Decollement tectonics in the Jura foreland fold-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 the Mesozoic and Cenozoic cover of the Jura fold-and-thrust belt has been deformed over a very weak basal decollement and displaced for many kilometers toward the NW, and that the basement is not involved. A depth to the basement map derived From depth conversion of the seismic lines; shows a rather smooth flat basement dipping 1-3 degrees 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 Chaine Jura, however, are related to NW- or SE-vergent thrusts with at least kilometric dipslip displacement that has doubled the entire Jurassic sequence. The Plateau Jura evaporite-related folds, located in the foreland, are interpreted as early-stage buckle folds. With further deformation, a fault ramp nucleates and develops into thrust-related folds as observed in the Haute Chaine. 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 elastic wedge. (C) 1999 Elsevier Science Ltd. All rights reserved.