The Evolutionof the Alps

PROFESSOR STEFAN SCHMALHOLZ

The evolution of the Alps

Professor Stefan Schmalholz describes his latest geoscience research project focusing on the numerical simulation of fold nappe dynamics and tectonic evolution in the Western Swiss Alps

Could you discuss the overall aims of your equations. Only linear equations can be solved current project? What are the ways in which by a computer. The FEM is well established you propose to investigate how nappes form and our understanding of fold nappes can be with respect to physical processes? advanced by improving the system of PDEs (eg. including equations coupling heat transfer and The aim of our project is to better understand deformation), the model confi guration or the the thermomechanical processes that fl ow laws that describe rock deformation. generated fold nappes and the tectonic evolution of the Alps which consist of a large What advantages do numerical methods number of nappes. Nearly all models explaining have over more traditional ones used to the formation of fold nappes are geometric, study the dynamic processes acting during ie. they are not based on fundamental laws of fold nappe formation? physics and do not consider fl ow laws for rocks. Such fl ow laws are mathematical equations In the past, mostly laboratory experiments which describe the strength and deformation were used to study the formation of fold behaviour of rocks and have been derived from nappes. Their advantage is that a real laboratory experiments. deformation process can be investigated and that all experiments are 3D. However, with In our project, we build on the basic laboratory experiments it is diffi cult to scale confi guration of existing geometric models. gravity stresses correctly, to use materials the participating researchers with a background Our models are mathematically based in the experiments exhibiting a similar in mechanics and rock deformation, there was a on the principles of thermomechanics dependence of strength on temperature as consensus that signifi cant pressure deviations are and take into consideration existing fl ow rocks, to quantify accurately stresses and possible and feasible. In contrast, in the geological laws. We perform numerical simulations, temperatures, and to consider the change and mineralogical community signifi cant ie. we solve the mathematical equations in material behaviour from ductile to deviations are generally considered unrealistic. describing the formation of fold nappes on brittle when the rocks cool during tectonic modern computers. We also determine the deformation. Therefore, more and more Are there wider implications that these confi guration and model parameters that numerical simulations (eg. based on the FEM) results will have for the fi eld of physics and provide results which agree best with the are used to study the dynamic processes for society more generally? observed geology of fold nappes. In addition, during fold nappe formation. we want to understand which processes are Our mathematical models are often based on the dominant processes during fold nappe With a particular focus on controversial existing models of mechanics and we apply formation for generating a simple and topics, what were the objectives of the them to tectonic processes which have particular comprehensible mathematical model. GeoMod conference, organised and held by time, length and stress scales. Typical results of your group in July of this year? Were some our work are dimensionless parameters which How does the fi nite element method (FEM) surprising results shared? consist of a number of physical parameters (eg. simulate three-dimensional (3D) fl uid fl ow temperature) and which control the tectonic and the formation of 3D fold nappes? Is it One particular controversial topic discussed in process. In the context of physics then, our work accurate or could improvements be made? the conference was the magnitude of pressure shows how to scale a basic physical process to in the crust and lithosphere during mountain geological processes. The deformation of material bodies such as building. Most geologists assume that the rocks can be described within the theory of pressure at any depth is simply determined by A better understanding of how rocks continuum mechanics. Continuum mechanics the load of the overlying rocks and therefore it deformed during tectonic processes is is based on fundamental laws of physics such is easy to correlate pressure with depth. Some important for constructing infrastructure as mass and energy conservation and provides researchers think that pressures can locally (eg. streets, tunnels, etc.), for geological risk a system of partial differential equations deviate strongly from lithostatic pressure, assessments (eg. rock fall, landslides, etc.), (PDEs) which are usually nonlinear. The FEM is especially in tectonically-active regions such as energy recovery (eg. geothermal energy) or a method for transforming these PDEs to linear continent collision zones. I found that, among for detecting ore deposits.

