2019 | 72/3 | 179–193 | 13 Figs. | 1 Tab. | www.geologia-croatica.hr Journal of the Croatian Geological Survey and the Croatian Geological Society 1. INTRODUCTION The Vinodol Valley is a 20 km long NW‒SE oriented valley which runs parallel to the NE Adriatic coastline, stretching from Križišće in the NW to Novi Vinodolski in the SE. The area of the Bakar Bay represents its geomorphological and geological extension to- wards the NW (Fig. 1). The most important geological investigation of the wider area of the Vinodol Valley and Bakar Bay was undertaken for the Basic Geological Map of SFRY at a 1:100,000 scale (ŠUŠNJAR et al., 1970). Accompanying explanatory notes (GRIMANI et al., 1973) also include a review of previous studies. In addition to these geological investigations associated with the Basic Geo- logical Map and its explanatory notes, the most important geo- logical and structural studies of the Vinodol Valley were those by BLAŠKOVIĆ (1991, 1997, 1998, 1999, 2005), in which he pro- posed a tectonic concept of the Vinodol Valley area. The area of the Vinodol Valley and Bakar Bay was investi- gated as a seismotectonically active area by PRELOGOVIĆ et al. (1981, 1982, 1995), HERAK et al. (1996, 2017), MARKUŠIĆ & Geological and structural setting of the Vinodol Valley (NW Adriatic, Croatia): insights into its tectonic evolution based on structural investigations Damir Palenik1*, Dubravko Matičec1, Ladislav Fuček1, Bojan Matoš2, Marijan Herak3 and Igor Vlahović2 1 Croatian Geological Survey, Department of Geology, Sachsova 2, HR-10000 Zagreb, Croatia; (*corresponding author: dpalenik@hgi-cgs.hr) 2 University of Zagreb, Faculty of Mining, Geology and Petroleum Engineering, Department of Geology and Geological Engineering, Pierottijeva 6, HR-10000 Zagreb, Croatia 3 University of Zagreb, Faculty of Science, Department of Geophysics, Horvatovac 95, HR-10000 Zagreb, Croatia doi: 10.4154/gc.2019.13 Abstract The area of the Vinodol Valley and Bakar Bay represents a NW‒SE oriented valley in the NW Adriatic characterised by prominent historical and instrumentally recorded seismicity. As part of the greater geodynamic domain including the Ilirska Bistrica–Rijeka–Senj seismogenic fault zone, new geological and structural data addressing the tectonic evolution of the area were col- lected in order to better understand the focal mechanisms of previous earthquakes and to en- able identification of potential seismogenic sources. Mapped informal lithostratigraphic units mostly correspond to the Upper Cretaceous, Palaeo- gene and Quaternary successions described in other parts of the External Dinarides. However, a shorter stratigraphic range of the Gornji Humac fm., the youngest Cretaceous unit in the study area, was determined and suggests that the uplifted area in the central NW part of the Adriatic Carbonate Platform already comprised several thousand km2 (from W and NW Istria to Krk and Vinodol area) at the end of the Turonian. Structural measurements of the fault planes in the study area generally correspond to the exist- ing structural model of the tectonic evolution of the Dinarides. However, in contrast to the SW vergences typical of the Dinarides, NE-vergent reverse structures are common, especially along the SW margin of the Vinodol Valley. Cross-cutting relationships suggest that transpressional (NW–SE and NE–SW striking dextral and sinistral faults) and extensional features (NW–SE and NE–SW striking normal faults) are structurally concurrent or younger than the reverse faults, suggesting a change in the palaeostress field during the Neogene–Quaternary, with prevalent transpression and radial extension. Comparison of results of the palaeostress field analysis and the constructed synthetic focal mechanisms on one side, with available focal mechanism solu- tions for earthquakes within the Ilirska Bistrica–Rijeka–Senj seismogenic fault zone on the oth- er, shows a favourable orientation of the observed NW‒SE and NE‒SW striking faults with re- spect to the recent compressional/transpressional stress field (N‒S oriented P-axis), indicating these as potential seismogenic sources within the study area. HERAK (1999), HERAK & HERAK (2017), and IVANČIĆ et al. (2018). These authors reported data on instrumental and his- torical seismic activity in the area and discussed the potential seismic sources. Research presented here was performed in the framework of two ongoing scientific projects funded by the Croatian Science Foundation (HRZZ): VELEBIT (Grant no. IP-2014-09-9666) and GEOSEKVA (Grant no. IP-2016-06-1854). The VELEBIT project represents a multidisciplinary research approach addressing cor- relation of seismological data and tectonic framework in the neighbouring Velebit Mt. region, which is characterised by low seismicity and weak to moderate magnitude (M ≤ 5) earthquakes at shallow depths. The main research objectives of the GEO- SEKVA project are the production of new geological maps (at 1:25,000 and 1:50,000 scales) and 3D geological models of se- lected structures in the wider Kvarner region, as well as the broadening of the seismic station network and construction of a relevant seismotectonic model of the Kvarner region. Article history: Manuscript received January 28, 2019 Revised manuscript accepted May 31, 2019 Available online October 31, 2019 Keywords: NW Adriatic, Vinodol Valley, Bakar Bay, SW and NE vergences, compression/transpression, neotectonic activity, focal mechanism solutions, seismicity G eo lo gi a C ro at ic a Geologia Croatica 72/3180 The main focus of the research in the Vinodol Valley was the geological mapping from Križišće in the NW to Kričina in the SE accompanied by detailed structural-geological research of the Vi- nodol Valley and Bakar Bay (its continuation towards the NW). Such investigation, together with new data on the focal mecha- nisms of previous earthquakes could contribute to greater under- standing of the tectonic evolution of the study area, including fault kinematics and identification of potential seismogenic structures. 2. METHODOLOGY 2.1. Geological mapping of the study area The first phase of the research was focused on production of the new geological map of the Vinodol Valley and its border areas based on lithostratigraphic principles. The central part of the Vi- nodol Valley has been mapped at 1:5,000 scale, while the mar- ginal parts were mapped at 1:25,000 scale. Due to the intense tectonics, thicknesses of individual lithostratigraphic units were estimated from geological cross-sec- tions and compared with contemporaneous lithostratigraphic units from neighbouring areas. 2.2. Structural-geological investigation and analysis Structural measurements were performed along the SW and NE edges of the Vinodol Valley as well as in the Bakar Bay. Within an approximately 30 km long and 1–5.5 km wide area, structural data on outcrop-scale fault slips were collected. During analysis, for determination of fault kinematics in relation Figure 1. Location map and schematic geological map of the investigated and wider areas of the Vinodol Valley and Bakar Bay. Modified from the Basic Geological Map of the Republic of Croatia 1:300,000 (HGI, 2009). G eologia C roatica Palenik et al.: Geological and structural setting of the Vinodol Valley (NW Adriatic, Croatia): insights into its tectonic evolution based on structural investigations 181 to the past and present stress fields, we used data of field meas- urements of dip direction and dip angle of fault planes and orien- tation of carbonate slickensides defined by azimuth and plunge, and, wherever possible, their sense of movement. Despite the relatively weathered outcrops and steep terrain, more than 120 sets of shear joint/fault planes data were gathered at 127 stations along the NE and SW margins of the Vinodol Valley and Bakar Bay. In some places, overlapping and crosscutting relationships between the observed striations enabled the relative temporal comparison of deformation. Based on kinematic criteria, the structural data were separated into compatible datasets and pro- cessed by Tectonics FP software (ORTNER et al., 2002). Using the P–T axis method (TURNER, 1953; MARRETT & ALL- MENDINGER, 1990) theoretical maximum (σ1), intermediate (σ2) and minimum stress axes (σ3) were calculated, whereas using the computed Right Dihedra Method (ANGELIER & MECH- LER, 1977) synthetic focal mechanisms for the analysed fault segments were determined, i.e., palaeo-synthetic focal mecha- nisms as representations of the palaeostress fields. 