Geophysical Study of the Fordongianus Geothermal Area (Sardinia Island, Italy) I

Bo1.1.r:rn1vo at Gsortstca Troatca so arr1.ic.»tr.=t Vol. to-cut, N. tea _ Juas taac

a. BALIA 1, ta. CIMINALE 2, M. Loono 3, D. P.-5.TELLA 4-, o. PECOHINI 5 and A. THAMACERE 4

GEOPHYSICAL STUDY OF THE FORDONGIANUS GEOTHERMAL AREA (SARDINIA ISLAND, ITALY) I-

Abstract. A geophysical survey using gravity and deep dipole geoclectrics was carried out in the area of Fordongianns (Sardinia), where low enthalpy geothermal manifestations are present. The survey al- lowed the reconstruction of the main structural units in the sttitly area and the identiﬁcation of the structures more closely related to the principal thermal waters.

INTRODUCTION

It is known that Sardinia is characterized by many hydrothermal manifestations. Some are potentially interesting such as those along the Sardinian Rift in the western part of the island (Balia et al., 1984). The Fordongianus area, located in the Tirso valley, is considered one of the most promising geothermal areas. I The most important manifestation is situated on the outskirts of the town of Fordongianus. It has a flow rate of 43 Us and a temperature of 54°C (Dettori ct al., 1982) and the ruins of the Roman bath indicate that it was well-known and utilised even in ancient times. The aim of the present paper is to present new geophysical data for the construction of a structural model of the area, and-to single out, also using available geological and geochem- ical information, the buried structures which could be potential reservoirs of geothermal fluids, with a view to a better utilization of this resource. ‘To this purpose, a geophysical survey over an area of about 700_l~:m 2, hetween latitudes 30°55’ and 40°37’ N and longitudes 8°35’ and 3°55’ E was carried out using gravity and electrical resistivity methods.

BRIEF GEOLOGICAL OUTLINE

The Fordongianus area (Fig. 1) is situated along the north eastern border of the Cam- pidano Graben, at the intersection of two fracture systems running in a NNE-SSW and a N-S direction, respectively. The latter system is related to the Tertiary distensive tectonics [Pala et al., I932].

@ Copyright 1990 by QC-S, Osservatorio C-eoﬁsico Sperimentale. All rights reserved. Manuscript received September 20, 1989; accepted June 21, 1990. llstituto di Giacimenti Mincrari, Ceolisica e Science Geologiche, University of Cagliari, Cagliari, Italy.

Zlilipartirnettto di Scienze della Terra, University of Calahria, Cosenra, Italy. 3Dipartimento di Geologia e Geoﬁsica, University of Bari, Bari, Italy. ‘inipartimento di Geofisica e Vuleanologia, University of Naples, Italy. 5Dipartimento di Scienze deila Terra, University of Cagliari, Cagiiari, Italy.

N I 130 BALIA ct al.

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I ‘Itr-H. I r 3950.:Cl - ' - -'*--- - - <5’ iiifiilu ' ' I I I In . I I I ~. I:IFHi!iE ‘I I_ __ _ ' ___"I ‘I _ _ _ _ ’ ‘_j ti ::I’lI_Ir -. .. -- - II *'._ _ I -I . I I III - .,tt- ,| - tr - ' i.-. i “'1 ll I __'. X“‘.-‘LEI ,;l-// - I ‘J’III /I,~T»aw.-1.-1/Vi rm‘? -- in it I. — . ~ =.:#;L'J‘.-a.I —-“‘-"--I-.2'-i' .|lIIIIIIll I?-Efl J __.I _.._ .I _ II _I _ _ "i‘ 5",.___!-15 . _ ___ ._ _ ._ ' ‘é=I__IIfl. ta I‘ 1 2 L ? B '3 19 I nlt T.“:.:::lal‘--''_-~I"\ F m ''.'7'"ti‘'1"-t- .""".3’-.n.-__.-.-___t':,l'1 tin % m mu /I," "- ras-*~4tr Fig. l - Schematic geologic map of the Fordongianus area. I: Paleozoic schists. 2: Granites and minor Paleozoic gneiss. 3: Triassic dolomites. 4: Oligocene-Miocene anrlesite formation andlor lavas and pyroclastic brec- - cias with minor ignimbritcs. 5: Mostly rhyodacitic igniinhrites and related pumice and ash tuffs of Oligocene- '-»:a;';.t;4'='=§et!*t'-_£1=-".f-1:» 4,-t Early Miocene age. 6: Mostly marine sandy or marly sediments with underlying pumice-ash continental 11'.*3 deposits and minor limestones of Miocene age. T: Mt. Arci rhyolites and trachytes of Pliocene age. 8: Plio- -u.'-'5 t‘;- I|I\. cene basalt flows. 9. Quaternary mostly alluvial deposits. I0: Observed and inferred faults. 11: Fordon- idL. ‘ . gianus thermal springs. .1'IF:- :I|_ "is .\l' In the entire region, three geologically distinct parts can be differentiated (Fanfani et al., so-:=t"-"1 1986): 1) Paleozoic metamorphic granites and schists, 2) ignimbrites, tuffs and marine or continental sediments (Oligo-Miocene] and 3) Plio-Quaternary basaltic rocks and alluvial sedi- 1.

