Abstract Serpentinite is widely assumed to constitute weak material in subduction zones and to play an essential role for the development of a subduction channel. Information on deformation mechanisms and appropriate rheological models to describe these large-scale flow processes can only be obtained from natural serpentinites exhumed from ancient subduction zones. We examine the microstructural record of HP-metamorphic ( P c. 2±0.5 GPa, T c. 550±50 °C) serpentinites exposed in the Zermatt–Saas zone, Western Alps, using optical and scanning electron microscopy with electron backscatter diffraction (EBSD). The schistose and compositionally layered rocks show pervasive small-scale folding. There is no evidence for any significant deformation by dislocation creep. Instead, the microfabrics including strain shadows and crenulation cleavage indicate that high strain is accumulated by dissolution precipitation creep. In terms of rheology, this suggests Newtonian behaviour and a low viscosity for the long-term flow of serpentinites in deeper levels of subduction zones. This does not preclude dislocation creep and a power law rheology at higher stress levels, as realized at local sites of stress concentration and transient episodes of post-seismic creep.

Shocked quartz from the Charlevoix impact structure has been investigated by optical and scanning electron microscopy, combined with electron backscatter diffraction techniques. The apparent shock pressure recorded by specific sets of planar deformation features (PDFs) in quartz shows a systematic variation with distance (0–10 km) from the center of the structure from ∼5–20 GPa. The occurrence of basal PDFs at distances of ∼2–10 km from the center of the structure indicates a high deviatoric stress component of the shock wave–associated stress tensor. Grain size effects and a greater mineralogical heterogeneity are proposed to be the main cause for slightly lower shock pressures recorded by PDFs in finer-grained granitic gneisses in the southeastern part of the structure, compared to coarse-grained charnockitic gneisses to the northwest at similar distances from the center of the structure. The influence of the crystallographic orientation of quartz on the orientation distribution of planar microstructures appears to superimpose an influence of the orientation of the impact-related stress field. Based on the appearance of Dauphiné twins that are associated with PDFs and the occurrence of PDFs with orientations that correspond to positive and negative rhombohedra, quartz is suspected to have locally been in the β-modification state. Dauphiné twinning is proposed to be mainly due to a reversion to α-quartz during cooling. These findings imply that the uplifted, preheated target rocks have locally been shock-heated to the α-β transition temperature.