Structural framework and polyphase kinematic evolution of the Gorski Kotar region, External Dinarides, Croatia – A cross-section view

The Dinarides fold-and-thrust belt formed due to the convergence between the Adriatic and Eurasian plates, culminating in continent–continent collision during the latest Cretaceous. Deformation first affected the internal Dinarides and subsequently propagated southwestward towards the External Dinarides during Eocene–Oligocene. Throughout most of the External Dinarides, up to 8 km thick carbonate-platform successions of predominantly Mesozoic age are deformed into NW–SE-striking, SW-vergent thrust sheets. In contrast, the Gorski Kotar region (NW Croatia) is characterized by a distinct change in the orientation of the fold-and-thrust belt, with predominantly N–S-striking structures and localized E- to NE-vergent thrusting. Field observations and cross-sections suggest that this structural anomaly is inherited from Early Jurassic E–W-directed extension, which caused a syn-rift succession up to ~2.7 km thick, approximately five times thicker than in adjacent regions.

To study these thickness variations and structural architecture in more detail, we constructed three geological cross-sections based on field observations, dip data, fault kinematic data, and geological maps. These cross-sections represent the deformed state of the region and were used in 2D kinematic forward models to simulate deformation over time.

The study area hosts extensive exposures of Permian siliciclastics, bounded to the west and to the south by thrust-related homoclines, which we interpret as relict back thrust ramps. To the north, the active dextral Idrija Fault bounds the Permian rocks. The Permian siliciclastics consistently occupy the structurally highest position and form the basal unit of a regionally continuous thrust sheet emplaced during Eocene–Oligocene shortening on top of the Mesozoic carbonate succession. The present-day distribution of this thrust sheet has been strongly modified and fragmented by subsequent post-orogenic extension and transpression.

Our results indicate that the present-day structural architecture was largely controlled by Early Jurassic extension accommodated by WSW-dipping normal faults. During subsequent Eocene–Oligocene shortening, this inherited rift architecture promoted the development of a top-to-the-NE/E-directed passive roof thrust sheet above a structurally complex triangle structure. Field observations and forward models further show that this thrust sheet was not emplaced atop a single stratigraphic horizon but instead overrode a structurally complex footwall that had already experienced folding, thrusting, and erosion prior to emplacement.