Lineament Domain Analysis to Infer Groundwater Flow Paths: Clues from the Pale Di San Martino Fractured Aquifer, Eastern Italian Alps GEOSPHERE; V

Research Paper

GEOSPHERE Lineament domain analysis to infer groundwater flow paths: Clues from the Pale di San Martino fractured aquifer, Eastern Italian Alps GEOSPHERE; v. 13, no. 5 G. Lucianetti, P. Cianfarra, and R. Mazza Dipartimento di Scienze, Università degli Studi Roma Tre, Rome 00146, Italy doi:10.1130/GES01500.1

13 figures; 1 table ABSTRACT et al., 2001) and at regional scales (Bell and Babcock, 1986; Mandl and Hark ness, 1987; Storti et al., 1997; Morin and Savage, 2003; Tondi et al., 2016). In re CORRESPONDENCE: paola.cianfarra@uniroma3.it An automatic lineament extraction was carried out on a processed digi sponse to the direction of the crustal stress field, fractures can be kept open or tal elevation model of a high-altitude Alpine fractured reservoir, the Pale di undergo closure (Gale et al., 2004). Typical methods to determine active stress CITATION: Lucianetti, G., Cianfarra, P., and Mazza, San Martino area (Dolomites) located in the southern sector of the Eastern directions include the analysis of seismological data and borehole breakouts R., 2017, Lineament domain analysis to infer groundwater flow paths: Clues from the Pale di San Martino Italian Alps. The strike of the main lineament domain indicates the direction (e.g., Montone et al., 1999, 2004, 2012; Montone and Mariucci, 2016) as well fractured aquifer, Eastern Italian Alps: Geosphere, of the principal crustal stress. This direction was compared with earthquake as the development of tectonic numerical models (e.g., Cianfarra and Maggi, v. 13, no. 5, p. 1729–1746, doi:10.1130/GES01500.1. focal mechanisms to confirm the orientation of the regional crustal stress. 2017). Nevertheless, these methods are seldom applicable, as they require the The two data sets provide similar results and show a NNW-SSE maximum presence of deep wells or of well-constrained earthquake focal mechanisms. Received 20 January 2017 horizontal crustal stress orientation, compatible with the direction of the last In this paper, we present an alternative approach to determine the direction Revision received 19 April 2017 Accepted 9 June 2017 Alpine compression reported by previous studies in the investigated region. of the present regional stress field, which is based on the automatic detection of Published online 9 August 2017 The orientation of the maximum horizontal compressive stress was then used lineaments derived from digital elevation models. Lineaments from synthetic to explore the ability of specific fracture and fault sets to enhance ground scaled images of the earth surface cluster around preferential orientations water flow. Subvertical strike-slip faults and joints oriented NW-SE to north- (domains) in response to the (recently) active crustal stress field, and the main south provide the greater contribution to the groundwater flow. The location lineament domain strikes parallel to the maximum horizontal compressive of the main springs and evidence from a dye tracer test conducted in the area stress (Wise et al., 1985; Pardo et al., 2009; Giordano et al., 2013; Pischiutta confirm this main drainage direction. This study demonstrates that automatic et al., 2013; Cianfarra and Salvini, 2014, 2015). Therefore, by detecting the lineament analysis is an efficient and inexpensive method to identify the tra main lineament domain, it is possible to identify the direction of the regional jectories of groundwater flow in fractured aquifers. crustal stress active in a given area. The use of this method, supported by field surveys, has relevant implications in hydrogeological investigations, because the present-day stress holds open the preexisting fractures and faults that INTRODUCTION are oriented nearly parallel to the maximum compressive stress (Nur et al., 1986; Gale et al., 2004), thus increasing the ability of the fracture network to The influence that brittle deformation plays on fluid flow is well known in transport fluids. On the contrary, fractures striking perpendicular to the main petroleum exploration (Odling et al., 1999; Aydin, 2000; Simpson et al., 2001; stress (i.e., to the main lineament domain) are likely closed due to their com Billi, 2005; Maggi et al., 2015), in the geothermal industry (Giordano et al., pression status, reducing the fluid transmissibility in that specific direction 2013), and in groundwater investigations (Mayer and Sharp, 1995, 1998; Neu (Lorenz et al., 1996; Sayers, 1990). man, 2005; Cooke et al., 2006). In rock reservoirs with low primary permea Various studies have been done in the past to relate lineaments with bility, the bulk mass permeability is dominantly controlled by the fracture groundwater flow paths (e.g., Sander, 2007, and references therein). In most of system (Larsen and Gudmundsson, 2010). Understanding and characterizing these works, lineaments are related directly to karst features or to faults. For fractures in terms of orientation, connectivity, aperture, spacing, and length instance, Kresic (1995) used remote sensing techniques to delineate karstic- are crucial elements for planning any type of fluid exploitation, whether in specific surface forms, such as uvalas and sinkholes, and derived a proba regards to oil, gas, or potable water (Cooke et al., 2006; Agosta et al., 2010; ble general direction of the groundwater flow. Other studies investigated the Giordano et al., 2013; Cianfarra and Salvini, 2016a). Depending on the atti connectivity of lineaments (Shaban et al., 2006) and their length and density tude and intensity of the principal component of the imposed stress, hydraulic (Teeuw, 1995; Hung et al., 2004) as critical factors controlling the permeability properties of the fracture system can change consistently, affecting the per of aquifers. Nevertheless, a direct relation between lineaments and tectoni For permission to copy, contact Copyright meability of the media (Aydin, 2000; Baghbanan and Jing, 2008; Tondi et al., cally controlled morphologies is still controversial because lineaments from Permissions, GSA, or [email protected]. 2016). This is valid at the scale of microfractures (Gentier et al., 2000; Simpson synthetic scaled images do not relate directly to faults or tectonic discontinui

