Rheology of eclogites: Comparison of HP-HT deformation experiments and nature

The rheology of rocks describes the complex pressure and temperature sensitive material behaviour, which controls if rocks deform by either frictional faulting, possibly through an earthquake, or by slow viscous creep. The rheology of rocks furthermore constrains if deformation tends to localize in narrow shear zones or if deformation is distributed over the whole deformed rock volume. Therefore, the rheological behaviour of rocks is one of the main controlling factors of deformation at plate boundaries and plate interiors. Besides pressure and temperature the rheological behaviour of rocks is largely controlled by the relative amount and spatial distribution of its minerals. Because the different minerals may have various direction depended different material properties, the rheological behaviour of most polyphase rocks is rather complex and largely unknown. In this project we will study the rheological behaviour of eclogites. Eclogites are rocks, which form under high-pressure at depths, greater than those typical of the crust of the Earth. They play an extremely important role in plate tectonics influencing the dynamics of continental and oceanic subduction zones and in driving convection within the solid Earth. Large eclogite bodies are commonly found in collision orogens like the Alps, Himalaya and Caledonides. Eclogites are mainly composed of sodium-rich pyroxene (omphacite) and garnet in varying fractions, where omphacite represents the rheologically weaker and garnet the stronger mineral. The rheology of such composite rocks can be studies directly by deformation tests in the laboratory with the challenge that temperature and pressures during the experiment must match the conditions of the formations of eclogites (i.e in the in excess of 1.2 GPa corresponding the 45 km depth at >550 C).During the experiments characteristic microstructures develop in the minerals in the deformed samples, which reflect the rheological behaviour of the composite rock sample. These microstructures can be studied and quantified by high-resolution analytical techniques like a scanning electron microscope equipped with several detectors. The major aim of the project is to quantitatively compare these synthetically generated microstructures, which formed under precisely known laboratory conditions, with naturally deformed eclogites from the Sau- and Koralpe in Austria in order to derive rheological key parameters, which are poorly known from the geological evolution of the natural examples. The results of the planned studies will help to better constrain the rheological behaviour of eclogites and their plate tectonic importance in localizing regional deformation.

Beside deformation-temperature and -rate, the flow behavior of rocks is largely controlled by the rock composition. In convergent zones, where tectonic plates collide and/or get subducted, eclogites form at high pressures. The formation, flow and deformation behavior of those high-pressure rocks therefore strongly influences the dynamic of the lithosphere. The only direct information we have of the deformation behavior of rocks in depth is based on their appearance. More specific by their microstructure. By applying high-resolution analysis, we can link the microfabric of a rock to its deformation behavior. In our project Rheology of Eclogite, we went one step further and performed high-pressure deformation experiments on synthesized eclogite. Experimental derived data provide information on the flow behavior of the rocks. Furthermore, by comparing microstructures of experimentally deformed eclogites to natural ones, we gain information on the flow behavior of the lithosphere in depth. Eclogites are predominantly composed of two minerals, omphacite and garnet, which can occur in varying fractions. Both minerals are known to be rather "strong" or "hard", meaning, they don't deform easily. Nevertheless, geologists observe frequently, that deformation localizes in these theoretically "strong" rocks. To unravel the discrepancy between theory and observation, we focused on the flow behavior of eclogite composed of varying volume fractions of garnet and omphacite. We further analyzed to what extend both minerals mutually influence their deformation behavior. In the course of the study, we showed that not only the flow behavior but also the deformation behavior of eclogite strongly varies with rock composition. The strength of eclogite increases with garnet content. To be more specific, we observe a log-linear relation between garnet-content and eclogite strength. A similar observation has previously been made for two-phase rocks. However, in our study we observe that eclogite becomes "weaker" after short period of steady state deformation. Such a mechanical weakening can potentially result in the localization of deformation in a small zone and influence large scale dynamic processes. We were able to document that mechanical weakening increases with increasing garnet content in eclogite. Further, could link the change in flow behavior to a switch in active deformation mechanism. While garnet-poor eclogite deforms in a continuously ductile manner, eclogites with a garnet fraction of 50% show onset of deformation localization in so called shear zones. Such shear zones are characterized by smaller grains of omphacite and garnet, and tend to deform easier and faster when compared to the surrounding rock. Eklogite with a garnet fraction of 75% deforms by the growth of discontinuous fractures. Our observation of omphacite-garnet interaction in the course of deformation, is most likely the key for to understand mechanical weakening in eclogite and the observation of deformation localization in the theoretically "hard" eclogite.