Three-dimensional thermo-mechanical numerical modelling of fold and thrust nappe stacking with application to the Helvetic nappe system (W Switzerland)


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PhD thesis: a PhD thesis.
Three-dimensional thermo-mechanical numerical modelling of fold and thrust nappe stacking with application to the Helvetic nappe system (W Switzerland)
Spitz Richard
Schmalholz Stefan M.
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Université de Lausanne, Faculté des géosciences et de l'environnement
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Fold and thrust belts are a common structural feature of orogens that form in response to compressional tectonics. They have been studied by geologists for at least a century with the aim to understand the structural style, tectonic evolution, dynamics and the control factors that govern their formation. Classically, we consider two distinctive large scale tectonic styles for a fold and thrust belt, namely thick-skinned and thin-skinned tectonic style. These styles are derived from the degree of basement-cover interaction during the fold and thrust belt formation. The former implies that the crystalline basement and overlying sedimentary cover-sequences accommodate an equal amount of deformation during crustal shortening. In contrast a thin-skinned style implies that most of the bulk shortening is accommodated in the deformation of sedimentary cover-sequences above the basement along a weak basal shear or detachment zone. Therefore, the basement remains mostly undeformed while the cover-sequences exhibit significant internal deformation and/or horizontal displacement. The structural style of fold and thrust belts is closely linked to inherited crustal structures, such as basins or variations in the lithostratigraphy. Hence, large amount of data on fold and thrust belts indicates that the structural style can be highly variable for the same belt. For example, studies report changes in style from the exterior of the belt to the interior or from one end to the other along-strike. Consequently, it is suggestible that the three-dimensional geometry of inherited pre-orogenic structures presents a prevailing control factor on the structural style of fold and thrust belts. In this thesis we focus on one of the preeminent fold and thrust belts belonging to the Western Swiss Alps,namely the Helvetic nappe system. The Helvetic nappe system has a long standing history in Alpine geology and was one of the major testing grounds for the evolution of the so-called nappe theory. In general, a tectonic nappe is defined as a coherent allochthonous rock unit/sheet that has been displaced away from its original position along a basal thrust or shear zone. Furthermore we distinguish between two end-member types, namely fold nappes and thrust nappes. Fold nappes are large recumbent folds with an amplitude of several kilometers contributing to a stratigraphic inversion of the same magnitude. In contrast thrust nappes are emplaced as coherent rock sheets along a basal thrust resulting in the superposition of stratigraphic older units on top of younger units. The Helvetic nappe system exhibits transition between these two the different nappe styles along-strike. The style changes form the famous Morcles fold nappe in the Southwest, to the Doldenhorn fold nappe in the center and the prominent Glarus thrust nappe in the Northeast. Morever, both the Morcles and Doldenhorn nappe are overlain by series of smaller thrusts sheets that are analogues to the Glarus nappe. Interestingly, the Doldenhorn fold nappe shows a less pronounced recumbent limb and internal folding than the Morcles nappe, whereas the Doldenhorn nappe shows greater shearing. Both of these nappes are derived from sediments that were once situated in a graben system that was inverted during the Alpine orogeny. Contrary reconstructions of the Glarus nappe do not point to a pronounced graben system. Hence, it is suggested that lateral variations in the basement structure had a major influence on the evolution of the Helvetic nappe systems. The aim of this thesis is to gain additional insights in the lateral transition between folding and thrusting and the evolution and emplacement of fold and thrust nappe stacking in three-dimensional space. To this aim we employ three-dimensional (3D) thermo-mechanical numerical models that we apply to the Helvetic nappes ystem. In our first study (Chapter 2) we implement a numerical algorithm to calculate and trace 3D finite strain in order to quantify the deformation. We further use a simple 3D viscous model consisting of a laterally changing mechanical stratigraphy (lithostratigraphy) to simulate the transition between thrusting and folding. Our results essentially show that the spatial distribution and gradient of mechanical stratigraphy is directly expressed in a change of the finite strain gradient along the hinge of the fold to the thrust sheet. In our second study (Chapter 3) we employ a 3D numerical model of a simplified passive margin with an inherited graben structure to simulate the formation of a fold nappe that is over-thrusted by a thrust sheet. Furthermore, the model parameters and configuration are adapted to mimic the initial conditions of the Helvetic nappe systems. We are able to reproduce several first order key features such as the nappe structure, temperature distribution,geologic timing and finite strain pattern. Continuing, we show that a relative simple graben systems can explain changes in fold nappe structure along-strike. Additionally our model results imply that large thrust sheets may propagate in horizontal direction without disturbance by underlying fold nappe formation. Finally our models also indicate that thrust and fold nappe formation in the Helvetic nappe systems likely occurred under a semi-brittle-ductile deformation regime
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26/02/2020 15:36
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02/10/2020 6:09
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