Surface-subsurface structural architecture and groundwater ... · tural geology techniques,...

ABSTRACT

A multidisciplinary integrated approach was used to study thestructural architecture influencing the circulation pattern of geother -mal fluids in the Equi Terme area (NW Alpi Apuane, Tuscany). Geo-logical-structural surveys were carried out to define the structuralsetting of the area and to characterize geometries and kinematics offault systems. Chemical (major components) and isotopic analyses(δ18O‰, δ2H‰, 3H, δ13C‰[DIC], δ34S‰[SO4]

) were performed onthermal water and cold springs. A geophysical survey was also con-ducted by means of both Magnetotelluric and Electrical ResistivityTomography methodologies, in order to gain insight into theresistivity distribution at depth and to indirectly image the subsur-face structure. This multidisciplinary approach proved to be apowerful tool, since it unravels the complexity of this naturalgeothermal system and provides useful suggestion for reconstructingthe fluid circulation outflowing at the Equi Terme thermal spring.Results pointed out how the E-W oriented fault system (the EquiTerme Fault) play a key role in controlling the thermal groundwateroutflow, and the chemical-physical features of this resource. Thisstructural lineament separates high permeability carbonate com-plexes (footwall), in which both shallow and deep flow pathsdevelop, from a medium-low permeability succession (hangingwall)that contains evaporitic formations from which thermal wateracquires a high salinity and a composition of the Na-Cl (Ca-SO4)type. During the uprising along the fault system, the thermal water isalso affected by a mixing with shallow fresh-cold waters that lead toa strong seasonal variation in the chemical-physical properties of thethermal springs.

KEY WORDS: Fault zone architecture, low temperaturehydrothermal systems, geophysics, hydrogeology, hydro-geochemistry, structural geology, Alpi Apuane, Italy.

INTRODUCTION

A comprehensive understanding of fault zone hydro-geology may be derived from integrated surface and sub-surface interpretation and modelling by combining struc-tural geology techniques, geophysical methodologies andhydrogeological and hydro-geochemical survey analyses.

In the geological literature is well established thatfault zones and related structures in shallow crust (< 2 Km)play a major role in introducing permeability hetero-geneity and anisotropy affecting and controlling regionaland local groundwater flow (see for a review BENSEet alii, 2013 and references therein). Fault zones mayhave, however, important control on subsurface fluidflow (MCCAIG, 1988; SIBSON, 1992, 1996; ANTONELLINI &

AYDIN, 1994; FOSSEN & BALE, 2007; MILLER et alii, 2008;LIOTTA et alii, 2009), acting as localized conduits, barriersand (or) combined conduit barrier (e.g. CAINE et alii,1996; KIRSCHNER & KENNEDY, 2001; RAWLING et alii,2001; CELLO et alii, 2001a,b; STORTI et alii, 2003;MICARELLI et alii, 2006; BROGI, 2008; FAULKNER et alii,2010; MOLLI et alii, 2010) since permeability may be sev-eral orders of magnitude different from the host rock(EVANS et alii, 1997; CAINE & FORSTER, 1999; AGOSTA etalii, 2007; BALSAMO et alii, 2010). In addition, fluids chan-nelized within fault zones strongly influence the mechani-cal behaviour and the mechanics of faulting, thus playinga fundamental role for earthquakes in the shallow andmiddle crust (e.g. SIBSON, 1992, 1996; SCHULTZ & EVANS,1998; FAULKNER & RUTTER, 2001; KENNEDY & WHITE,2001; LABAUME et alii, 2004; MICKLETHWAITE & COX,2004; WHIBBERLY et alii, 2010).

Major faults and related structures, as well as fluidpathways can be imaged by the so-called hydrogeophysicalmethods. Among these, electrical and electromagnetic (EM)methods are the most commonly used for determininghydrogeological parameters and processes, thanks to thestrong effect these latter have on the electrical propertiesof the subsurface (e.g., NOBES, 1996). EM methods arebased on the definition of electrical resistivity distributionon the subsurface, which in turn is strongly controlledby the lithology, water saturation, chemistry and flow ofgroundwater.

We present the results of a multidisciplinary inte-grated study performed to gain insights into surface andsubsurface architecture and circulation pattern of geot-hermal fluids in the Equi Terme area (NW Alpi Apuane,Tuscany). This area is characterized by high fluxes of coldgroundwater, as testified by karst springs (average flowrates in the range 50-200 L/s). The Equi Terme hydrother-mal system is drained by various springs and one, single,water well. The local thermal spa is fed by the mainspring, with a flow rate of the order of 10 L/s and anaverage temperature of about 23°C. Mixing processesinvolving shallow and deep groundwater lead to theseasonal variability of water temperature and salinity(CIONI et alii, 2007), with subsequent problems for thewater resource management by thermal spa.

Taking into account the normal thermal gradient of theApuane region, a groundwater circulation of several hun-dreds of meters of depth is required to justify the highertemperature values of thermal springs (about 27°C) withrespect to cold springs (10°C) in the dry season.

In order to provide useful results also in terms ofplanning of the thermal resource exploitation, our inter-

(*) Dipartimento di Scienze della Terra, Università di Pisa, Italy.(**) CNR - Istituto di Geoscienze e Georisorse, Pisa, Italy.(***) Via Pozzuoli, 2 - 54100 Massa (MS).

