Partial detachment of a lithospheric root under the Southeast Carpathians; toward a better definition of the detachment concept

The southeast corner of the Carpathian Mountains (Romania) provides a unique opportunity to study the very final and short-lived stage of plate convergence when a piece of subducted lithosphere finally detaches from the overriding plate and begins to sink into the mantle. Research efforts commonly focus on trying to image the cold lithospheric slab (tomographic techniques) and on trying to understand the tearing process from the earthquakes it generates. Here this issue is tackled from a different perspective emphasizing that the southeast Carpathians are extremely low considering crustal thickness; the crust there is almost as thick as the crust of the Alps (50-55 km), but the elevation is less than one-half of the elevation of the Alps. It is interesting that the Apennines (Italy) are just the opposite; they are rebounding after slab breakoff to reach elevations higher than expected from their crustal thickness (30-35 km like the crust of most passive-margin coasts). It is thus proposed that although seismologic studies indicate that the cold mantle body under the southeast Carpathians is already partly detached from the overriding lithosphere, it is still viscously coupled to the lithosphere and pulls it downward. By utilizing residual-topography analysis (observed topography minus the contribution of the crust to it), the subcrustal load is quantified and interpreted in terms of mantle-lithosphere thickness extending to a depth of approximately 300 km. A particularly interesting finding is that the seismically active region is shifted approximately 50 km east of the place of maximum pull-down, indicating that lithospheric tearing is now occurring at one side of the detaching root and not above it. This interpretation is consistent with the idea of lateral delamination of the lower lithosphere.