Apatite and zircon (U-Th)/He age constraints on exhumation phases of the westernmost Sava Suture Zone (Zrinska Gora, Croatia-Bosnia and Herzegovina boundary)

Wider Zrinska Gora area is located along the SW margin of the Pannonian Basin, within a Sava Suture Zone – a collision zone between the Tisza unit (European Plate) and the Internal Dinarides (Adria plate). Here, existing structures has been strongly affected by polyphase tectonics, characterized by repeated reactivations and tectonic inversions throughout the Mesozoic and Cenozoic in response to variable regional stresses and interactions between the European and African plates. This study focuses on constraining exhumation phases of the tectonostratigraphic units exposed in the Zrinska Gora area, westernmost SSZ area. The study area comprises four tectonostratigraphic units: the Palaeozoic–Triassic Pre-Karst Unit, the Jurassic Ophiolite Unit, the Upper Cretaceous Sava Zone Unit, and the Palaeogene Foreland Unit. Extensive field observations combined with vitrinite data suggests that the wider Zrinska Gora area generally experienced a low-temperature thermal overprint, sufficient to reset the apatite (U–Th)/He system. Vitrinite reflectance values indicate maximum post-depositional temperatures ranging from 120 to 150 °C (1.07 to 1.46 %Ro) for the Palaeogene foreland Unit, and 120 to 130 °C (0.92 %Ro) for the Jurassic Ophiolite Unit. In contrast, the Palaeozoic formations of Pre-Karst Unit exhibit a significantly higher thermal overprint, with maximum post-depositional temperatures estimated between 250 and 270 °C (4.64–5.94 %Ro), sufficient to fully reset the zircon (U–Th)/He system. Integration of the new zircon and apatite (U–Th)/He ages with vitrinite reflectance data suggests four tectonic phases in the thermal and structural evolution of the study area: i) Late Cretaceous thermal overprint of Sava Zone Unit deposits; ii) Palaeocene to early Eocene thrust propagation, nappe stacking, and partial exhumation caused by tectonic erosion; iii) post-collisional exhumation in the Late Eocene–Oligocene associated with Adria plate rollback; and iv) final Miocene extensional exhumation linked to reactivation of older thrusts due to lateral extrusion of the ALCAPA block and opening of the Pannonian Basin.

The presented work is supported by the Croatian Science Foundation project SECret (HRZZ IPS-2023-02-2683).