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ties (Goldscheider and Andreo, 2007; Cianfarra and Salvini, 2014). Previous and discharge of the main springs, it is possible to identify the main drainage studies have explored the relation between lineament domains and crustal directions. stress in the various geodynamic environments (Mazzarini and Salvini, 1994; This work proves that lineament analysis is a simple yet powerful tool to Mazzarini and D’Orazio, 2003; Pardo et al., 2009; Pischiutta et al., 2013; Gior assess the contribution of a given set of fractures to the conductivity of a frac dano et al., 2013; Cianfarra and Salvini, 2014, 2015, 2016b), but none of them tured aquifer, when coupled with field structural analyses and hydrogeological focused on groundwater exploration. The relationship between lineaments, evidence. stress field, and enhanced direction of fluid circulation is not sufficiently cov ered by the present-day literature. The aim of this paper is to fill this gap, pro posing a simple and efficient technique in the field of groundwater resources STUDY AREA investigations. The aquifer under investigation is a >1000-m-thick sequence of crystalline Tectonic Setting dolomite rocks located in the Dolomites region (Eastern Italian Alps). The ma trix porosity is <5% (Blendinger et al., 2015), whereas the secondary permea The Pale di San Martino is one of the numerous mountain groups of the bility is high due to the rock brittle deformation. This includes a complex net Dolomites, a well-known region located in the Eastern Italian Alps (Fig. 1). This work of fractures and faults generated from the poly-phased tectonic history region consists of a south-verging thrust belt (Doglioni, 1987) and is bordered that has affected this part of the Alpine orogen since Eocene times (Bosellini by two major, regionally sized tectonic structures, the Insubric Line to the north and Doglioni, 1986; Doglioni, 1987; Castellarin et al., 1992; Schönborn, 1999; and the Valsugana Line to the south. The uplift of the belt is ascribed to a Castellarin and Cantelli, 2000). Despite the fact that dolomite groundwater sys ramp-and-flat thrusting mechanism that was favored by inherited sedimentary tems usually show less karstification than limestone systems (Hartmann et al., features (Doglioni, 1985). Through the construction of balanced cross sections, 2010), the aquifer of the study area has several karst features. Their presence Schönborn (1999) proved that the Dolomites were likely uplifted by very long easily relates to the dense network of fractures and faults that favored dissolu décollements and by a number of blind thrusts. These thrusts also involve the tion and the enlargement of flow paths in specific directions, generating con crystalline basement and have provided an overall shortening below the east duits and caves. The strong influence that fractures play on karst development ern Southern Alps of at least 50 km since the late Miocene. highlights the need to study the structural setting of the area in order to prop From a tectonic point of view, the Dolomites are characterized by a complex erly characterize the groundwater flow paths. The coupling of fracture azimuth structural pattern. Mesostructural analysis carried out in the Southern Alps by measured at the outcrop scale with regional crustal stress orientations can various authors (Bosellini and Doglioni, 1986; Doglioni, 1987; Castellarin et al., provide indications of the ability of specific sets to transport groundwater. By 1992; Castellarin and Cantelli, 2000) documented several compressional tec merging these results with hydrogeological information, such as the location tonic events that can be summarized as follows.

13°E N 100 km

Figure 1. Tectonic map of the Alps (re drawn after Dal Piaz et al., 2003). Red square shows the location of the study area in the Eastern Italian Alps. EF—Euro pean foreland; M—Molasse foredeep; H—Helvetic-Dauphinois domain; EA— Eastern Austroalpine; otw—Ossola-Ticino tectonic window; ew—Engadine tectonic window, tw—Tauern tectonic window; SA—Southern Alps; PB—Pannonian basin; DI—Dinaric fold-and-thrust belt; PA—Po Valley–Adriatic forelands; AP—Apennines fold-and-thrust belt.

