New insights on the structure and dynamics of the intermediate-depth seismic zone located in the bending area of east Carpathians: INDEGEN project

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Besutiu, Lucian۱; Atanasiu, Ligia۱; Mitrofan, Horia۱; Zlagnean, Luminita۱; Radulian, Mircea۲; Popa, Mihaela۲; Rogozea, Maria۲; Sabau, Gavril۳; Negulescu, Maria۳; Baltres, Albert۳; Rus, Tiberiu۴; Danciu, Valentin۴; Moldoveanu, Constantin۴; Neuner, Joh
Holding Date 08 September 2008

Located within intra-continental environment, Vrancea active seismic zone represents, along with Bucaramanga (Colombia) and Hindu Kush (Afghanistan), one the famous worlds’ seismic nests. The INDEGEN project was aimed at approaching the problem from a genuine point of view. So far almost all researchers attempted to correlate intermediate seismicity with extensional mechanical processes within a subducted slab-relict, or in a lithosphere fragment generated by a mantle delamination. Due to evidence against the subduction-based model, and because presence of the brittle environment may be hardly assumed at the intermediate seismicity depths, an alternate geodynamic model for Vrancea region was implemented. It has been assumed that speed excess provided to Moesian Microplate by W Black Sea opening created an unstable FFT triple junction within the bending area of East Carpathians, and the central (colder) lithosphere compartment stretched between the three plates joining the area was pushed down, thus penetrating the (hotter) upper mantle. Consequently, thermo-baric accommodation phenomena (e.g. convective currents, thermal stress, phase transform processes with volume change and dehydrations) may generate seismicity right at the critical depths where the asthenosphere (as a thermal discontinuity) is located within the three neighbouring plates. The new project starts from the observation that seismic events focus at three levels in depth: 90 km, 130 km, and 150 km, exactly where MTS showed the top of asthenosphere within the neighbouring plates.
Complex studies related to the structure and dynamics of the seismic body, and eventual connections between lithosphere dynamics and thermo-baric accommodation phenomena were performed (e.g. seismologic studies (event location, focal mechanism), monitoring non-tidal gravity change, geodetic observations on the crust deformation, evaluation of the seismic contribution of the phase transform processes, numerical modelling of the geothermal evolution of the above mentioned geodynamic model, thermal stress estimate, watching crustal stress changes by monitoring water geochemistry of some deep originating emergences). Among the preliminary results, mention should be made to the following aspects: (i) non-tidal gravity changes revealed a vertical extension of the sunken seismic body, very likely generated by eclogitization of its lower part, to which earthquakes in its upper (brittle) segment may be associated; it is worth mentioning that focal mechanisms indicating vertical extension occur in the area of the assumed unstable triple junction only, unlike neighbouring zones where horizontal strike-slip are present; (ii) significant variations in water geochemistry were associated with larger events (Mw>6); (iii) numerical modelling of the thermal stress revealed large figures and focus in the area where intermediate earthquakes occur. Acknowledgement. The research was funded through the grant CEEX-MENER 732/2006.