GNGTS – Atti del 19° Convegno Nazionale / 14.02
F. Speranza (1), M. Chiappini (1), A. Meloni (1), E. Boschi (1), O. Faggioni (2),
N. Beverini (2), C. Carmisciano (2) and I. Marson (3)
(1)
Istitituto Nazionale di Geofisica, Roma
Ist. di Geofisica e Ambiente Marino, Consorzio Universitario, La Spezia
(3) DINMA, Università di Trieste e OGS, Trieste
(2)
TECTONIC IMPLICATIONS OF THE NEW MAGNETIC ANOMALY
MAP OF ITALY AND SURROUNDING SEA
The recent compilation of the magnetic observations made on shore the Italian
peninsula and all over the adjacent seas shows for the first time a new regional scale
view of the magnetic anomaly crustal field at sea level over the Italian territory. The
shaded relief map represents the anomalies of the total intensity of the Earth’s
magnetic field for the geomagnetic epoch 1979.0.
The most remarkable result of this new map is an unprecedented view of the
magnetic anomaly field of the whole area at ground level, which contains many
imprints of the major tectonic elements of Italy, and portrays their regional
characterization.
The map shows that the external Apennine belt has a clear magnetic signature.
A low intensity-long wavelength positive magnetic anomaly is observed along the
external belt, whereas negative anomalies are documented over the Adriatic-Apulian
foreland areas. The anomaly pattern over the Apennines indicates that the magnetic
basement located below the Triassic evaporites is ubiquitously incorporated in the
external compressive fronts, implying a thick-skinned tectonic style for the external
belt. At present the precise lithological nature of such a basement cannot be detailed,
because it is buried beneath a several km thick sedimentary cover. In principle the
magnetic basement may include upper Paleozoic-lower Trias continental clastics
(“Verrucano”) as well as underlying cristalline rocks.
Magnetic modeling in the northern Apennines supports a magnetic basement
southwestward upraising from 6-7 km depth in the Adriatic foreland to 2-3 km along
the belt axis, where it was reached by several exploration wells. At the range front
the basement uplift probably occurs by means of high-angle low-displacement thrust
faults inverting preorogenic normal faults.
In the southern Apennines, a remarkable positive magnetic anomaly runs
perfectly parallel to the belt front internally, at a few tens of km from it. Here seismic
data and deep wells show that the basement below the Apulian carbonates does not
undergo any significant uplift moving from the foreland to the belt. Therefore the
observed positive magnetic anomaly should be produced by a definitely more
susceptive basement beneath the belt, that we interpret as an internal crustal wedge
tectonically interposed between the Apulian carbonates and basement.