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 collected in order to better understand the focal mechanisms of previous earthquakes and to enable identification of potential seismogenic sources. Mapped informal lithostratigraphic units mostly correspond to the Upper Cretaceous, Palaeogene 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 existing 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 solutions for earthquakes within the Ilirska Bistrica–Rijeka–Senj seismogenic fault zone on the other, shows a favourable orientation of the observed NW‒SE and NE‒SW striking faults with respect to the recent compressional/transpressional stress field (N‒S oriented P-axis), indicating these as potential seismogenic sources within the study area.
Highlights
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
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, quite rare in the Dinarides, very common in the structures of the neighbouring Velebit Mt. and Krk island (VLAHOVIĆ et al, 2012; ŚRODOŃ et al, 2018; TOMLJENOVIĆ et al, 2018)
The lower part of the oldest Upper Cretaceous unit, the Cenomanian Milna fm., does not crop out in the study area, and 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 & JELASKA, 1990)
Summary
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 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) include a review of previous studies. In addition to these geological investigations associated with the Basic Geological Map and its explanatory notes, the most important geological and structural studies of the Vinodol Valley were those by BLAŠKOVIĆ (1991, 1997, 1998, 1999, 2005), in which he proposed a tectonic concept of the Vinodol Valley area. The area of the Vinodol Valley and Bakar Bay was investigated as a seismotectonically active area by PRELOGOVIĆ et al (1981, 1982, 1995), HERAK et al (1996, 2017), MARKUŠIĆ &
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