Structural framework and polyphase kinematic evolution of the Gorski Kotar region, External Dinarides, Croatia – A map View

The External Dinarides fold-and-thrust belt formed during the Middle Eocene–Oligocene as deformation propagated southwestward from the Internal Dinarides toward the Adriatic foreland during the Adria–Europe convergence and collision. Although the overall architecture of the External Dinarides is well established, the Gorski Kotar region remains one of its least understood segments. Here, the original contractional architecture has been extensively modified by pre- and post-orogenic extension and transpression, obscuring primary syn-orogenic thrust relationships and complicating the reconstruction of its tectonic evolution. We present the first results of comprehensive structural mapping, kinematic analyses, geological cross-sections, and Raman spectroscopy of carbonaceous material (RSCM) thermometry, integrated into an annotated geological map and short cross-sections to reconstruct the structural evolution of the region.

The study area exposes nearly complete Upper Triassic to Upper Cretaceous shallow-marine carbonate successions unconformably overlying Permian siliciclastic prodelta turbidites, incorporated in several thrust sheets. RSCM analyses of the Permian siliciclastics record maximum burial temperatures of approximately 180–230 °C, corresponding to burial depths of about 6–8 km, which exceeds the thickness of the preserved sedimentary cover (approximately 5 km), implying an additional component of tectonic loading beneath thrust sheets.

Structural mapping reveals two W- to SW-dipping thrust-related homoclines, bounded by the dextral Idrija Fault to the north and the Delnice Fault to the southeast, which separate the Gorski Kotar and Kvarner structural domains. Whereas deformation in the Kvarner domain primarily involves Cretaceous and Paleogene strata, the Gorski Kotar domain incorporates Permian to Upper Jurassic rocks into an E- to NE-vergent back-thrust system segmented by the NE-striking Delnice Fault. The juxtaposition of contrasting Jurassic facies across the Delnice Fault suggests that it represents an inherited pre-orogenic, extensional structure that influenced the development of the syn-orogenic contractional architecture.

Subsequently, the Dinaridic syn-orogenic thrust system was overprinted by polyphase deformation. High-angle normal faults related to E- to NE-directed post-orogenic extension dissect the thrust sheets and locally expose footwall units, resulting in the poor preservation of primary thrust contacts and fault kinematic data. In the northeastern part of the study area, younger (late Neogene?) SW-vergent thrusts locally overprint the earlier back-thrust system, producing apparent stratigraphic inversions by the emplacement of younger footwall units above older hanging wall units.

These results demonstrate that the tectonic evolution of the Gorski Kotar region reflects the superposition of pre-orogenic extension, syn-orogenic (back-) thrusting, post-orogenic extension, and younger (Neogene?) contractional overprint. The resulting structural framework provides new constraints on the tectonic evolution of the northern External Dinarides and the role of inherited structures controlling subsequence deformations.

Presented research was conducted in the scope of the internal research project GeoDRIFT at the Croatian Geological Survey, and Next Generation NPOO project GEO-RIZ, funded by the National Recovery and Resilience Plan 2021–2026 of the European Union – NextGenerationEU, and monitored by the Ministry of Science and Education of the Republic of Croatia.