Defining the formation mechanism of regional scale-folds

We aim to define a mechanism for the formation of outcrop-scale structures associated with the formation of regional asymmetric folds with overturned flanks crosscut by high-angle reverse faults in sediments characterized by contrasting rheological alternances. Although such regional “scale-folds” are often observed, a quantitative analysis of this deformation type is still needed. One place where outcrop-scale structures associated with the formation of regional scale-folds are well-observed is the Marginal Folds unit of the East Carpathians in Europe. Contrasting rheological alternances driving the formation of scale-folds are created by deep-water turbidites, pelagic sediments and more shallow-water deposits. We have combined new field structural observations with existing depth constraints from wells and seismic data to define the deformation history and analyse its relationship with the rheological stratification. Deformation starts with the formation of detachment folds and continues with the formation of regional asymmetric folds by gradual thinning along their flanks and thickening in hinges, accommodated by omission and duplication shears, as well as hinge-collapse structures. As shortening continues and deformation gradually exaggerates the regional scale folds, it undergoes reverse faulting that gradually steepens and truncate the overturned foreland flanks of folds in a forward-breaking sequence. This deformation is followed by out-of-sequence overthrusting and formation of hinterland-dipping duplexes and backthrusting. The result is a dominant sub-vertical position of the observed strata and closely spaced reverse faults that formed gradually during the intense folding. These findings demonstrate a new kinematic and structural genetic type of deformation that can be applied elsewhere for structural or applied studies. • Mechanism for shortening of sediments in contrasting rheological alternations. • Formation of closely spaced folds with overturned flanks associated with steep thrusts. • Novel definition of tectonic omission and tectonic duplication shears. • Significant elongation of folds' flanks and thickening of their hinges. • The mechanism explains the multi-stage shortening observed in the East Carpathians.