Dynamic evolution of thin-skinned fold-and-thrust belts

Thin-skinned fold-and-thrust belts related to convergent tectonics develop by scraping off a rock sequence along a weaker basal décollement, which is often formed by watersaturated shale layers or low-viscosity salt horizons. Although thin-skinned fold-andthrust belts are among the best and most investigated geological features on earth, there are many aspects that need further examination. The interplay between surface processes and the dynamic structural evolution is tested by magnetostratigraphy of progressive unconformities in the onshore Makran accretionary wedge and the Zagros foreland fold-and-thrust belt. The impact of décollement and wedge rheology on the long-term evolution of thin-skinned fold-and-thrust belts is investigated with numerical modelling and applied to the Makran and the Zagros. Comparison of numerical results to analytical solutions has been conducted. Progressive unconformities are predestined to link sedimentary and deformation processes in tectonically active settings. In the onshore Makran accretionary wedge, growth strata are found in large-wavelength, small-amplitude concentric synclines. The morphology of the growth strata indicates an underfilled basin where fold amplification was faster than sedimentation, forming typical sedimentary offlap structures. Magnetostratigraphy did not reveal any characteristic remanent magnetization. > 50% of the natural remanent magnetization was removed by step-wise thermal heating already above 200°C. The temporal evolution of deformation in the Zagros Simply Folded Belt is constrained by a magnetostratigraphic sequence containing a progressive unconformity in the Central Fars. The correlation of the magnetostratigraphic section with the geomagnetic polarity time scale constrains the transition from marine to fluvial sediment deposition at ∼ 6 Ma. This transition was accompanied by a change in accumulation rate from ∼ 15 cm/ka to ∼ 40 cm/ka. Alluvial river deposits first occurred at 3.2 − 3.1 Ma. In the Central Fars folding was initiated at 3.7 − 3.5 Ma. Comparing magnetostratigraphic sections and ages of growth strata in Central and NE Fars indicates a ∼ 1 cm/a migration rate of the deformation front towards the SW during the Middle and Late Miocene. A two-dimensional finite element model with a visco-elasto-plastic rheology is used to investigate the structural evolution of fold-and-thrust belts overlying different types of décollements and the influence of multiple weak layers within the stratigraphic column. Model shale décollements are frictional, with lower friction angles than the cover sequence. Model salt layers behave linear viscous, due to a lower viscosity than the cover sequence, or with a power law rheology. Fold-and-thrust belts with a single frictional basal décollement generate thrust systems ramping from the décollement to the surface. The structural evolution of simulations with an additional low-frictional layer depends on the strength relationship between the basal and the within-sequence décollement. Tectonic underplating and antiformal stacking, as observed in the Makran, occur if the within-sequence décollement is weaker than the basal one. Salt décollement with a viscosity of 1018 Pa·s leads to isolated box folds (detachment folds). Multiple salt layers (1018 Pa·s) result in long-wavelength folding. Latest developments in computer power and computational solutions open new ways to envision complex natural systems. The mechanics and dynamics of thin-skinned compressible thrust wedges with prescribed offsets in the backstop, i.e., transfer zones, are investigated using a three-dimensional finite difference numerical model with a viscobrittle/plastic rheology. The shorter the backstop offset, the earlier the two thrust planes connect, forming a curved frontal thrust along the entire width of the model. Younger, in-sequence thrusts are formed parallel to this curved shape. Long backstop offsets produce strongly curved thrust faults around the indenting corner. These simulations revealed that surface tapers of the wedge in front of the backstop promontory are larger than what the critical wedge theory predicts, whereas the tapers on the other side of the transfer zone are smaller than analytical values. This difference is amplified with increasing length of the backstop offset and/or strength of the décollement. Modelled surface elevation schemes reproduce well the topographic patterns of natural orogenic systems such as the topographic low along the Minab-Zendan transform/transfer fault between the Zagros and Makran. Advances in high-resolution 3D geodynamic modelling also allow investigating complex tectonic processes such as transpression. We present new answers to long lasting discussions. The Zagros Simply Folded Belt in Iran exhibits spatial variations of structural patterns from the homogeneously shortened, wide fold belt in the Fars to the narrower,