45°N

(1) The Mesoalpine (Eocene) and early Neoalpine (Oligo-Miocene) com From the geological point of view, the catchment is characterized by the pressional events responsible for the so-called “Dinaric compression”. oldest stratigraphic sequence of the Dolomites, ranging from Precambrian to According to Doglioni (1987), in the southeastern Dolomites this phase Late Triassic age. Quaternary sediments, mainly alluvial and glacial deposits, should be connected to a NE-SW to east-west compression and gener are widespread and cover the old sedimentary succession. The main forma ated thrusts and folds striking between N315° and N360°. tion cropping out is a >1000-m-thick sequence of dolomites deriving from (2) A marked compressional deformation, Serravallian to Tortonian in a Middle Triassic reef buildup, known as Sciliar Dolomites (Bosellini, 1984, age, trending approximately N340°. This phase is particularly evident 1996). These dolomites are underlain by clay-rich and evaporitic deposits of in the study area and is responsible for the Valsugana structural sys the Werfen and Bellerophon formations (Lower Triassic–Upper Permian). The tem with associated back-thrusts and for the main uplift of the Pale di boundary between the two different rock types is highlighted by the different San Martino. degree of erosion. Vertical rock faces and gullies characterize the dolomites, (3) The N300° to N330° compression, known as the Adriatic compres whereas more gentle slopes can be found at the outcrop of the clay-rich unit. sional event. This last phase, which occurred between Messinian and From the hydrogeological point of view, this lithological boundary serves as a Pliocene times, produced widespread deformation in the Southern no-flow limit for the groundwater flow (Lucianetti et al., 2016). The dolomitic Alps and the reactivation of previous structures. In the study area, rocks host the main regional aquifer of the area due to their high secondary the Adriatic compression mainly reactivated preexisting deformation permeability. Below the carbonates, the flow is limited by the low-permea structures. bility clay-rich deposits, which force the groundwater to outflow at the lith ological contact. The presence of Quaternary sediments accumulated in the Some authors also reported evidence of pre-Alpine tectonic events, such as a valleys and on the mountain slopes complicates this setting, masking the Middle Triassic sinistral transpression (Doglioni, 1984) and volcano tectonics emergence point and causing a diffused outflow from the debris. According related to the emplacement of Triassic magmatism, including thrusting, trans to spring cadasters, >500 springs are reportedly present in the area, but only pression, and strike-slip tectonics (Doglioni, 1984; Castellarin et al., 1988). 41 of them have a mean discharge >5 L/s (Lucianetti et al., 2016). In order to The Pale di San Martino mountain group is usually described as a synclino highlight the main permeability directions in this fractured reservoir, in 2016 rium structure formed by flexural slip or flexural flow, with a main décollement a regional dye tracer test was performed at the scale of the entire Pale di San in the evaporites of the Bellerophon Formation (Doglioni, 1987). The main Martino mountain group (Lucianetti, 2017). Na-fluorescein was injected in the structural element in the study area is the regional ENE-WSW–trending thrust central part of the Pale di San Martino Plateau, and the main springs fed by the fault known as the Valsugana thrust (Bosellini and Doglioni, 1986). dolomite aquifer were monitored (Fig. 2). Results from this test have shown A literature review revealed that most of the previous studies on the Pale di a main NNW-SSE to north-south groundwater flow direction (see Discussion San Martino mainly deal with stratigraphical aspects (Bosellini, 1984; Zampieri, section). 1987; Blendinger et al., 2015), whereas studies on structural setting are mainly done at the regional scale of the entire Dolomites (Doglioni, 1985, 1987; Schön born, 1999). Information on faults present in the investigated area is derived METHODS from the official Italian Geological Survey geological maps (SGN, 1970) and from the work of Zampieri (1987). Lineament Analysis

Lineaments appear in images as subtle, linear texture anisotropies that Geographical, Geological, and Hydrogeological Setting cluster around preferential orientations (domains, sensu Wise et al., 1985). We have performed automatic lineament analysis on a processed image from the The Pale di San Martino are a 250-km2 dolomitic massif reaching >3000 m Advanced Spaceborne Thermal Emission and Reflection Radiometer ASTER)( above sea level (a.s.l.) at its highest peak (Fig. 2) (Cima della Vezzana, 3192 m Global Digital Elevation Model (GDEM; http://gdex .cr .usgs.gov /gdex/). The a.s.l.). Steep rocky slopes and deeply incised valleys are typical geomorpho GDEM is generated from stereo image data from the ASTER sensor. This in logical elements of the landscape. Numerous valleys cut into the mountain strument has an along-track stereoscopic capability using its near-infrared massif, and in some cases, like in the San Lucano Valley, a difference in eleva spectral band and its nadir- and backward-viewing cameras. The GDEM is tion of >2000 m occurs between the valley floor and the surrounding ridges. A distributed in GeoTIFF format with geographic latitude and longitude coordi wide high-elevation karst plateau is present in the central part of the area (Pale nates and at a 1 arc-second grid corresponding to ~30 m at the latitude of the di San Martino Plateau with a mean elevation of 2600 m a.s.l.). It is character study area (Tachikawa et al., 2011). This resolution allows us to represent the ized by a large exposure of bare dolomites with numerous karst features such lineaments at the catchment scale and to avoid misleading interpretation of as dolines and karst shafts. local effects (such as anthropic features and local tectonic effects). An ad hoc

Figure 2. Hydrogeological setting of the Pale di San Martino, Eastern Italian Alps. Spring data were acquired by Lucianetti et al. (2016), and fault traces were drawn based on preexisting maps (SGN, 1970; Zampieri, 1979). Labels 1–4 refer to Pradi dali, Gares, Travignolo, and Angheraz springs, respectively. The WGS84 datum was adopted and the metric coordinates were referred to the UTM32N projection zone. Results of the 2016 dye tracer test performed in the study area are repre sented.

