Magnetic structural evidences of the 41° parallel zone (Tyrrhenian Sea) inferred from potential field data: the 3D model of the discontinuity

Potential field data hold a leading role in the geologic-structural application. Their use becomes even more important if applied to extremely inaccessible zones as oceanic basins or no-antropized area. By an areo-naval survey it is possible to cover large areas, in a short time, to define their deep crustal features that are otherwise not accessible by other direct methods. The analysis of the magnetic field data is particularly effective in the study of the crustal portions characterized by lateral variation of the magnetic susceptibility. The magnetic analysis is often applied to areas where sub-volcanic bodies or relic portion of oceanic nature in sedimentary deposits are located. The aim of this thesis is the development of a tridimensional model about the 41st parallel zone starting from potential field data. The name 41st parallel indicates a geographic zone aligned along N41st of latitude. This area is longitudinally defined between the Continental Campanian Margin (Naplean Gulf, Ischia and Procida island) and the northwestern portion of the Sardinia Island. From a geological point of view, the 41st parallel represents a complicate area. Several geological structures are located along this zone: submerged volcanic bodies such as the Etruschi, Vercelli and Cassinis seamounts, emerged edifices connected to the Pliocenic-Quaternary volcanism of the Central Tyrrhenian sea (Palmarola, Zenone, Ponza, Ischia and Procida) and several deep fault structures such as the E-W fault of Ponza. The formation and the development of this particular zone is not clear and is still object of discussion in literature. The structural setting of 41st parallel zone is highlighted only by magnetic field data. By the observation of the magnetic anomalous field it is possible to see an alignment of several magnetic anomalies along the N41st latitude. These anomalies take place on the main structural evidences of the area. The bathymetric data and information don't suggest these features. To this aim, I use the magnetic data to analyse in quantitative way, the 41st parallel zone. The magnetic data used for the development of the 3D magnetic model derived by the dataset of the Aeromagnetic Anomaly Map of Italy (Caratori Tontini et al., 2004). The original magnetic dataset includes the total intensity field of Italy and its surrounding seas acquired partly during the aeromagnetic surveys performed by Agip (now Eni-Spa) between 1971 and 1980 and during new surveys in the years 2001-2002(Eni,Exploration & Production Division - Igmar, La Spezia). The magnetic data were recorded, in a homogeneous way, by using a cesium-magnetometer. By the successive reprocessing of the row magnetic data the revised magnetic anomaly map is obtained showing a strong informative contribution and a good agreement with the sea-level map of Chiappini et al. (2002). In the first chapter of this thesis I describe the geological and structural features of the Tyrrhenian sea in general way. However, I analyse the 41st parallel zone starting from the literature data. In the second chapter I evaluate the Bouguer gravity field of the Tyrrhenian Sea by using two methodologies for an evaluation of the optimal Bouguer reduction density. Using a free-air gravimetric satellite data set of the Tyrrhenian sea, I perform a map of isostatic level of the central Tyrrhenian area. In the third chapter I describe the properties of the Geomagnetic field and its representation focusing my interest on the time and spatial dependencies of the field. The successive section provides information about the row magnetic data used for the quantitative elaboration describing the characteristics of the anomaly field of the studied area. In the fifth chapter, the properties of the magnetic signal is studied by using a statistical analysis of the power spectrum (Spector and Grant, 1970) and by the Continuous Wavelet Transform. After these analyses, in the sixth chapter I introduce the concept of magnetic basement and the relationship between magnetic signal and temperature. Starting from the regional heat-flow data (Della Vedova et al., 2001) of the Central Tyrrhenian area the Curie Isotherm surface is modeled defining the maximum depth of the magnetic-thermal basement. The boundaries of the magnetic sources (top and bottom) represents the base-line for the successive phases of quantitative analysis. By using a 2D inversion algorithm I obtain the map of apparent susceptibility. In the seventh chapter, I apply this algorithm to the magnetic evidence of the 41st parallel zone and to the Selli Line region. This procedure suggests a distribution of magnetization that permits to connect the 41st parallel zone and the structural elements of the Southern Tyrrhenian Sea such as the Magnaghi Basin and the Selli Line faults system. Starting form the results obtained by the apparent magnetization maps, I perform a 3D inversion of magnetic data providing information about the vertical distribution of the sources. In the eighth chapter, that represents an important part of the thesis, I introduce the inverse problem in the potential field analysis by a new 3D algorithm capable to evaluate the depth to the bottom of the source. Then, I apply this algorithm to the real magnetic dataset of the 41st and Selli Line regions. The recovered models show the shape, location in depth and direction of development of the magnetic generating sources suggesting the geometric relationship between the different sources. These information are important for evaluating the crustal setting of the study area. Finally, in the last chapter I interpret the results of inversion process evaluating the relationship between the 41 st parallel and the Selli Line region. Starting from the magnetic recovered models of these two regions I provide a chronological reconstruction of the geodynamic evolution of the Central Tyrrhenian Sea