Surface-subsurface structural architecture and groundwater flowof the Equi Terme hydrothermal area, northern Tuscany Italy

GIANCARLO MOLLI (*), MARCO DOVERI (**), ADELE MANZELLA (**), LIVIO BONINI (*), FLAVIA BOTTI (*),MATIA MENICHINI (**), DOMENICO MONTANARI (**), EUGENIO TRUMPY (**), ALINO UNGARI (*) & LUCA VASELLI (**) (***)

Ital. J. Geosci. (Boll. Soc. Geol. It.), Vol. 134, No. _ (2015), pp. 00-00, 12 figs. (doi: 10.3301/IJG.2014.25)© Società Geologica Italiana, Roma 2015

In acquires a high salinity and a composition of the Na-Cl (Ca-SO

In acquires a high salinity and a composition of the Na-Cl (Ca-SOtype. During the uprising along the fault system, the thermal water is

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In also affected by a mixing with shallow fresh-cold waters that lead toa strong seasonal variation in the chemical-physical properties of the

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Terme Fault) play a key role in controlling the thermal groundwateroutflow, and the chemical-physical features of this resource. This

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outflow, and the chemical-physical features of this resource. Thisstructural lineament separates high permeability carbonate com-pre

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a strong seasonal variation in the chemical-physical properties of the

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eral orders of magnitude different from the host rocket alii

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presset alii, 2010). In addition, fluids chan-

press, 2010). In addition, fluids chan-

nelized within fault zones strongly influence the mechani-

pressnelized within fault zones strongly influence the mechani-

cal behaviour and the mechanics of faulting, thus playing

presscal behaviour and the mechanics of faulting, thus playing

a fundamental role for earthquakes in the shallow and

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middle crust (e.g. S

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Major faults and related structures, as well as fluidpress

Major faults and related structures, as well as fluidpathways can be imaged by the so-called hydrogeophysicalpre

sspathways can be imaged by the so-called hydrogeophysicalmethods. Among these, electrical and electromagnetic (EM)pre

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disciplinary study focused on how the surface and sub-surface architecture influence on the mechanisms ofthermal-cold water outflow and mixing processes.

Geological-structural surveys were carried out todefine the structural setting of the area and to characterizegeometries and kinematics of fault systems. Water sam-pling was performed in different periods, during whichboth thermal and cold water samples were collected andanalysed to define chemical (major components) andisotopic (δ18O‰, δ2H‰, 3H, δ13C‰[DIC], δ34S‰[SO4])properties. A geophysical survey was also conducted bymeans of both MagnetoTelluric (MT) and ElectricalResistivity Tomography (ERT) methodologies, in order togain insights into the resistivity distribution at depth.

REGIONAL SETTING

The Equi Terme hydrothermal area, object of thisinvestigation, is located at the northern border of the AlpiApuane in the inner Apennines of NW Tuscany, Italy (fig. 1).

In the investigated area (fig. 1a,b) the completenorthern Apennines nappe stack can be observed fromthe lowermost Tuscan Metamorphic units (Massa andApuane units) of the Alpi Apuane tectonic window, to theoverlaying unmetamorphic Tuscan nappe, sub-Ligurianand Ligurian units. Whereas the latter are mainly formedby Cretaceous to Eocene deep water sediments includingremnants of Jurassic oceanic crust, the Tuscan unitsderived from the distal part of the Adriatic continentalmargin (e.g. ELTER, 1975; BERNOULLI, 2001).

The lowermost unit exposed in the studied area(figs. 2, 3) is represented by the Apuane unit (MOLLI &MECCHERI, 2012) which shows a lithostratigraphicsequence made up of a Palaeozoic basement (mainlyphyllites and metavolcanics CONTI et alii, 1993) uncon-formably overlain by an Permian to Oligocene-Miocene(?)metasedimentary succession. The Mesozoic cover includesTriassic continental to shallow marine water deposits(“Verrucano” Group) followed by Upper Triassic-Liassiccarbonate platform metasediments comprising dolo-stones (“Grezzoni” Fm.), dolomitic marbles and marbles(the “Carrara marbles” Fm.). These latter formationsare followed by Upper Liassic-Lower Oligocene chertymetalime stone, cherts, calcschists and sericitic phyllites.The Oligocene-Early Miocene(?) turbiditic metasand-stones (Pseudomacigno fm.) represent the youngest unitsof the metasedimentary sequence (PATACCA et alii, 2013and references therein).

The Tuscan nappe includes similar Mesozoic to Ter-tiary terms detached from their original basement alongthe former Upper Triassic basal level of evaporites, to becompared with the Anidriti di Burano (CIARAPICA &PASSERI, 2002 and references therein). The latter, formedby an original alternation of dolomite and anydriteincluding patches of halite, may be observed in most ofthe outcropping areas as carbonatic tectonic brecciacalled “Calcare Cavernoso” (ZACCAGNA, 1932; VIGHI,1958; BALDACCI et alii, 1967; GANDIN et alii, 2000).

The deformation structures within the lowermostmetamorphic unit may be related to Oligocene- earlyMiocene mid-crustal underplating and antiformal stack-ing and include a main axial-plane foliation of isoclinalfolds observable up to a kilometer-scale (figs. 1, 2, 3, 4)associated with a regionally NE-oriented stretching

lineation (D1 structures of CARMIGNANI et alii, 1978;MOLLI, 2008). The D1 structures were reworked by differentgenerations of folds and high-strain zones, related to syn-contractional exhumation of the metamorphic unit in theinner portion of the northern Apenninic wedge (D2 struc-tures of CARMIGNANI & KLIGFIELD, 1990; MOLLI & MEC-CHERI, 2012 and references therein). Available P, T, t dataof the Alpi Apuane metamorphism are summarized inMOLLI et alii (2002); FELLIN et alii (2007 and referencestherein). According to these authors, peak metamorphism(temperature around 450-350°C and pressure of 0.6 GPa)and early deformation D1 occurred during the earlyMiocene at 27-20 Ma (KLIGFIELD et alii, 1986), whereasthe syn-metamorphic D2 structures developed at T higherthan 250°C and predated 11 Ma according to zircon fission-track ages of FELLIN et alii (2007).

In contrast to the metamorphic units, the Tuscannappe was accreted at a shallow crustal level within thenorthern Apennines wedge beginning in early Miocene(CERRINA FERONI et alii, 2004; MOLLI, 2008). Burialoccurred under a sequence of thrust sheets now preservedin the overlying sub-Ligurian and Ligurian units (figs. 2, 3).Early thrusting is documented by top-to-the east smallscale shear zones and an early generation of east vergenttight to isoclinal folds within incompetent stratigraphiclayers (GIANMARINO & GIGLIA, 1990; CARTER, 1992; MOLLIet alii, 2011).

