Shear Stresses during Continental Deformation - an Integrated Investigation of the Geological and Petrological Record in Metamorphic Rocks.

Continents behave on geological time scales like highly viscous fluids. The magnitude of this viscosity determines how high mountains can grow and how large the horizontal stresses are that they exert on their surroundings. Thus, the strength, viscosity and deformation rates of rocks are important parameters in plate tectonics and a number of research teams around the world work on determining their magnitude. Some groups measure these parameters by deforming rocks in the laboratory and some constrain them with structural geological field methods. In this project we have used the chemical composition and the shape of individual garnet crystals in metamorphic rocks to determine (a) the strength of the crystals and the surrounding rock and (b) the strain rate of rocks. For our study we have selected two of the most fundamental shear zones in the world: the Plattengneiss shear zone in the eastern Alps and the Redbank Thrust in Central Australia. The Plattengneiss is a flat lying shear zone along which thick packages were thrust from south to north to form the Alps. The Redbank Thrust in Central Australia is a vertical shear zone that can be followed seismically down to the Earths mantle. Aside from a number of side studies that all have resulted in publications in internationally accepted top journals, our study focused on 4 major aspects: 1. In the Plattengneiss we have sampled a north south profile from the town of Leoben to the Slovenian Maribor. By analysing the chemistry of the minerals in the samples we could show that the central part of the profile was formed at a depth of more than 60 kilometres while the depth of formation of rocks decreases continuously to the north and south from there. We also could show that the ratio of formation temperature to formation depth does not reflect a normal geothermal gradient and that - therefore - other heat sources, for example friction heat - must have played a role in the formation of the rocks. 2. A common observation in the Plattengneiss concerns the formation of dark mica crystals in pressure shadows behind feldspar and garnet crystals. We used numerical calculations to estimate how large pressure differences in deforming rocks can be and if they are large enough to cause crystal growth. We could show that such pressure variation can be of the order of 10^8 Pascal which is sufficient to cause this reaction. 3. On the Redbank thrust we have sampled a profile vertical to the shear zone and used a series of radiometric dating methods to determine the age of rocks on both sides of the shear zone. We were able to constrain a detailed history of motion along this structure. In particular we showed that the shear zone was at least 4 times active in the last 2000 million years of Earths history. 4. A very exiting study concerned the determination of strain rates of rocks using single crystals. We used spiral shaped garnet crystals that formed due to simultaneous growth and rotation of the crystals. The growth rate for the crystals was determined using trace element zonation and the rotation rate was determined using a numerical model. By combining the results of both calculations, we were able to show that the crystals formed during a deformation rate of 10^-14 s^-1 .