3. RESULTS 3.1. Geological mapping of the Vinodol Valley Geological mapping determined that both margins of the Vinodol Valley are in general fault-bounded, whereas earlier proposals suggested that only the NE margin was fault bounded (ŠUŠNJAR et al., 1970). Along the NE edge of the Vinodol Valley reverse faults are characterised by tectonic transport to the SW, while those along the SW edge of the Vinodol Valley mostly indicate the opposite direction of tectonic transport, towards the NE, which is, although quite rare in the Dinarides, very common in the struc- tures of the neighbouring Velebit Mt. and Krk island (VLAHOVIĆ et al., 2012; ŚRODOŃ et al., 2018; TOMLJENOVIĆ et al., 2018). In addition to a completely new detailed lithostratigraphic subdi- vision of deposits, this is one of the key improvements in respect to the previous geological map of the area (ŠUŠNJAR et al., 1970). On the basis of the geological map (Fig. 2) five transverse geological cross-sections of the Vinodol Valley have been con- structed (Fig. 3). 3.2. Lithostratigraphic units of the Vinodol Valley Previous geological maps were based on the chronostratigraphic approach (ŠUŠNJAR et al., 1970; BLAŠKOVIĆ, 1999), so the map presented here is the first one based on lithostratigraphic principles. Upper Cretaceous lithostratigraphic units were named after, and compared to, the informal lithostratigraphic units al- ready defined in other areas of the External Dinarides (GUŠIĆ & JELASKA, 1990). A total of three Upper Cretaceous, three Pal- aeogene and four Quaternary units have been described in the study area. 3.2.1. Upper Cretaceous The oldest informal lithostratigraphic unit of the Vinodol Valley is the Cenomanian Milna fm., corresponding to the succession at the locus typicus – the island of Brač (GUŠIĆ & JELASKA, 1990). It is composed of well-bedded limestones of a light brown- Figure 2. Geological map and schematic geological column of the Vinodol Valley from Križišće to Kričina. G eo lo gi a C ro at ic a Geologia Croatica 72/3182 grey colour composed of mudstones to wackestones and peloid– bioclastic packstones and grainstones with variable amounts of radiolitid rudist debris and rare ostreid shells. Rudist floatstones and/or rudstones are relatively rare, as are layers containing chon- drodonts and LLH stromatolites, indicating deposition in shallow subtidal to peritidal environments. The microfossil assemblage includes the benthic foraminifera Chrysalidina gradata d’ORBIGNY, Nummuloculina regularis PHILIPPSON, Broe- ckina (P.) balcanica CHERCHI et al., Pseudolithuonella reicheli MARIE and alveolinids, indicating a Middle to Upper Cenoma- nian age. Macrofossils are relatively rare, represented by indeter- minate radiolitid rudists and chondrodont shells. The thickness of the Milna fm. may be estimated at c. 250 m (not including the lower part of the unit which does not crop out in the study area). Global relative sea-level rise during the Late Cenomanian‒ Early Turonian (HAQ et al., 1987; HARDENBOL et al., 1998) was also characterised by the widespread Oceanic Anoxic Event 2 Figure 3. Transverse geological cross-sections of the Vinodol Valley. G eologia C roatica Palenik et al.: Geological and structural setting of the Vinodol Valley (NW Adriatic, Croatia): insights into its tectonic evolution based on structural investigations 183 (OAE2; SCHLANGER & JENKYNS, 1976) which, in the area of the Adriatic Carbonate Platform, is recognized as an informal lithostratigraphic unit – the Sv. Duh fm. (GUŠIĆ & JELASKA, 1990). In the studied area the unit is composed of thick-bedded light brownish mudstones and mudstone‒wackestones. Besides a vari- able, but significant amount of pithonelomorphic calcispheres (Pithonella ovalis (KAUFMANN), Pithonella sphaerica (BONET) and Pithonella innominata (BONET)) rocks of this unit contain rare indeterminate planktonic foraminifera, and in the lower and upper part shallow-marine ostracods and miliolids as well as bio- clasts of gastropods and bivalves. The Sv. Duh fm. was deposited within temporary drowned platform environments, and transitions from the underlying and to the overlying shallow-marine deposits are gradual. Based on the superposition, microfossil assemblage and similar deposits in neighbouring Istria (VLAHOVIĆ et al., 2002) stratigraphic age of Sv. Duh fm. may be estimated as Upper Cenomanian to Lower Turonian. Due to intense post-sedimentary tectonics in the study area thickness of deposits cannot be meas- ured directly, but may be approximated at 130‒150 m. The Gornji Humac fm. (first established on the island of Brač by GUŠIĆ & JELASKA, 1990) represents the youngest Cre- taceous deposits in the study area, cropping out along the slopes of the Vinodol Valley and representing the footwall of the trans- gressive Palaeogene Foraminiferal limestones. It is composed of fenestral mudstones, mudstone‒wackestones with benthic fo- raminifera, calcareous algae, ostracods, small bioclasts and pe- loids, packstones with peloids and benthic foraminifera and rela- tively rare rudist floatstones. They were deposited in re-established peritidal environments and in places contain numerous Decas- tronema (Aeolisaccus) kotori (RADOIČIĆ) and Thaumato- porella parvovesiculifera (RAINERI), microfossils typical for this unit over the entire Adriatic Carbonate Platform area. Find- ings of the benthic foraminifera Pseudocyclammina sphaeroidea GENDROT indicates an Upper Turonian age for the deposits of this lithostratigraphic unit. In the study area, the uppermost part of the Gornji Humac fm. deposits is very karstified, more or less intensely recrystallized and coloured yellowish to reddish due to a very long stratigraphic hiatus (its duration may be estimated at c. 35–40 My). Therefore, the total thickness of the Gornji Humac fm. varies laterally from c. 70 to around 100 m. 3.2.2. The Palaeogene Eocene Foraminiferal limestones crop out along the slopes of the Vinodol Valley, transgressively overlying the Upper Creta- ceous deposits of the Gornji Humac fm. This unit is characterised by significant lateral and vertical facies diversity, probably caused by variable palaeorelief resulting in the mixing of organisms from different environments. Therefore, although a general deepening- upward trend is recognizable, Foraminiferal limestones in the study area cannot be separated into sub-units usually named af- ter predominant benthic foraminifera (from older and shallower to younger and deeper facies zones the usual sequence is from Miliolid, through Alveolinid and Nummulitid to Orthophragmid bearing limestones). These limestones are mostly represented by wackestones and/or packestones with variable amounts of fo- raminifera tests and other bioclasts. Younger part of the deposits are characterised by the predominance