Looking at Dauphiné twins in vein quartz from the High-Ardenne slate belt as a potential paleostress indicator

Paleostress studies commonly call upon (1) a fault slip data inversion technique, (2) a calcite twin stress inversion technique, (3) recrystallized grain size piezometry for quartz, or (4) direct measurements of residual lattice strain. Recent advances in orientation imaging microscopy (OIM) using electron backscatter diffraction (EBSD) on a scanning electron microscope (SEM) have revealed that Dauphiné twinning is very common in quartz in naturally deformed quartz-bearing rocks, in particular in low-grade tectonometamorphic conditions (~250-450°C) typical for the brittle-plastic transition zone at the base of the seismogenic crust (~7-15km). It has long been known that mechanical Dauphiné twinning in quartz can be stress-induced and thus potentially could be used as a (paleo-)stress indicator. To address this question, we performed an extensive EBSD-OIM analysis on well-studied quartz veins in the High-Ardenne slate belt (Belgium, Germany), which precipitated and deformed in low-grade metamorphic conditions during different stages of the orogeny. The vein quartz studied has only been weakly affected by low to moderate temperature (200 to 400°C) crystal-plastic deformation. The samples show grains with a high concentration of Dauphiné twin boundaries and others free of twin boundaries, thus being untwinned or completely twinned. This pattern depends on the crystallographic orientation. Twin boundaries are arrested by grain or subgrain boundaries, suggesting that Dauphiné twinning occurred on a pre-existing fabric that resulted from crystal-plastic deformation. An analysis of the orientation distribution of the rhombs in the twinned variant domains of individual quartz (sub-)grains reveals a particular preferred orientation of the poles to rhombs. Based on these results we will discuss the potential use of Dauphiné twins in vein quartz as a paleostress indicator, possibly completing our toolbox for reconstructing paleostresses in the deep crust.