Strain partitioning of scale-folds: In- versus out-of- sequence contractional deformation in the Southeastern Carpathians

Asymmetric regional folds with thickened hinges, thinned flanks and overturned strata truncated by foreland verging high-angle reverse faults are observed in thin-skinned thrust belts characterized by rheologically contrasting alternating layers in structures previously defined as scale-folds. This fold type shows substantial strain partitioning along the orogenic strike, although the understanding of deformation mechanisms involved is relatively limited. We aim to define such a strain partitioning mechanism of scale-folds by a kinematic study in the Vrancea tectonic half-window of the Southeastern Carpathians in Europe, which contains highly deformed Cretaceous - earliest Miocene strata of the Marginal Folds unit. The analysis of scale-fold structures correlated with available depth information and surface-to-depth projections demonstrates that their initial formation was followed by a gradual out-of-sequence partitioning of deformation along the orogenic strike. While a kilometres-scale décollement fold formed in the centre of the half-window, deformation is transferred towards its margins to low-angle thrusting and amplification of frontal splays. This transfer is accommodated by decreasing the amount of folding along a map-view regional structure, herewith defined as a tear fold. The correlation with modelling studies suggests that the transfer is controlled by mechanical decoupling within rheologically weak layers, associated with syn -kinematic exhumation. This correlation also demonstrates that the Southeastern Carpathians deformation took place in successive stages with different expressions than previously thought. The mechanism of deformation explains the differential exhumation observed in the tectonic half-window and sets the stage for better understanding the strain partitioning in sediments with a rheologically contrasting stratigraphy.