Slab break‐off – abrupt cut or gradual detachment? New insights from the Vrancea Region (SE Carpathians,Romania)

Slab break-off is a plate-tectonic process which does not only return lithospheric material into the deeper mantle, but also has severe effects on surface movements and seismic hazard: slab-pull induced seismicity is reduced when the subducted slab decouples from the overlying crust. In the Vrancea region (SE Carpathians), strong earthquakes frequently occur at intermediate depths (70–180 km) in a laterally small volume, while the crust shows low but distributed seismicity. The stress pattern shows similar partitioning with vertical extension in the slab and no preferred orientation in the overlying crust. Both features indicate a decoupling between slab and overlying crust, either by slab break-off or delamination. However, the strong vertical elongation of the slab requires that the upper end of the slab is fixed vertically. Thus, a 'soft' coupling is assumed that is strong enough to enable elongation of the slab, but weak enough to inhibit 'quasi-static' stress transfer to the overlying crust.