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|Authors: ||Cocchi, L.|
|Title: ||Magnetic structural evidences of the 41° parallel zone (Tyrrhenian Sea) inferred from potential field data: the 3D model of the discontinuity|
|Issue Date: ||May-2007|
|Keywords: ||Magnetic anomalies|
|Abstract: ||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
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
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.
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.|
|Appears in Collections:||04.05.03. Global and regional models|
04.05.04. Magnetic anomalies
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|Tesi_Cocchi_XIX_ciclo.pdf||PHd thesis with figures and references||17.27 MB||Adobe PDF||View/Open
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