it?‘-*13?.’-wt:-'3'; ments {Fig I). All hydrothermal manifestations in this area are located within the fault systems, which $1;1'', to the east delimit the Tertiary Sardinian Rift. In the most important spring, which is the one it‘. {-5 close to Fordongianus I), water comes out of fractures related to the fault marl-ung the ignimbrite-andesite contact. L .

III._.{El‘ It is worth ohserving that a few kilometers south east of the spring, the Paleozoic basement .-1-,1 outcrops (Dettori et al., 1982). "" J1

wit- en

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THE roaoomcumus czornaaiur .tar..t {troy} 131

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39¢-55 stars *2“ _.3Q"5E = -=51“/_—I' flmflflflfl 'Q- .,. DHISTAN inflfl I I //W *3aQ I/7/i;i;.; aguml :39°5u I +’t.D o ' 3 ff; ' / ::i .. . __ 1 _ , I1}3;3°40.. 5°51:

Fig. 2 - Bouguer gravity anomaly map of the Fordongianus area [contour interval: 2 mfl-al}-

GRAMITY SURVEY

Gravity measurements were made in 290 stations uniformly distributed over the whole study area, with a density of about 1 station every 2 l<;m2. A LaCoste dz Romberg mod. C gravity meter with a scale constant K: 1.116 mCal/div was ‘I! used. The instrumental drift was very-moderate and the estimated mean error of measurement was ::i:: 0-03 mCal. The elevations were determined by trigonometric levelling, using an ASA mod. 14 Geo- dimeter and a Wild Model 12 Theodolite. The gravity stations were linked to the reference stations of the ﬁrst gravity survey of Sar- dinia [Trudu, 1962), which were connected to the Italian 1st Order Gravity Network (Marson and Morelli, 1978) I-and therefore to the IGSNTI (Morelli et al., 1974). In order to obtain the Bouguer gravity anomaly, the usual corrections were applied to the observed values. The terrain correction was extended up to the Hayford L zone, since the to- lgft Y1

132 BaI.I.~'t ct .-ti. 1/ // // i am»i// it asst I III E “I t

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I ihl1* l a 1 t. ~..__g_-—--_ <4’

"25 I_ -l-i~\;'-'|..£‘i.1:|tl|=--‘rim-.. “SIr- gI "-ﬂ_r__-——*='_.I_I r IIIIJI.-"' y \. -B.l_1.§i..t;l.1l-—-J ' ' I '9': I I II’\\ 413 ° "B l ‘ I _,__,,.,-=1 40 ° mi t

37'? HT T l

I/I I Ei '| I E 5 ussa I so Samug so ii ssﬂai/"°( I IQ lasts Siarnann I I_-5‘ l E i Flu B-5

I .Z¢hn| "I- lo "I i ..'-“ I

.a\ I — B°4ti Et°tiii l

Fig. 3 - Regional gravity anomalies in the Fordongianus area [Ix T3’ 40 ltm; contottr interval: 1 inGal}. |

' 1I

pographic relief in the study area was relatively moderate. The calculation was carried out on I a computer by considering the vertical effect of prisms whose square section increases as the I distance from the station becomes larger (Nagy, 1966). t Bouguer gravity anomalies were deduced using the normal gravity values calculated by __,I employing the IGF (Moritz, 1984). The assumed density value was the mean crustal density of 2.67 glcm 3. The estimated mean error for Bouguer anomaly values is of the order of 0.1 mGal. 3.;-Isat . Ii 1| Ii

BOUGUER ANOMALY MAP .p.I_

The Bouguer anomaly map (Fig. 2) shows three gravity highs. The fn*st high is positioned II north of Solarussa, the second soutli of S'iamaniia and the third north of'Siamanna. The first I two fall in the areas covered by low density Miocene-Quaternary formations. The third high corresponds to the area with partially outcropping dense Paleozoic rocks. The gravity low in the northern part of the map falls in an area where a laterally extended dense basaltic cover occurs. ..

