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Tectonic evolution and identification of potential seismogenic sources of the Vinodol Valley (NW Adriatic, Croatia) based on geological mapping and structural investigations

The Vinodol valley is a NW‒SE oriented valley parallel to the NE Adriatic coastline, whereas area of the Bakar bay represents its geomorphological and geological extension towards NW. This complex, seismotectonically active area is characterised by prominent historical and instrumentally recorded seismicity. The area of the Vinodol valley and the Bakar bay is a part of the greater geodynamic domain comprising Ilirska Bistrica–Rijeka–Senj seismogenic fault zone. New geological and structural data in the study area were collected in order to better understand focal mechanisms of previous earthquakes and to possibly identify potential seismogenic sources. Conducted research in the Vinodol valley was focused on geological mapping accompanied with detailed structural-geological research of the valley and its continuation towards NW into the Bakar bay area. This investigation together with new data on focal mechanisms of previous earthquakes could help understanding of the complex tectonic evolution of this area, including fault kinematics within identified potentially seismogenic structures. Mapped informal lithostratigraphic units mostly correspond to the Upper Cretaceous, Palaeogene and Quaternary successions described in other parts of the External Dinarides (e.g. GUŠIĆ & JELASKA, 1990 ; FUČEK et al., 2015). Results of structural investigation generally correspond to the previous studies and existing knowledge about the tectonic evolution of the area and Dinarides in general (e.g. BLAŠKOVIĆ, 2005 and references therein). However, the investigation resulted with several new important findings within the study area. The results of geological and structural investigations indicate complex tectonic evolution of this area during Cretaceous, Palaeogene and Neogene, but also during the Quaternary. For example, the much shorter stratigraphic range of the youngest Upper Cretaceous unit, Gornji Humac fm. (of the Upper Turonian age) in the Vinodol valley suggests that the uplifted area within the central NW part of the ancient Adriatic Carbonate Platform (for more details see VLAHOVIĆ et al., 2005 and references therein) at the end of the Turonian already comprised several thousands km2 (from W and NW Istria to the Krk and Vinodol area). Additional new finding is that along the SW-vergences typical for the Dinarides in the study area there are common NE-vergent reverse structures, 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 compression ones (reverse faults), indicating change of the palaeostress field during Neogene–Quaternary, resulting in prevalent transpression and radial extension. Comparison of palaeostress field analysis and constructed synthetic focal mechanism with available data on focal mechanism solutions within the Ilirska Bistrica–Rijeka–Senj seismogenic fault zone (FMS-database, 2019 ; HERAK et al., 2017, 2018 and references therein) 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. The research has been partly supported by Croatian Science Foundation project IP-2016-06-1854, GEOSEKVA and partly by project IP-2014-09-9666, VELEBIT.

Vinodol valley, compression/transpression, neotectonic activity, focal mechanism solutions, seismicity

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