Thrust-top basins form and fill during their detachment from the foreland margin by active thrust sheets. Strike-normal (N–S) sections from the West Jaca thrust-top basin (Southwest Pyrenees) to the Ebro basin show that the Pena flexure, the local expression of the thrust front, can be interpreted as a passive-roof duplex. The duplex accommodated up to 7 km bulk shortening at the tip of the Gavarnie thrust sheet. Unravelling the timing of uplift and sedimentation in the West Jaca basin has revealed that development of the Pena flexure represented the climactic phase of deformation. Lateral (E–W) diachroneity of gross stratigraphy and thrust emergence throughout the southwest Pyrenees suggests that the thrust front propagated both toward the foreland and westward, parallel to the orogen axis.