.,I._-._.,I_.IIII_II.II,.,II:III

‘II I -_ _ - ,,,,I I-I - -.-:-. _. |_|1- '-._J-1-.I.__ _. - ._ .___....._._._ .

THE FOHDUNGIANUS GEU'I‘tiEFlM.-’tL AREA [IT.r'tLY] 133

B‘-‘tn B°m it lg I *“~t"' @ _ lq O I] /ll 49°“ l ® _l1B°uu /t-l/I i\~J :53) , g C) 9.:

0 - ' E _ 35.:-55' Cabraa 1 -I I _== lag,“ ‘ t . iman l t _' '\ y ISTAHU I H

|' 5- I (D

.l I ' L. l,_ QF‘I _;39°E-tt Hguﬁﬂi ‘ I /X I \ f\ /1"“, E‘-“tn Y B°su

Fig. 4 - Residual gravity anomaly map of the Fordongianus area {lt vi 40 lcm; contour interval: l mﬁall.

REGIONAL AND LOCAL GRAVITY ANOMALIES

The Bouguer map was separated into regional and residual components by means of two- dimensional filtering (Fuller, 1967; Bozzi Zadro and Caputo, 1968; Bath, 1974-), and using data taken from the gravity map of Sardina (Balia et al., 1983). The anomaly maps were obtained by employing a computer program which follows the methods suggested by Hessing et al. (1972) and Bolondi et al. (1977). Fig. 3 represents the regional anomaly map, where the extended gravity low can still be seen clearly ﬁll the NE border of the map. This low is probably due to a deep depression of a dense basement filled in by less dense rocks. The depression could be due to signiﬁcant local dislocations, especially along the Tirso river fault, the Margliine fault (about 20 km north of the study area) and a probable fault running approximately in the Paulilatino-Fordongianus direction. The residual anomaly map (Fig. 4) further emphasizes the characteristics already presented in the Bouguer map. In particular, the contraposition between the Ghilarza low and 134- B.*l.L.lA ct al.

I --‘ {-2 siiro _ 5°55; __ Ill - . I —______po _|-III! _ .JIlll I Ghiiarza ‘L'-'-..:=-t.-:.:E-:25---"'

. _ 5' i'.".n.'..-.A.A.'-_ FD9 4o°’oa~_ _4o‘*’os *" 0 Paulitatine , _..| <._| . |

i FDB , - . . A Etueach -.

. J /’ . 4oDoo,___ //,-/ I soU‘on ' -//:1 |=1:>1o _l ffgomongianus _

_-I-'-"_'-_'---PH-F.-F’-PH-_'-F.-F /

='_; FD2 I

A Solaruesa F05 li"""l""‘*-*"l:"i---'t‘="‘*"“""““"''1'-H-="-1-'_'=.:..'Ll‘s-§.~.;:£.'.'J,;“¢,u"=‘~'--|1~..=-.4:==;.-.-'-is-L-t-1--_=s.r=-I FD1 / ]

35335 |J_ 1--r-_.-'-.-'-_.-'-:-1-,-_-,_-J---;;-1.; '--.'-.-"-'\:'15--:'-_-1"’"-'--_-;_,..'.-..;|I...-",'-.|.;.i..,'-.-,-'t.‘.--5;.‘-"‘'-._.,.,_.'-'_'-.:|...... _-,_._r,.."'- .4 l. - B :| ll QFl$t3nD I

'i ..|

i -.

l s°-to 8°50 ‘- I

Fig. 5 -Location of the dipole-dipole geoelcctrical soundings along the two proﬁles AA‘ and BB’.

I the high of the central-southern part of the map is well shown. A 3D interpretation of the residual map will be presented after the discussion of the geo- 1 electrical survey. indeed, the geoelectrical data will be used as a guideline to the gravity m- I terpretation, in order to constrain both geophysical methods to a common structural model. t

1 ELECTRICAL SURVEY t

vr .