image processing of the GDEM was progressed for linear texture enhancing. Outcrop-Scale Surveys Shaded-relief images were created according to four, low-angle (20°) light ing conditions, namely from the north, northeast, east, and southeast. This Structural field measurements (i.e., fault and fracture system attitude analy allowed reduction of the bias induced by lighting conditions, which tend ses) were done in the investigated area to better understand and characterize to hide lineaments that lie nearly parallel to the lighting direction (Wise, the existing brittle deformation that contributes to the secondary permeability 1969). The image processing of each shaded-relief image included: (1) a of the aquifer. The comparison of the found fault and fracture populations (or preliminary low-pass filter to suppress the morphological variations with azimuthal sets) with the active stress field allows identification of the most wavelength <100 m related to local scale factors; (2) a Laplacian filter to em dilatant fracture and fault sets that enhance the groundwater flow paths within phasize the higher spatial frequency related to the presence of linear texture; the dolomite aquifer. (3) a threshold filter to select only significant pixels (i.e., pixels contributing A compass clinometer was used to measure dips and strikes of faults, frac to the image texture related to the presence of lineaments and in order to tures (e.g., extensional joints, veins, synthetic cleavage), and bedding planes. reduce the meaningful pixel number to ~10% of the image); and (4) the ap Structural measurements were transferred to a database and then were stereo plication of the Life filter (Cianfarra and Salvini, 2008) that allowed reduc graphically projected. Their azimuthal frequency was statistically analyzed by tion of random noise. The Life filter compares each significant pixel (i.e., a polymodal Gaussian fit using the Daisy3 software. Rose diagrams were also digital number value higher than the threshold slicing) with its surrounding created in order to ease the presentation and comparison with the main orien ones and clears it if the number of significant surrounding pixels is below tations of structures. a given threshold (four in the present analysis). The resulting images were A total of 157 measurements were collected from 20 field stations in the then automatically processed to recognize lineaments through the SID in- study area. The majority of the measurements were carried out near the main house (GeoQuTe Lab, Quantitative Geodynamics and Remote Sensing Lab springs of the area. at Roma Tre University) software (Mazzarini and Salvini, 1994; Cianfarra and Salvini, 2014, 2015). This software discovers lineaments as pixel alignments by the systematic search for all possible segments in a discrete image. The DATA PRESENTATION search was done for all possible azimuthal directions with a resolution of 1°. The automatic identification was performed by tuning the parameters that Regional-Scale Investigations: Lineament Analysis describe the lineament geometric characteristics (e.g., minimum and maxi mum length, width, sharpness) via a converging trial-and-error method. In Figure 3 the 319 lineaments automatically identified are represented. The This iteration was repeated until a satisfactory result was achieved, that is detected lineaments tend to align on morphological elements such as crests, the number of detected lineaments was meaningful to perform a statistical ridges, valleys, and tectonic structures. Results from the polymodal Gaussian analysis and the main azimuthal trends were persistent with the slight vari fit of the detected lineaments are shown in the rose diagram of Figure 4. ation of SID parameters. Both analyses of azimuth by cumulative frequency and azimuth by cumula In this study, a set of parameters was arranged to discover sharp linea tive length provide the same results. Three principal azimuth directions were ments >1.5 km long and >90 m wide. The detected lineaments were cumulated identified: (1) the main direction striking N340°; (2) a second direction striking into a database that was successively statistically analyzed through the Daisy3 N25°, and (3) a minor direction striking N80°. The capability of recognizing software (freely distributed at http://host .uniroma3 .it /progetti /fralab). Azi close azimuthal systems (domains) in the polymodal Gaussian fit is limited by muthal frequency analysis was performed following the techniques described their standard deviation (SD) due to the overlap between adjacent Gaussian in Wise et al. (1985) which included the polymodal Gaussian fit to identify the peaks. In the present analysis the lineament domains have a SD (see the table main azimuthal trends, corresponding to lineament domains. Azimuthal analy in Fig. 4) between 11° and 16°, and this may represent a conservative quan sis by cumulative length was also done. tification of the discriminating power of the analysis. On the other hand, the The results of lineament analysis were compared with the crustal stress azimuthal difference among the found systems (all >40°) is well above their data available in literature. Montone et al. (1999, 2004, 2012) and Montone and SD, thus avoiding a failure to identify other domains. Nevertheless we cannot Mariucci (2016) presented several updated versions of the Italian present-day exclude that a single peak might result from the contribution of multiple sys stress map. The latest version of the map consists of 737 data points, includ tems with azimuthal differences smaller than the corresponding peak SD. This ing borehole breakouts, seismicity, and active faults that were merged and latter hypothesis contrasts with the stress and deformation relations where the checked to depict the active crustal stress in the Italian region. Crustal stress development of a new fracture system is inhibited by the pre-existing fractures trajectories regarding the study area were extrapolated from this data set and until the new stress field rotates more than 30° (the internal friction angle; Nur were statistically analyzed by using the Daisy3 software to identify the main et al., 1986). This confirms that the detected lineaments are completely repre directions. sented by the three domains.

Figure 3. Results of the automatic lineament analysis. Detected lineaments were projected on a digital elevation model from Tarquini et al. (2007, 2012).