WWW.RESEARCHMEDIA.EU 113 PROFESSOR STEFAN SCHMALHOLZ

The Morcles fold nappe

A new study of Alpine fold nappes aims to extend our knowledge of the geology of the Alps, which may have profound implications for resource exploitation, construction and security in the region

processes that generated the Alps are still comprehensible and depends on a single incompletely understood. The reason is that parameter for the strain gradient: the the geology of the Alps is very complicated parameter β. This parameter is based on the and the rocks are highly heterogeneous. In activation energy of the applied fl ow law, the THE GEOLOGY OF the Alps has been addition, geological processes take place on temperature at the base of the shear zone and studied for over 200 years. Therefore, there vastly different scales: from the microscale the temperature difference across it. Bauville is a plethora of knowledge and information to kilometre scale. Most models that already and Schmalholz’s model describes the strain including geological maps and cross sections as exist, which explain the fi rst-order features of evolution and the fi rst-order features of the well as seismic observations. But the tectonic the Morcles nappe, are only kinematic. They Morcles fold nappe such as its geometry, evolution of these mountains still remains do not take into consideration the principles kinematics and dynamics, and has been used something of a mystery. This knowledge is of physics and completely ignore the rheology. to fi nd the main cause for the nonlinear fi nite necessary for the subsequent exploitation of Thus, it remains unclear whether they are strain gradient, which turned out to be the resources, the facilitation of constructions physically plausible, whether they can be temperature increase towards the shear zone’s and for security reasons for those who live or applied to the thermomechanical conditions base and the decrease in viscosity. However, travel through these mountains. of the Morcles nappe or whether they can be other factors not considered in the model such used for other nappes in the world. as the grain size reduction or viscous heating The surface of the Earth is made of must have played a role in the evolution of the lithospheric plates which are distinguished in ONE-DIMENSIONAL MODEL shear zone, especially close to its base. two types: the oceanic and the continental. The oceanic plates sink into the Earth’s The study of the thermomechanics of ductile HIGHER-DIMENSIONAL SIMULATIONS interior whereas continental plates stay on shear zones that are common deformations in the surface. When continental plates collide, orogenic belts can shed light on the dynamics Another new project led by Schmalholz they are shortened and compressed while of mountain building. Kilometre-scale shear focuses on the dynamics of fold nappe the rock units of their upper parts break or zones in the crust are characterised by a formation and, in particular, the evolution develop folds and move on top of each other, nonlinear increase of fi nite strain at the forming so-called nappes. The Alps are made shear zone’s base similar to fold nappes. For of such nappes which are of two kinds: the this reason, it has been suggested that fold fold nappes and thrust sheets. A thrust sheet nappes are formed by heterogeneous simple is a rock unit with a sheet-like geometry that shear found in these shear zones. Although behaves like a solid body and shows only the geometry and kinematics of shear zones slight internal deformation. A fold nappe, on have been investigated in recent years, there the other hand, is characterised by signifi cant have not been any studies that could make internal deformation and stratigraphic the connection between observed fi nite strain inversion at its base and behaves like a gradients and the thermomechanical process viscous body. Fold nappes show constant of shearing. shearing direction and a nonlinear increase of shear strain towards their overturned Therefore, a new study by Arthur Bauville limb with a mylonitic zone at their base. and Professor Stefan Schmalholz covers this They appear to be formed by heterogeneous gap in knowledge by providing a simple one- simple shear in crustal shear zones. dimensional (1D) thermomechanical model to explain the fi nite strain gradient in kilometre- One fold nappe is the Morcles nappe in the scale shear zones of various mountain belts. Western Alps. Although nappes have been Their model takes into account the dislocation studied in detail, the mechanics by which creep fl ow law and temperature dependent FIGURE 1. 