Analysis of systematic fracturing in Eocene flysch of the Slovenian coastal region Analiza sistematične razpokanosti eocenskih flišnih kamnin Slovenske obale Marko VRABEC & Galena JORDANOVA Univerza v Ljubljani, Naravoslovnotehniška fakulteta, Oddelek za geologijo, Privoz 11, SI –1000 Ljubljana, Slovenija; e-mail: marko.vrabec@geo.ntf.uni-lj.si Prejeto / Received 1. 3. 2017; Sprejeto / Accepted 10. 10. 2017; Objavljeno na spletu / Published online 22. 12. 2017 Dedicated to Professor France Šušteršič on the occasion of his 70th birthday Key words: Eocene flysch, systematic fractures, paleostress, joints, fracture spacing index, Istria, Slovenia Ključne besede: Eocenski fliš, sistematične razpoke, paleonapetosti, natezne razpoke, indeks oddaljenosti razpok, Istra, Slovenija Abstract We analyse systematic fractures occurring in sandstone beds in Eocene flysch of the Slovenian coastal area. Two nearly perpendicular fracture sets were identified: fractures F1 are generally NW-SE oriented, well- expressed and predominately planar, whereas fractures F2 are NE-SW-striking, shorter, more irregular in shape, and terminate against the F1 set. The average orientation of both sets does not change significantly in a coastal transect crossing all principal structural domains of the area. We analysed fracture spacing with respect to layer thickness and determined fracture spacing index for both fracture sets. We interpret both fracture sets as tensional (Mode I) joints originating in two distinct extensional episodes. Set F1 is older and formed in NE-SW directed tension which we correlate with the well-documented regional post-Dinaric orogen-perpendicular extension of presumably mid-Miocene age. Set F2 formed in NW-SE oriented tension, which is compatible with previously documented NE-SW-striking normal faults occurring in the area, but was so far not documented elsewhere. We interpret that F1 fractures predate folding and thrusting in the coastal belt. Earlier, Eocene-Oligocene Dinaric thrusting therefore did not significantly affect the coastal area, whereas post-F1 shortening, associated with northward indentation and underthrusting of the Adria microplate, did not commence before late Miocene. Izvleček Raziskali smo sistematične razpoke, ki se pojavljajo v plasteh peščenjaka v eocenskem flišu Slov- enske obale. Pojavljata se dve, medsebojno skoraj pravokotni družini razpok: razpoke družine F1 so generalno usmerjene v SZ-JV smeri in so dobro izražene in pretežno planarne, razpoke družine F2 pa imajo smer SV-JZ in so krajše, bolj nepravilno oblikovane in se zaključujejo na ploskvah razpok F1. Povprečna orientacija razpok obeh družin se bistveno ne spreminja vzdolž raziskanega profila, ki preči vse glavne strukturne domene ob Obali. Analizirali smo medsebojno oddaljenost razpok glede na debelino plasti in obema družinama določili indeks oddaljenosti razpok (fracture spacing index). Obe družini interpretiramo kot natezne razpoke tipa I, ki so nastale v dveh ločenih fazah natezne tek- tonike. Družina F1 je starejša in je nastala v SV-JZ usmerjeni tenziji in jo povezujemo z dobro pozna- no regionalno fazo post-Dinarske ekstenzije, ki je domnevno srednje miocenske starosti. Družina F2 je nastala v SZ-JV usmerjeni tenziji in je skladna z SV-JZ usmerjenimi normalnimi prelomi, ki so bili že prej dokumentirani v območju Obale, ni pa še bila odkrita v ostalih delih Slovenije. Po naši inter- pretaciji so razpoke družine F1 starejše od narivanja in gubanja v območju Obale. Zato sklepamo, da predhodno Dinarsko narivanje eocensko-oligocenske starosti ni bistveno prizadelo obalnega območja. Krčenje ozemlja po nastanku razpok F1, ki ga povezujemo s podrivanjem Jadranske mikroplošče proti severu, pa se ni začelo pred mlajšim miocenom. © Author(s) 2017. CC Atribution 4.0 LicenseGEOLOGIJA 60/2, 199-210, Ljubljana 2017 https://doi.org/10.5474/geologija.2017.014 200 Marko VRABEC & Galena JORDANOVA Introduction The territory of Slovenia was affected by polyphase tectonic deformation (e.g. vraBec et al. 2009 and references therein), which not only pro- duced a complex regional assemblage of tectonic units (Placer, 2008), but is also reflected in out- crop-scale structural geometry. It is therefore rare- ly possible to observe structurally