2D models exploring factors controlling N-S variation of external foreland fold and thrust belt of the Andes (Southern Bolivia -Northwest Argentina)

The structural style of foreland fold-and-thrust belts (FTBs) is highly sensitive to variations in pre-existing structures, three-dimensional décollement distribution, and syn-tectonic sedimentation. However, the relative importance of these factors and their influence on 2D and 3D structural variability remain poorly constrained. The pronounced along-strike variability of the Andean foreland FTB makes this region an ideal natural laboratory to investigate these interactions. We use the thermo-mechanically coupled tectonic model FANTOM 2D to produce high-resolution, fold-and-thrust belt–scale simulations that explore the interaction between internal properties of the wedge and surface processes. We systematically vary the strength of the two décollement horizons, combined with syn-tectonic sedimentation, and explore how this controls variability in structural styles of FTB formation. Our results show that a strong basal décollement combined with a weaker upper décollement leads to a steeper wedge taper and the development of an antiformal stack in the internal part of the fold-and-thrust belt and, in the foreland, all thrusts detach on the upper décollement, involving only the upper layer. In contrast, models with a weak basal and stronger upper décollement produce a lower-taper wedge, with thrust sheets detached solely on the basal décollement, propagating toward the foreland in a piggyback sequence. Structural complexity and kinematic variability increase when both décollements have similar low to intermediate strengths and interact with syn-tectonic sedimentation. Thrusts originate simultaneously in both décollements, involving the lower and upper layer in an alternating sequence, leading to a complex interaction between thrust propagation in the two layers. The onset of each structure—commonly pop-ups and triangular zones—and their subsequent diverse evolution can serve as diagnostic indicators of the relative strength between décollements. These modeled structures are comparable to the along-strike structural variability observed in the Bolivian fold-and-thrust belt. While syn-tectonic sedimentation primarily controls the number and length of thrusts, our results emphasize the first-order role of décollement rheology in shaping foreland fold-and-thrust belt architecture and its kinematic evolution.