Salt Tectonics, Fluids and Geohazards in the Mediterranean Sea - an Overview

COST-ANR MEDSALT Symposium, 24-28 October 2016, Palermo, Italy.-- 2 pagesThe Messinian Salinity Crisis (MSC) had a major impact on the subsequent evolution of the Mediterranean Basin and constrains Present-day geological processes. The presence of a thick salt layer, syn-depositional and post-depositional diapirism of the salt as well as large-scale mass-wasting during Messinian times play a major role in determining 1) fluid flow pathways, 2) tectonic movements caused by salt withdrawal beneath basinward prograding continental slopes 3) mass wasting, a possible threat for all kind of sea-floor installations and 4) oil and gas plays. Furthermore, brine fluids and the accompanying nutrient flux, which are controlled by evaporitic facies distribution, salt tectonics, faults and sediment load, strongly influence the deep biosphere above and beneath the salt. The general perception is that salt diapirs emerge by three mechanisms in an extensional setting: 1) the traditional model is the “active diapirism”. Driven by differential load the salt deforms and eventually pierces the overburden. Buoyancy forces due to density inversion play a minor role. 2) If sedimentation in the rim synclines keeps pace with the rise of a rather shallow diapir we call it “passive diapirism” or “downbuilding”. Once the tabular salt besides the diapir is depleted the diapir will be buried. 3) Regional extension due to tectonic strain may create accommodation space which will be filled by a rising diapir during the “reactive diapirism” phase. Passive diapirism is considered to play the major role. Another observation that has been proven a thousand times is that salt is an important stratigraphic seal owing to its almost total impermeability. It is noteworthy that these models and paradigms are based on research carried out in Mesozoic or Paleozoic salt giants. In spite of their global occurrence and general importance within the earth system, there is a significant lack in our knowledge about the early evolution of juvenile salt giants and its controlling factors.The less than 6 Ma old Messinian evaporites are very young compared to the Permian Zechstein salt or the Mesozoic evaporites in the North Atlantic. In some of the sub-basins, the salt structures are almost free of any overprinting by plate tectonic processes and thus reflect almost pure salt tectonics. During the last 15 years the structural evolution of the Messinian salt giant has been studied by means of newly released industrial and newly acquired academic high-resolution reflection seismic data. The recent findings challenged the above outlined paradigms and foster the hypothesis that emerging salt giants are features of a high intrinsic dynamics: 1) Layered and different evaporitic facies and intercalated clastics with trapped fluids cause multiple detachment layers and creeping along these layers during the salt precipitation and related basin floor tilt phase (“gravity gliding”). 2) Post-depositional salt tectonics is mainly attributed to differential load (“gravity spreading”) and may drive active seafloor deformation and gravity gliding; examples exist in southern Italy and eastern Spain. 3) The layering strongly influences the structural evolution of salt giants. 4) Embedded fluids have a significant impact on the early deformation history of salt giants. 5) Salt giants are not necessary seals, particularly were welds develop. Fluids may flow through or out of them. A quantitative reconstruction and in-depth understanding of the interplay between salt deposition, subsidence, salt burial, syn- and post-depositional salt tectonics, fluid through and outflow as well as mass wasting needs detailed geophysical imaging, parameterization and sediment coring, followed up by modeling. A compilation of basin-wide observations of salt-related structures will help defining the correct scientific questions and identifying the targets where scientific drilling could best address these questionsPeer Reviewe