of orthophragmids con- taining glauconite and a few other non-carbonate bioclasts. The estimated thickness of Foraminiferal limestones in the study area is c. 200 m, and they are of Lower–Middle Eocene age. Transitional deposits are a succession of blueish to grey marlstones with 20–30 cm clayey limestone intercalations, rep- resenting a gradual deepening, i.e. transition from shallow-ma- rine Foraminiferal limestones to deep-marine Flysch deposits. They are most clearly visible along the NE margin and in the SE part of the Vinodol Valley. This unit is characterised by glauco- nite grains, both detrital and diagenetic in origin. In the lower part, sea urchins are common in some beds, while the upper part of the unit often contains irregular chert nodules. The estimated thickness of the Transitional deposits in the study area is about 60 m, and based on superpositional relationships they are prob- ably of Middle Eocene age. Flysch deposits are represented by the alternation of me- dium-sized, well-sorted carbonate/quartz/mica sandstones com- monly showing normal gradation and grey-blue marls. However, sandstones are not regularly distributed since they are much more common in the NW part of the Vinodol Valley while in its SE part marls predominate. The Flysch deposits are usually consid- ered to be of the Middle to Upper Eocene age (GRIMANI et al., 1973), although some studies in neighbouring areas indicate a possibly younger age (ŠPARICA et al., 2005; MIKES et al., 2008). Although the Flysch deposits are mostly covered by Qua- ternary deposits their thickness in the study area probably does not exceed 350 m, as estimated from geological profiles. 3.2.3. Quaternary Rockfall breccias cover marginal areas of the Vinodol Valley and are characterised by a variable but generally high degree of lith- ification by carbonate cements. They consist mainly of unsorted limestone debris of typically variable stratigraphic ages, although some breccias contain clasts of only one lithostratigraphic unit, depending on their source area. The stratigraphic age of some completely lithified breccias may be probably even older than the Quaternary. Deluvial-proluvial deposits are represented by chaotic masses of angular to slightly rounded, unsorted limestone frag- ments ranging in size from mm- to metre-sized blocks within a mostly reddish clayey matrix derived mostly from the Mediter- ranean red soil (terra rossa). Alluvial deposits – river and stream beds of the Vinodol Val- ley are often filled with gravels characterised by variable degrees of roundness and different fragment sizes. The material for these deposits mainly originates from the carbonate hinterland (Creta- ceous and Palaeogene limestones) and partly from the sandstone horizons within the Flysch deposits. Flood plains are character- ised by the deposition of fine-grained sands, silty mud and clay. Rockfall deposits – active, unlithified rockfall deposits in the form of talus are located along the steep margins of the Vi- nodol Valley. They are more frequent and of larger dimensions along the NE valley margins. Their fragments are angular and of different sizes, being the result of erosion of the steep carbonate cliffs of the Vinodol Valley and its immediate hinterland. 3.3. Structural data analysis results within the Vinodol fault zone Within the structural field investigations in the study area, along the NE and SW margins of the Vinodol Valley and Bakar Bay, i.e. the Vinodol fault zone (VFZ; Fig. 4) more than 120 shear joint/ fault plane data on 127 geological stations have been collected (Fig. 4). According to the structural measurements, the area of the VFZ can be subdivided into the NE Vinodol Fault Zone (NEVFZ) and the SW Vinodol Fault Zone (SWVFZ) (Fig. 4). Faults are characterised by both dip-slip and strike-slip kinemat- ics, as well as oblique-slip kinematics characterised by a predom- G eo lo gi a C ro at ic a Geologia Croatica 72/3184 inant dip-slip reverse component. In the NE Vinodol (NEVFZ) and SW Vinodol (SWVFZ) fault zones (Fig. 4) kinematic criteria were used to separate the collected structural data into compat- ible structural datasets. Within the Vinodol fault zone, along the NE and SW mar- gins of the Vinodol Valley and Bakar Bay (NEVFZ and SWVFZ; Fig. 4), 48 and 72 fault planes were measured in the Upper Cre- taceous and Palaeogene carbonate rocks, respectively. Those fault planes show predominant NW‒SE and NE‒SW strike directions. With respect to the fault kinematics, data collected along the NEVFZ show 19 fault planes with strike-slip kinematics and 29 fault planes with dip-slip kinematics, out of which 8 have normal and 21 exhibit reverse kinematics. Field structural measurements in the SWVFZ area encompassed 36 observed fault planes with strike-slip kinematics and 36 fault planes with dip-slip kinemat- ics, i.e. 18 normal and 18 reverse kinematics. Taking into account the kinematic criteria, the three aforementioned main categories were subdivided into compatible fault groups and fault group sub- sets (Fig. 5). Results of structural analysis in the VFZ, i.e. NEVFZ and SWVFZ show that observed reverse fault planes can be separated into three fault groups and six group subsets (Fig. 5 and Table 1). The first group of observed reverse faults (VFZ/RF1) comprise data of similar fault kinematics measured both along the NE and SW margins of the VFZ. In general it is characterised by two fault group subsets i.e. conjugate fault pairs characterised by the NW‒ SE strike, dipping both towards the NE and SW (dip angle c. 60°) (Table 1). The first subset with an average NE dip direction and SW-directed tectonic transport (Figs. 5–7) indicates thrusting of the Upper Cretaceous deposits and Palaeogene Foraminiferal lime- stones over younger Palaeogene clastic deposits. It was predomi- nanty observed in the NW and central parts of the NEVFZ (Fig. 3; see structural diagrams for the VFZ/RF1 group in the Fig. 5 and Table 1 for details). The second subset of fault planes is character- ised by an average SW dip direction and NE-directed tectonic transport and was measured within the main thrusting fault zone along the SW margin of the Vinodol Valley, generally indicating NE tectonic transport (see cross-sections in the Fig. 3, structural diagrams for the group VFZ/RF1 in the Fig. 5 and Table 1). In the SE part and along the NE margin of the Vinodol Valley, older Up- per Cretaceous deposits (the Milna fm.) are thrust over the Palaeo- gene Foraminiferal limestones and clastic rocks, whereas along the SW margin of the Vinodol Valley Palaeogene Foraminiferal lime- stones are thrust over the Eocene Flysch deposits (see Figs. 3 and 8). The slight bending of the NEVFZ in its central part (in the vi- cinity of Tribalj; Figs. 2 and 3) implies also a slight change of the fault’s strike and slickenside orientation, which suggests the tran- sition from pure reverse dip-slip motion to locally reverse, dextral/ sinistral oblique-slip motions (see fault planes with SW-dipping directions on group NE/RF1 diagram in the Fig. 5). Structural analysis of the representative palaeostress field using the P–T axis method (TURNER, 1953; MARRETT & Figure 4. Topographic map of the Vinodol Valley and Bakar Bay, i.e. the Vinodol fault zone (VFZ) with 127 locations of structural measurements. The VFZ is subdi- vided into the NE Vinodol Fault Zone (NEVFZ), which is coloured with a blueish polygon, and the SW Vinodol Fault Zone (SWVFZ) coloured with a reddish polygon. G eologia C roatica Palenik et al.: Geological and structural setting of the Vinodol Valley (NW Adriatic, Croatia): insights into its tectonic evolution based on structural