According to Wise et al. (1985), the main lineament domain is parallel to the maximum horizontal compressive stress (SHmax). This stress is oriented N340° in the investigated area and tends to close the fracture planes that are oriented perpendicularly to it (average direction N70°). On the other hand, the found stress field tends to keep open fractures and faults lying nearly parallel to the detected SHmax (e.g., Heffer and Lean, 1993). These fractures and faults are likely the most conductive and would behave as preferential groundwater flow paths in the fractured reservoir. It should be considered that in the upper portion of the crust (~700 m) this effect overlaps with local topographic effects (McTigue, and Mei, 1981). However, due to the regional-scale analysis of this study, topographic effects were not considered by the present paper. A lineament density map was created using the line density tool imple mented in the ArcGIS software. This tool computes the density as units of length (km) per unit of area (km2). Lineament density is represented in Fig ure 5 using a color code from red (10.5–16.6 km of lineaments/km2) to blue tones (<2.9 km of lineaments/km2). The map shows the near coincidence of the higher-density values with the tectonic structures known from literature. Figure 4. Results of the polymodal Gaussian fit of the 319 automatically detected lineaments in the in In particular, most of the higher lineament densities are elongated consis vestigated area. The upper part of the wind rose diagram shows the azimuth by cumulative frequency; the lower part shows the azimuth by cumulative lengths. Both analyses show that the identified linea tently with the main structural trends, such as the NNW-SSE fault and mor ments cluster in three azimuthal sets called domains. The main one is N340° oriented and easily relates pho-structural trend aligned with the Cismon Valley, the NNW-SSE fault sys to the last tectonic event affecting the studied area and is consistent with the compression that produced tem cutting through the Pradidali Valley up to the western portion of the Pale the principal tectonic structure of the area, the Valsugana fault. The table with the statistical parameters di San Martino Plateau, the NNW-SSE subvertical fault system bordering the refers to the azimuth by frequency analysis and shows the highest values of the NorH/SD for the N340° main lineament domain. The NorH/SD parameter indicates the azimuthal scattering of lineaments with western side of the Angheraz Valley, and the ENE-WSW Valsugana fault sys respect to their frequency and it can be used to evaluate the relative youth of the lineament domains. tem. Areas of higher lineament density are also concentrated where the two NorH—Normalized Gaussian Height; SD—standard deviation. See text for details. % is the percentage of main lineament domains (see Fig. 4) have been automatically detected. The data falling within the 2 standard deviation interval around the given gaussian, including the overlapping zones. MaxH—Gaussian Height expressed as data number falling within 1% of the histogram. lineament density map shows that the location of the main springs does not coincide with the areas of higher lineament density, but appears to be shifted downslope. The presence of a thick sequence of Quaternary deposits, which The ratio between the normalized Gaussian height (NorH in the table of fills the valleys and masks the aquifer-aquiclude contact, may be responsible Fig. 4) and the SD of each Gaussian peak, NorH/SD, represents the peak’s for the observed shifting. sharpness (Cianfarra and Salvini, 2014), that is, the near parallelism among To further support the results obtained from automatic lineament domain lineaments belonging to the same domain. The morphology of the earth sur analysis, an independent data set was analyzed to highlight the crustal stress face results from the competition between tectonics and erosional processes. trajectories in the study area. Data for the Italian present-day stress map of Mon The first tends to enhance the sharp linear texture related to the structural set tone et al. (1999, 2004, 2012) and Montone and Mariucci (2016) were analyzed to ting of active regions. On the other hand, erosion smooths tectonically related detect the active stress in the area under investigation. According to the latest morphologies resulting in an increased azimuthal scattering of their originally update of the representation (Montone and Mariucci, 2016), the Eastern Italian sharp and elongated geometries. In this way, the NorH/SD parameter, indicat Alps are affected by both compressive and strike-slip regimes (Fig. 6). Minimum

ing the azimuthal scattering of lineaments with respect to their frequency, can horizontal stress orientations (Shmin, corresponding to either s2 or s3, with s1 >

be used to evaluate the relative youth of the lineament domains. In particular, s2 > s3) in the Eastern Italian Alps are ENE-WSW oriented with only minor rota comparatively higher values of NorH/SD indicate relatively younger lineament tions of a few degrees over long distances (>100 km). This consistency allowed domains (Salvini et al., 1979; Cianfarra and Salvini, 2014). The main N340° peak using this data set to investigate the active stress in the study area. has the highest NorH/SD value of 8.94, compared to values of 7.21 and 5.38 Six stress orientations were extrapolated from the Montone et al. (2004, for the N25° and N80° domains. According to this observation, the prevailing 2012) and Montone and Mariucci (2016) data sets. The stress orientations re NNW-SSE domain represents the youngest lineament population and relates sulted from the inversion of earthquake focal mechanisms from the area of to the most recent tectonic phase that occurred in the studied region. During homogeneous stress trajectories (Table 1). The data were statistically analyzed this tectonic event, the activity of the Valsugana fault system occurred followed by polymodal Gaussian fit (i.e., the same technique used for lineaments), and by the (still-active; Devoti et al., 2011) uplift of the Pale di San Martino area. a maximum horizontal stress SHmax striking N344° was identified (Fig. 7).

Figure 5. Lineament density map of the Pale di San Martino fractured aquifer.

sional deformation occurring between the Serravallian and the Tortonian and trending ~N340°. In the Pale di San Martino area this tectonic phase is likely responsible for most of the brittle deformation structures found. The follow ing Adriatic phase, which is nearly coaxial with the previous Valsugana phase, likely reactivated preexisting deformation structures.

Local-Scale Investigations: Structural Field Measurements

Faults are common in the study area and were observed all around the massif, although commonly their outcrops were not easily accessible due to the presence of steep rock slopes. Different types of faults were recognized in the field as the result of the succession of various tectonic phases. Strike-slip faults are the most common and persistent and can be followed for hundreds of meters in length and height (commonly beyond the bottom of the outcrops) and in some cases also for thousands of meters in length (Fig. 8A). Strike-slip faults are responsible for the main morphological alignments, such as valleys (e.g., Pradidali Valley) and steep rock faces. Frequently, strike-slip faults are related to spring emergence, and this is particularly evident at the intersec Figure 6. Crustal stress map (modified and redrawn after Montone et al., 2004). Symbols are colored by stress regime. a—study area; b—area of homogeneous stress orientations used for the stress analysis tion with thrust faults. This is due to the overlap of the associated deforma in the study area. SHmax—maximum horizontal stress; Shmin—minimum horizontal stress; Sv—ver tion of both the fault types which locally increases the permeability. A very tical stress. See Montone et al., 2004, for quality ranking system explanations. Gray shading indicates clear example is represented by the Treviso spring, located in the upper part of the elevations in meters a.s.l. The color code was added in the modified figure. Green half circles refer to strike-slip faulting regime; red and blue refer to normal and thrust faulting regime, respectively. the Canali Valley to the NE of the confluence with the Pradidali Valley (Figs. 2 and 8B), where groundwater outflows from a system of subvertical tear faults along the present-day tectonic contact. In general, a brecciated fault core is This direction is nearly parallel to the orientation of the main lineament domain rarely visible; nevertheless, the sense of movement is clearly recognizable (N340°). In this way, the two independent data sets (lineaments and earthquake from synthetic cleavages and slickenlines (Fig. 8C). focal mechanisms) confirm the present-day crustal stress field orientation in Although not quite as common as strike-slip faults, thrust and back-thrust the investigated area, which is characterized by a NNW-SSE compression. faults were also observed in the field (Fig. 9). Evidence of compression is The orientation of the crustal compressive stress correlates well with the provided by the numerous folds in the gypsum-rich layers of the Bellerophon maximum stress orientation reported in literature by Castellarin et al. (1992) for Formation (Fig. 9B) which likely forms one of the main detachment levels for the Valsugana structural system. Those authors indicated a marked compres the Pale di San Martino thrusting and back-thrusting (Doglioni, 1985).