homogeneous systematic sets of structures. One such exception occurs in the coastal region of Slovenia, which structurally represents the most external part of the Dinaric fold-and-thrust belt. Here, the gener- ally flat-lying to gently-dipping beds of Eocene fly- sch nearly everywhere have pervasive systematic fractures. We present a first structural analysis of these fracture sets with the aim to examine their geometry, to interpret their origin and tectonic significance, and to investigate whether meaning- ful statistical properties, specifically the fracture spacing index (FSI), can be derived to characterize the fracture sets. These data are useful not only for understanding the tectonic evolution of the area, but may also be relevant for various applications such as in hydrogeology, engineering geology, and in geothermal resources development. Geological setting The coastal region of Slovenia, geographi- cally situated in northwestern part of the Istria penninsula (Fig. 1), sits in the Adriatic – Apu- lian foreland of the fold-and-thrust belt of the northwestern External Dinarides (Placer, 2008; Placer et al., 2010). The coastal area was affect- ed by two major Tertiary shortening episodes: the Eocene-Oligocene SW-directed thrusting of the External Dinarides, and the subsequent un- derthrusting of the Adriatic microplate below the Dinarides and the Southern Alps, which is at present time NNW-directed (e.g. weBer et al, 2010). Major stratigraphic units of the area comprise Mesozoic – Palaeocene carbonates of the Adriat- ic carbonate platform, which are uncomfortably overlain by Eocene turbidites (flysch) deposited in SW-ward migrating foreland basin in front of the advancing Dinaric orogen (otoničar, 2007). Flysch in the Slovenian coastal area is of distal facies where well-bedded predominately cm-dm thick siliciclastic sandstones are interbedded with marls in approximately equal proportion (PavŠič & PeckMann, 1996). This monotonous se- quence is interrupted by a number of distinct 1 – 5 m thick calciturbiditic horizons, derived from the carbonate platform which persisted in front of the foreland basin throughout Eocene (PavŠič & PecKMann, 1996; Placer et al. 2004, vraBec & roŽič, 2014). In the offshore in the Gulf of Trieste, a several hundred m thick succession of Pliocene to Quaternary continental and marine sediments is covering the flysch sequence, with the topmost sedimentary cover represented by up to ten m of Holocene marine clay (Busetti et al., 2010). The transition from the inland karstic plateau of Kras where Mesozoic carbonate rocks prevail to the coastal lowland area dominated by Eocene flysch siliciclastics is structurally and morpho- logically defined by the NW-SE-striking Pal- manova Thrust System (Fig. 1), comprising sev- eral sub-parallel thrust faults with various local names, spanning from Palmanova in Italy to Mt. Učka in Croatia (Placer, 2008, carulli, 2011). A vertical displacement component of approx. 1000 m is inferred on this thrust system, hence the name Dinaric Frontal Ramp was also proposed (e.g. Busetti et al., 2010). External to this thrust boundary, between the Bay of Milje (Muggia) and Bay of Koper, careful structural mapping has revealed a series of low-angle thrust faults with km-scale displacements, cutting the Eocene flysch and locally producing vertical or even inverted DR1 IR1 IR2 SR4 SR3 SR2 SR1 FR1 FR2 FR3 FR4 FR5 Reka Trst Koper Poreč Pulj Tržič ISTRIA KRAS Postojna Gulf ofTrieste ADRIATIC SEA NG u l f o f T r i e s t e Bay of Milje Bay of Koper Strunjan structure Izola anticline Buzet Thrust Palmanova Thrust 5 km Mesozoic-Paleogene carbonates Eocene flysch thrust fault anticline asymmetric anticline with vergence direction unconformity data siteDR1 FIGURE 1 Fig. 1. Structural map of the investigated area with lo- cations of measurement si- tes (simplified from Placer, 2007, vraBec et al., 2014 & Placer, 2015). 