investigations 185 ALLMENDINGER, 1990) as well as derived synthetic structural focal mechanisms, indicated that the observed palaeostress com- pressional field is associated with a P-axis predominantly trend- ing NE‒SW, whereas the T-axis is subvertical, steeply dipping towards the NNW at an angle of 81° (see Table 1 for details). The second group of the observed reverse faults (NE/RF2) within the VFZ, along the NE margin of the study area, is gener- ally characterised by two fault group subsets (Fig. 5 and Table 1). The first subset is characterised by an average ESE dip direction and WNW-directed tectonic transport, whereas the second sub- Table 1. Mean geometric properties of the observed fault planes within the VFZ, i.e. along the NE and SW margins of the Vinodol Valley and Bakar Bay (NEVFZ and SWVFZ) with calculated kinematic indicators and parameters. Fault planes observed in the VFZ zone were delineated with respect to their geometrical properties and kinematic compatibility within the following groups (see Fig. 5): VFZ/RF1 – Reverse fault group 1; NE/RF2 – Reverse faults group; SW/RF3 – Reverse faults group 3; VFZ/SSF1 - Strike-slip faults group 1; E) NE/SSF2 – Strike-slip faults group 2; SW/SSF3 – Strike-slip faults group 3; G) VFZ/NF1 – Normal faults group 1; NE/NF2 – Nor- mal faults group 2; SW/NF3 – Normal faults group 3.Fault types: R – reverse; SS – strike-slip; N – normal. Orientation of the P and T-axis are based on constructed syn- thetic structural beach-ball diagrams. Fault group Fault group subset No. of fault data Dip azimuth (°) Dip angle (°) Pitch (°) Strike (°) Fault type Striation P-axis T-axis Trend (°) Plunge (°) Trend (°) Plunge (°) Trend (°) Plunge (°) VFZ/RF1 VFZ/RF1a 12 55 55 67 – R 82 43 222 10 3 81 VFZ/RF1b 15 219 63 74 – 216 54 NE/RF2 NE/RF2a 5 106 60 90 – R 106 60 121 12 250 80 NE/RF2b 3 323 63 60 – 200 47 SW/RF3 SW/RF3a 2 270 63 42 – R 202 37 63 02 158 70 SW/RF3b 2 334 52 62 – 297 43 VFZ/SSF1 VFZ/SSF1a 12 – – 12 44–224 S-S 111 11 175 11 266 05 VFZ/SSF1b 11 – – 10 136–316 268 9 NE/SSF2 NE/SSF2a 3 – – 32 42–222 S-S 124 32 221 06 131 04 NE/SSF2b 8 – – 0 131–311 112 0 SW/SSF3 SW/SSF3a 13 – – 13 76–256 S-S 144 12 018 03 288 02 SW/SSF3b 8 – – 16 168–348 185 25 VFZ/NF1 VFZ/NF1a 5 78 79 70 – N 135 64 338 77 234 03 VFZ/NF1b 11 230 63 75 – 229 55 NE/NF2 NE/NF2a 2 93 51 90 – N 93 51 193 68 79 09 NE/NF2b 2 286 42 35 – 223 23 SW/NF3 SW/NF3a 3 149 69 58 – N 208 51 246 76 339 01 SW/NF3b 3 317 67 55 – 258 48 Figure 5. Structural diagrams for the Vinodol fault zone (VFZ), i.e. the NE and SW margins of the Vinodol Valley and Bakar Bay (NEVFZ and SWVFZ – see Fig. 4). The white quadrants on the structural beach-ball diagrams represent compression, while the shaded quadrants represent tension. A) Reverse fault group 1 (VFZ/RF1); B) Reverse fault group 2 (NE/RF2); C) Reverse fault group 3 (SW/RF3); D) Strike-slip fault group 1 (VFZ/SSF1); E) Strike-slip fault group 2 (NE/SSF2); F) Strike-slip fault group 3 (SW/SSF3); G) Normal fault group 1 (VFZ/NF1); H) Normal fault group 2 (NE/NF2); I) Normal fault group 3 (SW/NF3). The red dots, rectangles, and blue tri- angles indicate σ1, σ2 and σ3, respectively. G eo lo gi a C ro at ic a Geologia Croatica 72/3186 set is characterised with an average NW dip direction and SE- directed tectonic transport. For this group of reverse faults (NE/ RF2), structural analysis of the representative palaeostress field indicates a compressional palaeostress field associated with a P- axis dominantly trending NW‒SE, whereas the T-axis is subver- tical, i.e. steeply dipping towards the WSW (Fig. 5 and Table 1). The third group of the reverse faults (SW/RF3) observed along the SW margin of the study area is represented by data from only four measured fault planes. This fault group is subdivided into two group subsets characterised by W-dipping (E-directed tectonic transport) and NW-dipping (SE-directed tectonic trans- port) fault subsets (Fig. 5 and Table 1). Structural analysis of the representative palaeostress field indicate that the observed fault planes are related to the compressional palaeostress field, associ- ated with the P-axis dominantly trending NE‒SW, whereas the T-axis is dipping towards SE (see Table 1 for details). Besides the reverse fault planes observed along the NEVFZ and SWVFZ, 55 strike-slip fault planes (Figs. 2, 5 and 9) were measured. The observed strike-slip fault planes were also sepa- rated according to their geometric properties and kinematic com- patibility into three fault groups and six group subsets i.e., three conjugate fault pairs (VFZ/SSF1, NE/SSF2 and SW/SSF3; see Fig. 5). Fault groups are characterised by steeply dipping geom- etry, with average dip angles of 65° and 80°, respectively. Meas- ured fault planes along the VFZ margins were dominantly strik- ing both NW–SE and NE–SW (VFZ/SSF1 and NE/SSF2; Fig. 5 and Table 1), while along the SW margin of the VFZ third strike- slip fault group resemble a conjugate fault pair with NNW–SSE and ENE–WSW strike (SW/SSF3; Fig. 5 and Table 1). Mapped strike-slip fault planes were characterised by struc- tural reactivation, with slickenside overgrowths indicating both dextral and sinistral movements (Fig. 10). This implies that for a few observed dextral or sinistral fault surfaces, in addition to the predominant movements they were characterised also by very poorly visible indicators of opposite movement that may indicate occasional interchange of principal stress axes σ1 and σ3 within the same stress field. Kinematic analysis of the observed strike-slip fault planes also supports the concept of structural reactivation, indicating that the identified fault planes may have been formed within two slightly different palaeostress fields: the first one associated with the N–S trending P-axis, and the T-axis trending E–W, and the second one associated with the NE‒SW trending P-axis and a T- axis trending NW–SE (see Fig. 5). Field observations and identi- fication of cross-cutting relationships between the mapped reverse and strike-slip faults show that strike-slip fault planes mostly cut across, thereby offsetting reverse fault planes, especially along the SWVFZ (see geological map in Fig. 2). This is particularly notice- able in cases where strike-slip faults (with NE‒SW and ENE‒ WSW strikes) are actually oriented perpendicular or at an oblique angle to the reverse faults of the NW‒SE strike (see geological map in Fig. 2), suggesting that the observed strike-slip fault plane systems are probably younger than the reverse ones. Along the NE margin of the VFZ, eight normal fault planes were measured, whereas along its SW margin 18 normal fault planes were observed. Along the NE margin of the Vinodol Val- ley, normal fault planes were predominantly identified in its SE part, and less in the NW and central parts. Measured normal fault planes within the VFZ characterised by both pure dip-slip and some oblique slip motions could be separated into three fault groups; VFZ/NF1, NE/NF2 and SW/NF3 and six subsets, i.e. three conjugate fault pairs (Fig. 5 and Table 1). Figure 6. Photograph of the reverse fault plane (Fp=37/48) in recrystallized limestones of the Upper Cretaceous Gornji Humac fm. along the NE margin of the Vinodol Valley. Location: 45°13’22.4’’N, 14°42’14.5’’E. Figure 7. Photograph of the reverse fault plane (Fp=24/62) in the recrystallized limestones of the Gornji Humac fm. along the NE margin of the Vinodol Valley. Location: 45°16’15.5’’N, 14°36’14.2’’E. Figure 8. Photograph of the reverse fault plane (Fp=72/58) in the Upper Creta- ceous Milna fm. near the thrust contact with Palaeogene clastic deposits cov- ered with the Quaternary rockfall and deluvial–proluvial deposits. Location: 45°15’32.1’’N, 14°37’7.8’’E. G eologia C roatica Palenik et al.: Geological and structural setting of the Vinodol Valley (NW Adriatic, Croatia): insights into its tectonic evolution based on structural investigations 187 Group 1 (VFZ/NF1; Fig. 5) is characterised by predomi- nantly NW‒SE striking fault plane subsets that are steeply dip- ping towards both NE and SW (Fig. 11 and Table 1). Kinematic analysis shows that these normal fault planes were formed within the palaeostress field characterised by a subvertical P-axis steeply dipping towards the WSW (P-axis orientation is 338/77; Table 1) and subhorizontal T-axis trending NE‒SW (see Table 1 for de- tails) that in overall resulted in the NE–SW directed extension. The Group 2 and group subsets of the normal fault planes (NE/NF2; Table 1) observed along the NE margin of the VFZ are characterised by a general N‒S strike (locally NNE‒SSW or NNW‒SSE), dipping to the W (WSW) and E with a dip angle of approx. 