The increasing amount of worldwide deep seismic data emphasizes the contrast between a seismically transparent upper crust and a highly reflective lower crust. Recent deep seismic profiling has revealed heterogeneous substrata in northwest European (North Sea, Southwest Britain, West France) sedimentary basins of late Paleozoic and Mesozoic age and with contrasting styles (half-grabens and symmetrical basins). Steeply dipping reflectors within the crust can be interpreted in all places as inclined and probably ductile shear zones relating to either extension or compression. With respect to their location, a good candidate for such a feature is to relate them as ghosts thrust planes inherited from Caledonian or Hercynian orogenies. These pre-existing zones of weakness induce heterogeneity in the upper crust, which controls the asymmetry of future extensional tectonics. During an extensional regime, the top of the lower crust acts as a mechanical decoupling boundary separating asymmetrical brittle failure within the upper crust and symmetrical ductile creep within the lower crust.