Deforming Balkans : Insights on Continental Collisional Processes from GPS Data.

The Balkans region that sits at the transition between stable Eurasia and highly straining Eastern Mediterranean, is a natural laboratory to study diffuse deformation of areas affected by continental collision. As in the Himalayan collision zone, it results in a widespread seismicity and underestimated seismic hazard (England & Jackson, 2011). Several Mw~6 shallow earthquakes occurred there in the last century producing large destructions and casualties. The completeness interval of the available historical seismicity catalogs is probably below the average recurrence of individual seismogenic structures, making them insufficient for accurate estimates of seismic hazard there. However, cGPS networks in the Balkans have been growing during the last few years mainly for civilian application, opening new opportunities to quantify the present-day rates of crustal deformation.

Here, we processed all the currently available data acquired on these new networks using the Gipsy-Oasis software and combined them with previously published velocities. We use this dense velocity field together with tensor moments from RCMT and CMT to derive an homogeneous map of the current-day strain using the methodology of Haines & Holt (1993). We image the deformation resulting from the boundary conditions around the Balkans : (i) the NE motion of Adria-Apulia blocks, (ii) the rigid Eurasian and Black sea backstops, (iii) the hellenic subduction zone and NAF rapid SE motion.

We show that the «Adria-Apulia push» is nearly entirely accommodated by compression across the Dinarides with minor propagation in the Pannonian basin. On the other hand, East of 20°E, the entire peninsula is moving southward, probably driven by the hellenic subduction pull. The transition between the SW-NE compressive and the N-S extensive regimes roughly corresponds to the Scutari-Pec transform zone, also known to be a boundary between paleomagnetic domains. Therefore, we show that the clockwise rotation pattern imaged by Perouse et al. (2012) in northern Greece extends up to northern Serbia (46°N), involving the whole Balkans. Finally, we calculate that the N-S extension does not account for the whole SW-NE compression over the Dinarides and that significant strain is stored over the Balkans, probably on multiple active structures that remain to be identified.