The early formed structures in the Tuscan nappe weresubsequently overprinted by small to large (kilometer-scale) refolding associated with sub-horizontal crenula-tion cleavage in pelitic rock units, low-angle normal faultsand later upright folds (CARMIGNANI et alii, 1995; STORTI,1995; CAROSI et alii, 2003). Metamorphism to anchizonegrade (CERRINA FERONI et alii, 1983; MOLLI et alii, 2011)during burial and deformation has been tectono-strati-graphically constrained to a maximum depth of 7 kmfor the Macigno sandstones (REUTTER et alii, 1978;MONTOMOLI et alii, 2001; MONTOMOLI, 2002; FELLIN etalii, 2007).

The youngest deformation structures affecting allunits of the nappe stack are represented by faults andrelated structures, developed since 4-5 Ma (FELLIN et alii,2007; MOLLI et alii, 2010) and still active today as aresponse of crustal-scale extension in the inner northernApennines (e.g. ELTER et alii, 1975; BENNETT et alii, 2012and references therein).

LOCAL GEOLOGICAL FRAMEWORK

In the study area different generations of deformationstructures are recognized on the base of overprinting rela-tionships as described as follows.

The lowermost Apuane unit is mainly represented byEarly Jurassic to Late Jurassic dolomitic marbles, mar-bles and cherty metalimestones. These metasediments areinvolved into hectometer to decimeter scale isoclinal D1folds (figs. 1, 2, 3) showing a medium to steep dippingaxial planar foliation (fig. 4c). The fold axes of thesestructures, although quite scattered, shows a mean trend(fig. 4d) sub-parallel to the main stretching direction(N60E), hence highlighting that the tectonic frame of thearea is related to a kilometre-scale hinge zone of thehighly non cylindric Vinca-Forno anticline (MOLLI &MECCHERI, 2012 and figs. 1b, 2, 3).

2 G. MOLLI ET ALII

In sequence made up of a Palaeozoic basement (mainly

In sequence made up of a Palaeozoic basement (mainly

et alii

In et alii, 1993) uncon-

In , 1993) uncon-

formably overlain by an Permian to Oligocene-Miocene(?)

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derived from the distal part of the Adriatic continental

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The lowermost unit exposed in the studied areapress

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in the overlying sub-Ligurian and Ligurian units (figs. 2, 3).Early thrusting is documented by top-to-the east small

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Early thrusting is documented by top-to-the east smallscale shear zones and an early generation of east vergent

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scale shear zones and an early generation of east vergenttight to isoclinal folds within incompetent stratigraphic

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SURFACE-SUBSURFACE STRUCTURAL ARCHITECTURE AND GROUNDWATER FLOW 3

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North ApuaneFault System

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QuaternarycoverLigurian unitsand Tuscan nappe

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Alpi Apuane Metamorphic Complex

Paleozoic basement

metaconglomerates,phyllites and schists(Ladinian-Carnian)

Paleozoic basement

dolomite (Norian)

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cherty limestone, radiolarianchert, schists, turbiditic flysch(Lower Lias pp-Oligocene)

Apuane unit

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Ligurian units

Pliocene-Pleistocenedeposits

Olocene deposits

Fig. 1 - a) Simplified tectonic sketch of the Alpi Apuane and surroundings with the location of the study area; b) Simplified geological map ofthe Alpi Apuane with the location of study area and detailed geological map of fig. 2. The dashed lines show the axial traces of the major D1and D2 regional folds.

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4 G. MOLLI ET ALII

Fig. 2 - Detailed geological map of the Equi Terme surroundings with ubication of the trace of cross-sections of fig. 3, overflow springs andwater well part of Equi Terme hydrothermal area.

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The steep attitude of D1 structures (figs. 3, 4e,f) is dueto a later refolding in a kilometer-scale D2 fold whoseminor structures are represented by pluri-decameter tocentimeter east-west trending, N-NW verging, open totight folds associated with a sub-horizontal crenulationcleavage (fig. 4e,f).

The main structure of the Tuscan nappe is repre-sented by its former basal thrust reactivated as a lowangle normal fault (fig. 1b,3) during syn- to post contrac-tional exhumation of the underlying metamorphic units(CARMIGNANI & KLIGFIELD, 1990; FELLIN et alii, 2007;MOLLI, 2008). The contact, locally trends approximatelyWSW-ENE and it is marked by cataclastic carbonatebreccias mapped as “Calcare Cavernoso Fm.”. Subsidiarylow-angle normal faults are also recognized through theTuscan Nappe, as suggested by local excision of strati-

graphic succession as well as by the variability in thick-ness of the different stratigraphic terms (e.g. the ScagliaToscana Fm.) affecting an original non-basinal (seamount-related) sequence (more details and references in FAZ-ZUOLI et alii, 2002; MOLLI & MECCHERI, 2012).

The Jurassic to Tertiary terms of the Tuscan Nappeare exposed along the Equi Terme valley in the low-angleN-dipping (figs. 3, 4a,b) normal limb of the regional kilo-meter-scale fold structure (the Tenerano Fold) wellexposed more south/south-east in the surrounding area ofCarrara (DECANDIA et alii, 1968; CARMIGNANI et alii, 2000;DEL TREDICI & PERILLI, 1998).

The Jurassic to Tertiary terms of the Tuscan Nappesuccession are tectonically juxtaposed to the Triassicbasal levels of the Calcare Cavernoso Fm. and/or to themetamorphic units (figs. 2, 3) through a pluridecameter


Fig. 3 - Vertical cross-sections across the Catenelle creek.

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thick N-NW dipping fault zone – the Equi Terme Fault –,representing the main structure controlling ground-waterflow of the hydrothermal system as described in detailsbelow.

