Exhumation of metamorphic core complexes of the internal Dinarides was triggered by the opening of the Pannonian Basin

The Sava suture zone of the internal Dinarides contains Maastrichtian trench-fill sediments, termed “Sava flysch” that record the closure of the northern branch of the Neotethys. Subsequent collision between Adria-derived thrust sheets and blocks of European affinity in Latest Cretaceous to Paleogene times culminated in the formation of the Dinarides fold-and-thrust belt. The suture zone hosts numerous Oligocene plutons of I-type granitic composition. Many of these intrusions are located in the center of metamorphic core complexes (MCCs) that were exhumed in early Miocene times. This phase of post-collisional extension was concomitant with the opening of the northerly adjacent Pannonian Basin and associated with granitic S-type magmatism. Both the processes responsible for extensional deformation and magmatic activity in the internal Dinarides are still a matter of debate.

Our Study contributes spatio-temporal constraints to better understand the tectono-magmatic processes of this area. We present field-kinematic, geochronological, and thermobarometric data from two MCCs at the transition between the internal Dinarides and the Pannonian Basin. Both MCCs are characterized by plutonic rocks in the center, surrounded by up to amphibolite-grade mylonites of exhuming shear zones. Heterogeneous extensional reactivation of formerly contractional structures that gave rise to these core complexes as low-angle detachments in the early Miocene is indicated by a variation in deformation ages of 3 Ma, obtained by Ar-Ar in-situ dating of white mica from deformed rocks of the respective shear zones. While Motajica MCC was exhumed from within the Sava zone during E-W extension at approximately 20 Ma, Cer MCC was exhumed as part of the underlying Adriatic basement during N-S extension between 17-16 Ma. For the Cer MCC, a concordia age of 17.6±0.1 Ma (2σ) obtained by U-Pb LA-ICP-MS on zircons from an S-type granite in combination with an Ar-Ar inverse isochron age of 16.6±0.2 Ma (2σ) obtained on white mica from the same sample, indicate a cooling rate of approximately 400°C/Ma.

Our results contribute to the idea of rapid exhumation of mid-crustal material in the form of MCCs in response to the opening of the Pannonian Basin. This is further corroborated by results of Raman spectroscopy on carbonaceous material, as the temperature profile across the shear zone implies extremely condensed isotherms of 250°C/km. Additionally, U-Pb analyses show that zircons of the I-type intrusion contain inherited cores with age maxima at 270 Ma and 516 Ma and newly formed rims with an age maximum at 31.7 Ma, indicating the timing of intrusion. The S-type granite of Cer in parts reworks the I-type intrusion, as inherited cores include ages of 31-32 Ma, while the rims show an age of 17-18 Ma, suggesting a syn-extensional emplacement. Our data further shows that zircons of the I-type intrusion contain a significant amount of inherited cores with an age spectrum that resembles the detrital age spectrum from sediments of the Sava zone. This challenges the idea that these I-type melts were solely generated from igneous protoliths, and rather suggests a formation from melting of Paleozoic to Mesozoic successions constituting tectonically buried nappes of the internal Dinarides.