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ABSTRACT

Regional Heat Flow and Geothermal Fields in Italy

FRANCESCO MONGELLIMARIANO LODDO

Istituto di Geodesia e Geofisica, Universith di Bari, Bari 70100, Italy

The relationship between heat flow, geothermal fields,and plate boundaries is today quite clear.

The Mediterranean area is the contact zone of the Africanand Eurasian plates, but here the situation is chaotic becauseof the fractioning of the principal plates which generatedseveral moving microplates.

Geophysical data point out subduction of the African andApulian plates under the Tyrrhenian Sea; less clear is thesituation in the northern and central Apennines.

A preliminary heat flow contour map of the Italianpeninsula and surrounding seas shows an arc of high heatflow which runs parallel to the Italian coast line from Tuscanyto Sicily.

The tentative conclusions are that: (1) the northern andcentral Apennines are probably an island arc but havegeothermal fields; ( 2 ) the southern Apennines and Calabriaalmost certainly make an island arc and very probably havegeothermal fields ; and (3) northern Sicily is probably anisland arc and may have geothermal fields.

INTRODUCTIONThe new global tectonic theory is a dynamic model which

today gives the best explanation of the anomalous heat flowareas distribution. There is, in fact, an evident relationshipbetween heat flow and recent boundaries of plates, andalso between heat flow and some tectonic units (Table 1 ).

Obviously, the existence of geothermal fields is subordi-nated to very high heat flow (>6-7 heat-flow units) andit is also connected with divergent and transcurrent plateboundaries, marginal basins, and internal rifts.

The aim of this work is to frame the actual and probablegeothermal fields of the Italian peninsula in this scheme.

Table 1. Relationships between heat flow and tectonic units.

Tectonic featuresDivergent plate boundariesConvergent plate boundariesTranscurrent plate boundariesMarginal basins and island arcsInternal riftsStable platesRecent orogenic areas

Heat flow in heatflow units (hfu)

high, > 2low, < 1

high, > 2high, > 2high, > 2low, 51

high, 1.5-2.0

HEAT FLOWANDGEODYNAMICOUTLINES IN ITALYThe Mediterranean area is the contact zone of the African

and Eurasian plates, but the boundaries of the convergentplates are not clear everywhere because of the existenceof some microplates generated from the principal ones (Fig.1 ). For example, referring to Italy, on the basis of seismo-logical data Lort (1971) delineated an Apulian (or Adriatic)microplate between the Apennines and Dinarides.The dis-tribution of heat flow (and the existence of geothermal fields)is connected with the interactions of these tectonic units.At the monent, even if the number of heat flow stationsis not high, it is possible to draw a preliminary map.

Figure 2 shows the position of the Italian areas of heatflow measurements on land and sea,numbered approximatelyfrom north to south and from east to west. In Table 2the observed mean values and the sources are reportedfor each area. Figure 3 shows the map of the observedheat flow; we have chosen an interval of 0.5 Fcal cm-2 sec-1(hfu) because of the small number of measurements.

From the general view of the map two positive anomaliesare evident, one on the western side of the Tuscan-LatialApennines and the other in the Tyrrhenian Sea. The negativeanomaly on the Adriatic side ( Romagna) complements tothe first, and the negative anomaly in the Bradano Troughin eastern Calabria and in Sicily complements the other.

But considering the geodynamic knowledge of the region,it is convenient to subdivide it in three parts: (1) middle-northern Apennines, (2) southern Apennines and Calabria,and (3) northern Sicily.

Figure 1. Plate tectonics in the Mediterranean region, afterLort ( 1971), mod ified.

5071/

1 1

1

495

EURASIAN PLATE

(1 ...-11

f

I , FAFACAN PLATE

496 MONGELLI AND LODDO

Figure 2. Location map of heat flow measurement areas in Figure 3.the Italian peninsula and Sicily.

Table 2. Observed mean values of heat flow ( in hfu ) inthe Italian peninsula and in Sicily.

Area*

425%S 22le W• NE

J&

31-1 i03Ii Authors

1 Romagna 3 0.7 in preparation2 Rosignano 3 2.4 Fanelli et al. (1974)3 Geothermal fields many very Boldizsar ( 1963); Cataldi et

high al., ( 1970); Cataldi andRendina (1973)

4 Garganot 3 3.0 Mongelli and Ricchetti(197Oa)

5 Fossa Bradanica A) 3 1.9 Mongelli and Ricchetti(197Ob)

6 Fossa Bradanica B) 6 0.9 Loddo and Mongell i(1974b)

7 Calabria 3 1.02 Loddo et al. (1973)8 Etna 3 0.82 Mongelli and Loddo (1975)9 Sicily 3 0.84 Loddo and Mongelli

(1974b)10 Tyrrhenian Sea 12 2.83 Erickson (1970)11 Sicily Channel 1 1.20 Erickson (1970)12 lonian Sea 8 0.79 Birch and Halunen ( 1966);

Erickson (1970)13 Southern Adriatic

Sea 11 1.32 Lavenia (1967)*Area numbers correspond to those in Figure 2.tThe small anomaly of Gargano is very probably due to rising ground water.