HYDROGEOLOGY AND GROUNDWATER FLOW

The Equi Terme hydrothermal system develops intothe north-western part of the Alpi Apuane, where sev-

eral hydrogeological studies have been carried out bydifferent authors (MASINI, 1960; PICCINI & PRANZINI,1989a,b; PICCINI, 1992; BALDACCI et alii, 1993; PICCINIet alii, 1997; PICCINI et alii, 1999; PICCINI, 2001;DOVERI, 2004; DOVERI et alii, 2013). The area is charac-terized by significant groundwater resources (CIVITA etalii, 1991; PICCINI et alii, 1999) due to high rainfall, themedium-to high permeability of the rock units alto-gether with an highly effective infiltration coefficient,which can reach the 75% (PICCINI et alii, 1997). The

6 G. MOLLI ET ALII

Fig. 4 - Equal area lower hemisphere stereograms showing: (a) poles of bedding of Ligurian units; (b) poles of bedding Tuscan Nappe; (c) polesof main foliation D1 (Sp) in the Apuane unit; (d) axis of main phase (D1) and stretching lineation (Ap/Lp) of Apuane unit; (e) poles of latephase (D2) crenulation cleavage (St) of Apuane unit; (f) axis of late phase (D2) and intersection lineation Sp/St of Apuane unit.

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main aquifers of the Apuane unit are hosted within car-bonatic units mainly represented by the dolostone andmarble (“Grezzoni” and “Carrara Marbles” Fms.), whichare characterized by an high secondary permeability, asresult of karstification processes. Medium-low to lowpermeability units (mainly metasandstones and slates)exist at the top of the carbonate units, and someaquitards (mainly metaradiolarites and calcschists) orminor aquifers (e.g. cherty metalimestones) are associ-ated to the main ones. In the general hydrogeologicalsuccession, the phyllites and metavolcanics of the Paleo -zoic basement play the role of an impermeable substra-

tum (fig. 5); due to the vertical attitude, these forma-tions can also represent lateral impermeable limits,thus leading to the distribution of groundwater flowinto different hydrogeological basins.

The carbonate units of the Tuscan Nappe (CalcareCavernoso, Calcare Massiccio and Maiolica Fms.) consti-tute potential aquifers. These carbonate units are how-ever associated and/or enclosed within medium-low tolow permeability complexes existing at the top (sand-stones and shales) of the succession, altogether withaquitards (radiolarites and marls) or minor aquifers (e.g.cherty limestones) (fig. 5).


Fig. 5 - Hydrogeological sketch map of theNW Apuan Alps: 1) faults and tectoniccontacts; 2) groundwater stream lines; 3) major groundwater divides; 4) trace ofcross-section showed in fig. 12.

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In the Alpi Apuane the groundwater flow is primarilycontrolled by structural grain related to large scale folds(D1 structures) producing confined aquifers enclosedwithin impermeable or less permeable units as well as bythe discontinuity related with fault surfaces. Moreover,karst phenomena significantly affect the groundwaterflow in carbonate lithologies (PICCINI et alii, 2007). Fig. 5shows the groundwater stream lines and some mainhydrogeological divides, achieved from various informa-tion such as springs altitudes, morpho-structures condi-tions, water level collected in karst conduits (in saturatedzone) and results of the dye tracing tests (PICCINI &PRANZINI, 1989; PICCINI, 1992; PICCINI et alii, 1999; PIC-CINI, 2001; PICCINI, 2002; RONCIONI, 2002). Stream linespath suggest a wide recharge area of the Equi Termesprings developing up to the mountains of the Tambura-Grondilice area (fig. 5).

According to the general scheme proposed by DOVERI(2004) and BALDACCI (1993), two types of groundwatersystems outflow in the investigated area. A shallow system,chiefly drained by three cold springs (ET8, ET10 andET11 in fig. 2) located close to the local karst base level; adeeper and thermal system, represented by various over-flow springs (e.g. ET1, ET3 and ET7 in fig. 2) and one well(ET2), which are distributed along the Catenella creek.

Chemical (major components) and isotopic analyses(δ18O‰, δ2H‰, 3H, δ13C‰[DIC], δ34S‰[SO4]) were per-formed in the 2007-2008 period on thermal water (ET1, 2,3, 7), stream water (ET4 and ET9, respectively upstreamand downstream in relation to the thermal water points)and cold springs (ET8, 10, 11). Samples were collected indry, wet and intermediate seasons. Sampling fields werein any case carried out after a significant period withoutrainfall, thus avoiding the short-term effect of the neo-infiltration water. Temperature, electrical conductivity(EC), pH, total alkalinity, and flow rate (only for ET1, themain thermal spring), were measured on site. Chemical,

water isotopes and δ13C‰[DIC] analyses were performed atthe laboratories of the Institute of Geosciences and EarthResources (IGG-CNR, Pisa, Italy), whereas δ34S‰[SO4]

was analysed at the Stable Isotope Laboratory of the Uni-versity of Reno (Nevada, USA).

As previously found by CIONI et alii (2007), the chemical-physical data obtained in the 2007-2008 surveys indicatea strong effect of the rainy regime on the quality of ther-mal groundwater that outflows at Equi Terme. The maxi-mum temperature (27°C) and ion concentrations(TDS=4900, Cl=2100 and SO4=800 mg/l) were measuredat the end of the dry season (early autumn), whereas aconsistent decrease in chemical values (TDS=3800,Cl=1700 and SO4,=600 mg/l) and temperature (21°C) wereobserved after the rainy period (late spring). ET1 dischargeswere in the range 9-12 L/s.

In the LL diagram (LANGELIER & LUDWIG, 1992) offig. 6a the thermal water belong to the NaCl facies,whereas the cold water shows a CaHCO3 composition.Also the diagram of fig. 6b clearly confirms that the fluxof the thermal water springs originates from a mixing ofCaHCO3 fresh water and NaCl saline water.