Ten electrical dipole-dipole soundings with axial electrode conﬁguration were carried out r ...r- in the zones where the main gravity anomalies occur. They followed two alignments oriented approximately N-S and E-W (see the sections AA’ and BB’ shown in Fig. 5). The measurements were carried out according to the method suggested by Patella et al. (1979). The voltage recordings were processed by means of the maximum likelihood spectral ana- -v lysis method (Lacoss, 1971; Loddo and Patella, 1977). Fig. 6 shows the apparent resistivity

experimental diagrams- |-IL-I-Mr-v-___

El! In order to carry out the quantitative interpretation, we smoothed the experimental dipolar 'i== ":'-!

l diagrams, generally affected by high wavenumber noise, by transforming them into equivalent .-1 -“§_| Schlumberger curves (Patella, 1974, 1984). ~-.-- Fig 7 shows the obtained electrical sections AA’ and BB’. Along the section AX’, between .i the soundings FD1 and FD4-, the layer having resistivity ranging between 8 and 32 Gm and thickness ran51'n5 between 2U and 250 rn can be associated with the alluvial deposits of mostly

Quaternary age and with the Miocene arenaceous sediments. Below, we find a conductive ho- -_--1-'-53

:-_|-_-

-L-uﬁ-ms-|-vi-Q.-4.-.-1-.-.-ti-I.-.'._.._._t,_._,,_,._'_-..-...... ; _-;_, _._J§F_';\'._;ig' .i" &_=,:.,i3_H I-,g‘.m___%,______,_ __, q ..||;|..aa:»..'.- =.--=-~-....--~-t-- -- '— ''''‘___ _ " ' ' ' _

I THE FORDONGIANUS GEOTHERMAL AREA [ITAL-Y) 135

1gi=-- -- 9 --- 10*‘-. ------twe — —+.

I 1"‘! I

I/'5

1031 xii 103 '1 ..--'\ ,' '1‘ E )1 E :

Q Ii. H. K ll gr s 1 l rse gs V i i

HQ |"\J -_H xi“--. -_HB“~ *1rr 5 4i— .. * *!_ h 4 In a—l C1 K ‘ f,*\;l-\O /‘J"-n..._* 1‘ J ii

:F1F' ityresisti 0.9.?‘ as ' : ,,, ,,_ 1, .. -t - resistivity ' ' ‘\ \. sass 6 sift is; l s s_ ,_mit 1 ~ Y7~a=~w/=,%;i- . I -l §r'*.:\ﬂ )6 Ii Y , ,9?) - H“, ‘git \, pp; t if lig--idle“ appa.rent trenappa 0‘ 31-‘{+"ﬂ ii’ 1'9: ‘ §‘I'2.- aw" J“ I

to s~.§¢.:§'-.8.“:s'i§#'¥‘H EH‘*as" i "__.- /*1 ‘ to - ‘Eiljt_ ;___,,1g__i»*ﬁ‘*s;_;' ;,5 m.__. " 7 "-.,,, ,asE’ )5 ~.\,1' Igt , r I ' -tI

1 . . Ema». t 1 » to? 103 10* 102 103 10* spacing lml s-"pa.ci.ng (ml

Fig. 6 - Experimental dipole-dipole apparent resistivity diagrams.

rizon (5 : 10 Qm) with significant thickness, varying from 300 to 1400 m, which can be iden- tified with the Miocene-Pliocene marly-clayey sediments of the Campidano graben. Underlying the conductive horizon, there is a deep resistive body (315-800 ilm), which can be associated with the massive Tertiary andesitic volcanites that filled the Sardinian Rift in Oligocene-Miocene times. Between FD4 and FD10, there is an abrupt lateral change of resistivity, which indicates the presence of an important discontinuity interpreted as a subvertical fault. ln FD10 a weakly conductive body (34 Qm), attributed to metamorphic schists, is clearly evident down to the depth of 1300 m, where the resistive basement (5500 Qrn), associated with dry and compact granites, is detected. The conductive body is also in lateral contact with the basement towards the east. Roughly midway between FD10 and FD5, the granitic basement emerges. Below FD5, a thin surficial, moderately resistive layer is found. it can be ascribed again to the granitic basement, which "has a lower resistivity due to extensive weathering. From surface geological evidence, the sequence of the electrical layers below FD6 can be given the same interpretation as below sounding station FDl0 (see section AA’). More spe- cifically, FD6 shoiws the presence of a conductive body (25 Um) down to more than 700 m of depth. This body can be ascribed to the Paleozoic schists, which outcrop all over the area occupied by Mt. Grighini, and probably overlie the granitic basement. ' The low resistivity may be due to high iron content (especially hematite). Below sounding stations FD7, FDB and FD9, a thickening of the very conductive [6-18 Qm) alluvial deposits and Miocene sediments, with probable underlying altered volcanites, is present down to a depth of about 1000 m. This is in good agreement with the gravity data,

i-—"'—I.v_»-.