TABLE 1. STRESS REGIME IN THE INVESTIGATED AREA*, PALE DI MARTINO, EASTERN ITALIAN ALPS Latitude Longitude SHmax strike No (°N) (°E) (°) Source 1 46.36 13.27345 Montone et al. (2004) 2 46.31 13 345Montone et al. (2004) 3 46.33 13.2 341Montone et al. (2004) 4 46.32 13.17339 Montone et al. (2004) 5 46.33 13 356Montone et al. (2004) 6 46.7 11.16345 Montone et al. (2012) *See Figure 6. Notes: SHmax—maximum horizontal stress. In this work we compare the orientation of the SHmax with the direction of the main lineament domain.The strike of the SHmax from each data was computed as perpendicular to the Shmin (minimum horizontal stress) reported by Montone et al, (2004, 2012). Reported data were derived from earthquake focal mechanisms.

S Hmax is not statistically analyzed. The dextral shear observed on the N290° set is compatible with a NW-SE compression. A wide range of fractures, including tensile-mode fractures, joints, veins, and pressure-solution and shear fractures, was observed and measured in the study area. As shown in the wind rose diagram (Fig. 10C), many directions were de tected from the polymodal Gaussian fitting. These multiple azimuthal sets may easily have resulted from the poly-phased tectonic history that affected this re gion of the Alpine orogen. The main strike is N300°, followed by a set striking ~N334°, a N18° set, and a sub-ordered set at N87°. This last east-west set is more prevailing in the eastern part of the Pale di San Martino Plateau. The trend of the first and second fracture sets (respectively N300° and Figure 7. Orientation of the maximum N334°) is similar to the orientation of the strike-slip faults, although a broader horizontal crustal stress SHmax in the investigated area as derived from the range of strikes is present when looking at the fracture orientations. This ob Montone et al. (2004, 2012) data set. servation is compatible with a tectonic origin for these fracture sets. The N18° See Figure 4 caption for an explana joint set is of uncertain origin. Commonly fractures from this set are filled up tion of the abbreviations in the table. with calcite suggesting that extension occurred. The majority of the fractures (45 of 76 measurements) are subvertical to vertically dipping (80°–90°). Steep joints are widespread over the study area and show non-stratabound characteristics, with heights (i.e., their along dip dimension) that reach hundreds of meters. In outcrop, a wide range of bedding dips was observed, as steep as 87° in the Upper Canali Valley. However, 57 (of 59) bedding dip measurements were <49°, and most of them can be considered subhorizontal (Fig. 11). The northern sector of the Pale di San Martino shows a SE bedding dip, whereas the southern portion shows generally NW-dipping to subhorizontal bedding. In the central part, corresponding to the plateau area, the bedding Normal faults with pure extensional indicators were rarely identified, and can be considered subhorizontal. The combination of the two main dip do in most cases transtensional structures were more common. Pure dip-slip mains striking NE-SW produces a syncline-like structure. In reality, various faults mainly involve the Permian porphyry plateau cropping out in the north fluctuations in this trend are present and are clearly visible on the plateau ern part of the Pale di San Martino. area (Fig. 12A). This observation is likely related to the ramp-and-flat thrusting Thrusts and strike-slip faults were grouped according to their kinematics. geometry, which creates variations in the bedding in correspondence with the Based on the polymodal gaussian fit represented in the rose diagrams, thrust ramp sectors (Doglioni, 1985; Arragoni et al., 2016). and back-thrust faults (Fig. 10A) are low dipping and show a main N40° strike, In some parts of the plateau, bedding is thin (<1 m) and shows a wavy possibly compatible with the regional direction of the main thrust faults of surface due to karstification (Figs. 12B and 12C), but most commonly the the region and morphologically related with the second lineament domain rocks appear massive and bedding is barely recognizable due to the extensive identified by the automatic analysis (Fig. 4). A minor family of compressive dolomitization. structures strikes approximately N345° and may be related to a ENE-WSW compression similar to the Dinaric compression. Based on this observation, these minor compressive structures are likely older than the other deformation DISCUSSION structures in the area, as confirmed by their higher scattering. Only faults with clear sense of slip were plotted in Figure 10. The use of automatic lineament analysis provided the direction of the main The main strike-slip fault population is oriented N290° (Fig. 10B), followed regional crustal stress active in the investigated area, corresponding to a N340° by a minor fault population striking N334°. Slickenlines, slickenfibers, and syn orientation. The statistical elaboration of the earthquake focal mechanism data thetic cleavages indicate that the first set is related to a dextral sense of shear from Montone et al. (2004, 2012) and Montone and Mariucci (2016) confirmed whereas the second set is composed mainly of sinistral strike-slip faults. A this direction, with the eastern part of the Italian Alpine belt characterized by a dextral movement was observed also in some east-west– to ENE-WSW–strik main horizontal stress orientation of N344° (Fig. 13). Geodetic studies based on ing strike-slip faults, but due to the small number of measurements, this set GPS measurements (Devoti et al., 2011) reported residual horizontal velocities