201Analysis of systematic fracturing in Eocene flysch of the Slovenian coastal region bedding (Placer, 2007). Of those, the prominent Buzet Thrust, reaching the coastline at Koper, was inferred to have a displacement in excess of 10 km (Placer et al. 2004; Placer, 2007) and can be traced on marine seismic reflection profiles to- wards the middle of the Gulf of Trieste (vraBec et al., 2014). The considerable amount of shortening deformation and the generally low-angle geome- try of these structures led Placer (2007) to inter- pret them as reflecting a younger phase of con- vergence related to Adria underthrusting and not as the frontal imbricate structures of the Dinaric thrust system, as previously believed. Further southwest of the Buzet Thrust, the degree of deformation is much lower. Here, the strata are gently folded forming the Izola Anti- cline with Paleocene carbonates outcropping in the fold core near the town of Izola (Fig.1). The most external deformation observed along the Slovenian coastline occurs at Strunjan, where several NE-verging reverse faults and asymmet- ric folds disrupt the flysch sequence (Strunjan structure of Placer, 2005). Still further south- west, from Fiesa to Piran, the beds are essential- ly undeformed and nearly flat-lying. At least two extensional episodes were also documented in the region. First is a NE-SW ori- ented extension postdating the Dinaric thrusting and occurring everywhere along the Dinarides. This extension is probably concurrent with mid-Miocene extension and basin subsidence in the Pannonian domain (vraBec & Fodor, 2006; ŽiBret & vraBec, 2016). A second episode, docu- mented so far only in the coastal area, exhibits NW-SE-oriented tension, which is manifested in normal faults with NE-SW strike, occurring both in map-scale (Placer, 2005) and in outcrop-scale (vraBec & roŽič, 2014). The youngest tectonic phase documented in the region is the ongoing NNW-SSE directed compression, which probably started in Pliocene (vraBec & Fodor, 2006; weBer et al., 2010; see also Moulin et al., 2016). This phase is mainly mani- fested by dextral slip on NW-SE-striking faults (ŽiBret & vraBec, 2016), which however are not common in the Slovenian coastal area. Methods We collected fracture data at 12 sites spanning all principal structural domains of the coastal region (Fig. 1): the low-angle thrust system be- tween the Palmanova Thrust and Buzet Thrust (site DR1), the southern limb of the Izola Anti- cline (sites IR1 and IR2), the Strunjan structure (sites SR1 to SR4), and the undeformed foreland (sites FR1 to FR5). A list of sites with their coor- dinates is presented in Table 1. At each site, we measured fractures in several sandstone beds of different thickness to provide representative bed thickness / fracture density data. We were pri- marily choosing beds with good 3D exposure to maximize measurement quality. Orientations of bedding planes and fractures were measured with digital geological compass GeoClino Digital Clinometer, which provides fast and reliable measuring procedure, and conven- iently facilitates data storage into internal mem- ory. To ensure measurement precision, digital compass was carefully calibrated according to manufacturer instructions at each measurement site. For additional control, digital readouts were checked several times against measurements with classical Freiberg-style geological compass. Fracture spacing and bed thickness was meas- ured with ordinary measuring tape. Fracture spacing was measured perpendicular to fracture strike. Sections of the outcrops with well-devel- oped straight and parallel fracture traces were exclusively considered for measuring. Orientation data were processed and analysed with Orient software, version 2.1.2.2 (vollMer, 2015). Software utility GeoCalculator version 4.9.8 written by Rob Holcombe was used to cal- culate intersecting angles between fracture sets. Fractures were separately analysed on a per-bed, per-site and per-domain basis. Structural analysis Strata at investigated sites predominately dip gently to the SSW (Table 1; Fig. 4a). Dip angles mostly range between 10° and 15°, except in the most external part of the study area (sites FR), where bedding is nearly flat-lying. At all inves- tigated sites, systematic fractures predominate- ly occur in thick (decimetre-scale) sandstone beds as planar to moderately curviplanar planes (Fig. 2). Only occasionally fractures exhibit more irregular configurations resembling polygonal structure, but such configurations appear to be limited to certain beds, whereas adjacent beds above and below in the stratigraphic succession normally have more regular fracture geometry. 