Décollement tectonics in the Jura forelandfold-and-thrust belt

Interpretation of more than 1500 km of industry seismic reflection lines from the Swiss and French Jura Mountains and the western Swiss Molasse Basin demonstrates that theMesozoic and Cenozoic cover of the Jura fold-and-thrust belt has been deformed over a veryweak basal décollement and displaced for many kilometers toward the NW, and that thebasement is not involved. A depth to the basement map derived from depth conversion of theseismic lines, shows a rather smooth flat basement dipping 1–3° to the S–SE. The low-amplitude broad folds of the Molasse Basin and the external Plateau Jura are controlled by evaporite pillows or anticlines of Middle Triassic age. The high amplitude folds of the Haute Chaıne Jura,however, are related to NW- or SE-vergent thrusts with at least kilometric dipslip displacement that has doubled the entire Jurassic sequence. <br> The Plateau Jura evaporite-related folds,located in the foreland, are interpreted as early-stage buckle folds. With further deformation, afault ramp nucleates and develops into thrust-related folds as observed in the Haute Chaıne. Thus, within the fold-and-thrust belt, deformation increases toward the hinterland. In the Molasse Basin, however, low-amplitude folds represent an early deformation stage which could not develop further owing to the load of the overlying thick Tertiary clastic wedge.