Figure 8. Field photos of strike-slip faults. (A) Waterfall outflowing from a strike-slip fault in the upper Liera Valley close to the emergence of the Gares spring (label 2 in Fig. 2). Visible height of waterfall is ~20 m. (B) Groundwater emerging from tear faults (dashed red lines) associated with the thrust plane (red line, triangles indi cate the over-thrusted side) in the upper Canali Valley. The measuring tape is 1 m long. (C) Strike-slip fault in the eastern A part of the Pale di San Martino Plateau; the fault plane is highlighted with the pale red color.

1 m B C

(<1 mm/yr, computed with respect to a fixed Eurasia) in the eastern portion of WSW fractures are held closed by the active crustal stress. Field surveys have the Alps along a roughly north-south direction and vertical uplift velocities of demonstrated that a complex and heterogeneous pattern of fractures is pres the order of few millimeters per year. These movements, which were recorded ent in the study area due to the succession of various tectonic phases. Ac in the time span 1998–2009, further support the orientation of the active stress cording to the crustal stress analysis, the main sets of joints (oriented N300° regime of the studied region. In particular, the general south-verging motion and N334°) and the N334° azimuthal set of strike-slip faults are the most open reported by Devoti et al. (2011) is compatible with the maximum horizontal sets and thus exhibit the greatest ability to transport water in the investigated crustal stress derived from the lineament domain analysis, namely N340°. fractured aquifer. Given their vertical persistency and widespread distribution Based on the above considerations, the fractures with similar strike direction, throughout the Pale di San Martino area, these high-angle fractures are ex such as N340° ± 30°, can be considered preferentially open and result in a pected to contribute to the vertical permeability of the groundwater system larger contribution to groundwater flow. On the contrary, east-west to ENE- and to enhance the groundwater flow direction along their strike. On the other

Figure 9. Field photos of thrust faults. (A) Waterfall located in correspondence with a nearly east-west back-thrust fault (red line, triangles indicate the over- thrusted side) in the San Lucano Valley. (B) Intense ductile deformation charac terized by recumbent folds in the Bellero phon Formation, which likely acted as a major décollement level for the Pale di San Martino thrusting and back-thrusting. (C) Thrust fault located near the Travignolo A spring (label 3 in Fig. 2) showing a top-to- the-north sense of tectonic transport (red arrow).

BBCC

hand, it is worth noticing that the described brittle deformation affects the west of the plateau, suggesting that groundwater flow is prevented or charac dolomite rocks whereas the underlying Bellerophon and Werfen Formations terized by a longer residence time in the east-west direction. Dye tracing tests are characterized by more ductile behavior. This prevented their significant conducted in the Pale di San Martino in 2016 (Lucianetti, 2017) also confirm fracturing, thus preserving their role as aquicludes also after the tectonic frac that groundwater drains along NW-SE– to north-south–trending structures turing events that involved the investigated area. (faults and fractures). Mean tracer velocities exceeded 100 m/h, and tracer was In the Pale di San Martino area most of the springs are located to the south detected at the Pradidali spring (spring 1 in Fig. 2), Gares spring (spring 2 in and to the north of the plateau. The plateau can be considered one of the main Fig. 2) and Travignolo spring (spring 3 in Fig. 2). Minor concentrations of the recharge areas of the study site due to the flat and bare exposure of karst tracer (<0.1 ppb) were detected in the Angheraz spring (spring 4 in Fig. 2) after rocks, as in other high-alpine catchments (Plan et al., 2009). This distribution >3 months from the injection date. These very low peak concentrations are of the springs shows that groundwater flows preferentially from the plateau close to the detection limit of the instrument (0.02 ppb; Schnegg and Bossy, in a latitudinal direction as testified by the location of many springs emerging 2001) and may indicate a possible groundwater drainage in the east-west direc in correspondence with NW-SE–trending faults reported in Italian Geological tion, but sub-ordered with respect to the main drainage direction. Survey geological maps (SGN, 1970) and Zampieri (1987) (e.g., springs 1 and With reference to the bedding planes, given their average flat dip and the 3 in Fig. 2). On the contrary, very few springs are located to the east and to the massive stratification, they are thought to be insignificant with respect to

Figure 10. Rose diagrams resulting from the polymodal Gaussian fit of the field-mea sured thrust faults (A), strike-slip faults (B), and joints (C). The right part of the figure shows contouring of poles to planes (Schmidt projection, lower hemisphere).

1987; Castellarin et al., 1992). Following Cianfarra and Salvini (2014), the azi muthal scattering of lineaments indicated that the main lineament domain (N340°) is the least scattered among the three directions identified and thus is directly related to the youngest tectonic phase active in the area. Field surveys suggested a similar conclusion and confirm the tectonic evolution sequence already identified by previous authors (Bosellini and Doglioni, 1986; Doglioni, 1987; Castellarin et al., 1992). Our field data (refer to Figs. 10 and 11) are com patible with the tectonic phases proposed by those authors.