202 Marko VRABEC & Galena JORDANOVA                               FIGURE 2      Fig. 2. Typical outcrop examples of investigated systematic fractures in Eocene flysch. a) and b) Debeli rtič (site DR1). c) Bele skale (site IR1). d) Rtič Strunjan (site SR3). e) Fiesa (site FR1). f) Fiesa (site FR2). g) Calcite-filled tension gash parallel to F1 fracture planes, Strunjan (close to Site SR1). h) Plumose ornamentations on F1 fracture plane, indicating horizontal (Mode I) propagation directed from right to left, Rt Ronek (between sites SR4 and IR2). 203Analysis of systematic fracturing in Eocene flysch of the Slovenian coastal region Systematic fractures appear in two well-de- fined sets, which we name F1 and F2 (Table 1; Figs. 2, 3, 4). The NW-SE-striking F1 fractures are normally longer with more straight traces. The generally NE-SW-trending F2 fractures are shorter and mainly appear to terminate against F1 set, although this relationship between the two sets is not everywhere clear. F2 fractures are also more curviplanar and irregular in orienta- tion. This is reflected in orientation distributions (Fig. 3), where F2 fractures show considerably greater spread of orientations compared to F1 fractures. The angle of intersection between the two sets is quite constant at around 80° on the average, but ranges from 65° to nearly perpendic- ular (Table 1). The NW-SE orientation of F1 fractures is rel- atively stationary between the sites and in all structural domains investigated in the study, with the exception of Strunjan structure (Figs. 3, 4b, 4d). There, a pronounced deviation to WNW- ESE or even E-W orientation is observed. Addi- tionally, a large variation in fracture sets orien- tation from bed to bed in the same section can be seen (see data for sites SR1 and SR2 in Fig. 2), reflecting perhaps the mechanical influence of intervening soft layers and the varying thickness of the sandstone beds and their interlayers. This was not noted at other sites. Within the Strunjan structure also F2 fracture orientations deviate from their general NE-SW trend towards a more N-S orientation, which is particularly clear at site SR2 (Fig. 2). site present-day orientation restored to horizontal position F1 fractures F2 fractures DR1 IR1 IR2 Fig. 3. Measured fracture orientations in spherical projection (equal-area projection, lower hemisphere). First column: pre- sent-day orientation. Bedding orientation is displayed with thick dark-grey traces. Second column: data backtilted to original (bedding-horizontal) position. Thick black traces indicate the average orientation (medium vector) of each fracture set. Third and fourth column: circular histogram of fracture strike directions (in backtilted orientation) for each fracture set. Bin size is 5°. 204 Marko VRABEC & Galena JORDANOVA site present-day orientation restored to horizontal position F1 fractures F2 fractures SR1 P1 SR1 P2 SR1 P3 SR2 P1 SR2 P2 SR3 205Analysis of systematic fracturing in Eocene flysch of the Slovenian coastal region site present-day orientation restored to horizontal position F1 fractures F2 fractures SR4 FR1 FR2 FR3 FR4 FR5 206 Marko VRABEC & Galena JORDANOVA206 arko VRABEC & Galena JORDANOVA avg. bedding = 192/08 F1AVG = 038/84 F2AVG = 306/82 σ3-F1 = 218/06 σ3-F2 = 126/08 a) b) c) F1AVG = 050/86 F2AVG = 320/83 σ3-F1 = 229/04 σ3-F2 = 139/07 d) e) F2 F1 σ3-F2σ3-F1 F2 F1 σ3-F2 σ3-F1 Fig. 4. Summary plots of orientations of investigated structures, averaged for each measurement site. a) Bedding. b) Circular histogram of fracture sets orientations. Bin size is 10°. c) Inferred directions of tensional stress axes from fracture sets F1 in and F2, assuming tension is perpendicular to the plane of average fracture orientation. d) and e): same as in b) and c), but excluding sites SR from the Strunjan structure. See text for discussion. In few places, weak plumose ornamentations were found on F1 and F2 fracture faces (Fig. 2h), suggesting Mode I opening with horizontal prop- agation direction. Additionally, up to several mm thick calcite-illed tensional gashes paralleling F1 fractures are occasionally found in sandstone beds (Fig. 2g). We therefore interpret F1 and F2 