40–50° (see Table 1). Kinematic analysis of collected structural data indicated that these fault planes were formed within the palaeostress field associated with the subvertical P- axis (orientation of P-axis is 193/68; see Table 1), and T-axis trending ENE‒WSW (see Table 1 for details), resulting in the NE–SW and E–W extension. Due to poor outcrops resulting in ambiguous structural relationships between the Cretaceous and Palaeogene units, identification of the cross-cutting relationships between the observed normal fault planes and reverse and strike- slip fault planes was rather unclear. Large bodies of the Quater- nary rockfall material and gravitational slides in the vicinity of measured normal fault planes suggest their potential genetic con- nection. The Group 3 and group subsets of the normal fault planes (SW/NF3; Table 1) were observed along the VFZ SW margin (Fig. 5). These conjugate fault pairs are characterised by the NE‒ SW striking fault plane subsets (Figs. 5 and 11) dipping both to- Figure 9. A) Photograph of the fault plane with NW–SE orientation (Fp=55/81) mapped along the SW margin of the Vinodol Valley; the fault is probably formed in the palaeostress field with the P-axis trending NE–SW, and T-axis trending NW–SE, and subsequently reactivated as a dextral fault within the palaeostress field char- acterised by the N–S trending P-axis, and E–W oriented T-axis. B) Detail of the fault plane – subhorizontal striations indicate dextral movement. Recrystallized lime- stones of the Upper Cretaceous Gornji Humac fm. Location: 45°11’27.4’’N, 14°42’4.9’’E. Orientation of bedding is 30/33. Figure 10. Photograph of the strike-slip fault plane (Fp=54/85) in the recrystal- lized limestones of the Upper Cretaceous Gornji Humac fm. along the SW mar- gin of the Vinodol Valley. Strike-slip fault plane with both dextral and sinistral movement indicators probably represents a reactivated older fault. Location: 45°11’27.6’’N, 14°42’4.4’’E. Figure 11. Photograph of the normal fault plane with the NW–SE strike (Fp=214/75; ls=80° from NW) within the Palaeogene Foraminiferal limestones along the SW margin of the Vinodol Valley. Location: 45°9’23.7’’N, 14°44’56.5’’E. G eo lo gi a C ro at ic a Geologia Croatica 72/3188 wards the NW and SE (SW/NF2) (see diagrams in Fig. 5). The NE–SW strike is normal to the general strike of the Dinarides (NW‒SE; Fig. 5). These normal fault planes were formed within the palaeostress field with a subvertical P-axis steeply dipping towards the SW, whereas the subhorizontal T-axis trends NW‒SE (see Table 1 for details). Such a palaeostress field resulted in the NW–SE directed extension. 4. DISCUSSION The results of the geological and structural investigations in the Vinodol Valley and Bakar Bay area indicate the complex tec- togenesis of this area through the Palaeogene, Neogene and Qua- ternary, which led to the formation of the recent geological struc- ture of the area. 4.1. Geology and palaeogeography of the Vinodol Valley and Bakar Bay The NW Adriatic domain, including the study area, represents part of the Mesozoic Adriatic Carbonate Platform (AdCP), and its Late Cretaceous dynamics was caused by the continuous con- vergence of the Adria Microplate and European Plate (see ŚRODOŃ et al., 2018 and references therein). Changes in the tec- tonically controlled subsidence between different blocks resulted in the thickness variation of the units, but their general charac- teristics can be more or less correlated over significant distances across the AdCP (VLAHOVIĆ et al., 2005). The lower part of the oldest Upper Cretaceous unit, the Cenomanian Milna fm., does not crop out in the study area, and therefore its estimated thickness of more than 250 m corresponds well with the >320 m thick succession determined for the entire unit in Northern Istria (VELIĆ & VLAHOVIĆ, 1994), 300–400 m in NW Slovenia (JURKOVŠEK et al., 1996), more than 400 m of shallow-marine Cenomanian deposits on the island of Cres (FUČEK et al., 2015), 300–350 m on the island of Unije (FUČEK et al., 2015), 300–350 m on the island of Vis (KORBAR et al., 2012) and c. 400 m thick Cenomanian shallow-marine limestones of the Milna fm. determined on the island of Brač (GUŠIĆ & JE- LASKA, 1990). The thickness of overlying limestones with pelagic influence of the Sv. Duh fm., deposited during a regional temporary drown- ing event near the Cenomanian/Turonian boundary was esti- mated at 130–150 m in the study area and also fits with thick- nesses determined in other areas: 110–140 m in Northern Istria (Ćićarija – BRČIĆ et al., 2017), 70–200 m in NW Slovenia (JURKOVŠEK et al., 1996), 30–100 m on the island of Cres (FUČEK et al., 2015), 40 m on the island of Vis (KORBAR et al., 2012) or c. 110 m at the locus typicus of the unit, on the island of Brač (GUŠIĆ & JELASKA, 1990). The youngest Cretaceous rocks belonging to the Gornji Hu- mac fm. are exclusively of Turonian age in the study area, since regional emergence between the Cretaceous and Palaeogene (re- corded in almost all parts of the Adriatic Carbonate Platform) in this area started relatively early, probably during the Late Turo- nian (c. 90 Ma), a bit later than in neighbouring N Istria (MATIČEC et al., 1996), the N part of the island of Cres and the island of Krk (ĆOSOVIĆ et al., 1994), as well as in the lower, au- tochthonous part of the Učka Mt. Such early emergence in the study area provides additional evidence that the area in the cen- tral NW part of the AdCP uplifted before the end-Turonian was relatively large, probably covering several thousand km2 (stretch- ing from W and NW Istria to Krk and the Vinodol area). Depos- its of the Gornji Humac fm. are consequently in the Vinodol Val- ley relatively thin, from 70 to c. 100 m (the variable thickness is due to intense karstification during the relatively long strati- graphic hiatus, probably 35 to 40 My). The top of the Gornji Hu- mac fm. is in other areas of the Adriatic Carbonate Platform usu- ally much younger, mostly Upper Santonian or even Campanian in age, resulting consequently in much thicker deposits – e.g. from 250 to almost 500 m at their type locality on the island of Brač (GUŠIĆ & JELASKA, 1990). The estimated thickness of Palaeogene Foraminiferal lime- stones of c. 200 m is thicker than the average thickness of these deposits presented on the Crikvenica sheet of the Basic Geologi- cal Map (120 m, ŠUŠNJAR et al., 1970). However, the thickness of Foraminiferal limestones is locally very variable, depending on the palaeorelief, local subsidence and duration of the strati- graphic hiatus – e.g. in the Čikola river section ŠPANIČEK et al. (2017) determined their thickness of 255 m, and DROBNE et al. (1991) measured thicknesses between 230 and 280 m in the Ravni Kotari area. The thickness of the Transitional deposits (Td) in the Vinodol Valley was estimated at 60 m, and thickness of overly- ing Flysch deposits (Fd) at >350 m, which is significantly greater than the average values presented on the Crikvenica sheet of the Basic Geological Map (the thickness of both units was estimated at 200 m – ŠUŠNJAR et al., 1970), but much less than the aver- age thicknesses represented on neighbouring maps (Labin: Td 300–700, Fd 450 m – ŠIKIĆ et al., 1969; Ilirska Bistrica: Td 200 m, Fd 400–650 m – ŠIKIĆ et al., 1972; Rab: Td 150 m, Fd 600 m – MAMUŽIĆ et al., 1969). 