Torn Between Two Plates: Exhumation of the Cer Massif (Internal Dinarides) as a Far-Field Effect of Carpathian Slab Rollback Inferred From 40Ar/39Ar Dating and Cross Section Balancing

Abstract

Extension across the southern Pannonian Basin and the internal Dinarides is characterized by Oligo-Miocene metamorphic core complexes (MCCs) exhumed along mylonitic low-angle extensional shear zones. Cer MCC at the transition between Dinarides and Pannonian Basin occupies a structural position within the distal-most Adriatic thrust sheet and originates from two different tectonic processes: Late Cretaceous-Paleogene nappe-stacking during a continent-continent collision with Adria in a lower plate position, and exhumation related to Miocene extension driven by the Carpathian slab-rollback. Structural data and a balanced cross section across the Cer massif show linking of the exhuming shear zone to a breakaway fault, which reactivated the early Late Cretaceous most internal nappe contact. Paleozoic greenschist-to amphibolite-grade lithologies surround a polyphase intrusion composed of I- and S-type granites and were exhumed along a shear zone characterized by top-N transport. Thermobarometric analyses indicate an intrusion depth of 7–8 km of the Oligocene I-type granite; cooling below ∼500°C occurred at 25.4 ± 0.6 Ma (1σ) yielded by 40Ar/39Ar dating of hornblende. Biotite and white mica from this intrusion as well as from the mylonitic shear zone yield 40Ar/39Ar cooling ages of 17–18 Ma independent of the used techniques (in situ laser ablation, single-grain total fusion, single-grain step heating, and multi-grain step heating). White mica from the S-type granite yield an 40Ar/39Ar cooling age of 16.7 ± 0.1 Ma (1σ). Associated dikes intruding the shear zone were also affected by N-S extension resulting in the exhumation of the MCC, which was triggered by the opening of the Pannonian back-arc basin in response to the Carpathian slab-rollback.