Control of synorogenic sedimentation on back and out-of-sequence thrusting: Insights from analog modeling of an orogenic front (Outer Carpathians, southern Poland)

Scaled sandbox models based on field and seismic data were used to simulate Miocene evolution of the Carpathian orogenic front. The experiments investigated the influence of synorogenic deposition on the geometry and kinematics of growing tectonic structures. The deposition introduced at the foreland of an active fold-thrust wedge resting on top of a weak ductile detachment horizon generally results in transferring the deformation to the rear part of the model. Depending on the rate and across-strike extent of synorogenic sedimentation and on the moment it starts, a single major back thrust above the rear edge of the ductile layer can form or out-of-sequence foreland verging thrusts can be initiated. Alternatively, older thrust faults can be reactivated at the rear part of the model thrust wedge, leading to destruction of the earlier formed structures. Our experiments explain the origin and evolution of three different structural geometries known from the Carpathian orogenic front, involving major back thrusts related to triangle zones and out-of-sequence breaching forethrusts that substantially reorganized deformation patterns in a growing thrust wedge. The development of these geometries was controlled by massive synorogenic sedimentation in the foreland. From the modeling and natural examples we infer a number of typical structural situations that can occur at an orogenic front underlain by a weak ductile horizon, depending on parameters controlling synorogenic sedimentation and tectonic push exerted by an approaching thrust wedge.