4.2. Structural relationships in the Vinodol Valley and Bakar Bay Structural investigations in the Vinodol Valley and its NW prolon- gation Bakar Bay based on structural data collected on fault planes within the NEVFZ and SWVFZ (Fig. 5) resulted in more than 120 measurements at 127 stations. These structural data were mostly collected along the tectonic contact between the Upper Cretaceous limestones and the Eocene carbonate and clastic deposits and pro- vided field evidence of the tectonic evolution of the study area, which may be also used in understanding the Quaternary evolution of the Vinodol Valley area. Kinematic analysis of fault planes and the overlapping relationships identified in the study area provided not only basic information of palaeostress field changes recorded along the mapped fault systems but also their potential neotectonic activity in respect to the recent stress field. In the area of the Vinodol Valley and Bakar Bay the first fault group (VFZ/RF1) is represented by the reverse fault planes ob- served along both margins, which are characterised by striations of both pure dip-slip and oblique-slip kinematics (Fig. 5, see dia- gram A). Kinematic analysis shows that the first reverse fault group (VFZ/RF1) was characterised by steep conjugate fault planes (c. 55° and 63°) dipping towards the NE and SW, indicat- ing SW- and NE-directed tectonic transport respectively (Fig. 5, diagram A; see Table 1 for details). With the NW‒SE strike, re- verse fault group subsets, i.e. conjugate fault pairs along the NE margin show dominant SW-directed tectonic transport, while the structural position of the lithostratigraphic units (Fig. 3) and measured fault planes along the main thrusting fault zone along the SW margin of the Vinodol Valley suggest predominant NE- directed tectonic transport (see cross-sections in the Fig. 3). The second reverse fault plane group (NE/RF2) observed along the NE margin of the VFZ was characterised by conjugate fault planes with a general NE–SW strike, dipping towards the NW and SE (c. 60° and 63°), indicating NW- and SE-directed G eologia C roatica Palenik et al.: Geological and structural setting of the Vinodol Valley (NW Adriatic, Croatia): insights into its tectonic evolution based on structural investigations 189 tectonic transport (Fig. 5, diagram B; see Table 1 for details). The third fault plane group (SW/RF3) of reverse faults along the SW margin of the Vinodol Valley and Bakar Bay resemble the reverse fault plane group NE/RF2 observed along the NE margin. The SW/RF3 fault group is characterised by subsets dipping gener- ally towards the NW (W) and ESE (c. 63° and 52°), with the tec- tonic transport towards SE (E) and WNW, respectively (diagram C in Fig. 5; see Table 1 for details). Palaeostress field analysis of measured reverse fault planes along the thrust contact of the Upper Cretaceous and Palaeogene carbonates (Foraminiferal limestones) over the younger Palaeo- gene clastic deposits suggest the compressional stress field and structures formed within both NE‒SW trending and NW‒SE trending compression (diagrams A, B and C in Fig. 5; see Table 1 for details). Structures related to the NE‒SW trending compres- sion probably correlate with the principal ‘Dinaridic tectonic phase’ (sensu BLAŠKOVIĆ, 2005 and references therein), which during the Late Cretaceous and Palaeogene resulted in the build up of the major fault and fold related NW‒SE striking structures in the Dinarides, being characterised by NE-dipping geometry and SW-directed tectonic transport (PRELOGOVIĆ et al., 1981, 1982, 1995; BLAŠKOVIĆ, 2005; ŽIBRET & VRABEC, 2016). Though typical NW‒SE striking, NE-dipping structures were previously observed within the Vinodol Valley and Bakar Bay (BLAŠKOVIĆ, 1991, 1997, 1999, 2005; PRELOGOVIĆ et al., 1981, 1982, 1995), this study identified numerous fault planes within the VFZ that are steeply dipping (c. 60°) towards the SW and are characterised by NE-directed tectonic transport. These new findings confirm that during the tectonic evolution of the External Dinarides, structures with NE-directed tectonic trans- port have been formed in some areas (as in the neighbouring Vel- ebit Mt. area – VLAHOVIĆ et al., 2012; ŚRODOŃ et al., 2018; TOMLJENOVIĆ et al., 2018) along with the predominant SW- directed tectonic transport. Such structures were formed either concurrently with SW-vergent Dinaridic structures or their for- mation may present a substage within the principal Dinaridic tec- tonic phase. The second observed reverse fault plane group in this work characterised by NW (W) and NE, ESE dip directions (diagrams B and C in Fig. 5; see Table 1 for details) correlates with results reported by BLAŠKOVIĆ (1998). In reported study author de- scribed existing Dinaridic structures as refolded and faulted structures which were deformed during the ‘late Dinaridic phase’ as a result of the regional stress field change from NE‒SW trend- ing compression to WNW–ESE trending compression Beside the measured reverse fault planes in the area of the Vinodol Valley and Bakar Bay numerous strike-slip fault planes were also observed in this study. In accordance with the geomet- ric properties and kinematics within the Vinodol fault zone de- lineated strike-slip fault groups were characterised by subvertical geometry and slickensides showing structural reactivation, i.e., both dextral and sinistral motions. Characterised by the NW‒SE strike (dipping towards both NE and SW) and perpendicular strike NE‒SW (dipping towards both NW and SE), kinematic analysis of the observed strike-slip fault planes indicated a simi- lar palaeostress field to the observed reverse fault plane groups, being characterised by N–S, NE‒SW, and NNE‒SSW trending P-axis (diagrams D, E, and F in Fig. 5; see Table 1 for details). These results may suggest that at least some of observed strike- slip fault planes in the study area resemble structurally reacti- vated inherited reverse fault planes within the transpressional/ transtensional stress field. However, at the same time, due to their subvertical geometry and present field observations of cross-cutting relationships be- tween mapped reverse and strike-slip fault planes, we believe that the observed strike-slip fault planes in the study area are probably younger than the reverse faults because most of them cross-cut and laterally displace measured reverse fault planes (Fig. 3). At the same time, the geometry of mapped strike-slip faults and computed stress field characterised by N–S, NE‒SW, and NNE‒ SSW trending P-axis is in accordance with the observed strike- slip and reverse fault planes in the NW External Dinarides of Slovenia, where N–S oriented compression during the Pliocene and Quaternary has also been indicated (ŽIBRET & VRABEC, 2016). According to PRELOGOVIĆ et al. (1981, 1982, 1995), the mapped NW‒SE and NE‒SW striking strike-slip faults in the Vinodol Valley and Bakar Bay are parts of the Ilirska