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A large-scale radial pattern of seismogenic slumping towards the Dead Sea Basin
Journal of the Geological Society, 2012
Although it has been tacitly assumed since the seminal work of Jones in the 1930s that slump folds bear a systematic and meaningful relationship to the slope upon which they were presumably created, there has in reality been very little attempt to objectively verify this association via the collection of regional slump data in a relatively controlled setting. The potential to walk around the intact Dead Sea Basin at c. 425 m below mean sea level provides a perhaps unparalleled opportunity to undertake such verification via the direct examination of slump fold relationships. The collection of slump data in this well-constrained environment, where the seismogenic trigger for slumping is established via earthquake records, and the palaeogeographical controls are also recognizable and clearly link to the present bathymetry and landscape, thereby permits an evaluation of the use of slump folds as indicators of palaeoslope. The Late Pleistocene Lisan Formation cropping out to the west of the Dead Sea contains superb examples of slump folds that systematically face (>95%) and verge (>90%) towards the east. This study employs and evaluates five statistical techniques, including a new mean axial-planar dip (MAD) method, to analyse relationships between the orientation of slump folds and palaeoslopes. We recognize for the first time that the direction of slumping inferred from slump folds and thrusts varies systematically along the entire c. 100 km length of the western Dead Sea Basin. SE-directed slumping is preserved in the north, easterly directed slumping in the central portion and NE-directed slumping at the southern end of the Dead Sea. They are interpreted to form part of a large-scale and newly recognized radial slump system directed towards the depocentre of the precursor to the Dead Sea, and to be triggered by earthquakes associated with seismicity along the Dead Sea Fault.
Soft-sediment deformation within seismogenic slumps of the Dead Sea Basin
Journal of Structural Geology, 2011
The Late Pleistocene Lisan Formation preserved next to the Dead Sea provides exceptional 3-D exposures of folds and faults generated during soft-sediment slumping and deformation. It is possible to generate a range of four different scenarios associated with overprinting in a single slump event. The progressive evolution of slump systems may be broadly categorised into initiation, translation, cessation, relaxation and compaction phases. Thrust packages typically define piggyback sequences during slump translation, with back-steepening of imbricate faults leading to collapse of folds back up the regional palaeoslope. Detailed evaluation of slumped horizons may also permit structures to be traced across apparently separate and distinct slumped units. The recognition that slumps may be reworked by younger seismically-triggered events suggests that in some cases the seismic recurrence interval may be shorter than previously anticipated.
Prehistoric Seismic Basin Effects in the Dead Sea Pull-apart
Site effect is the specific response to earthquakes that is characteristic of the attributes of a site. The two-and three-dimensional shape of sedimentary basins may constitute an important factor of site effects. In sediment-filled basins, in which a lens of soft sediments overlies rocks with higher seismic velocities, two-dimensional resonance patterns may prolong the duration of shaking, and induce a large amplification, much larger than the one predicted from the corresponding one-dimensional analysis. The main source of these phenomena is the development within the basins of surface waves, including the vertically and elliptically polarized Rayleigh waves, and horizontally polarized Love waves.
Holocene seismic and tectonic activity in the Dead Sea area
Tectonophysics, 1981
. Holocene seismic and tectonic activity in the Dead Sea area. In: R. Freund and Z. Garfunkel (Editors), The Dead Sea Rift. Tectonophysics, 80: 235-254. The Dead Sea is a large, active graben within the Dead Sea rift, which is bounded by two major strike-slip faults, the Jericho and the Arava faults. We investigated the young tectonic activity along the Jericho fault by excavating trenches, up to 3.5 m deep, across its trace. The trenches penetrate through Late Pleistocene and Holocene sediments. We found that a zone, up to 15 m wide, of disturbed sediments exists along the fault. These disturbed sediments provide evidence for two periods of intensive activity or more likely, for two major earthquakes, that occurred during the last 2000 years. The earthquakes are evident in small faults, vertical throw of a few layers, cracks, unconformities and wide fissures. We further documented evidence for recent sinistral shear along the Jericho fault in deformed sediments and damage to an 8th Century palace on a subsidiary fault. We suggest that the two earthquakes may be correlated with the 31 B.C. earthquake and the 748 A.D. earthquake, reported by the ancients. * Present address: Woodward-Clyde Consultants, Three Embarcadero Center, San Fransisco, Calif. 94111 (U.S.A.).
Holocene Earthquakes Inferred from a Fan-Delta Sequence in the Dead Sea Graben
Quaternary Research, 2000
The Holocene sequence of the fan-delta of Nahal Darga, in Israel, records deformation associated with earthquakes related to the Dead Sea Transform in general and to the Jericho Fault in particular. The fan-delta sequence is well exposed, and 20 radiocarbon ages help to date the earthquakes that are inferred from (a) displacement along faults, (b) liquefaction features associated with 11 separate sandy and silty layers, and (c) slumped allocthonous bodies of sediments located directly above one of the main splays of the Jericho Fault. On average, an earthquake larger than M 5.5 has occurred approximately every 600 years. This estimate is based on the earthquake record of the complete stratigraphic sequence, with erosional hiatuses omitted from the calculations. The most recently deformed layer is related to the 1927 Jericho (M L 6.2) earthquake. This layer provides a modern analog for the style of soft-sediment deformation associated with earthquakes in the late Pleistocene and Holocene silty sand beds of the fan-delta complexes of the Dead Sea and its predecessor, Lake Lisan.
The structure of the Dead Sea basin
Tectonophysics, 1996
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 Dead Sea fault (DSF) is one of the most active plate boundaries in the world. Understanding the Quaternary history and sediments of the DSF requires investigation into the Neogene development of this plate boundary. DSF lateral motion preceded significant extension and rift morphology by~10 Ma. Sediments of the Sedom Formation, dated here between 5.0 ± 0.5 Ma and 6.2 −2.1 +inf Ma, yielded extremely low 10 Be concentrations and 26 Al is absent. These reflect the antiquity of the sediments, deposited in the Sedom Lagoon, which evolved in a subdued landscape and was connected to the Mediterranean Sea. The base of the overlying Amora Formation, deposited in the terminal Amora Lake which developed under increasing relief that promoted escarpment incision, was dated at 3.3 −0.8 +0.9 Ma. Burial ages of fluvial sediments within caves (3.4 ± 0.2 Ma and 3.6 ± 0.4 Ma) represent the timing of initial incision. Initial DSF topography coincides with the earliest Red Sea MORB's and the East Anatolian fault initiation. These suggest a change in the relative Arabian-African plate motion. This change introduced the rifting component to the DSF followed by a significant subsidence, margin uplift, and a reorganization of relief and drainage pattern in the region resulting in the topographic framework observed today.