fractures as tensional joints which originated perpendicular to the axis of minimal stress σ3. According to this interpretation, we infer the σ3 direction at the time of F1 formation to match the normal to average fracture orientation, which implies that F1 joints formed in NE-SW-directed tension (Table 1; Figs. 4c, 4e). This matches well with the known regional extensional episode of probable mid-Miocene age (Vrabec & Fodor, 2006), which was documented in External Di- narides of central Slovenia by fault-slip data in- version (Žibret & Vrabec, 2016). From the observed ield relationships we in- terpret that F2 fractures postdate the F1 frac- tures. Inferred σ3 orientations (Table 1; Figs. 4c, 4e) suggest their origin in NW-SE-oriented tension. We therefore presume that F2 fractures formed in an independent extensional phase concurrently with the NE-SW-striking normal faults that were previously documented both in outcrop scale in the coastal cliff (Vrabec & roŽič, 2014) and in map scale in the interior (Rokava Fault of Placer, 2005). Fig. 4. Summary plots of orientations of investigated structures, averaged for each measurement site. a) Bedding. b) Circular histogra of fracture sets orientations. Bin size is 10°. c) Inferred directions of tensional stress axes from fracture sets F1 in and F2, assuming tension is perpendicular to the plane of average fracture orientation. d) and e): same as in b) and c), but excluding sites SR from the Strunjan structure. See text for discussion. In few places, weak plumose ornamentations were found on F1 and F2 fracture faces (Fig. 2h), suggesting Mode I opening with horizontal prop- agation direction. Additionally, up to several mm thick calcite-filled tensional gashes paralleling F1 fractures are occasionally found in sandstone beds (Fig. 2g). We therefore interpret F1 and F2 fractures as tensional joints which originated perpendicular to the axis of minimal stress σ3. According to this interpretation, we infer the σ3 directio at the time of F1 formation to match the normal to av rage fracture orientation, whi implies that F1 joints ormed in NE-SW-directed tension (Table 1; Figs. 4c, 4e). This matches w ll with the known regional exten ional episode of probable mid-Miocene age (vraBec & Fodor, 2006), which was documented in External Di- narides of central Slovenia by fault-slip data in- version (ŽiBret & vraBec, 2016). From the observed field relationships we in- terpret that F2 fractures postdate the F1 frac- tures. Inferred σ3 orientations (Table 1; Figs. 4c, 4e) suggest their origin in NW-SE-oriented tension. We therefore presume that F2 fractures formed in an independent extensional phase c ncurrently with the NE-SW-striking norm l faults that were previously documented both in outcrop scale in the coasta cliff (vraBec & roŽič, 2014) and in map scale in the interior (Rokava Fault of Placer, 2005). 207Analysis of systematic fracturing in Eocene flysch of the Slovenian coastal region site site coordinates bedding orientation fracture set F1 fracture set F2 angle between D1 and D2 φ λ N average orientation dispersion (°) average pole orientation N average orientation dispersion (°) average pole orientation DR1 45°35‘23.8“N 13°42‘12.1“E 022/13 73 213/78 2 033/12 88 320/85 2 140/05 70 IR1 45°32‘07.4“N 13°37‘29.2“E 185/12 47 046/78 3 226/12 26 311/79 11 131/11 87 IR2 45°32‘23.0“N 13°37‘02.1“E 186/16 30 062/80 4 242/10 29 317/71 7 137/19 79 SR1-1 45°31‘59.8“N 13°36‘03.7“E 187/11 9 354/79 5 174/11 12 267/86 5 087/04 86 SR1-2 45°31‘59.8“N 13°36‘03.7“E 184/13 29 015/81 11 195/09 20 277/85 6 097/05 83 SR1-3 45°31‘59.8“N 13°36‘03.7“E 159/08 13 217/85 19 037/05 23 282/83 39 102/07 65 SR2-1 45°31‘59.8“N 13°36‘03.7“E 202/13 24 051/79 3 231/09 20 318/81 5 138/09 89 SR2-2 45°31‘59.8“N 13°36‘03.7“E 180/06 10 204/77 126 024/13 16 293/83 142 113/07 87 SR3 45°32‘13.0“N 13°36‘07.2“E 214/32 11 026/53 4 206/37 9 313/78 44 133/12 69 SR4 45°32‘15.3“N 13°36‘15.8“E 190/11 40 020/83 3 200/07 42 268/80 11 088/10 70 FR1 45°31‘42.2“N 13°34‘27.4“E 188/07 81 049/89 32 229/01 69 314/87 5 134/03 85 FR2 45°31‘43.0“N 13°34‘25.8“E 205/05 22 050/85 11 230/05 29 326/84 164 146/06 87 FR3 45°31‘45.3“N 13°34‘15.7“E 184/03 49 053/86 2 233/04 58 327/83 3 147/07 86 FR4 45°31‘43.8“N 13°34‘22.8“E 172/02 9 052/82 