i i i

X I I | I I I

36 sstu. ct al. THE Foano;

FD? FD2 FD3 F64 Foe FD5 l I I I I I A _*‘§P . ,5 ' y . an ______." - ' -- -"___-,5 — _ _,:-».-a,,- __ ;'FH____.;—_;__,-,,g_F—'I--———---"- " to __ 5.1. ts _ _ - - , ' : 1» is . ; III | ' I - 7 II I I I i -’ em . I-011--I/. ______- ._F-"' , 1 iI ,u_= ' y’ I 40 9 MT, : ‘_ -‘I 1//rtI . ‘I M , i |. I I

II I I 1’, ’IIII i y - I 13Ju_.-_-’» '

II IIOCII I rIII I I II 55°F’ I 1664 i

375 I I ' iI

I‘ I I 40‘i'm|I‘\ . _,¢

-.. o_12a4,,m

F10? spa Flor FlD8 spa ' I Z

B 4 5“ BI I " 7 / _|- 45 ' ' my ‘ _1 _ - s-i. __ - . _ X — I0 _ "_‘ 5 , t as‘ an '9 tn F -. I ._---rm mm. I11. » 5 . w— -'“ -I I‘. -= ' := 39°'a"sg1 to l_ _ \.\ \ 6 5'50‘? . \ t I /.'-'I|II ___ ._?1s:.__-___I __ III ‘ix 2250 \ , B J sou »' . , _ , Ii I ‘ III~ I I"- _ -91“ !j— — —- —--______1555 I fr x I I \_ _i : '"“"--—- —, It

=|. II,II I=n2so_. \ ‘I' 1 t‘ -_ Fl€~ 3 - Synth

| II

Igiﬁ I |IJ4IIIi I559- I I 1 111 partit - crossed by t having heigl the data rep the rock unit then ﬂssigne I I Elf cours

- U 1 2 3 4 km I116 Sfifitflfg F

, _ _ _ , _ I 111 a some h, Fig. 7 - Ceoelectrtc cross-section along the proiiles All la) and BB (b) ofF1g. 5. Cuuld he I:IIak

especially in the area of F118 and FD9 where a wide extensive minimum occurs (Ghilarza low). I Table ' Rvﬁivlli Finally, the deep resistive body (400-650 Qrn) may be identiﬁed with the massive andesitic volcanites, as below soundings FD1, FD2 and FD3 along the section AA’ (see Fig. 7a).

QUANTITATIVE INTERPRETATION OF GRAVITY DATA

A 3D interpretation of the Bouguer residual map of Fig. 4 was performed by the trial- and-error forward approach, using the modelling technique suggested by Nagy (1966). The geometry of the geoelectrical bodies was assumed as input condition to the gravity modelling. The procedure of closely combining gravity and deep geoelectrical data has already been success- fully applied in the Logudoro basin, Sardinia (Pecorini et al., 1938) and in the Etna area, Si- I cily (Loddo et al., 1989).

- - ..- --.---- —~.——'-._,:._ - - — —_ I _ -1- — _ — _ ' -- -'.—I—- -I-|| ||.|.,.,,.-q,‘-I _ ..._....z...s~_..= -_-,- t..¢.._.=,._._._._._-._.-_l_.._,. ... _.._ _. ._

ran rosoortctwaus csorttsai-mi. .»tns_»-t qzratvi 131

___:5“e!ﬂ__ é _ _ cB°_m_ E

|' I I ' I or Q’ it == /l 4'3°°5+- "*‘4tJ"us

@ts *1/if ‘<1 C, rt /,, ?@@*\‘ Bchi 4Donn._\0 \ I ‘F ‘wow asi Man _

Lo @ F3Q /Q10 kmC'5 Q 3E|o5s|= Tl L ‘K: ll Y“ V __ ._.., . T39 55

v ‘ i 8°11 -_-_—ﬁ J-“H mm.“

Fig. 8 - Synthetic: residual gravity anomaly map of the Fordongianus area {contour interval: 1 mGal).