Despite their NaCl composition, the thermal waters arecharacterized by significant amounts of SO4 and Ca. Thesefeatures, together with the δ34S‰[SO4] value of 15.6 δ‰representative of Triassic rock (CORTECCI et alii, 1985;BOSCHETTI et alii, 2011), suggest a water interaction withTriassic evaporites at the bottom of the carbonate sequence.

Unlike the chemical parameters, δ18O‰ and δ2H‰values of thermal water were observed to be almost stableover time. Indeed, while the chemical dilution observedin the wet season was significant, the isotopic valuesshowed a minimal increase (close to the analytical errors)with respect to the dry season (from –7.64‰ to –7.49‰and from –47.8‰ to –44.7‰, for δ18O and δ2H, respec-tively). Such behaviour indicates a similar rechargeaverage altitude for shallow and deep groundwater flow

8 G. MOLLI ET ALII

Fig. 6 - a) LLCl square diagram; b) LLCl cross section: 1) thermal water sampled at ET1,3,7 springs and at ET2 well; 2) cold springs(ET8,10,11); 3) Catenelle Creek sampled upstream (ET4) and downstream (ET9) of thermal water points.

In et alii

In et alii

shows the groundwater stream lines and some main

In shows the groundwater stream lines and some mainhydrogeological divides, achieved from various informa-

In hydrogeological divides, achieved from various informa-tion such as springs altitudes, morpho-structures condi-In tion such as springs altitudes, morpho-structures condi-tions, water level collected in karst conduits (in saturatedIn tions, water level collected in karst conduits (in saturatedzone) and results of the dye tracing tests (PIn zone) and results of the dye tracing tests (P

ICCINI In ICCINI et aliiIn et alii, 1999; PIn , 1999; P, 2002). Stream linesIn , 2002). Stream lines

press

press

presscross section: 1) thermal water sampled at ET1,3,7 springs and at ET2 well; 2) cold springs

presscross section: 1) thermal water sampled at ET1,3,7 springs and at ET2 well; 2) cold springs

press

controlled by structural grain related to large scale folds

press

controlled by structural grain related to large scale folds(D1 structures) producing confined aquifers enclosed

press

(D1 structures) producing confined aquifers enclosedwithin impermeable or less permeable units as well as bypre

sswithin impermeable or less permeable units as well as bythe discontinuity related with fault surfaces. Moreover,pre

ssthe discontinuity related with fault surfaces. Moreover,karst phenomena significantly affect the groundwaterpre

sskarst phenomena significantly affect the groundwater

, 2007). Fig. 5press

, 2007). Fig. 5shows the groundwater stream lines and some mainpre

ssshows the groundwater stream lines and some mainhydrogeological divides, achieved from various informa-pre

sshydrogeological divides, achieved from various informa-

water isotopes and

presswater isotopes and δ

pressδ

the laboratories of the Institute of Geosciences and Earth

press

the laboratories of the Institute of Geosciences and EarthResources (IGG-CNR, Pisa, Italy), whereas

press

Resources (IGG-CNR, Pisa, Italy), whereas was analysed at the Stable Isotope Laboratory of the Uni-pre

sswas analysed at the Stable Isotope Laboratory of the Uni-versity of Reno (Nevada, USA).pre

ssversity of Reno (Nevada, USA).

As previously found by Cpress

As previously found by Cphysical data obtained in the 2007-2008 surveys indicatepre

ssphysical data obtained in the 2007-2008 surveys indicatea strong effect of the rainy regime on the quality of ther-pre

ssa strong effect of the rainy regime on the quality of ther-mal groundwater that outflows at Equi Terme. The maxi-pre

ssmal groundwater that outflows at Equi Terme. The maxi-pre

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ss(ET8,10,11); 3) Catenelle Creek sampled upstream (ET4) and downstream (ET9) of thermal water points.

press(ET8,10,11); 3) Catenelle Creek sampled upstream (ET4) and downstream (ET9) of thermal water points.

involved in the mixing process. From the values of δ18Oand the relations of elevation versus δ18O achieved byMUSSI et alii (1998) for the Alpi Apuane region, arecharge average elevation of about 1000-1200 m a.s.l.can be estimated.

The amounts of 3H registered for cold water werebetween 5 TU and 6 TU, or congruent with those of therecent rainfall (DOVERI et alii, 2005). Thermal waterpoints showed values slightly depleted values (3.5-4 TU),thus indicating that the deep hydrothermal flow involvedin the mixing process has a longer residence time.

This is also in agreement with the higher values ofδ13C [DIC] showed by the thermal water points comparedto the cold water (–5 ÷ –6‰ and –8 ÷ –9‰, respectively).Indeed, starting from values less than –10‰ in the soilzone (CLARK & FRITZ, 1997), groundwater δ13C[DIC]approaches δ13C[DIC] of local carbonate rocks (0 ÷ +3‰;CORTECCI et alii, 1994) and this process is also linked tothe time of water-rock interaction.

THE EQUI TERME FAULT

SURFACE STRUCTURES, FAULT CHARACTERIZATION AND

KINEMATICS

The Equi Terme Fault is part of a regional system(North Apuane Transfer Fault of BROZZETTI et alii, 2007;Marciaso-Tenerano Fault of SCANDONE, 2007 here afterNorth Apuane Fault System, in accordance with Molli2013, fig. 1a), bounding the Alpi Apuane and separatingthem from the northernmost tectonic depression of the Lunigiana intramontane basin which is filled byPliocene-upper/middle Pleistocene continental deposits(FEDERICI, 1978; RAGGI, 1985; BERNINI & PAPANI, 2002with references).

