Shortening and Multiple Detachments in Thick Salt: Insights from the Messinian Saline Giant, Eastern Mediterranean
Martin P. Jackson1, Joe A. Cartwright2, and Simon Higgins2
1Bureau of Economic Geology, Jackson School of Geosciences, University of Texas, Austin, TX
2School of Earth, Ocean & Planetary Sciences, Cardiff University, Cardiff, United Kingdom
The seismic character of salt is typically low-amplitude and incoherent. However, as seismic technology improves, we increasingly recognize salt as a multilayer sequence of halite interbedded with other rocks. Multilayers in salt elucidate the longstanding question of whether tectonic detachments form in the top, middle, or base of an evaporite sequence. This has implications for interpreting intrasalt seismic events and picking the base of salt above subsalt targets.
Using 3D seismic surveys from the Levant continental margin, we analyzed shortening within the Messinian (uppermost Miocene) evaporite and its overburden. Thrusting and folding began in the Pleistocene and continues today. Strain reached 7% shortening in the salt and 2% shortening in the overburden. Multilayered salt partitioned strain into four main detachment layers, whose seismic incoherency suggests they are halite-rich. The most effective evaporite is in the uppermost Messinian, followed by one near, but not at, the base of salt. Competent intrasalt layers shortened to form thrusts and sinusoidal, chevron, and kink folds. Conjugate strike-slip faults detached within the salt during the shortening. The structural pattern at different levels is robust. Half the structures stack vertically and the other half do not, which could be explained by the multiple detachment levels. Stacking of structures and a consistent shortening direction suggest a single phase of deformation in the Pleistocene. Vertical variation in the strain of competent layers shows a modified Couette flow profile in the Messinian sequence as a whole: salt displacement increased upward. These results are from a well-imaged evaporite sequence that is mildly deformed by salt-tectonic standards. Extrapolating the results predicts that multiple detachments are almost certainly present in more highly strained multilayer evaporites.
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