Bistrica– Rijeka–Senj seismogenic fault zone and resemble neotectonically active faults in the study area. Mapped normal faults in the Vinodol Valley and Bakar Bay were characterised by NW‒SE and NE‒SW (locally N–S, NNE‒ SSW and NNW‒SSE) striking planes that accommodated pure dip-slip/oblique motions (diagrams G, H and I in Fig. 5; see Table 1) and NE‒SW and NW‒SE (locally E‒W) trending extension. Though the structural relationships between reverse and strike- slip fault groups with respect to the normal faults are not clear, it can be assumed that NE‒SW, E‒W and NW‒SE directed exten- sion may be considered as a local feature associated with gravi- tational collapse of the hanging wall structural sequence, i.e. hinge zones of the hosting anticlines. In our opinion, such gravi- tationally conditioned extension (cf. TAVANI et al., 2012 and ref- erences therein) may be either potentially concurrent to active folding and formation of the Vinodol Valley and Bakar Bay area during the Late Cretaceous and Palaeogene or it was associated with the younger tectonic phase characterised by radial extension of the folded structure after/before compressional/transpressional tectonics which commenced in the Pliocene and Quaternary. Ac- cording to VAN UNEN et al. (2019) and their observations in the area of the Internal Dinarides, the extension of folded structures in the Dinarides occurred during the late Cretaceous–Oligocene, simultaneously or slightly after the main orogeny build up, but definitely before the Pliocene–Quaternary transpression. Fur- thermore, in the vicinity of the observed normal fault planes, large bodies of Quaternary rockfall material and gravitational slides have been mapped, which according to BLAŠKOVIĆ (1997) may suggest their potential role in the increasing rock in- stability and failure. Results of the structural study conducted and presented in this paper are generally in accord with the existing knowledge on the local/regional tectonic evolution in this part of the External Dinarides (PRELOGOVIĆ et al., 1981, 1982, 1995; BLAŠKOVIĆ, 2005; ŽIBRET & VRABEC, 2016; VAN UNEN et al., 2019). However, our structural results address the presence of compres- sional structures characterised by both SW and NE-directed tec- tonic transport. Our field observations of the cross-cutting rela- tionships between mapped reverse and strike-slip fault planes also suggest that the observed strike-slip fault planes in the study area are probably younger than the reverse fault planes. Alterna- tively, the measured normal fault planes may be associated with gravitational collapse within the hinge zones of the hosting anti- clines during the late Cretaceous–Oligocene, simultaneously or slightly after orogeny build up, however, before the Pliocene– Quaternary transpression. G eo lo gi a C ro at ic a Geologia Croatica 72/3190 4.3. Comparison of palaeostress field analysis and seismological data In order to address the potential neotectonic activity of faults within the VFZ and their possible favourable orientation within the recent stress field, we hereby compare our results of palae- ostress field analysis with the available seismological data from the wider study area. The ongoing tectonic activity in the greater Vinodol area (in- cluding the epicentral areas of Ilirska Bistrica–Klana, Rijeka, Bakar, Vinodol, Krk and Senj) is expressed by moderate seismic- ity, with hypocentres in the upper crust, mostly shallower than 20 km. It is well documented in the corresponding entries in the Croatian Earthquake Catalogue (CEC, first described by HERAK et al., 1996, updated annually thereafter) and by numerous seis- micity-related studies (e.g. MARKUŠIĆ & HERAK, 1999; IVANČIĆ et al., 2006, 2018; KASTELIC et al., 2013; HERAK & HERAK, 2017 and references therein). Figure 12 shows that significant earthquakes are known to have occurred here in the past. The most important ones (from the point of view of this study) are the events of 1323, 1750, 1870, and 1916. The strongest of them could have been that of 1323 (the ‘Vinodol earthquake’) which is often listed as causing intensity as high as IX° MSK. Although this earthquake probably indeed occurred and caused considerable damage, it is unfortunate that only very sparse information exists about it (merely a sentence in the glagolitic document from Grižane), so we treat both the loca- tion and the magnitude as unreliable. The large Bakar earthquake of November 28, 1750 (Io = VII–VIII MSK) has been studied in detail by HERAK et al. (2017), so here we will only mention the fact that about 3000 aftershocks were felt in the three years fol- lowing it. The Klana mainshock of 1870 (March 1, 1870, Io = VIII MSK, HERAK et al., 2018) was the last significant earthquake in the Ilirska Bistrica–Klana area. It is still vividly remembered by the local inhabitants, and is one of key events for understand- ing seismic hazard in the greater Rijeka area. The most recent one occurred on March 12, 1916 about 10 km east of Novi Vi- nodolski, with the epicentral intensity Io = VIII MSK. Most dam- age was reported from Grižane (over 100 houses damaged) and Bribir (in the ENE part of the Vinodol Valley, within the NE Vi- nodol Fault Zone (NEVFZ) area ‒ see Fig. 4). Contemporary seismicity follows the same pattern (Fig. 13), however, with no event exceeding magnitude 4.8 for over two decades. Nevertheless, significant densification of the network of seismological stations in Croatia and Slovenia in this time period, enabled lowering the detection threshold well below magnitude 1.0 in some areas. This, in turn, made it possible to collect infor- mation on details of seismogenic faults properties even during their periods of low activity. In particular, the observed spatial distribution of the first P-wave polarity could have been inverted for a number of rather small events (Fig. 13), and advanced loca- tion methods (e.g. HERAK & HERAK, 2017) were used to learn more about the geometry of active faults. Focal mechanism solutions (FMS, shown in Fig. 13) have been computed by inverting the observed spatial distribution of Figure 12. Historical seismicity (1323–1908) and epicentres of instrumentally recorded earthquakes (until 1995) with magnitudes ML > 4.5 in the wider Vinodol re- gion. The year of earthquake occurrence is indicated by each epicentre. The events of 1323, 1648, and 1776 are considered uncertain (see text). Magnitudes of the pre-instrumental earthquakes are estimated from the observed intensities. After the Croatian Earthquake Catalogue (first described by HERAK et al., 1996, updated annually thereafter). G eologia C roatica Palenik et al.: Geological and structural setting of the Vinodol Valley (NW Adriatic, Croatia): insights into its tectonic evolution based on structural investigations 191 the first longitudinal wave polarity for the best fit double-couple source geometry (FMS-Database, 2019). The P-axes computed from FMS are often taken as the first order proxy (within about ±15°) for the direction of local tectonic movement. Judging from their spatial distribution as presented in Fig. 13, the tectonic stress field is relatively homogeneous and horizontal (average plunge 1.5°), with a mean orientation close to N–S (N13°E). Individual deviations from this direction may be a consequence of local stresses, pre-existing weak fractures (especially for small earth- quakes), variations in pore pressure, influence of friction during faulting, or simply the inversion uncertainty. The suggested types of faulting, on the other hand, are far from homogeneous over the study region. While in the NW part of the considered area (in Slovenia) all FMS are