The North Apuane Fault (fig. 1) is striking for nearly20 km with a SW-NE trend, and includes three intercon-nected segments: a westernmost E-W trending FosdinovoFault, a central SW/NE trending Tenerano-Marciaso-Aiola Fault and a northeasternmost, E-W trending, seg-


Fig. 7 - a) Simplified tectonic map with the main faults part of the Equi Terme Fault system. OTO and ACC Ligurian and subLigurian units;FTOa,b Tuscan Nappe terms above Calcare Cavernoso (CCA); CLF, MAA Selcifero metalimestone and marble. Rose diagrams (b) and stereogramsof poles of main faults (c); P/T diagrams showing the kinematics for the oblique-transcurrent faults (d) and oblique-normal faults (e). In(b) direction classes counted within 10°; in (c) counturs at 0.50, 1.00, 1.50, 2.00, 2.50, 3.00, 3.50, 4.00, 4.50 and 5.00; maximum at 09/187;(d) and (e) P axis (circle), T axis (square). Total analysed data 37.

In In ) Simplified tectonic map with the main faults part of the Equi Terme Fault system. OTO and ACC Ligurian and subLigurian units;In ) Simplified tectonic map with the main faults part of the Equi Terme Fault system. OTO and ACC Ligurian and subLigurian units;FTOa,b Tuscan Nappe terms above Calcare Cavernoso (CCA); CLF, MAA Selcifero metalimestone and marble. Rose diagrams (In FTOa,b Tuscan Nappe terms above Calcare Cavernoso (CCA); CLF, MAA Selcifero metalimestone and marble. Rose diagrams (

); P/T diagrams showing the kinematics for the oblique-transcurrent faults (In ); P/T diagrams showing the kinematics for the oblique-transcurrent faults (In pre

ss

ment represented by the Equi Terme Fault. This lattersegment is associated with strong morphological evidencesof recent activity and has a primary seismogenic role assuggested by the recent June 2013 earthquake (M = 5.2)and previous small-magnitude instrumental and histori-cal activity occurred in 1837 (I = IX-X, M~5.7), 1778 and1767 (CSI and ISIDE data, CASTELLO et alii, 2006, ISDeWorking Group, 2010). Some authors (SOLA RINO, 2005;BROZZETTI et alii, 2007) also related the largest event ofthe Lunigiana-Garfagnana area (I = IX-X, M~6.5), with acomplex rupture involving the Equi Terme Fault plusanother structure at the northwest termination of Garfa -gnana (the Casciana-Sillicana fault).

The Equi Terme Fault includes a system of high anglefaults well developed within the footwall Apuane block aswell as in the composite hangingwall made up of Tuscan,sub-Ligurian and Ligurian units (figs. 2, 3, 7, 8a).

In the footwall, an array of anastomosed to sub-paral-lel relayed faults can be observed (figs. 7, 8b). They forma domain of medium to high angle structures with asouthwest/northeast to east/west trend and a spacing ofc. 50-100 m.

Individual faults are characterized by a decimeter tohalf of meter fault core associated with cataclasites andcataclastic breccias bounded by main-slip polished sur-faces in which slickenlines can be observed (fig. 8b,d).

The fault core is usually within a meter to pluridecameterdamage zone with fractured protoliths.

The faulted footwall domain is extendeds for a widthof c.500 m away from the main fault zone, which is carto-graphically characterized by juxtaxposition of the basal“Calcare Cavernoso Fm.” and Triassic Rhethian lime-stones to the uppermost stratigraphic levels of the TuscanNappe represented by the Tertiary Macigno sandstone.Locally (e.g. close to the Equi Terme village), the MacignoFm. is in direct contact with the footwall block repre-sented by the cherty metalimestones of the Apuane unit(fig. 8c).

In the hangingwall domain a system of hectometerscale, subparallel arrays of faults with a NW-SE andNE-SW trends can be recognized (fig. 7). They have anapparent cartographic displacement, which is of normal-type and well constrained in surface only where theycrosscut the Cretaceous to Jurassic terms of the TuscanNappe, east-north east of Equi Terme, toward the villageof Ugliancaldo (figs. 3, 7).

A kinematic analysis of the main slip surfaces in boththe hangingwall as well as the footwall block was per-formed by a systematic record of indicators such as stepson rock, slicknensides, striae-type ridges and grooves andgrowth fibers consisting essentially of calcite. The kine-matic analysis indicates predominant movements of tran-

10 G. MOLLI ET ALII

Fig. 8 - a) Panoramic view (toward W/SW) of the Equi Terme area with indication of the main structural domain across the Equi Termefault. The village in the valley is Equi Terme; b) minor oblique slip fault in the footwall domain of Equi Terme Fault. Striated fault plane withslicklines within marble protolith are clearly observable, scale bar c.10 m; c) fault surface with slickenside and fault rocks of an exposed segmentsof the Equi Terme Fault along the Catenelle creek, scale bar c.2 m; d) minor slip surface and fault breccia within footwall domain, scale bar c.0,2 m.

In ment represented by the Equi Terme Fault. This latterIn ment represented by the Equi Terme Fault. This lattersegment is associated with strong morphological evidencesIn segment is associated with strong morphological evidencesof recent activity and has a primary seismogenic role asIn of recent activity and has a primary seismogenic role asIn In

) Panoramic view (toward W/SW) of the Equi Terme area with indication of the main structural domain across the Equi Terme

In ) Panoramic view (toward W/SW) of the Equi Terme area with indication of the main structural domain across the Equi Terme

) minor oblique slip fault in the footwall domain of Equi Terme Fault. Striated fault plane with

In ) minor oblique slip fault in the footwall domain of Equi Terme Fault. Striated fault plane with

slicklines within marble protolith are clearly observable, scale bar c.10 m;

In slicklines within marble protolith are clearly observable, scale bar c.10 m; of the Equi Terme Fault along the Catenelle creek, scale bar c.2 m;

In of the Equi Terme Fault along the Catenelle creek, scale bar c.2 m;

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press

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) Panoramic view (toward W/SW) of the Equi Terme area with indication of the main structural domain across the Equi Termepress

) Panoramic view (toward W/SW) of the Equi Terme area with indication of the main structural domain across the Equi Terme) minor oblique slip fault in the footwall domain of Equi Terme Fault. Striated fault plane withpre

ss) minor oblique slip fault in the footwall domain of Equi Terme Fault. Striated fault plane with

slicklines within marble protolith are clearly observable, scale bar c.10 m; press

slicklines within marble protolith are clearly observable, scale bar c.10 m; cpress

c) fault surface with slickenside and fault rocks of an exposed segmentspress

) fault surface with slickenside and fault rocks of an exposed segments) minor slip surface and fault breccia within footwall domain, scale bar c.0,2 m.pre

ss) minor slip surface and fault breccia within footwall domain, scale bar c.0,2 m.

scurrent and normal type as well as normal to oblique(fig. 8b,c,d).

