Abstract: In this paper, we document the early stage of fault-zone development based on detailed observations of mesocale faults in layered rocks. The vertical propagation of the studied faults is stopped by layer-parallel faults contained in a weak layer. This restriction involves a flat-topped throw profile along the fault plane and modifications of the fault structures near the restricted tips, with geometries ranging from planar structures to fault zones characterized by abundant parallel fault segments. The ‘far-field’ displacement (i.e. the sum of the displacement accumulated by all the fault segments and the folding) measured along the restricted faults exhibiting this segmentation may have flat-topped shapes or triangular shapes when fault-related folding is observed above the layer-parallel faults. We develop a model from the observations. In this model, during the course of restriction, a fault forms as a simple isolated planar structure, then parallel fault segments successively initiate to accommodate the increasing displacement. We assume that, eventually, the fault propagates beyond the layer-parallel fault. This model implies first that fault widening is controlled by the fault capacity to propagate vertically in the layered section. Likewise, owing to restriction, fault growth occurs with non-linear increases in maximum displacement, length and thickness.

Abstract: The total offset across a fault zone may include offsets by discontinuous faulting as well as continuous deformation, including fault-related folding. This study investigates the relationships between these two components during fault growth. We established conceptual models for the distributions of displacement due to faulting (i.e. brittle component or near-field displacement), to folding (i.e. ductile component) and to the sum of both (i.e. far-field displacement) for different mechanisms of fault-related folding. We then compared these theoretical displacement profiles with those measured along mesoscale normal faults cutting carbonate-rich sequences in the Southeast Mesozoic sedimentary basin of France. The near-field and far-field displacement profiles follow either a flat-topped or a triangular shape. Several fold mechanisms were recognized, sometimes occurring together along the same fault and represent either fault-propagation folds, shear folds or coherent drag folds. In the last case, local deficit in the fault slip is balanced by folding so that the brittle and ductile components compose together a coherent fault zone. Common characteristics of these faults are a high folding component that can reach up to 75% of the total fault throw, a high displacement gradient (up to 0.5) and a strong fault sinuosity.