Localization of duplex thrust-ramps by buckling: analog and numerical modelling

Duplex structures in natural fold-thrust belts occur over a wide range of geometric scales. Duplex thrust ramps exhibit a regular spacing linearly related to the thickness of strata involved in the duplex. We suggest that buckling instability in layered systems can produce local stress concentrations which localize thrust ramps with regular spacing. This mechanism is demonstrated through analog (centrifuge) and numerical (finite element) modelling. fuge models containing finely-laminated multilayers composed of plasticine and silicone putty (simulating rocks such as limestone and shale) are compressed from one edge; folds propagate from hinterland to foreland. As shortening continues, the lowest competent unit is thrust into a blind duplex structure by breakthrusting. The duplex develops by serial nucleation of faults from hinterland to foreland; the ramp locations are inherited from the initial buckling instability. -element models based on the analog models and their natural prototypes demonstrate that stress concentrations develop in fore-limbs of anticlines within competent stratigraphie units. Models containing thrust discontinuities (at sites of calculated stress concentration) display additional stress concentrations in the forelimbs of unfaulted folds closer to the foreland. The locus of stress concentration thus propagates towards the foreland, consistent with foreland thrust propagation in nature. The location and regular spacing of ramps are inherited from early (possibly even incipient) buckle folds.