Rheology of fine-grained siliciclastic rocks in the middle crust-evidence from structural and numerical analysis

Constitutive models to describe the rheology of the crust are based on a combination of indirect methods, and many aspects are still unknown and controversial. Here we present a new method to quantify the rheology of fine-grained siliciclastic rocks, which are common in the middle crust and deform by solution-precipitation processes. We use a combination of structural analysis at different scales and a geomechanical model, to develop a parameter estimation scheme to calculate rheological parameters at 350-400 °C and geologic strain rates. The results of this study demonstrate that fine-grained siliciclastic rocks in the middle crust have a Newtonian viscous rheology, approximately 10 times weaker than wet quartz. This is in agreement with observed microstructures. Our results imply that the strength of polyphase quartz-rich rocks located in the middle crust is much lower than predicted by conventional models based on flow laws for dislocation creep. Because fine-grained siliciclastic rocks control the rheology of the middle crust in many sedimentary basins, our results provide new quantitative parameters for geodynamic modelling in settings where dissolution-precipitation creep is important.