Monitoring of Slab Detachment in the Carpathians

Abstract.

Detachment of descending oceanic lithosphere (slab) is considered to be an important geodynamic process returning lithospheric material into the deeper mantle. On the geological time scale the detachment process itself is a short-term procedure. Thus it can be studied only at a few localities on Earth today. The Vrancea region in the SE-Carpathians is one of these rare places. In the SE-Carpathians, Miocene subduction was followed by continental collision accompanied by slab steepening (rollback). At the present time the slab is in a nearly vertical position and we interpret its strong intermediate-depth seismicity (70-180 km depth) to be triggered by slab pull (vertical extension axes from focal mechanism solutions). Our results from seismic tomography revealed a high-velocity body beneath the SE-Carpathians extending to a depth of at least 350 km and showing a SW-NE orientation. Seismicity is restricted to the north-eastern part of the high-velocity body. The SW-part is aseismic and thus probably already detached from the overlying crust. Detachment of the NE-part is likely to happen at the moment or in near future at a depth range of 40 to 70 km where a zone of low seismicity is associated with a low-velocity sub-crustal layer detected in refraction seismic profiles. The seismicity is located at the lower surface of the descending slab and thus can be interpreted as the lower plane of a double seismic zone where earthquakes are triggered be dehydration reactions. The upper plane, which is not observed in Vrancea, might be missing due to different reaction times, water contents, or other parameters varying for the two different slab regions. We developed a new model for the regional tectonic evolution during the last 15 million years taking into consideration subduction zone retreat, slab rollback and finally detachment. In this model the geometry of the plate boundaries define the direction of subduction retreat and thus the present-day NE-SW orientation of the slab. The subdivision of the slab in a seismically active NE-part and an aseismic SW-part is explained by slab detachment of the SW-part. The NE-part is still (partly) attached so that slab-pull induced stress accumulations lead to the intermediate-depth seismicity. We tested different scenarios for the tectonic evolution of the SE-Carpathians by kinematic gravity modelling and found a best-fit of modelled and measured gravity anomaly data for slab delamination and a still attached slab in the Vrancea region.