DIACHRONOUS CENOZOIC EXTENSION AND RIFT-BASIN FORMATION IN THE SOUTHERN BALKANS EXEMPLIFIED BY THE PIRIN MTS. AND SANDANSKI BASIN, SW BULGARIA

More than 40 Ma of extension have shaped the Southern Balkans into a high-relief landscape of fault-bounded basins and ranges. The Southern Balkan extensional province is the northern continuation of the Aegean realm and attributed to dynamic causes including gravitational spreading of post-collisional hot and thick crust and back-arc extension above the retreating Hellenic trench. One of the best-studied extensional systems in the Southern Balkans, the South Rhodope core complex, records multiple stages of detachment faulting (e.g., Kerdilion and Strymon detachments). Syn-kinematic basins occur above the footwall and hanging wall of detachment faults; the spatial and temporal relationships between extension, footwall exhumation, and basin development are poorly understood and have led to a dispute about the timing of detachment faulting.

Our study focusses on the northern continuation of the Strymon detachment and Strymon basin, the Sandanski half-graben in SW Bulgaria. We combine structural, thermochronologic, paleomagnetic, and geomorphologic analyses, aiming for a more complete picture of Eocene to Quaternary extension and relief development. We present new (1) bedrock apatite and zircon U-Th/He ages from the exhumed footwall (Pirin Mts.), (2) mapping of Cenozoic faults and shear zones, (3) paleomagnetic age constraints on basin stratigraphy, (4) detrital apatite U-Th/He ages from the Sandanski basin, and (5) river profile analyses. Our integrated approach unravels two episodes of Eo-/Oligocene and early to middle Miocene detachment faulting under E-W extension that were probably not associated with significant relief or basin development. In contrast, the Sandanski basin formed between 10-6 Ma in the hanging wall of the West Pirin and Melnik normal faults. This late Miocene E-W extension resulted in uplift of the Pirin Mts. and the development of a mountain landscape similar to the modern topography. In the Quaternary, the stress field switched to N-S extension reflected in E-W trending faults and Quaternary basins. Our results advance our understanding of extension processes in the Southern Balkans in general and yield first constraints on the Neogene evolution of topography in the Southern Balkans with important implications on the evolution of regional climate and on Neogene paleoecology.