Constraints from Moho geometry and crustal thickness on the geodynamic origin of the Vrancea Seismogenic Zone (Romania)

Reprocessing of industry deep seismic reflection data (Ramnicu Sarat and Braila profiles) from the SE Carpathian foreland of Romania provides important new constraints on geodynamic models for the origin of the intermediate depth Vrancea Seismogenic Zone (VSZ). Mantle (70–200 km) earthquakes of the VSZ are characterized by high magnitudes (greater than 6.5), frequent occurrence rates (approximately 25 years), and confinement in a very narrow (30 × 70 × 200 km³) near vertical zone atypical for a Wadati–Benioff plane, located in front of the orogen. These two deep (20 s) seismic reflection profiles (70 km length across the foreland) reveal (1) a high-amplitude, gently east-dipping reflection across most of the section from what we interpret to be the Moho at  15 s (40–42 km) on the Ramnicu Sarat line to  16 s (47–48 km) on the Braila line, (2) a thick sedimentary cover increasing in thickness from east (1 s;  800 m) to west (7.5 s; 14 km), (3) an eastward increase in crustal thickness from 38 km (near VSZ) to  45 km, (4) seismic and topographic evidence for a newly imaged, possibly seismically active basement fault with a surface offset of 30 m observed on the Ramnicu Sarat line, (5) a lack of notable west-dipping structures in the crust and across the Moho, and (6) variable displacements on Peceneaga–Camena Fault of  5 km at Moho and  200 m at the basement–sedimentary cover contact.These observations appear to argue against recent models for west-dipping subduction of oceanic lithosphere at or in the vicinity of the Vrancea Seismogenic Zone given the lack of west-dipping fabrics in the lower crust and across the crust–mantle boundary. Consequently, one possible explanation for the geodynamic origin of VSZ could be partial delamination of the continental lithosphere in an intra-plate setting along a sub-horizontal lithospheric interface in the Carpathian hinterland that likely involves remnant lithospheric coupling between the crust and uppermost mantle in the foreland.