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RESEARCH PRODUCT

Control of 3D tectonic inheritance on fold-and-thrust belts: insights from 3D numerical models and application to the Helvetic nappe system

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Abstract. Fold-and-thrust belts and associated tectonic nappes are common in orogenic regions. They exhibit a wide variety of geometries and often a considerable along-strike variation. However, the mechanics of fold-and-thrust belt formation and the control of the initial geological configuration on this formation are still incompletely understood. Here, we apply three-dimensional (3D) thermo-mechanical numerical simulations of the shortening of the upper crustal region of a passive margin to investigate the control of 3D laterally variable inherited structures on the fold-and-thrust belt evolution and associated nappe formation. We consider tectonic inheritance by applying an initial model configuration with horst and graben structures having laterally variable geometry and with sedimentary layers having different mechanical strength. We use a visco-plastic rheology with temperature dependent flow laws and a Drucker-Prager yield criterion. The models show the folding, detachment and horizontal displacement of sedimentary units, which resemble structures of fold and thrust nappes. The models further show the stacking of nappes. The detachment of nappe-like structures is controlled by the initial basement and sedimentary layer geometry. Significant horizontal transport is facilitated by weak sedimentary units below these nappes. The initial half-graben geometry has a strong impact on the basement and sediment deformation. Generally, deeper half-grabens generate thicker nappes and stronger deformation of the neighboring horst while shallower half-grabens generate thinner nappes and less deformation in the horst. Horizontally continuous strong sediment layers, which are not restricted to inital graben structures, cause detachment folding and not overthrusting. The amplitude of the detachment folds is controlled by the underlying graben geometry. A mechanically weaker basement favors the formation of fold nappes while stronger basement favors thrust sheets. The applied model configuration is motivated by the application of the 3D model to the Helvetic nappe system of the French-Swiss Alps. Our model is able to reproduce several first-order structural features of this nappe system, namely (i) closure of a half-graben and associated formation of the Morcles and Doldenhorn nappes, (ii) the overthrusting of a nappe resembling the Wildhorn and Glarus nappes and (iii) the formation of a nappe pile resembling the Helvetic nappes resting above the Infrahelvetic complex. Furthermore, the finite strain pattern, temperature distribution and timing of the 3D model is in broad agreement with data from the Helvetic nappe system. Our model, hence, provides a first-order 3D reconstruction of the tectonic evolution of the Helvetic nappe system based on thermo-mechanical deformation processes.