Miocene extensional core complex in the SW Pannonian Basin: stratigraphic and thermochronological evidence from Hrastovička Gora Mt., Central Croatia

Extensional core complexes commonly develop in regions undergoing post-orogenic extension and lithospheric thinning, where extension exhumes rocks from deeper crustal levels. One such region is the southwestern margin of the Pannonian Basin System, where extension started shortly after the peak orogenic stage of the Dinarides. We present new stratigraphic and thermochronological evidence for the Early Miocene extensional exhumation of the Hrastovička Gora core complex, located at the southwestern margin of the Pannonian Basin System.

The study area exposes a complete geological record related to the Early to Middle Miocene rifting and opening of the Pannonian Basin System, comprising pre-rift, syn-rift and post-rift rock units. Hrastovička Gora Mt. lies in the center of the study area, currently separating the Sava Basin to the NE from the Glina Basin to the SW.

The oldest, Upper Cretaceous to Eocene pre-rift basement units are typically assigned to the so-called Sava Suture Zone of the Dinarides. They comprise Upper Cretaceous granitic and metamorphic rocks, as well as a volcano-sedimentary complex of intercalated spilites and pelagic Scaglia-type limestone, continuously overlain by Paleocene to Eocene, mostly siliciclastic, syn-orogenic flysch-type deposits.

The subsequent Ottnangian to upper Badenian syn-rift deposits consist of siliciclastic conglomerates, sands, marls and limestones recording a gradual transition from alluvial to marine environments. While the surrounding Sava and Glina basins preserve this complete syn-rift succession deposited on top of the basement, the syn-rift units are entirely absent on Hrastovička Gora Mt. itself. There, the post-rift stage begins with a major Late Badenian overstepping transgression, depositing Upper Badenian to upper Pannonian post-rift successions directly above the pre-rift basement. This post-rift thermal subsidence stage forms a complete transgressive–regressive cycle, beginning with basal conglomerates and passing upward into sandstones, limestones, and marls, followed by a gradual shallowing from deep-water marls back to alluvial conglomerates, sands, and clays.

Fission-track and (U–Th)/He dating of zircon and apatite from pre-rift and syn-rift rocks indicates the development of two distinct thermal paths following Late Eocene to Early Oligocene times. Thermochronological data indicate the exhumation of pre-rift basement units of Hrastovička Gora Mt. during Early to Middle Miocene times, whereas basement units of the surrounding Sava and Glina basins had already reached similar crustal levels during the Eocene. The integration of thermochronological and stratigraphic data indicate the existence of an extensional detachment, with Hrastovička Gora representing its exhumed footwall, and the Glina and Sava basins representing a supradetachment basins developed on top of hanging wall units. This detachment probably correlates with detachments previously discovered further southeast along the southwestern margin of the Pannonian Basin System in Prosara, Kozara, Motajica, Cer and Bukulja mountains.

Presented research was conducted in the scope of the internal research project GeoSAVAGE at the Croatian Geological Survey, 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.