Evidence for regional Dauphiné twinning in quartz from the Santa Rosa mylonite zone in Southern California. A neutron diffraction study

Preferred orientation in granitic mylonites from the Santa Rosa mylonite zone in Southern California is investigated with time-of-flight neutron diffraction. Quartz and biotite display strong preferred orientation, whereas, feldspar alignment is weak. For quartz, a c-axis maximum in the intermediate fabric direction is consistent with dynamic recrystallization. Pole figures for positive and negative rhombs, such as r={101¯1} and z={011¯1}, are distinct over a wide region, suggesting that the pattern reflects tectonic deformation and was not affected by local late-stage mechanical Dauphiné twinning. Indeed, modeling based on the elastic properties of quartz indicates that compression perpendicular to the foliation may have induced Dauphiné twinning and this mechanism may thus be useful as a paleo-piezometer to record tectonic stresses in quartz-bearing rocks.