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Towards understanding the dynamics of tectonic nappe stacking and overthrusting in the Helvetic nappe system

English title Towards understanding the dynamics of tectonic nappe stacking and overthrusting in the Helvetic nappe system
Applicant Schmalholz Stefan Markus
Number 149380
Funding scheme Project funding (Div. I-III)
Research institution Institut de géologie et de paléontologie Université de Lausanne
Institution of higher education University of Lausanne - LA
Main discipline Geology
Start/End 01.11.2013 - 31.10.2019
Approved amount 592'094.00
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Keywords (4)

Overthrusting; Western Swiss Alps; Nappe stacking; Numerical modeling

Lay Summary (German)

Lead
Die voralpinen tektonischen Strukturen, welche während der Bildung von Ozeanbecken durch Streckung der Erdkruste entstanden sind, hatten einen wichtigen Einfluss auf die tektonische Entwicklung der Alpen. Dieser Einfluss ist bis heute nicht vollständig verstanden. Wir wenden numerischen Modelle an, welche die thermomechanischen Prozesse während der Streckung und Kompression der Lithosphäre simulieren, um diesen Einfluss der voralpinen Strukturen besser zu verstehen und zu quantifizieren.
Lay summary
Die geodynamische Entwicklung der Alpen istimmer noch nicht vollständig verstanden. Die Alpen enthalten die Überreste vonzwei Ozeanbecken der Jura- bis Kreidezeit, dem sogenannten Walliser Trog unddem Piemontesischen Ozean. Während des Känozoikums wurden die gestrecktenKontinentalränder dieser Ozeanbecken in die obere tektonische Platte derAlpinen Gebirgsbildung eingearbeitet. Diese Kontinentalränder waren gekennzeichnetdurch Merkmale wie exhumierte kontinentale Mantelgesteine, Abscherhorizonte aufKrustenmassstab und stark gestreckte kontinentale Kruste. Der mechanische Einflussdieser voralpinen Merkmale auf die folgende Subduktion, auf die Deckenentwicklungund auf die Bildung und Exhumierung von Ultrahochdruckgesteinen währenden der AlpinenGebirgsbildung ist weitgehend unbekannt. In diesem Projekt führen wir zwei-dimensionale(2-D) numerische Simulationen durch, in denen die Lithsophäre zuerst gestrecktwird (um die magma-armen Kontinentalränder zu bilden) und danach verkürzt wird.Mit diesen Simulationen untersuchen wir den thermomechanischen Einfluss von voralpinenStrukturen auf die tektonische Entwicklung der Westalpen. Wir untersuchen (1) denSubduktionsbeginn an magma-armen Kontinentalrändern, (2) die Bildung und Exhumierungvon Ultrahochdruckgesteinen während der Kompression, (3) den Einfluss von verschiedenenMaterialschwächungsmechanismen, und (4) den Einfluss von voralpinen Schwächezoneenauf die Deformation während der Kompression. Die numerischen Resultate werden mitverfügbaren geologischen und geophysikalischen Daten verglichen. Das Ziel istes, eine numerische Simulation zu erstellen, welche die verfügbaren Daten ambesten erklären kann.
Direct link to Lay Summary Last update: 08.10.2013

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Associated projects

Number Title Start Funding scheme
131897 Towards understanding fold nappe dynamics in the Western Swiss Alps 01.10.2010 Project funding (Div. I-III)
163169 Towards a self-consistent dynamic orogenic wedge model for the Western Alps 01.11.2015 Project funding (Div. I-III)
144250 Towards understanding the tectonic evolution from magma-poor rifted margins to the Alpine orogen: Insights from 2D and 3D numerical modeling 01.11.2012 Project funding (Div. I-III)

Abstract

The Alps are a mountain range made of tectonic nappes and have long been a testing ground for revolutionary ideas in tectonics, such as the nappe theory. However, despite more than 100 years of research and an enormous amount of geological data the mechanisms by which nappes form are still one of the major unsolved problems in tectonics. To understand the mechanism of nappe formation, it is not enough to perform field work and data analysis, it is equally necessary to apply numerical models which relate the thermo-mechanical processes acting during nappe formation to the acquired data. This project studies the dynamics of nappe formation with numerical modeling and compares numerical results with available data from the Helvetic nappe system. In this project two ongoing PhD studies focusing on the formation of fold nappes in two (2D) and three (3D) dimensions will be finished and two new PhD studies will be started. The first new study applies a 2D thermo-mechanical numerical algorithm to investigate large-displacement overthrusting and nappe stacking. The stacking, or emplacement, of nappes in the Alps happened in an ordered succession such that the stacking order from top to bottom reflects the palaeogeographic position from south to north, respectively. However, the thermo-mechanical conditions (i.e. flow laws, geotherm, layer thickness etc.) necessary to generate such ordered nappe stacking have not been studied until now. The applied 2D algorithm is based on the finite element method, uses a viscoelastoplastic rheology, considers thermo-mechanical coupling, gravity and a free surface. The remeshing is done by a contour-line method that guarantees high numerical accuracy for the large strain deformation of layers with strongly varying strength representing the competent limestone units and the weak shale units in the Helvetic nappe system. The first sub-study will investigate systematically the necessary thermo-mechanical conditions for large-displacement overthrusting applying an initial weak zone in the strong layer from where overthrusting can initiate. The second sub-study will investigate the necessary conditions for the ordered stacking of several nappes. The third sub-study will provide a model configuration that can simulate the formation of the Morcles fold nappe together with the overthrusting Diablerets nappe and the deformation of the underlying basement. The second study will investigate the lateral transition from folding to overthrusting in 3D which is also characteristic for the Helvetic nappe system. The 3D algorithm is also based on the finite element method, considers viscoelastocplastic rheologies and gravity, and uses a remeshing strategy based on contour lines. The first sub-study will investigate the control of lateral variations in the thickness of a weak detachment horizon on the transition from folding (larger thickness of the detachment horizon) to overthrusting. The geometry, finite strain, strain rate and stress fields will be quantified and compared with available data from the Helvetic nappe system. The second sub-study investigates the impact of laterally varying strength in the strong layer on the transition between folding and overthrusting. The critical strength variation causing a transition from folding to overthrusting will be quantified. The results will be applied to available data for the Morcles nappe west of the Rhone valley because there different tectonic interpretations exist and the numerical results can be used to test these interpretations. The results of this project will improve our understanding of the dynamics of tectonic napes and of the tectonic evolution of the Alps. The project involves collaboration with other scientists of the Faculty of Geosciences and Environment at the University of Lausanne.
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