3D NUMERICAL RESULT OF A FOLD they are formed and the thermomechanical viscosity. Even though it is simplifi ed, it is NAPPE SIMULATION 114 INTERNATIONAL INNOVATION INTELLIGENCE TOWARDS UNDERSTANDING FOLD NAPPE DYNAMICS IN THE WESTERN SWISS ALPS OBJECTIVES • To identify the dominant process during fold nappe formation and the physical The dynamic models constructed in parameters that control this process the studies by Schmalholz and his • To develop a simple and comprehensible thermomechanical model for the Morcles collaborators can contribute to fold nappe a better understanding of the • To better understand the three- dimensional tectonic evolution of the formation of fold nappes which Morcles-Doldenhorn nappe system are not only found in the KEY COLLABORATORS Professor Jean-Luc Epard; Professor Yuri Alps but also in other Podladchikov, University of Lausanne, mountain belts Switzerland Professor Boris Kaus, University of Mainz, Germany PhD students: Arthur Bauville; Marina von Tscharner, University of Lausanne, of fold nappes in the Western Swiss Alps. Dynamic models work in accordance with Switzerland It is divided into two main studies which physical laws as opposed to geometric investigate and describe two processes: models, and offer less, though more plausible FUNDING fi rst, the strain formation of recumbent and answers about the tectonic evolution of a Swiss National Science Foundation, grant no. asymmetric folds with overturned limbs and geological structure. They also use proven 200021_131897 the parasitic fold in ductile multilayers; and concepts of continuum mechanics and fl ow second, the evolution of fi nite strain during laws for rocks. These models can identify University of Lausanne 3D fold nappe formation with an emphasis on both the mechanical process that dominates CONTACT fold-axis parallel extension as well as bending the fold nappe formation and the physical and rotation. parameters that control this process, and Professor Stefan Schmalholz therefore improve our understanding of the Project Coordinator In the fi rst study, performed by Bauville, dynamics of fold nappe formation. Faculty of Geosciences and Environment numerical simulations are used to observe the Institute of Earth Sciences impact that the initial geometry, boundary FUTURE RESEARCH ISTE conditions, rheology and material properties UNIL-Dorigny District have on the fold shapes, parasitic folds and the The dynamic models constructed in the Building Anthropole 3172 fold nappe geometry. The simulations yield studies by Schmalholz and his collaborators CH-1015 Lausanne the distribution of the fi nite strain ellipses and can contribute to a better understanding Switzerland the distribution of the shear around the folded of the formation of fold nappes which are layers, and these distributions are compared not only found in the Alps but also in other T +41 21 692 4302 with fi eld observations. mountain belts. In addition, the numerical E [email protected] algorithms can be extended for future The second study, performed by Marina von research and applied to orogenic dynamics in PROFESSOR STEFAN SCHMALHOLZ Tscharner, offers another numerical model to general and not just Alpine tectonics, and the received his PhD from ETH Zürich in 2000, explain the dynamics of ductile fold nappes 2D and 3D numerical models can be used to and then worked as a researcher at the in 3D and to determine the 3D deformation assess zones of intense deformation and for University of Oslo and ETH Zürich. He was and fi nite strain patterns. An improved risk analysis of constructions. They can also be appointed Full Professor of Tectonics and numerical algorithm tested by Schmalholz used in teaching and in the dissemination of Geodynamics at the University of Lausanne is used in the study. Numerical simulations research results. in 2010. His research focuses on applying model 3D ductile fold nappe formations, continuum mechanics and computational measure the evolution of 3D fi nite strain The models offer valuable information methods to study tectonic processes and the ellipsoids during folding and quantify the about the geometry, kinematic evolution, resulting deformation structures in rock. amount of fold-axis parallel extension, incremental and fi nite strain, foliation bending and rotation and the impact of orientation, temperature and pressure geometry, boundary conditions and material distributions that can eventually be properties on the fold axis deformation. compared with fi eld data. They can be combined with optimisation algorithms For the above studies, dynamic models are to determine the parameters that are applied based on numerical algorithms. in accordance with observations and, Field work and structural analyses are also ultimately, they can advance the study of the performed. The modelled and fi eld data rheology of rocks that form the continental are compared for the construction of valid lithosphere and explicate the formation of numerical models of fold nappe formation lithospheric deformation structures such that match the structural features of the as mountain ranges, continental rifts and natural fold nappes. sedimentary basins. WWW.RESEARCHMEDIA.EU 115