Pre-existing Basement Faults Controlling Deformation in the Jura Mountains Fold-and-Thrust Belt: Insights from Analogue Models

Pre-existing faults in the mechanical basement are believed to play an important role controlling deformation of thin-skinned Jura Mountains fold-and-thrust belt, which constitutes northernmost extension European Alps. We use brittle-viscous analogue models investigate influence frontal and oblique steps on subsequent evolution structures during shortening. Vertical offset between two rigid baseplates (simulating basement) causes formation reverse grabens overlying brittle layers that not reactivated However, baseplate localise deformation, causing a temporary frontward propagation early stage inhibiting afterwards. Downward induce very strong localisation foster fault-bend folds. Models featuring upward develop step-controlled pop-up with imbricated fronts viscous ramps shorten dynamically progressive contraction. find increases both higher step-throws lower obliquity (α) strike step (e.g. α = 0°). With increasing step-throws, 30° 45° upward-steps lead characteristic imbrication cover laterally confined thrust-slices step-parallel oblique-thrusts, rotate up 15° about vertical axis over time. Step-controlled backthrusts preceding do show notable rotation hence constitute excellent indicators for orientation upward-steps. The topographic patterns oblique-step resemble individual Internal (i.e. Pontarlier Vuache fault zones, nappe system SE Oyonnax Chasseral anticline), strongly suggesting pre-existing NNE-SSW NW-SE striking controlled cover. Our model results may be applied other belts worldwide formed above structures.