The activity of subhorizontal décollements mapped by geologists in extensional provinces is not explained by rock mechanics principles, which predict that only steep faults can slip. These exposed décollements are therefore suspected to be inactive; they may correspond to rotated, formerly active, high-angle faults. However, growing seismological evidence of earthquakes with low-angle fault-plane mechanisms has forced us to revisit this viewpoint. Using the example of the Gulf of Corinth in Greece, we propose a mechanical model that explains how a weak, high-angle fault may form a low-angle décollement at depth. This décollement is uplifted in a second stage of extension, which allows the exhumation of a metamorphic core complex such as the northern Snake Range, Nevada.