Dinaric thrust sheet over the Slovenian Basin. Where is the contact of Southern-Alps and Dinarides? Structural and stratigraphical constraints.

The transition from the Dinarides to the S-Alps and the position of the S-Alpine Trust-front east of Ljubljana is debated. Previous research agrees that the contact runs in the central Slovenian Sava Folds, they follow the analogy of western Slovenia, where the S-Alpine Thrust front (SATF) has been determined as the base of the Tolmin Thrust sheet (Placer 2008, Schmid et al. 2020). This tectonic unit incorporates the deep-water Middle Triassic to Cretaceous sedimentary succession of the Slovenian Basin (SB) which has been thrust over the Dinaric Platform. The presence of SB sediments east from Ljubljana, in the Sava Folds was suggested before (Buser 1989), but the presence of Jurassic deep-water sediments have only been proved recently (Rožič et al. 2022, Scherman et al. 2023). However, the area lying north of the discontinuous occurrence of the SB rocks was postulated to be of SB origin (Buser 2010, Placer 2008).
Detailed stratigraphic and structural observations confirmed the following succession: Ladinian siliciclastic sediments with volcanites (Pseudozilian Formation) are followed by Ladinian to Carnian Platform limestone (Schlern Formation). With a large gap Late Jurassic to Early Cretaceous pelagic limestone layers follow, containing resedimented limestone beds (Biancone Formation sl.). Covered by Early Cretaceous marlstone, with occasional calcarenitic interlayers (Lower Flyschoid Formation). The succession ends with pelagic limestones (Volče Limestone Formation).
The succession resembles the External Dinaric succession considered by Placer (2008) as the “Transitional zone” between the External and Internal Dinarides originally lying east from the Dinaric Carbonate Platform. With this discovery there is evidence for units of Dinaric origin north of the SATF, along the northern margin of the Sava Folds region, near the Sava Fault.
Tectonically this newly identified unit of External Dinaric origin is thrust over the SB succession in a south-vergent direction, which occurred likely during the Early Miocene, prior to the folding of the Sava Folds in a N-S contractional phase. The emplacement of the SB over the Dinaric units of the Sava folds is the oldest of the three events. Indirect evidence suggest that this thrusting was Palaeocene-Eocene, in SW direction (pre Oligocene post “mid” Cretaceous). The pre-Oligocene formations are folded. Over the erosional contact following Oligo-Miocene formations are folded in a following phase.
It is evident that the structural and the older palaeogeographical-stratigraphical boundaries have to deviate from each other east of Ljubljana. This also suggests that the SATF should be defined on structural basis (Schmid et al. 2020) and could represent a Miocene south-vergent thrust front. Emplacement of the deep-water Mesozoic basin over the platform seems to be a different deformation east of Ljubljana.

The research was supported by the OTKA (134873), Hantken and The Papp Simon Foundations.

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