Mechanical twinning in quartz: Shock experiments, impact, pseudotachylites and fault breccias

a r t i c l e i n f o Increasing use of diffraction methods to study preferred orientation of minerals has established that quartz in deformed rocks not only displays characteristic c-axis orientation patterns, but that there is also generally a distinct difference in the orientation of positive and negative rhombs. In the trigonal quartz crystal structure positive and negative rhombs are structurally different, and particularly negative rhombs (e.g. {0111}) are much stiffer than positive rhombs (e.g. {1011}). Here, we focus on the role of mechanical Dauphiné twinning under stress as a cause of this difference and illustrate with EBSD measurements ubiquitous twinning in quartz-bearing rocks subjected to high stresses. Characteristic twinning is observed in experimentally shocked sandstones and stishovite-bearing quartzites from the Vredefort meteorite impact site in South Africa. Similar twinning is documented for quartz associated with pseudotachylites from the Santa Rosa mylonite zone in Southern California, whereas quartz in underlying ductile mylonites are more or less twin-free. It suggests that twinning was produced by local seismic stresses that caused fracture and frictional melting on fault surfaces. Quartz-bearing breccias from the SAFOD (San Andreas Fault Observatory at Depth) drilling project also show evidence of twinning and suggest high seismic stresses in the currently creeping segment of the San Andreas Fault at Parkfield. From these observations it appears that Dauphiné twin microstructures can be diagnostic of high local and transient stresses. It has long been known that quartz undergoes mechanical twinning when exposed to high stresses (Schubnikov, 1930; Schubnikov and Zinserling, 1932). The significance of these twins in deformed quartz aggregates was first investigated by Tullis (1970) and Tullis and Tullis (1972). Mechanical twins occur in many materials (e.g., Klassen-Neklyudova, 1964) but Dauphiné twins in quartz are rather special compared, for example, with classical twins in carbonates (e.g. The twin–host relationship for Dauphiné twins is a 180° rotation about the c-axis of trigonal quartz. On the atomic scale, it is achieved by a slight distortion of the structure (Fig. 1), without significant change in macroscopic shape of the quartz crystal. Twinning does not change the orientation of the c-axis or a-axes but reverses positive rhombs such as {1011} and negative rhombs {0111}. This is of profound mechanical importance, as directions normal to positive rhombs are half as stiff as those normal to negative rhombs (e.g., McSkimin et al., 1965). In a compression experiment with a quartz aggregate, crystals …