NEOTECTONIC ACTIVITY AND EMISSION OF FLUIDS IN THE NORTHWEST SICILY CHANNEL

The southern Sicilian coast represents an important contribution to Italian tourism, the Sicily Channel is an important communication path because at the top of its seafloor there are many pipelines, and submarine communication cables which are laid to carry signals, and which are very important to the minor islands. In this work, we are presenting results of the detailed geomorphological and seismostratigraphic analysis based on new very high-resolution dataset (multibeam and CHIRP) acquired during the ACUSCAL 2015 Cruise. We also used low resolution bathymetric and seismic data provided from online database (ViDEPI, GEBCO, EMODnet). This study allows us to reconstruct the tectonic volcanic mass movement and bottom current processes and to identify and map several relative geological elements (e.g. fluid escape structures, gravity driven deposits, volcanic morphologies) which were used to describe the geological setting of the study area. The latter is a small area (approx 100 km2) located in a shallow sector of the north-western Sicily Channel, at a distance of 45 km from the Sicilian coastline, where seven seamounts (M1-M7) have been identified and studied between 10 and 350 m in depth. In the study area, the interaction of the sedimentary multilayer (e.g. Holocene sedimentary body, Messinian Evaporites) and the volcanic context (i.e. Graham Bank) with the tectonic activity (shallow and deep) creates a complex and highly geological framework hosting seafloor seepage. The main focus is the description of the fluid escape structures suggesting a possible model for the origin of the fluid and the mechanisms by which it is transferred to the seafloor. Another important aim is the reconstruction of the relationship between the deep tectonic structures and the fluid escape structures in the north-west Sicily Channel. Seafloor seepage is a widespread phenomenon in the global ocean forming important seafloor morphologies. It affects the global carbon cycle and provides the basis for abundant chemosynthetic organisms living in and on the seafloor. In this Ph.D. project, detailed investigation about the geological setting of such sites and the controlling factors for spatial and temporal distribution of the fluid escape structures were indispensable to understand the “fluid flow system functions” This work is a prime example of these interactions and their influence on seafloor seepage in the Western part of the Sicily Channel. Many pockmarks, mounds and some active seafloor seepages (i.e. hydro acoustic anomalies) have been identified and investigated using both stratigraphic information provided by AGIP/ENI Wells in conjunction with various geophysical data primarily multibeam and Chirp and secondarily multichannel seismic profiles and low resolution bathymetric data. The accurate bathymetric surveying by multibeam instruments allowed us to recognise several positive (mounds) and negative morphologies (pockmarks) linked at the fluid escape phenomena. The bathymetric data revealed an increase in size of the pockmarks with increasing distance to the elongated channel located in the central part of the study area and also that both pockmarks and mounds are isolated, aligned or clustered. CHIRP profiles imaged the presence of several recent faults affecting the Holocene sedimentary body and the occurrence of hydro-acoustic anomalies in the water column, located mainly at the top of the recognised volcanoes. The Multichannel seismic profiles that were correlated with the Agip/Eni wells, showed several acoustic anomalies (often from the Messinian Evaporites) and sub vertical normal fault systems, which affect gas migration in the sub-seafloor. In the western part of the study area, the Messinian Evaporites rise up to the seafloor with a jagged seafloor topography of the MES deforming the complete geological succession. Additionally, the volcanic context supplies large volumes of material to the seafloor, which are distributed as seamounts. The uplift of Messinian deposits concurrently with the formation of sub vertical normal faults leads to gases/fluids migration towards the seafloor forming a field at pockmarks of different sizes. In the eastern part, the rise up of volcanic material through the sub vertical normal faults leads to gases/fluids migration towards the top of the seamounts forming vertical plumes. The first study case of this work revealed a strong tectonic control on the development and distribution of the seepage features in the study area. The aim of the second study case of this work was the identification, description and classification of different examples of gravity driven deposits. The recognised gravity driven deposits in the study area using CHIRP and Multibeam data are: (i) soft-sediment depositional structures (SSDSs), (ii) rock falls and (iii) large debris avalanches. We however estimated the main pre-conditioning geomorphological factor, which could have induced sediment instability and the trigger mechanisms often linked to different geological processes. This second study case revealed that these geomorphological structures are characterised by a genesis probably linked to elevate slopes of the seabed as well as trigger different factors. Finally, mainly using the multibeam data and secondarily the CHIRP data, a cluster of seamounts that was recognised within the study area have been investigated and studied in detail. Their main dimensions and relative physical parameters have been measured and tabulated, allowing a morphometric and morphologic classification on the basis of their shapes. Several restrictive criteria were applied in order to be ceratin of their volcanic origin (conical shape with maximum height, basal ratio, aspect ratio) obtaining finally what can be classified as volcanic seamounts. These structures are very similar to submarine volcanoes described elsewhere on the seafloor. Some show at the top plain morphologies interpreted as marine terraces, others show a typical cone shape of the volcanoes one of which is characterised by a complex shape. On the basis of their morphological features these have been subdivided into three categories.