In particular, the area was divided into square sectors with sides 2 km long. The sectors crossed by the resistivity proﬁles AA’ and BB’ were shaped by a pile of right paralielepipeda having heights closely conforming to the thicknesses in the resistivity sections of Fig- 7. Using the data reported in Angenheister (1982), typical ranges of density values were attrihued to the rock units associated with the geoelectrical bodies, as in the table. The prismatic units were then assigned initial density values, chosen within the selected ranges.‘ Of course, the initial height and density values of all the prismatic elements belonging to the sectors placed outside the areas covered by the geoelectrical investigation, were chosen in a sornewhat arbitrary way, as only the qualitative pattern of the Bouguer residual anomalies could be taken into account.

Table - Resistivity and density ranges associated with the rock l;}'1lBB existing in the For-dongianus area

resistivity density _p (_Qm)p prick _pp{slf:ra__3) _ 5 F 18 Clayev and marly-clayey sediments; 1.6 - 2.5 altered volcanites and ignimbrites

B - 32 Quaternary alluvial deposits; L9 - 2.1} Miocene arenaceous sediments

25 - 250 Metamorphic schists 2.4- - 2.9

315 - BOO Massive Tertiary andcsitic volcanites 2.4 - 2.9

:I?r._1000 _ _ “___ Granites -v2.3 ||__|i|__|__||_|__:|__I_|__:__

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THE FORDONCIANUS GEOTHEHI‘-dal. ARIIA {IT.»‘tI.Y) 139

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Fig. 10 - Geologic cross-sections of the Fortlongianus area along the proﬁles AA’ la) and BB’ {bl of Fig. 5. 1: Locally altered Palaeozoic schists (Tirso Valley}. 2: Mostly phyllitio schists. 3: Granites and minor gneisses. 4: Uhgocene-Miocene andesitic lavas and breccias and some minor ignintbrites. 5: Locally very altered Oligocene-Early Miotzene ignirnbrites and pumice-ash tuffs. 6: Miocene mostly sandy-marly marine sediments and some calcareous deposits with underlying tufts and sandstones of lacustrine facies. 7: Middle- Upper Pliocene alluvial and lacustrine deposits {mostly conglomerates and niarls of the “samassi formation“). 8: Pliocene basalts flows. 9: ll/lostly alluvial deposits of Quaternary age.

By iterated adjustements of the initial density values, and in some cases, also by strong modification of the height of the prismatic units where no geoelectrical tracer could be followed, the synthetic gravity map of Fig. 8., matching the original residual map of Fig. 4-, was finally obtained. The gravitational effect of the prisms was computed by considering the density differences of the various bodies with respect to the mean crustal density used in the gravity corrections. The comparison shows that a very satisfactory fit was obtained. For a better visualization of the results from the combined gravity and geoelectrical modelling, the two sections AA’ and BB’ were reconstructed as in Fig, 9 (a and b). The final density values are reported in the corresponding legends.

GEOLOGICAL INTERPRETATION AND CONCLUSIONS

On the basis of the above geophysical results, the two sections AA’ and BB’ were given a geological representation down to the depth of about 2000 m, as in Fig. 10. The pattern of the granitic-metamorphic basement is of particular interest. It outcrops in the east and south and deepens sharply just near Fordongianus to give place to the extensive Oligocene-Miocene andesitic formation. This transition seems to coincide with the Tirso river fault, a well known tectonic line, whose throw is of the order of at least QOOU rn. The fault activity started either before or at the same time as the volcanism. The throw is therefore a fault scarp which supported the whole series of andesites, ignimbrites and sediments from the Oligocene throughout the Miocene. The lithotypes which were recognized in the area at the investigated depths are characterized by middle-to-low permeabilities (Caboi et al., 1982; Fanfani et al., 1986]. Thus, we may infer that the reservoir feeding the thermal springs should be located inside more or less deep fractures of the paleozoic rocks. The interpreted geological sections of Fig. 1O show that some of these fractures outcrop, but the majority of them are buried under an impermeable cover. In conclusion, the tectonic picture in the area where the Tirso river and the Fordongianus- Paulilatinofault alignment cross appears most interesting and warrants the application of more detailed methods of detection. 1 .,,,;, BALM a al.

Acknowledgements The authors wish to thank the referee prof. Francesco lltlongelli for his useful suggestions for improving the paper. Research carried out with financial contribution from the Italian National Research Council {Contract n- 8?-02223] and from the EEC {Contract n. EC)-'t 2-O53).

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