The uniform stretching model has been used to calculate the volume and composition of melt generated by adiabatic decompression during extension of the continental lithosphere. The consequences of this approach for melt generation by litbospheric simple shear were investigated. The results show that, given an initially planar detachment fault, it is extremely difficult to generate melt from the asthenosphere, under either normal or elevated asthenospheric potential temperatures. Our conclusion is independent of the initial dip of the detachment fault and also holds when the initial dip in the lithospheric mantle is double that in the crust. In the North Sea, lithospheric simple shear fails to account for the existence and location of magmatism. These conclusions may also apply, more generally, to passive margin development.