Structural styles and decoupling in stratigraphic sequences with double décollements during thin-skinned contractional tectonics: Insights from numerical modelling

Six series of particle-based numerical experiments were performed to simulate thin-skinned contractional tectonics in stratigraphic sequences with double décollements during horizontal shortening. The models were assigned with varying rock competence, depth and thickness of the upper décollement, which resulted in significantly different styles of deformation and decoupling characteristics above and below the upper décollement. The models composed of the least competent material produced distributed sinusoidal detachment folds, with many shallow structures profoundly decoupled from the deep-seated folds. The models composed of a more competent material are dominated by faulted box folds, with minor disharmonic folds developed in their limbs. In contrast, the results of models composed of the most competent material are characterised by localised piggyback thrusts, fault-bend folds and pop-up structures with tensile fractures developed in fold hinges. Depth of the upper décollements also plays an important role in controlling structural decoupling, i.e. the shallower the upper décollements, the higher the degree of decoupling becomes. Thicker upper décollements can provide sufficient mobile materials to fill fold cores, and contribute to the formation of secondary disharmonic folds, helping enhance structural decoupling. Our modelling results are comparable to the structural features exhibited in the Dezful Embayment of the Zagros Fold-and-Thrust Belt with the Miocene Gachsaran Formation acting as the shallow upper décollement, and the Fars with the Triassic Dashtak Formation as its intermediate décollement. This study demonstrates that rock competence, depth and thickness of the upper décollements can jointly affect the structural styles and decoupling. Our modelling results are instructive for structural interpretation of deep zones in fold-and-thrust belts that exhibit distinct structural decoupling features.