6 232/08 14 329/87 8 149/03 83 FR5 45°31‘35.6“N 13°34‘40.1“E 222/04 14 051/85 2 231/05 10 314/87 8 134/03 83 Table 1. Site coordinates and average orientation of fracture sets for each investigated site. Fracture Spacing Index It is commonly observed that the density, or spacing, of systematic fractures in layered rocks is roughly proportional to layer thickness, but may also be dependent on rock type (e.g. Davis & reYnolds, 1996). In our study area, clearly ex- pressed fractures only occur in sandstone beds, therefore the effect of lithology on their spac- ing should be negligible. For analysing fracture spacing – layer thickness relationship it is com- mon to use the median of fracture spacing for each measured bed since the frequency distribu- tion of interfracture distances is usually skewed from normal distribution (narr & suPPe, 1991). A summary of our fracture spacing measure- ments is presented in Table 2. For each joint set, we then plotted median joint spacing against the respective layer thickness (Fig. 5) to derive frac- ture spacing index (FSI), which is the slope of the regression line in this plot (narr & suPPe, 1991). Thus defined, larger FSI values indicate higher joint density. We find that spacing of joint sets F1 and F2 in Eocene flysch sandstone of the Slovenian coastal area is reasonably-well correlated to bed thick- ness (Fig. 5). Whereas the recorded range of joint spacing is quite high, the median values tend to stay at the lower end of the range, reflecting that modest joint spacings dominate the frequency distribution. For both joint sets, the dispersion in median joint spacings values appears to be lower for smaller layer thicknesses under 20 cm than for layers between 20 and 35 cm thick. For joint set F1 the derived FSI value is 0.483 with correla- tion coefficient R2 of 0.6331. For joint set F2 the FSI value is 0.616 with correlation coefficient R2 of 0.5035. We conclude that joint spacing in sets F1 and F2 is reasonably predictable. The joint density may additionally depend on the degree of de- formation. We could not conclusively determine this since the total number of measurements was relatively low, therefore FSI determinations for individual structural domains did not yield satis- factory high correlations. Moreover, the structur- al domains used in this study were defined with respect to the style and amount of shortening deformation, whereas the joints have originated in independent tensional episodes. Joint density is therefore not likely to spatially correlate with shortening domains. 208 Marko VRABEC & Galena JORDANOVA site bed bed thickness (cm) fracture set F1 spacing (cm) fracture set F2 spacing (cm) median average standard deviation minimum maximum median average standard deviation minimum maximum SR1 P1 17,0 26,3 28,5 15,4 6,9 64,8 34,3 42,2 30,8 16,1 76,1 SR1 P2 8,5 18,5 19,4 8,6 3,1 35,1 7,1 11,0 8,1 3,8 27,3 SR1 P3 11,1 21,3 23,0 11,4 8,1 52,3 15,9 17,5 8,1 4,2 33,0 SR2 P1 16,2 19,6 21,1 11,9 3,1 47,2 18,2 19,3 8,7 5,2 38,0 SR2 P2 13,0 19,2 25,3 16,5 6,9 62,9 26,0 26,3 15,7 2,5 50,9 SR3 P1 13,0 26,2 21,0 11,2 6,8 32,5 16,0 19,3 9,0 9,5 34,1 SR4 P1 10,0 16,1 11,7 7,7 2,6 21,6 11,2 16,3 4,6 3,9 28,6 SR4 P2 10,2 9,0 8,3 2,3 4,1 12,4 8,4 9,8 3,5 4,9 17,9 IR1 P1 5,0 7,6 7,6 2,5 3,1 12,5 10,2 11,0 5,2 2,9 21,0 IR1 P2 4,6 6,3 7,1 2,6 3,1 12,6 13,2 12,7 3,6 4,9 17,6 IR2 P1 24,0 54,9 58,7 13,6 41,7 81,3 33,2 31,0 6,1 24,0 36,8 IR2 P2 26,4 39,3 33,8 17,5 5,0 60,1 19,8 18,5 7,1 2,7 30,2 FR1 P1 32,0 33,4 36,6 21,1 7,9 89,0 33,6 35,4 10,9 23,4 57,3 FR1 P2 16,0 28,2 29,5 11,4 8,3 59,1 19,3 20,5 8,0 8,6 35,1 FR2 P1 9,0 25,5 25,2 7,4 12,4 39,3 15,0 16,2 6,1 8,2 31,4 FR3 P1 12,0 23,6 22,5 11,2 2,2 47,2 13,4 15,2 6,9 4,5 26,2 FR3 P2 10,5 16,0 17,7 7,7 2,2 47,2 11,2 11,3 5,5 1,8 30,0 FR4 P1 17,7 26,1 25,6 10,5 12,6 47,9 25,9 26,1 4,9 19,7 33,7 FR5 P1 23,2 47,7 47,4 20,5 14,8 81,8 13,3 14,5 6,4 8,8 21,5 DR1 P1 11,5 26,9 27,1 10,0 5,8 55,4 16,7 17,1 7,1 6,1 34,5 DR1 P2 24,8 31,9 36,8 17,7 12,3 74,4 29,0 27,2 8,6 9,6 39,1 DR1 P3 12,2 25,4 23,3 7,0 9,0 34,1 20,0 20,0 8,1 7,6 34,3 Table 2. Summary statistical data for fracture spacing measurements. Fig. 5. Fracture spacing index (FSI) determination in Eocene flysch sandstone layers according to narr & suPPe (1991). Plotted are median fracture spacing distances (black dots) with indicated range of measured spacing values against respective layer thicknesses. FSI value corresponds to the slope of the regression line. a) Fracture set F1; b) fracture set F2. 