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AAPG Geoscience Technology Workshop

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Salt Tectonics in the Levant Basin: When Two Deformation Systems Meet

Abstract

A slope-parallel normal fault system has been recognized offshore Israel more than 40 years ago. Then, in the early 2000s a system of folds and thrust faults was recognized farther west in the deep Levant basin. Initially the combination of updip extension and downdip contraction seemed to fit the classic paradigm of salt tectonics, where the sediments overriding salt slide bainwards and produce extension behind and contraction in front. However, later studies showed that the updip extension system and the downdip contractional system are not parallel and probably independent. The extensional system offshore Israel expresses slope-parallel downdip gliding, whereas the shortening system in the deep basin belongs to a circum-Nile fold belt propelled by out squeezing of salt from under the Nile overburden. At the moment the interrelationship between the two deformation systems remains elusive. The purpose of the current study is to fill up this knowledge gap by gaining better understanding of the relationship between the two systems. For this we examine (1) timing (beginning and rate) and (2) amplitude (extension vs. contraction) of deformation in the two systems seeking for possible mutual influence on one another. For examining the age of deformation we use a new chronostratigraphic subdivision of the Pliocene-Quaternary (PQ) section offshore Israel based on bio- and seismo-stratigraphy. The new chronostratigraphic scheme assigns absolute ages for 5 seismic reflectors, dividing the PQ section into four depositional units. Here we use these dated seismic reflectors to prepare structural and isopach maps. Based on numerous 2D time migrated surveys regional TWT were prepared for Israel’s exclusive economic zone (EEZ) and local high resolution depth maps were prepared for the Sara-Myra 3D depth migrated seismic survey. These maps are used for thickness analysis that identifies syn-tectonic units which are age indicators. For examining the amplitude of deformation we measured elongation and shortening along ten sections crossing both the extensional and the contractional systems. Our findings suggest the absence of any age correlation between extensional activity on the slope and contraction activity in the deep basin. We show that (1) folding activity postdates the extensional activity by ~0.8 million years; and (2) that elongation across the slope reaches 30%, whereas shortening across the deep basin folds reaches 50%. On the other hand, we also show that slope perpendicular elongation and fold perpendicular shortening both decrease southwards. This suggests that the two different systems restrain each other where they meet and northwestwards gliding of the extended belt is restrained by the northeastwards motion of the contractional belt. Interestingly, our isopach maps show that the establishment of a continuous continental shelf connecting the Nile delta to offshore Israel exactly coincides with the beginning of folding along the circum-Nile fold belt (1.8 Ma). We point out that these two apparently independent phenomena both express maturity of the Nile delta. We suggest that at 1.8 Ma the Nile exceeded a geometrical threshold that triggered salt out squeezing on one hand and coevally produced a continuous continental shelf from Egypt to Israel. This continuous shelf allowed alongshore sediment transport from the Nile to the Israeli coast which allowed the formation of the Israeli continental shelf.