5:15 PM - 7:15 PM
[SMP28-P09] Low-temperature ductile deformation of dolomite: example from the Eastern Alps
Keywords:dolomite, ductile deformation, The Eastern Alps
This study aims to clarify the conditions under which ductile deformation of dolomite rocks occurred during low-grade metamorphism in the Eastern Alps.
The microstructure of naturally deformed rocks resulting from high-strain ductile deformation is widely used to infer deformation conditions and mechanisms. These inferences are important to model the strength of the mid to lower crust. Monomineralic rocks such as dunite, quartzite, and calcite carbonate have been the focus of much research; such rocks represent relatively simple systems that can be compared to the results of laboratory-based rock deformation experiments. Carbonate rocks are a major component of passive continental margins and commonly form an important part of continental collisions zones. In addition to the commonly studied calcite-rich rocks (limestone), carbonate consisting dominantly of dolomite is also widespread, but has only received little attention. One reason for this is that observations of naturally deformed dolomite commonly indicate brittle deformation at moderate metamorphic conditions, and dolomite has not been thought of as important in causing strain localization generally represented by the formation of mylonites in the natural record. However, recent deformation experiments on dolomite suggest that, in natural settings, dolomite can deform at a temperature as low as 3000C and these results are supported by several recent studies of natural examples raising the possibility that dolomite mylonite may be more common than generally recognized and could be an additional important source of information on deformation conditions in orogenic belts.
To explore this possibility, samples of dolomite mylonite were collected from the Matrei Zone of the Eastern Alps and deformation microstructures were compared with the results from deformation experiments in previous studies. The Matrei Zone lies along a fundamental tectonic boundary between the Austroalpine domain representing the upper plate which has been thrust over a sequence of metamorphic rocks representing the Penninic oceanic domain. The Matrei Zone has undergone high-strain ductile deformation as shown by the development of kilometer-scale tight folds and metamorphic tectonites with strong planer fabrics and stretching lineations. The dominant rock units are metamorphic basement, quartz-schist, carbonate deposits dominated by dolomite, and a sequence of calcareous meta sediments with carbonaceous material and varying amounts of chlorite collectively referred to as Bündnerschiefer. The Matrei zone is generally assigned to the lower Austroalpine domain and its constituent rock types suggest it represents fragments of the former continental margin. Dolomite rocks generally show fracturing, boudinage and necking typical of dominantly brittle deformation. However, locally strongly foliated dolomite is also observed. These dolomite mylonite samples are generally associated with quartz schist layers. The deformation temperature was determined to be between 430 and 5500C from Raman carbonaceous material thermometry and quartz c-axis fabric thermometry, and by using the quartz piezometer, applied to closely adjacent samples of pelitic schist and quartzite, the differential stress was estimated to be 50–70 MPa. Observations of the dolomite microstructures revealed signs of bulging recrystallization and dislocation creep in grains with a size of around 100μm, and both grain-size-sensitive creep and dislocation creep in the smaller grains of around 10μm. Based on these results, it was concluded that bulging recrystallization reduced the grain size in dolomite to the point where grain-size-sensitive creep became activated, consistent with the results from deformation experiments. When the estimated grain size and differential stress are plotted on a deformation mechanism map at the appropriate temperature, the results suggest deformation in the observed dolomite mylonite occurred at the boundary between dislocation creep and grain size sensitive creep. This suggests that grain boundary migration had reached a balance between the competing processes driven by lattice strain energy and surface energy. The inferred strain rate is ~10-12/s. These findings suggest that monomineralic dolomite rock showing ductile deformation may be used to determine parameters such as strain rate and differential stress, which are essential for quantifying the mechanics of orogenic belts.
The microstructure of naturally deformed rocks resulting from high-strain ductile deformation is widely used to infer deformation conditions and mechanisms. These inferences are important to model the strength of the mid to lower crust. Monomineralic rocks such as dunite, quartzite, and calcite carbonate have been the focus of much research; such rocks represent relatively simple systems that can be compared to the results of laboratory-based rock deformation experiments. Carbonate rocks are a major component of passive continental margins and commonly form an important part of continental collisions zones. In addition to the commonly studied calcite-rich rocks (limestone), carbonate consisting dominantly of dolomite is also widespread, but has only received little attention. One reason for this is that observations of naturally deformed dolomite commonly indicate brittle deformation at moderate metamorphic conditions, and dolomite has not been thought of as important in causing strain localization generally represented by the formation of mylonites in the natural record. However, recent deformation experiments on dolomite suggest that, in natural settings, dolomite can deform at a temperature as low as 3000C and these results are supported by several recent studies of natural examples raising the possibility that dolomite mylonite may be more common than generally recognized and could be an additional important source of information on deformation conditions in orogenic belts.
To explore this possibility, samples of dolomite mylonite were collected from the Matrei Zone of the Eastern Alps and deformation microstructures were compared with the results from deformation experiments in previous studies. The Matrei Zone lies along a fundamental tectonic boundary between the Austroalpine domain representing the upper plate which has been thrust over a sequence of metamorphic rocks representing the Penninic oceanic domain. The Matrei Zone has undergone high-strain ductile deformation as shown by the development of kilometer-scale tight folds and metamorphic tectonites with strong planer fabrics and stretching lineations. The dominant rock units are metamorphic basement, quartz-schist, carbonate deposits dominated by dolomite, and a sequence of calcareous meta sediments with carbonaceous material and varying amounts of chlorite collectively referred to as Bündnerschiefer. The Matrei zone is generally assigned to the lower Austroalpine domain and its constituent rock types suggest it represents fragments of the former continental margin. Dolomite rocks generally show fracturing, boudinage and necking typical of dominantly brittle deformation. However, locally strongly foliated dolomite is also observed. These dolomite mylonite samples are generally associated with quartz schist layers. The deformation temperature was determined to be between 430 and 5500C from Raman carbonaceous material thermometry and quartz c-axis fabric thermometry, and by using the quartz piezometer, applied to closely adjacent samples of pelitic schist and quartzite, the differential stress was estimated to be 50–70 MPa. Observations of the dolomite microstructures revealed signs of bulging recrystallization and dislocation creep in grains with a size of around 100μm, and both grain-size-sensitive creep and dislocation creep in the smaller grains of around 10μm. Based on these results, it was concluded that bulging recrystallization reduced the grain size in dolomite to the point where grain-size-sensitive creep became activated, consistent with the results from deformation experiments. When the estimated grain size and differential stress are plotted on a deformation mechanism map at the appropriate temperature, the results suggest deformation in the observed dolomite mylonite occurred at the boundary between dislocation creep and grain size sensitive creep. This suggests that grain boundary migration had reached a balance between the competing processes driven by lattice strain energy and surface energy. The inferred strain rate is ~10-12/s. These findings suggest that monomineralic dolomite rock showing ductile deformation may be used to determine parameters such as strain rate and differential stress, which are essential for quantifying the mechanics of orogenic belts.