consistent with right-lateral strike-slip faulting along the SE–NW striking sub- vertical faults (e.g. the Raša fault), further to the SE there is a mixture of transpressive, almost pure dip-slip, and strike-slip faulting. In a compressional tectonic setting this is indicative of a generally transpressive regime, where the σ2 and σ3 principal stresses are of similar magnitude, and often only details deter- mine which one is vertical. Indeed, our study area (and the greater Kvarner‒Velebit region) may be regarded as the transitional zone of transpression where predominantly reverse and thrust mecha- nisms of the southern External Dinarides gradually switch into pure strike-slip in the NW Dinarides of southern Slovenia. In such environments, instead of oblique faulting, the stress release may also be decoupled between parallel striking thrust and strike-slip faults. According to MICHAEL (1990), assuming equality of resistance to sliding on both faults and the angle of convergence between 50° and 35° (as is the case here), decoupled faulting will be preferred over oblique faulting for thrusts with dip angles lower than 20–30°. BECK’s (1983) estimates are more permissive and favour decoupling even for steeper dipping faults. This process may be plausible in some cases here, especially for focal depths exceeding about 5 km or if the strike-slip fault is considerably weaker than the thrust. Synthetic focal mechanisms constructed by applying the Right Dihedra Method (ANGELIER & MECHLER, 1977) (see Fig. 5) show good correspondence with the available focal mech- anism solutions (Fig. 13). Both synthetic focal mechanism and available focal mechanism solutions for instrumentally recorded earthquakes within the larger Rijeka‒Vinodol area and Kvarner Bay suggest that the principal NW‒SE and NE‒SW striking seis- mogenic sources (defined by mapping as reverse and strike-slip faults) are compressional and transpressional structures, which are recently tectonically active within the compressional stress field, characterised by a generally N‒S oriented P-axis. 5. CONCLUSIONS In the area of the Vinodol Valley and Bakar Bay geological and structural investigations highlight the complex tectonic evolution of this area not only through the Palaeogene and Neogene, but also during the Quaternary period. Upper Cretaceous informal lithostratigraphic units mapped in the study area can be well correlated with units proposed by Figure 13. Seismicity in the period 1996–2017 (ML > 0.0). The hypocentral depth is represented by the symbol colour according to the colour scale on the right. Only events located by at least 8 reported phase onsets, and having azimuthal gap of reporting stations smaller than 120° are shown. The beach-ball diagrams (1986–2017) represent focal mechanism solutions (FMS) as lower hemisphere, stereographic projection of the sense of the P-wave first motion onto the focal sphere (compressive quadrants are shaded). See the legend for colour-coding of the mechanism type. Red lines show inferred orientation of the P-axes for each FMS. G eo lo gi a C ro at ic a Geologia Croatica 72/3192 GUŠIĆ & JELASKA (1990), except for the much shorter strati- graphic range of the youngest unit, the Gornji Humac fm. Upper Turonian age of the youngest Cretaceous rocks in the Vinodol Valley suggest that the uplifted area in the central NW part of the Adriatic Carbonate Platform at the end of Turonian already cov- ered several thousands km2 (from W and NW Istria to the Krk and Vinodol area). Results of the structural investigation in general correspond to those of previous studies and existing knowledge about the tectonic evolution of the Dinarides, including several important new findings. In the area of the Vinodol Valley and Bakar Bay, the major- ity of the observed reverse fault planes were characterised by both pure dip-slip and oblique-slip kinematics. Kinematic analysis shows that the first reverse fault groups were characterised by steep planes (c. 60°) dipping either towards the NE or the SW in- dicating SW- and NE-directed tectonic transport respectively. The NE-directed tectonic transport was determined along mapped structures within both the NE and SW margins of the Vinodol Valley. However, according to the structural and strati- graphic relationships of the mapped lithostratigraphic units along the SW margin of the Vinodol Valley we address faulted struc- tures as predominantly NE-vergent structures. Both NE and SW dipping structures are related to a NE‒SW trending compression stress field that probably formed within the Dinaridic tectonic phase (BLAŠKOVIĆ 2005; ŽIBRET & VRABEC, 2016). Newly mapped SW-dipping structures indicate that beside the predom- inant SW-vergent structures in the Dinarides, NE-vergent struc- tures formed either concurrent to the SW-vergent structures or were associated with a separate sub-stage within the principal Dinaridic phase that also existed in the study area. The second mapped reverse fault plane groups in the study area are characterised by both NW (W) and ESE dip directions, and SE (E) and WNW directed tectonic transport respectively. These fault plane subsets resemble conjugate fault pairs formed within both NE‒SW and NW‒SE trending compression. Accord- ing to BLAŠKOVIĆ (1998) these Dinaridic structures were formed in the ‘late Dinaridic phase’ of the refolding and faulting of existing Dinaridic structures due to a change in regional stress field from NE‒SW trending compression to WNW–ESE trend- ing compression. In the area of the Vinodol Valley and Bakar Bay, strike-slip fault planes were also observed. Strike-slip fault groups charac- terised by subvertical geometry show structural reactivation, i.e. both dextral and sinistral motions. NW‒SE striking (dipping both towards NE and SW) and NE‒SW striking fault planes indicate a palaeostress field characterised by NE‒SW, N‒S and a NNW‒ SSE trending P-axis, similar to the observed reverse fault planes. Orientation of the P-axis associated with the transpressional/tran- stensional stress field may suggest that mapped strike-slip fault planes in the study area resemble structurally reactivated older reverse fault planes. At the same time, the fault subvertical ge- ometry and cross-cutting relationships between mapped reverse and strike-slip fault planes suggests that the strike-slip structures are probably the younger ones. Measured normal fault planes in the study area characterised by generally NW‒SE and NE‒SW striking planes (NE‒SW and NW‒SE directed extension) and may have been caused by grav- itational collapse of hanging wall structures in the hinge zones of the anticline (TAVANI et al., 2012 and references therein) dur- ing the orogen build up (VAN UNEN et al., 2019) or slightly af- ter, however before the Pliocene–Quaternary transpression with dominant strike-slip motions. Comparison of palaeostress field analysis and constructed synthetic focal mechanism with available data on focal mecha- nism solutions within the Ilirska Bistrica–Rijeka–Senj seismo- genic fault zone shows the favourable orientation of observed NW‒SE and NE‒SW striking faults with respect to recent com- pressional/transpressional stress field (N‒S oriented P-axis) sug- gesting they may be potential seismogenic sources within the study area. ACKNOWLEDGEMENT This paper resulted from work on two scientific projects funded by the Croatian Science Foundation (HRZZ): VELEBIT (Grant no. IP-2014-09-9666, PI Professor Marijan HERAK) and GEO- SEKVA (Grant no. IP-2016-06-1854, PI Dr. Tvrtko KORBAR), and the authors are grateful for all support in the field and cabinet research. 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