209Analysis of systematic fracturing in Eocene flysch of the Slovenian coastal region Discussion and conclusions The first structural study of systematic frac- tures in Eocene flysch sandstones in the fore- land of the External Dinarides fold-and-thrust belt of the Slovenian coastal area has revealed that the fractures occur in two major sets. The first set F1 comprises NW-SE oriented tensional (Mode I) joints which originated in NE-SW di- rected tension. This direction is roughly paral- lel to the shortening direction implied from the orientation of thrusts and folds in the coastal area, therefore we assume that extension is in- dependent of thrusting (i.e., it does not represent shortening-perpendicular extension which is relatively common in thrustbelts). We correlate the origin of F1 joints to the well-documented regional extensional episode, which postdates Dinaric thrusting (see ŽiBret & vraBec, 2016 for outcrop-scale and map-scale evidence). In the Slovenian coastal area however, the F1 joints and associated tensional gashes clearly formed when bedding was still in horizontal orientation. This implies that Dinaric shortenting did not signifi- cantly (if at all) affect the Slovenian coastal area. The thrusting and folding which tilted the beds must therefore be both post-Dinaric and young- er than NE-SW extension which is presumably of mid-Miocene age (vraBec & Fodor, 2006). This conclusion corroborates well with the early inter- pretation of Placer (2007) that the coastal short- ening deformation reflects young underthrusting of the Adria microplate which is independent of and subsequent to Dinaric thrusting. Our ten- tative chronology constrains the start of under- thrusting to Late Miocene, as already presumed by Placer (2007), or even to Miocene – Pliocene transition. Fractures of the second set F2 are NE-SW-ori- ented and intersect F1 joints in a nearly perpen- dicular orientation, with the average intersection angle of 80°. In our interpretation F2 fractures are younger and probably originated as tensional joints in a distinct extensional episode with NW- SE oriented tension, which was so far not docu- mented in other parts of Slovenia. The relative timing of this second extension is not clear as the available data is contradictory. The orienta- tion of F2 joints and of mesoscopic normal faults originating in the same extensional regime (vra- Bec & roŽič, 2014) with respect to bedding im- plies that this extension also predates the tilting of the beds and is therefore probably older than shortening. On the other hand, the map-scale Rokava Fault, also implying NW-SE extension, is interpreted by Placer (2005) to cut and post- date the Buzet Thrust, a major thrust structure of the post-Dinaric coastal belt, but it apparent- ly does not cut other thrusts in the hinterland of Buzet Thrust. Finally, geodetic data from GNSS campaigns convincingly demonstrate ongoing NNW-SSE-directed shortening in the area with no indications for contemporaneous NW-SE ex- tension (weBer et al., 2010). Our study shows that the orientations of F1 and F2 fractures remain reasonably constant along the entire transect crossing all major struc- tural domains of the coastal belt. This implies that the fracture orientations probably reflect regional-scale paleostress fields and are largely independent of local influences. The only signif- icant deviations occur in the Strunjan structure, which is a frontal deformed area with reverse faults and asymmetric folds verging in the oppo- site direction to the overall SW-ward vergence of the coastal area structures. We also investigated the fracture spacing with respect to layer thickness to determine the fracture spacing index (FSI), and found that the correlation is reasonably good. The systematic relationships of fracture orientations and their spacing with respect to bedding thickness estab- lished in this study may be utilized in future en- gineering and resource utilization projects. 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