoret in the extremely arid region of the Negev desert, Israel, Reg soils developed on stable alluvial surfaces in colluvium along terrace risers and in colluvial units on a complex fault scarp were compared. The degree of soil profile development provided an age estimate of the surface that was faulted, a maximum age estimate of the scarp or terrace riser, and an estimate of the maximum recurrence intervals of faulting events. IRSL dates of the tectonically displaced alluvial surfaces and colluvial units at the same site enabled us to re-evaluate the use of Reg soils for relative dating. The major faulting event started at 34.8 ± 4.3 ka. It displaced a late Pleistocene surface of the Nahal Shehoret alluvial fan dated at 56 ± 10.8 ka. Renewed fluvial activity and deposition of alluvium on the downfaulted block had almost terminated by 13.6 ± 2.3 ka. The scarp was formed during the latest Pleistocene and was followed by smaller faulting events which had only minor effects on the fault scarp morphology. The similarity of age estimates from IRSL dating and the degree of soil development in this environment indicates that soils can be used to date surfaces and colluvial units.

Determination of escarpment age using morphologic analysis: An example from the Galilee, northern Israel

We used topographic and structural data and very limited age control to perform quantitative morphometric analyses and to determine relative ages of escarpments bounded by late Cenozoic normal faults in the Galilee, Israel. The Galilee is an extensional zone composed of a series of uplifted and tilted blocks forming large escarpments built mainly of carbonate rocks. Two parameters used to discriminate tectonic stages are the ratio between the height of the escarpment and the total stratigraphic displacement ( L ) and the degree of concavity of escarpment slopes relative to a reference slope. The only dated reference slope is Mount Tur9an, ∼300 m high and formed by the Tur9an fault system, which has a total stratigraphic displacement of 625 m. A basalt flow that delimits the age of the Tur9an escarpment is dated to 4.23 ± 0.23 Ma and displaced 300 m, which is identical to the present-day topographic expression of this escarpment. The L value for this escarpment is ∼0.5. The Tur9an fault system was active prior to 4.23 Ma at slow uplift rates that enabled erosion to maintain the gentle slope over which the basalt flowed. Increased offset rates following the basalt extrusion led to the formation of the escarpment. The preservation of the basalt at the top of the escarpment indicates that erosional lowering of the upper surface of the Tur9an block has been minor since its formation. The L values indicate two stages of uplift; an early stage during which offset rates were probably low enough that they did not form topography, and a later stage that formed topography, which is preserved. The timing of the change in displacement rates from a slow continuous stage to a fast, topography-forming stage was determined by comparing the shape of the dated slope of Tur9an to that of other slopes. We conclude the following: (1) generally, the topographic profiles of different parts of each individual escarpment have similar shapes indicating similar ages; (2) escarpments having slopes that are more concave or convex than the reference Tur9an escarpment are older or younger than 4 Ma, respectively; and (3) the Galilee escarpments did not form simultaneously. A few escarpments were already major morphologic features by the early to middle Pliocene, whereas the rest formed during the late Pliocene. Morphometric analysis is a useful method for studying the geologic history of a landscape controlled by normal fault uplift and characterized by the absence of sediment deposition and where carbonate dissolution is the main erosional process. This and similar approaches can be used to discriminate tectonic stages and understand the relationship between tectonic activity and surface processes in other extensional regions.