Original paper

Calcite microstructures as paleo-thermometer and indicator of shear zone width

Ebert, Andreas; Herwegh, Marco; Ramseyer, Karl; Decrouez, Danielle


In contrast to many previous studies, calcite microfabrics in carbonate mylonites from different large-scale shear zones yield astonishingly similar relationships between calcite grain size and deformation temperature. This consistency particularly results from a rigorous quantification of the effect of second-phase minerals on the matrix grain size and a correction of this effect. Although variations in strain rate, fluid flow, shear zone geometry, tectonic settings and deformation history between the observed natural shear zones exist, these parameters are closely linked with each other. In other words, for a given deformation temperature; stress, strain rate and shear zone width are adjusted to each other because of this coupling. The coupling is consistent for different geodynamic settings. Once calibrated with respect to the dynamically stabilized microfabric, predictions about changes of one of these parameters (e. g., temperature, shear zone width) is possible, simply based on the quantification of dynamically stabilized microfabrics within a shear zone. As an example, the calcite grain size can serve as a simple and stable geothermometer over a temperature range of200 - >700°C reflecting the temperature conditions of individual stages of shear zone evolution.