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

Abstract

Abstract. We apply three-dimensional (3-D) thermo-mechanical numerical simulations of the shortening of the upper crustal region of a passive margin in order to investigate the control of 3-D laterally variable inherited structures on fold-and-thrust belt evolution and associated nappe formation. We consider tectonic inheritance by employing an initial model configuration with basement horst and graben structures having laterally variable geometry and with sedimentary layers having different mechanical strength. We use a visco-plastic rheology with a temperature-dependent flow law and a Drucker–Prager yield criterion. The models show the folding, detachment (shearing off) and horizontal transport 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 neighbouring horst, while shallower half-grabens generate thinner nappes and less deformation in the horst. Horizontally continuous strong sediment layers, which are not restricted to initial graben structures, cause detachment (décollement) folding and not overthrusting. The amplitude of the detachment folds is controlled by the underlying graben geometry. A mechanically weaker basement favours the formation of fold nappes, while stronger basement favours thrust sheets. The model configuration is motivated by applying the 3-D model to the Helvetic nappe system of the Central Alps of France and Switzerland. Our model reproduces several first-order features of this nappe system, namely (1) closure of a half-graben and associated formation of the Morcles and Doldenhorn nappes, (2) overthrusting of a nappe resembling the Wildhorn and Glarus nappes, and (3) 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 3-D model is in broad agreement with data from the Helvetic nappe system. Our model, hence, provides a 3-D reconstruction of the first-order tectonic evolution of the Helvetic nappe system. Moreover, we do not apply any strain softening mechanisms. Strain localization, folding and nappe transport are controlled by initial geometrical and mechanical heterogeneities showing the fundamental importance of tectonic inheritance on fold-and-thrust belt evolution.

Buoyancy versus shear forces in building orogenic wedges

“…This way, the generated passive margin, the marine basin system and the subsequent convergence are modelled in an internally consistent manner. Subduction initiation is horizontally forced (Stern, 2004;Stern and Gerya, 2018;Crameri et al, 2020), and a major lithospheric shear zone forms around the transition from the distal to the proximal margin (see also the discussion in Candioti et al, 2020). The ad hoc parameterized layer of serpentinite is not relevant for subduction initiation, as subduction is initiated also in the reference model without serpentinite (see Fig.…”

“…Such wedge models have also been used to study the formation of viscous fold nappes during fold and thrust belt evolution (e.g. Bauville and Schmalholz, 2015;Spitz et al, 2020) or the impact of surface processes on wedge formation (e.g. Willett, 1999).…”

Physics-inspired pseudo-transient method and its application in modelling focused fluid flow with geological complexity

“…First, high-resolution 3-D modelling requires assembling large matrices that are challenging to solve even with the sparse matrix format (Burov et al 2014;May et al 2014;Morra 2020). As a result, the maximum degree of freedom is usually limited to <10 9 in published 3-D models (Zhong et al 2000;Settari & Walters 2001;Tackley 2008;May et al 2014;Moresi et al 2014;Dannberg & Heister 2016;Spitz et al 2020;Rinaldi et al 2021). Secondly, existing porous flow models in deformable rocks commonly use simplified equations that might give inaccurate results (Settari & Walters 2001;Prevost 2014).…”

Large thrusting and late faulting shape the Aiguilles Rouges crystalline massif (Western Alps), structural implications

“…These field observations are key to characterize the deformation within the orogenic wedge, and thus to discuss the outcome of rheological models (e.g. Spitz et al, 2020;Bauville et Schmalholz, 2015). Helvetic thrust, Alpine Sole Thrust, and late thrusts.…”