The kinematic analysis of the fault surfaces enabled usto identify the main directions of shortening and extension(P and T kinematic axes) associated with the developmentof the faults themselves. In particular, the system of tran-scurrent faults is compatible with a direction of maximumshortening (P axis) inclined at a low angle and directedapproximately SE-NW and a direction of maximum exten-sion (axis T) sub-horizontal and oriented SW-NE (fig. 7d).The system of normal faults and/or normal/oblique isinstead associated with a direction of maximum shorteningsub-vertical and a direction of sub-horizontal maximumextension and directed approximately NS (fig. 7e). As awhole the kinematic analysis suggests a dextral transten-sive setting for the Equi Terme Fault.

SUBSURFACE STRUCTURES: GEOPHYSICAL INVESTIGATIONS

Two kinds of geophysical methods were tested andproved effective in characterizing subsurface structures:Electrical Resistivity Tomography (ERT) and Magne-toTelluric (MT). The survey location is shown in fig. 9ERT, which is an active method, i.e. it relies on an artifi-cial source signal that guarantees its efficacy almosteverywhere, was very useful to gain insight into the shal-low (0-250 m) resistivity distribution. MT exploits a nat-

ural source signal and may be useless in areas of highelectromagnetic noise; where effective, in our study itprovided information on the distribution of resistivitydown to a depth of 2 km.

The ERT acquisition along two profiles was per-formed with an IRIS system and different arrays (Wen-ner, Schlumberger, Dipole-Dipole, Pole-Dipole). The twoprofiles were located on both sides of the Catenelle creek,had a length of 1 km and were made of 48 electrodes 20 mapart. The analysis of ERT data, performed using theRES2Dinv, a commercial software package based onLOKE et alii (2003), provided 2D models of the resistivitydistribution below the profiles, in particular the areaaround the thermal springs where the two profiles wereclose. The profiles were not straight lines both due to therough topography and logistics of the area and in order tofollow the thermal springs along the Catenelle creek.

The resistivity distribution along the north profileprovided the most interesting results in terms of thestructural investigation. Thanks to the strong resistivitycontrast of the clayey Scaglia fm. with respect to the car-bonate units, it was possible to locate a fault dislocatingthe different units. Fig. 10 top shows the geological inter-pretation of the resistivity section obtained by data inver-sion of combined array methods. The fault likely plays amain role in the hangingwall groundwater flow, by creat-ing an impermeable barrier.


Fig. 9 - ERT and MT site location. Only 6 of the 10 MT sites are shown, the other are far from the main study area.

In In scurrent and normal type as well as normal to obliqueIn scurrent and normal type as well as normal to oblique

The kinematic analysis of the fault surfaces enabled usIn The kinematic analysis of the fault surfaces enabled usto identify the main directions of shortening and extensionIn to identify the main directions of shortening and extensionIn In

- ERT and MT site location. Only 6 of the 10 MT sites are shown, the other are far from the main study area.

In - ERT and MT site location. Only 6 of the 10 MT sites are shown, the other are far from the main study area.pre

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ss- ERT and MT site location. Only 6 of the 10 MT sites are shown, the other are far from the main study area.pre

ss- ERT and MT site location. Only 6 of the 10 MT sites are shown, the other are far from the main study area.

The subsurface investigated by the south profileseems to be roughly divided in to two sectors of differentresistivity: a shallow level characterised by low resistivitywhich lies above a highly resistive level. Geologically,these two sectors are both related to carbonate units, theshallower being most probably highly fractured and per-meable. High water saturation and a low resistivity is theresult. Due to the strong lateral resistivity variation shownby data inversions of different configuration arrays, wefocused on dipole-dipole array data, the most robust forthis situation (fig. 10 bottom). Two vertical low resistivityanomalies are clearly displayed. Their location corre-sponds to the NE-SW trending normal faults mapped inthe north-east part of valley by the geological survey. Dueto their low resistivity, these faults appear fluid filled, andmay represent important pathways for deep fluid upflow.

Only 10 sites could be identified for MT acquisitiondue to the location of the investigated area in a narrowand steep valley. Although MT data were noisy due to themany power lines along the valley, they provided someinsight. Four horizontal electric and magnetic compo-nents of the MT field were simultaneously recorded witha Stratagem equipment. MT data were collected in thefrequency range 105-0.1 Hz, reaching an investigationdepth of almost 2 km. Since the chemical and isotopicanalysis of thermal waters highlighted their deep interac-tion with evaporitic rocks as previously documented and

these are supposed to be present in the deep part of thestudied area, a calibration of the MT data was also per-formed outside of the investigated area near to Sassalbovillage (fig. 1a), where the evaporitic unit extensivelycrops out. These Triassic deposits, mainly composed ofalternated gypsum/anhydrite levels and dolomite, provedhighly resistive (1000 ohm.m).

Apparent resistivity and phase values were obtainedusing classical tensorial processing techniques (SWIFT,1967). Singular- value decomposition of the impedanceswas then carried out, obtaining by eigenstate analysis themaximum (named xy) and minimum (named yx) impedanceamplitude and phase (LA TORRACA et alii, 1986). Smooth(CONSTABLE et alii, 1987) and rough layered earth inver-sions of the rotationally invariant MT data were com-puted, as well as a 2D inversion models, using RODI &MACKIE (2000) code. After 1D inversion of the invariantvalues and 2D inversion, the subsurface image obtainedby resistivity distribution appeared clearly three dimen-sional. Although there were not enough data to perform arefined 3D model, some features are very evident.

