Geophysical Study of the Fordongianus Geothermal Area (Sardinia Island, Italy) I
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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 struc- tures 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. 4 N I 130 BALIA ct al. s"4o also | """'l"'- 2'. -""'““Tl'.f"i"' —‘-“'3 "' I‘ I " - '. 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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 con- tinental 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 t, -.1 F-l.-ll “EH 1:2rt -5-=l :1‘ .3" ------1----rt-|—_|-or-rr-r-.-1-1.-It--_||-w'|.-?\t\ .-1 -. .I. .- -_.I-.;- I.I . -,-..I.-I.,-I-|,- I -' II ;.,|.- -_--:_.:- _ I. _I. I 1II:-II? I . -__ _-I II‘ . .-II. I ,I,-. I_ . 5___, I.I I I I . I_:I :- _III -.I - - - --‘--.- -._ .'-Lj.-'.-.. *.-_ -|-+'..-'-=:.'--.. .-. 1..-- ---.:-- ".---I‘-_.-.1-,I‘ .. ---=-_ IIt _.-5---1-..-.. I- - I._:-.-.-.j.-_.-,,+-t:'|I-I-. 4,- - . _ . ' -___.. ' .. -.=..-t.-tr.-.i._-._-____. .. -. THE roaoomcumus czornaaiur .tar..t {troy} 131 B°4u B°su E I C1 -ll" - 1' ' J- 1lfluBl._, 400“ it I '9 b I I 0 l I H111 I I I Bl-I I . kip _ IOIIII III | llflnllfl 051» ' ""'-~..___..r"j4go“ I 1’I Furttnn n EU 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 I I I I I I I it It “I Eh‘ I i MED 1 '1 4o=*e_It , t I II EI t*,I I /A-ttJ°ttltr I ihl1* l a 1 t. ~..__g_-—--_ <4’ "25 I_ -l-i~\;'-'|..£‘i.1:|tl|=--‘rim-.