(1) The east-west compression related to the pre-Neogene Alpine phase (Doglioni, 1987), which is partly obliterated by the subsequent tectonic phases. This compression is likely responsible for the formation of NNW-SSE thrust faults (N340°–N350°) and of the east-west joint set. It is likely related also to the few NE-SW dextral strike-slip faults analyzed in the eastern part of the plateau. (2) The N340° Neogene compression and its subsequent rotation toward the N300°–N330° Adriatic compression. This phase caused the main up lift of the area and formed thrust faults in the direction perpendicular to the main compression. It is also responsible for the formation of dextral and sinistral strike-slip faults that can be interpreted as a conjugate sub

Figure 11. Stereoplot (Schmidt projection, lower hemisphere) of the bedding planes and con set of faults generated from the maximum horizontal stress (s1) nearly touring of poles to bedding. oriented NNW to NW. These strike-slip faults were likely formed to gether with the uplift of the area and accommodated these movements, acting as tear faults. The main joints also formed in this phase and are the overall groundwater circulation, at least where the massive structure pre mainly tectonic joints and extensional joints with strike directions com vails. This is confirmed also by cave development data that were provided by a patible with the direction of the strike-slip faults. It should be consid local speleological group. The data show that a small number of subhorizontal ered that slight discrepancies in the dominant strike directions could caves is present compared to the relevant number of vertical karst shafts. In reflect changes in the horizontal stress direction during the transitions particular, a total of 312 caves have been explored and mapped by the local between the Dinaric, Alpine, and the Adriatic compressions. Moreover, speleological group. According to the cave cadaster (P. Mietto, from the Proteo these differences can also be explained by preexisting fractures being Vicenza Speleological Club, personal commun., 2016), 102 caves are subhor already present before this tectonic phase and causing a deviating ef izontal passages (i.e., caverns, rock shelters) and 210 are characterized by a fect from Anderson’s theory behavior. subvertical development (i.e., shafts, pits). Most of the caves show a small vertical and horizontal cumulative length, and only 23 exceed 50 m. In a plan The present-day syncline-like shape of the Pale di San Martino results view, these caves present a main NW-SE to NNE-SSW development direction. from the activity of a thrust and back-thrust system that is part of the regional Nevertheless, it should be taken into account that only the northern part of Valsugana fold-and-thrust structure. The marls, clays, and gypsum-rich the Pale di San Martino Plateau was surveyed, thus the reported speleological layers of the Bellerophon and Werfen Formations acted as a décollement for information is not necessarily representative for the entire study area. the superficial thrusting of the Pale di San Martino. These units are in fact Previous studies carried out in similar Alpine contexts showed that in folded characterized by intense ductile deformation (Fig. 9B). The thrusting is likely karst aquifers, groundwater follows the plunge axis of syncline structures characterized by a ramp-and-flat geometry according to the tectonic style (Goldscheider, 2005; Gremaud et al., 2009). Despite the similar syncline-like proposed by other authors for this part of the Alpine orogen (e.g., Doglioni, shape of the studied aquifer, our findings highlighted the prevailing control of 1985; Schönborn, 1999), and is responsible for the observed intense fractur N-S to NW-SE fracture and fault networks on groundwater circulation and the ing of the Pale di San Martino aquifer. The main lineament domain showing minor role exerted by the folded geometry of the structure. a N340° direction of regional compression thus indicates that the fracture All of the information acquired by outcrop-scale investigations and by the systems nearly parallel to this direction are the main water carrier. In this way more regional scale of the lineament analysis may be framed within the pro the orientation of the main lineament domain corresponds to the preferential posed tectonic evolution of the area (Bosellini and Doglioni, 1986; Doglioni, drainage direction.

B C

Figure 12. Field photos showing the bedding undulations in the Pale di San Martino Plateau. (A) Aerial view toward the southeast of the Pale di San Martino. Bedding is sub-parallel to the topography. (B) Bedding planes on the Cima Rosetta peak dipping to the east. (C) Karstified bedding planes.

CONCLUSIONS fracture apertures of the Pale di San Martino aquifer. The use of the automatic lineament analysis provided the direction of the main regional crustal stress ac In this study, two different approaches were compared in order to investigate tive in the investigated area, corresponding to a N340° orientation. the regional crustal stress orientations. Firstly, the automatic lineament analysis Given their orientation, the main sets of joints oriented N300° and N334° was carried out on an ASTER GDEM. From this analysis, following the approach and the N334° azimuthal set of strike-slip faults are expected to be the most of Wise et al. (1985), the strike of the main regional horizontal stress was iden open sets and thus exhibit the greatest ability to transport fluids. In addition, tified. The second methodology consisted of the analysis of earthquake focal their widespread distribution, high dip, and relevant length, which in some mechanisms from previous data sets (Montone et al., 2004, 2012). The two inde cases cross-cuts the entire aquifer thickness, further support the hypothesis pendent approaches provide very similar results, suggesting that lineament do that these fractures largely control infiltration and groundwater flow. The more main analysis is a low-cost but powerful tool to investigate active-recent crustal ductile behavior of the underlying Bellerophon and Werfen formations pre stress. Although the relation between lineaments and crustal stress orientations vented their significant fracturing, leaving them the role of aquicludes even was previously studied by several authors (Pischiutta et al., 2013; Cianfarra and after the tectonic fracturing events that involved the overlying carbonates. The Salvini, 2014, 2015), this study represents an original application to groundwater location of the main springs and the results of dye tracer tests conducted in exploration. In this paper, the orientation of the crustal stress was used to infer the area confirm a NNW-SSE main drainage direction.

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