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Geological and structural setting of the Vinodol valley (NW Adriatic, Croatia): insights into tectonic evolution based on structural investigations

The area of the Vinodol valley and the Bakar bay represents a NW‒SE oriented valley in the NW Adriatic characterised by prominent historical and instrumentally recorded seismicity. Being a part of the greater geodynamic domain including Ilirska Bistrica– Rijeka–Senj seismogenic fault zone, new geological and structural data addressing tectonic evolution of the area were collected in order to better understand 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, determined shorter stratigraphic range of the Gornji Humac fm., youngest Cretaceous unit in the study area, suggests that the uplifted area in the central NW part of the Adriatic Carbonate Platform at the end of the Turonian comprised already several thousand km2 (from W and NW Istria to Krk and Vinodol area). Structural measurements of the fault planes and/or shear joints in the study area generally correspond to the existing structural model of tectonic evolution of the Dinarides. However, opposite to the SW vergences typical for 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 reverse ones, suggesting change in palaeostress field during Neogene–Quaternary, with prevalent transpression and radial extension. Comparison of palaeostress field analysis and constructed synthetic focal mechanism with available focal mechanism solutions data within the Ilirska Bistrica– Rijeka–Senj seismogenic fault zone show favourable orientation of observed NW‒SE and NE‒SW striking faults in respect to recent compressional/transpressional stress field (N‒S oriented P-axis), indicating them as potential seismogenic sources within the study area.

NW Adriatic, Vinodol valley, Bakar bay, SW and NE vergences, compression/transpression, neotectonic activity, focal mechanism solutions, seismicity

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