Contour map of observed mean values of heat flowin the Italian peninsula and Sicily.

Middle-Northern ApenninesThe seismic activity in the middle-northern part of the

Apennines is purely superficial ( Giorgetti and Iaccarino,1971 ) and the expected geodynamic indications are not clear.Boccaletti, Elter, and Guazzone ( 1971), on the basis ofgeological evidence, connect the northern Apennines to theinversion of a subduction zone along the Alpine-to-Apenninicpolarity starting in upper Oligocene-lower Miocene.

From a gravimetric point of view, the present dynamicsituation is delineated, according to Mongelli, Loddo, andCalcagnile ( 1975), by the asymmetry between Bougueranomalies and topography, which agrees with the Apenninicpolarity.

The important geothermal fields of Larderello, MonteAmiata, and Vulsini Lakes can be explained as due tofrequent intrusions in the thinned lithosphere of marginalbasins behind the arc. The low heat flow values observedon the Adriatic side complement the above geothermal fieldsand may refer to a present subduction.

Southern Apennines and CalabriaMongelli, Loddo, and Calcagnile (1975) have shown that

the relation between topography and Bouguer anomaliesgives different parameters when passing from the northernto the southern Apennines; according to the authors thisfact is connected with different subduction stages.

The seismic activity clearly indicates the existence ofa Benioff plane ( Peterschmitt, 1956 ) dipping from theApennines toward the Tyrrhenian Sea, definitely down to350 km and probably down to 480 km. This was interpretedin terms of subduction by Ritsema (1969) and by others.The southern part of the Adriatic microplate, that is, thereal Apulian plate, would be in subduction.

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From the gravimetric point of view, the asymmetry

between topography and Bouguer anomalies, (Mongelli,

Loddo, and Calcagnile, 1975) and the interpretation of the

isostatic anomalies in this region are in good agreement

with the seismological deductions (Loddo and Mongelli,

1975). The petrochemical characteristics of volcanoes of

the Aeolian (Lipari) Islands are of calc-alkaline compressive

type and indicate a deepening zone (Barberi, et al., 1973).

Finally, the heat flow distribution in the Tyrrhenian Sea

and behind the arc is identical to that observed in a classic

marginal basin-arc-trench system (Loddo and Mongelli,

1974a). Unfortunately, there are no heat flow measurements

on the Tyrrhenian side of the southern Apennines and

Calabria, but the volcanic activity around the Gulf of Naples

seems to offer good promise for the discovery of geothermal

fields.

Northern Sicily

The seismic activity in northern Sicily is purely superficial

(Giorgetti and Iaccarino, 1971 ). The fault plane solution of

three earthquakes indicates a thrust mechanism on east-west

planes (McKenzie, 1972). Deep seismic sounding profiles

indicate that the crustal thickness increases from the Sicily

channel to the northern edge of Sicily up to about 40 km

and then sharply decreases when entering the Tyrrhenian

Sea (Cassinis, et al., 1969; Colombi, et al., 1973). Barberi,

etal. ( 1974), on the basis of paleomagneticand volcanological

evidence, maintain that Sicily is a part of the African plate,

and from the Mesozoic the African and European plates

have converged with consequent consumption of the litho-

sphere.Also, in this case the heat flow distribution in the Tyrrhen-

ian Sea and Sicily, even if based on few measurements,

seems similar to that of arc-trench systems. Unfortunately,

measurements from northern Sicily are lacking; but the

volcanic activity of the Aeolian Islands at the boundary

between the converging African and Apulian plates seems

to be very promising for the discovery of new geothermal

fields.

CONCLUSIONS

The arc from Tuscany to Sicily seems to be due to the

contact of plates of different sizes at different stages of

evolution.

Where the situation is less clear (northern Apennines)

because of the scarcity of information, very - important

geothermal fields exist.

The Campania-Calabrian arc has all the geophysical char-

acteristics of an island arc; heat flow measurements on

land near the seacoast might confirm the existence of

geothermal fields.

In northern Sicily and the Aeolian Islands the available

geophysical information has not been resolved; heat flow

measurements are necessary in order to define the thermal

field.

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MONGELLI AND LODDO