In the north-south inversion profiles (fig. 11) the val-ley coincides with an abrupt change of resistivity atdepth. On the south the units are resistive down to adepth of at least 2 km, whereas in the north conductiveunits cover resistive units at depth. By comparison withthe shallow investigation provided by ERT data and geo-

12 G. MOLLI ET ALII

Fig. 10 - Top: Interpreted ERT inversion model of the northern profile. The arrow points to the possible fault zone characterized by lowresistivity values. The interpreted geological top units correspond to the outcroppings. Bottom: ERT inversion model of southern profile.

In In - Top: Interpreted ERT inversion model of the northern profile. The arrow points to the possible fault zone characterized by low

In - Top: Interpreted ERT inversion model of the northern profile. The arrow points to the possible fault zone characterized by low

In The subsurface investigated by the south profileIn The subsurface investigated by the south profileseems to be roughly divided in to two sectors of differentIn seems to be roughly divided in to two sectors of differentresistivity: a shallow level characterised by low resistivityIn resistivity: a shallow level characterised by low resistivitywhich lies above a highly resistive level. Geologically,In which lies above a highly resistive level. Geologically,In In In resistivity values. The interpreted geological top units correspond to the outcroppings. Bottom: ERT inversion model of souther

In resistivity values. The interpreted geological top units correspond to the outcroppings. Bottom: ERT inversion model of southerpre

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ssresistivity values. The interpreted geological top units correspond to the outcroppings. Bottom: ERT inversion model of southerpre

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logical outcropping, the conductive values are related tosedimentary units, whereas the high resistivity on thesouth is related to the marbles and cherty metalime-stones. The main fault that gave rise to the valley appearsin the sections of fig. 11 as a steep, north dipping fault. Itis characterized by low resistivity values, possibly fluid-filled cataclastic units.

DISCUSSION AND CONCLUSIONS

Fig. 12 summarizes the main features of the EquiTerme geothermal area, whose groundwater flow istectonically controlled by a major structure – the Equi Terme Fault – and its hangingwall and footwall 3-Darchitecture.

As evidenced by geochemistry results, the Equi Termefluids derive from a mixing, at various ratios, of a karstshallow Ca-HCO3 footwall originated water and a deepNa-Cl (Ca-SO4) water derived from the interaction withTriassic evaporitic formations located at the base of a nonmetamorphic carbonate sequence in the hangingwalldomain. Their mixing ratio mainly depends on the effectof the rainwater which recharges the karst shallow circu-lation in the footwall of the Equi Terme Fault.

MT and ERT data have showed anomalies of resistivitythat are correlated with geological units and with zonesof higher permeability and fluid content representing thepossible fluid pathways. In both MT profiles (fig. 11) lowresistivity areas are well defined at a depth of about 500 mb.s.l.. A combined interpretation of geophysical, geologicaland hydrogeochemical (fig. 12) data suggests that the highly


Fig. 11 - North-south profile sections obtained by 2D inversion of TE-TM MT data. See fig. 9 for profile location.

1600

1200

400

0

-800

800

-400

400

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-400

-800

-1200

-1600

800

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M. Pisanino

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GROUNDWATER FLOW PATHScold water thermal waterSub-Tuscan Nappe

cover unit

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‰18O -7.8O ‰18 -7.5

‰18O -7.0AIV-R

AIV-R

AIV-R

ET10: T 8.9 ÷ 10.4 °CCaHCO3 ‰18O -7.3 ÷ -8.0‰18O -7.3 ÷ -8.0;

ET1: T 21.4 ÷ 27.0 °CNaCl(CaSO4 ‰18O ‰18O -7.5 ÷ -7.6;)

Fig. 12 - Regional cross-section with surface and subsurface structure of the Equi Terme Fault and groundwater flow paths schematization(AIV-R: average isotopic value in the rainfall, which is assessable from MUSSI et alii (1998); isotopic and chemical-physical data were achievedduring this study; for the section trace and the hydrogeological units see fig. 5)

In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In In HydraulicIn HydraulicgradientIn gradientgradientIn gradientIn In In

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- North-south profile sections obtained by 2D inversion of TE-TM MT data. See fig. 9 for profile location.

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- North-south profile sections obtained by 2D inversion of TE-TM MT data. See fig. 9 for profile location.

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conductive zones could represent areas of enhanced cir-culation within the hangingwall, where water is enrichedin Na-Cl.

ERT data provides more details of the local fluid cir-culation. The most interesting data come from Profile 2and the changes in resistivity determined by waterdrainage. Since the geological units below Profile 2 aremainly carbonatic units, we think that the conductivezones at a shallow level are related to areas where thesegments of the fault systems are more developed. Theconductive vertical discontinuities, crossing the deep andmost probably less fractured resistive layer, seem to becorrelated with the main faults identified from geological/structural surveys. These faults are interpreted as thepreferential pathway for the uprising of the thermalwaters after their interaction with the deep evaporiticrocks. The cold groundwaters, involved in the mixingprocess with hot thermal waters, flow within the highlyfractured carbonatic rocks affected by the fault and frac-ture pattern characterizing the area.

ACKNOWLEDGEMENTS

This paper is dedicated to the memory of Francesco Baldacci,who started with us this project as principal investigator. TheG.A.T.T. – Equi Terme and S. Trifirò (IGG-CNR Pisa) are thankedfor collaboration as well as the anonymous reviewers and the editorswhich provided useful comments and suggestions that improved theoriginal manuscript.

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ZACCAGNA

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ACCAGNA D. (1932) -

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D. (1932) - Descr. Carta Geol. d’Italia, press

Descr. Carta Geol. d’Italia, press

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Manuscript received 14 March 2014; accepted 26 June 2014; published online press

Manuscript received 14 March 2014; accepted 26 June 2014; published online 28 December2014press

28 December2014

Surface-subsurface structural architecture and groundwater ... · tural geology techniques,...

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