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

TLDR

Slab break‑off, a process that returns lithospheric material to the mantle and reduces slab‑pull induced seismicity, is relevant to the Vrancea region where intermediate‑depth earthquakes occur in a small volume and the vertically elongated slab must have a vertically fixed upper end. Stress analysis reveals vertical extension in the slab with no preferred orientation in the crust, indicating a decoupling that is best explained by a soft coupling strong enough for slab elongation yet weak enough to prevent quasi‑static stress transfer.

Abstract

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.