Mechanics of Deformation in the Eastern Alps

The kinematic evolution of the eastern Alpine orogen over the last 30 my is now largely known. It involves both lateral extrusion and extension of the orogen towards the east. This kinematic picture is beautifully documented by a range of structural and geomorphological observations and is largely confirmed by the geodetics community who are currently increasing the GPS network across the orogen. However, the interpretation of the mechanical controls of this kinematic evolution, as well as the linking of GPS information with the dynamic interpretations of the geologists is still in its early stages. In this project it is planned to integrate information from both schools to interpret the mechanical controls of alpine kinematics in the horizontal direction: It is planned to use the information obtained by structural geologists to assist the geodeticists positioning and interpretation of time series obtained from permanent GPS stations. Conversely, direct measurement of time series obtained by the geodeticists will be used to constrain the kinematic interpretations made by structural geologists. This general plan is divided into 2 specific goals: (i) The principle goal of the project is to perform plan view modeling of the mechanical evolution of the Alps on the scale of the orogen using as boundary conditions both the kinematic data obtained by structural geologist and GPS time series of the geodeticists. For this modeling it will also be necessary to remeasure GPS sites in key localities. (ii) The second goal of the project is to provide direct complementary work to the new GPS positioning involving detailed mapping of GPS sites and providing tectonic recommendations for new site selections. While the mechanics of the Alpine deformation is undoubtedly an inherently three-dimensional problem, mechanical modeling of the orogen on cross sections has been common, but plan-view studies as planned here are practically absent. Thus, the results of this project will provide a range of basic constraints crucial for a full 3-D understanding of the mechanical controls of the orogen.

This project set out to constrain the rheological properties of the European Alps, for example, how "hard" the mountain belt is as a whole, how "hard" or "soft" it is in comparison to the Adriatic - and the European plates and the magnitude of the forces that are were necessary to build up the mountain range. The idea for this project was based on the fact hat there is a rapidly growing body of GPS data from the European Alps that document the present deformation field of the range. These data give us detailed information on the movements of the Alps in vertical and lateral directions and they therefore provide a unique opportunity to model the mechanical properties. For the modelling we divided the central European region into 4 major sections: the European foreland, the Adriatic plate, the Pannonian basin and the Alps themselves. The GPS data constrain that the European foreland is quite stable and we assigned it to be 10 times as viscous ("hard") as the Alps. The Pannonian Basin is constrained by a corresponding logic to be somewhat less viscous than the Alps (80%). However, the viscosity contrast between the Alps and the Adriatic plate is not known and we used it as a variable. In both the Alps and the Adriatic plates there are significant mountains which seem to indicate that their hardness is similar, (for example, the Tauern range is in the Alps, but the Dolomites are on the Adriatic plate), but if they were, the width of the Alps would be much larger. Using a two dimensional finite element model, we were able to constrain that the Adriatic plate is in fact currently about as viscous as the Alps themselves, but that this is a fairly recent feature. Only 10 million years ago, it must have been at least twice as hard. In numbers, we showed that the viscosity of the Alps is about 10^23 Pas, (in comparison, cold honey has a viscosity of 4 Pas). Aside from these major findings, a series of minor studied paralleled the principle aims of the project. All results are published and can be found in the literature.